Kinetic modeling of photocatalytic oxidation-ozonation of metronidazole removal from water

This work presents a kinetic modeling study of the photocatalytic oxidation-ozonation process for the removal of the antibiotic metronidazole (MTZ) from water. The model is based on elemental reactions with a focus on understanding the role of various reactive species and the synergistic effects between ozonation and photocatalysis. The kinetic model accurately describes the photocatalytic oxidation and photocatalytic ozonation of metronidazole, with good agreement between experimental and calculated results, especially for the first 600 seconds of the reaction. Accounting for the generation of intermediates seems to be crucial to avoid overestimation of MTZ removal rate. The assumed intermediates were selected based on the likelihood of their formation during MTZ photocatalytic oxidation and photocatalytic ozonation processes as previously studied confirm. Compared to single ozonation and photocatalytic oxidation, photocatalytic ozonation achieves higher mineralization figures with TOC removal reaching 83 % after 2 hours. MTZ and TOC synergy factors of 9.4 and 2.3, respectively, highlight the enhanced interaction between ozonation and photocatalysis. Scavenger addition runs confirmed the significant role hydroxyl radicals play in photocatalysis, while direct reaction with generated holes was found to be negligible. The proposed mechanism considers the potential reactions developed both in the liquid phase and through adsorbed reactive species, while also accounting for photon absorption rates. The optimization process yielded key parameters such as the catalyst quantum yield, 0.35 molEinstein −1 , the apparent rate constant of positive hole decomposition in hydroxyl radicals, 6.6 x10 10 s −1 and the recombination rate constant of carriers, 1.6 × 10 18 M −1 s −1 . These values, obtained via non-linear regression analysis, demonstrate the reliability of the model in describing the kinetics of the process. • Amongst different processes, photocatalytic ozonation shows the lowest O 3 demand. • Contrarily to positive holes, HO radicals play a significant role in UVA/TiO 2 system. • Sinergy is found between O 3 and UVA/TiO 2 while no interaction is observed in O 3 /TiO 2 . • Inclusion of intermediates is of paramount importance when simulating oxidation processes.