Formation of Nitrophenolic Byproducts during UV-Activated Peroxydisulfate Oxidation in the Presence of Nitrate

UV-peroxydisulfate (UV/PDS) oxidation is a promising technology to degrade organic pollutants. In this study, we found that nitrate (NO3-) could lead to the formation of toxic nitrophenolic byproducts during UV/PDS oxidation of natural organic matter (NOM). At a UV fluence of 4.49 x 10(5) mJ L-1, the formation of 4-nitrophenol, 4-hydroxy-3-nitrobenzoic acid, and 2,4-dinitrophenol reached 0.0084, 0.0324, and 0.0046 mu M, respectively. NO2 center dot produced from the photolysis of NO3- acted as the nitrating agent. Meanwhile, the phenolic moieties of NOM molecules were oxidized by SO4 center dot- through single-electron transfer, giving rise to phenoxyl radicals. The phenoxyl radicals coupled with NO2 center dot to generate nitrated byproducts. Although (OH)-O-center dot was also formed in the UV/PDS process, theoretical computation suggests that the phenoxyl radicals were primarily ascribed to SO4 center dot-, because (OH)-O-center dot preferentially reacted with NOM via addition mechanism resulting in hydroxylated intermediates. In addition, the aromatic carboxyl moieties of NOM molecules could be decarboxylated upon reaction with SO4 center dot- and transformed to phenolic intermediates, which also contributed to the nitrated byproducts formation. This study reveals a novel nitration mechanism that is specific to the UV/PDS process and raises concerns to the potential risks when the UV/PDS is applied to wastewaters with high levels of NO3-.

Automated summary

UV/PDS oxidation is a promising technology for degrading organic pollutants. However, this study found that nitrate can lead to the formation of toxic nitrophenolic byproducts during the UV/PDS oxidation of natural organic matter. Nitrate is photolyzed to produce nitrite, which acts as a nitrating agent. Meanwhile, SO4•- oxidizes the phenolic moieties of NOM molecules, generating phenoxyl radicals. The phenoxyl radicals react with nitrite to generate nitrated byproducts. Additionally, the aromatic carboxyl moieties of NOM molecules can be decarboxylated upon reaction with SO4•-, contributing to the formation of nitrated byproducts. This study reveals a novel nitration mechanism specific to the UV/PDS process and raises concerns about the potential risks when this technology is used for wastewaters with high levels of nitrate.