Nitrite Oxidation during Ozonation Revisited: Mechanisms of Nitration Reactions

The formation of toxicologically relevant nitro compounds has been observed during ozonation of nitritecontaining secondary wastewater effluents, but their formation mechanism remains unknown. To identify key nitrating species, three reaction systems were investigated: ozonation of nitrite (O 3 / NO 2 −), peroxynitrite (ONOOH/ONOO −), and hydroxyl radical oxidation of nitrite with γ-radiolysis (γ/NO 2 −). Nitrite ozonation (O 3 /NO 2 −) yielded significant amounts of the nitrating agent nitrogen dioxide • NO 2 (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 • NO 2 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 O 3 /NO 2 − and γ-radiolysis (γ/NO 2 −) 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) × 10 6 M −1 s −1), suggest that a significant contribution of peroxynitrite is highly unlikely. Overall, nitration during ozonation involves nitrite oxidation to • NO 2 , mostly by hydroxyl radical and to a minor extent by ozone.