New evidence on the formation of oxidizing species in corona discharge in contact with liquid and their reactions with organic compounds.

The objective of these investigations is to understand in more detail how organic compounds in water are degraded during plasma treatment. The formation of oxidizing species (ozone (O), hydrogen peroxide (HO) and hydroxyl radicals (OH)) in a pulsed corona discharge in contact with liquid is investigated. The degradation of a target organic compound (methylparaben) in aqueous solution was increased when combining plasma treatment with ozonation, using the O generated in the discharge. Enhanced mass transfer of O obtained in this plasma+O configuration leads to a six fold increase of MeP oxidation rate. The evolution of oxidants concentration during treatment of MeP solutions provides information on their consumption in reactions with MeP and its oxidation products. The correlation of MeP degradation results (MeP removal and mineralization) with O consumption and the identified reaction products confirms that although O plays an important role in the degradation, for the mineralization OH radicals have an essential contribution. The concentration of OH radicals is diminished in the solutions containing MeP as compared to plasma-treated water, indicating OH consumption in reactions with the target compound and its degradation products. The concentration of HO in the liquid can be either increased or reduced in the presence of MeP, depending on its initial concentration. On the one hand, decomposition of HO by OH or O is suppressed in the presence of MeP, but on the other hand less OH radicals are available for its formation.