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

Abstract

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