Anisotropy and Effect of Salinity in Diffusion and Activation Energies of Cations and Anions in Compacted Bentonite

摘要

The diffusion experiments for Γ and Cs~+ in the parallel and perpendicular directions to the orientated direction of smectite particles were performed as a function of smectite's dry density, salinity and temperature. The anisotropies and the effect of salinity in the apparent diffusivities (D_a) and activation energies (ΔE_a) for both ions were additionally discussed. The D_a-values for both ions showed a tendency to be higher in the parallel direction than in the perpendicular direction. The D_a-values of Γ in the parallel direction decreased with increasing salinity at low-dry density, but those of Cs~+ increased with increasing salinity for all conditions. Based on this, it is interpreted that Γ mainly diffuses in interstitial pores and that Cs diffuses in interlayer and interstitial pores. The ΔE_a-values for Γ, similar levels to that for the diffusivity in free water (D~o) at low-dry density, increased with increasing dry density. The ΔE_a-values for Cs~+, higher than that for D~o even at low-dry density, increased with increasing dry density. Such high ΔE_a-values for Cs~+ are considered to be due to the effects of ion exchange enthalpy (ΔH~o) between Cs~+ and Na~+ and the decrease in the activity of porewater.