Performance improvement of polysulfone ultrafiltration membrane using PANiEB as both pore forming agent and hydrophilic modifier

Journal of Membrane Science 385–386 (2011) 251–262

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Performance improvement of polysulfone ultrafiltration membrane using PANiEB as both pore forming agent and hydrophilic modifier Song Zhao a,b,c , Zhi Wang a,b,c,∗ , Xin Wei a,b,c , Boran Zhao a,b,c , Jixiao Wang a,b,c , Shangbao Yang a,c , Shichang Wang a,c a

Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China State Key Laboratory of Chemical Engineering, Tianjin University, Tianjin 300072, PR China c Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University, Tianjin 300072, PR China b

a r t i c l e

i n f o

Article history: Received 30 July 2011 Received in revised form 30 September 2011 Accepted 1 October 2011 Available online 6 October 2011 Keywords: Polysulfone Emeraldine base polyaniline Ultrafiltration membrane Phase separation Enhanced permeability

a b s t r a c t Emeraldine base polyaniline (PANiEB), which is mostly soluble in N-methyl-2-pyrrolidone (NMP) and slightly soluble in water, was used as the additive to prepare polysulfone (PSf)/PANiEB membrane via immersion precipitation process. The behavior of PANiEB during membrane formation and the effect of PANiEB addition on membrane structure and performance were investigated. During membrane formation, a portion of PANiEB, located near the surface of the casting film, could diffuse out of the casting film along with NMP into the coagulation bath and act as pore forming agent. The other portion of PANiEB could remain in the prepared membrane and act as hydrophilic modifier. All the PSf/PANiEB membranes had higher porosity, larger surface pore size, more vertically interconnected finger-like pores and less macrovoids than PSf membrane. PSf/PANiEB membranes exhibited stable pure water flux during membrane compaction at 0.30 MPa TMP and slower pure water flux decline at 0.5 MPa TMP than PSf/polyvinylpyrrolidone (PVP) membranes. Pure water fluxes of PSf/PANiEB membranes were 1.7–2.8 times that of PSf membrane while rejection property including bovine serum albumin, egg albumin and trypsin rejections changed slightly. BSA ultrafiltration experiment showed that PSf/PANiEB membranes had higher flux and better antifouling property than PSf membrane. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Polysulfone (PSf) ultrafiltration (UF) membrane is widely used in ultrafiltration and support layer of composite membranes due to its excellent heat-aging resistance, chemical stability, mechanical property and wide pH value range [1,2]. However, PSf UF membrane has low permeability and suffers serious membrane fouling, which limits its application and shortens membrane life [3–5]. Generally, low permeability results from small membrane surface pore size, low porosity and surface hydrophobicity. Poor antifouling property is mainly caused by the hydrophobic surface [2,5,6]. In order to improve membrane permeability and antifouling property, many efforts have been devoted to membrane modification, including material modification, polymer blend and surface modification [7]. Among these approaches, polymer blend is considered as an effective and convenient approach for membrane modification due to its excellent modification efficiency and facile

∗ Corresponding author at: Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin University, Weijin Road 92#, Nankai District, Tianjin 300072, PR China. Tel.: +86 22 27404533; fax: +86 22 27404496. E-mail address: [email protected] (Z. Wang). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.10.006

operation. It has been reported that blending of PSf with hydrophilic additive in the casting solution can obtain excellent membrane performance with high permeability and antifouling property [8–18]. Polyvinylpyrrolidone (PVP) [8–10], poly (ethylene glycol) (PEG) [11,12], TiO2 nanoparticles [13,14], carbon nanotubes [15,16] and polyaniline (PANi) nanomaterials [17,18] have been used as the additives during membrane preparation via immersion precipitation process to improve PSf UF membrane performance. Generally, these additives can be classified as water-soluble polymers (such as PVP, PEG) and nanomaterials (such as TiO2 nanoparticles, PANi nanofibers). These two kinds of additives exhibit much different solubility in solvent (such as N-methyl-2-pyrrolidone (NMP)) or nonsolvent (such as water). Water-soluble polymers can be dissolved both in solvent and in nonsolvent while nanomaterials can be dissolved neither in solvent nor in nonsolvent. During membrane formation, phase separation is achieved by the exchange of solvent and nonsolvent across the interface between the casting film and the coagulation bath [12]. Water-soluble polymers are mostly leached out of the casting film during membrane formation and often defined as pore forming agent, resulting in the enhancement of membrane porosity, surface pore size, permeability [8,10–12]. Although a small portion of water-soluble polymers adsorbed on membrane surface is favorable to the improvement of

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membrane hydrophilicity and antifouling property, these residual water-soluble polymers suffer gradual loss during membrane usage [7]. Nanomaterials as the additive could exist in the prepared membranes stably and improve membrane hydrophilicity, permeability, antifouling property and thermal stability [14,17]. However, nanomaterials are difficult to be well dispersed in the casting solution due to the nanomaterial agglomeration and the high viscosity of the casting solution [15–17]. This problem makes the preparation process of nanocomposite membrane complicated, time-consuming and energy-consuming. More than this, the agglomerate of nanomaterials could block membrane pores and decrease membrane permeability [19]. PANi is one of the most promising conducting polymers and has been used to prepare membranes for gas separation, pervaporation, electrodialysis and ultrafiltration due to its simple synthesis, good environmental stability and simple acid–base doping chemistry [17,18,20–25]. The acid–base doping chemistry of PANi is reversible and has been clearly described in Heeger’ paper [26]. Generally, emeraldine salt PANi (PANiES) has better conductivity and hydrophilicity than emeraldine base PANi (PANiEB) due to the protonation of acid. However, PANiES has poor solubility in most of solvents and this restricts its application. In our previous work, PANiES with nanostructures was used as the additive to prepare PSf/PANi nanocomposite membrane [17]. Different with PANiES, highly processable PANiEB can be mostly soluble in NMP and slightly soluble in water [27–29]. Advantages of using PANiEB as the additive to modify PSf membrane are listed as follows. Firstly, due to the good solubility of PANiEB in NMP, it could be economical and convenient to prepare homogeneous casting solution using PANiEB as the additive; Secondly, a portion of PANiEB could be leached out of the casting film following the diffusion of NMP into the coagulation bath and thus it could act as pore forming agent during membrane formation; Thirdly, as water enters into the casting film during membrane formation, the other portion of PANiEB could exist in the prepared membranes stably due to the slight solubility of PANiEB in water, and thus this portion of PANiEB could act as hydrophilic modifier; Finally, PANiEB could evenly distribute in the prepared membrane. As mentioned above, PANiEB could be a feasible and suitable additive to improve membrane structure and performance. However, to the best of our knowledge, no experimental study about using PANiEB to modify PSf UF membrane or other UF membranes has been carried out. In this work, PSf/PANiEB membrane was prepared via immersion precipitation process using PSf as membrane material, PANiEB as the additive, NMP as the solvent and water as the coagulation bath. The objective of this work is to study the behavior of PANiEB during membrane formation and the effect of PANiEB addition on membrane structure and performance. Membrane surface chemical composition, surface hydrophilicity and morphology were characterized by X-ray photoelectron spectroscopy (XPS) analysis, water contact angle measurement and scanning electron microscope (SEM), respectively. Pure water fluxes of the membranes were tested to reflect membrane permeability. Proteins including bovine serum albumin (BSA), egg albumin (EA) and trypsin were used to estimate membrane rejection property. The antifouling property of membranes was evaluated through BSA ultrafiltration experiment. 2. Experimental 2.1. Materials PSf was purchased from Dalian Polysulfone Plastic Limited Co. (Dalian, China) and used as a membrane material. Hydrochloric acid (HCl), ammonia solution, ammonium peroxydisulfate (APS),

NMP and anhydrous ethanol were purchased from Kewei Chemical Reagent Co. Ltd. (Tianjin, China) and used without further purification. Aniline was purchased from Kewei Chemical Reagent Co. Ltd. (Tianjin, China) and distilled under reduced pressure to purify before use. BSA (molecular weight: 67 kDa) was electrophoresis pure and purchased from Zhengjiang High-technology Co. (Tianjin, China). EA (molecular weight: 43 kDa) and trypsin (molecular weight: 23 kDa) were supplied by Aladdin Reagent Co. (Shanghai, China). Deionized pure water having a conductivity of less than 12 ␮s/cm was produced by a reverse osmosis system. 2.2. Synthesis and characterization of PANiEB PANi was synthesized through chemical oxidative polymerization of aniline in aqueous HCl using APS as an oxidant, according to a well-established procedure [30]. After filtration and dryness, PANi was added in 0.5 M aqueous ammonia solution with constant stirring for 4 h. After that, the mixture was filtrated with microfiltration membrane (pore size: 0.22 ␮m), washed several times with deionized water and then dried under a vacuum at 40 ◦ C for 12 h to obtain the blue PANiEB powder [30,31]. PANiEB powder was characterized by SEM (Nova NanoSEM430, FEI, USA). Solubility of PANiEB in NMP or water was determined according to the method described in Xu’s paper [28]. 0.2 g of PANiEB powder was added to 20 mL of NMP or water with ultrasonication for 2 h and stir for 12 h at room temperature. The mixture was then let stand for more than seven days. After that, the supernatant liquid was carefully removed from the mixture and the residual liquid was evaporated to remove solvent (NMP or water). Through weighting the mass of final residual PANiEB, solubilities of PANiEB in NMP and in water could be measured. Fourier transform infrared (FTIR) spectroscopy and Ultraviolet–visible (UV–vis) spectroscopy were used to confirm the chemical composite of PANiEB. FTIR spectrum was recorded between 500 and 2000 cm−1 using a spectrometer (560 ESP, USA). UV–vis analysis was carried out by taking only the soluble portion of PANiEB in NMP or in water as the sample. UV–vis spectrum was recorded between 250 and 1000 nm using an UV–vis spectrophotometer (TU-1810DPC, China). 2.3. Preparation of PSf/PANiEB membrane PSf/PANiEB membranes were prepared via immersion precipitation process [11,32–34]. Taking the casting solution with 0.1 wt% PANiEB content as an example, 0.03 g of PANiEB was added and dissolved into 25.47 g of NMP with constant stirring for 2 h. 4.5 g of PSf was then added into the above solution and fully dissolved after stirring for about 12 h. The casting solution obtained was left still for about 12 h to allow complete release of bubbles. After that, it was cast onto a glass plate using a stainless-steel knife to get a casting film of 200 ␮m thickness, exposed to the atmosphere for 30 s, and then immersed into a coagulation bath of pure water. The membrane preparation process was performed in a constant temperature chamber (GT-TH-S-64G, China), keeping the temperature at 24 ± 1 ◦ C and the humidity at 27 ± 2%. The prepared membranes were kept in pure water for more than 12 h to remove residual solvent before test. For all the casting solutions, the mass content of PSf to total casting solution was 15 wt%, and PANiEB contents were varied from 0 to 1.0 wt%. The prepared membranes with 0, 0.01, 0.05, 0.1, 0.5 and 1.0 wt% PANiEB contents in the casting solutions were designated as M0, M0.01, M0.05, M0.1, M0.5 and M1.0, respectively. The viscosity of the casting solution was investigated using a rotating viscometer (Brookfield LVDV-C, USA) at 25 ◦ C with a rotating rate of 20 rpm.

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2.4. Membrane structure characterization The elemental compositions of membrane top surface were characterized by XPS (PHI-1600, USA) using Mg K␣ as the radiation source. Survey spectra were collected over a range of 0–1000 eV. The top surface and cross-sectional morphologies of the prepared membranes were observed by SEM (Nova NanoSEM430, FEI, USA). The SEM surface images of the membranes were observed at 200,000× magnification. Before SEM analysis, the membrane samples for surface morphology observation were dehydrated through graded ethanol series (50 vol% ETH aqueous solution, 70 vol% ETH aqueous solution, 80 vol% ETH aqueous solution, 90 vol% ETH aqueous solution, 95 vol% ETH aqueous solution, anhydrous ethanol, 5 min each step), and then dried at room temperature. After that, it was cut into appropriate size and sputter-coated with gold. The membrane samples for cross-sectional morphology observation were cut into appropriate size, frozen in liquid nitrogen, fractured and then sputter-coated with gold. Surface pore size and pore size distribution of the membranes were determined by the analysis of SEM surface images using ImageJ 1.38× software (National Institute of Health, http://rsb.info.nih.gov/ij). In order to clearly observe membrane surface pore, threshold images for SEM surface images were obtained with the aid of ImageJ 1.38× software following a procedure described by Masselin et al. [35]. For each sample, SEM surface images were taken from random locations. At least one hundred membrane pores were selected randomly from five SEM surface images to get average surface pore size and pore size distribution. In order to analyze the distribution of PANiEB in the membrane matrix, N element mapping was conducted with scanning microscope equipped with energy dispersive X-ray (EDX) spectrum Meter (EDAX, Genesis XM2 APEX 60SEM). The membrane porosity was determined by the mass loss of wet membrane after drying. The membrane sample being wetted thoroughly was mopped water on the surface and weighed under wet status. Then, the membrane sample was dried until a constant mass was obtained. Porosity ε, i.e., the ratio of pore volume to geometrical volume for the membranes was obtained by Eq. (1): ε=

(mw − md ) AL

(1)

where mw is the mass of wet membrane sample and md is the mass of dry state membrane sample; A, L, and  are the sample area, the sample thickness and pure water density, respectively. In order to evaluate membrane surface hydrophilicity, the static contact angle between water and membrane top surface was measured using a contact angle measurement instrument (OCA15EC, Dataphysics, Germany). Two microlitres of a water droplet was placed onto the membrane surface using a microsyringe, and then the contact angle was analyzed by SCA 202 software (Dataphysics, Germany). At least five water contact angles at different locations on one membrane surface were averaged to get a reliable value.

Fig. 1. 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.

where Jw (L/(m2 h)) 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. The rejection property of the membrane was tested at 0.16 MPa TMP using 1.0 g/L BSA, EA and trypsin aqueous solution, respectively. The protein concentrations in the feed and the permeate solutions were measured using an UV–vis spectrophotometer (TU1810DPC, China), at a wavelength of 280 nm. The protein rejections (%R) were calculated by Eq. (3):



%R =

1 − Cp Cf



× 100

(3)

where Cp and Cf (mg/mL) were BSA, EA or trypsin concentrations in permeate and feed solutions, respectively. The membrane fouling behavior was studied as follows. Firstly, pure water flux of the membrane Jw1 (L/(m2 h)) was tested at 0.16 MPa TMP. Then, 0.8 g/L BSA aqueous solution was fed into the ultrafiltration system. After BSA ultrafiltration for 90 min, the membrane was flushed with pure water for 10 min and then pure water flux of the membrane Jw2 (L/(m2 h)) was measured. The flux recovery ratio (FRR) was calculated using Eq. (4) to evaluate membrane antifouling property: FRR (%) =

Jw2 × 100 Jw1

(4)

3. Results and discussion 3.1. Characterization of PANiEB

2.5. Membrane performance characterization The pure water flux of the membrane with an effective area of 19.3 cm2 was tested using a cross-flow UF experimental apparatus [17]. The schematic diagram of the cross-flow UF experimental apparatus used in this study is show in Fig. 1. Initially, membrane compaction study was carried out at 0.30 MPa transmembrane pressure (TMP) for 1 h. Then, the pure water flux was measured at 0.20 MPa TMP, 23 ± 1 ◦ C and 0.22 m/s cross-flow velocity. The pure water flux was calculated by the following equation: Jw =

V A × t

(2)

Fig. 2 shows SEM image of synthesized PANiEB powder. It reveals that PANiEB produces an agglomerated polymer structure. Fig. 3 shows photographs of the mixtures of PANiEB in NMP and in water after let stand for seven days. It can be observed that there is almost no floating particle and no precipitate in the bottle (a) while there is lots of PANiEB powder settling down at the bottom of the bottle (b). This indicates that PANiEB could be mostly soluble in NMP and slightly soluble in water. Solubility of PANiEB in NMP is measured to be 95.0 ± 4.2 wt% while solubility of PANiEB in water is 2.6 ± 0.9 wt%. Fig. 4 shows FTIR spectrum of PANiEB. The bands at 1593 and 1504 cm−1 are assigned to C–C stretching for quinoid ring and

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Fig. 5. UV–vis spectra of PANiEB in NMP and PANiEB in water.

Fig. 2. The SEM image of synthesized PANiEB powder.

orbital of the benzenoid rings to the lowest unoccupied molecular orbital of the quinoid rings [38,39]. Two peaks at 342 and 674 nm can be found in the spectrum of PANiEB in water. Compared with the peaks in the spectrum of PANiEB in water, the peaks in the spectrum of PANiEB in NMP show slightly blue-shift, which may be due to the interaction between PANiEB and NMP through hydrogen bonding [40]. 3.2. The behavior of PANiEB during membrane formation

Fig. 3. Photographs of the mixtures, bottle (a) PANiEB in NMP and bottle (b) PANiEB in water.

benzenoid ring, respectively. The bands at 1315 and 1238 cm−1 attribute to C–N stretching mode of the benzenoid ring. The bands at 1165 and 833 cm−1 are due to the aromatic C–H bending in the plane and out of the plane aromatic ring. These characteristics are similar to the FTIR spectra of PANiEB reported in the papers [36,37]. Fig. 5 shows UV–vis spectra of PANiEB in NMP and PANiEB in water. The spectrum of PANiEB in NMP exhibits two distinct peaks. The first peak at 330 nm is due to ␲ → ␲* transition associated with the ␲ electrons in the benzene rings, and mainly a function of intra-chain interaction. The second peak at 630 nm is assigned to the excitation of an electron from the highest occupied molecular

Fig. 4. FTIR spectrum of PANiEB.

Water-soluble polymers have been considered as a pore-former during membrane formation. It is assumed that most of watersoluble polymers can be leached out of the casting film and the sites where the polymers exist become micropores in the prepared membrane [10–12]. In this work, the behavior of PANiEB during membrane formation is analyzed according to its solubility in NMP and water. The hypothetical behavior of PANiEB during membrane formation is schematically illustrated in Fig. 6. Due to the miscibility of nonsolvent (water) and solvent (NMP), a diffusion-driven exchange of solvent and nonsolvent formed and induced the casting film to phase separate. After the casting film was immersed into the coagulation bath, PSf immediately coagulated to form a membrane

Fig. 6. Schematic illustration of the behavior of PANiEB during membrane formation.

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Fig. 7. The liquids after the phase separation of the casting films: (a) for M0, (b) for M0.01, (c) for M0.05, (d) for M0.1, (e) for M0.5 and (f) for M1.0; (g) TEM image of PANiEB in the bottle (f); (h) SEM image of PANiEB in the bottle (f).

matrix, and PANiEB migrated spontaneously toward the coagulation bath to reduce interfacial energy between the casting film and water. Since PANiEB has good solubility in NMP, a portion of PANiEB, located near the surface of the casting film and having comparatively high diffusivity, could diffuse out along with NMP into the coagulation bath and perform as pore forming agent during the phase separation. This pore forming effect of PANiEB could enhance membrane surface pore size and porosity (detailed discussion can be found in Section 3.3.2). After this portion of PANiEB entered into water, it could be assembled into nanoparticles due to the slight solubility of PANiEB in water, which were directly observed by TEM (see Fig. 7(g)). Due to the immediate phase separation, the casting film was rapidly solidified and thus the other portion of PANiEB was embedded in the prepared membrane and assembled into nanoparticles with water entering into the casting film, which could be confirmed by the observation of the SEM image of membrane crosssectional morphology (see Fig. 11(f)). This portion of PANiEB could migrate toward the membrane/water interface and enrich near the membrane top surface, which may result in the improvement of vertical finger-like pore interconnection and membrane surface hydrophilicity (detailed discussion could be found in Sections 3.3.2 and 3.3.3). The above-mentioned behavior of PANiEB during membrane formation was further verified by the following experiments. During membrane preparation, it was observed that a portion of blue PANiEB was leached out of the casting film after immersed into the coagulation bath. To confirm this phenomenon, 1.2 mL of casting solution was cast evenly on a petri dish with an effective spread area of 56.7 cm2 . The thickness of casting film was 212 ␮m theoretically, which was similar to the thickness of the casting film (200 ␮m) during membrane preparation (see Section 2.3). 30 mL of pure water was then poured into the petri dish and the phase separation was performed for 3 min. After that, the solidified membrane was taken out and the liquid in the petri dish was well mixed and then analyzed using the UV–vis spectrometer with pure water as the reference. As shown in Fig. 7, the liquids after the phase separation are blue and become deepen gradually with increasing PANiEB content in the casting solution. The morphology of PANiEB in the liquid was examined by a transmission electron microscopy (TEM, JEOL 1000CX-II) and SEM after filtered on a filter membrane. Fig. 7(g) and (h) present that PANiEB exhibits spherical morphology, indicating that PANiEB molecules could assemble into nanoparticles with the diameter of about 150 nm after diffusing into the coagulation bath. Fig. 8 presents UV–vis spectra of the liquids after the phase separation of the casting films for M0.5 and M1.0. Two peaks at 342 and 674 nm are observed in the UV–vis spectra, which might be assigned to the ␲–␲* transition centered on the benzenoid unit and the quinoid exciton band of PANiEB, respectively. These UV–vis spectra are in accordance with the UV–vis spectrum of PANiEB in water (see Fig. 5). The absorbencies of the peak at 674 nm are 0.197

for M0.5 and 0.238 for M1.0. The increasing absorbance of the peak at 674 nm indicated that more PANiEB diffused out of the casting film into the coagulation bath with increasing PANiEB content in the casting solution. Joung et al. [41] quantitatively analyzed the residual amount of PVP (Mw 46k) in the prepared membrane using FTIR analysis and found that about 2% of PVP was left in the prepared membrane, which implied that 98% of PVP was leached out during membrane formation. In this work, the percentages of PANiEB diffusing out and remaining in the prepared membranes were measured through weighing method. The total mass of the casting solution (Mt ) was weighted using an electronic analytical balance. After phase separation, the liquid in the petri dish was filtrated through a filter membrane (surface pore size: about 20 nm, Vontron Technology Co. Ltd.), and the mass of PANiEB diffusing out of the casting film (M1 ) was weighted. Then the content of PANiEB diffusing out (W1 ) to the casting solution was calculated by M1 /Mt × 100. Since the content of PANiEB in the casting solution (WP ) was definite, the percentage of PANiEB diffusing out (P1 ) was calculated by W1 /WP × 100 and the percentage of PANiEB remaining in the prepared membranes (P0 ) was calculated by 1 − W1 /WP × 100. The calculation results are shown in Table 1. It can be seen that the content of PANiEB diffusing out of the casting film (W1 ) increases with increasing PANiEB content in the casting solution, which is consistent with the UV–vis analysis results shown in Fig. 8. It can also be seen from Table 1 that the percentage of PANiEB diffusing out decreases with increasing PANiEB content in the casting solution. 71.9% of PANiEB was leached out during membrane formation for M0.01 while only 6.0% of PANiEB was leached out during membrane formation for M1.0. This might be because the viscosity of the casting solution increases with increasing PANiEB content in the casting solution, which limits the movement of

Fig. 8. UV–vis spectra of the liquids after the phase separation of the casting films.

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Table 1 Quantization analysis of PANiEB remaining in the prepared membrane and diffusing out of the casting film after phase separation, and the viscosities of the casting solutions with different PANiEB contents. PANiEB content in the casting solution (WP ) (wt%)

W1 (wt%) P1 (%) P0 (%) Viscosity of the casting solution (mPa s)

0

0.01

0.05

0.1

0.5

1.0

– – – 282 ± 4

0.00719 71.9 28.1 295 ± 4

0.0284 56.8 43.2 313 ± 4

0.0448 44.8 55.2 346 ± 3

0.0505 10.1 83.9 390 ± 6

0.06 6.0 94.0 442 ± 8

Note: W1 : The content of PANiEB diffusing out of the casting film during phase separation to the casting solution (wt%). P0 : The percentage of PANiEB remaining in the prepared membrane (%). P1 : The percentage of PANiEB diffusing out of the casting film (%).

membranes was bluer than the bottom surface, which indicated that PANiEB migrated to top surface during membrane formation and more PANiEB congregated near the top surface instead of the bottom surface. The enrichment of hydrophilic component near the membrane top surface was also reported by Fan et al. [17].

Fig. 9. XPS spectra of the membranes: (a) M0, (b) M0.1 and (c) M0.5.

PANiEB in the casting film and decreases the percentage of PANiEB diffusing out during membrane formation. 3.3. Membrane structure 3.3.1. Membrane surface chemical composition Membranes surface chemical composition was characterized by XPS analysis. Fig. 9 shows XPS spectra of the membranes and Table 2 shows the element contents of membrane surface. It can be seen that the addition of PANiEB obviously increases the N atomic percent on membrane surface. With increasing PANiEB content from 0.1 to 0.5 wt%, the N atomic percent on PSf/PANiEB membrane surface increases from 2.2 to 2.9 at.%, indicating that more PANiEB exists near the membrane surface. Due to the lack of O and S atoms in PANiEB molecule, the O and S atomic percents on the PSf/PANiEB membrane surface decrease with increasing PANiEB content in the casting solution. During membrane formation, PSf immediately coagulated to form a membrane matrix, and the hydrophilic additive would spontaneously migrate and concentrate at the membrane/water interface in order to minimize the interfacial energy [17,42,43]. In our experiment, it was observed that the top surface of PSf/PANiEB

Table 3 Average surface pore size and porosity of the membranes with different PANiEB contents.

Table 2 XPS analysis results of the membranes. Membranes

M0 M0.1 M0.5

3.3.2. Membrane surface and cross-sectional morphology Membrane surface and cross-sectional morphologies were observed by SEM. As presented in Fig. 10, all the PSf/PANiEB membranes have larger surface pore size and higher porosity than PSf membrane. Quantitatively calculation results of average surface pore sizes are shown in Table 3. It can be seen that PSf/PANiEB membranes have average surface pore sizes in the range of 7.5–9.3 nm, which increase by 15–43% compared with that of PSf membrane. It can also be seen from Table 3 that with increasing PANiEB content, membrane porosity increases firstly and then decreases. Fig. 11 shows surface pore diameter distribution for each membrane. It can be seen that the surface pore diameter distributions of all the membranes are concentrated in the range of 5–11 nm. The surface pore diameter distributions of PSf/PANiEB membranes are clearly shifted toward larger pore diameter values compared with that of PSf membrane. As shown in Fig. 12, all of the membranes exhibit typical asymmetrical structure of ultrafiltration membrane including a dense top layer and a porous sublayer. PSf membrane has obvious macrovoids in the sublayer. The cross-sectional morphologies of PSf/PANiEB membranes display that macrovoids are suppressed and the finger-like pores become run through the cross-sectional structure, especially for M0.05, M0.1, M0.5 and M1.0. This indicates that the addition of PANiEB leads to the formation of longer fingerlike pores and less macrovoids in the sublayer. Besides, it can also be found from Fig. 12(f) that well-dispersed PANiEB nanoparticles absorb on the membrane pore wall without any large agglomerate, which is in conformity with the illustration of membrane formation shown in Section 3.2. The changes in membrane morphologies induced by the addition of PANiEB could be interpreted from membrane formation mechanism during the phase separation. The addition of PANiEB reduced the miscibility of the casting solution with water, causing the acceleration of the phase separation. When the casting film came into contact with the nonsolvent in the coagulation bath,

Atomic percent (at.%) C

O

S

N

80.3 79.9 79.9

16.9 14.7 14.8

2.8 2.7 2.4

– 2.7 2.9

Membranes

Average surface pore size (nm)

Porosity (%)

M0 M0.01 M0.05 M0.1 M0.5 M1.0

6.5 7.5 8.2 9.3 8.5 8.3

69.9 77.2 83.5 80.9 78.9 74.8

± ± ± ± ± ±

2.0 2.0 3.8 1.9 4.2 1.9

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Fig. 10. Surface morphologies of the membranes with different PANiEB contents: (a) M0, (b) M0.01, (c) M0.05, (d) M0.1, (e) M0.5 and (f) M1.0; threshold images (the upper right corner) was obtained through the analysis of SEM surface images (the dotted box) using ImageJ 1.38× software.

there was a rapid outflow of the solvent from the casting film to the coagulation bath, inducing the diffusion behavior of soluble additive into the coagulation bath. From the analysis in Section 3.2, a portion of PANiEB was leached out of the casting film along with NMP and thus acted as pore forming agent during membrane formation, resulting in the increase of membrane porosity and surface pore size. However, when PANiEB content was above 0.1 wt%, the high viscosity of the casting solution reduced the

percentage of PANiEB diffusing out of the casting film and consequently weakened the pore-forming effect of PANiEB. Moreover, the high viscosity of the casting solutions hindered the formation and development of membrane pore and caused the decrease of membrane porosity and surface pore size. Macrovoids are normally found in the membranes formed by immersion precipitation process [9]. The formation mechanisms of macrovoids were proposed by Kim and Lee [11] and Young and

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Fig. 11. Surface pore diameter distribution of the membranes with different PANiEB contents: (a) M0, (b) M0.01, (c) M0.05, (d) M0.1, (e) M0.5 and (f) M1.0.

Chen [44], that is, it was necessary for forming macrovoids that had a dense skinlayer to limit the large amount of nonsolvent into sublayer to induce many nuclei formations. When membrane surface pore size became larger and the toplayer became more porous, the macrovoids would become less sharp and the distance from the top surface to the starting point of the macrovoids formation would increase [11]. The dense surface of PSf membrane restricted nonsolvent diffusing through the skinlayer and thus produced little nuclei in the sublayer, which can fully develop into macrovoids. Since PSf/PANiEB membranes had larger surface pore size and higher porosity than PSf membrane, the porous surface of PSf/PANiEB membranes was favorable for the large amount of nonsolvent diffusing into the sublayer, and then many nuclei formed, which mutually limited their growth, resulting in the inhibition of macrovoids. Thus, PSf/PANiEB membranes had longer finger-like pores and less macrovoids than PSf membrane. Due to the migration of PANiEB during membrane formation, the finger-like pores of PSf/PANiEB membranes also had good interconnection and could run through the cross-sectional structure [17]. The PANiEB distribution in the cross-section of PSf/PANiEB membrane was detected by employing EDX mapping. Fig. 13(a) and (b) shows the cross-sectional SEM image of PSf/PANiEB membrane (M0.5) and its corresponding EDX mapping for N element. The N element is highlighted as the red spot. The EDX mapping image presents that N elements were well distributed in the membrane cross-section, which indicated that PANiEB nanoparticles were homogeneously distributed in the membrane matrix. 3.3.3. Membrane surface hydrophilicity As shown in Fig. 14, water contact angle of PSf membrane surface is 80.8◦ while water contact angles of PSf/PANiEB membranes are in the rang of 68.1–70.0◦ . This indicated that the addition of PANiEB enhanced membrane surface hydrophilicity, which might be attributed to the hydrophilic group of PANiEB (–NH–) and the existence of PANiEB in the prepared membranes.

3.4. Membrane performance 3.4.1. Membrane permeability and rejection As shown in Fig. 15, the pure water fluxes of membranes suffer gradual decrease during membrane compaction, which may be due to the mechanical deformation of the polymeric membrane matrix [7,45]. After about 50 min of compaction, pure water fluxes approach a steady value for all the membranes and the steady pure water fluxes were 79%, 75%, 88%, 81%, 91% and 95% of the initial pure water fluxes for M0, M0.01, M0.05, M0.1, M0.5 and M1.0, respectively. Generally, pure water fluxes of the membranes with hydrophilic additives (especially with water-soluble polymers) would suffer severe decrease during membrane compaction, which might be due to not only the immiscible between the additive and membrane material but also the existence of lots of macrovoids in the sublayer [10]. It was reported that the pure water fluxes of the membranes with PVP or PEG addition had more than 40% decrease during membrane compaction [7,10]. However, in this work, all of PSf/PANiEB membranes had the pure water fluxes decrement lower than 30%. Moreover, the pure water fluxes of M0.05, M0.5 and M1.0 were more stable than M0 during membrane compaction. This might be due to the good miscibility of PANiEB and PSf in the casting solution, the rigidity structure of PANiEB and the even distribution of PANiEB in the prepared membrane. In addition, the fact that PSf/PANiEB membranes had little macrovoids in the sublayer may also contribute to the stability of pure water flux during membrane compaction. In order to further investigate the stability of PSf/PANiEB membranes under high TMP, high pressure resistance experiment was carried out at 0.50 MPa TMP using PSf/PVP membranes as the comparison. Fig. 16 shows the time-dependent pure water fluxes of PSf/PVP membranes and PSf/PANiEB membranes at 0.50 MPa TMP. It can be found that the pure water fluxes of PSf/PVP membranes decrease rapidly with time while the pure water fluxes of PSf/PANiEB membranes decrease slowly with time and approach steady values after about 35 min of operation. The initial pure water

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Fig. 12. Cross-sectional morphologies of the membranes with different PANiEB contents: (a) M0, (b) M0.01, (c) M0.05, (d) M0.1, (e) M0.5 and (f) M1.0.

flux of PSf/PVP membrane with 0.5 wt% PVP is slightly larger than that of M0.5. However, after 12 min of operation, the pure water flux of PSf/PVP membrane with 0.5 wt% PVP is lower than that of M0.5. The initial pure water flux of M0.1 is slightly larger than PSf/PVP membrane with 5.0 wt% PVP and the flux gap increases over time. After 60 min of operation, the final pure water fluxes of PSf/PVP membrane with 0.5 wt% PVP, PSf/PVP membrane with 5.0 wt% PVP, M0.1 and M0.5 are 67%, 66%, 76% and 84% of the initial pure water fluxes, respectively. These results indicated that PSf/PANiEB membranes exhibited much more stable pure water flux under high TMP than PSf/PVP membranes. Fig. 17 presents the pure water fluxes of the membranes with different PANiEB contents. All of PSf/PANiEB membranes have higher pure water fluxes than PSf membrane. This was because PSf/PANiEB membranes had larger surface pore size, higher porosity, more hydrophilic surface and better vertically interconnected

finger-like pores than PSf membrane (see Section 3.3), which greatly weakened the resistance of water permeating through the membranes and thus increased membrane permeability. With increasing PANiEB content, pure water fluxes of PSf/PANiEB membranes increase firstly and then decrease. Pure water fluxes of M0.01, M0.05, M0.1, M0.5 and M1.0 are 1.7, 2.2, 2.8, 2.3 and 1.9 times that of PSf membrane, respectively. This might be caused by the following reasons. When PANiEB content in the casting solution was below 0.1 wt%, the increases of membrane porosity, surface pore size, hydrophilicity and finger-like pores interconnection with increasing PANiEB content were favorable to the enhancement of membrane permeability. However, when PANiEB content in the casting solution was above 0.1 wt%, membrane porosity and surface pore size decreased with increasing PANiEB content (see Section 3.3.2), resulting in the decrease of membrane permeability.

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Fig. 13. EDX mapping image of N element distribution in PSf/PANiEB membrane (M0.5) cross-section: (a) cross-sectional SEM images of the membrane and (b) cross-sectional EDX mapping image of the membrane.

Fig. 14. Water contact angles of the membranes with different PANiEB contents.

Fig. 16. The time-dependent pure water fluxes of PSf/PVP membranes and PSf/PANiEB membranes at 0.50 MPa TMP.

As shown in Fig. 18, membrane rejection property changes slightly with the addition of PANiEB. BSA, EA and trypsin rejections of the membranes are in the ranges of 98.2–98.6%, 97.1–98.0% and 64.9–67.7%, respectively. For all of the membranes, the rejections decrease in an order of BSA rejection > EA rejection > trypsin

rejection, which is due to the difference of protein molecular weights. UF membrane is generally characterized by its molecular weight cut-off (MWCO), which is defined as the lowest molecular weight of a solute that has a rejection of 95% [46,47]. According to

Fig. 15. The time-dependent pure water fluxes of the membranes during membrane compaction at 0.30 MPa TMP.

Fig. 17. Pure water fluxes of the membranes with different PANiEB contents, TMP, 0.20 MPa; temperature, 23 ± 1 ◦ C; cross-flow velocity, 0.22 m/s.

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Fig. 18. BSA, EA and trypsin rejections of the membranes with different PANiEB contents, protein concentration, 1.0 g/L; TMP, 0.16 MPa; temperature, 23 ± 1 ◦ C; cross-flow velocity, 0.22 m/s.

this, the MWCOs of PSf membrane and PSf/PANiEB membrane are in the range of 23–43 kDa.

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Fig. 20. Flux recovery ratio (FRR) values of the membranes.

thus the protein molecular adsorbed on PSf/PANiEB membranes surface could be easily washed away by hydraulic cleaning. 4. Conclusions

3.4.2. Membrane antifouling property Fig. 19 shows flux decline behavior of the membranes during BSA ultrafiltration. It can be seen that the fluxes of the membranes decrease dramatically in the initial five minutes due to the adsorption and deposition of protein molecules on membrane surface. All of PSf/PANiEB membranes exhibit higher fluxes than PSf membrane (M0). It is well known that membrane fouling could result in flux decline and shorten membrane life. Hydraulic cleaning is often used to recover membrane flux and the efficiency of hydraulic cleaning is assessed by FRR value to show membrane antifouling property [48]. Higher FRR value means higher efficiency of hydraulic cleaning and better antifouling property. The FRR values of all the membranes are shown in Fig. 20. Compared with PSf membrane, PSf/PANiEB membranes process higher FRR values, indicating that PSf/PANiEB membranes have better antifouling property than PSf membrane. This was because PSf/PANiEB membranes had better surface hydrophilicity (see Section 3.3.3), which weakened the interaction between BSA molecular and membrane surface and

Homogeneous PSf/PANiEB membrane can be easily prepared via immersion precipitation process due to the good solubility of PANiEB in NMP. During membrane formation, PANiEB performed as both pore forming agent and hydrophilic modifier. The addition of PANiEB increased membrane surface pore size, porosity, finger-like pore interconnection and hydrophilicity. PSf/PANiEB membranes displayed stable pure water flux during membrane compaction. Pure water fluxes of PSf/PANiEB membranes were 1.7–2.8 times that of PSf membrane with slightly changes of BSA, EA and trypsin rejections. PSf/PANiEB membranes had higher flux during BSA ultrafiltration and higher flux recovery after simple water flushing than PSf membrane. In this paper, we mainly investigated the effect of PANiEB addition on membrane structure and performance. All the experimental results suggested that PANiEB was a promising additive for modifying UF membrane. In the following study, surface hydrophilic modification and other parameters of membrane preparation, such as coagulation bath and membrane thickness, would be studied to obtain PSf/PANiEB membrane with better performance. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 20836006), 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). References

Fig. 19. Flux decline behavior of the membranes during BSA ultrafiltration, BSA concentration, 0.8 g/L; TMP, 0.16 MPa; temperature, 23 ± 1 ◦ C; cross-flow velocity, 0.22 m/s.

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