Role of pore volume and surface diffusion in the adsorption of aromatic compounds on activated carbon

The objective of this manuscript was to elucidate the main mechanism of mass transport involved in intraparticle diffusion during the adsorption of phenol, methylene blue (MB), and methyl blue (MTB) on granular activated carbon (AC F400) by using three diffusional models: (i) pore volume diffusion model (PVDM), (ii) pore volume and surface diffusion model (PVSDM), and (iii) surface diffusion model (SDM). Results revealed that the molecular size of the organic compounds as well as its adsorption capacity considerably affect their overall adsorption rate as well as controlling the mass transfer mechanism. The PVSDM model, which assumes that pore volume and surface diffusion are both important in intraparticle diffusion, satisfactorily fitted the experimental data. The percentage contribution of surface diffusion to overall intraparticle diffusion was > 82 % for phenol, 19 % for MTB and 5-95 % for MB. These results confirmed that the controlling mechanism of the overall adsorption rate on AC F400 is surface diffusion for phenol and pore volume diffusion for MTB, while both diffusion mechanisms are important in the adsorption of MB. The SDM modeled reasonably well the concentration decay data for phenol adsorption under different experimental conditions. Furthermore, it was found that surface diffusion coefficient values augmented as the amount of phenol adsorbed at equilibrium increased. The PVDM model fitted the experimental MTB decay data reasonably well, showing that the tortuosity factor of AC F400 ranged from 3 to 5. The external mass transport did not affect the overall rate of phenol, MB and MTB adsorption on AC F400.