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, Δ_rH₁ = 29.1 ± 4.0 kJ·mol⁻¹, log₁₀ K₂(25 °C) = 1.03 ± 0.04, and Δ_rH₂ = 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 Δ_rH₁ in previous calorimetric studies. In fact, one of the published calorimetric determinations of Δ_rH₁ is here supported by the TRLFS results only when reinterpreted with a more correct equilibrium constant value, which shifts the fitted Δ_rH₁ value up by 9 kJ·mol⁻¹. UO₂(SO₄)₃⁴⁻ was evidenced in a 3 M Na⁺ ionic medium: log₁₀ K₃(25 °C) = 0.76 ± 0.20 and Δ_rH₃ = 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.