The improvement on total nitrogen removal in nitrate reduction by using a prepared CuO-Co₃O₄/Ti cathode

In this work, a CuO-Co₃O₄/Ti composite was prepared via the coating-calcination method and employed as a cathode for the NO₃^--N reduction to increase the removal efficiency of total nitrogen (TN). SEM, EDS, and XRD characterization results indicated that CuO and Co₃O₄ were successfully introduced to the surface of Ti. The CuO-Co₃O₄/Ti electrode eventually removed NO₃^--N with the main products of N₂, NH₄⁺-N and NO₂^--N. In comparison to the Co₃O₄/Ti electrode, the better hydrogen evolution properties of the CuO-Co₃O₄/Ti electrode resulted in pH increase and NH₃ gas release, so the TN removal for CuO-Co₃O₄/Ti electrode was improved approximately 20%. The presence of Cl^- with the concentration up to 1000 mg L^-1 greatly promoted the removal of TN from 40.1% to 94.0%, as a result of NH₄⁺-N oxidation with free chlorine produced from the anode. Furthermore, the CuO-Co₃O₄/Ti electrode was applied to conduct three types of actual wastewater (biological effluent of municipal wastewater and industrial wastewater, and a regeneration concentrate from an anion exchange process) for nitrate removal. The highest TN removal efficiency (78.5%) and current efficiency (54.5%), and the lowest energy consumption (2 × 10^-4 kWh mg^-1 TN) were obtained for the regeneration concentrate, suggesting the feasibility of the CuO-Co₃O₄/Ti electrode to the water with high conductivity and high Cl^- concentration for removing TN by the reduction of nitrate.