Role of trace elements in the $^{226}$Radium incorporation in sulfate minerals (gypsum and celestite)
Résumé
Incorporation of $^{226}$Ra within gypsum (CaSO$_4$.2H$_2$O$_{(s)}$) and celestite (SrSO$_{4(s)}$) was assessed through dedicated batch experiments monitored over hundreds of days. Results indicate that the published value for the distribution coefficient of $^{226}$Ra between gypsum and an aqueous phase, D$_{Ra,}$$_{gypsum}$= 0.03 is an upper limit. On the other hand, celestite shows high incorporation of the radionuclide, with partition coefficient around 200. This high $^{226}$Ra uptake by celestite results from the existence of a solid solution between celestite and radium sulfate, as expected considering the similarities between the two solids, as between barite and radium sulphate. On the contrary, a solid solution between gypsum and radium sulfate (Ca,Ra)SO$_4$.2H$_2$O$_{(s)}$ cannot be considered per se, due mainly to the fact they do not behave in the same crystal family. However, $^{226}$Ra incorporation in gypsum can be enhanced by the presence of Sr impurities (from 0.1 molar %). In such conditions, the radium distribution coefficient is around 0.15 $\pm$ 0.09. This behavior can be explained by an ion-exchange mechanism between $^{226}$Ra and Sr. These results highlight the key role of trace elements in the incorporation of $^{226}$Ra in sulphate bearing minerals and bring new insights in our understanding of the $^{226}$Ra in environment.(as illustrated with an example in mining context)
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