Characterization of unknown iodinated disinfection byproducts during chlorination/chloramination using ultrahigh resolution mass spectrometry
Graphical abstract
Introduction
Formation of iodinated disinfection byproducts (I-DBPs) including iodine containing trihalomethanes (THMs) and iodinated acetic acids during disinfection of drinking water has recently caused wide attention because they exhibit greatly increased toxicological effects compared to their chlorinated and brominated analogues (Plewa et al., 2004, Cemeli et al., 2006, Richardson et al., 2007, Plewa et al., 2008, Richardson et al., 2008). Mammalian cell assay has shown that iodoacetic acid is 3 and 288 times more cytotoxic, and 2 and 47 times more genotoxic than bromoacetic acid and chloroacetic acid, respectively (Plewa et al., 2004). Most recently, it has been reported that I-DBPs present significantly higher developmental toxicity and growth inhibition than their brominated or chlorinated DBP analogues (Yang and Zhang, 2013, Liu and Zhang, 2014). Yang et al. (2014) investigated toxic impact of bromide and iodide on drinking water disinfection and found that both cytotoxicity and genotoxicity were correlated with total organic iodine (TOI), but not to total organic chlorine (TOCl). Therefore, the potential health impacts of the I-DBPs could not be neglected.
Richardson et al. (2006) surveyed over 100 distribution systems and found the presence of I-THMs in one plant with a concentration as high as 25 μg/L. At the same time, five iodinated acids, including iodoacetic acid, bromoiodoacetic acid, 3-bromo-3-iodopropenoic acid, and 2-iodo-3-methylbutenedioic acid, have been identified in drinking water disinfected with chloramine during a nationwide DBP occurrence study in the U.S. (Plewa et al., 2004, Krasner et al., 2006). These iodinated acids were also found in most drinking water samples in a 23-city DBPs occurrence study conducted in the U.S. and Canada, and the maximum concentration for iodoacetic acid was 1.7 μg/L (Richardson et al., 2008).
Several studies have investigated the formation and occurrence of I-DBPs during chlorination and chloramination, a substitution of chlorination to reduce the formation of the regulated THMs and haloacetic acids (HAAs), of iodide containing water samples (Karpel Vel Leitner et al., 1998, Bichsel and von Gunten, 1999, Bichsel and von Gunten, 2000, Hua and Reckhow, 2007, Ding and Zhang, 2009). It was found that chloramination favors the formation of I-DBPs because chloramines can oxidize I− to HOI but without formation of iodate (IO3−), which is an inert and non-toxic form of iodine (Bichsel and von Gunten, 1999, Bichsel and von Gunten, 2000). However, partially due to lack of appropriate analytical method for I-DBPs, most of these studies use iodine containing trihalomethanes (I-THMs) or the group parameter, total organic iodine (TOI) as the representative of the whole pool of I-DBPs. Considering the potential health effect of I-DBPs are directly depending on their molecule structures, it is necessary to extensively characterize and compare the molecular species of I-DBPs formed with different disinfectants.
To date, only a few I-DBPs have been identified by gas chromatography/mass spectrometry (GC/MS) (Richardson et al., 2007, Plewa et al., 2008, Richardson et al., 2008). However, recent studies have shown that a significant portion of unknown halogen containing DBPs would be polar/highly polar (Zhang et al., 2008, Ding and Zhang, 2009, Zhang et al., 2012a, Zhang et al., 2014). Ding and Zhang (2009) have successfully identified 17 polar/highly polar I-DBPs using the precursor ion scan (PIS) method via electrospray ionization-triple quadrupole mass spectrometry (ESI-tqMS). However, the unit mass resolution of tqMS is still too low to allow nominally isobaric ions to be mass-resolved and may bias the compositional interpretation of I-DBPs. Recently, ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has been successfully used to characterize the previously unknown Cl-DBPs and Br-DBPs (Zhang et al., 2012a, Zhang et al., 2012b, Lavonen et al., 2013, Zhang et al., 2014). The ultrahigh resolution and mass accuracy of FT-ICR MS combined with electrospray ionization (ESI) allows the determination of unambiguous and exact molecular formulas (Stenson et al., 2003, Kim et al., 2006a, Kim et al., 2006b, Hertkorn et al., 2008). It is thus anticipated that the ESI FT-ICR MS method could also be suitable for identifying the molecular formulas of unknown I-DBPs.
The main objective of this study was to characterize unknown I-DBPs in chlorinated/chloraminated water spiked with iodide and humic substances by using ESI FT-ICR MS. In addition, the species pattern of unknown I-DBPs was compared with that of Cl-DBPs formed during chlorination or chloramination to reveal the effect of different disinfectants on the formation of I-DBPs.
Section snippets
Materials
Suwannee River fulvic acid (SRFA) was obtained from the International Humic Substances Society. Methanol (LC-MS grade) was purchased from Merck (Darmstadt, Germany). Formic acid (99%) was purchased from Acros. Sodium hypochlorite solution (analytical grade, Sinopharm Chemical Reagent, Beijing) was diluted and used to prepare free chlorine. Ammonium chloride (p.a. grade) was obtained from Sinopharm Chemical Reagent (Beijing). Ultrapure water with a resistivity of 18.2 MΩ·cm− 1 was obtained from a
Detection of unknown I-DBPs
Fig. 1 exemplarily shows the detection of unknown Cl-DBPs and I-DBPs at the nominal mass m/z 403 in simulated drinking water samples after chlorination and chloramination. Newly formed one-iodine-containing DBPs and two iodine-containing DBPs were clearly observed in the mass spectrum of the SRFA+I+NH2Cl sample (Fig. 1e). The I-DBPs were observed at almost every odd nominal mass from m/z 247–499 in the SRFA+I+NH2Cl sample, and occasionally observed in the SRFA+I+NaClO sample. The very high mass
Conclusions
In this study, 206 formulas of previously unknown polar I-DBPs were identified in chlorinated/chloraminated simulated drinking water. Species of I-DBP formed during chloramination (206 formulas) were much more than that formed during chlorination (15 formulas). More than 68% of the 206 formulas of I-DBPs detected in the C18 extracts have aromatic structures or polycyclic aromatic structures, indicating precursor molecules with high aromaticity might be more reactive and preferential to form
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Nos. 21377150 and 51578530).
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