Aqueous uranium(IV) fluoride complexes: determination of fluorescence properties by means of TRLFS

Fluorescence properties of uranium(IV) complexes have only been little investigated up to now. Our research contributes first data on uranium(IV) fluoride complexes in an aqueous environment. Investigations were carried out using time-resolved laser-induced fluorescence spectroscopy (TRLFS). The excitation wavelength for all samples applied was 245 nm. Samples investigated comprised a constant uranium(IV) concentration. Fluoride was added with varying concentrations (stoichiometrical deficiency up to an excess with respect to uranium).
The fluorescence spectrum of a pure uranium(IV) solution without the addition of fluoride is in accordance with the ones described by [1] and [2]. Positions of 5 main peaks were determined: 291, 320, 335, 394 and 409 nm. Addition of a stoichiometrically deficient proportion of fluoride of one tenth the uranium concentration does not a result in a change of the fluorescence spectrum. There is no shift of peaks or increase or decrease of fluorescence intensity observable. However, raising the fluoride concentration up to one third the concentration of uranium(IV) affects the spectra in a way that all the peaks commence to diminish, best observable via the main peaks at 319 and 409 nm.
At the same time, a new peak in the region around 400 nm occurs. This peak broadens with increasing fluoride concentration. In addition, a peak around 320 nm starts to build up and superimpose the original uranium(IV) peak at 320 nm. Enhancing the fluoride concentration further on up to half the uranium(IV) concentration results in an increase of the fluorescence spectrum of the complex formed. A further increase of the fluoride concentration does not affect the spectrum. The position of the new formed peaks as well as the fluorescence intensity remain stable. Hence, the formation of the uranium(IV)-fluoride complex has been complete.
Future investigations aim at the determination of the complex formation constants of the uranium(IV) fluoride complexes.