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

Abstract

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 (Rd) = 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 Rd ranging from 0.02 mL g−1 to 1.25 mL g−1. The range of Rd 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.