Oxygen doped graphite carbon nitride as efficient metal-free catalyst for peroxymonosulfate activation: Performance,...

In this study, oxygen-doped graphitic carbon nitride (g-C₃N₄), named O-g-C₃N₄, was successfully fabricated and characterized, and its performance in activating peroxymonosulfate (PMS, HSO₅^-) for the removal of phenol, 2,4-dichlorophenol (2,4-DCP), bisphenol A (BPA), rhodamine B (RhB), reactive brilliant blue (RBB) and acid orange 7 (AO7) was evaluated. The catalytic performance of O-g-C₃N₄ for AO7 removal increased by 14 times compared to g-C₃N₄. In the presence of 0.2 g L^-1 O-g-C₃N₄, 3.5 mM PMS at natural pH 5.8, 96.4% of AO7 could be removed in 60 min, reduced toxicity of the treated AO7 solution was obtained, and the mineralization efficiency was 47.2% within 120 min. Density functional theory (DFT) calculations showed that the charge distribution changed after oxygen doping, and PMS was more readily adsorbed by O-g-C₃N₄ with the adsorption energy (E_ads) of -0.855 kcal/mol than that of the pristine g-C₃N₄ (E_ads: -0.305 kcal/mol). Mechanism investigation implied that AO7 was primarily removed by the sulfate radicals (SO₄^•-) and hydroxyl radicals (^•OH) on the surface of O-g-C₃N₄, but the role of singlet oxygen (¹O₂) to AO7 elimination was negligible. The results of cyclic experiments and catalyst characterization after reaction confirmed the favorable catalytic activity and structural stability of O-g-C₃N₄ particles. Furthermore, the O-g-C₃N₄/PMS system was very resistant to most of the environmental impacts, and AO7 removal was still acceptable in natural water environment. This study may provide an efficient metal-free carbonaceous activator with low dosage for PMS activation to remove recalcitrant organic pollutants (ROPs).