Nonreductive Biomineralization of Uranium(VI) Phosphate Via Microbial Phosphatase Activity in Anaerobic Conditions

The remediation of uranium from soils and groundwater at Department of Energy (DOE) sites across the United States represents a major environmental issue, and bioremediation has exhibited great potential as a strategy to immobilize U in the subsurface. The bioreduction of U(VI) to insoluble U(IV) uraninite has been proposed to be an effective bioremediation process in anaerobic conditions. However, high concentrations of nitrate and low pH found in some contaminated areas have been shown to limit the efficiency of microbial reduction of uranium. In the present study, nonreductive uranium biomineralization promoted by microbial phosphatase activity was investigated in anaerobic conditions in the presence of high nitrate and low pH as an alternative approach to the bioreduction of U(VI). A facultative anaerobe, Rahnella sp. Y9602, isolated from soils at DOE's Oak Ridge Field Research Center (ORFRC), was able to respire anaerobically on nitrate as a terminal electron acceptor in the presence of glycerol-3-phosphate (G3P) as the sole carbon and phosphorus source and hydrolyzed sufficient phosphate to precipitate 95% total uranium after 120 hours in synthetic groundwater at pH 5.5. Synchrotron X-ray diffraction and X-ray absorption spectroscopy identified the mineral formed as chernikovite, a U(VI) autunite-type mineral. The results of this study suggest that in contaminated subsurfaces, such as at the ORFRC, where high concentrations of nitrate and low pH may limit uranium bioreduction, the biomineralization of U(VI) phosphate minerals may be a more attractive approach for in situ remediation providing that a source of organophosphate is supplied for bioremediation.