Degradation of 4-nitrophenol using Ar/air thermal plasma in submerged conditions: plasma characteristics, degradation chemical kinetics and degradation pathways

This study explores the application of thermal plasma in submerged conditions for the efficient degradation of 4-nitrophenol (4-NP), a high-priority toxic pollutant. A customized 10 kW DC non-transferred arc thermal plasma torch was employed to generate the plasma using argon (Ar) and air as plasma-forming gases. The plasma jet was characterized through arc voltage fluctuations, optical emission spectroscopy, and submerged jet dynamics, offering detailed insights into the physical and chemical properties of the Ar/air plasma. The plasma treatment of an initial 50 mg l-1 concentration of 4-NP in an aqueous medium resulted in 94% degradation and 58% mineralization within 30 min at the plasma input power of 6.2 kW. The removal process followed a pseudo-first-order kinetic model with a reaction rate constant of 0.0684 min-1. The possible degradation intermediates and products were identified via Liquid chromatography-mass spectroscopy and Ion chromatography. The submerged thermal plasma technique demonstrated enhanced compound removal efficiency, making it a promising method for the treatment of toxic dissolved organic compounds in wastewater.