Environ. Sci. Technol. 2010, 44, 236–242
Uranium Transformations in Static Microcosms S H E L L Y D . K E L L Y , * ,†,‡ W E I - M I N W U , § FAN YANG,| CRAIG S. CRIDDLE,§ TERENCE L. MARSH,| EDWARD J. O’LOUGHLIN,† B R U C E R A V E L , †,⊥ D A V I D W A T S O N , 3 PHILIP M. JARDINE,3 AND KENNETH M. KEMNER† Biosciences Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439-4843, Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305-4020, Microbiology and Molecular Genetics, Michigan State University, 2215 Biomedical Physical Sciences, East Lansing, Michigan 48824-4320, and Environmental Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
Received July 21, 2009. Revised manuscript received October 21, 2009. Accepted November 11, 2009.
Elucidation of complex biogeochemical processes and their effects on speciation of U in the subsurface is critical for developing remediation strategies with an understanding of stability. We have developed static microcosms that are similar to bioreduction process studies in situ under laminar flow conditions or in sediment pores. Uranium L3-edge X-ray absorption near-edge spectroscopy analysis with depth in the microcosms indicated that transformation of UVI to UIV occurred by at least two distinct processes. Extended X-ray absorption fine structure (EXAFS) analysis indicated that initial UVI species associated with C- and P-containing ligands were transformed to UIV in the form of uraninite and U associated with Fe-bound ligands. Microbial community analysis identified putative FeIII and sulfate reducers at two different depths in the microcosms. The slow reduction of UVI to UIV may contribute the stability of UIV within microcosms at 11 months after a decrease in bioreducing conditions due to limited electron donors.
1. Introduction Uranium is a common groundwater contaminant at U.S. Department of Energy (DOE) sites and worldwide. Bioreduction through sequestering U has been investigated since the early 1990s (1) and tested in situ in recent years (2-6). Delivering an electron donor, such as ethanol, to Ucontaminated subsurface leads to groundwater concentrations below the U.S. Environmental Protection Agency (EPA) maximum contaminant level of 0.126 µM (30 µg L-1) for drinking water (6). Aqueous U is removed rapidly during * Corresponding author phone: (630) 270-8748; fax: (630) 2529793; email:
[email protected]. † Argonne National Laboratory. ‡ Present address: EXAFS Analysis, Bolingbrook, IL 60440. § Stanford University. | Michigan State University. ⊥ Present address: Synchrotron Methods Group at Brookhaven National Laboratory, National Institute of Standards and Technology, Bldg 535A, Upton, NY 11973. 3 Oak Ridge National Laboratory. 236
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conversion of ethanol to acetate and is associated with interspecies hydrogen transfer (7, 8) as follows: CH3CH2OH + H2O f CH3COO- + H+ + 2H2 (∆G° ) +9.6 kJ/reaction)
(1)
Until recently, reducing UVI to UIV under anaerobic conditions was commonly assumed to sequester U in the subsurface as uraninite, on the basis of the low solubility of uraninite (9) and its ease of formation in laboratory experiments (1, 10). Many studies attributed the loss of UVI from the aqueous phase during microbial stimulation to UIV precipitation without direct measurement. X-ray absorption spectroscopy (XAS) measurements for solid-phase products of biogeochemical systems have shown that the products are difficult to predict as the complexity of a system increases. Under some conditions, UIV reduction product exists in nanoparticulate or colloidal (11, 12) forms, while under other conditions UIV is associated with Fe or other transition metals (13-15). Other studies have shown that UIV can be solubilized by interaction with chelating agents (16, 17). In each study, the biostimulation product failed to follow the model of bulk uraninite formation leading to U sequestration. Our previous measurements (15) of sediment cores from the Oak Ridge National Laboratory Integrated Field-Research Challenge (IFRC) collected before biostimulation showed UVI bound through equatorial oxygen to P- and C-containing ligands. Sediments surged from wells at the same locations after a year of biostimulation showed U complexed with Fe and P ligands. No uraninite was detected in these sediments. These results illustrate the heterogeneity of the system and the complexity of U transformations in field materials. XAS is uniquely suited to monitoring changes in U speciation (U transformations) as they occur in an intact, undisturbed microcosm during the evolution of biogeochemical systems, which is similar to the bioreduction process in the subsurface under laminar flow or in sediment pores. Previous X-ray absorption near-edge spectroscopy (XANES) studies have demonstrated the utility of monitoring intact columns of sediment materials (18-20). In this study, we demonstrate that U and Fe speciation, aqueous chemical, and microbiology can be monitored in microcosms during the evolution of biogeochemical conditions over an 11-month period under slowly stimulated conditions that retain the stability of UIV.
2. Materials and Methods Additional Materials and Methods are given in the Supporting Information. The sediment was taken from well FW026 of Area 3 at the IFRC site adjacent to the former S-3 ponds at the Y-12 National Security Complex, Oak Ridge, TN. The geochemical properties of the sediment and groundwater in microcosms are described in Table S1 of Supporting Information. The sediment contained significant amounts of U (713 mg kg-1), HCl-extractable Fe (47 mg g-1), ferrous iron (7.0 mg g-1), and total organic carbon (7.6 mg g-1). The aqueous phase contained sulfate, 1.06 mM; bicarbonate, 1.0 mM; Ca2+, 1.37 mM; and Mn2+, 0.058 mM. Two duplicate 650-mL microcosms were prepared under sterile anoxic conditions in a Coy anaerobic chamber filled with 95% nitrogen and 5% H2 and were incubated in the Coy chamber (Figure 1). The initial H2 (0.42 mmol) in the microcosms is well suited to our measurement cycles at the synchrotron of approximately 3 months, allowing for monitoring of U transformations. Clone libraries of 16S rRNA sequences were constructed to identify predominant bacterial 10.1021/es902191s
2010 American Chemical Society
Published on Web 12/02/2009
FIGURE 1. Microcosm appearance at T0 (left), T3 (top right), and T11 (bottom right). groups (section 1.2, Supporting Information) after 5 and 11 months of incubation from sediment removed from the top layer and from 2-cm depth. Uranium XANES measurements (section 1.3, Supporting Information) were performed on the microcosms within 5 days of initialization of the microcosms at T0, then again at 3 months (T3), 5 months (T5), and 11 months (T11). Uranium EXAFS measurements were made on the microcosms and also on centrifuged sediment packaged in a plastic holder and sealed with Kapton film and Kapton tape at T0. Because of the low U concentrations in the microcosms, U EXAFS measurements were performed sparingly at T0 and again on specified regions of the microcosms at T11. Iron XANES and EXAFS measurements were made on the microcosms at T5. Details are found elsewhere (21). The parameters determined by the EXAFS modeling are given in Tables S2-S4 in Supporting Information, and the EXAFS spectra χ(k) are shown in Figure S1 in Supporting Information. Analytical methods are described in section 1.4, Supporting Information.
3. Results and Discussion 3.1. Groundwater and Sediment Geochemistry. The microcosms are pictured in Figure 1. The microcosm sediments were initially a uniform yellow-tan color. At T3, the top layer turned black, and black regions appeared throughout the microcosm. By T11, the entire sediment had turned dark gray-green, similar to the color of bioreduced sediment samples during field test (5), and the black regions had grown, consistent with bioreduction in both microcosms. The initial chemical composition of the aqueous phase and the solids is presented in Table S1 of Supporting Information. The sediment contained U at 713 mg kg-1 and Fe at 47.1 mg g-1, with 15% of the total Fe as FeII. Aqueous sulfate concentrations decreased from 1.06 to less than 0.02
mM by T11 (Figure 2A). During this time, the aqueous UVI concentration decreased from 11.85 to
