Academic Appointments

All Publications

  • 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


    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


    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


    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


    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


    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


    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


    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


    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


    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


    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


    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


    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


    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


    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