A geometric approach to determine adsorption and desorption kinetic constants

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 co nstant (k(d)(c)) is found from kinetic experiments and the other is calcula ted by their correlation with the equilibrium constant, i.e, k(d)(c) = K-co n/k(a)(c), where K-con has been known from equilibrium studies. The determi ned 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. Theref ore, the objectives of this study are applying a geometric approach to dire ctly determine Langmuir kinetic constants and describe adsorption behavior. In this approach, both adsorption kinetic constant (k(a)(g)) and desorptio n kinetic constant (k(d)(g)) are obtained only from data of kinetic experim ents, 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 accur acy of k(a)(g) and k(d)(g) which were determined by this method. This appro ach was applicable to selenate, selenite and Mg2+ adsorption onto SiO2 rega rdless of whether the adsorbate formed inner- or outer-sphere complexes. Ho wever, this method showed some deviation between K-con and K-geo for Mn2+ a dsorption because of the formation of surface Mn(II)-hydroxide clusters, wh ich was inconsistent with the basic assumption of this method of monolayer adsorption. (C) 2000 Elsevier Science Ltd. All rights reserved.