Se(-II, IV) interactions with Fe(II) species and phases under anoxic and reducing conditions

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

Selenium-79 (Se-79) will be a significant contributor to the long-term radiologic hazards in geological repositories for high-level wastes due to its long half-life. Thus, understanding its behavior in anoxic and reducing subsurface environments prevalent in geological repositories is a key concern in repository safety assessments. There is limited information on primary reaction products arising from the interaction of aqueous Se species with either aqueous or solid Fe(II) phases, which are present in either as geological repositories as products of steel overpack corrosion or as accessory phases in buffer materials or host rocks. In this work, we investigated the interaction of aqueous Se(-II) and Se(IV), potentially the most mobile Se species in repositories, with aqueous Fe(II) and solid Fe(OH) at circumneutral and alkaline conditions, respectively. Using electron microscopy and XAFS spectroscopy, we identified beta-FeSe as the primary reaction product at circumneutral and alkaline conditions between Se(-II) and either aqueous Fe(II) and Fe(OH). For Se(IV), elemental Se(0) was the main reaction product at circumneutral conditions while iron selenides only became dominant at alkaline conditions; these results show rapid and efficient reduction of Se(IV). Aging the solids at 90C for 1 week induced the transformation of beta-FeSe and elemental Se(0) to FeSe at circumneutral conditions, while a proportion of Se was released back into solution at alkaline conditions. These results provide constraints on the initial behavior of Se upon release from the waste, and may be useful in validating long-term thermodynamic predictions. *This work was part of "The project for validating near-field assessment methodology in geological disposal (FY2021, Grant Number: JPJ007597)" supported by the Ministry of Economy, Trade and Industry of Japan.