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Microbial Reduction of U(VI) under Alkaline Conditions: Implications for Radioactive Waste Geodisposal

DOI: 10.1021/es5017125 DOI Help

Authors: Adam Williamson (University of Manchester) , Katherine Morris (University of Manchester) , Gareth Law (Centre for Radiochemistry Research and Research Centre for Radwaste and Decommissioning, University of Manchester, U.K.) , Athanasios Rizoulis (University of Manchester) , John Charnock (University of Manchester) , Jonathan R. Lloyd (The University of Manchester)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Environmental Science & Technology , VOL 48 (22) , PAGES 13549 - 13556

State: Published (Approved)
Published: November 2014
Diamond Proposal Number(s): 7367 , 7593 , 8070

Open Access Open Access

Abstract: Although there is consensus that microorganisms significantly influence uranium speciation and mobility in the subsurface under circumneutral conditions, microbiologically mediated U(VI) redox cycling under alkaline conditions relevant to the geological disposal of cementitious intermediate level radioactive waste, remains unexplored. Here, we describe microcosm experiments that investigate the biogeochemical fate of U(VI) at pH 10–10.5, using sediments from a legacy lime working site, stimulated with an added electron donor, and incubated in the presence and absence of added Fe(III) as ferrihydrite. In systems without added Fe(III), partial U(VI) reduction occurred, forming a U(IV)-bearing non-uraninite phase which underwent reoxidation in the presence of air (O2) and to some extent nitrate. By contrast, in the presence of added Fe(III), U(VI) was first removed from solution by sorption to the Fe(III) mineral, followed by bioreduction and (bio)magnetite formation coupled to formation of a complex U(IV)-bearing phase with uraninite present, which also underwent air (O2) and partial nitrate reoxidation. 16S rRNA gene pyrosequencing showed that Gram-positive bacteria affiliated with the Firmicutes and Bacteroidetes dominated in the post-reduction sediments. These data provide the first insights into uranium biogeochemistry at high pH and have significant implications for the long-term fate of uranium in geological disposal in both engineered barrier systems and the alkaline, chemically disturbed geosphere.

Subject Areas: Environment, Energy, Earth Science

Instruments: B18-Core EXAFS