A geometric approach to determine adsorption and desorption kinetic constants

Abstract

A geometric method based on Langmuir kinetics has been derived to determine adsorption and desorption kinetic constants. In the conventional procedure, either the adsorption kinetic constant ( k a c) or desorption kinetic constant ( k d c) is found from kinetic experiments and the other is calculated by their correlation with the equilibrium constant, i.e, k d c= K con/ k a c, where K con has been known from equilibrium studies. The determined constants ( K con, k a c, k d c), if based only on the conventional procedure, may not be accurate due to their mathematical dependence. Therefore, the objectives of this study are applying a geometric approach to directly determine Langmuir kinetic constants and describe adsorption behavior. In this approach, both adsorption kinetic constant ( k a g) and desorption kinetic constant ( k d g) are obtained only from data of kinetic experiments, and a geometric equilibrium constant ( K geo) is calculated by K geo= k a g/ k d g. The deviation between K geo and K con can prove the accuracy of k a g and k d g which were determined by this method. This approach was applicable to selenate, selenite and Mg 2+ adsorption onto SiO 2 regardless of whether the adsorbate formed inner- or outer-sphere complexes. However, this method showed some deviation between K con and K geo for Mn 2+ adsorption because of the formation of surface Mn(II)-hydroxide clusters, which was inconsistent with the basic assumption of this method of monolayer adsorption.