Ionic transport in compacted bentonite: Preliminary equilibrium results

An attempt is made to decompose some quantities, most commonly integrated over the chemical composition, the structure of the pore network and the time scale, into their differential counterparts. Into this category typically fall, say, the apparent diffusivities of ionic species. To serve this objective, a two-dimensional unsteady molecular-level electrokinetic transport model for ionic species in bentonite clays will be developed. The model incorporates additional features to the conventional Gouy-Chapman (GC) theory: ionic hydration, dielectric saturation, and the volume exclusion of ions. The governing equations for the flow of electrolyte solution through the pores are solved by an iterative numerical scheme to relate the characteristics of the flow to the characteristics of the pores and to the composition of the external solution in contact with the clay. The pore geometry of the clay is modelled as an array of non-interconnected tortuous channels with no parallel or serial-type non-uniformities along the pathway. This roughly corresponds to the picture of clay particles of infinite extent aligned in parallel and spaced apart by a constant distance. The model aims to simulate and interpret equilibrium and transport experiments for bentonite clays containing different types of background electrolytes at various compactions. Specifically, emphasis is placed on quantifying the extent of co-ion exclusion and understanding the postulated surface diffusion mechanism on the basis of the well-established electric double-layer (EDL) theory. This contribution presents and discusses some preliminary results, based on the modified Boltzmann statistics, for the equilibrating part of the model.