Diffusion and sorption of caesium depend on the dry bulk density of bentonite

Smectite-rich natural clays, usually referred to as bentonite, are used as backfill material in disposal concepts for highly-radioactive wastes. The main component of bentonite is montmorillonite, characterised by a high cation exchange capacity resulting from isomorphous substitution and swelling due to the incorporation of water molecules between the stacked clay platelets, the interlayer. These properties render bentonite an ideally suited barrier material for cationic radionuclides. Caesium is such a radionuclide that is relevant in the context of nuclear waste disposal since it is highly soluble and can be incorporated in organisms.

Migration of caesium in MX-80 bentonite (Na-montmorillonite) was investigated for different dry bulk densities (1.3, 1.6 and 1.9 g/cm³) in long-term through-diffusion experiments running for up to 600 days. Diffusion experiments of tritiated water (HTO, non-sorbing) provided the transport accessible porosities. Batch sorption experiments at varying caesium concentrations should test the transferability between dispersed and compacted systems by means of the distribution coefficient K_d (m³/kg). The synthetic pore water compositions were calculated as a function of the dry bulk density. Caesium and HTO are expected to migrate via molecular diffusion through the compacted bentonite sample. Therefore, a one-dimensional numerical model was applied to determine the transport parameters, effective diffusion coefficient D_e (m²/s) and rock capacity factor α (-), from the temporal evolution of the diffusive flux and the accumulated activity.

In the dispersed systems, measured K_d values increase with decreasing caesium concentration and dry bulk density. The linear sorption isotherm indicates that sorption mainly occurred via cation exchange, predominantly with ions from the interlayer. Accordingly, the higher the cation concentration in the contacting pore water, less caesium is exchanged due to competing effects. At low concentrations (<10^-6 mol/L), however, the measured sorbed caesium concentrations do not match exactly with the isotherm. This can be attributed to impurities in form of illite-smectite mixed layers. With increasing caesium concentration, measured K_d decreases due to saturation of the high affinity sites at the illite surfaces. K_d values differ by a factor of up to ten between batch and compacted systems. With compaction, the amount of water in the interlayer decreases. This, in turn, affects the space and ability to hydrate sodium so that it is exchanged with the less hydrated caesium from the bulk solution. Accordingly, K_d values are not transferable due to thermodynamic changes of the exchange process with compaction. Consequently, caesium sorption depends on the ionic strength, the compaction and on the caesium concentration.

With increasing bulk density, the apparent diffusion coefficients of caesium decrease by a factor of ten. In general, diffusion of caesium was twice as high as that of HTO. This can be attributed to an additional transport pathway in the interlayer, which is not accessible for neutral species. For the transient phase, there was an offset between simulations and experiments, what might be explained by higher temperatures at the beginning of the experiments. Simulated and experimental steady-state phases are in line. With compaction, sorption increases, and thus diffusivity decreases.