Introduction

With population growth, industrial development, and climate change, freshwater scarcity has become a global challenge, affecting approximately 4 billion people and expected to worsen1,2. Chemicals containing electron-donating groups (EDG), such as -OH, -NH2, etc.3,4, are widely used in modern industries due to their high chemical reactivity, biocompatibility, and functionalization capabilities. Especially, industrial parks with intensive activities generate high-concentration wastewater containing electron-rich new pollutants (e-RNPs), which are difficult to remove and pose significant risks to biological treatment systems4,5,6. Conventional pre-oxidation process based on hydrolytic acidification can improve wastewater biodegradability7,8,9, but fails to meet increasingly stringent discharge standards due to long hydraulic retention times (HRT), odor production, sludge accumulation, and environmental sensitivity (temperature, pH, etc.). Advanced treatment methods are urgently needed to treat e-RNPs to increase freshwater supplies beyond the hydrological cycle and alleviate the water crisis. Recently, PMS-based advanced oxidation processes (AOPs) have gained attention as they can generate highly oxidizing radicals and selective nonradicals, enabling the efficient oxidation of e-RNPs without secondary pollution10,11,12,13. Unfortunately, complex water with coexisting ions can hinder the generation of non-selective radicals or cause false attacks, and the oxidative performance of single nonradicals in H2O is limited by their insufficient oxidation and extremely short lifetimes ( ~ 2 to 4 μs), thus restricting the degradation of target pollutants11,13.

To address these challenges, achieving nonradicals synergistic pathways through the regulation of PMS activation is an effective approach to avoid the inherent drawbacks of radical oxidation processes and improve the selective degradation efficiency of e-RNPs in complex aqueous matrices13,14,15,16. Nonradicals include: (1) 1O2, generated through the self-reaction of SO5•- (2SO5•- → 1O2 + 2SO42-) and the disproportionation of superoxide radicals (2O2•- + 2H₂O → 1O2 + H2 + 2OH-); (2) high-valent metal species, formed by the cleavage of O-H and O-O bonds within the M-PMS complex, accompanied by a two-electron transfer process; (3) mediated electron transfer (MET), in which the formation of a surface-adsorbed PMS* complex facilitates electron transfer while preventing excessive elongation of the O-O bond. For instance, Xu et al.17 increased the production of high-valent iron-oxo (FeIV=O) and 1O2 by precisely controlling Fe single-atom coordination, achieving efficient pollutant degradation in environmental matrices. Similarly, Xing et al.18 integrated Mo and Co active sites into a conjugated structure, where the catalyst promoted effective electron transfer and PMS activation, outperforming traditional Fenton-like systems. However, past systems often relied on single-atom catalysts to achieve two or more pathways, which are limited by the electron cycling of a single active center and the efficiency of carrier electron transfer. Compared to single-atom catalysts, adjacent heterogeneous metal atoms can effectively trigger synergistic effects between dual active centers, endowing them with new electronic structures and efficient reaction pathways. Recent studies have emphasized the critical role of dipole effects in regulating catalytic processes, where the uneven charge distribution within materials influences molecular interactions and reaction kinetics. In PMS activation, local dipoles within the catalyst can break electronic symmetry, forming positively and negatively polarized charge centers19,20,21. This not only facilitates charge transfer but also provides reactive sites for the redox reactions of PMS, further enabling a nonradicals synergistic pathway. Moreover, research on the promotion of PMS activation by dipole effects has not yet been conducted.

Enlightened by this, the work firstly breaks the pristine ZO charge symmetry via Co doping, thereby constructing a local dipole in the adjacent Zn and Co site regions for PMS activation. In situ characterization and theoretical calculations indicate that the Co active center with high charge density promotes a two-electron transfer to cleave the O-O and O-H bonds, leading to the formation of CoIV = O. Simultaneously, the positively charged polarized Zn site facilitates the self-decomposition of PMS molecules, generating 1O2. Consequently, relying on the synergistic action of 1O2 and CoIV = O, the ZOC/PMS system demonstrates high selectivity for e-RNPs, with a k-value as high as 73.93 min−1 M−1 that is 189.6 times higher than the ZO, which is significantly superior to the reported advanced catalysts. Furthermore, in an air-lifting internal circulating reactor (ALICR), the ZOC/PMS system exhibits excellent selective degradation for e-RNPs and significantly enhances the biodegradability of industrial parks wastewater from 0.29 to over 0.55, which also advances the development of efficient and sustainable water purification solutions.

Results

Characterization of ZOC

Scale-up synthesis of gram-level ZnO catalysts (25.5 g) was achieved using a sol-gel method with citric acid as a chelating agent. Under similar conditions, a series of ZnO with varying Co content was precisely synthesized by adjusting the amount of metal salt added (denoted as ZOCx, x represents the molar percentage of Co, x = 0, 1, 3, 5 and 7) (Fig. 1a) and the content of Co in the different synthetic samples was determined by the inductively coupled plasma-atomic emission spectroscopy (ICP) (Supplementary Table 1). Among all the samples, taking ZnO with 5% Co doping as the model sample (denoted as ZOC) due to its optimal performance, numerous characterizations were performed to investigate the structure and composition of the catalyst22.

Fig. 1: Synthesis and structural characterizations.
figure 1

a Synthesis procedure of ZOC. b Refined XRD of ZO and ZOC. c TEM image and d atomic resolution HAADF-STEM image of ZOC. e The line scan measured along the x-y rectangle region marked in d. f HAADF-STEM and corresponding EDS mapping images of ZOC. Green, red, and blue represent Zn, O, and Co elements, respectively. g Aberration-corrected HAADF-STEM image and corresponding atomic-resolution elemental maps (green and cyan correspond to Zn and Co atoms, respectively). h EELS mapping of the area enclosed by the red solid box in the inset of h (red pixels correspond to Co atoms). i EELS spectra acquired from the corresponding pixels in h. In g and h, the pixels highlighted by the yellow dashed and solid circles all correspond to Co atoms in the ZOC. j Normalized Co K-edge XANES spectra of ZOC sample, and standard Co foil, CoO, Co3O4, and Co2O3 as reference. k Normalized Zn K-edge XANES spectra of ZO, ZOC, and standard Zn foil and ZnO as reference. The inset shows the local enlargement of Zn K-edge XANES spectra. l FT k2 χ(R) Co K-edge EXAFS of ZOC, and standard Co foil, CoO, Co3O4, and Co2O3 as reference. m FT k2 χ(R) Zn K-edge EXAFS of ZO and ZOC, and standard Zn foil and ZnO as reference.

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The X-ray diffraction (XRD) was initially used to verify the phase structure, and all Bragg diffraction peaks of ZOC had the same peak positions as the ZO (P63mc, JCPDS No. 36-1451), which is a well-proven Wurtzite structured of ZO. XRD refinement shows that the cell volume of ZOC is slightly larger than that of ZO (from 47.639 to 47.677 Å3)23,24, which can be attributed to the difference in the radii of Co and Zn atoms and the lattice distortion caused by doping, indicating that Co atoms replaced Zn atoms in the lattice (Fig. 1b and Supplementary Table 2). In addition, FT-IR spectroscopy, Brunauer-Emmett-Teller (BET), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) analyses further revealed the morphology, structure, and surface area of ZOC and the influence of the Co introduction (Fig. 1c, Supplementary Figs. 1–8 and Supplementary Table 3). Further atomic-level structural information of ZOC was obtained using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). In Fig. 1d, the lattice fringes with interplanar spacings of 0.26 nm and 0.16 nm correspond to the (110) and (002) facets of wurtzite ZOC, respectively25. Notably, the atomic number is positively correlated with the brightness of atoms in the HAADF-STEM, but due to the similar atomic numbers of Co and Zn, it is challenging to distinguish between the two. Nevertheless, the selected area (x-y rectangle regions in Fig. 1d) surface and line intensity plots revealed a uniform distribution of atoms, further confirming that Co atoms substituted Zn in the lattice without forming nanoparticles or altering the original lattice structure (Fig. 1e). In addition, elemental mapping by energy-dispersive X-ray spectroscopy (EDS) reveals the presence and uniform distribution of Co, Zn, and O on ZOC (Fig. 1f, g). To further confirm the successful embedding of Co atoms, aberration-corrected transmission electron microscopy (ACTEM) scanning electron energy loss spectroscopy (EELS) mapping was applied. One of the isolated red pixels is marked in Fig. 1h, where the serial numbers match with that of extracted EELS spectra in Fig. 1i. Therein, a signal peak around 780.9 eV corresponding to the Co L edge is clearly revealed by the EELS, which directly confirms the incorporation of highly dispersed Co atoms in the ZnO.

Chemical state and atomic structure of ZOC

To further elucidate the electronic structure and coordination environment of Co and Zn atoms, X-ray absorption fine structure (XAFS) and X-ray photoelectron spectroscopy (XPS) measurements were conducted. As shown in Fig. 1j, the Co K edge X-ray absorption near-edge structures (XANES) reveals that the absorption edge of Co in ZOC is close to that of CoO and situated between CoO and Co3O4, indicating that the chemical state of Co is approximately +215,26,27. This conclusion is also supported by the high-resolution Co 2p XPS spectrum (Supplementary Figs. 9–11)26. The Zn K edge XANES shows that the absorption edges of Zn atom in both ZO and ZOC are between those of Zn foil and ZnO (Fig. 1k), indicating that the chemical states of Zn in both samples are between 0 and +225,28. Additionally, the energy absorption intensity of ZOC is significantly stronger than that of ZO, suggesting that the dipole effect induced by Co doping reduces the electron density at Zn sites. This conclusion is further confirmed by the main peak shifts in the Zn 2p XPS spectrum and the Zn Auger electron spectroscopy (Supplementary Fig. 12)29.

From the Fourier-transformed (FT) k2-weighted extended X-ray absorption fine structure (EXAFS) spectra, ZOC displays the first peak at 1.54 Å due to single scattering caused by the first coordinating oxygen around the central Co atom, while the second peak at ≈2.83 Å primarily originates from the interaction between the Co atom and its nearest neighboring Zn atoms (Fig. 1l). It is notable that the absence of Co-Co coordination shells at 2.17 Å unambiguously rules out metallic Co clusters in ZOC, demonstrating the Co atoms are atomically dispersed in the ZnO. Similarly, the two peaks observed in FT-EXAFS of Zn K-edge correspond to the interactions between Zn atoms and the surrounding O atoms, as well as the nearest neighboring metal atoms, respectively (Fig. 1m). Notably, the peak position at 2.90 Å in ZOC is slightly lower than that in ZO and the ZnO standard sample, which could be attributed to subtle differences between the Zn-O-Zn and Zn-O-Co structures. The above results are also evidenced by the wavelet transform (WT) analysis with significantly different maximum intensities distinguishing them from clusters (Supplementary Figs. 13–15). The subtle differences in the local coordination structures between the ZO and ZOC samples are further confirmed by quantitatively extracting the structural parameters of Co and Zn atoms using the EXAFS fitting analysis. The EXAFS fitting results of the first shell peaks for ZO and ZOC reveal that the Co atoms are coordinated with O atoms, with an average bond length of 1.96 Å, and the coordination number of Co atoms in ZOC is similar to that of Zn atoms in ZO and ZOC, indicating that Co atoms have substituted Zn atoms. (Supplementary Figs. 16–19 and Supplementary Tables 4 and 5). These results were consistent with the STEM findings, confirming the atomic dispersion of Co.

Stability and anti-interference performance of ZOC

AN, containing -NH2 on the benzene ring was initially chosen as a typical pollutant to evaluate the catalytic performance of ZO and ZOC in activating PMS to degrade e-RNPs (Supplemental Figs. 20–22 and Supplementary Table 6)1,30. The adsorption rates of ZO and ZOC with different Co loadings on AN are all <5% within 5 minutes, making their contribution to the degradation results. The optimal experimental parameters for the ZOC/PMS system were also determined (Fig. 2a and Supplemental Figs. 23 and 24). Astonishingly, could be completely removed within 5 minutes in the ZOC/PMS system, with a kobs of 1.4785 min−1, which is 598.6 times and 34.1 times higher than those of the PMS and ZO/PMS systems, respectively (Fig. 2b, c). Additionally, ZOC exhibits excellent structural integrity and PMS activation performance, maintaining over 98% degradation efficiency after five cycles (Supplemental Figs. 25–28).

Fig. 2: Stability and Anti-interference performance of ZOC.
figure 2

a The effect of catalyst and PMS dosage on the degradation efficiency of the ZOC/PMS system. b Degradation profiles of AN under different conditions and c corresponding first-order rate constants for the degradation of AN in different systems. d Comparison of k-values of advanced catalytic materials activating PMS for pollutant removal. e Removal of multiple contaminants within 5 min using ZOC activated PMS system. f Decomposition efficiency of PMS in ZO/PMS and ZOC/PMS systems, respectively. g Effects of environmental background cations, anions, and natural organic matter on the ZOC/PMS system. h Degradation rate and apparent rate constant of AN in the ZOC/PMS catalytic system under different pH conditions. Experimental conditions: [PMS]0 = 1 mM, [Catalysts]0 = 0.2 g/L, [pollutants]0 = 10 mg/L, T = 25 °C. All error bars in the figure represent the standard deviation from three replicate experiments.

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As shown in Fig. 2d, the k-values of different PMS-based advanced oxidation systems were calculated. Impressively, ZOC exhibits the highest catalytic performance using less PMS dosage in recently reported work (Supplementary Table 7), with a k-value as high as 73.93 min−1 M−1, which is 189.6 times higher than the ZO catalyst. In addition, a possible degradation pathway was proposed based on the identified reaction intermediates, and the toxicity of AN and its intermediates significantly decreases with degradation based on the Quantitative Structure-Activity Relationship (QSAR) (Supplementary Figs. 29–31 and Supplementary Table 8).

To verify the selective oxidation capability of the ZOC/PMS system for e-RNPs, a series of pollutants with EDG/electron-withdrawing groups (EDG/EWG) were employed (Fig. 2e and Supplementary Figs. 32 and 33). Interestingly, e-RNPs containing EDG (such as -OH, -NH2, -CH3, and organic sulfide groups) were more effectively decomposed, with removal rates ranging from 80.0% to 100%. In contrast, in the ZOC/PMS system, pollutants with EWG (such as -COOH and -NO2), specifically NB and BA, were more difficult to remove, with degradation efficiencies of only 16.7% and 12.2%, respectively. Additionally, pollutants containing both EDG and EWG, such as p-nitrophenol (p-NP) and o-nitrophenol (ONP), showed increased degradation compared to those containing only EWG. The above results indicate that ZOC/PMS has higher reactivity with pollutants containing EDG, which readily react with nonradicals, highlighting the potential application in overcoming the inherent limitations of radicals in complex water matrices. The degradation trend of AN was quite close to the apparent consumption trend of PMS by ZOC (up to more than 70% within 5 min), indicating that the rapid pollutant removal is a process of activating PMS to generate active species (Fig. 2f).

The water matrix generally contains abundant cations, anions, and natural organic matter represented by humic acid (HA). Therefore, it is necessary to investigate their impact on the removal of e-RNPs. As shown in Fig. 2g, it can be seen that apart from a slight inhibition by Ac- on the ZOC/PMS system, the other coexisting substances (i.e., Cl-, CO32-, NO3-, PO43-, and HA) do not significantly affect the degradation efficiency of AN, highlighting that this system is minimally affected by common environmental matrices in real water bodies. More interestingly, the ZOC/PMS system still achieved over 98% removal of AN in secondary effluent from a wastewater treatment plant (WWTP). In addition, the influence of the initial PH value and the Zeta potential of catalysts was also assessed, and the ZOC/PMS system showed excellent adaptability (Fig. 2h and Supplementary Figs. 34–36).

Generation mechanism of 1O2 and Co(IV) = O

Generally, the multiple activation pathways of PMS can produce various reactive species, including •OH, SO4•-, superoxide radicals (O2•-), 1O2, and high-valent metal species. Based on chemical quenching experiments and in situ electron paramagnetic resonance (EPR) spectroscopy, it was found that ZOC accelerated the 1O2 formation during the Fenton-like reaction (Fig. 3a and Supplementary Figs. 37–41), which is consistent with the significantly decreased removal rate after the TEMP addition31,32. Notably, the appearance of the 5,5-dimethyl-2-pyrrolidone-N-oxyl (DMPOX) and the inhibitory effect of dimethyl sulfoxide (DMSO) on the degradation prompted us to further explore the possible contribution of high-valent metal species. It has been reported that CoIV = O is formed by two-electron transfer in cobalt-based catalysts/peroxides (such as PMS and peracetic acid) and dominates the oxidation of pollutants14,15. Then, using methyl phenyl sulfoxide (PMSO) as a probe compound, the CoIV = O species was further investigated. According to the oxygen atom transfer (OAT) reaction, PMSO can be selectively converted to methyl phenyl sulfone (PMSO2) by CoIV = O. As expected, a much higher concentration of PMSO2 was detected in the ZOC/PMS system, further confirming that ZOC significantly promotes the formation of CoIV = O (Fig. 3b and Supplementary Fig. 40)33,34. The above results highlight that the 1O2 and CoIV = O in the ZOC/PMS system are the active species for e-RNPs degradation, accounting for 57% and 43%, respectively. Besides, the AN degradation was significantly inhibited when Co atoms were complexed with potassium thiocyanate (KSCN)35, suggesting that Co atoms are active sites for PMS activation (Supplementary Fig. 42).

Fig. 3: Investigation of PMS activation behaviors.
figure 3

a In-situ ESR spectroscopy for detecting 1O2. b PMSO consumption and PMSO2 generation in the two catalytic systems. Inset: Contribution rates of 1O2, CoIV = O, and other radicals to pollutant degradation rates in the corresponding Fenton-like system, respectively. The error bars in (b) represent the standard deviation of three parallel measurements. c In-situ XANES spectra of Co K-edge and Zn K-edge of ZOC. d Relationship between normalized absorption edge energy and Co-O as well as Zn-O coordination number during the Fenton-like process. e In-situ FT-IR measurements in the range of 1000-1400 cm−1 at different reaction times for the ZOC/PMS system during the Fenton-like reaction (The peaks at 1202/1031 and 1111 cm−1 correspond to the S-O bond and S = O bonds, respectively). f Dipole moment and its electron cloud distribution, and dipole field and its change with external forces. g Charge density difference for PMS adsorption on ZOC and the corresponding charge transfer. The yellow and blue regions represent the accumulation and depletion of electrons, respectively. h Formation pathway of high-valent Co-oxo and 1O2 in ZOC/PMS system. i The proposed mechanism of PMS activation by ZOC resulting in the generation of high-valent Co-oxo and 1O2. Experimental conditions: [PMS]0 = 1 mM, [Catalysts]0 = 0.2 g/L, [PMSO]0 = 1.5 mM, T = 25 °C.

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In situ characterization and local structure evolution analysis

To deeply elucidate the underlying mechanism responsible for the exceptional nonradicals selective generation of ZOC during Fenton-like processes, in situ XAFS measurements of the Zn K-edge and Co K-edge were carried out using a homemade in situ cell (Supplementary Figs. 43–46). In situ XANES spectra reveal that the absorption edges of Co and Zn exhibit distinctly different energy shift directions during the reaction compared to ex-situ conditions (Fig. 3c)36,37. The fact that Co exhibits a slight positive-energy shift while Zn exhibits a slight negative-energy shift within 5 min suggests that Co loses electrons and Zn gains electrons during the reaction, leading to a change in valence state. This corresponds to the electron transfer pathways in PMS activation, where CoIV = O and 1O2 are generated, which enhances the reliability of further analysis.

The fitting results of the Co K edge EXAFS spectra indicate that the coordination number of the first shell (Co-O) significantly increases from 3.3 to 4.1 after the reaction with PMS begins (Fig. 3d and Supplementary Tables 9-10). This increase in coordination number may be the reason for the elevated oxidation state of Co species1. Similarly, the fitting results of the Zn K-edge indicate that the coordination number of the Zn-O bond rises from 3.9 to 5.2 and then decreases to a value close to the coordination number in the Ex situ condition. The significant difference in the Co-O and Zn-O coordination numbers may stem from the regulation effect of oxygen-containing adsorbates. More notably, as the reaction progresses, the coordination number of Co-O elevated and does not show a significant decrease until the end of the reaction, but the coordination number of Zn-O experienced an increase and then a decrease, which also coincides with the fact that high-valent metal species tend to have longer lifetimes and the migration of 1O2 into the liquid phase after its generation on the catalyst surface, and also points to the existence of CoIV = O and 1O2. Additionally, in the XANES spectra, the shifts of the Co and Zn absorption edges and white lines towards the Ex situ environment after the reaction further illustrate the cyclic stability of the ZOC catalyst.

Although EXAFS can provide coordination numbers as well as metal-ligand bond lengths, it is limited to specific metal elements and fails to probe the activation behavior of PMS. Thus, to investigate the adsorption behavior of PMS and key intermediate species at Co and Zn sites, Fourier transform infrared (FT-IR) spectra were employed (Supplementary Figs. 47 and 48). As presented in Fig. 3e, the reflectance intensity heatmap obtained from in situ FT-IR spectroscopy of ZOC shows the stretching vibrational intensity of the key bonds in PMS, namely the bands at 1202/1031 and 1111 cm−1 can be indicated for the S-O bond (HSO5-) and S = O, respectively2,3,4,38,39,40. Notably, as the reaction progresses, the characteristic peak at 1202 cm−1 shifts toward lower wavenumbers, while the characteristic peak at 1031 cm−1 shifts toward higher wavenumbers. This indicates that PMS undergoes bonding on the ZOC surface and experiences a bidirectional electron transfer process, which aligns with the electron gain and loss of active sites as revealed by in situ synchrotron radiation analysis. Meanwhile, the intensity of the vibrational peaks decreased significantly with the reaction time, which should be the result of the rapid decomposition of PMS. In contrast, although PMS adsorption on the ZO surface was detected, its activation behavior is fundamentally different.

Mechanism investigation

A dipole is defined as a pair of opposite charges “q” and “-q” separated by a distance “d”, which are universal in catalysts, due to the uneven distribution of charge centers20,21. In this case, the different metal species in ZOC remind us of the charge imbalance between neighboring Zn and Co sites. Due to the difference in electronegativity between the metal and the O atoms, bond dipole moments are formed in the Zn-O and Co-O bonds, and the vectors superposition of the bond dipoles will form a localized dipole in the adjacent annular zone of a central metal site, leading to a charge density difference between the Co and Zn sites (Fig. 3f and Supplementary Data 1). Thereafter, electron localization function (ELF) calculations were performed to verify the above5, and compared to the original ZO, the charge density distribution in ZOC is asymmetric, with the Co site exhibiting a higher charge density distribution than the adjacent Zn site. Bader charge analysis confirmed that the Bader charges of the adjacent Co and Zn atoms are +0.805 and +1.056 e, respectively, and the Zn site in ZO is +0.906, indicating that the dipole effect effectively regulates the electron density of Co and the surrounding Zn atoms, which disrupts the original charge balance, forming a positively (Zn site) and negatively (Co site) polarized charge centers (Supplementary Figs. 49–54)41,42. Collectively, the excellent Fenton-like catalytic activity of ZOC originates from the dipole effect-induced electronic structure redistribution in the (002) crystal plane, which promotes the dual-pathway activation of PMS, generating CoIV = O and 1O2.

Electrochemical measurements and the density of states (DOS) analysis were used to verify the influence of the dipole effect on the electron transfer kinetics of the catalyst. In linear sweep voltammetry (LSV) analysis, the current density of both ZO and ZOC electrodes increased in the presence of PMS, with the increase being more pronounced in the ZOC system (Supplementary Fig. 55), which confirmed the coupled strong electron transfer between PMS and active sites. Additionally, cyclic voltammetry (CV) curves, Tafel polarization curves, and electrochemical impedance spectroscopy (EIS) also observed a marked improvement in electron transfer efficiency in ZOC compared to ZO (Supplementary Fig. 56). Furthermore, the density of states (DOS) analysis indicates that the dipole effect can effectively modulate electron distribution, resulting in more electrons near the Fermi level, which enhances the electron transfer efficiency and reactivity of ZOC (Supplementary Fig. 57)5. These results demonstrate that the presence of localized dipoles facilitates the formation of high-valent metal species, making electron transfer easier.

To elucidate the rationale behind the generation of 1O2 and CoIV = O species, DFT calculations were performed to investigate the mechanism underlying. First, the adsorption energies of two possible adsorption styles were compared to determine the PMS adsorption configuration: the Co site coordinates with the peroxyl O atom in PMS, while the Zn site adsorbs the terminal O atom of the PMS molecule (style 1) in an end-on mode and the peroxyl O atom (style 2), respectively. The negative adsorption energy of style 1 (−4.09 eV) is higher than that of the type style-2 configuration (−2.16 eV), implying the possibility that PMS dual-pathway activation generates 1O2 and CoIV = O (Supplementary Figs. 58 and 59).

The charge-density difference (CDD) and bader charge further confirmed electron distribution and the electron transfer between ZOC and PMS. After forming the inter-complex PMS-ZOC compound, a significant electron delocalization can be observed at the Co sites. However, there is a notable accumulation of charge at the Zn sites, which is direct evidence of bidirectional electron transfer between PMS molecules and ZOC (Fig. 3g). Additionally, bader charge analysis quantitatively revealed the charge transfer between the Co and Zn atoms and PMS, demonstrating that the dipole effect, which creates positively and negatively polarized charge centers, effectively enhances the oxidative and reductive activation of PMS.

The Gibbs free energy and rate-determining steps (RDS) for 1O2 and CoIV = O generation were calculated. As shown in Fig. 3h, different reaction intermediates along with three possible pathways for 1O2 were explored. The Zn site in ZOC tends to adsorb terminal O atoms, which promotes oxidation of PMS losing H atoms to SO5•-, and subsequent rapid self-reaction based to generate 1O2, S2O82-, and SO4- (path-2)6,7,31. Alternatively, 1O2 generation mediated by two PMS molecules at the Zn site is energetically favorable (path-1) (Supplementary Fig. 60). Path-3 describes CoIV = O formation: PMS is heterolytically cleaved at the Co site, forming a co-adsorption state of *OH and *SO4, followed by SO42- desorption and *OH deprotonation, consistent with previously reported mechanisms17,43,44. Notably, the enhanced local charge density induced by the dipole effect not only facilitates the two-electron transfer promoting O-O bond cleavage but also potentially driving subsequent heterolytic cleavage of the polar O-H bond. These findings reveal that the construction of positive and negative charge polarization centers is the origin of the high Fenton-like activity of ZOC. Specifically, the local dipole effect created by Co introduction increases the Co site charge density in the negative charge polarization center, which in turn promotes the generation of CoIV = O. Conversely, the Zn site, as the positive charge polarization center, makes it possible for PMS to be oxidized into 1O2. Thus, through the synergistic effect of high-valent metal species and 1O2, selective removal of e-RNPs in complex water bodies is achieved (Fig. 3i).

Overview of air-lifting internal circulating reactor

The ZOC/PMS system exhibited exciting performance in degrading e-RNPs in various anionic solutions and complex water matrices, as well as in pre-oxidizing real wastewater, which is attributed to the good interference resistance of ¹O₂ and CoIV = O. This prompted us to investigate the integration of water purification equipment and its practical effectiveness for treating real wastewater. The ALICR, a multiphase bioreactor known for its high mixing efficiency, low energy consumption, and uniform shear distribution, has been widely utilized in industrial processes such as hydrogenation, oxidation, and wastewater treatment45,46,47. Considering the treatment demands of industrial park sewage treatment plants (Fig. 4a), ALICR was employed to evaluate the practical application potential of ZOC/PMS for pre-oxidation removal of e-RNPs (Supplementary Fig. 61)9,48. Taking into account the catalyst circulation and the abrasive effects of water and air, hydrogel was used to load ZOC onto porous alumina ceramic balls (H-ZOC@PACB) to ensure stability during catalytic reactions. Images of the prepared H-ZOC@PACB are presented in Fig. 4b and Supplementary Fig. 62, with SEM analysis confirming the distribution of ZOC on the ceramic balls. The ALICR wastewater treatment process and its structural schematic are illustrated in Fig. 4c, d.

Fig. 4: Practical applications of the ALICR.
figure 4

a Process flow diagram of a WWTP after adding the ALICR pre-oxidation process. After undergoing primary treatment, the mixed wastewater enters the integrated pretreatment unit, where it undergoes pre-oxidation of pollutants through catalytic circulation before entering the subsequent treatment structures for further processing. b Schematic structure and SEM images of H-ZOC@PACB. c Schematic diagram of the operation principle of the ALICR. Wastewater enters from the bottom of the ALICR, where the introduced gas facilitates internal circulation to achieve pre-oxidation, and finally flows out through the overflow weir at the top of the reactor. d Photograph of experiment device (AN as indicator pollutant). e Hydraulic parameters during ALICR operation. f Comparison of water samples before and after ALICR treatment. g AN removal performance and BOD/COD changes during ALICR system operation with H-ZOC@PACB (The dashed line in the Fig. 4g represents the biodegradability threshold at a BOD/COD ratio of 0.3).

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Pre-oxidation performance of H-ZOC@PACB system for wastewater

To evaluate the performance of the reactor, influent samples were collected from a WWTP in an industrial park (Tianjin, China). The long-term stability was assessed through continuous operation for over 24 h (Fig. 4e, f, and Supplementary Table 11), and no significant fluctuations in wastewater flow were observed during the operation. The impact of this system on biodegradability was evaluated using the BOD/COD ratio. The effluent BOD/COD increased from 0.29 in raw water (below the biodegradability threshold of 0.3) to over 0.55 after treatment5,49, indicating both efficient e-RNPs removal and significant improvement in wastewater biodegradability9 (Fig. 4g). Another advantage of H-ZOC@PACB is the leaching resistance, leaking Zn and Co ions at concentrations well below the reclaimed water limit (50 μg/L) (Supplementary Fig. 63), indicating their potential for future water purification applications. Additionally, by adding e-RNPs as indicators to investigate the broad applicability, the results demonstrated that the ZOC/PMS system can effectively remove various common e-RNPs in industrial mixed wastewater, including AN, tetracycline (TC), doxycycline hydrochloride (DOXH), and o-aminophenol (O-AP) (Fig. 5a), further demonstrating the reliable and effective activation PMS by H-ZOC@PACB for pollutant degradation.

Fig. 5: Pre-oxidation performance of ZOC/PMS system for wastewater.
figure 5

a Degradation of different e-RNPs by H-ZOC@PACB. b and c 3D-EEM fluorescence spectra of wastewater and wastewater treated with the ZOC/PMS system (The four excitation/emission (EX/EM) peaks at 285/335 nm (peak I), 235/335 nm (peak II), 340/425 nm (peak III), and 240/425 nm (peak IV) correspond to tryptophan, aromatic proteins, humic acid, and fulvic acid in the wastewater, respectively). d Relative abundance of different classes of heteroatomic compounds in WWTP influent and pre-oxidized treatment effluent (Ox, S1-3Ox, and N1-3Ox compounds, respectively). e The relative intensity of the DOM classified according to substance (Substances include lipids, aliphatic/proteins, lignins/CRAM-like structures, carbohydrates, unsaturated hydrocarbons, aromatic structures, and tannins). The relative intensity of DOM: e element classification and f substance classification. g Van Krevelen diagrams of DOM in the influent of a WWTP: influent and effluent after pre-oxidation with the ZOC/PMS system. h Inhibition of Vibrio fischeri in AN solution and WWTP influent before and after treatment, respectively. Experimental conditions: [AN] = [TC] = [DOXH] = [O-AP] = 2 mg/L. The error bars in (a) and (h) represent the standard deviation of three parallel measurements.

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The composition and characteristics of dissolved organic matter (DOM), as the primary components of organic matter in wastewater, can undergo significant changes during the pre-oxidation process, potentially introducing unpredictable risks to subsequent treatment processes and effluent quality50,51. Conventional biological treatments often leave behind toxic and refractory soluble organic matter in effluents, necessitating DOM monitoring to implement targeted stabilization strategies. Figure 5b, c present the fluorescence signals of wastewater before and after treatment, as analyzed via three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy. At 285/335 nm (peak I), 235/335 nm (peak II), 340/425 nm (peak III), and 240/425 nm (peak IV), four excitation/emission (Ex/Em) peaks are observed, corresponding to tryptophan, aromatic proteins, humic and fulvic acid-like substances in the wastewater, respectively52,53. Astonishingly, after treatment with the ZOC/PMS system, the intensities of these EEM peaks significantly decreased, particularly for the peaks related to aromatic proteins, which indicates that the ZOC/PMS system treatment can effectively reduce recalcitrant substances. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was employed to assess the specific molecular composition of DOM before and after ZOC/PMS pre-oxidation. The number of detected chemical substances increased from 2,791 to 3,674 after treatment. According to previous literature reports, the H/C ratio and double bond equivalent (DBE) of DOM in the wastewater before and after the Fenton-like treatment were calculated, which reveals that the H/C ratio of DOM increased from 1.473 to 1.485, and the DBE of DOM decreased from 11.323 to 10.517. Previous studies have demonstrated that, in DOM, lower carbon numbers and DBE values correspond to a reduction in the total number of carbon atoms, double bonds, and aromatic rings, which indicates that the treated DOM has smaller molecular weight and unsaturated characteristics, showing that the pre-oxidation with the ZOC/PMS system significantly improved biodegradability50,51. The relative abundance of acidic CHO compounds, acidic CHOS compounds, basic CHN compounds, and CHONS compounds before and after pre-oxidation disclosed that pre-oxidation generated or retained more hydrophilic polyoxygenated compounds, which is beneficial for biological treatment and reduces the aggregation of lipid-structured compounds (Fig. 5d–f).

The molecular composition of DOM is visualized using a van Krevelen diagram (Fig. 5g), which categorizes compounds into seven classes, including lipids, aliphatic/proteins, lignins/carboxylic rich alicyclic molecules (CRAM)-like structures, carbohydrates, unsaturated hydrocarbons, aromatic structures, and tannins. Lipids are the largest group of DOM, followed by aliphatic/proteins and lignins/CRAM, which is consistent with previous studies. After pre-oxidation treatment, the content of refractory organic substances such as lignin/carboxyl-rich alicyclic molecules, aromatics, and tannins decreased, indicating enhanced biodegradability and reduced biotoxicity of the DOM5. As a process capable of disrupting the structural stability of organic compounds, oxidative pretreatment not only enhances the biodegradability of wastewater but also reduces biotoxicity, making them more accessible to biological systems and improving effluent quality.

To evaluate the adaptability of the ZOC catalyst, the photobacterium of Vibrio fischeri was examined to check the biotoxicity of the catalytic system53,54. After 15 min of treatment, the inhibition rate of AN and influent wastewater on Vibrio fischeri decreased significantly (Fig. 5h), indicating that the ZOC/PMS pre-oxidation system does not generate toxic byproducts. Instead, it improves the wastewater’s compatibility with microbial systems, further enhancing its potential for effective downstream biological treatment.

Discussion

In summary, this work breaks the charge symmetry of pristine ZnO by doping with Co, thereby constructing localized dipoles in the region of adjacent Zn and Co sites for efficient PMS activation. Multi-scale characterization and theoretical calculations show that Co active centers with a high charge density promote a two-electron transfer, which cleaves the O-O and O-H bonds to form CoIV = O. Simultaneously, positively charged Zn sites promote the self-decomposition of two PMS molecules to generate 1O2. Benefiting from the synergistic effects of these non-radical, the ZOC/PMS system achieves remarkable selectivity for e-RNPs, with a k-value as high as 73.93 min−1 M−1 that is 189.6 times higher than the ZO, and significantly outperforming state-of-the-art catalysts. Additionally, in an ALICR, the ZOC/PMS system exhibits excellent selective degradation for e-RNPs and significantly enhances the biodegradability of industrial parks wastewater from 0.29 to over 0.55, underscoring its potential for advancing efficient and sustainable water purification strategies.

Methods

Catalyst preparation

Preparation of ZnO

The ZnO catalysts were prepared by a conventional sol-gel method using citric acid as a chelating agent. Initially, 15 mmol of Zn(NO3)2•6H2O (4.462 g) and 45 mmol of citric acid (8.645 g) were dissolved in 25 ml of deionized water. The solution underwent sonication for 5 min at room temperature, followed by magnetic stirring for 1 h. Subsequently, the resulting precursor solution was subjected to heating in a water bath at 80 °C to remove excess water until a wet gel was formed. The wet gel was then dried at 170 °C for 3 h to obtain a dry gel, which was further calcined in a muffle furnace at 500 °C for 5 h with a heating rate of 5 °C/min. The calcined catalyst was then washed several times with ethanol and deionized water to remove soluble impurities. The sample was dried in a vacuum oven at 60 °C for 4 h. The obtained material was denoted as ZO.

Preparation of Co-ZnO

These x % Co-ZnO catalysts (x represents the molar percentage of Co, x = 0, 1, 3, 5, and 7) were synthesized employing the same methodology as for ZnO. Taking 5% Co-ZnO as an example, 15 mmol of Zn(NO3)2•6H2O (4.462 g), 0.75 mmol of Co(NO3)3•6H2O (0.218 g), and 45 mmol of citric acid (8.645 g) were dissolved in 25 ml of deionized water. The resulting solution underwent sonication for 5 minutes at room temperature, followed by stirring for 1 h. Subsequently, the precursor solution was heated in a water bath at 80 °C to remove excess water until a wet gel was formed. The wet gel was then dried at 170 °C for 3 h to obtain a dry gel, which was further calcined in a muffle furnace at 500 °C for 5 h with a heating rate of 5 °C/min. The calcined catalyst was then washed several times with ethanol and deionized water to remove soluble impurities. The sample was dried in a vacuum oven at 60 °C for 4 h. The obtained material was denoted as ZOCx (x = 1, 3, 5, and 7).

Characterizations

The characterization methods including XRD, BET, XPS, SEM, HR-TEM, HAADF-STEM, FTIR, EPR, XAS, and XAFS are described in detail in Supplementary Methods.

Analytical methods

The qualitative and quantitative analytical methods including HPLC, UPLC-MS measurement, boundaries of regions in Van Krevelen diagrams, and electrochemical tests are available in Supplementary Methods. Supplementary Data 1 provides the atomic coordinates of the optimized computational models.

Quantitation of reactive oxygen species (ROS)

Quantitative 1O2: FFA is used as an 1O2 probe with reaction rate constant of 1.0 × 108 M−1 s−1. The concentration of FFA was measured and determined by HPLC, and then the concentration of 1O2 was calculated based on the quantitative reaction relationship between FFA and 1O2. The concentration of FFA was 1 mM in our experiment.

Quantitative MIV = O: Based on the fact that methyl phenyl sulfoxide (PMSO) can be readily oxidized to the corresponding sulfone [methyl phenyl sulfone (PMSO2)] via an oxygen atom transfer mechanism, the presence of MIV = O was successfully identified in the Fenton-like system. In this study, the initial concentration of PMSO was 1 mM.

$${{{\rm{PMSO}}}}+{{{{\rm{M}}}}}^{({{{\rm{n}}}}+2)+}={{{\rm{O}}}}\to {{{{\rm{PMSO}}}}}_{2}+{{{{\rm{Mn}}}}}^{{{{\rm{n}}}}+}$$

In situ characterization

In situ XAFS spectra were measured on Table XAFS-500A (Specreation Instruments Co., Ltd. China). The data collection was performed in transmission mode. To ensure high-quality XAS spectra, the catalyst-coated carbon paper is tightly pressed against a window with micron-sized gaps to minimize light source loss. A pump circulates the PMS-containing reaction solution within in situ X-ray absorption spectroscopy reaction cell.

A custom-made ATR-SIRAS setup with a ZnSe crystal as the infrared transmission window was used to obtain all ATR-SIRAS spectra on a Fourier transform infrared spectrometer (FT-IR, Nicolet iS50, Thermo Fisher Scientific). A nanoscale Au film was deposited on the reflective surface of a silicon prism via electroless gold plating. Before depositing Au, the Si prism was polished with a 0.05 μm Al2O3 suspension and cleaned sequentially with ultrasonic treatment in acetone and deionized water. The catalyst was prepared by spraying a catalyst ink onto the Au film prepared as described. The catalyst was pressed tightly onto the ZnSe crystal window with a micron-scale gap to minimize infrared light loss. To ensure high-quality FT-IR spectra, the setup employed a reflection mode with infrared light incident perpendicularly. All measurements were obtained with a spectral resolution of 4 cm−1, averaged over 128 scans, and presented in absorbance after background subtraction.

Set-up and operation of the air-lifting internal circulating reactor

Before initiating formal experiments, H-ZOC@PACB were prepared. The PACB were soaked in 1 M NaOH for 12 h to remove impurities before loading. First, a suitable amount of ZOC catalyst is dispersed in an appropriate volume of deionized water using an ultrasonic disperser. Subsequently, the porous ceramic spheres are immersed in the ZOC catalyst dispersion, gently stirred to ensure uniform adsorption of the catalyst on both the surface and within the internal pores of the ceramic spheres. Next, the hydrogel is prepared according to previously reported methods, and the ZOC-loaded ceramic spheres are slowly added to the hydrogel solution, gently stirred or shaken to ensure uniform coating of the hydrogel on the ceramic spheres. The effective volume of the air-lift internal circulation reactor is 3.2 L, with a height of 400.0 mm and a diameter of 100.0 mm. The influent is driven by a diaphragm pump. The influent flow rate ranged from 0.40 ml to 570.00 ml per minute, corresponding to an effective HRT of 5.61–8000 min. Influent and effluent samples from the air-lifting internal circulating reactor were collected twice daily, filtered through 0.22 μm filter membranes and analysed. The COD and BOD were determined using the dichromate spectrophotometric method and the iodometric method, respectively. The concentrations of Zn and Co ions were determined through the ICP-MS (PerkinElmer ElAN DRC-e).