Freestanding 3-dimensional macro-porous SnO2 electrodes for efficient electrochemical degradation of antibiotics in wastewater

Electrochemical (EC) oxidation is an effective technology for treating wastewaters with emerging and persistent organic pollutants. However, conventional 2-dimensional (2D) film-type anodes have a low efficiency and short service life due to a limited electro-active surface area (EASA) and poor stability. In this study, novel freestanding and highly-stable 3-dimensional SnO2-Sb anodes with macro-pores (3D MP SnO2-Sb) were fabricated without the Ti base substrate by simply compressing the precursor of SnO2-Sb and carbon fibers followed by one-step sintering. Compared with the conventional 2D SnO2-Sb/Ti anode, the 3D MP SnO2-Sb anode featured a more than 100-fold increase in EASA, which greatly improved the EC efficiency for antibiotic oxidation and mineralization. Nearly 100% of ciprofloxacin (CIP, 20 mg L-1) in the synthetic wastewater was degraded by the 3D MP SnO2-Sb anode at a very low current density of 5 mA cm−2 within 120 min, which was significantly more effective than that by the 2D SnO2-Sb (74%) and a state-of-the-art boron-doped diamond anode (69%). The 3D MP SnO2-Sb anode exhibited a high stability (>200 cycles) and performed well in treating actual wastewater. The combined reaction kinetics analysis and porosimetry characterization indicated the importance of macro-pores to the excellent EC oxidation capacity and performance of the anode. The mechanism and pathways of electrocatalytic CIP degradation on the 3D anode were also elucidated from the density functional theory (DFT) calculations, molecular probe-based EC reaction analysis, and intermediate product detection.