Enhancing photocatalytic degradation of quinoline by ZnO:TiO_2 mixed oxide: Optimization of operating parameters and mechanistic study

摘要

This study focuses on the photocatalytic degradation of quinoline, a recalcitrant heterocyclic nitrogenous aromatic organic compound, using the mixed oxide ZnO-TiO_2 photo-catalyst. Photo-catalysts were synthesized by the solid-state reaction method at different calcination temperatures of 400℃, 600℃, and 800 ℃. Different analytical methods, including Field emission scanning electron microscope, Brunauer-Emmett-Teller surface area, X-ray diffraction, UV-vis diffuse reflectance spectroscopy, Fourier-transform infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy analysis were used for the catalyst characterization. The highest pore surface area of 57.9 m~2g~(-1) was obtained for the photo-catalyst calcined at 400 ℃. The effects of calcination temperature, solution pH, initial concentration, catalyst dose as well as irradiation time were studied. At the optimum condition, i.e., calcination temperature of 400 ℃, pH ≈8 and catalyst dose of 2.5 gL~(-1), maximum quinoline degradation and total organic carbon (TOC) removal efficiency of ≈92% and ≈78% were obtained after 240 min for initial quinoline amount of 50 mgL~(-1). The 1st, 2nd, and nth-order kinetic models were applied to analyze the quinoline degradation rate. The photocatalytic mechanism was studied by drawing energy level diagram with the help of the band-gap structures of the ZnO and TiO_2, potential of the free radicals like 'OH and •O_2 and HOMO-LUMO energy gap of the quinoline molecule. The proposed pathways of quinoline mineralization were suggested on the basis of the identified intermediates by the gas chromatograph-mass spectrometer analysis and scavenger study.