3D modeling of the overall adsorption rate of metronidazole on granular activated carbon at low and high concentrations in aqueous solution

In this work, the adsorption rate of metronidazole (MNZ) on activated carbon from aqueous solution at low and high concentrations was studied thoroughly by the application of a diffusional model in three dimensions. The diffusional model considers the external mass transport, intraparticle diffusion (pore volume diffusion and surface diffusion) and the adsorption on an active site. Additionally, the effect of the mass of MNZ adsorbed at the equilibrium (qe) on the magnitude and direction of surface diffusion and pore volume diffusion fluxes were analyzed in detail. The results revealed that the time to reach adsorption equilibrium drastically increases from 630 min to 19,000 min as the qe value diminished from 223.94 to 60.20 mg/g due to a decrease in the concentration gradient in the aqueous phase. The application of the diffusional model showed that surface diffusion is more relevant than pore volume diffusion, but its importance is a function of qe. At qe values less than 200 mg g−1, the intraparticle diffusion of MNZ is controlled by surface diffusion, whereas at values higher than 200 mg g−1, the contribution of pore volume diffusion is considerable at times close to equilibrium time. The values of Ds increased exponentially from 0.02 × 10−8 to 2.67 × 10−8 cm2/s as qe increased. Finally, the surface diffusion model can be applied to interpret the adsorption rate of MNZ at low concentrations of MNZ in water. © 2018 Elsevier B.V.

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