Uptake of iodide by calcium aluminate phases (AFm phases)

Abstract

Iodine-129 is an important dose-determining radionuclide emanating from low and intermediate level radioactive waste (L/ILW) repositories. A good candidate for the immobilization of the hazardous I− anions are AFm phases due to their interlayer anion exchange capacity. The ability of AFm phases to bind iodine was investigated in a set of co-precipitation experiments, in which the formation of solid solutions between the iodine AFm end member monoiodide (I-AFm) and the common AFm phases hemicarbonate (OH,CO3-AFm), monocarbonate (CO3-AFm) and hydroxy-AFm (OH-AFm), was examined. The samples were characterized by TGA, FTIR, XRPD and liquid phase analyses, and the obtained data were used to construct solid solution models with the help of the thermodynamic modelling program GEMS. The formation of extensive solid solutions was found between the AFm end member pairs monoiodide and hydroxy-AFm, and monoiodide and hemicarbonate. In these two cases, the progressive substitution of the interlayer anion is reflected by a gradual change of the interlayer distance, visible by a continuous peak shift of the basal reflexion from one end member towards the other. No solid solution forms between the end members monoiodide and monocarbonate. In the presence of small amounts of carbonate, a mixed (I,OH,CO3)-AFm phase precipitates, similar to the one found in the monoiodide-hemicarbonate set of experiments, indicated by the similarity in the observed d-values. With increasing carbonate concentration, monocarbonate is stabilized. The experimental data suggests that iodine can be incorporated in the AFm interlayer where anion exchange leads to the formation of solid solutions. The solid solution formation is favoured only between end members with similar crystal symmetry. The presence of carbonate favours the formation of monocarbonate in the long term, reducing the capability of AFm phases to immobilize iodide.