Thermodynamics of the Eu(iii)–Mg–SO4–H2O and Eu(iii)–Na–SO4–H2O systems. Part I: solubility experiments and the full dissociation Pitzer model

Abstract

The solubility of Eu(III) was investigated under undersaturated conditions in acidic, dilute to concentrated MgSO₄ and Na₂SO₄ 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 Eu₂(SO₄)₃–MgSO₄–H₂O (19 samples) and Eu₂(SO₄)₃–Na₂SO₄–H₂O (26 samples). Eu₂(SO₄)₃·8H₂O(cr) was found to control the solubility of Eu(III) in all investigated MgSO₄ solutions, as well as in dilute Na₂SO₄ systems. The transformation of Eu₂(SO₄)₃·8H₂O(cr) into the double salt Na₂Eu₂(SO₄)₄·2H₂O(cr) was observed at mNa₂SO₄ > 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 MgSO₄ and Na₂SO₄ 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^ϕ_ij, θ_ik, Ψ_ijk).