Transformation of ammonium to nitrophenolic byproducts by sulfate radical oxidation

Sulfate radical (SO4•−) based oxidation shows great promise in wastewater treatment and subsurface remediation. For the first time, we demonstrated that SO4•− could induce the transformation of ammonium (NH4+) to nitrophenolic byproducts. Using high-resolution mass spectrometry in combination with 15N labeling, mono-nitro and di-nitro phenolic byproducts were identified in a sample containing 1 mM NH4+ and 10 mg/L natural organic matter (NOM) following heat activated peroxydisulfate (PDS) oxidation. At PDS dose of 1 mM, the formation of p-nitrophenol and 5-nitrosalicylic acid reached 0.21 and 0.30 μM, respectively, in 12 h and then decreased; the formation of 2,4-dinitrophenol and 3,5-dinitrosalicylic acid increased monotonically, reaching 0.37 and 0.62 μM, respectively, in 24 h. One-electron oxidation of NH4+ to form aminyl radicals (•NH2) was the first step of the transformation. The reaction of •NH2 with oxygen was a key step in propagating radical chain reactions, leading to nitrogen dioxide radicals (NO2•) as a key nitrating agent. The reactive sites susceptible to nitrating in NOM molecules are not limited to phenolic moieties. We found that aromatic carboxylate moieties could be in situ transformed to phenolics by SO4•−, thus contributed to nitrophenolic byproducts formation as well. Considering the ubiquitous presence of NH4+ in the environment, formation of nitrophenolic byproducts will be widespread when SO4•− is applied for onsite remediation, which should be taken into consideration when evaluating the feasibility of this technology.