The influence of pH and reaction time on the formation of FeSe2 upon selenite reduction by nano-sized pyrite-greigite

Abstract The influence of pH and reaction time on the formation of FeSe 2 by reductive precipitation of Se(IV) with nano-sized pyrite-greigite was investigated. Reductive precipitation is an effective method of attenuating the mobility of 79 Se, which is foreseen to be a dangerous radioisotope for the geological disposal of high-level radioactive waste (HLW). The results indicated that Se(0)was formed at pH <4.05, whereas, at pH > 6.07, considerable amount of FeSe 2 was formed along with Se(0). These observations are in agreement with the thermodynamic predictions reported in this work. Furthermore, the formation of FeSe 2 was found to continue by increasing the reaction time, indicating that the Se(0) formed in the early reaction stage is gradually transformed to FeSe 2 upon the depletion of aqueous Se(IV). Since FeSe 2 has a stronger reactivity than pyrite, it was proposed that greigite, rather than pyrite, was responsible for the formation of FeSe 2 . The findings in this study are of interest for key geochemical processes governing the mobility of toxic 79 Se in the environment in presence of iron sulfides.