Incorporating electrical double layers into reactive-transport simulations of processes in clays by using the Nernst-Planck equation: A benchmark revisited

Owing to their low permeability clay formations are considered as potential host rocks for nuclear waste or as seals capping permeable reservoirs for storing unconventional gases. Clay materials such as bentonite are considered as backfill or buffer material in nuclear waste repositories forming barriers to fluid flow owing to their hydraulic and swelling properties. The low permeability of clays implies that solute transport in the pore water is dominated by diffusion. Another important characteristic of clays is the negative surface charge of clay minerals which affects transport and the distribution of ions in the pore space: cations are attracted to while anions are repelled from clay mineral surfaces. Models of reactive transport need to consider these electrostatic effects to be able to accurately simulate the transport of ions through clay. Here we use a new approach which is entirely based on the solution of the Nernst-Planck equation to incorporate the effect of diffuse layers (DLs) into reactive transport simulations. A simulation benchmark is used to validate this new approach. In variants of this benchmark problem the impact of different activity models on the pore water composition, Donnan equilibrium versus a kinetic exchange between the DL and free pore water and the effect of diffusive transport in the DL are explored.