Uranium(VI) speciation at elevated temperatures and pressures by time-resolved laser-induced fluorescence spectroscopy

A few experimental studies on uranium(VI) hydrolysis and complexation at elevated temperatures and pressures performed to date are obviously quite insufficient to depict a comprehensive picture of the species and the behavior in a wide variety of hydrothermal conditions in the geosphere. In this study, an optical cell system for spectroscopic speciation of metal ions in hydrothermal solutions was developed and combined with time-resolved laser-induced fluorescence spectroscopy for uranium(VI) speciation. Emission spectra and lifetimes of 1x10^-4 M(=mol·dm^-3) uranium(VI) in NaClO₄ solutions in the presence and absence of sulfate or fluoride ion were measured as a function of pH, ligand concentration, temperature (25-200degC), or pressure (0.1 or 40 MPa). The results were compared with speciation calculations at various conditions on the basis of thermodynamic model and data in the literature. The emission intensities and lifetimes of uranium(VI) decreased rapidly with increasing temperature. The pressure effect on the fluorescence properties could not be neglected, but it was rather small relative to the temperature effect. The temperature dependence of the lifetimes obeyed well the Arrhenius law, and the activation energy was characterized for uranyl(VI) ion and the hydroxide and fluoride complexes. (author)