Visible-light-driven photocatalytic degradation of tetracycline using citric acid and lemon juice-derived carbon quantum dots incorporated TiO2 nanocomposites

Tetracyclines (TC) are consistently found in aquatic ecosystems, contributing to the development and spread of antibiotic resistance genes and antibiotic-resistant bacteria. This poses adverse effects on aquatic organisms and humans, emphasizing the need for an effective approach to remove these contaminants. TiO2-based photocatalysts hold great potential for efficiently removing TC from water. This study aimed to synthesize TiO2 nanocomposites incorporated with carbon quantum dots (CQDs) derived from both artificial and natural sources. The nanocomposites were characterized using X-ray diffraction, electron microscopy, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and N2 adsorption-desorption test. The incorporation of CQDs into TiO2 resulted in the reduction of the band gap energy from 3.17 eV in pure TiO2 to 3.10 eV, broadening the light absorption range, and improving the charge carrier transfer efficiency. Continuously generated electrons led to the production of reactive oxygen species, predominantly center dot O2-, contributing to TC degradation. Holes and center dot OH radicals also played a significant role in TC degradation. As a result, CQDs/TiO2 nanocomposites exhibited 91.5 % TC removal under visible light irradiation (lambda >= 400 nm) for 120 min at pH 8. The active sites of TC molecules were determined through density functional theory calculations, and the corresponding degradation pathways of TC were proposed using liquid chromatography-mass spectrometry. Furthermore, this study attempted to synthesize CQDs using lemon juice, showcasing the potential of these eco-friendly photocatalysts for TC removal. The visible-light-driven degradation of TC by the natural-source-derived CQDs/TiO2 nanocomposites demonstrated their applicability in water treatment to effectively address current environmental challenges.