Evaluation of a liquid-phase plasma discharge process for ammonia oxidation in wastewater: Process optimization and...

Removing ammonia-nitrogen (NH₃N) from wastewater is of paramount importance for wastewater treatment. In this study, a novel continuous liquid plasma process (CLPD) was evaluated to remove NH₃N from synthetic wastewater. The Box-Behnken experimental design was used to optimize the main process parameters, including the initial NH₃N concentration (50-200 mg/L), power input (150-300 W), and gas-flow rate (1.5-2.5 L/min), for efficient NH₃N removal from wastewater. The gas-flow rate and power input were found to be significant factors affecting the removal efficiency of NH₃N, whereas the initial concentration of NH₃N played a vital role in determining the energy efficiency of the process. Under the optimal conditions of an initial NH₃N concentration of 200 mg/L, applied power of 223 W, and gas-flow rate of 2.4 L/min, 98.91% of NH₃N could be removed with a N₂ selectivity of 92.91%, and the corresponding energy efficiency was 0.527 g/kWh after 2 hrs of treatment. A small fraction of undesirable NO₃^--N (7.05 mg/L) and NO₂^--N (2.83 mg/L) were also produced. Kinetic modeling revealed that NH₃N degradation by the CLPD followed a pseudo-first-order reaction model, with a rate constant (k) of 0.03522 min^-1. Optical emission spectroscopy (OES) was used to gather information about the active chemical species produced during the plasma discharge. The obtained spectra revealed the presence of several highly oxidative radicals, including ‧OH, ‧O, and ‧O₂⁺. These results demonstrate the potential of liquid phase plasma discharge as a highly efficient technology for removing ammonia from aqueous solutions.