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• Due to the use of aromatic pollutants such as organic dyes, antibiotics and pesticides in various fields, these pollutants inevitably flow into natural groundwater, reservoir, and watercourse [1, 2]. Water pollution caused by the disposal of sewage with pollutants above has become an environmental issue of great concern. So as to treat wastewater, physics, chemistry, biological treatment technologies are generally adopted, while advanced oxidation processes (AOPs) have attracted increasing interest as promising powerful methods to efficiently removing persistent organic pollutants from water [3-6]. Fenton or Fenton-like technology is one of the most important AOPs [7-9] and has been widely used in biology, environment and other fields. This type of technology relies on the generation of various reactive oxygen species (ROSs), such as hydroxyl radicals (·OH), singlet oxygen (¹O₂) and superoxide radicals (·O₂^–) to degrade organic pollutants. However, practical Fenton technology used to degrade stable organic pollutants has certain drawbacks up to now. In the traditional Fenton reaction, Fe³⁺ generated after Fe²⁺ reacts with hydrogen peroxide (H₂O₂) cannot be recycled, which leads to the loss of iron source in the reaction and the reduction of Fenton reaction efficiency [10-12]. In addition, the opportune pH range in the conventional Fenton reaction is narrow [13]. High pH will make the generation of ROSs difficult and produce Fe(OH)₃ precipitation [14]. These objections severely hinder the further application of Fenton reaction.

  To overcome the above shortages, in recent years, MoS₂ as a cocatalyst in Fenton reaction has attracted extensive concern since it can overcome most of the shortcomings of Fenton reactions. This is derived from the discovery by Xing et al. in 2018 [10]. They found that commercial 2H-type MoS₂ (< 2.00 µm) without any additional treatment could serve as a cocatalyst to significantly accelerate the cycle of Fe²⁺/Fe³⁺ in Fenton reaction. Unsaturated S atoms on the surface of metal sulfides can capture protons from the solution to form H₂S, and expose reductive metallic active sites to greatly accelerate the rate-limiting step of Fe³⁺/Fe²⁺ conversion at the same time. In addition, the fast conversion of Fe³⁺/Fe²⁺ caused an increased amount of ·OH generated from the decomposition of H₂O₂, which was proved to rapid and efficient inactivate Escherichia coli K-12 (E. coli) and Staphylococcus aureus (S. aureus) [15]. In 2018, Xu et al. reported that flower-like MoS₂ powders were synthesized via a one-pot hydrothermal reaction and exhibited high-efficient adsorption performance to cationic dyes [16]. Due to strong electrostatic interaction between the positively charged pollutants molecules and electronegative MoS₂ nanoflowers, high-efficient adsorption capacity was very effective for improving the reaction efficiency of heterogeneous Fenton reaction. Next, Zhang et al. found that the performance of MoS₂ for activating Fe³⁺/persulfate system can be effectively enhanced by introducing S defects into MoS₂ through thermal annealing. And the mechanism of S defects in accelerating the catalytic degradation of contaminants was revealed to be the modified surface charge distribution and enhanced adsorption of Fe³⁺ [17]. Shen et al. developed a new and convenient biomimetic method to construct magnetic composites of MoS₂ and Fe₃O₄ magnetic nanoparticles (MoS₂-Fe₃O₄ MNPs) that effectively degraded methylene blue with H₂O₂ [18]. Indeed, it can be proved by current research that MoS₂ has a satisfactory effect in promoting the heterogeneous Fenton reaction, especially in solving the iron sludge problem and promoting the cycle of Fe²⁺/Fe³⁺. However, these researches tend to focus on designing more new catalysts based on MoS₂ to efficiently degrade pollutants in the different environment, while there are few studies on the effect of oxidant (H₂O₂ etc.)-MoS₂ interaction on the whole Fenton process. And the mechanics of the catalyst may have unexpected consequences for the whole Fenton reaction.

  Herein, we reported a new type of easily prepared composite catalyst (MoS₂-Fe_x, x/mg represents the mass of FeSO₄·7H₂O in the precursor) by using MoS₂ as a co-catalyst while bringing its own highly dispersed iron source (Fig. 1a). The structure and composition of the catalysts were thoroughly analyzed with multiple characterization methods. Through the tests of transmission electron microscopy (TEM), high resolution TEM (HRTEM) and scanning TEM (STEM), it could be clearly seen that the MoS₂ still maintained the original nanoflower shape, and iron was evenly dispersed in the center and edge petals of the MoS₂ nanoflowers. Through the combination of experiments and DFT calculations, the mechanism of the interaction between H₂O₂ and MoS₂ on the activity of the whole Fenton system was explained, especially the feeding way of H₂O₂ played an important role. The sulfadiazine (SD) was used as a model pollutant to study the effect of the catalyst in the degradation of organic pollutants, and ROSs in the system were verified through electron paramagnetic resonance (EPR) and sacrificial agent experiments. It was found that the MoS₂-Fe_x based Fenton system with a "small amount for multiple times" of H₂O₂ feeding way was suitable for treating organic pollutants in near neutral or even weakly alkaline environments. Finally, degradation experiments for other pollutants were used to prove the universality of the catalyst, and the stability of the catalyst was proved through cycle experiments and long-term experiments.

  Figure 1

  

  Figure 1.  Characterization of MoS₂-Fe₇₅. (a) The experimental device for preparing MoS₂-Fe₇₅. (b) SEM image of MoS₂-Fe₇₅. (c, d) TEM images of MoS₂-Fe₇₅. (e) HRTEM image of MoS₂-Fe₇₅. (f) XRD patterns of MoS₂-Fe₇₅ before and after reaction. (g) Infrared spectrum of MoS₂-Fe₇₅. (h) EDX mapping images of MoS₂-Fe₇₅.

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  In order to investigate the specific morphology of the catalyst, scanning electron microscope (SEM) and TEM were used first to characterize the morphology and structure of MoS₂-Fe_x. It can be clearly seen from Figs. 1b and c that MoS₂-Fe₇₅ still maintained the nanoflower shape of MoS₂ with an average diameter of 0.10-0.60 µm and massive petals. According to the results of inductively coupled plasma atomic emission spectrometry (ICP-AES) in Table S1 (Supporting information), the iron content in the catalyst was about 13.0%. As shown in Fig. 1d, Figs. S1a and b (Supporting information), no particles were observed at the edge thin layer of MoS₂-Fe₇₅ nanoflowers in the TEM image, differing from three-dimensional (3D) α-Fe₂O₃/MoS₂ hierarchical nano-heterostructure [19]. In the HRTEM images (Fig. 1e, Figs. S1c and d in Supporting information), only the lattice fringes of MoS₂ with low crystallinity were observed, which means that there was no crystalline iron or iron oxide embedded in MoS₂. In addition, the X-ray diffraction (XRD) pattern in Fig. 1f evidently showed the characteristic peaks of MoS₂ at 2θ of 14.53°, 33.02°, 34.06°, 38.37°, 41.10°, 58.31° and 60.50°correspond well to MoS₂ Bragg planes of (003), (101), (012), (104), (015), (110) and (113), respectively. No characteristic peak of iron or iron oxides was observed, which was consistent with the result in the above HRTEM images. Although the content of Fe is as high as 13%, it does not generate any iron oxide, indicating that our synthesis strategy can achieve a highly dispersion of Fe atoms on the MoS₂ surface. The XRD pattern of MoS₂-Fe₇₅ after reaction in Fig. 1f retained the characteristic peaks of MoS₂, which meant good stability of MoS₂-Fe₇₅.

  On the basis of infrared spectrum of MoS₂-Fe₇₅ in Fig. 1g, the peaks at 3430.15 cm⁻¹ and 1400.41 cm⁻¹ indicated the presence of –OH on the surface of MoS₂-Fe₇₅. Compared with the infrared spectrum of MoS₂ in Fig. S2a (Supporting information), the peak at 3434.33 cm⁻¹ was significantly enhanced, which indicated that the introduction of iron brought more surface hydroxyl groups (–OH). Therefore, it could be speculated that some of the iron atoms in the catalyst were connected to the –OH during the preparation process. The peak at 607.79 cm⁻¹ indicated the stretching vibration of Fe–S bond [20], which illustrated that Fe²⁺ was firstly coordinated with unsaturated S and then reduced and fixed on the surface of MoS₂, and it also explained why there was only a small peak belonging to Fe⁰ in the followed high resolution X-ray photoelectron spectroscopy (XPS) results. Besides, the peak at 480.69 cm⁻¹ indicated the presence of Metal–S or S–S bond [21]. According to the infrared spectrum of MoS₂-Fe₇₅ after degradation reaction (Fig. S2b in Supporting information), the –OH of MoS₂-Fe₇₅ was reduced, proving that the –OH on its surface would be consumed with the dissolution of iron. In addition, the infrared spectrum of TiO₂-Fe₇₅ (Fig. S2c in Supporting information) confirmed that the introduction of Fe in the same way also resulted in a large number of surface –OH and further improved the hydrophilicity of the catalyst. According to Fig. S3 (Supporting information), nitrogen adsorption and desorption isotherms of MoS₂ and MoS₂-Fe₇₅ show that their specific surface areas were 29.9572 m²/g and 19.4070 m²/g respectively, accompanied by the hysteresis loop caused by nano petals. With the introduction of iron, the specific surface area of the catalyst was slightly reduced.

  During the preparation process, Fe²⁺ was more easily coordinated with unsaturated S on the surface of MoS₂ [17], leading to good dispersion and adhesion on the surface of MoS₂. With the addition of NaBH₄, Fe²⁺ was reduced gradually. MoS₂-Fe_x were also analyzed using STEM coupled with energy dispersive X-ray spectroscopy (EDX) to confirm the composition of catalyst. The corresponding EDX mapping images of MoS₂-Fe₇₅ in Fig. 1h showed that the homogeneous distribution of Fe (indicated by yellow color), Mo (indicated by blue color), O (indicated by red color) and S (indicated by orange color) elements, demonstrating the uniform distribution of Fe atoms among the whole MoS₂ nanoflower.

  In order to investigate the effect of MoS₂-Fe_x in the degradation of organic pollutants, we firstly tested the catalytic effect of MoS₂-Fe_x prepared with different FeSO₄·7H₂O content in the precursor. When the mass of FeSO₄·7H₂O in the precursor was 75.0 mg, the catalytic effect of MoS₂-Fe_x was the best (Fig. S4a in Supporting information). During the exploration, it was found that the addition of a large amount of H₂O₂ at one time did not achieve a good effect on the degradation of SD. According to the results in Fig. 2a, when the amount of H₂O₂ was 1.0, 4.0, 10.0, 20.0 and 100.0 µL, the degradation rate reached 33.0%, 41.0%, 43.0%, 41.3% and 43.9%, respectively, which may be due to the presence of excessive H₂O₂ triggered a large number of competing reactions [22]. Therefore, a "small amount for multiple times" feeding way of H₂O₂ may achieve better degradation effect. When 1.0, 1.5, 2.0 and 3.0 µL of H₂O₂ was added for 3 times, the degradation efficiency of SD reached 48.6%, 53.5%, 60.0% and 50.6%, respectively (Fig. S4b in Supporting information). The time interval of adding H₂O₂ was also proved to affect the degradation efficiency. With the increase of the total amount of H₂O₂, the activity of MoS₂-Fe₇₅ for the degradation of SD increased first and then decreased, indicating that excessive H₂O₂ with MoS₂-Fe₇₅ as the catalyst inhibited the degradation process due to reaction between MoS₂ and H₂O₂. When the total amount of H₂O₂ is constant, the activity of the catalyst increases slightly with the increase of reaction time. For instance, the degradation rate of the MoS₂-Fe₇₅/1*6-60min (The Z/a*b-cmin means a µL of H₂O₂ was added to the solution every c/b min, a total of b times, and the total experiment time was c min with Z as catalyst) system was only 13.0% higher than that of the MoS₂-Fe₇₅/1*6-30min system (Fig. S4c in Supporting information). Interestingly, the feeding way of H₂O₂ has a significant effect on the Fenton activity of MoS₂-Fe_x. For example, when the total amount of H₂O₂ is fixed at 9.0 µL in MoS₂-Fe₇₅ based Fenton system, the degradation rate of the system adding H₂O₂ in 9 times is much higher than that of the system adding H₂O₂ in three or one time (Fig. S4c). In order to maximize the utilization rate of H₂O₂, a total of 9.0 µL H₂O₂ is added into the solution in 9 times (adding 1.0 µL H₂O₂ every 10 min), which is defined as MoS₂-Fe₇₅/1*9-90min under the optimal degradation condition for 10 mg/L of SD without adjusting pH here.

  Figure 2

  

  Figure 2.  Degradation rate of SD under different conditions with MoS₂-Fe₇₅. (a) Degradation rate at different dosages of H₂O₂ without adjusting pH (H₂O₂ is added at one time; SD: 10.0 mg/L, MoS₂-Fe₇₅: 100.0 mg/L, Reaction time: 30 min). (b) Changes in degradation rate over time under optimal conditions (SD: 10.0 mg/L, MoS₂-Fe₇₅: 100.0 mg/L, MoS₂-Fe₇₅/1*9-90min, pH_i 6.50). (c) The degradation of different dosages of catalyst and H₂O₂ under different pH (pH_e means the pH value at the end of reaction). (d) The degradation efficiency of other catalysts prepared by the same method (SD: 10.0 mg/L, catalyst: 100.0 mg/L, MoS₂-Fe₇₅/1*3-30min).

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  In addition, MoS₂-Fe₇₅ exhibited interesting wide pH adaptability. When 10 mg of MoS₂-Fe₇₅ was added to 10 mg/L of SD solution, the initial pH (pH_i) of the solution was 6.5. As shown in Fig. 2b, MoS₂-Fe₇₅ based Fenton system showed much higher efficiency in degrading SD than the blank catalyst without H₂O₂. When the pH_i of the MoS₂-Fe₇₅/1*9-90min system was close to neutral, the degradation rate was fast at first and then slowed down gradually. The degradation rate of the MoS₂-Fe₇₅/1*9-90min system can reach 93.2%, when the pH_i is 6.50 (Fig. 2c). Even when the pH_i was increased to 9.00, the degradation rate of the MoS₂-Fe₇₅/1*9-90min system remained at 77.9%. However, when the pH_i was 10.0, its degradation rate could only reach 24.9%. To solve this problem, the addition amounts of MoS₂-Fe₇₅ and H₂O₂ were increased, as shown in Fig. 2c. Obviously, the degradation rate of the MoS₂-Fe₇₅/3*9-90min system could reach 95.6% with 300.0 mg/L MoS₂-Fe₇₅ added, indicating that the increase of the amount of MoS₂-Fe₇₅ and H₂O₂ had varying degrees of impact on the degradation rate. As shown in Fig. S4d (Supporting information), when pH_i was 10.00, we also monitored the concentration of SD in the system in real time. Similar to the experiment of pH_i 6.50, the concentration of SD in the system also decreased with each addition of H₂O₂ instead of waiting until the pH dropped to a certain level before it started to decrease. Although the pH of the system continued to decrease with the addition of H₂O₂ and became less than 4.0 at the end of reaction (pH_e < 4.0, Fig. S5 in Supporting information), the degradation of SD was carried out with each addition of H₂O₂ (Fig. 2b), rather than when the pH reached the appropriate range for the Fenton reaction. Besides, the degradation rate reached the fastest at the beginning of the experiment and gradually slowed down as the pH decreased. Therefore, the degradation rate had no obvious correlation with the pH of the system but connected with the concentration of SD in the system. As for MoS₂-Fe₇₅, adsorption was not the main factor leading to the reduction of SD content. According to Fig. 2b, MoS₂-Fe₇₅ reached the adsorption equilibrium for SD in about 1 h, and the adsorption amount was about 10.0% of the total.

  The chemical interaction between MoS₂ and Fe was indispensable because independent MoS₂ or Fe was difficult to degrade SD effectively. As shown in Fig. 2d, the effect of TiO₂-Fe₇₅ synthesized by the same method as MoS₂-Fe₇₅ was only about half of MoS₂-Fe₇₅ when the pH_i was 4.00, while TiO₂-Fe₇₅ had almost no effect when the pH_i was 6.50. As for MoS₂, it was difficult to degrade SD with the addition of H₂O₂. The degradation rate of SD only reached 11.5%, no matter the pH_i of the system was 4.00 or 6.50, and the degradation rate here was probably due to the adsorption of MoS₂ itself, according to the previous adsorption test of MoS₂-Fe₇₅ in Fig. 2b.

  To further explore the oxidation mechanism of the MoS₂-Fe₇₅ catalytic Fenton reaction, radical quenching tests were conducted by using tert-butanol (TBA), p-benzoquinone (p-BQ) and 2,2,6,6-tetramethylplperldine (TEMP) as the corresponding sacrificial agents. According to Fig. 3a, the degradation rate of SD in the MoS₂-Fe₇₅/1*9-90min system decreased inordinately with the addition of sacrificial agents under optimal conditions. Therein, the addition of TEMP that could sacrifice ¹O₂ reduced the degradation rate of SD to 21.6%, which meant that ¹O₂ played the most crucial role in the reaction, and TBA and p-BQ also hindered the degradation in a minor way at the same time. There were the same results occurring in the MoS₂-Fe₇₅/9*1-90min system, demonstrating the similar oxidation mechanism. This indicates that the different adding ways of H₂O₂ may not change the decomposition process of H₂O₂, but only improve the decomposition efficiency of H₂O₂. Additionally, EPR test was employed to further detect the radicals present in the system with 5,5-dimethyl-1-pyrrolidine-N-oxide (DMPO) and TEMP as capture agents. Interestingly, the MoS₂-Fe₇₅ catalytic Fenton system exhibited obvious signal of ¹O₂ (Fig. 3b), while the signals of ·OH and ·O₂^– was not detected immensely shown in Fig. S6 (Supporting information) violating the traditional Fenton process. The electrostatic layer formed by the negative charge on MoS₂-Fe₇₅ surface greatly attracted iron ions and positively charged pollutants while non-electropositive substance like TEMP and DMPO cannot be adsorbed to the surface of MoS₂-Fe₇₅ very soon, which may greatly affect the capture of ·OH and ·O₂^– that formed near the surface of MoS₂-Fe₇₅.

  Figure 3

  

  Figure 3.  Detection of ROSs in the system and changes in catalysts before and after the reaction. (a) Degradation rate of SD in the presence of different radical scavengers including TBA (0.02 mol/L), p-BQ (0.70 mmol/L), and TEMP (0.14 mmol/L) for the quenching of ·OH, ·O₂^–, and ¹O₂, respectively, under the optimal conditions (SD: 10.0 mg/L, MoS₂-Fe₇₅: 100.0 mg/L, MoS₂-Fe₇₅/1*9-90min). (b) EPR spectra for the detection of ¹O₂ in the presence of TEMP (98.0%, GC), using water as the solvent. The details of the EPR tests are as follows: capture agent was added into the reaction solution after the reaction started for 1 min, and the EPR test was performed after mixing the two solutions for 30 s. (c) The XPS pattern of Mo 3d before and after co-catalytic Fenton reaction (left to right: 235.5, 232.9, 232.2, 231.4, 231.0, 229.7, 229.1, 228.3 eV). (d) The XPS pattern of Fe 2p before and after co-catalytic Fenton reaction (left to right: 732.5, 726.7, 723.8, 718.8, 713.3, 710.6, 708.7 eV). (e) The concentration of the dissolved iron ions in the system of MoS₂-Fe₇₅/1*9-90min and MoS₂-Fe₇₅/1*9-90min (pH_i 6.5). (f) Raman spectra of MoS₂-Fe₇₅ and MoS₂-Fe₇₅ after different H₂O₂ added in the way of MoS₂-Fe₇₅/1*9-90min and MoS₂-Fe₇₅/1*9-90min.

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  Additionally, methyl phenyl sulfoxide (PMSO) as a probe was employed to detected Fe(Ⅳ) in the reaction [23]. Fe(Ⅳ) is able to convert PMSO into its oxidation product (PMSO₂), markedly differing from the ·OH-induced products. Depending on the η which is the molar ratio of PMSO₂ produced to PMSO lost (Eq. 1 and Fig. S7 in Supporting information), its value increased to 0.16 in the first 15 min, which meant that Fe(Ⅳ) was not the primary oxidative species generated in the MoS₂-Fe₇₅/H₂O₂ system at the beginning of experiment and it got higher as the reaction went on. Compared with the degradation process of SD catalyzed by MoS₂-Fe₇₅ (Fig. 2b), SD was degraded fast at the beginning of the reaction and the reaction rate was getting slower, which was just opposite of the changing trend of η. Therefore, Fe(Ⅳ) was not the primary oxidative species generated in the MoS₂-Fe₇₅/H₂O₂ system during the whole process.

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  The XPS spectra of Mo 3d and Fe 2p were employed to study the surface chemical state of MoS₂-Fe₇₅, as is shown in Figs. 3c and d. As for the MoS₂-Fe₇₅ before reaction, there were four kinds of molybdenum ions and three kinds of iron ions on the surface. Compared with the original MoS₂ in Fig. S8 (Supporting information), the Mo⁵⁺ on the surface of the MoS₂ was significantly reduced, and replaced by Mo⁴⁺. Mo⁰ existed on the surface of MoS₂-Fe₇₅ after reduction while Mo⁶⁺ was still found on MoS₂-Fe₇₅. Additionally, there was Fe⁰, Fe²⁺ and Fe³⁺ on the surface of MoS₂-Fe₇₅, and Fe²⁺/Fe³⁺ occupied the majority among them (Fig. 3d). For the catalyst after the reaction, Mo⁰ and Mo⁴⁺ was greatly consumed while Mo⁶⁺ increased evidently. For the Fe in the catalyst, Fe⁰ was consumed while the other iron ions did not change significantly, suggesting that Fe⁰ was involved in the decomposition of H₂O₂ by the Fenton reaction.

  The concentration of Fe²⁺ was measured by 1,10-orthophenanthroline as a probe, while Fe³⁺ was reduced to Fe²⁺ first and then tested to calculate the total iron ions in the solution. According to Fig. 3e, the concentration of iron ions (10.07 mg/L) in the MoS₂-Fe₇₅/1*9-90min system was higher than that in the MoS₂-Fe₇₅/9*1-90min system (7.17 mg/L). However, the ratio of Fe³⁺ to Fe²⁺ in the former was 1.13, much higher than that in the latter (0.053). It meant that more Fe²⁺ participated in the Fenton reaction and then were oxidized to Fe³⁺, which made the degradation efficient. In other words, since H₂O₂ is added to the reaction system in a "small amount for multiple times" feeding way, the Fe²⁺ dissolved on the catalyst surface can decompose H₂O₂ more effectively, thus maintaining a stable iron cycle and keeping the ratio of Fe³⁺/Fe²⁺ at about 1.0. It was proved that excessive H₂O₂ added at one time did not speed up the process of Fenton reaction, which may be due to the direct reaction between H₂O₂ and MoS₂. The side reaction of excess H₂O₂ and MoS₂ made less amount of H₂O₂ involved in the Fenton reaction, resulting in a lower content of Fe³⁺ in the system of MoS₂-Fe₇₅/9*1-90min. Therefore, it is particularly important to understand the interaction between H₂O₂ and MoS₂.

  According to the above phenomena and analysis, the following mechanism can be inferred: Fe⁰ had strong reducibility that can reduce O₂ to form H₂O₂ in acidic environment (Eq. 2) [24]. MoS₂-Fe₇₅ only contained a small section of Fe⁰ so that it was difficult to generate a large amount of H₂O₂, and the consumption of Fe⁰ cannot be avoided. Part of iron ions on the surface of MoS₂-Fe₇₅ was dissolved with the addition of H₂O₂ (Eq. 3). As shown in Fig. 3d, the peak of Fe⁰ completely disappeared, and the peaks of other iron ions in MoS₂-Fe₇₅ did not changed greatly. For some iron ions dissolved in the system, classic Fenton reaction was bound to happen in the system (Eq. 4), and Fe²⁺ could also reduce the O₂ in the solution to ·O₂^– (Eq. 5). And the main ROS was still derived from H₂O₂ because the degradation rate did not have a great effect when the experiment was carried out in nitrogen atmosphere according to Fig. S9 (Supporting information). For the molybdenum ions of MoS₂-Fe₇₅, Mo⁴⁺ vastly facilitated the cycle of Fe²⁺ and Fe³⁺ (Eq. 6) during the reaction, preventing the formation of iron sludge and part of Mo⁴⁺ was oxidized to Mo⁶⁺ (Eq. 6) at the same time, which could be proved by the change of peaks in Mo 3d spectra. Mo⁶⁺ could be reduced to Mo⁴⁺ by H₂O₂, and O₂ was produced meanwhile (Eq. 7). Moreover, Mo⁰ on the surface of MoS₂-Fe₇₅ effectively quenched ·OH in the system and then generated Mo⁴⁺ and OH^– (Eq. 8 and Scheme 1), while part of the ·OH may further reacted with H₂O₂ to form ·O₂H (Eq. 9) [25] and H₂O. ·O₂H was likely to be converted into ·O₂^– (Eq. 10) [26] with the departure of H⁺. Finally, Mo⁶⁺ was reacted with ·O₂^– to form ¹O₂ for further oxidation reactions (Eq. 11). Significantly, Mo° can effectively quench ·OH in the system after the reaction between H₂O₂ and Fe²⁺ [27], and Mo⁶⁺ on the surface fleetly oxidized the ·O₂^– to ¹O₂, which made it difficult to capture ·OH and ·O₂^– in the EPR tests. Relatively, ¹O₂ has a longer lifetime and it was easier to be captured in the MoS₂-Fe₇₅/H₂O₂ system.

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  Scheme 1

  

  Scheme 1.  Possible mechanism of MoS₂-Fe_x for the decomposition of H₂O₂ in different feeding way.

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  As is known to all, traditional Fenton reaction is greatly limited by the narrow pH range. However, the degradation of SD in nearly neutral and alkaline was achieved effectively by using MoS₂-Fe₇₅ instead of Fe²⁺ (Figs. 2b and c). High degradation rate was realized by increasing the amount of MoS₂-Fe₇₅ and H₂O₂, even though the catalytic performance of MoS₂-Fe₇₅ decreased at higher pH. Depending on the test in Fig. 2b, SD started to degrade immediately with the addition of H₂O₂, rather than when the pH of the solution became acidic. Therefore, high pH did not hinder the degradation in the MoS₂-Fe₇₅/H₂O₂ system, which may be attributed to the existence of acidic environment on the surface of MoS₂-Fe₇₅. As mentioned above, iron ions of MoS₂-Fe₇₅ was gradually dissolved with the addition of H₂O₂. Due to the existence of the unsaturated S with negative charges on the surface of MoS₂-Fe₇₅, H⁺ and Fe²⁺ tended to accumulate in the electrostatic layer near the surface of MoS₂-Fe₇₅, which constructed an acidic environment near the surface of MoS₂-Fe₇₅, and the pollutant was able to be degraded in this environment, so that the MoS₂-Fe₇₅/H₂O₂ system was no longer sensitive to pH.

  Another interesting experimental phenomenon mentioned above was that compared with the addition of excessive H₂O₂ at once, the addition of H₂O₂ in batches with small amounts realized a better degradation effect. Fig. 3f exhibited the Raman spectra of MoS₂-Fe₇₅ before and after reaction. The E_2g¹ (384 cm⁻¹) and A_1g (408 cm⁻¹) modes appeared at the same position as the commercial MoS₂. The peak at 930 cm⁻¹ was attributed to polymolybdates and its red shift [28] might be ascribed to the structure of –OH groups reported by Stencel et al. [29]. It was inferred that MoS₂ can instantly react with high concentration of H₂O₂ because the addition of H₂O₂ to the system in different ways had substantially different effects on MoS₂-Fe₇₅ (Scheme 1). As shown in Fig. 3f, the peak of MoS₂ was still prominent with H₂O₂ added in stages while barely seen with H₂O₂ added at one time. That means that MoS₂ of MoS₂-Fe₇₅ was greatly consumed in the MoS₂-Fe₇₅/9*1-90min system but maintained the original peaks in the MoS₂-Fe₇₅/1*9-90min system. Combined with the dissolution of iron ions in Fig. 3e, it can be concluded that most of the H₂O₂ may directly react with MoS₂ if the feeding rate of H₂O₂ was too fast, and the quick consumption of H₂O₂ made the Fenton reaction greatly restricted.

  In addition, the prepared catalyst of MoS₂-Fe₇₅ was compared with the traditional heterogeneous Fenton catalysts, such as nano-iron powder (NIP) and Fe₂O₃. Interestingly, the experiment of NIP/9*1-90min got a degradability almost equivalent to MoS₂-Fe₇₅/1*9-90min while Fe₂O₃ could not be used to effectively degrade SD, as shown in Fig. 4a. In the system of MoS₂-Fe₇₅/1*9-90min, yellow iron mud was not found as is shown in Fig. S10 (Supporting information). However, it was easy for NIP/9*1-90min system to form a large amount of iron sludge after the reaction, causing considerable iron source loss (Fig. S11 in Supporting information). According to Fig. 4b, only a small amount of iron ions was left in the solution of NIP/9*1-90min system, and iron content in the yellow precipitate was very low, and more of precipitate was iron mud. In order to investigate the influence of H₂O₂ feeding way on Fe₂O₃ based heterogeneous Fenton system, we increased the amount of H₂O₂ to 63 µL to improve the degradation activity of the whole system, as shown in Fig. 4c. Unlike MoS₂-Fe₇₅ and NIP systems, Fe₂O₃ system is not sensitive to the feeding way of H₂O₂.The two feeding ways had no significant effect on the degradation activity of SD by Fe₂O₃ based Fenton system. The same phenomenon occurs in traditional homogeneous Fenton systems. When ferrous sulfate was used as the iron source, the activity of homogeneous Fenton reaction to degrade SD was also not affected by the feeding way of H₂O₂ (Fig. 4c). The above comparative experiments indicate that when researchers are developing new heterogeneous Fenton catalysts, they must consider the influence of H₂O₂ feeding way on the activity of the whole system. It is noted that MoS₂-Fe₇₅ can significantly reduce the amount of H₂O₂ compared with Fe₂O₃ based Fenton system; Compared with NIP, the formation of iron sludge was significantly reduced in MoS₂-Fe₇₅ system. Furthermore, the MoS₂-Fe₇₅ is fed with "small amount for multiple times" feeding way of H₂O₂, and its degradation activity is comparable to that of traditional homogeneous Fenton system.

  Figure 4

  

  Figure 4.  Comparative experiments of different Fenton systems (SD: 10.0 mg/L, catalyst: 100.0 mg/L, reaction time: 90 min) and the adsorption energy of H₂O₂ to the surface of catalyst at different distances and degradation universality and stability test: (a) pH after adding the catalyst and its degradation effect with different H₂O₂ adding ways. (b) Concentration of iron ions in the solution after the reaction (pH_i 4.0). (c) Comparison of homogeneous and heterogeneous Fenton systems. (d, e) The reaction of H₂O₂ on Mo and Fe sites of MoS₂-Fe₇₅ when H₂O₂ is close to the surface. (f, g) The adsorption energy of H₂O₂ on Mo and Fe sites of MoS₂-Fe₇₅ when H₂O₂ is far from the surface. (h) Degradation test for different pollutants with 100.0 mg/L MoS₂-Fe₇₅ under optimal degradation condition (Pollutant: 20.0 mg/L, MoS₂-Fe₇₅/1*9-90min for antibiotics and MoS₂-Fe₇₅/1*3-30min for dyes). (i) Cycle tests of the degradation of RhB (20.0 mg/L) in the MoS₂-Fe₇₅ + H₂O₂ system under the optimal conditions (MoS₂-Fe₇₅: 100.0 mg/L, MoS₂-Fe₇₅/1*3-30min). (j) 10-day continuous degradation of RhB (total concentration: 2500.0 mg/L).

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  To further understand the interaction between H₂O₂ and MoS₂, here, we have studied the adsorption of H₂O₂ on the surface of the catalyst by density functional theory (DFT) calculations. When the concentration of H₂O₂ is high, a large amount of H₂O₂ is close to the surface of MoS₂-Fe₇₅. At this time, the adsorption energy between the surface of MoS₂-Fe₇₅ and H₂O₂ cannot be calculated. In this case, a large amount of H₂O₂ will be decomposed into –OH and adsorbed on S atoms to form –S–O–H or –S–O, as shown in Figs. 4d and e. Therefore, only a small amount of H₂O₂ is decomposed to produce ROS, which also explains that in previous experiments, the MoS₂-Fe₇₅/9*1-90min system could not achieve a good degradation effect. However, when the concentration of H₂O₂ is low and its distance from the surface of MoS₂-Fe₇₅ is relatively far, different sites have different adsorption energies for H₂O₂. As shown in Figs. S12a and b (Supporting information) the adsorption energy of H₂O₂ on Mo site of MoS₂ is −1.643 eV, but it becomes smaller on Fe site (−1.112 eV) and on Mo site (−1.443 eV) of MoS₂-Fe₇₅, according to Figs. 4f and g. It means that the introduction of highly dispersed Fe weakens the adsorption of MoS₂ to H₂O₂ and reduces the amount of H₂O₂ that reacts directly with MoS₂ (Scheme 1), which also shows that the introduction of iron atoms improves the stability of MoS₂. Smaller absorption energy makes H₂O₂ dominant in the Fenton reaction when the amount of H₂O₂ is low. This point can also be confirmed in the iron dissolution experiment above (Fig. 3e). Compared with the one-time addition, more Fe²⁺ are oxidize to Fe³⁺ in the MoS₂-Fe₇₅/1*9-90min system, demonstrating that more H₂O₂ actually participate in the Fenton reaction when added in batches and lower concentration of H₂O₂ can better ensure the stability of MoS₂-Fe₇₅. There is still a big controversy as to whether the iron dissolved in the heterogeneous Fenton reaction or the iron in the solid phase plays a leading role [30]. According to the results of DFT calculations, the adsorption energy of H₂O₂ on Fe site of MoS₂-Fe_x is smaller than that on Mo site, so H₂O₂ cannot directly react well with the iron in the solid phase of MoS₂-Fe₇₅. Therefore, it can be inferred that the iron dissolved on the catalyst surface is in the leading position in the Fenton reaction (Scheme 1).

  In order to study the degradation pathway of SD in the MoS₂-Fe₇₅/H₂O₂ system, LC-MS/MS were used to determine the degradation intermediates. Sample was obtained through adding methanol when the MoS₂-Fe₇₅/1*9-90min system reacted for 10 min. The possible degradation intermediates are listed in Fig. S13 (Supporting information). Sixteen intermediates were identified, and a reasonable degradation pathway for SD by the MoS₂-Fe₇₅/H₂O₂ system was proposed. Due to the existence of sulfanyl and the pyrimidine ring in the SD, its degradation pathway was complicated. In the case of dominated by ¹O₂, C₃H₄O₄ (m/z 103) and C₂H₂O₄ (m/z 90) occupied a large proportion of the product, which was mainly obtained by the ring-opening reaction of C₁₀H₁₀N₄ (m/z 187), C₄H₅N₃ (m/z 111) and C₄H₄N₂ (m/z 81) [31, 32]. In addition, due to the good stability of the benzene ring, there were a large number of aromatic compound intermediates in the solution, such as C₆H₅NO₃ (m/z 139), C₆H₇N (m/z 93), C₆H₅NO₂ (m/z 124). These intermediates were further mineralized in the following reaction. It's worth mentioning that ·OH in the traditional Fenton system will attack the amino group of SD to produce phenylhydroxylamine derivatives and other intermediates [33]. Phenylhydroxylamine derivatives are typical toxic byproducts, which can cause methemoglobinemia, and their toxicity is even higher than that of the SD molecule itself. However, in this case, the degradation pathways of SD based on ¹O₂ produced by MoS₂-Fe₇₅/H₂O₂ system were significantly different. ¹O₂ will directly attack the pyrimidine ring and the S–N bond, thus reducing the risk of forming phenylhydroxylamine toxic byproducts.

  MoS₂-Fe₇₅ also exhibited excellent degradation performance on other model pollutants towards the removal of dyes and other antibiotics. Fig. 4h shows that the degradation rate of tetracycline hydrochloride, norfloxacin and enrofloxacin could reach 100.0%, 98.3% and 94.1% under the optimal degradation condition of SD. On the other hand, dyes such as new coccine, brilliant green, tartrazine and methyl orange were degraded in the system of MoS₂-Fe₇₅/1*3-30min and the degradation rates could reach 92.5%, 79.3%, 71.7% and 55.2%. This reflected that the MoS₂-Fe₇₅/H₂O₂ system had good degradability for different pollutants, which meant that this system had a certain degree of universality. As for the stability of MoS₂-Fe₇₅ during the degradation process, ten cycles of catalytic reaction tests were performed on the MoS₂-Fe₇₅/H₂O₂/RhB system under optimal conditions. As shown in Fig. 4i, the degradation rate of RhB maintained over 80.0% in the first 3 times with the system of MoS₂-Fe₇₅/1*3-30min and then decreased gradually. The decrease in degradation effect may be due to the degradation intermediates adsorbed on the surface of MoS₂-Fe₇₅ and this problem could be solved by UV lamp irradiation [34]. As a result, the organic intermediates on the surface of MoS₂-Fe₇₅ was further mineralized by ultraviolet light irradiation [35].

  Overall, MoS₂-Fe_x with highly dispersed iron as a novel catalyst has been successfully prepared through a simple reduction method. Specially, the MoS₂-Fe₇₅ based Fenton technology exhibits the ability to effectively degrade various organic pollutants by a "small amount for multiple times" feeding way of H₂O₂. In order to further verify the application prospect of the modified Fenton technology in the field of environment, we conducted a long-term degradation experiment of MoS₂-Fe₇₅/H₂O₂ system, as shown in Fig. 4j. When the concentration of RhB in the system was reduced from 100.0 mg/L to 30.0 mg/L or less, 100.0 mg/L of RhB was added to the reaction system to continue the reaction and the dosage of MoS₂-Fe₇₅ was 50.0 mg. In 10 days, a total of 25 times of 100.0 mg/L of RhB was added (total concentration: 2500 mg/L). The concentration of RhB in the system was reduced to 30 mg/L every time after the system of MoS₂-Fe₇₅/5*3-180min during the 10-day test. Combining with the results of the DFT calculations, when the concentration of H₂O₂ is low, the iron in the catalyst is not easy to directly react with H₂O₂, so MoS₂-Fe₇₅ has satisfactory stability when it interacts with H₂O₂. After drying and weighing, the weight of the remaining solid matter after the experiment is 40.5 mg and it could be calculated that 1.0 mg of RhB would be degraded for every 3.0 µL H₂O₂ and 0.076 mg MoS₂-Fe₇₅. This research provided a new option for overcoming the current drawbacks of Fenton reaction, including the problems of iron mud and the narrow pH range of Fenton reaction. Indeed, it provides more theoretical basis for the future application of MoS₂ in the various fields. By revealing the mechanism of action between H₂O₂ and MoS₂, the effect of "small amount for multiple times" H₂O₂ addition on Fenton activity is well explained, which has guiding significance for the future large-scale treatment of industrial wastewater by using improved Fenton technology.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was supported by the State Key Research Development Program of China (No. 2016YFA0204200). Project supported by Shanghai Municipal Science and Technology Major Project (No. 2018SHZDZX03) and the Program of Introducing Talents of Discipline to Universities (No. B16017). National Natural Science Foundation of China (No. 21822603), and the Science and Technology Commission of Shanghai Municipality (No. 20DZ2250400). Authors thank Research Center of Analysis and Test of East China University of Science and Technology for the help on the characterization.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2021.08.016.

  
  
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• 

  Figure 1  Characterization of MoS₂-Fe₇₅. (a) The experimental device for preparing MoS₂-Fe₇₅. (b) SEM image of MoS₂-Fe₇₅. (c, d) TEM images of MoS₂-Fe₇₅. (e) HRTEM image of MoS₂-Fe₇₅. (f) XRD patterns of MoS₂-Fe₇₅ before and after reaction. (g) Infrared spectrum of MoS₂-Fe₇₅. (h) EDX mapping images of MoS₂-Fe₇₅.

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  Figure 2  Degradation rate of SD under different conditions with MoS₂-Fe₇₅. (a) Degradation rate at different dosages of H₂O₂ without adjusting pH (H₂O₂ is added at one time; SD: 10.0 mg/L, MoS₂-Fe₇₅: 100.0 mg/L, Reaction time: 30 min). (b) Changes in degradation rate over time under optimal conditions (SD: 10.0 mg/L, MoS₂-Fe₇₅: 100.0 mg/L, MoS₂-Fe₇₅/1*9-90min, pH_i 6.50). (c) The degradation of different dosages of catalyst and H₂O₂ under different pH (pH_e means the pH value at the end of reaction). (d) The degradation efficiency of other catalysts prepared by the same method (SD: 10.0 mg/L, catalyst: 100.0 mg/L, MoS₂-Fe₇₅/1*3-30min).

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  Figure 3  Detection of ROSs in the system and changes in catalysts before and after the reaction. (a) Degradation rate of SD in the presence of different radical scavengers including TBA (0.02 mol/L), p-BQ (0.70 mmol/L), and TEMP (0.14 mmol/L) for the quenching of ·OH, ·O₂^–, and ¹O₂, respectively, under the optimal conditions (SD: 10.0 mg/L, MoS₂-Fe₇₅: 100.0 mg/L, MoS₂-Fe₇₅/1*9-90min). (b) EPR spectra for the detection of ¹O₂ in the presence of TEMP (98.0%, GC), using water as the solvent. The details of the EPR tests are as follows: capture agent was added into the reaction solution after the reaction started for 1 min, and the EPR test was performed after mixing the two solutions for 30 s. (c) The XPS pattern of Mo 3d before and after co-catalytic Fenton reaction (left to right: 235.5, 232.9, 232.2, 231.4, 231.0, 229.7, 229.1, 228.3 eV). (d) The XPS pattern of Fe 2p before and after co-catalytic Fenton reaction (left to right: 732.5, 726.7, 723.8, 718.8, 713.3, 710.6, 708.7 eV). (e) The concentration of the dissolved iron ions in the system of MoS₂-Fe₇₅/1*9-90min and MoS₂-Fe₇₅/1*9-90min (pH_i 6.5). (f) Raman spectra of MoS₂-Fe₇₅ and MoS₂-Fe₇₅ after different H₂O₂ added in the way of MoS₂-Fe₇₅/1*9-90min and MoS₂-Fe₇₅/1*9-90min.

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  Scheme 1  Possible mechanism of MoS₂-Fe_x for the decomposition of H₂O₂ in different feeding way.

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  Figure 4  Comparative experiments of different Fenton systems (SD: 10.0 mg/L, catalyst: 100.0 mg/L, reaction time: 90 min) and the adsorption energy of H₂O₂ to the surface of catalyst at different distances and degradation universality and stability test: (a) pH after adding the catalyst and its degradation effect with different H₂O₂ adding ways. (b) Concentration of iron ions in the solution after the reaction (pH_i 4.0). (c) Comparison of homogeneous and heterogeneous Fenton systems. (d, e) The reaction of H₂O₂ on Mo and Fe sites of MoS₂-Fe₇₅ when H₂O₂ is close to the surface. (f, g) The adsorption energy of H₂O₂ on Mo and Fe sites of MoS₂-Fe₇₅ when H₂O₂ is far from the surface. (h) Degradation test for different pollutants with 100.0 mg/L MoS₂-Fe₇₅ under optimal degradation condition (Pollutant: 20.0 mg/L, MoS₂-Fe₇₅/1*9-90min for antibiotics and MoS₂-Fe₇₅/1*3-30min for dyes). (i) Cycle tests of the degradation of RhB (20.0 mg/L) in the MoS₂-Fe₇₅ + H₂O₂ system under the optimal conditions (MoS₂-Fe₇₅: 100.0 mg/L, MoS₂-Fe₇₅/1*3-30min). (j) 10-day continuous degradation of RhB (total concentration: 2500.0 mg/L).

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