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Hydrophilic modification of polypropylene microfiltration membranes by ozone-induced graft polymerization
Journal of Membrane Science 169 (2000) 269–276
Hydrophilic modification of polypropylene microfiltration membranes by ozone-induced graft polymerization Young Wang, Jae-Hong Kim, Kwang-Ho Choo, Yoon-Sik Lee ∗ , Chung-Hak Lee School of Chemical Engineering, College of Engineering, Seoul National University, Kwanak-Gu, Shinlim-Dong, San 56-1, Seoul 151-742, South Korea Received 20 August 1999; accepted 2 November 1999
Abstract Hydrophilic modification of polypropylene microfiltration membranes was performed with the introduction of peroxide onto the membrane surface by ozone treatment followed by graft polymerization with hydroxyethyl methacrylate (HEMA). The grafting was initiated at a mild temperature by redox decomposition of the peroxide. The ozone treatment time was optimized in the range of 3–5 min to have a degree of grafting while keeping the mechanical strength. The HEMA grafting made the surface of the PP membrane hydrophilic and less adsorbable to BSA proteins, although its effects were dependent on the ozone treatment time. The grafted membrane with 5 min ozone treatment gave greater flux recoveries (up to approximately 95%) at the end of the MF of the BSA solution, suggesting that the protein fouling layer was reversible because of the hydrophilic nature of the modified membranes. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Ozone treatment; Graft polymerization; Microfiltration membrane; Flux recovery; Hydrophilicity
1. Introduction The major drawback in the extensive use of membranes is membrane fouling, which results in flux reduction during operation. In the case of fouling caused by the adsorption of proteins on the membrane, the hydrophobic interaction between membrane surfaces and proteins has been considered a dominating factor [1]. With respect to the prevention of membrane fouling, less hydrophobic (more hydrophilic) membranes are normally favored [1,2], but hydrophilic membranes are susceptible to chemical and thermal impacts in their applications [3].
∗ Corresponding author. Fax: +82-2-888-1604. E-mail address: [email protected] (Y.-S. Lee).
Recently, a technique for the chemical modification of polymeric surfaces such as graft polymerization has been extensively studied, particularly in biomedical fields. The graft polymerization method is very attractive in controlling the adhesion of particles and macromolecules onto the substrate surfaces. In graft polymerization, functional groups were first introduced to the polymeric substrate by UV irradiation [4], glow-discharge [5,6], or ozone [7,8] and then were decomposed and initiated for polymerization. For these studies, various polymeric substrates, such as fiber and powder [8], polyethylene film [5], polyurethane film [7], polysulfone membrane [4], polypropylene membrane and polyvinylidene fluoride membrane [9], were used. However, there are few studies on the modification of membrane surfaces in order to solve fouling problems in their application.
0376-7388/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 9 9 ) 0 0 3 4 5 - 2
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Fig. 1. Experimental setup for ozonation. 1. Oxygen tank; 2. Ozone generator; 3. Flow meter; 4. Water bath; 5.Ozonation reactor; 6. Membrane; 7. Ozone trap with KI.
In fact, little information is available on the characteristics of the modified membrane surfaces. In this study, the hydrophilic modification of polypropylene (PP) microfiltration (MF) membrane surfaces was investigated in terms of surface properties, protein adsorption characteristics and membrane fouling control. The oxidation of PP membranes was done by exposing ozone gas for predetermined times. The peroxide that was generated during the ozone treatment was decomposed by redox reaction to initiate graft polymerization at mild temperatures. The effects of ozone treatment time on the tensile strength of the membrane and the degree of graft polymerization were examined. The amount of proteins adsorbed and the reversibility of fouling layers were compared between virgin (unmodified) and modified membranes using a bovine serum albumin (BSA) protein solution.
2.2. Ozone treatment The ozone was generated by an ozone generator (Fisher Ozone 502, Germany) using pure oxygen gas as illustrated in Fig. 1. The ozone concentration and the oxygen gas flow rate were adjusted to about 2.3 mol% and 50 l/min, respectively. The ozone gas was passed through a water bath whose temperature was maintained at 30◦ C and then was supplied to the ozonation reactor, where the membrane sheets were placed. After ozone treatment for a given period of time, residual ozone in the reactor was removed with nitrogen gas purging and then the membrane was placed under vacuum for 30 min to make sure the physically absorbed ozone molecules were removed.
2.3. Graft polymerization 2. Experimental 2.1. Materials Flat sheets of polypropylene MF membranes (Accurel PP2E, Akzo Nobel, Germany) with a nominal pore size of 0.2 m were used as a substrate for this study. Commercially modified GVHP and GVWP MF membranes with 0.2 m pores (Millipore, USA) were also used for comparison. The 2-hydroxyethyl methacrylate (HEMA) monomer was vacuum-distilled at 67◦ C under −3.5 mmHg before use in order to remove the polymerization inhibitor, monomethyl ether hydroquinone (MEHQ).
The ozone-treated membrane was soaked in ethanol for 10 min and placed in HEMA solution. The grafting reaction was initiated by the addition of FeCl2 ·2H2 O and proceeded as shown in Fig. 2. Graft polymerization was allowed to proceed in a shaking incubator at 40◦ C with a shaking speed of 150 rpm for 2–12 h. The overall concentrations of HEMA and the catalyst (FeCl2 ·2H2 O ) were 5% (v/v) and 10−4 M, respectively. When the polymerization was finished, the membrane was washed in a Soxhlet extraction apparatus with hot methanol to remove residual monomers and homopolymers. The grafted membranes were stored at 4◦ C in ultra-pure water before use.
Y. Wang et al. / Journal of Membrane Science 169 (2000) 269–276
271
Fig. 2. Schematic illustration of ozone-induced graft polymerization.
2.4. Protein adsorption and cross-flow filtration
2.5. Analytical methods
Bovine serum albumin (BSA, Sigma, USA) was used as a model protein to evaluate the protein fouling characteristics of virgin and grafted membranes. The membranes were soaked in ethanol for 10 min for pre-wetting and then put into a BSA solution with a concentration of 2000 mg/l whose pH was adjusted to 7.0 with a 0.1 M phosphate buffer. The adsorption tests were carried out by placing a piece of membrane with a surface area of 20 cm2 in 20 ml BSA solution for 24 h at a temperature of 20◦ C and a shaking speed of 150 rpm. The concentrations of the BSA solution before and after adsorption were determined based on absorbance at 278 nm using a UV–Vis spectrophotometer (Hewlett Packard 8452A, Diode Array Spectrophotometer, USA). Cross-flow MF was performed with a plate-andframe module to check the extent of membrane fouling on the virgin and grafted membranes. The cross-flow velocity was maintained at 0.3 m/s using a peristaltic pump (Cole Parmer Model 7553-75, USA), while the transmembrane pressure was controlled at 0.31 bar (4.5 psi) by regulating the back-pressure valve. The BSA solution that has a concentration of 2000 mg/l and a pH 7.0 in 0.1 M phosphate buffer was circulated through the membrane module. The permeate flux was measured using a balance and then the permeate was returned to the feed tank to keep the total system volume constant. Before and after the filtration of the BSA solution, the pure water flux was measured to check the changes in membrane permeability and thus the extent of membrane fouling.
The mechanical property of the ozone-treated membranes was determined using a Universal Testing Machine (UTM, LLOYD Instrument LR 10 K, UK). Samples were prepared as ASTM D638 type V and the tensile strength was determined at a cross-head speed of 5 mm/min. The amount of peroxide groups produced on the membrane by the ozone treatment was determined using the DPPH (2,2-diphenyl-1-(2,4,6-trinitrophenyl)hydrazyl) method [7]. In this method, it was assumed that one equivalent of hydroperoxide reacts with two equivalents of DPPH. The PP membrane treated for a given period of time by ozone was put into a 30 ml DPPH solution. The sample solutions prepared were refluxed with stirring and N2 purging at 60◦ C for 24 h. Then, the samples were diluted to 50 ml with benzene and finally the remaining DPPH concentration was determined from the absorbance at a wavelength of 520 nm using a UV–Vis spectrophotometer (Hewlett Packard 8452A, Diode Array Spectrophotometer, USA). Based on this measurement, the amount of hydroperoxide consumed was calculated. For blank test, the same procedures were followed with the virgin membrane. Each test was repeated four times. Fourier transform infrared (FT-IR) analyses were taken on a Bomem MB-100 spectrometer (Canada) equipped with a DTGS detector in the micro-ATR mode. A germanium cell was used as the internal reflection element and the incident angle was fixed at 45◦ .
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Fig. 3. Concentration of peroxide on the membrane surface as a function of ozone treatment time.
The contact angle of the membrane surface was measured to quantify the change in its hydrophilicity. The membrane samples were placed on a Goniometer (Rame Hart, Model 100–00–115, USA), a drop of ultrapure water was placed on it, and then the advancing angle was measured.
Bubble point measurement is the most widely used and the simplest technique for the determination of membrane pore size. It was measured to detect any change in maximum pore size before and after
Fig. 4. Effect of ozone treatment time on tensile strength.
Fig. 5. FT-IR spectra of virgin and grafted membranes.
Y. Wang et al. / Journal of Membrane Science 169 (2000) 269–276
273
Fig. 6. SEM photos of (a) virgin and grafted membranes that had different ozone treatment times of (b) 3 min and (c) 5 min.
graft polymerization by the procedure described in American Society for Testing and Materials Standard (ASTM) Method F 316. Isopropylalcohol was used as a test liquid and the pressure was monitored through a digital pressure gauge (Cole-Parmer 94785-05, USA). SEM pictures of the virgin and grafted membranes were taken with a JSM-35 microscope (JEOL, Japan) at 25 kV. Samples were dried and precoated with gold at 0.2 Torr before the SEM analysis.
The longer the membrane was contacted by ozone gas, the more the peroxide group was generated. However, the excessive ozone treatment made the membrane mechanically fragile since ozone attacked the backbone of the polypropylene membrane. In Fig. 4, the maximum stress at break steadily decreased as the ozone treatment time increased. It was thought that a 10 min ozone treatment was too long because the tensile strength was decreased by approximately 40% compared to that of the virgin membrane.
3. Results and discussion
3.2. Graft polymerization
3.1. Ozone treatment
Fig. 5 shows the FT-IR spectra of the virgin and grafted PP membranes. The presence of HEMA grafted on the membrane surface was confirmed by the C=O vibration peak at 1725 cm−1 . The peak
The peroxide concentration on the membrane surface was measured and the results are shown in Fig. 3.
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Fig. 7. Changes in physicochemical properties of membranes before and after modification: (a) mass increment; (b) bubble point; and (c) contact angle.
intensity at 1725 cm−1 was increased with increasing the ozone treatment time, suggesting that a larger amount of HEMA was grafted with longer ozonation times. Additionally, the SEM pictures showed that the membrane surface became smoother by the grafting (Fig. 6). It also looked like that with grafting the surface porosity was somewhat reduced because poly-HEMA chains covered the original surfaces of PP membrane.
The mass increase of the membranes after graft polymerization is shown in Fig. 7(a). The mass of the membrane with grafting was increased by 4 to 23% with increasing the ozone treatment time, but there was no remarkable change in the bubble point (Fig. 7(b)), suggesting that physical characteristics such as pore size were not changed significantly by the grafting process. However, the contact angle of the grafted membranes decreased substantially compared to the
Y. Wang et al. / Journal of Membrane Science 169 (2000) 269–276
Fig. 8. Amount of protein adsorbed per unit area of the virgin (PP), grafted (1, 3, and 5 min ozone treated) and the commercial (GVHP and GVWP) membranes.
virgin membrane as shown in Fig. 7(c). The grafted membranes that had been treated for more than 3 min by ozone were wetted completely, supporting the fact that the membrane surface became highly hydrophilic with grafting. 3.3. Protein adsorption and filtration Batchwise BSA adsorption tests were carried out to find out the membrane fouling potential caused by the membrane surface–protein interaction. Fig. 8 compares the amount of proteins adsorbed on the virgin and modified PP membranes in the batch tests. The amount of proteins adsorbed on the grafted membranes decreased substantially with longer ozone treatment times. Furthermore, the grafted membranes obtained in this work had a smaller or similar adsorption capacity for BSA protein, compared to the commercially available hydrophilic GVWP membrane. Cross-flow filtration of the model protein solution was performed to compare the permeate flux and fouling tendencies between the unmodified and modified membranes. The flux during microfiltration of the BSA solution is shown in Fig. 9. The flux decline for the modified (grafted) membrane was nearly in the same trend as that of the unmodified (virgin) membrane. However, the flux recovery efficiency that is closely associated with the reversibility of membrane fouling layers increased markedly with increasing the ozone treatment time by up to 5 min (Fig. 10). The
275
Fig. 9. Fluxes for virgin and modified membranes during cross-flow MF of BSA solution.
grafted membrane obtained with 5 min ozone treatment gave the highest flux recovery of approximately 95%. However, the 1 min ozonation had no effect on an improvement of flux recovery, suggesting that only part of the membrane surfaces and pores were modified during short time ozonation. It appears that membrane fouling might be caused by the accumulation of protein aggregates at the membrane surface regardless of the membranes used but could be reversed more easily for the grafted membranes with an appropriate time of ozonation. In summary, the protein fouling layer became reversible with grafting,
Fig. 10. Flux recovery efficiencies with water flushing at the end of operation.
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Y. Wang et al. / Journal of Membrane Science 169 (2000) 269–276
being removed easily just by water flushing. This can be attributed to the more hydrophilic nature of the modified membrane surfaces. Also, the steric hindrance resulting from the poly-HEMA chains grafted may play a role in preventing the direct interaction between membrane surfaces and proteins [6]. 4. Conclusions Hydrophilic modification of the PP membrane surfaces was done by ozone treatment followed by graft-polymerization with HEMA. The longer ozone treatment led to an increase in the degree of graft polymerization and membrane hydrophilicity. However, excessive ozone treatment made the membrane brittle, so that the ozone treatment time should be optimized in the range of 3–5 min in this work. The grafted PP membranes had a relatively weak affinity to BSA proteins compared to the virgin membrane and even commercially available hydrophilic membranes. A great improvement in flux recovery was achieved with the grafted PP membranes, suggesting that the grafted membranes had a high reversibility of the protein fouling layer. This can be explained by the increase of hydrophilicity of the grafted membranes and the steric hindrance by the HEMA chains present at the membrane surfaces. Acknowledgements The authors are grateful to Ms. A. Sainbayar, Mr. W.J. Cho and Ms. H.G. Sohn for their assistance dur-
ing experiments at the School of Chemical Engineering, Seoul National University.
References [1] J.A. Howell, M. Nystrom, Fouling phenomena, in: J.A. Howell, V. Sanchez, R.W. field (Eds.), Membranes in Bioprocessing: Theory and Applications, Chapmann & Hall, London, 1993, pp. 203–244. [2] G. Belfort, R.H. Davis, A.L. Zydney, The behavior of suspensions and macromolecular solutions in cross-flow microfiltration, J. Membr. Sci. 96 (1996) 1–58. [3] Y. Osada, T. Nakagawa, Membrane Science and Technology, Vol. 290, Marcel Dekker, New York, 1992. [4] M. Nyström, P. Järvinen, Modification of polysulfone ultrafiltration membranes with UV irradiation and hydrophilicity increasing agents, J. Membr. Sci. 60 (1991) 275. [5] M. Suzuki, A. Kishida, H. Iwata, Y. Ikada, Graft copolymerization of acrylamide onto a polyethylene surface pretreated with a glow discharge, Macromolecules 19 (1986) 1804. [6] K. Fujimoto, Y. Takebayashi, H. Inoue, Y. Ikada, Polyurethane surface modification by graft polymerization of acrylamide for reduced protein adsorption and platelet adhesion, Biomaterials 14 (1993) 442. [7] K. Fujimoto, Y. Takebayashi, H. Inoue, Y. Ikada, Ozone-induced graft polymerization onto polymer surface, J. Polym. Sci. Polym. Chem. 31 (1993) 1035. [8] J. Yamauchi, A. Yamaoka, K. Ikemoto, T. Matsui, Graft copolymerization of methyl methacrylate onto polypropylene oxidized with ozone, J. Appl. Polym. Sci. 43 (1991) 1197. [9] D. Düputell, E. Staude, Heterogeneous modification of ultrafiltration membrane made from poly (vinylidene fluoride) and their characterization, J. Membr. Sci. 78 (1993) 45.
Hydrophilic modification of polypropylene microfiltration membranes by ozone-induced graft polymerization Young Wang, Jae-Hong Kim, Kwang-Ho Choo, Yoon-Sik Lee ∗ , Chung-Hak Lee School of Chemical Engineering, College of Engineering, Seoul National University, Kwanak-Gu, Shinlim-Dong, San 56-1, Seoul 151-742, South Korea Received 20 August 1999; accepted 2 November 1999
Abstract Hydrophilic modification of polypropylene microfiltration membranes was performed with the introduction of peroxide onto the membrane surface by ozone treatment followed by graft polymerization with hydroxyethyl methacrylate (HEMA). The grafting was initiated at a mild temperature by redox decomposition of the peroxide. The ozone treatment time was optimized in the range of 3–5 min to have a degree of grafting while keeping the mechanical strength. The HEMA grafting made the surface of the PP membrane hydrophilic and less adsorbable to BSA proteins, although its effects were dependent on the ozone treatment time. The grafted membrane with 5 min ozone treatment gave greater flux recoveries (up to approximately 95%) at the end of the MF of the BSA solution, suggesting that the protein fouling layer was reversible because of the hydrophilic nature of the modified membranes. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Ozone treatment; Graft polymerization; Microfiltration membrane; Flux recovery; Hydrophilicity
1. Introduction The major drawback in the extensive use of membranes is membrane fouling, which results in flux reduction during operation. In the case of fouling caused by the adsorption of proteins on the membrane, the hydrophobic interaction between membrane surfaces and proteins has been considered a dominating factor [1]. With respect to the prevention of membrane fouling, less hydrophobic (more hydrophilic) membranes are normally favored [1,2], but hydrophilic membranes are susceptible to chemical and thermal impacts in their applications [3].
∗ Corresponding author. Fax: +82-2-888-1604. E-mail address: [email protected] (Y.-S. Lee).
Recently, a technique for the chemical modification of polymeric surfaces such as graft polymerization has been extensively studied, particularly in biomedical fields. The graft polymerization method is very attractive in controlling the adhesion of particles and macromolecules onto the substrate surfaces. In graft polymerization, functional groups were first introduced to the polymeric substrate by UV irradiation [4], glow-discharge [5,6], or ozone [7,8] and then were decomposed and initiated for polymerization. For these studies, various polymeric substrates, such as fiber and powder [8], polyethylene film [5], polyurethane film [7], polysulfone membrane [4], polypropylene membrane and polyvinylidene fluoride membrane [9], were used. However, there are few studies on the modification of membrane surfaces in order to solve fouling problems in their application.
0376-7388/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 9 9 ) 0 0 3 4 5 - 2
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Y. Wang et al. / Journal of Membrane Science 169 (2000) 269–276
Fig. 1. Experimental setup for ozonation. 1. Oxygen tank; 2. Ozone generator; 3. Flow meter; 4. Water bath; 5.Ozonation reactor; 6. Membrane; 7. Ozone trap with KI.
In fact, little information is available on the characteristics of the modified membrane surfaces. In this study, the hydrophilic modification of polypropylene (PP) microfiltration (MF) membrane surfaces was investigated in terms of surface properties, protein adsorption characteristics and membrane fouling control. The oxidation of PP membranes was done by exposing ozone gas for predetermined times. The peroxide that was generated during the ozone treatment was decomposed by redox reaction to initiate graft polymerization at mild temperatures. The effects of ozone treatment time on the tensile strength of the membrane and the degree of graft polymerization were examined. The amount of proteins adsorbed and the reversibility of fouling layers were compared between virgin (unmodified) and modified membranes using a bovine serum albumin (BSA) protein solution.
2.2. Ozone treatment The ozone was generated by an ozone generator (Fisher Ozone 502, Germany) using pure oxygen gas as illustrated in Fig. 1. The ozone concentration and the oxygen gas flow rate were adjusted to about 2.3 mol% and 50 l/min, respectively. The ozone gas was passed through a water bath whose temperature was maintained at 30◦ C and then was supplied to the ozonation reactor, where the membrane sheets were placed. After ozone treatment for a given period of time, residual ozone in the reactor was removed with nitrogen gas purging and then the membrane was placed under vacuum for 30 min to make sure the physically absorbed ozone molecules were removed.
2.3. Graft polymerization 2. Experimental 2.1. Materials Flat sheets of polypropylene MF membranes (Accurel PP2E, Akzo Nobel, Germany) with a nominal pore size of 0.2 m were used as a substrate for this study. Commercially modified GVHP and GVWP MF membranes with 0.2 m pores (Millipore, USA) were also used for comparison. The 2-hydroxyethyl methacrylate (HEMA) monomer was vacuum-distilled at 67◦ C under −3.5 mmHg before use in order to remove the polymerization inhibitor, monomethyl ether hydroquinone (MEHQ).
The ozone-treated membrane was soaked in ethanol for 10 min and placed in HEMA solution. The grafting reaction was initiated by the addition of FeCl2 ·2H2 O and proceeded as shown in Fig. 2. Graft polymerization was allowed to proceed in a shaking incubator at 40◦ C with a shaking speed of 150 rpm for 2–12 h. The overall concentrations of HEMA and the catalyst (FeCl2 ·2H2 O ) were 5% (v/v) and 10−4 M, respectively. When the polymerization was finished, the membrane was washed in a Soxhlet extraction apparatus with hot methanol to remove residual monomers and homopolymers. The grafted membranes were stored at 4◦ C in ultra-pure water before use.
Y. Wang et al. / Journal of Membrane Science 169 (2000) 269–276
271
Fig. 2. Schematic illustration of ozone-induced graft polymerization.
2.4. Protein adsorption and cross-flow filtration
2.5. Analytical methods
Bovine serum albumin (BSA, Sigma, USA) was used as a model protein to evaluate the protein fouling characteristics of virgin and grafted membranes. The membranes were soaked in ethanol for 10 min for pre-wetting and then put into a BSA solution with a concentration of 2000 mg/l whose pH was adjusted to 7.0 with a 0.1 M phosphate buffer. The adsorption tests were carried out by placing a piece of membrane with a surface area of 20 cm2 in 20 ml BSA solution for 24 h at a temperature of 20◦ C and a shaking speed of 150 rpm. The concentrations of the BSA solution before and after adsorption were determined based on absorbance at 278 nm using a UV–Vis spectrophotometer (Hewlett Packard 8452A, Diode Array Spectrophotometer, USA). Cross-flow MF was performed with a plate-andframe module to check the extent of membrane fouling on the virgin and grafted membranes. The cross-flow velocity was maintained at 0.3 m/s using a peristaltic pump (Cole Parmer Model 7553-75, USA), while the transmembrane pressure was controlled at 0.31 bar (4.5 psi) by regulating the back-pressure valve. The BSA solution that has a concentration of 2000 mg/l and a pH 7.0 in 0.1 M phosphate buffer was circulated through the membrane module. The permeate flux was measured using a balance and then the permeate was returned to the feed tank to keep the total system volume constant. Before and after the filtration of the BSA solution, the pure water flux was measured to check the changes in membrane permeability and thus the extent of membrane fouling.
The mechanical property of the ozone-treated membranes was determined using a Universal Testing Machine (UTM, LLOYD Instrument LR 10 K, UK). Samples were prepared as ASTM D638 type V and the tensile strength was determined at a cross-head speed of 5 mm/min. The amount of peroxide groups produced on the membrane by the ozone treatment was determined using the DPPH (2,2-diphenyl-1-(2,4,6-trinitrophenyl)hydrazyl) method [7]. In this method, it was assumed that one equivalent of hydroperoxide reacts with two equivalents of DPPH. The PP membrane treated for a given period of time by ozone was put into a 30 ml DPPH solution. The sample solutions prepared were refluxed with stirring and N2 purging at 60◦ C for 24 h. Then, the samples were diluted to 50 ml with benzene and finally the remaining DPPH concentration was determined from the absorbance at a wavelength of 520 nm using a UV–Vis spectrophotometer (Hewlett Packard 8452A, Diode Array Spectrophotometer, USA). Based on this measurement, the amount of hydroperoxide consumed was calculated. For blank test, the same procedures were followed with the virgin membrane. Each test was repeated four times. Fourier transform infrared (FT-IR) analyses were taken on a Bomem MB-100 spectrometer (Canada) equipped with a DTGS detector in the micro-ATR mode. A germanium cell was used as the internal reflection element and the incident angle was fixed at 45◦ .
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Fig. 3. Concentration of peroxide on the membrane surface as a function of ozone treatment time.
The contact angle of the membrane surface was measured to quantify the change in its hydrophilicity. The membrane samples were placed on a Goniometer (Rame Hart, Model 100–00–115, USA), a drop of ultrapure water was placed on it, and then the advancing angle was measured.
Bubble point measurement is the most widely used and the simplest technique for the determination of membrane pore size. It was measured to detect any change in maximum pore size before and after
Fig. 4. Effect of ozone treatment time on tensile strength.
Fig. 5. FT-IR spectra of virgin and grafted membranes.
Y. Wang et al. / Journal of Membrane Science 169 (2000) 269–276
273
Fig. 6. SEM photos of (a) virgin and grafted membranes that had different ozone treatment times of (b) 3 min and (c) 5 min.
graft polymerization by the procedure described in American Society for Testing and Materials Standard (ASTM) Method F 316. Isopropylalcohol was used as a test liquid and the pressure was monitored through a digital pressure gauge (Cole-Parmer 94785-05, USA). SEM pictures of the virgin and grafted membranes were taken with a JSM-35 microscope (JEOL, Japan) at 25 kV. Samples were dried and precoated with gold at 0.2 Torr before the SEM analysis.
The longer the membrane was contacted by ozone gas, the more the peroxide group was generated. However, the excessive ozone treatment made the membrane mechanically fragile since ozone attacked the backbone of the polypropylene membrane. In Fig. 4, the maximum stress at break steadily decreased as the ozone treatment time increased. It was thought that a 10 min ozone treatment was too long because the tensile strength was decreased by approximately 40% compared to that of the virgin membrane.
3. Results and discussion
3.2. Graft polymerization
3.1. Ozone treatment
Fig. 5 shows the FT-IR spectra of the virgin and grafted PP membranes. The presence of HEMA grafted on the membrane surface was confirmed by the C=O vibration peak at 1725 cm−1 . The peak
The peroxide concentration on the membrane surface was measured and the results are shown in Fig. 3.
274
Y. Wang et al. / Journal of Membrane Science 169 (2000) 269–276
Fig. 7. Changes in physicochemical properties of membranes before and after modification: (a) mass increment; (b) bubble point; and (c) contact angle.
intensity at 1725 cm−1 was increased with increasing the ozone treatment time, suggesting that a larger amount of HEMA was grafted with longer ozonation times. Additionally, the SEM pictures showed that the membrane surface became smoother by the grafting (Fig. 6). It also looked like that with grafting the surface porosity was somewhat reduced because poly-HEMA chains covered the original surfaces of PP membrane.
The mass increase of the membranes after graft polymerization is shown in Fig. 7(a). The mass of the membrane with grafting was increased by 4 to 23% with increasing the ozone treatment time, but there was no remarkable change in the bubble point (Fig. 7(b)), suggesting that physical characteristics such as pore size were not changed significantly by the grafting process. However, the contact angle of the grafted membranes decreased substantially compared to the
Y. Wang et al. / Journal of Membrane Science 169 (2000) 269–276
Fig. 8. Amount of protein adsorbed per unit area of the virgin (PP), grafted (1, 3, and 5 min ozone treated) and the commercial (GVHP and GVWP) membranes.
virgin membrane as shown in Fig. 7(c). The grafted membranes that had been treated for more than 3 min by ozone were wetted completely, supporting the fact that the membrane surface became highly hydrophilic with grafting. 3.3. Protein adsorption and filtration Batchwise BSA adsorption tests were carried out to find out the membrane fouling potential caused by the membrane surface–protein interaction. Fig. 8 compares the amount of proteins adsorbed on the virgin and modified PP membranes in the batch tests. The amount of proteins adsorbed on the grafted membranes decreased substantially with longer ozone treatment times. Furthermore, the grafted membranes obtained in this work had a smaller or similar adsorption capacity for BSA protein, compared to the commercially available hydrophilic GVWP membrane. Cross-flow filtration of the model protein solution was performed to compare the permeate flux and fouling tendencies between the unmodified and modified membranes. The flux during microfiltration of the BSA solution is shown in Fig. 9. The flux decline for the modified (grafted) membrane was nearly in the same trend as that of the unmodified (virgin) membrane. However, the flux recovery efficiency that is closely associated with the reversibility of membrane fouling layers increased markedly with increasing the ozone treatment time by up to 5 min (Fig. 10). The
275
Fig. 9. Fluxes for virgin and modified membranes during cross-flow MF of BSA solution.
grafted membrane obtained with 5 min ozone treatment gave the highest flux recovery of approximately 95%. However, the 1 min ozonation had no effect on an improvement of flux recovery, suggesting that only part of the membrane surfaces and pores were modified during short time ozonation. It appears that membrane fouling might be caused by the accumulation of protein aggregates at the membrane surface regardless of the membranes used but could be reversed more easily for the grafted membranes with an appropriate time of ozonation. In summary, the protein fouling layer became reversible with grafting,
Fig. 10. Flux recovery efficiencies with water flushing at the end of operation.
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Y. Wang et al. / Journal of Membrane Science 169 (2000) 269–276
being removed easily just by water flushing. This can be attributed to the more hydrophilic nature of the modified membrane surfaces. Also, the steric hindrance resulting from the poly-HEMA chains grafted may play a role in preventing the direct interaction between membrane surfaces and proteins [6]. 4. Conclusions Hydrophilic modification of the PP membrane surfaces was done by ozone treatment followed by graft-polymerization with HEMA. The longer ozone treatment led to an increase in the degree of graft polymerization and membrane hydrophilicity. However, excessive ozone treatment made the membrane brittle, so that the ozone treatment time should be optimized in the range of 3–5 min in this work. The grafted PP membranes had a relatively weak affinity to BSA proteins compared to the virgin membrane and even commercially available hydrophilic membranes. A great improvement in flux recovery was achieved with the grafted PP membranes, suggesting that the grafted membranes had a high reversibility of the protein fouling layer. This can be explained by the increase of hydrophilicity of the grafted membranes and the steric hindrance by the HEMA chains present at the membrane surfaces. Acknowledgements The authors are grateful to Ms. A. Sainbayar, Mr. W.J. Cho and Ms. H.G. Sohn for their assistance dur-
ing experiments at the School of Chemical Engineering, Seoul National University.
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