Green Synthesis of Fe-N-C Single-Atom Catalysts <italic>via</italic> Solid-Phase Method and Their Application in Continuous Degradation of Acid Orange 7 Through Peroxymonosulfate Activation

The advanced oxidation process based on peroxymonosulfate （PMS） activation by Fe-N-C single-atom catalysts has garnered wide attention in the field of wastewater treatment. However， two challenges remain to be addressed for its practical engineering application： firstly， the green synthesis of highly active Fe-N-C catalysts， and secondly， the performance evaluation of the Fe-N-C/PMS system for continuous pollutant treatment. Using 2-methylimidazole and zinc/iron oxides as raw materials， a solvent-free green synthesis of Fe-N-C single-atom catalysts was achieved through solid-phase reaction and high-temperature thermal treatment. Furthermore， a conceptual setup for continuous wastewater treatment was designed， employing cotton-supported catalysts as a fixed-bed for PMS activation， achieving nearly 100% removal efficiency in the continuous degradation of acid orange 7 （AO7） over 280 h. Structural analysis revealed that the single-atom Fe in the synthesized Fe-N-C catalyst coordinates with either pyrrolic or pyridinic N to form Fe-N4 moieties. Studies indicate that these moieties serve as the primary catalytic sites for PMS activation and works synergistically with adjacent micropores and N-Cx sites to degrade AO7. Quenching experiments demonstrated that singlet oxygen （1O2） is the dominant reactive oxygen species responsible for AO7 degradation. The experimental results on AO7 degradation indicate that the Fe-N-C/PMS system has a broad pH application range （3.0~11.0） and exhibits excellent catalytic degradation performance for AO7 in different water qualities （ultrapure water， tap water， Foshan Qiandeng Lake water）. Furthermore， common anions in water bodies （Cl-、SO42-、HCO3-， etc.） have minimal impact on the removal efficiency of AO7 by this system. These results provide valuable insights for further research on the engineering application of Fe-N-C and other transition metal single-atom catalysts （TM-N-C） in advanced oxidation processes.