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

Apparent diffusivities (Da) for I^- and Cs⁺ in compacted Na-smectite were studied as a function of smectite's dry density (0.9-1.4 Mg/m³), ionic strength (IS: [NaCl]=0.01, 0.51 M), temperature (22-60°C) and diffusion direction to the orientated direction of smectite particles. The Da-values for both ions in parallel direction to the orientated direction showed a tendency to be higher than those in the perpendicular direction at low IS. The Da-values for I^- showed different trends depending on diffusion direction and dry density at high IS, but Da-values for Cs⁺ increased with increasing IS in both diffusion directions. The activation enthalpies (ΔEa) for I^-, slightly higher than that of diffusivity in free water (D^o) at low IS, similar level to that of D^o at high IS, increased with increasing dry density. While, ΔEa-values for Cs⁺, clearly higher than that of D^o for all conditions, increased with increasing dry density. Since it is known that interlayer distance depends on both dry density and IS, diffusive pathway is considered to differ depending on the charge of diffusion species.