Analytical models for describing cation adsorption/desorption kinetics as considering the electrostatic field from surface charges of particles

Abstract: Surface charges of particles together with the adsorbed counter ions in diffuse layer can set up a strong electrostatic field around the particles in aqueous solution. The existent kinetic models for describing cation exchange on solid/liquid interface were either empirical or semi-empirical, and in which the electrostatic field is not considered. In this paper, as considering the important effect of electrostatic field around particles on cations adsorption/desorption, for the first time the dynamic distribution equations of cations in diffuse layer for adsorption and desorption processes in both flow method and batch technique have been established. Those equations clearly show how the cation concentration changes with time in different position of diffuse layer during the cation exchange process, and the corresponding new kinetic models have been obtained upon them. The new models indicate that, in both flow method and batch technique, for the adsorption process, experimental results should appear zero order kinetic process caused by the strong force adsorption in the initial stage of adsorption, and then transform to the first order kinetic process of the weak force adsorption; and for the desorption process, however, only first order kinetic process may exist. The new models are essentially different from the classic apparent or empirical kinetic models since all the parameters have their defined physical meanings in the new models, thus the rate parameters in the new models have the potential to theoretically predict. Theoretical analyses also indicated that, the adsorption/desorption rate in flow method experiment will be much higher than that in batch technique experiment. [Copyright &y& Elsevier]