Endogenous iron-enriched biochar loaded nickel-foam cathode in electro-Fenton for ciprofloxacin degradation: performance, mechanism and DFT calculation

Abstract

Steel mill wastewater sludge, with abundant endogenous iron resources, could be pyrolyzed into iron-enriched biochar (SBC) in one step and loaded onto nickel foam cathode (Ni-Foam) to construct a heterogeneous electro-Fenton system (SBC@Ni-Foam/EF). The purpose of this study were to optimize SBC@Ni-Foam/EF for efficient ciprofloxacin (CIP) removal, and its degradation mechanism and pathway in the presence of endogenous iron were elucidated by DFT calculation. The results showed that the highest CIP degradation efficiency (95.11 %) was achieved at a SBC pyrolysis temperature of 800 °C, an initial solution pH of 3, a Na2SO4 concentration of 0.05 mol/L, a current density of 40 mA cm−2, and an aeration rate of 200 mL min−1. The degradation of CIP by SBC@Ni-Foam/EF was dominated by cathodic catalytic oxidation, and ·OH played a major role in the CIP degradation process. Combined with characterization and DFT calculation, the endogenous iron was dispersed in the stable Fe3O4 crystalline phase, which provided surface defects and catalytic sites for the cathode and promoted H2O2 activation to generate ·OH through iron cycling. The possible degradation pathways of CIP were proposed to include the cleavage of piperazine and quinolone rings, hydroxylation, and substitution or detachment of fluorine, etc., and the overall toxicity of the intermediates was alleviated after SBC@Ni-Foam/EF treatment. Therefore, this study was expected to achieve a win–win situation for iron-enriched sludge biomass resource utilization and CIP wastewater treatment.