Transformation of ammonium to nitrophenolic byproducts by sulfate radical oxidation

Sulfate radical (SO4 center dot-) based oxidation shows great promise in wastewater treatment and subsurface remediation. For the first time, we demonstrated that SO4 center dot- could induce the transformation of ammonium (NH4+) to nitrophenolic byproducts. Using high-resolution mass spectrometry in combination with N-15 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 mu M, 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 mu M, respectively, in 24 h. One-electron oxidation of NH4+ to form aminyl radicals ((NH2)-N-center dot) was the first step of the transformation. The reaction of (NH2)-N-center dot with oxygen was a key step in propagating radical chain reactions, leading to nitrogen dioxide radicals (NO2 center dot) 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 center dot-, 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 center dot- is applied for onsite remediation, which should be taken into consideration when evaluating the feasibility of this technology.

Automated summary

Sulfate radical (SO4 center dot-) based oxidation has great potential in wastewater treatment by transforming ammonium (NH4+) into nitrophenolic byproducts. The reaction mechanism leading to the formation of nitrophenolic byproducts, as well as the transformation of aromatic carboxylate moieties to phenolics by SO4 center dot-, has been demonstrated in the study. The widespread formation of nitrophenolic byproducts when using SO4 center dot- for onsite remediation highlights the importance of considering this factor in evaluating the feasibility of the technology.