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.

Enhanced formation of trichloronitromethane precursors during UV/monochloramine treatment.

This study systematically investigates the formation of trichloronitromethane (TCNM) from 2 natural waters, 6 humic substances and 16 phenolic compounds during UV/monochloramine (UV/NH2Cl) followed by post-chloramination. Using 15N-NH2Cl as an isotope tracer, we found that 15N-TCNM accounted for 70.7-76.5% of total TCNM during UV/NH2Cl treated 2 natural waters, which was significantly higher than the proportion of 15N-TCNM in chloramination (NH2Cl alone). This is a direct evidence that NH2Cl, rather than the nitrogenous matters in waters, was the predominant nitrogen source of TCNM during UV/NH2Cl treatment. Phenol derivatives with meta-substituents and with electron-withdrawing groups facilitated the formation of TCNM precursors during UV/NH2Cl treatment. Significant correlations were found between Hammett constants (σ) of substituents and TCNM formation potentials. The formation mechanisms of TCNM were revealed using resorcinol as a representative phenolic compound. During UV/NH2Cl treatment, HO•, reactive chlorine species and reactive nitrogen species contributed to 28.1%, 29.0% and 19.4% of resorcinol degradation. Five nitro(so)-intermediates were identified as the main TCNM precursors. The formation pathways of TCNM were proposed. Alkaline pH was recommended to reduce the formation of TCNM precursors during UV/NH2Cl treatment.

UV-induced activation of organic chloramine: Radicals generation, transformation pathway and DBP formation.

Organic chloramines of little disinfection efficacy commonly exist in disinfection process (chlor(am)ination) due to the wide presence of organic amines in water, of which N-chlorodimethylamine (CDMA) is a typical one. For the first time, UV photolysis for the activation of CDMA was investigated. UV photolysis caused the cleavage of N-Cl bond in CDMA to form Cl• and subsequently HO•, both of which are dominant contributors to the destruction of model contaminant bisphenol A (BPA). Typical spectra of HO• were detected by electron paramagnetic resonance (EPR) experiments, while spectra of reactive nitrogen species (RNS) were not detected during UV photolysis of CDMA. The increase of pH (6.0-8.0), HCO3-/CO32-, Cl- and nature organic matter inhibited the degradation of BPA. We proposed pathways of CDMA and BPA degradation based on the identified transformation products. UV photolysis of CDMA and BPA reduced the formation of N-nitrosodimethylamine (NDMA) at pH 8.0, but increased the formation of trichloronitromethane (TCNM) at pH 7.0 and 8.0. The increasing toxicity and the formation of TCNM and NDMA gave us a hint that formation of organic chloramines should be concerned.

Nitrogen conversion from ammonia to trichloronitromethane: Potential risk during UV/chlorine process.

In this study, the potential formation of trichloronitromethane (TCNM) from model organic compounds in ammonia-containing water treated by UV/chlorine process was evaluated. Monochloramine generated from the reaction of chlorine and ammonia can be photolyzed to produce NO2- and reactive nitrogen species (RNS), which play important roles in the formation of TCNM during the subsequent chlorination. The results showed that increase of nitrogen to chlorine molar ratio (from 0 to 1.0) and pH (from 6.5 to 8.0) enhanced the formation of TCNM, mainly due to the increased yield of NO2- and RNS from the photolyzed monochloramine. The formation of TCNM was interestingly found to be linearly correlated with Hammett constants of the model precursors, which is theoretically related to the rate constants of RNS with model compounds. Enhanced formation of TCNM was also observed during the treatment of natural organic matter by UV/chlorine process in ammonia-containing water. The toxicity assessment showed that TCNM significantly increased the genotoxicity of formed DBPs. Furthermore, the electrophilic substitution reaction of •NO2 was proved to more likely occur on the ortho and para position of phenol according to the calculation of Gaussian program, and a possible reaction pathway of phenol and •NO2 was proposed based on the calculated results.