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seawater separation
Author's Accepted Manuscript
Under seawater superoleophobic PVDF membrane inspired by polydopamine for efficient oil/seawater separation Yanhui Xiang, Fu Liu, Lixin Xue
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PII: DOI: Reference:
S0376-7388(14)00896-5 http://dx.doi.org/10.1016/j.memsci.2014.11.052 MEMSCI13340
To appear in:
Journal of Membrane Science
Received date: 16 October 2014 Revised date: 24 November 2014 Accepted date: 28 November 2014 Cite this article as: Yanhui Xiang, Fu Liu, Lixin Xue, Under seawater superoleophobic PVDF membrane inspired by polydopamine for efficient oil/seawater separation, Journal of Membrane Science, http://dx.doi.org/10.1016/j. memsci.2014.11.052 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Under seawater superoleophobic PVDF membrane inspired by polydopamine for efficient oil/seawater separation Yanhui Xiang, Fu Liu*, Lixin Xue* Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbo, 315201, China
*Please address correspondences to: Prof. Fu Liu Tel.: +8657486685256; Fax: +8657486685186 E-mail: [email protected] Prof. Lixin Xue Tel.: +8657486685831; Fax: +8657486685186 E-mail: [email protected]
1
Abstract Under seawater superoleophobic polyvinylidene fluoride (PVDF) membrane inspired by mussel is successfully fabricated for both surfactant-free and surfactant-stabilized oil/seawater separation. The conventional PVDF membrane is modified by a simple solution-immersion method, which was immersed in dopamine aqueous solution for 24 h. The morphology and chemistry of dopamine inspired PVDF membrane was characterized by scanning electron microscope (SEM), atomic force microscopy (AFM), attenuated total reflectance Fourier transform infrared spectra (FTIR-ATR) and X-ray photoelectron spectroscopy (XPS) respectively. The as-prepared membrane obtained stable superoleophobicity under seawater with an oil contact angle of 152±0.3 ° and extremely low oil-adhesion. Separation experiments for both surfactant-free and surfactant-stabilized emulsions showed that our membrane exhibited high oil/seawater separation efficiency (oil concentration in filtrate below 80 ppm) and substantially high permeability, which are several times higher than that current traditional filtration membrane. Fouling resistance to proteins was also investigated for long-term use purpose. This study provides a facile solution-immersion method to fabricate under seawater superoleophobic membrane, highlighting its great potential in practical oil/seawater separation. Keywords: PVDF membrane; dopamine; solution-immersion; under seawater superoleophobicity; oil/seawater separation Highlights:
PVDF membrane showed stable superoleophobicity under seawater. 2
Excellent separation performances for oil/seawater emulsions were obtained.
PVDF membrane showed good fouling resistance and flux recovery.
1. Introduction With increasing environmental awareness and tighter regulations, novel strategies to separate oils from industrial wastewater, polluted oceanic waters, and oil-spill mixtures, especially in the presence of surfactants, are highly desired [1, 2]. Because oil/seawater separation is an interfacial challenge, using special wettability to design novel materials is an effective and facile way [3-6]. Materials which are under oil superhydrophobic have incited broad attention in recent years. They realized filtration or absorption of oils from water selectively and effectively, which are the so called “oil-removing” type of materials, such as reduced graphene oxide coated polyurethane (rGPU) sponges [7], single-walled carbon nanotube network films [8], n-dodecyl mercaptan modified stainless steel mesh [9] and polydimethylsiloxane sponge [10]. However, these “oil-removing” materials are easily fouled or even blocked up by oil droplets or natural organic matters due to their intrinsic oleophilic property and therefore encountered permeability decline. The adhered foulants seriously decrease the separation efficiency after a limited number of times usage [11]. What’s more, adhered or absorbed oils are hard to expel, leading to secondary pollution as well as a waste of both the oils and oleophilic materials [12, 13]. Therefore, it is of great importance to develop novel materials for oil/seawater separation with high separation efficacy and excellent antifouling property. Nowadays, materials which are under 3
seawater superoleophobic play an important role in separating oil/seawater emulsions and show great potential in bio-adhesion [14, 15], microfluidic technology [16, 17], industrial metal cleaning [18] and marine antifouling coating [19-22]. Compared with traditional hydrophobic and oleophilic materials, these novel “water-removing” type materials overcome the easy-fouling and hard-recycling limitations in essence [23]. Previously,
our
group
reported
a
methodology
to
fabricate
superhydrohpilic-superoleophilic PVDF membrane which is a promising candidate for oil/water separation [2]. While the preparation method involved multi-processes such as phase inversion, template peeling process and in-situ crosslinking. Hydrophilic vinyl pyrrolidone together with crosslinking vinyltriethoxysilane is reacted under nitrogen atmosphere. Poly(acrylic acid)-grafted PVDF (PAA-g-PVDF) filtration membrane produced by a salt-induced phase inversion approach for efficient oil/water separation was discovered by Jin [24]. PAA-g-PVDF were first synthesized via 60Co γ-ray source irradiation. A nearly saturated concentration of salt in coagulation bath was played as nucleates to induce the assembly of PAA-g-PVDF micelles, thus the multi-scale structure and superwetting property was obtained. It can be seen that both methods above include multi-processes and chemical reactions, which are not easy to be controlled or scaled up. And also both method above applied pure water to prepare oil/water emulsions and ignored the influence of ions in sea on separation performances. Herein, we report a facile method to fabricate under seawater superoleophobic PVDF membrane inspired by mussel’s fouling resistant surface. Dopamine is a 4
versatile molecule containing catechol and amine functional groups and has received increasing attention due to its unique properties like self-polymerization, anchor capability, special recognition, and so on [25]. A plenty of studies have been reported about polydopamine (PDA) as a versatile platform for functional material applied in oil/water separation field. Xu reported a hydrophilization method via co-deposition of polydopamine and polyethyleneimine (PEI) on polypropylene microfiltration membranes [26].The PDA/PEI coating endows the membranes with ultra-high water permeability, allowing microfiltration separation of oil-in-water emulsions. Latterly, based on “bio-glue” property of dopamine, PDA/PEI deposited polypropylene membrane was induced to polycondensation of silicic acid via a biomineralization process [27]. Water permeation flux and membrane breakthrough pressure were increased. The substrates used in these experiments were commercial polypropylene microfiltration membrane and the separation aim originated from pure water. Different from that, we try to explore the effect of dopamine on PVDF membrane. Besides that, only dopamine is chosen as modifying monomer in our work, and the stability of PDA coating in seawater is measured as well. The as-prepared PVDF membrane can treat both surfactant-free and surfactant-stabilized oil/seawater emulsions with high efficiency (oil concentration in filtrate below 80 ppm). The conventional hydrophobic PVDF membrane as substrate was firstly produced by traditional non-solvent induced phase separation method. Afterwards, the membrane was coated with adhesive polydopamine layer by simple immersion in dopamine trihytdroxy methyl-aminomethane aqueous solution with a pH of 8.5 (adjusted by 5
hydrochloric acid) for a certain period of time. During the solution-immersion process, dopamine is able to undergo oxidant-induced polymerization, nano-sized spherical PDA product is generated in solution, and a tightly adhesive PDA film is formed on membrane surface [28, 29]. Compared with conventional processes for surface modification, including chemical vapor deposition (CVD), additional polymerization of the organic molecule, and electrostatic deposition [30-35], this approach is much more convenient, economical and environmentally friendly. No harsh conditions, sophisticated equipment, or an aggressive etching solution is involved, indicating its great potential in practical applications. Besides, most current studies related to oil/seawater separation are based on pure water, ignoring the influence of salts on the separation in case of oil spills in sea. Herein, we aim to offer a facile method to fabricate PVDF membrane for separating oil from seawater. The morphology, chemistry, wettability, permeability and separation performances are discussed in detail. 2. Experimental section 2.1. Materials PVDF (FR904) was supplied by Shanghai 3F New Material Co., Ltd. China. Dopamine hydrochloride, dichloroethane, trihytdroxy methyl-aminomethane and bovine serum albumin (BSA, for molecular biology) were purchased from Shanghai Aladdin Chemistry Co., Ltd. China. Polyethylene glycol (PEG, Mn = 600 g/mol), hydrochloric acid, chloroform, toluene and triethylphosphate (TEP) were obtained from Sino pharm Chemical Reagent Co., Ltd. China. All the chemicals were used as received. 6
2.2. Fabrication of polydopamine coated PVDF membrane The conventional hydrophobic PVDF membrane was produced by traditional non-solvent induced phase separation method. Detail is as following: 15 g PVDF together with 2 g PEG was dissolved in 100 g TEP at 70
℃ and stirred at a constant
speed for 24 h to achieve a homogenous casting solution. Next the homogenous casting solution was kept still overnight to remove any residual air bubbles and then cast uniformly onto glass plate using a casting knife with a knife gap of 200 µm. The nascent membrane was immediately immersed into an inert non-solvent bath composed of TEP/water mixture (v/v: 5/5) for 5 s, and then moved to deionized water bath to complete total solidification. After immersing in deionized water for 48 h to eliminate solvent, the pristine PVDF membrane was immersed in 2 g/L dopamine tris(hydroxymethl) aminomethane solution (pH = 8.5) for a certain period of time (including 0 h, 6 h, 12 h and 24 h) for self-polymerization. During this time, dopamine occurred for oxidant-induced polymerization to generate massive PDA on PVDF membrane surface. After this reaction, PDA coated PVDF membrane was thoroughly rinsed with deionized water and kept in seawater or dried out in air for further experiment. The membranes were named as M-P0
， M-P6 ， M-P12 and M-P24
respectively related to immersion time. 2.3. Membrane characterization The morphology of the as-prepared membrane was measured by a field-emission scanning electron microscope (S-4800, Hitachi, Japan) with an accelerating voltage of 4.0 kV and 7 µA. An atomic force microscopy (AFM, Veeco Dimension 3100V, US) 7
with tapping mode was also used to further study the surface roughness of the membranes. The through-pore sizes of the as-prepared membranes were determined by a liquid-liquid porometer (LLP-1200A, Porous Materials Inc. US). All the samples were wetted by silwick solution (Porous Materials Inc. US). The silwick is a standard wetting solution whose surface tension is adjusted to 20.1 dyn/cm (20
℃ ). The
functional groups on membrane surface were measured by attenuated total reflectance Fourier transform infrared spectra (ATR-FTIR, Thermo-Nicolet 6700, US). The chemical compositions in the near top surface of the as-prepared membranes were analyzed by an X-ray photoelectron spectroscopy (XPS, Shimadzu Axis Utltradld spectroscope, Japan) with Mg-Kα as radiation resource. The take-off angle of the photoelectron was set at 90 °. The thermal stability and the content of PDA in membrane were evaluated by thermogravimetric analysis (TGA, Mettler-Toledo, Switzerland) at 10 °C /min from 50 °C to 700 °C under a nitrogen flow (20 mL/min). Contact angles of membrane top surface were measured by a contact angle meter (OCA20, Dataphysics, Germany), and at least five measurements were taken at different positions on each sample. The purity of the purified filtrates was analyzed by Karl Fischer Titrator (Mettler Toledo DL31, Switzerland). 2.4. Preparation for oil/seawater emulsions For oil-in-water emulsions, tween80 with a hydrophile lipophile balance (HLB) value of 15 was chosen as emulsifier. Practical preparation method was as following: for surfactant-free toluene-in-water (T/W) emulsion, 1.022 g toluene was added into 80 mL seawater which was homemade by dissolving 3.5 g seawater crystals (mainly 8
composed of Ca2+, Mg2+, SO42-, Cl-, Na+, K+) in 96.5 g deionized water, and the seawater conductivity is 42.3 mS/cm at 15.9 °C; and for surfactant-stabilized T/W emulsion, based on the former emulsion, 0.269 g tween80 was applied. For surfactant-free chloroform-in-water (C/W) emulsion, 1g chloroform was added into 80 mL seawater; and for surfactant-stabilized C/W emulsion, based on the former emulsion, 0.187 g tween80 was applied. For surfactant-free dichloroethane-in-water (DCE/W) emulsion, 1 g dichloroethane was added into 80 mL seawater; and for surfactant-stabilized DCE/W emulsion, based on the former emulsion, 0.199 g tween80 was applied. All these mixtures were stirred for 4 h to obtain emulsions which were stable during separation experiment and no demulsification or precipitation was observed. 2.5. Filtration and antifouling performance assessment The as-prepared PVDF membrane was sealed between one vertical glass tube with an area of 3.14 cm2 and one conical flask. A certain volume (10 mL) of seawater was poured onto the PVDF membrane. The flux measurement was carried out under an extra pressure (-0.09 MPa) generated by a vacuum driven filtration system, and was calculated from the permeation volume as a function of time. The seawater flux Jw1
：
(L/m2 h) was defined as Equation (1)
J w1 =
V A∆t
(1)
where V (L) was the volume of permeated seawater, A (m2) was the membrane area and ∆t (h) was the permeation time. Then 10 mL 1.0 g/L BSA solution was applied to pass through the membrane. After 9
filtration of BSA solution, the membrane was rinsed with ethanol and water to recover the flux, and afterward 10 mL seawater poured onto the cleaned membrane again. The flux JBSA (L/m2 h) and Jw2 (L/m2 h) for BSA solution and second seawater were measured by Equation (1). To analyze the antifouling property in details, several ratios were defined. The flux recovery ratio (FRR) was calculated by Equation (2): FRR =
J w2 × 100% J w1
(2)
A higher value of FRR implies a better antifouling property of membrane. The reversible fouling ratio (DRr), irreversible fouling ratio (DRir) and total fouling ratio (DRt) were described as following: DRr =
J w 2 − J BSA ×100% J w1
(3)
DRir =
J w1 − J w 2 × 100% J w1
(4)
DRt = DRr + DRir
(5)
Where DRr describes the flux decline caused by cake layer formation; DRir indicates the non-recoverable flux decline induced by pore plugging and adsorption or deposition of foulants on membrane surface and pores; DRt shows the total flux decline arises from the reversible fouling and irreversible fouling. 2.6. Oil/seawater emulsions separation experiments The apparatus and operating condition are identical as described above. After fixing sample membrane M-P24, 10 mL freshly prepared emulsion (including surfactant-free and surfactant-stabilized emulsions) was poured onto M-P24. The fluxes for emulsions were determined by Equation (1). For every emulsion, at least five 10
membranes were measured to obtain an average value and the finally obtained filtrate was collected for purity tests.
3. Results and discussion 3.1. Morphology of PVDF membrane The morphology evolution of PVDF membrane modified by dopamine solution-immersion for different time was investigated by SEM, as shown in Fig. 1. The top surface experienced a significant change from porous to compact and dense structure with extending immersion time from 0 to 24 h, while the bottom surface almost kept unchanged. The average through-pore sizes were 42.3 nm, 39.7 nm, 36.6
，
nm, 28.2 nm for M-P0 M-P6, M-P12
，M-P24 respectively. The membrane thickness
increased from ~72 to ~100 µm with extending immersion time from 0 to 24 h, while the whole cross section is still quite uniform without any finger-like or macro-pores due to the delayed phase separation caused by inert non-solvent bath. Furthermore, a plenty of nano-fibers are distributed on the matrix of bicontinuous pores in PVDF membranes. Generally, the self-polymerization of dopamine was mainly happened on membrane top surface, while the bulk of the membrane was not influenced by the coating. Thereafter, the surface roughness was also modified by solution-immersion of polydopamine. AFM results are depicted in Fig. 2, the membrane roughness increased slightly with extending immersion time. For pristine PVDF membrane, the arithmetic mean of the surface roughness (Ra) is 197 nm; due to the formation of PDA agglomerates incorporated inhomogeneous stacking into PVDF surface deposit [36-38], after immersed in dopamine solution for 6, 12 or 24 h, the Ra value increases to 229, 11
233 and 242 nm respectively. 3.2. Chemical composition of PVDF membrane As observed from morphology evolution, the polydopamine coating was anchored on PVDF membrane surface. The persistent interaction between polydopamine and PVDF substrate through oxidant self-polymerization provided the possibility of long term usage even in saline environment. Chemical composition of the pristine and as-prepared membranes were investigated by FTIR-ATR and XPS experiments, as indicated in Fig. 3a and 3b. No characteristic peak for polydopamine was detected for M-P0, M-P6 and M-P12. However, two absorption peaks at 1622 cm-1 and 1502 cm-1 assigned to C=C resonance vibrations in the aromatic rings and N–H bending vibrations were identified in terms of M-P24, implying detectable amount of polydopamine was coated on PVDF membrane surface. The adhesiveness of polydopamine is attributable to amino, imino, hydroxyl and catechol functional groups and π−π interactions [39, 40]. To further evaluate the polydopamine amount in quantity, XPS measurement was carried out. As shown in Fig. 3b, for pristine membrane M-P0, only two peaks at 685 and 283 eV represented for F and C elements respectively are labeled. For other three membranes, signals stand for O and N elements originate from polydopamine are detected, indicating the successful immobilization of polydopamine coating. The amount of polydopamine on membrane top surface was calculated from Table 1, it can be found that the amount of O and N elements ascribed to polydopamine increased quantificationally with increasing immersion time. For M-P24, 37.93 wt.% F and 2.55 wt.% N was detected respectively, and the amount of dopamine on the 12
composite membrane top surface is calculated about 30.4 wt.%. TGA measurements were carried out to calculate the bulk content of PDA in M-P24 as shown in Fig. 3c, and ~8.6 wt.% PDA was measured. This value is much lower than the quantity evaluated by XPS measurement, implying the self-polymerization of dopamine is mainly occurred on PVDF membrane surface. 3.3. Wettability of PVDF membrane top surface The wettability of hydrophobic PVDF membrane was adjusted by the hydrophilic polydopamine coating. As shown in Fig. 4, water contact angle (WCA) of membrane top surface in air decreased from 118±1.5 ° to 53±2.3 ° after modification for 24 h, indicating a transition from hydrophobic membrane to hydrophilic one. In order to apply this modified membrane to separate oil-in-seawater emulsions, the under seawater oil contact angle (OCA) is a critical performance. The initial under seawater OCA was performed in Fig. 5a, 3 µL chloroform droplet was dropped on PVDF membrane top surface under seawater. It can be seen that M-P0, M-P6 and M-P12 exhibited under seawater initial OCA of 114±2 °, 114±1.5 ° and 134±1 ° respectively, while M-P24 demonstrated a under seawater OCA of 152±0.3 °, indicating its unique under seawater superoleophobicity. Variation of under seawater OCA depends on time was latterly carried out to testify the stability of membrane oleophobicity. The result is described in Fig. 5b, implying that stable under seawater oleophobicity can only be obtained for M-P24, which was immersed in dopamine solution for 24 h. For the top surfaces of M-P0 and M-P6, the under seawater OCA decreases to 0 ° within 9 s. It takes about 85 s for oil droplet to permeate into membrane M-P12. During the 13
measuring time, the under seawater OCA for M-P24 top surface keeps still at about 152 °, implying excellent under seawater superoleophobicity. More importantly, the long term stability of superwetting property in saline water was investigated, as shown in Fig. 5c. The under seawater OCA of M-P24 top surface maintains around 152 ° even after being immersed in saline water at room temperature (25
º C ) for 35 days,
indicating its resistance to saline ions corrosion and stable adhesion of polydopamine on PVDF membrane surface. Furthermore, the oil-adhesion property of M-P24 surface was also characterized. As discussed before, the dopamine coated PVDF membrane surface is hydrophilic and rough, which traps water molecules in a Wenzel state and therefore decreases the contact area between oil droplet and PVDF membrane surface. In this case, the oil-adhesion of the surface is extremely low, as performed in Fig. 6. As the needle approaching M-P24 top surface, the oil droplet (3 µL chloroform is taken as detecting oil) is squeezed onto membrane surface, and then the oil droplet is forced to sufficiently contact the membrane surface with an obvious deformation; afterwards, the needle is withdrew, the oil droplet sticks completely to the needle and no trace of oil is detected on the membrane surface. The result indicates that the polydopamine coated PVDF membrane exhibited an extremely low oil-adhesion behavior, indicating good antifouling performance during oil/seawater separation. 3.4. Separation performances of oil/seawater emulsions As mentioned above, M-P24 obtains excellent and stable under seawater superoleophobicity, which is necessary for separation of oil-in-seawater emulsions. To 14
testify this possibility, a series of surfactant-free and surfactant-stabilized oil-in-seawater emulsions were prepared. The oil/seawater separation experiment procedure was performed as shown in Fig. 7a. M-P24 was fixed between one vertical glass tube with an area of 3.14 cm2 and one conical flask, a certain volume (10 mL) of the as-prepared emulsion was poured on the membrane at 0.09 MPa. Continuous water phase permeated massively through the membrane while disperse phase was constrained in the feed side. Compared to the original milky white feed emulsion, the collected filtrates are totally transparent, indicating a high efficiency of M-P24 for separating oil-in-seawater emulsion. The permeability of M-P24 for different emulsions was measured. As shown in Fig. 7b, for surfactant-stabilized emulsion e.g. T/W, C/W and DCE/W, the flux of 1991, 3969 and 2351 L/m2 h was obtained respectively. As comparison, for surfactant-free emulsions, the corresponding flux was 15882, 14037 and 10183 L/m2 h respectively, which is much higher without the interference of surfactant. All these results are several times higher than those traditional filtration membrane [41], whose flux for emulsion with oil concentration of 0.1 g/L is less than 200 L/m2 h at 0.173 MPa. The purity of filtrates was measured by Karl Fischer Titrator and the result is shown in Fig. 7c. For surfactant-stabilized emulsions, the oil content in filtrates is below 80 ppm, and for surfactant-free emulsions, this value is even lower than 40 ppm. This result indicates a high separation efficiency of M-P24 with excellent permeability. The higher oil content obtained for surfactant-stabilized emulsions is probably caused by the dissolved surfactant in the filtrates [24]. 15
3.5. Antifouling properties of PVDF membrane During the separation of oil/seawater process, oils, salts and organic matters will cause unavoidable membrane fouling and hinder wider applications of membrane technology [42, 43]. For this reason, membrane antifouling property which closely related with membrane hydrophilicity and low surface free energy [44-47] is very important for long-term use. The water contact angle on PVDF membrane surface indicated that the hydrophilicity of M-P24 was significantly improved. The constantly high oil contact angle for long time indicated its stable superoleophobicity under seawater and excellent fouling resistance to oils and salts. To evaluate the antifouling property to proteins of M-P24, seawater and 1 g/L BSA solution permeated through membrane alternatively. After BSA solution permeated, the membrane was simply rinsed with ethanol and water to recover the flux. Each solution was measured three times and the variation of flux during the process is revealed in Fig. 8a. It can be seen that the water flux of M-P0 was ~20000 L/m2 h and BSA flux decreased substantially to ~9000 L/m2 h, and membrane cleaning recovered the flux to ~12300 L/m2 h. Although the water flux of M-P24 was ~14000 L/m2 h, lower than that of M-P0, BSA flux exhibited a high value of ~12000 L/m2 h, the further membrane cleaning recovered the flux to ~13600 L/m2 h, which is even higher than that of M-P0. In order to explain this phenomenon, membrane thickness and pore size should be considered. As discussed before, the polydopamine coating anchored on PVDF membrane surface suppressed the surface pore size and increased the membrane thickness, thus enhanced the flowing hindrance through the porous membrane. However, the polydopamine coating also 16
improved the biocompatibility and fouling resistance, therefore increased the BSA solution permeability and recovery as well. Further analysis of membrane antifouling property for M-P0 and M-P24 was carried out and the results are shown in Fig. 8b. The membrane flux recovery ratio (FRR) is 61.8% and 96.1% for M-P0 and M-P24 respectively. The reversible fouling ratio (DRr), irreversible fouling ratio (DRir) and total fouling ratio (DRt) is 14.5%, 38.2% and 52.7% for M-P0. However, after modified by PDA for 24 h, water molecules from the protein solution are capable of adsorbing on the membrane surface preferentially, which weakens the interactions of protein molecules with the membrane and reduces membrane fouling [48, 49]. As a result, these fouling ratios decrease to 12.6%, 3.9% and 16.5% for M-P24. Higher FRR value together with lower DRt value indicates a better antifouling property for M-P24. Decreased value of DRr and DRir for M-P24 showed that the polydopamie coating on PVDF membrane surface enhanced the membrane hydrophilicity and fouling resistance to proteins.
4. Conclusions In
summary,
mussel-inspired
PVDF
membrane
was
prepared
via
solution-immersion for oil/seawater emulsion separation. The hydrophilicity and superoleophobicity under seawater was achieved for M-P24 membrane which immersed in dopamine solution for 24 h. The polydopamine coating increased membrane thickness and minished pore size was revealed by SEM. The as-prepared M-P24 obtained stable superoleophobicity under seawater. And the membrane showed excellent permeability and selectivity, which is significantly higher than that of 17
traditional filtration membranes. Besides, the dopamine coated PVDF membrane demonstrated necessary fouling resistance to proteins and quick flux recovery after membrane cleaning. Our study provided a facile method to produce superwetting PVDF membranes for oil/seawater emulsion separation, and its process feasibility, easy scaling up, stable superoleophobicity under seawater and excellent separation performances show out great potential for separating oil/seawater emulsions from oil spill in sea.
Acknowledgements This work was supported by the National Natural Science Foundation of China (51273211, 51473177), and National 863 Foundation of China (No. 2012AA03A605).
18
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[19] Z. Xue, S. Wang, L. Lin, L. Chen, M. Liu, L. Feng, L. Jiang, A novel superhydrophilic and underwater superoleophobic hydrogel-coated mesh for oil/water separation, Adv. Mater. 23 (2011) 4270-4273. [20] R. Sheparovych, M. Motornov, S. Minko, Low adhesive surfaces that adapt to changing environments, Adv. Mater. 21 (2009) 1840-1844. [21] Y.C. Jung, B. Bhushan, Wetting behavior of water and oil droplets in three-phase interfaces for hydrophobicity/philicity and oleophobicity/philicity, Langmuir 25 (2009) 14165-14173. [22] L.D. Chambers, K.R. Stokes, F.C. Walsh, R.J. Wood, Modern approaches to marine antifouling coatings, Surf. Coat. Technol. 201 (2006) 3642-3652. [23] L. Zhang, Y. Zhong, D. Cha, P. Wang, A self-cleaning underwater superoleophobic mesh for oil-water separation, Sci. Rep. 3 (2013). [24] W. Zhang, Y. Zhu, X. Liu, D. Wang, J. Li, L. Jiang, J. Jin, Salt-induced fabrication of superhydrophilic and underwater superoleophobic PAA-g-PVDF membranes for effective separation of oil-in-water emulsions, Angew. Chem. Int. Ed. 53 (2014) 856-860. [25] H. Yang, Y. Lan, W. Zhu, W. Li, D. Xu, J. Cui, D. Shen, G. Li, Polydopamine-coated nanofibrous mats as a versatile platform for producing porous functional membranes, J. Mater. Chem. 22 (2012) 16994-17001. [26] H.C. Yang, K.J. Liao, H. Huang, Q.-. Wu, L.S. Wan, Z.K. Xu, Mussel-inspired modification of a polymer membrane for ultra-high water permeability and oil-in-water emulsion separation, J. Mater. Chem. 2 (2014) 10225-10230. [27] H.C. Yang, J.K. Pi, K.J. Liao, H. Huang, Q.Y. Wu, X.J. Huang, Z.K. Xu, Silica-decorated polypropylene microfiltration membranes with a mussel-inspired intermediate layer for oil-in-water emulsion separation, ACS Appl. Mater.Interfaces 6 (2014) 12566-12572. [28] K.Y. Ju, Y. Lee, S. Lee, S.B. Park, J.K. Lee, Bioinspired polymerization of dopamine to generate melanin-like nanoparticles having an excellent free-radical-scavenging property, Biomacromolecules 12 (2011) 625-632. [29] H. Lee, S.M. Dellatore, W.M. Miller, P.B. Messersmith, Mussel-inspired surface chemistry for multifunctional coatings, Science 318 (2007) 426-430. [30] C. Ruan, K. Ai, X. Li, L. Lu, A superhydrophobic sponge with excellent absorbency and flame retardancy, Angew. Chem. Int. Ed. 53 (2014) 5556-5560. [31] A. Li, H.X. Sun, D.Z. Tan, W.J. Fan, S.H. Wen, X.J. Qing, G.X. Li, S.Y. Li, W.Q. Deng, Superhydrophobic conjugated microporous polymers for separation and adsorption, Energy Environ. Sci. 4 (2011) 2062-2065. [32] H. Sun, A. Li, Z. Zhu, W. Liang, X. Zhao, P. La, W. Deng, Superhydrophobic activated carbon-coated sponges for separation and absorption, ChemSusChem, 6 (2013) 1057-1062. [33] Q. Zhu, Q. Pan, F. Liu, Facile removal and collection of oils from water surfaces through superhydrophobic and superoleophilic sponges, J. Phys. Chem. C 115 (2011) 17464-17470. [34] Q. Zhu, Y. Chu, Z. Wang, N. Chen, L. Lin, F. Liu, Q. Pan, Robust superhydrophobic polyurethane sponge as a highly reusable oil-absorption material, J. Mater. Chem. A 1 (2013) 5386-5393. [35] P. Calcagnile, D. Fragouli, I.S. Bayer, G.C. Anyfantis, L. Martiradonna, P.D. Cozzoli, R. Cingolani, A. Athanassiou, Magnetically driven floating foams for the removal of oil contaminants from water, ACS Nano 6 (2012) 5413-5419. [36] F. Bernsmann, A. Ponche, C. Ringwald, J. Hemmerlé, J. Raya, B. Bechinger, J.C. Voegel, P. Schaaf, V. Ball, Characterization of dopamine-melanin growth on silicon oxide, J. Phys. Chem. C 113 (2009) 20
8234-8242. [37] D.R. Dreyer, D.J. Miller, B.D. Freeman, D.R. Paul, C.W. Bielawski, Elucidating the structure of poly(dopamine), Langmuir 28 (2012) 6428-6435. [38] Q. Wei, F. Zhang, J. Li, B. Li, C. Zhao, Oxidant-induced dopamine polymerization for multifunctional coatings, Polym. Chem. 1 (2010) 1430-1433. [39] Z. Iqbal, E.P. Lai, T.J. Avis, Antimicrobial effect of polydopamine coating on Escherichia coli, J. Mater. Chem. 22 (2012) 21608-21612. [40] P. Henke, H. Kozak, A. Artemenko, P. Kubát, J. Forstová, J. Mosinger, Superhydrophilic polystyrene nanofiber materials generating O2(1∆g): Postprocessing surface modifications toward efficient antibacterial effect, ACS Appl. Mater. Interfaces 6 (2014) 13007-13014. [41] B. Chakrabarty, A.K. Ghoshal, M.K. Purkait, Ultrafiltration of stable oil-in-water emulsion by polysulfone membrane, J. Membr. Sci. 325 (2008) 427-437. [42] D. Rana, T. Matsuura, Surface modifications for antifouling membranes, Chem. Rev. 110 (2010) 2448-2471. [43] G. M. Geise, H.S. Lee, D.J. Miller, B.D. Freeman, J.E. McGrath, D.R. Paul, Water purification by membranes: The role of polymer science, J. Polym. Sci., Part B: Polym. Phys. 48 (2010) 1685-1718. [44] H. Karimnezhad, L. Rajabi, E. Salehi, A.A. Derakhshan, S. Azimi, Novel nanocomposite Kevlar fabric membranes: Fabrication characterization, and performance in oil/water separation, Appl. Surf. Sci. 293 (2014) 275-286. [45] M. Zhang, C. Wang, S. Wang, J. Li, Fabrication of superhydrophobic cotton textiles for water-oil separation based on drop-coating route, Carbohydr. Polym. 97 (2013) 59-64. [46] M. Abbasi, M. Mirfendereski, M. Nikbakht, M. Golshenas, T. Mohammadi, Performance study of mullite and mullite-alumina ceramic MF membranes for oily wastewaters treatment, Desalination 259 (2010) 169-178. [47] Y. Yang, H. Wang, J. Li, B. He, T. Wang, S. Liao, Novel functionalized nano-TiO2 loading electrocatalytic membrane for oily wastewater treatment, Environ. Sci. Technol. 46 (2012) 6815-6821. [48] B.D. McCloskey, H.B. Park, H. Ju, B.W. Rowe, D.J. Miller, B.D. Freeman, A bioinspired fouling-resistant surface modification for water purification membranes, J. Membr. Sci. 413-414 (2012) 82-90. [49] S. Kasemset, A. Lee, D.J. Miller, B.D. Freeman, M.M. Sharma, Effect of polydopamine deposition conditions on fouling resistance, physical properties, and permeation properties of reverse osmosis membranes in oil/water separation, J. Membr. Sci. 425-426 (2013) 208-216.
21
Figure and Table Captions: Fig. 1. Morphology characterization for M-P0, M-P6
，M-P12 and M-P24 by SEM.
，M-P12 and M-P24 by AFM. Fig. 3. Chemical composition for M-P0, M-P6，M-P12 and M-P24. (a) FTIR-ATR Fig. 2. Surface roughness for M-P0, M-P6
spectra ; (b) A wide scan XPS spectra; (c) TGA measurement for M-P24 and control M-P0. Fig. 4. Static water contact angle in air for top surfaces of M-P0, M-P6, M-P12 and M-P24, insert pictures show the shapes of water droplet on PVDF membrane surface. Fig. 5. Under seawater wettability characterization for top surfaces of M-P0, M-P6, M-P12 and M-P24. (a) Static under seawater oil (3 µL chloroform droplet) contact angle; (b) Variation of under seawater oil contact angle depends on time; (c) Static under seawater oil contact angle of M-P24 for different days. Fig. 6. Photographs of dynamic underwater oil-adhesion measurements on M-P24 top surface, 3 µL chloroform droplet is squeezed against the surface with a needle approaching surface and then allowed to leave. Fig. 7. Separation results for different oil/seawater emulsions. (a) Equipment for practical separation using surfactant-stabilized DCE/W as sample emulsion; (b) M-P24 permeate flux for a series of surfactant-stabilized and surfactant-free emulsions; (c) Oil concentration of corresponding filtrates, insert is the surfactant-stabilized DCE/W emulsion before (left) and after (right) filtration. Fig. 8. Membrane antifouling property for M-P24 and control M-P0. (a) Variation of flux and flux recovery by treating of seawater and BSA solution alternatively, after 22
permeating BSA solution, membrane was washed by ethanol and water, and each solution was measured three times; (b) Flux recovery ratio (FRR), reversible fouling ratio (DRr), irreversible fouling ratio (DRir) and total fouling ratio (DRt) for M-P24 and M-P0. Table 1. Elements mass concentration (%) for M-P0, M-P6 measured by XPS.
23
， M-P12 and M-P24
Fig. 1. Morphology characterization for M M-P0, M-P6
24
，M-P12 and M-P24 P24 by SEM.
Fig. 2. Surface roughness for M M-P0, M-P6
，M-P12 and M-P24 by AFM.
25
26
，
Fig. 3. Chemical composition for M-P0, M-P6 M-P12 and M-P24. P24. (a) FTIR FTIR-ATR spectra ; (b) A wide scan XPS spectra; spectra (c) TGA measurement for M-P24 P24 and control M-P0.
Fig. 4. Static water contact angle in air for top surfaces of M-P0, M-P6, P6, M-P12 M and M-P24, insert pictures show the shapes of water droplet on PVDF membrane surface surface.
27
Fig. 5. Under seawater water wettability characterization for top surfaces of M-P0, P0, M M-P6, M-P12 and M-P24. P24. (a) Static under seawater oil (3 µL chloroform droplet) droplet contact angle; (b) Variation of under seawater oil contact angle depends on time;; (c) Static under seawater water oil contact angle of M M-P24 for different days.
Fig. 6. Photographs of dynamic unde under seawater oil-adhesion adhesion measurements on M-P24 M top surface, 3 µL chloroform droplet is squeezed against the surface with a needle approaching surface and then allowed to leave.
28
Fig. 7. Separation results for different oil/seawater oil/ water emulsions. (a) Equipment for practical separation using surfactant surfactant-stabilized stabilized DCE/W as sample emulsion; (b) M-P24 P24 permeate flux for a series of surfactant-stabilized and surfactant-free free emulsions; (c) Oil concentration of corresponding filtrates, filtrates insert is the surfactant--stabilized DCE/W emulsion before (left) and after (right) filtration.
29
Fig. 8. Membrane antifouling property for M-P24 and control M-P0. (a) Variation of flux and flux recovery by treating of seawater and BSA solution alternatively, after permeating BSA solution, membrane was washed by ethanol and water, and each solution was measured three times; (b) Flux recovery ratio (FRR), reversible fouling ratio (DRr), irreversible fouling ratio (DRir) and total fouling ratio (DRt) for M-P24 and M-P0.
Table 1. Elements mass concentration (%) for M-P0, M-P6
， M-P12 and M-P24
measured by XPS. Sample name
F
C
O
N
M-P0
51.23
48.77
0
0
M-P6
46.97
45.62
6.54
0.87
M-P12
37.94
50.87
9.14
2.05
M-P24
37.93
49.90
9.62
2.55
30
Highlights:
PVDF membrane showed stable superoleophobicity under seawater.
Excellent separation performances for oil/seawater emulsions were obtained.
PVDF membrane showed good fouling resistance and flux recovery.
31
Under seawater superoleophobic PVDF membrane inspired by polydopamine for efficient oil/seawater separation Yanhui Xiang, Fu Liu, Lixin Xue
www.elsevier.com/locate/memsci
PII: DOI: Reference:
S0376-7388(14)00896-5 http://dx.doi.org/10.1016/j.memsci.2014.11.052 MEMSCI13340
To appear in:
Journal of Membrane Science
Received date: 16 October 2014 Revised date: 24 November 2014 Accepted date: 28 November 2014 Cite this article as: Yanhui Xiang, Fu Liu, Lixin Xue, Under seawater superoleophobic PVDF membrane inspired by polydopamine for efficient oil/seawater separation, Journal of Membrane Science, http://dx.doi.org/10.1016/j. memsci.2014.11.052 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Under seawater superoleophobic PVDF membrane inspired by polydopamine for efficient oil/seawater separation Yanhui Xiang, Fu Liu*, Lixin Xue* Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbo, 315201, China
*Please address correspondences to: Prof. Fu Liu Tel.: +8657486685256; Fax: +8657486685186 E-mail: [email protected] Prof. Lixin Xue Tel.: +8657486685831; Fax: +8657486685186 E-mail: [email protected]
1
Abstract Under seawater superoleophobic polyvinylidene fluoride (PVDF) membrane inspired by mussel is successfully fabricated for both surfactant-free and surfactant-stabilized oil/seawater separation. The conventional PVDF membrane is modified by a simple solution-immersion method, which was immersed in dopamine aqueous solution for 24 h. The morphology and chemistry of dopamine inspired PVDF membrane was characterized by scanning electron microscope (SEM), atomic force microscopy (AFM), attenuated total reflectance Fourier transform infrared spectra (FTIR-ATR) and X-ray photoelectron spectroscopy (XPS) respectively. The as-prepared membrane obtained stable superoleophobicity under seawater with an oil contact angle of 152±0.3 ° and extremely low oil-adhesion. Separation experiments for both surfactant-free and surfactant-stabilized emulsions showed that our membrane exhibited high oil/seawater separation efficiency (oil concentration in filtrate below 80 ppm) and substantially high permeability, which are several times higher than that current traditional filtration membrane. Fouling resistance to proteins was also investigated for long-term use purpose. This study provides a facile solution-immersion method to fabricate under seawater superoleophobic membrane, highlighting its great potential in practical oil/seawater separation. Keywords: PVDF membrane; dopamine; solution-immersion; under seawater superoleophobicity; oil/seawater separation Highlights:
PVDF membrane showed stable superoleophobicity under seawater. 2
Excellent separation performances for oil/seawater emulsions were obtained.
PVDF membrane showed good fouling resistance and flux recovery.
1. Introduction With increasing environmental awareness and tighter regulations, novel strategies to separate oils from industrial wastewater, polluted oceanic waters, and oil-spill mixtures, especially in the presence of surfactants, are highly desired [1, 2]. Because oil/seawater separation is an interfacial challenge, using special wettability to design novel materials is an effective and facile way [3-6]. Materials which are under oil superhydrophobic have incited broad attention in recent years. They realized filtration or absorption of oils from water selectively and effectively, which are the so called “oil-removing” type of materials, such as reduced graphene oxide coated polyurethane (rGPU) sponges [7], single-walled carbon nanotube network films [8], n-dodecyl mercaptan modified stainless steel mesh [9] and polydimethylsiloxane sponge [10]. However, these “oil-removing” materials are easily fouled or even blocked up by oil droplets or natural organic matters due to their intrinsic oleophilic property and therefore encountered permeability decline. The adhered foulants seriously decrease the separation efficiency after a limited number of times usage [11]. What’s more, adhered or absorbed oils are hard to expel, leading to secondary pollution as well as a waste of both the oils and oleophilic materials [12, 13]. Therefore, it is of great importance to develop novel materials for oil/seawater separation with high separation efficacy and excellent antifouling property. Nowadays, materials which are under 3
seawater superoleophobic play an important role in separating oil/seawater emulsions and show great potential in bio-adhesion [14, 15], microfluidic technology [16, 17], industrial metal cleaning [18] and marine antifouling coating [19-22]. Compared with traditional hydrophobic and oleophilic materials, these novel “water-removing” type materials overcome the easy-fouling and hard-recycling limitations in essence [23]. Previously,
our
group
reported
a
methodology
to
fabricate
superhydrohpilic-superoleophilic PVDF membrane which is a promising candidate for oil/water separation [2]. While the preparation method involved multi-processes such as phase inversion, template peeling process and in-situ crosslinking. Hydrophilic vinyl pyrrolidone together with crosslinking vinyltriethoxysilane is reacted under nitrogen atmosphere. Poly(acrylic acid)-grafted PVDF (PAA-g-PVDF) filtration membrane produced by a salt-induced phase inversion approach for efficient oil/water separation was discovered by Jin [24]. PAA-g-PVDF were first synthesized via 60Co γ-ray source irradiation. A nearly saturated concentration of salt in coagulation bath was played as nucleates to induce the assembly of PAA-g-PVDF micelles, thus the multi-scale structure and superwetting property was obtained. It can be seen that both methods above include multi-processes and chemical reactions, which are not easy to be controlled or scaled up. And also both method above applied pure water to prepare oil/water emulsions and ignored the influence of ions in sea on separation performances. Herein, we report a facile method to fabricate under seawater superoleophobic PVDF membrane inspired by mussel’s fouling resistant surface. Dopamine is a 4
versatile molecule containing catechol and amine functional groups and has received increasing attention due to its unique properties like self-polymerization, anchor capability, special recognition, and so on [25]. A plenty of studies have been reported about polydopamine (PDA) as a versatile platform for functional material applied in oil/water separation field. Xu reported a hydrophilization method via co-deposition of polydopamine and polyethyleneimine (PEI) on polypropylene microfiltration membranes [26].The PDA/PEI coating endows the membranes with ultra-high water permeability, allowing microfiltration separation of oil-in-water emulsions. Latterly, based on “bio-glue” property of dopamine, PDA/PEI deposited polypropylene membrane was induced to polycondensation of silicic acid via a biomineralization process [27]. Water permeation flux and membrane breakthrough pressure were increased. The substrates used in these experiments were commercial polypropylene microfiltration membrane and the separation aim originated from pure water. Different from that, we try to explore the effect of dopamine on PVDF membrane. Besides that, only dopamine is chosen as modifying monomer in our work, and the stability of PDA coating in seawater is measured as well. The as-prepared PVDF membrane can treat both surfactant-free and surfactant-stabilized oil/seawater emulsions with high efficiency (oil concentration in filtrate below 80 ppm). The conventional hydrophobic PVDF membrane as substrate was firstly produced by traditional non-solvent induced phase separation method. Afterwards, the membrane was coated with adhesive polydopamine layer by simple immersion in dopamine trihytdroxy methyl-aminomethane aqueous solution with a pH of 8.5 (adjusted by 5
hydrochloric acid) for a certain period of time. During the solution-immersion process, dopamine is able to undergo oxidant-induced polymerization, nano-sized spherical PDA product is generated in solution, and a tightly adhesive PDA film is formed on membrane surface [28, 29]. Compared with conventional processes for surface modification, including chemical vapor deposition (CVD), additional polymerization of the organic molecule, and electrostatic deposition [30-35], this approach is much more convenient, economical and environmentally friendly. No harsh conditions, sophisticated equipment, or an aggressive etching solution is involved, indicating its great potential in practical applications. Besides, most current studies related to oil/seawater separation are based on pure water, ignoring the influence of salts on the separation in case of oil spills in sea. Herein, we aim to offer a facile method to fabricate PVDF membrane for separating oil from seawater. The morphology, chemistry, wettability, permeability and separation performances are discussed in detail. 2. Experimental section 2.1. Materials PVDF (FR904) was supplied by Shanghai 3F New Material Co., Ltd. China. Dopamine hydrochloride, dichloroethane, trihytdroxy methyl-aminomethane and bovine serum albumin (BSA, for molecular biology) were purchased from Shanghai Aladdin Chemistry Co., Ltd. China. Polyethylene glycol (PEG, Mn = 600 g/mol), hydrochloric acid, chloroform, toluene and triethylphosphate (TEP) were obtained from Sino pharm Chemical Reagent Co., Ltd. China. All the chemicals were used as received. 6
2.2. Fabrication of polydopamine coated PVDF membrane The conventional hydrophobic PVDF membrane was produced by traditional non-solvent induced phase separation method. Detail is as following: 15 g PVDF together with 2 g PEG was dissolved in 100 g TEP at 70
℃ and stirred at a constant
speed for 24 h to achieve a homogenous casting solution. Next the homogenous casting solution was kept still overnight to remove any residual air bubbles and then cast uniformly onto glass plate using a casting knife with a knife gap of 200 µm. The nascent membrane was immediately immersed into an inert non-solvent bath composed of TEP/water mixture (v/v: 5/5) for 5 s, and then moved to deionized water bath to complete total solidification. After immersing in deionized water for 48 h to eliminate solvent, the pristine PVDF membrane was immersed in 2 g/L dopamine tris(hydroxymethl) aminomethane solution (pH = 8.5) for a certain period of time (including 0 h, 6 h, 12 h and 24 h) for self-polymerization. During this time, dopamine occurred for oxidant-induced polymerization to generate massive PDA on PVDF membrane surface. After this reaction, PDA coated PVDF membrane was thoroughly rinsed with deionized water and kept in seawater or dried out in air for further experiment. The membranes were named as M-P0
， M-P6 ， M-P12 and M-P24
respectively related to immersion time. 2.3. Membrane characterization The morphology of the as-prepared membrane was measured by a field-emission scanning electron microscope (S-4800, Hitachi, Japan) with an accelerating voltage of 4.0 kV and 7 µA. An atomic force microscopy (AFM, Veeco Dimension 3100V, US) 7
with tapping mode was also used to further study the surface roughness of the membranes. The through-pore sizes of the as-prepared membranes were determined by a liquid-liquid porometer (LLP-1200A, Porous Materials Inc. US). All the samples were wetted by silwick solution (Porous Materials Inc. US). The silwick is a standard wetting solution whose surface tension is adjusted to 20.1 dyn/cm (20
℃ ). The
functional groups on membrane surface were measured by attenuated total reflectance Fourier transform infrared spectra (ATR-FTIR, Thermo-Nicolet 6700, US). The chemical compositions in the near top surface of the as-prepared membranes were analyzed by an X-ray photoelectron spectroscopy (XPS, Shimadzu Axis Utltradld spectroscope, Japan) with Mg-Kα as radiation resource. The take-off angle of the photoelectron was set at 90 °. The thermal stability and the content of PDA in membrane were evaluated by thermogravimetric analysis (TGA, Mettler-Toledo, Switzerland) at 10 °C /min from 50 °C to 700 °C under a nitrogen flow (20 mL/min). Contact angles of membrane top surface were measured by a contact angle meter (OCA20, Dataphysics, Germany), and at least five measurements were taken at different positions on each sample. The purity of the purified filtrates was analyzed by Karl Fischer Titrator (Mettler Toledo DL31, Switzerland). 2.4. Preparation for oil/seawater emulsions For oil-in-water emulsions, tween80 with a hydrophile lipophile balance (HLB) value of 15 was chosen as emulsifier. Practical preparation method was as following: for surfactant-free toluene-in-water (T/W) emulsion, 1.022 g toluene was added into 80 mL seawater which was homemade by dissolving 3.5 g seawater crystals (mainly 8
composed of Ca2+, Mg2+, SO42-, Cl-, Na+, K+) in 96.5 g deionized water, and the seawater conductivity is 42.3 mS/cm at 15.9 °C; and for surfactant-stabilized T/W emulsion, based on the former emulsion, 0.269 g tween80 was applied. For surfactant-free chloroform-in-water (C/W) emulsion, 1g chloroform was added into 80 mL seawater; and for surfactant-stabilized C/W emulsion, based on the former emulsion, 0.187 g tween80 was applied. For surfactant-free dichloroethane-in-water (DCE/W) emulsion, 1 g dichloroethane was added into 80 mL seawater; and for surfactant-stabilized DCE/W emulsion, based on the former emulsion, 0.199 g tween80 was applied. All these mixtures were stirred for 4 h to obtain emulsions which were stable during separation experiment and no demulsification or precipitation was observed. 2.5. Filtration and antifouling performance assessment The as-prepared PVDF membrane was sealed between one vertical glass tube with an area of 3.14 cm2 and one conical flask. A certain volume (10 mL) of seawater was poured onto the PVDF membrane. The flux measurement was carried out under an extra pressure (-0.09 MPa) generated by a vacuum driven filtration system, and was calculated from the permeation volume as a function of time. The seawater flux Jw1
：
(L/m2 h) was defined as Equation (1)
J w1 =
V A∆t
(1)
where V (L) was the volume of permeated seawater, A (m2) was the membrane area and ∆t (h) was the permeation time. Then 10 mL 1.0 g/L BSA solution was applied to pass through the membrane. After 9
filtration of BSA solution, the membrane was rinsed with ethanol and water to recover the flux, and afterward 10 mL seawater poured onto the cleaned membrane again. The flux JBSA (L/m2 h) and Jw2 (L/m2 h) for BSA solution and second seawater were measured by Equation (1). To analyze the antifouling property in details, several ratios were defined. The flux recovery ratio (FRR) was calculated by Equation (2): FRR =
J w2 × 100% J w1
(2)
A higher value of FRR implies a better antifouling property of membrane. The reversible fouling ratio (DRr), irreversible fouling ratio (DRir) and total fouling ratio (DRt) were described as following: DRr =
J w 2 − J BSA ×100% J w1
(3)
DRir =
J w1 − J w 2 × 100% J w1
(4)
DRt = DRr + DRir
(5)
Where DRr describes the flux decline caused by cake layer formation; DRir indicates the non-recoverable flux decline induced by pore plugging and adsorption or deposition of foulants on membrane surface and pores; DRt shows the total flux decline arises from the reversible fouling and irreversible fouling. 2.6. Oil/seawater emulsions separation experiments The apparatus and operating condition are identical as described above. After fixing sample membrane M-P24, 10 mL freshly prepared emulsion (including surfactant-free and surfactant-stabilized emulsions) was poured onto M-P24. The fluxes for emulsions were determined by Equation (1). For every emulsion, at least five 10
membranes were measured to obtain an average value and the finally obtained filtrate was collected for purity tests.
3. Results and discussion 3.1. Morphology of PVDF membrane The morphology evolution of PVDF membrane modified by dopamine solution-immersion for different time was investigated by SEM, as shown in Fig. 1. The top surface experienced a significant change from porous to compact and dense structure with extending immersion time from 0 to 24 h, while the bottom surface almost kept unchanged. The average through-pore sizes were 42.3 nm, 39.7 nm, 36.6
，
nm, 28.2 nm for M-P0 M-P6, M-P12
，M-P24 respectively. The membrane thickness
increased from ~72 to ~100 µm with extending immersion time from 0 to 24 h, while the whole cross section is still quite uniform without any finger-like or macro-pores due to the delayed phase separation caused by inert non-solvent bath. Furthermore, a plenty of nano-fibers are distributed on the matrix of bicontinuous pores in PVDF membranes. Generally, the self-polymerization of dopamine was mainly happened on membrane top surface, while the bulk of the membrane was not influenced by the coating. Thereafter, the surface roughness was also modified by solution-immersion of polydopamine. AFM results are depicted in Fig. 2, the membrane roughness increased slightly with extending immersion time. For pristine PVDF membrane, the arithmetic mean of the surface roughness (Ra) is 197 nm; due to the formation of PDA agglomerates incorporated inhomogeneous stacking into PVDF surface deposit [36-38], after immersed in dopamine solution for 6, 12 or 24 h, the Ra value increases to 229, 11
233 and 242 nm respectively. 3.2. Chemical composition of PVDF membrane As observed from morphology evolution, the polydopamine coating was anchored on PVDF membrane surface. The persistent interaction between polydopamine and PVDF substrate through oxidant self-polymerization provided the possibility of long term usage even in saline environment. Chemical composition of the pristine and as-prepared membranes were investigated by FTIR-ATR and XPS experiments, as indicated in Fig. 3a and 3b. No characteristic peak for polydopamine was detected for M-P0, M-P6 and M-P12. However, two absorption peaks at 1622 cm-1 and 1502 cm-1 assigned to C=C resonance vibrations in the aromatic rings and N–H bending vibrations were identified in terms of M-P24, implying detectable amount of polydopamine was coated on PVDF membrane surface. The adhesiveness of polydopamine is attributable to amino, imino, hydroxyl and catechol functional groups and π−π interactions [39, 40]. To further evaluate the polydopamine amount in quantity, XPS measurement was carried out. As shown in Fig. 3b, for pristine membrane M-P0, only two peaks at 685 and 283 eV represented for F and C elements respectively are labeled. For other three membranes, signals stand for O and N elements originate from polydopamine are detected, indicating the successful immobilization of polydopamine coating. The amount of polydopamine on membrane top surface was calculated from Table 1, it can be found that the amount of O and N elements ascribed to polydopamine increased quantificationally with increasing immersion time. For M-P24, 37.93 wt.% F and 2.55 wt.% N was detected respectively, and the amount of dopamine on the 12
composite membrane top surface is calculated about 30.4 wt.%. TGA measurements were carried out to calculate the bulk content of PDA in M-P24 as shown in Fig. 3c, and ~8.6 wt.% PDA was measured. This value is much lower than the quantity evaluated by XPS measurement, implying the self-polymerization of dopamine is mainly occurred on PVDF membrane surface. 3.3. Wettability of PVDF membrane top surface The wettability of hydrophobic PVDF membrane was adjusted by the hydrophilic polydopamine coating. As shown in Fig. 4, water contact angle (WCA) of membrane top surface in air decreased from 118±1.5 ° to 53±2.3 ° after modification for 24 h, indicating a transition from hydrophobic membrane to hydrophilic one. In order to apply this modified membrane to separate oil-in-seawater emulsions, the under seawater oil contact angle (OCA) is a critical performance. The initial under seawater OCA was performed in Fig. 5a, 3 µL chloroform droplet was dropped on PVDF membrane top surface under seawater. It can be seen that M-P0, M-P6 and M-P12 exhibited under seawater initial OCA of 114±2 °, 114±1.5 ° and 134±1 ° respectively, while M-P24 demonstrated a under seawater OCA of 152±0.3 °, indicating its unique under seawater superoleophobicity. Variation of under seawater OCA depends on time was latterly carried out to testify the stability of membrane oleophobicity. The result is described in Fig. 5b, implying that stable under seawater oleophobicity can only be obtained for M-P24, which was immersed in dopamine solution for 24 h. For the top surfaces of M-P0 and M-P6, the under seawater OCA decreases to 0 ° within 9 s. It takes about 85 s for oil droplet to permeate into membrane M-P12. During the 13
measuring time, the under seawater OCA for M-P24 top surface keeps still at about 152 °, implying excellent under seawater superoleophobicity. More importantly, the long term stability of superwetting property in saline water was investigated, as shown in Fig. 5c. The under seawater OCA of M-P24 top surface maintains around 152 ° even after being immersed in saline water at room temperature (25
º C ) for 35 days,
indicating its resistance to saline ions corrosion and stable adhesion of polydopamine on PVDF membrane surface. Furthermore, the oil-adhesion property of M-P24 surface was also characterized. As discussed before, the dopamine coated PVDF membrane surface is hydrophilic and rough, which traps water molecules in a Wenzel state and therefore decreases the contact area between oil droplet and PVDF membrane surface. In this case, the oil-adhesion of the surface is extremely low, as performed in Fig. 6. As the needle approaching M-P24 top surface, the oil droplet (3 µL chloroform is taken as detecting oil) is squeezed onto membrane surface, and then the oil droplet is forced to sufficiently contact the membrane surface with an obvious deformation; afterwards, the needle is withdrew, the oil droplet sticks completely to the needle and no trace of oil is detected on the membrane surface. The result indicates that the polydopamine coated PVDF membrane exhibited an extremely low oil-adhesion behavior, indicating good antifouling performance during oil/seawater separation. 3.4. Separation performances of oil/seawater emulsions As mentioned above, M-P24 obtains excellent and stable under seawater superoleophobicity, which is necessary for separation of oil-in-seawater emulsions. To 14
testify this possibility, a series of surfactant-free and surfactant-stabilized oil-in-seawater emulsions were prepared. The oil/seawater separation experiment procedure was performed as shown in Fig. 7a. M-P24 was fixed between one vertical glass tube with an area of 3.14 cm2 and one conical flask, a certain volume (10 mL) of the as-prepared emulsion was poured on the membrane at 0.09 MPa. Continuous water phase permeated massively through the membrane while disperse phase was constrained in the feed side. Compared to the original milky white feed emulsion, the collected filtrates are totally transparent, indicating a high efficiency of M-P24 for separating oil-in-seawater emulsion. The permeability of M-P24 for different emulsions was measured. As shown in Fig. 7b, for surfactant-stabilized emulsion e.g. T/W, C/W and DCE/W, the flux of 1991, 3969 and 2351 L/m2 h was obtained respectively. As comparison, for surfactant-free emulsions, the corresponding flux was 15882, 14037 and 10183 L/m2 h respectively, which is much higher without the interference of surfactant. All these results are several times higher than those traditional filtration membrane [41], whose flux for emulsion with oil concentration of 0.1 g/L is less than 200 L/m2 h at 0.173 MPa. The purity of filtrates was measured by Karl Fischer Titrator and the result is shown in Fig. 7c. For surfactant-stabilized emulsions, the oil content in filtrates is below 80 ppm, and for surfactant-free emulsions, this value is even lower than 40 ppm. This result indicates a high separation efficiency of M-P24 with excellent permeability. The higher oil content obtained for surfactant-stabilized emulsions is probably caused by the dissolved surfactant in the filtrates [24]. 15
3.5. Antifouling properties of PVDF membrane During the separation of oil/seawater process, oils, salts and organic matters will cause unavoidable membrane fouling and hinder wider applications of membrane technology [42, 43]. For this reason, membrane antifouling property which closely related with membrane hydrophilicity and low surface free energy [44-47] is very important for long-term use. The water contact angle on PVDF membrane surface indicated that the hydrophilicity of M-P24 was significantly improved. The constantly high oil contact angle for long time indicated its stable superoleophobicity under seawater and excellent fouling resistance to oils and salts. To evaluate the antifouling property to proteins of M-P24, seawater and 1 g/L BSA solution permeated through membrane alternatively. After BSA solution permeated, the membrane was simply rinsed with ethanol and water to recover the flux. Each solution was measured three times and the variation of flux during the process is revealed in Fig. 8a. It can be seen that the water flux of M-P0 was ~20000 L/m2 h and BSA flux decreased substantially to ~9000 L/m2 h, and membrane cleaning recovered the flux to ~12300 L/m2 h. Although the water flux of M-P24 was ~14000 L/m2 h, lower than that of M-P0, BSA flux exhibited a high value of ~12000 L/m2 h, the further membrane cleaning recovered the flux to ~13600 L/m2 h, which is even higher than that of M-P0. In order to explain this phenomenon, membrane thickness and pore size should be considered. As discussed before, the polydopamine coating anchored on PVDF membrane surface suppressed the surface pore size and increased the membrane thickness, thus enhanced the flowing hindrance through the porous membrane. However, the polydopamine coating also 16
improved the biocompatibility and fouling resistance, therefore increased the BSA solution permeability and recovery as well. Further analysis of membrane antifouling property for M-P0 and M-P24 was carried out and the results are shown in Fig. 8b. The membrane flux recovery ratio (FRR) is 61.8% and 96.1% for M-P0 and M-P24 respectively. The reversible fouling ratio (DRr), irreversible fouling ratio (DRir) and total fouling ratio (DRt) is 14.5%, 38.2% and 52.7% for M-P0. However, after modified by PDA for 24 h, water molecules from the protein solution are capable of adsorbing on the membrane surface preferentially, which weakens the interactions of protein molecules with the membrane and reduces membrane fouling [48, 49]. As a result, these fouling ratios decrease to 12.6%, 3.9% and 16.5% for M-P24. Higher FRR value together with lower DRt value indicates a better antifouling property for M-P24. Decreased value of DRr and DRir for M-P24 showed that the polydopamie coating on PVDF membrane surface enhanced the membrane hydrophilicity and fouling resistance to proteins.
4. Conclusions In
summary,
mussel-inspired
PVDF
membrane
was
prepared
via
solution-immersion for oil/seawater emulsion separation. The hydrophilicity and superoleophobicity under seawater was achieved for M-P24 membrane which immersed in dopamine solution for 24 h. The polydopamine coating increased membrane thickness and minished pore size was revealed by SEM. The as-prepared M-P24 obtained stable superoleophobicity under seawater. And the membrane showed excellent permeability and selectivity, which is significantly higher than that of 17
traditional filtration membranes. Besides, the dopamine coated PVDF membrane demonstrated necessary fouling resistance to proteins and quick flux recovery after membrane cleaning. Our study provided a facile method to produce superwetting PVDF membranes for oil/seawater emulsion separation, and its process feasibility, easy scaling up, stable superoleophobicity under seawater and excellent separation performances show out great potential for separating oil/seawater emulsions from oil spill in sea.
Acknowledgements This work was supported by the National Natural Science Foundation of China (51273211, 51473177), and National 863 Foundation of China (No. 2012AA03A605).
18
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21
Figure and Table Captions: Fig. 1. Morphology characterization for M-P0, M-P6
，M-P12 and M-P24 by SEM.
，M-P12 and M-P24 by AFM. Fig. 3. Chemical composition for M-P0, M-P6，M-P12 and M-P24. (a) FTIR-ATR Fig. 2. Surface roughness for M-P0, M-P6
spectra ; (b) A wide scan XPS spectra; (c) TGA measurement for M-P24 and control M-P0. Fig. 4. Static water contact angle in air for top surfaces of M-P0, M-P6, M-P12 and M-P24, insert pictures show the shapes of water droplet on PVDF membrane surface. Fig. 5. Under seawater wettability characterization for top surfaces of M-P0, M-P6, M-P12 and M-P24. (a) Static under seawater oil (3 µL chloroform droplet) contact angle; (b) Variation of under seawater oil contact angle depends on time; (c) Static under seawater oil contact angle of M-P24 for different days. Fig. 6. Photographs of dynamic underwater oil-adhesion measurements on M-P24 top surface, 3 µL chloroform droplet is squeezed against the surface with a needle approaching surface and then allowed to leave. Fig. 7. Separation results for different oil/seawater emulsions. (a) Equipment for practical separation using surfactant-stabilized DCE/W as sample emulsion; (b) M-P24 permeate flux for a series of surfactant-stabilized and surfactant-free emulsions; (c) Oil concentration of corresponding filtrates, insert is the surfactant-stabilized DCE/W emulsion before (left) and after (right) filtration. Fig. 8. Membrane antifouling property for M-P24 and control M-P0. (a) Variation of flux and flux recovery by treating of seawater and BSA solution alternatively, after 22
permeating BSA solution, membrane was washed by ethanol and water, and each solution was measured three times; (b) Flux recovery ratio (FRR), reversible fouling ratio (DRr), irreversible fouling ratio (DRir) and total fouling ratio (DRt) for M-P24 and M-P0. Table 1. Elements mass concentration (%) for M-P0, M-P6 measured by XPS.
23
， M-P12 and M-P24
Fig. 1. Morphology characterization for M M-P0, M-P6
24
，M-P12 and M-P24 P24 by SEM.
Fig. 2. Surface roughness for M M-P0, M-P6
，M-P12 and M-P24 by AFM.
25
26
，
Fig. 3. Chemical composition for M-P0, M-P6 M-P12 and M-P24. P24. (a) FTIR FTIR-ATR spectra ; (b) A wide scan XPS spectra; spectra (c) TGA measurement for M-P24 P24 and control M-P0.
Fig. 4. Static water contact angle in air for top surfaces of M-P0, M-P6, P6, M-P12 M and M-P24, insert pictures show the shapes of water droplet on PVDF membrane surface surface.
27
Fig. 5. Under seawater water wettability characterization for top surfaces of M-P0, P0, M M-P6, M-P12 and M-P24. P24. (a) Static under seawater oil (3 µL chloroform droplet) droplet contact angle; (b) Variation of under seawater oil contact angle depends on time;; (c) Static under seawater water oil contact angle of M M-P24 for different days.
Fig. 6. Photographs of dynamic unde under seawater oil-adhesion adhesion measurements on M-P24 M top surface, 3 µL chloroform droplet is squeezed against the surface with a needle approaching surface and then allowed to leave.
28
Fig. 7. Separation results for different oil/seawater oil/ water emulsions. (a) Equipment for practical separation using surfactant surfactant-stabilized stabilized DCE/W as sample emulsion; (b) M-P24 P24 permeate flux for a series of surfactant-stabilized and surfactant-free free emulsions; (c) Oil concentration of corresponding filtrates, filtrates insert is the surfactant--stabilized DCE/W emulsion before (left) and after (right) filtration.
29
Fig. 8. Membrane antifouling property for M-P24 and control M-P0. (a) Variation of flux and flux recovery by treating of seawater and BSA solution alternatively, after permeating BSA solution, membrane was washed by ethanol and water, and each solution was measured three times; (b) Flux recovery ratio (FRR), reversible fouling ratio (DRr), irreversible fouling ratio (DRir) and total fouling ratio (DRt) for M-P24 and M-P0.
Table 1. Elements mass concentration (%) for M-P0, M-P6
， M-P12 and M-P24
measured by XPS. Sample name
F
C
O
N
M-P0
51.23
48.77
0
0
M-P6
46.97
45.62
6.54
0.87
M-P12
37.94
50.87
9.14
2.05
M-P24
37.93
49.90
9.62
2.55
30
Highlights:
PVDF membrane showed stable superoleophobicity under seawater.
Excellent separation performances for oil/seawater emulsions were obtained.
PVDF membrane showed good fouling resistance and flux recovery.
31