Formation of nitrated naphthalene in the sulfate radical oxidation process in the presence of nitrite

Abstract

Polycyclic aromatic hydrocarbons (PAHs) have become a global environmental concern due to their potential hazardous implication for human health. In this study, we found that sulfate radical (SO4•−) could effectively degrade naphthalene (NAP), a representative PAH in groundwaters, generating 1-naphthol. This intermediate underwent further degradation, yielding ring-opening products including phthalic acid and salicylic acid. However, the presence of nitrite (NO2−), a prevalent ion in subsurface environments, was observed to compete with NAP for SO4•−, thus slowing down the NAP degradation. The reaction between NO2− and SO4•− generated a nitrogen dioxide radical (NO2•). Concurrently, in-situ formed 1-naphthol underwent further oxidization to the 1-naphthoxyl radical by SO4•−. The coupling of 1-naphthoxyl radicals with NO2• gave rise to a series of nitrated NAP, namely 2-nitro-1-naphthol, 4-nitro-1-naphthol, and 2,4-dinitro-1-naphthol. In addition, the in-situ formed phthalic acid and salicylic acid also underwent nitration, generating nitrophenolic products, although this pathway appeared less prominent than the nitration of 1-naphthol. When 10 μΜ NAP was subjected to heat activated peroxydisulfate oxidation in the presence of 10 μΜ NO2−, the total yield of nitrated products reached 0.730 μΜ in 120 min. Overall, the presence of NO2− dramatically altered the behavior of NAP degradation by SO4•− oxidation and contributed to the formation of toxic nitrated products. These findings raise awareness of the potential environmental risks associated with the application of SO4•−-based oxidation processes for the remediation of PAHs-polluted sites in presence of NO2−.