Bicarbonate impact on?U(VI) bioreduction?in a shallow alluvial aquifer
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a?Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
b?Pacific Northwest National Laboratory, Richland, WA 99352, USA
c?Los Gatos Research, Mountain View, CA 94041-1529, USA
d?Akonni Biosystems, Inc., 400 Sagner Avenue, Suite 300, Frederick, MD 21701, USA
e?Haley & Aldrich, Inc., 501 River St. #100, Greenville, SC 29601, USA
f?Department of Molecular Biology and Genetics, 321 Biotechnology Building, Cornell University, Ithaca, NY 14853, USA
g?Earth and Planetary Science and Environmental Science, Policy, and Management, University of California-Berkeley, Berkeley, CA 94720, USA
h?Department of Microbiology, University of Massachusetts, Amherst, MA 01003, USA
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Abstract
Field-scale biostimulation and desorption tracer experiments conducted in a uranium (U) contaminated, shallow alluvial aquifer have provided insight into the coupling of microbiology, biogeochemistry, and hydrogeology that control U mobility in the subsurface. Initial experiments successfully tested the concept that Fe-reducing bacteria such as?Geobacter?sp. could enzymatically reduce soluble U(VI) to insoluble U(IV) during?in situ?electron donor amendment (Anderson et al., 2003; Williams et al., 2011). In parallel,?in situdesorption tracer tests using bicarbonate amendment demonstrated rate-limited U(VI) desorption (Fox et al., 2012). These results and prior laboratory studies underscored the importance of enzymatic U(VI)-reduction and suggested the ability to combine desorption and bioreduction of U(VI). Here we report the results of a new field experiment in which bicarbonate-promoted uranium desorption and acetate amendment were combined and compared to an acetate amendment-only experiment in the same experimental plot. Results confirm that bicarbonate amendment to alluvial aquifer sediments desorbs U(VI) and increases the abundance of Ca-uranyl-carbonato complexes. At the same time, the rate of acetate-promoted enzymatic U(VI) reduction was greater in the presence of added bicarbonate in spite of the increased dominance of Ca-uranyl-carbonato aqueous complexes. A model-simulated peak rate of U(VI) reduction was?～3.8 times higher during acetate-bicarbonate treatment than under acetate-only conditions. Lack of consistent differences in microbial community structure between acetate-bicarbonate and acetate-only treatments suggest that a significantly higher rate of U(VI) reduction in the bicarbonate-impacted sediment may be due to a higher intrinsic rate of microbial reduction induced by elevated concentrations of the bicarbonate oxyanion. The findings indicate that bicarbonate amendment may be useful in improving the engineered bioremediation of uranium in aquifers.
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