The activated persulfate (PS) process could produce sulfate radical (SO4·-) and rapidly degrade organic pollutants. The application of Fe3O4 as a promising PS activator was limited due to the rapid conversion of Fe2+ to Fe3+ on its surface. Mo4+ on MoS2 surface could be used as a reducing site to convert Fe3+ to Fe2+, but the separation and recovery of MoS2 was complex. In this study, MoS2/Fe3O4 was prepared to accelerate the Fe3+/Fe2+ cycle on Fe3O4 surface and achieved efficient separation of MoS2. The results showed that MoS2/Fe3O4 was more effective for PS activation compared to Fe3O4 or MoS2, with a removal efficiency of 91.8% for 20 mg·L−1 tetracycline (TC) solution under the optimal conditions. Fe2+ and Mo4+ on MoS2/Fe3O4 surface acted as active sites for PS activation with the generation of SO4•−, •OH, •O2, and 1O2. Mo4+ acted as an electron donor to promote the Fe3+/Fe2+ cycling and thus improved the PS activation capability of MoS2/Fe3O4. The degradation pathways of TC were inferred as hydroxylation, ketylation of dimethylamino group and C-N bond breaking. This study provided a promising activated persulfate-based advanced oxidation process for the efficient degradation of TC by employing MoS2/Fe3O4 as an effective activator.

  • MoS2/Fe3O4 was an effective, recoverable PS activator for tetracycline degradation.

  • TC was removed by free radical and non-free radical degradation pathways.

  • Mo4+ promoted the regeneration of Fe2+ on the Fe3O4 surface.

Tetracycline (TC), as a broad-spectrum drug, has been widely prescribed for the treatment of bacterial infections in humans and animals (Peng et al. 2022). However, only a small portion of TC is absorbed and digested by organisms, while the residue is discharged into the water ecosystem, leading to the occurrence of drug-resistant bacteria and posing a great threat to human health (Cheng & Ji 2022; Liu et al. 2022b). Consequently, it is necessary to remove TC from the wastewater (Neolaka et al. 2023; Zhang et al. 2023d).

Persulfate (PS)-based advanced oxidation processes (AOPs) are considered as an effective method for decomposing pollutants due to their ability to produce sulfate radical () (Wang et al. 2023). Compared with hydroxyl (), produced by the activated PS has a higher redox potential (2.5–3.1 V) (Zhou et al. 2023) and a longer lifetime (30–40 μs) (Wang et al. 2022; Shabanloo et al. 2023), which is conducive to the degradation and mineralization of the pollutants due to sufficient time provided for diffusion and contact with organic pollutants. can be produced by activating PS using various methods, including ultraviolet (UV), heat, ultrasound, transition metal ions (Diao et al. 2020; Du et al. 2020; Deng et al. 2023; Cabrera-Reina et al. 2023) and heterogeneous transition metal catalysts (Cai et al. 2022; Zhang et al. 2023a). The activation of PS by heterogeneous transition metals is independent of additional energy requirements. Additionally, these metals can be easily recovered after the reaction and thus secondary pollution is avoided, which is more favorable in practical applications (Li et al. 2022b, Dai et al. 2023).

Recently, Fe3O4 was frequently applied to PS activation for the removal of organic pollutants due to its excellent magnetism and environmental friendliness (Peng et al. 2018). Fe2+ had the ability to activate PS and generate more , which was efficient for the degradation of organic pollutants (Zhu et al. 2022). Nonetheless, Fe2+ on the surface of Fe3O4 tended to be oxidized to Fe3+ during the activation of PS, and agglomeration was likely to occur among Fe3O4 nanoparticles due to their inherent magnetic properties, leading to the reduction of their specific surface area and the efficiency of activating PS (Zhang et al. 2022a).

Molybdenum disulfide (MoS2) was widely utilized in the wastewater treatment due to its low toxicity, abundant active center and high electron mobility (Jlidi et al. 2021). It was proved that MoS2 could be used to activate PS to enhance the degradation of organic pollutants (Wang et al. 2021a). Mo4+ exposed due to S defect in MoS2 could be used as an active site to reduce Fe3+ to Fe2+, thus promoting the Fe3+/Fe2+ recycling (Lu et al. 2021). Song et al. found that MoS2 could be used as a cocatalyst of Fe2+ to activate PS for the degradation of sulfisoxazole (SIX). The regeneration of Fe2+ was accelerated because Mo4+ in MoS2 could convert Fe3+ to Fe2+, and the removal efficiency of SIX was as high as 97.1% within 40 min (Song et al. 2020a). Kuang et al. added MoS2 into the Fe3+/PS system to degrade p-chloroaniline (PCA), and the electron-rich Mo4+ could transform Fe3+ into Fe2+ through electron transfer, and thus Fe2+ could be continuously produced and continuous degradation of PCA was achieved (Kuang et al. 2021). However, the separation and recovery from the solution of MoS2 could only be performed by repeated filtration or centrifugation, which was a complicated and expensive process.

One possible strategy to solve the above problems is to combine MoS2 and Fe3O4 to prepare a magnetic MoS2/Fe3O4 composite, which can realize rapid separation and recovery of MoS2 under a magnetic field. Mo4+ in MoS2 can promote the Fe3+/Fe2+ cycle on the surface of Fe3O4 and improve its catalytic activity. However, there are few reports on MoS2/Fe3O4 as a PS activator, and the mechanisms are still unclear.

In this study, MoS2/Fe3O4 was prepared as a PS activator by solvothermal hydrothermal method for the effective removal of TC. The structure, morphology and chemical composition of MoS2/Fe3O4 were explored and its catalytic performance for PS was evaluated. Meanwhile, the effects of MoS2/Fe3O4 dosage, PS concentration, initial pH, temperature and common inorganic anions on TC degradation were investigated. The mechanisms of MoS2/Fe3O4 as a PS activator were proposed, the possible degradation pathways of TC in the MoS2/Fe3O4/PS system were revealed, and the risk of TC and its degradation intermediates to aquatic organism were predicted. It is anticipated that MoS2/Fe3O4 can serve as an efficient PS activator for the enhanced degradation of TC and simultaneously be separated and recovered by simple operations.

Chemicals

Detailed information on chemicals and reagents was shown in the Supporting Materials (Text S1).

Synthesis of catalysts

Synthesis of Fe3O4

Fe3O4 was prepared using a solvothermal method (Dolatabadi et al. 2023). Specifically, 1.2 g FeCl3·6H2O and 0.5 g Na3C6H5O7·2H2O were dissolved in 30 mL ethylene glycol (EG) solution at 35 °C by magnetic stirring. Then 3.04 g CH3COONa was added into the above solution and stirred for 1 h to obtain the precursor. The precursor was poured into a 50 mL Teflon-lined stainless steel autoclave and placed in a constant temperature oven at 200 °C for 12 h. The resulting solid products were collected, washed alternately with deionized water and anhydrous ethanol and then dried in a vacuum drying oven at 55 °C for 10 h to obtain Fe3O4 nanoparticles.

Synthesis of MoS2/Fe3O4

First, 1 mmoL of (NH4)6Mo7O24·4H2O was added to 35 mL deionized water by stirring. Different doses of Fe3O4 (0.1, 0.15 and 0.2 g) were added and stirred for 30 min. Then, 2.66 g thiourea was added to the above mixture and stirred again for 30 min. The mixture was then transferred into a 50 mL Teflon-lined stainless steel autoclave and placed in a constant temperature oven at 180 °C for 12 h. Then the solid products were collected and washed alternately with deionized water and anhydrous ethanol. After drying at 55 °C for 10 h in a vacuum drying oven, MoS2/Fe3O4 was obtained, which was labeled as FeM-0.1, FeM-0.15, and FeM-0.2 based on the dosage of Fe3O4, respectively.

Characterization

Detailed information on characterization was given in the Supporting Materials (Text S2).

Experimental procedure

400 mL TC (20 mg·L−1) was added into a 500 mL beaker and placed in a thermostatic water bath (25 °C) for magnetic agitation. The initial pH of the solution was adjusted using 0.1 mol·L−1 HCl or 0.1 mol·L−1 NaOH, and then 4 mmol·L−1 PS and 0.4 g·L−1 MoS2/Fe3O4 were added to initiate the reaction. At an interval of 10 min, a 5 mL aliquot sample was collected and filtered by a 0.22 μm filter. At the same time, the reaction was immediately terminated with 0.5 mL Na2S2O3 (0.1 mol·L−1). After the reaction, MoS2/Fe3O4 was recovered and washed alternately with deionized water and anhydrous ethanol three times. Finally, it was dried under vacuum conditions at 60 °C for the recycling experiment.

Analysis methods

TC concentration was detected using a UV-visible spectrophotometer (UV-1800, AUCY Scientific, China) at 357 nm (Fatimah et al. 2023). The pH meter (Five Easy Plus, Mettler-Toledo, China) was used to determine the pH of the solution. The inductively coupled plasma spectrometry (ICP, Agilent 700 ICP-OES, USA) was used to detect the concentrations of the leaching metal ions in the liquid in the reaction system. Electron paramagnetic resonance spectroscopy (EPR, Bruker A300, Germany) was used to analyze the active species for TC degradation. The degradation intermediates of TC were identified using a liquid chromatography-mass spectrometer (LC-MS, Waters 2695, USA) and the details are shown in Text S3. The calculation method and data for the Fukui function are presented in Text S4. Details of the degradation intermediates and their toxicity assessment are summarized in Text S5 and Table S3.

Effect of loading dose of Fe3O4 in MoS2/Fe3O4 on TC degradation

As presented in Fig. S1, the degradation efficiency of TC was 91.8% in the MoS2/Fe3O4 system, while it decreased to 85.5 and 81.1% in the FeM-0.1 and FeM-0.2 systems, respectively. It was reported that the specific surface area and the number of active sites increased with the rising addition of Fe3O4, consequently improving the TC degradation efficiency (Li et al. 2023e; Sun et al. 2020). However, excessive Fe3O4 might cover the reactive active site on the MoS2 surface and influence the electron transfer (Song et al. 2023). As a result, FeM-0.15 was chosen for further investigation, and MoS2/Fe3O4 was referred to FeM-0.15 in the following discussion.

Characterization of MoS2/Fe3O4

As shown in Figure 1, the peaks located at 18.31°, 30.12°, 35.48°, 43.12°, 57.03° and 62.62° corresponded to (111), (220), (311), (400), (511) and (440) planes of Fe3O4 (PDF#75-0033), respectively. The detected diffraction peaks at 14.38°, 32.68°, 33.51°, 39.54°, 49.79°, 58.33° and 60.14° matched with the (002), (100), (101), (103), (105), (110) and (008) crystal planes of MoS2 (PDF#37-1492), respectively. MoS2/Fe3O4 presented the characteristic peaks of MoS2 and Fe3O4. The absence of individual peaks could be attributed to the low crystallinity of MoS2/Fe3O4 (Lu et al. 2021), which demonstrated MoS2/Fe3O4 was successfully synthesized using a precipitation-hydrothermal method.
Figure 1

XRD pattern of MoS2/Fe3O4.

Figure 1

XRD pattern of MoS2/Fe3O4.

Close modal
Figure 2(a) shows that Fe3O4 appeared as uniform spherical nanoparticles with an approximate diameter of 100 nm. In Figure 2(b), MoS2 presented a flower-like microsphere structure assembled from nanosheets of different dimensions, and its average diameter was about 5 μm. Figure 2(c) shows that Fe3O4 nanoparticles were uniformly attached on the surface of MoS2 to form MoS2/Fe3O4, which could effectively avoid the agglomeration of Fe3O4 nanoparticles. As present in Figure 2(d), the MoS2 nanofakes and Fe3O4 nanoparticles were observed, and Fe3O4 nanoparticles were embedded into the nanofakes of MoS2. Figure 2(e) displays that two distinct lattice fringes were observed in the HRTEM image of MoS2/Fe3O4. The fringes with lattice spacing of 0.210 and 0.274 nm corresponded to the (400) planes of Fe3O4 and (100) planes of MoS2, respectively (Yi et al. 2021; Yu et al. 2022).
Figure 2

(a) SEM image of Fe3O4; (b) SEM image of MoS2; (c) SEM image of MoS2/Fe3O4; (d) TEM image of MoS2/Fe3O4; (e) HRTEM image of MoS2/Fe3O4.

Figure 2

(a) SEM image of Fe3O4; (b) SEM image of MoS2; (c) SEM image of MoS2/Fe3O4; (d) TEM image of MoS2/Fe3O4; (e) HRTEM image of MoS2/Fe3O4.

Close modal
As shown in Figure 3(a), only Mo, S, Fe and O elements were observed on the surface of MoS2/Fe3O4, with an atomic ratio of Fe/Mo of 1:1.5. EDS element mapping (Figure 3(b)–3(f)) revealed that Mo, S, Fe and O elements were uniformly distributed, further affirming that Fe3O4 were successfully anchored on MoS2 and thus achieved favorable PS activation. Additionally, Guo et al. (2023) also demonstrated that better dispersity provided more active sites on the surface of the composite, contributing to the synergistic effect between MoS2 and Fe3O4 and PS activation.
Figure 3

(a) EDS pattern of MoS2/Fe3O4; (b–f) EDS element mapping of Mo, Fe, O and S of MoS2/Fe3O4.

Figure 3

(a) EDS pattern of MoS2/Fe3O4; (b–f) EDS element mapping of Mo, Fe, O and S of MoS2/Fe3O4.

Close modal
As shown in Figure 4(a), the peaks detected at 710.6 and 724.3 eV corresponded to Fe2+ 2p3/2 and Fe2+ 2p1/2, respectively, while the peaks at 713.1 and 727.1 eV were attributed to Fe3+ 2p3/2 and Fe3+ 2p1/2, respectively. The peaks located at 719.1 and 733.3 eV were satellite peaks of Fe 2p3/2 and Fe 2p1/2, respectively (Huang et al. 2019; Tong et al. 2022). The two typical peaks at 228.5 and 231.8 eV in Figure 4(b) were assigned to Mo4+ 3d5/2 and Mo4+ 3d3/2, respectively (Li et al. 2022c). A weak peak at 235.3 eV was ascribed to Mo6+ because negligible MoS2 was oxidized into MoO3. The peaks at 229.5 and 232.80 eV corresponded to Mo5+ 3d5/2 and Mo5+ 3d3/2 of MoS2. A peak detected at 225.6 eV was attributed to S 2s of MoS2. In the O 1s spectra of MoS2/Fe3O4 (Figure 4(c)), two peaks located at 530.2 and 531.7 were defined as the lattice oxygen in metal oxides (M-O) and surface hydroxyl group (-OH), respectively (Huang et al. 2019). For the S 2p spectra of MoS2/Fe3O4 (Figure 4(d)), the peaks at 161.4 and 162.7 eV were assigned to S 2p3/2 and S 2p1/2, respectively (Luo et al. 2021a). Based on the above characteristics, MoS2/Fe3O4 had been successfully synthesized.
Figure 4

XPS spectra of the MoS2/Fe3O4: (a) Fe 2p, (b) Mo 3d, (c) O 1s and (d) S 2p.

Figure 4

XPS spectra of the MoS2/Fe3O4: (a) Fe 2p, (b) Mo 3d, (c) O 1s and (d) S 2p.

Close modal
The recovery of the magnetic compounds was simple, which could reduce the operating costs and the secondary pollution (Fernández-Velayos et al. 2022; Swami et al. 2023). As shown in Figure 5, MoS2/Fe3O4 exhibited superparamagnetism with a saturation magnetization of 9.73 emu g−1. In addition, MoS2/Fe3O4 could be separated from the solution within 120 s under the action of the magnet as shown in Figure 5, avoiding its unnecessary loss (Wang et al. 2021b).
Figure 5

MH curve of MoS2/Fe3O4.

Figure 5

MH curve of MoS2/Fe3O4.

Close modal

TC degradation

PS activation performance of MoS2/Fe3O4

As shown in Figure 6(a), only 6.1% of TC could be removed within 60 min in a single PS system, while 15.2% when MoS2/Fe3O4 was used due to the limited adsorption of TC by MoS2/Fe3O4. The removal efficiencies of TC in the Fe3O4/PS and MoS2/PS systems were 65.5 and 59%, respectively. These results showed that a single Fe3O4 or single MoS2 was invalid in activating PS to degrade TC. However, in the MoS2/Fe3O4/PS system, the removal efficiency of TC was significantly improved to 91.8% after 60 min reaction, which was much higher than the Fe3O4/PS or MoS2/PS systems as there might be a synergistic effect between MoS2 and Fe3O4 for PS activation. The degradation of TC followed the first-order reaction kinetic model (R2 > 0.92), as shown in Figure 6(b). The reaction rate constant (kobs) values for PS, Fe3O4/PS, MoS2/PS and MoS2/Fe3O4/PS systems were 0.00106, 0.01808, 0.01366 and 0.04187 min−1, respectively. The kobs values of the MoS2/Fe3O4/PS system were 39.5, 2.32 and 3.06 times higher than those of PS, Fe3O4/PS and MoS2/PS systems, respectively. As shown in Table 1, compared with similar studies, we used the hydrothermal method to load Fe3O4 nanoparticles onto MoS2 of flower-shaped microspheres, avoiding the aggregation of Fe3O4 nanoparticles and accelerating the surface Fe3+/Fe2+ cycling. The preparation method was simple and a small amount of MoS2/Fe3O4 could effectively degrade TC in a short time. In summary, MoS2/Fe3O4 exhibited efficient catalytic ability for TC degradation.
Table 1

Comparison of this work with previous reports on the use of different catalysts to degrade TC

CatalystPreparation methodStructureDosage (g·L−1)TC (mg·L1)Reaction time (min)Removal efficiency (%)Reference
Fe3O4 Commercial purchase Nanoparticles 100 90 89.0 Hou et al. (2012))  
Fe3O4@JDC Soaking and calcination method Nanoparticles anchored on the surface of tubular JDC 0.1 10 60 90.2 Zhang et al. (2023c
BC300-MoS2-1 Hydrothermal method Flower-like cluster framework 0.05 20 120 78 Su et al. (2022)  
FeOOH@MoS2 Hydrothermal-chemical deposition method Nanosheet structure 0.4 50 30 85 Yi et al. (2021)  
Fe3O4/CoS2 Hydrothermal method Regular polyhedron 0.1 20 20 88.3 Qiu et al. (2024)  
C@Fe3O4 One-pot hydrothermal method Hollow sphere structure 0.5 100 90 78.5 Peng et al. (2018)  
CoFe2O4@MoS2 One-pot hydrothermal method Nanoparticles distributed in the MoS2 flakes 0.2 10 30 80.4 Peng et al. (2022)  
Fe3O4/MoS2 Hydrothermal method Nanoparticles attached on the surface of MoS2 0.4 20 60 91.8 This work 
CatalystPreparation methodStructureDosage (g·L−1)TC (mg·L1)Reaction time (min)Removal efficiency (%)Reference
Fe3O4 Commercial purchase Nanoparticles 100 90 89.0 Hou et al. (2012))  
Fe3O4@JDC Soaking and calcination method Nanoparticles anchored on the surface of tubular JDC 0.1 10 60 90.2 Zhang et al. (2023c
BC300-MoS2-1 Hydrothermal method Flower-like cluster framework 0.05 20 120 78 Su et al. (2022)  
FeOOH@MoS2 Hydrothermal-chemical deposition method Nanosheet structure 0.4 50 30 85 Yi et al. (2021)  
Fe3O4/CoS2 Hydrothermal method Regular polyhedron 0.1 20 20 88.3 Qiu et al. (2024)  
C@Fe3O4 One-pot hydrothermal method Hollow sphere structure 0.5 100 90 78.5 Peng et al. (2018)  
CoFe2O4@MoS2 One-pot hydrothermal method Nanoparticles distributed in the MoS2 flakes 0.2 10 30 80.4 Peng et al. (2022)  
Fe3O4/MoS2 Hydrothermal method Nanoparticles attached on the surface of MoS2 0.4 20 60 91.8 This work 
Figure 6

(a) Removal efficiency of TC in various systems. (b) Kinetics of TC degradation. Reaction conditions: [MoS2/Fe3O4] = 0.4 g·L−1, [PS] = 4 mmoL, [TC] = 20 mg·L−1, initial pH = 7, T = 25 °C and reaction time = 60 min.

Figure 6

(a) Removal efficiency of TC in various systems. (b) Kinetics of TC degradation. Reaction conditions: [MoS2/Fe3O4] = 0.4 g·L−1, [PS] = 4 mmoL, [TC] = 20 mg·L−1, initial pH = 7, T = 25 °C and reaction time = 60 min.

Close modal

Effects of MoS2/Fe3O4 dosage and PS concentration

As the dosage was increased from 0.1 to 0.4 g·L−1, the removal efficiency of TC improved from 61.3 to 91.8% (Figure 7(a)), which could be attributed to the increase of reactive sites for PS activation with higher MoS2/Fe3O4 dosage (Yi et al. 2021). However, when the dosage was increased to 0.5 g·L−1, the removal efficiency of TC only increased by 1.5%, which suggested that excessive MoS2/Fe3O4 addition could not facilitate TC removal as PS was insufficient. Therefore, 0.5 g·L−1 MoS2/Fe3O4 was selected for further experiments.
Figure 7

Effects of reaction parameters on the TC degradation: (a) the dosage of MoS2/Fe3O4; (b) PS concentration; (c) initial pH; (d) Zeta potential of MoS2/Fe3O4 at different pH values; (e) temperature; (f) inorganic anions (Cl, , ) and HA. Reaction conditions: [MoS2/Fe3O4] = 0.4 g·L−1, [PS] = 4 mmoL, [TC] = 20 mg·L−1, initial pH = 7, T = 25 °C and reaction time = 60 min.

Figure 7

Effects of reaction parameters on the TC degradation: (a) the dosage of MoS2/Fe3O4; (b) PS concentration; (c) initial pH; (d) Zeta potential of MoS2/Fe3O4 at different pH values; (e) temperature; (f) inorganic anions (Cl, , ) and HA. Reaction conditions: [MoS2/Fe3O4] = 0.4 g·L−1, [PS] = 4 mmoL, [TC] = 20 mg·L−1, initial pH = 7, T = 25 °C and reaction time = 60 min.

Close modal
In Figure 7(b), as the PS concentration was increased from 1.0 to 4.0 mmol·L−1, the removal efficiency of TC elevated from 46.3 to 91.8%. When the PS concentration was further increased from 4.0 to 5.0 mmol·L−1, the degradation efficiency of TC decreased from 91.8 to 86.9%. It was inferred that the generation rate of free radicals increased with the rising PS concentration, thus enhancing the removal efficiency of TC (Li et al. 2023a; Sun et al. 2020). However, excessive PS inhibited the degradation of TC because the increased was quenched by itself or PS through the following equations (Li et al. 2022c; Liu et al. 2022a).
(1)
(2)

Effects of pH and temperature

At initial pH values of 3, 5 and 7, the removal efficiencies of TC were 94.8, 93.5 and 91.8%, respectively. However, at initial pH values of 9 and 10, the removal efficiencies of TC decreased to 84.2 and 42.9%, respectively (Figure 7(c)). It was thus inferred that the favorable TC degradation in the MoS2/Fe3O4/PS system could be achieved within the initial pH range of 3–7. The pH in the MoS2/Fe3O4/PS system was monitored during the reaction, as shown in Figure S2. When initial pH values were 3, 5 and 7, the pH dropped below 3.30 after 10 min and continued to decrease in subsequent reactions. When initial pH values were 9 and 10, the pH remained above 3.89 within 60 min. The zero charge point (pHPZC) of the MoS2/Fe3O4 surface was approximately 3.66 (Figure 7(d)). Therefore, the MoS2/Fe3O4 surface carried positive charges after 10 min at initial pH values lower than 7, and with negative charges was easily adsorbed on the surface of MoS2/Fe3O4, which promoted the activation of PS and improved the removal efficiency of TC (Zhu et al. 2022). However, the MoS2/Fe3O4 surface carried negative charges within 60 min at initial pH values higher than 9, and the electrostatic repulsion prevented the contact between MoS2/Fe3O4 and PS, inhibiting the degradation of TC (Luo et al. 2021b).

The effect of temperature on TC degradation in the MoS2/Fe3O4/PS system is shown in Figure 7(e). When the temperature was increased from 25 to 35 °C, the removal efficiencies of TC were higher than 90% after 40 min. When the temperature raised to 45 and 55 °C, the removal efficiency reached 90% after 30 min. It indicated that the elevated temperature could accelerate TC degradation in the MoS2/Fe3O4/PS system. Elevated temperature was conducive to the thermal decomposition of PS, promoting the generation of (Guo et al. 2016; Zhang et al. 2022b).

Effects of inorganic anions and humic acid

The effects of common inorganic anions (Cl, and ) and humic acid (HA) on TC degradation were investigated at a concentration of 3 mmol·L−1. As shown in Figure 7(f), when 3 mmol·L−1 Cl and were added into the MoS2/Fe3O4/PS system, the removal efficiency of TC increased from 37.5 to 55.4 and 70.1% after 10 min, and from 91.8 to 93.2 and 93.4% after 60 min, respectively, demonstrating that the introduction of Cl and accelerated the degradation of TC in the MoS2/Fe3O4/PS system. Cl could react with to generate Cl· and , and could be oxidized by to produce (Equations (3)–(5)) (Sun et al. 2020; Li et al. 2023d; Wang et al. 2023). Although the oxidation potential of , and was slightly lower than that of , they could compensate for the depletion of due to Cl and scavenging and enhance the degradation efficiency of TC, which was consistent with previous reports (Song et al. 2020a, 2020b). The removal efficiency of TC decreased from 91.8 to 47.3% after 60 min in the presence of . This was due to the fact that a part of was quenched by and generated by its ionization (Equations (6) and (7)) (Sun et al. 2020; Li et al. 2023b). The removal efficiency of TC decreased from 91.8 to 83.2% after adding 3 mmol·L−1 HA into the MoS2/Fe3O4/PS system, indicating that HA suppressed TC degradation. Previous studies had shown that HA could act as a free radical quencher by competing with and . In addition, the phenolic hydroxyl and carboxyl groups in HA were adsorbed on the surface of MoS2/Fe3O4, blocking the active site and hindering the activation of PS (Li et al. 2019).
(3)
(4)
(5)
(6)
(7)

Mechanisms

EPR testing and quenching experiment

The active species generated in the MoS2/Fe3O4/PS system were identified by EPR testing (Sun et al. 2022). As shown in Figure 8(a), no signals were observed in the PS system. In contrast, a DMPO-·OH characteristic signal with a peak intensity of 1:2:2:1 was detected in the MoS2/Fe3O4/PS system. This indicated that MoS2/Fe3O4 could effectively activate PS to produce . A signal formed by DMPO and was also observed in the MoS2/Fe3O4/PS system. The signal intensity of was much weaker than that of DMPO-, which was attributed to the fact that could react quickly with H2O to convert into ·OH through Equation (8) (Li et al. 2022a). In addition, DMPO and TEMP were applied to capture and . Figure 8(b) and 8(c) showed the characteristic signals of and TEMP-, indicating the generation of and . The Fe2+ on MoS2/Fe3O4 surface could provide an electron for dissolved oxygen to produce (Equation (9)), which was consistent with the research results reported by He et al. (2021) and Huang et al. (2021). Subsequently, the generated could interact or react with to generate (Equations (10) and (11)) (Li et al. 2019). Moreover, Mo6+ on MoS2/Fe3O4 surface could react with to produce (Equation (12)) (Zhang et al. 2020). The results of the EPR testing confirmed the presence of , , and in the MoS2/Fe3O4/PS system.
Figure 8

(a–c) EPR spectrum in the MoS2/Fe3O4/PS system; (d) Effects of radical scavengers on the degradation of TC. Reaction conditions: [MoS2/Fe3O4] = 0.4 g·L−1, [PS] = 4 mmoL, [TC] = 20 mg·L−1, initial pH = 7, T = 25 °C and reaction time = 60 min.

Figure 8

(a–c) EPR spectrum in the MoS2/Fe3O4/PS system; (d) Effects of radical scavengers on the degradation of TC. Reaction conditions: [MoS2/Fe3O4] = 0.4 g·L−1, [PS] = 4 mmoL, [TC] = 20 mg·L−1, initial pH = 7, T = 25 °C and reaction time = 60 min.

Close modal
To verify the above conclusions, methanol (MeOH) was used as a quencher for and , while tert-butanol (TBA); ascorbic acid (AA); L-histidine (L-H) were used as quenchers for , and , respectively (Li et al. 2022c; Peng et al. 2022; Liao et al. 2011). The removal efficiency of TC decreased from 91.8 to 61.8 and 73.6% with the addition of 1 moL MeOH and TBA, respectively (Figure 8(d)) (Li et al. 2022a; Peng et al. 2022). The results indicated the presence of and in the MoS2/Fe3O4/PS system, and the contribution of to TC degradation was higher than that of . However, MeOH or TBA did not completely inhibit the degradation of TC, indicating the presence of other active species in the MoS2/Fe3O4/PS system. When 10 mmoL AA and L-H were added into the MoS2/Fe3O4/PS system, the removal efficiency of TC decreased from 91.8 to 31.5 and 56.3%, respectively, confirming that and also played an important role in TC degradation (Yang et al. 2020, 2022), which was consistent with the EPR test results. According to the results of EPR testing and quenching experiments, , , and were present in the MoS2/Fe3O4/PS system, and , and were the predominant active species.
(8)
(9)
(10)
(11)
(12)

XPS analysis

In order to reveal the mechanism of MoS2/Fe3O4 activating PS, XPS was used to analyze its surface element valence before and after the activation reaction (Figure 9). As shown in Figure 9(a), the proportion of surface Fe3+ decreased from 44.7 to 40.4%, while that of Fe2+ increased from 55.3 to 59.6% after the activation reaction as a part of Fe3+ was reduced to Fe2+. Previous reports had shown that Mo4+ contributed to the regeneration of Fe2+ and thus accelerated the cycling of Fe3+/Fe2+ (Lu et al. 2021). Therefore, Fe2+ was first oxidized into Fe3+ by PS, and then the exposed Mo4+ continuously reduced Fe3+ to Fe2+, resulting in an increase of Fe2+ proportion. The regeneration of Fe2+ was a key step in efficient PS activation and the regenerated Fe2+ could participate again in the activation of PS and promote the generation of free radicals. As shown in Figure 9(b), the proportion of Mo4+ decreased from 59.6 to 53%, and the proportion of Mo6+ increased from 6.1 to 13.6%, which demonstrated that a portion of Mo4+ was oxidized into Mo6+. As shown in Figure 9(c), the proportion of M-O increased from 22.4 to 41.1% after the reaction, which might be due to the production of molybdenum oxide on the catalyst surface. The proportion of surface -OH decreased from 77.6 to 58.9% after the reaction, indicating that the surface -OH of the catalyst also participated in the reaction. In addition, two new peaks appeared at 163.4 and 169.0 eV after the reaction (Figure 9(d)), indicating the formation of S0 and on the catalyst surface (Fan et al. 2018). This was due to the oxidation of S2− by PS during the reaction process. These results confirmed that Fe3O4 and MoS2 had synergistic effects on PS activation and Mo4+ in MoS2/Fe3O4 could reduce Fe3+ to Fe2+, promoting the cycling of Fe3+/Fe2+ and the production of active species.
Figure 9

XPS spectra of MoS2/Fe3O4 before and after use: (a) Mo 3d; (b) Fe 2p; (c) O 1s; (d) S 2p.

Figure 9

XPS spectra of MoS2/Fe3O4 before and after use: (a) Mo 3d; (b) Fe 2p; (c) O 1s; (d) S 2p.

Close modal

Mechanisms of MoS2/Fe3O4 activating PS

Based on the active species identification and XPS analysis, the mechanisms for the PS activation by CuS/Fe3O4 were proposed. Firstly, Mo4+ and Fe2+ on the surface of MoS2/Fe3O4 acted as the active sites to activate PS and produce (Equations (13) and (14)). The generated was further converted to (Equation (15)), and Mo4+ and Fe2+ were oxidized to Mo6+ and Fe3+, respectively. Fe2+ on the surface of MoS2/Fe3O4 reacted with dissolved oxygen to form , and was further converted to by self-reaction or reacting with ·OH (Equations (9)–(11)). In addition, Mo6+ on MoS2/Fe3O4 surface could react with to produce (Equation (12)). Mo4+ could be used as an electron donor to reduce Fe3+ to Fe2+ (Equation (16)). This process promoted the cycle of Fe3+/Fe2+ and ensured sufficient Fe2+ on the surface of MoS2/Fe3O4 to improve the catalytic activity of MoS2/Fe3O4.
(13)
(14)
(15)
(16)

TC degradation pathway and toxicity assessment

Fukui index was employed to analyze the main sites for reactive species on TC molecule on the basis of the Density functional theory (DFT) calculation (Cheng & Ji 2022; Wu et al. 2022). In this study, , , and were the attack species participated in TC degradation in the MoS2/Fe3O4/PS system. Therefore, and were taken into account to predict the active site of TC in the degradation process (Chen et al. 2023). Fukui index (, and ) and the Natural Population Analysis (NPA) charges distribution are shown in Table S1, and the optimized structure of TC is listed in Fig. S3. Generally, the Fukui exponent values with larger and were more likely to be attacked by and free radicals (, ·OH, ), respectively (Cheng & Ji 2022). Hence, N14 (0.0581) and C7 (0.0751) with higher was susceptible to attack. C7 (0.0455) and C12 (0.0401) with larger values were vulnerable reaction sites for , , attack, which could undergo oxidation, hydroxylation, and double bond breaking (Zhang et al. 2023b). Although C12 had a larger value, the attack of free radicals was difficult because of the saturated sites and steric hindrance (Wu et al. 2023).

According to the detected intermediates (Table S2) and the Fukui index, the possible degradation pathways of TC in the MoS2/Fe3O4/PS system are shown in Figure 10. In path I, C7 was attacked by , ·OH, and and TC was hydroxylated to produce P1 (m/z = 461). P2 (m/z = 309) was also formed due to the loss of two dimethyl from P1, and then P2 was converted into P3 (m/z = 213) owing to the loss of two amino groups (Wang et al. 2017). In path Ⅱ, N14 tended to be attacked by because of its relative larger (0.0581) value. The dimethylamino in the TC was oxidized to the ketone group to form P4 (m/z = 417). P4 was hydroxylated to produce P5 (m/z = 433). P5 was further oxidized, causing the fracture of the aromatic ring to produce P6 (m/z = 481). In addition, P7 (m/z = 401) was formed by the dehydration of P4, and P7 was further oxidized by free radicals to form P8 (m/z = 332) (Pi et al. 2019). In path III, TC was transformed into P9 (m/z = 431) owing to the C-N cleavage and dethylation. P10 (m/z = 447) was generated owing to the hydroxylation of P9 and could be further oxidized to P11 (m/z = 349) by free radicals. P12 (m/z = 374) was stemmed from subsequent deamidation of P11 (m/z = 349), and was further opened in the ring to produce P13 (m/z = 181) (Guo et al. 2023). P3, P6, P8 and P13 was further degraded into small organic molecules including P14 (m/z = 57), P15 (m/z = 60), P16 (m/z = 100) and P17 (m/z = 129) (Dong et al. 2022). Eventually, these short-chain organic molecules were oxidized and decomposed into H2O and CO2.
Figure 10

Degradation pathway of TC.

Figure 10

Degradation pathway of TC.

Close modal
Furthermore, ECOSAR software was applied to predict the toxicity of TC and its intermediate products (Yin et al. 2018). The toxicity values for fish LC50 (96 h), daphnid LC50 (48 h) and green algae EC50 (48 h) are presented in Table S3 and the range of toxicity values is shown in Table S4. As demonstrated in Figure 11, the acute toxicity values for fish and green algae were calculated as 61.44 and 17.68 mg·L−1, which was ‘harmful’. The value for daphnid was 4.49 mg·L−1 in the ‘toxic’ range. The chronic toxicity value for fish was 0.96 mg·L−1, which was classified as ‘very toxic’. The values for daphnid and green algae were 1.04 and 2.52 mg·L−1, which indicated that TC had a certain toxic effect on aquatic organisms. It was worth noting that P3 and P9 were 6.29 and 12.12 mg·L−1, which indicated the slightly higher toxicity to green algae than TC. However, as the oxidation reaction occurred, toxicity values of other intermediates were lower than that of TC, which fell in the ‘no harmful’ or ‘harmful’ range, indicating that the MoS2/Fe3O4 system could not only effectively degrade TC, but also reduce certain toxic effects.
Figure 11

Acute toxicity of (a) fish LC50 (96 h); (b) daphnid LC50 (48 h); (c) green algae EC50 (48 h); chronic toxicity of (d) fish; (e) daphnid; (f) green algae.

Figure 11

Acute toxicity of (a) fish LC50 (96 h); (b) daphnid LC50 (48 h); (c) green algae EC50 (48 h); chronic toxicity of (d) fish; (e) daphnid; (f) green algae.

Close modal

Recyclability and stability of MoS2/Fe3O4

As shown in Figure 12(a), after three recycles, the degradation efficiency of TC was still higher than 86% after 60 min, which proved that the MoS2/Fe3O4 had excellent recyclability. The slight decrease of TC degradation efficiency during the recycle experiment might be due to the adsorption of TC degradation intermediates on MoS2/Fe3O4 surface, leading to the plugging of active sites (Sun et al. 2020; Li et al. 2023c). In addition, the reduction of the proportion of Mo4+ on the MoS2/Fe3O4 surface during the reaction resulted in the decrease in the conversion efficiency of Fe3+ to Fe2+. In order to investigate the stability of MoS2/Fe3O4, the concentration of Mo and Fe ions leached from MoS2/Fe3O4 was monitored by ICP. As shown in Figure 12(b), the concentration of Mo and Fe in the MoS2/Fe3O4/PS system was only 30.4 and 4.85 mg·L−1 after 60 min reaction, respectively, which demonstrated its high stability.
Figure 12

(a) Recyclability experiments of MoS2/Fe3O4 and (b) leaching concentration of metal ions.

Figure 12

(a) Recyclability experiments of MoS2/Fe3O4 and (b) leaching concentration of metal ions.

Close modal

In this work, MoS2/Fe3O4 was successfully prepared as PS activator to improve the removal of TC and the separation of MoS2. Compared to single Fe3O4 and MoS2, MoS2/Fe3O4 had higher activation activity for PS activation, achieving a removal efficiency of 91.8% for 20 mg·L−1 TC after 60 min under the conditions of 0.4 g·L−1 MoS2/Fe3O4, 4 mmoL PS and pH 7. MoS2/Fe3O4 also demonstrated favorable stability during recycling experiments. The degradation process of TC in the MoS2/Fe3O4 system could be divided into free radical decomposition pathway (, and ) and non-free radical decomposition pathway (), with , and identified as the predominant active species. The three major degradation pathways of TC in the MoS2/Fe3O4 system were proposed as hydroxylation, ketylation of dimethylamino group and C-N bond breaking. The toxicity of TC and its intermediates could be effectively reduced by using MoS2/Fe3O4 to activate PS. In conclusion, MoS2/Fe3O4 prepared was an efficient PS activator to promote the decomposition of refractory organic pollutants.

This work was supported by the Science and Technology Development Program of Jilin Province, China (Nos. 20230203168SF).

L. Z. arranged the resources, wrote the review and edited the article, and rendered support in funding acquisition. Q. Z. investigated the work, rendered support in data curation, and wrote the original draft. T. C. investigated the work and rendered support in data curation. C. W. investigated the work and rendered support in data curation. C. X. investigated the work and edited the article. J. G. supervised the work, arranged the resources, and wrote the review. X. P. investigated the work and edited the article. S. L. validated the article and visualized the data.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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Author notes

Lanhe Zhang and Qi Zhang are co-first authors.

This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY-NC-ND 4.0), which permits copying and redistribution for non-commercial purposes with no derivatives, provided the original work is properly cited (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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