A spectroscopic study of the effect of ligand complexation on the reduction of uranium(VI) by anthraquinone-2,6-disulfonate (AH₂DS)

Abstract

In this paper, the reduction rate of uranyl complexes with hydroxide, carbonate, EDTA, and desferriferrioxamine B (DFB) by anthraquinone-2,6-disulfonate (AH₂DS) is studied by stopped-flow kinetic technique under anoxic atmosphere. The apparent reaction rates varied with ligand type, solution pH, and U(VI) concentration. For each ligand, a single largest pseudo −1^st order reaction rate constant, k_obs, within the studied pH range was observed, suggesting the influence of pH-dependent speciation on the U(VI) reduction rate. The maximum reaction rate found in each case followed the order of OH⁻>CO₃²⁻>EDTA>DFB, in reverse order of the trend of the thermodynamic stability of the uranyl complexes and ionic sizes of the ligands. The pH-dependent rates were modeled using a second-order rate expression that was assumed to be dependent on a single U(VI) complex and an AH₂DS species. By quantitatively comparing the calculated and measured apparent rate constants as a function of pH, species AHDS³⁻ was suggested as the primary reductant in all cases examined. Species UO₂CO₃(aq), UO₂HEDTA⁻, and (UO₂)₂(OH)₂²⁺ were suggested as the principal electron acceptors among the U(VI) species mixture in each of the carbonate, EDTA, and hydroxyl systems, respectively.