Emerging Disinfection Byproducts, Halobenzoquinones: Effects of Isomeric Structure and Halogen Substitution on Cytotoxicity, Formation of Reactive Oxygen Species, and Genotoxicity

Formation of highly toxic p-benzoquinones byproducts during ozonation of cresols

This study aims to identify the highly non-specific toxic by-products during ozonation of three cresols in wastewater. In ozonated effluents, biotoxicity increased along with increasing reaction time, followed by a gradual decrease. The peak biotoxicity for ozonated o-cresol (o-C), m-cresol (m-C), and p-cresol (p-C) was estimated to be 17.4, 14.8 and 5.5 times higher than that of untreated wastewater, respectively. A redox-directed approach with high-resolution mass spectrometry detection and toxicity prediction revealed that monomeric para-benzoquinones (p-BQs), hydroxylated p-BQs, and dimeric p-BQs in ozonated cresols were the primary contributors to the increased toxicity. Calculations based on density functional theory indicated formation pathways of p-BQs byproducts, e.g., the formation of 2-methyl-p-benzoquinone was likely induced by ozone molecules rather than hydroxyl radicals in ozonated o-C and m-C, and the formation of p-BQs during ozonation of p-C was attributed to the oxidation of methyl group to carboxyl group and subsequent decarboxylation initiated by hydroxyl radicals. Electron paramagnetic resonance and spin density calculation showed that the presence of carbon-centered cresoxyl radicals was responsible for dimeric p-BQs formation. Collectively, these results underscore significant contribution of non-halogenated p-BQs to non-specific toxicity increase in ozonated effluents.

Organic magnetic nanoparticles catalyze CO2 capture in hydrogen-bonded nanocages via water-driven crystallization

Limiting global warming increasingly relies on the development of environmentally friendly CO2 capture strategies. Crystallization is renowned for versatile separation and purification, yet traditional compound crystallization-based CO2 capture still necessitates intricate preparation processes, stringent reaction conditions, and high regenerative energy consumption. As an ambitious sustainability goal, natural water could be used as a precursor of crystallization to construct hydrogen-bonded water cages for CO2 capture, but main obstacles are slow crystallization kinetics and low capture capacity. Here, a water-activation-induced crystallization strategy by organic magnetic nanoparticles (Methionine@Fe3O4) has been proposed for efficient CO2 capture. Local water ordering strengthened by hydrophobic amino acids and abundant nucleation sites provided by nanoparticles create hotspots for hydration phase transition and crystal growth, with a CO2 capture capacity of 118.7 v/v (22.7 wt%). Favorable biocompatibility and stable performance are conducive to the industrial application of this nanomaterial, and the excellent magnetic recyclable property enables simple separation from clean water. This strategy demonstrates an extraordinary CO2 capture potential compared to state-of-the-art systems, thus providing an inspiration for sustainable CO2 capture and storage with zero resource depletion (ZRD).

Oxidative stress induced by emerging halohydroxybenzonitrile disinfection byproducts and their binding interaction with catalase

Halohydroxybenzonitriles (HHBNs) were reported to exist in drinking water as an emerging group of nitrogen-containing aromatic disinfection byproducts (DBPs). The involvement of binding energy with catalase (CAT) as one of molecular descriptors in the quantitative structure-activity relationship model for HHBN cytotoxicity supported that oxidative stress is highly possible to be an essential cytotoxicity mechanism for HHBNs, and the disruption of cellular redox homeostasis may be partly attributed to their interactions towards CAT. However, the relevant evidence is lacking as yet. Thus, in this study, typical biomarkers for oxidative stress and damage in HHBN-treated cells were examined, and the binding interactions between HHBN and CAT were explored. Results indicated that upon exposure to 3,5-dibromo-2-hydroxybenzonitrile, the antioxidant system (e.g., CAT, superoxide dismutase, and glutathione) was disrupted, and excessive reactive oxygen species gave rise to oxidative DNA damage, which further resulted in cell apoptosis. Moreover, the binding interaction between CAT and 3,5-dibromo-2-hydroxybenzonitrile could change the structure and activity of CAT, with the formation of complexes primarily dependent on van der Waals forces and hydrogen bonding. A positive correlation was observed between HHBN cytotoxicity and inhibitory potency on the biological function of CAT, and some unidentified iodinated HHBNs warrant special attention in future owing to their potential high cytotoxicity. The above results contribute to a better understanding of the toxic mechanisms of emerging HHBN DBPs.

Endocrine disruptor identification and multitoxicity level assessment of organic chemicals: An example of multiple machine learning models

Endocrine-disrupting chemicals (EDCs) pollution is a major global environmental issue. Assessing the multiple toxic effects of EDCs is key to managing their risks. This study successfully developed an EDCs classification and recognition model based on recursive feature elimination and random forest coupling, which passed external validation. Furthermore, the study classified the hormonal effects of EDCs and elucidated their hormonal roles. Molecular dynamics simulations were employed to investigate the toxicity of EDCs, and a regression model for such toxicity was developed using neural networks. A multi-toxicity regression model for EDCs was also developed using the XGBoost algorithm. This model can evaluate carcinogenicity, teratogenicity, and potential developmental toxicity of EDCs. The Spearman and Kendall correlation coefficient methods were used to assess the relations between toxicities. This study combines data filtering with model optimisation to ensure the use of efficient and concise methods. This allows for a comprehensive assessment of EDCs toxicity. It also helps analyse the link between EDCs molecular structure and their toxic effects, providing ideas for designing new chemicals. However, the model exhibits high complexity, and some processes are difficult to fully explain.

Formation mechanisms of chlorinated disinfection byproducts chlorinated benzoquinones from free aromatic amino acids: In vitro and in silico study

Chlorinated benzoquinones (CBQs) are a class of disinfection byproducts (DBPs) that are widely present in aquatic environments. Due to their strong cytotoxicity and genotoxicity to aquatic organisms and humans, the identification of novel CBQs and the exploration of their formation mechanisms became the focus of recent research. In this study, a new group of chlorinated DBPs formed during chlorination of two aromatic amino acids, tyrosine (Tyr) and tryptophan (Trp), were identified for the first time. Four predominant formation pathways of the main chlorinated product from Tyr and Trp, dichloro-benzoquinone (DCBQ), were deduced based on the MASS spectrum data, which consist of reactions including electrophilic addition, chlorination, hydrolysis, decarboxylation, oxidation, dehydrochlorination, and cleavage of the pyrrole ring. The potential chlorination substituted sites of Tyr to form 2,6-DCBQ were further explored by quantum chemical calculations. Chlorination tended to occur at the ortho C-position of the -OH (hydroxyl) group in Tyr. The C1 site of Tyr was most prone to undergo monochlorination, and dichlorination was more likely to take place at C5 site of the monochlorinated product. In addition, the nitrogen atom (N18) of Tyr could also undergo chlorine substitution, but it was difficult to happen when the C1 and C5 sites were already substituted. By combining experimental and theoretical results, this study summarized the predominant pathway for the formation of 2,6-DCBQ during the chlorination process of amino acids. The outcome contributed to a deeper understanding of the implications for water safety and the potential health risks associated with chlorination byproducts.

Electroactivation of peroxymonosulfate (PMS)-coupled electrocoagulation for degradation of iodinated contrast media and reduction of I-DBPs formation: Dual control and QSAR modeling

Iodinated contrast medias (ICMs) can act as an iodine source to react with dissolved organic matter (DOM) in the chlorination process to form toxic iodinated disinfection byproducts (I-DBPs). In this study, a coupled process of electrically activated PMS and electrocoagulation (EO-PMS/EC) was constructed to simultaneously remove iopamidol (IPM) and DOM, which has a dual control effect on the formation of I-DBPs. This system achieved a 90.1 % degradation rate of IPM, as well as remarkable removal of DOM. Firstly, the effects of experimental conditions and water matrix on IPM degradation were investigated; 1O2, HO·, SO4·-, O2·- and Fe (IV) are the main contributors to IPM degradation. Moreover, the transformation pathways of IPM were inferred and most of the intermediates were less toxic compared to I-DBPs. The formation of I-DBPs was reduced by 82.7 % and was effectively inhibited in all three real water samples. Furthermore, the developed QSAR model illustrate that the degradation rate constants of ICMs are positively correlated with the values of their highest occupied molecular orbital energies and dipole moments. This study provided a more effective I-DBPs reduction strategy which reduced the environmental risk of I-DBPs to a certain extent.

In vivo oxidative stress responses and mechanism to chlorinated and methylated p-benzoquinone oxidation byproducts: A comparison study

Halogen-substituted para-benzoquinones (p-BQs) are emerging disinfection byproducts known to induce oxidative damage both in vitro and in vivo. However, as ubiquitous oxidation byproducts, the in vivo toxicity and transport mechanism of non-halogenated p-BQs with similar structure of α, β-unsaturated ketones to halogenated p-BQs have not been thoroughly investigated. In this study, the effect of substituents on toxicity and transportation of 2-chloro-1,4-benzoquinone (CBQ) and 2-methyl-1,4-benzoquinone (MBQ) was systematically investigated. The results show that MBQ exhibits slightly lower acute toxicity to zebrafish embryos compared to CBQ. Exposure to both CBQ and MBQ at concentration of 10 μg/L and 100 μg/L significantly increased the levels of reactive oxygen species, and enhanced the activities of total superoxide dismutase, catalase, and glutathione peroxidase, while malformations were primarily observed in the 100 μg/L exposure groups. The varying developmental toxicity was associated with significant upregulation of 10 genes by CBQ compared to only 6 by MBQ. Using the high-resolution mass spectrometry and electron paramagnetic resonance spectroscopy, the hydroxylation of both CBQ and MBQ, and the production of semiquinone radicals and hydroxyl radicals in aqueous environments have been revealed. This study has demonstrated that the toxicity of non-halogenated p-BQs should not be overlooked and contributes to the understanding of the generated radicals, leading to excessive oxidative-stress in vivo.

A Low-Cost Electrochemical Cell Sensor Based on MWCNT-COOH/α-Fe2O3 for Toxicity Detection of Drinking Water Disinfection Byproducts

The disinfection of drinking water is essential for eliminating pathogens and preventing waterborne diseases. However, this process generates various disinfection byproducts (DBPs), which toxicological research indicates can have detrimental effects on living organisms. Moreover, the safety of these DBPs has not been sufficiently assessed, underscoring the need for a comprehensive evaluation of their toxic effects and associated health risks. Compared to traditional methods for studying the toxicity of pollutants, emerging electrochemical sensing technologies offer advantages such as simplicity, speed, and sensitivity, presenting an effective means for toxicity research on pollutants. However, challenges remain in this field, including the need to improve electrode sensitivity and reduce electrode costs. In this study, a pencil graphite electrode (PGE) was modified with carboxylated multi-walled carbon nanotubes (MWCNT-COOH) and nano-iron (III) oxide (α-Fe2O3) to fabricate a low-cost electrode with excellent electrocatalytic performance for cell-active substances. Subsequently, a novel cellular electrochemical sensor was constructed for the sensitive detection of the toxicity of three drinking water DBPs. The half inhibitory concentration (IC50) values of 2-chlorophenylacetonitrile (2-CPAN), 3-chlorophenylacetonitrile (3-CPAN), and 4-chlorophenylacetonitrile (4-CPAN) for HepG2 cells were 660.69, 831.76, and 812.83 µM, respectively. This study provides technical support and scientific evidence for the toxicity detection and safety assessment of emerging contaminants.

Halobenzoquinone-induced potential carcinogenicity associated with p53-mediated cell cycle pathway

2,6-Dibromo-1,4-benzoquinone (2,6-DBBQ) and 2,6-dichloro-1,4-benzoquinone (2,6-DCBQ), two emerging halobenzoquinones (HBQs), have the highest detection frequencies and levels in drinking water among all HBQs. They are more toxic than the regulated disinfection byproducts. Quantitative structure toxicity relationship analysis predicted that HBQs are a class of potential bladder carcinogens. However, direct experimental evidence for the carcinogenicity of 2,6-DBBQ and 2,6-DCBQ is lacking and the associated toxicity mechanisms remain unclear. In this study, we confirmed the potential carcinogenicity of 2,6-DBBQ and 2,6-DCBQ using an in vitro malignant transformation assay, evaluated their cytotoxicity and genotoxicity, and investigated their toxicity mechanisms. The results showed that 2,6-DBBQ and 2,6-DCBQ significantly decreased the viability of human uroepithelial SV-HUC-1 cells and induced DNA damage in SV-HUC-1 cells, and chromosomal damage in HepG2 cells, and malignant transformation of SV-HUC-1 cells. Moreover, transcriptome sequencing revealed that 2,6-DBBQ and 2,6-DCBQ activated the p53-mediated cell cycle pathway in bladder cancer. In the p53-mediated cell cycle pathway, 2,6-DBBQ and 2,6-DCBQ induced cell cycle arrest at the S phase by downregulating p53 and upregulating p21. Additionally, 2,6-DBBQ and 2,6-DCBQ may have produced excessive reactive oxygen species, damaging DNA and chromosomes. These results not only first confirm the potential carcinogenicity of 2,6-DBBQ and 2,6-DCBQ but also provide an important reference for exploring the cytotoxicity and genotoxicity mechanisms of these HBQs.

Metabolite identification of emerging disinfection byproduct dibromo-benzoquinone in vivo and in vitro: Multi-strategy mass-spectrometry annotation and toxicity characterization

Halobenzoquinones (HBQs) are emerging disinfection byproducts (DBPs) of high toxicity and also are shared active toxic intermediates of multiple halogenated organic pollutants. Due to the strong oxidizing property and electrophilicity, HBQs exhibit extremely diverse metabolism pathways in organisms. The identification of toxic-decisive metabolites is pivotal, albeit challenging, for understanding the toxicity mechanisms of HBQs. We employed dibromo-benzoquinone (DBBQ) as a representative HBQ, and established a systematic analytical strategy using high-resolution mass spectrometry, which collectively coupled suspect screening (SS), mass defect filtering (MDF), product ion filtering (PIF), isotopic signature filtering (ISF), and molecular networking (MN). As a result, 20 biotransformation products of DBBQ were identified in vivo and in vitro, involving metabolism reactions such as hydroxylation, methylation, methoxylation, acetylation, sulfonation, glucuronidation, glutathionylation, dimerization, and conjugation with amino acids or fatty acids. Quantitative structure-activity relationship (QSAR) analysis and cytotoxicity experiments consistently demonstrated the significantly high toxicity of the fatty acid conjugate compared to the parent compound DBBQ and other metabolites, pinpointing the important role of the fatty acid conjugation in determining the metabolism and toxicity of HBQs. The research conducted a comprehensive evaluation of the metabolism of a typical HBQ with the combination of multiple analytical and toxicity characterization methods, therefore screen out the most important metabolism pathway of HBQs.

Biomimetic cytotoxicity control of select nitrogenous disinfection byproducts in water

Nitrogenous disinfection byproducts (N-DBPs) in water are carcinogenic, teratogenic, and mutagenic. In this work, we developed a biomimetic reduction approach based on the cysteine thiol that destructed the highly toxic, select nitrogenous haloacetamides (HAMs) and haloacetonitriles (HANs) while effectively controlling the cytotoxicity of the degradation products to serve as a basis for further technological applications (e.g. immobilized contact bed for terminal users). Mechanisms on toxicity control were elucidated. Results showed the degradation and cytotoxicity control of HAMs as more efficient than that of the HANs. The cytotoxicity of the chlorinated, brominated, and iodinated HAMs and HANs was reduced to 25 %- 0.25 % of the original after biomimetic reduction using a reasonable concentration ratio. Through a combination of thiol-specific reactivity, dehalogenation, and quantitative structure-activity relationship analyses, the major toxicity control mechanisms were found to be the reductive dehalogenation of the N-DBPs. The halogenated functional groups on the N-DBPs had a more pronounced effect than the amide and nitrile groups on the cytotoxicity and detoxification effect. Patterns of toxicity interaction variations with DBPs concentrations were identified to detect possible synergistic cytotoxicity interactions under various combinations of HAMs and HANs in the presence of the cysteine thiol. Results could benefit future N-DBPs control efforts.

A group of emerging heterocyclic nitrogenous disinfection byproducts: Formation and cytotoxicity of halopyridinols in drinking water

Recently, a new group of halopyridinol disinfection byproducts (DBPs) was reported in drinking water. The in vivo developmental and acute toxicity assays have shown that they were more toxic than a few commonly known aliphatic DBPs such as bromoform and iodoacetic acid. However, many pyridinol DBPs with the same main structures but different halogen substitutions were still unknown due to complicated water quality conditions and various disinfection methods applied in drinking water treatment plants. Studies on their transformation mechanisms in drinking water disinfection were quite limited. In this study, comprehensive detection and identification of halopyridinols were conducted, and five new halopyridinols were first reported, including 2-chloro-3-pyridinol, 2,6-dichloro-3-pyridinol, 2-bromo-5-chloro-3-pyridinol, 2,4,6-trichloro-3-pyridinol and 2,5,6-trichloro-3-pyridinol. Formation conditions and mechanisms of the halopyridinols were explored, and results showed that chlorination promoted their formation compared with chloramination. Halopyridinols were intermediate DBPs that could undergo further transformation/degradation with increasing contact time, disinfectant dose, bromide concentration, and pH. The in vitro cytotoxicity of the halopyridinols was evaluated using human hepatocellular carcinoma cells. Results showed that the cytotoxicity of 3,5,6-trichloro-2-pyridinol was the highest (EC50 = 474.3 μM), which was 13.0 and 1.6 times higher than that of 2-bromo-3-pyridinol (EC50 = 6214.5 μM) and tribromomethane (EC50 = 753.6 μM), respectively.

A ng/L Level LC-MS Method Using Membrane SPE as Sampling Technology: Determination of Nine Halobenzoquinones in Potable Water

A promising method was established for the determination of nine halobenzoquinones (HBQs) in potable water by membrane solid-phase extraction (MSPE) pretreatment and the liquid chromatography-mass spectrometry (LC-MS) method. A 500 mL water sample was taken for enrichment by the SDB-RPS membrane, which was previously activated by methanol and ultrapure water. The sample was eluted with methanol and re-dissolved with the initial mobile phase after nitrogen blowing. Then, it was detected in negative ion mode using the working curve, and HBQs were quantified by the external standard method. The linearity was satisfactory in the concentration range of 4-1000 ng/L, with correlation coefficients of 0.9963~0.9994. The recoveries were 73.5~126.6% at three spiked levels, with relative standard deviations (RSDs) of 6.8~15.5%. The limits of detection (LOD, S/N = 3) values were 0.1~0.7 ng/L. The results demonstrate that the MSPE-LC-MS method is reliable, rapid, and sensitive for the simultaneous analysis of nine HBPs in potable water.

Cytotoxicity of emerging halophenylacetamide disinfection byproducts in drinking water: Mechanism and prediction

Halophenylacetamides (HPAcAms) have been identified as a new group of nitrogenous aromatic disinfection byproducts (DBPs) in drinking water, but the toxicity mechanisms associated with HPAcAms remain almost completely unknown. In this work, the cytotoxicity of HPAcAms in human hepatoma (HepG2) cells was evaluated, intracellular oxidative stress/damage levels were analyzed, their binding interactions with antioxidative enzyme were explored, and a quantitative structure-activity relationship (QSAR) model was established. Results indicated that the EC50 values of HPAcAms ranged from 2353 μM to 9780 μM, and the isomeric structure as well as the type and number of halogen substitutions could obviously induce the change in the cytotoxicity of HPAcAms. Upon exposure to 2-(3,4-dichlorophenyl)acetamide (3,4-DCPAcAm), various important biomarkers linked to oxidative stress and damage, such as reactive oxygen species, 8‑hydroxy-2-deoxyguanosine, and cell apoptosis, exhibited a significant increase in a dose-dependent manner. Moreover, 3,4-DCPAcAm could directly bind with Cu/Zn-superoxide dismutase and induce the alterations in the structure and activity, and the formation of complexes was predominantly influenced by the van der Waals force and hydrogen bonding. The QSAR model supported that the nucleophilic reactivity as well as the molecular compactness might be highly important in their cytotoxicity mechanisms in HepG2 cells, and 2-(2,4-dibromophenyl)acetamide and 2-(3,4-dibromophenyl)acetamide deserved particular attention in future studies due to the relatively higher predicted cytotoxicity. This study provided the first comprehensive investigation on the cytotoxicity mechanisms of HPAcAm DBPs.

Comprehensive Toxicological Assessment of Halobenzoquinones in Drinking Water at Environmentally Relevant Concentration

Halobenzoquinones (HBQs), an emerging unregulated category of disinfection byproduct (DBP) in drinking water, have aroused an increasing concern over their potential health risks. However, the chronic toxicity of HBQs at environmentally relevant concentrations remains largely unknown. Here, the occurrence and concentrations of 13 HBQs in drinking water from a northern megacity in China were examined using ultrahigh performance liquid chromatography coupled with triple-quadrupole tandem mass spectrometry (UHPLC-MS/MS). Four HBQs, including 2,6-dichloro-1,4-benzoquinone (2,6-DCBQ), 2,6-dibromo-1,4-benzoquinone (2,6-DBBQ), 2,3,6-trichloro-1,4-benzoquinone (TriCBQ), and 2,5-dibromo-1,4-benzoquinone (2,5-DBBQ), were detected beyond 50% occurrence frequency and at median concentrations from 4 to 50 ng/L. The chronic toxicity of these four HBQs to normal human colon and liver cells (FHC and THLE-2) was investigated at these concentrations. After 90 days of exposure, 2,5-DBBQ and 2,6-DCBQ induced the highest levels of oxidative stress and deoxyribonucleic acid (DNA) damage in colon and liver cells, respectively. Moreover, 2,5-DBBQ and 2,6-DCBQ were also found to induce epithelial-mesenchymal transition (EMT) in normal human liver cells via the extracellular signal regulated kinase (ERK) signaling pathway. Importantly, heating to 100 °C (boiling) was found to efficiently reduce the levels of these four HBQs in drinking water. These results suggested that environmentally relevant concentrations of HBQs could induce cytotoxicity and genotoxicity in normal human cells, and boiling is a highly efficient way of detoxification for HBQs.

Formation of halogenated forms of bisphenol A (BPA) in water: Resolving isomers with ion mobility - mass spectrometry and the role of halogenation position in cellular toxicity

Halogenated BPA (XBPA) forms resulting from water chlorination can lead to increased toxicity and different biological effects. While previous studies have reported the occurrence of different XBPAs, analytical limitation have hindered the analysis and differentiation of the many potential isomeric forms. Using online solid-phase extraction - liquid chromatography - ion-mobility - high-resolution mass spectrometry (OSPE-LC-IM-HRMS), we demonstrated a rapid analysis method for the analysis of XBPA forms after water chlorination, with a total analysis time of less than 10 min including extraction and concentration and low detection limits (∼5-80 ng/L range). A multi in-vitro bioassay testing approach for the identified products revealed that cytotoxicity and bioenergetics impacts were largely associated with the presence of halogen atoms at positions 2 or 2' and the overall number of halogens incorporated into the BPA molecule. Different XBPA also showed distinct impacts on oxidative stress, peroxisome proliferator-activated receptor gamma - PPARγ, and inflammatory response. While increased DNA damage was observed for chlorinated water samples (4.14 ± 1.21-fold change), the additive effect of the selected 20 XBPA studied could not explain the increased DNA damage observed, indicating that additional species or synergistic effects might be at play.

New mechanistic insights into halogen-dependent cytotoxic pattern of monohaloacetamide disinfection byproducts

As highly toxic nitrogenous disinfection byproducts (DBPs), monohaloacetamides (monoHAcAms) generally exhibited a cytotoxic rank order of iodoacetamide ˃ bromoacetamide ˃ chloroacetamide. However, the mechanisms underlying the halogen-dependent cytotoxic pattern remain largely veiled as yet. In this work, oxidative stress/damage levels in monoHAcAm-treated Chinese hamster ovary cells were thoroughly analyzed, and binding interactions between monoHAcAms and antioxidative enzyme Cu/Zn-superoxide dismutase (Cu/Zn-SOD) were investigated by multiple spectroscopic techniques and molecular docking. Upon exposure to monoHAcAms, the intracellular levels of key biomarkers associated with oxidative stress/damage, including reactive oxygen species, malondialdehyde, lactate dehydrogenase, 8-hydroxy-2-deoxyguanosine, cell apoptosis, and G1 cell cycle arrest, were all significantly increased in a dose-response manner with the same halogen-dependent rank order as their cytotoxicity. Moreover, this rank order was also determined to be applicable to the monoHAcAm-induced alterations in the conformation, secondary structure, and activity of Cu/Zn-SOD, the microenvironment surrounding aromatic amino acid residues in Cu/Zn-SOD, as well as the predicted binding energy of SOD-monoHAcAm interactions. Our results revealed that the halogen-dependent cytotoxic pattern of monoHAcAms was attributed to their differential capacity to induce oxidative stress/damage and their interaction with antioxidative enzyme, which contribute to a better understanding of the halogenated DBP-induced toxicological mechanisms.

Formation and toxicity contribution of chlorinated and dechlorinated halobenzoquinones from dichlorophenols after ozonation

Halobenzoquinones (HBQs) are a class of disinfection byproducts with high cytotoxicity and potential carcinogenicity, which have been widely detected in chlorination of drinking water and swimming pool water. However, to date, the formation of HBQs upon ozonation and the HBQ precursors have been overlooked. This study investigated the formation of chlorinated and dechlorinated HBQs from six dichlorophenol (DCP) isomers. The monomeric and dimeric HBQs were identified in all the ozonation effluents, exhibiting 1-100 times higher toxicity levels than their precursors. The sum of detected HBQs intensity had a satisfactory linear relation with the maximum toxic unit (R2 = 0.9657), indicating the primary toxicity contribution to the increased overall toxicity of effluents. Based on density functional theory calculations, when ozone attacks the para carbon to the hydroxyl group of 2,3-DCP, the probability of producing chlorinated HBQs is 80.41 %, indicating that the para carbon attack mainly resulted in the formation of monomeric HBQs. 2,3-dichlorophenoxy radicals were successfully detected in ozonated 2,3-DCP effluent through electron paramagnetic resonance and further validated using theoretical calculation, revealing the formation pathway of dimeric HBQs. The results indicate that chlorinated phenols, regardless of the positions of chlorine substitution, can potentially serve as precursors for both chlorinated and dechlorinated HBQs formation during ozonation.

Effects of ozone dose on brominated DBPs in subsequent chlor(am)ination: A comprehensive study of aliphatic, alicyclic and aromatic DBPs

Ozone‒chlor(am)ine is a commonly used combination of disinfectants in drinking water treatment. Although there are quite a few studies on the formation of some individual DBPs in the ozone‒chlor(am)ine disinfection, an overall picture of the DBP formation in the combined disinfection is largely unavailable. In this study, the effects of ozone dose on the formation and speciation of organic brominated disinfection byproducts (DBPs) in subsequent chlorination, chloramination, or chlorination‒chloramination of simulated drinking water were investigated. High-molecular-weight, aliphatic, alicyclic and aromatic brominated DBPs were selectively detected and studied using a powerful precursor ion scan method with ultra performance liquid chromatography/electrospray ionization triple quadrupole mass spectrometry (UPLC/ESI-tqMS). Two groups of unregulated yet relatively toxic DBPs, dihalonitromethanes and dihaloacetaldehydes, were detected by the UPLC/ESI-tqMS for the first time. With increasing ozone dose, the levels of high-molecular-weight (m/z 300-500) and alicyclic and aromatic brominated DBPs generally decreased, the levels of brominated aliphatic acids were slightly affected, and the levels of dihalonitromethanes and dihaloacetaldehydes generally increased in the subsequent disinfection processes. Despite different molecular compositions of the detected DBPs, increasing ozone dose generally shifted the formation of DBPs from chlorinated ones to brominated analogues in the subsequent disinfection processes. This study provided a comprehensive analysis of the impact of ozone dose on the DBP formation and speciation in subsequent chlor(am)ine disinfection.

Non-regulated haloaromatic water disinfection byproducts act as endocrine and lipid disrupters in human placental cells

The disinfection of drinking water generates hundreds of disinfection byproducts (DBPs), including haloaromatic DBPs. These haloaromatic DBPs are suspected to be more toxic than haloaliphatic ones, and they are currently not regulated. This work investigates their toxicity and ability to interfere with estrogen synthesis in human placental JEG-3 cells, and their genotoxic potential in human alveolar A549 cells. Among the haloaromatic DBPs studied, halobenzoquinones (2,6-dichloro-1,4-benzoquinone (DCBQ) and 2,6-dibromo-1,4-benzoquinone (DBBQ)) showed the highest cytotoxicity (EC50: 18-26 μg/mL). They induced the generation of very high levels of reactive oxygen species (ROS) and up-regulated the expression of genes involved in estrogen synthesis (cyp19a1, hsd17b1). Increased ROS was linked to significant depletion of polyunsaturated lipid species from inner cell membranes. The other DBPs tested showed low or no significant cytotoxicity (EC50 ≥ 100 μg/mL), while 2,4,6-trichloro-phenol (TCP), 2,4,6-tribromo-phenol (TBP) and 3,5-dibromo-4-hydroxybenzaldehyde (DCHB) induced the formation of micronuclei at concentrations much higher than those typically found in water (100 μg/mL). This study reveals the different modes of action of haloaromatic DBPs, and highlights the toxic potential of halobenzoquinones, which had a significant impact on the expression of placenta steroid metabolism related genes and induce oxidative stress, implying potential adverse health effects.

Chloro- and bromo-benzoquinone formation and transformation mechanisms in a drinking water-distribution system

Halogenated benzoquinones (HBQs) are frequently detected in tap water. HBQ levels are correlated with water age. As the water-transmission distance (water age) increases, the levels of bromo-benzoquinones (Br-BQs) decreased and those of chloro-benzoquinones (Cl-BQs) remained relatively stable in drinking water-distribution system in the presence of residual chlorine. 2,3,5,6-Tetrachloro-1,4-benzoquinone (TCBQ) and 2,6-dibromo-1,4-benzoquinone (DBBQ) were the most abundant Cl-BQ and Br-BQ, with maximum concentrations of 60.2 and 181.4 ng/L, respectively. TCBQ and DBBQ were chosen as representatives of HBQs to investigate their reactions with chlorine, including kinetics, pathways, and changes in toxicity. The hydrolysis and chlorination rates of HBQs were significantly pH-dependent, and the kinetic rates of DBBQ were faster than TCBQ in the pH range of 5-10. Chlorination converted highly toxic TCBQ and DBBQ to less-toxic chlorinated/brominated aliphatic disinfection by-products (DBPs), thereby reducing the overall toxicity of water bodies. This study provides comprehensive insights into the distinct life cycles of TCBQ and DBBQ in drinking water, covering formation, transformation, and toxicity. These findings provide a nuanced understanding of the risks posed by HBQs at various locations within the drinking water distribution system, offering valuable guidance for improving the control of DBPs in drinking water.

The alternation of halobenzoquinone disinfection byproduct on toxicogenomics of DNA damage and repair in uroepithelial cells

Halobenzoquinones (HBQs) were recently discovered as an emerging class of drinking water disinfection byproducts with carcinogenic concern. However, the molecular mechanism underlying HBQs-induced DNA damage is not clear. In this study, we integrated in vitro genotoxicity, computational toxicology, and the quantitative toxicogenomic analysis of HBQs on DNA damage/repair pathways in human bladder epithelial cells SV-HUC-1. The results showed that HBQs could induce cytotoxicity with the descending order as 2,6-DIBQ > 2,6-DCBQ ≈ 2,6-DBBQ. Also, HBQs can increase DNA damage in SV-HUC-1 cells and thus generate genotoxicity. However, there is no significant difference in genotoxicity among the three HBQs. The results of molecular docking and molecular dynamics simulation further confirmed that HBQs had high binding fractions and stability to DNA. Toxicogenomic analysis indicated that HBQs interfered with DNA repair pathways, mainly affecting base excision repair, nucleotide excision repair and homologous recombination repair. These results have provided new insights into the underlying molecular mechanisms of HBQs-induced DNA damage, and contributed to the understanding of the relationship between exposure to DBPs and risks of developing bladder cancer.

Target analysis, occurrence and cytotoxicity of halogenated polyhydroxyphenols as emerging disinfection byproducts in drinking water

Halogenated aromatic disinfection byproducts (DBPs) in drinking water, such as halogenated phenols, have received widespread attention due to their high toxicity and ubiquitous occurrence in recent years. This study identified a group of emerging halogenated aromatic DBPs, known as halogenated polyhydroxyphenols (HPPs), and investigated their occurrence and cytotoxicity. We developed a highly sensitive solid-phase extraction ultra-performance liquid chromatography-tandem mass spectrometry (SPE-UPLC-MS/MS) method under multiple reaction monitoring (MRM) mode, with recoveries ranging from 86 to 115% and method detection limits (MDLs) ranging from 0.10 to 1.87 ng/L for the analysis of 15 HPPs. Eleven of these HPP DBPs were detected in collected drinking water samples using this method with detection frequencies ranging from 14 to 100% and a maximum concentration of 24 ng/L. The IC50 of the 15 HPPs in Chinese hamster ovary (CHO-K1) cells were ranged from 15.13 µM to 6.08×103 µM. The tested HPPs with -CHO substitution exhibited higher cytotoxicity compared to those with -COOH substitution. The TIC-Tox values of HPPs were calculated to be higher than those of HPs, indicating a potential necessity to pay attention to HPP DBPs. A quantitative structure-activity relationship (QSAR) model was developed for the cytotoxicity of HPPs, which was shown to be significantly associated with acid dissociation constant (pKa) and total valence connectivity (TVCon). To the best of our knowledge, this study reported the analysis, occurrence, and cytotoxicity of HPP DBPs in drinking water for the first time.

Investigation on toxicity and mechanism to Daphnia magna for 14 disinfection by-products: Enzyme activity and molecular docking

Exposure to disinfection by-products (DBPs) has been found to induce a range of toxic effects in aquatic organism. Previous studies have consistently demonstrated that a majority of DBPs have the ability to induce in vivo toxicity in aquatic organisms. However, the impact of DBPs on the metabolic processes of Daphnia magna (D. magna) and the underlying molecular toxicity mechanisms are still not well understood. Therefore, we investigated the effects of 14 DBPs on two oxidative stress enzymes and malondialdehyde (MDA) levels in D. magna. Additionally, we employed molecular docking to simulate the toxicity of DBPs to D. magna at the molecular level. This comprehensive analysis allowed us to gain further insights into the toxicity of DBPs on D. magna. The results showed that among the aliphatic DBPs, the more bromine substituents, the lower the toxicity effect, and it's opposite in the aromatic DBPs. In the detection of oxidative stress level, catalase (CAT) enzyme and superoxide dismutase (SOD) enzyme in D. magna under compound stress showed a low increase and decrease with the increase of concentration. The level of MDA showed a positive correlation with the concentration. In the last, molecular docking simulations have shown promise in predicting the toxicity of DBPs and providing insights into their toxic effects to a certain extent, and the docking situation of P53 is slightly different. Hence, it is imperative to further regulate the presence of aromatic DBPs due to their pronounced toxic effects on D. magna, and these simulations can be complemented with actual experiments to enhance our understanding of the toxicity mechanisms of DBPs.

Thiol Reactome: A Nontargeted Strategy to Precisely Identify Thiol Reactive Drinking Water Disinfection Byproducts

The precise identification of predominant toxic disinfection byproducts (DBPs) from disinfected water is a longstanding challenge. We propose a new acellular analytical strategy, the 'Thiol Reactome', to identify thiol-reactive DBPs by employing a thiol probe and nontargeted mass spectrometry (MS) analysis. Disinfected/oxidized water samples had reduced cellular oxidative stress responses of 46 ± 23% in Nrf2 reporter cells when preincubated with glutathione (GSH). This supports thiol-reactive DBPs as the predominant drivers of oxidative stress. This method was benchmarked using seven classes of DBPs including haloacetonitriles, which preferentially reacted with GSH via substitution or addition depending on the number of halogens present. The method was then applied to chemically disinfected/oxidized waters, and 181 tentative DBP-GSH reaction products were detected. The formulas of 24 high abundance DBP-GSH adducts were predicted, among which nitrogenous-DBPs (11) and unsaturated carbonyls (4) were the predominant compound classes. Two major unsaturated carbonyl-GSH adducts, GSH-acrolein and GSH-acrylic acid, were confirmed by their authentic standards. These two adducts were unexpectedly formed from larger native DBPs when reacting with GSH. This study demonstrated the "Thiol Reactome" as an effective acellular assay to precisely identify and broadly capture toxic DBPs from water mixtures.

Phthalate acid esters contribute to the cytotoxicity of mask leachate: Cell-based assay for toxicity assessment

After the COVID-19 outbreak, masks have become an essential part of people lives. Although several studies have been conducted to determine the release of hazardous substances from masks, how their co-presence poses a potential exposure risk to human health remains unexplored. In this study, we quantitatively compared the leaching of substances from six different common types of masks, including phthalate acid esters (PAEs), metals, and microplastics (MPs), and comprehensively evaluated the potential cytotoxicity of different leachates. MPs smaller than 3 µm were quantified by Py-GC-MS, and reusable masks showed greater releasing potentials up to 1504 µg/g. We also detected the prevalence of PAEs in masks, with the highest release reaching 42 μg/g, with dibutyl phthalate (DBP), diisobutyl phthalate (DiBP) and bis (2-ethylhexyl) phthalate (DEHP) being the predominant types. Moreover, the antimicrobial cloth masks released 173.0 µg of Cu or 4.5 µg of Ag, representing 2.7% and 0.04% of the original masks, respectively. Our cell-based assay results demonstrated for the first time that mask leachate induced nuclear condensation with DNA damage, and simultaneously triggered high levels of glutathione and reactive oxidative stress production, which exacerbated mitochondrial fragmentation, eventually leading to cell death. Combined with substance identification and correlation analysis, PAEs were found to be the contributors to cytotoxicity. Masks containing Cu or Ag led to acidification of lysosomes and alkalinization of cells. These results strongly suggested that the levels of PAEs in the production of regulatory masks should be strictly controlled.

Oxidative stress as a key event in 2,6-dichloro-1,4-benzoquinone-induced neurodevelopmental toxicity

2,6-dichloro-1,4-benzoquinone (DCBQ) has been identified as an emerging disinfection byproducts (DBPs) in drinking water and has the potential to induce neurodevelopmental toxicity. However, there is rarely a comprehensive toxicological evaluation of the neurodevelopmental toxicity of DCBQ. Here, neural differentiating SH-SY5Y cells were used as an in vitro model. Our results have found that DCBQ has decreased cell viability and neural differentiation, generated higher level of reactive oxygen species (ROS), increased the percentage of apoptosis and lowered the level of mitochondrial membrane potential, suggesting the neurodevelopmental toxicity of DCBQ. In addition, antioxidant N-acetyl-L-cysteine (NAC) could significantly attenuate these DCBQ-induced neurotoxic effects, supporting our hypothesis that the neurodevelopmental toxicity may be related with oxidative stress induced by DCBQ. We further demonstrated that DCBQ-induced neurodevelopmental toxicity could promote the mitochondrial apoptosis pathway and inhibit the prosurvival PI3K/AKT/mTOR pathway through inducing ROS, which ultimately inhibited cell proliferation and induced apoptosis in neural differentiating SH-SY5Y cells. These findings have provided novel insights into the risk of neurodevelopmental toxic effects associated with DCBQ exposure, emphasizing the importance of assessing the potential neurodevelopmental toxicity of DBPs.

Zerovalent Iron/Cu Combined Degradation of Halogenated Disinfection Byproducts and Quantitative Structure-Activity Relationship Modeling

Previous studies have reported that zerovalent iron (ZVI) can reduce several aliphatic groups of disinfection byproducts (DBPs) (e.g., haloacetic acids and haloacetamides) effectively, and the removal efficiency can be significantly improved by metallic copper. Information regarding ZVI/Cu combined degradation of different types of halogenated DBPs can help understand the fate of overall DBPs in drinking water distribution and storage systems consisting of unlined cast iron/copper pipes and related potential control strategies. In this study, we found that, besides aliphatic DBPs, many groups of new emerging aromatic DBPs formed in chlorinated and chloraminated drinking water can be effectively degraded by ZVI/Cu; meanwhile, total organic halogen and total ion intensity were reduced significantly after treatment. Moreover, a robust quantitative structure-activity relationship model was developed and validated based on the ZVI/Cu combined degradation rate constants of 14 typical aromatic DBPs; it can predict the degradation rate constants of other aromatic DBPs for screening and comparative purposes, and the optimized descriptors indicate that DBPs possessing a lower value of the lowest unoccupied molecular orbital energy and a higher value of dipole moment tend to present higher degradation rate constants. In addition, toxicity data of 47 DBPs (belonging to 18 groups) were predicted by two previously established toxicity models, demonstrating that, although most DBPs exhibit higher toxicity than their dehalogenated products, some DBPs show lower toxicity than their lowly halogenated analogs.

Impacts of disinfection byproduct exposures on male reproductive health: Current evidence, possible mechanisms and future needs

Disinfection byproducts (DBPs) are a class of ubiquitous chemicals in drinking water and inevitably result in widespread human exposures. Potentially adverse health effects of DBP exposures, including reproductive and developmental outcomes, have been increasing public concerns. Several reviews have focused on the adverse pregnancy outcomes of DBPs. This review summarized current evidence on male reproduction health upon exposure to DBPs from toxicological and epidemiological literature. Based on existing experimental studies, there are sufficient evidence showing that haloacetic acids (HAAs) are male reproductive toxicants, including reduced epididymal weight, decreased semen parameters and sperm protein 22, and declined testosterone levels. However, epidemiological evidence remains insufficient to support a link of DBP exposures with adverse male reproductive outcomes, despite that blood and urinary DBP biomarkers are associated with decreased semen quality. Eight potential mechanisms, including germ/somatic cell dysfunction, oxidative stress, genotoxicity, inflammation, endocrine hormones, folate metabolism, epigenetic alterations, and gut microbiota, are likely involved in male reproductive toxicity of DBPs. We also identified knowledge gaps in toxicological and epidemiological studies to enhance future needs.

Investigation of the nephrotoxicity of 2,6-dichloro-1,4-benzoquinone disinfection by-product in mice through a 28-day toxicity test

In recent years, 2,6-dichloro-1,4-benzoquinone (DCBQ) has become an emerging water disinfection by-product and widely distributed in disinfected water. Although kidney is a potential target of DCBQ, a systematic study of the in vivo nephrotoxicity of DCBQ is rare. In this study, a 28-day oral toxicity test was used to assess the nephrotoxic effects of DCBQ on mice. And the potential mechanisms of nephrotoxicity induced by DCBQ were explored through inflammation, oxidative stress, apoptosis and gut microbiota. The results showed that the kidney indexes of mice were not altered in DCBQ-exposed group in comparison with the control group. The histopathological investigation revealed that DCBQ caused swollen of renal tube, destruction of the renal structure, and infiltration of inflammatory cell in kidney. DCBQ has induced oxidative damage in kidney, as the observation of the increase of the renal superoxide dismutase (SOD) and catalase (CAT) activity. Also, DCBQ has triggered the inflammatory response in kidney through the increased expression of IL-1β, NF-κB and iNOS. Moreover, DCBQ has activated the apoptosis pathway, as indicated by the increased mRNA expression of Caspase-3 and Caspase-9. We eventually found an association between gut microbiota and nephrotoxic variables, demonstrating the importance of gut-kidney axis in DCBQ toxicity. Our results suggested that exposure to DCBQ in disinfected water might be a risk factor for kidney and provided novel insights into the underlying mechanisms of DCBQ-induced kidney injury, contributing to better interpretation of the health impact of the environmentally emerging contaminant DCBQ.

Methoxylated Modification of Glutathione-Mediated Metabolism of Halobenzoquinones In Vivo and In Vitro

Xenobiotics were generally detoxified in organisms through interaction with endogenous molecules, which may also generate metabolites of increased toxicity. Halobenzoquinones (HBQs), a group of highly toxic emerging disinfection byproducts (DBPs), can be metabolized by reacting with glutathione (GSH) to form various glutathionylated conjugates (SG-HBQs). In this study, the cytotoxicity of HBQs in CHO-K1 cells showed a wavy curve as a function of increased GSH dosage, which was inconsistent with the commonly recognized progressive detoxification curve. We hypothesized that the formation and cytotoxicity of GSH-mediated HBQ metabolites contribute to the unusual wave-shaped cytotoxicity curve. Results showed that glutathionyl-methoxyl HBQs (SG-MeO-HBQs) were identified to be the primary metabolites significantly correlated with the unusual cytotoxicity variation of HBQs. The formation pathway was initiated by stepwise metabolism via hydroxylation and glutathionylation to produce detoxified hydroxyl HBQs (OH-HBQs) and SG-HBQs, followed by methylation to generate SG-MeO-HBQs of potentiated toxicity. To further verify the occurrence of the aforementioned metabolism in vivo, SG-HBQs and SG-MeO-HBQs were detected in the liver, kidney, spleen, testis, bladder, and feces of HBQ-exposed mice, with the highest concentration quantified in the liver. The present study supported that the co-occurrence of metabolism can be antagonistic, which enhanced our understanding of the toxicity and metabolic mechanism of HBQs.

Molecular insights of 2,6-dichlorobenzoquinone-induced cytotoxicity in zebrafish embryo: Activation of ROS-mediated cell cycle arrest and apoptosis

2,6-dichloro-1,4-benzoquinone (2,6-DCBQ), as an emerging disinfection by-product, has been frequently detected in waters, posing potential health risk on public health. Although some studies have pointed out that 2,6-DCBQ exposure can induce cytotoxicity, limited information is available for underlying mechanism for 2,6-DCBQ-induced cytotoxicity. To explore this mechanism, we assessed the levels of reactive oxygen species (ROS), acridine orange (AO) staining, and the mRNA transcriptions of genes (Chk2, Cdk2, Ccna, Ccnb and Ccne) involved in cell-cycle and genes (p53, bax, bcl-2 and caspase 3) involved in apoptosis in zebrafish embryo, after exposed to different concentrations (10, 30, 60, 90 and 120 μg/L) of 2,6-DCBQ for 72 h. Our results indicated that 2,6-DCBQ exposure induced ROS generation and cell apoptosis, and disturbed the mRNA transcription of genes related to cell cycle and apoptosis in zebrafish embryo. Moreover, we also found that 30 ~ 60 μg/L 2,6-DCBQ is the important transition from cell-cycle arrest to cell apoptosis. These results provided novel insight into 2,6-DCBQ-induced cytotoxicity.

Persistent Cytotoxicity and Endocrine Activity in the First Oil Sands End-Pit Lake

Oil sands process-affected water (OSPW) is a byproduct of bitumen extraction that has persistent toxicity owing to its complex mixture of organics. A prominent remediation strategy that involves aging OSPW in end-pit lakes and Base Mine Lake (BML) is the first full-scale test. Its effectiveness over the first 5 years was investigated here using real-time cell analysis, yeast estrogenic and androgenic screens (YES/YAS), and ultra-high-resolution mass spectrometry. HepG2 cytotoxicity per volume of BML organics extracted decreased with age; however, the toxic potency (i.e., toxicity per mass of extract) was not significantly different between years. This was consistent with mass spectral evidence showing no difference in chemical profiles, yet lower total abundance of organics in field-aged samples, suggestive that dilution explains the declining cytotoxicity in BML. The IC50's of BML extracts for YES/YAS antagonism were at environmental concentrations and were similar despite differences in field-age. Persistent YES/YAS antagonism and cytotoxicity were detected in experimental pond OSPW field-aged >20 years, and while organic acids were depleted here, non-acid chemical classes were enriched compared to BML, suggesting these contribute to persistent toxicity of aged OSPW. To avoid a legacy of contaminated sites, active water treatment may be required to accelerate detoxification of end-pit lakes.

Emerging Disinfection Byproduct 2,6-Dichlorobenzoquinone-Induced Cardiovascular Developmental Toxicity of Embryonic Zebrafish and Larvae: Imaging and Transcriptome Analysis

Epidemiological studies have observed the potential association of water disinfection byproduct (DBP) exposure with cardiac defects. Aromatic DBPs represent a significant portion of total DBPs, but their effects on cardiovascular development are unclear. In this study, we examined the effects of an aromatic DBP, 2,6-dichlorobenzoquinone (DCBQ), on the cardiovascular development of zebrafish embryos. After exposure to 2, 4, and 8 μM DCBQ, morphological images of growing zebrafish embryos clearly showed cardiovascular malformation. Fluorescent images of transgenic zebrafish strains with fluorescently labeled heart and blood vessels show that DCBQ exposure resulted in deformed atrium-ventricle looping, degenerated abdomen and trunk vessels, pericardial edema, and decreased blood flow. Furthermore, the expression of the marker gene myl7 (essential for the differentiation and motility of cardiomyocytes) was inhibited in a dose-dependent manner by DCBQ exposure. Finally, transcriptome analysis found that in the 4 μM DCBQ exposure group, the numbers of differentially expressed genes (DEGs) were 113 (50 upregulated and 63 downregulated) at 24 hpf, 2123 (762 upregulated and 1361 downregulated) at 48 hpf, and 61 (11 upregulated and 50 downregulated) at 120 hpf; in the 8 μM DCBQ exposure group, the number of DEGs was 1407 (647 upregulated and 760 downregulated) at 120 hpf. The FoxO signaling pathway was significantly altered. The in vivo results demonstrate the effects of 2,6-DCBQ (0-8 μM) on cardiovascular development, contributing to the understanding of the developmental toxicity of aromatic DBP halobenzoquinones (HBQs).

Quantitative structure-toxicity relationships of halobenzoquinone isomers on DNA reactivity and genotoxicity

Halobenzoquinones (HBQs) are an emerging class of drinking water disinfection byproducts that have been predicted as bladder carcinogens. However, data on the genotoxicity of HBQs are still scarce. This study performed a quantitative structure-toxicity relationship (QSTR) analysis of HBQ isomers on DNA reactivity and genotoxicity. The interaction of HBQs with calf thymus DNA (ct-DNA) was studied using multi-spectroscopic and molecular docking techniques. UV-Vis absorption spectra observed a significant hyperchromic effect with the increase of HBQ concentration. The fluorescence intensity of both probe-ct-DNA decreased with the increasing concentration of HBQs, indicating that the interaction mode between each HBQs and DNA was quite complicated, and there were both minor groove binding and intercalation binding. Molecular docking showed that HBQs interacted with DNA predominantly via hydrogen bond at guanine-rich areas in the minor groove of DNA. The genotoxicity of HBQs on human hepatocytes (L-02) was evaluated by micronucleus test, and the results showed that HBQs could cause significant chromosomal damage. The rank order of HBQ isomers on DNA reactivity and genotoxicity was 2,5-HBQs > their corresponding 2,6-HBQs. QSTR analysis found that dipole moment is the key structural descriptor influencing both DNA reactivity and genotoxicity of HBQ isomers. This study suggested that HBQs have caused genotoxicity which was influenced by their isomeric effects, warranting a comprehensive understanding of the genotoxic and carcinogenic risks associated with HBQs exposure.

Oxidative Stress Responses and Gene Transcription of Mice under Chronic-Exposure to 2,6-Dichlorobenzoquinone

2,6-Dichlorobenzoquinone (2,6-DCBQ), as an emerging disinfection by-production, was frequently detected and identified in the drinking water; however, limited information is available for the toxic effect of 2,6-DCBQ on mice. In the present study, adult mice were used to assess the impact of 2,6-DCBQ via measuring the responses of antioxidant enzymes (superoxide dismutase (SOD) and catalase (CAT)), the key genes (Heme oxygenase-1 (HO-1), NADPH quinone oxidoreductase 1 (NQO1) and glutamate-L-cysteine ligase catalytic subunit (GCLC)) in the Nrf2-keap1 pathway, and lipid peroxidation (malonaldehyde, MDA). Our results clearly indicated that 2,6-DCBQ decreased the activities of SOD and CAT, repressed the transcriptional levels of key genes in Nrf2-keap1 pathway, further caused oxidative damage on mice. These results provided evidence for assessing the threat of 2,6-DCBQ on human.

Effects of ultrasonication on the DBP formation and toxicity during chlorination of saline wastewater effluents

Chlorine disinfection of saline wastewater effluents rich in bromide and iodide forms relatively toxic brominated and iodinated disinfection byproducts (DBPs). Ultrasonication is a relatively new water treatment technology, and it is less sensitive to suspended solids in wastewaters. In this study, we examined the effects of ultrasonication (in terms of reactor type and combination mode with chlorination) on the DBP formation and toxicity in chlorinated primary and secondary saline wastewater effluents. Compared with the chlorinated wastewater effluent samples without ultrasonication, ultrasonic horn pretreatment of the wastewater effluent samples reduced the total organic halogen (TOX) levels in chlorination by ∼30%, but ultrasonic bath pretreatment of the wastewater samples did not significantly change the TOX levels in chlorination, which might be attributed to the higher energy utilization and decomposition extent of organic DBP precursors in the ultrasonic horn reactor. Moreover, the TOX levels in the chlorinated samples with ultrasonic horn pretreatment (USH-chlorination), simultaneous treatment (chlorination+USH) and subsequent treatment (chlorination-USH) were also significantly reduced, with the maximum TOX reductions occurring in the samples with ultrasonic horn pretreatment. A toxicity index was calculated by weighting and summing the levels of total organic chlorine, total organic bromine and total organic iodine in each treated sample. The calculated toxicity index values of the chlorinated wastewater effluent samples followed a descending rank order of "chlorination" > "chlorination+USH" > "chlorination-USH" > "USH-chlorination", with the lowest toxicity occurring in the samples with ultrasonic horn pretreatment. Then, a developmental toxicity bioassay was conducted for each treated sample. The measured toxicity index values of the chlorinated wastewater samples followed the same descending rank order.

Halohydroxybenzonitriles as a new group of halogenated aromatic DBPs in drinking water: Are they of comparable risk to halonitrophenols?

Halogenated aromatic disinfection byproducts (DBPs) exhibited similar total organic halogen levels in chlorinated drinking water samples as compared with aliphatic ones, and they predominantly accounted for the overall toxicity of the samples. Among the reported halogenated aromatic DBPs, halonitrophenols (HNPs) have received particular attention in recent years due to the relatively high risk in drinking water. In this study, a new group of halogenated aromatic DBPs were detected and then proposed to be halohydroxybenzonitriles (HHBNs) by employing the ultra-performance liquid chromatography/tandem mass spectrometers. Thereafter, the specific HHBN species in drinking water were theoretically speculated and then thoroughly identified with standard compounds. Their occurrence in drinking water was investigated, their cytotoxicity was evaluated, and their stability in the presence of chlorine was assessed. Seven newly identified HHBNs, including 3,5-dichloro-4-hydroxybenzonitrile, 3,5-dichloro-2-hydroxybenzonitrile, 5-bromo-3-chloro-4-hydroxybenzonitrile, 5-bromo-3-chloro-2-hydroxybenzonitrile, 3,5-dibromo-4-hydroxybenzonitrile, 3,5-dibromo-2-hydroxybenzonitrile, and 3,5-diiodo-4-hydroxybenzonitrile, showed 100% detection frequency in the collected drinking water samples with concentrations up to 36 ng/L. HHBNs exhibited significantly higher cytotoxicity in Chinese hamster ovary cells than regulated DBPs (e.g., trihalomethanes and haloacetic acids), which might be contributed by their cellular uptake efficiency and nucleophilicity. The seven HHBNs were proved to undergo transformation during chlorination following pseudo-first-order decay with half-lives in the range of 9-63 h. More importantly, in comparison to HNPs, which showed relatively high toxicity and strong stability among the halogenated aromatic DBPs, HHBNs presented comparable concentration-cytotoxicity contribution (50%) and slightly weaker stability (43%), suggesting that HHBNs should be a new group of DBPs of concern in drinking water.

Halonaphthoquinones: A group of emerging disinfection byproducts of high toxicity in drinking water

Aromatic halogenated disinfection byproducts (DBPs) have received particular attention in recent years due to their high toxicity. However, most relevant researches at present focused merely on halo-monocyclic DBPs, while halo-polycyclic DBPs were scarcely explored. In this study, a new group of halo-bicyclic DBPs termed as halonaphthoquinones (HNQs) was systematically studied. By coupling with vacuum centrifugal concentrator, a SPE-UPLC-MS/MS method with high accuracy and sensitivity was developed to detect five semi-volatile HNQs in drinking water, which achieved the detection limits in the range of 0.05-0.24 ng/L. Five HNQs were identified using this method with 100% detection frequency at concentrations up to 136.7 ng/L in drinking water originated from seven water treatment plants. The cytotoxicity of the five tested HNQs in CHO-K1 cells (IC₅₀ from 3.17 to 13.18 μM) was comparable to the most toxic known carbonaceous DBP in drinking water, iodoacetic acid (IC₅₀=2.95 μM). Meanwhile, the cytotoxicity of five tested HNQs were also higher than 2,6-dichloro-1,4-benzoquinone (IC₅₀=21.73 μM) which is hundreds to thousands of times more toxic than regulated DBPs, indicating the significant toxicity risk of HNQ DBPs. To the best of our knowledge, this study presents the first analytical method for analysis of HNQ DBPs, and the first set of data on the occurrence and cytotoxicity of HNQ DBPs in drinking water. These findings are meaningful for probing deeply into the presence of varied halo-polycyclic DBPs in the aqueous environment.

Formation of chlorinated halobenzoquinones during chlorination of free aromatic amino acids

Chlorinated halobenzoquinones (HBQs) widely exist in drinking water as emerging disinfection byproducts (DBPs), which have attracted significant attention due to their wide occurrence and high toxicity. In this study, the formation of chlorinated HBQs from the three free aromatic amino acids, tryptophan (Trp), tyrosine (Tyr) and phenylalanine (Phe), during chlorination was investigated, the formation pathways of chlorinated HBQs were explained based on the detected intermediates and influence factors. The results revealed that four chlorinated HBQs, including 2,6-dichloro-1,4-benzoquinone (2,6-DCBQ), 2,3,5-trichloro-1,4-benzoquinone, 2,3,5,6-tetrachloro-1,4-benzoquinone and 2,6-dichloro-3-methyl-1,4-benzonquinone, were formed in chlorination of the three free aromatic amino acids, and 2,6-DCBQ was the dominant species among the formed chlorinated HBQs. Of the three free aromatic amino acids, Trp and Tyr presented relatively high yields of chlorinated HBQs than Phe. Moreover, ten intermediates were successfully detected (e.g., N,2-dichloroaniline from Trp, 2,4,6-trichlorophenol from Tyr) according to the isotope and fragment information obtained using high resolution mass spectrometry. The formation pathways of chlorinated HBQs from Trp and Tyr were proposed to include electrophilic addition, electrophilic substitution, oxidation, deacidification and dehydration reaction, and further validated using theoretical calculation. The yields of chlorinated HBQs during chlorination of the free aromatic amino acids were significantly affected by free chlorine dosage, pH and temperature.

Halogen substitution reactions of halobenzenes during water disinfection

Although being successfully applied all over the world for more than 100 years water disinfection by means of chlorination possesses certain drawbacks, first of all formation of hazardous disinfection by-products (DBP). Aromatic halogenated DBPs significantly contribute to the total organic halogen and developmental toxicity of chlorinated water. The present study deals with investigation of possible substitution of one halogen for another in aromatic substrates in conditions of aqueous chlorination/bromination. The reaction showed high yields especially in case of substrates with proper position of an activating group in the aromatic ring. Thus, ipso-substitution of iodine by chlorine is the main process of aqueous chlorination of para-iodoanisole. Oxidation of the eliminating I⁺ ions into non-reactive IO₃^- species facilitates the substitution. Oxidation of eliminating Br⁺ is not so easy while being highly reactive it attacks initial substrates forming polybrominated products. Substitution of iodine and bromine by chlorine may also involve migration of electrophilic species inside the aromatic ring resulting in larger number of isomeric DBPs. Substitution of chlorine by bromine in aromatic substrates during aqueous bromination is not so pronounced as substitution of bromine by chlorine in aqueous chlorination due to higher electronegativity of chlorine atom. However, formation of some chlorine-free polybrominated products proves possibility of that process.

Immune dysfunction induced by 2,6-dichloro-1,4-benzoquinone, an emerging water disinfection byproduct, due to the defects of host-microbiome interactions

2,6-dichloro-1,4-benzoquinone (DCBQ), as an emerging water disinfection byproducts (DBPs), has posed potential risks via the digestion system. However, little is known about the toxicity of DCBQ on the gut microbiome, which plays a critical role on human health. This study has comprehensively investigated the impact of DCBQ on the intestinal microbiome, metabolic functions, and immunity after the mice orally exposure to DCBQ at the concentration of 31.25, 62.5 and 125 mg/kg body weight for 28 days. Our results indicated that DCBQ exposure has perturbed the balance between T helper (Th) 1 mediated pro-inflammatory response and Th2 mediated anti-inflammatory response in mice, especially inducing the activation of immune system toward a Th2 response. DCBQ group has induced gut microbiota dysbiosis, and at phylum level, Proteobacteria was relatively less abundant compared with that in the control group. Furthermore, DCBQ exposure has dramatically perturbed metabolites profiles which were involved in 28 metabolic pathways, such as amino acids biosynthesis and metabolism, lipid metabolism. In particular, the altered gut microbiota showed strong correlations with both the altered metabolites and the altered immunological variables after DCBQ exposure. This study provides evidence on the adverse effects and mechanisms of water disinfection byproduct DCBQ through the interaction of immune-microbiome-metabolome, highlighting the importance to assess DBPs-associated risks.

Formation and Cytotoxicity of Halophenylacetamides: A New Group of Nitrogenous Aromatic Halogenated Disinfection Byproducts in Drinking Water

Nitrogenous aromatic halogenated disinfection byproducts (DBPs) in drinking water have received considerable attention recently owing to their relatively high toxicity. In this study, a new group of nitrogenous aromatic halogenated disinfection byproducts, halophenylacetamides (HPAcAms), were successfully identified for the first time in both the laboratory experiments and realistic drinking water. The formation mechanism of HPAcAms during chlorination of phenylalanine in the presence of Br^- and I^-, occurrence frequencies, and concentrations in authentic drinking water were investigated, and a quantitative structure-activity relationship (QSAR) model was developed based on the acquired cytotoxicity data. The results demonstrated that HPAcAms could be formed from phenylalanine in chlorination via electrophilic substitution, decarboxylation, hydrochloric acid elimination, and hydrolysis. The HPAcAm yields from phenylalanine were significantly affected by contact time, pH, chlorine dose, and temperature. Nine HPAcAms with concentrations in the range of 0.02-1.54 ng/L were detected in authentic drinking water samples. Most tested HPAcAms showed significantly higher cytotoxicity compared with dichloroacetamide, which is the most abundant aliphatic haloacetamide DBP. The QSAR model demonstrated that the cellular uptake efficiency and the polarized distributions of electrons of HPAcAms play essential roles in their cytotoxicity mechanisms.

Sequential ClO2-UV/chlorine process for micropollutant removal and disinfection byproduct control

This study systematically revealed the feasibility of the sequential ClO₂-UV/chlorine process for micropollutant removal and disinfection byproduct (DBP) control. The results demonstrated that the sequential ClO₂-UV/chlorine process was effective for the removal of 12 micropollutants. ClO₂ pre-treatment reduced the formation of disinfect byproducts (DBPs) in the UV/chlorine process. Compared to the UV/chlorine process, ClO₂ pre-treatment (1.0 mg L^-1) decreased the formation of the 6 DBPs by 25.1-72.2%; and decreased the formation potential of the 6 DBPs by 13.9-51.8%. Moreover, ClO₂ pre-treatment reduced the concentration of total organic chlorine by 19.8%. ClO₂ pre-treatment affected the UV/chlorine process in different ways. Firstly, ClO₂ pre-treatment generated chlorite, which dominantly served as a scavenger of chlorine radical (Cl) and hydroxyl radical (HO). Secondly, ClO₂ pre-treatment decreased the reactivity of natural organic matter (NOM) towards radicals. Finally, ClO₂ pre-treatment altered the properties of NOM, in terms of reducing the electron-donating capacity and aromaticity of NOM (SUVA₂₅₄), and slightly reducing the average molecular weight of NOM. Overall, ClO₂ pre-treatment effectively controlled the formation of DBPs in the UV/chlorine process. This study confirmed the sequential ClO₂-UV/chlorine process was an alternative strategy to balancing the micropollutant removal and DBP control.

Comparative Toxicity Analyses from Different Endpoints: Are New Cyclic Disinfection Byproducts (DBPs) More Toxic than Common Aliphatic DBPs?

In recent years, dozens of halogenated disinfection byproducts (DBPs) with cyclic structures were identified and detected in drinking water globally. Previous in vivo toxicity studies have shown that a few new cyclic DBPs possessed higher developmental toxicity and growth inhibition rate than common aliphatic DBPs; however, in vitro toxicity studies have proved that the latter exhibited higher cytotoxicity and genotoxicity than the former. Thus, to provide a more comprehensive toxicity comparison of DBPs from different endpoints, 11 groups of cyclic DBPs and nine groups of aliphatic DBPs were evaluated for their comparative in vitro and in vivo toxicity using human hepatoma cells (Hep G2) and zebrafish embryos. Notably, results showed that the in vitro Hep G2 cytotoxicity index of the aliphatic DBPs was nearly eight times higher than that of the cyclic DBPs, whereas the in vivo zebrafish embryo developmental/acute toxicity indexes of the cyclic DBPs were roughly 48-50 times higher than those of the aliphatic DBPs, indicating that the toxicity rank order differed when different endpoints were applied. For a broader comparison, a Pearson correlation analysis of DBP toxicity data from nine different endpoints was conducted. It was found that the observed Hep G2 cytotoxicity and zebrafish embryo developmental/acute toxicity in this study were highly correlated with the previously reported in vitro CHO cytotoxicity and in vivo toxicity in aquatic organisms (P < 0.01), respectively. However, the observed in vitro toxicity had no correlation with the in vivo toxicity (P > 0.05), suggesting that the toxicity rank orders obtained from in vitro and in vivo bioassays had large discrepancies. According to the observed toxicity data in this study and the candidate descriptors, two quantitative structure-activity relationship (QSAR) models were established, which help to further interpret the toxicity mechanisms of DBPs from different endpoints.

Transformation of Organic Compounds during Water Chlorination/Bromination: Formation Pathways for Disinfection By-Products (A Review)

The purity of drinking water is an important issue of the human life quality. Water disinfection has saved millions people from the diseases spread with water. However, that procedure has a certain drawback due to formation of toxic organic disinfection products. Establishing the structures of these products and the mechanisms of their formation and diminishing their levels in drinking water represent an important task for chemistry and medicine, while mass spectrometry is the most efficient tool for the corresponding studies. The current review throws light upon natural and anthropogenic sources of the formation of disinfection by-products (DBPs) and the mechanisms of their formation related to the structural peculiarities and the presence of functional groups. In addition to chlorination, bromination is discussed since it is used quite often as an alternative method of disinfection, particularly, for the purification of swimming pool water. The benefits of the contemporary GC/MS and LC/MS methods for the elucidation of DBP structures and study of the mechanisms of their formation are discussed. The reactions characteristic for various functional groups and directions of transformation of certain classes of organic compounds in conditions of aqueous chlorination/bromination are also covered in the review.

Synergistic cytotoxicity of bromoacetic acid and three emerging bromophenolic disinfection byproducts against human intestinal and neuronal cells

Halogenated disinfection byproducts (halo-DBPs) are drinking water contaminants of great public health concern. Nine haloaliphatic DBPs have been regulated by the U.S. Environmental Protection Agency and various halophenolic compounds have been identified as emerging DBPs. In this study, we evaluated the cytotoxic interactions of the regulated bromoacetic acid and three emerging bromophenolic DBPs, i.e., 2,4,6-tribromophenol, 3,5-dibromo-4-hydroxybenzoic acid, and 3,5-dibromo-4-hydroxybenzaldehyde. Cytotoxicity was measured for each DBP individually as well as each of their mixtures using in vitro human epithelial colorectal adenocarcinoma (Caco-2) and neuroblastoma (SH-SY5Y) cells. Concentration addition (CA) model and isobolographic analysis were employed to characterize the interactions among the DBPs. Our results show that the cytotoxicity of four bromo-DBPs against both cell-types followed the descending rank order of bromoacetic acid > 2,4,6-tribromophenol > 3,5-dibromo-4-hydroxybenzaldehyde > 3,5-dibromo-4-hydroxybenzoic acid. Compared with the toxicity data in literature, our finding that bromoacetic acid showed higher cytotoxicity than bromophenolic DBPs was consistent with the results from Chinese hamster ovary cells (a commonly used in vitro model of DBP toxicological studies); but different from the results obtained from in vivo biological models. Significantly, with CA model prediction, we found that mixtures of four bromo-DBPs exhibited synergistic cytotoxic effects on both human cell types. Isobolographic analysis of binary DBP mixtures revealed that, for Caco-2 cells, bromoacetic acid, 2,4,6-tribromophenol, and 3,5-dibromo-4-hydroxybenzoic acid induced synergism; for SH-SY5Y cells, bromoacetic acid induced synergism with all three bromophenolic DBPs. The production of reactive oxidative species (ROS) induced by DBP mixtures could be an important reason for the synergistic cytotoxicity.

Haloquinone Chloroimides as Toxic Disinfection Byproducts Identified in Drinking Water

Haloquinone chloroimides (HQCs) are suspected to be highly toxic contaminants, and their production during drinking water disinfection is predicted. However, HQC disinfection byproducts (DBPs) have not been reported in drinking water to date because of analytical limitations. In this study, we developed an analytical method to detect five HQCs, including 2,6-dichloroquinone-4-chloroimide (2,6-DCQC), 2,6-dibromoquinone-4-chloroimide (2,6-DBQC), 2-chloroquinone-4-chloroimide (2-CQC), 3-chloroquinone-4-chloroimide (3-CQC), and 2,6-dichloroquinone-3-methyl-chloroimide (2,6-DCMQC). This method combined a derivatization reaction of HQCs with phenol in alkaline solutions to produce halogenated indophenols, a solid-phase extraction pretreatment using hydrophilic-lipophilic balanced (HLB) cartridges, and a multiple reaction monitoring (MRM) method for quantification. The method was demonstrated to be sensitive and accurate with recoveries of 71-85% and limits of detection of 0.1-0.2 ng/L for the five tested HQCs. Using this method, five tested HQCs were identified in drinking water samples from nine water treatment plants and water distribution systems as new DBPs at concentrations of up to 23.1 ng/L. The cytotoxicity of the five tested HQCs in HepG2 cells was higher than or comparable to that of 2,6-dichloro-1,4-benzoquinone (2,6-DCBQ), an emerging DBP that was hundreds to thousands of times more toxic than regulated DBPs. This study presents the first analytical method for HQC DBPs in drinking water and the first set of occurrence and cytotoxicity data of HQC DBPs.

Identification of chloramination disinfection by-products from phenylalanine in tap drinking water

Phenylalanine (Phe) is widely present in natural water and serves as a precursor of disinfection by-products (DBPs). We reported the identification of chloramination DBPs from Phe in drinking water using ultra-high performance liquid chromatography (UHPLC) coupled with complementary high-resolution quadrupole time-of-flight (QTOF) and triple quadrupole (tQ) tandem mass spectrometry (MS/MS). In the chloraminated Phe water solution, sixteen new DBPs in a total of seventeen were identified based on their accurate mass, MS/MS spectra and ³⁵Cl/³⁷Cl isotopic patterns. Three of these DBPs were verified as benzamide, phenylacetamide, and p-hydroxyphenylacetamide with their standards, while the others were chlorinated derivatives of Phe, hydrazone, amidine, amide and peroxide, in which the unique structures of these DBPs were rarely reported. Their stability and formation process were investigated as well. Furthermore, a method consisting of solid phase extraction (SPE) and UHPLC-MS/MS using dynamic multiple reaction monitoring (dMRM) was developed to investigate these DBPs in authentic waters. Phe, benzamide, phenylacetamide, and N-Cl-2-phenylacetimidamide were detected in chlorinated tap water. Compared with the other identified DBPs, these three DBPs were exceptionally stable and could be formed in wide formation conditions. Our work not only provided ideas for the identification of new chloramination DBPs, but also demonstrated that some DBPs usually generated in the chloramination disinfection process could also be found in the chlorinated drinking water.