Impact of tryptophan on the formation of TCNM in the process of UV/chlorine disinfection

Formation of halonitromethanes from benzylamine during UV/chlorination: Impact factors, toxicity alteration, and pathways

Halonitromethanes (HNMs), a representative nitrogen-containing disinfection byproduct, have gained significant concerns due to their higher cytotoxicity and genotoxicity. UV/chlorination is considered a promising alternative disinfection technology for chlorination. This study aimed to investigate the HNMs formation from benzylamine (BZA) during UV/chlorination. The experimental results revealed that the yields of HNMs initially raised to a peak then dropped over time. Higher chlorine dosage and BZA concentration promoted the formation of HNMs, whereas alkaline pH inhibited their formation. The presence of bromine ion (Br-) not only converted chlorinated-HNMs (Cl-HNMs) to brominated (chlorinated)-HNMs Br (Cl)-HNMs) and brominated-HNMs (Br-HNMs) but also enhanced the total concentration of HNMs. Besides, the calculated cytotoxicity index (CTI) and genotoxicity index (GTI) of HNMs were elevated by 68.97% and 60.66% as Br- concentration raised from 2 to 6 µM. The possible formation pathways of HNMs from BZA were proposed based on the intermediates identified by a gas chromatography/mass spectrometry (GC/MS). In addition, the formation rules of HNMs in actual water verified the results in deionized water during UV/chlorination. The results of this study provide basic data and a theoretical basis for the formation and control of HNMs, which is conducive to applying UV/chlorination.

Bromide induced the formation of brominated halonitromethanes from aspartic acid in the UV/chlorine disinfection process

Brominated halonitromethanes (Br-HNMs) are generated in water disinfection processes and present high toxicity to human health. This work used aspartic acid (ASP) as the precursor to reveal that bromide (Br-) induced the production of Br-HNMs in the UV/chlorine disinfection process. Consequently, six Br-HNMs were identified, and their yields presented an increasing and then declining evolution over the reaction time from 0 to 15 min. Also, the total Br-HNMs yield reached the maximum of 251.1 μg L-1 at 5 min and then declined to 107.1 μg L-1. The total Br-HNMs yield increased from 2.40 to 251.14 μg L-1 with the increase of Cl2:Br- ratios from 0.25 to 3.0 by increasing free chlorine dosage with a fixed Br- concentration, and it increased from 207.59 to 251.14 μg L-1 and then decreased to 93.44 μg L-1 with the increase of Cl2:Br- ratio from 1.0 to 3.6 by increasing Br- concentration with a fixed free chlorine dosage. Besides, the total Br-HNMs yield reached the highest value (251.14 μg L-1) at pH 7.0 and the lowest value (74.20 μg L-1) at pH 8.0. Subsequently, the possible reaction mechanism of Br-HNMs generated from ASP was deduced, and the changes in toxicity of Br-HNMs also followed an increasing and then declining trend, closely relating to Br-HNMs yields and Br- utilization. This work explored and illustrated the yields, influence factors, reaction mechanisms, and toxicity of Br-HNMs formed from Br- containing ASP water during UV/chlorine disinfection, which might help to control Br-HNMs formation.

Formation and transformation of halonitromethanes from dimethylamine in the presence of bromide during the UV/chlorine disinfection

Halonitromethanes (HNMs) is a typical class of nitrogenous disinfection byproducts with high toxicity. The effect of Br^- on the formation and transformation of HNMs from dimethylamine (DMA) during the ultraviolet (UV)/chlorine disinfection has been investigated in current study. Results reveal that only chloronitromethane, dichloronitromethane and trichloronitromethane (TCNM) could be found during the UV/chlorine disinfection. Whereas in the presence of Br^-, nine species of HNMs could be observed simultaneously. When Br^- concentration increased from 0 to 15.0 mg L^-1, the predominant species of HNMs were gradually changed from TCNM to dibromonitromethane and tribromonitromethane, which contributed to 23.37% and 31.07% of total HNMs concentration at 15 mg L^-1 Br^-, respectively. The presence of Br^- not only shifted the chlorinated-HNMs (Cl-HNMs) towards brominated-HNMs (Br-HNMs) but also affected the dominant species and total concentration of HNMs. When Br^- concentration was 4.0 mg L^-1, the formation of HNMs decreased with the increase of pH from 6.0 to 8.0 and increased with the increase of free chlorine and DMA. When free chlorine concentration rose from 0.25 to 1.1 mmol L^-1, Br-HNMs were shifted to Br(Cl)-HNMs and then to Cl-HNMs. According to the findings, possible formation and transformation pathways of HNMs from DMA were proposed in the presence of Br^- during the UV/chlorine disinfection. Finally, it was proved that the effect of Br^- on the trend of HNMs in real water was similar to that in deionized water, but higher HNMs concentrations and delayed peak time were observed in real water. This study can provide the scientific evidence and fundamental data for the applications of UV/chlorine disinfection in the treatment of water containing Br^-.

Effects of amines on the formation and photodegradation of DCNM under UV/chlorine disinfection

Investigations were conducted to examine the effects of amine type and initial concentration, free chlorine concentration, UV light intensity, pH and tert-butyl alcohol (TBA) on the formation of dichloronitromethane (DCNM) under UV/chlorine. Methylamine (MA), dimethylamine (DMA) and poly-dimethyl diallyl ammonium chloride (PolyDADMAC) were selected as the amine precursors of DCNM. And the reaction products of amines were explored through observing the contents of various nitrogen under UV/chlorine. Experimental results indicated that the higher of the intensity of UV light, the concentration of amines and free chlorine, the greater of the amount of DCNM formation; the amine substance with simple structure is more likely oxidized to form DCNM, so the potential of MA to form DCNM is the largest among three amines; the formation of DCNM decreased with increasing pH from 6.0 to 8.0; due to adding TBA into the reaction solution, halogen and hydroxyl radicals were restrained which resulted the DCNM formation decreased. In the reaction process, the formation of DCNM from amines increased at the beginning, then decreased and almost disappeared due to photodegradation. During the formation and photodegradation of DCNM, the dissolved organic nitrogen could be transformed into the ammonia-nitrogen (NH₃-N) and nitrate-nitrogen (NO₃^--N).