Thermodynamic description of Eu(NO$_3$)$_3$-NaNO$_3$-H$_2$O and Eu(NO$_3$)$_3$-Mg(NO$_3$)$_2$-H$_2$O systems. Solubility experiments and full dissociation Pitzer models

Solubility studies in the systems Eu(NO$_3$)$_3$-NaNO$_3$-H$_2$O and Eu(NO$_3$)$_3$-Mg(NO$_3$)$_2$-H$_2$O 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 Na$_2$SO$_4$ and MgSO$_4$ 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.