Diffusion of 22Na+, 85Sr2+, 134Cs+ and 57Co2+ in bentonite clay compacted to different densities:: experiments and modeling

The diffusion of radionuclides in water-saturated porous media, such as compacted bentonite, is traditionally modeled assuming diffusion in the pore water and immobilization by adsorption on the clay surface. In reality there are several sorption mechanisms acting in the clay-water system. We have therefore carried out a careful diffusion study of the cations Na+, Sr2+, Cs+, which are predominantly sorbed by electrostatic interaction, and Co2+, for which the dominant sorption mechanism at pH > 7 is inner-sphere complexation and/or surface precipitation. The study has been performed at different densities of clay compaction (1.8, 1.6, 1.2, 0.8, 0.4 g cm(-3)) using a through diffusion technique. In each experiment, the apparent diffusivity and transport K-d were obtained by computer simulation of the experimentally measured cumulative flux through a plug of compacted bentonite and the concentration profile within the bentonite. Batch sorption experiments with Na+, Sr2+, Cs+ and Co2+ were also carried out. Measured batch K-d values were compared to diffusion-derived K-d values using the computer code ANADIFF. The obtained diffusivities of Cs+ and Sr2+ decreased as the density of bentonite increased. The diffusivity of Cs+ decreased from 1.5 x 10(-7) to 1.1 x 10(-8) cm(2)/s as the clay dry bulk density increased from 0.4 to 1.8g/cm(3) and the diffusivity for Sr2+ varied from 4.3 x 10(-7) to 9.0 x 10(-8) cm(2)/s. The diffusivities of Co2+ also decreased from 3.0 x 10(-9) to 1.0 x 10(-10) cm(2)/s in the same range of clay density. An interpretation of the diffusion data is proposed based on differences in porosity and tortuosity of the bentonite compacted to different densities, and on differences in binding energy, hydration energy, and hydrolysis constants for the cations studied.