Photochemical characterization of uranyl interaction with acetic acid

Abstract

Aqueous perchloric acid solutions containing 1 × 10−4 M of U(VI) and acetic acid at different concentrations (from 0 up to 0.6 M) are studied by time-resolved laser-induced luminescence spectroscopy (TRLLS) at pH 1.5, 2.0 and 2.5. The quenching of uranyl luminescence is observed. The graphs giving U(VI) luminescence intensity, as a function of dissociated and non-dissociated acetic acid form concentrations are constructed at the different pH values to point out the interacting ligand species. Acetate ions are demonstrated to be responsible for the luminescence quenching. Stern–Volmer analysis gives the dynamic quenching rate constant value (kq) (1.0 ± 0.2) × 109 L M−1 s−1 at ionic strength (μ) 0.05 M (pH 2.0). The bimolecular excited-state process is shown to be diffusion-controlled as kq is practically identical to the diffusion rate constant as calculated for uranyl and acetate species. With an increase of acetate concentration (from 1 × 10−4 M), static quenching occurs in addition to the dynamic quenching. The formation of a non-luminescent complex between uranyl and acetate is supported (with 1:1 stochiometry). The stability constant values for the first complex species (UO2CH3COO+) are obtained at different ionic strengths. Extrapolation to μ = 0 by the specific ion interaction theory (SIT) gives log β1° = 2.95 ± 0.08 at 293 K. From 1.5 × 10−3 M of acetate concentration, the decay curves became biexponential and shorter lifetime appears. The U(VI) luminescence intensity is not decreased anymore and the bathochromic shift of peak maxima is observed. Most probably, this indicates the formation of the second complex species of U(VI) with two acetate ligands. These species give luminescence emission.