Interface Stabilization of Undercoordinated Iron Centers on Manganese Oxides for Nature-Inspired Peroxide Activation

Linking atomic to mesoscopic scales in multilevel structural tailoring of single-atom catalysts for peroxide activation

A key challenge in designing single-atom catalysts (SACs) with multiple and synergistic functions is to optimize their structure across different scales, as each scale determines specific material properties. We advance the concept of a comprehensive optimization of SACs across different levels of scale, from atomic, microscopic to mesoscopic scales, based on interfacial kinetics control on the coupled metal-dissolution/polymer-growth process in SAC synthesis. This approach enables us to manipulate the multilevel interior morphologies of SACs, such as highly porous, hollow, and double-shelled structures, as well as the exterior morphologies inherited from the metal oxide precursors. The atomic environment around the metal centers can be flexibly adjusted during the dynamic metal-oxide consumption and metal-polymer formation. We show the versatility of this approach using mono- or bi-metallic oxides to access SACs with rich microporosity, tunable mesoscopic structures and atomic coordinating compositions of oxygen and nitrogen in the first coordination-shell. The structures at each level collectively optimize the electronic and geometric structure of the exposed single-atom sites and lower the surface *O formation barriers for efficient and selective peroxidase-type reaction. The unique spatial geometric configuration of the edge-hosted active centers further improves substrate accessibility and substrate-to-catalyst hydrogen overflow due to tunable structural heterogeneity at mesoscopic scales. This strategy opens up new possibilities for engineering more multilevel structures and offers a unique and comprehensive perspective on the design principles of SACs.

Superior performance of oxygen vacancy-enriched Cu-Co3O4/urushiol-rGO/peroxymonosulfate for hypophosphite and phosphite removal by enhancing singlet oxygen

The treatment of wastewater containing hypophosphite [P(I)] and phosphite [P(III)] is challenged by limitations of traditional Fenton oxidation such as low efficiency, secondary pollution and high costs. This study introduced a facile solvent-thermal method to synthesize Cu-Co3O4 nanoparticles uniformly loaded on graphene (Cu-Co3O4/U-rGO) through the reduction and coordination effects of urushiol (U). As prepared Cu-Co3O4/U-rGO exhibited excellent activity in activating peroxymonosulfate (PMS) for the oxidation of P(I)/P(III) to phosphate [P(V)] (0.229 min-1), along with high stability and reusability (91.5 % after 6 cycles), low metal leaching rate (Co: 0.2 mg/L, Cu: 0.05 mg/L), insensitivity to common anions in water and a wide pH range (3-11). The activation mechanism involved the synergistic effects from both urushiol and graphene, which promoted redox of Cu+/Cu2+ and Co2+/Co3+ and induced abundant oxygen vacancies for PMS activation to produce singlet oxygen. Furthermore, the Cu-Co3O4/U-rGO/PMS was also excellent in the oxidative removal of organic phosphorus. This study is expected to advance strategies for the treatment of P(I)/P(III)-rich wastewater and provide new insights for the development of low-cost, highly efficient heterogeneous catalysts with abundant oxygen vacancies.

Directional Formation of Reactive Oxygen Species Via a Non-Redox Catalysis Strategy That Bypasses Electron Transfer Process

A broad range of chemical transformations driven by catalytic processes necessitates the electron transfer between catalyst and substrate. The redox cycle limitation arising from the inequivalent electron donation and acceptance of the involved catalysts, however, generally leads to their deactivation, causing substantial economic losses and environmental risks. Here, a "non-redox catalysis" strategy is provided, wherein the catalytic units are constructed by atomic Fe and B as dual active sites to create tensile force and electric field, which allows directional self-decomposition of peroxymonosulfate (PMS) molecules through internal electron transfer to form singlet oxygen, bypassing the need of electron transfer between catalyst and PMS. The proposed catalytic approach with non-redox cycling of catalyst contributes to excellent stability of the active centers while the generated reactive oxygen species find high efficiency in long-term catalytic pollutant degradation and selective organic oxidation synthesis in aqueous phase. This work offers a new avenue for directional substrate conversion, which holds promise to advance the design of alternative catalytic pathways for sustainable energy conversion and valuable chemical production.

Lattice oxygen activation of MnO2 by CeO2 for the improved degradation of bisphenol A in the peroxymonosulfate-based oxidation

The structure of MnO2 was modified by constructing the composites CeO2/ MnO2 via a facile hydrothermal method. The catalytic performance of optimal composite (Mn-Ce10) in peroxymonosulfate (PMS) activation for the degradation of bisphenol A (BPA) is approximately three times higher than that of MnO2 alone. The average valence of manganese in CeO2/MnO2 is lowered compared to MnO2, which induces the generation of more free radicals, such as OH and SO4•-. In addition, the composite exhibits a higher concentration of oxygen vacancies than MnO2, facilitating bondingwith PMS to produce more singlet oxygen (1O2). Moreover, the incorporation of CeO2 activates the lattice oxygen of MnO2, improving its oxidative ability. Consequently, approximately 48% of BPA decomposition in 10min is attributed to direct oxidation in the Mn-Ce10/PMS system, whereas only 36% occurs in 30min for the MnO2/PMS system. Simulation results confirm weakened Mn-O covalency and elongated Mn-O bonds due to the activation of lattice oxygen in CeO2/MnO2, demonstrating that PMS tends to be adsorbed on the composite rather than on MnO2. This work establishes a relationship between lattice oxygen and the degradation pathway, offering a novel approach for the targeted regulation of catalytic oxidation.

Domain-limited thermal transformation preparation of novel graphitized carbon-supported layered double oxides for efficient tetracycline degradation

The resource utilization of industrial lignin to construct high-performance catalysts for wastewater treatment field is pioneering research. Herein, the novel graphitized carbon-supported CuCoAl-layered double oxides (LDOs-GC) were successfully designed by the domain-limited thermal transformation technology using sodium lignosulfonate (LS) self-assembled CuCoAl-layered double hydroxides as the precursor. The optimized LDOs-GC catalyst owned the excellent tetracycline (TC) degradation of 98.0% within 15 min by activated peroxymonosulfate (PMS) under optimal conditions (20 mg/L catalyst, 1.5 mM PMS, 30 mg/L TC). The density of metal ions in the catalyst and the synergistic interaction between graphitized carbon (GC) and metal ions played a major role in TC degradation. Based on a comprehensive analysis, the TC degradation in LDOs-GC/PMS system was proved to be accomplished by a combination of free radicals (SO4·- and HO·) and non-radicals (1O2). Meanwhile, the possible degradation pathways of TC were proposed by the analysis of TC degradation intermediates and a comprehensive analysis of the rational reaction mechanism for TC degradation by LDOs-GC/PMS system was also performed. This work provides a new strategy for developing novel high-performance catalysts from industrial waste, while offering a green, cheap and sustainable approach to antibiotic degradation.

Fe2O3/TiO2/reduced graphene oxide-driven recycled visible-photocatalytic Fenton reactions to mineralize organic pollutants in a wide pH range

Photocatalytic Fenton reactions combined the advantages from both photocatalysis and Fenton reaction in mineralizing organic pollutants. The key problems are the efficiency and recycling stability. Herein, we reported a novel Fe2O3/TiO2/reduced graphene oxide (FTG) nanocomposite synthesized by a facile solvothermal method. The TiO2 in FTG degraded organic pollutants and mineralized intermediates via photocatalysis under visible light irradiation, which could also promote Fenton reaction by accelerating Fe3+-Fe2+ recycle. Meanwhile, the Fe2O3 rapidly degraded organic pollutants via Fenton reactions, which also promoted photocatalysis by enhancing visible light absorbance and diminishing photoelectron-hole recombination. The high distribution of TiO2 and Fe2O3 on rGO, together with their strong interaction resulted in enhanced synergetic cooperation between photocatalysis and Fenton reactions, leading to the high mineralization efficiency of organic pollutants. More importantly, it could also inhibit the leaching of Fe species, leading to the long lifetime of FTG during photocatalytic Fenton reactions in a wide pH range from 3.4 to 9.2.

Polar electric field-modulated peroxymonosulfate selective activation for removal of organic contaminants via non-radical electron transfer process

Nonradical electron transfer process (ETP) in peroxomonosulfate (PMS) based advanced oxidation processes (AOPs) is regarded promising for selective degradation of organic contaminants in water, however, the subjective modulation strategy and the definitive mechanistic elucidation of ETP are still lacking. Herein, we proposed a heretofore unreported yet efficient ETP indution approach by construction of polar electrical field on biochar via nonmetallic elements co-doping. Physicochemical characterizations and density functional theory (DFT) calculations verified the electronegativity difference among boron, nitrogen, and sulfur elements bestowed robust local electric fields on biochar surface (BC-BNS), which effectively enhanced the adsorption complexation and charge transfer between biochar and PMS. Compared to the other single-doped or co-doped biochar, BC-BNS exhibited superior catalytic performance of PMS activation for degradation of atrazine (ATZ) (kobs=0.036 min-1), as well as various kinds of electron-rich organics. The remarkable catalytic degradation capacity was further verified in various aqueous matrices and background factors, representing the excellent selectivity. Analysis of contribution from reactive oxygen species and electrochemical testing together substantiated the role of polar electric fields in facilitating the modulation from singlet oxygen (1O2) to ETP as a prevailing mechanism. DFT calculations and apparent interactions revealed the dissociation of S-O bond was thermodynamically favored within this potent localized electric field, which further induced the cleavage of OO bond and ultimately promoted the dual electron transfer between ATZ and PMS. The superiority of BC-BNS/PMS system was further validated with the low ecotoxicity caused by enhanced dechlorination, the low energy consumption, and the long-term effectiveness. The novel modulation principle and atomic-level mechanism exploration gave suggestions for advancing ETP-dominated AOP to remove recalcitrant contaminants during water treatment and restoration.

Waste self-heating bag derived iron-based composite with abundant oxygen vacancies for highly efficient Fenton-like degradation of micropollutants

In this study, iron-rich waste self-heating bag was reutilized as the raw material to prepare oxygen vacancies (OV) functionalized iron-based composite (iron oxide (Fe₃O₄)-carbon-vermiculite, viz. OV-ICV), which exhibited excellent performance in the Fenton-like degradation of micropollutants via peroxydisulfate (PDS) activation. Above 95% of 1.0 mg/L carbaryl (CB) was efficiently eliminated in the presence of 0.1 g/L of OV-ICV and 0.5 mmol/L of PDS over a wide pH range of 3-10 within 30 min. Besides, OV-ICV also showed acceptable adaptability, stability, and renewability. Imbedding OV into Fe₃O₄ structure significantly generated more active iron sites and localized electrons, promoted the charge transfer ability, and assisted the redox cycle of ≡Fe(III)/≡Fe(II) for PDS activation. Mechanism investigation demonstrated that superoxide radicals (O₂^•-) derived from the activation of molecular oxygen mediated the generation of H₂O₂, and both of them further enhanced the formation of more sulfate radicals (SO₄^•-) and hydroxyl radicals (^•OH), which led to the efficient degradation and mineralization of CB. Furthermore, the degradation pathways of CB were proposed based on the intermediates identification. This work lays a foundation for the rational reutilization of iron-containing wastes modified with defect engineering in heterogeneous Fenton-like catalysis for the remediation of micropollutants wastewater.

Asymmetrically Coordinated CoB1 N3 Moieties for Selective Generation of High-Valence Co-Oxo Species via Coupled Electron-Proton Transfer in Fenton-like Reactions

High-valence metal species generated in peroxymonosulfate (PMS)-based Fenton-like processes are promising candidates for selective degradation of contaminants in water, the formation of which necessitates the cleavage of OH and OO bonds as well as efficient electron transfer. However, the high dissociation energy of OH bond makes its cleavage quite challenging, largely hampering the selective generation of reactive oxygen species. Herein, an asymmetrical configuration characterized by a single cobalt atom coordinated with boron and nitrogen (CoB₁ N₃ ) is established to offer a strong local electric field, upon which the cleavage of OH bond is thermodynamically favored via a promoted coupled electron-proton transfer process, which serves an essential step to further allow OO bond cleavage and efficient electron transfer. Accordingly, the selective formation of Co(IV)O in a single-atom Co/PMS system enables highly efficient removal performance toward various organic pollutants. The proposed strategy also holds true in other heteroatom doping systems to configure asymmetric coordination, thus paving alternative pathways for specific reactive species conversion by rationalized design of catalysts at atomic level toward environmental applications and more.

Efficient removal of 3,6-dichlorocarbazole with Fe0-activated peroxymonosulfate: performance, intermediates and mechanism

Nowadays, polyhalogenated carbazoles (PHCZs) are a major pollutant that has recently sparked widespread concern. In this work, peroxymonosulfate (PMS) was activated by zero valent iron (Fe⁰) to remove 3,6-dichlorocarbazole (3,6-CCZ). First, the key parameters influencing 3,6-CCZ degradation (PMS dosage, Fe⁰ dosage, initial pH, temperature and co-existing ions) were determined. Under the determined optimum conditions, the removal rate of 3,6-CCZ reached 100% within 1.5 h. Sulfate radicals (SO₄·^-), hydroxyl radicals (OH·), and singlet oxygen (¹O₂) generated in the reaction were directly identified with 0.1 M 5,5-dimethyl-1-pyrrolidine N-oxide (DMPO) by in-situ electron paramagnetic resonance (EPR) and indirectly identified by radical quenching experiments. The main reactive oxygen species (ROS) were different from most reported hydroxyl radicals (OH·) and sulfate radicals (SO₄·^-). In this study, it was found that OH· and ¹O₂ play a major role. Then, fresh and reacted Fe⁰ were characterized by XRD, SEM, and XPS. Iron corrosion products such as Fe₂O₃, Fe₃O₄, and FeOOH were generated. Finally, 3,6-CCZ degradation intermediates were identified by GC-MS and its degradation pathway was speculated. The intermediate pathway confirmed the combined action of (OH·) and (¹O₂) in 3,6-CCZ removal. This study provides new insight into the activation mechanism of Fe⁰-activated PMS and the removal mechanism of 3,6-CCZ.Highlights Fe⁰ is a long-lasting and efficient catalyst of PMS for the degradation of 3,6-CCZ.The key parameters influencing 3,6-CCZ degradation were determined.The degradation pathways of 3,6-CCZ were inferred.OH· and ¹O₂ were the main ROS in Fe⁰-activated PMS system.

Activation Mechanism of a Peroxymonosulfate-based Fenton-like System with Birnessite and Cu2+ Modified Birnessite

The peroxymonosulfate (PMS) activation mechanism is closely related to the structure/configuration of the "active site" of the Fenton-like catalysts. In this paper, it is shown that Cu²⁺ chemically bonds the surface of birnessite at the Mn vacancy site to form triple-corner-sharing inner-sphere surface complexes (Cu-Bir), which regulates the PMS activation mechanism. Birnessite/PMS Fenton-like system degrades phenol via a non-radical mechanism, which involves high valent Mn(V)=O generated from the surface reaction of Mn(III) with adsorbed PMS. While Cu-Bir initiates an additional radical mechanism that involves reactive oxygen species of ⋅O₂ ^- and ¹ O₂ , and the k value of 5Cu-Bir is 1.9 times higher than that of Bir. The Fenton-like reaction mechanism of the Cu-Bir/PMS system is thus proposed as a radical and non-radical cooccurrence mechanism, which involves a synergistic effect that the radical pathway accelerates the regeneration of Mn(III) for the non-radical pathway, and the non-radical pathway assists to produce ¹ O₂ for the radical pathway.

Generation of FeIV =O and its Contribution to Fenton-Like Reactions on a Single-Atom Iron-N-C Catalyst

Generating Fe^IV =O on single-atom catalysts by Fenton-like reaction has been established for water treatment; however, the Fe^IV =O generation pathway and oxidation behavior remain obscure. Employing an Fe-N-C catalyst with a typical Fe-N₄ moiety to activate peroxymonosulfate (PMS), we demonstrate that generating Fe^IV =O is mediated by an Fe-N-C-PMS* complex-a well-recognized nonradical species for induction of electron-transfer oxidation-and we determined that adjacent Fe sites with a specific Fe₁ -Fe₁ distance are required. After the Fe atoms with an Fe₁ -Fe₁ distance <4 Å are PMS-saturated, Fe-N-C-PMS* formed on Fe sites with an Fe₁ -Fe₁ distance of 4-5 Å can coordinate with the adjacent Fe^II -N₄ , forming an inter-complex with enhanced charge transfer to produce Fe^IV =O. Fe^IV =O enables the Fenton-like system to efficiently oxidize various pollutants in a substrate-specific, pH-tolerant, and sustainable manner, where its prominent contribution manifests for pollutants with higher one-electron oxidation potential.

Modified birnessite MnO2 as efficient Fenton-like catalysts through electron transfer process between the simultaneously surface-activated peroxymonosulfate and pollutants

The development of efficient and eco-friendly Mn-based hybrids for the degradation of biorefractory organic pollutants via peroxymonosulfate (PMS) activation is highly desired. In this study, a novel graphite nanosheet (GNs)-based Fe-Mn bimetallic oxide (Fe doped birnessite MnO₂, FeMn/GNs) was synthesized under mild conditions. Compared with monometallic Fe or Mn oxide on GNs, FeMn/GNs exhibited a higher surface area, decreased Mn oxidation states, stronger interaction with GNs, and more active sites for PMS adsorption. Among different Fe/Mn ratios, Fe₂Mn₁/GNs showed the optimum performance for bisphenol A (BPA) degradation with the first-order rate constant of 0.22 min^-1, which was about 8.5 and 12.9 times higher than that of Mn/GNs and Fe/GNs, respectively. Different from the pollutant-catalyst-PMS electron transfer mechanism for Mn/GNs, the direct two-electron transfer in FeMn/GNs+PMS system, was mainly processed between the simultaneously activated BPA and PMS. This was probably based on the double adsorption sites of Fe and Mn species on the same catalyst: PMS was adsorbed by Fe species through hydroxyl groups, while BPA was mainly coordinated with Mn species due to the layered structure and hydrophobicity of the Mn oxide. This study is expected to provide the rational design of efficient Mn-based hybrids for PMS activation.

Engineering of 3D graphene hydrogel-supported MnO2-FeOOH nanoparticles with synergistic effect of oxidation and adsorption toward highly efficient removal of arsenic

Iron-manganese-based adsorbent has been regarded as a promising candidate for arsenic purification from water, especially the inorganic As(III), due to its inherent advantage of low cost and large-scale producibility. However, the nanoparticle aggregation, metal leaching and insufficient removal efficiency remain the main challenges in the practical applications of the granular adsorbents. In this work, we develop a universal strategy for the fabrication of an active Fe(III) oxyhydroxide-Mn(IV) oxide/3D graphene oxide (GO) gel composite via a simple hydrothermal reaction. The successful immobilization of Fe-Mn oxyhydroxide/oxides on the interconnected GO gels was intuitively confirmed by the transmission electron microscopy and atomic force microscopy. The combinative characterizations of the X-ray absorption near edge structure and X-ray photoelectron spectroscopy clearly reveal the electron transfer from Fe atoms to Mn atoms. The optimized Fe-Mn/GO composites possess the superior performance with the removal efficiency of over 90% for As(III) at pH 7.0 and ∼97% for As(V) at pH 5.0 and the As(III, V) levels (100 μg l^-1) are reduced to below the WHO guideline of 10 μg l^-1. The sorption isotherm and kinetic experiments on the As removal were also carried out. The post characterizations are employed to better unveil the oxidation-adsorption mechanism. Notably, the application of Fe-Mn/GO composites in the treatment of As-simulated natural water demonstrated a stable and continuous operation for over 20 days and an effluent concentration of arsenic as low as the 10 μg l^-1 in a specially designed flow reactor.

Preparation of VCo-MOF@MXene composite catalyst and study on its removal of ciprofloxacin by catalytically activating peroxymonosulfate: Construction of ternary system and superoxide radical pathway

The synergistic effect between transition metal active centers and the generation of multiple removal pathways has a significant impact on the catalytic activation efficiency of peroxymonosulfate. In this work, a kind of composite catalyst was prepared by growing VCo-metal-organic frameworks (VCo-MOF) in-situ on the surface of Ti3C2Tx by a solvothermal method. The morphology and structure are characterized by Transmission Electron Microscope (TEM), Energy Dispersion Spectrum (EDS), Atomic Force Microscope (AFM), etc. Response surface methodology was used to optimize the experimental conditions. Only 5 mg catalyst can be used to effectively activate PMS and remove 96.14 % ciprofloxacin (CIP, 20 mg/L) within 30 min. The removal effect of catalyst on CIP in different actual water environment was explored. In addition, the fluorescence spectrum test also verified the effective removal of ciprofloxacin. V-Co-Ti ternary system provides a wealth of active sites for CIP removal. Cyclic voltammetry (CV) and lear sweep voltammetry (LSV) tests showed the existence of the electron transfer pathway. The results of density functional theory (DFT) calculation show that VCo-MOF@Ti3C2Tx has excellent adsorption and activation ability for PMS. At the same time, the hydrophilicity of the catalyst makes PMS more inclined to react with water molecules, which promotes the formation of a unique superoxide radical path.

Bicarbonate enhanced heterogeneous activation of peroxymonosulfate by copper ferrite nanoparticles for the efficient degradation of refractory organic contaminants in water

Nowadays, the treatment of residual refractory organic contaminants (ROCs) is a huge challenge for environmental remediation. In this study, a potential process is provided by copper ferrite catalyst (CuFe₂O₄) activated peroxymonosulfate (PMS, HSO₅^-) in the bicarbonate (HCO₃^-) enhanced system for efficient removal of Acid Orange 7 (AO7), 2,4-dichlorophenol, phenol and methyl orange (MO) in water. The impact of key reaction parameters, water quality components, main reactive oxygen species (ROS), probable degradation mechanism, rational degradation pathways and catalyst stability were systematically investigated. A 95.0% AO7 (C₀ = 100 mg L^-1) removal was achieved at initial pH (pH₀) of 5.9 ± 0.1 (natural pH), CuFe₂O₄ dosage of 0.15 g L^-1, PMS concentration of 0.98 mM, HCO₃^- concentration of 2 mM, and reaction time of 30 min. Both sulfate radical (SO₄^-•) and hydroxyl radical (^•OH) on the surface of catalyst were proved as the predominant radical species through radical quenching experiments and electron paramagnetic resonance (EPR) analysis. The buffer nature of HCO₃^- was partially contributed for the enhanced degradation of AO7 under CuFe₂O₄/PMS/HCO₃^- system. Importantly, according to ¹³C nuclear magnetic resonance (NMR) and EPR analysis, the positive effect of bicarbonate may be mainly attributed to the formation of peroxymonocarbonate (HCO₄^-), which may enhance the generation of ^•OH. The magnetic CuFe₂O₄ particles can be well recycled and the leaching concentration of Cu was acceptable (<1 mg L^-1). Considering the widespread presence of bicarbonate in water environment, this work may provide a safe, efficient, and sustainable technique for the elimination of ROCs from practical complex wastewater.

Porous boron nitride intercalated zero-valent iron particles for highly efficient elimination of organic contaminants and Cr (VI)

Developing novel bifunctional materials to high efficiently degrade organic pollutants and eliminate hexavalent chromium (Cr (VI)) is significantly desired in the wastewater treatment field. The porous boron nitride (p-BN) was fabricated by a two-stage calcination strategy and was innovatively employed to support zero-valent iron (ZVI), achieving the bifunctional material (p-BN@ZVI) to degrade carbamazepine (CBZ) and eliminate Cr (VI). p-BN@ZVI could degrade more than 98% CBZ in 6 min with the high apparent first-order constant (k_obs) of 0.536 min^-1, almost 5 times higher than that of the ZVI/PMS system and outperformed most previous reported ZVI supported catalysts, which was mainly ascribed to the fact that the introduction of p-BN with high surface area (793.97 m²/g) improved the dispersion of ZVI and exposed more active sites. Quenching tests coupled with electron paramagnetic resonance (EPR) suggested that •OH was the major reactive oxygen species with a contribution of 71.6%. Notably, the p-BN@ZVI/PMS system expressed low activation energy of 8.23 kJ/mol and reached a 65.69% TOC degradation in 20 min even at 0 °C. p-BN@ZVI possessed remarkable storage stability and could still degrade 92.3% CBZ despite three-month storage. More interestingly, p-BN@ZVI was capable to eliminate 98.1% of 50 mg/L Cr (VI) within 5 min through adsorption and reduction, where nearly 80% Cr (VI) was transformed to Cr (III), and exhibited the maximum Cr (VI) elimination capacity of 349 mg/g. This study provides new insights into the efficient organic contaminants degradation and Cr (VI) elimination in the treatment of wastewater.

A Review of Sulfate Radical-Based and Singlet Oxygen-Based Advanced Oxidation Technologies: Recent Advances and Prospects

In recent years, advanced oxidation process (AOPs) based on sulfate radical (SO4●−) and singlet oxygen (1O2) has attracted a lot of attention because of its characteristics of rapid reaction, efficient treatment, safety and stability, and easy operation. SO4●− and 1O2 mainly comes from the activation reaction of peroxymonosulfate (PMS) or persulfate (PS), which represent the oxidation reactions involving radicals and non-radicals, respectively. The degradation effects of target pollutants will be different due to the type of oxidant, reaction system, activation methods, operating conditions, and other factors. In this paper, according to the characteristics of PMS and PS, the activation methods and mechanisms in these oxidation processes, respectively dominated by SO4●− and 1O2, are systematically introduced. The research progress of PMS and PS activation for the degradation of organic pollutants in recent years is reviewed, and the existing problems and future research directions are pointed out. It is expected to provide ideas for further research and practical application of advanced oxidation processes dominated by SO4●− and 1O2.

Electron delocalization triggers nonradical Fenton-like catalysis over spinel oxides

Nonradical Fenton-like catalysis offers opportunities to overcome the low efficiency and secondary pollution limitations of existing advanced oxidation decontamination technologies, but realizing this on transition metal spinel oxide catalysts remains challenging due to insufficient understanding of their catalytic mechanisms. Here, we explore the origins of catalytic selectivity of Fe-Mn spinel oxide and identify electron delocalization of the surface metal active site as the key driver of its nonradical catalysis. Through fine-tuning the crystal geometry to trigger Fe-Mn superexchange interaction at the spinel octahedra, ZnFeMnO₄ with high-degree electron delocalization of the Mn-O unit was created to enable near 100% nonradical activation of peroxymonosulfate (PMS) at unprecedented utilization efficiency. The resulting surface-bound PMS* complex can efficiently oxidize electron-rich pollutants with extraordinary degradation activity, selectivity, and good environmental robustness to favor water decontamination applications. Our work provides a molecule-level understanding of the catalytic selectivity and bimetallic interactions of Fe-Mn spinel oxides, which may guide the design of low-cost spinel oxides for more selective and efficient decontamination applications.

Efficient organics heterogeneous degradation by spinel CuFe2O4 supported porous carbon nitride catalyst: Multiple electron transfer pathways for reactive oxygen species generation

Facilitating reactive oxygen species (ROS) generation is an effective way to promote the heterogeneous catalytic efficiency for organics removal. However, the metal leaching in metal-based catalysts and the low activity of non-metallic materials restrict ROS production. In this work, the purpose was achieved by loading a small amount of spinel CuFe₂O₄ onto porous carbon nitride substrate. The synthesized CuFe₂O₄@O-CN composite first to activate peroxymonosulfate (PMS), which produce a plenty of ROS (•OH, SO₄•^- and ¹O₂) for organics removal, leading to highly oxidation for diverse organics. Through the comparative analysis of the surface composition before and after reaction, we found that the interface multi-electron transfer routs, including surface Cu(II)/Cu(I), Fe(III)/Fe(II) and their cross interaction, participated in the redox cycle, giving rise to the rapid and massive production of ROS, so that DMPO and TEMP were instantly oxidized in electron paramagnetic resonance (ESR) detection. Importantly, the carrier of porous O-CN, which acted as the electron transfer mediator, not only favors PMS adsorption via surface -OH, but also facilitates the conversion between different metal species. As a result, the CuFe₂O₄@O-CN/PMS system can remove 99.1% BPA and achieve 52.6% mineralization under optimized conditions. Thus, this study not only sheds light on the tailored design of heterogeneous catalyst for organics removal and elucidates the interfacial catalytic mechanisms for PMS activation.

Construction of durable superhydrophilic activated carbon fibers based material for highly-efficient oil/water separation and aqueous contaminants degradation

Developing filtering materials with high permeation flux and contaminant removal rate is of great importance for oily wastewater remediation. Herein, a robust three-dimensional (3D) activated carbon fibers (ACFs) based composite with uniformly grown layered double hydroxide (LDH) on the surface was successfully constructed through a feasible hydrothermal strategy. The LDH with a high surface energy and vertically aligned structure could provide superhydrophilicity to ACFs. Systematic investigation confirmed that the 3D material could overcome the size mismatch between the ACFs macropores and tiny emulsified droplets through the combination of size-sieving filtration on the surface and oil droplet coalescence in the fiber network. This process efficiently separated the intractable surfactant-stabilized oil-in-water emulsions with high permeation flux (up to 4.16 × 10⁶ L m^-2 h^-1 bar^-1). Notably, the LDH also had well-dispersed catalytic active sites, which could initiate advanced oxidation processes (AOPs) to efficiently eliminate various types of water-soluble organic pollutants (e.g., pharmaceuticals, phenolic compounds and organic dyes). The resulting modified ACFs exhibited exceptional removal rates for both oil and organic pollutants in the complex sewage during the continuous filtration process. These versatile abilities integrated with the facile preparation method reported herein provide outstanding prospects for the large-scale treatment of oily wastewater.

Efficient Charge Transfer Channels in Reduced Graphene Oxide/Mesoporous TiO2 Nanotube Heterojunction Assemblies toward Optimized Photocatalytic Hydrogen Evolution

Interface engineering is usually considered to be an efficient strategy to promote the separation and migration of photoexcited electron-hole pairs and improve photocatalytic performance. Herein, reduced graphene oxide/mesoporous titanium dioxide nanotube heterojunction assemblies (rGO/TiO₂) are fabricated via a facile hydrothermal method. The rGO is anchored on the surface of TiO₂ nanosheet assembled nanotubes in a tightly manner due to the laminated effect, in which the formed heterojunction interface becomes efficient charge transfer channels to boost the photocatalytic performance. The resultant rGO/TiO₂ heterojunction assemblies extend the photoresponse to the visible light region and exhibit an excellent photocatalytic hydrogen production rate of 932.9 μmol h^-1 g^-1 under simulated sunlight (AM 1.5G), which is much higher than that of pristine TiO₂ nanotubes (768.4 μmol h^-1 g^-1). The enhancement can be ascribed to the formation of a heterojunction assembly, establishing effective charge transfer channels and favoring spatial charge separation, the introduced rGO acting as an electron acceptor and the two-dimensional mesoporous nanosheets structure supplying a large surface area and adequate surface active sites. This heterojunction assembly will have potential applications in energy fields.

Sustainably recycling spent lithium-ion batteries to prepare magnetically separable cobalt ferrite for catalytic degradation of bisphenol A via peroxymonosulfate activation

A selective separation-recovery process based on tuning organic acid was proposed to the resource recycling of spent lithium-ion batteries (LIBs) for the first time. The low-cost preparation of CoFe₂O₄, reuse of waste acid and recovery of Li can be realized in this process, simultaneously. Li and Co in spent LIBs can be leached efficiently using citric acid as a leaching agent, and separated effectively from leaching solution by tuning oxalic acid content. The results from the characterizations of the prepared CoFe₂O₄ (CoFe₂O₄-LIBs) show that it possesses higher ratio of Co(II)/Co(III) and Fe(II)/Fe(III), larger surface specific area and more number of acid sites in comparison with pure CoFe₂O₄. Besides, CoFe₂O₄-LIBs was used to activate peroxymonosulfate (PMS) for the degradation of bisphenol A (BPA). Interestingly, its degradation performance is superior to that of pure CoFe₂O₄ and the related Co-based catalysts. The excellent degradation performance can be maintained in presence of inorganic ions (e.g., Cl^-, HCO₃^-, H₂PO₄^- and NO₃^-) with high concentration or humic acid. Moreover, surface-bound SO₄^∙- is considered as the main reactive species for the degradation of BPA. More importantly, CoFe₂O₄-LIBs can be readily recycled by using an external magnet and own superior ability of regeneration.

Peroxymonosulfate activation by Fe3O4-MnO2/CNT nanohybrid electroactive filter towards ultrafast micropollutants decontamination: Performance and mechanism

Electrocatalytic peroxymonosulfate (PMS) activation is a promising advanced oxidation process for the degradation of micropollutants. Herein, we developed an electroactive carbon nanotube (CNT) filter functionalized with Fe₃O₄-MnO₂ hybrid (Fe₃O₄-MnO₂/CNT) to activate PMS towards ultrafast degradation of sulfamethoxazole (SMX). SMX was completely degraded via a single-pass through the nanohybrid filter (τ < 2 s). The ultrafast degradation kinetics were maintained across a wide pH range (from 3.0 to 8.0), in complicated matrices (e.g., tap water, lake water, WWTP effluent and pharmaceutical wastewater), and for the degradation of various persistent micropollutants. Compared with a conventional batch reactor, the flow-through operation provides an 9.2-fold higher SMX degradation kinetics by virtue of the convection-enhanced mass transport (1.47 vs. 0.16 min^-1). The efficient redox cycle of Fe²⁺/Fe³⁺ and Mn²⁺/Mn⁴⁺ facilitate the PMS activation to generate SO₄^•- under electric field. Meanwhile, the ketonic groups on the CNT provide active sites for the generation of ¹O₂. Both experimental and theoretical results revealed the superior activity of nanohybrid filter associated with the synergistic effects among Fe, Mn, CNT and electric field. Therefore, the electrocatalytic filter based PMS activation system provides a green strategy for the remediation of micropollutants in a sustainable manner.

Origins of Electron-Transfer Regime in Persulfate-Based Nonradical Oxidation Processes

Persulfate-based nonradical oxidation processes (PS-NOPs) are appealing in wastewater purification due to their high efficiency and selectivity for removing trace organic contaminants in complicated water matrices. In this review, we showcased the recent progresses of state-of-the-art strategies in the nonradical electron-transfer regimes in PS-NOPs, including design of metal and metal-free heterogeneous catalysts, in situ/operando characterization/analytical techniques, and insights into the origins of electron-transfer mechanisms. In a typical electron-transfer process (ETP), persulfate is activated by a catalyst to form surface activated complexes, which directly or indirectly interact with target pollutants to finalize the oxidation. We discussed different analytical techniques on the fundamentals and tactics for accurate analysis of ETP. Moreover, we demonstrated the challenges and proposed future research strategies for ETP-based systems, such as computation-enabled molecular-level investigations, rational design of catalysts, and real-scenario applications in the complicated water environment. Overall, this review dedicates to sharpening the understanding of ETP in PS-NOPs and presenting promising applications in remediation technology and green chemistry.

Facile preparation, catalytic performance and reaction mechanism of MnxCo1-xOδ/3DOM-m Ti0.7Si0.2W0.1Oy catalysts for the simultaneous removal of soot and NOx

In this work, three-dimensionally ordered macroporous-mesoporous Ti0.7Si0.2W0.1Oy (3DOM-m TiSiWO) supported MnxCo1-xOδ catalysts with different x values were prepared using the colloidal crystal template method and incipient wetness impregnation method. The...

O, S-Dual-Vacancy Defects Mediated Efficient Charge Separation in ZnIn2S4/Black TiO2 Heterojunction Hollow Spheres for Boosting Photocatalytic Hydrogen Production

Defective ZnIn₂S₄ nanosheets/mesoporous black TiO₂ heterojunction hollow spheres (H-ZIS/b-TiO₂) are prepared through hydrothermal and surface low-temperature hydrogenation strategies, which show broad-spectrum response and excellent charge separation efficiency. This H-ZIS/b-TiO₂ flower-like heterojunction hollow spheres with a narrow band gap of ∼1.88 eV expand the light response to visible light and show excellent photocatalytic hydrogen evolution rate (278 μmol h^-1 50 mg^-1) under visible-light irradiation, which is 1.5 times as high as that of ZnIn₂S₄/black TiO₂ heterojunction hollow spheres (ZIS/b-TiO₂) (181 μmol h^-1 50 mg^-1). The excellent photocatalytic performance is due to the formation of O, S dual vacancies in b-TiO₂ and H-ZIS providing more active sites for photocatalytic reaction and improving the charge separation efficiency, heterojunctions promoting transport of photogenerated carriers, and the hollow structure increasing light utilization by reflecting light. The novel heterojunction hollow sphere with high performance has broad application prospects in the field of energy.

Carbon aerogel from forestry biomass as a peroxymonosulfate activator for organic contaminants degradation

The carbon catalyst has been widely used as a peroxymonosulfate (PMS) activator to degrade organic contaminants. The biomass carbon aerogel (CA) derived from poplar powder was synthesized in this study. CA with three-dimensional structure exhibited an excellent degradation performance of PMS activation for different types of organic contaminants including bisphenol A (BPA), rhodamine 6 G, phenol, and p-chlorophenol with the removal efficiencies up to 91%, 100%, 100%, and 60% within 60 min, respectively. It was found that singlet oxygen (¹O₂) dominated the non-radical pathway worked for BPA removal in CA/PMS system. The possible mechanism for PMS activation was discussed. A portion of ¹O₂ was produced through the transformation of superoxide radical (O₂^•-) in CA/PMS system. Electronic impedance spectroscopy (EIS) proved that the hierarchical structure of CA contributed to the electron transfer process for PMS activation. The ketonic/carbonyl groups (C˭O) on the surface of CA could serve as a possible active site to facilitate the generation of ¹O₂. In addition, CA showed superior degradation performance in actual water bodies and reusability with high-temperature regeneration treatment. This study developed an efficient and environmentally benign catalyst for water remediation of organic pollutants.

Greatly enhanced oxidative activity of δ-MnO2 to degrade organic pollutants driven by dominantly exposed {-111} facets

The reactivity of oxidizing materials is highly related to the exposed crystal facets. Herein, δ-MnO₂ with different exposure facets were synthesized and the oxidative activities of the as-prepared materials were evaluated by degrading phenol in water without light. The degradation rate of phenol by δ-MnO₂-{-111} was significantly higher than that by δ-MnO₂-{001}. δ-MnO₂-{-111} also displayed high degradation efficiency to a variety of other organic pollutants, such as ciprofloxacin, bisphenol A, 3-chlorophenol and sulfadiazine. Comprehensive characterization and theoretical calculation verified that the {-111} facet had high density of Mn³⁺, thus displaying enhanced direct oxidative capacity to degrade organic pollutants. In addition, the dominant {-111} facet promoted adsorption/activation of O₂, thus favored the generation of superoxide radical (O₂^•-), which actively participated in the degradation of pollutants. The phenol degradation kinetics could be divided into two distinct phases: the rapid phase (k1_obs = 0.468 min^-1) induced by Mn³⁺ and the slower phase (k2_obs = 0.048 min^-1) dominated by O₂^•-. The synergistically promoted non-radical and radical based reactions resulted in greatly enhanced the oxidative activity of the δ-MnO₂-{-111}. These findings deepen the understanding of facet-dependent oxidative performance of materials and provided valuable insights into the possible practical application of δ-MnO₂ for water purification.

Interface-Promoted Direct Oxidation of p-Arsanilic Acid and Removal of Total Arsenic by the Coupling of Peroxymonosulfate and Mn-Fe-Mixed Oxide

As one of the extensively used feed additives in livestock and poultry breeding, p-arsanilic acid (p-ASA) has become an organoarsenic pollutant with great concern. For the efficient removal of p-ASA from water, the combination of chemical oxidation and adsorption is recognized as a promising process. Herein, hollow/porous Mn-Fe-mixed oxide (MnFeO) nanocubes were synthesized and used in coupling with peroxymonosulfate (PMS) to oxidize p-ASA and remove the total arsenic (As). Under acidic conditions, both p-ASA and total As could be completely removed in the PMS/MnFeO process and the overall performance was substantially better than that of the Mn/Fe monometallic system. More importantly, an interface-promoted direct oxidation mechanism was found in the p-ASA-involved PMS/MnFeO system. Rather than activate PMS to generate reactive oxygen species (i.e., SO₄^·-, ·OH, and ¹O₂), the MnFeO nanocubes first adsorbed p-ASA to form a ligand-oxide interface, which improved the oxidation of the adsorbed p-ASA by PMS and ultimately enhanced the removal of the total As. Such a direct oxidation process achieved selective oxidation of p-ASA and avoidance of severe interference from the commonly present constituents in real water samples. After facile elution with dilute alkali solution, the used MnFeO nanocubes exhibited superior recyclability in the repeated p-ASA removal experiments. Therefore, this work provides a promising approach for efficient abatement of phenylarsenical-caused water pollution based on the PMS/MnFeO oxidation process.

Nonradicals induced degradation of organic pollutants by peroxydisulfate (PDS) and peroxymonosulfate (PMS): Recent advances and perspective

Nonradical persulfate oxidation processes have emerged as a new wastewater treatment method due to production of mild nonradical oxidants, selective oxidation of organic pollutants, and higher tolerance to water matrixes compared with radical persulfate oxidation processes. Since the case of the nonradical activation of peroxydisulfate (PDS) was reported on CuO surface in 2014, nonradical persulfate oxidation processes have been extensively investigated, and much achievement has been made on realization of nonradical persulfate activation processes and understanding of intrinsic reaction mechanism. Therefore, in the review, nonradical pathways and reaction mechanisms for oxidation of various organic pollutants by PDS and peroxymonosulfate (PMS) are overviewed. Five nonradical persulfate oxidation pathways for degradation of organic pollutants are summarized, which include surface activated persulfate, catalysts-free or catalysts mediated electron transfer, ¹O₂, high-valent metals, and newly derived inorganic oxidants (e.g., HOCl and HCO₄^-). Among them, the direct oxidation processes by persulfate, nonradical based persulfate activation by inorganic/organic molecules and in electrochemical methods is first overviewed. Moreover, nonradical based persulfate activation mechanisms by metal oxides and carbon materials are further updated. Furthermore, investigation methods of interaction between persulfate and catalyst surface, and nature of reactive species are also discussed in detail. Finally, the future research needs are proposed based on limited understanding on reaction mechanism of nonradical based persulfate activation. The review can offer a comprehensive assessment on nonradical oxidation of organic pollutants by persulfate to fill the knowledge gap and provide better guidance for future research and engineering application of persulfate.

A novel electrocatalytic filtration system with carbon nanotube supported nanoscale zerovalent copper toward ultrafast oxidation of organic pollutants

In this study, we designed an integrated electrochemical filtration system for catalytic activation of peroxymonosulfate (PMS) and degradation of aqueous microcontaminants. Composites of carbon nanotube (CNT) and nanoscale zero valence copper (nZVC) were developed to serve as high-performance catalysts, electrode and filtration media simultaneously. We observed both radical and nonradical reaction pathways, which collectively contributed to the degradation of model pollutants. Congo red was completely removed via a single-pass through the nZVCCNT filter (τ <2 s) at neutral pH. The rapid kinetics of Congo red degradation were maintained across a wide pH range (from 3.0-7.0), in complicated matrixes (e.g., tap water and lake water), and for the degradation of a wide array of persistent organic contaminants. The superior activity of nZVCCNT stems from the boosted redox cycles of Cu²⁺/Cu⁺ in the presence of an external electric field. The flow-through design remarkably outperformed the conventional batch system due to the convection-enhanced mass transport. Mechanism studies suggested that the carbonyl group and electrophilic oxygen of CNT served as electron donor and electron acceptor, respectively, to activate PMS to generate •OH and ¹O₂via one-electron transport. The electron-deficient Cu atoms are prone to react with PMS via surface hydroxyl group to produce reactive intermediates (Cu²⁺-O-O-SO₃^-), and then ¹O₂ will be generated by breaking the coordination bond of the metastable intermediate. The study will provide a green strategy for the remediation of organic pollution by a highly efficient and integrated system based on catalytic oxidation, electrochemistry, and nano-filtration techniques.

Fe-Mn Oxides Based Multifunctional Adsorptive/Electrosensing Nanoplatforms: Dynamic Site Rearrangement for Metal Ion Selectivity

Achieving structural requirements for the exclusive selectivity of adsorbent to a specific metal remains challenging, as certain metal ions show similar adsorptive behaviors and preference toward a given site. We reported the morphology and oxidation state-dependent selectivity manipulating of layered oxides by controlling the dynamic evolution of different adsorptive sites. The computational investigation predicted the site-specific partitioning trends of metal ions at two sites of manganese oxide (MnO₂) layers: the lateral edge sites (LESs) and octahedral vacancy sites (OVSs). In contrast to the predominant occupation of the OVSs for other metal ions, the binding of lead (Pb) ions was energetically favored at both the sites. We assembled ultrathin MnO₂ nanosheets on the magnetic iron oxides to first enhance the accessibility of the LESs. A sequential ligand-promoted partial reduction of the atomic MnO₂ layers induced the edge-to-interlayer migration of Mn atoms to block the nonspecific OVSs and activate the LESs, enabling a superior selectivity to Pb. In addition, the iron oxides helped construct a multifunctional adsorptive/electrosensing platform for Pb regarding their facile magnetic separation and electrochemical activity. Simultaneous selective adsorption and on-site monitoring of Pb(II) were achieved on this nanoplatform, owing to its satisfactory stability and sensitivity without an obvious matrix effect.

Synergistic effect of dual sites on bimetal-organic frameworks for highly efficient peroxide activation

Active site engineering is of significant importance for developing high activity metal-organic frameworks (MOFs) for catalytic applications. Herein, we develop a one-pot strategy to construct bimetal organic frameworks with Fe-Co dual sites for Fenton-like catalysis. Density functional theory (DFT) demonstrated that the introducing Co heteroatoms into MIL-101(Fe) (MIL represents Matérial Institute Lavoisier) was favorable for the formation of electron-deficient centers around benzene rings and electron-rich centers around Fe/Co. This synergistic effect could effectively decrease the energy barrier of H₂O₂ activation. Due to the facilitated charge transfer in the coordinated structures, MIL-101(Fe,Co) with engineered dual sites exhibited exceptionally high efficiency for the degradation of ciprofloxacin (CIP). The reaction rate of MIL-101(Fe,Co)/H₂O₂ system was 0.12 min^-1, which was nearly 7.5 times higher than that of pristine MIL-101(Fe). The reaction mechanism of heterogeneous Fenton-like catalysis was fundamentally investigated by series of in-situ techniques, such as DRIFTS and Raman. ·OH radicals generated by H₂O₂ activation endowed the inspiring ability of MIL-101(Fe,Co) for water decontamination. This work offers a facile principle of exploring MOFs-based Fenton-like catalysts with a wide working pH range for environmental applications.

Manipulating Interfaces of Electrocatalysts Down to Atomic Scales: Fundamentals, Strategies, and Electrocatalytic Applications

Raising electrocatalysis by rationally devising catalysts plays a core role in almost all renewable energy conversion and storage systems. The principal catalytic properties can be controlled and improved well by manipulation of interfaces, ascribed to the interactions among different components/players at the interfaces. In particular, manipulating interfaces down to atomic scales is becoming increasingly attractive, not only because those atoms at around the interface are the key players during electrocatalysis, but also, understandings on the atomic level electrocatalysis allow one to gain deep insights into the reaction mechanism. With the feature down-sizing to atomic scales, there is a timely need to redefine the interfaces, as some of them have gone beyond the conventionally perceived interfacial concept. In this overview, the key active players participating in the interfacial manipulation of electrocatalysts are examined, from a new angle of "atomic interface," including those individual atoms, defects, and their interactions, together with the essential characterization techniques for them. The specific approaches and pathways to engineer better atomic interfaces are investigated, and thus to enable the unique electrocatalysis for targeted applications. Looking beyond recent progress, the challenges and prospects of the atomic level interfacial engineering are also briefly visited.

General synthesis of carbon and oxygen dual-doped graphitic carbon nitride via copolymerization for non-photochemical oxidation of organic pollutant

Earth-abundant, environmental-benign and durable catalysts are of paramount importance for remediation of organic pollutants, and graphitic carbon nitride (g-C₃N₄) is a promising nonmetallic material for this application. However, the catalytic oxidation on g-C₃N₄ suffers from low efficiency because of its chemical inertness if not irradiated with light. Herein, we develop a facile copolymerization strategy for the synthesis of carbon and oxygen dual-doped g-C₃N₄ using urea as g-C₃N₄ precursor and ascorbic acid (AA) as carbon and oxygen sources, which induces electronic structure reconfiguration. By replacing AA with other organic precursors, a series of C and O dual-doped g-C₃N₄ are successfully prepared, demonstrating the generality of the developed methodology. As a demonstration, the C and O dual-doped g-C₃N₄ using AA as the organic precursor (CN-AA_0.3) exhibits pronouncedly enhanced catalytic activity in peroxymonosulfate (PMS) activation for organic pollutant degradation without light irradiation compared with pristine g-C₃N₄ and single oxygen-doped g-C₃N₄. Experimental and theoretical results revealed the electron-poor C atoms and electron-rich O atoms as active sites for PMS activation in terms of simultaneous PMS oxidation and reduction. This work offers a universal approach to synthesize nonmetal dual-doped g-C₃N₄ with reconfigured electronic structure, stimulating the development of g-C₃N₄-based materials for diverse environmental applications.

Nitrogen-doped porous carbon encapsulating iron nanoparticles for enhanced sulfathiazole removal via peroxymonosulfate activation

Developing novel catalyst with both high efficiency and stability presents an enticing prospect for peroxymonosulfate (PMS) activation. In this paper, nitrogen-doped porous carbon encapsulating iron nanoparticles (CN-Fe) was fabricated by a facile carbothermal reduction process using polyaniline (PANI) and α-Fe₂O₃ as the precursors. The stubborn antibiotics, sulfathiazole (STZ), was employed as a target pollutant, demonstrating that CN-Fe coupled with PMS could achieve 96% removal efficiency and even 57% mineralization rate of STZ within 40 min. More importantly, the rate constant of CN-Fe was calculated to be 0.07665 min^-1, which was 6 times higher than that of the commercial α-Fe₂O₃ catalyst. Furthermore, CN-Fe also presented a favorable catalytic performance for removing other organic pollutants including phenolic compounds and organic dyes. Interestingly, the catalytic activity of the used CN-Fe catalyst could be regenerated after thermal treatment (600 °C) and the as-synthesized CN-Fe catalyst exhibited excellent long-term stability with almost no loss of activity after storage for three months. The catalytic mechanism in the CN-Fe/PMS system was elucidated by electron paramagnetic resonance (EPR), linear sweep voltammetry (LSV), radical and electron trapping tests, which confirmed that sulfate radicals (SO₄^-), hydroxyl radicals (OH), superoxide radicals (O₂^-) and singlet oxygen (¹O₂) were generated in the oxidation process with the assistance of electron transfer between PMS and catalyst. To our knowledge, this was the first attempt for the application of PANI-derived CN-Fe hybrid materials as PMS activators and the findings would provide a simple and promising strategy to fabricate highly efficient and environment-benign catalysts for wastewater remediation.

A Bimetallic Fe-Mn Oxide-Activated Oxone for In Situ Chemical Oxidation (ISCO) of Trichloroethylene in Groundwater: Efficiency, Sustained Activity, and Mechanism Investigation

Bimetallic Fe-Mn oxide (BFMO) has been regarded as a promising activator of peroxysulfate (PS), the sustained activity and durability of BFMO for long-term activation of PS in situ, however, is unclear for groundwater remediation. A BFMO (i.e., Mn_1.5FeO_6.35) was prepared and explored for PS-based in situ chemical oxidation (ISCO) of trichloroethylene (TCE) in sand columns with simulated/actual groundwater (SGW/AGW). The sustained activity of BFMO, oxidant utilization efficiency, and postreaction characterization were particularly investigated. Electron spin resonance (ESR) and radical scavenging tests implied that sulfate radicals (SO₄^•-) and hydroxyl radicals (HO^•) played major roles in degrading TCE, whereas singlet oxygen (¹O₂) contributed less to TCE degradation by BFMO-activated Oxone. Fast degradation and almost complete dechlorination of TCE in AGW were obtained, with reaction stoichiometry efficiencies (RSE) of ΔTCE/ΔOxone at 3-5%, much higher than those reported RSE values in H₂O₂-based ISCO (≤0.28%). HCO₃^- did not show detrimental effect on TCE degradation, and effects of natural organic matters (NOM) were negligible at high Oxone dosage. Postreaction characterizations displayed that the BFMO was remarkably stable with sustained activity for Oxone activation after 115 days of continuous-flow test, which therefore can be promising catalyst for Oxone-based ISCO for TCE-contaminated groundwater remediation.

Synergistic Features of Superoxide Molecule Anchoring and Charge Transfer on Two-Dimensional Ti3C2Tx MXene for Efficient Peroxymonosulfate Activation

The heterogeneous Fenton-like process is regarded as a promising approach to produce reactive oxygen species for water purification and environmental remediation. Here, we report a simple and rational strategy for the design of an efficient catalyst by reducing the dimensionality instead of changing the composition or structure. Based on theoretical and experimental evidence, considerable active sites were exposed on the low-dimensional Ti₃C₂T_x monolayer surface and showed outstanding reactivity toward peroxymonosulfate activation, which was mainly because of the superior compatibility between the highest occupied molecular orbital of catalysts and lowest unoccupied molecular orbital of Oxone. Stimulated emission depletion super-resolution microscopy innovatively provided visual insights into the spatiotemporal heterogeneous activation process and revealed that the unilaminar Ti₃C₂T_x nanosheet exhibited preferable reaction dynamics relative to its inert bulk counterpart, with an aqueous 2,4-dichlorophenoxyacetic acid degradation rate ∼376 times higher than that when using bulk Ti₃C₂T_x as the activator.

Tuning of Persulfate Activation from a Free Radical to a Nonradical Pathway through the Incorporation of Non-Redox Magnesium Oxide

Nonradical-based advanced oxidation processes for pollutant removal have attracted much attention due to their inherent advantages. Herein we report that magnesium oxides (MgO) in CuOMgO/Fe₃O₄ not only enhanced the catalytic properties but also switched the free radical peroxymonosulfate (PMS)-activated process into the ¹O₂ based nonradical process. CuOMgO/Fe₃O₄ catalyst exhibited consistent performance in a wide pH range from 5.0 to 10.0, and the degradation kinetics were not inhibited by the common free radical scavengers, anions, or natural organic matter. Quantitative structure-activity relationships (QSARs) revealed the relationship between the degradation rate constant of 14 substituted phenols and their conventional descriptor variables (i.e., Hammett constants σ, σ^-, σ⁺), half-wave oxidation potential (E_1/2), and pK_a values. QSARs together with the kinetic isotopic effect (KIE) recognized the electron transfer as the dominant oxidation process. Characterizations and DFT calculation indicated that the incorporated MgO alters the copper sites to highly oxidized metal centers, offering a more suitable platform for PMS to generate metastable copper intermediates. These highly oxidized metals centers of copper played the key role in producing O₂^•- after accepting an electron from another PMS molecule, and finally ¹O₂ as sole reactive species was generated from the direct oxidation of O₂^•- through thermodynamically feasible reactions.

Novel α-Fe2O3/MXene nanocomposite as heterogeneous activator of peroxymonosulfate for the degradation of salicylic acid

The development of non-cobalt-based heterogeneous catalysts with efficient catalytic activity, good stability and nontoxicity is very important for the application of peroxymonosulfate-based advanced oxidation processes (AOPs) in water treatment. In this work, with two dimensional MXene as the catalyst substrate, a novel α-Fe₂O₃/MXene (FM) nanocomposite was fabricated through a facile solvothermal method. Systematic characterization demonstrated that the MXene substrate could facilitate the size reduction and good dispersion of α-Fe₂O₃ nanoparticles. The FM nanocomposite achieved high efficiency and stability towards activating peroxymonosulfate (PMS) to produce free radicals for the degradation of salicylic acid (SA) in aqueous solution. The operating parameters, including catalyst dosage, PMS dosage, SA concentration and initial pH value, were evaluated and analysed. The co-existence of sulfate radicals (SO₄^-) and hydroxyl radicals (OH) was confirmed using electron paramagnetic resonance spectroscopy and radical scavenger tests, while SO₄^-was identified as the main reactive species in the FM/PMS catalytic system. The possible mechanisms for the electron transfer and radical generation during the process of PMS activation by the FM nanocomposite are further investigated using XPS and in situ Raman analysis. The results provide an avenue for rationally constructing and developing alternative catalysts for the treatment of organics in wastewater.

Mn-based catalysts for sulfate radical-based advanced oxidation processes: A review

Sulfate radical-based advanced oxidation processes (AOPs) have drawn increasing attention during the past two decades, and Mn-based materials have been proven to be effective catalysts for activating peroxymonosulfate (PMS) and peroxydisulfate (PDS) to degrade many contaminants. This article presents a comprehensive review of various Mn-based materials to activate PMS and PDS. The activation mechanisms of different Mn-based catalysts (i.e., Mn oxides MnO_x, MnO_x hybrids, and MnO_x‑carbonaceous material composites) were first summarized and discussed in detail. Besides the commonly reported free radicals (SO₄^-• and ^•OH), non-radical mechanisms such as singlet oxygen and direct electron transfer have also been discovered for selected materials. The effects of pH, inorganic ions, natural organic matter (NOM), dissolved oxygen content, temperature, and the crystallinity of the materials on the catalytic reactivity were also discussed. Then, important instrumentations and technologies employed to characterize Mn-based materials and to understand the reaction mechanisms were concisely summarized. Three common overlooks in the experimental designs for examining the PMS/PDS-MnO_x systems were also discussed. Finally, future research directions were suggested to further improve the technology and to provide a guidance to develop cost-effective Mn-based materials to activate PMS/PDS.

Calcined CoAl-layered double hydroxide as a heterogeneous catalyst for the degradation of acetaminophen and rhodamine B: activity, stability, and mechanism

Peroxymonosulfate (PMS) activated by nanomaterials presents one of the most promising strategies to generate reactive species for remediation of organic pollutant-contaminated water. In this study, CoAl-layered double hydroxide (CoAl-LDH) and calcined CoAl-LDH (CoAl-CLDH) were employed as catalysts for PMS activation towards aqueous organic pollutants degradation. Our experiments showed that the leaching of metal ions from catalyst can be significantly mediated by calcination treatment, which can avoid the secondary contamination. The stable CoAl-CLDH exhibited a high catalytic activity, which is comparable to that of the unstable CoAl-LDH. Importantly, reactive species quenching and electron paramagnetic resonance (EPR) results revealed that singlet oxygen (¹O₂) is the dominant reactive species and plays a crucial role in the catalytic oxidation process in CoAl-CLDH/PMS system. A possible mechanism was proposed for the activation of PMS on the CoAl-CLDH. We demonstrate that CoAl-CLDH is a highly active and stable heterogeneous catalyst for efficient catalytic oxidation of organic pollutants (such as acetaminophen and rhodamine B (RhB)) via activation of PMS.

Polyaniline: A New Metal-Free Catalyst for Peroxymonosulfate Activation with Highly Efficient and Durable Removal of Organic Pollutants

Metal-free heterogeneous catalysts are receiving more and more attention for wastewater remediation by activating peroxymonosulfate (PMS) due to their environmental benign. However, carbon-based materials as the most typical metal-free heterogeneous always suffer from poor durability. Inspired by the fact that a conjugated system may facilitate the electron transfer during PMS activation, we innovatively select polyaniline (PANI) as a new PMS activator and investigate its catalytic performance in detail. It is found that PANI can display better catalytic performance than traditional metal-based catalysts and popular N-doped carbocatalysts in methyl orange (MO) degradation. More importantly, PANI is not only universal for various pollutants degradation but also maintains its catalytic performance in repeated degradation experiments. The stable N sites in the conjugated chains and the oxidation-resistance benzene rings as the building units are considered to be responsible for such an excellent durability. In addition, the influences of some routine factors and actual water backgrounds are comprehensively checked and analyzed. The quenching experiments and electron paramagnetic resonance confirm that MO degradation is achieved through both radical and nonradical pathways, where SO₄^•- and ¹O₂ are primary reactive species. The reaction mechanism is also proposed with the assistance of X-ray photoelectron spectroscopy.