Ammonia Nitrogen Removal by Gas–Liquid Discharge Plasma: Investigating the Voltage Effect and Plasma Action Mechanisms

Abstract

Atmospheric pressure gas–liquid discharge plasma has garnered considerable attention for its efficacy in wastewater contaminant removal. This study utilized atmospheric oxygen gas–liquid discharge plasma for the treatment of ammonia nitrogen wastewater. The effect of applied voltage on the treatment of ammonia nitrogen wastewater by gas–liquid discharge plasma was discussed, and the potential reaction mechanism was elucidated. As the applied voltage increased from 9 kV to 17 kV, the ammonia nitrogen removal efficiency rose from 49.45% to 99.04%, with an N2 selectivity of 87.72%. The mechanism of ammonia nitrogen degradation by gas–liquid discharge plasma under different applied voltages was deduced through electrical characteristic analysis, emission spectrum diagnosis, and further measurement of the concentration of active species in the gas–liquid two-phase system. The degradation of ammonia nitrogen by gas–liquid discharge plasma primarily relies on the generation of active species in the liquid phase after plasma–gas interactions, rather than direct plasma effects. Increasing the applied voltage leads to changes in discharge morphology, higher energy input, elevated electron excitation temperatures, enhanced collisions, a decrease in plasma electron density, and an increase in rotational temperatures. The change in the plasma state enhances the gas–liquid transfer process and increases the concentration of H2O2, O3, and, ⋅OH in the liquid phase. Ultimately, the efficient removal of ammonia nitrogen from wastewater is achieved.