Spectroscopic studies on U(VI)-salicylate complex formation with multiple equilibria

摘要

This study investigates multiple equilibria related to the formation of the U(VI)-salicylate complex in a pH range of 3.0-5.5 using UV-Vis absorption and fluorescence measure-ment techniques. The absorbance changes at the characteris-tic charge-transfer bands of the complex were monitored, and the results indicated the presence of multiple equilibria and the formation of both 1 : 1 and 1 : 2 (U(VI) : salicylate) com-plexes possessing bi-dentate chelate structures. The determined step-wise formation constants (log K_(1:1) and log K_(1:2)) are as follows: 12.5 ± 0.1 and 11.4 ± 0.2 for salicylate, 11.2 ± 0.1 and 10.1 ± 0.2 for 5-sulfosalicylate, and 12.4 ± 0.1 and 11.4 ± 0.1 for 2,6-dihydroxybenzoate, respectively. The molar absorptivities of the complexes are also provided. Furthermore, time-resolved laser-induced luminescence spectra of U(VI) species demonstrate the presence of both a dynamic and static quenching process upon the addition of a salicylate ligand. Particularly for the luminescent hydroxouranyl species, a strong static quenching effect is observed. The results suggest that both the UO_2(HSal)~+ and the U(VI)-Sal chelate complexes serve as ground-state complexes that induce static quenching. The SternVolmer parameters were derived based on the measured luminescent intensity and lifetime data. The static quenching constants (log K_s) obtained are 3.3 ± 0.1, 4.9 ± 0.1, and 4.4 ± 0.1 for UO_2~(2+), (UO_2)_2(OH)_2~(2+) and (UO_2)_3(OH)_5~+, respectively.