Thermodynamics of the Eu(III)-Mg-SO₄-H₂O and Eu(III)-Na-SO₄-H₂O systems. Part II: spectroscopy experiments,...

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)₃^3- 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 (β0i), and the specific binary and ternary interaction parameters (ε, β(0)ij, β(1)ij, Cϕij, θ, Ψ) for both activity models. The thermodynamic constants determined in this work are discussed with reference to values available in the literature.