Effective Combination of MOF and MoS₂ Layers: A Novel Composite Material Capable of Rapidly Degrading Dyes

Abstract

This study successfully prepared MIL-101(Fe)@MoS₂ composite photocatalysts via hydrothermal methods to address the efficient removal of refractory organic dyes in dye wastewater. Characterization using X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) confirmed that molybdenum disulfide (MoS₂) was uniformly loaded onto the surface of MIL-101(Fe), forming a heterojunction that significantly enhanced light absorption capacity and charge separation efficiency. In a visible-light-driven photo-Fenton system, this material exhibited excellent degradation performance for Congo red (CR). At an initial CR concentration of 50 mg/L, a catalyst dosage of 0.2 g/L, 4 mL of added H₂O₂, and pH 7, CR was completely degraded within 30 min, with the total organic carbon (TOC) removal reaching 72.5%. The material maintained high degradation efficiency (>90%) across a pH range of 3–9, overcoming the traditional Fenton system’s dependency on acidic media. Radical-trapping experiments indicated that superoxide radicals (·O₂⁻) and photogenerated holes (·h⁺) were the primary active species responsible for degradation, revealing a synergistic catalytic mechanism at the heterojunction interface. Recyclability tests showed that the material retained 90.8% degradation efficiency after five cycles, and an X-ray photoelectron spectroscopy (XPS) analysis demonstrated the stable binding of Fe and Mo, preventing secondary pollution. This study provides a scientific basis for developing efficient, stable, and wide-pH adaptable photo-Fenton catalytic systems, contributing significantly to the advancement of green water treatment technologies.

1. Introduction

With industrial development and population growth, the problem of water pollution is becoming increasingly serious, especially regarding the wastewater discharged from the printing and dyeing industry, which contains a large amount of organic dyes that are difficult to degrade [1]. Congo red (CR), as a typical azo dye, is widely utilized in the textile industry [2,3] According to GB8978-1996 [4], the wastewater discharge standard stipulates that the concentration of CR in wastewater must not exceed 0.1 mg/L. CR is difficult to degrade, and it poses significant carcinogenic risks to humans and other organisms. Moreover, CR causes environmental harm and poses a serious threat to both the environment and human health [5,6,7]. Therefore, the development of efficient, economical, and convenient wastewater treatment technologies is particularly important [8].

Among many wastewater treatment technologies, dye removal using advanced oxidation processes (AOPs) stands out as one of the most promising and effective techniques. This method excels in achieving rapid decolorization and environmental protection [9,10,11]. Recent studies have highlighted the potential of alternative AOPs, such as ultrasonic irradiation for the disruption of complex dye molecules [12] and electrochemical oxidation for the mineralization of persistent organic pollutants [13] However, challenges including high energy consumption and secondary pollution limit the scalability of these techniques. Photo-Fenton, as the most typical and widely studied AOP, has attracted much attention due to its strong oxidizing effect, high efficiency, and environmental friendliness [14]. Photo-Fenton oxidation is an important AOP [15] that is suitable for treating wastewater containing refractory organic compounds. In the photo-Fenton reaction system, iron ions can catalyze hydrogen peroxide to produce various highly oxidized oxygen species, such as ¹O₂ and ·O₂⁻, which can convert the macromolecules in organic dyes into carbon dioxide and water [16].

Metal–organic frameworks (MOFs) are representative porous crystalline materials that are characterized by a highly ordered porosity, large specific surface areas, and abundant coordinated unsaturated metal sites [17,18,19,20]. They can also be customized and optimized according to specific needs [21]. Therefore, extensive research has focused on MOFs with the aim of discovering their potential applications in photo-Fenton [22,23,24]. MIL-101, as an MOF series material, is a promising iron-based MOF [25] that is well known for its excellent stability and adsorption capacity. The introduction of metal ions into the organic ligand structure of MIL-101 enables materials to exhibit diverse properties, and MIL-101 is widely utilized in many fields [26,27].

Molybdenum disulfide (MoS₂) is a common semiconductor material that has been widely applied in various fields, including lubrication, electronic transistors, batteries, and catalysis [28,29,30]. MoS₂ can act as a co-catalyst in photo-Fenton, promoting the decomposition of hydrogen peroxide into ·OH, accelerating the cycle of Fe²⁺ and Fe³⁺, and significantly reducing the consumption of H₂O₂ and Fe³⁺, thus improving the overall degradation efficiency of the photo-Fenton AOP [31,32].

In this study, a new composite photocatalytic material, MIL-101(Fe)@MoS₂, was successfully prepared using the hydrothermal method, and its photocatalytic performance in the removal of CR dyes from dyed wastewater was investigated in depth. The results showed that MIL-101(Fe)@MoS₂ exhibited excellent performance in CR degradation driven by visible light and good pH adaptability. The purpose of this study was to provide a scientific basis for the field of organic printing and dyed wastewater treatment, offering strong support for the further promotion of green and sustainable development.

2. Experimental Part

2.1. Experimental Test Materials

2.1.1. Chemical Reagents

Ferric chloride hexahydrate (FeCl₃·6H₂O); terephthalic acid (C₈H₆O₄, BDC); molybdenum disulfide (MoS₂); N, N-Dimethylformamide (C₃H₇NO, DMF); Congo red (C₃₂H₂₂N₆Na₂O₆S₂); hydrogen peroxide (H₂O₂, 30 wt%); ethanol (C₂H₅OH); ethylenediaminetetraacetic acid disodium salt dihydrate (C₁₀H₁₄N₂O₈Na₂·2H₂O, EDTA-2Na); n-Butanol (C₄H₁₀O, nBuOH); isopropyl alcohol ((CH₃)₂CHOH, IPA); benzoquinone (BQ, C₆H₄O₂); potassium hydrogen phthalate (for TOC standard curve); and distilled water.

2.1.2. Experimental Instruments

IKA RH basic KT/C magnetic stirrer (IKA, Staufen, Germany); FE28-Standard pH meter (Mettler-Toledo, Shanghai, China); DZF-6050 vacuum drying oven (Shanghai Experimental Instrument Factory, Shanghai, China); BSA224S-CW electronic balance (Shanghai Shuangxu Electronics, Shanghai, China); H1850 science benchtop high-speed centrifuge (Hunan Xiangyi Centrifuge, Changsha, China); SU8010 scanning electron microscope (Hitachi, Tokyo, Japan); XRD-6000 X-ray diffractometer (Shimadzu, Kyoto, Japan); TU-19-01 double-beam ultraviolet spectrophotometer (Beijing Purkinje General Instrument, Beijing, China); Shimadzu TOC-L analyzer (Shimadzu, Kyoto, Japan); Double-layer glass photocatalytic reactor (Custom equipment, Huamo, Hangzhou, China); High-resolution transmission electron microscope (HRTEM) (Hitachi, Tokyo, Japan); Fourier transform infrared spectrometer (FT-IR) (Thermo Fisher Scientific, Waltham, MA, USA); Energy-dispersive X-ray spectrometer (EDS) (Oxford Instruments, Abingdon, UK).

2.2. Catalyst Preparation

2.2.1. Preparation of MIL-101(Fe)

MIL-101(Fe) was synthesized using previously reported hydrothermal methods [33]. First, 1.3244 g FeCl₃·6H₂O and 0.412 g terephthalic acid (BDC) were dissolved in 30 mL of N, N-dimethylformamide (DMF) solution and sonicated for 20 min. Subsequently, the above mixture was transferred into a Teflon-lined autoclave and heated at 110 °C for 20 h. After cooling to 20 °C room temperature, the resulting precipitate was collected via centrifugation, washed thoroughly with DMF and ethanol until the supernatant became clear, and then dried at 60 °C under vacuum until the product reached a constant weight. The obtained product was then ground into fine brown–red MIL-101(Fe) powder using an agate mortar.

2.2.2. Preparation of MIL-101(Fe)@MoS₂

The synthesis of the MIL-101(Fe)@MoS₂ nanocomposite material was conducted using the hydrothermal method, with the preparation process as illustrated in Figure 1. Specifically, 1.3244 g of FeCl₃·6H₂O and 0.412 g of BDC were dissolved in 30 mL of DMF solution. After 20 min of ultrasonic treatment, MoS₂ powders of different masses (0.1042, 0.2084, and 0.4167 g) were added to the solution and ultrasonicated for 30 min. The subsequent steps were the same as those utilized for the synthesis of MIL-101(Fe), and the resulting samples were denoted as xMIL-101(Fe)@MoS₂ (where x represents the mass percentage of MoS₂ in the composite material, which are 6%, 12%, and 24% for the different composites).

2.3. Analytical Methods

2.3.1. Photo-Fenton Degradation of CR Experiment

The photo-Fenton catalytic performance of the catalyst samples using the degradation of CR was evaluated as a model reaction. Experiments were conducted in a double-layer glass photocatalytic reactor (Huamo, Hangzhou, China) at a constant temperature of 25 °C, where 20 mg of catalyst sample was added to 100 mL of CR solution (pH = 7). Irradiation was conducted using a 300 W xenon lamp (Perfect Light Co., Ltd., Beijing, China) equipped with a cut-off filter (λ > 420 nm). After turning on the xenon lamp, magnetic stirring was performed at a speed of 400 r/min, and H₂O₂ (30 wt%) was added to initiate the photo-Fenton reaction. At regular intervals, 3 mL of the reaction solution was drawn out and filtered through a 0.45 μm filter membrane, and the absorbance value was determined at a wavelength of 498 nm using spectrophotometry to ascertain the CR content in the solution. The degradation rate of the CR was calculated as follows:

C = (1 − C_t/C₀) × 100%,

(1)

where C₀ and C_t represent the initial concentration and real-time concentration of CR, respectively.

2.3.2. Free-Radical-Trapping Experiments

In the CR solution, fixed amounts (0.1 mol/L) of p-benzoquinone (BQ), ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), n-butanol (nBuOH), and isopropanol (IPA) were added as radical scavengers for superoxide anion radicals (·O₂⁻), photogenerated holes (·h⁺), hydroxyl radicals (·OH), and sulfate radicals (·SO₄⁻), respectively. This was conducted to capture the superoxide radicals ·O₂⁻, ·h⁺, ·OH, and ·SO₄⁻, which were produced during the photochemical reaction process. The remaining operations followed the steps of the photocatalytic degradation experiment.

2.3.3. Total Organic Carbon (TOC) Determination and Mineralization Rate Assessment

The TOC contents of the system before and after the reaction were determined using a Shimadzu TOC-L analyzer, and the mineralization rate was calculated using the difference method. A 10 mL CR degradation solution was filtered through a 0.45 μm filter membrane, and a standard curve was established using potassium hydrogen phthalate as the standard substance. Each sample was measured in triplicate, and the average value was taken. The mineralization rate η is calculated as follows:

η = (1 − C_t/C₀) × 100%,

(2)

where C₀ and C_t represent the TOC concentrations of the CR degradation solution at the initial time and at the conclusion of the reaction, respectively.

2.3.4. Catalyst Stability and Reproducibility Testing

The stability of the catalyst was evaluated through five cycles of experimental tests. Following each degradation experiment, the suspension was centrifuged at 3000 r/min for 5 min to recover the catalyst, which was then washed three times with anhydrous ethanol and ultrapure water in sequence. After drying in a vacuum at 60 °C for 8 h, the catalyst was reused. The crystalline phase structure and surface morphology of the catalyst after recycling were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The repeatability tests were completed under the same experimental conditions.

3. Results and Discussion

3.1. Catalyst Characterization

3.1.1. SEM/Energy-Dispersive X-Ray Spectroscopy (EDS) and High-Resolution Transmission Electron Microscopy Analysis

The morphology and microstructures of MIL-101(Fe), MoS₂, and the MIL-101(Fe)@MoS₂ composite were analyzed using SEM and high-resolution transmission electron microscopy (HRTEM). As shown in Figure 2a, pure MIL-101(Fe) exhibited regular octahedral morphology and a uniform size of approximately 500 nm [34]. Figure 2b depicts the structure of MoS₂, which is formed by the stacking and stratification of thin nanosheets with a thickness of 50 nm. After the introduction of MoS₂, the MIL-101(Fe)@MoS₂ maintained roughly the same shape as the pure MIL-101(Fe), but the surface became rougher (Figure 2c). Furthermore, the diameter of the MIL-101(Fe)@MoS₂ became larger compared to that of both MIL-101(Fe) and MoS₂, which was attributed to the highly dispersed MIL-101(Fe) in the MoS₂ [35]. HRTEM observations revealed that MoS₂ was evenly distributed on the surface of the MIL-101(Fe) octahedral morphology (Figure 2d), and the lattice spacing of the MIL-101(Fe)@MoS₂ composite reached 0.21 nm, which was consistent with the carbon (001) crystal surface [36], indicating that MoS₂ was successfully loaded into the MIL-101(Fe)@MoS₂ composite.

Figure 3 and Figure 4 depict the elemental maps of MIL-101(Fe) and MIL-101(Fe)@MoS₂, respectively, indicating that the MIL-101(Fe) samples contain Fe, O, C, Cl, Mo, and S elements, where Mo and S are mainly contributed by MoS₂. Furthermore, the elemental content obtained through EDS shows that the carbon and oxygen content in MIL-101(Fe)@MoS₂ varies significantly with increasing MoS₂ loading compared to the original MIL-101(Fe). The above characterization results further confirm the successful synthesis of MIL-101(Fe)@MoS₂ composed of MIL-101(Fe) and MoS₂.

3.1.2. XRD Analysis

The crystal structure and composition of MIL-101(Fe), MoS₂, and MIL-101(Fe)@MoS₂ were systematically characterized through XRD analysis. As illustrated in Figure 5, the MIL-101(Fe) samples exhibited distinctive characteristic peaks at 9.38°, 10.66°, 12.67°, 16.52°, 18.86°, and 21.98°, aligning precisely with the reference data from previous studies [37]. Concurrently, the MoS₂ sample exhibited six pronounced diffraction peaks at 2θ values of 14.4°, 32.7°, 39.6°, 49.8°, and 58.3°, which corresponded closely to established crystallographic patterns [38]. The sharpness of these peaks further confirmed the material’s exceptional crystalline quality and high structural order.

Notably, the diffraction peak of the MIL-101(Fe)@MoS₂ composite had more diffraction peaks characteristic of MoS₂, in addition to those of the original MIL-101(Fe) material, and the crystal surfaces were consistent, which confirmed that MoS₂ was successfully doped into the composite. The low diffraction peak intensity may have occurred because the prepared MIL-101(Fe)@MoS₂ had smaller crystal sizes [39].

Detailed crystallographic analysis revealed that the four attenuated diffraction peaks of MoS₂ at 2θ = 14.4°, 32.7°, 39.6°, 49.8°, and 58.3° corresponded to the (002), (100), (103), (105), and (110) planes (PDF#77–1716), respectively. The diffraction profile of MIL-101(Fe) exhibited perfect consistency with the simulated patterns (CCDC No. 605510) [40], further validating the structural integrity of the synthesized materials.

Based on the XRD data (Figure 5), this study applied the Debye–Scherrer formula to analyze the full width at half maximum of the characteristic diffraction peaks for MIL-101(Fe), MoS₂, and MIL-101(Fe)@MoS₂ composites to calculate the average grain size. The formula is as follows:

$$\mathrm{D}={\displaystyle \frac{\mathrm{k}\mathsf{\lambda }}{\mathsf{\beta }\cos \mathsf{\theta }}}$$

(3)

where k = 0.89 (the Scherrer constant), λ = 0.15406 nm (the Cu-Kα radiation wavelength), β is the half-width of the diffraction peak in radians, and θ is the Bragg angle.

Based on the calculation results, the average grain size of MIL-101(Fe) was 91.6 nm, which differed from the regular octahedral morphology observed using SEM (~500 nm), indicating its polycrystalline structure. The average grain size of MIL-101(Fe)@MoS₂ was 63.5 nm. However, the particle size distribution image (Figure 5b) revealed an average particle size of approximately 741.95 ± 16.88 nm. This anomaly may originate from the Scherrer method of XRD, which can only measure the size of individual grains and cannot reflect the grain size distribution or agglomeration effects. The larger particle size observed by the SEM discrepancies arises from agglomeration, lattice strain, and measurement limitations compared to pure MIL-101(Fe) [41]. The grain size of MIL-101(Fe) in the composite material was significantly reduced, and the smaller grain size and the close contact between the two phases contributed to the enhanced catalytic efficiency of the photo-Fenton reaction [42].

3.1.3. Fourier Transform Infrared (FT-IR) Spectroscopy Analysis

The functional groups and bonds of MIL-101(Fe), MIL-101, and MIL-101(Fe)@MoS₂ were studied using FT-IR spectroscopy. As shown in Figure 6, the characteristic peaks of the MIL-101(Fe)@MoS₂ sample almost matched the typical vibration modes of the original MIL-101(Fe), indicating that the introduction of MoS₂ did not alter the structural characteristics of MIL-101(Fe). The broad peak centered at 3440 cm⁻¹ may be related to the O-H longitudinal tensile vibration of the adsorbed water molecules. The narrow peak at 550 cm⁻¹ corresponds to the stretching vibration of the Fe-O bonds arising from the coordination between Fe³⁺ and the carbohydrate group (-COO⁻) of the ligated carbolic acid. The sharp peak at 1660 cm⁻¹ corresponds to the C=C aromatic ring stretching vibrations from the benzene backbone of the ligated BDC, as reported in the FT-IR features of MIL-101(Fe) [43] and the bending vibration of the H-O-H in water molecules. Meanwhile, the weak absorption at 748 cm⁻¹ originates from out-of-plane bending vibrations of the C-H bonds in the mono-/institutionalized benzene ring, further confirming the gangland’s structural integrity. The prominent peaks at 1600 cm⁻¹ and 1390 cm⁻¹ are assigned to the asymmetric and symmetric stretching vibrations of the carbohydrate groups (-COO⁻), respectively, which are characteristic of the encephalitic acid ligated in MIL-101(Fe). This dual-peak signature confirms the deprecation and coordination of carbohydrate groups to Fe³⁺ centers [44]. In the low-frequency region, the distinct peak at 550 cm⁻¹ is attributed to Fe-O stretching vibrations [45], reflecting the coordination interaction between Fe³⁺ ions and carbohydrate oxygen atoms. Notably, the intensity of peaks associated with O-H (3440 cm⁻¹), C=C (1660 cm⁻¹), and carbohydrate groups (1600/1390 cm⁻¹) are significantly enhanced in the MIL-101(Fe)@MS₂ composite compared to pristine MIL-101(Fe).

3.2. Analysis of the Photocatalytic Properties

The experiment employed the single-factor experimental design. MoS₂ was doped into MIL-101(Fe) at mass percentages of 0%, 6%, 12%, and 24% to study its effect on catalytic activity, and 30% H₂O₂ was added in volumes ranging from 0 to 6 mL to determine the optimal oxidant concentration. Initial CR concentrations (50–200 mg/L) were tested to evaluate the system’s capacity under high pollutant loads. The pH was adjusted (3–11) to assess the catalyst’s pH adaptability.

3.2.1. The Effect of Different MoS₂ Doping Ratios on the Degradation Efficiency

Figure 7a shows the effect of different MoS₂ doping ratios on the degradation efficiency of MIL-101(Fe)@MoS₂, with the initial CR concentration at 100 mg/L and H₂O₂ at 1 mL. The degradation rates of CR by MIL-101(Fe), MoS₂, and MIL-101(Fe)@MoS₂ were initially very high, with CR degradation occurring within 30 min. Among the different composites, MIL-101(Fe)@MoS₂ with a 12% MoS₂ mass ratio exhibited the optimal removal rate.

The experimental data was fitted using the pseudo-first-order model to further study the photocatalytic degradation process, with the results shown in Figure 7b. The rate constant (k_obs) for MoS₂, MIL-101(Fe), 6% MIL-101(Fe)@MoS₂, 12% MIL-101(Fe)@MoS₂, and 24% MIL-101(Fe)@MoS₂ were 0.00548, 0.091, 0.104, 0.139, and 0.071 min⁻¹, respectively. Among these, the 12% MoS₂ doping ratio of 12% MIL-101(Fe)@MoS₂ had the fastest degradation rate.

3.2.2. Effect of H₂O₂ Dosage on the Degradation Efficiency

Using the optimal composite MIL-101(Fe)@MoS₂ (12%) as a reference, the effects of different dosages of H₂O₂ on the degradation efficiency of CR were investigated. As shown in Figure 8a, with the increase in the H₂O₂ dosage from 0.5 mL to 6 mL, the CR removal rate gradually increased. According to the pseudo-first-order kinetic model, as shown in Figure 8b, under the dosage conditions of 0.5 mL and 2 mL, the reaction rate constants (k_obs) were 0.067 and 0.122 min⁻¹, respectively, which were significantly lower than the value at 4 mL (0.144 min⁻¹). Additionally, the R² value for 2 mL (0.97) was slightly inferior to that of 4 mL (0.99). When the H₂O₂ dosage was 4 mL, the reaction rate constant (k_obs = 0.144 min⁻¹) matched that of 1 mL, but the model-fitting accuracy (R² = 0.99) was significantly superior to the 1 mL condition (R² = 0.88). Although the 6 mL dosage achieved the highest (0.216 min⁻¹), considering the cost and the fact that excess H₂O₂ can scavenge hydroxyl radicals (·OH) produced by photo-Fenton oxidation, the H₂O₂ dosage of 4 mL was selected for subsequent experiments.

3.2.3. Effect of the Initial CR Concentration on the Degradation Efficiency

Using the optimal reaction conditions obtained from the previous single-factor experiments as the initial conditions, Figure 9 shows the experimental results for CR initial concentrations ranging from 50 to 200 mg/L. When the CR concentration was 50 mg/L, in the presence of H₂O₂, the CR degradation rate reached 96.7% after 15 min of reaction. As the CR concentration increased, the CR degradation rate decreased. At a CR concentration of 200 mg/L, the CR degradation rate was 82.7% after 15 min of reaction. The main reason for this was that, when the CR concentration was too high, there were insufficient active sites on the MIL-101(Fe)@MoS₂, leading to the insufficient activation of H₂O₂ to produce ·OH radicals. Therefore, it was not possible to achieve the rapid and efficient degradation of high concentrations of CR.

3.2.4. Effect of the Initial pH Value on Degradation Efficiency

Figure 10a depicts the CR degradation performance under different pH conditions. When the initial pH of the solution was 11, the degradation rate of CR reached 10.1% after 15 min of reaction, and the degradation of CR was significantly inhibited. However, when the initial pH of the solution was between three and nine, after 15 min of reaction, the degradation rate of CR could be maintained above 90%. Therefore, the degradation of CR by MIL-101(Fe)@MoS₂ photo-Fenton oxidation has a wide pH adaptation range (pH = 3–9).

A zeta potential analysis indicated that the surface charge characteristics of MIL-101(Fe)@MoS₂ were significantly correlated with the pH value of the solution (Figure 10b). When the pH value increased from 3 to 11, the material’s zeta potential linearly decreased from +29.3 mV to −39.3 mV and crossed the zero point at pH = 7 (pHzpc ≈ 7). This phenomenon was consistent with the typical surface charge response mechanism of MOF materials. Under acidic conditions (pH = 3), the material’s surface protonation resulted in abundant positive charge sites (zeta potential = +29.3 mV), which was favorable for the adsorption of negatively charged CR molecules through electrostatic attraction, thereby promoting its subsequent degradation. This charge-dependent adsorption behavior was consistent with the experimental results shown in Figure 10a, where the CR degradation efficiency at pH = 3 (over 90%) was significantly higher than that at pH = 11 (10.1%).

It is particularly noteworthy that, when pH > pHzpc (e.g., pH = 11), the material’s surface negative charge (zeta potential = −39.3 mV) caused electrostatic repulsion with the CR anionic groups, which directly inhibited their effective contact [46]. The pH-dependent speciation of CR plays a critical role in its interaction with the catalyst. Under acidic conditions (pH 3–5), partial protonation of CR’s sulfonic groups (-SO₃H) reduces electrostatic repulsion, promoting compact self-assembly via hydrophobic stacking [47]. This facilitates adsorption onto the positively charged MIL-101(Fe)@MoS₂ surface (zeta potential ≈ +29.3 mV). Conversely, at neutral to alkaline pH (7–9), fully deprotonated CR (-SO₃⁻) forms loose aggregates but interacts with MoS₂ through coordination, enhancing ·O₂⁻ generation [48].

3.3. Degradation Kinetics and Mechanism Analysis

3.3.1. Comparison of the Contribution of MIL-101(Fe)@MoS₂ to CR Degradation and Adsorption Under Different Conditions

Figure 11a shows the comparison of the contribution of MIL-101(Fe)@MoS₂ to CR degradation under different conditions. First, under dark conditions, in the absence of light and H₂O₂, the degradation rate of CR by MIL-101(Fe)@MoS₂ reached 64.5%, which was mainly due to the adsorption effect of MIL-101(Fe)@MoS₂ on CR. After adding H₂O₂, the degradation rate of CR significantly increased to 86.8%, indicating that MIL-101(Fe)@MoS₂ acted as a catalyst in the photo-Fenton reaction, promoting the oxidative degradation of CR. Under visible-light irradiation, the CR degradation rate further increased, reaching 72.2% and 100%, respectively. This demonstrates that MIL-101(Fe)@MoS₂ has not only an adsorption effect but also an oxidative effect, and under the conditions of the photo-Fenton reaction, MIL-101(Fe)@MoS₂ exhibits the highest CR degradation rate.

According to the pseudo-first-order kinetic model, the rate constants (k_obs) for the degradation of CR under four different conditions for MIL-101(Fe)@MoS₂ were fitted. As shown in Figure 11b, under dark conditions, the rate constant for MIL-101(Fe)@MoS₂ was 0.063 min⁻¹. After adding H₂O₂, the rate constant increased to 0.037 min⁻¹. Under visible light irradiation, the rate constant for MIL-101(Fe)@MoS₂ was 0.021 min⁻¹. In the photo-Fenton reaction, the rate constant was the highest at 0.204 min⁻¹. Based on these findings, it is evident that MIL-101(Fe)@MoS₂ mainly degrades CR through adsorption under dark conditions. After adding H₂O₂, MIL-101(Fe)@MoS₂ significantly improved the degradation rate of CR through the photo-Fenton reaction. Visible-light irradiation further enhanced the CR degradation effect, especially in the photo-Fenton reaction, where CR could be completely degraded within 30 min, demonstrating the highly efficient application potential of this system in environmental remediation.

As can be seen from Figure 12, with the passage of time, the mineralization efficiency of CR gradually increased, indicating that MIL-101(Fe)@MoS₂ had a significant degradation effect on CR in the photo-Fenton reaction. Notably, within 30 min, the mineralization efficiency of CR reached 75.2%, and the mineralization rate curve differs from Figure 11a. This discrepancy arises from the distinct two-stage mechanism of advanced oxidation processes. In the initial stage (0–3 min), the synergistic adsorption–catalysis effect dominated, and hydroxyl radicals (·OH) and superoxide radicals (·O₂⁻) generated via the photo-Fenton reaction cleaved the chromophoric azo bonds (-N=N-), leading to immediate decolorization. However, this step produced intermediate organic byproducts (e.g., aromatic amines and naphthalene derivatives) [49] that retained organic carbon content. After 3 min, the process transitioned to a catalysis-dominated phase, where intermediates were slowly oxidized into CO₂ and H₂O, further explaining the reason why the overall TOC curve maintains a stable trend [50].

This study conducted a comparative analysis with other similar studies in terms of degradation efficiency, degree of mineralization, and reaction rate constants. Detailed data are presented in

Table 1. Comparison between the performance of previously reported photocatalysts and MIL-101(Fe)@MoS₂.

Catalyst Type	Pollutants (Concentration)	Degradation Time	Degradation Efficiency	Reaction Rate Constant min−1	Repeatability Stability	Literature Source
TiO2-g-C3N4-10	Congo red (100 mg/L)	180 min	100%	0.0104	After five cycles: 83.7%	[51]
RGO-modified MIL-125 (Ti)	Congo red (10 mg/L)	180 min	92.60%	0.012	After four cycles: 65.2%	[52]
Ultrathin oxygen-doped g-C3N4 nanosheets	Congo red (20 mg/L)	120 min	83.10%	0.0131	-	[53]
Fe-doped BiOBr hollow spheres (7% Fe)	Congo red (50 mg/L)	180 min	89.66%	0.00935	-	[54]
GCN/CdO/CaFe2O4 ternary heterojunction	Congo red (5 × 10−6 M)	60 min	88%	-	After five cycles: 80%	[55]
MIL-101(Fe)@MoS2	Congo red (50 mg/L)	30 min	100%	0.204	After five cycles: 90.8%	This work

.

As can be seen in Table 1, MIL-101(Fe)@MoS₂ achieved 100% degradation efficiency for CR (50 mg/L) in just 30 min, which was a significantly shorter time than that of other catalysts, such as the TiO₂-g-C₃N₄-10 heterojunction (180 min to reach 100% degradation efficiency), RGO-modified MIL-125 (Ti) (180 min to reach 92.60% degradation efficiency), ultrathin oxygen-doped g-C₃N₄ nanosheets (120 min to reach 83.10% degradation efficiency), Fe-doped BiOBr hollow flower balls (7% Fe) (180 min to reach 89.66% degradation efficiency), and the GCN/CdO/CaFe₂O₄ ternary heterojunction (60 min to reach 88% degradation efficiency). The degradation time was greatly reduced, and the degradation efficiency was high, demonstrating the high efficiency of MIL-101(Fe)@MoS₂ in terms of catalytic performance. Moreover, the reaction rate constant of MIL-101(Fe)@MoS₂ was 0.204 min⁻¹, which was significantly higher than that of the TiO₂-g-C₃N₄-10 heterojunction (0.0104 min⁻¹), RGO-modified MIL-125 (Ti) (0.012 min⁻¹), ultrathin oxygen-doped g-C₃N₄ nanosheets (0.0131 min⁻¹), and Fe-doped BiOBr hollow flower balls (7% Fe) (0.00935 min⁻¹). This indicated that the catalytic reaction proceeded at a faster rate, further demonstrating the advantages of MIL-101(Fe)@MoS₂ in catalytic performance. In addition, MIL-101(Fe)@MoS₂, when degrading CR (50 mg/L), required a catalyst dosage of 20 mg/100 mL and could maintain 90.8% degradation efficiency after five cycles of use. Compared to other catalysts, such as the TiO₂-g-C₃N₄-10 heterojunction, which dropped to 83.7% degradation efficiency after five cycles, and RGO-modified MIL-125 (Ti), which dropped to 65.2% degradation efficiency after four cycles. MIL-101(Fe)@MoS₂ showed better repeatability stability and could maintain high catalytic activity over a long period of use. From an economic perspective, this reduces the cost of frequent catalyst replacement, offering an economic advantage.

3.3.2. Radical-Trapping Experiment and Electron Paramagnetic Resonance (EPR) Spectrum Analysis

The contribution of different active substances to the performance of photo-Fenton reactions was studied through active species-trapping experiments. Under the conditions of the photo-Fenton reaction, BQ, EDTA, nBuOH, and IPA were added to the CR solution as radical scavengers for superoxide anion radicals (·O₂⁻) [56], photogenerated holes (·h⁺) [57], hydroxyl radicals (·OH) [58], and sulfate radicals (·SO₄⁻), respectively [59]. As shown in Figure 13a, after adding BQ and EDTA, the CR degradation rate was effectively suppressed, decreasing to 60.8% and 53.1%, respectively, indicating that ·O₂⁻ and ·h⁺ are the main radicals in the MIL-101(Fe)@MoS₂ and H₂O₂ system. The degradation rate first increased and then decreased after the introduction of BQ. This suggests that ·O₂⁻ is the main radical in the degradation process, and the subsequent decrease may be due to the adsorption effect of MIL-101(Fe)@MoS₂ on CR, which is stronger than the inhibitory effect. Figure 13b further confirms the presence and role of the relevant free radicals.

3.3.3. Analysis of Band Structure and Band Gap Values (Tauc Formula)

The light absorption properties of MIL-101(Fe), MoS₂, and MIL-101(Fe)@MoS₂ were characterized using ultraviolet–visible diffuse reflectance spectroscopy (UV-Vis DRS), as shown in Figure 14a. MIL-101(Fe) exhibited a strong light absorption performance in the ultraviolet region, with an absorption edge at 515 nm. In contrast, MoS₂ showed a satisfactory light-harvesting performance over a wide range (200–800 nm), with the corresponding absorption edge around 700 nm. Compared to MIL-101(Fe), the light-harvesting capability of MIL-101(Fe)@MoS₂ was significantly improved, especially in the visible-light region.

According to the Kubelkae–Munk function, the band gap was determined based on the relationship curve between the square of the band gap ((αhν)²) and the energy (hv) of the excitation light, as shown in Figure 14b. The bandgaps of the three materials, MIL-101(Fe), MoS₂, and MIL-101(Fe)@MoS₂, were 2.63, 2.49, and 2.09 eV, respectively, indicating that the light absorption performance and photoelectric conversion ability of the MIL-101(Fe)@MoS₂ material synthesized from MIL-101(Fe) and MoS₂ were enhanced.

According to recent studies, the combination of transition metal composite oxides (such as CoFe₂O₄) with carbon-based materials can significantly enhance the activation efficiency of persulfate, with the synergistic factor reaching 2.75 [60]. Furthermore, the composite system of ZnO with magnetic cobalt ferrite (CoFe₂O₄) exhibits excellent stability over a wide pH range (3–9), which is highly consistent with the pH adaptability mechanism of the MIL-101(Fe)@MoS₂ discussed in the present study [61]. These findings further validate the key role of heterojunction interface charge separation and synergistic catalysis in enhancing the performance of the photo-Fenton processes.

3.4. Catalyst Stability

3.4.1. Loop Experiment and Efficiency Retention Rate

Reusability is an important indicator for measuring the practical application of catalysts. The cyclic degradation experimental data shown in Figure 15a fully confirmed that, after five consecutive catalytic cycles without additional regeneration treatment, MIL-101(Fe)@MoS₂ maintained a degradation efficiency of 90.8% for CR in the fifth cycle after 150 min. Synchronous characterization results revealed that the material retained its unique octahedral morphology features following the reaction (Figure 15b). More importantly, FTIR spectroscopy analysis (Figure 15c) did not detect any significant new characteristic peaks or the attenuation of the original functional group signals, indicating that the material’s chemical structure had excellent stability. In addition, the comparative XRD pattern (Figure 15d) showed that the positions of the characteristic diffraction peaks before and after the reaction were highly consistent, and the crystal phase structure did not undergo significant changes. The above multi-dimensional characterization results fully verified the excellent structural stability and recyclability performance of the MIL-101(Fe)@MoS₂ composite material.

3.4.2. Metal-Leaching Quantity Assessment and Environmental Safety

The leaching Fe concentrations after the reaction are shown in Figure 16, which is 1.88 mg/L after the first reaction, then decreases to 1.53 and 1.38 mg/L after the second and third, accounting for 0.94%, 0.76%and 0.69% of the MIL-101(Fe)@MoS₂ dosage (0.2 g/L), which was lower than that of previous reports [62]. After three reactions, the total dissolution rate of Fe was 2.41%. This phenomenon can be explained from two aspects. First, the magnetic stirring, redox conditions, and changes in pH value during the reaction process may cause slight damage to the MOF structure, releasing a small amount of iron ions. Second, the strong oxidizing power of H₂O₂ and photogenerated holes (h⁺) may initiate the redox cycle of Fe³⁺/Fe²⁺, and due to incomplete coordination, some iron ions dissolve. However, the overall performance and environmental safety still meet the requirements for practical applications [63].

4. Conclusions

This study successfully constructed MIL-101(Fe)@MoS₂ composite photocatalytic material and confirmed its optimized structural and morphological integration using characterization techniques including XRD, FT-IR, SEM, and EDS. The experiments demonstrated that the material exhibited excellent CR degradation performance in a visible-light-driven photo-Fenton system. Under neutral conditions with an initial CR concentration of 50 mg/L, a catalyst dosage of 0.2 g/L, an H₂O₂ addition of 4 mL, and pH = 7, CR could be completely degraded within 30 min, with the TOC removal rate reaching 72.5%, which significantly enhanced the mineralization efficiency of the organic pollutants. Moreover, the catalyst maintained an efficient degradation capacity (90%) over a broad pH range of 3–9, breaking through the traditional Fenton system’s dependence on acidic media and showing stronger practical application adaptability.

Radical-trapping experiments and EPR analysis clarified that superoxide radicals (·O₂⁻) and photogenerated holes (·h⁺) were the main active substances for CR degradation, revealing the reaction mechanism of heterojunction interface synergistic catalysis. Cyclic experiments indicated that the catalyst maintained a degradation efficiency of 90.8% after five repeated uses, and XPS analysis confirmed the stable combination of Fe and Mo, with an extremely low metal-leaching amount (the binding energy of Fe³⁺ did not significantly shift, and the peak intensity of Mo⁴⁺/Mo⁶⁺ remained stable), effectively avoiding the risk of secondary pollution. This study developed a novel photo-Fenton catalytic system with high activity, wide pH adaptability, and stability for the efficient treatment of dyed wastewater, providing a scientific basis for the development of green water treatment technology.