Journal of Nuclear Fuel Cycle and Waste Technology

In this study the complex formation reactions between uranium(VI) and 2,6-dihydroxybenzoate (DHB) as a model ligand of humic acid were investigated by using UV-Vis spectrophotometry and time-resolved laser-induced fluorescence spectroscopy (TRLFS). The analysis of the spectrophotometric data, i.e., absorbance changes at the characteristic charge-transfer bands of the U(VI)-DHB complex, indicates that both 1:1 and 1:2 (U(VI):DHB) complexes occur as a result of dual equilibria and their distribution varies in a pH-dependent manner. The stepwise stability constants determined (log K1 and log K2) are 12.4 0.1 and 11.4 0.1. Further, the TRLFS study shows that DHB plays a role as a fluorescence quencher of U(VI) species. The presence of both a dynamic and static quenching process was identified for all U(VI) species examined, i.e., UO22+, (UO2)2(OH)22+, and (UO2)3(OH)5+. The fluorescence intensity and lifetimes of each species were measured from the time-resolved spectra at various ligand concentrations, and then analyzed based on Stern-Volmer equations. The static quenching constants (log Ks) obtained are 4.2 0.1, 4.3 0.1, and 4.34 0.08 for UO22+, (UO2)2(OH)22+, and (UO2)3(OH)5+, respectively. The results of Stern-Volmer analysis suggest that both mono- and bi-dentate U(VI)-DHB complexes serve as groundstate complexes inducing static quenching.

Keywords

Uranium(VI),2,6-Dihydroxybenzoic acid,Chelate complex,Complex formation constant,Absorption spectroscopy,TRLFS