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

Abstract

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 M4-edge high-energy resolved fluorescence detection–X-ray absorption near-edge structure (XANES), and U L3-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.

Controlling the Reduction of Chelated Uranyl to Stable Tetravalent Uranium Coordination Complexes in Aqueous Solution

“…The chemically-mediated reduction of U, from +VI to +IV, is known in synthetic schemes, 25−27 biological systems, 28 and the environment, 29 yet the reduction is complicated by the known instability of U IV in the presence of molecular oxygen. 27,30 This complication makes the U VI /U IV redox couple hard to evaluate and has thus far limited the applicability of chelation-driven reduction as a means for U decorporation, although recent efforts have elucidated some details about U two-electron processes in both aqueous and nonaqueous systems.…”

Biological Reduction of a U(V)–Organic Ligand Complex

“… 19 Direct experimental evidence of uranyl(V) arose from EXAFS analysis of the incubation supernatants of G. sulfurreducens , 18 and recently from M 4 -edge HR-XANES on cell suspensions of S. oneidensis MR-1. 22 As for the second step of the biological reduction, these studies support the disproportionation of uranyl(V), which leads to U(VI) and U(IV). In fact, Renshaw et al justify uranyl(V) disproportionation in their system by substituting stable Np(V) to U(V) in their experiment and reporting its lack of reduction.…”

“… 18 By analogy, the authors conclude that U(V) would not either, and thus that U(V) disproportionates to form U(IV). Such observations were later supported by Jones et al 21 and more recently by Vettese et al, 22 who report a sawtooth profile for uranyl(VI) fluorescence in cell suspensions of G. sulfurreducens and S. oneidensis MR-1, respectively. 21 , 22 The repeated increases in the U(VI) fluorescence intensity, forming the sawtooth profile, are interpreted as U(V) disproportionation releasing uranyl(VI).…”

“… 10 − 13 In both mineral and biological cases, a pentavalent U(V) intermediate has been shown to form following a one-electron transfer to the U(VI) moiety. 14 − 22 Furthermore, U(V) appears to persist under certain environmentally relevant conditions; 14 , 15 however, it remains challenging to detect it, due to its sensitivity to oxidation and its ability to undergo rapid disproportionation to U(VI) and U(IV), through the interaction of two uranyl(V) cations (i.e., cation–cation interaction, CCI). 23 …”

“…However, these experiments were conducted in carbonate-rich buffer 18 , 22 in which the disproportionation of U(V) is rapid at circumneutral pH. 24 In the environment, biological U(VI) reduction often occurs in organic-rich environments.…”