Nitrite Oxidation during Ozonation Revisited: Mechanisms of Nitration Reactions

The formation of toxicologically relevant nitro compounds has been observed during ozonation of nitrite-containing secondary wastewater effluents, but their formation mechanism remains unknown. To identify key nitrating species, three reaction systems were investigated: ozonation of nitrite (O₃/NO₂^-), peroxynitrite (ONOOH/ONOO^-), and hydroxyl radical oxidation of nitrite with γ-radiolysis (γ/NO₂^-). Nitrite ozonation (O₃/NO₂^-) yielded significant amounts of the nitrating agent nitrogen dioxide ^•NO₂ (9.4% at pH 7 to 22% at pH 12) besides the main product nitrate. No peroxynitrite formation was detected during ozonation of nitrite-containing waters, suggesting that ^•NO₂ is the key nitrating species. A comparison of nitro compound formation from 20 aromatic compounds in the three reaction systems showed a consistent formation of nitro compounds with minor differences. Furthermore, the pH-dependence patterns of nitration for two micropollutants (diuron and carbendazim) revealed similarities between the O₃/NO₂^- and γ-radiolysis (γ/NO₂^-) systems, unlike peroxynitrite. These trends, combined with the lack of detection of peroxynitrite during ozonation, a fast reaction of peroxynitrite with ozone (k_pH=9 = (4.96 ± 0.40) × 10⁶ M^-1s^-1), suggest that a significant contribution of peroxynitrite is highly unlikely. Overall, nitration during ozonation involves nitrite oxidation to ^•NO₂, mostly by hydroxyl radical and to a minor extent by ozone.