Formation of nitro(so) and chlorinated products and toxicity alteration during the UV/monochloramine treatment of phenol

Abstract The UV/monochloramine (UV/NH2Cl) process is an emerging advanced oxidation process (AOP) to remove organic contaminants in water treatment with radicals including hydroxyl radicals (HO•), reactive chlorine species (RCS) and reactive nitrogen species (RNS). This study investigated the formation of nitro(so) and chlorinated products and toxicity alteration during the UV/NH2Cl treatment of phenol. RNS and/or RCS induced the formation of nitro(so), chlorinated and polymeric compounds during phenol transformation by UV/NH2Cl. These compounds dramatically increased the cytotoxicity to Chinese hamster ovary cells after 20 min UV/NH2Cl treatment, which was 10 times higher than that after 24 h chloramination. The increase of cytotoxicity in UV/NH2Cl was primarily attributable to 4-nitrosophenol, and the cytotoxicity followed the order of 4-nitrosophenol >> 4-nitrophenol > 2,4,6-trichlorophenol > 2,4-dichlorophenol > phenol. 4-Nitrosophenol was significantly generated by the combination of •NO and phenoxy radical, where the maximum conversion rates of phenol to 4-nitrosophenol increased from 4.9% to 62.4% when pH increased from 5 to 10. The highest conversion rate was at pH 10 because the •NO concentration increased with increasing pH from 5 to 10 in UV/NH2Cl, as verified by the electron paramagnetic resonance (EPR) analysis. Nitrophenols were also detected at much lower concentrations than 4-nitrosophenol, which were mainly formed by the oxidation of 4-nitrosophenol and the combination of •NO2 with phenoxy radicals. RCS was responsible for the formation of chlorinated products mainly through Cl• addition and the reactions of Cl•/Cl2•− with phenoxy radicals. Also, RCS and RNS significantly enhanced the formation of carbonaceous (i.e., chloroform and chloral hydrate) and nitrogenous disinfection byproducts (i.e., chloropicrin and dichloroacetonitrile) in UV/NH2Cl. This study indicates that the UV/NH2Cl treatment significantly increased toxicity and validates the roles of RNS and RCS in producing toxic nitro(so) and chlorinated products.