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 were studied in the micromolar range by a combined approach of optical spectroscopy (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 were calculated to be 61.7, 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 infinitely diluted solution were determined by thermodynamic modelling to be log β = 6.9 ± 0.3, log β = 11.8 ± 0.5 and log β = -(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.