Photocatalytic Decomposition of Paraquat Under Visible Light by Carbon Nitride and Hydrogen Peroxide

Carbon nitride is a newly discovered and probably only known organic photocatalyst that can be activated by visible light due to its relatively lower bandgap energy. In this research, carbon nitride was synthesized by thermal polymerization of dicyandiamide in air and in elevated temperature. Paraquat, or methyl viologen, is a commonly used herbicide that possesses several threats to the environment. In this research, photocatalytic decomposition of paraquat by pure carbon nitride in water and under visible light irradiation is observed to be negligible. Addition of only a small amount of hydrogen peroxide to the system remarkably enhances the overall photocatalytic decomposition of paraquat under visible light. The photodegradation of paraquat is also investigated as a function of the dose of photocatalyst, concentration of hydrogen peroxide, and initial concentration of paraquat. The dual role of hydrogen peroxide in enhancing the photocatalytic activity of carbon nitride can be attributed to the generation of additional amounts of highly reactive hydroxyl radical in the system and the prevention of recombination of electron-hole pairs in carbon nitride thereby stabilizing its prolonged photocatalytic activity. In this study, liquid chromatography-mass spectroscopy (LCMS) study was employed to understand the intermediate products of the photodegradation of paraquat. To the best of our knowledge, this is one of the few reports on the photocatalytic decomposition of paraquat under visible light irradiation and the results suggest that the carbon nitride-hydrogen peroxide system can be potentially used to decompose paraquat and its toxic effects in water.