Diffusion behavior for Se and Zr in sodium-bentonite

Apparent diffusion coefficients for Se and Zr in bentonite were measured by in-diffusion method at room temperature using water-saturated sodium-bentonite. KunigelV1qq* containing 50wt% Na-smectite as a major mineral was used as the bentonite material. The experiments were carried out in the dry density range of 400-1800 kg/m³. Bentonite samples were immersed with distilled water and saturated before the experiments. The experiments for Se were carried out under N₂ atmospheric condition (O₂: 2.5 ppm). Those for Zr were carried out under aerobic condition. The apparent diffusion coefficients decrease with increasing density of the bentonite. Since dominant species of Se in the pore water is predicted SeO₃^2-, Se may be retarded by anion-exclusion because of negative charge on the surface of the bentonite and little sorption. The dominant species of Zr in the porewater is predicted Zr(OH₃)₅^- or HZrO₃^-. Distribution coefficient measured for Zr on the bentonite was about 1.0 m³/kg from batch experiment. Therefore, the retardation may be caused by combination of the sorption and the anion-exclusion. A modelling for the diffusion mechanisms in the bentonite were discussed based on an electric double layer theory. Comparison between the apparent diffusion coefficients predicted by the model and the measured ones shows a good agreement.