Accelerated degradation of 4-nitrophenol using microplasma discharge: Processes and mechanisms

In this work, an atmospheric microplasma reactor has been constructed to evaluate, the influence of microplasma forming gases (air, oxygen, nitrogen, and argon) on 4-nitrophenol (4NP) degradation and mineralization efficiency as a function of treatment time. 92 % of 4NP in solution was degraded by air microplasma after 7 min, which was higher than nitrogen, oxygen, and argon microplasma treatments. The air microplasma treatment achieved a consistent mineralization percentage of 57 %. With the addition of hydroxyl radical scavenger during the treatment, irrespective of the gas medium, the degradation percentage got reduced; for air, it was 76 % reduced compared to without scavenger. This implies that the hydroxyl radical is primarily responsible for the degradation of 4NP. The amount of hydrogen peroxide produced during the treatment was measured, and its role in 4NP degradation was examined. A density functional theory calculation was carried out to identify reactive sites on the 4NP molecule. 4NP intermediates, including hydroquinone and benzoquinone, were identified using liquid chromatography-mass spectrometry, and a 4NP degradation pathway was proposed based on the detected intermediates. The phytotoxicity of plasma mineralized 4NP aqueous solution was analyzed by mung bean seed germination, and the results reveal that plasma treated 4NP aqueous solution has 11.5 % of higher germination percentage than untreated 4NP.