Fabrication of MoS₂@Fe₃O₄ Magnetic Catalysts with Photo-Fenton Reaction for Enhancing Tetracycline Degradation

Abstract

Tetracycline (TCs) is widely used in the treatment of human and animal infectious disease. TCs gives rise to a growing threat to the human health and environment protection due to its overuse. Therefore, it is important to remove TCs contaminants from waste effluents. In this work, MoS₂@Fe₃O₄ catalytic material was fabricated by the simple hydrothermal method, which was applied in the photo-Fenton system to degrade TCs. The crystal structure, surface morphology, elemental composition, chemical state, electrochemical properties, and separability of MoS₂@Fe₃O₄ catalytic materials were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), conventional and high-resolution transmission electron microscopy (TEM/HRTEM), X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), and vibrating sample magnetometry (VSM). Furthermore, MoS₂@Fe₃O₄ could degrade 98.6% of TCs within 60 min under the optimum reaction conditions (the catalyst dosage of 3 g/L, H₂O₂ concentration of 5 mmol/L, the initial TCs concentration of 50 mg/L, and the initial pH of 5), which was a significant increase compared with pure Fe₃O₄. MoS₂ can accelerate the Fe³⁺/Fe²⁺ cycle through electron transfer from Mo⁴⁺ to Fe³⁺, resulting in the improvement in the degradation efficiency of TCs. The quenching and electron paramagnetic resonance (EPR) results showed that OH^• and photogenic hole h+ was the main active species in the photo-Fenton system. What is more, MoS₂@Fe₃O₄ catalytic materials had remarkable stability and reusability, and can be handily regained via magnetic separation technology in a real scenario.

1. Introduction

Compared with other antibiotics, tetracycline (TCs) is widely used in animal husbandry, aquaculture industry, and the medical industry due to its advantages of low cost, good effects, and wide antibacterial range [1,2]. Because it is biologically active, stable, and non-biodegradable, TCs has now been detected in surface water, groundwater, and sediments [3]. Residual TCs in the environment can threaten the aquatic and microbial community structure and increase the risk posed by antibiotic-resistant pathogens, which can be transferred throughout the food chain, causing longer-term harm to the environment and human health [4,5]. Therefore, how to efficiently remove TCs pollutants has gradually become a research hotspot.

Several methods, including physical adsorption [6,7], membrane treatment [8], biodegradation [9,10], advanced oxidation processes (AOPs) such as photocatalytic degradation [11], ozonation [12], Fenton and Fenton-like technology [13,14], and photo-Fenton processes [15], are utilized to remove TCs from polluted water. Specifically, the heterogeneous photo-Fenton process, which combined the features of photocatalysis and Fenton method, can realize the rapid degradation of TCs. Due to the use of sunlight as a light source, the photocatalytic reaction process is green non-pollution and huge energy release. Meanwhile, the introduction of hydrogen peroxide (H₂O₂) can boost the yield of OH^•, form an iron cycle reaction, reduce the drainage of ferrous ions (Fe²⁺), and avoid the generation of iron sludge.

In recent years, thanks to the excellent catalytic properties and magnetic characteristics, transition elements hybrid oxides have become promising catalysts in oxidative degradation. Some studies have suggested that Fe₃O₄ nanoparticles can effectively catalyze the decomposition of H₂O₂ to produce OH^•, and their pH range is wide, playing a leading role in the Fenton reaction. Meanwhile, Fe₃O₄ nanoparticles after reaction can be quickly separated using its magnetic properties, which reduced the formation of iron sludge [16,17,18]. Due to the inverse spinel structure of Fe₃O₄, electrons can transmit between Fe²⁺ and Fe³⁺ at the octahedral location, promoting the mineralization of pollutants. However, after several reaction cycles, its catalytic activity gradually decreased, and due to the rapid recombination of photoelectron hole pairs, its photocatalytic performance was not ideal, so it is necessary to modify it to enhance their catalytic activity and increase the separation rate of photo generated electron hole pairs [16,19].

MoS₂ is a typical transition-metal dichalcogenide (TMD) which features a 2D layered structure similar to that of graphene, with high edge molecular activity and an adjustable band gap [20]. MoS₂ can be transformed from an indirect band gap semiconductor to a direct band gap semiconductor as the number of layers decreases. Correspondingly, the band gap increases from 1.29 eV to 1.97 eV, extending the absorption spectrum to the visible region [21]. At the same time, MoS₂ with small sizes or fewer layers has excellent peroxidase-like activity, through which H₂O₂ can be efficaciously converted to OH^• [22,23]. Therefore, MoS₂ exhibits amazing catalytic activity, excellent optical absorption, and high photothermal efficiency. Recent evidence demonstrated that MoS₂ can accelerate the Fe³⁺/Fe²⁺ cycle through electron transfer from Mo⁴⁺ to Fe³⁺, which was attributed that the standard redox potential of Mo⁴⁺ (E₀ = 0.65 V) being lower than that of Fe³⁺ (E₀ = 0.77 V) [24,25]. Therefore, the combination of Fe₃O₄ nanoparticles and MoS₂ enhances the H₂O₂ activation, and facilitates the Fe³⁺/Fe²⁺ rapid cycle by MoS₂.

In this study, a series of magnetic MoS₂ @Fe₃O₄ catalysts was fabricated by a simple hydrothermal process, and its catalytic performance was explored through photo-Fenton experiments used TCs as the target pollutant. The morphology, structure, chemical composition, and electrochemical properties of the catalysts were determined using XRD, SEM, TEM, XPS, EIS, and VSM. The effects of catalyst dosage, H₂O₂ concentration, TCs initial concentration, and pH on the TCs degradation performance were studied. The stability and reusability of MoS₂ @Fe₃O₄ catalysts were verified by repeatability experiments and magnetic recovery experiments. The possible degradation mechanism was also discussed.

2. Materials and Methods

2.1. Materials and Reagents

Analytical-grade FeSO₄·7H₂O, Na₂S₂O₃·5H₂O, H₂O₂, and NaOH were purchased from Sinopharm Chemical Reagents Co., Ltd. (Shanghai, China). Tetracycline hydrochloride (TCs, analytical reagent) was obtained from Rhawn Technology Development Co., Ltd. (Shanghai, China). Ammonium molybdate (H₈MoN₂O₄, analytical reagent) and L-cysteine (C₃H₇NO₂S, analytical reagent) were obtained from Tianjin Damao Chemical reagent Factory (Tianjin, China).

2.2. Catalyst Synthesis

Fe₃O₄ powder was fabricated by the hydrothermal method [26]. They were synthesized as follows: FeSO₄·7H₂O (2.78 g) and Na₂S₂O₃·5H₂O (2.48 g) were dissolved in 50 mL distilled water to form solution A. Then, 0.8 g of NaOH were dissolved in 10 mL distilled water to form solution B. Solution A and B were mixed and evenly stirred for 20–30 min. Subsequently, the mixture was added into 100 mL Teflon-lined autoclave chamber and hydrothermally treated at 140 °C for 12 h. After cooling to room temperature, the solid was separated from the mixture by centrifugal separation, then washed three times with distilled water and anhydrous ethanol, and dried in an oven at 60 °C for 12 h. The final product was crushed to obtain the Fe₃O₄ powder.

In the synthetic process of MoS₂@Fe₃O₄, ammonium molybdate (0.5 g) were dissolved in 70 mL distilled water, then L-cysteine (1.4 g) was added and stirred evenly for 20 min. A certain amount of Fe₃O₄ powder was immersed in the previous solution. After ultrasonic dispersion, the solution was filled into a 100 mL hydrothermal reactor and treated at 200 °C for 24 h. Afterward, the solid was centrifuged at 10,000 rpm and rinsed repeatedly in ethanol. Subsequently, the product was dried under the vacuum condition at 60 °C. According to the mass ratio of MoS₂ and Fe₃O₄ (2%, 4%, and 6%) in the MoS₂@Fe₃O₄, the samples were denoted as 2% MoS₂@Fe₃O₄, 4% MoS₂@Fe₃O₄, and 6%MoS₂@Fe₃O₄.

2.3. Characterization Method

The physical phase and crystal structure of the sample were characterized by D8-FOCUS X-ray diffraction (XRD) in the scanning range of 5° to 70° at 2°/min using Cu-Ka as the radiation source (Bruker, Bremen, Germany). Apreo 2C scanning electron microscopy (SEM) (Thermo Fisher Scientific, Shanghai, China) was used to observe the apparent morphology of the samples, as well as to conduct EDS (EDS XD-3) (Shimadzu, Shanghai, China) analyses. The microstructure of the material was characterized by transmission electron microscopy (TEM) (JEOL, Tokyo, Japan), and the lattice of the material was characterized by HRTEM with SEAD. The elemental and chemical valence states of the materials was characterized by ESCALAB 250 X-ray photoelectron spectrometer (XPS) with Al-Kα radiation (Thermo Fisher Scientific, Shanghai, China). The Mott–Schottky test was performed by the CHI760E electrochemical workstation (Austin, TX, USA). The magnetic strength of the material in a magnetic field of 10,000 Oe was analyzed by a magnetometer (Westerville, OH, USA).

2.4. Degradation Performance Evaluation

The photo-Fenton degradation experiment was conducted with 50 mg/L TCs as the target pollutant, and a 400 W xenon lamp bulb with cut-off filters for λ = 420 nm as the light source. A certain amount of catalyst was added to 200 mL of the TCs solution, and then a certain concentration of H₂O₂ was added to the solution; then, the light source was turned on to trigger the photo-Fenton reaction. Subsequently, 4.0 mL suspension was collected every 10 min and filtered through a 0.45 μm filter. Finally, the concentration of TCs was measured using a UV spectrophotometer at 357 nm. The reusability and comparative study of MoS₂@Fe₃O₄ were further examined under the same conditions for five runs.

3. Results

3.1. Characterization

The crystal structure and phase composition of the prepared samples were analyzed by XRD, which are shown in Figure 1. The diffraction peaks at 30.3°, 35.6°, 43.4°, 57.4°, and 63.0° for the pure phase Fe₃O₄ sample corresponded to the characteristic peaks of the (220), (311), (400), (422), (511), and (440) crystal planes of the cubic crystal system (equiaxial crystal system) magnetite Fe₃O₄ (JCPDS card 19-0629), respectively [27]. The characteristic diffraction peaks of each crystal plane of Fe₃O₄ can be clearly seen in the MoS₂@Fe₃O₄ samples synthesized with different mass ratios of MoS₂ and Fe₃O₄. The diffraction peak of Fe₃O₄ showed a gradually weakening trend with the increase in the MoS₂ mass, indicating that the introduction of MoS₂ did not affect the crystal plane structure of Fe₃O₄. The characteristic diffraction peak of MoS₂ was not recognized in the XRD patterns, implying that MoS₂ was amorphous in the sample. As shown in JCPDS card 37-1492, the diffraction peaks of pure MoS₂ at 2θ₍₀₀₂₎ = 14.39, 2θ₍₀₀₄₎ = 29.03, 2θ₍₁₀₀₎ = 32.69, 2θ₍₁₀₁₎ = 33.52, 2θ₍₁₀₂₎ = 35.87, 2θ₍₁₀₃₎ = 39.55, 2θ₍₀₀₆₎ = 44.16, 2θ₍₁₀₅₎ = 49.80, 2θ₍₁₀₆₎ = 55.98, 2θ₍₁₁₀₎ = 58.34, 2θ₍₁₁₄₎ = 64.14, 2θ₍₁₀₈₎ = 70.16, 2θ₍₂₀₃₎ = 72.79, and 2θ₍₁₁₆₎ = 75.99 corresponded to hexagonal structure having space group P6₃/mmc [28]. The peak intensity of MoS₂ phase is noticeably weak or absent in 2% MoS₂@Fe₃O₄; however, in case of 4% MoS₂@Fe₃O₄, the intensity of the XRD characteristic peaks of MoS₂ has been improved. In fact, the main peaks of MoS₂ corresponding to planes (100), (102), and (114) are clearly visible in 4% MoS₂@Fe₃O₄, which confirmed the MoS₂ phase was introduced into the Fe₃O₄ phase.

As depicted in Figure 2a,b, the apparent morphology of 4% MoS₂@Fe₃O₄ catalyst was analyzed by scanning electron microscopy (SEM). It can be observed that Fe₃O₄ presented a polyhedral structure with uniform particle size between 50 and 80 nm, and good dispersity. The elemental distribution of MoS₂@Fe₃O₄ was also investigated by EDS element mapping. Figure 2c shows that the O, Fe, Mo, and S elements were uniformly distributed in the surface of sample, which confirmed the successful preparation of MoS₂@Fe₃O₄. Because MoS₂ attached to the Fe₃O₄ surface, the map of Mo and S elements was clearer.

In order to further analyze the microstructure, 4% MoS₂@Fe₃O₄ catalyst was characterized by TEM and HRTEM, and the results are shown in Figure 3. From Figure 3a, it can be seen that the MoS₂ was successfully loaded on the surface of Fe₃O₄ particles, and the MoS₂ had better dispersity and no aggregation phenomenon. At the same time, Fe₃O₄ presented a polyhedral structure with a particle size of about 50–80 nm, which was consistent with the SEM spectra. In Figure 3b, two kinds of lattice fringes can be clearly observed by high-resolution HRTEM. The measured lattice spacing of 0.27 nm corresponded to the (100) crystallographic plane of MoS₂, while that of 0.297 nm corresponded to the (110) crystal plane of Fe₃O₄ [29], which confirmed that MoS₂@Fe₃O₄ catalyst was successfully synthesized.

An XPS measurement was conducted to reveal the surface elemental composition and chemical state of 4% MoS₂@Fe₃O₄. Figure 4a is the wide scan spectrum of MoS₂@Fe₃O₄ catalyst, indicating the existence of Mo, S, Fe, and O elements on the MoS₂@Fe₃O₄ surface. Meanwhile, the high-resolution XPS spectra of Fe 2p, O 1s, Mo 3d, and S 2p were detected. As shown in Figure 4b, the peaks at 712.64 eV and 726.44 eV were consistent with Fe³⁺ at Fe 2P_3/4 and Fe 2p_1/2, respectively, and the peaks at 709.44 eV and 723.1 eV were fitted with Fe²⁺ at Fe 2P_3/4 and Fe 2p_1/2, respectively [18,30]. The O 1s spectrum in Figure 4c exposed two obvious peaks at 530.3 eV and 531.6 eV, which belonged to Fe−O of Fe₃O₄ and Fe−OH/−OH groups, respectively [31]. The peaks at 232.2 V and 229.3 eV represented Mo 3d_5/2 and Mo 3d_3/2, respectively, which indicated the predominant presence of Mo⁴⁺ (Figure 4d) [32]. In the S 2p spectrum (Figure 4e), the presence of two peaks at 163.1 eV and 161.8 eV correspond to S 2p_3/2 and S 2p_1/2 stated in metal sulfides [33], respectively.

To investigate the electrochemical properties, pure Fe₃O₄, MoS₂, and MoS₂@Fe₃O₄ were tested by AC impedance. As shown in Figure 5, the Nyquist arc radius of 4% MoS₂@Fe₃O₄ was significantly smaller than that of pure Fe₃O₄ and MoS₂ sample, which implied a smaller charge transfer resistance as a result of the fast migration of photogenerated carriers after the introduction of MoS₂ [34]. During the photo-Fenton reaction, the conversion between Fe³⁺ and Fe²⁺ can be promoted, which in turn improved the catalytic activity.

3.2. Evaluation of MoS₂@Fe₃O₄ Photo-Fenton Catalytic Activity

3.2.1. Photo-Fenton Degradation Performance

To evaluate the effect of different mass ratios on the photo-Fenton catalytic performance of MoS₂@Fe₃O₄, three samples were tested under the same reaction conditions: the concentration of TCs was 50 mg/L, solution volume was 200 mL, H₂O₂ concentration was 5 mmol/L, catalyst dosage was 3 g/L, pH was 5.0, and temperature was room temperature. It can be seen from Figure 6a that the TCs degradation efficiency of 2% MoS₂@Fe₃O₄, 4% MoS₂@Fe₃O₄, and 6% MoS₂@Fe₃O₄ materials was 90.8%, 98.6%, and 93.9%, respectively, of which 4% MoS₂@Fe₃O₄ had the best catalytic activity. When the content of MoS₂ was too small, too high an Fe₃O₄content would hinder the absorption of visible light by MoS₂, so that the quantum yield of light was low and the photocatalytic efficiency was low. When the content of MoS₂ was too large, too low an Fe₃O₄ content would reduce the efficiency of the dominant Fenton reaction. Therefore, 4% MoS₂@Fe₃O₄ had the best photo-Fenton degradation efficiency of TCs, and the latter experiments were chosen 4% MoS₂@Fe₃O₄ as catalyst.

To further investigate the catalytic activity of 4% MoS₂@Fe₃O₄, the TCs degradation processes were compared with among different systems, involving vis-Fe₃O₄, Fe₃O₄-H₂O₂, vis-Fe₃O₄-H₂O₂, vis-MoS₂@Fe₃O₄, MoS₂@Fe₃O₄-H₂O₂, and vis-MoS₂@Fe₃O₄-H₂O₂ systems, and the results are shown in Figure 6b (Catalyst = 3 g/L, H₂O₂ = 5 mmol/L, TCs = 50 mg/L, pH = 5, 200 mL). As depicted in Figure 6b, the photocatalytic degradation of TCs was not obvious under visible light irradiation and pure Fe₃O₄ as catalyst. When the MoS₂@Fe₃O₄ catalyst was added, the TCs degradation efficiency during photocatalysis reached 28%. The Fenton system using pure Fe₃O₄ as catalyst exhibited significantly lower TCs degradation efficiency compared to using MoS₂@Fe₃O₄ (39%). Under the condition of photo-Fenton, using MoS₂@Fe₃O₄ as catalyst, the degradation efficiency of TCs reached 98.6%, which was significantly higher than that of pure Fe₃O₄. It indicated that visible light and the Fenton reaction had a synergistic effect, which significantly enhanced the degradation efficiency. Meanwhile, the introduction of MoS₂ can accelerate the Fe³⁺/Fe²⁺ cycle through electron transfer from Mo⁴⁺ to Fe³⁺, resulting in the improvement in degradation efficiency of TCs. At the same time, MoS₂ can enhance the transfer efficiency of photogenerated electron holes and restrain the recombination of electron holes, thus improving the performance of photo-Fenton process. This result was consistent with the previous result of AC impedance.

3.2.2. Different Influencing Factors

The influences of various reaction conditions such as catalyst dosage, H₂O₂ concentration, TCs concentration, initial pH on the photo-Fenton reaction were investigated. The catalytic efficiency of TCs at different catalyst dosage is depicted in Figure 7a (H₂O₂ = 5 mmol/L, TCs = 50 mg/L, pH = 5, 200 mL). The degradation efficiency of TCs increased from 85.1% to 98.6% when the catalyst dosage increased from 1 to 3 g/L, and then reduced to 89.4% with elevated 4% MoS₂@Fe₃O₄ dosage to 5 g/L. The excessive catalyst dosage inhibited the absorption of visible light, reduced the production of photogenerated electrons and holes, and thus lowered the catalytic efficiency. Meanwhile, the scavenging of OH^• radicals by excessive Fe²⁺ led to a decline in OH^• radical generation [35]. Therefore, the catalyst dosage of 3 g/L was selected in subsequent experiments.

Figure 7b demonstrated that the catalytic efficiency of TCs increased from 82% to 98.6% with the increase in H₂O₂ concentration from 3 mmol/L to 5 mmol/L, and then the efficiency had a slight decline with the further rise in the H₂O₂ concentration to 9 mmol/L (Catalyst = 3 g/L, TCs = 50 mg/L, pH = 5, 200 mL). This might be due to the fact that H₂O₂ was responsible for the formation of OH^•, and a higher concentration of H₂O₂ would promote to product more OH^•. However, excessed H₂O₂ may further react with OH^• and convert it into HO₂^•. which was in poor reactivity because of its lower oxidation potential [14]. Therefore, although the H₂O₂ concentration was high, the OH^• production decreased, resulting in a lower TCs degradation efficiency. Therefore, 5 mmol/L was selected as the addition concentration of H₂O₂ for subsequent experiments.

The concentration of TCs changed in the range of 40 to 60 mg/L, and the photo-Fenton experiment was carried out (Catalyst = 25 mg, H₂O₂ = 5 mmol/L, pH = 5, 200 mL). As displayed in Figure 7c, the photocatalytic performance of TCs reduced with the increase in TCs concentrations. This was because the amount of catalyst and oxidizer was fixed, so the same number of active species was formed. Meanwhile, high concentrations of TCs may reduce catalyst activity by adsorbing and blocking the surface of the catalyst, or consume H₂O₂ by depleting reactive oxygen species, and trigger side reactions [36]. Considering that the economy, 50 mg/L was determined as the appropriate TCs concentration.

Figure 7d shows the degradation curves of TCs at different pH (Catalyst = 3 g/L, H₂O₂ = 5 mmol/L, TCs = 50 mg/L, 100 mL). The TCs degradation increased from 90% to 98.6% after 60 min with the increase in pH from 3 to 5. According to the pH value of the solution, TCs exists in three forms, specifically, the cationic form when pH < 3.3, the zwitterionic form with a pH of 3.3–7.7, and the anionic form in alkali environment (pH > 7.7) [37]. The cationic form is less vulnerable to reactive oxygen than the zwitterionic form. Consequently, under the condition of pH = 3, TCs cations were formed by protonation, which reduced the degradation efficiency of TCs slightly. When pH further increased to 9 the degradation efficiency of TCs decreased to 62%. This might be due to the quick consumption of H₂O₂ in an alkaline environment. Therefore, pH = 5 was applied in the follow-up experiment.

3.2.3. Stability and Recyclability of MoS₂@Fe₃O₄

In order to evaluate the stability and recyclability of MoS₂@Fe₃O₄ catalyst, the degradation of TCs in recycling experiments was carried out (Catalyst = 3 g/L, H₂O₂ = 5 mmol/L, TCs = 50 mg/L, pH = 5, 200 mL), and the experimental results are displayed in Fig8a. The catalytic efficiency of TCs reduced from 98.6% to 92.1% after five cycles, demonstrating its remarkable stability and reusability. The reason for the slight decrease in degradation efficiency was that the active sites were blocked by the adsorbed TCs and TCs degradation intermediates after repeated use of the catalyst [38]. At the same time, ICP results showed the concentration of the leached iron and molybdenum were 0.021mg/L and 0.009 mg/L, also indicated the MoS₂@Fe₃O₄ catalyst had a better stability. In addition, the MoS₂@Fe₃O₄ catalyst could be easily separated by a magnet after use. To analyze the magnetic characteristics of the samples, hysteresis loop analysis was performed on 4% MoS₂@Fe₃O₄, as shown in Figure 8b. It can be seen that the magnetic saturation value of pure Fe₃O₄ was 63.0 emu/g under an external magnetic field of 10,000 Oe, while that of MoS₂@Fe₃O₄ sample was 43 emu/g, indicating that the magnetism of the catalyst reduced after the introduction of MoS₂. The inset of Figure 8b showed the dispersion of MoS₂@Fe₃O₄ sample in an aqueous solution and in solution after 1 min under an applied magnetic field. MoS₂@Fe₃O₄ sample can be adsorbed by a magnet and the solution became clarified, which indicated that the magnetic properties of the MoS₂@Fe₃O₄ were weaker than those of pure Fe₃O₄, but can still be easily separated and reused under the action of applied magnetic field.

3.3. Degradation Mechanism

In the MoS₂@Fe₃O₄ photo-Fenton system, tertiary-butanol (t-BA), p-benzoquinone (p-BQ), sodium azide (NaN₃) and ammonium oxalate (250 mmol/L) were used as OH^•, O₂^•−, O₂¹ and hole (h+) inhibitors, and dominant active species were evaluated by the quenching experiments. As described in Figure 9a, the order of inhibiting effect on TCs degradation was t-BA > ammonium oxalate > p-BQ > NaN₃. When t-BA was added to the degradation system, the degradation efficiency of TCs dropped nearly to 25.63%, illustrating that OH^• was the main free radical, because t-BA is a strong quenching agent of OH^•. When the degradation of TCs in the quenched h+ system was 72.68%, indicating that photogenerated holes were also the important part of active species in the photo-Fenton system. In contrast, the catalytic efficiency of TCs was hardly affected and was still 90.85% and 96.72% after inhibition of O₂^•− and O₂¹. Therefore, OH^• and h+ were the main active groups of TCs degradation in the MoS₂@Fe₃O₄ photo-Fenton system.

Electron paramagnetic resonance spectroscopy (EPR) trapping tests were used to identify the dominant radical species in in the photo-Fenton system, which further supported the above conclusions. 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was as a radical scavenger of OH^• and O₂^•− in the MoS₂@Fe₃O₄ photo-Fenton system, and the results are shown in Figure 9b,c. The signal with an intensity ratio of 1:2:2:1 observed under the ultrapure aqueous phase test conditions belonged to the characteristic peak of typical DMPO-OH^•, confirming the formation of OH^• in the MoS₂@Fe₃O₄ photo-Fenton system (Figure 9b). At the same time, six characteristic peaks with almost equal intensity ratio observed under the methanol phase test conditions belonged to the characteristic peaks of typical DMPO-O₂^•−, confirming the formation of O₂^•− in the MoS₂@Fe₃O₄ photo-Fenton system (Figure 9c). The results of ESR spectra were in good agreement with the quenching experimental results.

4. Conclusions

In this study, a serial of MoS₂@Fe₃O₄ magnetic catalysts were fabricated by a simple hydrothermal process, which had an excellent photo-Fenton catalytic performance. The optimal reaction conditions were evaluated to be the catalyst dosage of 3 g/L, H₂O₂ concentration of 5 mmol/L, initial TCs concentration of 50 mg/L, and pH of 5. The photocatalytic degradation of TCs reached 98.6% within 60 min under the optimal conditions, which was a remarkable growth compared to the pure Fe₃O₄. MoS₂ can accelerate the Fe³⁺/Fe²⁺ cycle through electron transfer from Mo⁴⁺ to Fe³⁺, resulting in the improvement in degradation efficiency of TCs. EPR analysis and free radical quenching experiments results showed that OH^• and photogenic hole h+ played a great distribution on TCs degradation. Furthermore, MoS₂@Fe₃O₄ catalytic materials had significant stability and reusability, and can be conveniently collected by a magnet in a real scenario, which would reduce the cost of catalyst separation and recovery.