Modelling reactive diffusion in clays with two-phase-informed pore networks

• Pore networks incorporating pore space and solid phase information
• Effects of anion exclusion and sorption on diffusion
• Simulations in agreement with experimental data
• Explanation of different species transport in porous media

Development of pore network models for reactive diffusion of various species in clays is presented. Networks are constructed using experimentally measured pore-space and solid-phase characteristics. This way incomplete experimental information for the topological (pore connectivity) and the geometrical (inter-pore distances) properties of a given pore system can be balanced by known solid-phase properties. Opalinus Clay (OPA) is selected to demonstrate the application and validate the proposed model. OPA is modelled as anisotropic porous medium, reflecting preferential orientation of meso-porosity along clay bedding direction. Bulk diffusivities of various species (HTO, Cl− and I−) are calculated to investigate the effects of pore structure, anion exclusion and adsorption. Adsorption is simulated by changing the pore space, which is more realistic compared to existing reactive transport models with assumed constant pore geometry. Anion exclusion effects are simulated by introducing diffuse double layer (DDL) in the model. Results agree well with experimentally measured diffusion coefficients for transport parallel and perpendicular to the bedding direction. The proposed model is applicable to a large class of geo-materials and suitable for linking to lattice models for deformation and damage.