Journal of Membrane Science 472 (2014) 185–193
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Preparation and characterization of emulsion poly(vinyl chloride) (EPVC)/TiO2 nanocomposite ultrafiltration membrane Hesamoddin Rabiee, Mohammad Hossein Davood Abadi Farahani, Vahid Vatanpour n Faculty of Chemistry, Kharazmi University, 15719-14911 Tehran, Iran
art ic l e i nf o
a b s t r a c t
Article history: Received 30 June 2014 Received in revised form 21 August 2014 Accepted 26 August 2014 Available online 6 September 2014
Emulsion poly(vinyl chloride)/titanium dioxide (EPVC/TiO2) nanocomposite ultrafiltration membranes were prepared using the phase inversion method with different TiO2 contents. Pure water flux through the membranes was investigated at a operating pressure of 2 bar and its antifouling properties were studied using bovine serum albumin (BSA) as a foulant. The results showed an increment in pure water flux with increasing content of TiO2 up to 2 wt%, and then it slightly decreased by addition of 4 wt% TiO2 due to agglomeration of the nanoparticles at this content. The static water contact angle test showed improvement in membrane hydrophilicity, due to hydrophilic behavior of the nanoparticles, which led to higher water flux. SEM and EDAX analyses were applied to investigate membrane morphological changes. EDAX analysis indicated that the nanoparticles are homogeneously dispersed in membrane structure at low concentrations. However, at high loading, the nanoparticles have a propensity to aggregate. SEM images showed that with TiO2 addition, initially finger-like structures change to macrovoids and after 1 wt% TiO2 loading, they return to finger-like construction with elongated finger-like pores. TiO2 addition also enhanced BSA rejection properties. BSA ultrafiltration experiments showed that the antifouling ability of nano-TiO2 embedded membranes was better than the unfilled EPVC membrane. & 2014 Elsevier B.V. All rights reserved.
Keywords: Mixed matrix membrane Poly(vinyl chloride) TiO2 Ultrafiltration Antifouling
1. Introduction Membrane separation processes have been taken into account due to their interesting features like separation efficiency and low operating costs, for the last two decades. Porous membranes, fabricated by different phase inversion methods such as nonsolvent induced phase separation (NIPS) and thermally introduced phase separation (TIPS), are widely being used for waste water treatment. Various polymers like poly(ethersulfone) (PES) [1], poly(sulfone) (PSf) [2], Poly(vinylidene fluoride) (PVDF) [3], cellulose acetate (CA) [4], poly(acrylonitrile PAN) [5,6] or their blends [7,8] have been considered to prepare membranes with desired structure. Poly(vinyl chloride) (PVC) is a promising membrane material for its suitable mechanical and chemical resistance, low cost and excellent thermal stability. This polymer could be easily dissolved in different organic solvents such as N-methyl-pyrrolidinone (NMP), N, N dimethylacetamide (DMAc), dimethylformamide (DMF) and tetrahydrofuran (THF), which makes it appropriate for industrial membrane separation application. In the recent years several studies have concentrated on preparation of ultrafiltration and microfiltration
n
Corresponding author. Tel./fax: þ 98 26 34551023. E-mail address:
[email protected] (V. Vatanpour).
http://dx.doi.org/10.1016/j.memsci.2014.08.051 0376-7388/& 2014 Elsevier B.V. All rights reserved.
membranes with PVC and different solvents. Liu et al. [9] investigated the effect of Pluronic F127, as a polymeric additive, on morphology and performance of PVC membrane. Their results showed reduction in pore size, pore density and flux of the membranes. However, antifouling properties of the membranes enhanced remarkably. Mei et al. [10] studied the effect of different additives (PVP, PEG and sucrose) on phase diagram of PVC/DMAc in the presence of water as the nonsolvent. They found that the velocity of gelation in the system with PVP is more than two others, and a small amount of sucrose addition can cause membrane structure causing it to turn into spongelike. However, in the case of PVP addition, finger-like extended into membrane body. Peng et al. [11] enhanced hydrophilicity properties of PVC membrane by the addition of poly(vinyl butyral) (PVB) which caused a reduction in the contact angle of PVC membranes from 781 to 42.31, along with increment in water flux. Zhang et al. evaluated using PVC ultrafiltration membrane as a pretreatment for a reverse osmosis (RO) system and obtained 131% and 129% enhancement for permeate flux and recovery of the RO system respectively [12]. Xu et al. prepared a hollow fiber ultrafiltration membrane with PVC/DMAc system in the presence of PVP and PEG as polymeric pore former additives [13]. They observed lower mechanical properties and rejection for protein, along with higher porosity and permeation flux as a result of PVP/PEG addition. It is noteworthy to mention that among different grades of PVC, emulsion polyvinyl chloride (EPVC) is desirable for membrane
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preparation due to its low temperature of processing, good thermal stability, low cost and high chemical resistance, which makes it a great option for membrane preparation. Recently, incorporation of different types of nanoparticles for modification of filtration membranes has attracted a great deal of attention. Various types of nanoparticles such as titanium oxide (TiO2) [14–16], zinc oxide (ZnO) [17–19], alumina (Al2O3) [20], ZrO2 [21], silica (SiO2) [22] and carbon nanotube [23,24] have been utilized in order to improve antifouling properties of fabricated membranes and prepare membranes with desired performance. Wu et al. showed that addition of TiO2 nanoparticles into PES, led to lower fouling ratio and consequently, water flux went up as TiO2 content increased [16]. They also found that hydrophilicity, thermal stability and mechanical strength of the membranes were enhanced with TiO2 addition. Shen et al. [17] fabricated PES/ZnO hybrid membrane and observed improvement in membrane porosity with ZnO addition. They also reported better antifouling ability, lower contact angle and higher porosity of the modified membranes. Maximous et al. investigated the separation property and morphology of Al2O3 entrapped-PES UF membranes. Their results showed higher porosity and better antifouling in comparison with unfilled PES UF membrane [20]. They also studied the influence of nano-ZrO2 on PES membrane and observed lower fouling resistance compared to neat PES membrane [21]. Shen et al. reported higher pure water flux, hydrophilicity and flux recovery with addition of SiO2 for the case of PES/SiO2 composite membranes [25]. Among all these nanoparticles, various types of TiO2 have been used by many researches to improve membrane performance, because of its suitable properties like desirable hydrophilicity and fine dispersion in polymeric solution [26]. In addition, nano-TiO2 has a tendency to absorb OH-groups and become hydrophilic. In previous work, we investigate the effect of different sizes and types of nano-TiO2 on antifouling and performance of PES membrane [27]. Results showed an increment in hydrophilicity and pure water flux with increasing TiO2 content. In addition, P25 type of TiO2 showed lower tendency to aggregate, resulting in more active membrane pores and higher pure water flux and water flux recovery ratio. Cao et al. also observed that smaller nano-TiO2 could more remarkably improve antifouling property of the PVDF membrane [28]. To the best of our knowledge, preparation of ultrafiltration membranes with EPVC and TiO2 has not been worked on. Thereby, the current study focuses on the effect of TiO2 nanoparticle concentration on performance of EPVC membrane, fabricated by the nonsolvent induced phase inversion. The membranes were synthesized with addition of TiO2 in five different loadings. Pure water flux and BSA flux were measured to calculate flux recovery of the membranes. The membranes were characterized by SEM to evaluate their morphology. Contact angle of the neat and nanocomposite membranes was measured to detect the change in their hydrophilicity.
2. Experimental 2.1. Materials Emulsion polyvinyl chloride (EPVC) was supplied from Arvand Petrochemical Co., Iran. TiO2 Degussa P25 nanoparticles were obtained from Degussa Co., Germany. 1-Methyl 2-pyrrolidone (NMP) and polyethylene glycol (PEG) with molecular weight of 6 kDa were purchased from Merck, Germany. Bovine serum albumin (BSA) was bought from Sigma. 2.2. Preparation of (EPVC)/TiO2 nanocomposite ultrafiltration membranes The immersion precipitation phase inversion method was used to prepare flat sheet asymmetric membranes. First of all, the
precise amounts of TiO2 were dispersed into NMP as a polymer solvent and sonicated for one hour to obtain homogeneous TiO2 suspensions. Based on literaure, TiO2 contents were selected as 0.2, 0.5, 1, 2 and 4 wt% of polymeric solution [17,27–30]. Subsequently, PEG (4 wt%) and then EPVC (15 wt%) were added to suspensions and stirred to achieve homogeneous solutions. The final polymer/ TiO2 casting solutions were again sonicated for 30 min to remove air bubbles. Following that, the membranes were casted with a 200 μm casting knife onto a glass plate at room temperature. Immediately after that, the glass plates were immersed in a deionized water coagulation bath at 25 1C. After 10 min, the fabricated films were put in the second bath with fresh distilled water for 24 h to ensure that the solvent is leached and phase inversion is completely done. Finally, the prepared membranes were dried by placing them between two filter papers. Table 1 shows the compositions of the prepared membranes. 2.3. Membrane characterization 2.3.1. SEM and EDAX Scanning electron microscopy (SEM) was applied to investigate the surface and cross-sectional morphology of the prepared membranes using VEGA\\TESCAN SEM, Czech Republic. For cross-sectional images, the membranes were fractured in liquid nitrogen. The samples were coated with gold before SEM analysis. In addition, since SEM device was equipped with dispersive X-ray analysis (EDAX) detector, this analysis was used to inspect dispersion of nano-TiO2 particles in the cross section of the fabricated membranes. 2.3.2. Contact angle The hydrophilicity of the membranes was examined by static water contact angle measurement, using OCA20, Dataphysics Instruments, Germany. Deionized water droplets were placed on the surface of membranes, and then the contact angle between the water and membrane was measured until no change was observed. At least, five measurements were taken at different locations on the membrane surface to ensure about the repeatability of the results of contact angle value. The tests were done at 25 1C. 2.3.3. Pore size and porosity The overall porosity (ε) of the nanocomposite membranes was calculated by the gravimetric method, as follows [15]:
ε¼
ω1 ω2 A l dw
ð1Þ
where ω1 and ω2 are the weights of the wet and dry membranes, respectively, l is the membrane thickness (m), A is the membrane area (m2) and dw is the water density (0.998 g/cm3). In order to measure porosity, first a piece of the membranes with known area is immersed in distillated water at least for 12 h to make sure that all the pores of the membranes are filled. Immediately after that, the sample should be weighted only after the water on the surface of the samples is cleaned cautiously. Subsequently, the samples are Table 1 Compositions of the prepared membranes. Membrane
EPVC (wt%) PEG 6 kDa (wt%) TiO2 (wt%) NMP (wt%)
Neat EPVC 0.2 wt% TiO2/EPVC 0.5 wt% TiO2/EPVC 1 wt% TiO2/EPVC 2 wt% TiO2/EPVC 4 wt% TiO2/EPVC
15 15 15 15 15 15
4 4 4 4 4 4
0 0.2 0.5 1 2 4
81 80.8 80.5 80 79 77
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placed in an oven for 2 h at 60 1C to evaporate water from membrane pores and weighted again. Mean pore radius (rm) of the membranes was also measured by the Guerout–Elford–Ferry equation, as follows [15]: rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð2:9 1:75εÞ 8ηlQ ð2Þ rm ¼ ε A ΔP where Q is the water flux (m3/s), η is the water viscosity (8.9 10 4 Pa s) and ΔP is the operation pressure (0.2 MPa). 2.4. Permeation experiments 2.4.1. Ultrafiltration test apparatus Permeation apparatus used in this study is schematically shown in Fig. 1. The membrane is located in the dead-end cell with effective area of 19.6 cm2. The feed (pure water or BSA) is poured into the feed tank and N2 pressure provides the needed driving force for the feed to pass the membrane. The liquid in the cell is magnetically stirred. The weight of permeation is measured by a scale, which is linked to a computer that records the weight of permeation in the given period of time. 2.4.2. Performance calculations Permeation experiments were carried out at pressure of 2 bar for pure water and BSA with concentration of 500 ppm to evaluate water flux, flux recovery and rejection. Prior to the main tests, the membranes were compacted by applying 3 bar pressure for approximately 30 min. First, pure water flux was passed for 90 min and calculated as follows: J¼
M AΔt
ð3Þ
where J, M, A and Δt are the flux, mass of permeated water, membrane effective area and permeation time, respectively. After water permeation, the feed tank is filled with 500 ppm BSA solution to measure its permeation for analyzing the membranes rejection and fouling resistance. BSA flux test was carried out for 90 min. Next, the membranes were brought out of the cell and were placed in fresh water for 1 h and the feed tank was cleaned with distilled water. Finally, second water flux was
Fig. 1. Schematic diagram of dead-end UF setup.
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measured to investigate the flux recovery ratio (FRR) of the membranes as follows: J FRR ð%Þ ¼ water;2 100 ð4Þ J water;1 where Jwater,2 and Jwater,1 are the flux of pure water after and before BSA flux, respectively. The percentage of BSA rejection (R) was calculated by Cp R ð%Þ ¼ 1 100 ð5Þ CF where Cp and Cf are the BSA concentrations in permeate and feed, respectively.
3. Results and discussion 3.1. Morphological properties of membranes The cross-sectional and surface morphology of the neat and the nanocomposite membranes were investigated by SEM, as shown in Figs. 2 and 3 at different magnifications. All the images indicate typically porous and asymmetric structure of the membranes, which have a dense top layer and a porous sub-layer. Addition of TiO2 as a hydrophilic additive affects thermodynamic and kinetics of the phase inversion process which leads to obtain different morphologies [31,32]. As it is obvious from Fig. 2, the neat membrane has large finger-like structure that is due to high affinity between solvent (NMP) and non-solvent (water) and instantaneous demixing during phase separation. However, with increasing the content of TiO2, first, the finger-like structures change to the macro-voids and next, return to finger-like construction with elongated finger-like pores extended throughout the membrane cross-section. Also, the top-layer pore size of the prepared membranes increases and then, diminishes with high TiO2 contents. Compared to the neat PVC dope solution, the presence of hydrophilic titanium dioxide nanoparticles in the dope solution strongly facilitates diffusion of water from coagulation bath to the cast polymer film (the faster exchange rate between solvent and non-solvent), causing the development of large finger-like or macro-voids and the improvement in overall porosity and mean pore size (Table 2) [33,34]. However, high concentration of TiO2 nanoparticles in the polymeric solution leads to increment in viscosity (can be visually seen). The kinetic of the phase inversion is influenced by viscosity, which decreases the exchange velocity in membrane formation. The velocity of phase inversion and solvent/non-solvent instantaneous demixing act as the main deterrent for macro-void formation and the membrane morphology turns to narrow finger-like [35]. In spite of changes in membrane structure with addition of TiO2, crosssectional morphology did not turn to sponge-like. This behavior has been seen by some researches for other polymers like PSf [36]. The observed results are fairly consistent with other studies about nanocomposite membranes [25,27,30,37]. The surface of the membranes was also visualized by SEM and results are presented in Fig. 3. The amount of the TiO2 nanoparticles on the surface of the prepared membranes has been increased by enlarging the nanoparticle concentration in the casting solution. However, it is apparent that increasing TiO2 content leads to more aggregation of the nanoparticles. It is well-known that the TiO2 nanoparticles exhibit a tendency to aggregate (formation of larger size TiO2 clusters) due to their high specific surface area and the hydroxyl groups on the TiO2 surface [38]. EDAX analysis was applied to observe nano-TiO2 dispersion in the membrane structures. Titanium (Ti) was selected for element mapping. As shown in Fig. 4, the amount of TiO2 in the membrane
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0.49 70.05, 0.94 70.11, 1.85 70.2 and 3.89 7 0.4 for 0.2, 0.5, 1, 2, and 4 wt% TiO2/EPVC membranes, respectively. 3.2. Permeation and rejection results
Fig. 2. Cross-sectional SEM images of the neat and TiO2/EPVC nanocomposite membranes.
matrix increases with increment in the weight of the nanoparticles. Also, at low concentrations, the nanofiller is homogeneously dispersed throughout the membrane with low aggregation. In spite of asymmetric structure of the membranes with different morphologies from top to bottom of the membranes, the nanoparticle distribution is completely uniform and in accordance with increasing weight of TiO2 in the nanocomposite membranes. Nevertheless, at higher loadings (particularly at 4 wt% TiO2), the nanoparticles show tendency to aggregate slightly that is common in other nanoparticle-modified membranes [17]. In addition, leach out of the nanoparticle from the prepared membranes was negligible. EDAX analysis showed the percentages of TiO2 nanoparticles in dried membranes were 0.21 70.03,
The effect of TiO2 content on pure water flux was investigated for 90 min at operating pressure of 2 barg. As shown in Fig. 5, the flux of water increases as the TiO2 content increases up to 2 wt% and next, it decreases. TiO2 addition can be two dissimilar effects on water permeation: one is improving of flux due to increase of the porosity and hydrophilicity and another is decreasing of flux due to pore blocking. The existence of TiO2 leads to more porous structure (Table 2) and also, more connectivity between top layer and bottom layer of the membrane, which reduce the resistance of water permeation through the membranes, as observed from the SEM images. On the other hand, based on the results of contact angle (Fig. 6), hydrophilicity of the membranes is improved, which is more favorable to the water flux. Thereby, the nanocomposite membranes tend to attract water molecules and facilitate water penetration more than that of the neat membrane, owing to higher hydrophilicity and enhanced structure, which means higher water flux [39]. However, at high TiO2 content, nanoparticles may cause pore blockage of the membranes, which leads to lower water flux. Thus, pore blockage can neutralize the positive effect of morphology and hydrophilicity on water permeation. In addition, as mentioned earlier, TiO2 addition at high values leads to increment in dope viscosity and formation of membranes with thicker skin layer, which acts as a resistance for water flux [37,39]. In this study, even though sonication of polymeric/nano-TiO2 solution was applied to disperse the nanoparticles, at high TiO2 content, the agglomeration took place, and at 4 wt% TiO2 the pure water flux reduced. However, the flux reduction is not very considerable and permeation of 4 wt% TiO2/EPVC membrane is much higher than the neat membrane and only slightly lower than 2 wt% TiO2/EPVC. This behavior shows that higher TiO2 content after 4 wt% will lead to serious pore blockage and permeation reduction. In the case of agglomeration of the nanoparticle in the polymer matrix and pore blockage, EDAX analysis presented in Fig. 4 obviously shows that the white dots at high TiO2 content membranes are accumulated and joined together specially in 4 wt% embedded membrane, showing aggregation of the nanoparticles. Similar results for other nanocomposite membranes have been reported in literature [25,27,37]. In order to investigate the hydrophilicity of the membranes, static water contact angle test was applied and the results are shown in Fig. 6. The contact angle of the prepared mixed matrix membranes decreased continuously as the content of TiO2 nanoparticles increased. For the unmodified EPVC, the contact angle was around 66.81 and it reduced to 55.41 for 4 wt% TiO2/EPVC membrane. It shows that the surface of the membranes becomes more hydrophilic by TiO2 addition. The reason for this observation is that during membrane preparation, it is possible for the nanoparticles to move towards the surface of the membrane. Therefore, as the nanoparticles are hydrophilic, the hydrophilicity of the membranes increases. This phenomena lead to higher water adsorption and consequently, higher water permeability. This surface migration of nanoparticles could be due to reduction in interface energy or the gravitational force [18,40]. Table 2 represents mean pore size and porosity of the prepared nanocomposite membranes. All the fabricated membranes have porosity between 68% and 79%, due to low polymer concentration and existence of polyethylene glycol additive in polymeric dope which acts as a pore former and leach out the membrane during demixing process in the coagulation bath [35]. However, as an overall trend, the porosity enhances because of faster precipitation
H. Rabiee et al. / Journal of Membrane Science 472 (2014) 185–193
189
Fig. 3. Surface SEM images of the neat and TiO2/EPVC nanocomposite membranes.
and exchange of solvent/non-solvent with addition of TiO2 nanoparticles [23]. Pore size of the membranes is also influenced by the nanoparticles which on the one hand lead to faster demixing due to its hydrophilic behavior, thus larger pores are formed. However,
as presented in Table 2, mean pore size again decreases at high loading. This could be due to slight nanoparticle aggregation. The BSA rejection properties, of the modified and unmodified membranes, are shown in Fig. 7. As can be seen, BSA rejection for
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Table 2 Porosity and mean pore size of the nanocomposite membranes. Membrane
Porosity (%)
Neat EPVC 0.2 wt% TiO2/EPVC 0.5 wt% TiO2/EPVC 1 wt% TiO2/EPVC 2 wt% TiO2/EPVC 4 wt% TiO2/EPVC
68.5 72.6 76.4 79.1 78.7 75.2
( 7 3.2) ( 7 3.8) ( 7 3.9) ( 7 4.2) ( 7 3.1) ( 7 3.2)
Mean pore radius, r (nm) 9.1 9.6 10.0 10.3 12.5 12.1
(7 0.6) (7 0.7) (7 0.7) (7 0.6) (7 0.8) (7 0.9)
all the membranes is more than 90%. The rejection enhances to almost 98% for 2 wt% TiO2/EPVC membrane. The incorporation of hydrophilic TiO2 nanoparticle with surface hydroxyl group caused to increase the interaction between BSA and membrane surface and improve BSA rejection. TiO2 addition increases surface hydrophilicity of the membranes, which results in lower affinity and interaction between BSA and membrane surface and higher rejection [41]. In addition, the thicker skin layer can be another reason for increment in BSA rejection for TiO2-modified membranes and that is why TiO2 addition causes higher BSA retention. However, similar to water flux order, BSA rejection for 4 wt% TiO2/ EPVC membrane decreased slightly, which can be attributed to the nanoparticles agglomeration at high content and formation of large pores that diminish the rejection [37,41].
3.3. Antifouling properties of the membranes The membranes, fabricated by hydrophobic materials like PES and PVC, suffer from fouling during the processes, in which components such as natural organic matter (NOM), whey or micro-organism exist as foulants. They lead to lower permeation due to fouling of membrane surface. Thereby, modified membranes should be able to reduce fouling as much as possible and make it reversible with water washing to recover the pure water flux after foulant permeation, sufficiently. Thus, the flux of BSA solution was performed to evaluate antifouling properties of the mixed matrix membranes. Fig. 8 shows the permeation results of the neat and the nanocomposite membranes at three different periods of time. First, pure water is passed through the membrane, then BSA is filtered and finally, second water flux is performed to compare water flux before and after BSA. As can be seen in Fig. 8, all the permeations in the three steps follow the same trends, and 2 wt% TiO2/EPVC membrane shows the highest permeation for water and BSA solution. The fluxes of all the fabricated membranes decreased till they almost reached to stable plateau. BSA molecules can interact with the surface of membrane, resulting in the formation of a layer which leads to the formation of gel layer, increase membrane resistance and reduce water flux. However, the presence of TiO2 on the surface of membrane and inner surface of membrane pores could reduce the interaction between BSA and membrane surface and prevent BSA adsorption and consequently, decrease the membrane fouling [27,39]. Fig. 9 presents flux recovery ratio (FRR) of the prepared membranes. The flux recovery ratio of the membranes increased with addition of TiO2 content and reduced slightly for 4 wt% TiO2. The FRR promoted from 69% for the unmodified membrane to 89% for 2 wt% TiO2/EPVC, which showed the highest antifouling properties of the nanocomposite membranes. This means that protein fouling becomes more reversible with increment in TiO2 addition. The observations are in good accordance with other
Fig. 4. EDAX of the cross-section of TiO2/EPVC nanocomposite membranes: (a) 0.2 wt%, (b) 0.5 wt%, (c) 1 wt%, (d) 2 wt% and (e) 4 wt%.
H. Rabiee et al. / Journal of Membrane Science 472 (2014) 185–193
500
550
435
4 wt.% TiO2/EPVC
403
450 Pure Water Flux (kg/m 2. h)
191
2 wt.% TiO2/EPVC
500
1 wt.% TiO2/EPVC
400 323
350
0.2 wt.% TiO2/EPVC
284
300
400
221
Flux (kg/m 2 h)
213
0.5 wt.% TiO2/EPVC
450
250 200 150 100
Neat EPVC
350 300 250 200
50
150 VC Ti O 2/ EP
VC
100
w t.%
50 0 0
4
2
1
w t.%
Ti O 2/ EP w t.%
Ti O 2/ EP
VC
VC Ti O 2/ EP w t.%
0. 5
0. 2
w t.%
N ea
tE
Ti O 2/ EP
PV
C
VC
0
30
60
90
120
150
180
210
240
270
300
Time (min)
Fig. 5. Pure water flux of EPVC/TiO2 nanocomposite membranes (results are average of three replicates). 80 66.8
70
64.6 61.03
59.1
100
57.2
60
89
55.4
84
90
86
79 75
80
50 Flux recovery ratio (%)
Static Water contact angle (°)
Fig. 8. Flux of water and BSA versus time for the modified and unmodified membranes as follows: the first 90 min for pure water flux, the second for BSA flux and the third for water flux after membrane washing with pure water for 30 min.
40 30 20
69
70 60 50 40 30 20
10
10
0
Rejection of BSA (%)
VC 2/ O Ti t%
Fig. 9. Flux recovery ratio (FRR) of the modified and unmodified membranes.
w
100
90
80
70
60
Unmodified EPVC
0.2 wt.% 0.5 wt.% 1 wt.% 2 wt.% 4 wt.% TiO2/EPVC TiO2/EPVC TiO2/EPVC TiO2/EPVC TiO2/EPVC
4
Fig. 6. Static water contact angle of the TiO2/EPVC membranes (average contact angle of five replicates is reported).
50
Neat EPVC
EP
VC O
2
w
t%
Ti t% w 1
Ti
O
2/
EP 2/
EP
VC
VC EP 2/ Ti t% w
5 0.
0.
2
w
t%
N
Ti
O
O
ea
2/
tE
EP
PV
C
VC
0
0.2 wt.% 0.5 wt.% 1 wt.% 2 wt.% 4 wt.% TiO2/EPVC TiO2/EPVC TiO2/EPVC TiO2/EPVC TiO2/EPVC
Fig. 7. BSA rejection of unmodified and TiO2-modified membranes.
membranes modified by nanoparticles for water treatment [27,37,42]. However, for the case of 4 wt% TiO2 content, the FRR decreases slightly that could be attributed to the nanoparticle agglomeration. Excessive loading of TiO2 nanoparticles leads to significant agglomeration on the substrate surface, reducing the contact area of hydroxyl groups carried by TiO2 nanoparticles and possibly decreases repulsion between protein and membrane surface [33].
The water flux and FRR of 4 wt% TiO2 membrane are very close to that of 2 wt% TiO2/EPVC and still more than that of 1 wt% TiO2, which confirms that agglomeration is not very remarkable. By incorporation of hydrophilic nanoparticles on the membrane surface, a hydrated layer is formed, which can effectively prevent the attachment of foulants [43]. In order to investigate the performance reproducibility of the prepared membranes, three-cycle water and BSA flux were performed for unmodified EPVC and 2 wt% TiO2/EPVC membranes, which showed the best result for flux recovery ratio. The fluxes are shown in Fig. 10 and as it can be seen, water and BSA fluxes decrease for both the membranes; therefore FRR reduces after the second and third BSA flux, compared to the first one. However, this reduction in FRR is not very significant, particularly for 2 wt% TiO2/EPVC membrane; its FRR are 89%, 85% and 81% related to the first water flux. First, second and third FRR for neat EPVC membrane are 69%, 65% and 61%, respectively. This observation indicates high efficiency of the TiO2-modified membranes in hydraulic cleaning. Accordingly, it can be said that the addition of TiO2 to emulsion polyvinyl chloride matrix for preparation of ultrafiltration membrane develops the antifouling characteristic of the prepared mixed matrix membranes.
4. Conclusion EPVC/TiO2 nanocomposite ultrafiltration membranes were fabricated via induced immersion precipitation technique with NMP
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Fig. 10. Reproducible characteristic of the neat EPVC and 2 wt% TiO2/EPVC membranes during three BSA filtrations.
as the solvent and water as the non-solvent. The SEM images showed finger-like pores in membrane structure with more connectivity between top and bottom of the membranes, after TiO2 addition. EDAX analysis showed fine and homogeneous dispersion of TiO2 nanoparticles in membrane matrix at low concentrations. The prepared mixed matrix membranes showed an increment in hydrophilicity, pure water flux and flux recovery ratio with increasing TiO2 content. At 4 wt% TiO2, a slight reduction in pure water flux and FRR was observed, which could be due to nanoparticles tendency to aggregate at this loading. TiO2 addition also led to more BSA rejection and antifouling properties of the prepared membranes. Flux recovery ratio of the nanocomposite membranes increased from 69% for the neat membrane to 89% for 2 wt% TiO2 nanocomposite membrane. This study showed that the incorporated TiO2 nanoparticle could improve antifouling properties of the emulsion poly (vinyl chloride) membrane to be used as an ultrafiltration membrane for treatment of high fouling wastewaters.
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