Retention of iodide by the Callovo-Oxfordian formation: An experimental study

• Iodine exists in the formation as iodide.
• The control of PCO2 is crucial for avoiding measurement artifacts.
• Iodide is sorbed with a Kd value ranging from 0.02 to 0.14 L/kg.

The Callovo-Oxfordian (COx) clayed formation is chosen in France as the possible site for an underground nuclear waste disposal. The question raised in this paper concerns the ability of the COx to sorb iodine. Experimental set-ups working in both batch and dynamic systems have been designed for the accurate determination of weak retention factors (i.e. < 0.1 L/kg) under conditions relevant to the geological formation. Adsorption data measurements were combined with speciation studies performed by anion exchange chromatography. Iodide was shown to be the only form of iodine present in the system regarding both the natural pristine iodine content of the samples and iodine added to the samples. A weak but significant adsorption was observed at 20 °C (Kd < 0.31 L/kg). The adsorption process is reversible when working under fixed PCO2 (1% CO2) with the necessity to consider a kinetic-controlled process under dynamic conditions. When the partial CO2 pressure was not controlled, a part of iodine was shown to be irreversibly bound, probably in a newly precipitated carbonate phase. A Kd value in the range of 0.01–0.14 L/kg can be given under controlled PCO2 to describe the adsorption of trace concentrations of iodine by the COx formation.