Effects of the orientation of smectite particles and ionic strength on diffusion and activation enthalpies of I− and Cs+ ions in compacted smectite

The apparent diffusion coefficients (Da) for I− and Cs+ ions in compacted Na-smectite which is a major constituent clay mineral of bentonite were studied as a function of smectite's dry density (0.9-1.4 Mg/m3), ionic strength ([NaCl] = 0.01, 0.51 M), temperature (22-60 °C) and diffusion direction to the orientated direction of smectite particles. The Na-smectite was prepared by ion-exchanging with Na+ ions a Na-bentonite, Kunipia-F®, of which smectite content is over 99 wt.%. The Da-values for both ions showed a tendency to be higher in the parallel direction than in the perpendicular direction to the orientated direction of smectite particles at a low-ionic strength of [NaCl] = 0.01 M. The Da-values for I− ions showed different trends depending on diffusion direction and dry density at a high-ionic strength of [NaCl] = 0.51 M. Namely, although the Da-values for I− ions showed a tendency to be higher in the parallel direction than in the perpendicular direction to the orientated direction of smectite particles at a high-dry density of 1.4 Mg/m3, these showed a reciprocal tendency at dry densities of 0.9-1.0 Mg/m3. The Da-values for Cs+ ions uniformly increased with an increase of ionic strength in both diffusion directions. Considering electrostatic effect from smectite surface and the change in tortuosity on dry density, ionic strength and diffusion direction to the orientated direction of smectite particles, I− ions are considered to mainly diffuse in interstitial pores. While, Cs+ ions can diffuse in both interlayer and interstitial pores, and the Da-values are considered to have elevated by competing with Na+ ions. The activation enthalpies (ΔEa) for I− ions, slightly higher (ΔEa = 19.8−20.0 kJ/mol) than that of the diffusion coefficient in free water (Do) for I− ions (ΔEa = 17.36 kJ/mol) at a low-ionic strength of [NaCl] = 0.01 M, decreased with an increase of ionic strength, became of similar level to that of the Do at a high-ionic strength of [NaCl] = 0.51 M, increased with an increase of dry density. On the contrary, the ΔEa-values for Cs+ ions, clearly higher (ΔEa = 25.6−28.4 kJ/mol) than that of the Do for Cs+ ions (ΔEa = 16.47 kJ/mol) even in low-dry density over the ionic strength, increased with an increase of dry density. The ΔEa-values for Cs+ ions are considered to be due to the decrease in the activity of porewater in addition to the effect of ion exchange enthalpy between Cs+ and Na+ ions in smectite.