A Novel Two-Resistance Model for Description of the Adsorption Kinetics onto Porous Particles

A novel two-resistance model for description of the sorption kinetics by porous particles has been proposed. The model takes into account two kinetic steps of different kinds which are involved in the overall sorption process rate: (i) the rate of solute diffusion in pores of the sorbent particles having uniform sizes and characterized by the homogeneous intraparticle diffusion coefficient and (ii) the rate of a direct adsorption/desorption process on the surface, described by applying the statistical rate theory (SRT) approach. Two different kinds of sorbent particles geometry are considered: the spherical and the plane particles, having their dimension characterized by radius and thickness, respectively. The meaning of the parameters which influence the sorption kinetics has been discussed. The results make it possible to judge which conditions have to be fulfilled to consider only one kinetic step and, thus, to simplify the theoretical description of a given system. The conclusion has been drawn that the concave character of kinetic sorption isotherms plotted in the function of the square root of time is of a general nature and is connected with the situation when at least two different processes are involved in controlling the sorption kinetics.