Mechanistic insights into norfloxacin removal in a novel riboflavin-mediated nanoscale zero-valent iron/peracetic acid system: Synergistic radical and non-radical pathways

In this study, a novel riboflavin-mediated nanoscale zero-valent iron/peracetic acid system (RF/nZVI/PAA) was constructed to increase the removal of norfloxacin. Under the optimal conditions (PAA=10 mg/L, nZVI=20 mg/L, RF= 1 mg/L, and initial pH =4), complete norfloxacin removal was achieved within 30 min, accompanied by a 70 % mineralization rate. Electron paramagnetic resonance spectroscopy combined with quenching experiments quantitatively identified hydroxyl radical, carbon-centered radical, and singlet oxygen as the predominant reactive oxidative species (ROS) responsible for norfloxacin removal, with contributions of 42 %, 44 %, and 10 %, respectively. In contrast, direct oxidation with PAA and adsorption onto nZVI demonstrated negligible contributions to norfloxacin removal in the system (only 4 %). Electrochemical characterization revealed that RF functioned as a pivotal catalytic mediator for ROS generation in the system, owing to its exceptional electron transfer capability. Seventeen transformation products (TPs) were identified during the degradation of norfloxacin via radical-mediated piperazine cleavage, nucleophilic piperazine oxidation, defluorination, and quinoline group oxidation. Density functional theory calculations revealed that the piperazine ring and fluorinated benzene moiety in nofloxacin serve as primary electron-donating sites vulnerable to radical/nucleophilic attacks, whereas the quinolone groups exhibit electrophilic susceptibility. Toxicity assessment revealed that the majority of TPs exhibited reduced toxicity compared to the parent norfloxacin after treatment. Furthermore, nZVI showed high reusability, sustaining a norfloxacin removal efficiency of 86 % over seven cycles. Therefore, this study not only elucidates the fundamental mechanisms underlying riboflavin-mediated advanced oxidation processes but also offers a promising technology for the efficient removal of emerging organic contaminants in (waste)water.