Activation energy for diffusion of chloride ions in compacted sodium montmorillonite

Diffusion of chloride ions in compacted montmorillonite was studied for a performance assessment as buffer material in geological disposal of high-level radioactive waste. The apparent diffusion coefficients were from 6.8×10−12 to 2.2×10−10 m2 s−1 for sodium montmorillonite at dry densities from 1.0 to 1.8 Mg m−3 at diffusion temperatures from 278 to 323 K. The activation energy increased from 14 kJ mol−1 (lower than that in free water, 17.4 kJ mol−1) to 25 kJ mol−1 (higher than that in free water), as the dry density increased from 1.0 to 1.8 Mg m−3. These facts can be attributed to changes in the predominant diffusion process as dry density increases. The dry density dependence of the activation energy cannot be explained by conventional diffusion models, such as the pore water diffusion model.