Comparative study of tracer diffusion of HTO, 22Na+ and 36Cl- in compacted kaolinite, illite and montmorillonite

The through-diffusion of HTO, Na-22(+) and Cl-36(-) in kaolinite, homo-ionic Na-illite and homo-ionic Na-montmorillonite was measured at a high degree of compaction as a function of the salt concentration in the 'external solution', i.e. in the solution in contact with the clay sample. The clays were chosen for this Study because of their differences in the number and nature of ion exchange sites leading to different proportions of interlayer-, inter-particle and free pore water. It was found that the diffusive mass transfer of Na+ in Na-montmorillonite and Na-illite increased with decreasing external salt concentration, while the opposite trend was observed for the diffusion of Cl-. These trends are more pronounced in the case of Na-montmorillonite than in Na-illite, while almost no salt effect was observed for kaolinite. Similarly no salt effect was observed for the diffusion of HTO through all of the clays tested. These observations are in agreement with a conceptual model where it is assumed that cations diffuse preferentially in the interlayer or diffuse double-layer porosity, while anions are almost completely excluded from these regions. In the case of Na+ diffusion, the salt effects can be explained by an influence on the concentration gradient of diffusing cations, while in the case of Cl- the external salt concentration has an effect on the accessible porosity. Effective diffusion coefficients of Cl- fulfil the same relationship to porosity as those of the uncharged HTO, when using accessible porosities for such a comparison. Furthermore it is shown that pore diffusion coefficients for the three tracers are fairly well correlated with the respective diffusion coefficients in bulk water, if the effective diffusion coefficients for Na+ are derived from calculated tracer concentration gradients in the interlayer or diffuse double-layer porosities. (C) 2010 Elsevier Ltd. All rights reserved.