Preparation, characterization, antifouling and

Desalination 278 (2011) 343–353

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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

TiO2 entrapped nano-composite PVDF/SPES membranes: Preparation, characterization, antifouling and antibacterial properties Ahmad Rahimpour a,⁎, Mohsen Jahanshahi a, Babak Rajaeian a, Mostafa Rahimnejad b a b

Membrane Research Group, Nanotechnology Institute, Babol University of Technology, Iran Faculty of Chemical Engineering, Babol University of Technology, Iran

a r t i c l e

i n f o

Article history: Received 12 April 2011 Received in revised form 18 May 2011 Accepted 19 May 2011 Available online 12 June 2011 Keywords: PVDF/SPES membrane TiO2 nano-particles Antifouling Antibacterial

a b s t r a c t In the current study, poly (vinylidene fluoride) (PVDF)/sulfonated polyethersulfone (SPES) blend membrane was modified using TiO2 nano-particles. Firstly, sulfonation of polyethersulfone (PES) was carried out, then PVDF/SPES blend membranes were prepared with and without TiO2 nano-particles in the casting solution using phase inversion induced by immersion precipitation technique. Polyvinylpirrolidone (PVP) with the concentration of 4 wt.% was added in the casting solution as pore former. The morphological studies were investigated using SEM, AFM, XRD, FTIR and contact angle goniometry. They showed that the average sizes of membrane pores in surface and sub-layer were reduced with addition of TiO2 nano particles in the casting solution. The presence of TiO2 nano-particles in the membrane structure was confirmed by XRD, FTIR and EDX analyses. The contact angle measurements demonstrated that the hydrophilicity of modified membranes was enhanced by addition of TiO2 in the casting solution. The experimental results demonstrated that the initial flux of TiO2 entrapped PVDF/SPES membranes was lower than the initial flux of neat PVDF/SPES membrane. The antifouling properties of membranes were improved by changing the membrane surface from hydrophobic to hydrophilic after TiO2 addition in the casting solution. Finally, neat and 4% TiO2 modified PVDF/SPES membranes were tested to possess the dramatic photo-bactericidal effect on Escherichia coli (E. coli). © 2011 Elsevier B.V. All rights reserved.

1. Introduction Phase inversion process has been widely adopted as a method for preparation of asymmetric polymeric ultrafiltration membrane [1–3]. Most of polymeric membranes used for micro- and ultrafiltration of liquids are prepared by phase inversion via immersion precipitation [4]. In this technique, a thin layer of polymer solution is immersed into a bath of non-solvent. At the same stage, precipitation (sometimes recognized as liquid–liquid demixing, phase inversion, crystallization) occurs leading to the formation of porous solid film. The structures of the formed membranes are very complex and dependent upon the composition of casting solution, coagulation bath [5–7]. Poly (vinylidene fluoride) (PVDF) is a semi-crystalline which shows outstanding thermal, chemical, and oxidation resistances against corrosive chemicals such as acids, bases, oxidants and halogens and exceptional hydrolytic stability. The crystalline phase of the polymer provides thermal stability while the amorphous phase accommodates the desired membrane flexibility and good mechanical and film-forming properties [8–11]. All the mentioned specifications make PVDF as an

⁎ Corresponding author. Tel.: + 98 912 8093155; fax: +98 111 3220342. E-mail addresses: [email protected], [email protected] (A. Rahimpour). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.05.049

excellent polymer for microfiltration [12,13], ultrafiltration [14,15], pervaporation [16,17] and membrane distillation [18,19]. Moreover, the hydrophilicity and the pore structure of a membrane play key roles on the membrane separation performance. A suitable membrane must have high permeability, good hydrophilicity and a superior chemical resistance to the feed [20]. Due to hydrophobic nature of PVDF, many studies have been carried out on improving the PVDF membrane hydrophilicity and performance. These studies include physical blending, chemical grafting and surface modifying. Blending of polymers presents the advantage of an easy preparation by the method of phase inversion [11]. There are two different options for modifying the membrane formation; one process is addition of proper additive to the casting solution [21,22] and another is immobilization of polymers with hydrophilic segments by photo- or plasma polymerization [23–25]. One of the efficient methods to produce membranes with optimum morphology and specified properties is changing the composition in the dope solution or the coagulation bath. There are different surfactants [26], polymer [27], mineral fillers [28] and non-solvents [29], which can be used as additives. The role of the additives is to suppress and/or excite the formation of macrovoids, enhance pore formation and improve pore interconnectivity and/or hydrophilicity [30]. Sulfonated polymers were considered to be much more resistant to fouling [31,32]. The sulfonated polymers must be capable of forming asymmetric membranes with required

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mechanical, thermal, chemical and separation properties. In a previous work [33], we prepared a modified PES membrane by blending of PES with SPES in the presence of polyvinylpirrolidone (PVP) as pore former in the casting solution. Results demonstrate that hydrophilicity, permeability, surface pore size and sub-layer porosity were significantly improved. Wang et al. [34] proved that the dispersion of inorganic particles in the polymer matrix have been useful in the improvement of membrane performance. There are many inorganic nano-particles which have been used to prepare polymeric membranes such as silica [35], Al2O3[36], Fe3O4[37], ZnO [38], ZrO2[39], TiO2[40–46], and CdS [47]. Nano titanium dioxide (TiO2 nano-particles) as a nano-material improves the permeability and antifouling properties, so it has been the focus of numerous studies in recent years due to its innocuity, resistivity, photo catalytic and superhydrophilicity effects. Therefore, it has been applied to surface modification of several membranes [48,49]. Kim et al. [50,51] prepared a hybrid composite membrane by selfassembly of TiO2 nanoparticles through interaction with the COOH functional group of an aromatic polyamide thin film layer. The membrane possessed significant photo-bactericidal effect on Escherichia coli under UV light illumination. Ebert et al. [52] discovered that PVDF and poly(amide-imide) (PAI) membranes blended with TiO2 as inorganic filler had improved temperature resistance and permeability. The main strategy of the present work was the preparation of PVDF membranes with high hydrophilicity. To obtain this goal at the first stage, the sulfonated polyethersulfone (SPES) was employed to blend with PVDF. Next step, TiO2 nano-particles were used due to their potential reduction in antifouling abilities by changing parameters such as membrane topography and specially, hydrophilicity as well as self-cleaning properties. The sulfonation of PES was performed by sulforic acid (98%) as sulfonating agent. PVDF/SPES blend membranes were prepared with and without commercial nano-sized Titania by phase separation via immersion precipitation technique. The concentrations of TiO2 nano-particles were 0.1, 0.5, 1, 2, 4 and 6 wt.% in the casting solution. The performance of prepared membranes was investigated by pure water flux, retention efficiency and flux of bovine serum albumin (BSA). The contact angles of the membranes were measured to evaluate the surface hydrophilicity. The surface and inner structures of the sample membranes were studied with several apparatus such as SEM, AFM and FTIR. X-ray diffraction was also employed to analyze the crystalline change of PVDF molecules. The antifouling and antibacterial properties of modified membranes with TiO2 nano-particles were also investigated. 2. Experimental 2.1. Materials Poly (vinylidene fluoride) (PVDF) from Alfa-Aesar as membrane polymer, polyethersulfone (PES Ultrason E6020P with Mw = 58,000 g/mol) and dimethylacetamide (DMAC) were supplied by BASF Aktiengesellschaft, Germany. Sulforic acid (98%) and polyvinylpyrrolidone (PVP, with Mw = 25,000 g/mol) from Merck were used. Titanium dioxide nanoparticles (TiO2, particle size of 20 nm) and bovine serum albumin powder [some properties are followed: assay: N96%, mol wt: 66 kDa, pH ~7, solubility N40 mg/mL in H2O] were obtained from Degussa and Sigma, respectively. Distilled water was used throughout this study. 2.2. Sulfonation of PES Sulfonation of PES was performed by sulforic acid (98%) as sulfonating agent and solvent [33]. A glass reactor equipped with magnetic stirrer and condenser was charged with PES powder and sulforic acid (98%). The polymer dissolution time and temperature

were 3 h and 20 °C, respectively. SPES was gradually precipitated into ice-cold distilled water under rapid stirring. The resulting precipitate was recovered by filtration and washing repeatedly with distilled water. Finally, the SPES was dried under vacuum at 40 °C overnight. 2.3. Preparation of PVDF membrane blended with the SPES and TiO2 nano-particles The flat sheet membranes were prepared by phase inversion via immersion precipitation technique. The blend homogeneous solutions based on PVDF and synthesized SPES polymers were prepared by dissolving two polymers at different concentrations of TiO2 nanoparticles (0–6 wt.%) in DMAC as solvent in the presence of 4 wt.% PVP as pore former at around 25 °C with magnetic stirrer at 200 rpm for 8 h. The homogeneous polymer solution was kept for the removal of bubbles. The compositions of casting solution are shown in Table 1. The solution was sprinkled and cast using a home-made casting knife with 75 μm thickness on polyester non-woven fabric. This was immediately moved to the non-solvent bath for immersion at room temperature without any evaporation. The non-solvent was only water. The prepared membranes were washed and stored in water for at least 1 day to completely leach out the residual solvents and additives. As the final stage, the membranes were dried by placing between two sheets of filter paper for 24 h at room temperature. 2.4. Characterization of membranes 2.4.1. Contact angle measurement In order to examine variations in the surface wetting characteristics of the PVDF/SPES membrane as a function of TiO2 nanoparticle concentration, water contact angle was measured for membrane surface using a contact angle measuring instrument [G10, KRUSS, Germany]. This represents the membrane hydrophilicity. De-ionized water was used as the probe liquid in all measurements. To minimize the experimental error, the contact angles were measured at five random locations for each sample and the average number was reported. 2.4.2. FT-IR analysis FTIR spectra of PVDF/SPES and PVDF/SPES/TiO2 blend membranes were obtained for spectroscopic investigation. All FTIR spectra we recorded by the attenuated total reflection (ATR) technique using Bruker-IFS 48 FTIR spectrometer (Ettlingen, Germany) with horizontal ATR device (Ge, 45°). 32 scans were taken with 4 cm − 1 resolution between 4000 and 500 cm − 1. 2.4.3. Morphological studies In order to inspect the top surface and cross-section of membranes, SEM (Philips-X130) was employed. The membranes were cut into pieces of small sizes and cleaned with filter paper. These pieces were immersed in liquid nitrogen for 10–15 s and were frozen. Frozen bits of the membranes were broken and kept in air for drying. These dry samples were gold sputtered for producing electric conductivity, and

Table 1 Compositions of casting solution. PVDF + SPESa (wt.%)

PVP (wt.%)

TiO2 (wt.%)

DMAC (wt.%)

16 16 16 16 16 16 16

4 4 4 4 4 4 4

0 0.1 0.5 1 2 4 6

80 79.9 79.5 79 78 76 74

a

55% PVDF + 45% SPES.


photomicrographs were taken in very high vacuum conditions at 17 kV. The non-woven polyester cannot be broken in liquid nitrogen. For this reason, the PVDF-SPES membrane layer was detached from the non-woven polyester fabric and then was broken in liquid nitrogen. Therefore, the non-woven polyester fabric is not seen in the cross-sectional SEM image. Atomic force microscopy was used to analyze the surface morphology and roughness of the prepared membranes. The AFM device was Nanosurf scanning probe-optical microscope (EasyScan II, Swiss). Small squares of the prepared membranes (approximately 1 cm 2) were cut and glued on glass substrate. The membrane surfaces were imaged in a scan size of 5 μm × 5 μm and 10 μm × 10 μm. The surface roughness parameters of the membranes which are expressed in terms of the mean roughness (Sa), the root mean square of the Z data (Sq) and the mean difference between the five highest peaks and lowest valleys (Sz) were calculated from AFM images using tapping mode method via Nanosurf EasyScan software at a scan area of 10 μm × 10 μm. The average roughness parameter, Sa, is the most used surface roughness parameter. It is the arithmetic mean or average of the absolute distances of the surface points from the mean plane. The equation that represents this algorithm is shown below:

Sa =

1 N M ∑ ∑ jzj xi ; yj MN j−1 i−1

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 N M 2 ∑ ∑ z xi ; yj : MN j−1 i−1

determine the peaks and the valleys. The equation that represents this algorithm is displayed below: 5 5 ∑ zpi + ∑ zvi

Sz =

i−1

i−1

5

ð3Þ

where zpi and zvi (i = 1,2,3,4,5) are the five highest peaks and five lowest valleys respectively. 2.4.4. X-ray diffraction (XRD) The crystal structure and the phase present in resulting powders were analyzed with X-ray diffraction (XRD). This instrument (Philips PW 3710) works with voltage and current settings of 35 kV and 28.40 mA respectively, and uses Cu–Kα radiation (1.540510 Å). For qualitative analysis, XRD diagrams were recorded in the interval 20° ≤ 2θ ≤ 50° at a scan speed of 2°/min. The mean crystallite sizes “D” were determined according to the Scherrer equation (D = 0.9λ/β cosθ, where λ is the X-ray wavelength (1.5405 Å), β is the full width at half maximum of the diffraction line, and θ is the diffraction angle). 2.5. Filtration performance and fouling analysis

ð1Þ

where M is the number of columns in the surface and N is the number of rows in the surface. The Root Mean Square (RMS) roughness parameter, Sq, is the root mean square of the surface departures from the mean plane within the sampling area. The equation that indicates this algorithm is represented in Eq. (2)

Sq =

345

ð2Þ

Mean of distance between the 5 highest peaks and the 5 deepest valleys. A region of 1 μm × 1 μm samples is taken into account to

All filtration experiments were carried out in a self-made dead-end ultrafiltration stirred cell with maximum pressure capacity of 5 bar as depicted in Scheme 1. The capacity and the effective surface area of the module were 300 ml and 20 cm 2, respectively. The top of the cell contained a gas inlet. Nitrogen gas was used to pressurize the cell to operating pressure. Fouling can be quantified by the resistance appearing during the filtration and cleaning can be specified by the removal of this resistance. The resistance is due to the formation of a cake or gel layer on the membrane surface. The flux (J) through the cake and the membrane may be described by the following equation:

J=

m ðA ΔtÞ

ð4Þ

Scheme 1. Scheme of the dead-end cell filtration apparatus: 1—cylinder with compressed nitrogen, 2—pressure regulator, 3—feed tank, 4—membrane, 5—magnetic stirrer, 6—magnet, 7—membrane cell, 8—manometer, 9—PSV (pressure safety valve), 10—permeate collector, 11—balance.

346


The fouling-resistant capability of the membrane was described by:

Rt ð%Þ =

Jwi −Jp × 100: Jwi

ð6Þ

Here, Rt is the degree of the total flux loss caused by total fouling. Rr and Rir, described by Eqs. (7) and (8)[53] show reversible deposition and irreversible fouling:

Fig. 1. X-ray diffraction patterns of neat and 1 wt.% TiO2 modified membranes.

Rr ð%Þ =

Jwc −Jp × 100 Jwi

ð7Þ

Rir ð%Þ =

Jwi −Jwc × 100 Jwi

ð8Þ

Rt = Rr + Rir : where m is the mass of permeated water, A the membrane area and Δt the permeation time. After pure water flux measurement (Jwi), the solution reservoir was refilled with a 500 ppm BSA solution and the flux (JP) was obtained. After 2 h of filtration, the membrane was washed using distilled water at high stirring rate (1200 rpm) in order to remove the loose bound BSA on the membrane surface for 10 min and the pure water flux of cleaned membranes was measured (Jwc). In order to evaluate the fouling-resistant capability of the membrane, the flux recovery ratio (FRR) was calculated using the following expression: J FRRð%Þ = WC : JWi

ð5Þ

ð9Þ

The flux and rejection of all membranes were determined under the 1 bar transmembrane pressure (TMP) at 25 °C using a dead-end cell at fixed speed of 500 rpm. The 500 ppm BSA was used as the feed for membrane performance and fouling evaluation. The BSA rejection ratio was calculated following the equation below:

Rð%Þ =

1−

Cpermeate BSA Cfeed BSA

! × 100

ð10Þ

permeate feed and CBSA represented BSA concentrations in permeate where CBSA and feed solutions, respectively, measured by Jenway 6305 UV/Visible range Spectrophotometer (190–1000 nm).

Fig. 2. Fourier transform infrared spectra of: (a) PVDF/SPES, (b) 1 wt.% TiO2 entrapped membranes.


347

Fig. 3. Effect of TiO2 on cross-sectional SEM images of PVDF/SPES membranes: (a) 0 wt.% TiO2 (b) 1 wt.% TiO2 (c) 2 wt.% TiO2 (d) 4 wt.% TiO2.

2.6. Antibacterial properties of TiO2 entrapped PVDF/SPES membranes The E. coli microorganism was anaerobically grown in an anaerobic jar. The medium consisted of yeast extract, glucose, KHPO4, MgSO4

and MnSO4 with concentration of 3, 30, 0.2, 0.2 and 0.05 g/l, respectively. The medium was sterilized in an autoclaved at 15 psig and temperature of 121 °C for 20 min. The medium pH was initially adjusted to 6.5 and the inoculums were introduced into the media

Fig. 4. Effect of TiO2 on surface SEM images of PVDF/SPES membranes: (a) 0 wt.% TiO2 (5000×) (b) 1 wt.% TiO2 (10,000×) (c) 2 wt.% TiO2 (5000×) (d) 4 wt.% TiO2 (5000×).

348


6.5 and the inoculums were introduced into the media at ambient temperature. The inoculated cultures were incubated at 32 °C. The bacteria were fully grown for the duration of 48 h. Substrate consumption was calculated base on determination of the remained sugars in the culture. Dinitrosalicylic acid [2-(OH)-3,5-(NO2 ) 2C6H2COOH] (DNS) method was used to detect the monomeric sugar. The samples were withdrawn in a time interval of 12 h. The antibacterial activity of the prepared membrane was investigated by an inhibition zone method. All types of membranes were cut into circular shape with 20 mm, autoclaved and put on the bacteria cultured Petri dishes for incubation at 35 °C for 24 h. The inhibition ring formed after 24 h served as an indicator for the antibacterial activity. 3. Results and discussion 3.1. XRD and ATR-FTIR analyses of TiO2 entrapped PVDF/SPES blend membranes The occurrence of TiO2 crystalline phase was ruled out according to the results obtained from the XRD analysis (Fig. 1). It indicates that

the crystallization behavior of amorphous PVDF/SPES membrane was affected by addition of TiO2 nano-particles in the casting solution. Previous works reported that TiO2 crystal powders have nearly peaks at 2θ of 38°, 48°, 55° and 63° [54,55]. Peaks shown with arrows presented differences between two diffraction patterns. It is clear that these differences are affected by adding TiO2 nanoparticles. The following peaks of PVDF/SPES/TiO2 composite membrane appear at 2θ of 37°, 47°, 53° and 62°. This slight movement of peak dispersions illustrates that there are interactions between polymers and TiO2 nano-particles [55]. Moreover, the peak intensities of TiO2 entrapped PVDF/SPES membrane reduced as compared with PVDF/SPES membrane. This reduction is due to decrease in polymer (PVDF and SPES) transparency which is caused by TiO2 nano-particle aggregation. The FTIR-ATR spectra of neat PVDF/SPES and 1 wt.% TiO2 entrapped membranes are shown in Fig. 2. The spectrum of neat PVDF/SPES membrane revealed a weak band at 3200–3400 cm − 1. This band is associated with the OH stretching of sulfonic group (SO3H). Analysis of the area of OH stretching in the spectrum of TiO2 entrapped PVDF/SPES membrane exhibits strong bands of asymmetric vibration at 3200–3400 cm − 1. The high intensity of these bands can be explained by the high affinity of TiO2 nano-particles

Fig. 5. EDX results of surface of (a) neat PVDF/SPES membrane (b) 6 wt.% TiO2 entrapped PVDF/SPES membrane.


entrapped on the surface structure of PVDF/SPES membrane to water. The strengthening of OH bonds in the TiO2 entrapped PVDF/SPES membrane is the main factor to settlement of TiO2 nano-particles on the membrane surface structure. 3.2. Morphological studies Upon immersion of the cast polymer solution (PVDF/SPES/PVP/ DMAC) in the non-solvent bath e.g. water, the fast solvent/nonsolvent exchange takes place across the interface of casting film and non-solvent. This is combined with the large repulsive forces between polymers (PVDF/SPES) and water (water is a good non-solvent for PVDF and SPES) leading to immediate precipitation of polymers at the interface. As a result, an asymmetric membrane with a thin skin layer and large pore structure in the sub-layer is formed (Fig. 3a). In order to understand the influence of TiO2 concentration on the final membrane structure, the cross-section of the prepared membranes were observed using SEM and shown in Fig. 3. All the membranes had large macrovoids in the sub layer. The cross sectional morphology of

349

the membranes prepared with the addition of 1, 2 and 4 wt.% of TiO2 is different slightly from the original PDVF/SPES membrane. A comparison between images in Fig. 3 shows that the increase of TiO2 concentration leads to membrane containing less porous sub-layer with more interconnectivity of sub-layer macrovoids. This indicates that the presence of hydrophilic TiO2 nano-particles in the casting solution may result in lower porosity in the membrane sub-layer leading to reduce the permeability. The top surface SEM images of PVDF/SPES membranes prepared without and with different concentrations of TiO2 in the casting solution are depicted in Fig. 4. TiO2 nano-particles are uniformly distributed on the membrane surface. However, some particles form large aggregates. The surface of PVDF membrane is composed of a micro-porous structure with relatively large pores. In such a micro-porous surface, both interconnected holes and networks are constructed with micro-spherical particles connected with each other. There was also an amount of TiO2 nano-particles put deeply and uniformly into pore-surface and outer-surface of PVDF/SPES/TiO2 blend membranes (Fig. 4b–d) in comparison with neat membrane (Fig. 4a).

Fig. 6. Two- and three-dimensional surface AFM images of PVDF/SPES membranes prepared with different concentrations of TiO2 in the casting solution: (a) 4 wt.% TiO2 (b) 2 wt.% TiO2 (c) 0 wt.% TiO2.

350


Table 2 Surface roughness parameters of neat and TiO2 blended membranes. TiO2 concentration

Roughness Sa (nm)

Sq (nm)

Sz (nm)

0 0.1 0.5 1 2 4

64.4 57.2 54.5 44.2 41.5 40.5

81.3 74.5 67.4 55.1 54.9 52.0

532.2 478.3 472.6 401.9 362.7 349.8

EDX analysis was also carried out to investigate the distribution of TiO2 nano-particles on the membrane structure. Fig. 5 illustrates the EDX spectra of neat and nano-TiO2 entrapped membranes. The

EDX profile was acquired on a single spot of the neat PVDF/SPES and 6 wt.% TiO2 entrapped PVDF/SPES membranes. It was observed that only titanium (Ti) and aurum (Au) elements were identified. The EDX analysis proved that white particles on modified membranes were TiO2 nano-particles, which had homogeneous dispersion within blend membrane surface. Fig. 6 demonstrates the two- and three-dimensional surface AFM images of neat and TiO2 modified PVDF/SPES membranes at a scan size of 5 μm × 5 μm. In these images, the brightest area presents the highest point of the membrane surface and the dark regions indicate valley or membrane pores. This is clear that the surface morphologies of membranes were influenced by addition of TiO2 nanoparticles in the casting solution, especially at higher concentration. It visually seems that the surface porosities of membranes prepared with TiO2 are low compared to unmodified

Fig. 7. Pore size distributions of PVDF/SPES membranes prepared with: (a) 0 wt.% TiO2, (b) 1 wt.% TiO2 (c) 4 wt.% TiO2.


351

Table 3 Contact angle of PVDF/SPES membrane with different TiO2 contents. Membrane

Contact angle (°)

PVDF PVDF/SPES PVDF/SPES/TiO2 PVDF/SPES/TiO2 PVDF/SPES/TiO2 PVDF/SPES/TiO2

92.3 ± 1.0 74.4 ± 1.7 72.2 ± 2.1 68.7 ± 2.8 65.1 ± 1.9 64.2 ± 1.5

(0.5%) (1%) (4%) (6%)

membrane. In this condition, the pore sizes of the membranes are decreased. The surface roughness parameters of the membranes were calculated at a scan size of 10 μm × 10 μm and presented in Table 2. The roughness parameters of membranes decreased with an increase in TiO2 concentration in the casting solution. Since the roughness parameters depend on the Z-value, which is the vertical distance that the piezoelectric scanner moves, this relationship is expected. When the surface includes deep depressions (pores) and high peaks (nodules), the tip moves up and down over a wide range and the roughness parameter of the surface is high. Fig. 7 indicated the pore size distribution of PVDF/SPES membranes prepared with different concentrations of TiO2 nano-particles (0, 1 and 4 wt.%). This fact can be explained through filling membrane pores because of entrapping inorganic n-particles in polymeric matrix which can improve membrane porosity and increase the number of small pores [55]. As a result, the pore size distributions shifted to the small pore diameters by increasing the TiO2 concentration in the casting solution. 3.3. Hydrophilicity, performance, and antifouling properties of membranes The hydrophilicity of unmodified and 0.1, 0.5, 1, 2, 4 and 6 wt.% of TiO2 modified membranes are given in Table 3. The contact angle of PVDF membrane decreased from 92.3° to 74.4° by blending with SPES. The higher hydrophilicity of PVDF/SPES membrane compared to PVDF membrane can be ascribed by the polar groups of SPES polymer. SPES has SO3H polar group that is strongly hydrophilic. Moreover, the contact angle of membranes decreased when the TiO2 nanoparticles were added in the casting solution. This is due to higher affinity of TiO2 to water. Therefore, a more hydrophilic surface for PVDF membrane is produced by blending with SPES and adding TiO2 in the casting solution. Fig. 8 shows the effect of TiO2 nano-particles on pure water flux before and after BSA filtration. The pure water flux of membranes before BSA solution filtration declined from 1068 to 616 kg/m 2.h with

Fig. 8. Effect of TiO2 concentration on pure water flux.

Fig. 9. Flux behavior of neat PVDF/SPES and TiO2 modified membranes during the BSA filtration.

addition of 4 wt.% TiO2 in the casting solution and then increased slightly from 616 to 670 kg/m 2.h with higher addition of TiO2. However, the decline in pure water flux of PVDF/SPES membrane after BSA solution filtration was higher compared to TiO2 modified membranes. Fig. 9 indicates the fluxes of different membranes during filtration of BSA solution at 120 min. The flux rapidly declined at the beginning of filtration for all membranes. Moreover, although the initial flux of neat PVDF/SPES membrane was high, the flux decline for this membrane during time was high compared to the TiO2 modified membranes, as shown in Fig. 9. The obtained results clearly reveal that the antifouling property of TiO2 modified membrane was significantly improved. Moreover the fouling mitigation effect was gradually enhanced by increasing TiO2 concentration in casting solution up to 4 wt.%. Further increment of TiO2 concentration did not change the flux decline. TiO2 nano-particles plug the membrane pores at high concentration and decrease the membrane performance, considerably. This can be considered as a lower fouling tendency for modified membranes. Fig. 10 shows the histories for BSA protein rejection as a function of TiO2 concentration in the casting solution. The rejection parameter was increased with addition of TiO2 nano-particles in the casting solution. This is due to formation of membranes with smaller surface pore size by addition of TiO2 in the casting solution. To evaluate the antifouling properties of membranes, the recovery ratios, total flux losses and irreversible resistance of unmodified and TiO2 modified membranes were calculated and represented in Table 4.

Fig. 10. Protein rejection of PVDF/SPES nano-composite membranes as a function of TiO2 concentration.

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Table 4 Flux recovery ratio and resistances of unmodified and TiO2-modified membranes. TiO2 concentration

FRR (%)

Rt (%)

Rr (%)

Rir (%)

0 0.1 0.5 1 2 4 6

64.6 76.0 76.4 77.1 80.4 86.2 82.2

44.8 42.4 35.5 34.7 27.5 19.6 26.3

9.4 18.4 12.0 11.9 7.9 5.8 8.6

35.4 24.0 23.5 22.8 19.6 13.8 17.7

It can be seen that the flux recovery ratios (FRR) of TiO2 blended membranes are high compared to the neat PVDF/SPES membrane. This indicates that the modified membranes have high recycling property. In addition, the total flux losses of modified membranes are lower than neat PVDF/SPES membrane. The irreversible resistance (Rir) for neat PVDF/SPES membrane is high compared to the other membranes modified with TiO2. The surface of TiO2 entrapped membrane can be more hydrophilic than that of neat polymeric membrane due to higher affinity of metal oxide to water. As shown in Table 3, the surfaces of TiO2 entrapped membranes are more hydrophilic compared to the neat PVDF/PES membrane. It is well known that the membranes with high hydrophilicity have lower tendency to the fouling. Therefore, hydrophobic adsorption between BSA protein and surface of TiO2 modified membranes were diminished and deposited foulants were readily removed during filtration. 3.4. Antibacterial activity 4 wt.% TiO2-modified membrane which has illustrated the best characterization and anti-fouling performance, was employed for antibacterial activity under Philips 57202-E170 125 W UV light exposure for about 1 h. The purpose of this test is to observe the bactericidal effect of UV-irradiated TiO2 nano-particles on the membranes. Fig. 11 shows the antibacterial effect of 4 wt.% TiO2 entrapped membrane against E. coli which has been proven by the formation of inhibition ring around this membrane. These results indicated that the PVDF/SPES/TiO2 membranes exposed to UV light are able to eliminate E. coli more efficiently than composite membrane with the same concentration of TiO2 without UV irradiation and

Fig. 11. Results of the antibacterial activity on the E. coli bacteria.

neat PVDF/SPES membranes due to the photocatalytic bactericidal effect of TiO2 catalyst. The bactericidal effect of UV/TiO2 photocatalysis is due to the presence of reactive oxygen species like O2−, H2O2 and HO generated by TiO2 or the direct UV illumination of the cells. Most of the studies have concluded that the main mechanism for the death of E. coli cells by bactericidal effect of TiO2 photocatalytic was HO attack and lipid peroxidation reaction [50,51,56,57]. 4. Conclusion A novel PVDF membrane with appropriate antifouling and antibacterial properties were manufactured using sulfonated polyethersulfone (SPES) and TiO2 nanoparticles in the presence of polyvinylpirrolidone (PVP) in the casting solution. The phase inversion induced by immersion precipitation technique was used for membrane preparation. The XRD, FTIR and SEM studies indicated that the TiO2 nano-particles were entrapped on the surface of PVDF/SPES membrane. The pore size distribution of TiO2 entrapped PVDF/SPES membranes obtained from AFM images showed that the pore size decreased with addition of TiO2 in the casting solution. The contact angle measurements demonstrated significant increment in surface hydrophilicity of TiO2 entrapped membranes. The permeability of initial pure water and BSA solution of TiO2 entrapped PVDF/SPES membranes were low compared to the neat PVDF/SPES membrane. However, the antifouling properties and longterm flux stability were significantly enhanced. The results of antibacterial study indicated that the TiO2 modified membrane under UV light had better results for bactericidal ability in comparison with two other tested membranes due to photocatalytic property of Tio2 nanoparticles which had influence on killing of E. coli bacteria. Acknowledgment The authors gratefully acknowledge the financial support by the Iran National Science Foundation (INSF) (Grant No. 88002471). References [1] Q.Z. Zheny, P. Wang, Y.N. Yang, Rheological and thermodynamic variation in polysulfone solution by PEG introduction and its effect on kinetics of membrane formation via phase-inversion process, J. Membr. Sci. 27 (2006) 230. [2] Y.N. Yang, P. Wang, Q.Z. Zheng, Preparation and properties of polysulfone/TiO2 composite ultrafiltration membrane, J. Polym. Sci. Part B: Polym. Phys. 44 (2006) 8790. [3] K. Scott, Handbook of Industrial Membranes, 2nd, Elsevier Advanced Technology, 1999, p. 2050. [4] A. Akthakul, W.F. McDonald, A.M. Mayes, Noncircular pores on the surface of asymmetric polymer membranes: evidence of pore formation via spinodal demixing, J. Membr. Sci. 208 (2002) 147. [5] T.H. Young, L.W. Chen, Roles of bio-molecular interaction and relative diffusion rate in membrane structure control, J. Membr. Sci. 83 (1993) 153. [6] T.H. Young, L.W. Chen, Pore formation mechanism of membranes from phase inversion processes, Desalination 103 (1995) 233. [7] T.H. Young, L.W. Chen, L.P. Cheng, Membranes with micro-particular morphology, Polymer 37 (1993) 1305. [8] A. Rahimpour, S.S. Madaeni, S. Zereshki, Y. Mansourpanah, Preparation and characterization of modified nano-porous PVDF membrane with high antifouling property using UV photo-grafting, J. Appl. Surf. Sci. 255 (2009) 7455. [9] A.J. Lovinger, in: D.C. Bassett (Ed.), Poly(vinylidene fluoride), Development in Crystalline Polymers, 1, Applied Science, London, 1982. [10] Panu Sukitpaneenit, Tai-Shung Chung, Molecular elucidation of morphology and mechanical properties of PVDF hollow fiber membranes from aspects of phase inversion, crystallization and rheology, J. Membr. Sci. 34 (2009) 192. [11] Yana Lu, Shui Lib Yu, Xiang Chai Bao, Preparation of poly (vinylidene fluoride) (pvdf) ultrafiltration membrane modified by nano-sized alumina (Al2O3) and its antifouling research, J. Polym. 4 (6) (2005) 7701. [12] B.J. Cha, J.M. Yang, Preparation of poly(vinylidene fluoride) hollow fiber membranes for microfiltration using modified TIPS process, J. Membr. Sci. 291 (2007) 191. [13] L.P. Cheng, Effect of temperature on the formation of micro-porous PVDF membranes by precipitation from 1-octanol/DMF/PVDF and water/DMF/PVDF systems, Macromolecules 32 (1999) 6668. [14] J. Kong, K. Li, Oil removal from oil-in-water emulsions using PVDF membranes, Sep. Purif. Technol. 16 (1999) 83. [15] W.D. Benzinger, B.S. Parekh, J.L. Eichelberger, High temperature ultrafiltration with Kynar poly(vinylidene fluoride) membranes, Sep. Sci. Technol. 15 (4) (1980) 1193.

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