Treatment of methyl orange by nitrogen non-thermal plasma in a corona reactor: The role of reactive nitrogen species

Highlights: • Nitration of methyl orange is one of the main processes in treatment with N₂-plasma. • MS/MS analysis shows preferred nitration of methyl orange in ortho position. • N₂ plasma, N₂-PAW, reaction with NO₂⁻ or NO₂⁻/H₂O₂ at pH 2 give the same products. - Abstract: Methyl orange (MO) azo dye served as model organic pollutant to investigate the role of reactive nitrogen species (RNS) in non-thermal plasma (NTP) induced water treatments. The results of experiments in which MO aqueous solutions were directly exposed to N₂-NTP are compared with those of control experiments in which MO was allowed to react with nitrite, nitrate and hydrogen peroxide, which are species formed in water exposed to N₂-NTP. Treatment of MO was also performed in PAW, Plasma Activated Water, that is water previously exposed to N₂-NTP. Both direct N₂-NTP and N₂-PAW treatments induced the rapid decay of MO. No appreciable reaction was instead observed when MO was treated with NO₃⁻ and H₂O₂ either under acidic or neutral pH. In contrast, in acidic solutions MO decayed rapidly when treated with NO₂⁻ and with a combination of NO₂⁻ and H₂O₂. Thorough product analysis was carried out by HPLC coupled with UV–vis and ESI–MS/MS detectors. In all experiments in which MO reaction was observed, the major primary product was a derivative nitro-substituted at the ortho position with respect to the N,N-dimethylamino group of MO. The reactions of RNS are discussed and a mechanism for the observed nitration products is proposed.