Bioinspired Peptide-Coated Superhydrophilic Poly(vinylidene fluoride

May 11, 2018 - The pristine PVDF membrane was coated with dopamine under mild alkaline ..... CTAB are around 2200 L·m–2·h–1, while for Tween 80 ...
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Cite This: Langmuir 2018, 34, 6621−6627

Bioinspired Peptide-Coated Superhydrophilic Poly(vinylidene fluoride) Membrane for Oil/Water Emulsion Separation Weiming Wu,† Renliang Huang,*,‡ Wei Qi,*,†,§,∥ Rongxin Su,†,§,∥ and Zhimin He† †

State Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology, ‡Tianjin Key Laboratory of Indoor Air Environmental Quality Control, School of Environmental Science and Engineering, §Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), and ∥Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University, Tianjin 300072, P. R. China S Supporting Information *

ABSTRACT: Polyvinylidene fluoride (PVDF) membranes are limited in the field of oil-in-water emulsion treatment because the intrinsic hydrophobicity of PVDF can cause serious membrane fouling. Here, a superhydrophilic PVDF membrane (PVDF@PDA−GSH) was fabricated using a facile, versatile, mussel-inspired method. The pristine PVDF membrane was coated with dopamine under mild alkaline conditions by a dip-coating method, followed by addition of glutathione (GSH) via a simple reaction. GSH was successfully coated onto the membrane surface and confirmed by X-ray photoelectron spectroscopy and energy dispersive X-ray spectrometry. Hierarchical surface structure and superhydrophilicity were examined by scanning electron microscopy and contact angle, respectively, giving the PVDF@PDA−GSH membrane excellent wettability and antifouling ability. The water flux of PVDF@PDA−GSH was several-fold higher than conventional filtration membranes, and the oil rejection ratio was nearly 99%. The PVDF@PDA−GSH membrane also showed favorable reusability because the flux recovery ratio (FRR) remained above 90% after five cycles. In general, these results indicated that this modification might provide a good method for the fabrication of superhydrophilic PVDF membranes with good prospects for water filtration applications.

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Polyvinylidene fluoride (PVDF) membranes are commonly used because of their outstanding properties, such as chemical resistance, mechanical robustness, and thermal stability.9 However, PVDF membranes also suffer from oil fouling because of their intrinsic hydrophobicity. Many studies focus on superhydrophilic modification of the PVDF membrane, which helps to reduce fouling during filtration.8−11 Yang et al.9 prepared a superhydrophilic PVDF membrane by a facile method in which dopamine (DA) copolymerizes with 3aminopropyltriethoxy-silane-functionalized multiwall carbon nanotubes, and the prepared PVDF membrane showed outstanding performance in the oil-in-water emulsion separation. Chew et al.12 developed a robust PVDF hollow-fiber membrane by single-step codeposition of the hydrophilic polydopamine (PDA)/polyethylenimine layer on the outer surface, which avoided oil fouling during direct-contact membrane distillation because of its superhydrophilicity. Zhu et al.5 reported a novel zwitterionic PMAPS-grafted PVDF via surface-initiated atom transfer radical polymerization, and this membrane thoroughly separated oil from water with ultrahigh

INTRODUCTION Every year, a large amount of oily wastewater is produced by frequent oil spills and industrial processes, including petrochemical, transportation, food, textile, and metallurgical processing.1 Oily wastewater seriously endangers the environment, natural landscapes, and human health.2,3 Oil in water, generally speaking, is classified as floated oil (Dp > 100 μm), dispersed oil (Dp = 100−10 μm), emulsified oil (Dp < 10 μm), and dissolved oil (Dp < 0.1 μm, in the form of molecules).4 Traditional treatment techniques for oily wastewater, including gravity separation, air floatation, adsorption, and coagulation, are effective for floating oil and dispersed oil but are useless for emulsified oil and dispersed oil, especially surfactant-stabilized emulsions.5,6 Thus, there is an urgent need for treatment methods for emulsified oil. Currently, membrane technologies and functional filtration materials with special wettability are considered promising candidates for the separation of emulsified oil because of high efficiency, low consumption, and easy operation.7 However, polymer membranes used in oily wastewater treatment suffer from fouling during the filtration process, causing reduced operating life and increased operation cost.8 Therefore, a solution for fouling in the filtration process is the basic premise of membrane technologies applied to oil-inwater emulsions. © 2018 American Chemical Society

Received: March 28, 2018 Revised: April 24, 2018 Published: May 11, 2018 6621

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membrane is referred to as PVDF@PDA−GSH). Finally, the modified membrane was placed in ultrapure water until further testing. Membrane Characterization. Surface morphologies and element mappings of pristine and modified PVDF membranes were observed using field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and energy-dispersive spectroscopy (EDS, Hitachi S-4800), respectively. The surface chemistry was investigated by X-ray photoelectron spectrometry (XPS, ULVAC-PHI, Japan). The water contact angles (CAs) were measured using an SL200KS optical CA measuring device (Kina, America). Preparation for Oil/Water Emulsions. A series of surfactantstabilized oil/water emulsions, including SDS/n-hexadecane/water emulsion, SDS/diesel oil/water emulsion, SDS/soybean oil/water emulsion, CTAB/n-hexadecane/water emulsion, CTAB/diesel oil/ water emulsion, CTAB/soybean oil/water emulsion, Tween 80/nhexadecane/water emulsion, Tween 80/diesel oil/water emulsion, and Tween 80/soybean oil/water emulsion were prepared by mixing oil and water (v/v = 1/100) at 20 000 rpm stirring for 15 min at a surfactant concentration of 0.2 mg/mL. Then, the floating oil was removed. All emulsions were stabilized for 24 h in a laboratory environment before use. Filtration and Antifouling Performance Assessment. The separation experiments occurred in a vacuum filter apparatus (Figure S1) and the effective separation area of PVDF membranes was 11.53 cm2. For each separation experiment, a certain volume (50 mL) of feed solution or BSA solution was poured onto the membrane at a pressure difference of 0.09 MPa, and the filtrate solution was collected. Micrographs of the feed solution and the filtrate solution of the oil-inwater emulsion were obtained using an optical microscope. The oil contents in the filtrates of mixtures and emulsions were tested using a total organic carbon analyzer (TOC, Shimadzu TOC-VCPH, Japan). The flux (J) is defined as eq 1

separation efficiency. Thus, surface modification can improve the antifouling ability of PVDF membranes. However, conventional materials applied in membrane surface modification such as polyethylene glycol, perfluoro compounds, and zwitterionic polymers have some disadvantages in practical industrial applications, such as damage to the environment by fluoride, requirements for complex surface modification methods, and lack of commercial availability. DA, inspired by the composition of adhesive proteins in mussels, can self-polymerize to form thin, surface-adherent PDA films onto a wide range of substrates.13 Nowadays, PDA is widely used as an adhesive layer for membrane coating.14−16 However, the limited hydrophilicity of PDA layer restricts its application for oil-in-water emulsion separation.17 Therefore, we attempt to construct a hydrophilic layer on the surface of PDA via the reaction between thiol/amine groups and PDA.18 Peptides, with protonated amine groups and deprotonated carboxyl groups, are fairly similar to zwitterionic materials and have been used in the construction of antifouling sensing interfaces.19,20 The excellent performance of peptides in antifouling sensing interfaces suggests that it is possible to use them in membrane modification. Glutathione (GSH) is a natural, hydrophilic, environmentally friendly, and commercially available tripeptide that can interact with water strongly because of its abundant polar groups and zwitterionic pair.21 In our previous work, GSH was used to form a composite layer that functions as a low-fouling coating that improved surface hydrophilicity and antifouling ability.22 Thus, we believe that GSH is the ideal material for membrane modification that avoids the shortcomings of conventional materials. Inspired by these findings, we developed a facile, versatile method to modify the PVDF membrane. First, the pristine PVDF membrane was coated with DA and bound to GSH via a simple reaction. The advantages of this modification: a simple process, commercially available materials, and greatly improved wettability and antifouling ability of the PVDF membrane. We studied the separation performance of the modified PVDF membrane and found outstanding performance in various oilin-water emulsion separations.

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J=

V AΔt

(1) 2

where V (L) is the volume of feed solution, A (m ) is the effective separation area, and Δt (h) is the permeation time. The oil rejection ratio (R) is defined as eq 2

⎛ C ⎞ R (%) = ⎜1 − 2 ⎟ × 100 C1 ⎠ ⎝

(2)

where C1 (mg/L) is the oil concentration of the feed solution and C2 (mg/L) is the oil concentration of the filtrate solution. The flux recovery rate (FRR) is defined as eq 3

EXPERIMENTAL SECTION

⎛J ⎞ FRR (%) = ⎜⎜ 2 ⎟⎟ × 100 ⎝ J1 ⎠

Materials. PVDF membranes were purchased from Merck Millipore Ltd., Ireland (0.22 μm). DA hydrochloride (98%), GSH (97%), bovine serum albumin (BSA), sodium dodecyl sulfate (SDS), hexadecyl trimethyl ammonium bromide (CTAB), Tween 80, and nhexadecane were purchased from Heowns, Tianjin, China. Tris(hydroxymethyl)aminomethane (Tris, 99%), ethyl alcohol, and sodium hydroxide were purchased from Aladdin, China. Diesel oil and soybean oil were purchased from a local factory. The solution pH was measured with an MP 220 pH meter (Mettler Toledo, Switzerland). The water used in the experiments was purified with a three-stage Millipore Milli-Q Plus 185 purification system (Millipore Corp., Bedford, MA). All chemicals were used as received. Fabrication of GSH Coated PVDF Membrane. PVDF membranes were soaked in ethanol for 2 h and rinsed with ultrapure water three times. The PVDF membranes were immersed in 2 mg/mL DA Tris solution (pH = 8.5) for a period of time (2, 4, 12, or 24 h) for self-polymerization, cleaned ultrasonically with ethanol for 20 min, and rinsed with ultrapure water three times (this membrane is referred to as PVDF@PDA). Finally, the PVDF@PDA membranes were immersed in 10 mg/mL GSH Tris solution (pH = 8.5) for a period of time (0, 2, 6, 12, or 24 h) for reaction under N2. The Tris solution was degassed with nitrogen for 30 min before use to avoid oxidation of the thiol groups (−SH) to disulfide bonds (−S−S−). Then, the GSHcoated PVDF membranes were rinsed with ultrapure water (this

−2

−1

(3) −2

−1

where J1 (L·m ·h ) and J2 (L·m ·h ) are the pure water flux before and after filter solution, respectively.9

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RESULTS AND DISCUSSION As shown in Scheme 1, the GSH-coated superhydrophilic PVDF membrane was fabricated using a facile dip-coating method (note: it is a simplified representation of the structure of PDA and the interaction between PDA and GSH in Scheme 1 according to the previous studies23−25). For excellent antifouling ability, the modified PVDF membrane must meet two crucial conditions: hierarchical surface structure and superhydrophilicity. A superhydrophilic surface with a hierarchical structure could be water-affinitive and trap water in slots on the membrane surface to reduce the contact area between the oil droplet and the coated membrane surface.7 The surface morphologies of the membranes before and after modification were revealed by FE-SEM images (Figure 1). As shown in Figure 1a−c, there are many large pores on the membrane surface, with small pores beneath the surface. The 6622

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O, and S elements across the surface of the PVDF@PDA−GSH membrane, also confirmed this conclusion. For other membranes prepared with various PDA and GSH treatment times, the elemental composition of membranes is summarized in Table S1. All of these demonstrated the presence of GSH on the membrane surface. The wettability of the PVDF membrane was shown by accurate measurements of the water CA. As shown in Figure 3, the CA of the pristine PVDF membrane was 120.6°, indicating the intrinsic superhydrophobicity of pristine PVDF membrane. The CA of PVDF@PDA was 69.3°, which revealed improved hydrophilicity, but it was not enough. With the modification of GSH, the hydrophilicity of PVDF@PDA was improved again, and the CA of PVDF@PDA−GSH was 11.1°. This value was close to the definition of superhydrophilicity as desired. As shown in Figure S5, with the increasing PDA treatment time from 2 to 4 h (the GSH treatment time is 24 h), the CA decreased from 64.3° to 21.6°. When the GSH treatment time increased from 2 to 12 h (the PDA treatment time is 4 h), the CA increased slightly. Eight seconds later, the CAs of pristine PVDF and PVDF@PDA were 119.6° and 65.7°, respectively. The decline in CA of the pristine PVDF and PVDF@PDA was not obvious, demonstrating the hydrophobicity of the PVDF membrane and the weak hydrophilicity of the PVDF@PDA membrane. However, when a droplet of water contacted the PVDF@PDA−GSH membrane surface, it spread out immediately and disappeared in 1 s. We saw a significant decline in the CA of PVDF@PDA−GSH, and the CA was 0° at 1 s because of the coating of GSH. This appearance indicated that PVDF@ PDA−GSH has excellent permeability for water, demonstrating that the PVDF membrane has changed from hydrophobic to superhydrophilic. Pure water flux was another important surface property. As shown in Figure 4, the pure water flux of the pristine PVDF membrane at 0.09 MPa was 0 L·m−2·h−1, which meant that water cannot penetrate because of superhydrophobicity, identical to the prior CA test. In comparison, the pure water fluxes of PVDF@PDA and PVDF@PDA−GSH under the same pressure increased to 8887 ± 347 and 9900 ± 562 L·m−2·h−1, respectively. As we know, the hydrophilicity of the membrane imparted the water a fast permeability velocity.27 Compared to pristine PVDF and PVDF@PDA membranes, the PVDF@ PDA−GSH membrane has a much higher pure water flux, suggesting the successful superhydrophilic coating on the surface and also the negligible pore blocking. This result is in

Scheme 1. Schematic Illustration for the Fabrication of PVDF@PDA−GSH Membrane

pores of membrane were not blocked after the coating of PDA. However, many nanosize particles, which were formed from the aggregation of PDA, were present on the surface of PVDF@ PDA and PVDF@PDA−GSH membranes. The different membranes were prepared with various PDA and GSH treatment times. As shown in Figures S2 and S3, the SEM images show that increasing the PDA treatment time could facilitate the aggregation of nanoparticles, and the differences in morphology could be observed. XPS and EDS were used to investigate the presence of GSH on the modified PVDF membrane. As displayed in Figure 2a, there were two peaks in the image of the pristine PVDF membrane, which represent C 1s (289 eV) and F 1s (686 eV). However, for PVDF@PDA−GSH membrane, as shown in Figure 2b, three other peaks represent O 1s (532 eV), N 1s (399 eV), and S 2p (165 eV).26 Comparing Figure 2c,d, we found that the signal of S 2p in PVDF@PDA−GSH was clearly higher than the signal in the pristine PVDF membrane. Table 1 summarizes the element change of the membranes before and after modification. The content of C, O, and N elements increased, and the S element appeared. The new peaks for O 1s and N 1s might belong to PDA and GSH, but elemental sulfur was present in GSH only throughout the fabrication of PVDF@ PDA−GSH membrane. Thus, we believe that GSH was successfully coated on the PVDF membrane. EDS mapping (Figure S4), which illustrates the uniform dispersion of C, F, N,

Figure 1. FE-SEM images of the surface of (a,d) PVDF, (b,e) PVDF@PDA, and (c,f) PVDF@PDA−GSH. The time of PDA and GSH treatment was 4 and 2 h, respectively. 6623

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Figure 2. XPS wide-scan and S 2p high-resolution spectrum of (a,c) PVDF and (b,d) PVDF@PDA−GSH. The time of PDA and GSH treatment was 4 and 2 h, respectively.

Table 1. Elemental Composition of Pristine PVDF and PVDF@PDA−GSH Membranes composition (%) membrane

C

F

O

N

S

pristine PVDF PVDF@PDA−GSH

50.62 65.56

49.26 16.62

0.12 12.25

0 4.8

0 0.28

Figure 4. Pure water flux of different membranes under 0.09 MPa. The time of PDA and GSH treatment was 4 and 2 h, respectively.

high BSA rejection ratio is suitable for practical water treatment because the organic matter (BSA is the representative substance) in practical oil-in-water emulsion will cause membrane fouling.28 We investigated the BSA rejection ratio, and the result is shown in Figure 5. The BSA rejection ratio of the membrane was increased after the modification of PDA and GSH; specifically, the BSA rejection ratio improved from 0 (pristine PVDF) to 75.7% (PVDF@PDA) and reached 97.2% (PVDF@PDA−GSH). As expected, the BSA rejection ratio of the PVDF membrane was effectively improved by the coating of PDA, and the coating of GSH led to further improvement

Figure 3. Water CAs of different membranes with different drop times. The time of PDA and GSH treatment was 4 and 2 h, respectively.

accordance with the previous CA test and SEM characterization. It also demonstrated that GSH modification makes the membrane hydrophilic, which is beneficial to enhancing the PVDF membrane’s antifouling property because of the formation of hydration layer on the surface.9 The BSA rejection ratio (FRR of BSA solution) was also important for membrane separation. The membrane with a

Figure 5. BSA rejection ratio of various membranes under 0.09 MPa. The time of PDA and GSH treatment was 4 and 2 h, respectively. 6624

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Langmuir because the formation of a hydration layer can intercept BSA molecules. It should be emphasized that the BSA rejection ratio of PVDF@PDA−GSH (97.2%) was higher than some of the other membranes, for example, the BSA rejection ratio of a poly(lysine methacrylamide)-grafted PVDF membrane was 86%.8 As described above, PVDF@PDA−GSH is superhydrophilic and may have an excellent ability to separate oil-in-water emulsions. To verify this point, a series of oil-in-water emulsions were prepared. Three different kinds of oil (nhexadecane, diesel oil, and soybean oil) and three different types of surfactants (SDS, CTAB, and Tween 80) were used. The water permeation flux, oil rejection ratios, and flux recovery ratios were measured to evaluate the separation performance of PVDF@PDA−GSH membranes. The surfactant-free diesel oil-in-water emulsion served as a feed solution. As shown in Figure 6a, the flux of PVDF@PDA4-

Figure 7. (a) Flux and (b) oil rejection ratio of various oil-in-water emulsions.

around 2200 L·m−2·h−1, while for Tween 80 it is about 1100 L· m−2·h−1. The results indicate that the surfactant type also affects the performance of the PVDF@PDA−GSH membrane. To understand these phenomena, we measured the droplet size of oil-in-water emulsion using SDS as the emulsifier. As shown in Figure S6, the average diameter of diesel-oil-in-water emulsion droplet is 2.42 μm, which was larger than those of n-hexadecane-in-water (1.51 μm) and soybean-oil-in-water (1.48 μm) emulsion systems. Compared to n-hexadecane-inwater and soybean-oil-in-water emulsion systems, the diesel-oilin-water emulsion had a much higher flux, which is probably attributed to the larger droplet size. The oil contents of the filtrate solutions of these oil-in-water emulsions were also measured (the numerical values are shown in Table S2). As shown in Figure 7b, all of the oil rejection ratios were greater than 97%. More importantly, we found that the type of oil and surfactant had no influence on the oil rejection ratio, indicating that PVDF@PDA−GSH is suitable for different kinds of oil-in-water emulsion systems. The optical microscopy images for the diesel-oil-in-water emulsion and its filtrate are shown in Figure 8. There are many oil droplets with an average diameter of 2.42 μm (Figure S6) in the emulsion. However, no droplets can be seen in the image of the filtrate. The results revealed the excellent separation efficiency of PVDF@PDA−GSH membranes. The antifouling performance of the PVDF@PDA−GSH membrane was investigated and expressed by the flux recovery ratio (FRR), divided by the pure water fluxes of PVDF@PDA− GSH before and after filter emulsion. Figure 9 shows the flux recovery ratios of different emulsions (the numerical values are shown in Table S2). As shown in Figure 9, all of the flux recovery ratios were greater than 90%. Additionally, there is no obvious change in the flux recovery ratios of various oil-in-water emulsions, suggesting that the PVDF@PDA−GSH membrane has good antifouling performance that was not affected by the type of oil and surfactant.

Figure 6. Oil-in-water emulsion flux of different (a) PDA coating time and (b) GSH coating time membranes.

GSH24 (4 and 24 represent that the time of DA and GSH treatment was 4 and 24 h, respectively) is lower than that of PVDF@PDA12−GSH24 in the beginning but higher in the end. The results indicated that the flux of PVDF@PDA4− GSH24 fell slowly, which means the slow fouling rate and the good antifouling ability. Similarly, as shown in Figure 6b, the flux of PVDF@PDA4−GSH2 reduced at a low rate, indicating the low fouling rate. Oil-in-water emulsion separation experiment was further conducted. Figure 7a shows the emulsion permeation fluxes of different oil-in-water emulsions (the numerical values were shown in Table S2). As shown in Figure 7a, the flux of dieseloil-in-water emulsion is clearly higher than that of n-hexadecane and soybean oil. Therefore, the type of oil can affect the flux of emulsions through the PVDF@PDA−GSH membranes. Meanwhile, the fluxes of emulsions with the same oil but different surfactants were also different. For example, in the hexadecanein-water emulsion system, the fluxes for SDS and CTAB are 6625

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from hydrophobic to superhydrophilic. We use GSH, a new material for use in oil-in-water emulsion separation, to modify the PVDF membrane and demonstrated its effectiveness. The modified membrane (PVDF@PDA−GSH) shows high pure water permeability and has good antifouling performance and reusability, which suits the membrane for long-term and efficient separation. Additionally, the PVDF@PDA−GSH membrane also has excellent separation performance that is not affected by the type of oil and surfactant; thus, it is suitable for various oil-in-water emulsions. Because of the simple, environmentally friendly fabrication procedure, this PVDF@ PDA−GSH membrane shows great potential for practical application.

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Figure 8. Digital images and micrographs of SDS stabilized diesel oilin-water emulsion and filtrate.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b01017. Photograph of the oil/water separation device; SEM and EDS scan images of the PVDF@PDA−GSH membranes; water CAs of different membranes; micrographs and size distribution of SDS-stabilized emulsions; elemental composition of different membranes; and flux, oil rejection ratio, and flux recovery ratio of different emulsions (PDF)

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Figure 9. Flux recovery ratio for various oil-in-water emulsions.

AUTHOR INFORMATION

Corresponding Authors

To evaluate the reusability of the PVDF@PDA−GSH membrane, a five-cyclic filtration experiment was performed using an SDS-stabilized diesel oil-in-water emulsion as a typical foulant, and the PVDF@PDA−GSH membrane was washed by ultrapure water after each cycle. As shown in Figure 10, for each

*E-mail: [email protected] (R.H.). *E-mail: [email protected]. Phone: +86 22 27407799. Fax: +86 22 27407599 (W.Q.). ORCID

Renliang Huang: 0000-0003-0797-3473 Wei Qi: 0000-0002-7378-1392 Rongxin Su: 0000-0001-9778-9113 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (nos. 21777112, 51773149, and 21621004). REFERENCES

(1) Shi, H.; He, Y.; Pan, Y.; Di, H.; Zeng, G.; Zhang, L.; Zhang, C. A modified mussel-inspired method to fabricate TiO2 decorated superhydrophilic PVDF membrane for oil/water separation. J. Membr. Sci. 2016, 506, 60−70. (2) Downs, C. A.; Shigenaka, G.; Fauth, J. E.; Robinson, C. E.; Huang, A. Cellular physiological assessment of bivalves after chronic exposure to spilled Exxon Valdez crude oil using a novel molecular diagnostic biotechnology. Environ. Sci. Technol. 2002, 36, 2987−2993. (3) Wirtz, K. W.; Baumberger, N.; Adam, S.; Liu, X. Oil spill impact minimization under uncertainty: Evaluating contingency simulations of the Prestige accident. Ecol. Econ. 2007, 61, 417−428. (4) Lu, D.; Zhang, T.; Gutierrez, L.; Ma, J.; Croué, J.-P. Influence of surface properties of filtration-layer metal oxide on ceramic membrane fouling during ultrafiltration of oil/water emulsion. Environ. Sci. Technol. 2016, 50, 4668−4674. (5) Zhu, Y.; Zhang, F.; Wang, D.; Pei, X. F.; Zhang, W.; Jin, J. A novel zwitterionic polyelectrolyte grafted PVDF membrane for thoroughly separating oil from water with ultrahigh efficiency. J. Mater. Chem. A 2013, 1, 5758−5765.

Figure 10. Flux recovery in the separation of SDS-stabilized diesel-oilin-water emulsion over five cycles under 0.09 MPa.

circle, the flux decreased in the process of filtration because of the oil retained on the surface or in the pores of membranes. However, the flux of emulsion almost returned to its initial level with washing with ultrapure water for approximately 10 min after each cycle, suggesting the good stability of PDA on PVDF membranes. The good reusability indicates that the PVDF@ PDA−GSH membrane is promising for practical oil-in-water emulsion separation.

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CONCLUSIONS We demonstrate a novel, facile, mussel-inspired method to modify PVDF membranes, which converts PVDF membranes 6626

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DOI: 10.1021/acs.langmuir.8b01017 Langmuir 2018, 34, 6621−6627