Uranium Redox Cycling in Sediment and Biomineral Systems

Abstract

Under anaerobic conditions, uranium solubility is significantly controlled by the microbially mediated reduction of relatively soluble U(VI) to poorly soluble U(IV). However, the reaction mechanism(s) for bioreduction are complex with prior sorption of U(VI) to sediments significant in many systems, and both enzymatic and abiotic U(VI) reduction pathways potentially possible. Here, we describe results from sediment microcosm and Fe(II)-bearing biomineral experiments designed to assess the relative importance of enzymatic vs. abiotic U(VI) reduction mechanisms and the long-term fate of U(IV). In oxic sediments representative of the UK Sellafield reprocessing site, U(VI) was rapidly and significantly sorbed to surfaces and during microbially-mediated bioreduction, XAS analysis showed that sorbed U(VI) was reduced to U(IV) commensurate with Fe(III)-reduction. Additional control experiments with Fe(III)-reducing sediments that were sterilized after bioreduction and then exposed to U(VI), indicated that U(VI) reduction was inhibited, implying that enzymatic as opposed to abiotic mechanisms dominated in these systems. Further experiments with model Fe(II)-bearing biomineral phases (magnetite and vivianite) showed that significant U(VI) reduction occurred in co-precipitation systems, where U(VI) was spiked into the biomineral precursor phases prior to inoculation with Geobacter sulfurreducens. In contrast, when U(VI) was exposed to pre-formed, washed biominerals, XAS analysis indicated that U(VI) was recalcitrant to reduction. Reoxidation experiments examined the long-term fate of U(IV). In sediments, air exposure resulted in Fe(II) oxidation and significant U(IV) oxidative remobilization. By contrast, only partial oxidation of U(IV) and no remobilization to solution occurred with nitrate mediated bio-oxidation of sediments. Magnetite was resistant to biooxidation with nitrate. On exposure to air, magnetite changed from black to brown in colour, yet there was limited mobilization of uranium to solution and XAS confirmed that U(IV) remained dominant in the oxidized mineral phase. Overall these results highlight the complexity of uranium biogeochemistry and highlight the importance of mechanistic insights into these reactions if optimal management of the global nuclear legacy is to occur.

Keywords:

• uranium
• sediment
• bioreduction
• biomineral
• redox

ACKNOWLEDGMENTS

We thank Rob Mortimer, Lesley Neve, James Begg, and Dave Hatfield (University of Leeds), Bob Bilsborrow (SRS Daresbury), and Christopher Boothman (University of Manchester) for help with data acquisition, and Vicky Coker (University of Manchester) and Mike Kelly (University of Leeds) for ESEM and TEM images. This work was supported by the UK Natural Environment Research Council (NERC) grants (NE/D00473X/1 & NE/D005361/1) and by STFC beam-time awards at SRS Daresbury.

Current affiliation for A. Geissler: Forschungszentrum Dresden-Rossendorf, Dresden Germany

Current affiliation for J. M. McBeth: Bigelow Laboratory for Ocean Sciences, West Boothbay Harbour, Maine, USA