A branched pore kinetic model for aqueous phase activated carbon adsorption is presented in which the carbon particle is separated into rapidly and slowly diffusing regions. The model was developed to overcome problems arising from a single rate parameter analysis and is shown to describe experimental data well. In addition to very different rates of transport in the two regions, parameters estimated by regression analysis indicated differences in the adsorptive characteristics. RUSSELL G. PEEL SCOPEActivated carbon is gaining ever-increasing usage as a general purpose adsorbent for organics from the aqueous phase. However, the actual mechanisms of transport and adsorption within the activated carbon particle are not yet well understood. Because of the complexity of the internal pore structure, activated carbon kinetics have generally been modelled with a single effective diffusion parameter active throughout the particle (Fleck, 1973;Crittenden and Weber, 1978a;Fritz et al, 1979), even though it is accepted that activated carbons have heterogeneous surface properties and a wide range of pore sizes.This single intraparticle parameter approach has led to inconsistencies between observed equilibrium and kinetic behavior. Consequently, it is the objective of this study to develop a model, based on present knowledge of the internal structure of activated carbon, which would enable a consistent treatment of both equilibrium and kinetic data. Such a model should give an accurate description of the adsorption process from initial contact through to equilibrium.In the phenolic adsorption studies of Snoeyink (1969) and Zogorski (1976), among others, a rapid initial uptake phase followed by a slow approach to equilibrium was noted. Such behavior is not consistent with the single effective diffusion parameter model. In addition, several authors (Beck and Schultz, 1970;Satterfield et al., 1973) have demonstrated that the effective diffusion parameter in small pores is a strong function of the pore diameter to solute diameter ratio, and that diffusion rates decrease with decreasing pore size. Since activated carbons have a wide range of pore sizes, a corresponding range of diffusion rates should be expected. Rapid saturation of the fast diffusing pores would result in a net decrease in effective diffusion rate as adsorption proceeds.As an approximation to the microscopic description of the diffusional process, a model has been developed in this work which divides the carbon particle into two regions of different diffusion rates. The regions are loosely termed macropores and micropores. (These terms should not be confused with their conventional uses to define certain pore size ranges.) Relatively rapid diffusion and adsorption occur in the macropores, and the remaining slow approach to equilibrium occurs in the micropores.The micropores are assumed to be homogeneously distributed throughout the particle and to branch off the larger macropore network which is responsible for radial transport. A schematic diagram of the propos...

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The use of two simplified driving force expressions are investigated for describing the intraparticle rate behaviour during adsorption onto activated carbon. A previously developed numerical solution to the surface diffusion model is used to compare the linear and quadratic driving force approximation when non‐linear isotherms and time variant boundary conditions are present. The results show that while the linear driving force equation is a poor approximation of the exact solution, the quadratic driving force is a good approximation, and its use in a kinetic model of activated carbon adsorption gives as good a description of adsorption behaviour as is given by the more detailed surface diffusion model. The driving force model has the advantage of being easier to solve, and the numerical solution requires significantly less computational effort.