Study of iodide sorption on the argilite of Tournemire.

K. Devivier, S. Savoye, I. Devol. Brown Actes du congrès GOLDSCHMIDT 2002, du 18 au 23 août 2002 à Davos (Suisse) numéro spécial de Geochimica and Cosmochimica Acta (2002), Volume 66, n°15-A, page A-180.

Introduction Hydraulically active fractures have been discovered in indurated claystones of the Tournemire site (Aveyron, France). An artificial tracer experiment with iodide is thus planned to estimate the extent of circulation in such networks (Savoye et al., 2001). Before carrying out the in situ experiments, the sorption properties of iodide were evaluated in laboratory with batch method to verify the conservative behaviour of this tracer. Materials and methods Our purpose is to estimate the extent of iodide uptake on three important phases for the argillite of Tournemire: the clay fraction, the calcite and the organic matter. A study of the influence of both pH and the iodide initial concentration was carried out on three solids, obtained by a method based on aqueous extractions (Devol-Brown et al., 1998). The solids are respectively a raw argillite, a calcite-free argillite and a purified-to-calcite and to-organic-matter argillite. The experiments were performed in controlled atmosphere. The results suggest that, whatever the considered solid-solution system, the iodide sorption is weak, and even almost null (iodide uptake < 17 %), which means that the iodide does not show any significant affinity towards the clay fraction, the organic matter and the calcite. The influence of both pH (between 5 and 9) and the iodide initial concentration is low on the iodide sorption in the case of the argillite of Tournemire.