Metal-polyphenol coordination networks: Towards engineering of antifouling hybrid membranes via in situ assembly

Journal of Membrane Science 563 (2018) 435–446

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Metal-polyphenol coordination networks: Towards engineering of antifouling hybrid membranes via in situ assembly ⁎

T

⁎

Xueting Zhaoa,b,c, , Ning Jiaa, Lijuan Chenga, Lifen Liua,b,c, , Congjie Gaoa,b,c a

Center for Membrane and Water Science & Technology, Ocean College, Zhejiang University of Technology, Hangzhou 310014, China Collaborative Innovation Center of Membrane Separation and Water Treatment of Zhejiang Province, Hangzhou 310014, China c Huzhou Institute of Collaborative Innovation Center for Membrane Separation and Water Treatment, Zhejiang University of Technology, Huzhou, Zhejiang 313000, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Metal-polyphenol coordination In situ assembly Hybrid membrane Antifouling Oil/water separation

Metal-polyphenol coordination networks have been actively explored as a facile, rapid and green platform for developing materials. In this study, novel antifouling hybrid membranes are successfully prepared via in situ assembling metal-polyphenol coordination networks and are proposed for oil/water separation application. Based on the coordination-driven cross-linking and assembling of TiIV and TA within polyvinylidene fluoride (PVDF) membrane matrix, TA-Ti coordination networks are successfully introduced and uniformly distributed in the as-prepared PVDF/TA-Ti membranes. The effects of the embedded TA-Ti coordination networks on both surface morphologies and pore structures of PVDF/TA-Ti membranes are investigated. The surface chemical compositions of PVDF/TA-Ti membranes are analyzed by energy-dispersive X-ray (EDX) and Fourier transform infrared spectroscopy (FTIR). The water contact angle analysis and DSC study on bound water content reveal the outstanding hydration capability of PVDF/TA-Ti membranes, indicating the higher underwater superoleophobicity and antifouling property of PVDF/TA-Ti membranes. The as-prepared PVDF/TA-Ti membranes exhibit remarkably improved antifouling performance with the flux recover ability increased to a maximum level about 100% for the filtration of oil-in-water emulsions. Overall, this study highlights the promising antifouling potential of TA-Ti coordination networks in designing antifouling membranes, and proposes a facile in situ hybridization method for preparing antifouling membranes derived from versatile metal-polyphenol coordination networks.

1. Introduction Membrane technology has been successfully and widely applied in sustainable wastewater treatment and reuse to alleviate the global water crisis [1,2]. Polymeric membrane is the dominant material in membrane technology, and is the core of high-performance membrane separation. Despite the relatively advanced application level of membrane industry, membrane fouling still remains one of the major bottlenecks for the further improvement of membrane efficiency and sustainable development of membrane technology [3–5]. Typical polymeric materials for fabricating porous membranes (such as ultrafiltration and microfiltration) are hydrophobic, and the as-prepared porous membranes are prone to fouling due to the inherent hydrophobic interaction between foulants and membrane surfaces, which leads to reduced permeability, increased energy consumption and deteriorated service life [6–8]. Therefore, it is highly desired to prevent membranes from fouling. The fundamental solution is to develop

membranes with superior antifouling capacity. The general design guideline of antifouling membranes lies in constructing hydrophilic membrane surfaces. Hydrophilic modification can promote the interactions between water molecules and membrane surfaces, and resist membrane fouling by forming hydration barrier layer between foulants and membrane surfaces [9,10]. Much effort has been devoted to engineering antifouling membranes either by surface coating [11,12], surface grafting [13,14] or in situ blending [9,15,16]. It is highlighted that the in situ blending method features one-step membrane formation and membrane modification, and shows its advantages in the three-dimensional modification of porous membranes without the restrictions of multi-step manufacturing or undesirable pore blocking [3,17]. Recently, abundant antifouling studies have been carried out by in situ blending amphiphilic copolymers [9,18,19] and hydrophilic nanomaterials [15,16,20] as antifouling additives into polymer matrixes. However, the access to most potential antifouling additives are still not straight-forward, and inherently limited by

⁎ Corresponding authors at: Center for Membrane and Water Science & Technology, Ocean College, Zhejiang University of Technology, No. 18, Chaowang Road, Xiacheng District, Hangzhou 310014, China. E-mail addresses: [email protected] (X. Zhao), [email protected] (L. Liu).

https://doi.org/10.1016/j.memsci.2018.06.014 Received 2 March 2018; Received in revised form 15 May 2018; Accepted 8 June 2018 0376-7388/ © 2018 Elsevier B.V. All rights reserved.

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2. Experimental

complicated synthesis route or harsh synthesis conditions. Thus, it is urgent to develop a facile and one-step synthetic pathway towards in situ antifouling membrane construction. Metal-phenolic coordination networks have become rising stars in the field of engineering functional materials. Typically, natural phenolic compound tannic acid (TA) and specific metal ions can one-step assemble into three dimensionally stabilized metal-phenolic coordination networks facilitated by one-step coordination-driven crosslinking [21]. Since the one-step coordination-driven assembly of natural polyphenols and FeIII ion was first reported in 2013 [22], metalphenolic coordination networks have been of interest for engineering a variety of functional materials [23,24], films/capsules [25–27], gels [28,29] and mesocrystals [30]. Recent studies also predicated the potential of the assembled TA-Fe complexes on membrane surfaces in tailoring membrane physicochemical properties (such as hydrophilicity, electric charge, fouling resistance, etc.) through post-modification [31–34]. It can be deductive that the integration of the onestep assembly of metal-phenolic coordination networks with the onestep modification of in situ blending method will unlocked a versatile, facile and straight-forward toolbox for the in situ construction of antifouling membranes. When applying non-covalent coordination interactions to construct antifouling membranes, the stability of coordination networks is a critical factor. The higher oxidation state of a metal ion always creates a more stable coordination network, and thus the coordination networks derived from TiIV ion with higher oxidation state and formal charge are more robust than those derived from FeIII ion. [26] Therefore, TA-Ti coordination networks will become a preferential choice for antifouling membrane design. In addition, the unique gelation behavior of TA-Ti coordination networks [28] will also affect the entanglement and movement capabilities of TA-Ti coordination networks and PVDF chain, which could potentially manipulate membrane structures and properties. In this study, metal ion TiIV and polyphenol TA were in situ introduced into membrane casting solutions, and antifouling PVDF/TA-Ti membranes derived from TA-Ti coordination networks were then obtained via non-solvent induced phase inversion (NIPS) (Scheme 1). In this strategy, TA molecules were assembled with TiIV ions and crosslinked into networks via coordination interactions, and the resulting in situ assembled TA-Ti coordination networks were hybridized and entangled within PVDF membrane matrix. Membrane morphology, surface chemistry, surface charge, hydration capability and underwater oleophobicity were investigated. Then, the separation performance and antifouling capability of PVDF/TA-Ti membranes were evaluated via a series of filtration experiments using oil-in-water emulsions as the separation system.

2.1. Materials PVDF (FR-904) was purchased from Shanghai 3F New Material Co. Ltd and dried at 60 °C for 24 h before use. Titanium(IV) bis(ammonium lactato) dihydroxide (TiBALDH, 50 wt% aqueous solution) was purchased from Sigma-Aldrich Co., China. Tannic acid (TA), tris(hydroxymethyl) amiomethane (Tris), sodium dodecylsulfate (SDS) and nhexadecane (HD) were purchased from Aladdin-reagent Co., China. Nmethylpyrrolidinone (NMP), hydrochloric acid (HCl) and other chemicals were purchased from local reagent corporations. All the above materials were used as received unless otherwise stated. 2.2. Fabrication of PVDF/TA-Ti membranes PVDF/TA-Ti membranes derived from metal (TiIV)-polyphenol (TA) coordination networks were fabricated via non-solvent induced phase inversion. PVDF and TA were dissolved in NMP to form homogenous PVDF/TA (NMP) solutions. Given quantities of TiBALDH (50 wt% aqueous solution) were mixed with NMP to form homogenous TiBALDH (NMP) solutions. Then, the TiBALDH (NMP) solutions were added dropwise into PVDF/TA(NMP) solutions and stirred for 12 h at 60 °C to form homogeneous casting solution. The final concentrations of PVDF and TA in casting solutions were 10.0 wt% and 30 mmol/L, respectively. The TiBALDH content in casting solutions was controlled from 60 to 150 mmol/L, with molar ratio of TA/TiIV from 1:2 to 1:5. After releasing air bubbles for another 12 h, the casting solutions were cast onto a glass plate using a casting knife with a gap height of 250 µm and immediately immersed into a water bath at 25 °C. After the full exchange of solvent (NMP) and non-solvent (water), the as-prepared membranes detached from the glass plates and were stored in deionized water before use. The as-prepared PVDF/TA-Ti membranes were named as PVDF/TA-Ti (m), with the letter m in membrane ID denotes the molar ratio of TA/TiIV in casting solutions. PVDF/TA membrane without adding TiBALDH was prepared for further comparison. The viscosities of degassed casting solutions were measured by a Brookfield Viscometer DV-2T. 2.3. Membrane characterization Scanning electron microscopy (SEM) images were acquired on a HitachiS-4700. The SEM was equipped with an energy-dispersive X-ray (EDX) detector for elemental analysis. Atomic force microscopy (AFM) was performed on a Bruker Dimension Icon using the peak force tapping mode. Fourier transform infrared spectroscopy (FTIR) measurements were recorded on Nicolet 6700 with ATR smart device. Zeta

Scheme 1. The schematic illustration of the fabrication process of PVDF/TA-Ti membranes. 436

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potential measurements were conducted on SurPASS Electrokinetic Analyzer. Water contact angle and underwater oil contact angle measurements were conducted on OCA-20 DataPhysics Contact Angle Analyzer. Differential scanning calorimetry (DSC) measurements were performed on TA Q20 (scanning at a temperature ranging from −40 °C to 40 °C at a heating rate of 5 °C/min under nitrogenous gas). Membrane bulk porosity was determined by the classic gravimetric method according to the following equation [35]:

ε=

Wwet − Wdry ρw Aδ0

× 100%

DRr =

DRir = DRt − DRr = 1−

(1)

Where

Jo ×100% Jw0

Cf

(7)

is the concentration ratio of oil in the permeate and feed

3.1. In situ assembly of TA-Ti coordination networks The assembly of TA-Ti coordination networks took place in situ in PVDF casting solution. In the in situ assembling process, gallol-rich TA could provide chelating sites for TiIV ion and facilitate efficient coordination-driven cross-linking [21], thereby resulting in TA-Ti coordination networks within PVDF casting solution. The high oxidation state TiIV ion led to the greater affinities with the highly electronegative pyrogallol ligand groups of TA and coordinated with catechol primarily through a tris-coordination state and partially through a bis-coordination (bis-μ-oxo bridged) state [36,37]. The coordination interaction between TA and TiIV ion caused a dramatic change in the color of the membrane casting solution from colorless to orange-red (Fig. 1a). The remarkable color change could be attributed to the ligand-to-metal charge-transfer band [27]. Upon the coordination-driven cross-linking and assembling of TiIV and TA, the viscosities of membrane casting solutions were dramatically increased, as shown in Table 1. The increased viscosities could be explained by the formation of coordinately cross-linked networks. In addition to coordinative crosslinking, the gelation behavior of TA-Ti coordination networks (Fig. 1b) might also be another critical factor. As the TA-Ti coordination networks were in situ assembled in membrane casting solutions, and PVDF/TA-Ti membranes were then fabricated via non-solvent induced phase inversion. The photographs of as-prepared PVDF/TA-Ti membranes derived from different TiIV/TA addition ratio were shown in the inset of Fig. 2a. Compared with the PVDF/TA control membrane, PVDF/TA-Ti membranes showed orange color, and the increase of TiIV/TA addition ratio darkened the orange color of PVDF/TA-Ti membranes. The color change indicated the formation of TiIV/TA coordination. It should be noted that no gelation behavior of TA-Ti coordination networks with lower TA-Ti ratio of 1:1 could be observed (Fig. S1), suggesting looser and lower level of crosslinking. Due to the lack of highly crosslinked structure, it is proposed that the TA-Ti coordination network with the TA-Ti ratio of 1:1 was unstable and easier to leach out of membrane matrix during the solvent/nonsolvent exchange and phase inversion process (Table S1).

(2)

where V (L) is the volume of permeated water, A (m ) is the effective membrane area and Δt (h) is the filtration time. For antifouling property evaluation, the initial water flux Jw0 was firstly recorded for 30 min (operating pressure: 0.05 MPa, stirring speed: 200 rpm). In the following step, the dead-end stirred cell was emptied and refilled with model oil-in-water emulsions. The filtration experiments of model oilin-water emulsions were performed under the same operating pressure and stirring speed (operating pressure: 0.05 MPa, stirring speed: 200 rpm). Model oil-in-water emulsions with oil droplet size 200–600 nm were prepared by adding 100 mg HD (or pump oil, soybean oil) and 10 mg SDS into 100 g water, and then the mixtures were sonicated with ultrasonic probe (JY92-IIN) under power of 100 W for 30 min to produce milky emulsions. The oil-in-water emulsion flux Jo of each membrane was recorded for 60 min and calculated according to Eq. (2). After the filtration of model oil-in-water emulsions, the deadend stirred cell was emptied and refilled with pure water for membrane cleaning (cleaning time: 30 min, stirring speed: 400 rpm). Finally, the dead-end stirred cell was emptied and refilled with pure water again, and the recovered water flux of as-cleaned membrane (Jw1) was measured. To evaluate antifouling properties of different membranes, several parameters were introduced, including flux recovery ratio (FRR), total flux decline ratio (DRt), reversible flux decline ratio (DRr) and irreversible flux decline ratio (DRir):

DRt = 1−

Cp

3. Results and discussion

2

Jw1 ×100% Jw0

(6)

solution.

The permeation antifouling performance of membranes for oil/ water separation was evaluated with a dead-end stirred cell (Millipore Model 8010) with effective membrane area of 4.1 cm2. Each membrane was first pre-compacted at 0.1 MPa to get a steady flux, and then the flux was measured at 0.05 MPa to evaluate the permeation performance and antifouling property of each membrane. The stirring speed was controlled at 200 rpm. For permeation performance evaluation, the initial water flux Jw0 (L/(m2 h)) of each membrane was recorded for 30 min and calculated using the following equation:

FRR =

Jw1 ×100% Jw0

Cp R = ⎜⎛1− ⎟⎞ ×100% ⎝ Cf ⎠

2.4. Permeation performance and antifouling property evaluation

V A∆t

(5)

Generally, higher flux recovery ratio, lower total flux decline ratio and lower DRir/DRt value indicated better antifouling properties of membranes during oil-in-water emulsion separation. The reported data was the average results from three independent experiments. The rejection ratios of emulsified oil droplets were calculated from the oil concentration in the feed and permeate solutions determined by UV–vis spectrophotometer (TU-1810PC) at wavelength of 530 nm and depicted as the following equation:

where Wwet (g) and Wdry (g) are the weights of the wet membrane and dry membrane (placed in an oven at 60 °C until a constant weight), respectively; ρw (g cm−3) is the density of pure water; A (cm2) and δ0 (cm) are the area and thickness of the wet membrane, respectively. Surface porosity and average surface pore size were determined by magnified SEM images using ImageJ software. Membrane samples for SEM, EDX, FTIR, water contact angle analysis were freeze-dried for 12 h. Membrane samples for porosity analysis, DSC analysis and underwater oil contact angle measurement were kept in water for at least 24 h before use.

Jw0 =

Jw1 − Jo ×100% Jw0

3.2. Membrane morphologies Fig. 2a and b shows the SEM surface and cross-section morphologies of the PVDF/TA control membrane and PVDF/TA-Ti membranes. As shown in Fig. 2a and Table 1, all membranes exhibited the porous surfaces with the average pore sizes about 20–22 nm and surface porosity about 2.5–2.9%. As shown in Fig. 2b, all membranes exhibited the

(3)

(4) 437

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Fig. 1. (a) Color change of membrane casting solution by forming TA-Ti coordination networks, and (b) gelation behavior of TA-Ti coordination networks in membrane casting solution. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

and gradually decreased the porosities of PVDF/TA-Ti membranes [40]. The roughness of the PVDF/TA control membrane and PVDF/TA-Ti membranes was investigated by AFM. The AFM results were shown in Fig. 2c. It was observed that PVDF/TA-Ti membranes exhibited rougher surface compared with the PVDF/TA control membrane. The surface roughness of PVDF/TA-Ti membranes was gradually increased with the increase of TiIV/TA addition ratio due to the presence of accumulated TA-Ti coordination networks within the skin layers. The higher roughness of PVDF/TA-Ti membranes could also contributed to the enhanced solvent/nonsolvent exchange rate [41] and the formation of larger gel grains [42].

asymmetric cross-section morphologies with a thin dense skin-layer and macrovoid structure in the sub-layer (Fig. 2b). It was clearly observed that the thickness of the membranes increased with the introduction of TA-Ti coordination networks. The increased thicknesses of PVDF/TA-Ti membranes as compared to the PVDF/TA control membrane could be attributed to the confined movement capabilities of PVDF chain by TATi coordination networks during phase inversion process. The PVDF/TA control membrane and PVDF/TA-Ti (1:2) membrane showed large macrovoids in the sub-layer, while the macrovoids of PVDF/TA-Ti membranes were further suppressed to some extent as the TiIV/TA addition ratio increased. The phenomenon could be explained by the rheological hindrance from increased casting solution viscosity due to TA-Ti cross-linking and gelation. The bulk porosity results of membranes were also given in Table 1 to quantitatively investigate the effect of TA-Ti coordination networks on the porous structures of membranes. The bulk porosity of PVDF/TA control membrane was found to be 76.2%. The enhanced bulk porosity was found for PVDF/TA-Ti (1:2) membrane and then gradually decreased with the further increase in the TiIV/TA addition ratio. This phenomenon could be attributed to the hydrophilic effect of TA-Ti coordination networks, the shrinkage stress difference of polymer and network, and the viscosity variation of casting solutions. When TA-Ti coordination networks (low TiIV/TA addition ratio) were incorporated in casting solution, there were two synergistic effects increasing membrane porosity during the solvent/ nonsolvent exchange process. On one hand, the hydrophilicity of TA-Ti coordination networks promoted the solvent/nonsolvent exchange rate and thus increased membrane porosity [38]. On the other hand, the difference in shrinkage stresses between PVDF and TA-Ti coordination networks also resulted in the increased porosity [39]. While, with the further increase of casting solution viscosity from TA-Ti cross-linking and gelation, the solvent/nonsolvent exchange rate was greatly limited

3.3. Membrane surface chemistry The surface chemical components of the PVDF/TA control membrane and PVDF/TA-Ti membranes were studied using EDX and FTIR analysis. For the PVDF/TA control membrane, the main elements on the membrane surface were C, F and O (Table 2). The presence of O element was ascribed to the residual TA molecules in membrane matrix, whereas most TA molecules leached out from the membrane matrix into coagulation bath during solvent/nonsolvent exchange process due to their low compatibility with PVDF polymer. For the PVDF/TA-Ti membrane, the main elements on the membrane surface were C, F, O and Ti (Table 2). The atomic fractions of O element and the O/C atomic ratios of PVDF/TA-Ti membranes were significantly increased by incorporating TA-Ti coordination networks in membrane matrix. With the in situ cross-linking and assembling of TA-Ti coordination networks within PVDF membrane matrix, TA molecules were embedded within PVDF/TA-Ti membranes by forming stable complexes with TiIV ions, thus increasing the O/C atomic ratios. With the increase of TiIV/TA addition ratio, the atomic fractions of Ti element and the Ti/C atomic

Table 1 The average surface pore size, surface porosity and bulk porosity of PVDF/TA and PVDF/TA-Ti membranes, and the viscosities of membrane casting solutions. Membrane PVDF/TA PVDF/TA-Ti PVDF/TA-Ti PVDF/TA-Ti PVDF/TA-Ti

(1:2) (1:3) (1:4) (1:5)

Average surface pore sizes (nm)

Surface porosity (%)

Bulk porosity (%)

Casting solution viscosity (mPa s)

20.4 21.4 21.5 20.8 21.4

2.6 2.9 2.9 2.7 2.5

76.2 84.6 81.2 79.8 75.7

2738 10,800 11,330 13,010 16,800

± ± ± ± ±

1.1 1.4 1.2 2.1 1.8

± ± ± ± ±

0.1 0.1 0.2 0.2 0.1

438

± ± ± ± ±

1.7 2.8 2.1 1.3 2.6

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Fig. 2. (a) Surface SEM and digital (insets) images, (b) cross-section SEM images and (c) AFM 3D images of PVDF/TA and PVDF/TA-Ti membranes.

anchor more TA-Ti coordination networks within membrane matrix. The plentiful ester and pyrogallol groups of TA would provide sufficient binding sites for TiIV ions to form the highly cross-linked coordination networks via different coordination modes (primarily tris-coordination

ratios of PVDF/TA-Ti membranes were gradually increased. It was also found that the higher TiIV/TA addition ratio resulted in the higher Ti/O atomic ratio, indicating higher coordination. These results indicated that higher TiIV ion content could embed more TA molecules and 439

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Table 2 The elemental composition of PVDF/TA and PVDF/TA-Ti membranes. Membrane

Elemental composition C (at%)

PVDF/TA PVDF/TA-Ti PVDF/TA-Ti PVDF/TA-Ti PVDF/TA-Ti

(1:2) (1:3) (1:4) (1:5)

52.0 53.2 51.1 51.1 50.2

± ± ± ± ±

2.2 1.1 0.5 1.1 2.0

O (at%)

F (at%)

0.5 4.7 5.0 5.4 5.8

47.5 41.6 42.9 42.2 42.4

± ± ± ± ±

0.2 0.5 0.3 0.2 0.2

± ± ± ± ±

2.7 1.5 0.5 1.1 1.6

Ti (at%)

O/C

– 0.5 1.0 1.3 1.6

0.010 0.090 0.098 0.107 0.115

± ± ± ±

0.1 0.1 0.1 0.1

Ti/C ± ± ± ± ±

0.003 0.007 0.011 0.002 0.005

– 0.010 0.021 0.027 0.031

Ti/O

± ± ± ±

0.001 0.002 0.005 0.004

– 0.117 0.210 0.251 0.285

± ± ± ±

0.008 0.012 0.010 0.021

Fig. 3. (a) The ATR-FTIR spectra of PVDF/TA and PVDF/TA-Ti membranes, and (b) proposed coordination modes for TA-Ti coordination networks.

the membranes.

and bis-coordination bridging states [43]). In addition, the enhanced gelation tendency of TA-Ti coordination networks at higher TiIV/TA molar ratio [28] might also favor the anchorage of TA-Ti coordination networks within PVDF membrane matrix. Fig. 3a showed the ATR-FTIR spectra of the PVDF/TA control membrane and PVDF/TA-Ti membranes. Characteristic peaks of PVDF were observed for all of the PVDF/TA control membrane and PVDF/TATi membranes at 1403 cm−1 for CH2 and CF2 deformation vibration, 1176 cm−1 for CF2 stretching vibration, 1070 cm−1 for C-C stretching vibration, 877 and 838 cm−1 for the vibration bands of β phase, and 750–760 cm−1 for the vibration bands of α phase [44]. For the PVDF/ TA control membrane, the peaks at 1717 and 1610 cm−1 were attributed to the C˭O stretching vibration and the vibration of substituted benzene rings from residual TA molecules [45], respectively. For PVDF/ TA-Ti membranes, the TiIV ions could coordinate with both ester and pyrogallol groups of TA molecules (Fig. 3b). The observed shift of C˭O stretching vibration from 1717 to 1701 cm−1 could be attributed to the electron-withdrawal from the formation of C˭O∙∙∙Ti complex. The observed shift of vibration of substituted benzene rings from 1610 to 1580 cm−1 could be attributed to the formation of five-membered pyrogallol-TiIV chelate rings. Absorption bands at 1486 cm−1 were associated with pyrogallol-TiIV coordination [46]. Absorption bands at 1637 cm−1 were related to the stretching vibration of Ti-O-Ti from the μ-oxo/hydroxo bridged multinuclear TiIV species (due to partial hydrolysis and gelation of TiIV in aqueous solution) in coordination networks [43,47]. The EDX elemental mapping was also used to analyze the distribution of TA-Ti coordination networks in PVDF/TA-Ti membranes. The EDX mapping images of the surface and cross-section of PVDF/TATi (1:5) membrane were shown in Fig. 4. The C and F elements were derived from PVDF, whereas the Ti and O elements were derived from TA-Ti coordination networks. A uniform distribution of Ti and O elements across the surface and cross-section of PVDF/TA-Ti membrane could be clearly revealed (Figs. 4 and S3). These results confirmed that the TA-Ti coordination networks were successfully formed throughout

3.4. Membrane surface charge The surface charge characteristics of the PVDF/TA control membrane and PVDF/TA-Ti membranes were investigated by determining the zeta potentials of membrane surfaces, as shown in the Fig. 5. Both the PVDF/TA control membrane and PVDF/TA-Ti membranes were negatively charged at all tested pH levels. Compared with PVDF/TA control membrane, the zeta potentials of PVDF/TA-Ti (1:2) membrane became much more negatively charged. The formation of TA-Ti coordination networks anchored more TA molecules and imparted more anionic pyrogallol groups on membrane surfaces. It was also found that the zeta potentials of PVDF/TA-Ti became less negatively charged with the increased TiIV/TA addition ratio. This phenomenon was probably due to the decrease in non-coordinated pyrogallol groups due to the further formation of more pyrogallol-Ti coordination bonding and more highly cross-linked structures in TA-Ti coordination networks. The zeta potentials of the PVDF/TA control membrane and PVDF/ TA-Ti membranes were both gradually decreased with increase of pH value from 4 to 8. With the further increase in pH value to 10, the PVDF/TA-Ti membranes exhibited sharp decrease in zeta potentials, while the zeta potential of PVDF/TA control membrane was not significantly decreased. For PVDF/TA-Ti membranes, the sharp decrease in the zeta potentials in alkaline solutions were attributed to the partial disassembly of TA-Ti coordination networks and the deprotonation of non-coordinated pyrogallol groups [26] (Fig. S2). The different decreasing tendency of zeta potentials between the PVDF/TA control membrane and PVDF/TA-Ti membranes confirmed that more TA molecules were anchored within PVDF/TA-Ti membranes by forming TATi coordination networks and only a small amount of TA molecules were retained in PVDF/TA control membrane. The results were in well accordance with those in the surface chemistry analysis.

440

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Fig. 4. The EDX mapping of various elements of the (a) surface and (b) cross-section of the PVDF/TA-Ti membranes. (a: the scale bars are 2 µm; b: the scale bars are 100 µm).

bound water content indicates a higher hydration capability. To investigate the effects of TA-Ti coordination networks on the hydration capability of membranes, both water contact angles and bound water contents of the PVDF/TA control membrane and PVDF/TA-Ti membranes were measured. The water contact angles of the PVDF/TA control membrane and PVDF/TA-Ti membranes were shown in Fig. 6a. Clearly, PVDF/TA-Ti membranes showed lower water contact angles than PVDF/TA control membrane with the water contact angles decreasing from about 67.9° to 39.6°, indicating that the incorporated TA-Ti coordination networks

3.5. Membrane hydration capability The hydration capability of membranes plays important roles in resist membrane fouling during oil-in-water emulsion separation process. High hydration capability is prone to forming compact hydration layers at membrane-water interfaces, which can inhibit the deposition or adsorption of oil droplets on membrane surfaces [48]. The hydration capability of membranes could be estimated from the water contact angles on the surfaces and the bound water (non-freezing bound water) contents in membranes. A lower water contact angle and a higher 441

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Fig. 5. The zeta potentials of PVDF/TA and PVDF/TA-Ti membranes.

Fig. 7. The pure water fluxes of PVDF/TA and PVDF/TA-Ti membranes.

were favorable to membrane hydrophilicity. The enhanced hydrophilicity of the PVDF/TA-Ti membranes was assumed to be related to the higher loading amount of TA-Ti coordination networks within PVDF/TA-Ti membranes. The phenolic hydroxyl groups or μ-oxo/hydroxo bridges from TA-Ti coordination networks could attract water molecules through hydrogen bonding, rendering PVDF/TA-Ti membranes with hydrophilic character. The higher surface roughness of the PVDF/TA-Ti membranes also enhanced the wetting property and hydrophilicity of the membranes. Besides water contact angles, the water contents (total, free and bound water content) in the PVDF/TA control membrane and PVDF/ TA-Ti membranes were also investigated. Total water content (TWC), free water content (FWC) and bound water content (BWC) was given in Fig. 6b. TWC was calculated according to the loss weight of water by freeze-drying. FWC was calculated comparing the enthalpy of melting endotherm (calculated from DSC cooling curves) with the melting enthalpy of pure water. BWC was the difference between the TWC and FWC. PVDF/TA-Ti membranes exhibited higher bound water contents than the PVDF/TA control membrane. This result was in agreement with the results obtained from water contact angle analysis. The bound water contents in PVDF/TA-Ti membranes were increased with the increased TiIV/TA addition ratio, likely due to the relatively higher hydrophilicity of PVDF/TA-Ti membranes and the stronger water binding ability of phenolic hydroxyl groups or μ-oxo/hydroxo bridges from TA-Ti coordination networks. The PVDF/TA-Ti membranes exhibited better hydration capability than PVDF/TA control membrane by incorporating TA-Ti coordination networks into membrane matrix. This outstanding hydration capability was predicted to be beneficial for PVDF/TA-Ti membranes to obtain

underwater oleophobicity. In Fig. 6c, the underwater oil contact angles of the PVDF/TA control membrane and the PVDF/TA-Ti (1:5) membrane were characterized by applying HD droplets onto the membrane surfaces. The underwater oil contact angle of the PVDF/TA control membrane was 125 ± 2°. In contrast, the underwater oil contact angles of the PVDF/TA-Ti membrane were increased to 158 ± 1.5°, showing underwater superoleophobicity. The higher hydration capability, the higher underwater superoleophobicity. The underwater superoleophobicity originated from the formation of hydration layer trapped at water/membrane interfaces. The hydration layer would repel oil fouling efficiently [49], which promised the potential antifouling properties of PVDF/TA-Ti membranes for oil/water separation.

3.6. Membrane permeation and antifouling property The pure water fluxes of the PVDF/TA control membrane and PVDF/TA-Ti membranes were shown in the Fig. 7. Compared with PVDF/TA membrane, the water permeate property of PVDF/TA-Ti membranes were enhanced by incorporating TA-Ti coordination networks. PVDF/TA-Ti (1:2) membrane had the highest water flux of approximately 228 L/(m2 h). The enhanced water permeate property of PVDF/TA-Ti membranes was attributed to the synergistic combination of higher porosities and enhanced hydrophilicity of PVDF/TA-Ti membranes (Fig. 6 and Table 1). The pure water fluxes of PVDF/TA-Ti membranes were also found to gradually decrease from 228 L/(m2 h) to 187 L/(m2 h) with the further increase in the TiIV/TA addition ratio. The gradual decrease in water fluxes of PVDF/TA-Ti membranes was mainly affected by the gradual decrease in porosities of PVDF/TA-Ti membranes.

Fig. 6. (a) Water contact angles, (b) total water content (TWC), free water content (FWC) and bound water content (BWC), and (c) underwater oil (HD) contact angles of PVDF/TA and PVDF/TA-Ti membranes. 442

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Fig. 8. Time-dependent membrane fluxes in oil-in-water emulsion filtration experiments and the summary of the average FRR, DRt and DRir/DRt parameters from three independent experiments: (a, b) PVDF/TA and PVDF/TA-Ti membranes for filtration of HD-in-water emulsion, (c, d) PVDF/TA-Ti (1:5) membrane for filtration of HD-in-water emulsion with different concentration, and (e, f) PVDF/TA-Ti (1:5) membrane for filtration of different oil-in-water emulsions.

the PVDF/TA control membrane, sever flux decline about 60.6% of the initial flux occurred during emulsion filtration, and recovered to only 56.9% of the initial flux after rinsing. The high ratio of DRir/DRt of 71.1% indicated severe oil fouling on PVDF/TA control membrane. For the PVDF/TA-Ti membranes, the antifouling property was gradually improved, as shown in Fig. 8b. By incorporating TA-Ti coordination networks into hydrophobic membrane matrix, the flux recovery ability of PVDF/TA-Ti membranes was significantly enhanced and the irreversible oil fouling on PVDF/TA-Ti membranes was almost averted. As the TiIV/TA addition ratio increased, PVDF/TA-Ti membranes showed an increasing trend in FRR (from 56.9% to 98.8%) and a decreasing trend in DRir/DRt (from 71.1% to 4.5%). Although the higher initial flux of PVDF/TA-Ti membranes could produce higher permeate drag force pushing of foulants to membrane surface than PVDF/TA control

The antifouling properties of the as-prepared PVDF/TA-Ti membranes were evaluated by dead-end filtration experiments using surfactant-stabilized oil-in-water emulsions as feed solution of model foulant. Fig. 8a showed the time-dependent fluxes of the PVDF/TA control membrane and PVDF/TA-Ti membranes during the filtration experiments using HD-in-water emulsion as model foulant system. Each filtration experiments included three stages: initial water permeation, emulsion filtration and water permeation after rinsing. During emulsion filtration, the emulsion fluxes (Jo) were firstly declined due to the deposition and accumulation of oil droplets on membrane surfaces driven by permeate drag force. Steady emulsion fluxes were then reached when the deposition/accumulation of oil droplets and the diffusion of oil droplets back to emulsion feeds were at an equilibrium under the specific near-surface shear force. It could be found in Fig. 8b that, for 443

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membrane, the flux decline was significantly alleviated, especially for PVDF/TA-Ti (1:4) and PVDF/TA-Ti (1:5) membranes. The remarkable alleviation of flux decline seemed to largely rely on the favorable surface properties of PVDF/TA-Ti membranes, and the adverse effects of high permeate drag force on fouling behavior could be generally offset. Since the average pore sizes of all membranes were one magnitude less than the sizes of oil droplets, all the PVDF/TA-Ti membrane exhibited desired oil rejection ratios higher than 99.9% (determined by UV–vis spectrophotometer). Therefore, membrane fouling was mainly caused by accumulation, adhesion and coalescence of rejected oil droplets on membrane surfaces. Both the surface charge and hydrophilicity/oleophobicity of membranes played important roles in inhibiting membrane fouling [50]. For PVDF/TA-Ti (1:2) membrane and PVDF/TA-Ti (1:3) membrane with higher negativity and lower hydrophilicity/oleophobicity, the enhanced antifouling performance could be attributed to the synergy of electrostatic repulsion and surface hydration effect. With the further increase in the TiIV/TA addition ratio, the surface negativity of membrane was weakened, and the surface hydrophilicity/oleophobicity of membrane was enhanced. Therefore, the hydrophilicity/oleophobicity of membranes became the leading factor for the favorable antifouling performance of PVDF/TA-Ti membranes. For PVDF/TA-Ti (1:4) membrane and PVDF/TA-Ti (1:5) membrane with lower negativity and higher hydrophilicity/superoleophobicity (underwater oil contact angle > 150°), the significant enhancement of the antifouling performance could be dominated by the effective hydration layer bounded tightly at the membrane-water interface. With the TA-Ti coordination networks compacting dense hydration layers on PVDF/TA-Ti membrane surfaces, the robust hydration energy barriers could be formed on membrane surfaces to repel the direct contact and interaction of oil droplets with membrane surfaces and effectively retard oil-adhesion [48,51]. As a result, the antifouling properties of PVDF/TA-Ti membranes were remarkably improved. Fig. 8c and d also illustrated that the PVDF/TA-Ti (1:5) membrane with the best antifouling performance could still maintain more than 95% FRR and less than 11% DRir/DRt even when challenged with HDin-water emulsions of 10-fold higher oil concentrations. Analogously, when applied to separate oil-in-water emulsions of higher viscosity oils (soybean oil and pump oil), the PVDF/TA-Ti (1:5) membrane exhibited considerable antifouling performance with FRR higher than 95% and DRir/DRt value lower than 9% (shown in Fig. 8e and f). These results indicated the wide-ranging antifouling property of PVDF/TA-Ti membranes and predicted the further extensive application in oil/water separation. The durability of antifouling performance of PVDF/TA-Ti membrane was also investigated through two-cycle long-time filtration experiments. The cycling performance of HD-in-water emulsion filtration

was conducted and recorded in Fig. 9. It could be clearly observed that the fluxes of PVDF/TA membrane declined dramatically after twocyclic filtration. The final FRR value was only 42.0% and the final DRir/ DRt value was as high as 71.6%, indicating severe oil fouling of PVDF/ TA membrane. However, for PVDF/TA-Ti (1:5) membrane, the FRR value in each cycle was higher than 95%, and the DRir/DRt value in each cycle was lower than 10%. After two-cyclic filtration, the fluxes of PVDF/TA-Ti membrane could still maintain at a sufficient high level with the final FRR value about 91.8%. Moreover, even after 30-day immersion in water, the PVDF/TA-Ti (1:5) membrane could also maintain more than 95% FRR during HD-in-water emulsion filtration experiment (Fig. S3), indicating the stability of antifouling performance. These results demonstrated the excellent and robust antifouling performance of PVDF/TA-Ti membranes for continuous oil-in-water emulsion separation. It should also be noted that the antifouling performance of PVDF/TA-Ti (1:5) membrane was declined after 1-h immersion in NaOH (pH = 10) solution (Fig. S3), which could be mainly attribute to the partial disassembly of TA-Ti coordination networks under high pH as discussed above. Considering the instability of PVDF/ TA-Ti membrane at high pH condition, the alkali cleaning reagents, as the most common chemicals for organic foulant removal, was inapplicable. The cleaning procedures of antifouling PVDF/TA-Ti membrane in practical applications were recommended to adopt primarily the physical hydraulic cleaning supplemented by the chemical surfactant cleaning. 4. Conclusions In this study, PVDF/TA-Ti membranes were prepared via non-solvent induced phase inversion with TiIV ion and TA in situ assembled in membrane casting solutions. The gallol-rich TA molecules coordinated with TiIV ions and cross-linked into TA-Ti coordination networks within PVDF membrane matrix. The hybridization of TA-Ti coordination networks into PVDF membrane matrix affected the morphologies and properties of PVDF/TA-Ti membranes, with enhanced membrane porosity, higher surface roughness, surface hydrophilicity, hydration capability and underwater superoleophobicity. Compared with the PVDF/ TA control membrane, the PVDF/TA-Ti membranes highly improved antifouling properties. During the filtration of oil-in-water emulsions, the PVDF/TA-Ti membranes derived from the maximum TiIV/TA addition ratio showed impressive, wide-ranging and durable antifouling performance with almost full flux recovery and greatly inhibited irreversible fouling. In summary, this study provided a facile in situ hybridization route to improve the antifouling capacity of PVDF membranes and highlight the potential of TA-Ti coordination networks in developing antifouling membranes. We also expected that this method

Fig. 9. (a) Time-dependent fluxes of PVDF/TA and PVDF/TA-Ti (1:5) membrane in two-cycle long-time filtration of HD-in-water emulsion and (b) the summary of the average FRR, DRt and DRir/DRt parameters from three independent experiments. 444

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would extended to other transition metal ions and polyphenol systems, and enabled the design of novel antifouling membranes or other hybrid materials.

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Acknowledgements

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This work was financially supported by National Natural Science Foundation of China (Grant No. 21706230), Zhejiang Provincial Natural Science Foundation of China (Grant No. LQ17B060002), National Key Research and Development Program-China (Grant No. 2016YFC0401508, 2017YFD0400604) and Zhejiang Provincial Collaborative Innovation Center Program 2011 (Grant No. G1504126001900)

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Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2018.06.014.

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