The Biogeochemical Fate of Se(VI) in Bentonite Systems Relevant to the Disposal of High-Level Radioactive Waste

Selenium-79, a radionuclide present in higher-activity radioactive wastes destined for deep geological disposal, is mobile under oxic conditions where Se(IV) and Se(VI) dominate. Anoxic batch microcosm incubations were constructed containing Wyoming MX80 bentonite (a candidate buffer material in geological disposal), and artificial groundwater with or without steel coupons to represent canister materials. Se(VI)(aq) bioreduced and removed by 7 days when lactate was added as an electron donor, after which sulfate reduction occurred. With H2 gas as the electron donor, Se(VI) bioreduction slowed, with complete removal at 14 days and minimal sulfate reduction thereafter. 16S rRNA gene sequencing highlighted the dominance of Anaerobacillus spp. (44 % at 28 days) during Se(VI)-reduction, and in the lactate-amended systems there was a subsequent enrichment in sulfate-reducing bacteria affiliated with Desulfosporosinus spp. (60% relative abundance at 84 days). Extended x-ray absorption fine structure (EXAFS) analyses identified monoclinic Se(0) as the bioreduction product after 28 days, but by 84 days this evolved to trigonal Se(0) in the absence of steel coupons, or was further reduced to FeSe2 with steel present. The reduction of Se(VI)(aq) to poorly soluble Se(0)/FeSe2 mediated by indigenous bentonite microbial communities highlights their potential importance in promoting Se-79 retention during deep geological disposal.