Effects of the Orientation of Clay Particles and Ionic Strength on Diffusion and Activation Enthalpies of I−and Cs+Ions in Compacted Bentonite (II)

Abstract

ABSTRACTThe apparent diffusivities (Da) and activation enthalpies (ΔEa) 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). TheDa-values for both ions tended to be higher in the parallel direction than in the perpendicular direction to the orientation of smectite particles. TheDa-values of 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 ΔEa-values forI−ions are at similar levels as those for the ionic diffusivity in free water (D°) of I−ions at low dry density and increased with increasing dry density. The ΔEa-values for Cs+ions are higher than those for theD° of Cs+ions even at low dry density and increased with increasing dry density. Such high ΔEa-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.