Simulations were used to study the diffusive transport properties of water and counter-ions in 1-, 2- and 3-layer hydrated Wyoming-type montmorillonite. The counter-ions considered were monovalent cations such as Li+, Na+, K+, Rb+, Cs+ and divalent cations such as Ca2+, Ni2+, Zn2+ and Pb2+. The simulation results showed that the diffusion coefficients of both water molecules and counter ions increased with water content. For a given hydrated state, the diffusion of cations depended strongly upon their size, weight and capacity to attract water. Monovalent cations always diffused much faster than divalent ones. Although increasing hydration could increase the diffusion coefficient of divalent heavy cations, the activity of the heavy counter ions was still very weak. The diffusion of cations in the interlayer space of clay was much weaker than that in bulk water. This meant that the presence of charged clay had a great influence upon the diffusion of cations and entirely changed their diffusion behaviour.

How Water and Counter Ions Diffuse into the Hydrated Montmorillonite. Zheng, Y., Zaoui, A.: Solid State Ionics, 2011, 203[1], 80-5