Enriched nitrogen-doped carbon derived from expired drug with dual active sites as effective peroxymonosulfate activator: Ultra-fast sulfamethoxazole degradation and mechanism insight

Abstract

It’s a win–win strategy to treat organic wastewater by activating peroxymonosulfate (PMS) with nitrogen-doped carbocatalysts derived from solid waste. However, the preparation of carbocatalysts with a high nitrogen content is still tricky. Herein, enriched nitrogen-doped carbon (NDC20) with dual active sites derived from expired drugs (999 cold medicine) was developed to improve PMS activation and ultra-fast sulfamethoxazole (SMX) degradation. NDC20 with high N-doping level (23.26 at %) adjusted the charge distribution of the catalyst surface, resulting in an excellent catalytic activation of PMS. Remarkably, the NDC20/PMS system fully eliminated SMX within 2 min (kobs = 4.912 min−1), which was higher than all previously reported N-doped carbon catalysts. Quenching and electron paramagnetic resonance (EPR) experiments verified that the main mechanism involved a dual nonradical pathway involving singlet oxygen (1O2) and direct electron transfer (DET). Combined with theoretical calculations and in situ FTIR spectra, the exclusive role of dual active sites (pyridinic N/pyrrolic N and electron-deficient carbon atoms) was identified. An unanticipated generation pathway of 1O2 involving electrons acquisition by dissolved oxygen to form a superoxide radical (O2·-) and PMS oxidation over electron-deficient carbon atoms was revealed. Moreover, the possible pathways of SMX degradation were proposed, and the toxicity of the intermediates was anticipated. Finally, the practical application experiments demonstrated the sustainability of the catalytic activity of NDC20, which completely removed SMX after 100 h. This work provides a neoteric idea for synthesizing high-level N-doped carbocatalysts and solid waste treatment, and deepens the mechanistic understanding of PMS activation over high-level N-doped carbocatalysts.