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Fouling-Free Membranes Obtained by Facile Surface Modification of Commercially Available Membranes Using the Dynamic Forming Method Kazuki Akamatsu,* Keita Mitsumori, Fang Han, and Shin-ichi Nakao Department of Environmental and Energy Chemistry, Faculty of Engineering, Kogakuin University, 2665-1 Nakano-machi, Hachioji-shi, Tokyo 192-0015, Japan ABSTRACT: The dynamic forming method was first employed for the preparation of fouling-free membranes. The copolymer of 1-carboxy-N,N-dimethyl-N-(20 -methacryloyloxyethyl) methanaminium inner salt and n-butyl methacrylate was selected for the surface modification polymer to realize the fouling-free property in this study, and its aqueous solution was filtered using a commercially available ultrafiltration membrane. The surface modification polymer was thus deposited on the ultrafiltration membrane in a facile manner over several minutes. Compared to the unmodified membrane, the dynamically formed membrane with the surface modification polymer layer proved to have excellent fouling resistance against 1000 ppm bovine serum albumin (BSA) solution. In particular, the dynamically formed membrane was hardly fouled and maintained its initial flux when the filtration flux was set below 6 106 m3 m2 s1.
1. INTRODUCTION Fouling is one of the most widely known and severe problems in membrane technology. When fouling occurs, the flux decreases in constant-pressure filtration or the transmembrane pressure increases in constant-flux filtration. Effective solutions to these problems have not yet been developed, because of the complexity of the fouling phenomena and invisibility of fouling phenomena in situ. Therefore, the fouling mechanism is not completely understood. To suppress fouling with dissolved organic compounds, many types of membranes have been developed. Among the methods employed for such development, surface modification of commercially available membranes is one of the most studied. This method involves grafting various polymers, such as poly(ethylene glycol),1 poly(acrylamide),2 poly(acrylic acid),3,4 poly(2-hydroxyethyl methacrylate),5,6 or a copolymer of acrylamide and acrylic acid,7 onto virgin substrate membranes. The monomer types have also been compared.8 These modification methods are based on tuning the hydrophilicityhydrophobicity of the membrane surfaces. Recently, surface modification with zwitterionic polymers has been proposed as a novel method to suppress fouling.9,10 Zwitterionic polymers have been extensively studied in the field of biomaterials, because some biomaterials require biocompatible, antithrombogenic surfaces. This characteristic of biomaterials can be referred to as an “antibiofouling” property, and it is demanded that proteins or organic compounds cannot be adsorbed onto the surfaces of biomaterials. One of the well-known polymers to meet this requirement is poly(2-methacryloyloxyethyl phosphorylcholine), which is synthesized with a methacrylate monomer having a zwitterionic phosphorylcholine headgroup in the side chain.11 In analogy with developments in the field of biomaterials, fundamental studies are also being conducted in the field of membranes. Sulfobetaine and carboxybetaine polymers are expected to have a function similar to that of the phosphobetaine polymer. Indeed, conventional modification methods, r 2011 American Chemical Society
including the grafting method, are suitable for membranes with a small area. However, these methods are not easily applied to modification over a large area, and therefore it is difficult to apply them as practical and industrial methods. The dynamic forming method was first proposed in 1966 by a group at the Oak Ridge National Laboratory;12 dynamically formed membranes are defined as membranes that are formed on porous supports via the filtering of membrane-forming materials such as inorganic hydrous oxides, colloidal materials, or polymers. These membranes generally can reject salts, because of the electric charge of the membrane-forming materials, and they were developed as novel types of reverse osmosis or nanofiltration membranes. Because of the large pure water permeability (Lp) in facile formation, various types of dynamically formed membranes were extensively developed in research works,1318 including practical studies.19,20 Since the 1990s, there have been few reports on dynamically formed membranes, probably because many types of reverse osmosis or nanofiltration membranes with excellent properties have been developed. However, the dynamic forming method is attractive in terms of facile surface modification over a large area in a single treatment. In addition, there has been no reports on the development of antifouling membranes employing the dynamic forming method. In this study, we prepare dynamically formed membranes with commercially available ultrafiltration (UF) membranes as the support and 1-carboxy-N,N-dimethyl-N-(20 -methacryloyloxyethyl) methanaminium inner salt (CMB)-based polymers as modifying polymers for the fouling-free property. The CMB-based polymer is now a well-known polymer for biomaterials.2123 We then investigate the effect of surface modification by conducting filtration Received: June 5, 2011 Accepted: September 21, 2011 Revised: September 7, 2011 Published: September 21, 2011 12281
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Figure 1. Chemical structure of poly(CMB-co-BMA).
Figure 3. Time course of flux during surface modification employing the dynamic forming method.
Figure 2. Schematic diagram of the experimental setup.
studies for aqueous bovine serum albumin (BSA) solution with a concentration of 1000 ppm, and we demonstrate the availability of this facile modification method to realize a fouling-free property.
2. EXPERIMENTAL SECTION 2.1. Materials. CMB monomer was kindly supplied by Osaka Organic Chemical Industry, Ltd., Japan. n-Butyl methacrylate (BMA), ethanol, 2,20 -azobis(isobutyronitrile) (AIBN), and BSA were purchased from Wako Pure Chemical Industries, Ltd., Japan. BMA was used after distillation. All other chemicals were used without further purification. 2.2. Synthesis of Poly(CMB-co-BMA). A 20% (by weight) monomer solution (CMB:BMA = 50:50 [mol:mol]) was prepared in N2-bubbled ethanol for radical polymerization. After adding AIBN as an initiator, the solution was heated to 70 °C using an oil bath and kept at that temperature for 24 h. The solution was then cooled to room temperature, and the polymers were purified using dialysis tubes with a molecular weight cutoff (MWCO) of 12 000 for more than 3 days. The chemical structure of poly(CMB-co-BMA) is shown in Figure 1. 2.3. Surface Modification Employing the Dynamic Forming Method. Figure 2 shows a conceptual illustration of the experimental apparatus. Polysulfone UF membranes with an MWCO of 10 000 were purchased from Toyo Roshi Kaisha, Ltd., Japan, and used as base membranes. Poly(CMB-co-BMA) aqueous solution with a concentration of 1000 ppm was used as the surface modification polymer solution, and was filtered with one membrane under pressure of 1 MPa at a flow rate of 3 L/min until the filtration flux became constant. This will be called the “dynamically formed membrane”. 2.4. Evaluation of the Fouling-Free Property. The same apparatus as that shown in Figure 2 was used to evaluate the fouling-free property. The dynamically formed membrane or
Figure 4. Relationship between the pure water flux and the applied pressure for the unmodified (open symbols) and dynamically formed membranes (solid symbols).
unmodified base membrane was set in the membrane cell, and the relationships between applied pressure and pure-water flux were first examined to decide Lp. Then, a 1000 ppm BSA aqueous solution was filtered at a flow rate of 5 L/min. The initial pressure condition for each membrane was decided on the basis of Lp to ensure that each flux level was the same, thereby eliminating the effect of the flux difference on the fouling behavior. This evaluation was conducted immediately after preparing the dynamically formed membrane.
3. RESULTS AND DISCUSSION 3.1. Dynamic Forming of the Poly(CMB-co-BMA) Layer on the Surface of Commercially Available UF Membrane. The
time course of filtration flux during the surface modification is shown in Figure 3. The filtration flux decreased drastically during the first several minutes and then became constant. The initial decrease was due to the increase in filtration resistance resulting from the adhesion of poly(CMB-co-BMA) on the support UF membrane. Figure 4 shows the relationship between the applied pressure and the pure water flux for the unmodified UF membrane and the dynamically formed membrane. The slopes of the two lines in the figure correspond to the Lp value for pure water. After the formation of poly(CMB-co-BMA) on the support UF membrane, the line became less steep, and Lp decreased from 6.2 1011 to 1.7 1011 m3 m2 s1 Pa1. 12282
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Figure 5. Time courses of flux during permeation tests using 1000 ppm BSA aqueous solution as feed for unmodified (open symbols) and dynamically formed membranes (solid symbols).
This suggests that the permeation resistance for pure water increased because of the adhesion of poly(CMB-co-BMA) on the UF membrane. The above results demonstrate that the dynamic forming method is a facile and rapid surface modification method for forming a poly(CMB-co-BMA) layer on a commercially available UF membrane. 3.2. Evaluation of the Fouling-Free Property Using 1000 ppm BSA Aqueous Solution. The time courses of filtration flux when 1000 ppm BSA aqueous solution was added are shown in Figure 5. First, the applied pressures for unmodified and dynamically formed membranes were set to 0.064 and 0.23 MPa, respectively (Condition 1), and pure water was initially supplied as feed solution. These pressure conditions were decided by considering the Lp value for pure water passing through the two membranes, as mentioned in the previous section, and it was intended that the pure water flux be the same for both the dynamically formed membrane and the unmodified membrane (4 106 m3 m2 s1). This consideration is based on the fact that the effect of BSA on fouling would differ if the initial flux differs. After 30 min, BSA was added such that the feed solution was 1000 ppm BSA aqueous solution, and we observed the extent of the flux decline for each membrane with time. In the case of the unmodified membrane, the flux dropped drastically, to 1.2 106 m3 m2 s1, whereas in the case of the dynamically formed membrane, the flux hardly decreased and remained as high as 3.7 106 m3 m2 s1. This means that the dynamically formed membrane was hardly fouled by the 1000 ppm BSA solution, while the unmodified membrane was severely fouled. We have thus observed the prominent effect of the dynamically formed poly(CMB-co-BMA) layer in realizing a fouling-free property. After 180 min, the applied pressures for the unmodified and dynamically formed membranes were raised to 0.096 and 0.34 MPa, respectively (Condition 2). These conditions were such that the pure water flux was 6 106 m3 m2 s1 for both the dynamically formed membrane and the unmodified membrane, considering the Lp values for pure water. The purpose of this examination was to investigate the effect of flux on fouling suppression. In the case of the unmodified membrane, the filtration flux did indeed increase. However, this increase was due to the increase in the applied pressure, and it did not mean that fouling was suppressed. On the other hand, in the case of the
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dynamically formed membrane, the flux increased to 5.8 106 m3 m2 s1, which was as high as the flux expected, considering the Lp value for pure water, and the flux remained high. This result means that the dynamically formed membrane had performance similar to that in the case of pure water under such a high flux condition, and the dynamically formed membrane was thus hardly fouled by BSA. After 210 min, the applied pressures for the unmodified and dynamically formed membranes were increased to 0.16 and 0.57 MPa, respectively (Condition 3). These conditions were such that the pure water flux was as high as 1 105 m3 m2 s1 for both the dynamically formed membrane and the unmodified membrane, considering the Lp values for pure water. In the case of the dynamically formed membrane, the flux increased to 9.5 106 m3 m2 s1, which was almost the same as the intended flux. However, the flux decreased gradually and then remained constant at ∼8.0 106 m3 m2 s1, probably because of the slight fouling by BSA. Compared with the unmodified membrane, excellent fouling suppression was observed; however, there was fouling by BSA to a slight extent under such a high flux condition. In addition, BSA was not detected in the permeates during all the experiments. Kitano’s group investigated biomaterials that had been prepared with poly(CMB-co-BMA) and reported that a surface prepared with poly(CMB-co-BMA) with 30 or 40 mol % CMB can effectively suppress the adsorption of various types of proteins.23,24 Therefore, the result obtained in this study—that the membrane dynamically formed using poly(CMB-co-BMA) with 50 mol % CMB had excellent fouling resistance against BSA aqueous solution—is reasonable. In addition, note that the dynamic forming method is facile for obtaining fouling-free UF membranes. With regard to the stability of the membrane, we should continue to study this phenomenon.
4. CONCLUSION The dynamic forming method was first applied to prepare fouling-free membranes by modifying the surface of a commercially available UF membrane using poly(CMB-co-BMA) with 50 mol % CMB. The surface modification was facile and was found to be complete within several minutes. In addition, compared to the unmodified membrane, the membrane dynamically formed with poly(CMB-co-BMA) had excellent fouling resistance against a 1000 ppm BSA aqueous solution. In particular, when the initial flux was less than 6 106 m3 m2 s1 in the permeation test, the dynamically formed membrane was found to be hardly fouled by BSA and maintained its initial flux, while severe fouling was observed in the case of the unmodified membrane. Therefore, this fouling-free property can be said to be due to the poly(CMBco-BMA) layer that formed on the surface of the UF support when employing the dynamic forming method. ’ AUTHOR INFORMATION Corresponding Author
*Tel.: +81-42-628-4584. Fax: +81-42-628-4542. E-mail: akamatsu@ cc.kogakuin.ac.jp.
’ ACKNOWLEDGMENT The CMB monomer was kindly supplied by Osaka Organic Chemical Industry Ltd., Japan. Part of this research was supported 12283
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Industrial & Engineering Chemistry Research by Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST).
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