Uranium(VI) Complexation with Carbonate studied by Cryo-TRLFS and PARAFAC

Cryogenic techniques in laser spectroscopy are currently used in scanning tunneling microscopy (STM) and single molecule spectroscopy. Recently such cryogenic devices have also been adapted to time-resolved laser-induced fluorescence spectroscopy (TRLFS) applied to uranium. We used TRLFS at low temperatures (cryo-TRLFS) to investigate the uranium(VI) complexation with carbonate. The luminescence spectra were analyzed with parallel factor analysis (PARAFAC). The PARAFAC modeling provided the luminescence spectra, decay lifetimes, and relative intensity profiles of four U(VI) species in presence of carbonate. The U(VI) species distribution calculated from the known stability constants of relevant U(VI) hydrolysis and carbonate species,compared well with the pH-depending U(VI) speciation derived from PARAFAC-analyzed TRLFS spectra. In the acidic pH range below pH 4.5 the U(VI) speciation is dominated by the free UO22+ and one UO22+/OH- species. With increasing pH we observed the formation of two different U(VI) carbonate species. Our results suggest that PARAFAC is a promising data analysis tool for TRLFS in the context of speciation studies of fluorescent metal ions.