Pore scale modeling of ion diffusion in variably saturated clays

Abstract:

The transport of ions in unsaturated porous media is an important fundamental process in many natural and technical settings. A realistic description of transport processes in unsaturated porous media relevant to the deep geological disposal of nuclear waste across scales is still an unresolved scientific challenge. A newly developed lattice Boltzmann based model for pore-scale transport simulations was applied to investigate ion diffusion in microscale and nanoscale pores of unsaturated clays. In traditional microscale pore-scale simulations ignoring the contribution from water films adsorbed on the surface of clay minerals due to the coarse resolution, the transport pathways related to large capillary pores depercolate under the condition of low saturation. Our improved pore-scale simulations shows that the water films covering on the surface of clay particles has non-negligible influences on ion transport as volumetric water content is less than 0.25. When water content lower than 0.1, the thin water film on clay surface controls solute diffusion. At nanoscale, electrical double layer (EDL) effects caused by the charged surface of clays can significantly enhance cation diffusion but reduce anion diffusion. Our simulations indicate that the influence from EDL on ion diffusion becomes stronger in the clay with lower water content. The present study could help improve the understanding of the diffusion mechanism of ions in variably saturated clays.