Speciation study on uranium(VI) hydrolysis at high temperatures and pressures

Hydrolysis behavior of U(VI) has been examined at elevated temperatures (up to 100 degreesC) and pressures (0.1-40 MPa) using time-resolved laser-induced fluorescence spectroscopy. Emission spectra and lifetimes of 5 x 10(-5) M uranium(VI) in 0.5 M NaClO4 at pH 3.8-6.0 were measured as a function of temperature and pressure. With increasing temperature, the emission intensities and lifetimes of uranium(VI) hydrolysis species decreased rapidly. On the other hand, the pressure effect on the luminescence properties was quite small as compared with the effect of temperature. The temperature dependence of the lifetimes followed well the Arrhenius law, enabling the activation energies to be estimated for the luminescent species. These experimental results were compared with the speciation calculations carried out at several temperatures by using the DQUANT equation together with the thermodynamic data in the literature. From this comparison, each component of luminescence could be assigned to the hydrolysis species such as UO2(OH)(+), (UO2)(2)(OH)(2)(2+), (UO2)(3)(OH)(5)(+) oruranyl(VI) ion indicating that the Arrhenius plot of luminescence lifetime is an effective method for in-situ speciation of uranyl hydrolysis product at elevated temperatures. (C) 2003 Elsevier B.V. All rights reserved.