Sorption–diffusion in heterogeneous systems. Part 5.—A general diffusion/immobilization model

Abstract

A general diffusion/immobilization model is presented. With the aid of a modified Fick's second equation in which the mobile diffusing species are distinguished from the immobilized substance, general practical criteria for the behaviour of the whole spectrum of sorptive systems are developed. With the help of these criteria the Fickian as well as the non-Fickian sorption behaviour can be easily understood and predicted. Furthermore, numerical calculations of the uptake kinetics for constant external boundary concentration and the procedure for numerical solutions are presented.