Calorimetric screening of co-operative effects in adsorption of Co(II) on γ-alumina surface in the presence of Co-complexing anions in aqueous solution

Abstract

The understanding of the mechanism of co-operative adsorption of Co(II) cations and acetate or citrate anions onto γ-Al2O3 from aqueous solutions has been refined on the basis of comparison between the enthalpies of displacement measured in single-solute and bi-solute systems by means of isothermal titration calorimetry. The data processing procedures were adapted to take into account the occurrence of a cobalt-ligand complex in the bulk solution. Considering the bridging role of the adsorbed cobalt cations as a starting point to reproduce the enthalpy of displacement in the cobalt-acetate system, each ligand unit was suggested to bind preferentially to more than one adsorbed metal species and to interact additionally with some electrically neutral binding sites on the oxide surface. In the case of cobalt-citrate couple, the formation of a 1:1 stoichiometry solid-ligand-metal complex and simultaneous adsorption of cobalt cations as bidentate inner-sphere complexes reproduced best the experimental data.