Anion diffusion in clay-rich sedimentary rocks - A pore network modelling

Mass transport in compacted clays is predominantly by diffusion, making them preferred candidates for barriers to radionuclide transport in future repositories for nuclear waste. Diffusivity of species depends on a number of factors: (1) the physical and chemical properties of the clay, such as the geometry and topology of the pore space and the functional groups on the surface; (2) the environmental system, e.g. pH, ionic strength, temperature and organic matter; (3) the nature of species, e.g. charges, sizes and their interactions. Existing models do not consider these characteristics integrally in diffusion simulations and analysis. In this work, a developed pore network model is used to study the diffusion behaviour of Cl− with different ionic strengths. The network is constructed based on experimental pore space information and mineral compositions, and captures the anisotropy and heterogeneity of clays. Opalinus clay is selected for testing the model. The effects of pore size distribution, porosity, cation exchange capacity and surface area on the transport behaviour of anions with different ionic strength are analysed. This improves the understanding of the effects of different factors on the species transport in Opalinus clay. The ionic strength of the pore water is varied between 0.01 and 5 M to evaluate its effect on the diffusion of Cl− in clays. It is shown that the model predictions are in good agreement with measured experimental data of Opalinus clay. The agreement demonstrated, both qualitatively and quantitatively, suggests that the proposed approach for integrating known effects on diffusivity is reliable and efficient.