Electrochemical oxidation metronidazole with Co modified PbO2 electrode: Degradation and mechanism

•Typical transition metal cobalt (Co) modified PbO2 electrode was prepared.•Co2+ improved the catalytic activity of PbO2 electrode.•Co-PbO2 electrode was characterized by the steady-state polarization curves and cyclic voltammetry.•Electrochemical method is proved to be a useful way of degrading metronidazole.•The main metronidazole electrochemical degradation pathway was proposed. This study presents an electrochemical method for the degradation of metronidazole (MNZ) in aqueous solution with novel PbO2 electrode, which was prepared via electrodeposition in nitrate solution. The influencing factors on electrochemical decomposition of pollutants were evaluated and the removal of MNZ and chemical oxygen demand (COD) reached 78.09% and 21.77% after 120min treatment under the optimal conditions with the concentration of supporting electrolyte, the initial concentration of MNZ, the pH and the current density were 0.10mol/L, 100mg/L, 6.4 and 10mA/cm2, respectively. Then, the Co doped PbO2 electrodes were prepared and used to degrade MNZ under the optimal conditions. Relatively good performance was achieved when the molar ratio of Pb and Co was 100:1. Compared to pure PbO2 electrode, the steady-state polarization curves and cyclic voltammetry analysis showed that 1.0% Co-PbO2 electrode had a higher oxygen evolution potential and greater current of reduction and oxidation peaks, which increased the electrochemical activity and decreased the energy consumption. Finally, based on the intermediate products identified by IC and GC/MS, the main degradation pathway of MNZ was proposed including ring open, denitrification and radical reactions.