Structure-Activity Relationships for Rates of Aromatic Amine Oxidation by Manganese Dioxide

A new perspective on contaminants as "activators": Aromatic amine groups promoted degradation of tetracycline by ferrate(VI)

Occasionally, our group found that the degradation of tetracycline by ferrate(VI) could be promoted by four co-exist contaminants, containing aromatic amines (ofloxacin, diatrizoic acid, sulfadiazine and alachlor). This study investigated the promotion of aromatic amine groups on tetracycline degradation by ferrate(VI) by using aniline as a model compound. The results implied that the presence of aniline increased the degradation rate of tetracycline by 2.76 times, and the enhancement was weakened gradually with the decrease of pH from 10 to 7.5. The generation of Fe(IV) and·OH by the reaction between ferrate(VI) and aniline was proposed to enhance the degradation of tetracycline, supported by quenching experiments, electron paramagnetic resonance (EPR) and theoretical calculations. A positive correlation was found between the rate constant of tetracycline degradation and the electron-donating ability of the substituted amines (quantified by the Hammett substituent constants). In addition, the degradation of tetracycline was remarkably inhibited by HA and some inorganic ions such as NO3-, SO42-, Cl-, Ca2+, and Mg2+, and the inhibition also happened in the Songhua River water and the secondary effluent. The present study provided an insight into the complex oxidation process for the degradation of micropollutants containing aromatic amine by ferrate in water treatment.

Characterization and rational engineering of an alkaline-tolerant azoreductase derived from Roseibium sp. H3510 for enhanced decolorization of azo dyes

rAzoR2326, an azoreductase derived from Roseibium sp. H3510, functions as an FMN-dependent homodimer utilizing NADH as cofactor. It demonstrated maximum activity at 45 °C and retained moderate activity above 50 °C, exhibiting stability from pH 7-10. Evolution and structure guided rational design of wild-type rAzoR2326 (WT) efficiently yielded 6 single-point mutants with improved thermostability and activity from a 22-variant library. Further combinatorial mutation led to mutant M20 with substantially enhanced thermostability (15-fold longer half-life at 50 °C) and activity (3.24-fold higher kcat/Km). M20 exhibited superior catalytic properties for decolorizing Allura Red compared to WT. Specifically, its decolorization capacity at pH 10.0 was 4.26-fold higher than WT. Additionally, M20 demonstrated remarkable thermostability, retaining 76.83 % decolorization activity for Allura Red after 120 min at 50 °C, whereas WT nearly lost all catalytic activity under the same conditions. Molecular dynamics simulations revealed the structural changes in M20, such as improved hydrogen bonding and a new C-H···π interaction, led to a more compact and rigid enzyme structure. This resulted in a more stable FMN-binding pocket and substrate tunnel, thereby improving the catalytic stability and activity of M20. Given its enhanced dye decolorization ability and alkaline tolerance, M20 shows promise as a biocatalyst for treating azo dye effluents.

Influence of Divalent Cation Inhibition and Dissolved Organic Matter Enhancement on Phenol Oxidation Kinetics by Manganese Oxides

Manganese oxides can oxidize organic compounds, such as phenols, and may potentially be used in passive water treatment applications. However, the impact of common water constituents, including cations and dissolved organic matter (DOM), on this reaction is poorly understood. For example, the presence of DOM can increase or decrease phenol oxidation rates with manganese oxides. Furthermore, the interactions of DOM and cations and their impact on the phenol oxidation rates have not been examined. Therefore, we investigated the oxidation kinetics of six phenolic contaminants with acid birnessite in ten whole water samples. The oxidation rate constants of 4-chlorophenol, 4-tert-octylphenol, 4-bromophenol, and phenol consistently decreased in all waters relative to buffered ultrapure water, whereas the oxidation rate of bisphenol A and triclosan increased by up to 260% in some waters. Linear regression analyses and targeted experiments demonstrated that the inhibition of phenol oxidation is largely determined by cations. Furthermore, quencher experiments indicated that radical-mediated interactions from oxidized DOM contributed to enhanced oxidation of bisphenol A. The variable changes between compounds and water samples demonstrate the challenge of accurately predicting contaminant transformation rates in environmentally relevant systems based on experiments conducted in the absence of natural water constituents.

Predicting reactivity dynamics of halogen species and trace organic contaminants using machine learning models

Reactions of reactive halogen species (Cl•, Br•, and Cl2•-) with trace organic contaminants (TrOCs) have received much attention in recent years, and their k values are fundamental parameters for understanding their reaction mechanisms. However, k values are usually unknown. In this study, we developed machine learning (ML)-based quantitative structure-activity relationship (QSAR) models to predict k values. We tested five algorithms, namely, random forest, neural network, XGBoost, support vector machine (SVM), and multilinear regression, using molecular descriptors (MDs) and molecular fingerprints (MFs) as inputs. The optimal algorithms were MD-XGBoost for Cl• and Br•, and MF-SVM for Cl2•-, respectively, with R2test values of 0.876, 0.743, and 0.853. We found that electron-withdrawing/donating groups tended to interfere with the reactivity of Cl2•- more than Cl• and Br•. This explains why MFs are better inputs for predictive models of Cl2•-, whereas MDs are more suitable for Cl• and Br•. Furthermore, we interpreted the models using SHAP analysis, and the results indicated that our models accurately predicted k values both statistically and mechanistically. Our models provide useful tools for obtaining unknown k values and help researchers understand the inherent relationships between the models.

Prediction of free radical reactions toward organic pollutants with easily accessible molecular descriptors

Machine learning (ML) is becoming an efficient tool for predicting the fate of aquatic contaminants owing to the preponderance of big data. However, whether ML can "learn" the differences in reactivity among different free radicals has not yet been tested. In this work, the effectiveness of combining ML algorithms with molecular fingerprints to predict the reactivity of three free radicals was evaluated. First, a dataset containing 211 organic pollutants and their respective rate constants with the carbonate radical (CO3•-) was used to develop predictive models using both linear regression and ML methods. The use of topological atomic alignment information, in the form of the molecular access system (MACCS) and Morgan Fingerprint, and the electronic structure features (energy levels of the lowest unoccupied and highest occupied molecular orbitals, ELUMO and EHOMO, and the energy gap between ELUMO and EHOMO) gave satisfactory predictive performances (ML model with Random Forest algorithm with MACCS: RMSEtest = 0.787; linear regression model with energy levels: RMSEtest = 0.641). Additionally, the model interpretation correctly described that the key reactivity features for CO3•- were relatively close to those for SO4•- rather than those for •OH. These results suggest that combination of ML algorithms with easily accessible molecular fingerprints would be a powerful tool to accurately predict the radical reactions towards organic compounds.

Thermodynamic and Kinetic Behaviors of Persulfate-Based Electron-Transfer Regime in Carbocatalysis

A carbon-based advanced oxidation process is featured for the nonradical electron-transfer pathway (ETP) from electron-donating organic compounds to activated persulfate complexes, enabling it as a green technology for the selective oxidation of organic pollutants in complex water environments. However, the thermodynamic and kinetic behaviors of the nonradical electron-transfer regime had been ambiguous due to a neglect of the influence of pH on the mechanisms. In this study, three kinds of organic pollutants were divided in the carbon-based ETP regime: (i) physio-adsorption, (ii) adsorption-dominated ETP (oxidation rate slightly surpasses adsorption rate), and (iii) oxidation-dominated ETP (oxidation rate outpaces the adsorption rate). The differential kinetic behaviors were attributed to the physicochemical properties of the organic pollutants. For example, the hydrophobicity, molecular radius, and positive electrostatic potential controlled the mass-transfer process of the adsorption stage of the reactants (peroxydisulfate (PDS) and organics). Meanwhile, other descriptors, including the Fukui index, oxidation potential, and electron cloud density regulated the electron-transfer processes and thus the kinetics of oxidation. Most importantly, the oxidation pathways of these organic pollutants could be altered by adjusting the water chemistry. This study reveals the principles for developing efficient nonradical systems to selectively remove and recycle organic pollutants in wastewater.

Neutral Phenolic Contaminants Are Not Necessarily More Resistant to Permanganate Oxidation Than Their Dissociated Counterparts: Importance of Proton-Coupled Electron Transfer

Despite decades of research on phenols oxidation by permanganate, there are still considerable uncertainties regarding the mechanisms accounting for the unexpected parabolic pH-dependent oxidation rate. Herein, the pH effect on phenols oxidation was reinvestigated experimentally and theoretically by highlighting the previously unappreciated proton transfer. The results revealed that the oxidation of protonated phenols occurred via proton-coupled electron transfer (PCET) pathways, which can switch from ETPT (electron transfer followed by proton transfer) to CEPT (concerted electron-proton transfer) or PTET (proton transfer followed by electron transfer) with an increase in pH. A PCET-based model was thus established, and it could fit the kinetic data of phenols oxidation by permanganate well. In contrast with what was previously thought, both the simulating results and the density functional theory calculation indicated the rate of CEPT reaction of protonated phenols with OH- as the proton acceptor was much higher than that of deprotonated phenols, which could account for the pH-rate profiles for phenols oxidation. Analysis of the quantitative structure-activity relationships among the modeled rate constants, Hammett constants, and pKa values of phenols further supports the idea that the oxidation of protonated phenols is dominated by PCET. This study improves our understanding of permanganate oxidation and suggests a new pattern of reactivity that may be applicable to other systems.

Electron-transfer-based peroxymonosulfate activation on defect-rich carbon nanotubes: Understanding the substituent effect on the selective oxidation of phenols

Nanocarbon-based persulfate oxidation technologies are promising for green elimination of phenolic pollutants. Previous studies revealed the electron transfer via defective carbon nanotube (CNTs) for selective oxidation of various phenols. However, an underlying relationship between the molecular structure of phenols and the selectivity of electron transfer-induced oxidation has not been well understood. Herein, we report that defect-rich CNTs could initiate electron-transfer regime from phenols to peroxymonosulfate (PMS), resulting in the efficient degradation of phenols. Further studies uncover a distinctive substituent group-dependent selective oxidation of phenols via the CNT-mediated electron transfer process. Specifically, the degradation rate of para-substituted phenols with electron-donating groups (e.g., -NH₂ and -OCH₃) is faster than those with electron-withdrawing groups (e.g., -NO₂ and -COOH). For a kind of substituted phenols, the substituent position has a great influence on the phenols degradation and their degradation rates follow this sequence: para > ortho > meta -position. Besides, increasing the number of the substituent group can accelerate the degradation of substituted phenols. This study elucidates the substituent effect on the electron transfer-dominated selective oxidation of phenols for the first time, which guides the application of carbon/persulfate system for the targeted remediation of phenols-polluted wastewater.

Phenoxyl mediators improve enzymatic degradation of organic pollutants: Effect and mechanism

A mediation strategy can effectively overcome the low reaction activity of enzymes with nonspecific substrates. In this study, we demonstrated how phenol compounds can mitigate the substrate limitation of HRP in catalytic degradation of various organic pollutants. In a classical HRP/H₂O₂ system, phenol and natural phenolic compounds (4-HBA & pHBA), exhibited up to over 100-fold enhancement in eliminating organic dyes and persistent antibiotics while the loading is only 2-5 wt%. A combination of molecular modelling, docking and frontier orbital energy analysis was employed to elucidate the catalytic performance and mechanism. We revealed that (1) generating phenoxyl radicals required the proximity of mediators to the HRP active centre, and (2) the subsequent efficient radical transfer to pollutants was determined by the large energy gap between the SOMO energy of phenoxyl radicals and the HOMO energy of phenols. When considering phenols as pollutants, we showed a synergistic effect on catalytic degradation of phenols, dyes, and tetracycline with a removal efficiency of 71-92 %. Overall, this work not only demonstrates that phenoxyl mediators can overcome the lower efficiency and substrate-specificity limitations of the HRP/H₂O₂ system but also revealed their structure-mediation relationship, implying great potential in the biodegradation of diverse pollutants and their mixtures.

Reusing Sulfur-Poisoned Palladium Waste as a Highly Active, Nonradical Fenton-like Catalyst for Selective Degradation of Phenolic Pollutants

Recycling of deactivated palladium (Pd)-based catalysts can not only lower the economic cost of their industrial use but also save the cost for waste disposal. Considering that the sulfur-poisoned Pd (Pd_xS_y) with a strong Pd-S bond is difficult to regenerate, here, we propose a direct reuse of such waste materials as an efficient catalyst for decontamination via Fenton-like processes. Among the Pd_xS_y materials with different poisoning degrees, Pd₄S stood out as the most active catalyst for peroxymonosulfate activation, exhibiting pollutant-degradation performance rivaling the Pd and Co²⁺ benchmarks. Moreover, the incorporated S atom was found to tune the surface electrostatic potentials and charge densities of the Pd active site, triggering a shift in catalytic pathway from surface-bound radicals to predominantly direct electron transfer pathway that favors a highly selective oxidation of phenols. The catalyst stability was also improved due to the formation of strong Pd-S bond that reduces corrosion. Our work paves a new way for upcycling of Pd-based industrial wastes and for guiding the development of advanced oxidation technologies toward higher sustainability.

In Situ-Formed Phenoxyl Radical on the CuO Surface Triggers Efficient Persulfate Activation for Phenol Degradation

Transition-metal oxide (M_xO_y)-based persulfate (PDS) activation processes have demonstrated enormous potential for pollutant degradation in water purification. However, the mechanistic insight of PDS activation by a M_xO_y catalyst concerning the mediate role of the organic substrate remains obscure. Here, we demonstrated that the in situ-formed phenoxyl radical on the CuO surface can trigger efficient persulfate activation for phenol degradation. The formation of the phenoxyl radical was an inner-sphere process, which involved the successive steps of chemisorption through surface hydroxyl group substitution and the subsequent spontaneous electron transfer reaction from adsorbed phenol to CuO. The organic substrate phenol can be oxidized by the PDS molecule and surface-bound SO₄^•- through the nonradical and free-radical pathways, respectively. Such a unique "half-radical" mechanism resulted in an extraordinarily high PDS utilization efficiency of 188.9%. More importantly, a general rule for phenoxyl radical formation was concluded; it can be formed in the cases of organic substrates with a Hammett constant σ⁺ lower than -0.02 and metal ion of a 3d subshell between half-filled and fully filled. This study clarifies the mediate role of the organic substrate for interfacial PDS activation on M_xO_y and also gives new insights into the rational design of a highly efficient M_xO_y catalyst for selective phenolic/aniline pollutant degradation in wastewater.

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.

Recent advances in the biodegradation of azo dyes

As dye demand continues to rapidly increase in the food, pharmaceutical, cosmetic, paper, textile, and leather industries, an industrialization increase is occurring. Meanwhile, the degradation and removal of azo dyes have raised broad concern regarding the hazards posed by these dyes to the ecological environment and human health. Physicochemical treatments have been applied but are hindered by high energy and economic costs, high sludge production, and chemicals handling. Comparatively, the bioremediation technique is an eco-friendly, removal-efficient, and cost-competitive method to resolve the problem. This paper provides scientific and technical information about recent advances in the biodegradation of azo dyes. It expands the biodegradation efficiency, characteristics, and mechanisms of various microorganisms containing bacteria, fungi, microalgae, and microbial consortia, which have been reported to biodegrade azo dyes. In addition, information about physicochemical factors affecting dye biodegradation has been compiled. Furthermore, this paper also sketches the recent development and characteristics of advanced bioreactors.

Ferrate(VI) Oxidation Mechanism of Substituted Anilines: A Density Functional Theory Investigation

Ferrate(VI) (Fe(VI)) is a promising oxidant coagulant and disinfectant for the degradation of organic micropollutants. However, it is hard to elucidate the detailed oxidation mechanism through the current experimental approaches. Substituted anilines (SANs) are important chemical compounds that are widely used in many industries. This paper presents the use of density functional theory (DFT) to understand the oxidation mechanism of SANs by Fe(VI) and the effect of substituents. The calculation results revealed that the primary oxidations of SANs follow the hydrogen atom transfer (HAT) mechanism. Interestingly, the hydroxyl oxygen of HFeO₄ ^- is more reactive than the carbonyl oxygen when reacting with SANs. The formation of the SAN radical is crucial, and all of the products are formed from it. Azobenzene is more favorable to generate the above products. In addition, the obtained results indicate that this kind of substituent has a much greater influence on the reaction rather than the position. Thus, the present study provides a valuable insight into the transformation pathways of SANs in the Fe(VI) oxidation process and the effects of the substituent on oxidation. These results will advance the understanding of Fe(VI) involved in wastewater treatment.

Biodegradation of azo dye-containing wastewater by activated sludge: a critical review

The effluent from the textile industry is a complex mixture of recalcitrant molecules that can harm the environment and human health. Biological treatments are usually applied for this wastewater, particularly activated sludge, due to its high efficiency, and low implementation and operation costs. However, the activated sludge microbiome is rarely well-known. In general, activated sludges are composed of Acidobacteria, Bacillus, Clostridium, Pseudomonas, Proteobacteria, and Streptococcus, in which Bacillus and Pseudomonas are highlighted for bacterial dye degradation. Consequently, the process is not carried out under optimum conditions (treatment yield). Therefore, this review aims to contextualize the potential environmental impacts of azo dye-containing wastewater from the textile industry, including toxicity, activated sludge microbiome identification, in particular using the matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) as a novel, rapid and accurate strategy for the identification of activated sludge microbiome (potential to enhance treatment yield).

Coupled Manganese Redox Cycling and Organic Carbon Degradation on Mineral Surfaces

Minerals, natural organic matter (NOM), and divalent manganese (Mn(II)) often coexist in suboxic/oxic environment. Multiple adsorption and oxidation processes occur in this ternary system, which are coupled to affect the fate of both OM and Mn therein and alter their chemical reactivity toward metals and other pollutants. However, the details about the coupling are poorly known although much has been gained for the binary systems. We determined the mutual influence of surface-catalyzed Mn(II) oxidation and humic acid (HA) adsorption and oxidation in a Fe(III) oxide (goethite)-HA-Mn(II) system at pH 5-8. The presence of Mn(II) substantially increased HA adsorption whereas HA greatly impaired the extent and rate of Mn(II) oxidation by O₂ on goethite surfaces. The impacts were more pronounced at higher pH. Mn(II) oxidation produced β-MnOOH, γ-MnOOH, and Mn₃O₄ which in turn oxidized HA, producing small organic acids. The presence of HA markedly altered the composition of Mn(II) oxidation products by inhibiting the formation of β-MnOOH while favoring the production of γ-MnOOH and Mn(II) adsorbed on the HA-mineral assemblage. Nonconducting γ-Al₂O₃ exhibited similar but weaker effects than semiconducting goethite in the above processes. Our results suggest that similar to Mn-oxidizing microorganisms, mineral surfaces can drive the coupling of the Mn redox cycle with NOM oxidative degradation under suboxic/oxic and circumneutral/alkaline conditions.

Reduction of 1,2,3-trichloropropane (TCP): pathways and mechanisms from computational chemistry calculations

The characteristic pathway for degradation of halogenated aliphatic compounds in groundwater or other environments with relatively anoxic and/or reducing conditions is reductive dechlorination. For 1,2-dihalocarbons, reductive dechlorination can include hydrogenolysis and dehydrohalogenation, the relative significance of which depends on various structural and energetic factors. To better understand how these factors influence the degradation rates and products of the lesser halogenated hydrocarbons (in contrast to the widely studied per-halogenated hydrocarbons, like trichloroethylene and carbon tetrachloride), density functional theory calculations were performed to compare all of the possible pathways for reduction and elimination of 1,2,3-trichloropropane (TCP). The results showed that free energies of each species and reaction step are similar for all levels of theory, although B3LYP differed from the others. In all cases, the reaction coordinate diagrams suggest that β-elimination of TCP to allyl chloride followed by hydrogenolysis to propene is the thermodynamically favored pathway. This result is consistent with experimental results obtained using TCP, 1,2-dichloropropane, and 1,3-dichloropropane in batch experiments with zerovalent zinc (Zn⁰, ZVI) as a reductant.

A deep neural network combined with molecular fingerprints (DNN-MF) to develop predictive models for hydroxyl radical rate constants of water contaminants

This work combined a Deep Neural Network (DNN) with molecular fingerprints (MF) to develop models to predict the OH radical rate constants of 593 organic contaminants. Molecular descriptors, most often used in establishing quantitative structural-activity relationships (QSARs), were not used here because of their complicated generation processes that rely on advanced physicochemical and computational knowledge. Instead, we only fed the most basic information of the contaminant structures, i.e., MF encoding the types of atoms and how they are connected, to DNN and DNN then developed predictive models automatically. Here, a dataset containing 457 contaminants and their OH rate constants was first used to develop predictive models by DNN-MF. The hence developed models showed comparable accuracy to the traditional QSARs. The root mean square error (RMSE) values of the test sets were 0.358-0.384. The length of 2048 bits for the MF and 3 hidden layers (each with 1024 neurons) were found to be the optimal parameters for DNN. The model containing additional 89 micorpollutants in the training set was then successfully applied to predict the OH rate constants of 17 organophosphorus flame retardants and 29 additional micropollutants, with comparable accuracy to the reported molecular descriptors-based QSARs.

Insights into the Electron-Transfer Regime of Peroxydisulfate Activation on Carbon Nanotubes: The Role of Oxygen Functional Groups

Carbon-driven advanced oxidation processes are appealing in wastewater purification because of the metal-free feature of the carbocatalysts. However, the regime of the emerging nonradical pathway is ambiguous because of the intricate carbon structure. To this end, this study was dedicated to unveil the intrinsic structure-performance relationship of peroxydisulfate (PDS) activation by carbon nanotubes (CNTs) toward nonradical oxidation of organics such as phenol (PE) via electron transfer. Eighteen analogical CNTs were synthesized and functionalized with different categories and contents of oxygen species. The quenching tests and chronopotentiometry suggest that an improved reactivity of surface-regulated CNTs was attributed to the reinforced electron-transfer regime without generation of free radicals and singlet oxygen. The quantitative structure-activity relationships were established and correlated to the Tafel equation, which unveils the nature of the nonradical oxidation by CNT-activated PDS complexes (CNT-PDS*). First, a decline in the concentration of oxygen groups in CNTs will make the zeta potential of the CNT become less negative in neutral solutions, which facilitated the adsorption of PDS because of weaker electrostatic repulsion. Then, the metastable CNT-PDS* was formed, which elevated the oxidation capacity of the CNT. Finally, PE would be oxidized over CNT-PDS* via electron transfer to fulfill the redox cycle. Moreover, the nonradical oxidation rate was uncovered to be exponentially related with the potential of the complexes, suggesting that the nonradical oxidation by the CNT-PDS* undergoes a mechanism analogous to anodic oxidation.

Mineralization Enhancement of Pharmaceutical Contaminants by Radical-Based Oxidation Promoted by Oxide-Bound Metal Ions

While the use of transition metal oxides in catalyzing advanced oxidation reactions has been widely investigated, very few reports have focused on how the preliminary contact of oxides with target compounds may affect the succession of reaction. In this study, we examined the adsorption and electron transfer reactions of two fluoroquinolones, flumequine (FLU), and norfloxacin (NOR), with goethite (α-FeOOH) or manganese (Mn) oxide, and their impact on the subsequent mineralization of target compounds using H₂O₂ or S₂O₈^2- under UVA irradiation. Intriguingly, higher total organic carbon (TOC) removal was achieved when antibiotics and metal oxides were allowed for preequilibration before starting the oxidation reaction. The rate and extent of TOC removal are strongly dependent on the molecule structure and the redox-active mineral used, and much less on the preequilibration time. This high efficiency can be ascribed to the presence of reduced metal ions, chemically or photochemically generated during the first stage, onto oxide minerals. Oxide-bound Mn^II plays a crucial role in catalyzing oxidant decomposition and then producing greater amounts of radical species through a photoassisted redox cycle, regardless of the underlying surface, Mn^IVO₂ or Mn^IIIOOH. This finding will be of fundamental and practical significance to Mn-based oxidation reactions and wastewater treatment processes.

Activation of Peroxydisulfate on Carbon Nanotubes: Electron-Transfer Mechanism

This study proposed an electrochemical technique for investigating the mechanism of nonradical oxidation of organics with peroxydisulfate (PDS) activated by carbon nanotubes (CNT). The electrochemical property of twelve phenolic compounds (PCs) was evaluated by their half-wave potentials, which were then correlated to their kinetic rate constants in the PDS/CNT system. Integrated with quantitative structure-activity relationships (QSARs), electron paramagnetic resonance (EPR), and radical scavenging tests, the nature of nonradical pathways of phenolic compound oxidation was unveiled to be an electron-transfer regime other than a singlet oxygenation process. The QSARs were established according to their standard electrode potentials, activation energy, and pre-exponential factor. A facile electrochemical analysis method (chronopotentiometry combined with chronoamperometry) was also employed to probe the mechanism, suggesting that PDS was catalyzed initially by CNT to form a CNT surface-confined and -activated PDS (CNT-PDS*) complex with a high redox potential. Then, the CNT-PDS* complex selectively abstracted electrons from the co-adsorbed PCs to initiate the oxidation. Finally, a comparison of PDS/CNT and graphite anodic oxidation under constant potentials was comprehensively analyzed to unveil the relative activity of the nonradical CNT-PDS* complex toward the oxidation of different PCs, which was found to be dependent on the oxidative potentials of the CNT-PDS* complex and the adsorbed organics.

Unusual Chemistry in an Uncatalyzed Bromate-Aniline Oscillator: Ring-Contraction Oxidation of Aniline with Pulsative CO2 Production

The bromate-aniline oscillatory reaction was discovered 4 decades ago, but neither the detailed mechanism nor the key products or intermediates of the reaction were described. We report herein a detailed study of this reaction, which yielded new insights. We found that oscillatory oxidation of aniline by acidic bromate proceeds, to a significant extent, via a novel reaction pathway with the periodic release of carbon dioxide. Several products were isolated, and their structures, not described so far, were justified on the basis of MS and NMR. One of the main products of the reaction associated with the CO₂ release route can be assigned to 2,2-dibromo-5-(phenylimino)cyclopent-3-en-1-one. A number of known compounds produced in the studied reaction, including unexpected brominated 1-phenylpyrroles and 1-phenylmaleimides, were identified by comparison with standards. A mechanism is suggested to explain the appearance of the detected compounds, based on coupling of the anilino radical with the produced 1,4-benzoquinone. We assume that the radical adduct reacts with bromine to form a cyclopropanone intermediate that undergoes a Favorskii-type rearrangement. Further oxidation and bromination steps including decarboxylation lead to the found brominated phenyliminocyclopentenones. The detected derivatives of 1-phenylpyrrole could be produced by a one-electron oxidation of a proposed intermediate 2-phenylamino-5-bromocyclopenta-1,3-dien-1-ol followed by β-scission with the abstraction of carbon monoxide. Such a mechanism is known from the combustion chemistry of cyclopentadiene. The proposed mechanism of this reaction provides a framework for understanding the observed oscillatory kinetics.

Mechanism-based structure-activity relationship (SAR) analysis of aromatic amines and nitroaromatics carcinogenicity via statistical analyses based on CPDB

Cancer is a leading cause of human mortality around the globe. In this study, mechanism-based SAR (Structure-Activity Relationship) was employed to investigate the carcinogenicity of aromatic amines and nitroaromatics based on CPDB. Principal component analysis and cluster analysis were used to construct the SAR model. Principle component analysis generated three principal components from 12 mechanism-based descriptors. The extracted principal components were later used for cluster analysis, which divided the selected 55 chemicals into six clusters. The three principal components were proposed to describe the "transport", "reactivity" and "electrophilicity" properties of the chemicals. Cluster analysis indicated that the relevant "transport" properties positively correlated with the carcinogenic potential and were contributing factors in determining the carcinogenicity of the studied chemicals. The mechanism-based SAR analysis suggested the electron donating groups, electron withdrawing groups and planarity are significant factors in determining the carcinogenic potency for studied aromatic compounds. The present study may provide insights into the relationship between the three proposed properties and the carcinogenesis of aromatic amines and nitroaromatics.

Quantitative structure-activity relationship for the oxidation of aromatic organic contaminants in water by TAML/H2O2

Tetra-amido macrocyclic ligand (TAML) activator is a functional analog of peroxidase enzymes, which activates hydrogen peroxide (H₂O₂) to form high valence iron-oxo complexes that selectively degrade persistent aromatic organic contaminants (ACs) in water. Here, we develop quantitative structure-activity relationship (QSAR) models based on measured pseudo first-order kinetic rate coefficients (k_obs) of 29 ACs (e.g., phenols and pharmaceuticals) oxidized by TAML/H₂O₂ at neutral and basic pH values to gain mechanistic insight on the selectivity and pH dependence of TAML/H₂O₂ systems. These QSAR models infer that electron donating ability (E_HOMO) is the most important AC characteristic for TAML/H₂O₂ oxidation, pointing to a rate-limiting single-electron transfer (SET) mechanism. Oxidation rates at pH 7 also depend on AC reactive indices such as f_min^- and qH⁺, which respectively represent propensity for electrophilic attack and the most positive net atomic charge on hydrogen atoms. At pH 10, TAML/H₂O₂ is more reactive towards ACs with a lower hydrogen to carbon atoms ratio (#H:C), suggesting the significance of hydrogen atom abstraction. In addition, lnk_obs of 14 monosubstituted phenols is negatively correlated with Hammett constants (σ) and exhibits similar sensitivity to substituent effects as horseradish peroxidase. Although accurately predicting degradation rates of specific ACs in complex wastewater matrices could be difficult, these QSAR models are statistically robust and help predict both relative degradability and reaction mechanism for TAML/H₂O₂-based treatment processes.

Oxidation of reduced daughter products from 2,4-dinitroanisole (DNAN) by Mn(IV) and Fe(III) oxides

Abiotic transformation of anthropogenic compounds by redox-active metal oxides affects contaminant fate in soil. The capacity of birnessite and ferrihydrite to oxidize the insensitive munitions compound, 2,4-dinitroanisol (DNAN), and its amine-containing daughter products, 2-methoxy-5-nitro aniline (MENA) and 2,4-diaminoanisole (DAAN), was studied in stirred reactors at controlled pH (7.0). Aqueous suspensions were reacted at metal oxide solid to solution mass ratios (SSR) of 0.15, 1.5 and 15 g kg^-1 and solutions were analyzed after 0-3 h by high performance liquid chromatography coupled with photodiode array or mass spectrometry detection. Results indicate that DNAN was resistant to oxidation by birnessite and ferrihydrite. Ferrihydrite did not oxidize MENA, but MENA was susceptible to rapid oxidation by birnessite, with nitrogen largely mineralized to nitrite. This is the first report on mineralization of nonphenolic aromatics and the release of mineralized N from aromatic amines following reaction with birnessite. DAAN was oxidized by both solids, but ca. ten times higher rate was observed with birnessite as compared to ferrihydrite at an SSR of 1.5 g kg^-1. At 15 g kg^-1 SSR, DAAN was removed from solution within 5 min of reaction with birnessite. CO_2(g) evolution experiments indicate mineralization of 15 and 12% of the carbon associated with MENA and DAAN, respectively, under oxic conditions with birnessite at SSR of 15 g kg^-1. The results taken as a whole indicate that initial reductive (bio)transformation products of DNAN are readily oxidized by birnessite. The oxidizability of the reduced DNAN products was increased with progressive (bio)reduction as reflected by impacts on the oxidation rate.

Transformation of Methylparaben by aqueous permanganate in the presence of iodide: Kinetics, modeling, and formation of iodinated aromatic products

This work investigated impacts of iodide (I^-) on the transformation of the widely used phenolic preservative methylparaben (MeP) as well as 11 other phenolic compounds by potassium permanganate (KMnO₄). It was found that KMnO₄ showed a low reactivity towards MeP in the absence of I^- with apparent second-order rate constants (k_app) ranging from 0.065 ± 0.0071 to 1.0 ± 0.1 M^-1s^-1 over the pH range of 5-9. The presence of I^- remarkably enhanced the transformation rates of MeP by KMnO₄ via the contribution of hypoiodous acid (HOI) in situ formed, which displayed several orders of magnitude higher reactivity towards MeP than KMnO₄. This enhancing effect of I^- was greatly influenced by solution conditions (e.g., I^- or KMnO₄ concentration or pH), which could be well simulated by a kinetic model involving competition reactions (i.e., KMnO₄ with I^-, KMnO₄ with MeP, HOI with KMnO₄, and HOI with MeP). Similar enhancing effect of I^- on the transformation kinetics of 5 other selected phenols (i.e., p-hydroxybenzoic acid, phenol, and bromophenols) at pH 7 was also observed, but not in the cases of bisphenol A, triclosan, 4-n-nonylphenol, and cresols. This discrepancy could be well explained by the relative reactivity of KMnO₄ towards phenols vs I^-. Liquid chromatography-tandem mass spectrometry analysis showed that iodinated aromatic products and/or iodinated quinone-like product were generated in the cases where I^- enhancing effect was observed. Evolution of iodinated aromatic products generated from MeP (10 μM) treated by KMnO₄ (50-150 μM) in the presence of I^- (5-15 μM) suggested that higher I^- or moderate KMnO₄ concentration or neutral pH promoted their formation. A similar enhancing effect of I^- (1 μM) on the transformation of MeP (1 μM) by KMnO₄ (12.6 μM) and formation of iodinated aromatic products were also observed in natural water. This work demonstrates an important role of I^- in the transformation kinetics and product formation of phenolic compounds by KMnO₄, which has great implications for future applications of KMnO₄ in treatment of I^--containing water.

Recent advancements in bioremediation of dye: Current status and challenges

The rampant industrialization and unchecked growth of modern textile production facilities coupled with the lack of proper treatment facilities have proliferated the discharge of effluents enriched with toxic, baleful, and carcinogenic pollutants including dyes, heavy metals, volatile organic compounds, odorants, and other hazardous materials. Therefore, the development of cost-effective and efficient control measures against such pollution is imperative to safeguard ecosystems and natural resources. In this regard, recent advances in biotechnology and microbiology have propelled bioremediation as a prospective alternative to traditional treatment methods. This review was organized to address bioremediation as a practical option for the treatment of dyes by evaluating its performance and typical attributes. It further highlights the current hurdles and future prospects for the abatement of dyes via biotechnology-based remediation techniques.

Oxidation Kinetics of Bromophenols by Nonradical Activation of Peroxydisulfate in the Presence of Carbon Nanotube and Formation of Brominated Polymeric Products

This work demonstrated that bromophenols (BrPs) could be readily oxidized by peroxydisulfate (PDS) activated by a commercial carbon nanotube (CNT), while furfuryl alcohol (a chemical probe for singlet oxygen (¹O₂)) was quite refractory. Results obtained by radical quenching experiments, electron paramagnetic resonance spectroscopy, and Fourier transform infrared spectroscopy further confirmed the involvement of nonradical PDS-CNT complexes rather than ¹O₂. Bicarbonate and chloride ion exhibited negligible impacts on BrPs degradation by the PDS/CNT system, while a significant inhibitory effect was observed for natural organic matter. The oxidation of BrPs was influenced by solution pH with maximum rates occurring at neutral pH. Linear free energy relationships (LFERs) were established between the observed pseudo-first-order oxidation rates of various substituted phenols and the classical descriptor variables (i.e., Hammett constant σ⁺, and half-wave oxidation potential E_1/2). Products analyses by liquid chromatography tandem mass spectrometry clearly showed the formation of hydroxylated polybrominated diphenyl ethers and hydroxylated polybrominated biphenyls on CNT surface. Their formation pathway possibly involved the generation of bromophenoxyl radicals from BrPs one-electron oxidation and their subsequent coupling reactions. These results suggest that the novel nonradical PDS/CNT oxidation technology is a good alternative for selectively eliminating BrPs with alleviating toxic byproducts in treated water effluent.

A novel model to predict gas-phase hydroxyl radical oxidation kinetics of polychlorinated compounds

In this study, a novel model based on aromatic meta-substituent grouping was presented to predict the second-order rate constants (k) for OH oxidation of PCBs in gas-phase. Since the oxidation kinetics are dependent on the chlorination degree and position, we hypothesized that it may be more accurate for k value prediction if we group PCB congeners based on substitution positions (i.e., ortho (o), meta (m), and para (p)). To test this hypothesis, we examined the correlation of polarizability (α), a quantum chemical based descriptor for k values, with an empirical Hammett constant (σ⁺) on each substitution position. Our result shows that α is highly linearly correlated to ∑σ_o,m,p⁺ based on aromatic meta-substituents leading to the grouping based predictive model. With the new model, the calculated k values exhibited an excellent agreement with experimental measurements, and greater predictive power than the quantum chemical based quantitative structure activity relationship (QSAR) model. Further, the relationship of α and ∑σ_o,m,p⁺ for PCDDs congeners, together with highest occupied molecular orbital (HOMO) distribution, were used to validate the aromatic meta-substituent grouping method. This newly developed model features a combination of good predictability of quantum chemical based QSAR model and simplicity of Hammett relationship, showing a great potential for fast and computational tractable prediction of k values for gas-phase OH oxidation of polychlorinated compounds.

In silico environmental chemical science: properties and processes from statistical and computational modelling

Quantitative structure-activity relationships (QSARs) have long been used in the environmental sciences. More recently, molecular modeling and chemoinformatic methods have become widespread. These methods have the potential to expand and accelerate advances in environmental chemistry because they complement observational and experimental data with "in silico" results and analysis. The opportunities and challenges that arise at the intersection between statistical and theoretical in silico methods are most apparent in the context of properties that determine the environmental fate and effects of chemical contaminants (degradation rate constants, partition coefficients, toxicities, etc.). The main example of this is the calibration of QSARs using descriptor variable data calculated from molecular modeling, which can make QSARs more useful for predicting property data that are unavailable, but also can make them more powerful tools for diagnosis of fate determining pathways and mechanisms. Emerging opportunities for "in silico environmental chemical science" are to move beyond the calculation of specific chemical properties using statistical models and toward more fully in silico models, prediction of transformation pathways and products, incorporation of environmental factors into model predictions, integration of databases and predictive models into more comprehensive and efficient tools for exposure assessment, and extending the applicability of all the above from chemicals to biologicals and materials.