Abstract
Recently a new general ion adsorption model has been developed for ion binding to mineral surfaces (Hiemstra and van Riemsdijk, 1996). The model uses the Pauling concept of charge distribution (CD) and is an extension of the multi-site complexation (MUSIC) approach. In the CD–MUSIC model the charge of an adsorbing ion that forms an inner sphere complex is distributed over its ligands, which are present in two different electrostatic planes. In this paper we have applied the CD–MUSIC model to the adsorption of metal cations, using an extended data set for cadmium adsorbing on goethite. The adsorption of cadmium and the cadmium–proton exchange ratio were measured as function of metal ion concentration, pH, and ionic strength. The data could be described well, taking into account the surface heterogeneity resulting from the presence of two different crystal planes (the dominant 110 face and the minor 021 face). The surface species used in the model are consistent with recent EXAFS data. In accordance with the EXAFS results, high-affinity complexes at the 021 face were used in the model.
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