Implications of recently derived thermodynamic data and specific ionic interaction theory parameters for (Mg/Ca)_nUO₂(CO₃)₃^(4-2n)- complexes on the predominance of the Mg²⁺-Ca²⁺-UO₂²⁺-OH^--CO₃^2- systems, and application to natural and legacy-mine waters

The formation of alkaline earth(II)triscarbonatouranyl(VI) (Ae_nUO₂(CO₃)₃^(4-2n)-) species that have been evidenced both in laboratory and in-field studies, is important from slightly acidic pH up to near degraded cementitious in carbonated waters. They are also showing distinctive luminescence properties with a hypsochromic shift relative to UO₂²⁺. The conditions of pH, activities of alkaline earth(II) free ions (mostly Mg²⁺ and Ca²⁺) and carbonate ions (HCO₃^-) can be predicted from the thermodynamic functions and constants. The predictive validity of the activity of major alkaline ions (mostly Na⁺) is determined from the models used to describe the ionic strength comportment of these species, particularly using coefficients from the specific ion interaction theory (SIT). The stability domains of these species are better defined as a function of the activity of the constituents, and applied to natural waters. In this work, using recently obtained complete thermodynamic data and SIT coefficients, we will draw the stability domains of the Ae_nUO₂(CO₃)₃^(4-2n)- species in combinations of activities of H⁺, HCO₃^-, Mg²⁺, Ca²⁺, and Na⁺ for a wide selection of water compositions from the literature. Water samples were collected near a French mining legacy-site (Site du Bosc, Lodève, France). After determining the major ion compositions, we will verify that the luminescence signal of uranium is in agreement with the predicted speciation in the stability domains.