Electrochemical nitrogen removal from ethylene washing wastewater

Abstract

Ethylene alkaline washing wastewater after wet catalytic oxidation typically contains low concentrations of ammonia and nitrate, while its high salinity and pH restrict the application of biological nitrogen removal processes. The principle of electrochemical removal of ammonia is to use the applied electric field to promote the redox reaction of ammonia; convert it into ammonia, nitrogen, or nitrogen oxides; and achieve the purpose of reducing the content of ammonia in the water body. This study proposes the use of electrochemical methods for the nitrogen removal from the wastewater, with a focus on comparing the electrochemical performance and nitrogen removal efficiency of three common commercial anodes (Ti/RuO₂-IrO₂, Ti/SnO₂-Sb₂O₃, and graphite plate). Although Ti/RuO₂-IrO₂ had a higher electrical impedance than the other two electrodes, its larger electrochemical active surface area resulted in higher current density under the same potential conditions. Meanwhile, the excellent chlorine evolution performance of the Ti/RuO₂-IrO₂ electrode ensured the complete oxidation of ammonia in wastewater with a low chloride concentration. Additionally, neutral pH favored ammonia oxidation on all electrodes, but Ti/RuO₂-IrO₂ electrodes could maintain higher ammonia oxidation efficiency and N₂ selectivity even under alkaline conditions. Increasing current density promoted the oxidation of ammonia, but 10 mA·cm⁻² was ideal as it offered relatively low energy consumption. During long-term continuous operation, the Ti/RuO₂-IrO₂ electrode was the most stable, whereas the Ti/SnO₂-Sb₂O₃ electrode lost activity after 30 h of reaction, and the mass of the graphite electrode decreased by 12% after 100 h of reaction.