Fe@Fe2O3/etched carbon felt as a cathode for efficient bisphenol a removal in a flow-through electro-Fenton system: Electron transfer pathway and underlying mechanism

The advanced oxidation processes based on the electro-Fenton reactions show great potentials as a wastewater treatment approach for removing emerging contaminants. In the electro-Fenton system, some factors still exist affecting the treatment efficiency, e.g., the formation of iron sludge, a narrow pH working range, and slow conversion rates between Fe2+ and Fe3+. In this research, a novel heterogeneous flow-through electro-Fenton system was constructed, and carbon felt was used as the template material for the preparation of an etched carbon felt cathode loaded with zero-valent iron and iron oxide (Fe@Fe2O3/ECF) for the degradation of bisphenol A, which addressed the aforementioned issues. The Fe@Fe2O3/ECF electrode showed excellent performance across a pH range between 3 and 11, with exceptional stability and resistance to other substances in water. For the three pollutants, bisphenol A, tetracycline and ciprofloxacin, the highest degradation efficiency can reach 99.3 %. Compared to traditional electro-Fenton systems, the novel process exhibited superior performances due to synergistic effects from feasible flow design, enhanced electrochemical reactivity and greater exposure of active sites. Through electron spin resonance and free-radical quenching experiments, it was determined that the primary species participating in the heterogeneous electro-Fenton process were the hydroxyl radical and superoxide anion. Specifically, the hydroxyl radical was identified as playing a pivotal role in the degradation of bisphenol A. After a continuous operation of 240 min, the electrode showed low metal ion leaching rate and good stability, with the concentrations of leached Fe and Co ions to be only 17 and 7 μg/L, respectively. Meanwhile, the removal efficiency was only decreased by 5.7 % during the process. Through density functional theory calculation and intermediate product analysis, three potential degradation pathways were proposed, with the cleavage of carbon bonds serving as the main degradation approach. The toxicity analysis demonstrated that the intermediates were generally less toxic than BPA during treatments. This research provides a fresh means for promoting a valence state cycling of ferric species in electro-Fenton system, which has potential applications in wastewater treatment containing emerging contaminants.