Diffusion and sorption of Cs+, Na+, I− and HTO in compacted sodium montmorillonite as a function of porewater salinity: Integrated sorption and diffusion model

Abstract

Montmorillonite clay is an important component of barrier materials such as bentonites and argillaceous rocks for the safe geological disposal of radioactive waste. The diffusion and sorption behaviors of cationic Cs+ and Na+, anionic I− and neutral tritiated water (HTO) tracers in compacted montmorillonite were investigated as a function of porewater salinity by experimental and modeling approaches. The effective diffusivities of these tracers were measured by the through-diffusion method coupled with multiple curve analysis including depth profiles, and typical salinity-dependent cation excess and anion exclusion effects were shown. The distribution coefficients for Cs+ and Na+ obtained by through-diffusion tests were sensitive to salinity because of competitive ion exchange and were in good agreement with those obtained by batch sorption tests. However, trends in the apparent diffusivities of Cs+ and Na+ were mostly independent of salinity.A diffusion model based on a simplified pore structure and electrical double layer theory, describing the change of ionic concentration and viscoelectric effects caused by the electrostatic interaction with negatively charged clay surfaces, could quantitatively account for the cation excess and anion exclusion behaviors and their salinity dependence. The trends in the distribution coefficients predicted by the ion exchange sorption model show good agreement with the measured values as a function of salinity. Salinity-independent apparent diffusivities could be interpreted as results of the coupled effects of the drastic dependence of diffusion and sorption on salinity. The difference in the diffusion and sorption behaviors between Cs+ and Na+ can be explained by the difference in the sorption mechanism (inner- and outer-sphere) and their representation in the diffusion model (cation excess diffusion). This integrated sorption and diffusion model was further demonstrated as being applicable to the diffusion data of these monovalent cations and anions in compacted bentonites over a wide range of compaction.