Single-Atom Fe Catalyst Outperforms Its Homogeneous Counterpart for Activating Peroxymonosulfate to Achieve Effective Degradation of Organic Contaminants

High-efficiency and sustainable peroxymonosulfate activation on Fe single-atom catalyst through incorporating complementary S species for enhanced water decontamination

Single-atom FeNC catalyst is a promising alternative for peroxymonosulfate (PMS) activation based water treatment, but the trade-off between PMS adsorption and products desorption on Fe single site limits its performance for achieving sustainable PMS activation. Herein, the thiophene-like S (CSC) and oxidized S (C-SOx) with different electron-donating/withdrawing properties were introduced into the second coordination shell of single-atom FeNC catalyst (Fe-NSBC) to optimize its Fe electronic structure for high-efficiency and sustainable PMS activation. The S species composition was also regulated to study its effect on catalytic performance and mechanism of Fe-NSBC. Results showed the Fe-NSBC displayed highest PMS catalytic efficiency and radical (SO4•- and •OH) yield at moderate CSC/C-SOx ratio, whose catalytic activity was 3.8 and 21.1 times higher than that of Fe-NBC without S doping and homogeneous Co2+ (metal-based benchmark), and greatly outperformed those of the state-of-the-art FeNC based catalysts. Moreover, the Fe-NSBC/PMS favored free radicals production for preferential removal of hydrophilic pollutants, and displayed unique anti-interference and long-term effectiveness in real water decontamination. The CSC and C-SOx played complementary role in promoting sustainable PMS activation: CSC raised d-band center and electron density of Fe for enhanced adsorption and reduction of PMS into radicals, while C-SOx lowered d-band center of Fe for favorable radical desorption and active site regeneration.

Selective activation of peroxymonosulfate through gating heteronuclear diatomic distance for flexible generation of high-valent cobalt-oxo species or sulfate radicals

Heteronuclear diatomic engineering has been widely applied to generate selective or nonselective active species in Fenton-like system for wastewater treatment. However, active species adapted to diverse wastewater were different, and flexible control of active species has remained elusive, often necessitating complex and repetitive atom modifications. Here, we proposed a diatomic distance gating strategy that adjusted the spintronic structure of cobalt site for flexible transformation of high-valent cobalt-oxo and sulfate radical for adapted wastewater treatment. Electron paramagnetic resonance spectra, magnetic susceptibility-temperatur curve and partial density of states revealed electron transfer from dx2-y2, dz2 and dyz orbitals of high-spin cobalt to peroxymonosulfate for high-valent cobalt-oxo generation at 3.8 nm, and from dz2 orbital of medium-spin cobalt to peroxymonosulfate for sulfate radical generation at 2.5 nm. The Fenton-like system with 3.8 nm of diatomic distance preferentially degraded contaminants with low n-octanol/water partition constant and high ionization potential, while Fenton-like system with 2.5 nm of diatomic distance readily degraded contaminants with high Hammett substituent constant and low dissociation constant. This study elucidated the effect of diatomic distance on Fenton-like chemistry and provided a blueprint for the design of intelligent Fenton-like system for treating diverse wastewater treatment scenarios.

Mo doping modulates peroxymonosulfate activation of cobalt carbon nanotube-based catalysts for efficient multi-pollutants removal: Oxygen vacancies trigger the evolution of high-valence cobalt-oxo species

Formation of defective catalysts with oxygen-rich vacancies via ion doping represents an advanced strategy for enhancing catalytic activity. Cobalt oxides supported on carbon nanotubes (CNTs) were strategically designed to enhance peroxomonosulfate (PMS) through molybdenum (Mo)-doped oxygen vacancies (Vo) management to achieve the application of high-valent cobalt oxygen (Co(IV)O) dominated degradation mechanism. The first-order rate constant for tetracycline hydrochloride degradation in the CoMo/CNTs/PMS system (0.081 min-1) was twice that of the Co/CNTs system. Additionally, the system exhibited high resistance to interference and excellent pH adaptability. The system demonstrated a high removal efficiency (91.3 %-100 %) for the emerging contaminant (sodium p-perfluorous nonenoxybenzene sulfonate), as well as other organic pollutants such as carbamazepine and imidacloprid. The prepared catalyst membranes exhibited stability and effectively degraded tetracycline hydrochloride over 10 h of continuous-flow experiments, highlighting their potential for practical applications. Theoretical calculations revealed that molybdenum doping reduced the formation energy of oxygen vacancies, while these vacancies shifted the d-band center of cobalt (Co) in CoMo/CNTs upward, bringing it closer to the Fermi energy level. Furthermore, enhanced charge transfer and stronger peroxide bond stretching were observed during PMS adsorption, thus promoting the chemical reaction of PMS adsorbed on CoMo/CNTs. More significantly, the oxygen-rich vacancies in CoMo/CNTs lowered the energy barrier for Co(IV)=O generation. This study provides insights into the mechanism of ionic doping in PMS activation by metal-based catalysts, thereby expanding the application of defective catalysts in environmental remediation.

Rapid charge transfer and O3 selective catalysis induced by B-doped nanoconfined reactor realized complete Cu-EDTA decomplexation: Significant role of BC3 conformation

Nanoconfinement strategy that overcomes the defects of conventional heterogeneous catalysts in electron and mass transport provides a new outlet to enhance REDOX processes. Nonetheless, limitations in the activity and selectivity of effective catalytic sites are still the drawbacks of nanoconfined catalysts. In this study, a B-doped carbon nanotubes-confined-FexOy catalyst (B500Fe200@CNTs-L) coupled with a dielectric barrier discharge (DBD) plasma system (DBD/B500Fe200@CNTs-L) was developed for Cu-EDTA removal. The DBD/B500Fe200@CNTs-L system realized 100% Cu-EDTA decomplexation within 3 min, which was 3.6 times kinetically faster than without B doping. The system emphasized extensive pH adaptability, maintaining 100% Cu-EDTA removal at a pH of 3-9. B doping increased the selectivity to O3 and promoted active species generation, in which •OH and O2•- prominently contributed to Cu-EDTA decomplexation, as well as FeIV=O. The strong electronic activity induced by BC3 conformation enhanced charge transfer, regulating the positive charge and d-band center of central Fe atoms to decline the energy barriers of H2O2 and O3 adsorption and active species formation. Moreover, this system emphasized the superior catalytic stability under different matrix water (Cl⁻, CO₃²⁻, NO₃⁻, SO₄²⁻, and PO₄³⁻).

Selective glyphosate degradation via oxygen activation using Fe-N-C: Critical role of size exclusion

Selective elimination of glyphosate (PMG) from complex water matrices remains a significant challenge. Metal-nitrogen-carbon (M-N-C) materials derived from metal-organic frameworks (MOFs) offer a promising platform due to their tunable porosity and abundant active sites. In this study, three Fe-N-C-x (x = 5, 10, 20) catalysts with varying pore sizes (2-4 nm) and no surface-active sites were synthesized for PMG degradation under interference with contaminants of different sizes. The results showed Fe-N-C-5 exhibited superior catalytic and anti-interference performance for PMG degradation compared to Fe-N-C-10 and Fe-N-C-20. This was attributed to the greater accessibility of smaller-sized PMG (molecular size 0.9 nm) to the internal active sites through the pore channels, while larger-sized pollutants were effectively excluded. Zeta potential measurements and in situ ATR-FTIR spectroscopy revealed that the entrance of PMG was driven by both electrostatic interaction and coordination bonding between phosphate and Fe in Fe-N-C-5. Quenching experiments combined with electron spin resonance (ESR) analysis confirmed that singlet oxygen (1O2) was the primary reactive oxygen species responsible for PMG degradation in the Fe-N-C-5/O2(Vis) system. This study highlights the robust anti-interference capability of Fe-N-C-5 and provides new insights into its potential applications in advanced water treatment technologies.

CO2 geological sequestration can remove emerging contaminants in groundwater: The important role of secondary mineral carbonates

Carbon dioxide (CO2) injection has been proposed as a strategy for carbon sequestration, while uncertainties persist regarding its effects on groundwater. Concerns have been raised that CO2 mineralization and sequestration could potentially lead to groundwater contamination. However, our study demonstrates its capability to mitigate pollution. The injection of CO2 facilitates the rapid dissolution of minerals, releasing Ca(II), Mg(II), and Fe(II) and forming secondary carbonate minerals, such as CaCO3, MgCO3, and FeCO3. The in-situ generated FeCO3 can activate oxygen to produce hydroxyl radicals (•OH) under oxic condition, thereby enhancing the degradation of emerging organic contaminants in groundwater, such as 2,4,6-tribromophenol, flurbiprofen, diclofenac, carbamazepine, phenol, and sulfamethoxazole. Mechanism studies suggest that this process is enhanced by the conversion of in-situ formed FeCO3 into a two-dimensional goethite nanosheet structure, which provides a larger specific surface area and enables more Fe(II) to be adsorbed on the mineral surface. The formation of Fe-O coordination bonds effectively reduces the loss of •OH at the interfacial reaction layer. The study further distinguishes and quantifies the contributions of different Fe(II) forms to •OH generation. The transformation pathways of the six contaminants and the toxicity of their intermediates are also analyzed. CaCO3 and MgCO3 do not exhibit the ability to degrade pollutants, but play a role in carbon mineralization. This work reveals that secondary minerals generated through the CO2 mineralization and sequestration process display simultaneous capabilities of contaminant degradation and carbon fixation. Such activities are pivotal not only for the environmental fate and transformation of emerging contaminants in groundwater but also for regulating the carbon cycle.

Boosting High-Valent Fe═O Formation via Fe Electron Localization in Asymmetric FeMo Dual-Atom Catalyst for Fenton-Like Reaction

High-valent Fe═O species, recognized as pivotal reactive oxygen intermediates in the catalyst-activated peroxymonosulfate (PMS) oxidation system, play a dominant role in contaminant degradation. However, the inherent correlation between the Fe 3d electronic structure of heterogeneous catalysts and the generation efficiency of high-valent Fe═O remains unclear, limiting the rational design of high-performance catalysts. To the end, Fe-Mo dual-atom catalysts (FeMoNC) with N3Fe-O-MoN2 configurations are constructed, which exhibit exceptional sulfadiazine (SDZ) degradation activity (k = 0.92 min-1). This performance surpasses that of monometallic FeNC (1.67 times), attributed to the optimized generation of high-valent Fe═O species. Combined XPS/XAS analysis and DFT calculations reveal that electron transfer from Mo to Fe upshifts the Fe d-band center by 0.144 eV, which facilitates Oγ-Oβ bond cleavage in PMS (energy barrier reduced by 31%) and stabilizes high-valent Fe═O species. The electronic modifications further confirm the promoted high-valent Fe═O formation. This work elucidates the electronic origin of high-valent Fe═O generation in heteronuclear dual-atom catalysts, providing a universal strategy for manipulating 3d-electron configurations to enhance high-valent metal-oxo chemistry in advanced oxidation processes.

Microwave synthesis of CMF@FexOy-CN: a highly dispersible magnetically recyclable photocatalyst for efficient Rh B degradation

Polluted water poses a significant threat to human health and ecological balance, necessitating the development of efficient and cost-effective water purification technologies. This study synthesized a composite photocatalytic material (CMF@FexOy-CN) using a microwave radiation synchrotron cooling technique. This material incorporates iron oxide-doped g-C3N4 and in-situ micro-fibrillated cellulose, exhibiting high dispersion stability and magnetic recyclability. The material demonstrates a bandgap energy (Eg) of 2.69 eV. Remarkably, 25 mg/20 mL of CMF@FexOy-CN (with 40 wt% FexOy-CN loading) achieved a 97.9 % degradation of Rhodamine B (Rh B) within 1 h. Even after 10 cycles, the degradation efficiency remained high at 89.4 %. The superoxide radical (·O2-) played a critical role in Rh B degradation, facilitated by the stable dispersion of CMF. The issue of low photocatalytic efficiency, often caused by poor dispersion of photocatalysts, was addressed through the in-situ micro-fibrillation of cellulose, offering a green and economical solution. This study provides a theoretical foundation for applying photocatalysis in water purification.

Impregnation of Single-Atom Iron from Metallosurfactants onto MOF-Derived Porous N-Doped Carbon for Efficient Wastewater Treatment via Peroxymonosulfate Activation

Herein, a Fe-MOF-derived single iron atom impregnated on porous N-doped carbon (FeSAC/NC) was synthesized using an iron-based metallosurfactant (FeCTAC) as the precursor. The formation of isolated single atoms and their surrounding environment after pyrolysis at 900 °C was confirmed by XPS and EXAFS analysis. The use of FeCTAC facilitated the exposure of more single iron atoms on the porous NC matrix and prevented their aggregation, as verified by elemental analysis and HAADEF-TEM. The resulting FeSAC/NC was applied in the Fenton reaction for the degradation of tetracycline (TC) by activating peroxymonosulfate (PMS). It achieved 92% TC removal in 40 min compared to only 23% with PMS alone. Free radical scavenging experiments indicated that a nonradical pathway, involving singlet oxygen and electron transfer, dominated the degradation process. Additionally, pyrrolic and graphitic nitrogen sites in the NC matrix contributed significantly to TC adsorption. This exceptional catalytic performance provides valuable insights for designing highly efficient catalysts for wastewater treatment.

Sulfur Bridge Geometry Boosts Selective FeIV═O Generation for Efficient Fenton-Like Reactions

High-valent iron-oxo species (FeIV═O) is a fascinating enzymatic agent with excellent anti-interference abilities in various oxidation processes. However, selective and high-yield production of FeIV═O remains challenging. Herein, Fe diatomic pairs are rationally fabricated with an assisted S bridge to tune their neighbor distances and increase their loading to 11.8 wt.%. This geometry regulated the d-band center of Fe atoms, favoring their bonding with the terminal and hydroxyl O sites of peroxymonosulfate (PMS) via heterolytic cleavage of O─O, improving the PMS utilization (70%), and selective generation of FeIV═O (>90%) at a high yield (63% of PMS) offers competitive performance against state-of-the-art catalysts. These continuous reactions in a fabricated device and technol-economic assessment further verified the catalyst with impressive long-term activity and scale-up potential for sustainable water treatment. Altogether, this heteroatom-bridge strategy of diatomic pairs constitutes a promising platform for selective and efficient synthesis of high-valent metal-oxo species.

Enhanced FeIV═O Generation via Peroxymonosulfate Activation by an Edge-Site Engineered Single-Atom Iron Catalyst

As an oxidant, the ferryl-oxo complex (FeIV═O) offers excellent reactivity and selectivity for degrading recalcitrant organic contaminants. However, enhancing FeIV═O generation on heterogeneous surfaces remains challenging because the underlying formation mechanism is poorly understood. This study introduces edge defects onto a single-atom Fe catalyst (FeNC-edge) to promote FeIV═O generation via peroxymonosulfate (PMS) activation. In the presence of PMS, the FeNC-edge catalyst at a low dose (20 mg L-1, equivalent to 0.14 mg L-1 Fe) exhibits unprecedented activity for organic contaminant degradation. Electrochemical analysis, in situ Raman spectroscopy, and FeIV═O probe experiments confirm that FeIV═O generation is enhanced on the surface of FeNC-edge. Density functional theory calculations reveal that the introduced edge sites concentrate electron density on active Fe atoms, facilitating charge transfer from Fe to PMS. Notably, FeNC-edge immobilized on a polymeric membrane functioned as a continuous-flow oxidation system with efficient catalyst recycling and minimal Fe leaching.

Non-metallic iodine single-atom catalysts with optimized electronic structures for efficient Fenton-like reactions

In this study, we introduce a highly effective non-metallic iodine single-atom catalyst (SAC), referred to as I-NC, which is strategically confined within a nitrogen-doped carbon (NC) scaffold. This configuration features a distinctive C-I coordination that optimizes the electronic structure of the nitrogen-adjacent carbon sites. As a result, this arrangement enhances electron transfer from peroxymonosulfate (PMS) to the active sites, particularly the electron-deficient carbon. This electron transfer is followed by a deprotonation process that generates the peroxymonosulfate radical (SO5•-). Subsequently, the SO5•- radical undergoes a disproportionation reaction, leading to the production of singlet oxygen (1O2). Furthermore, the energy barrier for the rate-limiting step of SO5•- generation in I-NC is significantly lower at 1.45 eV, compared to 1.65 eV in the NC scaffold. This reduction in energy barrier effectively overcomes kinetic obstacles, thereby facilitating an enhanced generation of 1O2. Consequently, the I-NC catalyst exhibits remarkable catalytic efficiency and unmatched reactivity for PMS activation. This leads to a significantly accelerated degradation of pollutants, evidenced by a relatively high observed kinetic rate constant (kobs ~ 0.436 min-1) compared to other metallic SACs. This study offers valuable insights into the rational design of effective non-metallic SACs, showcasing their promising potential for Fenton-like reactions in water treatment applications.

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.

Modulating the Iron Microenvironment for a Cooperative Interplay Between Fe-N-C Single Atoms and Fe3C Nanoclusters on the Oxygen Reduction Reaction

The coexistence of single atoms and nanoparticles is shown to increase the oxygen reduction performance in Fe-N-C electrocatalysts, but the mechanisms underlying this synergistic effect remain elusive. In this study, model Fe-N-C electrocatalysts with controlled ratios of FeN4 sites and Fe3C nanoclusters is systematically designed and synthesized. Experiments and density functional theory (DFT) computations reveal that Fe3C nanoclusters near FeN4 sites modulate the electron density of the Fe single-atom microenvironment through an electron withdrawing effect. This substantially alters the oxygen reduction reaction (ORR) mechanisms and boosts the catalytic performance of FeN4 sites. This study provides fundamental insights into the dynamic catalytic impact of single atoms and nanoparticle coexistence in advanced Fe-N-C electrocatalysts for the ORR, paving the way for further refinement through various combinations.

Low-peroxide-consumption fenton-like systems: The future of advanced oxidation processes

Conventional heterogeneous Fenton-like systems employing different peroxides have been developed for water/wastewater remediation. However, a large population of peroxides consumed during various Fenton-like systems with low utilization efficiency and associated secondary contamination have become the bottlenecks for their actual applications. Recent strategies for lowering the peroxide consumptions to develop economic Fenton-like systems are primarily devoted to the effective radical generation and subsequent high-efficiency radical utilization through catalysts/systems engineering, leveraging emerging nonradical oxidation pathways with higher selectivity and longer life of the reactive intermediate, as well as reactor designs for promoting the mass transfer and peroxides decomposition to improve the yield of radicals/nonradicals. However, a comparative review summarizing the mechanisms and pathways of these strategies has not yet been published. In this review, we endeavor to showcase the designated systems achieving the reduction of peroxides while ensuring high catalytic activity from the perspective of the above strategic mechanisms. An in-depth understanding of these aspects will help elucidate the key mechanisms for achieving economic peroxide consumption. Finally, the existing problems of these strategies are put forward, and new ideas and research directions for lowering peroxide consumption are proposed to promote the application of various Fenton-like systems in actual wastewater purification.

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.

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.

Coordination engineering of heterogeneous high-valent Fe(IV)-oxo for safe removal of pollutants via powerful Fenton-like reactions

Coordination engineering of high-valent Fe(IV)-oxo (FeIV=O) is expected to break the activity-selectivity trade-off of traditional reactive oxygen species, while attempts to regulate the oxidation behaviors of heterogeneous FeIV=O remain unexplored. Here, by coordination engineering of Fe-Nx single-atom catalysts (Fe-Nx SACs), we propose a feasible approach to regulate the oxidation behaviors of heterogeneous FeIV=O. The developed Fe-N2 SACs/peroxymonosulfate (PMS) system delivers boosted performance for FeIV=O generation, and thereby can selectively remove a range of pollutants within tens of seconds. In-situ spectra and theoretical simulations suggest that low-coordination Fe-Nx SACs favor the generation of FeIV=O via PMS activation as providing more electrons to facilitate the desorption of the key *SO4H intermediate. Due to their disparate attacking sites to sulfamethoxazole (SMX) molecules, Fe-N2 SACs mediated FeIV=O (FeIVN2=O) oxidize SMX to small molecules with less toxicity, while FeIVN4=O produces series of more toxic azo compounds through N-N coupling with more complex oxidation pathways.

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

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

Non-radical oxidation driven by iron-based materials without energy assistance in wastewater treatment

Chemical oxidation is extensively utilized to mitigate the impact of organic pollutants in wastewater. The non-radical oxidation driven by iron-based materials is noted for its environmental friendliness and resistance to wastewater matrix, and it is a promising approach for practical wastewater treatment. However, the complexity of heterogeneous systems and the diversity of evolutionary pathways make the mechanisms of non-radical oxidation driven by iron-based materials elusive. This work provides a systematic review of various non-radical oxidation systems driven by iron-based materials, including singlet oxygen (1O2), reactive iron species (RFeS), and interfacial electron transfer. The unique mechanisms by which iron-based materials activate different oxidants (ozone, hydrogen peroxide, persulfate, periodate, and peracetic acid) to produce non-radical oxidation are described. The roles of active sites and the unique structures of iron-based materials in facilitating non-radical oxidation are discussed. Commonly employed identification methods in wastewater treatment are compared, such as quenching, chemical probes, spectroscopy, mass spectrometry, and electrochemical testing. According to the process of iron-based materials driving non-radical oxidation to remove organic pollutants, the driving factors at different stages are summarized. Finally, challenges and countermeasures are proposed in terms of mechanism exploration, detection methods and practical applications of non-radical oxidation driven by iron-based materials. This work provides valuable insights for understanding and developing non-radical oxidation systems.

A General Descriptor for Single-Atom Catalysts with Axial Ligands

Decoration of an axial coordination ligand (ACL) on the active metal site is a highly effective and versatile strategy to tune activity of single-atom catalysts (SACs). However, the regulation mechanism of ACLs on SACs is still incompletely known. Herein, we investigate diversified combinations of ACL-SACs, including all 3d-5d transition metals and ten prototype ACLs. We identify that ACLs can weaken the adsorption capability of the metal atom (M) by raising the bonding energy levels of the M-O bond while enhancing dispersity of the d orbital of M. Through examination of various local configurations and intrinsic parameters of ACL-SACs, a general structure descriptor σ is constructed to quantify the structure-activity relationship of ACL-SACs which solely based on a few key intrinsic features. Importantly, we also identified the axial ligand descriptor σACL, as a part of σ, which can serve as a potential descriptor to determine the rate-limiting steps (RLS) of ACL-SACs in experiment. And we predicted several ACL-SACs, namely, CrN4-, FeN4-, CoN4-, RuN4-, RhN4-, OsN4-, IrN4- and PtN4-ACLs, that entail markedly higher activities than the benchmark catalysts of Pt and IrO2 for oxygen reduction reaction and oxygen evolution reaction, respectively, thereby supporting that the general descriptor σ can provide a simple and cost-effective method to assess efficient electrocatalysts.

Revisiting the synergistic oxidation of peracetic acid and permanganate(Ⅶ) towards micropollutants: The enhanced electron transfer mechanism of reactive manganese species

Synergistic actions of peroxides and high-valent metals have garnered increasing attentions in wastewater treatment. However, how peroxides interact with the reactive metal species to enhance the reactivity remains unclear. Herein, we report the synergistic oxidation of peracetic acid (PAA) and permanganate(Ⅶ) towards micropollutants, and revisit the underlying mechanism. The PAA-Mn(VII) system showed remarkable efficiency with a 28-fold enhancement on sulfamethoxazole (SMX) degradation compared to Mn(Ⅶ) alone. Extensive quenching experiments and electron spin resonance (ESR) analysis revealed the generation of unexpected Mn(V) and Mn(VI) beyond Mn(III) in the PAA-Mn(VII) system. The utilization efficiency of Mn intermediates was quantified using 2,2'-azino-bis(3-ethylbenzothiazoline)-6-sulfonate (ABTS), and the results indicated that PAA could enhance the electron transfer efficiency of reactive manganese (Mn) species, thus accelerating the micropollutant degradation. Density functional theory (DFT) calculations showed that Mn intermediates could coordinate to the O1 of PAA with a low energy gap, enhancing the oxidation capacity and stability of Mn intermediates. A kinetic model based on first principles was established to simulate the time-dependent concentration profiles of the PAA-Mn complexes and quantify the contributions of the PAA-Mn(III) complex (50.8 to 59.3 %) and the PAA-Mn(Ⅴ/Ⅵ) complex (40.7 to 49.2 %). The PAA-Mn(VII) system was resistant to the interference from complex matrix components (e.g., chloride and humic acid), leading to the high efficiency in real wastewater. This work provides new insights into the interaction of PAA with reactive manganese species for accelerated oxidation of micropollutants, facilitating its application in wastewater treatment.

Regulating iron center by defective MoS2 for superior Fenton-like catalysis in water purification: The key role of surface interaction and superoxide radical in accelerating metal redox-cycling

Transition metals exhibit high reactivity for Fenton-like catalysis in environmental remediation, but how to save consumption and reduce pollution is of great interest. In this study, rationally designed defect-engineered Fe@MoS2 (Fe@D-MoS2) was prepared by incorporating reactive iron onto structural defects generated from the chemical acid-etching, aiming to improve the energetic consumption of the catalyst in Fenton-like applications. Morphological and structural properties were elucidated in details, the Fenton-like reactivity was evaluated with five phenolic contaminants for oxidant activation, radical generation and environmental remediation. Compared to Fe@MoS2, Fe@D-MoS2 exhibited a 18.9-fold increase in phenol degradation (0.09 versus 1.79 min-1). Quenching experiments, electron paramagnetic resonance tests and electrochemical measurements revealed the dominant sulfate and superoxide radicals. Rendered by strong metal-substrate surface and electronic interactions from regulated chemical environment and coordination structure, the inert ≡ Fe(III) was reduced to the reactive ≡ Fe(II) accompanied by the ≡ Mo(IV) oxidation to ≡ Mo(V) in MoS2 lattice, with adjacent sulfur serving as the key electron transfer bridge. Therefore, this work shows that the incorporation of reactive centers is able to boost two-dimensional sulfide materials for environmental catalysis 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.

Atomic-Level Engineered Cobalt Catalysts for Fenton-Like Reactions: Synergy of Single Atom Metal Sites and Nonmetal-Bonded Functionalities

Single atom catalysts (SACs) are atomic-level-engineered materials with high intrinsic activity. Catalytic centers of SACs are typically the transition metal (TM)-nonmetal coordination sites, while the functions of coexisting non-TM-bonded functionalities are usually overlooked in catalysis. Herein, the scalable preparation of carbon-supported cobalt-anchored SACs (CoCN) with controlled Co─N sites and free functional N species is reported. The role of metal- and nonmetal-bonded functionalities in the SACs for peroxymonosulfate (PMS)-driven Fenton-like reactions is first systematically studied, revealing their contribution to performance improvement and pathway steering. Experiments and computations demonstrate that the Co─N3C coordination plays a vital role in the formation of a surface-confined PMS* complex to trigger the electron transfer pathway and promote kinetics because of the optimized electronic state of Co centers, while the nonmetal-coordinated graphitic N sites act as preferable pollutant adsorption sites and additional PMS activation sites to accelerate electron transfer. Synergistically, CoCN exhibits ultrahigh activity in PMS activation for p-hydroxybenzoic acid oxidation, achieving complete degradation within 10 min with an ultrahigh turnover frequency of 0.38 min-1, surpassing most reported materials. These findings offer new insights into the versatile functions of N species in SACs and inspire rational design of high-performance catalysts in complicated heterogeneous systems.

Mediated Peroxymonosulfate Activation at the Single Atom Fe-N3O1 Sites: Synergistic Degradation of Antibiotics by Two Non-Radical Pathways

The activation of persulfates to degrade refractory organic pollutants is a hot issue in advanced oxidation right now. Here, it is reported that single-atom Fe-incorporated carbon nitride (Fe-CN-650) can effectively activate peroxymonosulfate (PMS) for sulfamethoxazole (SMX) removal. Through some characterization techniques and DFT calculation, it is proved that Fe single atoms in Fe-CN-650 exist mainly in the form of Fe-N3O1 coordination, and Fe-N3O1 exhibited better affinity for PMS than the traditional Fe-N4 structure. The degradation rate constant of SMX in the Fe-CN-650/PMS system reached 0.472 min-1, and 90.80% of SMX can still be effectively degraded within 10 min after five consecutive recovery cycles. The radical quenching experiment and electrochemical analysis confirm that the pollutants are mainly degraded by two non-radical pathways through 1O2 and Fe(IV)═O induced at the Fe-N3O1 sites. In addition, the intermediate products of SMX degradation in the Fe-CN-650/PMS system show toxicity attenuation or non-toxicity. This study offers valuable insights into the design of carbon-based single-atom catalysts and provides a potential remediation technology for the optimum activation of PMS to disintegrate organic pollutants.

Asymmetrically Coordinated Mn-S1N3 Configuration Induces Localized Electric Field-Driven Peroxymonosulfate Activation for Remarkably Efficient Generation of 1O2

Singlet oxygen (1O2) species generated in peroxymonosulfate (PMS)-based advanced oxidation processes offer opportunities to overcome the low efficiency and secondary pollution limitations of existing AOPs, but efficient production of 1O2 via tuning the coordination environment of metal active sites remains challenging due to insufficient understanding of their catalytic mechanisms. Herein, an asymmetrical configuration characterized by a manganese single atom coordinated is established with one S atom and three N atoms (denoted as Mn-S1N3), which offer a strong local electric field to promote the cleavage of O─H and S─O bonds, serving as the crucial driver of its high 1O2 production. Strikingly, an enhanced the local electric field caused by the dynamic inter-transformation of the Mn coordination structure (Mn-S1N3 ↔ Mn-N3) can further downshift the 1O2 production energy barrier. Mn-S1N3 demonstrates 100% selective product 1O2 by activation of PMS at unprecedented utilization efficiency, and efficiently oxidize electron-rich pollutants. This work provides an atomic-level understanding of the catalytic selectivity and is expected to guide the design of smart 1O2-AOPs catalysts for more selective and efficient decontamination applications.

Single-Atom Iron Can Steer Atomic Hydrogen toward Selective Reductive Dechlorination: Implications for Remediation of Chlorinated Solvents-Impacted Groundwater

Atomic hydrogen (H*) is a powerful and versatile reductant and has tremendous potential in the degradation of oxidized pollutants (e.g., chlorinated solvents). However, its application for groundwater remediation is hindered by the scavenging side reaction of H2 evolution. Herein, we report that a composite material (Fe0@Fe-N4-C), consisting of zerovalent iron (Fe0) nanoparticles and nitrogen-coordinated single-atom Fe (Fe-N4), can effectively steer H* toward reductive dechlorination of trichloroethylene (TCE), a common groundwater contaminant and primary risk driver at many hazardous waste sites. The Fe-N4 structure strengthens the bond between surface Fe atoms and H*, inhibiting H2 evolution. Nonetheless, H* is available for dechlorination, as the adsorption of TCE weakens this bond. Interestingly, H* also enhances electron delocalization and transfer between adsorbed TCE and surface Fe atoms, increasing the reactivity of adsorbed TCE with H*. Consequently, Fe0@Fe-N4-C exhibits high electron selectivity (up to 86%) toward dechlorination, as well as a high TCE degradation kinetic constant. This material is resilient against water matrix interferences, achieving long-lasting performance for effective TCE removal. These findings shed light on the utilization of H* for the in situ remediation of groundwater contaminated with chlorinated solvents, by rational design of earth-abundant metal-based single-atom catalysts.

Single Atom Catalyst in Persulfate Oxidation Reaction: From Atom Species to Substance

With maximum utilization of active metal sites, more and more researchers have reported using single atom catalysts (SACs) to activate persulfate (PS) for organic pollutants removal. In SACs, single metal atoms (Fe, Co, Cu, Mn, etc.) and different substrates (porous carbon, biochar, graphene oxide, carbon nitride, MOF, MoS2, and others) are the basic structural. Metal single atoms, substances, and connected chemical bonds all have a great influence on the electronic structures that directly affect the activation process of PS and degradation efficiency to organic pollutants. However, there are few relevant reviews about the interaction between metal single atoms and substances during PS activation process. In this review, the SACs with different metal species and substrates are summarized to investigate the metal-support interaction and evaluate their effects on PS oxidation reaction process. Furthermore, how metal atoms and substrates affect the reactive species and degradation pathways are also discussed. Finally, the challenges and prospects of SACs in PS-AOPs are proposed.

Size-Dependent Catalysis in Fenton-like Chemistry: From Nanoparticles to Single Atoms

State-of-the-art Fenton-like reactions are crucial in advanced oxidation processes (AOPs) for water purification. This review explores the latest advancements in heterogeneous metal-based catalysts within AOPs, covering nanoparticles (NPs), single-atom catalysts (SACs), and ultra-small atom clusters. A distinct connection between the physical properties of these catalysts, such as size, degree of unsaturation, electronic structure, and oxidation state, and their impacts on catalytic behavior and efficacy in Fenton-like reactions. In-depth comparative analysis of metal NPs and SACs is conducted focusing on how particle size variations and metal-support interactions affect oxidation species and pathways. The review highlights the cutting-edge characterization techniques and theoretical calculations, indispensable for deciphering the complex electronic and structural characteristics of active sites in downsized metal particles. Additionally, the review underscores innovative strategies for immobilizing these catalysts onto membrane surfaces, offering a solution to the inherent challenges of powdered catalysts. Recent advances in pilot-scale or engineering applications of Fenton-like-based devices are also summarized for the first time. The paper concludes by charting new research directions, emphasizing advanced catalyst design, precise identification of reactive oxygen species, and in-depth mechanistic studies. These efforts aim to enhance the application potential of nanotechnology-based AOPs in real-world wastewater treatment.

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.

Mechanism and ecological environmental risk assessment of peroxymonosulfate for the treatment of heavy metals in soil

Oxidation technologies based on peroxymonosulfate (PMS) have been effectively used for the remediation of soil organic pollutants due to their high efficiency. However, the effects of advanced PMS-based oxidation technologies on other soil pollutants, such as heavy metals, remain unknown. In this study, changes in the form of heavy metals in soil after using PMS and the risk of pollution to the ecological environment were investigated. Furthermore, two risk assessment methods, the mung bean germination toxicity test and groundwater leaching soil column test, were employed to evaluate the soil before and after PMS treatment. The results showed that PMS has a strong ability to degrade complex compounds, enabling the transformation of heavy metals, such as Cd, Pb, and Zn, from stable to active states in the soil. The risk assessments showed that PMS treatment activated heavy metals in the soil, which delayed the growth of plants, increased heavy metal content in plant tissues and the risk of groundwater pollution. These findings provide a new perspective for understanding the effects of PMS on soil, thus facilitating the sustained and reliable development of future research in the field of advanced oxidation applied to soil treatment.

Degradation of Bisphenol A by Nitrogen-Rich ZIF-8-Derived Carbon Materials-Activated Peroxymonosulfate

Bisphenol A (BPA), representing a class of organic pollutants, finds extensive applications in the pharmaceutical industry. However, its widespread use poses a significant hazard to both ecosystem integrity and human health. Advanced oxidation processes (AOPs) based on peroxymonosulfate (PMS) via heterogeneous catalysts are frequently proposed for treating persistent pollutants. In this study, the degradation performance of BPA in an oxidation system of PMS activated by transition metal sites anchored nitrogen-doped carbonaceous substrate (M-N-C) materials was investigated. As heterogeneous catalysts targeting the activation of peroxymonosulfate (PMS), M-N-C materials emerge as promising contenders poised to overcome the limitations encountered with traditional carbon materials, which often exhibit insufficient activity in the PMS activation process. Nevertheless, the amalgamation of metal sites during the synthesis process presents a formidable challenge to the structural design of M-N-C. Herein, employing ZIF-8 as the precursor of carbonaceous support, metal ions can readily penetrate the cage structure of the substrate, and the N-rich linkers serve as effective ligands for anchoring metal cations, thereby overcoming the awkward limitation. The research results of this study indicate BPA in water matrix can be effectively removed in the M-N-C/PMS system, in which the obtained nitrogen-rich ZIF-8-derived Cu-N-C presented excellent activity and stability on the PMS activation, as well as the outstanding resistance towards the variation of environmental factors. Moreover, the biological toxicity of BPA and its degradation intermediates were investigated via the Toxicity Estimation Software Tool (T.E.S.T.) based on the ECOSAR system.

Insights into mechanism of peroxymonosufate activation by Mo single-atom catalysts: Singlet oxygen evolution and role of Mo-N coordination

Recently, the Fenton-like reaction using peroxymonosulfate (PMS) has been acknowledged as a potential method for breaking down organic pollutants. In this study, we successfully synthesized a highly efficient and stable single atom molybdenum (Mo) catalyst dispersed on nitrogen-doped carbon (Mo-NC-0.1). This catalyst was then utilized for the first time to activate PMS and degrade bisphenol A (BPA). The Mo-NC-0.1/PMS system demonstrated the ability to completely degrade BPA within just 20 min. Scavenging tests and density functional theory (DFT) calculations have demonstrated that the primary reactive oxygen species was singlet oxygen (1O2) produced by Mo-N4 sites. The self-cycling of Mo facilitated PMS activation and the transition from a free radical activation pathway to a non-radical pathway mediated by 1O2. Simultaneously, the nearby pyridinic N served as adsorption sites to immobilize BPA and PMS molecules. The exceptionally high catalytic activity of Mo-NC-0.1 derived from its unique Mo-N coordination, which markedly reduced the distance for 1O2 to migrate to the BPA molecules. The Mo-NC-0.1/PMS system effectively reduced the acute toxicity of BPA and exhibited excellent cycling stability with minimal leaching. This study presented a new catalyst with high selectivity for 1O2 generation and provided valuable insights for the application of single atom catalysts in PMS-based AOPs.

Controllable chemical redox reactions to couple microbial degradation for organic contaminated sites remediation: A review

Global environmental concern over organic contaminated sites has been progressively conspicuous during the process of urbanization and industrial restructuring. While traditional physical or chemical remediation technologies may significantly destroy the soil structure and function, coupling moderate chemical degradation with microbial remediation becomes a potential way for the green, economic, and efficient remediation of contaminated sites. Hence, this work systematically elucidates why and how to couple chemical technology with microbial remediation, mainly focused on the controllable redox reactions of organic contaminants. The rational design of materials structure, selective generation of reactive oxygen species, and estimation of degradation pathway are described for chemical oxidation. Meanwhile, current progress on efficient and selective reductions of organic contaminants (i.e., dechlorination, defluorination, -NO2 reduction) is introduced. Combined with the microbial remediation of contaminated sites, several consideration factors of how to couple chemical and microbial remediation are proposed based on both fundamental and practical points of view. This review will advance the understanding and development of chemical-microbial coupled remediation for organic contaminated sites.

How hetero-single-atom dispersion reconstructed electronic structure of carbon materials and regulated Fenton-like oxidation pathways

Single-atom catalysts (SACs) have emerged as competitive candidates for Fenton-like oxidation of micro-pollutants in water. However, the impact of metal insertion on the intrinsic catalytic activity of carrier materials has been commonly overlooked, and the environmental risk due to metal leaching still requires attention. In contrast to previous reports, where metal sites were conventionally considered as catalytic centers, our study investigates, for the first time, the crucial catalytic role of the carbon carrier modulated through hetero-single-atom dispersion and the regulation of Fenton-like oxidation pathways. The inherent differences in electronic properties between Fe and Co can effectively trigger long-range electron rearrangement in the sp2-carbon-conjugated structure, creating more electron-rich regions for peroxymonosulfate (PMS) complexation and initiating the electron transfer process (ETP) for pollutant degradation, which imparts the synthesized catalyst (FeCo-NCB) with exceptional catalytic efficiency despite its relatively low metal content. Moreover, the FeCo-NCB/PMS system exhibits enduring decontamination efficiency in complex water matrices, satisfactory catalytic stability, and low metal leaching, signifying promising practical applications. More impressively, the spatial relationship between metal sites and electron density clouds is revealed to determine whether high-valent metal-oxo species (HVMO) are involved during the decomposition of surface complexes. Unlike single-type single-atom dispersion, where metal sites are situated within electron-rich regions, hetero-single-atom dispersion can cause the deviation of electron density clouds from the metal sites, thus hindering the in-situ oxidation of metal within the complexes and minimizing the contribution of HVMO. These findings provide new insights into the development of carbon-based SACs and advance the understanding of nonradical mechanisms underpinning Fenton-like treatments.

Singlet Oxygen Formation Mechanism for the H2O2-Based Fenton-like Reaction Catalyzed by the Carbon Nitride Homojunction

The singlet oxygen (1O2) oxidation process activated by metal-free catalysts has recently attracted considerable attention for organic pollutant degradation; however, the 1O2 formation remains controversial. Simultaneously, the catalytic activity of the metal-free catalyst limits the practical application. In this study, carbon nitride (HCCN) containing an intramolecular homojunction, a kind of metal-free catalyst, exhibits excellent activity compared to g-C3N4 (CN) and crystalline carbon nitride (HCN) for tetracycline hydrochloride degradation through the H2O2-based Fenton-like reaction. The rate constant for HCCN increased about 16.1 and 8.9 times than that of CN and HCN, respectively. The activity of HCCN was enhanced, and the dominant reactive oxygen species (ROS) changed from hydroxyl radicals (•OH) to 1O2 with an increase in pH from 4.5 to 11.5. A novel formation pathway of 1O2 was revealed. This result is different from the normal reference, in which •OH is always the primary ROS in the H2O2-based Fenton-like reaction. This study may provide a possible strategy for the investigation on the nonradical oxidation process in the Fenton-like reaction.

Influence mechanism of anions on iron doping into swine bone char: Promoting non-radical oxidation of acetaminophen in a Fenton-like system

The application of iron-doped biochar in peroxymonosulfate (PMS) activation systems has gained increasing attention due to their effectiveness and environmental friendliness in addressing environmental issues. However, the behavioral mechanism of iron doping and the detailed 1O2 generation mechanism in PMS activation systems remain ambiguous. Here, we investigated the effects of three anions (Cl-, NO3-and SO42-) on the process of iron doping into bone char, leading to the synthesis of three iron-doped bone char (Fe-ClBC, Fe-NBC and Fe -SBC). These iron-doped bone char were used to catalyze PMS to degrade acetaminophen (APAP) and exhibited the following activity order: Fe-ClBC > Fe-NBC > Fe-SBC. Characterization results indicated that iron doping primarily occurred through the substitution of calcium in hydroxyapatite within BC. In the course of the impregnation, the binding of SO42- and Ca2+ hindered the exchange of iron ions, resulting in lower catalytic activity of Fe-SBC. The primary reactive oxygen species in the Fe-ClBC/PMS and Fe-NBC/PMS systems were both 1O2. 1O2 is produced through O2•- conversion and PMS self-dissociation, which involves the generation of metastable iron intermediates and electron transfer within iron species. The presence of oxygen vacancies and more carbon defects in the Fe-ClBC catalyst facilitates 1O2 generation, thereby enhancing APAP degradation within the Fe-ClBC/PMS system. This study is dedicated to in-depth exploration of the mechanisms underlying iron doping and defect materials in promoting 1O2 generation.

Tailoring d-band center of high-valent metal-oxo species for pollutant removal via complete polymerization

Polymerization-driven removal of pollutants in advanced oxidation processes (AOPs) offers a sustainable way for the simultaneous achievement of contamination abatement and resource recovery, supporting a low-carbon water purification approach. However, regulating such a process remains a great challenge due to the insufficient microscopic understanding of electronic structure-dependent reaction mechanisms. Herein, this work probes the origin of catalytic pollutant polymerization using a series of transition metal (Cu, Ni, Co, and Fe) single-atom catalysts and identifies the d-band center of active site as the key driver for polymerization transfer of pollutants. The high-valent metal-oxo species, produced via peroxymonosulfate activation, are found to trigger the pollutant removal via polymerization transfer. Phenoxyl radicals, identified by the innovative spin-trapping and quenching approaches, act as the key intermediate in the polymerization reactions. More importantly, the oxidation capacity of high-valent metal-oxo species can be facilely tuned by regulating their binding strength for peroxymonosulfate through d-band center modulation. A 100% polymerization transfer ratio is achieved by lowering the d-band center. This work presents a paradigm to dynamically modulate the electronic structure of high-valent metal-oxo species and optimize pollutant removal from wastewater via polymerization.

Iron containing sludge-derived carbon towards efficient peroxymonosulfate activation: Active site synergy, performance and alternation mechanism

Converting industrial sludge into catalytic materials for water purification is a promising approach to simultaneously realize effective disposal of sludge and resource of water. However, manipulating the high efficiency remains a huge challenge due to the difficulty in the active sites control of the sludge. Herein, we proposed a constitutive modulation strategy by the combination of hydrothermal and pyrolysis (HTP) for the fabrication of defects-assistant Fe containing sludge-derived carbon catalysts on upgrading performance in peroxymonosulfate (PMS) activation for pollutant degradation. Adjustable defects on dyeing sludge-derived carbon catalysts (DSCC) were achieved by introducing oxygen or nitrogen functional precursors (hydroquinone or p-phenylenediamine) during hydrothermal processes and by further pyrolysis, where O was detrimental while N was beneficial to defect generation. Compared to the DSCC with less defects (DHSC-O), the defect-rich sample (DHSC-2N) exhibited superior catalytic performance of PMS activation for bisphenol A (BPA) elimination (k = 0.45 min-1, 2.52 times of DHSC-O), as well as 81.4% total organic carbon (TOC) removal. Meanwhile, the degradation capacity was verified in wide pH range (2.1-8.1) and various aqueous matrices, reflecting the excellent adaptability and anti-interference performance. Furthermore, the continuous-flow experiments on industrial wastewater showed synchronous BPA and chemical oxygen demand (COD) removal, implying great potential for practical application. Solid electron paramagnetic resonance (EPR) and 57Fe Mösssbauer spectra analysis indicated that the defects acted as secondary active sites for Fe sites, which were beneficial to accelerating the electron transfer process. The only Fe active sites preferred the radical pathway. The controllable reaction tendency provides possibilities for the on-demand design of sludge-based catalysts to meet the requirements of practical wastewater treatment under Fenton-like reaction.

Converting peracetic acid activation by Fe3O4 from nonradical to radical pathway via the incorporation of L-cysteine

Recently, peracetic acid (PAA) based Fenton (-like) processes have received much attention in water treatment. However, these processes are limited by the sluggish Fe(III)/Fe(II) redox circulation efficiency. In this study, L-cysteine (L-Cys), an environmentally friendly electron donor, was applied to enhance the Fe3O4/PAA process for the sulfamethoxazole (SMX) abatement. Surprisingly, the L-Cys incorporation was found not only to enhance the SMX degradation rate constant by 3.2 times but also to switch the Fe(IV) dominated nonradical pathway into the •OH dominated radical pathway. Experiment and theoretical calculation result elucidated -NH2, -SH, and -COOH of L-Cys can increase Fe solubilization by binding to the Fe sites of Fe3O4, while -SH of L-Cys can promote the reduction of bounded/dissolved Fe(III). Similar SMX conversion pathways driven by the Fe3O4/PAA process with or without L-Cys were revealed. Excessive L-Cys or PAA, high pH and the coexisting HCO3-/H2PO4- exhibit inhibitory effects on SMX degradation, while Cl- and humic acid barely affect the SMX removal. This work advances the knowledge of the enhanced mechanism insights of L-Cys toward heterogeneous Fenton (-like) processes and provides experimental data for the efficient treatment of sulfonamide antibiotics in the water treatment.

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.

Synchronously improved permeability, selectivity and fouling resistance of Fe-N-C functionalized ceramic catalytic membrane for effective water treatment: The critical role of Fe

Constructing catalytic membrane simultaneously displaying high permeability, selectivity and antifouling performance in water treatment remains challenging. Herein, the surface and pore channels of the ceramic membrane were co-functionalized with nitrogen doped carbon supported Fe catalyst (CN-F), and the Fe content was varied to investigate its effect on performance of CN-F coupled with peroxymonosulfate (PMS) activation (CN-F/PMS) for water treatment. Results confirmed the introduced Fe (in Fe-N coordination form) greatly enhanced the permeability, selectivity and fouling resistance of CN-F. Optimal CN-F3/PMS achieved 96.5% removal and 52.1% mineralization of sulfamethoxazole in short retention duration (2.7 min), whose performance was 5.4 and 6.7 times higher than that of nitrogen doped carbon functionalized ceramic catalytic membrane (CN/PMS) and CN-F3 filtration alone, respectively. CN-F3/PMS also efficiently inhibited fouling on both surface and pores with 2.8 and 2.4 times lower flux loss than that of CN/PMS and CN-F3 filtration alone, respectively. Moreover, CN-F3/PMS displayed superior performance in long-term treatment of real coking wastewater. The outstanding performance of CN-F was mainly attributed to the dual role of supported Fe, which served as hydrophilic site for enhanced water permeation and major active site for PMS adsorption and reduction into reactive species (mainly high-valent Fe(IV)=O species) towards pollutant elimination.

Precise coordination of high-loading Fe single atoms with sulfur boosts selective generation of nonradicals

Nonradicals are effective in selectively degrading electron-rich organic contaminants, which unfortunately suffer from unsatisfactory yield and uncontrollable composition due to the competitive generation of radicals. Herein, we precisely construct a local microenvironment of the carbon nitride-supported high-loading (~9 wt.%) Fe single-atom catalyst (Fe SAC) with sulfur via a facile supermolecular self-assembly strategy. Short-distance S coordination boosts the peroxymonosulfate (PMS) activation and selectively generates high-valent iron-oxo species (FeIV=O) along with singlet oxygen (1O2), significantly increasing the 1O2 yield, PMS utilization, and p-chlorophenol reactivity by 6.0, 3.0, and 8.4 times, respectively. The composition of nonradicals is controllable by simply changing the S content. In contrast, long-distance S coordination generates both radicals and nonradicals, and could not promote reactivity. Experimental and theoretical analyses suggest that the short-distance S upshifts the d-band center of the Fe atom, i.e., being close to the Fermi level, which changes the binding mode between the Fe atom and O site of PMS to selectively generate 1O2 and FeIV=O with a high yield. The short-distance S-coordinated Fe SAC exhibits excellent application potential in various water matrices. These findings can guide the rational design of robust SACs toward a selective and controllable generation of nonradicals with high yield and PMS utilization.

Iron 3D-Orbital Configuration Dependent Electron Transfer for Efficient Fenton-Like Catalysis

Transition metals are excellent active sites to activate peroxymonosulfate (PMS) for water treatment, but the favorable electronic structures governing  reaction mechanism still remain elusive. Herein, the authors construct typical d-orbital configurations on iron octahedral (FeOh ) and tetrahedral (FeTd ) sites in spinel ZnFe2 O4 and FeAl2 O4 , respectively. ZnFe2 O4 (136.58 min-1 F-1 cm2 ) presented higher specific activity than FeAl2 O4 (97.47 min-1 F-1 cm2 ) for tetracycline removal by PMS activation. Considering orbital features of charge amount, spin state, and orbital arrangement by magnetic spectroscopic analysis, ZnFe2 O4 has a larger bond order to decompose PMS. Using this descriptor, high-spin FeOh is assumed to activate PMS mainly to produce nonradical reactive oxygen species (ROS) while high-spin FeTd prefers to induce radical species. This hypothesis is confirmed by the selective predominant ROS of 1 O2 on ZnFe2 O4 and O2 •- on FeAl2 O4 via quenching experiments. Electrochemical determinations reveal that FeOh has superior capability than FeTd for feasible valence transformation of iron cations and fast interfacial electron transfer. DFT calculations further suggest octahedral d-orbital configuration of ZnFe2 O4 is beneficial to enhancing Fe-O covalence for electron exchange. This work attempts to understand the d-orbital configuration-dependent PMS activation to design efficient catalysts.

Active Center Size-Dependent Fenton-Like Chemistry for Sustainable Water Decontamination

Accurately controlling catalytic activity and mechanism as well as identifying structure-activity-selectivity correlations in Fenton-like chemistry is essential for designing high-performance catalysts for sustainable water decontamination. Herein, active center size-dependent catalysts with single cobalt atoms (CoSA), atomic clusters (CoAC), and nanoparticles (CoNP) were fabricated to realize the changeover of catalytic activity and mechanism in peroxymonosulfate (PMS)-based Fenton-like chemistry. Catalytic activity and durability vary with the change in metal active center sizes. Besides, reducing the metal size from nanoparticles to single atoms significantly modulates contributions of radical and nonradical mechanisms, thus achieving selective/nonselective degradation. Density functional theory calculations reveal evolutions in catalytic mechanisms of size-dependent catalytic systems over different Gibbs free energies for reactive oxygen species generation. Single-atom site contact with PMS is preferred to induce nonradical mechanisms, while PMS dissociates and generates radicals on clusters and nanoparticles. Differences originating from reaction mechanisms endow developed systems with size-dependent selectivity and mineralization for treating actual hospital wastewater in column reactors. This work brings an in-depth understanding of metal size effects in Fenton-like chemistry and guides the design of intelligent catalysts to fulfill the demand of specific scenes for water purification.

Unraveling the single-atom Fe-N4 catalytic site selectivity generate singlet oxygen via activation of persulfate: Polarizing electric fields changes the electron transfer pathway

Single-atom catalysts have been proved to be an effective material for the removal of organic pollutants from water and wastewater, and yet, the relationship between their internal structures and their roles still remains elusive. In this work, a catalyst Fe (MIL)-SAC with single-atom Fe-N4 active site was prepared. Fe (MIL)-SAC/Peroxydisulfate (PDS) system was able to achieve complete degrade of the Sulfamethoxazole (SMX) with kobs at 0.466 min-1, which was faster than the Fenton system under the same conditions (kobs = 0.422 min-1) and 16 times faster than Fe (MIL) (kobs = 0.029 min-1). Density functional calculations reveal that the Fe-N4 structure will affect the electron transport path and lead to selective generation of 1O2 by triggering S-O breakage and O-O polarization in PDS. Furthermore, Fe (MIL)-SAC/PDS system exhibits strong resistance to common influencing factors and has good application prospects. This work provides a new approach for the selectively generation of 1O2 for the efficient treatment of organic pollutants in aqueous environment.