Influence of redox conditions on iodide migration through a deep clay formation (Toarcian argillaceous rock,Tournemire, France)

With a half-life of 15.7 Ma, a high mobility and the potential to accumulate in the biosphere, ¹²⁹I is considered, in safety assessment calculations for radioactive waste repositories, to be one of the main contributors to the radiological dose. Several authors have reported that, at low concentration, I⁻ is weakly retained on argillaceous rocks. This process is not yet well-understood and different hypotheses have been put forward as to whether reactive phases or experimental artifacts (e.g. pyrite oxidation) could be the reason for the retention of I⁻ observed at low concentration. The aim of this study was to investigate the effect on I⁻ mobility of (i) the redox conditions and (ii) the amount of pyrite and natural organic matter (NOM) contents of the rock. These questions were addressed by performing batch sorption, through-diffusion and out-diffusion experiments on rock samples of Toarcian argillaceous rock from Tournemire (Aveyron, France). One of the challenges faced during this study was to distinguish actual transport properties from experimental artifacts. A especially elaborate experimental set-up allowed limiting the (i) oxidation of both argillaceous rock and I⁻, and (ii) carbonate precipitation. A comparison of the batch sorption results obtained for two Toarcian clay specimens, that differed in their amount of pyrite and NOM, allowed relating I⁻ sorption to pyrite oxidation. However, no evidence was found to associate the I⁻ behavior to the NOM amounts. While the through-diffusion experiments showed a very slight sorption (distribution ratio (R_d) = 0.016 mL g⁻¹) for the lowest I⁻ concentration under oxic conditions, the out-diffusion tests performed after the through-diffusion experiments on the same cells showed significant sorption under both oxic and anoxic conditions, resulting in R_d ranging from 0.02 mL g⁻¹ to 1.25 mL g⁻¹. The range of R_d values was higher for the upstream reservoir under oxic conditions. The discrepancies observed between the through-diffusion and the out-diffusion experiments suggest a kinetic control of the I⁻ uptake by argillaceous rocks under oxic and anoxic conditions.