Transformation of persulfate to free sulfate radical over granular activated carbon: Effect of acidic oxygen functional groups

Abstract In this research, modification of activated carbon using nitric acid was studied in order to investigate the role of acidic oxygen functional groups on the transformation of persulfate to free sulfate radicals for metronidazole antibiotic removal from an aqueous medium. The physicochemical properties of original and modified activated carbons were characterized by Elemental analysis, nitrogen sorption isotherms, scanning electronic microscope, X-ray photoelectron spectroscopy, Raman spectra, Fourier transform infrared spectra, and Boehm titration. Due to the acid treatment, it was found that modification process generated a large number of acidic oxygen functional groups on the surface of activated carbon, especially carboxylic acid. Also, the increase in nitrogen content of surface was very low compared to oxygen content. After modification, the specific surface area and point of zero charge of modified carbon reduced by 36% and 6.2 unit, respectively. Although the adsorption capacity of the two samples was approximately the same, the increase of persulfate decomposition and metronidazole removal after surface modification of catalyst were related to the increase in acidic oxygen functional groups on the surface. Moreover, the affinity of metronidazole removal to catalyst dosage as well as persulfate concentration was grown after modification. In the optimal condition, about 71% of metronidazole was removed after 60 min and gradually reached 87% at the end of the process. Scavenging study showed that the surface reactive radicals were the main responsible in the redox reaction. The reusability of modified carbon showed no obvious decline in the efficiency of the process which indicated the feasible potential of nitric acid modification for the preparation of high quality granular activated carbon.