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Singh A.,Washington University in St. Louis | Singh A.,Pacific Northwest National Laboratory | Catalano J.G.,Washington University in St. Louis | Ulrich K.-U.,Washington University in St. Louis | And 2 more authors.
Environmental Science and Technology | Year: 2012

The molecular-scale immobilization mechanisms of uranium uptake in the presence of phosphate and goethite were examined by extended X-ray absorption fine structure (EXAFS) spectroscopy. Wet chemistry data from U(VI)-equilibrated goethite suspensions at pH 4-7 in the presence of ~100 μM total phosphate indicated changes in U(VI) uptake mechanisms from adsorption to precipitation with increasing total uranium concentrations and with increasing pH. EXAFS analysis revealed that the precipitated U(VI) had a structure consistent with the meta-autunite group of solids. The adsorbed U(VI), in the absence of phosphate at pH 4-7, formed bidentate edge-sharing, ≡Fe(OH) 2UO 2, and bidentate corner-sharing, (≡FeOH) 2UO 2, surface complexes with respective U-Fe coordination distances of ∼3.45 and ∼4.3 Å In the presence of phosphate and goethite, the relative amounts of precipitated and adsorbed U(VI) were quantified using linear combinations of the EXAFS spectra of precipitated U(VI) and phosphate-free adsorbed U(VI). A U(VI)-phosphate-Fe(III) oxide ternary surface complex is suggested as the dominant species at pH 4 and total U(VI) of 10 μM or less on the basis of the linear combination fitting, a P shell indicated by EXAFS, and the simultaneous enhancement of U(VI) and phosphate uptake on goethite. A structural model for the ternary surface complex was proposed that included a single phosphate shell at ∼3.6 Å (U-P) and a single iron shell at ∼4.3 Å (U-Fe). While the data can be explained by a U-bridging ternary surface complex, (≡FeO) 2UO 2PO 4, it is not possible to statistically distinguish this scenario from one with P-bridging complexes also present. (Figure Presented). © 2012 American Chemical Society.

Singh A.,Washington University in St. Louis | Singh A.,Pacific Northwest National Laboratory | Ulrich K.-U.,Washington University in St. Louis | Ulrich K.-U.,BGD Soil and Groundwater Laboratory | Giammar D.E.,Washington University in St. Louis
Geochimica et Cosmochimica Acta | Year: 2010

Past mining, processing, and waste disposal activities have left a legacy of uranium-contaminated soil and groundwater. Phosphate addition to subsurface environments can potentially immobilize U(VI) in-situ through interactions with uranium at mineral-water interfaces. Phosphate can induce the precipitation of low solubility U(VI)-phosphates, and it may enhance or inhibit U(VI) adsorption to iron(III) (oxy)hydroxide surfaces. Such surfaces may also facilitate the heterogeneous nucleation of U(VI)-phosphate precipitates. The interactions among phosphate, U(VI), and goethite (α-FeOOH) were investigated in a year-long series of experiments at pH 4. Reaction time, total U(VI), total phosphate, and the presence and absence of goethite were systematically varied to determine their effects on the extent of U(VI) uptake and the dominant uranium immobilization mechanism. Dissolved U(VI) and phosphate concentrations were interpreted within a reaction-based modeling framework that included dissolution-precipitation reactions and a surface complexation model to account for adsorption. The best available thermodynamic data and past surface complexation models were integrated to form an internally consistent framework. Additional evidence for the uptake mechanisms was obtained using scanning electron microscopy and X-ray diffraction. The formation and crystal growth of a U(VI)-phosphate phase, most likely chernikovite, UO2HPO4·4H2O(s), occurred rapidly for initially supersaturated suspensions both with and without goethite. Nucleation appears to occur homogeneously for almost all conditions, even in the presence of goethite, but heterogeneous nucleation was likely at one condition. The U(VI)-phosphate solids exhibited metastability depending on the TOTU:TOTP ratio. At the highest phosphate concentration studied (130μM), U(VI) uptake was enhanced due to the likely formation of a ternary surface complex for low (∼1μM) to intermediate (∼10μM) TOTU concentrations and to U(VI)-phosphate precipitation for high TOTU (∼100μM) concentrations. For conditions favoring precipitation, the goethite surface acted as a sink for dissolved phosphate that resulted in higher dissolved U(VI) concentrations relative to goethite-free conditions. Based on the total uranium and available sorption sites, a critical phosphate concentration between 15μM and 130μM was required for preferential precipitation of uranium phosphate over U(VI) adsorption. © 2010 Elsevier Ltd.

Campbell K.M.,U.S. Geological Survey | Veeramani H.,Ecole Polytechnique Federale de Lausanne | Veeramani H.,University of Virginia | Ulrich K.-U.,Washington University in St. Louis | And 13 more authors.
Environmental Science and Technology | Year: 2011

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 (UO 2), 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 Ca 2+ 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. © 2011 American Chemical Society.

Sharp J.O.,Ecole Polytechnique Federale de Lausanne | Lezama-Pacheco J.S.,Stanford University | Schofield E.J.,Stanford University | Junier P.,Ecole Polytechnique Federale de Lausanne | And 11 more authors.
Geochimica et Cosmochimica Acta | Year: 2011

It has generally been assumed that the bioreduction of hexavalent uranium in groundwater systems will result in the precipitation of immobile uraninite (UO2). In order to explore the form and stability of uranium immobilized under these conditions, we introduced lactate (15mM for 3months) into flow-through columns containing sediments derived from a former uranium-processing site at Old Rifle, CO. This resulted in metal-reducing conditions as evidenced by concurrent uranium uptake and iron release. Despite initial augmentation with Shewanella oneidensis, bacteria belonging to the phylum Firmicutes dominated the biostimulated columns. The immobilization of uranium (∼1mmolU per kg sediment) enabled analysis by X-ray absorption spectroscopy (XAS). Tetravalent uranium associated with these sediments did not have spectroscopic signatures representative of U-U shells or crystalline UO2. Analysis by microfocused XAS revealed concentrated micrometer regions of solid U(IV) that had spectroscopic signatures consistent with bulk analyses and a poor proximal correlation (μm scale resolution) between U and Fe. A plausible explanation, supported by biogeochemical conditions and spectral interpretations, is uranium association with phosphoryl moieties found in biomass; hence implicating direct enzymatic uranium reduction. After the immobilization phase, two months of in situ exposure to oxic influent did not result in substantial uranium remobilization. Ex situ flow-through experiments demonstrated more rapid uranium mobilization than observed in column oxidation studies and indicated that sediment-associated U(IV) is more mobile than biogenic UO2. This work suggests that in situ uranium bioimmobilization studies and subsurface modeling parameters should be expanded to account for non-uraninite U(IV) species associated with biomass. © 2011 Elsevier Ltd.

Wang Z.,Stanford University | Ulrich K.-U.,BGD Soil and Groundwater Laboratory | Pan C.,Washington University in St. Louis | Giammar D.E.,Washington University in St. Louis
Environmental Science and Technology Letters | Year: 2015

Chemical or biological reduction of U(VI) produces a variety of poorly soluble U(IV) species. In addition to uraninite (UO2) and biomass-associated noncrystalline U(IV), recent research has found adsorbed U(IV) species on mineral surfaces. To build on these observations, we evaluated equilibrium adsorption of U(IV) to magnetite and rutile as a function of pH and total U(IV) loading. Surface complexation models that could successfully simulate the uptake of U(IV) by accounting for UO2 precipitation and adsorption of U(IV) to both the minerals and the reactor surfaces were developed. Application of the models could determine the conditions under which adsorption as opposed to precipitation would dominate U(IV) uptake with solids. The model-predicted U(IV) surface coverages of the minerals were consistent with a recent spectroscopic study. Such models advance our ability to predict the equilibrium speciation of U(IV) in the subsurface. © 2015 American Chemical Society.

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