Single Fe atom anchored by N vacancy of CN activates PMS for efficient degradation of refractory organics: The key role of non-radical pathway through O and Fe(IV)=O.

Fenton-like technology based on peroxymonosulfate activation has shown great potential in refractory organics degradation. In this work, single Fe atom catalysts were synthesized through facile ball milling and exhibited very high performance in peroxymonosulfate activation. The Fe single-atom filled an N vacancy on the triazine ring edge of CN, as confirmed through X-ray absorption fine structure, density functional calculation and electron paramagnetic resonance. The SAFeCN/PMS system could completely remove phenol (20 mg/L) within 10 min and its first-order kinetic constant was 12.3 times that of the FeO/PMS system. Under different initial pH levels and in various anionic environments, SAFeCN still demonstrated excellent catalytic activity, achieving a removal rate of over 90 % for phenol within 12 min. In addition, SAFeCN exhibited outstanding selectivity in reaction systems with different pollutants, showing excellent degradation effects on electron-rich pollutants only. Hydroxyl radicals (OH), singlet oxygen (O) and high-valent iron oxide (Fe(Ⅳ)=O) were detected in the SAFeCN/PMS system through free radical capture experiments. Further experiments on the quenching of active species and a methyl phenyl sulfoxide probe confirmed that O and Fe(Ⅳ)=O played dominant roles. Additionally, the change in the current response after adding PMS and phenol in succession proved that a direct electron transfer path between organic matter and the catalyst surface was unlikely to exist in the SAFeCN/PMS/Phenol degradation system. This study provides a new demonstration of the catalytic mechanism of single-atom catalysts.