In many adsorbents surface ditlusion is the governing mechanism of transport. The coefficient of surface difksion may be concentration dependent. A model based on the assumption that the coe5cient of diffusion is exponentially dependent on concentration in the adsorbent is used. Filmresistance is included. The model equations for adsorption in tinite bath and in countercurrent Row have been solved for some cases for a range of parameter values which may apply for many adsorption systems where activated carbon is used for adsorbing organic compounds from aqueous solution. The results are presented in generalized diagrams. A comparison is made with other models with constant coefficient of diffusion. It is shown that by proper choice of variables most models give very similar results. The very simple model based on pore djtfusion and a saturation isotherm approximately describes other cases such as pore diffusion and surface diflirsion with and without a concentration dependent coefficient of diffusion. The saturation isotherm model has a simple analytical solution which makes it very simple to use.
lNl'RODUcFION
The recent years much work has been done on adsorption on activated carbon theoretically [ l] as well as experimentally. The process is in practice used for removing organic compounds from water and from air. In most of the theoretical work it has been assumed that the coefficient of diffusion is independent of concentration, be it pore diffusion or surface diffusion. In a recent paper by Krtlckels [2] working with water vapor on silicagel, it was shown that the coefficient of diffusion is dependent on the concentration and on the concentration gradient. Gilliland[3] and Sladek[4] and Garg[5] have shown that for many systems the coefficient of diffusion is strongly concentration dependent. Many investigations[6-81 have found that by using pore diffusion models to describe adsorption experiments the pore diffusion coefficient DP is little or not at all dependent on concentration. For many of the systems investigated DP is much higher[l 111 than could be explained by the mechanism of pore diBusion.
It has been suggested [8,10,11] that this is due to surface ditIusion which acts in parallel to pore ditIusion. If, however, the surface diffusion mechanism is assumed and D, would have been evaluated from the experiments, the determined D, would be strongly dependent on the concentration. This can be seen from eqn (15) and diagrams 12-15 in [12]. The models used for determining 4, D, will then in many cases not be applicable and may lead erroneous conclusions.
In this paper it is assumed that the coefficient of surface diffusion in the particle is exponentially dependent on the concentration. This assumption is based on the papers of Gilliland and SladekD, 41.