Solid-aqueous equilibrium in the BaSO4-RaSO4-H2O system: First-principles calculations and a thermodynamic assessment

Phase relations in the BaSO4-RaSO4-H2O system are important for understanding the role of barite-type minerals in controlling the concentration of Ra2+ in natural water reservoirs. These relations are extremely sensitive to the difference in the solubility products of the end-members and to the degree of non-ideality of the solid solution phase. Experimental constraints to the standard entropy of RaSO4 and the regular interaction parameter of the barite-RaSO4 solid solution are ambiguous. This study is focused on determination of these parameters from first principles. The phonon density of states of RaSO4 is computed with the aid of the density functional perturbation theory. The regular interaction parameter in the BaSO4-RaSO4 solid solution, WBaRa, is interpreted as the slope of the enthalpy of mixing in the limit of infinite dilution (x(Ra) = 0) and is calculated from the change in the total energy of a 2 x 2 x 2 supercell of BaSO4 due to the insertion of a single substitutional defect of Ra. The method is validated by computing W values for a wider range of binary solid solutions with barite and aragonite structures. The computed value of W-BaRa = 2.50 +/- 1.00 kJ/mol implies that the solid-aqueous equilibrium in the BaSO4-RaSO4-H2O system may have an alyotropic point in close proximity to the BaSO4 end-member. The assessment of available data on re-crystallization of barite in Ra-bearing aqueous solutions suggests that the barite crystals may fully equilibrate on the time scale of hundred days. (C) 2013 Elsevier Ltd. All rights reserved.