Diffusion Behavior of Ca²⁺ Ions in Compacted Na-Montmorillonite

Abstract

Diffusion behavior of Ca²⁺ ions in water-saturated, compacted Na-montmorillonite was studied for the safety assessment of geological disposal of high-level radioactive waste. The diffusion coefficients of Ca²⁺ ions in compacted Na-montmorillonite obtained in this study were from 1.7×10^-11 to 6.0×10^-12 m² s^-1 with increasing dry density of 1.0 to 1.8 Mg m^-3. These values were approximately a quarter those of Na⁺ ions at each dry density. This suggests that the diffusion of Ca²⁺ ions could be the rate-determining mechanism for the alteration of Na-montmorillonite into Ca-montmorillonite. The activation energy for diffusion of Ca²⁺ ions was almost equal to that in free water (17.3 kJ mol^-1) at dry densities of 1.0 to 1.6 Mg m^-3. However, the activation energy suddenly increased to 25.1 kJmol^-1 at a dry density of 1.8 Mg m^-3. This increase cannot be explained using a single-diffusion process model, such as the pore-water diffusion model. Diffusion behavior of Ca²⁺ ions in compacted montmorillonite along with the experimental data on the basal spacing and water content of montmorillonite, and the activation energy for diffusion of Ca²⁺ and several other ions are discussed.