Analysis of mononuclear uranyl(VI) hydroxo complexes using TRLFS and PARAFAC

Speciation analysis of fast equilibrium processes is often challenging. Metal hydrolysis is one example of such a system. It is the basis for more complex aquatic systems and thus a deep understanding of those systems is indispensable. In the case of uranyl(VI) hydrolysis spectroscopic studies are hindered by low solubility over an wide pH range. Additionally occurrence of polynuclear complexes further complicates the system when working close to the solubility limit (e.g. 10-5 M uranyl(VI)). In a previous study we demonstrated that a combination of luminescence spectroscopic methods together with state of the art data analysis (parallel factor analysis; PARAFAC) and quantum chemical calculations is a powerful setup to gain information on that system [1]. Since uranyl(VI) has high affinities to several minerals and biopolymers systems containing trace metal concentrations have to be considered. In the present study we focus on this low concentration range (10-8 M uranyl(VI)).
We were able to extract thermodynamic constants for this system [(UO2(OH)(H2O)4+), (UO2(OH)2(H2O)3), (UO2(OH)3(H2O)2-)] using optimized data processing. Furthermore, advanced deconvolution of individual luminescence spectra demonstrates the correlation of luminescence spectroscopy and vibrational spectroscopy. Raman frequencies result from symmetrical stretching of the uranyl(VI) unit. We could show that features of individual emission spectra are separated by the same energy. Thus a further luminescence signal to chemical structure correlation is demonstrated which was missing for that system.