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 literature with many different surface complexation models. In the present study, five of these models were used to systematically compare their description of basic charging behavior and cation adsorption phenomena using an extended data set for proton and cadmium adsorption on goethite, 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 the best results. This model was able to describe the pristine charging behavior, the pH dependency of cadmium adsorption, and the salt dependency of cadmium adsorption, using five adjustable parameters; however, the H/Cd exchange ratio was significantly less than the observed ratio. (C) 1996 Academic Press, Inc.