For a porous water-saturated material where diffusion in the pore water, sorption on the solid material and diffusion of the sorbed ions (surface diffusion) occur, a diffusion equation can be derived where the apparent diffusivity includes two terms. One represents diffusion in the pore-water, the other surface diffusion.
In this research diffusion mechanisms were studied. The apparent diffusivities of strontium, cesium and cobalt in compacted sodium bentonite were measured by a non-steady state method. The sorption factors were adjusted using different sodium chloride solutions, groundwater and addition of EDTA for saturation of the bentonite samples. The corresponding sorption factors were measured by a batch method.
The results suggest that cations diffuse also while being sorbed. A combined pore diffusion-surface diffusion model has been used to explain the transport and the corresponding diffusivities have been evaluated. The surface diffusivities (Ds) of Sr and Cs were 8-9·10-12 m2/s and 4-7·10-13 m2 /s respectively. The pore diffusivity eD of Cs was 3.5.10-11 m2 /s which has been used also for Sr.
The sorption mechanism of Co seems to be different from that of Sr or Cs and the results allow no specific conclusions of the diffusion mechanism of Co. The apparent diffusivity of Co ranged from 2·10-14 to 7·10-14 m2/s.
The anionic Co-EDTA seems to follow some other diffusion mechanism than the cations.