Thermodynamic description of Eu(NO3)3-NaNO3-H2O and Eu(NO3)3-Mg(NO3)2-H2O systems. Solubility experiments and full dissociation Pitzer models

Abstract

Solubility studies in the systems Eu(NO₃)₃-NaNO₃-H₂O and Eu(NO₃)₃-Mg(NO₃)₂-H₂O were conducted at T = (22 ± 2) °C including solid phase identification by X-ray powder diffraction and Schreinemakers' method. Iso-water activity data of both systems at 25 °C were determined by iso-water activity (IWA) and dynamic vapor sorption (DVS) experiments. These experimental results were collectively used to verify and complement existing sets of Pitzer interaction parameters for the ternary systems. The resulting thermodynamic models assume the full dissociation of all involved salts. The geochemical code PhreeSCALE was used together with the parameter estimation software PEST to determine new ternary interaction parameters Ψ_{Eu³⁺,Na⁺,NO3⁻}, and Ψ_{Eu³⁺,Mg²⁺,NO3⁻}. In combination with our previous work on Na₂SO₄ and MgSO₄ solutions [F. dos Santos et al., 2024, Dalton Trans., 53, 6289–6299], the updated model allows an accurate description of Eu(III) solubility in complex nitrate–sulphate systems of relevance for radioactive waste disposal.