The overall rate of adsorption of pharmaceutical compounds on activated carbon (AC) from aqueous solution was investigated as this information is required to design fixed-bed adsorbers. In a recent study, it was found that metronidazole (MNZ) can be adsorbed considerably on granular AC F400 and powder mesoporous AC (CMK-3). The latter was synthesized using a template mesoporous silica (SBA-15). In several studies, it has been reported that the rate of adsorption of organic compounds on mesoporous AC is much faster than on conventional AC; however, the rate of adsorption on granular AC (F400) was compared to that on powder mesoporous AC. The fast adsorption rate observed in the CMK-3 was attributed to its mesoporosity, but not to its particle size. The aim of this work is to compare the rate of adsorption of MNZ on granular AC with that on powder mesoporous AC. It is very well known that the rate of adsorption of MNZ on AC depends on particle size. Hence, the effect of particle size on the rate of adsorption on granular AC was studied in this work.
The concentration decay curves of MNZ during adsorption on CMK-3 and F400 carbons, were obtained in a batch stirred tank adsorber. The concentration decay curves of MNZ were interpreted using diffusional models assuming that the overall rate of adsorption of MNZ was due to external mass transfer and intraparticle diffusion. Moreover, the latter may occur by pore volume diffusion (Fick diffusion) and surface diffusion. Independent of the operating conditions, the results revealed that the surface diffusion model (MDS) successfully interpreted the experimental kinetic data and the surface diffusion controlled the intraparticle diffusion of MNZ in both ACs. The particle size of F400 carbon was reduced by grinding to study the effect of particle size on the rate of adsorption. The results revealed that the time to reach adsorption equilibrium decreased from 720 min to 20 min when the particle diameter was reduced from 1.02 to 0.081 mm. Independent of particle size, the MDS model fitted the experimental data quite well and the surface diffusion coefficient, Ds, of MNZ in F400 was 2.54×10-8 cm2/s and not depended on particle size. As expected the values of the external mass transfer coefficient, kL, increased when the particle size of F400 was reduced. The time to reach adsorption equilibrium in CMK-3 is very similar to that in powdered F400 carbon when the particle diameter of CMK-3 (dp = 0.081 mm) is very similar to that of powered F400 (dp = 0.081 mm). It was concluded that the adsorption rate of MNZ on CMK-3 was faster than that on F400 because the particle size of CMK-3 was very small and not to their larger mesoporosity.
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