carboxymethyl cellulose acetate blend ultrafiltration membranes

carboxymethyl cellulose acetate blend ultrafiltration membranes

Desalination 311 (2013) 80–89 Contents lists available at SciVerse ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal Prepa...

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Desalination 311 (2013) 80–89

Contents lists available at SciVerse ScienceDirect

Desalination journal homepage: www.elsevier.com/locate/desal

Preparation and characterization of cellulose acetate/carboxymethyl cellulose acetate blend ultrafiltration membranes Baixin Han, Dalun Zhang ⁎, Ziqiang Shao, Linlin Kong, Shaoyi Lv School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China

H I G H L I G H T S ► ► ► ► ►

CMCA was first synthesized from CMC-Na by acidifying and esterifying. CA/CMCA blend UF membranes were prepared via phase inversion process. PEG 600 was used as additive in the cast film promoting the formation of pore. The permeation flux was increased by addition of CMCA in casting solution. The modified membranes showed better antifouling property than CA membranes.

a r t i c l e

i n f o

Article history: Received 1 August 2012 Received in revised form 28 October 2012 Accepted 2 November 2012 Available online 20 December 2012 Keywords: Ultrafiltration membrane Cellulose acetate Carboxymethyl cellulose acetate Membrane morphology Anti-fouling property

a b s t r a c t A carboxymethyl cellulose acetate (CMCA)/cellulose acetate (CA) blend ultrafiltration (UF) membrane was prepared via phase inversion in the absence and presence of 2.5 wt.% additive, namely polyethylene glycol 600 (PEG 600). CMCA was firstly synthesized from carboxymethyl cellulose sodium (CMC-Na) by acidifying and esterifying. The as-prepared blend membranes were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), mechanical analysis, contact angle, pure water flux (PWF), water content (WC), rejection and molecular weight cut off (MWCO) to understand the influence of polymer blend composition and additive on the properties of the modified membranes. SEM and AFM surface roughness analysis showed that CA/CMCA blend membranes possessed large size pore in the top layer and porous structures in the cross-section. Compared to the pure CA membrane, blending of CA with CMCA resulted in novel blend membranes with enhanced ultrafitration membrane characteristics such as lower contact angle and higher PWF coupled with higher water content. Bovine serum albumin (BSA) was subjected to rejection by the blend membranes. Meanwhile, the fouling resistant ability was studied and the UF experiments suggested that the modified membranes had more positive influence on membrane anti-fouling performance. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Nowadays, membrane separation processes have become one of the major emerging technologies that have been generally applied in many industries such as chemical, food, pharmaceutical industries [1]. Ultrafiltration (UF) is a widely used membrane separation method used to separate and concentrate high molecular weight species (such as proteins) present in solution [2]. This is mainly due to its distinct advantages such as non-destructive process, low operating pressure and ambient or relatively low operation temperature [3]. However, the serious fouling, which is caused by the aggregation and adsorption of bio-macromolecules on the membrane surface or inside the membrane pores, has been a serious obstacle for efficient use of membrane [4,5]. To overcome this obstacle, many efforts have been made and results have shown that the hydrophilic CA membrane is a promising membrane to ⁎ Corresponding author. Tel./fax: +86 10 68912307. E-mail address: [email protected] (D. Zhang). 0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2012.11.002

resist fouling. CA is a versatile material and has attracted much attention due to its excellent performances such as good toughness, high biocompatibility, good desalting and relatively low cost [6–8]. In addition, CA membranes also have outstanding hydrophilic characteristic and are mostly used in minimizing fouling [9–13]. However, a major bottleneck of CA membrane is the lack of reactive functional groups on the polymer backbones in order to enhance the separation efficiency of the membranes [14]. Furthermore, CA membrane has been often modified by blending with other polymers to obtain an excellent separation performance, such as higher flux and better selectivity. Blending CA with an appropriate polymer maybe improves the performance of CA membrane owing to the fact that polymer blends have provided a desirable way to fulfill new requirements for practical applications [15,16]. Carboxymethyl cellulose sodium (CMC-Na), one of the important cellulose derivatives, possesses many desirable properties, such as good filming, water maintaining, bind and emulsification [17]. It is expected that CMC-Na can play an important role as a blend polymer to increase the hydrophilicity of the CA membrane by introducing

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reactive sites (carboxymethyl groups). However, the low degree of substitution of carboxymethyl group in CMC-Na gives rise to poor solubility in some organic solvents so that its application is limited [18,19]. Therefore, it becomes more urgent to modify CMC-Na. For this purpose carboxymethyl cellulose acetate (CMCA) comes into mind. CMCA is a derivative from CMC-Na after acetylation and is expected to have better solubility than CMC-Na in some organic solvents such as N, N-Dimethylformamide and N-methyl-2-pyrrolidone. The hydrophilic CMCA has potential as a material of ultrafiltration membranes. However, to our best of knowledge there is no literature available about the CA and CMCA blend ultrafiltration membranes. The presence of additive into the casting solution is one of the key factors. The addition of polymeric additives, such as polyethylene glycol (PEG), has been demonstrated to increase the hydrophilic property of associated membrane and fulfill new requirements [20–23]. In this study, we have prepared a modified blend membrane by blending CA with CMCA at different compositions in the presence of additive PEG 600. The effect of polymer blend composition and additive in the casting solution on the as-prepared membrane was discussed by membrane morphology, mechanical property, contact angle, pure water flux, water content, rejection and molecular weight cut off. In addition, the effect of CMCA content and additive in the casting solution on fouling resistant ability was also investigated. 2. Experimental 2.1. Materials Cellulose acetate (CA, Mw 40,000 Da, acetyl content 39.8%) was purchased from Huibao Chemical Co. (Beijing, China). Carboxymethyl cellulose sodium (CMC-Na, with substitution degrees of carboxymethyl 0.30) of average molecular weight of approximately 400,000 Da was supplied by Dongfang Chemical Co. (Beijing, China). N,N-Dimethylformamide (DMF), sulphuric acid, acetic acid, acetic anhydride and magnesium sulfate were purchased from Beijing Chemical Works. Polyethylene glycol 600 (PEG 600, Mw 600 Da) was obtained from Hengcheng Chemical Co. (Beijing, China). Bovine serum albumin (BSA, 69,000 Da) was purchased from Nuoqiya Biotech Co. (Beijing, China). Trypsin (20 kDa), egg albumin (45 kDa) was purchased from Ruikang Biotech Co. (Beijing, China). Phosphate buffer solution (pH 7.0), which was prepared from anhydrous sodium monobasic phosphate and sodium dibasic phosphate heptahydrate (Huaye Chemical Co. Beijing, China).

2.3. Membrane preparation The phase inversion technique was employed for the preparation of CA/CMCA blend membranes as reported by other researchers [26,27]. The mixture based on CA and CMCA (total polymer concentration, 17.5 wt.%) was dissolved in an organic solvent (DMF) in the presence and absence of PEG 600 with stirring for 4 h in a round bottomed flask. The obtained homogeneous solution was then kept for at least 12 h to get rid of air bubbles. After casting, the solvent in the cast film was allowed to evaporate for 30 s, and the casting film was subsequently immersed in a coagulation bath of deionized water for 2 h. The formed membrane was peeled off from the glass plate slowly and washed thoroughly with distilled water to remove all residual solvent. Thickness of the blend membranes was maintained at 0.22± 0.02 mm and they were stored in 0.1% formalin solution to prevent microbial growth. The stepwise preparation of the polymeric membrane is shown in Fig. 1. A series of polymer solutions were also prepared by varying the blend membrane compositions, as shown in Table 1. 2.4. Membrane characterization 2.4.1. Scanning electron microscopy (SEM) analysis The surface and cross sectional images of the CA/CMCA blend membranes were observed by scanning electron microscope (SEM, Philips XL30E). These membranes were frozen under liquid nitrogen for 60 s, and then frozen fragments were broken and coated with gold by sputtering for producing electric conductivity. SEM images of the membrane surface were taken in a low vacuum conditions operating at 5 kV, while SEM photographs of the membrane cross-section were taken in a high vacuum conditions operating at 20 kV and at 300× magnifications.

2.2. Synthesis of CMCA Similar to the preparation of CA [24,25], CMCA has been prepared from CMC-Na after acetylation. A brief description is presented below: CMC-Na (5 g) was first dissolved in 10 wt.% aqueous sulphuric acid with stirring for 30 min and transformed into CMC-H by reacting with sulphuric acid. Impurities can be removed by displacing and filtrating with acetic acid. By adding 20 g acetic acid to the system, the mixture was activated for 2 h and cooled to 10 °C. Then esterification started with the addition of esterifying agent, which consisted of acetic acid (10 g), acetic anhydride (20 g) and sulphuric acid (0.2 g). Esterifying agent was added dropwise to the CMC-H solution using a dropping funnel within 1 h with vigorous stirring, and the reaction temperature rose to 72 °C for 30 min. When the mixture became homogeneous, the hydrolysis proceeded at an appropriate rate for 3 h by the addition of hydrolyzed solution, which consisted of acetic acid (10 g), water (10 g) and sulphuric acid (0.2 g). A mixture, which consisted of acetic acid, water and magnesium sulfate, was used to neutralize the sulfuric acid after reaction periods of 3 h. The obtained CMCA was precipitated by introducing the mixture into 5 wt.% aqueous acetic acid and subsequently washed. The substitution degrees of acetyl were determined by acid–base titration and the value was 2.56 (±0.04).

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Fig. 1. Scheme showing the preparation method of CA/CMCA UF membrane.

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where Ww and Wd (mg) were the wet and dry weight of the membranes, respectively.

Table 1 Compositions of CA/CMCA blend membranes. Blend composition Cellulose acetate (wt.%)

CMCA (wt.%)

100 90 80 70 100 90 80 70

0 10 20 30 0 10 20 30

Additive, PEG 600 (wt.%)

Solvent, DMF (wt.%)

0 0 0 0 2.5 2.5 2.5 2.5

82.5 82.5 82.5 82.5 80.0 80.0 80.0 80.0

2.4.2. Atomic force microscopy (AFM) analysis Atomic force microscopy (Multimode SPM-Veeco) was used to analyze the surface morphology and roughness of the membranes selective layer. The membrane surface roughness parameters were measured in tapping mode. The membrane surfaces were imaged in a small scan size (approximately 1 μm2). 2.4.3. Mechanical properties After removing water present in the membranes, tensile strength and elongation at break for prepared blend membrane were measured according to ASTM D882 using a Shimazu AG-10-TB Universal material-testing machine. All samples were cut to the standard shape of 10 mm wide and 30 mm gauge length. The measurement was performed in the ambient conditions (25 °C, relative humidity of 48± 2%). For each membrane, three samples were evaluated and the average values were reported for accuracy. 2.4.4. Contact angle The contact angle measurement was carried out with a contact angle meter (JC2000C1, Zhongyi Co., beijing, China). The frozen dried membrane was first tiled on the platform of contact angle meter, and then 5 μl deionized water was dropped onto membrane surface. The contact angle of the droplet was calculated by the software of contact angle meter. The reported contact angle of each sample membrane was the averages of the contact angles of ten droplets. 2.4.5. Pure water flux (PWF) After each membrane was initially pressurized for 1 h at 200 kPa, each membrane was subjected to a trans-membrane pressure of 100 kPa to measure PWF. The steady permeability was measured by Millipore 8200 ultrafiltration model with an effective membrane area of 28.7 cm2. The pure water flux was determined using the following equation:

J w1 ¼

Q AðΔt Þ

ð1Þ

where Jw1 was water flux (in lm−2 h−1), Q was the quantity of permeate (l), Δt was the permeation time (h), and A was the effective membrane area (m2). 2.4.6. Water content (WC) The water content of the membranes was obtained as follows. The weights of the wet membranes were first measured, and the wet membranes were then placed in a vacuum drier at 75 °C for 48 h. The weights of the dry membranes were determined later [28]. The percent of water content (WC) was calculated using the following equation:

WC ð%Þ ¼

Ww  Wd  100% Ww

ð2Þ

2.4.7. Protein rejection studies The rejection experiments were carried out using the protein solution of bovine serum albumin (BSA). The ultrafiltration kit capacity was 200 ml and the effective membrane surface area was 28.7 cm2. The protein (BSA) was dissolved in phosphate buffer (0.2 M, pH 7.0) solution and 0.1 wt.% BSA solution was used as a standard feed solution for rejection studies of protein. The UF cell was filled with 0.1 wt.% BSA solution and maintained at a constant pressure of 100 kPa. The concentration of the feed solution and permeate solution was estimated using UV–vis spectrophotometer at a wavelength of 278 nm. The solute rejection was calculated using the following equation: "

# Cp SR ð% Þ ¼ 1   100% Cf

ð3Þ

where Cp and Cf were BSA concentrations of permeate and feed solutions, respectively. In general, protein rejection was related to molecular weight cut-off (MWCO). The MWCO of a membrane was determined by the identification of an inert solute, which had the lowest molecular weight and had a solute rejection of 80–100% in steady state UF experiments [29]. Therefore, proteins of different molecular weights such as trypsin (20 kDa), egg albumin (45 kDa) and BSA (69 kDa) were chosen for the estimation of MWCO. All of the proteins were dissolved (0.1 wt.%) in phosphate buffer (0.2 M, pH 7.0) solution and filtered through each membrane individually. The solute rejection was calculated using the Eq. (3). 2.4.8. Flux recovery ratio (FRR) The membrane fouling behavior was studied and a brief description was presented below. PWF of the membrane Jw1 was first obtained at 100 kPa. After BSA buffer solution (0.1 wt.%) was then fed into the ultrafiltration cell for 1.5 h of ultrafiltration study, the membranes were washed with deionized water for 20 min and the pure water flux of the cleaned membranes (Jw2) was measured at 100 kPa again. In order to evaluate the antifouling ability of the membranes, flux recovery ratio (FRR) was introduced [30] and calculated using the following expression: FRRð% Þ ¼

J w2  100% J w1

ð4Þ

3. Results and discussion 3.1. Scanning electron microscopy (SEM) analysis In order to investigate the influence of CMCA concentration and additive (PEG 600) on the morphology of the resulting membrane, the top surface and cross sections of the as-prepared membranes were taken by using scanning electron microscopy (SEM). Fig. 2 shows the top surface of the pure CA and CA/CMCA blend membranes in the absence and presence of the pore former, PEG 600. When the cast film is immersed into the distilled water bath, phase inversion process starts due to the low miscibility between the polymer (CA) and the nonsolvent (water). Simultaneously, miscibility between the solvent (DMF) and the nonsolvent (water) causes diffusional flow of the solvent and the nonsolvent (exchange of solvent and nonsolvent) in several points of film top layer and sublayer which subsequently leads to formation of nucleuses of polymer phase. These nucleuses continue to grow until the polymer concentration at the pores/solution interface becomes too high so that solidification occurs (demixing process is completed) [31]. The rate of the demixing process affects the morphology of membranes. Instantaneous demixing often leads to formation of macrovoids in the

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Fig. 2. SEM images of the top surface of CA/CMCA blend membranes (w/w): (a) 100/0 without additive (b) 100/0 with 2.5 wt.% additive (c) 80/20 without additive (d) 80/20 without additive with 2.5 wt.% additive.

membrane structure, whereas slow demixing results in a denser structure. From Fig. 2a, it can be observed that the smooth membrane surface was formed and few pores were found in the dense top layer for pure CA membranes. With increasing CMCA concentration within the casting solution, dispersed pores increase proportionately and a rough membrane surface can be found in the skin of the CA/CMCA blend membranes. It is possible that an increase in CMCA concentration probably caused the thermodynamic membrane-forming system unstable so that it accelerated the precipitation rate in the coagulation bath and the formation of porous membranes [21]. Similar observations were achieved by Reza et al. [32]. From Fig. 2c and d, we can see that upon increase of 2.5 wt.% PEG 600 in the blend polymer caused the formation of more pores on the CA membrane surface compared to that of prepared in the absence of PEG 600. This is mainly due to the leaching process of the additive during the gelation [33]. Fig. 3 depicts the SEM cross-sectional photographs of the membranes. The membrane prepared from pure CA exhibits a finger like cavities and all small pores are not fully developed in the sub-layer. Compared to the pure CA membranes, higher CMCA concentration caused the formation of macrovoids and more porous structures beneath the skin layer of the membranes (Fig. 3a and c). The changes in the morphologies can be attributed to the changes in the blend composition by the addition of CMCA. We can expect an enhancement of the membrane surface hydrophilicity by blending with CMCA, due to the preferential orientation of carboxymethyl groups towards water during the membrane formation process [34]. In this study, the presence of CMCA as a hydrophilic composition may intensify thermodynamic instability of the cast film solution and this results in intensive increase of mutual diffusivities between the nonsolvent (water) and the solvent (DMF) in the system during solidification of the casting solution. Thus, using higher values of CMCA, the precipitation rate can be accelerated in the coagulation bath and consequently causes instantaneous demixing [35] in the coagulation bath. This facilitates the formation of macrovoids in the membrane structure.

When PEG 600 content increased from 0 to 2.5 wt.%, the sub-layer seems to have finger like cavities as well as greater macrovoids in Fig. 3c and d. The hydrophilic additives PEG 600, also a non-solvent, which increases thermodynamic instability of the cast film and promotes the instantaneous demixing in the coagulation bath. From the other point of view, presence of PEG 600 increases viscosity of the cast film due to higher molecular weight of PEG 600 than DMF and intra-molecular aggregations or entanglements of the polymer chains in the presence of PEG 600 [21]. Viscosity increase of the cast film slows down the diffusional exchange rate of solvent (DMF) and nonsolvent (water) during solidification process and consequently prevents instantaneous demixing. This leads to suppression of macrovoids and the formation of denser structure. Hence, adding hydrophilic additives, such as PEG 600, to the casting solution has a dual effect on the membrane morphology. When the concentration of PEG 600 is 2.5 wt.%, the effect of thermodynamic instability of the casting solution was obvious than the effect of increasing viscosity of the casting solution, consequently facilitates the formation of macrovoids structure [36,37]. It is believed that the PEG 600 can play a crucial role in changing the characteristics of CA membranes as well as improving the selectivity and permeability.

3.2. Atomic force microscopic (AFM) analysis AFM is one of the most useful and emerging research tools for providing overall appearance of membrane surface. To study the effects of CMCA concentration and additive on the membrane surface roughness, AFM analysis was carried out in the tapping mode and roughness parameters of these membranes could also be obtained through AFM analysis software. Three important parameters of the surface roughness in a scan size of 1 μm × 1 μm (such as Ra-the arithmetic mean roughness, Rq-the square average roughness and Rz-the mean depth with the given surface area [38]) were calculated.

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Fig. 3. SEM images of the cross-section of CA/CMCA blend membranes (w/w): (a) 100/0 without additive (b) 100/0 with 2.5 wt.% additive (c) 80/20 without additive (d) 80/20 without additive with 2.5 wt.% additive.

Figs. 4 and 5 represent the three dimensional AFM images of pure CA and CA/CMCA blend membranes in the presence and absence of PEG 600, respectively. In these images, the bright peaks may be attributed to the high points or nodules, whereas the valleys or pores are considered as dark depressions. Roughness parameters of these membranes are listed in Table 2. From Fig. 4, we can conclude that the surface roughness parameters of the relevant membrane reveal an increase by increasing the concentration of CMCA in the blend membranes. This may be caused by the addition of CMCA, which intensified thermodynamic instability of the cast film solution and accelerated the precipitation rate in the coagulation bath, consequently caused instantaneous demixing process. A similar trend for the surface roughness was observed by Yong Wei [39] and S. Rajesh [27], and they also have forecasted the same tendency for the mean pore size. The increase of mean distance between the highest peaks and lowest valleys indicated the increase in mean pore size in the membrane surface, which was in good agreement with the top surface images of scanning electron microscopy (Fig. 2a and c). It was clear that the smooth membrane surface became rough and the surface roughness parameters increased significantly by incorporating PEG 600 into membranes, as shown in Fig. 5. This may be attributed to the addition of hydrophilic additives PEG 600, which increased thermodynamic instability of the cast film and promoted instantaneous demixing process in the coagulation bath. The increasing pore number of the membranes was consistent with the top surface images revealed by Scanning electron microscopy (Fig. 2b and d).

3.3. Mechanical properties The mechanical property of the ultrafiltration membrane was another major concern for the practical application. The testing results of

mechanical stability including tensile strength and elongation at break values are listed in Table 3. As shown in Table 3, the tensile strength of the CA/CMCA blend membranes in the absence and presence of PEG 600 was lower than that of the pure CA membrane, and the tensile strength of the blend membranes decreased with the increase of CMCA content. The result might be due to the formation of macrovoids in the membrane structure. The tensile strength of the blend membranes decreased as the PEG 600 concentration increased from 0 to 2.5 wt.% in the casting solution. This result may be attributed to PEG 600 incorporation into the membranes that increased the flexibility of the CA chains and consequently weakened the mechanical strength of blend membranes [40]. However, the modified membranes could still meet the mechanical requirements for practical ultrafiltration.

3.4. Contact angle Hydrophilicity is one of the important properties of membranes and can be evaluated by measuring static water contact angle. It is noticeable that the contact angle is smaller, whereas the hydrophilicity is greater [41]. In the present study, membranes were subjected to the static water contact angle experiment ten times and the average values are showed in Fig. 6a. It can be seen from Fig. 6a that the contact angle was 59° for pure CA membrane, corresponding to the low hydrophilicity, while the contact angle decreased with the enhancement of CMCA concentration in the modified membranes. This is mainly due to the carboxylmethyl functional groups in CMCA providing additional sites for hydrogen bonding and the polar surface was obtained by increasing the carboxymethyl functional groups. As expected, the presence of carboxylmethyl functional groups in the blend membrane

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Fig. 4. AFM three dimensional images of CA/CMCA blend membranes (w/w) without additive: (a) 100/0 (b) 90/10 (c) 80/20 (d) 70/30.

was responsible for the increased hydrophilicity. Fig. 6a shows that virgin CA membrane with PEG 600 possessed smaller contact angle value (52°) than the corresponding CA membrane without PEG 600 (59°) in the cast solution. The similar trend could be found in other

CA/CMCA blend composition, and this implied that the addition of PEG 600 promoted membrane surface hydrophilicity, which might be ascribed to the hydrophilic group of PEG 600 (–OH) and PEG 600 remained on the membrane surface.

Fig. 5. AFM three dimensional images of CA/CMCA blend membranes (w/w) with 2.5 wt.% additive: (a) 100/0 (b) 90/10 (c) 80/20 (d) 70/30.

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Table 2 Surface parameters of the CA/CMCA blend membranes at various compositions. Polymer blend composition (17.5 wt.%)

PEG 600 (wt.%)

AFM surface roughness data Ra (nm)

Rq (nm)

Rz (nm)

100/0 90/10 80/20 70/30 100/0 90/10 80/20 70/30

0 0 0 0 2.5 2.5 2.5 2.5

0.59 ± 0.12 1.80 ± 0.56 5.72 ± 0.45 6.27 ± 0.52 3.11 ± 0.17 4.72 ± 0.51 5.57 ± 0.94 8.35 ± 0.59

0.84 ± 0.21 2.79 ± 0.78 7.22 ± 0.83 8.38 ± 1.10 3.93 ± 0.32 6.03 ± 0.76 7.11 ± 0.15 10.64 ± 0.32

10.28 ± 0.92 31.93 ± 1.32 48.70 ± 1.27 69.92 ± 1.38 30.92 ± 1.12 50.99 ± 2.94 60.07 ± 1.25 78.53 ± 1.41

3.5. Pure water flux After initial compaction of the membranes for 1 h at 200 kPa, the membranes were thoroughly washed with deionized water and subjected to a pressure of 100 kPa to measure the pure water flux. The low pure water flux value of 15 lm−2 h−1 was obtained for pure CA membrane, as shown in Fig. 6b. This is due to the forming of a very tight polymer matrix for pure CA, thus the pores formed on the skin of membrane showed smaller size. When the concentration of CMCA increased from 10 to 30 wt.% in CA/CMCA blend membranes, the PWF showed an increased value from 35.8 to 67.6 lm −2 h−1. It can be observed that the PWF of all blend (CA/CMCA) membranes exhibited higher values than pure CA membranes. This linear trend of PWF with an enhancement to CMCA content may be attributed to carboxymethyl functional groups in CMCA promoting the formation of abundant hydrogen bonding, consequently, the hydrophilicity of the blend membranes increased, which in turn exhibited enhanced water sorption and a higher PWF. The effect of additive PEG 600 on PWF within CA and CA/CMCA blend membranes were investigated. As shown in Fig. 6b, when CA and CMCA were blended at composition of 80/20 wt.% in the absence of PEG 600, the PWF of blend membrane was 56.4 lm−2 h−1. The enhancement in the concentration of additive PEG 600 from 0 to 2.5 wt.% at 80/20 wt.% blend membrane, which promoted the PWF of blend membrane from 56.4 to 73.2 lm −2 h−1. The membrane prepared in the presence of additive PEG 600, which yielded enhanced permeation flux. A similar trend was observed for various CA/CMCA compositions, which implied the leachability of PEG 600 during the gelation and the formation of numerous pores. 3.6. Water content Water content is correlated with hydrophilicity of the membrane [42]. The water content of membrane was evaluated by measuring the change in mass between their wet and dry weights. CA/CMCA membranes prepared at various proportions of constituent were subjected to water content studies in the presence and absence of Table 3 Mechanical stability and MWCO (molecular weight cut off) of the CA/CMCA blend membranes at various compositions. Polymer blend composition (17.5 wt.%)

PEG 600 (wt.%)

MWCO (kDa)

Mechanical properties Tensile strength (MPa)

Elongation at break (%)

100/0 90/10 80/20 70/30 100/0 90/10 80/20 70/30

0 0 0 0 2.5 2.5 2.5 2.5

20 20–45 45 69 20–45 45 45–69 69

3.68 3.02 3.27 2.98 3.39 3.14 3.06 2.74

12.58 10.39 11.42 9.45 11.02 9.71 10.13 9.97

Fig. 6. The characteristic parameters of the CA/CMCA blend membranes at various compositions in the absence and presence of 2.5 wt.% additive: (a) contact angle (b) pure water flux.

pore-former PEG 600, as shown in Fig. 7a. The water content of virgin CA membrane showed a result of 76.5%, when the CMCA composition increased from 10 to 30 wt.%, corresponding water content values were 79.5, 80.2, and 81.4%, respectively. For the CA/CMCA blend membrane, the water content value was higher than pure CA membrane. The increase in the water content with the enhancement of CMCA concentration implied that the hydrophilicity was just proportional to water content. This may be ascribable to carboxymethyl groups in CMCA towards improving the membranes hydrophilicity, and therefore the interaction between water and the membrane became appreciable resulting in higher water content with CMCA as the blend component. From Fig. 7a, it is found that the addition of PEG 600 to casting solution enhanced the water content of membranes in definite CA/CMCA blend composition. This is attributed to the leaching process of PEG 600 during gelation, resulting in formation of large pores and occupation of water molecules. It is also possible that hydrophilic PEG 600 absorbed more water molecules inside the blend membrane. 3.7. Protein rejection studies The blend membranes prepared in the present study were subjected to the rejection of BSA buffer solution as shown in Fig. 7b. The composition of the casting solution plays a crucial role in the separation of protein. Pure CA membrane subjecting to separation of BSA was found to be a higher separation 98.7%, as shown in Fig. 7b. From the rejection values, it can be revealed that as CMCA content increased from 0 to 30 wt.% in

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all the CA and CA/CMCA blend membranes. Thus, for pure CA (100%) membranes, when the additive concentration was increased from 0 to 2.5 wt.%, the MWCO values enhanced from 20 kDa to greater than 20 kDa. Similar observations were also observed in the other blend composition. Similar results were reported for cellulose acetate/sulfonated poly(ether imide) blend membranes by A. Nagendran et al. [43]. 3.8. Flux recovery ratio Membrane fouling could reduce the permeation efficiency and restrict the wide application of ultrafiltration membrane. Hydraulic cleaning is often used to recover membrane flux and FRR value is introduced to evaluate membrane antifouling property: the capability of antifouling property is more effective, the higher FRR value is obtained [44]. The flux decline behavior of the CA/CMCA blend membranes during BSA ultrafiltration at 100 kPa were depicted in Fig. 8a and b. It could be seen that the protein fluxes of the membranes declined slowly due to concentration polarization and membrane fouling, i.e. the adsorption of protein on membrane surface. All of CA/CMCA membranes showed higher protein fluxes than pure CA membrane. The FRR values of various membranes compositions were shown in Fig. 9. In the absence of PEG 600, when the CMCA concentration increased, the FRR value has obviously increased. This result demonstrated that the modification of membranes with CMCA had better flux recovery property and improved the antifouling property in agreement with surface hydrophilicity of membranes. The increasing hydrophilicity of membranes weakened the interaction between protein and membrane surface, thus hydraulic cleaning can wash away the protein adsorbed on CA/CMCA membrane surface. In the presence of the additive PEG 600 in the blend component, the FRR

Fig. 7. The characteristic parameters of the CA/CMCA blend membranes at various compositions in the absence and presence of 2.5 wt.% additive: (a) water content (b) BSA rejection.

CA/CMCA blend (absence of additive), the percentage of rejection decreased to 82.3%. This may be due to the higher CMCA content that created a mixed casting solution uneven and inhomogeneity, resulting in the formation of larger pores within the membranes. The larger pores at higher CMCA concentration was also confirmed by observing macroporous sublayer in SEM. Introduction of the pore former PEG 600 into the casting solution exhibited considerable influence on the separation efficiency as shown in Fig. 7b. Pure CA membrane exhibited a protein rejection of 98.7% and reduced to 92.5% by increasing the additive to 2.5 wt.%. The similar trend can be obtained for all the blend membranes with various CMCA concentrations. With the addition of additive PEG 600, the rejection of BSA reduced obviously and this is ascribed to rapid leaching process of PEG 600 during gelation, resulting in formation of large pores and supporting by the images of SEM (Fig. 2). The molecular weight cut-off (MWCO) is a key parameter for membranes and are determined using inert, stable molecules having various higher molecular weights. It is also evident from Table 3 that the MWCO values are dependent on the polymer and additive composition. Thus, in the CA/CMCA blend membranes, in the absence of additive, as the CMCA content increased, the MWCO value also increased from 20 kDa for 0% CMCA to 69 kDa for 30% CMCA. This result had good correlation with the permeability results of the membrane. It was believed that the incorporation of additive in casting solution would alter the MWCO of

Fig. 8. Permeate flux (BSA aqueous solution) of the CA/CMCA blend membranes at various compositions: (a) without additive (b) with 2.5 wt.% additive.

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Fig. 9. Flux recovery ratio of the CA/CMCA blend membranes at various compositions in the absence and presence of 2.5 wt.% additive.

value increased due to the increasing hydrophilicity. The higher FRR value of membrane implies that the modified membrane has better antifouling property. 4. Conclusion In the present investigation, CA/CMCA modified ultrafiltration membranes have been prepared by phase inversion technique using PEG 600 as the additive. The morphological studies were conducted using SEM and AFM. It was observed that the pore formation increased with the addition of CMCA or PEG 600 to the polymer membranes and morphological structure also changed. The as-prepared membranes were evaluated for the ultrafitration characteristic parameters such as contact angle, pure water flux and water content. The UF performance of CA/CMCA membranes illustrates that the pure water flux and water content were increased, while the membrane contact angle was decreased, as the concentration of CMCA in the casting solution was increased. In general, all the modified membranes prepared from CA/CMCA polymer blends in the presence of PEG 600 exhibited improved permeation flux for protein separation compared to the pure CA membranes. Permeation flux increased as a function of concentration of CMCA and PEG 600. However, increasing concentrations of CMCA or PEG 600 in the membrane casting solution resulted in decreased rejection of protein. This is due to the formation of pores by the addition of hydrophilic CMCA and additive PEG 600. The CMCA content in CA/CMCA blend membranes enhanced the antifouling property thereby improving flux recovery ability of the CA/ CMCA blend membrane with increased FRR value. The optimal combination of blend components (CA/CMCA) was 80/20 wt.% in the presence of PEG 600 with available pure water flux (73.2 lm−2 h−1) and high protein rejection (86.3%). Furthermore, permeation flux of 0.1 wt.% BSA solutions declined slowly and FRR value was high (78.2%) when the blend component (CA/CMCA) was 80/20 wt.% in the presence of PEG 600. Overall results suggest that morphology, hydrophilicity, permeability property, and antifouling properties of the prepared CA/CMCA blend membranes improved significantly by the incorporation of CMCA. References [1] C.J. Sajitha, D. Mohan, Studies on cellulose acetate-carboxylated polysulfone blend ultrafiltration membranes. III, J. Appl. Polym. Sci. 97 (2005) 976–988. [2] A. Aliane, N. Bounatiro, A.T. Cherif, D.E. Akretche, Removal of chromium from aqueous solution by complexation ultrafiltration using a water-soluble macroligand, Water Res. 35 (2001) 2320–2326.

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