Effect of modified attapulgite addition on the performance of a PVDF ultrafiltration membrane

Desalination 344 (2014) 71–78

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Effect of modified attapulgite addition on the performance of a PVDF ultrafiltration membrane Yalei Zhang a, Jing Zhao a, Huaqiang Chu a,⁎, Xuefei Zhou a, Yong Wei b a b

State Key Laboratory of Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Tongji University, Shanghai 200092, China School of Environmental and Safety Engineering, Changzhou University, Changzhou 213164, China

H I G H L I G H T S • • • • •

Attapulgite is a promising inorganic nano-additon for membranes preparation. Attapulgite modified with APTES perform uniformly distribution in the PVDF matrix. Attapulgite with rich –OH group enhances the hydrophilicity of the PVDF membranes. The hybrid APT/attapulgite membranes exhibit better performance than the pure one. Membrane antifouling and rejection have been improved with the APT addition.

a r t i c l e

i n f o

Article history: Received 31 October 2013 Received in revised form 15 February 2014 Accepted 4 March 2014 Available online xxxx Keywords: PVDF APT Modification Hybrid UF membrane Characterization

a b s t r a c t Attapulgite (APT), which is a highly hydrophilic fibrillar mineral in nature, is a promising new additive for preparation of ultrafiltration (UF) membranes. In this work, APT particles were first grafted with \NH2 using the silane coupling agent APTES to detach the crystal bundles to single crystal and enhance the uniform dispersion in an organic polymer matrix; these components were subsequently incorporated into a polyvinylidene fluoride (PVDF) matrix to develop a hybrid membrane via a phase inversion method. The modified APT mineral was characterized using SEM and FTIR to confirm the presence of the amine group on the surface. Other techniques (i.e., FTIR-ATR, XRD and TGA) and parameters (i.e., porosity, the average pore size, contact angle and filtration measurements of water and bovine serum albumin (BSA)) were applied and measured to study the performance of the UF membranes. Compared with the pure PVDF sample, the APT particle blended membranes exhibited better thermal stability, smaller pore size and higher hydrophilicity. More importantly, the values for pure flux, flux recovery ratio (FRR) and rejection of BSA solution all indicated that the PVDF/APT hybrid membranes offer excellent hydrophilicity, water permeability and good antifouling performance. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Membrane technologies consisting of nanofiltration (NF), microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO) and forward osmosis (FO) provide powerful methods for addressing water treatment issues [1]. Therefore, UF membranes are widely applied in industrial and drinking water treatments or as a pre-filtration stage prior to NF and RO processes [2,3]. The UF membranes are generally synthesized with such polymers as polysulfone, polyethersulfone, polypropylene and polyvinylidene fluoride (PVDF) [4,5]. Due to excellent antioxidation properties and good thermal and hydrophilicity stabilities as well as outstanding mechanical and membrane formation properties [6,7], PVDF has been extensively used in the production of UF membranes.

⁎ Corresponding author. Tel./fax: +86 21 65985811. E-mail address: [email protected] (H. Chu).

http://dx.doi.org/10.1016/j.desal.2014.03.007 0011-9164/© 2014 Elsevier B.V. All rights reserved.

Recently, many researchers have focused on organic–inorganic nanocomposite membranes to improve such polymer membrane properties as hydrophilicity, selectivity and thermal stability [8,9]. The common additive inorganic substances are TiO2, Al2O3, SiO2, ZnO, CuO, Fe3O4, ZrO2 and CdS nanoparticles [10–18]. However, to the best of our knowledge, reports on the application of an attapulgite (APT) mineral blend are rare. As a fibrillar clay mineral, the natural nanometer-scale hydrated magnesium aluminum silicate known as APT (i.e., palygorskite) consists of two double chains of the pyroxene (SiO3)2−-like amphibole (Si4O11)6− oriented parallel to the fiber axis [19,20]. This material also possesses reactive Si\OH groups on its surface and exchangeable cations in its framework channels [21]. Due to its excellent thermal stability and high mechanical strength and aspect ratio (as shown in recent years), APT is a good reinforcement filler candidate for an inorganic constituent in polymer nanocomposite membranes. Yin et al. prepared PAN/APT composite membranes with different APT concentrations [22]. Huang et al. developed chitosan/poly(vinyl alcohol) films reinforced with APT

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using high-pressure homogenization [23]. Li et al. prepared a novel polyimide/APT hybrid nanocomposite [24]. These previous studies proved that a small amount of APT nanorods incorporated into the membranes could significantly enhance the hydrophilicity property. Although the properties of inexpensive and rich hydroxyl groups on the surface make APT more readily compatible with a polymer matrix, they also tend to aggregate in the membranes due to van der Waals interactions among the APT rods [25]. Therefore, modification of APT powder immediately prior to UF membrane preparation is essential. Many attempts have been made to achieve high-efficiency disaggregation of APT and can be summarized in terms of five groups of processes [26–28]: (1) a high-temperature roasting activation method, (2) ultrasonic wave processing, (3) coupling agent surface treatment, (4) surfactant processing and (5) acidified processing. In the current work, (3-aminopropyl) triethoxysilane (APTES), a silane coupling agent consisting of amino (\NH2) and exthoxy (\OC2H5) groups, was chosen to facilitate the uniform dispersion of inorganic fillers in the PVDF matrix, and PVDF/APT hybrid UF membranes were synthesized using different dosages of APT particles via a phase inversion method. Membranes with various APT contents were investigated in terms of morphology, hydrophilicity and permeability, crystalline structure and thermal stability as well as water flux, antifouling performance and stability of APT in membranes.

Table 1 Composition and viscosity of casting solutions. PVP (wt.%)

APT (wt.%)

Viscosity (MPa·s)

APT-0 APT-1 APT-3 APT-5 APT-7

20 20 20 20 20

77 77 77 77 77

3 3 3 3 3

0 0.2 0.6 1.0 1.4

7509 10,371 12,858 14,272 15,367

The PVDF/APT composite membranes were prepared using the phase inversion method via an immersion precipitation technique. Modified APT aliquots of different concentrations (0, 1, 3, 5 and 7 wt.% by weight of PVDF labeled as APT-0, APT-1, APT-3, APT-5 and APT-7, respectively) were added into casting dopes consisting of PVDF, DMAc (as a solvent) and PVP (as a pore former). The composition and viscosity of the casting solution are illustrated in Table 1. After constant stirring for 24 h in a water bath at 50 °C, the polymer solution was held at 12 h for bubble removal. The homogeneous solutions were uniformly cast onto a glass plate with the membranes maintained at a height of 250 μm with an Automatic Film Applicator cast-knife (Elometer 4340, U.K.) and immersed into a 25 °C ethanol/water (15/85, v/v) non-solvent bath immediately without any evaporation for one day. The membranes were subsequently held in glycerin/water (10/90, v/v) for further usage.

The PVDF (Solef®1015) was purchased from Solvay Advanced Polymers. L.L.C., polyvinylpyrrolidone (PVP) was obtained from BASF (Germany), and BSA (Mw = 67,000 Da) and (3-aminopropyl) triethoxysilane (APTES, Mw = 221.37) were supplied by Aladdin Reagent Co., Ltd. (Shanghai, China) and used as received. Dimethylacetamide (DMAc), ethanol and glycerin of AR grade were provided by Shanghai Chemical Reagent Company. Additionally, the APT additive was kindly provided by Jiangsu Huayuan Mineral Co., Ltd.

2.4. Characterization of membranes The viscosity of the casting solution played an important role in the formation of the membrane microstructures by influencing the exchange rate of solvent and non-solvent. The viscosities of dopes with different APT concentrations were determined by digital viscometer (NDJ-8S, Shanghai Fangrui Equipment Co., Ltd., China) in a 25 °C water bath. All images of the surface and cross-section of the membranes were observed using a scanning electron microscope (SEM, Philips XL30, Netherlands). To obtain clear morphological structures, the membranes were immersed into liquid nitrogen, subsequently fractured, and sputtered with a thin layer of gold step-by-step before scanning. An FTIR-ATR spectrometer (Nicolet 5700, USA) was used to identify the chemical compositions on the surface of the UF membranes over a wavenumber range of 600–4000 cm−1 at a resolution of 4 cm−1. A wide angle XRD (Bruker D8 Advance, Germany), an effective technique for study of the existence and distribution of APT minerals, was operated at 40 kV voltage and 40 mA current using Cu Kα radiation (λ = 0.154 nm) from 5° to 90° (2θ). The thermal stability of the UF membranes with various APT contents was characterized by TGA analysis (TGA7, Perkin Elmer, USA), performed under a nitrogen atmosphere by heating of the membranes from 50 °C to 700 °C at a rate of 10 °C/min. The pore-size distribution of the UF membranes was calculated via the Barrett–Joyner–Halenda (BJH) method using a Micromeritics Tristar

2.2. Modification of APT The APT was modified according to the procedure previously reported in the literature [29]. A coupling agent (APTES) was selected to construct the linkage between the inorganic hosts and organic guests. The concrete grafting process is described as follows. The APTES (20 mL) was added to a 250-mL breaker container, followed by ethanol (72 mL) and deionized water (8 mL). Next, the APTES was hydrolyzed at 20 °C until a stable conductivity value was reached, as characterized with a conductivity meter (DDSJ-308F, Shanghai Yidian Equipment Co., Ltd., China). Disperse APT powder (6 g) was placed in a triangle flask containing deionized water (50 mL) and mixed with the assistance of ultrasound (KQ2200DV, Shanghai Precision Instruments Co., Ltd., China) for 2 h. When the fully hydrolyzed APTES and APT were prepared, the APTES mixture liquor was added into the triangle flask containing the APT solution and stirred uniformly for 8 h at 70 °C. The reaction mixture was centrifuged at a speed of 5000 rpm for 10 min and washed three times each with pure ethanol and deionized water

OH Si OH OH

DI water, ethanol °C

DMAc (wt.%)

2.3. Membrane preparation

2.1. Materials

O Si O O

PVDF (wt.%)

to leach out the residual APTES. Finally, the resulting product was dried in a desiccator at 80 °C for 12 h. The steps related to the surface grafting of APT are illustrated in Scheme 1 [29].

2. Experimental

H2 N

Membrane no.

H2 N +

Stirring °C

OH OH OH

Scheme 1. Schematic reaction route for synthesis of APT-NH2.

O O Si O

NH2

Y. Zhang et al. / Desalination 344 (2014) 71–78

flux of the membrane was re-measured as Jr. The permeation flux was defined as shown in Formula (2):

3000 instrument. The contact angle between the membrane upper surface and deionized water was used to represent the membrane hydrophilicity via a contact angle measurement instrument (JC2000A, Shanghai Zhongcheng Digital Equipment Co., Ltd., China). Similarly, each sample was measured at five different points, and the arithmetic average value was calculated. The membrane porosity (%) was calculated according to the method of dry-wet weight and Formula (1), as follows: ε¼

mw −md ρw ·A·δ

73

J¼

V A·t

ð2Þ

where J is the flux for pure water or BSA solution (L/(m2·h)), V is the volume for permeate water or BSA solution (L), A is the effective membrane area (m2), and t is the permeation time (h). The flux recovery ratio (FRR) was calculated according Formula (3), and the experimental rejection of the BSA solution (R) was calculated by Formula (4):

ð1Þ

where mw is the wet membrane weight (g), md is the dry membrane weight (g), ρw is the water density (0.998 g/cm3), A is the effective area (cm2), and δ is the thickness of the wet membrane (cm).

Jr  100% Jw

ð3Þ

C f− Cp  100% Cf

ð4Þ

FRR ¼

2.5. Filtration measurements R¼

All membrane filtration tests were performed with an UF experimental set-up [30] consisting of a Millipore 8050 UF cup (Millipore Co., Ltd., USA), a nitrogen gas cylinder, a pressure controller and a magnetic stirrer. Each of the dead-end filtration measurements was collected in four steps. First, the samples were compacted at 0.15 MPa for 30 min with pure water. Second, the pressure was held at 0.10 MPa, and the stable pure water permeation flux was recorded as Jw. Third, BSA solution (1 g/L) was fed at 0.10 MPa, and the permeation flux (JBSA) was measured for rejection of the BSA solution (R). Fourth, the UF membrane was back-flushed using deionized water, and the water

where Cf and Cp are the BSA content in the feed and permeate solutions, respectively. The stability of modified APT was an extremely essential factor considered in the process of water treatment. An ultrafiltration experimental set-up was used to investigate the stability of particles. The set-up was fed with deionized water at a pressure of 0.10 MPa and ran for 12, 24 and 48 h. Then an X-ray photoelectron spectrophotometer (XPS) (Kratos AXIS Ultra DLD, Japan) was applied to study the content

(a) (a a’) Pure P e AP PT 3276 3398 3548

Transmitance

3613

1649 2925

(b b’) APT-NH H2 1642 1605

(c c’) APTES 1601 3357 3291 2 2973 4000

3500

3000

2929 2500

2000

1500

1000

Wavenumbers (cm-1)

(b)

(c)

Fig. 1. FTIR spectra and SEM images of APT: (a) FTIR spectra of APT before and after modification, (b) unmodified APT and (c) modified APT.

74

Y. Zhang et al. / Desalination 344 (2014) 71–78

of membranes before and after the filtration operation. The stability of APT can be determined from the mass ratio of Si to F (Si/F). 3. Results and discussion 3.1. Characterization of modified APT As described above, the APT filler modified with APTES was characterized using the FTIR spectroscopy and SEM (Fig. 1). Fig. 1(a–c′) clearly shows two typical bands at 3291 cm−1 and 3357 cm−1, which can be assigned to the amine groups of the APTES [31]. The two new peaks located at 2925 cm− 1 (\CH2) and 1605 cm− 1 (\NH2) in Fig. 1(a–b′) confirmed the presence of the grafted APTES in the APT. The bands associated with the surface hydroxyl groups (Si\OH at 3613 cm−1, crystalline water at 3540–3560 cm−1, zeolite water at 3400–3440 cm−1, and adsorption water at 3240–3280 cm−1), as shown in Fig. 1(a–a′),

(a)

were weakened in Fig. 1(a–b′) following the grafting process, suggesting that a condensation reaction indeed occurred between APTES and \OH on the surface of the APT. Altogether, these observations indicated that the surface modification of APT was achieved. Fig. 1(b) and (c) exhibits the APT morphologies before and after the grafting process. The state of the APT dispersion in the PVDF matrix was known to be an important factor for the nanocomposite performance. According to Fig. 1(b), the natural APT obviously appeared as a bundle or aggregation of randomly oriented rod crystals. In contrast, most of the APT rods dispersed uniformly and individually after modification, as shown in Fig. 1(c). Similar to the previous work [22], we observed that APT was a typical one-dimensional (1D) nanostructured material, and the basic structural unit was a fibrillar or rod-like single crystal. However, the length was not uniform and varied from 1 μm to 2 μm, and the diameter was approximately 10–25 nm.

(b)

(d)

(c)

(e)

(a’)

(b’)

(c’)

(d’)

(e’)

(f)

Fig. 2. SEM images of the bottom surface, cross-section and inner porous structure of the prepared membranes: (a) APT-0, (b) APT-1, (c) APT-3, (d) APT-5 and (e) APT-7; (a′) APT-0, (b′) APT-1, (c′) APT-3, (d′) APT-5, (e′) APT-7 and (f) inner porous structure of APT-3 sample.

Y. Zhang et al. / Desalination 344 (2014) 71–78

75

3.2. Morphology studies of UF membranes

3.3. Analyses of FTIR-ATR, XRD and TGA The FTIR-ATR spectrum of the PVDF copolymer and the nanocomposites can be seen in Fig. 3. The new peak at 987 cm− 1 (Si\\Si stretching) showed much more significantly with the increase of modified APT, implying the successful attachment of additives and PVDF membrane. Peaks at 840 cm−1 and 879 cm− 1 were α-crystal and β-crystal of PVDF, respectively. Bands of 1176–1273 cm−1 can be ascribed to C\F stretching, and the absorption peaks at 1406 cm−1 and 2978 cm−1 were due to C\H deformation and C\H stretching. The peak at 1670 cm−1 represented a typical amide carbonyl (C_O) stretching vibration, which confirmed the PVP (K30) in the matrix [33, 34]. The C_O peak at 1670 cm−1 was observed to shift to a low wavenumber region (1662 cm− 1) when the amount of APT increased to 7 wt.% in the membrane. These results were due to H-bonding between \OH of the APT and amide carbonyl of the PVDF copolymer [35]. Furthermore, the \OH shifted from 3416 cm− 1 in the pure PVDF/PVP membrane to 3493 cm−1 for the APT-7 wt.% sample, and the vibration peak of the \OH band in the APT-7 sample broadened to a greater extent than that in the pure PVDF/PVP membrane, which might be due

Pure APT APT-7

Itensity

The surface and cross-sectional morphologies of the PVDF/APT blended membranes were characterized using SEM (Fig. 2). The first five graphs show that the pure PVDF membrane and the PVDF/APT membranes exhibited similar smooth surface morphologies. However, additional pores can clearly be observed on the membrane surface with increasing APT in the matrix. All of the membranes showed the typical asymmetrical structure consisting of a skin layer as the selective layer and a thick finger-like sub-layer. The low viscosity of the doping solution enhanced the phase separation, and instantaneous de-mixing occurs, leading to a porous skin-layer and finger-like pores in the sublayer [32]. As shown in Fig. 2(a′), large slanting macro-voids and a porous top layer were observed. However, the formation of macro-voids in the membranes was suppressed for APT-7. The increase in the APT amount leading to a high viscosity of the polymer solution could affect the exchange between DMAC and water during the preparation process and subsequently affect the morphology, which was consistent with the viscosity of the casting solution shown in Table 1. The inner porous structure of the APT-3 sample is shown in Fig. 2(f). The rod-like APT was evenly dispersed in the polymer matrix, resulting in no obvious agglomeration in the hybrid membrane, which was apparently beneficial to the enhancement of the membrane properties [22].

APT-5 APT-3 APT-1 APT-0

10

20

30

40

50

60

70

80

90

2θ (°) Fig. 4. XRD patterns of neat PVDF membrane and nanocomposites with different APT concentrations.

to the rich \OH on the surface of the unmodified APT as much more enforced filler was introduced in the matrix [36]. The crystallization degrees of PVDF/APT nanocomposites were investigated by X-ray diffraction (XRD), and the results are shown in Fig. 4. It can be observed that the diffraction peak (2θ = 8.4°) remained unchanged in the pure PVDF samples, of which the basal spacing (110) was approximately 10.4 nm [37]. The peak positions (2θ) at 19.7°, 34.5° and 42.3° in pure APT were consistent with the conclusion in the previous work [38]. It was clear that the intensity of the peak at 2θ = 8.4° generally increased with increasing APT content, suggesting that the nucleating effect of the APT nanorods played a major role in crystallinity. However, in the APT-5 sample with high clay content, the crystallinity showed a downward trend but was still better than the neat PVDF sample. This decrease may result from the confinement effect of the APT network, which will prevent the motion of the polymer chains from fully incorporating into the growing crystalline lamella [22]. However, other new peaks were not observed in the nanocomposites after blending with APT, and this behavior can be explained by the fact that polymer chains cannot intercalate into the APT layers [23]. As shown in Fig. 5, the PVDF/APT hybrid membranes exhibited a higher thermal decomposition temperature Td (defined as the temperature at 3% weight loss) compared with those of the PVDF membranes with no APT, which revealed that the introduction of APT to the polymer

100

(e) APT-7

1662

(d) APT-5

1662

(c) APT-3

1662

(b) APT-1

1666

(a) APT-0 3416 3020 2978

1670

Transmitance

3477 3428 3440

80 987

1406 1273 1234 1176

Weight (%)

3493

840

879

60

APT-0 APT-1 APT-3 APT-5 APT-7

40

20

0 0

4000

3500

3000

2500

2000

1500

Wavenumbers(cm-1) Fig. 3. FTIR-ATR spectra of PVDF membranes.

1000

100

200

300

400

500

600

700

800

Temperature (°C) Fig. 5. TGA curves of PVDF/APT hybrid membranes: APT-0, Td = 318.14 °C; APT-1, Td = 338.62 °C; APT-3, Td = 359.01 °C; APT-5, Td = 355.87 °C and APT-7, Td = 360.74 °C.

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Y. Zhang et al. / Desalination 344 (2014) 71–78

100

Table 2 Porosity, pore size and contact angle of PVDF/APT hybrid membranes. Porosity ε (%)

Pore size d (nm)

Contact angel (°)

80.17 75.38 73.24 75.67 75.85

147.86 125.43 99.04 109.85 101.97

74.37 70.32 67.84 65.47 63.01

± ± ± ± ±

0.25 0.68 0.31 0.15 0.57

network resulted in an increase in thermal stability. This improvement in thermal stability could be attributed to the high thermal stability and uniform dispersion of APT fillers as well as interactions between the PVDF chains and the APT surface groups. 3.4. Porosity, pore size and contact angle

3.5. Filtration measurement and fouling analysis 3.5.1. Ultrafiltration test analysis Ultrafiltration experiments were performed to explore the performance of a series of hybrid PVDF/APT membranes. Fig. 6 shows the time-dependent fluxes of pure water and BSA solution for the membranes. The test at 0.1 MPa concluded three phases, the first phase proceeds with deionized water, the second one with BSA solution and the last one was backwashed membrane with deionized water. All of the modified membranes with higher additions exhibited a higher 500

APT-0 APT-1 APT-3 APT-5 APT-7

Flux (L/(m2 .h)

300

80

70

60

The porosity and pore size information for the hybrid membranes are listed in Table 2. The porosity and pore size first decreased to 73.24% and 3.65 nm, respectively, at an APT dosage of 3 wt.%, and subsequently increased slightly. The general tendency to generate membranes with a lower porosity and a higher number of pores of smaller size than those of unmodified membranes was possibly attributed to the increase in stress between the polymer and APT as a result of organic shrinkage that occurred during the precipitation process of wet-casting polymeric membranes. Another plausible reason was pore blocking due to the precipitation of APT on the pore wall, which may further reduce the pore size. Generally, the hydrophilicity is characterized by the water contact angle [39], and a higher hydrophilicity indicates a smaller contact angle [40]. Table 2 indicates that the contact angle of the UF membrane decreased from 74.37° to 63.01° by blending with APT, and therefore its hydrophilicity was clearly enhanced. This result was due to the presence of partially unmodified APT additions that generated a higher hydrophilicity surface with hydroxyl groups.

400

BSA Rejection FRR

90

(%)

Membrane no. APT-0 APT-1 APT-3 APT-5 APT-7

200

100

APT-0

APT-1

APT-3

APT-5

APT-7

Fig. 7. BSA rejection and flux recovery ratio (FRR) for different membranes.

water flux than that of the pure PVDF membrane. In the first phase, water flux of the APT-7 sample attained a maximum value of 401.16 L/(m2·h), while the value was just 318.25 L/(m2·h) for the pure PVDF membrane. The flux variation can be explained as follows. Incorporation of mineral APT enhanced the hydrophilicity of the membranes from 74.37° to 63.01° (Table 2), which improved the water permeation to a certain extent. The second phase showed that the fluxes of BSA solution were lower than those of pure water and dropped significantly at the same time. Similarly, the highest protein flux was 188.81 L/(m2·h) for the APT-7 membrane and the minimum protein flux was 94.44 L/(m2·h). During the filtration process, certain protein molecules can be deposited on the membrane surface, which caused an abrupt drop in flux. Additionally, protein molecules can be swept from the surface due to stirring, which resulted in an equilibrium between deposition and sweeping [41]. The third phase exhibited the results of water flux of membranes backwashed with deionized water. It was clear that the values were larger than those of process with BSA solution and smaller than those of the first phase. However, the trend of water flux was similar, with 224.5 L/(m2·h) for APT-0 sample and 8301.38 L/(m2·h) for APT-7 sample, which proved that even with backwashing management membranes had been polluted with BSA solute and a certain of substance adhered to the pores in the membranes under the pressure of 0.1 MPa.

3.5.2. Antifouling analysis The fouling resistance of a membrane plays a key role in practical application. In this work, the BSA rejection (R) and flux recovery ratio (FRR) were used to evaluate the antifouling properties. It was clear that the rejection was enhanced due to the increase of APT. Fig. 7 shows that the highest rejection was 88.68% as the additive loading rose to 7%, an increase of 12.43% compared with the 78.87% from the pure PVDF membrane. This behavior was consistent with the improvement of membrane hydrophilicity. This result can be explained by introduction of APT, which could induce the formation of smaller pores and was advantageous to BSA rejection [42]. At the same time, the hydrophilic surface of the membranes hindered protein adsorption because a large amount of water could bind to the surface and the inner walls of the membranes and thus enhance protein adsorption [43] Moreover, the FRR increased slightly from 70.57% to 75.13% (Fig. 7). The reason for Table 3 Effect of different times on the mass ratio of Si/F for APT-3 and APT-7 membranes.

0 0

20

40

60

80

100

t (min) Fig. 6. Water flux recovery of various membranes at 0.1 MPa.

120

Operating time

0h

12 h

24 h

48 h

Mass ratio of Si/F for APT-3 Mass ratio of Si/F for APT-7

0.00449 0.0116

0.00446 0.0109

0.00445 0.0107

0.00445 0.0107

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this increase was that the trapped protein in the pores cannot be easily washed out. Additionally, the higher FRR values proved the efficiency of the embedded APT particles. 3.5.3. Stability of APT in membranes XPS was introduced to investigate the stability of nano-APT on the surface of the hybrid membranes. Table 3 clearly shows the results of XPS measurements. It can be seen that the mass ratio of Si to F of APT-3 was 0.00449, while that of APT-7 was 0.0116. When the addition dosage was lower, APT particles played as the crystal nuclei in the crystallization process. Besides, the PVDF molecular chains rearranged around them, leading to most particle entrapment in the matrix. In contrast, higher contents of APT refrained the crystallization process of PVDF when the dosage of APT was higher, which led to agglomeration of APT on the surface of hybrid membranes and high Si/F ratio obtained [44]. Table 3 showed a little APT loss in both of the samples during water treatment. The value of Si/F kept in a stable level after 24 h operation for APT-3 sample and 48 h operation for APT-7 sample, and the reduction were about 0.89% and 7.76% Si loss respectively (from 0.00449 to 0.00445, from 0.0116 to 0.0107). Furthermore, more loss occurred during the first 12 h. It can conclude that APT particles could be flushed away from the membrane surface. Most of the APT particles on the surface were stable. Membranes prepared with lower APT dosage had a relatively higher APT stability. 4. Conclusion A novel organic–inorganic PVDF/APT hybrid UF membrane was fabricated using the phase inversion method by dispersing modified APTNH2 in the casting solution, and the effects of APT additive were studied. First, the saline coupling agent APTES achieved a more uniform presence of additives in the polymer matrix than that of the natural APT such that the hybrid membranes exhibited better performance not only in microstructure and hydrophilicity but also in thermal stability, permeation and antifouling properties. The surface and cross-section of the nanocomposite membranes characterized by SEM indicated that greater amount of additives promoted the formation of smallersize pores and finger-like pores. The same trend was observed for the hydrophilicity with a greater number of hydroxyl-group-rich APT particles entrapped in the casting solution. Furthermore, the thermal decomposition temperature attained a maximum value of 318.14 °C at a dosage of 7 wt.%, an increase of 13.39% compared with that of the APT-0 sample. Most importantly, the filtration experiments demonstrated that novel hybrid membranes showed excellent pure water filtration, good protein fouling resistance and high stability of flux recovery and APT additives. All of these results show that APT is a favorable additive for fabrication of ultrafiltration membranes. Acknowledgments This research was financially supported by the National Key Technologies R&D Program (Project #s 2012BAJ25B02, 2012BAJ25B04). References [1] B. Chakrabarty, A. Ghoshal, M. Purkait, Cross-flow ultrafiltration of stable oil-inwater emulsion using polysulfone membranes, Chem. Eng. J. 165 (2010) 447–456. [2] K.J. Howe, M.M. Clark, Fouling of microfiltration and ultrafiltration membranes by natural waters, Environ. Sci. Technol. 36 (2002) 3571–3576. [3] P.H. Wolf, S. Siverns, S. Monti, UF membranes for RO desalination pretreatment, Desalination 182 (2005) 293–300. [4] M. de Souza Araki, C. de Morais Coutinho, L.A. Gonçalves, L.A. Viotto, Solvent permeability in commercial ultrafiltration polymeric membranes and evaluation of the structural and chemical stability towards hexane, Sep. Purif. Technol. 71 (2010) 13–21. [5] J. Lindau, A.-S. Jönsson, Adsorptive fouling of modified and unmodified commercial polymeric ultrafiltration membranes, J. Membr. Sci. 160 (1999) 65–76. [6] L.S. Wu, J.F. Sun, Q.R. Wang, Poly(vinylidene fluoride)/polyethersulfone blend membranes: effects of solvent sort, polyethersulfone and polyvinylpyrrolidone concentration on their properties and morphology, J. Membr. Sci. 285 (2006) 290–298.

77

[7] L.Y. Yu, Z.L. Xu, H.M. Shen, H. Yang, Preparation and characterization of PVDF–SiO2 composite hollow fiber UF membrane by sol–gel method, J. Membr. Sci. 337 (2009) 257–265. [8] B.D. Freeman, Basis of permeability/selectivity tradeoff relations in polymeric gas separation membranes, Macromolecules 32 (1999) 375–380. [9] L.M. Robeson, Correlation of separation factor versus permeability for polymeric membranes, J. Membr. Sci. 62 (1991) 165–185. [10] S.J. Oh, N. Kim, Y.T. Lee, Preparation and characterization of PVDF/TiO2 organic– inorganic composite membranes for fouling resistance improvement, J. Membr. Sci. 345 (2009) 13–20. [11] A. Rahimpour, M. Jahanshahi, B. Rajaeian, M. Rahimnejad, TiO2 entrapped nanocomposite PVDF/SPES membranes: preparation, characterization, antifouling and antibacterial properties, Desalination 278 (2011) 343–353. [12] F. Liu, M. Abed, K. Li, Preparation and characterization of poly(vinylidene fluoride)(PVDF) based ultrafiltration membranes using nano γ-Al2O3, J. Membr. Sci. 366 (2011) 97–103. [13] G. Arthanareeswaran, T. Sriyamuna Devi, M. Raajenthiren, Effect of silica particles on cellulose acetate blend ultrafiltration membranes: part I, Sep. Purif. Technol. 64 (2008) 38–47. [14] S. Liang, K. Xiao, Y.H. Mo, X. Huang, A novel ZnO nanoparticle blended polyvinylidene fluoride membrane for anti-irreversible fouling, J. Membr. Sci. 394 (2012) 184–192. [15] R. Molinari, T. Poerio, Preparation, characterisation and testing of catalytic polymeric membranes in the oxidation of benzene to phenol, Appl. Catal. A 358 (2009) 119–128. [16] P. Jian, H. Yahui, W. Yang, L. Linlin, Preparation of polysulfone–Fe3O4 composite ultrafiltration membrane and its behavior in magnetic field, J. Membr. Sci. 284 (2006) 9–16. [17] A. Bottino, G. Capannelli, A. Comite, Preparation and characterization of novel porous PVDF-ZrO2 composite membranes, Desalination 146 (2002) 35–40. [18] C.E.L. Trigo, A.O. Porto, G.M. de Lima, Characterization of CdS nanoparticles in solutions of P(TFE-co-PVDF-co-Prop)/N, N-dimethylformamide, Eur. Polym. 40 (2004) 2465–2469. [19] P. Liu, Polymer modified clay minerals: a review, Appl. Clay Sci. 38 (2007) 64–76. [20] Q.H. Fan, X.L. Tan, J.X. Li, X.K. Wang, W.S. Wu, G. Montavon, Sorption of Eu(III) on attapulgite studied by batch, XPS, and EXAFS techniques, Environ. Sci. Technol. 43 (2009) 5776–5782. [21] A. Li, J.P. Zhang, A.Q. Wang, Preparation and slow-release property of a poly(acrylic acid)/attapulgite/sodium humate superabsorbent composite, J. Appl. Polym. Sci. 103 (2007) 37–45. [22] H. Yin, H.F. Chen, D.J. Chen, Morphology and mechanical properties of polyacrylonitrile/attapulgite nanocomposite, J. Mater. Sci. 45 (2010) 2372–2380. [23] D.J. Huang, W.B. Wang, J.X. Xu, A.Q. Wang, Mechanical and water resistance properties of chitosan/poly(vinyl alcohol) films reinforced with attapulgite dispersed by high-pressure homogenization, Chem. Eng. J. 210 (2012) 166–172. [24] A. Li, Y.Z. Pan, X.W. Shen, H.B. Lu, Y.L. Yang, Rod-like attapulgite/polyimide nanocomposites with simultaneously improved strength, toughness, thermal stability and related mechanisms, J. Mater. Chem. 18 (2008) 4928–4941. [25] J.X. Xu, A.Q. Wang, Electrokinetic and colloidal properties of homogenized and unhomogenized palygorskite in the presence of electrolytes, J. Chem. Eng. Data 57 (2012) 1586–1593. [26] Z.J. Du, J.F. Rong, W. Zhang, Z.H. Jing, H.Q. Li, Polyethylene/palygorskite nanocomposites with macromolecular comb structure via in situ polymerization, J. Mater. Sci. 38 (2003) 4863–4868. [27] S.Q. Xue, M. Reinholdt, T.J. Pinnavaia, Palygorskite as an epoxy polymer reinforcement agent, Polymer 47 (2006) 3344–3350. [28] J. Veerman, M. Saakes, S. Metz, G. Harmsen, Reverse electrodialysis: performance of a stack with 50 cells on the mixing of sea and river water, J. Membr. Sci. 327 (2009) 136–144. [29] R.G. Wang, Z. Li, Y.M. Wang, W.B. Liu, L.B. Deng, W.C. Jiao, F. Yang, Effects of modified attapulgite on the properties of attapulgite/epoxy nanocomposites, Polym. Compos. 34 (2013) 22–31. [30] G.P. Wu, S.Y. Gan, L.Z. Cui, Y.Y. Xu, Preparation and characterization of PES/TiO2 composite membranes, Appl. Surf. Sci. 254 (2008) 7080–7086. [31] T. Banerjee, S. Mitra, A. Kumar Singh, R. Kumar Sharma, A. Maitra, Preparation, characterization and biodistribution of ultrafine chitosan nanoparticles, Int. J. Pharm. 243 (2002) 93–105. [32] M.J. Han, S.T. Nam, Thermodynamic and rheological variation in polysulfone solution by PVP and its effect in the preparation of phase inversion membrane, J. Membr. Sci. 202 (2002) 55–61. [33] M. Masuelli, M. Grasselli, J. Marchese, N. Ochoa, Preparation, structural and functional characterization of modified porous PVDF membranes by γ-irradiation, J. Membr. Sci. 389 (2012) 91–98. [34] M. Nasir, H. Matsumoto, M. Minagawa, A. Tanioka, T. Danno, H. Horibe, Preparation of porous PVDF nanofiber from PVDF/PVP blend by electrospray deposition, Polym. J. 39 (2007) 1060–1064. [35] X.H. Dai, J. Xu, X.L. Guo, Y.L. Lu, D.Y. Shen, N. Zhao, X.D. Luo, X.L. Zhang, Study on structure and orientation action of polyurethane nanocomposites, Macromolecules 37 (2004) 5615–5623. [36] V. Panwar, K. Cha, J.-O. Park, S. Park, High actuation response of PVDF/PVP/PSSA based ionic polymer metal composites actuator, Sensors Actuators B 161 (2012) 460–470. [37] X.P. Yuan, C.C. Li, G.H. Guan, X.Q. Liu, Y.N. Xiao, D. Zhang, Synthesis and characterization of poly(ethylene terephthalate)/attapulgite nanocomposites, J. Appl. Polym. Sci. 103 (2007) 1279–1286. [38] J. Xu, Z.H. Sun, L. Jia, B. Li, L. Zhao, X. Liu, Y.F. Ma, H. Tian, Q. Wang, W.S. Liu, Y. Tang, Visible light sensitized attapulgite-based lanthanide composites: microstructure, photophysical behaviour and biological application, Dalton Trans. 40 (2011) 12909–12916.

78

Y. Zhang et al. / Desalination 344 (2014) 71–78

[39] L. Palacio, J. Calvo, P. Pradanos, A. Hernandez, P. Väisänen, M. Nyström, Contact angles and external protein adsorption onto UF membranes, J. Membr. Sci. 152 (1999) 189–201. [40] Y.N. Yang, H.X. Zhang, P. Wang, Q.Z. Zheng, J. Li, The influence of nano-sized TiO2 fillers on the morphologies and properties of PSF UF membrane, J. Membr. Sci. 288 (2007) 231–238. [41] M.P. Sun, Y.L. Su, C.X. Mu, Z.Y. Jiang, Improved antifouling property of PES ultrafiltration membranes using additive of silica-PVP nanocomposite, Ind. Eng. Chem. Res. 49 (2009) 790–796.

[42] F. Zhang, W.B. Zhang, Y. Yu, B. Deng, J.Y. Li, J. Jin, Sol–gel preparation of PAA-g-PVDF/ TiO2 nanocomposite hollow fiber membranes with extremely high water flux and improved antifouling property, J. Membr. Sci. 432 (2013) 25–32. [43] M. Hashino, K. Hirami, T. Ishigami, Y. Ohmukai, T. Maruyama, N. Kubota, H. Matsuyama, Effect of kinds of membrane materials on membrane fouling with BSA, J. Membr. Sci. 384 (2011) 157–165. [44] F.M. Shi, Y.X. Ma, J. Ma, P.P. Wang, W.X. Sun, Preparation and characterization of PVDF/TiO2 hybrid membranes with different dosage of nano-TiO2, J. Membr. Sci. 389 (2012) 522–531