Nonreductive biomineralization of uranium(VI) as a result of microbial phosphatase activity

Uranium contamination of soils and groundwater at Department of Energy facilities across the United States is a primary environmental concern and the development of effective remediation strategies is a major challenge. Bioremediation, or the use of microbial enzymatic activity to facilitate the remediation of a contaminant, offers a promising in situ approach that may be less invasive than traditional methods, such as pump and treat or excavation.;Environmental conditions at uranium contaminated waste sites impose unique biogeochemical obstacles that must be addressed to successfully remediate uranium contamination. Low pH and high nitrate concentrations in soils and groundwater affect microbial activity and the speciation and mobility of uranium. Microorganisms that are able to survive in such conditions may possess unique abilities that can be exploited in developing new bioremediation strategies. The overall objective of this research is to explore new ways to promote indigenous bacterial processes in contaminated soils and groundwater that immobilize uranium at low pH and high nitrate concentrations.;This study demonstrates for the first time the successful biomineralization of uranium phosphate minerals as a result of microbial phosphatase activity at low pH in both aerobic and anaerobic conditions using pure cultures and soils from a contaminated waste site. Pure cultures of microorganisms isolated from soils of a low pH, high uranium- and nitrate-contaminated waste site, expressed constitutive phosphatase activity in response to an organophosphate addition in aerobic (Rahnella sp. and Bacillus sp.) and anaerobic (Rahnella sp.) incubations. Sufficient phosphate was hydrolyzed to precipitate 73 to 95% total uranium as chernikovite, an autunite-type uranium phosphate mineral, identified by synchrotron X-ray absorption spectroscopy and X-ray diffraction. Highest rates of uranium precipitation and phosphatase activity were observed between pH 5.0 and 7.0.;Indigenous microorganisms were also stimulated by organophosphate amendment in soils from a contaminated waste site using flow-through reactors. A continuous supply of organophosphate, nitrate, and uranium in synthetic groundwater of the same composition as that at the site was pumped through both high and low pH soils for 30 and 75 days. High phosphate concentrations (0.5 to 3 mmol L-1) in pore water effluents were observed within days of organophosphate addition and throughout the course of the experiment. Highest rates of phosphatase activity occurred at pH 5.5 in naturally low pH soils in the presence of high uranium and nitrate concentrations. Uranyl phosphate precipitation occurred in organophosphate-amended soils at pH 5.5 and 7 as a result of the favorable reaction between negatively-charged phosphate and positively-charged uranyl. The precipitation of uranium phosphate in both soils was identified by a combination of pore water measurements, solid phase extractions, synchrotron-based X-ray spectroscopy, and a reactive transport model.;The results of this study demonstrate that uranium is biomineralized to a highly insoluble uranyl phosphate mineral as a result of enzymatic hydrolysis of an organophosphate compound as sole carbon and phosphorus source over a wide range of pH, in both aerobic and anaerobic conditions, and in the presence of high uranium and nitrate concentrations. The nonreductive biomineralization of U(VI) provides a promising new approach for in situ uranium bioremediation in low pH, high nitrate, and aerobic conditions that could be complementary to U(VI) bioreduction in high pH, low nitrate, and reducing environments.