Electroreduces nitrate to ammonia and converts to nitrogen through chloride ions by Co₃O₄/GF cathode: Performance, regulation and mechanism

With the development of cities, the issue of excess nitrate in wastewater has become increasingly severe. Electrochemical technology has garnered significant attention due to its straightforward operation and environmental sustainability. A Co₃O₄/GF cathode was successfully prepared by depositing Co₃O₄ onto Graphite felt (GF) using an electrochemical deposition-calcination method. The effects of electrochemical reduction of nitrate (NO₃^--N) using Co₃O₄/GF cathodes were investigated. The removal rate of NO₃^--N achieved an impressive 98.6% with the Co₃O₄/GF cathode, and the formation rate of reduced ammonia (NH₄⁺-N) was 100% within 2 h at -1.61 V vs. Ag/AgCl voltage. The optimal removal efficiency for NO₃^--N occurred at a pH of 5. In addition, the electrode demonstrated excellent recyclability and stability. In the presence of Cl^-, N₂ was produced instead of NH₄⁺ through mediated oxidation. NH₄⁺-N was oxidized to N₂ under the action of Cl^-. When the concentration of KCl reached 3000 mg/L, the total nitrogen removal rate achieved 98.63%. The reduction mechanism for NO₃^- reduction was confirmed through electrochemical analysis, scavenging experiments and XPS analysis: on the one hand, it was caused by the Co²⁺ -Co³⁺ -Co²⁺ process. On the other hand, it was caused by indirect reduction mediated by H*. This study presents an efficient and environmentally friendly method for converting NO₃^--N to NH₄⁺-N while simultaneously controlling NH₄⁺-N production through chlorine addition, providing a theoretical foundation for the degradation of NO₃^--N from wastewater.