Mechanisms of uranium interactions with hydroxyapatite: implications for groundwater remediation

The speciation of U(VI) sorbed to synthetic hydroxyapatite was investigated using a combination of U LIII-edge XAS, synchrotron XRD, batch uptake measurements, and SEM-EDS. The mechanisms of U(VI) removal by apatite were determined in order to evaluate the feasibility of apatite-based in-situ permeable reactive barriers (PRBs). In batch U(VI) uptake experiments with synthetic hydroxyapatite (HA), near complete removal of dissolved uranium (>99.5%) to <0.05 microM was observed over a range of total U(VI) concentrations up to equimolar of the total P in the suspension. XRD and XAS analyses of U(VI)-reacted HA at sorbed concentrations < or = 4,700 ppm U(VI) suggested that uranium(VI) phosphate, hydroxide, and carbonate solids were not present at these concentrations. Fits to EXAFS spectra indicate the presence of Ca neighbors at 3.81 A. U-Ca separation, suggesting that U(VI) adsorbs to the HA surfaces as an inner-sphere complex. Uranium(VI) phosphate solid phases were not detected in HA with 4700 ppm sorbed U(VI) by backscatter SEM or EDS, in agreement with the surface complexation process. In contrast, U(VI) speciation in samples that exceeded 7000 ppm sorbed U(VI) included a crystalline uranium(VI) phosphate solid phase, identified as chernikovite by XRD. At these higher concentrations, a secondary, uranium(VI) phosphate solid was detected by SEM-EDS, consistent with chernikovite precipitation. Autunite formation occurred at total U:P molar ratios > or = 0.2. Our findings provide a basis for evaluating U(VI) sorption mechanisms by commercially available natural apatites for use in development of PRBs for groundwater U(VI) remediation.     

Influence of sediment bioreduction and reoxidation on uranium sorption

The influence of sediment bioreduction and reoxidation on U(VI) sorption was studied using Fe(II) oxide-containing saprolite from the U.S. Department of Energy (DOE) Oak Ridge site. Bioreduced sediments were generated by anoxic incubation with a metal-reducing bacterium, Shewanella putrefaciens strain CN32, supplied with lactate as an electron donor. The reduced sediments were subsequently reoxidized by air contact. U(VI) sorption was studied in NaNO3-HCO3 electrolytes that were both closed and open to atmosphere and where pH, U(VI), and carbonate concentration were varied. M?ssbauer spectroscopy and chemical analyses showed that 50% of the Fe(III)-oxides were reduced to Fe(II) that was sorbed to the sediment during incubation with CN32. However, this reduction and subsequent reoxidation of the sorbed Fe(II) had negligible influence on the rate and extent of U sorption or the extractability of sorbed U by 0.2 mol/L NaHCO3. Various results indicated that U(VI) surface complexation was the primary process responsible for uranyl sorption by the bioreduced and reoxidized sediments. A two-site, nonelectrostatic surface complexation model best described U(VI) adsorption under variable pH, carbonate, and U(VI) conditions. A ferrihydrite-based diffuse double layer model provided a better estimation of U(VI) adsorption without parameter adjustment than did a goethite-based model, even though a majority of the Fe(III)-oxides in the sediments were goethite. Our results highlight the complexity of the coupled U-Fe redox system and show that sorbed Fe(II) is not a universal reductant for U(VI) as commonly assumed.     

Use of spectroscopic techniques for uranium(VI)/montmorillonite interaction modeling

To experimentally identify both clay sorption sites and sorption equilibria and to understand the retention mechanisms at a molecular level, we have characterized the structure of hexavalent uranium surface complexes resulting from the interaction between the uranyl ions and the surface retention groups of a montmorillonite clay. We have performed laser-induced fluorescence spectroscopy (LIFS) and X-ray photoelectron spectroscopy (XPS) on uranyl ion loaded montmorillonite. These structural results were then compared to those obtained from the study of uranyl ions sorbed onto an alumina and also from U(VI) sorbed on an amorphous silica. This experimental approach allowed for a clear determination of the reactive surface sites of montmorillonite for U(VI) sorption. The lifetime values and the U4f XPS spectra of uranium(VI) sorbed on montmorillonite have shown that this ion is sorbed on both exchange and edge sites. The comparison of U(VI)/clay and U(VI)/oxide systems has determined that the interaction between uranyl ions and montmorillonite edge sites occurs via both [triple bond]AlOH and [triple bond]SiOH surface groups and involves three distinct surface complexes. The surface complexation modeling of the U(VI)/montmorillonite sorption edges was determined using the constant capacitance model and the above experimental constraints. The following equilibria were found to account for the uranyl sorption mechanisms onto montmorillonite for metal concentrations ranged from 10(-6) to 10(-3) M and two ionic strengths (0.1 and 0.5 M): 2[triple bond]XNa + UO2(2+) <==> ([triple bond]X)2UO2 + 2Na+, log K0(exch) = 3.0; [triple bond]Al(OH)2 + UO2(2+) <==> [triple bond]Al(OH)2UO2(2+), log K0(Al) = 14.9; [triple bond]Si(OH)2 + UO2(2+) <==> [triple bond]SiO2UO2 + 2H+, log K0(Si1) = -3.8; and [triple bond]Si(OH)2 + 3UO2(2+) + 5H2O <==> [triple bond]SiO2(UO2)3(OH)5- + 7H+, log K0(Si2) = -20.0.     

Carbonate effects on hexavalent uranium adsorption by iron oxyhydroxide

Carbonate dramatically affects the adsorption of uranium (U(VI)) onto iron hydroxides and its mobility in the natural environment. Batch tests, zeta potential measurements, and Fourier transform infrared (FTIR) spectroscopic studies were utilized to characterize the nature of U(VI) adsorption on ferrihydrite. Adsorption isotherms demonstrated that carbonate had a negative effect on U(VI) adsorption on ferrihydrite at pH > 6. Zeta potential measurements indicated that U(VI) was adsorbed as a cationic species (SO-UO2+) in the absence of carbonate and as anionic U(VI) complexes in the presence of carbonate at neutral pH. FTIR spectroscopic measurement of adsorbed U(VI) suggested that it was retained as uranyl carbonate complexes in the presence of carbonate. An increase in carbonate concentration caused a shift in the antisymmetric stretching vibration of the uranyl (UO2(2+)) U-O bond toward lower wavenumbers, which indicated an increasing carbonate effect in the adsorbed uranyl carbonate complexes. The adsorbed U(VI) species were successfully incorporated into a surface complexation model to describe the adsorption of U(VI) by ferrihydrite from artificial solutions and contaminated water.     

Influence of calcium carbonate on U(VI) sorption to soils

The high stability of calcium uranyl carbonate complexes in the circumneutral pH range has a strong impact on U(VI) sorption in calcareous soils. To quantify this influence, sorption of U(VI) to soils in the presence of naturally occurring calcium carbonate was investigated by conducting batch experiments in which either U(VI) concentration or solution pH was varied. Two soils containing different calcium carbonate concentrations were selected, one from Oak Ridge, TN, and another from Altamont Pass, CA. The results show that the presence of calcium carbonate in soils strongly affects U(VI) sorption. Higher concentrations of soil calcium carbonate lead to a pronounced suppression of the pH-dependent sorption curve in the neutral pH range because of the formation of a very stable neutral complex of calcium uranyl carbonate in solution. A surface complexation model considering both strong and weak sites for ferrihydrite and ionizable hydroxyl sites for clay minerals was compared with experimental results, and U(VI) binding parameters were reasonably estimated. Fair agreement was found between the model predictions and sorption data, which span a wide range of U(VI) concentrations and pH. The results also show that appropriate solution-to-solid ratios need to be used when measuring distribution coefficients in calcareous soils to avoid complete CaCO3 dissolution and consequent dilution of calcium uranyl carbonate complexes.     

Determination of uranyl incorporation into biogenic manganese oxides using x-ray absorption spectroscopy and scattering

Biogenic manganese oxides are common and an important source of reactive mineral surfaces in the environment that may be potentially enhanced in bioremediation cases to improve natural attenuation. Experiments were performed in which the uranyl ion, UO2(2+) (U(VI)), at various concentrations was present during manganese oxide biogenesis. At all concentrations, there was strong uptake of U onto the oxides. Synchrotron-based extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray diffraction (XRD) studies were carried out to determine the molecular-scale mechanism by which uranyl is incorporated into the oxide and how this incorporation affects the resulting manganese oxide structure and mineralogy. The EXAFS experiments show that at low concentrations (<0.3 mol % U, <1 microM U(VI) in solution), U(VI) is present as a strong bidentate surface complex. At high concentrations (>2 mol % U, >4 microM U(VI) in solution), the presence of U(VI) affects the stability and structure of the Mn oxide to form poorly ordered Mn oxide tunnel structures, similar to todorokite. EXAFS modeling shows that uranyl is present in these oxides predominantly in the tunnels of the Mn oxide structure in a tridentate complex. Observations by XRD corroborate these results. Structural incorporation may lead to more stable U(VI) sequestration that may be suitable for remediation uses. These observations, combined with the very high uptake capacity of the Mn oxides, imply that Mn-oxidizing bacteria may significantly influence dissolved U(VI) concentrations in impacted waters via sorption and incorporation into Mn oxide biominerals.     

TRLFS evidence for precipitation of uranyl phosphate on the surface of alumina: environmental implications

We studied the ligand-enhanced sorption of uranyl ions (1-12 μM) on α-alumina colloids suspended in (and pre-equilibrated with) solutions at various concentrations of phosphate ions (P(T) = 0-900 μM). A highly sensitive technique, time resolved laser-induced fluorescence spectroscopy (TRLFS), was used to examine the chemical speciation of uranyl sorbed at trace concentrations (0.4-4 μmol U·g?1). The suspensions with P(T) ≥ 100 μM exhibited high uranyl adsorption, and a very high intensity of fluorescence that increased with the sorbed amounts of phosphate and uranyl. These samples exhibited similar spectral and temporal characteristics of fluorescence emission, evidencing a uniform speciation pattern and a single coordination environment for sorbed U, despite large variation in parameters such as aqueous uranyl speciation, U loading, and extent of coverage of alumina by secondary Al phosphates precipitating on the surface. The results pointed formation of surface precipitates of uranyl phosphates, which are characterized by high quantum yield, peak maxima at positions similar to those of U(VI) phosphate minerals and four lifetimes indicating distortions, in-homogeneities or varying number of water molecules in the lattice. The findings have major implications for our understanding of the mechanisms of immobilization of U at trace levels on surfaces of oxides submitted to phosphated solutions in soils with low pH.     

Cryogenic laser induced fluorescence characterization of U(VI) in Hanford Vadose Zone pore waters

Ambient and liquid helium temperature laser-induced time-resolved uranyl fluorescence spectroscopy was applied to study the speciation of aqueous uranyl solutions containing carbonate and phosphate and two porewater samples obtained by ultracentrifugation of U(VI)-contaminated sediments. The significantly enhanced fluorescence signal intensity and spectral resolution found at liquid helium temperature allowed, for the first time, direct fluorescence spectroscopic observation of the higher aqueous uranyl complexes with carbonate: UO2(CO3)2(2-), UO2(CO3)3(4-), and (UO2)2(OH)3CO3-. The porewater samples were nonfluorescent at room temperature. However, at liquid helium temperature, both porewater samples displayed strong, well-resolved fluorescence spectra. Comparisons of the spectroscopic characteristics of the porewaters with those of the standard uranyl-carbonate complexes confirmed that U(VI) in the porewaters existed primarily as UO2(CO3)3(4-) along with a small amount of other minor components, such as dicalcium-urano-tricarbonate complex, Ca2UO2(CO3)3, consistent with thermodynamic calculation. The U(VI)-carbonate complex is apparently the mobile species responsible for the subsurface migration of U(VI), even though the majority of the in-ground U(VI) inventory at the site from which the samples were obtained exists as intragrain U(VI)-silicate precipitates.     

Evidence of a stable uranyl site in ancient organic-rich calcite

The mechanism of uranium (U) incorporation into calcite (calcium carbonate) is of fundamental importance to the fate and transport of U at the surface and in the shallow subsurface and has implications for (a) the accuracy of U-Pb and U-series isotope ratio methods used to determine the ages of ancient deposits and (b) potential remediation strategies based on sequestration of U in the subsurface. Extended X-ray absorption fine structure (EXAFS) spectroscopy is uniquely suited to the study of U-calcite systems. The sensitivity of the EXAFS spectrum to the local atomic Ca coordination about U(VI) in the calcite structure results in an increase in the number and amplitude of Ca signals as the U(VI) becomes more ordered within the crystal structure. Our X-ray microprobe (10-microm) measurements of an ancient 298 million-year-old organic-rich calcite (calcrete) clearly revealed three coordination shells of Ca atoms, defining a well-ordered calcite structure about uranyl to a distance of approximately 6.5 angstroms. These results indicate that uranyl is incorporated at the Ca2+ site in calcite and that the uranyl environment may evolve over long time scales, becoming more calcite-like and more stable for long-term sequestration of uranium. These results therefore validate U-related dating methods and show that calcite can be effective at sequestering U in vadose zone sediments.     

Molecular-scale structure of uranium(VI) immobilized with goethite and phosphate

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)(2)UO(2), and bidentate corner-sharing, (≡ FeOH)(2)UO(2), surface complexes with respective U-Fe coordination distances of ~3.45 and ~4.3 ?. 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 ? (U-P) and a single iron shell at ~4.3 ? (U-Fe). While the data can be explained by a U-bridging ternary surface complex, (≡ FeO)(2)UO(2)PO(4), it is not possible to statistically distinguish this scenario from one with P-bridging complexes also present.     

Sorption of uranium(VI) onto ferric oxides in sulfate-rich acid waters

The mechanisms of the uranium(VI) sorption on schwertmannite and goethite in acid sulfate-rich solutions were studied by Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. The samples were prepared under N2 atmosphere and initial uranium(VI) concentrations of 1 x 10(-5) (pH 6.5) to 5 x 10(-5) M (pH 4.2). The ionic strength was adjusted using 0.01 M Na2SO4 or 0.01 M NaClO4, respectively. The EXAFS structural parameters for uranium(VI) sorbed on goethite in sulfate-rich, acid and near-neutral solutions indicate that uranium(VI) forms an inner-sphere, mononuclear, bidentate surface complex. This complex is characterized by a uranium-ferric-iron distance of approximately 3.45 A. Uranium(VI) sorbed onto schwertmannite in acid and sulfate-rich solution is coordinated to one or two sulfate molecules with a uranium-sulfur distance of 3.67 A. The EXAFS results indicate formation of binuclear, bidentate surface complexes and partly of mononuclear, monodentate surface complexes coordinated to the structural sulfate of schwertmannite. The formation of ternary uranium(VI)-sulfate surface complexes could not be excluded because of the uncertainty in assigning the sulfate either to the bulk structure or to adsorption reactions. The uranium(VI) adsorption onto schwertmannite in perchlorate solution occurs predominantly as a mononuclear, bidentate complexation with ferric iron due to the release of sulfate from the substrate.     

Interaction of uranyl with calcite in the presence of EDTA

Adsorption of uranyl at the surface of calcite was investigated by using batch sorption experiments and synchrotron X-ray standing wave (XSW) measurements. Aqueous solutions containing 236U(VI) (4.5 x 10(-7) to 1.0 x 10(-4) M) and EDTA (5.0 x 10(-7) to 1.1 x 10(-4) M) were reacted for 90 s to 60 min with freshly cleaved calcite (104) surfaces and calcite powders. Surface exchange coefficients, sorption kinetics, and influence of powder surface area/solution volume (SA/V) ratio were investigated by alpha-counting of 236U. Powder sorption results at SA/V = 870 cm2/mL fit a Freundlich isotherm [log [U]surface (in monolayers) = log K + n log [U]aq (in moles/L)], where K = 1.9+/-0.5 and n = 0.9+/-0.1, consistent with uptake of U(VI) by a specific surface reaction where the availability of sorption sites is nonlimiting in the U concentration range measured. Measured U(VI) coverages along this isotherm, based on the calcite (104) surface Ca site density, ranged from 0.04% to 5.4% of a monolayer. Steady state surface coverages were obtained within 90 s. Sorption of U(VI) on calcite (104) single-crystal cleavage surfaces using identical solutions yielded higher coverages, because of increased step density induced by dissolution at the relatively low SA/V ratio (approximately 1) of these measurements. The crystallographic location of the sorbed U(VI) was examined with the synchrotron XSW technique. Measurements were performed at the Advanced Photon Source on fresh calcite (104) cleavage surfaces reacted for 90 s with U(VI) solutions. Coherent fractions for sorbed U ranged from 0.14 to 0.62, and the mean value of the U coherent position was 0.84+/-0.02. This position was independent of dissolved U(VI) concentration and corresponds to a distance between the U atom and the calcite (104) plane of 2.55+/-0.06 A. These results are consistent with U(VI) adsorption atthe calcite surface as an inner-sphere uranyl-carbonate surface complex bonded with the outer oxygen atom(s) of a single surface carbonate group. Steric considerations allow this observed U(VI) surface complex to occur both at step sites ((441)_ and (481)_) and on terrace areas adjacent to Ca vacancies.     

U(VI)-kaolinite surface complexation in absence and presence of humic acid studied by TRLFS

Time-resolved laser-induced fluorescence spectroscopy (TRLFS) was applied to study the U(VI) surface complexes on kaolinite in the presence and absence of humic acid (HA). Two uranyl surface species with fluorescence lifetimes of 5.9 +/- 1.4 and 42.5 +/- 3.4 micros and 4.4 +/- 1.2 and 30.9 +/- 7.2 micros were identified in the binary (U(VI)-kaolinite) and ternary system (U(VI)-HA-kaolinite), respectively. The fluorescence spectra of adsorbed uranyl surface species are described with six and five fluorescence emission bands in the binary and ternary system, respectively. The positions of peak maxima are shifted significantly to higher wavelengths compared to the free uranyl ion in perchlorate medium. HA has no influence on positions of the fluorescence emission bands. In the binary system, both surface species can be attributed to adsorbed bidentate mononuclear surface complexes, which differ in the number of water molecules in their coordination environment. In the ternary system, U(VI) prefers direct binding on kaolinite rather than via HA, but it is sorbed as a uranyl-humate complex. Consequently, the hydration shell of the U(VI) surface complexes is displaced with complexed HA, which is simultaneously distributed between kaolinite particles. Aluminol binding sites are assumed to control the sorption of U(VI) onto kaolinite.     

Fate of uranyl in a quaternary system composed of uranyl,citrate, goethite,and Pseudomonas fluorescens

This study investigated the partitioning of uranyl within a quaternary system made up of uranyl, citrate, goethite, and the bacterium Pseudomonas fluorescens. In the absence of cells, uranyl was sorbed to goethite as a complex involving surface groups and/or citrate. Measurements of the evolution of CO2 indicated that the addition of bacterial cells lead to the gradual biodegradation of citrate. Throughout the biodegradation process, uranyl remained sorbed to the insoluble fraction comprised of goethite and cells. EXAFS (Extended X-ray Absorption Fine Structure) measurements showed that bacterial cells outcompeted goethite for uranyl under the experimental conditions and caused the repartitioning of uranyl from goethite to cell matter, independently from citrate degradation. Citrate degradation caused further release of uranyl from goethite surfaces, followed by subsequent association of uranyl with cells. At long equilibration times (3 months), cell lysis and phosphate release resulted in the precipitation of an autunite-like phase. This work suggests that bacterial degradation of uranyl-complexing ligands in contaminated subsurface media containing iron oxides should not necessarily lead to an increase in the mobility of uranyl.     

Surface complexation modeling of U(VI) sorption to Hanford sediment with varying geochemical conditions

A series of U(VI) sorption experiments with varying pH, ionic strength, concentrations of dissolved U(VI), and alkalinity was conducted to provide a more realistic database for U(VI) sorption onto near-field vadose zone sediments at the proposed Integrated Disposal Facility (IDF) on the Hanford Site, Washington. The distribution coefficient (Kd) for U(VI) in a leachate that is predicted to result from the weathering of vitrified wastes disposed in the IDF is 0 mL/g due to the high sodium and carbonate concentrations and high pH of the glass leachate. However, when the pH and alkalinity of the IDF sediment native pore water increases during mixing with the glass leachate, U(VI) uptake is observed and the value of the U(VI) Kd increases 4.3 mL/g, because of U(VI) coprecipitation with newly formed calcite. A nonelectrostatic, generalized composite approach for surface complexation modeling was applied and a combination of two U(VI) surface species, monodentate (SOUO2+), and bidentate (SO2UO2(CO3)2-), simulated the measured U(VI) sorption data very well. The generalized composite surface complexation model, when compared to the constant or single-valued Kd model, more accurately predicted U(VI) sorption under the varying geochemical conditions expected at the IDF.     

Extraction of oxidized and reduced forms of uranium from contaminated soils: effects of carbonate concentration and pH

Uranium may present in soil as precipitated, sorbed, complexed, and reduced forms, which impact its mobility and fate in the subsurface soil environment. In this study, a uranium-contaminated soil was extracted with carbonate/ bicarbonate at varying concentrations (0-1 M), pHs, and redox conditions in an attempt to evaluate their effects on the extraction efficiency and selectivity for various forms of uranium in the soil. Results indicate that at least three differentforms of uranium existed in the contaminated soil: uranium(VI) phosphate minerals, reduced U(IV) phases, and U(VI) complexed with soil organic matter. A small fraction of U(VI) could be sorbed onto soil minerals. The mechanism involved in the leaching of U(VI) by carbonates appears to involve three processes which may act concurrently or independently: the dissolution of uranium(VI) phosphate and other mineral phases, the oxidation-complexation of U(IV) under oxic conditions, and the desorption of U(VI)-organic matter complexes at elevated pH conditions. This study suggests that, depending on site-specific geochemical conditions, the presence of small quantities of carbonate/bicarbonate could result in a rapid and greatly increased leaching and the mobilization of U(VI) from the contaminated soil. Even the reduced U(IV) phases (only sparingly soluble in water) are subjected to rapid oxidation and therefore potential leaching into the environment.     

Pilot-scale in situ bioremedation of uranium in a highly contaminated aquifer. 2. Reduction of u(VI) and geochemical control of u(VI) bioavailability

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

Uranium biomineralization as a result of bacterial phosphatase activity: insights from bacterial isolates from a contaminated subsurface

Uranium contamination is an environmental concern at the Department of Energy's Field Research Center in Oak Ridge, Tennessee. In this study, we investigated whether phosphate biomineralization, or the aerobic precipitation of U(VI)-phosphate phases facilitated by the enzymatic activities of microorganisms, offers an alternative to the more extensively studied anaerobic U(VI) bioreduction. Three heterotrophic bacteria isolated from FRC soils were studied for their ability to grow and liberate phosphate in the presence of U(VI) and an organophosphate between pH 4.5 and 7.0. The objectives were to determine whether the strains hydrolyzed sufficient phosphate to precipitate uranium, to determine whether low pH might have an effect on U(VI) precipitation, and to identify the uranium solid phase formed during biomineralization. Two bacterial strains hydrolyzed sufficient organophosphate to precipitate 7395% total uranium after 120 h of incubation in simulated groundwater. The highest rates of uranium precipitation and phosphatase activity were observed between pH 5.0 and 7.0. EXAFS spectra identified the uranyl phosphate precipitate as an autunite/meta-autunite group mineral. The results of this study indicate that aerobic heterotrophic bacteria within a uranium-contaminated environment that can hydrolyze organophosphate, especially in low pH conditions, may play an important role in the bioremediation of uranium.     

Effects of phosphate on uranium(VI) adsorption to goethite-coated sand

U(VI)-phosphate interactions are important in governing the subsurface mobility of U(VI) in both natural and contaminated environments. We studied U(VI) adsorption on goethite-coated sand (to mimic natural Fe-coated subsurface materials) as a function of pH in systems closed to the atmosphere, in both the presence and the absence of phosphate. Our results indicate that phosphate strongly affects U(VI) adsorption. The effect of phosphate on U(VI) adsorption was dependent on solution pH. At low pH, the adsorption of U(VI) increased in the presence of phosphate, and higher phosphate concentration caused a larger extent of increase in U(VI) adsorption. Phosphate was strongly bound by the goethite surface in the low pH range, and the increased adsorption of U(VI) at low pH was attributed to the formation of ternary surface complexes involving both U(VI) and phosphate. In the high pH range, the adsorption of U(VI) decreased in the presence of phosphate at low total Fe concentration, and higher phosphate concentration caused a larger extent of decrease in U(VI) adsorption. This decrease in U(VI) adsorption was attributed to the formation of soluble uranium-phosphate complexes. A surface complexation model (SCM) was proposed to describe the effect of phosphate on U(VI) adsorption to goethite. This proposed model was based on previous models that predict U(VI) adsorption to iron oxides in the absence of phosphate and previous models developed to predict phosphate adsorption on goethite. A postulated ternary surface complex of the form of (>FePO4UO2) was included in our model to account for the interactions between U(VI) and phosphate. The model we established can successfully predict U(VI) adsorption in the presence of phosphate under a range of conditions (i.e., pH, total phosphate concentration, and total Fe concentration).     

Reduction of uranium(VI) by mixed iron(II)/iron(III) hydroxide (green rust): formation of UO2 nanoparticles

Green rusts, which are mixed ferrous/ferric hydroxides, are found in many suboxic environments and are believed to play a central role in the biogeochemistry of Fe. Analysis by U LIII-edge X-ray absorption near edge spectroscopy of aqueous green rust suspensions spiked with uranyl (U(VI)) showed that U(VI) was readily reduced to U(IV) by green rust The extended X-ray absorption fine structure (EXAFS) date for uranium reduced by green rust indicate the formation of a UO2 phase. A theoretical model based on the crystal structure of UO2 was generated by using FEFF7 and fitted to the data for the UO2 standard and the uranium in the green rust samples. The model fits indicate that the number of nearest-neighbor uranium atoms decreases from 12 for the UO2 structure to 5.4 forthe uranium-green rust sample. With an assumed four near-neighbor uranium atoms per uranium atom on the surface of UO2, the best-fit value for the average number of uranium atoms indicates UO2 particles with an average diameter of 1.7 +/- 0.6 nm. The formation of nanometer-scale particles of UO2, suggested by the modeling of the EXAFS data, was confirmed by high-resolution transmission electron microscopy, which showed discrete particles (approximately 2-9 nm in diameter) of crystalline UO2. Our results clearly indicate that U(VI) (as soluble uranyl ion) is readily reduced by green rust to U(IV) in the form of relatively insoluble UO2 nanoparticles, suggesting that the presence of green rusts in the subsurface may have significant effects on the mobility of uranium, particularly under iron-reducing conditions.