Speciation study on complex formation of uranium(VI) with phosphate and fluoride at high temperatures and pressures by time-resolved laser-induced fluorescence spectroscopy

The complex formation of U(VI) with phosphate and with fluoride has been studied at elevated temperatures (20-150 °C) and pressures (0.1-40 MPa) using time-resolved laser-induced fluorescence spectroscopy (TRLFS). Emission spectra and lifetimes of 5 × 10^-5 M uranium(VI) in 0.5 M NaClO₄ solutions in the presence and absence of ligand ions were measured as a function of the pH, ligand concentration, temperature and pressure. The emission intensities and lifetimes of uranium(VI) decreased rapidly with increasing temperature. In contrast, a pressure effect on the fluorescence properties was quite small. The fluorescence decay curves could be resolved into several fluorescent species. The temperature dependence of the lifetimes followed the Arrhenius law, and the activation energies were estimated for the phosphate and fluoride complexes. These results were compared with speciation calculations at several temperatures obtained by using the DQUANT equation and thermodynamic data in the literature. From these comparisons, each component of the fluorescence was identified as a complexed species or a free uranyl ion at several temperatures, indicating that the Arrhenius plot of fluorescence lifetime is an effective method for in-situ speciation of uranyl complexation at elevated temperatures.