A branched pore kinetic model for activated carbon adsorption

Abstract

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...

A Comparison of Chemisorption Kinetic Models Applied to Pollutant Removal on Various Sorbents

“…Homogeneous solid surface diffusion models have been developed for the adsorption of phenol onto activated carbon using the Crank-Nickolson ® nite difference method to solve the diffusion equation 53 , the adsorption of phenolic compounds onto carbon using orthogonal collocation to solve the diffusion equation 54 and the adsorption of basic dyes onto silica 55 and basic dyes onto activated carbon 56 , using a semi-analytical solution to solve the diffusion equation. Branched pore diffusion models based on macro-, meso-and micropore diffusion have been developed for phenol adsorption 57 and dye adsorption on activated carbon 58 . The application of variable surface diffusivity to the HSDM has been studied in several cases 59 61 .…”

Mathematically modeling fixed-bed adsorption in aqueous systems

“…Transfer rate of solute from macropore to micropore: (Peel et al, 1981;Yang and Al-Duri, 2001;Ko et al, 2002) Shrinking core theory model This model used to describe the intrapellet adsorption is controlled by pore diffusion, supposing adsorption firstly occurs at the outer region of adsorbent, then the mass transfer zone moves inward together with the extending of saturated outer region and shrinking of unloaded core (Fig. 5) The mass transfer flux at the solid-liquid phase interface: Du et al, 2007;2008) To be continued A review of surface diffusion is available elsewhere (Medved and Cerny, 2011), which might provide some ideas to modify the intrapellet diffusion models.…”

Adsorption of pyridine from aqueous solutions by polymeric adsorbents MN 200 and MN 500. Part 2: Kinetics and diffusion analysis

“…Study of pore size heterogeneity revealed the variation of adsorbate diffusivity within the porous adsorbent [23,39,40]. These processes have been modelled as a fractal-like dependence of diffusion coefficient [34,41,42], or to involve correlated variables, by categorising the adsorption kinetics into macropore and micropore diffusional regions, respectively [32,39,43]. As a key element for assessing the adsorption performance, the importance of surface diffusion in contributing to the total adsorbent mass transfer in porous materials has been highlighted in many liquid-surface adsorption systems [44,45].…”