Single Fe atom anchored by N vacancy of C3N4 activates PMS for efficient degradation of refractory organics: The key role of non-radical pathway through 1O2 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 C₃N₄, as confirmed through X-ray absorption fine structure, density functional calculation and electron paramagnetic resonance. The SAFe_0.4C₃N₄/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 Fe₃O₄/PMS system. Under different initial pH levels and in various anionic environments, SAFe_0.4C₃N₄ still demonstrated excellent catalytic activity, achieving a removal rate of over 90 % for phenol within 12 min. In addition, SAFe_0.4C₃N₄ exhibited outstanding selectivity in reaction systems with different pollutants, showing excellent degradation effects on electron-rich pollutants only. Hydroxyl radicals (^•OH), singlet oxygen (¹O₂) and high-valent iron oxide (Fe(Ⅳ)=O) were detected in the SAFe_0.4C₃N₄/PMS system through free radical capture experiments. Further experiments on the quenching of active species and a methyl phenyl sulfoxide probe confirmed that ¹O₂ 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 SAFe_0.4C₃N₄/PMS/Phenol degradation system. This study provides a new demonstration of the catalytic mechanism of single-atom catalysts.