A SURFACE STRUCTURAL APPROACH TO ION ADSORPTION - THE CHARGE-DISTRIBUTION (CD) MODEL

An ion adsorption model for metal hydroxides has been developed which deals with the observation that in the case of inner sphere complex fo rmation only part of the surface complex is incorporated into the surf ace by a ligand exchange reaction while the other part is located in t he Stern layer. The charge distribution (CD) concept of Pauling, used previously in the multi site complexation (MUSIC) model approach, is e xtended to account for adsorbed surface complexes. In the new model, s urface complexes are not treated as point charges, but are considered as having a spatial distribution of charge in the interfacial region, The new CD model can describe within a single conceptual framework all important experimental adsorption phenomena, taking into account the chemical composition of the crystal surface. The CD model has been app lied to one of the most difficult and challenging ion adsorption pheno mena, i.e,, PO4 adsorption on goethite, and successfully describes sim ultaneously the basic charging behavior of goethite, the concentration , pH, and salt dependency of adsorption, the shifts in the zeta potent ials and isoelectric point (IEP), and the OH/P exchange ratio, This is all achieved within the constraint that the experimental surface spec iation found from in situ IR spectroscopy is also described satisfacto rily. (C) 1996 Academic Press, Inc.