All the starting chemicals used in the experiment were of analytical grade without further purification. Metal nitrates, including Fe(NO₃)₃•9H₂O and Co(NO₃)₂•6H₂O, were all provided by Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Disodium ethylenediaminetetraacetate (EDTA), hydroxylamine (HA), and tert-butanol (TBA) were purchased from Sinopharm Chemical Reagent Co., Ltd. Ethanol (EtOH), hydrogen peroxide (H₂O₂) at 30% concentration, sodium hydroxide (NaOH), and chloroform (CF) were purchased from Xilong Chemical Co., Ltd (Shantou, China). Methyl orange (MO), rhodamine B (RhB), L-histidine (L-his), 5,5-dimethyl-pyrroline N-oxide (DMPO), and 2,2,6,6-tetramethyl-4-piperidone (TEMP) were obtained from Aladdin Chemical Reagent Co., Ltd (Shanghai, China). All aqueous solutions used in this study were deionized water.

The formation of CoFe₂O₄ nanoparticles (NPs) was synthesized using the solvothermal method. In detail, 0.015 M of Co(NO₃)₂·6H₂O and 0.03 M of Fe(NO₃)₃•9H₂O (the mole ratio of Co to Fe was 1:2) were dissolved in 300 mL of ethanol under magnetic stirring at 300 rpm for 1 h. Then, the pH of the solution was adjusted to 13 by adding 4 M NaOH, and the solution was continuously stirred for 1 h. After that, the dark brown mixture was transferred into a 100 mL Teflon-lined stainless-steel autoclave and hydrothermally treated at 180 °C for 20 h. After natural cooling, the dark brown precipitate was collected via vacuum filtration and thoroughly washed several times using EtOH and deionized water until the eluate was neutral. Finally, the obtained product was dried at 60 °C for 24 h. The preparation method of CoFe₂O₄ NPs is shown in Fig. 1, and the obtained products were denoted as CoFe₂O₄. For comparison, CoFe₂O₄ was synthesized using the solution combustion method from the literature and labeled as SCS-CoFe₂O₄ [16].

For MO and RhB degradation experiments, all batch were carried out in a 100 mL beaker filled with 80 mL of solution. The pH of the solutions was adjusted using 0.1 M sodium hydroxide or 0.1 M sulfuric acid solutions. Initially, 80 mg of CoFe₂O₄ NPs was added to the solution. Subsequently, a certain amount of EDTA and HA were sequentially added to the reaction system. After adjusting the pH to the desired initial pH (3–11), a certain amount of H₂O₂ was added to start the reaction. At certain time intervals, 4 mL of the suspension was withdrawn with a syringe from the beaker and filtered through a 0.45 μm filter. Then, the absorbance of the filtered samples was measured at the corresponding maximum absorption wavelength using a UV-vis spectrophotometer.

TBA, L-his, and CF were used as •OH, ¹O₂, and •O₂⁻ quenching agents, respectively, to investigate which free radical dominated dye degradation. Furthermore, in order to check the catalytic stability of CoFe₂O₄ NPs, cycling experiments were conducted. After each cycle, the used catalyst was recycled via centrifugation (10,000 rpm), washed with deionized water and ethanol three times, and then dried at 60 °C overnight for the next reaction. Six degradation cycles were conducted consecutively.

The structures and phases of CoFe₂O₄ NPs before and after the reaction were characterized via X-ray diffractometry (XRD, Shimadzu, XRD-6100) using a D/Max-2550 diffractometer with Cu Kα radiation at a scan rate of 4 (°)/min. The surface morphology and inner structures of the specimens were analyzed via field emission scanning electron microscopy (SEM, Zeiss, Gemini 300) with energy dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM, JEOL JEM-2100F). An X-ray photoelectron spectroscopy (XPS) system (PHI 5000 VersaProbe spectrometer) was applied to investigate the valence states of metal ions. Moreover, the Brunauer-Emmett-Teller method was used to estimate the specific surface areas and pore size. Electron paramagnetic resonance (EPR) experiments were performed on a Bruker EMX-E8/2.7 spectrometer with DMPO and TEMP as the spin-trapping agents.

The XRD was used to characterize the evolution of the CoFe₂O₄ NPs catalysts as shown in Fig. 2(a). There were nine well-defined diffraction peaks that appeared at 18.2°, 30.3°, 35.6°, 36.9°, 43.5°, 54.1°, 57.5°, 62.8°, and 74.6°, corresponding to the Bragg planes of (111), (220), (311), (222), (400), (422), (511), (440), and (533), respectively [17]. These diffraction peaks are consistent with the CoFe₂O₄ standard card (JCPDS: 22-1086) [18]. The results indicate that the CoFe₂O₄ NPs prepared by using the solvothermal method have a typical spinel structure [19].

The SEM was used to characterize the surface morphology of CoFe₂O₄ NPs. As depicted in Fig. 2(b), the CoFe₂O₄ NPs were composed of polyhedral particles and spherical nanoclusters randomly distributed on the surfaces of the polyhedron. This surface morphology could expose more reaction sites, which was conducive to the subsequent degradation of organic pollutants [20]. Besides, the elemental distribution and composition of the CoFe₂O₄ NPs were detected via EDS [21–22]. As shown in Fig. 2(c), the catalyst that was prepared comprised evenly distributed Co, Fe, and O elements. The atomic percentages of Co and Fe were 17.60% and 36.51%, respectively, with an atomic ratio of approximately 1:2 (Fig. S1). In addition, the lattice structure of the CoFe₂O₄ NPs was characterized via TEM and HR-TEM (Fig. 2(d) and 2(e)). The lattice fringe values of 0.299 nm, 0.254 nm, 0.208 nm, and 0.149 nm were consistent with the (220), (311), (400), and (440) Bragg planes of CoFe₂O₄ [23], respectively. The selected-area electron diffraction (SAED) was carried out to further explore the lattice structure. As shown in Fig. 2(f), four diffraction rings with an interplanar distance of 0.149 nm, 0.163 nm, 0.254 nm, and 0.299 nm were observed. These values corresponded to the lattice spacing of the (440), (511), (311), and (220) planes of CoFe₂O₄ [24], respectively. The above results fully prove that the prepared dark brown product is an ideal CoFe₂O₄ catalyst.

The effects of various experimental conditions, such as H₂O₂ concentrations, EDTA, and HA dosages on the degradation of MO in CoFe₂O₄/EDTA/H₂O₂ and CoFe₂O₄/HA/H₂O₂ systems were investigated, as shown in Fig. 3.

As illustrated in Fig. 3(a), the degradation efficiencies of MO increased as the dosage of H₂O₂ varied from 61.69 mM to 305.98 mM. The addition of H₂O₂ dosage at 305.98 mM, CoFe₂O₄ exhibited the highest MO removal rate (k₁ = 0.81×10⁻² min⁻¹, Fig. 3(b)). With the enhanced concentration of H₂O₂, CoFe₂O₄ was able to fully react with H₂O₂, resulting in the generation of a significant quantity of free radicals (such as •OH) for the degradation of pollutants [20]. However, it is widely recognized that using a vast amount of H₂O₂ is not economically feasible in practical applications. Therefore, a dosage of 122.39 mM H₂O₂ was selected as the optimum parameter for further investigation.

The initial concentration of EDTA has a significant impact on the degradation efficiency of MO in the CoFe₂O₄/H₂O₂/EDTA system. As depicted in Fig. 3(c), the degradation efficiency of MO increased first and then decreased as the dosage of EDTA was increased from 0.1 mM to 0.5 mM within 180 min. Specifically, the degradation efficiency of MO reached its maximum value (96.17%) at an EDTA concentration of 0.3 mM. Meanwhile, the k value increased by nearly eight times compared to that at an EDTA concentration of 0.1 mM (k₂ = 1.78×10⁻² min⁻¹, Fig. 3(d)). Increasing the dosage of EDTA appropriately promoted the production of Fe²⁺-EDTA and Co²⁺-EDTA, resulting in more ROS for enhanced MO degradation [25]. However, when the initial EDTA concentration exceeded 0.3 mM, both the degradation efficiency and k value of MO decreased rapidly to 83.54% and 1.02×10⁻² min⁻¹, respectively. Excessive EDTA not only competed with MO in consuming the produced ROS but also formed inert surface complexes on the CoFe₂O₄ surface [26].

With an increasing initial content of HA added into the CoFe₂O₄/H₂O₂/HA system, the removal rate of MO dramatically increased from 2.96×10⁻² (mM)⁻¹•min⁻¹ to 181×10⁻² (mM)⁻¹•min⁻¹ (Fig. 3(f)). HA could promote the redox cycles of Fe³⁺/Fe²⁺ and Co³⁺/Co²⁺ due to its high reducibility, which facilitated the continuous recovery of Fe²⁺ and Co²⁺ for the catalytic reaction [27–28]. As shown in Fig. 3(e), when the HA concentration was 1.5 mM, 2.0 mM, and 2.5 mM, the degradation efficiency of MO exceeded 95% within 90 min. However, using a large amount of HA is not economically feasible in practical applications. Therefore, a dosage of 1.0 mM HA was selected as the optimal parameter for further investigation.

In addition to the effects mentioned above regarding the different amounts of H₂O₂, EDTA, and HA, this paper also investigates the degradation effect of CoFe₂O₄/H₂O₂, CoFe₂O₄/H₂O₂/EDTA, CoFe₂O₄/H₂O₂/HA, and CoFe₂O₄/H₂O₂/EDTA/HA on MO at various initial pH values (3, 5, 7, 9, 11, and blank (i.e., pH=6.4)).

The degradation efficiency of MO in the CoFe₂O₄/H₂O₂ system reached 78.61% at pH 3 within 180 min. However, the degradation efficiency of MO at other pH values was less than 10% (Fig. 4(a)). This observation is consistent with the optimal reaction pH (2–3) in the classical Fenton system [29–30]. The degradation efficiency of MO in the CoFe₂O₄/H₂O₂/EDTA system reached its minimum value (77.69% within 180 min) at pH 3, which is lower than at other pH values (exceeding 90% within 90 min) (Fig. 4(b)). The poor activity of the CoFe₂O₄/H₂O₂/EDTA system at acidic pH can be attributed to the formation of perhydroxyl radicals (•OOH), which strongly inhibit the regeneration of Fe2+ surf/Co2+ surf-EDTA [31]. With the addition of HA, the CoFe₂O₄/H₂O₂ system transformed into the CoFe₂O₄/H₂O₂/HA system. The degradation efficiency of MO at a pH below 6.4 was approximately 6 times higher than that at a pH above 6.4 within 180 min (Fig. 4(c)). This discrepancy can be attributed to the pH-dependent morphology of HA (NH₃OH⁺, NH₂OH, and NH₂O⁻), which impacts the efficiency of electron transfer [32].

It is considered that the CoFe₂O₄/H₂O₂/EDTA and CoFe₂O₄/H₂O₂/HA systems have certain limitations within different initial pH ranges. Therefore, this study innovatively added EDTA and HA into the CoFe₂O₄/H₂O₂ system simultaneously. As illustrated in Fig. 4(d), the degradation efficiency of MO in the CoFe₂O₄/H₂O₂/EDTA/HA system exceeded 92% within 90 min at initial pH values ranging from 3 to 11. Specifically, at pH 7, the degradation efficiency of MO in this system was 88.52% within 180 min, slightly lower compared to the efficiency observed at other pH values. To investigate the reasons for this phenomenon, the zeta potential of CoFe₂O₄ NPs was measured at different pH values (Fig. S2). As shown in Fig. S2, the absolute value of the zeta potential increased with increasing pH value. At pH 7, the active sites on the surface of CoFe₂O₄ NPs may be in a transitional state, resulting in relatively few active sites. This could explain the decrease in the degradation efficiency of MO in the CoFe₂O₄/H₂O₂/EDTA/HA system when the initial pH value is 7. The degradation target of MO (anionic dye) was replaced with RhB (cationic dye). In the CoFe₂O₄/H₂O₂/EDTA/HA system, the degradation efficiency of RhB exceeded 96% across an initial pH range of 3 to 11 (Fig. S3). It is found that the initial pH range of the CoFe₂O₄/H₂O₂/EDTA/HA system was broader than that of various reported Fenton-like systems (Table S1).

The predominant forms of EDTA and HA in the initial pH range of the aforementioned study are further discussed. As depicted in Fig. 5(a), EDTA primarily exists in the forms of H₃Y⁻, H₂Y²⁻, HY³⁻, and Y⁴⁻ at the corresponding initial pH values. As the pH value increases, the process of deprotonation gradually intensifies, leading to a stronger electrostatic attraction to cations [33]. When the pH value was 3, the decrease in MO degradation efficiency may be related to the weakening of H₃Y⁻ complex ability in addition to the influence of •OOH. HA mainly exists in the forms of NH₃OH⁺ and NH₂OH (Fig. 5(b)). When the pH is below 5.96, HA undergoes protonation to form the cation NH₃OH⁺, which generates a significant amount of •OH and singlet oxygen (¹O₂) [28]. This phenomenon further confirms that HA only functions effectively under acidic conditions.

The chemical speciation model (Visual MINTEQ software, version 3.1) was used to simulate and analyze the iron and cobalt species in the Fe²⁺/Co²⁺-EDTA complex solution to clarify the influence of pH value, as shown in Fig. 5(c). The results demonstrated that CoEDTA²⁻ played a crucial role in the degradation of organic pollutants in the CoFe₂O₄/H₂O₂/EDTA and CoFe₂O₄/H₂O₂/EDTA/HA systems. However, CoHEDTA⁻ was more likely to function in acidic conditions, whereas FeOHEDTA³⁻ tended to act in alkaline conditions. It is worth noting that within the pH range of 6 to 8, the percentage of CoEDTA⁻ and FeEDTA²⁻ remained relatively constant at approximately 93% and 7%, respectively. This indicated that the main composition of the solution remained stable within this pH range and confirmed that the pH value affects the CoFe₂O₄ powder, resulting in a slight decrease in the degradation efficiency of MO in the CoFe₂O₄/H₂O₂/EDTA/HA system at pH 7.

Additionally, the promotional effects of EDTA and HA on the heterogeneous Fenton-like system are closely related to powder characteristics. A comparative study was conducted to fabricate CoFe₂O₄ particles using the solution combustion method, and the as-obtained products were denoted as SCS-CoFe₂O₄. The degradation efficiency of MO by the SCS-CoFe₂O₄/H₂O₂/EDTA/HA system reached 78.27% within 180 min, which was almost 7-fold higher than that of the reaction system without EDTA and HA (Fig. S4(a)). Under the same experimental conditions (as shown in Fig. S4), the CoFe₂O₄/H₂O₂/EDTA/HA system was almost 11-fold higher than the SCS-CoFe₂O₄/H₂O₂/EDTA system (Fig. S4(b)). This result was attributed to the smaller grain size and much larger specific surface area of CoFe₂O₄ NPs (110.72 m²•g⁻¹) compared to those of SCS-CoFe₂O₄ (only 4.35 m²•g⁻¹) (Fig. S4(c)-(d)) [16]. Therefore, the promotional effects of EDTA and HA depend on the specific surface area of the Fenton-like catalyst.

Chemical quenching experiments were conducted to identify the contributions of diverse ROS for MO degradation. TBA was selected as the scavenger of •OH [34]. L-his and CF were usually employed as the quenching agents of ¹O₂ and •O₂⁻, respectively [35–36]. As illustrated in Fig. 6(a), the addition of 0.5 M TBA resulted in a decrease in the degradation efficiency of MO from 96.19% to 75.61%, suggesting that the CoFe₂O₄/H₂O₂/EDTA/HA system generated a quantity of •OH. However, TBA was unable to completely inhibit the degradation of MO, indicating that other ROS, in addition to •OH, also had a role in the reaction. As observed, the degradation of MO was remarkably decreased by 61.88% with the addition of 1.0 mM L-his, providing evidence that ¹O₂ also played an important role in the removal of MO. It has been reported that L-his and •OH exhibit a high reaction rate constant (k₃ = 5×10⁹ M⁻¹S⁻¹) [31]. Therefore, the masking effect of L-his cannot only be attributed to ¹O₂, and the presence of •OH should not be ignored [37]. Interestingly, when CF was used in the quenching experiment, the degradation rate did not decrease.

To further investigate the existence of ROS, EPR spectra were recorded using DMPO or TEMP as trapping agents. It is widely recognized that DMPO functions as a spin-trapping agent for •OH and •O₂⁻, while TEMP has the ability to react with ¹O₂ and generate TEMP-¹O₂ [38–39]. As depicted in Fig. 6(c) and Fig. 6(b), the detection of characteristic peak signals of DMPOY−•OH and TEMP−¹O₂ confirms that the CoFe₂O₄/H₂O₂ system, coupled with EDTA and/or HA, was a multi-ROS process in which •OH and ¹O₂ coexist. It was consistent with the quenching experiments (as shown in Fig. 6(a)). The order of signal intensity of DMPO−•OH and TEMP−¹O₂ were suggested to follow the order of CoFe₂O₄/H₂O₂/EDTA/HA > CoFe₂O₄/H₂O₂/EDTA > CoFe₂O₄/H₂O₂/HA > CoFe₂O₄/H₂O₂. It can be considered semi-quantitatively that the large amount of •OH and ¹O₂ produced by the addition of both EDTA and HA to the CoFe₂O₄/H₂O₂ system is greater than that of EDTA and HA added to the CoFe₂O₄/H₂O₂ system alone. As shown in Fig. 6(d), when the solvent was replaced with methanol, the characteristic peak signals of DMPO-•O₂⁻ did not appear. It has been reported that •O₂⁻ converts to ¹O₂ at a high reaction rate constant (k₄ = 1.01×10¹⁰ M⁻¹S⁻¹) [40]. Therefore, the •O₂⁻ generated may be an intermediate in the ¹O₂ generation pathway [41–42].

Furthermore, quenching experiments were conducted on MO at pH 3 and 11. As indicated in Table S2, the degradation of MO in the CoFe₂O₄/H₂O₂/EDTA/HA system was mainly attributed to •OH at pH 3. Similarly, RhB quenching experiments carried out under identical conditions also demonstrated that •OH was the dominant factor in the degradation of RhB (Fig. S5(a)). Under the pH value of 11, the degradation of both MO and RhB was primarily attributed to •OH, with other ROS playing a negligible role (Fig. S5(b)). In comparison to the pH value of 6.4, the quenching agent L-his did not inhibit the degradation of MO or RhB in the CoFe₂O₄/H₂O₂/EDTA/HA system at pH 3 and 11. This phenomenon provided evidence that under highly acidic or alkaline conditions, the reaction between •O₂⁻ and •OH to generate ¹O₂ was hindered [43]. Based on the results of the quenching experiments and EPR test, it could be inferred that •OH was the dominant ROS for MO or RhB degradation, and ¹O₂ played an auxiliary role in the degradation.

The degradation of MO in the CoFe₂O₄/H₂O₂/EDTA/HA system may involve three components: adsorption, homogeneous reaction (ion leaching component), and heterogeneous reaction (catalyst surface component) [44]. To assess the homogeneity/heterogeneity of the reaction in the CoFe₂O₄/H₂O₂/EDTA/HA system, this paper compares the contribution of the radical reactions taking place on the surface and in the solution. After leaching CoFe₂O₄ NPs in D.I. water for 3 h, the eluate was mixed with EDTA, HA, and H₂O₂. As observed from Fig. 7(a), the reaction in a heterogeneous system showed that around 93.79% of MO was degraded compared to the homogeneous (2.98%) and adsorption (10.29%) degradation. The results indicated that the effect of the CoFe₂O₄ NPs surface was greater than the leached iron and cobalt.

The reusability of CoFe₂O₄ in the CoFe₂O₄/H₂O₂/EDTA/HA system is an essential factor in its practical application. As depicted in Fig. 7(b), the degradation efficiency of MO remained at 75.86% after 1080 min following six cycles of reaction. The decrease in the degradation efficiency of MO by CoFe₂O₄ was possibly attributed to the loss of active sites due to the absorption of intermediates on the surface of CoFe₂O₄ [45–46]. At the same time, there is no obvious difference in the XRD pattern between the fresh CoFe₂O₄ NPs and the reacted CoFe₂O₄ NPs, indicating that the structure of CoFe₂O₄ NPs remains stable (Fig. 7(c)).

The elemental state of Co, Fe, and O in the catalyst before the reaction and after three cycles was analyzed via XPS, as illustrated in Fig. 7(d–i). The XPS full spectrum of CoFe₂O₄ NPs before and after the reaction is shown in Fig. S6. The C 1s peak (284.84 eV) was used to correct the binding energy of each element [47]. In the O 1s spectrum, peaks at 529.78, 531.43, and 533.38 eV corresponded to lattice oxygen (O_latt), surface oxygen (O_surf), and adsorbed oxygen (O_ads), respectively (Fig. 6(d)) [48–50]. After the synergistic activation of H₂O₂ by EDTA and HA in the presence of CoFe₂O₄, the relative content of O_surf decreased from 36.19% to 20.62% (Fig. 6(g)). The decrease in O_surf could be due to the pollutants being adsorbed on the catalyst surface by hydrophilic oxygen-containing groups [51].

In the Fe 2p spectrum (Fig. 7(e)), the peaks observed at 710.34 and 723.59 eV were attributed to Fe²⁺, and the peaks at 712.34 and 725.64 eV were attributed to Fe³⁺. The peaks at 718.85 and 732.46 eV were identified as satellite peaks [52–53]. After three cycles, the relative content of Fe³⁺ increased from 37.55% to 41.37%, and Fe²⁺ slightly decreased from 62.45% to 58.61% (Fig. 7(h)), indicating the excellent cyclic redox reaction between Fe²⁺ and Fe³⁺ [54]. The Co 2p XPS spectra of the fresh CoFe₂O₄ contained six peaks, which could be ascribed to Co³⁺ (779.83 and 795.69 eV), Co²⁺ (781.87 and 797.09 eV), and two satellite peaks, respectively (Fig. 6(f)) [55–57]. Similarly, the relative contents of Co²⁺ and Co³⁺ showed little change (<4%) before and after three reactions (Fig. 7(i)). These findings demonstrate that HA enhances the electron transfer in the Fe²⁺/Fe³⁺ and Co²⁺/Co³⁺ cycles, promoting the cyclic redox reaction of Fe²⁺ and Co²⁺ with H₂O₂ to continuously activate H₂O₂ [58].

Based on the results and discussion presented above, a possible mechanism of the CoFe₂O₄/H₂O₂/EDTA/HA systems is elucidated (Fig. 8). The surface of CoFe₂O₄ serves as an important catalytic center for the activation of H₂O₂. First, EDTA chelates with Fe²⁺ and Co²⁺ on the surface of CoFe₂O₄ to form Fe2+ surf-EDTA and Co2+ surf-EDTA, which then react with H₂O₂ to produce •OH (Eq. (3)) [59].

The •OH can attack organic contaminants adsorbed on the surface of CoFe₂O₄ and degrade them. The generating Fe3+ surf-EDTA and Co3+ surf-EDTA complexes can exist in alkaline environments instead of forming a precipitate [25], resulting in the applicable pH range being extended to circumneutral/alkaline levels. Then, Fe3+ surf-EDTA and Co3+ surf-EDTA can again react with H₂O₂ or •OOH to regenerate Fe2+ surf-EDTA and Co2+ surf-EDTA (Eqs. (4), (5)) [60]. These two steps typically occur at a slow rate.

As a typical reductant, HA can accelerate the reduction reactions of Fe3+ surf-EDTA and Co3+ surf-EDTA (Eq. (6)) [61].

Remarkably, HA is introduced to induce the generation of a secondary intermediate (•NH₂O), which exhibits higher reactivity than HA [62]. •NH₂O can reduce Fe3+ surf-EDTA and Co3+ surf-EDTA into Fe2+ surf-EDTA and Co2+ surf-EDTA (Eq. (7)) [63].

Additionally, the electron density of Fe3+ surf-EDTA and Co3+ surf-EDTA shifts toward the carboxyl group, resulting in greater electron deficiency in the active centers of Fe³⁺ and Co³⁺ [64]. This phenomenon also promotes the transfer of electrons from electron donors (such as H₂O₂, HA, and •NH₂O) to Fe³⁺ and Co³⁺, forming Fe²⁺ and Co²⁺ [65]. This enables Fe2+ surf-EDTA and Co2+ surf-EDTA to continuously activate H₂O₂, generating more •OH and enhancing the efficiency of pollutant removal. Thus, a sustainable cycle reaction system is established.