Chloride adsorption on Fe- and Al-(hydr)oxide: estimation of Gibbs free energies

In this study, we used chemical quantum methods to analysis the adsorption of chloride on Al and Fe-(hydr)oxide clusters. Inner and outer sphere complexes were the generating complexes during the adsorption process on variably charged Al- and Fe-(hydr)oxide clusters. For the chloride adsorption on Al-(hydr)oxide, the outer sphere complexes-H-bonded-were favored for all clusters, while the adsorption modes as inner sphere complexes-BB or MM-were not favored. It was found, that the H-bonded complex on neutral clusters was the most thermodynamically favored with an adsorption energy of - 63.4 kJ/mol. For iron clusters, thermodynamic favorability was observed for both outer (- 70.5 kJ/mol) and inner monodentate (- 65.8 kJ/mol) sphere complexes. These theoretical results indicated that the thermodynamic favorability of chloride adsorption on Fe and Al-(hydr)oxide was directly related to positive surface charge.