Experimental study on the solubility of selenium under simulated disposal conditions

Tachikawa, Hirokazu*; Kitao, Hideo*; Katsurai, Kiyomichi*; Yanagisawa, Ichiro*; Shibata, Masahiro ; Suyama, Tadahiro*; Yui, Mikazu

In an evaluation of high level waste (HLW) repository performance, Se-79 is one of important elements to be analyzed. Selenium solubility and solubility limiting solid phase is not clear. Then, we performed solubility measurement tests from over saturation direction under reducing conditions considering the repository conditions in deep underground. In some cases, bentonite (Kunigel V1) or pyrite coexisted in the experimental system to simulate the repository conditions. Se bearing solids were determined by XRD analysis, and solubility limiting solid phase was discussed. FeSe (Ferroselite) and Se (hexagonal) were identified in the simple condition test, in which Fe(II) solution and Se solution were mixed. SeS was also identified when S(-II) solution was added. The Se concentrations in aqueous phase were approximately 10 mol/l at neutral pH and approximately 10 mol/l at pH9 in the bentonite coexisting tests and pyrite coexisting tests. The solid phases identified in the pyrite coexisting tests were mainly Se(hexagonal) and FeSe (Ferroselite) in one of the samples. Further, the possibility of Fe(S,Se) solid solution formation was presumed on the pyrite surface dipping in the test solutions. In addition, we performed another selenium solubility measurement to accelerate the transformation of Se bearing solid phase at elevated temperature (80C). The concentration of Se decreases with time and reached to the detection limit of ICP-MS (410 mol/1) in 3 months. At first, Se(hexagonal) is dominant in the precipitation, however this solid phase was gradually transformed to Fe-Se solids (FeSe, FeSe) with time. Therefore it is strongly suggested that FeSe which is the thermodynamically most stable phase will be a solubility limiting solid phase under repository conditions in long term. As the experimental system was confirmed as sulfate reducing bacteria free, it should be noted that whole observed reactions ...