COMPARISON OF DIFFERENT SITE BINDING MODELS FOR CATION SORPTION - DESCRIPTION OF PH DEPENDENCY, SALT DEPENDENCY, AND CATION-PROTON EXCHANGE

Ion adsorption on metal (hydr)oxides has been described in the literat ure with many different surface complexation models. In the present st udy, five of these models were used to systematically compare their de scription of basic charging behavior and cation adsorption phenomena u sing an extended data set for proton and cadmium adsorption on goethit e, The scope and limitations of the models are discussed, None of the models gave a completely satisfactory description of all of the data, A 1 pK model in combination with three electrostatic planes yielded th e best results. This model was able to describe the pristine charging behavior, the pH dependency of cadmium adsorption, and the salt depend ency of cadmium adsorption, using five adjustable parameters; however, the H/Cd exchange ratio was significantly less than the observed rati o. (C) 1996 Academic Press, Inc.