How mobile is iodide in the Callovo–Oxfordian claystones under experimental conditions close to the in situ ones?

The iodide behaviour towards the Callovo–Oxfordian claystone was studied using batch and diffusion experiments under conditions which limited the artefacts cited in the literature to be responsible for the iodide uptake (i.e. the experiments were carried out under anoxic conditions with N2/CO2 atmosphere with a monitoring of the iodine redox-state). The results show that all the radioactive iodine was 125I−, with no measurable activity for 125IO3−, which is known to have a higher affinity for the rock than iodide. Moreover, the batch experiments revealed no sorption, independently of the initial iodide concentration (from 10− 6 to 10− 3 mol L− 1) and the contact time (up to 106 days). Conversely, the diffusion experiments indicated a weak but measurable retention. The through-diffusion experiments led to distribution ratio values only significant (RD ~ 0.05 mL g− 1) for initial iodide concentration ≤ 10− 4 mol L− 1. Higher RD values were estimated from out-diffusion experiments, ranging from about 0.05 mL g− 1 for an initial concentration of 10− 3 mol L− 1 to 0.14 mL g− 1 for the lowest one. A retention phenomenon that could be reversible and kinetically-controlled was proposed to explain the differences in the extent of the iodide retardation of the two types of diffusion experiments.

► A weak but measurable iodide uptake was estimated in the Callovo–Oxfordian claystone.
► Given the care paid, the I uptake would not originate from some experimental artefacts.
► Out-diffusion method showed a reversible and kinetically controlled iodide uptake.
► Batch method was not enough sensitive for a very weakly-sorbed species, as iodide.