Uranium speciation studies at alkaline pH and in the presence of hydrogen peroxide using time-resolved laser-induced fluorescence spectroscopy

Time-resolved laser-induced fluorescence spectroscopy (TRLFS) was used to study the speciation of uranium(VI) at very alkaline pH (11–13), at room temperature and in the absence of CO₂. In this case, at pH = 11, two different fluorescence lifetimes appeared, which were attributed to the species UO₂(OH)₃ ⁻ and (UO₂)₃(OH)₇ ⁻. At pH = 13, no fluorescence was detected, indicating that the predominant species, UO₂(OH)₄ ²⁻, is not fluorescent. At pH = 12, the lifetime obtained is attributed to the predominant species UO₂(OH)₃ ⁻.

Because of the absence of fluorescence of the UO₂(OH)₄ ²⁻ species at room temperature, measurements at 10 K were made, obtaining two different lifetimes in the pH range between 12 and 13.5, indicating the presence of two different species: UO₂(OH)₃ ⁻ and UO₂(OH)₄ ²⁻. The difference between the lifetimes allowed the calculation of the contribution of each species to the total fluorescence signal intensity.

From the experiments carried out in the presence of hydrogen peroxide, it was observed that hydrogen peroxide produces a quenching effect to the fluorescence of the uranium species. At pH 12 the quenching is static, which points to the formation of a non-fluorescent complex between U(VI) and hydrogen peroxide. Using the Stern–Volmer equation for static quenching, the equilibrium formation constant of the first species, UO₂O₂(OH)₂ ²⁻, was calculated to be logK⁰ = 28.7 ± 0.4, which is similar to the one determined using UV–Visible spectrometry, 28.1 ± 0.2 Meca et al. (2011).