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

Comprehensive comparison of water quality risk and microbial ecology between new and old cast iron pipe distribution systems

The effects of cast iron pipe corrosion on water quality risk and microbial ecology in drinking water distribution systems (DWDSs) were investigated. It was found that trihalomethane (THMs) concentration and antibiotic resistance genes (ARGs) increased sharply in the old DWDSs. Under the same residual chlorine concentration conditions, the adenosine triphosphate concentration in the effluent of old DWDSs (Eff-old) was significantly higher than that in the effluent of new DWDSs. Moreover, stronger bioflocculation ability and weaker hydrophobicity coexisted in the extracellular polymeric substances of Eff-old, meanwhile, iron particles could be well inserted into the structure of the biofilms to enhance the mechanical strength and stability of the biofilms, hence enhancing the formation of THMs. Old DWDSs significantly influenced the microbial community of bulk water and triggered stronger microbial antioxidant systems response, resulting in higher ARGs abundance. Corroded cast iron pipes induced a unique interaction system of biofilms, chlorine, and corrosion products. Therefore, as the age of cast iron pipes increases, the fluctuation of water quality and microbial ecology should be paid more attention to maintain the safety of tap water.

An effective and rapidly degradable disinfectant from disinfection byproducts

Chloroxylenol is a worldwide commonly used disinfectant. The massive consumption and relatively high chemical stability of chloroxylenol have caused eco-toxicological threats in receiving waters. We noticed that chloroxylenol has a chemical structure similar to numerous halo-phenolic disinfection byproducts. Solar detoxification of some halo-phenolic disinfection byproducts intrigued us to select a rapidly degradable chloroxylenol alternative from them. In investigating antimicrobial activities of disinfection byproducts, we found that 2,6-dichlorobenzoquinone was 9.0-22 times more efficient than chloroxylenol in inactivating the tested bacteria, fungi and viruses. Also, the developmental toxicity of 2,6-dichlorobenzoquinone to marine polychaete embryos decreased rapidly due to its rapid degradation via hydrolysis in receiving seawater, even without sunlight. Our work shows that 2,6-dichlorobenzoquinone is a promising disinfectant that well addresses human biosecurity and environmental sustainability. More importantly, our work may enlighten scientists to exploit the slightly alkaline nature of seawater and develop other industrial products that can degrade rapidly via hydrolysis in seawater.

Dihalogenated nitrophenols in drinking water: Prevalence, resistance to household treatment, and cardiotoxic impact on zebrafish embryo

Dihalogenated nitrophenols (2,6-DHNPs), an emerging group of aromatic disinfection byproducts (DBPs) detected in drinking water, have limited available information regarding their persistence and toxicological risks. The present study found that 2,6-DHNPs are resistant to major drinking water treatment processes (sedimentation and filtration) and households methods (boiling, filtration, microwave irradiation, and ultrasonic cleaning). To further assess their health risks, we conducted a series of toxicology studies using zebrafish embryos as the model organism. Our findings reveal that these emerging 2,6-DHNPs showed lethal toxicity 248 times greater than that of the regulated DBP, dichloroacetic acid. Specifically, at sublethal concentrations, exposure to 2,6-DHNPs generated reactive oxygen species (ROS), caused apoptosis, inhibited cardiac looping, and induced cardiac failure in zebrafish. Remarkably, the use of a ROS scavenger, N-acetyl-l-cysteine, considerably mitigated these adverse effects, emphasizing the essential role of ROS in 2,6-DHNP-induced cardiotoxicity. Our findings highlight the cardiotoxic potential of 2,6-DHNPs in drinking water even at low concentrations of 19 μg/L and the beneficial effect of N-acetyl-l-cysteine in alleviating the 2,6-DHNP-induced cardiotoxicity. This study underscores the urgent need for increased scrutiny of these emerging compounds in public health discussions.

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.

Toxicity of urban stormwater on Chlorella pyrenoidosa: Implications for reuse safety

Urban stormwater is an alternative water source used to mitigate water resource shortages, and ensuring the safety of stormwater reuse is essential. An in-depth understanding of both individual pollutant concentrations/loads in stormwater and holistic stormwater quality can be used to comprehensively evaluate how safely stormwater can be reused. The toxicity test takes all pollutants present in water samples into account, and the results reflect the integrated effect of these pollutants. In this study, the influence of urban stormwater sourced from different land uses on microalgae (Chlorella pyrenoidosa) and the possible toxicity mechanisms were investigated. The results showed that urban stormwater, particularly residential road stormwater, significantly inhibited microalgal growth. The chlorophyll contents of microalgae exposed to residential road stormwater were relatively lower, while the corresponding values were relatively higher for microalgae exposed to grassland road stormwater. Additionally, the antioxidant-related metabolism of microalgae could be dysregulated due to exposure to urban stormwater. A possible toxicity mechanism is that urban stormwater influences metabolic pathways related to chlorophyll synthesis and further hinders photosynthesis and hence microalgal growth. To resist oxidative stress and maintain regular microalgal cell activities, the ribosome metabolism pathway was upregulated. The research results contribute to elucidating the toxicity effects of urban stormwater and hence provide useful insight for ensuring the safety of stormwater reuse. It is also worth noting that the study outcomes can only represent the influence of land use on stormwater toxicity, while the impacts of other factors (particularly rainfall-runoff characteristics) have not been considered. Therefore, the consideration of all influential factors of stormwater is strongly recommended to generate more robust results in the future and provide more effective guidance for real practices related to stormwater reuse safety.

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.

ChloroDBPFinder: Machine Learning-Guided Recognition of Chlorinated Disinfection Byproducts from Nontargeted LC-HRMS Analysis

High-resolution mass spectrometry (HRMS) is a prominent analytical tool that characterizes chlorinated disinfection byproducts (Cl-DBPs) in an unbiased manner. Due to the diversity of chemicals, complex background signals, and the inherent analytical fluctuations of HRMS, conventional isotopic pattern (37Cl/35Cl), mass defect, and direct molecular formula (MF) prediction are insufficient for accurate recognition of the diverse Cl-DBPs in real environmental samples. This work proposes a novel strategy to recognize Cl-containing chemicals based on machine learning. Our hierarchical machine learning framework has two random forest-based models: the first layer is a binary classifier to recognize Cl-containing chemicals, and the second layer is a multiclass classifier to annotate the number of Cl present. This model was trained using ∼1.4 million distinctive MFs from PubChem. Evaluated on over 14,000 unique MFs from NIST20, this machine learning model achieved 93.3% accuracy in recognizing Cl-containing MFs (Cl-MFs) and 92.9% accuracy in annotating the number of Cl for Cl-MFs. Furthermore, the trained model was integrated into ChloroDBPFinder, a standalone R package for the streamlined processing of LC-HRMS data and annotating both known and unknown Cl-containing compounds. Tested on existing Cl-DBP data sets related to aspartame chlorination in tap water, our ChloroDBPFinder efficiently extracted 159 Cl-containing DBP features and tentatively annotated the structures of 10 Cl-DBPs via molecular networking. In another application of a chlorinated humic substance, ChloroDBPFinder extracted 79 high-quality Cl-DBPs and tentatively annotated six compounds. In summary, our proposed machine learning strategy and the developed ChloroDBPFinder provide an advanced solution to identifying Cl-containing compounds in nontargeted analysis of water samples. It is freely available on GitHub (https://github.com/HuanLab/ChloroDBPFinder).

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.

Differences in the degradation behavior of disinfection by-products in UV/PDS and UV/H2O2 processes and the effect of their chemical properties

In this work, sixteen typical chlorinated and brominated aromatic disinfection by-products (DBPs) were selected as examples to investigate their different degradation mechanisms initiated by HO• and SO4•-. Addition reactions were the main mode of degradation of DBPs by HO•, while SO4•- dominated H-abstraction reactions and single electron transfer reactions. Chlorinated compounds had higher reactivity than brominated compounds. Furthermore, substituents with stronger electron-donating effects promoted the electrophilic reaction of DBPs with the two radicals. In addition, we developed a model based on the chemical properties LUMO, fmax-, and hardness for predicting the average reaction energy barriers for the initial reactions of DBPs with HO• and SO4•-. The model had good predictive performance for the difficulty of degradation of different DPBs by HO• and SO4•-, with R2 values of 0.85 and 0.87, respectively. Through the degradation efficiency simulation, we found that longer reaction times, higher oxidant concentrations and lower pollutant concentrations were more favorable for the removal of DBPs. The UV/PDS process showed better degradation of DBPs than the UV/H2O2 process. In addition, most degradation products of DBPs exhibited less toxicity to aquatic organisms than their parent compounds. This study provided theoretical guidance for the degradation and removal of other aromatic DBPs at the molecular level.

Recent progress in identification of water disinfection byproducts and opportunities for future research

Numerous disinfection by-products (DBPs) are formed from reactions between disinfectants and organic/inorganic matter during water disinfection. More than seven hundred DBPs that have been identified in disinfected water, only a fraction of which are regulated by drinking water guidelines, including trihalomethanes, haloacetic acids, bromate, and chlorite. Toxicity assessments have demonstrated that the identified DBPs cannot fully explain the overall toxicity of disinfected water; therefore, the identification of unknown DBPs is an important prerequisite to obtain insights for understanding the adverse effects of drinking water disinfection. Herein, we review the progress in identification of unknown DBPs in the recent five years with classifications of halogenated or nonhalogenated, aliphatic or aromatic, followed by specific halogen groups. The concentration and toxicity data of newly identified DBPs are also included. According to the current advances and existing shortcomings, we envisioned future perspectives in this field.

Occurrence, stability and cytotoxicity of halobenzamides: A new group of nitrogenous disinfection byproducts in drinking water

Exploring disinfection byproducts (DBPs) with adverse health effects in drinking water is a constant challenge. Halobenzamides (HBZAMs) are suspected to be a new group of nitrogenous DBPs but have not been reported in drinking water to date. In this study, by coupling SPE and UPLC‒MS/MS, a sensitive method was established to detect eight HBZAMs in drinking water with recoveries and limits of detection of 80-103% and 0.01-0.04 ng/L, respectively. Subsequently, distinct fragments of HBZAMs were extended to the development of a pseudotargeted method for the analysis of the fourteen HBZAMs that were speculated and lack chemical standards. Using the developed method, eight HBZAMs were quantified in ten drinking water samples with concentrations ranging from 2.4 to 7.2 ng/L and a detection frequency of 100%, among which five HBZAMs were stable with half-lives over 72 h under real chlorine levels. Twelve HBZAMs without standards were identified in three to ten drinking water samples with comparable levels. The cytotoxicity of eight quantified HBZAMs in CHO-K1 cells varied with disparity, in which the cytotoxicity of 3,5-DBBZAM was over 10-fold higher than that of aliphatic dichloroacetamide. Considering their diversity, toxicity and stability, the occurrence of HBZAMs in drinking water deserves attention.

Facile synthesis of Ag/ZIF-8@ZIF-67 as an electrochemical sensing platform for sensitive detection of halonitrophenols in drinking water

Halonitrophenols (HNPs) are an emerging type of aromatic disinfection byproduct, with detected concentrations of ∼nmol L-1 in source water and drinking water. Currently, there are no standard methods for identifying HNPs, and most of the reported methods are time-consuming and equipment-dependent. A core-shell metal-organic framework (MOF) based electrochemical sensor (Ag/ZIF-8@ZIF-67) capable of detecting 2,6-dichloro-4-nitrophenol (2,6-DCNP) is reported in this study. The electrochemical sensor obtains the concentration of 2,6-DCNP by detecting the peak current passing through the sensor. In this sensor, Ag nanoparticles (AgNPs) play a key role in electrochemical sensing by reducing nitro groups via electron transfer, and porous structure with a large surface area is offered by ZIF-8@ZIF-67. The cyclic voltammetry (CV) response of Ag/ZIF-8@ZIF-67 was found to be approximately 1.75 times and 2.23 times greater than that of Ag/ZIF-8 and Ag/ZIF-67, respectively, suggesting an ideal synergistic effect of the core-shell structures. The Ag/ZIF-8@ZIF-67 sensor exhibited exceptional sensitivity to 2,6-DCNP, exhibiting a broad linear response range (R2 = 0.992) from 240 nmol L-1 to 288 μmol L-1 and a low detection limit of 20 nmol L-1. Furthermore, the sensor exhibited good anti-interference for isomers and common distractors in water, excellent stability and reproducibility, and high recovery in actual water samples. Our reported sensor gives a novel strategy for sensitive, selective, and in situ detection of 2,6-DCNP in practical analysis.

Formation characteristics and acute toxicity assessment of THMs and HAcAms from DOM and its different fractions in source water during chlorination and chloramination

The formation characteristics of trihalomethanes (THMs) and haloacetamides (HAcAms) from dissolved organic matter and its fractions were investigated during chlorine-based disinfection processes. The relationships between water quality parameters, fluorescence parameters, and the formation levels of THMs and HAcAms were analyzed. The fractions contributing most to the acute toxicity were identified. The trichloromethane (TCM) generation level (72 h) generally followed the order of Cl₂ > NH₂Cl > NHCl₂ process. The NHCl₂ process was superior to the NH₂Cl process in controlling TCM formation. Hydrophobic acidic substance (HOA), hydrophobic neutral substance (HON), and hydrophilic substance (HIS) were identified as primary precursors of 2,2-dichloroacetamide and trichloroacetamide during chlorination and chloramination. The formation of TCM mainly resulted from HOA, HON and HIS fractions relatively uniformly, while HOA and HIS fractions contributed more to the formation of bromodichloromethane and dibromomonochloromethane. UV₂₅₄ could be used as an alternative indicator for the amount of ΣTHMs formed during chlorination and chloramination processes. Dissolved organic nitrogen was a potential precursor of 2,2-dichloroacetamide during chlorination process. The fractions with the highest potential acute toxicity after the chlorination were water-dependent.

Comparative cytotoxicity analyses of disinfection byproducts in drinking water using dimensionless parameter scaling method: Effect of halogen substitution type and number

Up to date, over 700 disinfection byproducts (DBPs) have been detected and identified in drinking water. It has been recognized that cytotoxicity of DBPs varied significantly among groups. Even within the same group, cytotoxicity of different DBP species was also different due to different halogen substitution types and numbers. However, it is still difficult to quantitatively determine the inter-group cytotoxicity relationships of DBPs under the effect of halogen substitution in different cell lines, especially when a large number of DBP groups and multiple cytotoxicity cell lines are involved. In this study, a powerful dimensionless parameter scaling method was adopted to quantitatively determine the relationship of halogen substitution and the cytotoxicity of various DBP groups in three cell lines (i.e., the human breast carcinoma (MVLN), Chinese hamster ovary (CHO), and human hepatoma (Hep G2) cell cytotoxicity) with no need to consider their absolute values and other influences. By introducing the dimensionless parameters D_{x-orn-species}^cellline and D¯_{x-orn-species}^cellline, as well as their corresponding linear regression equation coefficients k_typeornumber^cellline and k¯_typeornumber^cellline, the strength and trend of halogen substitution influences on the relative cytotoxic potency could be determined. It was found that the effect of halogen substitution type and number on the cytotoxicity of DBPs followed the same patterns in the three cell lines. The CHO cell cytotoxicity was the most sensitive cell line to evaluate the effect of halogen substitution on the aliphatic DBPs, whereas the MVLN cell cytotoxicity was the most sensitive cell line to evaluate the effect of halogen substitution on the cyclic DBPs. Notably, seven quantitative structure activity relationship (QSAR) models were established, which could not only predict the cytotoxicity data of DBPs, but also help to explain and verify the patterns of halogen substitution effect on cytotoxicity of DBPs.

Role of UV-based advanced oxidation processes on NOM alteration and DBP formation in drinking water treatment: A state-of-the-art review

Oxidative treatment of drinking water has been practiced for more than a century. UV-based advanced oxidation processes (UV-AOPs) have emerged as promising oxidative treatment technologies to eliminate recalcitrant chemicals and biological contaminants in drinking water. UV-AOPs inevitably alter the properties of natural organic matter (NOM) and affect the disinfection byproduct (DBP) formation in the post-disinfection. This paper provides a state-of-the-art review on the effects of UV-AOPs on the changes of NOM properties and the consequent impacts on DBP formation in the post-chlorination process. A tutorial review to the connotations of NOM properties (e.g., bulk properties, fractional constituents, and molecular structures) and the associated state-of-the-art analytical methods are firstly presented. The impacts of different radical-based AOPs on the changes of NOM properties together with the underlying NOM-radical reaction mechanisms are discussed. The impacts of alteration of NOM properties on DBP formation in the post-chlorination process are then reviewed. The current knowledge gaps and future research needs are finally presented, with emphases on the needs to strengthen the comparability of research data in literature, the accuracy in quantifying the reactive moieties of NOM, and the awareness of unknown DBPs in oxidative water treatment processes. The review and discussion improve the fundamental understanding of NOM-radical and NOM-chlorine chemistry. They also provide useful implications on the engineering design and operation of next-generation drinking water treatment plants.

Occurrence and Cytotoxicity of Aliphatic and Aromatic Halogenated Disinfection Byproducts in Indoor Swimming Pool Water and Their Incoming Tap Water

Disinfection byproducts (DBPs) in swimming pool water are of wide concern for public health. In this study, the occurrence of five categories of aliphatic halogenated DBPs, i.e., trihalomethanes (THMs), haloacetic acids (HAAs), haloacetonitriles (HANs), halonitromethanes (HNMs), and haloketones (HKs), and six categories of aromatic halogenated DBPs, i.e., halophenols (HPs), halonitrophenols (HNPs), halohydroxy-benzaldehydes (HBALs), halohydroxybenzoic acids (HBAs), halobenzoquinones (HBQs), and haloanilines (HAs), was examined in seven indoor swimming pool water and their incoming tap water. The correlations between the DBP concentrations and water quality parameters were explored. Moreover, the cytotoxicity of the aliphatic and aromatic halogenated DBPs was tested with human hepatoma (HepG2) cells, and the concentration-cytotoxicity contributions of different DBP categories were calculated. The results demonstrate that 24 aliphatic (5 THMs, 8 HAAs, 5 HANs, 4 HNMs, and 2 HKs) and 50 aromatic halogenated DBPs (9 HPs, 8 HNPs, 9 HBALs, 8 HBAs, 11 HBQs, and 5 HAs) were present in the swimming pool water, among which 41 aromatic halogenated DBPs were detected in swimming pool water for the first time. The average concentrations of the five categories of aliphatic halogenated DBPs in the swimming pool water were in the order of HAAs > HANs > HKs > THMs > HNMs, while those in their incoming tap water were in the order of THMs > HAAs > HKs > HANs > HNMs. The average concentrations of the aromatic halogenated DBPs in the swimming pool water were significantly lower than those of the aliphatic halogenated DBPs, following the order of HBQs > HPs > HBAs > HBALs > HAs > HNPs, while those in their incoming tap water were in the order of HBALs > HBQs > HPs > HBAs > HAs > HNPs. The average concentration-cytotoxicity contributions of different DBP categories in the swimming pool water followed the order of HAAs > HANs > HNMs > HKs > HBQs > THMs > HPs > HNPs > HBAs > HBALs > HAs, with HAAs, HANs, and HNMs possessing the main concentration-cytotoxicity contributions (93.2% in total) among all DBP categories.

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.