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

Abstract

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.

Keywords:

• biomineralization
• bioremediation
• microbial phosphatase
• uranium (VI)

This research was supported by the Office of Science (BER), U. S. Department of Energy Grant No. DE-FG02-04ER63906. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a national user facility operated by Stanford University on behalf of the U.S. 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. We thank Hong Yi at the Emory School of Medicine Electron Microscopy Core for transmission electron microscopy sample preparation and imaging and Dr. Joan S. Hudson at the Clemson University Electron Microscope Facility for variable-pressure scanning electron microscopy imaging, transmission electron microscopy imaging, and EDX analyses. We also thank the two anonymous reviewers for their valuable comments to improve the manuscript.

Notes

∗MS denotes multiple scattering paths.

^†PO dist is the distance between the P-O in phosphate coordination (used for the MS paths).