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

Isotope Techniques in Chemical Wastewater Treatment: Opportunities and Uncertainties

A comprehensive and in-depth analysis of reaction mechanisms is essential for advancing chemical water treatment technologies. However, due to the limitations of conventional experimental and analytical methods, the types of reactive species and their generation pathways are commonly debatable in many aqueous systems. As highly sensitive diagnostic tools, isotope techniques offer deeper insights with minimal interference from reaction conditions. Nevertheless, precise interpretations of isotope results remain a significant challenge. Herein, we first scrutinized the fundamentals of isotope chemistry and highlighted key changes induced by the isotope substitution. Next, we discussed the application of isotope techniques in kinetic isotope effects, presenting a roadmap for interpreting KIE in sophisticated systems. Furthermore, we summarized the applications of isotope techniques in elemental tracing to pinpoint reaction sites and identify dominant reactive species. Lastly, we propose future research directions, highlighting critical considerations for the rational design and interpretation of isotope experiments in environmental chemistry and related fields.

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

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

Phenols coupled during oxidation upstream of water treatment would generate higher toxic coupling phenolic disinfection by-products during chlorination disinfection

Phenolic compounds usually produce coupling phenols during the upstream oxidation treatment of water, but the disinfection by-products (DBPs) of coupling phenols in the subsequent disinfection process have been overlooked. Herein, we demonstrated the generation and higher toxicity of these DBPs. In Songhua River water, 0.355 ng/L 4-(4-bromophenoxy)phenol and 122.67 ng/L phenol were detected. Pre-oxidation with K2FeO4 and KMnO4 resulted in the formation of 0.68 ng/L and 0.506 ng/L 4-(4-iodophenoxy)phenol in subsequent disinfection, respectively, which were 2-3 times that without pre-oxidation. Coupling of phenolic compounds during pre-oxidation and then halogenated during chlorination was shown to be the main pathway for the generation of coupling phenolic DBPs. The maximum rate constants for the reactions of hypochlorous acid with phenol and its coupling products (4-phenoxyphenol, 2,2'-biphenol, and [1,1'-biphenyl]-2,4'-diol) were 115.61 M-1s-1, 65.85 M-1s-1, 143.13 M-1s-1, and 212.52 M-1s-1, respectively, with 2,2'-biphenol and [1,1'-biphenyl]-2,4'-diol breaking through lower energy barriers and releasing more energy than phenol. This indicated coupling phenols have a higher potential to form DBPs. Additionally, coupling phenolic DBPs (4-(4-chlorophenoxy)phenol, 4-(4-bromophenoxy)phenol, and 4-(4-iodophenoxy)phenol) was 1-3 orders of magnitude more toxic than their precursors (4-phenoxyphenol, 2,2'-biphenol, [1,1'-biphenyl]-2,4'-diol, and phenol) and uncoupled DBPs (4-iodophenol). Therefore, the formation and hazards of coupling phenolic DBPs require more attention.

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.

Predicting reaction kinetics of reactive bromine species with organic compounds by machine learning: Feature combination and knowledge transfer with reactive chlorine species

Reactive bromine species (RBS) such as bromine atom (Br•) and dibromine radical (Br2•-) are important oxidative species accounting for the transformation of organic compounds in bromide-containing water. This study developed quantitative structure-activity relationship (QSAR) models to predict second order rate constants (k) of RBS by machine learning (ML) and conducted knowledge transfer between RBS and reactive chlorine species (RCS, e.g., Cl• and Cl2•-) to improve model performance. The ML-based models (RMSEtest = 0.476 -0.712) outperformed the multiple linear regression-based models (RMSEtest = 0.572 -3.68) for predicting k of RBS. In addition, the combination of molecular fingerprints (MFs) and quantum descriptors (QDs) as input features improved the performance of ML-based models (RMSEtest = 0.476 -0.712) compared to those developed by MFs (RMSEtest = 0.524 -0.834) or QDs (RMSEtest = 0.572 -0.806) alone. EHOMO and Egap were identified to be the most important features affecting k of RBS based on SHAP analysis. A unified model integrating the datasets of four reactive halogen species (RHS, e.g., Br•, Br2•-, Cl• and Cl2•-) was further developed (R2test = 0.802), which showed better predictive performance than the individual models (R2test = 0.521 -0.776). Meanwhile, the model performance changed differently by employing knowledge transfer among RHS, which was improved for Br•/Cl•, mixed for Br•/Br2•- and Cl•/Cl2•-, but worse for Br2•-/Cl2•-. This study provides useful tools for predicting k of RHS in aqueous environments.

Comprehensive Insight into the Common Organic Radicals in Advanced Oxidation Processes for Water Decontamination

Radical-based advanced oxidation processes (AOPs) are among the most effective technologies employed to destroy organic pollutants. Compared to common inorganic radicals, such as •OH, O2•-, and SO4•-, organic radicals are widespread, and more selective, but are easily overlooked. Furthermore, a systematic understanding of the generation and contributions of organic radicals remains lacking. In this review, we systematically summarize the properties, possible generation pathways, detection methods, and contributions of organic radicals in AOPs. Notably, exploring organic radicals in AOPs is challenging due to (1) limited detection methods for generated organic radicals; (2) controversial organic radical-mediated reaction mechanisms; and (3) rapid transformation of organic radicals as reaction intermediates. In addition to their characteristics and reactivity, we examine potential scenarios of organic radical generation in AOPs, including during the peroxide activation process, in water matrices or with coexisting organic pollutants, and due to the addition of quenching agents. Subsequently, we summarize various methods for organic radical detection as reported previously, such as electron paramagnetic resonance spectroscopy (EPR), 31P nuclear magnetic resonance spectroscopy (31P NMR), liquid/gas chromatography-mass spectroscopy (GC/LC-MS), and fluorescence probes. Finally, we review the contributions of organic radicals to decontamination processes and provide recommendations for future research.

BTXs removals by modified clay during mitigation of Karenia brevis bloom: Insights from adsorption and transformation

Harmful algal blooms (HABs), especially those caused by toxic dinoflagellates, are spreading in marine ecosystems worldwide. Notably, the prevalence of Karenia brevis blooms and potent brevetoxins (BTXs) pose a serious risk to public health and marine ecosystems. Therefore, developing an environmentally friendly method to effectively control HABs and associated BTXs has been the focus of increasing attention. As a promising method, modified clay (MC) application could effectively control HABs. However, the environmental fate of BTXs during MC treatment has not been fully investigated. For the first time, this study revealed the effect and mechanism of BTX removal by MC from the perspective of adsorption and transformation. The results indicated that polyaluminium chloride-modified clay (PAC-MC, a typical kind of MC) performed well in the adsorption of BTX2 due to the elevated surface potential and more binding sites. The adsorption process was a spontaneous endothermic process that conformed to pseudo-second-order adsorption kinetics (k2 = 6.8 × 10-4, PAC-MC = 0.20 g L-1) and the Freundlich isotherm (Kf = 55.30, 20 °C). In addition, detailed product analysis using liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS) indicated that PAC-MC treatment effectively removed the BTX2 and BTX3, especially those in the particulate forms. Surprisingly, PAC-MC could promote the transformation of BTX2 to derivatives, including OR-BTX2, OR-BTX3, and OR-BTX-B5, which were proven to have lower cytotoxicity.

Development of an Elementary Reaction-Based Kinetic Model to Predict the Aqueous-Phase Fate of Organic Compounds Induced by Reactive Free Radicals

ConspectusAqueous-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•), peroxyl radicals (ROO•), and singlet oxygen (1O2); reactive halogen species such as chlorine radicals (Cl•) and bromine radicals (Br•); reactive nitrogen species (NO2•); and carbonate radicals (CO3•-). 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• 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, 1O2, and HO•, 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.

Enzymatic degradability of diclofenac ozonation products: A mechanistic analysis

The treatment of waterborne micropollutants, such as diclofenac, presents a significant challenge to wastewater treatment plants due to their incomplete removal by conventional methods. Ozonation is an effective technique for the degradation of micropollutants. However, incomplete oxidation can lead to the formation of ecotoxic by-products that require a subsequent post-treatment step. In this study, we analyze the susceptibility of micropollutant ozonation products to enzymatic digestion with laccase from Trametes versicolor to evaluate the potential of enzymatic treatment as a post-ozonation step. The omnipresent micropollutant diclofenac is used as an example, and the enzymatic degradation kinetics of all 14 detected ozonation products are analyzed by high-performance liquid chromatography coupled with high-resolution mass spectrometry (HPLC-HRMS) and tandem mass spectrometry (MS2). The analysis shows that most of the ozonation products are responsive to chemo-enzymatic treatment but show considerable variation in enzymatic degradation kinetics and efficiencies. Mechanistic investigation of representative transformation products reveals that the hydroxylated aromatic nature of the ozonation products matches the substrate spectrum, facilitating their rapid recognition as substrates by laccase. However, after initiation by laccase, the subsequent chemical pathway of the enzymatically formed radicals determines the global degradability observed in the enzymatic process. Substrates capable of forming stable molecular oxidation products inhibit complete detoxification by oligomerization. This emphasizes that it is not the enzymatic uptake of the substrates but the channelling of the reaction of the substrate radicals towards the oligomerization of the substrate radicals that is the key step in the further development of an enzymatic treatment step for wastewater applications.

Oxidation processes and me

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