Synergistic ternary porous CN-PPy-MMt nanocomposite for efficient photocatalytic metronidazole mineralization: Performance, mechanism, and pathways

Carbon nitride based photocatalysts are widely used to decontaminate aqueous solutions by eliminating toxic and non-biodegradable compounds. It is desirable to develop a photocatalyst with high charge separation and migration efficiency. In this study, synergistic ternary porous carbon nitride-polypyrrole-montmorillonite (CN-PPy-MMt) was successfully synthesized via an in situ oxidative polymerization method. The photocatalytic performance towards mineralization of metronidazole (MZ) under visible light was studied, where the CN-PPy-MMt (10%) nanocomposite exhibited the best performance compared to CN-PPy-MMt (5%, 15%, 20%), CN-PPy, pure CN and other nanocomposites reported. This superior photocatalytic mineralization performance was attributed to synergistic inter-constituent interactions within the CN-PPy-MMt nanocomposite, which effectively enhanced the light absorption capacity and charge transfer, and reduced the recombination of electron-hole pairs. The results were confirmed by UV-DRS, photocurrent, impedance, and photoluminescence measurements. The effect of interfering anions was examined and the results indicated that the MZ mineralization efficiency was significantly inhibited by the addition of HCO3- and PO43-. The reusability and stability of the photocatalyst were excellent even after five repeated photocatalytic reactions. Analysis of the radical scavenger properties indicated that superoxide radicals (O2-) and holes (h+) played a major role in the mineralization of MZ. The intermediate products were confirmed using liquid chromatography-mass spectrometry, which provided an insight into the MZ mineralization mechanism. This work suggested that the design of a ternary nanocomposite based on CN with conducting polymers could be an effective strategy to improve the photocatalytic mineralization of antibiotics and energy related applications.