Multiple antifouling capacities of hybrid membranes derived from multifunctional titania nanoparticles

Journal of Membrane Science 495 (2015) 226–234

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Multiple antifouling capacities of hybrid membranes derived from multifunctional titania nanoparticles Xueting Zhao a,b, Yanlei Su a,b, Yanan Liu a,b, Runnan Zhang a,b, Zhongyi Jiang a,b,n a Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, No. 92, Weijin Road, Nankai District, Tianjin 300072, China b Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China

art ic l e i nf o

a b s t r a c t

Article history: Received 16 June 2015 Received in revised form 10 August 2015 Accepted 11 August 2015 Available online 14 August 2015

Hybridization has evolved a powerful toolbox for hierarchical structure manipulation of antifouling membrane surface. In this study, zwitterionic and fluorine-containing moieties are immobilized onto the surface of TiO2 nanoparticles through the mediation of polydopamine, and the resultant nanoparticles are incorporated into poly(vinylidene fluoride) (PVDF) matrix to simultaneously manipulate the chemical and topological structures of membrane surfaces. The hierarchical topographies of the hybrid membranes arise from the morphology of nanoparticles. The surface heterogeneity of the hybrid membranes arises from the different chemical moieties on nanoparticles, endowing the membrane surfaces with both hydrophilic zwitterionic segments and low surface energy fluorine-containing segments. Due to the favorable hierarchical chemical and physical structures, the hybrid membranes display a remarkable enhancement in oil-fouling-resistant and oil-fouling-release capacities during oil-in-water emulsion filtration: the flux declines at the minimum level of 16.7% and recovers to the maximum level about 100%. It can be anticipated that the present study will offer a physico-chemical coordinated antifouling mechanisms to control membrane oil-fouling for the efficient oil-containing wastewater treatment. & 2015 Elsevier B.V. All rights reserved.

Keywords: Hybrid membranes Multifunctional inorganic nanoparticles Surface heterogeneity Hierarchical topographies Multiple anti-oil-fouling mechanisms

1 Introduction Global water crisis poses growing challenge to sustainable development, especially for developing countries. For the last five years, the total volume of wastewater discharge in China is calculated as more than 60 billion ton/year, which far exceeds environmental capacity. Membrane technology is regarded as the most efficient approach for advanced wastewater treatment and reuse in refinery and petrochemical industries. However, most of membranes suffer from inevitable fouling in the implementation [1–3]. Membrane fouling is caused by complicated interactions (hydrogen-bond, electrostatic, hydrophobic, etc. [4–6]) between foulants and membranes, resulting in extra cost and energy requirements. For effective wastewater treatment, antifouling membranes that are capable of preventing or minimizing the direct interactions between foulants and membranes are becoming imperative requirement. The rational design of high-performance antifouling membranes n Corresponding author at: Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, No. 92, Weijin Road, Nankai District, Tianjin 300072, China. Fax: þ86 22 27406646. E-mail address: [email protected] (Z. Jiang).

http://dx.doi.org/10.1016/j.memsci.2015.08.026 0376-7388/& 2015 Elsevier B.V. All rights reserved.

relies heavily on the innovation in reliable fabrication methods and efficient antifouling mechanisms. To date, hybridization approach has attracted remarkable attention for the construction of antifouling membrane surfaces. For example, hydrophilic inorganic nanoparticles (NPs) and carbon-based nanomaterials have been incorporated in membrane modification to improve surface hydrophilicity and antifouling capacities [7–11]. More importantly, inorganic nanomaterials have also been used as robust carriers to immobilize a variety of antifouling moieties on membrane surfaces. Neutral and hydrophilic brushes, such as poly(2-hydroxyethyl methacrylate) [12–14], poly (ethylene glycol) [15,16], poly(zwitterionic methacrylate) [17–20], poly(1-vinylpyrrolidone) [21] and polyamines [22,23], have been grafted on inorganic nanomaterials and successfully anchored on the surfaces of hybrid membranes. These hydrophilic brushes facilitate the surface hydration of hybrid membranes via hydrogen-bond or electrostatic interactions, which endows hybrid membranes with outstanding antifouling performance. The efficient antifouling mechanisms are considered as the critical issues for the construction of antifouling membranes. In general, antifouling mechanisms have been catalogued as foulingresistant mechanism and fouling-release mechanism. The core of fouling-resistant mechanism is to generate compact hydration layer barrier and robust steric repulsion effects between foulants and surfaces so that the attachment of foulants onto hydrophilic

X. Zhao et al. / Journal of Membrane Science 495 (2015) 226–234

surfaces can be prevented [24,25]. The core of fouling-release mechanism is to weaken or minimize the interfacial polar or hydrogen bonding interactions between foulants and low surface energy surfaces so that the attached foulants can be more readily removed by hydraulic shear forces [26–28]. Recently, the pursuit of robust antifouling capacities triggers the transformation of antifouling mechanisms from single mode to multiple mode. Pioneering works of Wooley et al. [29,30] and Ober et al. [31] reported the superior antifouling capacities of marine coating with surface heterogeneity by coordinating the fouling-resistant mechanism of poly(ethylene glycol) chains and the fouling-release mechanism of fluorinated chains. Advances in antifouling polymeric membranes with tunable fouling-release and fouling-resistant attributes contribute to increased flux recovery and decreased flux decline during membrane filtration [32–35]. However, the effective routes to engineer the multiple antifouling mechanisms of membranes are still limited. Thanks to the multiple functionalities and multiscale structures of hybrid membranes, it can be envisioned that hybridization manipulation strategy can provide a competitive approach to confer multiple antifouling capacities to membranes for enhanced separation performance. Oil is usually consider as one of the major foulants in membrane processes for industrial oil-containing wastewater treatment. The low surface tension of oil foulants makes oil-fouling more complicated. Although previously reported hybrid membranes have shown superior anti-oil-fouling capacities by coconstructing the oil-fouling-resistant mechanism of TiO2 NPs and the oil-fouling-release mechanism of fluorinated group [36,37], the topological structure was rarely taken into account. Consequently, the synergistic effects of surface chemical and topological structures on the anti-oil-fouling capacities of hybrid membranes are essential and should also be investigated more extensively. In this study, polydopamine-coated TiO2 NPs (TiO2@PDA NPs) are synthesized by one-step deposition and self-polymerization of dopamine, which serve as the carrier to anchor both zwitterionic and fluorine-containing antifouling moieties. The modified TiO2@PDA NPs are incorporated into PVDF membrane matrix to endow membrane surfaces with both surface heterogeneity and hierarchical topographies. The topographies of membranes are evaluated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The heterogeneity of the membranes are assessed by X-ray photoelectron spectroscopy (XPS), contact angle measurement and surface energy analysis. The synergistic effects of surface heterogeneity and nano-structured surface topology on the anti-oil-fouling property of membranes during oil-in-water emulsion filtration have been investigated and discussed in detail.

2 Experimental 2.1 Materials Poly(vinylidene fluoride) (PVDF, FR-904) was obtained from Shanghai 3F New Material Co. Ltd. and dried at 110 °C for at least 12 h before use. 2,2,3,4,4,4-Hexafluorobutyl methacrylate (HFBM) was supplied by Xeogia Fluorine-Silicon Chemical Co. Ltd. and washed twice with a sodium hydroxide solution (1 mol/L) and deionized water to remove the inhibitor before use. [2-(Methacryloxyl)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide inner salt (SBMA) and tris(hydroxymethyl) amiomethane (Tris) were purchased from Sigma-Aldrich Co. N-vinylformamide (NVF) was purchased from Tokyo Chemical Industry Co. Ltd. 2,2′-Azobis (2methylpropion-amidine) dihydrochloride (AIBA) was purchased from J&K Scientific Ltd. Dopamine hydrochloride (DA) was purchased from Yuancheng Technology Development Co. Ltd. Titanium dioxide nanoparticles (TiO2 NPs) was purchased from

227

Aladdin Industrial Inc. High-speed vacuum pump oil (GS-1) was purchased from Beijing Sifang Special Oil Factory. Polyethylene glycols (PEG), N-methyl-2-pyrrolidone (NMP), sodium dodecylsulfate (SDS) and other chemicals were obtained from Tianjin Guangfu Fine Chemical Research Institute. 2.2 Synthesis of zwitterionic and fluoride-containing modifiers H2N-PVNF-Z and H2N-PVNF-F copolymers were synthesized via AIBA initiated free radical polymerization in aqueous solution followed by acidic hydrolysis. The synthetic procedure was shown in Fig. S1. NVF (20 mmol) was added into water (50 ml) and purged with N2 for 30 min to remove oxygen. AIBA (0.5 mmol) was dissolved in water (6 ml) and added into the NVF solution. The polymerization was carried out at 52 °C for 2 h under nitrogen atmosphere. After 2 h, the SBMA (Z) aqueous solution (1.43 mol/L, 14 ml) or SDS-stabilized HFBM (F) emulsion (1.43 mol/L, 14 ml) were added dropwise into the above solution. Polymerization was performed at 52 °C for another 8 h under nitrogen atmosphere and then terminated by cooling and exposing to air. After polymerization, hydrochloric acid (37 wt%, 7 ml) was added into the solution of PVNF-Z or PVNF-F copolymer and hydrolysis was performed at 70 °C for 2 h to convert amide group into amine group. The resultant polymer solution was dialyzed in dialysis tubing (molecular weight cut-off, 3500 Da) against water for at least 72 h and then dried under vacuum. The obtained H2N-PVNF-Z and H2N-PVNF-F copolymers were characterized by Nicolet 6700 Fourier transform infrared (FTIR) spectrometry and INVOA-500 1H nuclear magnetic resonance (1H NMR) spectrometer (see the Supporting information). 2.3 Preparation of TiO2@PDA NPs and modified TiO2@PDA NPs TiO2 NPs (1.0 g) and PEG 400 (50 mg) were pre-dispersed in Tris–HCl buffer (50 mM, pH ¼8.5, 100 ml) and sonicated for 30 min for uniform dispersion. Then, the dispersion was subjected to high-speed stirring and DA (200 mg) was added. After oxidantinduced surface polymerization of DA for 24 h at 30 °C, the asprepared TiO2@PDA NPs were collected and washed by centrifugation. To prepare modified TiO2@PDA NPs with different functional segments, the obtained TiO2@PDA NPs were dispersed in Tris–HCl buffer (50 mM, pH ¼8.5, 45 ml) by sonicator, followed by mixing with the aqueous solution of H2N-PVNF-Z (100 mg/ml, 5 ml), H2N-PVNF-F (100 mg/ml, 5 ml) or both. The mixture was subjected to high-speed stirring at 60 °C for 2 h. The H2N-PVNF-Z and H2N-PVNF-F copolymers could be covalent linked with the PDA shell on TiO2 NPs by primary amine groups. The as-prepared TiO2@PDA-Z, TiO2@PDA-F and TiO2@PDA-ZF NPs were collected and washed by centrifugation. Fig. 1 presents a schematic diagram of PDA-based modification procedure. The obtained TiO2@PDA NPs and modified TiO2@PDA NPs were characterized by Nicolet 6700 FTIR spectrometry, NETZSCH TG209 F3 thermogravimetric analysis (TGA, air atmosphere), Nanosem 430 energy dispersive X-ray spectroscopy (EDS) and JEM-2100F transmission electron microscope (TEM). 2.4 Fabrication of hybrid membranes Hybrid membranes derived from TiO2@PDA NPs and modified TiO2@PDA NPs were fabricated by non-solvent induced phase inversion (NIPS). The TiO2@PDA NPs or modified TiO2@PDA NPs (20 wt% versus the weight of PVDF) were added to the NMP sonicated for 30 min for uniform dispersion. Then, PVDF (12 wt%) and PEG2000 (6%) were added to the dispersions and stirred for 12 h at 60 °C to form homogeneous casting solutions. After degassing for 12 h, the casting solutions were cast onto glass plates

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γl(1 + cos θl ) = 2 γl dγsd + 2 γl pγsp

(2)

where θl is the contact angle of test liquid, γl, γl p and γld are the total, polar and dispersive surface energy of test liquid, respectively. 2.5 Evaluation of anti-oil-fouling performance for hybrid membranes The anti-oil-fouling performance of hybrid membranes was evaluated with a dead-end stirred cell (effective membrane area of 28.7 cm2) and conducted at the pressure of 0.1 MPa and the stirring speed of 200 rpm. The fluxes (J) of membranes were calculated by the following formula:

J=

Fig. 1. The scheme for the PDA-based modification procedure of TiO2@PDA NPs and modified TiO2@PDA NPs.

using a casting knife with a gap height of 240 μm and immediately immersed in 25 °C coagulation bathes. The as-prepared membranes were rinsed with deionized water and stored in deionized water for at least 24 h before use. The resultant membranes derived from TiO2@PDA, TiO2@PDA-Z, TiO2@PDA-F and TiO2@PDA-ZF NPs were denoted as MTP, MTP-Z, MTP-F and MTP-ZF, respectively. PVDF/PEG and PVDF/PEG/PSBMA-PHFBM membranes were prepared from the casting solution with PVDF (12 wt%), PEG2000 (6 wt%) and copolymer (3.6 wt% versus the weight of PVDF, PSBMA:PHFBM ¼1/2 mol/mol) for further comparison, and denoted as M-0 and M-ZF, respectively. The obtained hybrid membranes were characterized by a Nanosem 430 field emission scanning electron microscope (FESEM), Bruker Atomic force microscopy (AFM, Multimode 3) Kratos Axis Ultra X-ray photoelectron spectroscopy (XPS), Nicolet 6700 FTIR spectrometry and tensile testing machine. JC2000C contact angle goniometer was used to investigate the contact angles of water, diiodomethane and captive air bubbles on membrane surface. The dynamic advancing contact angle (θa) and receding contact angle (θr) of water were also measured by adding or removing liquid from the water drops. The difference between advancing and receding contact angles was defined as the contact angle hysteresis. The total surface energy (γs) of membrane surfaces, as well as the polar ( γsp ) and dispersive ( γsd ) components, were calculated from the Owens and Wendt's method [38] employing a polar test liquid (water) and a nonpolar test liquid (diiodomethane).

γs = γsd + γsp

(1)

V AΔt

(3)

where V (L) is the permeated volume, A (m2) is the effective membrane area, and ΔT (h) was the operation time. For antifouling property evaluation, oil-in-water emulsion (GS-1 highspeed vacuum pump oil 0.9 g L  1 and SDS 0.1 g L  1, average diameter  540 nm, see the Supporting information) was employed as the model foulant solutions. Before testing, each membrane was compacted at the pressure of 0.1 MPa with deionized water for 1 h to obtain a steady flux. The membrane was firstly filtrated with deionized water solutions for 30 min and the initial flux (J1) was recorded. Then, the membrane was subsequently filtrated with oilin-water emulsion for 1 h and the flux for feed solutions (Jf) was recorded. After the filtration of oil-in-water emulsion, the membrane was rinsed with deionized water for 30 min and the flux of cleaned membrane (J2) was recorded. To evaluate the antifouling performance of hybrid membranes, the flux recovery ratio (FRR ¼J2/J1), total flux decline ratio (DRt ¼1  Jf/J1), reversible flux decline ratio (DRr ¼(J2  Jf)/J1) and irreversible flux decline ratio (DRir ¼1  J2/J1) were calculated and analyzed in detail. The solute rejections (R) were calculated using the following formula:

R = (1 −

Cp Cf

) × 100%

(4)

where Cp and Cf (g/L) are the concentration of permeate and feed solutions respectively. The concentrations of oil-in-water emulsion were quantified using a Hitachi UV-2800 UV–vis spectrophotometer at 530 nm.

3 Results and discussion 3.1 Characterization of TiO2@PDA NPs and modified TiO2@PDA NPs TiO2@PDA NPs were fabricated by DA self-polymerization on the surface of TiO2 NPs. DA bound strongly to the surface of TiO2 NPs via catechol groups in either monodentate or bidentate configurations, followed by oxidizing and polymerizing under alkaline conditions to form a thin adherent PDA shell on the surface of TiO2 NPs. Fig. 2 illustrates the TEM images of TiO2@PDA NPs. The obtained TiO2@PDA NPs showed clear shell-core structure with PDA shell of 10–15 nm in thickness. The strength of PDA binding to TiO2 NPs was attributed to the diverse modes of interactions such as covalent, coordination, hydrogen bond, van der Waals, hydrophobic, charge transfer and π–π stacking interactions [39–41]. PDA-mediated immobilization of functional moieties had become a powerful tool box for material manipulating. Catechol-containing PDA coated NPs could serve as carriers for antifouling modifiers based on catechol-amine or catechol-mercapto couplings. To load hydrophilic segments and low surface energy segments on TiO2@PDA NPs, zwitterionic H2N-PVNF-Z copolymer and fluoride-

X. Zhao et al. / Journal of Membrane Science 495 (2015) 226–234

229

Fig. 2. TEM images of TiO2@PDA NPs at different magnifications.

containing H2N-PVNF-F copolymer were synthesized and grafted onto TiO2@PDA NPs. The primary amine groups in H2N-PVNF-Z and H2N-PVNF-F copolymers were able to react with unsaturated indole rings and catechol groups in PDA via Michael and/or Schiffbase reactions, and form covalent linkages with the PDA shell on TiO2 NPs (Fig. 1) [40–42]. Fig. 3a shows the FTIR spectra for TiO2@PDA, TiO2@PDA-Z, TiO2@PDA-F and TiO2@PDA-ZF NPs. After the self-polymerization of DA, the vibration absorption of C ¼C resonance and N–H bending at 1603 cm  1 appeared, as well as the vibration absorption of N–H shearing at 1509 cm  1. After grafting zwitterionic H2N-PVNF-Z copolymer, the stretching vibration of C ¼O appeared at 1725 cm  1, the stretching vibration of þ N(CH3)3 appeared at 1484 cm  1, and the symmetric SO−3 and asymmetric SO−3 stretching vibration appeared at 1046 cm  1 and 1171 cm  1, respectively. After grafting fluoride-containing H2N-PVNF-F copolymer, the stretching vibration of C ¼O appeared at 1747 cm  1, and the stretching vibration of CFx appeared at 1100 cm  1, 1190 cm  1 and 1290 cm  1. For TiO2@PDA-ZF NPs, the vibration absorption of C ¼O, þ N(CH3)3, SO3  and CFx appeared, which confirmed the coexistence of zwitterionic and fluoride-containing units. TGA was used to evaluate the graft ratio of modified TiO2@PDA NPs, and the results are displayed in Fig. 3b. The thermal degradation of polymeric shell structure on the surface of TiO2 NPs was observed. The weight losses of TiO2@PDA NPs, TiO2@PDA-Z, TiO2@PDA-F and TiO2@PDA-ZF NPs were 7.1%, 12.5%, 22.5%, and 25.3%, respectively. The graft degrees of H2N-PVNF-Z and H2NPVNF-F copolymers on the surface of TiO2@PDA NPs were 5.4%, 15.4%, and 18.2% for TiO2@PDA-Z, TiO2@PDA-F and TiO2@PDA-ZF NPs, respectively. The EDS analysis of the different NPs confirmed the presence of S element on TiO2@PDA-Z and TiO2@PDA-ZF NPs and the presence of S element on TiO2@PDA-F and TiO2@PDA-ZF

NPs (see the Supporting information). All the results showed that TiO2@PDA NPs and modified TiO2@PDA NPs were successfully synthesized. 3.2 Characterization of hybrid membranes The cross-section morphologies of as-prepared hybrid membranes derived from TiO2@PDA NPs (MTP) and PVDF control membrane (M-0) are shown in Fig. 4. It could be observed that both MTP and M-0 membranes exhibited asymmetric structures with top skin layers supported by macrovoid sub-layers. For MTP membrane, the TiO2@PDA NPs were obviously dispersed in the PVDF matrix. No significant influence was observed for the asymmetric structures after incorporating TiO2@PDA NPs. The element distribution of MTP membrane across the thickness was characterized via EDS mapping analysis (Fig. 4c). Ti element, as well as N and O element, was evenly dispersed in scanning spectra without obvious aggregation, suggesting that good distribution of TiO2@PDA NPs within MTP membrane. The surface morphologies of as-prepared M-0 and MTP membranes are shown in Fig. 5. With the introduction of TiO2@PDA NPs in membrane matrix, the surface pore size of MTP membrane was significantly decreased compared with that of M-0 membrane, which was attributed to the confined chain motion of PVDF membrane-forming polymer by imparted NPs. The AFM image analysis was applied to investigate the surface morphological differences between M-0 and MTP membranes (Fig. 5). Comparing with the flat morphology of M-0 membrane surface, clear microprotrusions with the size about 150–250 nm were observed on MTP membrane surface. The MTP membrane with TiO2@PDA NPs decorated on the surface appeared to be much rougher than M-0 membrane, with the RMS roughness increased from 10.34 nm to

100

TiO2 95

C=C N-H

Weight (%)

TiO2@PDA TiO2@PDA-Z TiO2@PDA-F TiO2@PDA-ZF

O-C=O +N(CH ) 33 N-C=O

90 85 80 75

CFx

SO3-

TiO2@PDA TiO2@PDA-Z TiO2@PDA-F TiO2@PDA-ZF

70

2000 1800 1600 1400 1200 1000 800 600 -1

Wavenumber (cm )

100 200 300 400 500 600 700 800

Temperature(oC)

Fig. 3. (a) FTIR spectra and (b) TGA curves of TiO2@PDA NPs and modified TiO2@PDA NPs.

230

X. Zhao et al. / Journal of Membrane Science 495 (2015) 226–234

Fig. 4. Cross-section SEM images of (a) M-0 and (b) MTP membranes, and (c) EDS mapping analysis of MTP membrane.

Fig. 5. Surface SEM and AFM images of (a–c) M-0 and (d–f) MTP membranes.

15.90 nm. TiO2@PDA NPs were further used as carriers for zwitterionic and fluoride-containing moieties to tailor not only topological structure but also chemical structure of membrane surface. The apparent surface composition of MTP-Z, MTP-F and MTP-ZF membranes were characterized by XPS using the high resolution XPS scans of C 1s and N 1s shown in Fig. 6. As shown in Fig. 6a and d, the MTP-Z membrane featured five C 1s deconvoluted peaks corresponding to CH at 284.9 eV, CH2(PVDF) at 286.3 eV, C–O(ester) at 287.9 eV, C ¼O(C–F) at 289.1 eV, and CF2 at 290.7 eV, and two N 1s deconvoluted peaks corresponding to C–N at 400.4 eV and quaternary ammonium N þ at 403.3 eV. The XPS results clearly

proved the existence of zwitterionic segments on MTP-Z membrane surface. As shown in Fig. 6b and e, the MTP-F membrane featured six C 1s deconvoluted peaks corresponding to CH at 285.0 eV, CH2(PVDF) at 286.3 eV, C–O(ester) at 287.8 eV, C ¼ O(C– F) at 289.0 eV, CF2 at 290.7 eV, and CF3 at 293.1 eV, which clearly confirmed the existence of fluoride-containing segments on MTP-F membrane surface. As shown in Fig. 6c and f, it was obvious that MTP-ZF membrane also exhibited deconvoluted characteristic peaks corresponding to CF3 at 293.4 eV and quaternary ammonium N þ at 403.3 eV. This indicated that the successful attachment of both zwitterionic and fluoride-containing segments on MTP-ZF membrane surface.

X. Zhao et al. / Journal of Membrane Science 495 (2015) 226–234

CH2(PVDF)

CH

CF/C=O 296

294

292

290

286

CH

CF2 CF/C=O

C-O

288

284

282

296

294

292

290

404

402

CF2

286

CH

CF/C=O

C-O

288

284

282

296

294

292

290

C-O 288

NH

NH

NH2

NH2

400

398

406

404

402

400

284

282

N

+ N

NH NH2

398

Bending Energy (eV)

Bending Energy (eV)

286

Bending Energy (eV)

N

N

406

CF3

Bending Energy (eV)

Bending Energy (eV)

+ N

CH2(PVDF)

CH2(PVDF)

CF3

CF2

231

406

404

402

400

398

Bending Energy (eV)

Fig. 6. XPS C 1s and N 1s spectra of (a, d) MTP-Z, (b, e) MTP-F and (c, f) MTP-ZF membrane surfaces.

Based on XPS signal intensities and differentiation, the chemical composition and surface coverage of zwitterionic segments (Φz) and fluoride-containing segments (Φf) could be calculated from following expressions:

Φz = Ν% × ΑN + × 18

(5)

Φf = C % × ACF3 × 16

(6)

where N% and C% are the atom percentage of N and C elements on membrane surfaces determined by XPS respectively, AN þ is the area ratio of the peak for N þ in N 1s XPS spectra, ACF3 is the area ratio of the peak for CF3 in C 1s XPS spectra, the factor 16 accounted for the 16 atoms in each HFBM unit, and the factor 18 accounted for the 18 atoms in each SBMA unit. The detailed information on Φz and Φf was listed in Table 1. Using TiO2@PDA NPs as carriers for zwitterionic and fluoride-containing moieties was proved to be effective in tailoring the surface heterogeneity of membrane surfaces. The excellent stability of NPs in MTP-ZF membrane also indicated the durability of surface heterogeneity and antifouling performance (see the Supporting Information). The resultant membrane surfaces with zwitterionic or/and fluoride-containing segments could manipulate both the wetting characteristics and the surface energy of membrane surfaces. Fig. 7a shows the water and oil contact angles of different membrane surfaces. The water contact angle of MTP-Z membrane was smaller than that of MTP membrane, indicating that the presence of zwitterionic segments increased the hydrophilicity of

membrane surface. The water contact angle of MTP-F membrane was increased to 89.2° in contrast to 70.5° for MTP membrane, which indicated a notable hydrophobic conversion due to the presence of fluoride-containing segments. For heterogeneous MTP-ZF membrane, the synergistic effect of hydrophilic zwitterionic segments and hydrophobic fluoride-containing segments resulted in the decrease of water contact angle to about 80.7°. With the introduction of the zwitterionic segments on membrane surface, the underwater air contact angles of MTP-Z and MTP-ZF membranes were obviously increased, indicating the enhanced hydration capacity (see the Supporting Information). Moreover, with the introduction of the fluoride-containing segments on membrane surface, the oil contact angle of MTP-F and MTP-ZF membranes were obviously increased, indicating the enhanced oleophobicity. Regarding dynamic water contact angle (Fig. 7b), the thermodynamic contact angle hysteresis was clearly detected by introducing surface heterogeneity. Due to the presence of the zwitterionic segments, a decrease in receding contact angle of 25.3° for MTP-Z membrane with respect to MTP membrane was observed. Due to the presence of the fluoride-containing segments, an increase in advancing contact angle of 89.3° for MTP-F membrane with respect to MTP membrane was also observed. With zwitterionic and fluoride-containing segments simultaneously decorated on membrane surface, water contact angle hysteresis exhibited a considerable increase from 36.5° for the MTP membrane to 67.0° for the MTP-ZF membrane. The decrease in receding contact angle was attributed to the hydrophilic feature

Table 1 The elemental percentage and surface coverage of zwitterionic segments (Φz) and fluoride-containing segments (Φf) on membrane surfaces derived from XPS. Membranes

C (at%)

N (at%)

O (at%)

F (at%)

Ti (at%)

S (at%)

Φz (at%)

Φf (at%)

MTP-Z MTP-F MTP-ZF

53.54 61.10 55.76

1.17 1.21 1.23

1.91 6.53 4.09

43.25 31.09 38.59

N.A. 0.08 0.11

0.14 – 0.22

4.1 – 4.8

– 14.4 11.5

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X. Zhao et al. / Journal of Membrane Science 495 (2015) 226–234

Water Contact Angle ( o)

80 60 40 20 0

MTP

MTP-Z

MTP-F

MTP-ZF

Water Contact Angle ( o)

120 Water Contact Angle Oil Contact Angle

100

100

Advancing contact angle Receding contact angle Contact angle hysteresis

80 60 40 20 0

MTP

MTP-Z

MTP-F

MTP-ZF

Fig. 7. (a) Water/oil contact angles and (b) dynamic water contact angle hysteresis of MTP, MTP-Z, MTP-F and MTP-ZF membranes.

Surface free energy (mJ/m2)

50

d

γs

45

p

γs

γs

40

Furthermore, the MTP-ZF membrane combined both of the hydration and low surface energy effects, and showed the surface energy of 34.5 mJ/m2 with the polar components of 5.35 mJ/m2 and the dispersive components of 28.7 mJ/m2. 3.3 Integration of multiple anti-oil-fouling mechanisms on heterogeneous hybrid membrane surfaces

35 30 25 10 5 0

MTP

MTP-Z

MTP-F

MTP-ZF

Fig. 8. Surface energy parameters of MTP, MTP-Z, MTP-F and MTP-ZF membranes.

of zwitterionic segments, and the advancing contact angle was sensitive to the low surface energy feature of fluoride-containing segments [43]. The surface energy parameters of MTP, MTP-Z, MTP-F and MTPZF membranes are summarized in Fig. 8. The surface energy of MTP membrane was 38.3 mJ/m2, and that of MTP-Z membrane was slightly increased by the highly polar zwitterionic segments. The higher polar components of MTP-Z membrane with respect to MTP membrane indicated the electrostatically induced hydration potential of zwitterionic segments [25,44]. In comparison, both the surface energy and polar components of MTP-F membrane were notably decreased due to the nonpolar low surface energy nature of fluoride-containing segments, which indicated the thermodynamic unfavorable adhesion with membrane foulants [26,45].

Flux (L/m 2 h)

120

oil-in-water emulsion water

Percentage(%)

water

240 200 160 120

MTP MTP-Z MTP-F MTP-ZF

80 40 0 0

20

The experiential and effective strategies to deter membrane fouling included inhibiting the direct interaction by constructing hydration layer barrier (fouling resistant mechanism) and minimizing the interaction strength by constructing low surface energy barrier (fouling release mechanism). Using emulsified vacuum pump oil as model foulant, the dead-end ultrafiltration results of oil-in-water emulsion and the anti-oil-fouling capacities of membranes are shown in Fig. 9. The MTP membrane showed large flux decline ( 65.3%) during the oil-in-water emulsion filtration and almost no flux recovery was observed after rinsed with deionized water. The TiO2@PDA NPs on membrane surfaces had no antifouling capacities against oil-foulant. In contrast, enhanced flux recovery ability was observed from MTP-Z membrane and intensified flux-decline resistant ability was observed from MTP-F membrane. The zwitterionic segments on MTP-Z membrane surface could provide a hydration layer barrier against the direct adhesion of oil droplets, and the reversibility of flux decline significantly improved. The FRR of MTP-Z membrane was 75.0%, which was obviously higher than that of MTP membrane (40.5%). The fluoride-containing segments on MTP-F membrane surface could build a low surface energy barrier against the tough adhesion and spreading of oil droplets, and the flux decline caused by irreversible oil-fouling was significantly reduced. The DRt of MTP-F membrane was 30.5%, which was dramatically lower than that of MTP membrane (65.3%). Aiming at binary cooperative

40

FRR

DRt

DRr

DR ir

100 80 60 40 20 0

60

80

100

120

MTP

MTP-Z

MTP-F

MTP-ZF

Time (min) Fig. 9. (a) Time-dependent fluxes and (b) a summary of the corresponding fouling parameters of MTP, MTP-Z, MTP-F and MTP-ZF membranes during the oil-in-water emulsion filtration.

X. Zhao et al. / Journal of Membrane Science 495 (2015) 226–234

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Fig. 10. Time-dependent fluxes and AFM images of (a) MTP-ZF membrane with nano-structured surface topology and (b) M-ZF membrane with micro-structured surface topology during the oil-in-water emulsion filtration.

anti-oil-fouling, heterogeneous MTP-ZF membrane was designed and applied to oil-in-water emulsion filtration, exhibiting almost complete flux recovery and only 16.7% total flux decline. The outstanding anti-oil-fouling capacities of MTP-ZF membrane, compared with either MTP-Z or MTP-F membrane, came from the synergistic effect of oil-fouling-resistant and oil-fouling-release mechanisms on the surfaces. The hierarchical topographies of MTP-ZF membrane surface also played vital roles in deterring oil-fouling. Polymeric M-ZF membrane derived from PSBMA-PHFBM copolymer modifier without the anchor of TiO2@PDA NPs was employed to evaluate the effect of surface topology on the anti-oil-fouling capacities of membranes. Fig. 10 shows the anti-oil-fouling capacities of MTP-ZF and M-ZF membranes during the oil-in-water emulsion filtration. MTP-ZF membrane with nano-structured surface topology tailored by TiO2@PDA NPs exhibited flux decline of only 16.7% and flux recovery of 100%, while M-ZF membrane with micro-structured surface topology showed notable flux decline of 37.5% and limited flux recovery of 79.8%. For heterogeneous membranes with roughly the same amount of zwitterionic and fluoride-containing antifouling moieties, the anti-oil-fouling properties of membrane were greatly intensified by patterned nano-structured surface topology. Similar phenomena were also found in the recent literatures [46,47]. These results suggested that membranes synergistically manipulated with surface heterogeneity and hierarchical topographies were potential and promising candidates to eliminate membrane oil-fouling. On one hand, the hydrophilic zwitterionic segments imparted the formation of hydration layer barrier to avoid the direct contact between emulsified oil droplets and membrane surfaces; on the other hand, the fluorine-containing segments favored the formation of low surface energy barrier to decrease the interaction between emulsified oil droplets and membrane surfaces. Both effects effectively induced the confused wetting behavior and affinity state of oil droplets. Moreover, the surface topological transition from micro-scale to nano-scale also provided promoting effects on the anti-oil-fouling performance of membranes. The hierarchical topographies would influence the near-surface flow regime and thus the increasing disturbance would prevent the coalescence of oil droplets [48]. The scale of surface topology, which were much smaller than oil droplets, would also provide fewer attachment points for the deposition of oil droplets on membrane surface. In summary, the optimal integration of oil-fouling-resistant mechanism, oil-fouling-release mechanism and hierarchical topographies provided nonequilibrium repulsive interaction between neighboring topographies, which favored the unstable attachment and easy detachment of oil droplets from membrane surface (Fig. 11).

Fig. 11. Tentative illustration of the multiple antifouling mechanisms on heterogeneous hybrid membrane surfaces.

4 Conclusions In this study, TiO2@PDA NPs were prepared and employed as the carrier of zwitterionic and fluoride-containing antifouling moieties to tailor both chemical and topological structures of hybrid membranes. The TiO2@PDA NPs were well dispersed in membrane matrix and endow membrane with nano-structured surface topology. Derived from multifunctional TiO2@PDA NPs, both hydrophilic zwitterionic segments and low surface energy fluorine-containing segments were decorated on membrane surfaces to render desirable surface heterogeneity. The zwitterionic segments endowed membranes with oil-fouling-resistant mechanism, and simultaneously, the fluorine-containing segments endowed membranes with oil-fouling-release mechanism. The combination of surface heterogeneity and hierarchical topographies synergistically improved the anti-oil-fouling capacities of membranes. The hybrid membranes displayed excellent anti-oilfouling properties (about 16.7% flux decline and 100% flux recovery) when utilized for oil and water separation. This study would provide a facile method for the controllable construction of heterogeneous and hierarchical membrane surfaces, and achieved the synergistic intensification of multiple anti-oil-fouling capacities of membrane surfaces.

Acknowledgements This work was financially supported by National Science Fund

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for Distinguished Young Scholars (21125627) and Tianjin Natural Science Foundation (14JCZDJC37400 and 13JCYBJC20500).

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2015.08. 026.

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