Deconvolution of composite fluorescence spectra and related decay curves for Uv' in the pH ranges &5 and 9-1 1 at different total U concentrations, yielded decay times and individual fluorescence spectra for various hydroxocomplexes (U02),(0H),c2" '-") , 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 t h e uranyl ion, whereas the latter is ca. 3 times more fluorescent than t h e (1, 1) complex. Thus, relatively small concentrations of polynuclear species contribute appreciably to t h e overall fluorescence of aqueous Uv' solutions. The fairly large difference between decay times and the high sensitivity of t h e equipment used made it possible to detect even small amounts of t h e hydroxo complexes. In the alkaline pH range 10-12, t h e (1, 3) hydroxo-complex was found to predominate. In t h e 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(v1) phosphate complexes at high pH.
Fluorescence measurements were performed at different temperatures to increase the resolving capabilities of time-resolved laser-induced fluorescence for uranium speciation. This spectroscopic technique was used to determine the fluorescence lifetimes of the uranyl ion and the species UO2OH+, (UO2)2(OH)2+2, and (UO2)3 (OH)5+ in 0.5 M NaClO4 solutions at different pH and total uranium concentration and in the temperature range from 10 to 60 °C. The temperature dependence of the lifetimes was found to follow the Arrhenius law, and the activation energy of the process was determined for each species. The influence of temperature on the equilibrium constants of the uranium hydroxo-complexes was also considered.
The reduction of U(VI) to U(IV) in aqueous suspensions of
TiO2 (pH 4.5−7) was investigated by time-resolved laser-induced fluorescence (TRLIF). The laser irradiation was
used both to provide the energy necessary for the
photogeneration of charge carriers and to monitor the
kinetics of the oxidation state change directly in the
measurement cuvette by exciting the U(VI) fluorescence.
The U(VI) photoreduction was found to involve only adsorbed
uranium species. The TRLIF measurements of U(VI)
fluorescence lifetimes and individual emission fluorescence
spectra allowed a selective identification of the various
U(VI) hydro complexes present in the aqueous phase and
adsorbed on the mineral surface. A reaction mechanism
is proposed based on the kinetics results obtained at pH 7
where adsorption of the hydroxocomplex (UO2)3(OH)5
- on
TiO2 is complete.
The effects of humic acids (HA) on the photoinduced
reduction of U(VI) in aqueous suspensions containing
titanium dioxide particles was investigated by time-resolved
laser induced fluorescence in the pH range 5 to 7. The
fluorescence response of both aqueous phase and adsorbed
U(VI) species was also analyzed. The rate of the aqueous
phase U(VI) photoreduction was found to be enhanced
by U(VI) complexation with HA. A photosensitizing effect
of HA on the interface reaction was evidenced at high
irradiation intensity, with the HA prefilter effect prevailing
at low irradiation. A kinetic model was developed,
which takes into account U(VI) photoreduction occurring
both in the aqueous phase and at the semiconductor oxide−water interface as well as adsorption and reoxidation
reactions.
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