Study of iodide sorption to the argillite of Tournemire in alkaline media

Abstract Argillaceous rocks are considered potential host rocks for radioactive waste repositories. The concrete matrix that could be used as a barrier could react with the groundwater of the geological site, inducing a drastic change in its chemical composition and its pH (10–13). Consequently, the physicochemical properties of the rock in contact with this alkaline solution may be modified and, in turn, may induce modification on the behaviour of radioelements. This study, applied to the argillite of Tournemire, involves characterizing I − sorption to an argillaceous rock in alkaline media in batch experiments under N 2 -controlled conditions. I − was added as a 125 I radiotracer and measured by γ spectrometry. Preliminary experiments were conducted with different solution/solid ratios (v/m=2.5, 5 and 20 ml g −1 ) and contact times (1–14 days) in order to determine the optimal experimental conditions. The chosen v/m ratio was 5 ml g −1 as the best compromise between a high K d value and a low error of the measure. The chosen experiment duration was 1 day because I − sorption was highest and to limit the effects of pyrite oxidation. One of the experiments, performed with a radio-sterilized sample to test possible effects from microorganisms, showed that they could enhance iodide retention, particularly during the first 2 contact days. The influence of pH on I − sorption was tested using solutions between values of 8.3 and 12.8. The K d values were independent of pH and very low (0.3 ml g −1 ). Finally, the influence of the chemical composition of concrete fluids was also tested. Three solution compositions corresponding to different steps in the evolution of fluids in contact with altering concrete were used: fluid in contact with fresh concrete (pH 13.2), with moderately degraded concrete (pH 12.1) and with strongly degraded concrete (pH 11.5). Each solution contained variable amounts of sodium, potassium, calcium, silica and sulphate. I − sorption was also very low ( K d ∼0.2 ml g −1 ). Additional experiments were conducted with alkaline solutions containing different amounts of SO 4 2− ions (10 −3 –10 −2 M) to test sulphate–iodide sorption competition. I − retention was independent of the sulphate concentration.