Diffusion of iodide and iodate through Opalinus Clay: Monitoring of the redox state using an anion chromatographic technique

The through- and out-diffusion behaviour of 125I− and 125image were separately studied in an intact sample of Opalinus Clay (OPA) from the Benken drilling site (Switzerland) using purified tracer solutions. The concentrations of the two radioactive I species, 125I− and 125image, were monitored using a high performance anion exchange chromatographic technique. An effective diffusion coefficient, De = (4.5 ± 0.5) × 10−13 m2 s−1, and a rock capacity factor, α = 0.04 ± 0.01, were measured in the through-diffusion experiments with 125I−. The latter result shows that no sorption of I− takes place on OPA because in previous work the same rock capacity factor was measured for the non-sorbing Cl− tracer. The out-diffusion of 125I− was characterised by an unexpectedly high flux from the surface adjacent to the high concentration reservoir in the through-diffusion experiment. The present experiments resolved former inconsistencies with respect to the sorption of I− on OPA, where unpurified tracers were used. However, they confirmed the inconsistencies with respect to the observed De values between through- and out-diffusion experiments. No break-through of 125image could be observed in the through-diffusion experiment with 125image as the source species. Instead a flux of 125I− into the target reservoir was substantiated. This observation, together with the speciation analysis in the feed reservoir, showed that image was rapidly converted to I− upon contact with OPA. The transformation was presumably mediated by unidentified reducing components of OPA because such fast reactions are not observed in homogeneous aqueous solutions. The rapid conversion of image to I− in the OPA sample was corroborated in the profile analysis performed after the through-diffusion experiment, where a strongly one-sided distribution of the tracer ion was observed.