2013 Volume 52 Issue 1 Pages 9-22
Dissolution rates of montmorillonite in bentonite under hyperalkaline conditions have been obtained to evaluate the long-term performance of engineered barriers for radioactive waste disposal. The saturation state of pore water with respect to montmorillonite in bentonite has been demonstrated as an important factor controlling montmorillonite dissolution rate. Pore water chemistry including the saturation state may be significantly affected by the dissolution of accessory minerals in bentonite. The bentonite "Kunigel V1", which is being considered for use in radioactive disposal barriers in Japan, actually contains a large amount (〜50 %) of accessory silica minerals, such as chalcedony and quartz. Dissolution of the silica minerals may inhibit the dissolution of montmorillonite in the bentonite by increasing the silica concentration and hence the saturation state with respect to montmorillonite in the pore water. Therefore, the objectives of this study are to examine the dissolution kinetics of the silica minerals and the effect of dissolved silica on the dissolution of montmorillonite in a compacted bentonite using X-ray computed tomography (CT) and geochemical modeling. Advective alteration experiments of a compacted bentonite (Kunigel V1) with a dry density of 0.3 Mg/m^3 was conducted with 0.3M NaOH solution at 70℃ for 360 days. X-ray CT images, which were taken every 10 days, showed that the volume of a light colored material decreased as the interaction between the bentonite and hyperalkaline-fluid progressed during the experiments. This is attributed to the dissolution of accessory silica minerals in the bentonite. XRD analyses of altered bentonite after the experiments identified that the accessory mineral was mainly chalcedony. The kinetic data for dissolution of chalcedony was obtained by developing the methodology to quantify the volume of accessory minerals in the CT images. These results showed that chalcedony was almost completely dissolved in the area close to the fluid input within 80 days. The geochemical transport model consistent with the experimental results indicates that the pore water in the bentonite became near saturation with respect to montmorillonite due to the dissolution of silica minerals in bentonite, inhibiting the dissolution of montmorillonite in bentonite. The model also indicates that the inhibition of montmorillonite dissolution will not be sustained beyond the experimental duration under the same experimental conditions. However, a compacted bentonite with higher dry density such as 1.6 Mg/m^3, where diffusion is the dominant mass transport mechanism, has been considered for use in actual radioactive disposal barriers in Japan. In the much more compacted bentonite system, dissolution of montmorillonite will be inhibited for a much longer term. Therefore, it is important to consider the dissolution behavior of silica minerals to sufficiently evaluate the long-term performance of bentonite as a component of engineered barriers for radioactive waste disposal.
