The relationship between molecular structure and ion adsorption on variable charge minerals

Ion adsorption modeling is influenced by the presumed binding structure of surface complexes. Ideally, surface complexes determined by modeling should correspond with those derived from spectroscopy, thereby assuring that the mechanistic description of ion binding scales from the nanoscopic molecula r structure to the macroscopic adsorption behavior. Here we show that the s tructure of adsorbed species is a major factor controlling the pH dependenc y of adsorption. An important aspect of the pH dependency is the macroscopi c proton-ion adsorption stoichiometry. A simple and accurate experimental m ethod was developed to determine this stoichiometry. With this method, prot on-ion stoichiometry ratios for vanadate, phosphate, arsenate, chromate, mo lybdate, tungstate, selenate and sulfate have been characterized at I or 2 pH values. Modeling of these data shows that the macroscopic proton-ion ads orption stoichiometry is almost solely determined by the interfacial charge distribution of adsorbed complexes. The bond valence concept of Pauling ca n be used to estimate this charge distribution from spectroscopic data. Con versely, the experimentally determined proton-ion adsorption stoichiometry allows us to successfully predict the spectroscopically identified structur es of, for example, selenite and arsenate on goethite. Consequently, we hav e demonstrated a direct relationship between molecular surface structure an d macroscopic adsorption phenomena. Copyright (C) 1999 Elsevier Science Ltd .