Sector-zoned radionuclide incorporation in natural calcite

Deep geological repositories for radioactive waste necessitate safety evaluations, which encompass assessing the possible leakage of radionuclides from the waste containers into groundwater. In this context, the retention of radionuclides through water–mineral interactions is considered. Calcite is of significant importance in this regard, as it (re)crystallizes under relevant physico-chemical conditions in groundwater and can incorporate significant amounts of radioactive elements, such as trivalent actinides (Am, Cm). In contrast to laboratory experiments, which are conducted over short time periods and under controlled conditions, this study investigates radionuclide uptake during solid solution formation in natural systems. This approach offers the opportunity to evaluate the stability of solid solution phases over geological timescales.

To investigate the incorporation of trivalent actinides into calcite, as well as their analog element La, natural calcite samples were obtained from the Wenzel mine in the Black Forest, SW Germany. High-resolution LA-ICP-MS analyses reveal that the enrichment of La in specific sectors of the crystal can exceed that of other sectors by more than a factor of over 200. A mass balance of monovalent and trivalent ions within the different crystal sectors shows that the incorporation of La is not compensated by monovalent ion substitution. Our results suggest that trivalent ions are more likely compensated by vacancies on divalent ion sites on specific crystal faces. The study shows that calcite plays a significant role as a sink for trivalent actinides, but this role is highly specific and depends on the characteristics of the respective crystal surfaces.