Thermodynamics of the Eu(III)-Mg-SO 4 -H 2 O and Eu(III)-Na-SO 4 -H 2 O systems. Part I: solubility experiments and the full dissociation Pitzer model.

The solubility of Eu(III) was investigated under undersaturated conditions in acidic, dilute to concentrated MgSO 4 and Na 2 SO 4 solutions at T = (22 ± 2) °C. After attaining equilibrium conditions, solid phases were characterized by a multi-method approach, including X-ray diffraction (XRD), Raman and infrared (IR) spectroscopy, quantitative chemical analysis (ICP-OES) and thermogravimetric analysis (TG-DTA). A total of 45 solubility samples were investigated for the systems Eu 2 (SO 4 ) 3 -MgSO 4 -H 2 O (19 samples) and Eu 2 (SO 4 ) 3 -Na 2 SO 4 -H 2 O (26 samples). Eu 2 (SO 4 ) 3 ·8H 2 O(cr) was found to control the solubility of Eu(III) in all investigated MgSO 4 solutions, as well as in dilute Na 2 SO 4 systems. The transformation of Eu 2 (SO 4 ) 3 ·8H 2 O(cr) into the double salt Na 2 Eu 2 (SO 4 ) 4 ·2H 2 O(cr) was observed at mNa 2 SO 4 > 0.01 mol kg -1 . The latter phase is characterized by significantly lower solubility. Based on these experimental solubility measurements, thermodynamic and activity models were proposed based on the Pitzer equations considering the full dissociation of the Eu(III) species in MgSO 4 and Na 2 SO 4 aqueous solutions, i.e. deliberately excluding Eu(III)-sulfate complex formation. A combination of the geochemical calculation code PhreeSCALE and the parameter estimation code PEST was used to determine the values of solubility products and binary and ternary specific interaction parameters ( β (0)ij, β (1)ij, C ϕij, θ ik , Ψ ijk ).