Axially engineered single atoms in enzyme-mimic-binding pocket steering dehalogenation–polymerization pathways toward water pollutant upcycling

Domain: neuroscience
Published: 2026-02-05
Editorial outcome: minor_revision

Abstract
Polymerization-based advanced oxidation processes (P-AOPs) represent a promising strategy for simultaneous pollutant abatement and water resource recovery. However, selective removal of halogenated organic pollutants via P-AOP remains challenging due to uncontrollable oligomerization pathways and radical chain inhibition induced by the chlorine substituent. Here, we develop an axial-nitrogen-coordinated single-atom iron catalyst (Fe-NCN) that triggers an electron-transfer pathway (ETP) via enhanced electronic pulling on 2,4,6-trichlorophenol (TCP), achieving near-complete dechlorination and catalytic transformation into polymeric products. Spectroscopic characterization and theoretical calculations reveal that the axial coordination of Fe-NCN regulates peroxymonosulfate (PMS) activation to form surface PMS* complex, which oxidizes the adjacent TCP via a short-range ETP regime due to the charge-constrained nature of the carbon nitride substrate. Upon electron extraction from TCP, PMS* is transformed into an active surface hydroxyl intermediate (OH−*), which attains nucleophilic substitution of organochlorine, subsequently triggering C-O crosslinking of the hydroxylated by-products with ultra-high electron utilization efficiency (~353%). This integrated dechlorination-hydroxylation-polymerization process can be scaled up for device-scale wastewater treatment with excellent stability. The collected oligomers can be processed to manufacture industrial-grade plastic products. This study advances the understanding of the complex polymerization regime of chlorinated micropollutants by using engineered atomic catalysts for sustainable and low-emission water purification.

Introduction
Chemical oxidation technologies utilize generated reactive oxygen species (ROS) for effective degradation and mineralization of recalcitrant organic pollutants, demonstrating considerable potential for securing potable and drinking water supplies1,2. However, conventional mineralization-based advanced oxidation processes are limited by excessive oxidant dosages, toxic by-products (e.g., disinfection by-products), and excessive carbon emissions, collectively leading to higher treatment costs, secondary pollution risks, and excessive chemical consumption3–5. In contrast, polymerization-based advanced oxidation processes (P-AOPs) emerge as a sustainable alternative via transforming aqueous organic pollutants into recyclable solid-phase products through selective catalytic pathways, enabling concurrent contaminant removal and resource recovery6–9. Nevertheless, the technical bottlenecks that constrain their practical implementation lie in the precise control of pollutant and product selectivity as well as long-term stability of the catalytic system.

Conversion efficiency in P-AOPs systems is predominantly governed by the electronic characteristics of substituents in parent compounds10. For example, halogenated phenols demonstrated substantially lower polymerization rates compared with their alkyl-substituted analogs6,11. This is because the strong electron-withdrawing (electronegativity) nature of halogen functionalities greatly delocalizes π-electron density away from the aromatic benzene, thereby destabilizing organic radical intermediates and kinetically impeding chain propagation of halogenated organic pollutants (HOPs)10,11. Furthermore, halogen substitution on the aromatic rings preferentially induced C-C coupling over C-O linkages6,12–14. The resultant C-C coupled products exhibit weaker interfacial interactions with catalytic surfaces because of the exposure of more polar oxygen functional groups14, leading to enhanced aqueous phase mobility (Supplementary Fig. 1). Critically, these oligomeric species retain significant halogen content stemming from precursor molecules, amplifying their ecotoxicological potential through bioaccumulation6,15. Developing C-O linkages via dehalogenation synergistically enhances polymerization and detoxification of HOPs, while precise catalyst engineering to achieve controlled dehalogenation to produce high-quality polymerization substrate remains a critical bottleneck.

In hydrolytic enzymes, such as esterases in nature, the active centers use deprotonated H2O (OH−) as a nucleophilic reagent to attack electrophilic atoms in the substrate molecule to generate alcohols or acids16. Notably, halogen substituents amplify the electrophilicity of the adjacent carbon, creating activated (positively charged) sites for targeted nucleophilic substitution17. Similarly, surface-confined hydroxyl species, generated from catalytic cleavage of the peroxide bond in peroxymonosulfate (PMS), exhibit exceptional nucleophilicity, enabling direct nucleophilic attack or mediated indirect nucleophilic substitution of phenolic pollutants to yield hydroxyl groups18. The formation of phenolic hydroxyl groups not only enhances the stability of organic radical intermediates but also provides additional reactive crosslinking sites, promoting the subsequent C-O coupling-mediated polymerization19. Thus, modulating the electron transfer pathway (ETP) via the dual-electron process can activate PMS to generate surface-confined OH− species (OH−*), which may achieve oxidative conversion of recalcitrant chlorinated pollutants and intermediates via successive dechlorination-hydroxylation-polymerization (DHP) processes.

Herein, we synthesize a mimic-enzyme axially ligand-modified single-atom iron catalyst (Fe-SACs, designated Fe-NCN), which achieves near-complete dehalogenation (99.3%) and polymerization transfer (98.2%) of 2,4,6-trichlorophenol (TCP). The electron-pulling effect of the axial nitrogen ligand modulates the electronic structure of the central Fe atoms, induces PMS coordination to form a complex intermediate (PMS*), and achieves rapid TCP removal by short-range ETP under the charge confinement of the substrates (kNor = 3.97 min−1 L g−1). Integrated product identification, spectroscopic analysis, and density functional theory (DFT) calculations elucidate the role of axial ligands in regulating the catalytic pathway transition from single toxygenation to ETP, which facilitates the C-O crosslinking via the DHP path. In addition, mimicking the hydrophobic and polar microenvironment of the enzyme-binding pocket drives the separation of polar group-containing but macroscopic hydrophobic oligomers in the active site, thereby maintaining the catalyst activity. Combined techno-economic analysis (TEA), life cycle assessment (LCA), and continuous-flow reactor trials demonstrate the feasibility of industrial-scale manufacturing of Fe-NCN, robust wastewater treatment performance (TOC removal >80% within 60 days), carbon recovery potential (product recovery ≈45%) and environmental friendliness (carbon footprint decreased by 78%) in practical wastewater remediation.

Results
Synthesis and characterization of Fe-SACs
The structure and function of CYP152 peroxidase were fully dissected to design dehalogenation-polymerization catalysts. Microbial-mediated hydrolytic dechlorination occurs at the active center of heme iron modified by weakly axial sulfate ligands, using surface-activated peroxides to reconfigure the local hydrogen-bonding network for hydroxylated dehalogenation of the substrate (Fig. 1a)18. Binding pockets consisting of hydrophobic amino acids and polar-interacting residues provide substrate-trapping zones. B12-dependent reductive dehalogenases have also been reported to have similar structures20 (Supplementary Fig. 2). Inspired by the structure of the natural enzyme, the introduction of axial ligands to modify the metal center may be beneficial for regulating the switch in oxidative mechanisms in the DHP pathway.

A pre-coordination-assisted strategy was used to prepare Fe-NCN, as shown in Fig. 1b. Initially, Fe2+ ions were complexed with 1,10-phenanthroline in aqueous solution to form a metal-organic precursor as pre-coordination templates. Subsequently, this complex was introduced into a graphitized carbon nitride (g-C3N4, CN) dispersion and subjected to ultrasonic treatment, followed by overnight aging to achieve oriented assembly of nanostructures. The products were treated by pyrolysis under NH3/HCl atmosphere (generated from in situ hydroxylamine hydrochloride decomposition) to yield the porous carbon-supported single-atom iron catalyst (Fe-NCN). For comparative analysis, Fe-CN was synthesized in parallel using only iron salts as precursors. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM) images revealed a hierarchical architecture of Fe-SACs with no visible Fe clusters or nanoparticles (Supplementary Fig. 3). Brunauer-Emmett-Teller (BET) analysis confirmed the incorporation of Fe species maintained the mesoporous structure of the CN substrate and significantly increased the specific surface area (SSA) of Fe-SACs (Supplementary Fig. 4). X-ray diffraction (XRD) patterns showed characteristic CN diffraction peaks without crystalline iron phases, ruling out the existence of Fe-based nanoparticles (Supplementary Fig. 5a)21. Atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) directly visualized isolated Fe atoms as bright spots on the CN matrix (Fig. 1d and Supplementary Fig. 5b). Elemental mapping images verified homogeneous distribution of C, N, and Fe atoms throughout the CN framework (Fig. 1c and Supplementary Fig. 5d). Besides, the atom-overlapping Gaussian-function-fitting results from AC-HAADF-STEM displayed additional spatial information concerning the atomically dispersed Fe centers (Fig. 1e and Supplementary Fig. 5c). Inductively coupled plasma optical emission spectroscopy (ICP-OES) quantified Fe contents as 1.85 wt% and 2.06 wt% for Fe-CN and Fe-NCN, respectively (Supplementary Table 1). Notably, Fe-NCN synthesized using a pre-coordinated template showed a product yield of up to 45.0% and a more compact stacking of structures compared to Fe-CN (32.2%) (Supplementary Fig. 3). Thus, the pre-coordinated templating approach not only inhibits the loss of carbon and nitrogen at high temperatures through interlayer crosslinking, but also enables precise tuning of catalyst structures.

The coordination structure of Fe species was investigated using X-ray photoelectron spectroscopy (XPS). The absence of Fe–C and Fe–O bonds in Fe-SACs was confirmed by C 1s and O 1s XPS spectra (Supplementary Figs. 6 and 7). Deconvoluted N 1s spectra revealed characteristic Fe–N peaks at 399.2 eV in Fe-SACs with diminished pyridinic N signals compared to pristine CN, implying their chemical bonding with Fe species (Supplementary Fig. 8a)22. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) at m/z=67.93 and 71.94 corresponded to weak characteristic signals for [FeC]− and [FeO]−, respectively. In contrast, pronounced signals for the fragment [FeN]− (m/z=69.94) were observed, indicating the presence of only Fe–N bonds in Fe-SACs (Supplementary Fig. 8c). X-ray absorption spectroscopy (XAS) was employed to probe the chemical states and coordination environments of Fe-SACs. The Fe K-edge X-ray absorption near-edge structure (XANES) spectra demonstrated that the absorption edge intensities of Fe-SACs lie between those of FeO and Fe2O3, closely resembling iron phthalocyanine (FePc) (Fig. 1f). Quantitative analysis of XANES derivative spectra yielded average Fe valence states of +2.39 and +2.76 for Fe-CN and Fe-NCN, respectively, consistent with Fe 2p XPS results (Fig. 1f inset and Supplementary Fig. 8b). Compared to Fe-CN, Fe-NCN exhibited reduced pre-edge intensity and enhanced white-line intensity, indicative of decreased symmetry and charge redistribution in the coordination environment23. Fourier-transform extended X-ray absorption fine structure (FT-EXAFS) spectra showed a prominent Fe-N coordination peak at ~1.5 Å without metallic Fe–Fe bonds, confirming single-atom dispersion (Fig. 1g). This conclusion was further supported by wavelet transform (WT)-EXAFS analysis (Supplementary Fig. 9). Notably, the Fe-NCN primary peak exhibited a positive shift compared to Fe-CN and Fe foil, suggesting increased N coordination number (n>4)24. EXAFS fitting on k-space and R-space revealed average Fe-N coordination numbers of 4.28 and 5.13 for Fe-CN and Fe-NCN, respectively (Supplementary Fig. 10 and Supplementary Table 2), which was also supported by TOF-SIMS analysis (Supplementary Fig. 8d). The comparable coordination environments between Fe-CN and FePc confirmed the planar Fe-N4 configuration in Fe-CN. In contrast, the significantly higher planar distortion index (IA/IB = 1.12 vs. 0.98) observed in Fe-NCN compared to Fe-CN (Fig. 1h)25, combined with excellent agreement between experimental data and simulated XANES spectra for the axial coordination model (Supplementary Fig. 11), conclusively demonstrated that isolated Fe atoms in Fe-NCN adopt a distorted square pyramidal configuration with four in-plane N atoms and one axial N ligand (FeN4-Naxial).

Fenton-like catalytic reactions for pollutant removal
Typically, the oxidative removal of TCP via P-AOPs is challenging because the substitution of active hydrogens in the ortho and para sites of TCP by organochlorines prohibits the generation of critical phenoxy intermediates triggered by the proton-coupled electron transfer pathway; furthermore, the shielding of the Cl atoms blocks the active sites on TCP to proceed the polymerization reaction via conventional radical and ETP pathways6,11. In this work, PMS alone exhibited negligible TCP removal (Fig. 2a). In contrast, Fe-NCN achieved complete removal of TCP within 2.5 min, with its pseudo-first-order rate constant (kobs) reaching 157, 59, and 17-fold higher than those of metal-free N-C (direct pyrolysis of the chelating agent), CN, and Fe-CN (Supplementary Fig. 12a). The adsorptive removal of TCP for Fe-NCN was only 2.4% within 10 min, indicating that TCP was removed through catalytic oxidation (Supplementary Fig. 12b). The iron leaching from Fe-NCN after reaction was approximately 7.02 µg/L, while Fe2+ at the same concentration only achieved 3.1% TCP removal with the presence of PMS, excluding the contribution of homogeneous reactions (Supplementary Fig. 12c).

The intrinsic catalytic activity was assessed using surface area-normalized rate constant (kSSA), turnover frequency (TOF), and reaction parameter-normalized rate constant (kNor) (Fig. 2b and Supplementary Table 3). Obviously, Fe-NCN demonstrated overwhelming superiority over other materials across all these metrics. During cycling tests, the Fe-NCN/PMS system maintained high removal efficiency across six consecutive cycles with low iron leaching (Supplementary Fig. 12d). The Fe-NCN/PMS system also maintained superior TCP removal efficiency over a wide initial pH range of 1.0–9.0. However, when the pH was increased to 12.0, Fe-NCN exhibited a significantly inhibited removal efficiency, and this alkaline inhibition could be attributed to the electrostatic repulsion between the PMS anion and the hydroxylated and negatively charged catalyst surface10 (Supplementary Fig. 13). Furthermore, intrinsic activity of Fe-NCN based on TOF and kNor surpassed state-of-the-art atomic and nanoparticle Fenton-like catalysts (Fig. 2c and Supplementary Tables 4 and 5).

Furthermore, Fe-NCN also demonstrated significantly enhanced oxidation efficiencies compared to Fe-CN for removing other micropollutants such as halogenated/dehalogenated phenolic compounds and sulfonamide compounds (Supplementary Fig. 14). Of particular interest, both Fe-NCN and Fe-CN exhibited slower removal kinetics toward electron-deficient organic pollutants, such as atrazine (ATZ), nitrobenzene (NB), and benzoic acid (BA), which are common radical probes, implying the selective non-radical nature of the two systems26. Such selectivity of non-radical oxidation can effectively mitigate interference from inorganic ions and organic humic substances in water matrices24,27. As shown in Supplementary Fig. 15, the presence of 10 mM inorganic anions showed a negligible impact on TCP removal. Even with 50 mg/L humic acid, TCP removal was inhibited by less than 10%. Additionally, Fe-NCN exhibited excellent TCP removal efficiency in tap water, surface water, and secondary effluent of wastewater treatment plants (Supplementary Fig. 16).

Acute toxicity was evaluated by calculating the relative inhibition of P. phosphoreum luminescence intensity before and after the reaction. The Fe-CN/PMS system showed rapidly increasing acute toxicity levels during the reaction, while the Fe-NCN/PMS system exhibited significantly reduced toxicity (Supplementary Fig. 17a). This observation strongly correlated with the dechlorination trends (Supplementary Fig. 17b), reflecting rapid TCP dechlorination into less/non-chlorinated products of low toxicity in the Fe-NCN/PMS system28. As shown in Supplementary Fig. 18a, although Fe-CN removed 64.1% of TCP within 10 min, only ~20% of total organic carbon (TOC) was eliminated in the bulk solution. In contrast, Fe-NCN achieved near-complete TOC removal (~98%). Chemical oxygen demand (COD) analysis after TCP removal revealed ~80% residual COD in bulk solution and ~7% COD remained on the catalyst in the Fe-CN/PMS system, whereas the Fe-NCN system retained <5% COD in bulk solution and ~93% on the catalyst (Fig. 2d and Supplementary Fig. 18b). These results collectively suggest the oxidative transformation of organic pollutants through a non-mineralization pathway in the Fe-NCN/PMS system with the polymeric products accumulated on the catalyst surface6,10,29. Thermogravimetric analysis (TGA) further confirmed this conclusion (Supplementary Fig. 19). In the Fe-NCN/PMS/TCP system, the theoretical maximum electron equivalents obtained by PMS (0.6 mM) were substantially lower than those provided for pollutant mineralization (2.12 mM), achieving an electron utilization efficiency of ~354% (Fig. 2e), while the electron utilization efficiency of the Fe-CN/PMS system was only 48.1% (Supplementary Table 6). These results imply the distinct pollutant transformation pathways in Fe-CN and Fe-NCN-based systems.

Reaction pathway regulation by axial coordination
Quenching experiments and electron paramagnetic resonance (EPR) spectroscopy were employed to identify potential ROS in catalytic systems. As shown in Supplementary Fig. 20, excess scavengers, including methanol (MeOH), tert-butanol (TBA) and isopropanol (IPA), exhibited negligible inhibitory effects in the Fe-SACs/PMS system, eliminating the contribution of hydroxyl radicals (•OH) and sulfate radicals (SO4•−) as primary oxidative species27,30. The absence of radical species was further corroborated by radical trapping and chemical probing tests with EPR and photoluminescence detection (Supplementary Figs. 21 and 22). Additionally, both the absence of a featured bisguanidine product in the chemical tests probed with nitroblue tetrazolium (NBT) and the negligible TCP removal in the O2 atmosphere ruled out the oxidative contribution of superoxide radicals (O2•−) (Supplementary Fig. 23)24. These findings conclusively validate the non-radical oxidation pathways in the Fe-SACs/PMS system.

High-valent metal-oxo species, singlet oxygen (1O2), and surface-bound PMS complexes (PMS*) are widely recognized as typical non-radical reactive species in Fenton-like catalysis30–32. However, dimethyl sulfoxide (DMSO) quenching experiments and methyl phenyl sulfoxide (PMSO) probing revealed no presence of high-valent Fe-oxo intermediates in the Fe-SACs/PMS system (Supplementary Fig. 24). Since the intermetallic Fe-O species can exchange an oxygen atom with water, we further applied 18O isotope labeling experiments to exclude the presence of high-valent Fe-oxo intermediates33. If Fe-oxo species exist in the system, their isotopic oxygen exchange with H218O results in the formation of 18O-containing Fe-oxo species, and such labeled Fe-oxo species can transfer 18O to the coexisting PMSO to generate detectable PMSO18O. However, when the reaction system was transferred to the H218O matrix, the two characteristic peaks of PMS16O16O (m/z=157) and PMS16O18O (m/z=159) were hardly detected, and the two characteristic fragments of PMS16O18O with two mass unit deviations were not observed in the MS2 spectra.

Furfuryl alcohol (FFA) and 2,2,6,6-tetramethyl-4-piperidinol (TEMP), as single-linear oxygen (1O2) scavengers, severely inhibited pollutant removal after addition to the Fe-CN/PMS system (Supplementary Fig. 20). Furthermore, EPR test using TEMP as the 1O2 probe revealed apparent triplet signals for 1O2 which was augmented in D2O, further supporting that 1O2 was generated in this system (Fig. 2f and Supplementary Fig. 25)8,34. Trace amounts of FFA were used as competitive kinetic probes to further evidence the dominant contribution of 1O2 to TCP oxidation in Fe-CN/PMS (86.9%) (Supplementary Fig. 26). In contrast, the Fe-NCN/PMS system exhibited negligible 1O2 signals, indicating alternative non-radical pathways. No significant fluorescence signal was detected in Fe-NCN/PMS system after the addition of singlet oxygen sensor green (SOSG), further excluding 1O2 generation (Supplementary Fig. 27)35. These findings collectively demonstrate that 1O2 is the primary reactive species in Fe-SACs/PMS, whereas it was not produced in the Fe-NCN/PMS system.

To probe the formation of surface PMS* complex to coordinate a non-radical ETP regime for TCP removal, the interaction between Fe-NCN and PMS was investigated using in situ Fourier transform infrared spectroscopy (IS-FTIR) (Fig. 2g). Upon introducing Fe-NCN, the stretching vibration peaks corresponding to HSO5− (1061 cm−1) and SO42− (1275 cm−1) shifted to 1052 cm−1 and 1263 cm−1, respectively, with marked attenuation in peak intensity. This shift corresponds to reduced electron density in S=O bonds, confirming the formation of metastable PMS* complexes36. In contrast, no such peak shifts were observed in 1O2-based Fe-CN/PMS system (Supplementary Fig. 28). Similarly, in situ Raman spectroscopy (IS-Raman) detected a transient PMS-associated peak at ~831 cm−1 upon PMS addition, which disappeared following TCP introduction, mirroring PMS complex (PMS*) formation and consumption dynamics (Supplementary Fig. 29)37. The reactivity of pre-oxidized catalyst by PMS and post-oxidation-reaction catalyst was also assessed (Supplementary Fig. 30)38. Pre-oxidized Fe-NCN (stored in PMS solution for 3 h) rapidly removed ~93% of TCP within 1 min, while the reacted catalysts still retained ~26% TCP removal, indicating the formation of long-lived PMS* intermediates on Fe-NCN surface that contributed to TCP oxidation via a non-radical manner. The PMS consumption profile in Fe-NCN systems exhibited strong pollutant dependence, with PMS decomposition increasing drastically after TCP addition, as evidenced by the ETP mechanism (Supplementary Fig. 31). This is different from the Fe-CN system, where the presence of the pollutant did not affect PMS consumption, as PMS was continuously decomposed into 1O2 29,39.

As expected, the open loop potential (OCP) and timing currents of Fe-NCN showed sharp changes in response to PMS and TCP addition (Fig. 2h and Supplementary Fig. 32a)40, but no significant changes were found in Fe-CN. Linear voltammetry curves (LSV) showed that the positive current density of the Fe-NCN electrode was significantly increased in the presence of PMS, confirming that the continuous electron transfer from the catalyst to PMS (Supplementary Fig. 33)27. Conversely, Fe-CN exhibited a predominant cathodic current at negative potentials, suggesting that electrons tend to transfer from PMS to Fe-CN41. This process can trigger the dissociation of O–H bond, thereby resulting in the formation of persulfate radicals (SO5•−) that serve as a critical precursor for 1O2 (2SO5•− + 2e− → 2SO42− + 1O2)42. Furthermore, rotating ring-disk electrode (RRDE) measurements confirmed the dual-electron transfer process during TCP oxidation via ETP in Fe-NCN/PMS, in contrast with one-electron transfer for Fe-CN to mediate SO5•− (Fig. 2i and Supplementary Fig. 34)42. Collectively, Fe-NCN with axial N coordination triggers the PMS activation pathway, transitioning from 1O2 generation (FeN4, Fe-CN) to ETP (FeN4-Naxial, Fe-NCN) in Fe-SACs/PMS.

Theoretical revelation of oxidation mechanisms
In the ETP pathway, contaminants can be oxidized either by PMS* via direct inner/outer-shell interactions (denoted as short-range ETP, featured as point oxidation) or indirectly through the conductive catalyst that act as an electron shuttle (denoted as long-range ETP, featured as domain-based/anode-like oxidation) (Fig. 3a)6,43. Three customized reactor configurations elucidated these pathways in the Fe-NCN/PMS/TCP system (Fig. 3c and Supplementary Text 16). In the dual-chamber reactor (Mod. 1) with separated TCP and PMS, the Fe-NCN electrode showed low transient current (~21 μA) and limited TCP removal efficiency (~17%) (Supplementary Fig. 35a). The electrode plate-separated dual-chamber reactor (Mod. 2) showed marginally improved removal efficiency (~22%) when PMS and pollutant are separated, while the single-chamber reactor (Mod. 3) achieved rapid and complete TCP elimination within 2.5 min (Supplementary Fig. 35b). The significant difference between the different reactors demonstrated that the necessity of concurrent presence of PMS and Fe-NCN, suggesting the direct oxidation feature of short-range ETP in the Fe-NCN/PMS system (Fig. 3d). This pathway was enabled by the direct inner-sphere interactions of pollutant and surface activated PMS*, as evidenced by its great immunity to NaClO interference (Supplementary Fig. 20)28,43.

Indeed, the semiconducting properties of CN, with its limited charge-transfer capacity, disabled the substrate as an electron shuttle to mediate charge transfer from the contaminant to surface-activated PMS, leading to short-range ETP-dominated contaminant oxidation (Supplementary Figs. 32b and 36)42. In contrast, the highly graphitic and conductive carbon nanotube (CNT) electrode exhibited a significant transient current (~194 μA) and achieved ~73% TCP removal in the galvanic cell, suggesting the long-range ETP dominated oxidation (Supplementary Fig. 37). However, the CNT/PMS system exhibited a low dechlorination efficiency (~4%), with predominant products as polychlorinated biphenyl compounds that typically exhibit higher toxicity than the parent TCP (Supplementary Fig. 38). DFT calculations elucidated the different dechlorination behaviors of the short-range and long-range ETPs for TCP removal (Supplementary Fig. 39). When the distance between TCP and CNT/PMS* was between 10 and 20 nm, the C–Cl bond can be hardly activated. However, when the distance decreases to 2–10 nm, the interfacial electron transfer between the aromatic π system of TCP and the electrophilic complex PMS* is significantly enhanced. This intensifies the shift of the electron density on the C–Cl bond towards the chlorine atom, resulting in the weakening and elongation of the C–Cl bond (1.72→1.75 Å). These findings emphasized that close spatial matching between PMS* and pollutants is a crucial factor in regulating C–Cl bond cleavage over catalysts (Fig. 3b).

Remarkably, regulating in-plane coordination number from FeN4 to FeN4+1 slightly enhanced PMS adsorption energy (Eads = −3.96 vs. −4.02 eV) and electron transfer (−0.88e vs. −0.91e) (Supplementary Fig. 40). However, although axial N coordination significantly reduced Eads to −2.98 eV with moderated electron transfer (−0.85e), it favored metastable low-spin Fe-PMS* complex formation (Fig. 3e, Supplementary Figs. 41 and 42 and Supplementary Table 7). The projected crystal orbital Hamilton population (pCOHP) analyzed the bond strength between the central Fe atom and the O atom of the O–O bond during PMS activation. Axial coordination lifted the antibonding orbitals and decreased the electronic occupancy in these regions, leading to significantly increased IpCOHP for the Fe–O bond (−1.10 → −1.83 eV), which facilitated the formation of more stable PMS* intermediates (Fig. 3f). After adsorption of PMS, the center of the d band of Fe-NCN was closer to the Fermi energy level than that of Fe-CN (−2.07 eV vs. −2.28 eV), indicating that axial N coordination broadens the d-electronic energy level distribution of the central Fe atoms and promote its interactions with PMS to coordinate an electron transfer process (Fig. 3e and Supplementary Fig. 43)44.

Temperature-dependent magnetization measurements, EPR tests and spin-orbit filling calculations further determined the spin state of the Fe atom (Supplementary Figs. 44−47 and Supplementary Table 8)45. The lower ratio of spin states to unpaired electrons in Fe-NCN compared to Fe-CN suggests that the axial ligand exerts an electron-pulling effect on the Fe site by manipulating the spin electrons. Specifically, due to the geometrical distortion (D4h→C4v) induced by the axial coordination configuration, the simplified dxz and dyz orbitals of Fe-NCN split into non-simplified orbitals, leading to a staggered energy rearrangement of the dx2-y2 and dz2 orbitals, and consequently to decreased unpaired electrons in eg orbitals, giving rise to the low spin state46 (Fig. 3g). The change in orbital configuration and the reduction of half-filled orbitals of Fe-NCN promote the charge transfer from eg orbital centers the Fe site to PMS molecules to form PMS* intermediates to initiate ETP (Supplementary Fig. 48). In contrast, the mid-spin state of Fe-CN underwent strong coordination and local transfer of unpaired electrons with PMS, resulting in significantly stretched O–H bond in PMS (0.98 Å → 1.03 Å), leading to a low-energy barrier for deprotonation to produce SO5•−, two of which subsequently dismutate into 1O2 (Fig. 3h and Supplementary Table 7). Compared to Fe-NCN, Fe-CN only required overcoming a lower reaction energy barrier (1.95 vs. 2.87 eV) to achieve dissociation of the high-bond-energy O–H bond, thereby driving the entire process into a thermodynamically favorable state (ΔG = –1.26 eV). As expected, the energy difference of charge transfer from the eg center in Fe-CN to the PMS increases significantly, reversing the direction of electron transfer from PMS to the catalyst, yielding SO5•−.

Steered polymerization reaction mechanism
To elucidate the short-range ETP-driven contaminant polymerization mechanism, we systematically examined the compositional change of the catalyst before and after reactions. XPS and FTIR spectra revealed intensified C−O−C (1065 cm−1), C−C (1582 cm−1), and C−O (1237 cm−1) vibrational modes on reacted Fe-NCN, indicating substantial polyphenylether accumulation (Fig. 4a and Supplementary Figs. 49 and 50). Subsequently, the possible small molecule products (~14%) and large molecule products (~83%) were eluted by stepwise solubilization using ethanol and tetrahydrofuran (THF) in sequence, and the eluates appeared as clear and yellowish-brown colors, respectively (Supplementary Fig. 51). The ethanol-eluted small molecule products were analyzed using high-performance liquid chromatography-mass spectrometry (HPLC-MS) testing and identified as the C-C coupled products (i.e. Biphenyl-2,2′,5,5′-tetrol (m/z=217)). Structural analysis of the THF-eluted solubilised products by nuclear magnetic resonance (NMR), gel permeation chromatography (GPC) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) identified the macromolecular products as crosslinked polyphenylene ethers formed mainly via C-O linkage, with an average molecular weight (Mn) of 4292 Da (Supplementary Figs. 52−56)6,10. Notably, more products were collected using THF as the eluent than with toluene, suggesting that the products may be predominantly polar, medium-molecular-weight oligomers (Supplementary Fig. 57a).

Remarkably, most of the macropolymers did not retain organochlorines, coinciding with the rapid and complete dechlorination of the system (Supplementary Fig. 17b), accompanied by the formation of many phenolic hydroxyl groups. IS-Raman spectroscopy directly confirmed hydroxyl substitution of aromatic chlorine during TCP polymerization, as evidenced by time-dependent intensification of the Fe–OH stretching vibration at 578 cm−1 and concomitant attenuation of the C–Cl bond signal at 667 cm−1 (Fig. 4b)47,48. This process was also confirmed by the increased content of hydroxyl groups on the catalyst surface (Supplementary Fig. 57b–d).

Based on these results, we further investigated the key intermediates for TCP polymerization transfer in the catalytic system. The inner-sphere nature of short-range ETP precludes outer-sphere single-electron abstraction from TCP by PMS* to form chlorophenoxyl radical cations (TCP•+)7. The inner-sphere oxidation pathway between the surface complex PMS* and the substrate was identified using 3-ethylbenzothiazoline-6-sulfonic acid (ABTS) as a probe compound. Component quantification calculations showed that ABTS was rapidly oxidized to the green persistent radical of ABTS•+ in the Fe-CN/PMS system (Supplementary Fig. 58), featured by the four characteristic absorption peaks at 415, 643, 728, and 818 nm (Supplementary Fig. 59), via either the single electron transfer (SET) or hydrogen atom abstraction (HAA) pathway28,39. This is consistent with the singlet oxygenation regime, in which, because of the unoccupied π* orbital, 1O2 can attack and reduce electron-rich organic compounds via SET49. In contrast, Fe-NCN/PMS systems generated ABTS2+ species with distinct absorption signatures at 216 and 293 nm50, evidencing dual-electron transfer processes from ABST to PMS*. For deuterated phenol (C6H5OD), where the BDE of the O−D bond is distinctly higher than that of the O−H bond, the HAA pathway with phenolic hydroxyl groups as reaction sites is expected to lead to a significantly large KIE value28. However, insignificant kinetic isotope effects (KIE=1.32) in Fe-NCN/PMS systems using C6H5OD as the substrate ruled out the HAA pathway as the dominant reaction pathway (Supplementary Fig. 60). The reaction system with D2O as the solvents showed a primary KIE, which reached a value of 1.71 (the isotope effect is usually considered to play a dominant influence on the reaction rate when the KIE>1.5)51. This result suggests that the cleavage/formation of C–H bonds plays a crucial role in the reactions, while the C atoms on the aromatic ring undergo a transition from sp3 hybridization to sp2 hybridization, providing strong evidence for the presence of metastable carbocationic intermediates during the reaction (Supplementary Fig. 25)6,52. Thus, the generation of chlorophenoxy radicals with varied resonance states (TCP-(O)•) was ruled out, which is the typical polymerization intermediate generated from SET via inner-sphere interactions7.

Reference experiments with ferulic acid (FA, phenyl radical-specific probe) and EPR analyzes of DMPO as the spin trap did not show inhibitory effects, and characteristic signals in the quintuple state (1:4:6:4:1) further support the absence of the TCP-(O)• intermediates (Supplementary Fig. 61)7,53. Subsequent trapping experiments with acetonitrile as a nucleophilic scavenger identified chlorophenoxonium ions (TCP-(O)+) as crucial intermediates generated via two-electron transfer from TCP to metastable Fe-NCN/PMS* complex (Supplementary Fig. 62)54. The reactive TCP-(O)+ intermediates further underwent C-C coupling to form dimers or C-O polymerization to yield oligomers through nucleophilic aromatic substitution.

To further unravel the PMS*-mediated DHP path activated by Fe-NCN, combined DFT calculations and molecular dynamics (MD) simulations were conducted. Initial, the axial ligand makes the adsorption of Fe-SACs/PMS* with para-chlorine in the TCP molecule significantly stronger than phenolic hydroxyls, providing the feasibility of C-Cl bond dissociation for the dual-electron transfer process (Supplementary Fig. 63). Based on this favorable conformation, the inner-sphere double electron transfer between TCP and the catalyst surface complex PMS* induces deprotonation of the phenolic hydroxyl group to form a chlorophenoxonium ion intermediate (TCP-(O)+) as well as a pronounced stretching of the C–Cl bond (1.72 Å → 1.74 Å) with an exothermic release of −1.18 eV (IV′) or −1.97 eV (IV) (Fig. 4c, Supplementary Fig. 64 and Supplementary Table 9). While transient resonance delocalization may stabilize TCP-(O)+ on catalytic surfaces, generating meta-carbocationic centers through aromatic polarization, the synergistic steric-electronic constraints imposed by chlorine substituents render these stabilized chlorophenoxonium ions kinetically inert toward subsequent coupling or polymerization pathways55,56. Since the bulk solution is weakly acidic, TCP-(O)+ is rapidly protonated to TCP-(OH2)+. Note that the electronic effects and spatial site resistance of the TCP-(OH2)+ molecule, the C4 site tended to be preferentially attacked by nucleophilic OH− (Supplementary Fig. 65, Fuki index f+=0.015), which led to the substitution of para-chlorine (SNAr mechanism), giving rise to 2,6-dichlorobenzenediphenol (OH-DCP-(OH2)+, m/z=178) (Supplementary Figs 66–68). The newly formed phenolic hydroxyl group donated electrons to the aromatic ring via a conjugated molecular network, stabilizing the radical intermediate. To validate the origin of the para-chloro-substituted hydroxyl atom, isotopic tracer experiments were conducted using H218O, revealing that the hydroxyl oxygen of OH-DCP-(OH2)+ does not originate from PMS but water, as evidenced by LC-MS analysis (Supplementary Fig. 69). As the rate-determining step, water-induced hydroxylation of the cation intermediate experienced a much lower energy barrier than the direct substitution of para-chlorine with OH−* (1.04 vs. 2.03 eV).

MD simulations indicate that OH-DCP-(OH2)+ has a tighter hydrogen bonding network and a larger diffusion coefficient compared to TCP, which facilitates its adsorption and stabilization with the terminal oxygen atoms of PMS* via a stable water bridge, releasing energy of 0.65 eV (Fig. 4d−f and Supplementary Figs. 70 and 71). Notably, this stable hydrogen bonding network effectively stabilized OH-DCP-(OH2)+, preventing the π-electron delocalization and resonance stabilization of radicals/ions required for quinone formation (Supplementary Fig. 72). Subsequently, under the charge-induced effect of the terminal oxygen atom, PMS* will release energy of 1.47 eV to capture a proton from H2O after overcoming an energy barrier of 0.98 eV. The deprotonated H2O will then undergo a low-energy barrier (0.12 eV) for nucleophilic attack on the ortho-chlorine, leading to spontaneous dehalogenation and hydroxylation. This mechanism of synergistically induced hydroxyl substitution of o-chlorine on halogenated phenols by electrophilic intermediates and water bridges has also been similarly reported in dehalogenated peroxidases18. After undergoing a further hydroxylation process in a similar manner, TCP was finally completely dehalogenated, yielding the more stable trihydroxyphenoxonium ion (Supplementary Figs. 73 and 74). Subsequently, these electron-rich and stable phenoxyeonium ion intermediates were readily linked via the phenolic hydroxyl groups of another (chloro)phenols, leading to spontaneous C-O polymerization.

Practical application investigation
To evaluate the applicability of Fe-NCN for real wastewater treatment, understanding the impact of oligomer accumulation on catalytic stability is critical. Previous studies have demonstrated that polymeric products’ adhesion and accumulation on catalyst surfaces induce self-inhibitory effects, leading to rapid catalytic deactivation14,57. Organic solvent elution effectively removed the adsorbed oligomers on the catalyst surface, refreshing “deactivated” catalysts with 98.3% of the initial activity (Fig. 2d and Supplementary Fig. 75). To understand the competition between oligomers and reactants, 4,4′-bisphenol was used as a competing probe for polymerization reactions (Supplementary Fig. 77a), as 4,4′-bisphenol is a potential coupling product with similar hydrophobicity to TCP but has a higher polarity and highest occupied molecular orbital (HOMO, more susceptible to oxidation).

As shown in Supplementary Fig. 76, the Fe-NCN/PMS system maintained TCP removal efficiency when 4,4′-bisphenol was added, while the Fe-CN/PMS system exhibited a loss of 40% activity. Comparative adsorption analyzes (Supplementary Fig. 77b) showed that the adsorption capacity of the materials exhibits a strong positive correlation with the surface zeta potential and hydrophobicity. The higher zero-charge point favors the specific adsorption of oligomers with polar functional groups by polarity matching (Supplementary Fig. 77c). In addition, enhanced hydrophobicity promotes interactions between the substrate and the hydrophobic structure of the oligomer, reducing the competition of the product for the active site (Supplementary Fig. 77d). Indeed, this is primarily attributed to the introduction of the chelator 1,10-phenanthroline, whose pyrolytic formation of N-C confers a hydrophobic and polar microenvironment in the vicinity of the active centers in Fe-NCN. This hydrophobic/polar microenvironment extensively exists in the binding pocket of natural enzymes, which is a key prerequisite for specific catalysis and efficient cycling18,20,58 (Fig. 1a and Supplementary Fig. 2). Therefore, mimicking the enzyme hydrophobic/polar binding pocket microenvironment can effectively retard the product coverage of the active site based on the DHP path to maintain the catalytic stability of the atomic catalyst.

Bleached wastewater collected from the pulp and paper industry was used to evaluate the practicality of the Fe-NCN/PMS system. The results showed that the catalyst performed well in treating the actual wastewater, removing more than 85.7% of COD (Fig. 5a). The excitation emission matrix (EEM) spectra showed a significant reduction in the fluorescence intensity of the solution after the reaction, suggesting that organics such as microbial by-products in the wastewater samples were readily removed (Fig. 5b).

We further assessed Fe-NCN for practical applications at the device level. A membrane-catalyzed microreactor was employed to evaluate the long-term stability of DHP technology by continuously treating simulated wastewater. As shown in Supplementary Fig. 78, the catalyst coating of Fe-NCN (20 mg) was uniformly covered on the microfiltration membrane, which could effectively remediate 6 L of TCP wastewater (2.0 ppm) at a flow rate of 0.5 L/h. TOC removal and dechlorination were maintained above 80% and 90%, respectively, and the leaching of Fe ions was very low (<7 µg/L). The DHP-based technology is further scaled up using a larger fluid-bed reactor configuration embedded with the Fe-NCN catalyst. The river stones with an average size of 10–20 mesh were used to fix 5.0 g of catalyst in the reaction layer, and the inlet and outlet ports were isolated by quartzite to form an enlarged reaction unit (Fig. 5c). The fluidized bed reactor was still able to achieve close to ~100% TCP removal, above 80% TOC removal and 90% dechlorination over 60 days (2 mL/min flow rate), equivalent to 34.56 L/g purification capacity (Fig. 5d). Given these results, a tonne of TCP wastewater can be treated at the expense of $USD 0.37 (Supplementary Table 10).

The post-reaction catalysts were regenerated via ethanol/THF elution washing and to recover organic solid products (Fig. 5e and Supplementary Fig. 79). By adding a methanol-water mixture to the THF solution, a powdery product was precipitated, taking advantage of the difference in solubility. Uniformly sized plastic pellets were obtained by the extrusion-condensation-granulation process after drying treatment (Fig. 5f). The excellent and stable properties of these pellets, which are mainly composed of phenolic hydroxyl carbon and ether-bonded aromatic rings (Fig. 5g), allow further injection and molded into a wider range of high-performance plastic products with great potential for real-world applications (Supplementary Text 20, Supplementary Fig. 80 and Supplementary Table 11).

Besides the excellent performance of the material, the potential for scale-up synthesis is also one of the most important concerns in practical applications. Thus, we conducted TEA and LCA evaluations to assess the economic-environmental feasibility of the per-tonne scale-up process. The results show that the plant-gate levelized cost per tonne of Fe-NCN macroscale, based on our synthesis method, is projected to be lower than the current market prices of activated carbon and cobalt oxide (Fig. 5h), which serve as representative adsorbents and P-AOP catalysts, respectively6. Once optimized chemical input ratios, the cost could be further reduced in the future (Supplementary Text 21 and Supplementary Tables 12–14)57. Considering further the profitability of polymer recycling, the cost decreased significantly by 31.9% to US$ 6739.79 (Supplementary Text 22). Crucially, the quantitative LCA results for the scaled-up synthesis of Fe-NCN and Fenton-like performance demonstrated significantly lower environmental impacts compared to typical mineralization-based AOP catalysts (i.e., zeolite azolium salt framework-based iron monoatom and commercial nano zero-valent iron) across both materials synthesis and treatment processes (Supplementary Fig. 81 and Supplementary Tables 15 and 16)24,59. The product recovery step after the polymerization process reduced the global warming potential (GWP) of the Fe-NCN/PMS system by ~78%, indicating a substantially decreased carbon footprint. Overall, these results suggested that Fe-NCN is suitable for industrialization with excellent long-term stability, lower production cost, and significantly reduced environmental impacts.

Methods
Synthesis of Fe-SACs
Generally, 0.18 mmol FeSO4∙6H2O and 1,10-phenanthroline (0.027 g) were added to 10.0 mL of deionized water for 30.0 min ultrasonic treatment, and stirred at room temperature for 1.0 h to quickly form Fe2+-phen chelate (red). Subsequently, it was added to a 20.0 mL solution containing dispersed 1.0 g CN (10.0 mL deionized water + 10.0 mL ethanol), stirred at room temperature for another 2.0 h, left for self-assembly overnight, and dried at 60 °C. The dried sample was thoroughly ground with 0.0347 g hydroxylamine hydrochloride in a mortar. The resulting mixture is then heated in a tube furnace to 550 °C in N2 atmosphere at a rate of 3.3 °C/min for 3.0 h. Finally, washing with ultra-pure water and ethanol for 3 times to remove soluble iron, drying overnight at 60 °C, Fe-NCN was collected. Fe-CN was prepared under the same conditions except that no chelating agent was added.
Melamine was heated in a tube furnace at 3.3 °C/ min in N2 atmosphere to 550 °C for 3 h. The product collected by washing three times is named CN. Under the same pyrolysis conditions, the nitrogen-rich carbon material was prepared using phen as the precursor and named N-C.

Characterizations
XRD patterns were performed on a Rigaku MiniFlex 600 X-ray diffractometer. SEM images were collected on a FEI Quanta 450 FEG environmental scanning electron microscope. TEM images were collected on FEI Tecnai G2 Spirit TEM. HAADF-STEM images with EDS analysis were obtained on FEI Titan Themis 80-200 operating at 200 kV. Energy-dispersive X-ray spectroscopy (EDS) mapping was performed using a JEM-2100F field-emission high-resolution transmission electron microscope operated at 200 kV. Single-atom Co was characterized by AC-HAADF-STEM, and the images were captured using a JEOL JEM-2010 LaB6 high-resolution transmission electron microscope operated at 200 kV. The N2 adsorption-desorption isotherms were obtained on a Micromeritics ASAP 2020 system to determine the Brunauer-Emmett-Teller (BET) specific surface area and pore size of catalysts. The surface elemental composition of the samples was determined by ESCALab 250Xi (Thermo, USA) x-ray photoelectron spectroscopy (XPS), and the Al Kα X-ray source was used for analysis, and all binding energies were calibrated using the C 1s peak (284.8 eV). Raman spectra were measured using a Raman spectrometer (LabRAM HR Evolution, Horiba Co., Japan) equipped with a 532 nm laser. FT-IR spectroscopy spectra were collected by IR microscopy (Nicolet iN10, Thermo Fisher Inc.). The EPR signals spin-trapped by DMPO and TEMP were recorded on a Bruker spectrometer (A300, Bruker, Karlsruhe, Germany) with the following settings: center field=3512 G, microwave frequency=9.86 GHz, and power=6.36 mW.

Oxidation experiments
Fenton-like oxidation experiments of organic pollutants were carried out in a beaker. Specifically, the catalyst (0.1 g/L) was added to 50.0 mL of TCP solution (0.1 mM) mixed with 0.3 mM PMS to initiate the reaction. Unless otherwise stated, reactions were carried out at room temperature and neutral initial pH. Samples were taken periodically with a syringe, filtered through a 0.22 μm polyethersulfone membrane (1.0 mL of sample mixed with 20.0 μL sodium thiosulfate solution (0.15 mM)), and transferred to an HPLC vial. In particular, the polyethersulfone membrane needs to be pretreated to remove organic contaminants adsorbed on the membrane surface, reducing the interference of membrane adsorption on the detection of organics.

Electrochemical analysis tests
Experimental details of electrochemical analytical methods, including open circuit potentiometry (OCPT), linear scanning voltammetry (LSV), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), chronoamperometry (i-t), rotating ring-disk electrode experiment, and electrooxidation experiments, can be found in the Supplementary Information.

Theoretical calculation
All DFT calculations were performed using the Vienna ab initio simulation package (VASP v6.3.2)60,61. All MD simulations were performed by the Forcite module of the Materials Studio 20.1 software. The specific calculation details and operational parameters can be found in the Supplementary Information.

Data availability
The data supporting the findings of this study are included within the main text and the Supplementary Information files. All the raw data relevant to the study can be obtained from the corresponding author upon request. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript presents an innovative and well-structured study on the development of an enzyme-mimicking functionalized single-atom catalyst for the efficient polymerization and dehalogenation and detoxification of halogenated organic compounds. The findings highlight that axial nitrogen induces the electronic structure and spin states of the Fe sites and induces short-range ETP processes for the dehalogenation-polymerisation reaction of halogenated phenols in the semiconducting nature of the substrate. A comprehensive systematic integration of in situ characterization, isotope experiments and theoretical simulation calculations systematically reveals the key intermediates of the polymerization reaction as well as a continuous dehalogenation-hydroxylation-coupling pathway. The superior performance and stability of the catalytic process was confirmed by comprehensive characterization and systematic evaluation in both model and real aqueous systems. Interestingly, this study confirms for the first time that the value-added processing and economic value of oligomers in aqueous chemical polymerization processes does not remain only a discussion at the level of advanced catalyst synthesis and mechanism revelation. This study provides valuable insights to advance the understanding of cost-effective polymerization oxidation processes and selective polymerization mechanisms. Therefore, I suggest that after resolving the following minor issues, the manuscript be published in Nature Communications.\n1. Introduction. Line 103: \"TCP removal > 80% within 60 days\", but in line 554-555 it is described as \"~100% TCP removal, above 80% TOC removal and 90% dechlorination over 60 days\". There is a contradiction in terms. Please make revisions and double-check such issues in full manuscript.\n2. The submitted manuscript contains two errors regarding the confusion of catalyst signatures. The first is in Line 163-165: \"The comparable coordination environments between Fe-CN and FePc confirmed the planar Fe-N4 configuration in Fe-CN\". It needs to be modified to \"the planar Fe-N4 configuration in Fe-NCN\". The second is in Line 305: \"As expected, the open loop potential (OCP) and timing currents of Fe-CN\".\n3. A variety of quenching agents were used to verify the active species of the SACs/PMS system, but the quenching effects of reagents such as OA, KSCN and K2CrO7 on the catalytic system were not discussed (in Fig. S19). It would enhance the mechanistic exploration if their quenching effect during the reaction could be complemented.\n4. PMS utilization efficiency is an important indicator for evaluating catalyst performance. The authors are encouraged to provide a comparison of the PMS utilization efficiency with other advanced nano-atomic scale catalysts to highlight the superiority of this catalyst.\n5. The authors can complement the toxicity evaluation of Polychlorinated biphenyls compounds produced in CNT/PMS systems by emphasizing the need for dehalogenation to reduce polymer toxicity (Line 346-349).\n6. Some key statements in the manuscript would benefit from supporting references to enhance their validity. Such as experiments related to ferulic acid and DMPO (Line 451-455) and \"typical mineralization-based AOP catalysts (i.e. zeolite azolium salt framework-based iron monoatom and commercial nano zero-valent iron)\" (Line 582-583).\n7. Since the P-AOP system only oxidatively transfers organic pollutants to the catalyst surface, the term \"degradation\" clearly does not apply to this catalytic system (Line 522-523). It is suggested that it be modified to \"removal\", as it is elsewhere in the manuscript.\n8. Please clarify how and under what conditions continuous flow reactors operate. For packed-bed reactors, why are river rocks chosen as fillers? Do these fillers promote PMS activation?\n9. Figure. Reaction conditions should be added to the corresponding legend to facilitate the reader's quick understanding of these details. Particularly, some in situ characterization experiments and molecular probing experiments.", "reviewer_2": "This study developed an axial-nitrogen-coordinated single-atom iron catalyst (Fe-NCN), which triggers an electron-transfer pathway (ETP) by enhancing the electronic pulling effect on 2,4,6-trichlorophenol (TCP). This approach achieves near-complete dechlorination of TCP and its catalytic conversion into polymeric products. Overall, this study provides novel insights in polymerization-based advanced oxidation processes; however, there remain shortcomings in the generalizability of the mechanism and the in-depth exploration of its details. The detailed comments are as follows:\n1. The source of the XlogP (hydrophobicity index) data in Supplementary Fig. 1 should be clearly specified.\n2. The authors mention that the pre-coordinated template exhibited a product yield of up to 45.0%, which is attributed to interlayer cross-linking. It is necessary to clarify: (1) How does this interlayer cross-linking process occur? (2) Why can it increase the loss of C and N? (3) At high temperatures, will the loss of C and N still be constrained by the cross-linking effect?\n3. It is inappropriate for the authors to rule out the existence of Fe-C and Fe-O bonds solely through XPS. In fact, XPS peak-splitting results may correspond to multiple chemical states of Fe. Therefore, it is overly arbitrary for the authors to assume that all coordinating atoms of Fe are N during EXAFS fitting, and further conclude that the Fe-N coordination numbers are 4.28 (for Fe-CN) and 5.13 (for Fe-NCN). Additional characterization methods should be employed to probe the actual coordinating atoms of Fe, such as Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS).\n4. The results in Supplementary Fig. 13b show that electron-deficient organic pollutants (e.g., ATZ) can still be effectively removed (with approximately 70% degradation efficiency for ATZ). If this reaction is dominated by non-radical species, the authors need to explain this phenomenon.\n5. The amount of chloride ions released during the reaction should be detected to further confirm the dechlorination-polymerization process of TCP.\n6. In Fig. 3c, the concentrations of the organic substrate (TCP) and PMS, as well as the catalyst loading, should be clearly stated. Additionally, all reaction conditions should be specified in the figure caption.\n7. The authors claim that \"the greater the distance between TCP and CNT/PMS*, the stronger the C-Cl bond\". What is the theoretical basis for this assertion?\n8. The IpCOHP analysis indicates that axial N coordination enhances both the Fe-O bond strength and the d-band center of Fe, which are interpreted as evidence for strengthened PMS adsorption. However, the adsorption energy calculations show the opposite trend (weakened PMS adsorption). The authors must explain this contradiction.\n9. In Lines 364–365, the authors state that DFT calculations reveal that axial N coordination reduces both the number of transferred electrons and the adsorption energy. This result is inconsistent with the authors’ conclusion—this inconsistency requires clarification.\n10. The authors claim that \"axial N coordination leads to a staggered energy rearrangement of the dx2-y2 and dz2 orbitals, consequently reducing the number of unpaired electrons in the eg orbitals\". Please explain the underlying mechanism of this process.\n11. For Fe-CN-activated PMS, the key intermediate for 1O2 generation is the SO5•⁻, and its formation requires electron transfer from PMS to the catalyst. The authors mention that \"the energy difference for charge transfer from the eg orbital center of Fe-CN to PMS increases significantly, reversing the direction of electron transfer (from PMS to the catalyst) and thus generating SO₅•⁻\". However, Fig. 3h clearly shows that the reaction process has a positive energy barrier, which indicates that the reaction cannot proceed spontaneously. This discrepancy needs to be explained.\n12. What characteristics of Fe-NCN enable its high-efficiency polymerization performance? Can the authors propose a general structural descriptor for this performance, or discuss relevant design principles? Additionally, can this structural descriptor be applied to other single-atom transition metal materials (e.g., Co-based, Cu-based single-atom catalysts)?\n13. Regarding the claim of \"mimicking the enzyme’s hydrophobic/polar binding pocket microenvironment\", the authors should address the following: (1) Why does Fe-NCN possess a hydrophobic and polar microenvironment? (2) The paper focuses on enzyme-mimetic catalysis—what similarities exist between the structural features of natural enzymes and those of Fe-NCN? (3) Why does Fe-CN not exhibit enzyme-mimetic functionality? (4) What are the decisive factors that determine whether the catalyst’s active center has a hydrophobic/polar microenvironment? These points should be specifically discussed in the revised manuscript.", "reviewer_3": "This study successfully designed and synthesized an axial nitrogen-coordinated single-atom iron catalyst (Fe–NCN) for efficient activation of peroxymonosulfate (PMS), achieving nearly complete dechlorination and directed polymerization conversion of a typical halogenated organic compound, 2,4,6-trichlorophenol (TCP), into plastic products with potential application value. The work not only provides in-depth insights into the unique \"dehalogenation–hydroxylation–polymerization\" reaction mechanism but also demonstrates the application potential of this technology through techno-economic analysis and life cycle assessment. Spanning multiple levels from atomic-scale catalyst design and mechanistic studies to practical application evaluation, the research is well-supported by detailed data and robust argumentation. It holds significant importance for advancing advanced oxidation processes based on resource recovery and is recommended for acceptance after minor revisions.\n1.The formatting of section headings is inconsistent. For example, the heading style of “Results” is the same as that of its subheadings such as “Synthesis and characterization of Fe-SACs”, while the heading “Methods” differs from its subheadings like “Material Preparation”. It is recommended to unify the heading styles and ensure distinction between main headings and subheadings.\n2.There are spelling errors in the manuscript, such as “1,10 phenanthrene” in line 625 of the main text and in the supporting information. The correct term should be “1,10-phenanthroline”.\n3.The phrase “mono-atomic dispersion” is unconventional. A more precise expression would be “single-atom dispersion”.\n4.The study confirms the two-electron transfer process in the Fe–NCN/PMS system through RRDE tests. However, the data processing procedure and fitting error range for the electron transfer number are not presented in sufficient detail. It is recommended to include necessary explanations and data in the supporting information.\n5.The price of the recovered modified polyphenylene oxide particles is set at 0.9 times that of commercial polyphenylene oxide. A brief rationale for this factor should be provided.\n6.The techno-economic analysis (TEA) is detailed, but potential costs such as solvent recovery and recycling are not included in the calculation. It is recommended to address this in the TEA discussion, noting that the current cost assessment does not account for solvent recovery and reuse, to make the analysis more comprehensive.\n7.The article describes that the TCP removal and dechlorination rates remained high during 60 days of continuous operation. However, it is not clarified whether this stable performance was due to the inherent stability of the catalyst or the application of periodic regeneration operations. It is suggested to specify in the supporting information whether any catalyst regeneration was performed."}, "author_response": "General comments: This manuscript presents an innovative and well-structured study on the development of an enzyme-mimicking functionalized single-atom catalyst for the efficient polymerization and dehalogenation and detoxification of halogenated organic compounds. The findings highlight that axial nitrogen induces the electronic structure and spin states of the Fe sites and induces short-range ETP processes for the dehalogenation-polymerisation reaction of halogenated phenols in the semiconducting nature of the substrate. A comprehensive systematic integration of in situ characterization, isotope experiments and theoretical simulation calculations systematically reveals the key intermediates of the polymerization reaction as well as a continuous dehalogenation-hydroxylation-coupling pathway. The superior performance and stability of the catalytic process was confirmed by comprehensive characterization and systematic evaluation in both model and real aqueous systems. Interestingly, this study confirms for the first time that the value-added processing and economic value of oligomers in aqueous chemical polymerization processes does not remain only a discussion at the level of advanced catalyst synthesis and mechanism revelation. This study provides valuable insights to advance the understanding of cost-effective polymerization oxidation processes and selective polymerization mechanisms. Therefore, I suggest that after resolving the following minor issues, the manuscript be published in Nature Communications.\nResponse: Many thanks for the Reviewer’s very positive comments on the novelty and significance of this manuscript. We accept all these valuable suggestions and have revised the manuscript accordingly. We hope the revision could be satisfactory.\nComment 1. Introduction. Line 103: \"TCP removal > 80% within 60 days\", but in line 554-555 it is described as \"~100% TCP removal, above 80% TOC removal and 90% dechlorination over 60 days\". There is a contradiction in terms. Please make revisions and double-check such issues in full manuscript.\nResponse: Many thanks to the reviewer for the careful review and gentle reminder. The sentence in the Introduction originally mixed metrics (TCP removal vs. TOC removal). We have corrected it to report TOC removal only, aligning the introductory summary with the continuous-flow results reported later. We have also re-checked the main text, figure captions and SI for similar inconsistencies.\nChange:\n“Combined techno-economic analysis (TEA), life cycle assessment (LCA), and continuous-flow reactor trials demonstrated Fe-NCN's industrial-scale synthesis feasibility, robust wastewater treatment performance (TOC removal > 80% within 60 days)…” (Revised manuscript, Page 4, Lines 99−102)\nComment 2. The submitted manuscript contains two errors regarding the confusion of catalyst signatures. The first is in Line 163-165: \"The comparable coordination environments between Fe-CN and FePc confirmed the planar Fe-N4 configuration in Fe-CN\". It needs to be modified to \"the planar Fe-N4 configuration in Fe-NCN\". The second is in Line 305: \"As expected, the open loop potential (OCP) and timing currents of Fe-CN\".\nResponse: We thank the Reviewer for carefully identifying these typographical errors in the catalyst labels. We have corrected both instances to ensure consistency throughout the manuscript as below:\nChange:\n“In contrast, the significantly higher planar distortion index (I /I = 1.12 vs. 0.98) observed A B in Fe-NCN compared to Fe-CN (Fig. 1h)…” (Revised manuscript, Page 6, Lines 178−179)\n“…the open loop potential (OCP) and timing currents of Fe-NCN showed sharp changes…” (Revised manuscript, Page 11, Lines 315−316)\nComment 3. A variety of quenching agents were used to verify the active species of the SACs/PMS system, but the quenching effects of reagents such as OA, KSCN and K Cr O 2 2 7 on the catalytic system were not discussed (in Fig. S19). It would enhance the mechanistic exploration if their quenching effect during the reaction could be complemented.\nResponse: Many Thanks for the Reviewer’s kind reminder. To enrich the exploration of the active species in the SACs/PMS system, we have supplemented the discussions on the effects of these quenchers in the oxidation process.\nChange:\n“Note: As typical metal complexing agents, oxalic acid (OA) and potassium thiocyanide (KSCN) can be coordinated with Fe species to form metal-carboxylate complexes. In the presence of OA and KSCN, TCP removal was almost greatly inhibited in both catalytic systems. These results indicate that the Fe site is the main active site on the Fe-SACs. Furthermore, the addition of K Cr O as a scavenger of electrons had a strong inhibitory 2 2 7 effect, suggesting that electron transfer is the main pathway for TCP removal20.” (SI, Pages 43−44, Supplementary Fig. 20)\nComment 4. PMS utilization efficiency is an important indicator for evaluating catalyst performance. The authors are encouraged to provide a comparison of the PMS utilization efficiency with other advanced nano-atomic scale catalysts to highlight the superiority of this catalyst.\nResponse: We appreciate the Reviewer’s insightful comment regarding the importance of PMS utilisation efficiency (PUE) as an important performance metric. In response, we have incorporated a detailed comparison of PUE values for our Fe–NCN catalyst with other reported nano-atomic-scale catalysts (Table 1, Revision comments). Interestingly, our analysis reveals that polymerization-based advanced oxidation systems (including the present Fe–NCN/PMS system) exhibit PUE values exceeding 100%, markedly higher than those observed in mineralization-dominated systems.\nThis phenomenon arises from the intrinsic difference in electron requirements between polymerization and mineralization pathways. The polymerization route requires only 1–2 moles of electrons to activate 1 mole of PMS, whereas full mineralization to CO demands 2 substantially more electrons (for example, the complete oxidation of phenol requires approximately 14 moles of electrons per mole of substrate). Consequently, the significantly reduced electron demand in polymerization-oriented reactions leads to apparent PUE values greater than 100%. We have therefore clarified in the revised text that, while PUE remains an informative comparative metric, it may not be directly applicable to polymerization-based oxidation systems, where the goal is catalytic conversion and upcycling rather than complete mineralization. Please see Pages 119–120 in Supporting Information.\nTable 1. Compared with the oxidant utilization rate of the currently reported nano/atomic scale Fenton catalyst in removing pollutants.\nPollutant Catalyst Pollutant Oxidant PUE Catalyst concentration dosage Ref. s (mM) (%) (mM) (g/L)\nCu -MgO phenol 0.2 0.2 0.98 25.6 1 sur\nMnN 4-CP 0.078 0.5 1 65.0 2 5\nFe S/CN 4-CP 0.01 0.5 0.5 36.0 3 1\nCo CNCl/S phenol 0.2 0.05 0.4 58.8 4 1\nCu-N /C-B BPA 0.088 0.1 0.3 67.3 5 4\nCuSA/MXene BPA 0.04 0.5 2 2.0 6\nFeCo-N/C-3 phenol 0.16 0.05 1.3 23.7 7\nCoN O -2 BPA 0.016 0.1 1 4.0 8 3 1\nFeNx-C-600 BPA 0.087 0.2 0.5 34.8 9\n1.2Fe-N-C phenol 0.213 0.15 0.65 36.4 10\nCu-In O /O TC 0.045 0.5 1 9.5 11 2 3 v\nCuO 4-CP 0.078 0.2 0.2 50.5 12\nNCB800 4-CP 0.78 0.5 3.25 56.5 13\nZMCs APAP 0.1 0.05 0.5 25.0 14\nUiO-66-NH - 2 phenol 0.1 0.1 6 121.3 15 (Zr/Fe)/GA\nMn O @55ACNT phenol 0.2 0.075 2 166.5 16 3 4\nHEA phenol 0.1 0.1 0.25 280 17\nBC800 TCP 0.025 0.5 0.2 150.3 18\nFe-MnO phenol 0.106 0.2 1 161.7 19 2\nFe-NCN TCP 0.101 0.1 0.3 140.4 This work\n[1] Liu, Y. et al. Magnesium Oxide-Supported Single Atoms with Fine-Modulated Steric Location for Polymerization Transfer Removal of Water Pollutants. Environ. Sci. Technol.59(1), 880−891 (2024).\n[2] Miao J, et al. Single-Atom MnN5 Catalytic Sites Enable Efficient Peroxymonosulfate Activation by Forming Highly Reactive Mn(IV)-Oxo Species. Environ. Sci. Technol. 57, 4266−4275 (2023).\n[3] Jiang, X. et al. Precise coordination of high-loading Fe single atoms with sulfur boosts selective generation of nonradicals. Proc. Natl. Acad. Sci. USA.121(4), e2309102121 (2024).\n[4] Zhu, Z. et al. Multidimensional engineering of single-atom cobalt catalysts for ultrafast Fenton-like reactions. Nat. Water3(2), 211−221 (2025).\n[5] Zhou, X. et al. Identification of Fenton-like active Cu sites by heteroatom modulation of electronic density. Proc. Natl. Acad. Sci. USA. 119(8), e2119492119 (2022).\n[6] Yang, P. et al. Single-atom copper embedded in two-dimensional MXene toward peroxymonosulfate activation to generate singlet oxygen with nearly 100% selectivity for enhanced Fenton-like reactions. Appl. Catal. B Environ.324, 122245 (2023).\n[7] Zhao, Z. et al. Construction of dual active sites on diatomic metal (FeCo−N/C-x) catalysts for enhanced Fenton-like catalysis. Appl. Catal. B Environ.309, 121256 (2022).\n[8] Wang, Z. et al. Cobalt Single Atoms Anchored on Oxygen-Doped Tubular Carbon Nitride for Efficient Peroxymonosulfate Activation: Simultaneous Coordination Structure and Morphology Modulation. Angew. Chem. Int. Ed.134(29), e202202338 (2022).\n[9] Zhang, B et al. Elucidating the Mechanistic Origin of a Spin State-Dependent FeN -C Catalyst toward x Organic Contaminant Oxidation via Peroxymonosulfate Activation. Environ. Sci. Technol. 56, 1321−1330 (2022).\n[10] He, J. et al. ZIF-8 derived Fe‒N coordination moieties anchored carbon nanocubes for efficient peroxymonosulfate activation via non-radical pathways: Role of FeN sites. J. Hazard. Mater.405, 124199 x (2021).\n[11] Zhao, Z. et al. Enhanced Interfacial Electron Transfer by Asymmetric Cu-Ov-In Sites on In O for 2 3 Efficient Peroxymonosulfate Activation. Angew. Chem. Int. Ed.62(11), e202216403 (2023).\n[12] Wei, Y. et al. Ultrahigh peroxymonosulfate utilization efficiency over CuO nanosheets via heterogeneous Cu (III) formation and preferential electron transfer during degradation of phenols. Environ. Sci. Technol.56(12), 8984−8992 (2022).\n[13] Wan, Z. et al. Critical impact of nitrogen vacancies in nonradical carbocatalysis on nitrogen-doped graphitic biochar. Environ. Sci. Technol.55(10), 7004−7014 (2021).\n[14] Qiu, X. et al. Different peroxymonosulfate activation and utilization pathways of typical cobalt oxides, cobalt-carbon and carbonaceous composites derived from metal-organic frameworks for pollutant oxidation in wastewater. Chem. Eng. J.475, 146234. (2023).\n[15] Zhang, X. et al. Nanoconfinement-triggered oligomerization pathway for efficient removal of phenolic pollutants via a Fenton-like reaction. Nat. Commun.15(1), 917 (2024).\n[16] Gao, X. et al. Carbon redirection via tunable Fenton-like reactions under nanoconfinement toward sustainable water treatment. Nat. Commun.15(1), 2808 (2024).\n[17] Yao, Z, et al. High-entropy alloys catalyzing polymeric transformation of water pollutants with remarkably improved electron utilization efficiency. Nat. Commun.16(1), 148 (2025).\n[18] Dou, J. et al. Neglected but efficient electron utilization driven by biochar-coactivated phenols and peroxydisulfate: Polyphenol accumulation rather than mineralization. Environ. Sci. Technol. 57(14), 5703−5713 (2023).\n[19] Zeng, G. et al. Enhanced peroxymonosulfate activation by δ-MnO nanocatalyst enriched with 2 oxygen vacancies for phenolic pollutants polymerization. Appl. Catal. B Environ. 357, 124327 (2024).\nComment 5. The authors can complement the toxicity evaluation of Polychlorinated biphenyls compounds produced in CNT/PMS systems by emphasizing the need for dehalogenation to reduce polymer toxicity (Line 346−349).\nResponse: We thank the reviewer for this valuable suggestion. To better illustrate the environmental significance of dehalogenation, we have complemented the study with a detailed toxicity evaluation of halogenated polymeric products generated in the CNT/PMS system. Using the Toxicity Estimation Software Tool (TEST), we calculated the bioaccumulation factor and Ames mutagenicity indices for the chlorinated oligomers, enabling a direct comparison with the parent compound (TCP). These new results have been incorporated into the Supporting Information as Supplementary Fig. 38m and discussed accordingly.\nChange:\nm Supplementary Figure 38 | The removal performance of TCP by CNT-activated PMS and product identification. (m) Bioaccumulation factors and Ames mutagenicity values for TCP and halogenated oligomers in the CNT/PMS system were predicted based on toxicological data.\n“Note: It can be found that the Bioaccumulation factor and Ames mutagenicity values of TCP as well as halogenated oligomers in the CNT/PMS system are significantly higher than those of the parent compound (TCP) suggesting that incomplete removal or excessive accumulation of the halogens can produce more toxic oligomers (Figure S38m). Although all of these oligomers could theoretically be enriched and immobilised by the CNT surface, the low-polymerization-degree of the products and the risk of fragmentation of the material could still allow them to be released from the catalyst into water, posing a potential secondary contamination risk.” (Supporting Information, Page 64)\nComment 6. Some key statements in the manuscript would benefit from supporting references to enhance their validity. Such as experiments related to ferulic acid and DMPO (Line 451-455) and \"typical mineralization-based AOP catalysts (i.e. zeolite azolium salt framework-based iron monoatom and commercial nano zero-valent iron)\" (Line 582-583).\nResponse: We appreciate the reviewer’s kind suggestion. As advised, we have appropriate supporting references for the experiments involving ferulic acid and DMPO, as well as for the examples of mineralization-based AOP catalysts (Fe-doped zeolite and nano ZVI). In addition, we have carefully reviewed and standardized the formatting and citation style of all references throughout the manuscript and Supporting Information.\nChange:\n“Reference experiments with ferulic acid (FA, phenyl radical-specific probe) and EPR analyses of DMPO as the spin trap did not show inhibitory effects, and characteristic signals in the quintuple state (1:4:6:4:1) further support the absence of the TCP-(O)• intermediates (Supplementary Fig. 61)7,53.” (Revised manuscript, Page 17, Lines 454−457)\n7. Gao, X. et al. Carbon redirection via tunable Fenton-like reactions under nanoconfinement toward sustainable water treatment. Nat. Commun. 15(1), 2808 (2024).\n53. Yao, Z, et al. High-entropy alloys catalyzing polymeric transformation of water pollutants with remarkably improved electron utilization efficiency. Nat. Commun. 16(1), 148 (2025).\n“Crucially, the quantitative LCA results for the scaled-up synthesis of Fe-NCN and Fenton-like performance demonstrated significantly lower environmental impacts compared to typical mineralization-based AOP catalysts (i.e., zeolite azolium salt framework-based iron monoatom and commercial nano zero-valent iron) across both material synthesis and treatment processes (Supplementary Fig. 81 and Supplementary Tables 14 and 15)24,59.” (Revised manuscript, Page 21, Lines 598−603)\n24. Wu, Z. et al. Facilely tuning the first-shell coordination microenvironment in iron single-atom for Fenton-like chemistry toward highly efficient wastewater purification. Environ. Sci. Technol. 57(37), 14046−14057 (2023).\n59. Pi, Z. et al. Complete degradation of 2, 4-dichlorophenol in a sequential sulfidated nanoscale zero-valent iron/peroxydisulfate system: dechlorination, mineralization and mechanism. Environ. Sci. Nano 12(2), 1414−1430 (2025).\nComment 7. Since the P-AOP system only oxidatively transfers organic pollutants to the catalyst surface, the term \"degradation\" clearly does not apply to this catalytic system (Line 522-523). It is suggested that it be modified to \"removal\", as it is elsewhere in the manuscript.\nResponse: We appreciate and accept the Reviewer’s valuable suggestion. We fully agree that the term “degradation” is not appropriate for polymerization-oriented AOP systems, in which organic pollutants undergo surface-mediated oxidative coupling/polymerization processes rather than complete mineralization into CO . Accordingly, we have replaced 2 “degradation” with “removal” throughout the main text and Supporting Information to ensure conceptual and terminological consistency.\nChange:\n“However, when the pH was increased to 12.0, Fe-NCN exhibited a significantly inhibited removal efficiency, and this alkaline inhibition could be attributed to the electrostatic repulsion between the PMS anion and the hydroxylated and negatively charged catalyst surface10 (Supplementary Fig. 13).” (Revised manuscript, Page 8, Lines 207−210)\n“As shown in Supplementary Fig. 76, the Fe-NCN/PMS system maintained TCP removal efficiency when 4,4'-bisphenol was added…” (Revised manuscript, Page 19, Lines 538−539)\nFig. 3 | Mechanistic investigation of axial ligand-induced catalytic pathway transitions. d Performance of TCP removal by Fe-SACs activated PMS in three model reactors. (Revised manuscript, Page 34, Lines 898−899)\nComment 8.Please clarify how and under what conditions continuous flow reactors operate. For packed-bed reactors, why are river rocks chosen as fillers? Do these fillers promote PMS activation?\nResponse: We thank the Reviewer for the very valuable suggestions, which have significantly contributed to improving the scientific clarification of our manuscript. Detailed justifications for the operational mode and condition parameters of the reactor and for the choice of river rock as fluidised bed packing are supplemented, please see Supplementary Text 19. We also confirm, based on experimental results in Supplementary Fig. 78c, neither microfiltration membranes nor river rocks were effective in activating PMS for TCP removal. Specific modifications were marked in red in the Supporting Information.\nChange:\n“Text S19. Device-level long-term continuous flow wastewater treatment using Fe-NCN\nA membrane-catalysed microreactor and an enlarged fluidised bed reactor were used to evaluate the treatment performance of Fe-NCN in a device-scale reactor. To prepare the catalytic membranes, 20 mg of Fe-NCN powder was ultrasonically dispersed in 50 mL of water, and the catalyst was subsequently loaded onto the surface of a polypropylene microfiltration membrane (pore size 0.2 μm) by vacuum filtration. After overnight drying, the Fe-NCN-loaded catalytic membrane was obtained. The catalytic membrane was fixed and stabilised with a porous vinyl chloride sieve plate for water flow, and chlorophenol-containing wastewater (2.0 ppm) and PMS solution (0.1 mM) were passed through the membrane module at a flow rate of 0.5 L/h. In addition, a custom-designed fluidised bed column (~10 cm in diameter) with a bed depth of 25 cm was filled with 5 g of Fe-NCN powder, where river rock with an average particle size of 10−20 mesh was used as a fluidised bed filler to immobilise and load the catalyst powder in the reaction layer. The inlet and outlet were isolated with quartz sand and polyethylene sieve panels to form a separate, scaled-up reaction unit. In addition, chlorophenol-containing tap water was used as simulated wastewater, and the flow reactor was operated with a peristaltic pump at a flow rate of 2 mL/min. The concentrations of TCP and PMS in the feed solution were 1.0 ppm and 0.1 mM, respectively. Samples were collected at selected intervals, and TCP concentration in the effluent water was determined by UPLC to evaluate the treatment performance of the microreactor and the amplified reaction system. No catalyst regeneration was carried out during the operating cycle of the fluidised bed reaction unit.\nThe main components of natural river rock, calcium carbonate and silica, are stable in nature and do not readily undergo side reactions with oxidising agents and organic pollutants. In addition, due to the advantages of low cost, abrasion resistance and compression resistance, the choice of river rock as filler is green and economical, and also effectively mitigates catalyst loss or reactor clogging caused by filler abrasion.” (Supporting Information, Page 11−12)\nSupplementary Figure 78 | Membrane microreactor. (c) Removal efficiency of TCP by microfiltration membranes and river rock activated PMS.\n“Note: The activation of PMS by microfiltration membranes and river rock was negligible in terms of the performance of TCP removal efficiency.” (Supporting Information, Page 111)\nComment 9. Figure. Reaction conditions should be added to the corresponding legend to facilitate the reader's quick understanding of these details. Particularly, some in situ characterization experiments and molecular probing experiments.\nResponse: We thank the Reviewer for this very considerate suggestion. We have included the experimental conditions in the captions of Figs. 2−5 and Fig. S17, Fig. S18, Fig. S21, Figs. S24−26, Fig. 30−33, Fig. S35, Fig. S58, Figs. S60−62, and Fig. S76. Detailed modifications are highlighted in the Revised manuscript and Supporting Information.\nChange:\nRevised manuscript:\n“Fig. 2 | Catalytic activity of catalyst and identification of oxidation mechanisms. a Removal of TCP by different catalysts/PMS systems. b A critical evaluation of Fe-SACs/PMS in different aspects. c Intrinsic catalytic activity of the Fe-NCN in comparison with the state-of-the-art SACs and metal-based material in PMS-based Fenton-like oxidation. d Residual COD of the reaction solution and of the reacted Fe-SACs after elution with different solvents. e Electronic utilization efficiency of Fe-SACs/PMS/TCP system. Experimental condition: [pollutant] = 0.1 mM, [PMS] = 0.3 mM, [catalyst] = 0.1 g L−1, [pH] = 6.7, initial T = 25 °C. 0 0 0 Data are presented as mean values ± s.d. (n = 3). f EPR spectra of TEMP adducts recorded during PMS activation in Fe-SACs systems. g In situ FTIR spectra of Fe-NCN system. h OCP changes upon sequential addition of PMS and TCP for Fe-SACs electrodes. Experimental condition: [PMS] = 5.0 mM, [catalyst] = 0.4 g, [pH] = 6.7, initial T = 25 °C. 0 0 i The number of electron transfers fitted by the rotating ring disc electrode (RRDE) experiment. Source data are provided as a Source Data file.”(Revised manuscript, Page 33, Lines 878−892)\n“Fig. 3 | Mechanistic investigation of axial ligand-induced catalytic pathway transitions. a Schematic diagrams of the two different ETP paths. b Short-range ETP mechanism of oxidized pollutants in Fe-NCN/PMS system. c Three different galvanic model reactors. d Performance of TCP removal by Fe-SACs activated PMS in three model reactors. Experimental condition: [pollutant] = 0.1 mM, [PMS] = 0.5 mM, [catalyst coat] = 0.5 mg cm−2. 0 0 Data are presented as mean values ± s.d. (n = 2). e The adsorption configuration, adsorption energy of Fe-SACs/PMS and the d band center of catalyst before and after PMS adsorption. f pCOHP between the Fe atom and the O atom as PMS adsorption. Corresponding IpCOHP values are shown in the figures. g Spin states of Fe-CN (Fe-N ), Fe-NCN (Fe-N ), Fe- 4 4+axial CN/PMS, and Fe-NCN/PMS. h Pathways and corresponding thermodynamic change curves of 1O generation from PMS activated by Fe-SACs. Source data are provided as a Source 2 Data file.” (Revised manuscript, Pages 34−35, Lines 893−905)\n“Fig. 4 | Identification of polymerization products and mechanism of C-O polymerization of pollutants in DHP path. a XPS spectra of Fe-NCN in the original, after activation of PMS, and after reaction with TCP. b Chemical evolution of the Fe-NCN surface based on in situ Raman spectra after sequential addition of PMS and TCP. Experimental condition: [PMS] = 5.0 mM, [catalyst] = 0.5 g, [TCP] = 0.5 mM, [pH] = 6.7, initial T = 25 °C. 0 0 c Thermodynamic potential energy curve of hydroxylation-dechlorine reaction path based on short-range ETP induced by axial ligands. d Radial distribution function g(r) for the organic molecule−O distance between organic molecule and the O atom of water. w e Coordination number (CN) of water. f Synergistic modulation of hydroxylation substitution reaction pathways and thermodynamic potential energy profiles of ortho-chlorine on chlorophenoxonium ions by the electrophilic intermediate PMS* and water bridges. Source data are provided as a Source Data file.” (Revised manuscript, Page 36, Lines 906−919)\n“Fig. 5 | Application potential, value-added processing of products and economic and environmental feasibility of Fenton-like oxidation process. a, COD changes and EEM spectra b, before and after treatment of secondary domestic effluent from pulp and paper industry by Fe-NCN/PMS system. Experimental condition: [pollutant] = 0.1 mM, 0 [PMS] = 0.3 mM, [catalyst] = 0.1 g L−1, [reaction solution] = 200.0 mL. Data are presented 0 as mean values ± s.d. (n = 2). c, A schematic illustration of the size-enlarged catalytic reactor. d, TCP (2.0 ppm) removal efficiency, TOC removal and dechlorination over with days in a size-enlarged catalytic reactor (inset: Fe leaching within reaction time). e, Precipitation separation and value-added production of oligomers. f, The industrial-grade plastic pallets are obtained through the extrusion-condensation-granulation process. g, Solid-state 13C nuclear magnetic resonance (13CNMR) spectroscopy of plastic granules. h, The economic feasibility of the amplified synthesis of Fe-NCN was demonstrated by technical and economic analysis (TEA). Source data are provided as a Source Data file.” (Revised manuscript, Pages 37−38, Lines 920−934)\nSupporting Information:\n“Supplementary Figure 13 | The removal of TCP in the Fe-NCN/PMS system under different conditions. Under the different (a−b) catalyst doses, (c−d) oxidant concentrations, (e−f) pollutant concentrations, and (g−h) solution pH, the removal profiles and the first-order oxidation kinetics of TCP in Fe-NCN/PMS systems. Experimental condition: [pollutant] = 0 0.05−0.15 mM, [PMS] = 0.1−0.8 mM, [catalyst] = 0.02−0.2 g L−1, [pH] = 1.0−12.0 (if any), 0 0 initial T = 25 °C.” (Supporting Information, Page 36)\na b\nSupplementary Figure 17 | Effects of different real water matrices. (a) Dechlorination performance of TCP removal in the Fe-SACs/PMS system. (b) Antibacterial activity of the samples for Photobacterium phosphoreum in Fe-SACs/PMS systems (inset: release of chloride ions during the reaction). Experimental condition: [pollutant] = 0.1 mM, [PMS] = 0.3 0 0 mM, [catalyst] = 0.1 g L−1, [pH] = 6.7, initial T = 25 °C. (Supporting Information, Page 0)\na b\nSupplementary Figure 18 | Residual TOC and COD content in bulk solution and catalyst surface. (a) TOC in bulk solution after Fe-SACs/PMS system reaction to remove different organic pollutants. (b) COD content in the bulk solution and in the catalyst after TCP adsorption on Fe-SACs. Experimental condition: [pollutant] = 0.1 mM, [PMS] = 0.3 mM, 0 0 [catalyst] = 0.1 g L−1, [pH] = 6.7, initial T = 25 °C. (Supporting Information, Page 41) 0\nR14\na b\nSupplementary Figure 21 | EPR tests. EPR spectra of Fe-SACs/PMS system using DMPO as trapping agent in (a) aqueous solution and (b) MeOH solution, respectively. Experimental condition: [PMS] = 2.0 mM, [catalyst] = 75 mg L−1, [TCP] = 0.1 mM, [DMPO] = 20.0 mM, 0 0 0 [pH] = 6.7, initial T = 25 °C. (Supporting Information, Page 45) 0\n“Supplementary Figure 24 | Isotope experiments. (a) Chromatogram of the 18O-labeled or unlabeled PMSO generated in the Fe-SACs/PMS system. (b) Corresponding MS2 2 spectrum of PMS16O16O and PMS16O18O. (c) PMSO consumption, PMSO generation and 2 PMSO/PMSO values in catalytic systems. Experimental condition: [PMS] = 0.3 mM, 2 0 [catalyst] = 0.1 g L−1, [TCP] = [PMSO] = 0.1 mM, [pH] = 6.7, initial T = 25 °C.” 0 0 0\n“Supplementary Figure 25 | EPR test and Solvent exchange experiment verification of 1O .(a,b) EPR spectrum of Fe-SACs/PMS/TCP system. (c,d) Removal performances of TCP 2 in H O and D O solvents Fe-SACs/PMS system. Experimental condition: [PMS] = 2.0 mM, 2 2 0 [catalyst] = 75 mg L−1, [TCP] = 0.1 mM, [TEMP] = 2.0 mM, [pH] = 6.7, initial T = 25 °C.” 0 0 0\n“Supplementary Figure 26 | Competitive dynamics experiment using FFA as microprobe. Removal efficiency and kinetics curves of (a, b) FFA probe and (c, d) removal efficiency and kinetics curves of TCP in Fe-SACs/PMS system. (e) 1O steady-state 2 concentration and oxidation contribution estimated by competitive dynamics experiments using FFA as microprobe. Experimental condition: [PMS] = 0.3 mM, [catalyst] = 0.1 g L−1, 0 [TCP] = 0.1 mM, [FFA] = 10.0 μM, [pH] = 6.7, initial T = 25 °C.” (Supporting Information, 0 0 0 Pages 48−51)\nR15\na b\nSupplementary Figure 30 | Verification of PMS* complexes. (a) The removal effect of TCP on catalysts that were stored for 0.5, 1.0 and 3.0 hours, respectively, after pre-oxidation. (b) Controlled experiments of adsorption, pre-oxidation after 24 h and reacted Fe-SACs. Experimental condition: [PMS] = 0.3 mM, [catalyst] = 0.1 g L−1, [TCP] = 0.1 mM, [pH] = 6.7, 0 0 0 initial T = 25 °C. (Supporting Information, Page 55)\na b\nSupplementary Figure 31 | Detection of PMS concentration. (a) Changes in PMS concentration over time in the Fe-SACs/PMS system before and after adding TCP. (b) Decomposition rate of PMS in Fe-SACs/PMS system. Experimental condition: [PMS] = 0.3 0 mM, [catalyst] = 0.1 g L−1, [TCP] = 0.1 mM, [pH] = 6.7, initial T = 25 °C. (Supporting 0 0 Information, Page 56)\nR16\nSupplementary Figure 32 | I−t (current-time) and Electrochemical impedance spectroscopy (EIS) test. (a) Changes of the I−t curves upon sequential additions of PMS and TCP in Fe-SACs system. (b) EIS of Fe-SACs. Experimental condition: [PMS] = 5.0 mM, 0 [TCP] = 0.2 mM, [catalyst coat] = 0.5 mg cm−2, [pH] = 6.7, initial T = 25 °C. (Supporting 0 0 Information, Page 52)\na b\nSupplementary Figure 33 | Linear voltammetry curves (LSV). After adding PMS and TCP successively, the current changes of the Fe-CN and Fe-NCN electrodes at (a) positive potential and (b) negative potential, respectively. Experimental condition: [PMS] = 2.0 mM, 0 [TCP] = 0.2 mM, [catalyst coat] = 0.5 mg cm−2, [pH] = 6.7, initial T = 25 °C. (Supporting 0 0 Information, Page 58)\nR17\nSupplementary Figure 35 | Different model reactors test. Removal of TCP in Fe-SACs/PMS system in (a) Mod. 1, (b) Mod. 2 and Mod. 3 electrochemical reactors. Experimental condition: [TCP] = 0.1 mM, [PMS] = 0.5 mM, [catalyst coat] = 0.5 mg cm−2. 0 0 Data are presented as mean values ± s.d. (n = 2). (Supporting Information, Page 60)\na b\nSupplementary Figure 58 | Compositional changes of ABTS and its product. Compositional changes of ABTS during the activation of PMS by Fe-CN and Fe-NCN. Experimental condition: [ABTS] = 50 μM, [PMS] = 0.5 mM, [catalyst] = 0.1 g L−1. 0 0 (Supporting Information, Pages 85−86)\n“Supplementary Figure 60 | Kinetic isotope experiment. Kinetic isotope experiment using deuterated phenol (C H OD) as the organic substrate. Experimental condition: 6 5 [pollutant] = 0.05 mM, [PMS] = 0.2 mM, [catalyst] = 0.05 g L−1, [pH] = 6.7, initial T = 25 °C.” 0 0 0\n“Supplementary Figure 61 | Identification of TCP-O•.(a) Burst mechanism of FA on phenyl oxygen radicals. (b) Effect of FA addition on TCP removal by Fe-NCN/PMS system. (c) EPR spectra of DMPO–TCP-O• adducts before and after filtration in Fe-NCN/PMS sytem. Experimental condition: [pollutant] = 0.1 mM, [PMS] = 0.3 mM, [catalyst] = 0.1 g L−1, [ferulic 0 0 acid] = 0.05 or 0.20 g L−1, [DMPO] = 20 mM, [pH] = 6.7, initial T = 25 °C.” “Supplementary 0 Figure 62 | Identification of TCP-O+. Competitive inhibition of acetonitrile on TCP removal in Fe-NCN/PMS system. Experimental condition: [pollutant] = 0.1 mM, [PMS] = 0.3 mM, 0 0 [catalyst] = 0.1 g L−1.” (Supporting Information, Pages 88−90)\na b\nOH\nOH\nSupplementary Figure 76 | Effect of prepolymerization products on removal of TCP from Fe-SACs. (a) The removal efficiency of TCP by the Fe-SACs/PMS system using 4,4'-Biphenol as the competing probe for the polymerization reaction and (b) the corresponding apparent removal rate constant. Experimental condition: [TCP] = 0.1 mM, [PMS] = 0.3 mM, 0 0 [4,4'-Biphenol] = 10.0 or 50.0 μM, [catalyst] = 0.1 g L−1. (Supporting Information, Page 0 108)\nR19\nReviewer #2:\nGeneral comments: This study developed an axial-nitrogen-coordinated single-atom iron catalyst (Fe-NCN), which triggers an electron-transfer pathway (ETP) by enhancing the electronic pulling effect on 2,4,6-trichlorophenol (TCP). This approach achieves near-complete dechlorination of TCP and its catalytic conversion into polymeric products. Overall, this study provides novel insights in polymerization-based advanced oxidation processes; however, there remain shortcomings in the generalizability of the mechanism and the in-depth exploration of its details. The detailed comments are as follows:\nResponse: We thank the reviewers for their valuable comments and constructive suggestions, all of which are accepted. We have made substantial revisions based on the comments, adding the necessary discussions and experiments. We hope that these revisions will be satisfactory.\nComment 1. The source of the XlogP (hydrophobicity index) data in Supplementary Fig. 1 should be clearly specified.\nResponse: Many thanks to the reviewers for their helpful reminders. We have meticulously reviewed the manuscript and corrected erroneous data. In the revised version of the Supporting Information, we have supplemented the data sources for the molecular descriptors of hydrophobicity index (X ), topological polar surface area (TPSA), acid-base logP dissociation constant (pK ). Please see Pages 23 and 109 in Supporting Information. a\nChange:\n“Supplementary Figure 1 | Comparison of hydrophobicity of products in coupling and polymerization pathways. X (hydrophobicity index) of the intermediate products logP transformed by phenol and C-C coupling and C-O polymerization pathways. The data for the X descriptor was obtained from the PubChem database logP (https://pubchem.ncbi.nlm.nih.gov).” (Supporting Information, Page 23)\na b\n- TCP 4,4'-biphenyl\nXLogP 3.7 3.6\nTPSA 40.5 20.2\npKa 6.23 9.74\nHOMO -5.13 -4.01\nc d\n13.9° 121.1°\nCN N-C\n15.6° 29.9°\nFe-CN Fe-NCN\n“Supplementary Figure 77 | Properties of different catalysts. (a) Comparison of the properties of 4,4'-biphenol with TCP. The data for X , topological polarity surface area logP (TPSA) and acid-base dissociation constant (pK ) descriptor was obtained from the a PubChem database (https://pubchem.ncbi.nlm.nih.gov/).” (Supporting Information, Page 109)\nComment 2. The authors mention that the pre-coordinated template exhibited a product yield of up to 45.0%, which is attributed to interlayer crosslinking. It is necessary to clarify: (1) How does this interlayer crosslinking process occur? (2) Why can it increase the loss of C and N? (3) At high temperatures, will the loss of C and N still be constrained by the crosslinking effect?\nResponse: We sincerely thank the reviewers for their thoughtful consideration of interlayer crosslinking mechanisms in catalyst synthesis. The associated discussions are added in Supplementary Fig. 8. Please see Pages 30−31 in the Supplementary Information.\nChange:\n“As shown in Figures S6 and S8a, 1,10-phen decomposes at elevated temperatures to iminyl (−C=N−) and pyridinic N. These reactive species can undergo ligand reactions or form covalent bonds with the amino (−NH ) and cyano (−C≡N) of the carbon nitride (CN), 2 facilitating interlayer crosslinking between CNs17. This interlayer crosslinking decreases the voids between the CN layers, leading to reduced specific surface area and pore volume in Fe-NCN and the formation of a denser crosslinked structure (Figure S4). XRD spectra also support this result (Figure. S5a). The introduction of 1,10-phen further reduces the intensity of the characteristic diffraction peaks, indicating that the ordered lamellar structure and stacking pattern of the CN substrate are disrupted. The tighter interlayer bonding prevents CN from decomposing the diffusion of small molecules such as HCN and NH , which in turn 3 inhibits C and N loss18. In addition, hydroxylamine hydrochloride can provide an NH /HCl 3 atmosphere that may also inhibit N loss and C oxidation of crosslinked structures. If the carbon nitride substrate is replaced with other carriers (e.g., carbon black), this crosslinking effect can still inhibit C and N loss at high temperatures (>600 °C), but the effect may diminish with increasing temperatures19.”\n17. Xiang, J. et al. NIR-enhanced Pt single atom/g-C N nanozymes as SOD/CAT mimics 3 4 to rescue ATP energy crisis by regulating oxidative phosphorylation pathway for delaying osteoarthritis progression. Bioactive Mat. 36, 1 (2025).\n18. Yu, C. et al. FeNC shell-stabilized L10-PtFe intermetallic nanoparticles for high-performance oxygen reduction. Chinese J. Cata. 75, 125-136 (2025).\n19. Wang, X. et al. Intensified atomic utilization efficiency of single-atom catalysts for nitrate conversion via electrified nanoporous membrane. Sci. Adv. 11(28), eads6943 (2025).\nComment 3. It is inappropriate for the authors to rule out the existence of Fe-C and Fe-O bonds solely through XPS. In fact, XPS peak-splitting results may correspond to multiple chemical states of Fe. Therefore, it is overly arbitrary for the authors to assume that all coordinating atoms of Fe are N during EXAFS fitting, and further conclude that the Fe-N coordination numbers are 4.28 (for Fe-CN) and 5.13 (for Fe-NCN). Additional characterization methods should be employed to probe the actual coordinating atoms of Fe, such as Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS).\nResponse: We thank the reviewers for their valuable feedback and fully agree that XPS alone cannot conclusively exclude the presence of Fe−C and Fe−O bonds. Following the reviewer’s suggestion, we conducted time-of-flight secondary ion mass spectrometry (TOF-SIMS) to detect the actual ligand atoms of Fe. As shown in Supplementary Fig. 8c, the signals of [FeC]− and [FeO]− corresponding to m/z= 67.93 and 71.94, respectively, in the Fe-SACs are weak (respectively), whereas the characteristic signal of the [FeN]− fragment (m/z=69.94) is significant. The results confirm the dominant Fe–N coordination environment in both Fe–CN and Fe–NCN catalysts.\nFurthermore, compared with the weaker fragment [FeN ]− in Fe-CN, [FeN ]− of Fe-NCN 5 5 showed a substantially higher signal intensity, indicating that the FeN site of Fe-NCN 4 synthesised by the pre-coordination self-assembly strategy was converted to the FeN site 5 (Supplementary Fig. 8d). This additional characterisation effectively compensates for the limitations of the XPS technique and validates that isolated Fe centers in Fe-NCN are coordinated to five nitrogen atoms (FeN ). Please see Lines 142-146 and Lines 161−167 5 in Revised manuscript and Pages 30−31 in Supporting Information.\nChange:\n“Time-of-flight secondary ion mass spectrometry (TOF-SIMS) at m/z = 67.93 and 71.94 corresponded to weak characteristic signals for [FeC]- and [FeO]-, respectively. In contrast, pronounced signals for the fragment [FeN]- (m/z = 69.94) were observed, indicating the presence of only Fe–N bonds in Fe-SACs (Supplementary Fig. 8c).” (Revised manuscript, Page 6, Lines 154-158)\n“EXAFS fitting on k-space and R-space revealed average Fe-N coordination numbers of 4.28 and 5.13 for Fe-CN and Fe-NCN, respectively (Supplementary Fig. 10 and Supplementary Table 2), which was also supported by TOF-SIMS analysis (Supplementary Fig. 8d).” (Revised manuscript, Page 7, Lines 173−176)\nc d\nSupplementary Figure 8 | XPS spectra and TOF-SIMS of different catalysts. (c, d) TOF-SIMS mass spectra of Fe-CN and Fe-NCN. (Supplementary Information, Page 30)\nComment 4. The results in Supplementary Fig. 13b show that electron-deficient organic pollutants (e.g., ATZ) can still be effectively removed (with approximately 70% degradation efficiency for ATZ). If this reaction is dominated by non-radical species, the authors need to explain this phenomenon.\nResponse: We thank the reviewers for their insightful comments and providing the opportunity to clarify the observed removal of the electron-deficient pollutant (ATZ) under a predominantly nonradical ETP regime. To elucidate this phenomenon, we performed DFT analyses, including electrostatic potential mapping, frontier molecular orbital distribution, and Fukui function calculations (Figure R1).\nThe results reveal that although ATZ is overall electron-deficient, its ethylamino (−NHCH CH ) and isopropylamino (−NHCH(CH ) ) side chains contain localized electron- 2 3 3 2 rich regions associated with the highest occupied molecular orbital (HOMO). The Fukui function further identified several electrophilic-susceptible sites (N , N , N ) that can interact 6 7 9 with oxidizing intermediates such as Fe–O* or PMS-complexes, even under non-radical conditions. Consequently, ATZ can still undergo partial oxidation and transformation through electrophilic attack by surface-bound reactive species with moderate redox potential. This interpretation aligns with previous reports that non-radical ETP processes can effectively contribute to ATZ removal, despite its low electron density, due to the participation of surface-confined charge-transfer complexes rather than free radicals (Applied Cat. B Environ., 343, 123526; Water Res., 246, 120678; ACS EST Eng., 4(11), 2676−2686).\na b d\n26 14 18\n2 2 5 2 2 2 3 7 12 16 20 15 19 2 2 1 4 1 1 1 3 17\n9 6 7\n2 1\n5 4\n3\n28\nc\nFigure R1. (a) Optimised molecular structure, (b) electrostatic potential, (c) frontier orbitals and (d) Fukui function of ATZ.\nComment 5. The amount of chloride ions released during the reaction should be detected to further confirm the dechlorination-polymerization process of TCP.\nResponse: We accept the reviewer’s great suggestion to assess the release of chloride ions during the reaction. The supplemented data has been provided in the inset of Supplementary Fig. 17b. Please see Page 40 in Supporting Information.\na b\nSupplementary Figure 17 | Effects of different real water matrices. (a) Dechlorination performance of TCP removal in the Fe-SACs/PMS system. (b) Antibacterial activity of the samples for Photobacterium phosphoreum in Fe-SACs/PMS systems (inset: release of chloride ions during the reaction). Experimental condition: [pollutant] = 0.1 mM, [PMS] = 0.3 0 0 mM, [catalyst] = 0.1 g L−1, [pH] = 6.7, initial T = 25 °C. 0\nComment 6. In Fig. 3c, the concentrations of the organic substrate (TCP) and PMS, as well as the catalyst loading, should be clearly stated. Additionally, all reaction conditions should be specified in the figure caption.\nResponse: We thank the reviewer for this valuable suggestion. As suggested, we have supplemented the experimental conditions in the captions of Fig. 2−5 in Revised manuscript and Fig. S17, Fig. S18, Fig. S21, Figs. S24−26, Fig. 30−33, Fig. S35, Fig. S58, Figs. S60−62, and Fig. S76 in Supporting Information.\nChange:\nRevised manuscript:\n“Fig. 2 | Catalytic activity of catalyst and identification of oxidation mechanisms. a Removal of TCP by different catalysts/PMS systems. b A critical evaluation of Fe-SACs/PMS in different aspects. c Intrinsic catalytic activity of the Fe-NCN in comparison with the state-of-the-art SACs and metal-based material in PMS-based Fenton-like oxidation. d Residual COD of the reaction solution and of the reacted Fe-SACs after elution with different solvents. R25 e Electronic utilization efficiency of Fe-SACs/PMS/TCP system. Experimental condition: [pollutant] = 0.1 mM, [PMS] = 0.3 mM, [catalyst] = 0.1 g L−1, [pH] = 6.7, initial T = 25 °C. 0 0 0 Data are presented as mean values ± s.d. (n = 3). f EPR spectra of TEMP adducts recorded during PMS activation in Fe-SACs systems. g In situ FTIR spectra of Fe-NCN system. h OCP changes upon sequential addition of PMS and TCP for Fe-SACs electrodes. Experimental condition: [PMS] = 5.0 mM, [catalyst] = 0.4 g, [pH] = 6.7, initial T = 25 °C. 0 0 i The number of electron transfers fitted by the rotating ring disc electrode (RRDE) experiment. Source data are provided as a Source Data file.”(Revised manuscript, Page 33, Lines 878−892)\n“Fig. 3 | Mechanistic investigation of axial ligand-induced catalytic pathway transitions. a Schematic diagrams of the two different ETP paths. b Short-range ETP mechanism of oxidized pollutants in Fe-NCN/PMS system. c Three different galvanic model reactors. d Performance of TCP removal by Fe-SACs activated PMS in three model reactors. Experimental condition: [pollutant] = 0.1 mM, [PMS] = 0.5 mM, [catalyst coat] = 0.5 mg cm−2. 0 0 Data are presented as mean values ± s.d. (n = 2). e The adsorption configuration, adsorption energy of Fe-SACs/PMS and the d band center of catalyst before and after PMS adsorption. f pCOHP between the Fe atom and the O atom as PMS adsorption. Corresponding IpCOHP values are shown in the figures. g Spin states of Fe-CN (Fe-N ), Fe-NCN (Fe-N ), Fe- 4 4+axial CN/PMS, and Fe-NCN/PMS. h Pathways and corresponding thermodynamic change curves of 1O generation from PMS activated by Fe-SACs. Source data are provided as a Source 2 Data file.” (Revised manuscript, Pages 34−35, Lines 893−905)\n“Fig. 4 | Identification of polymerization products and mechanism of C-O polymerization of pollutants in DHP path. a XPS spectra of Fe-NCN in the original, after activation of PMS, and after reaction with TCP. b Chemical evolution of the Fe-NCN surface based on in situ Raman spectra after sequential addition of PMS and TCP. Experimental condition: [PMS] = 5.0 mM, [catalyst] = 0.5 g, [TCP] = 0.5 mM, [pH] = 6.7, initial T = 25 °C. 0 0 c Thermodynamic potential energy curve of hydroxylation-dechlorine reaction path based on short-range ETP induced by axial ligands. d Radial distribution function g(r) for the organic molecule−O distance between organic molecule and the O atom of water. w e Coordination number (CN) of water. f Synergistic modulation of hydroxylation substitution reaction pathways and thermodynamic potential energy profiles of ortho-chlorine on chlorophenoxonium ions by the electrophilic intermediate PMS* and water bridges. Source data are provided as a Source Data file.” (Revised manuscript, Page 36, Lines 906−919)\n“Fig. 5 | Application potential, value-added processing of products and economic and environmental feasibility of Fenton-like oxidation process. a, COD changes and EEM spectra b, before and after treatment of secondary domestic effluent from pulp and paper industry by Fe-NCN/PMS system. Experimental condition: [pollutant] = 0.1 mM, 0 [PMS] = 0.3 mM, [catalyst] = 0.1 g L−1, [reaction solution] = 200.0 mL. Data are presented 0 as mean values ± s.d. (n = 2). c, A schematic illustration of the size-enlarged catalytic reactor. d, TCP (2.0 ppm) removal efficiency, TOC removal and dechlorination over with days in a size-enlarged catalytic reactor (inset: Fe leaching within reaction time). e, Precipitation separation and value-added production of oligomers. f, The industrial-grade plastic pallets are obtained through the extrusion-condensation-granulation process. g, Solid-state 13C nuclear magnetic resonance (13CNMR) spectroscopy of plastic granules. h, The economic feasibility of the amplified synthesis of Fe-NCN was demonstrated by technical and economic analysis (TEA). Source data are provided as a Source Data file.” (Revised manuscript, Pages 37−38, Lines 920−934)\nSupporting Information:\n“Supplementary Figure 13 | The removal of TCP in the Fe-NCN/PMS system under different conditions. Under the different (a−b) catalyst doses, (c−d) oxidant concentrations, (e−f) pollutant concentrations, and (g−h) solution pH, the removal profiles and the first-order oxidation kinetics of TCP in Fe-NCN/PMS systems. Experimental condition: [pollutant] = 0 0.05−0.15 mM, [PMS] = 0.1−0.8 mM, [catalyst] = 0.02−0.2 g L−1, [pH] = 1.0−12.0 (if any), 0 0 initial T = 25 °C.” (Supporting Information, Page 36)\na b\nSupplementary Figure 17 | Effects of different real water matrices. (a) Dechlorination performance of TCP removal in the Fe-SACs/PMS system. (b) Antibacterial activity of the samples for Photobacterium phosphoreum in Fe-SACs/PMS systems (inset: release of chloride ions during the reaction). Experimental condition: [pollutant] = 0.1 mM, [PMS] = 0.3 0 0 mM, [catalyst] = 0.1 g L−1, [pH] = 6.7, initial T = 25 °C. (Supporting Information, Page 0 40)\na b\nSupplementary Figure 18 | Residual TOC and COD content in bulk solution and catalyst surface. (a) TOC in bulk solution after Fe-SACs/PMS system reaction to remove different organic pollutants. (b) COD content in the bulk solution and in the catalyst after TCP adsorption on Fe-SACs. Experimental condition: [pollutant] = 0.1 mM, [PMS] = 0.3 mM, 0 0 [catalyst] = 0.1 g L−1, [pH] = 6.7, initial T = 25 °C. (Supporting Information, Page 41) 0\nR28\na b\nSupplementary Figure 21 | EPR tests. EPR spectra of Fe-SACs/PMS system using DMPO as trapping agent in (a) aqueous solution and (b) MeOH solution, respectively. Experimental condition: [PMS] = 2.0 mM, [catalyst] = 75 mg L−1, [TCP] = 0.1 mM, [DMPO] = 20.0 mM, 0 0 0 [pH] = 6.7, initial T = 25 °C. (Supporting Information, Page 45) 0\n“Supplementary Figure 24 | Isotope experiments. (a) Chromatogram of the 18O-labeled or unlabeled PMSO generated in the Fe-SACs/PMS system. (b) Corresponding MS2 2 spectrum of PMS16O16O and PMS16O18O. (c) PMSO consumption, PMSO generation and 2 PMSO/PMSO values in catalytic systems. Experimental condition: [PMS] = 0.3 mM, 2 0 [catalyst] = 0.1 g L−1, [TCP] = [PMSO] = 0.1 mM, [pH] = 6.7, initial T = 25 °C.” 0 0 0\n“Supplementary Figure 25 | EPR test and Solvent exchange experiment verification of 1O .(a,b) EPR spectrum of Fe-SACs/PMS/TCP system. (c,d) Removal performances of TCP 2 in H O and D O solvents Fe-SACs/PMS system. Experimental condition: [PMS] = 2.0 mM, 2 2 0 [catalyst] = 75 mg L−1, [TCP] = 0.1 mM, [TEMP] = 2.0 mM, [pH] = 6.7, initial T = 25 °C.” 0 0 0\n“Supplementary Figure 26 | Competitive dynamics experiment using FFA as microprobe. Removal efficiency and kinetics curves of (a, b) FFA probe and (c, d) removal efficiency and kinetics curves of TCP in Fe-SACs/PMS system. (e) 1O steady-state 2 concentration and oxidation contribution estimated by competitive dynamics experiments using FFA as microprobe. Experimental condition: [PMS] = 0.3 mM, [catalyst] = 0.1 g L−1, 0 [TCP] = 0.1 mM, [FFA] = 10.0 μM, [pH] = 6.7, initial T = 25 °C.” (Supporting Information, 0 0 0 Pages 48−51)\nR29\na b\nSupplementary Figure 30 | Verification of PMS* complexes. (a) The removal effect of TCP on catalysts that were stored for 0.5, 1.0 and 3.0 hours, respectively, after pre-oxidation. (b) Controlled experiments of adsorption, pre-oxidation after 24 h and reacted Fe-SACs. Experimental condition: [PMS] = 0.3 mM, [catalyst] = 0.1 g L−1, [TCP] = 0.1 mM, [pH] = 6.7, 0 0 0 initial T = 25 °C. (Supporting Information, Page 55)\na b\nSupplementary Figure 31 | Detection of PMS concentration. (a) Changes in PMS concentration over time in the Fe-SACs/PMS system before and after adding TCP. (b) Decomposition rate of PMS in Fe-SACs/PMS system. Experimental condition: [PMS] = 0.3 0 mM, [catalyst] = 0.1 g L−1, [TCP] = 0.1 mM, [pH] = 6.7, initial T = 25 °C. (Supporting 0 0 Information, Page 56)\nR30\nSupplementary Figure 32 | I−t (current-time) and Electrochemical impedance spectroscopy (EIS) test. (a) Changes of the I−t curves upon sequential additions of PMS and TCP in Fe-SACs system. (b) EIS of Fe-SACs. Experimental condition: [PMS] = 5.0 mM, 0 [TCP] = 0.2 mM, [catalyst coat] = 0.5 mg cm−2, [pH] = 6.7, initial T = 25 °C. (Supporting 0 0 Information, Page 52)\na b\nSupplementary Figure 33 | Linear voltammetry curves (LSV). After adding PMS and TCP successively, the current changes of the Fe-CN and Fe-NCN electrodes at (a) positive potential and (b) negative potential, respectively. Experimental condition: [PMS] = 2.0 mM, 0 [TCP] = 0.2 mM, [catalyst coat] = 0.5 mg cm−2, [pH] = 6.7, initial T = 25 °C. (Supporting 0 0 Information, Page 58)\nR31\nSupplementary Figure 35 | Different model reactors test. Removal of TCP in Fe-SACs/PMS system in (a) Mod. 1, (b) Mod. 2 and Mod. 3 electrochemical reactors. Experimental condition: [TCP] = 0.1 mM, [PMS] = 0.5 mM, [catalyst coat] = 0.5 mg cm−2. 0 0 Data are presented as mean values ± s.d. (n = 2). (Supporting Information, Page 60)\na b\nSupplementary Figure 58 | Compositional changes of ABTS and its product. Compositional changes of ABTS during the activation of PMS by Fe-CN and Fe-NCN. Experimental condition: [ABTS] = 50 μM, [PMS] = 0.5 mM, [catalyst] = 0.1 g L−1. 0 0 (Supporting Information, Pages 85−86)\n“Supplementary Figure 60 | Kinetic isotope experiment. Kinetic isotope experiment using deuterated phenol (C H OD) as the organic substrate. Experimental condition: 6 5 [pollutant] = 0.05 mM, [PMS] = 0.2 mM, [catalyst] = 0.05 g L−1, [pH] = 6.7, initial T = 25 °C.” 0 0 0\n“Supplementary Figure 61 | Identification of TCP-O•.(a) Burst mechanism of FA on phenyl oxygen radicals. (b) Effect of FA addition on TCP removal by Fe-NCN/PMS system. (c) EPR spectra of DMPO–TCP-O• adducts before and after filtration in Fe-NCN/PMS sytem. Experimental condition: [pollutant] = 0.1 mM, [PMS] = 0.3 mM, [catalyst] = 0.1 g L−1, [ferulic 0 0 acid] = 0.05 or 0.20 g L−1, [DMPO] = 20 mM, [pH] = 6.7, initial T = 25 °C.” “Supplementary 0 Figure 62 | Identification of TCP-O+. Competitive inhibition of acetonitrile on TCP removal in Fe-NCN/PMS system. Experimental condition: [pollutant] = 0.1 mM, [PMS] = 0.3 mM, 0 0 [catalyst] = 0.1 g L−1.” (Supporting Information, Pages 88−90)\na b\nOH\nOH\nSupplementary Figure 76 | Effect of prepolymerization products on removal of TCP from Fe-SACs. (a) The removal efficiency of TCP by the Fe-SACs/PMS system using 4,4'-Biphenol as the competing probe for the polymerization reaction and (b) the corresponding apparent removal rate constant. Experimental condition: [TCP] = 0.1 mM, [PMS] = 0.3 mM, 0 0 [4,4'-Biphenol] = 10.0 or 50.0 μM, [catalyst] = 0.1 g L−1. (Supporting Information, Page 0 108)\nComment 7. The authors claim that \"the greater the distance between TCP and CNT/PMS*, the stronger the C-Cl bond\". What is the theoretical basis for this assertion?\nResponse: We sincerely appreciate the reviewers' insightful question. The statement regarding the relationship between the TCP–CNT/PMS* distance and C–Cl bond strength is supported by DFT calculations and charge-density analyses (Supplementary Fig. 39). The results show that the C–Cl bond length of TCP progressively decreases as the TCP molecule moves farther from the CNT/PMS* complex, eventually approaching the bond length of free TCP. This behaviour arises from the combined effects of electron transfer and electrostatic polarization between TCP and the electrophilic CNT/PMS* intermediate. When TCP is positioned close to CNT/PMS*, strong interfacial electron transfer occurs from the aromatic π-system of TCP to the electrophilic PMS*-derived complex, leading to a reduction in the electron density of the aromatic ring. Consequently, the C–Cl bond electron cloud shifts toward the chlorine atom, weakening and elongating the bond. As the separation distance increases, orbital overlap and charge-transfer coupling diminish exponentially, consistent with Marcus electron-transfer theory, resulting in weaker polarization and a partial restoration of the C–Cl bond electron density. This re-stabilization returns the C–Cl bond to a shorter and stronger configuration similar to that in unbound TCP.\nChange:\n“When the distance between TCP and CNT/PMS* was between 10 to 20 nm, the C–Cl bond can be hardly activated. However, when the distance decreases to 2–10 nm, the interfacial electron transfer between the aromatic π system of TCP and the electrophilic complex PMS* is significantly enhanced. This intensifies the shift of the C–Cl bond's electron density towards the chlorine atom, resulting in the weakening and elongation of the C–Cl bond (1.72 → 1.75 Å).” (Lines 348-353, Page 13 in Revised manuscript)\na\n5 nm 5 nm 5 nm 5 nm\n2 nm\n1.7147 Å\n1.7198 Å\n2 6\n4\n1.7234 Å\n2 nm 5 nm 10 nm 15 nm 20 nm\nC-Cl2 (Å) 1.7381 1.7289 1.7218 1.7209 1.7207\nC-Cl4 (Å) 1.7492 1.7366 1.7247 1.7241 1.7239\nC-Cl6 (Å) 1.7317 1.7234 1.7160 1.7155 1.7154\nb\nSupplementary Figure 39 | The variation in the bond length of C−Cl. The variation of the C−Cl bond length of TCP on the CNT surface with the distance between it and the PMS. (Supporting Information, Page 65)\nComment 8. The IpCOHP analysis indicates that axial N coordination enhances both the Fe-O bond strength and the d-band center of Fe, which are interpreted as evidence for strengthened PMS adsorption. However, the adsorption energy calculations show the opposite trend (weakened PMS adsorption). The authors must explain this contradiction.\nResponse: We appreciate the Reviewer's very careful review and insightful question. To clarify the apparent contradiction between the ICOHP, the d-band center and the trend of adsorption energy calculations, we would like to first clarify the distinct physical meanings of these parameters.\nCOHP quantifies the effect of bonding states (negative contribution) and antibonding states (positive contribution) on bonding stability by decomposing the interaction energies of atomic orbitals in the energy band structure; the larger the absolute value indicates stronger covalent bonding (more bonding, fewer antibonding states).\nThe d-band center model proposed by Hammer and Nørskov suggests that the position of the d-band center with respect to the Fermi energy level directly affects the bonding strength of the adsorbate to the metal (Adv. Catal. 45, 71, 2024): an upward shift of the d-band center toward the Fermi level implies enhanced orbital overlap and bonding between metal d orbitals and adsorbate orbitals, corresponding to stronger Fe–O bond interactions. Therefore, both ICOHP and d-band analyses describe bonding strength at the atomic scale, namely the intrinsic Fe–O covalency within a stable adsorption configuration.\nBy contrast, the adsorption energy (E ) in DFT calculations reflects the energy change ads of reaction intermediates or small molecules (e.g., PMS) upon adsorption on the catalyst surface and depends on multiple factors, such as electronic structure, charge transfer, and spatial coordination. Although the Fe center in Fe-NCN has a higher positive charge (lower electron density, Fig. 1f in Revised manuscript), which theoretically should favour electrostatic attraction with the negatively charged PMS, it has fewer spin electrons (fewer unpaired electrons, Supplementary Fig. 44 in Supporting Information), resulting in a reduced degree of charge transfer to PMS. Furthermore, axially coordinated N atoms create an off-plane geometry at the Fe center, which changes the adsorption configuration of PMS from \"horizontal-type adsorption\" to \"inclined-type adsorption\" (Figure R2) and increases the spatial separation between the PMS molecules and the substrate.\nConsequently, Fe–NCN exhibits a lower adsorption energy than Fe–CN, not because of weaker Fe–O bonding, but due to geometric and spin-related factors that lower the total binding energy. The local Fe–O bond itself remains stronger and more covalent, as evidenced by the higher ICOHP value and upward-shifted d-band center. Therefore, there is no fundamental contradiction, as Fe–NCN simultaneously exhibits stronger Fe–O bonds and more stable PMS complexes*, even though the overall adsorption energy is lower.\na b\nFigure R2. Adsorption configurations of PMS on (a) Fe-CN and (b) Fe-NCN.\nComment 9. In Lines 364–365, the authors state that DFT calculations reveal that axial N coordination reduces both the number of transferred electrons and the adsorption energy. This result is inconsistent with the authors’ conclusion—this inconsistency requires clarification.\nResponse: Thank you for the insightful comment on the link between adsorption energies and electron transfer quantities and the formation of substable PMS* complexes. Although axial coordination reduces the number of transferred electrons and the adsorption energy, this does not contradict our conclusion that axial ligands favor the formation of a low-spin and metastable PMS complex intermediate.\nFirstly, although the adsorption strength of Fe-NCN to PMS decreases, the axial N coordination effectively lifts the antibonding orbital occupation of the Fe−O bond and reduces its electronic bushing, leading to a significant elevation of pCOHP to −1.83 eV, which results in a more stable structure of the Fe-PMS* intermediate (Fig. 3f). On the other hand, compared to Fe-CN (0.88 e), 1.32 e was transferred out of the surface upon PMS adsorption on Fe-NCN, of which 0.85 e was transferred to PMS, and 0.47 e was transferred to the axial N ligand (Supplementary Fig. 40). This indicates electron redistribution through the axial coordination pathway. Additionally, smaller changes in O–O and O–H bond lengths (Supplementary Table 7) suggest that the reduced charge transfer induced by the axial ligand helps preserve the molecular integrity of PMS, thereby promoting the stabilization of the Fe–PMS* intermediate. Enhanced orbital overlap (Fe 3d–O 2p, 5.91 vs. 4.82; Supplementary Fig. 43) and an upward-shifted d-band center (−2.07 eV vs. −2.28 eV; Fig. 3e) further confirm strengthened Fe–O orbital hybridization under axial coordination.\nIn summary, while axial N ligands slightly lower E and electron transfer, they ads concurrently enhance Fe–O bond covalency and d–p orbital coupling, thus favouring the formation of metastable low-spin Fe–PMS* intermediates. The relevant section has been restructured to clarify this nuanced relationship. Please see Page 13 in Revised manuscript.\nChange:\n“However, although axial N coordination significantly reduced E to −2.98 eV with ads moderated electron transfer (−0.85 e), it favored metastable low-spin Fe-PMS* complex formation (Fig. 3e, Supplementary Figs. 41 and 42 and Supplementary Table 7).” (Revised manuscript, Page 14, Lines 372-375)\ne f\nFig. 3 | e The adsorption configuration, adsorption energy of Fe-SACs/PMS and the d band center of catalyst before and after PMS adsorption. f pCOHP between the Fe atom and the O atom as PMS adsorption. Corresponding IpCOHP values are shown in the figures. (Revised manuscript, Page 34, Lines 898−902)\na c\n0.85 e-\n0.88 e-\n0.47 e-\nSupplementary Figure 40 | Adsorption configurations and differential charge density maps of PMS adsorbed on (a) Fe-CN (FeN , four nitrogen atoms coordinating one iron atom) and 4 (c) Fe-NCN (FeN ). (Supporting Information, Page 66) 4+axial\nR37\nc d\nSupplementary Figure 43 | Theoretical simulation of Fe-SACs. PDOS for (c) Fe-CN and (d) Fe-NCN after adsorption of PMS. (Supporting Information, Page 69)\nSupplementary Table 7. Comparison of the apparent electron utilization efficiency of different systems. (Supporting Information, Page 124)\nPMS TCP Fe-CN/PMS* Fe-NCN/PMS* Fe-CN/PMS*/TCP Fe-NCN/PMS*/TCP\nO-O 1.4643 - 1.5147 1.4717 1.5148 1.5027\nO-H 0.9834 - 1.0316 0.9796 1.0317 0.9859\nComment 10. The authors claim that \"axial N coordination leads to a staggered energy rearrangement of the dx2-y2 and dz2 orbitals, consequently reducing the number of unpaired electrons in the eg orbitals\". Please explain the underlying mechanism of this process.\nResponse: Many thanks to the reviewers for their careful review and insightful question, which allows us to elaborate on the orbital-level mechanism underlying the observed rearrangement of the Fe d-orbitals upon axial N coordination.\nFor the plane-symmetric Fe-CN (D 4h ), the d x2-y2 orbitals stretch along the x- and y-axis directions and have the strongest orbital overlap and electrostatic repulsion with the in-plane ligands. Consequently, its energy level of d lies significantly higher than that of d in the x2-y2 z2 e orbitals (Fig. 3g). When the axial N ligand is introduced, the symmetry of the system is g reduced to the C point group, and the orbital splitting pattern is subsequently adjusted 4v (Angew. Chem. Int. Ed. 63(43), e202406030). The interactions of the axial N atoms with the z-axis oriented d orbitals are significantly enhanced, leading to a staggered rearrangement z2 of the d and d energy levels (Supplementary Fig. 45). Meanwhile, the strong z2 x2-y2 coordination ability of the N atoms further increases the splitting energy of the d orbitals, resulting in an overall downward shift of the energy levels of the t orbitals in Fe-NCN (J. Am. 2g Chem. Soc. 146(21), 14875-14888). Given the high electronegativity of the N atoms, the Fe center tends to transfer electrons to the axial N atoms. Since the d orbital is at a higher z2 energy level, its electrons are more easily excited to the N atom, which ultimately leads to a decreased number of unpaired electrons in the e orbital and a transition toward a more g stabilized and lower-spin configuration.\nNotably, despite this reduction in unpaired spins, the overall orbital spin filling increased from 1.99 to 2.11 (Supplementary Fig. 46), suggesting enhanced spin delocalization and electronic coupling between Fe and the coordinated ligands. The intensified overlap between Fe-e and PMS O–2p orbitals (Supplementary Fig. 48) further supports the formation of g more stable Fe–PMS complexes* with stronger electronic interactions (Angew. Chem. Int. Ed. 64, e202512925 (2025)). This enhanced orbital hybridization facilitates interfacial electron transfer in the Fe–NCN/PMS system, consistent with our mechanistic conclusions.\nFig. 3 | g Spin states of Fe-CN (Fe-N ), Fe-NCN (Fe-N ), Fe-CN/PMS, and Fe-NCN/PMS. 4 4+axial (Revised manuscript, Page 34, Lines 903)\nSupplementary Figure 45 | The projection density of states (PDOS). The PDOS of Fe-CN, Fe-NCN, Fe-CN/PMS*, and Fe-NCN/PMS*. (Supporting Information, Page 72)\nR39\nygrenE\nMedium spin (S=1) low spin (S=1/2)\nFe-CN Fe-NCN Fe-CN/PMSFe-NCN/PMS\ne\nD4h\nAxial N\nC4v\ng\nAxial N\nt\n2g\nt 4 e2 t 4 e1 t 4 e2 t 4 e1\n2g g 2g g 2g g 2g g\na b\nc d\nSupplementary Figure 46 | Spin filling calculation of Fe 3d orbitals. e orbital filling of Fe g sites for (a) Fe-CN, (b) Fe-NCN, (c) Fe-CN/PMS* and (d) Fe-NCN/PMS*. (Supporting Information, Page 73)\na b\nc\nSupplementary Figure 48 | Charge transfer energy. (a,b) Orbital overlap of the Fe e of g catalyst and O p-band centers of PMS. (Supporting Information, Page 75)\nComment 11. For Fe-CN-activated PMS, the key intermediate for 1O generation is the SO •-, 2 5 and its formation requires electron transfer from PMS to the catalyst. The authors mention that \"the energy difference for charge transfer from the eg orbital center of Fe-CN to PMS increases significantly, reversing the direction of electron transfer (from PMS to the catalyst) and thus generating SO •-\". However, Fig. 3h clearly shows that the reaction process has a 5 positive energy barrier, which indicates that the reaction cannot proceed spontaneously. This discrepancy needs to be explained.\nResponse: We thank the reviewer for the insightful comment and providing us the opportunity to clarify the apparent inconsistency between the observed positive reaction energy barrier and the proposed electron transfer direction from PMS to the Fe–CN catalyst.\nAs shown by our electrochemical analyses and DFT calculations, the activation of PMS on Fe–CN involves electron transfer from PMS to the Fe center, forming the key intermediate SO •- responsible for 1O generation. However, the calculated positive Gibbs free energy 5 2 change (ΔG > 0) indicates that this deprotonation and charge-transfer process is endothermic and therefore non-spontaneous under standard conditions. This phenomenon does not contradict the proposed mechanism as it reflects the intrinsic energetic barrier associated with O–H bond cleavage during PMS activation.\nThe relatively high bond dissociation energy of the O–H bond (370–463 kJ mol⁻¹) compared with that of the adjacent O–O bond (140–213 kJ mol⁻¹) necessitates external energy input, in this case (supplied through local thermal excitation, electronic polarization, or catalytic surface fields) to overcome the activation barrier (J. Am. Chem. Soc. 140, 8611–8623 (2018); Adv. Mater. 35, 2209552 (2023)). Once the O–H bond dissociates, PMS readily donates an electron to the Fe center, producing SO •- as the key intermediate for 1O 5 2 generation. Such endothermic but kinetically feasible activation steps are well documented in non-homogeneous oxidation systems, including oxygen evolution and peroxymonosulfate activation reactions (Nat. Commun. 15, 5314 (2024); Nat. Commun. 16, 800 (2025); Environ. Sci. Technol. 57, 14046–14057 (2023); Angew. Chem. Int. Ed. 137, e202504426 (2025); Proc. Natl. Acad. Sci. USA 120, e2300281120 (2023)). Therefore, while PMS activation over Fe-CN is endothermic, it can proceed under experimental conditions owing to the thermal energy provided by the system and surface polarization that facilitates O–H bond cleavage and electron transfer. The relevant amendments have been incorporated, please see Page 15 in Revised manuscript.\nChange:\n“Compared to Fe-NCN, Fe-CN only required overcoming a lower reaction energy barrier (1.95 vs. 2.87 eV) to achieve dissociation of the high-bond-energy O−H bond, thereby driving the entire process into a thermodynamically favourable state (ΔG = –1.26 eV).” (Revised manuscript, Page 15, Lines 402-405)\nComment 12. What characteristics of Fe-NCN enable its high-efficiency polymerization performance? Can the authors propose a general structural descriptor for this performance, or discuss relevant design principles? Additionally, can this structural descriptor be applied to other single-atom transition metal materials (e.g., Co-based, Cu-based single-atom catalysts)?\nResponse: We thank the reviewer for the insightful questions regarding the structural features and design principles governing the high-efficiency polymerization performance of Fe–NCN and related single-atom catalysts.\nThe reaction selectivity of the active center is the key factor enabling the exceptional polymerization performance of Fe–NCN. The introduction of an axial N ligand modulates both the electronic configuration and spin state of the Fe center, thereby tuning the non-radical electron-transfer pathway (ETP) toward the selective formation of the electrophilic PMS intermediate*. This Fe–PMS* complex exhibits a moderate oxidative potential, strong enough to drive electrophilic substitution and coupling reactions, yet mild enough to prevent over-oxidation or mineralization. Consequently, organic pollutants are selectively converted and immobilized as stable oligomeric species on the catalyst surface, achieving both detoxification and upcycling.\nWe propose several structural descriptors that capture these reactivity characteristics: pCOHP reflects the Fe–O bond covalency and serves as a descriptor for the ease of PMS* complex formation, while HOMO and the d-band center describe the redox potential and electron-donating ability of the Fe center within PMS* complexes. These parameters collectively provide a quantitative framework for correlating the catalyst’s electronic structure with its polymerization efficiency.\nAs the reviewer advised, to assess the universality of this design principle, we further synthesized a series of axially coordinated single-atom catalysts using a pre-coordination self-assembly strategy, yielding N-, S-, and O-axially coordinated Fe-, Co-, and Cu-based catalysts (denoted Fe–NCN, Co–NCN, Cu–NCN, Fe–SCN, Co–SCN, Cu–SCN, Fe–OCN, Co–OCN, Cu–OCN). As shown in Supplementary Figure 82, all catalysts demonstrated excellent pollutant removal and polymerization performances. A positive linear correlation was observed between the Bader charge of the metal center and both the reaction rate and TOC removal rate, highlighting that axial coordination reduces the spin density and promotes the formation of low-spin PMS* intermediates, which is the essential factor for controlled and high-efficiency polymerization. Moreover, as indicated by the Fe–NCN free-energy diagram (Fig. 4c), the desorption of OH* intermediates plays a key role in catalytic turnover. Thus, the free energy of oxygen intermediate desorption (ΔG ) may serve as an additional OH* universal descriptor for polymerization performance, analogous to its established role in oxygen evolution reaction (OER) catalysis (Angew. Chem. Int. Ed. 63, e202407812 (2024)).\nAlthough these proposed descriptors (e.g., pCOHP, d-band center, HOMO level, Bader charge, and ΔG ) offer promising criteria for rational catalyst design, we acknowledge that OH* further theoretical integration and systematic data-driven correlation analyses will be necessary, which form the focus of our ongoing and future work.\nChange:\n“The proposed pre-coordination-assisted strategy can be extended to the synthesis of other SACs (such as axially O- and S-coordinated SACs and other transition metal-based SACs), with all catalysts exhibiting excellent pollutant removal and polymerisation performance (Supplementary Fig. 82).” (Revised manuscript, Page 22, Lines 630–633)\na b c\nd e f\n“Supplementary Figure 82 | The removal and polymerisation performance of PMS on TCP activated by SACs with different axial ligands. (a-c) Performance of activated PMS with monatomic catalysts modified with different axial ligands for TCP removal. (d) Reaction rate and organic carbon removal efficiency of TCP removal by different catalyst/PMS systems. (e, f) Linear relationship of reaction rate and organic carbon removal efficiency with bader charge.\nNote: Axially O- and S-modified SACs were synthesised via identical procedures, substituting 1,10-phenanthroline with tannic acid and penicillamine respectively. Similarly, by replacing only the metal source, SACs featuring axially modified ligands based on different transition metals can be synthesised through the chelating action of the complexing agent. All catalysts demonstrated excellent pollutant removal and polymerization performances. A positive linear correlation was observed between the Bader charge of the metal center and both the reaction rate and TOC removal rate, highlighting that axial coordination reduces the spin density and promotes the formation of low-spin PMS* intermediates, which is the essential factor for controlled and high-efficiency polymerization. Future work may further investigate and integrate generic descriptors related to SACs structures to develop catalysts with highly efficient aggregation properties.” (Supporting Information, Page 116)\nComment 13. Regarding the claim of \"mimicking the enzyme’s hydrophobic/polar binding pocket microenvironment\", the authors should address the following: (1) Why does Fe-NCN possess a hydrophobic and polar microenvironment? (2) The paper focuses on enzyme-mimetic catalysis—what similarities exist between the structural features of natural enzymes and those of Fe-NCN? (3) Why does Fe-CN not exhibit enzyme-mimetic functionality? (4) What are the decisive factors that determine whether the catalyst’s active center has a hydrophobic/polar microenvironment? These points should be specifically discussed in the revised manuscript.\nResponse: We thank the reviewers for their valuable suggestions for refining the topic of mimicking enzyme catalysis. In response, we have substantially expanded the discussion on the enzyme-mimetic hydrophobic/polar binding-pocket microenvironment to address all four specific questions. Specifically, we complemented the active center structures and binding pocket microenvironments of CYP152 peroxidase and B12-dependent dehalogenase PceA to explain the structural similarity of Fe-NCN to the natural enzyme (Fig. 1a and Supplementary Fig. 2). Indeed, this ingenious microenvironment of hydrophobic cavities and polar residues within the bioenzyme binding pocket also encourages the application of advanced materials in inert gas capture and biomedical contexts (Angew. Chem. Int. Ed. e20094 (2025); Nat. Chem. Bio. 1-11 (2025); Adv. Mat. 35(15), 2211237 (2023)). The origin and determinants of the hydrophobic and polar chemical microenvironment of Fe-NCN were elucidated by a combination of XRD, BET, XPS, WCA and zeta potential techniques (Supplementary Fig. 77). The inability of Fe-CN to mimic enzyme catalytic function is discussed in terms of both physicochemical structure and oxidation mechanism, highlighting the importance of axial ligands and polar/hydrophobic binding pocket microenvironments in the design of enzyme-mimetic catalysts. Please see Pages 4, 8, 11 and 13−14 in Revised manuscript and Pages 8, 9, 29, 42−48, 56 and 58 in Supporting Information.\nChange:\n“The structure and function of CYP152 peroxidase were fully dissected to design dehalogenation-polymerisation catalysts. Microbial-mediated hydrolytic dechlorination occurs at the active center of heme iron modified by weakly axial sulphate ligands, using surface-activated peroxides to reconfigure the local hydrogen-bonding network for hydroxylated dehalogenation of the substrate (Fig. 1a)18. Binding pockets consisting of hydrophobic amino acids and polar-interacting residues provide substrate-trapping zones. B12-dependent reductive dehalogenases have also been reported to have similar structures20 (Supplementary Fig. 2). Inspired by the structure of the natural enzyme, the introduction of axial ligands to modify the metal center may be beneficial for regulating the switch in oxidative mechanisms in the DHP pathway.” (Revised manuscript, Page 5, Lines 111−116)\na\nCYP152 POD\nPolar residue\nHydrophobic\nAxial ligand\nBinding pocket\nFig. 1 | Composition characteristics, local coordination environment and electronic structure of catalyst. a Overall structure of CYP152 Peroxygenases (POD, P450 BS beta mutant F46A, Protein Data Bank ID: 9IY1). Amino acid fragments act as axial ligands that modulate the oxidative valence of the metal center, facilitating the promotion of oxidant activation, substrate recognition binding and maintenance of enzyme activity. The binding pocket consists of hydrophobic amino acid residues together with amino acid residues involved in polar interactions, providing a favourable microenvironment that not only promotes oxidant activation but also facilitates substrate binding and product release. (Revised manuscript, Page 31, Lines 862−869)\nPceA\nPolar residue\nHydrophobic\nAxial ligand Binding pocket\nSupplementary Figure 2 | Overall structure of the polygonal sulfur spirochete dehalogenation prototype enzyme PceA. Vitamin B12-dependent reductive dehalogenase, Protein Data Bank ID: 4UR0. Consists of weakly axial ligand-modified cobalamins as the active center and binding pockets including conserved polar and aromatic residues wrapped around it. (Supporting information, Page 24)\n“The higher zero-charge point favours the specific adsorption of oligomers with polar functional groups by polarity matching (Supplementary Fig. 77c). In addition, enhanced hydrophobicity promotes interactions between the substrate and the hydrophobic structure of the oligomer, reducing the competition of the product for the active site (Supplementary Fig. 77d). Indeed, this is largely attributed to the introduction of the chelator 1,10-phenanthroline, whose pyrolytic formation of N-C confers a hydrophobic and polar microenvironment in the vicinity of the active centers in Fe-NCN. This hydrophobic/polar microenvironment extensively exists in the binding pocket of natural enzymes, which is a key prerequisite for specific catalysis and efficient cycling18,20,58 (Fig. 1a and Supplementary Fig. 2).” (Revised manuscript, Pages 19−20, Lines 546−551)\na b\n- TCP 4,4'-biphenyl\nXLogP 3.7 3.6\nTPSA 40.5 20.2\npKa 6.23 9.74\nHOMO -5.13 -4.01\nc d\n13.9° 121.1°\nCN N-C\n15.6° 29.9°\nFe-CN Fe-NCN\n“Supplementary Figure 77 | Properties of different catalysts. (a) Comparison of the properties of 4,4'-biphenol with TCP. The data for XlogP, topological polarity surface area (TPSA), acid-base dissociation constant (pK ) descriptor was obtained from the a PubChem database (https://pubchem.ncbi.nlm.nih.gov/). Where XlogP corresponds to hydrophobicity, pka corresponds to the ability of the molecule to release protons, and the HOMO energy level responds to the ability of the molecule to be oxidised. The larger these metrics are, the more strongly the corresponding properties behave. The relative Topological Surface Area reflects the polarity of the molecule, the smaller the value, the greater the polarity. (b) Zeta potentials of different catalysts varying with pH. (c) Contact angles of different catalysts. (d) Linear correlation between the adsorption capacity of different materials for 4,4'-biphenyldiol and their zeta potentials and contact angles.\nNote: The C 1s XPS high-resolution mapping showed a greater proportion of polar groups (N-C=N and C-N/C-O, 83.89%) in N-C compared to CN (24.46%) (Figure S6). The introduction of monatomic iron sites slightly increased the content of polar functional groups in the Fe-CN structure (28.43%). Apparently, the content of polar functional groups in Fe-NCN increased significantly to 55.34% when 1,10-phen was introduced as a pre-coordinated chelator of Fe. This phenomenon suggests that the introduction of 1,10-phen effectively increases the polarity of the Fe-SACs structure, which is consistent with the trend in zeta potential (Figure S77b). In addition, the test results of WCA showed that the introduction of monoatomic iron sites did not significantly increase the hydrophobicity of the Fe-CN structure (15.6° vs. 13.9°, Figure S77c). Of interest, the overall structure of N-C exhibits significant hydrophobicity (water contact angle, WCA = 121.1°), despite containing highly polar functional groups, a property that effectively enhances the hydrophobicity of Fe-NCN (13.9° → 29.9°). The key mechanism is that by introducing 1,10-phen as a Fe chelator into the CN substrate, it successfully induces the formation of a microenvironment around the metal active center that is both polar and hydrophobic.\nInterestingly, this hydrophobic/polar microenvironment is widely observed in the binding pockets of hydrolases/dehalogenases, a key prerequisite for specific catalysis and efficient cycling. For example, CYP152 peroxidase in the cytochrome P450 enzyme subfamily consists of a combination of an active center constructed from the major heme iron center and an axial sulphate ligand, and a surrounding binding pocket made up of hydrophobically interacting hydrophobic amino acids (e.g., long-chained palmitic acid) and protein residues involved in polar interactions. Sulfurrospirillum PceA, a prototypical dehalogenase in multivorans, consists of an active center (vitamin B12) constructed from cobalamin (Co-corrin) and a weakly axial ligand (dimethylbenzimidazole), as well as a binding pocket constructed from an arrangement of aromatic ring residues in a hydrophobic cavity and adjacent highly conserved polar residues. This polar but hydrophobic microenvironment provides natural enzymes with a highly variable substrate capture region to support functional activity of the catalytic site and efficient operation of the reaction cycle.” (Supporting information, Pages 109−111)\n“In contrast, Fe-CN lacks not only enzyme-like axial ligands to regulate the dehalogenation-polymerisation transformation of TCP by metal-center-activated PMS, but also a microenvironment of enzyme-like binding pockets to regulate the transfer of the product at the active site, resulting in its lower mineralisation capacity and recyclability.” (Revised manuscript, Pages 22−23, Lines 636−640)\nThe expanded discussion integrates structural, spectroscopic, and physicochemical evidence to elucidate the enzyme-mimetic nature of Fe–NCN and clearly distinguishes it from Fe–CN. We hope these comprehensive revisions satisfactorily address the reviewer’s concerns.\nReviewer #3:\nGeneral comments: This study successfully designed and synthesized an axial nitrogen-coordinated single-atom iron catalyst (Fe–NCN) for efficient activation of peroxymonosulfate (PMS), achieving nearly complete dechlorination and directed polymerization conversion of a typical halogenated organic compound, 2,4,6-trichlorophenol (TCP), into plastic products with potential application value. The work not only provides in-depth insights into the unique \"dehalogenation–hydroxylation–polymerization\" reaction mechanism but also demonstrates the application potential of this technology through techno-economic analysis and life cycle assessment. Spanning multiple levels from atomic-scale catalyst design and mechanistic studies to practical application evaluation, the research is well-supported by detailed data and robust argumentation. It holds significant importance for advancing advanced oxidation processes based on resource recovery and is recommended for acceptance after minor revisions.\nResponse: We thank the reviewers for their encouraging comments on the novelty, scientific rigour and significance of this work, and we accept all of these minor comments and revise accordingly as follows.\nComment 1. The formatting of section headings is inconsistent. For example, the heading style of “Results” is the same as that of its subheadings such as “Synthesis and characterization of Fe-SACs”, while the heading “Methods” differs from its subheadings like “Material Preparation”. It is recommended to unify the heading styles and ensure distinction between main headings and subheadings.\nResponse: We thank the reviewer for the thoughtful suggestions. As advised, we standardized the manuscript title style to distinguish between main and subheadings. In addition, we checked the formatting throughout the manuscript and added experiment-related content in the revised manuscript.\nComment 2. There are spelling errors in the manuscript, such as “1,10 phenanthrene” in line 625 of the main text and in the supporting information. The correct term should be “1,10-phenanthroline”.\nResponse: Thanks for your careful review and corrections of these typographical errors. We have carefully checked the entire manuscript and Supporting Information and corrected all instances of “1,10 phenanthrene” to the correct chemical name “1,10-phenanthroline.” In total, four occurrences were corrected to ensure accuracy and consistency throughout the text and figures.\nChange:\n“Initially, Fe2+ ions were complexed with 1,10-phenanthroline in aqueous solution to form a metal-organic precursor as pre-coordination templates.” (Revised manuscript, Page 5, Line 105)\n“Generally, 0.18 mmol FeSO ∙6H O and 1,10-phenanthroline (0.027 g) were added to 10.0 4 2 mL of deionized water for 30.0 min ultrasonic treatment, and stirred at room temperature for 1.0 h to quickly form Fe2+-phen chelate (red).” (Revised manuscript, Page 22, Line 645)\n“1,10-phenanthroline (phen), water-18O (H 18O), potassium dihydrogen phosphate (KH PO ), 2 2 4 sodium bicarbonate (NaHCO ), sodium nitrate (NaNO ) and sodium sulfate (Na SO ) were 3 3 2 4 purchased from Sinopharm Chemical Reagent Co.” (Supporting Information, Page 1)\nComment 3. The phrase “mono-atomic dispersion” is unconventional. A more precise expression would be “single-atom dispersion”.\nResponse: Thank you very much for the reminder and suggestion from the reviewer. Based on the suggestions, we have replaced the unconventional phrase \"mono-atomic dispersion\" to \"single-atom dispersion\" to ensure the scientific precision and clarity of the manuscript.\nChange:\n“Fourier-transform extended X-ray absorption fine structure (FT-EXAFS) spectra showed a prominent Fe-N coordination peak at ~1.5 Å without metallic Fe–Fe bonds, confirming single-atom dispersion (Fig. 1g).” (Revised manuscript, Page 7, Lines 155-157).”\nComment 4. The study confirms the two-electron transfer process in the Fe–NCN/PMS system through RRDE tests. However, the data processing procedure and fitting error range for the electron transfer number are not presented in sufficient detail. It is recommended to include necessary explanations and data in the supporting information.\nResponse: We appreciate the reviewer’s considerate suggestion. As suggested, we have supplemented the Supporting Information with the necessary explanations on RRDE test data processing. Additional details regarding this aspect have been incorporated into Supplementary Text 14 (Supporting Information, Page 8−9).\nChange:\n“Text S14. Rotating Ring Disc Electrode (RRDE) test\nFirst, 5.0 mg of the material was added to 1.0 mL of 5% Nafion solution (containing 20% ethanol) and sonicated for 5.0 min for uniform suspension. Then, 2.5 μL of the suspension containing the material was dropped onto a glassy carbon disc 5 times to obtain a uniformly coated rotating disc electrode. The RRDE assembly consists of a glassy carbon rotating disc electrode (0.234 cm2 disc area) and a platinum ring with a theoretical collection efficiency of 31%. The disc electrode, platinum ring, and Ag/AgCl electrode were used as the working, counting, and reference electrodes, respectively. RRDE measurements were carried out at room temperature using a CHI 760 electrochemical station. All solutions used were degassed with nitrogen. The samples were electrochemically activated before RRDE experiments.\nPrior to the LSV scan tests, 1.0 mM PMS and 0.1 mM TCP were successively added to 0.1 M Na SO electrolyte (pH = 7), and the LSV curves were collected (three times) at a 2 4 certain electrode rotational speed with a scanning rate of 10 mV s−1 at voltages of −1.3, −1.1 and −0.9 V (versus Ag/AgCl), respectively. Scanning potentials from −0.5 V to 1.8 V (versus Ag/AgCl) under negative scanning mode and recording of test data. The number of electron transfers (n) on the RRDE was calculated using the following equation:\nn = 4 × I /(I +I /N ) D D R C\nwhere I , I and N are the ring current, disc current and theoretical collection efficiency, D R C respectively.”\nComment 5. The price of the recovered modified polyphenylene oxide particles is set at 0.9 times that of commercial polyphenylene oxide. A brief rationale for this factor should be provided.\nResponse: Thanks so much for this thought-provoking question.The reasons for setting the price of recovered modified polyphenylene (PPO) oxides pellets at 0.9 times the price of commercial PPO pellets are based on the performance and cost feedstock considerations.\nIn terms of performance, the synthetic PPO plastic pellets have slightly lower mechanical properties (compressive strength, 844 σM < 897 σM) and poorer thermal stability (glass transition temperature, 168 °C < 210 °C) than commercially available commercial PPO pellets (purchased from Macklin Co., Ltd) (Supplementary Fig. 80). However, considering that the synthesised plastic granules are rich in hydroxyl functional groups, they are more compatible with resins such as polystyrene and have higher processability during injection moulding. As a result, recycled synthetic plastic pellets are theoretically less expensive than commercial PPO pellets, which have better properties. These characteristics support the adoption of a moderately discounted valuation relative to commercial PPO.\nHowever, in terms of cost feedstock, the commercial PPO used in industrial applications is usually in a modified form, such as PPO co-blended with polystyrene (typically 30%). This suggests that the synthesis of commercial PPO pellets involves three processes (including extrusion-chopping of the polystyrene pellets) and requires the addition of additional modifiers. In contrast, the recovered modified plastic pellets in this study are oxidatively converted from chlorophenol polymerization and can be stored and used after a simple extrusion-pelletisation process at a much lower cost. Taking into consideration of slightly lower intrinsic performance but lower feedstock and production costs, the recovered PPO pellets were conservatively priced at 0.9 × the market value of commercial PPO. This benchmark reflects a realistic balance between material properties and cost efficiency. Future optimization of the P-AOP process to improve polymer purity, tailor microstructure, and enhance yield is expected to further increase product value. We have supplemented a concise rationale to discuss the formulation of this factor. Please see Supplementary Text 22 in Supporting Information.\nChange:\n“Taking into account performance differences and feedstock costs, we initially assumed that the price of recycled synthetic PPO is 0.9 times that of commercial PPO. In terms of performance, the mechanical properties (such as compressive strength) and thermal stability of recycled synthetic PPO are slightly inferior to commercially available PPO. However, owing to its high hydroxyl content, it exhibits excellent resin compatibility and enhanced injection moulding processability, theoretically improving its cost advantage. Regarding cost, commercial PPO is predominantly modified (e.g., blended with Polystyrene), necessitating multi-step processing and additives, resulting in higher costs. Recycled synthetic PPO are produced via chlorophenol oxidation conversion and can be readily used after simple extrusion pelletisation, yielding significantly lower costs.” (Supporting Information, Page 18)\nComment 6. The techno-economic analysis (TEA) is detailed, but potential costs such as solvent recovery and recycling are not included in the calculation. It is recommended to address this in the TEA discussion, noting that the current cost assessment does not account for solvent recovery and reuse, to make the analysis more comprehensive.\nResponse: We thank the reviewers for their thoughtful consideration of optimising the techno-economic analysis (TEA). We agree with the reviewers that addressing potential cost issues, such as solvent recovery and recycling, is important for refining the overall process cost assessment of the P-AOP process. The refined TEA can provide a valuable reference for the development of P-AOP processes. Please see Supplementary Text 22 (Supporting Information, Pages 16−20) for the revisions.\nChange:\n“Text S22. The profit that can be made from recycled and processed modified PPO pellets.\nConsidering that the collected polymer powder can be further processed into usable, industrial-grade modified PPO granules (injection-molding precursors), the recycling value of the product was further estimated.\nRevenue from polymers. Each tonne of Fe-NCN can process about 10 tonnes of TCP, with a recovery rate of ~45%. However, accounting for process losses, the product recovery rate is ~42%, i.e., 4.2 tonnes of synthetic PPO particles can be recovered per tonne of Fe-NCN. In addition, ~10,700 L of tetrahydrofuran (~9.5 tonnes) is used in the elution process. Specific chemical prices are shown in Table S11. The aqueous methanol solution served as an undesirable solvent, with approximately 1070 L of methanol consumed (~1.35 tonnes).\nTaking into account performance differences and feedstock costs, we initially assumed that the price of recycled synthetic PPO is 0.9 times that of commercial PPO. In terms of performance, the mechanical properties (such as compressive strength) and thermal stability of recycled synthetic PPO are slightly inferior to commercially available PPO. However, owing to its high hydroxyl content, it exhibits excellent resin compatibility and enhanced injection moulding processability, theoretically improving its cost advantage. Regarding cost, commercial PPO is predominantly modified (e.g., blended with PS), necessitating multi-step processing and additives, resulting in higher costs. Recycled synthetic PPO are produced via chlorophenol oxidation conversion and can be readily used after simple extrusion pelletisation, yielding significantly lower costs.\nThe profit of 1 tonne of Fe-NCN-treated chlorophenol wastewater that can be recycled and processed as PPO can be calculated as:\nSynthetic PPO profit = Income of PPO – eluent cost–undesirable solvent cost\n= 4.2 × 4000 × 0.9 – 9.5 × 1200 – 405\n= 3315 [US$ per tonne of Fe-NCN]\nSubsequently, we calculated the plant-gate levelized cost of processing recycled oligomer powder into usable plastic pellets using the extrusion-condensation-pelletization (ECP) process. Specifically, it is divided into equipment cost, installation cost, electricity cost and other operational costs.\nEquipment costs and installation costs. A miniature twin-screw extruder with a small condensation tank and a small pelletiser were selected for the equipment to save on purchase and installation costs and electricity costs, and the specific technical data and equipment prices are shown in Table S12. The cost of purchasing the equipment was converted to the cost of producing one tonne of modified PPO. Similarly, the equipment is assumed to have a life of 20 years with no salvage value at the end of the plant's service life, a plant capacity factor of 0.5, and a discount rate (i) of 6%. The capital recovery factor (CRF) is 0.087. The cost of the modified PPO treatment plant can be calculated as:\nCRF × equipment price\nEquipment cost = capacity factor × 365 days × production capacity\n0.087 × 25000\n= = 11.92 [US$ per tonne of PPO]\n0.5 × 365 × 1\nBased on empirical estimates of the magnitude of material discharges, we assume that the cost of equipment installation equals the cost of the equipment. The economic cost of equipment and installation is US$ 23.84 per ton.\nElectricity costs. The cost of electricity comprises two parts: the operation of the ECP process equipment and the Fenton-like reaction. The cost of electricity used for the ECP process equipment can be calculated as:\nECP process electricity cost = all equipment power consume × 24h × electricity price ∑\n= 11.35 [US$ per tonne of PPO]\nThe economic cost of electricity need to be was calculated to be about US$ 11.35 per tonne.\nOther operational costs. Other operational costs (such as labor, maintenance and waste treatment) are assumed to be 10% of the total capital costs calculated above.\nThe plant-gate levelized cost of ECP process was calculated to be US$ 38.71 per tonne.\nTheoretically, 4.2 tonnes of recycled synthetic PPO can be produced per tonne of Fe-NCN. Therefore, the economic benefits of the recycled synthetic PPO pellets was calculated to be about US$ (3315 – 4.2 × 38.71) = 3152.42 per tonne of Fe-NCN. Excitingly, the profitability of polymer recycling reduced the plant-gate levelised cost per tonne of Fe-NCN by 31.87%, lowering it to US$ 6,739.79.\nSolvent recovery and recycling costs. The separation, recovery and recycling of THF as an eluent and methanol as an undesirable solvent is necessary to reduce pollution emissions, lower feedstock costs and prevent toxic leakage. The THF-alcohol-containing wastewater was extracted and separated using an extraction tower. Among them, the THF-containing wastewater was distilled by a pressurised tower to obtain THF finished products, and the alcohol-containing wastewater was separated from methanol by dealcoholization in dealcoholization tower, which ultimately realised the separation and recovery of THF and methanol. The recovered THF and methanol can still be recycled. The THF-Methanol wastewater separation and recovery unit can be simplified into an extraction tower, a pressurised tower, a multi-stage condenser and a preheater. The specific technical data and equipment prices are shown in Table S12.\nSimilar to the ECP process, the plant-gate levelized cost of organic solvent recovery and recycling are categorised into equipment cost, installation cost, electricity cost and other operational costs.\nEquipment costs and installation costs. The cost of purchasing the equipment can be converted to the cost of producing one tonne of wastewater containing organic solvents.\nSimilarly, the equipment is assumed to have a life of 20 years with no salvage value at the end of the plant's life, a plant capacity factor of 0.9, and a discount rate (i) of 4%. The capital recovery factor (CRF) is 0.074. The cost of the modified PPO treatment plant can be calculated as:\nCRF × equipment price\nEquipment cost = capacity factor × 365 days × production capacity\n0.074 × 25000\n= = 7.51 [US$ per tonne of organic solvents]\n0.9 × 365 × 1\nBased on empirical estimates of the magnitude of material discharges, we assume that the cost of equipment installation equals the cost of the equipment. The economic cost of equipment and installation is US$ 15.02 per tonne of organic solvents.\nElectricity costs. The electricity prices were determined from the average industrial electricity prices in Heilongjiang, China, in 2024 (0.086 US$ per kWh). The electricity costs can be calculated by:\nElectricity cost = all equipment power consume × 24h × electricity price ∑\n= 29.10 [US$ per tonne of organic solvents]\nOther operational costs. Other operational costs (such as labor, maintenance and waste treatment) are assumed to be 10% of the total capital costs calculated above.\nIn fact, the treatment of one tonne of post-reaction Fe-NCN to collect the oligomer powder requires 9.2 tonnes of THF as eluent and 2.42 tonnes of aqueous methanol-containing solution (50:50 (v:v)). Therefore, the solvent recovery and recycling cost required to treat one tonne of Fe-NCN is calculated as US$ 578.53.\nTaking into account the cost of the input chemicals (PMS) and the electricity used for operation, the overall economic cost of the process can be further calculated as total cost = catalyst production cost + input chemical cost + operation electricity cost − plastic profit.\nWhere, the PMS cost can be calculated as:\nPMS cost. The economic cost of PMS is US$ 1379.3 per tonne, and the required PMS consumption is 1.35 tonnes per Fe-NCN-treated TCP. The cost of the PMS can be calculated as:\nPMS cost = PMS consumption × cost\n= 1.35 × 1379.3\n= 1862.06 [US$ per tonne of Fe-NCN]\nFurtheremore, the electricity cost required for the Fenton-like reaction process is calculated as:\nFenton-like reaction electricity cost = power consume× electricity price ∑\n= 22.36 [US$ per tonne of Fe-NCN]\nTherefore, the economic cost of the DHP process was calculated to be about US$ 9202.74 per tonne of Fe-NCN.\nThe DHP process total cost = catalyst production + chemical consumption+ operation electricity + solvent recovery and recycling – plastic profit\n= 9892.21 + 1862.06 + 22.36 + 578.53 – 3315+162.58\n= 9202.74 [US$ per tonne of Fe-NCN]\nDue to the value-added profit of plastics, the cost of wastewater treatment has been reduced by [(3315 – 162.58) / (9892.21 + 1862.06 + 22.36 + 578.53)] ×100% = 25.51%.” (Supporting Information, Pages 16−20)\nSupplementary Table 12. Input chemical amount and costs of 1 tonne Fe-NCN production.\nChemcial Price ($/t) Amount (t) Total cost ($)\nFeSO ∙6H O 157.32 0.19 29.16 4 2\n1,10-Phen 440.0 0.12 52.8\nHONH Cl 1504.8 0.09 139.44 3\nUrea 1641.6 5.0 8208.0\nWater 0.65 45.0 29.25\nN 83.44 0.18 15.02 2\nTotal - - 8473.67\nCommercial PPO 4000 - - \nMethanol 300 1.35 405\nNote: The cost datas for the above compounds were obtained from https://www.made-in-china.com. (Supporting Information, Page 130)\nSupplementary Table 13. Capacity, electricity power and price of equipment used in the TEA analysis. (Supporting Information, Page 131)\nElectrical power\nEquipment cost Capacity Price ($)\n(kW)\nWater purification 4 m3/h 13.3 36892.13\nAgitator tank 3*10 m3 3*5.5 2781.4\nMagnetic spinning machine 48 m3/h 3 8205.3\nVacuum drying 1 m3 8 4561.6\nTwin Screw Extruder 5~20 kg/H 5.5 25000\nTubular heat treatment furnace - 20 16422.24\nExtraction Tower (attach pump) 45−50 kg/h 2.5 11000\nHigh Pressure Packed Towers 2.1 m3/h 6.2 18000 (attach pump)\nMultistage condenser 0.2−0.6 m/s 1.6 1400\nPreheater - 3.8 3160\n“Once optimized chemical input ratios, this cost could be further reduced in the future (Supplementary Text 21 and Supplementary Tables 12 and 13)57. Considering further the profitability of polymer recycling, the cost decreased significantly by 31.9% to US$ 6739.79 (Supplementary Text 22).” (Revised manuscript, Page 20, Line 580−584)\n“Excitingly, the economic cost of the DHP process was reduced by 25.5%, attributable to profits from the plastics (Supplementary Text 22).”(Revised manuscript, Page 22, Line 631−633)\nComment 7. The article describes that the TCP removal and dechlorination rates remained high during 60 days of continuous operation. However, it is not clarified whether this stable performance was due to the inherent stability of the catalyst or the application of periodic regeneration operations. It is suggested to specify in the supporting information whether any catalyst regeneration was performed.\nResponse: We thank the reviewer for the considerate reminder. The sustained TCP removal and dechlorination efficiencies observed over 60 days of continuous operation, mainly due to the intrinsic stability of Fe-NCN as well as the robust activity of its Fe active centers and the hydrophobic yet polar-like binding pocket microenvironment. These factors jointly ensured the dehalogenation-polymerization of halogenated phenols and prolonged catalytic activity through the specific separation of products (Revised manuscript, Pages 7−9 and 19−20). In fact, no cyclic regeneration procedures were conducted during the entire operation period. To enhance clarity for readers, the operational details of the continuous-flow fluidized bed reactor, including catalyst loading, flow conditions, and sampling protocol, have been explicitly described in Supplementary Text 19 (Pages 11−12, SI).\nChange:\n“…Samples were collected at selected intervals, and TCP concentration in the effluent water was determined by UPLC to evaluate the treatment performance of the microreactor and the amplified reaction system. No catalyst regeneration was carried out during the operating cycle of the fluidised bed reaction unit.” (Supporting Information, Pages 11−12)"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The raised concerns have been well addressed. Thus, I would suggest to accept the revised manuscript for publication.", "reviewer_2": "All my concerns have been adequately resolved in this revision. The manuscript is now ready for publication in Nature Communications.", "reviewer_3": "I have reviewed the revised manuscript submitted by the authors in response to my comments.\nOverall, the authors have thoroughly addressed all seven review comments with considerable attention and detailed responses. The revisions not only resolve the specific issues raised but also significantly enhance the scientific rigor, precision, and clarity of the manuscript. The authors have demonstrated a conscientious and responsible approach to revision, and their reply letter is well-structured and convincingly argued, reflecting excellent academic competence.\nBelow is a point-by-point assessment of the revisions:\nRegarding Comment 1: The authors have standardized the heading format throughout the manuscript as suggested, ensuring a clear hierarchy between main and subheadings. This issue has been fully resolved.\nRegarding Comment 2: The authors have thoroughly checked the entire manuscript and supplementary materials, correcting all instances of “1,10 phenanthrene” to the accurate “1,10-phenanthroline.” This issue has been completely addressed.\nRegarding Comment 3: The authors have appropriately adopted the suggested terminology, revising it to the more standard and scientific term “single-atom dispersion.” This matter has been satisfactorily resolved.\nRegarding Comment 4: The authors have supplemented Text S14 in the supplementary information with a detailed description of the RRDE testing procedures, parameters, and the specific formula for calculating the electron transfer number. This addition greatly improves the reproducibility and credibility of the data. This issue has been adequately resolved.\nRegarding Comment 5: The authors have provided an excellent explanation in Text S22 of the supplementary information, convincingly justifying the 0.9 pricing factor from the perspectives of both performance differences and raw material costs. This response exceeds expectations, and the issue has been excellently resolved.\nRegarding Comment 6: This represents one of the most notable improvements in this revision. The authors have proactively incorporated a comprehensive cost analysis for solvent recovery and reuse into the TEA model, including costs related to equipment, installation, electricity, and operation, and clearly illustrated the impact on the overall cost. This renders the techno-economic analysis substantially more comprehensive and reliable. The issue has been thoroughly and commendably resolved.\nRegarding Comment 7: The authors have clearly stated in Text S19 of the supplementary information that no catalyst regeneration was performed during the 60-day continuous operation, attributing the long-term stability to the intrinsic structural stability of the catalyst. This clarification is crucial, as it eliminates potential reader confusion and reinforces the key conclusion regarding the superior performance of the catalyst. This issue has been clearly resolved.\nGiven that the authors have comprehensively and thoroughly addressed all review comments, resulting in substantial improvements to the manuscript, I am highly satisfied with this revised version. I hereby recommend acceptance of the manuscript."}, "author_response": "Response: Thank you for your thorough and highly positive final assessment. We are very grateful that you found all concerns comprehensively addressed and that the revisions have significantly strengthened the manuscript. Your insightful comments were invaluable in improving our work. We are honored by your strong recommendation for acceptance and sincerely thank you for your time and expert guidance throughout the review process."}]

Decision letter