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

Abstract

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