Multiple Lines of Evidence Identify U(V) as a Key Intermediate during U(VI) Reduction by MR1

As the dominant radionuclide by mass in many radioactive wastes, the control of uranium mobility in contaminated environments is of high concern. U speciation can be governed by microbial interactions, whereby metal-reducing bacteria are able to reduce soluble U(VI) to insoluble U(IV), providing a method for removal of U from contaminated groundwater. Although microbial U(VI) reduction is widely reported, the mechanism(s) for the transformation of U(VI) to relatively insoluble U(IV) phases are poorly understood. By combining a suite of analyses, including luminescence, U M$_{4}$-edge high-energy resolved fluorescence detection–X-ray absorption near-edge structure (XANES), and U L$_{3}$-edge XANES/extended X-ray absorption fine structure, we show that the microbial reduction of U(VI) by the model Fe(III)-reducing bacterium, Shewanella oneidensis MR1, proceeds via a single electron transfer to form a pentavalent U(V) intermediate which disproportionates to form U(VI) and U(IV). Furthermore, we have identified significant U(V) present in post reduction solid phases, implying that U(V) may be stabilized for up to 120.5 h.