Distinguishing homogeneous-heterogeneous degradation of norfloxacin in a photochemical Fenton-like system (Fe3O4/UV/oxalate) and the interfacial reaction mechanism.

This study demonstrated the efficient degradation of a typical bio-refractory antibiotic norfloxacin (NOR) in a photochemical iron oxides/oxalate system adopting magnetic catalyst (Fe3O4/UV/Ox). It was found that the in-situ generated HO was the main reactive oxygen species (ROS) but CO2(-) could also participate in the NOR degradation to form formylate organic intermediates. Besides, NOR would be degraded via an interesting pathway comprising an initial lag and a subsequent rapid period, where the former could be eliminated by introducing the pre-dissolution of Fe3O4 particles. Furthermore, specific comparative investigations and surface characterizations of pre-adsorbed Fe3O4 particles had evidenced that the existence of surface-bound iron-Ox complexes would be critical for the heterogeneous photochemical dissolution of Fe3O4 and effectively initiated the subsequent homogeneous-heterogeneous NOR degradation. Finally, a comprehensive distinguishing reaction mechanism was proposed including a homogeneous-heterogeneous iron cycle on the solid-water interface and a series of homogeneous radical reactions. Therein, complexation instead of photochemical reduction would be dominant during the whole dissolution process even under UV irradiation. Rapid electrons exchange would occur photochemically between Fe(II) and Fe(III) in the octahedral sites, further weakening the surface Fe-O bonds and accelerating its breakaway from the bulk Fe3O4 structure. This work could distinguish the complex heterogeneous/homogeneous reactions in the photochemical in-situ chemical oxidation systems that utilize naturally abundant iron oxides and polycarboxylic acids.