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 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 0i), and the specific binary and ternary interaction parameters ( ε ij , β (0)ij, β (1)ij, C ϕij, 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.