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

Removing ammonia-nitrogen (NH3-N) from wastewater is of paramount importance for wastewater treatment. In this study, a novel continuous liquid plasma process (CLPD) was evaluated to remove NH3-N from synthetic wastewater. The Box-Behnken experimental design was used to optimize the main process parameters, including the initial NH3-N concentration (50-200 mg/L), power input (150-300 W), and gas-flow rate (1.5-2.5 L/min), for efficient NH3-N removal from wastewater. The gas-flow rate and power input were found to be significant factors affecting the removal efficiency of NH3-N, whereas the initial concentration of NH3-N played a vital role in determining the energy efficiency of the process. Under the optimal conditions of an initial NH3-N concentration of 200 mg/L, applied power of 223 W, and gas-flow rate of 2.4 L/min, 98.91% of NH3-N could be removed with a N-2 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 NH3-N 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 center dot OH, center dot O, and center dot O-2(+). These results demonstrate the potential of liquid phase plasma discharge as a highly efficient technology for removing ammonia from aqueous solutions.