Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Formation of Iodinated Disinfection Byproducts (I-DBPs) in Drinking Water: Emerging Concerns and Current Issues Published as part of the Accounts of Chemical Research special issue “Water for Two Worlds: Urban and Rural Communities”. Huiyu Dong,†,‡ Zhimin Qiang,‡ and Susan D. Richardson*,† †
Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100085, China
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‡
CONSPECTUS: Formation of iodinated disinfection byproducts (I-DBPs) in drinking water has become an emerging concern. Compared to chlorine- and bromine-containing DBPs, I-DBPs are more toxic, have different precursors and formation mechanisms, and are unregulated. In this Account, we focus on recent research in the formation of known and unknown I-DBPs in drinking water. We present the state-ofthe-art understanding of known I-DBPs for the six groups reported to date, including iodinated methanes, acids, acetamides, acetonitriles, acetaldehyde, and phenols. I-DBP concentrations in drinking water generally range from ng L−1 to low-μg L−1. The toxicological effects of I-DBPs are summarized and compared with those of chlorinated and brominated DBPs. I-DBPs are almost always more cytotoxic and genotoxic than their chlorinated and brominated analogues. Iodoacetic acid is the most genotoxic of all DBPs studied to date, and diiodoacetamide and iodoacetamide are the most cytotoxic. We discuss I-DBP formation mechanisms during oxidation, disinfection, and distribution of drinking water, focusing on inorganic and organic iodine sources, oxidation kinetics of iodide, and formation pathways. Naturally occurring iodide, iodate, and iodinated organic compounds are regarded as important sources of I-DBPs. The apparent second-order rate constant and half-lives for oxidation of iodide or hypoiodous acid by various oxidants are highly variable, which is a key factor governing the iodine fate during drinking water treatment. In distribution systems, residual iodide and disinfectants can participate in reactions involving heterogeneous chemical oxidation, reduction, adsorption, and catalysis, which may eventually affect I-DBP levels in finished drinking water. The identification of unknown I-DBPs and total organic iodine analysis is also summarized in this Account, which provides a more complete picture of I-DBP formation in drinking water. As organic DBP precursors are difficult to completely remove during the drinking water treatment process, the removal of iodide provides a cost-effective solution for the control of I-DBP formation. This Account not only serves as a reference for future epidemiological studies to better assess human health risks due to exposure to I-DBPs in drinking water but also helps drinking water utilities, researchers, regulators, and the general public understand the formed species, levels, and formation mechanisms of I-DBPs in drinking water. and chlorite.3 Continued efforts seek to control regulated DBPs, mostly via use of alternative disinfectants. For example, many drinking water treatment plants (DWTPs) in the U.S. have switched from chlorination to chloramination to lower regulated DBP formation. However, iodinated DBPs (I-DBPs) and nitrogenous DBPs, which are more cytotoxic, genotoxic, and developmentally toxic than regulated DBPs, can increase in formation with chloramine (NH2Cl).4−9 The first I-DBP, dichloroiodomethane, was identified in 1977 and was referred to as “the 5th trihalomethane”.10 One
1. INTRODUCTION Disinfection of drinking water is arguably the greatest public health achievement of the last century. However, an unintended consequence is disinfection byproducts (DBPs), formed by reaction of disinfectants with natural organic matter (NOM), anthropogenic pollutants, and bromide/iodide (Br−/ I−). More than 700 DBPs have been chemically characterized since the detection of chloroform in 1974.1,2 The discovery of DBPs forced regulators and the drinking water industry to seek a balance between risks associated with pathogen exposure and risks pertinent to toxic DBPs. Only 11 DBPs are currently regulated in the U.S.: 4 trihalomethanes (THMs), 5 haloacetic acids (HAAs), bromate, © XXXX American Chemical Society
Received: December 15, 2018
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DOI: 10.1021/acs.accounts.8b00641 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research
Figure 1. Chemical structures of representative known I-DBPs.
still a long way to understanding their formation mechanisms and potential risks. This Account summarizes the observed species and concentrations, potential precursors, oxidation kinetics, and formation mechanisms of known and unknown IDBPs.
limitation in the study of iodo-trihalomethanes (I-THMs) was the lack of chemical standards. In 2000, Cancho et al. synthesized five I-THMs and quantified them in chlorinated drinking water from Spain.11 I-THMs have been associated with taste and odor issues in drinking water, with low threshold levels of 0.02−0.5 μg L−1.12 However, it was not until the discovery of iodinated haloacetic acids (I-HAAs)5,13 that IDBPs were tested for toxicity.5,6 Although toxicity evidence from in vivo assays and molecular epidemiology studies for IDBPs is still lacking, in vitro studies on 103 DBPs demonstrate that I-DBPs are more cytotoxic and genotoxic than their brominated and chlorinated analogues.5−7,14,15 Since their discovery, I-DBPs have been widely detected in finished drinking waters, with the most extensive data collected from a 23 city survey in the U.S. and Canada.6 Although I-DBPs are generally found at lower concentrations relative to regulated THMs and HAAs, they can be greater drivers of the overall toxicity. For example, a new metric called “TIC-Tox”, which multiplies a chemical’s concentration (or semiquantitative peak area) in water by its cytotoxicity or genotoxicity index value, can be used to determine chemical drivers of overall toxicity in water.16 The TIC-Tox analysis of a drinking water sample from the U.S. Nationwide DBP Occurrence Study reveals that the TIC-Tox value (concentration × cytotoxicity) for I-THMs was 2.1-fold higher than that of the regulated THMs.16,17 Ding et al. also found that the TIC-Tox value for I-THMs in drinking water samples from China was comparable to those of regulated THMs and halonitromethanes.18 Thus, whereas I-THMs are generally present at lower concentrations, they can more effectively drive the overall toxicity compared to regulated THMs. Other priority unregulated DBPs, such as haloacetonitriles, haloamides, and halonitromethanes, can also be important drivers of overall toxicity, with TIC-Tox values even higher than those of I-DBPs.16 Even though recent efforts have been made toward understanding the occurrence and toxicity of I-DBPs, there is
2. KNOWN I-DBP SPECIES, CONCENTRATIONS, AND TOXICITY As shown in Figure 1, six groups of I-DBPs have been previously reported in drinking water or in simulated drinking water impacted by different sources of iodine: iodo-methanes (I-methanes) (including dichloroiodomethane (DCIM), bromochloroiodomethane (BCIM), dibromoiodomethane (DBIM), chlorodiiodomethane (CDIM), bromodiiodomethane (BDIM), iodoform (TIM), chloroiodomethane (CIM), and trichloroiodomethane (TCIM)); iodo-acids (I-acids) (including iodoacetic acid (IAA), bromoiodoacetic acid (BIAA), chloroiodoacetic acid (CIAA), diiodoacetic acid (DIAA), triiodoacetic acid (TIAA), (Z)-3-bromo-3-iodopropenoic acid, (E)-3-bromo-3-iodo-propenoic acid, and (E)2-iodo-3-methylbutenedioic acid); iodo-haloacetamides (IHAMs) (including chloroiodoacetamide (CIAM), diiodoacetamide (DIAM), iodoacetamide (IAM), and bromoiodoacetamide (BIAM)); iodo-haloacetonitriles (I-HANs) (including iodoacetonitrile (IAN) and chloroiodoacetonitrile (CIAN)); iodo-haloaldehydes (I-HALs) (iodoacetaldehyde (IAL)); and iodo-phenols (I-phenols) (including 2-iodophenol, 4-iodophenol, 2,4,6-triiodophenol, and 4-iodo-2-methylphenol).5−7,10,11,13,17−31,33,35 I-DBP concentrations in drinking water range from ng L−1 to low-μg L−1 (Table 1). The median concentrations of I-THMs and I-HAAs in chloraminated and chlorinated drinking waters from 23 cities in the U.S. and Canada were 1.4 and 0.04 μg L−1, respectively,6 and the median concentration of I-THMs detected from the 12 DWTPs of the U.S. was 2.0 μg L−1.17 Although I-DBP concentrations are usually lower than chlorinated and B
DOI: 10.1021/acs.accounts.8b00641 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research
THMs exhibit higher mammalian cell cytotoxicity than their brominated and chlorinated analogues (Figure 2).6 TIM is the most cytotoxic, with a rank order of: TIM > BDIM > DBIM > CDIM ≈ BCIM > DCIM.6 Only one I-THM, CDIM, is genotoxic.6 In addition to I-THMs, two other I-methanes were recently reported but not yet quantified. CIM was identified in laboratory chloraminated source waters from Spain,34 and TCIM was tentatively identified in chlorinated source waters containing iopamidol, an iodinated X-ray contrast media (ICM).35
Table 1. Concentrations of I-Methanes, I-Acids, I-HAMs, and I-HALs in Drinking Water Worldwide I-DBPs I-methanes
I-acids
I-HAMs I-HANs I-HALs I-phenols
species DCIM
location
U.S. and Canada DCIM U.S. DCIM China DCIM China DCIM Canada BCIM U.S. and Canada BCIM U.S. BCIM China BCIM China BCIM Canada CDIM China CDIM U.S. CDIM Canada DBIM U.S. DBIM Canada BDIM U.S. BDIM Canada TIM China TIM Canada IAA U.S. and Canada IAA China IAA U.S. BIAA U.S. and Canada CIAA China CIAA U.S. ((Z)-3-bromo-3U.S. and iodopropenoic acid Canada (E)-3-bromo-3U.S. and iodopropenoic Canada ((E)-2-iodo-3U.S. and methylbutenedioic Canada acid), BIAM U.S. CIAM China IAN China IAL U.S. 2,4,6-triiodophenol China
concentration levels (μg L−1)
reference
