Plant-based synthesis of TiO2 – rGO nanocomposite for the efficient photocatalytic degradation of an emerging contaminant: Experimental and theoretical investigations

Abstract

The need to develop sustainable and efficient photocatalysts for the degradation of contaminants of emerging concern has gained considerable attention. In this study, plant-based titanium dioxide-reduced graphene oxide (TiO2-rGO) photocatalyst was synthesized by a hydrothermal method using Azadirachta indica (AI) leaf extract. Morphological and chemical properties of the AI-TiO2-rGO photocatalyst were evaluated in detail. The clusters of AI-TiO2 wrapped in AI-rGO were identified with TEM analysis, and from the XRD, the purity of AI-TiO2-rGO was confirmed. The low band gap (2.80 eV) of AI-TiO2-rGO made it a suitable photocatalyst under visible light. The X-ray photoelectron spectroscopy (XPS) results indicated the establishment of strong Ti-O-C chemical linkage within the AI-TiO2-rGO photocatalyst. The efficiency of AI-TiO2-rGO was assessed for the degradation of carbamazepine (CBZ) under UV-A irradiation (94.28%) and visible light (59.97%) in a batch photocatalytic reactor for 60 min. The optimized conditions such as AI-TiO2-rGO dosage (50 mg/L), initial CBZ concentration (4.5 mg/L), and light source (UV-A (60 W)), were validated with response surface methodology (RSM). The mechanism of degradation of CBZ by AI-TiO2-rGO was explained with the identification of intermediate compounds, and three plausible CBZ degradation pathways were proposed. The reusability study suggested that the AI-TiO2-rGO photocatalyst could be effectively used for 8 cycles. The evaluation of toxicity of treated water with E.Coli growth curve (reduction in Iind to 30.12% (UV) and 27.65% (visible)) signified that the treated water was less toxic. The comparison of AI-TiO2-rGO with chemically synthesized TiO2-rGO indicated that even though AI-TiO2-rGO exhibited a slightly higher band gap (2.80 eV), it can be the better alternative for the photocatalysts synthesized by chemical route. The economic analysis of the overall process proved that the application of AI-TiO2-rGO was more cost-effective and sustainable than the conventional TiO2 photocatalyst. Overall, the study provides valuable insight into synthesizing recyclable plant-based photocatalysts to degrade recalcitrant emerging contaminants in the water.