Efficient denitrification of pharmaceutical wastewater using a coral-like CuO-ZIF-Co/NF cathode through reasonable matching of nitrate and nitrite reduction

[Display omitted] •ZIF-Co skeleton can achieve efficient composite of CuO and Co3O4 after annealing.•The cathode exhibits both the advantages of CuO and Co3O4 in NO3RR simultaneously.•The matching of NO3− and NO2− reductions prevents the accumulation of NO2−.•High NO3− removal and NH4+ generation rates are achieved at low current density.•The total nitrogen in actual pharmaceutical wastewater can be efficiently removed. Electrochemical NO3− reduction reaction (NO3RR) is an effective method for removing nitrate from industrial wastewater. The commonly used Cu based cathode can effectively reduce NO3− to NO2−. However, how to achieve a reasonable match between NO3− reduction and subsequent NO2− reduction is a key scientific issue in improving the efficiency of electrochemical NO3RR. A coral-like CuO-ZIF-Co/NF cathode is synthesized through the electrodeposition of Cu particles on the surface of ZIF-Co/NF skeleton combined with annealing. Excessive 2-methylimidazole in ZIF-Co is prone to self-deprotonation which enables effective binding of Co3O4 and CuO on the surface. Electrochemical characterizations reveal that compared to NO3−-N reduction, CuO-ZIF-Co/NF has lower over potential and faster reduction rate for NO2−-N reduction. Further kinetic studies also exhibit that the removal rate of NO2−-N by CuO-ZIF-Co/NF is 1.4 times higher than that of NO3−-N. This difference in reduction rate results in CuO-ZIF-Co/NF exhibiting higher rates of NO3−-N removal (0.97 mg-N (L∙min)-1) and NH4+-N generation (0.37 mg-N (L∙min)-1) than other reported materials at the current density as low as 10 mA cm−2, without any accumulation of NO2−-N during the NO3RR. Moreover, the electrode has shown excellent advantages in the actual pharmaceutical wastewater treatment process. It can not only remove the total nitrogen below the WHO emission limit standard, but also remove hardness and COD from the wastewater simultaneously.