Tracer diffusion of water and inorganic ions in compacted saturated sodium bentonite

Compacted saturated bentonite barriers are in use worldwide in the isolation of waste landfills, and may become used for the long-term isolation of high-level radioactive waste in deep geological repositories. The tracer diffusion of water and ions of simple chemistry (sodium, strontium, cesium, chloride, iodide) through compacted saturated bentonite has been widely studied, as a function of compaction, temperature, and the ionic strength of bulk solutions in contact with the compacted clay. No predictive model of the available experimental data is currently known.; In the present dissertation, I have developed a model describing the diffusion of water and ionic tracers along parallel diffusion paths through nanometer-thick interlayer pores (between montmorillonite lamellae) and through larger pores (macropores) of compacted saturated Na-bentonite. This model predicted the ratio of the relative apparent diffusivity of sodium and strontium tracers to that of water tracers in saturated compacted Na-bentonite, in a broad range of experimental conditions.; Using the diffusion model developed in this dissertation, I showed that the one-dimensional compaction of bentonite causes the tortuosity of diffusion paths to become greater in the direction parallel vs. normal to compaction, at least in some bentonites, but that the 'isotropic' (direction-averaged) tortuosity of compacted bentonite is independent of compaction (with a value of tau = 1.9 +/- 0.4) at partial montmorillonite dry densities greater than 0.2 kg dm-3. I also showed that the diffusion of cations through montmorillonite interlayers may be affected by the pH-dependent charge carried by the edge surfaces of montmorillonite lamellae, and that the slow diffusion of cesium vs. sodium ions in montmorillonite interlayers is consistent with a temporary immobilization of cesium ions adsorbed as inner-sphere surface complexes in the interlayers. The model developed in this dissertation over-predicted the apparent diffusivity of anionic tracers at high partial montmorillonite dry densities and low ionic strengths, perhaps because anions are not only excluded from the interlayers, but are also partly excluded from the macropores; this partial exclusion must vary along macropore diffusion paths, causing a decrease in the apparent diffusivity of anions, unless all macropores have the same size.