The critical role of the surface iron-oxalate complexing species in determining photochemical degradation of norfloxacin using different iron oxides

Abstract

In the past decades, in-situ generation of reactive oxygen species (ROS) in the photochemical iron oxides/oxalate system (UV/IOs/Ox) has drawn a lot of attentions, while understanding the reaction mechanism upon the solid-liquid surface complexing behaviors/species is still scarcely. In this study, comparative degradation of norfloxacin (NOR) was investigated in the UV/IOs/Ox systems adopting four different common iron oxides. It was found that the type of IOs would lead to rather different NOR degradation patterns following the order of goethite (α-FeOOH) > hematite (α-Fe2O3) > maghemite (γ-Fe2O3) ≈ magnetite (Fe3O4). •OH was the main ROS and effects of parameters (dosage of IOs, NOR and Ox, pH) on the pseudo-first-order kobs(NOR) were evaluated in the four systems. Results showed that the surface structures of IOs instead of catalyst amounts would be more responsible for the degradation efficiency. Evolutions of •OH, H2O2 and Fe2+ indicated the surface interfacial reactions would also contribute for the NOR degradation, but depended on the type of IOs. ATR-FTIR examinations demonstrated that catalytic activity of IOs correlated highly (R2 = 0.999) with the amounts of bidentate mononuclear iron-Ox surface complex but correlated poorly with the amounts of monodentate mononuclear and outer sphere complexes. Different intrinsic properties of IOs such as the species of surface hydroxyl groups (-OH) would result in changing the proportion of the three surface complexes. Besides, the minor reduction transformation of NOR by the carbon dioxide anion radical (CO2•−) was verified by density function theory (DFT) calculation, further confirmed the involvement of both •OH and CO2•− for NOR degradation.