Microscopic study of transport properties in clay
Résumé
A better understanding of the diffusive transport of solutes and water in compacted clays is of prime interest for the disposal of radioactive waste. In this work, microscopic simulations are undertaken in interlayer spacings of Na- and Cs-montmorillonites in mono- and bihydrated states corresponding to very compacted clays. Water is rather structured towards sheets' surfaces, Cs+ exhibits a site-to-site diffusion on the surface but preferential sites are less defined for Na+. Calculated diffusion coefficients are ten times lower than in bulk water for monohydrated state but increase with hydration and with temperature. Increasing temperature does not modify the main features of each cation behavior. Self-diffusion coefficients of water are in agreement with short time coefficients measured by neutron scattering. For the ions, the experimental values are related to macroscopic long time motions and are not directly comparable with simulations. But the introduction of simulated values in macroscopic models gives results quite in agreement with tracers' experiments.