Selenium reactions in ferrous and sulfide-rich environments; Effects of sulfide speciation

Francisco, P. C. M.  ; 石寺 孝充 ; 塩飽 秀啓   ; 菊池 亮佑*; 舘 幸男  

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

As a long-lived fission product, Se-79 is predicted to account for a significant proportion of radioactivity long after the closure of a geological repository for high-level wastes. Predicting its behavior for long-term safety assessment of repositories thus requires a detailed understanding of its reactions with various geological repository components. Interaction with ferrous and sulfide aqueous species and solid phases could help attenuate Se concentrations under anoxic and reducing conditions in repositories. We performed four sets of coprecipitation experiments where Se(-II) was reacted with aqueous Fe(II) and with either monosulfides or polysulfides, at Fe/Stot ratios of 1 and 0.5. The precipitates were aged under anoxic conditions at ~25C for up to 5 months. The monosulfide coprecipitation experiments produced mackinawite and efficiently sequestered Se(-II) from solutions. XANES analyses showed that Se retained its -II oxidation state and EXAFS fitting results were consistent with Se being incorporated in the mackinawite lattice. Uptake was similarly efficient in the polysulfide experiments, though lesser Se was sequestered than in the monosulfide experiments. Unlike in the monosulfide experiments, Se was initially oxidized and precipitated as elemental Se(0). In the experiment at Fe/S 1, Se became incorporated in pyrite. Despite the very low abundance of pyrite, it was clear that it hosted most of the Se. In summary, monosulfide coprecipitation sequesters and retains Se in mackinawite, while reaction with polysulfides sequesters Se first as Se(0), then incorporates it in pyrite. Because sulfide speciation is tied to redox conditions, the results provide fundamental insights into the processes governing Se retention across a broad range of redox environments, which may aid in predicting Se migration behavior in geological repositories.