Enhancing the permeation and fouling resistance of PVDF microfiltration membranes by constructing an auto-soak surface

Abstract

A new kind of hydrophilic polyvinylidene fluoride (PVDF) microfiltration membrane with an auto-soak surface was fabricated by plasma treatment and interfacial crosslinking to improve the separation efficiency and fouling resistance in water treatment. Based on the anchored hydrophilic polyvinyl alcohol (PVA) network and the porous structure, the modified PVDF membrane was able to exhibit an auto-soak ability. The separation test results showed that to obtain the same separation efficiency, the operating pressure of the modified PVDF membranes was significantly reduced as compared with that of the neat PVDF membrane, due to the decreased permeation resistance. The fouling test results also demonstrated that this auto-soak surface of the PVDF membrane was able to alleviate the irreversible fouling problem during dynamic filtration, and the water flux recovery ratio (FRR) remained above 75% even after three fouling cycles. All these results indicated that the modified PVDF membrane with the auto-soak surface provided a new means for the reduction of the permeation resistance, as well as the membrane fouling problem of the hydrophobic porous membranes.

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1. Introduction

Membrane separation, as a common technology to solve water resource, environmental and energy crises, exhibits great application significance.^1–3 Commercial porous membranes, including polyvinylidene fluoride (PVDF), polyethersulfone (PES) and polypropylene (PP), have been employed as one of the most important means in water and wastewater treatment. However, the hydrophobic properties of these porous membranes have become the main disadvantage for separation applications due to the increased penetration resistance and severe membrane fouling problem, which significantly decreases the separation efficiency, shortens the service life of the membrane module, and increases the application cost. In order to improve the separation efficiency and the antifouling performances of the commercial hydrophobic membranes, hydrophilic modification of the hydrophobic membranes has been a research focus in the field of membrane preparation and applications.^4–6

The separation process of the hydrophobic porous membrane is a completely passive process, because the permeation of water molecules needs to overcome the dual resistance of the interfacial tension and the physical channels with the assistance of the operating pressure. As a result, a larger operating pressure is needed to obtain a suitable water flux which on the other hand favors the interaction of the organic pollutants with the membrane surface and cause the membrane fouling problems, which is inevitable in the pressure-driven membrane filtration process due to the multiple physical and chemical effects between pollutants and the membrane materials.^4,7,8 Therefore, the lower operating pressure can significantly reduce the membrane fouling problem and the power consumption. It is of great economic value to enhance the separation efficiency of the porous membrane with lower operating pressure in a wide range of applications. It is generally known that the hydrophilic modification of the hydrophobic membrane is favorable to reduce the operating pressure across the membrane due to the reduced mass transfer resistance.^9,10 As the common modification methods to improving the hydrophilicity or fouling resistance, blending amphiphilic polymers^11–16 and surface anchoring of hydrophilic media^17–20 have been used as the effective means to enhance the separation efficiency and antifouling properties of the hydrophobic membranes. Blending method can be applied to the hydrophilic modification of the entire membrane material including surfaces and pores. However, the introduction of the hydrophilic media in the membrane matrix will decrease the mechanical properties of the prepared membranes.^4,21,22 Surface modification methods such as surface grafting and self-assembly of hydrophilic components are able to enhance the hydrophilicity of the membrane surface without significantly affecting the hydrophobic nature of the membrane matrix and pores.^4,23 It is noteworthy that improving the hydrophilicity of the membrane surface and pore structures is equally important for the reduction of permeation resistance and pollutants fouling.

The construction of an auto-soak surface of the membranes will be able to alleviate these problems. As far as we know, membranes with an auto-soak surface can be used in various fields, including oil and water separation, fresh water collection and antifouling coating due to the super-hydrophilic property.^24–26 The auto-soak interface has the self-wetting ability to water. Water molecules can be preferentially passed through the auto-soak surface though the aid of gravity without other assistance, which will greatly reduce the energy consumption requirements in the membrane separation process.^27–31 In addition, membranes with an auto-soak surface should satisfy the following two requirements. Firstly, the membrane surfaces and pores should be hydrophilic enough. Secondly, the membrane has a large porosity and pore size. The porous structure of the membrane itself provides the foundation to build a stable auto-soak surface via the interface hydrophilic modification techniques.^32–34 Due to the reduced penetration resistance, the water molecules in the feed can spontaneously pass through the pores of the membrane with an auto-soak surface. Therefore, the permeation of water changes from a passive mode with the hydrophobic porous membrane to an active mode with the hydrophilic membrane, resulting in the decrease of both the operating pressure and the membrane fouling tendency.

In this paper, a simple and feasible method, which combined plasma treatment with interface crosslinking, was used to improve the hydrophilicity of the PVDF porous membrane by constructing an auto-soak surface. The separation performance and fouling resistance of the modified membranes were investigated. The aim of this paper is to provide a simple and efficient way to improve the hydrophilicity and the application performance of the hydrophobic porous membranes.

2. Experimental

2.1 Materials

Polyvinyl alcohol (PVA, M_w ∼ 75 000, the degree of alcoholysis is 99%), bovine serum albumin (BSA, M_w ∼ 67 000) and trimesoyl chloride (TMC, 97%) were purchased from Aladdin (China). The polyvinylidene fluoride (PVDF) microfiltration (MF) membrane with an average pore size of 0.22 μm was purchased from a local company.

2.2 Membrane fabrication and characterization

The PVDF membranes were pretreated with plasma apparatus (CTP-2000K, Nanjing Suman, China) with the power of 200 W for 60 seconds through dielectric barrier discharge in air atmosphere, and the treated PVDF membrane (denoted as PVDF-P) was placed in a Buchner funnel for suction filtration, with ethanol as a pore-open agent and PVA solutions (0.2 wt%, 0.5 wt% and 1 wt%, 100 mL) as the filtration media. After the completion of suction filtration, the membranes were placed in TMC (0.3 wt%) solution for crosslinking. Finally, the modified membranes were washed with ethanol and deionized water successively, and dried for future use. The modified membrane was labeled as M-0.2, M-0.5 and M-1, respectively, according to the concentrations of PVA solutions.

Surface hydrophilicity of the neat and modified PVDF membranes were evaluated by water contact angle (CA, OCA40Micro, Germany) at room temperature. Membrane surface chemical composition was tested using X-ray photoelectron spectroscopy (XPS, Shimadzu AXIS UltraDLD, Japan) with the resolution of 0.68 eV/(C 1s), and the incidence angle of photoelectron radiation was set at 90°. Surface morphologies of the neat and modified PVDF membranes were examined with the field emission scanning electron microscope (FESEM, Hitachi SU8010, Japan).

2.3 Separation test

The separation properties of membranes were measured by a laboratory-made filtration unit with an effective area of 12.5 cm². The volume of the permeated water was collected and the flux (J, L m⁻² h⁻¹) was calculated by the following eqn (1), until the water volume kept constant for a certain time. The operating pressure can be adjusted according to the test requirement.where J (L m⁻² h⁻¹) is the flux of the permeated water, V (L) is the permeated water volume, t (h) is the permeation time and A (m²) is the effective area for filtration.

The mixture of water and n-hexane (v/v = 90/10), mixed by ultrasonic pretreatment, was used as feed solution for the oil–water separation test. The composition of permeation solution was calculated according to the observation via the polarized optical microscope (DM750P, Leica) and analyzed using the software Image J.

2.4 Fouling test

For the dynamic filtration fouling, BSA was used as the model pollutant due to strong hydrophobic–hydrophobic interactions with the PVDF membrane. The fouled flux recovery ratio (FRR) was employed to evaluate the antifouling ability. It should be worth noting that the membrane with higher value of FRR indicates the better antifouling properties. The executed dynamic fouling process consisted of three steps. Firstly, the stable flux (J) was obtained by pure water filtration. Secondly, the feed solution was changed to pollutant solution (BSA, 1 g L⁻¹, pH = 7.4) and another stable flux was obtained. Then a washing process was introduced and the membrane was taken out from the filter system to immerse in the phosphate butter for 10 min, rinsed with pure water for 20 min, and reinstalled into the filtration unit, for the water flux (J₂) of pure water to be recorded at the end. Dynamic fouling process can be repeated to obtain the final water flux (J_n). The FRR was calculated by eqn (2). The operating pressure can be adjusted according to the test requirements.

3. Results and discussion

The preparation process of the modified PVDF membrane was shown in Fig. 1. After plasma treatment, a large number of active groups were generated on the PVDF membrane surface and pores,³⁵ which are conducive to improve the hydrophilicity of the PVDF membrane and show good adsorption effect to PVA molecules. PVA can be adsorbed on the membrane surface and pores based on the assistance of pressure-driven suction filtration. Then, with the rapid reaction of acyl chloride of TMC and the hydroxyl groups of PVA, the PVA molecules were fixed by TMC in the corresponding position. In addition, the TMC and the active sites of the membrane surface also showed crosslinking and anchoring effects. Due to the double anchoring effects of suction filtration and TMC crosslinking in the preparation process, hydrophilic PVA molecules were fixed on the membrane surface and the pore surfaces, changing the original hydrophobic surface of the PVDF membrane into the hydrophilic surface.³⁶

Fig. 1 Schematic diagram of the preparation process of the modified PVDF membrane.

The contact angle test results were shown in Fig. 2. The contact angle of the neat PVDF membrane is 93°, which showed the inherent hydrophobicity of the PVDF material. The contact angle of the PVDF-P membrane decreased rapidly after plasma treatment, and the M-0.5 and M-1 membrane showed a super hydrophilic property with the contact angle of 0°, exhibiting the auto-soak ability. The digital photos in Fig. 3 also demonstrated the auto-soak property. On the surface of the neat PVDF membrane, the water drops formed a ball-type droplet; while ethanol as a PVDF membrane pore-open agent could enter the membrane pores, and auto-soaked on the neat PVDF membrane. In comparison, on the surface of the PVDF-P membrane, the water drops quickly spread with much decreased contact angle values, showing great enhancement of the hydrophilicity. The water droplets on the surface of the M-1 membrane totally soaked in the membrane matrix and wetted the membrane, which was similar to the phenomenon of ethanol molecules on the neat PVDF membrane. These results showed that the modified PVDF membranes have excellent wetting properties with an auto-soak surface.

Fig. 2 Contact angles of the neat and modified PVDF membranes.

Fig. 3 Digital photos of the droplets on the neat and modified PVDF membranes (the diameter of the membrane is 5 cm, the volume of the water drop is 10 μL).

The XPS characterizations of the PVDF membranes before and after modification were shown in Fig. 4. According to the XPS wide-scan spectra displayed in Fig. 4A, the neat PVDF membrane surface mainly contained fluorine (F) and carbon (C) elements. Compared, the oxygen (O) element appeared in the spectra of M-0.5 and M-1 membranes and the O contents were greatly increased. However, a large number of F elements could still be detected on M-0.5 and M-1 membranes, indicating that the thickness of the crosslinked layer was thinner than the XPS detection depth (about 10 nm). The C 1s core-level spectrum of M-1 (Fig. 4B) could be curve-fitted into four peaks, which belonged to C–C (285.4 eV), C–O (286.8 eV), C=O (288.8 eV) and C–F (291.1 eV),¹⁷ respectively. The existence of the C–O bond indicated the presence of PVA, and the presence of O–C=O bond indicated that PVA is crosslinked by TMC (see Fig. 1). The change of the membrane surface composition confirmed that the crosslinked PVA was successfully anchored onto the surface of the neat PVDF membrane.

Fig. 4 XPS analyses of the neat and modified PVDF membranes ((A) wide scan, (B) C 1s spectra of M1).

Fig. 5 showed the surface morphologies of the PVDF membranes before and after modification. It could be seen that the surface morphology of the modified membranes (M-0.2, M-0.5 and M-1) and the neat PVDF membrane were basically the same, suggesting that the interfacial crosslinking method barely changed the original porous structure of the neat PVDF microfiltration membranes. The viscosity of the used PVA solution was very low and the membrane pore size was large. PVA molecules would be adsorbed on the pore walls as close as possible under the driving force of the pressure, instead of covering on the membrane surface to form a coverage layer. As a result, the porous morphology of the original PVDF membrane was maintained. Based on the contact angle and XPS characterization with the detection depth of about 10 nm, it was assumed that the hydrophobic and porous PVDF membrane was transformed into a porous structure with a hydrophilic layer at the interface after modification. This method for the preparation of the porous membrane with an auto-soak surface could maintain the physical and chemical properties of the PVDF membrane matrix, and endow a functional ultra-thin layer on the membrane surfaces and pores.

Fig. 5 The SEM images of the neat and modified PVDF membranes.

The separation performance test results of all the membranes at 0.1 MPa were shown in Fig. 6. The pure water flux of the neat PVDF MF membrane is 1210 L m⁻² h⁻¹, and the water flux of the modified PVDF membranes increased with the increase of the crosslinked PVA concentration (e.g., the flux of M-1 membrane increased to 2550 L m⁻² h⁻¹). As the pore size was not changed, the increase in flux was mainly attributed to the auto-soak surface, which significantly increased in the hydrophilicity of the modified membrane.^37,38

Fig. 6 The pure water fluxes of the neat and modified PVDF membranes (at 0.1 MPa).

The improved hydrophilicity of the modified membrane was favorable to the improvement of the separation efficiency. In order to achieve the equivalent water flux of the pure PVDF membrane, the theoretical operation pressure of the modified membrane would be significantly reduced, which was verified by the experimental results. The required test pressure of the neat and the modified PVDF membrane to obtain the same flux were shown in Fig. 7. The operating pressure of the modified PVDF membrane decreased with the increase of the PVA concentration during preparation. For the M-1 membrane, only 0.045 MPa was needed to achieve the same separation efficiency with that of the neat PVDF membrane. Obviously, the modified PVDF membrane with an auto-soak surface could decrease the required operating pressure, which was beneficial for the reduction of the energy consumption.

Fig. 7 The test pressures of the neat and modified PVDF membranes to obtain the same flux.

During the separation process, water molecules passing through the hydrophobic MF membranes need to overcome a large permeation resistance, including the critical pressure of the hydrophobic interface and the mass transfer resistance of the pores. The water permeation across the hydrophobic membrane was a passive process with the assistance of the operation pressure. As contrast, the water molecules could be spontaneously transferred into the membrane pores in the self-wetting surface, which was an active process. The applied pressure on the hydrophobic interface and membrane pores of the PVDF MF membrane were labeled as P₁ (P₁ = P_critical) and P₂ respectively, according to eqn (3). It was assumed that all the membranes are in accordance with the all pores flow model. According to Darcy's law and Hagen–Poiseuille equation, flux (J) was proportional to the pressure (P) applied to the membrane (eqn (4), where r is the pore radius, μ is the dynamic viscosity of the liquid material, τ is the warp factor, ε is the porosity, and L is the membrane thickness). It could be derived from eqn (4) that the flux was affected by both the permeation characteristics of the hydrophobic interface and the membrane pores. For the neat PVDF membrane, based on the permeation resistance of these two parts, the permeation was a passive process. As shown in Fig. 8, for the neat PVDF membrane, it was known that P₁ (P_critical) > 0 and P₂ < P, and only part of the operating pressure acted on the water molecules passing through the membrane. For the modified PVDF membrane, the auto-soak surface and the hydrophilic channels could be formed after hydrophilic modification, and the filtration of water molecules become an active process with spontaneous properties due to the excellent hydrophilicity of the whole membrane interface. It could be derived that P₁ = 0 and P₂ = P, and most of the applied pressure acted on the water molecules passing through the membrane pores. Therefore, water molecules could pass through the modified PVDF membranes under the action of almost all the applied pressure. As a result, the same pressure led to the differences in the separation flux between the neat and the modified PVDF membranes. To achieve the same separation efficiency, the modified membranes only needed a smaller operating pressure, which was favorable for the enhancement of the permeate flux at lower pressure and the reduction of the energy consumption.

P = P₁ + P₂ (3)

Fig. 8 The proposed auto-soak mechanism of the modified PVDF membrane.

Oil–water separation process generally required extreme hydrophilic or hydrophobic interfaces to get the excellent separation performance.^27–29 In this study, due to the presence of the hydrophilic auto-soak surface, the modified PVDF membranes were used for oil–water separation applications. The separation results of oil–water mixture were shown in Fig. 9. The modified PVDF membrane with the auto-soak surface could effectively separate oil–water mixture, achieving the preferential permeation of the water molecules. Fig. 9B showed that the emulsion state of the feed almost disappeared in the permeate solution. The successful separation of the oil–water mixture was closely related to the interface characteristics. As a hydrophobic membrane material, the neat PVDF membrane tended to preferentially filter the oily solvent. Therefore, it was easier for the hexane to contact with and across the membranes. In the experiment, the modified PVDF membrane could effectively block the oil solvent molecules from penetrating the membranes, and the water molecules preferentially penetrated, which proved that the modified membranes with the hydrophilic auto-soak surface can be applied for the oil/water separation applications.

Fig. 9 The separation performance of the modified PVDF membranes using hexane–water mixture as feed ((A) separation performance, (B) digital photo).

As an important parameter to evaluate the application performance of the PVDF membranes, antifouling ability played an important role concerning the service life of the membrane module. The fouling resistance of the PVDF membranes was investigated by using the common pollutant BSA, and the results were shown in Fig. 10. After the filtration of BSA feed, the FRR values of PVDF, PVDF-P, M-0.2, M-0.5 and M-1 membranes are 31, 36, 49, 60 and 66%, respectively. The results indicated that the FRR values of the modified membranes are significantly improved, which showed that the auto-soak surface is conducive to improve the antifouling performance. Due to the crosslinked PVA layer present on the surface and pores of the PVDF membrane as the auto-soak layer, the enhancement of the interfacial hydrophilicity would be able to significantly inhibit the hydrophobic interactions between BSA molecules and the modified PVDF membranes.^4,36 Therefore, the adsorption of BSA molecules would be significantly reduced, which in turn alleviated the membrane fouling problems.

Fig. 10 The fouling test results of the neat and modified PVDF membranes at 0.1 MPa.

The operating pressure was an important influencing factor on the membrane fouling properties.^39,40 For the modified membrane with an auto-soak surface, the same separation efficiency could be obtained with the decreased operating pressure. To maintain the same separation efficiency, the antifouling effects with different operating pressures were investigated, as shown in Fig. 11. After a cycle of fouling, the FRR values of the modified membranes were significantly increased as compared with that of the neat PVDF membrane. The FRR value of the M-1 membrane was 91%; while the original PVDF membrane was only 31% for the first cycle. Even after 3 cycles, the FRR value of the M-1 membrane was still higher than 75%; while the FRR value of the neat PVDF membrane was significantly reduced to lower than 10%, which demonstrated the advantages of the antifouling ability of the modified PVDF membrane with the auto-soak surface. The operating pressure directly affects the degree of pollution, and the pollutant molecules are prone to enter the membrane pore under high operating pressures. Based on the double anchoring effects of the physical squeezing and hydrophobic interaction, a large number of BSA molecules could block the neat PVDF membrane pores and cause serious membrane fouling problems. However, for the modified PVDF membrane, the probability of BSA molecule being adsorbed and anchored in pores was greatly reduced since the auto-soak surface could effectively reduce the operating pressure and increase the interfacial energy due to the increased hydrophilicity.^41,42 The decreased hydrophobic–hydrophobic interactions between the membrane and BSA were unfavourable for the persistence of BSA molecules in the membrane pore as the pollutant. Therefore, BSA molecules were more likely to be washed away, and the proportion of reversible fouling would be greatly increased. Overall, the probability of the membrane fouling caused by BSA adsorption was greatly reduced. Therefore, even after repeated pollution during filtration cycles, the proportion of the irreversible fouling in the modified membrane was still lower, which would favor a longer service life of the porous membrane.

Fig. 11 The multiple fouling test results of the neat and modified PVDF membranes with different operating pressures.

4. Conclusions

In this paper, the hydrophilic PVDF membrane with an auto-soak surface has been constructed by plasma treatment and interfacial crosslinking. The self-wetting ability of the modified PVDF membrane based on the anchored PVA layer can significantly improve the permeability and reduce the operating pressure, as well as decrease the irreversible fouling tendency to enhance the antifouling performance of the membranes. The optimization of the interface properties of the membranes integrates the advantages of the membrane itself and the functional properties of the modified media, which is beneficial to the extension and improvement of the membrane applications in water and wastewater treatment.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21307083), the Research Foundations of Shenzhen (JCYJ20160308105200725, JCYJ20140418095735550, KQCX20140519103908550, ZDSYS20160606153007978), and China Postdoctoral Science Foundation (2016M602533).

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