Predicting chloroform production from organic precursors

Aromatic Structures Govern the Formation of Chlorinated Byproducts in Dichlorine Radical Reactions

Radical-induced disinfection byproduct (DBP) formation is drawing attention with increasing applications of advanced oxidation processes (AOPs). Cl2•- represents one of the extensively generated radicals in AOPs, whose behavior in DBP formation remains unknown. In this study, we found that aromatic structures serve as the main DBP precursors in Cl2•- reactions by employing diverse groups of model compounds. At a typical Cl2•- exposure of 1.2 × 10-9 M·s, the sum concentrations of 7 regulated aliphatic DBPs (e.g., trichloromethane, chloroacetic acids) are ∼0.10 to 0.48 μM for aromatic precursors and <0.05 μM for aliphatic ones. The DBP formation mechanisms from Cl2•- reactions involved the formation of chlorinated aromatics, radical-induced oxygen incorporation followed by ring cleavage, and the interactions of Cl2•- with ring-cleavage intermediates. In reacting with DOM, Cl2•- reactions produced much fewer aliphatic DBPs (5% of the total organochlorine vs 40% for chlorination) and chloroacetic acids dominated the aliphatic DBPs (usually trihalomethane for chlorination), which can be well interpreted by the precursors and mechanisms proposed. This work comprehensively reveals the precursors, formation patterns, and mechanisms of DBPs during the less-studied Cl2•- reactions, highlighting the importance of eliminating the aromatic structures of DOM before the AOPs.

One-Electron Oxidant-Induced Transformations of Aromatic Alcohol to Ketone Moieties in Dissolved Organic Matter Increase Trichloromethane Formation

Radicals in advanced oxidation processes (AOPs) degrade micropollutants during water and wastewater treatment, but the transformation of dissolved organic matter (DOM) may be equally important. Ketone moieties in DOM are known disinfection byproduct precursors, but ketones themselves are intermediates produced during AOPs. We found that aromatic alcohols in DOM underwent transformation to ketones by one-electron oxidants (using SO4•- as a representative), and the formed ketones significantly increased trichloromethane (CHCl3) formation potential (FP) upon subsequent chlorination. CHCl3-FPs from aromatic ketones (Ar-CO-CH3, average of 22 mol/mol) were 6-24 times of CHCl3-FPs from aromatic alcohols (Ar-CH(OH)-CH3, average of 0.85 mol/mol). At a typical SO4•- exposure of 7.0 × 10-12 M·s, CHCl3-FPs from aromatic alcohol transformation increased by 24.8%-112% with an average increase of 53.4%. Notably, SO4•- oxidation of aliphatic alcohols resulted in minute changes in CHCl3-FPs due to their low reactivities with SO4•- (∼107 M-1 s-1). Other one-electron oxidants (Cl2•-, Br2•-,and CO3•-) are present in AOPs and also lead to aromatic alcohol-ketone transformations similar to SO4•-. This study highlights that subtle changes in DOM physicochemical properties due to one-electron oxidants can greatly affect the reactivity with free chlorine and the formation of chlorinated byproducts.

Impact of prevalent chlorine quenchers on phenolic disinfection byproducts in drinking water and potential reaction mechanisms

To prevent the reactions of disinfection byproducts (DBPs) or natural organic matters with residual chlorine in drinking water in the course of the water store, residual chlorine is quenched by chlorine quenchers, while some chlorine quenchers may result in dechlorination of DBPs. Phenolic compounds are a group of highly toxic DBPs compared to regulated aliphatic DBPs (trihalomethanes (THMs) and haloacetic acids (HAAs)), which might be a great threat to drinking water safety. Nevertheless, impact of popular chlorine quenchers on phenolic DBPs is less understanding. In this study, the influences of ammonium chloride, ascorbic acid, sodium thiosulfate, and sodium sulfite on phenolic DBPs are assessed. Total concentration of 19 phenolic DBPs in drinking water from 7 Chinese cities was 145-1821 ng/L, suggesting a widely occurrence of these pollutants. Four assessed chlorine quenchers have not impacts on mass spectra of studied phenolic DBPs. Additionally, when the storage time ≤24 h, recoveries of 19 phenolic DBPs using four assessed chlorine quenchers are within the accept levels (70-130 %). However, when the storage time increased to 168 h, ascorbic acid and sodium thiosulfate satisfied the recovery requirement of phenolic DBPs during the sample analysis, and ammonium chloride and sodium sulfite showed a unacceptable impact on bromo-chloro-phenols. In general, ascorbic acid and sodium thiosulfate are recommended to be the ideal chlorine quenchers of phenolic DBPs. Mechanism study indicated that sodium sulfite induced the dechlorination of 2-chloro-4-bromophenol via nucleophilic reaction. This study is the first attempt to provide the impact of chlorine quenchers on phenolic DBPs and corresponding reaction mechanism.

The convergence of 2,6-dichloro-1,4-benzoquinone in the whole process of lignin phenol precursor chlorination

The formation and decomposition of 2,6-dichloro-1,4-benzoquinone, an emerging disinfection byproduct (DBP), was studied in the chlorination of lignin phenol precursors. The results show that DCBQ and the related hydroxyl DCBQ (DCBQ-OH) acts as the intermediate products of the chlorination process of the three typical lignin phenol precursors (p-hydroxybenzoic acid, protocatechuic acid, and gallic acid). The contributions of lignin phenol precursors to the overall formation of the targeted DBPs were determined based on the observed abundances of individual lignin phenols and their DBP yields. DCBQ and DCBQ-OH were generated within 2-6 h, the relative abundance of the yields of mol carbon atoms in DCBQ corresponding to the mol carbon atoms in the three model precursors (DCBQ-C) was about 0.01%-14.37% under different pH conditions. With the chlorination reaction time increased (after two or four h), the concentrations of DCBQ and DCBQ-OH entirely decreased, and the decomposition of DCBQ do not follow a pseudo-first-order kinetics during chlorination. Conversely, the decomposition of DCBQ generated from p-hydroxybenzoic acid followed a pseudo-second-order kinetics. Moreover, the formation of trichloromethane (TCM), dichloroacetic acid (DCAA), and trichloroacetic acid (TCAA) was also detected during the chlorination. The contribution of the decomposed DCBQ was mainly to TCAA and the unknown DBPs within 2-12 h, and DCBQ decomposition pathway was affected by pH. Moreover, except for DCBQ/DCBQ-OH and TCM/HAAs, there were still 73.6%-92.41% unknown products (including non-halogenated aromatic DBPs and chlorine-substituted DBPs) needing to identify during the chlorination process for lignin phenols. Overall, revealing the formation and decomposition of DCBQ during the chlorination of lignin phenol precursors would contribute to the effective development of drinking water treatment processes for the removal of highly toxic intermediates generated during disinfection.

Kinetics and Transformations of Diverse Dissolved Organic Matter Fractions with Sulfate Radicals

Dissolved organic matter (DOM) scavenges sulfate radicals (SO₄^•-), and SO₄^•--induced DOM transformations influence disinfection byproduct (DBP) formation when chlorination follows advanced oxidation processes (AOPs) used for pollutant destruction during water and wastewater treatment. Competition kinetics experiments and transient kinetics experiments were conducted in the presence of 19 DOM fractions. Second-order reaction rate constants for DOM reactions with SO₄^•- (k_DOM,SO4^•-) ranged from (6.38 ± 0.53) × 10⁶ M^-1 s^-1 to (3.68 ± 0.34) × 10⁷ M_C^-1 s^-1. k_DOM,SO4^•- correlated with specific absorbance at 254 nm (SUVA₂₅₄) (R² = 0.78) or total antioxidant capacity (R² = 0.78), suggesting that DOM with more aromatics and antioxidative moieties reacted faster with SO₄^•-. SO₄^•- exposure activated DBP precursors and increased carbonaceous DBP (C-DBP) yields (e.g., trichloromethane, chloral hydrate, and 1,1,1-trichloropropanone) in humic acid and fulvic acid DOM fractions despite the great reduction in their organic carbon, chromophores, and fluorophores. Conversely, SO₄^•--induced reactions reduced nitrogenous DBP yields (e.g., dichloroacetonitrile and trichloronitromethane) in wastewater effluent organic matter and algal organic matter without forming more C-DBP precursors. DBP formation as a function of SO₄^•- exposure (concentration × time) provides guidance on optimization strategies for SO₄^•--based AOPs in realistic water matrices.

Changes in structural characteristics of humic and fulvic acids under chlorination and their association with trihalomethanes and haloacetic acids formation

The effects of chlorination on 16 humic and fulvic acids (HAs and FAs, respectively) extracted from six different soil samples from Korea and two purchased soil samples (Canadian peat moss, Elliott Silt Loam Soil) were investigated to identify the changes in their structural characteristics and their effects on trihalomethane formation potential (THMFP) and haloacetic acid formation potential. The effect of chlorination was also investigated in fractionated samples (Aldrich HA, F₁-F₅) based on molecular weight (MW). Total organic carbon (TOC), specific UV absorbance (SUVA), fulvic-like fluorescence (%FLF), terrestrial humic-like fluorescence (%THLF), weight-average molecular weight (MW_w), and carbon structures (¹³C NMR) were measured for each sample before and after chlorination, and factors relating to the chlorination mechanism were examined using principal component analysis (PCA). The results showed that the changes in the structural characteristics and the disinfection by-product formation of chlorinated HA and FA differed critically. For chlorinated HA, TOC and %FLF decreased due to oxidation, whereas %THLF was reduced via incorporation; MW also affected the structural changes and THMFP generation. In the PCA results, high SUVA, low MW, low N/C, and low O groups of aromatic C were associated with high THMFP production in HA, whereas low O groups of aliphatic C in FA were associated with both oxidation and incorporation in terms of THMFP. These results elucidate the mechanisms associated with the effects of chlorination in HA and FA and will support the prediction of THMFP generation in HA and FA based on their specific structural characteristics.

Hydrolysis characteristics and risk assessment of a widely detected emerging drinking water disinfection-by-product-2,6-dichloro-1,4-benzoquinone-in the water environment of Tianjin (China)

Halobenzoquinones (HBQs) are an emerging class of drinking water disinfection byproducts (DBPs) that have been frequently detected in drinking water and are highly relevant to bladder cancer. Among the studied HBQs, 2,6-dichloro-1,4-benzoquinone (DCBQ) had the highest detection frequency and concentrations in drinking water. However, compared to other countries, the studies on HBQs that are being conducted in China, especially those on HBQs in drinking water, are not sufficient. Therefore, the concentrations of DCBQ in the Tianjin drinking water supply system were investigated in two seasons (winter and summer), and the risk that is posed by DCBQ in drinking water was evaluated for the first time. In addition, since HBQs are prone to hydrolysis in neutral and alkaline environments, identification of the hydrolytic characteristics of DCBQ at various pH values and in the real water environment is essential for better describing the environmental behavior of DCBQ; hence, the hydrolysis characteristics of DCBQ in phosphate buffers with various pH values and in four water samples were also examined in our study. The results demonstrated that DCBQ was widely detected in the drinking water treatment process and distribution systems, and the average concentration in our study (12.0 ng/L) was at a moderately high level compared with the reported concentration of DCBQ in the drinking water distribution networks. The risk quotient (RQ) of DCBQ is equivalent to that of trihalomethanes (THMs); thus, the relatively low concentrations of DCBQ should also be considered. Furthermore, the results demonstrated that the hydrolysis of DCBQ follows first-order reaction kinetics, the reaction rate accelerates as the pH of the phosphate buffer system increases, and the rate of hydrolysis of DCBQ in drinking water is affected not only by the pH but also by other environmental factors, such as the organic matter concentration. Therefore, further investigation is necessary to identify the main factor of DCBQ hydrolysis in real water environments.

Predicting formation of haloacetic acids by chlorination of organic compounds using machine-learning-assisted quantitative structure-activity relationships

The presence of disinfection byproducts (DBPs) in drinking water is a major public health concern, and an effective strategy to limit the formation of these DBPs is to prevent their precursors. In silico prediction from chemical structure would allow rapid identification of precursors and could be used as a prescreening tool to prioritize testing. We present models using machine learning algorithms (i.e., support vector regressor, random forest regressor, and multilayer perceptron regressor) and chemical descriptors as features to predict the formation of haloacetic acids (HAAs). A robust model with good predictivity (i.e., leave-one-out cross-validated Q² > 0.5) to predict the formation of trichloroacetic acid (TCAA) was developed using a random forest regressor. The number of aromatic bonds, hydrophilicity, and electrotopological descriptors related to electrostatic interactions and the atomic distribution of electronegativity were identified as important predictors of TCAA formation potentials (FPs). However, the prediction of dichloroacetic acid was less accurate, which is congruent with the presence of different types of precursors exhibiting distinct mechanisms. This study demonstrates that nonlinear combinations of general chemical descriptors can adequately estimate HAAFPs, and we hope that our study can be used to predict precursors of other disinfection byproducts based on chemical structures using a similar workflow.

Addition of lemon before boiling chlorinated tap water: A strategy to control halogenated disinfection byproducts

Chlorine disinfection is required to inactivate pathogens in drinking water, but it inevitably generates potentially toxic halogenated disinfection byproducts (halo-DBPs). A previous study has reported that the addition of ascorbate to tap water before boiling could significantly decrease the concentration of overall halo-DBPs in the boiled water. Since the fruit lemon is rich in vitamin C (i.e., ascorbic acid), adding it to tap water followed by heating and boiling in an effort to decrease levels of halo-DBPs was investigated in this study. We examined three approaches that produce lemon water: (i) adding lemon to tap water at room temperature, termed "Lemon"; (ii) adding lemon to boiled tap water (at 100 °C) and then cooling to room temperature, termed "Boiling + Lemon"; and (iii) adding lemon to tap water then boiling and cooling to room temperature, termed "Lemon + Boiling". The concentrations of total and individual halo-DBPs in the resultant water samples were quantified with high-performance liquid chromatography-tandem mass spectrometry and the cytotoxicity of DBP mixtures extracted from the water samples was evaluated using human epithelial colorectal adenocarcinoma Caco-2 cells and hepatoma HepG2 cells. Our results show that the "Lemon + Boiling" approach substantially decreased the concentrations of halo-DBPs and the cytotoxicity of tap water. This strategy could be applied to control halo-DBPs, as well as to lower the adverse health effects of halo-DBPs on humans through tap water ingestion.

Formation of emerging iodinated disinfection by-products during ballast water treatment based on ozonation processes

Disinfection by-products (DBPs) generated by ballast water treatment pose a potential threat to marine environment which aroused widespread concern. In recent years, emerging iodinated DBPs have attracted widespread attention because of their stronger cytotoxicity and genotoxicity than brominated/chlorinated DBPs. In this study, the effects of different natural organic matter species, total residual oxidant (TRO) concentrations, storage time, temperature, pH, bromide and iodide concentrations on the generation of iodinated trihalomethanes (I-THMs) during ozonation process of ballast water were investigated. The results showed that bromochloroiodomethane and diiodochloromethane (DICM) were not detected under all conditions during ozonation of humaic acid (HA). Different kinds of precursors had a significantly effect on the formation of I-THMs. For algal cells as precursor, DICM were detected (1.22 μg/L), while DICM were not detected from oxidation of 1,3-etonedicarboxylic acid, fulvic acid (FA), phenol, resorcinol, hydroquinone and HA as precursors. The yields of I-THMs from oxidation of algal cells, FA and phenol were higher than other precursors. Linear relationships were observed between the formation of I-THMs and TRO concentrations. The yields of I-THMs reached a peak at 48 h (180 μg/L) after ozonation treatment of ballast water, and then decreased with storage time extension. An increase in temperature enhanced the formation of dibromoiodomethane and bromodiiodomethane, while wakened the formation of iodoform and dichloroiodomethane. The formation of I-THMs was complicatedly affected by different pH values in the range from 4 to 9. The more bromide concentrations, the more brominated I-THMs were formed. The concentrations of I-THMs increased with increasing iodide concentrations, and low concentrations of iodide had greater effect on the production of I-THMs than high concentrations of iodide.

Ferrate(VI) pretreatment before disinfection: An effective approach to controlling unsaturated and aromatic halo-disinfection byproducts in chlorinated and chloraminated drinking waters

Disinfection is an essential process of drinking water treatment to eliminate harmful pathogens, but it generates potentially toxic disinfection byproducts (DBPs). Ferrate (FeO₄^2-, Fe(VI)) was used to pre-oxidize natural organic matter (NOM, the precursor of DBPs) in source water to control DBP formation in subsequent chlorine or chloramine disinfection. Currently, it is unclear how Fe(VI) changes the structure of NOM, and no information details the effect of Fe(VI) pretreatment on the aromatic DBPs or the speciation of overall DBPs generated in subsequent disinfection of drinking water. In the present paper, Fe(VI) was applied to pretreat simulated source water samples at a Fe(VI) to dissolved organic carbon mole ratio of 1:1 at pH 8.0. ¹³C nuclear magnetic resonance spectroscopy was newly employed to characterize NOM in simulated source waters with and without Fe(VI) treatment, and it was demonstrated that Fe(VI) converted unsaturated aromatic C functional groups in NOM to saturated aliphatic ones. High-resolution mass spectrometry (HRMS) and high performance liquid chromatography/triple quadrupole MS were applied to analyze the DBPs generated in chlorination and chloramination of the source waters with and without Fe(VI) pretreatment. It was confirmed that Fe(VI) pretreatment followed by chlorination (or chloramination), generated DBPs containing less unsaturated, halogenated, and aromatic moieties than chlorination (or chloramination) without pretreatment by Fe(VI). Finally, the cytotoxicity of disinfected drinking water samples were assessed with the human epithelial colorectal adenocarcinoma Caco-2 cell line (a model of the intestinal barrier for ingested toxicants), and the results show that Fe(VI) pretreatment detoxified the chlorinated and chloraminated drinking waters.

A comprehensive review of mathematical models developed for the estimation of organic disinfection byproducts

In this review, we present comparative and comprehensive views on the foundations, potentials and limitations of the previously reported mathematical models for the estimation of the concentration of disinfection byproducts (DBPs) generated during the chlor(am)ination of water. To this end, DBPs models were divided into two major categories: static variable (SV) and dynamic variable (DV) or differential models. In SV models, variables remain in their original form throughout a chlor(am)ination modelling period while DV models consider the changes driven by a chlor(am)ination treatment as the variables. This classification and the comparative study of the two types of models led to a better understanding of the assumptions, potentials, and limitations of the existing DBP models. In opposition to several claims in the literature, certain DV models based on UV absorbance/fluorescence failed to selectively track the chromophores responsible for DBP formation. In this critical review, a conceptual model for the photophysics of dissolved organic matter (DOM) based on the theory of electron delocalization was proposed to explain some inconsistent spectroscopic properties of DOM following chlor(am)ination and several unique photophysical properties of DOM. New insights for the development and deployment of mathematical models were also provided to estimate DBPs in various settings.

Nonhalogenated Aromatic DBPs in Drinking Water Chlorination: A Gap between NOM and Halogenated Aromatic DBPs

Halogenated disinfection byproducts (DBPs) are generated via reactions with natural organic matter (NOM) in chlorine disinfection of drinking water. How large NOM molecules are converted to halogenated aliphatic DBPs during chlorination remains a fascinating yet largely unresolved issue. Recently, many relatively toxic halogenated aromatic DBPs have been identified in chlorinated drinking waters, and they behave as intermediate DBPs to decompose to halogenated aliphatic DBPs. There is still one gap between NOM and halogenated aromatic DBPs. In this study, nine nonhalogenated aromatic compounds were identified as new intermediate DBPs in chlorination, including 4-hydroxybenzaldehyde, 4-hydroxybenzoic acid, 3-formyl-4-hydroxybenzoic acid, salicylic acid, 5-formyl-2-hydroxybenzoic acid, 4-hydroxyphthalic acid, 4'-hydroxyacetophenone, 4-methylbenzoic acid, and 4-hydroxy-3-methylbenzaldehyde. These nonhalogenated aromatic DBPs formed quickly and reached the maximum levels at relatively low chlorine doses within a short contact time, and their formation pathways were proposed. The formation kinetics of three nonhalogenated aromatic DBPs and their corresponding monochloro-/dichloro-substitutes during chlorination were then modeled. The nonhalogenated aromatic DBPs contributed up to 84% of the formed monochloro-substitutes and 22% of the formed dichloro-substitutes, demonstrating that they somewhat acted as intermediates between NOM and halogenated aromatic DBPs. Furthermore, the formed nonhalogenated aromatic DBPs were found to be removed by >50% by granular activated carbon adsorption.

Different cytotoxicity of disinfection by-product haloacetamides on two exposure pathway-related cell lines: Human gastric epithelial cell line GES-1 and immortalized human keratinocyte cell line HaCaT

Humans are exposed to disinfection by-products (DBPs) mainly through drinking water ingestion and dermal contact. As an emerging class of nitrogenous DBPs (N-DBPs), haloacetamides (HAcAms) have been found to have significantly higher cytotoxicity than regulated DBPs. In this study, we investigated the cytotoxicity of HAcAms on two exposure pathway-related cell lines: human gastric epithelial GES-1 cells and immortalized keratinocytes HaCaT. Our results showed that the ranking order of cytotoxicity of 13 HAcAms was different between HaCaT and GES-1 cells. In addition, the 50% inhibitive concentration in HaCaT was 1.01-3.29 times that in GES-1. Further comparison among GES-1, HaCaT and CHO cell lines confirmed that different cell lines exhibited different sensitivity to the same compound. Importantly, HAcAms showed 5.83-7.13 × 104 times higher toxicity than the well-clarified DBP chloroform, clearly demonstrating the increased toxicity of HAcAms. Finally, using a novel high-content screening (HCS) analysis, we found that 39.29% of chlorinated HAcAms, 42.86% of brominated HAcAms and 16.07% of iodinated HAcAms significantly affected at least one of the cell-health parameters, such as nuclear size, membrane permeability, mitochondrial membrane potential, or cytochrome c release, in GES-1 or HaCaT cells. Thus, brominated HAcAms appear to have stronger effects under the sublethal exposure dose, possibly causing cytotoxicity via apoptosis. Together, our study provides new insights to the toxicity of HAcAms and a comprehensive toxicology dataset for health risk assessment.

Identification, Formation, and Predicted Toxicity of Halogenated DBPs Derived from Tannic Acid and Its Biodegradation Products

Humic substances are commonly known disinfection byproduct (DBP) precursors. Tannic acid is one precursor of humic substances in organic degradation, and it occurs ubiquitously in both source water and wastewater. In this study, the biological degradation process was simulated under laboratory conditions, and the characteristics of DBP formation generated from the chlorination of tannic acid samples with different biodegradation times were explored. Twenty-six emerging halogenated DBPs were identified, and the formation pathways of the tannic acid-derived DBPs were tentatively proposed. Moreover, results demonstrated that the profile of the chlorinated DBP formation was significantly different from its brominated counterpart during biodegradation, and a general increasing trend of the ratio of TOBr/TOX or TII_PIS79/(TII_PIS79+TII_PIS35) as biodegradation time increasing was noticeable. The observed trend could be mainly ascribed to the reactive sites of tannic acid shifting from relatively fast to slow sites during biodegradation. In addition, the comparative toxicity of the detected DBPs derived from tannic acid was predicted by using two quantitative structure-activity relationship models established previously. On the basis of both the two toxicity metrics (involving developmental toxicity and growth inhibition potency), the predicted toxicity data indicated that the emerging DBP group trihalo-(di)hydroxycyclopentane-1,3-diones may possess extremely high toxic potencies.

Disinfection byproducts potentially responsible for the association between chlorinated drinking water and bladder cancer: A review

Epidemiological studies have consistently associated the consumption of chlorinated drinking water with an enhanced risk of bladder cancer. While this suggests that some disinfection byproducts (DBPs) are bladder carcinogens, causal agents are unknown. This study aims to highlight likely candidates. To achieve this, structures of known and hypothesised DBPs were compared with 76 known bladder carcinogens. The latter are dominated by nitrogenous and aromatic compounds; only 10 are halogenated. Under 10% of the chlorine applied during drinking water treatment is converted into identified halogenated byproducts; most of the chlorine is likely to be consumed during the generation of unidentified non-halogenated oxidation products. Six nitrosamines are among the nine most potent bladder carcinogens, and two of them are known to be DBPs: N-nitrosodiphenylamine and nitrosodibutylamine. However, these and other nitrosamines are formed in insufficiently low concentrations in chlorinated drinking water to account for the observed bladder cancer risk. Furthermore, although not proven bladder carcinogens, certain amines, haloamides, halocyclopentenoic acids, furans and haloquinones are potential candidates. At present, most identified bladder carcinogens are nitrogenous, whereas >90% of natural organic matter is not. Therefore, non-nitrogenous DBPs are likely to contribute to the bladder cancer risk. Given the high proportion of DBPs that remains uncharacterised, it is important that future research prioritises compounds believed to be potent toxicants.

Application of (LC/)MS/MS precursor ion scan for evaluating the occurrence, formation and control of polar halogenated DBPs in disinfected waters: A review

Water disinfection can result in the unintended formation of halogenated disinfection byproducts (DBPs), which have been the subject of intensive investigation over the past 40 years. Robust methods for evaluating and characterizing the formation of halogenated DBPs are prerequisites for ultimately controlling the formation of DBPs and ensuring quality and safe disinfected water. Only a fraction of the total organic halogen (TOX) formed during disinfection has been chemically identified or even well characterized by the classical (derivatization-)gas chromatography/mass spectrometry (GC/MS) method. Such a method may not be amenable to the detection of polar halogenated DBPs, which constitute a major portion of the TOX that is still unaccounted for. Accordingly, a novel precursor ion scan (PIS) method using (liquid chromatography/) electrospray ionization-triple quadrupole mass spectrometry was developed for the rapid selective detection of all polar halogenated DBPs-no matter whether the DBPs are known or unknown-in water. This article reviews recent literature on the application of the PIS method for evaluating the occurrence, formation and control of polar halogenated DBPs in disinfected waters. The challenges in developing the PIS method were briefly summarized. Application of the powerful method pinpointed >150 previously unknown DBPs and revealed the formation, speciation and transformation of halogenated DBPs in disinfected drinking water, wastewater effluents, and swimming pool water. For the same source water, positive correlations were found between the total ion intensity (TII) levels in the PIS spectra of m/z 35/79/126.9 and the total organic chlorine/bromine/iodine levels in the disinfected water sample, and a disinfected sample with a higher TII level generally showed a higher toxic potency. Accordingly, the TII value can be used as a surrogate to comparatively reflect the water quality and assess the efficiency of a DBP control approach. To achieve a more comprehensive and systematic understanding of the DBP compositions in different waters and thus better control the DBP formation and reduce their overall toxicity, topics for future work were discussed.

Formation of brominated trihalomethanes during chlorination or ozonation of natural organic matter extracts and model compounds in saline water

Oxidation experiments (chlorine, ozone and bromine) were carried out with synthetic saline waters containing natural organic matter (NOM) extracts and model compounds to evaluate the potential of these surrogates to mimic the formation of brominated trihalomethanes (Br-THMs) in natural saline waters. Synthetic saline water with Pony Lake fulvic acid (PLFA) showed comparable results to natural brackish and sea water for Br-THMs formation during chlorination and ozonation for typical ballast water treatment conditions ([Cl₂]₀ ≥ 5 mg/L or [O₃]₀ ≥ 3 mg/L). The molar CHBr₃ yield in synthetic saline waters is higher for chlorination than for ozonation, since ozone reacts slower with bromide and faster with THM precursors. For bromination, the molar yields of CHBr₃ for the NOM model compounds phenol, resorcinol, 3-oxopentanedioic acid and hydroquinone are 28, 62, 91 and 11%, respectively. CHBr₃ formation is low during chlorination or ozonation of resorcinol-containing synthetic saline waters due to the faster reaction of resorcinol with these oxidants compared to the bromine formation from bromide. Oxidation experiments with mixtures of hydroquinone and phenol (or resorcinol) were conducted to mimic various functional groups of NOM reacting with Cl₂ (or O₃) in saline water. With increasing hydroquinone concentrations, the CHBr₃ formation increases during both chlorination and ozonation of the mixtures, except for chlorination of the mixture of hydroquinone and resorcinol. The formation of THMs during chlorination of the mixture of hydroquinone and resorcinol is similar to that of resorcinol alone due to the much faster reaction of HOX with resorcinol compared to hydroquinone. In general, PLFA seems to be a reasonable DOM surrogate to simulate CHBr₃ formation for realistic ballast water treatment. During chlorination, CHBr₃ formations from phenol- and PLFA-containing synthetic brackish waters are comparable, for similar phenol contents.

Evaluating the Comparative Toxicity of DBP Mixtures from Different Disinfection Scenarios: A New Approach by Combining Freeze-Drying or Rotoevaporation with a Marine Polychaete Bioassay

The unintended formation of disinfection byproducts (DBPs) may compromise the safety of drinking water. Since no specified DBPs have been found to be responsible for the overall adverse effects and over half of total organic halogen (TOX) remains unidentified, DBP mixture toxicity is gaining increasing interest as a potential indicator of how risky drinking water might be. In this study, a new approach to evaluating the toxicity of drinking water DBP mixtures was developed by combining freeze-drying or rotoevaporation pretreatment with an in vivo high-salinity-tolerance bioassay with the embryos of a marine polychaete Platynereis dumerilii. The DBP recoveries by freeze-drying or rotoevaporation were compared with those by commonly applied liquid-liquid-extraction (LLE). For drinking water subjected to typical disinfection processes (i.e., chlorination, chloramination, chlorine dioxide treatment, and ozonation with or without postchlorination), LLE led to the lowest TOX recovery (11-18%) and the loss of all inorganic DBPs, while freeze-drying and rotoevaporation recovered 28-58% and 35-61% of TOX, respectively, and effectively recovered 81-99% and 85-104% of inorganic DBPs, respectively. Thus, LLE caused an underestimation of the toxicity of DBP mixtures compared with freeze-drying and rotoevaporation. Besides, the comparative toxicity varied significantly for water samples pretreated with different methods due to the effect of inorganic DBPs and a synergistic effect of organic and inorganic DBPs. The new approach revealed that the bromide-rich source water disinfected with ozone caused the highest developmental toxicity, followed by those disinfected with chlorine, chlorine dioxide, and chloramine in that order.