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

Sulfate radical (SO4 center dot-)-based advanced oxidation is a viable in situ remediation technology for degrading organic contaminants in the subsurface. In this study, we demonstrated that SO4 center dot- 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 N-15 nuclear magnetic resonance analysis revealed that the inorganic nitrite was incorporated into natural organic matter (NOM) to form organic nitrogen upon SO4 center dot- 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 N-15 labeling technique. Formation of nitrated byproducts was ascribed to the scavenging of SO4 center dot- by nitrite, which not only generated the nitrating agent NO2 center dot but also inhibited the degradation of organic compounds, making them more available to the reactions with NO2 center dot. The phenolic moieties in NOM served as the main reactive sites for NO2 center dot attack. The nitration begins with H abstraction on the phenoxy oxygen, followed by the addition of another NO2 center dot to its ortho or para site. Decarboxylation followed by NO2 center dot 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 SO2 center dot--based oxidation. It sheds light on the potential risks of this technology in subsurface remediation practices.