Heterogeneous response to biostimulation for U(VI) reduction in replicated sediment microcosms
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A field-scale experiment to assess biostimulation of uranium reduction is underway at the Natural and Accelerated Bioremediation Research Field Research Center (FRC) in Oak Ridge, Tennessee. To simulate the field experiment, we established replicate batch microcosms containing well-mixed contaminated sediment from a well within the FRC treatment zone, and we added an inoculum from a pilot-scale fluidized bed reactor representing the inoculum in the field experiment. After reduction of nitrate, both sulfate and soluble U(VI) concentration decreased. X-ray absorption near edge structure (XANES) spectroscopy confirmed formation of U(IV) in sediment from biostimulated microcosms, but did not detect reduction of solid-phase Fe(III). Two to three fragments dominated terminal restriction fragment length polymorphism (T-RFLP) profiles of the 16S rDNA gene. Comparison to a clone library indicated these fragments represented denitrifying organisms related to Acidovorax, and Acidovorax isolates from the inoculum were subsequently shown to reduce U(VI). Investigation using the T-RFLP Analysis Program (TAP T-RFLP) and chemical analyses detected the presence and activity of fermenting and sulfate-reducing bacteria after 2 weeks. These organisms likely contributed to uranium reduction. In some microcosms, soluble U(VI) concentration leveled off or rebounded, indicating microbial and/or mineralogical heterogeneity among samples. Sulfate, acetate, and ethanol were depleted only in those microcosms exhibiting a rebound in soluble U(VI). This suggests that rates of U(VI) desorption can exceed rates of U(VI) reduction when sulfate-reducing bacteria become substrate-limited. These observations underscore the importance of effective chemical delivery and the role of serial and parallel processes in uranium reduction.
Keywords16S rDNA bioremediation subsurface sulfate-reducing bacteria T-RFLP uranium reduction
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This work was funded from the National Science Foundation graduate fellowship program and a United States Department of Energy (DOE) Natural and Accelerated
Bioremediation Research (NABIR) Biological and Environmental Research (BER) grant (#DE-F603-00ER63046). The authors would like to thank David Watson of the Oak Ridge Field Research Center for providing sediment samples, and Dr. Wei-Min Wu for providing denitrified synthetic groundwater. Portions of this research were carried out at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the United States Department of Energy, Office of Basic Energy Sciences. The SSRL Structural Molecular Biology Program is supported by the Department of Energy, Office of Biological and Environmental Research, and by the National Institutes of Health, National Center for Research Resources, Biomedical Technology Program. Other portions were performed at GeoSoilEnviroCARS (Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation – Earth Sciences (EAR-0217473), Department of Energy – Geosciences (DE-FG02-94ER14466) and the State of Illinois. Use of the APS was supported by the US Department of Energy, Basic Energy Sciences, Office of Energy Research, under Contract No. W-31-109-Eng-38.
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