Speciation Studies of Metals in Trace Concentrations: The Mononuclear Uranyl(VI) Hydroxo Complexes

A direct luminescence spectroscopic experimental setup for the determination of complex stability constants of mononuclear uranyl(VI) hydrolysis species is presented. The occurrence of polynuclear species is prevented by using a low uranyl(VI) concentration of 10–8 M (2.4 ppb). Time-resolved laser-induced fluorescence spectra were recorded in the pH range from 3 to 10.5. Deconvolution with parallel factor analysis (PARAFAC) resulted in three hydrolysis complexes. A tentative assignment was based on thermodynamic calculations: UO22+, UO2(OH)+, UO2(OH)2, UO2(OH)3–. An implementation of a Newton–Raphson algorithm into PARAFAC allowed a direct extraction of complex stability constants during deconvolution yielding log(β1M,1°C)1:1 = −4.6, log(β1M,1°C)1:2 = −12.2, log(β1M,1°C)1:3 = −22.3. Extrapolation to standard conditions gave log(β0)1:1 = −3.9, log(β0)1:2 = −10.9, and log(β0)1:3 = −20.7. Luminescence characteristics (band position, lifetime) of the individual mononuclear hydroxo species were derived to serve as a reference data set for further investigations. A correlation of luminescence spectroscopic features with Raman frequencies was demonstrated for the mononuclear uranyl(VI) hydroxo complexes for the first time. Thereby a signal-to-structure correlation was achieved and the complex assignment validated.