Polyaniline Nanocomposite Membranes: Effect of

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Polyethersulfone/polyaniline nanocomposite membranes: Effect of nanofiber size on membrane morphology and properties song zhao, Zhi Wang, Jixiao Wang, and Shichang Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 20 Jun 2014 Downloaded from http://pubs.acs.org on June 20, 2014

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Industrial & Engineering Chemistry Research

Polyethersulfone/polyaniline nanocomposite membranes: Effect of nanofiber size on membrane morphology and properties

Song Zhao, Zhi Wang*, Jixiao Wang and Shichang Wang Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China

Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin 300072, PR China

State Key Laboratory of Chemical Engineering (Tianjin University), Synergetic Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, PR China

1

ACS Paragon Plus Environment


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ABSTRACT: Polyaniline (PANI) nanoparticles of different sizes were synthesized and

embedded

into

membranes

to

prepare

polyethersulfone

(PES)/PANI

nanocomposite ultrafiltration membranes. The size and dispersion of PANI nanoparticles were determined using transmission electron microscopy and the dynamic light scattering method. The effects of PANI nanoparticle size and content on membrane characteristics and performance were investigated using fourier transform infrared spectroscopy, scanning electron microscope, water contact angle measurement and cross-flow filtration experiments. Overall, PANI nanoparticles that were smaller in size had better dispersion in the casting solution and more uniform distribution in the blended membrane. The nanoparticle size had a significant influence on the modification efficiency of membrane surface pore structure and permeability. The antifouling performance of the membranes was investigated by BSA solution filtration. To evaluate membrane fouling, the filtration resistances were analyzed using the resistance-in-series model.

KEYWORDS: Polyethersulfone, Polyaniline nanoparticles, Particle size, Permeability, Membrane fouling

2


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INTRODUCTION Ultrafiltration (UF), a promising membrane separation process, has a wide range of applications, including wastewater treatment and separation in the food, dairy, textile and chemical industries 1. The key component behind the performance of UF is the membrane itself. As the demand for UF is growing, the efforts to improve UF membrane performance have been gaining importance. Polyethersulfone (PES), one of the most important polymeric membrane materials available, has been commonly used due to its outstanding thermal and hydrolytic stability and good mechanical properties 2, 3. PES UF membrane is typically prepared using phase inversion. Membrane performance can be enhanced via adjustments to the casting solution, such as through changes in concentration, solvents or additives. Although PES and PES-based UF membranes are commercially produced, they still have disadvantages and suffer serious fouling during use, thus restricting their application. Three approaches have typically been applied to modify a PES UF membrane, bulk modification of the PES material, surface modification of prepared PES membrane and modification by physical blending of additional agents into the membrane matrix

2, 4

. Blending is considered to be the simplest and most effective

method of modification due to its positive effects on both flux enhancement and fouling reduction. Water-soluble polymers, amphiphilic copolymers, polymeric nanoparticles and inorganic nanoparticles have been used in blending to modify UF membranes. Among these additives, polymeric membranes modified using nanoparticles have received 3



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much attention in recent years. Many types of inorganic nanoparticles, including multi-walled carbon nanotubes (MWCNs), nano-TiO2, nano-silica and nano-silver, have been used to modify polymeric UF membranes

5-18

. It has previously been

demonstrated that the addition of nanoparticles may increase membrane permeability and resistance to fouling. In a study by Akar et al. 6, PES based UF membranes were prepared using Se or Cu nanoparticles via the phase inversion method, resulting in nanocomposite membranes that had good antifouling properties for both protein separation and activated sludge filtration studies when compared with unmodified PES membranes. Celik et al.

8, 9

prepared PES/MWCNs nanocomposite membranes,

and used bovine serum albumin and ovalbumin as model proteins to assess the protein fouling behavior of these membranes. The authors found a reduction in the irreversible fouling ratio and an increase in the flux recovery ratio owing to the introduction of MWCNs into the membranes. Furthermore, Kim and Bruggen et al.

19

reviewed the use of nanoparticles in

polymeric membrane synthesis and discussed the role of engineered nanomaterials in pressure driven membrane technology for water treatment. They therefore concluded that the physicochemical aspects of nanoparticles, such as dispersion, particle size, hydrophilicity and surface charge, had a significant influence on the performance of nanocomposite membranes. Razmjou et al.

12, 13

studied the effects of mechanical

and chemical modification of TiO2 nanoparticles on their dispersion and performance in modified membranes. Here, the authors revealed that the problem in dispersion of these nanoparticles had been solved and membrane flux was enhanced. Aside from 4


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the modification of nanoparticles, membrane structure and performance could also be regulated by changing the size of the nanoparticle. In a study by Vatanpour et al. 14, three types of TiO2 nanoparticles of various sizes were applied in the preparation of mixed matrix membranes. They determined that smaller nanoparticles enhanced membrane flux and reduced membrane fouling more effectively due to their higher surface area and water adsorption capacity. Mollahosseini et al.

20

synthesized

nanocomposite UF membranes by blending silver nanoparticles of different sizes into the casting solutions. The membranes blended with smaller silver nanoparticles had an increase in their hydrophilic surface, flux and antibacterial behavior due to the higher silver release and larger surface to volume ratio. In the study of Zhang et al. 21, the much smaller particle size of silver nanoparticles could facilitate direct contact between the bacterial surface and the nanoparticles and thus enhance their antimicrobial activity. PANI, one of the most useful conducting polymers, has applications in synthesizing and modifying separation membranes, including UF membranes, gas separation membranes and pervaporation membranes

21-32

. In our previous studies, nano-PANI

including nanofibers and nanospheres were used to modify polysulfone UF membranes and enhanced membrane permeability and fouling resistance

27, 31, 32

.

Nano-PANI and its composite have also been used by other researchers to synthesize nanocomposite UF membranes with high permeability and low fouling 21, 28-30. As mentioned above, the shape, size and dispersion of nanoparticles are critical factors in the efficiency of membrane modification. It was reported that 5



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one-dimensional nanostructures (nanotubes, nanofibers and nanorods) are of both relevant theoretical and technological interest 33, 34. Here, TEM images of nanparticles with one-dimensional nanostructures are illustrated in Figure 1. However, to our knowledge, there is a lack in considerable research exploring the effect of the size and dispersion of one-dimensional nanoparticles on the efficiency of UF membrane modification. Thus, this work was carried out to investigate the effect of size and dispersion of one-dimensional nano-PANI on the structure and performance of UF membranes. Nano-PANI of different sizes and dispersion were synthesized and used to prepare PES/PANI nanocomposite membranes. The membrane properties were then examined, including dynamic hydrophilicity, morphology, pure water flux and resistance to fouling.

EXPERIMENTAL SECTION

Materials. Polyethersulfone (PES, commercial Radel®) was obtained from Solvay Advanced Polymers and used as membrane material. Aniline was purchased from Kewei Chemical Reagent Co. Ltd. (Tianjin, China) and purified by distillation under reduced pressure before use. BSA (molecular weight: 67 kDa) was electrophoresis pure and purchased from Zhengjiang High-technology Co. (Tianjin, China). PVP with different molecular weights were supplied by Aladdin Reagent Co. (Shanghai, China). Hydrochloric acid (HCl), ammonium peroxydisulfate (APS), 1-methyl-2-pyrrolidone (NMP) and acetone purchased from Kewei Chemical Reagent Co. Ltd. (Tianjin, China) were used without further purification. Pure water having a conductivity of 6


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less than 15 µs/cm was produced by a reverse osmosis system. Synthesis and characterization of nano-PANI. PANI nanoparticles were synthesized using the dispersion polymerization method using PVP of different molecular weights for steric stabilizer

32, 35

. After 12h, the polymerization was

considered complete. PANI nanoparticles were then isolated by precipitating the dispersion with an excess of acetone, washing with water and acetone repeatedly, and then drying at 60◦C for approximately 12h. PANI nanoparticles of different sizes were marked as PANI-I, PANI-II and PANI-III. The nanoparticle sizes were measured using transmission electron microscopy (TEM, JEM-100 CX, JEOL). Drops of the dispersion were placed on copper grids and then dried for TEM analysis. The diameter and length of nano-PANI were determined by analyzing TEM images using ImageJ software (1.38×, National Institute of Health). Dynamic Light Scattering (DLS) is a technique able to detect such small particles in dispersion in a fast, routine manner with little sample preparation

36

. Thus,

dispersability and particle size distribution of nano-PANI were also determined based on DLS method using a MasterSizer Laser Diffraction Particle Size Analyzer (Nano ZS, Malvern Instr., UK). Preparation of the PES/PANI nanocomposite membrane. Both flat sheet asymmetric PES and PES/PANI nanocomposite membranes were prepared using phase inversion induced by immersion precipitation. Prior to making the membrane, a casting solution was prepared containing 15 wt% PES, nano-PANI and NMP. Precise 7



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amount of nano-PANI was firstly dispersed in NMP by sonicating for 30 min and stirring for about 2 h. Then, PES was dissolved in the above dispersion by sustaining stirring at room temperature for 12 h. The resultant casting solution was left still for at least 12 h to allow a complete release of bubbles. After that, the casting solution was cast onto a glass plate using a stainless-steel knife to get a 200 µm thick film, exposed to the atmosphere for 30 s, and immersed into a coagulation bath of pure water. After it was manufactured, the membrane was stored in distilled water for at least 12 h to remove residual solvent. The prepared membranes were assigned as PES/PANI-X-Y in the following section. X and Y are the type and amount of nano-PANI in the membrane, respectively. For instance, the membrane marked as PES/PANI-I-0.1 refers to the PES/PANI nanocomposite membrane prepared using a casting solution containing 0.1 wt% of PANI-I nanoparticles. Membrane characterization. An ATR-FTIR spectrometer (FTS-6000, Bio-Rad, USA) was used to analyze the chemical composition of the membrane surface. Each ATR-FTIR spectrum was collected at 2 cm-1 resolution over the range from 2000 to 800 cm-1. The membrane sample was dried at 40 ◦C before the analysis. The photographic image of the membrane sample was taken using a digital camera to observe the evenness of membrane top surface. Morphology of the prepared membranes was observed using a scanning electron microscope (SEM, Nova NanoSEM430, FEI, USA). In order to avoid the shrink or collaps of surface pore, the membrane sample for top surface morphology was dehydrated by immersing into ethanol-water mixture for 10 seconds and then dried at room temperature. The 8


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membrane sample for cross-sectional morphology observation was fractured in liquid nitrogen and cut into appropriate size. Prior to the SEM analysis, the membrane samples were sputter coated with gold using an Emitech K575-XD argon ion sputterer. The surface pore size and distribution in the membrane was determined using ImageJ software and analytical processing. The hydrophilic character of the membranes was determined by measuring the dynamic contact angle of the membrane surface using a video based optical contact angle measuring instrument (OCA15EC, Dataphysics, Germany). The measurements were carried out using the sessile drop method at room temperature. 2.0 µL of water droplet was carefully placed on the top surface using a syringe, and the dynamic contact angles were recorded using the high speed optimum video analysis system. The change of the contact angles with the drop age was analyzed by SCA 202 software (Dataphysics, Germany). The overall membrane porosity, ε, which is the ratio of pore volume to geometrical volume for the membranes, was calculated using Eq. 1, ε=

( mw − md ) ρ

(1)

AL

where mw is the mass of the membrane sample in wet state and md is the mass of membrane sample in dry state. A, L, and ρ are the sample area, the sample thickness and pure water density, respectively. Pure water flux and protein rejection. The pure water flux of membrane with an effective area of 19.3 cm2 was tested using a cross-flow UF experimental apparatus, which is shown in Figure 2. The values on pressure gauges 6 and 8 are averaged to 9



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represent the transmembrane pressure. In the cross-flow membrane cell, the flow channel dimension (the height between membrane top surface and cell) is about 1.5mm. Through the adjustment of valves 4 and 9, the flow rate is set as 1.0L/min and the average crossflow velocity is calculated as 0.18m/s. Initially, membrane compaction was performed at a transmembrane pressure (TMP) of 0.30 MPa for approximately 30 min. Then, the pure water flux was measured at 0.20 MPa TMP. The pure water flux was calculated using Eq. 2, J W1 =

V A × ∆t

(2)

where Jw1 (L.m-2.h-1) is the pure water flux, V (L) is the volume of permeated water, A (m2) is the effective membrane area and ∆t (h) is the permeation time. Protein rejection of the membrane was tested using 1.0 g/L BSA aqueous solution. The protein concentrations in the feed solution and the permeate solution were measured using a UV-vis spectrophotometer at a wavelength of 280nm. Protein rejection (%R) was calculated using Eq. 3,

%R = (1 −

Cp Cf

) ×100

(3)

where Cp and Cf are the concentration of BSA in the permeate solution and the feed solution, respectively. Membrane fouling analysis. In order to examine membrane fouling, the pure water flux of the membrane (Jw1) was tested at 0.20 MPa TMP until the flux remained stable. Then, a model protein solution composed of a 200 mg/L BSA aqueous solution was fed into the filtration system and the flux (JP) was recorded for 1 h. Following 10


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BSA filtration, the membrane was cleaned with pure water for 10 min and then the water flux of the cleaned membrane (Jw2) was measured again. To evaluate the antifouling property of the membrane, the flux recovery ratio (FRR) was calculated according to Eq. 4.

FRR (%) =

J W2 ×100 J W1

(4)

To describe flux decline, the filtration resistance of the membranes was determined in accordance with the resistance-in-series model

37

. The permeation flux can be

described as a function of transmembrane pressure (∆P), the viscosity of permeates (η) and the total filtration resistance (Rt). When filtering pure water, the resistance against mass transport only involves intrinsic membrane resistance (Rm). During filtration of the feed solution, the reduction of flux can be ascribed to two separate parts: concentration polarization and membrane fouling

38

. Concentration polarization

resistance arises due to solute retention and leads to an accumulation of solutes in a mass transfer boundary layer adjacent to the membrane surface

35, 38

. In our

experiment, the feed solution volume is set as 2.0L and the flow velocity in the membrane cell is about 0.18m/s. Thus, during the test, the osmotic effect and concentration polarization was considered as a minor effect compared with membrane fouling. Membrane fouling (Rf) is a complex phenomenon that depends on the membrane material properties, solute properties and operating parameters. In general, Rf can be classified into reversible resistance (Rr), formed by loose attachment of foulants on the surface of the membrane, and irreversible resistance (Rir), caused by 11



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adsorption of foulants on membrane pore wall or surface

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11, 12, 39

. All of these

membrane resistances that involve the total filtration resistance (Rt) can be calculated as follows: Rm = Rir =

Rr =

∆P

η J W1

(5)

∆P − Rm η JW2

(6)

∆P

− Rm − Rir

(7)

Rt = Rm + R f = Rm + Rir + Rr

(8)

ηJp

RESULTS AND DISCUSSION Characterization of nano-PANI of different sizes. The TEM images of nano-PANI shown in Figure 3 exhibit the morphology, size and dispersion of nano-PANI. Both PANI-I and PANI-II show typical nanofiber morphology, but PANI-I nanofibers are significantly more tangled than PANI-II nanofibers, which may be due to their increased length as compared to PANI-II nanofibers. PANI-III has morphological nanorods and good dispersion. The average diameter and length of the nanoparticles were analyzed using ImageJ software and it was found that nano-PANI had similar diameters but different lengths. When combining the diameter and length, PANI-III is the smallest among these three types of nano-PANI. To further explore the dispersion of nano-PANI, the particle size distribution were measured by DLS method and presented in Figure 4. The average particle size is calculated by the software from the particle size distribution measured. The polydispersity index (PDI) given is a measure of the broadness of the particle size distribution, ranging from 0 for monodisperse particles to 1 for extremely 12


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polydisperse ones

36, 40

. PANI-I dispersion has a PDI of 0.79, indicating the presence

of aggregates in the dispersion of PANI-I. The PDIs of the PANI-II and PANI-III nanoparticles are 0.69 and 0.60, respectively. The average particle sizes measured by the DLS method are 889nm, 490nm and 368nm for PANI-I, PANI-II and PANI-III nanoparticles, respectively. These particle sizes were larger than those measured by TEM analysis due to the swollen state of the nanoparticles when in bulk solution. According to the particle size distribution shown in Figure 4, it can be considered that the

particle

sizes

of

nano-PANI

are

in

the

following

order

of

PANI-I>PANI-II>PANI-III.

Surface chemical composition of the membranes. Figure 5 shows ATR-FTIR spectra of PES membrane and PES/PANI nanocomposite membranes incorporating nano-PANI of different sizes. The absorption bands corresponding to the PES structure are observed at 1580 cm-1 (benzene ring stretching), 1488 cm-1 (C–C bond stretching) and 1244 cm-1 (aromatic ether stretching), respectively

41

.. Due to the

similar strong absorption bands of sulfones and secondary aromatic amines, the characteristic absorption band of PANI is not found in the range of 1120~1600 cm-1 in the spectra of PES/PANI nanocomposite membranes. However, the carbonyl absorption band at 1674 cm-1 appears and results from PVP molecules adsorbed on nano-PANI.

Morphology of the membranes. The macroscopic morphology of the nanocomposite membranes is shown in Figure 6. Physical properties of the membrane surface are investigated by observations of surface evenness and morphology. It was 13



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reported that the agglomeration of nanoparticles had an influence on the surface hydrophilicity by reducing the effective surface of nanoparticles

14

. To some extent,

the surface evenness could reflect the uniformity of nanoparticles distribution, and has coherence to the surface hydrophilicity

14, 42

. It can be seen that the surface of

PES/PANI-I-1.5 membranes was not even, while the surface of the PES/PANI-II-1.5 and PES/PANI-III-1.5 membranes was relatively uniform. The distribution of nanoparticles in a membrane matrix is affected by the dispersion and stability of the nanoparticles in the casting solution. It was observed that PANI-I nanoparticles easily aggregated in the casting solution and deposited on the bottom of the bottle after standing for a number days. While the casting solutions incorporated PANI-II and PANI-III nanoparticles remained stable without any precipitation, even after standing for weeks. Figure 7 shows the microscopic images of the surface morphology and the distribution of different pore sizes in the nanocomposite membranes. Surface SEM images demonstrate that all of the membranes have a fine pore structure with dimensions in the nanometer range. The size of the pores at the top surface of the membrane was measured and analyzed using ImageJ software. It can be seen from the SEM images and pore size distribution curves that the introduction of nanoparticles into the polymeric solution has increased the pore size and number of pores in the membrane top surface. Furthermore, nanoparticles with different sizes affect the microstructure of membrane top surface differently. As can be seen in Figure 7, PES/PANI nanocomposite membranes exhibit slightly larger surface pore sizes and have more surface pores than PES membrane. Moreover, PES/PANI-II-0.5 and 14


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PES/PANI-III-0.5 membranes have slightly larger surface pore sizes than PES/PANI-I-0.5 membrane. The porosity values are 62.1%, 79.6% and 80.5% for PES, PES/PANI-II-1.5 and PES/PANI-III-1.5 membranes, respectively. From these results, we conclude that PANI-III nanoparticles, due to being smallest in size, exert the largest influence on membrane pore structure. During the phase separation process, hydrophilic nanoparticles migrate from the PES matrix towards the non-solvent water to reduce interfacial energy between the casting film and water. As the size of the incorporated nanoparticles decreases, the nanoparticles segregate more easily to the film surface due to the low resistance to migration. Besides, the decrease in particle size results in an increase of the specific surface area of nanoparticles, providing more contact between nanoparticles and the polymer chains in the modified membrane structure

11

. Furthermore, it has been postulated that nanoparticles can act as pore

forming agents due to their hindrance effect in the interphase polymer–solvent during the membrane formation process

11

. In this work, the nanoparticle sizes were much

larger than the pore sizes at the top surface of the blended membranes. In view of this, almost all the nanoparticles were stably embedded in the membranes. Nevertheless, a small portion of the nanoparticles, located near the surface of the casting film, may migrate towards the film/water interface and diffuse into the coagulation bath. This could be viewed as a pore-forming effect. The reduction in nanoparticle size and the improvement of nanoparticle dispersion could enhance this pore-forming effect during membrane formation, thus resulting in an increase in membrane surface pore size, pore number and porosity. 15



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Figure 8 presents the cross-sectional morphology of the membranes. Here, it can be seen that all the membranes exhibit a typical asymmetric structure, consisting of a thin skin layer, a thick finger-like or macrovoid sub-layer, and a sponge-like bottom layer. For the unmodified PES membrane, it can be seen that the finger-like pores under the skin layer are compact. For the PES/PANI nanocomposite membranes, the finger-like pores and macrovoids intermingle with each another, making the entire cross-section more porous. The cross-section morphologies of PES/PANI nanocomposite membranes with different types of nano-PANI were similar and no agglomerates of nano-PANI were observed from the SEM images of the membranes.

Hydrophilicity of nano-PANI and membranes. The surface wettability of the membranes is affected by the addition of nano-PANI, as well as the size and dispersion of the nanoparticles. The contact angle of nano-PANI was firstly measured using the sessile drop method using the following procedure. Droplets of PANI aqueous dispersion were placed on a silicone mat (water contact angle of 92°) to prepare a PANI film. After the dryness, the relatively smooth PANI film was formed and then used to measure water contact angle of nano-PANI. As can be seen in Figure 9(a), PANI-I exhibits a contact angle of 58° while PANI-II and PANI-III show the contact angles of about 42°. During the dispersion polymerization of nano-PANI using PVP as the stabilizer, a certain PVP molecules would adsorb on nano-PANI. As the molecular weight of stabilizer increases, the size of nano-PANI decreases gradually 43. In the present study, PVP with molecular weights of 8k, 24k and 58k was used to prepare PANI-I, PANI-II and PANI-III, respectively. When using stabilizer 16


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with low molecular weight, the stabilizer is easily been removed during the separation procedure, which may be the reason that PANI-I shows a higher contact angle than PANI-II and PANI-III. To evaluate the surface hydrophilicity of the prepared membranes, the dynamic water contact angles of the membranes were evaluated. The time dependence of water contact angles of the membranes is displayed in Figure 9. It can be observed that the water contact angles of the membranes decrease with the drop age. The initial water contact angle of the PES membrane is about 78° and decreases to 59°-70° with the incorporation of nano-PANI. Furthermore, the water contact angle of the PES membranes decreased slightly, while those of the PES/PANI nanocomposite membranes decreased rapidly with the drop age. PES/PANI nanocomposite membranes with different types

of nano-PANI

possess

different

surface

hydrophilicities. It is apparent that PES/PANI-II and PES/PANI-III membranes had a more hydrophilic surface than PES/PANI-I membranes. This may be due to the uneven distribution of PANI-I nanoparticles on PES/PANI-I membrane surface. Table 1 shows the change in water contact angles of the PES/PANI nanocomposite membranes that occurred as the drop ages increased from 0 to 120 s. The final water contact angles of the PES, PES/PANI-II-1.5 and PES/PANI-III-1.5 membranes were approximately 72°, 46° and 48°, respectively. These results indicate that addition of nano-PANI improved not only membrane surface hydrophilicity, but also pore hydrophilicity. The water droplets spread easily and quickly on the surface of PES/PANI nanocomposite membranes. This may be because that the large surface 17



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pore size and porosity of PES/PANI nanocomposite membranes facilitate water diffusion into the membrane pores, and thus improve the wettability of the membranes 44, 45. The phenomenon was in agreement with the results of other studies, which indicated that the modified hydrophilic membrane with higher porosity exhibited a lower contact angle using the analysis by the modified Washburn equation 45

.

Porosity of the membranes. The overall membrane porosity is shown in Table 1. All of PES/PANI nanocomposite membranes possess higher porosity than unmodified PES membrane. Porosity values of PES/PANI nanocomposite membranes increase with increasing incorporated nanoparticle contents. Compared with PES/PANI-II nanocomposite

membranes,

the

porosities

of

PES/PANI-III

nanocomposite

membranes were much higher, which indicats that the porosity of nanocomposite membranes increased as the size of the incorporated nanoparticles decreased. Generally, the membranes with larger surface pore and more macrovoid in sub-layer created large free volumes in the polymeric matrix. Thus, changes in membrane porosity were consistent with the changes in membrane surface and cross-sectional morphologies.

Permeation flux and protein rejection of the membranes. UF experiments were conducted to study the permeability of membranes. As can be seen in Figure 10, all the nanocomposite membranes had a higher pure water flux than the PES membrane. In general, as a consequence of enhancement in surface pore size, porosity and hydrophilicity, the pure water fluxes increased markedly. The water flux of 18


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unmodified PES membranes was about 220 L.m-2.h-1, while the water flux of PES/PANI-III-0.5 membranes increased to almost 800 L.m-2.h-1. For PES/PANI-I membranes, the membrane surface became uneven and BSA rejection decreased to below 96% when the nano-PANI content was above 0.5 wt%. Thus, the fluxes of PES/PANI-I membranes with a high nanoparticle content were not considered. Making a comparison between PES/PANI-II and PES/PANI-III nanocomposite membranes, it was found that the membranes with PANI-III nanoparticles had higher water fluxes than those with PANI-II nanoparticles. This indicates that nanoparticles of a smaller size and better dispersion have more influence on the increase of membrane pure water flux. This might be because these nanoparticles can easily migrate towards the membrane surface and exert a greater impact on membrane surface pore size, porosity and hydrophilicity. Furthermore, the cross-section of membranes with smaller nano-PANI seemed to be looser and smoother, promoting water penetration into the polymer matrix. As shown in Table 2, BSA rejections of all the membranes are above 95%, indicating that there is no defect on the membrane surface. With the addition of nano-PANI, BSA rejection decreases a little from 97.6% to 95.5%, which may be ascribed to the increase of surface pore size shown in Figure 7.

Antifouling performance of the membranes. Flux decline and flux recovery ratio were used to assess the antifouling properties of the membranes. The flux decline behavior of the membranes during BSA filtration is shown in Figure 11. Between the 0-30th min for pure water filtration, the water fluxes tended to be stable after a slight 19



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decrease caused by membrane compaction or collapse. During protein filtration, the fluxes of the BSA solution during the 30th-90th min declined dramatically and were lower than pure water fluxes. The flux decline during protein filtration can be attributed to the cumulative effects of several mechanisms, including adsorption of foulant by the membrane surface, pore blocking and cake filtration

12

. Table 2

presents the BSA fluxes of the membranes. The BSA flux has increased from 91 to 218 L.m-2.h-1 with the additon of nano-PANI. After simple hydraulic cleaning, the fluxes of the membranes recovered to an extent and preserved a steady-state value. An FRR value is commonly used to estimate the antifouling properties of membranes. It can be seen from Table 3 that PES/PANI nanocomposite membranes have much higher FRR values than PES membranes. To evaluate membrane fouling, the filtration resistance of PES and PES/PANI nanocomposite membranes was calculated using Eqs. 5-8. Table 3 lists the intrinsic membrane resistance (Rm), fouling resistance (Rf), including reversible resistance (Rr) and irreversible resistance (Rir), and total resistance (Rt). Generally, Rm is due to factors related to membrane properties, Rr is due to the loose attachment of foulants onto the membrane surface that can be easily removed by hydraulic cleaning, and Rir is due to the adsorption of foulants on the membrane pore wall or surface. PES/PANI-II and PES/PANI-III membranes had much lower Rm values than PES membrane due to an improvement in membrane structure. PES membrane shows similar Rr and Rir through the analysis of the fouling resistance. Following the incorporation of nano-PANI, the Rir of the membrane decreased substantially and the 20


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Rr of the membrane only had a small decrease. Compared with PES membrane, the Rir decreased for the PES/PANI-II-1.5 membrane by 76% and the PES/PANI-III-1.5 membrane by 84%. This indicates that foulants are not easily adsorbed on the pore wall or surface of nanocomposite membrane, which may be due to the positive effect of the enhancement of surface pore size or hydrophilicity. The main fouling resistance of PES/PANI nanocomposite membranes, Rr, resulted from reversible protein adsorption, which could be removed by simple hydraulic cleaning. After comparing the filtration resistances of the PES/PANI-II-1.5 membranes and PES/PANI-III-1.5 membranes, it was found that the size of the nanoparticles had a great influence on the intrinsic membrane resistance and a slight influence on the fouling resistance. We therefore conclude from these results that membrane permeability can be effectively enhanced by using small and well-dispersed nanoparticles. The size and dispersity of incorporated nano-PANI have little correlation with fouling resistance.

CONCLUSION

In this study, PES/PANI nanocomposite membranes were prepared using nano-PANI of different sizes, and the effects of size and dispersity of nano-PANI on the membrane morphology and properties were evaluated. The results show that the size of the nanoparticles, as well as their dispersity, significantly influenced the uniformity of nanocompiste membranes. Upon the addition of nanoparticles, the membrane surface properties increased, including pore size, porosity and hydrophilicity. The filtration experiments showed that membrane permeability and 21



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antifouling property can be effectively enhanced using nano-PANI of small size and well dispersion. Filtration resistances of the membranes, including membrane resistance and fouling resistance, were significantly reduced upon the addition of nano-PANI. Comparing PES/PANI nanocomposite membranes with different nano-PANI, the size and dispersity of incorporated nano-PANI had a great influence on the membrane resistance while little correlation with fouling resistance.

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AUTHOR INFORMATION

Corresponding Author

E-mail address: [email protected] (Z. Wang). Tel.: +86-022-27404533. Fax: +86-022-27404496.

ACKNOWLEDGMENTS

This research was supported by grants from the National Natural Science Foundation of China (No. 21306130), the Major State Basic Research Development Program of China (973 Program, No. 2009CB623405), the Science & Technology Pillar Program of Tianjin (No. 10ZCKFSH01700) and the Program of Introducing Talents of Discipline to Universities (No. B06006).

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Captions of figures and tables

Figures Figure 1. TEM images of nanoparticles with one-dimensional nanostructures, (a) nanotubes and (b) nanofibers. Figure 2. Schematic diagram of cross-flow ultrafiltration experimental apparatus: (1) temperature adjustment system; (2) feed tank; (3) pump; (4) and (9) valve; (5) rotermeter; (6) and (8) pressure gauge; (7) membrane cell; (10) electronic balance; (11) computer. Figure 3. TEM images and the particle size distributions of three types of nano-PANI. Figure 4. The particle size distributions of nano-PANI measured by Malvern Master Sizer. Figure 5. ATR-FTIR spectra of PES membrane and PES/PANI nanocomposite membranes incorporating nano-PANI of different sizes. Figure 6. Pictures of PES/PANI nanocomposite membranes incorporated with 1.5 wt.% nano-PANI. Figure 7. SEM images of the top surface morphology of membranes and the corresponding surface pore size distribution. Figure 8. SEM images of the cross-sectional morphology of the membranes. Figure 9. (a) water contact angles of PANI nanoparticles and (b) the time dependence of water contact angles of the membranes. Figure 10. Pure water flux of the membranes, TMP: 0.20 MPa. Figure 11. Time-dependent fluxes for PES membrane and PES/PANI nanocomposite membranes during the ultrafiltration. The entire process includes three steps: 0-30th min for pure water filtration, 30th-90th min for BSA solution filtration, 90th-110th min for pure water flux after 10 min water cleaning. BSA concentration: 200 mg/L, TMP: 0.20 MPa.

Tables Table 1. Dynamic water contact angle (DWCA) and overall porosity (ε) of PES/PANI nanocomposite membranes. Table 2. The properties of the membranes during BSA filtration. Table 3. Filtration resistances of PES membrane and PES/PANI nanocomposite membranes during BSA filtration.

29



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Figure 1. TEM images of nanoparticles with one-dimensional nanostructures, (a) nanotubes and (b) nanofibers.

Figure 2. Schematic diagram of cross-flow ultrafiltration experimental apparatus: (1) temperature adjustment system; (2) feed tank; (3) pump; (4) and (9) valve; (5) rotermeter; (6) and (8) pressure gauge; (7) membrane cell; (10) electronic balance; (11) computer.

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Figure 3. TEM images and the particle size distributions of three types of nano-PANI.

Figure 4. The particle size distributions of nano-PANI measured by Malvern Master Sizer.

31



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Figure 5. ATR-FTIR spectra of PES membrane and PES/PANI nanocomposite membranes incorporating nano-PANI of different sizes.

Figure 6. Pictures of PES/PANI nanocomposite membranes incorporated with 1.5 wt.% nano-PANI.

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Figure 7. SEM images of the top surface morphology of membranes and the corresponding surface pore size distribution.

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Figure 8. SEM images of the cross-sectional morphology of the membranes.

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Figure 9. (a) water contact angles of PANI nanoparticles and (b) the time dep endence of water contact angles of the membranes.

Figure 10. Pure water flux of the membranes, TMP: 0.20 MPa.

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Figure 11. Time-dependent fluxes for PES membrane and PES/PANI nanocomposite membranes during the ultrafiltration. The entire process includes three steps: 0-30th min for pure water filtration, 30th-90th min for BSA solution filtration, 90th-110th min for pure water flux after 10 min water cleaning. BSA concentration: 200 mg/L, TMP: 0.20 MPa.

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Table 1. Average top surface pore size, dynamic water contact angle (DWCA) and overall porosity (ε) of PES/PANI nanocomposite membranes. Nano-PANI content (0.5wt%) Membrane

o

Avg. pore

DWCA( )

size

(0→120 s)

PES/PANI-I

(nm) 8.1±2.2

(0→120 s) 73.2→57.2

PES/PANI-II

8.8±2.5

PES/PANI-III

8.9±2.3

Nano-PANI content (1.5wt%) ε

Avg. pore

DWCA(o)

ε

size

(0→120 s)

(%)

(%) 66.3

(nm) --

69.9→54.2

--

63.2→39.7

72.8

8.9±1.8

59.7→45.8

78.5

65.2→41.8

79.5

10.0±2.3

60.7→48.1

82.6

*Note: The average top surface pore size of unmodified PES membrane is 8.1±1.9nm. The water contact angle of unmodified PES membrane decreases from 77.6o to 72.1o with increasing the drop ages from 0 to 120 s. The overall porosity of unmodified PES membrane is 62.9%.

Table 2. The properties of the membranes during BSA filtration. Membrane

BSA rejection (%)

BSA flux (L.m-2.h-1)

FRR (%)

PES

97.6±0.6

91.5±2.1

50.8±2.5

PES/PANI-II-1.5

96.3±0.8

181.4±10.5

66.8±1.9

PES/PANI-III-1.5

95.5±0.5

218.8±9.6

65.9±1.1

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Table 3. Filtration resistances of PES membrane and PES/PANI nanocomposite membranes during BSA filtration. Membrane

Rm (×1012) m-1

Rir (×1012) m-1

Rr (×1012) m-1

Rf (×1012) m-1

Rt (×1012) m-1

PES

2.89±0.17

2.80±0.11

2.14±0.11

4.94±0.10

7.83±0.18

PES/PANI-II-1.5

1.26±0.03

0.63±0.02

2.07±0.18

2.70±0.20

3.96±0.23

PES/PANI-III-1.5

0.86±0.01

0.45±0.02

1.97±0.15

2.42±0.13

3.28±0.14

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Polyaniline Nanocomposite Membranes: Effect of

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