Cl/S co-doped carbon nitride nanotube clusters effectively drive the metal-free photo-Fenton reaction under visible light: A new ROS conversion mechanism

In this work, we used Cl/S co-doped carbon nitride nanotube clusters (Cl/S-TCN) photocatalyst to effectively drive the metal-free photo-Fenton (MF-PF) reaction. The synergistic effect of Cl/S-TCN and H2O2 made MF-PF(Cl/S-TCN) system have excellent degradation performance for organic wastewater in a wide pH range (3–9). The optimal amount of H2O2 was only 10 mM, the degradation rate of p-nitrophenol (p-NP) by MF-PF(Cl/S-TCN) system under neutral conditions can be increased from 50.3% to 98.2%, the degradation rate of tetracycline (TC) reached more than 90% within 20 min, and the COD removal rates of domestic sewage and oilfield produced water reached 70.3% and 82.6% accordingly. MF-PF(Cl/S-TCN) system had excellent stability, and the performance loss was less than 0.7% after four cycles of experiments. Density functional theory calculations showed that Cl and S dopants accelerated the separation of photogenerated electrons and holes on the surface of Cl/S-TCN. These photogenerated electrons and holes, were used as electron donors and electron acceptors for reacting with H2O2, destroyed the O–O bond and O–H bond of H2O2, and finally converted H2O2 to •OH and •O2− with strong oxidation, which is the key to enhance the performance of MF-PF(Cl/S-TCN) system. Such reaction pathway was rarely reported in previous studies.