Development, characterization and separation performance of organic–inorganic membranes

Separation and Purification Technology 67 (2009) 271–281

Contents lists available at ScienceDirect

Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

Development, characterization and separation performance of organic–inorganic membranes Part II. Effect of additives G. Arthanareeswaran ∗ , T.K. Sriyamuna Devi, D. Mohan Department of Chemical Engineering, A.C. College of Technology, Anna University, Chennai, India

a r t i c l e

i n f o

Article history: Received 23 May 2007 Received in revised form 17 March 2009 Accepted 18 March 2009 Keywords: Organic–inorganic membranes Additives Lithium chloride Fouling-resistant ability Protein separation

a b s t r a c t The effects polyethylene glycol 600 (PEG 600) and lithium chloride (LiCl) as additives on the preparation of cellulose acetate (CA) and silica (SiO2 ) blend ultrafiltration membranes were investigated in terms of water content, hydraulic membrane resistance, permeation performance, membrane morphology, and mechanical property. The addition of the additives into the casting solution changed the structure of the resultant membranes, which was believed to be associated with the change of the thermodynamic and kinetic properties of the system in the phase inversion technique. The pure water flux (PWF) of the membranes improved from 29.60 to 34.28 l m−2 h−1 when 5 wt.% LiCl and PEG was used as the additives in the pure CA membrane, respectively. The average pore size and porosity of the membranes were revealed by pore statistics and molecular weight cut-off (MWCO) studies using different molecular weight of proteins. Among the two additives, the membrane made from the casting containing PEG possessed the highest average pore size and porosity. The separation of proteins with the membranes was also studied. In particular, it was found that CA/SiO2 blend membranes with additives exhibit improved permeate flux of proteins. The fouling-resistant ability and the recycling potential of the UF membranes were found using bovine serum albumin (BSA) as model protein and the results are discussed. The results indicated that presence of pore formers (PEG and LiCl) in pure CA and all composition CA/SiO2 blend membrane, the total fouling resistance decreased resulting in enhanced flux recovery ratio which improved the life time membranes. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.

1. Introduction Every part of chemical process involves at least one separation or purification stage and the chemical industry has developed a variety of separation processes to facilitate removal and recovery of the required products. In recent years, membrane separation processes have developed from laboratory device to an industrial process with considerable technical and commercial impact. In many cases, membrane processes are faster, more efficient and economical than conventional separation techniques. Membrane separation processes are classified as microfiltration, ultrafiltration, nanofiltration and reverse osmosis, etc. Amongst all the membranes process, ultrafiltration has the major variety of applications in different industries, because it is as a separation technology of high efficiency and low energy consumption [1]. Nowadays, the

∗ Corresponding author. Present address: Department of Chemical Engineering, National Institute of Technology, Tiruchirappalli 620015, India. Tel.: +91 431 2503118; fax: +91 431 2501811. E-mail address: [email protected] (G. Arthanareeswaran).

commercial membranes are prepared from synthetic polymers, copolymers or blends by the phase inversion method. Phase inversion method is commonly used to prepare symmetric and asymmetric polymeric membranes for a wide range of applications [2–4]. Cellulose acetate (CA) membrane was the first high performance membrane material which is widely used for reverse osmosis (RO), microfiltration (MF) and gas separation [5]. CA membranes have excellent hydrophilicity that is very important in minimizing fouling, good resistance to chlorine and solvent. At the same time, the addition of organic or inorganic additives as the third component to the casting solution has been one of the most important techniques used in membrane preparation to control the morphology and performance of membranes. In order to increase the effectiveness of the CA, and polysulfone (PSf) membranes, hydrophilicity or surface wettability is an important membrane characteristic which needs to be improved. Based on the fundamental concept that the surface layer of the asymmetric polymeric membrane is strongly influenced by the additives or that of their aggregates which are in the casting solution, there is always an ongoing research in finding new suitable additives for membrane making. For example, several authors [6–9]

1383-5866/$ – see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2009.03.037

272

G. Arthanareeswaran et al. / Separation and Purification Technology 67 (2009) 271–281

have reported that adding a second polymer (polyvinylpyrrolidone, PVP) in the solutions of polysulfone, polyethylene glycol (PEG) polyetherimide (PEI) and poly(vinyl chloride) (PVC) produced high porous membranes, well-interconnected pores and surface properties. Besides, Xu et al. studied the effect of PVP for different molecular weight on morphology of polyetherimide hollow fiber membrane [7]. Kim and Lee investigated the effect of PEG additive as a pore former on the structure formation of membranes and their permeation of thermodynamic and kinetic properties in phase inversion process [8]. Munari et al. [9] obtained the microporous polysulfone membranes prepared from casting solutions of polysulfone, solvent and PVP as additive. Addition of inorganic additives such as monovalent, divalent, or trivalent salts in the dope solutions of various polymers to improve the permeability and selectivity of ultrafiltration membranes have been investigated [10]. Bottino et al. [11] prepared poly(vinylidene fluoride) (PVDF) membranes by adding lithium chloride (LiCl) as dope to the casting solution. It was observed that the presence of LiCl in the dope solution tended to increase the coagulation rate and form membranes with good interconnectivity and porosity, but the enhanced membrane permeation performance was gained at a cost of reduction in its mechanical strength [11,12]. Lin et al. [13] observed the enhanced gelation phase behaviour of PVDF in water/N,N-dimethyl formamide (DMF) solutions with the addition of lithium percholate (LiClO4 ) as additive. Similarly Wang et al. [14] obtained high gas permeability and good mechanical strength for PVDF membranes when small amount of LiCl was added as additive. Further, Fontananova et al. [15] and Curcio et al. [16] found that the addition of LiCl in the PVDF/DMAc dope function as permeate flux enhancer at a low concentration of 2.5 wt.% but it suppressed macrovoid formation at a high concentration of 7.5% and resulted in a decrease of permeation flux. In our previous work [17], the mechanism for the formation of CA/SiO2 blend membranes by the immersion–precipitation was described and the enhanced performance of CA membrane with the addition of SiO2 particles was also reported. In the present study, the organic and inorganic additives like PEG 600 and LiCl were added to the pure CA and CA/SiO2 blend membranes at 5 wt.% concentration. The effects of adding additives to the blend membranes were determined by ultrafiltration studies such as compaction, pure water flux, water uptake and membrane hydraulic resistance. The molecular weight cut-off (MWCO) and pore statistics of the prepared membranes have been determined by using different molecular weight proteins. The morphology of the resulted membranes has also been studied by scanning electron microscopy (SEM). The antifouling ability and the recycling potential of the CA/SiO2 blend membranes in the presence of different additives were studied using bovine serum albumin (BSA) as model protein.

2. Experimental procedure 2.1. Materials Commercial grade CA was procured from Mysore Acetate and Chemical Co. Ltd., Mysore, India. CA was recrystallized from acetone and then dried in a vacuum oven at 70 ◦ C for 24 h prior to use. N,Ndimethyl formamide was obtained from Central Drug House, India and sieved through molecular sieves for removing moisture and stored in dried condition. Fumed silica powders (hydrophilic silica powder, average primary particle size = 0.014 mm), lithium chloride, and polyethylene glycol 600 were obtained from Central Drug House, India. The fumed silica was dried at 80 ◦ C before use. Sodium lauryl sulfate (SLS) and proteins like trypsin, pepsin, and bovine serum albumin were obtained from Sisco Research Limited, India. Egg albumin was obtained from Central Drug House, India. Phos-

phate buffers like Mono-sodium di-hydrogen ortho phosphate and di-sodium hydrogen ortho phosphate were procured from Central Drug House, India. 2.2. Solution blending and membrane formation The casting solutions (17.5 wt.%) were prepared by blending of cellulose acetate and silica particles of different compositions in presence of different pore formers such as LiCl and PEG 600 of 5 wt.% concentration in DMF under constant stirring for 4 h at 90 ◦ C and the membrane was cast by phase inversion technique as followed in our earlier study [17]. The solution was then casted over a glass plate with the help of doctor blade. The cast membranes were evaporated for 30 s, followed by immersion in a gelation bath containing water, surfactants and DMF kept at 18 ◦ C. The thickness of prepared membranes was measured in different places of membrane, the average membrane thickness was 0.2 mm ± 0.02 mm. This thickness is maintained for all the membranes. The formed membranes were stored in the 0.1 wt.% formalin solution to avoid microbial attack. 2.3. Experimental set up The permeation experiments were carried out in a batch type, dead end, stirred, ultrafiltration cell (UF cell-S76-400-Model, Spectrum, USA) with a diameter of 76 mm and effective membrane filtration area of 38.5 cm2 fitted with Teflon coated magnetic paddle. While carrying out the protein rejection studies, a constant agitation of speed 300 rpm was used in to order to reduce concentration polarization of the membranes. The cell was connected to a nitrogen cylinder with the pressure control valve and gauge through a feed reservoir. The experimental set up is shown in Fig. 1. 2.4. UF characterization The prepared membranes were characterized for pure water flux, membrane hydraulic resistance, protein rejection studies, fouling-resistant ability and recycling of the blend membranes using dead end UF experiment. The water content, MWCO, pore statistics, mechanical stability and morphological studies were also investigated. 2.4.1. Pure water flux (PWF) The thoroughly washed membrane was cut into desired shape and fitted in UF kit. The distilled water was fed into the UF kit from the pressure reservoir and the initial water flux was taken, 20 s after the pressurization at 414 kPa. The permeate was collected for every 1 h of time interval till it attains steady state. After compaction, the experiment was carried out at a differential system pressure of 345 kPa and permeate was collected. The PWF was calculated using the equation Jw =

Q tA

(1)

where Jw is the pure water flux (l m−2 h−1 ), Q is the amount of permeates collected (l), t is the sampling time (h) and A is the membrane area (m2 ). 2.4.2. Water content Water content of the membranes was obtained after soaking membranes in water for 24 h and the membranes were weighed followed by mopping it with blotting paper. The wet membranes were placed in vacuum drier at 75 ◦ C for 48 h and the dry weights of the membranes were determined [18]. The percent water content

G. Arthanareeswaran et al. / Separation and Purification Technology 67 (2009) 271–281

273

Fig. 1. Experimental set up of UF process.

was calculated given by the equation %Water content =

culated from the following equation [20,22]

(wet sample weight − dry sample weight) × 100 wet sample weight (2)

2.4.3. Membrane hydraulic resistance (Rm ) The membrane resistance (Rm ) was calculated by measuring the pure water flux at different transmembrane pressures (TMP) (p) viz., at 69, 138, 207, 276 and 345 kPa and slope of water flux vs. transmembrane pressure (p) using the following equation. The hydraulic resistances of the membranes were determined from the inverse of slopes using the following equation Rm =

p Jw

(3)

2.4.4. Molecular weight cut-off Proteins, e.g. BSA, EA, pepsin and trypsin were dissolved (0.1 wt.%) in phosphate buffer (0.05 M, pH 7.2) solution. The pH of the buffer solution was maintained at 7.2, since any change in the pH may lead to adsorptive fouling of the membrane surface [19]. The separation of individual proteins was conducted at 345 kPa in N2 atmosphere and the concentration of the feed solution was kept as constant. Molecular weight cut-off of the membrane was determined by identifying an inert solute of the lowest molecular weight which has solute rejection (SR) of 80–100% in steady state UF experiments [20]. The solute rejection of proteins were calculated by determining the concentration of proteins in the feed and permeate by UV spectrophotometer (Hitachi-2000) at max = 280 nm [21] and calculated by the equation %SR = 1 −

Cp × 100 Cf

(4)

where Cp and Cf are the concentrations of the solute in permeate and feed solutions. 2.4.5. Pore statistics For studying the pore statistics, the proteins of different molecular weight such as trypsin, pepsin, egg albumin and BSA were used. From the protein rejection studies described below, the average pore radius, surface porosity and pore density of the membrane were studied. The pores statistics like average pore radius was cal-

R¯ =

˛ ¯ × 100 SR%

(5)

where R¯ is the average pore radius (Å) of the membrane, ˛ ¯ is the average solute radius (Å) and is constant for each molecular weight. The average solute radii is known as “stoke radii” and the value of ˛ ¯ can be found from the plot between the solute radius and molecular weight of the solute given by Sarbolouki [20] which is shown in Table 2a. Assuming the membrane to be asymmetric skin type, the surface porosity of the membrane was found using the equation [23] ε=

3Jw ¯ RP

(6)

where ε is the surface porosity,  is the viscosity of the permeate (g cm−1 s−1 ), Jw is the pure water flux (cm s−1 ) and P is the applied pressure (dyn cm−2 ). From the known values of ε and R¯ (in cm), the pore density in the membrane surface can be calculated based on following equation [20] n=

ε R¯ 2

(7)

where n is number of pores cm−2 . 2.5. Mechanical stability The tensile intensity, tensile stress and break elongation ratio of the membranes were examined to investigate the mechanical stability. Two dumbbell shaped specimen of 5 mm wide and 10 mm long, were punched out of the membrane film. Mechanical studies such as tensile strength, tensile stress and percentage break elongation ratio were carried out using Instron 4500 model tensile testing system at an extension rate of 2 mm min−1 , after fixing the samples in the holders [24]. 2.6. Morphological studies The top surface and cross sectional morphology of the CA/SiO2 blend membranes in the presence of 5 wt.% of different additives like LiCl and PEG 600 were studied using scanning electron microscopy (JEOL JSM-6360). The samples were mounted on the platinum sputtered sample holders to provide electrical conductivity to very thin layers of polymeric membranes and

274

G. Arthanareeswaran et al. / Separation and Purification Technology 67 (2009) 271–281

photomicrographs were taken in very high vacuum conditions operating at 20 kV and observed at a magnification of 5000× for the top surface view of the membranes.

2.7. Protein separation studies The protein rejection study was carried out using the protein solutions of different molecular weight such as trypsin (20 kDa), pepsin (35 kDa), egg albumin (45 kDa) and BSA (69 kDa). The experimental pressure was maintained at 345 kPa to carry out the further study. The protein of lowest molecular weight such as trypsin was used first in order to reduce the fouling of the membranes with higher molecular weight protein molecules. The % solute rejection was found for each protein solution using the prepared membranes individually and was calculated by the Eq. (4) and their individual protein flux was also calculated based on the Eq. (1).

2.8. Fouling-resistant ability and recycling of blend membranes After 4 h of ultrafiltration, the membranes were washed with deionized water for 20 min and the water flux of the cleaned membranes was measured (Jw2 ) at 345 kPa. In order to evaluate the fouling-resistant ability of the blend membranes, flux recovery ratio (FRR) was introduced [25] and calculated using the following expression: %FRR =

Jw2 × 100 Jw1

(8)

To analyze the fouling process in details, we defined several ratios to describe the fouling-resistant ability of the blend membrane. The first ratio was rt as in equation rt = 1 −

Jp Jw1

(9)

Here, rt was the degree of total flux loss caused by total fouling. rr and rir were also defined to distinguish reversible fouling and irreversible fouling. Reversible fouling ratio (rr ) describes the fouling caused by concentration polarization and irreversible fouling ratio (rir ) describes the fouling caused by adsorption or deposition of protein molecules on the membrane surface. They are defined by equations rr =

(Jw2 − Jp ) Jw1

(10)

rir =

(Jw1 − Jw2 ) Jw1

(11)

Obviously, rt was the sum of rr and rir : rt = rr + rir

(12)

In order to test the recycling potential of CA/SiO2 membranes, three repetitive UF operations were carried out using BSA solution at 345 kPa. The BSA solutions of 0.1 wt.% were prepared by dissolving in (0.5 M, pH 7.2) phosphate buffer that were used as standard feed solutions. This recycling process includes four times run of pure water flux and three times run of BSA solution flux using dead end UF experiment cell. The water cleaning processes were carried out after every time of BSA solution flux, and again the pure water flux of cleaned membranes was measured. The process was stopped with four runs, because the BSA molecules foul on the membrane surface and the flux remained constant.

3. Results and discussion 3.1. Effect of exchange of additives in membrane preparation Additive, a forming pore agent, plays an important role in membrane preparation. It could change the solubility and the dissolution status of polymer. Furthermore, the chemical potential of solvent and the exchange rate between solvent and coagulating agent were also influenced and thereby the membrane performance could be enhanced. When a cast film of CA/SiO2 /LiCl solution is immersed in DMF and SLS gelation bath, exchange of DMF from the surface of film starts taking place, i.e. the solvent diffuses in to the gelation bath whereas the nonsolvent (water) diffuses in to the cast film. After a given period of time a stage comes when the solution becomes thermodynamically unstable and demixing takes place leading to precipitation of the polymer at the top surface of the film. Finally this precipitated film permits further exchange by leads to accomplishment of additives (PEG and LiC1) from the film surface which will result in an enhance of the water permeation rate of the resulting membrane. In order to investigate the amount of LiC1 that leaches out during the exchange process with time, a simple experiment was carried out. Membranes were casted with known amounts of casting solution and the solvent exchange process was carried out for a set of fixed times starting from t = 0 to t = 5 min. After the exchange, this film was immersed in a known quantity of water. The total quantity of LiC1 which was leached out in the water was estimated by Cl− titration with AgNO3 solution. Knowing the original concentration of LiC1 in the film (i.e. quantity of LiC1 at t = 0) the percentage of LiC1 lost in the gelation bath was calculated. LiCl lost in the gelation bath at 5 min was 5.9%. There is no quantitative method available to calculate the amount of PEG that leaches out during the exchange process. 3.2. Pure water flux After completion of compaction at 414 kPa, the pure water flux of blend membranes were studied at 345 kPa pressure and shown in Table 1. The PWF of CA/SiO2 blend membranes in the presence of LiCl as additive is compared with the CA/SiO2 blend membranes in the presence of PEG 600 as additive. It is seen that the pure water flux of 100 wt.% CA membrane and CA/SiO2 blend membranes with different composition at steady state is comparable to the values obtained in our previous study [17]. As it is seen from Table 1, the PWF of 100 wt.% CA membrane in the presence of 5 wt.% LiCl as additive is 29.60 l m−2 h−1 , which is higher than that of membrane prepared in the absence of LiCl (15.6 l m−2 h−1 ) and it increases to 62.32 l m−2 h−1 when the concentration of CA is reduced to 60 wt.% in the casting solution. The results clearly indicate that LiCl when used as an additive enhances the hydrophilic property of the membrane and this is exhibited by the improved PWF. Wang et al. [14] observed a similar increasing trend in terms of permeability for PVDF membranes with the addition of LiCl as additive. According to Kesting [26] the increase in water flux is attributed to the following factors; first, a complex between the salt cations and the nonsolvent water molecules in the casting solution is formed (i.e. a hydration effect). Second, this complex caused subsequent swelling of the polymer gel structure. Similarly, the PWF of 100 wt.% CA membrane is 34.28 l m−2 h−1 and increased to 81.02 l m−2 h−1 with the addition of 40 wt.% of silica particles in the casting solution in the presence of 5 wt.% PEG 600 as additive. Being a polymer additive, PEG is dissolved out by water and the sites where PEG exists become micropores. Xu and Xu [27] obtained a similar result for PVC hollow fiber UF membranes in the presence of PEG 600 as pore former. The PWF results indicate that the CA/SiO2 blend membranes in the presence of PEG 600 as additive shows a higher PWF than when LiCl

G. Arthanareeswaran et al. / Separation and Purification Technology 67 (2009) 271–281

275

Table 1 PWF, water content, membrane resistance of the blend membranes with different additives. Concentration (17.5 wt.%)

Solvent (wt.%)

CA

DMF

SiO2

100 90 80 70 60

0 10 20 30 40

PWF at 345 kPa (l m−2 h−1 ) Additives (5 wt.%)

77.5 77.5 77.5 77.5 77.5

Rm (kPa (l m−2 h−1 )−1 )

Water content (%) Additives (5 wt.%)

Additives (5 wt.%)

LiCl

PEG 600

LiCl

PEG 600

LiCl

PEG 600

29.60 40.50 45.18 49.86 62.32

34.28 46.74 65.44 76.34 81.02

81.77 83.21 84.94 86.26 87.98

84.00 85.28 87.36 92.35 98.57

11.89 8.19 7.34 6.44 5.24

9.90 7.03 4.98 4.51 3.90

was used as an additive in CA/SiO2 blend membranes. This may be due to the enhanced surface porosity of the blend membranes. 3.3. Water content Water content of the membrane relates hydrophilicity of the membrane [28]. The water content was calculated using Eq. (2) and tabulated in Table 1. As it is evident from Table 1, the % water content of 100 wt.% CA membranes in the presence of LiCl as pore former showed 81.77%, which is higher than that of membrane prepared in the absence of LiCl. Water content value of 76.6% of pure CA membrane without any additives was reported in our earlier work [17]. Similarly, in the presence of PEG 600 as additive, the 100 wt.% CA membrane showed increased % water content of 84%. Effect of PEG 600 on CA/SiO2 blend membranes prepared from 10 to 40 wt.% of SiO2 have considerably higher water content compared to effect LiCl additive membranes studied. PEG molecules can quickly diffuse out to the coagulant during dry and wet precipitation process which may lead to a higher water content (looser structure) while LiCl molecules were more difficult to be removed which may delay the exchanging rate of solvent and nonsolvent and result in lower water content [29]. We obtained a similar increase in % water content for CA/LCD PSf blend membrane in the presence of 5 wt.% PEG 600 as additive [28]. It can also be explained by the fact that as PEG 600 is a highly water soluble polymer, it can absorb more water molecules inside the membrane matrix [30]. 3.4. Membrane hydraulic resistance (Rm ) Membrane hydraulic resistance (Rm ) is an indication of tolerance of membrane towards hydraulic pressure. The resistance in ultrafiltration membranes is offered by or due the dense top “skin” layer and the porosity. Generally for pure water feed, the flux is proportional to the transmembrane pressure. In our case, the Rm value was measured by subjecting the membranes to various pressures from 69 to 414 kPa and measuring the pure water flux. Thus, all the membranes such as pure cellulose acetate and cellulose acetate/silica blends prepared in the presence of different additives such as LiCl and PEG 600, were subjected to pure water flux study. The Rm value was calculated from the inverse of the slope of the corresponding transmembrane pressure vs. pure water flux plots are presented in Table 1. From Table 1, the membrane prepared from pure cellulose acetate offers the highest membrane hydraulic resistance of

Table 2a Average solute radius of the proteins. Protein

Molecular weight (kDa)

Average solute radius (Å)a

Trypsin Pepsin Egg albumin BSA

20 35 45 69

21.5 28.5 33.0 45.0

a

Values are reported by Sarbolouki [20].

20 kPa (l m−2 h−1 )−1 compared to other membranes prepared with additives. For the comparison sake, the membrane with 80/20 wt.% of CA/SiO2 blend membrane in the presence of different additives of equal composition is discussed here. It is observed from Table 1 that in the presence of LiCl as additive to CA/SiO2 blend membrane offered a higher membrane hydraulic resistance membrane formed in the presence of PEG 600 as additive. With 5 wt.% of LiCl as additive to 80/20 wt.% CA/SiO2 blend membrane, the membrane hydraulic resistance value decreased to 7.34 kPa (l m−2 h−1 )−1 and this value decreased to 4.98 kPa (l m−2 h−1 )−1 when 5 wt.% PEG 600 was added as an additive. This decrease may be due to the formation of dense skin layer in the presence of LiCl as additive to the CA/SiO2 blend membrane that resulted in the increased membrane resistance. 3.5. Molecular weight cut-off The MWCO of a membrane corresponds to molecular weight of solutes that has the rejection of more than 80% [20]. MWCO of all the membranes of varying composition of CA/SiO2 was determined individually based on the study of protein rejection using proteins of different molecular weight and tabulated in Table 2b. MWCO of the CA/SiO2 blend membranes in the presence of LiCl as additive is compared with CA/SiO2 blend membranes in the presence of PEG 600 as additive. It is evident from Table 2b, the MWCO of 100 wt.% CA membrane did not differ in the presence of 5 wt.% of different additives and showed a value of 45 kDa. As the SiO2 concentration in the casting solution increased to 40 wt.% in the presence of 5 wt.% LiCl as additive, the MWCO increased to 69 kDa. Similarly, in the presence of PEG 600 as additive to the CA/SiO2 blend membrane, the MWCO increased to 69 kDa. It is well known that fast leaching of additives in a coagulation bath leads to more porous membranes as in the case of instantaneous L–L demixing [31] (Table 2a).

Table 2b Molecular weight cut-off of CA/SiO2 blend membranes with different additives. Concentration (17.5 wt.%)

Solvent (wt.%)

Molecular weight cut-off (MWCO) (kDa)

CA

SiO2

DMF

LiCl

PEG 600

100 90 80 70 60

0 10 20 30 40

77.5 77.5 77.5 77.5 77.5

45 45 45 45 69

45 45 69 69 >69

276

G. Arthanareeswaran et al. / Separation and Purification Technology 67 (2009) 271–281

3.6. Pore statistics Pore statistics includes average membrane pore radius (Å), sur¯ of the face porosity, and pore density. Average pore radius (R) membrane is found from the % solute rejection using the protein solutions prepared from trypsin, pepsin, EA, and BSA of 0.1 wt.% in phosphate buffers. The pure 100 wt.% CA membrane was compared with the CA/SiO2 blend composition of 90/10, 80/20, 70/30, and 60/40 wt.% in the presence of different additives. It was observed from Fig. 2(a), 100 wt.% CA membrane in the presence of LiCl as additive showed a average pore radius of 38.37 Å and with the addition of 40 wt.% of inorganic particles (silica), the average pore radius increased to 53 Å. The above observations were believed to be associated with the change of the thermodynamic and kinetic properties of the system before and after LiCl addition. In the presence of 5 wt.% PEG 600 as additive, the average pore radius of 100 wt.% showed a higher value of 40.24 Å. The average pore radius increased further to 55.56 Å with the addition of 40 wt.% inorganic particles to the casting solution. Surface porosity (ε) gives the detail about total pore area per unit surface area of the membrane and calculated using Eq. (6). In case of 100 wt.% CA membrane in the presence of LiCl as additive, the surface porosity showed 6.78 × 10−5 . When the additive added was PEG 600, the surface porosity increased to 7.64 × 10−5 . This increase in surface porosity may be due to the reduced interaction between the CA and SiO2 particles in the presence of additives. The effect of additives on membrane pore density of CA/SiO2 blend membrane is shown in Fig. 2(c) where the graph is drawn between the additives concentration vs. pore density (number of pores cm−2 ) and calculated using Eq. (7). The pore density of 100 wt.% CA membrane in the presence of LiCl as additive is 7.21 × 109 and increased to 8.78 × 109 with the addition of 30 wt.% of SiO2 particles to CA membrane. With the 5 wt.% of LiCl in 60/40 wt.% of CA/SiO2 blend membrane, the pore density decreased to 8.54 × 109 . This may be due to the fact that the excessive inorganic filler concentration can cause the particles aggregation and make them not to disperse uniformly in polymeric matrix enhancing the formation of larger pores thereby reducing the pore density. Similar result was obtained for 60/40 wt.% of CA/SiO2 blend membrane in the presence of additive PEG 600 in 5 wt.% concentration. From Fig. 2(c), the pore density increased initially with the addition both additives to the casting solution of 70/30 wt.% of CA/SiO2 blend membranes, and with the addition of 5 wt.% additives to 60/40 wt.% of CA/SiO2 blend membrane, the pore density decreased. The addition of additives to excessive presence of inorganic components (40 wt.% SiO2 ), it generates a thin skin layer that limits the exchange of the solvent and additive system for nonsolvents and thus, suppresses the pore formation in the membranes.

Fig. 2. (a) Effect of additives on membrane average pore radius for CA/SiO2 membrane at 100/0 wt.%

3.7. Mechanical stability

brane at 100/0 wt.%

The testing results of mechanical stability including tensile intensity, tensile stress and break elongation ratio are listed in Table 3. It is observed that the mechanical stability of 100 wt.% CA membrane in the presence of 5 wt.% LiCl as additive showed a value of 4.12 N, 2.75 N mm−2 , and 12% of tensile intensity, tensile stress, and break elongation ratio, respectively. Further, addition of 5 wt.% of LiCl to 90/10 wt.% of CA/SiO2 blend membrane, the mechanical stability decreased to 3.09 N, 2.06 N mm−2 and 9.38% of tensile intensity, tensile stress, and break elongation ratio, respectively. Similar trend was observed for other blend compositions with LiCl additive. The decrease in mechanical stability may be due to the fact that in the presence of LiCl as additive, the void fraction increased resulting in the decline of mechanical stability. Similarly in the presence of 5 wt.% of PEG 600 as additive to 100 wt.%

, 90/10 wt.%

, 80/20 wt.%

, 70/30 wt.%

and 60/40 wt.%

blend compositions. (b) Effect of additives on surface porosity for CA/SiO2 mem, 90/10 wt.%

, 80/20 wt.%

, 70/30 wt.%

and 60/40 wt.%

blend compositions. (c) Effect of additives on pore density for CA/SiO2 membrane at 100/0 wt.% , 90/10 wt.% blend compositions.

, 80/20 wt.%

, 70/30 wt.%

and 60/40 wt.%

CA membrane, the mechanical stability showed values of 4.25 N, 3.20 N mm−2 and 9.53% in terms of tensile intensity, tensile stress, and break elongation ratio, respectively. With the addition of 5 wt.% of organic additive to the CA/SiO2 membrane, the mechanical stability decreased gradually. This may be due to that polymer chains are repelled from the inclusions during membrane casting. The accompanying increase in free volume, and hence decline of mechanical stability that are in good agreement with experiments [24]. We can conclude that the repulsive force of Si–O bonds increased in poly-

G. Arthanareeswaran et al. / Separation and Purification Technology 67 (2009) 271–281

277

Table 3 Mechanical stability of the CA/SiO2 blend membranes with different additives. Polymer concentration (17.5 wt.%)

Solvent (wt.%)

CA

DMF

100 90 80 70 60

SiO2

0 10 20 30 40

77.5 77.5 77.5 77.5 77.5

Tensile stress (N mm−2 )

Tensile intensity (N)

Break elongation ratio (%)

Additive concentration (5 wt.%)

Additive concentration (5 wt.%)

Additive concentration (5 wt.%)

LiCl

PEG 600

LiCl

PEG 600

LiCl

PEG 600

4.12 3.09 2.98 2.32 1.98

4.25 4.05 3.48 2.62 2.05

2.75 2.06 1.52 1.37 1.28

3.20 2.73 2.29 1.75 1.37

12.00 9.38 8.84 7.97 8.27

9.53 17.40 13.96 10.67 8.66

mer which enhanced porosity of the membrane while decline the mechanical stability membrane [32]. 3.8. Morphological studies To attain a better performance of membranes, the morphological structure of the membrane has to be manipulated. It is observed that the 100 wt.% CA membrane in the presence of LiCl as additive and in the presence of PEG 600 as additive, the membrane morphologies changed gradually. LiCl has good affinity with water. Addition of LiCl, increased the solution thermodynamic instability in the gelation bath (nonsolvent), which facilitated a rapid phase demixing and resulted in macrovoid formation. On the other hand,

LiCl possesses weak interactions with the polymer and solvent. The weak interactions of polymer and solvent of the casting solution tended to progress the film precipitation. As a result, the size of the macrovoids was increased. It is seen that with the addition of LiCl as additive to CA/SiO2 blend membranes, the surface roughness on the membrane surface increases gradually resulting in the formation of numerous pores (Fig. 3(a) and (b)). The morphological changes of the CA/SiO2 blend membranes in the presence of 5 wt.% PEG 600 molecules is seen in Fig. 3(c) and (d). The fact that adding additives tend to increase the number of membrane pore has been proven and obviously shown by the membranes surface structure. As 5 wt.% of PEG and LiCl are added, the number of membranes pore was also found to increase which in turn resulted in higher flux and

Fig. 3. (a) SEM of top surface view of pure CA membrane in presence of 5 wt.% LiCl as additive. (b) SEM of top surface view of 80/20 wt.% CA/SiO2 blend UF membrane in presence of 5 wt.% LiCl as additive. (c) SEM of top surface view of pure CA membrane in presence of 5 wt.% PEG 600 as additive. (d) SEM of top surface view of 80/20 wt.% CA/SiO2 blend UF membrane in presence of 5 wt.% PEG 600 as additive.

278

G. Arthanareeswaran et al. / Separation and Purification Technology 67 (2009) 271–281

Fig. 4. (a) Effect of LiCl on solute rejection of proteins for CA/SiO2 membrane at 100/0 wt.% , 90/10 wt.% , 80/20 wt.% , 70/30 wt.% and 60/40 wt.% blend compositions. (b) Effect of PEG 600 on solute rejection of proteins for CA/SiO2 membrane at 100/0 wt.% 60/40 wt.%

, 90/10 wt.%

, 80/20 wt.%

, 70/30 wt.%

and

blend compositions.

Fig. 5. (a) Effect of LiCl on permeate flux of proteins for CA/SiO2 membrane at 100/0 wt.% , 90/10 wt.% , 80/20 wt.% , 70/30 wt.% and 60/40 wt.% blend compositions. (b) Effect of PEG 600 on permeate flux of proteins for CA/SiO2 membrane at 100/0 wt.%

lower rejection. In fact, the change of membranes porosity by addition of additives is quite noticeable. Similar observation was made elsewhere also [33]. 3.9. Separation of proteins 3.9.1. Rejection of protein Fig. 4(a) and (b) gives the rejection of proteins like BSA, EA, pepsin and trypsin through the membranes in presence of LiCl and PEG 600 of CA/SiO2 blend membranes in the composition of 100/0, 90/10, 80/20, 70/30 and 60/40 wt.%. The proteins rejection study was used in the increasing order of their molecular weight hence, the lowest molecular weight trypsin was used first to avoid the blocking of the pores by higher molecular weight protein molecules that might lead to fouling of the membranes. For CA (100 wt.%) membranes at 5 wt.% LiCl additive, the BSA rejection was found to be 90%, which decreased to 81% upon increase of the SiO2 to 40 wt.% in CA. Similar results were also observed for the other proteins, with varying magnitudes. This might be due to the exchange of solvent and additives to nonsolvent, from the membrane during gelation, thus creating pores on the membrane surface. From Fig. 4(b), it is evident that rejection of BSA decreases from 87 to 60% with PEG content in CA casting solution from 0 to 5 wt.%.

, 90/10 wt.%

, 80/20 wt.%

, 70/30 wt.%

and 60/40 wt.%

blend compositions.

Other proteins also exhibited the similar trend. Further, BSA shows higher percent rejection than EA, pepsin and trypsin. This may be due to the fact that the rejection mechanism in the UF of an individual protein is based on sieving mechanism which is related to the pore size and solute size as well as molecular weight of the solute [34]. PEG 600 was a water solubility nonionic surfactant and had strong affinity with water. Hydrogen bond could be formed between PEG 600 and DMF. This would reduce activity of DMF and increase thermodynamics stability of the casting solution. The characteristic of LiCl was similar to that of PEG 400, but obstruction of LiCl was larger than that of PEG 400. Therefore, the leakage rate of LiCl from the casting solution was slower, phase separation delay time was longer, and skin layer was form easier on membrane [35]. Thus, pore formation capability of LiCl was poorer than that of PEG 600. It was noted from Fig. 4(a) and (b) that effect of LiCl and PEG 600 on CA/silica membranes exhibited decrease in protein rejection using LiCl was higher than that using PEG 600. As a result, the pure water flux of PEG 600 was higher than that of LiCl at the 5 wt.% additive concentration.

G. Arthanareeswaran et al. / Separation and Purification Technology 67 (2009) 271–281

279

Table 4a Total fouling resistance (rf ), reversible resistance (rr ), irreversible resistance (rr ) and FRR value of CA/SiO2 blend membranes with 5 wt.% LiCl additive. Composition (17.5 wt.%)

Solvent (wt.%)

Flux (l m−2 h−1 )

CA

SiO2

DMF

Jw1

Jw2

Jp

rr

rir

rt

100 80 60

0 20 40

77.5 77.5 77.5

29.6 45.18 62.32

19.54 33.43 49.86

12.12 15.77 20.03

0.38 0.41 0.39

0.34 0.26 0.2

0.72 0.67 0.59

Fouling resistance ratio

3.9.2. Permeate flux of protein Protein flux is the measure of the product rate of the membrane for given protein solution. The effect of two additives such as LiCl and PEG 600 in varying SiO2 concentration in the blend composition on the protein flux is shown in Fig. 5(a) and (b). The permeate flux of pure CA was 5.19 l m−2 h−1 [17], pure CA in the presence of 5 wt.% of LiCl showed a value of 12.12 l m−2 h−1 for BSA and with other proteins showed comparatively higher permeate flux of 12.97, 13.77, 21.33 l m−2 h−1 for EA, pepsin and trypsin, respectively. The BSA flux value of 21 l m−2 h−1 for 60/40/5 wt.% of CA/silica/LiCl membranes are higher than (19 l m−2 h−1 ) the CA/silica membranes without LiCl additive. The protein flux for CA/silica membranes without additives has been reported in our earlier work [17]. The formation of the LiCl and DMF complexes creates a hydration effect and causes swelling in polymer gel. Similar findings were reported by Kesting [36] in the effect of inorganic salt additives on the formation and properties of cellulose acetate membranes where it was revealed that the permeation rate of cellulose acetate membranes significantly increases when salts are added to the casting solutions. In the presence of PEG 600 as additive to 60/40 wt.% CA/SiO2 membrane, the permeate flux showed an increased value of 22.71 l m−2 h−1 for BSA molecules. However, BSA permeate flux of 60/40 wt.% CA/SiO2 membrane without PEG 600 was 19.1 l m−2 h−1 [17]. This result was consistent with the MWCO with 5 wt.% of LiCl addition, which was discussed in Section 3.4. At a relatively 5 wt.% LiCl concentration in the dope solution, the macrovoid formation was suppressed because of the enhanced kinetic effect [35].

FRR (%)

66 74 80

ence of PEG 600 as additive to the CA/SiO2 blend membranes, the fouling-resistant ability increased which results in the increased fouling-resistant ability. It can be seen that the blend membranes in the presence of PEG 600 offers more fouling-resistance than in the presence of LiCl as additive. This may be due to the increased hydrophilicity of the CA/SiO2 blend membranes with the addition of PEG 600 as additive. Since long time operation with the product rate was of great importance for practical application of membranes, FRR was introduced to evaluate the recycling property of the blend membranes. The FRR values for the CA/SiO2 blend membranes were calculated using Eq. (8) and are presented in Tables 4a and 4b. In the presence of LiCl as additive to 60/40 wt.% of CA/SiO2 blend membrane, the FRR value showed 80% and in the presence of PEG 600 as pore former,

3.10. Analysis of membrane fouling during ultrafiltration Flux decline in membrane processes is due to two main sources: concentration polarization and membrane fouling. Concentration polarization decreases the driving force of water flow across the membrane due to a local increase in foulant concentration. This effect is completely reversible, and can be reduced by modifying the flow over the membrane [33,37]. The fouling of membranes may be due to the reversible fouling and irreversible fouling. Reversible fouling corresponds to the building up of protein solutes on the membrane surface which can be reduced with the increased stirring near the membrane surface. Irreversible fouling accounts for deposition or aggregation of protein molecules in the membrane which results in the decline of flux. The flux changes due to the fouling and cleaning of BSA were observed for 100/0, 80/20 and 60/40 wt.% of CA/SiO2 blend membranes in the presence of 5 wt.% LiCl as additive is shown in Table 4a. It is evident from table that the irreversible fouling-resistant ratio of 100 wt.% CA membrane presence of 5 wt.% LiCl showed 0.34, 60/40 wt.% of CA/SiO2 blend membrane presence of 5 wt.% LiCl, the irreversible fouling-resistant ratio was decreased to 0.20. This decreased irreversible fouling-resistant results in the increased anti-fouling ability of the membranes with association of LiCl additive. This may be due to the increased hydrophilicity of CA/SiO2 blend membranes with the addition of LiCl as additive. The reason behind is that the hydrophilic free O–H groups decrease as the CA/SiO2 was associated during membrane making. When the 5 wt.% of LiCl content is added, the high water sorption potential of LiCl can balance the decrease in O–H groups, therefore, the hydrophilicity increases. From Table 4b, it is evident that in the pres-

Fig. 6. (a) Recycling potential of CA/SiO2 blend UF membrane in the presence of 5 wt.% LiCl as additive. (b) Recycling potential of CA/SiO2 blend UF membrane in the presence of 5 wt.% PEG 600 as additive.

280

G. Arthanareeswaran et al. / Separation and Purification Technology 67 (2009) 271–281

Table 4b Total fouling resistance (rf ), reversible resistance (rr ), irreversible resistance (rir ) and FRR value of CA/SiO2 blend membranes with 5 wt.% PEG 600 additive. Composition (17.5 wt.%)

Solvent (wt.%)

Flux (l m−2 h−1 )

Fouling resistance ratio

FRR (%)

CA

SiO2

DMF

Jw1

Jw2

Jp

rr

rir

rt

100 80 60

0 20 40

77.5 77.5 77.5

34.28 65.44 81.02

24.33 49.73 68.87

12.62 15.95 22.74

0.39 0.41 0.42

0.29 0.24 0.15

0.68 0.65 0.57

the FRR value increased to 85%. It is reported that PEG molecules are more polydisperse hydrophilic polymers used as probe solutes to minimize fouling [38]. The increase in FRR value results in the increasing recycling potential of the CA/SiO2 blend membranes with additives. 3.11. Recycling of the blend membranes The excellent flux recovery property of blend membranes will provide the long time membrane run without significant decline of separation performances. To study the effect of additives on recycling property of the CA/SiO2 blend membranes, the graph is plotted between time vs. flux and the results are shown in Fig. 6(a) and (b). For the sake of comparison, the CA/SiO2 blend membranes in the presence of LiCl with the composition 80/20 wt.% is compared with the 80/20 wt.% CA/SiO2 blend membrane in the presence of PEG 600 as additive of 5 wt.% composition. The initial flux of the 80/20 wt.% of CA/SiO2 blend membrane in the presence of LiCl showed a value of 45.18 l m−2 h−1 and after three runs of BSA flux, the PWF decreased to 17.5 l m−2 h−1 . This decrease may be due to the deposition of BSA molecules on the membrane surface. In the presence of PEG 600 as additive, the 80/20 wt.% of CA/SiO2 blend membrane showed an increased initial flux of 44.5 l m−2 h−1 and declined to 10.5 l m−2 h−1 after three runs of BSA flux. This increase in PWF may be due to the decreased hydrophobicity with the addition of more hydrophilic component (PEG 600) to the casting solution. 4. Conclusions In the present work, the effects of two distinctive additives, PEG 600 and LiCl and, on the performance of CA/SiO2 blend UF membranes were studied and compared with the impact of addition of additives in pure CA and CA/SiO2 membranes. It is observed that the blend membranes in the presence of PEG 600 showed an increased value of PWF, water content than the presence of additive LiCl. The mechanical stability of 100/0 to 60/40 wt.% of CA/SiO2 blend membranes was increased in presence 5 wt.% of PEG 600 and LiCl in all blend composition. The viscosity of the CA/SiO2 casting solution with the additive also increased compared with the casting solution without any additive. Thus, the morphological structure changes of the resultant membranes caused by the additives were held to be coupled with the change of the thermodynamic and kinetic properties of the system in the phase separation process. The addition of additives PEG 600 and LiCl into the organic–inorganic casting solution also made the resultant membranes exhibit a increased average pore radius and surface porosity compared to the CA/SiO2 membrane with out additives. Furthermore, the proteins solute rejection of the membranes was influenced by the two additives in the sequence of LiCl > PEG 600. The presence of PEG 600 and LiCl in the dope solution improved the permeate flux of proteins. With the addition of PEG 600 as pore former, reversible fouling resistance ratio decreased and offered more resistance to total fouling thereby increasing the flux recovery ratio and recycling potential of the CA/SiO2 blend membranes when compared with LiCl as additive.

71 76 85

References [1] S.P. Nunes, K.V. Peinemann, Membrane Technology in the Chemical Industry, 2nd ed., WILEY-VCH Verlag GmbH & Co. KGaA, 2006 (Chapters 1–3). [2] E. Bernt, H. Bengt, Membrane applications in raw water treatment with and without reverse osmosis desalination, Desalination 98 (1994) 3. [3] K. Kohichi, High-performance immunoaffinity chromatography, an immunoaffinity membrane for selective removal of plasma components, and safety evaluation of the latter system, J. Biochem. Biophys. Methods 49 (2001) 241. [4] S. Toshikatsu, Anion exchange membrane with viologen moiety as anion exchange groups and generation of photo-induced electrical potential from the membrane, J. Membr. Sci. 118 (1996) 121. [5] T. Matsuura, S. Sourirajan, Fundamentals of Reverse Osmosis, National Research Council Canada, Ottawa, 1985. [6] M.J. Han, S.T. Nam, Thermodynamic and rheological variation in polysulfone solution by PVP and its effect in the preparation of phase inversion membrane, J. Membr. Sci. 202 (2002) 55. [7] Z.L. Xu, T.S. Chung, Y. Huang, Effect of polyvinylpyrrolidone molecular weights on morphology, oil/water separation, mechanical and thermal properties of polyetherimide/polyvinylpyrrolidone hollow fiber membranes, J. Appl. Polym. Sci. 74 (1999) 2220. [8] J. Kim, K. Lee, Effect of PEG additive on membrane formation by phase inversion, J. Membr. Sci. 138 (1998) 153. [9] S. Munari, A. Bottino, G. Capannelli, P. Moretti, P. Petit Bon, Preparation and characterization of polysulfone-polyvinylpyrrolidone based membranes, Desalination 70 (1988) 265. [10] M.H. Shinde, S.S. Kulkarni, D.A. Musale, S.G. Joshi, Improvement of the water purification capability of poly(acrylonitrile) ultrafiltration membranes, J. Membr. Sci. 162 (1999) 9. [11] A. Bottino, G. Capannelli, S. Munari, A. Turturro, High performace ultrafiltrafion membrane cast from LiCl doped solutions, Desalination 68 (1988) 167. [12] M. Tomaszewska, Preparation and properties of flat-sheet membranes from poly(vinylidene fluoride) for membrane distillation, Desalination 104 (1996) 1. [13] D.J. Lin, C.L. Chang, F.M. Huang, L.P. Cheng, Effect of salt additive on the formation of microporous poly (vinylidene fluoride) membranes by phase inversion from LiClO4 /Water/DMF/PVDF system, Polymer 44 (2003) 413. [14] D. Wang, K. Li, W.K. Teo, Porous PVDF asymmetric hollow fiber membranes prepared with use of small molecular additives, J. Membr. Sci. 178 (2000) 13. [15] E. Fontananova, J.C. Jansen, A. Cristiano, E. Curcio, E. Drioli, Effect of additives in the casting solution on the formation of PVDF membranes, Desalination 192 (2006) 190. [16] E. Curcio, E. Fontananova, G. Di Profio, E. Drioli, Influence of the structural properties of poly(vinylidene fluoride)membranes on the heterogeneous nucleation rate of protein crystals, J. Phys. Chem. B 110 (2006) 12438. [17] G. Arthanareeswaran, T.K. Sriyamuna Devi, M. Raajenthiren, Effect of silica particles on cellulose acetate blend ultrafiltration membranes: Part I, Sep. Purif. Technol. 64 (2008) 38. [18] M. Tamura, T. Uragami, M. Sugihara, Studies on syntheses and permeabilities of special polymer membranes. 30. Ultrafiltration and dialysis characteristics of cellulose nitrate-poly (vinyl pyrrolidone) polymer blend membranes, Polymer 22 (1981) 829. [19] J. Brinck, A.S. Jönsson, B. Jönsson, J. Lindau, Influence of pH on the adsorptive fouling of ultrafiltration membranes by fatty acid, J. Membr. Sci. 164 (2000) 187–194. [20] M.N. Sarbolouki, General diagram for estimating pore size of UF and RO membranes, Sep. Sci. Technol. 17 (1982) 381. [21] R. Mahendran, R. Malaisamy, D. Mohan, Preparation, characterization and effect of annealing on performance of cellulose acetate/sulfonated polysulfone and cellulose acetate/epoxy resin blend ultrafiltration membranes, Eur. Polym. J. 40 (2004) 623. [22] R. Malaisamy, R. Mahendran, D. Mohan, Cellulose acetate and sulfonated polysulfone blend ultrafiltration membranes. II. Pores statistics, molecular weight cutoff, and morphological studies, J. Appl. Polym. Sci. 84 (2002) 430. [23] O. Velicangil, J.A. Howell, Estimation of the properties of high-flux ultrafiltration membranes, J. Phys. Chem. 84 (1980) 2991. [24] Y. Yang, H. Zhang, P. Wang, Q. Zhang, J. Li, The influence of nano-sized TiO2 fillers on the morphologies and properties of polysulfone UF membranes, J. Membr. Sci. 288 (2007) 231. [25] Y.Q. Wang, Y.L. Su, X.L. Ma, Q. Sun, Z.Y. Jiang, Pluronic polymers and polyethersulfone blend membranes with improved fouling-resistant ability and ultrafiltration performance, J. Membr. Sci. 283 (2006) 440.

G. Arthanareeswaran et al. / Separation and Purification Technology 67 (2009) 271–281 [26] R.E. Kesting, Semipermeable membranes of cellulose acetate for desalination in the process of reverse osmosis. Part 1. Lytropic swelling of secondary cellulose acetate, J. Appl. Polym. Sci. 9 (1965) 663. [27] J. Xu, Z.L. Xu, Poly(vinyl chloride) hollow fiber ultrafiltration membranes prepared from PVC/additives/solvent, J. Membr. Sci. 208 (2002) 203. [28] G. Arthanareeswaran, C.S. Latha, D. Mohan, M. Raajenthiren, Studies on cellulose acetate/low cyclic dimmer polysulfone blend ultrafiltration membranes and their applications, Sep. Sci. Technol. 41 (2006) 2895. [29] W.L. Chou, D.G. Yu, M.C. Yang, C.H. Jou, Effect of molecular weight and concentration of PEG additives on morphology and permeation performance of cellulose acetate hollow fibers, Sep. Purif. Technol. 57 (2007) 209. [30] R. Malaisamy, R. Mahendran, D. Mohan, M. Rajendran, V. Mohan, Cellulose acetate and sulfonated polysulfone blend ultrafiltration membranes. Part I: Preparation and characterization, J. Appl. Polym. Sci. 86 (2002) 1749. [31] Y. Liu, G.H. Koops, H. Strathmann, Characterization of morphology controlled polyethersulfone hollow fiber membranes by the addition of polyethylene glycol to the dope and bore liquid solution, J. Membr. Sci. 223 (2003)187.

281

[32] C.S. Yang, Y.H. Yu, K.M. Lee, H.J. Lee, C.K. Choi, Investigation of low dielectric carbon-doped silicon oxide films prepared by PECVD using methyltrimethoxysilane precursor, Thin Solid Films 506 (2006) 50. [33] J.L. Zeman, L.A. Zydney, Microfiltration and Ultrafiltration: Principles and Applications, Marcel Dekker, New York, NY, 1996. [34] G. Grund, C.W. Robinson, B.R. Glick, Protein type effects on steady state cross flow membrane ultrafiltration fluxes and protein transmission, J. Membr. Sci. 70 (1992) 177. [35] L. Shi, R. Wang, Y. Cao, D.T. Liang, J.H. Tay, Effect of additives on the fabrication of poly (vinylidene fluoride-co-hexafluropropylene) (PVDF-HFP) asymmetric microporous hollow fiber membranes, J. Membr. Sci. 315 (2008) 195. [36] R.E. Kesting, Synthetic Polymeric Membranes, Wiley, New York, 1985, p. 255. [37] N. Hilal, O.O. Ogunbiyi, N.J. Miles, R. Nigmatullin, Methods employed for control of fouling in MF and UF membranes: a comprehensive review, Sep. Sci. Technol. 40 (2005) 1957. [38] P. Mikulasek, Methods to reduce concentration polarization and fouling in membrane filtration, Collect. Czech. Chem. Commun. 59 (1994) 7375.