Anisotropy in diffusion and activation energies of I- and Cs+ ions in compacted smectite

Sato, Haruo

The anisotropies in the diffusion and activation energies (Ea) of I- and Cs+ ions in compacted smectite and the effect of salinity on the diffusion of both ions were studied. A Na-smectite prepared by ion-exchanging with Na+ ions a Na-bentonite, was used. The diffusion experiments were performed as a function of smectite's dry density, salinity, temperature and diffusion direction to the orientated direction of smectite particles. Considering electrostatic effect from the surface of smectite aggregates and the change in tortuosity on dry density, salinity and diffusion direction, I- ions are presumed to mainly diffuse in interstitial pores, and Cs+ ions are presumed to diffuse in both interlayer and interstitial pores. The change in Ea-values on dry density is considered to be due to the effct of the activity of porewater in addition to the effect of electrostatic effect from smectite surface. Particularly, the diffusion behaviour of Cs+ ions can be explained by considering ion exchange enthalpy between Cs+ and Na+ ions at low-dry density, but it is presumed to be affected by both the decrease in the activity of porewater and the effect of the ion exchange enthalpy at high-dry density.