Improved Antifouling Properties of Poly(vinyl chloride) Ultrafiltration

Aug 21, 2014 - Qi Xue , Hao Cao , Fanning Meng , Miao Quan , Yong-Kuan Gong ... Guan Liu , Lijuan Zhang , Shimin Mao , Sohrab Rohani , Chibun Ching ...
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Improved Antifouling Properties of Poly(vinyl chloride) Ultrafiltration Membranes via Surface Zwitterionicalization Junao Zhu, Yanlei Su,* Xueting Zhao, Yafei Li, Jiaojiao Zhao, Xiaochen Fan, and Zhongyi Jiang Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, China ABSTRACT: Poly(vinyl chloride) (PVC) ultrafiltration membranes with improved antifouling properties were prepared by using a non-solvent-induced phase inversion process with in situ amination and subsequent surface zwitterionicalization. PVC was directly reacted with triethylenetetramine (TETA) in casting solutions. The introduction of amino groups not only enhanced the hydrophilic property of PVC membranes but also provided the active chemical sites on the membrane surfaces. The aminated PVC membranes were then immersed into the sodium chloroacetate solution to carry out quaternary amination reaction. The zwitterionic groups were formed on the PVC membrane surfaces and pore walls. The surface chemical compositions of modified PVC membranes were confirmed by energy dispersive X-ray spectroscopy (EDX), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Water contact angle and zeta-potential measurement were employed to explore surface property. In the ultrafiltration experiment of bovine serum albumin (BSA) solution, the zwitterionic PVC membranes exhibited better permeability and fouling resistance ability than the initial PVC membrane. The corresponding resistances of membrane, cake formation, adsorption, and pore blocking were calculated to explore the membrane fouling mechanism. membranes.13 Poly(acrylic acid) (PAA) is a hydrophilic material that was grafted onto PVC chains to fabricate ultrafiltration membranes. The obtained PVC membranes with grafted PAA had better defined fouling resistance than that of the pure PVC membranes.16 The potential of polyzwitterions used for antifouling has been clearly demonstrated.8,9,18−22 Ultrathin antifouling coatings with stable surface zwitterionic functionality were synthesized by chemical vapor deposition (CVD) technique.20 The coated surfaces exhibited very low adsorption of various foulants including bovine serum albumin (BSA), humic acid (HA), and sodium alginate (SA). A zwitterionic polyelectrolyte was grafted onto poly(vinylidene difluoride) (PVDF) membranes. The modified PVDF membranes were superhydrophilic and underwater superoleophobic.18 Membranes coated with poly(MPC-co-BMA) exhibited higher antifouling properties than bare membranes.19 The hydrophilicity and fouling resistance of polysulfone (PS) ultrafiltration membranes were improved via surface zwitterionicalization mediated by PSbased triblock copolymer additive.21 PPO-b-PSBMA copolymers with various zwitterionic PSBMA lengths were coated by self-assembling process. The obtained membranes presented a high hydration capability and improved the hemocompatible character significantly.22 In this work, the main objective was to improve the antifouling ability of PVC ultrafiltration membranes through the introduction of zwitterionic groups. The aminated PVC membranes were first prepared via in situ amination with

1. INTRODUCTION Ultrafiltration has been widely applied in many fields, such as wastewater treatment, environmental protection, and the food and pharmaceutical industries, in recent decades due to its convenience, low cost, and energy saving.1−3 However, membrane fouling caused by adsorption of cell debris, colloidal particles, and natural organic matter (NOM) has already been the major obstacle limiting the further use of ultrafiltration in industrial operations, because the membrane fouling has a strong impact on membrane performance, for example, in permeability and selectivity.4−6 It has been found that membrane fouling can be alleviated by hydrophilic polymers such as poly(ethylene glycol) (PEG) or polyzwitterions grafted on membrane surfaces.7 Particularly, polyzwitterions, which bear an equimolar number of homogeneously distributed anionic and cationic groups, can endow membranes with a high antifouling property due to the electrostatically induced hydration layer.8−11 The strongly bound water molecules in the hydration layer on membrane surfaces act as a buffer to the deposition and adsorption of foulants. Poly(vinyl chloride) (PVC) is one of the most versatile plastics and widely used in preparing ultrafiltration membranes due to its nontoxicity and economic benefits.12−14 However, membrane fouling owing to foulant deposition and adsorption on the PVC membrane surface should be solved.15,16 A few studies focus on the modification of PVC membranes to improve antifouling ability. Peng and Sui fabricated PVC/ poly(vinyl butyral) (PVB) blend membranes. PVB was introduced as the second polymer component to hydrophilize PVC ultrafiltration membranes. The hydrophilicity of the PVC/ PVB blend membranes was significantly improved.17 Liu et al. prepared PVC/chlorinated poly(vinyl chloride) (CPVC) blend ultrafiltration membranes, which were more hydrophilic and had better antifouling property than the pure PVC © 2014 American Chemical Society

Received: Revised: Accepted: Published: 14046

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Figure 1. Schematic illustration for the preparation of the zwitterionic PVC ultrafiltration membranes.

membranes (PVC-A) were washed completely by water to remove the residual solvent and additive and then stored in water before use. The zwitterionic groups were incorporated into the aminated PVC membranes via the quaternary amination reaction between amino groups and sodium chloroacetate. Figure 1 presents the schematic illustration for the preparation of the zwitterionic PVC ultrafiltration membranes. A series of modified PVC membranes obtained for different reactive conditions are listed in Table 1. The aminated PVC membranes

triethylenetetramine (TETA) in the casting solutions, which was a convenient and feasible method to modify polymer material for membrane fabrication.23−25 Amino groups on the PVC chains provided more active reaction sites, which were beneficial for the next modification. The subsequent surface zwitterionicalization was carried out through the chemical reaction between amino groups and the sodium chloroacetate. It is well-known that most methods to synthesize the polyzwitterions are via conventional free radical polymerization techniques and ring opening of lactone.26,27 Herein, the zwitterionic groups were synthesized via quaternary amination reaction, which was less costly than the methods mentioned above and easily achieved in industrial application.

Table 1. Modified PVC Membranes Fabricated under Different Reaction Conditions

2. EXPERIMENTAL SECTION 2.1. Materials. PVC resin with a mean degree of polymerization of 800 was purchased from Dagu Chemical factory (Tianjin, China). PEO−PPO−PEO block copolymer, Pluronic F127, was purchased from Sigma. TETA and N,Ndimethylacetamide (DMAc) were purchased from Kewei Chemicals Co. (Tianjin, China). BSA (Mw 66.4 kDa), fibrinogen (Mw 340 kDa), and lysozyme (Mw 14 kDa) were purchased from Institute of Hematology, Chinese Academy of Medical Science (Tianjin, China). Other chemicals were of commercial analytical grade and used without further purification. Water used in all experiments was purified by reverse osmosis equipment. 2.2. Preparation of Membranes. PVC membranes were prepared by the non-solvent-induced phase separation method (NIPS) with PVC as membrane material, and Pluronic F127 was used as additive.13 PVC membranes with a wet thickness of about 240 μm were kept in water before use. The aminated PVC membrane material was synthesized by chemical reaction between PVC and TETA in DMAc casting solution. The weight ratio of TETA/PVC is fixed at 10.0%. TETA was directly added into the PVC homogeneous mixing casting solution and then stirred at a temperature of 60 °C for 10 h. After complete release of the bubbles for 6 h, the casting solution was cast on glass plates with a steel knife and immersed in a water coagulation bath. The aminated PVC

membrane

quaternary amination time (h)

quaternary amination temperature (°C)

PVC-A PVC-A-NaOH PVC-A-Z-1 PVC-A-Z-2 PVC-A-Z-3 PVC-A-Z-4 PVC-A-Z-5

4 8 12 12 12

70 70 70 60 80

were immersed into 1.0 M aqueous sodium chloroacetate solution with 1.0 M NaOH as the catalyst for various quaternary amination reaction temperatures and times. Afterward, the modified PVC membranes with zwitterionic groups were taken out and rinsed entirely with water to remove the residual sodium chloroacetate. Finally, the zwitterionic PVC membranes (PVC-A-Z) were stored in water prior to utilization. 2.3. Characterization of Membranes. The cross-section morphologies of the modified PVC membranes were observed by scanning electron microscopy (SEM, Philips XL30E scanning microscope). The membranes frozen in liquid nitrogen were broken and sputtered with gold before SEM analysis. The compositions of the modified PVC membrane surfaces were also analyzed by energy dispersive X-ray (EDX) spectroscopy during SEM observation. X-ray photoelectron 14047

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Figure 2. Cross-sectional SEM morphologies of PVC (a), PVC-A (b), PVC-A-Z-1 (c), PVC-A-Z-2 (d), and PVC-A-Z-3 (e) membranes.

MPa to obtain a stable flux; then the pressure was reduced to the operation pressure of 0.1 MPa. The pure water flux J1 (L/ m2h) is calculated by using the equation

spectroscopy (XPS, PerkinElmer Phi 1600 ESCA system) using Mg Kα (1254.0 eV) as the radiation source was also used to evaluate the surface compositions of membranes. Survey scans were taken in the range of 0−1100 eV at a takeoff angle of 90°. FT-IR analysis was carried out on a FT-IR spectrometer (VERTEX70) to confirm the functional groups on the membranes. The static contact angles of water were measured by a contact angle goniometer (JC-2000C Contact Angle Meter, Powereach Co., Shanghai, China). The average value of static contact angle on each membrane is calculated with at least five different locations. Zeta potentials of the modified PVC membranes were determined from streaming potential measurements by electrokinetic analyzer (Anton Paar KG, Austria) equipped with a plated sample cell. The measurements were carried out at a temperature of 20 °C in KCl solution (1.0 mM, pH 6.5). Prior to use, the samples were immersed in 1.0 mM KCl solution for at least 24 h. Zeta potentials are calculated using the Helmholtz−Smoluchowski equation. 2.4. Separation Performance Evaluation. A filtration test cell (model 8200, Millipore Co., USA) connected with a N2 gas cylinder and solution reservoir was used to evaluate the separation performance of the modified PVC membranes. Each membrane was initially compacted for about 30 min at 0.15

J1 =

V AΔt

(1)

where V (L) is the volume of permeated pure water, A (m2) is the membrane effective aream and Δt (h) is the operation time. Subsequently, the cell and solution reservoir were refilled rapidly with 1.0 mg/mL model foulant feed solution. BSA at a concentration of 1.0 g/L was dissolved in 0.1 M citric acid or 0.1 M Na2HPO4 buffer solutions with a pH of 4.0, 4.8, 6.5, or 9.0, respectively. The flux for the feed solution was recorded as Jp (L/m2h). The rejection (R) of model foulant was calculated according to the equation ⎛ Cp ⎞ R = ⎜1 − ⎟ × 100% Cf ⎠ ⎝

(2)

where Cp and Cf (mg/mL) refer to the concentrations of BSA in permeate and feed solutions, respectively. The concentrations of BSA were measured via a UV−vis spectrophotometer (UV-2800, Hitachi Co., Japan) at a wavelength of 278 nm. 14048

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Figure 3. EDX spectra of PVC (a) and PVC-A (b).

filter cell for about 20 min, and then the pure water flux of the cleaned membrane (J2) was measured again. Ra and Rp, due to irreversible membrane fouling, still existed for the cleaned membrane. Rp was calculated using the following equation:

2.5. Resistance Determination. To further study the fouling resistant mechanism of the modified PVC ultrafiltration membranes, a four-parameter membrane resistance in series model was used here to analyze the resistance by the equation28−30 ΔP Jp = μ[R m + R c + R a + R p]

Rp = (3)

ΔP μJ1

Rc =

ΔP − Rm μJ0

ΔP − R m − Ra − R p μJp

(7)

3. RESULTS AND DICUSSION 3.1. Properties of the Aminated PVC Ultrafiltration Membrane. Many studies on PVC material mainly focused on the substitute modification of Cl atoms in PVC main chains.31−33 In this study, TETA was used as a nucleophile to aminate the PVC main chains through the SN2 reaction, which would be favored in polar aprotic solvent and low reaction temperature. As illustrated in Figure 1, TETA would substitute the liable Cl atoms in PVC chains to get the aminated PVC in DMAc solution, which is so-called in situ amination. In situ amination is an effective and convenient method to modify the membrane material.23−25 The aminated PVC ultrafiltration membranes were directly fabricated by NIPS method. The morphology of PVC-A membrane is presented in Figure 2. It was noted that PVC-A membrane had a dense skin layer and a support layer with finger-like structure as that of PVC membrane, which was the typical structure of asymmetric membrane. The surface main elements of the PVC-A membrane are given in Figure 3. The appearance of oxygen

(4)

where J1 is the pure water flux of the clean membrane. After this step, the same membrane was immersed in feed solution with model foulant for 6 h. This step allows the adsorption of model foulant on the membrane surface. After removal of the model foulant feed solution, the membrane with the adsorbed foulant was installed in the filter cell again. Ra was calculated by subtracting the hydraulic membrane resistance from the total resistance:

Ra =

(6)

Rc, due to additional hydraulic resistance of cake formation, was calculated by subtracting all other resistances to the total resistance in feed solution with model foulant using the following equation:

where Jp is the flux of feed solution with model foulants, ΔP is the transmembrane pressure (TMP) drop, Rm is the membrane hydraulic resistance (m−1), Rc is the resistance due to cake formation (m−1), Ra is the resistance due to the adsorption resistance (m−1), Rp is the resistance due to the pore blocking (m−1), and μ is the viscosity of the feed solution (kg/m·s). Rm is determined by filtering pure water through an ultrafiltration membrane with the same operating conditions as mentioned in section 2.4, which was calculated using the equation Rm =

ΔP − R m − Ra μJ2

(5)

J0 is the water flux through the modified PVC membrane with adsorbed foulant. After filtration of the feed solution for at least 1 h, the used membrane was directly washed with water in the 14049

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rejection was decreased from 79.5 to 64.8%, whereas the pure water flux was increased from 171.7 to 181.8 L/m2h. This implied that NaOH treatment probably enlarged the pore sizes of the PVC-A membrane. The cross-sectional morphologies of the zwitterionic PVC membranes are shown in Figure 2. All of the zwitterionic PVC membranes exhibited typical asymmetric cross-section structure consisting of a dense skin layer and a finger-like support layer as that of PVC membranes, which meant no appreciable structural and morphological changes after the modification. The surface main elements of the zwitterionic PVC membranes are shown in Table 3. After quaternary amination

element on the membrane surfaces was attributed to the segregation of PEO chains from additive Pluronic F127, which was used as both pore-forming agent and surface modifier to improve the permeability.13 N element also appeared on the PVC-A membrane surface. Because TETA was the only source of nitrogen element, EDX analysis proved that the amino groups were grafted on the membrane surfaces. In addition, PVC membrane had a zeta-potential value of −33.76 mV. This might be due to the strong adsorption of OH− ions onto polymer membrane surfaces.34 After grafting the amino groups, the zeta potential of PVC-A membrane was increased to −25.58 mV, which meant that part of the negative charges on the membrane surface were neutralized by the positive charged amino groups. The pore sizes of PVC-A membrane were evaluated by BSA as molecular probe, and the results are listed in Table 2. It was

Table 3. EDX Analysis of the PVC and the Modified PVC Membranes

Table 2. Properties of PVC, Aminated PVC, and Zwitterionic PVC Membranes membrane PVC PVC-A PVC-A-NaOH PVC-A-Z-1 PVC-A-Z-2 PVC-A-Z-3 PVC-A-Z-4 PVC-A-Z-5

water contact angle (deg)

zeta potentiala (mV)

± ± ± ± ± ± ± ±

−33.76 −25.58 −26.43 −31.45 −34.05 −33.28 −30.13 −33.77

65.8 55.3 56.5 42.2 39.9 39.4 43.9 36.1

1.3 3.1 2.7 1.8 2.1 1.3 1.8 2.6

pure water fluxb (L/m2h) 137.5 171.7 181.8 205.5 240.1 259.6 203.4 159.4

± ± ± ± ± ± ± ±

9.5 6.8 5.6 4.5 4.9 3.1 4.5 5.1

BSA rejectionb (%) 81.6 79.5 64.8 77.3 75.2 76.3 77.5 63.6

membrane

C (at. %)

N (at. %)

O (at. %)

Cl (at. %)

PVC PVC-A PVC-A-Z-1 PVC-A-Z-2 PVC-A-Z-3

82.52 89.62 89.35 89.06 88.39

3.28 2.93 2.75 2.57

3.39 2.67 2.76 2.82 3.29

14.08 4.42 4.96 5.37 5.76

reaction, the atomic fraction of the oxygen element increased with the increase of treatment time. The increased oxygen element might come from the sodium chloroacetate. In addition, FT-IR for PVC, PVC-A, and PVC-A-Z-2 was also employed to probe the surface chemistry of the membranes. As shown in Figure 4, a new peak at 1648 cm−1 corresponding to

Zeta potential was measured at pH 6.5. bThe filtration operation was carried out in the pressure of 0.1 MPa. a

observed that the BSA rejection ratios for PVC and PVC-A membrane were 81.6 and 79.5%, respectively. Time-dependent fluxes of PVC-A membrane during BSA solution ultrafiltration are shown in Figure 6. The fluxes of feed solution were lower than those of pure water due to the existence of concentration polarization and membrane fouling. A four-parameter membrane resistance in series model was used to analyze the membrane fouling process. As presented in Figure 7, although Rm and Rc of the PVC-A membrane were decreased, which might be owing to the enhancement of hydrophilicity, Ra and Rp of the PVC-A membrane (4.36 × 1011 and 1.55 × 1011 m−1) were higher than those of the PVC membrane (3.60 × 1011 and 1.44 × 1011 m−1). This phenomenon might be due to the electrostatic adsorption between the negatively charged BSA and the positively charged amino groups. Therefore, the antifouling ability of PVC-A membrane needs further improvement. 3.2. Properties of the Zwitterionic PVC Ultrafiltration Membrane. It has been widely accepted that zwitterionic materials can endow surfaces with excellent antifoulng ability.35,36 As shown in Figure 1, zwitterionic functional groups were formed on the aminated PVC membrane surface and pore walls. NaOH solution used in this process can not only catalyze the quaternary amination reaction but also neutralize the generated HCl to accelerate the reaction. PVC-A membrane was treated by sodium chloroacetate solutions with various quaternary amination temperatures and times to obtain the zwitterionic PVC membranes. PVC-A membrane was treated by only 1.0 M NaOH solution; the properties of the PVC-A−NaOH membrane are also listed in Table 2. The BSA

Figure 4. FT-IR spectrum of PVC, PVC-A, and PVC-A-Z-2 membranes.

the bending vibration of the N−H bonds was observed on the PVC-A membrane, suggesting the introduction of amino groups on PVC membrane surface. The spectrum of PVC-AZ-2 exhibited a strong absorption peak at 1592 cm −1 corresponding to the stretching vibration of carboxylate groups. Because sodium chloroacetate was the only source of carboxylate groups, this result indicated that zwitterionic groups were formed on PVC-A membrane surface and pore walls. The chemical feature in the near-surface region of the zwitterionic PVC membranes was further determined by XPS. Figure 5 shows the XPS C1s high-resolution scans of the 14050

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Figure 7. Resistance analysis of PVC and PVC-A membranes during BSA solution ultrafiltration.

Lorentzian−Gaussian functions. The C1s component peak at the binding energy of 284.9 was attributed to CC. The peaks observed at 286.3 eV were attributed to CCl, CO and C N, respectively. The peak at 288.5 eV was attributed to CO (Figure 5b). Therefore, the combination of FT-IR with XPS data could demonstrate the successful formation of zwitterionic groups. The hydrophilic property of the zwitterionic PVC membranes was further enhanced. As shown in Table 2, the water contact angles of the zwitterionic PVC membranes were continuously decreased to 39.4° with an increase of reaction time at a quaternary amination temperature of 70 °C. Moreover, when the reaction temperature attained 80 °C, the PVC-A-Z-5 membrane had the lowest contact angle of 36.1°. More zwitterionic groups were synthesized at higher temperature, which remarkably enhanced the membrane’s hydrophilicity. As presented in Table 2, the zeta potentials of all zwitterionic PVC membranes were close to that of PVC membranes, because the zwitterionic groups possessed an equimolar number of anionic and cationic groups. The introduction of negatively charged groups from the carboxylic groups of chloroacetate sodium made the zeta potential of the zwitterionic PVC membranes more negative than that of the PVC-A membrane. Figure 8 presents the influence of quaternary amination time on membrane performance during BSA solution ultrafiltration. It was obvious that pure water fluxes of the zwitterionic membranes were increased from 205.5 to 259.6 L/m2h when the reaction time was prolonged from 4 to 12 h. Higher pure water fluxes might be explained by the lower membrane resistance due to their higher hydrophilicity. However, BSA rejection of the zwitterionic PVC membranes was close to that of the PVC-A membrane. The influence of the quaternary amination temperature on membrane performance is also presented in Table 2. Pure water fluxes of the zwitterionic PVC membranes were increased from 203.4 to 259.6 L/m2h when the reaction temperature was increased from 60 to 70 °C; the higher hydrophilicity on the membrane surface led to the increase of the water flux. However, pure water fluxes were decreased to 159.4 L/m2h when the quaternary amination reaction temperature was further increased to 80 °C. The membrane pores in the skin layer of the asymmetric membrane might be destroyed at a high temperature of 80 °C. The BSA rejection ratio of the PVC-A-Z-

Figure 5. High-resolution XPS spectra of C1s for PVC (a) and PVC-AZ-2 (b) membranes.

Figure 6. Time-dependent fluxes of PVC and PVC-A membranes during BSA solution ultrafiltration at pH 6.5. The ultrafiltration process included four steps: pure water filtration, 0−0.5 h; BSA filtration, 0.5−1.5 h; water cleaning and pure water filtration of cleaned membrane, 1.5−2.0 h.

zwitterionic PVC membrane (PVC-A-Z-2). The C1s core level spectra could be further fitted with three peaks using a sum of 14051

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14.04 × 1011 m−1 for the PVC-A-Z-3 membrane. The gradual decrease of Rm was due to the improved hydrophilic property of the zwitterionic PVC membranes after longer reaction time. At the same time, the Rc value of PVC-A-Z-3 was decreased to 6.12 × 1011 m−1, which was significantly lower than that of the PVC membrane of 28.08 × 1011 m−1. The lower Rc meant that there was loose protein cake on the membrane surface for the PVC-A-Z-3 membrane. Additionally, Ra was decreased from 3.61 × 1011 m−1 for the PVC membrane to 0.18 × 1011 m−1 for the PVC-A-Z-3 membrane. Membrane fouling due to BSA adsorption was really inhibited by the zwitterionic PVC membranes. Another interesting note in Figure 9 was that Rp increased from 0.50 × 1011 m−1 for the PVC-A-Z-2 membrane to 2.08 × 1011 m−1 for the PVC-A-Z-3 when the quaternary amination reaction time was increased from 8 to 12 h. According to the Rm values of the PVC-A-Z-2 membrane (15.12 × 1011 m−1) and PVC-A-Z-3 membrane (14.04 × 1011 m−1), water would more easily pass through the channel of the PVC-A-Z-3 membrane. As a result, more BSA molecules would be blocked in the channel of the PVC-A-Z-3 membrane. This implied that lower Rm and incomplete BSA rejection would make pore blocking more severe. The time-dependent fluxes of the zwitterionic membranes with different quaternary amination reaction temperatures during BSA solution ultrafiltration at pH 6.5 are shown in Figure 10. The resistance analysis results are plotted in Figure

Figure 8. Time-dependent fluxes of the PVC and the zwitterionic membranes with different reaction times during BSA solution ultrafiltration at pH 6.5. The ultrafiltration process included four steps: pure water filtration, 0−0.5 h; BSA filtration, 0.5−1.5 h; water cleaning and pure water filtration of cleaned membrane, 1.5−2.0 h.

5 membrane is even lower than that of the PVC-A-Z-4 membrane. 3.3. Improved Separation Performance of the Zwitterionic PVC Ultrafiltration Membrane. Membrane fouling is a significant challenge in water purification application, which is affected by hydrodynamic conditions, membrane properties, and feedwater characteristics.3 The initial membrane fouling is largely determined by foulant−membrane surface interactions, which are strongly dependent on membrane surface properties.6 BSA was chosen as the protein model foulant to explore the antifouling mechanism of the zwitterionic PVC membranes. Protein fouling could result from cake formation, adsorption and pore blocking due to the hydrophobic interaction, electrostatic interactions, and other causes.6,37 The time-dependent fluxes of the zwitterionic membranes with different quaternary amination reaction times during BSA solution ultrafiltration at pH 6.5 are shown in Figure 8. The corresponding resistance analysis is shown in Figure 9. Rm was decreased from 26.28 × 1011 m−1 for the PVC membrane to

Figure 10. Time-dependent fluxes of the PVC and the zwitterionic PVC membranes with different reaction temperatures during BSA solution ultrafiltration at pH 6.5. The ultrafiltration process included four steps: pure water filtration, 0−0.5 h; BSA filtration, 0.5−1.5 h; water cleaning and pure water filtration of cleaned membrane, 1.5−2.0 h.

11. When the reaction temperature was raised from 60 to 70 °C, Rc was decreased from 10.08 × 1011 to 6.12 × 1011 m−1 and Ra was decreased from 0.61 × 1011 to 0.18 × 1011 m−1. However, all of the resistances were increased when the quaternary amination reaction temperature was further raised to 80 °C. The value of Rm was increased from 14.04 × 1011 m−1 for the PVC-A-Z-3 membrane to 22.68 × 1011 m−1 for the PVC-A-Z-5 membrane, which might be related to the membrane shrinkage at the higher reaction temperature of 80 °C. Optimized reaction conditions for the surface zwitterionicalization are that quaternary amination reaction time is about 8 h and reaction temperature is at about 70 °C. On the basis of the above analysis, all of the zwitterionic PVC membranes

Figure 9. Resistance analysis of the PVC and the zwitterionic PVC membranes with different reaction times during BSA solution ultrafiltration. 14052

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Figure 13. Resistance analysis of PVC-A-Z-2 membrane during BSA solution ultrafiltration under various pH values.

Figure 11. Resistance analysis of the PVC and the zwitterionic PVC membranes with different reaction temperatures during BSA solution ultrafiltration.

which was the highest among all feed solutions with different pH values. The reason might be that the PVC-A-Z-2 membrane was negatively charged, whereas the BSA surface was surrounded by positively charged amino acid residues as the environmental pH was less than the BSA isoelectric point (pI = 4.8). Hence, strong electrostatic adsorption occurred between the membrane surface and BSA molecules, which led to the maximum irreversible fouling. When the pH value of the feed solution was 4.8, the irreversible fouling was still heavy. The Ra of the PVC-A-Z-2 membrane was 2.9 × 1011 m−1 at pH 4.8, which was much higher than that at pH 6.5 of 0.19 × 1011 m−1 and at pH 9.0 of 0.49 × 1011 m−1, respectively. At pH 4.8, BSA molecules were expected to be electrically neutralized. Therefore, the neutrality of BSA molecules involves a higher compaction of the polarization layer and a faster adsorption of protein onto the membrane surface. However, when the pH value was increased to 6.5, the antifouling performance of the PVC-A-Z-2 membrane was improved, because electrostatic repulsion occurred and the zwitterionic PVC membrane could form the hydration layer on the membrane surface to reduce the protein adsorption. In addition, the higher hydrophilicity of the membrane led to the lowest Rm of 15.12 × 1011 m−1. When the pH value was further increased to 9.0, Ra of the PVC-A-Z-2 membrane was 0.49 × 1011 m−1. This might be owing to positive amino groups being partially neutralized by OH− in the alkaline solution and the negatively charged membrane surface being mutually repulsive with the negative charge BSA molecules, leading to less protein adsorption. Furthermore, the electrostatic repulsion could also alleviate the concentration polarization resistance, as Rc of the PVC-A-Z-2 membrane was 4.23 × 1011 m−1 at pH 9.0, which was the lowest resistance of cake formation of all feed solutions with different pH values. The high Rm of 18.75 × 1011 m−1 of the PVC-A-Z-2 membrane at pH 9.0 might be due to pore shrinkage, which was caused by the extended zwitterionic chains, whereas carboxyl groups were negatively charged and mutually exclusive.

showed better antifouling ability than the initial PVC membrane. Most BSA adsorption was inhibited by the repulsion interaction from the negative charge of the membrane surfaces and the antifouling ability from the grafted zwitterionic groups. Pore blocking is the reason for slightly irreversible fouling for the zwitterionic PVC membranes during BSA ultrafiltration. Moreover, the zwitterionic PVC membranes also showed much better antifouling performance with lower hydraulic resistance than PVC membrane during ultrafiltration of fibrinogen and lysozyme, which were both dissolved in buffer solution at pH 6.5. 3.4. Antifouling Performance of the Zwitterionic PVC Ultrafiltration Membrane at Different pH Values. Ultrafiltration experiment with pH values of feed solutions ranging from 4.0 to 9.0 were carried out to explore the antifouling mechanism of the zwitterionic PVC ultrafiltration membrane under different pH values. The results are shown in Figure 12, and the correspondent resistance analysis is listed in Figure 13. Ra of the PVC-A-Z-2 membrane at pH 4.0 was 4.6 × 1011 m−1,

4. CONCLUSIONS A facile approach to construct zwitterionic PVC ultrafiltration membranes by in situ amination and subsequent surface zwitterionicalization was presented in this work. The zwitterionic PVC ultrafiltration membranes displayed higher water fluxes and better permeability for BSA solution than the PVC membrane. The zwitterionic PVC membranes also

Figure 12. Time-dependent flux for PVC-A-Z-2 membrane during the BSA ultrafiltration process as a function of pH. The ultrafiltration process included four steps: pure water filtration, 0−0.5 h; BSA filtration, 0.5−1.5 h; water cleaning and pure water filtration of cleaned membrane, 1.5−2.0 h. 14053

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exhibited a desirable antifouling property for filtration of BSA solution. Pore blocking was the main reason for slightly irreversible fouling for the zwitterionic PVC membranes. When the pH value of the BSA solution was 6.5, the zwitterionic PVC membranes (PVC-A-Z-2) showed best fouling resistance ability with a high feed solution flux of 137.5 L/m2h and a low adsorption resistance of 0.19 × 1011 m−1. Due to lower cost, easier preparation, and better separation performance, the zwitterionic PVC ultrafiltration membranes have potential industrial application.

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AUTHOR INFORMATION

Corresponding Author

*(Y.S.) Fax: 86-22-27890882. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is financially supported by National Science Fund for Distinguished Young Scholars (21125627) and Tianjin Natural Science Foundation (No. 13JCYBJC20500).

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