Single-Atom MnN5 Catalytic Sites Enable Efficient Peroxymonosulfate Activation by Forming Highly Reactive Mn(IV)-Oxo Species

MnO/MnS nanoparticles encapsulated in Lycopodium spores derived nitrogen-doped porous carbon as a cost-effective peroxymonosulfate activator for pollutant decontamination: Insight into the mechanism of electron transfer-dominated non-radical pathway

The rational design and exploitation of cost-effective and robust catalysts for peroxymonosulfate (PMS) activation is of great significance. Herein, MnO/MnS nanoparticles encapsulated in Nitrogen-doped porous carbon skeleton (abbreviated as MnO/MnS@NPC) were first constructed through an easy two-step of impregnation along with subsequent pyrolysis technique and utilized to activate PMS for the elimination and mineralization of tetracycline (TC). Benefiting from the strong coupling of transition metal Mn with carbon-based material, the co-doping of heteroatom N and S, the enhanced electrical conductivity, and the hierarchical porous microarchitecture, the obtained MnO/MnS@NPC composite has been expected to present superior PMS activation capacity and pollutant elimination efficiency to its benchmark NPC and MnO@NPC, with 92.5 % degradation rate of TC within 60 min. Comprehensive investigations, involving quenching experiments, electron paramagnetic resonance (EPR) studies, in situ Raman identification, and electrochemical tests, jointly indicated that the non-radical pathways including electron-transfer, single oxygen (1O2) and the high-valent Mn-oxo species, especially the electron transfer process (ETP) from TC molecule to the metastable MnO/MnS@NPC-PMS* complex were dominantly responsible for PMS activation and further decomposition of TC, which greatly enhanced the selective removal of TC and the anti-interference capacity of the PMS system. Furthermore, the possible TC degradation routes were predicted by Density Functional Theory (DFT) calculation and the toxicity of degradation intermediates were also analyzed by toxicity assessment software. In addition, the heterogeneous catalyst displayed outstanding stability and reusability owing to the shield effect of NPC framework to MnO/MnS nanoparticles. Overall, this work proposed a prospective strategy for rationally designing and exploring heterogeneous PMS activator towards wastewater purification.

State-Of-The-Art Structural Regulation Methods and Quantum Chemistry for Carbon-Based Single-Atom Catalysts in Advanced Oxidation Process: Critical Perspectives into Molecular Level

Advanced oxidation processes (AOPs) by carbon-based single-atom catalysts (SACs) are recognized as an attractive scientific frontier for water treatment, with the outstanding benefits of ultra-effective and anti-interference capability. However, most of the research has paid more attention to the performance of SACs, while the in-depth understanding of catalytic regulation by molecular interaction is relatively deficient. This critical review delves into deciphering the catalytic mechanism through a micro-level, which makes it more convenient to interpret apparent catalytic phenomena. It first summarizes basic theories of quantum chemistry, which provide mechanism interpretation and prediction for molecular-oxidation systems. Additionally, corresponding oxidation pathways of common oxidants are underscored. Following the oxidants, state-of-the-art regulation methods are discussed with special attention to involved molecular interactions and pollutants. Particularly, the preliminary insights into the "oxidant-catalyst-pollutants" internal relationships are provided to help construct the SAC-AOP system from a molecular standpoint. Meanwhile, some cutting-edge laboratory devices and pilot-scale engineering are presented to illustrate the ultimate purpose of scientific molecular exploration. Eventually, relative challenges of SACs-AOPs upon the design of catalytic systems and investigation methods are provided. This review aims to promote the large-scale potential of SACs-based AOPs in practical water treatment by emphasizing the pivotal role of micro-insights.

Site-Specific Spin State Modulation in Spinel Oxides for Enhanced Nonradical Oxidation

Spinel oxides hold tremendous potential for driving advanced oxidation processes, yet the underlying mechanism for maximizing their activity remains unclear. In this study, we leverage tetrahedral and octahedral site interactions in MnxCo3-xO4 to modulate the spin states, specifically spin alignment and spin moment, thereby enhancing periodate (PI) activation and catalytic performance in contaminant degradation. Through combined experimental and density functional theory (DFT) analyses, we elucidate the role of spin alignment at synergetic tetrahedral and octahedral sites in facilitating quantum spin exchange interactions (QSEI) with an efficient electronic spin channel for charge transfer. Meanwhile, the engineered high spin configuration in CoMn2O4 raises the d-band center, favoring stable PI* surface complex formation and accelerating the rate-determining desorption of IO3 - with a lower-ICOHP value during the catalytic degradation of ciprofloxacin. As a result, the fine-tuned spin state of CoMn2O4 leads to enhanced overall reaction kinetics, with a 2.5-fold increase compared to MnCo2O4 and up to 22-fold increase compared to the octahedrally-active only catalysts. Such a site-specific modulation has been found applicable to other spinel oxides, enlightening fine-tuned electronic structure for maximizing catalytic performance.

Isotope Techniques in Chemical Wastewater Treatment: Opportunities and Uncertainties

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

One Heterogeneous Catalyst Drives Two Selective Fenton-like Reaction Modes for Sustainable Water Decontamination

Heterogeneous Fenton-like reactions based on nonradical reactive oxygen species (ROS) are desirable for selective water decontamination, and different pollutants coexisting in real scenarios necessitate a rational combination of multiple ROS for efficient and sustainable decontamination. However, the general one-catalyst-for-one-ROS strategy toward selective ROS generation inevitably renders the combinational process lengthy and cost ineffective. Herein, we developed a new approach to enable the separate but selective generation of two distinct ROS in one catalyst via peroxymonosulfate activation. The unique catalyst is comprised of a graphitic layer bottom-wrapped Fe@Fe3C encapsulated inside nitrogen-doped carbon nanotubes. The Fe3C shell facilitates selective formation of surface-bound FeIV═O with up to 96.0% selectivity, and the applied electric field could switch ROS generation toward free 1O2 with 90.5% selectivity, as enabled by C atoms adjacent to graphite N. One dual-site catalyst enables both high cumulative concentration for FeIV═O and 1O2 up to 16605 and 7674 μM at 30 min, respectively. Based on such a simple electricity on/off switch mode, a tandem process operated in one unit was proposed to efficiently degrade mixed pollutants of distinct adsorption properties. This study presents a simple but very effective strategy to modulate selective ROS generation that simplifies tandem Fenton-like systems for sustainable water decontamination.

Ultrahigh Peroxymonosulfate Utilization Over a Single-Atom Iron-N-C Catalyst for Efficient Fenton-Like Chemistry via Surface-Bound Reactive Complexes

Transition metal single-atom catalysts (SACs) find extensive application in peroxymonosulfate (PMS)-based advanced oxidation processes (AOPs). Yet, the disparity in intrinsic activity is often attributed to thermodynamics, but few studies focused on the electronic structure between different metals. Herein, transition metal catalysts in the form of single-atom M-N4 moieties moored to graphitic carbon nitride (denoted MSA CN, M = Fe, Co, and Cu) are developed and used for activating PMS for the degradation of 4-chlorophenol. Remarkably, FeSA CN achieves a catalyst-dose-normalized kinetic rate constant of 34.2 L min-1 g-1, surpassing reported systems by 2-551 times ─ even at ultralow catalyst (0.06 mg L-1) and PMS (0.2 mm) concentration. The in situ formation of surface-bound PMS* complexes enabled the degradation of 4-chlorophenol to achieve unprecedented utilization efficiency (≈100%) through highly efficient non-radical pathways. Density functional theory calculations revealed that large spin polarization of Fe-N-C sites facilitated the d orbitals to overlap with the PMS on the metal active sites and promoted electron transport, thereby facilitating PMS adsorption and enhancing the oxidation capacity. This work establishes a mechanistic foundation for designing a single Fe-atom catalyst/PMS system in Fenton-like water treatment.

Spin State Regulation for Peroxide Activation: Fundamental Insights and Regulation Mechanisms

Peroxides are widely used in environmental applications due to their strong oxidizing properties, however, traditional activation methods often face challenges such as uncontrolled reactive oxygen species (ROS) generation and high energy barriers. Recent advancements in spin state regulation provide a promising alternative to enhance the efficiency of peroxide activation. This review provides an overview of spin fundamentals and discusses the key factors affecting spin state in catalytic materials, including crystal field configuration, ligand environment, and valence changes. Subsequently, the role of electron spin state in peroxide activation is comprehensively analyzed, with a focus on how spin state regulation can tune adsorption energy, lower energy barriers, facilitate electron transfer between transition metals and peroxides, and promote selective ROS generation. Finally, this review briefly outlines the practical applications of peroxide activation in water treatment and concludes with a summary and perspectives on future research directions. This review aims to provide a comprehensive perspective on the role of spin state regulation in advancing peroxide activation strategies.

Precise manipulation of iron spin states in single-atom catalytic membranes for singlet oxygen selective production

Heterogeneous single-atom catalysts are attracting substantial attention for selectively generating singlet oxygen (1O2). However, precise manipulation of atom coordination structures remains challenging. Here, the fine coordination structure of iron single-atom carbon-nitride catalysts (Fe-CNs) was manipulated by precisely tuning the heating rate with 1 °C min-1 difference. Multiple techniques in combination with density functional theory (DFT) calculations reveal that FeN6 coordination sites with high Fe spin states promote the adsorption, electron transfer, and dissociation of peroxymonosulfate (PMS), resulting in nearly 100% selection of 1O2 generation. A lamellar single atom catalytic membrane is constructed, exhibiting high permeance, high degradation, high-salinity resistance and sustained operation stability. This work provides ideas for regulating spin states of the metal site to fabricate catalysts with selective 1O2 generation for membrane separation and environment catalysis applications.

Robust Fe-N4-C6O2 single atom sites for efficient PMS activation and enhanced FeIV = O reactivity

The microenvironment regulation of Fe-N4 single atom catalysts (SACs) critically governs peroxymonosulfate (PMS) activation. Although conventional heteroatom substitution in primary coordination enhances activity, it disrupts Fe-N4 symmetry and compromises stability. Herein, we propose oxygen doping in the secondary coordination shell to construct Fe-N4-C6O2 SAC, which amplifies the localized electric field while preserving the pristine coordination symmetry, thus trading off its activity and stability. This approach suppresses Fe-N bond structural deformation (bond amplitude reduced from 0.875-3.175 Å to 0.925-2.975 Å) during PMS activation by lowering Fe center electron density to strengthen Fe-N bond, achieving extended catalytic durability (>240 h). Simultaneously, the weakened coordination field lowers the Fe=O σ* orbital energy, promoting electrophilic σ-attack of high-valent iron-oxo towards bisphenol A, and increasing its degradation rate by 41.6-fold. This work demonstrates secondary coordination engineering as a viable strategy to resolve the activity-stability trade-off in SAC design, offering promising perspectives for developing environmental catalysts.

Switchable surface FeII/III sites for water/sediment remediation through enhanced selective oxidation and ROS regulation: Performance, mechanism and application

Selective oxidation relying on high-valent iron-oxo species (Fe(IV/V)) is a promising way of effective organic decontamination. However, Fe(IV/V) formation and further purposeful reinforcement production are commonly insufficient and unsustainable. Herein, cerium (Ce) modification strategy was adopted for efficient micropollutants removal through boosting Fe(IV/V) generation. Kinetic rate of sulfamethoxazole (SMX) removal through peracetic acid (PAA) activation by FeCe-O-CN is 4.1-fold of that without Ce doping. Ce modification lowered energy barrier of the key reaction pathway (*OH→*O) during Fe(IV/V) formation and accelerated the exposure of the surface FeII site for Fe(IV) production. Steady-state concentration of Fe(IV) and Fe(V) in FeCe-O-CN/PAA process is 2.5 × 10-8 and 9.7 × 10-11 M with its corresponding contribution to SMX removal as 64 % and 36 %. Not only intensified SMX removal, Ce modification significantly reduced the toxicity of transformation products. Furthermore, FeCe-O-CN/PAA system satisfies favorable decontaminant in long-term runs, anti-interference, and significantly alleviated bioaccumulation in plants. This study provides a new insight into the association between Ce modification and Fe(IV/V) generation in PAA activation and offered a feasible way for enhanced selective oxidation.

An S-Scheme MOF-on-MXene Heterostructure for Enhanced Photocatalytic Periodate Activation

Fully understanding the periodate (PI) activation system is still a great challenge, which calls for efficient heterogeneous catalysts with a sophisticated structure. Herein, we developed "MOF-on-MXene" heterostructures. By constructing S-scheme heterostructures MXene/Z67450, the internal electric field is generated via the Ti-O-Co bonds at the interface, favoring the excitation of the photogenerated electrons, providing a driving force for accelerating the charge transfer, and enhancing redox performances. Further contributed by the synergy of Ti-O-Co and Co-N4 bonds, the MXene/Z67450 composites exhibit enhanced ability for activating the periodate system to degrade organic pollutants via building a donor-catalyst-acceptor system. In the presence of periodate and light, MXene/Z67450 degraded ∼100% of tetracycline hydrochloride (TCH) in only 10 min. The active sites of the heterostructures can react with the periodate and give the intermediate MXene/Z67450-PI (*PI). As a result, it efficiently reduced the PI adsorption energy and promoted the decomposition of PI and the formation of holes/electrons, singlet oxygen (1O2) as well as hydroxyl radical (•OH). In addition, the MXene/Z67450 composites exhibit high stability, reusability, selectivity, and environmental robustness. Our study provides a research direction for rationally designing MXene-based heterojunctions and applying them in the periodate activation system.

Trimetallic MOF-derived Fe-Mn-Sn oxide heterostructure enabling exceptional catalytic degradation of organic pollutants

Developing efficient and environmentally benign heterogeneous catalysts that activate peroxymonosulfate (PMS) for the degradation of persistent organic contaminants remains a challenge. Metal-organic frameworks (MOFs)-derived metal oxide catalysts in advanced oxidation processes (AOPs) have received considerable attention research fraternity. Herein, we report an innovative magnetic trimetallic MOF-derived Fe-Mn-Sn oxide heterostructure (FeMnO@Sn) with adjustable morphology, size and Sn content, prepared through an impregnation-calcination strategy. The formation of a novel magnetic Fe2O3/Fe3O4/Mn3O4 heterostructure induces the generation of abundant Fe2+ and Mn2+ sites on the FeMnO@Sn surface. Meanwhile, the introduction of SnO2 into the Fe2O3/Fe3O4/Mn3O4 heterostructure facilitates the cleavage of the OO bond in adsorbed PMS. The synergy among the different functionalities of each metal oxide plays a vital role in the swift and effective degradation of pollutants. In addition, the uniquely designed catalyst exhibits magnetic properties that facilitate easy recycling and repeated use, thereby meeting environmental protection requirements. Overall, this research highlights the design of heterogeneous catalysts for the effective activation of PMS and provides valuable insights for the advancement of future environmental catalysts.

Insights into excitonic behavior in single-atom covalent organic frameworks for efficient photo-Fenton-like pollutant degradation

The generation of radicals through photo-Fenton-like reactions demonstrates significant potential for remediating emerging organic contaminants (EOCs) in complex aqueous environments. However, the excitonic effect, induced by Coulomb interactions between photoexcited electrons and holes, reduces carrier utilization efficiency in these systems. In this study, we develop Cu single-atom-loaded covalent organic frameworks (CuSA/COFs) as models to modulate excitonic effects. Temperature-dependent photoluminescence and ultrafast transient absorption spectra reveal that incorporating acenaphthene units into the linker (CuSA/Ace-COF) significantly reduces exciton binding energy (Eb). This modification not only enhances peroxymonosulfate adsorption at Cu active sites but also facilitates rapid electron transfer and promotes selective hydroxyl radical generation. Compared to CuSA/Obq-COF (Eb = 25.6 meV), CuSA/Ace-COF (Eb = 12.2 meV) shows a 39.5-fold increase in the pseudo-first-order rate constant for sulfamethoxazole degradation (0.434 min-1). This work provides insights into modulating excitonic behavior in single-atom catalysts via linker engineering for EOCs degradation.

Unexpected chloride-triggered organics removal in the zirconium oxide activated peroxymonosulfate system

Chloride ion (Cl-) is ubiquitous in diverse water bodies, yet poses a longstanding challenge in water pollution control by hindering the efficiency of pollutant degradation. Herein, we proposed a novel concept involving the direct utilization of endogenous Cl- ions in water for rapid water purification within a non-redox zirconium oxide (ZrO2)-activated peroxymonosulfate (PMS) system. In this process, PMS was complexed on the ZrO2 surface through inner-sphere coordination, and effectively activated by the partial electron cloud deviation from Zr(IV) sites to PMS, thereby forming a metastable surface complex with an elevated redox potential. Afterwards, the coexistence of Cl- could trigger the transformation of the reactive complex into free chlorine species, thus leading to a 255.0-fold enhancement in the elimination rate of micropollutants compared with the ZrO2/PMS system. Quantitative structure-activity relationship analysis revealed that the ZrO2/PMS/Cl- system displayed strong target-dependence towards electron-rich compounds, showcasing a faster oxidation rate for pollutants with higher EHOMO energy levels. Significantly, the novel system performed robust resistance to complex water matrices, achieved low oxidant consumption for pollutant removal, and demonstrated adaptation across a broad range of Cl- concentrations (1.0-100.0 mM). Overall, our findings provide new mechanistic insights into the influence of Cl- ions on PMS activation, which refresh the understanding of the role of Cl- ions on pollutant degradation, and help to guide the treatment design for chloride-containing wastewater.

Atomically Dispersed Fe-Mo Catalysts Mediate Fenton-Like Reaction to Efficiently Degrade Chlorophenol Pollutants Through Synergistic Oxidation and Dechlorination Reactions

Chlorophenols are difficult to degrade and mineralize by traditional advanced oxidation processes due to the strong electronegativity of chlorine. Here, a dual-site atomically dispersed catalyst (FeMoNC) is reported, which Fe/Mo supported on mesoporous nitrogen-doped carbon is prepared through high-temperature migration. The FeMoNC exhibits a high dechlorination rate of 93.3% within 1 min. Theoretical calculation suggested that the doping of high-valence Mo6+ as the electron reservoir, promoted electronic delocalization at Fe sites, thereby enhancing the adsorption and dissociation of peroxymonosulfate (PMS), subsequent generation of Fe (IV) = O and singlet oxygen (1O2) species. An interesting finding is that Mo sites can adsorb chlorine sites in 4-chlorophenol (4-CP) and induce C─Cl bond fracture. Thus, the FeMoNC/PMS system has high catalytic performance due to the synergistic effects of Mo-induced dechlorination and non-radical species (Fe(IV) = O and 1O2) as the degradation pathways, the degradation efficiency of 99.1% of 4-CP within 5 min without significant performance decline after 168 h ≈15,120-bed volumes. These findings can advance mechanistic understanding of PMS activation at the molecular level and guide the rational design of efficient eco-friendly single-atom catalysts (SACs) catalysts with bimetallic atomic sites.

Magnesium Oxide-Supported Single Atoms with Fine-Modulated Steric Location for Polymerization Transfer Removal of Water Pollutants

Organic pollutants removal via a polymerization transfer (PT) pathway based on the use of single-atom catalysts (SACs) promises efficient water purification with minimal energy/chemical inputs. However, the precise engineering of such catalytic systems toward PT decontamination is still challenging, and the conventional SACs are plagued by low structural stability of carbon material support. Here, we adopted magnesium oxide (MgO) as a structurally stable alternative for loading single copper (Cu) atoms to drive peroxymonosulfate-based Fenton-like reactions. Through fine-tuning the Cu atom steric location from lattice-embedding to surface-loading, the system exhibited a fundamental transition in the catalytic pathways toward the PT process and drastically improved decontamination efficiency. The catalytic pathway change was mainly ascribed to a downshifted d-band center of the Cu atoms. The optimized catalyst achieved complete, rapid removal of phenolic compounds from water via nearly 100% PT pathway, accompanied by high oxidant utilization efficiency surpassing most state-of-the-art SACs. Moreover, it showed excellent structural stability and environmental robustness and was successfully used for the treatment of lake water and industrial coking wastewater. The adaptability of the spatial engineering strategy to other MgO-supported single atoms, including Fe, Co, and Ni SACs, was also demonstrated. Our work lays a foundation for further advancing SACs-based advanced oxidation technologies toward sustainable water purification applications.

Support work-function dependent Fenton-like catalytic activity of Co single atoms for selective cobalt(IV)=O generation

In Fenton-like reactions, high-valent cobalt-oxo (CoIV=O) has attracted increasing interests due to high redox potential, long lifetime, and anti-interference properties, but its generation is hindered by the electron repulsion between the electron rich oxo- and cobalt centers. Here, we demonstrate CoIV=O generation from peroxymonosulfate (PMS) activation over cobalt single-atom catalysts (Co-SACs) using in-situ Co K-edge X-ray absorption spectra, and discern that CoIV=O generation is dependent on the support work-function (WF) due to the strong electronic metal-support interaction (EMSI). Supports with a high WF value like anatase-TiO2 facilitate the binding of PMS-terminal oxo-ligand to Co sites by extracting Co-d electrons, thus decreasing the generation barrier for the critical intermediate (Co-OOSO32-). The Co atoms anchored on anatase-TiO2 (Co-TiO2) exhibited enhanced CoIV=O generation and superior activity for sulfamethoxazole (SMX) degradation during PMS activation. The normalized steady-state concentration of CoIV=O in Co-TiO2/PMS system was three orders of magnitude higher than that of free radicals, and 1.3- to 11-fold higher than that generated in other Co-SACs/PMS systems. Co-TiO2/PMS sustained efficient removal of SMX with minimal Co2+ leaching under continuous flow operation, suggesting its attractive water purification potential. Overall, these results underscore the significance of support selection for enhanced generation of high-valent metal-oxo species and efficient PMS activation in supported metal SACs.

Revealing the Overlooked Catalytic Ability of γ-Al2O3: Efficient Activation of Peroxymonosulfate for Enhanced Water Treatment

Activated alumina (γ-Al2O3) is one of the few nanomaterials manufactured at a ton-scale and successfully implemented in large-scale water treatment. Yet its role in advanced oxidation processes (AOPs) has primarily been limited to functioning as an inert carrier due to its inherently nonredox nature. This study, for the first time, presents the highly efficient capability of γ-Al2O3 to activate peroxymonosulfate (PMS) for selectively eliminating electron-rich organic pollutants in the presence of Cl-. Through experimental and theoretical analysis, we revealed that γ-Al2O3, characterized by uniquely strong Lewis acid sites, enabled robust inner-sphere complexation between PMS and Al(III) sites, triggering the oxidation of Cl- to free chlorine through a distinctive, low-energy-barrier Eley-Rideal pathway. Such a unique pathway resulted in a 42.7-fold increase in free chlorine generation, culminating in a remarkable 145.9-fold enhancement in the degradation of carbamazepine (CBZ) compared with the case without γ-Al2O3. Furthermore, this catalyst exhibited high oxidant utilization efficiency, stable performance in real-world environmental matrices, and sustained long-term activation for over 1206 bed volumes (BV) with a CBZ removal rate exceeding 90% in fixed-bed experiments. These favorable features render γ-Al2O3 an extremely promising nanomaterial for sustainable water treatment initiatives.

Mesoporous Atomically Dispersed Fe Catalysts with Enhanced Nonradical Pathways in Fenton-like Reactions: The Role of SiO2 Templates

Single-atom catalysts (SACs) are extensively applied in Fenton-like catalytic processes to treat water pollutants. However, the role of the porous structures of SACs supports in catalytic reactions is often overlooked despite its significant contribution to mass diffusion during the reaction. Herein, we adopted a hard-template-assisted approach to fabricate Fe-based SACs (Fe-SACs) featuring a mesoporous architecture. The SiO2 template not only adjusts the pore architecture of the support but also facilitates the conversion of active sites from nanoscale sites to single-atom sites, thereby improving the selectivity for pollutant degradation via nonradical pathways (singlet oxygen and electron transfer mechanism). The experimental results demonstrated that using large-sized SiO2 (∼200 nm) as a template leads to metal aggregation on its surface, forming Fe nanoparticles (Fe-NPs). Fe-NPs exhibit narrow pore structures that prevent peroxymonosulfate (PMS) from being activated, resulting in a slow degradation of pollutants primarily through radical pathways. In contrast, employing small-sized SiO2 (∼10 nm) as a hard template not only produces supports with mesoporous structures but also promotes the building of single-atom active sites. The prepared Fe-SACs effectively activated PMS through nonradical pathways and removed contaminants at a rate k of 0.89 min-1, 33 times faster than Fe-NPs. This template-assisted method sheds light on the synthesis of effective Fenton-like catalysts with porous structures that enhance the efficient breakdown of contaminants in wastewater.

Revealing the essence of anion ligands in regulating amorphous MnOx to activate peracetic acid for micropollutant removal

How the anion ligands of manganese precursors affect the catalytic activity of amorphous manganese oxides (MnOx) in Fenton-like process is poorly understood. Here, five amorphous MnOx synthesized by Mn(II) precursors with different ligands were characterized and adopted to activate peracetic acid (PAA) for bisphenol A (BPA) degradation. Although > 90 % BPA removal was achieved in the five MnOx/PAA processes via both adsorption and oxidation, the oxidation kobs greatly differentiates by the ligands types with the order of MnOx-N > MnOx-S > MnOx-Cl > MnOx-AA > MnOx-OA. Ligands types would affect the specific surface area of MnOx and their ability to adsorb BPA, however which is not the decisive factor in determining the contaminant oxidation efficiency. Multiple experimental results indicate that the generation of oxygen vacancies induced by the ligands alters the Mn(III)/Mn(IV) ratio, ultimately contributing to the different efficiency of BPA oxidation driven by the direct electron transfer mechanism. Moreover, amorphous MnOx holds the promise of practical applications in catalytic PAA of various micropollutants with good stability. This study advances the fundamental understanding of ligand-regulated amorphous MnOx-catalyzed PAA process.

Molecular level decontamination of trace quinolones and Serratia marcescens in wastewater via in situ Cu(III) complexes mediated Fenton-like oxidation

Co-pollution caused by antibiotics and antibiotic-resistant bacteria (ARB) in wastewater has led to widespread concerns. Hence, their targeted and synergistic decontamination is urgently required. A homogeneous Fenton-like oxidation system comprising cupric complexes-activated peroxymonosulfate (PMS) was demonstrated to synergistically decontaminate trace quinolones (QNs) and QNs-resistant Serratia marcescens (QRSM) in wastewater. More than 99 % of QNs were degraded within 60 min under alkaline condition, and the degradation efficiency was only slightly influenced by humic acid (up to 1 %) and various anions (up to 20 %), furthermore, the degraded pathway was proposed and the environmental risk after QNs degradation were also reduced. The activation of PMS via cupric complexes coupling in situ Cu(III) complexes generation promoted intramolecular electron transfer (IET) featuring the targeted oxidation of QNs. The produced Cu(III) and •OH played primary and secondary roles in the synergistic inactivation of QRSM by destroying the cell membranes and walls, DNA bases (T, A, C, and G), antibiotic resistance genes (ARGs, including intracellular ARGs and extracellular ARGs), and total DNA (including intracellular DNA and extracellular DNA). This study demonstrates a successful strategy and provides an innovative perspective for the molecular level decontamination of trace antibiotics and ARB using a homogeneous cupric complexes-activated Fenton-like oxidation system from metal ions inherent in breeding wastewater under alkaline condition.

Advancements on Single-Atom Catalysts-Mediated Persulfate Activation: Generating Reactive Species for Contaminants Elimination in Water

The single-atom catalyst (SAC) activated persulfate process has emerged as a highly efficient technology for eliminating refractory organic compounds in aqueous environments. This review delves into the intricacies of utilizing SACs for the effective removal of various contaminants in water. The common supports and the preparation procedures of SACs are summarized at first. The synthesis methods of SACs (i.e., wet chemical method, one-pot hydrothermal method, and high-temperature pyrolysis method) are also described. Then, a comprehensive overview of the diverse reaction mechanisms in SAC-activated persulfate systems is presented, including a radical oxidation process via sulfate or hydroxyl radicals and superoxide radicals, or a nonradical process via single oxygen, surface active complex, and high-valent metal-oxo species oxidation. The impact of key factors such as peroxides concentration, SAC dosage, reaction pH, inorganic anions, organic matter, operando stability, and real water is also delved. The removal of various pollutants (i.e., azo dyes, phenolic compounds, pharmaceuticals, and bacteria) by this process is further summarized. Finally, the challenges and perspectives in the field of water treatment utilizing SACs are discussed.

Unraveling the Fundamentals of Axial Coordination FeN4+1 Sites Regulating the Peroxymonosulfate Activation for Fenton-Like Activity

Precise modulation of the axial coordination microenvironment in single-atom catalysts (SACs) to enhance peroxymonosulfate (PMS) activation represents a promising yet underexplored approach. This study introduces a pyrolysis-free strategy to fabricate SACs with well-defined axial-FeN4+1 coordination structures. By incorporating additional out-of-plane axial nitrogen into well-defined FeN4 active sites within a planar, fully conjugated polyphthalocyanine framework, FeN4+1 configurations are developed that significantly enhance PMS activation. The axial-FeN4+1 catalyst excelled in activating PMS, with a high bisphenol A (BPA) degradation rate of 2.256 min-1, surpassing planar-FeN4/PMS systems by 6.8 times. Theoretical calculations revealed that the axial coordination between N and the Fe sites forms an optimized axial FeN4+1 structure, disrupting the electron distribution symmetry of Fe and optimizing the electron distribution of the Fe 3d orbital (increasing the d-band center from -1.231 to -0.432 eV). Consequently, this led to an enhanced perpendicular adsorption energy of PMS from -1.79 to -1.82 eV and reduced energy barriers for the formation of the key reaction intermediate (O*) that generates 1O2. This study provides new insights into PMS activation through the axial coordinated engineering of well-defined SACs in water purification processes.

Fundamental Insights into the Direct Electron Transfer Mechanism on Ag Atomic Cluster

The nonradical oxidation pathway for pollutant degradation in Fenton-like catalysis is favorable for water treatment due to the high reaction rate and superior environmental robustness. However, precise regulation of such reactions is still restricted by our poor knowledge of underlying mechanisms, especially the correlation between metal site conformation of metal atom clusters and pollutant degradation behaviors. Herein, we investigated the electron transfer and pollutant oxidation mechanisms of atomic-level exposed Ag atom clusters (AgAC) loaded on specifically crafted nitrogen-doped porous carbon (NPC). The AgAC triggered a direct electron transfer (DET) between the terminal oxygen (Oα) of surface-activated peroxodisulfate and the electron-donating substituents-containing contaminants (EDTO-DET), rendering it 11-38 times higher degradation rate than the reported carbon-supported metal catalysts system with various single-atom active centers. Heterocyclic substituents and electron-donating groups were more conducive to degradation via the EDTO-DET system, while contaminants with high electron-absorbing capacity preferred the radical pathway. Notably, the system achieved 79.5% chemical oxygen demand (COD) removal for the treatment of actual pharmaceutical wastewater containing 1053 mg/L COD within 30 min. Our study provides valuable new insights into the Fenton-like reactions of metal atom cluster catalysts and lays an important basis for revolutionizing advanced oxidation water purification technologies.

Synergistic Coordination in Cu Single-Atom Catalysts Enhances High-Valent Copper-Oxo Species for Efficient PMS Activation

In the domain of heterogeneous catalytic activation of peroxymonosulfate (PMS), high-valent metal-oxo (HVMO) species are widely recognized as potent oxidants for the abatement of organic pollutants. However, the generation selectivity and efficiency of HVMO are often constrained by stringent requirements for catalyst adsorption sites and electron transfer efficiency. In this study, a single-atom catalyst, CuSA/CNP&S, is synthesized featuring multiple types (planar/axial) of heteroatom coordination via an H-bond-assisted self-assembly strategy. It is confirmed that CuN3 active centers with axial S coordination are uniformly distributed in a carbon matrix modified by planar P atoms. CuSA/CNP&S activated PMS to selectively generate Cu(III)═OH species as the primary reactive oxygen species (ROS). The pseudo-first-order kinetic rate for bisphenol A degradation reached 1.51 min-1, a 17.57-fold increase compared to the unmodified CuSA/CN catalyst. Additionally, the CuSA/CNP&S catalyst demonstrates high efficiency and durability in removing contaminants from various aqueous matrices. Theoretical calculations and experimental results indicate that the intrinsic electric field generated by distal planar P atoms enhances electron transfer efficiency within the carbon matrix. Meanwhile, axial S coordination elevates the d-band center and tunes the eg * band broadening of Cu, thereby enhancing the adsorption selectivity for the terminal oxygen of PMS. This multitype coordination synergistically mitigates the issues of low selectivity and yield of HVMO species.

Low-coordinated Mn-N2 sites in graphene oxide induce peroxydisulfate activation for tetracycline degradation: Process optimization and theoretical calculation

Atom-dispersed low-coordinated transition metal-Nx catalysts exhibit excellent efficiency in activating peroxydisulfate (PDS) for environmental remediation. However, their catalytic performance is limited due to metal-N coordination number and single-atom loading amount. In this study, low-coordinated nitrogen-doped graphene oxide (GO) confined single-atom Mn catalyst (Mn-SA/NGO) was synthesized by molten salt-assisted pyrolysis and coupled to PDS for degradation of tetracycline (TC) in water. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) and X-ray absorption fine structure spectroscopy (XAFS) analysis showed the successful doping of single-atom Mn (weight percentage 1.6%) onto GO and the formation of low-coordinated Mn-N2 sites. The optimized parameters obtained by Box-Behnken Design achieved 100% TC removal in both prediction and experimental results. The Mn-SA/NGO + PDS system had strong anti-interference ability for TC removal in the presence of anions. Besides, Mn-SA/NGO possessed good reusability and stability. O2•-, •OH, and 1O2 were the main active species for TC degradation, and the TC mineralization reached 85.1%. Density functional theory (DFT) calculations confirmed that the introduction of single atoms Mn could effectively enhance adsorption and activation of PDS. The findings provide a reference for the synthesis of high-performance single-atom catalysts for effective removal of antibiotics.

MnCe-based catalysts for removal of organic pollutants in urban wastewater by advanced oxidation processes - A critical review

With Advanced oxidation processes (AOPs) widely promoted, MnCe-based catalysts have received extensive attention under the advantages of high efficiency, stability and economy for refractory organic pollutants present in urban wastewater. Driven by multiple factors such as environmental pollution, technological development, and policy promotion, a systematic review of MnCe-based catalysts is urgently needed in the current research situation. This research provides a critical review of MnCe-based catalysts for removal of organic pollutants in urban wastewater by AOPs. It is found that co-precipitation and sol-gel methods are more appropriate methods for catalyst preparation. Among a host of influence factors, catalyst composition and pH are crucial in the catalytic oxidation processes. The synergistic effect of the free radical pathway and surface catalysis results in better pollutants degradation. It is more valuable to utilize multiple systems for oxidation (e.g., photo-Fenton technology) to improve the catalytic efficiency. This review provides theoretical guidance for MnCe-based catalysts and offers a reference direction for future research in the AOPs of organic pollutants removal from urban wastewater.

A single-atom manganese nanozyme mediated membrane reactor for water decontamination

Single-atom nanozymes possess high catalytic activity and selectivity, and are emerging as advanced heterogeneous catalysts for environmental applications. Herein, we present the innovative synthesis and characterization of a single-atom manganese-doped carbon nitride (SA-Mn-CN) nanozyme, integrated into a polyvinylidene fluoride (PVDF) membrane for advanced water treatment applications. The SA-Mn-CN nanozyme demonstrates high peroxidase-like activity, efficiently catalyzing the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) and generating reactive oxygen species (ROS) for effective antibacterial action. Notably, the SA-Mn-CN/PVDF membrane showcases enhanced water permeability, superior antifouling properties, and ultra-fast degradation kinetics of organic pollutants. Mechanistic studies reveal that the nanozyme selectively generates Mn(IV)-oxo species via peroxymonosulfate (PMS) activation, crucial for the efficient oxidation processes. Our integrated membrane system effectively removes (within 1 min, > 92 % removal) a variety of organic micropollutants in continuous-flow operations, demonstrating excellent stability and minimal manganese leaching. Compared to conventional advanced oxidation process (AOPs)/membrane system, the SA-Mn-CN/PVDF/PMS system holds the advantages of high catalytic activity and selectivity for generation of reactive species, wide working pH range (pH3-11) and excellent stability and reusability under the backwashing conditions. The developed device-scale AOPs/membrane system was proven to be effective in bacterial inactivation and pollutants degradation, verifying the vast application potential of the SA-Mn-CN/PVDF membrane for practical water decontamination. This work pioneers the development of enzyme-mimicking nanozyme membranes, offering a sustainable and high-performance solution for wastewater treatment, and sets a new benchmark for the design of nanozyme-based catalytic membranes in environmental applications.

Optimal Oxophilicity at the Fe-Nx Interface Enhances the Generation of Singlet Oxygen for Efficient Fenton-Like Catalysis

In the pursuit of efficient singlet oxygen generation in Fenton-like catalysis, the utilization of single-atom catalysts (SACs) emerges as a highly desired strategy. Here, a discovery is reported that the single-atom Fe coordinated with five N-atoms on N-doped porous carbon, denoted as Fe-N5/NC, outperform its counterparts, those coordinated with four (Fe-N4/NC) or six N-atoms (Fe-N6/NC), as well as state-of-the-art SACs comprising other transition metals. Thus, Fe-N5/NC exhibits exceptional efficacy in activating peroxymonosulfate for the degradation of organic pollutants. The coordination number of N-atoms can be readily adjusted by pyrolysis of pre-assembly structures consisting of Fe3+ and various isomers of phenylenediamine. Fe-N5/NC displayed outstanding tolerance to environmental disturbances and minimal iron leaching when incorporated into a membrane reactor. A mechanistic study reveals that the axial ligand N reduces the contribution of Fe-3d orbitals in LUMO and increases the LUMO energy of Fe-N5/NC. This, in turn, reduces the oxophilicity of the Fe center, promoting the reactivity of *OO intermediate-a pivotal step for yielding singlet oxygen and the rate-determining step. These findings unveil the significance of manipulating the oxophilicity of metal atoms in single-atom catalysis and highlight the potential to augment Fenton-like catalysis performance using Fe-SACs.

Deciphering the Novel Picolinate-Mn(II)/peroxymonosulfate System for Sustainable Fenton-like Oxidation: Dominance of the Picolinate-Mn(IV)-peroxymonosulfate Complex

A highly efficient and sustainable water treatment system was developed herein by combining Mn(II), peroxymonosulfate (PMS), and biodegradable picolinic acid (PICA). The micropollutant elimination process underwent two phases: an initial slow degradation phase (0-10 min) followed by a rapid phase (10-20 min). Multiple evidence demonstrated that a PICA-Mn(IV) complex (PICA-Mn(IV)*) was generated, acting as a conductive bridge facilitating the electron transfer between PMS and micropollutants. Quantum chemical calculations revealed that PMS readily oxidized the PICA-Mn(II)* to PICA-Mn(IV)*. This intermediate then complexed with PMS to produce PICA-Mn(IV)-PMS*, elongating the O-O bond of PMS and increasing its oxidation capacity. The primary transformation mechanisms of typical micropollutants mediated by PICA-Mn(IV)-PMS* include oxidation, ring-opening, bond cleavage, and epoxidation reactions. The toxicity assessment results showed that most products were less toxic than the parent compounds. Moreover, the Mn(II)/PICA/PMS system showed resilience to water matrices and high efficiency in real water environments. Notably, PICA-Mn(IV)* exhibited greater stability and a longer lifespan than traditional reactive oxygen species, enabling repeated utilization. Overall, this study developed an innovative, sustainable, and selective oxidation system, i.e., Mn(II)/PICA/PMS, for rapid water decontamination, highlighting the critical role of in situ generated Mn(IV).

Overcoming metals redox rate limitations in spinel oxide-driven Fenton-like reactions via synergistic heteroatom doping and carbon anchoring for efficient micropollutant removal

The transition metals redox rate limitations of spinel oxides during Fenton-like reactions hinder its efficient and sustainable treatment of actual wastewater. Herein, we propose to optimize the electronic structure of Co-Mn spinel oxide (CM) via sulfur doping and carbon matrix anchoring synergistically, enhancing the radicals-nonradicals Fenton-like processes for efficient water decontamination. Activating peroxymonosulfate (PMS) with optimised spinel oxide (CMSAC) achieved near-complete removal of ofloxacin (10 mg/L) within 6 min, showing 8.4 times higher efficiency than CM group. Significantly higher yields of SO4·- and high-valent metal species in CMSAC/PMS system provided exceptional resistance to co-existing anions, enabling efficient removal of various emerging contaminants in high salinity leachate. Specifically, sulfur coordination and carbon anchoring-induced oxygen vacancy synergistically improved the electronic structure and electron transfer efficiency of CMSAC, thus forming highly reactive Co sites and significantly reducing the energy barrier for Co(IV)=O generation. The reductive sulfur species facilitated the conversion of Co(III) to Co(II), thereby maintaining the stability of the catalytic activity of CMSAC. This work developed a synergistic optimization strategy to overcome the metals redox rate limitations of spinel oxides in Fenton-like reactions, providing deep mechanistic insights for designing Fenton-like catalysts suitable for practical applications.

Advanced Characterization Techniques and Theoretical Calculation for Single Atom Catalysts in Fenton-like Chemistry

Single-atom catalysts (SACs) have attracted extensive attention due to their unique catalytic properties and wide range of applications. Advanced characterization techniques, such as energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, scanning electron microscopy, and X-ray absorption fine-structure spectroscopy, have been used to investigate the elemental compositions, structural morphologies, and chemical bonding states of SACs in detail, aiming at unraveling the catalytic mechanism. Meanwhile, theoretical calculations, such as quantum chemical calculations and kinetic simulations, were used to predict the catalytic reaction pathways, active sites, and reaction kinetic behaviors of SACs, providing theoretical guidance for the design and optimization of SACs. This review overviews advanced characterization techniques and theoretical calculations for SACs in Fenton-like chemistry. Moreover, this work highlights the importance of advanced characterization techniques and theoretical calculations in the study of SACs and provides perspectives on the potential applications of SACs in the field of environmental remediation and the challenges of practical engineering.

Single-atom catalysts activate persulfate to degrade emerging organic contaminants in aqueous environments

Single-atom catalysts (SACs) exhibit outstanding catalytic activity due to their highly dispersed metal centers. Activating persulfates (PS) with SACs can generate various reactive oxygen species (ROS) to efficiently degrade emerging organic contaminants (EOCs) in aqueous environments, offering unique advantages such as high reaction rates and excellent stability. This technique has been extensively researched and holds enormous potential applications. In this paper, we comprehensively elaborated on the synthesis methods of SACs and their limitations, and factors influencing the catalytic performance of SACs, including metal center characteristics, coordination environment, and types of substrates. We also analyzed practical considerations for application. Subsequently, we discussed the mechanism of SACs activating PS for EOCs degradation, encompassing adsorption processes, radical pathways, and non-radical pathways. Finally, we provide prospects and outline our vision for future research, aiming to guide advancements in applying this technique.

Correlating active sites and oxidative species in single-atom catalyzed Fenton-like reactions

Single-atom catalysts (SACs) have gained widespread popularity in heterogeneous catalysis-based advanced oxidation processes (AOPs), owing to their optimal metal atom utilization efficiency and excellent recyclability by triggering reactive oxidative species (ROS) for target pollutant oxidation in water. Systematic summaries regarding the correlation between the active sites, catalytic activity, and reactive species of SACs have rarely been reported. This review provides an overview of the catalytic performance of carbon- and metal oxide-supported SACs in Fenton-like reactions, as well as the different oxidation pathways induced by the metal and non-metal active sites, including radical-based pathways (e.g., ·OH and SO4˙-) and nonradical-based pathways (e.g. 1O2, high-valent metal-oxo species, and direct electron transfer). Thereafter, we discuss the effects of metal types, coordination environments, and spin states on the overall catalytic performance and the generated ROS in Fenton-like reactions. Additionally, we provide a perspective on the future challenges and prospects for SACs in water purification.

Efficient reduction-oxidation coupling degradation of nitroaromatic compounds in continuous flow processes

Nitroaromatic compounds (NACs) with electron-withdrawing nitro (-NO2) groups are typical refractory pollutants. Despite advanced oxidation processes (AOPs) being appealing degradation technologies, inefficient ring-opening oxidation of NACs and practical large-scale applications remain challenges. Here we tackle these challenges by designing a reduction-oxidation coupling (ROC) degradation process in LaFe0.95Cu0.05O3@carbon fiber cloth (LFCO@CFC)/PMS/Vis continuous flow system. Cu doping enhances the photoelectron transfer, thus triggering the -NO2 photoreduction and breaking the barriers in the ring opening. Also, it modulates surface electronic configuration to generate radicals and non-radicals for subsequent oxidation of reduction products. Based on this, the ROC process can effectively remove and mineralize NACs under the environmental background. More importantly, the LFCO catalyst outperformed most of the recently reported catalysts with lower cost (13.72 CNY/ton) and higher processing capacity (3600 t/month). Furthermore, the high scalability, material durability, and catalytic activity of LFCO@CFC under various realistic environmental conditions prove the potential ability for large-scale applications.

Bioinspired axial S-coordinated single-atom cobalt catalyst to efficient activate peroxymonosulfate for selective high-valent Co-Oxo species generation

The efficient activation and selective high-valent metal-oxo (HVMO) species generation remain challenging for peroxymonosulfate (PMS)-based advanced oxidation processes (PMS-AOPs) in water purification. The underlying mechanism of the activation pathway is ambiguous, leading to a massive dilemma in the control and regulation of HVMO species generation. Herein, bioinspired by the bio-oxidase structure of cytochrome P450, the axial coordination strategy was adopted to tailor a single-atom cobalt catalyst (CoN4S-CB) with an axial S coordination. CoN4S-CB high-selectively generated high-valent Co-Oxo species (Co(IV)=O) via PMS activation. Co(IV)=O demonstrated an ingenious oxygen atom transfer (OAT) reaction to achieve the efficient degradation of sulfamethoxazole (SMX), and this allowed robust operation in various complex environments. The axial S coordination modulated the 3d orbital electron distribution of the Co atom. Density functional theory (DFT) calculation revealed that the axial S coordination decreased the energy barrier for PMS desorption and lowered the free energy change (ΔG) for Co(IV)=O generation. CoN4S-PMS* had a narrow d-band close to the Fermi level, which enhanced charge transfer to accelerate the cleavage of O-O and O-H bonds in PMS. This work provides a broader perspective on the activator design with natural enzyme structure-like active sites to efficient activate PMS for selective HVMO species generation.

In situ single iron atom doping on Bi2WO6 monolayers triggers efficient photo-fenton reaction

Developing an efficient photocatalytic system for hydrogen peroxide (H2O2) activation in Fenton-like processes holds significant promise for advancing water purification technologies. However, challenges such as high carrier recombination rates, limited active sites, and suboptimal H2O2 activation efficiency impede optimal performance. Here we show that single-iron-atom dispersed Bi2WO6 monolayers (SIAD-BWOM), designed through a facile hydrothermal approach, can offer abundant active sites for H2O2 activation. The SIAD-BWOM catalyst demonstrates superior photo-Fenton degradation capabilities, particularly for the persistent pesticide dinotefuran (DNF), showcasing its potential in addressing recalcitrant organic pollutants. We reveal that the incorporation of iron atoms in place of tungsten within the electron-rich [WO4]2- layers significantly facilitates electron transfer processes and boosts the Fe(II)/Fe(III) cycle efficiency. Complementary experimental investigations and theoretical analyses further elucidate how the atomically dispersed iron induces lattice strain in the Bi2WO6 monolayer, thereby modulating the d-band center of iron to improve H2O2 adsorption and activation. Our research provides a practical framework for developing advanced photo-Fenton catalysts, which can be used to treat emerging and refractory organic pollutants more effectively.

Were Persulfate-Based Advanced Oxidation Processes Really Understood? Basic Concepts, Cognitive Biases, and Experimental Details

Persulfate (PS)-based advanced oxidation processes (AOPs) for pollutant removal have attracted extensive interest, but some controversies about the identification of reactive species were usually observed. This critical review aims to comprehensively introduce basic concepts and rectify cognitive biases and appeals to pay more attention to experimental details in PS-AOPs, so as to accurately explore reaction mechanisms. The review scientifically summarizes the character, generation, and identification of different reactive species. It then highlights the complexities about the analysis of electron paramagnetic resonance, the uncertainties about the use of probes and scavengers, and the necessities about the determination of scavenger concentration. The importance of the choice of buffer solution, operating mode, terminator, and filter membrane is also emphasized. Finally, we discuss current challenges and future perspectives to alleviate the misinterpretations toward reactive species and reaction mechanisms in PS-AOPs.

The "4 + 1" strategy fabrication of iron single-atom catalysts with selective high-valent iron-oxo species generation

Single-atom catalysts (SACs) with atomic dispersion active sites have exhibited huge potentials in peroxymonosulfate (PMS)-based Fenton-like chemistry in water purification. However, four-N coordination metal (MN4) moieties often suffer from such problems as low selectivity and narrow workable pH. How to construct SACs in a controllable strategy with optimized electronic structures is of great challenge. Herein, an innovative strategy (i.e., the "4 + 1" fabrication) was devised to precisely modulate the first-shell coordinated microenvironment of FeN4 SAC using an additional N (SA-FeN5). This leads to almost 100% selective formation of high-valent iron-oxo [Fe(IV)═O] (steady-state concentration: 2.00 × 10-8 M) in the SA-FeN5/PMS system. In-depth theoretical calculations unveil that FeN5 configuration optimizes the electron distribution of monatomic Fe sites, which thus fosters PMS adsorption and reduces the energy barrier for Fe(IV)═O generation. SA-FeN5 was then attached to polyvinylidene difluoride membrane for a continuous flow device, showing long-term abatement of the microcontaminant. This work furnishes a general strategy for effective PMS activation and selective high-valent metal-oxo species generation by high N-coordination number regulation in SACs, which would provide guidance in the rational design of superior environmental catalysts for water purification.

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.

Synergetic Manipulation Mechanism of Single-Atom M-N4 and M-OH (M = Mn, Fe, Co, Ni) Sites for Ozone Activation: Theoretical Prediction and Experimental Verification

Carbon-based single-atom catalysts (SACs) have been gradually introduced in heterogeneous catalytic ozonation (HCO), but the interface mechanism of O3 activation on the catalyst surface is still ambiguous, especially the effect of a surface hydroxyl group (M-OH) at metal sites. Herein, we combined theoretical calculations with experimental verifications to comprehensively investigate the O3 activation mechanisms on a series of conventional SAC structures with N-doped nanocarbon substrates (MN4-NCs, where M = Mn, Fe, Co, Ni). The synergetic manipulation effect of the metal atom and M-OH on O3 activation pathways was paid particular attention. O3 tends to directly interact with the metal atom on MnN4-NC, FeN4-NC, and NiN4-NC catalysts, among which MnN4-NC has the best catalytic activity for its relatively lower activation energy barrier of O3 (0.62 eV) and more active surface-adsorbed oxygen species (Oads). On the CoN4-NC catalyst, direct interaction of O3 with the metal site is energetically infeasible, but O3 can be activated to generate Oads or HO2 species from direct or indirect participation of M-OH sites. The experimental results showed that 90.7 and 82.3% of total organic carbon (TOC) was removed within 40 min during catalytic ozonation of p-hydroxybenzoic acid with MnN4-NC and CoN4-NC catalysts, respectively. Phosphate quenching, catalyst characterization, and EPR measurement further supported the theoretical prediction. This contribution provides fundamental insights into the O3 activation mechanism on SACs, and the methods and ideals could be helpful for future studies of environmental catalysis.

Advances in Atomically Dispersed Metal and Nitrogen Co-Doped Carbon Catalysts for Advanced Oxidation Technologies and Water Remediation: From Microenvironment Modulation to Non-Radical Mechanisms

Atomically dispersed metal and nitrogen co-doped carbon catalysts (M-N-C) have been attracting tremendous attentions thanks to their unique MNx active sites and fantastic catalytic activities in advanced oxidation technologies (AOTs) for water remediation. However, precisely tailoring the microenvironment of active sites at atomic level is still an intricate challenge so far, and understanding of the non-radical mechanisms in persulfate activation exists many uncertainties. In this review, latest developments on the microenvironment modulation strategies of atomically dispersed M-N-C catalysts including regulation of central metal atoms, regulation of coordination numbers, regulation of coordination heteroatoms, and synergy between single-atom catalysts (SACs) with metal species are systematically highlighted and discussed. Afterwards, progress and underlying limitations about the typical non-radical pathways from production of singlet oxygen, electron transfer mechanism to generation of high-valent metal species are well demonstrated to inspire intrinsic insights about the mechanisms of M-N-C/persulfate systems. Lastly, perspectives for the remaining challenges and opportunities about the further development of carbon-based SACs in environment remediation are also pointed out. It is believed that this review will be much valuable for the further design of active sites in M-N-C/persulfate catalytic systems and promote the wide application of SACs in various fields.

Investigating the evolution of reactive species in the CuO-mediated peroxymonosulfate activation process

The utilization of copper oxide (CuO) as a catalyst in the peroxymonosulfate (PMS) activation process holds great promise for effectively degrading aqueous organic pollutants, while the relevant mechanism remains inadequately understood. In this study, we delve into the evolution pathways of reactive species in the CuO/PMS system through a comprehensive series of experimental analyses. Our findings indicate that various reactive species are generated in the CuO/PMS system with the specific sequence, where the decomposition of surface Cu(II)-OOSO3- leads to the formation of surface Cu(III) species, which are responsible for the subsequent generation of HO•. The reactivity of these reactive species and the sequence of their generation explain the distinct oxidation behaviors of pollutants with different values of ionization potential (IP). In addition, singlet oxygen (1O2) may be produced during the PMS activation process, while its involvement in the oxidation of substrates is deemed negligible. This investigation presents a novel perspective, enhancing our comprehension of the mechanism underlying transition metal-mediated PMS activation processes. ENVIRONMENTAL IMPLICATION: The removal of refractory organic contaminations in water constitutes a fundamental concern within the realm of environmental pollution management. Peroxymonosulfate activation induced by transition metal oxides has garnered significant recognition as a promising technological approach for the degradation of aqueous organic contaminants, while the underlying mechanism remains enigmatic. In this study, we systematically investigate the evolution pathways of reactive species in the CuO/peroxymonosulfate system to reveal the mystery of the reaction mechanism between CuO and peroxymonosulfate. The outcomes of our study contribute to enhancing the practical applicability of transition metal-triggered PMS activation processes.

Plastic wastes-derived N-doped carbon nanotubes for efficient removal of sulfamethoxazole in high salinity wastewater via nonradical peroxymonosulfate activation

Peroxymonosulfate (PMS) catalytic activation is effective to eliminate organic pollutants from water, thus the development of low-cost and efficient catalysts is significant in applications. The resource conversion of plastic wastes (PWs) into carbon nanotubes (CNTs) is a promising candidate for PMS-based advanced oxidation processes (AOPs), and also a sustainable strategy to realize plastic management and reutilization. Herein, cost-effective PWs-derived N-doped CNTs (N-pCNTs) were synthesized, which displayed efficient activity for PMS activation through an electron transfer pathway (ETP) for sulfamethoxazole (SMX) degradation in high salinity water. The pyrrolic N induced the positively charged surface of N-pCNTs, favoring the electrostatic adsorption of PMS and subsequent generation of active PMS* . A galvanic oxidation process was developed to prove the electron-shuttle dominated ETP for SMX oxidation. Combined with theoretical calculations, the efficiency of ETP was determined by the potential difference between HOMO of SMX and LUMO of N-pCNTs. Such oxidation produced low-toxicity intermediates and resulted in selective degradation of specific sulfonamide antibiotics. This work reveals the feasibility of low-cost N-pCNTs catalysts from PWs serving as an appealing candidate for PMS-AOPs in water remediation, providing a new solution to alleviate environmental issues caused by PWs and also advances the understanding of ETP during PMS activation.

Regulating electronic structure of Fe single-atom site by S/N dual-coordination for efficient Fenton-like catalysis

The activity of single-atom catalysts in peroxymonosulfate activation process is bound up with the local electronic state of metal center. However, the large electronegativity of N atoms in Metal-N4 restricts the electron transfer between center metal atom and peroxymonosulfate. Herein, we constructed Fe-SN-C catalyst by incorporating S atom in the first coordination sphere of Fe single-atom site (Fe-S1N3) for Fenton-like catalysis. The Fe-SN-C with a low valent Fe is found to exhibit excellent catalytic activity for bisphenol A degradation, and the corresponding rate constant reaches 0.405 min-1, 11.9-fold higher than the original Fe-N-C. Besides, the Fe-SN-C/PMS system exhibits ideal catalytic stability under the effect of wide pH range and background substrates by the fast generation of high-valent Fe species. Experimental results and theoretical calculations reveal that the dual coordination of S and N atoms notably increases the local electron density of Fe atoms and electron filling in eg orbital, causing a d band center shifting close to the fermi level and thereby optimizes the activation energy for peroxymonosulfate decomposition via Fe 3d-O 2p orbital interaction. This work provides further development of promising SACs for the efficient activation of peroxymonosulfate based on direct regulation of the coordination environment of active center metal atoms.

Selective electrophilic attack towards organic micropollutants with superior Fenton-like activity by biochar-supported cobalt single-atom catalyst

The global shortage of freshwater and inadequate supply of clean water have necessitated the implementation of robust technologies for wastewater purification, and Fenton-like chemistry is a highly-promising approach. However, realizing the rapid Fenton-like chemistry for high-efficiency degradation of organic micropollutants (OMs) remains challenging. Herein, one novel system was constructed by a Co single-atom catalyst activating peroxymonosulfate (PMS), and the optimal system (SA-Co-NBC-0.2/PMS) achieved unprecedented catalytic performance towards a model OM [Iohexol (IOH)], i.e., almost 100% decay ratio in only 10 min (the observed rate constant: 0.444 min-1) with high electrophilic species 1O2 (singlet oxygen) generation. Theoretical calculations unveiled that Co-N4 sites preferred to adsorb the terminal-O of PMS (more negative adsorption energy than other O sites: -32.67 kcal/mol), promoting the oxidation of PMS to generate 1O2. Iodine (I)23 (0.1097), I24 (0.1154) and I25 (0.0898) on IOH with higher f- electrophilic values were thus identified as the main attack sites. Furthermore, 16S ribosomal RNA high-throughput sequencing and quantitative structure-activity relationship analysis illustrated the environmentally-benign property of the SA-Co-NBC-0.2 and the tapering ecological risk during IOH degradation process. Significantly, this work comprehensively checked the competence of the SA-Co-NBC-0.2/PMS system for organics abatement in practical wastewater.

Understanding the Distance Effect of the Single-Atom Active Sites in Fenton-Like Reactions for Efficient Water Remediation

Emerging single-atom catalysts (SACs) are promising in water remediation through Fenton-like reactions. Despite the notable enhancement of catalytic activity through increasing the density of single-atom active sites, the performance improvement is not solely attributed to the increase in the number of active sites. The variation of catalytic behaviors stemming from the increased atomic density is particularly elusive and deserves an in-depth study. Herein, single-atom Fe catalysts (FeSA-CN) with different distances (dsite) between the adjacent single-atom Fe sites are constructed by controlling Fe loading. With the decrease in dsite value, remarkably enhanced catalytic activity of FeSA-CN is realized via the electron transfer regime with peroxymonosulfate (PMS) activation. The decrease in dsite value promotes electronic communication and further alters the electronic structure in favor of PMS activation. Moreover, the two adjacent single-atom Fe sites collectively adsorb PMS and achieve single-site desorption of the PMS decomposition products, maintaining continuous PMS activation and contaminant removal. Moreover, the FeSA-CN/PMS system exhibits excellent anti-interference performance for various aquatic systems and good durability in continuous-flow experiments, indicating its great potential for water treatment applications. This study provides an in-depth understanding of the distance effect of single-atom active sites on water remediation by designing densely populated SACs.

Photo-self-Fenton Reaction Mediated by Atomically Dispersed Ag-Co Photocatalysts toward Efficient Degradation of Organic Pollutants

Achieving the complete mineralization of persistent pollutants in wastewater is still a big challenge. Here, we propose an efficient photo-self-Fenton reaction for the degradation of different pollutants using the high-density (Ag: 22 wt %) of atomically dispersed AgCo dual sites embedded in graphic carbon nitride (AgCo-CN). Comprehensive experimental measurements and density functional theory (DFT) calculations demonstrate that the Ag and Co dual sites in AgCo-CN play a critical role in accelerating the photoinduced charge separation and forming the self-Fenton redox centers, respectively. The bimetallic AgCo-CN exhibited excellent photocatalytic performance toward the phenol even under extreme conditions due to an efficient degradation pathway and in situ generation of the hydrogen peroxide producing the main active oxygen species (⋅OH and 1 O2 ) and showed long-term activity in a self-design photo-Filter reactor for the purification of the phenol. Our discoveries pave the way for the design of efficient single-atoms photocatalysts-based photo-self-Fenton reaction for recalcitrant pollutant treatment.

Constructed electron-dense Mn sites in nitrogen-doped Mn3O4 for efficient catalytic ozonation of pyrazines: Degradation and odor elimination

In this study, N-doped Mn3O4 catalysts (Mn-nN) with electron-dense Mn sites were synthesized and employed in heterogeneous catalytic ozonation (HCO). These catalysts demonstrated excellent performance in pyrazines degradation and odor elimination. The synthesis of Mn-nN was achieved through a facile urea-assisted heat treatment method. Experimental characterization and theoretical analyses revealed that the MnN structures in Mn-nN, played a crucial role in facilitating the formation of electron-dense Mn sites that served as the primary active sites for ozone activation. In particular, Mn-1N exhibited excellent performance in the HCO system, demonstrating the highest 2,5-dimethylpyrazine (2,5-DMP) degradation efficiency. •OH was confirmed as the primary reactive oxygen species involved in the HCO process. The second-order rate constants for 2,5-DMP degradation with O3 and •OH, were determined to be (3.75 ± 0.018) × 10-1 and (6.29 ± 0.844) × 109 M-1 s-1, respectively. Seventeen intermediates were identified through GC-MS analysis during the degradation of 2,5-DMP via HCO process with Mn-1N. The degradation pathways were subsequently proposed by considering these identified intermediates. This study introduces a novel approach to synthesize N-doped Mn3O4 catalysts and demonstrates their efficacy in HCO for the degradation of pyrazines and the elimination of associated odors. The results show that the catalysts are promising for addressing odor-related environmental issues and provide valuable insights about the broader significance of catalytic ozonation processes.

Self-carbon-thermal-reduction strategy for boosting the Fenton-like activity of single Fe-N4 sites by carbon-defect engineering

Carbon-defect engineering in metal single-atom catalysts by simple and robust strategy, boosting their catalytic activity, and revealing the carbon defect-catalytic activity relationship are meaningful but challenging. Herein, we report a facile self-carbon-thermal-reduction strategy for carbon-defect engineering of single Fe-N4 sites in ZnO-Carbon nano-reactor, as efficient catalyst in Fenton-like reaction for degradation of phenol. The carbon vacancies are easily constructed adjacent to single Fe-N4 sites during synthesis, facilitating the formation of C-O bonding and lowering the energy barrier of rate-determining-step during degradation of phenol. Consequently, the catalyst Fe-NCv-900 with carbon vacancies exhibits a much improved activity than the Fe-NC-900 without abundant carbon vacancies, with 13.5 times improvement in the first-order rate constant of phenol degradation. The Fe-NCv-900 shows high activity (97% removal ratio of phenol in only 5 min), good recyclability and the wide-ranging pH universality (pH range 3-9). This work not only provides a rational strategy for improving the Fenton-like activity of metal single-atom catalysts, but also deepens the fundamental understanding on how periphery carbon environment affects the property and performance of metal-N4 sites.