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Separation and antifouling properties of hydrolyzed PAN hybrid membranes prepared via in-situ sol-gel SiO2 nanoparticles growth
Author’s Accepted Manuscript Separation and antifouling properties of hydrolyzed PAN hybrid membranes prepared via in-situ sol-gel SiO2 nanoparticles growth Yutao Hu, Zhenhua Lü, Chao Wei, Sanchuan Yu, Meihong Liu, Congjie Gao www.elsevier.com/locate/memsci
PII: DOI: Reference:
S0376-7388(17)31082-7 https://doi.org/10.1016/j.memsci.2017.09.081 MEMSCI15621
To appear in: Journal of Membrane Science Received date: 16 April 2017 Revised date: 25 June 2017 Accepted date: 26 September 2017 Cite this article as: Yutao Hu, Zhenhua Lü, Chao Wei, Sanchuan Yu, Meihong Liu and Congjie Gao, Separation and antifouling properties of hydrolyzed PAN hybrid membranes prepared via in-situ sol-gel SiO2 nanoparticles growth, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2017.09.081 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Separation and antifouling properties of hydrolyzed PAN hybrid membranes prepared via in-situ sol-gel SiO2 nanoparticles growth
Yutao Hua, b, Zhenhua Lüa, b, Chao Weia, b, Sanchuan Yua, b, *, Meihong Liub, Congjie Gaoc
a
Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, People’s Republic of China
b
Engineering Research Center for Eco-Dyeing & Finishing of Textiles, Ministry of Education of China, Zhejiang Sci-Tech University, Hangzhou 310018, People’s Republic of China c
Development Center of Water Treatment Technology，SOA，Hangzhou 310012, People’s Republic of China
* Corresponding author: Zhejiang Sci-Tech University, Department of Chemistry, Hangzhou 310018, People’s Republic of China. Tel.:+86-571-86843217, Fax: +86-571-86843217, E-mail: [email protected] (S. Yu). Abstract:
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In this work, a novel approach of hydrolysis followed by in-situ sol-gel of tetraethyl orthosilicate (TEOS) was developed to modify the polyacrylonitrile (PAN) porous membrane prepared via non-solvent induced phase separation technique for improved separation and antifouling properties. PAN/SiO2 hybrid membranes prepared using different contents of TEOS were characterized through ATR-FTIR, XPS, SEM, EDS, water contact angle measurement, cross-flow permeation test, static protein adsorption test and dynamic cross-flow protein fouling experiment. It was found that the sol-gel of TEOS took placed both on membrane surface and within membrane pore and the generated SiO2 particles affected both membrane surface and permeation properties through the roles of surface deposition and pore-filling, respectively. The in-situ sol-gel of TEOS could efficiently improve membrane separation property through tuning pore size and narrowing pore size distribution. Compared with the base and hydrolyzed PAN membranes of same water permeability, the PAN/SiO2 hybrid membrane exhibited a much lower molecular weight cut-off and thus enhanced rejection performance. Furthermore, both static protein adsorption and dynamic cross-flow fouling experiments with bovine serum albumin aqueous solution demonstrated that the surface deposition of SiO2 could appreciably improve membrane antifouling property through making membrane surface more hydrophilic, negatively charged and enriched with hydroxyl groups.
Keywords: Organic-inorganic hybrid membrane; In-situ sol-gel; Anti-fouling; Silica nanoparticle; Pore size distribution 1. Introduction
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As a green, powerful and state-of-art technology, membrane separation process has become a well-developed and the most promising technology for seawater desalination, drinking water purification and wastewater reclamation [1-5]. Polymeric porous membrane prepared through phase-inversion technique has become the dominant semi-permeable membrane for the pressure-driven membrane processes of microfiltration (MF), ultrafiltration (UF) and nanofiltration (NF) for its relatively lower manufacture cost compared with the inorganic membrane [6, 7]. However, poor controllability of pore size, wide pore size distribution and susceptibility to fouling are the major obstacles that limit the extended application of the phase-inversed polymeric porous membranes [8, 9]. Many efforts have been devoted to address the aforementioned drawbacks through surface modification [10, 11], blending a relatively hydrophilic polymer [12], adding an amphiphilic compound [13] or incorporating an inorganic nanomaterial [14]. Of these approaches, the incorporation of inorganic component seems to be more attractive since the inorganic component will produce synergistic effects and thus endow the formed organic-inorganic hybrid membrane with the desired properties of organic polymer and inorganic nanoparticle. The incorporation of inorganic nanoparticle into the polymeric porous membrane was first investigated through adding the bare nanoparticles directly into the membrane casting solution [15]. However, this method usually faces two major problems of the agglomeration of nonoparticles and the poor compatibility between inorganic component and polymer matrix [16]. A modified approach was then developed to improve the dispersion and compatibility of the nanoparticle through in-situ formation of inorganic nanoparticles inside membrane matrix by adding the inorganic precursor into the casting solution. For example, tetraethyl
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orthosilicate (TEOS) was added into the poly(vinylidene fluoride) (PVDF) casting solution by Liang et al. [17] to prepare PVDF/SiO2 hybrid membrane through the thermally induced phase separation technique. It was demonstrated that the generated SiO2 nanoparticles were uniformly distributed inside the membrane matrix, and the surface hydrophilicity, pure water permeability and anti-fouling property of the obtained membrane were appreciably improved. However, it is difficult to assure the synchronism between the sol-gel process and the phase separation process and to avoid the elution of TEOS from the membrane matrix. Chen et al. [18] also demonstrated that the generated SiO2 nanoparticles played as a pore-forming agent during phase separation and their out-diffusion from membrane matrix into the coagulation bath usually resulted in a high porosity and large average pore size. They reported that the pure water flux was dramatically increased from about 1.7 l/m2 h of the pure cellulose acetate (CA) membrane to 435.2 l/m2 h of the CA/SiO2 hybrid membrane at the expense of solute rejection ability. Furthermore, the above-mentioned approach still bears the additional drawback of the limiting content of nanoparticle that can be incorporated [19]. Therefore, research interest still remains in developing novel approaches to simultaneously improve the separation and surface properties of the phase-inversed polymeric porous membranes through incorporating inorganic material. It is known from literature that the method of in-situ growth of nanoparticles can be used to implement the incorporation of inorganic silica network within the pores of asymmetric polymeric membranes [20, 21]. Therefore, in this work, the novel approach of hydrolysis followed by in-situ sol-gel was developed to simultaneously tune the separation and surface properties of the polymeric porous membrane fabricated through non-solvent
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induced phase separation (NIPS). The synthesis strategy is schematically shown in Fig. 1. Polyacrylonitrile (PAN) was chosen as the support membrane for its superior thermal, chemical stability [22, 23] and the enrichment of nitrile group (CN) for hydrolysis [24]. The sol-gel process of TEOS would take place both on top surface and within pores of the hydrolyzed PAN porous membrane and the carboxyl groups of the hydrolyzed PAN matrix would play as the functional sites for the deposition of the generated SiO2 particle through the reaction with the silanol groups of SiO2 particle. It could be expected that the hydrolysis and surface deposition of SiO2 particles would improve membrane surface hydrophilicity and thus enhance membrane antifouling property, and the pore-filling role of the generated SiO2 particles would improve membrane pore size distribution and thus enhance membrane separation property. In the experiments, PAN/SiO2 hybrid membranes were prepared by varying the TEOS content. The tunability of membrane pore size and surface property through the developed approach of hydrolysis followed by sol-gel were investigated through rigorously characterizations of membrane physic-chemical property, permeation performance, porosity, pore size distribution and anti-fouling property.
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Fig.1. Schematic diagram for the fabrication of PAN/SiO2 hybrid porous membrane through hydrolysis followed by in-situ sol-gel of TEOS.
2. Experimental 2.1. Materials and reagents Polyacrylonitrile (PAN, MW=75,000 g/mol) was supplied by Shangyu Baisheng Chemicals Co., Ltd., of China and was dried to constant weight before use. N, N’-dimethylacetamide (DMAc) and tetraethyl orthosilicate (TEOS) of analytical grade were purchased from Tianjin Guanfu Fine Chemical Research Institute (China). Lithium chloride anhydrous (LiCl) was obtained from Tianjin Fengchuan Chemical Reagent Technologies Co., Ltd., of China. Bovine serum albumin (BSA, MW=67,000 g/mol) was purchased from Sinopharm Chemical Reagent Co., Ltd., of China. Phosphate-buffered saline (PBS, pH=7.4) 6/35
aqueous solution containing 0.03M KH2PO4, 0.03M Na2HPO4 and 0.03M NaOH was used as a buffer solution. Polyethylene glycol (PEG) fractions with molecular weights of 600, 1000, 2000, 4000 and 6000 g/mol, respectively, were purchased from Sigma-Aldrich. De-ionized (DI) water was used throughout the experiments. All other reagents were of analytical grade and used without further purification.
2.2. Preparation and hydrolysis of PAN porous membrane Flat-sheet polyacrylonitrile (PAN) porous membranes were prepared via the non-solvent induced phase separation (NIPS) method as reported previously [25]. The brief fabrication procedure is as follow. To begin with, materials such as PAN, LiCl and H2O were dissolved in the solvent of DMAc at about 80 ℃ under constant mechanical stirring of 2000 rpm to form a homogeneous membrane casting solution composing of 15.0% PAN, 3.0% LiCl, 81.0% DMAc and 1.0% H2O by weight. After de-aeration at 80 ℃ for 5.0 h, the prepared casting solution was cooled to about 40.0 ℃ and membrane fabrication was conducted on a self-made lab-scale membrane-casting machine by casting the polymer solution onto the hard surface of the reinforced support of polyester nonwoven fabric with a thickness of about 90 μm (Provided by Mitsubishi Co., Ltd., Japan). The nascent casted membrane with a total thickness of around 200 μm was then immediately immersed into the coagulation bath of de-ionized water of 20.0 ± 1.0 ℃ for phase separation. After complete precipitation, the membranes were transferred and rinsed thoroughly with de-ionized water. All the membranes thus obtained were designated as membrane PAN and kept in de-ionized water for further experiments.
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Hydrolysis of the polyacrylonitrile porous membrane was conducted by soaking the above prepared PAN membrane into the 1.0 M NaOH aqueous solution of 60 ℃ for 1 h. The hydrolysis mechanism was schematic illustrated in Fig. 2 [26]. The hydrolyzed membrane was then rinsed thoroughly with de-ionized water to remove all the residual NaOH and stored in de-ionized water. The hydrolyzed membrane exhibits a color of yellowish white and was designated as membrane PAN-COOH.
Fig.2. Hydrolysis mechanism of PAN porous membrane.
2.3. Preparation of PAN/SiO2 hybrid membranes Firstly, the displacement of water by ethanol was conducted with the hydrolyzed PAN membrane through successively soaking the membrane sample into ethanol/water solutions with preset volume ratios of 25/75, 50/50, 75/25 and 100/0, respectively, at room temperature. The soaking time of each ethanol/water solution was 15 min. After air-drying for about 10 min, the hydrolyzed PAN membrane was immersed into the TEOS/C2H5OH solution with a preset volume ratio of 25/75, 50/50 or 100/0 for 1.0 h. Then the membrane sample was air-dried and soaked into a hydrochloride acid/water mixture solution of room temperature and pH=1.0 for overnight, during which the sol-gel of TEOS took place both on surface and within the pores of the hydrolyzed PAN membrane. Finally, the membrane sample was rinsed thoroughly with de-ionized water and heat treated in a hot air dryer of 105 ℃ for 8 min to 8/35
form the final PAN/SiO2 hybrid membrane, which was designated as PAN/SiO2-X, where X indicates the TEOS content used in the membrane fabrication process.
2.4. Characterization of membranes Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was employed to analyze the surface composition of the dried membrane samples. The ATR-FTIR spectra were recorded on a Nicolet Aratar 370 FTIR spectrometer with a ZnSe crystal as the internal refection element with an angle of incidence of 45 degree. Membrane surface chemical composition was also characterized through X-ray photoelectron spectroscopy (XPS) by employing a PerkinElmer PHI 5000C ESCA System with Mg/Al Dual Anode Hel/Hell ultra violet source (400 W, 15 kV, 1253.6 eV). The spectra were taken with the electron emission angle at 54° to give a sampling depth of 10 nm, by a concentric hemispherical energy electron analyzer operating in the constant pass energy mode at 100.0 eV, using a 400 μm diameter analysis area. The data obtained was analyzed through PHI ACCESS ESCA-V6.0F software package. Scanning electron microscopy (SEM) carried out with a field emission scanning electron microscopy (FE-SEM) (HitachiS-4800, Japan) was adopted to observe membrane surface and cross-section morphological structure after gold sputter-coating. The cross-section was obtained by fracturing the membrane samples in liquid nitrogen. Energy-dispersive spectroscopy X-ray (EDS) analysis employing the FESEM with a 20 keV energy beam was used to map the chemical composition of the membrane. Membrane surface hydrophilicity was evaluated through the measurement of air/water
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contact angle performed with a DSA10-MK2 contact angle analyzer (KRUSS BmbH, Germany). Static contact angle was measured by sessile drop method under 25.0 ℃ as described in other studied [27, 28]. After introducing a de-ionized water drop of about 3.0 μl onto the dry membrane surface, the images of the droplet were recorded in equal time interval of 1.0 s for 120 s and contact angles were calculated from these images with calculation software. All results presented were an average of at least five membrane samples. Membrane porosity (ε, %) was determined by the wet-dry gravimetric method [29] and calculated through the following equation of (1):
（%）
WW WD 100 W Al
(1)
where WW is the weight of the wet membrane (g), WD is the weight of the dry membrane (g),
W is the water density (0.998 g/cm3), A is the wet membrane area (cm2) and l is the wet membrane thickness (cm). The reinforced non-woven fabric was peeled off from membrane sample before porosity determination. Membrane pore size distribution was estimated by using a LLP-1500A capillary flow porometer (Porous Materials, Inc., USA). The data was obtained with the aid of the computer software coupled to CFP.
2.5. Evaluation of membrane permeation property Membrane permeation properties in terms of pure water flux and solute rejection were evaluated through cross-flow permeation tests employing a lab-scale filtration setup with a permeation cell having an effective membrane area of 141.0 cm2. During the test, the retentate stream was circulated back to the feed tank through pressure regulator, and the permeate 10/35
stream was collected for detecting volume and solute content and then returned to the feed tank to maintain a constant feed concentration. Membrane coupon loaded in the permeation cell was pressurized under 2.0 bar with DI water for at least 5.0 h before test to ensure stable flux. After measurement of pure water flux using DI water under the pressure of 1.0 bar, model solute of PEG was added to the feed tank to make a solute concentration of 50 mg/l and to determine membrane PEG rejection under the same pressure. Water flux (J) was determined by measuring the volume of the permeate water collected over a certain period in terms of liter per square meter per hour (l/m2·h) and calculated through the following equation of (2): J
V A t
(2)
where V is the volume of the permeated water (l), A is the membrane filtration area (m2) and
t is the permeation time (h). The observed solute rejection (R) was calculated using the following equation of (3): C R(%) 1 P 100 CF
(3)
where Cf and Cp are the PEG concentrations in feed and permeate, respectively, which were determined using a spectrometric titration after iodine complexation [30] with an ultraviolet-visible spectrophotometer (UV759, Shanghai). All permeation tests were conducted under a constant of 25.0 ℃ and feed pH of 7.0 ± 0.2. The results presented were average data from at least three samples of each membrane type.
2.6. Static protein adsorption experiments Membrane anti-adsorption performance was evaluated through static protein adsorption 11/35
experiment. In the experiment, the surface of a 6 cm×6 cm membrane sample was first contacted with 50 ml 1.0 g/L BSA/PBS solution of pH 7.4 for 12 h at room temperature for static adsorption equilibrium of BSA molecules on membrane surface. Then the BSA solution was removed from membrane surface and the membrane sample was rinsed thoroughly with DI water to wash off all the loosely adsorbed BSA molecules. The amount of BSA that has adsorbed on the membrane surface was determined through measuring the BSA content of the aqueous solution before and after adsorption. The BSA content was determined by measuring the absorption peak intensity at 280 nm with an ultraviolet-visible spectrophotometer (UV759, Shanghai) and comparing the calibration plot drawn between BSA concentration and absorption peak intensity [31].
2.7. Dynamic cross-flow fouling experiments The anti-fouling property of the hybrid membrane was further investigated through cross-flow fouling experiments with BSA as model foulant by employing membrane PAN/SiO2-25. Since the pure water permeability of the above-prepared PAN and PAN-COOH membranes are much higher than that of membrane PAN/SiO2-25, new base and hydrolyzed polyacrylonitrile porous membranes with almost the same water permeability of membrane PAN/SiO2-25 were tailor fabricated through increasing the PAN content of the membrane casting solution up to 20% and 16.5% by weight, respectively (the contents of LiCl and H2O remain unchanged and the DMAc content decreases accordingly). The obtained base and hydrolyzed membranes were designed as PAN-20 and PAN-COOH-16.5, respectively and employed in the fouling experiments for comparison. Thus, all the tested membranes could be
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operated under the same pressure and initial water flux. In the fouling experiment, after filtration with de-ionized water of 25.0 ℃ under 2.0 bar for at least 5.0 h for stable membrane flux and under 1.0 bar for 10 min for the determination of membrane pure water flux (Jw), BSA was added to the feed tank to make a 1.0 g/l BSA aqueous solution and fouling test was then conducted under the recirculation model and the pressure of 1.0 bar and cross-flow velocity of 0.5 m/s by periodically motoring water flux until a steady-state flux (Js) was reached. After which, the membrane sample loaded in the permeation cell was flushed with de-ionized water of 25 ℃ for 30 min under the cross-flow velocity of about 2.0 m/s and the pressure of lower than 0.1 bar to remove all the BSA molecules loosely deposited on the membrane surface. Finally, the pure water flux of the water-cleaned membrane (Jc) was re-evaluated with de-ionized water of 25 ℃ at 1.0 bar. The relative flux reduction (RFR) and the flux recovery ratio (FRR) were determined according to the following equations of (4) and (5), respectively:
J RFR (%) 1 s 100 Jw FRR (%)
Jc 100 JW
(4)
(5)
3. Results and discussion 3.1. Membrane surface chemical structure The surface chemical structure of the fabricated membrane was first analyzed by ATR-FTIR spectroscopy. The spectra of the base (PAN), hydrolyzed (PAN-COOH) and hybrid (PAN/SiO2-X) membranes are presented in Fig. 3. The spectrum of base PAN
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membrane (Fig. 3a) clearly shows the typical absorption peaks of C≡N group at about 2243 and 1453 cm−1, the characteristic peak of C=O of the copolymerization component poly(methyl acrylate) (PMA) at 1731 cm−1 [32], the O-H bending of water at 1635 cm-1 [33] and the asymmetric stretching of methylene groups at about 2978 cm-1 [34]. After hydrolysis with NaOH solution, the new peaks at 1563 cm-1 and 1409 cm-1 (Fig. 3b) ascribed to the asymmetric and symmetric stretching vibrations of C=O, respectively, indicate the existence of carboxyl groups on the surface of the hydrolyzed PAN membrane [26, 35]. In the spectra of the PAN/SiO2 hybrid membranes (Fig. 3c-e), both the new peaks at 1076 and 797 cm-1 ascribed to Si-O-Si group and the new peak at 970 cm-1 ascribed to Si-OH group illustrate the existence of SiO2 nanoparticles on the surface of the hybrid membrane [36], while the new peak at 1166 cm-1 associated with the ester group [37, 38] reveals the reaction between the silanol group of the generated SiO2 nanoparticle and the carboxyl group of the hydrolyzed PAN membrane, which can be further confirmed by the weakening trend of the broad peak at 3200-3600 cm-1 of carboxylic acid group and the strengthening trend of the peak at 1730 cm-1 of ester group with increasing TEOS content [39].
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-1
1076 cm
-1
970 cm -1 797 cm
-1
1166 cm
-1
2978 cm
-1
2243 cm
e
-1
1730 cm
d c -1
1563 cm
-1
1409 cm
b -1
1635 cm -1 1453 cm -1 1731 cm
a
4000
3500
3000
2500
2000
1500
1000
-1
Wavenumbers(cm )
Fig.3. ATR-FTIR spectra of membranes PAN (a), PAN-COOH (b), PAN/SiO2-25 (c), PAN/SiO2-50 (d) and PAN/SiO2-100 (e).
Membrane surface chemical composition was further characterized through XPS. Fig. 4 shows the XPS spectra of the base, hydrolyzed and hybrid membranes. Compared with the spectra of the base and hydrolyzed PAN membranes, the new peaks of the PAN/SiO 2 hybrid membranes at 101.6 and 153.1 eV are attributed to the binding energy of Si 2p and 2s, respectively. Furthermore, the atomic percentages listed in Table 1 clearly demonstrate the increasing tendency of the atomic percentages of O and Si elements with the increase of TEOS content. These results clearly reveal the formation of SiO2 particles on membrane surface via the sol-gel process of TEOS.
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Counts / s
e
N
Si 2S Si 2p
O C
d
c b a
1200
1000
800
600
400
200
0
Binding Energy (ev)
Fig.4. XPS spectra of membranes PAN (a), PAN-COOH (b), PAN/SiO2-25 (c), PAN/SiO2-50 (d) and PAN/SiO2-100 (e).
Table 1 Surface atomic contents of the base, hydrolyzed and hybrid membranes by XPS. Atomic content (%) Membrane C
N
O
Si
O/C
Si/C
PAN
73.06
15.91
11.03
−
0.1510
−
PAN-COOH
67.83
19.49
12.68
−
0.1869
−
PAN/SiO2-25
58.96
15.84
17.06
8.14
0.2893
0.1381
PAN/SiO2-50
39.80
7.51
32.61
20.08
0.8193
0.5045
PAN/SiO2-100
36.59
5.07
35.44
22.91
0.9686
0.6261
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3.2. Membrane morphological structure Membrane morphological structure was observed through SEM. The surface and cross-section micrographs and the EDS spectra of the cross section of the near-surface skin layer are provided in Fig. 5. The SEM surface images (left) clearly prove the existence of SiO2 particles with nano-scale size on the surface of the PAN/SiO2 hybrid membranes and the decrease of surface pore size with increasing TEOS content. The decrease of membrane pore size of the PAN/SiO2 hybrid membrane is mainly due to the pore-filling role of the generated SiO2 nanoparticle and the contractile role during the step of heat treatment [40]. The SEM cross-section images (middle) illustrate that all the membranes exhibit an asymmetric structure comprising a relatively dense skin layer and a porous sub-layer with finger-like pores. However, compared with the base and hydrolyzed PAN membranes, the hybrid membranes exhibit a denser top skin layer as the result of the deposition of SiO2 nanoparticles. The EDS spectra of the cross section of the near surface skin layer (right) also illustrate the existence of SiO2 nanoparticle near the skin layer of the membrane and its increasing trend with the increase of TEOS content.
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Fig.5. SEM surface (30,000×, left) and cross-section (10,000×, middle) images and EDS spectra (red area of cross-section image, right) of membranes PAN, PAN-COOH and PAN/SiO2.
3.3. Membrane surface hydrophilicity The time dependent static water contact angles of the base PAN, hydrolyzed PAN (PAN-COOH) and PAN/SiO2 hybrid membranes are depicted in Fig. 6. It can be found from the figure that the surface water contact angle of each membrane shows an overall decreasing trend with drop age. The onset contact angles for membranes PAN, PA-COOH, PAN/SiO2-25, PAN/SiO2-50 and PAN/SiO2-100 are 61.8°, 38.3°, 41.7°, 45.7° and 51.2°, respectively, while the water contact angles of all the three PAN/SiO2 hybrid membranes decline faster than those of the base or hydrolyzed PAN membrane. The water droplets took about 66, 71, 75 and 90s 18/35
to penetrate into pores and disappear completely from the surfaces of membranes PAN/SiO2-100 PAN/SiO2-50, PAN/SiO2-25 and PAN-COOH, respectively, while the water droplet on the surface of membrane PAN remained unchanged when the drop age exceeds 120 s. The accelerated digressive rate of surface water contact angle reveals the improvement of surface hydrophilicity, since water drop will penetrate into the pores more quickly for the hydrophilic membrane surface than that of the hydrophobic one [27]. The improvement of surface hydrophilicity of the hydrolyzed membrane is due to the introduction of carboxylic groups through hydrolysis, while the further improvement of hydrophilicity of the hybrid membranes is probably attributed to the fact that the positive effect of the introduction of hydroxyl groups from SiO2 nanoparticles on hydrophilicity surpasses the negative effect of the reduction of carboxyl groups on membrane surface through esterification. The higher onset contact angle of PAN/SiO2 hybrid membrane compared with the hydrolyzed membrane is mainly due to the increased surface roughness resulting from the surface deposition of SiO2 particle as illustrated Fig. 5 [41, 42].
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65 60
PAN PAN-COOH PAN/SiO2-25 PAN/SiO2-50 PAN/SiO2-100
Surface water contact angle (°)
55 50 45 40 35 30 25 20 15 10 5 0 0
10
20
30
40
50
60
70
80
90
100
110
120
Drop age (s)
Fig.6.
Time dependence of water contact angle for membranes PAN (♦), PAN-COOH (■), PAN/SiO2-25 (▲), PAN/SiO2-50 (▼) and PAN/SiO2-100 (●).
3.4. Membrane porosity and pore size distribution Membrane porosity was determined by the gravimetric method and the calculated values are listed in Table 2. It can be found from the table that both the hydrolysis and in-situ sol-gel processes result in a decrease of membrane porosity, and the porosity of the PAN/SiO2 hybrid membrane decreases gradually with increasing TEOS content. The decline of porosity of the hydrolyzed PAN membrane is possibly due to the formation of the carboxyl groups from nitrile groups, since PAN with a certain content of hydrophilic carboxyl groups is easily swollen when exposed to aqueous medium and the hydrolyzed PAN molecular chain becomes more mobile to move to surface of the pores [24, 26, 43]. The further decrease of porosity of the PAN/SiO2 hybrid membrane is attributed to the deposition of SiO2 nanoparticles in the membrane pores.
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Table 2 Porosity values of membranes PAN, PAN-COOH and PAN/SiO2. Membrane
Porosity ( % )
PAN
78.6 ± 1.3
PAN-COOH
74.8 ± 1.1
PAN/SiO2-25
72.3 ± 0.8
PAN/SiO2-50
69.8 ± 1.0
PAN/SiO2-100
67.2 ± 0.7
The effects of the process of hydrolysis followed by sol-gel on both membrane pore size and pore size distribution were further studied by using a capillary flow porometer. The pore size distributions of membranes of PAN, PAN-COOH and PAN/SiO2-25 are depicted in Fig. 7. It is apparent from the graphs that the process of hydrolysis followed by sol-gel results in a reduced pore size and narrowed pore size distribution. The average pore diameters of the tested membranes PAN, PAN-COOH and PAN/SiO2-25 are around 0.073, 0.032 and 0.006 μm, respectively. Compared with the base PAN membrane, the pore size of the PAN/SiO2-25 hybrid membrane reduces by approximately 12 times. The results reveal that the approach of hydrolysis followed by in-situ sol-gel can be effectively used to tune the pore size and to improve the pore size distribution of the porous membrane.
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100
PAN PAN-COOH PAN/SiO2-25
Pore Size Distribution
90 80 70 60 50 40 30 20 10 0 0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
0.11
Average Diameter (microns)
Fig.7. Pore size distributions of membranes PAN (♦), PAN-COOH (■) and PAN/SiO2-25 (▲).
3.5. Membrane permeation properties Membrane permeation properties in terms of pure water flux and solute rejection were investigated through cross-flow permeation tests and the results of the fabricated membranes under 1.0 bar and 25.0 ℃ are tabulated in Table 3. The process of hydrolysis increases membrane BSA rejection from 72.77% to 98.26% and the following process of in-situ sol-gel process even makes the membrane fully reject BSA. The PEG retention cures of the hydrolyzed and hybrid membranes are illustrated in Fig. 8. The MWCO values taken as the molecular weight of PEG fraction rejected by the membrane to 90% [44] are about 6000, 4600 and 1800 g/mol for the hybrid membranes PAN/SiO2-25, PAN/SiO2-50 and PAN/SiO2-100, respectively. The decreasing tendency of the MWCO with increasing TEOS
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concentration also illustrates the controllability of the pore size of the PAN/SiO2 hybrid membrane through the simple control of TEOS content. Both the decreased porosity and pore size make attributions to the declined pure water flux of the hydrolyzed and hybrid membranes.
Table 3 Pure water fluxes and solute rejections of membranes PAN, PAN-COOH and PAN/SiO2. Solute Rejection b ( % )
Pure water Flux a
PEG
PEG
PEG
PEG
PEG
( l/m2 h )
600
1000
2000
4000
6000
PAN
605.4 ± 3.2
−
−
−
−
0.15
72.27
PAN-COOH
239.8± 2.6
−
0.56
1.02
4.88
10.26
98.26
PAN/SiO2-25
58.7 ± 2.0
1.27
5.24
46.56
79.11
90.18
≈100
PAN/SiO2-50
36.3 ± 1.1
9.22
41.09
67.52
88.08
96.96
≈100
PAN/SiO2-100
7.9 ± 0.5
55.46
80.59
91.94
98.44
99.02
≈100
Membrane
a
Tested with de-ionized water under 1.0 bar and 25.0 ℃;
b
Tested with 50 mg/l solute aqueous solution under 1.0 bar, 25.0 ℃ and pH 7.0.
BSA
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100
PAN/SiO2-100
PAN/SiO2-50
PAN/SiO2-25
PAN-COOH
90 80
PEG rejection (%)
70 60 50 40 30 20 10 0 600
1000
2000
4000
6000
Molecular weight of PEG (g/mol)
Fig.8. PEG rejection as a function of molecular weight for the hydrolyzed and hybrid membranes tested with 50 mg/l PEG aqueous solution at 1.0 bar and 25.0 °C.
To further illustrate the role of pore-filling on membrane separation property. Base membrane PAN-20 and hydrolyzed membrane PAN-COOH-16.5 with nearly the same water permeability of the hybrid membrane PAN/SiO2-25 were tailor fabricated and characterized for their MWCO values through PEG retention experiments. As tabulated in Table 4, one can find that, compared with the base and hydrolyzed PAN membranes of the same water permeability, the PAN/SiO2 hybrid membrane exhibits a much lower molecular weight cut-off. In other word, PAN/SiO2 hybrid membrane prepared through the approach developed in this work will have higher water permeability than those of the base and hydrolyzed PAN membranes of the same value of MWCO (membrane mean pore size).
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Table 4 Properties of the base, hydrolyzed and hybrid membranes of nearly the same water permeability.
*
Membrane
Jw (l/m2 h)
MWCO* (g/mol)
PAN-20
58.2±1.6
20,000±200
PAN-COOH-16.5
59.3±1.4
16,000±150
PAN/SiO2-25
58.7±1.8
6,000±80
Determined using PEG fractions according to the method described in Section 2.5.
3.6. Membrane anti-adsorption property Foulants can adsorb onto the membrane surface through hydrophobic interaction, hydrogen bonding, Van Der Waals attraction, and electrostatic interaction [45]. Therefore, an effective method to reduce membrane fouling is to improve membrane anti-adsorption ability through minimizing these adsorptive interactions between foulants and membrane surface. Membrane anti-adsorption ability was evaluated by static adsorption experiment with BSA as the model protein. As shown in Fig. 9, the step of hydrolysis can effectively mitigate the adsorption of BSA molecules on membrane surface, which is further reduced by the following step of in-situ sol-gel. Along with the improved surface hydrophilicity [46], the increased surface negative charge of the hydrolyzed and hybrid membranes as illustrated in Fig. 10 is the another reason for the improvement of anti-adsorption ability [47]. For the hybrid membrane, the enrichment of hydroxyl groups on membrane surface due to the deposited SiO2 nanoparticles is the other additional reason for their further improvement of anti-adsorption ability [48].
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120
2
BSA adsorption (μg/cm )
100
80
60
40
20
0
PAN
PAN PAN PAN PAN -CO /SiO /SiO /SiO OH 2 -100 5 2 -25 2 0
Fig.9. Static BSA adsorption values of membranes PAN, PAN-COOH and PAN/SiO2 under pH 7.4. -60
Surface zeta potential (mV)
-50
-40
-30
-20
-10
0
PAN
PAN PAN PAN PAN -CO /SiO /SiO /SiO OH 2 -100 2 -25 2 -50
Fig.10. Surface zeta potentials of membranes PAN, PAN-COOH and PAN/SiO2 tested with 0.001 mol/l KCl under pH 7.0.
The effect of the process of hydrolysis followed by sol-gel on membrane anti-fouling property was further investigated through dynamic cross-flow fouling experiments.
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Membranes listed in Table 4 were employed to assure the equal effect of the transverse hydrodynamic force (permeation drag) on the membrane fouling and the same degree of compactness of the BSA layer that deposited on membrane surface through being filtrated under the same pressure and initial permeate flux. The time-dependent fluxes of the three tested membranes in a three-cycle filtration of BSA aqueous solution followed by physical flushing with de-ionized water were illustrated in Fig. 11. It can be found from the figure that, in each cycle, the water flux of each tested membrane decreases quickly in the initial filtration time of 10 min as the result of the concentration polarization and membrane fouling and then reaches to a steady-state value after filtration of about 60 min, and the flux of the fouled membrane can be resume to a certain extent through physical flushing. However, the extents of fouling and flux recovery are different.
PAN-20
70 65
PAN-COOH-16.5
PAN/SiO2-25
Physical flushing
Physical flushing
60
Cycle 3
Cycle 2
Cycle 1
75
Physical flushing
2
Flux (l/m h)
55 50 45 40 35 30 25 20 15
0
20
40
60
80
100
120
140
160
180
200
220
Time (min)
Fig.11. Time-dependent flux of the tested membranes in a three-cycle filtration of 1.0 g/l BSA solution followed by physical cleaning with pure water under 1.0 bar, 25.0 ℃ and pH of 7.0. 27/35
As illustrated in Table 5, membrane PAN/SiO2-25 exhibits a lower RFR value and higher FRR value in each cycle compared with membranes PAN-20 and PAN-COOH-16.5. The lower RFR value indicates slight deposition or adsorption of BSA molecules on membrane surface, while higher FRR value indicates most of adsorbed and deposited BSA molecules can be washed away by simple hydraulic washing. The enhanced anti-fouling property of the hybrid membrane is mainly due to its improved surface hydrophilicity, enhanced surface negative charge as well as enrichment of surface hydroxyl groups [47]. Additionally, the three-cycle fouling experiment also demonstrate the reliability and durability of the hybrid PAN/SiO2 membrane prepared in this work.
Table 5 RFR and FRR values of membranes PAN-20, PAN-COOH-16.5 and PAN/SiO2-25 in the three-cycle fouling experiment. Membrane
First cycle
Second cycle
Third cycle
RFR (%)
FRR (%)
RFR (%) FRR (%)
RFR (%)
FRR (%)
58.1
80.8
58.9
77.3
59.1
76.5
PAN-COOH-16.5 53.5
88.3
54.2
85.4
54.7
84.6
50.8
91.3
51.8
90.6
51.8
90.3
PAN-20
PAN/SiO2-25
4. Conclusions PAN/SiO2 hybrid membrane with enhanced both separation and anti-fouling properties has been successfully prepared through adopting the approach of hydrolysis followed by in-situ sol-gel of TEOS. The following conclusions could be obtained from the experimental
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results: (1) ATR-FTIR and XPS analysis confirmed the formation of SiO2 particles on membrane surface and the reaction between the silanols of the SiO2 particle and the carboxyl groups of the hydrolyzed membrane. SEM surface and cross-section images verified the generation of SiO2 particles with nano-scale size and their depositions both on membrane surface and in membrane pores. (2) The SiO2 nanoparticles on surface endowed the hybrid membrane with enhanced surface hydrophilicity and abundant hydroxyl groups, and thus improved anti-fouling property. The SiO2 nanoparticles in membrane pores reduced membrane mean pore size and narrowed membrane pore size distribution, and thus improving separation property. Hybrid membrane possessed a much lower MWCO compared with those of the base and hydrolyzed PAN membranes of the same water permeability. (3) Both separation and surface properties of the prepared hybrid membrane could be tuned through the control of TEOS content, and the hybrid PAN/SiO2 membrane prepared possessed good reliability and durability. Therefore, the approach developed in this study is of potential application in fabricating organic-inorganic hybrid porous membranes with controlled pore size, narrowed pore size distribution and tailored surface property.
Acknowledgments The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China (NNSFC) (Grant nos. 21476213 and 21676256) and the 521 personnel training plan of Zhejiang Sci-Tech University.
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Research Highlights:
►Hydrolysis followed by sol-gel of TEOS was performed with PAN porous membrane. ►Silica nanoparticles deposited both on membrane surface and in membrane pores. ►Hybrid membrane showed reduced mean pore size and narrowed pore size distribution. ► Membrane antifouling property to BSA could be appreciably enhanced. ►Membrane property could be tuned through the simple control of TEOS content.
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PII: DOI: Reference:
S0376-7388(17)31082-7 https://doi.org/10.1016/j.memsci.2017.09.081 MEMSCI15621
To appear in: Journal of Membrane Science Received date: 16 April 2017 Revised date: 25 June 2017 Accepted date: 26 September 2017 Cite this article as: Yutao Hu, Zhenhua Lü, Chao Wei, Sanchuan Yu, Meihong Liu and Congjie Gao, Separation and antifouling properties of hydrolyzed PAN hybrid membranes prepared via in-situ sol-gel SiO2 nanoparticles growth, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2017.09.081 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Separation and antifouling properties of hydrolyzed PAN hybrid membranes prepared via in-situ sol-gel SiO2 nanoparticles growth
Yutao Hua, b, Zhenhua Lüa, b, Chao Weia, b, Sanchuan Yua, b, *, Meihong Liub, Congjie Gaoc
a
Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, People’s Republic of China
b
Engineering Research Center for Eco-Dyeing & Finishing of Textiles, Ministry of Education of China, Zhejiang Sci-Tech University, Hangzhou 310018, People’s Republic of China c
Development Center of Water Treatment Technology，SOA，Hangzhou 310012, People’s Republic of China
* Corresponding author: Zhejiang Sci-Tech University, Department of Chemistry, Hangzhou 310018, People’s Republic of China. Tel.:+86-571-86843217, Fax: +86-571-86843217, E-mail: [email protected] (S. Yu). Abstract:
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In this work, a novel approach of hydrolysis followed by in-situ sol-gel of tetraethyl orthosilicate (TEOS) was developed to modify the polyacrylonitrile (PAN) porous membrane prepared via non-solvent induced phase separation technique for improved separation and antifouling properties. PAN/SiO2 hybrid membranes prepared using different contents of TEOS were characterized through ATR-FTIR, XPS, SEM, EDS, water contact angle measurement, cross-flow permeation test, static protein adsorption test and dynamic cross-flow protein fouling experiment. It was found that the sol-gel of TEOS took placed both on membrane surface and within membrane pore and the generated SiO2 particles affected both membrane surface and permeation properties through the roles of surface deposition and pore-filling, respectively. The in-situ sol-gel of TEOS could efficiently improve membrane separation property through tuning pore size and narrowing pore size distribution. Compared with the base and hydrolyzed PAN membranes of same water permeability, the PAN/SiO2 hybrid membrane exhibited a much lower molecular weight cut-off and thus enhanced rejection performance. Furthermore, both static protein adsorption and dynamic cross-flow fouling experiments with bovine serum albumin aqueous solution demonstrated that the surface deposition of SiO2 could appreciably improve membrane antifouling property through making membrane surface more hydrophilic, negatively charged and enriched with hydroxyl groups.
Keywords: Organic-inorganic hybrid membrane; In-situ sol-gel; Anti-fouling; Silica nanoparticle; Pore size distribution 1. Introduction
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As a green, powerful and state-of-art technology, membrane separation process has become a well-developed and the most promising technology for seawater desalination, drinking water purification and wastewater reclamation [1-5]. Polymeric porous membrane prepared through phase-inversion technique has become the dominant semi-permeable membrane for the pressure-driven membrane processes of microfiltration (MF), ultrafiltration (UF) and nanofiltration (NF) for its relatively lower manufacture cost compared with the inorganic membrane [6, 7]. However, poor controllability of pore size, wide pore size distribution and susceptibility to fouling are the major obstacles that limit the extended application of the phase-inversed polymeric porous membranes [8, 9]. Many efforts have been devoted to address the aforementioned drawbacks through surface modification [10, 11], blending a relatively hydrophilic polymer [12], adding an amphiphilic compound [13] or incorporating an inorganic nanomaterial [14]. Of these approaches, the incorporation of inorganic component seems to be more attractive since the inorganic component will produce synergistic effects and thus endow the formed organic-inorganic hybrid membrane with the desired properties of organic polymer and inorganic nanoparticle. The incorporation of inorganic nanoparticle into the polymeric porous membrane was first investigated through adding the bare nanoparticles directly into the membrane casting solution [15]. However, this method usually faces two major problems of the agglomeration of nonoparticles and the poor compatibility between inorganic component and polymer matrix [16]. A modified approach was then developed to improve the dispersion and compatibility of the nanoparticle through in-situ formation of inorganic nanoparticles inside membrane matrix by adding the inorganic precursor into the casting solution. For example, tetraethyl
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orthosilicate (TEOS) was added into the poly(vinylidene fluoride) (PVDF) casting solution by Liang et al. [17] to prepare PVDF/SiO2 hybrid membrane through the thermally induced phase separation technique. It was demonstrated that the generated SiO2 nanoparticles were uniformly distributed inside the membrane matrix, and the surface hydrophilicity, pure water permeability and anti-fouling property of the obtained membrane were appreciably improved. However, it is difficult to assure the synchronism between the sol-gel process and the phase separation process and to avoid the elution of TEOS from the membrane matrix. Chen et al. [18] also demonstrated that the generated SiO2 nanoparticles played as a pore-forming agent during phase separation and their out-diffusion from membrane matrix into the coagulation bath usually resulted in a high porosity and large average pore size. They reported that the pure water flux was dramatically increased from about 1.7 l/m2 h of the pure cellulose acetate (CA) membrane to 435.2 l/m2 h of the CA/SiO2 hybrid membrane at the expense of solute rejection ability. Furthermore, the above-mentioned approach still bears the additional drawback of the limiting content of nanoparticle that can be incorporated [19]. Therefore, research interest still remains in developing novel approaches to simultaneously improve the separation and surface properties of the phase-inversed polymeric porous membranes through incorporating inorganic material. It is known from literature that the method of in-situ growth of nanoparticles can be used to implement the incorporation of inorganic silica network within the pores of asymmetric polymeric membranes [20, 21]. Therefore, in this work, the novel approach of hydrolysis followed by in-situ sol-gel was developed to simultaneously tune the separation and surface properties of the polymeric porous membrane fabricated through non-solvent
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induced phase separation (NIPS). The synthesis strategy is schematically shown in Fig. 1. Polyacrylonitrile (PAN) was chosen as the support membrane for its superior thermal, chemical stability [22, 23] and the enrichment of nitrile group (CN) for hydrolysis [24]. The sol-gel process of TEOS would take place both on top surface and within pores of the hydrolyzed PAN porous membrane and the carboxyl groups of the hydrolyzed PAN matrix would play as the functional sites for the deposition of the generated SiO2 particle through the reaction with the silanol groups of SiO2 particle. It could be expected that the hydrolysis and surface deposition of SiO2 particles would improve membrane surface hydrophilicity and thus enhance membrane antifouling property, and the pore-filling role of the generated SiO2 particles would improve membrane pore size distribution and thus enhance membrane separation property. In the experiments, PAN/SiO2 hybrid membranes were prepared by varying the TEOS content. The tunability of membrane pore size and surface property through the developed approach of hydrolysis followed by sol-gel were investigated through rigorously characterizations of membrane physic-chemical property, permeation performance, porosity, pore size distribution and anti-fouling property.
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Fig.1. Schematic diagram for the fabrication of PAN/SiO2 hybrid porous membrane through hydrolysis followed by in-situ sol-gel of TEOS.
2. Experimental 2.1. Materials and reagents Polyacrylonitrile (PAN, MW=75,000 g/mol) was supplied by Shangyu Baisheng Chemicals Co., Ltd., of China and was dried to constant weight before use. N, N’-dimethylacetamide (DMAc) and tetraethyl orthosilicate (TEOS) of analytical grade were purchased from Tianjin Guanfu Fine Chemical Research Institute (China). Lithium chloride anhydrous (LiCl) was obtained from Tianjin Fengchuan Chemical Reagent Technologies Co., Ltd., of China. Bovine serum albumin (BSA, MW=67,000 g/mol) was purchased from Sinopharm Chemical Reagent Co., Ltd., of China. Phosphate-buffered saline (PBS, pH=7.4) 6/35
aqueous solution containing 0.03M KH2PO4, 0.03M Na2HPO4 and 0.03M NaOH was used as a buffer solution. Polyethylene glycol (PEG) fractions with molecular weights of 600, 1000, 2000, 4000 and 6000 g/mol, respectively, were purchased from Sigma-Aldrich. De-ionized (DI) water was used throughout the experiments. All other reagents were of analytical grade and used without further purification.
2.2. Preparation and hydrolysis of PAN porous membrane Flat-sheet polyacrylonitrile (PAN) porous membranes were prepared via the non-solvent induced phase separation (NIPS) method as reported previously [25]. The brief fabrication procedure is as follow. To begin with, materials such as PAN, LiCl and H2O were dissolved in the solvent of DMAc at about 80 ℃ under constant mechanical stirring of 2000 rpm to form a homogeneous membrane casting solution composing of 15.0% PAN, 3.0% LiCl, 81.0% DMAc and 1.0% H2O by weight. After de-aeration at 80 ℃ for 5.0 h, the prepared casting solution was cooled to about 40.0 ℃ and membrane fabrication was conducted on a self-made lab-scale membrane-casting machine by casting the polymer solution onto the hard surface of the reinforced support of polyester nonwoven fabric with a thickness of about 90 μm (Provided by Mitsubishi Co., Ltd., Japan). The nascent casted membrane with a total thickness of around 200 μm was then immediately immersed into the coagulation bath of de-ionized water of 20.0 ± 1.0 ℃ for phase separation. After complete precipitation, the membranes were transferred and rinsed thoroughly with de-ionized water. All the membranes thus obtained were designated as membrane PAN and kept in de-ionized water for further experiments.
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Hydrolysis of the polyacrylonitrile porous membrane was conducted by soaking the above prepared PAN membrane into the 1.0 M NaOH aqueous solution of 60 ℃ for 1 h. The hydrolysis mechanism was schematic illustrated in Fig. 2 [26]. The hydrolyzed membrane was then rinsed thoroughly with de-ionized water to remove all the residual NaOH and stored in de-ionized water. The hydrolyzed membrane exhibits a color of yellowish white and was designated as membrane PAN-COOH.
Fig.2. Hydrolysis mechanism of PAN porous membrane.
2.3. Preparation of PAN/SiO2 hybrid membranes Firstly, the displacement of water by ethanol was conducted with the hydrolyzed PAN membrane through successively soaking the membrane sample into ethanol/water solutions with preset volume ratios of 25/75, 50/50, 75/25 and 100/0, respectively, at room temperature. The soaking time of each ethanol/water solution was 15 min. After air-drying for about 10 min, the hydrolyzed PAN membrane was immersed into the TEOS/C2H5OH solution with a preset volume ratio of 25/75, 50/50 or 100/0 for 1.0 h. Then the membrane sample was air-dried and soaked into a hydrochloride acid/water mixture solution of room temperature and pH=1.0 for overnight, during which the sol-gel of TEOS took place both on surface and within the pores of the hydrolyzed PAN membrane. Finally, the membrane sample was rinsed thoroughly with de-ionized water and heat treated in a hot air dryer of 105 ℃ for 8 min to 8/35
form the final PAN/SiO2 hybrid membrane, which was designated as PAN/SiO2-X, where X indicates the TEOS content used in the membrane fabrication process.
2.4. Characterization of membranes Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was employed to analyze the surface composition of the dried membrane samples. The ATR-FTIR spectra were recorded on a Nicolet Aratar 370 FTIR spectrometer with a ZnSe crystal as the internal refection element with an angle of incidence of 45 degree. Membrane surface chemical composition was also characterized through X-ray photoelectron spectroscopy (XPS) by employing a PerkinElmer PHI 5000C ESCA System with Mg/Al Dual Anode Hel/Hell ultra violet source (400 W, 15 kV, 1253.6 eV). The spectra were taken with the electron emission angle at 54° to give a sampling depth of 10 nm, by a concentric hemispherical energy electron analyzer operating in the constant pass energy mode at 100.0 eV, using a 400 μm diameter analysis area. The data obtained was analyzed through PHI ACCESS ESCA-V6.0F software package. Scanning electron microscopy (SEM) carried out with a field emission scanning electron microscopy (FE-SEM) (HitachiS-4800, Japan) was adopted to observe membrane surface and cross-section morphological structure after gold sputter-coating. The cross-section was obtained by fracturing the membrane samples in liquid nitrogen. Energy-dispersive spectroscopy X-ray (EDS) analysis employing the FESEM with a 20 keV energy beam was used to map the chemical composition of the membrane. Membrane surface hydrophilicity was evaluated through the measurement of air/water
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contact angle performed with a DSA10-MK2 contact angle analyzer (KRUSS BmbH, Germany). Static contact angle was measured by sessile drop method under 25.0 ℃ as described in other studied [27, 28]. After introducing a de-ionized water drop of about 3.0 μl onto the dry membrane surface, the images of the droplet were recorded in equal time interval of 1.0 s for 120 s and contact angles were calculated from these images with calculation software. All results presented were an average of at least five membrane samples. Membrane porosity (ε, %) was determined by the wet-dry gravimetric method [29] and calculated through the following equation of (1):
（%）
WW WD 100 W Al
(1)
where WW is the weight of the wet membrane (g), WD is the weight of the dry membrane (g),
W is the water density (0.998 g/cm3), A is the wet membrane area (cm2) and l is the wet membrane thickness (cm). The reinforced non-woven fabric was peeled off from membrane sample before porosity determination. Membrane pore size distribution was estimated by using a LLP-1500A capillary flow porometer (Porous Materials, Inc., USA). The data was obtained with the aid of the computer software coupled to CFP.
2.5. Evaluation of membrane permeation property Membrane permeation properties in terms of pure water flux and solute rejection were evaluated through cross-flow permeation tests employing a lab-scale filtration setup with a permeation cell having an effective membrane area of 141.0 cm2. During the test, the retentate stream was circulated back to the feed tank through pressure regulator, and the permeate 10/35
stream was collected for detecting volume and solute content and then returned to the feed tank to maintain a constant feed concentration. Membrane coupon loaded in the permeation cell was pressurized under 2.0 bar with DI water for at least 5.0 h before test to ensure stable flux. After measurement of pure water flux using DI water under the pressure of 1.0 bar, model solute of PEG was added to the feed tank to make a solute concentration of 50 mg/l and to determine membrane PEG rejection under the same pressure. Water flux (J) was determined by measuring the volume of the permeate water collected over a certain period in terms of liter per square meter per hour (l/m2·h) and calculated through the following equation of (2): J
V A t
(2)
where V is the volume of the permeated water (l), A is the membrane filtration area (m2) and
t is the permeation time (h). The observed solute rejection (R) was calculated using the following equation of (3): C R(%) 1 P 100 CF
(3)
where Cf and Cp are the PEG concentrations in feed and permeate, respectively, which were determined using a spectrometric titration after iodine complexation [30] with an ultraviolet-visible spectrophotometer (UV759, Shanghai). All permeation tests were conducted under a constant of 25.0 ℃ and feed pH of 7.0 ± 0.2. The results presented were average data from at least three samples of each membrane type.
2.6. Static protein adsorption experiments Membrane anti-adsorption performance was evaluated through static protein adsorption 11/35
experiment. In the experiment, the surface of a 6 cm×6 cm membrane sample was first contacted with 50 ml 1.0 g/L BSA/PBS solution of pH 7.4 for 12 h at room temperature for static adsorption equilibrium of BSA molecules on membrane surface. Then the BSA solution was removed from membrane surface and the membrane sample was rinsed thoroughly with DI water to wash off all the loosely adsorbed BSA molecules. The amount of BSA that has adsorbed on the membrane surface was determined through measuring the BSA content of the aqueous solution before and after adsorption. The BSA content was determined by measuring the absorption peak intensity at 280 nm with an ultraviolet-visible spectrophotometer (UV759, Shanghai) and comparing the calibration plot drawn between BSA concentration and absorption peak intensity [31].
2.7. Dynamic cross-flow fouling experiments The anti-fouling property of the hybrid membrane was further investigated through cross-flow fouling experiments with BSA as model foulant by employing membrane PAN/SiO2-25. Since the pure water permeability of the above-prepared PAN and PAN-COOH membranes are much higher than that of membrane PAN/SiO2-25, new base and hydrolyzed polyacrylonitrile porous membranes with almost the same water permeability of membrane PAN/SiO2-25 were tailor fabricated through increasing the PAN content of the membrane casting solution up to 20% and 16.5% by weight, respectively (the contents of LiCl and H2O remain unchanged and the DMAc content decreases accordingly). The obtained base and hydrolyzed membranes were designed as PAN-20 and PAN-COOH-16.5, respectively and employed in the fouling experiments for comparison. Thus, all the tested membranes could be
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operated under the same pressure and initial water flux. In the fouling experiment, after filtration with de-ionized water of 25.0 ℃ under 2.0 bar for at least 5.0 h for stable membrane flux and under 1.0 bar for 10 min for the determination of membrane pure water flux (Jw), BSA was added to the feed tank to make a 1.0 g/l BSA aqueous solution and fouling test was then conducted under the recirculation model and the pressure of 1.0 bar and cross-flow velocity of 0.5 m/s by periodically motoring water flux until a steady-state flux (Js) was reached. After which, the membrane sample loaded in the permeation cell was flushed with de-ionized water of 25 ℃ for 30 min under the cross-flow velocity of about 2.0 m/s and the pressure of lower than 0.1 bar to remove all the BSA molecules loosely deposited on the membrane surface. Finally, the pure water flux of the water-cleaned membrane (Jc) was re-evaluated with de-ionized water of 25 ℃ at 1.0 bar. The relative flux reduction (RFR) and the flux recovery ratio (FRR) were determined according to the following equations of (4) and (5), respectively:
J RFR (%) 1 s 100 Jw FRR (%)
Jc 100 JW
(4)
(5)
3. Results and discussion 3.1. Membrane surface chemical structure The surface chemical structure of the fabricated membrane was first analyzed by ATR-FTIR spectroscopy. The spectra of the base (PAN), hydrolyzed (PAN-COOH) and hybrid (PAN/SiO2-X) membranes are presented in Fig. 3. The spectrum of base PAN
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membrane (Fig. 3a) clearly shows the typical absorption peaks of C≡N group at about 2243 and 1453 cm−1, the characteristic peak of C=O of the copolymerization component poly(methyl acrylate) (PMA) at 1731 cm−1 [32], the O-H bending of water at 1635 cm-1 [33] and the asymmetric stretching of methylene groups at about 2978 cm-1 [34]. After hydrolysis with NaOH solution, the new peaks at 1563 cm-1 and 1409 cm-1 (Fig. 3b) ascribed to the asymmetric and symmetric stretching vibrations of C=O, respectively, indicate the existence of carboxyl groups on the surface of the hydrolyzed PAN membrane [26, 35]. In the spectra of the PAN/SiO2 hybrid membranes (Fig. 3c-e), both the new peaks at 1076 and 797 cm-1 ascribed to Si-O-Si group and the new peak at 970 cm-1 ascribed to Si-OH group illustrate the existence of SiO2 nanoparticles on the surface of the hybrid membrane [36], while the new peak at 1166 cm-1 associated with the ester group [37, 38] reveals the reaction between the silanol group of the generated SiO2 nanoparticle and the carboxyl group of the hydrolyzed PAN membrane, which can be further confirmed by the weakening trend of the broad peak at 3200-3600 cm-1 of carboxylic acid group and the strengthening trend of the peak at 1730 cm-1 of ester group with increasing TEOS content [39].
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-1
1076 cm
-1
970 cm -1 797 cm
-1
1166 cm
-1
2978 cm
-1
2243 cm
e
-1
1730 cm
d c -1
1563 cm
-1
1409 cm
b -1
1635 cm -1 1453 cm -1 1731 cm
a
4000
3500
3000
2500
2000
1500
1000
-1
Wavenumbers(cm )
Fig.3. ATR-FTIR spectra of membranes PAN (a), PAN-COOH (b), PAN/SiO2-25 (c), PAN/SiO2-50 (d) and PAN/SiO2-100 (e).
Membrane surface chemical composition was further characterized through XPS. Fig. 4 shows the XPS spectra of the base, hydrolyzed and hybrid membranes. Compared with the spectra of the base and hydrolyzed PAN membranes, the new peaks of the PAN/SiO 2 hybrid membranes at 101.6 and 153.1 eV are attributed to the binding energy of Si 2p and 2s, respectively. Furthermore, the atomic percentages listed in Table 1 clearly demonstrate the increasing tendency of the atomic percentages of O and Si elements with the increase of TEOS content. These results clearly reveal the formation of SiO2 particles on membrane surface via the sol-gel process of TEOS.
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Counts / s
e
N
Si 2S Si 2p
O C
d
c b a
1200
1000
800
600
400
200
0
Binding Energy (ev)
Fig.4. XPS spectra of membranes PAN (a), PAN-COOH (b), PAN/SiO2-25 (c), PAN/SiO2-50 (d) and PAN/SiO2-100 (e).
Table 1 Surface atomic contents of the base, hydrolyzed and hybrid membranes by XPS. Atomic content (%) Membrane C
N
O
Si
O/C
Si/C
PAN
73.06
15.91
11.03
−
0.1510
−
PAN-COOH
67.83
19.49
12.68
−
0.1869
−
PAN/SiO2-25
58.96
15.84
17.06
8.14
0.2893
0.1381
PAN/SiO2-50
39.80
7.51
32.61
20.08
0.8193
0.5045
PAN/SiO2-100
36.59
5.07
35.44
22.91
0.9686
0.6261
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3.2. Membrane morphological structure Membrane morphological structure was observed through SEM. The surface and cross-section micrographs and the EDS spectra of the cross section of the near-surface skin layer are provided in Fig. 5. The SEM surface images (left) clearly prove the existence of SiO2 particles with nano-scale size on the surface of the PAN/SiO2 hybrid membranes and the decrease of surface pore size with increasing TEOS content. The decrease of membrane pore size of the PAN/SiO2 hybrid membrane is mainly due to the pore-filling role of the generated SiO2 nanoparticle and the contractile role during the step of heat treatment [40]. The SEM cross-section images (middle) illustrate that all the membranes exhibit an asymmetric structure comprising a relatively dense skin layer and a porous sub-layer with finger-like pores. However, compared with the base and hydrolyzed PAN membranes, the hybrid membranes exhibit a denser top skin layer as the result of the deposition of SiO2 nanoparticles. The EDS spectra of the cross section of the near surface skin layer (right) also illustrate the existence of SiO2 nanoparticle near the skin layer of the membrane and its increasing trend with the increase of TEOS content.
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Fig.5. SEM surface (30,000×, left) and cross-section (10,000×, middle) images and EDS spectra (red area of cross-section image, right) of membranes PAN, PAN-COOH and PAN/SiO2.
3.3. Membrane surface hydrophilicity The time dependent static water contact angles of the base PAN, hydrolyzed PAN (PAN-COOH) and PAN/SiO2 hybrid membranes are depicted in Fig. 6. It can be found from the figure that the surface water contact angle of each membrane shows an overall decreasing trend with drop age. The onset contact angles for membranes PAN, PA-COOH, PAN/SiO2-25, PAN/SiO2-50 and PAN/SiO2-100 are 61.8°, 38.3°, 41.7°, 45.7° and 51.2°, respectively, while the water contact angles of all the three PAN/SiO2 hybrid membranes decline faster than those of the base or hydrolyzed PAN membrane. The water droplets took about 66, 71, 75 and 90s 18/35
to penetrate into pores and disappear completely from the surfaces of membranes PAN/SiO2-100 PAN/SiO2-50, PAN/SiO2-25 and PAN-COOH, respectively, while the water droplet on the surface of membrane PAN remained unchanged when the drop age exceeds 120 s. The accelerated digressive rate of surface water contact angle reveals the improvement of surface hydrophilicity, since water drop will penetrate into the pores more quickly for the hydrophilic membrane surface than that of the hydrophobic one [27]. The improvement of surface hydrophilicity of the hydrolyzed membrane is due to the introduction of carboxylic groups through hydrolysis, while the further improvement of hydrophilicity of the hybrid membranes is probably attributed to the fact that the positive effect of the introduction of hydroxyl groups from SiO2 nanoparticles on hydrophilicity surpasses the negative effect of the reduction of carboxyl groups on membrane surface through esterification. The higher onset contact angle of PAN/SiO2 hybrid membrane compared with the hydrolyzed membrane is mainly due to the increased surface roughness resulting from the surface deposition of SiO2 particle as illustrated Fig. 5 [41, 42].
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65 60
PAN PAN-COOH PAN/SiO2-25 PAN/SiO2-50 PAN/SiO2-100
Surface water contact angle (°)
55 50 45 40 35 30 25 20 15 10 5 0 0
10
20
30
40
50
60
70
80
90
100
110
120
Drop age (s)
Fig.6.
Time dependence of water contact angle for membranes PAN (♦), PAN-COOH (■), PAN/SiO2-25 (▲), PAN/SiO2-50 (▼) and PAN/SiO2-100 (●).
3.4. Membrane porosity and pore size distribution Membrane porosity was determined by the gravimetric method and the calculated values are listed in Table 2. It can be found from the table that both the hydrolysis and in-situ sol-gel processes result in a decrease of membrane porosity, and the porosity of the PAN/SiO2 hybrid membrane decreases gradually with increasing TEOS content. The decline of porosity of the hydrolyzed PAN membrane is possibly due to the formation of the carboxyl groups from nitrile groups, since PAN with a certain content of hydrophilic carboxyl groups is easily swollen when exposed to aqueous medium and the hydrolyzed PAN molecular chain becomes more mobile to move to surface of the pores [24, 26, 43]. The further decrease of porosity of the PAN/SiO2 hybrid membrane is attributed to the deposition of SiO2 nanoparticles in the membrane pores.
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Table 2 Porosity values of membranes PAN, PAN-COOH and PAN/SiO2. Membrane
Porosity ( % )
PAN
78.6 ± 1.3
PAN-COOH
74.8 ± 1.1
PAN/SiO2-25
72.3 ± 0.8
PAN/SiO2-50
69.8 ± 1.0
PAN/SiO2-100
67.2 ± 0.7
The effects of the process of hydrolysis followed by sol-gel on both membrane pore size and pore size distribution were further studied by using a capillary flow porometer. The pore size distributions of membranes of PAN, PAN-COOH and PAN/SiO2-25 are depicted in Fig. 7. It is apparent from the graphs that the process of hydrolysis followed by sol-gel results in a reduced pore size and narrowed pore size distribution. The average pore diameters of the tested membranes PAN, PAN-COOH and PAN/SiO2-25 are around 0.073, 0.032 and 0.006 μm, respectively. Compared with the base PAN membrane, the pore size of the PAN/SiO2-25 hybrid membrane reduces by approximately 12 times. The results reveal that the approach of hydrolysis followed by in-situ sol-gel can be effectively used to tune the pore size and to improve the pore size distribution of the porous membrane.
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100
PAN PAN-COOH PAN/SiO2-25
Pore Size Distribution
90 80 70 60 50 40 30 20 10 0 0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
0.11
Average Diameter (microns)
Fig.7. Pore size distributions of membranes PAN (♦), PAN-COOH (■) and PAN/SiO2-25 (▲).
3.5. Membrane permeation properties Membrane permeation properties in terms of pure water flux and solute rejection were investigated through cross-flow permeation tests and the results of the fabricated membranes under 1.0 bar and 25.0 ℃ are tabulated in Table 3. The process of hydrolysis increases membrane BSA rejection from 72.77% to 98.26% and the following process of in-situ sol-gel process even makes the membrane fully reject BSA. The PEG retention cures of the hydrolyzed and hybrid membranes are illustrated in Fig. 8. The MWCO values taken as the molecular weight of PEG fraction rejected by the membrane to 90% [44] are about 6000, 4600 and 1800 g/mol for the hybrid membranes PAN/SiO2-25, PAN/SiO2-50 and PAN/SiO2-100, respectively. The decreasing tendency of the MWCO with increasing TEOS
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concentration also illustrates the controllability of the pore size of the PAN/SiO2 hybrid membrane through the simple control of TEOS content. Both the decreased porosity and pore size make attributions to the declined pure water flux of the hydrolyzed and hybrid membranes.
Table 3 Pure water fluxes and solute rejections of membranes PAN, PAN-COOH and PAN/SiO2. Solute Rejection b ( % )
Pure water Flux a
PEG
PEG
PEG
PEG
PEG
( l/m2 h )
600
1000
2000
4000
6000
PAN
605.4 ± 3.2
−
−
−
−
0.15
72.27
PAN-COOH
239.8± 2.6
−
0.56
1.02
4.88
10.26
98.26
PAN/SiO2-25
58.7 ± 2.0
1.27
5.24
46.56
79.11
90.18
≈100
PAN/SiO2-50
36.3 ± 1.1
9.22
41.09
67.52
88.08
96.96
≈100
PAN/SiO2-100
7.9 ± 0.5
55.46
80.59
91.94
98.44
99.02
≈100
Membrane
a
Tested with de-ionized water under 1.0 bar and 25.0 ℃;
b
Tested with 50 mg/l solute aqueous solution under 1.0 bar, 25.0 ℃ and pH 7.0.
BSA
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100
PAN/SiO2-100
PAN/SiO2-50
PAN/SiO2-25
PAN-COOH
90 80
PEG rejection (%)
70 60 50 40 30 20 10 0 600
1000
2000
4000
6000
Molecular weight of PEG (g/mol)
Fig.8. PEG rejection as a function of molecular weight for the hydrolyzed and hybrid membranes tested with 50 mg/l PEG aqueous solution at 1.0 bar and 25.0 °C.
To further illustrate the role of pore-filling on membrane separation property. Base membrane PAN-20 and hydrolyzed membrane PAN-COOH-16.5 with nearly the same water permeability of the hybrid membrane PAN/SiO2-25 were tailor fabricated and characterized for their MWCO values through PEG retention experiments. As tabulated in Table 4, one can find that, compared with the base and hydrolyzed PAN membranes of the same water permeability, the PAN/SiO2 hybrid membrane exhibits a much lower molecular weight cut-off. In other word, PAN/SiO2 hybrid membrane prepared through the approach developed in this work will have higher water permeability than those of the base and hydrolyzed PAN membranes of the same value of MWCO (membrane mean pore size).
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Table 4 Properties of the base, hydrolyzed and hybrid membranes of nearly the same water permeability.
*
Membrane
Jw (l/m2 h)
MWCO* (g/mol)
PAN-20
58.2±1.6
20,000±200
PAN-COOH-16.5
59.3±1.4
16,000±150
PAN/SiO2-25
58.7±1.8
6,000±80
Determined using PEG fractions according to the method described in Section 2.5.
3.6. Membrane anti-adsorption property Foulants can adsorb onto the membrane surface through hydrophobic interaction, hydrogen bonding, Van Der Waals attraction, and electrostatic interaction [45]. Therefore, an effective method to reduce membrane fouling is to improve membrane anti-adsorption ability through minimizing these adsorptive interactions between foulants and membrane surface. Membrane anti-adsorption ability was evaluated by static adsorption experiment with BSA as the model protein. As shown in Fig. 9, the step of hydrolysis can effectively mitigate the adsorption of BSA molecules on membrane surface, which is further reduced by the following step of in-situ sol-gel. Along with the improved surface hydrophilicity [46], the increased surface negative charge of the hydrolyzed and hybrid membranes as illustrated in Fig. 10 is the another reason for the improvement of anti-adsorption ability [47]. For the hybrid membrane, the enrichment of hydroxyl groups on membrane surface due to the deposited SiO2 nanoparticles is the other additional reason for their further improvement of anti-adsorption ability [48].
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120
2
BSA adsorption (μg/cm )
100
80
60
40
20
0
PAN
PAN PAN PAN PAN -CO /SiO /SiO /SiO OH 2 -100 5 2 -25 2 0
Fig.9. Static BSA adsorption values of membranes PAN, PAN-COOH and PAN/SiO2 under pH 7.4. -60
Surface zeta potential (mV)
-50
-40
-30
-20
-10
0
PAN
PAN PAN PAN PAN -CO /SiO /SiO /SiO OH 2 -100 2 -25 2 -50
Fig.10. Surface zeta potentials of membranes PAN, PAN-COOH and PAN/SiO2 tested with 0.001 mol/l KCl under pH 7.0.
The effect of the process of hydrolysis followed by sol-gel on membrane anti-fouling property was further investigated through dynamic cross-flow fouling experiments.
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Membranes listed in Table 4 were employed to assure the equal effect of the transverse hydrodynamic force (permeation drag) on the membrane fouling and the same degree of compactness of the BSA layer that deposited on membrane surface through being filtrated under the same pressure and initial permeate flux. The time-dependent fluxes of the three tested membranes in a three-cycle filtration of BSA aqueous solution followed by physical flushing with de-ionized water were illustrated in Fig. 11. It can be found from the figure that, in each cycle, the water flux of each tested membrane decreases quickly in the initial filtration time of 10 min as the result of the concentration polarization and membrane fouling and then reaches to a steady-state value after filtration of about 60 min, and the flux of the fouled membrane can be resume to a certain extent through physical flushing. However, the extents of fouling and flux recovery are different.
PAN-20
70 65
PAN-COOH-16.5
PAN/SiO2-25
Physical flushing
Physical flushing
60
Cycle 3
Cycle 2
Cycle 1
75
Physical flushing
2
Flux (l/m h)
55 50 45 40 35 30 25 20 15
0
20
40
60
80
100
120
140
160
180
200
220
Time (min)
Fig.11. Time-dependent flux of the tested membranes in a three-cycle filtration of 1.0 g/l BSA solution followed by physical cleaning with pure water under 1.0 bar, 25.0 ℃ and pH of 7.0. 27/35
As illustrated in Table 5, membrane PAN/SiO2-25 exhibits a lower RFR value and higher FRR value in each cycle compared with membranes PAN-20 and PAN-COOH-16.5. The lower RFR value indicates slight deposition or adsorption of BSA molecules on membrane surface, while higher FRR value indicates most of adsorbed and deposited BSA molecules can be washed away by simple hydraulic washing. The enhanced anti-fouling property of the hybrid membrane is mainly due to its improved surface hydrophilicity, enhanced surface negative charge as well as enrichment of surface hydroxyl groups [47]. Additionally, the three-cycle fouling experiment also demonstrate the reliability and durability of the hybrid PAN/SiO2 membrane prepared in this work.
Table 5 RFR and FRR values of membranes PAN-20, PAN-COOH-16.5 and PAN/SiO2-25 in the three-cycle fouling experiment. Membrane
First cycle
Second cycle
Third cycle
RFR (%)
FRR (%)
RFR (%) FRR (%)
RFR (%)
FRR (%)
58.1
80.8
58.9
77.3
59.1
76.5
PAN-COOH-16.5 53.5
88.3
54.2
85.4
54.7
84.6
50.8
91.3
51.8
90.6
51.8
90.3
PAN-20
PAN/SiO2-25
4. Conclusions PAN/SiO2 hybrid membrane with enhanced both separation and anti-fouling properties has been successfully prepared through adopting the approach of hydrolysis followed by in-situ sol-gel of TEOS. The following conclusions could be obtained from the experimental
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results: (1) ATR-FTIR and XPS analysis confirmed the formation of SiO2 particles on membrane surface and the reaction between the silanols of the SiO2 particle and the carboxyl groups of the hydrolyzed membrane. SEM surface and cross-section images verified the generation of SiO2 particles with nano-scale size and their depositions both on membrane surface and in membrane pores. (2) The SiO2 nanoparticles on surface endowed the hybrid membrane with enhanced surface hydrophilicity and abundant hydroxyl groups, and thus improved anti-fouling property. The SiO2 nanoparticles in membrane pores reduced membrane mean pore size and narrowed membrane pore size distribution, and thus improving separation property. Hybrid membrane possessed a much lower MWCO compared with those of the base and hydrolyzed PAN membranes of the same water permeability. (3) Both separation and surface properties of the prepared hybrid membrane could be tuned through the control of TEOS content, and the hybrid PAN/SiO2 membrane prepared possessed good reliability and durability. Therefore, the approach developed in this study is of potential application in fabricating organic-inorganic hybrid porous membranes with controlled pore size, narrowed pore size distribution and tailored surface property.
Acknowledgments The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China (NNSFC) (Grant nos. 21476213 and 21676256) and the 521 personnel training plan of Zhejiang Sci-Tech University.
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Research Highlights:
►Hydrolysis followed by sol-gel of TEOS was performed with PAN porous membrane. ►Silica nanoparticles deposited both on membrane surface and in membrane pores. ►Hybrid membrane showed reduced mean pore size and narrowed pore size distribution. ► Membrane antifouling property to BSA could be appreciably enhanced. ►Membrane property could be tuned through the simple control of TEOS content.
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