Defect-induced electric field effects direct Fenton-like oxidation pathways towards polymerization for sustainable water treatment

Abstract

Polymerization-oriented Fenton-like oxidation of organic pollutants offers a promising method for energy harvesting while lowering carbon emissions. However, altering the organic pollutant removal route from molecular fragmentation to polymerization is challenging. Here we report that defect engineering, i.e., tailoring defect density in carbon catalysts, can strengthen polymeric decontamination in Fenton-like oxidation reactions. Theoretical and experimental results show that the vacancy defect-induced electric field on carbon nanotubes accelerates electron transfer from 4-chlorophenol to surface-bound peroxymonosulfate, increasing the formation of polymeric precursors (i.e., phenoxonium) via the two-electron transfer route. The defects suppress the strong oxidizing species generation and enhance the precursor adsorption, simultaneously promoting the stabilization and aggregation of phenoxonium precursors for polymerization. The established oxidative system results in complete phenolic pollutant removal, with electron utilization efficiency reaching 551%. Life cycle assessment, toxicity assessments, and continuous operation tests demonstrate the system’s applicability in real-world scenarios. Overall, this work provides a feasible approach to direct organic pollutant removal towards polymerization for sustainable water treatment.

By shifting carbon conversion from oxidative degradation to polymerization, defect engineering provides a feasible design strategy to develop advanced Fenton-like systems for sustainable water treatment.

Introduction

Fenton-like advanced oxidation processes are recognized as an efficient approach for treating emerging organic pollutants^1–4. Efforts to improve Fenton-like reactions have focused on the effective degradation and/or mineralization of organic pollutants to minimize environmental risks. However, conventional oxidation processes have intrinsic drawbacks, including excessive energy and/or chemical demand, and undesirable carbon emissions. The current adaptive oxidation paradigm redirects organic contaminant removal from molecular fragmentation to polymerization, which improves energy harvesting and lowers the carbon footprint^5–7. Unlike conventional chemical oxidation, the polymerization strategy does not focus on destroying the chemical bonds of target organics, but rather on triggering oxidative polymerization to produce polymer products^8–10. Since polymerization reactions do not decompose the carbon backbone of the organic compounds, such a strategy minimizes energy input while retaining the embedded chemical energy⁶. In addition, the low solubility of polymer products allows them to be readily separated from the aqueous phase for subsequent energy harvesting and value-added resource recovery^11–13.

Despite the promise of organic pollutant removal via polymerization reactions, research on their application in water treatment is still at an early stage, with insufficient understanding of the underlying mechanisms. A recent study summarized kinetic and thermodynamic factors that determine the conversion pathway of target molecules¹⁴. Unlike endothermic bond-breaking reactions (ΔG > 0) in complete mineralization processes, the coupling and chain growth reaction of oxidative polymerization is spontaneous (ΔG < 0)¹⁵. The polymerization reaction is thermodynamically favored, requiring mild oxidants to trigger it. Kinetically, a higher concentration of precursor facilitates a faster polymerization reaction, since it allows more efficient molecular collision^16,17. This requires rapid generation and accumulation of precursors during the reaction, while avoiding oxidative decomposition of the monomers and the new polymers. In addition, improving the ability to enrich and stabilize polymer precursors can overcome kinetic limitations when dealing with low-concentration pollutants^6,18. Based on thermodynamic and kinetic considerations, the main effort to achieve oxidative polymerization should be to improve the generation, stabilization, and aggregation of the polymeric precursors¹⁴.

To date, several strategies have been employed to achieve organic pollutant removal by polymerization rather than by complete oxidation. For example, Yang et al. manipulated the interlayer structure of iron oxychloride to selectively generate nonradical ferryl species during H₂O₂ activation, thus enhancing polymer precursor formation¹³. Yu et al. tailored the d-band center of high-valent metal-oxo species to control their oxidation process, preventing decomposition of polymer intermediates¹⁹. Pan et al. utilized nanoconfinement for the enrichment and stabilization of reactants and products, thereby promoting oxidative polymerization^6,7. Inspired by these studies, we considered the possibility of using defect engineering to allow the polymerization pathway to remove target organics. Defect engineering has been widely utilized to fine-tune Fenton-like reactions by heterogeneous catalysts, especially carbon-based catalysts^20–22. However, previous defect engineering^23–26 studies mainly focused on optimizing and tailoring catalytic properties, neglecting the potential to directly influence pollutant conversion pathways. We hypothesize that, beyond providing functional active sites, defect-induced polarization may exert an additional electric field effect^27–29 that modifies surface binding energy, thus potentially favoring the generation, stabilization, and aggregation of polymer precursors. To our knowledge, there is still a lack of comprehensive investigation into whether defect engineering may directly modify organic pollutant conversion from complete oxidation to polymerization.

In this work, we combine experiments with theoretical calculations to understand how defect engineering directs reaction pathways toward polymerization. The oxidation system consists of carbon nanotubes (CNTs) as catalysts, peroxymonosulfate (PMS) as an oxidant, and 4-chlorophenol (4-CP) as a model pollutant. Nitrogen compounds are used as dopants to regulate the density of defects on CNTs. We show that the introduction of defects not only improves organic removal performance but also alters the reaction pathways. The defect-induced electric field is found to be the key driver that enhances the generation, stabilization, and enrichment of polymer precursors, thus favoring the polymerization pathway both kinetically and thermodynamically. Our work offers an approach to regulating the pollutant transformation pathway that will be useful in polymerization-oriented water treatment technologies.

Results

Defect engineering of CNT catalysts

Defect-engineered CNTs (DCNTs) were fabricated by annealing nitrogen-doped CNTs (NCNTs) under an inert atmosphere to remove the nitrogen species via pyrolysis/evolution of N‑containing gases³⁰. NCNTs were initially synthesized by ball milling industrial-grade CNT with different nitrogen sources (i.e., melamine, dicyandiamide, or urea). The resultant NCNTs and DCNTs were termed as x-NCNTs and x-DCNTs, respectively, where x is M for melamine, D for dicyandiamide, and U for urea. In the transmission electron microscopy (TEM) images (Supplementary Fig. 1), CNT and DCNTs exhibit a typical tubular microstructure. After the defect engineering treatment, a significant difference in micro-shape is observed. The high-resolution TEM images show that all DCNTs possess amorphous carbon structures due to disordered impurities. (Fig. 1a–c). The X-ray diffraction (XRD) patterns (Fig. 1d) show the characteristic hexagonal graphite configuration³¹ of CNT, with a major peak at 25.9° corresponding to the (002) facet and a minor peak (43.1°) to the (111) facet. However, closer examination of the full width at half maximum (FWHM) of the diffraction peak (25.9°) shows a subtle difference in peak width, which could result from increased disorder of the graphite network³¹. Nitrogen adsorption/desorption analysis (Supplementary Table 3) shows that the specific surface area increased from 183 m² g⁻¹ (CNT) to 288-348 m² g⁻¹ for DCNTs, which is advantageous for catalytic surface reactions.

Fig. 1. Defect engineering of CNT catalysts.

TEM images of M-DCNT (a), D-DCNT (b), and U-DCNT (c). XRD patterns (d), Raman spectra (e), and (f) EPR spectra of M-DCNT, D-DCNT, U-DCNT, and CNT. XPS spectra of M-DCNT (g), D-DCNT (h), U-DCNT (i), and CNT (j). In (g–j), A_C-sp³/A_C-sp² refers to the ratio of the area of the sp³ carbon peak to the area of the sp² carbon peak.

X-ray photoelectron spectroscopy (XPS) surface characterization results show that heat treatment at 800 °C burned off almost all N species (Supplementary Fig. 2 and Supplementary Table 4). In the high-resolution C 1 s spectrum, two dominant peaks at ~284.80 eV and ~285.50 eV, corresponding to C-sp² and C-sp³, are identified (Fig. 1g–j and Supplementary Fig. 3). Since the presence of C-sp³ signifies lattice disorder/defects in carbon-based materials, the atomic ratios of C-sp³ and C-sp² can be used as indicators to assess the relative defect content^20,21. The calculated A_C-sp3/A_C-sp2 was 0.39 for CNT, 0.66 for U-DCNT, 0.71 for D-DCNT, and 0.83 for M-DCNT. The order of defect density in DCNTs was further confirmed by Raman spectroscopy (Fig. 1e and Supplementary Fig. 4). The D and D’ signals in the Raman spectra of DCNTs correspond to defective C-sp³ configuration³², and the intensity ratio of the D and G bands (I_D/I_G) indicates the degree of defect. Data show that I_D/I_G significantly increased from 1.11 (CNT) to 1.32 (U-DCNT), 1.39 (D-DCNT), and 1.44 (M-DCNT), consistent with the degree of defects analyzed by XPS. It is noteworthy that the Raman intensity ratio of I_D/I_D’ is less than 7.0 for all DCNTs, verifying that the defects are mainly vacancy type^21,25, which could include Stone-Wales defect, single vacancy defect, double vacancy (DV) defect, and pentagon defect. Considering that Stone-Wales defects require stricter preparation protocols while single vacancy and pentagon defects have poor thermal stability under our synthesis temperatures, and readily convert to the more stable DV configuration^21,33, DV defects are deemed to be predominant in our synthesized DCNTs.

Electron paramagnetic resonance (EPR) spectra (Fig. 1f) show that signal peaks at the g-factor of 2.003 (another indicator of C defects) emerge in DCNTs, with the signal intensity of M-DCNT markedly surpassing those of the other catalysts. It is well-documented that unpaired electrons associated with defects can modulate the distribution of valence-electron density and interfacial electric field of the materials, thereby facilitating electron transfer at the interfacial regions²¹. Thus, the higher defect density is indicative of potentially superior catalytic performance. These results conclude that thermal treatment of NCNTs is effective in forming defects in CNTs, the type of N used can regulate the defect density of CNTs, and a maximum defect density has been generated with melamine as the N source.

Evaluation of catalytic activity

Fenton-like catalytic performance of DCNTs was evaluated by activating PMS for 4-CP removal, and compared with CNT and NCNTs. Control experiments suggest that the oxidation of 4-CP by PMS alone was negligible (Supplementary Fig. 5a), and the adsorption of 4-CP on DCNTs was less than 10% (Supplementary Fig. 5b, c). Compared to CNT and NCNTs, DCNTs dramatically increased the pseudo-first-order rate of 4-CP removal by PMS (Supplementary Fig. 5d, e). In addition, XPS spectra in Supplementary Fig. 2 show that almost all N species were removed during annealing treatment of NCNTs to generate DCNTs, ruling out the primary role of N-doping in the enhanced reactivity observed with DCNTs (Supplementary Fig. 5b). The removal of 4-CP increased with defect content in DCNTs (in the order M-DCNT > D-DCNT > U-DCNT > CNT, Supplementary Fig. 5c). The observed rate constant, k_obs, in reaction with M-DCNT reached 0.095 min⁻¹, 44.5 times higher than that of CNT (Fig. 2a), indicating substantial contributions of lattice defects to the activity of DCNTs. The correlation analysis between k_obs and defect indicators (i.e., A_C-sp³/A_C-sp² and I_D/I_G) further supports this conclusion. As shown in Fig. 2b and Supplementary Fig. 6, ln(k_obs) values are positively correlated with A_C-sp³/A_C-sp² and I_D/I_G, and the coefficient R² values are up to 0.95 and 0.93, respectively. All these correlation results emphasize the role of lattice defects, which can lead to substantial changes in electron transfer efficiency and, consequently, in Fenton-like catalytic performance.

Fig. 2. Evaluation of catalytic activity and defect effect on product selectivity.

a Observed rate constants for 4-CP removal by PMS catalyzed with CNT and DCNTs. b Correlation analysis between A_C-sp³/A_C-sp² of DCNTs and ln(k_obs) of corresponding PMS systems. c Comparison of the electron utilization efficiency (EUE) of the M-DCNT/PMS system reported in this study with chemical oxidation systems reported by other studies (Supplementary Table 5 for detailed information). (a) ref. ³⁷; (b) ref. ³⁸; (c) ref. ³⁹; (d) ref. ³⁹; (e) ref. ⁴⁰; (f) ref. ⁴¹; (g) ref. ³⁶; (h) ref. ⁴². d Schematic diagram of the process of extracting small molecule oxidation products and polymerization products from the reacted M-DCNT surface. e Possible transformation pathways of 4-CP in M-DCNT/PMS. f Gel permeation chromatogram (GPC) of 4-CP oxidized products obtained by elution from the surface of reacted DCNTs using toluene. g MALDI-TOF-MS spectra of 2,6-M-PhOH oxidized products obtained by elution with toluene from the reacted CNT and M-DCNT surfaces. h Correlation analysis between I_D/I_G of CNT, NCNTs, and DCNTs with DP_avg of 2,6-M-PhOH in the corresponding PMS oxidation system. Reaction conditions: [Cat.] = 0.1 g L⁻¹, [PMS] = 1.0 mM, [4-CP] = [2,6-M-PhOH] = 0.5 mM, initial pH = 7.4, and T = 25 ± 2°C. The error bars in a represent the s.d. (mean values ± s.d., n = 3), obtained from three repeated experiments.

The effects of pH and anions on the performance of M-DCNT/PMS in removing 4-CP were investigated. As shown in Supplementary Fig. 7a, removal of 4-CP by M-DCNT/PMS was highest at pH 9 and 7, and remained effective at pH 3 and 11. Anions (NO₃⁻, SO₄²⁻, Cl⁻, and HCO₃⁻) had little inhibitory effect on 4-CP removal (Supplementary Fig. 7b). Although HA (10 ppm HA) exerted some inhibition^34,35, 74.3% of 4-CP removal was achieved by M-DCNT/PMS within 60 min. The excellent performance of M-DCNT/PMS under wide-ranging pH and environmental conditions is significant for its application in real water treatment. Using surface water and groundwater (Supplementary Table 1 for detailed chemistry parameters) as the reaction matrix, experiments show that the efficiency of M-DCNT/PMS for 4-CP removal approached 99.9% within 60 min (Supplementary Fig. 7c).

To evaluate the reusability of the catalysts, M-DCNT was tested in multi-cycle experiments. After three consecutive treatment cycles, M-DCNT showed minor deactivation (Supplementary Fig. 7d), presumably due to the occupation of active sites or competitive reaction with the accumulating oxidative intermediates. Nevertheless, catalytic activity was fully recovered after annealing at 800 °C. Furthermore, it is important to note that the consumption of the oxidant in the M-DCNT/PMS system was only 0.881 mM when 74% total organic carbon (TOC) removal was achieved with 0.5 mM 4-CP. The electron utilization efficiency^33,36 can reach 551%, surpassing most reported Fenton-like catalytic systems^36–42 (Fig. 2c, Supplementary Methods 4, 5, and Supplementary Table 5).

Defect effect on product selectivity

Product type and distribution were identified during 4-CP oxidation experiments. The aqueous TOC linearly decreased with the removal of 4-CP (Slope = 0.79) (Supplementary Fig. 8), which differs from the typical degradation of phenolic compounds in advanced oxidation processes⁴³. Similar phenomena are frequently reported in polymerization-dominant oxidation systems, and such unusual TOC removal could be due to the transfer and accumulation of oxidative intermediates from the solution to the catalyst surface^9,19,44. The chemical oxygen demand (COD) results show that ~76.9% degradation products accumulated on the catalyst surface (Supplementary Fig. 9). As shown by thermal gravimetric analysis (TGA), the content of catalyst-bound organic products was 7.81% in M-DCNT (shown as weight loss, Supplementary Figs. 10, 11), suggesting that 4-CP was oxidized and attached to the catalyst surface. The weight loss is proportional to the defect density (in the order M-DCNT > D-DCNT > U-DCNT > CNT) (Supplementary Fig. 11), implying that an increase in defect sites enhances the generation of polymeric products. The oxidative products that accumulated on the catalyst fall into three categories—quinones, organic acids, and oligomers—identified by liquid chromatography-mass spectrometry (LC-MS) in the ethanol extracts (Supplementary Data 1 and Supplementary Figs. 12–14). The oligomers are mainly dimer products, typically produced via C–C/C–O coupling of phenolic compounds (Fig. 2e)^8,9,11.

In the toluene extracts, a brown gelatinous solid was obtained after drying (Fig. 2d). Gel permeation chromatography (GPC) results showed that the molecular weight of these gelatinous solids was extremely high and increased with defect density, ranging from 1501 Da for CNT to 2572 Da for M-DCNT (Fig. 2f). These data agree with the reported compounds generated from crosslinking polymerization of phenolic compounds⁴⁵. To further identify the role of defect density on polymer selectivity, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was used to analyze the crosslinked products. Unfortunately, the presence of more than one active hydrogen atoms⁹ on the benzene ring in 4-CP (i.e., ortho-positions of the hydroxyl group) leads to complex crosslinking modes, making the identification of the resulting polymer structures difficult, except for a small amount of partially identifiable chain-like polymers (Supplementary Fig. 15). Nonetheless, the higher m/z signals, corresponding to polymers with larger molecular weight, were highest with high-defect M-DCNT. This indicates that the defects in the catalyst promote the formation of longer-chain and higher-molecular-weight polymeric products from 4-CP, consistent with the GPC results (Fig. 2f).

To further understand the effect of defect engineering on product selectivity by DCNTs/PMS, 2,6-dimethylphenol (2,6-M-PhOH) was used as a chemical probe in product analysis. With only one active hydrogen site, 2,6-M-PhOH facilitates product identification and analysis⁴⁶. MALDI-TOF-MS analysis of oxidation products of 2,6-M-PhOH by DCNTs/PMS showed polyphenylene oxide (repeat m/z units of 120) resulting from specific C-O polymerization (Fig. 2g and Supplementary Fig. 16). In Fourier transform infrared spectroscopy (FTIR), the signals corresponding to aromatic C-O-C and aliphatic C-H carbon bonds were detected, further confirming the generation of polyphenylene oxide (Supplementary Fig. 17). For 2,6-M-PhOH, the oxidation products that can be readily eluted from the catalysts are mainly non-crosslinked. As shown in MALDI-TOF-MS (Fig. 2g and Supplementary Fig. 16), the chain lengths of polyphenylene oxide were significantly enhanced as the defects in DCNTs increased, as in 4-CP reactions. GPC confirmed the formation of highly polymerized products with an average molecular weight of ~2725 Da in the M-DCNT/PMS system, much higher than that in the CNT/PMS system (1810 Da) (Supplementary Fig. 18). To reveal the intrinsic relationship between defect density and the degree of 2,6-M-PhOH oxidative polymerization by DCNTs/PMS, the defect indicators (i.e., I_D/I_G and A_C-sp³/A_C-sp²) of CNT, NCNTs, and DCNTs were correlated with the average degree of polymerization (DP_avg) (defined in the Supplementary Method 6). DP_avg is positively correlated with I_D/I_G (Fig. 2h) and with A_C-sp³/A_C-sp² (Supplementary Fig. 19), with R² values of 0.92 and 0.81, respectively, indicating that a higher degree of defects induces a higher degree of polymerization.

Mechanism investigation

PMS activation mechanism

The nature of active oxidants and elementary steps for polymerization were investigated. Using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin-trapping reagent, signals corresponding to hydroxy radical (^•OH) and sulfate radical (SO₄^•−) were observed in the U-DCNT/PMS and D-DCNT/PMS systems (Fig. 3a). These radicals, however, were not detected in M-DCNT/PMS. Since M-DCNT had the highest defect density, a mechanistic shift from a radical to a nonradical pathway may have occurred. Experiments using methanol (MeOH) and tertiary butanol (TBA) show that 4-CP removal was decreased by the quenching of radicals ^•OH/ SO₄^•− in the U-DCNT/PMS and D-DCNT/PMS systems (Fig. 3b and Supplementary Fig. 20). In contrast, 4-CP removal was not affected by MeOH and TBA in the M-DCNT/PMS system. It can be concluded that the defect engineering of CNTs enhances PMS activation via the nonradical pathway. It is noteworthy that although the radical pathway contributes to 4-CP oxidation in the U-DCNT/PMS and D-DCNT/PMS systems, the nonradical pathway is dominant in all DCNTs/PMS systems, accounting for more than 80% 4-CP removal (Fig. 3b and Supplementary Fig. 20).

Fig. 3. Mechanism investigation.

a Electron paramagnetic resonance (EPR) spectra of DMPO-trapped radicals at 1 min and (b) effect of various scavengers on 4-CP removal in DCNTs/PMS systems. c EPR signal of CNT/PMS and DCNTs/PMS at 1 min using TEMP as the ¹O₂ trapping agent. d Amperometric i–t curves and (e) open-circuit potential curves upon adding PMS and 4-CP in DCNTs/PMS systems. f In situ Raman spectra of M-DCNT/PMS/4-CP in the liquid solution. g Schematic diagram of the single-chamber contact system (marked as system 1) and the two-chamber galvanic cell system (marked as system 2). h Concentration changes of 4-CP in the two systems that are shown in (g). In Fig. 3h, the abbreviation “w/o” represents “without”. (i) Relative current changes in the two-chamber galvanic cell system with DCNTs/PMS. Reaction conditions: [Cat.] = 0.1 g L⁻¹, [PMS] = 1.0 mM, [4-CP] = 0.5 mM, [DMPO] = [TEMP] = 0/100 mM (in a, c), [MeOH] = [TBA] = 60 mM (in b), [NaSO₄] = 20 mM (in d–e and g–i), initial pH = 7.4, and T = 25 ± 2°C. The error bars in (b, h) represent the s.d. (mean values ± s.d., n = 3), obtained from three repeated experiments.

While the free radicals enable strong oxidation, nonradical oxidation favors the polymerization pathway¹⁴. Since free radicals were not the main mechanism for 4-CP oxidation, 2,2,6,6-tetramethyl-4-piperidinol (TEMP) was employed as the trapping agent to confirm the occurrence of singlet oxygen (¹O₂), which is widely reported for nonradical pathway⁴⁷. As shown in Fig. 3c and Supplementary Fig. 21, typical triplet EPR spectra assigned to TEMP − ¹O₂ were detected in all DCNTs/PMS systems, suggesting the generation of ¹O₂. However, the signal intensity exhibited an opposite trend to the rate of 4-CP removal, indicating that ¹O₂ cannot be the main reactive oxygen species (ROS) for oxidation, at least not in the M-DCNT/PMS system (with the lowest TEMP − ¹O₂ signal). The same conclusion is substantiated by quenching experiments employing furfuryl alcohol (FFA) (Supplementary Method 2 and Supplementary Fig. 22). Beyond the ¹O₂, the CNT-mediated electron transfer pathway was explored as an alternative nonradical pathway implicated in heterogeneous Fenton-like catalysis^48,49. Electrochemical experiments were conducted to track charge migration using chronoamperometric i − t curves. As shown in Fig. 3d, PMS addition resulted in a significant negative current response, but no distinct current change occurred at the U-DCNT and D-DCNT electrodes as 4-CP was added. In contrast, at the M-DCNT electrode, a pronounced positive current was observed in response to 4-CP addition. These results imply that M-DCNT can serve as an electron mediator to facilitate electron transfer from 4-CP to PMS. Enhanced electron mobility was also supported by electrochemical impedance spectroscopy (EIS) Nyquist plots (Supplementary Fig. 23), in which M-DCNT had the smallest arc radii among the tested DCNTs. Defect engineering enhances the electron transfer ability of CNT: the more defects created in the material, the better electron transfer it achieves.

To confirm that electron transfer occurred via surface-bound PMS species, quenching experiments were conducted using potassium iodide (KI, a typical scavenger of surface-bound ROS) during 4-CP removal by M-DCNT/PMS (Supplementary Fig. 24). In the presence of KI, catalytic oxidation was completely inhibited, supporting the dominant role of surface-activated PMS. Additionally, chronopotentiometry was conducted to verify the involvement of the surface species. As shown in Fig. 3e, the open-circuit potential at the DCNT electrodes increased upon PMS addition and subsequently reached equilibrium potential, suggesting the formation of surface species by the interaction of DCNTs and PMS. As 4-CP was added, the potential immediately declined due to a reaction between the reactive surface species and 4-CP. Thus, the chronopotentiometry results confirm the involvement of reactive surface species in 4-CP removal. Additionally, surface species on M-DCNT had a lower potential than D-DCNT and U-DCNT (Fig. 3e), helping to prevent the further oxidative decomposition of intermediates and consequently favoring the polymerization reaction.

To identify the surface species, an in-situ Raman study was performed. Figure 3f shows that in addition to the characteristic peaks of HSO₅⁻ (at 878/1060 cm⁻¹) and SO₄²⁻ (at 982 cm⁻¹), an additional peak at 832 cm⁻¹ appeared as M-DCNT was added to the PMS solution. The new peak is assigned to the activated peroxo-species and has been reported in various PMS/catalyst systems^34,50. However, this 832 cm⁻¹ peak vanished once 4-CP was added to the M-DCNT/PMS system, supporting the involvement of surface PMS species during oxidation (Fig. 3f). To further validate the role of surface species, galvanic cell experiments were conducted. Although a significant current response was recorded in a galvanic cell upon the addition of PMS (i.e., oxidant) and 4-CP (reductant) into the cathode and anode chambers, respectively (Fig. 3g–i), the removal of 4-CP via redox reaction was negligible. In contrast, 4-CP removal occurred in a single chamber (not a galvanic cell system), where PMS and 4-CP came into contact. Therefore, in contrast to the conventional catalyst-mediated electron transfer pathway for oxidation, the M-DCNT/PMS system requires simultaneous contact of oxidant and reductant on the same catalyst surface.

Elementary steps of the polymerization reaction

Electron transfer between PMS and 4-CP can proceed via either one-electron transfer or two-electron transfer to generate polymerization precursors^51,52. If one-electron transfer occurred, direct abstraction of an electron from the aromatic ring would yield aromatic radical cations, which can further undergo electron-H elimination to produce aromatic radicals (i.e., phenoxy radicals). Phenoxy radicals are key intermediates to initiate coupling and polymerization reactions through C–O or C–C^6,53. However, in-situ EPR experiments showed no signal corresponding to phenoxy radical in the 4-CP/M-DCNT/PMS system (Supplementary Fig. 25), excluding the possible role of one-electron transfer. Alternatively, the two-electron transfer step would lead to the generation of phenoxonium ions^9,44. Through instantaneous resonance transfer, phenoxonium ions can form positively charged carbon centers on the catalysts, spontaneously triggering either C–O polymerization or C–C coupling reactions. To prove the involvement of phenoxonium ions, nucleophilic acetonitrile was selected as a scavenger, which readily reacts with electrophilic phenoxonium ions⁵⁴. The addition of acetonitrile partly inhibited 4-CP removal (from 100% to 49%) by M-DCNT/PMS, supporting the involvement of phenoxonium precursors (Supplementary Fig. 26). Additional spectroscopic evidence was obtained by monitoring the UV-vis spectrum change in 2,6-M-PhOH during oxidation because phenoxenium ions are known to exhibit strong UV-vis absorption. As shown in Supplementary Fig. 27, a significant signal at 414 nm appeared upon M-DCNT/PMS oxidation of 2,6-M-PhOH in MeOH solution (note that the signal was absent in aqueous solution due to the instability of phenoxonium in water⁵⁵). The same characteristic signals were detected in the Co₃O₄/PMS system, where phenoxenium ions are proven to be the major precursors for 2,6-M-PhOH oxidative polymerization⁹. These results suggest that two-electron transfer is the elementary step and that the phenoxonium ions serve as intermediates in the polymerization process.

Origin of the defect effect on the catalytic activity

To gain insight into the role of defects in Fenton-like activity and the polymerization process, density functional theory (DFT) calculations were performed. Owing to the large disparity in size (diameter of CNT ~ 20 nm versus the size of the PMS or 4-CP molecule at <1 nm), the curvature effect of the CNT structure is assumed to be negligible. Accordingly, defect-containing graphene was selected as a model system for calculations in this work. In addition, the DV defect was chosen as the model defect configuration, which has been verified as the predominant, stable, and most active site among various defect configurations of CNT in previous studies^21,33. The configurations with no, one, two, and four defect sites are denoted as no-defect, low-defect, middle-defect, and high-defect, respectively (Fig. 4a). The optimized configurations show that the introduction of defects can lead to distinct surface relaxation and C-rearrangement, enhancing the disordered C-sp³ configuration (Supplementary Fig. 28). The theoretical results are consistent with those obtained by TEM, Raman, and XPS experiments. Electrostatic potential (ESP) diagrams show that the no-defect graphene was negatively charged, except for the regions surrounding the carbon matrix and edge-H. With an increase in defect sites, the carbon support became more charge inhomogeneous, resulting in charge redistribution (Fig. 4a). Uneven charge distribution also induces polarization, presumably leading to the formation of a built-in electric field^27,29,56. Compared to the calculated dipole moment of 0.0004 D for the ‘no-defect’ model, a robust dipole moment of 3.8455 D was obtained for the ‘high-defect’ model (Fig. 4a). The strong electric field effect enables countering of the Coulombic forces to provide more free-flowing p-electrons, thereby facilitating electron mobility. Such a strong interfacial electric field (IEF) was further determined experimentally using Kelvin probe force microscopy (KPFM) and Zeta potential measurement. KPFM results (Fig. 4c-f) show that the surface potential of all DCNTs exceeded the CNT, with M-DCNT reaching a maximum of 53.4 mV. In the Zeta potential study, M-DCNT had the highest potential values among all DCNTs (Supplementary Method 7 and Supplementary Fig. 29). The increased surface charge density and Zeta potential collectively suggest the presence of a robust built-in electric field in DCNTs²⁸. Thus, the defect engineering of CNT enables an extra electric field effect, contributing to electron transfer and PMS activation. Critically, the correlations between IEF intensity and catalytic performance were established. As shown in Supplementary Fig. 30a, a strong positive correlation (R² = 0.87) exists between IEF intensity and 4-CP removal efficiency, confirming enhanced PMS activation under IEF. The linear relationship between IEF intensity and DP_avg (R² = 0.70) also suggests that defect-induced IEF promotes chain growth for polymerization (Supplementary Fig. 30b).

Fig. 4. Origin of the defect effect on the catalytic activity.

a Electrostatic potential maps for structures with zero, one, two, and four defect sites (left to right), plotted on the 0.001 a.u. (atomic units) isodensity surface. b Calculated frontier orbital diagrams. Yellow and blue contours correspond to electron accumulation and deletion, respectively. Surface potentials of CNT (c), U-DCNT (d), D-DCNT (e), and M-DCNT (f) detected by Kelvin probe force microscopy (KPFM).

Molecular orbital energy analysis also shows that the highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO-LUMO) gap for all models with defects is narrower compared to the no-defect model. The more defects in the material, the lower the gap energies in the material (Fig. 4b). Thus, the introduction of defects can substantially perturb electron distribution and decrease the HOMO − LUMO gap. Given that the HOMO of 4-CP (−0.59 eV) is more negative than the LUMO of M-DCNT (−3.67 eV), the electron transfer from 4-CP to M-DCNT is energetically favorable. Similarly, the HOMO of M-DCNT (−4.27 eV) is more negative than the LUMO of PMS (−2.61 eV), thus the electrons localized at the M-DCNT can be readily transferred to the PMS. The decrease in the HOMO–LUMO gap triggered by the defect also facilitates electron migration from the LUMO of DCNTs to their HOMO²¹. Thus, the difference in molecular orbital energy suggests electron transfer from 4-CP to PMS through DCNTs, where defects serve as an “expressway” to accelerate electron delivery. Collectively, the narrower HOMO−LUMO energy gap, the more free-flowing p-electrons, and the enhanced electron mobility induced by the field effect make DCNT-mediated electron transfer from 4-CP to PMS easier and faster, boosting Fenton-like activity.

Additionally, local electron attachment energy (LEAE)⁵⁷ calculation reveals that, except for edge regions, the introduction of defects enables the creation of additional electrophilic sites in the vicinity of defects (Fig. 5a). The optimized adsorption configuration indicates that PMS can be adsorbed only at the edge in the no-defect model (termed as Mode I). In contrast, the PMS molecule can be adsorbed not only at the edge regions (Mode II), but also at the surface over defect sites (Mode III) of DCNTs (four defect sites) (Fig. 5e). To shed light on electron transfer from the catalyst surface to PMS, charge density difference analysis was performed. The results demonstrate that much stronger charge transfer occurs at the defect-type electrophilic sites (Mode III) compared to the edge sites (i.e., Mode I and Mode II) (Fig. 5e). The corresponding Bader charge analysis shows that the number of electrons transferred from the carbon fragment to PMS is 0.08 e, 0.10 e, and 0.14 e for Modes I, II, and III, respectively. These results match well with the charge density difference. Moreover, the free-energy diagram (Fig. 5b–d) suggests that, in the no-defect model (Mode I), the potential to activate PMS through the one-electron transfer pathway to produce SO₄^•− is favorable, whereas in defect mode (Mode II and III), it is more favorable for PMS oxidation to form ¹O₂ via SO₅^•− and PMS self-decomposition through two-electron transfer. Mode III shows maximum energy release during PMS transformation compared to Modes I and II. These findings align well with the experimental observations and verify that defect density affects Fenton-like reaction mechanisms. In all Modes, the two-electron transfer pathway is preferred and is responsible for generating the polymerization precursor (i.e., phenoxonium ion).

Fig. 5. DFT calculations.

a Local electron attachment energy corresponding to models with zero, one, two, and four defect sites (left to right), plotted on the 0.004 a.u. isodensity surface. b–d) Free energy of PMS activation at different adsorption sites, represented by Mode I—edge site, defect-free; Mode II—edge site, and Mode III—defect site (four defect sites). e Charge density difference between PMS and CNT/DCNTs at different adsorption sites and Bader charge analysis with an isosurface level of 0.003 eÅ⁻¹. Yellow and blue contours correspond to electron accumulation and deletion, respectively. f The interaction between PMS and adsorption sites using a gradient model based on Hirshfeld partition (IGMH). The color bar indicates the sign(λ₂)ρ values, with blue representing prominent attractive interactions (e.g., hydrogen bonding), red indicating steric repulsion, and green exhibiting very weak van der Waals interactions. g Schematic surface catalytic pathways for oxidative coupling and polymerization by DCNT/PMS. In (b–f), atoms are represented by the following color code: S (yellow), O (red), C (gray), H (white), Cl (orange).

The analysis of noncovalent interactions in Fig. 5f indicates that defect-triggered electric fields can also control the behavior of adsorbed phenoxonium ions. An independent gradient model based on Hirshfeld partition (IGMH)^58,59 indicates that the phenoxonium ion is adsorbed on the carbon site of the no-defect CNT as a result of π − π conjugation and C-O interaction. Phenoxonium ion is also sorbed on defect-containing CNTs, but with much higher adsorption energy (Fig. 5f). Such differences in noncovalent interactions could stem from the defect-induced additional electric field, which enhances the adsorption potential⁶⁰ of the catalyst and retains/stabilizes the adsorbed phenoxonium ion near defect locations²⁷. The stabilized precursors experience increased local concentration at the defect sites, overcoming the kinetic limitation of low concentrations and promoting intermolecular coupling (chain growth). Based on the above discussion, defect-induced electric field can promote two-electron transfer by DCNTs and stabilize/retain phenoxonium precursors, thus directing phenol evolution toward polymerization during the catalytic oxidation process (Fig. 5g).

Environmental sustainability assessment and wastewater purification applications

The potential environmental impacts of using M-DCNT/PMS and other mineralization-driven oxidation systems for wastewater treatment were evaluated using life cycle assessment (LCA) analysis, from catalyst preparation to pollutant degradation (Supplementary Method 8, Fig. 6a, and Supplementary Fig. 31)⁶¹. The input materials and energy required to treat wastewater containing 1 kg 4-CP are listed based on lab-scale data (Supplementary Tables 6–9). Sensitivity analyses show that wastewater volume and catalyst preparation are the two main factors determining the environmental sustainability (expressed as global warming potential, GWP) of the M-DCNT/PMS system (Fig. 6b). Compared to the mineralization-driven oxidation processes, including Fenton (1786.91 kg CO₂ eqL⁻¹), oxygen doped CNT(O-CNT)/PMS²¹ (3555.87 kg CO₂ eqL⁻¹) and CuO/PMS⁶² (4721.45 kg CO₂ eqL⁻¹), the M-DCNT/PMS system (1014.54 kg CO₂ eqL⁻¹) produces the lowest carbon emissions (Fig. 6c, d and Supplementary Datas 2–7), benefiting from its low requirement of materials (catalyst and PMS) and high removal efficiency.

Fig. 6. Environmental sustainability assessment and wastewater purification applications.

a System boundaries for life cycle assessment (LCA) analysis of M-DCNT/PMS system. b Sensitivity analysis of changes in global warming potential (GWP) with varying input factors (see Supplementary Tables 6 to 9 for specific categories and values) in M-DCNT/PMS. c Comparison of carbon emissions of M-DCNT/PMS with the other three typical chemical oxidation systems (i.e., Fenton (Fe²⁺/H₂O₂), O-CNT/PMS, and CuO/PMS) a, ref. ²¹; b, ref. ⁶². d Impact scores of various descriptors in LCA for the four oxidation systems. Photographs (e), root length (f), and shoot length (g) of the wheat seedlings after 7 days of growth in DI water (blank), 4-CP solution, M-DCNT/PMS/4-CP treated solution, 7-day leaching solution of the reacted M-DCNT, and in water containing M-DCNT and reacted M-DCNT at 1.0 g/L. For the violin plots in (f, g): the circle represents the median; the thick bar represents the interquartile range (IQR); the whiskers extend to 1.5 times the IQR; individual points show outliers. Sample sizes: Blank (n = 19), 4-CP (n = 17), Treated (n = 19), Leaching (n = 19), M-DCNT (n = 19), Reacted M-DCNT (n = 19). h 72 h development of zebrafish embryos in the above six solutions. i Survival of E. coli in DI water (blank), 4-CP solution, M-DCNT/PMS/4-CP treated solution, and 7-day leaching solution of the reacted M-DCNT. j Diagram of the experimental equipment for a continuous flow system. k The removal of 4-CP by PVDF membrane only, M-DCNT/PVDF, and M-DCNT/PVDF/PMS systems, and TOC removal by M-DCNT/PVDF/PMS systems. Water flux results indicate that M-DCNT does not change the permeability of the membrane. l Schematic illustration of the M-DCNT/PMS system for the treatment of industrial phenol-containing wastewater and potentials for byproduct applications. Reaction conditions: [PMS] = 1.0 mM, [4-CP] = 0.5 mM in DI water with initial pH 7.4.

To assess the ecological compatibility of the M-DCNT/PMS system, toxicity assays were conducted using multiple biological models (Supplementary Method 9). Encouragingly, the toxicity of the treated solution was significantly decreased compared to the highly toxic 4-CP parent compound. In wheat seed germination experiments, the growth-promoting effects were observed in solutions containing the reacted M-DCNT and in the 7-day leaching solution, comparable to the effect of pristine M-DCNT, indicating the low toxicity and high stability of the polymerized products (Fig. 6e–g, Supplementary Table 10, and Supplementary Figs. 32, 33). The spent catalyst coated with polymeric products even promoted plant growth compared to the control, suggesting that it may potentially function as an artificial fertilizer. Zebrafish embryos exposed to the treated solutions showed no abnormalities or deaths over 72 h, unlike the lethal effects observed with 4-CP (Fig. 6h). Escherichia coli assays further verified the minimized toxicity of the treated solution, with a survival rate of 90.5% ± 5.57%, near that of the control. This is in stark contrast to the high lethality of the untreated 4-CP solution (10.4% ± 1.73% survival) (Fig. 6i). The 7-day leaching of the reacted DCNTs in DI water released negligible Cl^- (Supplementary Fig. 34) and organics that are detectable by LC-MS. All the results collectively demonstrate that the polymerization-driven strategy effectively reduces the ecotoxicological risks associated with 4-CP contamination.

The applicability of the M-DCNT/PMS system was also explored. A continuous flow reactor was constructed with M-DCNT/PVDF membranes (Fig. 6j, k), which were formed through vacuum-assisted filtration (Supplementary Method 10 and Supplementary Fig. 35). The M-DCNT/PVDF/PMS system was able to achieve >84% 4-CP removal after 18 h of continuous operation, with small flux variations and stable TOC removal. Energy dispersive spectra (EDS) characterization of the catalyst revealed the accumulation of a significant amount of chlorine on the catalyst surface after the reaction, suggesting the attachment of 4-CP transformation products onto the catalyst surface (Supplementary Fig. 36). Therefore, the M-DCNT/PMS system has shown efficient and stable operation performance, and thus has potential for polymerization removal of phenolic compounds from wastewater in a scaled-up system. While polymer/catalyst reuse (e.g., valorization, plant growth promotion) and safe disposal are currently conceptual (Fig. 6l) and the protocols are lacking, it provides future research directions in wastewater treatment, enabling simultaneous pollution abatement and resource recovery towards a carbon-neutral economy.

Discussion

In this work, defect engineering, i.e., tailoring the vacancy defect density of CNT catalyst, is shown to be a feasible technique to improve catalytic performance and regulate the chlorinated phenol removal pathway during Fenton-like reactions. In addition to reactive sites, defect-induced polarization provides an additional electric field to accelerate electron transfer, regulate ROS generation, and modify adsorbate binding energies. These combined effects, in turn, result in the regulation of product selectivity during the oxidation of organic pollutants. By enhancing the generation, stabilization, and aggregation of the precursor (i.e., phenoxonium ion), the defect-rich M-DCNT/PMS system enables the polymerization reaction to produce long-chain molecules, both kinetically and thermodynamically. More importantly, the DCNT/PMS system holds great potential for efficient chlorinated phenol removal at low oxidant consumption, high electron utilization efficiency (551%), and lower carbon emission (74% TOC attached to the catalyst surface), compared to existing mineralization technologies. In addition, LCA analysis, toxicity assessments, and continuous operation tests demonstrate that the developed oxidative polymerization system is environmentally sustainable and applicable in real-world scenarios. The polymerization products may also have potential as value-added resources. Despite all the advantages, the proposed polymerization strategy is currently far from being practical. Future studies on polymer selectivity, product separation and collection, and environmental safety are needed to facilitate its transition from the laboratory to engineering applications.

Methods

Chemicals

Details of the sources and purities of chemical reagents are provided in Supplementary Method 1.

Preparation of catalysts

Several types of NCNTs were synthesized using a ball-milling method. Briefly, industrial-grade CNT (0.45 g) and nitrogen compounds (1.35 g), namely, melamine, dicyandiamide, and urea, were placed in a grinding jar containing agate beads (36 g). The materials were mixed in a ball mill rotating at 450 r/min for 20 h, in forward and reverse directions every 30 min. The milled products were washed with alcohol and water to obtain NCNTs. To make DCNTs, the NCNTs were annealed at 800 °C for 60 min in a tube furnace with pressurized N₂ flow, heated at a rate of 10 °C · min⁻¹. The resulting materials were washed, dried, and ground for use.

Oxidation experiments

The catalytic oxidation experiments were conducted in a brown wide-mouth bottle (250 mL) at 25 °C. Catalysts (NCNTs, DCNTs, and CNT) (0.1 g/L) were added to 120 mL 4-CP solution (0.5 mM) and stirred for 30 min to achieve adsorption and desorption equilibrium. Oxidation began by adding PMS at predetermined concentrations to the reaction solution (with the catalysts). In some experiments, 2,6-M-PhOH was used as the target contaminant, and anions (NO₃⁻, SO₄²⁻, Cl⁻, and HCO₃⁻) and humic acid (HA) were used to study their interference in the degradation of 4-CP. Surface water from a lake at the Moganshan Campus of Zhejiang University of Technology, and groundwater from an ancient well in Ziyang Street, Hangzhou City (Supplementary Table 1 for details), were also used in the study. At given time intervals, samples were collected and immediately filtered through a 0.22 μm PTFE membrane for chemical analysis by high-performance liquid chromatography (Supplementary Table 2 for details). All kinetic experiments were conducted in duplicate or triplicate to ensure reproducibility.

Characterization and analytical methods

TEM was used to observe changes in morphology and structure resulting from defect engineering, and XRD was used to examine catalyst crystallinity. XPS was used to provide insights into changes in surface chemical composition and, together with Raman spectroscopy, for the atomic/lattice configuration of the DCNTs.

EPR measurements were employed to probe and quantify the generated ROS, with TEMP or DMPO as the spin trapping agent. The effect of radical removal was investigated by using MeOH and TBA in chemical quenching experiments. Surface-bound ROS were identified using KI and electrochemical analysis. Detailed information regarding instrument operating conditions of EPR measurements and electrochemical tests is provided in Supplementary Method 2.

Degradation product identification

The reaction system was enlarged by a factor of 5 to collect degradation products. After 90 min of reaction, the catalyst was collected by vacuum filtration. The products attached to the catalyst were sequentially eluted using ethanol and toluene to obtain eluent A and eluent B, respectively. Oxidation products in eluent A were analyzed using LC-MS. The brown soft solid obtained after drying eluate B was re-dissolved in tetrahydrofuran (THF), and the molecular mass of the dissolved product was determined by GPC. The THF solution was mixed with matrix compound, 2,5-dihydroxybenzoic acid (DHB). Subsequently, MALDI-TOF MS was used to analyze the crosslinked products dissolved in THF under negative ionization mode. The brown soft solid was also analyzed by FTIR. To quantify the amounts of organic products accumulated on the catalysts, TGA was conducted by increasing the temperature to 800 °C at a rate of 10 °C/min. Additional details are provided in Supplementary Method 2.

Computational details

All DFT calculations were performed using Gaussian 09 package (Revision D.01). Detailed information is in Supplementary Method 3.

Author contributions

F.H., Z.Y.W., and B.H.L. conceived and designed the study and supervised the project. C.K.Y. and X.P.H. performed the experiments and analyzed the data. Y. P. carried out the TEM and in situ Ramman characterizations. B.H.L. and C.K.Y. drafted the manuscript. C.C. and W.Q.G. provided constructive suggestions. F.H. and L.Y.L. revised the manuscript. All authors discussed the results and contributed to the final manuscript.

Peer review

Peer review information

Nature Communications thanks Mingjie Huang, Wen Liu and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Data availability

The data generated in this study are provided in the Supplementary Information/Source Data file. All data underlying this study are available from the corresponding author upon request. Source data are provided with this paper.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-025-65966-8.