Diffusion modeling in compacted bentonite based on modified Gouy-Chapman model

Yotsuji, Kenji; Tachi, Yukio  ; Nishimaki, Yuichiro*

The integrated sorption and diffusion (ISD) model has been developed to quantify these processes in compacted bentonite. The ISD model assuming averaged narrow porespace and the electric double layer (EDL) theory could give quantitative agreement with diffusion data of monovalent cations and anions under wide range of conditions. However, the systematic disagreements were observed for multivalent cations, anions and complex species. In this study, the excluded volume effect and the dielectric saturation effect were introduced into the current ISD model and the modified Poisson-Boltzmann equations were numerically solved. The ionic distributions were influenced at the near surface, however, these changes were canceled by averaging for the porespace. As a result, these modified models had little effect on the effective diffusivity. On the other hand, the modified model considering hydrated ions with the effective electric charge could give good reproducibility of diffusion data.