Identification of unknown disinfection byproducts in drinking water produced from Taihu Lake source water

https://doi.org/10.1016/j.jes.2021.05.040Get rights and content

Highlights

  • The pollution of Taihu Lake source water shaped the DBP distribution.

  • Taihu Lake source water is polluted with phenolic compounds.

  • Formulas of ninety-one unknown DBPs were identified.

  • The occurrence of 5 aromatic and heterocyclic brominated DBPs was confirmed.

  • Nineteen possible precursors of the discovered DBPs were detected.

Abstract

Although disinfection byproducts (DBPs) in drinking water have been suggested as a cancer causing factor, the causative compounds have not yet been clarified. In this study, we used liquid chromatography quadrupole-time-of-flight spectrometry (LC-QTOF MS) to identify the unknown disinfection byproducts (DBPs) in drinking water produced from Taihu Lake source water, which is known as a convergence point for the anthropogenic pollutants discharged from intensive industrial activities in the surrounding regions. In total, 91 formulas of DBPs were discovered through LC-QTOF MS nontarget screen, 81 of which have not yet been reported. Among the 91 molecules, 56 only contain bromine, 15 only contain chlorine and 20 DBPs have both bromine and chlorine atoms. Finally, five DBPs including 2,4,6-tribromophenol, 2,6-dibromo-4-chlorophenol, 2,6-dichloro-4-bromophenol, 4-bromo-2,6-di-tert-butylphenol and 3,6-dibromocarbazole were confirmed using standards. The former three compounds mainly formed in the predisinfection step (maximum concentration, 0.2-2.6 µg/L), while the latter two formed in the disinfection step (maximum concentration, 18.2-33.6 ng/L). In addition, 19 possible precursors of the discovered DBPs were detected, with the aromatic compounds being a major group. 2,6-di-tert-butylphenol as the precursor of 4-bromo-2,6-di-tert-butylphenol was confirmed with standard, with a concentration of 20.3 µg/L in raw water. The results of this study show that brominated DBPs which are possibly formed from industrial pollutants are relevant DBP species in drinking water produced form Taihu source water, suggesting protection of Taihu Lake source water is important to control the DBP risks.

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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