ClO2 pre-oxidation changes dissolved organic matter at the molecular level and reduces chloro-organic byproducts and toxicity of water treated by the UV/chlorine process

The Role of Natural Organic Matter in the Degradation of Phenolic Pollutants by Sulfate Radical Oxidation: Radical Scavenging vs Reduction

Dissolved natural organic matter (NOM) significantly influences the performance of water treatment processes. It is generally recognized that NOM acts as a radical scavenger, thus inhibiting the degradation of organic pollutants in advanced oxidation processes (AOPs). This study examined the impacts of 8 different NOM isolates on the degradation of 4-chlorophenol (CP), a representative phenolic pollutant, in sulfate radical (SO4•-)-based AOPs. We developed an improved probe method to measure the steady-state concentration of SO4•- ([SO4•-]ss) in both the absence and presence of NOM. Results show that adding 1.00 mgC L-1 NOM resulted in only a 1.3-3.4% decrease in [SO4•-]ss. However, the apparent rate constants of CP degradation decreased by 76-88%. This discrepancy indicates that radical scavenging cannot be the primary mechanism for observed inhibition. We proposed NOM primarily acts as a reducing agent, reacting with the phenoxy radical intermediates generated from the single-electron oxidation of CP by SO4•-. Based on this hypothesis, we developed and validated a kinetic model using experimental data. The reductive capacity of NOM, as determined by the kinetic model, correlates positively with its electron-donating capacity. These findings enhance the understanding of NOM's role in SO4•--based AOPs and provide a foundation for developing strategies to mitigate its adverse effects.

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

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

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

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

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

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

Molecular insight of dissolved organic matter and chlorinated disinfection by-products in reclaimed water during chlorination with permanganate preoxidation

Permanganate is a common preoxidant applied in water treatment to remove organic pollutants and to reduce the formation of disinfection by-products. However, the effect of permanganate preoxidation on the transformation of dissolved effluent organic matter (dEfOM) and on the formation of unknown chlorinated disinfection by-products (Cl-DBPs) during chlorination remains unknown at molecular level. In this work, the molecular changes of dEfOM during permanganate preoxidation and subsequent chlorination were characterized using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Permanganate preoxidation was found to decrease the DBE (double bond equivalent) and AImod (modified aromaticity index) of the dEfOM. The identity and fate of over 400 unknown Cl-DBPs during KMnO4-chlorine treatment were investigated. Most Cl-DBPs and the precursors were found to be highly unsaturated aliphatic and phenolic compounds. The Cl-DBPs precursors with lower H/C and lower O/C were preferentially removed by permanganate preoxidation. Additionally, permanganate preoxidation decreased the number of unknown Cl-DBPs by 30% and intensity of unknown Cl-DBPs by 25%. One-chlorine-containing DBPs were the major Cl-DBPs and had more CH2 groups and higher DBEw than Cl-DBPs containing two and three chlorine atoms. 60% of the Cl-DBPs formation was attributed to substitution reactions (i.e., +Cl-H, +2Cl-2H, +3Cl-3H, +ClO-H, +Cl2O3-2H). This work provides detailed molecular level information on the efficacy of permanganate preoxidation on the control of overall Cl-DBPs formation during chlorination.

Bisphenol A degradation by chlorine dioxide (ClO2) and S(IV)/ClO2 process: Mechanism, degradation pathways and toxicity assessment

Recently, it has been reported that chlorine dioxide (ClO2) and (bi)sulfite/ClO2 showed excellent performance in micropollutant removal from water; however, the degradation mechanisms and application boundaries of the two system have not been identified. In this study, bisphenol A (BPA) was chosen as the target contaminant to give multiple comparisons of ClO2 and S(IV)/ClO2 process regarding the degradation performance of contaminant, generation of reactive species, transformation of products and toxicity variation. Both ClO2 and S(IV)/ClO2 can degrade BPA within 3 min. The BPA degradation mechanism was mainly based on direct oxidation in ClO2 process while it was attributed to radicals (especially SO4·-) generation in S(IV)/ClO2 process. Meanwhile, the effect of pH and coexisting substances (Cl-, Br-, HCO3- and HA) were evaluated. It was found that ClO2 preferred the neutral and alkaline condition and S(IV)/ClO2 preferred the acidic condition for BPA degradation. An unexpected speed-up of BPA degradation was observed in ClO2 process in the presence of Br-, HCO3- and HA. In addition, the intermediate products in BPA degradation were identified. Three exclusive products were found in ClO2 process, in which p-benzoquinone was considered to be the reason of the acute toxicity increase in ClO2 process.

Efficient purification of a nitrate and chlorate mixture in water via photoredox activated intermediate coupling-decoupling pathway

Nitrate (NO3-) is a widespread contaminant that threatens human health and ecological safety. Meanwhile, the disinfection byproducts chlorate (ClO3-) is generated inevitably in conventional wastewater treatment. Therefore, the contaminants mixture of NO3- and ClO3- are universal in common emission units. Photocatalysis technology is a feasible approach for the synergistic abatement of contaminant mixture, where matching suitable oxidation reactions is a potential strategy to improve the photocatalytic reduction reactions. Herein, formate (HCOOH) oxidation is introduced to facilitate the photocatalytic reduction of the NO3- and ClO3- mixture. As a result, high purification efficiency of NO3- and ClO3- mixture are achieved, evidenced by 84.6% e--dependent removal of the mixture at a reaction time of 30 min, with 94.5% N2 selectivity and 100% Cl- selectivity, respectively. Specifically, by the close combination of in-situ characterizations and theoretical calculations, the detailed reaction mechanism is revealed, in which the intermediate coupling-decoupling route from NO3- reduction and HCOOH oxidation is established by the chlorate-induced photoredox activation, leading to the significantly enhanced efficiency for the wastewater mixture purification. The practical application of this pathway is established for simulated wastewater to show its wide applicability. This work provides new insights into photoredox catalysis technology for its environmental application.

Toxicological aspect of water treated by chlorine-based advanced oxidation processes: A review

As well established in the literature, residual toxicity is an important parameter for evaluating the sanitary and environmental safety of water treatment processes, and this parameter becomes even more crucial when chlorine-based processes are applied for water treatment. Eliminating initial toxicity or preventing its increase after water treatment remains a huge challenge mainly due to the formation of highly toxic disinfection by-products (DBPs) that stem from the degradation of organic contaminants or the interaction of the chlorine-based oxidants with different matrix components. In this review, we present a comprehensive discussion regarding the toxicological aspects of water treated using chlorine-based advanced oxidation processes (AOPs) and the recent findings related to the factors influencing toxicity, and provide directions for future research in the area. The review begins by shedding light on the advances made in the application of free chlorine AOPs and the findings from studies conducted using electrochemical technologies based on free chlorine generation. We then delve into the insights and contributions brought to the fore regarding the application of NH₂Cl- and ClO₂-based treatment processes. Finally, we broaden our discussion by evaluating the toxicological assays and predictive models employed in the study of residual toxicity and provide an overview of the findings reported to date on this subject matter, while giving useful insights and directions for future research on the topic.

Polyethylene pipes exposed to chlorine dioxide in drinking water supply system: A critical review of degradation mechanisms and accelerated aging methods

Polyethylene (PE) pipes have been widely used in drinking water distribution systems across the world. In many cases, chlorine dioxide (ClO₂) is used to maintain a residual disinfectant concentration in potable water. Practical experiences have shown that the lifetime of PE pipes is significantly reduced due to exposure to drinking water with ClO₂. Recently, many companies have proposed new PE pipes with a modified formulation, which are more resistant to chlorine dioxide. However, a standardized test method for evaluating the long-term performances of PE pipes is still missing. This literature review was performed to provide a description of chlorine dioxide uses and degradation mechanisms of polyethylene pipes in real water distribution systems. Current accelerated aging methods to evaluate long-term performances of PE pipes exposed to ClO₂ are described and discussed along with the common technics used to characterize the specimens. Accelerate aging methods can be distinguished in immersion aging tests and pressurized pipe loop tests. Wide ranges of operational conditions (chlorine dioxide concentration, water pressure, water temperature, etc.) are applied, resulting in a great variety of results. It was concluded that pressurized looping tests applying semi-realistic operational conditions could better replicate the aging mechanisms occurring in service. Despite this, the acceleration and the evaluation of the long-term performance are still difficult to determine precisely. Further experimentation is needed to correlate chemical-mechanical characterization parameters of PE pipes with their lifetime in service.

Formation of emerging disinfection byproducts from agricultural biomass-derived DOM: Overlooked health risk source

Carbon-derived dissolved organic matter (CDOM) are inevitably released to surface water during returning agricultural biomass carbon to farmland, which are potential precursors of disinfection byproducts (DBPs). In this study, CDOM was extracted from aerobic incineration ("OX") and anoxic pyrolysis ("PY") of three kinds of straw (wheat, corn, and rice), and the emerging DBPs from them were deciphered. The CDOM with molecular weight < 1 kDa in the OX and PY groups accounted for 53-87%, and it was higher in the PY group. A total 1343-2107 of CHO and 641-1761 of CHNO formulas were detected in the CDOM derived from the OX group, among which 74%-83% contained aromatic structures rich in oxygen containing groups. 1919-3289 of CHO and 785-1954 of CHNO formulas were observed in the PY group, and 77%-86% of them were lignins/CRAM-like compounds. Surprisingly, 765-2158 and 895-1648 of emerging DBPs were identified in the OX and PY groups, and the proportions of N-DBPs were 20.3-54.8% and 2.8-4.8%, respectively. Based on HOCl addition and Cl substitution mechanisms, the H/C ratios of the DBP precursors in the OX and PY groups were in the range of 0.2-1.5 and 0.6-2.0, respectively. The DBPs derived from the OX group exhibited higher cytotoxicity and genotoxicity due to the higher aromaticity and more N-DBPs. Thus, returning agricultural biomass carbon, particularly that produced by direct combustion, to farmland brought potential threat to drinking water safety.

Dominant Dissolved Oxygen-Independent Pathway to Form Hydroxyl Radicals and the Generation of Reactive Chlorine and Nitrogen Species in Breakpoint Chlorination

Due to the complexities of the interactions between ammonia, chlor(am)ine, and intermediate species such as ONOOH, the radical formation in breakpoint chlorination and the consequential removal of micropollutants remain largely unexplored. In this study, the dominant generation pathway of HO^•, as a primary radical in breakpoint chlorination, was examined, and the generations of HO^•, reactive chlorine species (RCS), and reactive nitrogen species (RNS) were quantitatively evaluated. A dissolved oxygen (DO)-independent pathway was verified by ¹⁸O labeling and contributed over 90% to HO^• generation. The commonly believed pathway, the decomposition of ONOOH involving DO, contributed only 7% to HO^• formation in breakpoint chlorination. The chlorine to nitrogen (Cl/N) ratio and pH greatly affected the generations and speciations of the reactive species. An optimum Cl/N mass ratio for HO^•, Cl₂^•-, and RNS generations occurred at the breakpoint (i.e., Cl/N mass ratio = 9), whereas excessive free chlorine shifted the radical speciation toward ClO^• at Cl/N mass ratios above the breakpoint. Basic conditions inhibited the generations of HO^• and RNS but significantly promoted that of ClO^•. These findings improved the fundamental understanding of the radical chemistry of breakpoint chlorination, which can be extended to estimate the degradations of micropollutants of known rate constants toward the reactive species with influences from the Cl/N ratio and pH in real-world applications.

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

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

Molecular transformation of algal organic matter during sequential ozonation-chlorination: Role of pre-ozonation and properties of chlorinated disinfection byproducts

Formation of unknown chlorinated disinfection byproducts (Cl-DBPs) during chlorination gradually raised great concern, and pre-oxidation was considered as an efficient method to minimize Cl-DBP formation. In this study, pre-ozonation of algal organic matter was investigated, to explore its impacts on Cl-DBP formation and acute toxicity during subsequent chlorination. With fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis, the conversion of algal organic matter in chlorination with/without pre-ozonation was tracked. The results show that pre-ozonation reduced the formation of trichloromethane (TCM), yet the species and intensity of unknown Cl-DBPs were significantly increased in subsequent chlorination. Meanwhile, the solution acute toxicity was higher in chlorination with pre-ozonation than in chlorination only. Besides, molecular properties of these unknown Cl-DBPs were further explored and featured. One-chlorine-containing DBPs were unsaturated high molecular-weight compounds with more CH₂ structures, while two or three-chlorine-containing DBPs were mainly oxidized or saturated compounds. Of note, large amounts of one-chlorine-containing DBPs related to polycyclic aromatics and polyphenols compositions were generated, which may contribute to the high potential toxicity. Overall, the findings of this study could provide new insights into the impacts of pre-ozonation on the formation of unknown Cl-DBPs and potential toxicity during chlorination for actual application.

Chlorine dioxide-based oxidation processes for water purification：A review

Chlorine dioxide (ClO₂) has emerged as a broad-spectrum, safe, and effective disinfectant due to its high oxidation efficiency and reduced formation of organochlorinated by-products during application. This article provides an updated overview of ClO₂-based oxidation processes used in water treatment. A systematic review of scientific information and experimental data on ClO₂-based water purification procedures is presented. Concerning ClO₂-based oxidation derivative problems, the pros and cons of ClO₂-based combined processes are assessed and disinfection by-product (DBP) control approaches are proposed. The kinetic and mechanistic data on ClO₂ reactivity towards micropollutants are discussed. ClO₂ selectively reacts with electron-rich moieties (anilines, phenols, olefins, and amines) and eliminates certain inorganic ions and microorganisms with high efficiency. The formation of chlorite and chlorate during the oxidation process is a crucial concern when utilizing ClO₂. Future applications include the combination of ClO₂ with ferrous ions, activated carbon, ozone, UV, visible light, or persulfate processes. The combined process can reduce by-product generation while still ensuring ClO₂ sterilization and disinfection. Overall, this research could provide useful information and new insights into the application of ClO₂-based technologies.