Mixed ad/desorption kinetics unraveled with the equilibrium adsorption isotherm

Abstract

Predicting ad/desorption rates at solid/solution interfaces is important for environmental and industrial processes. For non-porous surfaces the adsorption process can be divided in film diffusion and surface reactions. Classical models consider situations where one of these processes is rate limiting, but by considering a steady state situation of film diffusion and surface reactions a simple mixed kinetic model for ad/desorption of solutes or nano particles is obtained. With this model, the driving force for ad/desorption can be predicted when the equilibrium isotherm is known. The model can be combined either with experimentally meausured isotherms or with a large variety of equilibrium adsorption isotherm equations for mono- and (pseudo) multicomponent adsorption of charged and uncharged solutes on homogeneous and heterogeneous surfaces. The kinetic model is illustrated by using the Langmuir equation, and the relation between the adsorption affinity and the driving force for ad/desorption is discussed for controlled-flow and stirred batch experiments. With high-affinity isotherms adsorption is relatively fast and desorption is very slow. In batch systems the ad/desorption rates are smaller than in flow systems because the bulk solute concentration decreases with adsorption and increases with desorption. Experimental results for both flow and batch systems are used for a semi-quantitative comparison of the observed kinetics with model predictions based on the driving force for ad/desorption.