A Ligand Exchange Model for the Adsorption of Inorganic and Organic Ligands at Hydrous Oxide Interfaces

Specific adsorption of organic and inorganic weak acids and of anions on hydrous oxide surfaces and the concomitant influences upon surface charge can be interpreted as ligand exchange reactions at the reactive surface sites. Direct (inner sphere) binding of the ligands to the surface is postulated. The extent of adsorption and its pH dependence can be explained by considering the affinity of the surface sites and those of the ligands. Surface equilibrium constants have been determined experimentally for various surface reactions; they can be used to predict extent of adsorption and resulting surface charge. The adsorption of simple weak acids or their anions is largest around the pH value of pH = pK. The surface complex formation constants show the same trend in stability as the corresponding solute complex formation constants; thus surface coordination equilibrium constants can be estimated from the corresponding complex formation constants in solution