Sorption behavior of cesium on calcium silicate hydrate gel formed as secondary mineral under the coexistence condition of sulfate ions

In the assessment of radionuclide migration underground, the Ca/Si ratio of calcium silicate hydrate (C-S-H) gel formed as a secondary mineral is a key factor, because the sorption behavior of cationic nuclides such as cesium on the C-S-H gel depends on the Ca/Si ratio. In addition, this Ca/Si ratio of the C-S-H gel is changed by the formation of various compounds upon the reaction of Ca ions leaching from the C-S-H gel with coexisting anions. In this study, the effect of sulfate ions (up to 500 mM) on the sorption of Cs ions onto the C-S-H gel (Ca/Si ratios of 0.4, 0.8, 1.2 and 1.6) was examined and compared with that of carbonate ions. The obtained results show that the sorption ratio of Cs ions on the C-S-H gel slightly decreased with the increase in the concentration of sulfate ions under any Ca/Si ratio and that the effect of the copresence of sulfate ions on the decrease in the Ca/Si ratio of the C-S-H gel was extremely small compared with that of carbonate ions. That is, the C-S-H gel formed as a secondary mineral contributed to the retardation of the migration of Cs ions, even though the plume containing a high concentration of sulfate ions flowed around the repository. © 2021 Atomic Energy Society of Japan, All Rights Reserved.