Reactions of chlorine with inorganic and organic compounds during water treatment-Kinetics and mechanisms: a critical review

Efficient phosphonate degradation by Ti4O7 electrode under realistic conditions: Key contribution of HOCl and long-term stability

Using phosphorus-containing scale inhibitors (i.e., phosphonates) in cooling water poses a significant challenge to water pollution and eutrophication during discharge. Here we report an efficient electrochemical oxidation (EO) process based on a low-cost titanium suboxide (Ti4O7) inert anode. We found that ubiquitous chloride ions (Cl-) in cooling water significantly promoted the transformation of methylene phosphonic acid (NTMP, a representative phosphonate) into orthophosphate. In the Cl--Ti4O7-AO system, hypochlorous acid (HOCl) is the primary reactive species responsible for the sequential cleavage of C-P and C-C bonds, converting organic phosphorus into inorganic phosphate. Furthermore, the presence of HCO3- or humic acid (HA) under realistic conditions showed negligible effects on the degradation of NTMP. In long-term operation, the Ti4O7-AO system sustained 100% conversion efficiency of NTMP over continuous-flow operation mode for more than 720 h, even though the cathode was almost entirely covered with CaCO3 deposits. The results revealed that cathode scaling does not profoundly affect the conversion of organic phosphorus to phosphate in the Ti4O7-AO system. Overall, our study elucidates the mechanism and robust efficiency of the Cl--Ti4O7-AO system in treating phosphonate-laden wastewater and offers a new solution for dealing with cooling water by coupling electrochemical oxidation with ubiquitous chloride ions.

Breakpoint electrochlorination in ammonia removal: Unveiling the impact of convective mass transfer

Breakpoint chlorination, the point at which ammonia is completely oxidized by chlorine to nitrogen gas, may occur during electrochemical water treatment due to the simultaneous abundance of inorganic nitrogen species and chloride ions in many water matrices. Nevertheless, little is known about the difference between the chemical breakpoint chlorination and electrochemical ammonia abatement as well as the impact of the electrode-electrolyte interface that drives the breakpoint electrochlorination. This study investigates the influence of the interface on ammonia oxidation by comparing indirect breakpoint electrochlorination with the chemical approach and by examining the impact of varying convective mass transfer on breakpoint electrochlorination. Our results revealed that, under identical conditions and bulk pH, breakpoint electrochlorination releases much lower residual chlorine species in the bulk solution before ammonia is oxidized, as compared to chemical breakpoint chlorination. It was observed that lower convective mass transfer not only accelerates ammonia removal but also increases the chlorine evolution reaction. Results from a closed divided cell experiment confirmed that chlorine evolution is enhanced under lower convective mass transfer, which suggests a relevant role of species distribution within electrode-electrolyte interface. We hypothesize that this effect may be due to a more acidic local pH under lower mass transfer conditions, which favors chlorine evolution over oxygen evolution reaction. These findings provide insights into the fundamental differences of chemical breakpoint chlorination and indirect breakpoint electrochlorination. The results can guide operating strategies for electrochemical water treatment that can potentially reduce energy consumption by lowering flow speeds, while achieving higher chlorine yield and faster ammonia removal.

Transformation of cyclic amides and uracil-derived nitrogen heterocycles during chlorination

Nitrogen heterocycles are important structural components in biomolecules and anthropogenic chemicals, yet their transformation during chlorine disinfection remains poorly understood. This study investigated chlorination kinetics and product formation for six nitrogen heterocycles of increasing structural complexity, including cyclic amides (2-piperidone, glutarimide, 5,6-dihydrouracil) and uracil derivatives (uracil, uridine, and 1,3-dimethyluracil) to determine how structural variations influence reaction pathways. Apparent second-order rate constants varied widely from 9.2 × 10-3 M-1 s-1 (2-piperidone) to >103 M-1 s-1 (uracil, uridine), largely influenced by the nitrogen pKa values. Chlorination proceeded through initial N-chlorination, forming organic chloramides. While most organic chloramides were transient, that derived from 2-piperidone persisted for days under excess chlorine conditions. For saturated heterocyclic imides (glutarimide, 5,6-dihydrouracil), hydrolysis of the organic chloramides between the imide nitrogen and an adjacent acyl group rapidly formed ring-opened organic acids. Among uracil derivatives, chlorine added across the double bond. For uracil, the resulting 5-chlorouracil rapidly fragmented between the C-4 and C-5 position to release trichloroacetaldehyde at ∼100 % yield. Substitution at heterocyclic nitrogens in uridine and 1,3-dimethyluracil limited such fragmentation, forming more stable C-chlorinated heterocyclic or ring-opened products. The reaction patterns observed for these six nitrogen heterocycles were further validated using phthalimide and thymine, demonstrating the broader applicability of the identified reaction trends. These findings enhance our understanding of nitrogen heterocycle chlorination mechanisms and their implications for drinking water disinfection, offering insights into minimizing the formation of potentially harmful DBPs during chlorination.

Why hypochlorite in water cannot be detected by ion chromatography?

Free chlorine (FC), primarily existing as hypochlorite anion in alkaline water (pKa 7.5), could theoretically be analyzed by an ion chromatography (IC) system. However, the feasibility of direct FC detection by IC remains controversial in previous researches. To address this issue, this study systematically examined the fate of FC under varying IC eluent pH, anion exchange column (AEC) types, detectors, and suppression conditions. The results ultimately disprove it and show that the primary obstacle to direct FC analysis with IC system is its transformation: 31 %-41 % became Cl- and 59 %-69 % became immobile organic chloramines in AEC, due to its reaction with quaternary ammonium compounds (QACs) in AECs. Moreover, IC analysis of Cl- might be interfered by coexisting FC, although FC concentrations are typically much lower than Cl-. Eluent suppressor can also consume FC even when residual FC after AEC is limited. In contrast, variations in water pH do not affect FC stability, and a UV detector effectively distinguishes FC from other anions. This study hence for the first time elucidates several FC consumption mechanisms in IC system, which represent key barriers hindering FC analysis by IC, and accordingly provide some guidance on future efforts to develop novel AEC to overcome these barriers.

Insights into chlorination-induced degradation of sulfonylurea herbicides: Unraveling kinetics and intermediates during water treatment

Chlorination is a common method for drinking water disinfection due to its efficiency and low cost. The strong oxidative properties of chlorine can lead to reactions with dissolved organic compounds, resulting in various transformation products. This study investigates the chlorination-induced degradation of the sulfonylurea herbicides metsulfuron-methyl and chlorimuron-ethyl, which are frequently found in surface and groundwater. The degradation of these herbicides follows a second-order kinetic model. The apparent second-order rate constants for metsulfuron-methyl range from 3.2 to 244 M⁻¹ s⁻¹, while those for chlorimuron-ethyl range from 2.2 to 287.7 M⁻¹ s⁻¹ within a pH range of 4 to 9. Reaction with HClO effectively reduced the concentration of pesticides. Under acidic conditions, the reaction was significantly enhanced, likely due to hydrolysis or changes in the speciation of the organic compounds. In fact, the rate constant under acidic conditions was approximately 35 and 27 times higher than the reaction rate at more neutral pH for chlorimuron-ethyl and metsulfuron-methyl, respectively. The reaction rate with ClO⁻ approached zero for both herbicides, suggesting a minor or negligible pathway involving the hypochlorite anion. Mass spectrometry identified six chlorination products for metsulfuron-methyl and five for chlorimuron-ethyl. Although the specific reaction mechanisms were not fully elucidated, these products provided valuable insights into the fate of sulfonylureas under chlorination. Under typical disinfection conditions (pH 7 and 4 mg L⁻¹ chlorine), the half-lives of 17.8 minutes for metsulfuron-methyl and 26.6 minutes for chlorimuron-ethyl demonstrate the potential for effective degradation in relatively short timeframes. This study underscores the potential for effective removal of these herbicides in drinking water treatment and highlights the importance of evaluating degradation products over time, as they remain detectable even after seven days.

Diatomaceous organic matter is overlooked but forms disinfection byproducts of high cytotoxicity during chlorination

Diatomaceous organic matter (DAOM) has the potential to be the main precursor of disinfection byproducts (DBPs) in multiple drinking water sources during diatom blooms. However, characterization of DAOM and subsequent formation of chlorination DBPs, especially at the molecular level, have rarely been studied, let alone the links between DAOM chemodiversity and DBPs' cytotoxicity. Herein three types of DAOM derived from Cyclotella meneghiniana, Synedra ulna, and Fragilaria nanana - which are common species during diatom blooms in drinking water sources - were selected. Cyanobacterial organic matter (CAOM) that originated from the common bloom-forming cyanobacterium Microcystis aeruginosa was also selected to thoroughly compare the differences. Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR/MS) revealed that DAOM contained more lipids, while the content of proteins in CAOM was higher. Compared with CAOM, DAOM had fewer molecules with CHON formulas, but more with CHO formulas, lower molecular weight, fewer aromatic and unsaturated compounds, and higher hydrogen saturation. High-molecular-weight DBPs (more than two carbon atoms) made up the majority of DBPs (67.7-78.7 %). DAOM generated more high-molecular-weight carbonaceous DBPs, but CAOM formed more macromolecular nitrogenous DBPs after chlorination. Chlorination of DAOM mainly occurred through chlorine substitution, and the proportion of precursors associated with substitution reactions in DAOM was larger than that in CAOM. Furthermore, the cytotoxicity of chlorinated DAOM was obviously higher than that of CAOM at the same algal density (about 2.3-3.1 times). This study provides new insights into the formation of DBPs, especially the unknown macromolecular DBPs, and the potential cytotoxicity of DBPs during chlorination of DAOM-containing water.

Removal of antibiotic resistance genes by Cl2-UV process: Direct UV damage outweighs free radicals in effectiveness

Antibiotic resistance genes (ARGs) pose significant environmental health problems and have become a major global concern. This study investigated the efficacy and mechanism of the Cl2-UV process (chlorine followed by UV irradiation) for removing ARGs in various forms. The Cl2-UV process caused irreversible damage to nearly all ARB at typical disinfectant dosages. In solutions containing only extracellular ARGs (eARGs), the Cl₂-UV process achieved over 99.0 % degradation of eARGs. When both eARGs and intracellular ARGs (iARGs) were present, the process reached a 97.2 % removal rate for iARGs. While the abundance of eARGs initially increased due to the release of iARGs from lysed cells during pre-chlorination, subsequent UV irradiation rapidly degraded the released eARGs, restoring their abundance to near-initial levels by the end of the Cl₂-UV process. Analysis of the roles in degrading eARGs and iARGs during the Cl2-UV process revealed that UV, rather than free radicals, was the dominant factor causing ARG damage. Pre-chlorination enhanced direct UV damage to eARGs and iARGs by altering plasmid conformation and promoting efficient damage to high UV-absorbing cellular components. Furthermore, no further natural transformation of residual ARGs occurred following the Cl2-UV treatment. This study demonstrated strong evidence for the effectiveness of the Cl2-UV process in controlling antibiotic resistance.

Chlorination of Ammonium Ion under Acidic Conditions

The kinetics and mechanism of the reaction between NH4+ and hypochlorous acid was studied under acidic conditions (25 °C, I = 1.0 M NaClO4). The rate-determining step is the second order formation of monochloramine (MCA): v = k1cNH4+cHOCl. MCA is converted into trichloramine (TCA) via dichloramine (DCA) in fast consecutive chlorination steps. The reactions of MCA and DCA with hypochlorous acid are second order processes; the active forms of the chlorinating agent are HOCl and Cl2O, respectively. The rates of both reactions increase by decreasing the pH. This implies that MCA and DCA are involved in fast acid-base equilibria and the protonated species have considerably higher reactivity compared to the corresponding base forms. In contrast to earlier assumptions, the chlorination steps are irreversible for all practical purposes. The evaporation of TCA from the reaction mixture may significantly contribute to its decay. The chemical decomposition of TCA is a slow process yielding N2 and nitrate ion. The formation of NO3-, was monitored by ion chromatography. A coherent mechanism is proposed that postulates the rate determining hydrolysis of TCA into dichloro-hydroxylamine (Cl2NOH), which is followed by the elimination of HCl and HOCl in subsequent reaction steps.

Rapid urea decomposition via bromination for ultrapure water production: New insights into pH-dependent reaction kinetics and mechanisms compared to chlorination

Urea, a persistent organic contaminant, poses significant challenges in ultrapure water production due to its resistance to conventional treatment processes. This study comprehensively investigated the reaction kinetics and mechanisms of urea bromination, with a comparative analysis of urea chlorination. Bromination proved significantly faster than chlorination, with optimal urea removal occurring at pH 10.5 for bromination and pH 6.0 for chlorination. The initial halogenation of urea to mono-halogenated urea was identified as a key step in facilitating total urea decomposition. Successive halogenation decreased the pKa of halogenated ureas, enhancing their deprotonation and promoting further transformation via hydrolysis. Acidic conditions favored the first halogenation step, while neutral to alkaline conditions facilitated multi-halogenation and subsequent mineralization. Complete urea decomposition required three bromine molecules at pH ≥ 6, whereas four to eight bromine molecules at pH ≤ 5, with bromine consumption increasing under acidic conditions. Treatment efficiency improved with higher oxidant-to-urea molar ratios but declined as initial urea concentrations decreased. Elevated oxidant dosages were necessary for rapid urea removal at ppb levels, typical for ultrapure water production. Urea reactions with bromine and chlorine produced halogenated ureas, which subsequently underwent hydrolysis and/or further halogen attack, forming haloamines, NH4+, NO2-, NO3-, N2, N2O and CO2. This study presents the first detailed comparison of pH-dependent kinetic and mechanistic differences between urea bromination and chlorination, revealing the distinct and superior efficiency of bromination at µg/L levels and new insights into mineralization pathways. The findings highlight the potential of free bromine as a more reactive and cost-effective alternative for water treatment.

Formation and aqueous phase leaching of organic compounds following thermal degradation of commercial drinking water plastic pipes

After a wildfire, drinking water quality may be impacted by the thermal degradation of polyethylene pipes in drinking water distribution systems. Volatile organic compounds (VOC) and semi-VOCs (SVOC) have been detected in at least fifteen water distribution systems following wildfires between 2017 and 2024. This study investigated if plastics could potentially contaminate water directly by submerging four commercially available plastic drinking water pipes (crosslinked polyethylene (PEX)-a, PEX-b, PEX-c, and high density polyethylene (HDPE)) and one HDPE resin in water and heating them to various temperatures (100-285°C) in a continuously stirred tank reactor (CSTR). After cooling, clean water was pumped through the CSTR to assess flushing's efficacy as a decontamination strategy. Each plastic leached up to 10 VOCs out of 36 VOC/SVOCs examined at the highest exposure temperature of 285°C, including various monoaromatic and phenolic compounds. Benzene, a carcinogen, leached from all plastics at temperatures of 150°C and above. PEX-a leached the greatest concentrations of most detected VOCs, where the number and magnitude of compounds leached increased with increasing exposure temperature. Flushing removed the compounds over time, but flushing was slower than expected for the more hydrophobic compounds and not so for the more hydrophilic ones, due to their continuous leaching from the plastics. Similar compounds (7 of the 12 total found) were extracted from exhumed materials from wildfire impacted water systems, including several polyaromatic hydrocarbons which were not detected in the laboratory experiments. Benzene was found to leach from all exhumed plastic pipes above maximum contaminant levels. Results confirm that plastics may be a source of contamination, flushing removes contaminants over time, and procedures must be optimized to ensure their complete removal from water distribution systems post-wildfire. SYNOPSIS: Wildfires have caused contamination of drinking water systems. This study found that the thermal degradation of plastic drinking water pipes may be one source of detected contaminants and evaluated the efficacy of flushing as a decontamination strategy.

Generation, fate and transport of volatile chlorine compounds following hypochlorite discharges to municipal sewers

The presence of volatile chlorine compounds in the headspaces of sewers receiving hypochlorite discharges is of concern because of their potential to cause corrosion of infrastructure and risks associated with human exposures. In the current study their presence was investigated using a commercial gas phase Cl2 sensor. NCl3 was identified as the most likely gas phase species when ammonia is present and it was found that the Cl2 sensor was cross-sensitive to this species. The Cl/N ratio substantially impacted gaseous NCl3 concentrations with maximum values observed at a Cl:N ratio of 12:1. Gaseous NCl3 concentrations decreased as pH increased (6.5-7.5) and temperature decreased (20-15oC). A model that included liquid-gas mass transfer (KLa) and first order decay in liquid (kdl) and gas (kdg) phases was calibrated. The value of kdl decreased as the Cl/N ratio increased (10:1-14:1) but increased as pH and temperature increased. Values of kdl and KLa were lower in real wastewater than in synthetic wastewater. Simulations with a sewer model revealed that with a fixed hypochlorite loading, peak headspace NCl3 concentrations decreased as wastewater flow increased due to changes in Cl/N ratio. Increased air flow reduced headspace NCl3 concentrations immediately downstream of the discharge due to dilution but had little effect further downstream. Wastewater pH impacted headspace NCl3 concentrations by controlling the concentration of NCl3 in the wastewater. Temperature influenced peak headspace concentrations through its impact on mass transfer and gas phase decay rates. This study provides insights that can be employed to develop sewer use bylaws that regulate hypochlorite discharges.

Fate and transformation of psychotropic drugs in urban wastewater systems and receiving rivers via the integration of targeted and suspect screening analysis

Psychotropic drugs rank among the most prescribed pharmaceuticals in the world. The ubiquitous occurrence of psychotropic drugs in the environment evoked rising concerns due to their various toxic effect on non-target organisms at low concentrations. However, the removal, transformation, and discharging of these drugs throughout wastewater treatment plants (WWTPs) have rarely been reported. Based on the targeted analysis and suspected screening, this study investigated the distribution of psychotropic drugs and their transformation products (TPs) within the entire treatment processes in WWTPs and their receiving rivers. The results indicated that 13 out of 47 psychotropic drugs are widely observed across wastewater, sludge, receiving river water, and sediment, respectively. The aqueous removal efficiencies of most psychotropic drugs exhibited their significant recalcitrance in wastewater treatment processes. For instance, venlafaxine (VEL) was slightly removed by 2.64 % and 10.8 % in these two WWTPs. The concentrations of oxazepam (OZP) and lamotrigine (LMT) dramatically increased after the overall treatment processes due to their metabolic conversion and regeneration processes, respectively. Given the recalcitrance of psychotropic drugs, the identified TPs generated within WWTPs were not abundant, but a wider variety of TPs were identified from human metabolites. A total of 25 TPs were identified via the suspect screening analysis, of which nine were newly identified. In receiving rivers, the risk quotient (RQ) presented OZP, sertraline (SER), and VEL posed high potential risks; the integration of the toxicological priority index (ToxPi) and the toxicity-weighted concentration (TWC) suggested TP-CIT-322 and TP-OCX-195 as the high-priority contaminants. Given the recalcitrance and environmental risks of psychotropic drugs and their TPs in WWTPs and environments, it is crucial for the further exploration of their effective treatment technologies and emission control strategies .

Low formation of trihalomethanes in the disinfection of anaerobic effluent by electroactivated water

This study investigated the effect of chlorinating an anaerobic reactor effluent with a high dosage of electroactivated water (EAW) and using the breakpoint strategy. The resulting effluent quality was evaluated based on the resolutions of the Brazilian Environmental Council. Disinfection by-products (DBPs) formation was measured as total trihalomethane (THM4), i.e. the sum of chloroform (TCM), bromodichloromethane (BDCM), dibromochloromethane (DBCM), and bromoform (TBM), which are the four major THM4 species. The results for the formation of THM4 were 217, 171 and 63 μg L-1 at dosages of 55, 60, and 65 mg Cl2 L-1, respectively, which are even lower than the maximum value of 1 mg TCM L-1, the only THM established in the Brazilian standards. The TCM, BDCM, DBCM and TBM concentrations determined in this study for those three dosages also comply with the discharge limits established for the classes II and III of surface wasters, by the Florida Department of Environmental Protection (USA).

Treatment of Alfalfa Seeds With Food-grade Organic Acid Mixtures Reduces Loads of Pathogenic Escherichia coli O157:H7 and Salmonella Typhimurium on Sprouts Without Reducing Germination Percentage or Sprout Mass

Sprouts are regarded as premier health foods due to their high content of vitamins and minerals. Unfortunately, numerous outbreaks of foodborne illness have been linked to raw sprouts due to their capacity to harbor bacterial pathogens combined with growing conditions favoring microbial growth. One commonly utilized practice to reduce microbial hazards on fresh sprouts is treatment of seeds with 20,000 ppm sodium hypochlorite (NaClO) or calcium hypochlorite (Ca(ClO)2). However, these traditional chlorine-based treatments have a limited capacity to reduce the population growth of pathogens during sprout development and might affect the safety of workers and health of consumers; thus, alternative treatments are needed. Promising alternatives to chlorine-based sanitizers are organic acids. We have investigated the capacity of novel matrices comprised of organic acids (OAM), consisting of unique mixtures of zinc acetate (Zn(CH3CO2)2), citric acid (HOC(CH2CO2H)2), malic acid (HO2CCHCH2CO2H), and lactic acid (C3H6O3) to reduce loads of human pathogenic bacteria Escherichia coli O157:H7 and Salmonella Typhimurium inoculated on alfalfa sprouts (∼6 log CFU/ml). Seed treatment with OAM formulations prior to germination resulted in approximately 100-fold reductions and was superior to treatment with solutions containing only the base organic acid ingredients of the OAMs, but not as significant as the 20,000 ppm NaClO treatments which consistently reduced pathogen loads over 1000-fold. However, all OAM treatments resulted in significantly increased germination percentages (89.0-95.3%) compared to 20,000 ppm NaClO treatments (85%). Fresh sprout weight measured after 5 days for 3 of the 4 tested OAMs (100 sprout batches; 2.31-2.62 g) was also significantly higher than fresh weights after NaClO treatment (∼2.0 g). Our results indicate a promising step towards implementing treatments that decrease sickness risks from consuming fresh sprouts without compromising production yield.

Prediction of chlorination degradation rate of emerging contaminants based on machine learning models

Assessing the degradation of emerging contaminants in water through chlorination is crucial for regulatory monitoring of these contaminants. In this study, we developed a machine learning model to predict the apparent second-order reaction rate constants for organic pollutants undergoing chlorination. The model was trained using second-order reaction rate constants for 587 organic pollutants, with 314 data points obtained from actual experiments, the other data points 273 came from previous studies. We evaluated ten machine learning algorithms with Modred molecular descriptors and MACCS molecular fingerprints, optimizing the hyperparameters through Bayesian optimization to enhance the predictive capability of the model. The optimized model GPR algorithm combined with molecular fingerprint model achieved R2train = 0.866 and R2test = 0.801. Subsequently, the model was fed with chemical features of four organic pollutants, and the predicted results were compared with experimentally obtained values, the deviations between predicted and experimental values were found to be 2.12%, 0.37%, 0.15%, and 14.8%, respectively, further validating the accuracy of the predictive model. SHAP analysis showed that the amino-methyl group CN(C)C had the highest feature value, demonstrating the interpretability of the model in predicting chlorine-degraded pollutants The model established in this study is more representative of real chlorination environments, providing preliminary guidance for chlorination plants on the degradation of numerous emerging contaminants lacking treatment standards and facilitating the refinement of strategies for the prevention and control of emerging contaminants.

Toilet Bowl Cleaning Tablets as Sources of Chlorine, Bromine, and Disinfection Byproducts in Wastewater

Commercial toilet bowl cleaning tablets were examined in laboratory systems to characterize their release of active halogens and their potential to form trihalomethanes (THMs) when combined with synthetic sewage. Active halogens (e.g., HOCl, HOBr, and reactive halamines) were quantified via derivatization with 1,3,5-trimethoxybenzene prior to analysis by liquid chromatography. The effects of several variables on halogen release profiles were examined, including pH, ionic strength, temperature, tank solution volume, flushing frequency, and tablet brand. Changes in pH resulted in modest or no appreciable changes in halogen release profiles. Release of active halogens increased as ionic strength decreased and as temperature increased. Tank volume, flushing frequency, and tablet brand had pronounced impacts on halogen release profiles. Maximum measured active chlorine and bromine concentrations in toilet tank water were 189 mg/L as Cl2 and 164 mg/L as Cl2, respectively. Active halogens persisted in toilet bowl water for >24 h. When toilet-tablet-treated water was combined with synthetic sewage, THMs formed at up to 219 ppb with bromine incorporation factors up to 2.86. Active halogens and highly brominated THMs released into wastewater from toilet tablets could have implications for downstream microbial ecology, septic system performance, and overall water quality.

Comparisons on the evaluation methods of chlorine resistance fungi in drinking water

Chlorine disinfection is fundamental for ensuring microbial safety in drinking water systems. However, fungi pose significant pathogenic risks due to their substantially higher chlorine resistance compared to bacteria. Existing approaches for evaluating fungal chlorine resistance face challenges, including the absence of standardized protocols, labor-intensive procedures, prolonged experimental durations, and limited real-time detection capabilities. Moreover, the mechanisms underlying fungal chlorine resistance remain inadequately understood. This study provides a comprehensive and systematic comparison of the primary methods used to assess fungal chlorine resistance, including log reduction, concentration-time (CT) values, and minimum inhibitory concentration (MIC). The CT value method incorporates disinfectant decay, contact time, and experimental conditions to reflect the dynamics of the disinfection process. In contrast, the log reduction method focuses on endpoint inactivation, while MIC provides a retrospective evaluation. Therefore, the CT method is recommended as the most effective method. This study investigates the underlying mechanisms of fungal chlorine resistance, emphasizing the critical roles played by fungal cell wall components, such as melanin and chitosan, the antioxidant enzyme systems, and the formation of biofilms in conferring enhanced resistance to chlorine exposure. The findings provide a theoretical foundation for the development of standardized methods and more effective strategies for controlling fungal contamination in water treatment process.

Effectiveness of sanitizers on different biofilm-forming microorganisms associated with the poultry drinking water system

The sanitation of the poultry drinking water system (DWS) is essential to controlling pathogens and biofilms in the DWS. Intervention approaches including several sanitizers have been developed, but there is limited information on the efficacy of some of these sanitizers. The aim of this study was to evaluate the effectiveness of peracid-based (PAB), peroxide-based (PB), and hypochlorite-based (HB) sanitizers against field-isolated Salmonella (10), E. coli (2) and Bacillus (2), along with their antibiofilm effects on six of these bacterial strains on polyvinylchloride (PVC), a common DWS pipe material. The minimum inhibitory and bactericidal concentrations (MIC and MBC) were determined using the microdilution broth method. For biofilm production, PVC rings were inoculated (5-6 Log10 CFU/mL) in buffered peptone water, incubated at 30°C for 48 h, and detached with cotton swabs for quantification. The antibiofilm effect of the sanitizers was further assessed at MIC, 2X-MIC, 4X-MIC, and water (control). Data was analyzed using ANOVA and Least squares in JMP Pro 18. The MIC and MBC of PAB for all isolates ranged from 11.36 to 28.42 ppm, PB from 15.26 to 71.21 ppm, and HB was 106.67 to 350 ppm. Bacillus licheniformis formed the most biofilm (5.39 Log10 CFU/mL) as single-species bacteria while Salmonella attached more (6.36 Log10 CFU/mL) than E. coli (5.41 Log10 CFU/mL) and Bacillus (2.08 Log10 CFU/mL) when grown together in mixed cultures. PAB and HB eliminated the biofilms of all strains tested at MIC in mixed-species cultures while PB had no significant effect. Overall, PAB demonstrated the greatest potential as a DWS sanitizer, showing superior efficacy against planktonic and biofilm cells compared to PB and HB. This research highlights the importance of targeted microbial profiling and sanitizer efficacy testing for pre-harvest pathogen control, providing valuable insights for enhancing food safety in poultry production systems.

The discharge of chlorinated effluent from wastewater treatment plants enhances dissolved oxygen in the receiving river: From laboratory study to practical application

Dissolved oxygen (DO) is essential for the health of aquatic ecosystems, supporting biogeochemical cycles and the decomposition of organic matter. However, continuous untreated external inputs from illicit discharges or sewer overflows, coupled with inadequate ecological base flow, have led to widespread river deoxygenation and serious ecological crises. This study demonstrates that chlorinated wastewater treatment plant (WWTP) effluent can significantly enhance DO levels in downstream rivers, particularly in areas with high pollution loads or poor ecological base flow. Notably, DO increases in receiving waters were positively correlated with initial chorine doses. Residual chlorine in WWTP effluent reduced inorganic nitrogen and dissolved organic matter (DOM). Analysis of DOM and molecular properties showed that residual chlorine preferentially reacts with low-molecular-weight organics like amino acids, increasing their hydrophobicity and electrophilicity. These molecular changes inhibit enzyme interactions, reducing the bioavailability of these compounds for oxygen-consuming processes. Field studies demonstrated that through on-site optimization of the full-scale WWTP disinfection process, specifically by controlling residual chlorine levels in effluents, DO levels downstream increased by an average of 15 %, with a maximum of 48 % compared to upstream levels, while typical disinfection byproducts (i.e., trihalomethanes, haloacetic acids and haloacetonitriles) remained below regulatory thresholds. This work provides new insights into the positive effects of chlorinated WWTP effluent on DO levels in receiving waters.

Enhanced abatement of phenolic compounds by chlorine in the presence of CuO: Absence of electrophilic aromatic substitution

It has been demonstrated that chlorine predominately reacts with phenolic compounds through an electrophilic aromatic substitution, yielding chlorinated phenols. Previous studies showed that copper oxide (CuO), a water pipe corrosion product, can catalytically enhance the reactivity of chlorine and its disproportionation. In this study, kinetics and mechanisms for the reactions of chlorine with phenolic compounds in the presence of CuO were investigated. CuO at 100 mg/L increases the apparent second-order rate constants (kapp) for reactions of chlorine with phenol, chlorophenols, bromophenols, iodophenols, 2,6-dimethylphenol, acetaminophen, and 4-hydroxybenzoic acid at pH 7.6 and 21 °C by up to 50 times. For the same reaction conditions, increasing CuO concentrations from 0 to 200 mg/L increase the kapp of phenol chlorination from ∼42 to 608 M-1 s-1. In general, a stronger enhancement of the chlorine reactions with phenols was observed in the pH range of 6.6-7.6 than 7.6-9.0, indicating that CuO more readily activates hypochlorous acid. Moreover, CuO significantly changes the pathway for phenol chlorination. Yields of chlorophenols decreased from 98 % to < 5 % as the CuO concentration increased from 0 to 100 mg/L. Non-chlorinated compounds (e.g., catechol, 2,3-dihydroxymuconic acid, maleic acid, and oxalic acid) are major transformation products. Model simulations suggest a pre-equilibrium step with the formation of a CuO-HOCl complex as the rate-limiting step for the overall reactions. Heterogeneous chlorination processes with limited formation of chlorinated phenols tend to be predominant for CuO concentrations > ∼ 5 - 36 mg/L for various phenols. These findings have implications for the transformation of phenolic compounds during chlorination in copper-containing water distribution systems.

Chlorine-substituted transformation products generated during chlorination of the organophosphorus insecticide disulfoton induce anti-acetylcholine esterase activity

Global concern regarding transformation products (TPs) derived from contaminants, including pesticides, in the environment and during water treatment has been growing markedly. In the present study, we investigated the anti-acetylcholinesterase (AChE) activity of an aqueous solution of the organophosphorus insecticide disulfoton, a toxicological endpoint for determining the acceptable daily intake of disulfoton, both in the presence and the absence of metabolism during chlorination. Disulfoton rapidly reacted with free chlorine and completely disappeared within 0.25 h. Although the aqueous disulfoton solution did not induce anti-AChE activity before chlorination, the chlorinated samples did induce anti-AChE activity, both with (indirect toxicity) and without (direct toxicity) metabolism. These observations clearly indicated that disulfoton was converted into toxic TPs through reactions with free chlorine. Liquid chromatographic fractionation followed by an anti-AChE activity assay revealed that three TPs were responsible for the observed direct toxicity. Further mass spectrometric analyses showed that these TPs were disulfoton-oxon-sulfone, and mono- and dichloro-substituted derivatives of disulfoton-oxon-sulfoxide (O-(1-chloroethyl) S-[2-(ethanesulfinyl)ethyl] O-ethyl phosphorothioate and O-(1,2-dichloroethyl) S-[2-(ethanesulfinyl)ethyl] O-ethyl phosphorothioate, respectively), none of which were simply oxon. Results of the anti-AChE activity assay on the chemical standard of disulfoton-oxon-sulfone after metabolism and quantification of the disulfoton-oxon-sulfone in the chlorinated samples revealed that the observed indirect toxicity was solely induced by this TP. It is recommend that drinking water treatment plants that use free chlorine as a disinfectant monitor the concentrations of at least disulfoton-oxon-sulfone, which is commercially available, in finished water in addition to disulfoton itself, to ensure the safety of tap water.

Reactions of oxazepam with two typical water treatment oxidants in aqueous solutions: Results based on density functional theory

Pharmaceutical residues in the environment and their transformation mechanism are important challenges in environmental pollution research. The present study investigated the transformation mechanisms and reaction kinetics of oxazepam, a representative of benzodiazepine pharmaceutical, with two typical water treatment oxidants including HOCl and ∙OH in aqueous solution through theoretical calculations and experimental verification. The results showed that oxazepam is a chiral molecule with two enantiomers in equal proportions. The reactions between oxazepam and HOCl can be classified into Cl-substitution, OH-substitution, and bond-fission reactions. Among these substitutions, the Cl-substitution reaction at the N23 site was most likely to occur. The bond-fission reactions were predominated by the cleavage of the C27-N29 bond, which could lead to further bond cleavage reactions. The reactions between oxazepam and ∙OH involved the addition and H-abstraction pathways, with the addition reactions at the C5, C13, and C17 sites being the top three major reaction pathways. The kinetics rate constants obtained by the density functional theory (DFT) calculation were 0.16 and 1.78 × 1011 M-1 s-1 for the reactions of oxazepam with HOCl (kHOCl, M-1 s-1) and ∙OH (k·OH, M-1 s-1) respectively, which are basically consistent with the experimental results. This comprehensive understanding of the reaction mechanisms of oxazepam with HOCl and ∙OH based on quantum chemical calculations is crucial for exploring the chlorination and advanced oxidation of benzodiazepine pharmaceuticals.

Boosting hypochlorite's disinfection power through pH modulation

PURPOSE

Hypochlorite-based formulations are widely used for surface disinfection. However, the efficacy of hypochlorite against spore-forming bacteria varies significantly in the literature. Although neutral or low pH hypochlorite solutions are effective sporicides due to the formation of hypochlorous acid (HOCl), their optimal conditions and the specific role of pH in disinfection remain unclear. These conditions also increase the solution's corrosiveness and compromise its shelf life. Therefore, further research is needed to identify the pH conditions that balance solution stability and effective hypochlorite-based spore disinfection.

RESULTS

This study investigates the impact of neutral to alkaline pH on the sporicidal efficiency of hypochlorite against a pathogenic Bacillus cereus strain. We apply a 5,000 ppm hypochlorite formulation for 10-min across a pH range of 7.0-12.0, simulating common surface decontamination practices. Our results demonstrate that hypochlorite is largely ineffective at pH levels above 11.0, showing less than 1-log reduction in spore viability. However, there is a significant increase in sporicidal efficiency between pH 11.0 and 9.5, with a 4-log reduction in viability. This pH level corresponds to 2 - 55 ppm of the HOCl ionic form of hypochlorite. Further reduction in pH slightly improves the disinfection efficacy. However, the shelf life of hypochlorite solution decreases exponentially below pH 8.5. To explore the pH-dependent efficacy of hypochlorite, Raman spectroscopy and fluorescence imaging were used to investigate the biochemical mechanisms of spore decontamination. Results showed that lower pH enhances spore permeability and promotes calcium dipicolinic acid (CaDPA) release from the core.

CONCLUSION

Our results highlight the complex relationship between pH, sporicidal efficacy of hypochlorite, and its shelf life. While lower pH enhances the sporicidal efficiency, it compromises the solution's shelf life. A pH of 9.5 offers a balance, significantly improving shelf life compared to previously suggested pH ranges 7.0-8.0 while maintaining effective spore inactivation. Our findings challenge the common practice of diluting sodium hypochlorite with water to a 5,000 ppm solution, as this highly alkaline solution (pH of 11.9), is insufficient for eliminating B. cereus spores, even after a 10-min exposure. These findings are critical for improving disinfection practices, highlighting the importance of optimizing sodium hypochlorite effectiveness through pH adjustments before application.

Unveiling the reaction chemistry of sulfoxides during water chlorination

Species-specific second-order rate constants for the reactions of eight model sulfoxides with hypochlorous acid (kHOCl) were determined to be in the range of 2.7 M-1 s-1 to 5.8 × 103M-1 s-1. A quantitative structure-activity relationships (QSAR) with Taft σ* constants was developed based on eight measured kHOCl-values, showing a good linear correlation (R2 = 0.89) with a negative slope ρ = -1.5 typical for electrophilic reactions. The reaction is mainly controlled by HOCl, with a minor contribution of OCl-. The contributions of other reactive chlorine species (e.g., Cl2 and Cl2O) to the overall kinetics are only 7 % for Cl2O and 5 % for Cl2 under typical drinking water treatment conditions. A combination of several analytical methods (HPLC-MS/MS, HPLC-ICP-MS/MS, and NMR) was applied for the identification of transformation products. Major transformation products from the reactions of chlorine with sulfoxides are sulfones, Cl-substituted sulfoxides, aldehydes, and sulfonic acids potentially formed via a transient chlorosulfonium cation. In general, sulfoxides react more readily with chlorine compared to bromine. This might be caused by a partial positive charge on the sulfur which leads to a stronger interaction with Cl in HOCl having a smaller partial positive charge than Br in HOBr. The ratios of the species-specific second-order rate constants for the reactions of the selected sulfoxides with chlorine or bromine (kHOCl/kHOBr) range from 6 to 480. For sulfoxide compounds with strong electron-withdrawing substituents the reaction occurs most likely via a carbanion intermediate for which the reaction with HOBr is preferred, resulting in a kHOCl/kHOBr = 0.8.

Revealing Molecular Connections between Dissolved Organic Matter in Surface Water Sources and Their Cytotoxicity Influenced by Chlorination Disinfection

Dissolved organic matter (DOM) is the primary precursor of disinfection products (DBPs) during chlorination. However, the compositional characteristics of DOM transformation during the chlorination process in different source waters and its relationship to cytotoxicity remain understudied. Here, we used high-resolution mass spectrometry to evaluate chlorination-induced molecular-level changes in DOM derived from different surface water sources. We correlated DOM components with the cytotoxicity profiles of selected DBPs using new alternative methods with predictive toxicological assessments. Our findings indicate a selective chlorination of DOM in natural waters and a tendency for lignin and protein conversion during the manual chlorination process. The reactivity of bioactive compounds decreased in the order of lignin > protein > tannin or ConAC. The cytotoxicity of DOM from source waters is mainly attributed to lignin- and protein-like compounds within the CHO and CHNO groups. Additionally, mitochondrial damage is a highly sensitive indicator of DOM-induced cytotoxicity. The toxicity profiles of DBPs revealed 37 common toxicity-driving components characterized by low mass, medium H/C ratio, low O/C ratio, reduction state, and hydrophobicity. Our findings highlight the need to exploit the health effects of DOM and provide substantial experimental evidence for the necessity to remove potential toxicants.

Phenols coupled during oxidation upstream of water treatment would generate higher toxic coupling phenolic disinfection by-products during chlorination disinfection

Phenolic compounds usually produce coupling phenols during the upstream oxidation treatment of water, but the disinfection by-products (DBPs) of coupling phenols in the subsequent disinfection process have been overlooked. Herein, we demonstrated the generation and higher toxicity of these DBPs. In Songhua River water, 0.355 ng/L 4-(4-bromophenoxy)phenol and 122.67 ng/L phenol were detected. Pre-oxidation with K2FeO4 and KMnO4 resulted in the formation of 0.68 ng/L and 0.506 ng/L 4-(4-iodophenoxy)phenol in subsequent disinfection, respectively, which were 2-3 times that without pre-oxidation. Coupling of phenolic compounds during pre-oxidation and then halogenated during chlorination was shown to be the main pathway for the generation of coupling phenolic DBPs. The maximum rate constants for the reactions of hypochlorous acid with phenol and its coupling products (4-phenoxyphenol, 2,2'-biphenol, and [1,1'-biphenyl]-2,4'-diol) were 115.61 M-1s-1, 65.85 M-1s-1, 143.13 M-1s-1, and 212.52 M-1s-1, respectively, with 2,2'-biphenol and [1,1'-biphenyl]-2,4'-diol breaking through lower energy barriers and releasing more energy than phenol. This indicated coupling phenols have a higher potential to form DBPs. Additionally, coupling phenolic DBPs (4-(4-chlorophenoxy)phenol, 4-(4-bromophenoxy)phenol, and 4-(4-iodophenoxy)phenol) was 1-3 orders of magnitude more toxic than their precursors (4-phenoxyphenol, 2,2'-biphenol, [1,1'-biphenyl]-2,4'-diol, and phenol) and uncoupled DBPs (4-iodophenol). Therefore, the formation and hazards of coupling phenolic DBPs require more attention.

Chlorination of quorum sensing molecules: Kinetics and transformation pathways

The impact of chlorination on quorum sensing molecules (QSMs) is not often addressed in disinfection research. Yet pathogenicity and biofilm formation are controlled by quorum sensing (QS) in many bacteria. Chemical transformation of the compounds could have an impact on all of these processes. For this reason, our study elucidated the reaction kinetics and transformation pathways of several N-acyl homoserine lactones (AHLs) and 2-heptyl-4-quinolone (HHQ) in contact with free available chlorine (FAC), a potent QS inhibitor. Both AHLs and HHQ, are known as QSMs for Gram-negative bacteria. Using FAC, a complete degradation of the target compound was observed for p-coumaroyl AHL (pC-AHL), C14:1-AHL, HHQ and 3-Oxo-C14-AHL. The reaction order for FAC varied between 1.19 (±0.07) (pC-AHL) to 1.62 (±0.13) (HHQ). This means that different reactive species (e.g. hypochlorous acid and dichlorine monoxide) are likely to be involved in the reaction mechanism. The first-order rate constants were strongly pH-dependent. For C14:1-AHL and HHQ, the first-order rate constants decreased from pH 6.0 to pH 8.5. A maximum was observed for pC-AHL at pH 8.5 ranging from pH 6.0 to 10. In addition to the distribution of the reactive species, the phenol/phenolate ratio strongly influenced the first-order rate constants for pC-AHL. In total, at pH 7 (phosphate buffered) 29 transformation products were identified and the related transformation pathways were proposed via non-target and suspect screening using high-resolution mass spectrometry. The observed reaction mechanisms can be transferred to structurally similar QSMs to further understand QS-controlled processes during chlorination. We assumed that the transformation of the QSMs affects QS of the bacteria, thereby blocking QS-controlled processes such as biofilm formation.

Investigation of Transformation Pathways of Polyfluoroalkyl Substances during Chlorine Disinfection

Recent regulations on perfluorinated compounds in drinking water underscore the need for a deeper understanding of the formation of perfluorinated compounds from polyfluoroalkyl substances during chlorine disinfection. Among the compounds investigated in this study, N-(3-(dimethylaminopropan-1-yl)perfluoro-1-hexanesulfonamide (N-AP-FHxSA) underwent rapid transformation during chlorination. Within an hour, it produced quantitative yields of various poly- and per-fluorinated products, including perfluorohexanoic acid (PFHxA). Sixteen reactions involving chlorine with N-AP-FHxSA and its quaternary ammonium analog were investigated; seven were confirmed, while the remainder were either disproved or found to be insignificant. The quaternary ammonium moiety did not determine a polyfluoroalkyl substance's reactivity toward chlorine. For example, while 6:2 fluorotelomer sulfonamide betaine transformed rapidly to PFHxA, other quaternary-ammonium-containing polyfluoroalkyl substances, such as 5:1:2 and 5:3 fluorotelomer betaines, showed significant resistance to chlorination. Further investigation identified potential sites for electrophilic attacks near the amine region by examining the highest occupied molecular orbitals of the polyfluoroalkyl substances. Visualization techniques helped pinpoint electron-deficient and electron-rich sites as potential targets for nucleophilic and electrophilic attacks, respectively. Increasing the solution pH from 6 to 10 did not diminish the apparent degradation of the studied polyfluoroalkyl substances, likely due to the greater reactivity of the deprotonated forms compared to the conjugate acids. Finally, we also examined the hydrolysis of polyfluoroalkyl substances at pH 6 to 11 in the absence of chlorine.

Heterogeneous Activation of NaClO by Nano-CoMn2O4 Spinel for Methylene Blue Decolorization

In this study, the nano-spinel CoMn2O4 was synthesized by coprecipitation pyrolysis and employed to heterogeneously activate hypochlorite (NaClO) for the oxidative decolorization of methylene blue (MB). The crystal structure, elemental composition, surface morphology, and microstructure of the prepared CoMn2O4 nano-spinel were analyzed using a series of characterization techniques. The pyrolysis temperature was screened on the basis of MB decolorization efficiency and the leaching of metal ions during the reaction. The MB decolorization efficiency was compared using different catalysts and process. The impacts of CoMn2O4 dosage, effective chlorine dose, MB concentration, and initial pH on MB decolorization were explored. The catalytic mechanism of MB oxidation was elucidated through quenching experiments combined with radical identification. The degradation pathway of MB was preliminarily proposed based on the detection of the intermediates. The reusability of recycled CoMn2O4 was finally investigated. The results revealed that maximal MB oxidation efficiency and minimal leaching of Co and Mn ions were achieved at the calcination temperature of 600 °C. Complete oxidative decolorization of MB within 40 min was obtained at an initial MB concentration of 50 mg/L, a CoMn2O4 dosage of 1 g/L, an effective chlorine dose of 0.1%, and an initial pH of 4.3. Superoxide radical (O2•-) was found to be dominantly responsible for MB decolorization according to the results of radical scavenging experiments and electron paramagnetic resonance. The CoMn2O4 spinel can be recycled for five cycles with the MB removal in the range of 90.6~98.7%.

Insight into Iron(III)-Tannate Biosorbent for Adsorption Desalination and Tertiary Treatment of Water Resources

An innovative biosorbent-based water remediation unit could reduce the demand for freshwater while protecting the surface and groundwater sources by using saline water resources, such as brine, brackish water, and seawater for irrigation. Herein, for the first time, we introduce a simple, rapid, and cost-effective iron(III)-tannate biosorbent-based technology, which functions as a stand-alone fixed-bed filter system for the treatment of salinity, heavy-metal contaminants, and pathogens present in a variety of water resources. Our approach presents a streamlined, cost-efficient, energy-saving, and sustainable avenue for water treatment, distinct from current adsorption desalination or conventional membrane techniques supplemented with chemical and UV treatments for disinfection. The proof of feasibility for effective treatment of heavy metals, adsorption desalination, and cleansing of pathogens is demonstrated using synthetic water, brine, and field-collected seawater. The adsorption equilibrium and adsorption kinetic isotherm models, and mass transfer diffusion models confirmed the sorbent's function for sieving heavy-metal ions-silver (Ag+), cadmium (Cd2+), and lead (Pb2+)-from water. The maximum adsorption capacities (q m) of the sorbent for Ag+, Cd2+, and Pb2+ reach 96.25, 66.54, and 133.83 mg/g at neutral pH. The sorbent's affinity for heavy-metal-ion adsorption significantly increased, yielding q m of 116.57 mg/g for Ag+, 104.04 mg/g for Cd2+, and 165.66 mg/g for Pb2+, at pH 9, respectively, due to the sorbent's amphoteric nature. The pristine sorbents exhibit exceptional adsorption desalination efficacy (>70%) for removing salinity from brine and seawater, promoting heterogeneous adsorption. Fe(III)-TA's ability to disinfect seawater, with 67% efficacy over a very short contact time (∼15 min), confirms its remarkable antimicrobial properties for contact active mode pathogens cleansing. By preventing the release of salts, heavy-metal contaminants, and pathogens into the environment, our results proved that this novel multiplex biobased sorbent approach directly contributes to the water quality of surface and groundwater resources.

The sorption of algal organic matter by extracellular polymeric substances: Trade-offs in disinfection byproduct formation influenced by divalent ions

Disinfection by-products (DBPs), formed from biofilm extracellular polymeric substances (EPS) and organic matter during regular disinfection practices in drinking water distribution systems, poses a potential threat to drinking water safety. However, the diverse DBP formations induced by the intertwined algal organic matter (AOM) and bacterial EPS remains elusive. In this study, we show substantial variations in EPS and DBP formation patterns driven by AOM biosorption with divalent ions (Ca2+ and Mg2+). Divalent ions in bulk water can significantly inhibit carbonaceous DBPs (C-DBPs) and nitrogenous DBPs (N-DBPs) formation. Mechanistically, divalent ions promote the complexation of negative charged groups and thus inhibit C-DBP formation, while the hindering chlorine substitution of hydrogen atoms on α‑carbon and amine groups reduces N-DBP formation. Conversely, Ca2+ and Mg2+ could facilitate biosorption processes that increased the yields of C-DBPs and N-DBPs. Both EPS and AOM provide halogenated reactive sites for DBP formation, exhibiting diverse aromatic substances and unsaturated (lignin and tannins) compounds. Our results highlight divalent ions acting as a fundamental driving force in DBP formation, suggesting the need for cautious monitoring of divalent ions in karst water.

Characterizing the precursors of byproducts formed by chlorine and chlorine dioxide disinfection using unknown screening analysis with Orbitrap mass spectrometry

Chlorine (Cl2) and chlorine dioxide (ClO2) are commonly used to disinfect water but unfavorable interactions with dissolved organic matter (DOM) result in the formation of disinfection byproducts (DBPs). This study investigated the formation of organic DBPs arising from Cl2 and ClO2 disinfections under different contact times in two surface waters in Thailand and Suwannee River natural organic matter with/without bromide using unknown screening analysis with Orbitrap mass spectrometry. Many CHOCl-DBPs and CHOBr-DBPs intermediates were rapidly formed during the initial period of contact (5-30 min). Subsequently, the number of DBPs either decreased or increased (60-1440 min) due to the ongoing formation and decomposition of intermediate DBPs reacting with disinfectants. Over one hundred newly formed chlorinated DBPs were produced by Cl2 and ClO2 disinfections (CHOCl Cl2-DBPs and CHOCl ClO2-DBP, respectively). At least 40 % of the chlorinated DBPs were commonly found in the presence of both disinfectants, probably due to HOCl impurity formed by ClO2. In addition, CHO features with high degree of unsaturation ([DBE-O]/C) and moderate degree of carbon oxidation state (Cos) were found to be statistically correlated with several CHOCl-DBP and CHOBr-DBP features in Cl2 and ClO2 disinfections, and are therefore considered as putative precursors. Furthermore, the putative CHOBr-DBP precursors showed a more highly oxidized character than the putative CHOCl-DBP precursors. By tracking precursors from reactions using mass difference analysis, Cl2 preferentially reacts with saturated precursors via electrophilic substitution reaction, where the Cl2 addition reaction occurs more favorably in the presence of unsaturated precursors.

Synergistic effects of a sequential recirculation electrochemical system combined with low-cost UV-LEDs on the gram-negative bacteria inactivation

In this work, an electrochemical system combined with low-cost UV-LEDs was implemented for the inactivation of Escherichia coli and Pseudomonas aeruginosa. The individual elimination of these bacteria was followed by plate counting and flow cytometry, as complementary techniques to establish the cell inactivation and non-viability, respectively. The contribution of the different parts of the disinfection system (anode, cathode, and light) was determined. In addition, the efficiency of the UV-LEDs/GDE/DSA system in the disinfection of an irrigation water sample was studied. It was found that the combination of the electrochemical system with UV-LEDs was highly synergistic (φ > 7), having low electric energy consumptions per order of magnitude (EEO: 1.13 × 10-2 and 1.55 × 10-2 kWh/m3 order). Moreover, some differences in the inactivation kinetics and synergy between E. coli and P. aeruginosa were observed and linked to the structural/morphological characteristics of the two bacteria. Remarkably, the electrochemical system combined with low-cost UV-LEDs inactivated both target microorganisms after only 2 min of treatment. The flow cytometry analyses evidenced the damage to the cell membrane of the bacteria by the simultaneous and synergistic action of the electrogenerated H2O2 and active chlorine species (ACS), plus the attacks of photo-generated reactive oxygen species. This synergistic combination in the UV-LEDs/GDE/DSA system demonstrated remarkable efficiency in the disinfection of an irrigation water sample, achieving the elimination of culturable bacteria in 45 min of treatment. The results of this research demonstrated the capacity and great potential of an easy combination of electrochemistry with UV-LEDs as an alternative system for the elimination of gram-negative bacteria in water.

Modeling ClO2-NOM Reactions for Predicting Byproduct Formation and Micropollutant Degradation in Surface Water

Chlorine dioxide (ClO2) is a promising alternative disinfectant/oxidant to free chlorine in drinking water treatment, while it reacts with natural organic matter (NOM) to form free chlorine, chlorite ions (ClO2-), and chlorate ions (ClO3-) as byproducts. Predicting the ClO2 consumption and the formation of these byproducts using a kinetic model helps to balance the trade-off between disinfection/oxidation efficiency and byproduct formation. This study establishes a summative equation to describe the reaction between ClO2 and ClO2-reactive moieties in the NOM (CRNOM). The average molar yields of ClO2-, free chlorine, Cl-, and ClO3- from the reactions between ClO2 and nine NOM isolates are determined to be 0.576 ± 0.017, 0.258 ± 0.022, 0.141 ± 0.010, and 0.039 ± 0.002 per consumed ClO2, respectively. The bimolecular rate constants of CRNOM toward ClO2 (kCRNOM-ClO2) are comparable among nine NOM isolates (683 ± 57 M-1·s-1 at pH 7.0). The CRNOM concentrations and kCRNOM-ClO2 increase by 2-fold and 1.3-fold, respectively, as pH increases from 6.0 to 9.0, while pH barely affects the molar yields of inorganic products. A kinetic model is established and enables the accurate prediction of ClO2- and ClO3- formation and ofloxacin degradation during ClO2 oxidation in surface water.

Molecular Alterations of Algal Organic Matter in Oxidation Processes: Implications to the Formation of Disinfection Products

Seasonal algal blooms in surface waters can adversely impact drinking water quality. Oxidative treatment has been demonstrated as an effective measure for the removal of algal cells. However, this, in turn, leads to the release of algal organic matter (AOM). Effects of oxidative treatment using chlorine, bromine, chloramine, ozone, and permanganate on the molecular alterations of the AOM were studied using Fourier transform ion cyclotron resonance mass spectrometry. Increased chemodiversity, decreased aromaticity, and elevated average oxidation state of carbon () were observed after oxidation. Of the oxidants, ozone caused the most pronounced changes. There was a positive correlation between the increases in and reduction potentials of oxidants (i.e., ozone > chlorine ≈ bromine > permanganate > chloramine). Oxygen transfer and oxidative dehydrogenation were major pathways (42.3-52.8%) for AOM oxidation, while other pathways (e.g., deamination, dealkylation, decarboxylation, and halogen substitution/addition) existed. Moreover, the halogen substitution/addition pathway only accounted for 1.3-10.3%, even for chlorine or bromine treatment. Oxidative treatment could decrease the reactivity of AOM in postchlorination, thereby decreasing the trichloromethane formation. However, the formation of oxygen-rich disinfection byproducts (DBPs, e.g., trichloronitromethane) could be favored, especially for ozonation. This study provides molecular-level insights into the effects of oxidative treatment on AOM and derived DBP formation in water treatment.

Transformation of dissolved organic matter leached from biodegradable and conventional microplastics under UV/chlorine treatment and the subsequent effect on contaminant removal

The ultraviolet (UV)/chlorine process has been widely applied for water treatment. However, the transformation of microplastic-leached dissolved organic matter (MP-DOM) in advanced treatment of real wastewater remains unclear. Here, we investigated alterations in the photoproperties of MP-DOM leached from biodegradable and conventional microplastics (MPs) and their subsequent effects on the degradation of sulfamethazine (SMT) by the UV/chlorine process. Spectroscopy was used to assess photophysical properties, focusing on changes in light absorption capacity, functional groups, and fluorescence components, while photochemical properties were determined by calculating the apparent quantum yields of reactive intermediates (ΦRIs). For photophysical properties, our findings revealed that the degree of molecular structure modification, functional group changes, and fluorescence characteristics during UV/chlorine treatment are closely linked to the type of MPs. For photochemical properties, the ΦRIs increased with higher chlorine dosages due to the formation of new functionalities. Both singlet oxygen (1O2) and hydroxyl radicals (•OH) formation were strongly correlated with excited triplet state of DOM (3DOM*) in the UV/chlorine treatment. Additionally, we found that the four types of MP-DOM inhibit the degradation of SMT and elucidated the mechanisms behind this inhibition. We also proposed degradation pathways for SMT and assessed the ecotoxicity of the resulting intermediates. This study provides important insights into how the characteristics and transformation of MP-DOM affect contaminant degradation, which is critical for evaluating the practical application of UV-based advanced oxidation processes (UV-AOPs).

First insight into the environmental fate of N-acetylated sulfonamides from wastewater disinfection to solar-irradiated receiving waters

The worldwide detection of emerging transformation products of organic micropollutants has raised accumulating concerns owing to their unknown environmental fate and undesired toxicity. This work first explored the reaction kinetics and mechanisms of the prevalent N-acetylated sulfonamides (N4-AcSAs, the typical sulfonamide metabolites) from wastewater disinfection to solar-irradiated receiving waters. The transformation scenarios included chlorination/bromination, photodegradation, and solar/chlorine treatment. The halogenations of two N4-AcSAs (N4-acetylated sulfadiazine, N4-AcSDZ; N4-acetylated sulfamethoxazole, N4-AcSMX) were pH-dependent at pH 5.0-8.0, and the reactions between the neutral forms of oxidants and anionic N4-AcSAs dominated the process. Furthermore, solar-based photolysis significantly eliminated N4-AcSAs in small water bodies with low dissolved organic carbon levels, while the indirect photolysis mediated by hydroxyl radicals and carbonate radicals contributed the most. The presence of chlorine residues in solar-irradiated wastewater effluents promoted the decay of N4-AcSAs, in which the generated hydroxyl radicals and ozone played a major role. Product analysis suggested the main transformation patterns of N4-AcSAs during the above scenarios included electrophilic attack, bond cleavage, SO2 extrusion, hydroxylation, and rearrangement. Multiple secondary products maintained higher persistence, mobility, and toxicity to aquatic organisms than N4-AcSAs. Overall, the natural and engineered transformations of such micropollutants underlined the necessity of including their degradation products in future chemical management and risk assessment.

Molecular composition and formation mechanism of chlorinated organic compounds in biological waste leachate treated by electrochemical oxidation with a boron-doped diamond anode

The use of electrochemical oxidation with boron-doped diamond (BDD) as an anode has been demonstrated to be an effective means of removing dissolved organic matter (DOM) from biologically treated waste leachate. However, in the presence of chloride ions, undesired chlorine evolution occurs on the anode; this forms chlorinated DOM, mostly of unknown molecular composition. We investigate the molecular composition and formation mechanism of chlorinated DOM during electrochemical oxidation process of biologically treated leachate DOM. At a current density of 8 mA/cm2, after 120 min of electrolysis, 479 unknown chlorinated DOMs were detected in the treated effluent, comprising 21.55% of the total. The unknown species are dominated by oxygen-rich, highly unsaturated structures, and exhibit higher oxidation degrees, lower unsaturation, and lower aromaticity compared to the removed nonchlorinated DOM. An additional 43.63 mg/L of known chlorinated DOM species, predominantly dichloroacetic and trichloroacetic acids, also accumulate in the treated effluent. Introducing hydroxyl radicals (HO•) to the anode surface forms reactive chlorine species including chlorine radical (Cl•), dichlorine radical (Cl2•-), and hypochlorous acid/hypochlorite (HOCl/OCl-); the concentration of HOCl/OCl- reaches 529.2 mg/L. These species react with reduced and aromatic dissolved organic matter via reaction pathways such as chlorine substitution for hydrogen (Cl+H-) and the HOCl addition reaction (HO+Cl+) to generate unknown chlorinated DOM species; the known chlorinated DOM are formed afterward via ring opening and dealkylation pathways. Our results provide a theory for the prevention and control of chlorinated DOM during treatment of chlorine-laden organic wastewater by an electrochemical oxidation system with a boron-doped diamond anode.

Estimation of suspected estrogenic transformation products generated during preservative butylparaben chlorination using a simplified effect-based analysis approach

Estrogenic transformation products (TPs) generated after water chlorination can be considered as an environmental and health concern, since they can retain and even increase the estrogenicity of the parent compound, thus posing possible risks to drinking water safety. Identification of the estrogenic TPs generated from estrogenic precursor during water chlorination is important. Herein, butylparaben (BuP), which was widely used as preservative in food, pharmaceuticals and personal care products (PPCPs), was selected for research. A simplified effect-based analysis (EDA) approach was applied for the identification of estrogenic TPs generated during BuP chlorination. Despite the removal of BuP corresponds to the decrease of estrogenicity in chlorinated samples, an significant increase of estrogenicity was observed (at T = 30 min, presented an estrogenicity equivalent to 17β-estradiol). Chemical analysis of the estrogenic chlorinated samples that have been previously subjected to biological analysis (in vitro assays), in combination with the principal component analysis (PCA) evaluation, followed by validating the estrogenic potency of most relevant estrogenic TPs through an in silico approach (molecular dynamics simulations), identified that the halogenated TP3 (3,5-Dichloro-butylparaben) increased by 62.5 % and 61.8 % of the estrogenic activity of the parent compound in samples chlorinated with 30 min and 1 h, respectively being classified as a potentially estrogenic activity driver after BuP chlorination. This study provides a scientific basis for the more comprehensive assessment of the environmental and health risk associated with BuP chlorination, highlighting the necessity of identifying the unknown estrogenic TPs generateded from estrogenic precursors chlorination.

Quantifying the effects of chlorine disinfection on microplastics by time-resolved inductively coupled plasma-mass spectrometry

Using current water treatment systems, significant amounts of microplastics (MPs) are passing through and being released into the aquatic environment. However, we do not clearly know what effects disinfection processes have had on these particles. In this study, we applied inductively coupled plasma-mass spectrometry (ICP-MS) operating in time-resolved analysis (TRA) mode for quantifying changes in the chlorine (Cl) content of MPs under a variety of water treatment scenarios. Our results illustrated that time-resolved ICP-MS offers a potential method for sensitive and direct analysis of Cl content, including Cl mass and chlorine association (%Cl/C), of discrete particles in the MP suspension by the fast sequential measurements of signals from 35Cl1H2 and 12C1H. Our research, across various water treatment scenarios, also showed that polystyrene (PS) MPs exhibited greater reactivity to Cl disinfectant after being pre-disinfected with UV light and in mildly acidic to neutral pH environments. It is noteworthy that about half of the particles in MP suspension exposed to 10 mg Cl2/L, a typical Cl dose applied in water treatment, were chlorinated, and had a Cl content comparable to that of particles subjected to extreme conditions. Of even greater concern is the fact that our cell viability tests revealed that chlorinated MPs induced considerably higher rates of cell death in both human A549 and Caco-2 cells, and that the effects were Cl dose- and polymer type-dependent. Overall, this study demonstrates the potential of time-resolved ICP-MS as a valuable technique for quantifying the Cl content of MP particles, which is crucial to assessing the fate and transformation of MPs in our water supply and treatment systems.

Enhanced disinfestation in grain spawn production through cold plasma and sodium hypochlorite synergy

Heat-resistant fungal conidia are a common source of contamination and can cause significant difficulties in producing spawns. Through the use of PCR method, Aspergillus tubingensis and Aspergillus flavus as common microbial contaminants found in wheat grain spawn were identified that had been sterilized at 120 ºc for 2 h. Since these conidia are highly resistant to standard sterilization techniques, alternative methods were used to treat them with NaOCl and cold plasma and evaluate their effectiveness in reducing contamination. Optical emission spectroscopy (OES) analysis of the plasma showed dominant emissions from the N2 second positive system and N2+ first negative system, while reactive oxygen species (ROS) spectral lines were undetected due to collision-induced quenching effects. Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive X-ray Spectroscopy (EDXS) analyses revealed notable alterations in the elemental makeup of conidia surfaces, as evidenced by a marked rise in levels of Na, O, Cl (in the case of NaOCl treatment) and N (in the case of plasma treatment). The conidia size was reduced at lower levels of NaOCl, but with increased concentrations and plasma treatment, the conidia underwent rupture and, in some cases, pulverization. The research suggests that utilizing a combined approach can be highly effective in eliminating heat-resistant fungal conidia and drastically cutting down the sterilization time for producing wheat spawn to only 30 s.

Promoting Cl2O Generation from the HOCl + HOCl Reaction on Aqueous/Frozen Air-Water Interfaces

Hypochlorous acid (HOCl) is considered a temporary reservoir of dichlorine monoxide (Cl2O). Previous studies have suggested that Cl2O is difficult to generate from the reaction of HOCl + HOCl in the gas phase. Here, we demonstrate that Cl2O can be generated from the HOCl + HOCl reaction at aqueous/frozen air-water interfaces, which is confirmed by ab initio molecular dynamic calculations. Distinct from the one-step reaction in the gas phase, our results show that Cl2O generation from HOCl + HOCl on aqueous/frozen interfaces involves two elementary steps, namely, one HOCl deprotonation and one Cl-abstraction from the other HOCl. Specifically, the mechanisms of neutral/acidic catalysis from interfacial water/nitric acid and base catalysis from ammonia, methylamine and dimethylamine have been examined. For the former, HOCl deprotonation is the rate-limiting step, and the total k of Cl2O generation increases to 9.23 × 10-9-9.10 × 10-1 M-1 s-1 at the aqueous interface and 3.20 × 10-7-4.10 × 10-3 M-1 s-1 at the frozen interface, which is at least 23 and 25 orders of magnitude greater than that of gaseous k (3.31 × 10-32 M-1 s-1). For the latter, the rate-limiting step is changed to Cl-abstraction, whose total k dramatically increases to 1.40-8.97 × 107 M-1 s-1 at the aqueous interface and 7.12-9.99 × 106 M-1 s-1 at the frozen interface. Interestingly, the Cl2O production rates ranked in the order of dimethylamine < methylamine < ammonia and decreased with increasing catalytic alkalinity. These findings provide new insights for understanding other Cl2O sources beyond the ClONO2 + HOCl reaction.

Reaction Mechanism of Oxytetracycline Degradation by Electrogenerated Reactive Chlorine: The Influence of Current Density and pH

A binary dimensionally stable anode Ti/TiO2-RuO2 electrode was used to abate the antibiotic oxytetracycline (OTC) (C22H24N2O9) in chloride water. The anode was prepared using the Pechini method and subsequently characterized by X-ray diffraction, scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS), and cyclic voltammetry (CV). The optimum values of the operational parameters affecting removal efficiency were determined using a 2 × 3 factorial design by screening j (6.0, 10, and 20 A m-2) and pH (3, 6.5, and 10). The textural analysis revealed the formation of active oxides (RuO2 and TiO2 coating rutile-type P42/mnm, space group 136), with a cracked surface and good dispersion of metal components. A contour graph verified that the most suitable condition for contaminant degradation was 20 A m-2 at a circumneutral pH of 6.5, resulting in approximately 97% degradation after 20 min of electrolysis according to pseudo-first-order reaction kinetics and the loss of the antibiotic activity of OTC. In addition, the results of oxidant formation and CV indicate that the best electrochemical activation of the anode to form Cl2-active mainly depended on pH. Liquid chromatography-mass spectrometry (LC-MS) and density functional theory were employed to propose a reaction pathway for OTC degradation. Three byproducts with m/z 426, 256, and 226 were identified corresponding to the removal of amide and amine groups, which are susceptible sites to electrophilic attack by active chlorine species. The findings from this work stand out for prospective applications of anodic electrochemical oxidation to efficiently eliminate antibiotics with similar chemical structures in wastewater containing chlorides.

Principles, challenges and prospects for electro-oxidation treatment of oilfield produced water

The oil industry is facing substantial environmental challenges, especially in managing waste streams such as Oilfield Produced Water (OPW), which represents a significant component of the industrial ecological footprint. Conventional treatment methods often fail to effectively remove dissolved oils and grease compounds, leading to operational difficulties and incomplete remediation. Electrochemical oxidation (EO) has emerged as a promising alternative due to its operational simplicity and ability to degrade pollutants directly and indirectly, which has already been applied in treating several effluents containing organic compounds. The application of EO treatment for OPW is still in an initial stage, due to the intricate nature of this matrix and scattered information about it. This study provides a technological overview of EO technology for OPW treatment, from laboratory scale to the development of large-scale prototypes, identifying design and process parameters that can potentially permit high efficiency, applicability, and commercial deployment. Research in this domain has demonstrated notable rates of removal of recalcitrant pollutants (>90%), utilizing active and non-active electrodes. Electro-generated active species, primarily from chloride, play a pivotal role in the oxidation of organic compounds. However, the highly saline conditions in OPW hinder the complete mineralization of these organics, which can be improved by using non-active anodes and lower salinity levels. The performance of electrodes greatly influences the efficiency and effectiveness of OPW treatment. Various factors must be considered when selecting the electrode material, such as its conductivity, stability, surface area, corrosion resistance, and cost. Additionally, the specific contaminants present in the OPW, and their electrochemical reactivity must be considered to ensure optimal treatment outcomes. Balancing these considerations can be challenging, but it is crucial for achieving successful OPW treatment. Active electrode materials exhibit a high affinity for chloride molecules, generating more active species than non-active materials, which exhibit more significant degradation potential due to the production of hydroxyl radicals. Regarding scale-up, key challenges include low current efficiency, the formation of by-products, electrode deactivation, and limitations in mass transfer. To address these issues, enhanced mass transfer rates and appropriate residence times can be achieved using flow-through mesh anodes and moderate current densities, which have proven to be the optimal configuration for this process.

Kinetics and mechanisms of non-radically and radically induced degradation of bisphenol A in a peroxymonosulfate-chloride system

Bisphenol A, a hazardous endocrine disruptor, poses significant environmental and human health threats, demanding efficient removal approaches. Traditional biological methods struggle to treat BPA wastewater with high chloride (Cl-) levels due to the toxicity of high Cl- to microorganisms. While persulfate-based advanced oxidation processes (PS-AOPs) have shown promise in removing BPA from high Cl- wastewater, their widespread application is always limited by the high energy and chemical usage costs. Here we show that peroxymonosulfate (PMS) degrades BPA in situ under high Cl- concentrations. BPA was completely removed in 30 min with 0.3 mM PMS and 60 mM Cl-. Non-radical reactive species, notably free chlorine species, including dissolved Cl2(l), HClO, and ClO- dominate the removal of BPA at temperatures ranging from 15 to 60 °C. Besides, free radicals, including •OH and Cl2 •-, contribute minimally to BPA removal at 60 °C. Based on the elementary kinetic models, the production rate constant of Cl2(l) (32.5 M-1 s-1) is much higher than HClO (6.5 × 10-4 M-1 s-1), and its degradation rate with BPA (2 × 107 M-1 s-1) is also much faster than HClO (18 M-1 s-1). Furthermore, the degradation of BPA by Cl2(l) and HClO were enlarged by 10- and 18-fold at 60 °C compared to room temperature, suggesting waste heat utilization can enhance treatment performance. Overall, this research provides valuable insights into the effectiveness of direct PMS introduction for removing organic micropollutants from high Cl- wastewater. It further underscores the critical kinetics and mechanisms within the PMS/Cl⁻ system, presenting a cost-effective and environmentally sustainable alternative for wastewater treatment.

Improving biodegradability of dissolved organic matter (DOM) in old landfill leachate by electrochemical pretreatment: The effect mechanism of polarity

Enhancing the biodegradability of old landfill leachate is vital for the efficient treatment or resource utilization of municipal solid waste. Electrochemical pretreatment emerges as a promising technology for transformation of refractory dissolved organic matter (DOM). However, the specific impact of polarity on improving biodegradability of DOM remains unclear. In this study, a divided electrolyzer was used to explore the changes in the biodegradability of DOM in old landfill leachate during electrolysis. Meanwhile, the correlation mechanism between BOD5 variation and DOM evolution was explored by spectroscopy and Maldi-TOF-MS analysis. Results shown that different polarities all have positive effect on enhancing the biodegradability of DOM, while the structural changes related with BOD5 are depending on the polarity. In the anode chamber, electrochemical oxidation (EO) generates and eliminates carboxyl groups. Additionally, EO concurrently eliminates humic-like substances, which are challenging for microorganisms to degrade, and protein-like substances, which are easily degradable by microorganisms. This creates a competitive mechanism that coexist the promotion and inhibition for biodegradability. In the cathode chamber, electrochemical reduction (ER) transforms DOM components, accumulating easily useable protein components for microorganisms. Kinetic studies show that EO related BOD5 changes are aptly described by a competition model, considering both generation and removal of bioavailable components. ER related BOD5 changes suit a pseudo-first-order kinetic model. These insights into the transformation of old leachate DOM support the development of methods predicting BOD5 evolution, crucial for future process optimization.

Acute toxicity assessment and QSAR modeling of zebrafish embryos exposed to methyl paraben and its halogenated byproducts

Halogenated methyl parabens are formed readily during water chlorination, with or without bromide ion presence. However, research gaps persist in in vivo toxicological assessments of vertebrates exposed to halo-MePs. To address this gap, this study evaluated acute toxicities at 24-96 h-post-fertilization in zebrafish embryos exposed to methyl paraben and its mono- or di-halogenated derivatives, using various apical endpoints. Significant enhanced toxic effects were confirmed for halo-MePs compared to MeP on embryo coagulation (3-19 fold), heartbeat rate decrement (11-80 fold), deformity rate increment (9-68 fold), and hatching failure (4-33 fold), with parentheses indicating the determined toxic potency ratios. Moreover, halo-MePs showed a significantly higher increase in biochemical levels of reactive oxygen species, catalase, superoxide dismutase, and malondialdehyde, while acetylcholinesterase activity was inhibited compared to NT and MeP. The experimental toxic potencies (log(1/EC50 or LC50)) were compared with the predicted ones (log(1/EC50 or LC50, baseline)) using the baseline toxicity Quantitative Structure-Activity Relationship previously established for zebrafish embryos. Halo-MePs were specific (or reactive) toxicants based on their toxic ratios of more than 10 for apical endpoints including heartbeat rate, deformity rate, and hatching rate, while MeP acted as a baseline toxicant. Overall, this study presents the comprehensive toxicological assessment of halo-MePs in zebrafish embryos, contributing to an essential in vivo toxicity database for halogenated phenolic contaminants in aquatic ecosystems.

Insight into the photodegradation of methylisothiazolinone and benzoisothiazolinone in aquatic environments

Methylisothiazolinone (MIT) and Benzisothiazolinone (BIT) are two widely used non-oxidizing biocides of isothiazolinones. Their production and usage volume have sharply increased since the pandemic of COVID-19, inevitably leading to more release into water environment. However, their photochemical behaviors in water environment are still unclear. Therefore, this study investigated photodegradation properties of MIT and BIT in natural water under simulated sunlight. The results demonstrated that direct photolysis was mainly responsible for their photodegradation which occurred through their excited singlet states rather than triplet states. The quantum yields of MIT and BIT photodegradation were 11 - 13.6 × 10-4 and 2.43 - 5.79 × 10-4, respectively. pH had almost no effect on the photodegradation of MIT, while the photodegradation of BIT was significantly promoted under alkaline condition due to abundance of BIT in its deprotonated form (BIT-N-). Cl-, NO3- and dissolved organic matter (DOM) in natural water inhibited the photodegradation of both MIT and BIT, with the light screening effect of DOM being the most significantly inhibitory factor. The addition of other isothiazolinones, which possibly coexisted with MIT and BIT in actual condition, slightly inhibited the photodegradation of MIT and BIT. The estimated half-life under natural sunlight at a 30°N latitude was estimated to be approximately 1.1 days. The photodegradation pathways of MIT and BIT are similar, primarily initiated from the ring-opening at the N-S bond, with Frontier electron densities (FED) calculations suggesting the likelihood of oxidation and ·OH addition reactions at the O, N, and S sites. While the photodegradation products exhibited significantly reduced acute toxicity compared to their parent compounds, they nonetheless posed substantial chronic toxicity. These insights are vital for assessing the ecological impacts of MIT and BIT in aquatic environments.

A convenient reduction method for the detection of low concentration free available chlorine--utilizing sodium sulfite as a quencher

Chlorine, serving as the mainstream disinfectant, can react with dissolved organic matter (DOM) to form undeserved disinfection by-products (DBPs). Free available chlorine (FAC) concentration is crucial to ensure effective disinfection while minimizing the formation of toxic DBPs. In this study, we propose a convenient method using sodium sulfite (Na2SO3) to reduce oxidized chlorine in FAC. The molar concentration of reduced chloride ion (Cl-) was quantified directly by ion chromatography to reflect FAC concentration. Compared with common FAC detection techniques including DPD colorimetry, iodometry, and UV methods, this novel reduction method exhibits a lower detection limit and is more resistant to interference. Common water matrices, such as DOM and anions did not affect the method accuracy (< 3.6%). Furthermore, carbonaceous DBPs (C-DBPs) like regulated trihalomethanes and halogenacetic acids, unregulated aromatic chlorophenols, did not interfere with the determination of FAC by using this reduction method. This lack of interference can be attributed to the low redox potential of Na2SO3, which does not readily react with these C-DBPs. However, nitrogenated DBPs (N-DBPs) like dichloroacetonitrile displayed slight interference (the effect of common dichloroacetonitrile concentration in water on FAC was less than 0.0007 μM). This suggests that this method is well-suited for determining FAC in chlorination processes where the C-DBPs predominated. Overall, the reduction method enables precise determination of FAC and proves valuable in assessing residual chlorine levels in both laboratory and real disinfected water samples dominated by C-DBPs.

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

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