Stoichiometries and thermodynamic stabilities for aqueous sulfate complexes of U(VI)

The formation constants of UO2SO4 (aq), UO2(SO4)(2)(2-), and UO2(SO4)(3)(4-) were measured in aqueous solutions from 10 to 75 degrees C by time-resolved laser-induced fluorescence spectroscopy (TRLFS). A constant enthalpy of reaction approach was satisfactorily used to fit the thermodynamic parameters of stepwise complex formation reactions in a 0.1 M Na+ ionic medium: log(10) K-1(25 degrees C) = 2.45 +/- 0.05, Delta H-r(1) = 29.1 +/- 4.0 kJ.mol(-1), log(10) K-2(25 degrees C) = 1.03 +/- 0.04, and Delta H-r(2) = 16.6 +/- 4.5 kJ.mol(-1). While the enthalpy of the UO2(SO4)(2)(2-) formation reaction is in good agreement with calorimetric data, that for UO2SO4 (aq) is higher than other values by a few kilojoules per mole. Incomplete knowledge of the speciation may have led to an underestimation of Delta H-r(1) in previous calorimetric studies. In fact, one of the published calorimetric determinations of Delta H-r(1) is here supported by the TRLFS results only when reinterpreted with a more correct equilibrium constant value, which shifts the fitted Delta H-r(1) value up by 9 kJ . mol(-1). UO2(SO4)(3)(4-) was evidenced in a 3 M Na+ ionic medium: log(10) K-3(25 degrees C) = 0.76 +/- 0.20 and Delta H-r(3) = 11 +/- 8 kJ . mol(-1) were obtained. The fluorescence features of the sulfate complexes were observed to depend on the ionic conditions. Changes in the coordination mode (mono- and bidentate) of the sulfate ligands may explain these observations, in line with recent structural data.