Characterization of iodide retention on Callovo-Oxfordian argillites and its influence on iodide migration

Iodine-129 is commonly considered to belong to the largest contributors to the calculated impact on health associated with the long-term nuclear underground waste disposal. Its behaviour in Callovo-Oxfordian argillites, an argillaceous host rock studied by ANDRA, therefore must be fully characterized. Investigated cores have been extracted from a place outside the Meuse/Haute Marne underground laboratory. Thus, batch and through-diffusion experiments were performed to investigate iodide sorption and transport, on two different test samples. Batch experiments give evidence of a kinetically limited slight retention of iodide at low total iodine concentration. This sorption, dependent on the oxic/anoxic conditions, has been modelled with an initial quick reaction and a long-term steady loss of iodide from the solution. Through-diffusion experiments have confirmed this retention but to a lower extent than observed in batch experiments and differences were found between both samples. These differences have been explained by anionic exclusion, which potentially blocks sorption site access.