Effect of exchangeable cations on apparent diffusion of Ca2+ ions in Na- and Ca-montmorillonite mixtures

Compacted Na-bentonite, of which the major mineral is montmorillonite, is a candidate buffer material for the geological disposal of high-level radioactive waste. A potential alteration of the bentonite in a repository is the partial replacement of the exchangeable cations of Na+ with Ca2+. The Ca2+ cations could be released from cementitious materials and diffuse into the buffer material in the repository. In this study, to evaluate the alteration that could reduce the performance of the bentonite buffer, the apparent diffusion coefficients of HTO and Ca2+ ions were determined from non-steady, one-dimensional diffusion experiments using Na- and Ca-montmorillonite mixtures with different ionic equivalent fractions of Ca2+ ions. The apparent diffusion coefficient of HTO at a dry density of 1.0 Mg m−3 slightly increased with an increase in the ionic equivalent fraction of Ca2+ ions. However, the apparent diffusion coefficient of Ca2+ and the activation energy for diffusion at the same dry density were independent of the ionic equivalent fraction of Ca2+ ions. These findings suggest that unlike HTO, which can be postulated to diffuse mainly in pore water, Ca2+ ion diffusion could occur predominantly in interlayer spaces, of which the basal spacing was determined to be constant by the XRD technique.