Investigation of Uranyl Sulfate Complexation under Hydrothermal Conditions by Quantitative Raman Spectroscopy and Density Functional Theory

Quantitative first and second formation constants of aqueous uranyl sulfate complexes were obtained from Raman spectra of solutions in fused silica capillary cells at 25 MPa, at temperatures ranging from 25 to 375 °C. Temperature-dependent values of the symmetric O-U-O vibrational frequencies of UO₂²⁺(aq), UO₂SO₄⁰(aq), and UO₂(SO₄)₂^2-(aq) were determined from the high-temperature spectra. Temperature-independent Raman scattering coefficients of UO₂²⁺(aq) were calculated directly from uranyl triflate spectra from 25 to 300 °C, while those of UO₂SO₄⁰(aq) and UO₂(SO₄)₂^2-(aq) were derived from spectroscopic data at 25 °C and concentrations calculated using the formation constants of Tian and Rao ( J. Chem. Thermodyn. 2009 , 41 , 569 - 574 ), together with the Specific Ion Interaction Theory (SIT) activity coefficient model. Chemical structures and vibrational frequencies predicted from Density Functional Theory (Gaussian 09) were employed to interpret the Raman spectra. Values of the cumulative formation constants ranged from log β₁ = 3.23 ± 0.08 and log β₂ = 4.22 ± 0.15 at 25 °C, to log β₁ = 12.35 ± 0.22 and log β₂ = 14.97 ± 0.02 at 350 °C. This is the first reported use of high-pressure fused silica capillary cells to determine formation constants of metal ligand complexes from their reduced isotropic Raman spectra under hydrothermal conditions.