Iron-tannin-framework complex modified PES ultrafiltration membranes with enhanced filtration performance and fouling resistance

Iron-tannin-framework complex modified PES ultrafiltration membranes with enhanced filtration performance and fouling resistance

Journal of Colloid and Interface Science 505 (2017) 642–652 Contents lists available at ScienceDirect Journal of Colloid and Interface Science journ...

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Journal of Colloid and Interface Science 505 (2017) 642–652

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis

Iron-tannin-framework complex modified PES ultrafiltration membranes with enhanced filtration performance and fouling resistance Xiaofeng Fang a, Jiansheng Li a,⇑, Xin Li a,b, Shunlong Pan a, Xiuyun Sun a, Jinyou Shen a, Weiqing Han a, Lianjun Wang a, Bart Van der Bruggen b,c,⇑ a Key Laboratory of Jiangsu Province for Chemical Pollution Control and Resources Reuse, School of Environment and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, China b Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium c Faculty of Engineering and the Built Environment, Tshwane University of Technology, Private Bag X680, Pretoria 0001, South Africa

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Iron-tannin-framework complex

450 100

(ITF)/polyethersulfone (PES) membranes were synthesized.  ITF can regulate the porous structure and surface properties of PES membranes.  ITF/PES membranes exhibit simultaneous enhancement of permeability and selectivity.  ITF/PES membranes have long-term hydrophilicity and excellent antifouling ability.

i n f o

Article history: Received 12 May 2017 Revised 19 June 2017 Accepted 19 June 2017 Available online 20 June 2017 Keywords: Polyethersulfone Ultrafiltration membranes Iron-tannin-framework complex Hydrophilic performance Antifouling property

350

Flux J0(L/m .h.bar)

2

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250 200

40

150 100

0.00

0.15

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0.75

0.90

20

ITF content (g)

a b s t r a c t In this work, an iron-tannin-framework (ITF) complex was introduced to a poly (ether sulfone) (PES) casting solution as a hydrophilic additive to fabricate ITF/PES ultrafiltration (UF) membranes via non-solventinduced phase separation (NIPS). The structure and performance of the PES membranes with ITF concentrations ranging from 0 to 0.9 wt.% were systematically investigated by scanning electron microscopy, water contact angle, permeability, protein rejection and fouling resistance measurements. The results indicate that the pore structure and surface properties of PES UF membranes can be regulated by incorporating the ITF complex. Compared with classical PES membranes, ITF/PES membranes were found to have an increased hydrophilicity and porosity and reduced surface pore size. Importantly, a simultaneous enhancement of permeability and separation performance was observed for the blend membranes, which indicates that the introduction of the ITF complex can break through the trade-off between permeability and selectivity of UF membranes. When the ITF content was 0.3 wt.%, the permeability reached a maximum of 319.4 (L/m2 h) at 0.1 MPa, which is 1.6 times higher than that of the classical PES membrane. Furthermore, the BSA rejection increased from 25.9% for the PES membrane to 95.9% for the enhanced membrane. In addition, the same membrane showed an improved fouling resistance (higher flux recovery and lower adhesion force) and stable hydrophilicity (unchanged after incubation in deionized water for 30 days). The simple, green and cost-effective preparation process and the outstanding filtration performance highlight the potential of ITF/PES membranes for practical applications. Ó 2017 Elsevier Inc. All rights reserved.

⇑ Corresponding authors. E-mail addresses: [email protected] (J. Li), [email protected] be (B. Van der Bruggen). http://dx.doi.org/10.1016/j.jcis.2017.06.067 0021-9797/Ó 2017 Elsevier Inc. All rights reserved.

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X. Fang et al. / Journal of Colloid and Interface Science 505 (2017) 642–652

1. Introduction The lack of clean and fresh water is increasing due to population growth, enhanced living standards, as well as the expansion of agricultural and industrial activities [1,2]. To address this challenge, membrane filtration, especially using polymer membranes, is considered a highly competitive and promising approach for supplying water owing to the high efficiency, easy scale-up, stable effluent quality, and low energy consumption [3,4]. However, most polymeric membranes are relatively hydrophobic, which increases the resistance to water permeation and intensifies membrane fouling caused by irreversible deposition of foulants on the hydrophobic membrane surface, degenerating the comprehensive performance of the membranes [5,6]. In addition, polymeric membranes are limited by the trade-off between permeability and selectivity [7,8]. Therefore, the modification of polymeric membranes is required to enhance the anti-fouling properties and filtration performance before practical application. Numerous modification strategies for polymeric membranes have been developed [9]. These methods may include (1) surface modification, such as surface grafting [10–13] and surface coating [14–16], and (2) bulk modification, such as membrane materials modification with hydrophilic polymers by radical polymerization [17,18] and blending hydrophilic additives [19–22]. Surface coating and grafting are the commonly used techniques to enhance the membrane hydrophilicity and fouling resistance. However, they usually involve substantial post-treatment processes in membrane fabrication and modify only the outer surface of the membrane and not the internal pores [23]. In comparison, blending is simple, and no additional step is needed during the preparation of composite membranes. In addition, membranes with a tailored range of performances can be obtained simply by varying the blend composition to control the membrane structure. For this method, different types of hydrophilic materials including inorganic nanomaterials [24–27] and hydrophilic polymers [28–30] have been incorporated into polymer matrices as additives to improve the membrane performance. It has been demonstrated that the hydrophilicity, water flux and antifouling ability of membranes can be improved by blending inorganic fillers [31]. Nevertheless, the poor interfacial compatibility between polymer matrices and nano-sized additives is a major technical problem for the application of inorganic fillers to design composite membranes. This leads to a non-uniform dispersion of the additives in the membrane matrix and often a gradual detachment from the membrane, which deteriorates the antifouling efficiency of membrane and leads to

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secondary pollution [32,33]. Polymer blending is another important method to modify polymeric membranes with desired properties. Different types of homopolymers [28,34] and amphiphilic copolymers [35,36] have been used in the fabrication of blend membranes. Hydrophilic homopolymers such as poly (vinyl pyrrolidone) (PVP) and poly (ethylene glycol) (PEG) have been used as additives during membrane preparation to improve the membrane performance [28,37]. However, most homopolymers are dissolved and extracted during membrane formation and filtration, which may deteriorate antifouling properties. In contrast, amphiphilic copolymers are composed of both hydrophilic and hydrophobic segments, which can improve the membrane hydrophilicity and the compatibility with host polymers. It has been demonstrated that blending amphiphilic copolymers in a membrane matrix has lower leakage than homopolymers [22]. Nevertheless, the complex synthesis conditions and high cost of copolymers make it difficult to produce such modified membranes on a large scale. Therefore, it is highly desirable to seek a suitable additive for preparing a high-performance polymeric membrane in a facile, low cost and eco-friendly approach. Recently, iron-tannin-framework (ITF) complexes, a new kind of material constructed from iron ion (FeIII) and tannic acid (TA), has attracted widespread interest because of the coupled benefits of inorganic and organic building blocks [38,39]. TA is a low-cost and environmentally friendly polyphenol and can be directly extracted from plants including tea, wood and Chinese galls. Three galloyl groups from TA can react with each FeIII ion to form a stable octahedral complex [40], shown in Fig. 1. Additionally, TA can adhere to various substrates as it can strongly bind to substrates with different shapes and surface properties through covalent and noncovalent interactions [39,41]. The ITF complex has been widely used as a coating material for surface modification [42,43]. Because there are abundant phenolic hydroxyl groups (hydrophilic units) on the surface of TA, the ITF complex makes the surface of the modified material more hydrophilic. Studies have been carried out to modify the polymeric membranes by surface coating with ITF complexes to enhance the membrane hydrophilicity and fouling resistance [44]. Nevertheless, the procedures involved are complex and time-consuming. Furthermore, the water fluxes are relatively low due to pore plugging by the coating layer formed on the membrane surface. In contrast, blending of ITF complexes in the casting solutions may be a much simpler strategy for membrane modification to obtain controllable structures by varying the blend composition. Importantly, ITF complexes combine the benefits of inorganic and organic building blocks, which

Fig. 1. Schematic illustration of the procedure for the preparation of mixed matrix membranes with addition of ITF complex.

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isopropanol were purchased from Nanjing Chemical Reagent Co., Ltd (China).

Table 1 Composition of casting solutions used in this study. Membrane

PES (%)

PEG (%)

ITF (%)

DMF (%)

PES-0 PES-0.15 PES-0.3 PES-0.6 PES-0.9

16.5 16.5 16.5 16.5 16.5

8 8 8 8 8

0 0.15 0.3 0.6 0.9

75.5 75.35 75.2 74.9 74.6

2.2. Membrane preparation The PES membranes were prepared by the classical phase inversion method. The formulations of all membrane casting solutions are described in detail in Table 1. The casting solution of pristine PES membrane was prepared as follows: PES (16.5 g) and PEG (8 g) were completely dissolved in DMF (75.5 g) at 60 °C for 6 h. ITF/PES blend membranes were prepared as follows: 1) TA and FeCl3 were dissolved in DMF at 30 °C, respectively. Then the iron chloride solution was added into the tannic acid solution under stirring to obtain the ITF solution. Four different concentrations of TA/FeCl3 (0.125/0.025, 0.25/0.05, 0.5/0.1, 0.75/0.15 g) were dissolved in DMF (20 ml). 2) PES (16.5 g) and PEG (8 g) were completely dissolved in residual DMF at 60 °C for 6 h. 3) Then ITF solutions with different concentrations were dispersed in the PES solution under mechanical stirring at 30 °C for 2 h to obtain a homogeneous casting solution (Fig. 2a). The casting solutions were stored at room temperature for 12 h to ensure a complete release of bubbles and then coated on a flat glass plate using an automated film applicator (MRX-TM300, Shenzhen, China) with a gap of 350 lm. Subsequently, the cast films were immersed into a 25% isopropanol coagulation bath at room temperature after being exposed to the atmosphere for 30 s. Then the prepared membranes were immersed in pure water for at least 24 h to leach out the residual solvent before testing. The prepared membranes with different ITF contents are referred to as PES-0, PES-0.15, PES-0.3, PES0.6, PES-0.9, respectively (Table 1). The procedures for the preparation of the hybrid membranes are schematically illustrated in Fig. 1.

may enhance their compatibility and stability in membrane matrices. However, to the best of our knowledge, the effect of the addition of an ITF complex on the morphology and properties of polymeric membranes has not been reported. Herein, we developed a simple, green, and low-cost blending process to modify poly (ether sulfone) (PES) UF membranes by the phase inversion method using ITF complexes as the functional additive. The aim of this study is to explore the effects of ITF complex addition into the casting solution on the performance of PES membranes. The morphology, hydrophilicity, water permeability and bovine serum albumin (BSA) rejection of ITF/PES membranes were investigated in detail. The antifouling performance of prepared membranes was determined using BSA and humic acid (HA) as model foulants. To further understand the fouling resistance behavior of modified membrane surface, interaction forces between membrane surfaces and foulants were measured by atomic force microscopy (AFM) [45,46].

2. Experimental 2.1. Materials Polyethersulfone (PES Ultrason E6020P with Mw = 58 kDa) was supplied by BASF (Germany). Tannic acid (TA) and iron (III) chloride (FeCl3) were purchased from Aladdin (Shanghai, China). Polyethylene glycol (PEG, 20 kDa), bovine serum albumin (BSA, Mw = 67 kDa) and humic acid (HA) were supplied by Sinopharm Chemical Reagent Co., Ltd. N, N-dimethylformamide (DMF) and

2.3. Casting solution viscosity measurement The rheological characteristics of the casting solutions were investigated using a rotating viscometer (Brookfield LVDV-II + P, USA) at room temperature with a rotating rate of 20 rpm. To

(b)

Viscosity of casting solution (cP)

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(c) 1400

1350

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1200 0.00

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0.75

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ITF content (g) Fig. 2. (a) Digital photos and (b) viscosity of membrane casting solutions with different ITF contents; (c) Photographs of PES/ITF composite membranes prepared with the following concentrations: 0, 0.15, 0.3, 0.6, and 0.9 wt.% of ITF complex in a dry state.

X. Fang et al. / Journal of Colloid and Interface Science 505 (2017) 642–652

ensure reproducibility of the data, the experiments were performed three times. 2.4. Membrane characterization Fourier transform-infrared (FT-IR) spectra (Perkin Elmer 100) were used to investigate the chemical structure of the membranes. The spectrum of each membrane was observed in the wavenumber range from 500 to 2500 cm1. Thermogravimetric analysis (TGA, SDT Q600 USA) was conducted with a heating rate of 10 °C/min from 25 °C to 700 °C under 100 mL/min nitrogen. The contact angle change with the drop age of the prepared membranes was recorded to examine the degree of hydrophilicity by using a contact angle goniometer (Krüss DSA30, Germany). The surface and cross-section morphology of the prepared membrane was characterized by field emission scanning electronic microscopy (FESEM, Quanta 250, FEI). The membrane samples were dried in a super-critical drying apparatus (Leica, EM CPD300, Germany) and then pre-treated with gold ion sputtering for making the surface conductive. The mean pore size distribution of the prepared membrane was measured by the wet-up/dry-up method with the help of a Capillary Flow Porometer (Porolux 1000, IB-FT GmbH, Germany). The membrane samples (diameter: 2 cm) were wetted with commercial porefil liquid (surface tension of 16 dyn cm1) and pressed by pure nitrogen gas. The permeation flow rates and nitrogen pressure through the membrane were recorded with an automated capillary flow porometer system software. And the mean flow pore size was determined with results of the nitrogen flow rates through the membrane. The membrane porosity e, the ratio of pore volume to geometrical volume for the membranes, was obtained according to the gravimetric method by Eq. (1) [19]:

ðm  md Þ=q e¼ w Al

tested at 0.1 MPa using 1.0 g/L PEG aqueous solution. The solute concentrations were measured by the total organic carbon (TOC) analyzer (TOC-VCSH, Shimadzu, Japan) in both feed and permeate solutions. The rejections (R) were calculated by Eq. (3). Considering metal ions in natural water, the effect of metal ions on the membrane filtration performance and surface hydrophilicity were investigated. The 13.4 cm2 membranes were immersed in 100 ml solution of Pb2+, Cu2+ or Cd2+ ions with 20 ppm at pH = 6.0, respectively, and were kept on a shaker under 180 rpm for 24 h. The concentration of metal ions was analyzed by inductively coupled plasma mass spectrometry (ICP-OES, PerkinElmer Optima 7000DV). The pure water flux, BSA rejection and contact angle of the membrane after being immersed were tested again, as described before. 2.6. Antifouling experiments To characterize the fouling resistant ability of PES-0 and PES-0.3 membranes, two model foulant solutions (1 g/L BSA and 50 mg/L HA aqueous solutions) were used. The antifouling experiments can be divided into four steps: 1) the pure water flux J0 was measured for 45 min; 2) feed solution was then replaced by the model foulant solutions and the flux for the foulant solution Jf was measured for 60 min at a stirring speed of 300 rpm; 3) the membrane samples were washed with deionized water for 30 min; 4) the pure water flux (Jw) was measured again for another 45 min. Several indices, including the flux recovery ratio (FRR), total fouling ratio (Rt), reversible fouling ratio (Rr) and irreversible fouling ratio (Rir), were calculated to evaluate the antifouling property of the membrane using the following equations [47]:

Jw  100% J0

ð4Þ

Rt ¼

J0  Jf  100% J0

ð5Þ

Rr ¼

Jw  Jf  100% J0

ð6Þ

Rir ¼

J0  Jw  100% J0

ð7Þ

FRR ¼

ð1Þ

where mw and md are the weights of a membrane (g) at the wet and dry state, respectively. A is the sample area (cm2), l is the sample thickness (cm) and q is the density of pure water (g/cm3). 2.5. Ultrafiltration experiments All filtration experiments were conducted on a dead-end filtration apparatus (Model 8050, Millipore Corporation, effective area of 13.4 cm2) at 25 °C. The Membranes were initially compacted at 0.15 MPa transmembrane pressure (TMP) to obtain a steady permeation and then the pressure was lowered to 0.1 MPa. The pure water flux J0 (L/(m2 h)) was calculated by the following equation:

J0 ¼

V A  Dt

ð2Þ

where V (L) is the volume of permeated water, A (m2) is the effective membrane area and Dt (h) is the permeation time. The rejection of the membrane was carried out with a BSA solution (1.0 g/L in phosphate buffer solution at pH 7.4) at 0.1 MPa TMP. The BSA concentrations in the feed and the permeate solutions were determined using a UV–vis spectrophotometer (Lambda 25, PerkinElmer, USA), at a wavelength of 280 nm. The protein rejections (R) were calculated by Eq. (3):

R ¼ ð1 

Cp Þ  100% Cf

ð3Þ

where Cp and Cf (mg/mL) denote BSA concentrations in permeate and feed solutions, respectively. To further probe the performance of these membranes, a PEG molecular weight cut-off (MWCO) curve was done. The PEG rejections of membrane PES-0.3 were tested. The rejection of PEG was

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To further investigate the antifouling properties of the membrane, the interaction forces between the membrane surface and foulant-immobilized tips were measured with an atomic force microscope (AFM, Dimension Icon, Bruker, Germany). The foulant (BSA and HA) AFM tips were prepared by immersing a commercial V-shaped SiN probe (spring constant of 0.06 N m1, Novascan Technologies, Inc., USA) modified with a 5.0 lm carboxyl-SiO2 particle on the end of the cantilever in the foulant solutions, as described in our previous work [46,48]. The AFM adhesion force measurements were performed in a fluid cell under contact mode, following the procedures described by Elimelech and Li [45,48]. For each type of membrane sample, force measurements were conducted at five different locations, with more than 20 force curves collected at each location. 2.7. Stability and durability of the membranes To study the stability and durability of the ITF/PES composite membranes, the release of iron ions from the blend membranes was evaluated via static immersion well as dynamic filtration experiments. In the static immersion test, the PES-0.3 membrane samples were incubated in a shaken water bath at 20 °C for up to 30 days (the ratio was 4 cm2 membrane sample: 100 ml DI water).

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The water was replaced every two days. All water samples were collected and analyzed by ICP-OES. After 30 days, the water contact angles of membrane were tested again, as described before. The degree of iron release under the dynamic filtration was assessed by filtering 2.0 L of pure water through the membrane at 0.1 MPa and permeate was collected every 200 ml. The iron concentrations in the collected samples were analyzed by ICP-OES. To further evaluate the stability of ITF complex in acidic environment, the PES-0.3 membranes were immersed into aqueous solutions with different pH (2–7) for 10 h (the ratio was 4 cm2 membrane sample: 100 ml DI water). The release of iron ions was analyzed by ICP-OES.

3. Results and discussion 3.1. Chemical structure and morphology of PES hybrid membrane Composite membranes containing a range of contents of ITF complex were fabricated using the NIPS precipitation technique. Photographs of each membrane in a dry state are shown in Fig. 2 c. It can be observed that the pure PES membrane exhibits a white color, while the ITF/PES membranes have a grey color. As the complex loading increases, the membrane color successively changes from light grey to dark grey, indicating the successful introduction of ITF. To further confirm the existence of ITF, FTIR-ATR spectra were recorded to qualitatively analyze the surface functional groups. This is presented in Fig. 3 a. Compared with the pure PES membrane, the ATR-FTIR spectrum of the ITF/PES composite membranes showed an additional adsorption band at 1720 cm1, which could be attributed to the carboxyl absorption of TA [49]. These results confirm the successful incorporation of the ITF complex into the blend membranes. The presence of ITF complexes in the membrane matrix was also explored by TGA analysis. As presented in Fig. 3b, the TGA curves indicate that the residual weight increases gradually with an increase of the ITF concentration in the ITF/PES hybrid membranes. For a complete carbonization of the pure PES membrane, the residual weight should be related to the organic content of PES. The increase of residual mass should correspond to the ITF complex fraction in the ITF/PES membranes. This further confirms the presence of ITF complexes in the blend membrane composition. Fig. 4 shows the cross-sectional and surface morphologies of pure PES membrane and the blend membranes, as observed by

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FE-SEM. From the cross-section images, it was found that all membranes have the same hierarchical structure with a dense skin layer on a finger-like voids substructure, indicating that the addition of ITF complex would hardly affect the morphology of the cross section. However, the ITF content has a significant impact on the surface morphology, although pores were observed on the top surface for all membranes. With the increase in the ITF content, the surface pore size becomes smaller. Using the NIH Image J analysis software on the SEM images of the membrane top surface, the membrane surface pore size distribution was obtained (histograms in Fig. 4). It can be seen that the surface pore diameters of all the membranes are mainly distributed in the range of 5–33 nm. The blend membranes with addition of ITF complex exhibited smaller pore diameter values compared to the pure PES membrane. In addition, to further identify the effect of ITF on the pore structure, the porosity and mean pore size of membranes were determined. The relevant data are given in Table 2. The results showed that all ITF/PES composite membranes have a higher porosity than the pure PES membrane. However, the membrane porosity increases first and then slightly decreases with an increase in ITF content from 0.15 wt.% to 0.9 wt.%. The mean pore size decreased from 16.1 ± 0.9 nm to 13.7 ± 0.6 nm with increased ITF loading, which is consistent with the FESEM analysis. The changes in membrane pore structure can be explained by the phase separation mechanism during the membrane formation process. The addition of ITF complex reduces the thermodynamic stability of the casting solution, which increases the immiscibility of the casting solution with water and enhances the inflow rate of water when the casting film contacts the nonsolvent in the coagulation bath. This improves the water content in the nascent film, thereby increasing the porosity. Furthermore, some ITF complex is released from the nascent films into the water bath in the NIPS process, forming pores in the resultant ITF/PES blend membranes, which may be another reason for the increase of the membrane porosity. However, the addition of ITF in the casting solution also leads to an increase in the viscosity of the solution (Fig. 2b). The high viscosity of the casting solutions decreases the mass transfer rate of non-solvent and retards the movement of the additives [50], resulting in a decrease of surface pore size and porosity. Therefore, the membrane structures are determined by the tradeoff between the thermodynamic enhancement and the kinetic hindrance during the NIPS process, which is similar to previous studies [51,52]. The specific deposition location of ITF in the blend membrane was detected by elemental mapping analysis. The cross-sectional SEM image of PES-0.3 membrane and the corresponding

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PE S-0 .15

Weight (%)

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Fig. 3. (a) ATR-FTIR spectra and (b) TGA curves of the pure and blend modified PES membranes.

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C

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J

Fig. 4. FE-SEM images of top surfaces and cross sections for PES-0 (A-B), PES-0.15 (C-D), PES-0.3 (E-F), PES-0.6 (G-H) and PES-0.9 (I-J). The insets of each histogram show the corresponding surface pore diameter distribution.

Table 2 Porosity and mean pore size of the prepared membranes. Membrane

Porosity (%)

Mean pore size (nm)

PES-0 PES-0.15 PES-0.3 PES-0.6 PES-0.9

85.3 89.5 89.7 89.2 88.5

16.1 15.7 14.5 14.3 13.7

(±3.9) (±2.7) (±3.2) (±3.5) (±2.8)

(±0.9) (±0.7) (±0.8) (±0.7) (±0.6)

SEM-EDX mapping scanning spectra for carbon (C), oxygen (O), sulfur (S) and iron (Fe) elements, respectively, are shown in Fig. 5. It is noted that Fe distributes homogeneously in the membrane cross section. This clearly manifests that ITF complex is evenly distributed in the ITF/PES membrane matrix. 3.2. Membrane hydrophilicity and filtration performance Generally, an improved hydrophilicity is beneficial for the permeation flux and antifouling ability of membranes [53]. In this work, the dynamic water contact angle is used as a qualitative measurement of surface hydrophilicity. Fig. 6 shows the time dependence of the water contact angles of the membranes. The pure PES membrane (PES-0) has the highest initial contact angle

(75°) and there is no significant change with drop aging (120 s), revealing a poor hydrophilicity of PES. The initial water contact angles for PES-0.15, PES-0.3, PES-0.6 and PES-0.9 were 62°, 57°, 55° and 53°, respectively, which are lower than that of the pure PES membrane. The initial water contact angles of the membranes decrease with increasing content of ITF complex, suggesting an improved surface hydrophilicity. Moreover, the water contact angle on the blend membrane has an apparent declining trend within the measurement time. At a drop age of 120 s, water contact angles of PES-0.15, PES-0.3, PES-0.6, and PES-0.9 are 56°, 45°, 38°, and 34°, respectively. The decreasing tendency of water contact angles increases with the increase of ITF complex content. These results indicate that the addition of ITF complex improves not only membrane surface hydrophilicity but also the pore hydrophilicity. This implies that the blend membranes have a higher hydrophilicity and should achieve a higher water flux through the addition of ITF complex. The pure water flux and BSA rejection for PES and ITF/PES composite membranes were examined as shown in Fig. 7. The membrane PES-0 has minimum water flux (197.8 L/(m2 h) and BSA rejection (25.9%) at 0.1 MPa. However, after the addition of ITF complex, the flux first increases drastically and then decreases slightly, whereas the BSA rejection also significantly increases and then levels off. The highest flux (374.3 L/m2h) was observed

Fig. 5. (a) SEM image, (b) overlapped elemental distribution mapping and the elemental mapping scanning spectra of (c) C, (d) S, (e) O, and (f) Fe on the cross section of PES0.3 membrane.

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80

PES-0 BSA PES-0.3 BSA PES-0 HA PES-0.3 HA

1.0

Normalized flux (J/J0)

70

ºC

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PES-0 PES-0.15 PES-0.3 PES-0.6 PES-0.9

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Time (min)

Time(s) Fig. 6. The curves of water contact angle decaying with drop age for the pure and blend modified PES membranes.

Fig. 8. Time-dependent normalized flux for PES-0 and PES-0.3 membranes during the filtration of BSA and HA solution.

at 0.15 wt.% ITF complex. With a further increase of ITF complex contents from 0.3 wt.% to 0.9 wt.%, the water flux decreased from 319.4 L/(m2 h) to 246.4 L/(m2 h). However, the minimum flux (246.4 L/(m2 h)) was still about 24.6% higher than that of the pure PES membrane. Generally, the pure water flux of a membrane is significantly affected by the membrane hydrophilicity and pore structure (pore size, and porosity). It is well known that the improved hydrophilicity of the membranes can enhance the water permeation by attracting water molecules inside the membrane matrix and promoting them to transport more quickly through the membrane [31,54]. The addition of ITF complex yields a more hydrophilic surface and higher porosity for a modified membrane compared to a pure PES membrane. As a result, the water flux is considerably higher for ITF/PES membranes compared with pure PES membrane, although the membrane pore size kept decreasing. When the ITF complex content further increases from 0.3 to 0.9 wt. %, the hydrophilicity improves while the membrane surface pore size and porosity are reduced. In this concentration interval, the water flux of ITF/PES membranes is mainly determined by the membrane pore structure changes instead of hydrophilicity improvements. Consequently, the water flux decreases gradually with a further increase of the ITF complex content, compared with the PES-0.15 membrane. The BSA rejection increases from 44.5% to

Table 3 Antifouling indices (Rt, Rr, Rir and FRR) of the composite membrane during BSA and HA solution filtration measurements.

450 100 400

Flux J0(L/m .h.bar)

80

2

300 60

250 200

40

BSA Rejection(%)

350

150 20

100 0.00

0.15

0.30

0.45

0.60

0.75

0.90

ITF content (g) Fig. 7. Pure water flux and BSA rejection of the pure and blend modified PES membranes at 0.1 MPa.

Antifouling index (%)

FRR Rt Rr Rir

BSA

HA

PES-0

PES-0.3

PES-0

PES-0.3

66.05 92.36 58.42 33.94

88.29 88.81 77.10 11.71

70.05 87.56 57.62 29.94

90.81 62.05 52.87 9.18

97.9% with increasing ITF content from 0.15 to 0.9 wt.%, which is ascribed to the reduced pore size. Overall, both the permeability and selectivity were simultaneously enhanced by incorporating the ITF complex into the PES membrane, compared to the values of the pristine PES membrane. Comprehensively considering the water flux and BSA rejection, the content of ITF was fixed at 0.3 wt.% in the following experiments. The membrane with 0.3 wt.% ITF has a flux of 319.4 L/(m2 h) and BSA rejection of 95.9% at 0.1 MPa. To further probe the separation performance of the membrane, the molecular weight cut-off (MWCO) curves of PES-0.3 membrane was measured with a series of PEG under 0.1 MPa, as shown in Fig. S1. The membrane MWCO was reported as the PEG molecular weight at which the rejection was more than 90%. The MWCO of PES-0.3 membrane was 250 kDa, which was in the range of ultrafiltration. The metal ions such as Pb2+, Cu2+ and Cd2+ are expected to be present in natural water and could further react with the residue phenolic hydroxyl groups on the membrane. Thus, we determined the influence of metal ions (Pb2+, Cu2+ and Cd2+) on the performance of ITF/PES membrane. The water flux, BSA rejection and contact angle for PES-0.3 membrane were conducted after adsorbing metal ions, and the results are depicted in Table S1. It can be seen that the PES-0.3 membrane shown low adsorption capacities for Pb2+, Cu2+ and Cd2+ (23.1 lg Pb/cm2, 4.6 lg Cu/cm2 and 13.5 lg Cd/cm2, respectively). Compared with the performance of the initial PES-0.3 membrane, the water fluxes, BSA rejections and contact angles almost remain unchanged after adsorbing metal ions, which might be ascribed to the low adsorptive capacity and small ionic radius of the metal ions. These results demonstrated that the metal ions have negligible effect on the membrane surface property and filtration performance.

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(b)

(a) 0.4

30

0.2

25

-0.2

Frequency Count

Force (mN/m)

0.0

PES-0 BSA Avg.force= 1.18 mN/m PES-0.3 BSA Avg.force= 0.45mN/m

-0.4

PES-0 BSA PES-0.3 BSA

-0.6 -0.8 -1.0

20 15 10

-1.2

5

-1.4 -1.6 0

300

600

900

1200

1500

1800

0

2100

-1.6

-1.4

Extension (nm)

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

Adhesion Force (mN/m)

(c)

(d)

0.2

30

0.0

PES-0 HA Avg.force= 0.96 mN/m PES-0.3 HA Avg.force= 0.39 mN/m

25

PES-0 HA PES-0.3 HA

-0.2 -0.4 -0.6 -0.8

Frequency Count

Force (mN/m)

-1.2

20 15 10

-1.0

5

-1.2

0 0

300

600

900

1200

1500

1800

-1.4

2100

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

Adhesion Force (mN/m)

Extension (nm)

Fig. 9. Representative force-distance curves of (a) BSA-membrane; (b) the frequency distribution; (c) HA-membrane; (d) the frequency distribution.

ºC

60 50 40 30 20 10

1.0

1.0

(b)

0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8

PES-0.3

(c)

Total amout of Fe3+:4.21±0.22 g cm-2 Membrane area:13.4cm2

0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8

-1.0 0

0

0.8

Total amout of Fe3+:4.21±0.22 g cm-2

Concentration of Fe3+ ( g/L)

(a)

The releasing rate of Fe3+ ( g cm-2day-1)

70

5

10

PES-0.3-2

15

Time (day)

20

25

30

-1.0 0.0

0.4

0.8

1.2

1.6

2.0

Volume of Permeate (L)

Fig. 10. Stability test: the contact angle before and after incubating membrane samples in deionized water at 25 °C for 30 days (a); iron ion release from the batch (b) and flow-through (c) tests.

3.3. Antifouling properties The increase in hydrophilicity of the membrane matrix has a positive effect on the improvement of the antifouling performance [22]. In order to confirm this impact, antifouling experiments for membranes were conducted using BSA and HA as model foulants. Fig. 8 illustrates the time-dependent normalized flux curves for the PES-0 and PES-0.3 membranes during the filtration process. The filtration recycle can be divided three phases: deionized water flitra-

tion for 45 min, pollutant solution filtration for 60 min and deionized water filtration for another 45 min. When the feed solution was changed to the foulant solution, flux values of all the membranes declined promptly, which is due to the adsorption of BSA/HA on the membrane surface or membrane pores. Subsequently, the membrane was cleaned by hydraulic washing and the flux of cleaned membrane was recovered to a certain extent. It is worth noting that the relative flux of the PES-0 membrane declines faster than that of the ITF modified membrane (PES-0.3)

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Table 4 Comparative performance of the blended PES membranes with polymeric additive. Membrane type

Pressure (MPa)

Pure water flux (L/m2 h)

BSA rejection (%)

FRR (%)

Ref.

PES/P(H-P-A)-2 wt% PES/PVFM- 9 wt% PES/ PEI-0.3 wt% PES/MTB-5 wt% PES/PSA-1 wt% PES/[email protected] PES/ITF-0.3 wt%

0.1 0.1 0.1 0.1 0.4 0.2 0.1

186.9 1024.8 359.0 180.8 244.2 141.8 319.4

33.3 86.5 96.1 100 97.5 94.6 95.9

95 98.0 67.3 98.4 60.2 84.4 88.3

[35] [47] [52] [55] [56] [57] This work

during filtration of the pollutant solution. Due to the lower membrane hydrophilicity, the pure PES membrane was prone to the adhesion and deposition of foulant (BSA/HA), which resulted in more fouling and a larger flux drop. This demonstrates that the blend membrane has better antifouling properties. The antifouling properties of membranes were also evaluated by calculating the FRR, Rt, Rr and Rir. As shown in Table 3, the FRR was as low as 66.1% and an Rt value of 92.4% (Rr about 58.5%, Rir about 33.9%) was observed for the PES-0 membrane during BSA solution filtration. Compared with the neat PES membrane, the PES-0.3 membrane showed a larger FRR (88.3%). The Rt, and Rir values decreased to 88.8% and 11.7%, respectively. During filtration of a HA solution, FRR value was 70.1% and the corresponding Rt was as high as about 87.5% (Rr about 57.6%, Rir about 29.9%) for the PES-0 membrane. The PES-0.3 membrane has a reduced level of fouling (Rt 62.1%) and increased flux recovery (FRR about 90.8%). The Rir value was only 9.2%. As described above, the blend membrane has a higher FRR and lower Rir by compared with a pure PES membrane (PES-0) after the membrane cleaning process. These results suggest that the fouling resistance was improved remarkably via incorporating the ITF complex. The enhanced antifouling performance of blend membrane was closely related to the high hydrophilicity of the membrane. To further understand the antifouling behaviour of the membranes, interaction forces between the foulants and the membrane surfaces were measured by atomic force microscopy (AFM). Adhesion force measurements by AFM have been widely used to investigate membrane-foulant interactions leading to membrane fouling [45,48]. The adhesion force experiments were carried out to study the interactions between the two model pollutants (BSA and HA) and two kinds of membranes (PES-0 and PES-0.3). Fig. 9 (a) shows typical adhesion force curves when a BSA-tethered tip interacts with the pure PES and ITF/PES membrane surfaces in water, and the frequency distributions of the corresponding forces are displayed in Fig. 9 (b). It is observed that the average adhesion force of the PES-0.3 membrane and BSA (0.45 mN/m) is lower than that of PES-0 and BSA (1.18 mN/m), which shows that the blend membrane has a weaker interaction with BSA. Similarly, when a HA-tethered tip is used, the PES-0 membrane shows a higher average adhesion force (0.96 mN/m) compared with the PES-0.3 membrane (0.39 mN/m) (Fig. 9(c), (d)). These phenomena further indicate that the antifouling properties of blend membrane have been enhanced. This result is in agreement with the conclusion from the BSA and HA solution filtration experiments. 3.4. Stability of the ITF complex in/on the hybrid membranes The stability and durability of the ITF/PES composite membranes were examined by measuring the water contact angle of the samples before and after washing in a shaken water bath, which is presented in Fig. 10(a). It can be seen that the surface water contact angles of PES-0.3 membranes increased only slightly, within the range of 2°, after washing in deionized water for

30 days, suggesting a permanent hydrophilicity and good durability. Simultaneously, the release of iron ions from the blend membranes was measured via static immersion well as dynamic filtration experiments; this is presented in Fig. 10(b) and (c). No iron ions were detected in the water samples, which further confirm that the ITF complex can be stably immobilized in the PES membranes for long-term operation in aqueous environment. In order to further evaluate the stability of ITF complex in acidic environment, the PES-0.3 membranes were immersed into aqueous solutions with different pH for 10 h and the leaching of iron ion was analyzed by ICP-OES. As can be seen from Fig.S2, the releasing amount of Fe3+ decreased with the increase of pH in the soak solution. The releasing amounts of Fe3+ were 4.25, 4.18 and 0.43 lg/cm2 at pH 2, pH 3 and pH 4, respectively. When the pH was above 5, no iron ions were detected in the water samples. These results demonstrate that the ITF complexes are disassembled at low pH (<4), whereas ITF/PES membranes are stable at a solution pH higher than 5. 3.5. Comparison of ITF blended PES membrane with literature Table 4 compares pure water flux, BSA rejection and FRR values of the present membranes with other PES membranes with different polymeric additives [35,47,52,55–57]. Although several membranes show higher FRR, their corresponding water fluxes or BSA rejections are relatively low. Comprehensive comparison, the ITF/ PES membrane in this work synchronously exhibits excellent separation performance and antifouling properties under similar experimental conditions. 4. Conclusions Novel blend PES ultrafiltration membranes were prepared by addition of ITF complex in the casting solution. The influence of blending the ITF complex on the morphology and performance of the fabricated composite membranes was examined by measuring the pure water flux, BSA removal and fouling. The results showed that the addition of ITF complex improved the membrane hydrophilicity and porosity, and diminished the pore size. Importantly, the blend membranes with ITF complex have a high water permeability, superior BSA rejection and excellent anti-fouling ability. When the ITF content was 0.3 wt.%, the membrane PES0.3 exhibited a pure water flux of 319.4 L/m2 h and a BSA rejection of 95.9% at 0.1 MPa. This is 1.6 and 3.7 times that of the pure PES membrane. In addition, the same membrane showed an improved fouling resistance (higher flux recovery and lower adhesion force) and stable hydrophilicity (unchanged after incubation in deionized water for 30 days). In summary, our study provided a new way to solve the tradeoff between permeability and selectivity and enhance the antifouling property of PES UF membranes by the embedding of ITF complex. This can also be extended to prepare other composite membranes via phase inversion for water treatment applications.

X. Fang et al. / Journal of Colloid and Interface Science 505 (2017) 642–652

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 51678307, 51278247), the priority academic program development of Jiangsu higher education institutions, and Research Innovation Grant for Graduate of Jiangsu Common High School (Grant No. KYZZ16_0200).

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