N, S co-doped porous carbon to accelerate Fe3+/Fe2+ redox cycle for peroxymonosulfate activation

Fenton-like process has important implications for water treatment due to its effective degradation of refractory contaminants. However, sluggish conversion of Fe3+ back to Fe2+ strongly restricts its practical application. Herein, a novel N, S co-doped porous carbon (S-ZCN) derived from ZIF-8 was prepared as a metal-free heterogeneous catalyst to accelerate the redox cycle of Fe3+/Fe2+, leading to more efficient peroxymonosulfate (PMS) activation. With only 20 mg/L S-ZCN added, sulfamethoxazole (SMX) could be removed completely by the S-ZCN/Fe3+/PMS system within 40 min. Reactive oxygen species (ROS) were identified by quenching experiments, electron paramagnetic resonance (EPR), and probe experiments. It was revealed that hydroxyl radical (HO•) was the primary ROS, which contributed 79.2 % to SMX degradation. Verified by Fe concentration detection, chelating agent experiments and the results of XPS and FT-IR spectra, the synergistic effect of N, S co-doping and formation of Fe-Nx sites promoted the electron transfer from sp2-hybridized C to surface-associated Fe3+ to reconstruct Fe2+/PMS system. Based on the detection results of intermediates by Q-TOF analysis, potential degradation pathways for SMX in the S-ZCN/Fe3+/PMS system were proposed. Wide range of pH applicability and strong degradation to various pollutants exhibited the advantages in practical applications. This work provides a novel approach involving metal-free catalysts to break the bottleneck in the field of traditional Fenton-like reaction.