Roles of nitrogen dioxide radical (•NO2) in the transformation of aniline by sulfate radical and hydroxyl radical systems with the presence of nitrite

Sulfate radicals (SO4 center dot-)- and hydroxyl radicals (HO center dot)-based advanced oxidation processes (AOPs) are efficient for the destruction of contaminants in water treatment, while the ubiquitous presence of nitrite (NO2-) in aquatic environments quickly transforms SO4 center dot- and HO center dot to nitrogen dioxide radicals ((NO2)-N-center dot). This study investigated the roles of (NO2)-N-center dot in the transformation of aniline by SO4 center dot- and HO(center dot)in the presence of NO2-. By laser flash photolysis, aniline radical cations (C6H5NH2 center dot+) were identified to be the primary intermediates by SO4 center dot- through electron transfer, while anilino radicals (C6H5NH center dot) and HO-adduct radicals were predominant by HO center dot via hydrogen abstraction and addition, respectively. Also, (NO2)-N-center dot attacked aniline via electron transfer to form C6H5NH2 center dot+, which contributed 42.3 % and 69.2 % to the overall degradation rates of aniline in the SO4 center dot- and HO center dot systems at the experimental conditions, respectively. C6H5NH2 center dot+ could be reversibly converted to C6H5NH center dot with pKa at 7, and HO-adduct radicals could be also quickly transformed to C6H5NH center dot via water splitting. Therefore, nitmanilines were comparably generated in the SO4 center dot- and HO center dot systems via the rapid combination of C6H5NH2 center dot+/C6H5NH center dot and (NO2)-N-center dot, which conversion yields from aniline to nitroanilines were 40.0 % and 29.8 % at 20 min, respectively. Additionally, the comparable formation of C6H5NH2 center dot+/C6H5NH center dot results in the similar transformation pathways of aniline by SO4 center dot- and HO center dot with the presence of NO2-. This study demonstrates the important roles of (NO2)-N-center dot in the transformation of aniline in SO4 center dot- and HO center dot-based AOPs with the presence of NO2-.

Automated summary

The role of nitrite (NO2-) in the transformation of aniline by sulfate radicals (SO4•-) and hydroxyl radicals (HO•) was investigated in this study. It was found that NO2- played an important catalytic role in both SO4•- and HO• systems, contributing to the overall degradation rates of aniline.