Combining Luminescence Spectroscopy, Parallel Factor Analysis and Quantum Chemistry to Reveal Metal Speciation - a Case Study of Uranyl (VI) Hydrolysis

This study of aqueous metal speciation is an advanced combination of theoretical and experimental methods. Continuous wave (CW) and time-resolved laser-induced fluorescence spectroscopy (TRLFS) data of uranyl(VI) hydrolysis were analyzed using parallel factor analysis (PARAFAC). Distribution patterns of five major species were thereby derived under a fixed uranyl concentration (10-5 M) over a wide pH range from 2 to 11. UV (180 nm to 370 nm) excitation spectra were extracted for individual species. Time-dependent density functional theory (TD-DFT) calculations revealed ligand excitation (water, hydroxo, oxo) in this region and ligand to metal charge transfer (LMCT) responsible for luminescence. Thus excitation in the UV region is extreme ligand sensitive and not unspecific. Combining findings from PARAFAC and DFT the UO22+ cation (aquo complex 1:0) and four hydroxo complexes (1:1, 3:5, 3:7 and 1:3) were identified.