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

Abstract

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.