Characterisation of hydroxide complexes of uranium(VI) by time-resolved fluorescence spectroscopy

Abstract

Deconvolution of composite fluorescence spectra and related decay curves for U^VI in the pH ranges 0–5 and 9–11 at different total U concentrations, yielded decay times and individual fluorescence spectra for various hydroxo-complexes (UO₂)_m(OH)_n^(2m–n), where (m, n)=(1, 1); (1, 3); (2, 2) and (3, 5). Depending on the emission wavelength, the fluorescence efficiencies of (2, 2) and (3, 5) were found to be, respectively, 7 to 85, and 3 to 4 times higher than that of the uranyl ion, whereas the latter is ca. 3 times more fluorescent than the (1, 1) complex. Thus, relatively small concentrations of polynuclear species contribute appreciably to the overall fluorescence of aqueous U^VI solutions. The fairly large difference between decay times and the high sensitivity of the equipment used made it possible to detect even small amounts of the hydroxo complexes. In the alkaline pH range 10–12, the (1, 3) hydroxo-complex was found to predominate. In the presence of 0.05 mol dm^–3 phosphate and in the same pH range, there was a pronounced change in the fluorescence spectrum indicating that the chemical speciation in the system had changed. These observations make it necessary to revise the current equilibrium data for uranium (VI) phosphate complexes at high pH.