Zwitterionic SiO₂ nanoparticles as novel additives to improve the antifouling properties of PVDF membranes

Abstract

Hybrid polyvinylidene fluoride (PVDF) ultrafiltration (UF) membranes with excellent antifouling properties were prepared by non-solvent-induced phase separation through blending zwitterionic SiO₂ nanoparticles. Lysine was used to modify SiO₂ nanoparticles to generate a surface zwitterion of the amino acid type. Zwitterionic SiO₂ nanoparticles were distributed uniformly in the membrane bulk to avoid massive agglomeration and to significantly improve the hydrophilicity and separation performance of PVDF UF membranes. The amount of BSA adsorbed on a hybrid ZP-5% membrane surface of static fouling test decreased to 10 μg cm⁻², and the secondary water flux recovery rate (FRR) increased to more than 95% for the dynamic antifouling test of BSA and HA. The addition of zwitterionic SiO₂ nanoparticles enhanced the antifouling ability of the membrane through inhibiting irreversible fouling and prolonging the service life of the PVDF UF membrane.

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1. Introduction

In recent years, membrane technologies have attracted a lot of attention as they play an important role in water treatment, industries, pharmacy, food, and other fields.¹ However, the problem of membrane fouling is inevitable for the practical application of all membrane separation processes, leading to an increase in application costs and reduction in service life. Therefore, the preparation of low fouling or non-fouling membranes have been a hot issue in the field of membrane modification.^2–5

Numerous studies have demonstrated that the hydrophilic modification of hydrophobic membranes is an effective way to improve the fouling resistance properties. As we all know, water molecules are preferentially adsorbed at the hydrophilic interface during separation processes to form a hydration shell, which can effectively block the hydrophobic pollutants from contacting the membrane surface and alleviate the subsequent deposition of pollutants, enhancing the antifouling performance of the hydrophobic membrane.^6–10 Zwitterionic materials with biomimetic properties are considered to be one of the best fouling-resistant materials due to their excellent hydration capacity based on the positive and negative groups with strong hydrophilicity, and the water molecules can be preferentially adsorbed to form a protective layer in the film application process.^11–15 However, the strong hydrophilicity of zwitterionic materials limit their solubility in conventional organic solvents, thus zwitterionic materials cannot be directly blended with commercially available hydrophobic membrane materials. Therefore, the convenient use of zwitterionic materials remains a challenge, and the currently used method of surface grafting and preparation of block copolymers using harsh conditions and multi-step reactions still limits the large-scale industrial applications of zwitterionic materials.^16–19

Hydrophilic SiO₂ nanoparticles with large surface areas and strong hydrophilic features are usually used as modified additives for coatings, pharmaceuticals, environmental protection, separation membranes and in other fields.^20–23 In addition, due to the particle surface being covered with reactive active sites, SiO₂ nanoparticles are ideal carriers of functional materials, but agglomeration is a major factor limiting their application.^24–28 Zwitterionic nanoparticles can be prepared though surface modification, which will overcome the problems with application such as the agglomeration of nanomaterials and insolubility of zwitterionic materials in organic solvents. Using SiO₂ nanoparticles with good dispersibility as carriers makes it possible to enhance the fouling resistance of hydrophobic materials within a zwitterionic medium.

Recently, studies found that amino acids are potentially zwitterionic materials that come from a wide range of sources and have both excellent design characteristics and good antifouling effects.^29–34 Therefore, in this study, lysine was used for modifying the SiO₂ nanoparticles to generate a zwitterionic surface, and the hybrid PVDF UF membranes were prepared by blending membranes with this new additive of zwitterionic nanoparticles (ZP). The effect of adding zwitterionic nanoparticles for the separation performance and fouling resistant properties of the PVDF membranes were investigated. This study aims to provide a convenient method for the preparation and large-scale production of antifouling PVDF membranes using zwitterionic materials.

2. Experimental

2.1 Materials

PVDF (MG105) was purchased from Arkema. Bovine serum albumin (BSA, 67000 Da), PEG (M_w ∼ 20 000) and lysine were purchased from Sinopharm Chemical Reagent Co. (China). Humic acid (HA, fulvic acid >90%), 3-glycidyloxypropyl (dimethoxy) methylsilane (GPMS, KH560), and hydrophilic SiO₂ nanoparticles (30 nm) were purchased from Aladdin Co. (China). Ethanol, hydrochloric acid and dimethylacetamide (DMAC) were purchased from a local corporation at commercial analytical grade. All the reagents were used without any purification.

2.2 Modification of SiO₂ nanoparticles

1 g SiO₂ nanoparticles were dispersed into 20% (v/v) alcohol aqueous solution by ultrasound for 30 min, then 0.3 g GPMS and 1 drop of hydrochloric acid were added into the abovementioned solution to react for 6 hours under magnetic stirring at 60 °C. After that, 5 ml of lysine aqueous solution (10%) was added into the SiO₂ dispersion solution and kept for another 3 h to react. Finally, the reaction products were centrifuged, washed and dried for subsequent characterization and modification.

2.3 Membrane preparation

A certain amount of modified SiO₂ nanoparticles was added to the mixed solution of 40 g DMAC and 10 g H₂O and subjected to ultrasound for 1 h to promote the dispersion of nanoparticles. 5 g PEG was added to further induce the uniform dispersion of modified SiO₂ nanoparticles under magnetic stirring for 2 h. 10 g PVDF powder was added to the abovementioned solution and dissolved completely as the membrane casting solution, which was used to prepare PVDF hybrid membranes by non-solvent induced phase separation (NIPS) method with ice water as the coagulating bath. The formed PVDF membranes were placed in water for more than 12 h to remove excess solvent and porogen. The dosages of modified SiO₂ nanoparticles were 2% and 5% for the PVDF mass, and the prepared hybrid membranes were labeled ZP-2% and ZP-5%, respectively. A PVDF membrane with 5% addition of pristine SiO₂ was used as a reference sample.

2.4 Characterization

Particle size distribution and zeta potential of SiO₂ nanoparticles were tested by the nanoparticle size and a potential analyzer (Nano ZS, Malvern). The chemical compositions of modified SiO₂ nanoparticles were characterized by Fourier transform infrared spectroscopy (FTIR, Nicolet 8700) and X-ray photoelectron spectroscopy (XPS, Shimadzu AXIS UltraDLD). Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR, Nicolet 8700, USA) was used to examine the surface composition of PVDF membranes. The membranes were analyzed on a ZnSe crystal as the internal refection element with an aperture angle of 45°. The ATR-FTIR spectra were obtained by the accumulated average of 32 scans at 4 cm⁻¹ resolution. Surface hydrophilicities of neat and modified membranes were evaluated by water contact angle (CA, OCA40Micro, Germany) on the membranes surface at room temperature. A total of about 3 μl of deionized water was dropped onto the membrane surfaces using a micro syringe, and the results were obtained using the drop shape image analysis system to evaluate the surface wetting ability. In addition, the wetting property of membrane matrix (interior of the membrane) was examined though a novel method. Hybrid membranes without skin layer were directly obtained using the casting solution by spin coating (5000 rpm) and thermosetting (80 °C), which imitated the membrane matrix structure. Then, the contact angle can be directly used to test the hydrophilicity of mimetic matrix with the same method of membrane surface. The surface and cross-section morphology of membranes were viewed with the field emitting scanning electron microscope (SEM, Hitachi SU8010, Japan), all the samples were coated with gold before observation and cross-section of membranes were acquired using liquid nitrogen. The surface microstructure of the membranes was characterized with an Agilent5500 atomic force microscope (AFM, Agilent Technologies Inc., USA) in the tapping mode, and the roughness of membranes were characterization with their R_q value.

2.5 Filtration

The separation performance of pristine and hybrid PVDF membranes were investigated using a dead-end filtration system with an effective membrane area of 12.5 cm². The filtration experiments were carried out with the compacting pressure at 0.15 MPa for 30 min using pure water as feed solution, and the pressure was lowered to 0.1 MPa to obtain a stable flux by comparing the flux values every 5 minutes. The pure water flux was noted as J₁ (L m⁻² h⁻¹) and calculated by eqn (1).

The rejection of all membrane was tested with a BSA solution (1 g L⁻¹, pH7.4) as a feed solution. The experiment process was referred to the water flux test method and the permeation concentration was examined via a UV spectrophotometer (UV-1800, Shimadzu) with the characteristic wavelength at 280 nm, the rejection was calculated as the following eqn (2).

J = V/(A × t) (1)

R = (1 − C_p/C) × 100% (2)

where V (L) is the volume of permeated water, t (h) is the permeation time, A (m²) is effective area for filtration, and C_p and C are the BSA concentrations of permeate and feed solutions, respectively.

2.6 Fouling test

The static fouling test was carried out by immersing the hybrid PVDF membranes (3 × 3 cm²) in a pollutant solution (BSA, 0.5 g L⁻¹, pH 7.4) without additional pressure-assisted BSA into the membrane pore. After adsorption–desorption equilibrium for 12 h at room temperature, the concentrations of the BSA solution before and after the adsorption by PVDF membranes were measured with a UV-1800, and the mass (μg cm⁻²) of adsorbed BSA on the membrane surface was calculated. In order to test the stability of nanoparticles during separation process in the aquatic environment, the water flux of long time (8 h) of the hybrid membrane was tested.

The dynamic fouling experiments were executed using a circulating mode with an alternate feed solution of pure water and pollutants solution. BSA and HA were used as the pollutants in the feed solution. First, stable pure water flux was recorded as J₁, then the feed was replaced with a pollutant solution and a stable flux was obtained and recorded as J_p. After that, the tested membranes were rinsed with pure water for 20 min and secondary pure water flux was obtained as J₂. Water flux recover ratio (FRR) values and irreversible fouling resistance (IFR) values were calculated by eqn (3) and (4). In addition, the dynamic fouling experiment was repeated several times to evaluate the antifouling stability of zwitterionic nanoparticles in the membrane matrix.

FRR = J₂/J₁ × 100% (3)

IFR = 1 − FRR (4)

3. Results and discussion

The zwitterionic modification method of SiO₂ nanoparticles is shown in Fig. 1. The silane coupling agent KH560 was used to react with SiO₂ nanoparticles under acidic conditions to prepare the intermediate products with active epoxy groups. Using the ring-opening reaction between the amino group of lysine and epoxy groups on the SiO₂ particle surface, lysine with a pair of amino and carboxyl was grafted onto SiO₂ nanoparticles to obtain zwitterionic nanoparticles with amino acids.

Fig. 1 Scheme for the zwitterionic modification of SiO₂ nanoparticles.

FTIR spectra of SiO₂ nanoparticles before and after modification are shown in Fig. 2a. Comparing the spectra of the pristine and lysine-modified SiO₂ nanoparticles, three new peaks at 1410 cm⁻¹, 1510 cm⁻¹ and 2960 cm⁻¹ were observed, which were attributed to the plane bending vibration absorption peak of the carboxyl OH bond, shear vibration absorption of amino, and the methylene absorption peak. Moreover, the hydroxyl absorption at 3400 cm⁻¹ peak was significantly broadened, suggesting an increase of carboxyl and amino groups on the surface of SiO₂ nanoparticles. All these results indicate that the lysine with amino group and carboxyl group were successfully grafted onto the SiO₂ nanoparticle surface, and novel modified SiO₂ nanoparticles were prepared with a zwitterionic surface.

Fig. 2 Characterization of pristine and lysine modified SiO₂ nanoparticles ((a) FTIR, (b) particle size, (c) zeta potential, (d) XPS of SiO₂-lysine).

To investigate the effect of zwitterionic modification on the agglomeration problem of SiO₂ nanoparticles, the particle size distributions of SiO₂ nanoparticles before and after modification were measured and are shown in Fig. 2b. It was observed that the size distribution of modified SiO₂ nanoparticles had a narrower range and the average diameter was smaller than that of the pristine SiO₂ nanoparticles. The average diameter decreased from 80 nm to about 40 nm, which was close to the diameter (30 nm) of the pristine SiO₂ nanoparticles. From this, we can conclude that the agglomeration problem of SiO₂ nanoparticles with zwitterionic modification was significantly alleviated. The appropriate reason should be attributed to surface modification strengthening the interfacial stability of nanoparticles, the ionogenic pair of amino and carboxyl groups reduce the absolute value of nanoparticles surface charge, leading to a better dispersion effect.

Because lysine includes a pair of positive and negative ions, the zeta potential of the modified SiO₂ nanoparticles was measured (Fig. 2c). The SiO₂-lysine exhibited a negative charge at high pH and positive charge, which is a characteristic of zwitterionic materials. The characteristic elements N and S belonging to lysine were detected in the XPS spectra of SiO₂-lysine nanoparticles (Fig. 2c), and the atomic ratio was almost 1 : 1. All these results demonstrated the successful preparation of zwitterionic SiO₂ nanoparticles.

The effect of zwitterionic SiO₂ nanoparticle addition for the morphology of PVDF ultrafiltration membranes were characterized by SEM, as shown in Fig. 3. As can be seen from the cross-sectional view (A, B, C) of the PVDF, ZP-2% and ZP-5% membranes, PVDF and hybrid membranes exhibited a conventional asymmetric structure with a cortex layer and finger holes in the polymer membranes, and for the cross-sectional view images, the massive particle agglomeration problem was not observed clearly. In addition, from the surface images (a, b, c) of the PVDF, ZP-2% and ZP-5% membranes, a large number of nanoparticles were observed on the ZP-2% and ZP-5% membrane surface, and the observed number of nanoparticles on the hybrid membrane surface increased with the additional amounts of zwitterionic SiO₂ nanoparticles to the PVDF membrane. It can be seen that the plentiful nanoparticles are uniformly distributed on the hybrid membrane surface without an obvious agglomeration phenomenon from the surface SEM image of ZP-5%. All these SEM results showed that the modified nanoparticles could be dispersed evenly in the hybrid PVDF membranes, which provided a foundation for improving the hydrophilicity of hybrid PVDF membranes. In addition, it should be noted that there will be significant agglomeration of nanoparticles when the added amount is more than 15% in the experiment.

Fig. 3 SEM images of PVDF and hybrid PVDF membranes (A, (a) PVDF, B, (b) ZP-2%, C, (c) ZP-5%).

To further investigate the hybrid influence of ZP for the morphology of PVDF membranes, the surface roughness of hybrid PVDF membranes was measured by AFM. As shown in Fig. 4, the membrane surface roughness of nanoparticles changed significantly after zwitterionic SiO₂ nanoparticles were added into the PVDF membranes, and the different addition amount affected the surface roughness of the hybrid membrane. When the additional amount of zwitterionic SiO₂ nanoparticles was 5%, the highest roughness value of 33.361 nm was obtained.

Fig. 4 The AFM images of PVDF and hybrid PVDF membranes.

To examine the impact of nanoparticles on the physical and chemical properties of the PVDF membranes, ATR-FTIR was used to characterize the surface composition of hybrid PVDF membranes. Fig. 5 shows the ATR-FTIR spectra of PVDF, ZP-2% and ZP-5% membranes. It could be confirmed that there were no significant differences between the spectra of PVDF and ZP-2%, indicating that the membrane surface composition did not change significantly when the added amount of zwitterionic SiO₂ nanoparticles was less than 2%. However, there was an apparent peak at 1103 cm⁻¹ that was attributed to the Si–O bond characteristic absorption on the spectra of ZP-5% membrane, and the peak intensity of the hydroxyl group at 3400 cm⁻¹ increased significantly, indicating the presence of SiO₂ nanoparticles on the membrane surface. There are numerous hydroxyl, amino and carboxyl groups on the zwitterionic SiO₂ nanoparticles surface, which increased the number of polar groups on membrane surface thereby enhancing the 3400 cm⁻¹ peak intensity. Increased numbers of polar groups, such as hydroxy, carboxy and amino groups, were beneficial to improve the hydrophilic performance of PVDF membranes.

Fig. 5 ATR-FTIR spectra of PVDF and hybrid PVDF membranes.

Spin coating is one of the most used methods for evenly spreading a single polymer or nanoparticle onto a substrate.³⁵ Thus, a hybrid membrane solution could be spread over the substrate uniformly, then thermosetting directly to suppress the generation of a skin layer on the membrane surface, to produce a mimetic porous structure in the membrane matrix. As can be seen from Fig. 6, the contact angle of the membrane surface and mimetic matrix for hybrid PVDF membranes decreased distinctly, the surface contact angle of ZP-5% membrane decreased from 95° to about 50°, and the contact angle of mimetic membrane matrix lowered to 10°. Obviously, the presence of zwitterionic SiO₂ nanoparticles in hybrid PVDF membranes improved the wetting ability of entire hydrophobic PVDF membrane, especially on the interior of the membrane.

Fig. 6 Water contact angle of membrane surface (a) and mimetic membrane matrix (b).

While the effects of added zwitterionic nanoparticles on the physicochemical properties and microstructure were detailed above, their effects on the separation performance of PVDF membrane were also studied. Fig. 7 shows the separation performance of PVDF membranes before and after hybrid modification. The separation data showed that the pure water flux of ZP-5% hybrid membrane increased from 75 L m⁻² h⁻¹ to 113 L m⁻² h⁻¹ compared with pristine PVDF membrane with the increased ratio of 50%, while its rejection rate was also improved from 80% to 93%. This suggests that the addition of hydrophilic nanoparticles improved the separation efficiency of hydrophobic PVDF membrane and broke the traditional trade-off phenomenon between flux and rejection ratio. The exhibited separation performance of ZP-2% and ZP-5% membranes were all better than the SiO₂-5% membrane. The added ZP particles improved the hydrophilicity of the PVDF membranes, leading to the improvement of water flux and sieve capacity to BSA molecules of PVDF hybrid membranes. BSA molecules were repulsed from the hydrophilic surface and the water molecules were preferentially adsorbed and allowed to penetrate the hydrophilic membrane, thus improving the separation of flux and rejection. The stability of ZP in the membrane matrix was tested through the long-term test in a water environment, as shown in Fig. 8. There was no obvious change in water flux after the test of eight hours, indicating that the nanoparticles would not be lost in the separation process.

Fig. 7 Separation performance of PVDF and hybrid PVDF membranes.

Fig. 8 Time dependent water flux of ZP-5% membrane at 0.1 MPa.

The antifouling abilities of PVDF hybrid ultrafiltration membranes were evaluated by static fouling adsorption and dynamic fouling with common pollutants. The static fouling test results for the PVDF membrane before and after hybridization are shown in Fig. 9. From the adsorption mass date of BSA solution on PVDF membranes, it can be seen that BSA adsorption per unit area on ZP-modified PVDF hybrid membranes decreased rapidly, and the BSA adsorption mass of ZP-5% membrane was only 10 μg cm⁻², suggesting that the improvement of the surface wetting ability by zwitterionic nanoparticles enhanced the anti-adsorption capacity for hydrophobic pollutants and conduced to improve the static antifouling ability of PVDF membranes. When the antifouling characteristics of zwitterionic nanomaterials on the membrane surface were active used, the non-specific adsorption of hydrophobic contaminants in the material interface were suppressed.

Fig. 9 Static adsorption of BSA solution of PVDF and hybrid PVDF membranes.

Fig. 10 shows the results of the dynamic fouling test of PVDF and hybrid PVDF membranes with BSA and HA as two types of typical pollutants. As can be seen from the FRR and IFR values of BSA dynamic fouling in Fig. 10a, the FRR values of ZP-2% and ZP-5% hybrid membranes were 81% and 95%, respectively, and were significantly higher than the 55% corresponding to the FRR value of the pristine PVDF membrane. With the addition of more zwitterionic nanoparticles, greater FRR values were obtained, and the relative IFR values were smaller, indicating that the added zwitterionic SiO₂ nanoparticles were beneficial to reduce the loss of water flux caused by irreversible fouling and that hydrophobic pollutants were more likely to be cleaned such that the accumulation of pollutants in the pores and on the membrane surface decreased rapidly. The dynamic fouling results of the HA solution are shown in Fig. 10b. The results showed the same trend as the BSA fouling test, and the FRR value increased to 97% for the ZP-5% membrane. The irreversible fouling caused by HA was significantly reduced due to the addition of the zwitterionic SiO₂ nanoparticles. Hybrid PVDF membranes exhibited higher water flux recovery rate and reusability. It can be that the FRR values of SiO₂-5% membrane were not significantly improved, which should be due to the reunification problem of nanoparticles. Fig. 11 shows the long-term antifouling effect of ZP, and even after the dynamic fouling test of 3 times, the FRR value of ZP-5% membrane was no less than 85%, showing that the hybrid membrane would have a longer service life.

Fig. 10 Dynamic fouling performance of PVDF and hybrid PVDF membranes (a) BSA solution, (b) HA solution.

Fig. 11 Repeated antifouling performance of PVDF and ZP-5% membrane (3 times, BSA as pollutant).

All these results demonstrated that the hybrid of PVDF membrane with zwitterionic nanoparticles enhanced the antifouling performance of hydrophobic PVDF membranes and improved the membrane separation efficiency though reducing the irreversible fouling caused by irremovable pollutants. The addition of zwitterionic SiO₂ nanoparticle fabricated hydrophilic membrane surface and membrane matrix. Based on the hydrophilic and antifouling properties of modified zwitterionic SiO₂ nanoparticle, the wettability of entire hydrophobic PVDF membrane was significantly improved. Water molecules were preferentially adsorbed on the hydrophilic membrane surface and the pore surface, so that the anchored effect of pollutants on the hydrophobic porous membrane was weakened, leading to the pollutants being washed away easily.

4. Conclusions

Zwitterionic SiO₂ nanoparticles were blended with PVDF to prepare hybrid PVDF ultrafiltration membranes. The distribution of zwitterionic SiO₂ nanoparticles was extremely uniform in hybrid PVDF ultrafiltration membranes, indicating that the agglomeration of nanoparticles was effectively inhibited. Furthermore, the surface wetting properties and separation performance of PVDF membranes were all significantly improved due to the addition of the zwitterionic SiO₂ nanoparticles. Moreover, the blended zwitterionic SiO₂ nanoparticles significantly enhanced the antifouling performance of PVDF membranes. The mass of BSA adsorption on to the ZP-5% membrane decreased to 10 μg cm⁻² in the static fouling test, and the FRR values of secondary water flux were no less than 95% for the dynamic fouling test using BSA and HA pollutants. The excellent antifouling performance of PVDF membranes indicates that the hybrid modification with anchored zwitterionic material is an effective and convenient method to enhance the fouling resistance of hydrophobic PVDF membranes.

Acknowledgements

This study is supported by the grants from the National Science Foundation of China (no. 21174027), the Program for New Century Excellent Talents in University (no. NCET-12-0827), the Program of Introducing Talents of Discipline to Universities (no. 111-2-04) and the Innovation Funds for PhD Students of Donghua University (CUSF-DH-D-2015027).

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Footnote

† These authors contributed equally.

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