Antifouling Property of a Weak Polyelectrolyte Membrane Based on

Key Laboratory for Green Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China...
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Ind. Eng. Chem. Res. 2009, 48, 3136–3141

Antifouling Property of a Weak Polyelectrolyte Membrane Based on Poly(acrylonitrile) during Protein Ultrafiltration Yanlei Su,* Chunxia Mu, Chao Li, and Zhongyi Jiang Key Laboratory for Green Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, China

The permeability of a weak polyelectrolyte ultrafiltration membrane based on poly(acrylonitrile and 2-dimethylaminoethyl methacrylate) (PAN-DMAEMA) copolymer was measured at various conditions of solution pH and ionic strength. Bovine serum albumin (BSA) and lysozyme were used as model proteins to investigate the antifouling property of PAN-DMAEMA ultrafiltration membrane. Electrostatic interactions, the conformation of PDMAEMA chains, and the nature of protein play important roles in enhancing the antifouling property of PAN-DMAEAM membrane. During BSA ultrafiltration, BSA rejection ratio is decreased, flux recovery ratio is remarkably increased, and the total membrane fouling and irreversible membrane fouling are decreased at higher pH value and ionic strength. During lysozyme ultrafiltration, the PAN-DMAEAM membrane displays excellent antifouling property in a broad range of pH and ionic strength. 1. Introduction Ultrafiltration membrane processes are widely used in wastewater treatment, reverse osmosis pretreatment, separation and purification of proteins in food industry, and biotechnology.1-6 Low energy cost, the ability to operate near ambient conditions, and easy use are the advantages of this technique. One of the major problems associated with ultrafiltration technique is membrane fouling, which is ascribed to the accumulation of matter on the membrane surface due to concentration polarization, nonspecific adsorption, and deposition.3-6 Membrane fouling results in substantial flux decline, increase of energy consumption and operation cost, modification of the retention characteristics, and need of washing and replacement of membranes. Protein ultrafiltration is usually used in laboratories to characterize the permeability and evaluate the antifouling property of ultrafiltration membranes. Membrane fouling is strongly dependent on protein-membrane and protein-protein interactions, which are affected by a series of factors, such as the surface chemistry of membranes, pH value and ionic strength of feed solution, the nature of protein, and the hydrodynamics of the process.5-9 The protein-membrane interactions mainly affect irreversible adsorption onto the membrane surface and within membrane pores, while protein-protein interactions affect the structure of the cake layer that forms on the membrane surface. It is generally accepted that increasing membrane surface hydrophilicity could effectively inhibit membrane fouling.10-14 Polyelectrolytes are highly hydrophilic, and many studies have been carried out for surface modification of membranes with polyelectrolytes to enhance hydrophilicity and antifouling capacity. Polyethersulfone ultrafiltration membranes were surface modified by preadsorption of poly(sodium 4-styrenesulfonate) (PSS) which showed better antifouling properties compared to unmodified membranes.12 Polysulfone/sulfonated poly(ether ether ketone) blend membranes had substantially higher water flux, salt rejection, porosity, and greatly reduced particle adhesion compared to the polysulfone-based membrane.13 We have prepared an ultrafiltration membrane using * Corresponding author. Fax: 86-22-27890882. E-mail: suyanlei@ tju.edu.cn.

poly(acrylonitrile and 2-dimethylaminoethyl methacrylate) (PAN-DMAEMA) copolymer, which has higher hydrophilicity than a PAN membrane.15,16 Water flux of PAN-DMAEMA ultrafiltration membrane is tunable due to the switch of stretched and collapsed states of PDMAEMA chains at different pH values and NaCl concentrations in the feed solutions.15 Weak polyelectrolyte membranes have potential applications in flow control, size-selective filtration, biomolecular separation, water treatment, and so on.17-19 But few attempts have been devoted to study the antifouling property of weak polyelectrolyte membranes. In the present paper, the antifouling property of PAN-DMAEMA ultrafiltration membrane in protein ultrafiltration was carefully examined under various conditions of solution pH and ionic strength. It was found that electrostatic interactions, the conformation of PDMAEMA chains, and the nature of protein play important roles in enhancing antifouling property of PAN-DMAEAM membrane. A major goal in the present study is to elucidate some antifouling mechanism for PANDMAEMA ultrafiltration membrane and promote the application of weak polyelectrolyte membranes in protein separation and purification. 2. Experimental Section 2.1. Materials. Acrylonitrile (AN) was purchased from Damao Chemical Co. (Tianjin, China) and distilled before use. 2-Dimethylaminoethyl methacrylate (DMAEMA) was purchased from Xinyu Chemical Co. (Wuxi, China). Azobisisobutyronitrile (AIBN), N,N-dimethylformamide (DMF), and poly(vinyl alcohol) (PVA, the degree of polymerization is 1700) were purchased from Kewei Chemical Reagent Co. (Tianjin, China). Bovine serum albumin (BSA, MW ) 67 000) and hen egg white lysozyme (MW ) 14 600) were purchased from Institute of Hematology, Chinese Academy of Medical Sciences (Tianjin, China). Other reagents were all of analytical grade and used without further purification. 2.2. Synthesis of PAN-DMAEMA Copolymer. A random copolymer containing AN and DMAEMA monomers was synthesized by water phase precipitation polymerization. PVA was added as the dispersed agent, and AIBN was added to initiate the copolymerization reaction; the detailed synthesis has been report in previous work.15,16 The synthesized polymer was

10.1021/ie801393z CCC: $40.75  2009 American Chemical Society Published on Web 02/09/2009

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collected, washed, and dried in a vacuum. According to the result of elemental analysis, the composition of PAN-DMAEMA copolymer was 91.6 mol % AN units and 8.4 mol % DMAEMA units. 2.3. Preparation of PAN-DMAEMA Ultrafiltration Membrane. PAN-DMAEMA copolymer was dissolved in DMF at concentration of 18 wt % to prepare the casting solution. The solution was stirred for 4 h at temperature of 60 °C and then left for 6 h to allow complete release of bubbles. After cooling to room temperature, the solution was cast on steel plate and immediately immersed in a coagulation bath of deionized water. The membrane was peeled off from the steel plate and rinsed with water to remove the residual DMF. The membrane was kept in water before use. The cross-section morphology of PAN-DMAEMA ultrafiltration membrane was observed by a scanning electron microscopy (SEM, Philips XL30E). The membrane was frozen in liquid nitrogen, broken, and sputtered with gold before SEM analysis. Water contact angles of the membrane surface were measured by the captive air bubble technique using a contact angle goniometer (JC2000C contact angle meter, Powereach Co., Shanghai, China).15,16 At least five measurements at different locations were averaged to obtain contact angles. 2.4. Protein Adsorption Experiment. Circular pieces cut from PAN-DMAEMA membrane with area of 18.15 cm2 were put into vials containing 10 mL volume of 1.0 mg/mL BSA or lysozyme aqueous solutions and incubated at room temperature of 20 ( 1 °C. The pH value of protein aqueous solution was adjusted with HCl or NaOH solutions, while the ionic strength was adjusted with added NaCl. After incubation of 5 h at room temperature, protein concentrations in the solutions were analyzed with a UV spectrophotometer (Hitach UV-2800, Japan), and then the amounts of adsorbed proteins on PANDMAEMA membrane were calculated. 2.5. Ultrafiltration Experiment. A dead-end stirred cell filtration system connected with a filtration cell (model 8200, Millipore Co.), a nitrogen gas cylinder, and a solution reservoir was designed to characterize the flux of the membrane. After fixing the membrane in the cell, the stirred cell and solution reservoir were filled with water. The feed side of the system was pressed by nitrogen gas. Each membrane was initially pressurized for 30 min at 150 kPa; then the pressure was reduced to the operating pressure of 100 kPa. Water flux (Jw1) at pressure of 100 kPa was first measured by the following equation: Jw1 )

V A∆t

(1)

where V is the volume of permeated water (L), A is membrane area (m2), and ∆t is permeation time (h). All the ultrafiltration experiments were carried out at stirring speed of 400 rpm and temperature of 20 ( 1 °C. The cell and solution reservoir were then emptied and refilled rapidly with 1.0 mg/mL BSA or lysozyme solutions at a given pH value and NaCl concentration, and the fluxes of BSA or lysozyme solutions were recorded (Jp). Protein rejection ratio was calculated at ultrafiltration time of 0.5 h by the following equation:

( )

R) 1-

Cp × 100% Cf

(2)

where Cp and Cf (mg/mL) are protein concentrations of permeate and feed solutions, respectively. After 2 h of operation, the protein solution was discharged and 50 mL of water was added. The membrane was washed under stirring for 20 min, the cell

Figure 1. Cross-sectional SEM photograph of PAN-DMAEMA ultrafiltration membrane.

was then emptied and refilled with water, and water flux (Jw2) was measured again. The water used for Jw1 and Jw2 measurement has the same pH value and ionic strength as protein solution. The flux recovery ratio (FRR) was calculated by eq 3: FRR )

( )

Jw2 × 100% Jw1

(3)

The FRR value can be used to reflect the antifouling capacity of ultrafiltration membrane: the higher the FRR value, the better the antifouling property of the membrane. 3. Results and Discussion 3.1. Protein Adsorption on PAN-DMAEMA Membrane. A weak polyelectrolyte ultrafiltration membrane based on PAN-DMAEMA copolymer was fabricated through a simple aqueous-based immersion precipitation process. PAN is the membrane matrix, and PDMAEMA is a weak polyelectrolyte to modify the membrane. The surface segregation during aqueous-based coagulation rendered the arrangement of hydrophilic PDMAEMA on the surfaces of membrane and pores.15,16,20 Figure 1 shows the cross-sectional morphology of PANDMAEMA membrane obtained by SEM. The membrane has the typical structure of asymmetric membrane with a dense skin layer, and a support layer with sponge-like structure, and macrovoids appear in the middle of the membrane. The water contact angle of PAN membrane is 57.4° ( 2°. However, the water contact angle of PAN-DMAEMA membrane is only 42.5° ( 2°. The smaller water contact angle means that the weak polyelectrolyte membrane has higher hydrophilicity. 3.1.1. pH Influence. The electrostatic interactions provide the dominating driving force for protein adsorption on the polyelectrolyte brush.21,22 The pKa for PDMAEMA in aqueous solution is 7.5, which is defined as the pH at which 50% of the amino groups in the polymer chains are protonated.21 Adjusting solution pH can easily control the degree of ionization of weak polyelectrolyte. The degree of protonation (R) of amino groups in PDMAEMA can be calculated by the following equation: R)

([H+] ⁄ Ka) (1 + [H+] ⁄ Ka)

(4)

The isoelectric point is the pH at which protein molecule carries no net electrical charge. Below the isoelectric point, proteins carry a net positive charge and above it a net negative charge.

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Figure 2. Effect of pH value on the amount of adsorbed protein on PANDMAEAM ultrafiltration membrane. NaCl concentration in solution is 0.01 mol/L.

Figure 3. Effect of NaCl concentration on the amount of adsorbed protein on PAN-DMAEAM ultrafiltration membrane. The solution pH value is 5.8.

The isoelectric point for BSA is 4.7, and for lysozyme it is 11.0.21,22 Figure 2 presents the effect of pH value on the amount of adsorbed BSA on PAN-DMAEMA membrane. At pH 5.8 and 6.5, BSA molecules carry negative charge, but PDMAEMA chains carry positive charge. The favored electrostatic attractions provide the driving force for BSA adsorption. The amounts of adsorbed BSA are 202.2 and 191.3 µg/cm2 at pH 5.8 and 6.5, respectively. The degree of protonation in the amino groups of PDMAEMA is about 98.1% at pH 5.8, and 90.9% at pH 6.5. The higher degree of ionization results in more strongly electrostatic repulsions between individual segments of PDMAEMA at pH 5.8 and 6.5, which leads to the stretched conformation of PDMAEMA chains. The extended conformation and favored electrostatic attractions, as well as the porous structure and complicated internal surface of PAN-DMAEMA membrane, provide the contribution of the higher amount of adsorbed BSA on PAN-DMAEMA membrane.16,21 In Figure 2 at pH 4.0, slightly below the BSA isoelectric point, BSA molecules carry a net positive charge. However, the positively charged BSA can adsorb on PAN-DMAEMA membrane, but the amount of adsorbed BSA at pH 4.0 is relatively lower than that at pH 5.8 and 6.5. There are electrostatic attractions between the negatively charged amino acid residues in BSA molecules and positively charged DMAEMA groups in the polymer chains, which may be the reason for BSA adsorption. At higher pH value in Figure 2, the amount of BSA adsorption on PAN-DMAEMA membrane is dramatically decreased. PANDMAEMA membrane can resist protein adsorption under basic condition. The degree of protonation in the amino groups of PDMAEMA is about 24.0% at pH 8.0, and 1.0% at pH 9.5. Most DMAEMA groups are neutral at pH 8.0 and 9.5, so that the electrostatic attractions for BSA adsorption are dramatically decreased. On the other hand, PDMAEMA chains adopt a shrunk conformation at higher pH value; the shrunk PDMAEMA chains expose more ester groups on the membrane surface (the increased oxygen content on the membrane surface has been observed by X-ray photoelectron spectroscopy (XPS) analysis).15,16 The neutral ester groups formed hydrogen bonds with water which generates an energetic barrier to prevent protein adsorption.23,24 The isoelectric point of lysozyme is 11.0, and therefore lysozyme carries a net positive charge over the pH range from 4.0 to 9.5 in the present study. As shown in Figure 2, the amount

of adsorbed lysozyme was much lower on PAN-DMAEMA membrane. The amounts of adsorbed lysozyme are 0 and 3.2 µg/cm2 at pH 5.8 and 9.5, respectively. The electrostatic repulsions are the main factor for preventing the approach of positively charged lysozyme molecules to the surface of PANDMAEMA membrane at lower pH value.21 PDMAEMA chains forming a shrunk conformation and exposing more ester groups on the membrane surface are used to explain the resistance of lysozyme adsorption at higher pH value.16 3.1.2. Ionic Strength Influence. Figure 3 shows the effect of NaCl concentration on the amount of adsorbed BSA at pH 5.8 on PAN-DMAEMA membrane. The amount of adsorbed BSA on PAN-DMAEMA membrane is dramatically decreased at higher NaCl concentration. At higher ionic strength in aqueous solution, the charges of amino groups in the PDMAEMA chains and amino acid groups in BSA are screened, so that the electrostatic attractions for BSA adsorption on PAN-DMAEMA membrane are dramatically decreased. Another reason for resistance of BSA adsorption comes from the reorientation of DMAEMA groups, which occurs when electrostatic repulsions between individual segments of PDMAEMA are strongly reduced at higher ionic strength. The shrunk PDMAEMA chains expose more ester groups on the membrane surface, which enhances the resistance of BSA adsorption.15,16 Figure 3 also shows that there is no lysozyme adsorption on PAN-DMAEAM membrane in a broad range of ionic strength at pH 5.8. The positively charged lysozyme is completely repulsed by the positively charged PDMAEAM chains on the membrane surface, which is consistent with results reported in the literature.21 3.2. BSA Ultrafiltration with PAN-DMAEAM Membrane. 3.2.1. pH Influence. Dead-end stirred cell devices are commonly used in laboratories to characterize separation property of ultrafiltration membranes.1-6 The effect of pH on water flux of PAN-DMAEMA ultrafiltration membrane during BSA ultrafiltration is given in Figure 4. Water flux of PANDMAEMA ultrafiltration membrane is increased with an increase of pH value. The shrinkage of PDMAEMA chains in the pore channels at higher pH value would expand pore sizes in the skin layer of the weak polyelectrolyte membrane, which leads to an increase of water flux.17-19 The flux of protein solution is lower than the flux of water at the same pH value. We thought that membrane fouling, due to protein adsorption or deposition on the surface and in pores, mostly causes the flux decline. The mixing induced by the stirrer

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Figure 4. Time-dependent flux of PAN-DMAEMA membrane during BSA ultrafiltration at different pH values. In feed solution, BSA concentration is 1.0 mg/mL and NaCl concentration is 0.01 mol/L. The water used for Jw1 and Jw2 measurement has the same pH value and ionic strength as protein solution. Table 1. Rejection Ratio and Flux Recovery Ratio of PAN-DMAEAM Membrane at Various pH Valuesa BSA

lysozyme

pH value

R (%)b

FRR (%)c

R (%)b

FRR (%)c

4.0 5.8 6.5 8.0 9.5

99.0 98.3 96.6 85.5 84.3

77.8 42.9 44.3 88.4 91.5

42.6 39.3 36.4 22.3 21.2

94.6 93.2 94.3 96.0 95.6

a The NaCl concentration in feed solution is 0.01 mol/L. ratio. c Flux recovery ratio.

b

not recovered to original water values (Jw1) due to the irreversible adsorption of BSA on the membrane surface. FRR values were calculated to evaluate the membrane antifouling property and are given in Table 1. The higher FRR value means the better antifouling property of the membrane. It is seen that there are lower FRR values at pH 5.8 and 6.5, meaning that PANDMAEAM membrane has weak antifouling ability. FRR values are higher at pH 4.0, 8.0, and 9.5. The maximum FRR value can reach 91.5% at pH 9.5 for PAN-DMAEMA membrane displaying excellent antifouling property. Membrane fouling is usually composed of reversible fouling and irreversible fouling.5-9 Reversible protein adsorption or deposition causes reversible fouling that can be removed by hydraulic cleaning, e.g., backwashing and cross flushing. However, irreversible protein adsorption causes irreversible fouling that can only be eliminated by chemical cleaning. To further investigate the antifouling property in detail, several ratios are introduced and defined. The first ratio Rt is calculated by eq 5 Rt ) 1 -

may actually sweep the deposited protein molecules away from membrane surface. The deposition and sweeping of protein may reach equilibrium in the subsequent operation, so that a relatively steady flux (Jp) is retained during BSA solution ultrafiltration. The effect of pH on BSA rejection ratio is given in Table 1. BSA rejection ratio is gradually decreased with increase of pH value, indicating that pore sizes in the skin layer of the weak polyelectrolyte ultrafiltration membrane are enlarged at higher pH value. After BSA ultrafiltration operation, the PAN-DMAEAM membrane was rinsed with water and water flux (Jw2) was measured again. In all given conditions, water fluxes (Jw2) are

(5)

which describes the degree of total flux loss caused by total fouling. Rr is calculated by eq 6 Rr )

Jw2 - Jp Jw1

(6)

which describes the degree of flux loss due to reversible fouling; the reversible protein deposition (cake formation) can be eliminated through hydraulic cleaning. Rir is defined by eq 7

Rejection

Figure 5. Effect of pH value on fouling ratio of PAN-DMAEMA ultrafiltration membrane during BSA ultrafiltration. NaCl concentration in solution is 0.01 mol/L.

Jp Jw1

Rir )

Jw1 - Jw2 Jw1

(7)

which describes the degree of flux loss caused by irreversible fouling, the irreversible protein adsorption can not be eliminated through simple hydraulic cleaning. Rt is the sum of Rr and Rir. A summary of Rt, Rir, and Rr of PAN-DMAEMA ultrafiltration membrane as a function of pH value is shown in Figure 5. It can be seen that Rt values are higher at pH values of 5.8 and 6.5. However, lower Rt values appear at pH values of 4.0, 8.0, and 9.5. The lower Rt indicates lower total flux loss, corresponding to less protein adsorption or deposition on the membrane surface and better antifouling property. Figure 5 also shows that PAN-DMAEMA ultrafiltration membrane has not only lower Rt but also lower Rir at higher pH values of 8.0 and 9.5 than that at pH values of 5.8 and 6.5. When solution pH is increased from 5.8 to 9.5, Rir is dramatically decreased from 0.57 to 0.09. The increase of solution pH reduces total membrane fouling, especially irreversible membrane fouling. In other words, at higher pH value, reversible fouling is the dominant factor responsible for the flux loss of PAN-DMAEMA ultrafiltration membrane. 3.2.2. Ionic Strength Influence. It is known that water flux of PAN-DMAEMA ultrafiltration membrane is increased due to the addition of NaCl in the feed solution,15 since the electrostatic repulsions are screened and PDMAEMA chains are shrunk at higher ionic strength. The effect of NaCl concentration on the flux of PAN-DMAEMA membrane during BSA ultrafiltration operation at pH 5.8 was studied, and the results are presented in Figure 6. The fluxes of protein solutions (Jp) are all lower than fluxes of water (Jw1) at the same pH value. Membrane fouling, due to BSA adsorption and deposition on the membrane surface and in the pores, results in the flux decline.

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Figure 6. Time-dependent flux of PAN-DMAEMA membrane during BSA ultrafiltration at different NaCl concentrations. In feed solution, BSA concentration is 1.0 mg/mL and solution pH value is 5.8. The water used for Jw1 and Jw2 measurement has the same pH value and ionic strength as protein solution.

Figure 7. Effect of NaCl concentration on fouling ratio of PAN-DMAEMA ultrafiltration membrane during BSA ultrafiltration. The solution pH value is 5.8.

Table 2. Rejection Ratio and Flux Recovery Ratio of PAN-DMAEAM Membrane at Various NaCl Concentrationsa BSA

lysozyme

NaCl concn (mol/L)

R (%)b

FRR (%)c

R (%)b

FRR (%)c

0.01 0.1 0.5 1.0

97.5 89.2 77.5 72.1

44.6 71.3 89.8 89.7

39.3 32.6 20.4 18.2

93.2 93.6 95.5 94.8

a The pH value of feed solution is 5.8. recovery ratio.

b

Rejection ratio.

c

Flux

The effect of NaCl concentration on BSA rejection ratio and FRR of PAN-DMAEMA ultrafiltration membrane is given in Table 2. The rejection ratio of BSA is gradually decreased with an increase of NaCl concentration, since the pore sizes in the skin layer of PAN-DMAEAM membrane are enlarged at higher NaCl concentration. FRR value of PAN-DMAEMA ultrafiltration membrane is increased obviously with an increase of NaCl concentration. FRR value is as high as 89.8% when NaCl concentration in feed solution is 0.50 mol/L. PAN-DMAEAM membrane has better antifouling property at higher ionic strength, this is in agreement with the results of protein adsorption. A summary of Rt, Rir, and Rr of PAN-DMAEMA ultrafiltration membrane as a function of ionic strength at pH 5.8 is shown in Figure 7. It can be seen that Rt is decreased from 0.79 to 0.56 with an increase of NaCl concentration from 0.01 to 1.0 mol/L. The smaller Rt indicates lower total flux loss, corresponding to less protein adsorption or deposition on the membrane surfaces. When NaCl concentration is increased from 0.01 to 1.0 mol/L, Rir is dramatically decreased from 0.55 to 0.10. The increase of NaCl concentration in feed solution reduces total membrane fouling, especially irreversible membrane fouling, and enhances antifouling property. 3.3. Lysozyme Ultrafiltration with PAN-DMAEAM Membrane. 3.3.1. pH Value Influence. Since there is few lysozyme adsorbed on PAN-DMAEAM membranes, the weak polyelectrolyte membranes possess excellent antifouling property during lysozyme ultrafiltration. The lysozyme rejection ratios and FRR values at various pH values are given in Table 1. Lysozyme rejection ratio is gradually decreased with an increase of pH value, indicating that pore sizes in the skin layer of the weak polyelectrolyte ultrafiltration membrane are enlarged at higher pH value. FRR values for lysozyme are all higher

Figure 8. Effect of pH value on fouling ratio of PAN-DMAEMA ultrafiltration membrane during lysozyme ultrafiltration. NaCl concentration in solution is 0.01 mol/L.

than that for BSA. The maximum FRR value can reach 96.0% at pH value of 8.0 for PAN-DMAEMA membrane displaying excellent antifouling property. Figure 8 presents the data of Rt, Rir, and Rr of PANDMAEMA ultrafiltration membrane as a function of pH value during lysozyme ultrafiltration. PAN-DMAEMA ultrafiltration membrane has not only lower Rt but also lower Rir during lysozyme ultrafiltration than that during BSA ultrafiltration. The reversible fouling is the dominant factor responsible for the flux loss during lysozyme ultrafiltration. Since few positively charged lysozyme can adsorb on positively charged PAN-DMAEMA ultrafiltration membrane at lower pH value, and PDMAEMA adopts a shrunk conformation and resists protein adsorption at higher pH value, PAN-DMAEMA ultrafiltration membrane has excellent antifouling property in a broad pH range during lysozyme ultrafiltration. 3.3.2. Ionic Strength Influence. The rejection ratios and FRR values of PAN-DMAEMA ultrafiltration membrane at different ionic strengths during lysozyme ultrafiltration are given in Table 2. The rejection ratio of lysozyme is gradually decreased with an increase of NaCl concentration at pH 5.8. The higher FRR values during lysozyme ultrafiltration mean that PAN-DMAEAM membrane has better antifouling property in a broad range of ionic strength.

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Figure 9. Effect of NaCl concentration on fouling ratio of PAN-DMAEMA ultrafiltration membrane during lysozyme ultrafiltration. The solution pH value is 5.8.

A summary of Rt, Rir, and Rr of PAN-DMAEMA ultrafiltration membrane as a function of ionic strength during lysozyme ultrafiltration at pH 5.8 is shown in Figure 9. The lower Rt values mean that less lysozyme adsorption or deposition on the membrane surface during lysozyme ultrafiltration. The lower Rir values means that the fouling may be largely attributed to reversible membrane fouling in the case of higher ionic strength, a simple water washing can clean the reversible fouling. The controlled permeability and excellent antifouling property under given conditions endow PAN-DMAEAM membrane with potential applications in protein separation and purification. 4. Conclusion PAN-DMAEAM membrane displays excellent antifouling property for BSA at higher pH value and ionic strength and for lysozyme in a broad range of pH and ionic strength. Electrostatic interactions, the conformation of PDMAEMA chains, and the nature of protein play important roles in enhancing the antifouling property of PAN-DMAEAM membrane. At lower pH value and ionic strength, the higher degree of ionization and the stretched conformation of PDMAEMA chains can resist lysozyme adsorption, but BSA can easily be adsorbed and deposited on PAN-DMAEAM membrane. At higher pH value and ionic strength, PDMAEMA chains adopt a shrunk conformation and expose more ester groups on the membrane surface. The neutral ester groups formed hydrogen bonds with water generates an energetic barrier to prevent protein adsorption and enhances antifouling property. Acknowledgment This research was funded by Tianjin Natural Science Foundation (No. 07JCYBJC00900), the Program of Introducing Talents of Discipline to Universities, No. B06006, and Doctoral Fund of Ministry of Education of China for New Teachers (No. 20070056041). Literature Cited (1) Lee, S.; Choo, K.; Lee, C. H.; Lee, H.; Hyeon, T.; Choi, W.; Kwon, H. Use of ultrafiltration membranes for the separation of TiO2 phorocatalysts in drinking water treatment. Ind. Eng. Chem. Res. 2001, 40, 1712.

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ReceiVed for reView September 16, 2008 ReVised manuscript receiVed November 25, 2008 Accepted January 13, 2009 IE801393Z