Effects of the orientation of clay particles and ionic strength on diffusion and activation enthalpies of i^- and Cs⁺ ions in compacted bentonite (II)

The apparent diffusivities (D_a) and activation enthalpies (ΔE_a) for I^- and Cs⁺ ions in compacted Na-smectite were obtained in the parallel and perpendicular directions to the orientation of smectite particles as a function of smectite's dry density (0.9-1.4 Mg/m³), salinity ([NaCl]=0.01, 0.51 M) and temperature (295-333 K). The D_a-values for both ions tended to be higher in the parallel direction than in the perpendicular direction to the orientation of smectite particles. The D_a-values off I^- ions in the parallel direction decreased with increasing salinity only at low dry density, but those of Cs⁺ ions increased with increasing salinity for all conditions. Based on this, it is interpreted that I^- ions predominantly diffuse in external pores and Cs⁺ ions diffuse in both interlayer and external pores. The ΔE_a-values for I ions are at similar levels as those for the ionic diffusivity in free water (D ^o) of I^- ions at low dry density and increased with increasing dry density. The ΔE_a-values for Cs⁺ ions are higher than those for the D^o of Cs⁺ ions even at low dry density and increased with increasing dry density. Such high ΔE _a-values for Cs⁺ ions are considered to be due to the combined effects of the ion exchange enthalpy between Cs⁺ and Na ⁺ ions in smectite and the lowering in the activity of porewater.