Constructing Bimetallic Cobalt-Iron MOF-derived Carbon for Efficiently Activating Peroxydisulfate to Improve Degradation of Organic Pollutants

1. Introduction

The outflow of wastewater in China has gotten more complex due to the rapid industrial and medical development of the country, making wastewater treatment a more challenging undertaking. One of the most urgent environmental issues that needs to be solved is treating dyeing wastewater and antibiotic wastewater [1,2]. Due to its high mineralization degree and effective pollutant degradation, sulfate-based high-level oxidation technology, a common advanced oxidation technology, offers promising application potential. Therefore, it is crucial to locate a good catalyst that does not produce secondary pollutants.

The activation of sulfate-based high-level oxidation technology using carbon compounds produced from MOFs is an effective technique for treating wastewater containing organic contaminants and has the potential for usage in real-world settings. For instance，Li et al. constructed a co-mixed empty carbon nitrate (ZCCN) catalyst loaded with metal organic skeleton MOF by eroding the boiling stone cyanide ZIF-67, which has excellent visible light capture and electron transfer performance, achieving 99% tetrahedron (TC) degradation and 65.9% peroxyl monosulfate (PMS) breakdown in 40 minutes[3].Zhang et al. obtained the MOFs derived carbon material NDHC with good structure and composition properties by simply thermal dissolving the boiling stone mimetic skeleton (ZIF) particles of phenolide resin (PR) coating, resulting in the NDHC removing 98% of BPA (20 ppm) in 5 minutes, which is superior to many other peroxide monosulfate (PMS) catalysts[4].Wang and others synthesized two Mn/C compounds (MnOx-NA and Mn Ox-A) from the mercury-metal organic skeleton (MOF) by combustion of nitrogen atmosphere and atmosphere[5].The prepared MnOx-NA table shows better activation properties than Mn Ox-A peroxide monosulfate (PMS) and can remove almost 100% of the uranium dihydroxide (SMT).Liu et al.reported the Hofmann-MOF-derived CoFeNi nanoalloy@CNT as a magnetic activator for peroxymonosulfate to degrade benzophenone-1 in water, CFNC could also be reusable and remain highly effective, stable[6].For the application of persulfate technology, it is crucially important theoretically and practically to improve the catalytic performance and stability of these derived carbon materials, address secondary pollution brought on by metal leaching, and explain the mechanism and pathway for activating PMS to degrade organic pollutants.

The activation of PMS for destroying organic contaminants in carbon materials is strongly promoted by a large number of transition metal ions.Carbon compounds generated from bimetallic MOFs are superior to monometallic MOFs. Co²⁺ has been reported to have the best effect on activating PMS for degradation when compared to other transition metal ions (including Ag⁺, Ce³⁺, Co²⁺, Fe²⁺, Fe³⁺, Mn²⁺, Ni²⁺, Ru³⁺, and V³⁺)[7,8]. metal-induced activation of PMS is validated as a convenient and efficient method[9], and Co is an extremely efficient metal for activating PMS[10,11,12,13]. So, utilizing NH₂-MIL-101(Fe/Co) as the substrate, this paper doped the Co²⁺ into the compound, and the carbon material was manufactured using a one-step calcination technique. In order to breakdown organic contaminants in water, the mechanism primarily activates PMS through the redox cycle of Fe²⁺/Fe³⁺ or Co²⁺/Co³⁺ in Fe-based MOFs and Co-based MOFs. The phase composition, microstructure, and surface properties of the synthesized Fe/Co-CNs were examined using a variety of characterisation techniques, and a series of degradation studies were carried out to investigate how well the Fe/Co-CNs activated PMS to break down TC. New concepts for effective water treatment were offered by investigating the potential degradation mechanism utilizing free radical capture experiments and electron paramagnetic resonance.

2. Experimental

2.1. Experimental Reagents and Instruments

Experimental instruments: FCD-3000 constant temperature air drying oven (Shanghai Langxuan Laboratory Equipment Co., Ltd.), ATY-224 electronic analytical balance (Shanghai Yueping Scientific Instrument Co., Ltd.), TG16G centrifuge (Hulankaida Scientific Instrument Co., Ltd.), OTF-1200X tubular furnace (Hefei Kejing Material Technology Co., Ltd.), HT-100*1 reactor (Anhui Kemi Machinery Technology Co., Ltd.), UV-1900PC UV-Vis spectrophotometer (Shanghai Aoyi Instrument Co., Ltd.), D/MAX-RBX X-ray photoelectron spectrum (Japan Rigaku Corporation), JEOL JEM 2100 transmission electron microscope (Japan Electronics Corporation), ESCALAB IIX X-ray Photoelectron Spectroscopy (XPS, USA Thermo Fisher Scientific Corporation), Nicolet 6700 Fourier transform infrared spectrometer (USA Thermo Fisher Scientific Corporation), ASAP2020 HD88 surface area and porosity analyzer (Micromeritics Instrument Corporation), EMX nano electron paramagnetic resonance spectrometer (Germany Bruker Corporation)

Reagents: Acetone, anhydrous ethanol, anhydrous methanol, N,N-dimethylformamide (DMF), iron(III) chloride hexahydrate (FeCl₃∙6H₂O), cobalt(II) nitrate hexahydrate (Co(NO₃)₂·6H₂O), 2-aminoterephthalic acid (NH₂-H₂BDC), and deionized water (DI water) were used in the experiment.

2.2. Materials Preparation

2.2.1. Synthesis of Metal Organic Frameworks (MOFs)

Through a straightforward solvothermal procedure, NH₂-MIL-101(Fe/Co) was effectively synthesized. In a 100 mL reactor with 15 mL of DMF, NH₂-H₂BDC (224.6 mg, 1.24 mmol) was added together with specific amounts of FeCl₃∙6H₂O and Co(NO₃)₂∙6H₂O (quantities listed in Table 1.1). Sonication was used to dissolve the solid entirely. The combination was then put into a high-pressure autoclave, maintained at 110 °C for 24 hours, and then allowed to cool down naturally. Centrifugation was used to separate the resulting dark brown powder for 10 minutes at 8000 rpm. To create the NH₂-MIL-101(Fe/Co) product, the product was next immersed in DMF for 14 hours, followed by ethanol for 24 hours, and lastly dried in a vacuum oven. Figure 1 depicts the precise synthesis procedure.

Figure 1. Preparation of Metal organic skeleton material NH₂-MIL-101 (Fe/Co).

Figure 1. Preparation of Metal organic skeleton material NH₂-MIL-101 (Fe/Co).

2.2.2. Synthesis of Metal Organic Framework-Derived Carbon Materials

It was possible to synthesize Fe/Co-CNs with a single carbonization step. In a tube furnace, operating at 700 °C with a heating rate of 5 °C/min and a dwell period of 3 hours, NH₂-MIL-101(Fe/Co) was calcined. To synthesize the final Fe/Co-CNs product, the resultant material was cooled to room temperature, three times washed by DI water and ethanol, and then dried in a vacuum oven.

2.3. Activation of Fe/Co-CNs for Degradation of Organic Pollutants by Peroxydisulfate

The reaction volume was 100 mL in a 250 mL beaker with magnetic stirring during the experiment, which was carried out in a water bath with a constant temperature of 25 °C. The steps are as follows: For 30 minutes, 10 mg of catalyst was mixed into 100 mL of 20 mg/L TC solution to create an adsorption-desorption equilibrium with the pollutant. To assess the adsorption capacity of the catalyst for TC, 3 mL of the solution were taken, centrifuged, and the supernatant's absorbance at 356 nm was determined. Following the addition of 20 mg of PMS to the solution, samples were taken at predetermined intervals, centrifuged, and the absorbance was calculated. The degradation rate was calculated from the following formula,degradation rate =(C₀-C_t)/C₀×100% . Where C₀ represents the starting concentration, C_t represents the concentration at time t, and denotes the degradation rate. 1 M NaOH and 1 M HNO₃ were used to change the pH in experiments. In tests involving catalyst recycling, the catalyst was cleaned with ethanol and DI water before being dried and put to use in the following test.

3. Results and Discussion

3.1. Structural and Morphological Characterization

3.1.1. Structural Characterization

The XRD patterns of Fe/Co-CNs-1, Fe/Co-CNs-2, Fe/Co-CNs-3, and Fe/Co-CNs-4 are revealed in Figure 2. As shown in Figure 2a, in the XRD pattern of Fe/Co-CNs-2, peaks located at 26.4°, 37.7°, 39.8°, 40.6°, 42.9°, 43.7°, 44.6°, 45.0°, 45.9°, 48.6°, 49.1°, 51.8°, 54.4°, 56.0°, 58.0°, 59.7°, 61.4°, 64.8°, 70.8°, 77.9°, 78.6°, 79.4°, 80.8°, 83.0°, 83.9°, 85.8°, and 88.1° correspond to the (020), (210), (002), (201), (211), (102), (220), (031), (112), (131), (221), (122), (230), (212), (301), (311), (141), (321), (123), (401), (133), (411), (250), (332), (251), (004), and (430) faces of Fe₃C (JCPDS#35-0772). This outcome signifies the presence of Fe₃C in the Fe/Co-CNs-2 material, exhibiting a high degree of crystalline quality [14]. Compared with the standard card of Co₂C (JCPDS#05-0704), the peaks at 56.6°, 59.4°, 64.4°, 76.3°, 79.4°, 82.5°, and 86.3° in Fe/Co-CNs-2 may correspond to the (121), (220), (002), (131), (022), and (212) faces of Co₂C, indicating the presence of a small amount of Co₂C in Fe/Co-CNs-2 [15]. As shown in Figure 2b, the peaks of the four materials with different ratios are basically the same, showing the derived peaks of Fe₃C and Co₂C.

Figure 1. (a) XRD patterns of Fe/Co-CNs-2; (b) XRD patterns of Fe/Co-CNs-1, Fe/Co-CNs-2, Fe/Co-CNs-3 and Fe/Co-CNs-4.

Figure 1. (a) XRD patterns of Fe/Co-CNs-2; (b) XRD patterns of Fe/Co-CNs-1, Fe/Co-CNs-2, Fe/Co-CNs-3 and Fe/Co-CNs-4.

3.1.2. Morphology and Structural Characterization

Fe/Co-CNs-2 was characterized by SEM for its morphology, as shown in Figure 2. From Figure 2-2(a), there were polygonal structures, which were unburned NH2-MIL-101(Fe/Co). The red circle in the magnified image of Figure 2-2(a) indicates this area, and Figure 2-2(b, c) show that the unburned bimetallic MOFs had internal cavities. This indicates that although the external structure of the bimetallic MOFs was intact, they had collapsed internally. Figure 2 d-f show another part of the surface morphology of Fe/Co-CNs-2, which indicates that the burned Fe/Co-CNs-2 had a block-like structure and was larger than Fe-CNs-7. To further verify the doping of Co, EDS elemental analysis was performed on Fe/Co-CNs-2. Figure 3-3(g-l) shows the element distribution of C, N, O, Fe, and Co in Fe/Co-CNs-2, indicating that these elements were all present and evenly distributed, there by demonstrating the successful doping of Co.

The TEM images of Fe/Co-CNs-2 were exhibited in Figure 3. From Figure 2(a-c) and Figure 3(a-c), it can be observed that there were large black spherical particles, indicating the presence of two metals, Fe and Co, which proves successful Co doping. Meanwhile the carbon layer was thicker, which is consistent with the larger bulk seen in SEM. Further magnification in Figure 3d revealed edge-like structures similar to hexagonal prisms, which may be structures retained by MOFs during baking. As shown in Figure 2(e, f) and Figure 3(e, f), the black large spherical particles were wrapped in a thick carbon layer, indicating that Fe and Co in Fe/Co-CNs-2 were tightly encapsulated in the carbon layer, preventing the leaching of metals.

3.1.4. XPS Analysis

To further analyze the types of surface elements, the valence states, and bonding behavior, Fe/Co-CNs-2 was subjected to XPS analysis. As shown in Figure 4a, Fe/Co-CNs-2 composite material contains five elements: C, Fe, Co, O, and N. The C 1s spectrum is shown in Figure 4b. The peak at 284.8 eV corresponds to the C-C bond, and the peak at 285.6 eV corresponds to the C-N bond of Fe/Co-CNs-2, which was calibrated with a binding energy correction of 284.8 eV. The Fe 2p spectrum is shown in Figure 2-4(c), and the characteristic peaks correspond to Fe²⁺ (710.7 and 723.9 eV) and Fe³⁺ (712.8 and 725.7 eV). The Co 2p XPS spectrum in Figure 2-4(d) shows that multiple valence states of Co (Co²⁺ and Co³⁺) in Fe/Co-CNs-2 were observed. Peaks at 780.6 eV, 785.2 eV, 796.4 eV, and 801.4 eV represent Co 2p_3/2 and Co 2p_1/2 orbitals of Co³⁺ and Co²⁺, respectively. The satellite peaks of Co 2p were observed at 788.8 eV and 805.3 eV [16,17]. This confirms that Co ions were successfully doped.

Figure 5 shows the Fourier transform infrared (FTIR) spectrum of Fe/Co-CNs-2. The peak at 560 cm^-1 corresponds to the Fe-O vibration, and the peak at 680 cm^-1 corresponds to the stretching vibration of Co-O, indicating the successful doping of Co [18]. The peak at 1200 cm^-1 corresponds to the asymmetric stretching vibration of C-O, and the peak at 1650 cm^-1 corresponds to the stretching vibration of the benzene ring, which may be from the unburned MOFs. The peak at 2990 cm^-1 corresponds to the stretching vibration of C-H. The peak at 3341 cm^-1 corresponds to the C-N group.

3.1.6. Calculation of Surface Areas

The nitrogen adsorption/desorption isotherm of Fe/Co-CNs-2 is shown in Figure 6. The specific surface area of Fe/Co-CNs-2 was determined to be 214.86 m²/g with an average pore size of 5.25 nm and an average pore volume of 0.14 cm²/g. According to the IUPAC classification, all the materials exhibited a typical type IV isotherm, and the hysteresis loop could be classified as H₄ [19]. The sharp increase in adsorption at low relative pressure verified the existence of micropores, and the hysteresis characteristics of the parallel adsorption/desorption curve represented capillary condensation in mesopores. The inset of the graph shows the pore size and pore volume distribution, indicating that the pore size of Fe/Co-CNs-2 was mainly distributed around 3 nm.

3.1.7. Raman Analysis

Figure 7 shows the Raman spectra of Fe/Co-CNs-2 and Fe/Co-CNs-4. A relatively broad D peak (1350 cm^-1) and a G peak (1590 cm^-1) were observed in all Raman spectra. The I_D/I_G of Fe/Co-CNs-4 was about 1.12, while Fe/Co-CNs-2 had an I_D/I_G ratio as high as 2.01. This indicates that Fe/Co-CNs-2 increases the degree of defects in the material, which can better activate PMS [20]. Therefore, the degradation performance of catalysts with different doping ratios may vary significantly [21].

3.2. Evaluation of Fe/Co-CNs' Performance in Pollutant Degradation

To find the ideal ratio, the catalytic activities of Fe/Co-CNs with various relative mass ratios were investigated because the quantity of doping Co²⁺ may alter the synergistic effects of Fe³⁺ and Co²⁺. The degradation efficiencies of Fe/Co-CNs-1, Fe/Co-CNs-2, Fe/Co-CNs-3, Fe/Co-CNs-4, and Fe/Co-CNs-5 were 80.54%, 93.34%, 86.47%, and 83.51%, respectively, as shown in Figure a. The best treatment efficacy was attained with an increase in doping amount of the Co ion when the Co ion doping amount reached 20% within 30 minutes. However, when the Co ion doping level increased, the degradation rate reduced. This could be attributed to an excessive presence of Co ions, which may have disrupted the formation of MOFs and led to diminished performance of the resulting carbonized materials. Furthermore, Fe/Co-CNs-2 greatly outperformed Fe-CNs-7 in its ability to degrade TC by activating PMS at 78.83%. This suggests that PMS and the synergistic impact of Co ions and Fe ions can effectively degrade TC [22]. The degradation rate of TC by Fe/Co-CNs-2 alone was only 28.39% during 30 min, as shown in Figure 8(b). The degradation rate of TC when PMS was present alone was 40.54% within 30 minutes, demonstrating that the removal capacity was constrained when catalyst and PMS operated separately. The degradation rate of TC by the Fe/Co-CNs-2/PMS system was much higher than that of the Fe/Co-CNs-2/PDS system. This suggests that Fe/Co-CNs-2's ability to activate PMS is superior than that of Fe/Co-CNs-2's ability to activate PDS.

4. Factors Affecting Pollutant Degradation by Fe/Co-CNs-2

4.1. Catalyst Dosage

We investigated the impact of different Fe/Co-CNs-2 dosages (0.05 g/L, 0.1 g/L, and 0.2 g/L) on the degradation of TC by activating PMS since catalyst dosage has a significant impact on the efficiency of the degradation process. According to Figure 9, when the Fe/Co-CNs-2 dosage was 0.05 g/L, the degradation rate of TC within 30 min was 87.92%, and when the dosage was increased to 0.1 g/L, the degradation rate reached 93.69% within 30 min . This is mostly because the system has more catalytic sites that can be used directly for the reaction. Adsorption-assisted catalysis can further improve the catalytic degradation process because as the catalyst concentration rises, so does the amount of TC adsorbed. The degrading efficiency was marginally increased over 0.1 g/L by gradually increasing the catalyst dosage to 0.2 g/L. This is most likely caused by the small amount of TC in the solution. Therefore, 0.1 g/L Fe/Co-CNs-2 was determined to be the best dosage for additional studies from an economic standpoint.

4.2. PMS Dosage

The amount of PMS used has been optimized to obtain the best catalytic activity of the Fe/Co-CNs-2/PMS system for the degradation of TC. According to Figure 10, when the PMS dosage was increased from 0.1 g/L to 0.2 g/L under the conditions of a Fe/Co-CNs-2 dosage of 0.1 g/L and an initial concentration of TC of 20 mg/L, the degradation rate of TC increased from 80.9% to 93.3%. When the PMS dosage was raised to 0.5 g/L, the degradation efficiency only slightly improved from 0.2 g/L to 0.5 g/L (2.13%), and the degradation rate did not significantly rise. This can be the outcome of excessive PMS brought on by upping the PMS dosage [23]. This study determined that 0.2 g/L was the ideal PMS dosage for subsequent tests.

4.3. TC Concentration

The ability of Fe/Co-CNs-2/PMS system's to degrade TC is significantly influenced by the TC concentration. The degradation rate of TC by the Fe/Co-CNs-2/PMS system was 93.16% when the TC concentration was 10 mg/L, as shown in Figure 11. When the TC concentration was increased to 20 mg/L, the degradation rate was 93.34%, and no significant change was observed compared to the concentration of 10 mg/L. The degradation rate dropped to 80.26% as the TC concentration increased further to 50 mg/L, and the degradation rate also slowed down as a result. Even though Fe/Co-CNs-2 has a great ability to degrade TC by activating PMS, the effect of the degradation likewise diminishes as the pollutant concentration rises. Therefore, a lower TC content would improve the system's ability to degrade. The ideal TC concentration was found to be 20 mg/L after taking the degrading performance of the catalyst into consideration.

4.4. Initial pH of the Solution

Along with the aforementioned experimental variables, initial pH of the solution has a significant impact on the degradation of TC over Fe/Co-CNs-2/PMS system. The effectiveness of TC degradation by the Fe/Co-CNs-2/PMS system was examined throughout a broad pH range (3.06–9.11), as shown in Figure 12. The degradation rate of TC during 30 minutes was 78.32% at a pH of 3.06. This occurs because, in acidic environments, PMS's H⁺ interacts with O-O to form an H-bond, which prevents PMS from decomposition [24]. The degradation efficiency of TC by the Fe/Co-CNs-2/PMS system increased to 92.14% when the pH value was raised to 5.12. The degradation efficiency of TC was 93.34% when the pH value climbed to 7.33. The efficiency of TC did not differ noticeably from neutral conditions when the pH level was 9.11. This suggests that a wide pH range for TC breakdown exists in the Fe/Co-CNs-2/PMS system.

4.5. Anions

Inorganic anions such as CO₃^2-, Cl^-, HPO₄^2-, HCO^3-, and SO₄^2- are frequently found in natural water and have been shown to significantly affect PMS degradation mechanisms. Because of this, understanding how these anions affect FeCo-CNs-2/PMS is crucial for their practical use in wastewater treatment [25].

As the concentration of CO₃^2- grew from 10 to 50 mmol, as shown in Figure 13(a), the inhibitory impact on the Fe/Co-CNs-2/PMS system in degrading TC increased steadily. This is such that the formation of active components is inhibited by the addition of CO₃^2-. Figure 13(b) demonstrates that Cl- inhibits the Fe/Co-CNs-2/PMS system's ability to break down TC and that this inhibitory impact increases somewhat as Cl- concentration rises from 10 to 50 mM. As the concentration of HPO₄^2- rises from 0 to 10 mM, the ability of the Fe/Co-CNs-2/PMS system to degrade TC decreases, as shown in Figure 13(c), hence reducing the breakdown of TC. The inhibitory effect on the system's ability to degrade TC gradually wanes when the HPO₄^2- concentration rises to 50 mmol, and once it reaches a specific level, it starts to encourage TC to be degraded by the Fe/Co-CNs-2/PMS system. This could be as a result of HPO₄^2- reacting with ¹O₂ initially to produce species with lower activity, preventing ¹O₂ from taking part in the reaction and preventing the degradation of TC . Figure 13 (d) demonstrates how the inhibitory effect of the system on TC degradation gradually declines as SO₄^2- concentration rises from 10 to 50 mM. At 50 mM, there is no discernible influence on TC degradation. This is so that more active components can be produced in the system when SO₄^2- concentration rises [26,27,28]. Figure 13 (e) demonstrates that adding 10 mM to 50 mM of HPO₄^2- to the solution had no impact on the rate.

5. Stability of Fe/Co-CNs-2 in Pollutant Degradation

Stability is additional significant marker for a catalyst's capacity to be used more effectively in practice. Through cycling studies, the stability of the Fe/Co-CNs-2 catalyst is examined in this section. Figure 14 depicts the cyclic degradation of TC by the Fe/Co-CNs-2/PMS system (TC initial concentration of 20 mg/L, Fe/Co-CNs-2 dosage of 0.1 g/L, PMS dosage of 0.2 g/L). We carried out numerous parallel trials, precipitated the catalyst after degradation, washed and dried the deteriorated material to acquire the material for the subsequent cycle since the catalyst material has a specific magnetic. The findings demonstrated that TC's degrading efficiency did not significantly decline after four cycles of use, peaking at 88.7% in the fourth cycle, demonstrating the catalyst's strong catalytic stability. Another element for gauging the stability of the catalyst is the degree of metal dissolution. It was discovered that 0.10 mg/L and 0.08 mg/L of iron and cobalt ions, respectively, dissolved in the Fe/Co-CNs-2/PMS system in 100 mL of 20 mg/L TC, proving that the activation of PMS by Fe/Co-CNs-2 is not activated by metal ion dissolution, but the catalyst.

6. Mechanism Analysis of Pollutant Degradation by Fe/Co-CNs-2

6.1. Free Radical Quenching Experiment of Fe/Co-CNs-2

This work carried out quenching tests by introducing various free radical scavengers and assessed the roles of free radicals in the degradation of TC in order to identify the active species in the FeCo-CNs-2/PMS system. We employed the following substances as scavengers of •OH, SO₄^•-, singlet oxygen (¹O₂), and superoxide (O₂^•-): tert-butanol (TBA), ethanol (EtOH), ferulic acid (FFA), and CHCl₃.

The degradation rate of TC did not significantly alter when 10 mM CHCl₃ was introduced to the solution, as seen in Figure 15. Even with a 100 mM addition of CHCl₃, the rate of TC breakdown only dropped by 1.6%. This suggests that the impact of Cl^- is negligible.

6.2. XPS Spectra of Fe/Co-CNs-2 after Degradation

Figure 16 shows the XPS spectra of the catalyst before and after degradation. After calibration with 284.8 eV for C, the characteristic peaks in the Fe 2p spectrum correspond to Fe²⁺ (710.8 and 723.9 eV) and Fe³⁺ (712.7 and 725.8 eV), while the peaks in the Co 2p_3/2 and Co 2p_1/2 orbitals of Co³⁺ and Co²⁺ are represented at 780.6 eV, 785.2 eV, 796.4 eV, and 801.4 eV, respectively, and the satellite peaks of the Co 2p are at 789.2 eV and 805.3 eV. The characteristic peaks of the Fe 2p and Co 2p spectra of the degraded Fe/Co-CNs-2 catalyst are slightly shifted compared to the before one. These results indicate that both Fe(II)/Fe(III) and Co(II)/Co(III) redox participate in the degradation of TC by the Fe/Co-CNs-2/PMS system.

By generating SO₄^•- and OH^-, Fe²⁺ and Co²⁺ lose an electron to be oxidized to Fe³⁺ and Co³⁺. In addition, as shown in Equation (12), Fe³⁺ and Co³⁺ can also be reduced back to Fe²⁺ and Co²⁺, respectively, while generating SO₅^•- and H⁺, which also ensures the catalytic reaction (Equations (5-3)-(5-6)) [29,30]. The SO₅^•- free radicals in the solution can react with each other to produce S₂O₈^2- and ¹O₂ (Equation (5-7)), or can react with HSO₅- to form ¹O₂ (Equation (5-8)) [31,32]. Finally, the resulting SO₄^•- free radical and ¹O₂ non-free radical degrade TC into water and carbon dioxide.

HSO₅^-→ SO₅^2-+ H⁺

(5-1)

HSO₅^-+ SO₅^2-→ 2SO₄^2-+ ¹O₂+ H⁺

(5-2)

Fe (Ⅱ) + PMS(HSO₅^-) → Fe(Ⅲ) + SO₄^•- + OH^-

(5-3)

Co (Ⅱ) + PMS(HSO₅^-) → Co (Ⅲ) + SO₄^•- + OH^-

(5-4)

Fe (Ⅲ) + PMS(HSO₅^-) → Fe(Ⅱ)+ SO₅^•-+H⁺

(5-5)

Co (Ⅲ) + PMS(HSO₅^-) → Co (Ⅱ)+ SO₅^•-+H⁺

(5-6)

SO₅^•-+SO₅^•-→ S₂O₈^2-+ ¹O₂

(5-7)

SO₅^2- + HSO₅^-→ HSO₄^2- + ¹O₂

(5-8)

7. Conclusion

In this study, a series of Fe/Co-CNs samples were successfully synthesized using straight forward solvent-thermal and one-step carbonization techniques. Several characterization techniques, including XRD, FTIR, Raman, SEM, TEM, XPS, and BET were used to determine the phase composition, microstructure, and surface characteristics of Fe-CNs. Meanwhile, the impacts of various parameters on the TC degradation in the Fe/Co-CNs/PMS system were discussed. Furthermore, the synthesized Fe/Co-CNs was utilized to activate PMS for TC degradation in simulated wastewater. The Fe/Co-CNs-2 present greater catalytic activity in the degradation of TC in the Fe/Co-CNs/PMS system, and under the ideal conditions (Fe/Co-CNs-2: 0.1 g/L, PMS: 0.2 g/L, pH=7.33), it was able to degrade 93.34% of 20 mg/L of TC in 30 min, outperforming other catalysts with different mass ratios under the same conditions. The system of Fe/Co-CNs-2/PMS showed remarkable stability. Moreover, radical capture experiments revealed that ¹O₂ was the primary free radical species participating in the reaction system.