Environ. Sci. Technol. 2007, 41, 7343-7348
Speciation-Dependent Microbial Reduction of Uranium within Iron-Coated Sands JIM NEISS,† BRANDY D. STEWART,† P E T E R S . N I C O , ‡ A N D S C O T T F E N D O R F * ,† Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, and Environmental Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94702
Transport of uranium within surface and subsurface environments is predicated largely on its redox state. Uranyl reduction may transpire through either biotic (enzymatic) or abiotic pathways; in either case, reduction of U(VI) to U(IV) results in the formation of sparingly soluble UO2 precipitates. Biological reduction of U(VI), while demonstrated as prolific under both laboratory and field conditions, is influenced by competing electron acceptors (such as nitrate, manganese oxides, or iron oxides) and uranyl speciation. Formation of Ca-UO2-CO3 ternary complexes, often the predominate uranyl species in carbonatebearing soils and sediments, decreases the rate of dissimilatory U(VI) reduction. The combined influence of uranyl speciation within a mineralogical matrix comparable to natural environments and under hydrodynamic conditions, however, remains unresolved. We therefore examined uranyl reduction by Shewanella putrefaciens within packed mineral columns of ferrihydrite-coated quartz sand under conditions conducive or nonconducive to CaUO2-CO3 species formation. The results are dramatic. In the absence of Ca, where uranyl carbonato complexes dominate, U(VI) reduction transpires and consumes all of the U(VI) within the influent solution (0.166 mM) over the first 2.5 cm of the flow field for the entirety of the 54 d experiment. Over 2 g of U is deposited during this reaction period, and despite ferrihydrite being a competitive electron acceptor, uranium reduction appears unabated for the duration of our experiments. By contrast, in columns with 4 mM Ca in the influent solution (0.166 mM uranyl), reduction (enzymatic or surface-bound Fe(II) mediated) appears absent and breakthrough occurs within 18 d (at a flow rate of 3 pore volumes per day). Uranyl speciation, and in particular the formation of ternary Ca-UO2-CO3 complexes, has a profound impact on U(VI) reduction and thus transport within anaerobic systems.
Introduction Historic waste disposal methods from nuclear-production activities, including mining, weapons manufacturing, and energy production, have released radionuclides, heavy metals, and co-contaminants into the environment. Having * Corresponding author phone: (650) 723-5238; fax: (650) 7252199; e-mail:
[email protected]. † Stanford University. ‡ Lawrence Berkeley National Laboratory. 10.1021/es0706697 CCC: $37.00 Published on Web 09/25/2007
2007 American Chemical Society
migrated into surface waters and groundwaters, uranium has become a contaminant of environmental concern. To predict, and in some cases control, the hazard of uranium, comprehension of biogeochemical processes and environmental conditions governing its fate and transport is critical. In soils and sediments, the oxidized state of uranium (U(VI)) is typically mobile, particularly in carbonate-bearing waters where uranyl-carbonato complexes dominate the speciation of uranium. The reduced state of uranium (U(IV)), in contrast, forms phases of low solubility, such as uraninite, and thus typically has limited mobility in waters (1). Like other toxic metals and radionuclides (e.g., Cr and Tc), the fate and transport of uranium is in part dictated by microbially mediated reduction, which results in the subsequent precipitation of UO2 (2-5). Reductive stabilization is, in fact, being explored as a means to limit the transport of uranium by generating phases of limited solubility (6-8). Although abiotic reduction pathways (via reaction with Fe(II), for example) exist (9-13), uranyl reduction is largely predicated on dissimilatory metal-reducing bacteria (DMRB), coupling U(VI) reduction with the oxidation of H2 or organic carbon (14-17). The extent of bacterially mediated reduction of uranium may be impeded by the local geochemistry, however. Nitrate, manganese(III/IV) (hydr)oxides, and iron(III) (hydr)oxides, for example, all of which are ubiquitous in the soils and sediments, may serve as competitive terminal electron acceptors, thus retarding uranyl reduction (18-20). Potentially equally important as competing electron acceptors, however, is the influence of uranyl speciation on the extent of reduction (21). In the presence of calcium, ternary Ca-UO2-CO3 complexes become the dominant aqueous species and have the capacity to limit biotic reduction of uranium (21). The importance of considering Ca-UO2-CO3 species is illustrated by groundwater from the Tuba City, AZ, uranium mill tailing remedial action (UMTRA) site.Excludingcalciumcomplexes,UO2(CO3)22- andUO2(CO3)34account for 94% of aqueous U(VI) (22), while including formation constants for Ca2UO2(CO3)3 (log β113 ) 27.18) and CaUO2(CO3)32- (log β213 ) 30.70) (23) indicates the former species account for 99.3% and 0.3%, respectively, of the U(VI) species in solution. Moreover, the impact of calcium-uranylcarbonato complexes on biogeochemical reactions involving uranium can be appreciable. In two batch studies, the reduction of U(VI) by Shewanella putrefaciens coupled with the electron donors lactate and H2 resulted in a ca. 50% decrease in the extent of uranium reduction in the presence of calcium (with concentrations ranging from 2.5 to 5 mM) compared to that of parallel systems containing no calcium (21), illustrating the importance of considering the specific species of uranyl in reaction fate (24). Iron has a unique relation with uranium, being an oxidant or reductant depending on the specific geochemical conditions (25); Fe may even alternate between being an oxidant and reductant during a single incubation (see, for example, ref 26). The redox couples of iron(III/II) and uranium(VI/IV) are comparable, and thus, the specific element species and chemical gradients determine the reaction direction. Depending on the specific species of uranyl, the U(IV/VI) redox couple can change more than 200 mV, with UO22+ leading to the highest and Ca2UO2(CO3)3 the lowest; a shift in U(VI) speciation toward the calcium ternary complex thus makes U(IV) most susceptible to oxidation. Ferrihydrite has the highest redox potential of the iron(III) (hydr)oxides and consequently has the greatest likelihood to compete as an electron acceptor in microbial respiration and serve as an oxidant of U(IV). Wielinga et al. (18), for example, observed VOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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a 52% decrease in uranyl reduction by Shewanella alga strain BrY in the presence of ferrihydrite, compared to only uranyl in solution, over a period of 10 h (18). Despite the important findings revealed by examining dissimilatory reduction of uranyl under static-flow conditions in the presence and absence of calcium, what remains inconclusive is the impact Ca may have under more complex regimes composed of a mineral matrix and advective flow conditions. Accordingly, in this study, we examine the impact of calcium on dissimilatory uranyl reduction within a system representing a more complex mineralogical environment composed of an alternate electron acceptor and under hydrodynamic conditions.
Materials and Methods Uranyl reduction by S. putrefaciens was examined in systems containing two-line-ferrihydrite-coated sand under advective flow conditions, similar to that described previously (27). Two iron-coated sand columns were run in parallel and received simulated groundwater containing 170 µM U(VI) (as uranyl acetate); one column had no calcium and the other 4 mM Ca in the simulated groundwater. The columns where maintained at ca. pH 7. The system with calcium was buffered using 3 mM bicarbonate, and the system without calcium was buffered using 10 mM PIPES (1,4-piperazinediethanesulfonic acid). Influent lactate concentrations for both columns averaged ca. 2.9 mM. A constant flow rate of ca. 3.5 pore volumes per day was maintained for 16 d for the column with calcium and 54 d for the column without calcium. Preparation of Bacterial Cultures and Reaction Media. S. putrefaciens, strain CN32, cell suspensions were prepared from frozen stocks (27), grown aerobically on tryptic soy broth, and resuspended in an artificial groundwater medium. Artificial groundwater medium, as described previously (28), contained the following ingredients (mg/L): KHCO3, 380; KCl, 5; MgSO4, 50; NaCl, 30; NH4Cl, 0.95; KH2PO4, 0.95; 1 mL of mineral solution (29). Lactate (sodium lactate) was added to provide a final concentration of 3 mM. The medium was buffered at pH 7 with either 10 mM PIPES, made anoxic by boiling and cooling under a stream of O2-free N2/CO2 (80:20) gas, or by equilibrating with calcite (0.4 g/L) and CO2 at 0.02 atm (pH 7). Two-line-ferrihydrite-coated quartz sand was prepared as described by Brooks et al. (30). A ferrihydrite floc was prepared by titrating ferric chloride with NaOH to a pH of 7.5 rapidly (
