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

Boosting Ammonium Oxidation in Wastewater by the BiOCl-Functionalized Anode

Mixed metal oxide (MMO) anodes are commonly used for electrochlorination of ammonium (NH4+) in wastewater treatment, but they suffer from low efficiency due to inadequate chlorine generation at low Cl- concentrations and sluggish reaction kinetics between free chlorine and NH4+ under acidic pH conditions. To address this challenge, we develop a straightforward wet chemistry approach to synthesize BiOCl-functionalized MMO electrodes using the MMO as an efficient Ohmic contact for electron transfer. Our study demonstrates that the BiOCl@MMO anode outperforms the pristine MMO anode, exhibiting higher free chlorine generation (24.6-60.0 mg Cl2 L-1), increased Faradaic efficiency (75.5 vs 31.0%), and improved rate constant of NH4+ oxidation (2.41 vs 0.76 mg L-1 min-1) at 50 mM Cl- concentration. Characterization techniques including electron paramagnetic resonance and in situ transient absorption spectra confirm the production of chlorine radicals (Cl• and Cl2•-) by the BiOCl/MMO anode. Laser flash photolysis reveals significantly higher apparent second-order rate constants ((4.3-4.9) × 106 M-1 s-1 at pH 2.0-4.0) for the reaction between NH4+ and Cl•, compared to the undetectable reaction between NH4+ and Cl2•-, as well as the slower reaction between NH4+ and free chlorine (102 M-1 s-1 at pH < 4.0) within the same pH range, emphasizing the significance of Cl• in enhancing NH4+ oxidation. Mechanistic studies provide compelling evidence of the capacity of BiOCl for Cl- adsorption, facilitating chlorine evolution and Cl• generation. Importantly, the BiOCl@MMO anode exhibits excellent long-term stability and high catalytic activity for NH4+-N removal in a real landfill leachate. These findings offer valuable insights into the rational design of electrodes to improve electrocatalytic NH4+ abatement, which holds great promise for wastewater treatment applications.

Rice husk-based pyrogenic carbonaceous material efficiently promoted peroxymonosulfate activation toward the non-radical pathway for the degradation of pharmaceuticals in water

Pristine pyrogenic carbonaceous material (BRH) obtained from rice husk and modified with FeCl3 (BRH-FeCl3) were prepared and explored as carbocatalysts for the activation of peroxymonosulfate (PMS) to degrade a model pharmaceutical (acetaminophen, ACE) in water. The BRH-FeCl3/PMS system removed the pharmaceutical faster than the BRH/PMS. This is explained because in BRH-FeCl3, compared to BRH, the modification (iron played a role as a structuring agent mainly) increased the average pore diameter and the presence of functional groups such as -COO-, -Si-O-, or oxygen vacancies, which allowed to remove the pollutant through an adsorption process and significant carbocatalytic degradation. BRH-FeCl3 was reusable during four cycles and had a higher efficiency for activating PMS than another inorganic peroxide (peroxydisulfate, PDS). The effects of BRH-FeCl3 and PMS concentrations were evaluated and optimized through an experimental design, maximizing the ACE degradation. In the optimized system, a non-radical pathway (i.e., the action of singlet oxygen, from the interaction of PMS with defects and/or -COO-/-Si-O- moieties on the BRH-FeCl3) was found. The BRH-FeCl3/PMS system generated only one primary degradation product that was more susceptible to biodegradation and less active against living organisms than ACE. Also, the BRH-FeCl3/PMS system induced partial removals of chemical oxygen demand and dissolved organic carbon. Furthermore, the carbocatalytic system eliminated ACE in a wide pH range and in simulated urine, having a low-moderate electric energy consumption, indicating the feasibility of the carbocatalytic process to treat water polluted with pharmaceuticals.

Developing a hybrid model for predicting the reaction kinetics between chlorine and micropollutants in water

Understanding the reactivities of chlorine towards micropollutants is crucial for assessing the fate of micropollutants in water chlorination. In this study, we integrated machine learning with kinetic modeling to predict the reaction kinetics between micropollutants and chlorine in deionized water and real surface water. We first established a framework to predict the apparent second-order rate constants for micropollutants with chlorine by combining Morgan molecular fingerprints with machine learning algorithms. The framework was tuned using Bayesian optimization and showed high prediction accuracy. It was validated through experiments and used to predict the unreported apparent second-order rate constants for 103 emerging micropollutants with chlorine. The framework also improved the understanding of the structure-dependence of micropollutants' reactivity with chlorine. We incorporated the predicted apparent second-order rate constants into the Kintecus software to establish a hybrid model to profile the time-dependent changes of micropollutant concentrations by chlorination. The hybrid model was validated by experiments conducted in real surface water in the presence of natural organic matter. The hybrid model could predict how much micropollutants were degraded by chlorination with varied chlorine contact times and/or initial chlorine dosages. This study advances fundamental understanding of the reaction kinetics between chlorine and emerging micropollutants, and also offers a valuable tool to assess the fate of micropollutants during chlorination of drinking water.

Ipso Substitution of Aromatic Bromine in Chlorinated Waters: Impacts on Trihalomethane Formation

Parabens and salicylates were examined as disinfection byproduct (DBP) precursors to explore the possible influence of ipso substitution (i.e., halogen exchange) on the yield and speciation of trihalomethanes (THMs) formed during water chlorination. Substoichiometric conversion of C-Br bonds into C-Cl bonds was confirmed for several parabens and salicylates. The co-occurrence of (mono)brominated and nonhalogenated precursors in the presence of free chlorine (but in the absence of added Br-) generated polybrominated THMs, implicating ipso substitution. The THM molar yield, bromine incorporation, and bromine recovery from brominated and nonhalogenated precursor mixtures were commensurate with those observed from equimolar additions of NaBr, indicating efficient displacement of aromatic bromine by free chlorine followed by reincorporation of liberated HOBr into DBP precursors. The THM molar yield from brominated precursors was enhanced by a factor of ≤20 relative to that from nonhalogenated precursors. Trends in THM molar yields and bromine incorporation differed between brominated parabens and brominated salicylates, suggesting that the influence of ipso substitution on THM formation varies with the structure of the organic precursor. Collectively, these results provide new evidence of the often-overlooked role ipso substitution can play in promoting halogen exchange and bromine enrichment among DBPs in chlorinated waters.

Unravelling High-Molecular-Weight DBP Toxicity Drivers in Chlorinated and Chloraminated Drinking Water: Effect-Directed Analysis of Molecular Weight Fractions

As disinfection byproducts (DBPs) are ubiquitous sources of chemical exposure in disinfected drinking water, identifying unknown DBPs, especially unknown drivers of toxicity, is one of the major challenges in the safe supply of drinking water. While >700 low-molecular-weight DBPs have been identified, the molecular composition of high-molecular-weight DBPs remains poorly understood. Moreover, due to the absence of chemical standards for most DBPs, it is difficult to assess toxicity contributions for new DBPs identified. Based on effect-directed analysis, this study combined predictive cytotoxicity and quantitative genotoxicity analyses and Fourier transform ion cyclotron resonance mass spectrometry (21 T FT-ICR-MS) identification to resolve molecular weight fractions that induce toxicity in chloraminated and chlorinated drinking waters, along with the molecular composition of these DBP drivers. Fractionation using ultrafiltration membranes allowed the investigation of <1 kD, 1-3 kD, 3-5 kD, and >5 kD molecular weight fractions. Thiol reactivity based predictive cytotoxicity and single-cell gel electrophoresis based genotoxicity assays revealed that the <1 kD fraction for both chloraminated and chlorinated waters exhibited the highest levels of predictive cytotoxicity and direct genotoxicity. The <1 kD target fraction was used for subsequent molecular composition identification. Ultrahigh-resolution MS identified singly charged species (as evidenced by the 1 Da spacing in 13C isotopologues), including 3599 chlorine-containing DBPs in the <1 kD fraction with the empirical formulas CHOCl, CHOCl2, and CHOCl3, with a relative abundance order of CHOCl > CHOCl2 ≫ CHOCl3. Interestingly, more high-molecular-weight CHOCl1-3 DBPs were identified in the chloraminated vs chlorinated waters. This may be due to slower reactions of NH2Cl. Most of the DBPs formed in chloraminated waters were composed of high-molecular-weight Cl-DBPs (up to 1 kD) rather than known low-molecular-weight DBPs. Moreover, with the increase of chlorine number in the high-molecular-weight DBPs detected, the O/C ratio exhibited an increasing trend, while the modified aromaticity index (AImod) showed an opposite trend. In drinking water treatment processes, the removal of natural organic matter fractions with high O/C ratio and high AImod value should be strengthened to minimize the formation of known and unknown DBPs.

Mechanistic Insight for Disinfection Byproduct Formation Potential of Peracetic Acid and Performic Acid in Halide-Containing Water

Peracetic acid (PAA) and performic acid (PFA) are two major peroxyacid (POA) oxidants of growing usage. This study reports the first systematic evaluation of PAA, PFA, and chlorine for their disinfection byproduct (DBP) formation potential in wastewater with or without high halide (i.e., bromide or iodide) concentrations. Compared with chlorine, DBP formation by PAA and PFA was minimal in regular wastewater. However, during 24 h disinfection of saline wastewater, PAA surprisingly produced more brominated and iodinated DBPs than chlorine, while PFA effectively kept all tested DBPs at bay. To understand these phenomena, a kinetic model was developed based on the literature and an additional kinetic investigation of POA decay and DBP (e.g., bromate, iodate, and iodophenol) generation in the POA/halide systems. The results show that PFA not only oxidizes halides 4-5 times faster than PAA to the corresponding HOBr or HOI but also efficiently oxidizes HOI/IO- to IO3-, thereby mitigating iodinated DBP formation. Additionally, PFA's rapid self-decay and slow release of H2O2 limit the HOBr level over the long-term oxidation in bromide-containing water. For saline water, this paper reveals the DBP formation potential of PAA and identifies PFA as an alternative to minimize DBPs. The new kinetic model is useful to optimize oxidant selection and elucidate involved DBP chemistry.

Reaction Mechanisms of Chlorine Dioxide with Phenolic Compounds─Influence of Different Substituents on Stoichiometric Ratios and Intrinsic Formation of Free Available Chlorine

Chlorine dioxide (ClO2) is an oxidant applied in water treatment processes that is very effective for disinfection and abatement of inorganic and organic pollutants. Thereby phenol is the most important reaction partner of ClO2 in reactions of natural organic matter (NOM) and in pollutant degradation. It was previously reported that with specific reaction partners (e.g., phenol), free available chlorine (FAC) could form as another byproduct next to chlorite (ClO2-). This study investigates the impact of different functional groups attached to the aromatic ring of phenol on the formation of inorganic byproducts (i.e., FAC, ClO2-, chloride, and chlorate) and the overall reaction mechanism. The majority of the investigated compounds reacted with a 2:1 stoichiometry and formed 50% ClO2- and 50% FAC, regardless of the position and kind of the groups attached to the aromatic ring. The only functional groups strongly influencing the FAC formation in the ClO2 reaction with phenols were hydroxyl- and amino-substituents in ortho- and para-positions, causing 100% ClO2- and 0% FAC formation. Additionally, this class of compounds showed a pH-dependent stoichiometric ratio due to pH-dependent autoxidation. Overall, FAC is an important secondary oxidant in ClO2 based treatment processes. Synergetic effects in pollutant control and disinfection might be observable; however, the formation of halogenated byproducts needs to be considered as well.

Bromination of Quorum Sensing Molecules: Vanadium Bromoperoxidase and Cerium Dioxide Nanocrystals via Free Active Bromine Transform Bacterial Communication

The halogenation of quorum sensing molecules (QSMs) is known to be catalyzed by enzymes such as haloperoxidase (HPO) as well as cerium dioxide nanocrystals (NC), which mimic enzymes. Those enzymes and mimics can influence biological processes such as biofilm formation, where bacteria use QSMs for the "chemical" communication between each other and the coordination of surface colonization. However, not much is known about the degradation behavior of a broad spectrum of QSMs, especially for HPO and its mimics. Therefore, in this study, the degradation of three QSMs with different molecule moieties was elucidated. For this purpose, different batch experiments were carried out with HPOs, NCs and free active bromine (FAB). For N-β-ketocaproyl-homoserine lactone (3-Oxo-C6-AHL), N-cis-tetradec-9Z-enoyl-homoserine lactone (C14:1-AHL) and 2-heptyl-4-quinolone (HHQ) a fast degradation and moiety-specific transformations were observed. The HPO vanadium bromoperoxidase as well as cerium dioxide NCs catalyzed the formation of the same brominated transformation products (TPs). Since the same TPs are formed in batch experiments with FAB it is very likely that FAB is playing a major role in the catalytical reaction mechanism leading to the transformation of QSMs. In this study in total 17 TPs could be identified in different levels of confidence and the catalytic degradation processes for two QS groups (unsaturated AHLs and alkyl quinolones) with cerium dioxide NCs and vanadium bromoperoxidase were expanded.

CuO Promotes the Formation of Halogenated Disinfection Byproducts during Chlorination via an Enhanced Oxidation Pathway

Previous studies showed that cupric oxide (CuO) can enhance the formation of trihalomethanes (THMs), haloacetic acids, and bromate during chlorination of bromide-containing waters. In this study, the impact of CuO on the formation kinetics and mechanisms of halogenated disinfection byproducts (DBPs) during chlorination was investigated. CuO does not enhance the formation of DBPs (i.e., 1,1,1-trichloropropanone, chloroform, and trichloroacetaldehyde (TCAL) /dichloroacetonitrile) during chlorination of acetone, 3-oxopentanedioic acid (3-OPA), and aspartic acid, respectively. This indicates that the halogen substitution pathway cannot be enhanced by CuO. Instead, CuO (0.1 g L-1) accelerates the second-order rate constants for reactions of chlorine (HOCl) with TCAL, citric acid, and oxalic acid at pH 8.0 and 21 °C from <0.1 to 29.4, 7.2, and 15.8 M-1 s-1, respectively. Oxidation pathway predominates based on the quantification of oxidation products (e.g., a trichloroacetic acid yield of ∼100% from TCAL) and kinetic modeling. CuO can enhance the formation of DBPs (e.g., THMs, haloacetaldehydes, and haloacetonitriles) during chlorination of model compounds and dissolved organic matter, of which both halogen substitution and oxidation pathways are required. Reaction rate constants of rate-limiting steps (e.g., citric acid to 3-OPA, aromatic ring cleavage) could be enhanced by CuO via an oxidation pathway since CuO-HOCl complex is more oxidative toward a range of substrates than HOCl in water. These findings provide novel insights into the DBP formation pathway in copper-containing distribution systems.

Bacteria and Virus Inactivation: Relative Efficacy and Mechanisms of Peroxyacids and Chlor(am)ine

Peroxyacids (POAs) are a promising alternative to chlorine for reducing the formation of disinfection byproducts. However, their capacity for microbial inactivation and mechanisms of action require further investigation. We evaluated the efficacy of three POAs (performic acid (PFA), peracetic acid (PAA), and perpropionic acid (PPA)) and chlor(am)ine for inactivation of four representative microorganisms (Escherichia coli (Gram-negative bacteria), Staphylococcus epidermidis (Gram-positive bacteria), MS2 bacteriophage (nonenveloped virus), and Φ6 (enveloped virus)) and for reaction rates with biomolecules (amino acids and nucleotides). Bacterial inactivation efficacy (in anaerobic membrane bioreactor (AnMBR) effluent) followed the order of PFA > chlorine > PAA ≈ PPA. Fluorescence microscopic analysis indicated that free chlorine induced surface damage and cell lysis rapidly, whereas POAs led to intracellular oxidative stress through penetrating the intact cell membrane. However, POAs (50 μM) were less effective than chlorine at inactivating viruses, achieving only ∼1-log PFU removal for MS2 and Φ6 after 30 min of reaction in phosphate buffer without genome damage. Results suggest that POAs' unique interaction with bacteria and ineffective viral inactivation could be attributed to their selectivity toward cysteine and methionine through oxygen-transfer reactions and limited reactivity for other biomolecules. These mechanistic insights could inform the application of POAs in water and wastewater treatment.

Reactions of Monobromamine and Dibromamine with Phenolic Compounds and Organic Matter: Kinetics and Formation of Bromophenols and Bromoform

Monobromamine (NH2Br) and dibromamine (NHBr2) produced from reactions of hypobromous acid (HOBr) with ammonia can react with phenolic structures of natural organic matter (NOM) to produce disinfection byproducts such as bromoform (CHBr3). The reactivity of NH2Br was controlled by the reaction of the bromoammonium ion (NH3Br+) with phenolate species, with specific rate constants ranging from 6.32 × 102 for 2,4,6-tribromophenol to 1.22 × 108 M-1 s-1 for phenol. Reactions of NHBr2 with phenol and bromophenols were negligible compared to its self-decomposition; rate constants could be determined only with resorcinol for pH > 7. At pH 8.1-8.2, no formation of CHBr3 was observed from the reaction of NH2Br with phenol while the reaction of NH2Br with resorcinol produced a significant concentration of CHBr3. In contrast to NH2Br, a significant amount of CHBr3 produced with an excess of NHBr2 over phenol was explained by the reactions of HOBr produced from NHBr2 decomposition. A comprehensive kinetic model including the formation and decomposition of bromamines and the reactivity of HOBr and NH2Br with phenolic compounds was developed at pH 8.0-8.3. Furthermore, the kinetic model was used to evaluate the significance of the NH2Br and NHBr2 reactions with the phenolic structures of two NOM isolates.

Predicting Transformation Products during Aqueous Oxidation Processes: Current State and Outlook

Water quality and its impacts on human and ecosystem health presents tremendous global challenges. While oxidative water treatment can solve many of these problems related to hygiene and micropollutants, identifying and predicting transformation products from a large variety of micropollutants induced by dosed chemical oxidants and in situ formed radicals is still a major challenge. To this end, a better understanding of the formed transformation products and their potential toxicity is needed. Currently, no theoretical tools alone can predict oxidatively induced transformation products in aqueous systems. Coupling experimental and theoretical studies has advanced the understanding of reaction kinetics and mechanisms significantly. This perspective article highlights the key progress made concerning experimental and computational approaches to predict transformation products. Knowledge gaps are identified, and the research required to advance the predictive capability is discussed.

Insights into C-C Bond Cleavage Mechanisms in Dichloroacetonitrile Formation during Chlorination of Long-Chain Primary Amines, Amino Acids, and Dipeptides

Dichloroacetonitrile (DCAN) as one of the potentially prioritized regulated DBPs has drawn great attention; however, understanding its formation, especially the C-C bond cleavage mechanisms, is limited. In this study, DCAN formation mechanisms from long-chain primary amines, amino acids, and dipeptides during chlorination were investigated by a combined computational and experimental approach. The results indicate that nitriles initially generate for all of the above precursors, then they undergo β-C-hydroxylation or/and α-C-chlorination processes, and finally, DCAN is produced through the Cα-Cβ bond cleavage. For the first time, the underlying mechanism of the C-C bond cleavage was unraveled to be electron transfer from the O- anion into its attached C atom in the chlorinated nitriles, leading to the strongly polarized Cα-Cβ bond heterocleavage and DCAN- formation. Moreover, DCAN molar yields of precursors studied in the present work were found to be determined by their groups at the γ-site of the amino group, where the carbonyl group including -CO2-, -COR, and -CONHR, the aromatic group, and the -OH group can all dramatically facilitate DCAN formation by skipping over or promoting the time-consuming β-C-hydroxylation process and featuring relatively lower activation free energies in the C-C bond cleavage. Importantly, 4-amino-2-hydroxybutyric acid was revealed to possess the highest DCAN yield among all the known aliphatic long-chain precursors to date during chlorination. Additionally, enonitriles, (chloro-)isocyanates, and nitriles can be generated during DCAN formation and should be of concern due to their high toxicities.

A Bipolar Membrane-Integrated Electrochlorination Process for Highly Efficient Ammonium Removal in Mature Landfill Leachate: The Importance of ClO• Generation

Electrochemical oxidation has been demonstrated to be a useful method for removing biorefractory organic pollutants in mature landfill leachate but suffers from low efficiency in eliminating ammonium because of its resistance to being oxidized by HO• or free chlorine (FC) at decreased pH. Here, we propose a new bipolar membrane-electrochlorination (BPM-EC) process to address this issue. We found that the BPM-EC system was significantly superior to both the undivided and divided reactors with monopolar membranes in terms of elevated rate of ammonium removal, attenuated generation of byproducts (e.g., nitrate and chloramines), increased Faradaic efficiency, and decreased energy consumption. Mechanistic studies revealed that the integration of BPM was helpful in creating alkaline environments in the vicinity of the anode, which facilitated production of surface-bound HO• and FC and eventually promoted in situ generation of ClO•, a crucial reactive species mainly responsible for accelerating ammonium oxidation and selective transformation to nitrogen. The efficacy of BPM-EC in treating landfill leachates with different ammonium concentrations was verified under batch and continuous-flow conditions. A kinetic model that incorporates the key parameters was developed, which can successfully predict the optimal number of BPM-EC reactors (e.g., 2 and 5 for leachates containing 589.4 ± 5.5 and 1258.1 ± 9.6 mg L-1 NH4+-N, respectively) necessary for complete removal of ammonium. These findings reveal that the BPM-EC process shows promise in treating ammonium-containing wastewater, with advantages that include effectiveness, adaptability, and flexibility.

Production of Dichloroacetonitrile from Derivatives of Isoxaflutole Herbicide during Water Treatment

The herbicide isoxaflutole has the potential to contaminate drinking water directly, as well as upon hydrolyzing to its active form diketonitrile. Diketonitrile also may impact water quality by acting as a precursor for dichloroacetonitrile (DCAN), which is an unregulated but highly toxic disinfection byproduct (DBP). In this study, we investigated the reaction of diketonitrile with free chlorine and chloramine to form DCAN. We found that diketonitrile reacts with free chlorine within seconds but reacts with chloramine on the time scale of hours to days. In the presence of both oxidants, DCAN was generated at yields up to 100%. Diketonitrile reacted fastest with chlorine at circumneutral pH, which was consistent with base-catalyzed halogenation involving the enolate form of diketonitrile present at alkaline pH and electrophilic hypochlorous acid, which decreases in abundance above its pKa (7.5). In contrast, we found that diketonitrile reacts faster with chloramine as pH values decreased, consistent with an attack on the enolate by electrophilic protonated monochloramine that increases in abundance at acidic pH approaching its pKa (1.6). Our results indicate that increasing isoxaflutole use, particularly in light of the recent release of genetically modified isoxaflutole-tolerant crops, could result in greater occurrences of a high-yield DCAN precursor during disinfection.

A Novel Source of Radicals from UV/Dichloroisocyanurate for Surpassing Abatement of Emerging Contaminants Versus Conventional UV/Chlor(am)ine Processes

Ultraviolet (UV)/chlor(am)ine processes are emerging advanced oxidation processes (AOPs) for water decontamination and raising continuous attention. However, limitations appear in the UV/hypochlorite and UV/monochloramine for removing specific contaminants ascribed to the differences in the sorts and yields of free radicals. Here, this study reports UV/dichloroisocyanurate (NaDCC) as a novel source of radicals. NaDCC was demonstrated to be a well-balanced compound between hypochlorite and monochloramine, and it had significant UV absorption and a medium intrinsic quantum yield. The UV/NaDCC produced more substantial hydroxyl radicals (·OH) and reactive chlorine species (RCSs, including Cl·, ClO·, and Cl2·-) than conventional UV/chlor(am)ine, thereby generating a higher oxidation efficiency. The reaction mechanisms, environmental applicability, and energy requirements of the UV/NaDCC process for emerging contaminants (ECs) abatement were further investigated. The results showed that ·OH and ·NH2 attacked ECs mostly through hydrogen atom transfer (HAT) and radical adduct formation, whereas Cl· destroyed ECs mainly through HAT and single electron transfer, with ClO· playing a certain role through HAT. Kinetic model analyses revealed that the UV/NaDCC outperformed the conventional UV/chlor(am)ine in a variety of water matrices with superior degradation efficiency, significantly saving up to 96% electrical energy per order. Overall, this study first demonstrates application prospects of a novel AOP using UV/NaDCC, which can compensate for the deficiency of the conventional UV/chlor(am)ine AOPs.

Contributions of Pharmaceuticals to DBP Formation and Developmental Toxicity in Chlorination of NOM-containing Source Water

Pharmaceuticals have been considered a priority group of emerging micropollutants in source waters in recent years, while their role in the formation and toxicity of disinfection byproducts (DBPs) during chlorine disinfection remains largely unclear. In this study, the contributions of natural organic matter (NOM) and pharmaceuticals (a mixture of ten representative pharmaceuticals) to the overall DBP formation and toxicity during drinking water chlorination were investigated. By innovatively "normalizing" chlorine exposure and constructing a kinetic model, we were able to differentiate and evaluate the contributions of NOM and pharmaceuticals to the total organic halogen (TOX) formation for source waters that contained different levels of pharmaceuticals. It was found that at a chlorine contact time of 1.0 h, NOM (2 mg/L as C) and pharmaceuticals (total 0.0062-0.31 mg/L as C) contributed 79.8-99.5% and 0.5-20.2%, respectively, of TOX. The toxicity test results showed that the chlorination remarkably increased the toxicity of the pharmaceutical mixture by converting the parent compounds into more toxic pharmaceutical-derived DBPs, and these DBPs might contribute significantly to the overall developmental toxicity of chlorinated waters. This study highlights the non-negligible role of pharmaceuticals in the formation and toxicity of overall DBPs in chlorinated drinking water.

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

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

Formation Mechanisms of Nitro Products from Transformation of Aliphatic Amines by UV/Chlorine Treatment

Formation of nitrogenous disinfection byproducts from aliphatic amines is a widespread concern owing to the serious health risks associated with them. However, the mechanisms of transforming aliphatic amines and forming nitro products in the UV/chlorine process have rarely been discussed, which are investigated in this work. Initially, secondary amines (R1R2NH) are transformed into secondary organic chloramines (R1R2NCl) via chlorination. Subsequently, radicals, such as HO• and Cl•, are found to contribute predominantly to such transformations. The rate constants at which HO•, Cl•, and Cl2•- react with R1R2NCl are (2.4-5.1) × 109, (1.5-3.8) × 109, and (1.2-6.1) × 107 M-1 s-1, respectively. Consequently, R1R2NCl are transformed into primary amines (R1NH2/R2NH2) and chlorinated primary amines (R1NHCl/R2NHCl and R1NCl2/R2NCl2) by excess chlorine. Furthermore, primarily driven by UV photolysis, chlorinated primary amines can be transformed into nitroalkanes with conversion rates of ∼10%. Dissolved oxygen and free chlorine play crucial roles in forming nitroalkanes, and post-chlorination can further form chloronitroalkanes, such as trichloronitromethane (TCNM). Radicals are involved in forming TCNM in the UV/chlorine process. This study provides new insights into the mechanisms of transforming aliphatic amines and forming nitro products using the UV/chlorine process.

Reaction of Amino Acids with Ferrate(VI): Impact of the Carboxylic Group on the Primary Amine Oxidation Kinetics and Mechanism

Ferrate (Fe(VI)) is a novel oxidant that can be used to mitigate disinfection byproduct (DBP) precursors. However, the reaction of Fe(VI) with organic nitrogen, which is a potential precursor of potent nitrogenous DBPs, remains largely unexplored. The present work aimed to identify the kinetics and products for the reaction of Fe(VI) with primary amines, notably amino acids. A new kinetic model involving ionizable intermediates was proposed and can describe the unusual pH effect on the Fe(VI) reactivity toward primary amines and amino acids. The Fe(VI) oxidation of phenylalanine produced a mixture of nitrile, nitrite/nitrate, amide, and ammonia, while nitroalkane was an additional product in the case of glycine. The product distribution for amino acids significantly differed from that of uncarboxylated primary amines that mainly generate nitriles. A general reaction pathway for primary amines and amino acids was proposed and notably involved the formation of imines, the degradation of which was affected by the presence of a carboxylic group. In comparison, ozonation led to higher yields of nitroalkanes that could be readily converted to potent halonitroalkanes during chlor(am)ination. Based on this study, Fe(VI) can effectively mitigate primary amine-based, nitrogenous DBP precursors with little formation of toxic halonitroalkanes.

Identification and Quantification of Aqueous Disinfectants Using an Array of Carbon Nanotube-Based Chemiresistors

Disinfection of water is essential to prevent the growth of pathogens, but at high levels, it can cause harm to human health. Therefore, accurate monitoring of disinfectant concentrations in water is essential to ensure safe drinking water. The use of multiple disinfectants at different stages in water treatment plants makes it necessary to also identify the type and concentrations of all of the disinfectant species present. Here, we demonstrate an effective approach to identify and quantify multiple disinfectants (using the example of free chlorine and potassium permanganate) in water using single-walled carbon nanotube (SWCNT)-based reagent-free chemiresistive sensing arrays. Facile fabrication of chemiresistive devices makes them a popular choice for the implementation of sensor arrays. Our sensing array consists of functionalized and unfunctionalized (blank) SWCNT sensors to distinguish the disinfectants. The distinct responses from the different sensors at varying concentrations and pH can be fitted to the mathematical model of a Langmuir adsorption isotherm separately for each sensor. Blank and functionalized sensors respond through different mechanisms that result in varying responses that are concentration- and pH-dependent. Chemometric techniques such as principal component analysis (PCA) and partial least-squares-discriminant analysis (PLS-DA) were used to analyze the sensor data. PCA showed an excellent separation of the analytes over five different pHs (5.5, 6.5, 7.5, 8.5, and 9.5). PLS-DA provided excellent separability as well as good predictability with a Q2 of 94.26% and an R2 of 95.67% for the five pH regions of the two analytes. This proof-of-concept solid-state chemiresistive sensing array can be developed for specific disinfectants that are commonly used in water treatment plants and can be deployed in water distribution and monitoring facilities. We have demonstrated the applicability of chemiresistive devices in a sensor array format for the first time for aqueous disinfectant monitoring.

The secondary metabolite hydrogen cyanide protects Pseudomonas aeruginosa against sodium hypochlorite-induced oxidative stress

The high pathogenicity of Pseudomonas aeruginosa is attributed to the production of many virulence factors and its resistance to several antimicrobials. Among them, sodium hypochlorite (NaOCl) is a widely used disinfectant due to its strong antimicrobial effect. However, bacteria develop many mechanisms to survive the damage caused by this agent. Therefore, this study aimed to identify novel mechanisms employed by P. aeruginosa to resist oxidative stress induced by the strong oxidizing agent NaOCl. We analyzed the growth of the P. aeruginosa mutants ΔkatA, ΔkatE, ΔahpC, ΔahpF, ΔmsrA at 1 μg/mL NaOCl, and showed that these known H2O2 resistance mechanisms are also important for the survival of P. aeruginosa under NaOCl stress. We then conducted a screening of the P. aeruginosa PA14 transposon insertion mutant library and identified 48 mutants with increased susceptibility toward NaOCl. Among them were 10 mutants with a disrupted nrdJa, bvlR, hcnA, orn, sucC, cysZ, nuoJ, PA4166, opmQ, or thiC gene, which also exhibited a significant growth defect in the presence of NaOCl. We focussed our follow-up experiments (i.e., growth analyzes and kill-kinetics) on mutants with defect in the synthesis of the secondary metabolite hydrogen cyanide (HCN). We showed that HCN produced by P. aeruginosa contributes to its resistance toward NaOCl as it acts as a scavenger molecule, quenching the toxic effects of NaOCl.

Cobalt-based metal-organic framework-functionalized graphene oxide modified electrode as a new electrochemical sensing platform for detection of free chlorine in aqueous solution

This work reports the profit of using a MOF compound for developing a sensitive electrochemical sensor to free chlorine detection in an aqueous solution. Co-MOF and FGO composites were synthesized and combined with the carbon paste (CP) to prepare an efficient electrochemical sensor with high sensing ability. The fabricated Co-MOF and FGO composites were characterized by SEM, EDX, FT-IR, and XRD techniques. Meanwhile, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were utilized to assess the electrochemical performance of the Co-MOF-FGO/CP modified electrode. Under the optimized condition, the amperometric detection showed that the reduction current of free chlorine increased linearly with a coefficient determination of 0.995 during its wide concentration range of 0.1-700 ppm. Also the detection limit (LOD) (S/N = 3) was 0.01 ppm. The selectivity of the sensor was tested with possible interferences, and satisfactory results were obtained. The proposed sensor was successfully used to determine the free chlorine in tap water and swimming pool water real samples. The results suggested that this proposed sensor could pave the way for developing the electrochemical sensor of free chlorine in aqueous media with MOFs.

Characterization of halogenated organic compounds by the Fourier transform ion cyclotron resonance mass spectrometry: A critical review

Halogenated organic compounds (HOCs), widely present in various environments, are generally formed by natural processes (e.g., photochemical halogenation) and anthropogenic activities (e.g., water disinfection and anthropogenic discharge of HOCs), posing health and environmental risks. Therefore, in-depth knowledge of the molecular composition, transformation, and fate of HOCs is crucial to regulate and reduce their formation. Because of the extremely complex nature of HOCs and their precursors, the molecular composition of HOCs remains largely unknown. The Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) offers the most powerful resolution and mass accuracy for the simultaneous molecular-level characterization of HOCs and their precursors. However, there is still a paucity of reviews regarding the comprehensive characterization of HOCs by FT-ICR MS. Based on the FT-ICR MS, the formation mechanism, sample pretreatment, and analysis methods were summarized for two typical HOCs classes, namely halogenated disinfection byproducts and per- and polyfluoroalkyl substances in this review. Moreover, we have highlighted data analysis methods and some typical applications of HOCs using FT-ICR MS and proposed suggestions for current issues. This review will deepen our understanding of the chemical characterization of HOCs and their formation mechanisms and transformation at the molecular level in aquatic systems, facilitating the application of the state-of-the-art FT-ICR MS in environmental and geochemical research.

Nanoclays and mineral derivates applied to pesticide water remediation

Although pesticides are vital in agroecosystems to control pests, their indiscriminate use generates innumerable environmental problems daily. Groundwater and surface water networks are the most affected environmental matrices. Since these water basins are mainly used to obtain water for human consumption, it is a challenge to find solutions to pesticide contamination. For these reasons, development of efficient and sustainable remedial technologies is key. Based on their unique properties including high surface area, recyclability, environmental friendliness, tunable surface chemistry and low cost, nanoclays and derived minerals emerged as effective adsorbents towards environmental remediation of pesticides. This study provides a comprehensive review of the use of nanoclays and mineral derivatives as adsorbents for pesticides in water. For this purpose, the characteristics of existing pesticides and general aspects of the relevant clays and minerals are discussed. Furthermore, the study provides insightful discussion on the potential application of nanoclays and their derivatives toward the mitigation of pesticide pollution in the environment. Finally, the outlook and future prospects on nanoclay implications and their environmental implementation are elucidated.

Alkaline chlorination of drinking water: A trade-off between genotoxicity control and trihalomethane formation

The pH of chlorination is an important factor affecting the formation of disinfection byproducts (DBPs). In this study, we discovered that the genotoxicity induced by chlorination can be effectively reduced under alkaline conditions. As the pH of chlorination increased from 6.5 to 8.5, the genotoxicity of investigated waters reduced by ∼30-90 %. By assessing the genotoxicity of the mixture of measured DBPs, it was found that the contribution of measured DBPs to the overall genotoxicity was lower than 5 %, and the significant reduction of genotoxicity was largely associated with unknown DBPs. The result of Pearson's correlation analysis indicated that humified organics and soluble microbial byproducts were likely responsible for the genotoxicity, and their derived genotoxic compounds (i.e., unknown DBPs) tended to decompose during alkaline chlorination. However, the control of genotoxicity by alkaline chlorination was achieved at the expense of promoting trihalomethane (THM) formation. The highest genotoxicity reduction (93 %) was observed for chlorinated granular activated carbon-treated waters, but the formation of THMs was promoted to a level approaching that in untreated waters. The inconsistent trend of overall genotoxicity and THM concentration during alkaline chlorination suggested the inadequacy of THMs as metric for DBP exposure, and considerations should also be given to the toxicity of bulk water in addition to regulated DBPs.

Colorimetric and fluorometric dual-mode determination of hypochlorite based on redox-mediated quenching

We have successfully created a dual-modal probe, labeled as of iron(ii)-ortho-phenanthroline/N, S@g-CDs, which combines both fluorometric and colorimetric techniques for the accurate and sensitive detection of hypochlorite (ClO-). The mechanism behind this probe involves the fluorescence quenching interaction between nitrogen and sulfur co-doped green emissive carbon dots (N, S@g-CDs) and the iron(ii)-ortho-phenanthroline chelate, utilizing both the inner filter effect and redox processes. As the concentration of ClO- increases, the iron(ii) undergo oxidation to iron(iii) as follows: Fe(ii) + 2HClO = Fe(iii) + Cl2O + H2O, leading to the restoration of N, S@g-CDs fluorescence. Simultaneously, the color of the system transitions gradually from red to colorless, enabling colorimetric measurements. In the fluorometric method with an excitation wavelength of 370 nm, the ClO- concentration exhibits a wide linear correlation with fluorescence intensity ranging from 0.07 to 220 μM. The detection limit achieved in this method is 0.02 μM (S/N = 3). In contrast, the colorimetric method exhibits a linear range of 1.12 to 200 μM, with a detection limit of 0.335 μM (S/N = 3). The proposed selective absorbance for this method is 510 nm. The probe has been effectively utilized for the detection of ClO- in various samples, including water and milk samples. This successful application showcases its potential for determining ClO- in complex matrices, highlighting its broad range of practical uses.

Kinetics of hypochlorous acid reactions with organic and chloride-containing tropospheric aerosol

Chlorine plays an important role in tropospheric oxidation processes, in both marine and continental environments. Although modeling studies have explored the importance of halogen chemistry, uncertainty remains in associated chemical mechanisms and fundamental kinetics parameters. Prior kinetics measurements of multiphase halogen recycling reactions have been largely performed with dilute, bulk solutions, leaving unexplored more realistic chemical systems which have high solute concentrations and are internally mixed with both halide and organic components. Here, we address the multiphase kinetics of gaseous HOCl using an aerosol flow tube and aerosol mass spectrometer to study its reactions with particulate chloride, using atmospherically relevant particle acidity, solute concentrations, and ionic strength. We also investigate the chemistry that results when biomass burning (BB) aerosol components and chloride are internally mixed. Using pH-buffered deliquesced particles, we show that the rate constant for reaction of dissolved HOCl with H+ and Cl- at high relative humidity (RH) (80-85%) is within a factor of two of the literature value for bulk phase conditions. However, at lower RH values (60-70%) where the particles are considerably more concentrated, the rate constant for chloride loss from the particles is an order of magnitude higher. For pure organic compounds commonly found in biomass burning (BB) aerosol, such as coniferaldehyde, salicylic acid and furfural, an increase in the aerosol chlorine content occurs with HOCl exposure, indicating the formation of organochlorine species. Together, these independent findings explain results for internally mixed aerosol particles with both chloride and BB components present where we observed behavior consistent with both chloride loss and organochlorine formation occurring simultaneously upon HOCl exposure. Our results indicate that chlorine recycling via HOCl uptake by chloride-containing particles will occur in the atmosphere efficiently over a wide range of RH conditions, even when reactive organic compounds are present in the same particles as chloride. Simultaneously, formation of organochlorine compounds, which are commonly toxic, is likely occurring when reactive organic components are present.

Realism-based assessment of the efficacy of potassium peroxymonosulphate on Stenotrophomonas maltophilia biofilm control

The potential of pentapotassium bis(peroxymonosulphate) bis(sulphate) (OXONE) to control biofilms in drinking water distribution systems (DWDS) was evaluated and compared to chlorine disinfection. Mature biofilms of drinking water (DW)-isolated Stenotrophomonas maltophilia were formed using a simulated DWDS with a rotating cylinder reactor (RCR). After 30 min of exposure, OXONE at 10 × minimum bactericidal concentration (MBC) caused a significant 4 log reduction of biofilm culturability in comparison to the unexposed biofilms and a decrease in the number of non-damaged cells below the detection limit (4.8 log cells/cm2). The effects of free chlorine were restricted to approximately 1 log reduction in both biofilm culturability and non-damaged cells. OXONE in synthetic tap water (STW) at 25 ºC was more stable over 40 days than free chlorine in the same conditions. OXONE solution exhibited a disinfectant decrease of about 10% of the initial concentration during the first 9 days, and after this time the values remained stable. Whereas possible reaction of chlorine with inorganic and organic substances in STW contributed to free chlorine depletion of approximately 48% of the initial concentration. Electron paramagnetic resonance (EPR) spectroscopy studies confirmed the presence of singlet oxygen and other free radicals during S. maltophilia disinfection with OXONE. Overall, OXONE constitutes a relevant alternative to conventional DW disinfection for effective biofilm control in DWDS.

Self-repair and resuscitation of viable injured bacteria in chlorinated drinking water: Achromobacter as an example

Chlorine disinfection for the treatment of drinking water can cause injury to the membrane and DNA of bacterial cells and may induce the surviving injured bacteria into a viable but non-culturable (VBNC) state. It is difficult to monitor viable injured bacteria by heterotrophic plate counting (HPC), and their presence is also easily miscalculated in flow cytometry intact cell counting (FCM-ICC). Viable injured bacteria have a potential risk of resuscitation in drinking water distribution systems (DWDSs) and pose a threat to public health when drinking from faucets. In this study, bacteria with injured membranes were isolated from chlorinated drinking water by FCM cell sorting. The culture rate of injured bacteria varied from 0.08% to 2.6% on agar plates and 0.39% to 6.5% in 96-well plates. As the dominant genus among the five identified genera, as well as an opportunistic pathogen with multiple antibiotic resistance, Achromobacter was selected and further studied. After treatment with chlorine at a concentration of 1.2 mg/L, Achromobacter entered into the intermediate injured state on the FCM plot, and the injury on the bacterial surface was observed by electron microscopy. However, the CTC respiratory activity assay showed that 75.0% of the bacteria were still physiologically active, and they entered into a VBNC state. The injured VBNC Achromobacter in sterile drinking water were resuscitated after approximately 25 h. The cellular repair behavior of injured bacteria was studied by Fourier transform infrared attenuated total reflectance (FTIR-ATR) and comet assays. It was found that DNA injury rather than membrane injury was repaired first. The expression of Ku and ligD increased significantly during the DNA repair period, indicating that non-homologous end-joining (NHEJ) played an important role in repairing DNA double-strand breaks. This study deepened the understanding of the effect of chlorine disinfection on bacterial viability in drinking water and will provide support for the improvement of the chlorine disinfection process for the treatment of drinking water.

Mixed chlorine/chloramines in disinfected water and drinking water distribution systems (DWDSs): A critical review

Mixed chlorine/chloramines are commonly occurring in real drinking water distribution systems (DWDSs) but often overlooked. This review provides a comprehensive overview of the occurrences, characteristics, analysis methods, and control strategies of mixed chlorine/chloramines in DWDSs. The characteristics of mixed chlorine/chloramine species are summarized for treated water in drinking water treatment plants (DWTPs), secondary disinfection facilities, and DWDSs where different disinfectants could be blended. The key to differentiating and quantifying mixed chlorine/chloramine species is to separate organic chloramines (OCs) from di/tri-chloramines and overcome certain interferences. The complex interactions between water matrixes and chlorine/chloramine species could accelerate pipeline corrosions, enhance emerging disinfection by-products risks, lead to off-flavors in drinking water, and induce bio-instability issues (such as nitrification, microorganism regrowth, and promotion of horizontal gene-transfers). Three promising strategies for alleviating mixed chlorine/chloramine species are recommended, which include (i) removing precursors intensively and reconditioning the treated water, (ii) combining UV irradiation to eliminate undesired chlorine/chloramines species, and (iii) strengthening monitoring, operation, and maintenance management of DWDSs. Finally, the challenges for gaining insights into the mechanisms of mixed chlorine/chloramine species conversion are discussed and promising research directions are proposed.

Formation of dichloroacetic acid and dichloroacetamide from phenicol antibiotic abatement during ozonation and post-chlor(am)ination

This study investigated the formation of dichloroacetamide (DCAM) and dichloroacetic acid (DCAA) from the abatement of three phenicol antibiotics (PABs, chloramphenicol, thiamphenicol, and florfenicol) during ozonation and post-chlor(am)ination. Results show that the three PABs have a low ozone reactivity (kO3 = 0.11‒0.12 M-1 s-1), and therefore are mainly abated through the hydrogen abstraction mechanism by hydroxyl radicals (•OH) during ozonation. During PAB degradation, the carboxamide moiety in the parent molecules can be cleaved off by •OH attack and thus gives rise to DCAM. The formed DCAM can then be further oxidized by O3 and/or •OH to DCAA as a more stable transformation product (TP). When the three PABs were adequately abated (abatement efficiency of ∼82 %‒95 %), the molar yields of DCAM and DCAA were determined to be 2.79 %‒4.71 % and 32.9 %‒37.2 %, respectively. Furthermore, post-chloramination of the ozonation effluents increased the yields of DCAM and DCAA slightly to 4.20 %‒6.45 % and 39.0 %‒41.1 %, respectively. In comparison, post-chlorination eliminated DCAM in the solutions, but significantly increased DCAA yields to ∼100 % due to the further conversion of DCAM and other ozonation TPs to DCAA by chlorine oxidation. The results of this study indicate that high yields of DCAM and DCAA can be generated from PAB degradation during ozonation, and post-chlorination and post-chloramination will result in very different fates of DCAM and DCAA in the disinfected effluent. The formation and transformation of DCAM and DCAA during PAB degradation need to be taken into account when selecting multi-barrier treatment processes for the treatment of PAB-containing water.

Effects of dissolved organic matter removal and molecular transformation in different water treatment processes on formation of disinfection byproducts

Alterations in molecular composition of dissolved organic matter (DOM) during water treatments can influence the composition and toxicity of disinfection by-products (DBPs) in subsequent chlorination disinfection process. In this study, the impacts of DOM composition after various water treatment techniques (coagulation, adsorption, nanofiltration, biological aerated filter (BAF), and their integrated processes) on the generation mechanisms of DBPs were comprehensively explored by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) in combination with GC-MS and LC-MS analysis. The results indicated that coagulation preferentially removed unsaturated (low H/C) and oxidized (high O/C) compounds, whereas adsorption was prone to remove the reduced (low O/C) component that was more reactive with chlorine, leading to lower yields (μg DBP/mg DOC) of trihalomethanes (THMs) and haloacetic acids (HAAs) during subsequent chlorination. The coagulation-adsorption technique exhibited a relatively high removal of both known and unknown DBPs, demonstrating that coagulation and adsorption were complementary for DOM removal at the molecular level. Nanofiltration selectively removed molecules with relatively high O/C, however, those with very low O/C that were more reactive with chlorine could pass through the nanofiltration membrane, resulting in the highest yields of THMs and HAAs. Although BAF was inefficient in removing DBPs precursors, it could convert molecules with low degree of oxidation and unsaturation into highly oxidized and unsaturated ones, thereby significantly enhancing the removal of DBPs precursors in the subsequent coagulation-adsorption process. These findings are instrumental in developing and selecting more effective techniques to minimize the formation of DBPs in water treatment.

Selective catalytic synthesis of new terpenic chlorides using NaDCC as an eco-friendly and highly stable FAC agent

A simple, mild and efficient scope pathway for a selective catalytic chlorination of terpenic olefins is investigated in the presence of a highly efficient chlorination agent and a readily available Lewis acid catalyst. The sodium dichloroisocyanurate (NaDCC) used in the present work as an easy handling, sustainable and cost-effective chlorine donor due to its high free available chlorine (FAC), exhibits a high efficiency for selective catalytic chlorination. Herein, we report for the first-time the FeCl2/NaDCC combination system for the selective catalytic chlorination towards new functionalized terpenic olefins. In order to examine the general features of this catalytic reaction, the effects of pH, solvent, dilution, chlorination agent nature, stoichiometry and reaction kinetics are optimized using carvone as a model substrate. Among the studied parameters, catalyst stoichiometry was found to be determinant for highly controllable chlorination selectivity towards new allylic and vinylic chlorides. Indeed, the oxidation state, ligand and metal effects of the catalyst are examined using various Lewis acids, where the chlorinated ones (MClx), such as FeCl2, FeCl3 and SnCl2, exhibit a comprehensive approach for a controllable chlorination reaction. In addition, the homogeneous catalytic system shows good reusability with significant catalytic conversion depending on the FAC content in the reaction medium. The reaction proceeds under mild conditions with shorter reaction time and high selectivity towards new high added value allylic and vinylic chlorinated derivatives of naturally occurring terpenic olefins in good to excellent yields.

Leaching of organic matter and iodine, formation of iodinated disinfection by-products and toxic risk from Laminaria japonica during simulated household cooking

Iodinated disinfection by-products (I-DBPs) exhibited potential health risk owing to the high toxicity. Our recent study demonstrated that I-DBPs from Laminaria japonica (Haidai), the commonly edible seaweed, upon simulated household cooking condition were several hundred times more than the concentration of drinking water. Here, the characterization of Haidai and its leachate tandem with the formation, identification and toxicity of I-DBPs from the cooking of Haidai were systemically investigated. The dominant organic matter in Haidai leachate were polysaccharides, while the highest iodine specie was iodide (∼90% of total iodine). Several unknown I-DBPs generated from the cooking of Haidai were tentatively proposed, of which 3,5-diiodo-4-hydroxybenzaldehyde was dominant specie. Following a simulated household cooking with real chloraminated tap water, the presence of Haidai sharply increased aggregate iodinated trihalomethanes, iodinated haloacetic acids, and total organic iodine concentrations to 97.4 ± 7.6 μg/L,16.4 ± 2.1 μg/L, and 0.53 ± 0.06 mg/L, respectively. Moreover, the acute toxicity of Haidai soup to Vibrio qinghaiensis sp.-Q67 was around 7.3 times higher than that of tap water in terms of EC50. These results demonstrated that the yield of I-DBPs from the cooking of Haidai and other seaweed should be carefully considered.

Chlorination of bisphenols in water: Understanding the kinetics and formation mechanism of 2-butene-1,4-dial and analogues

While it is widely accepted that 2-butene-1,4-dial (BDA) is a toxic metabolite with genotoxic and carcinogenic properties, little is known about BDA and its analogues (BDAs) formation during water disinfection. In this study, the effects of different chlorination conditions on the formation of BDAs from bisphenol and its analogues (BPs analogues) were evaluated. A transformation pathway for the formation of BDAs upon chlorination of BPs analogues is proposed. The time profile of the transformation of BPs analogues into BDAs reveals that the generation of dichlorohydroquinone, dichloro-hydroxybenzenesulfonic acid and 2,4,6-trichlorophenol, are significantly associated with the formation of BDAs in the disinfected water. Owing to the different bridging groups contributing to the electrophilicity of BPs analogues in varying degrees, the stronger the electrophilicity of BPs analogues the more BDAs are formed. In addition, the type of BDAs produced is also affected. Four types of BDAs were detected in this study, one of which was newly identified. This study confirms that BPs analogues are an important source of BDAs and provides more insights into the formation of BDAs during chlorination. Greater attention should be given to the formation of BDAs in chlorinated water and their potential threat to humans and the ecosystem.

Hidden risks: Simulated leakage of domestic sewage to secondary water supply systems poses serious microbiological risks

There are continuous reports about the pollution of the secondary water supply systems (SWSSs), among which domestic sewage leakage is the most serious. In this study, a pilot experiment lasting 70 days was conducted to explore the changes in physicochemical water quality and the microbial profiles in SWSSs polluted by different doses of domestic sewage through qPCR and high-throughput sequencing methods. The results showed that when domestic sewage entered the simulated water storage tank, a large amount of organic matter brought by domestic sewage quickly consumed chlorine disinfectants. High pollution levels (pollution index ≥ 1/1000) were accompanied by significant increases in turbidity and ammonia nitrogen concentration (p < 0.05) and by abnormal changes in sensory properties. Although different microbial community structures were found only at high pollution levels, qPCR results showed that the abundance of the bacterial 16S rRNA gene and some pathogenic gene markers in the polluted tank increased with the pollution level, and the specific gene marker of pathogens could be detected even at imperceptible pollution levels. In particular, the high detection frequency and abundance of Escherichia coli and Enterococcus faecails in polluted tank water samples demonstrated that they can be used for early warning. Moreover, it seems that the microorganisms that came with the domestic sewage lost their cultivability soon after entering SWSSs but could recover their activities during stagnation. In addition, the biofilm biomass in the polluted tank with high pollution levels increased faster at the initial stage, while after a longer contact time, it tended to remain at the same level as the control tank. This study emphasized the high microbial risk introduced by sewage water leakage even at imperceptible levels and could provide scientific suggestions for early warning and prevention of pollution to SWSSs.

Waterborne transmission of protozoan parasites: a review of worldwide outbreaks - an update 2017-2022

The current study presents a comprehensive review of worldwide waterborne parasitic protozoan outbreaks reported between 2017 and 2022. In total, 416 outbreaks were attributed to the waterborne transmission of parasitic protozoa. Cryptosporidium accounted for 77.4% (322) of outbreaks, while Giardia was identified as the etiological agent in 17.1% (71). Toxoplasma gondii and Naegleria fowleri were the primary causes in 1.4% (6) and 1% (4) of outbreaks, respectively. Blastocystis hominis, Cyclospora cayetanensis, and Dientamoeba fragilis were independently identified in 0.72% (3) of outbreaks. Moreover, Acanthamoeba spp., Entamoeba histolytica, Vittaforma corneae, and Enterocytozoon bieneusi were independently the causal agents in 0.24% (1) of the total outbreaks. The majority of the outbreaks (195, 47%) were reported in North America. The suspected sources for 313 (75.2%) waterborne parasitic outbreaks were recreational water and/or swimming pools, accounting for 92% of the total Cryptosporidium outbreaks. Furthermore, 25.3% of the outbreaks caused by Giardia were associated with recreational water and/or swimming pools. Developing countries are most likely to be impacted by such outbreaks due to the lack of reliable monitoring strategies and water treatment processes. There is still a need for international surveillance and reporting systems concerning both waterborne diseases and water contamination with parasitic protozoa.

A Reflection on Sustainable Anode Materials for Electrochemical Chloride Oxidation

Chloride oxidation is a key industrial electrochemical process in chlorine-based chemical production and water treatment. Over the past few decades, dimensionally stable anodes (DSAs) consisting of RuO2 - and IrO2 -based mixed-metal oxides have been successfully commercialized in the electrochemical chloride oxidation industry. For a sustainable supply of anode materials, considerable efforts both from the scientific and industrial aspects for developing earth-abundant-metal-based electrocatalysts have been made. This review first describes the history of commercial DSA fabrication and strategies to improve their efficiency and stability. Important features related to the electrocatalytic performance for chloride oxidation and reaction mechanism are then summarized. From the perspective of sustainability, recent progress in the design and fabrication of noble-metal-free anode materials, as well as methods for evaluating the industrialization of novel electrocatalysts, are highlighted. Finally, future directions for developing highly efficient and stable electrocatalysts for industrial chloride oxidation are proposed.

Polyphosphate promotes oxidation resistance of ppk-expressing transgenic rice in low phosphorus culture

Phosphorus (P) plays a crucial role in plant growth. Insufficient availability of inorganic phosphate (Pi) can significantly impact crop yields. To address this, we previously developed transgenic rice expressing the low polyphosphate kinase gene (ppk) - known as ETRS - to enhance the efficiency of P resource utilization. Previous studies have shown that ETRS thrives and presents high yields in the low P culture. ETRS and wild-type rice (WT) were cultivated to the heading stage at 15 μM of P in the low P (LP) culture and 300 μM of P in the normal culture (CK) to identify the molecular pathways behind low P tolerance. Our findings revealed that polyphosphate (polyP) significantly enhanced the growth performance of ETRS in the LP culture. This enhanced tolerance can be attributed to polyP's capacity to mitigate oxidative damage induced by LP. This was evidenced by the reduction in levels of superoxide radicals, hydrogen peroxide, and malondialdehyde. PolyP also improved the antioxidant capacity of ETRS under LP stress by regulating enzymatic antioxidants viz., superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), as well as non-enzymatic antioxidants such as ascorbate (AsA) and glutathione (GSH). In addition, transcriptomics analysis suggested that polyP synthesis positively promoted the expressions of SOD, POD, and CAT related genes and played an active role in regulating the expression of AsA-GSH cycle system related genes in ETRS in the LP culture. These results strongly support the notion that polyP within ETRS mitigates oxidative damage through enhancement of the antioxidant system, ultimately bolstering tolerance to LP conditions.

Disinfection by-product precursors introduced by sandstorm events: Composition, formation characteristics and potential risks

Sandstorms, a natural meteorological event, occur repeatedly during the dry season and can accumulate large amounts of natural/anthropogenic pollutants during the deposition process, potentially introducing disinfection by-product (DBP) precursors into surface waters. In this study, the characteristics of sandstorm-derived dissolved organic matter (DOM) and its DBP formation potential were elucidated. Overall, sandstorm-derived DOM mainly consisted of low-molecular-weight, low-aromaticity, high-nitrogen organic matter, with a dissolved organic carbon (DOC) release yield of 14.4 mg-DOC/g. The halogenated DBP formation potential (calculated as total organic halogen) of sandstorm-derived DOM was comparable to that of surface water, while the normalized DBP-associated toxicity was 1.96 times higher. Similar to DOM introduced by other depositional pathways, sandstorm-derived DOM also had higher yields of highly cytotoxic DBPs (haloacetaldehydes [HALs], haloacetonitriles [HANs] and halonitromethanes [HNMs]). The average atmospheric deposition flux for DOM during the sandstorm event (50.4 ± 2.1 kg km-2 day-1) was 6.95 times higher than that of dry deposition, indicating a higher probability of contaminant input. Simultaneously, the estimation revealed that the sandstorm will increase the formation potential of toxicity forcing agents, such as HALs, HANs and HNMs, in surface water by 3.87%, 2.39% and 9.04%, respectively. Considering the high frequency of sandstorm events and the sorption of other organic pollutants by sand and dust, the impact of sandstorms on surface water quality should be of concern.

Fabricating modified carbon sesame straw for adsorption of acetaminophen and ibuprofen from aqueous media: isotherm and kinetic models

As acetaminophen (ACT) and ibuprofen (IBP) have serious environmental impacts, despite their widespread use in many countries, the present research examined the effectiveness of activated carbon made from straw and sesame stubble in removing ACT and IBP from water. To that end, the as-synthesized adsorbent was functionalized using zinc chloride. The maximum adsorption capacities were found to be 51.7 mg g-1 for ACT and 63.7 mg g-1 for IBP. The adsorption kinetics and isotherm results showed that the pseudo-second-order (PSO) kinetics and Langmuir isotherm fit the data obtained from this study better than the other experimental models do. Also, the adsorption reached equilibrium within 120 min, and the optimal adsorbent dose and temperature were obtained as 1.0 mg and 25 °C, respectively. The mechanisms involved in the adsorption process would include acid-base, hydrogen bonding, electrostatic forces, and π-π interaction. Reusability studies revealed that the adsorbent still preserved about 89% and 82% of the adsorption performance for ACT and IBP, respectively, after seven repeated adsorption cycles. As the findings indicated, CSS/Zn could be accepted as a hopeful adsorbent to be used in pharmaceutical treatment.

Resource utilization of rice straw to prepare biochar as peroxymonosulfate activator for naphthalene removal: Performances, mechanisms, environmental impact and applicability in groundwater

Herein, biochar was prepared using rice straw, and it served as the peroxymonosulfate (PMS) activator to degrade naphthalene (NAP). The results showed that pyrolysis temperature has played an important role in regulating biochar structure and properties. The biochar prepared at 900°C (BC900) had the best activation capacity and could remove NAP in a wide range of initial pH (5-11). In the system of BC900/PMS, multi-reactive species were produced, in which 1O2 and electron transfer mainly contributed to NAP degradation. In addition, the interference of complex groundwater components on the NAP removal rate must get attention. Cl- had a significant promotional effect but risked the formation of chlorinated disinfection by-products. HCO3-, CO32-, and humic acid (HA) had an inhibitory effect; surfactants had compatibility problems with the BC900/PMS system, which could lead to unproductive consumption of PMS. Significantly, the BC900/PMS system showed satisfactory remediation performance in spiked natural groundwater and soil, and it could solve the problem of persistent groundwater contamination caused by NAP desorption from the soil. Besides, the degradation pathway of NAP was proposed, and the BC900/PMS system could degrade NAP into low or nontoxic products. These suggest that the BC900/PMS system has promising applications in in-situ groundwater remediation.

Electrolyzed Hypochlorous Acid (HOCl) Aqueous Solution as Low-Impact and Eco-Friendly Agent for Floor Cleaning and Sanitation

Recently, the use of disinfectants has been becoming a diffused and sometimes indiscriminate practice of paramount importance to limit the spreading of infections. The control of microbial contamination has now been concentrated on the use of traditional agents (i.e., hypochlorite, ozone). However, their prolonged use can cause potential treats, for both human health and environment. Currently, low-impact but effective biocides that are prepared in a way that avoids waste, with a very low toxicity, and safe and easy to handle and store are strongly needed. In this study, produced electrochemically activated hypochlorous (HOCl) acid solutions are investigated and proposed, integrated in a scrubbing machine for floor cleaning treatment. Such an innovative machine has been used for floor cleaning and sanitation in order to evaluate the microbial charge and organic dirt removal capacity of HOCl in comparison with a machine charged with traditional Ecolabel standard detergent. The potential damage on floor materials has also been investigated by means of Scanning Electron Microscope (SEM). A comparative Life Cycle Assessment (LCA) analysis has been carried out for evaluating the sustainability of the use of the HOCl-based and detergent-based machine.

Formation of halogenated chloroxylenols through chlorination and their photochemical activity

Trace organic contaminants usually go through multiple treatment units in a modern water treatment train. Structural modification triggered by pretreatment (e.g., prechlorination) may influence the further transformation and fate of contaminants in downstream units. However, knowledge on this aspect is still limited. In this contribution, we investigated the chlorination of chloroxylenol (PCMX), an antimicrobial agent extensively used during COVID-19 pandemic, and the photoreactivity of its halogenated derivatives. Results indicate that chlorination of PCMX mainly proceeded through electrophilic substitution to give chlorinated products, including Cl- and 2Cl-PCMX. The presence of bromide (Br-) resulted in brominated analogues. Owing to the bathochromic and "heavy atom" effects of halogen substituents, these products show increased light absorption and photoreactivity. Toxicity evaluation suggest that these halo-derivatives have higher persistence, bioaccumulation, and toxicity (PBT) than the parent PCMX. Results of this contribution advance our understanding of the transformation of PCMX during chlorination and the photochemical activity of its halogenated derivatives in subsequent UV disinfection process or sunlit surface waters.

Kinetics study of chloride-activated peracetic acid for purifying bisphenol A: Role of Cl2/HClO and carbon-centered radicals

Peracetic acid is an emerging oxidant and disinfectant for wastewater purification. In this study, we first developed a comprehensive and accurate model to elucidate the reaction mechanisms and simulate reaction kinetics of peracetic acid (PAA, CH3C(=O)OOH) activated by chloride (Cl-) based on experimental results and literature. A diversity of experiments methods (e.g., quenching experiments, probe compounds degradation, electron paramagnetic resonance (EPR) measurements) and kinetic modeling were used to determine the reactive species. As a result, carbon-centered radicals and free chlorine reactive species (Cl2 and HClO) were devoted to BPA degradation in the PAA/Cl- system. The carbon-centered radicals CH3C(=O)OO•, CH3C(=O)O•, CH3OO•, and •CH3 greatly accelerated BPA degradation with their corresponding kinetics of kCH3C(=O)OO•, BPA = 2 × 108 M-1 s-1, kCH3C(=O)O•, BPA = 2 × 107 M-1 s-1, k•CH3, BPA = 2 × 106 M-1 s-1 and kCH3OO•, BPA = 2 × 104 M-1 s-1. Dissolved Cl2(l) species was also important for BPA degradation with kCl2, BPA of 2 × 107 M-1 s-1, much higher than HClO/ClO- of kHClO, BPA = 1.2 × 101 M-1 s-1 and kClO-, BPA = 9 × 10-3 M-1 s-1. While free chlorine tends to transform BPA to estrogenic chlorinated organic products, the primary degradation of BPA by carbon-centered radicals results in chlorine-free products, reducing the production of disinfection byproducts during the treatment of saline wastewater. This study improves the knowledge of reaction kinetics and mechanism and reactive species generation in the PAA/Cl- system.

Enhanced oxidation of urea by pH swing during chlorination: pH-dependent reaction mechanism

Urea reacts with chlorine to form chlorinated ureas (chloroureas), and fully chlorinated urea (tetrachlorourea) is further hydrolyzed into CO2 and chloramines. This study found that the oxidative degradation of urea by chlorination was enhanced by the pH swing, wherein the reaction proceeded under an acidic pH (e.g., pH = 3) in the first stage, and the solution pH was subsequently increased to a neutral or alkaline value (e.g., pH > 7) in the second-stage reaction. The degradation rate of urea by pH-swing chlorination increased with increasing chlorine dose and pH during the second-stage reaction. The pH-swing chlorination was based on the opposite pH dependence of sub-processes comprising urea chlorination. The formation of monochlorourea was favored under acidic pH conditions; however, the subsequent conversion into di- and trichloroureas was favored under neutral or alkaline pH conditions. The deprotonation of monochlorourea (pKa = 9.7 ± 1.1) and dichlorourea (pKa = 5.1 ± 1.4) was suggested to be responsible for the accelerated reaction in the second stage under increased pH conditions. pH-swing chlorination was also effective in degrading urea at low concentrations (micromolar levels). In addition, the total nitrogen concentration significantly decreased during the degradation of urea because of the volatilization of chloramines and the release of other gaseous nitrogen compounds.

Bayesian Optimization of Booster Disinfection Scheduling in Water Distribution Networks

Chlorine remains the most widely used disinfectant in drinking water treatment and distribution systems worldwide. To maintain a minimum residual throughout the distribution network, chlorine dosage needs to be regulated by optimizing the locations of chlorine boosters and their scheduling (i.e., chlorine injection rates). Such optimization can be computationally expensive since it requires numerous evaluations of water quality (WQ) simulation models. In recent years, Bayesian optimization (BO) has garnered considerable attention due to its efficiency in optimizing black-box functions in a wide range of applications. This study presents the first attempt to implement BO for the optimization of WQ in water distribution networks. The developed python-based framework couples BO with EPANET-MSX to optimize the scheduling of chlorine sources, while ensuring the delivery of water that satisfies water quality standards. Using Gaussian process regression to build the BO surrogate model, a comprehensive analysis was conducted to evaluate the performance of different BO methods. To that end, systematic testing of different acquisition functions, including the probability of improvement, expected improvement, upper confidence bound, and entropy search, in conjunction with different covariance kernels, including Matérn, squared-exponential, gamma-exponential, and rational quadratic, was conducted. Additionally, a thorough sensitivity analysis was performed to understand the influence of different BO parameters, including the number of initial points, covariance kernel length scale, and the level of exploration vs exploitation. The results revealed substantial variability in the performance of different BO methods and showed that the choice of the acquisition function has a more profound influence on the performance of BO than the covariance kernel.

Chlorination of amides: Kinetics and mechanisms of formation of N-chloramides and their reactions with phenolic compounds

Amides are common constituents in natural organic matter and synthetic chemicals. In this study, we investigated kinetics and mechanisms of the reactions of chlorine with seven amides, including acetamide, N-methylformamide, N-methylacetamide, benzamide, N-methylbenzamide, N-propylbenzamide, and N-(benzoylglycyl)glycine amide. Apparent second-order rate constants for the reactions of the amides with chlorine at pH 8 are in the range of 5.8 × 10-3 - 1.8 M-1s-1 and activation energies in the range of 62-88 kJ/mol. The second-order rate constants for the reactions of chlorine with different amides decrease with increasing electron donor character of the substituents on the amide-N and N-carbonyl-C in the amide structures. Hypochlorite (‒OCl) dominates the reactions of chlorine with amides yielding N-chloramides with species-specific second-order rate constants in the range of 7.3 × 10-3 - 2.3 M-1s-1. Kinetic model simulations suggest that N-chlorinated primary amides further react with HOCl with second-order rate constants in the order of 10 M-1s-1. The chlorination products of amides, N-chloramides are reactive towards phenolic compounds, forming chlorinated phenols via electrophilic aromatic substitution (phenol and resorcinol) and quinone via electron transfer (hydroquinone). Meanwhile, N-chloramides were recycled to the parent amides. At neutral pH, apparent second-order rate constants for the reactions between phenols and N-chloramides are in the order of 10-4-0.1 M-1s-1, comparable to those with chloramine. The findings of this study improve the understanding of the fate of amides and chlorine during chlorination processes.

Fluorescent Dissolved Organic Matter Components as Surrogates for Disinfection Byproduct Formation in Drinking Water: A Critical Review

Disinfection byproduct (DBP) formation, prediction, and minimization are critical challenges facing the drinking water treatment industry worldwide where chemical disinfection is required to inactivate pathogenic microorganisms. Fluorescence excitation-emission matrices-parallel factor analysis (EEM-PARAFAC) is used to characterize and quantify fluorescent dissolved organic matter (FDOM) components in aquatic systems and may offer considerable promise as a low-cost optical surrogate for DBP formation in treated drinking waters. However, the global utility of this approach for quantification and prediction of specific DBP classes or species has not been widely explored to date. Hence, this critical review aims to elucidate recurring empirical relationships between common environmental fluorophores (identified by PARAFAC) and DBP concentrations produced during water disinfection. From 45 selected peer-reviewed articles, 218 statistically significant linear relationships (R2 ≥ 0.5) with one or more DBP classes or species were established. Trihalomethanes (THMs) and haloacetic acids (HAAs), as key regulated classes, were extensively investigated and exhibited strong, recurrent relationships with ubiquitous humic/fulvic-like FDOM components, highlighting their potential as surrogates for carbonaceous DBP formation. Conversely, observed relationships between nitrogenous DBP classes, such as haloacetonitriles (HANs), halonitromethanes (HNMs), and N-nitrosamines (NAs), and PARAFAC fluorophores were more ambiguous, but preferential relationships with protein-like components in the case of algal/microbial FDOM sources were noted. This review highlights the challenges of transposing site-specific or FDOM source-specific empirical relationships between PARAFAC component and DBP formation potential to a global model.