A microwave atmospheric plasma strategy for fast and efficient degradation of aqueous p-nitrophenol

Plasma technology has received intensive research interest in pollutants degradation. However, conventional plasma generator suffers from erosion of electrodes and consequent short life time and pollution. In this work, an electrodeless high-flow microwave atmospheric plasma jet is developed for fast degradation of p-nitrophenol. With the assistance of injection locking technology, stable plasma is managed to be generated by low-cost magnetron. 100% removal of 100 mg/L PNP is achieved after 12 min, with a TOC removal efficiency of 57.6%. The fast degradation is probably due to the high cross section (around 153 mm²) of plasma gas. Change in the removal efficiency are less than 4% and 5% as the pH of the solution changes from 2.02 to 12.07 and conductivity varies between 5^.38 × 10^-2 and 43.6 mS/cm, respectively. Moreover, optical emission spectroscopy spectra of the microwave plasma and a hydroxyl radical scavenger (t-butanol) are employed to identify the generated oxidizing species, which indicates that ^•OH is the key factor during the degradation process. The hydroxylated intermediates and organic acid transformed from PNP were revealed. Based on the examined intermediate products, the possible degradation mechanism and pathway are analyzed.