Diffusion mechanism of cesium ions in compacted montmorillonite

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

“…for Ca-montmorillonite (Kozaki et al, 1999a). The specific surface areas determined by the BET method have a clear difference between Na-and Ca-montmorillonite, whereas there is a small difference in the specific surface areas determined by the EGME method.…”

“…and the previous studies (Ca 2+ ionic equivalent fraction of 0, Kozaki et al, 2001a). (Kozaki et al, 2005) and Cs + (Kozaki et al, 1999a). Kozaki et al, 1999b, n is the number of data) Figure 5…”

“…That implies that the change in the apparent diffusion coefficient of HTO could be caused by the structural changes in the pore spaces rather than in the interlayer spaces where the basal spacing was found to remain constant. Table 2, and the dependency of these values on the Ca 2+ ionic equivalent fractions is shown in Figure 4, along with those of the activation energies for Na + diffusion (Kozaki et al, 2005) and Cs + diffusion (Kozaki et al, 1999a). The activation energy for Ca 2+ diffusion is found to be independent of the ionic equivalent fractions of Ca 2+ , although the activation energies for Na + and Cs + diffusion vary as a function of the Ca 2+ ionic equivalent fraction.…”

“…Therefore, for Na-and Ca-montmorillonite mixture samples, the degree of the contribution of each diffusion pathway to the total diffusivity of radionuclides could vary with the ionic equivalent fraction of Ca 2+ in the sample. For example, the dependency of the activation energy for Cs + diffusion on the ionic equivalent fraction of Ca 2+ (as indicated in Figure 5) was explained by assuming that the diffusion of Cs + in Ca-montmorillonite becomes predominant at the high ionic equivalent fractions of Ca 2+ (Kozaki et al, 1999a). Similarly, the alterations of the predominant diffusion process among three diffusion pathways were proposed as a possible explanation for the changes in the activation energy for Na + diffusion as a function of the Ca 2+ ionic equivalent fraction (Kozaki et al, 2005).…”

“…These predictions should be based on studies of the diffusivities of Ca 2+ ions in both the Na-bentonite and the altered bentonite. Although extensive studies have been performed for the diffusion of radionuclides in bentonite (Bourg et al, 2008, Glaus et al, 2007, Gonzalez-Sanchez et al, 2008, Kozaki et al, 2008, Melkior et al, 2009, there are few studies on the diffusivities of Ca 2+ ions in compacted bentonite (Kozaki et al, 2001a) and the effects of exchangeable cations on diffusion of radionuclides (Choi and Oscarson, 1996, Kozaki et al, 1999a, 2005 The diffusion mechanism of Ca 2+ ions in compacted montmorillonite discussed in this paper is based on these experimental data.…”