Diffusion of radiosulphate and radiocaesium in kaolinite clay (KGa-2): Testing the applicability of the pore water diffusion model

The effective diffusion coefficients (De) and rock capacity factors (α) for radiosulphate and radiocaesium in compacted kaolinite (KGa-2) were measured as a function of the concentration of NaClO4 in the contacting solution to test the effect of salinity on diffusion of these species. Solid-liquid distribution coefficients (Kd) were derived from α and compared with Kd values from batch sorption experiments using dispersed clay suspensions. De values for radiosulphate (3.3-8.0 × 10−11 m2 s−1) showed no substantial dependence on the concentration of NaClO4. The diffusion Kd values (9-40 L kg−1) indicated a weak sorption process, confirmed by the low Kd values measured in the batch sorption experiments. No unambiguous conclusion could be drawn regarding a potential salinity effect on radiocaesium De values (1.4-3.0 × 10−10 m2 s−1). The diffusion-derived Kd of 134Cs (53-498 L kg−1) varied inversely with the NaClO4 concentration, consistent with a cation exchange process, and substantially decreased in the presence of stable Cs, suggesting the presence of specific sites for 134Cs that become saturated at high Cs concentrations. Structural analyses indicated an illite content of around 1.7%, which explained the generally large Kd values for radiocaesium. Finally, the correlation between measured De values and the respective diffusion coefficients in bulk water (Dw) was examined for a number of species, including radiosulphate and radiocaesium in the presence of stable Cs, along with additional data gathered from other species. Both variables were highly correlated, confirming the validity of the pore water diffusion model, but only in the absence of specific interaction with the clay.