The reduction of selenite by Fe(II)-containing minerals

Abstract

The radioactive isotope 79Se, with a half-life of 2.95×105 years, is presently considered as the key mobile fission product for the disposal of spent fuel and high-level radioactive waste. The solubility of selenium is significantly controlled by its oxidation state, which depends on the redox conditions of the surrounding environment. Tetravalent and hexavalent Se are very soluble and mobile aqueous oxyanions, while Se species with oxidation states 0, -I, and -II are low solubility solids. Due to the weak adsorption of Se(IV) and Se(VI) on natural minerals, in particular on granite or claystone minerals, which are considered as host rocks for nuclear waste disposal, reductive precipitation is considered to be the most effective way to immobilize 79Se. On the other hand, Fe(II)-bearing minerals (such as pyrite, pyrrhotite, magnetite, etc.) are ubiquitous in geological environment. Due to the reducing capacity, reduction of selenite by Fe(II)-containing minerals has received extensive attention worldwide. In this paper, we gave a critical review on the research progress in this field, along with the thermodynamic calculations on the reaction feasibility and the controlling-factor of the reaction products.