Dynamic oxygen vacancy engineering on CuO via refreshable catalytic surface for high-efficient water decontamination

Domain: chemistry
Published: 2026-01-07
Editorial outcome: major_revision

Abstract
Oxygen vacancies (Ov) on metal oxide surfaces exhibit high catalytic activity for activating peroxymonosulfate (PMS) in wastewater decontamination, yet their in-situ regeneration remains a significant challenge. This study successfully achieves in-situ real-time regeneration of Ov on CuO surfaces through simple alkali etching without interrupting the contaminant removal process. The surface hydroxyl groups introduced by alkali treatment significantly reduce the formation energy of Ov on CuO surfaces from 1.60 eV to 0.38 eV. Both experimental results and density functional theory calculations reveal that the high activity of CuO relies on the synergy of surface hydroxyl groups and Ov. This synergy increases the antibonding states below the Fermi level and the electron spin density of Cu near Ov, thereby promoting electron transfer from CuO to PMS. As a result, by just adding an equimolar amount of alkali relative to PMS in CuO/PMS system, the degradation rate constant of sulfamethoxazole (SMX) greatly increases by 42 times. The primary reactive oxygen species in this system are sulfate radicals and hydroxyl radicals. Furthermore, OH-/CuO/PMS system exhibits a long-term stability (>300 h) for SMX removal in a real water matrix. This work provides a highly executable method to in-situ real-time regenerate Ov on CuO surfaces, representing significant progress in the critical yet underappreciated field of catalyst regeneration.

Introduction
Residual pharmaceuticals in the secondary effluent of wastewater treatment plants constitute a significant source of toxic organic compounds in natural water bodies1–4. Peroxymonosulfate (PMS)-based advanced oxidation processes (PMS-AOPs) have attracted considerable attention due to their effectiveness in eliminating these residual pharmaceuticals5–8. However, the practical application of PMS-AOPs is limited by catalyst deactivation, which results from issues such as coverage by intermediate products, difficulties in regenerating active sites, and metal leaching9,10. Although significant efforts have been devoted to catalyst regeneration, most current methods involve off-site high-temperature treatments or the in-situ addition of cleaning agents after halting the reaction, resulting in complex and costly regeneration procedures11–13. Therefore, addressing the bottleneck of catalyst deactivation is pivotal for the practical application of heterogeneous activation of PMS.

A primary cause of catalyst deactivation in PMS-AOPs is the adsorption of organic compounds on the catalyst surface, particularly in the case of carbon-based materials14. These materials inherently exhibit strong adsorption capacities for organic compounds and can activate PMS to initiate the polymerization of organic pollutants, leading to the formation of tenacious adhered layers15. In contrast, in radical-dominated PMS-AOPs (e.g., •OH and SO4•−), the adsorbed organic compounds can be mineralized and removed, thereby helping to maintain the exposure of active sites16,17.

Compared to carbon-based materials, transition metal oxides demonstrate higher effectiveness in activating PMS to generate •OH and SO4•−, along with advantages in cost efficiency and ease of preparation18–20. The catalytic activity of metal oxides in PMS activation originates from low-valent metals, surface hydroxyl groups, and oxygen vacancies (Ov)21–24. However, regenerating low-valent metals requires consuming a significant amount of oxidant7,25, while the relatively weak activity of surface hydroxyl groups often requires large catalyst dosages26,27. Alternatively, highly active Ov offer a more efficient alternative, requiring reduced amounts of both oxidant and catalyst28. As intrinsic anion defects, Ov are rich in localized electrons and exhibit high activity towards triggering PMS. Nevertheless, these vacancies are susceptible to occupation by external oxygen atoms, which can lead to their deactivation29,30.

Achieving in-situ continuous regeneration of Ov during PMS-AOPs operation, while simultaneously generating highly reactive species to remove organic deposits from the catalyst surface, could substantially lower catalyst regeneration costs. However, current research efforts remain predominantly focused on mitigating Ov poisoning rather than enabling their in-situ continuous regeneration29. Existing methods for generating Ov include high-temperature treatment31, chemical reduction32,33, heteroatom doping34, acid/alkali etching35, mechanical treatment36, and ultraviolet irradiation28, etc. Of these, only chemical reduction and acid/alkali etching can be conducted in-situ. However, chemical reduction generally necessitates interrupting the reaction and often involves hazardous reducing agents.

Clearly, acid/alkali etching represents the most promising approach for achieving in-situ continuous regeneration of Ov. Previous studies have reported that surface hydroxyl groups facilitate the formation of Ov30,37. Thus, the in-situ addition of alkali for the regeneration of Ov during the operation of PMS-AOPs is pre-explored. After screening a series of metal oxides, it is found that even a low alkali addition can significantly increase the number of Ov on the CuO surface. Based on this discovery, our study systematically investigates the effectiveness of the alkali/CuO/PMS system in removing organic pollutants, the mechanism of Ov formation on the CuO surface under alkaline conditions, and the mechanism of PMS activation in this system. The applicability of this system is also evaluated. This research offers particular insights and methodologies for the in-situ continuous regeneration of active sites in PMS-AOPs catalysts, thereby enhancing the practical applicability of PMS-AOPs.

Results
NaOH induced superior performance of CuO in PMS activation
The target CuO was commercial product with monoclinic structure (JCPDS no. 45-0937, Fig. 1a). Combined with additional evidence from the high-resolution transmission electron microscope (HR-TEM) image (Fig. 1b) and the corresponding fast Fourier transform (FFT) pattern (Fig. 1c), CuO was found to possess the main exposed crystal planes of (002), (111), and (311)25. Other characterizations, including scanning electron microscope (SEM, Supplementary Fig. 1), X-ray photoelectron spectroscopy (XPS, Supplementary Fig. 2) and N2 adsorption-desorption isotherm (Supplementary Fig. 3), provided additional detailed properties of this commercial CuO.

CuO rather than other metal oxides (such as Fe3O4 and Co3O4, Supplementary Fig. 4), was selected as the catalyst because only the performance of CuO in PMS activation could be greatly enhanced in the presence of NaOH. Using the commonly studied sulfamethoxazole (SMX) as the target pollutant, almost 100% of SMX was removed in the OH−/CuO/PMS system, while in the control groups, the removal rates of SMX were all lower than 20% (Fig. 1d). The poor performance of control groups excluded that the enhancement of SMX removal was due to adsorption or alkali-induced PMS activation. The apparent rate constant (kapp) of SMX removal in OH−/CuO/PMS was 4.2×10−3 s−1, which was 210, 70, and 42 times greater than that in the reaction system of PMS alone, OH−/PMS, and CuO/PMS, respectively (Supplementary Table 1). The dosage influence of alkali, CuO, and PMS was also explored to confirm optimal experimental conditions with the detailed results in Supplementary Fig. 5. Compared to the reported catalysts in recent years (Fig. 1e38–49 and Supplementary Data 1), OH−/CuO showed significant superiority in SMX removal. Apparently, OH−/CuO has great ability in PMS activation, which was further evidenced by the PMS consumption rate. At 20 min, the consumption rate of PMS in OH−/CuO/PMS system reached 84.52%, which was remarkably higher than that in OH−/PMS (3.43%) and CuO/PMS (15.86%) system (Supplementary Fig. 6a).

OH−/CuO showed a more significant performance than most of the studies in Fig. 1e and Supplementary Data 1 in terms of reusability and mineralizing SMX. In the OH−/CuO/PMS system, the TOC removal rate of SMX was as high as 65.9% after 20 min of treatment (Supplementary Fig. 7). In the presence of alkali, CuO exhibited satisfactory reusability, and specifically, the degradation rate of SMX decreased by only 5.28% in the fifth cycle (Fig. 1f). Besides, the kapp of SMX removal and PMS consumption rate also remained a high level during the reuse experiment (Supplementary Figs. 8 and 9). The introduction of alkali significantly reduced the leaching of Cu ions from 2.68 mg/L to below 0.05 mg/L, a concentration far lower than the standard limit of 1.0 mg/L set in China (GB3838-2002) (Fig. 1g). After five cycles, the CuO had no significant change based on the characterization by SEM (Supplementary Fig. 10a, b) and XRD (Supplementary Fig. 10c), indicating the good structural stability of CuO in the OH−/CuO/PMS system.

The OH−/CuO/PMS system demonstrated exceptional performance in removing a variety of organic pollutants, including known refractory compounds such as ibuprofen (IBP), quinoline (QNL), carbamazepine (CBZ), and atrazine (ATZ), implying that this was a radical-dominated system. As displayed in Fig. 1h, the removal efficiency of IBP, QNL, CBZ, and ATZ reached 94%, 100%, 100%, and 99%, respectively. The corresponding fitted first-order kinetic curves yielded apparent rate constants of 2.5×10−3 s−1, 4.0×10−3 s−1, 7.4×10−3 s−1, and 3.3×10−3 s−1 (Supplementary Fig. 11). In addition, when treating simulated wastewater containing 10 types of organic pollutants, all of them exhibited removal rates greater than 95% (Supplementary Fig. 12).

Active sites: surface oxygen vacancy from alkali in-situ etching
The enhancement of alkali on the performance of OH−/CuO/PMS system was not attributable to alkali activation and electrostatic adsorption (Supplementary Figs. 13 and 14). Although OH− in-situ etching increased BET surface area of CuO (Supplementary Fig. 3b–f), the enhanced activity of CuO did not originate from the change of BET surface area, as even after increasing the CuO dosage from 0.1 g/L to 0.3 g/L to increase the total BET surface area, CuO (0.3 g/L) still exhibited much lower activity than OH−/CuO (0.1 g/L) in triggering PMS to remove SMX (Supplementary Fig. 15). In addition, 0.2 mM OH− in-situ etching bring about similar BET surface area improvement for CuO with 0.4 mM OH−. These two points indicate that OH− enhanced activity of CuO was not due to the change of BET surface area.

In the presence of alkali, surface hydroxyl groups (−OH) tend to form on metal oxides, which are capable of activating PMS50,51. For the alkali-etched CuO, the Fourier transform infrared spectroscopy (FTIR) analysis revealed a positive correlation between the intensity of the −OH stretching vibration band (observed at approximately 3400 cm−1) and the increasing alkali concentration (Fig. 2a). A strong positive correlation was observed between the signal intensity of surface −OH and the degradation rate constant (kapp) of SMX, suggesting that surface −OH likely serve as the primary active sites for PMS activation (Fig. 2c Left). However, despite its abundant hydroxyl groups, Cu(OH)2 (characterized by XRD, Supplementary Fig. 16a, b) demonstrated significantly lower catalytic activity compared to OH−/CuO (Supplementary Fig. 16c). This observation strongly suggests the existence of additional crucial active sites beyond surface hydroxyl groups in OH−/CuO.

Previous studies have shown that surface −OH could help in the generation of Ov on the TiO2 surface30. Therefore, solid electron paramagnetic resonance (EPR) spectroscopy was carried out to detect Ov on CuO surface, revealing a positive correlation between the signal intensity of Ov (g=2.003) and alkali dosage (Fig. 2b)34. To quantify the relative concentration of Ov on the CuO surface, XPS analysis was further employed (Supplementary Fig. 17). The relative ratio of Ov was summarized in Supplementary Fig. 18 Left24. As the alkali concentration increased, the relative content of Ov increased from 24.62% (for 0 mM OH−) to 25.24% (for 0.2 mM OH−), 28.93% (for 0.3 mM OH−), and 37.58% (for 0.4 mM OH−), respectively. The ratio of low-valence copper increased from 65.26% to 66.90%, 73.43% and 74.37%, respectively (Supplementary Fig. 19), which is attributed to the loss of lattice oxygen in the formation of Ov28,52. Furthermore, the formation of Ov was also evidenced by thermogravimetric analysis that the mass loss of CuO decreased along with the OH− concentration (Supplementary Fig. 20), since the oxidation process of Ov in O2 atmosphere under high-temperature would reduce the mass loss53–55.

The strong positive correlation between the kapp of SMX removal and Ov content provides further evidence of Ov as the primary active site for PMS decomposition (Fig. 2c Right and Supplementary Fig. 18 Right). In contrast, Cu(OH)2 with no observed Ov (Supplementary Fig. 21), had much lower activity than OH−/CuO (Supplementary Fig. 16c). However, the above evidence was still insufficient to fully establish the key role of Ov. Thus, an Ov quenching experiment was further carried out. After pre-aerating O2 for 2 h to consume Ov in OH−/CuO system, the removal of SMX was significantly inhibited (Fig. 2d), proving that Ov was indeed the main active site. This also indicates that surface −OH alone was not the active site, since its concentration remained unchanged while Ov was significantly reduced under an O2 atmosphere (Supplementary Fig. 22). Furthermore, no significant inhibition of SMX removal was observed after the pre-aeration of N2, reflecting that the generation of reactive oxygen species (ROSs) for SMX removal did not depend on the dissolved oxygen in water56. Based on the results of density functional theory (DFT) calculations, the surface −OH significantly reduced the formation energy of Ov from 1.60 to 0.38 eV (Fig. 2e). Therefore, based on the experimental and theoretical results, it is reasonable to conclude that alkali in-situ etching leads to the formation of surface –OH groups (Fig. 2a), which in turn facilitate the generation of Ov (Fig. 2e). Additionally, another CuO sample, denoted as CuO(002) with a higher (002)/(111) crystal plane intensity ratio in the XRD pattern than the original CuO, was prepared (Supplementary Fig. 23a). CuO(002) exhibited similar activity with the present CuO in the removal of SMX (Supplementary Fig. 23b), indicating that the ratio decreases of (111) plane did not significantly change the activity of CuO. The addition of OH− also enhanced the formation of surface −OH and Ov for CuO(002) (Supplementary Fig. 24), and OH− can enhance the formation of Ov on (111) plane as well (Supplementary Fig. 25), indicating that both the (002) and (111) planes interact effectively with OH−. Consequently, the simple addition of alkali enables the in-situ continuous regeneration of Ov, ensuring the high activity of CuO. Based on the quantitative analysis of Ov concentrations (Supplementary Table 2), the Ov concentration at 0.4 mM OH− was about 1.6-fold than that without OH−. Furthermore, the Ov formation energy on CuO surface with the assistance of OH− was just 0.38 eV (Fig. 2e), while it was 1.69 and 1.60 eV for Fe3O4 (Supplementary Fig. 26) and Co3O4 (Supplementary Fig. 27), respectively. EPR tests also confirmed that Ov was much easier to form on the surface of CuO than on the surface of Fe3O4 and Co3O4 (Supplementary Fig. 28 and Supplementary Table 2), which should be the reason that the activity of Fe3O4 and Co3O4 cannot be enhanced by OH− (Supplementary Fig. 4).

The pre-treated CuO, namely alkali off-site etching, could enhance the removal of SMX as well, but its activity was much lower than that of alkali in-situ etching (Fig. 2f). This could be attributed to the fact that after the consumption of pre-formed surface −OH and Ov, there was no alkali to regenerate Ov and surface −OH (Fig. 2g, h). It is noteworthy that the used CuO (OH− off-site etching) still had relative EPR signal intensity of Ov, which was similar to that of CuO subjected to 0.3 mM OH− in-situ etching, but the latter achieved a significantly higher removal rate of SMX (Supplementary Fig. 29). The results of the XPS assay and analysis further verified the above conclusion (Supplementary Figs. 30 and 31). Compared to the remarkable residual of Ov (Fig. 2g), the surface −OH of the used CuO (both OH− in-situ etching and OH− off-site etching) was almost consumed (Fig. 2h). It seemed that although Ov served as the key active site, its high catalytic activity required the synergistic assistance of surface −OH. This mechanism will be further elucidated in the DFT-based mechanistic study of PMS activation. In addition, higher OH− dosage bring about higher surface −OH and Ov on CuO (Supplementary Fig. 32), but resulted in relative lower activity, as evaluated by kapp (Supplementary Fig. 5a) and PMS consumption (Supplementary Fig. 6b). This is likely due to the electrostatic repulsion, since PMS (HSO5−) is negative charged, and more OH− on the CuO surface imparts a more negative charge to it.

Identification of major reactive oxygen species and their contributions
In OH−/CuO/PMS system, •OH and SO4•− were identified as the key ROS. The potential roles of other ROS commonly involved in PMS-based advanced oxidation processes (AOPs) were systematically excluded through a series of experiments. No EPR signal of O2•− was found in all conditions, and there were similar EPR signal intensities of 1O2 for all conditions (including background value, namely PMS itself without catalyst) (Supplementary Figs. 33 and 34), indicating that O2•− and 1O2 were not the key ROS for SMX removal. Even in the atmosphere of O2, there was no signal of O2•− and no enhanced signal of 1O2, indicating that O2 indeed did not contribute the ROS formation (Supplementary Fig. 35). In the presence of alkali, there was a much stronger EPR signal intensity of •OH and SO4•− (Fig. 3a). To further verify the role of •OH and SO4•−, quenching experiments were carried out, in which, the dosage of quenchers was calculated based on Eq. (1) to achieve completely quenching23.

kquencher,ROS × Cquencher ≥ 500 × kantibiotic,ROS × Cantibiotic (1)

Tert-butyl alcohol (TBA, quencher of •OH) and methanol (MeOH, quencher of both •OH and SO4•−) could make the degradation rate of SMX decrease from near 100% to 51.8% and 7.4%, respectively, indicating that both •OH and SO4•− contributed a lot in the removal of SMX. Although furfuryl alcohol (FFA, quencher of 1O2) had a certain degree of inhibition on SMX removal, this inhibition effect should be ascribed to the consumption of •OH and SO4•− by FFA since the high reaction rate between FFA and these two radicals (Supplementary Table 3). Tetrachloromethane (CCl4), as a quencher of O2•− (kCCl4, O2•− = 1.1×109)57, did not affect the reaction rate, further excluding the involvement of O2•− (Fig. 3b). In the studied range of OH− dosage (0.2–0.6 mM), the main ROSs had no change based on the results of quenching experiments (Supplementary Fig. 36) and EPR tests (Supplementary Fig. 37). The generation of •OH and SO4•− increased along with the OH− concentration from 0 to 0.4 mM. A further increase in OH− concentration resulted in a slight decrease in SMX removal and a reduction in the generation of •OH and SO4•−. This decline may be attributed to enhanced electrostatic repulsion between the negatively charged PMS and the CuO surface, as higher OH− concentrations lead to a more negatively charged surface of CuO58–60.

Besides the aforementioned ROS (•OH, SO4•−, 1O2 and O2•−), high-valent metal and adsorbed PMS (PMS*) are often proposed as important contributors in PMS-based AOPs23,61. As the possible high-valent metal in OH−/CuO/PMS system, Cu(III) was detected by periodate colorimetric method (Cu(III)-periodate complex: 400~450 nm)62,63 and in-situ Raman spectroscopy (Cu(III): 600~620 cm−1)64,65, but the related characteristic peak was not observed (Supplementary Fig. 39 and Fig. 3c). Thus, the generation of Cu(III) could be ruled out. Regarding the electron transfer process (ETP) mediated by PMS*, this pathway was also excluded based on pre-mixing experiments, since the pre-mixing of OH−, CuO, and PMS led to significant inhibition of SMX removal (Supplementary Fig. 40). In this process, OH−, CuO, and PMS were pre-mixed at different times, and then SMX was added to initiate the reaction. ETP-dominated process usually does not consume the PMS in the absence of target pollutants; thus, the removal rate of pollutants will not decrease in the pre-mixing test66–70. The observed rapid decrease in SMX removal with increasing pre-mixing time indicates that ETP did not contribute to SMX degradation in the OH⁻/CuO/PMS system. This point was further verified by the i-t curve (Fig. 3d) since there was no observable current change after SMX addition. In fact, the quenching effect of methanol could rule out the role of Cu(III) and PMS*, but the above results provided more solid evidence.

To quantify the contribution of •OH, SO4•− and PMS direct oxidation to SMX degradation, the integral areas under the reaction kinetic curves—plotted as 1–C/C0 (Y axis) versus time (X axis)—were calculated and compared based on quenching experiment results. Herein, •OH, SO4•−, and PMS contributed 52.6%, 42.6%, and 5.1% to the removal of SMX, respectively (Fig. 3e).

The activation mechanisms of PMS in OH−/CuO/PMS system
The formation of Ov on the CuO surface through OH− etching constitutes a reduction process, while the consumption of Ov during PMS activation represents an oxidation process. Thus, the addition of alkali and PMS would cause an opposite current, which was proved well by i-t curves in Fig. 3d. The introduction of defect sites was usually favourable for electron transfer at the material surface71. The arc radius of Nyquist curves of CuO after being in-situ etched by alkali were all smaller than that of the initial CuO, in the order of CuO > 0.2 mM OH−+CuO > 0.3 mM OH−+CuO > 0.4 mM OH−+CuO (Supplementary Fig. 41a). According the fitted equivalent circuit model, the main electrical resistance was from the charge transfer resistance of the electrode, which decreased in the range of OH− concentrations from 0 to 0.4 mM (Supplementary Fig. 41b and Supplementary Table 4). It could be inferred that the existence of Ov attenuated the charge transfer resistance and boosted carrier transport72. A related key intermediate was further found at about 809 cm−1 in in-situ Raman tests (Fig. 3f). This new characteristic peak corresponded to the vibration of the O–O bond belonging to metastable Ov−•OOSO3, and this metastable species would be readily decomposed to produce SO4•−73,74. Noting that, although Ov existed in the untreated CuO, Ov−•OOSO3 was not observed. It should be that in the absence of OH−, Ov cannot efficiently regenerate after being consumed. It is noteworthy that although Ov sites were present in the untreated CuO, the Ov−•OOSO3 intermediate was not detected. This is likely because, in the absence of OH−, Ov cannot be efficiently regenerated once consumed.

The above points consolidated the important role of Ov and surface −OH. To further investigate their synergistic effect, four configurations (CuO, OH-CuO, Ov-CuO, OH-Ov-CuO in Supplementary Fig. 42) were employed in the DFT calculations. According to the projected crystal orbital Hamiltonian preoccupation (pCOHP) without adsorption of PMS (Fig. 4a Top), OH-Ov-CuO had the most antibonding states under Fermi level. This mean that OH-Ov-CuO is prone to lose electron for reducing the antibonding states to get stable state. In other words, OH-Ov-CuO is expected to have the strongest interaction with PMS, since PMS is an electron-deficient molecule that tend to accept electrons. This point was well reflected after PMS adsorption (Fig. 4a Bottom) because there was highest absolute value of integrated pCOHP (IpCOHP) (−1.32 eV) and shortest Cu-O (Cu and the O of PMS) bond length (1.82 Å). These results clearly demonstrate that OH-Ov-CuO has the strongest interaction with PMS.

As is well known, the d-band center is a robust descriptor for evaluating the interaction between a catalyst and an adsorbed molecule. Therefore, the d-band center of Cu was calculated to further analyze the interaction between the catalyst and PMS (Fig. 4b). In the absence of PMS, the Cu d-band center of OH-Ov-CuO was located closest to the Fermi level (–1.49 eV), indicating that the combination of surface −OH and Ov enhances the interaction between Cu sites and PMS (Fig. 4b Left)75. Upon PMS adsorption, the d-band center of the Cu atom in OH-Ov-CuO shifted the farthest, decreasing to −2.211 eV (Fig. 4b Right), demonstrating that both of surface −OH and Ov are responsible for the enhanced interaction of catalyst and PMS. In the presence of both surface −OH and Ov, there were the highest PMS adsorption energy (−4.13 eV) and the longest O–O bond of PMS (1.481 Å) (Fig. 4c), which were much higher than that being triggered by Ov or surface −OH itself. This further indicated that the synergy of surface −OH and Ov played key roles in PMS activation.

As reported, high electron spin density can facilitate PMS activation to produce radicals76,77. Analysis of the electron spin density distribution (Fig. 4d) revealed that the introduction of surface −OH significantly increased the electron spin density on Cu atoms near Ov sites. Metal centers with high spin electron density, characterized by delocalized unpaired electrons, promote electron transfer between the metal and PMS76,77. Based on charge density difference and Bader charge analysis (Fig. 4e), the synergy of surface −OH and Ov indeed promoted the electron transfer (0.237 e−) from catalyst to PMS. The results of transition state calculations showed that the synergy of OH-Ov-CuO had the lowest free energy barrier in triggering PMS to form radicals (Fig. 4f and Supplementary Fig. 43), which strongly indicate the synergistic effect of surface −OH and Ov in PMS activation. Increasing the number of surface −OH and Ov can result in lower energy barrier of •OH and SO4•− formation (Fig. 4f and Supplementary Fig. 44), which had a similar tendency to experimental results, namely the positive correlation between the activity of CuO and the content of surface −OH and Ov in the range of 0 to 0.4 mM OH−. Based on all of the results of DFT calculations, it is reasonably to deduce the process of PMS activation as that after grabbing electron from CuO, the O–O bond of PMS would be broken with the formation of •OH or SO4•− (Fig. 4g). On the whole, there were four steps in the process of OH− enhanced •OH and SO4•− formation, including OH− adsorption, Ov formation, PMS adsorption, and •OH and SO4•− formation, namely the combination of Figs. 2e and 4f. Compared to the other three steps, the energy barrier of •OH and SO4•− formation was highest, which can be regarded as the rate limited step.

Evaluation of application feasibility of OH−/CuO/PMS system
As known, •OH and SO4•− dominated AOPs usually deserve low efficiency to remove target pollutants in the application. In OH−/CuO/PMS system, HCO3− and NOM had significant inhibition on SMX removal among the studied coexisting substances (Cl−, SO42−, NO3−, HPO42−, HCO3−, Ca2+, Mg2+, and NOM) (Supplementary Fig. 45a–i). To guarantee the efficient removal of SMX in the practical application, the dosage of PMS (0.4–0.5 mM), OH− (0.4–0.5 mM) and CuO (100–400 mg/L) was increased (Supplementary Fig. 46). The need for remarkably higher CuO dosage suggested that the inhibition of HCO3− and NOM on SMX removal was due to their coverage of catalytic sites. Under the updated condition, the degradation rate of SMX could be up to 100%, 99.7%, and 99.1% within 40 min in tap water, Qinghe River water (surface water of Beijing), and secondary effluent of waste water treatment plant (WWTP), respectively (Supplementary Fig. 47a and Supplementary Table 5). In these three waters, the consumed PMS were 78.09%, 67.58%, and 69.89%, respectively (Supplementary Fig. 47b), and concentrations of leached Cu ions were all below 0.1 mg/L (Supplementary Fig. 47c). In addition, OH− itself (0.5 mM) or CuO itself (400 mg/L) still could not efficiently trigger PMS to remove SMX (Supplementary Fig. 48).

In the absence of externally added pollutants, various pharmaceutical and personal care products (PPCPs) were detected in the secondary effluent of a WWTP. Nearly all of these compounds were completely removed after treatment with the OH−/CuO/PMS system (Fig. 5a1 and a2). The characterization with 3D excitation-emission-matrix (EEM) spectra showed that proteins (IV region) and humic acid (V region) were the main organics in the secondary effluent of WWTP, which were effectively removed by the treatment of OH−/CuO/PMS system but not by other systems (OH−/PMS, CuO/PMS) (Supplementary Fig. 49). The TOC removal rate of secondary effluent was up to 60.9% (Supplementary Fig. 50a), and the indicator UV254, representing the natural humic macromolecules and aromatic compounds with C=C double bonds and C=O double bonds78, reduced by 51.67% (Supplementary Fig. 50b). In summary, the organics in secondary effluent could be well removed by OH−/CuO/PMS system. Furthermore, the leached Cu ions and consumed PMS were about 0.08 mg/L and 71.1%, respectively (Supplementary Fig. 50c). OH−/CuO/PMS system can efficiently remove the trace organics from other three actual secondary effluents of municipal WWTPs, without extra addition of organic pollutants as well (Supplementary Figs. 51, 52, and 54), even using a lower concentration of PMS (0.25 mM) (Supplementary Fig. 53). In addition, OH−/CuO/PMS system also perform well in treating actual secondary effluent from an industrial park (Supplementary Fig. 55). Thus, OH−/CuO/PMS system had good applicability in treating actual wastewaters.

To further evaluate the practical applicability of OH−/CuO/PMS system for treating the secondary effluent of WWTP, a continuous-flow reactor was employed (hydraulic residence time, HRT≈40 min) (Fig. 5b). In the secondary effluent, an extra 5 mg/L SMX was added. The results showed that this reactor operated stably for about 300 h, during which more than 95% removal of SMX was achieved (Fig. 5c) with average 64.3% TOC removal rate (Fig. 5d) and lower than 0.1 mg/L of leached Cu ions (Fig. 5e). The consumption rate of PMS was maintained at more than 84% (Supplementary Fig. 56), and the pH of the effluent was stabilized at around 7 (Supplementary Fig. 57). Obviously, OH−/CuO/PMS had the desired performance in treating the secondary effluent of WWTP. The used CuO for 15 d in a continuous-flow reactor was characterized. The micro-morphology and phase structure of used CuO did not change significantly, but the Ov and surface −OH groups were heavily depleted. (Supplementary Fig. 58 and Supplementary Table 2).

The toxicity of the effluent of the continuous-flow reactor was detected via luminescent bacteria (Vibrio fischeri), some plants (Brassica napus, Mung bean, Rice), and zebrafish embryos. Compared to the untreated one, the effluent behaved much lower inhibition to luminescent intensity of Vibrio fischeri (Fig. 5f) and the growth of plants (Fig. 5c inset for Brassica napus, and Supplementary Fig. 59 for Mung bean and Rice). As for zebrafish embryos, the untreated actual wastewater had no inhibition on its growth (Supplementary Fig. 60). Thus, there was no significant difference cultured in the untreated and treated actual wastewater, but which can indicate that the toxicity had no increase at least. Owing to the complex components of the secondary effluent of WWTP, it was hard to detect the degradation products of SMX. Thus, the analysis of the degradation products was carried in the simulated wastewater, namely, with a background of ultrapure water. According to electrospray ionization quadrupole time-of-flight mass spectrometry (ESIQTOFMS) qualitative analysis results (Supplementary Data 2 and Supplementary Table 6), 12 possible intermediates were detected, and three different degradation pathways were deduced according to the reaction sites (N-atom, aniline group, and S-N sites) (Supplementary Fig. 61). The toxicity of these intermediates was predicted by the Toxicity Estimation Software Tool (T.E.S.T.). All the intermediates of SMX had reduced developmental toxicity, while most of them were of negative mutagenicity (Supplementary Fig. 62a, b). And all the intermediates had lower bioaccumulation factors as well (Supplementary Fig. 62c). In brief, OH−/CuO/PMS system could significantly reduce the toxicity of SMX.

Methods
Chemicals
The target CuO (≥99.0%, CAS: 1317-38-0) used in this study was purchased from Macklin Biochemical Technology Co., Ltd. more details about characterization methods were described in Supplementary Text 1. PMS (Peroxymonosulfate, KHSO5•0.5KHSO4•0.5K2SO4), sodium thiosulfate pentahydrate (Na2S2O3•5H2O), tertiary butyl alcohol (TBA), methanol (MeOH), ethanol (EtOH), furfuryl alcohol (FFA), tetrachloromethane (CCl4), 5,5-dimethyl-1-pyrrolidinium N-oxide (DMPO), 2,2,6,6-tetramethyl-4-piperidinol (TEMP) were provided by Aladdin Reagent Co., Ltd. Sulfamethoxazole (SMX), atrazine (ATZ), ibuprofen (IBP), carbamazepine (CBZ), quinoline (QNL), potassium iodide (KI), and naphthol were purchased from Sinopharm Chemical Reagent Co., Ltd. For details of other reagents, please refer to Supplementary Text 2. All reagents used in the experiments were of analytical grade without further purification, which were prepared with ultrapure water (18.2 MΩ·cm) as solvent within the validity period. Stock solutions of PMS (0.1 M) and Na2S2O3 (0.1 M) were reconstituted once a week.

Catalytic activity experiments
Batch experiments were conducted with mechanical stirring (350 rpm) in a 100 mL beaker at 25 °C. The initiation of the reaction involved the sequential addition of NaOH, CuO, and PMS with predetermined concentrations to the solution containing the target contaminants. Samples were taken at given time intervals and filtered through polyethersulfone membranes (0.22 μm), quenched with excess Na2S2O3 (2 mM). The CuO after use was separated by a solvent filter, collected, and washed three times each with ultrapure water and ethanol, followed by drying for subsequent repeat experiments.

Statistics and reproducibility
All experiments and characterizations, including microscopy analyses, were performed in duplicate to ensure reproducibility and obtain reliable mean values with standard deviation. The linear relationship between the active site detection signal intensity and the catalytic reaction activity was fitted within a 95% confidence interval using a linear function of the form “y=ax+b”. The direction of the correlation (positive or negative) was determined by the coefficient “a”.

Corresponding analytical methods
The concentrations of the compounds of interest were determined on a high-performance liquid chromatography (HPLC, 1260 infinity III, Agilent, USA) equipped with an XDB-C18 chromatography column (4.6×150 mm, 5 μm), the specific assays were listed in Supplementary Table 7. The PMS concentration was determined according to the method recorded in Supplementary Text 3, and Cu ion dissolution was determined by a high-resolution inductively coupled plasma emission spectrometer (ICP, Plasma Quant PQ9000). Mineralization of contaminants was determined by a TOC meter at a reaction temperature of 850 °C and an oxygen pressure setting of 0.1 MPa. Detailed information on pollutant degradation efficiency and reaction kinetic calculations was provided in Supplementary Text 4. Other experimental procedures, including in-situ Raman tests, electrochemical measurements, reactive oxygen species identification, quantification and contribution calculations, and identification of intermediates, could be searched in Supplementary Text 5–14.

Calculation and DFT models
All the density functional theory (DFT) calculations were conducted using the Vienna Ab initio Simulation Package (VASP)79,80 to obtain relaxed geometries and total energies. The projector augmented wave (PAW) method was implemented to describe nuclei–electron interactions81,82. For exchange-correlation energy calculations, we employed the Perdew-Burke-Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) framework with a kinetic energy cutoff of 420 eV83–85. The computational model consisted of a four-layer p(3×3) CuO slab with periodic boundary conditions, where the two uppermost layers were allowed to relax during structural optimization. A 15 Å vacuum layer along the Z-axis was incorporated to minimize inter-slab interactions. Convergence criteria were established as follows: the electronic self-consistent field (SCF) iterations were terminated when energy differences fell below 1×10−5 eV/atom, while atomic positions were optimized until residual forces became less than 0.04 eV/Å. Charge density difference analysis was subsequently performed using the VESTA software package. The calculation method for formation energy of surface Ov, projected electronic structure analysis, and transition state reaction energy barrier were offered in Supplementary Text 15–17.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In the manuscript of 'Dynamic oxygen vacancy engineering on CuO via refreshable catalytic surface for high-efficient water decontamination (NCOMMS-25-28930)', an innovative alkali in-situ etching strategy was developed for the sustainable generation of oxygen vacancies on low-cost commercial CuO, enabling highly efficient peroxymonosulfate activation for degrading refractory pollutants. The in-situ etching approach using alkali has been demonstrated to be a cost-effective method of achieving continuous Ov replenishment on CuO surfaces. This approach has been shown to be effective in preventing catalyst deactivation through dynamic vacancy regeneration. The constructed OH-/CuO/PMS system demonstrated exceptional catalytic performance, attaining complete SMX removal and remarkably low metal leaching. The synergy of Ov and surface -OH is the origin of the enhanced catalytic activity of CuO with alkali in-situ etching, promoting electron transfer from CuO to PMS and reducing the formation energy barriers of radicals. Furthermore, a continuous-flow system with CuO demonstrated high SMX removal efficiency over 300 h in the secondary effluent from WWTP. This manuscript presents a method that is both practical and achievable, with the potential to regenerate Ov on CuO surfaces in situ and in real time. This development represents a significant advancement in the field of catalyst regeneration, which is of critical importance for environmental science.\nSpecific comments:\n1. The SMX concentration (5 mg/L) exceeds typical wastewater levels (ng/L - µg/L). Testing at environmentally relevant concentrations would strengthen applicability.\n2. Why CuO exhibited different phenomenon of Fe3O4 and Co3O4? This specific phenomenon requires further in-depth discussion to clarify the reasons.\n3. Pre-aeration with N2/O2 (Fig. 2d) suggests dissolved oxygen is irrelevant, but its role in radical propagation (e.g., O2→ O2•⁻) is not explored.\n4. Supplementary Fig. 4d, It is imperative to ascertain whether the dissolved metal ions are total Fe and Co.\n5. The DFT calculations in Fig. 4 show a synergistic effect, but the experiments have not directly verified the dynamic change of surface hydroxyl groups in the reaction (e.g., in situ FTIR), and additional relevant data are needed.\n6. Supplementary Fig. 18, more Ov were created with OH- in-situ etching. Thus, different concentration of OH- in-situ etching should be investigated to support the conclusion.\n7. Does higher OH- concentration than 0.4 mM in-situ etching lead to more oxygen vacancy in CuO? More efficient PMS activation and SMX degradation?\n8. Supplementary Fig. 32, different concentration of inorganic ions and NOM on SMX removal in the system of OH-/CuO/PMS should be investigated.\n9. Through the DFT calculations, the rate limited step of radicals generation in OH⁻/CuO/PMS system should be clarified.\n10. The EPR signal intensity of the oxygen vacancy (g = 2.003) in Fig. 2b is only expressed in terms of relative intensity and lacks absolute quantitative data (e.g., spin density calculations), which needs to be supplemented with quantitative analysis to be more convincing.\n11. DFT calculations assumed CuO(002) as the only exposed crystalline surface, but the material may show coexistence of multiple crystalline surfaces in the experiments (Fig. 1b), and it needs to be discussed whether the effect of crystalline surfaces on the oxygen vacancy formation energy is significant.\n12. The EIS Nyquist plots need to be fitted.\n13. Supplementary Fig. 26 excluded Cu³⁺ by UV absorption but did not consider the possibility of transient intermediates. Additional evidence is recommended for more rigorous validation.\n14. The abbreviation “OH-/CuO/PMS system” is used in the text and figures, while “NaOH/CuO/PMS” is used in the abstract), and the terminology needs to be harmonized throughout the manuscript.\n15. Supplementary Fig. 41, Figure should be numbered, mass spectral data for some degradation products do not show fragment ion match maps, secondary mass spectra are required to confirm structure. Supplementary Fig. 43, bottom left panel P13 m/z=98 does not match the mass spectrometry data m/z=90.85.\n16. Does the surface area and pore size distribution of OH- in-situ etching CuO change? The effect of this on the catalytic performance should be discussed.\n17. The pH of the system changed during the degradation process, would it be better if the alkaline pH is maintained by a buffer solution?", "reviewer_2": "In this manuscript, the author achieved in-situ real-time regeneration of Ov on CuO surfaces through alkali etching methods. Experiments and DFT calculations revealed the synergy effects of surface −OH and Ov, which promoted electron transfer in CuO/PMS system. Identification and contribution of reactive oxygen species were conducted and calculated. Generally, we personally think that the innovation of the article is limited, and the significance of the topic is not deep enough. Also, there are several questions which need to be considered.\n1. Alkali etching method was adopted to ensure the in-situ regeneration of Ov during the operation of PMS-AOPs. According to the experiment conditions, the ratio of oxidant (PMS) and alkali (NaOH) dosage was 1:1, and the concentration of PMS seemed to exceed the average level. Personally, the significance and the necessity of this work needs to be further considered.\n2. In NaOH Induced Superior Performance of CuO in PMS Activation section, the author compared the performance of different catalysts/PMS systems, and the -OH/CuO system shown competitive k(min-1) value. The question is that, based on what criteria are these catalysts selected for comparison.\n3. In figure 2f, the degradation efficiency of SMX with PMS activated by 0.4 mM OH− off-site etching CuO, during 20 min’s reaction, the removal of SMX was less than 60%. However, in Supplementary Fig.4c, in the unmodified Co3O4/PMS system, the removal of SMX was nearly 60%. Why does the author choose Cu system to explore the PMS activated topic? In fact, previous research has reported many types of high activity catalysts.\n4. In OH-/CuO/PMS system, •OH and SO4•- were identified as the key ROS. MeOH and TBA were selected as quenchers of reactive species. Basing on the quenching experiments, the author calculated the relative contributions of main ROS (Fig 3e). However, previous reports have revealed the controversial roles of organic quenchers (alcohols) in radical scavenging process (10.1021/acs.est.1c03796; 10.1021/acs.est.1c01578, et al). Moreover, competitive dynamics methods were used to calculate the contribution of •OH and SO4•- using NB and BA as probes. The question is whether this calculation method is suitable in heterogeneous catalytic system?\n5. In Evaluation of Application Feasibility of OH-/CuO/PMS System section, the author predicted and evaluated the toxicity changes through intermediates and Toxicity Estimation Software Tool (T.E.S.T.). However, without the detailed concentrations of different intermediates, this method of predicting toxicity changes has little practical significance.\n6. The authors need to review the entire document to correct multiple grammatical errors and polish the language of the translation.", "reviewer_3": "In this work, an innovative alkali in-situ etching strategy was developed for the sustainable generation of Ov on CuO, enabling highly efficient PMS activation for degrading refractory pollutants. The active species responsible for the SMX decontamination were identified, and the activation process was proposed using theoretical calculations. The authors attributed the superior catalytic activity of CuO to the synergy of surface hydroxyl groups and Ov. However, several issues remain to be addressed, mainly related to catalyst characterization and the proposed mechanism. Therefore, this reviewer can’t recommend the publication of this work in Nature Communications. The detailed comments are as follows:\n1. Is the number of hydroxyl groups on Cu(OH)2 the same as on OH-/CuO? If not, how can it be determined that it is a function of the other active sites?\n2. The surface -OH could help in the generation of Ov on the TiO2 surface. The author also found that there was a positive correlation between the signal intensity of Ov and alkali dosage. Please further explain the reasons for this enhancement.\n3. The surface -OH played an important role in decreasing the formation energy of Ov, however, the author emphasized that the high activity of CuO depended on the synergy of surface hydroxyl groups and Ov. I think it’s kind of misleading, because the surface -OH do not directly activate PMS.\n4. Line 214-216, it is difficult to understand why the inhibited removal of SMX with increasing premixing time would suggest that ETP was not the way of SMX removal in OH-/CuO/PMS system.\n5. Pollutants often co-occur in the secondary effluent from the WWTP. Since the practical applicability of the OH-/CuO/PMS system is being considered, the simultaneous removal of multiple pollutants should be conducted.\n6. What is the basis of the optimized models of CuO, OH-CuO, Ov-CuO, OH-Ov-CuO? How do you quantify the number of hydroxyl and oxygen vacancies?\n7. As the alkali concentration increased, whether an alteration in the types and concentrations of ROS occurs concomitantly?"}, "author_response": "Response to the comments from reviewers\nDear Reviewers:\nThank you for your comments concerning our manuscript entitled “Dynamic oxygen vacancy engineering on CuO via refreshable catalytic surface for high-efficient water decontamination” (Manuscript Number: NCOMMS-25-28930). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval.\nWe also respond point by point to the reviewer’s comments as listed below. Please find the comments in black, followed by our responses in blue and red for each.\nReviewer #1\nIn the manuscript of 'Dynamic oxygen vacancy engineering on CuO via refreshable catalytic surface for high-efficient water decontamination (NCOMMS-25-28930)', an innovative alkali in-situ etching strategy was developed for the sustainable generation of oxygen vacancies on low-cost commercial CuO, enabling highly efficient peroxymonosulfate activation for degrading refractory pollutants. The in-situ etching approach using alkali has been demonstrated to be a cost-effective method of achieving continuous O replenishment on CuO surfaces. This approach has been shown to be v effective in preventing catalyst deactivation through dynamic vacancy regeneration. The constructed OH-/CuO/PMS system demonstrated exceptional catalytic performance, attaining complete SMX removal and remarkably low metal leaching. The synergy of O and surface -OH is the origin of the enhanced catalytic activity of v CuO with alkali in-situ etching, promoting electron transfer from CuO to PMS and reducing the formation energy barriers of radicals. Furthermore, a continuous-flow system with CuO demonstrated high SMX removal efficiency over 300 h in the secondary effluent from WWTP. This manuscript presents a method that is both practical and achievable, with the potential to regenerate O on CuO surfaces in situ v and in real time. This development represents a significant advancement in the field of catalyst regeneration, which is of critical importance for environmental science.\nThanks for the positive evaluation in general. Now, we have carefully revised the manuscript to address all of your comments listed below.\n1. The SMX concentration (5 mg/L) exceeds typical wastewater levels (ng/L - µg/L). Testing at environmentally relevant concentrations would strengthen applicability.\nResponse: Thanks for your meticulous consideration in the application. In fact, this is a problem widely existing in the studies of this filed, namely using an organic concentration at mg/L levels. From where we stand, it is because the detection of organics at a level of ng/L - µg/L need HPLC/MS, which is not available for us when to detect thousands of samples. Thus, only in the experiments to treat actual wastewater, HPLC/MS was used to detect the change of trace organic pollutants in our original manuscript, and other two kinds of actual wastewater have been newly studied in the revised manuscript.\n(1) The related experiments in original manuscript.\nNoting that, although 5 mg/L of SMX was used in the main content of our manuscript, there was no extra addition of SMX in the experiment to treat actual secondary effluent of wastewater treatment plant (Fig. 5a1 and a2), in which the concentrations of the detected organic pollutants were at the levels of ng/L - µg/L. The results showed that most of the detected pollutants had high removal rate. The related content in the original manuscript is as follows (Line 341-344, Page 9): In the absence of externally added pollutants, various pharmaceutical and personal care products (PPCPs) were detected in the secondary effluent of a WWTP. Nearly all of these compounds were completely removed after treatment with the OH-/CuO/PMS system (Fig. 5a1 and a2).\nFig. 5 Practical performance evaluation and environmental impact assessment of the OH⁻/CuO/PMS system. a Treatment effect on residual PPCPs in secondary effluent from WWTP 618 with OH-/CuO/PMS system: a1 The detected concentration of PPCPs in secondary effluent before 619 and after treatment. a2 The removal rate of PPCPs in secondary effluent after the treatment.\n(2) The newly added experiments in the revised manuscript.\nTo further test the applicability of OH-/CuO/PMS system, actual wastewaters were taken from other two wastewater treatment plants as the studied object. And one of them was treated with two concentrations of PMS (0.5 and 0.25 mM, respectively). The new results have been added into the revised manuscript as follows (Line 352-355, Page 9):OH-/CuO/PMS system can efficiently remove the trace organics from other two actual secondary effluents without extra addition of organic pollutants as well (Supplementary Fig. 51 and Supplementary Fig. 52), even using a lower concentration of PMS (0.25 mM) (Supplementary Fig. 53).\n[... content truncated for brevity ...]"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have provided satisfactory responses to all the reviewers' comments and revised the manuscript accordingly.\nThe paper now meets the scientific and editorial standards of Nature Communications thus is ready for acceptance.", "reviewer_2": "Thanks to the authors for their efforts in addressing all my concerns and I think it could be accepted in Nature Communications.", "reviewer_3": "Following a thorough revision process, the manuscript was deemed to be in a more satisfactory state. The following issues are still under consideration prior to acceptance by Nature Communications:\n1. Since the author explains that the surface hydroxyl groups act as co-catalysts here, the phenomenon of surface hydroxyl depletion after the reaction must be considered. In theory, co-catalysts should not be consumed unless they become poisoned or deactivated during the reaction.\n2. Only the performance of CuO in PMS activation could be greatly enhanced in the presence of NaOH. So, how should the author consider the applicability of this technology?"}, "author_response": "Response to the comments from reviewers\nDear Reviewers:\nWe sincerely appreciate your time and insightful comments on our manuscript entitled “Dynamic oxygen vacancy engineering on CuO via refreshable catalytic surface for high-efficient water decontamination” (Manuscript Number: NCOMMS-25-28930A). We are particularly grateful for the overall positive feedback and the highly valuable new suggestions provided by Reviewer #3.\nWe have carefully considered all the points raised and have provided a point-by-point response below. The original comments are presented in black, and our corresponding responses followed in blue and red.\nReviewer #1 (Remarks to the Author):\nThe authors have provided satisfactory responses to all the reviewers' comments and revised the manuscript accordingly. The paper now meets the scientific and editorial standards of Nature Communications thus is ready for acceptance.\nResponse: We sincerely appreciate your positive feedback on our manuscript.\nReviewer #2 (Remarks to the Author):\nThanks to the authors for their efforts in addressing all my concerns and I think it could be accepted in Nature Communications.\nResponse: Thank you very much for your recognition on our manuscript.\nReviewer #3 (Remarks to the Author):\nFollowing a thorough revision process, the manuscript was deemed to be in a more satisfactory state. The following issues are still under consideration prior to acceptance by Nature Communications:\nResponse: We thank you very much for your thorough review and the valuable new comments. We appreciate the opportunity to further improve our manuscript.\n1. Since the author explains that the surface hydroxyl groups act as co-catalysts here, the phenomenon of surface hydroxyl depletion after the reaction must be considered. In theory, co-catalysts should not be consumed unless they become poisoned or deactivated during the reaction.\nResponse: We appreciate this valuable comment. As known, the commonly used PMS is a kind of complex salt (KHSO 0.5KHSO 0.5K SO ), which can release H+ into 5 4 2 4 water. The increased the surface hydroxyl groups caused by OH- addition will be consumed by the released H+ from PMS.\nIn fact, in the field of advanced oxidation processes, not all of the co-catalyst are stable, and some of them will be consumed (Cell Reports Physical Science 1, 100149, August 26, 2020). For instance, some organic acids (protocatechuic acid and ascorbic acid, etc.) have good co-catalytic effect in Fenton or Fenton-like systems, in which these organic acids play role to reduce Fe3+ to Fe2+. These organic acids can be easily degraded by the generated reactive oxygen species, such as •OH.\n2. Only the performance of CuO in PMS activation could be greatly enhanced in the presence of NaOH. So, how should the author consider the applicability of this technology?\nResponse: Thank you for raising this important question regarding the applicability of this technology. We agree that the OH--enhanced strategy is currently most effective for a part of Cu-based catalysts. Our evaluation of applicability thus focuses on two key aspects: (1) The applicability of OH-/CuO/PMS system in treating different wastewater, and (2) the applicability of OH- for enhancing the activity of different catalyst. Thus, this question is answered from the above two aspects.\n(1) The applicability of OH-/CuO/PMS system in treating different wastewater\nAs being stated in the Introduction, the aim of our study is to develop a technology for the advanced treatment of the secondary effluent of wastewater treatment plants. In our current manuscript, three kinds of actual secondary effluent from different wastewater treatment plants have been used to study the applicability of OH-/CuO/PMS system. In these actual wastewaters, OH-/CuO/PMS had desired performance in removing the residual organic pollutants, which can reflect the good applicability of OH-/CuO/PMS system in the advanced treatment of the secondary effluent.\nTo further validate the applicability of the OH-/CuO/PMS system in treating real wastewater, two additional actual wastewaters from Huanggang (Hubei Province) and Xinxiang (Henan Province) were investigated. Since the three previously studied real wastewaters were collected from southern (Guangzhou) and northern (Beijing) China, Huanggang—located in the central region—was selected to improve geographical representation. Moreover, whereas the wastewaters from Guangzhou, Beijing, and Huanggang are of urban origin, the Xinxiang sample was chosen as it originates from an industrial park.\nThe trace organic pollutants in the newly studied two kinds of wastewaters can be efficiently removed as well, further confirming the good applicability of OH-/CuO/PMS system in the advanced treatment of secondary effluent from wastewater treatment plants.\nAnd the related content in the revised manuscript is as follows (Line 352-358, Page 9): OH-/CuO/PMS system can efficiently remove the trace organics from other three actual secondary effluents of municipal WWTPs, without extra addition of organic pollutants as well (Supplementary Fig. 51, Supplementary Fig. 52, and Supplementary Fig. 54), even using a lower concentration of PMS (0.25 mM) (Supplementary Fig. 53). In addition, OH-/CuO/PMS system also perform well in treating actual secondary effluent from an industrial park (Supplementary Fig. 55). Thus, OH-/CuO/PMS system had good applicability in treating actual wastewaters.\n(2) The applicability of OH- for enhancing the activity of different catalyst\nTo be honest, this kind of applicability is weak, since we only found that the activity of very few catalysts (such as Cu2+, Cu(OH) , CuO) can be enhanced by OH-. Since 2 Cu2+ as homogeneous catalyst is hard to be recovered and the activity of Cu(OH) is 2 much lower than that of CuO in the presence of OH-, CuO was selected as the studied catalyst.\nAlthough the applicability of OH- enhanced method to other catalysts is weak, it brings about good enlightenment in developing high-efficient catalyst system based on the common and cheap catalysts. Namely, there might be other good chemicals or materials having ability to enhance the activity of common and cheap catalysts.\nWe are grateful for your insightful comments, which have been invaluable in improving our manuscript. We have carefully addressed all points raised and conducted the requested additional experiments. The revisions have significantly strengthened the paper, and we hope the manuscript now could get your approval."}, {"round": 3, "reviewer_comments": {"reviewer_3": "The paper has been significantly improved through revisions and is now acceptable for publication."}, "author_response": ""}]

Decision letter