Solubility measurement of iron-selenium compounds under reducing conditions

Kitamura, Akira  ; Shibata, Masahiro ; Kitao, Hideo*

Chemical behavior of selenium(Se), which was one of the important elements for performance assessment of geological disposal of high-level radioactive waste, was investigated under reducing and iron-containing conditions. A washing method for an iron diselenide(FeSe(cr)) reagent with acidic and basic solutions (0.1 and 1 M HCl and 1 M NaOH) was carried out for the purification of FeSe reagent, which was considered to be a solubility limiting solid for Se under the geological disposal conditions. Furthermore, solubility of FeSe(cr) was measured in alkaline solution (pH: 11 - 13) under reducing conditions (E vs SHE: -0.4 - O V), and thermodynamic data on equilibrium reactions between Se in solution and Se precipitate were obtained. The dependencies of solubility values on pH and redox potential (Eh: vs. standard hydrogen electrode) were best interpreted that the solubility limiting solid was not FeSe(cr) but Se(cr) and the aqueous species was SeO in the present experimental conditions. The equilibrium constant between Se(cr) and SeO at zero ionic strength was determined and compared with literature values. The chemical behavior of Se under geological disposal conditions was discussed.