Carbonization of camphor sulfonic acid and melamine to N,S-co-doped carbon for sulfamethoxazole degradation via persulfate activation: Nonradical dominant pathway

A nonradical dominant degradation process via N,S-co-doped carbon-catalyzed persulfate (PS) activation was reported. The camphor sulfonic acid acted as both carbon and sulfur source, and melamine served as nitrogen source. The introduction of N atom into S-doped carbon created new defective sites and N active species. Accordingly, the resulting NSC-750 showed an enhanced catalytic activity (0.0348 min⁻¹), with 5.8-folds higher SMX removal efficiency than that of SC-650 (0.006 min⁻¹). 96% removal, 69% mineralization, and 98% PS decomposition rate were achieved. The NSC-750/PS system could work effectively over a wide pH range of 3–9. While carbonate (CO₃^2–) played an opposite role in degrading SMX. Both radical and nonradical pathway were involved in SMX degradation. Therein, ¹O₂ played a critical role while .OH, SO₄.^-, and electron transfer exerted a minor contribution. It was assumed that pyridine N and thiophene S are responsible for the production of SO₄.^-; structural defect and C = O contributed to the formation of ¹O₂ and electron-transfer process, respectively; partial ¹O₂ was originated from the conversation of O₂.^-. In addition, NSC-750/PS system was capable of degrading other contaminants (bisphenol A, ciprofloxacin, and rhodamine B et al). The obtained hybrid EPR spectra, coupling with ¹O₂ and unpaired electron signal of catalyst, broadens our horizon to better distinguish the reactive species. This work facilitates the application of N,S-co-doped carbon in nonradical-dominated antibiotics remediation, and deepens the understanding of catalytic mechanism and active sites.