Scott Fendorf

All Publications

• First Insight into the Mobilization and Sequestration of Arsenic in a Karstic Soil during Redox Changes. Environmental science & technology Wang, J., Shaheen, S. M., Swertz, A. C., Liu, C., Anderson, C. W., Fendorf, S., Wang, S. L., Feng, X., Rinklebe, J. 2024

  Abstract

  Karst terrains provide drinking water for about 25% of the people on our planet, particularly in the southwest of China. Pollutants such as arsenic (As) in the soil can infiltrate groundwater through sinkholes and bedrock fractures in karst terrains. Despite this, the underlying mechanisms responsible for As release from karst soils under redox changes remain largely unexplored. Here, we used multiple synchrotron-based spectroscopic analyses to explore As mobilization and sequestration in As-polluted karstic soil under biogeochemical conditions that mimic field-validated redox conditions. We observed that As in the soil exists primarily as As(V), which is mainly associated with Fe(oxyhydr)oxides. The concentration of the dissolved As was high (294 μM) and As(III) was dominant (∼95%) at low Eh (≤-100 mV), indicating the high risk of As leaching under reducing conditions. This As mobilization was attributed to the fact that the dissolution of ferrihydrite and calcite promoted the release and reduction of associated As(V). The concentration of the dissolved As was low (17.0 μM) and As(V) was dominant (∼68%) at high Eh (≥+100 mV), which might be due to the oxidation and/or sequestration of As(III) by the recrystallized ferric phase. Our results showed that the combined effects of the reductive release of As(V) from both ferric and nonferric phases, along with the recrystallization of the ferric phase, govern the redox-induced mobilization and potential leaching of As in soils within karst environments.

  View details for DOI 10.1021/acs.est.4c02989

  View details for PubMedID 39319747

• Seasonal Controls on Microbial Depolymerization and Oxidation of Organic Matter in Floodplain Soils. Environmental science & technology Anderson, C. G., Tfaily, M. M., Chu, R. K., Tolić, N., Fox, P. M., Nico, P. S., Fendorf, S., Keiluweit, M. 2024

  Abstract

  Floodplain soils are vast reservoirs of organic carbon often attributed to anaerobic conditions that impose metabolic constraints on organic matter degradation. What remains elusive is how such metabolic constraints respond to dynamic flooding and drainage cycles characteristic of floodplain soils. Here we show that microbial depolymerization and respiration of organic compounds, two rate-limiting steps in decomposition, vary spatially and temporally with seasonal flooding of mountainous floodplain soils (Gothic, Colorado, USA). Combining metabolomics and -proteomics, we found a lower abundance of oxidative enzymes during flooding coincided with the accumulation of aromatic, high-molecular weight compounds, particularly in surface soils. In subsurface soils, we found that a lower oxidation state of carbon coincided with a greater abundance of chemically reduced, energetically less favorable low-molecular weight metabolites, irrespective of flooding condition. Our results suggest that seasonal flooding temporarily constrains oxidative depolymerization of larger, potentially plant-derived compounds in surface soils; in contrast, energetic constraints on microbial respiration persist in more reducing subsurface soils regardless of flooding. Our work underscores that the potential vulnerability of these distinct anaerobic carbon storage mechanisms to changing flooding dynamics should be considered, particularly as climate change shifts both the frequency and extent of flooding in floodplains globally.

  View details for DOI 10.1021/acs.est.4c05109

  View details for PubMedID 39279153

• Microbial Proxies for Anoxic Microsites Vary with Management and Partially Explain Soil Carbon Concentration. Environmental science & technology Lacroix, E. M., Gomes, A., Heitmann, G. B., Schuler, D., Dekas, A. E., Liptzin, D., Aberle, E., Watts, D. B., Nelson, K. A., Culman, S., Fendorf, S. 2024

  Abstract

  Anoxic microsites are potentially important but unresolved contributors to soil organic carbon (C) storage. How anoxic microsites vary with soil management and the degree to which anoxic microsites contribute to soil C stabilization remain unknown. Sampling from four long-term agricultural experiments in the central United States, we examined how anoxic microsites varied with management (e.g., cultivation, tillage, and manure amendments) and whether anoxic microsites determine soil C concentration in surface (0-15 cm) soils. We used a novel approach to track anaerobe habitat space and, hence, anoxic microsites using DNA copies of anaerobic functional genes over a confined volume of soil. No-till practices inconsistently increased anoxic microsite extent compared to conventionally tilled soils, and within one site organic matter amendments increased anaerobe abundance in no-till soils. Across all long-term tillage trials, uncultivated soils had ∼2-4 times more copies of anaerobic functional genes than their cropland counterparts. Finally, anaerobe abundance was positively correlated to soil C concentration. Even when accounting for other soil C protection mechanisms, anaerobe abundance, our proxy for anoxic microsites, explained 41% of the variance and 5% of the unique variance in soil C concentration in cropland soils, making anoxic microsites the strongest management-responsive predictor of soil C concentration. Our results suggest that careful management of anoxic microsites may be a promising strategy to increase soil C storage within agricultural soils.

  View details for DOI 10.1021/acs.est.4c01882

  View details for PubMedID 38875507

• Molecular insights and impacts of wildfire-induced soil chemical changes NATURE REVIEWS EARTH & ENVIRONMENT Lopez, A., Avila, C. E., Vanderroest, J. P., Roth, H. K., Fendorf, S., Borch, T. 2024

  View details for DOI 10.1038/s43017-024-00548-8

  View details for Web of Science ID 001222400000001

• Water Supply Planning in the Face of Drought and Ecosystem Flows: Examining the Impact of the Bay-Delta Plan on Bay Area Water Supply. Environmental science & technology Gile, B. C., Sherris, A. R., Holmes, R. T., Fendorf, S., Luthy, R. G. 2024

  Abstract

  In California, recent Bay-Delta Plan legislation attempts to balance water supply and ecosystem protection by requiring 40% of the flow to remain in-stream in the Tuolumne River from February through June. Serious questions remain about what this means for the Bay Area water supply, especially during drought. Our work develops a new approach to analyze how in-stream flow policy coupled with climate change could impact regional water supply over the coming decades. Results show that the new in-stream flow demand would exceed urban water deliveries in a typical year. In wet years, water supply performance is minimally impacted, but in drought, the policy can lead to less water in storage, delayed reservoir recovery, and increased time at critically low storage. Storage impact exceeding 50 000 acre-feet (60 million m3) is anticipated with at least 18% frequency, demonstrating that, climate uncertainty notwithstanding, this impact must be planned for and managed to ensure a reliable future water supply.

  View details for DOI 10.1021/acs.est.3c07398

  View details for PubMedID 38390867

• X-ray chemical imaging for assessing redox microsites within soils and sediments FRONTIERS IN ENVIRONMENTAL CHEMISTRY Noel, V., Boye, K., Naughton, H. R., Lacroix, E. M., Aeppli, M., Kumar, N., Fendorf, S., Webb, S. M. 2024; 5

  View details for DOI 10.3389/fenvc.2024.1329887

  View details for Web of Science ID 001238057600001

• Mid-season drain severity impacts on rice yields, greenhouse gas emissions and heavy metal uptake in grain: evidence from on-farm studies FIELD CROPS RESEARCH Perry, H., Carrijo, D. R., Duncan, A. H., Fendorf, S., Linquist, B. A. 2024; 307

  View details for DOI 10.1016/j.fcr.2024.109248

  View details for Web of Science ID 001165460100001

• Metal toxin threat in wildland fires determined by geology and fire severity. Nature communications Lopez, A. M., Pacheco, J. L., Fendorf, S. 2023; 14 (1): 8007

  Abstract

  Accentuated by climate change, catastrophic wildfires are a growing, distributed global public health risk from inhalation of smoke and dust. Underrecognized, however, are the health threats arising from fire-altered toxic metals natural to soils and plants. Here, we demonstrate that high temperatures during California wildfires catalyzed widespread transformation of chromium to its carcinogenic form in soil and ash, as hexavalent chromium, particularly in areas with metal-rich geologies (e.g., serpentinite). In wildfire ash, we observed dangerous levels (327-13,100 µg kg-1) of reactive hexavalent chromium in wind-dispersible particulates. Relatively dry post-fire weather contributed to the persistence of elevated hexavalent chromium in surficial soil layers for up to ten months post-fire. The geographic distribution of metal-rich soils and fire incidents illustrate the broad global threat of wildfire smoke- and dust-born metals to populations. Our findings provide new insights into why wildfire smoke exposure appears to be more hazardous to humans than pollution from other sources.

  View details for DOI 10.1038/s41467-023-43101-9

  View details for PubMedID 38086795

  View details for PubMedCentralID 7812759

• Consider the Anoxic Microsite: Acknowledging and Appreciating Spatiotemporal Redox Heterogeneity in Soils and Sediments ACS EARTH AND SPACE CHEMISTRY Lacroix, E. M., Aeppli, M., Boye, K., Brodie, E., Fendorf, S., Keiluweit, M., Naughton, H. R., Noel, V., Sihi, D. 2023: 1592-1609

  Abstract

  Reduction-oxidation (redox) reactions underlie essentially all biogeochemical cycles. Like most soil properties and processes, redox is spatiotemporally heterogeneous. However, unlike other soil features, redox heterogeneity has yet to be incorporated into mainstream conceptualizations of soil biogeochemistry. Anoxic microsites, the defining feature of redox heterogeneity in bulk oxic soils and sediments, are zones of oxygen depletion in otherwise oxic environments. In this review, we suggest that anoxic microsites represent a critical component of soil function and that appreciating anoxic microsites promises to advance our understanding of soil and sediment biogeochemistry. In sections 1 and 2, we define anoxic microsites and highlight their dynamic properties, specifically anoxic microsite distribution, redox gradient magnitude, and temporality. In section 3, we describe the influence of anoxic microsites on several key elemental cycles, organic carbon, nitrogen, iron, manganese, and sulfur. In section 4, we evaluate methods for identifying and characterizing anoxic microsites, and in section 5, we highlight past and current approaches to modeling anoxic microsites. Finally, in section 6, we suggest steps for incorporating anoxic microsites and redox heterogeneities more broadly into our understanding of soils and sediments.

  View details for DOI 10.1021/acsearthspacechem.3c00032

  View details for Web of Science ID 001063451500001

  View details for PubMedID 37753209

  View details for PubMedCentralID PMC10519444

• Molecular Nature of Mineral-Organic Associations within Redox-Active Mountainous Floodplain Sediments ACS EARTH AND SPACE CHEMISTRY Anderson, C. G., Goebel, G. M., Tfaily, M. M., Fox, P. M., Nico, P. S., Fendorf, S., Keiluweit, M. 2023

  View details for DOI 10.1021/acsearthspacechem.3c00037

  View details for Web of Science ID 001052106200001

• Quantitative Separation of Unknown Organic-Metal Complexes by Liquid Chromatography-Inductively Coupled Plasma-Mass Spectrometry. Analytical chemistry Dewey, C., Kaplan, D. I., Fendorf, S., Boiteau, R. M. 2023

  Abstract

  Dissolved organic matter (DOM) is widely recognized to control the solubility and reactivity of trace metals in the environment. However, the mechanisms that govern metal-DOM complexation remain elusive, primarily due to the analytical challenge of fractionating and quantifying metal-organic species within the complex mixture of organic compounds that comprise DOM. Here, we describe a quantitative method for fractionation and element-specific detection of organic-metal complexes using liquid chromatography with online inductively coupled plasma mass spectrometry (LC-ICP-MS). The method implements a post-column compensation gradient to stabilize ICP-MS elemental response across the LC solvent gradient, thereby overcoming a major barrier to achieving quantitative accuracy with LC-ICP-MS. With external calibration and internal standard correction, the method yields concentrations of organic-metal complexes that were consistently within 6% of their true values, regardless of the complex's elution time. We used the method to evaluate the effects of four stationary phases (C18, phenyl, amide, and pentafluoroylphenyl propyl) on the recovery and separation of environmentally relevant trace metals (Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb) in Suwannee River Fulvic Acid and Suwannee River Natural Organic Matter. The C18, amide, and phenyl phases generally yielded optimal metal recoveries (>75% for all metals except Pb), with the phenyl phase separating polar species to a greater extent than C18 or amide. We also fractionated organic-bound Fe, Cu, and Ni in oxidized and reduced soils, revealing divergent metal-DOM speciation across soil redox environments. By enabling quantitative fractionation of DOM-bound metals, our method offers a means for advancing a mechanistic understanding of metal-organic complexation throughout the environment.

  View details for DOI 10.1021/acs.analchem.3c00696

  View details for PubMedID 37163723

• Uranium surface processes with sandstone and volcanic rocks in acidic and alkaline solutions. Journal of colloid and interface science Kenney, J. P., Lezama-Pacheco, J., Fendorf, S., Alessi, D. S., Weiss, D. J. 2023; 645: 715-723

  Abstract

  Understanding the behaviour of uranium waste, for disposal purposes, is crucial due to the correlation between pH values and the disposal of distinct types of waste, with low level waste typically associated with acidic pH values, and higher and intermediate level waste commonly related to alkaline pH values. We studied the adsorption of U(VI) on sandstone and volcanic rock surfaces at pH 5.5 and 11.5 in aqueous solutions with and without bicarbonate (2 mM HCO3-) using XAS and FTIR. In the sandstone system, U(VI) adsorbs as a bidentate complex to Si at pH 5.5 without bicarbonate and as uranyl carbonate species with bicarbonate. At pH 11.5 without bicarbonate, U(VI) adsorbs as monodentate complexes to Si and precipitates as uranophane. With bicarbonate at pH 11.5, U(VI) precipitated as a Na-clarkeite mineral or remained as a uranyl carbonate surface species. In the volcanic rock system, U(VI) adsorbed to Si as an outer sphere complex at pH 5.5, regardless of the presence of bicarbonate. At pH 11.5 without bicarbonate, U(VI) adsorbed as a monodentate complex to one Si atom and precipitated as a Na-clarkeite mineral. With bicarbonate at pH 11.5, U(VI) sorbed as a bidentate carbonate complex to one Si atom. These results provide insight into the behaviour of U(VI) in heterogeneous, real-world systems related to the disposal of radioactive waste.

  View details for DOI 10.1016/j.jcis.2023.04.174

  View details for PubMedID 37172481

• Seasonal Oxygenation of Contaminated Floodplain Soil Releases Zn to Porewater. Environmental science & technology Dewey, C., Juillot, F., Fendorf, S., Bargar, J. R. 2023

  Abstract

  Soil zinc contamination is a major threat to water quality and sensitive ecosystems. While Zn itself is not redox-active in soils, transitions in soil redox conditions may promote mobilization of Zn from common Zn hosts, including Mn(IV)/Fe(III)-(hydr)oxides and sulfide precipitates, leading to elevated concentrations of dissolved Zn in surface and groundwater and thus a potential increase in Zn transport and uptake. Here, we examined the impacts of hydrologic fluctuations and coupled redox transitions on Zn partitioning in contaminated riparian soil in a mountain watershed. We found that oxygenation of the soil profile during low water conditions caused a spike in porewater Zn concentrations, driven by oxidative dissolution of amorphous ZnS and weak partitioning of Zn to Fe(III)-(hydr)oxides, hydroxy-interlayer vermiculite, and vermiculite. In contrast to Pb, released Zn did not immediately adsorb to Fe(III)-(hydr)oxides or particulate organic matter due to less-favorable sorption of Zn than that of Pb and, further, decreased Zn sorption at slightly acidic pH. As aridification intensifies and groundwater levels decline throughout the western United States, contaminated floodplain soils in mountain watersheds may be frequently oxygenated, leading to increased mobilization of dissolved Zn, which will amplify the threat Zn poses to water quality and ecosystem health.

  View details for DOI 10.1021/acs.est.2c08764

  View details for PubMedID 36917499

• Reactive iron, not fungal community, drives organic carbon oxidation potential in floodplain soils SOIL BIOLOGY & BIOCHEMISTRY Naughton, H. R., Tolar, B. B., Dewey, C., Keiluweit, M., Nico, P. S., Fendorf, S. 2023; 178

  View details for DOI 10.1016/j.soilbio.2023.108962

  View details for Web of Science ID 000926685100001

• Iron Reduction in Profundal Sediments of Ultraoligotrophic Lake Tahoe under Oxygen-Limited Conditions. Environmental science & technology Aeppli, M., Schladow, G., Lezama Pacheco, J. S., Fendorf, S. 2023

  Abstract

  Increased periods of bottom water anoxia in deep temperate lakes due to decreasing frequency and depth of water column mixing in a warming climate may result in the reductive dissolution of iron minerals and increased flux of nutrients from the sediment into the water column. Here, we assessed the sediment properties and reactivities under depleted oxygen concentrations of Lake Tahoe, a deep ultraoligotrophic lake in the Sierra Nevada mountain range. Using whole-core incubation experiments, we found that a decrease in dissolved oxygen concentration in the top 2 cm of the sediment resulted in an extension of the microbial iron reduction zone from below 4.5 to below 1.5 cm depth. Concentrations of reactive iron generally decreased with sediment depth, and microbial iron reduction seemingly ceased as concentrations of Fe(II) approximated concentrations of reactive iron. These findings suggest that microorganisms preferentially utilized reactive iron and/or iron minerals became less reactive due to mineral transformation and surface passivation. The estimated release of iron mineral-associated phosphorus is not expected to change Lake Tahoe's trophic state but will likely contribute to increased phytoplankton productivity if mixed into surface waters.

  View details for DOI 10.1021/acs.est.2c05714

  View details for PubMedID 36633549

• Beaver dams overshadow climate extremes in controlling riparian hydrology and water quality. Nature communications Dewey, C., Fox, P. M., Bouskill, N. J., Dwivedi, D., Nico, P., Fendorf, S. 2022; 13 (1): 6509

  Abstract

  Hydrologic extremes dominate chemical exports from riparian zones and dictate water quality in major river systems. Yet, changes in land use and ecosystem services alongside growing climate variability are altering hydrologic extremes and their coupled impacts on riverine water quality. In the western U.S., warming temperatures and intensified aridification are increasingly paired with the expanding range of the American beaver-and their dams, which transform hydrologic and biogeochemical cycles in riparian systems. Here, we show that beaver dams overshadow climatic hydrologic extremes in their effects on water residence time and oxygen and nitrogen fluxes in the riparian subsurface. In a mountainous watershed in Colorado, U.S.A., we find that the increase in riparian hydraulic gradients imposed by a beaver dam is 10.7-13.3 times greater than seasonal hydrologic extremes. The massive hydraulic gradient increases hyporheic nitrate removal by 44.2% relative to seasonal extremes alone. A drier, hotter climate in the western U.S. will further expand the range of beavers and magnify their impacts on watershed hydrology and biogeochemistry, illustrating that ecosystem feedbacks to climate change will alter water quality in river systems.

  View details for DOI 10.1038/s41467-022-34022-0

  View details for PubMedID 36347847

• Assessing Analytical Methods for the Rapid Detection of Lead Adulteration in the Global Spice Market. Environmental science & technology Lopez, A. M., Nicolini, C. M., Aeppli, M., Luby, S. P., Fendorf, S., Forsyth, J. E. 2022

  Abstract

  Lead adulteration of spices, primarily via Pb chromate compounds, has been documented globally as a growing public health concern. Currently, Pb detection in spices relies primarily on expensive and time-consuming laboratory analyses. Advancing rapid Pb detection methods, inclusive of their accuracy and precision, would improve field assessments by food safety inspectors, stakeholders, and the public in the hope of reducing Pb exposure risks at its source. Here, we present two field procedures for Pb detection: portable X-ray fluorescence analysis (pXRF) and a simple colorimetric test. We assess their efficacy to detect Pb and its chemical form in seven spice types, including powders, spice-salt mixtures, and dried roots, compared to the proven laboratory technique, inductively coupled plasma mass spectrometry (ICP-MS). Lead concentrations measured using pXRF and ICP-MS were within 5% of each other for spice powders and 24% for dried roots. By pXRF, spice samples were analyzed within collection plastic bags without preparation, resulting in a detection limit of 2 mg Pb/kg for spice powders, which is comparable to national food standards. The colorimetric test utilized here targets hexavalent chromium, making the method selective to Pb chromate adulteration assuming that this is its dominant source in spices. Color development, and thus detection, was observed when Pb concentrations exceeded approximately 5-70 mg/kg in dried turmeric roots and 1000 mg/kg in spice powders; however, it was ineffective for the spice-salt mixture. We show that pXRF analysis and a colorimetric assay provide information that may improve field decisions about Pb adulteration in a range of spice types, helping to minimize Pb exposure.

  View details for DOI 10.1021/acs.est.2c03241

  View details for PubMedID 36343212

• Redox Properties of Solid Phase Electron Acceptors Affect Anaerobic Microbial Respiration under Oxygen-Limited Conditions in Floodplain Soils. Environmental science & technology Aeppli, M., Thompson, A., Dewey, C., Fendorf, S. 2022

  Abstract

  Mountain floodplain soils often show spatiotemporal variations in redox conditions that arise due to changing hydrology and resulting biogeochemistry. Under oxygen-depleted conditions, solid phase terminal electron acceptors (TEAs) can be used in anaerobic respiration. However, it remains unclear to what degree the redox properties of solid phases limit respiration rates and hence organic matter degradation. Here, we assess such limitations in soils collected across a gradient in native redox states from the Slate River floodplain (Colorado, U.S.A.). We incubated soils under anoxic conditions and quantified CO2 production and microbial Fe(III) reduction, the main microbial metabolic pathway, as well as the reactivity of whole-soil solid phase TEAs toward mediated electrochemical reduction. Fe(III) reduction occurred together with CO2 production in native oxic soils, while neither Fe(II) nor CO2 production was observed in native anoxic soils. Initial CO2 production rates increased with increasing TEA redox reactivity toward mediated electrochemical reduction across all soil depths. Low TEA redox reactivity appears to be caused by elevated Fe(II) concentrations rather than crystallinity of Fe(III) phases. Our findings illustrate that the buildup of Fe(II) in systems with long residence times limits the thermodynamic viability of dissimilatory Fe(III) reduction and thereby limits the mineralization of organic carbon.

  View details for DOI 10.1021/acs.est.2c05797

  View details for PubMedID 36342198

• Contributions of anoxic microsites to soil carbon protection across soil textures GEODERMA Lacroix, E. M., Mendillo, J., Gomes, A., Dekas, A., Fendorf, S. 2022; 425

  View details for DOI 10.1016/j.geoderma.2022.116050

  View details for Web of Science ID 000847217000011

• Nitrate Controls on the Extent and Type of Metal Retention in Fine-Grained Sediments of a Simulated Aquifer. Environmental science & technology Engel, M., Noel, V., Kukkadapu, R. K., Boye, K., Bargar, J. R., Fendorf, S. 2022

  Abstract

  Aquifer groundwater quality is largely controlled by sediment composition and physical heterogeneity, which commonly sustains a unique redox gradient pattern. Attenuation of heavy metals within these heterogeneous aquifers is reliant on multiple factors, including redox conditions and redox-active species that can further influence biogeochemical cycling. Here, we simulated an alluvial aquifer system using columns filled with natural coarse-grained sediments and two domains of fine-grained sediment lenses. Our goal was to examine heavy metal (Ni and Zn) attenuation within a complex aquifer network and further explore nitrate-rich groundwater conditions. The fine-grained sediment lenses sustained reducing conditions and served as a sink for Ni sequestration─in the form of Ni-silicates, Ni-organic matter, and a dominant Ni-sulfide phase. The silicate clay and sulfide pools were also important retention mechanisms for Zn; however, Ni was associated more extensively with organic matter compared to Zn, which formed layered double hydroxides. Nitrate-rich conditions promoted denitrification within the lenses that was coupled to the oxidation of Fe(II) and the concomitant precipitation of an Fe(III) phase with higher structural distortion. A decreased metal sulfide pool also resulted, where nitrate-rich conditions generated an average 20% decrease in solid-phase Ni, Zn, and Fe. Ultimately, nitrate plays a significant role in the aquifer's biogeochemical cycling and the capacity to retain heavy metals.

  View details for DOI 10.1021/acs.est.2c03403

  View details for PubMedID 36206030

• Field science in the age of online learning: Dynamic instruction of techniques to assess soil physical properties FRONTIERS IN EDUCATION Hinckley, E. S., Fendorf, S. 2022; 7

  View details for DOI 10.3389/feduc.2022.959776

  View details for Web of Science ID 000877102900001

• Mechanism of Arsenic Partitioning During Sulfidation of As-Sorbed Ferrihydrite Nanoparticles. ACS earth & space chemistry Kumar, N., Noël, V., Besold, J., Planer-Friedrich, B., Boye, K., Fendorf, S., Brown, G. E. 2022; 6 (7): 1666-1673

  Abstract

  Knowledge of how arsenic (As) partitions among various phases in Fe-rich sulfidic environments is critical for understanding the fate and mobility of As in such environments. We studied the reaction of arsenite and arsenate sorbed on ferrihydrite nanoparticle surfaces with dissolved sulfide at varying S/Fe ratios (0.1-2.0) to understand the fate and transformation mechanism of As during sulfidation of ferrihydrite. By using aqueous As speciation analysis by IC-ICP-MS and solid-phase As speciation analysis by synchrotron-based X-ray absorption spectroscopy (XAS), we were able to discern the mechanism and pathways of As partitioning and thio-arsenic species formation. Our results provide a mechanistic understanding of the fate and transformation of arsenic during the codiagenesis of As, Fe, and S in reducing environments. Our aqueous-phase As speciation data, combined with solid-phase speciation data, indicate that sulfidation of As-sorbed ferrihydrite nanoparticles results in their transformation to trithioarsenate and arsenite, independent of the initial arsenic species used. The nature and extent of transformation and the thioarsenate species formed were controlled by S/Fe ratios in our experiments. However, arsenate was reduced to arsenite before transformation to trithioarsenate.

  View details for DOI 10.1021/acsearthspacechem.1c00373

  View details for PubMedID 35903782

  View details for PubMedCentralID PMC9310089

• Mechanism of Arsenic Partitioning During Sulfidation of As-Sorbed Ferrihydrite Nanoparticles ACS EARTH AND SPACE CHEMISTRY Kumar, N., Noel, V., Besold, J., Planer-Friedrich, B., Boye, K., Fendorf, S., Brown Jr, G. E. 2022

  View details for DOI 10.1021/acsearthspacechem.1c00373

  View details for Web of Science ID 000826418100001

• Sulfur Biogeochemical Cycling and Redox Dynamics in a Shale-Dominated Mountainous Watershed JOURNAL OF GEOPHYSICAL RESEARCH-BIOGEOSCIENCES Fox, P. M., Carrero, S., Anderson, C., Dewey, C., Keiluweit, M., Conrad, M., Naughton, H. R., Fendorf, S., Carroll, R., Dafflon, B., Malenda-Lawrence, H., Dwivedi, D., Gilbert, B., Christensen, J. N., Boye, K., Beutler, C., Brown, W., Newman, A., Versteeg, R., Williams, K. H., Nico, P. S. 2022; 127 (6)

  View details for DOI 10.1029/2021JG006769

  View details for Web of Science ID 000812194100001

• Mineral Protection and Resource Limitations Combine to Explain Profile-Scale Soil Carbon Persistence JOURNAL OF GEOPHYSICAL RESEARCH-BIOGEOSCIENCES Lacroix, E. M., Masue-Slowey, Y., Dlott, G. A., Keiluweit, M., Chadwick, O. A., Fendorf, S. 2022; 127 (4)

  View details for DOI 10.1029/2021JG006674

  View details for Web of Science ID 000783134900001

• Residual As(V) in Aqueous Solutions After Its Removal by Synthetic Minerals WATER AIR AND SOIL POLLUTION Dias, A., Ferreira Fontes, M., Ferreira, M., Vergutz, L., Fendorf, S. 2022; 233 (4)

  View details for DOI 10.1007/s11270-022-05576-y

  View details for Web of Science ID 000774004600004

• Export of Organic Carbon from Reduced Fine-Grained Zones Governs Biogeochemical Reactivity in a Simulated Aquifer. Environmental science & technology Aeppli, M., Babey, T., Engel, M., Lacroix, E. M., Tolar, B. B., Fendorf, S., Bargar, J. R., Boye, K. 1800

  Abstract

  Sediment interfaces in alluvial aquifers have a disproportionately large influence on biogeochemical activity and, therefore, on groundwater quality. Previous work showed that exports from fine-grained, organic-rich zones sustain reducing conditions in downstream coarse-grained aquifers beyond the influence of reduced aqueous products alone. Here, we show that sustained anaerobic activity can be attributed to the export of organic carbon, including live microorganisms, from fine-grained zones. We used a dual-domain column system with ferrihydrite-coated sand and embedded reduced, fine-grained lenses from Slate River (Crested Butte, CO) and Wind River (Riverton, WY) floodplains. After 50 d of groundwater flow, 8.8 ± 0.7% and 14.8 ± 3.1% of the total organic carbon exported from the Slate and Wind River lenses, respectively, had accumulated in the sand downstream. Furthermore, higher concentrations of dissolved Fe(II) and lower concentrations of dissolved organic carbon in the sand compared to total aqueous transport from the lenses suggest that Fe(II) was produced in situ by microbial oxidation of organic carbon coupled to iron reduction. This was further supported by an elevated abundance of 16S rRNA and iron-reducing (gltA) gene copies. These findings suggest that organic carbon transport across interfaces contributes to downstream biogeochemical reactions in natural alluvial aquifers.

  View details for DOI 10.1021/acs.est.1c04664

  View details for PubMedID 35072465

• Perchlorate and Agriculture on Mars SOIL SYSTEMS Oze, C., Beisel, J., Dabsys, E., Dall, J., North, G., Scott, A., Lopez, A., Holmes, R., Fendorf, S. 2021; 5 (3)

  View details for DOI 10.3390/soilsystems5030037

  View details for Web of Science ID 000702098200001

• Nitrate in Drinking Water during Pregnancy and Spontaneous Preterm Birth: A Retrospective Within-Mother Analysis in California. Environmental health perspectives Sherris, A. R., Baiocchi, M., Fendorf, S., Luby, S. P., Yang, W., Shaw, G. M. 2021; 129 (5): 57001

  Abstract

  BACKGROUND: Nitrate is a widespread groundwater contaminant and a leading cause of drinking water quality violations in California. Associations between nitrate exposure and select adverse birth outcomes have been suggested, but few studies have examined gestational exposures to nitrate and risk of preterm birth (before 37 wk gestation).OBJECTIVE: We investigated the association between elevated nitrate in drinking water and spontaneous preterm birth through a within-mother retrospective cohort study of births in California.METHODS: We acquired over 6 million birth certificate records linked with Office of Statewide Health Planning and Development hospital discharge data for California births from 2000-2011. We used public water system monitoring records to estimate nitrate concentrations in drinking water for each woman's residence during gestation. After exclusions, we constructed a sample of 1,443,318 consecutive sibling births in order to conduct a within-mother analysis. We used separate conditional logistic regression models to estimate the odds of preterm birth at 20-31 and 32-36 wk, respectively, among women whose nitrate exposure changed between consecutive pregnancies.RESULTS: Spontaneous preterm birth at 20-31 wk was increased in association with tap water nitrate concentrations during pregnancy of 5 to <10mg/L [odds ratio (OR)=1.47; 95% confidence interval (CI): 1.29, 1.67] and ≥10mg/L (OR=2.52; 95% CI: 1.49, 4.26) compared with <5mg/L (as nitrogen). Corresponding estimates for spontaneous preterm birth at 32-36 wk were positive but close to the null for 5 to <10mg/L nitrate (OR=1.08; 95% CI: 1.02, 1.15) and for ≥10mg/L nitrate (OR=1.05; 95% CI: 0.85, 1.31) vs. <5mg/L nitrate. Our findings were similar in several secondary and sensitivity analyses, including in a conventional individual-level design.DISCUSSION: The results suggest that nitrate in drinking water is associated with increased odds of spontaneous preterm birth. Notably, we estimated modestly increased odds associated with tap water nitrate concentrations of 5 to <10mg/L (below the federal drinking water standard of 10mg/L) relative to <5mg/L. https://doi.org/10.1289/EHP8205.

  View details for DOI 10.1289/EHP8205

  View details for PubMedID 33949893

• Porewater Lead Concentrations Limited by Particulate Organic Matter Coupled With Ephemeral Iron(III) and Sulfide Phases during Redox Cycles Within Contaminated Floodplain Soils. Environmental science & technology Dewey, C., Bargar, J. R., Fendorf, S. 2021

  Abstract

  Lead contamination in soils and sediments is a major threat to water quality. In surface and near-surface environments, Pb is not redox active; however, common Pb hosts, including Fe(III)-(hydr)oxides and sulfides, dissolve and precipitate as redox conditions change. Dissolution of Pb hosts may release Pb to porewater, leading to spikes in dissolved Pb concentrations and potential transport into surface or groundwater. Here, we examine the impacts of hydrologically coupled redox transitions on Pb partitioning in contaminated floodplain soils. We find that the affinity of Pb for particulate organic matter (POM), inclusive of mineral-associated organic matter, ensures that across redox transitions Pb is retained in the solid phase, despite host-phase (Fe(III)-(hydr)oxide and sulfide) dissolution. As seasonal hydrologic dynamics shift porewater redox conditions, Pb-bearing Fe(III)-(hydr)oxides (Pb-HFO) and sulfides (PbS) are dissolved and (re)precipitated. However, despite these shifts in redox conditions and associated host-phase transformations, Pb retention on POM, coupled with the formation of PbS and Pb-HFO, maintains dissolved Pb concentrations below 17 mug L-1. Importantly, the predominance of Pb adsorbed on POM alongside low dissolved Pb concentrations indicates that Pb released from HFO and PbS is retained by POM. Thus, despite host-phase dissolution during redox transitions, partitioning of Pb to the aqueous phase is minimal and, therefore, transport of dissolved Pb is unlikely.

  View details for DOI 10.1021/acs.est.0c08162

  View details for PubMedID 33899483

• The effect of porewater ionic composition on arsenate adsorption to clay minerals. The Science of the total environment Fakhreddine, S., Fendorf, S. 2021; 785: 147096

  Abstract

  Adsorption of arsenate on clay minerals can control the partitioning and mobility of arsenic and subsequent contamination of groundwater. While the effect of ionic strength on arsenic adsorption to phyllosilicate minerals has been evaluated for various clay minerals, the specific ionic composition of the surrounding porewater can play a critical role in promoting adsorption (or desorption) of arsenate (HxAsO4x-4). We conducted a series of adsorption isotherms to evaluate the adsorption of arsenate to various phyllosilicates in the presence of monovalent (K+), divalent (Mg2+, Ca2+), and trivalent (La3+) cations while maintaining constant ionic strength and pH. Adsorption isotherms were combined with surface complexation modeling to examine retention processes of arsenate as a function of ionic composition in the surrounding solution. The higher charge density of greater valent cations results in stronger outer-sphere bridging complexes between negatively charged phyllosilicate mineral surfaces and negatively charged arsenate oxyanions. Higher valent cations thus enhance the propensity for arsenate adsorption on phyllosilicate minerals. We further deciphered surface complexation processes by conducting adsorption isotherms on various clay minerals including smectite, illite, and pyrophyllite to evaluate the role of interlayer, permanent charge, and terminal edge sites. We conclude that arsenate is most likely retained largely on the planar surface where structural negative charge emanates allowing cation bridging complexes to develop. Our findings illustrate that clay mineralogy of soils and sediments can combine with porewater ionic composition (and specifically the proportion of divalent cations) to describe arsenic transport, particularly in iron- or aluminum-oxide poor systems.

  View details for DOI 10.1016/j.scitotenv.2021.147096

  View details for PubMedID 33932669

• Development of energetic and enzymatic limitations on microbial carbon cycling in soils BIOGEOCHEMISTRY Naughton, H. R., Keiluweit, M., Tfaily, M. M., Dynes, J. J., Regier, T., Fendorf, S. 2021; 153 (2): 191–213

  View details for DOI 10.1007/s10533-021-00781-z

  View details for Web of Science ID 000635930700005

• Effects of moisture and physical disturbance on pore-scale oxygen content and anaerobic metabolisms in upland soils. The Science of the total environment Lacroix, E. M., Rossi, R. J., Bossio, D., Fendorf, S. 2021; 780: 146572

  Abstract

  Soils are the largest dynamic stock of carbon (C) on Earth, and microbial respiration of soil organic C accounts for over 25% of global carbon dioxide (CO2) emissions. Zones of oxygen depletion in upland soils (anaerobic microsites) are increasingly recognized as an important control on soil microbial respiration rates, but the factors governing the volume and distribution of anaerobic microsites are relatively unknown. We measured the dissolved oxygen (DO) content of porewater from incubated soil cores of varying moisture contents (<80% and >80% water saturation) and degrees of disturbance (undisturbed, conventionally tilled, and physically disturbed). Porewater was extracted sequentially from pores constrained by three effective pore diameters, ≥3.0mum, 3.0-1.0mum, and 1.0-0.6mum, from cores incubated for 7, 14, or 28days, using a modified Tempe cell extraction system. We observed a parabolic pattern in mean dissolved oxygen (DO) concentrations across pore sizes, independent of soil moisture and degree of disturbance. Specifically, DO values within the largest and smallest pore domains were relatively depleted (155±10muM and 160±11muM, respectively), while DO values within medium pores were closer to saturation (214±8muM). The observed DO pattern provides insight into the balance of microbial oxygen demand versus oxygen supply across pore domains within upland soils. Additionally, we observed iron and manganese reduction in all soils except samples subjected to disturbance and incubated at <80% water saturation, suggesting that disturbance enhances aeration and diminishes anaerobic metabolisms within upland soils. Our findings highlight the influence of soil moisture and management on soil redox and CO2 efflux rates.

  View details for DOI 10.1016/j.scitotenv.2021.146572

  View details for PubMedID 33774307

• Geochemical signatures and natural background values of rare earth elements in soils of Brazilian Amazon. Environmental pollution (Barking, Essex : 1987) Ferreira, M. d., Fontes, M. P., Bellato, C. R., Marques Neto, J. d., Lima, H. N., Fendorf, S. 2021; 277: 116743

  Abstract

  Rare earth elements (REEs) are generally defined as a homogenous group of elements with similar physical-chemical properties, encompassing Y and Sc and the lanthanides elements series. Natural REEs contents in soils depend on the parent material, the soil genesis processes and can be gradually added to the soil by anthropogenic activities. The REEs have been considered emerging pollutants in several countries, so the establishment of regulatory guidelines is necessary to avoid environmental contamination. In Brazil, REE soils data are restricted to some regions, and knowledge about them in the Amazon soils is scarce, although this biome covers more than 40% of the Brazilian territory. Thus, the objectives of this study were to determine the REE content in soils of two hydrographic basins (Solimoes and Rio Negro) of the Amazon biome, establish their Quality Reference Values (QRV) and to investigate the existence of enrichment of REEs in urban soils. The SigmaREE(Y+Sc) content of Solimoes surface samples was 109.28mgkg-1 and the SigmaREE(Y+Sc) content in the subsurface samples was 94.11mgkg-1. In soils of Rio Negro basin, the SigmaREE(Y+Sc) was 43.95 15mgkg-1 surface samples and 38.40mgkg-1 in subsurface samples. The SigmaREE(Y+Sc) in urban topsoils samples was 38.62mgkg-1. The REEs contents pattern in three studied areas are influenced in different amplitude by natural soil properties. The REEs content in urban topsoils were slightly higher than the Rio Negro pristine soils, but the ecological risk was low. QRVs recommend for Solimoes soils ranged from 0.01 (Lu) to 145.6mgkg-1 (Ce) and for Rio Negro soils ranged from 0.05 (Lu) to 15.8mgkg-1 (Ce).

  View details for DOI 10.1016/j.envpol.2021.116743

  View details for PubMedID 33640811

• Simulated Aquifer Heterogeneity Leads to Enhanced Attenuation and Multiple Retention Processes of Zinc. Environmental science & technology Engel, M., Boye, K., Noel, V., Babey, T., Bargar, J. R., Fendorf, S. 2021

  Abstract

  Alluvial aquifers serve as one of the main water sources for domestic, agricultural, and industrial purposes globally. Groundwater quality, however, can be threatened by naturally occurring and anthropogenic metal contaminants. Differing hydrologic and biogeochemical conditions between predominantly coarse-grained aquifer sediments and embedded layers or lenses of fine-grained materials lead to variation in metal behavior. Here, we examine processes controlling Zn partitioning within a dual-pore domain-reconstructed alluvial aquifer. Natural coarse aquifer sediments from the Wind River-Little Wind River floodplain near Riverton, WY, were used in columns with or without fine-grained lenses to examine biogeochemical controls on Zn concentrations, retention mechanisms, and transport. Following the introduction of Zn to the groundwater source, Zn preferentially accumulated in the fine-grained lenses, despite their small volumetric contributions. While the clay fraction dominated Zn retention in the sandy aquifer, the lenses supported additional reaction pathways of retention-the reducing conditions within the lenses resulted in ZnS precipitation, overriding the contribution of organic matter. Zinc concentration in the groundwater controlled the formation of Zn-clays and Zn-layered double hydroxides, whereas the extent of sulfide production controlled precipitation of ZnS. Our findings illustrate how both spatial and compositional heterogeneities govern the extent and mechanisms of Zn retention in intricate groundwater systems, with implications for plume behavior and groundwater quality.

  View details for DOI 10.1021/acs.est.0c06750

  View details for PubMedID 33570404

• Human health risk assessment and geochemical mobility of rare earth elements in Amazon soils. The Science of the total environment da Silva Ferreira, M., Fontes, M. P., Lima, M. T., Cordeiro, S. G., Wyatt, N. L., Lima, H. N., Fendorf, S. 2021: 151191

  Abstract

  Rare earth elements (REEs) are a grouping of elements that encompasses lanthanides, yttrium and scandium due to their similar chemical properties and occurrence in ore deposits. Over the past few decades, economic interest in REEs has increased due to their use in several types of industries such as high-tech, medicine and agriculture. Extraction of REEs has been followed, in general, by incorrect disposal of tailing and waste, creating hazardous conditions in several countries. However, the magnitude of the possible impacts on ecosystem and human health are relatively unknown, especially in tropical systems. Thus, the objectives of this study were to assess the geochemical mobility and the bioaccessibility of REEs based on a series of chemical extractions and in vitro essay. We also tested two promising simple protocols (0.01 mol L-1 CaCl2 and 0.43 mol L-1 HNO3) for measuring REE bioaccessible fractions through a single extraction. Our findings show that the bioavailable fractions represent less than 20% of the ΣREEs fraction in all soil samples examine. Similarly, the oral bioaccessibility obtained by two in vitro methods (Gastric protocol and Gastric-Intestinal protocol) and by the single extraction tests represented less than 20% of the ΣREE contents. The non-carcinogenic risks and the carcinogenic risks associated to REEs oral exposure were low for children and adults. The extractions with 0.01 mol L-1 CaCl2 showed great potential as a method for measuring the REEs bioaccessible fraction.

  View details for DOI 10.1016/j.scitotenv.2021.151191

  View details for PubMedID 34710416

• Bone manganese is a sensitive biomarker of ongoing elevated manganese exposure, but does not accumulate across the lifespan. Environmental research Conley, T. E., Richardson, C., Pacheco, J., Dave, N., Jursa, T., Guazzetti, S., Lucchini, R. G., Fendorf, S., Ritchie, R. O., Smith, D. R. 2021: 112355

  Abstract

  Studies have established associations between environmental and occupational manganese (Mn) exposure and executive and motor function deficits in children, adolescents, and adults. These health risks from elevated Mn exposure underscore the need for effective exposure biomarkers to improve exposure classification and help detect/diagnose Mn-related impairments. Here, neonate rats were orally exposed to 0, 25, or 50 mg Mn/kg/day during early life (PND 1-21) or lifelong through ∼ PND 500 to determine the relationship between oral Mn exposure and blood, brain, and bone Mn levels over the lifespan, whether Mn accumulates in bone, and whether elevated bone Mn altered the local atomic and mineral structure of bone, or its biomechanical properties. Additionally, we assessed levels of bone Mn compared to bone lead (Pb) in aged humans (age 41-91) living in regions impacted by historic industrial ferromanganese activity. The animal studies show that blood, brain, and bone Mn levels naturally decrease across the lifespan without elevated Mn exposure. With elevated exposure, bone Mn levels were strongly associated with blood Mn levels, bone Mn was more sensitive to elevated exposures than blood or brain Mn, and Mn did not accumulate with lifelong elevated exposure. Elevated early life Mn exposure caused some changes in bone mineral properties, including altered local atomic structure of hydroxyapatite, along with some biomechanical changes in bone stiffness in weanlings or young adult animals. In aged humans, blood Mn ranged from 5.4 to 23.5 ng/mL; bone Mn was universally low, and decreased with age, but did not vary based on sex or female parity history. Unlike Pb, bone Mn showed no evidence of accumulation over the lifespan, and may not be a biomarker of cumulative long-term exposure. Thus, bone may be a useful biomarker of recent ongoing Mn exposure in humans, and may be a relatively minor target of elevated exposure.

  View details for DOI 10.1016/j.envres.2021.112355

  View details for PubMedID 34774504

• Effect of Bicarbonate, Calcium, and pH on the Reactivity of As(V) and U(VI) Mixtures. Environmental science & technology Gonzalez-Estrella, J., Meza, I., Burns, A. J., Ali, A. S., Lezama-Pacheco, J. S., Lichtner, P., Shaikh, N., Fendorf, S., Cerrato, J. M. 2020

  Abstract

  Natural or anthropogenic processes can increase the concentration of uranium (U) and arsenic (As) above the maximum contaminant levels in water sources. Bicarbonate and calcium (Ca) can have major impacts on U speciation and can affect the reactivity between U and As. We therefore investigated the reactivity of aqueous U and As mixtures with bicarbonate and Ca for acidic and neutral pH conditions. In experiments performed with 1 mM U and As mixtures, 10 mM Ca, and without added bicarbonate (pCO2 = 3.5), aqueous U decreased to <0.25 mM at pH 3 and 7. Aqueous As decreased the most at pH 3 (0.125 mM). Experiments initiated with 0.005 mM As and U showed similar trends. X-ray spectroscopy (i.e., XAS and EDX) and diffraction indicated that U-As-Ca- and U-Ca-bearing solids resemble uranospinite [Ca(UO2)2(AsO4)2·10H2O] and becquerelite [Ca(UO2)6O4(OH)6·8(H2O)]. These findings suggest that U-As-Ca-bearing solids formed in mixed solutions are stable at pH 3. However, the dissolution of U-As-Ca and U-Ca-bearing solids at pH 7 was observed in reactors containing 10 mM bicarbonate and Ca, suggesting a kinetic reaction of aqueous uranyl-calcium-carbonate complexation. Our study provides new insights regarding U and As mobilization for risk assessment and remediation strategies.

  View details for DOI 10.1021/acs.est.9b06063

  View details for PubMedID 32176846

• Controlling Arsenic Mobilization during Managed Aquifer Recharge: The Role of Sediment Heterogeneity. Environmental science & technology Fakhreddine, S. n., Prommer, H. n., Gorelick, S. M., Dadakis, J. n., Fendorf, S. n. 2020; 54 (14): 8728–38

  Abstract

  Managed aquifer recharge (MAR) enhances freshwater security and augments local groundwater supplies. However, geochemical and hydrological shifts during MAR can release toxic, geogenic contaminants from sediments to groundwater, threatening the viability of MAR as a water management strategy. Using reactive transport modeling coupled with aquifer analyses and measured water chemistry, we investigate the causal mechanisms of arsenic release during MAR via injection in the Orange County Groundwater Basin. Here, injection water is oxygenated, highly purified recycled water produced by advanced water treatment. Injection occurs via a well screened at several depth intervals ranging from 160-365 m, allowing recharge into multiple confined horizons (zones) of the aquifer system. However, these zones are characterized by varying degrees of prior oxidation due to historic, long-term infiltration from the overlying aquifer. The resulting sediment geochemical heterogeneity provides a critical control on the release (or retention) of arsenic. In zones with prior oxidation, As mobilization occurs via arsenate desorption from Fe-(hydr)oxides, primarily associated with shifts in pH; within zones that remain reduced prior to injection, As release is attributed to the oxidative dissolution of As-bearing pyrite. We find that As release can be attributed to various geochemical mechanisms within a single injection well owing to geochemical heterogeneity across the aquifer system.

  View details for DOI 10.1021/acs.est.0c00794

  View details for PubMedID 32516527

• Arsenic Fate in Peat Controlled by the pH-Dependent Role of Reduced Sulfur. Environmental science & technology Eberle, A. n., Besold, J. n., Kerl, C. F., Lezama-Pacheco, J. S., Fendorf, S. n., Planer-Friedrich, B. n. 2020

  Abstract

  Reduced sulfur (S) has a contrasting role in the fate of arsenic (As) in peatlands. Sulfur bridges provide efficient binding of As to organic carbon (C), but the formation of aqueous As-S species, so-called thioarsenates, leads to a low to no sorption tendency to organic C functional groups. Here, we studied how pH changes the role of reduced S in desorption and retention of presorbed As in model peat. Control desorption experiments without S addition revealed that As was mobilized, predominantly as arsenite, in all treatments with relative mobilization increasing with pH (4.5 < 7.0 < 8.5). Addition of sulfide or polysulfide caused substantial As retention at acidic conditions but significantly enhanced As desorption compared to controls at neutral to alkaline pH. Thioarsenates dominated As speciation at pH 7.0 and 8.5 (maximum, 79%) and remained in solution without (re)sorption to peat. Predominance of arsenite in control experiments and no evidence of surface-bound thioarsenates at pH 7.0 suggest mobilization to proceed via arsenite desorption, reaction with dissolved or surface-bound reduced S, and formation of thioarsenates. Our results suggest that natural or management-related increases in pH or increases in reduced S in near-neutral pH environments can turn organic matter from an As sink into a source.

  View details for DOI 10.1021/acs.est.0c00457

  View details for PubMedID 32347724

• Calcium-Uranyl-Carbonato Species Kinetically Limit U(VI) Reduction by Fe(II) and Lead to U(V)-Bearing Ferrihydrite. Environmental science & technology Dewey, C. n., Sokaras, D. n., Kroll, T. n., Bargar, J. R., Fendorf, S. n. 2020

  Abstract

  Reaction conditions and mechanisms promoting or inhibiting U reduction exert a central control on U solubility and, therefore, U transport and its associated risks. Here, we vary and track common aqueous uranium species to show that a kinetic restriction inhibits homogeneous reduction of the calcium-uranyl-carbonato species (CaUO2(CO3)32- and Ca2UO2(CO3)3) by Fe(II)(aq), while ferrihydrite surface-catalyzed reduction of all aqueous uranyl by Fe(II) proceeds. Using U L3 high energy resolution fluorescence detection (HERFD) X-ray absorption near edge structure (XANES) spectroscopy, U L3 extended X-ray absorption fine structure (EXAFS) spectroscopy, and transmission electron microscopy (TEM), we also show that U(V) is generated and incorporated into ferrihydrite formed from homogeneous U(VI) reduction by Fe(II)(aq). Through elucidation of the mechanisms that inhibit reduction of the calcium-uranyl-carbonato species and promote stabilization of U(V), we advance our understanding of the controls on U solubility and thus improve prediction of U transport in surface and subsurface systems.

  View details for DOI 10.1021/acs.est.9b05870

  View details for PubMedID 32315524

• Contribution of clay-aquitard to aquifer iron concentrations and water quality. The Science of the total environment Liu, Y. n., Ma, T. n., Chen, J. n., Xiao, C. n., Liu, R. n., Du, Y. n., Fendorf, S. n. 2020; 741: 140061

  Abstract

  The contribution of aquitards to aquifer water quality can be pronounced but is rarely considered. The aims of this study were to delineate the spatial distribution of iron in a shallow aquitard-aquifer system within Jianghan Plain (JHP) of central China and to identify the origin of high iron within aquifers. Infiltration, hydraulic gradients and sediment chemistry influence the distribution of iron in the aquitard pore water which has a significant effect on the underlying aquifer. Chemical equilibrium modeling of pore water was used to simulate chemical processes influencing aquifer chemistry and determined the possible precipitation of FeCO3, FeS minerals (FeSx) and Fe-oxides (representing hydroxides, oxyhydroxides, and oxides of ferric iron). We presented a conceptual chemical-physical scenario to explain the observed Fe distributions: (1) Increasing iron concentrations with low-level sulfide in aquitard pore water. (2) Increasing iron concentrations with low-level sulfide in aquitard pore water underlying ponded water. (3) Decreasing iron concentrations with high-level sulfide in aquitard pore water. In combination, our findings illustrate the influence of aquitards on aquifer chemistry using Fe within the Jianghan Plain as an example.

  View details for DOI 10.1016/j.scitotenv.2020.140061

  View details for PubMedID 32603935

• Organic compounds alter the preference and rates of heavy metal adsorption on ferrihydrite. The Science of the total environment Engel, M. n., Lezama Pacheco, J. S., Noël, V. n., Boye, K. n., Fendorf, S. n. 2020; 750: 141485

  Abstract

  The availability of heavy metals in terrestrial environments is largely controlled by their interactions with minerals and organic matter, with iron minerals having a particularly strong role in heavy metal fate. Because soil organic matter contains a variety of compounds that differ in their chemical properties, the underlying impact organic matter-soil mineral associations bestow on heavy metal binding is still unresolved. Here, we systematically examine the binding of Cd, Zn and Ni by a suite of organic-ferrihydrite assemblages, chosen to account for various compound chemistries within soil organic matter. We posited that organic compound functionality would dictate the extent of association with the organic-ferrihydrite assemblages. Increased heavy metal binding to the assemblages was observed and attributed to the introduction of additional binding sites by the organic functional groups with differing metal affinities. The relative increase depended on the metal's Lewis acidity and followed the order Cd > Zn > Ni, whereas the reverse order was obtained for metal binding by pristine ferrihydrite (Ni > Zn > Cd). Citric acid-, aspartic acid- and cysteine-ferrihydrite assemblages also enhanced the metal binding rate. X-ray absorption spectroscopy revealed that the organic coating contributed significantly to Zn binding by the assemblages, despite relatively low organic surface coverage. Our findings provide valuable information on the nature of heavy metal-organic-mineral interactions and metal adsorption processes regulating their bioavailability and transport.

  View details for DOI 10.1016/j.scitotenv.2020.141485

  View details for PubMedID 32862002

• Complexation by Organic Matter Controls Uranium Mobility in Anoxic Sediments. Environmental science & technology Bone, S. E., Cliff, J. n., Weaver, K. n., Takacs, C. J., Roycroft, S. n., Fendorf, S. n., Bargar, J. R. 2020

  Abstract

  Uranium contamination threatens the availability of safe and clean drinking water globally. This toxic element occurs both naturally and as a result of mining and ore-processing in alluvial sediments, where it accumulates as tetravalent U [U(IV)], a form once considered largely immobile. Changing hydrologic and geochemical conditions cause U to be released into groundwater. Knowledge of the chemical form(s) of U(IV) is essential to understand the release mechanism, yet the relevant U(IV) species are poorly characterized. There is growing belief that natural organic matter (OM) binds U(IV) and mediates its fate in the subsurface. In this work, we combined nanoscale imaging (nano secondary ion mass spectrometry and scanning transmission X-ray microscopy) with a density-based fractionation approach to physically and microscopically isolate organic and mineral matter from alluvial sediments contaminated with uranium. We identified two populations of U (dominantly +IV) in anoxic sediments. Uranium was retained on OM and adsorbed to particulate organic carbon, comprising both microbial and plant material. Surprisingly, U was also adsorbed to clay minerals and OM-coated clay minerals. The dominance of OM-associated U provides a framework to understand U mobility in the shallow subsurface, and, in particular, emphasizes roles for desorption and colloid formation in its mobilization.

  View details for DOI 10.1021/acs.est.9b04741

  View details for PubMedID 31886668

• Redox Heterogeneities Promote Thioarsenate Formation and Release into Groundwater from Low Arsenic Sediments. Environmental science & technology Kumar, N. n., Noël, V. n., Planer-Friedrich, B. n., Besold, J. n., Lezama-Pacheco, J. n., Bargar, J. R., Brown, G. E., Fendorf, S. n., Boye, K. n. 2020

  Abstract

  Groundwater contamination by As from natural and anthropogenic sources is a worldwide concern. Redox heterogeneities over space and time are common and can influence the molecular-level speciation of As, and thus, As release/retention but are largely unexplored. Here, we present results from a dual-domain column experiment, with natural organic-rich, fine-grained, and sulfidic sediments embedded as lenses (referred to as "reducing lenses") within natural aquifer sand. We show that redox interfaces in sulfur-rich, alkaline aquifers may release concerning levels of As, even when sediment As concentration is low (<2 mg/kg), due to the formation of mobile thioarsenates at aqueous sulfide/Fe molar ratios <1. In our experiments, this behavior occurred in the aquifer sand between reducing lenses and was attributed to the spreading of sulfidic conditions and subsequent Fe reductive dissolution. In contrast, inside reducing lenses (and some locations in the aquifer) the aqueous sulfide/Fe molar ratios exceeded 1 and aqueous sulfide/As molar ratios exceeded 100, which partitioned As(III)-S to the solid phase (associated with organics or as realgar (As4S4)). These results highlight the importance of thioarsenates in natural sediments and indicate that redox interfaces and sediment heterogeneities could locally degrade groundwater quality, even in aquifers with unconcerning solid-phase As concentrations.

  View details for DOI 10.1021/acs.est.9b06502

  View details for PubMedID 32069033

• Soil and Aquifer Properties Combine as Predictors of Groundwater Uranium Concentrations within the Central Valley, California Environmental Science & Technology Lopez, A. M., Wells, A., Fendorf, S. 2020: 10

  View details for DOI 10.1021/acs.est.0c05591

• Governing Constraints of Chromium(VI) Formation from Chromium(III)-Bearing Minerals in Soils and Sediments SOIL SYSTEMS Hausladen, D., Fakhreddine, S., Fendorf, S. 2019; 3 (4)

  View details for DOI 10.3390/soilsystems3040074

  View details for Web of Science ID 000505592400014

• Lithologic and redox controls on hexavalent chromium in vadose zone sediments of California's Central Valley GEOCHIMICA ET COSMOCHIMICA ACTA McClain, C. N., Fendorf, S., Johnson, S. T., Menendez, A., Maher, K. 2019; 265: 478–94

  View details for DOI 10.1016/j.gca.2019.07.044

  View details for Web of Science ID 000488959900028

• Rice production threatened by coupled stresses of climate and soil arsenic. Nature communications Muehe, E. M., Wang, T., Kerl, C. F., Planer-Friedrich, B., Fendorf, S. 2019; 10 (1): 4985

  Abstract

  Projections of globalrice yieldsaccount for climate change. They do not, however, consider the coupled stresses of impending climate change and arsenic in paddy soils. Here, we show in a greenhouse study that future conditions cause a greater proportion of pore-water arsenite, the more toxic form of arsenic, in the rhizosphere of Californian Oryza sativa L. variety M206, grown on Californian paddy soil. As a result, grain yields decrease by 39% compared to yields at today's arsenic soil concentrations. In addition, future climatic conditions cause a nearly twofold increase of grain inorganic arsenic concentrations. Our findings indicate that climate-induced changes in soil arsenic behaviour and plant response will lead to currently unforeseen losses in rice grain productivity and quality. Pursuing rice varieties and crop management practices that alleviate the coupled stresses of soil arsenic and change in climatic factors are needed to overcome the currently impending food crisis.

  View details for DOI 10.1038/s41467-019-12946-4

  View details for PubMedID 31676771

• Sources of Blood Lead Exposure in Rural Bangladesh ENVIRONMENTAL SCIENCE & TECHNOLOGY Forsyth, J. E., Weaver, K. L., Maher, K., Islam, M., Raqib, R., Rahman, M., Fendorf, S., Luby, S. P. 2019; 53 (19): 11429–36

  Abstract

  Lead (Pb) exposure is a major public health problem worldwide. Although high levels of Pb in blood in Bangladesh have been documented, the dominant Pb sources contributing to human exposure in rural Bangladesh have not been determined. Here, we first obtained blood from pregnant women from three rural Bangladeshi districts who were previously assessed by a case-control and sampling study, and we then conducted semistructured in-depth interviews to understand Pb exposure behavior and finally collected samples of the suspected Pb sources. We measured the Pb isotopic composition of both potential Pb sources and 45 blood samples in order to understand which of three sources predominate: (1) food from Pb-soldered cans, (2) turmeric, or (3) geophagous materials (clay, soil, or ash). The Pb isotope ratios of the three sources are distinct (p = 0.0001) and blood isotope ratios are most similar to turmeric. Elevated lead and chromium (Cr) concentrations in turmeric and a yellow pigment used in turmeric processing are consistent with reported consumption behavior that indicated turmeric as a primary contributor to blood Pb. The Pb isotopic composition analyses combined with a case-control and sampling approach provides evidence that turmeric adulterated with the yellow Pb-bearing pigment is the main Pb exposure source in these districts and illustrates the need to assess drivers and practices of turmeric adulteration, as well as the prevalence of adulteration across South Asia.

  View details for DOI 10.1021/acs.est.9b00744

  View details for Web of Science ID 000488993500041

  View details for PubMedID 31525910

• Predicting drivers of groundwater Cr(VI) contamination in the Central Valley, CA: Integrated multivariate statistical & geospatial approach Lopez, A., Caers, J., Fendorf, S. AMER CHEMICAL SOC. 2019

  View details for Web of Science ID 000525061500595

• Protecting groundwater quality from geogenic and emerging contaminants in actively managed aquifers Fakhreddine, S., Sherris, A., Lopez, A., Wells, A., Holmes, R., Nico, P., Babbitt, C., Fendorf, S. AMER CHEMICAL SOC. 2019

  View details for Web of Science ID 000525061500718

• How natural organic compounds influence zinc retention by iron oxides Engel, M., Fendorf, S. AMER CHEMICAL SOC. 2019

  View details for Web of Science ID 000525061500842

• Influence of redox interfaces on metal(loid) contaminant mobility in shallow alluvial groundwater aquifers Boye, K., Kumar, N., Noel, V., Bargar, J., Fendorf, S. AMER CHEMICAL SOC. 2019

  View details for Web of Science ID 000525061500662

• Simplex-Centroid mixture design applied to arsenic (V) removal from waters using synthetic minerals JOURNAL OF ENVIRONMENTAL MANAGEMENT Dias, A., Ferreira Fontes, M., Reis, C., Bellato, C., Fendorf, S. 2019; 238: 92–101

  View details for DOI 10.1016/j.jenvman.2019.02.099

  View details for Web of Science ID 000464296400011

• Antimonite Complexation with Thiol and Carboxyl/Phenol Groups of Peat Organic Matter ENVIRONMENTAL SCIENCE & TECHNOLOGY Besold, J., Kumar, N., Scheinost, A. C., Pacheco, J., Fendorf, S., Planer-Friedrich, B. 2019; 53 (9): 5005–15

  View details for DOI 10.1021/acs.est.9b00495

  View details for Web of Science ID 000467641800037

• Simplex-Centroid mixture design applied to arsenic (V) removal from waters using synthetic minerals. Journal of environmental management Dias, A. C., Fontes, M. P., Reis, C., Bellato, C. R., Fendorf, S. 2019; 238: 92–101

  Abstract

  Arsenic (As) is a toxic and carcinogenic element. Therefore, it is necessary to carry out research on As-contaminated water management in order to achieve the World Health Organization (WHO) standard for drinking water (0.010 mg L-1). A Simplex-Centroid mixture design (SCMD) was used to determine the best mineral composition for both maximum adsorption capacity of As(V) (MAC-As) and residual concentration of As(V) (RC-As), using synthetic poorly crystallized aluminum hydroxide (pAlHyd), calcined layered double hydroxide (cLDH), and two-line ferrihydrite (2ℓFh). The analysis of variance results and the predicted values of models showed a good agreement with the experimental data, indicating that SCMD is a reliable method to optimize As removal through determination of the best mineral composition. The ability of pure synthetic minerals to remove As from water was different among those mixtures thereof, which indicate that the mineral components interacted with each other. Results showed that cLDH was the best As adsorbent. However, it showed a RC-As higher than the WHO standard. The pAlHyd and 2ℓFh exhibited smaller MAC-As, but they lowered RC-As to below 0.010 mg L-1, showing no direct relationship between high MAC-As and low RC-As. Therefore, mineral compositions which combine high adsorption capacity with low residual concentration should work better for removing As from drinking water, ensuring it meets the WHO potability standard. Ternary diagrams for MAC-As and RC-As showed that the best combination for maximizing MAC-As and reducing RC-As should be a mixture of 75-90% of cLDH, 10-20% of pAlHyd, and 0-5% of 2ℓFh.

  View details for PubMedID 30849602

• Sedimentogenesis and hydrobiogeochemistry of high arsenic Late Pleistocene-Holocene aquifer systems EARTH-SCIENCE REVIEWS Wang, Y., Pi, K., Fendorf, S., Deng, Y., Xie, X. 2019; 189: 79–98

  View details for DOI 10.1016/j.earscirev.2017.10.007

  View details for Web of Science ID 000462692600005

• Antimonite Binding to Natural Organic Matter: Spectroscopic Evidence from a Mine Water Impacted Peatland. Environmental science & technology Besold, J. n., Eberle, A. n., Noël, V. n., Kujala, K. n., Kumar, N. n., Scheinost, A. C., Pacheco, J. L., Fendorf, S. n., Planer-Friedrich, B. n. 2019

  Abstract

  Peatlands and other wetlands are sinks for antimony (Sb), and solid natural organic matter (NOM) may play an important role in controlling Sb binding. However, direct evidence of Sb sequestration in natural peat samples is lacking. Here, we analyzed solid phase Sb, iron (Fe), and sulfur (S) as well as aqueous Sb speciation in three profiles up to a depth of 80 cm in a mine water impacted peatland in northern Finland. Linear combination fittings of extended X-ray absorption fine structure spectra showed that Sb binding to Fe phases was of minor importance and observed only in the uppermost layers of the peatland. Instead, the dominant (to almost exclusive) sequestration mechanism was Sb(III) binding to oxygen-containing functional groups, and at greater depths, increasingly Sb(III) binding to thiol groups of NOM. Aqueous Sb speciation was dominated by antimonate, while antimonite concentrations were low, further supporting our findings of much higher reactivity of Sb(III) than Sb(V) toward peat surfaces. Insufficient residence time for efficient reduction of antimonate to antimonite currently hinders higher Sb removal in the studied peatland. Overall, our findings imply that Sb(III) binding to solid NOM acts as an important sequestration mechanism under reducing conditions in peatlands and other high-organic matter environments.

  View details for DOI 10.1021/acs.est.9b03924

  View details for PubMedID 31436960

• Turmeric means "yellow" in Bengali: Lead chromate pigments added to turmeric threaten public health across Bangladesh. Environmental research Forsyth, J. E., Nurunnahar, S. n., Islam, S. S., Baker, M. n., Yeasmin, D. n., Islam, M. S., Rahman, M. n., Fendorf, S. n., Ardoin, N. M., Winch, P. J., Luby, S. P. 2019; 179 (Pt A): 108722

  Abstract

  Adulteration is a growing food safety concern worldwide. Previous studies have implicated turmeric as a source of lead (Pb) exposure due to the addition of lead chromate (PbCrO4), a yellow pigment used to enhance brightness. We aimed to assess the practice of adding yellow pigments to turmeric and producer- consumer- and regulatory-factors affecting this practice across the supply chain in Bangladesh. We identified and visited the nine major turmeric-producing districts of Bangladesh as well as two districts with minimal turmeric production. In each district, we conducted semi-structured interviews and informal observations with individuals involved in the production, consumption, and regulation of turmeric. We explored perceptions of and preferences for turmeric quality. We collected samples of yellow pigments and turmeric from the most-frequented wholesale and retail markets. We collected samples of turmeric, pigments, dust, and soil from turmeric polishing mills to assess evidence of adulteration. Interviews were analyzed through an inductive, thematic coding process, with attention focused on perceptions of and preferences for turmeric quality. Samples were analyzed for Pb and chromium (Cr) concentrations via inductively coupled plasma mass spectrometry and x-ray fluorescence. In total, we interviewed 152 individuals from across the supply chain and collected 524 samples of turmeric, pigments, dust, and soil (Table S3, Table S4). Turmeric Pb and Cr concentrations were highest in Dhaka and Munshiganj districts, with maximum turmeric powder Pb concentrations of 1152 μg/g, compared to 690 μg/g in the 9 major turmeric-producing districts. We found evidence of PbCrO4-based yellow pigment adulteration in 7 of the 9 major turmeric-producing districts. Soil samples from polishing mills contained a maximum of 4257 μg/g Pb and yellow pigments contained 2-10% Pb by weight with an average Pb:Cr molar ratio of 1.3. Turmeric wholesalers reported that the practice of adding yellow pigments to dried turmeric root during polishing began more than 30 years ago and continues today, primarily driven by consumer preferences for colorful yellow curries. Farmers stated that merchants are able to sell otherwise poor-quality roots and increase their profits by asking polishers to adulterate with yellow pigments. Adulterating turmeric with lead chromate poses significant risks to human health and development. The results from this study indicate that PbCrO4 is being added to turmeric by polishers, who are unaware of its neurotoxic effects, in order to satisfy wholesalers who are driven by consumer demand for yellow roots. We recommend immediate intervention that engages turmeric producers and consumers to address this public health crisis and ensure a future with Pb-free turmeric.

  View details for DOI 10.1016/j.envres.2019.108722

  View details for PubMedID 31550596

• Antimonite Complexation with Thiol and Carboxyl/Phenol Groups of Peat Organic Matter. Environmental science & technology Besold, J. n., Kumar, N. n., Scheinost, A. C., Lezama Pacheco, J. n., Fendorf, S. n., Planer-Friedrich, B. n. 2019

  Abstract

  Peatlands and other wetlands with abundant natural organic matter (NOM) are important sinks for antimony (Sb). While formation of Sb(III) sulfide phases or Sb(III) binding to NOM are discussed to decrease Sb mobility, the exact binding mechanisms remain elusive. Here, we reacted increasing sulfide concentrations with purified model peat at pH 6, forming reduced organic sulfur species, and subsequently equilibrated the reaction products with 50 μM of antimonite under anoxic conditions. Sulfur solid-phase speciation and the local binding environment of Sb were analyzed using X-ray absorption spectroscopy. We found that 85% of antimonite was sorbed by untreated peat. Sulfide-reacted peat increased sorption to 98%. Shell-by-shell fitting of Sb K-edge X-ray absorption fine structure spectra revealed Sb in untreated peat bound to carboxyl or phenol groups with average Sb-carbon distances of ∼2.90 Å. With increasing content of reduced organic sulfur, Sb was progressively coordinated to S atoms at distances of ∼2.45 Å and Sb-carbon distances of ∼3.33 Å, suggesting increasing Sb-thiol binding. Iterative target factor analysis allowed exclusion of reduced inorganic Sb-sulfur phases with similar Sb-sulfur distances. In conclusion, even when free sulfide concentrations are too low for formation of Sb-sulfur precipitates, peat NOM can sequester Sb in anoxic, sulfur-enriched environments.

  View details for PubMedID 30973221

• Experimental constrains on redox-induced arsenic release and retention from aquifer sediments in the central Yangtze River Basin. The Science of the total environment Duan, Y., Schaefer, M. V., Wang, Y., Gan, Y., Yu, K., Deng, Y., Fendorf, S. 2018; 649: 629–39

  Abstract

  The consumption of arsenic (As) contaminated groundwater affects the health of almost 20 million people in China. Unlike the preponderance of observations within the deltas of South and Southeast Asia, groundwater As concentrations in the central Yangtze River Basin, China, vary by up to an order of magnitude seasonally. In order to decipher the cause of seasonal release and retention of As between sediments and groundwater, we conducted batch sediment incubations under varying (imposed) redox conditions. Incubations were conducted under both N2 and O2 gas purges to simulate conditions observed within the field. In all cases, anoxic conditions resulted in As release to solution while As was removed from solution under oxic conditions. These experiments confirm that anoxia is a prerequisite for As mobilization into groundwater from Yangtze River Basin sediments. Alternating redox conditions resulted in Fe minerals dissolution, transformation, crystallization, and precipitation, and subsequent As release and retention in the system. More importantly, aquifer sediments at depths >15 m release As through multiple redox cycles without an exogenous electron donor (carbon source), organic matter in the sediments is sufficiently reactive to support microbial reduction of As(V) and Fe(III). These results provide direct evidence for previously described mechanisms explaining the observed seasonal variation of groundwater As concentrations in the central Yangtze River Basin, where seasonal changes in surface and groundwater levels drive changes in redox conditions and thus As concentrations.

  View details for PubMedID 30176474

• Quantifying biogeochemical heterogeneity in soil systems GEODERMA Wanzek, T., Keiluweit, M., Baham, J., Dragila, M. I., Fendorf, S., Fiedler, S., Nico, P. S., Kleber, M. 2018; 324: 89–97

  View details for DOI 10.1016/j.geodenna.2018.03.003

  View details for Web of Science ID 000431159500010

• Hexavalent Chromium Sources and Distribution in California Groundwater ENVIRONMENTAL SCIENCE & TECHNOLOGY Hausladen, D. M., Alexander-Ozinskas, A., McClain, C., Fendorf, S. 2018; 52 (15): 8242–51

  Abstract

  Groundwater resources in California represent a confluence of high-risk factors for hexavalent chromium contamination as a result of industrial activities, natural geology, and, potentially, land use. Here, we examine state-wide links in California between groundwater Cr(VI) concentrations and chemicals that provide signatures for source attribution. In environmental monitoring wells, Cr(VI) had the highest co-occurrence and also clustered with 1,4-dioxane and several chlorinated hydrocarbons indicative of the metal plating industry. Additionally, hotspots of Cr(VI) co-occurring with bromoform result from volatile organic compound remediation using in situ chemical oxidation that inadvertently oxidizes naturally occurring Cr(III). In groundwater supply wells, which are typically free of industrial inputs, Cr(VI) correlates with dichlorodiphenyldichloroethylene (DDE), vanadium, and ammonia and clusters with nitrate and dissolved oxygen, suggesting potential links between agricultural activities and Cr(VI). Specific controls on Cr(VI) vary substantially by region: from the metal plating industry around Los Angeles and the San Francisco Bay areas to natural redox conditions along flow paths in the Mojave Desert and to correlations with agricultural practices in the Central Valley of California. While industrial uses of Cr lead to the most acute cases of groundwater Cr(VI) contamination, oxidation of naturally occurring Cr affects a larger area, more wells, and a greater number of people throughout California.

  View details for DOI 10.1021/acs.est.7b06627

  View details for Web of Science ID 000441477600022

  View details for PubMedID 29949365

• Overpumping leads to California groundwater arsenic threat. Nature communications Smith, R., Knight, R., Fendorf, S. 2018; 9 (1): 2089

  Abstract

  Water resources are being challenged to meet domestic, agricultural, and industrial needs. To complement finite surface water supplies that are being stressed by changes in precipitation and increased demand, groundwater is increasingly being used. Sustaining groundwater use requires considering both water quantity and quality. A unique challenge for groundwater use, as compared with surface water, is the presence of naturally occurring contaminants within aquifer sediments, which can enter the water supply. Here we find that recent groundwater pumping, observed through land subsidence, results in an increase in aquifer arsenic concentrations in the San Joaquin Valley of California. By comparison, historic groundwater pumping shows no link to current groundwater arsenic concentrations. Our results support the premise that arsenic can reside within pore water of clay strata within aquifers and is released due to overpumping. We provide a quantitative model for using subsidence as an indicator of arsenic concentrations correlated with groundwater pumping.

  View details for PubMedID 29872050

• Prevalence of elevated blood lead levels among pregnant women and sources of lead exposure in rural Bangladesh: A case control study. Environmental research Forsyth, J. E., Saiful Islam, M., Parvez, S. M., Raqib, R., Sajjadur Rahman, M., Marie Muehe, E., Fendorf, S., Luby, S. P. 2018; 166: 1–9

  Abstract

  Prenatal and early childhood lead exposures impair cognitive development. We aimed to evaluate the prevalence of elevated blood lead levels (BLLs) among pregnant women in rural Bangladesh and to identify sources of lead exposure. We analyzed the BLLs of 430 pregnant women randomly selected from rural communities in central Bangladesh. Fifty-seven cases were selected with the highest BLLs, ≥ 7 mug/dL, and 59 controls were selected with the lowest BLLs, < 2 mug/dL. An exposure questionnaire was administered and soil, rice, turmeric, water, traditional medicine, agrochemical, and can samples were analyzed for lead contamination. Of all 430 women, 132 (31%) had BLLs > 5 mug/dL. Most women with elevated BLLs were spatially clustered. Cases were 2.6 times more likely than controls to consume food from a can (95% CI 1.0-6.3, p = 0.04); 3.6 times more likely to use Basudin, a specific brand of pesticide (95% CI 1.6-7.9, p = 0.002); 3.6 times more likely to use Rifit, a specific brand of herbicide (95% CI 1.7-7.9, p = 0.001); 2.9 times more likely to report using any herbicides (95% CI 1.2-7.3, p = 0.02); and 3.3 times more likely to grind rice (95% CI 1.3-8.4, p = 0.01). Five out of 28 food storage cans were lead-soldered. However, there was minimal physical evidence of lead contamination from 382 agrochemical samples and 129 ground and unground rice samples. Among 17 turmeric samples, one contained excessive lead (265 mug/g) and chromium (49 mug/g). Overall, we found evidence of elevated BLLs and multiple possible sources of lead exposure in rural Bangladesh. Further research should explicate and develop interventions to interrupt these pathways.

  View details for PubMedID 29804028

• Discerning Microbially Mediated Processes During Redox Transitions in Flooded Soils Using Carbon and Energy Balances FRONTIERS IN ENVIRONMENTAL SCIENCE Boye, K., Herrmann, A. M., Schaefer, M., Tfaily, M. M., Fendorf, S. 2018; 6

  View details for DOI 10.3389/fenvs.2018.00015

  View details for Web of Science ID 000452106300001

• Oxidative uranium release from anoxic sediments under diffusion-limited conditions Bone, S., Cahill, M., Jones, M., Fendorf, S., Davis, J., Williams, K., Bargar, J. AMER CHEMICAL SOC. 2018

  View details for Web of Science ID 000435539901659

• Vertical transport of uranium in the unsaturated zone: A likely plume persistence mechanism Roycroft, S., Noel, V., Boye, K., Johnson, R., Dam, W., Fendorf, S., Bargar, J. AMER CHEMICAL SOC. 2018

  View details for Web of Science ID 000435539901660

• Understanding the natural mechanisms for chromium mobilization in groundwater Houlihan, M., Lopez, A., Fendorf, S. AMER CHEMICAL SOC. 2018

  View details for Web of Science ID 000435539902497

• Intimate and complex coupling of carbon and iron cycles within terrestrial systems Fendorf, S., Keiluweit, M., Schaefer, M., Masue-Slowey, Y., Chadwick, O. AMER CHEMICAL SOC. 2018

  View details for Web of Science ID 000435539901020

• Anoxic microsites in upland soils dominantly controlled by clay content SOIL BIOLOGY & BIOCHEMISTRY Keiluweit, M., Gee, K., Denney, A., Fendorf, S. 2018; 118: 42–50

  View details for DOI 10.1016/j.soilbio.2017.12.002

  View details for Web of Science ID 000428490500006

• Arsenic leaching from ceramic water filters ENVIRONMENTAL SCIENCE-WATER RESEARCH & TECHNOLOGY Schaefer, M. V., Shantz, A., Fendorf, S., Ying, S. C. 2018; 4 (2): 234–40

  View details for DOI 10.1039/c7ew00176b

  View details for Web of Science ID 000424010900010

• Synchrotron X-Ray Fluorescence Analysis ENCYCLOPEDIA OF GEOCHEMISTRY: A COMPREHENSIVE REFERENCE SOURCE ON THE CHEMISTRY OF THE EARTH Pacheco, J., Fendorf, S. E., White, W. M. 2018: 1415–17

  View details for Web of Science ID 000473130400295

• Fate of arsenic before and after chemical-enhanced washing of an arsenic-containing soil in Hong Kong SCIENCE OF THE TOTAL ENVIRONMENT Beiyuan, J., Li, J., Tsang, D. W., Wang, L., Poon, C., Li, X., Fendorf, S. 2017; 599: 679–88

  Abstract

  This study evaluated the feasibility of 2-h chemical-enhanced washing of As-containing soil resulting from geogenic sources in Hong Kong and the fate of As before and after remediation. The soil morphology and As speciation in soil was elucidated by scanning electron microscopy with energy dispersive X-ray spectroscopy, X-ray diffractometer, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy. Integrated analysis of the results suggests that the As (>90%) resides predominantly as arsenate bound to ferric iron oxides, with a minor contribution (<10%) from an AsIII-sulphide phase. This accounts for the marginal leachability, mobility, and bioaccessibility of geogenic As in the untreated soil despite its high concentration. Among the five types of reagents (organic ligands, reductants, alkaline solvents, inorganic acids, and chelants), only dithionite-citrate-bicarbonate method and alkaline solvents (NaOH and Na2CO3) extracted 37-78% and 26-42% of the As by mineral dissolution. However, these extraction methods notably increased the leachability, mobility, and bioaccessibility of remaining As after soil washing, suggesting that a significant proportion of newly released As is prone to re-adsorption onto the soil surface and becomes highly mobile and bioaccessible. While inorganic acids and chelating agents had marginal effects on the fate of remaining As, organic ligands (citrate, oxalate, or pyrophosphate) probably destabilized the bonding of geogenic As and increased its mobility/bioaccessibility despite marginal extraction. The applicability of chemical extraction of geogenic As is questionable regardless of chemical agents, thus proper management of the As-containing soil by containment or physical encapsulation may be considered before land development.

  View details for PubMedID 28494293

• Anaerobic microsites have an unaccounted role in soil carbon stabilization NATURE COMMUNICATIONS Keiluweit, M., Wanzek, T., Kleber, M., Nico, P., Fendorf, S. 2017; 8

  View details for DOI 10.1038/s41467-017-01406-6

  View details for Web of Science ID 000416292800005

• Anaerobic microsites have an unaccounted role in soil carbon stabilization. Nature communications Keiluweit, M., Wanzek, T., Kleber, M., Nico, P., Fendorf, S. 2017; 8 (1): 1771

  Abstract

  Soils represent the largest carbon reservoir within terrestrial ecosystems. The mechanisms controlling the amount of carbon stored and its feedback to the climate system, however, remain poorly resolved. Global carbon models assume that carbon cycling in upland soils is entirely driven by aerobic respiration; the impact of anaerobic microsites prevalent even within well-drained soils is missed within this conception. Here, we show that anaerobic microsites are important regulators of soil carbon persistence, shifting microbial metabolism to less efficient anaerobic respiration, and selectively protecting otherwise bioavailable, reduced organic compounds such as lipids and waxes from decomposition. Further, shifting from anaerobic to aerobic conditions leads to a 10-fold increase in volume-specific mineralization rate, illustrating the sensitivity of anaerobically protected carbon to disturbance. The vulnerability of anaerobically protected carbon to future climate or land use change thus constitutes a yet unrecognized soil carbon-climate feedback that should be incorporated into terrestrial ecosystem models.

  View details for DOI 10.1038/s41467-017-01406-6

  View details for PubMedID 29176641

  View details for PubMedCentralID PMC5701132

• Partitioning of uranyl between ferrihydrite and humic substances at acidic and circum-neutral pH GEOCHIMICA ET COSMOCHIMICA ACTA Dublet, G., Pacheco, J., Bargar, J. R., Fendorf, S., Kumar, N., Lowry, G. V., Brown, G. E. 2017; 215: 122–40

  View details for DOI 10.1016/j.gca.2017.07.013

  View details for Web of Science ID 000409107900008

• Oxidative Uranium Release from Anoxic Sediments under Diffusion-Limited Conditions ENVIRONMENTAL SCIENCE & TECHNOLOGY Bone, S. E., Cahill, M. R., Jones, M. E., Fendorf, S., Davis, J., Williams, K. H., Bargar, J. R. 2017; 51 (19): 11039–47

  Abstract

  Uranium (U) contamination occurs as a result of mining and ore processing; often in alluvial aquifers that contain organic-rich, reduced sediments that accumulate tetravalent U, U(IV). Uranium(IV) is sparingly soluble, but may be mobilized upon exposure to nitrate (NO3-) and oxygen (O2), which become elevated in groundwater due to seasonal fluctuations in the water table. The extent to which oxidative U mobilization can occur depends upon the transport properties of the sediments, the rate of U(IV) oxidation, and the availability of inorganic reductants and organic electron donors that consume oxidants. We investigated the processes governing U release upon exposure of reduced sediments to artificial groundwater containing O2 or NO3- under diffusion-limited conditions. Little U was mobilized during the 85-day reaction, despite rapid diffusion of groundwater within the sediments and the presence of nonuraninite U(IV) species. The production of ferrous iron and sulfide in conjunction with rapid oxidant consumption suggested that the sediments harbored large concentrations of bioavailable organic carbon that fueled anaerobic microbial respiration and stabilized U(IV). Our results suggest that seasonal influxes of O2 and NO3- may cause only localized mobilization of U without leading to export of U from the reducing sediments when ample organic carbon is present.

  View details for PubMedID 28876920

• Arsenic-containing soil from geogenic source in Hong Kong: Leaching characteristics and stabilization/solidification CHEMOSPHERE Li, J., Beiyuan, J., Tsang, D. W., Wang, L., Poon, C., Li, X., Fendorf, S. 2017; 182: 31–39

  Abstract

  Geogenic sources of arsenic (As) have aroused extensive environmental concerns in many countries. This study evaluated the vertical profiles, leaching characteristics, and surface characteristics of As-containing soils in Hong Kong. The results indicated that elevated levels of As (486-1985 mg kg-1) were mostly encountered in deeper layer (15-20 m below ground). Despite high concentrations, geogenic As displayed a high degree of chemical stability in the natural geochemical conditions, and there was minimal leaching of As in various leaching tests representing leachability, mobility, phytoavailability, and bioaccessibility. Microscopic/spectroscopic investigations suggested that As in the soils was predominantly present as As(V) in a coordination environment with Fe oxides. Sequential extraction indicated that the majority of As were strongly bound with crystalline Fe/Al oxides and residual phase. Yet, uncertainties may remain with potential As exposure through accidental ingestion and abiotic/biotic transformation due to changes in geochemical conditions. Hence, the effectiveness of stabilization/solidification (S/S) treatment was evaluated. Although the leached concentrations of As from the S/S treated soils increased to varying extent in different batch leaching tests due to the increase in alkalinity, the mobility of As was considered very low based on semi-dynamic leaching test. This suggested that As immobilization in the S/S treated soils was predominantly dependent on physical encapsulation by interlocking framework of hydration products, which could also prevent potential exposure and allow controlled utilization of S/S treated soils as monolithic materials. These results illustrate the importance of holistic assessment and treatment/management of As-containing soils for enabling flexible future land use.

  View details for PubMedID 28486153

• Depth Stratification Leads to Distinct Zones of Manganese and Arsenic Contaminated Groundwater ENVIRONMENTAL SCIENCE & TECHNOLOGY Ying, S. C., Schaefer, M. V., Cock-Esteb, A., Li, J., Fendorf, S. 2017; 51 (16): 8926–32

  Abstract

  Providing access to safe drinking water is a global challenge, for which groundwater is increasingly being used throughout the world. However, geogenic contaminants limit the suitability of groundwater for domestic purposes over large geographic areas across most continents. Geogenic contaminants in groundwater are often evaluated individually, but here we demonstrate the need to evaluate multiple contaminants to ensure that groundwater is safe for human consumption and agricultural usage. We compiled groundwater chemical data from three aquifer regions across the world that have been reported to have widespread As and Mn contamination including the Glacial Aquifer in the U.S., the Ganges-Brahmaputra-Mehta Basin within Bangladesh, and the Mekong Delta in Cambodia, along with newly sampled wells in the Yangtze River Basin of China. The proportion of contaminated wells increase by up to 40% in some cases when both As and Mn contaminants are considered. Wilcoxon rank-sum analysis indicates that Mn contamination consistently occurs at significantly shallower depths than As contaminated wells in all regions. Arsenic concentrations in groundwater are well predicted by redox indicators (Eh and dissolved oxygen) whereas Mn shows no significant relationship with either parameter. These findings illustrate that the number of safe wells may be drastically overestimated in some regions when Mn contamination is not taken into account and that depth may be used as a distinguishing variable in efforts to predict the presence of groundwater contaminants regionally.

  View details for PubMedID 28695739

• Thermodynamically controlled preservation of organic carbon in floodplains NATURE GEOSCIENCE Boye, K., Noel, V., Tfaily, M. M., Bone, S. E., Williams, K. H., Bargar, J. R., Fendorf, S. 2017; 10 (6): 415-+

  View details for DOI 10.1038/NGEO2940

  View details for Web of Science ID 000402579200009

• Understanding controls on redox processes in floodplain sediments of the Upper Colorado River Basin. The Science of the total environment Noël, V., Boye, K., Kukkadapu, R. K., Bone, S., Lezama Pacheco, J. S., Cardarelli, E., Janot, N., Fendorf, S., Williams, K. H., Bargar, J. R. 2017

  Abstract

  Floodplains, heavily used for water supplies, housing, agriculture, mining, and industry, are important repositories of organic carbon, nutrients, and metal contaminants. The accumulation and release of these species is often mediated by redox processes. Understanding the physicochemical, hydrological, and biogeochemical controls on the distribution and variability of sediment redox conditions is therefore critical to developing conceptual and numerical models of contaminant transport within floodplains. The Upper Colorado River Basin (UCRB) is impacted by former uranium and vanadium ore processing, resulting in contamination by V, Cr, Mn, As, Se, Mo and U. Previous authors have suggested that sediment redox activity occurring within organic carbon-enriched bodies located below the groundwater level may be regionally important to the maintenance and release of contaminant inventories, particularly uranium. To help assess this hypothesis, vertical distributions of Fe and S redox states and sulfide mineralogy were assessed in sediment cores from three floodplain sites spanning a 250km transect of the central UCRB. The results of this study support the hypothesis that organic-enriched reduced sediments are important zones of biogeochemical activity within UCRB floodplains. We found that the presence of organic carbon, together with pore saturation, are the key requirements for maintaining reducing conditions, which were dominated by sulfate-reduction products. Sediment texture was found to be of secondary importance and to moderate the response of the system to external forcing, such as oxidant diffusion. Consequently, fine-grain sediments are relatively resistant to oxidation in comparison to coarser-grained sediments. Exposure to oxidants consumes precipitated sulfides, with a disproportionate loss of mackinawite (FeS) as compared to the more stable pyrite. The accompanying loss of redox buffering capacity creates the potential for release of sequestered radionuclides and metals. Because of their redox reactivity and stores of metals, C, and N, organic-enriched sediments are likely to be important to nutrient and contaminant mobility within UCRB floodplain aquifers.

  View details for DOI 10.1016/j.scitotenv.2017.01.109

  View details for PubMedID 28359569

• Hexavalent Chromium Generation within Naturally Structured Soils and Sediments ENVIRONMENTAL SCIENCE & TECHNOLOGY Hausladen, D. M., Fendorf, S. 2017; 51 (4): 2058-2067

  Abstract

  Chromium(VI) produced from the oxidation of indigenous Cr(III) minerals is increasingly being recognized as a threat to groundwater quality. A critical determinant of Cr(VI) generation within soils and sediments is the necessary interaction of two low-solubility phases-Cr(III) silicates or (hydr)oxides and Mn(III/IV) oxides-that lead to its production. Here we investigate the potential for Cr(III) oxidation by Mn oxides within fixed solid matrices common to soils and sediments. Artificial aggregates were constructed from Cr(OH)3- and Cr0.25Fe0.75(OH)3-coated quartz grains and either mixed with synthetic birnessite or inoculated with the Mn(II)-oxidizing bacterium Leptothrix cholodnii. In aggregates simulating low organic carbon environments, we observe Cr(VI) concentrations within advecting solutes at levels more than twenty-times the California drinking water standard. Chromium(VI) production is highly dependent on Cr-mineral solubility; increasing Fe-substitution (x = 0 to x = 0.75) decreases the solubility of the solid and concomitantly decreases total Cr(VI) generation by 37%. In environments with high organic carbon, reducing conditions within aggregate cores (microbially) generate sufficient Fe(II) to suppress Cr(VI) efflux. Our results illustrate Cr(VI) generation from reaction with Mn oxides within structured media simulating soils and sediments and provide insight into how fluctuating hydrologic and redox conditions impact coupled processes controlling Cr and Mn cycling.

  View details for DOI 10.1021/acs.est.6b04039

  View details for Web of Science ID 000394724300019

  View details for PubMedID 28084730

• Quantifying Cr(VI) Production and Export from Serpentine Soil of the California Coast Range ENVIRONMENTAL SCIENCE & TECHNOLOGY McClain, C. N., Fendorf, S., Webb, S. M., Maher, K. 2017; 51 (1): 141-149

  Abstract

  Hexavalent chromium (Cr(VI)) is generated in serpentine soils and exported to surface and groundwaters at levels above health-based drinking water standards. Although Cr(VI) concentrations are elevated in serpentine soil pore water, few studies have reported field evidence documenting Cr(VI) production rates and fluxes that govern Cr(VI) transport from soil to water sources. We report Cr speciation (i) in four serpentine soil depth profiles derived from the California Coast Range serpentinite belt and (ii) in local surface waters. Within soils, we detected Cr(VI) in the same horizons where Cr(III)-minerals are colocated with biogenic Mn(III/IV)-oxides, suggesting Cr(VI) generation through oxidation by Mn-oxides. Water-extractable Cr(VI) concentrations increase with depth constituting a 7.8 to 12 kg/km(2) reservoir of Cr(VI) in soil. Here, Cr(VI) is produced at a rate of 0.3 to 4.8 kg Cr(VI)/km(2)/yr and subsequently flushed from soil during water infiltration, exporting 0.01 to 3.9 kg Cr(VI)/km(2)/yr at concentrations ranging from 25 to 172 μg/L. Although soil-derived Cr(VI) is leached from soil at concentrations exceeding 10 μg/L, due to reduction and dilution during transport to streams, Cr(VI) levels measured in local surface waters largely remain below California's drinking water limit.

  View details for DOI 10.1021/acs.est.6b03484

  View details for Web of Science ID 000391346900016

  View details for PubMedID 27935688

• Anoxic oxidation of chromium GEOLOGY Oze, C., Sleep, N. H., Coleman, R. G., Fendorf, S. 2016; 44 (7): 543-546

  View details for DOI 10.1130/G37844.1

  View details for Web of Science ID 000379358300019

• Aquifer Arsenic Cycling Induced by Seasonal Hydrologic Changes within the Yangtze River Basin ENVIRONMENTAL SCIENCE & TECHNOLOGY Schaefer, M. V., Ying, S. C., Benner, S. G., Duan, Y., Wang, Y., Fendorf, S. 2016; 50 (7): 3521-3529

  Abstract

  Consumption of groundwater containing >10 μg L(-1) arsenic (As) adversely impacts more than 100 million people worldwide. Multiyear trends in aquifer As concentrations have been documented, but strong seasonal variations are not commonly observed. Here we report dramatic seasonal changes in As concentrations and aquifer chemistry within the Jianghan Plain of the Yangtze River, China. At some wells, concentrations fluctuate by more than an order of magnitude within a single year (100-1200 μg L(-1)). Groundwater extraction and sustained water levels of surface channels during the dry season induces a strong downward hydraulic gradient, seasonally supplying oxidizing (oxygen, nitrate) water to the otherwise anoxic aquifer. Oxygen and/or nitrate addition promotes a transient drop in As concentrations for 1-3 months. When recharge ceases, reducing, low-arsenic conditions are reestablished by reactive, endogenous organic carbon. Temporal variability in As concentrations is especially problematic because it increases the probability of false-negative well testing during low-arsenic seasons. However, periods of low As may also provide a source of less toxic water for irrigation or other uses. Our results highlight the vulnerability and variability of groundwater resources in the Jianghan Plain and other inland basins within Asia to changing geochemical conditions, both natural and anthropogenic, and reinforce that continued monitoring of wells in high-risk regions is essential.

  View details for DOI 10.1021/acs.est.5b04986

  View details for Web of Science ID 000373655800027

  View details for PubMedID 26788939

• Numerical Modeling of Arsenic Mobility during Reductive Iron-Mineral Transformations ENVIRONMENTAL SCIENCE & TECHNOLOGY Rawson, J., Prommer, H., Siade, A., Carr, J., Berg, M., Davis, J. A., Fendorf, S. 2016; 50 (5): 2459-2467

  Abstract

  Millions of individuals worldwide are chronically exposed to hazardous concentrations of arsenic from contaminated drinking water. Despite massive efforts toward understanding the extent and underlying geochemical processes of the problem, numerical modeling and reliable predictions of future arsenic behavior remain a significant challenge. One of the key knowledge gaps concerns a refined understanding of the mechanisms that underlie arsenic mobilization, particularly under the onset of anaerobic conditions, and the quantification of the factors that affect this process. In this study, we focus on the development and testing of appropriate conceptual and numerical model approaches to represent and quantify the reductive dissolution of iron oxides, the concomitant release of sorbed arsenic, and the role of iron-mineral transformations. The initial model development in this study was guided by data and hypothesized processes from a previously reported,1 well-controlled column experiment in which arsenic desorption from ferrihydrite coated sands by variable loads of organic carbon was investigated. Using the measured data as constraints, we provide a quantitative interpretation of the processes controlling arsenic mobility during the microbial reductive transformation of iron oxides. Our analysis suggests that the observed arsenic behavior is primarily controlled by a combination of reductive dissolution of ferrihydrite, arsenic incorporation into or co-precipitation with freshly transformed iron minerals, and partial arsenic redox transformations.

  View details for DOI 10.1021/acs.est.5b05956

  View details for Web of Science ID 000371371700039

  View details for PubMedID 26835553

• Imaging geochemical heterogeneities using inverse reactive transport modeling: An example relevant for characterizing arsenic mobilization and distribution ADVANCES IN WATER RESOURCES Fakhreddine, S., Lee, J., Kitanidis, P. K., Fendorf, S., Rolle, M. 2016; 88: 186-197

  View details for DOI 10.1016/j.advwatres.2015.12.005

  View details for Web of Science ID 000371311800017

• Are oxygen limitations under recognized regulators of organic carbon turnover in upland soils? BIOGEOCHEMISTRY Keiluweit, M., Nico, P. S., Kleber, M., Fendorf, S. 2016; 127 (2-3): 157-171

  View details for DOI 10.1007/s10533-015-0180-6

  View details for Web of Science ID 000371606000001

• Physico-Chemical Heterogeneity of Organic-Rich Sediments in the Rifle Aquifer, CO: Impact on Uranium Biogeochemistry ENVIRONMENTAL SCIENCE & TECHNOLOGY Janot, N., Pacheco, J. S., Pham, D. Q., O'Brien, T. M., Hausladen, D., Noel, V., Lallier, F., Maher, K., Fendorf, S., Williams, K. H., Long, P. E., Bargar, J. R. 2016; 50 (1): 46-53

  Abstract

  The Rifle alluvial aquifer along the Colorado River in west central Colorado contains fine-grained, diffusion-limited sediment lenses that are substantially enriched in organic carbon and sulfides, as well as uranium, from previous milling operations. These naturally reduced zones (NRZs) coincide spatially with a persistent uranium groundwater plume. There is concern that uranium release from NRZs is contributing to plume persistence or will do so in the future. To better define the physical extent, heterogeneity and biogeochemistry of these NRZs, we investigated sediment cores from five neighboring wells. The main NRZ body exhibited uranium concentrations up to 100 mg/kg U as U(IV) and contains ca. 286 g of U in total. Uranium accumulated only in areas where organic carbon and reduced sulfur (as iron sulfides) were present, emphasizing the importance of sulfate-reducing conditions to uranium retention and the essential role of organic matter. NRZs further exhibited centimeter-scale variations in both redox status and particle size. Mackinawite, greigite, pyrite and sulfate coexist in the sediments, indicating that dynamic redox cycling occurs within NRZs and that their internal portions can be seasonally oxidized. We show that oxidative U(VI) release to the aquifer has the potential to sustain a groundwater contaminant plume for centuries. NRZs, known to exist in other uranium-contaminated aquifers, may be regionally important to uranium persistence.

  View details for DOI 10.1021/acs.est.5b03208

  View details for Web of Science ID 000367866300006

• Physico-Chemical Heterogeneity of Organic-Rich Sediments in the Rifle Aquifer, CO: Impact on Uranium Biogeochemistry. Environmental science & technology Janot, N., Lezama Pacheco, J. S., Pham, D. Q., O'Brien, T. M., Hausladen, D., Noël, V., Lallier, F., Maher, K., Fendorf, S., Williams, K. H., Long, P. E., Bargar, J. R. 2016; 50 (1): 46-53

  Abstract

  The Rifle alluvial aquifer along the Colorado River in west central Colorado contains fine-grained, diffusion-limited sediment lenses that are substantially enriched in organic carbon and sulfides, as well as uranium, from previous milling operations. These naturally reduced zones (NRZs) coincide spatially with a persistent uranium groundwater plume. There is concern that uranium release from NRZs is contributing to plume persistence or will do so in the future. To better define the physical extent, heterogeneity and biogeochemistry of these NRZs, we investigated sediment cores from five neighboring wells. The main NRZ body exhibited uranium concentrations up to 100 mg/kg U as U(IV) and contains ca. 286 g of U in total. Uranium accumulated only in areas where organic carbon and reduced sulfur (as iron sulfides) were present, emphasizing the importance of sulfate-reducing conditions to uranium retention and the essential role of organic matter. NRZs further exhibited centimeter-scale variations in both redox status and particle size. Mackinawite, greigite, pyrite and sulfate coexist in the sediments, indicating that dynamic redox cycling occurs within NRZs and that their internal portions can be seasonally oxidized. We show that oxidative U(VI) release to the aquifer has the potential to sustain a groundwater contaminant plume for centuries. NRZs, known to exist in other uranium-contaminated aquifers, may be regionally important to uranium persistence.

  View details for DOI 10.1021/acs.est.5b03208

  View details for PubMedID 26651843

• Arsenic release metabolically limited to permanently water-saturated soil in Mekong Delta NATURE GEOSCIENCE Stuckey, J., Schaefer, M. V., Kocar, B. D., Benner, S. G., Fendorf, S. 2016; 9 (1): 70-?

  View details for DOI 10.1038/NGEO2589

  View details for Web of Science ID 000367200300020

• Delineating the Convergence of Biogeochemical Factors Responsible for Arsenic Release to Groundwater in South and Southeast Asia ADVANCES IN AGRONOMY, VOL 140 Stuckey, J. W., Sparks, D. L., Fendorf, S. 2016; 140: 43-74

  View details for DOI 10.1016/bs.agron.2016.06.002

  View details for Web of Science ID 000394566200003

• Reactivity and speciation of mineral-associated arsenic in seasonal and permanent wetlands of the Mekong Delta GEOCHIMICA ET COSMOCHIMICA ACTA Stuckey, J. W., Schaefer, M. V., Benner, S. G., Fendorf, S. 2015; 171: 143-155

  View details for DOI 10.1016/j.gca.2015.09.002

  View details for Web of Science ID 000364822100009

• Indigenous arsenic(V)-reducing microbial communities in redox-fluctuating near-surface sediments of the Mekong Delta GEOBIOLOGY Ying, S. C., DAMASHEK, J., Fendorf, S., Francis, C. A. 2015; 13 (6): 581-587

  Abstract

  Arsenic (As) cycling within soils and sediments of the Mekong Delta of Cambodia is affected by drastic redox fluctuations caused by seasonal monsoons. Extensive flooding during monsoon seasons creates anoxic soil conditions that favor anaerobic microbial processes, including arsenate [As(V)] respiration-a process contributing to the mobilization of As. Repeated oxidation and reduction in near-surface sediments, which contain 10-40 mg kg(-1) As, lead to the eventual downward movement of As to the underlying aquifer. Amplification of a highly conserved functional gene encoding dissimilatory As(V) reductase, arrA, can be used as a molecular marker to detect the genetic potential for As(V) respiration in environmental samples. However, few studies have successfully amplified arrA from sediments without prior enrichment, which can drastically shift community structure. In the present study, we examine the distribution and diversity of arrA genes amplified from multiple sites within the Cambodian Mekong Delta as a function of near-surface depth (10, 50, 100, 200, and 400 cm), where sediments undergo seasonal redox fluctuations. We report successful amplification of 302 arrA gene sequences (72 OTUs) from near-surface Cambodian soils (without prior enrichment or stimulation with carbon amendments), where a large majority (>70%) formed a well-supported clade that is phylogenetically distinct from previously reported sequences from Cambodia and other South and Southeast Asian sediments, with highest sequence similarity to known Geobacter species capable of As(V) respiration, further supporting the potentially important role of Geobacter sp. in arsenic mobilization in these regions.

  View details for DOI 10.1111/gbi.12152

  View details for Web of Science ID 000362958500005

  View details for PubMedID 26466963

• Assessment of human-natural system characteristics influencing global freshwater supply vulnerability ENVIRONMENTAL RESEARCH LETTERS Padowski, J. C., Gorelick, S. M., Thompson, B. H., Rozelle, S., Fendorf, S. 2015; 10 (10)

  View details for DOI 10.1088/1748-9326/10/10/104014

  View details for Web of Science ID 000367180300019

• Geochemical Triggers of Arsenic Mobilization during Managed Aquifer Recharge ENVIRONMENTAL SCIENCE & TECHNOLOGY Fakhreddine, S., Dittmar, J., Phipps, D., Dadakis, J., Fendorf, S. 2015; 49 (13): 7802-7809

  Abstract

  Mobilization of arsenic and other trace metal contaminants during managed aquifer recharge (MAR) poses a challenge to maintaining local groundwater quality and to ensuring the viability of aquifer storage and recovery techniques. Arsenic release from sediments into solution has occurred during purified recycled water recharge of shallow aquifers within Orange County, CA. Accordingly, we examine the geochemical processes controlling As desorption and mobilization from shallow, aerated sediments underlying MAR infiltration basins. Further, we conducted a series of batch and column experiments to evaluate recharge water chemistries that minimize the propensity of As desorption from the aquifer sediments. Within the shallow Orange County Groundwater Basin sediments, the divalent cations Ca(2+) and Mg(2+) are critical for limiting arsenic desorption; they promote As (as arsenate) adsorption to the phyllosilicate clay minerals of the aquifer. While native groundwater contains adequate concentrations of dissolved Ca(2+) and Mg(2+), these cations are not present at sufficient concentrations during recharge of highly purified recycled water. Subsequently, the absence of dissolved Ca(2+) and Mg(2+) displaces As from the sediments into solution. Increasing the dosages of common water treatment amendments including quicklime (Ca(OH)2) and dolomitic lime (CaO·MgO) provides recharge water with higher concentrations of Ca(2+) and Mg(2+) ions and subsequently decreases the release of As during infiltration.

  View details for DOI 10.1021/acs.est.5b01140

  View details for Web of Science ID 000357840300038

• Geochemical Triggers of Arsenic Mobilization during Managed Aquifer Recharge. Environmental science & technology Fakhreddine, S., Dittmar, J., Phipps, D., Dadakis, J., Fendorf, S. 2015; 49 (13): 7802-9

  Abstract

  Mobilization of arsenic and other trace metal contaminants during managed aquifer recharge (MAR) poses a challenge to maintaining local groundwater quality and to ensuring the viability of aquifer storage and recovery techniques. Arsenic release from sediments into solution has occurred during purified recycled water recharge of shallow aquifers within Orange County, CA. Accordingly, we examine the geochemical processes controlling As desorption and mobilization from shallow, aerated sediments underlying MAR infiltration basins. Further, we conducted a series of batch and column experiments to evaluate recharge water chemistries that minimize the propensity of As desorption from the aquifer sediments. Within the shallow Orange County Groundwater Basin sediments, the divalent cations Ca(2+) and Mg(2+) are critical for limiting arsenic desorption; they promote As (as arsenate) adsorption to the phyllosilicate clay minerals of the aquifer. While native groundwater contains adequate concentrations of dissolved Ca(2+) and Mg(2+), these cations are not present at sufficient concentrations during recharge of highly purified recycled water. Subsequently, the absence of dissolved Ca(2+) and Mg(2+) displaces As from the sediments into solution. Increasing the dosages of common water treatment amendments including quicklime (Ca(OH)2) and dolomitic lime (CaO·MgO) provides recharge water with higher concentrations of Ca(2+) and Mg(2+) ions and subsequently decreases the release of As during infiltration.

  View details for DOI 10.1021/acs.est.5b01140

  View details for PubMedID 26057865

• Stable Isotopes and Iron Oxide Mineral Products as Markers of Chemodenitrification ENVIRONMENTAL SCIENCE & TECHNOLOGY Jones, L. C., Peters, B., Pacheco, J. S., Casciotti, K. L., Fendorf, S. 2015; 49 (6): 3444-3452

  Abstract

  When oxygen is limiting in soils and sediments, microorganisms utilize nitrate (NO3(-)) in respiration-through the process of denitrification-leading to the production of dinitrogen (N2) gas and trace amounts of nitrous (N2O) and nitric (NO) oxides. A chemical pathway involving reaction of ferrous iron (Fe(2+)) with nitrite (NO2(-)), an intermediate in the denitrification pathway, can also result in production of N2O. We examine the chemical reduction of NO2(-) by Fe(II)-chemodenitrification-in anoxic batch incubations at neutral pH. Aqueous Fe(2+) and NO2(-) reacted rapidly, producing N2O and generating Fe(III) (hydr)oxide mineral products. Lepidocrotite and goethite, identified by synchrotron X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopy, were produced from initially aqueous reactants, with two-line ferrihydrite increasing in abundance later in the reaction sequence. Based on the similarity of apparent rate constants with different mineral catalysts, we propose that the chemodenitrification rate is insensitive to the type of Fe(III) (hydr)oxide. With stable isotope measurements, we reveal a narrow range of isotopic fractionation during NO2(-) reduction to N2O. The location of N isotopes in the linear N2O molecule, known as site preference, was also constrained to a signature range. The coexistence of Fe(III) (hydr)oxide, characteristic (15)N and (18)O fractionation, and N2O site preference may be used in combination to qualitatively distinguish between abiotic and biogenically emitted N2O-a finding important for determining N2O sources in natural systems.

  View details for DOI 10.1021/es504862x

  View details for Web of Science ID 000351324400022

  View details for PubMedID 25683572

• Peat formation concentrates arsenic within sediment deposits of the Mekong Delta GEOCHIMICA ET COSMOCHIMICA ACTA Stuckey, J. W., Schaefer, M. V., Kocar, B. D., Dittmar, J., Pacheco, J. L., Benner, S. G., Fendorf, S. 2015; 149: 190-205

  View details for DOI 10.1016/j.gca.2014.10.021

  View details for Web of Science ID 000346744500013

• Competing retention pathways of uranium upon reaction with Fe(II) GEOCHIMICA ET COSMOCHIMICA ACTA Massey, M. S., Lezama-Pacheco, J. S., Jones, M. E., Ilton, E. S., Cerrato, J. M., Bargar, J. R., Fendorf, S. 2014; 142: 166-185

  View details for DOI 10.1016/j.gca.2014.07.016

  View details for Web of Science ID 000342622400012

• Uranium incorporation into aluminum-substituted ferrihydrite during iron(ii)-induced transformation. Environmental science. Processes & impacts Massey, M. S., Lezama-Pacheco, J. S., Michel, F. M., Fendorf, S. 2014; 16 (9): 2137-2144

  Abstract

  Uranium retention processes (adsorption, precipitation, and incorporation into host minerals) exert strong controls on U mobility in the environment, and understanding U retention is therefore crucial for predicting the migration of U within surface and groundwater. Uranium can be incorporated into Fe (hydr)oxides during Fe(ii)-induced transformation of ferrihydrite to goethite. However, ferrihydrite seldom exists as a pure phase within soils or sediments, and structural impurities such as Al alter its reactivity. The presence of Al in ferrihydrite, for example, decreases the rate of transformation to goethite, and thus may impact the retention pathway, or extent of retention, of U. Here, we investigate the extent and pathways of U(vi) retention on Al-ferrihydrite during Fe(ii)-induced transformation. Ferrihydrite containing 0%, 1%, 5%, 10%, and 20% Al was reacted with 10 μM U and 300 μM Fe(ii) in the presence of 0 mM and 4 mM Ca(2+) and 3.8 mM carbonate at pH 7.0. Solid reaction products were characterized using U L3-edge EXAFS spectroscopy to differentiate between adsorbed U and U incorporated into the goethite lattice. Uranium incorporation into Al-ferrihydrite declined from ∼70% of solid-phase U at 0% and 1% Al to ∼30% of solid phase U at 20% Al content. The decrease in U incorporation with increasing Al concentration was due to two main factors: (1) decreased transformation of ferrihydrite to goethite; and, (2) a decrease of the goethite lattice with increasing Al, making the lattice less compatible with large U atoms. However, uranium incorporation can occur even with an Al-substituted ferrihydrite precursor in the presence or absence of Ca(2+). The process of U incorporation into Al-goethite may therefore be a potential long-term sink of U in subsurface environments where Al-substituted iron oxides are common, albeit at lower levels of incorporation with increasing Al content.

  View details for DOI 10.1039/c4em00148f

  View details for PubMedID 25124142

• Uranium Incorporation into Amorphous Silica ENVIRONMENTAL SCIENCE & TECHNOLOGY Massey, M. S., Lezama-Pacheco, J. S., Nelson, J. M., Fendor, S., Maher, K. 2014; 48 (15): 8636-8644

  Abstract

  High concentrations of uranium are commonly observed in naturally occurring amorphous silica (including opal) deposits, suggesting that incorporation of U into amorphous silica may represent a natural attenuation mechanism and promising strategy for U remediation. However, the stability of uranium in opaline silicates, determined in part by the binding mechanism for U, is an important factor in its long-term fate. U may bind directly to the opaline silicate matrix, or to materials such as iron (hydr)oxides that are subsequently occluded within the opal. Here, we examine the coordination environment of U within opaline silica to elucidate incorporation mechanisms. Precipitates (with and without ferrihydrite inclusions) were synthesized from U-bearing sodium metasilicate solutions, buffered at pH ∼ 5.6. Natural and synthetic solids were analyzed with X-ray absorption spectroscopy and a suite of other techniques. In synthetic amorphous silica, U was coordinated by silicate in a double corner-sharing coordination geometry (Si at ∼ 3.8-3.9 Å) and a small amount of uranyl and silicate in a bidentate, mononuclear (edge-sharing) coordination (Si at ∼ 3.1-3.2 Å, U at ∼ 3.8-3.9 Å). In iron-bearing synthetic solids, U was adsorbed to iron (hydr)oxide, but the coordination environment also contained silicate in both edge-sharing and corner-sharing coordination. Uranium local coordination in synthetic solids is similar to that of natural U-bearing opals that retain U for millions of years. The stability and extent of U incorporation into opaline and amorphous silica represents a long-term repository for U that may provide an alternative strategy for remediation of U contamination.

  View details for DOI 10.1021/es501064m

  View details for Web of Science ID 000340080600039

  View details for PubMedID 24984107

• Arsenic in the Multi-aquifer System of the Mekong Delta, Vietnam: Analysis of Large-Scale Spatial Trends and Controlling Factors. Environmental science & technology Erban, L. E., Gorelick, S. M., Fendorf, S. 2014; 48 (11): 6081-6088

  Abstract

  Groundwater exploitation is rising in the Mekong Delta, Vietnam, potentially exacerbating arsenic contamination from natural sources. We investigate trends and controls on contamination patterns throughout the Delta's multi-aquifer system as observed in a spatially exhaustive data set of arsenic measured in >40,000 wells, 10.5% of which exceed the WHO drinking water standard for arsenic (10 μg/L). We relate strong trends in the distribution of contamination among well samples to explanatory variables derived from 3D ancillary physicochemical data sets using logistic regression models. Parsimonious models describe much of the observed variability in arsenic occurrence, which differs considerably between subsets of wells tapping shallow versus deeper aquifer groups. In the shallowest Holocene-Pleistocene aquifers, arsenic occurrence is best described by distance to the Mekong river channels and delta front, depth, and location within fault-bounded zones of the region. The same model, however, fails to explain observations in the deeper group of Pliocene-Miocene aquifers. Among these deeper units, arsenic occurrence is rare except among older wells in near-river, heavily pumped areas. Our analysis is the first to examine both natural and anthropogenically mediated contributions to the distribution of arsenic throughout the Mekong Delta's multi-aquifer system, with implications for management of similarly affected basins throughout Southeast Asia.

  View details for DOI 10.1021/es403932t

  View details for PubMedID 24849074

• Arsenic Concentrations in Paddy Soil and Rice and Health Implications for Major Rice-Growing Regions of Cambodia ENVIRONMENTAL SCIENCE & TECHNOLOGY Seyfferth, A. L., McCurdy, S., Schaefer, M. V., Fendorf, S. 2014; 48 (9): 4699-4706

  Abstract

  Despite the global importance of As in rice, research has primarily focused on Bangladesh, India, China, and the United States with limited attention given to other countries. Owing to both indigenous As within the soil and the possible increases arising from the onset of irrigation with groundwater, an assessment of As in rice within Cambodia is needed, which offers a "base-case" comparison against sediments of similar origin that comprise rice paddy soils where As-contaminated water is used for irrigation (e.g., Bangladesh). Here, we evaluated the As content of rice from five provinces (Kandal, Prey Veng, Battambang, Banteay Meanchey, and Kampong Thom) in the rice-growing regions of Cambodia and coupled that data to soil-chemical factors based on extractions of paddy soil collected and processed under anoxic conditions. At total soil As concentrations ranging 0.8 to 18 μg g(-1), total grain As concentrations averaged 0.2 μg g(-1) and ranged from 0.1 to 0.37 with Banteay Meanchey rice having significantly higher values than Prey Veng rice. Overall, soil-extractable concentrations of As, Fe, P, and Si and total As were poor predictors of grain As concentrations. While biogeochemical factors leading to reduction of As(V)-bearing Fe(III) oxides are likely most important for predicting plant-available As, husk and straw As concentrations were the most significant predictors of grain-As levels among our measured parameters.

  View details for DOI 10.1021/es405016t

  View details for Web of Science ID 000335720100008

  View details for PubMedID 24712677

• Constraints on Precipitation of the Ferrous Arsenite Solid H7Fe4(AsO3)(5) JOURNAL OF ENVIRONMENTAL QUALITY Masue-Slowey, Y., Slowey, A. J., Michel, F. M., Webb, S. M., Fendorf, S. 2014; 43 (3): 947-954

  Abstract

  Formation of Fe(II)-As(III) solids is suspected to limit dissolved As concentrations in anaerobic environments. Iron(II) precipitates enriched in As(III) have been observed after microbial reduction of As(V)-loaded lepidocrocite (γ-FeOOH) and symplesite (Fe(II)(As(V)O)]·8HO) and upon abiotic reaction of Fe(II) with As(III). However, the conditions favorable for Fe(II)-As(III) precipitation and the long-term stability (relative to dissolution) of this phase are unknown. Here we examine the composition, local structure, and solubility of an Fe(II)-As(III) precipitate to determine environments where such a solid may form and persist. We reveal that the Fe(II)-As(III) precipitate has a composition of HFe(AsO) and a log of 34 for the dissolution reaction defined as: HFe(AsO) + 8H = 4Fe + 5HAsO. Extended X-ray absorption fine structure spectroscopic analysis of HFe(AsO) shows that the molecular environment of Fe is dominated by edge-sharing octahedra within an Fe(OH) sheet and that As is dominated by corner-sharing AsO pyramids, which are consistent with previously published structures of As(III)-rich Fe(II) solids. The HFe(AsO) solid has a pH-dependent solubility and requires millimolar concentrations of dissolved Fe(II) and As(III) to precipitate at pH <7.5. By contrast, alkaline conditions are more conducive to formation of HFe(AsO); however, a high concentration of Fe(II) is required, which is unusual under alkaline conditions.

  View details for DOI 10.2134/jeq2013.08.0340

  View details for Web of Science ID 000336275700017

  View details for PubMedID 25602823

• Deciphering and predicting spatial and temporal concentrations of arsenic within the Mekong Delta aquifer ENVIRONMENTAL CHEMISTRY Kocar, B. D., Benner, S. G., Fendorf, S. 2014; 11 (5): 579-594

  View details for DOI 10.1071/EN13244

  View details for Web of Science ID 000342891400011

• Seasonal dynamics of dissolved silicon in a rice cropping system after straw incorporation GEOCHIMICA ET COSMOCHIMICA ACTA Seyfferth, A. L., Kocar, B. D., Lee, J. A., Fendorf, S. 2013; 123: 120-133

  View details for DOI 10.1016/j.gca.2013.09.015

  View details for Web of Science ID 000326871400008

• Influence of Soil Geochemical and Physical Properties on Chromium(VI) Sorption and Bioaccessibility ENVIRONMENTAL SCIENCE & TECHNOLOGY Jardine, P. M., Stewart, M. A., Barnett, M. O., Basta, N. T., Brooks, S. C., Fendorf, S., Mehlhorn, T. L. 2013; 47 (19): 11241-11248

  Abstract

  The Department of Defense (DoD) is faced with the daunting task of possible remediation of numerous soil-Cr(VI) contaminated sites throughout the continental U.S. The primary risk driver at these sites is hand-to-mouth ingestion of contaminated soil by children. In the following study we investigate the impact of soil geochemical and physical properties on the sorption and bioaccessibility of Cr(VI) in a vast array of soils relevant to neighboring DoD sites. For the 35 soils used in this study, A-horizon soils typically sorbed significantly more Cr(VI) relative to B-horizon soils. Multiple linear regression analysis suggested that Cr(VI) sorption increased with increasing soil total organic C (TOC) and decreasing soil pH. The bioaccessibility of total Cr (CrT) and Cr(VI) on the soils decreased with increasing soil TOC content. As the soil TOC content approached 0.4%, the bioaccessibility of soil bound Cr systematically decreased from approximately 65 to 10%. As the soil TOC content increased from 0.4 to 4%, the bioaccessibility of Cr(VI) and CrT remained relatively constant at approximately 4% and 10%, respectively. X-ray absorption near edge structure (XANES) spectroscopy suggested that Cr(VI) reduction to Cr(III) was prevalent and that the redox transformation of Cr(VI) increased with increasing soil TOC. XANES confirmed that nearly all bioaccessible soil Cr was the Cr(VI) moiety. Multiple linear regression analysis suggested that the bioaccessibility of Cr(VI) and its reduced counterpart Cr(III), decreased with increasing soil TOC and increasing soil pH. This is consistent with the observation that the reduction reaction and formation of Cr(III) increased with increasing soil TOC and that Cr(III) was significantly less bioaccessible relative to Cr(VI). The model was found to adequately describe CrT bioaccessibility in soils from DoD facilities where Cr(VI) contaminated sites were present. The results of this study illustrate the importance of soil properties on Cr(VI) sorption and bioassessability and help define what soil types have the greatest risk associated with Cr(VI) exposure.

  View details for DOI 10.1021/es401611h

  View details for Web of Science ID 000330094900062

  View details for PubMedID 23941581

• Release of arsenic to deep groundwater in the Mekong Delta, Vietnam, linked to pumping-induced land subsidence. Proceedings of the National Academy of Sciences of the United States of America Erban, L. E., Gorelick, S. M., Zebker, H. A., Fendorf, S. 2013; 110 (34): 13751-13756

  Abstract

  Deep aquifers in South and Southeast Asia are increasingly exploited as presumed sources of pathogen- and arsenic-free water, although little is known of the processes that may compromise their long-term viability. We analyze a large area (>1,000 km(2)) of the Mekong Delta, Vietnam, in which arsenic is found pervasively in deep, Pliocene-Miocene-age aquifers, where nearly 900 wells at depths of 200-500 m are contaminated. There, intensive groundwater extraction is causing land subsidence of up to 3 cm/y as measured using satellite-based radar images from 2007 to 2010 and consistent with transient 3D aquifer simulations showing similar subsidence rates and total subsidence of up to 27 cm since 1988. We propose a previously unrecognized mechanism in which deep groundwater extraction is causing interbedded clays to compact and expel water containing dissolved arsenic or arsenic-mobilizing solutes (e.g., dissolved organic carbon and competing ions) to deep aquifers over decades. The implication for the broader Mekong Delta region, and potentially others like it across Asia, is that deep, untreated groundwater will not necessarily remain a safe source of drinking water.

  View details for DOI 10.1073/pnas.1300503110

  View details for PubMedID 23918360

  View details for PubMedCentralID PMC3752228

• Dependence of Arsenic Fate and Transport on Biogeochemical Heterogeneity Arising from the Physical Structure of Soils and Sediments JOURNAL OF ENVIRONMENTAL QUALITY Masue-Slowey, Y., Ying, S. C., Kocar, B. D., Pallud, C. E., Fendorf, S. 2013; 42 (4): 1119-1129

  View details for DOI 10.2134/jeq2012.0253

  View details for Web of Science ID 000321822300018

• Dependence of arsenic fate and transport on biogeochemical heterogeneity arising from the physical structure of soils and sediments. Journal of environmental quality Masue-Slowey, Y., Ying, S. C., Kocar, B. D., Pallud, C. E., Fendorf, S. 2013; 42 (4): 1119-1129

  Abstract

  Reduction of As(V) and Fe(III) is commonly the dominant process controlling the fate and transport of As in soils and sediments. However, the physical structure of such environments produces complex heterogeneity in biogeochemical processes controlling the fate and transport of As. To resolve the role of soil and sediment physical structure on the distribution of biogeochemical processes controlling the fate and transport of As, we examined the biogeochemical transformations of As and Fe within constructed aggregates-a fundamental unit of soil structure. Spherical aggregates were made with As(V)- or As(III)-bearing, ferrihydrite-coated quartz that was fused with agarose and placed in a cylindrical reactor; advective flow of anoxic solutes was then initiated around the aggregates to examine As release from a dual-pore domain system. To examine the impact of biotic As(V) and Fe(III) reduction, constructed aggregates having As(V)-bearing, ferrihydrite-coated quartz inoculated with sp. ANA-3 were placed in flow-through reactors under anoxic and aerated advective flow. Consistent with desorption from advective columns, As(III) is released to advecting water more prevalently than As(V) within abiotic aggregate systems, indicating a greater lability and concomitant enhanced propensity for transport of As(III) relative to As(V). During reaction with , As release to advecting water was similar between anoxic and aerated systems for the first 20 d; thereafter, the anoxic advecting solutes increased As release relative to the aerated counterpart. With aerated advecting solutes, Fe remained oxidized (or was oxidized) in the aggregate exterior, forming a protective barrier that limited As release to the advective channel. However, anaerobiosis within the aggregate interior, even with aerated advective flow, promotes internal repartitioning of As to the exterior region.

  View details for DOI 10.2134/jeq2012.0253

  View details for PubMedID 24216363

• Distributed microbially- and chemically-mediated redox processes controlling arsenic dynamics within Mn-/Fe-oxide constructed aggregates GEOCHIMICA ET COSMOCHIMICA ACTA Ying, S. C., Masue-Slowey, Y., Kocar, B. D., Griffis, S. D., Webb, S., Marcus, M. A., Francis, C. A., Fendorf, S. 2013; 104: 29-41

  View details for DOI 10.1016/j.gca.2012.08.020

  View details for Web of Science ID 000314664500003

• Morphological Adaptations for Digging and Climate-Impacted Soil Properties Define Pocket Gopher (Thomomys spp.) Distributions. PloS one Marcy, A. E., Fendorf, S., Patton, J. L., Hadly, E. A. 2013; 8 (5)

  Abstract

  Species ranges are mediated by physiology, environmental factors, and competition with other organisms. The allopatric distribution of five species of northern Californian pocket gophers (Thomomys spp.) is hypothesized to result from competitive exclusion. The five species in this environmentally heterogeneous region separate into two subgenera, Thomomys or Megascapheus, which have divergent digging styles. While all pocket gophers dig with their claws, the tooth-digging adaptations of subgenus Megascapheus allow access to harder soils and climate-protected depths. In a Northern Californian locality, replacement of subgenus Thomomys with subgenus Megascapheus occurred gradually during the Pleistocene-Holocene transition. Concurrent climate change over this transition suggests that environmental factors - in addition to soil - define pocket gopher distributional limits. Here we show 1) that all pocket gophers occupy the subset of less energetically costly soils and 2) that subgenera sort by percent soil clay, bulk density, and shrink-swell capacity (a mineralogical attribute). While clay and bulk density (without major perturbations) stay constant over decades to millennia, low precipitation and high temperatures can cause shrink-swell clays to crack and harden within days. The strong yet underappreciated interaction between soil and moisture on the distribution of vertebrates is rarely considered when projecting species responses to climatic change. Furthermore, increased precipitation alters the weathering processes that create shrink-swell minerals. Two projected outcomes of ongoing climate change-higher temperatures and precipitation-will dramatically impact hardness of soil with shrink-swell minerals. Current climate models do not include factors controlling soil hardness, despite its impact on all organisms that depend on a stable soil structure.

  View details for DOI 10.1371/journal.pone.0064935

  View details for PubMedID 23717675

  View details for PubMedCentralID PMC3663803

• Silicate Mineral Impacts on the Uptake and Storage of Arsenic and Plant Nutrients in Rice (Oryza sativa L.) ENVIRONMENTAL SCIENCE & TECHNOLOGY Seyfferth, A. L., Fendorf, S. 2012; 46 (24): 13176-13183

  Abstract

  Arsenic-contaminated rice grain may threaten human health globally. Since H₃AsO₃⁰ is the predominant As species found in paddy pore-waters, and H₄SiO₄⁰ and H₃AsO₃⁰ share an uptake pathway, silica amendments have been proposed to decrease As uptake and consequent As concentrations in grains. Here, we evaluated the impact of two silicate mineral additions differing in solubility (+Si(L), diatomaceous earth, 0.29 mM Si; +Si(H), Si-gel, 1.1 mM Si) to soils differing in mineralogy on arsenic concentration in rice. The +Si(L) addition either did not change or decreased As concentration in pore-water but did not change or increased grain-As levels relative to the (+As--Si) control. The +Si(H) addition increased As in pore-water, but it significantly decreased grain-As relative to the (+As--Si) control. Only the +Si(H) addition resulted in significant increases in straw- and husk-Si. Total grain- and straw-As was negatively correlated with pore-water Si, and the relationship differed between two soils exhibiting different mineralogy. These differing results are a consequence of competition between H₄SiO₄⁰ and H₃AsO₃⁰ for adsorption sites on soil solids and subsequent plant-uptake, and illustrate the importance of Si mineralogy on arsenic uptake.

  View details for DOI 10.1021/es3025337

  View details for Web of Science ID 000312432200020

  View details for PubMedID 23153302

• Oxidation and competitive retention of arsenic between iron- and manganese oxides GEOCHIMICA ET COSMOCHIMICA ACTA Ying, S. C., Kocar, B. D., Fendorf, S. 2012; 96: 294-303

  View details for DOI 10.1016/j.gca.2012.07.013

  View details for Web of Science ID 000310427200019

• Intra-particle migration of mercury in granular polysulfide-rubber-coated activated carbon (PSR-AC) CHEMOSPHERE Kim, E., Masue-Slowey, Y., Fendorf, S., Luthy, R. G. 2012; 86 (6): 648-654

  Abstract

  The depth profile of mercuric ion after the reaction with polysulfide-rubber-coated activated carbon (PSR-AC) was investigated using micro-X-ray fluorescence (μ-XRF) imaging techniques and mathematical modeling. The μ-XRF results revealed that mercury was concentrated at 0-100 μm from the exterior of the particle after 3 months of treatment with PSR-AC in 10 ppm HgCl(2) aqueous solution. The μ-X-ray absorption near edge spectroscopic (μ-XANES) analyses indicated HgS as a major mercury species, and suggested that the intra-particle mercury transport involved a chemical reaction with PSR polymer. An intra-particle mass transfer model was developed based on either a Langmuir sorption isotherm with liquid phase diffusion (Langmuir model) or a kinetic sorption with surface diffusion (kinetic sorption model). The Langmuir model predicted the general trend of mercury diffusion, although at a slower rate than observed from the μ-XRF map. A kinetic sorption model suggested faster mercury transport, which overestimated the movement of mercuric ions through an exchange reaction between the fast and slow reaction sites. Both μ-XRF and mathematical modeling results suggest mercury removal occurs not only at the outer surface of the PSR-AC particle but also at some interior regions due to a large PSR surface area within an AC particle.

  View details for DOI 10.1016/j.chemosphere.2011.11.012

  View details for Web of Science ID 000301096200014

  View details for PubMedID 22133913

  View details for PubMedCentralID PMC3265622

• Native and Non-Native Community Assembly through Edaphic Manipulation: Implications for Habitat Creation and Restoration RESTORATION ECOLOGY Bonebrake, T. C., Navratil, R. T., Boggs, C. L., Fendorf, S., Field, C. B., Ehrlichl, P. R. 2011; 19 (6): 709-716

  View details for DOI 10.1111/j.1526-100X.2010.00768.x

  View details for Web of Science ID 000297078600006

• Defining the distribution of arsenic species and plant nutrients in rice (Oryza sativa L.) from the root to the grain GEOCHIMICA ET COSMOCHIMICA ACTA Seyfferth, A. L., Webb, S. M., Andrews, J. C., Fendorf, S. 2011; 75 (21): 6655-6671

  View details for DOI 10.1016/j.gca.2011.06.029

  View details for Web of Science ID 000296566600024

• Geochemical Processes Governing the Fate and Transport of Chromium(III) and Chromium(VI) in Soils VADOSE ZONE JOURNAL Jardine, P. M., Mehlhorn, T. L., Bailey, W. B., Brooks, S. C., Fendorf, S., Gentry, R. W., Phelps, T. J., Saiers, J. E. 2011; 10 (3): 1058-1070

  View details for DOI 10.2136/vzj2010.0102

  View details for Web of Science ID 000294007600022

• Competitive Microbially and Mn Oxide Mediated Redox Processes Controlling Arsenic Speciation and Partitioning ENVIRONMENTAL SCIENCE & TECHNOLOGY Ying, S. C., Kocar, B. D., Griffis, S. D., Fendorf, S. 2011; 45 (13): 5572-5579

  Abstract

  The speciation and partitioning of arsenic (As) in surface and subsurface environments are controlled, in part, by redox processes. Within soils and sediments, redox gradients resulting from mass transfer limitations lead to competitive reduction-oxidation reactions that drive the fate of As. Accordingly, the objective of this study was to determine the fate and redox cycling of As at the interface of birnessite (a strong oxidant in soil with a nominal formula of MnO(x), where x ≈ 2) and dissimilatory As(V)-reducing bacteria (strong reductant). Here, we investigate As reduction-oxidation dynamics in a diffusively controlled system using a Donnan reactor where birnessite and Shewanella sp. ANA-3 are isolated by a semipermeable membrane through which As migrates. Arsenic(III) injected into the reaction cell containing birnessite is rapidly oxidized to As(V). Arsenic(V) diffusing into the Shewanella chamber is then reduced to As(III), which subsequently diffuses back to the birnessite chamber, undergoing oxidation, and establishing a continuous cycling of As. However, we observe a rapid decline in the rate of As(III) oxidation owing to passivation of the birnessite surface. Modeling and experimental results show that high [Mn(II)] combined with increasing [CO(3)(2-)] from microbial respiration leads to the precipitation of rhodochrosite, which eventually passivates the Mn oxide surface, inhibiting further As(III) oxidation. Our results show that despite the initial capacity of birnessite to rapidly oxidize As(III), the synergistic effect of intense As(V) reduction by microorganisms and the buildup of reactive metabolites capable of passivating reactive mineral surfaces-here, birnessite-will produce (bio)geochemical conditions outside of those based on thermodynamic predictions.

  View details for DOI 10.1021/es200351m

  View details for Web of Science ID 000292075100019

  View details for PubMedID 21648436

• Reduction of Uranium(VI) by Soluble Iron(II) Conforms with Thermodynamic Predictions ENVIRONMENTAL SCIENCE & TECHNOLOGY Du, X., Boonchayaanant, B., Wu, W., Fendorf, S., Bargar, J., Criddle, C. S. 2011; 45 (11): 4718-4725

  Abstract

  Soluble Fe(II) can reduce soluble U(VI) at rapid rates and in accordance with thermodynamic predictions. This was established by initially creating acidic aqueous solutions in which the sole oxidants were soluble U(VI) species and the sole reductants were soluble Fe(II) species. The pH of the solution was then increased by stepwise addition of OH(-), thereby increasing the potential for electron transfer from Fe(II) to U(VI). For each new pH value resulting from addition of base, values of ΔG for the Fe(II)-mediated reduction of U(VI) were calculated using the computed distribution of U and Fe species and possible half reaction combinations. For initial conditions of pH 2.4 and a molar ratio of Fe(II) to U(VI) of 5:1 (1 mM Fe(II) and 0.2 mM U(VI)), ΔG for U(VI) reduction was greater than zero, and U(VI) reduction was not observed. When sufficient OH(-) was added to exceed the computed equilibrium pH of 5.4, ΔG for U(VI) reduction was negative and soluble Fe(II) species reacted with U(VI) in a molar ratio of ∼2:1. X-ray absorption near-edge structure (XANES) spectroscopy confirmed production of U(IV). A decrease in pH confirmed production of acidity as the reaction advanced. As solution pH decreased to the equilibrium value, the rate of reaction declined, stopping completely at the predicted equilibrium pH. Initiation of the reaction at a higher pH resulted in a higher final ratio of U(IV) to U(VI) at equilibrium.

  View details for DOI 10.1021/es2006012

  View details for Web of Science ID 000291128700011

  View details for PubMedID 21553877

• Dehalogenation of Polybrominated Diphenyl Ethers and Polychlorinated Biphenyl by Bimetallic, Impregnated, and Nanoscale Zerovalent Iron ENVIRONMENTAL SCIENCE & TECHNOLOGY Zhuang, Y., Ahn, S., Seyfferth, A. L., Masue-Slowey, Y., Fendorf, S., Luthy, R. G. 2011; 45 (11): 4896-4903

  Abstract

  Nanoscale zerovalent iron particles (nZVI), bimetallic nanoparticles (nZVI/Pd), and nZVI/Pd impregnated activated carbon (nZVI/Pd-AC) composite particles were synthesized and investigated for their effectiveness to remove polybrominated diphenyl ethers (PBDEs) and/or polychlorinated biphenyls (PCBs). Palladization of nZVI promoted the dehalogenation kinetics for mono- to tri-BDEs and 2,3,4-trichlorobiphenyl (PCB 21). Compared to nZVI, the iron-normalized rate constants for nZVI/Pd were about 2-, 3-, and 4-orders of magnitude greater for tri-, di-, and mono-BDEs, respectively, with diphenyl ether as a main reaction product. The reaction kinetics and pathways suggest an H-atom transfer mechanism. The reaction pathways with nZVI/Pd favor preferential removal of para-halogens on PBDEs and PCBs. X-ray fluorescence mapping of nZVI/Pd-AC showed that Pd mainly deposits on the outer part of particles, while Fe was present throughout the activated carbon particles. While BDE 21 was sorbed onto activated carbon composites quickly, debromination was slower compared to reaction with freely dispersed nZVI/Pd. Our XPS and chemical data suggest about 7% of the total iron within the activated carbon was zerovalent, which shows the difficulty with in-situ synthesis of a significant fraction of zerovalent iron in the microporous material. Related factors that likely hinder the reaction with nZVI/Pd-AC are the heterogeneous distribution of nZVI and Pd on activated carbon and/or immobilization of hydrophobic organic contaminants at the adsorption sites thereby inhibiting contact with nZVI.

  View details for DOI 10.1021/es104312h

  View details for Web of Science ID 000291128700035

  View details for PubMedID 21557574

  View details for PubMedCentralID PMC3122486

• Alteration of ferrihydrite reductive dissolution and transformation by adsorbed As and structural Al: Implications for As retention GEOCHIMICA ET COSMOCHIMICA ACTA Masue-Slowey, Y., Loeppert, R. H., Fendorf, S. 2011; 75 (3): 870-886

  View details for DOI 10.1016/j.gca.2010.11.016

  View details for Web of Science ID 000286367900011

• Influence of Natural Organic Matter on As Transport and Retention ENVIRONMENTAL SCIENCE & TECHNOLOGY Sharma, P., Rolle, M., Kocar, B., Fendorf, S., Kappler, A. 2011; 45 (2): 546-553

  Abstract

  Natural organic matter (NOM) can affect the behavior of arsenic within surface and subsurface environments. We used batch and column experiments to determine the effect of peat humic acids (PHA), groundwater fulvic acids (GFA), and a soil organic matter (SOM) extract on As sorption/transport in ferrihydrite-coated sand columns. A reactive transport model was used to quantitatively interpret the transport of As in flow-through column (breakthrough) experiments. We found that As(III) breakthrough was faster than As(V) by up to 18% (with OM) and 14% (without OM). The most rapid breakthrough occurred in systems containing SOM and GFA. Dialysis and ultrafiltration of samples from breakthrough experiments showed that in OM-containing systems, As was transported mostly as free (noncomplexed) dissolved As but also as ternary As-Fe-OM colloids and dissolved complexes. In OM-free systems, As was transported in colloidal form or as a free ion. During desorption, more As(III) desorbed (23-37%) than As(V) (10-16%), and SOM resulted in the highest and OM-free systems the lowest amount of desorption. Overall, our experiments reveal that (i) NOM can enhance transport/mobilization of As, (ii) different fractions of NOM are capable of As mobilization, and (iii) freshly extracted SOM (from a forest soil) had greater impact on As transport than purified GFA/PHA.

  View details for DOI 10.1021/es1026008

  View details for Web of Science ID 000286090500034

  View details for PubMedID 21142173

• Transport Implications Resulting from Internal Redistribution of Arsenic and Iron within Constructed Soil Aggregates ENVIRONMENTAL SCIENCE & TECHNOLOGY Masue-Slowey, Y., Kocar, B. D., Jofre, S. A., Mayer, K. U., Fendorf, S. 2011; 45 (2): 582-588

  Abstract

  Soils are an aggregate-based structured media that have a multitude of pore domains resulting in varying degrees of advective and diffusive solute and gas transport. Consequently, a spectrum of biogeochemical processes may function at the aggregate scale that collectively, and coupled with solute transport, determine element cycling in soils and sediments. To explore how the physical structure impacts biogeochemical processes influencing the fate and transport of As, we examined temporal changes in speciation and distribution of As and Fe within constructed aggregates through experimental measurement and reactive transport simulations. Spherical aggregates were made with As(V)-bearing ferrihydrite-coated sand inoculated with Shewanella sp. ANA-3; aerated solute flow around the aggregate was then induced. Despite the aerated aggregate exterior, where As(V) and ferrihydrite persist as the dominant species, anoxia develops within the aggregate interior. As a result, As and Fe redox gradients emerge, and the proportion of As(III) and magnetite increases toward the aggregate interior. Arsenic(III) and Fe(II) produced in the interior migrate toward the aggregated exterior and result in coaccumulation of As and Fe(III) proximal to preferential flow paths as a consequence of oxygenic precipitation. The oxidized rind of aggregates thus serves as a barrier to As release into advecting pore-water, but also leads to be a buildup of this hazardous element at preferential flow boundaries that could be released upon shifting geochemical conditions.

  View details for DOI 10.1021/es1027663

  View details for Web of Science ID 000286090500039

  View details for PubMedID 21158450

• Immobilization of Hg(II) in water with polysulfide-rubber (PSR) polymer-coated activated carbon WATER RESEARCH Kim, E., Seyfferth, A. L., Fendorf, S., Luthy, R. G. 2011; 45 (2): 453-460

  Abstract

  An effective mercury removal method using polymer-coated activated carbon was studied for possible use in water treatment. In order to increase the affinity of activated carbon for mercury, a sulfur-rich compound, polysulfide-rubber (PSR) polymer, was effectively coated onto the activated carbon. The polymer was synthesized by condensation polymerization between sodium tetrasulfide and 1,2-dichloroethane in water. PSR-mercury interactions and Hg-S bonding were elucidated from x-ray photoelectron spectroscopy, and Fourier transform infra-red spectroscopy analyses. The sulfur loading levels were controlled by the polymer dose during the coating process and the total surface area of the activated carbon was maintained for the sulfur loading less than 2 wt%. Sorption kinetic studies showed that PSR-coated activated carbon facilitates fast reaction by providing a greater reactive surface area than PSR alone. High sulfur loading on activated carbon enhanced mercury adsorption contributing to a three orders of magnitude reduction in mercury concentration. μ-X-ray absorption near edge spectroscopic analyses of the mercury bound to activated carbon and to PSR on activated carbon suggests the chemical bond with mercury on the surface is a combination of Hg-Cl and Hg-S interaction. The pH effect on mercury removal and adsorption isotherm results indicate competition between protons and mercury for binding to sulfur at low pH.

  View details for DOI 10.1016/j.watres.2010.08.045

  View details for Web of Science ID 000286790500005

  View details for PubMedID 20965542

• Effect of Uranium(VI) Speciation on Simultaneous Microbial Reduction of Uranium(VI) and Iron(III) JOURNAL OF ENVIRONMENTAL QUALITY Stewart, B. D., Amos, R. T., Fendorf, S. 2011; 40 (1): 90-97

  Abstract

  Uranium is a pollutant of concern to both human and ecosystem health. Uranium's redox state often dictates whether it will reside in the aqueous or solid phase and thus plays an integral role in the mobility of uranium within the environment. In anaerobic environments, the more oxidized and mobile form of uranium (UO2(2+) and associated species) may be reduced, directly or indirectly, by microorganisms to U(IV) with subsequent precipitation of UO. However, various factors within soils and sediments, such as U(VI) speciation and the presence of competitive electron acceptors, may limit biological reduction of U(VI). Here we examine simultaneous dissimilatory reduction of Fe(III) and U(VI) in batch systems containing dissolved uranyl acetate and ferrihydrite-coated sand. Varying amounts of calcium were added to induce changes in aqueous U(VI) speciation. The amount of uranium removed from solution during 100 h of incubation with S. putrefaciens was 77% in absence of Ca or ferrihydrite, but only 24% (with ferrihydrite) and 14% (without ferrihydrite) were removed for systems with 0.8 mM Ca. Dissimilatory reduction of Fe(III) and U(VI) proceed through different enzyme pathways within one type of organism. We quantified the rate coefficients for simultaneous dissimilatory reduction of Fe(III) and U(VI) in systems varying in Ca concecentration (0-0.8 mM). The mathematical construct, implemented with the reactive transport code MIN3P, reveals predominant factors controlling rates and extent of uranium reduction in complex geochemical systems.

  View details for DOI 10.2134/jeq2010.0304

  View details for Web of Science ID 000285715300011

  View details for PubMedID 21488497

• Influence of Uranyl Speciation and Iron Oxides on Uranium Biogeochemical Redox Reactions GEOMICROBIOLOGY JOURNAL Stewart, B. D., Amos, R. T., Nico, P. S., Fendorf, S. 2011; 28 (5-6): 444-456

  View details for DOI 10.1080/01490451.2010.507646

  View details for Web of Science ID 000296189400007

• Short-term fates of high sulfur inputs in Northern California vineyard soils NUTRIENT CYCLING IN AGROECOSYSTEMS Hinckley, E. S., Fendorf, S., Matson, P. 2011; 89 (1): 135-142

  View details for DOI 10.1007/s10705-010-9383-3

  View details for Web of Science ID 000285365100012

• Competitive Mn-oxide and microbially mediated redox process controlling arsenic speciation and partitioning Environmental Science & Technology Ying, S. C., Kocar, B. D., Griffis, S., Fendorf, S. 2011; 45: 5572-5577

  View details for DOI 10.1021/es200351m

• Arsenic Localization, Speciation, and Co-Occurrence with Iron on Rice (Oryza sativa L.) Roots Having Variable Fe Coatings ENVIRONMENTAL SCIENCE & TECHNOLOGY Seyfferth, A. L., Webb, S. M., Andrews, J. C., Fendorf, S. 2010; 44 (21): 8108-8113

  Abstract

  Arsenic contamination of rice is widespread, but the rhizosphere processes influencing arsenic attenuation remain unresolved. In particular, the formation of Fe plaque around rice roots is thought to be an important barrier to As uptake, but the relative importance of this mechanism is not well characterized. Here we elucidate the colocalization of As species and Fe on rice roots with variable Fe coatings; we used a combination of techniques--X-ray fluorescence imaging, μXANES, transmission X-ray microscopy, and tomography--for this purpose. Two dominant As species were observed in fine roots-inorganic As(V) and As(III) -with minor amounts of dimethylarsinic acid (DMA) and arsenic trisglutathione (AsGlu(3)). Our investigation shows that variable Fe plaque formation affects As entry into rice roots. In roots with Fe plaque, As and Fe were strongly colocated around the root; however, maximal As and Fe were dissociated and did not encapsulate roots that had minimal Fe plaque. Moreover, As was not exclusively associated with Fe plaque in the rice root system; Fe plaque does not coat many of the young roots or the younger portion of mature roots. Young, fine roots, important for solute uptake, have little to no iron plaque. Thus, Fe plaque does not directly intercept (and hence restrict) As supply to and uptake by rice roots but rather serves as a bulk scavenger of As predominantly near the root base.

  View details for DOI 10.1021/es101139z

  View details for Web of Science ID 000283484000024

  View details for PubMedID 20936818

• Spatial and Temporal Variations of Groundwater Arsenic in South and Southeast Asia SCIENCE Fendorf, S., Michael, H. A., van Geen, A. 2010; 328 (5982): 1123-1127

  Abstract

  Over the past few decades, groundwater wells installed in rural areas throughout the major river basins draining the Himalayas have become the main source of drinking water for tens of millions of people. Groundwater in this region is much less likely to contain microbial pathogens than surface water but often contains hazardous amounts of arsenic--a known carcinogen. Arsenic enters groundwater naturally from rocks and sediment by coupled biogeochemical and hydrologic processes, some of which are presently affected by human activity. Mitigation of the resulting health crisis in South and Southeast Asia requires an understanding of the transport of arsenic and key reactants such as organic carbon that could trigger release in zones with presently low groundwater arsenic levels.

  View details for DOI 10.1126/science.1172974

  View details for PubMedID 20508123

• Aggregate-scale spatial heterogeneity in reductive transformation of ferrihydrite resulting from coupled biogeochemical and physical processes GEOCHIMICA ET COSMOCHIMICA ACTA Pallud, C., Masue-Slowey, Y., Fendorf, S. 2010; 74 (10): 2811-2825

  View details for DOI 10.1016/j.gca.2010.01.032

  View details for Web of Science ID 000276989400001

• Impact of Uranyl-Calcium-Carbonato Complexes on Uranium(VI) Adsorption to Synthetic and Natural Sediments ENVIRONMENTAL SCIENCE & TECHNOLOGY Stewart, B. D., Mayes, M. A., Fendorf, S. 2010; 44 (3): 928-934

  Abstract

  Adsorption on soil and sediment solids may decrease aqueous uranium concentrations and limit its propensity for migration in natural and contaminated settings. Uranium adsorption will be controlled in large part by its aqueous speciation, with a particular dependence on the presence of dissolved calcium and carbonate. Here we quantify the impact of uranyl speciation on adsorption to both goethite and sediments from the Hanford Clastic Dike and Oak Ridge Melton Branch Ridgetop formations. Hanford sediments were preconditioned with sodium acetate and acetic acid to remove carbonate grains, and Ca and carbonate were reintroduced at defined levels to provide a range of aqueous uranyl species. U(VI) adsorption is directly linked to UO(2)(2+) speciation, with the extent of retention decreasing with formation of ternary uranyl-calcium-carbonato species. Adsorption isotherms under the conditions studied are linear, and K(d) values decrease from 48 to 17 L kg(-1) for goethite, from 64 to 29 L kg (-1) for Hanford sediments, and from 95 to 51 L kg(-1) for Melton Branch sediments as the Ca concentration increases from 0 to 1 mM at pH 7. Our observations reveal that, in carbonate-bearing waters, neutral to slightly acidic pH values ( approximately 5) and limited dissolved calcium are optimal for uranium adsorption.

  View details for DOI 10.1021/es902194x

  View details for Web of Science ID 000273950100015

  View details for PubMedID 20058915

• Arsenic repartitioning during biogenic sulfidization and transformation of ferrihydrite GEOCHIMICA ET COSMOCHIMICA ACTA Kocar, B. D., Borch, T., Fendorf, S. 2010; 74 (3): 980-994

  View details for DOI 10.1016/j.gca.2009.10.023

  View details for Web of Science ID 000273781100013

• Microbial and metal water quality in rain catchments compared with traditional drinking water sources in the East Sepik Province, Papua New Guinea JOURNAL OF WATER AND HEALTH Horak, H. M., Chynoweth, J. S., Myers, W. P., Davis, J., Fendorf, S., Boehm, A. B. 2010; 8 (1): 126-138

  Abstract

  In Papua New Guinea, a significant portion of morbidity and mortality is attributed to water-borne diseases. To reduce incidence of disease, communities and non-governmental organizations have installed rain catchments to provide drinking water of improved quality. However, little work has been done to determine whether these rain catchments provide drinking water of better quality than traditional drinking water sources, and if morbidity is decreased in villages with rain catchments. The specific aim of this study was to evaluate the quality of water produced by rain catchments in comparison with traditional drinking water sources in rural villages in the East Sepik Province. Fifty-four water sources in 22 villages were evaluated for enterococci and Escherichia coli densities as well as 14 health-relevant metals. In addition, we examined how the prevalence of diarrhoeal illness in villages relates to the type of primary drinking water source. The majority of tested metals were below World Health Organization safety limits. Catchment water sources had lower enterococci and E. coli than other water sources. Individuals in villages using Sepik River water as their primary water source had significantly higher incidence of diarrhoea than those primarily using other water sources (streams, dug wells and catchments).

  View details for Web of Science ID 000275310700014

  View details for PubMedID 20009255

• Arsenic Chemistry in Soils and Sediments Synchrotron-Based Techniques in Soils and Sediments. Elsevier Publishing Fendorf, S., Nico, P. S., Kocar, B. D., Masue, Y., Tufano, K. J. edited by Singh, B., Grafe, M. 2010: 357–378
• Arsenic in South Asia Groundwater Geography Compass Benner, S. G., Fendorf, S. 2010; 4: 1532-1552

  View details for DOI 10.1111/j.1749-8198.2010.00387.x

• Spatial Patterns and Modeling of Reductive Ferrihydrite Transformation Observed in Artificial Soil Aggregates ENVIRONMENTAL SCIENCE & TECHNOLOGY Pallud, C., Kausch, M., Fendorf, S., Meile, C. 2010; 44 (1): 74-79

  Abstract

  Within soils, biogeochemical processes controlling elemental cycling are heterogeneously distributed owing, in large part, to the physical complexity of the media. Here we quantify how diffusive mass-transfer limitation at the soil aggregate scale controls the biogeochemical processes governing ferrihydrite reductive dissolution and secondary iron mineral formation. Artificial cm-scale aggregates made of ferrihydrite-coated sand inoculated with iron-reducing bacteria were placed in flow-through reactors, mimicking macro- and microporous soil environments. A reactive transport model was developed to delineate diffusively and advectively controlled regions, identify reaction zones and estimate kinetic parameters. Simulated iron (Fe) breakthrough-curves show good agreement with experimental results for a wide-range of flow rates and input lactate concentrations, with only a limited amount (< or =12%) of Fe lost in the reactor outflow over a 31 day period. Model simulations show substantial intra-aggregate, mm-scale radial variations in the secondary iron phase distributions, reproducing the trends observed experimentally where only limited transformation of ferrihydrite was found near the aggregate surface, whereas extensive formation of goethite/lepidocrocite and minor amounts of magnetite and/or siderite were observed toward the aggregate center. Our study highlights the important control of variations in transport intensities on microbially induced iron transformation at the soil aggregate scale.

  View details for DOI 10.1021/es901736t

  View details for Web of Science ID 000273267000017

  View details for PubMedID 20039736

• Kinetic and Mechanistic Constraints on the Oxidation of Biogenic Uraninite by Ferrihydrite ENVIRONMENTAL SCIENCE & TECHNOLOGY Ginder-Vogel, M., Stewart, B., Fendorf, S. 2010; 44 (1): 163-169

  Abstract

  The oxidation state of uranium plays a major role in determining uranium mobility in the environment. Under anaerobic conditions, common metal respiring bacteria enzymatically reduce soluble U(VI) to U(IV), resulting in the formation of sparingly soluble UO(2(bio)) (biogenic uraninite). The stability of biologically precipitated uraninite is critical for determining the long-term fate of uranium and is not well characterized within soils and sediments. Here, we demonstrate that biogenic uraninite oxidation by ferrihydrite, an environmentally ubiquitous, disordered Fe(III) (hydr)oxide, appears to proceed through a soluble U(IV) intermediate and results in the concomitant production of Fe(II) and dissolved U(VI). Uraninite oxidation rates are accelerated under conditions that increase its solubility and decrease uraninite surface passivation, which include high bicarbonate concentration and pH values deviating from neutrality. Thus, our results demonstrate that UO(2(bio)) oxidation by Fe(III) (hydr)oxides is controlled by the rate of uraninite dissolution and that this process may limit uranium(IV) sequestration in the presence of Fe(III) (hydr)oxides.

  View details for DOI 10.1021/es902452u

  View details for Web of Science ID 000273267000030

  View details for PubMedID 20039747

• Aggregate-Scale Heterogeneity in Iron (Hydr)oxide Reductive Transformations VADOSE ZONE JOURNAL Tufano, K. J., Benner, S. G., Mayer, K. U., Marcus, M. A., Nico, P. S., Fendorf, S. 2009; 8 (4): 1004-1012

  View details for DOI 10.2136/vzj2008.0090

  View details for Web of Science ID 000271936900018

• Incorporation of Oxidized Uranium into Fe (Hydr)oxides during Fe(II) Catalyzed Remineralization ENVIRONMENTAL SCIENCE & TECHNOLOGY Nico, P. S., Stewart, B. D., Fendorf, S. 2009; 43 (19): 7391-7396

  Abstract

  The form of solid phase U after Fe(II) induced anaerobic remineralization of ferrihydrite in the presence of aqueous and absorbed U(VI) was investigated under both abiotic batch and biotic flow conditions. Experiments were conducted with synthetic ground waters containing 0.168 mM U(VI), 3.8 mM carbonate, and 3.0 mM Ca2+. In spite of the high solubility of U(VI) under these conditions, appreciable removal of U(VI) from solution was observed in both the abiotic and biotic systems. The majority of the removed U was determined to be substituted as oxidized U (U(VI) or U(V)) into the octahedral position of the goethite and magnetite formed during ferrihydrite remineralization. It is estimated that between 3 and 6% of octahedral Fe(III) centers in the new Fe minerals were occupied by U. This site specific substitution is distinct from the nonspecific U coprecipitation processes in which uranyl compounds, e.g., uranyl hydroxide or carbonate, are entrapped within newly formed Fe oxides. The prevalence of site specific U incorporation under both abiotic and biotic conditions and the fact that the produced solids were shown to be resistant to both extraction (30 mM KHCO3) and oxidation (air for 5 days) suggest the potential importance of sequestration in Fe oxides as a stable and immobile form of U in the environment.

  View details for DOI 10.1021/es900515q

  View details for Web of Science ID 000270136500039

  View details for PubMedID 19848151

• Stability of Uranium Incorporated into Fe (Hydr)oxides under Fluctuating Redox Conditions ENVIRONMENTAL SCIENCE & TECHNOLOGY Stewart, B. D., Nico, P. S., Fendorf, S. 2009; 43 (13): 4922-4927

  Abstract

  Reaction pathways resulting in uranium-bearing solids that are stable (i.e., having limited solubility) under aerobic and anaerobic conditions will limit dissolved concentrations and migration of this toxin. Here, we examine the sorption mechanism and propensity for release of uranium reacted with Fe (hydr)oxides under cyclic oxidizing and reducing conditions. Upon reaction of ferrihydrite with Fe(II) under conditions where aqueous Ca-UO2-CO3 species predominate (3 mM Ca and 3.8 mM total CO3), dissolved uranium concentrations decrease from 0.16 mM to below detection limit (BDL) after 5-15 d, depending on the Fe(II) concentration. In systems undergoing 3 successive redox cycles (14 d of reduction, followed by 5 d of oxidation) and a pulsed decrease to 0.15 mM total CO3, dissolved uranium concentrations varied depending on the Fe(II) concentration during the initial and subsequent reduction phases. U concentrations resulting during the oxic "rebound" varied inversely with the Fe(II) concentration during the reduction cycle. Uranium removed from solution remains in the oxidized form and is found adsorbed onto and incorporated into the structure of newly formed goethite and magnetite. Our results reveal that the fate of uranium is dependent on anaerobic/ aerobic conditions, aqueous uranium speciation, and the fate of iron.

  View details for DOI 10.1021/es803317w

  View details for Web of Science ID 000267435500048

  View details for PubMedID 19673286

• Thermodynamic Constraints on Reductive Reactions Influencing the Biogeochemistry of Arsenic in Soils and Sediments ENVIRONMENTAL SCIENCE & TECHNOLOGY Kocar, B. D., Fendorf, S. 2009; 43 (13): 4871-4877

  Abstract

  Arsenic is a widespread environmental toxin having devastating impacts on human health. A transition to anaerobic conditions is a key driver for promoting As desorption through either the reduction of As(V) or the reductive dissolution of Fe(III) (hydr)oxides. However, a disparity in the reported release sequence for As and Fe to the aqueous solution hinders our ability to determine the controlling factors liberating As to the aqueous environment. Accordingly, we performed a thermodynamic analysis of Fe, using a range of Fe-(hydr)oxides, and As reduction coupled with hydrogen, acetate, and lactate oxidation for a range of relevant field conditions. The favorability of sulfate reduction is also evaluated. Our results illustrate that As reduction is favorable over a wide-range of field conditions, and Fe reduction is differentially favorable depending on the buildup of metabolites (mainly Fe2+) and the Fe (hydr)oxide being reduced; reduction of As(V) is thermodynamically favorable under most environmental conditions and almost always more favorable than goethite and hematite reduction. Sulfate reduction is favorable over a range of conditions, and may occur concomitantly with Fe reduction depending on the Fe (hydr)oxides present. Thus, on a thermodynamic basis, the general sequence of microbial reduction should be As(V) followed by Fe(III) or sulfate.

  View details for DOI 10.1021/es8035384

  View details for Web of Science ID 000267435500040

  View details for PubMedID 19673278

• Time-lapse geophysical imaging of soil moisture dynamics in tropical deltaic soils: An aid to interpreting hydrological and geochemical processes WATER RESOURCES RESEARCH Robinson, D. A., Lebron, I., Kocar, B., Phan, K., Sampson, M., Crook, N., Fendorf, S. 2009; 45

  View details for DOI 10.1029/2008WR006984

  View details for Web of Science ID 000264688800001

• BIOGEOCHEMICAL PROCESSES CONTROLLING THE FATE AND TRANSPORT OF ARSENIC: IMPLICATIONS FOR SOUTH AND SOUTHEAST ASIAm ADVANCES IN AGRONOMY, VOLUME 104 Fendorf, S., Kocar, B. D. 2009; 104: 137-164

  View details for DOI 10.1016/S0065-2113(09)04003-6

  View details for Web of Science ID 000271012200003

• Incorporation of uranium(VI) into Fe(hydr)oxides during Fe(II) catalyzed remineralization Environmental Science & Technology Nico, P. S., Stewart, B. D., Fendorf, S.. 2009; 43: 7391-7396
• Spatial patterns of iron transformations within artificial soil aggregates: Experimental and modeling analysis of diffusion limited iron cycling Environmental Science & Technology Pallud, C., Kausch, M., Fendorf, S., Meile, C. 2009; 43: 74-79
• Reductive Processes Controlling Arsenic Retention: Revealing the Relative Importance of Iron and Arsenic Reduction ENVIRONMENTAL SCIENCE & TECHNOLOGY Tufano, K. J., Reyes, C., Saltikov, C. W., Fendorf, S. 2008; 42 (22): 8283-8289

  Abstract

  The fate and transport of arsenic is regulated, in part, by its strong affinity for iron (hydr)oxides. A transition from aerobic to anaerobic conditions resulting in concomitant reduction of both As(V) and iron (hydr)oxides can thus have a pronounced influence on As partitioning. However, it is presently unclear whether As desorption under anaerobic conditions results predominantly from a transformation from As(V) to As(III) or from mineralogical changes as a consequence of iron and manganese reduction. Here, we examine desorption of both As(III) and As(V) from ferrihydrite-, goethite-, and hematite-coated sand under hydrodynamic conditions. Furthermore, to resolve the relative role of Fe(III) and/or As(V) reduction in regulating dissolved As concentrations, we also examined As desorption from ferrihydrite- and goethite-coated sands presorbed with As(V) using wild type or mutants of Shewanella sp. ANA-3, capable of Fe(III)- and/or As(V)-reduction. We reveal substantial differences in As(III) and As(V) desorption from ferrihydrite, goethite, and hematite. Despite being adsorbed to a greater extent than As(V), As(III) is desorbed more rapidly and extensively from all oxides, suggesting weaker binding of As(III) than As(V). When As(V) and Fe(III) reduction are decoupled, As(V) reduction appears to be the dominant process controlling As release. Our results also suggest the importance of appreciating physical properties of specific Fe (hydr)oxides when predicting the potential for As desorption.

  View details for DOI 10.1021/es801059s

  View details for Web of Science ID 000260921400020

  View details for PubMedID 19068807

• Groundwater flow in an arsenic-contaminated aquifer, Mekong Delta, Cambodia APPLIED GEOCHEMISTRY Benner, S. G., Polizzotto, M. L., Kocar, B. D., Ganguly, S., Phan, K., Ouch, K., Sampson, M., Fendorf, S. 2008; 23 (11): 3072-3087

  View details for DOI 10.1016/j.apgeochem.2008.06.013

  View details for Web of Science ID 000261541500011

• Integrated biogeochemical and hydrologic processes driving arsenic release from shallow sediments to groundwaters of the Mekong delta APPLIED GEOCHEMISTRY Kocar, B. D., Polizzotto, M. L., Benner, S. G., Ying, S. C., Ung, M., Ouch, K., Samreth, S., Suy, B., Phan, K., Sampson, M., Fendorf, S. 2008; 23 (11): 3059-3071

  View details for DOI 10.1016/j.apgeochem.2008.06.026

  View details for Web of Science ID 000261541500010

• Depositional influences on porewater arsenic in sediments of a mining-contaminated freshwater lake ENVIRONMENTAL SCIENCE & TECHNOLOGY Toevs, G., Morra, M. J., Winowiecki, L., Strawn, D., Polizzotto, M. L., Fendorf, S. 2008; 42 (18): 6823-6829

  Abstract

  Arsenic-containing minerals mobilized during mining activities and deposited to Lake Coeur d'Alene (CDA), Idaho sediments represent a potential source of soluble As to the overlying water. Our objective was to delineate the processes controlling porewater As concentrations within Lake CDA sediments. Sediment and porewater As concentrations were determined, and solid-phase As associations were probed using X-ray absorption near-edge structure (XANES) spectroscopy. Although maximum As in the sediment porewaters varied from 8.4 to 16.2 microM, As sorption on iron oxyhydroxides at the oxic sediment-water interface prevented flux to overlying water. Floods deposit sediment containing variable amounts of arsenopyrite (FeAsS), with majorfloods depositing large amounts of sediment that bury and preserve reduced minerals. Periods of lower deposition increase sediment residence times in the oxic zone, promoting oxidation of reduced minerals, SO4(2-) efflux, and formation of oxide precipitates. Depositional events bury oxides containing sorbed As, transitioning them into anoxic environments where they undergo dissolution, releasing As to the porewater. High Fe:S ratios limit the formation of arsenic sulfides in the anoxic zone. As a result of As sequestration at the sediment-water interface and its release upon burial, decreased concentrations of porewater As will not occur unless As-bearing erosional inputs are eliminated.

  View details for DOI 10.1021/es800937t

  View details for Web of Science ID 000259139400012

  View details for PubMedID 18853795

• Near-surface wetland sediments as a source of arsenic release to ground water in Asia NATURE Polizzotto, M. L., Kocar, B. D., Benner, S. G., Sampson, M., Fendorf, S. 2008; 454 (7203): 505-U5

  Abstract

  Tens of millions of people in south and southeast Asia routinely consume ground water that has unsafe arsenic levels. Arsenic is naturally derived from eroded Himalayan sediments, and is believed to enter solution following reductive release from solid phases under anaerobic conditions. However, the processes governing aqueous concentrations and locations of arsenic release to pore water remain unresolved, limiting our ability to predict arsenic concentrations spatially (between wells) and temporally (future concentrations) and to assess the impact of human activities on the arsenic problem. This uncertainty is partly attributed to a poor understanding of groundwater flow paths altered by extensive irrigation pumping in the Ganges-Brahmaputra delta, where most research has focused. Here, using hydrologic and (bio)geochemical measurements, we show that on the minimally disturbed Mekong delta of Cambodia, arsenic is released from near-surface, river-derived sediments and transported, on a centennial timescale, through the underlying aquifer back to the river. Owing to similarities in geologic deposition, aquifer source rock and regional hydrologic gradients, our results represent a model for understanding pre-disturbance conditions for other major deltas in Asia. Furthermore, the observation of strong hydrologic influence on arsenic behaviour indicates that release and transport of arsenic are sensitive to continuing and impending anthropogenic disturbances. In particular, groundwater pumping for irrigation, changes in agricultural practices, sediment excavation, levee construction and upstream dam installations will alter the hydraulic regime and/or arsenic source material and, by extension, influence groundwater arsenic concentrations and the future of this health problem.

  View details for DOI 10.1038/nature07093

  View details for Web of Science ID 000257860300048

  View details for PubMedID 18650922

• Confounding impacts of iron reduction on arsenic retention ENVIRONMENTAL SCIENCE & TECHNOLOGY Tufano, K. J., Fendorf, S. 2008; 42 (13): 4777-4783

  Abstract

  A transition from oxidizing to reducing conditions has long been implicated to increase aqueous As concentrations, for which reductive dissolution of iron (hydr)oxides is commonly implicated as the primary culprit. Confounding our understanding of processes controlling As retention, however, is that reductive transformation of ferrihydrite has recently been shown to promote As retention rather than release. To resolve the role iron phases have in regulating arsenic concentrations, here we examine As desorption from ferrihydrite-coated sands presorbed with As(III); experiments were performed at circumneutral pH under Fe-reducing conditions with the dissimilatory iron reducing bacterium Shewanella putrefaciens strain CN-32 over extended time periods. We reveal that with the initial phase of iron reduction, ferrihydrite undergoes transformation to secondary phases and increases As(III) retention (relative to abiotic controls). However, with increased reaction time, cessation of the phase transitions and ensuing reductive dissolution result in prolonged release of As(III) to the aqueous phase. Our results suggest that As(III) retention during iron reduction is temporally dependent on secondary precipitation of iron phases; during transformation to secondary phases, particularly magnetite, As(III) retention is enhanced even relative to oxidized systems. However, conditions that retard secondary transformation (more stable iron oxides or limited iron reducing bacterial activity), or prolonged anaerobiosis, will lead to both the dissolution of ferric (hydr)oxides and release of As(III) to the aqueous phase.

  View details for DOI 10.1021/es702625e

  View details for Web of Science ID 000257220600031

  View details for PubMedID 18678005

• Changes in bacterial and archaeal community structure and functional diversity along a geochemically variable soil profile APPLIED AND ENVIRONMENTAL MICROBIOLOGY Hansel, C. M., Fendorf, S., Jardine, P. M., Francis, C. A. 2008; 74 (5): 1620-1633

  Abstract

  Spatial heterogeneity in physical, chemical, and biological properties of soils allows for the proliferation of diverse microbial communities. Factors influencing the structuring of microbial communities, including availability of nutrients and water, pH, and soil texture, can vary considerably with soil depth and within soil aggregates. Here we investigated changes in the microbial and functional communities within soil aggregates obtained along a soil profile spanning the surface, vadose zone, and saturated soil environments. The composition and diversity of microbial communities and specific functional groups involved in key pathways in the geochemical cycling of nitrogen, Fe, and sulfur were characterized using a coupled approach involving cultivation-independent analysis of both 16S rRNA (bacterial and archaeal) and functional genes (amoA and dsrAB) as well as cultivation-based analysis of Fe(III)-reducing organisms. Here we found that the microbial communities and putative ammonia-oxidizing and Fe(III)-reducing communities varied greatly along the soil profile, likely reflecting differences in carbon availability, water content, and pH. In particular, the Crenarchaeota 16S rRNA sequences are largely unique to each horizon, sharing a distribution and diversity similar to those of the putative (amoA-based) ammonia-oxidizing archaeal community. Anaerobic microenvironments within soil aggregates also appear to allow for both anaerobic- and aerobic-based metabolisms, further highlighting the complexity and spatial heterogeneity impacting microbial community structure and metabolic potential within soils.

  View details for DOI 10.1128/AEM.01787-07

  View details for Web of Science ID 000253792700037

  View details for PubMedID 18192411

  View details for PubMedCentralID PMC2258623

• Decreasing lead bioaccessibility in industrial and firing range soils with phosphate-based amendments Journal of Environmental Quality Mosely, R. A., Barnett, M. O., Stewart, M. A., Mehlhorn, T. L., Jardine, P. M., Ginder-Vogel, M., Fendorf, S. 2008; 37: 2116-2124
• Speciation-dependent microbial reduction of uranium within iron-coated sands ENVIRONMENTAL SCIENCE & TECHNOLOGY Neiss, J., Stewart, B. D., Nico, P. S., Fendorf, S. 2007; 41 (21): 7343-7348

  Abstract

  Transport of uranium within surface and subsurface environments is predicated largely on its redox state. Uranyl reduction may transpire through either biotic (enzymatic) or abiotic pathways; in either case, reduction of U(VI) to U(IV) results in the formation of sparingly soluble UO2 precipitates. Biological reduction of U(VI), while demonstrated as prolific under both laboratory and field conditions, is influenced by competing electron acceptors (such as nitrate, manganese oxides, or iron oxides) and uranyl speciation. Formation of Ca-UO2-CO3 ternary complexes, often the predominate uranyl species in carbonate-bearing soils and sediments, decreases the rate of dissimilatory U(VI) reduction. The combined influence of uranyl speciation within a mineralogical matrix comparable to natural environments and under hydrodynamic conditions, however, remains unresolved. We therefore examined uranyl reduction by Shewanella putrefaciens within packed mineral columns of ferrihydrite-coated quartz sand under conditions conducive or nonconducive to Ca-UO2-CO3 species formation. The results are dramatic. In the absence of Ca, where uranyl carbonato complexes dominate, U(VI) reduction transpires and consumes all of the U(VI) within the influent solution (0.166 mM) over the first 2.5 cm of the flow field for the entirety of the 54 d experiment. Over 2 g of U is deposited during this reaction period, and despite ferrihydrite being a competitive electron acceptor, uranium reduction appears unabated for the duration of our experiments. By contrast, in columns with 4 mM Ca in the influent solution (0.166 mM uranyl), reduction (enzymatic or surface-bound Fe(III) mediated) appears absent and breakthrough occurs within 18 d (at a flow rate of 3 pore volumes per day). Uranyl speciation, and in particular the formation of ternary Ca-UO2-CO3 complexes, has a profound impact on U(VI) reduction and thus transport within anaerobic systems.

  View details for DOI 10.1021/es0706697

  View details for Web of Science ID 000250556100028

  View details for PubMedID 18044509

• In situ bioreduction of uranium (VI) to submicromolar levels and reoxidation by dissolved oxygen ENVIRONMENTAL SCIENCE & TECHNOLOGY Wu, W., Carley, J., Luo, J., Ginder-Vogel, M. A., Cardenas, E., Leigh, M. B., Hwang, C., Kelly, S. D., Ruan, C., Wu, L., Van Nostrand, J., Gentry, T., Lowe, K., Mehlhorn, T., Carroll, S., Luo, W., Fields, M. W., Gu, B., Watson, D., Kemner, K. M., Marsh, T., Tiedje, J., Zhou, J., Fendorf, S., Kitanidis, P. K., Jardine, P. M., Criddle, C. S. 2007; 41 (16): 5716-5723

  Abstract

  Groundwater within Area 3 of the U.S. Department of Energy (DOE) Environmental Remediation Sciences Program (ERSP) Field Research Center at Oak Ridge, TN (ORFRC) contains up to 135 microM uranium as U(VI). Through a series of experiments at a pilot scale test facility, we explored the lower limits of groundwater U(VI) that can be achieved by in-situ biostimulation and the effects of dissolved oxygen on immobilized uranium. Weekly 2 day additions of ethanol over a 2-year period stimulated growth of denitrifying, Fe(III)-reducing, and sulfate-reducing bacteria, and immobilization of uranium as U(IV), with dissolved uranium concentrations decreasing to low levels. Following sulfite addition to remove dissolved oxygen, aqueous U(VI) concentrations fell below the U.S. Environmental Protection Agengy maximum contaminant limit (MCL) for drinking water (< 30/microg L(-1) or 0.126 microM). Under anaerobic conditions, these low concentrations were stable, even in the absence of added ethanol. However, when sulfite additions stopped, and dissolved oxygen (4.0-5.5 mg L(-1)) entered the injection well, spatially variable changes in aqueous U(VI) occurred over a 60 day period, with concentrations increasing rapidly from < 0.13 to 2.0 microM at a multilevel sampling (MLS) well located close to the injection well, but changing little at an MLS well located further away. Resumption of ethanol addition restored reduction of Fe(III), sulfate, and U(VI) within 36 h. After 2 years of ethanol addition, X-ray absorption near-edge structure spectroscopy (XANES) analyses indicated that U(IV) comprised 60-80% of the total uranium in sediment samples. Atthe completion of the project (day 1260), U concentrations in MLS wells were less than 0.1 microM. The microbial community at MLS wells with low U(VI) contained bacteria that are known to reduce uranium, including Desulfovibrio spp. and Geobacter spp., in both sediment and groundwater. The dominant Fe(III)-reducing species were Geothrix spp.

  View details for DOI 10.1021/es062657b

  View details for Web of Science ID 000248886000026

  View details for PubMedID 17874778

• Genesis of hexavalent chromium from natural sources in soil and groundwater PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Oze, C., Bird, D. K., Fendorf, S. 2007; 104 (16): 6544-6549

  Abstract

  Naturally occurring Cr(VI) has recently been reported in ground and surface waters. Rock strata rich in Cr(III)-bearing minerals, in particular chromite, are universally found in these areas that occur near convergent plate margins. Here we report experiments demonstrating accelerated dissolution of chromite and subsequent oxidation of Cr(III) to aqueous Cr(VI) in the presence of birnessite, a common manganese mineral, explaining the generation of Cr(VI) by a Cr(III)-bearing mineral considered geochemically inert. Our results demonstrate that Cr(III) within ultramafic- and serpentinite-derived soils/sediments can be oxidized and dissolved through natural processes, leading to hazardous levels of aqueous Cr(VI) in surface and groundwater.

  View details for DOI 10.1073/pnas.0701085104

  View details for Web of Science ID 000245869200012

  View details for PubMedID 17420454

  View details for PubMedCentralID PMC1871822

• Quantifying constraints imposed by calcium and iron on bacterial reduction of uranium(VI) JOURNAL OF ENVIRONMENTAL QUALITY Stewart, B. D., Neiss, J., Fendorf, S. 2007; 36 (2): 363-372

  Abstract

  Uranium is a redox active contaminant of concern to both human health and ecological preservation. In anaerobic soils and sediments, the more mobile, oxidized form of uranium (UO(2)(2+) and associated species) may be reduced by dissimilatory metal-reducing bacteria. Despite rapid reduction in controlled, experimental systems, various factors within soils or sediments may limit biological reduction of U(VI), inclusive of competing electron acceptors and alterations in uranyl speciation. Here we elucidate the impact of U(VI) speciation on the extent and rate of reduction, and we examine the impact of Fe(III) (hydr)oxides (ferrihydrite, goethite, and hematite) varying in free energies of formation. Observed pseudo first-order rate coefficients for U(VI) reduction vary from 12 +/- 0.60 x 10(-3) h(-1) (0 mM Ca in the presence of goethite) to 2.0 +/- 0.10 x 10(-3) h(-1) (0.8 mM Ca in the presence of hematite). Nevertheless, dissolved Ca (at concentrations from 0.2 to 0.8 mM) decreases the extent of U(VI) reduction by approximately 25% after 528 h relative to rates without Ca present. Imparting an important criterion on uranium reduction, goethite and hematite decrease the dissolved concentration of calcium through adsorption and thus tend to diminish the effect of calcium on uranium reduction. Ferrihydrite, in contrast, acts as a competitive electron acceptor and thus, like Ca, decreases uranium reduction. However, while ferrihydrite decreases U(VI) in solutions without Ca, with increasing Ca concentrations U(VI) reduction is enhanced in the presence of ferrihydrite (relative to its absence)-U(VI) reduction, in fact, becomes almost independent of Ca concentration. The quantitative framework described herein helps to predict the fate and transport of uranium within anaerobic environments.

  View details for DOI 10.2134/jeq2006.0058

  View details for Web of Science ID 000244979300003

  View details for PubMedID 17255623

• Micro-scale heterogeneity in biogeochemical uranium cycling 13th International Conference on X-Ray Absorption Fine Structure (XAFS13) Ginder-Vogel, M., Wu, W., Kelly, S., Criddle, C. S., Carley, J., Jardine, P., Kemner, K. A., Fendorf, S. AMER INST PHYSICS. 2007: 190–192

  View details for Web of Science ID 000245805900049

• Biogeochemical processes controlling the cycling of arsenic in soils and sediments Biophysico-Chemical Processes of Heavy Metals and Metalloids in Soil Environments. IUPAC Division VI-Chemistry and the Environment Fendorf, S. M., Herbel, J., Tufano, K. J., Kocar, B. edited by Violante, A., Huang, P. M., Gadd, G. John Wiley & Sons, Chichester, England. 2007: 313–338
• Phosphate imposed limitations on biological reduction and alteration of ferrihydrite ENVIRONMENTAL SCIENCE & TECHNOLOGY Borch, T., Masue, Y., Kukkadapu, R. K., Fendorf, S. 2007; 41 (1): 166-172

  Abstract

  Biogeochemical transformation (inclusive of dissolution) of iron (hydr)oxides resulting from dissimilatory reduction has a pronounced impact on the fate and transport of nutrients and contaminants in subsurface environments. Despite the reactivity noted for pristine (unreacted) minerals, iron (hydr)oxides within native environments will likely have a different reactivity owing in part to changes in surface composition. Accordingly, here we explore the impact of surface modifications induced by phosphate adsorption on ferrihydrite reduction by Shewanella putrefaciens under static and advective flow conditions. Alterations in surface reactivity induced by phosphate changes the extent, decreasing Fe(Ill) reduction nearly linearly with increasing P surface coverage, and pathway of iron biomineralization. Magnetite is the most appreciable mineralization product while minor amounts of vivianite and green rust-like phases are formed in systems having high aqueous concentrations of phosphate, ferrous iron, and bicarbonate. Goethite and lepidocrocite, characteristic biomineralization products at low ferrous-iron concentrations, are inhibited in the presence of adsorbed phosphate. Thus, deviations in iron (hydr)oxide reactivity with changes in surface composition, such as those noted here for phosphate, need to be considered within natural environments.

  View details for DOI 10.1021/es060695p

  View details for Web of Science ID 000243124600030

  View details for PubMedID 17265943

• Reduction of Cr(VI) under acidic conditions by the facultative Fe(III)-reducing bacterium Acidiphilium cryptum ENVIRONMENTAL SCIENCE & TECHNOLOGY Cummings, D. E., Fendorf, S., Singh, N., Sani, R. K., Peyton, B. M., Magnuson, T. S. 2007; 41 (1): 146-152

  Abstract

  The potential for biological reduction of Cr(VI) under acidic conditions was evaluated with the acidophilic, facultatively metal-reducing bacterium Acidiphilium cryptum strain JF-5 to explore the role of acidophilic microorganisms in the Cr cycle in low-pH environments. An anaerobic suspension of washed A. cryptum cells rapidly reduced 50 microM Cr(VI) at pH 3.2; biological reduction was detected from pH 1.7-4.7. The reduction product, confirmed by XANES analysis, was entirely Cr(III) that was associated predominantly with the cell biomass (70-80%) with the residual residing in the aqueous phase. Reduction of Cr(VI) showed a pH optimum similar to that for growth and was inhibited by 5 mM HgCl2, suggesting that the reaction was enzyme-mediated. Introduction of O2 into the reaction medium slowed the reduction rate only slightly, whereas soluble Fe(III) (as ferric sulfate) increased the rate dramatically, presumably by the shuttling of electrons from bioreduced Fe(II) to Cr(VI) in a coupled biotic-abiotic cycle. Starved cells could not reduce Cr(VI) when provided as sole electron acceptor, indicating that Cr(VI) reduction is not an energy-conserving process in A. cryptum. We speculate, rather, that Cr(VI) reduction is used here as a detoxification mechanism.

  View details for DOI 10.1021/es061333k

  View details for Web of Science ID 000243124600027

  View details for PubMedID 17265940

• Elucidating biogeochemical reduction of chromate via carbon amendments and soil sterilization GEOMICROBIOLOGY JOURNAL Bank, T. L., Vishnivetskaya, T. A., Jardine, P. M., Ginder-Vogel, M. A., Fendorf, S., Baldwin, M. E. 2007; 24 (2): 125-132

  View details for DOI 10.1080/01490450701266639

  View details for Web of Science ID 000248266100005

• Biogeochemical uranium redox transformations: Potential oxidants of uraninite Adsorption to Geomedia Ginder-Vogel, M., Fendorf, S. edited by Barnett, M. A., Kent, D. B. Academic Press, NY. 2007: 293–321
• Phosphate interactions with iron (hydr)oxides: Mineralization pathways and phosphorus retention upon bioreduction Adsorption to Geomedia Borch, T., Fendorf, S. edited by Barnett, M. A., Kent, D. B. Academic Press, NY . 2007: 322–348
• Contrasting effects of dissimilatory iron(III) and arsenic(V) reduction on arsenic retention and transport ENVIRONMENTAL SCIENCE & TECHNOLOGY Kocar, B. D., Herbel, M. J., Tufano, K. J., Fendorf, S. 2006; 40 (21): 6715-6721

  Abstract

  Reduction of arsenate As(V) and As-bearing Fe (hydr)- oxides have been proposed as dominant pathways of As release within soils and aquifers. Here we examine As elution from columns loaded with ferrihydrite-coated sand presorbed with As(V) or As(III) at circumneutral pH upon Fe and/or As reduction; biotic stimulated reduction is then compared to abiotic elution. Columns were inoculated with Shewanella putrefaciens strain CN-32 or Sulfurospirillum barnesii strain SES-3, organisms capable of As (V) and Fe (III) reduction, or Bacillus benzoevorans strain HT-1, an organism capable of As(V) but not Fe(III) reduction. On the basis of equal surface coverages, As(III) elution from abiotic columns exceeded As(V) elution by a factor of 2; thus, As(III) is more readily released from ferrihydrite under the imposed reaction conditions. Biologically mediated Asreduction induced by B. benzoevorans enhances the release of total As relative to As (V) under abiotic conditions. However, under Fe reducing conditions invoked by either S. barnesii or S. putrefaciens, approximately three times more As (V or III) was retained within column solids relative to the abiotic experiments, despite appreciable decreases in surface area due to biotransformation of solid phases. Enhanced As sequestration upon ferrihydrite reduction is consistent with adsorption or incorporation of As into biotransformed solids. Our observations indicate that As retention and release from Fe (hydr)oxide(s) is controlled by complex pathways of Fe biotransformation and that reductive dissolution of As-bearing ferrihydrite can promote As sequestration rather than desorption under conditions examined here.

  View details for DOI 10.1021/es061540k

  View details for Web of Science ID 000241628800036

  View details for PubMedID 17144301

• Heterogeneous response to biostimulation for U(VI) reduction in replicated sediment microcosms BIODEGRADATION Nyman, J. L., Marsh, T. L., Ginder-Vogel, M. A., Gentile, M., Fendorf, S., Criddle, C. 2006; 17 (4): 303-316

  Abstract

  A field-scale experiment to assess biostimulation of uranium reduction is underway at the Natural and Accelerated Bioremediation Research Field Research Center (FRC) in Oak Ridge, Tennessee. To simulate the field experiment, we established replicate batch microcosms containing well-mixed contaminated sediment from a well within the FRC treatment zone, and we added an inoculum from a pilot-scale fluidized bed reactor representing the inoculum in the field experiment. After reduction of nitrate, both sulfate and soluble U(VI) concentration decreased. X-ray absorption near edge structure (XANES) spectroscopy confirmed formation of U(IV) in sediment from biostimulated microcosms, but did not detect reduction of solid-phase Fe(III). Two to three fragments dominated terminal restriction fragment length polymorphism (T-RFLP) profiles of the 16S rDNA gene. Comparison to a clone library indicated these fragments represented denitrifying organisms related to Acidovorax, and Acidovorax isolates from the inoculum were subsequently shown to reduce U(VI). Investigation using the T-RFLP Analysis Program (TAP T-RFLP) and chemical analyses detected the presence and activity of fermenting and sulfate-reducing bacteria after 2 weeks. These organisms likely contributed to uranium reduction. In some microcosms, soluble U(VI) concentration leveled off or rebounded, indicating microbial and/or mineralogical heterogeneity among samples. Sulfate, acetate, and ethanol were depleted only in those microcosms exhibiting a rebound in soluble U(VI). This suggests that rates of U(VI) desorption can exceed rates of U(VI) reduction when sulfate-reducing bacteria become substrate-limited. These observations underscore the importance of effective chemical delivery and the role of serial and parallel processes in uranium reduction.

  View details for DOI 10.1007/s10532-005-9000-3

  View details for Web of Science ID 000238773600002

  View details for PubMedID 16491308

• Pilot-scale in situ bioremedation of uranium in a highly contaminated aquifer. 2. Reduction of U(VI) and geochemical control of U(VI) bioavailability ENVIRONMENTAL SCIENCE & TECHNOLOGY Wu, W., Carley, J., Gentry, T., Ginder-Vogel, M. A., Fienen, M., Mehlhorn, T., Yan, H., Caroll, S., Pace, M. N., Nyman, J., Luo, J., Gentile, M. E., Fields, M. W., Hickey, R. F., Gu, B., Watson, D., Cirpka, O. A., Zhou, J., Fendorf, S., Kitanidis, P. K., Jardine, P. M., Criddle, C. S. 2006; 40 (12): 3986-3995

  Abstract

  In situ microbial reduction of soluble U(VI) to sparingly soluble U(IV) was evaluated at the site of the former S-3 Ponds in Area 3 of the U.S. Department of Energy Natural and Accelerated Bioremediation Research Field Research Center, Oak Ridge, TN. After establishing conditions favorable for bioremediation (Wu, et al. Environ. Sci. Technol. 2006, 40, 3988-3995), intermittent additions of ethanol were initiated within the conditioned inner loop of a nested well recirculation system. These additions initially stimulated denitrification of matrix-entrapped nitrate, but after 2 months, aqueous U levels fell from 5 to approximately 1 microM and sulfate reduction ensued. Continued additions sustained U(VI) reduction over 13 months. X-ray near-edge absorption spectroscopy (XANES) confirmed U(VI) reduction to U(IV) within the inner loop wells, with up to 51%, 35%, and 28% solid-phase U(IV) in sediment samples from the injection well, a monitoring well, and the extraction well, respectively. Microbial analyses confirmed the presence of denitrifying, sulfate-reducing, and iron-reducing bacteria in groundwater and sediments. System pH was generally maintained at less than 6.2 with low bicarbonate level (0.75-1.5 mM) and residual sulfate to suppress methanogenesis and minimize uranium mobilization. The bioavailability of sorbed U(VI) was manipulated by addition of low-level carbonate (< 5 mM) followed by ethanol (1-1.5 mM). Addition of low levels of carbonate increased the concentration of aqueous U, indicating an increased rate of U desorption due to formation of uranyl carbonate complexes. Upon ethanol addition, aqueous U(VI) levels fell, indicating that the rate of microbial reduction exceeded the rate of desorption. Sulfate levels simultaneously decreased, with a corresponding increase in sulfide. When ethanol addition ended but carbonate addition continued, soluble U levels increased, indicating faster desorption than reduction. When bicarbonate addition stopped, aqueous U levels decreased, indicating adsorption to sediments. Changes in the sequence of carbonate and ethanol addition confirmed that carbonate-controlled desorption increased bioavailability of U(VI) for reduction.

  View details for DOI 10.1021/es051960u

  View details for Web of Science ID 000238217200052

  View details for PubMedID 16830572

• Thermodynamic constraints on the oxidation of biogenic UO2 by Fe(III) (hydr) oxides ENVIRONMENTAL SCIENCE & TECHNOLOGY Ginder-Vogel, M., Criddle, C. S., Fendorf, S. 2006; 40 (11): 3544-3550

  Abstract

  Uranium mobility in the environment is partially controlled by its oxidation state, where it exists as either U(VI) or U(IV). In aerobic environments, uranium is generally found in the hexavalent form, is quite soluble, and readily forms complexes with carbonate and calcium. Under anaerobic conditions, common metal respiring bacteria can reduce soluble U(VI) species to sparingly soluble UO2 (uraninite); stimulation of these bacteria, in fact, is being explored as an in situ uranium remediation technique. However, the stability of biologically precipitated uraninite within soils and sediments is not well characterized. Here we demonstrate that uraninite oxidation by Fe(III) (hydr)oxides is thermodynamically favorable under limited geochemical conditions. Our analysis reveals that goethite and hematite have a limited capacity to oxidize UO2(biogenic) while ferrihydrite can lead to UO2(biogenic) oxidation. The extent of UO2(biogenic) oxidation by ferrihydrite increases with increasing bicarbonate and calcium concentration, but decreases with elevated Fe(II)(aq) and U(VI)(aq) concentrations. Thus, our results demonstrate that the oxidation of UO2(biogenic) by Fe(III) (hydr)oxides may transpire under mildly reducing conditions when ferrihydrite is present.

  View details for DOI 10.1021/es052305p

  View details for Web of Science ID 000237921200023

  View details for PubMedID 16786692

• Solid-phases and desorption processes of arsenic within Bangladesh sediments Annual Conference of the Soil-Science-Society-of-America Polizzotto, M. L., Harvey, C. F., Li, G. C., Badruzzman, B., Ali, A., Newville, M., Sutton, S., Fendorf, S. ELSEVIER SCIENCE BV. 2006: 97–111

  View details for DOI 10.1016/j.chemgeo.2005.11.026

  View details for Web of Science ID 000236997800009

• Biogeochemical processes controlling the speciation and transport of arsenic within iron coated sands Annual Conference of the Soil-Science-Society-of-America Herbel, M., Fendorf, S. ELSEVIER SCIENCE BV. 2006: 16–32

  View details for DOI 10.1016/j.chemgeo.2005.11.016

  View details for Web of Science ID 000236997800003

• Introduction: Controls on arsenic transport in near-surface aquatic systems CHEMICAL GEOLOGY Ford, R. G., Fendorf, S., Wilkin, R. T. 2006; 228 (1-3): 1-5

  View details for DOI 10.1016/j.chemgeo.2005.11.014

  View details for Web of Science ID 000236997800001

• Metal(loid) diagenesis in mine-impacted sediments of Lake Coeur d'Alene, Idaho ENVIRONMENTAL SCIENCE & TECHNOLOGY Toevs, G. R., Morra, M. J., Polizzotto, M. L., Strawn, D. G., Bostick, B. C., Fendorf, S. 2006; 40 (8): 2537-2543

  Abstract

  Mining activity along the South Fork of the Coeur d' Alene River in northern Idaho has resulted in fluvial mine tailings enriched in Pb, As, Ag, Sb, Hg, Cd, and Zn deposited on the lakebed of Lake Coeur d'Alene, thus serving as a potential benthic source of inorganic contaminants. Our objective was to characterize the dominant solid phase materials and diagenetic processes controlling metal(loid) solubilities, and thus their potential release to the overlying water column. Aqueous and solid concentrations of metal(loid) contaminants were examined along with distinct species of Fe and S within sediments and interstitial water. A gradient from oxic conditions at the sediment-water interface to anoxic conditions below 15 cm exists at all sites, resulting in a dynamic redox environment that controls the partitioning of contaminants. Fluvial deposition from frequent seasonal flood events bury ferric oxides residing at the sediment-water interface leading to reductive dissolution as they transition to the anoxic zone, consequently releasing associated metal(loids) to the interstitial water. Insufficient sulfur limits the formation of sulfidic minerals, but high carbonate content of this mining region buffers pH and promotes formation of siderite. Diagenetic reactions create chemical gradients encouraging the diffusion of metal(loids) toward the sediment--water interface, thereby, increasing the potential for release into the overlying water.

  View details for DOI 10.1021/es051781c

  View details for Web of Science ID 000236992700012

  View details for PubMedID 16683589

• Arsenic cycling within surface and subsurface environments: Impact of iron mineralogy and bioreduction processes Chemical Geology Herbel, M., Fendorf, S. 2006; 228: 16-32
• Pilot-scale bioremediation of uranium in a highly contaminated aquifer II: Reduction of U(VI) and geochemical control of U(VI) bioavailability Environmental Science & Technology Wu, W., Carley, J., Gentry, T., Ginder-Vogel, M. A., Fienen, M., Mehlhorn, T., Yan, H., Carroll, S., Nyman, J., Luo, J., Gentile, M. E., Fields, M. W., Hickey, R. F., Watson, D., Cirpka, O. A., Fendorf, S., Zhou, J., Kitanidis, P., Jardine, P. M., Criddle, C. S. 2006; 40: 3986-3995
• Processes conducive to the release and transport of arsenic into aquifers of Bangladesh PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Polizzotto, M. L., Harvey, C. F., Sutton, S. R., Fendorf, S. 2005; 102 (52): 18819-18823

  Abstract

  Arsenic is a contaminant in the groundwater of Holocene aquifers in Bangladesh, where approximately 57 million people drink water with arsenic levels exceeding the limits set by the World Health Organization. Although arsenic is native to the sediments, the means by which it is released to groundwater remains unresolved. Contrary to the current paradigm, ferric (hydr)oxides appear to dominate the partitioning of arsenic in the near surface but have a limited impact at aquifer depths where wells extract groundwater with high arsenic concentrations. We present a sequence of evidence that, taken together, suggest that arsenic may be released in the near surface and then transported to depth. We establish that (i) the only portion of the sediment profile with conditions destabilizing to arsenic in our analysis is in the surface or near-surface environment; (ii) a consistent input of arsenic via sediment deposition exists; (iii) retardation of arsenic transport is limited in the aquifers; and (iv) groundwater recharge occurs at a rate sufficient to necessitate continued input of arsenic to maintain observed concentrations. Our analyses thus lead to the premise that arsenic is liberated in surface and near-surface sediments through cyclic redox conditions and is subsequently transported to well depth. Influx of sediment and redox cycling provide a long-term source of arsenic that when liberated in the near surface is only weakly partitioned onto sediments deeper in the profile and is transported through aquifers by groundwater recharge.

  View details for DOI 10.1073/pna5.0509539103

  View details for Web of Science ID 000234350000010

  View details for PubMedID 16357194

  View details for PubMedCentralID PMC1323201

• Chromate reduction and retention processes within arid subsurface environments ENVIRONMENTAL SCIENCE & TECHNOLOGY Ginder-Vogel, M., Borch, T., Mayes, M. A., Jardine, P. M., Fendorf, S. 2005; 39 (20): 7833-7839

  Abstract

  Chromate is a widespread contaminantthat has deleterious impacts on human health, the mobility and toxicity of which are diminished by reduction to Cr(III). While biological and chemical reduction reactions of Cr(VI) are well resolved, reduction within natural sediments, particularly of arid environments, remains poorly described. Here, we examine chromate reduction within arid sediments from the Hanford, WA site, where Fe(III) (hydr)oxide and carbonate coatings limit mineral reactivity. Chromium(VI) reduction by Hanford sediments is negligible unless pretreated with acid; acidic pretreatment of packed mineral beds having a Cr(VI) feed solution results in Cr(III) associating with the minerals antigorite and lizardite in addition to magnetite and Fe(II)-bearing clay minerals. Highly alkaline conditions (pH > 14), representative of conditions near high-level nuclearwaste tanks, result in Fe(II) dissolution and concurrent Cr(VI) reduction. Additionally, Cr(III) and Cr(VI) are found associated with portlandite, suggesting a secondary mechanism for chromium retention at high pH. Thus, mineral reactivity is limited within this arid environment and appreciable reduction of Cr(VI) is restricted to highly alkaline conditions resulting near leaking radioactive waste disposal tanks.

  View details for DOI 10.1021/es050535y

  View details for Web of Science ID 000232758400015

  View details for PubMedID 16295844

• In situ analysis of thioarsenite complexes in neutral to alkaline arsenic sulphide solutions Conference on Environmental Mineralogy, Geochemistry and Human Health Bostick, B. C., Fendorf, S., Brown, G. E. MINERALOGICAL SOC. 2005: 781–95

  View details for DOI 10.1180/0026461056950288

  View details for Web of Science ID 000235009100021

• Adsorption, oxidation, and bioaccessibility of As(III) in soils ENVIRONMENTAL SCIENCE & TECHNOLOGY Yang, J. K., Barnett, M. O., Zhuang, J. L., Fendorf, S. E., Jardine, P. M. 2005; 39 (18): 7102-7110

  Abstract

  At As-contaminated sites, where the ingestion of soil by children is typically the critical human-health exposure pathway, information on the bioavailability of soil-bound As is often limited. The influence of various soil physical and chemical properties (iron and manganese oxides, pH, cation exchange capacity, total inorganic and organic carbon, and particle size) on As(III) adsorption, sequestration, bioaccessibility (as a surrogate for oral bioavailability), and oxidation was investigated in 36 well-characterized soils by use of a physiologically based extraction test (PBET). These results were compared to an earlier published study with As(V) on the same set of soils. The properties of the soils were able to explain >80% of the variability in the adsorption and sequestration (as measured by the reduction in bioaccessibility over time) of As(III) in these soils. The initial bioaccessibility of As(III) was significantly higher than the initial bioaccessibility of As(V) on the same set of soils. However, over a 6-month period of aerobic aging, a significant portion of the solid-phase As(III) on these soils was oxidized to As(V), decreasing its bioaccessibility markedly. A multivariable linear regression model previously developed to predict the steady-state bioaccessibility of As(V) in soils was able to predict the bioaccessibility in As(III)-spiked soils within a root-mean-square error (RMSE) of 16.8%. Generally, soils having a higher iron oxide content and lower soil pH exhibited lower bioaccessibility. This model was also able to predict the in vivo bioavailability of As in contaminated soils previously used in an independent juvenile swine dosing trial within an RMSE of 15.5%, providing a greatly improved yet conservative estimate of bioavailability relative to the typical default assumption of 100%. However, the model was not able to accurately predict the bioavailability of As in a different set of contaminated soils previously used in an independent Cebus monkey dosing trial, consistently overpredicting the bioavailability, resulting in an RMSE of 42.7%. This model can be used to provide an initial estimate of As bioavailability in soil to aid in screening sites and justifying expensive site-specific animal feeding studies. Further, as the model is based on major soil properties, the resulting estimates are valid as long as the major soil properties do not change, thus providing some confidence in the long-term applicability of the estimates.

  View details for DOI 10.1021/es0481474

  View details for Web of Science ID 000231941700030

  View details for PubMedID 16201635

• Competing Fe(II)-induced mineralization pathways of ferrihydrite ENVIRONMENTAL SCIENCE & TECHNOLOGY Hansel, C. M., Benner, S. G., Fendorf, S. 2005; 39 (18): 7147-7153

  Abstract

  Owing to its high surface area and intrinsic reactivity, ferrihydrite serves as a dominant sink for numerous metals and nutrients in surface environments and is a potentially important terminal electron acceptor for microbial respiration. Introduction of Fe (II), by reductive dissolution of Fe(III) minerals, for example, converts ferrihydrite to Fe phases varying in their retention and reducing capacity. While Fe(II) concentration is the master variable dictating secondary mineralization pathways of ferrihydrite, here we reveal thatthe kinetics of conversion and ultimate mineral assemblage are a function of competing mineralization pathways influenced by pH and stabilizing ligands. Reaction of Fe(II) with ferrihydrite results in the precipitation of goethite, lepidocrocite, and magnetite. The three phases vary in their precipitation extent, rate, and residence time, all of which are primarily a function of Fe(II) concentration and ligand type (Cl, SO4, CO3). While lepidocrocite and goethite precipitate over a large Fe(II) concentration range, magnetite accumulation is only observed at surface loadings greater than 1.0 mmol Fe(II)/g ferrihydrite (in the absence of bicarbonate). Precipitation of magnetite induces the dissolution of lepidocrocite (presence of Cl) or goethite (presence of SO4), allowing for Fe(III)-dependent crystal growth. The rate of magnetite precipitation is a function of the relative proportions of goethite to lepidocrocite; the lower solubility of the former Fe (hydr)oxide slows magnetite precipitation. A one unit pH deviation from 7, however, either impedes (pH 6) or enhances (pH 8) magnetite precipitation. In the absence of magnetite nucleation, lepidocrocite and goethite continue to precipitate at the expense of ferrihydrite with near complete conversion within hours, the relative proportions of the two hydroxides dependent upon the ligand present. Goethite also continues to precipitate at the expense of lepidocrocite in the absence of chloride. In fact, the rate and extent of both goethite and magnetite precipitation are influenced by conditions conducive to the production and stability of lepidocrocite. Thus, predicting the secondary mineralization of ferrihydrite, a process having sweeping influences on contaminant/nutrient dynamics, will need to take into consideration kinetic restraints and transient precursor phases (e.g., lepidocrocite) that influence ensuing reaction pathways.

  View details for DOI 10.1021/es050666z

  View details for Web of Science ID 000231941700036

  View details for PubMedID 16201641

• Effects of a diel oxygen cycle on nitrogen transformations and greenhouse gas emissions in a eutrophied subtropical stream AQUATIC SCIENCES Harrison, J. A., Matson, P. A., Fendorf, S. E. 2005; 67 (3): 308-315

  View details for DOI 10.1007/s00027-005-0776-3

  View details for Web of Science ID 000232030000008

• Bioreduction of uranium in a contaminated soil column ENVIRONMENTAL SCIENCE & TECHNOLOGY Gu, B. H., Wu, W. M., Ginder-Vogel, M. A., Yan, H., Fields, M. W., Zhou, J., Fendorf, S., Criddle, C. S., Jardine, P. M. 2005; 39 (13): 4841-4847

  Abstract

  The bioreduction of soluble uranium [U(VI)] to sparingly soluble U(IV) species is an attractive remedial technology for contaminated soil and groundwater due to the potential for immobilizing uranium and impeding its migration in subsurface environments. This manuscript describes a column study designed to simulate a three-step strategy proposed for the remediation of a heavily contaminated site at the U.S. Department of Energy's NABIR Field Research Center in Oak Ridge, TN. The soil is contaminated with high concentrations of uranium, aluminum, and nitrate and has a low, highly buffered pH (approximately 3.5). Steps proposed for remediation are (i) flushing to remove nitrate and aluminum, (ii) neutralization to establish pH conditions favorable for biostimulation, and (iii) biostimulation for U(VI) reduction. We simulated this sequence using a packed soil column containing undisturbed aggregates of U(VI)-contaminated saprolite that was flushed with an acidified salt solution (pH 4.0), neutralized with bicarbonate (60 mM), and then biostimulated by adding ethanol. The column was operated anaerobically in a closed-loop recirculation setup. However, during the initial month of biostimulation, ethanol was not utilized, and U(VI) was not reduced. A bacterial culture enriched from the site groundwaterwas subsequently added, and the consumption of ethanol coupled with sulfate reduction immediately ensued. The aqueous concentration of U(VI) initially increased, evidently because of the biological production of carbonate, a ligand known to solubilize uranyl. After approximately 50 days, aqueous U(VI) concentrations rapidly decreased from approximately 17 to <1 mg/L. At the conclusion of the experiment,the presence of reduced solid phase U(IV) was confirmed using X-ray absorption near edge structure spectroscopy. The results indicate that bioreduction to immobilize uranium is potentially feasible at this site; however, the stability of the reduced U(IV) and its potential reoxidation will require further investigation, as do the effects of groundwater chemistry and competitive microbial processes, such as methanogenesis.

  View details for Web of Science ID 000230245500032

  View details for PubMedID 16053082

• Uranium hydrogeochemistry of the Hanford caliche layer 15th Annual V M Goldschmidt Conference Mayes, M. A., Pace, M. N., Fendorf, S. E., Jardine, P. M., Yin, X. L. PERGAMON-ELSEVIER SCIENCE LTD. 2005: A424–A424

  View details for Web of Science ID 000229399701234

• Ca-UO2-CO3 complexation - Implications for Bioremediation of U(VI) PHYSICA SCRIPTA Kelly, S. D., Kemner, K. M., Brooks, S. C., Fredrickson, J. K., Carroll, S. L., Kennedy, D. W., Zachara, J. M., Plymale, A. E., Fendorf, S. 2005; T115: 915-917

  View details for Web of Science ID 000204272100275

• Transformation and transport of arsenic within ferric hydroxide coated sands upon dissimilatory reducing bacterial activity Symposium on Advances in Arsenic Research held at the 226th ACS National Meeting Herbel, M., Fendorf, S. AMER CHEMICAL SOC. 2005: 77–90

  View details for Web of Science ID 000236279300006

• Temporal changes in soil partitioning and bioaccessibility of arsenic, chromium, and lead JOURNAL OF ENVIRONMENTAL QUALITY Fendorf, S., La Force, M. J., Li, G. C. 2004; 33 (6): 2049-2055

  Abstract

  The hazard imposed by trace element contaminants within soils is dependent on their ability to migrate into water systems and their availability for biological uptake. The degree to which a contaminant may dissociate from soil solids and become available to a target organism (i.e., bioaccessibility) is therefore a determining risk factor. We used a physiologically based extraction test (PBET) to estimate the bioaccessible fraction of arsenic-, chromium-, and lead-amended soil. We investigated soils from the A and B horizons of the Melton Valley series, obtained from Oak Ridge National Laboratory site, to address temporal changes in bioaccessibility. Additionally, common extractions that seek to define reactive pools of metals were employed and their correlation to PBET levels evaluated. With the exception of Pb amended to the A horizon, all other treatments exhibited an exponential decrease in bioaccessibility with incubation time. The bioaccessible fraction was less than 0.2 mg kg(-1) within 30 d of incubation for As and Cr in the A horizon and for As and Pb within the B horizon; Cr in the B horizon declined to nearly 0.3 mg kg(-1) within 100 d of aging. The exchangeable fraction declined with incubation period and, with the exception of Pb, was highly correlated with the decline in bioaccessibility. Our results demonstrate limited bioaccessibility in all but one case and the need to address both short-term temporal changes and, most importantly, the soil physiochemical properties. They further reveal the importance of incubation time on the reactivity of such trace elements.

  View details for Web of Science ID 000225240900010

  View details for PubMedID 15537927

• Chemical structure of arsenic and chromium in CCA-treated wood: Implications of environmental weathering ENVIRONMENTAL SCIENCE & TECHNOLOGY Nico, P. S., Fendorf, S. E., Lowney, Y. W., Holm, S. E., Ruby, M. V. 2004; 38 (19): 5253-5260

  Abstract

  Chromated copper arsenate (CCA) has been used to treat lumber for over 60 years to increase the expected lifetime of CCA-treated wood. Because of the toxicity of the arsenic and chromium used in CCA treatment, regulatory and public attention has become focused on the potential risks from this exposure source. In particular, exposure of children to arsenic from CCA-treated wood used in decks and play sets has received considerable attention. X-ray Absorption Spectroscopy (XAS) was used to evaluate the chemical structure of As and Cr in three samples of CCA-treated materials: newly treated wood, aged wood (5 years as decking), and dislodgeable residue from aged (1-4 years as decking) CCA-treated wood. The form of the Cr and As in CCA-treated material is the same in fresh and aged samples, and between treated wood and dislodged residue. In all cases, the dominant oxidation state of the two elements is As(V) and Cr(III), and the local chemical environment of the two elements is best represented as a Cr/As cluster consisting of a Cr dimer bridged by an As(V) oxyanion. Long-term stability of the As/Cr cluster is suggested by its persistence from the new wood through the aged wood and the dislodgeable residue.

  View details for DOI 10.1021/es0351342

  View details for Web of Science ID 000224234100055

  View details for PubMedID 15506225

• Structural constraints of ferric (hydr)oxides on dissimilatory iron reduction and the fate of Fe(II) Meeting on Microbial Geochemistry held at the 2002 Fall Meeting of the AGU Hansel, C. M., Benner, S. G., Nico, P., Fendorf, S. PERGAMON-ELSEVIER SCIENCE LTD. 2004: 3217–29

  View details for DOI 10.1016/j.gca.2003.10.041

  View details for Web of Science ID 000222909900007

• Biomineralization of As(V)-hydrous ferric oxyhydroxide in microbial mats of an acid-sulfate-chloride geothermal spring, Yellowstone National Park Meeting on Microbial Geochemistry held at the 2002 Fall Meeting of the AGU Inskeep, W. P., Macur, R. E., Harrison, G., Bostick, B. C., Fendorf, S. PERGAMON-ELSEVIER SCIENCE LTD. 2004: 3141–55

  View details for DOI 10.1016/j.gca.2003.09.020

  View details for Web of Science ID 000222909900002

• Arsenite retention mechanisms within estuarine sediments of Pescadero, CA ENVIRONMENTAL SCIENCE & TECHNOLOGY Bostick, B. C., Chen, C., Fendorf, S. 2004; 38 (12): 3299-3304

  Abstract

  Arsenic, a toxic metalloid, is commonly associated with sulfide minerals in anoxic sediments. Here we characterize arsenic(III) retention on sediments from a sulfidic estuarine marsh using a series of sorption experiments, and probe the structure of retained arsenite with X-ray absorption spectroscopy. Although the extent of sorption varied with sampling locations, several adsorption characteristics were apparent. A fraction of arsenite adsorbed over the entire pH range examined, although it was most extensive at pH greater than 7, and conformed to a Langmuir isotherm. Iron sulfide phases were responsible for As partitioning in these sediments. Initially, an FeAsS-like precipitate formed with a structure similar to those reported for As(III) sorbed on iron sulfides, a complex that is highly reactive. Following reaction for 21 d, much of the FeAsS-like precipitate was converted to As2S3. A drop in the redox potential accompanied this conversion, suggesting that the evolution of sulfide and other reduced species stabilizes bound arsenic. Processes discerned in this study reveal the importance of sulfide minerals in As sequestration within anoxic environments.

  View details for DOI 10.1021/es035006d

  View details for Web of Science ID 000222051400023

  View details for PubMedID 15260327

• Chrominium geochemistry of serpentine soils INTERNATIONAL GEOLOGY REVIEW Oze, C., Fendorf, S., Bird, D. K., Coleman, R. G. 2004; 46 (2): 97-126

  View details for Web of Science ID 000220348700001

• Chromium geochemistry in serpentinized ultramafic rocks and serpentine soils from the Franciscan Complex of California AMERICAN JOURNAL OF SCIENCE Oze, C., Fendorf, S., Bird, D. K., Coleman, R. G. 2004; 304 (1): 67-101

  View details for Web of Science ID 000189188000003

• Soil chemistry and mineralogy: Kinetics of redox reactions Encyclopedia of Soils in the Environment Nico, P. S., Fendorf, S. edited by Hillel, D. Academic Press. 2004
• Enrichment of Mo in hydrothermal Mn precipitates: possible Mo sources, formation process and phase associations CHEMICAL GEOLOGY Kuhn, T., Bostick, B. C., Koschinsky, A., Halbach, P., Fendorf, S. 2003; 199 (1-2): 29-43

  View details for DOI 10.1016/S0009-2541(03)00054-8

  View details for Web of Science ID 000183713600002

• Secondary mineralization pathways induced by dissimilatory iron reduction of ferrihydrite under advective flow GEOCHIMICA ET COSMOCHIMICA ACTA Hansel, C. M., Benner, S. G., Neiss, J., Dohnalkova, A., Kukkadapu, R. K., Fendorf, S. 2003; 67 (16): 2977-2992

  View details for DOI 10.1016/S0016-7037(03)00276-X

  View details for Web of Science ID 000184609300006

• Kinetics and structural constraints of chromate reduction by green rusts ENVIRONMENTAL SCIENCE & TECHNOLOGY Bond, D. L., Fendorf, S. 2003; 37 (12): 2750-2757

  Abstract

  Green rusts, ferrous-ferric iron oxides, occur in many anaerobic soils and sediments and are highly reactive, making them important phases impacting the fate and transport of environmental contaminants. Despite their potential importance in environmental settings, reactions involving green rusts remain rather poorly described. Chromate is a widespread contaminant having deleterious impacts on plant and animal health; its fate may in part be controlled by green rust. Here we examine chromate reduction by a series of green rust phases and resolve the reaction kinetics at pH 7. The overall kinetics of the reactions are well described by the expression d[Cr(VI)]/dt = -k[Cr(VI)][GR], and this model was successfully used to predict rates of reaction at varying chromium concentrations. The rates of reduction are controlled by the concentration of ferrous iron, surface area, and chemical structure of the green rust including layer spacing. On a mass basis, green rust (GR) chloride is the most rapid reductant of Cr(VI) followed by GRCO3 and GRSO4, with pseudo-first-order rate coefficients (k(obs)) (with respect to Cr(VI) concentration) ranging from 1.22 x 10(-3) to 3.7 x 10(-2) s(-1). Chromium(III)-substituted magnetite and lepidocrocite were identified as the major oxidation products. The nature of the oxidation products appears to be independent of the anionic class of green rust, but their respective concentrations display a dependence on the initial GR. The mole fraction of Fe(III) in the Cr(x),Fe(1-x)(OH)3 x nH2O reaction product ranged from 17% to 68%, leading to a highly stabilized (low solubility) phase.

  View details for DOI 10.1021/es026341p

  View details for Web of Science ID 000183504900018

  View details for PubMedID 12854715

• Inhihition of bacterial U(VI) reduction by calcium ENVIRONMENTAL SCIENCE & TECHNOLOGY Brooks, S. C., Fredrickson, J. K., Carroll, S. L., Kennedy, D. W., Zachara, J. M., Plymale, A. E., Kelly, S. D., Kemner, K. M., Fendorf, S. 2003; 37 (9): 1850-1858

  Abstract

  The rapid kinetics of bacterial U(VI) reduction and low solubility of uraninite (UO2,cr) make this process an attractive option for removing uranium from groundwater. Nevertheless, conditions that may promote or inhibit U(VI) reduction are not well-defined. Recent descriptions of Ca-UO2-CO3 complexes indicate that these species may dominate the aqueous speciation of U(VI) in many environments. We monitored the bacterial reduction of U(VI) in bicarbonate-buffered solution in the presence and absence of Ca. XAFS measurements confirmed the presence of a Ca-U(VI)-C03 complex in the initial solutions containing calcium. Calcium, at millimolar concentrations (0.45-5 mM), caused a significant decrease in the rate and extent of bacterial U(VI) reduction. Both facultative (Shewanella putrefaciens strain CN32) and obligate (Desulfovibrio desulfuricans, Geobacter sulfurreducens) anaerobic bacteria were affected by the presence of calcium. Reduction of U(VI) ceased when the calculated system Eh reached -0.046 +/- 0.001 V, based on the Ca2UO2(CO3)3 --> UO2,cr couple. The results are consistent with the hypothesis that U is a less energetically favorable electron acceptor when the Ca-UO2-CO3 complexes are present. The results do not support Ca inhibition caused by direct interactions with the cells or with the electron donor as the reduction of fumarate or Tc(VII)O4- under identical conditions was unaffected by the presence of Ca.

  View details for DOI 10.1021/es0210042

  View details for Web of Science ID 000182635200039

  View details for PubMedID 12775057

• Arsenite sorption on troilite (FeS) and pyrite (FeS2) Topical Symposium on Advances in Oxide and Sulfide Mineral Surface Geochemistry Bostick, B. C., Fendorf, S. PERGAMON-ELSEVIER SCIENCE LTD. 2003: 909–21

  View details for DOI 10.1016/S0016-7037(02)01170-5

  View details for Web of Science ID 000181126800012

• Arsenite adsorption on galena (PbS) and sphalerite (ZnS) Topical Symposium on Advances in Oxide and Sulfide Mineral Surface Geochemistry Bostick, B. C., Fendorf, S., Manning, B. A. PERGAMON-ELSEVIER SCIENCE LTD. 2003: 895–907

  View details for Web of Science ID 000181126800011

• Structural and compositional evolution of Cr/Fe solids after indirect chromate reduction by dissimilatory iron-reducing bacteria GEOCHIMICA ET COSMOCHIMICA ACTA Hansel, C. M., Wielinga, B. W., Fendorf, S. R. 2003; 67 (3): 401-412

  View details for Web of Science ID 000180692600006

• Differential adsorption of molybdate and tetrathiomolybdate on pyrite (FeS2) ENVIRONMENTAL SCIENCE & TECHNOLOGY Bostick, B. C., Fendorf, S., Helz, G. R. 2003; 37 (2): 285-291

  Abstract

  Molybdenum is a nutrient important for a variety of biological functions, most notably nitrogen fixation. Molybdenum availability is limited through sorption reactions, particularly in environments rich in sulfide minerals. This study examines the sorption of two major molybdenum species, molybdate (MoO4(2-)) and tetrathiomolybdate (MoS4(2-)), on synthetic pyrite (FeS2) as a function of solution composition. Both MoO4(2-) and MoS4(2-) partitioned strongly on FeS2 under a range of conditions and ionic strengths. Molybdate and tetrathiomolybdate adsorption obeyed a Langmuir isotherm with a calculated site density between 2 and 3 sites/nm2 under acidic and circumneutral conditions, which decreased to less than 1 site/ nm2 at pH 9. Although both MoO4(2-) and MoS4(2-) adsorbed most strongly under moderately acidic conditions, MoO4(2-) readily desorbed while MoS4(2-) remained adsorbed even at high pH. The reversibility of MoO4(2-) adsorption suggests the formation of labile surface complexes while MoS4(2-) likely forms strong inner-sphere complexes. X-ray absorption spectroscopy was used to determine the structure of the surface complexes. Molybdate formed bidentate, mononuclear complexes on FeS2. The Mo-S and Mo-Fe distances for tetrathiomolybdate on pyrite are consistent with the formation of Mo-Fe-S cubane-type clusters. The high affinity of MoS4(2-) for FeS2, as well as its resistance to desorption, supports the hypothesis that thiomolybdate species are the reactive Mo constituents in reduced sediments and may control Mo enrichment in anoxic marine environments.

  View details for DOI 10.1021/es027487

  View details for Web of Science ID 000180501500012

  View details for PubMedID 12564899

• Arsenic(III) complexation and oxidation reactions on soil Symposium on Biogeochemistry of Trace Elements held at 221st National Meeting of the American-Chemical-Society Manning, B. A., Fendorf, S. E., Suarez, D. L. AMER CHEMICAL SOC. 2003: 57–69

  View details for Web of Science ID 000180815400005

• Arsenite adsorption on galena (PbS) and sphalerite (ZnS) Geochimica et Cosmochimica Acta Bostick, B. C., Fendorf, S., Manning, B. A. 2003; 37: 285-291
• Effects of contaminant concentration, aging, and soil properties on the bioaccessibility of Cr(III) and Cr(VI) in soil SOIL & SEDIMENT CONTAMINATION Stewart, M. A., Jardine, P. M., Brandt, C. C., Barnett, M. O., Fendorf, S. E., McKay, L. D., Mehlhorn, T. L., Paul, K. 2003; 12 (1): 1-21

  View details for Web of Science ID 000186637000001

• Seasonal transformations of manganese in a palustrine emergent wetland SOIL SCIENCE SOCIETY OF AMERICA JOURNAL La Force, M. J., Hansel, C. M., Fendorf, S. 2002; 66 (4): 1377-1389

  View details for Web of Science ID 000176588300034

• Spatial and temporal association of As and Fe species on aquatic plant roots ENVIRONMENTAL SCIENCE & TECHNOLOGY Hansel, C. M., La Force, M. J., Fendorf, S., Sutton, S. 2002; 36 (9): 1988-1994

  Abstract

  The formation of an Fe(III) precipitate (plaque) on the surface of aquatic plant roots may provide a means of attenuation and external exclusion of metals. Presently, the mechanisms of metal(loid) sequestration at the root surface are unresolved. Accordingly, we investigated the mechanisms of Fe and As attenuation and association on the roots of two common aquatic plant species, Phalaris arundinacea (reed canarygrass) and Typha latifolia (cattail) using X-ray absorption spectroscopy and X-ray fluorescence microtomography. Iron plaque of both P. arundinacea and T. latifolia consist predominantly of hydrated iron oxides (ferrihydrite) with lesser amounts of goethite and minor levels of siderite. Typha latifolia, however, differs from P. arundinacea by having a significant contribution from lepidocrocite as well as a greater proportion of crystalline minerals. Coexistence of goethite and lepidocrocite suggests the presence of chemically diverse microenvironments at the root surface. Arsenic exists as a combination of two sorbed As species, being comprised predominantly of arsenate- (approximately 82%) with lesser amounts (approximately 18%) of As(III)-iron (hydr)oxide complexes. Furthermore, both spatial and temporal correlations between As and Fe on the root surfaces were observed. While the iron (hydr)oxide deposits form a continuous surficial rind around the root, As exists in isolated regions on the exterior and interior of the root. Root surface-associated As generally corresponds to regions of enhanced Fe levels and may therefore occur as a direct consequence of Fe phase heterogeneity and preferential As sorption reactions.

  View details for DOI 10.1021/es015647d

  View details for Web of Science ID 000175311900022

  View details for PubMedID 12026982

• Reductive dissolution and biomineralization of iron hydroxide under dynamic flow conditions ENVIRONMENTAL SCIENCE & TECHNOLOGY Benner, S. G., Hansel, C. M., Wielinga, B. W., Barber, T. M., Fendorf, S. 2002; 36 (8): 1705-1711

  Abstract

  Iron cycling and the associated changes in solid phase have dramatic implications for trace element mobility and bioavailability. Here we explore the formation of secondary iron phases during microbially mediated reductive dissolution of ferrihydrite-coated sand under dynamic flow conditions. An initial period (10 d) of rapid reduction, indicated by consumption of lactate and production of acetate and Fe-(II) to the pore water in association with a darkening of the column material, is followed by much lower rate of reduction to the termination of the experiment after 48 d. Although some Fe (<25%) is lost to the effluent pore water, the majority remains within the column as ferrihydrite (20-70%) and the secondary mineral phases magnetite (0-70%) and goethite (0-25%). Ferrihydrite converts to goethite in the influent end of the column where dissolved Fe(II) concentrations are low and converts to magnetite toward the effluent end where Fe(III) concentrations are elevated. A decline in the rate of Fe(II) production occurs concurrent with the formation of goethite and magnetite; at the termination of the experiment, the rate of reduction is <5% the initial rate. Despite the dramatic decrease in the rate of reduction, greater than 80% of the residual Fe remains in the ferric state. These results highlight the importance of coupled flow and water chemistry in controlling the rate and solid-phase products of iron (hydr)oxide reduction.

  View details for DOI 10.1021/es0156441

  View details for Web of Science ID 000174976300010

  View details for PubMedID 11993867

• Arsenic(III) oxidation and arsenic(V) adsorption reactions on synthetic birnessite ENVIRONMENTAL SCIENCE & TECHNOLOGY Manning, B. A., Fendorf, S. E., Bostick, B., Suarez, D. L. 2002; 36 (5): 976-981

  Abstract

  The oxidation of arsenite (As(III)) by manganese oxide is an important reaction in both the natural cycling of As and the development of remediation technology for lowering the concentration of dissolved As(III) in drinking water. This study used both a conventional stirred reaction apparatus and extended X-ray absorption fine structure (EXAFS) spectroscopy to investigate the reactions of As(III) and As(V) with synthetic birnessite (MnO2). Stirred reactor experiments indicate that As(III) is oxidized by MnO2 followed by the adsorption of the As(V) reaction product on the MnO2 solid phase. The As(V)-Mn interatomic distance determined by EXAFS analysis for both As(III)- and As(V)-treated MnO2 was 3.22 A, giving evidence for the formation of As(V) adsorption complexes on MnO2 crystallite surfaces. The most likely As(V)-MnO2 complex is a bidentate binuclear corner sharing (bridged) complex occurring at MnO2 crystallite edges and interlayer domains. In the As(III)-treated MnO2 systems, reductive dissolution of the MnO2 solid during the oxidation of As(III) caused an increase in the adsorption of As(V) when compared with As(V)-treated MnO2. This suggested that As(III) oxidation caused a surface alteration, creating fresh reaction sites for As(V) on MnO2 surfaces.

  View details for DOI 10.1021/es0110170

  View details for Web of Science ID 000174136400038

  View details for PubMedID 11918029

• Uranyl surface complexes formed on subsurface media from DOE facilities SOIL SCIENCE SOCIETY OF AMERICA JOURNAL Bostick, B. C., Fendorf, S., Barnett, M. O., Jardine, P. M., Brooks, S. C. 2002; 66 (1): 99-108

  View details for Web of Science ID 000173219300013

• Arsenic (V/III) cycling in soils and natural waters: Chemical and microbiological processes Environmental Chemistry of Arsenic Inskeep, , W. P., McDermott, T. R., Fendorf, S. E. edited by Frankenberger, Jr., W. T. Spinger-Verlag. 2002: 183–215
• Soil Geochemical Processes of Radionuclides Soil Sci. Soc. Am. Special Publication Fendorf, S., Wielinga, B. W., Hansel, C. M. edited by Zhang, P. C., Brady, P. Soil Science Society of America. 2002
• Characterization of Fe plaque and associated metals on the roots of mine-waste impacted aquatic plants ENVIRONMENTAL SCIENCE & TECHNOLOGY Hansel, C. M., Fendorf, S., Sutton, S., Newville, M. 2001; 35 (19): 3863-3868

  Abstract

  Iron plaque on aquatic plant roots are ubiquitous and sequester metals in wetland soils; however, the mechanisms of metal sequestration are unresolved. Thus, characterizing the Fe plaque and associated metals will aid in understanding and predicting metal cycling in wetland ecosystems. Accordingly, microscopic and spectroscopic techniques were utilized to identify the spatial distributions, associations, and chemical environments of Fe, Mn, Pb, and Zn on the roots of a common, indigenous wetland plant (Phalaris arundinacea). Iron forms a continuous precipitate on the root surface, which is composed dominantly of ferrihydrite (ca. 63%) with lesser amounts of goethite (32%) and minor levels of siderite (5%). Although Pb is juxtaposed with Fe on the root surface, it is complexed to organic functional groups, consistent with those of bacterial biofilms. In contrast, Mn and Zn exist as discrete, isolated mixed-metal carbonate (rhodochrosite/hydrozincite) nodules on the root surface. Accordingly, the soil-root interface appears to be a complex biochemical environment, containing both reduced and oxidized mineral species, as well as bacterial-induced organic-metal complexes. As such, hydrated iron oxides, bacterial biofilms, and metal carbonates will influence the availability and mobility of metals within the rhizosphere of aquatic plants.

  View details for DOI 10.1021/es0105459

  View details for Web of Science ID 000171352500020

  View details for PubMedID 11642445

• Seasonal fluctuations in zinc speciation within a contaminated wetland ENVIRONMENTAL SCIENCE & TECHNOLOGY Bostick, B. C., Hansel, C. M., La Force, M. J., Fendorf, S. 2001; 35 (19): 3823-3829

  Abstract

  The cycling of common sorbents such as metal (hydr)- oxides, carbonates, and sulfides in redox-active environments influences the partitioning of associated trace elements such as zinc. Consequently, fluctuations in redox status may in part determine the availability and mobility of Zn and other trace elements. This research examines changes in Zn speciation in a contaminated wetland soil that undergoes seasonal flooding. X-ray absorption spectroscopy (XAS) was employed to identify and quantify Zn species from soil cores collected over a 1-year cycle as a function of water depth, location, and soil depth. Zinc associated with (hydr)oxide phases in dry, oxidized soils and with sulfides and carbonates in flooded systems. An increase in water level was accompanied by a reversible change in Zn fractionation toward ZnS and ZnC03. However, a small, recalcitrant fraction of Zn associated with (hydr)oxides remained even when the soils were exposed to highly reducing conditions. Water depth and redox potential were the most important factors in determining Zn speciation, although spatial variation was also important. These data indicate that zinc sorption is a dynamic process influenced by environmental changes.

  View details for DOI 10.1021/es010549d

  View details for Web of Science ID 000171352500014

  View details for PubMedID 11642439

• Co(II0I)EDTA(-) reduction by Desulfovibrio vulgaris and propagation of reactions involving dissolved sulfide and polysulfides ENVIRONMENTAL SCIENCE & TECHNOLOGY Blessing, T. C., Wielinga, B. W., Morra, M. J., Fendorf, S. 2001; 35 (8): 1599-1603

  Abstract

  The migration of 60Co, dominantly via transport of Co-EDTA complexes, into surface water and groundwater is a recognized concern at many nuclear production and storage sites. Reduction of CoIIIEDTA- to CoIIEDTA2- should decrease the mobility of 60Co in natural environments by stimulating ligand displacement with Fe(III) or Al(III) or by precipitation of CoSx in sulfidic environments. In this study, we examine direct (enzymatic) and indirect (metabolite) reduction processes of CoIIIEDTA- by the sulfate-reducing bacterium Desulfovibrio vulgaris. D. vulgaris reduces CoIIIEDTA- to CoIIEDTA2-, but growth using it as a terminal electron acceptor was not demonstrated. Rather than acting as a competing electron acceptor and limiting cobalt reduction, introducing sulfate with D. vulgaris enhances the reduction of CoIIIEDTA- as a result of sulfide production. Sulfide reduces CoIIIEDTA- in a pathway involving polysulfide formation and leads to a CoS precipitate. Thus, both direct and indirect (i.e., through the production of sulfide) microbial reduction pathways of CoIIIEDTA- may help to retard its migration within soils and waters.

  View details for Web of Science ID 000168099000007

  View details for PubMedID 11329708

• Iron promoted reduction of chromate by dissimilatory iron-deducing bacteria ENVIRONMENTAL SCIENCE & TECHNOLOGY Wielinga, B., Mizuba, M. M., Hansel, C. M., Fendorf, S. 2001; 35 (3): 522-527

  Abstract

  Chromate is a priority pollutant within the U.S. and many other countries, the hazard of which can be mitigated by reduction to the trivalent form of chromium. Here we elucidate the reduction of Cr(VI) to Cr(III) via a closely coupled, biotic-abiotic reductive pathway under iron-reducing conditions. Injection of chromate into stirred-flow reactors containing Shewanella alga strain BrY and iron (hydr)oxides of varying stabilities results in complete reduction to Cr(III). The maximum sustainable Cr(VI) reduction rate was 5.5 micrograms CrVI.mg-cell-1.h-1 within ferric (hydr)oxide suspensions (surface area 10 m2). In iron limited systems (having HEPES as a buffer), iron was cycled suggesting it acts in a catalytic-type manner for the bacterial reduction of Cr(VI). BrY also reduced Cr(VI) directly; however, the rate of direct (enzymatic) reduction is considerably slower than by Fe(II)(aq) and is inhibited within 20 h due to chromate toxicity. Thus, dissimilatory iron reduction may provide a primary pathway for the sequestration and detoxification of chromate in anaerobic soils and water.

  View details for Web of Science ID 000166727700024

  View details for PubMedID 11351723

• Ecosystem dynamics of zinc and manganese within a mine-waste impacted wetland Crerar Volume Hansel, C. M., LaForce, M. J., Sutton, S. E., Fendorf, S. edited by Wood, S., Hellmann, R. Geochemical Society of America. 2001
• Kinetics of arsenate reduction by dissolved sulfide ENVIRONMENTAL SCIENCE & TECHNOLOGY Rochette, E. A., Bostick, B. C., Li, G. C., Fendorf, S. 2000; 34 (22): 4714-4720

  View details for Web of Science ID 000165315900009

• Multispecies transport of metal-EDTA complexes and chromate through undisturbed columns of weathered fractured saprolite JOURNAL OF CONTAMINANT HYDROLOGY Mayes, M. A., Jardine, P. M., Larsen, I. L., Brooks, S. C., Fendorf, S. E. 2000; 45 (3-4): 243-265

  View details for Web of Science ID 000089919000002

• Arsenic speciation, seasonal transformations, and co-distribution with iron in a mine waste-influenced palustrine emergent wetland ENVIRONMENTAL SCIENCE & TECHNOLOGY La Force, M. J., Hansel, C. M., Fendorf, S. 2000; 34 (18): 3937-3943

  View details for DOI 10.1021/es0010150

  View details for Web of Science ID 000089315600018

• Solid-phase iron characterization during common selective sequential extractions SOIL SCIENCE SOCIETY OF AMERICA JOURNAL La Force, M. J., Fendorf, S. 2000; 64 (5): 1608-1615

  View details for Web of Science ID 000089688600009

• Chromium transformations in natural environments: The role of biological and abiological. processes in chromium(VI) reduction INTERNATIONAL GEOLOGY REVIEW Fendorf, S., Wielinga, B. W., Hansel, C. M. 2000; 42 (8): 691-701

  View details for Web of Science ID 000089507500003

• Inhibition of bacterially promoted uranium reduction: Ferric (hydr)oxides as competitive electron acceptors ENVIRONMENTAL SCIENCE & TECHNOLOGY Wielinga, B., Bostick, B., Hansel, C. M., Rosenzweig, R. F., Fendorf, S. 2000; 34 (11): 2190-2195

  View details for Web of Science ID 000087394400031

• Purification to homogeneity and characterization of a novel Pseudomonas putida chromate reductase APPLIED AND ENVIRONMENTAL MICROBIOLOGY Park, C. H., Keyhan, M., Wielinga, B., Fendorf, S., Matin, A. 2000; 66 (5): 1788-1795

  Abstract

  Cr(VI) (chromate) is a widespread environmental contaminant. Bacterial chromate reductases can convert soluble and toxic chromate to the insoluble and less toxic Cr(III). Bioremediation can therefore be effective in removing chromate from the environment, especially if the bacterial propensity for such removal is enhanced by genetic and biochemical engineering. To clone the chromate reductase-encoding gene, we purified to homogeneity (>600-fold purification) and characterized a novel soluble chromate reductase from Pseudomonas putida, using ammonium sulfate precipitation (55 to 70%), anion-exchange chromatography (DEAE Sepharose CL-6B), chromatofocusing (Polybuffer exchanger 94), and gel filtration (Superose 12 HR 10/30). The enzyme activity was dependent on NADH or NADPH; the temperature and pH optima for chromate reduction were 80 degrees C and 5, respectively; and the K(m) was 374 microM, with a V(max) of 1.72 micromol/min/mg of protein. Sulfate inhibited the enzyme activity noncompetitively. The reductase activity remained virtually unaltered after 30 min of exposure to 50 degrees C; even exposure to higher temperatures did not immediately inactivate the enzyme. X-ray absorption near-edge-structure spectra showed quantitative conversion of chromate to Cr(III) during the enzyme reaction.

  View details for Web of Science ID 000086805500003

  View details for PubMedID 10788340

• Influence of cadmium sorption on FeS2 oxidation ENVIRONMENTAL SCIENCE & TECHNOLOGY Bostick, B. C., Fendorf, S., Bowie, B. T., Griffiths, P. R. 2000; 34 (8): 1494-1499

  View details for Web of Science ID 000086456100019

• Constructing simple wetland sampling devices SOIL SCIENCE SOCIETY OF AMERICA JOURNAL LaForce, M. J., Hansel, C. M., Fendorf, S. 2000; 64 (2): 809-811

  View details for Web of Science ID 000089446200050

• Disulfide disproportionation and CdS formation upon cadmium sorption on FeS2 GEOCHIMICA ET COSMOCHIMICA ACTA Bostick, B. C., Fendorf, S., Fendorf, M. 2000; 64 (2): 247-255

  View details for Web of Science ID 000085089600008

• Evidence for microbial Fe(III) reduction in anoxic, mining-impacted lake sediments (Lake Coeur d'Alene, Idaho) APPLIED AND ENVIRONMENTAL MICROBIOLOGY Cummings, D. E., March, A. W., Bostick, B., Spring, S., Caccavo, F., Fendorf, S., Rosenzweig, R. F. 2000; 66 (1): 154-162

  Abstract

  Mining-impacted sediments of Lake Coeur d'Alene, Idaho, contain more than 10% metals on a dry weight basis, approximately 80% of which is iron. Since iron (hydr)oxides adsorb toxic, ore-associated elements, such as arsenic, iron (hydr)oxide reduction may in part control the mobility and bioavailability of these elements. Geochemical and microbiological data were collected to examine the ecological role of dissimilatory Fe(III)-reducing bacteria in this habitat. The concentration of mild-acid-extractable Fe(II) increased with sediment depth up to 50 g kg(-1), suggesting that iron reduction has occurred recently. The maximum concentrations of dissolved Fe(II) in interstitial water (41 mg liter(-1)) occurred 10 to 15 cm beneath the sediment-water interface, suggesting that sulfidogenesis may not be the predominant terminal electron-accepting process in this environment and that dissolved Fe(II) arises from biological reductive dissolution of iron (hydr)oxides. The concentration of sedimentary magnetite (Fe(3)O(4)), a common product of bacterial Fe(III) hydroxide reduction, was as much as 15.5 g kg(-1). Most-probable-number enrichment cultures revealed that the mean density of Fe(III)-reducing bacteria was 8.3 x 10(5) cells g (dry weight) of sediment(-1). Two new strains of dissimilatory Fe(III)-reducing bacteria were isolated from surface sediments. Collectively, the results of this study support the hypothesis that dissimilatory reduction of iron has been and continues to be an important biogeochemical process in the environment examined.

  View details for Web of Science ID 000084585800023

  View details for PubMedID 10618217

• Evidence for microbial Fe(III) reduction in anoxic, mining-impacted lake sediments (Lake Coeur d'Alene, USA) Applied and Environmental Microbiology Cummings, D. E., March, A. W., Bostick, B. C., Spring, S., Caccavo, F., Jr., S. Fendorf, Rosenzweig, R. F. 2000; 66: 154-162
• Pyrolusite surface transformations measured in real-time during the reactive transport of Co(II)EDTA(2-) GEOCHIMICA ET COSMOCHIMICA ACTA Fendorf, S., Jardine, P. M., Patterson, R. R., TAYLOR, D. L., Brooks, S. C. 1999; 63 (19-20): 3049-3057

  View details for Web of Science ID 000083824200018

• Fate and transport of hexavalent chromium in undisturbed heterogeneous soil ENVIRONMENTAL SCIENCE & TECHNOLOGY Jardine, P. M., Fendorf, S. E., Mayes, M. A., Larsen, I. L., Brooks, S. C., Bailey, W. B. 1999; 33 (17): 2939-2944

  View details for Web of Science ID 000082367400035

• Arsenic sorption in phosphate-amended soils during flooding and subsequent aeration SOIL SCIENCE SOCIETY OF AMERICA JOURNAL Reynolds, J. G., Naylor, D. V., Fendorf, S. E. 1999; 63 (5): 1149-1156

  View details for Web of Science ID 000083839800014

• Redistribution of trace elements from contaminated sediments of Lake Coeur d'Alene during oxygenation JOURNAL OF ENVIRONMENTAL QUALITY La Force, M. J., Fendorf, S., Li, G. C., Rosenzweig, R. F. 1999; 28 (4): 1195-1200

  View details for Web of Science ID 000081345400019

• Arsenic mobilization by the dissimilatory Fe(III)-reducing bacterium Shewanella alga BrY ENVIRONMENTAL SCIENCE & TECHNOLOGY Cummings, D. E., Caccavo, F., Fendorf, S., Rosenzweig, R. F. 1999; 33 (5): 723-729

  View details for Web of Science ID 000078929000025

• Arsenic mobilization by the dissimilatory Fe(III) reducing bacterium Shewanella alga BrY Environmental Science & Technology Cummings, D., Caccavo, F., Fendorf, S., Rosenzweig, R. F. 1999; 33: 723-729
• Fundamental aspects and applications of x-ray absorption spectroscopy in clay and soil science Applications of synchrotron radiation in clay science Fendorf, S. E. edited by Schulze, D. G., Bertsch, P. M. Clay Mineral Society, Ottawa, Canada. 1999: 31–74
• Stability of arsenate minerals in soil under biotically generated reducing conditions SOIL SCIENCE SOCIETY OF AMERICA JOURNAL Rochette, E. A., Li, G. C., Fendorf, S. E. 1998; 62 (6): 1530-1537

  View details for Web of Science ID 000077836400008

• Reaction sequence of Nickel(II) with kaolinite: Mineral dissolution and surface complexation and precipitation SOIL SCIENCE SOCIETY OF AMERICA JOURNAL Eick, M. J., Fendorf, S. E. 1998; 62 (5): 1257-1267

  View details for Web of Science ID 000076500900016

• Biotic generation of arsenic(III) in metal(loid)-contaminated freshwater lake sediments ENVIRONMENTAL SCIENCE & TECHNOLOGY Harrington, J. M., Fendorf, S. E., Rosenzweig, R. F. 1998; 32 (16): 2425-2430

  View details for Web of Science ID 000075380800010

• Surface structures and stability of arsenic(III) on goethite: Spectroscopic evidence for inner-sphere complexes ENVIRONMENTAL SCIENCE & TECHNOLOGY Manning, B. A., Fendorf, S. E., Goldberg, S. 1998; 32 (16): 2383-2388

  View details for Web of Science ID 000075380800004

• Alteration of arsenic sorption in flooded-dried soils SOIL SCIENCE SOCIETY OF AMERICA JOURNAL McGeehan, S. L., Fendorf, S. E., Naylor, D. V. 1998; 62 (3): 828-833

  View details for Web of Science ID 000074698500042

• A laboratory evaluation of trace element mobility from flooding and nutrient loading of Coeur d'Alene River sediments JOURNAL OF ENVIRONMENTAL QUALITY La Force, M. J., Fendorf, S. E., Li, G. C., Schneider, G. M., Rosenzweig, R. F. 1998; 27 (2): 318-328

  View details for Web of Science ID 000072723300011

• Phase associations and mobilization of iron and trace elements in Coeur d'Alene Lake, Idaho ENVIRONMENTAL SCIENCE & TECHNOLOGY Harrington, J. M., LaForce, M. J., Rember, W. C., Fendorf, S. E., Rosenzweig, R. F. 1998; 32 (5): 650-656

  View details for Web of Science ID 000072307100040

• Phase associations and mobilization of iron and trace metals in sediments of Lake Coeur d'Alene, Idaho Environmental Science & Technology Harrington, J. M., Rosenzweig, R. F., Rember, W. C., Fendorf, S. E. 1998; 32: 650-656
• Kinetics and mechanisms of reactions at the mineral/water interface American Chemical Society Special Publication Fendorf, S. E., Jardine, P. M., Taylor, D. L., Brooks, S. C., Rochette, E. A. edited by Sparks, D. L., Grundl, T. American Chemical Society. 1998
• Stability of arsenate minerals in soils under biotically-generated reducing conditions Soil Science Society of America Journal Rochette, E. A., Li, G. C., Fendorf, S. E. 1998; 62: 1530-1537
• Mobility of trace-element contaminants upon flooding of the Coeur d'Alene River Journal of Environmental Quality LaForce, M. J., Fendorf, S. E., Li, G. C., Schneider, M., Rosenzweig, R. F. 1998; 27: 318-328
• Biotic generation of arsenic(III) in metal contaminated lake sediments Environmental Science & Technology Harrington, J. M., Fendorf, S. E., Rosenzwieg, R. F. 1998; 32: 2425-2430
• Alteration of arsenic sorption in flooded-dried soils Soil Science Society of America Journal McGeehan, S. L., Fendorf, S. E., Naylor, D. V. 1998; 62: 828-833
• Surface structures and stability of arsenic(III) on goethite: Spectroscopic evidence for inner-sphere complexes Environmental Science & Technology Manning, B. A., Fendorf, S. E., Goldberg, S. 1998; 32: 2383-2388
• Reaction sequence of nickelsorption on kaolinite Soil Science Society of America Journal Eick, M. J., Fendorf, S. E. 1998; 62: 1257-1267
• Reduction of hexavalent chromium by amorphous iron sulfide ENVIRONMENTAL SCIENCE & TECHNOLOGY Patterson, R. R., Fendorf, S., Fendorf, M. 1997; 31 (7): 2039-2044

  View details for Web of Science ID A1997XH48300051

• Arsenate and chromate retention mechanisms on goethite .1. Surface structure ENVIRONMENTAL SCIENCE & TECHNOLOGY Fendorf, S., Eick, M. J., Grossl, P., Sparks, D. L. 1997; 31 (2): 315-320

  View details for Web of Science ID A1997WG34300023

• Imaging a pseudomonad in mineral suspensions with scanning force and electron microscopy SOIL SCIENCE SOCIETY OF AMERICA JOURNAL Fendorf, S. E., Li, G. C., Morra, M. J., Dandurand, L. M. 1997; 61 (1): 109-115

  View details for Web of Science ID A1997WH68000017

• Sorption mechanisms of lanthanum on oxide minerals CLAYS AND CLAY MINERALS Fendorf, S., Fendorf, M. 1996; 44 (2): 220-227

  View details for Web of Science ID A1996UL99400007

• APPLICATIONS OF X-RAY-ABSORPTION FINE-STRUCTURE SPECTROSCOPY TO SOILS SOIL SCIENCE SOCIETY OF AMERICA JOURNAL Fendorf, S. E., Sparks, D. L., Lamble, G. M., Kelley, M. J. 1994; 58 (6): 1583-1595

  View details for Web of Science ID A1994PT17400001

• MECHANISMS OF CHROMIUM(III) SORPTION ON SILICA .2. EFFECT OF REACTION CONDITIONS ENVIRONMENTAL SCIENCE & TECHNOLOGY Fendorf, S. E., Sparks, D. L. 1994; 28 (2): 290-297

  View details for Web of Science ID A1994MV34700020

  View details for PubMedID 22176175

• MECHANISMS OF CHROMIUM(III) SORPTION ON SILICA .1. CR(III) SURFACE-STRUCTURE DERIVED BY EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE SPECTROSCOPY ENVIRONMENTAL SCIENCE & TECHNOLOGY Fendorf, S. E., Lamble, G. M., Stapleton, M. G., Kelley, M. J., Sparks, D. L. 1994; 28 (2): 284-289

  View details for Web of Science ID A1994MV34700019

  View details for PubMedID 22176174

• Mechanism of aluminum sorption on birnessite: Influences on chromium (III) oxidation 15th World Congress of Soil Science Fendorf, S. E., Sparks, D. L., Fendorf, M. INTERNATIONAL SOCIETY SOIL SCIENCE. 1994: 129–130

  View details for Web of Science ID A1994BD59Z00008

• Application of surface spectroscopies and microscopies to elucidate sorption mechanisms on oxide surfaces 15th World Congress of Soil Science Fendorf, S. E., Sparks, D. L. INTERNATIONAL SOCIETY SOIL SCIENCE. 1994: 182–199

  View details for Web of Science ID A1994BD59Z00014

• ELECTRON-PARAMAGNETIC RESONANCE STOPPED-FLOW KINETIC-STUDY OF MANGANESE(II) SORPTION DESORPTION ON BIRNESSITE SOIL SCIENCE SOCIETY OF AMERICA JOURNAL Fendorf, S. E., Sparks, D. L., Franz, J. A., Camaioni, D. M. 1993; 57 (1): 57-62

  View details for Web of Science ID A1993KW02200010

• INHIBITORY MECHANISMS OF CR(III) OXIDATION BY DELTA-MNO2 JOURNAL OF COLLOID AND INTERFACE SCIENCE Fendorf, S. E., Fendorf, M., Sparks, D. L., Gronsky, R. 1992; 153 (1): 37-54

  View details for Web of Science ID A1992JP47700005