All Publications


  • Effect of Natural Organic Matter on the Fate of Cadmium During Microbial Ferrihydrite Reduction. Environmental science & technology Zhou, Z., Muehe, E. M., Tomaszewski, E. J., Lezama-Pacheco, J., Kappler, A., Byrne, J. M. 2020

    Abstract

    Natural organic matter (NOM) is known to affect the microbial reduction and transformation of ferrihydrite, but its implication toward cadmium (Cd) associated with ferrihydrite is not well-known. Here, we investigated how Cd is redistributed when ferrihydrite undergoes microbial reduction in the presence of NOM. Incubation with Geobacter sulfurreducens showed that both the rate and the extent of reduction of Cd-loaded ferrihydrite were enhanced by increasing concentrations of NOM (i.e., C/Fe ratio). Without NOM, only 3-4% of Fe(III) was reduced, but around 61% of preadsorbed Cd was released into solution due to ferrihydrite transformation to lepidocrocite. At high C/Fe ratio (1.6), more than 35% of Fe(III) was reduced, as NOM can facilitate bioreduction by working as an electron shuttle and decreased aggregate size, but only a negligible amount of Cd was released into solution, thus decreasing Cd toxicity and prolonging microbial Fe(III) reduction. No ferrihydrite transformation was observed at high C/Fe ratios using Mossbauer spectroscopy and X-ray diffraction, and X-ray absorption spectroscopy indicated the proportion of Cd-OM bond increased after microbial reduction. This study shows that the presence of NOM leads to less mobilization of Cd under reducing condition possibly by inhibiting ferrihydrite transformation and recapturing Cd through Cd-OM bond.

    View details for DOI 10.1021/acs.est.0c03062

    View details for PubMedID 32633952

  • FeS colloids - formation and mobilization pathways in natural waters ENVIRONMENTAL SCIENCE-NANO Noel, V., Kumar, N., Boye, K., Barragan, L., Lezama-Pacheco, J. S., Chu, R., Tolic, N., Brown, G. E., Bargar, J. R. 2020; 7 (7): 2102–16

    View details for DOI 10.1039/c9en01427f

    View details for Web of Science ID 000549099000015

  • 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

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

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

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

  • Multivalence states and optical band-tail effect in heavily V-doped ZnS thin films MATERIALS LETTERS Colina-Ruiz, R. A., Hoy-Benitez, J. A., Lezama-Pacheco, J. S., Mustre de Leon, J., Espinosa-Faller, F. J. 2019; 257
  • Effect of Bicarbonate and Oxidizing Conditions on U(IV) and U(VI) Reactivity in Mineralized Deposits of New Mexico. Chemical geology Avasarala, S., Torres, C., Ali, A. S., Thomson, B. M., Spilde, M. N., Peterson, E. J., Artyushkova, K., Dobrica, E., Lezama-Pacheco, J. S., Cerrato, J. M. 2019; 524: 345–55

    Abstract

    We investigated the effect of bicarbonate and oxidizing agents on uranium (U) reactivity and subsequent dissolution of U(IV) and U(VI) mineral phases in the mineralized deposits from Jackpile mine, Laguna Pueblo, New Mexico, by integrating laboratory experiments with spectroscopy, microscopy and diffraction techniques. Uranium concentration in solid samples from mineralized deposit obtained for this study exceeded 7000 mg kg-1, as determined by X-ray fluorescence (XRF). Results from X-ray photoelectron spectroscopy (XPS) suggest the coexistence of U(VI) and U(IV) at a ratio of 19:1 at the near surface region of unreacted solid samples. Analyses made using X-ray diffraction (XRD) and electron microprobe detected the presence of coffinite (USiO4) and uranium-phosphorous-potassium (U-P-K) mineral phases. Imaging, mapping and spectroscopy results from scanning transmission electron microscopy (STEM) indicate that the U-P-K phases were encapsulated by carbon. Despite exposing the solid samples to strong oxidizing conditions, the highest aqueous U concentrations were measured from samples reacted with 100% air saturated 10 mM NaHCO3 solution, at pH 7.5. Analyses using X-ray absorption spectroscopy (XAS) indicate that all the U(IV) in these solid samples were oxidized to U(VI) after reaction with dissolved oxygen and hypochlorite (OCl-) in the presence of bicarbonate (HCO3 -). The reaction between these organic rich deposits, and 100% air saturated bicarbonate solution (containing dissolved oxygen), can result in considerable mobilization of U in water, which has relevance to the U concentrations observed at the Rio Paguate across the Jackpile mine. Results from this investigation provide insights on the reactivity of carbon encapsulated U-phases under mild and strong oxidizing conditions that have important implication in U recovery, remediation and risk exposure assessment of sites.

    View details for DOI 10.1016/j.chemgeo.2019.07.007

    View details for PubMedID 31406388

  • Organic Functional Group Chemistry in Mineralized Deposits Containing U(IV) and U(VI) from the Jackpile Mine in New Mexico ENVIRONMENTAL SCIENCE & TECHNOLOGY Velasco, C. A., Artyushkova, K., Ali, A. S., Osburn, C. L., Gonzalez-Estrella, J., Lezama-Pacheco, J. S., Cabaniss, S. E., Cerrato, J. M. 2019; 53 (10): 5758–67
  • 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
  • Organic Functional Group Chemistry in Mineralized Deposits Containing U(IV) and U(VI) from the Jackpile Mine in New Mexico. Environmental science & technology Velasco, C. A., Artyushkova, K., Ali, A. S., Osburn, C. L., Gonzalez-Estrella, J., Lezama-Pacheco, J. S., Cabaniss, S. E., Cerrato, J. M. 2019

    Abstract

    We investigated the functional group chemistry of natural organic matter (NOM) associated with both U(IV) and U(VI) in solids from mineralized deposits exposed to oxidizing conditions from the Jackpile Mine, Laguna Pueblo, NM. The uranium (U) content in unreacted samples was 0.44-2.6% by weight determined by X-ray fluorescence. In spite of prolonged exposure to ambient oxidizing conditions, ≈49% of U(IV) and ≈51% of U(VI) were identified on U LIII edge extended X-ray absorption fine structure spectra. Loss on ignition and thermogravimetric analyses identified from 13% to 44% of NOM in the samples. Carbonyl, phenolic, and carboxylic functional groups in the unreacted samples were identified by fitting of high-resolution X-ray photoelectron spectroscopy (XPS) C 1s and O 1s spectra. Peaks corresponding to phenolic and carbonyl functional groups had intensities higher than those corresponding to carboxylic groups in samples from the supernatant from batch extractions conducted at pH 13, 7, and 2. U(IV) and U(VI) species were detected in the supernatant after batch extractions conducted under oxidizing conditions by fitting of high-resolution XPS U 4f spectra. The outcomes from this study highlight the importance of the influence of pH on the organic functional group chemistry and U speciation in mineralized deposits.

    View details for PubMedID 30998849

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

  • Reactivity of As and U co-occurring in Mine Wastes in northeastern Arizona. Chemical geology Blake, J. M., Avasarala, S., Ali, A. S., Spilde, M., Lezama-Pacheco, J. S., Latta, D., Artyushkova, K., Ilgen, A. G., Shuey, C., Nez, C., Cerrato, J. M. 2019; 522: 26–37

    Abstract

    The reactivity of co-occurring arsenic (As) and uranium (U) in mine wastes was investigated using batch reactors, microscopy, spectroscopy, and aqueous chemistry. Analyses of field samples collected in proximity to mine wastes in northeastern Arizona confirm the presence of As and U in soils and surrounding waters, as reported in a previous study from our research group. In this study, we measured As (< 0.500 to 7.77 μg/L) and U (0.950 to 165 μg/L) in waters, as well as mine wastes (< 20.0 to 40.0 mg/kg As and < 60.0 to 110 mg/kg U) and background solids (< 20.0 mg/kg As and < 60.0 mg/kg U). Analysis with X-ray fluorescence (XRF) and electron microprobe show the co-occurrence of As and U with iron (Fe) and vanadium (V). These field conditions served as a foundation for additional laboratory experiments to assess the reactivity of metals in these mine wastes. Results from laboratory experiments indicate that labile and exchangeable As(V) was released to solution when solids were sequentially reacted with water and magnesium chloride (MgCl2), while limited U was released to solution with the same reactants. The predominance of As(V) in mine waste solids was confirmed by X-ray absorption near edge (XANES) analysis. Both As and U were released to solution after reaction of solids in batch experiments with HCO3-. Both X-ray photoelectron spectroscopy (XPS) and XANES analysis determined the predominance of Fe(III) in the solids. Mössbauer spectroscopy detected the presence of nano-crystalline goethite, Fe(II) and Fe(III) in (phyllo)silicates, and an unidentified mineral with parameters consistent with arsenopyrite or jarosite in the mine waste solids. Our results suggest that As and U can be released under environmentally relevant conditions in mine waste, which is applicable to risk and exposure assessment.

    View details for DOI 10.1016/j.chemgeo.2019.05.024

    View details for PubMedID 31371834

    View details for PubMedCentralID PMC6675030

  • Copper Mobilization and Immobilization along an Organic Matter and Redox Gradient-Insights from a Mofette Site ENVIRONMENTAL SCIENCE & TECHNOLOGY Mehlhorn, J., Besold, J., Pacheco, J., Gustafsson, J., Kretzschmar, R., Planer-Friedrich, B. 2018; 52 (23): 13698–707

    Abstract

    Mofettes (natural geogenic CO2 exhalations) represent excellent sites to study the behavior of Cu in soils and the co-occurrence of different mobilization and immobilization processes since they exhibit both a gradient in redox conditions (oxic to permanently anoxic) and in soil organic matter (SOM; low to high contents). Soil and pore water samples from an 18 m-transect over a mofette showed a complex behavior of Cu, with highest mobility in the transition between oxic and anoxic conditions. Cu(II) sorption experiments on SOM-rich topsoil revealed that Cu mobility under oxic conditions was confined by adsorption to SOM while in the oxygen-free mofette center reduction and precipitation of sulfides was the dominating Cu-sequestering process. In transition areas with low amounts of oxygen (<10%), there was no mineral precipitation, instead high dissolved-to-soil organic carbon ratios strongly increased Cu mobility. Our results show that low stability of SOM formed under oxygen-limited conditions leads to increased Cu mobility unless sulfur-reducing conditions cause Cu sequestration by sulfide precipitation. The interplay of these (im)mobilization processes and especially the unexpectedly high mobility under suboxic conditions have to be considered when assessing Cu mobility along spatial or temporal redox gradients, e.g., at contamination sites or periodically flooded soils.

    View details for PubMedID 30199245

  • The coordination chemistry of Cm-III, Am-III, and Ac-III in nitrate solutions: an actinide L-3-edge EXAFS study CHEMICAL SCIENCE Ferrier, M. G., Stein, B., Bone, S. E., Cary, S. K., Ditter, A. S., Kozimor, S. A., Lezama Pacheco, J. S., Mocko, V., Seidler, G. T. 2018; 9 (35): 7078–90

    View details for DOI 10.1039/c8sc02270d

    View details for Web of Science ID 000445777400006

  • Physical and chemical mechanisms of uranium (U) and arsenic (As) sorption onto limestone Estrella, J., Meza, I., Ali, A., Lezama-Pacheco, J., Fendorr, S., Cerrato, J. AMER CHEMICAL SOC. 2018
  • Redox-Active vs Redox-Innocent: A Comparison of Uranium Complexes Containing Diamine Ligands INORGANIC CHEMISTRY Pattenaude, S. A., Mullane, K. C., Schelter, E. J., Ferriere, M. G., Stein, B. W., Bone, S. E., Pacheco, J., Kozimor, S. A., Fanwick, P. E., Zeller, M., Bart, S. C. 2018; 57 (11): 6530–39

    Abstract

    Uranium complexes (MesDAE)2U(THF) (1-DAE) and Cp2U(MesDAE) (2-DAE) (MesDAE = [ArN-CH2CH2-NAr]; Ar = 2,4,6-trimethylphenyl (Mes)), bearing redox-innocent diamide ligands, have been synthesized and characterized for a full comparison with previously published, redox-active diimine complexes, (MesDABMe)2U(THF) (1-DAB) and Cp2U(MesDABMe) (2-DAB) (MesDABMe = [ArN═C(Me)C(Me)═NAr]; Ar = Mes). These redox-innocent analogues maintain an analogous steric environment to their redox-active ligand counterparts to facilitate a study aimed at determining the differing electronic behavior around the uranium center. Structural analysis by X-ray crystallography showed 1-DAE and 2-DAE have a structural environment very similar to 1-DAB and 2-DAB, respectively. The main difference occurs with coordination of the ene-backbone to the uranium center in the latter species. Electronic absorption spectroscopy reveals these new DAE complexes are nearly identical to each other. X-ray absorption spectroscopy suggests all four species contain +4 uranium ions. The data also indicates that there is an electronic difference between the bis(diamide)-THF uranium complexes as opposed to those that only contain one diamide and two cyclopentadienyl rings. Finally, magnetic measurements reveal that all complexes display temperature-dependent behavior consistent with uranium(IV) ions that do not include ligand radicals. Overall, this study determines that there is no significant bonding difference between the redox-innocent and redox-active ligand frameworks on uranium. Furthermore, there are no data to suggest covalent bonding character using the latter ligand framework on uranium, despite what is known for transition metals.

    View details for PubMedID 29749729

  • The coordination chemistry of CmIII, AmIII, and AcIII in nitrate solutions: an actinide L3-edge EXAFS study. Chemical science Ferrier, M. G., Stein, B. W., Bone, S. E., Cary, S. K., Ditter, A. S., Kozimor, S. A., Lezama Pacheco, J. S., Mocko, V., Seidler, G. T. 2018; 9 (35): 7078–90

    Abstract

    Understanding actinide(iii) (AnIII = CmIII, AmIII, AcIII) solution-phase speciation is critical for controlling many actinide processing schemes, ranging from medical applications to reprocessing of spent nuclear fuel. Unfortunately, in comparison to most elements in the periodic table, AnIII speciation is often poorly defined in complexing aqueous solutions and in organic media. This neglect - in large part - is a direct result of the radioactive properties of these elements, which make them difficult to handle and acquire. Herein, we surmounted some of the handling challenges associated with these exotic 5f-elements and characterized CmIII, AmIII, and AcIII using AnIII L3-edge X-ray absorption spectroscopy (XAS) as a function of increasing nitric acid (HNO3) concentration. Our results revealed that actinide aquo ions, An(H2O) x 3+ (x = 9.6 ± 0.7, 8.9 ± 0.8, and 10.0 ± 0.9 for CmIII, AmIII, and AcIII), were the dominant species in dilute HNO3 (0.05 M). In concentrated HNO3 (16 M), shell-by-shell fitting of the extended X-ray fine structure (EXAFS) data showed the nitrate complexation increased, such that the average stoichiometries of Cm(NO3)4.1±0.7(H2O)5.7±1.3(1.1±0.2)-, Am(NO3)3.4±0.7(H2O)5.4±0.5(0.4±0.1)-, and Ac(NO3)2.3±1.7(H2O)8.3±5.2(0.7±0.5)+ were observed. Data obtained at the intermediate HNO3 concentration (4 M) were modeled as a linear combination of the 0.05 and 16 M spectra. For all three metals, the intermediate models showed larger contributions from the 0.05 M HNO3 spectra than from the 16 M HNO3 spectra. Additionally, these efforts enabled the Cm-NO3 and Ac-NO3 distances to be measured for the first time. Moreover, the AnIII L3-edge EXAFS results, contribute to the growing body of knowledge associated with CmIII, AmIII, and AcIII coordination chemistry, in particular toward advancing understanding of AnIII solution phase speciation.

    View details for PubMedID 30310628

    View details for PubMedCentralID PMC6137438

  • 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
  • Redox Controls over the Stability of U(IV) in Floodplains of the Upper Colorado River Basin ENVIRONMENTAL SCIENCE & TECHNOLOGY Noel, V., Boye, K., Pacheco, J., Bone, S. E., Janot, N., Cardarelli, E., Williams, K. H., Bargar, J. R. 2017; 51 (19): 10954–64

    Abstract

    Aquifers in the Upper Colorado River Basin (UCRB) exhibit persistent uranium (U) groundwater contamination plumes originating from former ore processing operations. Previous observations at Rifle, Colorado, have shown that fine grained, sulfidic, organic-enriched sediments accumulate U in its reduced form, U(IV), which is less mobile than oxidized U(VI). These reduced sediment bodies can subsequently act as secondary sources, releasing U back to the aquifer. There is a need to understand if U(IV) accumulation in reduced sediments is a common process at contaminated sites basin-wide, to constrain accumulated U(IV) speciation, and to define the biogeochemical factors controlling its reactivity. We have investigated U(IV) accumulation in organic-enriched reduced sediments at three UCRB floodplains. Noncrystalline U(IV) is the dominant form of accumulated U, but crystalline U(IV) comprises up to ca. 30% of total U at some locations. Differing susceptibilities of these species to oxidative remobilization can explain this variability. Particle size, organic carbon content, and pore saturation, control the exposure of U(IV) to oxidants, moderating its oxidative release. Further, our data suggest that U(IV) can be mobilized under deeply reducing conditions, which may contribute to maintenance and seasonal variability of U in groundwater plumes in the UCRB.

    View details for PubMedID 28873299

  • Polyoxovanadate-Alkoxide Clusters as a Redox Reservoir for Iron. Inorganic chemistry Li, F., Carpenter, S. H., Higgins, R. F., Hitt, M. G., Brennessel, W. W., Ferrier, M. G., Cary, S. K., Lezama-Pacheco, J. S., Wright, J. T., Stein, B. W., Shores, M. P., Neidig, M. L., Kozimor, S. A., Matson, E. M. 2017

    Abstract

    Inspired by the multielectron redox chemistry achieved using conventional organic-based redox-active ligands, we have characterized a series of iron-functionalized polyoxovanadate-alkoxide clusters in which the metal oxide scaffold functions as a three-dimensional, electron-deficient metalloligand. Four heterometallic clusters were prepared through sequential reduction, demonstrating that the metal oxide scaffold is capable of storing up to four electrons. These reduced products were characterized by cyclic voltammetry, IR, electronic absorption, and (1)H NMR spectroscopies. Moreover, Mössbauer and X-ray absorption spectroscopies suggest that the redox events involve primarily the vanadium ions, while the iron atoms remained in the 3+ oxidation state throughout the redox series. In this sense, the vanadium portion of the cluster mimics a conventional organic-based redox-active ligand bound to an iron(III) ion. Magnetic coupling within the hexanuclear cluster was characterized using SQUID magnetometry. Overall, the results suggest extensive electronic delocalization between the metal centers of the cluster core. These results demonstrate the ability of electronically flexible, reducible metal oxide supports to function as redox-active reservoirs for transition-metal centers.

    View details for DOI 10.1021/acs.inorgchem.7b00650

    View details for PubMedID 28548499

  • 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

  • Synthesis and Characterization of the Actinium Aquo Ion. ACS central science Ferrier, M. G., Stein, B. W., Batista, E. R., Berg, J. M., Birnbaum, E. R., Engle, J. W., John, K. D., Kozimor, S. A., Lezama Pacheco, J. S., Redman, L. N. 2017; 3 (3): 176-185

    Abstract

    Metal aquo ions occupy central roles in all equilibria that define metal complexation in natural environments. These complexes are used to establish thermodynamic metrics (i.e., stability constants) for predicting metal binding, which are essential for defining critical parameters associated with aqueous speciation, metal chelation, in vivo transport, and so on. As such, establishing the fundamental chemistry of the actinium(III) aquo ion (Ac-aquo ion, Ac(H2O) x(3+)) is critical for current efforts to develop (225)Ac [t1/2 = 10.0(1) d] as a targeted anticancer therapeutic agent. However, given the limited amount of actinium available for study and its high radioactivity, many aspects of actinium chemistry remain poorly defined. We overcame these challenges using the longer-lived (227)Ac [t1/2 = 21.772(3) y] isotope and report the first characterization of this fundamentally important Ac-aquo coordination complex. Our X-ray absorption fine structure study revealed 10.9 ± 0.5 water molecules directly coordinated to the Ac(III) cation with an Ac-OH2O distance of 2.63(1) Å. This experimentally determined distance was consistent with molecular dynamics density functional theory results that showed (over the course of 8 ps) that Ac(III) was coordinated by 9 water molecules with Ac-OH2O distances ranging from 2.61 to 2.76 Å. The data is presented in the context of other actinide(III) and lanthanide(III) aquo ions characterized by XAFS and highlights the uniqueness of the large Ac(III) coordination numbers and long Ac-OH2O bond distances.

    View details for DOI 10.1021/acscentsci.6b00356

    View details for PubMedID 28386595

  • Synthesis and Characterization of the Actinium Aquo Ion ACS CENTRAL SCIENCE Ferrier, M. G., Stein, B. W., Batista, E. R., Berg, J. M., Birnbaum, E. R., Engle, J. W., John, K. D., Kozimor, S. A., Pacheco, J. S., Redman, L. N. 2017; 3 (3): 176-185

    Abstract

    Metal aquo ions occupy central roles in all equilibria that define metal complexation in natural environments. These complexes are used to establish thermodynamic metrics (i.e., stability constants) for predicting metal binding, which are essential for defining critical parameters associated with aqueous speciation, metal chelation, in vivo transport, and so on. As such, establishing the fundamental chemistry of the actinium(III) aquo ion (Ac-aquo ion, Ac(H2O) x(3+)) is critical for current efforts to develop (225)Ac [t1/2 = 10.0(1) d] as a targeted anticancer therapeutic agent. However, given the limited amount of actinium available for study and its high radioactivity, many aspects of actinium chemistry remain poorly defined. We overcame these challenges using the longer-lived (227)Ac [t1/2 = 21.772(3) y] isotope and report the first characterization of this fundamentally important Ac-aquo coordination complex. Our X-ray absorption fine structure study revealed 10.9 ± 0.5 water molecules directly coordinated to the Ac(III) cation with an Ac-OH2O distance of 2.63(1) Å. This experimentally determined distance was consistent with molecular dynamics density functional theory results that showed (over the course of 8 ps) that Ac(III) was coordinated by 9 water molecules with Ac-OH2O distances ranging from 2.61 to 2.76 Å. The data is presented in the context of other actinide(III) and lanthanide(III) aquo ions characterized by XAFS and highlights the uniqueness of the large Ac(III) coordination numbers and long Ac-OH2O bond distances.

    View details for DOI 10.1021/acscentsci.6b300356

    View details for Web of Science ID 000397182100009

    View details for PubMedCentralID PMC5364452

  • Evaluating the electronic structure of formal LnII ions in LnII(C5H4SiMe3)31- using XANES spectroscopy and DFT calculations. Chemical science Fieser, M. E., Ferrier, M. G., Su, J., Batista, E., Cary, S. K., Engle, J. W., Evans, W. J., Lezama Pacheco, J. S., Kozimor, S. A., Olson, A. C., Ryan, A. J., Stein, B. W., Wagner, G. L., Woen, D. H., Vitova, T., Yang, P. 2017; 8 (9): 6076–91

    Abstract

    The isolation of [K(2.2.2-cryptand)][Ln(C5H4SiMe3)3], formally containing LnII, for all lanthanides (excluding Pm) was surprising given that +2 oxidation states are typically regarded as inaccessible for most 4f-elements. Herein, X-ray absorption near-edge spectroscopy (XANES), ground-state density functional theory (DFT), and transition dipole moment calculations are used to investigate the possibility that Ln(C5H4SiMe3)31- (Ln = Pr, Nd, Sm, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb and Lu) compounds represented molecular LnII complexes. Results from the ground-state DFT calculations were supported by additional calculations that utilized complete-active-space multi-configuration approach with second-order perturbation theoretical correction (CASPT2). Through comparisons with standards, Ln(C5H4SiMe3)31- (Ln = Sm, Tm, Yb, Lu, Y) are determined to contain 4f6 5d0 (SmII), 4f13 5d0 (TmII), 4f14 5d0 (YbII), 4f14 5d1 (LuII), and 4d1 (YII) electronic configurations. Additionally, our results suggest that Ln(C5H4SiMe3)31- (Ln = Pr, Nd, Gd, Tb, Dy, Ho, and Er) also contain LnII ions, but with 4f n 5d1 configurations (not 4f n+1 5d0). In these 4f n 5d1 complexes, the C3h-symmetric ligand environment provides a highly shielded 5d-orbital of a' symmetry that made the 4f n 5d1 electronic configurations lower in energy than the more typical 4f n+1 5d0 configuration.

    View details for PubMedID 28989638

    View details for PubMedCentralID PMC5625586

  • Examining the Effects of Ligand Variation on the Electronic Structure of Uranium Bis(imido) Species. Journal of the American Chemical Society Kiernicki, J. J., Ferrier, M. G., Lezama Pacheco, J. S., La Pierre, H. S., Stein, B. W., Zeller, M., Kozimor, S. A., Bart, S. C. 2016: -?

    Abstract

    Arylazide and diazene activation by highly reduced uranium(IV) complexes bearing trianionic redox-active pyridine(diimine) ligands, [Cp(P)U((Mes)PDI(Me))]2 (1-Cp(P)), Cp*U((Mes)PDI(Me))(THF) (1-Cp*) (Cp(P) = 1-(7,7-dimethylbenzyl)cyclopentadienide; Cp* = η(5)-1,2,3,4,5-pentamethylcyclopentadienide), and Cp*U((t)Bu-(Mes)PDI(Me)) (THF) (1-(t)Bu) (2,6-((Mes)N═CMe)2-p-R-C5H2N, Mes = 2,4,6-trimethylphenyl; R = H, (Mes)PDI(Me); R = C(CH3)3, (t)Bu-(Mes)PDI(Me)), has been investigated. While 1-Cp* and 1-Cp(P) readily reduce N3R (R = Ph, p-tolyl) to form trans-bis(imido) species, Cp(P)U(NAr)2((Mes)PDI(Me)) (Ar = Ph, 2-Cp(P); Ar = p-Tol, 3-Cp(P)) and Cp*U(NPh)2((Mes)PDI(Me)) (2-Cp*), only 1-Cp* can cleave diazene N═N double bonds to form the same product. Complexes 2-Cp*, 2-Cp(P), and 3-Cp(P) are uranium(V) trans-bis(imido) species supported by neutral [(Mes)PDI(Me)](0) ligands formed by complete oxidation of [(Mes)PDI(Me)](3-) ligands of 1-Cp(P) and 1-Cp*. Variation of the arylimido substituent in 2-Cp* from phenyl to p-tolyl, forming Cp*U(NTol)2((Mes)PDI(Me)) (3-Cp*), changes the electronic structure, generating a uranium(VI) ion with a monoanionic pyridine(diimine) radical. The tert-butyl-substituted analogue, Cp*U(NTol)2((t)Bu-(Mes)PDI(Me)) (3-(t)Bu), displays the same electronic structure. Oxidation of the ligand radical in 3-Cp* and 3-(t)Bu by Ag(I) forms cationic uranium(VI) [Cp*U(NTol)2((Mes)PDI(Me))][SbF6] (4-Cp*) and [Cp*U(NTol)2((t)Bu-(Mes)PDI(Me))][SbF6] (4-(t)Bu), respectively, as confirmed by metrical parameters. Conversely, oxidation of pentavalent 2-Cp* with AgSbF6 affords cationic [Cp*U(NPh)2((Mes)PDI(Me))][SbF6] (5-Cp*) from a metal-based U(V)/U(VI) oxidation. All complexes have been characterized by multidimensional NMR spectroscopy with assignments confirmed by electronic absorption spectroscopy. The effective nuclear charge at uranium has been probed using X-ray absorption spectroscopy, while structural parameters of 1-Cp(P), 3-Cp*, 3-(t)Bu, 4-Cp*, 4-(t)Bu, and 5-Cp* have been elucidated by X-ray crystallography.

    View details for PubMedID 27731988

  • 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

  • Spectroscopic and computational investigation of actinium coordination chemistry. Nature communications Ferrier, M. G., Batista, E. R., Berg, J. M., Birnbaum, E. R., Cross, J. N., Engle, J. W., La Pierre, H. S., Kozimor, S. A., Lezama Pacheco, J. S., Stein, B. W., Stieber, S. C., Wilson, J. J. 2016; 7: 12312-?

    Abstract

    Actinium-225 is a promising isotope for targeted-α therapy. Unfortunately, progress in developing chelators for medicinal applications has been hindered by a limited understanding of actinium chemistry. This knowledge gap is primarily associated with handling actinium, as it is highly radioactive and in short supply. Hence, Ac(III) reactivity is often inferred from the lanthanides and minor actinides (that is, Am, Cm), with limited success. Here we overcome these challenges and characterize actinium in HCl solutions using X-ray absorption spectroscopy and molecular dynamics density functional theory. The Ac-Cl and Ac-OH2O distances are measured to be 2.95(3) and 2.59(3) Å, respectively. The X-ray absorption spectroscopy comparisons between Ac(III) and Am(III) in HCl solutions indicate Ac(III) coordinates more inner-sphere Cl(1-) ligands (3.2±1.1) than Am(III) (0.8±0.3). These results imply diverse reactivity for the +3 actinides and highlight the unexpected and unique Ac(III) chemical behaviour.

    View details for DOI 10.1038/ncomms12312

    View details for PubMedID 27531582

    View details for PubMedCentralID PMC4992055

  • 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 PubMedID 26651843

  • Monomers, Dimers, and Helices: Complexities of Cerium and Plutonium Phenanthrolinecarboxylates. Inorganic chemistry Cary, S. K., Ferrier, M. G., Baumbach, R. E., Silver, M. A., Lezama Pacheco, J., Kozimor, S. A., La Pierre, H. S., Stein, B. W., Arico, A. A., Gray, D. L., Albrecht-Schmitt, T. E. 2016; 55 (9): 4373–80

    Abstract

    The reaction of Ce(III) or Pu(III) with 1,10-phenanthroline-2,9-dicarboxylic acid (PDAH2) results in the formation of new f-element coordination complexes. In the case of cerium, Ce(PDA)(H2O)2Cl·H2O (1) or [Ce(PDAH)(PDA)]2[Ce(PDAH)(PDA)] (2) was isolated depending on the Ce/ligand ratio in the reaction. The structure of 2 is composed of two distinct substructures that are constructed from the same monomer. This monomer is composed of a Ce(III) cation bound by one PDA(2-) dianionic ligand and one PDAH(-) monoanionic ligand, both of which are tetradentate. Bridging by the carboxylate moieties leads to either [Ce(PDAH)(PDA)]2 dimers or [Ce(PDAH)(PDA)]1∞ helical chains. For plutonium, Pu(PDA)2 (3) was the only product isolated regardless of the Pu/ligand ratio employed in the reaction. During the reaction of plutonium with PDAH2, Pu(III) is oxidized to Pu(IV), generating 3. This assignment is consistent with structural metrics and the optical absorption spectrum. Ambiguity in the assignment of the oxidation state of cerium in 1 and 2 from UV-vis-near-IR spectra invoked the use of Ce L3,2-edge X-ray absorption near-edge spectroscopy, magnetic susceptibility, and heat capacity measurements. These experiments support the assignment of Ce(III) in both compounds. The bond distances and coordination numbers are also consistent with these assignments. 3 contains 8-coordinate Pu(IV), whereas the cerium centers in 1 and 2 are 9- and/or 10-coordinate, which correlates with the increased size of Ce(III) versus Pu(IV). Taken together, these data provide an example of a system where the differences in the redox behavior between these f elements creates more complex chemistry with cerium than with plutonium.

    View details for PubMedID 27070401

  • Long-Term in Situ Oxidation of Biogenic Uraninite in an Alluvial Aquifer: Impact of Dissolved Oxygen and Calcium ENVIRONMENTAL SCIENCE & TECHNOLOGY Lezama-Pacheco, J. S., Cerrato, J. M., Veeramani, H., Alessi, D. S., Suvorova, E., Bernier-Latmani, R., Giammar, D. E., Long, P. E., Williams, K. H., Bargar, J. R. 2015; 49 (12): 7340-7347

    Abstract

    Oxidative dissolution controls uranium release to (sub)oxic pore waters from biogenic uraninite produced by natural or engineered processes, such as bioremediation. Laboratory studies show that uraninite dissolution is profoundly influenced by dissolved oxygen (DO), carbonate, and solutes such as Ca(2+). In complex and heterogeneous subsurface environments, the concentrations of these solutes vary in time and space. Knowledge of dissolution processes and kinetics occurring over the long-term under such conditions is needed to predict subsurface uranium behavior and optimize the selection and performance of uraninite-based remediation technologies over multiyear periods. We have assessed dissolution of biogenic uraninite deployed in wells at the Rifle, CO, DOE research site over a 22 month period. Uraninite loss rates were highly sensitive to DO, with near-complete loss at >0.6 mg/L over this period but no measurable loss at lower DO. We conclude that uraninite can be stable over decadal time scales in aquifers under low DO conditions. U(VI) solid products were absent over a wide range of DO values, suggesting that dissolution proceeded through complexation and removal of oxidized surface uranium atoms by carbonate. Moreover, under the groundwater conditions present, Ca(2+) binds strongly to uraninite surfaces at structural uranium sites, impacting uranium fate.

    View details for DOI 10.1021/acs.est.5b00949

    View details for Web of Science ID 000356755200028

    View details for PubMedID 26001126

  • Multiscale Speciation of U and Pu at Chernobyl, Hanford, Los Alamos, McGuire AFB, Mayak, and Rocky Flats ENVIRONMENTAL SCIENCE & TECHNOLOGY Batuk, O. N., Conradson, S. D., Aleksandrova, O. N., Boukhalfa, H., Burakov, B. E., Clark, D. L., Czerwinski, K. R., Felmy, A. R., Lezama-Pacheco, J. S., Kalmykov, S. N., Moore, D. A., Myasoedov, B. F., Reed, D. T., Reilly, D. D., Roback, R. C., Vlasova, I. E., Webb, S. M., Wilkerson, M. P. 2015; 49 (11): 6474-6484

    Abstract

    The speciation of U and Pu in soil and concrete from Rocky Flats and in particles from soils from Chernobyl, Hanford, Los Alamos, and McGuire Air Force Base and bottom sediments from Mayak was determined by a combination of X-ray absorption fine structure (XAFS) spectroscopy and X-ray fluorescence (XRF) element maps. These experiments identify four types of speciation that sometimes may and other times do not exhibit an association with the source terms and histories of these samples: relatively well ordered PuO2+x and UO2+x that had equilibrated with O2 and H2O under both ambient conditions and in fires or explosions; instances of small, isolated particles of U as UO2+x, U3O8, and U(VI) species coexisting in close proximity after decades in the environment; alteration phases of uranyl with other elements including ones that would not have come from soils; and mononuclear Pu-O species and novel PuO2+x-type compounds incorporating additional elements that may have occurred because the Pu was exposed to extreme chemical conditions such as acidic solutions released directly into soil or concrete. Our results therefore directly demonstrate instances of novel complexity in the Å and μm-scale chemical speciation and reactivity of U and Pu in their initial formation and after environmental exposure as well as occasions of unexpected behavior in the reaction pathways over short geological but significant sociological times. They also show that incorporating the actual disposal and site conditions and resultant novel materials such as those reported here may be necessary to develop the most accurate predictive models for Pu and U in the environment.

    View details for DOI 10.1021/es506145b

    View details for PubMedID 25815708

  • Investigation of the Electronic Ground States for a Reduced Pyridine(diimine) Uranium Series: Evidence for a Ligand Tetraanion Stabilized by a Uranium Dimer. Journal of the American Chemical Society Anderson, N. H., Odoh, S. O., Williams, U. J., Lewis, A. J., Wagner, G. L., Lezama Pacheco, J., Kozimor, S. A., Gagliardi, L., Schelter, E. J., Bart, S. C. 2015; 137 (14): 4690-4700

    Abstract

    The electronic structures of a series of highly reduced uranium complexes bearing the redox-active pyridine(diimine) ligand, (Mes)PDI(Me) ((Mes)PDI(Me) = 2,6-(2,4,6-Me3-C6H2-N═CMe)2C5H3N) have been investigated. The complexes, ((Mes)PDI(Me))UI3(THF) (1), ((Mes)PDI(Me))UI2(THF)2 (2), [((Mes)PDI(Me))UI]2 (3), and [((Mes)PDI(Me))U(THF)]2 (4), were examined using electronic and X-ray absorption spectroscopies, magnetometry, and computational analyses. Taken together, these studies suggest that all members of the series contain uranium(IV) centers with 5f (2) configurations and reduced ligand frameworks, specifically [(Mes)PDI(Me)](•/-), [(Mes)PDI(Me)](2-), [(Mes)PDI(Me)](3-) and [(Mes)PDI(Me)](4-), respectively. In the cases of 2, 3, and 4 no unpaired spin density was found on the ligands, indicating a singlet diradical ligand in monomeric 2 and ligand electron spin-pairing through dimerization in 3 and 4. Interaction energies, representing enthalpies of dimerization, of -116.0 and -144.4 kcal mol(-1) were calculated using DFT for the monomers of 3 and 4, respectively, showing there is a large stabilization gained by dimerization through uranium-arene bonds. Highlighted in these studies is compound 4, bearing a previously unobserved pyridine(diimine) tetraanion, that was uniquely stabilized by backbonding between uranium cations and the η(5)-pyridyl ring.

    View details for DOI 10.1021/ja511867a

    View details for PubMedID 25830409

  • X-ray Accelerated Photo-Oxidation of As(III) in Solution JOURNAL OF PHYSICAL CHEMISTRY A Canche-Tello, J., Cristina Vargas, M., Hernandez-Cobos, J., Ortega-Blake, I., Leclercq, A., Solari, P. L., Lezama-Pacheco, J., Den Auwer, C., Mustre de Leon, J. 2015; 119 (12): 2829-2833

    Abstract

    We performed near edge X-ray absorption spectroscopy (XANES) measurements on the arsenic K-edge of As(III) in solution under acidic and basic conditions, after exposure of the solutions to air. Spectra were recorded for increasing exposure times to the X-rays used to perform absorption spectroscopy measurements. We did not find changes for the solution under acidic conditions, whereas we observed significant changes in the case of solution under alkaline conditions. To interpret these changes, we compared the obtained spectra with XANES spectra of As(III) and As(V) solutions under alkaline conditions, not exposed to air, and used as standards. Principal component fits using these standards indicate an accelerated conversion of As(III) to As(V) due to the exposure to X-rays.

    View details for DOI 10.1021/jp510596p

    View details for Web of Science ID 000351971400010

    View details for PubMedID 25730736

  • 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

  • Speciation and Reactivity of Uranium Products Formed during in Situ Bioremediation in a Shallow Alluvial Aquifer ENVIRONMENTAL SCIENCE & TECHNOLOGY Alessi, D. S., Lezama-Pacheco, J. S., Janot, N., Suvorova, E. I., Cerrato, J. M., Giammar, D. E., Davis, J. A., Fox, P. M., Williams, K. H., Long, P. E., Handley, K. M., Bernier-Latmani, R., Bargar, J. R. 2014; 48 (21): 12842-12850

    View details for DOI 10.1021/es502701u

    View details for Web of Science ID 000344449100044

  • 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
  • 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

  • The product of microbial uranium reduction includes multiple species with U(IV)-phosphate coordination GEOCHIMICA ET COSMOCHIMICA ACTA Alessi, D. S., Lezama-Pacheco, J. S., Stubbs, J. E., Janousch, M., Bargar, J. R., Persson, P., Bernier-Latmani, R. 2014; 131: 115-127
  • Biogeochemical Controls on the Product of Microbial U(VI) Reduction ENVIRONMENTAL SCIENCE & TECHNOLOGY Stylo, M., Alessi, D. S., Shao, P. P., Lezama-Pacheco, J. S., Bargar, J. R., Bernier-Latmani, R. 2013; 47 (21): 12351-12358

    View details for DOI 10.1021/es402631w

    View details for Web of Science ID 000326711300059

  • Relative Reactivity of Biogenic and Chemogenic Uraninite and Biogenic Noncrystalline U(IV) ENVIRONMENTAL SCIENCE & TECHNOLOGY Cerrato, J. M., Ashner, M. N., Alessi, D. S., Lezama-Pacheco, J. S., Bernier-Latmani, R., Bargar, J. R., Giammar, D. E. 2013; 47 (17): 9756-9763

    Abstract

    Aqueous chemical extractions and X-ray absorption spectroscopy (XAS) analyses were conducted to investigate the reactivity of chemogenic uraninite, nanoparticulate biogenic uraninite, and biogenic monomeric U(IV) species. The analyses were conducted in systems containing a total U concentration that ranged from 1.48 to 2.10 mM. Less than 0.02% of the total U was released to solution in extractions that targeted water-soluble and ion exchangeable fractions. Less than 5% of the total U was solubilized via complexation with a 0.1 M solution of NaF. Greater than 90% of the total U was extracted from biogenic uraninite and monomeric U(IV) after 6 h of reaction in an oxidizing solution of 50 mM K2S2O8. Additional oxidation experiments with lower concentrations (2 mM and 10 mM) of K2S2O8 and 8.2 mg L(-1) dissolved oxygen suggested that monomeric U(IV) species are more labile than biogenic uraninite; chemogenic uraninite was much less susceptible to oxidation than either form of biogenic U(IV). These results suggest that noncrystalline forms of U(IV) may be more labile than uraninite in subsurface environments. This work helps fill critical gaps in our understanding of the behavior of solid-associated U(IV) species in bioremediated sites and natural uranium ore deposits.

    View details for DOI 10.1021/es401663t

    View details for Web of Science ID 000330094700026

    View details for PubMedID 23906226

  • Uranium redox transition pathways in acetate-amended sediments PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Bargar, J. R., Williams, K. H., Campbell, K. M., Long, P. E., Stubbs, J. E., Suvorova, E. I., Lezama-Pacheco, J. S., Alessi, D. S., Stylo, M., Webb, S. M., Davis, J. A., Giammar, D. E., Blue, L. Y., Bernier-Latmani, R. 2013; 110 (12): 4506-4511
  • Adsorption of Uranium(VI) to Manganese Oxides: X-ray Absorption Spectroscopy and Surface Complexation Modeling ENVIRONMENTAL SCIENCE & TECHNOLOGY Wang, Z., Lee, S., Catalano, J. G., Lezama-Pacheco, J. S., Bargar, J. R., Tebo, B. M., Giammar, D. E. 2013; 47 (2): 850-858

    Abstract

    The mobility of hexavalent uranium in soil and groundwater is strongly governed by adsorption to mineral surfaces. As strong naturally occurring adsorbents, manganese oxides may significantly influence the fate and transport of uranium. Models for U(VI) adsorption over a broad range of chemical conditions can improve predictive capabilities for uranium transport in the subsurface. This study integrated batch experiments of U(VI) adsorption to synthetic and biogenic MnO(2), surface complexation modeling, ζ-potential analysis, and molecular-scale characterization of adsorbed U(VI) with extended X-ray absorption fine structure (EXAFS) spectroscopy. The surface complexation model included inner-sphere monodentate and bidentate surface complexes and a ternary uranyl-carbonato surface complex, which was consistent with the EXAFS analysis. The model could successfully simulate adsorption results over a broad range of pH and dissolved inorganic carbon concentrations. U(VI) adsorption to synthetic δ-MnO(2) appears to be stronger than to biogenic MnO(2), and the differences in adsorption affinity and capacity are not associated with any substantial difference in U(VI) coordination.

    View details for DOI 10.1021/es304454g

    View details for Web of Science ID 000313667400025

    View details for PubMedID 23227949

  • Effect of diffusive transport limitations on UO2 dissolution WATER RESEARCH Giammar, D. E., Cerrato, J. M., Mehta, V., Wang, Z., Wang, Y., Pepping, T. J., Ulrich, K., Lezama-Pacheco, J. S., Bargar, J. R. 2012; 46 (18): 6023-6032

    Abstract

    The effects of diffusive transport limitations on the dissolution of UO(2) were investigated using an artificial groundwater prepared to simulate the conditions at the Old Rifle aquifer site in Colorado, USA. Controlled batch, continuously-stirred tank (CSTR), and plug flow reactors were used to study UO(2) dissolution in the absence and presence of diffusive limitations exerted by permeable sample cells. The net rate of uranium release following oxidative UO(2) dissolution obtained from diffusion-limited batch experiments was ten times lower than that obtained for UO(2) dissolution with no permeable sample cells. The release rate of uranium to bulk solution from UO(2) contained in permeable sample cells under advective flow conditions was more than 100 times lower than that obtained from CSTR experiments without diffusive limitations. A 1-dimensional transport model was developed that could successfully simulate diffusion-limited release of U following oxidative UO(2) dissolution with the dominant rate-limiting process being the transport of U(VI) out of the cells. Scanning electron microscopy, X-ray diffraction, and extended X-ray absorption fine structure spectroscopy (EXAFS) characterization of the UO(2) solids recovered from batch experiments suggest that oxidative dissolution was more evident in the absence of diffusive limitations. Ca-EXAFS spectra indicate the presence of Ca in the reacted UO(2) solids with a coordination environment similar to that of a Ca-O-Si mineral. The findings from this study advance our overall understanding of the coupling of geochemical and transport processes that can lead to differences in dissolution rates measured in the field and in laboratory experiments.

    View details for DOI 10.1016/j.watres.2012.08.034

    View details for Web of Science ID 000311130100019

    View details for PubMedID 22980573

  • Quantitative Separation of Monomeric U(IV) from UO2 in Products of U(VI) Reduction ENVIRONMENTAL SCIENCE & TECHNOLOGY Alessi, D. S., Uster, B., Veeramani, H., Suvorova, E. I., Lezama-Pacheco, J. S., Stubbs, J. E., Bargar, J. R., Bernier-Latmani, R. 2012; 46 (11): 6150-6157

    Abstract

    The reduction of soluble hexavalent uranium to tetravalent uranium can be catalyzed by bacteria and minerals. The end-product of this reduction is often the mineral uraninite, which was long assumed to be the only product of U(VI) reduction. However, recent studies report the formation of other species including an adsorbed U(IV) species, operationally referred to as monomeric U(IV). The discovery of monomeric U(IV) is important because the species is likely to be more labile and more susceptible to reoxidation than uraninite. Because there is a need to distinguish between these two U(IV) species, we propose here a wet chemical method of differentiating monomeric U(IV) from uraninite in environmental samples. To calibrate the method, U(IV) was extracted from known mixtures of uraninite and monomeric U(IV) and tested using X-ray absorption spectroscopy (XAS). Monomeric U(IV) was efficiently removed from biomass and Fe(II)-bearing phases by bicarbonate extraction, without affecting uraninite stability. After confirming that the method effectively separates monomeric U(IV) and uraninite, it is further evaluated for a system containing those reduced U species and adsorbed U(VI). The method provides a rapid complement, and in some cases alternative, to XAS analyses for quantifying monomeric U(IV), uraninite, and adsorbed U(VI) species in environmental samples.

    View details for DOI 10.1021/es204123z

    View details for Web of Science ID 000304783000067

    View details for PubMedID 22540966

  • Effect of Ca2+ and Zn2+ on UO2 Dissolution Rates ENVIRONMENTAL SCIENCE & TECHNOLOGY Cerrato, J. M., Barrows, C. J., Blue, L. Y., Lezama-Pacheco, J. S., Bargar, J. R., Giammar, D. E. 2012; 46 (5): 2731-2737

    Abstract

    The dissolution of UO(2) in a continuously stirred tank reactor (CSTR) in the presence of Ca(2+) and Zn(2+) was investigated under experimental conditions relevant to contaminated groundwater systems. Complementary experiments were performed to investigate the effect of adsorption and precipitation reactions on UO(2) dissolution. The experiments were performed under anoxic and oxic conditions. Zn(2+) had a much greater inhibitory effect on UO(2) dissolution than did Ca(2+). This inhibition was most substantial under oxic conditions, where the experimental rate of UO(2) dissolution was 7 times lower in the presence of Ca(2+) and 1450 times lower in the presence of Zn(2+) than in water free of divalent cations. EXAFS and solution chemistry analyses of UO(2) solids recovered from a Ca experiment suggest that a Ca-U(VI) phase precipitated. The Zn carbonate hydrozincite [Zn(5)(CO(3))(2)(OH)(6)] or a structurally similar phase precipitated on the UO(2) solids recovered from experiments performed in the presence of Zn. These precipitated Ca and Zn phases can coat the UO(2) surface, inhibiting the oxidative dissolution of UO(2). Interactions with divalent groundwater cations have implications for the longevity of UO(2) and the mobilization of U(VI) from these solids in remediated subsurface environments, waste disposal sites, and natural uranium ores.

    View details for DOI 10.1021/es203751t

    View details for Web of Science ID 000301023700035

    View details for PubMedID 22304297

  • Uranium speciation and stability after reductive immobilization in aquifer sediments GEOCHIMICA ET COSMOCHIMICA ACTA Sharp, J. O., Lezama-Pacheco, J. S., Schofield, E. J., Junier, P., Ulrich, K., Chinni, S., Veeramani, H., Margot-Roquier, C., Webb, S. M., Tebo, B. M., Giammar, D. E., Bargar, J. R., Bernier-Latmani, R. 2011; 75 (21): 6497-6510
  • Oxidative Dissolution of Biogenic Uraninite in Groundwater at Old Rifle, CO ENVIRONMENTAL SCIENCE & TECHNOLOGY Campbell, K. M., Veeramani, H., Urich, K., Blue, L. Y., Giammar, D. E., Bernier-Latmani, R., Stubbs, J. E., Suvorova, E., Yabusaki, S., Lezama-Pacheco, J. S., Mehta, A., Long, P. E., Bargar, J. R. 2011; 45 (20): 8748-8754

    Abstract

    Reductive bioremediation is currently being explored as a possible strategy for uranium-contaminated aquifers such as the Old Rifle site (Colorado). The stability of U(IV) phases under oxidizing conditions is key to the performance of this procedure. An in situ method was developed to study oxidative dissolution of biogenic uraninite (UO₂), a desirable U(VI) bioreduction product, in the Old Rifle, CO, aquifer under different variable oxygen conditions. Overall uranium loss rates were 50-100 times slower than laboratory rates. After accounting for molecular diffusion through the sample holders, a reactive transport model using laboratory dissolution rates was able to predict overall uranium loss. The presence of biomass further retarded diffusion and oxidation rates. These results confirm the importance of diffusion in controlling in-aquifer U(IV) oxidation rates. Upon retrieval, uraninite was found to be free of U(VI), indicating dissolution occurred via oxidation and removal of surface atoms. Interaction of groundwater solutes such as Ca²⁺ or silicate with uraninite surfaces also may retard in-aquifer U loss rates. These results indicate that the prolonged stability of U(IV) species in aquifers is strongly influenced by permeability, the presence of bacterial cells and cell exudates, and groundwater geochemistry.

    View details for DOI 10.1021/es200482f

    View details for Web of Science ID 000295704500022

    View details for PubMedID 21910475

  • Local Structure Instability Across the Martensitic Transition in Nb3Sn JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM Acosta-Alejandro, M., Lezama-Pacheco, J., Falconi, R., Escudero, R., Mustre de Leon, J. 2011; 24 (3): 1219-1223
  • Non-uraninite Products of Microbial U(VI) Reduction ENVIRONMENTAL SCIENCE & TECHNOLOGY Bernier-Latmani, R., Veeramani, H., Vecchia, E. D., Junier, P., Lezama-Pacheco, J. S., Suvorova, E. I., Sharp, J. O., Wigginton, N. S., Bargar, J. R. 2010; 44 (24): 9456-9462

    Abstract

    A promising remediation approach to mitigate subsurface uranium contamination is the stimulation of indigenous bacteria to reduce mobile U(VI) to sparingly soluble U(IV). The product of microbial uranium reduction is often reported as the mineral uraninite. Here, we show that the end products of uranium reduction by several environmentally relevant bacteria (Gram-positive and Gram-negative) and their spores include a variety of U(IV) species other than uraninite. U(IV) products were prepared in chemically variable media and characterized using transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS) to elucidate the factors favoring/inhibiting uraninite formation and to constrain molecular structure/composition of the non-uraninite reduction products. Molecular complexes of U(IV) were found to be bound to biomass, most likely through P-containing ligands. Minor U(IV)-orthophosphates such as ningyoite [CaU(PO(4))(2)], U(2)O(PO(4))(2), and U(2)(PO(4))(P(3)O(10)) were observed in addition to uraninite. Although factors controlling the predominance of these species are complex, the presence of various solutes was found to generally inhibit uraninite formation. These results suggest a new paradigm for U(IV) in the subsurface, i.e., that non-uraninite U(IV) products may be found more commonly than anticipated. These findings are relevant for bioremediation strategies and underscore the need for characterizing the stability of non-uraninite U(IV) species in natural settings.

    View details for DOI 10.1021/es101675a

    View details for Web of Science ID 000285266900035

    View details for PubMedID 21069950