Abstract:
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Time-Resolved Laser-Induced Fluorescence Spectroscopy (TRLFS) has been used to
study the speciation of uranium(VI) at very alkaline pH. The hydroxocomplex UO2(OH)3 has
been reported to be responsible for all uranium fluorescence at pH 12. In this conditions
hydrogen peroxide has a dramatic influence in the fluorescence of UO2(OH)3, known as
quenching. Observing fluorescence lifetime we have concluded that only static quenching
occurs. The study of the variation of the fluorescence as a function of hydrogen peroxide
concentration in solution, has allowed the determination of the formation constant of the
uranyl-peroxide complex UO2(OH)2(O2)-, with logK0=29.3 ± 0.4.
In addition to TRLFS experiments, formation of uranyl-peroxide complexes has been
studied with UV-Vis (UV-Visible light) spectrophotometry. Two complexes have been found
in the H2O2/U(VI) ratio < 2. Graphical and numerical methods have been used to determine
the equilibrium constants. For the numerical determination program STAR has been chosen,
due to his capacity of treating multiwavelength absorbance data and refining equilibrium
constants. The logarithm of the equilibrium constant is found to be 27.9 ± 0.1 for the first
specie (UO2O2(OH)2
2-) and 37.1 ± 0.2 for the second specie (UO2(O2)2(OH)2
4-). The
formation of those complexes in solution has a very important impact on the speciation of
uranium(VI), because it is predominant at hydrogen peroxide concentrations higher than the
total uranium concentration, at alkaline pH and, for example, increases the solubility of the
uranyl peroxide studtite more than two orders of magnitude depending on the solution
conditions. |