Iron–Cobalt Bimetallic Metal–Organic Framework-Derived Carbon Materials Activate PMS to Degrade Tetracycline Hydrochloride in Water

Abstract

Organic pollutants entering water bodies lead to severe water pollution, posing a threat to human health. The activation of persulfate advanced oxidation processes using carbon materials derived from MOFs as substrates can efficiently treat wastewater contaminated with organic pollutants. This research uses NH₂-MIL-101(Fe) as a substrate, doped with Fe²⁺ and Co²⁺, to prepare Fe/Co-CNs through a one-step carbonization method. The surface morphology, pore structure, and chemical composition of Fe/Co-CNs were investigated using characterization techniques such as XRD, SEM, TEM, XPS, FT-IR, BET, and Raman. A comparative study was conducted on the performance of catalysts with different Fe/Co ratios in activating PMS for the degradation of organic pollutants, as well as the effects of various influencing factors (the dosage of Fe/Co-CNs, the amount of peroxymonosulfate (PMS), the initial pH of the solution, the TC concentration, and inorganic anions) on the catalyst’s activation of persulfate for TC degradation. Through radical quenching experiments and post-degradation XPS analysis, the active radicals in the FeCo-CNs/PMS system were investigated to explain the possible mechanism of TC degradation in the Fe/Co-CNs/PMS system. The results indicate that Fe/Co-CNs-2 (with a Co²⁺ doping amount of 20%) achieves a degradation rate of 93.34% for TC (tetracycline hydrochloride) within 30 min when activating PMS, outperforming other Co²⁺ doping amounts. In addition, singlet oxygen (¹O₂) is the main reactive species in the reaction system.

1. Introduction

With the rapid development of industries such as manufacturing and pharmaceuticals, the composition of discharged wastewater has become increasingly complex, making wastewater treatment more challenging. Among the pressing environmental issues that need to be addressed is the treatment of dye wastewater and antibiotic wastewater [1,2]. Tetracycline hydrochloride (TC) is a commonly used veterinary antibiotic in daily life, with the molecular formula C₂₂H₂₄N₂O₈∙HCl and a molecular weight of 480.9 g/mol. The maximum absorption peak wavelength of hydrochloric acid tetracycline appears at 365 nm in UV full-spectrum scanning. TC is widely used in the livestock and aquaculture industries. Due to the stability of the TC structure, it is difficult for animals to absorb it, and most of it is excreted in the form of waste. Additionally, it is hard to degrade in the environment. Most untreated waste will flow into water bodies, thereby causing pollution. It will then threaten human health through drinking water, the food chain, and other means [3,4]. Providing an effective method for the degradation of TC is crucial for maintaining the balance of the water environment.

Currently, the treatment technologies for hydrochloric acid tetracycline mainly include physical–chemical methods, biological methods, and advanced oxidation processes. Compared to traditional methods, advanced oxidation processes have widespread applicability, can effectively degrade most pollutants, can avoid secondary pollution, and have shorter reaction times, addressing the shortcomings of traditional methods. Advanced oxidation processes (AOPs) catalyze the generation of highly oxidative active species under certain conditions, leading to a series of reactions such as ring-opening cleavage of large, hard-to-degrade organic pollutants, ultimately degrading them into low-toxicity or non-toxic small molecules [5,6,7]. Advanced oxidation processes have attracted the attention of many researchers due to their ability to generate strong oxidizing radicals that can oxidatively degrade most organic pollutants [8]. In advanced oxidation processes, strong oxidizing species are not limited to •OH; SO₄^•− has gradually garnered the attention of researchers due to its stronger redox potential.

Persulfates mainly include two types: peroxydisulfate and peroxymonosulfate. Among them, persulfate ions (PDS, S₂O₈²⁻) and peroxymonosulfate ions (PMS, HSO₅⁻) are the primary sources of sulfate radicals [9]. PDS and PMS have symmetrical and asymmetrical structures, respectively. Compared to PDS, PMS has a shorter bond length (1.46 Å), which results in higher O-O bond energy. Due to the symmetrical structure of PDS, it possesses strong stability, requiring a higher input of energy to break the O-O bond in the structure into free radicals, whereas PMS is relatively simpler.

Under normal conditions, PMS and PDS can hardly react with organic pollutants; they need to be activated to generate free radicals or other reactive substances in order to be effective. General methods of activation include the use of external energy, chemical activators, and transition metals or metal oxides, among others. Compared to other activation methods, carbon materials have characteristics such as a large specific surface area, high porosity, and good conductivity, making them suitable as both adsorbents and catalysts. Therefore, carbon-based materials are widely used in the activation of persulfates. Carbon-based materials have good electrical conductivity, so they can provide electrons to PMS, thereby enabling it to release free radicals. The surface of carbon materials is inert, and its electron transfer efficiency and ability to adsorb pollutants can be affected. Therefore, the modification of carbon materials has become a current research hotspot. Common modification methods include nitrogen doping, sulfur doping, and metal oxide doping. After modification, the oxygen-containing functional groups on the surface may serve as the catalytic active sites [10].

Metal–organic framework (MOFs) are porous materials with a certain periodic network structure, synthesized through the self-assembly of organic ligands (such as aromatic compounds) and metal elements (such as Zn, Cu, Cd, Al, Zr, Fe, and Co) [11,12]. Due to advantages such as a large specific surface area, uniform pore size, and abundant active sites, it has received widespread attention in energy storage conversion, adsorption, and catalysis. MOFs mainly include the Uio series [13], ZIFs series [14], and MIL series [15], among others. According to the classification based on MOF components, they can be divided into single-component MOF materials, MOF composites, and MOF-derived materials. MOF-derived materials refer to various micro-nano materials (such as metal oxides, metal sulfides, metal phosphides, metal carbides, and carbon materials) that are prepared using MOF materials as precursors through methods like calcination, sulfidation, and phosphidation [16].

Derivatives of metal-carbon materials from MOFs have received increasing attention in recent years. During the pyrolysis process of MOFs, the distribution and uniformity of pore size can be optimized. As a result, the derived metal–carbon materials generally exhibit high porosity, uniformity, and adjustable pore sizes, along with well-defined characteristics of active sites. In addition, adjusting the thermal decomposition temperature of the MOF precursors can control the morphology of the derived metal–carbon composite materials. Due to the aforementioned characteristics, metal–carbon materials derived from MOFs have broad development prospects as raw materials for various water treatment processes. Therefore, numerous studies have reported on the application of metal–carbon materials derived from MOFs in advanced oxidation. Li et al. constructed a metal–organic framework (MOF)-supported cobalt-doped hollow carbon nitride (ZCCN) catalyst by etching the zeolitic imidazolate framework ZIF-67. This catalyst exhibits excellent visible light-capture and electron-transfer properties, achieving 99% degradation of tetracycline (TC) and 65.9% decomposition of peroxymonosulfate (PMS) within 40 min [17]. Zhang et al. obtained MOF-derived carbon material NDHC with good structural and compositional characteristics by simply pyrolyzing phenolic resin (PR)-coated zeolitic imidazolate framework (ZIF) particles. The results showed that NDHC removed 98% of BPA (20 ppm) within 5 min, outperforming many other peroxymonosulfate (PMS) catalysts [18]. Wang et al. synthesized two Mn/C composites (MnOx-NA and MnOx-A) from manganese–metal organic frameworks (MOF) through calcination in nitrogen and air atmospheres [19]. The prepared MnOx-NA exhibits superior peroxymonosulfate (PMS) activation performance compared to MnOx-A, and MnOx can remove nearly 100% of sulfanilamide (SMT). The magnetic carbon studied by Mao and others achieved a degradation rate of over 89% within 15 min [20]. After conducting 10 cycle experiments, it was proven that the synthesized efficient nano-carbon-based Co@C-600 exhibits high stability and reusability.

Therefore, the application of MOF-derived metal–carbon materials in advanced oxidation is very extensive. This study also selected NH₂-MIL-101(Fe) as a precursor for the preparation of carbon materials, as many transition metal ions significantly promote the activation of PMS for the degradation of organic pollutants in carbon materials. Bimetallic MOF-derived carbon materials are even superior to those derived from monometallic MOFs. Among various transition metal ions (such as Ag⁺, Ce³⁺, Co²⁺, Fe²⁺, Fe³⁺, Mn²⁺, Ni²⁺, Ru³⁺, and V³⁺), Co²⁺ was found to have the best activation effect on PMS for the degradation of pollutants [21]. Therefore, this article prepares carbon materials by doping metal ions Co²⁺ into the compound, using NH₂-MIL-101(Fe/Co) as the substrate, through a one-step calcination method. Mainly focusing on Fe-based MOFs and Co-based MOFs, the mechanism primarily involves the redox cycling of Fe²⁺/Fe³⁺ or Co²⁺/Co³⁺ to activate PMS, generating various reactive oxygen species (ROS) such as SO₄^•− to degrade organic pollutants in water bodies. The phase composition, microstructure, and surface characteristics of the prepared Fe/Co-CNs were analyzed through various representations, and a series of degradation experiments were conducted to explore the performance of Fe/Co-CNs in activating PMS for the degradation of TC. Studying the possible degradation mechanisms through free radical-capture experiments and electron paramagnetic resonance can provide new ideas for practical water treatment.

2. Experiment

2.1. Experimental Reagents

Ferric chloride hexahydrate (FeCl₃·6H₂O), N,N-dimethylformamide (DMF), acetone, and anhydrous ethanol were obtained from Sinopharm Group Chemical reagent Co., Ltd., located in Shanghai, China. 2-aminoterephthalic acid (NH₂-H₂BDC), cobalt(II) nitrate hexahydrate (Co(NO₃)₂·6H₂O), and anhydrous methanol was procured from Shanghai Aladdin Biochemical Technology Co., Ltd., located in Shanghai, China. Tetracycline hydrochloride was procured from Beijing Biosharp Biotechnology Co., Ltd., located in Beijing, China. The purity of all the chemical reagents was analytical grade, and water used in the experiment was deionized water.

2.2. Material Preparation

Add NH₂-H₂BDC (224.6 mg, 1.24 mmol) and a certain amount of FeCl₃·6H₂O and Co(NO₃)₂·6H₂O (amounts are shown in

Table 1. Dosage of experimental raw materials.

Co Doping Amount	NH2-H2BDC	FeCl3·6H2O	Co(NO3)2·6H2O	Labeled as
10%	1.24 mmol	2.25 mmol	0.25 mmol	Fe/Co-CNs-1
20%	1.24 mmol	2 mmol	0.5 mmol	Fe/Co-CNs-2
30%	1.24 mmol	1.75 mmol	0.75 mmol	Fe/Co-CNs-3
40%	1.24 mmol	1.5 mmol	1 mmol	Fe/Co-CNs-4
50%	1.24 mmol	1.25 mmol	1.25 mmol	Fe/Co-CNs-5
70%	1.24 mmol	0.75 mol	1.75 mmol	Fe/Co-CNs-7

) to a 100 mL glass liner containing 15 mL of DMF, and sonicate until the solid is completely dissolved. Then, place it in a pressure cooker, raise the temperature to 110 °C, and maintain it for 24 h, then allow it to cool naturally. Collect the brown-black powder by centrifuging at 8000 rpm for 10 min. Soak the product in DMF for 14 h, then soak it in ethanol (99.7%) for 24 h; finally, dry it in a vacuum oven to obtain the product NH₂-MIL-101(Fe/Co).

Under an Ar atmosphere, the prepared NH₂-MIL-101(Fe/Co) is calcined in a tube furnace at 700 °C, with a heating rate of 5 °C/min. After maintaining the temperature for 3 h, it is cooled to room temperature (20 °C), washed with water and alcohol three times, and then dried in a vacuum drying oven to obtain the final product Fe/Co-CNs. The specific preparation process is shown in Figure 1.

2.3. Material Characterization

An X-ray diffractometer (XRD, D/MAX-RB, Tokyo, Japan) was used to analyze the aggregated structure of the samples in the 2θ scanning range of 10–80° with a scanning speed of 0.02°. The test was operated with Cu-Kα radiation at a voltage of 40 kV and a current of 50 mA.

A scanning electron microscope (SEM, TalOs F200X, Thermo Fischel, Waltham, MA, USA) was used to observe the surface micromorphology of the samples, which were sprayed with gold at an acceleration voltage of 20 kV.

A Transmission Electron Microscope (TEM, TalOs F200X, Thermo Fischel, Waltham, MA, USA) was used to analyze the morphology and structure of composite materials and the distribution of different elements.

X-ray photoelectron spectroscopy (XPS, ESCALABII, Thermo Fischel, Waltham, MA, USA) was used to analyze chemical compositions and metal valence states, with the binding energy of C1s (284.8 eV) as the control standard and Al Ka as the X-ray source.

A Fourier transform infrared spectrometer (FTIR, Nicolet 6700, Thermo Fischel, Waltham, MA, USA) was used to characterize the functional groups of the samples in the wavelength range of 400–4000 cm⁻¹.

A specific surface area porosity analyzer (BET, ASAP2020 HD88, Mack Instruments, Arlington, VT, USA) was used to measure the specific surface area and aperture. The material was heat-treated at 120 °C under nitrogen atmosphere for 2 h, and then the sample was subjected to nitrogen adsorption and desorption under liquid nitrogen environment to measure the specific surface area and aperture.

A Raman spectrometer (Raman, Renishaw 2000, Shanghai, China) was used to analyze the surface molecular structure, with a spectral scanning range of 800–3000 cm⁻¹.

2.4. Activation of Fe/Co-CNs for Degradation of Organic Pollutants Using Peroxydisulfate

Under the conditions of a 25 °C constant temperature water bath, dissolve 10 mg of catalyst into 100 mL of a 20 mg/L TC solution, with 600 rpm magnetic stirring for 30 min, then centrifuge and take 3 mL of the supernatant to measure the absorbance of the solution at a wavelength of 356 nm. Subsequently, add 20 mg of PMS to the solution, and take samples at fixed time intervals, followed by centrifugation to measure the absorbance. The degradation efficiency formula is as follows:

$$\mathsf{\eta }=({\mathrm{C}}_0-{\mathrm{C}}_t)/{\mathrm{C}}_0\times 100\%,$$

(1)

where η is the degradation rate, C₀ is the initial concentration, and C_t is the concentration at time t.

In the pH influence experiment, adjust the pH using a 1 M NaOH solution and a 1 M HNO₃ solution. In the catalyst cycling experiment, the catalyst undergoes centrifugation, water washing, alcohol washing, and drying before the next experiment.

3. Results and Discussion

3.1. Characterization of Materials

3.1.1. XRD Analysis

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 spectrum of Fe/Co-CNs-2, compared to the Fe₃C standard card (JCPDS#35-0772), the peaks at 37.7°, 42.9°, 43.7°, 44.6°, 49.1°, 64.8°, and 83.0°correspond to the (210), (201), (211), (102), (131), (321), and (332) crystal planes of Fe₃C, consistent with the positions reported in the literature. This result indicates that Fe₃C exists in the Fe/Co-CNs-2 material and has good crystallinity [22]. Compared to the standard card of Co₂C (JCPDS#05-0704), the Fe/Co-CNs-2 material exhibits peaks at 56.6°, 59.4°, 64.4°, 76.3°, 82.5°, and 86.3°, which may correspond to the (121), (220), (002), (131), (022), and (212) crystal planes of Co₂C. This result suggests that there may be a small amount of Co₂C present in the Fe/Co-CNs-2 material [23]. As shown in Figure 2b, the peak shapes of the four materials with different ratios are basically consistent, all exhibiting derivative peaks of Fe3C and Co2C. The main characteristic peak of Fe/Co-CNs-2 is the strongest, indicating that when the cobalt ion doping amount is 20%, the crystallinity is better and the relative abundance is higher. Fe/Co-CNs-2 and Fe/Co-CNs-3 exhibit two additional distinct characteristic peaks near 2θ of 65° and 83° compared to Fe/Co-CNs-4 and Fe/Co-CNs-1, which may be attributed to the formation of FeO phases during the material-synthesis process.

3.1.2. SEM Analysis

The morphological characteristics of Fe/Co-CNs-2 were characterized using SEM, as shown in Figure 3. In Figure 3a, the polygonal structure can be seen, which is the unsintered NH2-MIL-101(Fe/Co). The red circle in the magnified image of Figure 3a indicates this area, while Figure 3b,c show that the unsintered bimetallic MOFs have internal cavities. This indicates that, although the external structure of the bimetallic MOFs is intact, their internal structure has collapsed. Figure 3d–f shows another part of the surface morphology of Fe/Co-CNs-2, indicating that the sintered Fe/Co-CNs-2 has a blocky structure. In order to further verify the Co doping, EDS elemental analysis was conducted on Fe/Co-CNs-2. Figure 3g–k shows the elemental distribution of C, N, O, Fe, and Co in Fe/Co-CNs-2, indicating that these elements are evenly distributed and all present. The quantitative results in

Table 2. The elemental composition of Fe/Co-CNs-2.

Element	Wt%	At%
C	81.62	86.74
O	3.90	5.41
Fe	9.47	4.89
Co	4.76	2.45

(Quant Result) show that both iron and cobalt have been successfully doped.

3.1.3. TEM Analysis

The TEM images of Fe/Co-CNs-2 are exhibited in Figure 4. The average particle size of Fe/Co-CNs-2 is between 100 and 200 nm. In Figure 4a–c, it can be observed that there are 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 reveals edge-like structures similar to hexagonal prisms, which may be structures retained by MOFs during baking. As shown in Figure 4e,f, the large black 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, valence states, and bonding behavior, Fe/Co-CNs-2 was subjected to XPS analysis. As shown in Figure 5a, the Fe/Co-CNs-2 composite material contains five elements: C, Fe, Co, O, and N. The C 1s spectrum is presented in Figure 5b, where 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. These values were calibrated with a reference binding energy of 284.8 eV. Regarding the Co 2p XPS spectrum in Figure 5d, multiple valence states of Co (Co²⁺ and Co³⁺) are evident. The peaks at 780.6 eV, 785.2 eV, 796.4 eV, and 801.4 eV represent Co 2p3/2 and Co 2p1/2 orbitals for Co³⁺ and Co²⁺. The observed satellite peaks at 788.8 eV and 805.3 eV are typically associated with Co³⁺, supporting the presence of cobalt in this oxidation state. The intensity of these satellite peaks can be related to the strong correlation effects in transition metals, indicating that they may contribute to both Co²⁺ and Co³⁺ states, although their exact attribution remains a subject of ongoing debate in the literature [24,25].

3.1.5. FTIR Analysis

Figure 6 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 [26]. Notably, we observe that the intensity of the Co-O peak increases significantly compared to the undoped sample, suggesting effective incorporation of cobalt into the structure. Furthermore, the peak at 1200 cm⁻¹ corresponds to the asymmetric stretching vibration of C-O, while the peak at 1650 cm⁻¹ corresponds to the stretching vibration of the benzene ring, which may arise from unburned MOFs. The intensity and position of these peaks can also indicate changes in the molecular environment due to Co doping. For instance, any shifts in the C-O or C-H stretching vibrations may reflect alterations in the interactions between the organic framework and the metal ions. The peak at 2990 cm⁻¹ corresponds to the stretching vibration of C-H, and the peak at 3341 cm⁻¹ corresponds to the C-N group. Any variations in these peaks can provide additional insights into the structural changes during the transition from the original compound to the doped material, further supporting the effectiveness of Co incorporation.

3.1.6. BET Analysis

The nitrogen adsorption/desorption isotherm of Fe/Co-CNs-2 is shown in Figure 7. 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 [27]. 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 8 shows the Raman spectra of Fe/Co-CNs-2 and Fe/Co-CNs-4. In these spectra, a relatively broad D peak can be observed at around 1350 cm⁻¹ and a G peak at approximately 1590 cm⁻¹. The D peak is related to defects and disorder in the carbon structure, while the G peak is associated with the stretching of sp² carbon atoms in graphite materials. These peaks are crucial for assessing the structural quality of carbon-based materials. The intensity ratio of the D peak to the G peak (ID/IG) can provide deep insights into the degree of disorder in the carbon matrix. The ID/IG ratio of Fe/Co-CNs-4 is approximately 1.12, indicating a moderate level of defects. In comparison, the ID/IG ratio of Fe/Co-CNs-2 is significantly higher at 2.01, indicating that the increase in defect levels can enhance the activation of peroxymonosulfate (PMS) [28]. This higher defect density can enhance catalytic performance. It is worth noting that in the spectrum of Fe/Co-CNs-2, there are signs of splitting in the D peak. This kind of splitting may be due to the presence of different types of defects or variations in the local electronic environment within the material. It is known that the heterogeneity of this structure can affect the vibration modes and provide additional active sites for catalytic reactions. Therefore, understanding the nature of the D peak splitting is crucial, as it reflects the material’s ability to activate PMS more effectively than Fe/Co-CNs-4, leading to the potential enhanced degradation performance indicated by previous studies [29].

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 9a. The best treatment efficacy was attained with an increase in the doping amount of the Co ion when the Co ion doping amount reached 20% within 30 min. 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 (materials without doped Co ions) 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 [30]. The degradation rate of TC by Fe/Co-CNs-2 alone was only 28.39% for 30 min, as shown in Figure 9b. The degradation rate of TC when PMS was present alone was 40.54% within 30 min, demonstrating that the removal capacity was constrained when the 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 to that of Fe/Co-CNs-2’s ability to activate PDS.

4. Degradation Experiment Research

4.1. Decomposing Impact Factors

4.1.1. 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 the catalyst dosage has a significant impact on the efficiency of the degradation process. According to Figure 10, 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.

4.1.2. PMS Dosage

The amount of PMS used was optimized to obtain the best catalytic activity of the Fe/Co-CNs-2/PMS system for the degradation of TC. According to Figure 11, 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 [31]. This study determined that 0.2 g/L was the ideal PMS dosage for subsequent tests.

4.1.3. TC Concentration

The ability of Fe/Co-CNs-2/PMS systems 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 12. 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.

4.1.4. Initial pH of the Solution

The initial pH of the solution also has an important impact on the degradation of TC in the Fe/Co-CNs-2/PMS system. As shown in Figure 13, the degradation efficiency of the Fe/Co-CNs-2/PMS system for TC was investigated over a wide pH range (3.06–9.11). When the pH value is 3.06, the degradation rate of TC within 30 min is 78.32%. This phenomenon occurs because, under acidic conditions, the H⁺ in PMS interacts with O-O to form hydrogen bonds, thereby inhibiting the decomposition of PMS [32]. When the pH value is 5.12, the degradation efficiency of TC by the Fe/Co-CNs-2/PMS system increases to 92.14%. As the pH value continues to rise to 7.33, the degradation efficiency of TC reaches 93.34%. At a pH value of 9.11, there is no significant change in TC degradation efficiency compared to a neutral environment. This indicates that the Fe/Co-CNs-2/PMS system can effectively degrade TC within a wide pH range.

4.1.5. Anions

Inorganic anions such as CO₃²⁻, Cl⁻, HPO₄²⁻, HCO³⁻, and SO₄²⁻ 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 [33].

As the concentration of CO₃²⁻ grew from 10 to 50 mmol, as shown in Figure 14a, 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 CO₃²⁻. Figure 14b 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 the Cl⁻ concentration rises from 10 to 50 mM. As the concentration of HPO₄²⁻ rises from 0 to 10 mM, the ability of the Fe/Co-CNs-2/PMS system to degrade TC decreases, as shown in Figure 14c, hence reducing the breakdown of TC. The inhibitory effect on the system’s ability to degrade TC gradually wanes when the HPO₄²⁻ 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 a result of HPO₄²⁻ 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 14d demonstrates how the inhibitory effect of the system on TC degradation gradually declines as SO₄²⁻ 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 the SO₄²⁻ concentration rises [34,35]. Figure 14e demonstrates that adding 10 mM to 50 mM of HPO₄²⁻ to the solution had no impact on the rate.

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

Stability is an 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 15 depicts the cyclic degradation of TC by the Fe/Co-CNs-2/PMS system (initial TC concentration of 20 mg/L, Fe/Co-CNs-2 dosage of 0.1 g/L, and PMS dosage of 0.2 g/L). We carried out numerous parallel trials, precipitated the catalyst after degradation, and 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 by the catalyst.

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

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

To identify the active species in the FeCo-CNs-2/PMS system, this study conducted quenching experiments by adding different radical scavengers and evaluated the contribution of radicals in the degradation process of TC. We used tert-butanol (TBA), ethanol (EtOH), furfuryl alcohol (FFA), and CHCl₃ as scavengers for •OH, SO₄^•−, singlet oxygen (¹O₂), and superoxide (O₂^•−), respectively. As shown in Figure 16, when 10 mM of CHCl₃ is added to the system, the degradation rate of TC remains essentially unchanged. Even when CHCl₃ is further added to the system up to 100 mM, the degradation rate of TC only decreases by 1.6%. This indicates that Cl⁻ has only a minimal role in the system. When 10 mM of tert-butanol was added to the system, the degradation rate of TC decreased by 4.5%. Continuing to increase tert-butanol to 100 mM, the degradation rate of TC only dropped by 5.3%, showing no significant change compared to 10 mM. This indicates that the contribution of •OH in the system to the degradation efficiency of TC is limited. When 10 mM of ethanol was added to the system, it was observed that the degradation rate of TC decreased by 20%. Further increasing the ethanol concentration to 100 mM resulted in a 31% decrease in the degradation rate. Although ethanol does have a certain capturing effect on hydroxyl radicals, the contribution of hydroxyl radicals to the degradation efficiency of the system is quite limited. This indicates that SO₄^•− plays a significant role in the degradation of TC in the system. When 10 mM of furfural was added to the system, the degradation rate of TC decreased by 50.4%. Continuing to increase the furfural concentration to 100 mM resulted in an 86.7% decrease in the degradation rate of TC, with its catalytic activity nearly completely suppressed. Therefore, it can be demonstrated that in this system, ¹O₂ plays a dominant role.

4.3.2. XPS Spectra of Fe/Co-CNs-2 After Degradation

Figure 17 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.

Based on the experimental results mentioned above and the research conclusions reported in previous literature [36,37], the following catalytic mechanism ideas can be proposed. First, when PMS (with ${HSO}_5^{-}$ as the main active ingredient) dissolves in the solution, it generates a small amount of ${SO}_5^{2-}$ (Formula (2)). ${SO}_5^{2-}$ can react with ${HSO}_5^{-}$ to form a small amount of ¹O₂ (Formula (3)) [38]. Then, the metal ions activate PMS to generate free radicals. As shown in Figure 16, the Fe element exhibits +2 and +3 oxidation states, while the Co element shows +2 and +3 oxidation states, primarily existing in the form of Co²⁺. Fe²⁺ and Co²⁺ catalyze the HSO₅⁻ component in PMS to generate ${SO}_4^{\bullet -}$ and OH⁻, while Fe²⁺ and Co²⁺ lose one electron and are oxidized to Fe³⁺ and Co³⁺, respectively. In addition, Fe³⁺ and Co³⁺ can also be reduced to Fe²⁺ and Co²⁺, respectively, while generating ${SO}_5^{\bullet -}$ and H⁺, which also ensures the catalytic reaction of the catalyst (Formulas (4)–(7)) [39,40]. The SO₅^•− radical in the solution can react in pairs to produce S₂O₈²⁻ and ¹O₂ (Formula (8)), while ${SO}_5^{\bullet -}$ reacts with ${HSO}_5^{-}$ to form ¹O₂ (Formula (9)) [41,42]. Ultimately, the obtained ${SO}_4^{\bullet -}$ radical and ¹O₂ non-radical degrade TC into water and carbon dioxide.

$${HSO}_5^{-}\to {SO}_5^{2-}+H^{+}$$

(2)

$${HSO}_5^{-}+{SO}_5^{2-}\to {2SO}_4^{2-}+{}^1{O}_2+H^{+}$$

(3)

$$Fe\left(\mathrm{II}\right)+PMS\left({HSO}_5^{-}\right)\to Fe\left(\mathrm{III}\right)+{SO}_4^{\bullet -}+{OH}^{-}$$

(4)

$$Co\left(\mathrm{Ⅱ}\right)+PMS\left({HSO}_5^{-}\right)\to Co\left(\mathrm{III}\right)+{SO}_4^{\bullet -}+{OH}^{-}$$

(5)

$$Fe\left(\mathrm{III}\right)+PMS\left({HSO}_5^{-}\right)\to Fe\left(\mathrm{Ⅱ}\right)+{SO}_5^{\bullet -}+H^{+}$$

(6)

$$Co\left(\mathrm{III}\right)+PMS\left({HSO}_5^{-}\right)\to Co\left(\mathrm{Ⅱ}\right)+{SO}_5^{\bullet -}+H^{+}$$

(7)

$${SO}_5^{\bullet -}+{SO}_5^{\bullet -}\to S_2{O}_8^{2-}+{}^1{O}_2$$

(8)

$${SO}_5^{2-}+{HSO}_5^{-}\to {HSO}_4^{2-}+{}^1{O}_2$$

(9)

5. Conclusions

In this study, a series of Fe/Co-CNs samples were successfully synthesized using straightforward 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. Fe/Co-CNs-2 presents 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. In summary, the research developed an efficient activator for persulfate activation using carbonized bimetallic MOFs. By loading metal iron and cobalt, it enhanced the ability of the reaction system to treat pollutants while significantly improving the stability of the catalyst. This provides a new solution for expanding the application of metal–organic framework compounds and their derivatives.