Modified Polyethersulfone Ultrafiltration Membrane for Enhanced Antifouling Capacity and Dye Catalytic Degradation Efficiency

Abstract

Catalytic membranes, as a combination of heterogeneous advanced oxidation and membrane technology reaction systems, have important application prospects in the treatment of dyes and other organics. In practical applications, it is still challenging to construct catalytic membranes with excellent removal efficiency and fouling mitigation. Herein, molybdenum disulfide-iron oxyhydroxide (MoS₂-FeOOH) was fabricated using iron oxide and MoS₂ nanoflakes, which were synthesized by the hydrothermal method. Furthermore, by changing the concentration of MoS₂-FeOOH, the MoS₂-FeOOH/polyethersulfone (PES) composite ultrafiltration membrane was obtained with improved hydrophilicity, permeability, and antifouling capacity. The pure water flux of the composite membrane reached 385.3 L/(m²·h), which was 1.7 times that of the blank PES membrane. Compared with the blank membrane, with the increase of MoS₂-FeOOH content, the MoS₂-FeOOH/PES composite membranes had better adsorption capacity and catalytic performance, and the membrane with 3.0% MoS₂-FeOOH content (M4) could be achieved at a 60.2% methylene blue (MB) degradation rate. In addition, the membrane flux recovery ratio (FRR) of the composite membrane also increased from 25.6% of blank PES membrane (M0) to more than 70% after two cycles of bovine serum albumin (BSA) filtration and hydraulic cleaning. The membrane with 2.25% MoS₂-FeOOH content (M3) had the best antifouling performance, with the largest FRR and the smallest irreversible ratio (R_ir). Catalytic self-cleaning of the composite membrane M3 recovered 95% of the initial flux with 0.1 mol/L H₂O₂ cleaning. The MoS₂-FeOOH/PES composite membranes with the functions of excellent rejection and antifouling capacity have a good prospect in the treatment of printing and dyeing wastewater composed of soluble dyes.

1. Introduction

Ultrafiltration (UF) membranes have been widely used in water and wastewater treatment processes, including drinking water advanced treatment, seawater desalination and industrial wastewater reclamation [1,2,3], due mainly to their high efficiency, high selectivity, and wide application range [4,5,6]. However, reversible and irreversible fouling on porous membranes [7,8] inevitably decreases its permeability and service life [9,10], which significantly confines the application of UF membranes.

As a kind of advanced oxidation process (AOP), the Fenton method has obvious inherent advantages such as short reaction time, low cost, energy-saving, and environmental protection, which has been widely used in the field of sewage treatment [11]. Combined with this AOP, the catalytic membrane has become a candidate that is expected to simultaneously separate particulate pollutants and degrade organic pollutants. Modified organic membranes such as polyvinylidene fluoride (PVDF) [12], polysulfone (PSF) [13] and polyethersulfone (PES) [14] have been reported, while modified inorganic membranes include ceramic membranes [15], carbon nanotubes [16], and molybdenum disulphide membranes [17]. The organic catalytic membrane process has the advantages of simple synthesis, low cost of raw materials, large-scale production, etc.

PES is one of the most common polymeric materials used in the synthesis of composite membranes for water engineering due to its high heat resistance, chemical stability, and good mechanical properties [18,19,20]. Recently, catalytic PES membranes combined with nano-sized catalytic materials, such as FeOCl, FeOOH, and Fe₃O₄/SiO₂, have been developed to give the membrane a certain catalytic ability [21]. For instance, a composite UF membrane was prepared by a coating nano-FeOCl catalyst, and had higher water flux and enhanced flux recovery about 100% by facile H₂O₂ cleaning on the membrane surface [22]. Chen et al. [23] incorporated iron phthalocyanine (FePc) into PVDF to fabricate the composite UF membrane (FePc loading of 2.5 wt%) that displayed an enhanced pure water flux and BSA rejection efficiency. However, the existing Fenton catalysts tend to aggregate due to their nanometer size, resulting in uneven catalyst distribution in the catalytic membrane, which may further lead to a reduced Fenton or Fenton-like efficiency and poor membrane performance [24,25]. Therefore, the synthesis of highly efficient and stable iron-based heterogeneous Fenton catalyst remains a challenge.

In recent years, FeOOH has had broad application prospects as a heterogeneous Fenton catalyst due to its abundant natural resources, low cost, and environmental friendliness. FeOOH also has high hydrophilicity and exhibits efficient catalytic activity for degrading contaminants through the Fenton reaction in the presence of H₂O₂. The catalytic function of the membrane can be achieved by mixing FeOOH with the membrane material. However, the agglomeration of FeOOH hinders its application. MoS₂ is one kind of good co-catalyst with excellent adsorption capacity, large specific surface area, and stable chemical stability [26]. MoS₂ is expected to avoid agglomeration of FeOOH and increase active exposure sites that could promote the reaction with reactants. Moreover, MoS₂ can also accelerate the reduction of Fe³⁺ to Fe²⁺ and thus increase the overall reaction rate. Hence, introducing MoS₂ as a co-catalyst in FeOOH-based catalytic membranes might continuously initiate oxidation for foulants removal.

In this study, a novel MoS₂-FeOOH/PES membrane was designed and prepared to simultaneously improve the membrane permeability and obtain the catalytic self-cleaning ability. The MoS₂-FeOOH nanoparticles were prepared and incorporated in the PES membranes during the nonsolvent-induced phase separation method. The adsorption and catalytic properties of the resultant membranes were investigated under different H₂O₂ content, pH, and pressures. In the filtration experiments of bovine serum albumin (BSA) solution and methylene blue (MB), oxidation-initiated membrane self-cleaning and degradation performance was evaluated with the addition of H₂O₂. In addition, antifouling mechanisms were analyzed by stepwise quenching trials on the catalyst, aiming to understand catalytic self-cleaning function for sustainable fouling mitigation. This method for contaminant removal and fouling control has potential for membrane application in water engineering.

2. Experimental

2.1. Materials and Reagents

Analytical grade chemicals were used for the catalytic membrane preparation without any further purification. Specifically, ammonium molybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O, 99%), thiourea (CH₄N₂S, 99.0%), and anhydrous ethanol (C₂H₆O, 99%) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Ferric chloride hexahydrate (FeCl₃ 6H₂O, 99.0%) and polyethylene glycol with an MW of 20,000 g/mol as porogen for membrane preparation were bought from Macklin Reagent Co., (Shanghai, China). Polysulfone (Mn 3000), N,N-dimethylacetamide (DMAC, CH₃C(O)N(CH₃)₂, 99.0%), and hydrogen peroxide (H₂O₂, 30 wt%) were obtained from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Methylene blue (MB, C₁₆H₁₈C_lN₃S₃H₂O, Aladdin) and bovine serum albumin (BSA, Sigma Aldrich) were used as the model pollutants for membrane filtration tests.

2.2. Synthesis of MoS₂-FeOOH Nanoparticles

MoS₂ nanosheets were fabricated by ammonium molybdate and thiourea using a one-step hydrothermal method according to a previous report [27], and the optimized preparation conditions included the molar ratio of Mo:S of 1:4, the hydrothermal temperature of 220 °C, and the hydrothermal time of 24 h. Upon the preparation of MoS₂ nanosheets, FeOOH was loaded onto MoS₂ to fabricate MoS₂ composite material according to previous research [28]. In addition, 0.16 g of MoS₂ and 3 × 10⁻⁴ mol of FeCl₃ 6H₂O were dispersed in 40 mL of absolute ethanol for 5 min. Furthermore, 0.158 g of NH₄HCO₃ was added to the mixed solution and stirred on a magnetic stirrer for 8 h. After the reaction, the catalyst was centrifugally washed with deionized water at 10,000 r/min for 3 times and dried under vacuum conditions for 24 h to obtain the MoS₂-FeOOH catalyst.

2.3. Preparation of MoS₂-FeOOH/PES Composite Membrane

MoS₂-FeOOH/PES composite UF membranes were synthesized by the phase inversion method in which the optimized MoS₂-FeOOH was blended [29]. In brief, the cast solution for membrane fabrication was comprised of 15% PES, 20% polyethylene glycol (PEG), and 65% DMA_C, which respectively took a role as a membrane material matrix, porogen, and solvent. At first, a predetermined amount of MoS₂-FeOOH was added into DMAc, and the formed mixture was dispersed homogeneously with the assistance of proper sonication. A certain amount of PEG, PES, and DMAc was then separately added into a flask and stirred thoroughly at 60 °C for 1.5 h in a water bath until the PES was fully dissolved. Afterward, the DMAc solvent containing MoS₂-FeOOH was homogeneously mixed and was added into the flask, which was then continuously stirred for 1.5 h until MoS₂-FeOOH was evenly dispersed in the casting solution. Subsequently, the obtained casting solution was left standing for 24 h until being completely degassed. Finally, the composite UF membranes were cast and immersed in water to remove excess solvent with the deionized (DI) water and refilled every 8 h for phase inversion. The prepared membranes were tested after 24 h of stabilization (Figure 1a).

Four kinds of MoS₂-FeOOH/PES composite membranes with different compositions of MoS₂-FeOOH, including 0.05, 0.10, 0.15, and 0.20, were marked as M1, M2, M3, and M4, respectively (

Table 1. Composition of the membrane casting solution.

Membrane Name	PES (%)	DMAc (%)	PEG (%)	MoS2-FeOOH (%)
M0	15.00	65.00	20.00	0.00
M1	15.00	64.25	20.00	0.75
M2	15.00	63.50	20.00	1.50
M3	15.00	62.75	20.00	2.25
M4	15.00	62.00	20.00	3.00

). A control membrane without MoS₂-FeOOH addition was also fabricated and characterized comparatively with composite membrane.

2.4. Characterization of MoS₂-FeOOH Nanoparticles and Membranes

2.4.1. Characterization of Catalysts and Membranes

Scanning electron microscope (SEM, Hitachi SU8010, Tokyo, Japan) was used to study the morphology and microstructures of MoS₂-FeOOH nanoparticles (NPs) and MoS₂-FeOOH/PES composite membranes. X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Waltham, MA, USA) was used to analyze the chemical elements of the composite membrane. The major functional groups on the membrane were qualified by Fourier transform infrared spectrometer (FTIR, IRAffinity-1, Kyoto, Japan) with a resolution of 4 cm⁻¹ and 32 scans. The crystal structure of MoS₂-FeOOH NPs was observed by powder X-ray diffraction (XRD, Bruker D8 Advance, Karlsruhe, Germany).

The water contact angles of MoS₂-FeOOH NPs and MoS₂-FeOOH/PES composite membranes were characterized using a contact angle measurement instrument (SL200 KB, KINO, Shanghai, China) with a video-based high-speed device at room temperature, and the data were analyzed with the Young–Laplace equation and CAST 3.0 software fitting method. Commonly, a water droplet was dropped on a dry membrane surface and the high-velocity video started recording. The average value was obtained from at least three different positions of each membrane, in order to indicate membrane surface hydrophilicity.

The gravimetric method was used to calculate the total porosity ($\epsilon$) of obtained UF membrane [30]. The MoS₂-FeOOH/PES composite membranes were firstly placed in a 60 °C blast drying oven for 3 h to remove their moisture and then transferred into deionized water for 3 h until it was completely wet. The porosity of UF membranes was measured by weighing the dry and wet membrane following Equation (1):

$$\epsilon =\frac{{(m}_1{-m}_2{)/\rho }_w}{{(m}_1{-m}_2{)/\rho }_w{+m}_2{/\rho }_m}\times 100\%$$

(1)

where $m_1$ and $m_2$ are the weight of the wet and dry membrane, respectively, ${\rho }_w$ is the deionized water density (0.998 g/cm³), and ${\rho }_m$ is the composite membrane density (1.37 g/cm³).

The average pore size of the membrane was measured by the filtration rate method using the Guerout–Elford–Ferry formula, which was calculated according to Equation (2) [31]:

$$r_m=\sqrt{\frac{\left(2.9,-,1.75,\times ,\epsilon \right)\times 8\eta lJ}{\epsilon \times \Delta P}}$$

(2)

where $\epsilon$ and $l$ are the membrane porosity and thickness, respectively, $\eta$ is the viscosity of water (8.94 × 10⁻⁴ Pa·s), $J$ is pure water flux (L/(m²·h)), and $\Delta P$ is the applied transmembrane pressure (TMP) in the cross-flow filtration (0.1 MPa).

2.4.2. Permeability and Rejection Tests

Membrane filtration tests were carried out in a homemade cross-flow filtration set-up (Figure 1b). The feeding solution was pressurized into the filtration cell by a gear pump and passed through the membrane under suction pressure, which was controlled by adjusting the valve. The membrane was fixed in the membrane module with an effective filtration area of 19.6 cm². Constant TMP of 0.1 MPa during the filtration was kept for 90 min to run a filtration cycle, and the membrane flux was recorded by an electronic balance. The residence time (t, s) was adjusted by controlling the operating pressure and calculated using Equation (3) based on membrane thickness (ζ, μm) and membrane permeability (v, μm/s):

$$t\left(s\right)=\frac{\xi }{\nu }$$

(3)

The rejection efficiency, R (%), was then calculated according to the concentration of the target pollutant in feed and permeate solutions according to Equation (4):

$$R=\left(1-\frac{C_p}{C_0}\right)\times 100\%$$

(4)

where C_p and C₀ are solute concentrations of permeate and feed solution, respectively.

2.4.3. Fouling Tests of the Membranes

The filtration-fouling test was conducted in the cross-flow filtration set-up, both TMP and membrane flux were recorded to indicate membrane fouling development. Bovine serum albumin (BSA) was used as a model organic foulant to evaluate the antifouling capability of the composite membrane. At a fixed room temperature and specific TMP, the pure water flux ($J_w$) and BSA permeate flux ($J_P$) were measured with the feed of pure water and 500 mg/L BSA solution, respectively. After the membrane fouled, the membrane was backwashed physically by DI water for 30 min, and the flux was marked as $J_{wf}$. Accordingly, flux recovery ratio (FRR), total fouling ratio (R_t), reversible ratio (R_r), and irreversible ratio (R_ir) were calculated by Equations (5)–(8) [14]:

$$FRR=\frac{J_{wf}}{J_w}\times 100\%$$

(5)

$$R_t=\left(\frac{J_w-J_p}{J_w}\right)\times 100\%$$

(6)

$$R_r=\left(\frac{J_{wf}-J_p}{J_w}\right)\times 100\%$$

(7)

$$R_{ir}=\left(\frac{J_w-J_{wf}}{J_w}\right)\times 100\%$$

(8)

2.5. Catalytic Characterization of Catalysts and Membranes

2.5.1. Catalytic Activity of the MoS₂-FeOOH

Methylene blue (MB), as one of the most common dyes, is a member of the thiazine family of dyes, and has been widely used in dye manufacture, food testing, organic synthesis, biological treatment, and other applications [31]. In this experiment, MB was used as a model organic contaminant to assess the catalytic activity of MoS₂-FeOOH NPs by degradation of MB molecules in an aqueous solution [32]. First, a desired amount of catalyst (MoS₂-FeOOH NPs) (0.0–1.2 g/L) was added to a 30 mL methyl blue solution of 40 mg/L, and mixed with a magnetic stirrer (IKA, BigSquid, Staufen, Germany) at room temperature. The catalyst was contacted with MB for 20 min to reach the adsorption–desorption equilibrium. After that, a certain amount of 30 wt% H₂O₂ solution was poured to conduct oxidation reaction to degrade MB, and the best dosage for the catalytic reaction was determined. No catalyst was added in the control experiment, while the rest steps were the same. The concentration of MB was determined by an ultraviolet-visible spectrophotometer (UV-2600, Shimadzu, Kyoto, Japan) at 664 nm absorbance. At different time intervals, 2.5 mL of suspension was extracted and filtered by a 0.22 μm filter to measure the concentration of residual MB concentration in the filtrate.

2.5.2. Catalytic Property of MoS₂-FeOOH Membranes

The Fenton-like catalytic performance of MoS₂-FeOOH membranes was evaluated by taking MB degradation rate as an indicator. In a typical process, a MoS₂-FeOOH membrane with an area of 2 × 5 cm² was placed flat in a 100 mL beaker, and 50 mL of MB solution (10 mg/L) were added. Keep the MB solution and membrane in a beaker for 20 min to reach adsorption equilibrium. Then, a certain amount of H₂O₂ aqueous solution (30 wt%) was added to provide hydroxyl radicals to initiate the reaction. During the reaction, 2 mL of MB solution were extracted at a certain interval, and then mixed with 2 mL of methanol to dilute the MB solution twice. In this process, the change of MB solution with time under the action of the membrane was investigated.

2.5.3. Filtration and Antifouling Performance of MoS₂-FeOOH Membranes

The blank and MoS₂-FeOOH/PES composite membranes were tested for dye removal efficiency at room temperature. The effective filtration area of the membrane was 19.6 cm². In addition, 5.5 L of MB (10 mg/L) and H₂O₂ (0.15 mol/L) mixed solution were pumped to permeate through the membrane, and the gear pump re-circulated the permeate solution to the feed solution. MB concentrations in the feed solution and permeate solution were measured at an interval of 5 min to evaluate MB removal efficiency. As a comparison, MB adsorption performances of membranes were also investigated under the same conditions except the addition of H₂O₂.

3. Results and Discussion

3.1. Characterization of MoS₂ and MoS₂-FeOOH Nanoparticles

The morphology of the original MoS₂ and MoS₂-FeOOH was shown in Figure 2a and Figure 2b, respectively. MoS₂ had a layered structure with a smooth surface and uniform dispersion, with a size of about 300 nm. It can be seen from Figure 2b that there were many particles on the surface of MoS₂, which showed that FeOOH was successfully loaded on the surface of MoS₂. Furthermore, XRD was used to test the crystal structure of the original MoS₂ and MoS₂-FeOOH composite material. The results were shown in Figure 2c. It can be seen that the diffraction peak intensity and the full width of the original MoS₂ and MoS₂-FeOOH composites do not change significantly, indicating that the loading of FeOOH did not affect the crystal structure of MoS₂. Hence, FeOOH should be tightly fixed on the surface of MoS₂, without altering its structure. Figure 2d presented that the contact angle of the original MoS₂ and MoS₂-FeOOH corresponding 36.1° and 29.6°, respectively, indicating that the hydrophilicity of the catalyst MoS₂-FeOOH was enhanced by the doping of FeOOH. The contact angle of MoS₂-FeOOH was smaller than that of the original MoS₂, possibly because the addition of FeOOH has introduced OH functional groups which reduced the contact angle [33].

The effect of MoS₂-FeOOH concentrations on the degradation ability of organic dye MB was investigated. Firstly, the catalysts of different concentrations ranged 0.0 g/L to 1.2 g/L were added in the MB solution for 20 min to explore the adsorption effect, and then H₂O₂ was added for catalytic degradation experiments. As shown in Figure 2e, both the adsorption removal rate and degradation removal rate of MB gradually increased with the increase of catalyst concentration, and the total removal rates increased from 5.7% to 98.6%. The increase in catalyst concentration led to an increment in the number of exposed catalyst active sites, which could significantly improve the catalytic performance, thereby promoting MB degradation [34]. The stability and reusability of the catalyst were of great significance to its practical application. Thus, the reusability of MoS₂-FeOOH particles was investigated. The prepared MoS₂-FeOOH (1.0 g/L) and H₂O₂ (0.05 mol/L) were added into the 40 mg/L MB solution for catalytic degradation for 5 min, then centrifuged, washed, dried and recovered, and then put into the MB solution for the second round degradation. It can be seen from Figure 2f that the catalytic degradation activity of the sample decreased slightly with the increasing number of cycles. After four cycles, the catalytic degradation efficiency of MB dropped from 94.1% in the first cycle to 80.5% in the fourth cycle. The decrease in the catalytic activity of MoS₂-FeOOH particles may be owing to the particle loss during recovery, and slight poisoning and inactivation of particles during the degradation process [35].

3.2. Characterization of MoS₂-FeOOH/PES Composite Membranes

The MoS₂-FeOOH/PES composite membranes were fabricated according to the formulas in Table 1. Figure 3 showed the surface and cross-section structure of these catalytic membranes with different MoS₂-FeOOH contents. Compared with the PES membrane M0, all the MoS₂-FeOOH/PES composite membranes exhibited some particles on the surfaces (Figure 2a M0-1-M4-1). Moreover, with the increase of MoS₂-FeOOH addition in the cast solution, there were clearly more particles deposited on the prepared PES membrane surface and pores, which confirmed the successful loading of MoS₂-FeOOH on/in the PES membrane matrix. The cross-sectional structure of the prepared membrane all displayed asymmetric finger-like structures, which was a typical UF membrane structural property.

The porosity and average pore size of the prepared membranes were summarized in

Table 2. Porosity and average pore size of membranes.

Membrane Type	Porosity (%)	Average Aperture (nm)	Contact Angle (°)
M0	77.6	27.0	49.2
M1	82.5	27.6	37.6
M2	83.5	28.8	35.1
M3	85.1	30.4	31.6
M4	85.8	31.6	34.3

. The MoS₂-FeOOH/PES membranes showed an increasing trend with the increment of MoS₂-FeOOH content, concurring well with the SEM observation. One reason was that MoS₂-FeOOH particles were more hydrophilic than PES, and in the process of phase separation, the diffusion of nonsolvent into the membrane pore and that of solvent from PES matrix would be accelerated. In addition, the introduction of nanoparticles would reduce the thermodynamic stability of the cast solution, which would also enhance the phase inversion rate and form a more porous structure [36]. Consequently, the MoS₂-FeOOH/PES composite membranes presented a slightly loose membrane structure with larger pores.

XPS analysis of membrane surfaces in Figure 3b showed that, distinct from the control membrane M0, the binding energies peaks of 231.4 eV and 713.1 eV on the MoS₂-FeOOH/PES composite membranes were considered as the signal peak of Mo and Fe element, respectively. The phenomenon demonstrated that MoS₂-FeOOH was stably introduced into the PES matrix, corresponding well with the SEM images. The infrared spectra of all the MoS₂-FeOOH/PES composite membranes in Figure 3c displayed that the absorbance peak at 1102 cm⁻¹ and 1148 cm⁻¹ was mainly attributed to the stretching vibration of O=S=O, and the peak at 1236 cm⁻¹ was due to the asymmetric stretching vibration of C-O-C in PES polymer [37]. It was worth noting that MoS₂-FeOOH nanoparticles were mainly physically blending with PES, as no new peaks were found on the MoS₂-FeOOH/PES composite membranes.

From the water contact angle results of prepared membranes in Table 2, it was found that the introduction of MoS₂-FeOOH nanoparticles improved the membrane hydrophilicity, as the M1 with 0.75% MoS₂-FeOOH addition had a rather low contact angle of 37.6° that was rather improved compared to that of M0 of 49.2°. The increased dosage of hydrophilic MoS₂-FeOOH from 0.75% to 2.25% resulted in continuous improvement of membrane surface hydrophilicity [38]. However, with the increase of catalyst further to over 3.0%, excessive agglomeration of catalyst particles showed negative impacts on the membrane hydrophilicity [39]. Overall, it can be concluded that MoS₂-FeOOH nanoparticles have been successfully incorporated into the PES membrane with the enlarged pore size and enhanced surface hydrophilicity, which may promote the filtration and antifouling capability.

3.3. Ultrafiltration Performance of MoS₂-FeOOH/PES Composite Membranes

The pure water flux and BSA rejection efficiency of the MoS₂-FeOOH/PES composite UF membranes were evaluated under constant operating pressure of 0.1 MPa. As shown in Figure 4, all the MoS₂-FeOOH/PES composite membranes showed an enhanced BSA rejection efficiency compared to the PES membrane M0. With the increase of MoS₂-FeOOH content, the pure water flux of composite UF membranes increased significantly from 228 L/m²·h to 385.3 L/m²·h, due to the enhancement of the surface hydrophilicity and the increase of membrane porosity and average pore size. Meanwhile, the rejection efficiency of the BSA decreased slightly with the increment in the MoS₂-FeOOH dosage. Nonetheless, all the membranes had excellent BSA rejection efficiency ranging from 91.1% to 96.3%. It showed that the MoS₂-FeOOH/PES composite membranes have upgraded pure water flux and BSA rejection rate compared with some reports and commercial UF membranes [40].

3.4. Catalysis Performance of the MoS₂-FeOOH/PES Membranes

The MB degradation by MoS₂-FeOOH/PES composite membranes was investigated in the presence of H₂O₂ according to the method in Section 2.5.2. It can be seen from Figure 5a that, compared with the PES membrane M0, the adsorption capacity of the composite catalytic membrane was obviously enhanced. In particular, M4 adsorbed nearly 20% MB before degradation, about 6.3 times that of M0 adsorption. There may be two reasons: on the one hand, new pore channels were formed due to the increased pore size and porosity of the composite membranes, thereby improving the adsorption capacity [41], and on the other hand, the MoS₂-FeOOH catalyst showed adsorption effect as shown in Figure 2d. In the 180 min degradation experiment, it was found that the MoS₂-FeOOH/PES membranes could effectively decompose MB. In addition, as the content of MoS₂-FeOOH increased from 0.8% to 3.0%, the MB degradation efficiency increased from 24.1% to 60.2%, indicating that the addition of MoS₂-FeOOH enhanced the Fenton catalytic performance of the membrane. The more the MoS₂-FeOOH particles of composite membranes participated in the Fenton catalytic reaction, the more the hydroxyl radicals were generated and the higher the degradation efficiency [42]. During the catalytic process, the nanoparticles may escape from the MoS₂-FeOOH/PES composite membranes, and the released concentration of Fe was examined and shown in Figure 5b. Under neutral conditions, iron will not escape from the membrane. However, under acidic conditions, a small number of iron components would dissolve in the solution, and the leaching concentration increased with the amount of MoS₂-FeOOH. However, within the reaction time, the cumulative release of Fe was only 2.1 µg at most at pH = 4, which was relatively low compared to the iron content in groundwater, and had little impact on environmental health.

According to the results of Section 3.2, an M3 membrane with balanced properties was selected for testing the catalysis ability during the cross-flow experiment. The experimental results in Figure 5c showed that the MB removal rate of the PES membrane M0 did not change much with or without H₂O₂, and both were less than 10%. The degradation rate of M3 without H₂O₂ was close to 20%, which may be due to membrane interception and catalyst adsorption. After adding H₂O₂, the degradation rate of MB increased rapidly due to the Fenton effect. After 60 min, the degradation effect was 6.2 times and 4.2 times of the blank membrane with and without H₂O₂, respectively.

As indicated in Figure 5d, the lower the filtration pressure, the higher the MB removal efficiency by the M3 membrane, which indicated that a sufficiently long contact time is required to effectively adsorb and remove micro-pollutants. When the operating pressure was 25, 50, and 75 kPa, the membrane flux was 69.4 L/(m²·h), 134.3 L/(m²·h), and 193.7 L/(m²·h). The residence time of MB in the membrane was 10.4 s, 5.4 s, and 3.7 s, respectively. When the operating pressure was 25 kPa, the residence time of pollutants in the membrane increased by about four times compared with that under 75 kPa, and the degradation rate increased by 1.39 times. These results show that reducing membrane flux and extending contact time can significantly improve the degradation effect of pollutants.

3.5. Antifouling Performance of Membranes with H₂O₂ Dosing

Using 500 mg/L BSA solution as the feed solution, two cross-flow filtration experiments were carried out after pressurizing pure water for 1 h, and their fouling evolution rate and flux recovery rate were compared. After each cycle of filtering the BSA solution for 1 h, the membrane was backwashed with deionized water for 30 min. It can be seen from Figure 6a that the initial pure water flux of MoS₂-FeOOH/PES membranes were all significantly higher in contrast to the M0 membrane, which was mainly due to the increase of the membrane surface hydrophilic groups, porosity and average pore size. After switching the feed to BSA, the water flux of the membrane dropped from 200–400 L/(m²·h) to 50–150 L/(m²·h), which mainly resulted from the BSA accumulation and adsorption on the membrane surface. After simple hydraulic cleaning, the pure water flux could only be partially restored, as there was irreversible fouling. Under the same filtration conditions, the pure water flux of all MoS₂-FeOOH/PES composite UF membranes was kept higher than that of the blank PES membrane. After two filtration cycles, these MoS₂-FeOOH/PES composite membranes displayed a consistently slower fouling rate than the control PES membrane. Especially for M3 and M4 membranes, after the second cycle, the flux was 4.6 times and 4.4 times that of M0, respectively.

The fouling parameters, including FRR, R_t, R_r, and R_ir, were calculated according to the filtration-fouling tests. The higher the FRR, the better the antifouling performance of the membrane. Figure 6b illustrated that the FRR of M0 was much lower than that of MoS₂-FeOOH/PES composite membranes, and the FRR value of M1-M4 after two cycles were 2.8, 2.9, 3.1, and 2.8 times that of M0, respectively. It was observed that the flux recovery trend was consistent with the hydrophilicity of the membrane. In addition, M0 had the highest total fouling ratio R_t and irreversible ratio R_ir, and R_ir was the main component of R_t (up to ≥90%). Irreversible fouling cannot be removed by simple cleaning, so a smaller R_ir indicated that the anti-fouling capacity of the membrane had been improved. For the composite membranes, the R_t and R_ir were significantly reduced with the increase of MoS₂-FeOOH content, and the R_ir of M3 only accounted for 17% of the R_t. However, when the amount of MoS₂-FeOOH added reached 3% (M4), the decrease of R_t and R_ir was not obvious. This result indicated that the antifouling capability of the prepared UF membranes were improved by adding MoS₂-FeOOH, but excessive catalyst could not further enhance its function. Overall, the membranes containing MoS₂-FeOOH composite exhibited higher antifouling performance compared to M0.

Afterward, M3 was employed to filtrate MB at 0.1 MPa, during which the permeation flux and flux recovery ratio were displayed in Figure 7. Throughout the experiment, the water flux of membrane M3 was always higher than M0, and the flux of the first and second filtration MB was about 1.8 times that of M0 membrane. After the first hour of membrane compaction, the feed was changed from pure water to MB, and a rapid drop in permeate flux was observed for both membranes. During the first MB filtration process, M3 had relatively smaller R_t (22.7%) and R_ir (11.7%). After the first cycle, membrane fluxes increased after hydraulic cleaning, and the FRR of M0 and M3 was 78.8% and 88.3%, respectively. In the second cycle, 0.1 mol/L H₂O₂ was used as the cleaning solution instead of pure water. For M0, H₂O₂ had little effect on flux recovery, while M3 had a significant improvement on flux, and the FRR can be increased to 95%. After the second round of MB filtration, the interaction between the catalyst in the membrane and H₂O₂, i.e., Fenton-like reaction, could generate •OH, which could effectively remove the reversible and irreversible pollutants on the membrane surface and membrane pores. Compared with M0, the irreversible fouling of M3 was significantly reduced to 5%, which indicates that the catalytic membrane has a better anti-fouling effect. The self-cleaning ability and stable performance of MoS₂-FeOOH/PES composite membranes could extend the membrane life about 15%, and has broad application prospects in the treatment of dye wastewater.

Table 3. Research done to improve composite PES membranes in dye wastewater treatment.

Polymer Nanofiller		Flux (Lm−2h−1)	Rejection (%)	Goal	Refs.
PES	Cu(tpa)@GO	130 (0.18 MPa)	Congo Red ≥ 50 Methylene Blue <20	Antifouling and dye separation	[41]
PES	Fe3O4@SiO2	2084 (0.1 MPa)	Methylene Blue =90	High-flux, antifouling and dye separation	[43]
PES	GO	289.63 (0.4 MPa)	Methylene read =67 Methylene orange =55 Congo read =95	Antifouling and dye separation	[44]
PES	TiO2	142 (1 MPa)	Methylene Blue =30	Effective treatment of dye polluted wastewater	[45]
PES	MoS2-FeOOH	385 (0.1 MPa)	Methylene Blue =60	High-flux, antifouling and dye separation	This work

compares the MoS₂-FeOOH/PES membrane in this study with other works of PES membranes incorporating different nanomaterials for dye removal. As shown in the table, different types of inorganic nanomaterials have been used to improve PES membranes for increasing the resistance to various types of foulants and removing different dyes. This work endowed the composite membrane with the catalytic self-cleaning ability, thus presenting excellent water flux and antifouling capacity with reasonable dye rejection. Therefore, we expect that, by further adjusting the method of applying MoS₂-FeOOH, higher performance membranes can be produced for dye removal.

3.6. Catalytic Self-Cleaning Mechanism in the MoS₂-FeOOH/PES System

MB was used to study its degradation mechanisms in MoS₂-FeOOH/PES membrane. The inhibition experiment of •OH was carried out to verify the role of •OH in the catalytic degradation of MB. Tert-butyl alcohol (TBA) was selected as a scavenger for the quenching test [46]. Figure 8a showed the effects of the capture agent on the catalytic performance after 20 min of static adsorption. After TBA was added, the removal efficiency of the MB was reduced from 98.1% to 31.5% after 10 min reaction, as MB was oxidized by •OH that was produced by MoS₂-FeOOH in the Fenton-like reaction. To verify this mechanism, DMPO was used as a free radical scavenger, and electron paramagnetic resonance (EPR) technology was used to identify active free radicals. Figure 8b clearly shows four characteristic peaks with an intensity ratio of 1:2:2:1, corresponding to the primary DMPO-·OH adduct.

The production paths of •OH were shown in Equations (9)–(12). Firstly, Fe³⁺ on FeOOH surface formed a surface complex Fe³⁺–H₂O₂ with H₂O₂. In this complex, electron transfer occurred between Fe³⁺ and H₂O₂ to form •O₂H and reduced Fe²⁺, and the generated Fe²⁺ further reacted with H₂O₂ to form •OH [47]. During the whole heterogeneous Fenton-like process, the reduction of Fe³⁺ by H₂O₂ was the rate-limiting step [48]:

Fe³⁺ + H₂O₂ → Fe³⁺−H₂O₂

(9)

Fe³⁺–H₂O₂ → Fe²⁺ + •O₂H + H⁺

(10)

Fe²⁺ + H₂O₂ → Fe³⁺ + •OH + OH⁻

(11)

Fe³⁺ + •O₂H → Fe²⁺ + O₂/H⁺

(12)

As shown in Equations (13) and (14), MoS₂ can promote the formation rate of Fe²⁺ from Fe³⁺. Fe³⁺ in the solution was exposed to the active site Mo⁴⁺ of MoS₂ and reduced to Fe²⁺. The regenerated Fe²⁺ ions could be redistributed into H₂O₂ aqueous solution to produce •OH. Meanwhile, the oxidized Mo⁶⁺ was reduced by H₂O₂ to form Mo⁴⁺ [49]:

Mo⁴⁺ + Fe³⁺ → Mo⁶⁺ + Fe²⁺

(13)

Mo⁶⁺ + H₂O₂ → Mo⁴⁺ + H₂O + O₂

(14)

In summary, the catalytic self-cleaning mechanism was summarized in Figure 9. The MoS₂-FeOOH/PES composite membrane could facilitate H₂O₂ to convert to •OH radicals by a Fenton-like reaction. The •OH radicals could further decompose dyes and other potential foulants such as BSA, resulting in an enhanced removal efficiency and antifouling capacity.

4. Conclusions

In this study, a simple method was used to synthesize MoS₂-FeOOH and then the synthesized nanoparticles were mixed with PES casting solution to prepare a MoS₂-FeOOH/PES ultrafiltration composite membrane. XPS technology and infrared mapping confirmed that MoS₂-FeOOH was successfully introduced into the PES matrix by blending. It was found that the MoS₂-FeOOH/PES composite membranes showed a positive influence in porosity and hydrophilicity, increasing the permeability and the antifouling capability of the PES membrane. The composite membrane M3 had a balanced performance when the ultrasonic time was 1.5 h and the mass ratio of MoS₂-FeOOH to PES was 0.15. With the increase of MoS₂-FeOOH concentration, the pure water flux of the composite ultrafiltration membrane was gradually increased. The pure water flux of M3 was 337.0 L/(m²·h), which was 1.5 times that of M0. In addition, antifouling experiments show that the BPA rejection rate of M3 was 91%, and the flux recovery ratio had increased from 25.6% of M0 to 78.9%. The irreversible resistance R_ir of the composite membrane was significantly reduced with the increase of added MoS₂-FeOOH content when the doping amount of MoS₂-FeOOH was less than 3%. M3 composite ultrafiltration membrane was also used to treat dye simulated wastewater. It was found that the highest FRR of the M3 membrane could reach 95% after catalytic self-cleaning by H₂O₂, indicating that the composite ultrafiltration membrane has excellent antifouling performance and unique catalytic self-cleaning ability.