Diffusion Behavior of Ca 2+ Ions in Compacted Na-Montmorillonite
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Tamotsu Kozaki, Yoshifusa Adachi, Koichi Inada, Seichi Sato and Hiroshi Ohashi Division of Quantum Energy Engineering, Graduate School of Engineering, Hokkaido University, Sapporo, 060-8628, Japan
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.
INTRODUCTION Bentonite is a candidate buffer material for geological disposal of high-level radioactive waste [1]. The evaluation of the long-term stability of a bentonite buffer is therefore required for the safety assessment of the geological disposal of such waste. An important factor that affects the long-term stability is the alteration of Na-bentonite into Ca-bentonite, which could reduce the performance of a bentonite buffer. Since the alteration could be caused by the introduction of Ca2+ ions to the buffer from cementitious materials and/or groundwater in a repository, the diffusion behavior of Ca2+ ions in Na-bentonite must be understood to grasp the impact. The authors believe that there are few reports addressing the diffusion coefficients of Ca2+ ions in highly compacted
bentonite [2]. In this study, therefore, the diffusion coefficients of Ca2+ ions were experimentally determined in compacted Na-montmorillonite, which is the major clay mineral of bentonite. Another aim of this study was to investigate the diffusion mechanisms of radionuclides in compacted bentonite. Although Ca2+ ions are not important for safety analysis of geological disposal, they can be used as analogues for other divalent-cations. In previous papers, the unique dry density dependences of activation energies were described for Cs+ [3], Na+ [4], Sr2+ [5], and Clions [6, 7]. The pore-water diffusion model assumes that ions diffuse in the pore water of bentonite. However, the results of these previous works cannot be explained by this model since the same value of activation energy as that in free water can be expected if the ions diffuse in the pore water. In thi
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