Microbial Reduction of Intragrain U(VI) in Contaminated Sediment

The slower bioreduction rate was attributed to (1) the release of calcium from the desorption/dissolution of ... The host granitic clasts comprised on...
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Environ. Sci. Technol. 2009, 43, 4928–4933

Microbial Reduction of Intragrain U(VI) in Contaminated Sediment C H O N G X U A N L I U , * ,† J O H N M . Z A C H A R A , † LIRONG ZHONG,† STEVE M. HEALD,‡ ZHEMING WANG,† BYONG-HUN JEON,§ A N D J A M E S K . F R E D R I C K S O N * ,† Pacific Northwest National Laboratory, Richland, Washington 99352, Argonne National Laboratory, Argonne, Illinois 60439, and Yonsei University, Wonju Kangwon-Do, Korea 220-710

Received October 15, 2008. Revised manuscript received February 9, 2009. Accepted March 3, 2009.

The accessibility of precipitated, intragrain U(VI) in a contaminated sediment to microbial reduction was investigated to ascertain geochemical and microscopic transport phenomena controlling U(VI) bioavailability. The sediment was collected from the U.S. Department of Energy Hanford site, and contained uranyl precipitates within the mm-sized granitic lithic fragments in the sediment. Bioreduction was investigated in a culture of a dissimilatory metal-reducing bacterium, Shewanella oneidensis strain MR-1. Measurements of uranium concentration, speciation, and valence in aqueous and solid phases indicated that microbial reduction of intragrain U(VI) proceeded by two mechanisms: (1) sequentially coupled dissolution of intragrain uranyl precipitates, diffusion of dissolved U(VI) from intragrain regions, and microbial reduction of dissolved U(VI); and (2) U(VI) reduction in the intragrain regions by soluble, diffusible biogenic reductants. The bioreduction rate in the first pathway was over 3 orders of magnitude slower than that in comparable aqueous solutions containing aqueous U(VI) only. The slower bioreduction rate was attributed to (1) the release of calcium from the desorption/dissolution of calcium-containing minerals in the sediment, which subsequently altered U(VI) aqueous speciation and slowed U(VI) bioreduction and (2) alternative electron transfer pathways that reduced U(VI) in the intragrain regions and changed its dissolution and solubility behavior. The results implied that the overall rate of bioreduction of intragrain U(VI) will be influenced by the reactive mass transfer of U(VI) and biogenic reductants within intragrain regions, and geochemical reactions controlling major ion concentrations.

Introduction Dissimilatory metal-reducing bacteria (DMRB) can reduce various types of uranium(VI), including aqueous (1), adsorbed (2), and precipitated phases as terminal electron acceptors (3-5). The apparent rates of microbial reduction of U(VI) to U(IV) are, however, variable and depend on U(VI) speciation and accessibility. Microbial reduction of aqueous U(VI) is the fastest among different U(VI) phases, with a half-life ranging from hours in pure cultures (6-8) to days in * Corresponding author phone: (509) 371-6350; fax (509) 371-6354; e-mail: [email protected]. † Pacific Northwest National Laboratory. ‡ Argonne National Laboratory. § Yonsei University. 4928

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groundwater with bacterial stimulation (9, 10). The accessibility and mechanisms of bioreduction of adsorbed and precipitated uranyl species have been infrequently studied in spite of their equilibrium or kinetic linkage with aqueous U(VI). Limited study indicates that the microbial reduction of uranyl precipitates is slow, with a rate controlled by dissolution kinetics and enzymatic electron transfer to dissolved U(VI) (5, 11). The adsorption of U(VI) to iron oxide surfaces had no effect on the rate of microbial reduction (2). Bioreduction rates of U(VI) associated with microbe-inaccessible micropores was limited by the mass transfer to the aqueous phase (5). Bioreduction of adsorbed U(VI) in sediments was, however, significantly inhibited relative to the aqueous phase (2, 12). The inhibition mechanism was unclear because of unknown aqueous and sorbed uranyl speciation, sorption/desorption mechanism and kinetics, and mass transfer rate from the adsorbed to aqueous phase. Here, we report an investigation of microbial reduction of U(VI) that exists in a contaminated sediment as intragrain precipitates of Na-boltwoodite (13-16). Previous investigations of this sediment had shown that U(VI) release was controlled by a coupled dissolution/diffusion process within intragrain fractures (16-18). This well-characterized sediment in terms of uranyl speciation, dissolution, and intragrain diffusion properties provided an ideal system to investigate whether and how DMRB access physically inaccessible electron acceptors.

Materials and Experimental Procedures Contaminated Sediment. The uranium-contaminated sediment was collected from borehole 299-E33-45 at the Hanford BX tank farm that was drilled through a vadose zone U(VI) plume proximate to tank BX-102 (19). The sediment stratigraphy and U concentration profile in the borehole have been described elsewhere (16). The sediment was collected at approximately 40 m below ground surface and contained 1.37 µmol/g of U. The sediment U(VI) existed as micronsized precipitates of Na-boltwoodite that was associated with intragrain microfracture regions in plagioclase domains of granitic clasts (13-16). The width, length, and connectivity of the fractures were variable, forming a complex network of intragrain porous medium (18). The host granitic clasts comprised only 4% of the sediment that was dominated by basaltic lithic fragments. Microorganism. Bacterium S. oneidensis strain MR-1 (20) was used as a representative DMRB. Strain MR-1 was cultured with tryptic soy broth (TSB), 27.5 g/L (Difco Laboratories, Detroit, MI), harvested by centrifugation, washed twice with Na-PIPES (Na-1,4-piperazine-N,N′-bis-2-ethanesulfonic acid) buffer (10 mmol/L, pH 6.8). Cell density was determined by measuring the absorbance at 600 nm. The washed cells were resuspended in PIPES buffer and purged with O2-free N2 for bioreduction experiments. An MR-1 mutant that was depleted of proteins MtrC and OmcA, two external membraneassociated c-type cytochromes implicated in U(VI) enzymatic reduction (21) was also used to investigate the influence of alternative electron transport pathways in microbial reduction of U(VI). Microbial Reduction. Microbial reduction experiments were conducted using glass serum bottles as reactors that contained the U(VI)-contaminated sediment with a solidsolution ratio of 50 g/L. Anoxic solutions of Na-PIPES (10 mmol/L), Na-lactate (10 mmol/L), and NaHCO3 (10 mmol/L were added to the reactors, and the sediment suspension pH was adjusted to 6.8 with 0.1 mol/L HCl or NaOH. The reactors were capped with thick butyl rubber stoppers and crimp 10.1021/es8029208 CCC: $40.75

 2009 American Chemical Society

Published on Web 05/21/2009

sealed, and were allowed to equilibrate for select times before addition of microorganisms, which were injected by a needle and syringe. All experiments were performed in replicates in an anaerobic chamber (Coy Laboratory Products, Inc.) filled with 97% Ar and 3% H2. The suspensions were gently shaken on an orbital shaker at room temperature. At select times, 0.5 mL of solution was withdrawn from each reactor with a needle and syringe, filtered (0.2 µm), and acidified (0.01 mol/L HNO3 final concentration) in the anaerobic chamber, and then analyzed for dissolved U(VI) with a kinetic phosphorescence analyzer (KPA; Chemcheck Instruments, Richland, WA) with a detection limit of 0.001 µmol/L. Fe(II) that was extracted by 0.5 M HCl was also analyzed at selected times to determine the extent of bacterial Fe(III) reduction. The total solution volume withdrawn from each bottle during the experiment was less than 10% of total solution volume. Solution pH was measured using an Orion 250A+pH meter. Aqueous U(VI) Speciation. Laser-induced fluorescence spectroscopy (LIFS) was used to determine aqueous U(VI) speciation in the sediment suspensions. The LIFS measurements were performed in a Cryo Industries RC-152 cryostat near liquid He-temperature (5.5 ( 1.0 K) as described previously (22). Data were analyzed using the IGOR and the Globals program (23). Aqueous U(VI) speciation was also calculated using an equilibrium speciation model and a thermodynamic database that were assembled by the authors. Relevant U(VI) species, reactions, and reaction constants were compiled from literature (24, 25). Solid Phase Analysis. Sediment samples after microbial reduction were isolated by centrifugation, rinsed with PIPES buffer three times to remove residual electrolyte, and airdried in the anaerobic chamber. The samples were then imbedded in epoxy, wafered using a diamond saw, and prepared as 100 µm thin sections on fused quartz slides. The samples were analyzed by X-ray microprobe (XMP) and X-ray adsorption near-edge spectroscopy (XANES) on beamline 20-ID in sector 20 of the Advanced Photon Source (APS) at the Argonne National Laboratory. False-color (blue to red) abundance maps were constructed for selected areas after normalizing detected X-rays to a measured current that was proportional to the primary flux. Micro-XANES spectra were collected from U-containing regions of the thin section after the spatial distribution of U had been mapped with XMP. The incident energy was varied while monitoring the X-ray fluorescence. The XANES data were analyzed using the Athena and Artemis interfaces to the IFEFFIT program package (26). The valence of both U and Fe in these regions was obtained using linear combination fits of normalized standard spectra to the normalized data.

Results and Discussion Aqueous U(VI) Evolution. The dissolved U(VI) concentrations in the reactors increased with time as intragrain Na-boltwoodite dissolved within and released from the sediment (Figure 1, labeled as control data). The rate of U(VI) release from the sediment decreased with time as indicated by the decreasing slope in the temporal profile of U(VI) concentration. The solubility of Na-boltwoodite was calculated to be 570 µmol/L under the experimental conditions, which was 1 order of magnitude higher than the measured aqueous U(VI) concentrations. Thus, the bulk solutions were under-saturated with respect to the sediment precipitate over the entire course of the experiment. The under-saturation indicated that the decreasing rate of U(VI) release from the sediment was not caused by solubility limitation, but by the diffusive mass transfer of dissolved U(VI) from the sediment to the bulk solutions. Mass transfer flux was slowed by a decreasing concentration gradient with time as the U(VI) concentrations increased in the fluid phase.

FIGURE 1. U(VI) release from the sediment and microbial reduction of dissolved U(VI). Symbols and lines are experimental and model results, respectively. Models were described in text and Table 1. The addition of bacterium MR-1 after 5 to 7 days of incubation resulted in a decrease in aqueous U(VI) concentration with a rate dependent on bacterial concentration (Figure 1). The effect of MR-1 was minor within the experimental durations when cell concentration was less than 107 cells/mL. When cell concentration increased to and above 108 cells/mL, the dissolved U(VI) concentrations decreased rapidly. The results implied that the dissolved U(VI) concentrations were controlled by the interactive effects of mass transfer controlled desorption that increased aqueous U(VI) concentration; and microbial reduction that decreased aqueous U(VI) concentration in proportion to cell number. In the suspensions with low cell concentrations (e107 cells/ mL), U(VI) release from the sediment was fast enough to compensate for bioreductive U(IV) precipitation. In the high cell suspensions (g108 cells/mL), the bioreduction rate exceeded the mass transfer limited desorption. Aqueous U(VI) concentrations in the 108 cells/mL suspension displayed two apparent kinetic regions: fast decrease within 10 days of cell spike followed by slow decrease. Such kinetic behavior was attributed to the changes in the rates of U(VI) release from the sediment and microbial reduction. The rate of mass transfer limited desorption increased with decreases in aqueous U(VI) because of an enhanced concentration gradient between sediment and solution. At the same time, the bioreduction rate decreased with decreasing dissolved U(VI) concentration because its rate was proportional to electron acceptor concentration (5). When the bacterial concentration increased to 109 cells/mL, aqueous U(VI) concentrations decreased to below detection limit after one hour of incubation (Figure 1), indicating that bioreduction consistently outcompeted the rate of U(VI) release from the sediment. Mass balance calculations revealed that less than 10% of U(VI) was released from the sediment in the first 7 days, verifying that the low aqueous U(VI) concentration was not due to depletion of precipitated U(VI) in the sediment. The aqueous U(VI) concentration profiles were slightly affected by the replacement of MR-1 wild type with a mutant that was deleted of MtrC and OmcA, two membraneassociated C-type cytochromes that are important in electron transfer to insoluble electron acceptors, such as Fe(III) oxides (21). This observation was consistent with a literature report (21) that MR-1 exhibits multiple U(VI) bioreduction pathways including one mediated by membrane-associated and another by perisplasmic cytochromes. The minor difference of U(VI) concentration profiles in the suspensions containing wild type MR-1 and its mutant confirmed the expectation that direct contact between outer membrane cytochromes and the uranyl precipitates were not a requirement for U(VI) bioreduction in this system (21). Modeling. The rate of U(VI) release from the sediment was consistent with a mass transfer limited, dissolution VOL. 43, NO. 13, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Models and Parameters Used in This Study (1) dissolution of Na-boltwoodite: Na[UO2(Si3OH)](H2O)1.5 + 3H+ ) UO22+ + Na+ + H4SiO4 + 1.5H2O log Ksp ) 5.85

R)

d[U(UVI)]aq [HCO3-] [1 - (IAP/Ksp)n] ) kAFb dt Kbc + [HCO3-]

k: rate constant (2.05 × 10-6 mol/m2/h); A: Na-boltwoodite surface area (30.8 m2/g), Fb: Na-boltwoodite concentration (g/L); Kbc: half-rate constant with respect to bicarbonate (2.74 × 10-3 mol/L), IAP, and Ksp: ion activity product and solubility constant (5.85) for Na-boltwoodite, respectively; n: reaction order of nonlinearity (0.2); [HCO3-]: bicarbonate concentration (mol/L). The rate constants and solubility were independently characterized in refs 18, 27 (2) intragrain diffusion of dissolved U(VI): fracture domain: ∂Cf(l, t) Df ∂2Cf(l, t) + Rf(l, t) - f2km(Cf(l, t) - Cm(l, t)) ) 2 ∂t Lf ∂l2 matrix domain: ∂Cm(l, t) ) Rm(l, t) + km(Cf(l, t) - Cm(l, t)) ∂t where C and R are the aqueous concentration and Na-boltwoodite dissolution rate, respectively, subscript f and m refer to the fracture and matrix domains, respectively, Df/Lf2 ()6.8 h-1) is the diffusivity normalized to fracture length, and km () 3.5 × 10-3 h-1) is the mass transfer coefficient between fracture and matrix domains, f2 is the matrix and fracture porosity ratio (0.176). The fracture porosity (θf) in the U(VI)-containing grain was 0.1%. The parameters in the diffusion model was estimated from ref 16. (3) reduction of dissolved U(VI) by DMRB: lactate- + 2UO22+ + 2H2O ) acetate- + 2UO2(s) + HCO3- + 5H+ logK ) 21.49 R)

dCb VmCb X ) dt K s + Cb

where Cb is the total aqueous U(VI) concentration, Vm: maximum rate; Ks: half-rate constant with respect to total aqueous U(VI) concentration; and X: cell concentration. kinetic model involving structured intragrain microfracture domains in the granitic fragments of the sediment (solid line in Figure 1). The models of dissolution and diffusion in the intragrain regions are described elsewhere (16, 18, 27), and their mathematical equations and parameters are provided in Table 1 as reference. After solving the coupled intragrain dissolution and diffusion equations (Table 1), the dissolved U(VI) concentration in the control suspension was described by Vb

dCb θfVsDf ∂Cf(l)0,t) ) dt Lf ∂l

(1)

where Vb and Cb are the aqueous volume and concentration in the bulk solution, θf is the intragrain fracture porosity, Vs is the solid volume, and Lf is the fracture length. The right side is the diffusive flux, which was calculated by solving diffusion equations in Table 1 with all parameters indepen4930

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FIGURE 2. LIFS spectra showed Ca2UO2(CO3)3(aq) as a dominant aqueous U(VI) species in the sediment suspensions. In a control, without sediment, but with spiked U(VI), U(VI) species was dominated by UO2(CO3)34-. Vertical lines denote peak maxima of Ca2UO2(CO3)3(aq). dently determined. The model (eq 1) slightly under-predicted the experimental measurements. The aqueous chemical composition in the current experiments was significantly different from those that were used in determining the rate parameters (16, 18, 27). The close match between the calculated and measured U(VI) release (Figure 1) provided additional validation of the model robustness that involves a bicarbonate promoted, surface complex dissolution mechanism (16). The aqueous U(VI) concentrations after bioreduction were described by adding a microbial reduction term in eq 1: Vb

θfVsDf ∂Cf(l)0,t) dCb VmCb X ) - Vb dt Lf ∂l K s + Cb

(2)

where Vm is the maximum rate of microbial reduction, Ks is the half-rate constant of bioreduction, and X is the bacterial cell concentration. Saturation-type kinetics (the second right term in eq 2) has been widely used to describe the microbial reduction of dissolved U(VI) by various DMRB (6, 7, 28, 29). Previously determined rate constants for microbial reduction in a similar culture as used in this study [Ks ) 0.05 mM and Vm ) 4.0 × 10-15 mol/cell/h (8)] were used first to model U(VI) bioreduction. These rate constants were of the same order of magnitude as those reported in literature for DMRB in pure cultures (8, 28, 29). Model simulations using eq 2 and rate constants from ref 8 yielded calculated aqueous U(VI) concentrations that decreased more rapidly than measured values in all the suspensions after the spikes of microorganisms. Previous studies have shown that Ca2+ can inhibit U(VI) bioreduction through formation of ternary calcium-U(VI)-carbonate species, which inhibited microbial reduction (30). LIFS analysis of our experimental medium showed that Ca2UO2(CO3)3(aq) was an unexpected dominant aqueous U(VI) species (Figure 2) immediately before bacterial inoculation. Calcium was not added in the initial electrolyte, but it was apparently released from the sediment through chemical interactions between the solutions and sediment through ion exchange reactions or dissolution of calcite or plagioclase (17). The rate constants for the bioreduction of Ca2UO2(CO3)3(aq) by MR-1 were determined to be Vm ) 4.23 × 10-17 mol/cell/h and Ks ) 3.7 × 10-4 mol/L (11), in which the value of Vm was 2 orders of magnitude less than that in solutions dominated by UO2(CO3)34-. Remodeling the data in Figure 1 with the set of rate constants for microbial reduction of Ca2UO2(CO3)3(aq), however, still overpredicted the rate of bioreduction. Consequently we adjusted Vm by trial-and-error to fit the measured

FIGURE 3. Concentration of Fe(II) in the suspension that was extractable by 0.5 M HCl.

FIGURE 5. XANES spectra showing mixed valence of U and Fe in the intragrain domains (S1-2 and S2-1) and on the grain surfaces (S1-1 and S2-2). The spatial locations were shown in Figure 4.

TABLE 2. Relative Percentage of U(VI) and U(IV) in the Bioreduced Sedimenta FIGURE 4. XRM images showing grain morphology (top images), U (middle images) and Fe (bottom images) abundance in the intragrain regions. Sample S1 was collected after 30 days of microbial reduction, and Sample 2 was collected after 37 days of microbial reduction in the suspensions containing 108 cells/mL. dissolved U(VI) concentrations as a function of time and cell concentration in Figure 1. The best fit value of Vm was 1.2 × 10-18 mol/cell/h, which was over 30 times slower than the value (4.23 × 10-17 mol/cell/h) for Ca2UO2(CO3)3(aq)dominated aqueous solutions without sediment. The fitted profiles in Figure 1 matched the data reasonably well although the discrepancy existed. The sediment contains Fe(III)-oxides as a minor component (19), that may have competed with U(VI) as an alternative electron acceptor for DMRB (3, 31, 32). Such a mechanism was consistent with the increased concentration of Fe(II) that was extractable by 0.5 M HCl within 5 days after cell spike (Figure 3). However, the lack of significant difference of Fe(II) concentrations with time, and in the suspensions containing 106 and 107 cells/mL suggested a minor effect of microbial reduction on the apparent Fe(II) production. Over 30 times decrease of the rate constant (Vm) suggested that other mechanisms were responsible for the decreased rate of microbial reduction of dissolved U(VI). U in Solid Phase. Residual U in the sediment after 30 days of microbial incubation was primarily associated with the granitic lithic fragment interiors (Figure 4). In contrast, U in the original sediment was distributed relatively homogeneously from the intragrain interior to the particle edge (16). The preferential association of residual U toward the

sample location

U(VI)

U(IV)

S1-1 S1-2 S2-1 S2-2

0 0.46 0.70 0

1.0 0.54 0.30 1.0

(exterior) (interior) (interior) (exterior)

a

Sample locations are correspondent to the location marked in Figure 4.The percentage was determined from fitting the edges of XANES spectra shown in Figure 5.

grain interiors in the bioreduced sediment (Figure 4) indicated the importance of diffusion in regulating U(VI) release from the sediment during microbial reduction. The diffusion process supplied necessary reactants for surface-promoted dissolution (HCO3-) and removed dissolution products [Si, Na, U(VI)] to avoid local saturation and support continuous dissolution. The diffusion process enhanced precipitate dissolution in regions near and directly contacting the bulk solution. The inhomogeneous distribution of residual U in the intragrain regions (Figure 4) was attributed to a heterogeneous distribution of pore diffusion coefficients between different grains and within individual grains in the lithic fragments (18). The fracture regions with smaller diffusion coefficients released U(VI) at slower rates because of mass transfer resistance. XANES analysis of the thin sections revealed variations in U valence (Figure 5a) at different particle locations. At the center of U-containing grains, both U(VI) and U(IV) existed at significant concentration with variable ratios (Figure 5a and Table 2). In contrast, U at the grain edges was dominated by U(IV) (Table 2). The presence of U(IV) in the center of the intragrain regions was a surprising finding given that (1) only U(VI) was present in the contaminated sediment (14), (2) VOL. 43, NO. 13, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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fracture sizes were too small for DMRB to directly access intragrain U(VI), and (3) U(VI) precipitates were partially soluble under the experimental conditions (Figure 1). The result implied the bioproduction of diffusible reductants that reduced U(VI) in the intragrain regions. The reductants could be organic biomolecules of various identities (31, 33, 34) or biogenic products, such as Fe(II), that were generated at the grain surface and diffused into intragrain regions. Although both Fe(II) and Fe(III) were present in the intragrain regions (Figure 5b), Fe(II)/U(VI) redox reaction mechanism was not supported by the poor spatial correlation between Fe and U (Figure 4). The reduction of U(VI) within the intragrain regions provided an explanation for the slower rate of microbial reduction of aqueous U(VI) as required to model Figure 1 because electrons were diverted to intragrain U(VI), most likely in the form of biogenic organic reductants. The intragrain U(IV) production and precipitation may also have slowed the rates of boltwoodite dissolution and U(VI) mass transfer. Implication. Microbial reduction of sediment-associated U(VI) was slower than that observed in microbial reduction of aqueous U(VI) (2, 9, 10, 12), and such effect was interpreted to result from an inhibition in the enzymatic process (2, 12). Research described herein has shown that microbial reduction of U(VI) may be regulated by physical and biogeochemical processes that control (1) the concentrations of major ions (Ca in this case) that affect U(VI) speciation, which in turns affects the rate of microbial reduction; and (2) dissolution kinetics (diffusive mass transfer in this case) that control the bioavailable, dissolved U(VI) concentrations, alter physiochemical state of sorbed U, and modify its release mechanism. These processes can act in a complex manner and are strongly affected by geochemical conditions, sediment property and mineralogy, and biochemistry of active microorganism. The apparent rate and extent of microbial U(VI) reduction in sediments will therefore be variable depending on the relative importance of these individual processes and their coupling. This study demonstrated that aqueous U(VI) speciation evolved as solution composition changed from solution/ sediment interactions. The speciation change subsequently affected the bioreduction rate parameters (e.g., Vm in eq 2). In this study, the change of U(VI) aqueous speciation occurred before microbial reduction, allowing a single set of rate parameters (e.g., Vm and Ks in eq 2) to describe the microbial reduction kinetics. In natural environments with timevariable geochemical conditions, the rate constants for saturation-type kinetics (the second right term in eq 2) will be temporally variable depending on the extent of change in U(VI) aqueous speciation. This presents a significant challenge for characterizing and simulating microbial reduction rates under time-varying geochemical conditions. This sensitivity results from the empirical nature of the kinetic formulation used for microbial reduction. A more fundamental rate formulation is therefore needed with its rate parameters independent of U(VI) aqueous speciation. Reductive precipitation of U has been proposed as a remediation technology to immobilize groundwater U. This study demonstrated two reduction pathways for sedimentsorbed U(VI). The fast pathway involved intragrain dissolution and diffusion to the bulk fluid phase where soluble U(VI) was bioavailable for bacterial reduction. The second involved intragrain U(VI) reduction by diffusible, biogenic reductants such as flavins. The success of reductive remediation techniques will depend on the stability of reductively precipitated U(IV). The oxidative remobilization of U is a major concern when groundwater returns to oxic conditions because U(IV) precipitates are not stable in presence of O2 (35, 36). For the contaminant-sediment association studied herein, the second pathway is more desirable because 4932

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intragrain mass transfer would limit the delivery of oxidants to the intragrain regions, consequently decreasing the potential rate of U oxidative remobilization.

Acknowledgments This research was supported by DOE through the Environmental Remediation Science Program (ERSP). We acknowledged the help from Dave Kennedy for culturing bacterial cells, and Tom Resch in preparing slides for XMP and X-ray absorption analysis. Florescence measurements were performed at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at the Pacific Northwest National Laboratory (PNNL). PNNL is operated for the DOE by Battelle Memorial Institute under Contract DE-AC05-76RLO1830.

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