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Incorporation of Ra-226 within gypsum (CaSO4 center dot 2H(2)O((s))) and celestite (SrSO4(S)) was assessed through dedicated batch experiments monitored over hundreds of days. Dissolution/recrystallization and coprecipitation experiments were carried out to investigate a range of chemical conditions, close to or far from equilibrium conditions. These data were used to establish the ability of gypsum and celestite to incorporate Ra-226. Celestite shows a high Ra-226 incorporation, with a partition coefficient around 105. On the contrary, Ra-226 is not significantly incorporated into pure gypsum. A very low value of (8.8 +/- 1.5) x 10(-4) has been estimated, which is much lower than the values reported in the literature. This may explain why Ra-226 incorporation in gypsum is enhanced when Sr impurities are present (above 0.1 molar %). Under such conditions, the radium distribution coefficient is around 0.15 +/- 0.09. This behavior can be explained by an ion exchange mechanism between Ra-226 and Sr, consistently with the existence of a solid solution between celestite and radium sulfate, similarly to the barite (BaSO4(s)) and radium sulfate solid solution. These results highlight the key role of trace compounds in the incorporation of Ra-226 in sulfate bearing minerals and bring new insights in our understanding of the Ra-226 behavior in the environment. This is illustrated by the radium content observed and modeled within the pore waters of uranium mining tailings.

Role of Trace Elements in the 226-Radium Incorporation in Sulfate Minerals (Gypsum and Celestite)