Oxidative Oligomerization of Phenolic Endocrine Disrupting Chemicals Mediated by Mn(III)-L Complexes and the Role of Phenoxyl Radicals in the Enhanced Removal: Experimental and Theoretical Studies

Reductive Dissolution Mechanisms of Manganese Oxide Mediated by Algal Extracellular Organic Matter and the Effects on 17α-Ethinylestradiol Removal

Reductive dissolution of manganese oxide (MnOx) is a major process that improves the availability of manganese in natural aquatic environments. The extracellular organic matter (EOM) secreted by algae omnipresent in eutrophic waters may affect MnOx dissolution thus the fate of organic micropollutants. This study investigates the mechanisms of MnOx reductive dissolution mediated by EOM and examines the effects of this process on 17α-ethinylestradiol degradation. The influences of EOM concentration (1.0-20.0 mgC/L) and pH (6.0-9.0) in both dark and irradiated conditions were assessed. In the dark, EOM was found to facilitate MnOx reductive dissolution via the ligand-to-metal charge transfer (LMCT). The dissolution was further enhanced under irradiation, with the participation of superoxide ions (O2•-). Higher EOM concentrations increased the contents of available reducing substances and O2•-, accelerating the reductive dissolution. Higher pH slowed the photoreductive dissolution rates, while O2•--mediated reduction became more important. Polyphenols and highly unsaturated carbon and phenolic formulas in EOM were found to drive the reductive dissolution. Soluble reactive Mn(III) formed through reductive dissolution of MnOx effectively removed 17α-ethinylestradiol in solution. Overall, the findings regarding the mechanisms behind reductive dissolution of MnOx have broad implications for Mn geochemical cycles and organic micropollutant fate.

Transformation of dissolved organic matter leached from biodegradable and conventional microplastics under UV/chlorine treatment and the subsequent effect on contaminant removal

The ultraviolet (UV)/chlorine process has been widely applied for water treatment. However, the transformation of microplastic-leached dissolved organic matter (MP-DOM) in advanced treatment of real wastewater remains unclear. Here, we investigated alterations in the photoproperties of MP-DOM leached from biodegradable and conventional microplastics (MPs) and their subsequent effects on the degradation of sulfamethazine (SMT) by the UV/chlorine process. Spectroscopy was used to assess photophysical properties, focusing on changes in light absorption capacity, functional groups, and fluorescence components, while photochemical properties were determined by calculating the apparent quantum yields of reactive intermediates (ΦRIs). For photophysical properties, our findings revealed that the degree of molecular structure modification, functional group changes, and fluorescence characteristics during UV/chlorine treatment are closely linked to the type of MPs. For photochemical properties, the ΦRIs increased with higher chlorine dosages due to the formation of new functionalities. Both singlet oxygen (1O2) and hydroxyl radicals (•OH) formation were strongly correlated with excited triplet state of DOM (3DOM*) in the UV/chlorine treatment. Additionally, we found that the four types of MP-DOM inhibit the degradation of SMT and elucidated the mechanisms behind this inhibition. We also proposed degradation pathways for SMT and assessed the ecotoxicity of the resulting intermediates. This study provides important insights into how the characteristics and transformation of MP-DOM affect contaminant degradation, which is critical for evaluating the practical application of UV-based advanced oxidation processes (UV-AOPs).

Cross-Coupling of 1,2,3,4-Tetrachlororodibenzo-p-dioxin with Six Coexisting Polycyclic Aromatic Hydrocarbons during Photodegradation on a Fly Ash Surface

The adverse conditions of the garbage incineration process can lead to the generation of dioxins and polycyclic aromatic hydrocarbons (PAHs). This study aimed to investigate the removal efficiency and possible cross-coupling effect of 1,2,3,4-tetrachlorodibenzo-p-dioxin (1,2,3,4-TCDD) and six coexisting low-molecular-weight PAHs during photodegradation on the fly ash surface. Due to their higher photoreactivity and light-shielding effect, the six PAHs exhibited inhibitory effects on the photodegradation of 1,2,3,4-TCDD, causing a reduction of 4.1%-21.2% in the removal efficiency. Common degradation products of 1,2,3,4-TCDD and PAHs were identified by LC-MS and GC-MS, and the formation of primary products was verified by theoretical calculations of bond dissociate energies, excitation energy, frontier electron densities, and transition states. In addition, high-molecular-weight coupling products of 1,2,3,4-TCDD and its interaction products with PAHs were observed in the mixed irradiation samples, and two coupling elimination mechanisms were proposed to illustrate their formation through C-O-C bonding and -COO- bonding, respectively. According to toxicity prediction analysis, the developmental toxicity and mutagenicity of most interaction products were higher than 1,2,3,4-TCDD. This study provided some new insights into the transformation, interaction, and related ecological risks of dioxins and PAHs coexisting on the surface of fly ash during the waste incineration process.

Trivalent manganese in dissolved forms: Occurrence, speciation, reactivity and environmental geochemical impact

The cycling processes of elemental manganese (Mn), including the redox reactions of dissolved Mn(III) (dMn(III)), directly and indirectly influences the biogeochemical processes of many elements. Though increasing evidence indicates the widespread presence of dMn(III) mediates the fate of many elements, its role may be currently underestimated. There is both a lack of clear understanding of the historical research framework of dMn(III) and a systematic overview of its geochemical properties and detection methods. Therefore, the primary aim of this review is to outline the understanding of dMn(III) in multiple fields, including soil science, analytical chemistry, biochemistry, geochemistry, and water treatment, and summarize the formation pathways, species forms, and detection methods of dMn(III) in aquatic systems. This review considers how the characteristics of dMn(III), the intermediate formed in the single-electron reaction processes of Mn(II) oxidation and Mn(IV) reduction, determines its participation in environmental geochemical processes. Its widespread presence in diverse water systems and active redox properties coupling with various elements confirm its significant role in natural elemental geochemistry cycle and artificial water treatment processes. Therefore, further investigation into the role of dissolved Mn(III) in aquatic systems is warranted to unravel unexplored coupled elemental redox reaction processes mediated by dissolved Mn(III), filling in the gaps in our understanding of manganese environmental geochemistry, and providing a theoretical basis for recognizing the role of dMn(III) role in water treatment technologies.

BhrPETase catalyzed polyethylene terephthalate depolymerization: A quantum mechanics/molecular mechanics approach

Polyethylene terephthalate (PET) is a widely used material in our daily life, particularly in areas such as packaging, fibers, and engineering plastics. However, PET waste can accumulate in the environment and pose a great threat to our ecosystem. Recently enzymatic conversion has emerged as an efficient and green strategy to address the PET crisis. Here, using a theoretical approach combining molecular dynamics simulation and quantum mechanics/molecular mechanics calculations, the depolymerization mechanism of the thermophilic cutinase BhrPETase was fully deciphered. Surprisingly, unlike the previously studied cutinase LCCICCG, our results indicate that the first step, catalytic triad assisted nucleophilic attack, is the rate-determining step. The corresponding Boltzmann weighted average energy barrier is 18.2 kcal/mol. Through extensive comparison between BhrPETase and LCCICCG, we evidence that key features like charge CHis@N1 and angle APET@C1-Ser@O1-His@H1 significantly impact the depolymerization efficiency of BhrPETase. Non-covalent bond interaction and distortion/interaction analysis inform new insights on enzyme engineer and may aid the recycling of enzymatic PET waste. This study will aid the advancement of the plastic bio-recycling economy and promote resource conservation and reuse.

Contribution of complexed Fe(Ⅱ) oxygenation to norfloxacin humification and stabilization: Producing and trapping of more humified products

Organic pollutants polymerization in advanced oxidation processes or environmental matrices has attracted increasing attention, but little is known about stabilization of the polymerization products. The results in this work revealed the contribution of Fe(Ⅱ) oxygenation to stabilization of the products from norfloxacin (NOR) humification. It was found that upon oxygenation of Fe(Ⅱ) complexed by catechol (CT), NOR polymerized into the products with larger molecular weight through nucleophilic addition. Around 83.9-89.7 % organic carbon (OC) can be retained in the reaction solution and the precipitates at different Fe(II)/CT molar ratio. In this system with humification potential, the produced hydroxyl radical (HO•) dominantly modified, instead of decomposed, the structure of transformation products (TPs). TPs with diversified side chains were formed through hydroxylation and ring-opening, leading to the more humified products. In the subsequent Fe(Ⅱ) oxidative precipitation, Fe-TPs composites were formed as spherical particle clusters, which could steadily incorporate OC species with molecular fractionation. Specifically, lignin-like, tannins-like, condensed aromatic and high-molecular-weight TPs were preferentially preserved in the precipitates, while the recalcitrant aliphatic products mainly retained in the solution. These findings shed light on the role of Fe(Ⅱ) oxygenation in stabilizing the products from pollutants humification, which could strengthen both decontamination and organics sequestration.

Role of low-molecular-weight organic compounds on photochemical formation of Mn(III)-ligands in aqueous systems: Implications for BPA removal

Aqueous trivalent manganese [Mn(III)], an important reactive intermediate, is ubiquitous in natural surface water containing humic acid (HA). However, the effect of low-molecular-weight organic acids (LMWOAs) on the formation, stability and reactivity of Mn(III) intermediate is still unknown. In this study, six LMWOAs, including oxalic acid (Oxa), salicylic acid (Sal), catechol (Cat), caffeic acid (Caf), gallic acid (Gal) and ethylene diamine tetraacetic acid (EDTA), were selected to investigate the effects of LMWOAs on the degradation of BPA induced by in situ formed Mn(III)-L in the HA/Mn(II) system under light irradiation. The chromophoric constituents of HA could absorb light radiation and generate superoxide radical to promote the oxidation of Mn(II) to form Mn(III), which was further involved in transformation of BPA. Our results implied that different LMWOAs did significantly impact on Mn(III) production and its degradation of BPA due to their different functional group. EDTA, Oxa and Sal extensively increased the Mn(III) concentration from 50 to 100 μM compared to the system without LMWOAs, following the order of EDTA > Oxa > Sal, and also enhanced the degradation of BPA with the similar patterns. In contrast, Cat, Caf and Gal had an inhibitory effect on the formation of Mn(III), which is likely because they consumed the superoxide radicals generated from irradiated HA, resulting in the inhibition of Mn(II) oxidation and further BPA removal. The product identification and theoretical calculation indicated that a single electron transfer process occurred between Mn(III)-L and BPA, forming BPA radicals and subsequent self-coupling products. Our results demonstrated that the LMWOAs with different structures could alter the cycling process of Mn via complexation and redox reactions, which would provide new implications for the removal of organic pollutants in surface water.

Algal Extracellular Organic Matter Induced Photochemical Oxidation of Mn(II) to Solid Mn Oxide: Role of Mn(III)-EOM Complex and Its Ability to Remove 17α-Ethinylestradiol

Photosensitizer-mediated abiotic oxidation of Mn(II) can yield soluble reactive Mn(III) and solid Mn oxides. In eutrophic water systems, the ubiquitous algal extracellular organic matter (EOM) is a potential photosensitizer and may have a substantial impact on the oxidation of Mn(II). Herein, we focused on investigating the photochemical oxidation process from Mn(II) to solid Mn oxide driven by EOM. The results of irradiation experiments demonstrated that the generation of Mn(III) intermediate was crucial for the successful photo oxidization of Mn(II) to solid Mn oxide mediated by EOM. EOM can serve as both a photosensitizer and a ligand, facilitating the formation of the Mn(III)-EOM complex. The complex exhibited excellent efficiency in removing 17α-ethinylestradiol. Furthermore, the complex underwent decomposition as a result of reactions with reactive intermediates, forming a solid Mn oxide. The presence of nitrate can enhance the photochemical oxidation process, facilitating the conversion of Mn(II) to Mn(III) and then to solid Mn oxide. This study deepens our grasp of Mn(II) geochemical processes in eutrophic water and its impact on organic micropollutant fate.

Comparative study of the photooxidation of arsenite mediated by dissolved and mineral-associated humic acid under light irradiation

The photochemical processes of dissolved humic acid and its potential contribution to As(III) oxidation in natural water has received considerable attention. However, the role of mineral-humic complexes in As(III) conversion is rarely studied. Herein, two simulated mineral-humic complexes were prepared by coating humic acid on hydrous aluminum oxide, HA@HAO, or montmorillonite, HA@SWy, respectively, and batch experiments at circumneutral pH were performed under light irradiation. Our findings showed that the light-assisted oxidation of As(III) increased with increasing fractions of organic carbon in mineral-humic complexes, and As(III) photooxidation with HA@HAO or HA@SWy was up to 18.2 or 3.5-fold higher compared to that measured in the presence of equivalent amount of free HA, respectively. The reactive triplet state of HA and hydroxyl radicals in HA@HAO and HA@SWy system made a primary contribution to As(III) oxidation under irradiation. The results indicated that mineral-humic complexes have dual roles, an adsorbent and a photosensitizer, to promote As(III) access to reactive intermediates at the particle surfaces. This process was important for As(III) conversion in paddy water as colloidal particles, composed of both minerals and HA, could greatly promote As(III) oxidation and As(V) immobilization. This study provides a previously overlooked, important mechanism of As(III) phototransformation mediated by mineral-associated humic acid in natural environment.

Unraveling Multiple Pathways of Electron Donation from Phenolic Moieties in Natural Organic Matter

Natural organic matter (NOM) exhibits a distinctive electron-donating capacity (EDC) that serves a pivotal role in the redox reactions of contaminants and minerals through the transformation of electron-donating phenolic moieties. However, the ambiguity of the molecular transformation pathways (MTPs) that engender the EDC during NOM oxidation remains a significant issue. Here, MTPs that contribute to EDC were investigated by identifying the oxidized products of phenolic model compounds and NOM samples in direct or mediated electrochemical oxidation (DEO or MEO, respectively) using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). It was found that the oxidation of newly formed phenolic-OH (ArOH) and the oxidative coupling reaction of the phenoxy radical are the main MTPs that directly contribute to EDC, in addition to the transformation of hydroquinones to quinones. Notably, the oxidative coupling reaction of ArOH contributed at least 22-42% to the EDC. Ferulic acid-like structures can also directly contribute to EDC by incorporating H2O into their acrylic substituents. Furthermore, the opening of C rings can indirectly attenuate the EDC through structural alterations in the electron-donating process of NOM. Decarboxylation can either weaken or enhance the EDC depending on the structure of the phenolic moieties in NOM. These findings suggest that the EDC of NOM is a comprehensive result of multiple NOM MTPs, involving not only ArOH oxidation but also the addition of H2O to olefinic bonds and bond-breaking reactions. Our work provides molecular evidence that aids in the comprehension of the multiple EDC-associated transformation pathways of NOM.

Trade-off effect of dissolved organic matter on degradation and transformation of micropollutants: A review in water decontamination

The degradation of micropollutants by various treatments is commonly affected by the ubiquitous dissolved organic matter (DOM) in the water environment. To optimize the operating conditions and decomposition efficiency, it is necessary to consider the impacts of DOM. DOM exhibits varied behaviors in diverse treatments, including permanganate oxidation, solar/ultraviolet photolysis, advanced oxidation processes, advanced reduction process, and enzyme biological treatments. Besides, the different sources (i.e., terrestrial and aquatic, etc) of DOM, and operational circumstances (i.e., concentration and pH) fluctuate different transformation efficiency of micropollutants in water. However, so far, systematic explanations and summaries of relevant research and mechanism are rare. This paper reviewed the "trade-off" performances and the corresponding mechanisms of DOM in the elimination of micropollutants, and summarized the similarities and differences for the dual roles of DOM in each of the aforementioned treatments. Inhibition mechanisms typically include radical scavenging, UV attenuation, competition effect, enzyme inactivation, reaction between DOM and micropollutants, and intermediates reduction. Facilitation mechanisms include the generation of reactive species, complexation/stabilization, cross-coupling with pollutants, and electron shuttle. Moreover, electron-drawing groups (i.e., quinones, ketones functional groups) and electron-supplying groups (i.e., phenols) in the DOM are the main contributors to its trade-off effect.

Synthesis of MRGO@ZIF-7-Based Molecular Imprinted Polymer by Surface Polymerization for the Fast and Selective Removal of Phenolic Endocrine-Disrupting Chemicals from Aqueous Environments

In this study, Zn(NO3)2·6H2O was selected as the metal source, and ZIF-7-modified magnetic graphene-based matrix materials (MRGO@ZIF-7) were prepared by in situ growth. ZIF-7 modified magnetic graphene-based molecular imprinting complexes (MRGO@ZIF7-MIP) were successfully synthesized by a surface molecular imprinting technique using bisphenol A (BPA) as the template molecule. The obtained experimental materials were characterized by X-ray diffraction (XRD), Brunner–Emmet–Teller (BET) analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), vibrating sample magnetometry (VSM), and X-ray photoelectron spectroscopy (XPS). The proper adsorption and selective recognition ability of the MRGO@ZIF7-MIP were studied by an equilibrium adsorption method. The obtained MRGO@ZIF7-MIP showed significant molecular recognition of bisphenol A (BPA) and good selectivity and reproducibility for BPA in different aqueous environments such as drinking water, river water, and lake water. These properties make this material potentially applicable for the efficient removal of phenolic endocrine disruptors in real water environments.

Roles of MnO2 Colloids and Mn(III) during the Oxidation of Organic Contaminants by Permanganate

Although intermediate manganese species can be generated during the reactions of permanganate (Mn(VII)) with organic pollutants in water, the role of the in situ generated MnO₂ colloids in the Mn(VII) oxidation process remained controversial and the contribution of Mn(III) was largely neglected. This study showed that the apparent second-order rate constants (k_app) of Mn(VII) oxidation of methyl phenyl sulfoxide and carbamazepine remained constant with time. However, the degradation of four selected phenolic contaminants by Mn(VII) exhibited an autoaccelerating trend and a linear trend at pH 3.0-6.0 and pH 7.0-9.0, respectively. Multiple lines of evidence revealed that the occurrence of the autoaccelerating trend in the Mn(VII) oxidation process was ascribed to the oxidation of the phenolic organics by MnO₂ colloids. The influence of pyrophosphate on the oxidation of different organic contaminants by MnO₂ colloids suggests that Mn(III) was also responsible for the autoaccelerating oxidation of organic contaminants by Mn(VII) under specific reaction conditions. The kinetic models revealed that the overall contributions of MnO₂ colloids and Mn(III) ranged within 6.6-67.9% during the autoaccelerating oxidation of phenolic contaminants by Mn(VII). These findings advance the understanding of the roles of MnO₂ colloids and Mn(III) in the Mn(VII) oxidation process.

Molecular Design and Mechanism Analysis of Phthalic Acid Ester Substitutes: Improved Biodegradability in Processes of Sewage Treatment and Soil Remediation

Phthalic acid esters (PAEs) have the characteristics of environmental persistence. Therefore, improving the biodegradability of PAEs is the key to reducing the extent of ecological harm realized. Firstly, the scoring function values of PAEs docking with various degrading enzymes in sewage treatment were calculated. Based on this, a 3D-quantitative structure-activity relationship (3D-QSAR) model for PAE biodegradability was built, and 38 PAE substitutes were created. By predicting the endocrine-disrupting toxicity and functions of PAE substitutes, two types of PAE substitutes that are easily degraded by microorganisms, have low toxicity, and remain functional were successfully screened. Meanwhile, the differences in the mechanism of molecular degradation difference before and after PAE modification were analyzed based on the distribution characteristics of amino acid residues in the molecular docking complex. Finally, the photodegradability and microbial degradability of the PAE substitutes in the soil environment was evaluated. From the 3D-QSAR model design perspective, the modification mechanism of PAE substitutes suitable for sewage treatment and soil environment degradation was analyzed. We aim to improve the biodegradability of PAEs at the source and provide theoretical support for alleviating the environmental hazards of using PAEs.

Degradation of Organic Contaminants by Reactive Iron/Manganese Species: Progress and Challenges

Many iron(II, III, VI)- and manganese(II, IV, VII)-based oxidation processes can generate reactive iron/manganese species (RFeS/RMnS, i.e., Fe(IV)/Fe(V) and Mn(III)/Mn(V)/Mn(VI)), which have mild and selective reactivity toward a wide range of organic contaminants, and thus have drawn significant attention. The reaction mechanisms of these processes are rather complicated due to the simultaneous involvement of multiple radical and/or nonradical species. As a result, the ambiguity in the occurrence of RFeS/RMnS and divergence in the degradation mechanisms of trace organic contaminants in the presence of RFeS/RMnS exist in literature. In order to improve the critical understanding of the RFeS/RMnS-mediated oxidation processes, the detection methods of RFeS/RMnS and their roles in the destruction of trace organic contaminants are reviewed with special attention to some specific problems related to the scavenger and probe selection and experimental results analysis potentially resulting in some questionable conclusions. Moreover, the influence of background constituents, such as organic matter and halides, on oxidation efficiency of RFeS/RMnS-mediated oxidation processes and formation of byproducts are discussed through their comparison with those in free radicals-dominated oxidation processes. Finally, the prospects of the RFeS/RMnS-mediated oxidation processes and the challenges for future applications are presented.

Enhanced degradation of emerging contaminants by permanganate/quinone process: Case study with bisphenol A

Permanganate (Mn(VII)) is widely used as a mild oxidant in water treatment. However, the reaction rates of some emerging contaminants with Mn(VII) are extremely low. In this study, benzoquinone (BQ), a redox mediator with the important component in dissolved organic matter (DOM), enhanced the oxidation of bisphenol A (BPA) by Mn(VII) in a wide pH range of 4.0-10.0. The redox cycle of BQ would produce semiquinone radicals, which could act as ligands to stabilize the formed Mn(III) in the system to promote the oxidation of BPA. Notably, the presence of BQ might promote the formation of MnO₂. A novel mechanism was proposed that singlet oxygen (¹O₂), Mn(III)-ligands (Mn(III)-L) and in-situ formed MnO₂ were the main contributors to accelerate BPA degradation in the Mn(VII)/BQ system. Under acidic conditions, the in-situ formed MnO₂ involved in the redox reaction and part of the Mn(IV) was reduced to Mn(III), indicating that the electron transfer of BQ promoted the formation of active Mn species and enhanced the Mn(VII) oxidation performance. Semiquinone radicals generated by BQ transformation would couple with the hydrogen substitution products of BPA to inhibit BPA self-coupling and promote the ring-opening reactions of BPA. Mn(VII)/BQ had better effect in raw water than in pure water, indicating that the Mn(VII)/BQ system has high potential for practical application. This study provided insights into the role of DOM in enhancing the Mn(VII) oxidation in water treatment.

Bisulfite activated permanganate for oxidative water decontamination

Recently, bisulfite-activated permanganate (MnO₄^-; Mn(VII)) process has attracted considerable attention as a novel class of advanced oxidation technology for destruction of organic contaminants in water. However, disputes over the underlying activation mechanism as well as reactive species generated in the Mn(VII)/bisulfite system remain for a long period due to the fairly complex chemistry involved in this system. This article aims to present a critical review on scientific development of the Mn(VII)/bisulfite system, with particular focus on the generation and contribution of various reactive intermediates. Both reactive manganese species (RMnS) (i.e., soluble Mn(III), Mn(V), and Mn(VI)) and radical species (primarily SO₄^•-) are identified as the oxidizing components responsible for enhanced degradation of organic contaminants by the Mn(VII)/bisulfite system. Bisulfite plays a dual role of being an activating agent for reactive intermediates generation and acting as a complexing agent to stabilize RMnS. Solution chemistry (e.g., the [Mn(VII)]/[bisulfite] molar ratio, solution pH, the type of contaminants, ligands, and water matrix components) greatly impacts the generation and consumption of RMnS and radicals, thus influencing the degradation kinetics and pathways of organics. Particularly, dissolved oxygen (DO) is a vital factor for driving the oxidation of organics since the absence of DO can block the generation of SO₄^•- and meantime causes the consumption of RMnS by excess SO₃^•- as a strong reductant. Interestingly, ferrate (FeO₄^2-, Fe(VI)) and hexavalent chromium (CrO₄^2-/HCrO₄^-, Cr(VI)) that are high-valent metal oxyanions analogous to Mn(VII) can be activated by bisulfite via a similar pathway (i.e. both high-valent metal-oxo intermediates and reactive radicals are involved). Furthermore, key knowledge gaps are identified and future research needs are proposed to address the potential challenges encountered in practical application of the Mn(VII)/bisulfite oxidation technology.

Overlooked environmental risks deriving from aqueous transformation of bisphenol alternatives: Integration of chemical and toxicological insights

Owing to the widespread prevalence and ecotoxicity of bisphenol alternatives such as bisphenol S, bisphenol F, and bisphenol AF, the past decade has witnessed the publication of a remarkable number of studies related to their transformation and remediation in natural waters. However, the reactivity, removal efficiency, transformation products (TPs), and mechanisms of such emerging pollutants by different treatment processes have not been well elucidated. Particularly, the transformation-driven environmental risks have been mostly overlooked. Therefore, we present a review to address these issues from chemical and toxicological viewpoints. Four degradation systems can be largely classified as catalytic persulfate (PS) oxidation, non-catalytic oxidation, photolysis and photocatalysis, and biodegradation. It was found that bisphenol alternatives possess distinct reactivities with different oxidizing species, with the highest performance for hydroxyl radicals. All systems exhibit superior elimination efficiency for these compounds. The inadequate mineralization suggests the formation of recalcitrant TPs, from which the overall reaction pathways are proposed. The combined experimental and in silico analysis indicates that many TPs have developmental toxicity, endocrine-disrupting effects, and genotoxicity. Notably, catalytic PS systems and non-catalytic oxidation result in the formation of coupling products as well as halogenated TPs with higher acute and chronic toxicity and lower biodegradability than the parent compounds. In contrast, photolysis and photocatalysis generate hydroxylated and bond-cleavage TPs with less toxicity. Overall, this review highlights the secondary environmental risks from the transformation of bisphenol alternatives by conventional and emerging treatment processes. Finally, future perspectives are recommended to address the knowledge gaps of these contaminants in aquatic ecosystems.

Computational biotransformation of polyethylene terephthalate by depolymerase: A QM/MM approach

Despite increasing environmental concerns on ever-lasting Polyethylene Terephthalate (PET), its global production is continuously growing. Effective strategies that can completely remove PET from environment are urgently desired. Here biotransformation processes of PET by one of the most effective enzymes, leaf-branch compost cutinase (LCC), were systematically explored with Molecular Dynamics and Quantum Mechanics/Molecular Mechanics approaches. We found that four concerted steps are required to complete the whole catalytic cycle. The last concerted step, deacylation, was determined as the rate-determining step with Boltzmann-weighted average barrier of 13.6 kcal/mol and arithmetic average of 16.1 ± 2.9 kcal/mol. Interestingly, unprecedented fluctuations of hydrogen bond length during LCC catalyzed transformation process toward PET were found. This fluctuation was also observed in enzyme IsPETase, indicating that it may widely exist in other catalytic triad (Ser-His-Asp) containing enzymes as well. In addition, possible features (bond, angle, dihedral angle and charge) that influence the catalytic reaction were identified and correlations between activation energies and key features were established. Our results present new insights into catalytic mechanism of hydrolases and shed light on the efficient recycling of the ever-lasting PET.

Enhanced transformation of sulfamethoxazole by birnessite in the presence of gallic acid: Kinetics and pathways

The emergence of antibiotic agents like sulfamethoxazole (SMX) in soils and surface water can cause serious threat to human and animal health. In this work, mechanisms for the promotive effect of gallic acid (GA) on the transformation of SMX by birnessite were studied. In the absence of GA, the observed pseudo-first-order reaction constants (k_obs) decreased as the pH increased from 4.0 to 8.0, in agreement with the decrease in redox potential of birnessite with increasing pH. The changes in chemical state of surface Mn atoms suggested that Mn(IV) played a major role in SMX transformation. SMX was transformed mainly by the bond cleavage of SN and SC, NH oxidation, and self- or cross-coupling reactions, with the formation of seven transformation products. The presence of GA could significantly promote the transformation of SMX, which was more pronounced at pH 6.0 comparing to the effect observed at 4.0. This promotive effect was attributed to both the addition reactions between SMX and GA quinones and the condensation reactions between -COOH of quinones and -NH₂ of SMX or its transformation products. Moreover, the addition reaction between SMX and OH coming from the GA oxidation by birnessite was also proposed, while its contribution to SMX transformation was small. Furthermore, pyrogallol and methyl gallate that do not contain electron-withdrawing substituent like -COOH group are less effective in promoting SMX transformation than GA, suggesting the electron-density of β‑carbon is key to the occurrence of addition reaction. Our results demonstrate the important role of birnessite and naturally occurring phenolic acids in abiotic transformation SMX, which will profoundly affect its mobility and bioavailability in environments.

Engineered Nanoconfinement Accelerating Spontaneous Manganese-Catalyzed Degradation of Organic Contaminants

Manganese(III/IV) oxide minerals are known to spontaneously degrade organic pollutants in nature. However, the kinetics are too slow to be useful for engineered water treatment processes. Herein, we demonstrate that nanoscale Mn₃O₄ particles under nanoscale spatial confinement (down to 3-5 nm) can significantly accelerate the kinetics of pollutant degradation, nearly 3 orders of magnitude faster compared to the same reaction in the unconfined bulk phase. We first employed an anodized aluminum oxide scaffold with uniform channel dimensions for experimental and computational studies. We found that the observed kinetic enhancement resulted from the increased surface area of catalysts exposed to the reaction, as well as the increased local proton concentration at the Mn₃O₄ surface and subsequent acceleration of acid-catalyzed reactions even at neutral pH in bulk. We further demonstrate that a reactive Mn₃O₄-functionalized ceramic ultrafiltration membrane, a more suitable scaffold for realistic water treatment, achieved nearly complete removal of various phenolic and aniline pollutants, operated under a common ultrafiltration water flux. Our findings mark an important advance toward the development of catalytic membranes that can degrade pollutants in addition to their intrinsic function as a physical separation barrier, especially since they are based on accelerating natural catalytic pathways that do not require any chemical addition.

Oxidative transformation of emerging organic contaminants by aqueous permanganate: Kinetics, products, toxicity changes, and effects of manganese products

Permanganate (Mn(VII)) has been widely studied for removal of emerging organic contaminants (EOCs) in water treatment and in situ chemical oxidation process. Studies on the reactive intermediate manganese products (e.g., Mn(III) and manganese dioxide (MnO₂)) generated from Mn(VII) reduction by EOCs in recent decades shed new light on Mn(VII) oxidation process. The present work summarizes the latest research findings on Mn(VII) reactions with a wide range of EOCs (including phenols, olefins, and amines) in detailed aspects of reaction kinetics, oxidation products, and toxicity changes, along with special emphasis on the impacts of intermediate manganese products (mainly Mn(III) and MnO₂) in-situ formed. Mn(VII) shows appreciable reactivities towards EOCs with apparent second-order rate constants (k_app) generally decrease in the order of olefins (k_app = 0.3 - 2.1 × 10⁴ M^-1s^-1) > phenols (k_app = 0.03 - 460 M^-1s^-1) > amines (k_app = 3.5 × 10^-3 - 305.3 M^-1s^-1) at neutral pH. Phenolic benzene ring (for phenols), (conjugated) double bond (for olefins), primary amine group and the N-containing heterocyclic ring (for amines) are the most reactive sites towards Mn(VII) oxidation, leading to the formation of products with different structures (e.g., hydroxylated, aldehyde, carbonyl, quinone-like, polymeric, ring-opening, nitroso/nitro and C-N cleavage products). Destruction of functional groups of EOCs (e.g., benzene ring, (conjugated) double bond, and N-containing heterocyclic) by Mn(VII) tends to decrease solution toxicity, while oxidation products with higher toxicity than parent EOCs (e.g., quinone-like products in the case of phenolic EOCs) are sometimes formed. Mn(III) stabilized by model or unknown ligands remarkably accelerates phenolic EOCs oxidation by Mn(VII) under acidic to neutral conditions, while MnO₂ enhances the oxidation efficiency of phenolic and amine EOCs by Mn(VII) at acidic pH. The intermediate manganese products participate in Mn(VII) oxidation process most likely as both oxidants and catalysts with their generation/stability/reactivity affecting by the presence of NOM, ligand, cations, and anions in water matrices. This work presents the state-of-the-art findings on Mn(VII) oxidation of EOCs, especially highlights the significant roles of manganese products, which advances our understanding on Mn(VII) oxidation and its application in future water treatment processes.

Selective Reactivity and Oxidation of Dissolved Organic Matter by Manganese Oxides

Dissolved organic matter (DOM) varies widely across natural and engineered systems, but little is known about the influence of DOM composition on its reactivity with manganese oxides. Here, we investigate bulk and molecular transformations of 30 diverse DOM samples after reaction with acid birnessite (MnO₂), a strong oxidant that may react with DOM in Mn-rich environments or engineered treatment systems. The reaction of DOM with acid birnessite reduces Mn and forms DOM that is generally more aliphatic and lower in apparent molecular weight. However, the extent of reaction depends on the water type (e.g., wastewater, rivers) and highly aromatic DOM undergoes greater changes. Despite the variability in reactivity due to the DOM composition, aqueous products attributable to the oxidation of phenolic precursors are identified in waters analyzed by high-resolution mass spectrometry. The number of matched product formulas correlates significantly with indicators of DOM aromaticity, such as double-bond equivalents (p = 2.43 × 10^-4). At the molecular level, highly aromatic, lignin-like carbon reacts selectively with acid birnessite in all samples despite the variability in initial DOM composition, resulting in the formation of a wide range of aqueous products. These findings demonstrate that DOM oxidation occurs in diverse waters but also suggest that reactivity with acid birnessite and the composition of the resulting aqueous DOM pool are composition-dependent and linked to the DOM source and initial aromaticity.

Enhanced and selective phototransformation of chlorophene on aluminum hydroxide-humic complexes

Mineral-humic complexes, known as mineral-associated organic matter (MAOM), are ubiquitous in natural waters. However, the interaction between organic pollutants and MAOM remains elusive, which may affect their degradation process. In this study, photochemical transformation of chlorophene (CP) in the presence of MAOM, prepared by coating aluminum hydroxide with humic acid (HA-HAO), was investigated. Our results showed that the degradation of CP was significantly enhanced in the presence of HA-HAO, and the degradation rate constant was ~5 times as that with HA only. It was because the adsorption of CP to HA-HAO particles was greatly enhanced, and concentration of reactive oxygen species (ROS) was increased on HA-HAO surfaces, which further promoted the reactions between CP and ROS. The quenching experiments combined with EPR technology confirmed that superoxide anion (O₂^·-) was the primary reactive radical on CP photodegradation. More importantly, the degradation of CP with HA-HAO followed a hydroxylation process, rather than the oligomerization reaction with HA only. Spectroscopic analysis provided direct evidence for the formation of hydrogen bonding between CP phenolic hydroxyl group and surface oxygen of HAO, which would suppress the reactivity of phenolic hydroxyl group, consequently the ortho- and meta-positions of CP became more facile for the hydroxylation reaction. This study shows the importance of MAOM in altering the photochemical behavior and transformation pathway of organic contaminants.

Persistent Free Radicals from Low-Molecular-Weight Organic Compounds Enhance Cross-Coupling Reactions and Toxicity of Anthracene on Amorphous Silica Surfaces under Light

Polycyclic aromatic hydrocarbon (PAH) contamination has raised great environmental concerns, while the effects of low-molecular-weight organic compounds (LMWOCs) on PAH photodegradation at amorphous silica (AS)/air interfaces have been largely ignored. In this study, the phototransformation of anthracene (ANT) at amorphous silica (AS)/air interfaces was investigated with the addition of LMWOCs. ANT removal was attributed to ^•OH attacking and the energy transfer process via ³ANT*. Light irradiation induced the fractured ≡SiO^• or ≡Si^• generation on AS surfaces, which could react with absorbed H₂O and O₂ to generate ^•OH and further yield a series of hydroxylated products of ANT. The presence of citric acid and oxalic acid improved ^•OH generation and enhanced ANT removal by 1.0- and 2.2-fold, respectively. For comparison, the presence of catechol and hydroquinone significantly decreased ANT removal and produced coupling products. The results of density functional theory calculations suggest that persistent free radicals (PFRs) on AS surfaces from catechol or hydroquinone after ^•OH attacking prefer to cross-couple with ANT via C-C bonding rather than self-couple. Dianthrone and cross-coupling products might possess higher ecotoxicity, while hydroxylated products were less ecotoxic than their parent compounds based on Ecological Structure Activity Relationships (ECOSAR) estimation. The results of this study revealed the potential ecotoxicity of PAH-adsorbed particulates coexisting with LMWOCs and also provided a new insight into PAH transformation through PFR pathways.

Transformation of bromophenols by aqueous chlorination and exploration of main reaction mechanisms

Bromophenols (BPs) are ubiquitous phenolic contaminants and typical halogenated disinfection byproducts (DBPs) that are commonly detected in aquatic environments. The transformation of 2,4-dibromophenol (2,4-DBP) during chlorination process was fully explored in this research. It was found that active chlorine can react with 2,4-DBP effectively in a wide pH range of 5.0-11.0, with an apparent second-order rate constant (k_app) varying from 0.8 M^-1 s^-1 to 110.3 M^-1 s^-1. The addition of 5 mM ammonium ions almost completely suppressed the reaction via competitive consumption of free chlorine. With the concentration of HA increasing from 1.0 to 10.0 mg L^-1, the inhibition on the degradation of 2,4-DBP increased from 8.7% to 63.4%. By contrast, bromide ions at a concentration of 5 mM accelerated the process by about 4 times, due to the formation of hypobromous acid. On the basis of the eleven products (with eight nominal masses) identified by LC-TOF-MS, electrophilic substitution reactions and single-electron transfer reactions were mainly involved in the chlorination process. The concentration of primary chlorine-substituted products was about 4 times that of the dimer products, demonstrating that electrophilic substitution reaction was predominant during chlorination of 2,4-DBP. Density functional theory (DFT) based calculations revealed that HOCl is the dominant active oxidizing species for elimination of 2,4-DBP and coupling reaction occurs more easily at para and ortho position of hydroxyl group in the phenolic moiety. These findings could provide some new insights into the environmental fate of bromophenols during chlorine disinfection of water and wastewaters.

A new insight into the mechanism of carbamazepine oxidation by MnO2: Crystalline structure versus Mn(III)

Mn(III) has been regarded as the origin of oxidative reactivity of MnO₂ recently, however this remains controvertible. Herein, carbamazepine (CBZ), a typical refractory pharmaceutical, was treated by δ-, α-, β-, and γ-MnO₂ and the role of Mn(III) was investigated. After the removal of Mn(III) by pyrophosphate washing, the δ-MnO₂ exhibited a higher kinetics rate (0.180 min^-1) than the sample before washing (0.075 min^-1). Dissolved Mn(III) in the forms of acetate-complex Mn(III), newly acid-dissolved Mn(III) from MnO₂ solid, and in-situ generated Mn(III) showed negligible oxidative reactivity towards the oxidation of CBZ. These evidenced that Mn(III) did not play a critical role in the oxidation of CBZ. The oxidative reactivity of MnO₂ with different structures for the oxidation of CBZ followed the order: δ-MnO₂ >> > α-MnO₂ ≈ γ-MnO₂ > β-MnO₂. Density functional theory calculations suggested that the crystalline plane of δ-MnO₂ significantly contributed to the oxidation of CBZ, thus leading to the superior performance of δ-MnO₂. A new surface reaction dominated mechanism was proposed, which implies that the oxidative reactivity of MnO₂ may not result from Mn(III) as previously believed. These findings could shed light on the understanding of MnO₂-involved oxidation in water treatment and natural processes.

Oxidation of Mn(III) Species by Pb(IV) Oxide as a Surrogate Oxidant in Aquatic Systems

Dissolved Mn(III) species have been recognized as a significant form of Mn in redox transition environments, but a holistic understanding of their geochemical properties still lacks the characterization of their reactivity as reductants. Through using PbO₂ as a surrogate oxidant and pyrophosphate (PP) as a model ligand, we evaluated the thermodynamic and kinetic constrains of dissolved Mn(III) oxidation under environmentally relevant pH. Without disproportionation, Mn(III) complexes could be directly oxidized by PbO₂ to produce Mn oxides. The reaction rates decreased with increasing PP:Mn(III) ratio and became negligible when the ratio was above a threshold value. Particulate manganite could also be oxidized by PbO₂ with detectable production of Pb(II). The favorability of Mn(III) oxidation by PbO₂ as a function of the PP:Mn ratio could be predicted by the stability constant of the Mn(III)-PP complex. We developed kinetic models that couple multiple pathways of Mn oxidation by PbO₂ to simulate the dynamics of Pb release, loss of dissolved Mn, as well as Mn(III) production and consumption. Beyond the context of Mn geochemistry, the interactions between Pb and various Mn species, including its trivalent forms, may also have important implications to the water quality in lead service lines within distribution systems.

Removal of 1,4-Naphthoquinone by Birnessite-Catalyzed Oxidation: Effect of Phenolic Mediators and the Reaction Pathway

This study investigated the birnessite (δ-MnO₂) catalyzed oxidative removal of 1,4-naphthoquinone (1,4-NPQ) in the presence of phenolic mediators; specifically, the kinetics of 1,4-NPQ removal under various conditions was examined, and the reaction pathway of 1,4-NPQ was verified by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The removal rate of 1,4-NPQ by birnessite-catalyzed oxidation (pH = 5) was faster in the presence of phenolic mediators with electron-donating substituents (pseudo-first-order initial stage rate constant (k₁) = 0.380-0.733 h^-1) than with electron-withdrawing substituents (k₁ = 0.071-0.244 h^-1), and the effect on the substituents showed a positive correlation with the Hammett constant (Σσ) (r² = 0.85, p < 0.001). The rate constants obtained using variable birnessite loadings (0.1-1.0 g L^-1), catechol concentrations (0.1-1.0 mM), and reaction sequences indicate that phenolic mediators are the major limiting factor for the cross-coupling reaction of 1,4-NPQ in the initial reaction stages, whereas the birnessite-catalyzed surface reaction acts as the major limiting factor in the later reaction stages. This was explained by the operation of two different reaction mechanisms and reaction products identified by LC-MS/MS.