Mechanistic Study on the Formation of Cl-/Br-/I-Trihalomethanes during Chlorination/Chloramination Combined with a Theoretical Cytotoxicity Evaluation

Abating a micropollutant epinastine by UV-based advanced oxidation processes: Comparison for UV/hydrogen peroxide, UV/persulfate, and UV/chlorine, impacts of bromide contents, and formation of DBPs during post-chlorination

Anthropogenic organic compounds, such as pharmaceuticals and personal care products, contaminate water, posing toxicological risks caused by either their parent compounds or transformation products. This study compares ultraviolet (UV)-based advanced oxidation processes (UV/hydrogen peroxide, UV/persulfate, and UV/chlorine) for the abatement of an antihistamine drug epinastine. UV light at 254 nm was irradiated upon solutions containing 10 μM epinastine and 100 μM oxidant. UV/chlorine degraded epinastine most effectively at pH 6.0-8.0; considerable contributions by reactive chlorine species and hydroxyl radicals were quantified using probe compounds. Furthermore, the degradation efficiency of the UV/chlorine treatment persisted with a halved chlorine dosage. Additionally, the types and concentrations of disinfection byproducts (DBPs) produced during UV/chlorine treatment with or without post-chlorination varied depending on the concentrations of chlorine or bromide. By comparing estimated DBP formations at a constant degradation rate of epinastine, UV/chlorine formed smaller concentrations of DBPs. Consequently, this study experimentally revealed that UV/chlorine is superior to UV/hydrogen peroxide and UV/persulfate for degrading epinastine at the possible pH and bromide content in the environment and controlling toxicological risks caused by disinfection DBPs formation by optimising chlorine dosage and UV fluence.

Statistical modeling for iodinated trihalomethanes: Preformed chloramination versus prechlorination followed by ammonia addition

Developing predictive models for iodo-trihalomethane (I-THM) formation in water is needed and valuable to minimize extensive and costly analysis. The main objective of this study was to develop a statistical model for the formation of six types of I-THMs under uniform formation conditions. Prediction of I-THM formation in two different water sources (natural organic matter [NOM] and algal organic matter [AOM]) were comprehensively evaluated during both preformed chloramination and prechlorination followed by ammonia addition conditions. In addition, the prediction of THM10 (sum of six I-THM and THM4) formation was conducted during both oxidation strategies for NOM waters. In total, 460 experimental results were compiled from the literature and our own database. The results showed the coefficient of determination (R2) values for the six I-THM species ranged between 0.53-0.68 and 0.35-0.79 in the preformed NH2Cl and perchlorinated NOM waters, respectively. Among all independent variables, the I- exhibited the most significant influence on the formation of all I-THM species in the preformed NH2Cl, while SUVA254 was the most influential parameter for perchlorinated NOM water. When the preformed chloramination was compared with prechlorination followed by ammonia addition, the R2 value for I-THMs (0.93) was higher than for THM4 formation (0.79) in preformed chloramination. In the prechlorination followed by ammonia addition condition, the model prediction of I-THMs (R2= 0.45) formation was lower than THM4 (R2= 0.96). Overall, the pH, I-, SUVA254, and oxidant type are all played crucial roles in determining the I-THM formation, impacting the overall effectiveness and predictability of the models.

Control of chlorination disinfection by-products in drinking water by combined nanofiltration process: A case study with trihalomethanes and haloacetic acids

Disinfection by-products (DBPs) are prevalent contaminants in drinking water and are primarily linked to issues regarding water quality. These contaminants have been associated with various adverse health effects. Among different treatment processes, nanofiltration (NF) has demonstrated superior performance in effectively reducing the levels of DBPs compared to conventional processes and ozone-biological activated carbon (O3-BAC) processes. In this experiment, we systematically investigated the performance of three advanced membrane filtration treatment schemes, namely "sand filter + nanofiltration" (SF + NF), "sand filter + ozone-biological activated carbon + nanofiltration" (SF + O3-BAC + NF), and "ultrafiltration + nanofiltration" (UF + NF), in terms of their ability to control disinfection by-product (DBP) formation in treated water, analyzed the source and fate of DBP precursors during chlorination, and elucidated the role of precursor molecular weight distribution during membrane filtration in relation to DBP formation potential (DBPFP). The results indicated that each treatment process reduced DBPFP, as measured by trihalomethane formation potential (THMFP) and haloacetic acid formation potential (HAAFP), with the SF + O3-BAC + NF process being the most effective (14.27 μg/L and 14.88 μg/L), followed by the SF + NF process (21.04 μg/L and 16.29 μg/L) and the UF + NF process (26.26 μg/L and 21.75 μg/L). Tyrosine, tryptophan, and soluble microbial products were identified as the major DBP precursors during chlorination, with their fluorescence intensity decreasing gradually as water treatment progressed. Additionally, while large molecular weight organics (60-100,000 KDa) played a minor role in DBPFP, small molecular weight organics (0.2-5 KDa) were highlighted as key contributors to DBPFP, and medium molecular weight organics (5-60 KDa) could adhere to the membrane surface and reduce DBPFP. Based on these findings, the combined NF process can be reasonably selected for controlling DBP formation, with potential long-term benefits for human health.

Oxidation processes and me

This publication summarizes my journey in the field of chemical oxidation processes for water treatment over the last 30+ years. Initially, the efficiency of the application of chemical oxidants for micropollutant abatement was assessed by the abatement of the target compounds only. This is controlled by reaction kinetics and therefore, second-order rate constant for these reactions are the pre-requisite to assess the efficiency and feasibility of such processes. Due to the tremendous efforts in this area, we currently have a good experimental data base for second-order rate constants for many chemical oxidants, including radicals. Based on this, predictions can be made for compounds without experimental data with Quantitative Structure Activity Relationships with Hammet/Taft constants or energies of highest occupied molecular orbitals from quantum chemical computations. Chemical oxidation in water treatment has to be economically feasible and therefore, the extent of transformation of micropollutants is often limited and mineralization of target compounds cannot be achieved under realistic conditions. The formation of transformation products from the reactions of the target compounds with chemical oxidants is inherent to oxidation processes and the following questions have evolved over the years: Are the formed transformation products biologically less active than the target compounds? Is there a new toxicity associated with transformation products? Are transformation products more biodegradable than the corresponding target compounds? In addition to the positive effects on water quality related to abatement of micropollutants, chemical oxidants react mainly with water matrix components such as the dissolved organic matter (DOM), bromide and iodide. As a matter of fact, the fraction of oxidants consumed by the DOM is typically > 99%, which makes such processes inherently inefficient. The consequences are loss of oxidation capacity and the formation of organic and inorganic disinfection byproducts also involving bromide and iodide, which can be oxidized to reactive bromine and iodine with their ensuing reactions with DOM. Overall, it has turned out in the last three decades, that chemical oxidation processes are complex to understand and to manage. However, the tremendous research efforts have led to a good understanding of the underlying processes and allow a widespread and optimized application of such processes in water treatment practice such as drinking water, municipal and industrial wastewater and water reuse systems.

Comparative formation of chlorinated and brominated disinfection byproducts from chlorination and bromination of amino acids

Amino acids are the main components of dissolved organic nitrogen in algal- and wastewater-impacted waters, which can react with chlorine to form toxic halogenated disinfection by-products (DBPs) in the disinfection process. In the presence of bromide, the reaction between amino acids and secondarily formed hypobromous acid can lead to the formation of brominated DBPs that are more toxic than chlorinated analogues. This study compares the formation of regulated and unregulated DBPs during chlorination and bromination of representative amino acids (AAs) (e.g., aspartic acid, asparagine, tryptophan, tyrosine, and histidine). In general, concentrations of brominated DBPs (trihalomethanes, haloacetonitriles, and haloacetamides, 24.9-5835.0 nM) during bromination were higher than their chlorinated analogues (9.3-3235.3 nM) during chlorination. This indicates the greater efficacy of bromine as a halogenating agent. However, the formation of chlorinated haloacetic acids during chlorination was higher than the corresponding brominated DBPs from bromination. It is likely that an oxidation pathway is required for the formation of haloacetic acids and chlorine is a stronger oxidant than bromine. Moreover, chlorine forms higher levels of haloacetaldehydes (74.4-1077.8 nM) from amino acids than bromine (1.0-480.2 nM) owing to the instability of brominated species. The DBP formation yields depend on the types of functional groups in the side chain of AAs. Eight intermediates resulting from chlorination/bromination of tyrosine were identified by triple quadrupole mass spectrometer, including N-chlorinated/brominated tyrosine, 3-chloro/bromo-tyrosine, and 3,5-dichloro/dibromo-tyrosine. These findings provided new insights into the DBP formation during the chlorination of algal- and wastewater-impacted waters with elevated bromide.

Formation of Targeted and Novel Disinfection Byproducts during Chlorine Photolysis in the Presence of Bromide

Chlorine photolysis is an advanced oxidation process that relies on the combination of direct chlorination by free available chlorine, direct photolysis, and reactive oxidants to transform contaminants. In waters that contain bromide, free available bromine and reactive bromine species can also form. However, little is known about the underlying mechanisms or formation potential of disinfection byproducts (DBPs) under these conditions. We investigated reactive oxidant generation and DBP formation under dark conditions, chlorine photolysis, and radical-quenched chorine photolysis with variable chlorine (0-10 mg-Cl2/L) and bromide (0-2,000 μg/L) concentrations, as well as with free available bromine. Probe loss rates and ozone concentrations increase with chlorine concentration and are minimally impacted by bromide. Radical-mediated processes partially contribute to the formation targeted DBPs (i.e., trihalomethanes, haloacetic acids, haloacetonitriles, chlorate, and bromate), which increase with increasing chlorine concentration. Chlorinated novel DBPs detected by high-resolution mass spectrometry are attributable to a combination of dark chlorination, direct halogenation by reactive chlorine species, and transformation of precursors, whereas novel brominated DBPs are primarily attributable to dark bromination of electron-rich formulas. The formation of targeted and novel DBPs during chlorine photolysis in waters with elevated bromide may limit treatment applications.

Co-Occurrence of Bromine and Iodine Species in US Drinking Water Sources That Can Impact Disinfection Byproduct Formation

Bromine and iodine species are precursors for forming disinfection byproducts in finished drinking waters. Our study incorporates spatial and temporal data to quantify concentrations of inorganic (bromide (Br-), iodide (I-), and iodate (IO3-)), organic, and total bromine (BrT) and iodine (IT) species from 286 drinking water sources and 7 wastewater effluents across the United States. Br- ranged from <5-7800 μg/L (median of 62 μg/L in surface water (SW) and 95 μg/L in groundwater (GW)). I- was detected in 41% of SW (1-72 μg/L, median = <1 μg/L) and 62% of GW (<1-250 μg/L, median = 3 μg/L) samples. The median Br-/I- ratio in SW and GW was 22 μg/μg and 16 μg/μg, respectively, in paired samples with detect Br- and I-. BrT existed primarily as Br-, while IT was present as I-, IO3-, and/or total organic iodine (TOI). Inorganic iodine species (I- and IO3-) were predominant in GW samples, accounting for 60-100% of IT; however, they contributed to only 20-50% of IT in SW samples. The unknown fraction of IT was attributed to TOI. In lakes, seasonal cycling of I-species was observed and was presumably due to algal productivity. Finally, Spearman Rank Correlation tests revealed a strong correlation between Br- and IT in SW (RBr-,IT = 0.83) following the log10 (Br-, μg/L) = 0.65 × log10 (IT, μg/L) - 0.17 relationship. Br- and I- in treated wastewater effluents (median Br- = 234 μg/L, median I- = 5 μg/L) were higher than drinking water sources.

Trihalomethanes in water samples: Recent update on pretreatment and detection methods

Trihalomethanes (THMs) are classified as volatile organic compounds, considered to be a disinfection by-product during water disinfection process. THMs have been shown to be cytotoxic, genotoxic and mutagenic, with a risk of cancer when they contact with people directly. To protect public health and monitor water quality, it is important to monitor and measure THMs in drinking water. Therefore, it is crucial to develop fast, accurate, highly sensitivity and green analysis methods of THMs in various complicated matrices. Here, this review presents an overall summary of the current state of the pretreatment and detection methods for THMs in various sample matrices since 2005. In addition to the traditionally used pretreatment methods for THMs (such as headspace extraction, microwave-assisted extraction, liquid-liquid extraction), the new-developed methods, including solid-phase extraction, QuEChERS and different microextraction methods, have been summarized. The detection methods include gas chromatography-based methods, sensors and several other approaches. Additionally, benefits and limitations of different techniques were also discussed and compared. This study is anticipated to offer fruitful insights into the further advancement and widespread applications of pretreatment and detection technologies for THMs as well as for related substances.

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.

Cytotoxicity Comparison between Drinking Water Treated by Chlorination with Postchloramination versus Granular Activated Carbon (GAC) with Postchlorination

Granular activated carbon treatment with postchlorination (GAC/Cl2) and chlorination followed by chloramination (Cl2/NH2Cl) represent two options for utilities to reduce DBP formation in drinking water. To compare the total cytotoxicity of waters treated by a pilot-scale GAC treatment system with postchlorination (and in some instances with prechlorination upstream of GAC (i.e., (Cl2)/GAC/Cl2)) and chlorination/chloramination (Cl2/NH2Cl) at ambient and elevated Br- and I- levels and at three different GAC ages, we applied the Chinese hamster ovary (CHO) cell cytotoxicity assay to whole-water extracts in conjunction with calculations of the cytotoxicity contributed by the 33 (semi)volatile DBPs lost during extractions. At both ambient and elevated Br- and I- levels, GAC/Cl2 and Cl2/NH2Cl achieved comparable reductions in the formation of regulated trihalomethanes (THMs) and haloacetic acids (HAAs). Nonetheless, GAC/Cl2 always resulted in lower total cytotoxicity than Cl2/NH2Cl, even at up to 65% total organic carbon breakthrough. Prechlorination formed (semi)volatile DBPs that were removed by the GAC, yet there was no substantial difference in total cytotoxicity between Cl2/GAC/Cl2 and GAC/Cl2. The poorly characterized fraction of DBPs captured by the bioassay dominated the total cytotoxicity when the source water contained ambient levels of Br- and I-. When the water was spiked with Br- and I-, the known, unregulated (semi)volatile DBPs and the uncharacterized fraction of DBPs were comparable contributors to total cytotoxicity; the contributions of regulated THMs and HAAs were comparatively minor.

Removal and transformation of disinfection by-products in water during boiling treatment

Disinfection by-products (DBPs) remain an ongoing issue because of their widespread occurrence and toxicity. Boiling is the most popular household water treatment method and can effectively remove some DBPs. However, the transformation behavior of DBPs during boiling is still unclear, and the key contributors to toxicity have not been identified. In this study, the changes in the concentrations of DBPs in the single-DBP systems and the multi-DBP systems during boiling were monitored, and in-depth discussions on the removal and transformation of DBPs in both systems were carried out. The results showed that boiling was effective in removing volatile DBPs (over 90% for TCAL, TCAN, and DCAN, and over 60% for TCM), but ineffective for non-volatile DBPs (around 20% for TCAA and below 10% for DCAA and MCAA). By hydrolysis and decarboxylation, the transformation occurred among DBPs, i.e., 55% TCAL to TCM, followed by 23% DCAN to DCAA, 22% TCAN to TCAA, and 10% TCAA to TCM. The transformations were found to be significantly influenced by other co-existing DBPs. In multi-DBP systems, the transformations of DCAN to DCAA and TCAN to TCAA were both promoted, while the transformation of TCAN to TCAA was inhibited. Transformation and volatilization are the two processes responsible for DBP removal. Toxicity estimates indicated that boiling was effective in reducing the toxicity of DBPs and improving the safety of the water, despite the interconversion of DBPs in drinking water during boiling. This study emphasized the importance of studying the interconversion behaviors of DBPs in drinking water during boiling and provided practical information for end-use drinking water safety.

Sequential combination of pre-chlorination and powdered activated carbon adsorption on iodine removal and I-THMs control in drinking water

Combining pre-oxidation with activated carbon adsorption was explored as an ideal approach for removing iodine from water source to eliminate the formation of Iodinated trihalomethanes (I-THMs). Compared with permanganate and monochloramine, chlorine is more suitable as pre-oxidant to obtain higher active iodine species (HOI/I₂). Active iodine species adsorption using both powdered activated carbon (PAC) and granular activated carbon (GAC) can be well fitted the pseudo-second-order kinetic model indicating that chemical adsorption was the dominant mechanism for HOI/I₂ adsorption. The average pore size of activated carbons was the most strongly correlated with the adsorption capacity (R² > 0.98), followed by methylene blue (R² > 0.76), pore volume (R² > 0.70) and iodine number (R² > 0.67). Moreover, three models, including intraparticle diffusion, Byod kinetic, and diffusion-chemisorption were used to illustrate the mechanisms of HOI/I₂ adsorption. Chemical adsorption was the dominant mechanism for HOI/I₂ adsorption. In summary, at the molar ratio of [NaClO] and [I^-] as 1.2, pre-chloriantion time of 5 min, subsequently dosage of 15 mg/L of PAC E with 20 min adsorption can remove 79.8% iodine. In addition, the combined process can eliminate 61%-87.2% of I-THMs in the subsequent chlor(am)ination. The results indicate that pre-chlorination combined with PAC can effectively removed HOI/I₂ and attenuate I-THMs formation in the subsequent disinfection process.

Effects of biological activated carbon filter running time on disinfection by-product precursor removal

Biological activated carbon (BAC) filtration is usually considered to be able to decrease formation potentials (FPs) of disinfection by-products (DBPs) in drinking water treatment plant (DWTP). However, BAC filters with long running time may release microbial metabolites to effluents and therefore increase FPs of nitrogenous DBPs with high toxicity. To verify this hypothesis, this study continuously tracked BAC filters in a DWTP for one year, and assessed effects of old (running time 8-9 years) and new (running time 0-13 months) BAC filters on FPs of 15 regulated and unregulated DBPs. Results revealed that dissolved organic carbon (DOC) removal was slightly higher in the new BAC than the old one. All fluorescent components of dissolved organic matter evidently declined after new BAC filtration, but fulvic acid-like and soluble microbial product-like substances increased after old BAC filtration, which could be caused by microbial leakage. Correspondingly, new BAC filter generally removed more DBP FPs than the old one. 46.5% HAA₇ FPs from chlorination and 44.3% THM₄ FPs from chloramination were removed by new BAC filter. However, some DBP FPs, especially HAN FPs, were poorly removed or even increased by the old BAC filter. Proteobacteria could be a main contributor for DBP precursor removal in BAC filters. Herminiimonas, most abundant genera in new BAC filter, may explain its better DOC and UV₂₅₄ removal performance and lower DBP FPs, while Bradyrhizobium, most abundant genera in old BAC filter, might produce more extracellular polymeric substances and therefore increased N-DBP FPs in old BAC effluent. This study provided insight into variations of DBP FPs and microbial communities in the new and old BAC filters, and will be helpful for the optimization of DWTP design and operation for public health.

Do DBPs swim in salt water pools? Comparison of 60 DBPs formed by electrochemically generated chlorine vs. conventional chlorine

Disinfectants are added to swimming pools to kill harmful pathogens. Although liquid chlorine (sodium hypochlorite) is the most commonly used disinfectant, alternative disinfection techniques like electrochemically generated mixed oxidants or electrochemically generated chlorine, often referred to as salt water pools, are growing in popularity. However, these disinfectants react with natural organic matter and anthropogenic contaminants introduced to the pool water by swimmers to form disinfection byproducts (DBPs). DBPs have been linked to several adverse health effects, such as bladder cancer, adverse birth outcomes, and asthma. In this study, we quantified 60 DBPs using gas chromatography-mass spectrometry and assessed the calculated cytotoxicity and genotoxicity of an indoor community swimming pool before and after switching to a salt water pool with electrochemically generated chlorine. Interestingly, the total DBPs increased by 15% upon implementation of the salt water pool, but the calculated cytotoxicity and genotoxicity decreased by 45% and 15%, respectively. Predominant DBP classes formed were haloacetic acids, with trichloroacetic acid and dichloroacetic acid contributing 57% of the average total DBPs formed. Haloacetonitriles, haloacetic acids, and haloacetaldehydes were the primary drivers of calculated cytotoxicity, and haloacetic acids were the primary driver of calculated genotoxicity. Diiodoacetic acid, a highly toxic iodinated DBP, is reported for the first time in swimming pool water. Bromide impurities in sodium chloride used to electrochemically generate chlorine led to a 73% increase in brominated DBPs, primarily driven by bromochloroacetic acid. This study presents the most extensive DBP study to-date for salt water pools.

Insight into the formation of iodinated trihalomethanes during chlorination, monochloramination, and dichloramination of iodide-containing water

In this study, the formation of iodinated trihalomethanes (I-THMs) was systematically evaluated and compared for three treatment processes - (i) chlorination, (ii) monochloramine, and (iii) dichloramination - under different pH conditions. The results demonstrated that I-THM formation decreased in the order of monochloramination > dichloramination > chlorination in acidic and neutral pH. However, the generation of I-THMs increased in the dichloramination < chlorination < monochloramination order in alkaline condition. Specifically, the formation of I-THMs increased as pH increased from 5 to 9 during chlorination and monochloramination processes, while the maximum I-THM formation occurred at pH 7 during dichloramination. The discrepancy could be mainly related to the stability of the three chlor (am) ine disinfectants at different pH conditions. Moreover, in order to gain a thorough insight into the mechanisms of I-THM formation during dichloramination, further investigation was conducted on the influencing factors of DOC concentration and Br^-/I^- molar ratio. I-THM formation exhibited an increasing and then decreasing trend as the concentration of DOC increased from 1 to 7 mg-C/L, while the yield of I-THMs increased with increasing Br^-/I^- molar ratio from 5:0 to 5:10. During the three processes mentioned above, similar I-THM formation results were also obtained in real water, which indicates that the excessive generation of I-THMs should be paid special attention during the disinfection of iodide-containing water.

Formation of regulated and unregulated disinfection byproducts during chlorination and chloramination: Roles of dissolved organic matter type, bromide, and iodide

Algal blooms and wastewater effluents can introduce algal organic matter (AOM) and effluent organic matter (EfOM) into surface waters, respectively. In this study, the impact of bromide and iodide on the formation of halogenated disinfection byproducts (DBPs) during chlorination and chloramination from various types of dissolved organic matter (DOM, e.g., natural organic matter (NOM), AOM, and EfOM) were investigated based on the data collected from literature. In general, higher formation of trihalomethanes (THMs) and haloacetic acids (HAAs) was observed in NOM than AOM and EfOM, indicating high reactivities of phenolic moieties with both chlorine and monochloramine. The formation of haloacetaldehydes (HALs), haloacetonitriles (HANs) and haloacetamides (HAMs) was much lower than THMs and HAAs. Increasing initial bromide concentrations increased the formation of THMs, HAAs, HANs, and HAMs, but not HALs. Bromine substitution factor (BSF) values of DBPs formed in chlorination decreased as specific ultraviolet absorbance (SUVA) increased. AOM favored the formation of iodinated THMs (I-THMs) during chloramination using preformed chloramines and chlorination-chloramination processes. Increasing prechlorination time can reduce the I-THM concentrations because of the conversion of iodide to iodate, but this increased the formation of chlorinated and brominated DBPs. In an analogous way, iodine substitution factor (ISF) values of I-THMs formed in chloramination decreased as SUVA values of DOM increased. Compared to chlorination, the formation of noniodinated DBPs is low in chloramination.

Iodide sources in the aquatic environment and its fate during oxidative water treatment - A critical review

Iodine is a naturally-occurring halogen in natural waters generally present in concentrations between 0.5 and 100 µg L^-1. During oxidative drinking water treatment, iodine-containing disinfection by-products (I-DBPs) can be formed. The formation of I-DBPs was mostly associated to taste and odor issues in the produced tap water but has become a potential health problem more recently due to the generally more toxic character of I-DBPs compared to their chlorinated and brominated analogues. This paper is a systematic and critical review on the reactivity of iodide and on the most common intermediate reactive iodine species HOI. The first step of oxidation of I^- to HOI is rapid for most oxidants (apparent second-order rate constant, k_app > 10³ M^-1s^-1 at pH 7). The reactivity of hypoiodous acid with inorganic and organic compounds appears to be intermediate between chlorine and bromine. The life times of HOI during oxidative treatment determines the extent of the formation of I-DBPs. Based on this assessment, chloramine, chlorine dioxide and permanganate are of the highest concern when treating iodide-containing waters. The conditions for the formation of iodo-organic compounds are also critically reviewed. From an evaluation of I-DBPs in more than 650 drinking waters, it can be concluded that one third show low levels of I-THMs (<1 µg L^-1), and 18% exhibit concentrations > 10 µg L^-1. The most frequently detected I-THM is CHCl₂I followed by CHBrClI. More polar I-DBPs, iodoacetic acid in particular, have been reviewed as well. Finally, the transformation of iodide to iodate, a safe iodine-derived end-product, has been proposed to mitigate the formation of I-DBPs in drinking water processes. For this purpose a pre-oxidation step with either ozone or ferrate(VI) to completely oxidize iodide to iodate is an efficient process. Activated carbon has also been shown to be efficient in reducing I-DBPs during drinking water oxidation.

Preferential Halogenation of Algal Organic Matter by Iodine over Chlorine and Bromine: Formation of Disinfection Byproducts and Correlation with Toxicity of Disinfected Waters

The increasing occurrence of harmful algal blooms (HABs) in surface waters may increase the input of algal organic matter (AOM) in drinking water. The formation of halogenated disinfection byproducts (DBPs) during combined chlorination and chloramination of AOM and natural organic matter (NOM) in the presence of bromide and iodide and haloform formation during halogenation of model compounds were studied. Results indicated that haloform/halogen consumption ratios of halogens reacting with amino acids (representing proteins present in AOM) follow the order iodine > bromine > chlorine, with ratios for iodine generally 1-2 orders of magnitude greater than those for chlorine (0.19-2.83 vs 0.01-0.16%). This indicates that iodine is a better halogenating agent than chlorine and bromine. In contrast, chlorine or bromine shows higher ratios for phenols (representing the phenolic structure of humic substances present in NOM). Consistent with these observations, chloramination of AOM extracted from Microcystis aeruginosa in the presence of iodide produced 3 times greater iodinated trihalomethanes than those from Suwannee River NOM isolate. Cytotoxicity and genotoxicity of disinfected algal-impacted waters evaluated by Chinese hamster ovary cell bioassays both follow the order chloramination > prechlorination-chloramination > chlorination. This trend is in contrast to additive toxicity calculations based on the concentrations of measured DBPs since some toxic iodinated DBPs were not identified and quantified, suggesting the necessity of experimentally analyzing the toxicity of disinfected waters. During seasonal HAB events, disinfection practices warrant optimization for iodide-enriched waters to reduce the toxicity of finished waters.

Formation and control of disinfection by-products from iodinated contrast media attenuation through sequential treatment processes of ozone-low pressure ultraviolet light followed by chlorination

Different groups of disinfection by-products (DBPs) were studied through the degradation of iopamidol by the sequential oxidation process of ozone-low pressure ultraviolet light (O₃-LPUV) followed by chlorination. This paper investigates the attenuation of iopamidol under this sequential treatment and the effect of chlorine contact time (30 min versus 3 days) to control the formation potential of DBPs: trihalomethanes (THMs), haloacetonitriles (HANs) and haloacetamides (HAMs). Thirty target DBPs among the 9 iodinated-DBPs (I-DBPs), were monitored throughout the sequential treatment. Results showed that O₃-LPUV removed up to 99% of iopamidol, while ozone and LPUV alone removed only 90% and 76% respectively. After chlorine addition, O₃-LPUV yielded 56% lower I-DBPs than LPUV. Increasing chlorine contact time resulted in higher concentrations of all DBP groups (THMs, HANs, and HAMs), with the exception of I-DBPs. One new iodinated-haloacetamide, namely chloroiodoacetamide (CIACM) and one iodoacetonitrile (IACN) were detected. These results suggest the iodine incorporated in iopamidol may be a precursor for iodinated-nitrogenous-DBPs, which are currently not well studied.

Comprehensive review on iodinated X-ray contrast media: Complete fate, occurrence, and formation of disinfection byproducts

Iodinated contrast media (ICM) are drugs which are used in medical examinations for organ imaging purposes. Wastewater treatment plants (WWTPs) have shown incapability to remove ICM, and as a consequence, ICM and their transformation products (TPs) have been detected in environmental waters. ICM show limited biotransformation and low sorption potential. ICM can act as iodine source and can react with commonly used disinfectants such as chlorine in presence of organic matter to yield iodinated disinfection byproducts (IDBPs) which are more cytotoxic and genotoxic than conventionally known disinfection byproducts (DBPs). Even highly efficient advanced treatment systems have failed to completely mineralize ICM, and TPs that are more toxic than parent ICM are produced. This raises issues regarding the efficacy of existing treatment technologies and serious concern over disinfection of ICM containing waters. Realizing this, the current review aims to capture the attention of scientific community on areas of less focus. The review features in depth knowledge regarding complete environmental fate of ICM along with their existing treatment options.

Treating disinfection byproducts with UV or solar irradiation and in UV advanced oxidation processes: A review

This review focuses on the degradation kinetics and mechanisms of disinfection byproducts (DBPs) under UV and solar irradiation and in UV-based advanced oxidation processes (AOPs). A total of 59 such compounds are discussed. The processes evaluated are low pressure, medium pressure and vacuum UV irradiation, solar irradiation together with UV/hydrogen peroxide, UV/persulfate and UV/chlorine AOPs. Under UV and solar irradiation, the photodegradation rates of N-nitrosamines are much higher than those of halogenated DBPs. Among halogenated DBPs, those containing iodine are photodegraded more rapidly than those containing bromine or chlorine. This is due to differences in their bond energies (E_N-N < E_C-I < E_C-Br < E_C-Cl). Molar absorption coefficients at 254 nm and energy gaps can be used to predict the photodegradation rates of DBPs under low pressure UV irradiation. But many DBPs of interest cannot be degraded to half their original concentration with less than a 500 mJ cm^-2 dose of low pressure UV light. HO^• generally contributes to less than 30% of the degradation of DBPs except iodo-DBPs in UV/H₂O₂ AOPs. Reaction mechanisms under UV irradiation and in HO^•-mediated oxidation are also summarized. N-N bond cleavage initiates their direct UV photolysis of N-nitrosamines as C-X cleavage does among halogenated compounds. HO^• generally initiates degradation via single electron transfer, addition and hydrogen abstraction pathways. Information on the reaction rate constants of SO₄^•- and halogen radicals with DBPs is rather limited, and little information is available about their reaction pathways. Overall, this review provides improved understanding of UV, solar and AOPs.

Formation and control of C- and N-DBPs during disinfection of filter backwash and sedimentation sludge water in drinking water treatment

Drinking water treatment plants (DWTPs) produce filter backwash water (FBW) and sedimentation sludge water (SSW) that may be partially recycled to the head of DWTPs. The impacts of key disinfection conditions, water quality parameters (e.g., disinfection times, disinfectant types and doses, and pH values), and bromide concentration on controlling the formation of trihalomethanes (THMs), haloacetic acids (HAAs), haloacetonitriles (HANs), and haloacetamides (HAMs) during disinfection of FBW and SSW were investigated. Concentrations of most disinfection byproducts (DBPs) and associated calculated toxicity increased with extended chlorination for both FBW and SSW. During chlorination of both FBW and SSW, elevated chlorine doses significantly increased THM yields per unit dissolved organic carbon (DOC), but decreased HAN and HAM yields, with minimum effect on HAA yields. Chloramine disinfection effectively inhibited C-DBP formation but promoted N-DBPs yields, which increased with chloramine dose. Calculated toxicities after chloramination increased with chloramine dose, which was opposite to the trend found after free chlorine addition. An examination of pH effects demonstrated that C-DBPs were more readily generated at alkaline pH (pH=8), while acidic conditions (pH=6) favored N-DBP formation. Total DBP concentrations increased at higher pH levels, but calculated DBP toxicity deceased due to lower HAN and HAM concentrations. Addition of bromide markedly increased bromo-THM and bromo-HAN formation, which are more cytotoxic than chlorinated analogues, but had little impact on the formation of HAAs and HAMs. Bromide incorporation factors (BIFs) for THMs and HANs from both water samples all significantly increased as bromide concentrations increased. Overall, high bromide concentrations increased the calculated toxicity values in FBW and SSW after chlorination. Therefore, while currently challenging, technologies capable of removing bromide should be explored as part of a strategy towards controlling cumulative toxicity burden (i.e., hazard) while simultaneously lowering individual DBP concentrations (i.e., exposure) to manage DBP risks in drinking water.

Chlorination decreases acute toxicity of iodophenols through the formation of iodate and chlorinated aliphatic disinfection byproducts

Highly toxic iodinated phenolic by-products were frequently detected in the oxidative treatment and disinfection of iodine-containing water. Herein, it was found that three model iodinated phenolic disinfection byproducts (DBPs), 2-iodophenol, 4-iodophenol and 2,4,6-triiodophenol, were reactive with HOCl, and the reaction rate constants (at pH 7.0 and 25℃) were 1.86 ×10², 1.62 ×10² and 7.5 ×10¹ M^-1s^-1, respectively. When HOCl was in excess (HOCl/iodophenol = 40/1, [iodophenol]₀ = 20 μM), acute toxicity of water sample containing iodophenols could be largely eliminated (> 85%), with the conversion of iodophenols into stable and non-toxic iodate (IO₃^-) and iodinated and chlorinated aliphatic DBPs. Besides IO₃^-, seven kinds of aromatic intermediate products including iodophenols, chloroiodophenols, iodoquinones, chloroiodoquinones, chloroquinones, chlorophenols, and coupling products were detected. C-I bond of iodophenols was cleaved in the reaction and the resulted aromatic products were further transformed into chlorinated aliphatic DBPs [trichloromethane (TCM), trichloroacetic acid (TCAA), dichloroacetic acid (DCAA), and chloral hydrate (CH)] (mg/L level) and iodinated trihalomethanes (μg/L level). HOCl was effective for converting iodophenols into IO₃^- and less toxic chlorinated aliphatic DBPs. Considering that chlorine was widely used as disinfectant, transformation and toxicity alteration of emerging DBPs during chlorination/booster chlorination warrant further investigations.

Formation of disinfection by-products in a UV-activated mixed chlorine/chloramine system

In recent years, ultraviolet (UV) irradiation coupled with chlor(am)ination process is ubiquitous in secondary water supply systems in many cities of China. However, the disinfection by-products (DBPs) formation in a UV-activated mixed chlorine/chloramine system (MCCS) still remains unclear. In this study, the DBPs formation in a UV-activated MCCS was systematically investigated, considering influencing factors including the mass ratios of free chlorine to NH₂Cl, UV irradiation, pH values, NOM types, Br^- concentration and toxicity of the DBPs. Results indicated that DBPs formation decreased remarkably as mass ratio of free chlorine to NH₂Cl changed from 5:0 to 0:5. The DBPs formation in humic acid (HA)-containing water was the highest, followed by those in fulvic acid (FA) and algal organic matter (AOM). Besides, better control of the DBP-related calculated toxicity can be achieved in acidic conditions regardless of the UV irradiation. Furthermore, in the presence of Br^-, a significant reduction of DBPs formation could be achieved in a UV-activated MCCS. The findings also demonstrated that DBPs formation in real water can be effectively reduced at high UV fluence in a MCCS.

Formation and degradation mechanisms of CX3R-type oxidation by-products during cobalt catalyzed peroxymonosulfate oxidation: The roles of Co3+ and SO4·

Sulfate radical (SO₄^·-)-based advanced oxidation processes (AOPs) attract increasing attention in the control of micropollutants. However, SO₄^·- can react with other chemicals present in water and result in undesired oxidation by-products (OBPs) generation. The formation and degradation mechanisms of CX₃R-type OBPs during cobalt catalyzed peroxymonosulfate (Co²⁺/PMS) oxidation were investigated. In the formation of CX₃R-type OBPs, both Co³⁺ and SO₄^·- could convert chloride to free chlorine that then reacted with natural organic matter, leading to the formation of CX₃R-type OBPs. The concentrations of trichloromethane, chloral hydrate, dichloroacetonitrile, dichloroacetamide and trichloroacetamide after 15 min reaction were 9.8, 3.9, 1.2, 5.9 and 22.3 nM, respectively. Compared to SO₄^·-, Co³⁺ played a more significant role in the CX₃R-type OBP formation and calculated toxicity values of CX₃R-type OBPs. CX₃R-type OBPs could not only be formed but also be degraded at the same time during Co²⁺/PMS oxidation. As for the degradation of CX₃R-type OBPs, both Co³⁺ and SO₄^·- could transform CX₃R-type OBPs to chloride. Compared to Co³⁺, SO₄^·- played a more important role in the degradation of CX₃R-type OBPs and the conversion from chloride to final by-product chlorate. The adverse effects that results from Co³⁺ need more attention in SO₄^·--based AOPs application.

Source and drinking water organic and total iodine and correlation with water quality parameters

Iodinated disinfection by-products (I-DBPs) have recently emerged as part of the pool of DBPs of public health concern. Due to limitations in measuring individual I-DBPs in a water sample, the surrogate measure of total organic iodine (TOI) is often used to account for the sum of all I-DBPs. In this study, TOI and total iodine (TI) are quantified in raw and treated waters in treatment trains at three sites in the Northeast United States. The occurrence, magnitude, and seasonality of these species was investigated within each sampling train and across the different sites. A regression model was developed to explore how TOI occurrence varies with routinely measured physical and chemical parameters in a water sample. The TOI and TI concentration at the three sites ranged from below the method detection limit to 18 µg/L and from 3 and 18.9 µg/L, respectively. There was substantial inter-monthly variability in TOI without a clear seasonal signal, and the concentration of TOI did not increase upon treatment. The results of the multivariate regression model showed that dissolved organic carbon (DOC), specific UV₂₅₄ absorbance (SUVA), combined chlorine residual (TCl₂), and pH were all significantly related to TOI concentration to varying degrees. A Tobit model was fit to show TOI predictions against observed (measured) TOI values. The model could explain approximately 46% of the variance of TOI concentrations in the treated waters.

Efficiency of pre-oxidation of natural organic matter for the mitigation of disinfection byproducts: Electron donating capacity and UV absorbance as surrogate parameters

Pre-oxidation is often used before disinfection with chlorine to decrease the reactivity of the water matrix and mitigate the formation of regulated disinfection byproducts (DBPs). This study provides insights on the impact of oxidative pre-treatment with chlorine dioxide (ClO₂), ozone (O₃), ferrate (Fe(VI)) and permanganate (Mn(VII)) on Suwannee River Natural Organic Matter (SRNOM) properties characterized by the UV absorbance at 254 nm (UV₂₅₄) and the electron donating capacity (EDC). Changes in NOM reactivity and abatement of DBP precursors are also assessed. The impact of pre-oxidants (based on molar concentration) on UV₂₅₄ abatement ranked in the order O₃ > Mn(VII) > Fe(VI)/ClO₂, while the efficiency of pre-oxidation on EDC abatement followed the order Mn(VII) > ClO₂ > Fe(VI) > O₃ and two phases were observed. At low specific ClO₂, Fe(VI) and Mn(VII) doses corresponding to < 50% EDC abatement, a limited relative abatement of UV₂₅₄ compared to the EDC was observed (~ 8% EDC abatement per 1% UV₂₅₄ abatement). This suggests the oxidation of phenolic-type moieties to quinone-type moieties which absorb UV₂₅₄ and don't contribute to EDC. At higher oxidant doses (> 50% EDC abatement), a similar abatement of EDC and UV₂₅₄ (~ 0.9-1.2% EDC abatement per 1% UV₂₅₄ abatement) suggested aromatic ring cleavage. In comparison to the other oxidants, O₃ abated the relative UV₂₅₄ more effectively, due to a more efficient cleavage of aromatic rings. For a pre-oxidation with Mn(VII), ClO₂ and Fe(VI), similar correlations between the EDC abatement and the chlorine demand or the adsorbable organic halide (AOX) formation were obtained. In contrast, O₃ pre-treatment led to a lower chlorine demand and AOX formation for equivalent EDC abatement. For all oxidants_, trihalomethane formation was poorly correlated with both EDC and UV_254. The EDC abatement was found to be a pre-oxidant-independent surrogate for haloacetonitrile formation. These results emphasize the benefits of combining EDC and UV₂₅₄ measurement to understand and monitor oxidant-induced changes of NOM and assessing DBP formation.

Controlling disinfection byproducts from treated wastewater using adsorption with granular activated carbon: Impact of pre-ozonation and pre-chlorination

This study measured chlorine- and chloramine-reactive precursors using formation potential (FP) tests of nine U.S. Environmental Protection Agency (EPA) regulated and 57 unregulated disinfection byproducts (DBPs) in tertiary-filtered wastewater before and after pilot-scale granular activated carbon (GAC) adsorption. Using breakthrough of precursor concentration and of concentration associated calculated cytotoxicity and genotoxicity (by correlating known lethal concentrations reported elsewhere), the performance of three parallel GAC treatment trains were compared against tertiary-filtered wastewater: ozone/GAC, chlorine/GAC, and GAC alone. Results show GAC alone was the primary process, versus ozone or chlorine alone, to remove the largest fraction of total chlorine- and chloramine-reactive DBP precursors and calculated cytotoxicity and genotoxicity potencies. GAC with pre-ozonation removed the most chlorine- and chloramine-reactive DBP precursors followed by GAC with pre-chlorination and lastly GAC without pre-treatment. GAC with pre-ozonation produced an effluent with cytotoxicity and genotoxicity of DBPs from FP that generally matched that of GAC without pre-oxidation; meanwhile removal of toxicity was greater by GAC with pre-chlorination. The cytotoxicity and genotoxicity of DBPs from FP tests did not scale with DBP concentration; for example, more than 90% of the calculated cytotoxicity resulted from 20% of the DBPs, principally from haloacetaldehydes, haloacetamides, and haloacetonitriles. The calculated cytotoxicity and genotoxicity from DBPs associated with FP-chloramination were at times higher than with FP-chlorination though the concentration of DBPs was five times higher with FP-chlorination. The removal of DBP precursors using GAC based treatment was at least as effective as removal of DOC (except for halonitromethanes for GAC without pre-oxidation and with pre-chlorination), indicating DOC can be used as an indicator for DBP precursor adsorption efficacy. However, the DOC was not a good surrogate for total cytotoxicity and genotoxicity breakthrough behavior, therefore, unregulated DBPs could have negative health implications that are disconnected from general water quality parameters, such as DOC, and regulated classes of DBPs. Instead, cytotoxicity and genotoxicity correlate with the concentration of specific classes of unregulated DBPs.

Monohaloacetic Acids and Monohaloacetamides Attack Distinct Cellular Proteome Thiols

Disinfection byproduct (DBP) exposure has been linked to multiple adverse health outcomes. However, the molecular initiating events by which DBPs induce their toxicities remain unclear. Herein, we combined reporter cell lines and activity-based protein profiling (ABPP) chemical proteomics to identify the protein targets of three monohaloacetic acids (mHAAs) and three monohaloacetamides (mHAMs), at the proteome-wide level. While mHAAs and mHAMs have similar potencies in reducing MTT activity, mHAMs induced greater Nrf2-mediated oxidative stress responses, demonstrating their distinct toxicity pathways. ABPP on crude cell lysates suggested that general proteome thiol reactivity correlates with cytotoxicity. Interestingly, live cell ABPP results revealed class-specific proteins attacked by mHAMs or mHAAs. Subsequent proteomic analysis identified >100 unique targets per DBP. mHAMs preferentially react with redox proteins including disulfide oxidoreductase enzymes, accounting for their stronger Nrf2 responses. To further probe alkylation mechanisms, we directly monitored protein adducts and identified 120 and 37 unique peptides with iodoacetamide and iodoacetic acid adducts, respectively. Of the latter, we confirmed glyceraldehyde-3-phosphate dehydrogenase as a key target of IAA, specifically attacking the catalytic Cys 152. This is the first study reporting the distinct cellular protein targets of mHAAs and mHAMs at the proteome-wide level, which highlights their different toxicity pathways despite their similar structures.

Haloacetonitriles and haloacetamides precursors in filter backwash and sedimentation sludge water during drinking water treatment

Haloacetonitriles (HANs) and haloacetamides (HAMs) are nitrogenous disinfection byproducts that are present in filter backwash water (FBW) and sedimentation sludge water (SSW). In many cases FBW and SSW are recycled to the head of drinking water treatment plants. HAN and HAM concentrations in FBW and SSW, without additional oxidants, ranged from 6.8 to 11.6 nM and 2.9 to 3.6 nM of three HANs and four HAMs, respectively. Upon oxidant addition to FBW and SSW under formation potential conditions, concentrations for six HANs and six HAMs ranged from 92.2 to 190.4 nM and 42.2 to 95.5 nM, respectively. Therefore, at common FBW and SSW recycle rates (2 to 10% of treated water flows), the precursor levels in these recycle waters should not be ignored because they are comparable to levels present in finished water. Brominated HAN and chlorinated HAM were the dominant species in FBW and SSW, respectively. The lowest molecular weight ultrafiltration fraction (< 3 kDa) contributed the most to HAN and HAM formations. The hydrophilic (HPI) organic fraction contributed the greatest to HAN precursors in sand-FBW and SSW and were the most reactive HAM precursors in both sand- or carbon-FBWs. Fluorescence revealed that aromatic protein-like compounds were dominant HAN and HAM precursors. Therefore, strategies that remove low molecular weight hydrophilic organic matter and aromatic protein-like compounds will minimize HAN and HAM formations in recycled FBW and SSW.

Transformation of X-ray contrast media by conventional and advanced oxidation processes during water treatment: Efficiency, oxidation intermediates, and formation of iodinated byproducts

X-ray contrast media (ICM), as the most widely used intravascular pharmaceuticals, have been frequently detected in various environmental compartments. ICM have attracted increasingly scientific interest owing to their role as an iodine contributor, resulting in the high risk of forming toxic iodinated byproducts (I-BPs) during water treatment. In this review, we present the state-of-the-art findings relating to the removal efficiency as well as oxidation intermediates of ICM by conventional and advanced oxidation processes. Moreover, formation of specific small-molecular I-BPs (e.g., iodoacetic acid and iodoform) during these processes is also summarized. Conventional oxidants and disinfectants including chlorine (HOCl) and chloramine (NH₂Cl) have low reactivities towards ICM with HOCl being more reactive. Iodinated/deiodinated intermediates are generated from reactions of HOCl/NH₂Cl with ICM, and they can be further transformed into small-molecular I-BPs. Types of disinfectants and ICM as well as solution conditions (e.g., presence of bromide (Br^-) and natural organic matters (NOM)) display significant impact on formation of I-BPs during chlor(am)ination of ICM. Uncatalyzed advanced oxidation process (AOPs) involving ozone (O₃) and ferrate (Fe(VI)) exhibit slow to mild reactivities towards ICM, usually leading to their incomplete removal under typical water treatment conditions. In contrast, UV photolysis and catalyzed AOPs including hydroxyl radical (HO^•) and/or sulfate radical (SO₄^.-) based AOPs (e.g., UV/hydrogen peroxide, UV/persulfate, UV/peroxymonosulfate (PMS), and CuO/PMS) and reactive chlorine species (RCS) involved AOPs (e.g., UV/HOCl and UV/NH₂Cl) can effectively eliminate ICM under various conditions. Components of water matrix (e.g., chloride (Cl^-), Br^-, bicarbonate (HCO₃^-), and NOM) have great impact on oxidation efficiency of ICM by catalyzed AOPs. Generally, similar intermediates are formed from ICM oxidation by UV photolysis and AOPs, mainly resulting from a series reactions of the side chain and/or C-I groups (e.g. cleavage, dealkylation, oxidation, and rearrange). Further oxidation or disinfection of these intermediates leads to formation of small-molecular I-BPs. Pre-oxidation of ICM-containing waters by AOPs tends to increase formation of I-BPs during post-disinfection process, while this trend also depends on the oxidation processes applied and solution conditions. This review summarizes the latest research findings relating to ICM transformation and (by)products formation during disinfection and AOPs in water treatment, which has great implications for the practical applications of these technologies.

Mechanistic study on chlorine/nitrogen transformation and disinfection by-product generation in a UV-activated mixed chlorine/chloramines system

The conversion mechanisms of chlorine species (including free chlorine, monochloramine (NH₂Cl), dichloramine, and total chlorine), nitrogen species (including ammonium (NH₄⁺), nitrate (NO₃^-), and nitrite (NO₂^-)) as well as the formation of disinfection by-products (DBPs) in a UV-activated mixed chlorine/chloramines system in water were investigated in this work. The consumption rates of free chlorine and NH₂Cl were significantly promoted in a HOCl/NH₂Cl coexisting system, especially in the presence of UV irradiation. Moreover, the transformation forms of nitrogen in both ultrapure and HA-containing waters were considerably affected by UV irradiation and the mass ratio of free chlorine to NH₂Cl. NO₃^- and NO₂^- can be easily produced under UV irradiation, and the removal efficiency of total nitrogen with UV was obvious higher than that without UV when the initial ratio of HOCl/NH₂Cl was less than 1. The roles of different radicals in the degradation of free chlorine, NH₂Cl and NH₄⁺ were also considered in such a UV-activated mixed chlorine/chloramines system. The results indicated that OH• was important to the consumption of free chlorine and NH₂Cl, and showed negligible influence on the consumption of NH₄⁺. Besides, the changes of DOC and UV₂₅₄ in HA-containing water in UV-activated mixed chlorine/chloramines system indicated that the removal efficiency of DOC (24%) was much lower than that of UV₂₅₄ (94%). The formation of DBPs in a mixed chlorine/chloramines system was also evaluated. The yields of DBPs decreased significantly as the mass ratio of HOCl/NH₂Cl varied from 1 : 0 to 0 : 1. Moreover, compared to the conditions without UV irradiation, higher DBPs yields and DBP-associated calculated toxicity were observed during the UV-activated mixed chlorine/chloramine process.

Development and Application of a High-Precision Algorithm for Nontarget Identification of Organohalogens Based on Ultrahigh-Resolution Mass Spectrometry

Brominated and/or chlorinated organic compounds (referred to as organohalogens) are frequently detected in natural and engineered environments. However, ultrahigh-resolution mass spectrometry (UHR-MS)-based nontargeted identification of organohalogens remains challenging because of the coexistence of a vast number of halogenated and nonhalogenated organic molecules. In this study, a new algorithm, namely, the NOMDBP code, was developed to simultaneously identify organohalogens and non-organohalogens from the UHR-MS spectra of natural and engineered waters. In addition to isotopic patterns, for the first time, three optional filter rules [i.e., selection for minimum nonoxygen heteroatoms, inspection of the presence of newly formed halogenated disinfection byproducts (Xn-DBPs), and of their precursors] were incorporated into our code, which can accurately identify DBP-associated peaks and further elucidate Xn-DBP generation and transformation mechanisms. The formula assignment ratio against 2815 previously reported organohalogens, and their 11,583 isotopologues exceeded 97%. Application of our algorithm to disinfected natural organic matter indicated that oxygen-containing Xn-DBP species accounted for a majority of the Xn-DBPs. Furthermore, brominated Xn-DBPs (Br-DBPs) were characterized by a higher degree of unsaturation compared to chlorinated Xn-DBPs. In addition to electrophilic substitution and electrophilic addition reactions, the decomposition/transformation pathway was found to be another important mechanism in Br-DBP formation. The results of this study highlight the superior potential of our code for the efficient detection of yet unknown organohalogens (including organohalogens bearing nonoxygen heteroatoms) in a nontargeted manner and for the identification of their generation mechanism occurring during the disinfection process.

Enzymatic assays for the assessment of toxic effects of halogenated organic contaminants in water and food. A review

Halogenated organic compounds are a particular group of contaminants consisting of a large number of substances, and of great concern due to their persistence in the environment, potential for bioaccumulation and toxicity. Some of these compounds have been classified as persistent organic pollutants (POPs) under The Stockholm Convention and many toxicity assessments have been conducted on them previously. In this work we provide an overview of enzymatic assays used in these studies to establish toxic effects and dose-response relationships. Studies in vivo and in vitro have been considered with a particular emphasis on the impact of halogenated compounds on the activity of relevant enzymes to the humans and the environment. Most information available in the literature focuses on chlorinated compounds, but brominated and fluorinated molecules are also the target of increasing numbers of studies. The enzymes identified can be classified as enzymes: i) the activities of which are affected by the presence of halogenated organic compounds, and ii) those involved in their metabolisation/detoxification resulting in increased activities. In both cases the halogen substituent seems to have an important role in the effects observed. Finally, the use of these enzymes in biosensing tools for monitoring of halogenated compounds is described.

Micro-Polluted Surface Water Treated by Yeast-Chitosan Bio-Microcapsules

Ammonia nitrogen and natural organic matter (NOM) seriously degrade the quality of surface waters. In this study, the optimum preparation conditions of a yeast-chitosan bio-microcapsule of the Candida tropicalis strain, used to treat micro-polluted surface water, were investigated. Fourier transform infrared spectroscopy and scanning electron microscopy were used to characterize the bio-microcapsules. A continuous laboratory-scale reaction apparatus was built to evaluate the engineering applications of the bio-microcapsules and their treatment efficiency for major pollutants in micro-polluted raw water. The yeast-chitosan bio-microcapsules were found to rapidly and effectively remove suspended solids and ammonia nitrogen. Moreover, the bio-microcapsule pre-treatment process was capable of resisting impact loads and fluctuations in water quality. Even at low temperatures (12 °C), the removal rate of ammonia nitrogen still reached 79%. The treatment did not lead to a temporary increase in nitrite concentration, nor to the excessive accumulation of nitrogen. The application of bio-microcapsules is simple; it only requires aeration and certain nutrient substrates, and can be adapted to treat raw drinking water with a poor nutrient substrate, therefore showing promise for future use in engineering applications.

Role of Reactive Halogen Species in Disinfection Byproduct Formation during Chlorine Photolysis

The multiple reactive oxidants produced during chlorine photolysis effectively degrade organic contaminants during water treatment, but their role in disinfection byproduct (DBP) formation is unclear. The impact of chlorine photolysis on dissolved organic matter (DOM) composition and DBP formation is investigated using lake water collected after coagulation, flocculation, and filtration at pH 6.5 and pH 8.5 with irradiation at three wavelengths (254, 311, and 365 nm). The steady-state concentrations of hydroxyl radical and chlorine radical decrease by 38-100% in drinking water compared to ultrapure water, which is primarily attributed to radical scavenging by natural water constituents. Chlorine photolysis transforms DOM through multiple mechanisms to produce DOM that is more aliphatic in nature and contains novel high molecular weight chlorinated DBPs that are detected via high-resolution mass spectrometry. Quenching experiments demonstrate that reactive chlorine species are partially responsible for the formation of halogenated DOM, haloacetic acids, and haloacetonitriles, whereas trihalomethane formation decreases during chlorine photolysis. Furthermore, DOM transformation primarily due to direct photolysis alters DOM such that it is more reactive with chlorine, which also contributes to enhanced formation of novel DBPs during chlorine photolysis.

Kinetics of the reaction between hydrogen peroxide and aqueous iodine: Implications for technical and natural aquatic systems

Oxidative treatment of iodide-containing waters can lead to a formation of potentially toxic iodinated disinfection byproducts (I-DBPs). Iodide (I^-) is easily oxidized to HOI by various oxidation processes and its reaction with dissolved organic matter (DOM) can produce I-DBPs. Hydrogen peroxide (H₂O₂) plays a key role in minimizing the formation of I-DBPs by reduction of HOI during H₂O₂-based advanced oxidation processes or water treatment based on peracetic acid or ferrate(VI). To assess the importance of these reactions, second order rate constants for the reaction of HOI with H₂O₂ were determined in the pH range of 4.0-12.0. H₂O₂ showed considerable reactivity with HOI near neutral pH (k_app = 9.8 × 10³ and 6.3 × 10⁴ M^-1s^-1 at pH 7.1 and 8.0, respectively). The species-specific second order rate constants for the reactions of H₂O₂ with HOI, HO₂^- with HOI, and HO₂^- with OI^- were determined as k_H2O2+HOI = 29 ± 5.2 M^-1s^-1, k_HO2^-_+HOI = (3.1 ± 0.3) × 10⁸ M^-1s^-1, and k_HO2^-_+OI^- = (6.4 ± 1.4) × 10⁷ M^-1s^-1, respectively. The activation energy for the reaction between HOI and H₂O₂ was determined to be E_a = 34 kJ mol^-1. The effect of buffer types (phosphate, acetate, and borate) and their concentrations was also investigated. Phosphate and acetate buffers significantly increased the rate of the H₂O₂-HOI reaction at pH 7.3 and 4.7, respectively, whereas the effect of borate was moderate. It could be demonstrated, that the formation of iodophenols from phenol as a model for I-DBPs formation was significantly reduced by the addition of H₂O₂ to HOI- and phenol-containing solutions. During water treatment with the O₃/H₂O₂ process or peracetic acid in the presence of I^-, O₃ and peracetic acid will be consumed by a catalytic oxidation of I^- due to the fast reduction of HOI by H₂O₂. The O₃ deposition on the ocean surface may also be influenced by the presence of H₂O₂, which leads to a catalytic consumption of O₃ by I^-.

GAC to BAC: Does it make chloraminated drinking water safer?

Biological activated carbon (BAC) is widely used as a polishing step at full-scale drinking water plants to remove taste and odor compounds and assimilable organic carbon. BAC, especially with pre-ozonation, has been previously studied to control regulated disinfection by-products (DBPs) and DBP precursors. However, most previous studies only include regulated or a limited number of unregulated DBPs. This study explored two full-scale drinking water plants that use pre-chloramination followed by BAC and chloramine as the final disinfectant. While chloramine generally produces lower concentrations of regulated DBPs, it may form increased levels of unregulated nitrogenous and iodinated DBPs. We evaluated 71 DBPs from ten DBP classes including haloacetonitriles, haloacetamides, halonitromethanes, haloacetaldehydes, haloketones, iodinated acetic acids, iodinated trihalomethanes, nitrosamines, trihalomethanes, and haloacetic acids, along with speciated total organic halogen (total organic chlorine, bromine and iodine) across six different BAC filters of increasing age. Most preformed DBPs were well removed by BAC with different ages (i.e., operation times). However, some preformed DBPs were poorly removed or increased following treatment with BAC, including chloroacetaldehyde, dichloronitromethane, bromodichloronitromethane, N-nitrosodimethylamine, dibromochloromethane, tribromomethane, dibromochloroacetic acid, and tribromoacetic acid. Some compounds, including dibromoacetaldehyde, bromochloroacetamide, and dibromoacetamide, were formed only after treatment with BAC. Total organic halogen removal was variable in both plants and increases in TOCl or TOI were observable on one occasion at each plant. While calculated genotoxicity decreased in all filters, decreases in overall DBP formation did not correlate with decreases in calculated cytotoxicity. In three of the six filters, calculated toxicity increased by 4-27%. These results highlight that DBP concentration alone may not always provide an adequate basis for risk assessment.

Formation and removal of disinfection by-products in a full scale drinking water treatment plant

In this case study, high sensitivity simple methods for the analysis of trihalomethanes (THM4), iodinated-trihalomethanes (I-THMs), haloacetic acids (HAAs), bromide, iodide and iodate have been developed. A one-step procedure for the analysis of haloacetic acids by head-space GC-MS provides good reproducibility and low limits of quantification (≤50 ng L^-1). These methods were applied to characterize the formation of disinfection by-products (DBPs) in a full scale drinking water treatment plant. In this treatment plant, the incorporation of bromine into THMs increases throughout the water treatment line, due to the formation of bromine reactive species favored by the decrease of competition between dissolved organic carbon (DOC) and bromide towards chlorine. A linear correlation has been observed between the bromine incorporation factor and the Br^-/DOC mass ratio. The conversion of iodine to iodate by chlorination occurs in this water due to the relatively high bromide concentration. Moreover, a higher formation of iodate compared to iodide levels in the raw water is observed indicating a degradation of organic iodinated compounds. The formation of I-THMs was constant in terms of quantity and speciation between campaigns despite fluctuating concentrations of DOC and total iodine in the raw water. A preferential removal of DBPs formed by the intermediate chlorination in the order I-DBPs > Br-DBPs > Cl-DBPs occurs during the subsequent activated carbon filtration. The removal rates range from 25 to 36% for the regulated THM4, from 82 to 93% for the ∑I-THMs and 95% for haloacetic acids. The assessment of the relative toxicity shows that despite a much lower concentration of HAAs (<10% of the total mass of measured DBPs) compared to THMs, these compounds are responsible for 75% of the relative cytotoxicity of the treated water. Bromoacetic acid on its own accounts for more than 60% of the overall toxicity of the 17 compounds included in this study.

Chloramination of iodide-containing waters: Formation of iodinated disinfection byproducts and toxicity correlation with total organic halides of treated waters

The formation of iodinated disinfection byproducts (I-DBPs) in drinking waters is of a concern due to their higher cyto- and genotoxicity than their chlorinated and brominated analogues. This study investigated the formation of I-DBPs under chloramination conditions using preformed chloramine and associated cyto- and geno-toxicities obtained with Chinese Hamster Ovary (CHO) cell assay. Cyto- and geno-toxicity of the samples were also calculated using DBP toxicity index values and correlated with total organic halide (TOX) formation. In low iodide (I^-) (0.32 μM, 40 μg L^-1) water, increasing dissolved organic carbon (DOC) concentration of selected waters from 0.1 to 0.25 mg L^-1 increased the formation of iodinated trihalomethanes (I-THMs), while further increases from 0.25 to 4 mg L^-1 produced an opposite trend. In high iodide water (3.2 μM, 400 μg L^-1), increasing DOC from 0.5 to 4 mg L^-1 gradually increased the I-THM formation, while a decrease was observed at 5.4 mg L^-1 DOC. Iodoform was the most influenced species from the changes in DOC concentration. While increasing the initial iodide concentration from 0 to 5 μM increased the formation of iodoform, it did not make any considerable impact on the formation of other I-THMs. The measured cytotoxicity of samples was significantly correlated with increasing DOC concentration. Unknown TOCl and TOI showed a high correlation with measured cytotoxicity, while calculated total organic chlorine (TOCl) and total organic iodine (TOI) did not correlate. The comparison of measured and calculated cytotoxicity values showed that the calculated values do not always represent the overall cytotoxicity, since the formation of unknown DBPs are not taken into consideration during the toxicity calculations.

Metabisulfite as an Unconventional Reagent for Green Oxidation of Emerging Contaminants Using an Iron-Based Catalyst

In this work, contaminants of emerging concern were catalytically degraded in the homogeneous phase with the use of unconventional green reagents. Three reagents, namely, sulfite, metabisulfite, and persulfate, were tested and compared with conventional hydrogen peroxide in the degradation process activated by Fe-TAML. The latter is a biodegradable, homogeneous tetra-amido macrocyclic ligand catalyst containing iron(III). Metabisulfite showed the highest efficiency among the three tested reagents, and its reactivity was similar to that of H₂O₂. However, metabisulfite is a safer and cleaner reagent compared to H₂O₂. A comprehensive study of the activity of metabisulfite with Fe-TAML was carried out toward the oxidative degradation of eight contaminants of emerging concern. The catalytic process was tested at different pH values (7, 9, and 11). Metabisulfite showed the highest activity at pH 11, completely degrading some of the tested micropollutants, but in several cases, the system was active at pH 9 as well. In particular, metabisulfite showed the best efficiency toward phenolic compounds. A preliminary study on the reaction mechanism and the nature of the active species in the Fe-TAML/metabisulfite system was also conducted, highlighting that a high-valent iron-oxo species might be involved in the degradation pathways.

Iodination of Dimethenamid in Chloraminated Water: Active Iodinating Agents and Distinctions between Chlorination, Bromination, and Iodination

Few studies have elucidated the agent(s) that generate iodinated disinfection byproducts during drinking water treatment. We present a kinetic investigation of iodination of dimethenamid (DM), a model compound lacking acid-base speciation. Water chemistry parameters (pH, [Cl^-], [Br^-], [I^-], and [pH buffer]) were systematically varied. As pH increased (4-9), DM iodination rate decreased. Conventional wisdom considers hypoiodous acid (HOI) as the predominant iodinating agent; nevertheless, HOI (pK_HOI = 10.4) could not have produced this result, as its concentration is essentially invariant from pH 4-9. In contrast, [H₂OI⁺] and [ICl] both decrease as pH increases. To distinguish their contributions to DM iodination, [Cl^-] was added at constant pH and ionic strength. Although chloride addition did increase the iodination rate, the reaction order in [Cl^-] was fractional (≤0.36). The contribution of ICl to DM iodination remained below 47% under typical drinking water conditions ([Cl^-] ≤ 250 mg/L), implicating H₂OI⁺ as the predominant iodinating agent. Distinctions between DM iodination versus chlorination or bromination include a more pronounced role for the hypohalous acidium ion (H₂OX⁺), negligible contributions by hypohalous acid and molecular halogen (X₂), and a more muted influence of XCl, leading to lesser susceptibility to catalysis by chloride.

Disinfection Byproducts in Rajasthan, India: Are Trihalomethanes a Sufficient Indicator of Disinfection Byproduct Exposure in Low-Income Countries?

The implementation of chlorine disinfection in low-income countries reduces the risk of waterborne illness but initiates exposure to disinfection byproducts (DBPs). Like high-income countries, low-income countries typically are adopting regulations focusing on trihalomethanes (THM4) as an indicator of overall DBP exposure. However, the use of impaired water sources can decouple the formation of THM4 from other DBP classes that are more potent toxins. The documentation of DBP species other than THM4 is rare in low-income countries, where water sources may be degraded by inadequate sanitation infrastructure and other uncontrolled wastewater discharges. We measured THM4 and 21 unregulated DBPs in tap waters and laboratory-treated source waters from two cities in northwestern India. The contribution of each DBP class to the cumulative toxicity was estimated by weighting each species by metrics of toxic potency; haloacetonitriles typically were the dominant contributor, while the contribution of THM4 was negligible. THM4 concentrations did not correlate with the total toxic potency-weighted DBP concentrations. Although THM4 rarely exceeded international guidelines, DBPs of greater toxicological concern were observed in high concentrations. The total toxic potency-weighted DBP concentrations in some waters were elevated compared to conventional drinking waters in high-income countries and more closely resembled chlorine-disinfected wastewater effluents. Artificial sweeteners confirmed widespread contamination of both surface and groundwaters by domestic sewage. The results suggest that THM4 may not be an adequate indicator of overall DBP exposure in impaired water supplies prevalent in some low-income nations.

Formation of iodinated trihalomethanes and noniodinated disinfection byproducts during chloramination of algal organic matter extracted from Microcystis aeruginosa

The increasing occurrence of harmful algal blooms in surface waters may increase the input of algal organic matter (AOM) to the dissolved organic matter pool. The formation of iodinated trihalomethanes (I-THMs) and noniodinated disinfection byproducts (DBPs) in synthetic waters containing AOM extracted from Microcystis aeruginosa was investigated in chloramination (preformed and in-situ formed chloramine, NH₂Cl and Cl₂-NH₂Cl, respectively) and chlorination (Cl₂) processes. AOM is much more favorable for iodine incorporation than natural organic matter (NOM). For example, the formation of I-THM from AOM is much higher than NOM isolate extracted from treated water (e.g., 3.5 times higher in the NH₂Cl process), and thus higher iodine utilization and substitution factors from AOM were observed. Short contact time (2 min) chlorination in Cl₂-NH₂Cl process leading to the formation of halogenated intermediates favored I-THM formation, compared with NH₂Cl process. However, further increasing chlorine contact time from 5 min to 24 h facilitated the conversion from iodide to iodate and thus I-THM formation decreased. Meanwhile, the formation of noniodinated THM4, haloacetonitriles (HANs), and haloacetaldehydes (HALs) increased. Factors including concentrations of AOM and bromide, pH, and chlorine/nitrogen ratios influenced the formation of I-THMs and noniodinated DBPs. To evaluate the benefit of mitigating I-THM formation over the risk of noniodinated DBP formation, measured DBPs were weighed against their mammalian cell toxicity indexes. Increasing the chlorine exposure increased the calculated cytotoxicity based on concentrations of measured I-THMs and noniodinated DBPs since unregulated HANs and HALs were the controlling agents.

Disinfection byproducts and their toxicity in wastewater effluents treated by the mixing oxidant of ClO2/Cl2

Mixing oxidant of chlorine dioxide (ClO₂) and chlorine (Cl₂) often applied in water disinfection. Two secondary wastewater effluents at different ammonium-N levels (0.1 and 1.6 mg N L^-1) were treated with the mixing oxidant (ClO₂/Cl2) to evaluate the formation of disinfection byproducts (DBPs) and the associated cytotoxicity of treated wastewaters. The total chlorine concentrations of ClO₂ and Cl₂ were maintained at 10 mg L^-1 as Cl₂ with varied mixing ratios of ClO₂ to Cl₂. The formation of 37 halogenated DBPs, including nitrogenous, brominated and iodinated analogues, and 2 inorganic DBPs (chlorite and chlorate) was examined. The sum concentrations of the halogenated DBPs were reduced remarkably with decreasing Cl₂ percentages, but each individual DBP group had distinct features. The regulated trihalomethanes reduced the most when ClO₂ was present in chlorination, but decreasing Cl₂ percentage from 70% to 30% was not quite effective to reduce the formation of iodinated trihalomethanes, haloacetic acids and haloacetontriles in low ammonium-N wastewater. The bromine and iodine substitution factors tend to increase with decreasing Cl₂ percentages, indicating that destruction of DBP precursors by ClO₂ favored bromine and iodine incorporation. Additionally, decreasing Cl₂ percentages in the mixing oxidant (ClO₂/Cl₂) was often accompanied with lower chlorate formation but higher chlorite formation. The toxicity of treated wastewaters was evaluated through two approaches: the calculated cytotoxicity based on the concentrations of detected DBPs and the experimental cytotoxicity using the Chinese hamster ovary (CHO) cells. The calculated cytotoxicity decreased with decreasing Cl₂ percentages, with haloacetonitriles and haloacetaldehydes as predominate contributors. However, the experimental cytotoxicity tests showed that treatment of high ammonium-N wastewater with ClO₂/Cl₂ exhibited considerable higher (> 3 times) cytotoxicity potency than using single disinfectant.

Comparison of ferrate and ozone pre-oxidation on disinfection byproduct formation from chlorination and chloramination

This study investigated the effects of ferrate and ozone pre-oxidation on disinfection byproduct (DBP) formation from subsequent chlorination or chloramination. Two natural waters were treated at bench-scale under various scenarios (chlorine, chloramine, each with ferrate pre-oxidation, and each with pre-ozonation). The formation of brominated and iodinated DBPs in fortified natural waters was assessed. Results indicated ferrate and ozone pre-oxidation were comparable at molar equivalent doses for most DBPs. A net decrease in trihalomethanes (including iodinated forms), haloacetic acids (HAAs), dihaloacetonitrile, total organic chlorine, and total organic iodine was found with both pre-oxidants as compared to chlorination only. An increase in chloropicrin and minor changes in total organic bromine yield were caused by both pre-oxidants compared to chlorination only. However, ozone led to higher haloketone and chloropicrin formation potentials than ferrate. The relative performance of ferrate versus ozone for DBP precursor removal was affected by water quality (e.g., nature of organic matter and bromide concentration) and oxidant dose, and varied by DBP species. Ferrate and ozone pre-oxidation also decreased DBP formation from chloramination under most conditions. However, some increases in THM and dihaloacetonitrile formation potentials were observed at elevated bromide levels.