Se immobilization in pyrite by reaction with Fe(II) and polysulfides; Impacts of aging conditions

Francisco, P. C. M.  ; Shiwaku, Hideaki   ; Ishidera, Takamitsu ; Tachi, Yukio  

The long-lived fission product Se-79 is a significant contributor to predicted radioactivity in geological repositories long after their closure. One possible retardation in repositories is by incorporation into Fe-sulfides like pyrite (FeS2). In this study, we clarify the role of polysulfides in the incorporation of Se into pyrite. Polysulfides are intermediate-oxidation sulfide species that are likely produced by diagenetic and microbial processes in repository host rocks. The present results show that reaction of Se(-II) with polysulfides resulted in the initial formation of elemental selenium. Aging at anoxic conditions led to the incorporation of Se into pyrite. These results highlight the role of polysulfides in retaining Se. We also investigate the effects of aging in air to determine the effects of changes in the redox conditions. The results show that oxidation did not affect Se retention.