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

The formation constants of UO2SO4 (aq), UO2(SO4)22−, and UO2(SO4)34− were measured in aqueous solutions from 10 to 75 °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: log10 K1(25 °C) = 2.45 ± 0.05, ΔrH1 = 29.1 ± 4.0 kJ·mol−1, log10 K2(25 °C) = 1.03 ± 0.04, and ΔrH2 = 16.6 ± 4.5 kJ·mol−1. While the enthalpy of the UO2(SO4)22− 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 ΔrH1 in previous calorimetric studies. In fact, one of the published calorimetric determinations of ΔrH1 is here supported by the TRLFS results only when reinterpreted with a more correct equilibrium constant value, which shifts the fitted ΔrH1 value up by 9 kJ·mol−1. UO2(SO4)34− wa...