Diffusion and sorption of Sr in compacted sodium montmorillonite as a function of porewater salinity

Tachi, Yukio  ; Yotsuji, Kenji

Sorption and diffusion of radionuclides in compacted bentonite are key processes in the safe geological disposal. The effects of porewater salinity on Sr diffusion and sorption in compacted montmorillonite were investigated by experimental and modeling approaches. Effective diffusion coefficients (De) and distribution coefficients (Kd) of Sr in montmorillonite compacted to dry density of 800 kg/m were measured under four different salinities (0.01-0.5 M NaCl). The De decreased drastically with increasing porewater salinity. The Kd values decreased with increasing salinity, which are consistent with Kd values obtained by batch sorption. These results were interpreted by the ISD model coupling the thermodynamic sorption model and the EDL diffusion model in narrow pores. The ISD model can be basically applicable for divalent cations, however is needed to modify for better agreement at low salinity conditions by considering additional factors.