Dynamic in-situ reconstruction of active site circulators for photo-Fenton-like reactions

Computational prediction of constrained site insertion and in-situ catalyst reconfiguration

Advanced computational simulations elucidated the enhancement of the intrinsic properties of in-situ catalyst reconfiguration, underscoring the crucial role of polyiodinated species intercalation into carbon nitride (CN) during photoreaction. Initially, four distinct model systems were conceptualized: bulk CN, CN with K⁺ and I⁻ insertion (CN-KI), CN with K⁺ and I₃⁻ insertion (CN-KI₃), and CN with K⁺, I⁻/I₃⁻ redox constrained interlayer insertion (CN-KI-I₃). These models underwent optimization via preliminary theoretical calculations (Supplementary Fig. 1) to assess the feasibility of in-situ CN reconstruction using I⁻/I₃⁻ redox mediators and to evaluate the impact and functionality of potassium and various iodine species within the CN interlayers. Electrostatic potentials and two-dimensional charge density maps were generated to illustrate the electron cloud distribution across these systems (Fig. 1b, c). The highly symmetric structure of bulk CN resulted in a uniform distribution of electron-deficient and electron-rich regions, correlating with its lackluster catalytic performance due to restricted carrier migration. In contrast, the CN-KI and CN-KI₃ configurations demonstrated that the strong electronegativity of I⁻ or I₃⁻ shifted electron clouds from the center of the heptazine ring to these ions, disrupting the original uniform charge distribution and creating new negative electron-rich regions. This modification significantly enhanced photo-generated carrier migration and intrinsic photocatalytic activity. Moreover, in the CN-KI-I₃ system, where both I⁻ and I₃⁻ were simultaneously introduced, the charge distribution varied even more markedly compared to CN-KI and CN-KI₃, indicating enhanced photo-generated carrier separation. These findings substantiate that multiple iodine species-constrained insertion significantly improves CN’s photoactivity, facilitating more effective oxidant activation.

Work functions for the various model systems were assessed to deepen our understanding of electron dynamics. The measured work functions for CN, CN-KI, CN-KI₃, and CN-KI-I₃ were 5.59, 4.31, 4.84, and 4.39 eV, respectively (Fig. 1d–g). These measurements reveal that the insertion of different iodine species between CN layers promotes charge transfer and electron excitation, resulting in increased carrier concentrations. Notably, clever modulation was achieved through the simultaneous introduction of I⁻ and I₃⁻. This process must be meticulously managed to prevent weak catalyst binding to free electrons, potentially leading to surface recombination of carriers if not promptly utilized. Density of States (DOS) calculations were conducted to analyze the variations in electronic band structures and band gaps across these model systems (Fig. 1h–k). The conduction band (CB) of bulk CN was formed by the hybridization of C and N atom 2p orbitals, while the valence band (VB) consisted primarily of N 2p orbitals. The introduction of iodine species significantly altered the band structure of modified CN.

For instance, introducing I⁻ or I₃⁻ in CN-KI and CN-KI₃ resulted in a downward shift of the CB and induced midgap energy levels within the band gap. CN-KI-I₃, which incorporates both I⁻ and I₃⁻, maintained a suitable downward shift of the CB while generating additional midgap energy levels. These new midgap states facilitated the transition of photo-generated electrons from the VB to the CB by lowering transition barriers, thereby enhancing the efficient participation of photo-generated carriers in the catalytic reaction. The formation energy of these model systems was further explored to verify their structural stability and the feasibility of in-situ reconstruction (Fig. 1l). The formation energies for CN, CN-KI, CN-KI₃, and CN-KI-I₃ were −3.81, −2.95, −3.16, and −5.52 eV, respectively. Higher formation energies for CN-KI and CN-KI₃ indicated less structural stability, presenting excellent opportunities for in-situ reconstruction. As expected, CN-KI-I₃ exhibited the lowest formation energy among the models, suggesting that the preparation of a “pre-catalyst” by inserting I⁻ and K⁺ into CN interlayers, followed by the in-situ introduction of I₃⁻ through subsequent photo-Fenton-like reactions for catalyst reconstruction, is thermodynamically favorable. Differential charge density calculations illustrated electron transfer pathways for CN-KI, CN-KI₃, and CN-KI-I₃ (Fig. 1m–o and Supplementary Fig. 2), with electron accumulation and consumption depicted in red-brown and green colors, respectively. CN-KI-I₃ demonstrated more robust and diverse electron movement compared to CN-KI and CN-KI₃, indicating enhanced intrinsic photoactivity. This enhancement could provide more electrons for oxidant activation, facilitating the generation of active species necessary for the efficient purification of contaminated water bodies.

In summary, these computational insights affirm the strategic design of “pre-catalysts” by constraining K⁺ and I⁻ between CN layers, demonstrating their robust intrinsic photocatalytic activity and the feasibility of in-situ introduction of I₃⁻ for effective catalyst reconstruction.

Comprehensive assessment of the activity and stability of predetermined systems

To validate the exceptional catalytic activity and superior photo-stability of the predetermined reaction system, synthesized samples were tested for their ability to remove the antibiotic sulfamethoxazole (SMX) by activating periodate (PI) (Supplementary Fig. 3). Notable variations in SMX removal efficiency under different experimental conditions were observed (Fig. 2a and Supplementary Table 2). SMX adsorption on CN-KI was minimal (Supplementary Fig. 4), and PI alone exhibited no oxidation capability, underscoring the challenge of spontaneously generating active species. In the PI/Vis and CN-KI/PI systems, SMX removal efficiencies were 11.02% and 12.05%, respectively, indicating that neither visible light alone nor CN-KI effectively activated PI (Supplementary Fig. 5a). Furthermore, the CN/PI/Vis system achieved a modest 15.42% efficiency for SMX, underscoring the low activity of bulk CN. However, the CN-KI/Vis system demonstrated a removal efficiency of 32.18%, attributed to the enhanced photocatalytic performance of the catalyst, albeit still limited. Remarkably, the CN-KI/PI/Vis system achieved an impressive 99.14% removal efficiency for SMX, demonstrating the substantial formation of reactive oxygen species (ROS). Additionally, the SMX removal correlated positively with PI consumption, confirming a close relationship between catalytic oxidation and PI activation (Supplementary Fig. 5b). The quantitative kinetic assessment revealed that the degradation pseudo-first-order rate constant (K) for SMX in the CN-KI/PI/Vis system was 0.5681 min⁻¹, 12.32 to 6.48 times higher than that in other comparative systems. Moreover, the mineralization capacity in the CN-KI/PI/Vis system was found to be 7.65–61.99 times greater than in other systems (Supplementary Fig. 6). SMX removal in the CN-KI/PI/Vis system also exceeded that in individual iodine-doped and potassium-doped samples by 5.21 and 56.81 times, respectively, highlighting the synergistic effect of iodine and potassium in enhancing the activity of bulk CN (Supplementary Fig. 7). These results indicate that dual-site constrained interlayer insertion of K⁺ and I⁻ could significantly enhance CN’s ability to activate PI under visible light irradiation.

a Degradation efficiency and kinetic rate constants for SMX in various systems. b, c Effect of different ions and oxidants on SMX removal in the CN-KI/PI/Vis system and efficiency across various pollutants. The bar chart represents the removal efficiency of SMX in the presence of various coexisting substances. All error bars in the figure represent the standard deviation from three replicate experiments. d, e Cyclical degradation of SMX in the CN-KI/PI/Vis system without oxidant replenishment. f, g Cyclical degradation of SMX in the CN-KI/PI/Vis system without catalyst replenishment. h Cyclical degradation of SMX in the CN-KI/PI/Vis system with replenishment of both oxidant and catalyst.

Further analysis of the effect of KI co-polymerization amount on the photocatalytic activation of PI showed that within a specific range, SMX removal was positively correlated with the amount of KI co-polymerization (Supplementary Fig. 8). The optimal K value for SMX removal was identified at CN-KI-4. However, higher levels of KI co-polymerization reduced efficiency, likely due to excessive KI disrupting CN’s intrinsic structure. Subsequent tests to assess the effects of catalyst and PI doses on SMX removal exhibited a similar trend (Supplementary Figs. 9, 10). Considering kinetic constants and cost-efficiency, the optimal conditions for future experiments were determined to be a catalyst concentration of 0.2 g L⁻¹ and an oxidant concentration of 1 mM.

In the context of optimized process parameters, the practical application potential of the CN-KI/PI/Vis system was extensively evaluated. This system demonstrated remarkable adaptability across a broad pH range of 5.0–11.0 (Supplementary Fig. 11), maintaining high and consistent removal efficiency across various water matrices, with different concentrations of co-existing anions and in diverse organic matter environments (Fig. 2b and Supplementary Figs. 12–20 and Supplementary Tables 7–10). Notably, it achieved rapid decolorization of Rhodamine B (RhB) under natural sunlight (Supplementary Fig. 21), and effectively activated various categories of structurally asymmetric oxidants to remove multiple types of organic pollutants (Fig. 2c and Supplementary Fig. 22). Additionally, the degradation kinetic constant of the CN-KI/PI/Vis system surpassed those reported for similar systems in the literature (Fig. 2h illustration and Supplementary Table 13), showcasing its robust activity and adaptability for wastewater remediation.

Subsequent evaluations focused on the recycling performance of the CN-KI/PI/Vis system under various conditions to assess the balance between catalytic activity and stability during the photoactivation of PI. Successive SMX degradation experiments, conducted without replenishing the oxidant, demonstrated that the CN-KI samples sustained five cycles of stable and efficient recycling (Fig. 2d). A notable decrease in efficiency during the sixth cycle was attributed to diminished oxidant concentration within the system. Intriguingly, the degradation rate constant (K) for SMX exhibited an increasing and then decreasing trend across these cycles, consistently surpassing that of the control system with the same oxidant amount (Fig. 2e, Supplementary Figs. 23, 24 and Table 14), highlighting a dynamic enhancement in the catalytic activity that effectively compensated for activity loss due to oxidant depletion. Additional recycling experiments, conducted without refreshing the catalyst, mirrored this pattern (Fig. 2f). Over 11 cycles, the system maintained an SMX removal efficiency exceeding 97.39%. The K value displayed an initial increase, followed by a decrease, with the ninth cycle still exhibiting higher activity than the first (Fig. 2g and Supplementary Table 15). These results confirmed the dynamic activity enhancement in the CN-KI samples, where the increased activity sufficiently offsets the loss caused by reduced catalyst content.

Encouraged by these promising results, the photo-stability of the CN-KI/PI/Vis system was further assessed under conditions of simultaneous catalyst and oxidant renewal. Throughout 30 consecutive cycles, the system consistently achieved SMX removal efficiencies exceeding 94.66% (Fig. 2h and Supplementary Table 16). The K for SMX removal demonstrated an initial increase, followed by a decrease and eventual stabilization, yet it remained above initial levels throughout the test period (Fig. 2h). This exemplary cycling performance redefines the concept of ‘stability’ within the realm of photo-Fenton-like wastewater purification, highlighting the CN-KI/PI/Vis system’s exceptional efficacy and resilience.

Identification of catalysts for precise synthesis and in-situ reconstruction properties

To ascertain whether the simultaneous enhancement of photoactivity and stability was associated with dynamic catalyst reconstruction, a comprehensive series of characterizations was performed on various samples. Scanning electron microscopy (SEM) was utilized to evaluate morphological differences among CN, CN-KI, and CN-KI-I₃ (used CN-KI) (Supplementary Fig. 25). The morphologies of CN and the various CN-KI samples were similar, and no significant morphological differences were observed in CN-KI-I₃, suggesting that morphological changes did not contribute to enhanced activity. X-ray diffraction (XRD) analysis further investigated the crystal structures of these samples (Fig. 3a and Supplementary Fig. 27). The CN diffraction peaks at 12.92° (100) and 27.48° (002) represent the in-plane and interlayer stacking of heptazine units, respectively⁴⁰. Upon the introduction of K⁺ and I⁻, the (100) plane’s diffraction peak disappeared, and the (002) plane’s peak shifted and weakened. These noticeable changes suggest that K⁺ and I⁻ were intercalated into the in-plane and interlayers of CN, disrupting the ordered structure of in-plane heptazine units and altering the interlayer interactions. Excessive K⁺ and I⁻ introduction severely weakened the (002) plane diffraction peak, indicating a significant structural degradation of CN and a corresponding decrease in photocatalytic activity.

a XRD patterns of CN, CN-KI, and CN-KI-I₃. b, c UV-vis DRS and PL spectra of CN and CN-KI. d Electrochemical impedance spectra of CN, CN-KI, and CN-KI-I₃. e, f Semi-in-situ XPS spectra of CN-KI. g, h Semi-in situ CV curves and electrochemically active surface area (ECSA) measurements for CN and CN-KI. Semi-in-situ photocurrent curves (i) and semi-in-situ UV-vis absorption spectra (j) of CN-KI. The error bands in the figure represent the standard deviation between three different time points. k Schematic illustration of dynamic reconstruction processes in the pre-catalyst CN-KI during the photoreaction.

Fourier Transform Infrared (FT-IR) spectra across various samples showed similar vibrational peaks, with characteristic vibrations at 820, 1100–1700, and 3000–3500 cm⁻¹, attributed to triazine ring units, C=N heterocycles, and N-H groups, respectively⁴¹ (Supplementary Fig. 28). A distinct peak at ~2178 cm⁻¹ in modified CN, indicative of the cyano group (-C≡N), likely resulted from co-polymerization with KI. Although FT-IR spectra revealed no significant differences between CN-KI and CN-KI-I₃, XRD analysis showed weaker diffraction peaks associated with the (002) crystal plane in CN-KI-I₃, suggesting altered interlayer interactions. Moreover, a smaller BET surface area and pore size in CN-KI compared to CN ruled out an increase in active sites (Supplementary Fig. 29 and Supplementary Table 17). Further Ab Initio Molecular Dynamics (AIMD) simulations confirmed the thermal stability of CN-KI and CN-KI-I₃. Both materials maintained their crystal structure integrity throughout simulations lasting up to 20 ps, effectively ruling out any alterations in activity caused by structural depolymerization of the catalyst (Supplementary Fig. 30). Therefore, the remarkable simultaneous enhancement of activity and stability in CN-KI-I₃ could be attributed to the catalyst’s internal dynamic reconstruction.

X-ray photoelectron spectroscopy (XPS) was used to further analyze the chemical composition and bonding structure of the samples. The XPS spectrum of modified CN, unlike that of pure CN, displayed the presence of K and I elements, along with C and N, indicating successful integration into the CN structure through co-polymerization with KI (Supplementary Figs. 31–35 and Supplementary Table 18–21). SEM-energy dispersive X-ray spectroscopy (SEM-EDS) confirmed a uniform distribution of C, N, K, and I across the samples (Supplementary Fig. 36). Additionally, high-resolution X-ray photoelectron spectroscopy (XPS) spectra of C 1s and N 1s provided detailed insights into the chemical environment within the samples (Supplementary Figs. 31–35). Distinct convolution peaks were observed for C 1s at ~288, 286, and 284 eV, corresponding to the N-C=N, C-NH_x/C≡N, and C-C bonds, respectively⁴². For N 1s, prominent peaks at about 400, 399, and 398 eV were associated with C-N-H, N-C₃, and C-N=C bonds, respectively¹⁵. These slight fluctuations in binding energy among different catalysts underscore the effective optimization of the catalyst’s electron density through modification, while preserving the fundamental structure of CN intact—a crucial factor in maintaining its performance. Notably, the modified CN exhibited an increased percentage of C-NH_x/C≡N compared to bulk CN, a change attributed to enhanced deprotonation kinetics of terminal amino groups. These observations highlight the impact of KI modification on CN, corroborating findings from FT-IR analyses. Additionally, shifts in the binding energies of K 2p and I 3d spectra across the various modified CN samples suggest that the K and I elements were homogeneously integrated into the CN structure, rather than existing as separate KI compounds. This integration was crucial in enhancing the photocatalytic activity and stability of the modified CN, demonstrating the transformative effect of dual-site insertion on the material’s properties.

Assessing the optical properties and carrier transfer kinetics of pre-catalysts

Optical properties are fundamentally connected to the catalytic activity of photocatalysts. The influence of KI co-polymerization on the electronic and optical properties of photocatalysts was elucidated using UV-visible diffuse reflectance spectroscopy (UV-vis DRS) (Fig. 3b). Compared to bulk CN, CN-KI exhibited significantly enhanced light absorption due to n-π* and π-π* electronic transitions. This enhancement suggests that the introduction of K⁺ and I⁻ effectively improves the catalyst’s light capture capabilities⁴³. Analysis of Tauc plots and Mott-Schottky (MS) plots revealed significant alterations in the bandgap structure and band edge positions of CN-KI (Supplementary Figs. 37, 38). Specifically, CN-KI exhibited a narrower bandgap (2.67 eV compared to CN’s 2.77 eV) and a lower CB position (−0.99 eV for CN-KI versus −0.73 eV for CN). These modifications demonstrate that KI co-polymerization not only enhances the photocatalyst’s light capture ability but also boosts its electron-donating capacity, thereby increasing the driving force for oxidant activation.

To further investigate the efficiency of photo-generated carrier separation, the impact of KI co-polymerization on PI activation by modified CN was assessed. Notably, CN-KI showed virtually negligible photoluminescence (PL) emission peaks compared to CN, clearly indicating a significant reduction in the recombination rate of photo-generated carriers (Fig. 3c). Time-resolved photoluminescence (TR-PL) measurements revealed that the average lifetimes of carriers were significantly reduced in CN-KI (0.62 ns) compared to CN (3.17 ns). This reduction in lifetime indicates rapid exciton dissociation, enhanced by the interlayer insertion of K⁺ and I⁻. Such enhancement could improve the migration and diffusion of photo-generated carriers, thereby boosting catalytic activity. Additionally, electrochemical measurements indicated a higher photocurrent response and a smaller arc radius in CN-KI compared to CN (Fig. 3d and Supplementary Fig. 39), confirming more efficient carrier transfer. The enhanced mobility of photo-generated carriers suggests that more electrons participated in generating active species, which facilitated the rapid purification of polluted wastewater.

Deciphering the dynamic I⁻/I₃
⁻ redox mediator reconstruction process

To elucidate the mechanism behind the dynamic activity enhancement of CN-KI, a series of semi-in-situ characterization analyses were conducted. Utilizing semi-in-situ XPS technology, we observed the formation and operation of I⁻/I₃⁻ redox mediators within the catalyst, providing crucial evidence for its dynamic reconstruction (Fig. 3e–h, Supplementary Fig. 40 and Tables 22–24). Specifically, we analyzed changes in binding energy, where an increase indicates electron loss, and a decrease suggests electron gain⁴⁴. Initially, the electron dynamics within the system were primarily driven by carbon and nitrogen, as evidenced by symmetric changes in their binding energies, reflecting a balanced electron gain and loss. Upon initiation of the catalytic reaction, I⁻ was oxidized by photo-generated holes to form I₃⁻. This oxidation process initially intensified, as indicated by an increasing percentage of I₃⁻, but eventually decreased and stabilized as the reaction progressed.

The observed fluctuations in the system, alongside the minimal leaching of iodine and the maintenance of a relatively stable total iodine content throughout the catalytic cycle, suggest that the I₃⁻, produced by the oxidation of I⁻, were reduced back to I⁻ instead of being expelled from the catalyst (Supplementary Fig. 41). The consistent increase in binding energy for I⁻ and the corresponding decrease for I₃⁻ provided direct evidence of their mutual conversion. Furthermore, the reduction rate was influenced by the concentration of I₃⁻, highlighting a dynamic interplay between the formation and conversion of I₃⁻. This interconversion resulted in significant changes in electron flow within the system during the mid-reaction stages, characterized by a sharp decrease in electron output from carbon and electron intake by nitrogen. In the later stages of the reaction, electron flow stabilized and was predominantly maintained by the interconversion of I⁻ and I₃⁻, as their binding energy changes remained relatively constant. Unlike the initial fluctuations observed for carbon and nitrogen, the binding energy changes for I⁻ and I₃⁻ exhibited a sustained symmetrical distribution, indicating a stable conversion between these species as the reaction progressed.

These findings validate the cyclical process where I⁻ generating I₃⁻, stabilizing into a dynamic I⁻/I₃⁻ redox mediator in the later stages of the reaction, culminating in the catalyst’s dynamic reconstruction. This reconstruction improved the migration efficiency and utilization of photo-generated carriers within the catalyst. The marked increase followed by a decrease in electrons gained by I₃⁻ clearly demonstrates the ability of the I⁻/I₃⁻ redox mediators to temporarily store electrons not immediately involved in the reaction and release them in subsequent cycles. This mechanism significantly enhances the photocatalytic activity and stability of the system.

The semi-in-situ cyclic voltammetry (CV) analysis provided compelling evidence for the dynamic reconstruction of the catalyst (Fig. 3i). The CV curves for CN remained consistent across various time scales, indicating minimal changes in the electrochemical active surface area (ECSA) (Fig. 3j). In stark contrast, CN-KI exhibited clear oxidation peaks indicative of the oxidation of I⁻ to I₃⁻, and reduction peaks corresponding to the conversion of I₃⁻ back to I⁻, all within the same time frame. As the photoactivation of PI progressed, these peaks became more pronounced and underwent significant changes, accompanied by a gradual expansion of the ECSA. Additionally, a stepwise enhancement of the photocurrent response was observed in the semi-in-situ i-t curves (Fig. 3k), vividly demonstrating the dynamic reconstruction of the “pre-catalyst” during the reaction through the spontaneous formation of I⁻/I₃⁻ redox mediators. This dynamic reconstruction of the catalyst led to a secondary photoactivity enhancement, evidenced by a more robust photocurrent response and reduced impedance in CN-KI-I₃ compared to CN-KI.

To further clarify the relationship between the in-situ formation of I⁻/I₃⁻ redox mediators and the catalyst’s activity and stability, we monitored the dynamic changes in I₃⁻ concentration across different cycles using semi-in-situ UV-Vis absorption spectroscopy (Fig. 3i and Supplementary Fig. 42). Initially, the I₃⁻ concentration increased and subsequently decreased, reaching equilibrium after 8 cycles. This confirmed the dynamic transformation of I⁻/I₃⁻ redox mediators, aligning with the semi-in-situ XPS findings. Furthermore, the dynamic trend of I₃⁻ concentration closely mirrored the changes in the K value of SMX degradation during the cycling treatment, confirming a positive correlation between these variables.

Overall, these results substantiate that the dynamically enhanced activity and exceptional photo-stability of the CN-KI/PI/Vis system stem from the in-situ reconstruction of the photocatalyst by the spontaneously formed I⁻/I₃⁻ redox mediators within CN-KI. This discovery provides valuable insights and opens more opportunities for designing efficient and durable CN-based photocatalysts.

Determination and quantification of active species in reconstructed systems

The satisfactory catalytic efficiency led us to identify the dominant active species within the system. We used EDTA-2Na, L-histidine, p-BQ, and TBA as scavengers, specifically targeting photo-generated holes (h⁺), singlet oxygen (¹O₂), superoxide anions radicals (^•O₂⁻), and hydroxyl radicals (^•OH), respectively⁴⁵ (Supplementary Fig. 43). When EDTA-2Na, L-histidine, and p-BQ were individually introduced to the CN-KI/PI/Vis system, SMX removal was significantly reduced by 60.82%, 86.47%, and 91.58%, respectively, highlighting the critical roles of h⁺, ¹O₂, and ^•O₂⁻. Conversely, the addition of TBA had no significant effect, suggesting a minor role for ^•OH. This pattern was replicated in the CN-KI/Vis system using different quenchers, reinforcing these findings (Supplementary Fig. 44).

Electron spin resonance (ESR) measurements further corroborated these findings. The TEMPO-h⁺ signal intensity decreased after illumination and increased upon adding SMX, demonstrating h⁺ generation in the CN-KI/PI/Vis system post-irradiation and its subsequent utilization in oxidizing SMX. Similar trends were observed with TEMP-¹O₂ and DMPO-^•O₂⁻ (Fig. 4a–c). The distinct signal peaks observed under both dark and illuminated conditions significantly enhanced the catalyst’s activation for PI. The addition of SMX notably reduced the signal intensities of ¹O₂ and ^•O₂⁻, confirming their role in SMX oxidation. Further investigation explored the potential formation of iodine-active species, such as IO₃^•, HOI, I₂, I₃⁻, and iodine disinfection by-products (I-DBPs) during PI activation (Supplementary Figs. 45–48). Among these, only IO₃^• was detected, and its concentration remained consistently low, confirming its limited role in the removal of SMX. Importantly, this minimized the possibility of I-DBPs formation, as iodine-active species like HOI, I₂, I₃⁻ were more reactive in oxidizing natural organic matter to form I-DBPs^46,47,48,49. To substantiate the limited formation of I-DBPs further, we analyzed the degradation products of the CN-KI/PI system both with and without the presence of humic acid. The results indicated that the degradation products were nearly identical under both conditions, effectively ruling out the likelihood of I-DBPs formation during SMX degradation in the CN-KI/PI/Vis system.

a–c EPR spectra depicting active species in the CN-KI/PI/Vis system. d–f Utilization rates of PI and ROS production at different stages of catalyst reconstruction. Electron density difference diagrams for CN/PI (g), CN-KI/PI (h), CN-KI₃/PI (i), and CN-KI-I₃/PI (j). k Energy diagrams for ¹O₂ production from PI decomposition on CN and CN-KI-I₃; I-O bond lengths in CN/PI and CN-KI-I₃/PI; adsorption energies for various PI-catalyst complexes.

To comprehensively assess the impact of dynamic catalyst reconstruction, we quantified PI consumption and the concentration of ROS generated during the process (Fig. 4d–f and Supplementary Figs. 49–52). Although the types of dominant active species remained consistent, PI consumption initially increased and then decreased as reconstruction progressed. Correspondingly, the concentration of the dominant active species, ^•O₂⁻, initially increased and then stabilized. These trends were attributed to the in-situ introduction of I⁻/I₃⁻ redox mediators in the reconstructed system, which enhanced the photocatalyst’s intrinsic activity. In contrast, the concentration of ¹O₂ initially increased, then decreased, and finally stabilized, a key factor in the observed reduction of the SMX degradation rate after the fourth cycle. Overall, these trends correlate well with the fluctuations in I₃⁻ concentration during various cycles (at different reconstruction times) and the changes in the K value for SMX degradation across cycles. These results illustrate the in-situ dynamic reconstruction of CN-KI during reactions, which directly enhances its activity. This enhancement enables the newly formed catalytic system to generate more ROS than the pre-catalytic system, ensuring both high activity and exceptional stability.

Using theoretical calculations, we further explored the mechanisms behind the enhanced catalytic activity and photo-stability of the CN-KI(I₃)/PI/Vis system. Key factors such as the adsorption energy (E_ads) of PI molecules on the catalyst and the I-O bond length are critical for PI activation. E_ads indicate the thermodynamic feasibility of PI binding to the catalyst, reflecting the strength of their interaction, while the I-O bond length suggests the ease with which PI can decompose. The adsorption energies (E_ads) for CN, CN-KI, CN-KI₃, and CN-KI-I₃ with PI were measured at −1.41, 1.94, −2.08, and −2.19 eV, respectively (Fig. 4k). Notably, PI exhibited a suitable E_ads on CN-KI-I₃, suggesting that the moderate binding affinity between PI and the catalyst played a crucial role in enhancing catalytic activity and stability. Differential charge density maps visually illustrated these interactions, revealing more localized electron clouds and diverse electron movement pathways in CN-KI-I₃/PI (Fig. 4g–j and Supplementary Fig. 53). These observations suggest that the CN-KI-I₃/PI model exhibited higher electron mobility and faster reaction rates post-binding. However, the shortest I-O bond length in the CN-KI-I₃/PI model indicated a less favorable pathway for PI decomposition (Supplementary Fig. 54). Additionally, further energy barrier calculations for the production of ¹O₂ from PI decomposition challenged initial assumptions. In the rate-determining step (*IO₄⁻ → *IO₃⁻ + *O), the energy required for CN-KI-I₃ was unexpectedly higher compared to CN, CN-KI, and CN-KI₃. This finding contrasted with the high reaction rate constants observed in the CN-KI-I₃/PI/Vis system (Fig. 4k and Supplementary Fig. 55). Interestingly, the utilization efficiency of PI remained low across all designs. Additionally, the production trends of ^•O₂⁻ and ¹O₂ in the CN-KI(I₃)/PI/Vis system were closely aligned. Crucially, quantitative analysis of IO₃⁻ (IO₃^•) revealed that the consumed PI was almost entirely converted to IO₃^•, with no contribution to ^•O₂⁻ and ¹O₂ production (Supplementary Fig. 45). This suggested an alternative catalytic mechanism where the primary sources of the dominant active species, ^•O₂⁻ and ¹O₂, originated from the reduction processes of dissolved oxygen rather than PI decomposition. The contrasting behavior in SMX degradation under N₂ and O₂ atmospheres observed in the CN-KI/PI/Vis system further supported this catalytic mechanism (Supplementary Fig. 56).

Given these surprising findings, we proposed a revised catalytic mechanism for the reconstructed systems (Eqs. 1–5 and Supplementary Fig. 58). In this updated model, PI functioned not as a traditional oxidant but as an electron acceptor. This role was substantiated by the increased electrical current observed upon its complexation with CN-KI-I₃ (Supplementary Fig. 59). The van der Waals interaction between PI and CN-KI-I₃ formed a complex that significantly enhanced the intrinsic catalytic activity, facilitating the oxygen reduction reaction to generate more ^•O₂⁻ and ¹O₂. Together with the accumulated photo-generated holes, these reactive species effectively purify contaminated water, providing a robust and efficient mechanism for wastewater treatment.

Environmental adaptability assessment and outdoor scale-up applications

After elucidating the mechanisms behind the remarkable photoactivity and stability of the CN-KI/PI/Vis system, we evaluated the toxicity of SMX degradation products. Degradation pathways were inferred from liquid chromatography-mass spectrometry (LC-MS) analyses (Supplementary Figs. 60, 61 and Supplementary Table 29). Furthermore, the ecotoxicity of all degradation intermediates was predicted using the ECOSAR model and the Toxicity Estimation Software Tool (T.E.S.T)⁵⁰ (Fig. 5e and Supplementary Fig. 62 and Supplementary Table 30). The findings demonstrated that the acute (LC₅₀), chronic (CHV) toxicity, mutagenicity, bioaccumulative toxicity, and developmental toxicity of most intermediates were lower than those of the parent pollutant, indicating that the SMX removal process in the CN-KI/PI/Vis system effectively reduced toxicity. Additionally, real toxicity evaluations of the degradation products were conducted through biological assays, including cultures of Escherichia coli, Zebrafish embryo cultivation, and wheat seed germination tests (Fig. 5a–d and Supplementary Figs. 63–65). Zebrafish embryo hatching experiments, utilizing water treated by various systems as the exposure solution, revealed lethal symptoms in embryos exposed to the original SMX and those treated with the CN/PI/Vis and CN-KI/Vis systems. This indicates substantial animal toxicity in the original SMX and inadequate detoxification by these systems. In contrast, Zebrafish embryos developed comparably to the control group in the CN-KI/PI/Vis system, affirming its efficacy from a zoological perspective. Similarly, in wheat seed germination tests using water treated by various systems, only seeds irrigated with water from the CN-KI/PI/Vis system exhibited root and shoot lengths comparable to the control, further validating its detoxification efficiency. Additionally, in Escherichia coli culturing experiments using detoxified water as the medium, the survival rates in water treated with the original SMX, CN/PI/Vis, CN-KI/Vis, and CN-KI/PI/Vis were 12.66%, 19.82%, 33.21%, and 89.27%, respectively. These comprehensive evaluations from predictive, animal, plant, and bacterial perspectives confirmed the effective detoxification of SMX by the CN-KI/PI/Vis system.

The survival rate of Zebrafish embryo development (a), growth of wheat (b) and the survival rate of Escherichia coli (c, d) in different systems. e Toxicity indicators of parent SMX and its degradation products. In the violin plot, the box represents the interquartile range (IQR) between the first and third quartiles, with the points inside the box indicating the mean value. The whiskers represent the minimum or maximum values within 1.5 times the IQR from the first or third quartile. Additionally, the error bars in (c) represent the standard deviation of three replicate experiments. f, g Indoor continuous flow photocatalytic reactors and its photocatalytic purification performance for RhB and SMX.

To address the challenge of recovering powdered catalysts from liquid phases, 10 mg of the CN-KI catalyst was immobilized on a sponge within a micro continuous-flow device that served as the reactor (25 cm²). Wastewater was channeled through “U”-shaped tubing at a flow rate of 11.56 mL min⁻¹ to assess photocatalytic oxidation (Supplementary Fig. 66). Benefiting from CN-KI’s favorable in-situ dynamic reconstruction properties and the functionalization mechanism of PI, this setup consistently achieved 100% removal efficiency for RhB and at least 94% SMX removal during a continuous 24-h operation. This performance demonstrates CN-KI’s exceptional long-term operational potential in immobilized systems (Fig. 5f, g).

To further evaluate the application potential of CN-KI in real-world settings, a larger continuous flow reaction device was constructed, measuring 900 cm² with a flow rate of 20.21 mL min⁻¹, and its oxidation performance was tested under natural sunlight (Supplementary Fig. 68). The optimized wastewater treatment system consistently achieved over 90% SMX removal across a sunlight irradiance range of 15.53–78.38 mW cm⁻² (10 AM-8 PM). After sunset, despite the light intensity dropping to 2.23 mW cm⁻², the catalytic degradation of SMX continued, supported by the enhanced light absorption capacity of CN-KI and improvements in photocatalytic performance due to in-situ dynamic reconstruction. This capability was further validated by effective degradation under LED illumination at 4.41 mW cm⁻² (Supplementary Fig. 69), demonstrating that LED-driven removal could effectively address the limitations of photocatalytic technology’s nocturnal inoperability.

Evaluating the purification efficiency of real wastewater is essential for validating and optimizing proposed photocatalytic systems, offering significant reference value for industrial scale-up. To this end, simulated chemical wastewater was initially prepared using high-performance liquid chromatography effluent. The real-time removal efficiency of SMX was assessed using a plate-type continuous flow reactor with an effective area of 1 m², powered solely by sunlight (Fig. 6a–c and Supplementary Figs. 70, 71). Over a continuous 12-h operation, the system achieved satisfactory purification results at various flow rates, treating up to 28.8 L of wastewater and consistently maintaining SMX removal above 80% and TOC removal efficiencies above 48.67%.

a Solar-powered square meter-scale plate continuous flow reactor. b, c The removal efficiency of simulated chemical wastewater in a square-meter-scale plate continuous flow reactor at different flow rates. d–f Pilot treatment devices for simulated chemical, real medical, and coking wastewater. g Cyclic purification performance of the solar-driven pilot system for simulated chemical wastewater. h Purification performance of different catalytic systems for simulated chemical wastewater. i Removal of COD, NH₃-N, and TOC medical and coking wastewater by the pilot treatment unit. All error bars in the figure indicate the standard deviation from three replicate experiments. j Comparison of EE/O for different catalytic systems. k System boundaries for LCA analysis of CN-KI/PI/Vis system. l Sensitivity analysis of different stages in the CN-KI/PI/Vis system’s lifecycle. m Comparison of carbon emissions of different catalytic systems.

Subsequently, a pilot-scale reactor was developed to further evaluate the industrial scalability of the CN-KI/PI/Vis system. This reactor, which included a 20 L tank and 300 mg of floating CN-KI catalysts, was tested using simulated chemical wastewater, medical wastewater from a hospital in Chongqing, China, and coking wastewater from Chongqing Iron and Steel Company (Fig. 6d–f and Supplementary Fig. 72). The floating design of the CN-KI catalysts significantly enhanced light penetration and mass transfer at the gas-liquid-solid three-phase interface, promoting rapid generation of active oxygen species via the oxygen reduction reaction. Within 30 min of reaction time, 97.01% of SMX was removed from the simulated chemical wastewater, with a TOC removal rate of 74.94% (Fig. 6g–i and Supplementary Table 31). Due to the enhanced mass transfer and the dynamic reconstruction of the CN-KI/PI system, the reactor demonstrated the ability to continuously treat at least 80 L of wastewater without needing to replace the catalyst or oxidant, maintaining a removal efficiency above 87.98%, thereby demonstrating significant potential for industrial application.

After 60 min of treatment with medical wastewater, the TOC removal rate reached 73.79%, and the chemical oxygen demand (COD) decreased from 100.95 mg L⁻¹ to 20.95 mg L⁻¹, achieving a removal efficiency of 79.25% and meeting discharge standards. Ammonia nitrogen (NH₃-N) was reduced from 6.32 mg L⁻¹ to 4.89 mg L⁻¹, with a removal efficiency of 22.57%, which further mitigated environmental risks (Fig. 6i and Supplementary Fig. 73). After 2 h of treatment in the pilot-scale reactor, the removal efficiencies for TOC, COD, and NH₃-N in coking wastewater were 57.25%, 61.56%, and 68.77%, respectively. TOC decreased from 355.6 mg L⁻¹ to 152 mg L⁻¹, COD from 8650 mg L⁻¹ to 3325 mg L⁻¹, and NH₃-N from 200.40 mg L⁻¹ to 62.58 mg L⁻¹. Additionally, there were notable improvements in watercolor and salinity (Fig. 6i, Supplementary Fig. 74 and Table 31). These significant advancements relieved technical and economic pressures on subsequent treatment processes, confirming the industrial feasibility and scalability of the CN-KI/PI system.

In conclusion, the CN-KI/PI system, leveraging the advantages of in-situ dynamic restructuring and effectively overcoming light penetration limitations through its innovative floating design, demonstrated high activity and broad applicability across various scenarios. Furthermore, the dynamic reconstruction of CN-KI could be extended to H₂O₂ photocatalytic production, demonstrating its potential for multifunctional applications, particularly in the synthesis of high-value products (Supplementary Fig. 75).

Sustainability evaluation and cost analysis for industrial applications

To assess the primary costs and environmental impacts of the constructed catalytic system, an economic analysis was conducted based on the Electrical Energy per Order (EE/O) concept, accompanied by a preliminary Life Cycle Assessment (LCA). These assessments aimed to provide strategic optimization guidelines for potential industrial expansion. For comparative purposes, classical Fenton and O₃-based systems were utilized to purify simulated and real chemical wastewater. Notably, EE/O has been defined as the electrical energy required to reduce the concentration of low-level pollutants by one order of magnitude^51,52. Although the total cost for indoor purification using the CN-KI/PI system was higher, the operational cost for the CN-KI/PI/sunlight system in the outdoor pilot-scale reactor significantly decreased to 0.0297 kWh L⁻¹, which was lower than that of the widely recognized O₃ (0.085 kWh L⁻¹) and H₂O₂/O₃ systems (0.064 kWh L⁻¹) (Fig. 6j and Supplementary Table 32). It was important to note that while the Fenton system had a lower operational cost due to its reliance solely on reagents, its purification efficiency was less effective (Fig. 6h and Supplementary Figs. 77, 78). This lower efficiency was primarily due to the high content of alcohol species in the wastewater, which competed with the ^•OH, diminishing their effectiveness. Moreover, this comparison highlighted that the CN-KI/PI system has a broader application range in water pollution control than the Fenton system. Focusing solely on the energy input from the light source, a comprehensive cost comparison was conducted between the CN-KI/PI/Vis system and reported photo-Fenton-like, iodine-based, and chlorine-based oxidation processes (Fig. 6j and Supplementary Fig. 79). The treatment cost for the CN-KI/PI/Vis system was notably low at only 0.405 kWh L⁻¹, significantly less than those reported for other photocatalytic systems. These comparative data underscored the CN-KI/PI/Vis system’s practical feasibility, particularly for large-scale applications.

The LCA⁵³ further quantified the environmental impact of the CN-KI/PI system throughout the water purification process, from catalyst preparation to pollutant degradation (Fig. 6k and Supplementary Fig. 80). Catalyst preparation and light source energy input stages exhibited relatively high environmental impacts, primarily due to the operation of high-power equipment. Sensitivity analysis pinpointed catalyst preparation as the dominant factor affecting the overall environmental pressure of the process (Fig. 6l and Supplementary Fig. 81). For future industrial applications, it is advisable to operate electrical equipment at full capacity and produce large batches of catalysts to minimize environmental impacts. The minor environmental impacted associated with light source energy input could be mitigated by adopting solar illumination. In practice, the developed outdoor pilot-scale reactor using the CN-KI/PI/Sunlight system significantly reduced all emission indicators. Carbon emissions for the CN-KI/PI system were substantially lower when compared with those for the Fenton, O₃, and H₂O₂/O₃ systems during the purification of the same wastewater, demonstrating its superior dynamic catalytic activity (Fig. 6m, Supplementary Figs. 82–89 and Tables 33–39). During the purification of same wastewater, the carbon emissions for the Fenton, O₃, H₂O₂/O₃, and CN-KI/PI/Sunlight systems were 2.11 × 10⁻⁴kg CO₂ eq L⁻¹, 0.120 kg CO₂ eq L⁻¹, 0.0721 kg CO₂ eq L⁻¹, and 0.137 kg CO₂ eq L⁻¹, respectively. This enhanced activity allowed the system to continuously treat at least 80 L of simulated chemical wastewater using free solar energy, further reducing its carbon footprint to 0.0376 kg CO₂ eq L⁻¹. Additionally, the CN-KI/PI/Sunlight system avoided the safety risks and subsequent costs associated with O₃ leakage and iron sludge precipitation and removal, positioning it as a preferred option for sustainable water purification technologies.

These detailed economic and environmental sustainability assessments confirmed the promising wastewater treatment potential of the CN-KI/PI system and provided strategic insights for future industrial-scale applications. By synthesizing large quantities of catalysts in a single batch, the long-term energy consumption of high-power equipment could be reduced. Furthermore, recycling the remaining PI in the catalytic system could enhance daily water treatment capacities and minimize environmental risks. Unreacted PI could also be adsorbed and reused to boost the photocatalytic activity of the CN substrate for superior H₂O₂ photosynthesis (Supplementary Fig. 90), aligning with global circular economy initiatives.