Diffusion of anions and cations in compacted sodium bentonite: Dissertation

This thesis presents the results of studies on the diffusion mechanisms of anions and cations in compacted sodium bentonite, which is planned to be used as a buffer material in nuclear waste disposal in Finland. The diffusivities and sorption factors were determined by tracer experiments. The pore volume accessible to chloride, here defined as effective porosity, was determined as a function of bentonite density and electrolyte concentration in water, and the Stern-Gouy double-layer model was used to explain the observed anion exclusion. The exclusion of chloride and anionic uranium complexes could be explained satisfactorily with the double-layer model when the differences in the charge of the ions were taken into account. Exponential expression could be developed to describe the relation between effective porosity, and apparent and effective diffusivity. The sorption of Cs+ and Sr2+ was studied in loose and compacted bentonite samples as a function of the electrolyte concentration in solution. The dependence of sorption factors on the electrolyte concentration indicated a sorption reaction of cation exchange type. In order to obtain evidence of the diffusion of exchangeable cations, defined as surface diffusion, the diffusivities of Cs+ and Sr2+ in compacted bentonite were studied as a function of the sorption factor, which was varied by electrolyte concentration in solution. The measurements were performed both by a non-steady state method and by a through-diffusion method. The apparent diffusivities obtained were, to a large extent, independent of the sorption factor, and the effective diffusivities were found to increase with an increasing sorption factor. This indicates that sorbed cations diffuse, which increases the diffusion capacity, compared to a situation where diffusion occurs only in pore water and sorbed species are immobile. The relation between the diffusivity and sorption of cations could be explained satisfactorily by a model, where diffusion consists of two components, one representing diffusion in pore water and the other surface diffusion. The differences in the apparent diffusion coefficients of cations could be explained rather well by the hydration tendency which depends on the charge and ionic radius of the unhydrated ion. The neutral species seem to be immobile while being sorbed and their diffusion is retarded according to the sorption factor.