Sorption and diffusion of Ni in compacted montmorillonite; Effects of pH and salinity of porewater

Tachi, Yukio  ; Yotsuji, Kenji; Suyama, Tadahiro; Nakazawa, Toshiyuki*; Shimoda, Satoko*; Kuno, Yoshio*

Sorption and diffusion of radionuclides in compacted bentonite are key processes in the safe geological disposal. The effects of porewater pH and salinity on Ni sorption and diffusion in compacted montmorillonite were investigated by experimental and modeling approaches. Distribution coefficients (Kd) and effective diffusion coefficients (De) of Ni in montmorillonite compacted to dry density of 800 kg/m were measured under four conditions with different pH (6, 8) and salinities (0.05, 0.5 M NaCl). The Kd increased with increasing pH and with decreasing salinity of porewater, which are consistent with batch-derived Kd trends. The De values changed with porewater salinity, which are independent of pH. Sorption and diffusion behaviors were interpreted by coupling the thermodynamic sorption model, and the diffusion model based on EDL theory in narrow pores. The mechanistic model could be useful in predicting the sorption and diffusion of high sorbing species in compacted systems.