Studies on U(VI)-salicylate Charge Transfer Complex Formation by Using Time-resolved Laser Fluorescence Spectroscopy

Organic ligands, such as humic and fulvic acids, play important roles in dissolution and migration of actinide radionuclide species. They can form stable actinide complexes in the presence of inorganic ions like hydroxides and carbonates. Therefore, the structural mimics of such ligands containing carboxylic and phenolic functional groups have been targets of studies to understand their chemical behaviors migrating actinides under geological groundwater conditions. Among many carboxylic ligands salycylate (SA) is useful to examine the role of phenolic groups on humic substances. Salicylate ion can form ligand-to-metal chargetransfer (LMCT) complexes with various metal ions. It has been shown that the fluorescence of SA is suppressed in the presence of Cu(II) or Eu(III) at pH 4 by forming ground state complexes. While uranyl ions also can form complex with SA, the dominant species at pH 4-7 region is known to be an 1:1 complex. However, the chemical structures reported on this 1:1 species are different among [UO2SA]0, [UO2(OH)SAH]0 and [UO2(OH)SA]-. For α- hydroxycarboxylates cyclic chelate structures were suggested where both carboxylate and phenolate groups simultaneously bind to uranyl ion. In the present study we investigate aqueous U(VI)- SA complex systems by using UV-Vis absorbance measurement and highly sensitive time-resolved laserinduced fluorescence spectroscopic (TRLFS) technique. The U(VI)-SA complex shows two characteristic charge-transfer (CT) bands at higher pH (> 4), which we found useful to examine the complexation equilibrium. Further, the TRLFS method is used to study the fluorescence (FL) quenching of U(VI) species, particularly for hydroxouranyl species at pH 4.5 of which FL is significantly suppressed as SA concentration elevates in aqueous solution. Fluorescence quenching mechanism in conjunction with the formation of U(VI)-SA CT complex is discussed