Thermodynamics of the Eu(III)-Mg-SO4-H2O and Eu(III)-Na-SO4-H2O 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)-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 (β0i), and the specific binary and ternary interaction parameters (εij, β(0)ij, β(1)ij, Cϕij, θik, Ψijk) for both activity models. The thermodynamic constants determined in this work are discussed with reference to values available in the literature.

Speciation, thermodynamics and structure of Np(V) oxalate complexes in aqueous solution

The speciation, thermodynamics and structure of the Np(v) (as the NpO2+ cation) complexes with oxalate (Ox2-) are studied by different spectroscopic techniques. Near infrared absorption spectroscopy (Vis/NIR) is used to investigate complexation reactions as a function of the total ligand concentration ([Ox2-]total), ionic strength (Im = 0.5-4.0 mol kg-1 Na+(Cl-/ClO4-)) and temperature (T = 20-85 °C) for determination of the complex stoichiometry and thermodynamic functions (log β0n(T), ΔrH0n, ΔrS0n). Besides the solvated NpO2+ ion, two NpO2+ oxalate species (NpO2(Ox)n1-2n; n = 1, 2) are identified. With increasing temperature a decrease of the molar fractions of the 1 : 1 - and 1 : 2 - complexes is observed. Application of the law of mass action yields the temperature dependent conditional stability constants log β'n(T) at a given ionic strength which are extrapolated to IUPAC reference state conditions (Im = 0) according to the specific ion interaction theory (SIT). The log β0n(T) values of both complex species (log β01(25 °C) = 4.53 ± 0.12; log β02(25 °C) = 6.22 ± 0.24) decrease with increasing temperature confirming an exothermic complexation reaction. The temperature dependence of the thermodynamic stability constants is described by the integrated van't Hoff equation yielding the standard reaction enthalpies (ΔrH01 = -1.3 ± 0.7 kJ mol-1; ΔrH02 = -8.7 ± 1.4 kJ mol-1) and entropies (ΔrS01 = 82 ± 2 J mol-1 K-1; ΔrS02 = 90 ± 5 J mol-1 K-1) for the complexation reactions. In addition, the sum of the specific binary ion-ion interaction coefficients Δε0n(T) for the complexation reactions are obtained from SIT modelling as a function of the temperature. The structure of the complexes and the coordination mode of oxalate are investigated using EXAFS spectroscopy and quantum chemical calculations. The results show, that in case of both species NpO2(Ox)- and NpO2(Ox)23-, chelate complexes with 5-membered rings are formed.

Determination of thermodynamic functions and structural parameters of NpO2+ lactate complexes

This work is a detailed spectroscopic and quantum chemical study on the complexation of Np(v) with the α-hydroxy carboxylate lactate giving information on the complex stoichiometries and thermodynamics (log β, ΔH, ΔS).