Thermodynamic and structural aspects of the aqueous uranium(IV) system – hydrolysis vs. sulfate complexation

The aquatic species of U(IV) in acidic aqueous solution in the presence of sulfate was studied in the micromolar range by a combined approach of optical spectroscopies (UV/vis and mid-IR), quantum-chemical calculations (QCC), and thermodynamic modelling. The number of species occurring in solution within the pH range 0–2 was assessed by decomposition and fitting of photometric spectra using HypSpec and Geochemist’s Workbench software. Single component spectra of U⁴⁺, UOH³⁺, USO₄²⁺ and U(SO₄)₂ were obtained and extinction coefficients ελ have been calculated to be 58.8, 19.2, 47.6 and 40.3 L mol ⁻ ¹ cm ⁻ ¹, respectively. Complex formation constants of two U(IV) sulfate species and the first hydrolysis species UOH³⁺ in infinite diluted solution were determined by thermodynamic modelling to be log β⁰₁₀₁ = 6.9 ± 0.3, log β⁰₁₀₂ = 11.8 ± 0.5 and log β⁰₁₁₀ = − (0.36 ± 0.1), respectively. No further U(IV) sulfate and hydrolysis species were observed under the prevailing conditions. Molecular structural information of the sulfate species was derived from vibrational spectra and QCC exhibiting a predominant monodentate coordination of the sulfate ions.