Enhanced electrochlorination of ammonium to N2 mediated by pH self-buffering effect in the cathodic coagulation and anodic oxidation synergistic system

Abstract

Electrochlorination of NH₄⁺ to N₂ has been widely used for wastewater treatment, but it suffers from inefficiency because of the solution acidification caused by the protons releasing from NH₄⁺-N oxidation. This study developed an electrochemical NH₄⁺-N oxidation system employing Al cathode paired with an IrO₂-RuO₂/Ti anode, where the acidification effect could be effectively spontaneously alleviated by the hydroxyl-aluminum species produced during the reaction. The solution pH maintained 5.1 and gradually increased to 7 after 120 min. In contrast, the solution pH dropped below 3 when SS or Ni foam was used as the cathode. The NH₄⁺-N oxidation system with Al cathode showed NH₄⁺-N oxidation efficiency of 99% at a current density of 20 mA cm⁻² and Cl^- concentration of 30 mM, which outperformed the systems with Ni foam (20 %) and SS cathode (67 %). Additionally, the NH₄⁺-N oxidation system with Al cathode had appreciable resistance to organic pollutants, achieving 90 % COD removal efficiency while maintaining high NH₄⁺-N oxidation efficiency (88 %), which was much higher than SS (23 %) and Ni foam cathode (13.7 %). The NH₄⁺-N oxidation efficiency decreased 12 % by the addition of TBA to remove the Cl^• and OH^•. This result indicates that the free chlorine was the primary oxidizing species for NH₄⁺-N oxidation and could be well utilized to oxidize NH₄⁺ to N₂ at nearly neutral pH spontaneously regulated by cathode coagulation reactions. This study offers a promising method for NH₄⁺-N wastewater treatment and promotes the industrial application.