Diffusion in crushed rock and in bentonite clay: Dissertation

Diffusion theories for porous media with sorption are reviewed to serve as a basis for considering diffusion in simple systems like sand or crushed rock. A Fickian diffusion and linear sorption model is solved both by analytical Laplace transform and Green's function methods and by numerical methods, and then applied to small-scale experiments for Finnish low- and medium-level operating waste repositories. The main properties of bentonite are reviewed. The hydraulic conductivity of compacted bentonite is so low that the major transport mechanism is diffusion. A Fickian diffusion and linear sorption model is applied to bentonite. The main component of bentonite, montmorillonite, has a high ion-exchange capacity and thus, transport in bentonite consists of interactive chemical and diffusion phenomena. A chemical equilibrium model, CHEQ, is developed for ion-exchange reactions in bentonite water systems. CHEQ is applied to some bentonite experiments with success, especially for monovalent ions. The fitted log-binding constants for sodium exchange with potassium, magnesium, and calcium were 0.27, 1.50, and 2.10, respectively. A coupled chemical and diffusion model, CHEQDIFF, is developed to take account of diffusion in pore water, surface diffusion and ion-exchange reactions. The model is applied to the same experiments as CHEQ, and validation is partly successful. In the diffusion case, the above-mentioned values for binding constants are used. The apparent diffusion (both for anions and cations) and surface diffusion (only for cations) constants used are 3.0.10-11 m2/s and 6.0.10-12 m2/s, respectively, but these values are questionable, as experimental results good enough for fitting are not available.