Enrofloxacin degradation in a heterogeneous electro-Fenton system using a tri-metal-carbon nanofibers composite cathode

Abstract In this study, enrofloxacin (ENR) degradation by heterogeneous electro-Fenton (hetero-EF) system using Fe/Co/Zn-tri-metal co-doped carbon nanofibers (Fe/Co/Zn@C-NCNFs-800) modified cathode, which was fabricated by simply carbonization of the electrospun Fe-Co-Zn-ZIF@PAN. It was observed that the Fe/Co/Zn@C-NCNFs-800 electrode showed rougher surface, better crystal shape, more graphitic structures, larger specific surface area (108.0131 m2 g−1) and pore volume (0.1109 cm3 g−1) after calcination. Further electrochemical analysis revealed that the calcinated electrode possessed greater catalytic activity towards two-electron oxygen reduction reactions (2e-ORR), smaller electrochemical impedance, and higher corrosion potential compared to the initial Fe-Co-Zn-ZIF@PAN precursor. Under an optimized condition (e.g., pH = 3, and current = 40 mA), this hetero-EF system achieved an extraordinary ENR degradation efficiency of 99.08% within 30 min and a TOC removal efficiency of 47.13% within 150 min. Meanwhile, a remarkable yield and generation rate of H2O2 (129.029 mg L−1 and 0.8602 mg L−1 min−1) and ·OH (36.388 mg L−1 and 0.2426 mg L−1 min−1) were obtained. ENR degradation process involved the electron transfer between ≡FeII/III and ≡CoII/III redox couples as well as capture electron directly from cathode, which reduced the consumption of H2O2 and promoted the utilization of H2O2. Radical scavenging and electron paramagnetic resonance (EPR) tests verified that 1O2 and HO2·/O2·- served as the dominant reactive oxygen species (ROS) for ENR degradation. The possible ENR degradation pathway was deduced to be the open rings of piperazine and quinolone moiety as well as transformation into ciprofloxacin (CIP) according to the identification of produced intermediates. The relatively low energy consumption (3.07 kWh·m−3 and 2.54 kWh·m−3·order−1) and low intermediates toxicity made the practical applications of this hetero-EF system a real possibility.