Enhancing photocatalytic degradation of quinoline by ZnO:TiO2 mixed oxide: Optimization of operating parameters and mechanistic study

This study focuses on the photocatalytic degradation of quinoline, a recalcitrant heterocyclic nitrogenous aromatic organic compound, using the mixed oxide ZnO-TiO2 photo-catalyst. Photo-catalysts were synthesized by the solid-state reaction method at different calcination temperatures of 400 degrees C, 600 degrees C, and 800 degrees C. 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(2)g(-1) was obtained for the photo-catalyst calcined at 400 degrees C. 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 degrees C, pH approximate to 8 and catalyst dose of 2.5 gL(-1) maximum quinoline degradation and total organic carbon (TOC) removal efficiency of approximate to 92% and approximate to 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 TiO2, potential of the free radicals like (OH)-O-center dot and O-center dot(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.