Removal of sulfamethoxazole from water via activation of persulfate by Fe3C@NCNTs including mechanism of radical and nonradical process

In this work, the nitrogen-doped carbon nanotubes with encapsulated Fe3C nanoparticles (Fe3C@NCNTs) were prepared through a simple process by pyrolyzing the precursor of ferric trichloride hexahydrate and melamine composite. It was studied systematically that multiple Fe3C@NCNTs synthesized under 600-1000 degrees C activated persulfate (PS) for the degradation of sulfamethoxazole (SMX), a widely used antibiotic of sulfonamides. Raman spectrum, X-Ray Diffraction (XRD), X-ray photoelectron spectroscopy (XPS) were used to characterize the transformation of catalyst with the increase of annulling temperature. The Fe3C@NCNTs synthesized under 800, 900 and 1000 degrees C showed similar degradation ability to SMX with degradation rates of 0.650-0.893 min(-1). During catalytic process, the specific structure of Fe3C encapsulated NCNTs could both benefit the electron transfer and prevent the Fe3C dissolving in the solution. The results of quenching experiments coupling with electron paramagnetic resonance (EPR) spectra suggested the (OH)-O-center dot, SO4 center dot- and O-1(2) contributed to the degradation of SMX and the singlet oxygen took effect for longer time. The degradation intermediates were mainly derived from the reactions of hydroxylation, oxidation, aniline ring opening, deamination, desulfonation, as well as bond cleavage. In terms of the density functional theory (DFT) calculations, the graphitic N exhibited the highest Delta E-ads (-0.437 eV) for the PS molecule, and it was followed by the pyrrolic N (-0.304 eV), pristine graphene (-0.146 eV) and pyridine N (-0.072 eV), which means that the graphitic N is more favorable for the binding affinity with PS, forming the reactive complexes on the surface of the NCNTs catalysts to promote the nonradical oxidation.