Negative charge excess at the surface of clay minerals coupled with their high specific surface area implies that clay-rich media have transport properties very favourable to radioactive waste storage. Given their low hydraulic conductivity, diffusion is the main transport mechanism in these media. The goal of this research was to experimentally realize and validate a mechanistic model of transport in compact clay-rich media including a triple layer model. We have developed a transport model without the thin diffuse layer assumption. Our triple layer model is based on a crystallographic consideration and a complexation model including silanols, aluminols, and SiOAl sites. We have successfully used the predictions of this mechanistic model with hydraulic and ionic diffusivities. This was determined by measuring the relaxation of the electrical fields associated with the hydraulic (streaming potential) and ionic (membrane potential) disturbances. The diffusivity of salt increases with ionic strength because the diffusion of the salt depends on the variation of the membrane potential. We have successfully predicted the values for the effective diffusion coefficients of some monovalent ionic tracers such as 22Na and 36Cl at different dry densities and ionic strengths.
