U⁴⁺ Speciation in Acidic Aqueous Solution: Insights from UV–Vis, EXAFS, XANES, and Quantum-Statistical Simulations

This theoretical study investigates the UV-vis absorption properties of U⁴⁺-containing aqueous solutions and their relationship with the nature of aqua-complexes present at varying acidic levels. High-level quantum-mechanical calculations─accounting for relativistic effects, spin-orbit coupling, and both dynamic and nondynamic correlation─were combined with classical Molecular Dynamics simulations. EXAFS, XANES, and UV-vis spectra of U⁴⁺-containing aqueous solutions were used as experimental reference data and compared with the corresponding theoretical predictions. UV-vis spectra were available at various pH values. Theoretical spectra were generated as averages of individual spectra computed from the structures statistically generated. Coordination numbers ranging from 8 to 10 for the aqua ion were explored. Although the theoretical-experimental comparison of the EXAFS and XANES spectra allows us to reject ten-coordination, assigning the octa- or nine- (ennea-)coordination to the U⁴⁺ aqua ion is difficult. However, UV-vis spectroscopy provided some evidence supporting a preference for the ennea-coordination. Spectra for aqueous solutions up to pH 2.22 were compared with simulated spectra of hydrolyzed forms of the aqua ion, in which up to two water molecules were replaced by hydroxyl anions. Spectra obtained as simulated mixtures of the aqua ion and hydrolyzed species in varying ratios produced a spectral evolution with pH that closely resembles experimental observations.