Chemical scavengers are frequently used to quantify the contribution of target radicals to contaminant removal in natural and engineered waters. While favored for their ease of use and versatility across systems, improper selection can lead to significant kinetic and mechanistic misinterpretations. This study presents a critical evaluation of chemical scavengers in radical-induced reactions across various environmental scenarios. Specifically, we demonstrate that in systems containing both target and coexisting radicals, commonly used scavengers can react with both species, complicating the measurement of reaction kinetics and leading to misinterpretation of target radical contributions. In addition, we discuss the challenges associated with applying scavengers in heterogeneous systems, where the distribution of scavengers and target compounds across interfaces significantly impacts the evaluation of radical contributions. Further, our insights from non-steady-state systems into radicals' dynamic behavior and transient phenomena are often overlooked in other steady-state conditions. We address interactions between scavengers and triplet excited-state compounds in photochemical systems, emphasizing the importance of selecting appropriate scavengers to ensure accurate kinetic profiling and radical quantification. These findings hold significant implications for advancing scavenger research across a broad range of chemical research and practical applications.

The formation of toxicologically relevant nitro compounds has been observed during ozonation of nitrite-containing secondary wastewater effluents, but their formation mechanism remains unknown. To identify key nitrating species, three reaction systems were investigated: ozonation of nitrite (O3/NO2-), peroxynitrite (ONOOH/ONOO-), and hydroxyl radical oxidation of nitrite with γ-radiolysis (γ/NO2-). Nitrite ozonation (O3/NO2-) yielded significant amounts of the nitrating agent nitrogen dioxide •NO2 (9.4% at pH 7 to 22% at pH 12) besides the main product nitrate. No peroxynitrite formation was detected during ozonation of nitrite-containing waters, suggesting that •NO2 is the key nitrating species. A comparison of nitro compound formation from 20 aromatic compounds in the three reaction systems showed a consistent formation of nitro compounds with minor differences. Furthermore, the pH-dependence patterns of nitration for two micropollutants (diuron and carbendazim) revealed similarities between the O3/NO2- and γ-radiolysis (γ/NO2-) systems, unlike peroxynitrite. These trends, combined with the lack of detection of peroxynitrite during ozonation, a fast reaction of peroxynitrite with ozone (kpH=9 = (4.96 ± 0.40) × 106 M-1s-1), suggest that a significant contribution of peroxynitrite is highly unlikely. Overall, nitration during ozonation involves nitrite oxidation to •NO2, mostly by hydroxyl radical and to a minor extent by ozone.

The escalating release of emerging organic contaminants into aquatic systems necessitates rigorous investigation of their reactivity with chlorine, a ubiquitous disinfectant in water treatment. This study establishes the most comprehensive dataset to date on chlorine-organic reaction kinetics, integrating novel high-throughput microplate assays with literature data to compile 354 compounds. Advanced machine learning models-a regression-based extreme gradient boosting and a classification-based random forest, both utilizing modified molecular fingerprints-achieved superior predictive accuracy (validation-set R2 = 0.82, balanced accuracy = 0.903), outperforming prior quantitative structure-activity relationship (QSAR) models by 35-50%. To address frequency-dependent artifacts in SHAP interpretation, a novel reactivity index (RI) was developed, enabling unbiased quantification of substructure-specific contributions to chlorination kinetics. RI captured reactive low-frequency moieties (e.g., sulfur), which were overlooked by SHapley Additive exPlanations (SHAP) due to frequency bias. Application of these models to 10,589 environmentally relevant organic compounds revealed critical reactivity trends: 43% of pesticides exhibited slow reactivity (half-life > 6.8 h), while 22% of endocrine-disrupting chemicals showed fast degradation (half-life < 4.1 min). Mechanistic analysis identified electron-rich functional moieties. Notably, density functional theory calculations demonstrated that -COO- lowers reaction barriers by 11.6 kcal/mol compared to -COOH, challenging conventional electron-withdrawing classifications. Sulfur moieties, overlooked in prior studies, emerged as potent reactivity drivers. These findings provide a framework for predicting contaminant fate during chlorination, with direct implications for optimizing water treatment processes and prioritizing environmental monitoring of persistent pollutants. The integration of explainable machine learning with kinetic modeling advances molecular-level understanding of chlorine-organic interactions, establishing a foundation for next-generation contaminant mitigation strategies.

Aqueous-phase free radicals such as reactive oxygen, halogen, and nitrogen species play important roles in the fate of organic compounds in the aqueous-phase advanced water treatment processes and natural aquatic environments under sunlight irradiation. Predicting the fate of organic compounds in aqueous-phase advanced water treatment processes and natural aquatic environments necessitates understanding the kinetics and reaction mechanisms of initial reactions of free radicals with structurally diverse organic compounds and other reactions. Researchers developed conventional predictive models based on experimentally measured transformation products and determined reaction rate constants by fitting with the time-dependent concentration profiles of species due to difficulties in their measurements of unstable intermediates. However, the empirical treatment of lumped reaction mechanisms had a model prediction limitation with respect to the specific parent compound's fate. We use ab initio and density functional theory quantum chemical computations, numerical solutions of ordinary differential equations, and validation of the outcomes of the model with experiments. Sensitivity analysis of reaction rate constants and concentration profiles enables us to identify an important elementary reaction in formating the transformation product. Such predictive elementary reaction-based kinetics models can be used to screen organic compounds in water and predict their potentially toxic transformation products for a specific experimental investigation. Over the past decade, we determined linear free energy relationships (LFERs) that bridge the kinetic and thermochemical properties of reactive oxygen species such as hydroxyl radicals (HO center dot), peroxyl radicals (ROO center dot), and singlet oxygen (O-1(2)); reactive halogen species such as chlorine radicals (Cl-center dot) and bromine radicals (Br-center dot); reactive nitrogen species (NO2 center dot); and carbonate radicals (CO3 center dot-). We used literature-reported experimental rate constants as kinetic information. We considered the theoretically calculated aqueous-phase free energy of activation or reaction to be a kinetic or a thermochemical property, and obtained via validated ab initio or density functional theory-based quantum chemical computations using explicit and implicit solvation models. We determined rate-determining reaction mechanisms involved in reactions by observing robust LFERs. The general accuracy of LFERs to predict aqueous-phase rate constants was within a difference of a factor of 2-5 from experimental values. We developed elementary reaction-based kinetic models and predicted the fate of acetone induced by HO center dot in an advanced water treatment process and methionine by photochemically produced reactive intermediates in sunlit fresh waters. We provided mechanistic insight into peroxyl radical reaction mechanisms and critical roles in the degradation of acetone and the formation of transformation products. We highlighted different roles of triplet excited states of two surrogate CDOMs, O-1(2), and HO center dot, in methionine degradation. Predicted transformation products were compared to those obtained via benchtop experiments to validate our elementary reaction-based kinetic models. Predicting the reactivities of reactive halogen and nitrogen species implicates our understanding of the formation of potentially toxic halogen- and nitrogen-containing transformation products during water treatment processes and in natural aquatic environments.

In oxygenated aquatic environments, the predominant scavenging of the triplet excited state of chromophoric dissolved organic matter ((CDOM)-C-3*) involves dissolved ground-state oxygen, diverting attention away from the scavenging mechanisms of (CDOM)-C-3* mediated through specific organic compounds. Previous studies demonstrated that model (CDOM)-C-3* exhibited quantum yields (i.e., 1-56%) in the formation of radical ions, resulting from the competition between physical and chemical quenching through a common exciplex intermediate. Physical quenching was rationalized through the reverse intersystem crossing of the exciplex, followed by back electron transfer, yielding ground-state reactants. Despite this, direct experimental evidence for exciplex involvement has been elusive, owing to detection challenges. Herein, employing density functional theory (DFT) and time-dependent DFT specifically for excited state surrogate CDOM and organic scavengers, we unveil, for the first time, the underlying mechanisms responsible for the quenching of Rose Bengal through oxidative and reductive scavengers. Our computational findings provide evidence for the involvement of exciplexes during the quenching process of the excited triplet state of Rose Bengal, highlighting the impact of electronic coupling between Rose Bengal and quenchers on the quantum yield for radical ion formation.

N-Nitrosamines are potential human carcinogens frequently detected in natural and engineered aquatic systems. This study sheds light on the role of carbonyl compounds in the formation of N-nitrosamines by nitrosation of five secondary amines via different pathways. The results showed that compared to a control system, the presence of formaldehyde enhances the formation of N-nitrosamines by a factor of 5-152 at pH 7, depending on the structure of the secondary amines. Acetaldehyde showed a slight enhancement effect on N-nitrosamine formation, while acetone and benzaldehyde did not promote nitrosation reactions. For neutral and basic conditions, the iminium ion was the dominant intermediate for N-nitrosamine formation, while carbinolamine became the major contributor under acidic conditions. Negative free energy changes (<-19 kcal mol-1) and relatively low activation energies (<18 kcal mol-1) of the reactions of secondary amines with N2O3, iminium ions with nitrite and carbinolamines with N2O3 from quantum chemical computations further support the proposed reaction pathways. This highlights the roles of the iminium ion and carbinolamine in the formation of N-nitrosamines during nitrosation in the presence of carbonyl compounds, especially in the context of industrial wastewater.

Bromine radical (Br•) has been hypothesized to be a key intermediate of bromate formation during ozonation. Once formed, Br• further reacts with ozone to eventually form bromate. However, this reaction competes with the reaction of Br• with dissolved organic matter (DOM), of which reactivity and reaction mechanisms are less studied to date. To fill this gap, this study determined the second-order rate constant (k) of the reactions of selected organic model compounds, a DOM isolate, and monochloramine (NH2Cl) with Br• using γ-radiolysis. The kBr• of all model compounds were high (kBr• > 108 M–1 s–1) and well correlated with quantum-chemically computed free energies of activation, indicating a selectivity of Br• toward electron-rich compounds, governed by electron transfer. The reaction of phenol (a representative DOM moiety) with Br• yielded p-benzoquinone as a major product with a yield of 59% per consumed phenol, suggesting an electron transfer mechanism. Finally, the potential of NH2Cl to quench Br• was tested based on the fast reaction (kBr•, NH2Cl = 4.4 × 109 M–1 s–1, this study), resulting in reduced bromate formation of up to 77% during ozonation of bromide-containing lake water. Overall, our study demonstrated that Br• quenching by NH2Cl can substantially suppress bromate formation, especially in waters containing low DOC concentrations (1–2 mgC/L).

The benchmark advanced oxidation technology (AOT) that uses UV/H2O2 integrated with hypochlorous species exhibits great potential in removing micropollutants and enhancing wastewater treatability for reclamation purposes. Although efforts have been made to study the reactions of H2O2 with hypochlorous species, there exist great discrepancies in the order of reaction kinetics, the rate constants, and the molecule-level mechanisms. This results in an excessive use of hypochlorous reagents and system underperformance during treatment processes. Herein, the titled reaction was investigated systematically through complementary experimental and theoretical approaches. Stopped-flow spectroscopic measurements revealed a combination of bi- and trimolecular reaction kinetics. The bimolecular pathway dominates at low H2O2 concentrations, while the trimolecular pathway dominates at high H2O2 concentrations. Both reactions were simulated using direct dynamics trajectories, and the pathways identified in the trajectories were further validated by high-level quantum chemistry calculations. The theoretical results not only supported the spectroscopic data but also elucidated the molecule-level mechanisms and helped to address the origin of the discrepancies. In addition, the impact of the environmental matrix was evaluated by using two waters with discrete characteristics, namely municipal wastewater and ammonium-rich wastewater. Municipal wastewater had a negligible matrix effect on the reaction kinetics of H2O2 and the hypochlorous species, making it a highly suitable candidate for this integration technique. The obtained in-depth reaction mechanistic insights will enable the development of a viable and economical technology for safe water reuse.

The abiotic fate of dissolved free amino acids considerably contributes to the cycling of dissolved sulfur and nitrogen in natural aquatic environments. However, the roles of the functional groups of chromophoric dissolved organic matter (CDOM) and the fate of free amino acids under sunlight irradiation in fresh waters are not fully understood. This study aims to elucidate the fate of photolabile methionine in the presence of three CDOM surrogate compounds, i.e., 1,4-naphthoquinone, 2-naphthaldehyde, and umbelliferone, and two standard CDOM by coupling experimental measurement, quantum chemical computations, and kinetic modeling. Results indicate that excited triplet-state CDOM and hydroxyl radicals are able to cleave the C-S bond in methionine, resulting in the formation of smaller amino acids and volatile sulfur-containing compounds. Singlet oxygen forms methionine sulfoxide and methionine sulfone. The distribution of phototransformation products offers an improved understanding of the fate of nitrogen- and sulfur-containing compounds and their uptake by microorganisms in natural aquatic environments.

Hydrated electrons produced in aqueous-phase advanced reduction processes (ARPs) effectively destroy oxidized forms of environmentally relevant organic contaminants, including alkyl halides. Although the rate constants of hydrated electrons with various organic compounds have been experimentally measured and compiled in the literature, no mechanistic prediction tools have been developed. Given that numerous organic compounds are used in commercial production, a prediction tool for the fate of organic compounds in the aqueous-phase ARPs will be useful. This study focused on developing a group contribution method for hydrated electrons (GCMe) to predict the second-order rate constants with aliphatic and aromatic compounds. The GCMe includes 262 organic compounds undergoing four major reaction mechanisms. The GCMe fragments the structure of a given functional group of an organic compound based on the base structure that represent the major reaction with hydrated electrons and the neighboring functional group(s) that impact the main reaction. A total of 37 group rate constants and 69 group contribution factors were calibrated with 189 experimentally determined rate constants of single functional group compounds. Then, the parameters were validated with 73 multiple functional group compounds. Overall, the accuracy of GCMe in predicting the rate constants is within a difference of a factor of two from the experimental values. This predictive tool requiring only structural information of compounds can be used to screen hundreds of compounds in the prior assessment for experimental investigation in ARPs.

Single-electron transfer is a major aqueous-phase reaction mechanism commonly used in environmental engineering and natural processes such as aquatic photochemistry and advanced oxidation processes. While the Marcus theory is frequently used to analyze single-electron transfers, many previous studies appear to have overlooked its application, with uncertain energy values being reported without validation. Herein, using the carbonate radical as the oxidant, we analyze the validity of the Marcus theory to aqueous-phase reactions involving aromatic compounds. We highlight the impact of charged targeted molecules by comparing the reactivity with phenolate and aniline. Further, we expand the validated methodology to a wide range of structurally diverse organic compounds and reveal the underlying reaction mechanisms, such as outer-/inner-sphere single-electron transfer and proton coupled electron transfer. Our research outlines the next steps to be taken in Marcus theory calculations to investigate aqueous-phase environmental reactions.

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

The concept of circular water economy has emerged for the sustainable and resilient use of water and for mitigating attendant greenhouse gas (GHG) emissions. Although macroscale circular water economy has been widely addressed, analysis of microscale household circular water economy, including water-energy nexus aspects, has not been thoroughly developed and documented. In this study, we quantify the contribution of household water and energy use to water consumption and GHG emissions. We develop a comprehensive spreadsheet-based input-output model to calculate the net GHG emission expressed as carbon dioxide equivalent (CO(2)e) and water consumption levels of household appliances for various supply- and demand-side scenarios. We quantitatively evaluate the impact of temporally and spatially varying emission factors of energy production on the GHG emissions and water consumption for a model household that employs appliances with various efficiency levels. To advance the concept of a circular water economy, we develop a framework for considering water reduction, reuse, recycling, and recovery, along with retrofitting/remanufacturing of water and energy appliances and fixtures. Finally, we illustrate the choices in household water and energy systems through a trade-off analysis based on the capital and annual costs of each appliance.

ADVERTISEMENT RETURN TO ISSUEPREVCorrespondence/Rebut...Correspondence/RebuttalNEXTResponse to Comment on "Mechanistic Understanding of Superoxide Radical-Mediated Degradation of Perfluorocarboxylic Acids"Lu BaiLu BaiInstitute of Environmental Engineering, School of Metallurgy and Environment, Central South University, Changsha 410083, ChinaChinese National Engineering Research Center for Control & Treatment of Heavy Metal Pollution, Changsha 410083, ChinaMore by Lu Bai, Ying JiangYing JiangInstitute of Environmental Engineering, School of Metallurgy and Environment, Central South University, Changsha 410083, ChinaChinese National Engineering Research Center for Control & Treatment of Heavy Metal Pollution, Changsha 410083, ChinaMore by Ying Jiang, Deming XiaDeming XiaKey Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, ChinaMore by Deming Xia, Zongsu WeiZongsu WeiCentre for Water Technology (WATEC) & Department of Engineering, Aarhus University, Hangøvej 2, Aarhus N DK-8200, DenmarkMore by Zongsu Weihttps://orcid.org/0000-0001-8747-2251, Richard SpinneyRichard SpinneyDepartment of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, United StatesMore by Richard Spinneyhttps://orcid.org/0000-0002-8074-3386, Dionysios D. DionysiouDionysios D. DionysiouEnvironmental Engineering and Science Program, University of Cincinnati, Cincinnati, Ohio 45221, United StatesMore by Dionysios D. Dionysiouhttps://orcid.org/0000-0002-6974-9197, Daisuke MinakataDaisuke MinakataDepartment of Civil and Environmental Engineering, Michigan Technological University, Houghton, Michigan 49931, United StatesMore by Daisuke Minakatahttps://orcid.org/0000-0003-3055-3880, Ruiyang Xiao*Ruiyang XiaoInstitute of Environmental Engineering, School of Metallurgy and Environment, Central South University, Changsha 410083, ChinaChinese National Engineering Research Center for Control & Treatment of Heavy Metal Pollution, Changsha 410083, China*Email: [email protected]. Phone: +86-731-88830875. Fax: +86-731-88710171.More by Ruiyang Xiaohttps://orcid.org/0000-0001-9516-2202, Hong-Bin Xie*Hong-Bin XieKey Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China*Email: [email protected]. Phone: +86-411-84707251. Fax: +86-411-84707251.More by Hong-Bin Xiehttps://orcid.org/0000-0002-9119-9785, and Liyuan ChaiLiyuan ChaiInstitute of Environmental Engineering, School of Metallurgy and Environment, Central South University, Changsha 410083, ChinaChinese National Engineering Research Center for Control & Treatment of Heavy Metal Pollution, Changsha 410083, ChinaMore by Liyuan Chaihttps://orcid.org/0000-0001-8641-9774Cite this: Environ. Sci. Technol. 2022, 56, 8, 5289–5291Publication Date (Web):March 23, 2022Publication History Published online23 March 2022Published inissue 19 April 2022https://pubs.acs.org/doi/10.1021/acs.est.2c01335https://doi.org/10.1021/acs.est.2c01335article-commentaryACS PublicationsCopyright © 2022 American Chemical Society. This publication is available under these Terms of Use. Request reuse permissions This publication is free to access through this site. Learn MoreArticle Views2033Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail PDF (803 KB) Get e-AlertscloseSUBJECTS:Acidity,Chemical reactions,Degradation,Kinetic parameters,Oxides Get e-Alerts

Perfluorocarboxylic acids (PFCAs) exhibit strong persistence in sunlit surface waters and in radical-based treatment processes, where superoxide radical (O2•-) is an important and abundant reactive oxygen species. Given that the role of O2•- during the transformation of PFCAs remains largely unknown, we investigated the kinetics and mechanisms of O2•--mediated PFCAs attenuation through complementary experimental and theoretical approaches. The aqueous-phase rate constants between O2•- and C3-C8 PFCAs were measured using a newly designed in situ spectroscopic system. Mechanistically, bimolecular nucleophilic substitution (SN2) is most likely to be thermodynamically feasible, as indicated by density functional theory calculations at the CBS-QB3 level of theory. This pathway was then investigated by ab initio molecular dynamics simulation with free-energy samplings. As O2•- approaches PFCA, the C-F bond at the alpha carbon is spontaneously stretched, leading to the bond cleavage. The solvation mechanism for O2•--mediated PFCA degradation was also elucidated. Our results indicated that although the less polar solvent enhanced the nucleophilicity of O2•-, it also decreased the desolvation process of PFCAs, resulting in reduced kinetics. With these quantitative and mechanistic results, we achieved a defined picture of the O2•--initiated abatement of PFCAs in natural and engineered waters.

Novel linear free energy relationships were determined between experimentally measured rate constants of solvated electrons and theoretically calculated one electron reduction potential for the elucidation of reductive reaction mechanisms.

Singlet oxygen (1O2) is a selective reactive oxygen species that plays a key role for the fate of various organic compounds in the aquatic environment under sunlight irradiation, engineered water oxidation systems, atmospheric water droplets, and biomedical systems. While the initial rate-determining charge-transfer reaction mechanisms and kinetics of 1O2 have been studied extensively, no comprehensive studies have been performed to elucidate the reaction mechanisms with organic compounds that have various functional groups. In this study, we use density functional theory calculations to determine elementary reaction mechanisms with a wide variety of organic compounds. The theoretically calculated aqueous-phase free energies of activation of single electron transfer and 1O2 addition reactions are compared to the experimentally determined rate constants in the literature to determine linear free-energy relationships. The theoretically calculated free energies of activation for the groups of phenolates and phenols show excellent correlations with the Hammett constants that accept electron densities by through-resonance. The dominant elementary reaction mechanism is discussed for each group of compounds. As a practical implication, we demonstrate the fate of environmentally relevant organic compounds induced by photochemically produced intermediate species at different pH and evaluate the impact of predicting rate constants to the half-life.

UV photolysis is an effective process to remove nitrosamines from contaminated water resources.

Contemporary studies emphasize that superoxide radical (O-2(center dot-)) exhibits the potential to degrade organic contaminants, but practical application of this radical in engineered waters require an in-depth understanding of its kinetic profiles in a quantitative way. Here, we developed, for the first time, a convenient and reliable approach to generate micromolar level O-2(center dot-) in aqueous solution by photolysis of formate and H2O2. The presence of O-2(center dot-) was confirmed by comparing the UV spectra under pulse radiolysis and chromogenic reaction. We then constructed an in situ long-path spectroscopy to investigate the kinetics and mechanisms of O(2)(center dot-)mediated degradation of carbon tetrachloride (CCl4), a halogenated model contaminant. The rate constant for the reaction of O-2(center dot-) and CCl4 was determined to be 478 M-1 s(-1). In addition, we employed the transition state theory to model the reaction rate constants. Both results show that O center dot-2 exhibited low reactivity towards CCl4 with bimolecular rate constant lower by at least one order of magnitude than those radicals generated in typical advanced oxidation processes such as hydroxyl and sulfate radicals. Our results also indicate that nucleophilic substitution is the major pathway, and the solvation effect plays an important role in the reaction. The complementary experimental and theoretical approaches provide a mechanistic basis for better understanding aqueous-phase O-2(center dot-) chemistry and a holistic evaluation on the application of O-2(center dot-) for the degradation of organic contaminants of emerging concern.