title: Microbial reduction of U(VI) under alkaline conditions: implications for radioactive waste geodisposal.
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.
authors.affiliation: Research Centre for Radwaste Disposal and Williamson Research Centre for Molecular Environmental Science, School of Earth, Atmospheric and Environmental Sciences, The University of Manchester , Manchester, M13 9PL, United Kingdom.
authors.forename: Adam J
authors.initials: AJ
authors.lastname: Williamson
authors.forename: Katherine
authors.initials: K
authors.lastname: Morris
authors.forename: Gareth T W
authors.initials: GT
authors.lastname: Law
authors.forename: Athanasios
authors.initials: A
authors.lastname: Rizoulis
authors.forename: John M
authors.initials: JM
authors.lastname: Charnock
authors.forename: Jonathan R
authors.initials: JR
authors.lastname: Lloyd
mesh_terms: D065638:Absorption, Radiation; D041963:Bacteroidetes / Q000235:genetics / Q000378:metabolism*; D001483:Base Sequence; D001673:Biodegradation, Environmental; D004739:England; D005290:Ferric Compounds / Q000378:metabolism; D052203:Ferrosoferric Oxide / Q000378:metabolism; D019015:Geologic Sediments / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008969:Molecular Sequence Data; D009566:Nitrates / Q000378:metabolism; D010084:Oxidation-Reduction; D012336:RNA, Ribosomal, 16S / Q000235:genetics; D011850:Radioactive Waste / Q000032:analysis*; D017422:Sequence Analysis, DNA; D014501:Uranium / Q000737:chemistry / Q000378:metabolism*; D056928:X-Ray Absorption Spectroscopy
chemical_list: D005290:Ferric Compounds; D009566:Nitrates; D012336:RNA, Ribosomal, 16S; D011850:Radioactive Waste; D014501:Uranium; C092844:ferric oxyhydroxide; D052203:Ferrosoferric Oxide