Development of Accurate, Non-Concentration-Restricted Models for Predicting Solution Properties and Solid–Liquid Equilibrium in Binary Nitrate Systems from Low to Very High Concentrations (up to ≈ 30 mol.kg^-1) at 25^oC

In this study, we developed well-validated, Pitzer ion-interaction-approach-based thermodynamic models for solution behavior and solid–liquid equilibrium in 17 binary nitrate systems [of the type 1-1 (HNO₃-H₂O, LiNO₃-H₂O, NaNO₃-H₂O, KNO₃-H₂O, RbNO₃-H₂O, CsNO₃-H₂O, and NH₄NO₃-H₂O), of the type 2-1 (Mg(NO₃)₂-H₂O, Ca(NO₃)₂-H₂O, Ba(NO₃)₂-H₂O, Sr(NO₃)₂-H₂O, and UO₂(NO₃)₂-H₂O), 3-1 (Cr(NO₃)₃-H₂O, Al(NO₃)₃-H₂O, La(NO₃)₃-H₂O, Lu(NO₃)₃-H₂O), and 4-1 (Th(NO₃)₄-H₂O)] from low to very high concentrations at T = 25^oC. To parameterize the models for the binary systems, we used all available raw experimental osmotic coefficients data (φ) for the entire concentration range of solutions, including the supersaturation zone. To construct the models, we used different versions of the standard molality-based Pitzer approach. The predictions of the newly developed models presented here are in excellent agreement with experimental osmotic coefficient data, as well as with recommendations on mean activity coefficients given in the literature for binary solutions from low to very high concentrations. It was established that, for seven of the systems under study, the application of the extended approach with four parameters (β⁰, β¹, β², and C^φ) and variation of the a₂ term in the fundamental Pitzer equations leads to the lowest values of the standard model–experiment deviation. The Deliquescence Relative Humidity (DRH), thermodynamic solubility product (expressed as ln K^o_sp), and standard molar Gibbs free energy of formation (D_fG^o_m) of 18 nitrate solid phases have been determined on the basis of evaluated binary parameters and using m(sat) solubility data. The models for nitrate systems described in this study are of high importance, especially for the development of strategies and programs for nuclear waste geochemical storage.

Acknowledgement: This study is funded by the European Union–NextGenerationEU, project № BG-RRP-2.013-0001.