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 $(β^0\ _i)$, and the specific binary and ternary interaction parameters $(ε_{ij}, β^{(0)}\ _{ij}, β^{(1)}\ _{ij}, C^ϕ\ _{ij}, θ_{ik}, Ψ_{ijk})$ for both activity models. ... mehrThe thermodynamic constants determined in this work are discussed with reference to values available in the literature.