Anisotropy and effect of salinity in diffusion and activation energies of cations and anions in compacted bentonite

The diffusion experiments for I- 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 (Da) and activation energies (ΔEa) for both ions were additionally discussed. The Da-values for both ions showed a tendency to be higher in the parallel direction than in the perpendicular direction. The Da-values of I- 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 I- mainly diffuses in interstitial pores and that Cs+ diffuses in interlayer and interstitial pores. The ΔEa-values for I-, similar levels to that for the diffusivity in free water (Do) at low-dry density, increased with increasing dry density. The ΔEa-values for Cs+, higher than that for Do even at low-dry density, increased with increasing dry density. Such high ΔEa-values for Cs+ are considered to be due to the effects of ion exchange enthalpy (ΔHo) between Cs+ and Na+ and the decrease in the activity of porewater. (author)