Anisotropy and Effect of Salinity in Diffusion and Activation Energy of Cs⁺ Ions in Compacted Smectite

Abstract

In-diffusion experiments for Cs⁺ ions in compacted Na-smectite were performed in the directions parallel and perpendicular to the orientation of the smectite particles as a function of smectite dry density (between 0.9 and 1.4Mg/m³), salinity ([NaCl] =0.01 and 0.51M) and temperature (295-333K).
The apparent diffusion coefficient (D_a) tended to be higher in the direction parallel than in the direction perpendicular to the orientation of the smectite particles. Higher salinity leads to an increase of the D_a-values for all conditions. The activation energies (ΔE_a) are independent of the orientation of the smectite particles. Higher dry densities correspond to higher ΔE_a-values. For a dry density of 1.0Mg/m³, the corresponding ΔE_a-value agrees well to a predicted value ΔE_a-value=24.66±0.96kJ/mol. This value is obtained as the difference of the activation energy for diffusion of cesium in pure water (16.47kJ/mol) and the Cs/Na ion exchange enthalpy ΔH⁰ (-11.10kJ/mol). For a dry density of 1.4Mg/m³, ΔE_a is 35.20±0.83kJ/mol. The higher ΔE_a-values found at higher dry densities are attributed to complex effects of ΔH⁰ and the lowering of porewater activity at high-dry densities.