Thermodynamics of the Eu(iii)–Mg–SO4–H2O and Eu(iii)–Na–SO4–H2O systems. Part II: spectroscopy experiments, complexation and Pitzer/SIT models

Abstract

A time-resolved laser fluorescence spectroscopy (TRLFS) study was carried out to investigate the Eu(III)–SO₄ complexation at room temperature over a wide range of Na₂SO₄ concentrations (0–2 mol kg⁻¹). Spectroscopic observations confirm the step-wise formation of the aqueous complexes Eu(SO₄)⁺, Eu(SO₄)₂⁻ and Eu(SO₄)₃³⁻ over the investigated Na₂SO₄ concentrations. Combining TRLFS data obtained in this study and solubility data reported in Part I of this work for the Eu₂(SO₄)₃–Na₂SO₄–H₂O and Eu₂(SO₄)₃–MgSO₄–H₂O 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 of Eu₂(SO₄)₃·8H₂O(cr) and Na₂Eu₂(SO₄)₄·2H₂O(cr), stability constants of the Eu(III)–SO₄ complexes (β⁰_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. The thermodynamic constants determined in this work are discussed with reference to values available in the literature.