Influence of pH and competitive adsorption on the kinetics of ligand-promoted dissolution of aluminum oxide

The kinetics of sigma-Al2O3 dissolution were examined in the presence of 8-hydroxyquinoline-5-sulfonate (HQS) and salicylate over the pH range 3-9. The greatest effects of both of these ligands on sigma-Al2O3 dissolution were observed at pH Values higher than those corresponding to maximal adsorbed ligand concentrations. Thus, calculated rate constants were pH dependent. For HQS, correlation between the fluorescence of the surface complex and the adsorbed HQS concentration indicates that the pH dependence of the rate constant cannot be explained by a change in the structure of the metal-organic surface complex. Rather, it is proposed that the rate-determining step in the dissolution reaction involves a mixed surface complex in which aluminum is coordinated by both the organic ligand and hydroxide. Similarly, dissolution rates in the presence of the competing adsorbates HQS and fluoride suggest a synergistic action of these two ligands. Dissolution rates predicted from measured adsorbed concentrations of both ligands assuming independent, parallel pathways for HQS- and fluoride-promoted dissolution underpredict observed dissolution rates at some adsorbed ligand concentrations. In contrast, dissolution rates in the presence of the competing adsorbates HQS and arsenate could he predicted simply by accounting for the displacement of HQS from the oxide surface by arsenate.