Enzymatic reduction of U60 nanoclusters by Shewanella oneidensis MR-1

Abstract

Abstract In this study, a series of reduction experiments were conducted using a representative uranyl peroxide nanocluster, U60 (K16Li44[UO2(O2)OH]60) and a bacterial species, Shewanella oneidensis MR-1, that is capable of enzymatic U(VI) reduction. U60 was reduced by S. oneidensis in the absence of O2, but the reduction kinetics for U60 were significantly slower than was observed in this study for aqueous uranyl acetate, and were faster than was reported in previous studies for solid phase U(VI). Our results indicate that U60 aggregates bigger than 0.2 μm 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 h of control experiments open to the atmosphere, indicating that the breakdown and dissolution of U60 aggregates is caused by the reduction of U60, and that S. oneidensis is capable of direct reduction of the U(VI) within the U60 nanoclusters, likely due to the adsorption of U60 aggregates onto bacterial cells. This study is first to show the reduction capacity of bacteria for uranyl peroxide nanoclusters, and the results yield a better understanding of the long term fate of uranium in environmental systems in which uranyl peroxide nanoclusters are present.