Influence of initial iodide concentration on the iodide uptake by the argillite of Tournemire

Among the radionuclides that can migrate from radioactive wastes buried within consolidated argillaceous rocks, 129I poses a particular problem due to its high mobility in geomedia and its very long half-life of 1.6 107 years. Previous studies carried out on such rocks showed the existence of a low iodide retardation regarding the other anion species. The purpose of the present study was two-fold. It was hoped (1) to confirm this retardation by means of radial diffusion experiments and (2) to estimate the influence of the initial iodide concentrations towards the iodide uptake. The results obtained from in-diffusion stage show that (1) no iodide uptake occurs at high initial concentrations and (2) an uptake at low initial concentrations cannot be clearly evidenced.Finally, the most singular behaviour of iodide was highlighted for all the initial iodide concentrations, from the out-diffusion stage, during which some amount of iodide, available for out-diffusion, were not involved in the diffusion process. Although the origin of such phenomenon remains unclear, a kinetic effect for the iodide uptake was proposed, assuming that the period of 55days (corresponding to the in-diffusion stage duration) would be necessary for inducing the iodide uptake. This kinetic effect, already shown elsewhere by (Descostes, M., Blin, V., Bazer-Bachi, F., Meier, P., Grenut, B., Radwan, J., Schlegel, M.L., Buschaert, S., Coelho, D, Tevissen, E. Diffusion of anionic species in Callovo-Oxfordian argillites and Oxfordian limestones. In press for Appl. Geochem.) on similar materials, needs to be deepened.