Transformation of Selenium Solid Phase in the Presence of Iron under Reducing Conditions

Abstract

To evaluate the long-term performance of a geological disposal system for high-level radioactive wastes (HLW), selenium-79 (Se) solubility and its solubility-limiting solid phase(s) were investigated in the presence of iron (Fe) under reducing conditions. Se is one of the key radionuclides in the safety assessment of a HLW repository because Se forms anions and has a low coefficient of sorption onto geological materials. The formation of Fe-Se solids is expected due tothe release of Fe(II) by Fe overpack corrosion. Experiments in both bentonite-equilibrated and pure waters were performed in the presence of Fe under reducing conditions. With bentonite (Kunigel V1®), the experimental conditions were within the bounds of the FeSe2(cr) stability field when plotted on an Eh-pH diagram; however, no Se solid phases could be identified in the experiments by X-ray diffraction (XRD). With pure water, the experimental conditions were in the same FeSe2(cr) stability field. After 1 month, the experimental system was under a transitional condition which moved toward true equilibrium from oversaturation and Se concentration decreased as a likely result of the precipitation of FeSe2(cr), although it remains possible that FeSe(cr) controlled Se concentration if the water had an initial pH = 7. Se(cr) could not control Se concentration because Se concentration decreased to below Se solubility, even though XRD identified Se(cr) as the dominant solid phase. After 2 and 3.5 months, the transformation from Se(cr) to the Fe-Se solid phase (FeSe2(cr), FeSe(cr)) could be identified by XRD.