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Diffusion Behavior of Ca2+ Ions in Compacted Na-Montmorillonite

Published online by Cambridge University Press:  21 March 2011

Tamotsu Kozaki,
Yoshifusa Adachi,
Koichi Inada,
Seichi Sato and
Hiroshi Ohashi
Tamotsu Kozaki
Affiliation:
Division of Quantum Energy Engineering, Graduate School of Engineering, Hokkaido University, Sapporo, 060-8628, Japan
Yoshifusa Adachi
Affiliation:
Division of Quantum Energy Engineering, Graduate School of Engineering, Hokkaido University, Sapporo, 060-8628, Japan
Koichi Inada
Affiliation:
Division of Quantum Energy Engineering, Graduate School of Engineering, Hokkaido University, Sapporo, 060-8628, Japan
Seichi Sato
Affiliation:
Division of Quantum Energy Engineering, Graduate School of Engineering, Hokkaido University, Sapporo, 060-8628, Japan
Hiroshi Ohashi
Affiliation:
Division of Quantum Energy Engineering, Graduate School of Engineering, Hokkaido University, Sapporo, 060-8628, Japan
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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.

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References

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