Treatment of metronidazole pharmaceutical wastewater using pulsed switching peroxi-coagulation combined with electro-Fenton process

The aim of this study was to investigate the metronidazole (MNZ) degradation and real MNZ pharmaceutical wastewater treatment in the pulsed switching peroxi-coagulation (PSPC) process. Different pulsed switching frequencies and running times of H2O2 and Fe2+ productions were tested in the PSPC process. Results demonstrated that MNZ removal of 96.9 +/- 1.2 % was realized in the PSPC process with a 200 mg/L MNZ and 0.1 M Na2SO4 solution within 80 min under a pulsed switching frequency of 6s: 1s and a current density of 20 mA/cm2 (H2O2) and 20 mA/cm2 (Fe2+). High MNZ removal could be attributed to efficient center dot OH production with the highest center dot OH concentration reached 321 +/- 15 mu M in the PSPC process. The hydroxyl and carboxyl groups of MNZ were sequentially oxidated by center dot OH and mineralized based on seven identified intermediates during the MNZ degradation. However, only 56.9 +/- 6.7 % of COD was removed in the real MNZ wastewater treatment by the PSPC process within 90 min. A PSPC combined with electro-Fenton (EF) process was developed to enhance the COD removal in the MNZ wastewater. With MNZ wastewater as electrolytes, 3.3 +/- 0.3 g/L of H2O2 was produced in a conventional EF reactor. The final COD removal reached 86-90 % using the mixture of effluent from the PSPC, the anode, and cathode chambers of the EF reactor, resulting in less than 80 mg/L COD in the effluent. Results from this study should provide a useful way to enhance real MNZ pharmaceutical wastewater treatment.