Improved Antifouling Property of PES Ultrafiltration Membranes Using

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Ind. Eng. Chem. Res. 2010, 49, 790–796

Improved Antifouling Property of PES Ultrafiltration Membranes Using Additive of Silica-PVP Nanocomposite Mengping Sun, Yanlei Su,* Chunxia Mu, and Zhongyi Jiang Key Laboratory for Green Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, People’s Republic of China

To improve the surface coverage of polyvinylpyrrolidone (PVP) on membrane surfaces and further enhance the antifouling property, a silica-PVP nanocomposite was synthesized and used as a novel hydrophilic additive to modify polyethersulfone (PES) membranes. Transmission electron microscopy (TEM) observation showed that PES membranes, using additives of PVP and silica-PVP nanocomposites, have similar asymmetric structures. X-ray photoelectron spectroscopy (XPS) measurement indicated that the near-surface coverage of PVP for PES membrane with a silica-PVP nanocomposite additive is greater than that with a PVP additive. Protein ultrafiltration experiment also showed that the antifouling ability of PES membrane with a silica-PVP nanocomposite additive is stronger than that with a PVP additive. The hydrophilic modification with a silica-PVP nanocomposite is an appropriate method for improved antifouling property of PES ultrafiltration membranes. 1. Introduction Polyethersulfone (PES) is one of especially useful materials for preparing ultrafiltration membranes, because PES membranes have excellent temperature, pH, and chemical stability, as well as excellent mechanical strength.1-9 PES ultrafiltration membranes are widely used in the biotechnology industry for the recovery, diafiltration, and final concentration of proteins. However, their application is often limited by the inherent hydrophobic property of PES membranes; the hydrophobic interactions between PES membranes and protein molecules in feed solutions often cause serious membrane fouling, because of irreverisible protein adsorption.10-12 Membrane fouling reduces productivity (because of longer filtration times), shortens membrane life (because of the harsh chemical cleaning), and alters membrane selectivity (because of the change of pore size). It is generally accepted that increasing membrane surface hydrophilicity could effectively reduce membrane fouling. Many works have been reported that the enhancement of hydrophilic property to PES ultrafiltration membranes is an effective method to increase the membrane resistance toward fouling.2-9 Three different approaches, including (i) direct PES material modification before membrane preparation (pre-modification), (ii) blending of PES with a modifying agent in casting solutions during membrane preparation (additive), and (iii) surface modification after PES membrane preparation (post-modification) have been proposed. For the pre-modification method, the modified PES has been synthesized via a reaction of chlorosulfonated PES with oligomeric poly(ethylene glycol) (PEG), which was utilized as a hydrophilic modifier for fabricated antifouling ultrafiltration membranes.13 PEG, polyvinylpyrrolidone (PVP), poly(1-vinylpyrrolidone-co-styrene) copolymer, amphiphilic Pluronic F127, etc. have been used as additives during membrane preparation via phase separation methods to improve the antifouling property of PES ultrafiltration membranes.14-16 For the post-modification method, UV-assisted graft polymerization of N-vinyl-2-pyrrolidinone onto PES ultrafiltration membranes was achieved through dip and immersion modification techniques to increase surface wettability and decrease adsorptive * To whom correspondence should be addressed. Fax: 86-2227890882. E-mail address: [email protected].

fouling during the constant volume diafiltration of bovine serum albumin (BSA).8 The hydrophilic modification of PES ultrafiltration membranes through a blending technique is simple, and no additional step is needed during membrane preparation. The additives (usually hydrophilic polymers) in a casting solution are also used to increase both pore size and porosity and to suppress macrovoid formation. PVP, because of its nontoxicity, water solubility, and excellent biocompatibility, has been widely used as an additive for the fabrication of polymer membranes to enhance the blood compatibility and antifouling ability.4,5,14-17 During ultrafiltration operation, the attachment of PVP on the membrane surface enhances wettability, decreases protein adsorption, and prevents pore blockage.5 To improve surface coverage of PVP on membrane surface and further enhance antifouling property, silica-PVP nanocomposite was synthesized and used as a hydrophilic additive to modify PES membranes. To our knowledge, this is the first report about the hydrophilic modification for ultrafiltration membranes with inorganic polymer nanocomposite. Measurement of the water contact angle was performed and X-ray photoelectron spectroscopy (XPS) analysis was introduced to measure the surface property of PES ultrafiltration membranes. BSA was used as a model protein to probe the separation performance of PES membranes. Based on the experimental results, the hydrophilic modification with silica-PVP nanocomposites is an appropriate method for improved antifouling property of PES ultrafiltration membranes. 2. Experimental Section 2.1. Materials. PES (Ultrason 6020P, BASF Co., Germany) was dried at 110 °C for 12 h prior to use. PVP K30, tetraethyl orthosilicate (TEOS), N,N-dimethyl formamide (DMF), ammonia-water, and ethanol were purchased from Kewei Chemical Reagent Co. (Tianjin, PRC). BSA was obtained from the Institute of Hematology, at the Chinese Academic of Medical Sciences (Tianjin, PRC). Other regents were all of analytical grade and used without further purification. The water used in all experiments was deionized water at pH 6.0. BSA solution

10.1021/ie900560e  2010 American Chemical Society Published on Web 12/01/2009

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(1.0 mg/mL, pH 7.0) was prepared using a 0.1 M phosphate buffer solution. 2.2. Synthesis of Silica-PVP Nanocomposite. The silicaPVP nanocomposite was synthesized using the following method. First, 2 mL of TEOS and 6 mL of ammonia-water were added into 25 mL of ethyl alcohol under stirring for 3 h to generate silica nanoparticles. Then, 100 mL of 2 wt % PVP aqueous solution was added into the reaction mixture. The solution was heated and refluxed for ∼6 h to form robust PVP coating on silica nanoparticles. At last, the solution was dried through evaporation in air to obtain the silica-PVP nanocomposite solid. The synthesized silica-PVP nanocomposite is easily dissolved in water and DMF. A copper transmission electron microscopy (TEM) grid was dipped into an aqueous solution of silica-PVP nanocomposite and then dried at room temperature. The size of the silica-PVP nanocomposite was observed using TEM analysis (JEOL Model 2010 electron microscope). 2.3. Preparation of PES Ultrafiltration Membrane. To prepare casting solutions, 3.6 g of PES and a certain amount of the silica-PVP nanocomposite were added into 16.4 g of DMF. The solutions were stirred for 4 h at a temperature of 60 °C to ensure a complete dissolution of the polymers. The casting solution was then degassed at 60 °C for another 4 h without stirring to allow a complete release of bubbles. The viscosity of casting solutions was measured at room temperature, using a viscometer (Brookfield, Model DV-I Prime). The solutions were cast on glass plates using a stainless-steel knife, and then the glass plates were immersed in a coagulation bath of deionized water. The pristine membranes with a wet thickness of ∼240 µm were peeled off and subsequently rinsed with water to remove the residual solvent and pore-forming agent. The resultant membranes were kept in water prior to ultrafiltration operation. 2.4. Membrane Characterization. The cross-sectional morphologies of PES ultrafiltration membranes were observed by scanning electron microscopy (SEM) (Philips Model XL30E). The membranes were frozen in liquid nitrogen, broken, and sputtered with gold prior to SEM observation. Element mapping was conducted with the Philips Model XL30E SEM microscope equipped with energy-dispersive X-ray spectroscopy (EDX) of ISIS300 (Oxford). Thermogravimetric analysis (TGA) of PES membranes with additive silica-PVP nanocomposite was conducted under air with a Perkin-Elmer thermogravimetric analyzer at a heating rate of 10 °C/min. Static water contact angles of PES membranes were measured with a contact angle goniometer (JC2000C Contact Angle Meter, Powereach Co., Shanghai, PRC). At least six water contact angles at different locations on one surface were averaged to get a reliable value. The surface chemical compositions of PES membranes were analyzed via XPS analysis (Model PHI-1600 X-ray photoelectron spectrometer) using Mg KR radiation (1253.6 eV) as the radiation source. The takeoff angle of photoelectron was set to 90°. Surface spectra were collected over a range of 0-1100 eV. 2.5. Ultrafiltration Experiments. A dead-end stirred cell filtration system connected with a N2 gas cylinder and solution reservoir was designed to evaluate the filtration performance of membranes. All ultrafiltration experiments were performed using a filtration test cell (Model 8200, Millipore Co., Bedford, MA) whose volume capacity was 200 mL. The effective area of the membrane was 28.7 cm2. The operation pressure in the system was maintained by nitrogen gas. All the ultrafiltration

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experiments were performed at a stirring speed of 400 rpm and a temperature of 20 ( 1 °C. After fixing the membrane in the cell, the stirred cell and solution reservoir were filled with deionized water. Each membrane was initially compacted for 0.5 h at 150 kPa; the pressure then was reduced to an operating pressure of 100 kPa. The pure water flux (Jw1) is calculated using the following equation: Jw1 )

V A∆t

(1)

where V is the volume of permeated water (in liters), A the membrane area (expressed in units of m2), and ∆t the permeation time (in hours). The cell was emptied and refilled rapidly with 1.0 mg/mL BSA in a 0.1 M phosphate buffer solution at pH 7.0; the solution reservoir was refilled with phosphate buffer solution. The flux of protein solution (Jp) was recorded. The BSA rejection ratio (R) is calculated using the following equation:

(

R (%) ) 1 -

)

Cp × 100 Cf

(2)

where Cp and Cf (each expresed in terms of mg/mL) are BSA concentrations of permeate and feed solutions measured with a UV-vis spectrophotometer, respectively. After 1 h of protein ultrafiltration, the membranes were cleaned with deionized water under magnetic stirring for 20 min; the cell then was emptied and the pure water flux was measured again (now denoted as Jw2). To evaluate the antifouling property of PES membranes, the flux recovery ratio (FRR) is calculated using the following expression: FRR (%) )

( )

Jw2 × 100 Jw1

(3)

The higher FRR value, the better the antifouling property of the membrane. 3. Results and Discussion 3.1. Silica-PVP Nanocomposite. The inorganic polymer nanocomposite is a novel material where a combination of polymer and inorganic nanoparticles forms a unique composite with properties that neither of the two components provides indiviudally. The inorganic polymer nanocomposites have received special attention in recent years, because of their versatility in composition, structure, and properties, leading to potential applications in many fields such as optics, electronics, sensors, and catalysis.18-22 In the present work, silica-PVP nanocomposites were selected as an additive to fabricate antifouling ultrafiltration membranes. Silica nanoparticles were first prepared in ethanol solution through the hydrolyzation of TEOS. A more robust polymer coating forms upon refluxing silica colloids in an aqueous solution of PVP. Silica-PVP nanocomposites are easily prepared, because of the strong hydrogen bonds that can form between carbonyl groups of PVP and the hydroxyls on the silica surface.18 A slightly yellowcolored silica-PVP nanocomposite solid was finally obtained after drying. Since a PVP coating prevents the direct aggregation among silica nanoparticles, the silica-PVP nanocomposite is easily redissolved in water and DMF without ultrasonic treatment. Figure 1 showed SEM images of the prepared silica-PVP nanocomposite; the size of the silica-PVP nanocomposite is ∼30 ( 5 nm. The silica-PVP nanocomposite can be steadily

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Figure 1. Transmission electron microscopy (TEM) photomicrograph of the silica-PVP nanocomposite.

dispersed in casting solutions for PES membrane preparation. This is the obvious advantage of an inorganic polymer nanocomposite, versus inorganic nanoparticles, the latter of which requires a strong ultrasonic treatment to disperse inorganic nanoparticles in casting solutions. 3.2. Preparation of PES Ultrafiltration Membranes with a Silica-PVP Nanocomposite Additive. To enhance the hydrophilicity of the PES membranes, the PVP or silica-PVP nanocomposite additives were blended with PES in the casting solutions for membrane preparation. The weight ratios of PVP/ PES or silica-PVP nanocomposite/PES were first fixed at 1.0. It was proposed that the interactions between PES and PVP could be either between pyrrolidone groups in PVP and sulfone groups in PES or between side cyclic groups of PVP and aromatic ring of PES through investigation of viscoelastic behavior of their polymer mixtures.14 The casting solution of PES and silica-PVP nanocomposite in DMF is homogeneous without observable phase separation, which is similar to the casting solution of PES and PVP. The viscosity value of the casting solution with a PVP additive is 3531 cP, and that with a silica-PVP nanocomposite additive is 1050 cP. Most PVP molecules are strongly adsorbed on the silica nanoparticles for the silica-PVP nanocomposite, so that the number of PVP molecules interacted with PES are decreased and the viscosity of the casting solution with a silica-PVP nanocomposite additive is smaller than that with a PVP additive. The lower viscosity is another advantage for the casting solution with a silica-PVP nanocomposite additive, which can be easily cast on glass plates. During phase inversion for membrane preparation, PES is immediately coagulated to form a membrane matrix, and the hydrophilic additive is segregated spontaneously to the membrane/ water interface to decrease interface energy.23,24 Since PVP is water-soluble, most of the PVP is washed out during the phase inversion process and there is only a small portion of PVP entrapped in the membrane matrix and attached onto the membrane surface, because of the interactions between PVP and PES, which creates a hydrophilic antifouling surface.2,5,7,24 In the similar way, a portion of the silica-PVP nanocomposite is embedded into the PES membrane matrix and a portion of the silica-PVP nanocomposite is exposed on the membrane surface. The schematic illustration of the existence of the PVP and silica-PVP nanocomposite on the PES membrane surfaces is presented in Figure 2.

The cross-sectional SEM morphologies of the prepared PES membranes (the weight ratios of PVP/PES and silica-PVP nanocomposite/PES are 1.0) are shown in Figure 3. PES membranes using PVP and silica-PVP nanocomposite additives have similar asymmetric structures, which is the typical structure of ultrafiltration membranes, with a top dense layer, a porous sublayer, and fully developed macropores at the bottom. There is no observable morphology change between PES membranes using PVP and silica-PVP nanocomposite additives. The energy-dispersive X-ray (EDX) Si mapping on the surface of the PES membrane with the silica-PVP nanocomposite additive is also shown in Figure 3. It can be seen that elemental silicon from the nanocomposite (the bright spots) is evenly distributed in the membrane surface. The efficiency of incorporation of the silica-PVP nanocomposite in the PES membrane was investigated via TGA. The total weight loss below 800 °C is ∼93.6 wt %, which was attributed to polymer decomposition. The residue is the inorganic silica (6.4 wt %). It is estimated that 60.6 wt % of the added silica-PVP nanocomposite in the casting solution is retained in the membrane matrix during membrane fabrication. Measurement of the water contact angle is commonly used to estimate the hydrophilicity and wetting characteristics of polymer surfaces. A large water contact angle represents a hydrophobic surface, whereas a small water contact angle implies a hydrophilic surface. PES membrane with a PEG2000 additive (the molecular weight of PEG2000 is 2000, the weight ratio of PEG2000/PES is 1.0) has a contact angle of 64.5° ( 2°.25 The PES membrane with a PVP additive (the weight ratio of PVP/PES is 1.0) has a contact angle of 57.6° ( 2°. The PES membrane with a PVP additive is more hydrophilic than that with a PEG additive.14 The PES membrane with a silica-PVP nanocomposite additive (the weight ratio of silica-PVP nanocomposite/PES is 1.0) has a contact angle of 52.6° ( 2°. The smaller water contact angle indicates that the PES membrane with the silica-PVP nanocomposite additive is more hydrophilic than that of PES membranes with PEG2000 and PVP additives. The surface compositions of PES membranes were carefully characterized by XPS analysis. Figure 4 presented the XPS spectra of PES membranes. The atomic percentages of elemental C, O, N, and S are 75.8, 17.9, 3.8, and 2.5 at.%, respectively, for a PES membrane with a PVP additive (the weight ratio of PVP/PES is 1.0). PVP is the only source of elemental N; XPS measurement gave valuable information to confirm the attachment of PVP on the membrane surface. The atomic percentages of elemental C, O, N, S, and Si are 72.4, 18.6, 5.2, 2.4, and 1.3 at.%, respectively, for a PES membrane with the silica-PVP nanocomposite additive (the weight ratio of silica-PVP nanocomposite/PES is 1.0). The XPS signals of N and Si clearly indicated the existence of the silica-PVP nanocomposite on the membrane surface. The degree of PVP near-surface coverage, C, was introduced to evaluate surface modification, which is calculated using the following expressions: C (%) )

AmN × 100 APVP

(4)

where AmN is the nitrogen molar percentage on the membrane surface measured by XPS, and APVP is the nitrogen molar percentage of PVP under the condition of membrane surface completely covered with PVP polymer. APVP has a value of 12.5%, based on the total atomic composition of elemental C, N, and O in neat PVP. If the membrane surface were completely covered with PVP chains, the value of C would be equal to 100%. The PES membrane with a PVP additive has a C value

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Figure 2. Schematic illustration of the attachment of PVP and silica-PVP nanocomposite on the PES membrane surfaces.

Figure 3. Cross-sectional SEM micrographs of PES ultrafiltration membranes with (a) PVP additive and (b) silica-PVP nanocomposite additive. (c) Energydispersive X-ray (EDX) Si-mapping on the surface of PES membrane with the silica-PVP nanocomposite addditive. The weight ratios of PVP/PES and silica-PVP nanocomposite/PES are 1.0 in the casting solutions.

of 30.4%, meaning that 30.4% of the surface of the PES membrane is covered with PVP chains. The PES membrane with a silica-PVP nanocomposite additive has a C value of 41.6%, meaning that 41.6% of the surface of the PES membrane is covered with PVP chains. The PVP coating of the nanocomposite favors retention of a higher PVP chain density on the membrane surface, because of the existence of a silica-PVP nanocomposite on the membrane surface. The higher nearsurface coverage of PVP for the PES membrane with a silica-PVP nanocomposite additive enhances its hydrophilicity, which is consistent with the measurement of the water contact angle. 3.3. Improved Antifouling Property of PES Ultrafiltration Membranes. Ultrafiltration experiments were performed to study the permeability of PES membranes using PVP and silica-PVP nanocomposite additives, respectively. Figure 5 presented the time-dependent fluxes of PES membranes during BSA ultra-

filtration operation. The pure water flux (Jw1) of PES membranes using a PVP additive is 142.4 L/(m2 h). Pure water fluxes of the PES membrane using a silica-PVP nanocomposite have a value of 168.4 L/(m2h), which is higher than that with a PVP additive. In Figure 5, the fluxes of protein solution (Jp) are lower than that of pure water (Jw1). Some protein molecules in the feed can deposit and adsorb on the membrane surface (cake formation) at the initial BSA ultrafiltration operation. This deposition and adsorption causes an abrupt drop in flux in the first few minutes of operation. In the stirred cell, the mixing induced by the stirrer may actually sweep protein molecules away from the membrane surface. The deposition and sweeping of protein may reach equilibrium, so that a relatively steady flux (Jp) is retained in the final operation of BSA solution ultrafiltration. BSA rejection ratio of PES membranes with additive of PVP (the weight ratio of PVP/PES is 1.0) is 98.6%, with a silica-PVP nanocomposite

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Figure 6. Summary of the flux recovery ratio (FRR), the total fouling ratio (Rt), the reversible fouling ratio (Rr), and the irreversible fouling ratio (Rir) of PES ultrafiltration membranes with PVP and silica-PVP nanocomposite additives.

Figure 4. XPS spectra of PES membranes using PVP and silica-PVP nanocomposite additives. The weight ratios of PVP/PES and silica-PVP nanocomposite/PES are 1.0 in the casting solutions.

(85.6%), suggesting that deposited and adsorbed protein on the membrane surfaces can be easily washed away. The hydrophilic and dense PVP chains can prevent direct contact of BSA molecules with PES membrane surface, the deposited proteins can be washed away by water cleaning. The antifouling ability of PES with a silica-PVP nanocomposite additive, which possesses a higher PVP chain density on the membrane surface, is stronger than that with a PVP additive, based on the FRR values. The membrane fouling causes the flux loss (Jw1 - Jp). Membrane fouling is composed of reversible and irreversible fouling. Reversible protein adsorption and deposition causes reversible fouling that can be removed by hydraulic cleaning, while irreversible protein adsorption causes irreversible fouling that can only be eliminated by chemical cleaning or enzymatic degradation.1-9 To study the antifouling property in more detail for the prepared PES membranes, we defined several ratios to describe the fouling process. The first ratio (Rt) is defined as

(

Rt (%) ) 1 -

Figure 5. Time-dependent fluxes of PES membranes with additives of PVP and silica-PVP nanocomposite during protein ultrafiltration. The ultrafiltration process includes four steps: pure water flux measurement, BSA solution ultrafiltration, water washing, and pure water flux measurement of the cleaned membranes. BSA concentration is 1.0 mg/mL in 0.1 M PBS solution at pH 7.0.

additive (the weight ratio of silica-PVP nanocomposite/PES is 1.0) is 96.3%. The PES membrane with a silica-PVP nanocomposite additive may possess larger pore size than that with a PVP additive under a similar added amount. After 1 h of protein ultrafiltration, PES membranes were cleaned with stirred water under magnetic stirring; the water fluxes of the cleaned membranes (Jw2) were measured again. FRR values were calculated and presented in Figure 6. The FRR value is only 45.6% for the PES membrane with a PEG2000 additive, meaning the existence of serious membrane fouling. The role of PEG2000 is only as a pore-forming agent; there are few residual PEG2000 molecules on the membrane surface to resist membrane fouling.25 The FRR value is 70.4% for the PES membrane with a PVP additive, which means that membrane fouling is alleviated in comparison with that using the PEG2000 additive. The residual PVP on the membrane surface or pore wall effectively improves the hydrophilic property of PES membranes.16,23 The PES membrane using a silica-PVP nanocomposite additive has a larger FRR value

)

Jp × 100 Jw1

(5)

which is the degree of total flux loss caused by total fouling. A high value of Rt corresponds to a large flux decay and serious membrane fouling. The parameters Rr and Rir were used to describe reversible fouling and irreversible fouling, respectively. The degree of reversible flux loss caused by reversible fouling (Rr) is calculated using the following equation: Rr (%) )

(

)

Jw2 - Jp × 100 Jw1

(6)

The degree of irreversible flux loss caused by irreversible fouling (Rir) is calculated using the following equation: Rir (%) )

(

)

Jw1 - Jw2 × 100 Jw1

(7)

A summary of Rt, Rr, and Rir values of PES membranes is given in Figure 6. It can be seen that the Rt value of the PES membrane with a PVP additive is larger than that with a silica-PVP nanocomposite additive. It is known that the strongly bound water of PVP is responsible for its good biocompatibility and antifouling properties.4,5,14-17 The PES membrane with a silica-PVP nanocomposite additive has a stronger ability to resist membrane fouling than that with a PVP additive. The PES membrane with a silica-PVP nanocomposite additive has not only a smaller Rt value, but also a smaller Rir value, than that with additive of PVP. The existence of

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Figure 7. Time-dependent fluxes of PES membranes with different amounts of silica-PVP nanocomposite in the casting solutions. Figure 9. Summary of the total fouling ratio (Rt), reversible fouling ratio (Rd), and irreversible fouling ratio (Rir) of PES membranes with different amount of silica-PVP nanocomposite in casting solutions.

Figure 8. Summary of BSA rejection ratio and flux recovery ratio (FRR) of PES membranes with different amounts of silica-PVP nanocomposite in the casting solutions.

silica-PVP nanocomposite on the PES membrane surfaces increases the surface coverage of PVP and further reduces protein adsorption and deposition, especially irreversible protein adsorption and deposition. Moreover, the membrane fouling of the PES membrane with a silica-PVP nanocomposite additive may be largely attributed to reversible membrane fouling, which can be easily removed by simple water washing.23,25 3.4. Influence of the Amount of Silica-PVP Nanocomposite. Different amounts of silica-PVP nanocomposite were added into casting solutions as a novel additive to fabricate PES ultrafiltration membranes. The permeability of a series of PES membranes was measured and given in Figure 7. The water flux is ∼0 for a PES membrane without additive. PES membrane with a silica-PVP nanocomposite/PES weight ratio of 0.2 has a water flux of 54.4 L/(m2 h). The water flux was gradually increased with an increase of the added weight of silica-PVP in the casting solutions. A PES membrane with a silica-PVP nanocomposite/PES weight ratio of 1.0 has a higher water flux (168.4 L/(m2 h)). The permeability shows that the silica-PVP nanocomposite is an excellent pore-forming agent. In Figure 8, BSA rejection ratios are slightly decreased from 100% to 96.3% as the silica-PVP nanocomposite/PES weight ratio increases from 0.2 to 1.0, indicating that the pore sizes of PES membranes are increased with an increase of the added silica-PVP content in casting solutions. FRR values of PES

membranes with different amounts of added silica-PVP nanocomposite are all higher than that of a PES membrane with a PVP additive. These data suggested that the addition of silica-PVP nanocomposite in casting solutions is an appropriate technique to improve flux recovery of PES membranes. The highest FRR value is 90.4% for a PES membrane with a silica-PVP nanocomposite/PES weight ratio of 0.4. The probable reason for the lower FRR values for PES membranes with silica-PVP nanocomposite/PES weight ratios of 0.6, 0.8, and 1.0 (BSA rejection ratios of those membranes are lower than 100%) than that with a silica-PVP nanocomposite/PES weight ratio of 0.4 is due to pore blockage. The trapped protein in the membrane pores cannot be washed out easily, so that the plugged PES membranes have lower flux recovery. Membrane fouling of PES membranes with different amounts of silica-PVP nanocomposite additive was investigated in detail. A summarize of the Rt, Rr, and Rir values of PES membranes, plotted as a function of the silica-PVP nanocomposite/PES weight ratio, is given in Figure 9. A PES membrane with a silica-PVP nanocomposite/PES weight ratio of 0.2 has the smallest Rt value, but its Rir value is higherl; therefore, the antifouling ability of this membrane is weak. The PES membrane with a silica-PVP nanocomposite/PES weight ratio of 0.4 in the casting solution has the highest Rr value and the lowest Rir value. The complete rejection of BSA (prevent pore blockage) of a PES membrane with a silica-PVP nanocomposite/PES weight ratio of 0.4 may improve its resistance to irreversible fouling. All Rt, Rr, and Rir values of PES membranes with silica-PVP nanocomposite/PES weight ratios from 0.6 to 1.0 are decreased as the silica-PVP nanocomposite content increases, meaning that the antifouling ability is further enhanced with an increase in the content of silica-PVP nanocomposite. These results demonstrated that the modification of PES membranes with a silica-PVP nanocomposite additive surely improved the antifouling property. 4. Conclusions Silica-PVP nanocomposites (where PVP denotes polyvinylpyrrolidone) were synthesized and used as a hydrophilic additive to modify polyethersulfone (PES) membranes. The PES membrane with a silica-PVP nanocomposite additive has higher PVP surface coverage and hydrophilic properties than that with

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a PVP additive. The PES membrane with a silica-PVP nanocomposite additive has higher flow recovery and lower membrane fouling. Moreover, the membrane fouling of the PES membrane with a silica-PVP nanocomposite additive may largely attribute to reversible membrane fouling, which can be easily removed by simple water washing. Acknowledgment This work was supported by State Key Laboratory of Precision Measuring Technology and Instruments, the Programme of Introducing Talents of Discipline to Universities (No. B06006), and the Drug Separation and Purification Project in Programme for Development of Novel Drug (No. 2009ZX09301008). Literature Cited (1) Slgin, S.; Takac, S.; Ozdamar, T. H. Adsorption of bovine serium albumin on polyether sulfone ultrafiltration membranes: Determination of interfacial interaction energy and effective diffusion coefficient. J. Membr. Sci. 2006, 278, 251. (2) Roux, S. P.; Jacobs, E. P.; van Reenen, A. J.; Morkel, C.; Meincken, M. Hydrophilization of polysulphone ultrafiltration membranes by incorporation of branched PEO-block-PSU copolymers. J. Membr. Sci. 2006, 276, 8. (3) Susanto, H.; Ulbricht, M. Influence of ultrafiltration membrane characteristics on adsorptive fouling with dextrans. J. Membr. Sci. 2005, 266, 132. (4) Qin, J.; Oo, M. H.; Li, Y. Development of high flux polyethersulfone hollow fiber ultrafiltration membranes from a low critical solution temperature dope via hypochlorite treatment. J. Membr. Sci. 2005, 247, 137. (5) Marchese, J.; Ponce, M.; Ochoa, N. A.; Pradanos, P.; Palacio, L.; Hernandez, A. Fouling behavior of polyethersulfone UF membranes made with different PVP. J. Membr. Sci. 2003, 211, 1. (6) Jones, K. L.; O’Melia, C. R. Ultrafiltration of protein and humin substances: Effect of solution chemistry on fouling and flux decline. J. Membr. Sci. 2001, 193, 163. (7) Hancock, L. F.; Fagan, S. M.; Ziolo, M. S. Hydrophilic semipermeable membranes fabricated with poly(ethylene oxide)-polysulfone block copolymer. Biomaterials 2000, 21, 725. (8) Pieracci, J.; Wood, D. W.; Crivello, J. V.; Belfort, G. UV-assisted graft polymerization of N-vinyl-2-pyrrolidinone onto poly(ether sulfone) ultrafiltration membranes: Comparison of dip versus immersion modification techniques. Chem. Mater. 2000, 12, 2123. (9) Taniguchi, M.; Belfort, G. Correcting for surface roughness: Advancing and receding contact angles. Langmuir 2002, 18, 6465. (10) Beier, S. P.; Enevoldsen, A. D.; Kontogeorgis, G. M.; Hansen, E. B.; Jonsson, G. Adsorption of amylase enzyme on ultrafiltration membranes. Langmuir 2007, 23, 9341.

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ReceiVed for reView April 6, 2009 ReVised manuscript receiVed October 17, 2009 Accepted November 16, 2009 IE900560E