Study of uranyl(VI)malonate complexation by time-resolved laser-induced fluorescence spectroscopy (TRLFS)

The uranyl(VI) malonate complex formation was studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS) at pH 4 and an ionic strength of 0.1 M NaClO₄. The uranium concentration was 5 × 10^-6 M at ligand concentrations from 1 × 10^-5 to 1 × ^-2 M.

The measured fluorescence lifetimes of the 1:1 and 1:2 uranyl(VI) malonate complexes are 1.24 ± 0.02 µs and 6.48 ± 0.02 µs, respectively. The fluorescence lifetime of the uranyl(VI) ion is 1.57 ± 0.06 µs in 0.1 M perchloric media. The main fluorescence bands of the malonate complexes show a bathochromic shift compared to the uranyl(VI) ion and are centered at 494 nm, 515 nm and 540 nm for the 1:1 complexes and at 496 nm, 517 nm and 542 nm for the 1:2 complex. The spectra of the individual uranyl(VI) malonate complexes were calculated using a multi exponential fluorescence decay function for each intensity value at each wavelength, covering the entire wavelength range. Stability constants were determined for the complexes UO₂C₃H₂O₄°_(aq) and UO₂(C₃H₂O₄)₂ ^2- from results of spectra deconvolution using a least square fit algorithm (logβ₁° = 4.48 ± 0.06, logβ₂° = 7.42 ± 0.06 or logK₂° = 2.94 ± 0.04). The results are compared with literature values obtained by potentiometric measurements.