The anthropogenic use of uranium has resulted in widespread uranium contamination of groundwater and sediment. Microbial reduction of soluble U(VI) to insoluble U(IV) can strongly affect the mobility and partitioning of uranium in the environment, and it has also been viewed as a promising strategy for the remediation of uranium contaminated sites. However, the microbial reduction of uranyl peroxide nanoclusters, a class of newly-discovered uranium compounds that may be prevalent in peroxide-generating geologic and/or engineered systems, has not been studied. In this study, we conducted a series of reduction experiments using a representative uranyl peroxide nanocluster, U60 (K16Li44[UO2(O2)OH]60) and a uranium-reducing bacterium, Shewanella oneidensis MR-1. The results show that U60 was successfully reduced by Shewanella oneidensis MR-1 in the absence of O2, and that the U(IV) that is produced from the reduction reaction can be fully re-oxidized after exposure to air. Although S. oneidensis can reduce U60, the reduction kinetics is significantly slower than that for aqueous uranyl acetate, and at least half of the U(VI) present in the U60 nanoclusters was unavailable for reduction during the course of our experiments. Our results indicate that U60 aggregates bigger than 450 nm formed immediately upon mixing with the bacterial growth medium, and that these aggregates were gradually broken down during the process of reduction. Neither reduction nor dissolution of U60 was observed during 72 hours of control experiments in the air using the same conditions as the reduction experiments, indicating that the breakdown of U60 aggregates is likely caused by the reduction of U60, and that Shewanella oneidensis MR-1 is able to directly reduce the U(VI) within the U60 nanoclusters, likely through adsorption of the nanocluster onto the bacterial cell envelope. This study is first to show the reduction capacity of bacteria for uranium nanoclusters, and will result in a better understanding and prediction of the long term fate of uranium in the environment.

Microbial reduction of U60 nanoclusters by Shewanella oneidensis MR-1

251st National Meeting (2016)

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