Efficient persulfate activation catalyzed by pyridinic N, C–OH, and thiophene S on N,S-co-doped carbon for nonradical sulfamethoxazole degradation: Identification of active sites and mechanisms

Nitrogen- and sulfur-co-doped carbons were prepared by pyrolyzing chitosan and aminoethanesulfonic acid sodium, and used to activate persulfate (PS) for sulfamethoxazole (SMX) degradation. Sulfur incorporation into nitrogen-doped carbon (NC) enhanced the catalysis. Moreover, the catalytic properties were largely affected by the N/S atomic ratio. At the N/S ratio of 8.06, the best performance could be achieved (NSC-0.5, 0.08317 min⁻¹). Correspondingly, complete SMX removal, 80% mineralization, and 74% PS utilization were found within 120 min. The NSC-0.5/PS system worked effectively at strong acidic and weak basic conditions (pH ≈ 3–9). The pyridinic N, C–OH, and thiophene S were identified to be the active sites. The above three units could transfer electron to PS to generate superoxide radical (O₂^•−). The resulting O₂^•− reacted with H₂O or self-recombination with H⁺ to generate singlet oxygen (¹O₂). However, the structural defects might not work in the course of PS activation. Nonradical singlet oxygen (¹O₂) was mainly in charge of SMX oxidation. The toxicity of degradation system, influence of real water matrix, and reusability of the catalyst were investigated comprehensively. Nine possible degradation pathways were proposed. In addition, the NSC-0.5/PS system was also capable of degrading other antibiotics (ciprofloxacin, sulfamonomethoxine) and non-antibiotics (bisphenol A, rhodamine B). This work highlights the significant roles of pyridinic N, C–OH, and thiophene-S in PS activation for catalytic degradation organic contaminants, and presents a feasible approach to construct efficient carbon-based persulfate activators.