Enhanced trichloronitromethane formation during chlorine-UV treatment of nitrite-containing water by organic amines

This study explored the risk of trichloronitromethane (TCNM) formation during chlorination of the nitrite-containing water after pre-chlorination and subsequent UV irradiation (i.e., the chlorine-UV process). The competitive reaction between amino acid (AA) and NO₂^- for chlorine produced organic chloramine and reduced the oxidation from NO₂^- to NO₃^-, resulting in a significant enhancement of TCNM in the presence of AA (>5.52 μg L^-1) as compared to the absence of AA (0.42 μg L^-1). The generation of HO^• during UV photolysis of organic chloramines was confirmed. Among the process parameters, pre-chlorination time (from 5 min to 30 min) had no significant effect on TCNM formation; the highest TCNM formation occurred at pH 7 (from pH 6 to pH 8); prolonged UV irradiation time (from 5 min to 30 min) and increased chlorine to AA ratio (Cl₂:AA) (from 1 to 3) decreased the TCNM formation. The hydroxylated, chlorinated and nitrosated products were detected. The quantum chemical calculation results indicated the attack of NO₂^• was more likely to occur at the meta and para positions of benzoic acid (BZA), because of the steric hindrance of the carboxylic group in BZA to the ortho position. Based on the results of the toxicity assessment, pre-chlorination with a higher chlorine dosage could be an effective method of controlling both TCNM formation and acute toxicity. Overall, the results of this study contributed to the understanding of the TCNM formation mechanism as well as optimizing the parameters of the chlorine-UV process to reduce the risk of TCNM formation.