Formation of Nitrophenolic Byproducts during Heat-Activated Peroxydisulfate Oxidation in the Presence of Natural Organic Matter and Nitrite.

Sulfate radical (SO4•-)-based advanced oxidation is a viable in situ remediation technology for degrading organic contaminants in the subsurface. In this study, we demonstrated that SO4•- could induce the activation of nitrite, an anion commonly present in the subsurface environment, leading to the formation of nitrophenolic byproducts. Fourier-transform infrared spectroscope and 15N nuclear magnetic resonance analysis revealed that the inorganic nitrite was incorporated into natural organic matter (NOM) to form organic nitrogen upon SO4•- oxidation. Nitrophenolic byproducts, including 2-hydroxy-5-nitrobenzoic acid, 4-nitrophenol, and 2,4-dinitrophenol, were identified using high-resolution mass spectrometry in combination with a 15N labeling technique. Formation of nitrated byproducts was ascribed to the scavenging of SO4•- by nitrite, which not only generated the nitrating agent NO2• but also inhibited the degradation of organic compounds, making them more available to the reactions with NO2•. The phenolic moieties in NOM served as the main reactive sites for NO2• attack. The nitration begins with H abstraction on the phenoxy oxygen, followed by the addition of another NO2• to its ortho or para site. Decarboxylation followed by NO2• addition can also generate nitrophenolic byproducts. To the best of our knowledge, this is the first study reporting the nitration of NOM and formation of toxic nitrophenolic byproducts during SO4•--based oxidation. It sheds light on the potential risks of this technology in subsurface remediation practices.