Thermodynamics of the Eu( iii )–Mg–SO${}_4$ –H${}_2$O and Eu( iii )–Na–SO${}_4$–H${}_2$O systems. Part II: spectroscopy experiments, complexation and Pitzer/SIT models

A time-resolved laser fluorescence spectroscopy (TRLFS) study was carried out to investigate the Eu(III)–SO${}_4$ complexation at room temperature over a wide range of Na${}_2$SO${}_4$ concentrations (0–2 mol kg${}^{-1}$). Spectroscopic observations confirm the step-wise formation of the aqueous complexes Eu(SO${}_4$)${}^{+}$, Eu(SO${}_4$)${}_2$${}^{-}$ and Eu(SO${}_4$)${}_3$${}^{3-}$ over the investigated Na${}_2$SO${}_4$ concentrations. Combining TRLFS data obtained in this study and solubility data reported in Part I of this work for the Eu${}_2$(SO${}_4$)${}_3$–Na${}_2$SO${}_4$–H${}_2$O and Eu${}_2$(SO${}_4$)${}_3$–MgSO${}_4$–H${}_2$O systems, thermodynamic and activity models were derived based on the SIT and Pitzer formalisms. A combination of the geochemical calculation codes PhreeqC (SIT), PhreeSCALE (Pitzer) and the parameter estimation code PEST was used to determine the solubility products $(K_{s,0}^{°})$ of Eu${}_2$(SO${}_4$)${}_3$·8H${}_2$O(cr) and Na${}_2$Eu${}_2$(SO${}_4$)${}_4$·2H${}_2$O(cr), stability constants of the Eu(III)–SO${}_4$ complexes $({\beta }^0{}_i)$, and the specific binary and ternary interaction parameters $({\epsilon }_{ij},{\beta }^{(0)}{}_{ij},{\beta }^{(1)}{}_{ij},C^{\varphi }{}_{ij},{\theta }_{ik},{\Psi }_{ijk})$ for both activity models. ... mehrThe thermodynamic constants determined in this work are discussed with reference to values available in the literature.