Identification of unknown disinfection byproducts in drinking water produced from Taihu Lake source water
Graphical abstract
Introduction
Disinfection byproducts (DBPs) in drinking water have attracted wide attention and have become an important category of drinking water pollutants for regulation worldwide (WHO, 1995). However, the known DBPs in drinking water, including regulated trihalomethane (THM) and haloacetic acid (HAA), have been found to not account for the cancer risk predicted by bladder cancer epidemiology (Hrudey and Fawell, 2015). Since the discovery of the potential carcinogenic effects of drinking chlorine-disinfected water (Richardson et al., 2007), searching for possible causative compounds has become a hot research topic. Over 600 DBPs have been identified so far, with a couple of them being regulated internationally. Among the regulated DBPs, bromodichloromethane, bromate, bromoform, chloroform, and dichloroacetic acid are listed as possible or probable human carcinogens by the IARC or U.S. EPA (Richardson et al., 2007). Researchers have found that brominated DBPs normally generate higher toxicity than the corresponding chlorinated DBPs (Plewa et al., 2008). The toxicities of bromoacetaldehyde and dibromoacetaldehyde are 2- and 16-fold higher than those of chloroacetaldehyde and dichloroacetaldehyde, respectively, according to a single-cell gel electrophoresis (SCGE) assay (Postigo et al., 2015).
Until now, most of the identified DBPs are chlorination products of natural organic matter (NOM) (Wang et al., 2021b; Zhang et al., 2020a) because NOM is the major organic component in source water (Jeong et al., 2007). Though normally present in source water at a much lower concentration, some chemical pollutants, such as polyphenols, are more easily transformed into halogenated DBPs, such as halophenolic compounds (Jiang et al., 2018). More important, halophenolic DBPs are generally more toxic than haloaliphatic DBPs (Liu and Zhang, 2014). For example, the toxicities induced by 2,4,6-trichlorophenol and 2,4,6-tribromophenol are 12-fold and 36-fold higher, respectively, than those induced by iodoacetic acid in an algal toxicity test (Liu and Zhang, 2014).
Taihu Lake, the third-largest freshwater lake in China, plays an important role in providing source water to the surrounding cities, which are well known for their intensive industrial activities. Taihu Lake has become a convergence point for anthropogenic pollutants, raising serious concerns over drinking water quality (Zheng et al., 2017). Nnonylphenol (NP), 4-tert-octylphenol (4-OP), 2,4-di-tert-pentylphenol, 4-n-heptylphenol, 4-n-hexylphenol, and 4-tert-butylphenol were all found in the aquatic environment of Taihu Lake. Phenolic compounds including phenol, 2,4-dichlorophenol (2,4-DCP), 2,4,6-trichlorophenol (2,4,6-TCP), and pentachlorophenol (PCP) have been frequently detected in Taihu Lake (Wang et al., 2020).At the same time, Taihu Lake source water is also known to contain high concentrations of bromide (0.17-0.2 mg/L) (Shi et al., 2015; Xiao et al., 2017). Thus, brominated aromatic DBPs may be generated during disinfection (Hoonsik et al., 2018; Li et al., 2020a), inducing higher cytotoxicity and genotoxicity on animal mammalian cells (Richardson et al., 2007).
Known DBPs are mostly identified by gas chromatography (GC)-mass spectrometry (MS) (Richardson, 2011). However, GC-MS requires suitable analytes that are volatile/semivolatile and have limitations in molecular weight (<600 Da) (Tao et al., 2020). Liquid chromatography (LC)-MS can detect both high polar and high-molecular-weight DBPs (Richardson and Postigo, 2018). With the rapid development of liquid chromatography-quadrupole time-of-flight mass spectrometry (LC-QTOF MS), exploration of unknown compounds in water is becoming possible using a nontarget analysis approach through the evaluation of accurate mass and isotope patterns (Grapp et al., 2018). The presence of halogen atoms makes it easier to identify the unknown DBPs by searching halogen isotopes (Lu et al., 2021).
In this study, nontarget analysis using LC-QTOF MS was employed to identify the unknown DBPs in a drinking water treatment system fed source water from Taihu Lake. In total, formulas of 91 possible chlorinated and brominated DBPs were identified, among which 24 molecules had structures that matched measured fragments with an intensity over 50%, and 5 were verified using standards. Finally, the possible precursors were also explored to elucidate the impact of source water pollution on the formation of DBPs.
Section snippets
Chemicals and reagents
HPLC-grade acetonitrile, methanol and acetone were purchased from Fisher Scientific (Fair Lawn, NJ, USA). Standards of 2,6-dibromo-4-chlorophenol (2,6-DBCP) and 2,6-dichloro-4-bromophenol (2,6-DCBP) were purchased from Aladdin Industrial Inc. (China). 4-Bromo-2,6-di-tert-butylphenol, 2,6-di-tert-butylphenol, and carbazole were purchased from Tokyo Chemical Industry (Japan). Standards of 3,6-dibromocarbazole (3,6-DBCZ) and phenol were purchased from Sigma-Aldrich (Missouri, USA).
Characterization of Br-DBPs and Cl-DBPs
As shown in Appendix A Table S2, 91 DBPs were detected, with 54 detected in the disinfection step and 37 detected in the predisinfection step. 70 DBPs were detected in both the disinfection and predisinfection effluents. The majority of the discovered DBPs were brominated products (84%), with 56 molecules containing only bromine atoms and 20 molecules containing both bromine and chlorine atoms. In total, 24 DBPs had suspected structures matching the MS/MS fragments in the database at more than
Conclusion
In this study, we used LC-QTOF MS to detect the extracts of a treatment effluent and performed nontarget analysis on the data. A total of 1351 peaks were discovered as formulas, and 471 formulars containing Br or Cl elements were confirmed according to the specific isotopic pattern of the MS spectrum. A total of 91 formulas of DBPs were discovered. Eighty-one DBPs have not yet been reported. Five DBPs, including TBP, 2,6-DBCP, 2,6-DCBP, 4-bromo-2,6-di-tert-butylphenol and 3,6-DBCZ, were
Acknowledgment
This work was supported by Major Science and Technology Program for Water Pollution Control and Treatment (No. 2017ZX07502003) and the National Key R&D Program of China (No. 2018YFE0204101).
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