Diffusion of Water through the Dual-Porosity Swelling Clay Mineral Vermiculite

Prediction of water and solute migration in natural clay-based materials requires a detailed understanding of the roles played by different porosity types (around or inside clay particles) on the overall transfer process. For smectite, a reference material for the design of migration models, this discrimination is complex because of osmotic swelling of the structure under water-saturated conditions. Diffusion experiments with a water tracer (HDO) were conducted on 0.1-0.2, 1-2, and 10-20 μm size fractions of Na-vermiculite, a swelling clay mineral with no osmotic swelling. Results obtained for the two finest fractions suggest that osmotic swelling and the associated impact on pore structure are responsible for the low D_e values reported in the literature for smectite compared to those of vermiculite. When considering only interparticle porosity, D_e values for vermiculite are similar to those reported for nonporous grains (Na-kaolinite and Na-illite). This indicates that interparticle porosity has a primary effect on the overall water diffusion process, whereas interlayer porosity is shown to imply a small proportion of HDO adsorption. This study provides evidence that vermiculite is a promising reference mineral for the understanding of the roles played by pore structure and mineral-water interaction in the transport properties of water in claystones and for associated refinement of dual-porosity diffusion models.