Enhanced Treatment of Municipal Wastewater Effluents by Fe-TAML/H2O2: Efficiency of Micropollutant Abatement

Synergy of F-Fe dual sites in KFeF3 promoting Fenton-like cycle and refractory organics degradation through direct electron transfer

The electron cycling of single active site on catalyst was commonly restrict in Fenton-like reactions, thus limits the degradation of refractory organics in water treatment. In this work, a novel strategy for inducing efficient Fenton-like reaction through F-Fe dual sites on the surface of perovskite fluoride KFeF3 was practiced. The KFeF3 and a series of perovskite fluoride material were synthesized by simple hydrothermal method. The KFeF3 exhibited very high performance for remove of multiple refractory organics and chemical oxygen demand in lignin wastewater. Rhodamine B and phenol could be completely removed within 2 s and 16 s respectively and the mineralization rate of phenol reached ∼90 % within 5 min. Detection of ROS and in situ analysis combining density functional theory calculation of the interface proved that the rapid degradation of phenol was attribute to the synergistic effect of F-Fe dual sites and degradation pathway through direct electron transfer. F site serves as the activation site of H2O2 and reduce H2O2 to form OH, whereas, OH attacking was not the dominating pathway for degradation. The redox reaction between F site and H2O2 triggered the direct electron transfer from the Fe-phenol (or hydroxylation intermediates of phenol) complex to F site thus avoided the passivation of Fe site and accelerated Fenton-like cycle. This work revealed a fire-new mechanism of Fenton-like reaction and was expected to impulse treatment of refractory wastewater.

Regulation of phenol oxidation into polymeric derivatives ready for flocculation using polyaluminum chloride

Phenol, a toxic compound commonly found in wastewater, can be removed using the iron-tetraamidomacrocyclic ligand (Fe-TAML) and H2O2. However, it incurs high costs for Fe-TAML and H2O2, while treated water retains high chemical oxygen demand (COD) and CO2 emissions. To address these challenges, this study proposes converting phenol into polymeric derivatives followed by flocculation. Mass spectrometry (MS) reveals that phenol polymerization precedes polyphenol oxidation in the reaction, with slower reactions favoring phenol polymerization over polyphenol oxidation. It further demonstrates that reducing Fe-TAML dosage can slow down the reaction, thereby increasing the formation of polymeric derivatives at pH 10. Subsequent flocculation with polyaluminum chloride (PAC) effectively precipitates these products. When phenol concentration increases from 100 to 2500 ppm (mass ratio of H2O2 : phenol : PAC = 10 : 10 : 1), COD rises from 10% to 19%, while CO2 emissions decrease by over 45%. Meanwhile, the cost is reduced from 4.616 to 3.416 $ per kg phenol, as the Fe-TAML/phenol mass ratio decreases from 0.08% to 0.056%. Overall, this strategy is more cost-effective than conventional methods, requiring less Fe-TAML and H2O2 while significantly reducing COD and CO2 emissions.

Ozone-based advanced oxidation processes in water treatment: recent advances, challenges, and perspective

Water pollution, driven by a variety of enduring contaminants, poses considerable threats to ecosystems, human health, and biodiversity, highlighting the urgent need for innovative and sustainable treatment approaches. Ozone-based advanced oxidation processes (AOPs) have demonstrated significant efficacy in breaking down stubborn pollutants, such as organic micropollutants and pathogens, that are not easily addressed by traditional treatment techniques. This review offers an in-depth analysis of ozonation mechanisms, covering both the direct oxidation by ozone and the indirect reactions facilitated by hydroxyl radicals, emphasizing their effectiveness and adaptability across various wastewater matrices. Significant progress in the combination of ozonation with additional technologies, including UV irradiation, hydrogen peroxide (H₂O₂), catalytic systems, and biological treatments, is examined, highlighting their effectiveness in enhancing pollutant breakdown, increasing biodegradability, and reducing secondary pollution. Hybrid methods, including catalytic ozonation and ozone-biological treatment, show significant enhancements in process efficiency and cost-effectiveness, while effectively tackling challenges associated with energy use and byproduct generation. Despite the promising possibilities, obstacles remain, such as scalability issues, high operational costs, and the risk of generating potentially harmful transformation products. Cutting-edge approaches, including the creation of sophisticated catalysts, integration of processes, and refinement of reactor designs, are suggested to address these challenges and improve the real-world implementation of ozone-based advanced oxidation processes. This review highlights the significant potential of ozone-based advanced oxidation processes as sustainable approaches for wastewater treatment, providing an essential route to environmental conservation and safeguarding public health.

Hyper-Expandable Cross-Linked Protein Crystals as Scaffolds for Catalytic Reactions

Scaffolding catalytic reactions within porous materials is a powerful strategy to enhance the reaction rates of multicatalytic systems. However, it remains challenging to develop materials with high porosity, high diversity of functional groups within the pores, and guest-adaptive tunability. Furthermore, it is challenging to capture large catalysts such as enzymes within porous materials. Protein-based materials are promising candidates to overcome these limitations, owing to their large pore sizes and potential for stimuli-responsive adaptability. In this work, hydrogel beads were generated from cross-linked lysozyme crystals. These swellable lysozyme cross-linked crystals (SLCCs) expand more than 10 mL per gram of crystal following a simple treatment in ethanol, followed by the addition of water. SLCCs are sensitive to the solution environment and change their extent of swelling from adjusting the concentration and identity of the ions in the solution, or by changing the flexibility of the protein backbone, such as adding dithiothreitol to reduce the protein disulfide bonds. SLCCs can adsorb a wide range of catalysts ranging from transition metal complexes to large biomacromolecules, such as the 160 kDa enzyme glucose oxidase (GOx). Transition metal catalysts and enzymes captured within SLCCs maintained their catalytic activity and exhibited minimal leaching. We performed a cascade reaction by adsorbing GOx and the transition metal catalyst Fe-TAML into SLCCs, resulting in enhanced activity compared to a free-floating reaction. SLCCs offer a promising combination of attributes as scaffolds for multicatalytic reactions, including gram-scale batch preparation, tunable expansion to greater than 20-fold in volume, guest-responsive adaptable behavior, and facile capture of a wide array of small molecule and enzyme-catalysts.

Boosting singlet oxygen generation for salinity wastewater treatment through co-activation of oxygen and peroxymonosulfate in photoelectrochemical process

High concentrations of inorganic ions in saline wastewater pose adverse effects on hydroxyl radical (HO•)-dominated technologies. Here, we report a unique strategy for boosting singlet oxygen (1O2) generation via coactivation of oxygen and peroxymonosulfate (PMS) by regulating the electron transfer regime in the photoelectrochemical process. The Fe-N bridge in atomic Fe-modified graphitic carbon nitride (denoted SA-FeCN) favors the construction of electron-defective Fe and electron-rich N vacancies (Nvs) to accelerate directional electron transfer. The produced intermediate (HSO4-O···Fe-Nvs···O-O) as a chemical channel accelerates the directional electron transfer from PMS to further reduce O2 to form activated products (SO5 •-, O2 •-), thereby transforming O2 into 1O2. An optimized 1O2 generation rate of 39.4 μmol L - 1 s - 1 is obtained, which is 15.7-945.0 times higher than that in traditional advanced oxidation processes. Fast kinetics are achieved for removing various phenolic pollutants in a nonradical oxidation pathway, which is less susceptible to the coexistence of natural organic matter and inorganic ions. The COD removal for coal wastewater and complex industrial wastewater in real scenarios is found to reach a value of 90%-96% in 3 h. This work provides a new direction for boosting the 1O2 generation rate, especially for the selective degradation of target electron-rich contaminants in saline wastewater.

Overlooked Complexation and Competition Effects of Phenolic Contaminants in a Mn(II)/Nitrilotriacetic Acid/Peroxymonosulfate System: Inhibited Generation of Primary and Secondary High-Valent Manganese Species

Organic contaminants with lower Hammett constants are typically more prone to being attacked by reactive oxygen species (ROS) in advanced oxidation processes (AOPs). However, the interactions of an organic contaminant with catalytic centers and participating ROS are complex and lack an in-depth understanding. In this work, we observed an abnormal phenomenon in AOPs that the degradation of electron-rich phenolics, such as 4-methoxyphenol, acetaminophen, and 4-presol, was unexpectedly slower than electron-deficient phenolics in a Mn(II)/nitrilotriacetic acid/peroxymonosulfate (Mn(II)/NTA/PMS) system. The established quantitative structure-activity relationship revealed a volcano-type dependence of the degradation rates on the Hammett constants of pollutants. Leveraging substantial analytical techniques and modeling analysis, we concluded that the electron-rich phenolics would inhibit the generation of both primary (Mn(III)NTA) and secondary (Mn(V)NTA) high-valent manganese species through complexation and competition effects. Specifically, the electron-rich phenolics would form a hydrogen bond with Mn(II)/NTA/PMS through outer-sphere interactions, thereby reducing the electrophilic reactivity of PMS to accept the electron transfer from Mn(II)NTA, and slowing down the generation of reactive Mn(III)NTA. Furthermore, the generated Mn(III)NTA is more inclined to react with electron-rich phenolics than PMS due to their higher reaction rate constants (8314 ± 440, 6372 ± 146, and 6919 ± 31 M-1 s-1 for 4-methoxyphenol, acetaminophen, and 4-presol, respectively, as compared with 671 M-1 s-1 for PMS). Consequently, the two-stage inhibition impeded the generation of Mn(V)NTA. In contrast, the complexation and competition effects are insignificant for electron-deficient phenolics, leading to declined reaction rates when the Hammett constants of pollutants increase. For practical applications, such complexation and competition effects would cause the degradation of electron-rich phenolics to be more susceptible to water matrixes, whereas the degradation of electron-deficient phenolics remains largely unaffected. Overall, this study elucidated the intricate interaction mechanisms between contaminants and reactive metal species at both the electronic and kinetic levels, further illuminating their implications for practical treatment.

Amperometry for real-time and on-site monitoring of phenol and H2O2 during the treatments

In conventional wastewater treatment processes, a predetermined quantity of chemicals is introduced at the onset, without ongoing monitoring of the treatment progress. Thus, it is difficult to perform timely intervention in the treatment process. Herein, we develop an amperometry-guided wastewater treatment strategy based on a green oxidation process with H2O2 and an iron-tetraamidomacrocyclic ligand (Fe-TAML) catalyst. During the process, users can monitor both phenol and H2O2 concentrations in real time and then intervene by adding more H2O2 to accelerate the reaction. As a proof of concept, a wastewater sample containing 9.3 ppm of phenol is treated by using the amperometry-guided strategy with 1 dosage of Fe-TAML (0.45 ppm) and 3 dosages of H2O2 (1.86 ppm). After the treatment, phenol concentration in the wastewater decreases to 0 ppm after 21 min. In contrast, with only 1 dosage of Fe-TAML (0.45 ppm) and 1 dosage of H2O2 (1.86 ppm), the reaction slows down after 5 min and stops prematurely. After that, the reaction kinetics of ppb-level phenol are investigated, in which the phenol rate and the rate constant are estimated. Compared to conventional detections, the designed amperometry shows faster response, lower limit of detection (LOD, phenol: 11 ppb, H2O2: 80 ppb) and consumable cost, easier operation, and no pollution generated. This example demonstrates the importance of early intervention during wastewater treatment with the help of real-time information.

Current trends in textile wastewater treatment-bibliometric review

A bibliometric study using 1992 to 2021 database of the Science Citation Index Expanded was carried out to identify which are the current trends in textile wastewater treatment research. The study aimed to analyze the performance of scholarly scientific communications in terms of yearly publications/citations, total citations, scientific journals, and their categories in the Web of Sciences, top institutions/countries and research trends. The annual publication of scientific articles fluctuated in the first ten years, with a steady decrease for the last twenty years. An analysis of the most common terms used in the authors' keywords, publications' titles, and KeyWords Plus was carried out to predict future trends and current research priorities. Adsorbent nanomaterials would be the future of wastewater treatment for decoloration of the residual dyes in the wastewater. Membranes and electrolysis are important to demineralize textile effluent for reusing wastewater. Modern filtration techniques such as ultrafiltration and nanofiltration are advanced membrane filtration applications.

Packed OV-SnO2-Sb bead-electrodes for enhanced electrocatalytic oxidation of micropollutants in water

Electrocatalytic oxidation is an appealing treatment option for emerging micropollutants in wastewater, however, the limited reactive surface area and short service lifetime of planar electrodes hinder their industrial applications. This study introduces an innovative electrochemical wastewater treatment technology that employs packed bead-electrodes (PBE) as a dynamic electrocatalytic filter on a dimensionally stable anode (DSA) acting as a current collector. By using PBE, the electroactive volume is expanded beyond the vicinity of the common planar anode to the thick porous media of PBE with a vast electrocatalytic surface area. This greatly enhances the efficiency of electrochemical degradation of micropollutants. The OV-SnO2-Sb PBE filter achieved a nearly 100 % degradation of moxifloxacin (MOX) in under 2 min of single-pass filtration, with a degradation rate over an order of magnitude higher than the conventional electrochemical oxidation processes. The generation of abundant radical species (•OH) and non-radical species (1O2 and O3), along with the enhanced direct oxidation, led to the outstanding performance of the charged PBE system in MOX degradation. The OV-SnO2-Sb PBE was remarkably stable, and the separation between the electroactive PBE layer and the base Ti anode allows for easy renewal of the bead-electrode materials and scaling up of the system for practical applications. Overall, our study presents a dynamic electroactive PBE that advances the electrocatalytic oxidation technology for effective control of emerging pollutants in the water environment. This technology has the potential to revolutionize electrochemical wastewater treatment and contribute to a more sustainable future environment.

Thin Zinc Oxide Layer Passivating Bismuth Vanadate for Selective Photoelectrochemical Water Oxidation to Hydrogen Peroxide

Selective photoelectrochemical (PEC) water oxidation to hydrogen peroxide is an underexplored option as opposed to the mainstream oxygen reduction reaction. Albeit interesting, selective H2 O2 production via oxidative pathway is plagued by the noncontrollable two-electron transfer reaction and the overoxidation of the thus-formed H2 O2 to O2 . Here, ZnO passivator-coated BiVO4 photoanode is reported for selective PEC H2 O2 production. Both the H2 O2 selectivity and production rate increase in the range of 1.0-2.0 V versus RHE under simulated sunlight irradiation. The photoelectrochemical impedance spectra and open-circuit potentials suggest a flattened band bending and positively shifted quasi-Fermi level of BiVO4 upon ZnO coating, facilitating H2 O2 generation and suppressing the competitive reaction of O2 evolution. The ZnO overlayer also inhibits H2 O2 decomposition, accelerates charge extraction from BiVO4 , and serves as a hole reservoir under photoexcitation. This work offers insights into surface states and the role of the coating layer in manipulating two/four-electron transfer for selective H2 O2 synthesis from PEC water oxidation.

Selective Removal of Emerging Organic Contaminants from Water Using Electrogenerated Fe(IV) and Fe(V) under Near-Neutral Conditions

Fe(IV) and Fe(V) are promising oxidants for the selective removal of emerging organic contaminants (EOCs) from water under near-neutral conditions. The Fe(III)-assisted electrochemical oxidation system with a BDD anode (Fe(III)-EOS-BDD system) has been employed to generate Fe(VI), while the generation and contributions of Fe(IV) and Fe(V) have been largely ignored. Thus, we examined the feasibility and involved mechanisms of the selective degradation of EOCs in the Fe(III)-EOS-BDD system under near-neutral conditions. It was found that Fe(III) application selectively accelerated the electro-oxidation of phenolic and sulfonamide organics and made the oxidation system be resistant to interference from Cl^-, HCO₃^-, and humic acid. Several lines of evidence indicated that EOCs were decomposed via direct electron-transfer process on the BDD anode and by Fe(IV) and Fe(V) but not Fe(VI), besides HO^•. Fe(VI) was not generated until the exhaustion of EOCs. Furthermore, the overall contributions of Fe(IV) and Fe(V) to the oxidation of phenolic and sulfonamide organics were over 45%. Our results also revealed that Fe(III) was oxidized primarily by HO^• to Fe(IV) and Fe(V) in the Fe(III)-EOS-BDD system. This study advances the understanding of the roles of Fe(IV) and Fe(V) in the Fe(III)-EOS-BDD system and provides an alternative for utilizing Fe(IV) and Fe(V) under near-neutral conditions.

An extra-chelator-free fenton process assisted by electrocatalytic-induced in-situ pollutant carboxylation for target refractory organic efficient treatment in chemical-industrial wastewater

For traditional Fenton processes, the quenching behavior of radical contenders (e.g., most aliphatic hydrocarbons) on hydroxyl radicals (·OH) usually hinders the removal of target refractory pollutants (aromatic/heterocyclic hydrocarbons) in chemical industrial wastewater, leading to excess energy consumption. Herein, we proposed an electrocatalytic-assisted chelation-Fenton (EACF) process, with no extra-chelator addition, to significantly enhance target refractory pollutant (pyrazole as a representative) removal under high ·OH contender (glyoxal) levels. Experiments and theoretical calculations proved that superoxide radical (·O₂^-) and anodic direct electron transfer (DET) effectively converted the strong ·OH-quenching substance (glyoxal) to a weak radical competitor (oxalate) during the electrocatalytic oxidation process, promoting Fe²⁺ chelation and therefore increasing radical utilization for pyrazole degradation (reached maximum of ∼43-fold value upon traditional Fenton), which appeared more obviously in neutral/alkaline Fenton conditions. For actual pharmaceutical tailwater treatment, the EACF achieved 2-folds higher oriented-oxidation capability and ∼78% lower operation cost per pyrazole removal than the traditional Fenton process, demonstrating promising potential for future practical applications.

Oxidation of Pharmaceuticals by Ferrate(VI)-Amino Acid Systems: Enhancement by Proline

The occurrence of micropollutants in water threatens public health and ecology. Removal of micropollutants such as pharmaceuticals by a green oxidant, ferrate(VI) (FeVIO42-, Fe(VI)) can be accomplished. However, electron-deficient pharmaceuticals, such as carbamazepine (CBZ) showed a low removal rate by Fe(VI). This work investigates the activation of Fe(VI) by adding nine amino acids (AA) of different functionalities to accelerate the removal of CBZ in water under mild alkaline conditions. Among the studied amino acids, proline, a cyclic AA, had the highest removal of CBZ. The accelerated effect of proline was ascribed by demonstrating the involvement of highly reactive intermediate Fe(V) species, generated by one-electron transfer by the reaction of Fe(VI) with proline (i.e., Fe(VI) + proline → Fe(V) + proline•). The degradation kinetics of CBZ by a Fe(VI)-proline system was interpreted by kinetic modeling of the reactions involved that estimated the rate of the reaction of Fe(V) with CBZ as (1.03 ± 0.21) × 106 M-1 s-1, which was several orders of magnitude greater than that of Fe(VI) of 2.25 M-1 s-1. Overall, natural compounds such as amino acids may be applied to increase the removal efficiency of recalcitrant micropollutants by Fe(VI).

Enhanced Direct Electron Transfer Mediated Contaminant Degradation by Fe(IV) Using a Carbon Black-Supported Fe(III)-TAML Suspension Electrode System

Iron complexes of tetra-amido macrocyclic ligands (Fe-TAML) are recognized to be effective catalysts for the degradation of a wide range of organic contaminants in homogeneous conditions with the high valent Fe(IV) and Fe(V) species generated on activation of the Fe-TAML complex by hydrogen peroxide (H₂O₂) recognized to be powerful oxidants. Electrochemical activation of Fe-TAML would appear an attractive alternative to H₂O₂ activation, especially if the Fe-TAML complex could be attached to the anode, as this would enable formation of high valent iron species at the anode and, importantly, retention of the valuable Fe-TAML complex within the reaction system. In this work, we affix Fe-TAML to the surface of carbon black particles and apply this "suspension anode" process to oxidize selected target compounds via generation of high valent iron species. We show that the overpotential for Fe(IV) formation is 0.17 V lower than the potential required to generate Fe(IV) electrochemically in homogeneous solution and also show that the stability of the Fe(IV) species is enhanced considerably compared to the homogeneous Fe-TAML case. Application of the carbon black-supported Fe-TAML suspension anode reactor to degradation of oxalate and hydroquinone with an initial pH value of 3 resulted in oxidation rate constants that were up to three times higher than could be achieved by anodic oxidation in the absence of Fe-TAML and at energy consumptions per order of removal substantially lower than could be achieved by alternate technologies.

Mechanistic Insights into the Markedly Decreased Oxidation Capacity of the Fe(II)/S2O82- Process with Increasing pH

The poor oxidation capacity of the Fe(II)/S₂O₈^2- [Fe(II)/PDS] system at pH > 3.0 has limited its wide application in water treatment. To unravel the underlying mechanism, this study systematically evaluated the possible influencing factors over the pH range of 1.0-8.0 and developed a mathematical model to quantify these effects. Results showed that ∼82% of the generated Fe(IV) could be used for pollutant degradation at pH 1.0, whereas negligible Fe(IV) contribution was observed at pH 7.5. This dramatic decline of Fe(IV) contribution with increasing pH dominantly accounted for the pH-dependent performance of the Fe(II)/PDS process. Unexpectedly, Fe(II) could consume ∼80% of the generated SO₄^•- non-productively under both acidic and near-neutral conditions, while the larger formation of Fe(III) precipitates at high pH inhibited the SO₄^•- contribution mildly. Moreover, the strong Fe(II) scavenging effect was difficult to be compensated for by slowing down the Fe(II) dosing rate. The competition of dissolved oxygen with PDS for Fe(II) was insignificant at pH ≤ 7.5, where the second-order rate constants for reactions of Fe(II) with oxygen were much lower than or comparable to that between Fe(II) and PDS. These findings could advance our understanding of the chemistry and application of the Fe(II)/PDS process.

Oxidation of organic micropollutant surrogate functional groups with peracetic acid activated by aqueous Co(II), Cu(II), or Ag(I) and geopolymer-supported Co(II)

Peracetic acid (PAA) in combination with transition metals has recently gained increasing attention for organic micropollutant abatement. In this study, aqueous Co(II), Cu(II), and Ag(I) were compared for their capacity to activate PAA. Co(II) outperformed Cu(II) or Ag(I) and the optimum conditions were 0.05 mM of Co(II), 0.4 mM of PAA, and pH 3. However, due to a wider applicability in water treatment, pH 7 (i.e., bicarbonate buffer) was selected for detailed investigations. The abatement of different micropollutant surrogates could be described with a second-order rate equation (observed second-order rate constants, k_obs were in the range of 42-132 M^-1 s^-1). For the para-substituted phenols, there was a correlation between the observed second-order rate constants of the corresponding phenolates and the Hammett constants (R² = 0.949). In all oxidation experiments, the reaction rate decreased significantly after 1-2 min, which coincided with the depletion of PAA but also with the deactivation of the Co(II) catalyst by oxidation to Co(III) and subsequent precipitation. It was demonstrated that Co(II) immobilized on a geopolymer-foam performed approximately similarly as aqueous Co(II) but without deactivation due to Co(III) precipitation. This provides a potential option for the further development of heterogeneous catalytic Co(II)/PAA advanced oxidation processes utilizing geopolymers as a catalyst support material.

Preferential Destruction of Micropollutants in Water through a Self-Purification Process with Dissolved Organic Carbon Polar Complexation

Removing micropollutants in real water is a scientific challenge due to primary dissolved organic carbon (DOC) and high energy consumption of current technologies. Herein, we develop a self-purification process for the preferential destruction of various micropollutants in municipal wastewater, raw drinking water, and ultrapure water with humic acid (HA) driven by the surface microelectronic field of Fe⁰-Fe_yC_z/Fe_x-GZIF-8-rGO without any additional input. It was verified that a strongly polar complex consisting of an electron-rich HA/DOC area and an electron-poor micropollutant area was formed between HA/DOC and micropollutants, promoting more electrons of micropollutants in the adsorbed complex to delocalizing to electron-rich Fe species area and be trapped by O₂, which resulted in their surface cleavage and hydrolyzation preferentially. The higher micropollutant degradation efficiency observed in real wastewaters was due to the greater complex polarity of DOC. Moreover, the electron transfer process ensured the stability of the surface microelectronic field and continuous water purification. Our findings provide a new insight into low-energy combined-micropollution water treatment.

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.

Chlorination of para-substituted phenols: Formation of α, β-unsaturated C4-dialdehydes and C4-dicarboxylic acids

Despite the widespread occurrence of phenols in anthropogenic and natural compounds, their fate in reactions with hypochlorous acid (HOCl), one of the most common water treatment disinfectants, remains incompletely understood. To close this knowledge gap, this study investigated the formation of disinfection by-products (DBPs) in the reaction of free chlorine with seven para-substituted phenols. Based on the chemical structures of the DBPs and the reaction mechanisms leading to their formation, the DBPs were categorized into four groups: chlorophenols, coupling products, substituent reaction products, and ring cleavage products. In contrast to previous studies that investigated the formation of early-stage chlorophenols, the primary focus of this study was on the elucidation of novel ring cleavage products, in particular α, β-unsaturated C₄-dialdehydes, and C₄-dicarboxylic acids, which, for the first time, were identified and quantified in this study. The molar yields of 2-butene-1,4-dial (BDA), one of the identified α, β-unsaturated C₄-dialdehydes, varied among the different phenolic compounds, reaching a maximum value of 10.4% for bisphenol S. Molar yields of 2-chloromaleic acid (Cl-MA), one of the identified C₄-dicarboxylic acids, reached a maximum value of 30.5% for 4-hydroxy-phenylacetic acid under given conditions. 2,4,6-trichlorophenol (TCP) was shown to be an important intermediate of the parent phenols and the C₄-ring cleavage products. Based on the temporal trends of α, β-unsaturated C₄-dialdehydes and C₄-dicarboxylic acids, their formation is likely attributable to two separate ring cleavage pathways. Based on the obtained results, an overall transformation pathway for the reaction of para-substituted phenols with free chlorine leading to the formation of novel C₄ ring cleavage products was proposed.

Ozonation of organic compounds in water and wastewater: A critical review

Ozonation has been applied in water treatment for more than a century, first for disinfection, later for oxidation of inorganic and organic pollutants. In recent years, ozone has been increasingly applied for enhanced municipal wastewater treatment for ecosystem protection and for potable water reuse. These applications triggered significant research efforts on the abatement efficiency of organic contaminants and the ensuing formation of transformation products. This endeavor was accompanied by developments in analytical and computational chemistry, which allowed to improve the mechanistic understanding of ozone reactions. This critical review assesses the challenges of ozonation of impaired water qualities such as wastewaters and provides an up-to-date compilation of the recent kinetic and mechanistic findings of ozone reactions with dissolved organic matter, various functional groups (olefins, aromatic compounds, heterocyclic compounds, aliphatic nitrogen-containing compounds, sulfur-containing compounds, hydrocarbons, carbanions, β-diketones) and antibiotic resistance genes.

Toward Selective Oxidation of Contaminants in Aqueous Systems

The presence of diverse pollutants in water has been threating human health and aquatic ecosystems on a global scale. For more than a century, chemical oxidation using strongly oxidizing species was one of the most effective technologies to destruct pollutants and to ensure a safe and clean water supply. However, the removal of increasing amount of pollutants with higher structural complexity, especially the emerging micropollutants with trace concentrations in the complicated water matrix, requires excessive dosage of oxidant and/or energy input, resulting in a low cost-effectiveness and possible secondary pollution. Consequently, it is of practical significance but scientifically challenging to achieve selective oxidation of pollutants of interest for water decontamination. Currently, there are a variety of examples concerning selective oxidation of pollutants in aqueous systems. However, a systematic understanding of the relationship between the origin of selectivity and its applicable water treatment scenarios, as well as the rational design of catalyst for selective catalytic oxidation, is still lacking. In this critical review, we summarize the state-of-the-art selective oxidation strategies in water decontamination and probe the origins of selectivity, that is, the selectivity resulting from the reactivity of either oxidants or target pollutants, the selectivity arising from the accessibility of pollutants to oxidants via adsorption and size exclusion, as well as the selectivity due to the interfacial electron transfer process and enzymatic oxidation. Finally, the challenges and perspectives are briefly outlined to stimulate future discussion and interest on selective oxidation for water decontamination, particularly toward application in real scenarios.

Accelerating FeIII-Aqua Complex Reduction in an Efficient Solid-Liquid-Interfacial Fenton Reaction over the Mn-CNH Co-catalyst at Near-Neutral pH

The sluggish regeneration rate of Fe^II and low operating pH still restrict the wider application of classical Fenton process (Fe^II/H₂O₂) for practical water treatment. To overcome these challenges, we exploit the Mn-CNH co-catalyst to construct a solid-liquid interfacial Fenton reaction and accelerate the Fe^III/Fe^II redox cycle at the interface for sustainably generating ^•OH from H₂O₂ activation. The Mn-CNH co-catalyst exhibits an excellent regeneration rate of Fe^II (∼65%) and a high tetracycline removal rate (K_obs) of 0.0541 min^-1, which is 19.0 times higher than that of the Fe^II/H₂O₂ system (0.0027 min^-1) at a near-neutral pH (pH ≈ 5.8), and it also attains 100% degradation of sulfamethoxazole, rhodamine B, and methyl orange. The cyclic mechanism of Fe^III/Fe^II is further elucidated in an atomic scale by combining characterizations and density functional theory calculations, including Fe_aq^III specific adsorption and the electron-transfer process. Mn active sites can accumulate electrons from the matrix and adsorb Fe_aq^III to form Mn-Fe bonds at the solid-liquid interface, which accelerate electron transfer from Mn-CNH to Fe_aq^III and promote the regeneration of Fe^II at a wide pH range with a lower energy barrier. The regeneration rate of Fe^II in the Mn-CNH/Fe^II/H₂O₂ system outperforms the benchmark Fenton system and other typical metal nanomaterials, which has great potential to be widely applied in actual environment remediation.

Water Purification of Classical and Emerging Organic Pollutants: An Extensive Review

The main techniques used for organic pollutant removal from water are adsorption, reductive and oxidative processes, phytoremediation, bioremediation, separation by membranes and liquid–liquid extraction. In this review, strengths and weaknesses of the different purification techniques are discussed, with particular attention to the newest results published in the scientific literature. This study highlighted that adsorption is the most frequently used method for water purification, since it can balance high organic pollutants removal efficiency, it has the possibility to treat a large quantity of water in semi-continuous way and has acceptable costs.