Comparison and Characterization of Nitrogen/Sulfur-Doped Activated Carbon for Activating Peroxydisulfate to Degrade Acid Orange 7: An Experimental and Theoretical Study

Nitrogen-doped (NAC), sulfur-doped (SAC), and nitrogen/sulfur co-doped activated carbon (NSAC) were prepared to activate peroxydisulfate (PDS) for degrading acid orange 7 (AO7) in water, where urea, sulfur element, and thiourea were used as N/S sources. Greater degradation of AO7 was achieved by all modified samples with PDS compared with that of pristine AC, where the NSAC/PDS system achieved the highest degradation performance. Quenching tests, electrochemical analysis, and theoretical calculations suggested that the degradation of AO7 led by modified carbon catalysts and PDS was mainly attributed to the non-radical pathway (e.g., singlet oxygen formation and electron-transfer mechanism). The doping of N or S resulted in the generation of pyridinic N and thiophenic S. These groups significantly changed the charge distribution, creating more abundant edge defects and active sites, which enhanced the catalytic degradation of AO7. Co-doping of AC with N and S produced a higher abundance of graphitic N groups and edge defect, which likely promoted AO7 degradation compared with the other catalysts. The results of this study can provide theoretical support for the application of N/S-modified carbon materials for activating persulfates to effectively treat dyeing wastewater.