Ozone Promotes Chloropicrin Formation by Oxidizing Amines to Nitro Compounds

Far-UVC 222 nm Treatment: Effects of Nitrate/Nitrite on Disinfection Byproduct Formation Potential

Irradiation at far ultraviolet C (far-UVC) 222 nm by krypton chloride (KrCl*) excilamps can enhance microbial disinfection and micropollutant photolysis/oxidation. However, nitrate/nitrite, which absorbs strongly at 222 nm, may affect the formation of disinfection byproducts (DBPs). Herein, we evaluated model organic matter and real water samples and observed a substantial increase in the formation potential for trichloronitromethane (chloropicrin) (TCNM-FP), a nitrogenous DBP, by nitrate or nitrite after irradiation at 222 nm. At a disinfection dose of 100 mJ·cm-2, TCNM-FP of humic acids and fulvic acids increased from ∼0.4 to 25 and 43 μg·L-1, respectively, by the presence of 10 mg-N·L-1 nitrate. For the effect of nitrate concentration, the TCNM-FP peak was observed at 5-10 mg-N·L-1. Stronger fluence caused a greater increase of TCNM-FP. Similarly, the increase of TCNM-FP was also observed for wastewater and drinking water samples containing nitrate. Pretreatment using ozonation and coagulation, flocculation, and filtration or the addition of H2O2 can effectively control TCNM-FP. The formation potential of other DBPs was minorly affected by irradiation at 222 nm regardless of whether nitrate/nitrite was present. Overall, far-UVC 222 nm treatment poses the risk of increasing TCNM-FP of waters containing nitrate or nitrite at environmentally relevant concentrations and the mitigation strategies merit further research.

Disinfection by-products control in wastewater effluents treated with ozone and biological activated carbon followed by UV/Chlor(am)ine processes

Sequential utilization of ozone (O3) and biological activated carbon (BAC) followed by UV/chlor(am)ine advanced oxidation process (AOP) has drawn attention in water reuse. However, the formation of disinfection by-products (DBPs) in this process is less evaluated. This study investigated the DBP formation and the relevant toxicity during the O3-BAC-UV/chlor(am)ine treatment of sand-filtered municipal secondary effluent. DBP formation in UV/chlorine and UV/dichloramine (NHCl2) processes were compared, where the impact of key operational parameters (e.g., UV wavelength, pH) on DBP formation were comprehensively evaluated. O3-BAC significantly reduced DBP formation potential (DBPFP) (58.2 %). Compared to UV/chlorine AOP, UV/NHCl2 AOP reduced DBP formation by 29.7 % in short-time treatment, while insignificantly impacting on DBPFP (p > 0.05). UV/NHCl2 AOP also led to lower calculated cytotoxicity (67.7 %) and genotoxicity (55.9 %) of DBPs compared to UV/chlorine AOP. Compared to 254 nm UV light, the utilization of 285 nm UV light decreased the formation of DBPs in wastewater treated with the UV/chlorine AOP and UV/NHCl2 AOP by 31.3 % and 19.2 %, respectively. However, the cytotoxicity and genotoxicity in UV/NHCl2 AOP using 285 nm UV light increased by 83.4 % and 58.5 %, respectively, compared to 254 nm. The concentration of DBPs formed in the UV/NHCl2 AOP at pH 8 was 54.3 % lower than that at pH 7, suggesting a better control of DBPs at alkaline condition. In the presence of bromide, UV/NHCl2 AOP tended to generate more brominated DBPs than UV/chlorine AOP. Overall, UV/NHCl2 AOP resulted in lower concentration and toxicity of DBPs compared to UV/chlorine AOP.

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

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

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.

Ozonation of Gabapentin in Water─Investigating Reaction Kinetics and Transformation Mechanisms of a Primary Amine Using Isotopically Labeled Ozone

Aliphatic amines are abundant micropollutants in wastewater treatment plant effluents. In order to mitigate such micropollutants, ozonation is one of the most commonly employed advanced treatment processes. Current research regarding ozone efficiency is heavily focusing on reaction mechanisms of different contaminant groups, including structures with amine moieties as reactive sites. This study analyzes pH-dependent reaction kinetics and pathways of gabapentin (GBP), an aliphatic primary amine with an additional carboxylic acid group. The transformation pathway was elucidated applying a novel approach using isotopically labeled ozone (18O) and quantum chemistry calculations. While the direct reaction of GBP with ozone is highly pH-dependent and slow at pH 7 (13.7 M-1 s-1), the rate constant of the deprotonated species (1.76 × 105 M-1 s-1) is comparable to those of other amine compounds. Pathway analysis based on LC-MS/MS measurements revealed that ozonation of GBP leads to the formation of a carboxylic acid group and simultaneous nitrate formation, which was also observed in the case of the aliphatic amino acid glycine. Nitrate was formed with a yield of approximately 100%. Experiments with 18O-labeled ozone demonstrated that the intermediate aldehyde does most likely not include any oxygen originating from ozone. Furthermore, quantum chemistry calculations did not provide an explanation for the C-N scission during GBP ozonation without ozone involvement, although this reaction was slightly more favorable than for respective glycine and ethylamine reactions. Overall, this study contributes to a deeper understanding of reaction mechanisms of aliphatic primary amines during wastewater ozonation.

Chloramination of Nitromethane: Incomplete Chlorination and Unexpected Substitution Reaction

Ozone is commonly used as a predisinfectant in potable water reuse treatment trains. Nitromethane was recently found as a ubiquitous ozone byproduct in wastewater, and the key intermediate toward chloropicrin during subsequent secondary disinfection of ozonated wastewater effluent with chlorine. However, many utilities have switched from free chlorine to chloramines as a secondary disinfectant. The reaction mechanism and kinetics of nitromethane transformation by chloramines, unlike those for free chlorine, are unknown. In this work, the kinetics, mechanism, and products of nitromethane chloramination were studied. The expected principal product was chloropicrin, because chloramines are commonly assumed to react similarly to, although more slowly than, free chlorine. Different molar yields of chloropicrin were observed under acidic, neutral, and basic conditions, and surprisingly, transformation products other than chloropicrin were found. Monochloronitromethane and dichloronitromethane were detected at basic pH, and the mass balance was initially poor at neutral pH. Much of the missing mass was later attributed to nitrate formation, from a newly identified pathway involving monochloramine reacting as a nucleophile rather than a halogenating agent, through a presumed SN2 mechanism. The study indicates that nitromethane chloramination, unlike chlorination, is likely to produce a range of products, whose speciation is a function of pH and reaction time.

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.

Reduction of byproduct formation and cytotoxicity to mammalian cells during post-chlorination by the combined pretreatment of ferrate(VI) and biochar

Ferrate [Fe(VI)] can efficiently degrade various pollutants in wastewater. Biochar application can reduce resource use and waste emission. This study investigated the performance of Fe(VI)/biochar pretreatment to reduce disinfection byproducts (DBPs) and cytotoxicity to mammalian cells of wastewater during post-chlorination. Fe(VI)/biochar was more effective at inhibiting the cytotoxicity formation than Fe(VI) alone, reducing the cytotoxicity from 12.7 to 7.6 mg-phenol/L. The concentrations of total organic chlorine and total organic bromine decreased from 277 to 130 μg/L and from 51 to 39 μg/L, compared to the samples without pretreatment. Orbitrap ultra-high resolution mass spectrometry revealed that the number of molecules of DBPs decreased substantially from 517 to 229 by Fe(VI)/biochar, with the greatest reduction for phenols and highly unsaturated aliphatic compounds. In combination with the substantial reduction of 1Cl-DBPs and 2Cl-DBPs, 1Br-DBPs and 2Br-DBPs were also reduced. Fluorescence excitation-emission matrix coupled with parallel factor analysis suggested that fulvic acid-like substances and aromatic amino acid was obviously reduce likely due to the enhanced oxidation of Fe(IV)/Fe(V) produced by Fe(VI)/biochar and adsorption of biochar. Furthermore, the DBPs generated by electrophilic addition and electrophilic substitution of precursors were reduced. This study shows that Fe(VI)/biochar pretreatment can effectively reduce cytotoxicity formation during post-chlorination by transforming DBPs and their precursors.

Relating algal-derived extracellular and intracellular dissolved organic nitrogen with nitrogenous disinfection by-product formation

The dissolved organic nitrogen (DON) pool from algal-derived extracellular and intracellular organic matter (EOM and IOM) comprises proteins, peptides, free amino acids and carbohydrates, of which, proteins can contribute up to 100% of the DON. Previous reports of algal-derived DON character have focused on bulk properties including concentration, molecular weight and hydrophobicity. However, these can be similar between algal species and between the EOM and IOM even when the inherent molecular structures vary. A focus on bulk character presents challenges to the research on algal-derived nitrogenous-disinfection by-product (N-DBP) formation as N-DBP formation is sensitive to the changes in molecular structure. Hence, the main aim of this study was to characterize algal EOM and IOM-derived DON, specifically proteinaceous-DON, using a combination of bulk and molecular characterization techniques to enable a more detailed exploration of the relationship between the character of algal-derived proteins and the N-DBP formation potential. DON from the EOM and IOM of four commonly found algae and cyanobacteria in natural waters were evaluated, namely Chlorella vulgaris, Microcystis aeruginosa, Dolichospermum circinale, and Cylindrospermopsis raciborskii. It was observed that 77-96% of total DON in all EOM and IOM samples was of proteinaceous origin. In the proteins, DON concentrations were highest in the high molecular weight fraction of IOM-derived bulk proteins (0.13-0.75 mg N L^-1) and low to medium molecular weight fraction of EOM-derived bulk proteins (0.15-0.63 mg N L^-1) in all species. Similar observations were also made via sodium dodecyl sulphate polyacrylamide gel electrophoresis and liquid chromatography-high resolution mass spectrometry. Solid-state ¹⁵N nuclear magnetic resonance (NMR) spectroscopy of the EOM and IOM revealed the existence of common aliphatic and heterocyclic N-groups in all samples, including a dominant 2° amide peak. Species dependent variability was also observed in the spectra, particularly in the EOM; e.g. nitro signals were found only in the Cylindrospermopsis raciborskii EOM. Dichloroacetonitrile (DCAN) and N-nitrosamine concentrations from the EOM of the species evaluated in this study were lower than the guideline limits set by regulatory agencies. It is proposed that the dominant 2° amide in all samples decreased N-DBP formation upon chlorination. For chloramination, the presence of nitro groups and aliphatic and heterocyclic N-DBP precursors could cause variable N-nitrosamine formation. Compared to non-algal impacted waters, algae-laden waters are characterised by low organic carbon: organic nitrogen ratios of ∼7-14 and elevated DON and protein concentrations. Hence, relying only on bulk characterization increases the perceived risk of N-DBP formation from algae-laden waters.

Computational Studies of Rubber Ozonation Explain the Effectiveness of 6PPD as an Antidegradant and the Mechanism of Its Quinone Formation

The discovery that the commercial rubber antidegradant 6PPD reacts with ozone (O₃) to produce a highly toxic quinone (6PPDQ) spurred a significant research effort into nontoxic alternatives. This work has been hampered by lack of a detailed understanding of the mechanism of protection that 6PPD affords rubber compounds against ozone. Herein, we report high-level density functional theory studies into early steps of rubber and PPD (p-phenylenediamine) ozonation, identifying key steps that contribute to the antiozonant activity of PPDs. In this, we establish that our density functional theory approach can achieve chemical accuracy for many ozonation reactions, which are notoriously difficult to model. Using adiabatic energy decomposition analysis, we examine and dispel the notion that one-electron charge transfer initiates ozonation in these systems, as is sometimes argued. Instead, we find direct interaction between O₃ and the PPD aromatic ring is kinetically accessible and that this motif is more significant than interactions with PPD nitrogens. The former pathway results in a hydroxylated PPD intermediate, which reacts further with O₃ to afford 6PPD hydroquinone and, ultimately, 6PPDQ. This mechanism directly links the toxicity of 6PPDQ to the antiozonant function of 6PPD. These results have significant implications for development of alternative antiozonants, which are discussed.

Permanganate preoxidation affects the formation of disinfection byproducts from algal organic matter

During harmful algal blooms (HABs), permanganate may be used as a preoxidant to improve drinking water quality by removing algal cells and degrading algal toxins. However, permanganate also lyses algal cells, releasing intracellular algal organic matter (AOM). AOM further reacts with permanganate to alter the abundance of disinfection byproduct (DBP) precursors, which in turn affects DBP formation during disinfection. In this study, we evaluated the impacts of preoxidation by permanganate applied at commonly used doses (i.e., 1-5 mg/L) on DBP generation during chlorination and chloramination of AOM. We found that permanganate preoxidation increased trichloronitromethane (TCNM) formation by up to 3-fold and decreased dichloroacetonitrile (DCAN) formation by up to 40% during chlorination, indicating that permanganate oxidized organic amines in AOM to organic nitro compounds rather than organic nitrile compounds. To test this proposed mechanism, we demonstrated that permanganate oxidized organic amines in known DBP precursors (i.e., tyrosine, tryptophan) to favor the production of TCNM over DCAN during chlorination. Compared to the decreased formation of DCAN during chlorination, permanganate increased DCAN formation by 30-50% during chloramination of AOM. This difference likely arose from monochloramine's ability to react with non-nitrogenous precursors (e.g., organic aldehydes) that formed during permanganate preoxidation of AOM to generate nitrogen-containing intermediates that go on to form DCAN. Our results also showed that permanganate preoxidation favored the formation of dichlorobromomethane (DCBM) over trichloromethane (TCM) during chlorination and chloramination. The increased formation of DBPs, especially nitrogenous DBPs that are more toxic than carbonaceous DBPs, may increase the overall toxicity in finished drinking water when permanganate preoxidation is implemented.

Comparison of nitrate formation mechanisms from free amino acids and amines during ozonation: a computational study

Nitrate as a potential surrogate parameter for abatement of micropollutants, oxidant exposure, and characterizing oxidant-reactive DON during ozonation has attracted extensive attention, however, understanding of its formation mechanisms is still limited. In this study, nitrate formation mechanisms from amino acids (AAs) and amines during ozonation were investigated by the DFT method. The results indicate that N-ozonation initially occurs to produce competitive nitroso- and N,N-dihydroxy intermediates, and the former is preferred for both AAs and primary amines. Then, oxime and nitroalkane are generated during further ozonation, which are the important last intermediate products for nitrate formation from the respective AAs and amines. Moreover, the ozonation of the above important intermediates is the nitrate yield-controlling step, where the relatively higher reactivity of the CN moiety in the oxime compared to the general C_α atom in the nitroalkane explains why the nitrate yields of most AAs are higher than those from general amines, and it is the larger number of released C_α^- anions, which are the real reaction sites attacked by ozone, that leads to the higher nitrate yield for nitroalkane with an electron-withdrawing group bound to the C_α atom. The good relationship between nitrate yields and activation free energies of the rate-limiting step (ΔG≠rls) and nitrate yield-controlling step (ΔG≠nycs) for the respective AAs and amines verifies the reliability of the proposed mechanisms. Additionally, the bond dissociation energy of C_α-H in the nitroalkanes formed from amines was found to be a good parameter to evaluate the reactivity of the amines. The findings here are helpful for further understanding nitrate formation mechanisms and predicting nitrate precursors during ozonation.

Control of Pre-formed Halogenated Disinfection Byproducts with Reuse Biofiltration

Disinfection byproduct (DBP) pre-formation is a major issue when prechlorination is used before or during advanced treatment of impacted drinking water sources. Control strategies for pre-formed DBPs before final disinfection, especially for currently nonregulated although highly toxic DBP species, are not yet established. This study evaluated the biodegradation potential of pre-formed DBPs, including haloacetonitriles (HANs), haloacetamides (HAMs), and haloacetaldehydes (HALs), during biofiltration with sand, anthracite, and biological activated carbon of three wastewater effluents under potable reuse conditions. Up to 90%+ removal of di- and trihalogenated HANs, HAMs, and HALs was observed, and removal was associated with active heterotrophic biomass and removal of biodegradable organic carbon. Unlike the microbial dehalogenation pathway of haloacetic acids (HAAs), removal of HANs and HAMs appeared to result from a biologically mediated hydrolysis pathway (i.e., HANs to HAMs and HAAs) that may be prone to inhibition. After prechlorination, biofiltration effectively controlled pre-formed DBP concentrations (e.g., from 271 μg/L to as low as 22 μg/L in total) and DBP-associated calculated toxicity (e.g., 96%+ reduction). Abiotic residual adsorption capacity in biological activated carbon media was important for controlling trihalomethanes. Overall, the toxicity-driving DBP species exhibited high biodegradation potential and biofiltration showed significant promise as a pre-formed DBP control technology.

Oxidative treatment of NOM by selective oxidants in drinking water treatment and its impact on DBP formation in postchlorination

Natural organic matter (NOM), as a ubiquitous component in aqueous environments, has raised continuous scientific concerns due to its role as an organic precursor to disinfection by-products (DBPs) in the subsequent chlorination process. Selective oxidants, including ozone (O₃), chlorine dioxide (ClO₂), permanganate (Mn(VII)), and ferrate (Fe(VI)) are widely used in the preoxidation stage in drinking water treatment. The selective reactivity of those oxidants toward NOM is expected to alternate NOM properties and consequently DBP formation in postchlorination. Despite extensive studies on the interactions of NOM with selective oxidants, there is currently a lack of an overview of this area. To fill this gap, this study presents the current knowledge of the modification of NOM properties by selective oxidants and its impact on DBP formation in postchlorination. The NOM property changes in three aspects, including bulk property (e.g., total organic carbon, ultraviolet absorbance), fractional constituent (e.g., molecular size, hydrophilicity/hydrophobicity), and elemental composition (e.g., functional group) by the four selective oxidants (i.e., O₃, ClO₂, Mn(VII), and Fe(VI)) were discussed. Thereafter, the impacts of alteration of NOM properties by those selective oxidants on DBP formation in the subsequent chlorination were summarized, wherein the key influencing factors were discussed. Finally, the future perspectives in this area were forwarded, which highlighted the significance of process optimization, the attention to the less studied but more toxic DBPs, and the need for the identification of unknown DBPs. This review presented a state-of-the-art knowledge pool of the fate of NOM in oxidation and chlorination processes, promoted our understanding of the relationship between NOM properties and DBP formation, and identified further research needs in this area.

Peracetic Acid Enhances Micropollutant Degradation by Ferrate(VI) through Promotion of Electron Transfer Efficiency

Ferrate(VI) and peracetic acid (PAA) are two oxidants of growing importance in water treatment. Recently, our group found that simultaneous application of ferrate(VI) and PAA led to much faster degradation of micropollutants compared to that by a single oxidant, and this paper systematically evaluated the underlying mechanisms. First, we used benzoic acid and methyl phenyl sulfoxide as probe compounds and concluded that Fe(IV)/Fe(V) was the main reactive species, while organic radicals [CH₃C(O)O^•/CH₃C(O)OO^•] had negligible contribution. Second, we removed the coexistent hydrogen peroxide (H₂O₂) in PAA stock solution with free chlorine and, to our surprise, found the second-order reaction rate constant between ferrate(VI) and PAA to be only about 1.44 ± 0.12 M^-1s^-1 while that of H₂O₂ was as high as (2.01 ± 0.12) × 10¹ M^-1s^-1 at pH 9.0. Finally, further experiments on ferrate(VI)-bisulfite and ferrate(VI)-2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic)acid systems confirmed that PAA was not an activator for ferrate(VI). Rather, PAA could enhance the oxidation capacity of Fe(IV)/Fe(V), making their oxidation outcompete self-decay. This study, for the first time, reveals the ability of PAA to promote electron transfer efficiency between high-valent metals and organic contaminants and confirms the benefits of co-application of ferrate(VI) and PAA for alkaline wastewater treatment.

Emerging investigator series: contributions of reactive nitrogen species to transformations of organic compounds in water: a critical review

Reactive nitrogen species (RNS) pose a potential risk to drinking water quality because they react with organic compounds to form toxic byproducts. Since the discovery of RNS formation in sunlit surface waters, these reactive intermediates have been detected in numerous sunlit natural waters and engineered water treatment systems. This critical review summarizes what is known regarding RNS, including their formation, contributions to contaminant transformation, and products resulting from RNS reactions. Reaction mechanisms and rate constants have been described for nitrogen dioxide (˙NO₂) reacting with phenolic compounds. However, significant knowledge gaps remain regarding reactions of RNS with other types of organic compounds. Promising methods to quantify RNS concentrations and reaction rates include the use of selective quenchers and probe compounds as well as electron paramagnetic resonance spectroscopy. Additionally, high resolution mass spectrometry methods have enabled the identification of nitr(os)ated byproducts that form via RNS reactions in sunlit surface waters, UV-based treatment systems, treatment systems that employ chemical oxidants such as chlorine and ozone, and certain types of biological treatment processes. Recommendations are provided for future research to increase understanding of RNS reactions and products, and the implications for drinking water toxicity.

Ozonation of organic compounds in water and wastewater: A critical review

Ozonation has been applied in water treatment for more than a century, first for disinfection, later for oxidation of inorganic and organic pollutants. In recent years, ozone has been increasingly applied for enhanced municipal wastewater treatment for ecosystem protection and for potable water reuse. These applications triggered significant research efforts on the abatement efficiency of organic contaminants and the ensuing formation of transformation products. This endeavor was accompanied by developments in analytical and computational chemistry, which allowed to improve the mechanistic understanding of ozone reactions. This critical review assesses the challenges of ozonation of impaired water qualities such as wastewaters and provides an up-to-date compilation of the recent kinetic and mechanistic findings of ozone reactions with dissolved organic matter, various functional groups (olefins, aromatic compounds, heterocyclic compounds, aliphatic nitrogen-containing compounds, sulfur-containing compounds, hydrocarbons, carbanions, β-diketones) and antibiotic resistance genes.

Reactions of amines with ozone and chlorine: Two novel oxidative methods to evaluate the N-DBP formation potential from dissolved organic nitrogen

The composition of oxidant-reactive dissolved organic nitrogen (DON) is poorly characterized, although its ozonation is likely to form a great variety of disinfection by-products containing a nitrogen-oxygen bond (N-DBPs). In this study, two chemical oxidation procedures were developed: continuous ozonation at pH 7.0 and free available chlorine (FAC) titrations at pH 9.2. The formation of two oxidation products (nitrate (NO3-) and chloramines, respectively) was used to quantify and characterize oxidant-reactive nitrogenous moieties in DON. In addition, batch experiments were conducted to study the NO3- yields of 30 selected nitrogenous model compounds upon ozonation. The NO3- yields of 12 primary and secondary amines were highly variable (17-100%, specific ozone dose of 20 molO3/molN), 7 amino acids had high NO3- yields (≥90%), and tertiary amines as well as pyrrole, acetamide and urea had low NO3- yields (≤15%). The mechanisms of NO3- formation were further examined with benzylamine and N-methylbenzylamine as model compounds. Our results show that nitroalkanes are the last intermediate products before the formation of NO3-, both for primary and secondary amines. The presence of an electron-withdrawing group in the vicinity of the N-atom facilitates the formation of NO3- from nitroalkanes. Therefore, the formation of NO3- is attributed to amino acids and activated primary and secondary amines. In contrast, all primary and secondary amines were transformed to chloramines upon chlorination, which was determined by a novel oxidative titration with chlorine. To further support the selectivity of this assay, it was demonstrated by derivatization of amine moieties that chloramine formation could be inhibited. 13-45% of the DON of 4 dissolved organic matter isolates and 2 wastewater effluents formed NO3- and 0-39% formed chloramines, indicating that the potential for N-DBP formation is high (µMN/mgC-level). From differences in the formation of NO3- and chloramines the nature of the precursors can be hypothesized (e.g., activated or non-activated primary and secondary amines, partially oxidized nitrogenous compounds). This study highlights the capacity of two novel methods to characterize the oxidant-reactive DON fraction. Our results suggest that this fraction is significant and could form a variety of potentially toxic N-DBPs.

Nitriles as main products from the oxidation of primary amines by ferrate(VI): Kinetics, mechanisms and toxicological implications for nitrogenous disinfection byproduct control

Ferrate (Fe(VI)), a promising water treatment oxidant, can be used for micropollutant abatement or disinfection byproduct mitigation. However, knowledge gaps remain concerning the interaction between Fe(VI) and dissolved organic matter structures, notably primary amines. This study investigated degradation kinetics and products of several aliphatic primary amines by Fe(VI). Primary amines showed appreciable reactivity toward Fe(VI) (2.7-68 M-1s-1 at pH 7-9), ranking as follows: benzylamine > phenethylamine > phenylpropylamine > methylamine ≈ propylamine. Nitriles were the main oxidation products of the primary amines, with molar yields of 61-103%. Minor products included aldehydes, carboxylic acids, nitroalkanes, nitrite, nitrate, and ammonia. The buffering conditions were important. Compared to phosphate, borate enhanced the reactivity of the amines and shifted the products from nitriles to carbonyls. An evaluation of the effect potency of some cyano-compounds by an in vitro bioassay for oxidative stress response and cytotoxicity suggested that non-halogenated nitriles are unlikely to pose a significant threat because they were only toxic at high concentrations, acted as baseline toxicants and did not cause oxidative stress, unlike halonitroalkanes or halonitriles. The formation of non-halogenated nitriles is preferable to the formation of nitroalkanes arising from the ozonation of primary amines (other than amino acid N-terminals) because, during chlorination, nitriles remain unreactive while nitroalkanes lead to potent halonitroalkanes.

Formation mechanism of chloropicrin from amines and free amino acids during chlorination: A combined computational and experimental study

Chloropicrin as one of the most frequently detected N-DBPs has drawn great attention due to its high toxicity. However, our understanding of its formation mechanism is still very limited. A combined computational and experimental approach was used in this study to reveal chloropicrin formation mechanism during chlorination. Ethylamine, n-propylamine, alanine and tryptophan along with the above two amines and their four derivatives substituted by -OH or/and -NO₂ groups were chosen as computational and experimental model precursors, respectively. The results indicate that primary amines and free amino acids are more likely to share the same chloropicrin formation pathway including N-chlorination, imidization, β-C-alcoholization, N-nitration, α-C-chlorination and dealdehydation processes. Moreover, elimination of hydrochloric acid from N,N-dichloro-amine and electrophilic addition of N-chloroalkylimide with hypochlorous acid were found to be the rate-limiting steps among all the elementary reactions. By skipping over both of the above rate-limiting steps, RCH(OH)CH₂NO₂ and RCH(OH)CH₂NH(OH) compounds were proposed to be potent chloropicrin precursors, and experiments confirmed that 2-nitroethanol and N-methylhydroxylamine have the highest chloropicrin yields in the chlorination among all the precursors reported to date. The findings of this work are helpful for expanding the knowledge of chloropicrin formation mechanisms and predicting the potential chloropicrin precursors.

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.

N-nitrosodimethylamine formation potential (NDMA-FP) of ranitidine remains after chlorination and/or photo-irradiation: Identification of transformation products in combination with NDMA-FP test

N-nitrosodimethylamine (NDMA), a probable carcinogenic disinfection by-product, can be formed with high molar yields following chloramination of ranitidine (RNTD), a histamine H₂-receptor antagonist. Although RNTD and some of its transformation products (TPs) have been studied under chlorination and photo-irradiation, the relationship between RNTD TPs and NDMA formation potential (NDMA-FP) remaining after those processes is still unclear. This study investigated the effects of chlorination and/or photo-irradiation on NDMA-FP derived from RNTD, simulating an urban water environment receiving treated wastewater. After chlorination and/or photo-irradiation of RNTD, ten TPs including five new ones were identified by liquid chromatography-quadrupole time-of-flight mass spectrometry (LC-QTof-MS). In addition, important RNTD TPs responsible for NDMA-FP (e.g., chlorinated and hydroxylated RNTD: TP-364) were also confirmed by the relationship between detected peak area and NDMA-FP. The results showed that NDMA-FP remained due to the presence of RNTD TPs, although RNTD itself was significantly removed by chlorination and/or photo-irradiation. TP-364 was only formed by chlorination of RNTD and could not be removed by photo-irradiation. TP-314 (a stereoisomer of RNTD), -299, and -286, which were mainly formed by photo-irradiation of RNTD but not by photo-irradiation after chlorination, had strong positive correlations with NDMA-FP (R² > 0.90; F-test, P < 0.01).

Degradation mechanisms of simple aliphatic amines under ozonation: a DFT study

Aliphatic amines as common constituents of dissolved organic nitrogen (DON) exhibit high reactivity during ozonation; however, our understanding of their degradation mechanisms is very limited. In this study, methylamine (MA) and ethylamine (EA), as well as their secondary and tertiary amines (DMA, DEA, TMA and TEA) were chosen as aliphatic amine models and their degradation mechanisms during ozonation were investigated by using the DFT method. The oxygen-transfer reaction occurs initially and rapidly in the ozonation of all the above amines with a ΔG^≠ value of 8-10 kcal mol^-1 in great agreement with the experimental rate constant of 10⁴ to 10⁷ M^-1 s^-1. Moreover, N-oxide as the main degradation product for tertiary amines directly forms after oxygen-transfer, while nitroalkanes as main products for secondary and primary amines are yielded after a series of reactions mediated by hydroxylamine and nitrosoalkane with a ΔG^≠ value of 10-13 kcal mol^-1. Regarding the minor N-dealkylated products for all amines, alkylamino alcohol is an important intermediate possibly generated via a radical reaction pathway with a ΔG^≠ value of 21-34 kcal mol^-1. Additionally, comparison of the reactivity of aliphatic amines, hydroxylamines and alkylamino alcohols with ozone was made and elucidated in this study. The results are expected to expand our understanding of the degradation mechanisms for nitrogenous compounds during ozonation.

Evaluation of Histidine Reactivity and Byproduct Formation during Peptide Chlorination

The covalent modifications resulting from chlorine reactions with peptide-bound amino acids contribute to pathogen inactivation and disinfection byproduct (DBP) formation. Previous research suggested that histidine is the third most reactive of the seven chlorine-reactive amino acids, leading to the formation of 2-chlorohistidine, 2-oxohistidine, or low-molecular-weight byproducts such as trihalomethanes. This study demonstrates that histidine is less reactive toward formation of chlorine transformation products (transformation time scale of hours to days) than five of the seven chlorine-reactive amino acids, including tyrosine (transformation time scale of minutes). Chlorine targeted tyrosine in preference to histidine within peptides, indicating that chlorine reactions with tyrosine and other more reactive amino acids could contribute more to the structural modifications to proteins over the short time scales relevant to pathogen inactivation. Over the longer time scales relevant to disinfection byproduct formation in treatment plants or distribution systems, this study identified β-cyanoalanine as the dominant transformation product of chlorine reactions with peptide-bound histidine, with molar yields of ∼50% after 1 day. While a chlorinated histidine intermediate was observed at lower yields (maximum ∼5%), the cumulative concentration of the conventional low-molecular-weight DBPs (e.g., trihalomethanes) was ≤7%. These findings support the need to identify the high-yield initial transformation products of chlorine reactions with important precursor structures to facilitate the identification of unknown DBPs.

Factors affecting removal of NDMA in an ozone-biofiltration process for water reuse

N-nitrosodimethylamine (NDMA) is a carcinogen and a disinfection byproduct that is formed by ozone and combined chlorine. Various factors affecting NDMA formation and removal were examined at pilot-scale for a treatment train consisting of ozone, biologically-active carbon (BAC) filtration, and granular activated carbon (GAC) adsorption applied to two distinct feed waters. High concentrations of ozone and monochloramine were added to the influent, demonstrating that ozone removed monochloramine precursors of NDMA. Further, longer empty bed contact times (EBCTs) of 10 min for BAC and 10 and 20 min for GAC removed NDMA to <10 ng/L for both feed waters. NDMA removal by the BAC process was most favorable >22 °C, presumably due to elevated microbial activity. A monochloramine residual of 3 mg/L-Cl₂ in the BAC influent reduced NDMA removal in the 5 min EBCT BAC from 79% to 36% and in the 10 min EBCT BAC from 88.5% to 73.7%. The absence of ozone in the treatment process significantly reduced NDMA formed post ozone, but decreased NDMA removal in BAC, probably due to lower NDMA concentration in the BAC influent. Finally, adding 5 mg/L of allylthiourea, an inhibitor of ammonia-oxidizing bacteria, indicated that removal mechanisms for ammonia and NDMA are distinct. However, nitrification is still a good indicator for NDMA biodegradation potential, because nitrifying bacteria appear to flourish under similar EBCT, temperature. and monochloramine residual conditions during BAC filtration.

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.

Precursors of typical nitrogenous disinfection byproducts: Characteristics, removal, and toxicity formation potential

The emergence of nitrogenous disinfection byproducts (N-DBPs) in drinking water has become a widespread concern. In this study, dichloroacetonitrile (DCAN), dicholoacetamide (DCAcAm) and trichloronitromethane (TCNM) were chosen as representatives to clarify the characteristics of N-DBP precursors in the raw waters of Taihu Lake, the Yangtze River, and Gaoyou Lake. Removal of DCAN and DCAcAm precursors must focus on nonpolar and positively charged organics, but more attention should be paid to micromolecular, polar and non-positively charged organics as TCNM precursors. Compared to molecular weight (MW) and hydrophilicity fractionation, polarity and electrical classification have higher selectivity to intercept N-DBP precursors. The properties of N-DBP precursors are relatively fixed and traceable in water systems, which could contribute to their targeted removal. Based on investigation of their characteristics, the removal efficiency and preferences of organic precursors under different processes were studied in three drinking water treatment plants (DWTPs). The TCNM precursors produced in preozonation can be effectively removed during coagulation. The cumulative removal efficiency of conventional processes on N-DBP precursors was approximately 20-30%, but O₃/BAC process improved removal by about 40%. The key to improving the removal efficiency of N-DBP precursors by O₃/BAC is that it can significantly remove low-MW, nonpolar, positively charged, hydrophilic and transphilic organics. In combined toxicity trials, both cytotoxicity and genotoxicity showed a synergistic effect when DCAN, DCAcAm, and TCNM coexisted, which means that low-level toxicity enhancement in the actual water merits attention. DCAN precursors dominated in the toxicity formation potential (TFP), followed by TCNM precursors. In addition, the removal rate of total N-DBP precursors may be higher than that of TFP, leading to overly optimistic evaluation of precursor removal in water treatment practice. Therefore, the removal effect on TFP must also be considered.

Impact of chlorination and pre-ozonation on disinfection by-products formation from aqueous suspensions of cyanobacteria: Microcystis aeruginosa, Anabaena aequalis and Oscillatoria tenuis

The influence of the pre-ozonization on the formation of disinfection by-products (DBPs) upon chlorination for fresh waters containing three common cyanobacteria, namely Microcystis aeruginosa, Anabaena aequalis and Oscillatoria tenuis at 10,000 cells/mL is reported. Specifically, the formation carbonaceous-DBPs (C-DBPs) (trihalomethanes (THMs), haloacetic acids (HAAs) and haloketones (HKs)) and nitrogenous-DBPs (N-DBP) (haloacetonitriles (HAN) and trichloronitromethane (TCNM)) has been determined as a function of the pH (6.5 or 8.0 and bromide ion concentration (300 μg/L). The main C-DBPs were THMs and HAAs with negligible formation of HKs accompanied by minor amounts of HANs in the absence of TCNM. Pre-ozonation of the aqueous cyanobacteria suspensions does not allow a control over all the DBPs. In fact, pre-ozonation increases THM formation and generates TCNM, has low influence on HAAs and only decreases the formation of HANs. The overall conclusion of this work is that pre-ozonation of waters containing a relatively low concentration of common fresh water cyanobacteria is not an appropriate process to decrease DBP formation from chlorine. Cyanobacteria removal from raw water before chlorination or ozonation should reduce DBP formation.

Transformation of N-Methylamine Drugs during Wastewater Ozonation: Formation of Nitromethane, an Efficient Precursor to Halonitromethanes

Potable reuse of wastewater is expanding, and ozonation for water reuse is becoming more common, either as a preoxidant before membranes or as part of ozone/biological activated carbon (O₃/BAC) systems. However, previous research has demonstrated that ozone drastically increases the formation potential of genotoxic halonitromethanes (HNMs), including during O₃/BAC. Chloropicrin, the most common HNM, is synthesized by chlorinating nitromethane, suggesting that nitromethane may be the immediate precursor of chloropicrin, although nitromethane is unlikely to occur naturally in wastewater. We hypothesized that wastewater ozonation forms nitromethane, which would be the key intermediate toward HNMs. Ozonation of wastewater effluent was shown to form abundant nitromethane, which explained the majority (in one case, all) of subsequent chloropicrin formation. Next, we investigated a suspected category of nitromethane precursor: stimulant drugs, such as ephedrine and methamphetamine, and certain antidepressants. These drugs all feature N-methylamine functional groups, and certain N-alkylamines have been shown to produce primary nitroalkanes upon ozonation. Ozonation of N-methylamine drugs ubiquitously formed nitromethane, typically at >50% yield. Subsequent chlorination converted nitromethane to chloropicrin. The reaction mechanism was investigated to understand the variation in nitromethane yield between different precursors. These results suggest that nitromethane fate during reuse and nitromethane control should be investigated.

Effects of Pre-Oxidation on Haloacetonitrile and Trichloronitromethane Formation during Subsequent Chlorination of Nitrogenous Organic Compounds

The reaction between organic matter and disinfectants leads to the formation of disinfection byproducts (DBPs) in drinking water. With the improvement of detection technology and in-depth research, more than 1000 kinds of DBPs have been detected in drinking water. Nitrogenous DBPs (N-DBPs) are more genotoxic and cytotoxic than the regulated DBPs. The main methods are enhanced coagulation, pretreatment, and depth technologies which based are on conventional technology. Amino acids (AAs) are widely found in surface waters and play an important role by providing precursors from which toxic nitrogenous disinfection by-products (N-DBPs) are generated in chlorinated drinking water. The formation of N-DBPs, including dichloroacetonitrile, trichloroacetonitrile, and trichloronitromethane (TCNM), was investigated by analyzing chlorinated water using ozone (OZ), permanganate (PM), and ferrate (Fe(VI)) pre-oxidation processes. This paper has considered the control of pre-oxidation over N-DBPs formation of AAs, OZ, PM, and Fe(VI) pre-oxidation reduced the haloacetonitrile formation in the downstream chlorination. PM pre-oxidation decreased the TCNM formation during the subsequent chlorination, while Fe(VI) pre-oxidation had no significant influence on the TCNM formation, and OZ pre-oxidation increased the formation. OZ pre-oxidation formed the lowest degree of bromine substitution during subsequent chlorination of aspartic acid in the presence of bromide. Among the three oxidants, PM pre-oxidation was expected to be the best choice for reducing the estimated genotoxicity and cytotoxicity of the sum of the measured haloacetonitriles (HANs) and TCNM without bromide. Fe(VI) pre-oxidation had the best performance in the presence of bromide.

Formation potential of emerging disinfection by-products during ozonation and chlorination of sewage effluents

This study investigates the formation potential of emerging DBPs (haloacetonitriles, halonitromethanes and halopropanones) during ozonation and ozonation/hydrogen peroxide treatment and subsequent chlorination of sewage effluent under various experimental conditions. Estimation of possible risk due to DBPs by calculation of cytotoxicity and genotoxicity was attempted. The studied DBPs showed different formation behavior during chlorination, with maximum yields within 0.5-48 h. Maximum cytotoxicity and genotoxicity was observed after 4 h of chlorination with dibromoacetonitrile being the major contributor. Ozonation and O₃/H₂O₂ treatment resulted in increase of trichloronitromethane followed by a decline at higher doses, and reduction of haloacetonitriles. High ozone doses reduced cytotoxicity and genotoxicity of treated effluents. The presence of bromide shifted to bromo-DBPs formation and enhanced both cytotoxicity and genotoxicity. Particulate fraction in effluents significantly contributed to the formation of DBPs and consequently to the their toxicity.

DBP alteration from NOM and model compounds after UV/persulfate treatment with post chlorination

The UV/persulfate process is an effective advanced oxidation process (AOP) for the abatement of a variety of micropollutants via producing sulfate radicals (SO₄^•-). However, when this technology is used to reduce target pollutants, the precursors of disinfection byproducts (DBPs), such as natural organic matter (NOM) and organic nitrogen compounds, can be altered. This study systematically investigated the DBP formation from NOM and five model compounds after UV/H₂O₂ and UV/persulfate treatments followed with 24 h chlorination. Compared to chlorination alone, the yields of trichloromethane (TCM) and dichloroacetonitrile (DCAN) from NOM decreased by 50% and 54%, respectively, after UV/persulfate treatment followed with chlorination, whereas those of chloral hydrate (CH), 1,1,1-trichloropropanone (1,1,1-TCP) and trichloronitromethane (TCNM) increased by 217%, 136%, and 153%, respectively. The effect of UV/H₂O₂ treatment on DBP formation shared a similar trend to that of UV/persulfate treatment, but the DBP formation was higher from the former. As the UV/persulfate treatment time prolonged or the persulfate dosage increased, the formation of TCM and DCAN continuously decreased, while that of CH, 1,1,1-TCP and TCNM presented an increasing and then decreasing pattern. SO₄^•- activated benzoic acid (BA) to form phenolic compounds that enhanced the formation of TCM and CH, while it deactivated resorcinol to decrease the formation of TCM. SO₄^•- reacted with aliphatic amines such as methylamine (MA) and dimethylamine (DMA) to form nitro groups, which significantly increased the formation of TCNM in post chlorination, and the rate was determined to be higher than that of HO^•. This study illuminated the diverse impacts of the structures of the precursors on DBP formation after UV/persulfate treatment, and DBP alteration depended on the reactivity between SO₄^•- and specific precursor.

Reactions of aliphatic amines with ozone: Kinetics and mechanisms

Aliphatic amines are common constituents in micropollutants and dissolved organic matter and present in elevated concentrations in wastewater-impacted source waters. Due to high reactivity, reactions of aliphatic amines with ozone are likely to occur during ozonation in water and wastewater treatment. We investigated the kinetics and mechanisms of the reactions of ozone with ethylamine, diethylamine, and triethylamine as model nitrogenous compounds. Species-specific second-order rate constants for the neutral parent amines ranged from 9.3 × 10⁴ to 2.2 × 10⁶ M^-1s^-1 and the apparent second-order rate constants at pH 7 for potential or identified transformation products were 6.8 × 10⁵ M^-1s^-1 for N,N-diethylhydroxylamine, ∼10⁵ M^-1s^-1 for N-ethylhydroxylamine, 1.9 × 10³ M^-1s^-1 for N-ethylethanimine oxide, and 3.4 M^-1s^-1 for nitroethane. Product analyses revealed that all amines were transformed to products containing a nitrogen-oxygen bond (e.g., triethylamine N-oxide and nitroethane) with high yields, i.e., 64-100% with regard to the abated target amines. These findings could be confirmed by measurements of singlet oxygen and hydroxyl radical which are formed during the amine-ozone reactions. Based on the high yields of nitroethane from ethylamine and diethylamine, a significant formation of nitroalkanes can be expected during ozonation of waters containing high levels of dissolved organic nitrogen, as expected in wastewaters or wastewater-impaired source waters. This may pose adverse effects on the aquatic environment and human health.

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.

The formation mechanism of chloropicrin from methylamine during chlorination: a DFT study

Chloropicrin (TCNM) as one of the most frequently detected nitrogenous disinfection byproducts (N-DBPs) has attracted extensive attention due to its high toxicity. Although much research work on TCNM has been done, its formation mechanism during chlorination has not been known clearly yet. In this study, TCNM formation mechanisms from methylamine (MA) during chlorination, including N-chlorination of MA by hypochlorous acid to generate dichloromethylamine (DCMA) first and then oxidation of DCMA to form nitromethane (NM) and chloronitromethane (CNM), and finally TCNM formation from C-chlorination of NM and CNM, were investigated by using the DFT method. The calculated results show that in N-chlorination of MA, 2-3 water molecules involved in the reaction facilitate Cl+ and proton transfer with the activation free energies (ΔG≠) for the first and second chlorination in the range of 4-7 and 14-17 kcal mol-1, respectively, which are in good agreement with the experimental results. Formation of NM and CNM proceeds through a series of elimination, addition, and oxidation reactions with ΔG≠ of the rate-limiting steps being around 34-37 kcal mol-1, and the subsequent C-chlorination of methyl in NM and CNM by hypochlorous acid is a rapid process with ΔG≠ below 7 kcal mol-1. This infers that the TCNM formation mechanism from DCMA is more likely to undergo first N-oxidation and then C-chlorination. These results can explain the experimental findings that the molar yield of TCNM from MA during chlorination is low (<0.1%) whereas that from NM is rather high (∼45%). This work will be helpful to elucidate formation mechanisms of all the halonitromethanes during chlorination.

Effects of Sunlight on the Trichloronitromethane Formation Potential of Wastewater Effluents: Dependence on Nitrite Concentration

This study examined the effects of sunlight irradiation on the trichloronitromethane formation potential (TCNM-FP) of wastewater effluents and the roles of nitrite and nitrate in this process. Using disinfected secondary effluents from four treatment plants, we observed that sunlight irradiation (320 W/m²) for 8 h attenuated the TCNM-FP by 17-47% for 9 of 14 samples but increased the TCNM-FP for two of the other samples. A longer irradiation time (≤36 h) further reduced the TCNM-FP in a non-nitrified effluent with low nitrite and nitrate concentrations but increased the TCNM-FP in two nitrified effluents by 2-3-fold. When nitrite (0.1-2 mg N/L) was spiked into effluent samples, an increase in the TCNM-FP after irradiation was observed. The higher the nitrite concentration, the greater the increase in the TCNM-FP. In the presence of ∼1 mg N/L of nitrite, sunlight irradiation for 8 h increased the TCNM-FP of four wastewater samples by 0.3-3.6 μg/mg C. In contrast, the spike of nitrate up to 20 mg N/L had no effect. The nitrite-sunlight effect was also observed for four model precursors (humic acid, tryptophan, tyrosine, and phenol). Humic acid and tryptophan featured larger increases in the TCNM-FP compared to those of tyrosine and phenol after sunlight irradiation.

Ozone and chlorine reactions with dissolved organic matter - Assessment of oxidant-reactive moieties by optical measurements and the electron donating capacities

Oxidation processes are impacted by the type, concentration and reactivity of the dissolved organic matter (DOM). In this study, the reactions between various types of DOM (Suwannee River fulvic acid (SRFA), Nordic Reservoir NOM (NNOM) and Pony Lake fulvic acid (PLFA)) and two oxidants (ozone and chlorine) were studied in the pH range 2-9 by using a combination of optical measurements and electron donating capacities. The relationships between residual electron donating capacity (EDC) and residual absorbance showed a strong pH dependence for the ozone-DOM reactions with phenolic functional groups being the main reacting moieties. Relative EDC and absorbance abatements (UV₂₅₄ or UV₂₈₀) were similar at pH 2. At pH 7 or 9, the relative abatement of EDC was more pronounced than for absorbance, which could be explained by the formation of UV-absorbing products such as benzoquinone from the transformation of phenolic moieties. An increase in fluorescence abatement with increasing pH was also observed during ozonation. The increase in fluorescence quantum yields could not be attributed to formation of benzoquinone, but related to a faster abatement of phenolic moieties relative to fluorophores with low ozone reactivity. The overall ^•OH yields as a result of DOM-induced ozone consumption increased significantly with increasing pH, which could be related to the higher reactivity of phenolic moieties at higher pH. The ^•OH yields for SRFA and PLFA were proportional to the phenolic contents, whereas for NNOM, the ^•OH yield was about 30% higher. During chlorination of DOM at pH 7 an efficient relative EDC abatement was observed whereas the relative absorbance abatement was much less pronounced. This is due to the formation of chlorophenolic moieties, which exert a significant absorbance, and partly lose their electron donating capacity. Pre-ozonation of SRFA leads to a decrease of chloroform and haloacetic acid formation, however, only after a threshold of > ∼50% abatement of the EDC and under conditions which are not precursor limited. The decrease in chloroform and haloacetic acid formation after the threshold EDC abatement was proportional to the relative residual EDC.

Removal of precursors of typical nitrogenous disinfection byproducts in ozonation integrated with biological activated carbon (O3/BAC)

The O₃/BAC process has been widely used in drinking water treatment to improve the removal of dissolved organic matters (DOMs), including the precursors of nitrogenous disinfection byproducts (N-DBPs). In this study, the removal of N-DBP precursors by biological activated carbon (BAC) filters with different usage time of granular activated carbon (GAC) was investigated. Results showed that the BAC filter with 6 years of usage time of GAC (old BAC filter) had a poor performance in the removal of precursors of N-DBPs such as dichloroacetonitrile (DCAN; an average of only 4.7%), dichloroacetamide (DCAcAm), and trichloronitromethane (TCNM) when compared with the BAC filter with 1 year of usage time of GAC (new BAC filter). Particularly, the organic fraction >10 kDa and the percentage of autochthonous substances were increased in the effluent of the old BAC filter. The red shift of the fluorescence peaks was evident in the excitation-emission matrix spectrum of the effluent from the old BAC filter. The abiotic adsorption of precursors by the old BAC filter was less. In addition, less amino acids and polysaccharides were removed, but more amino sugars and proteins were produced because of microbial metabolism. The metabolism strength of the attached biofilm decreased with increased operation time of the BAC filter. The relative abundance of Sphingomonas significantly decreased in the biofilm of the old BAC filter. The diversity of microbial community in the old BAC filter was higher, but the equitability was lower than those of the new BAC filter. The less removal of N-DBP precursors by the old BAC filter was attributed to the changes in abiotic adsorption capacity and microbial metabolism properties, in which soluble microbial products played an important role.

Impact of ClO2 pre-oxidation on the formation of CX3R-type DBPs from tyrosine-based amino acid precursors during chlorination and chloramination

ClO₂ is frequently used as a pre-oxidant in water treatment plants. However, the effects of ClO₂ pre-oxidation on disinfection by-product (DBP) formation, especially the highly toxic nitrogenous DBPs, during subsequent chlor (am)ination have not been studied thoroughly. There is also limited information about DBP formation from combined amino acids (AAs), which are more abundant than free AAs in source waters. Many typical DBPs (including representative N-DBPs) have a similar structure of "CX₃R" (X = H, Cl, Br or I). In the study, tyrosine and forms representing its reactivity in combined AAs (tyrosine tert-butyl ester and Boc-tyrosine) were selected as model precursors. The formation of various regulated and unregulated CX₃R-type DBPs from ClO₂ pre-oxidation and subsequent chlor (am)ination were studied at a wide-range of ClO₂ and chlor (am)ine doses (ClO₂/precursors and chlor (am)ine/precursors are at the range of 0-2.5 and 1-20 [Mol/Mol], respectively). Chloroform and chloral hydrate (CH) yields increased with chlorine dose, while haloacetonitrile and haloacetamide maximized at median chlorine dose (Cl₂/Precursors = 10). All DBP yields increased with chloramine dose. ClO₂ pre-oxidation increased chloroform, haloacetonitrile, trichloronitromethane and CH yields during chlorination, but ClO₂ increased chloroform, CH, trichloroacetamide while decreased dichloroacetonitrile and trichloronitromethane yields during chloramination. The overall toxicity of the formed DBPs was evaluated by cytotoxicity index (CTI). ClO₂ pre-oxidation increased CTI from all precursors during post-chlorination while reduced it during post-chloramination. Results imply that ClO₂ is probably more suitable for use in combination with chloramination disinfection, rather than chlorination, in the integrated control of CX₃R-type DBPs from source waters abundant in AAs.

Drinking Water Disinfection Byproducts (DBPs) and Human Health Effects: Multidisciplinary Challenges and Opportunities

While drinking water disinfection has effectively prevented waterborne diseases, an unintended consequence is the generation of disinfection byproducts (DBPs). Epidemiological studies have consistently observed an association between consumption of chlorinated drinking water with an increased risk of bladder cancer. Out of the >600 DBPs identified, regulations focus on a few classes, such as trihalomethanes (THMs), whose concentrations were hypothesized to correlate with the DBPs driving the toxicity of disinfected waters. However, the DBPs responsible for the bladder cancer association remain unclear. Utilities are switching away from a reliance on chlorination of pristine drinking water supplies to the application of new disinfectant combinations to waters impaired by wastewater effluents and algal blooms. In light of these changes in disinfection practice, this article discusses new approaches being taken by analytical chemists, engineers, toxicologists and epidemiologists to characterize the DBP classes driving disinfected water toxicity, and suggests that DBP exposure should be measured using other DBP classes in addition to THMs.

Degradation of nitro-based pharmaceuticals by UV photolysis: Kinetics and simultaneous reduction on halonitromethanes formation potential

This study investigated the degradation kinetics and halonitromethanes formation potential (HNMsFP) of two nitro-based pharmaceuticals (i.e., ranitidine (RNTD) and nizatidine (NZTD)) during ultraviolet (UV) photolysis. It was found that the degradation kinetics of RNTD and NZTD exhibited pH-dependent trends, in accordance with their deprotonation equilibria. The neutral species of RNTD and NZTD were more photo-reactive than their corresponding deprotonated species, with their specific fluence-based first-order rate constants varying in the range of 5.64-31.90 m² E^-1. Both the RNTD and NZTD were prone precursors of HNMs (with molar yields of 5.6± 0.3% and 4.7± 0.4%, respectively at pH 7.0). Acidic and neutral circumstances facilitated the HNMs formation. The UV photolysis of RNTD and NZTD could reduce their HNMsFP simultaneously. Positive linear relationships between residual RNTD or NZTD concentration and HNMsFP were observed and the denitration during the UV photolysis accounted for the HNMsFP reduction. With the mandatory UV disinfection fluences in China (i.e. 20-80 mJ cm^-2), the effective abatement of RNTD and NZTD and their HNMsFP could not be fully achieved, highlighting the necessity of increasing UV fluence or developing UV-based advanced oxidation process in future.

Predicting reactivity of model DOM compounds towards chlorine with mediated electrochemical oxidation

Chlorine demand of a water sample depends on the characteristics of dissolved organic matter (DOM). It is an important parameter for water utilities used to assess oxidant and/or disinfectant consumption of source waters during treatment and distribution. In this study, model compounds namely resorcinol, tannic acid, vanillin, cysteine, tyrosine, and tryptophan were used to represent the reactive moieties of complex DOM mixtures. The reactivity of these compounds was evaluated in terms of Cl₂ demand and electron donating capacity (EDC). The EDC was determined by mediated electrochemical oxidation (MEO) which involves the use of 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) as an electron shuttle. The Cl₂ demand of readily oxidizable compounds (resorcinol, tannic acid, vanillin, and cysteine) was found to correlate well with EDC (R² = 0.98). The EDC values (mol e^-/mol C) of the model compounds are as follows: 1.18 (cysteine) > 0.77 (resorcinol) > 0.59 (vanillin) > 0.52 (tannic acid) > 0.36 (tryptophan) > 0.19 (tyrosine). To determine the effect of pre-oxidation on EDC, ozone was added (0.1 mol O₃/mol C) into each model compound solution. Ozonation caused a general decrease in EDC (10-40%), chlorine demand (10-30%), and UV absorbance (10-40%), except for tyrosine which showed both increased UV₂₇₅ and EDC. Before and after ozonation, 24 h disinfection byproduct (DBP) formation potential tests (Cl₂ residual = 1.5 mg/L) were conducted to evaluate the use of EDC for DBP formation prediction. The results indicate that there was no significant correlation between the EDC of the model compounds and the formation potentials of adsorbable organic chlorine, trichloromethane, and trichloroacetic acid. This suggests that while EDC correlates with Cl₂ demand, chlorine consumption may not directly translate to DBP formation because oxidation reactions may dominate over substitution reactions. Overall, this study provides useful insights on the reactions of ABTS⁺ and HOCl with model DOM compounds, and highlights the potential application of MEO for rapid determination of Cl₂ demand of a water sample.

Combined UV treatment and ozonation for the removal of by-product precursors in swimming pool water

Both UV treatment and ozonation are used to reduce different types of disinfection by-products (DBPs) in swimming pools. UV treatment is the most common approach, as it is particularly efficient at removing combined chlorine. However, the UV treatment of pool water increases chlorine reactivity and the formation of chloro-organic DBPs such as trihalomethanes. Based on the similar selective reactivity of ozone and chlorine, we hypothesised that the created reactivity to chlorine, as a result of the UV treatment of dissolved organic matter in swimming pool water, might also be expressed as increased reactivity to ozone. Moreover, ozonation might saturate the chlorine reactivity created by UV treatment and mitigate increased formation of a range of volatile DBPs. We found that UV treatment makes pool water highly reactive to ozone. The subsequent reactivity to chlorine decreases with increasing ozone dosage prior to contact with chlorine. Furthermore, ozone had a half-life of 5 min in non-UV treated pool water whereas complete consumption of ozone was obtained in less than 2 min in UV treated pool water. The ozonation of UV-treated pool water induced the formation of some DBPs that are not commonly reported in this medium, in particular trichloronitromethane, which is noteworthy for its genotoxicity, though this issue was removed by UV treatment when repeated combined UV/ozone treatment interchanging with chlorination was conducted over a 24-h period. The discovered reaction could form the basis for a new treatment method for swimming pools.

Effect of Ozonation and Biological Activated Carbon Treatment of Wastewater Effluents on Formation of N-nitrosamines and Halogenated Disinfection Byproducts

Ozonation followed by biological activated carbon (O₃/BAC) is being considered as a key component of reverse osmosis-free advanced treatment trains for potable wastewater reuse. Using a laboratory-scale O₃/BAC system treating two nitrified wastewater effluents, this study characterized the effect of different ozone dosages (0-1.0 mg O₃/mg dissolved organic carbon) and BAC empty bed contact times (EBCT; 15-60 min) on the formation after chlorination or chloramination of 35 regulated and unregulated halogenated disinfection byproducts (DBPs), 8 N-nitrosamines, and bromate. DBP concentrations were remarkably similar between the two wastewaters across O₃/BAC conditions. Ozonation increased bromate, TCNM, and N-nitrosodimethylamine, but ozonation was less significant for other DBPs. DBP formation generally decreased significantly with BAC treatment at 15 min EBCT, but little further reduction was observed at higher EBCT where low dissolved oxygen concentrations may have limited biological activity. The O₃/BAC-treated wastewaters met regulatory levels for trihalomethanes (THMs), haloacetic acids (HAAs), and bromate, although N-nitrosodimethylamine exceeded the California Notification Level in one case. Regulated THMs and HAAs dominated by mass. When DBP concentrations were weighted by measures of their toxic potencies, unregulated haloacetonitriles, haloacetaldehydes, and haloacetamides dominated. Assuming toxicity is additive, the calculated DBP-associated toxicity of the O₃/BAC-treated chloraminated effluents were comparable or slightly higher than those calculated in a recent evaluation of Full Advanced Treatment trains incorporating reverse osmosis.

Biodegradability of DBP precursors after drinking water ozonation

Ozonation is known to generate biodegradable organic matter, which is typically reduced by biological filtration to avoid bacterial regrowth in distribution systems. Post-chlorination generates halogenated disinfection byproducts (DBPs) but little is known about the biodegradability of their precursors. This study determined the effect of ozonation and biofiltration conditions, specifically ozone exposure and empty bed contact time (EBCT), on the control of DBP formation potentials in drinking water. Ozone exposure was varied through addition of H₂O₂ during ozonation at 1 mgO₃/mgDOC followed by biological filtration using either activated carbon (BAC) or anthracite. Ozonation led to a 10% decrease in dissolved organic carbon (DOC), without further improvement from H₂O₂ addition. Raising H₂O₂ concentrations from 0 to 2 mmol/mmolO₃ resulted in increased DBP formation potentials during post-chlorination of the ozonated water (target Cl₂ residual after 24 h = 1-2 mg/L) as follows: 4 trihalomethanes (THM4, 37%), 8 haloacetic acids (HAA8, 44%), chloral hydrate (CH, 107%), 2 haloketones (HK2, 97%), 4 haloacetonitriles (HAN4, 33%), trichloroacetamide (TCAM, 43%), and adsorbable organic halogen (AOX, 27%), but a decrease in the concentrations of 2 trihalonitromethanes (THNM2, 43%). Coupling ozonation with biofiltration prior to chlorination effectively lowered the formation potentials of all DBPs including CH, HK2, and THNM2, all of which increased after ozonation. The dynamics of DBP formation potentials during BAC filtration at different EBCTs followed first-order reaction kinetics. Minimum steady-state concentrations were attained at an EBCT of about 10-20 min, depending on the DBP species. The rate of reduction in DBP formation potentials varied among individual species before reaching their minimum concentrations. CH, HK2, and THNM2 had the highest rate constants of between 0.5 and 0.6 min^-1 followed by HAN4 (0.4 min^-1), THM4 (0.3 min^-1), HAA8 (0.2 min^-1), and AOX (0.1 min^-1). At an EBCT of 15 min, the reduction in formation potential for most DBPs was less than 50% but was higher than 70% for CH, HK2, and THNM2. The formation of bromine-containing DBPs increased with increasing EBCT, most likely due to an increase in Br^-/DOC ratio. Overall, this study demonstrated that the combination of ozonation and biofiltration is an effective approach to mitigate DBP formation during drinking water treatment.

Impacts of ferrate oxidation on natural organic matter and disinfection byproduct precursors

This study investigated the effectiveness of ferrate (Fe(VI)) oxidation in combination with ferric chloride coagulation on the removal of natural organic matter (NOM) and disinfection byproduct (DBP) precursors. Twelve natural waters were collected and four treatment scenarios were tested at bench-scale. Results showed that intermediate-ferrate treatment (i.e., coagulation and particle removal followed by ferrate oxidation) was most effective followed by pre-ferrate treatment (i.e., ferrate oxidation followed by coagulation and particle removal (conventional treatment)) or conventional treatment alone (i.e., no oxidation), and the least effective was ferrate oxidation alone (i.e., no coagulation). At typical doses, direct ferrate oxidation of raw water decreased DBP formation potentials (DBPFPs) by about 30% for trihalomethanes (THMs), 40% for trihaloacetic acids (THAAs), 10% for dihaloacetic acids (DHAAs), 30% for dihaloacetonitriles (DHANs), and 5% for haloketones (HKs). The formation potential of chloropicrin (CP) consistently increased after direct ferrate oxidation. Pre-ferrate followed by conventional treatment was similar to conventional treatment alone for NOM and DBP precursor removal. Ferrate pre-oxidation had positive effects on subsequent coagulation/particle removal for THM and THAA precursor removal and may allow the use of lower coagulant doses due to the Fe(III) introduced by ferrate decomposition. On the other hand, intermediate-ferrate resulted in substantially improved removal of NOM and DBP precursors, which can be attributed to initial removal by coagulation and particle removal, leaving precursors that are particularly susceptible to oxidation by ferrate. The Fe(III) resulting from ferrate decay during intermediate-ferrate process was primarily present as particulate iron and could be effectively removed by filtration.