Degradation of p-nitrophenol by DBD plasma/Fe2+/persulfate oxidation process

Dielectric barrier discharge (DBD) plasma in situ generates reactive species (e*, OH, O, H, H2O2, O3, etc.), UV irradiation and local high temperature. These physiochemical effects can non-selectively destroy aqueous organic contaminants, but the energy efficiency of DBD plasma for water decontamination can be further improved. Persulfate (S2O82-, PS) and ferrous ions (Fe2+) were employed for enhancing the degradation of p-nitrophenol (PNP). An addition of both S2O82- (<2.5 mM) and Fe2+ (<36 µM) in PNP solution apparently promoted the degradation efficiency of PNP and energy efficiency compared to discharge plasma alone, due to a more production of active OH and SO4·- through Fenton-like reactions and PS activation process. Moreover, the tertiary system of plasma/PS/Fe2+ (81.1%) presented higher PNP degradation efficiency after 50 min treatment than discharge plasma alone (34.8%) and the binary systems of plasma/PS (63.6%), plasma/Fe2+ (69.6%) and PS/Fe2+ (13.7%). However, an excessive addition of S2O82- (2.5–3.6 mM) and Fe2+ (36–360 µM) had unobvious or even harmful influence on PNP degradation, possibly due to the enhanced recombination or consumption of OH and SO4·- by radical side reactions. The effect of radical scavengers on PNP degradation indicates that OH was the most important radical species in plasma system for PNP degradation, but SO4·- was also very important for PNP degradation when PS was present in solution. Besides Fe2+ and the heat effect from discharge plasma, the electrons produced by electric discharge may also be one of the important contributors to the activation of PS.