- Email: [email protected]
water
Journal of Membrane Science 218 (2003) 173–183
Cellulose acetate hollow fiber ultrafiltration membranes made from CA/PVP 360 K/NMP/water Jian-Jun Qin a,∗ , Ying Li b , Leng-Siang Lee c , Hsiaowan Lee a a
c
Centre for Advanced Water Technology, Singapore Utilities International Pte Ltd., Blk 2, #241, Innovation Centre (NTU), 18 Nanyang Drive, Singapore 637723, Singapore b Institute of Environmental Science and Engineering, Blk 2, #237, Innovation Centre (NTU), 18 Nanyang Drive, Singapore 637723, Singapore Department of Civil Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore Received 1 March 2003; accepted 4 April 2003
Abstract Hydrophilic hollow fiber ultrafiltration (UF) membranes have been prepared from a new dope solution containing cellulose acetate (CA)/poly(vinyl pyrrolidone) (PVP 360 K)/N-methyl-2-pyrrolidone (NMP)/water with a mass ratio of 19.0/5.0/74.8/1.2 by using a dry-jet wet spinning process. The effect of air-gap length was studied. The as-spun fibers were post-treated by means of a hypochlorite solution of 200 mg l−1 (200 ppm) over different duration. The morphology of the membranes was investigated by Scanning Electron Microscopy (SEM). Pure water fluxes and solute retentions of the membranes were measured and different fouling resistances were analyzed. The experimental results showed that water flux of a membrane decreased while retention increased with increasing air gap. Fluxes of hypochlorite treated membranes were three times higher than that of an untreated membrane and retentions of the treated membranes were much lower. A 6 h treatment with 200 mg l−1 hypochlorite solution could achieve a CA UF membrane with a high pure water flux of 220 × 10−5 l m−2 h−1 Pa−1 . SEM images revealed that, PVP additive would favor the suppression of macrovoids and the thickness of inner skin increased with increasing air gap. PVP contents in the blend membrane could be significantly removed with the hypochlorite treatment. As a result, the treated CA membrane experienced higher fouling tendency than the untreated membrane. It was concluded that hypochlorite treatment altered the pore size of hollow fiber UF membranes made from the blend of CA and PVP 360 K. This is an extended conclusion to the published results and an attempt at explaining some differences were provided. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Cellulose acetate; Poly(vinyl pyrrolidone); Hollow fiber spinning; Hydrophilic ultrafiltration membranes; Hypochlorite treatment
1. Introduction The cellulose acetate (CA) membrane was the first high performance asymmetric membrane [1]. It has been widely used for reverse osmosis (RO), microfil∗ Corresponding author. Tel.: +65-67941507; fax: +65-67942791. E-mail address: [email protected] (J.-J. Qin).
tration (MF) and gas separation [2–4]. CA membranes have excellent hydrophilicity that is very important in minimizing fouling, good resistance to chlorine and solvent [5–7]. A regenerated CA membrane that was hydrolyzed from cellulose acetate has significantly improved solvent-resistance and thermostability [8]. Commercial CA membranes are either flat sheet or spiral-wound modules [2,3]. The hollow fiber configuration has become a favorite choice because the
0376-7388/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0376-7388(03)00170-4
174
J.-J. Qin et al. / Journal of Membrane Science 218 (2003) 173–183
hollow fiber membrane has three major advantages: (1) hollow fiber modules have much larger ratio of membrane area to unit volume (several thousand M2 /M3 ) compared to flat and spiral-wound modules (a few hundred M2 /M3 ), and hence higher productivity per unit volume of membrane module; (2) they are self-supporting which can be back-washed to recover the flux; and (3) they have good flexibility in the mode of operation. Hao and coworkers [9,10] have studied CA hollow fiber membrane preparation by the dry-jet wet spinning for gas separation and for ultra low pressure RO application. There are a number of patents on the fabrication of CA hollow fiber MF membranes [11–13]. An activated carbon-filled cellulose acetate hollow fiber membrane for cell immobilization and phenol degradation has been studied recently [14]. The air-gap length during spinning affects the performance of final fibers and this has been studied by a number of researchers [15–19]. Chung and Hu [15] reported that an increase in air-gap distance resulted in a significant decrease in permeation of the membrane, most likely due to a higher gravity-induced elongational stress on the fibers spun from a higher air-gap distance, resulting in a greater orientation and tighter molecular packing. Porter [17] stated that an increase in the air-gap length would result in tighter fibers with lower permeability and increased retentivity because of an increase in the degree of orientation due to the added weight of the fiber below and the extended polymer chains. Liu et al. [18] demonstrated that the average size of pores on the inner surface of polysulfone/polyethersulfone membranes decreased when the air-gap length increased due to the nascent fiber being stretched and elongated by its own weight, resulting in the polymer aggregates moving closer together and rearranging themselves into a state of greater stability. However, very few studies on CA hollow fiber ultrafiltration (UF) membranes have been published. PVP as an additive is often blended into the dope solution to increase hydrophilicity and flux of membranes [19–23]. However, the addition of PVP with high molecular mass results in a low flux membrane due to swelling of the residual PVP at the surface of pore walls when water passes through the membrane pores [19,21]. Therefore, hypochlorite treatment has been applied to increase flux of MF/UF membranes made from a blend of PVP and hydrophobic poly-
mers [21–25]. Wienk et al. [22] investigated the effect of hypochlorite treatment on a hollow fiber UF membrane made from a blend of PES and PVP 360 K and concluded that hypochlorite treatment reduced the swelling of PVP in the pores of the membranes without substantially changing the pore structure. Recently, Xu et al. [23] studied the effect of hypochlorite treatment on PEI/PVP hollow fiber UF membranes for oil/water separation. They reported that the treated membranes showed an obvious lower solute retention or larger pores than that of untreated membranes. They also found that the addition of PVP with high molecular mass would suppress the formation of macrovoids. More recently, the authors [24,25] have reported an optimum treatment time with hypochlorite and an optimum hypochlorite concentration to achieve high flux membranes spun from a blend of polysulfone and PVP. To our best knowledge, it does not appear to report how hypochlorite treatment impacts on a hollow fiber UF membrane fabricated from blends of CA and PVP of high molecular mass. This paper explores the effects of air-gap length and hypochlorite treatment on properties and morphology of CA hollow fiber UF membranes prepared from a new dope solution containing CA/PVP 360 K/NMP/water. 2. Experimental 2.1. Materials The membrane material, cellulose acetate (CA398-30) with acetyl content of 39.7 wt.% was purchased from Eastman Chemical Company (Kingsport, USA). Poly(vinyl pyrrolidone) (PVP 360 K, average MW 360,000) polymer was supplied by Sigma. The solvent, N-methyl-2-pyrrolidone (NMP, >99%) was supplied by Merck (Darmstadt, Germany). Poly(vinyl pyrrolidone) [PVP 10 K (average MW 10,000) and PVP 24 K (average MW 24,000)] from Sigma–Aldrich and bovine serum albumin (BSA, MW 66,000) from Calbiochem® Biosciences, Inc. (La Jolla, CA, USA) were used to characterize the separation performance of the hollow fiber UF membranes. Sodium hypochlorite solution (10–12%) from SINO Chemical Co. (Singapore) was used in the post-treatment of the UF hollow fiber membranes.
J.-J. Qin et al. / Journal of Membrane Science 218 (2003) 173–183
2.2. Fabrication of hollow fiber UF CA membranes 2.2.1. Preparation of a spinning dope CA and PVP 360 K were dried in an oven at 65 ◦ C to remove the moisture in the respective polymer samples. The weight loss of the polymers was monitored for 11 days until constant weight was achieved. The amount of water content in both CA and PVP 360 K could be determined from the final weight loss and was used to design the formulation of a spinning dope containing water. The spinning dope of CA/PVP 360 K/NMP/water was prepared by the following procedure. First, the solvent, NMP and the additive, undried PVP 360 K were well mixed in a glass bottle. The undried membrane polymer CA was then added into the mixture with the amount of water in the undried CA and PVP taken into consideration. After that, the mixture was stirred for at least 48 h to ensure the dope was a true solution, which was supported by the observation that the dope was still transparent after a period of 3-month storage in a 50 ml triangle flask. Finally, the formulated dope in a solution tank and the bore fluid in a metal canister were degassed to remove any gas bubbles before spinning.
175
different periods of time up to 48 h. Before the treatment, pH of the hypochlorite solution had to be adjusted from 11.4 to 7.0 by using H2 SO4 as CA membranes could not tolerate high pH. The treated fibers were then rinsed again in flowing town water. After that, the same procedure as described above was followed to characterize the treated fibers. 2.4. Characterization of hollow fiber UF membranes
2.3. Post-treatment of hollow fiber UF membranes
2.4.1. Measurements of flux and retention The experiments to measure flux and retention of hollow fiber UF membranes were carried out at temperature 25 ± 0.5 ◦ C in a cross-flow filtration setup as shown in Fig. 1. Six fibers with the length of around 30 cm were assembled into a test module. Since the hollow fibers had an inner dense skin, the feed was pumped into the lumen side of the fibers. The permeate was then collected at the bottom of the U type modules. In the tests, ultra pure water was first used to characterize the pure water flux of a fresh membrane. Feed solutions containing 100 mg l−1 of different solutes in ultra pure water were then used to test the separation performance of the membrane. After that, the fouled membrane was flushed with ultra pure water and the pure water flux of the fouled membrane was measured again. The average transmembrane pressure was around 100 kPa and the feed flow rate was adjusted to allow a feed velocity of 1 m s−1 . The solute concentration in the feed or in the permeate was determined by a TOC-5000A Analyser (Shimadzu). The permeate flux for an inner skin membrane was calculated by Eq. (1). The retention was calculated by Eq. (2). Q Q = (1) Flux = A P Nπdi P
The as-spun fibers were rinsed in flowing town water at room temperature for 16 h. After rinsing, the fibers were immersed in a 50% aqueous glycerol solution for 48 h and then dried in air at room temperature before used for making test modules. The pure water fluxes and solute retentions of the modules were then measured. To investigate the effect of hypochlorite treatment on membrane properties, the rinsed fibers were immersed in a hypochlorite solution of 200 mg l−1 over
in which, Cf (mg l−1 ) and Cp (mg l−1 ) represent the solute concentrations in the feed and permeate, respectively.
2.2.2. Spinning of hollow fibers Hollow fiber UF membranes were spun using the well-known dry-jet wet spinning process [15,25,26]. The dope solution and the bore fluid were fed into the spinneret by means of pressure from a nitrogen gas cylinder. The flow rates of the dope solution and the bore fluid could be regulated using valves that were provided. A flow meter was attached to the bore fluid setup to measure the flow rate.
where Q is the volume flow rate of permeate (l h−1 ), A the effective membrane area (m2 ), P the transmembrane pressure (Pa), N the number of fibers, di the inner diameter of fiber (m), and the effective length of fiber (m). Cp × 100% (2) Retention = 1 − Cf
176
J.-J. Qin et al. / Journal of Membrane Science 218 (2003) 173–183
Fig. 1. Experimental setup in testing of hollow fiber UF membranes.
2.4.2. Study of fouling tendency of hollow fiber membranes A UF membrane would show a reduction in flux with fouling. One of the most significant flux reduction mechanisms is concentration–polarization at the membrane surface. This phenomenon would lead to an increase in the osmotic pressure. Membrane fouling due to adsorption of solute molecules on the membrane surface and in the membrane pores would then occur. The osmotic pressure-adsorption model [27] was used in this study. Under normal operating conditions, the flux would decrease due to two main mechanisms: (i) reduction in hydrodynamic driving force by osmotic pressure (σ Π) and (ii) increase in fouling resistance (Ra ) from surface adsorption and pore plugging. The pure water flux through a fresh membrane (Jvw ) can be described by Darcy’s law (3). The solution flux (Jv ) and pure water flux through a fouled membrane (Jva ) can be represented as in Eqs. (4) and (5). Jvw = Jv =
P µRm
P − σ Π µ(Rm + Ra )
(3)
(4)
Jva =
P µ(Rm + Ra )
(5)
where P is the transmembrane pressure, Rm the hydraulic membrane resistance, µ the viscosity of the solution, Ra the hydraulic resistance of the adsorbed layer and σ Π the osmotic pressure. For comparison, the normalization of fluxes and flux reductions are usually computed with respect to the individual membrane resistances of the individual membrane. The relative flux (Jr ) is obtained by normalizing Jv by Jvw and represented as Eq. (6): Jr =
1 − Π 1 + Ra
(6)
where the normalized resistance parameters are Ra =
Ra Rm
and
Π =
σ Π P
Thus, the total flux reduction (Jrt ) can be represented as Π + Ra Jrt = 1 − Jr = (7) 1 + Ra In the case where no fouling occurs, then Jrt = 0. This implies that no flux reduction relative to pure water is observed. As Jrt → 1, the flux reduction is very large and it would mean that serious fouling has occurred.
J.-J. Qin et al. / Journal of Membrane Science 218 (2003) 173–183
177
2.4.3. Morphology study by SEM For Scanning Electron Microscopy (SEM) study, the as-spun fiber samples were immersed in liquid nitrogen and fractured to obtain tidy cross-section of fibers, followed by an ethanol–hexane–air drying procedure [28]. The fibers were then dried in a vacuum oven for at least 3 h at room temperature. Following which, they were immediately sputtered with gold using JEOL® JEE-400 Ion Sputtering Device. To investigate the structure and morphology of the fibers, these samples were studied using a JEOL® JSM 5310 LV scanning microscope. Fig. 2. Weight loss of polymers vs. drying time.
3. Results and discussion
3.2. Morphology of hollow fiber membranes
3.1. Formulation design of a spinning dope containing water
3.2.1. Effect of air gap Figs. 3 and 4 show SEM images of hollow fiber membranes spun from an air gap of 550 and 880 mm, respectively. The fiber diameters measured from the SEM images are shown in Table 2. It can be seen that the outer diameter or wall thickness of a fiber decreased with increasing air gap due to the enhanced elongation of a nascent fiber in an increased air gap while the inner diameter remained the same. Figs. 3a and b and 4a and b show that the cross-sections of the as-spun fibers regardless the air gap had sponge-like structures. The observed morphology confirmed that addition of high MW PVP in the spinning dope would favor the suppression of macrovoids [21,23,25]. Figs. 3c and 4c indicate that
CA and PVP 360 K polymers at room temperature absorb moisture due to their hydrophilic properties. Normally, the moisture has to be removed before making a spinning dope. The step to remove moisture may be omitted if the dope is purposely designed to contain water and the moisture contents in the polymers remain constant in the environment of almost unchanged temperature and humidity, which is the case in this study. Two advantages are apparent. One is savings on process time and energy (normally the moisture is removed by heating). Another is the ease in mixing polymers with the non-solvent water and the solvent. However, the quantity of the moisture contents in CA and PVP 360 K polymers have to be determined before designing the dope formulation. Fig. 2 shows the loss in weight of the polymers over time during heating. The weight loss could be attributed to the removal of moisture. It could be observed that weight loss of the polymers would tend to stabilize after the 8th day. The moisture content of CA and PVP 360 K was found to be 5.51 and 2.22%, respectively. As a consequence, 60.60 g CA, 225 g NMP and 15.30 g PVP 360 K could be mixed in practice to obtain a designed dope formulation of CA/NMP/PVP 360 K/water in a ratio of 19.0/74.8/5.0/1.2. Experimental parameters used in the spinning of hollow fiber membrane are summarized in Table 1.
Table 1 Experimental parameters used in spinning of CA hollow fiber UF membranes Parameters
Range of variables
Dope solution composition
CA/NMP/PVP 360 K/water (19/74.8/5/1.2) 0.29 Water 24 0.85 550 or 880 0.71 30 Water 1.0/0.6 (o.d./i.d.) 24 60–62
Dope flow rate (g min−1 ) Bore fluid composition Bore fluid temperature (◦ C) Bore fluid flow rate (cm3 min−1 ) Air-gap distance (mm) Take-up speed (m min−1 ) Coagulant temperature (◦ C) External coagulant Dimensions of spinneret (mm) Spinneret temperature (◦ C) Room relative humidity (%)
178
J.-J. Qin et al. / Journal of Membrane Science 218 (2003) 173–183
Fig. 3. SEM images of CA hollow fiber membrane spun from an air gap of 550 mm. (a) Overall, magnification: 75×; (b) partial cross-section, magnification: 1000×; (c) inner edge, magnification: 5000×; (d) outer edge, magnification: 10,000×.
the inner edges of the fibers were dense but Fig. 3d (where the outer layer faces the top-right) and Fig. 4d show that the outer edges were fully microporous, which suggested that the hollow fiber membranes had an inner dense skin. The as-spun fibers with an inner dense skin and outer porous surface were formed due to the fact that an instantaneous phase separation started from the inner surface of the nascent fiber immediately after the spinning dope exited from the
spinneret and the demixing of the dope solution continued toward to the outer surface as the fiber fell through an air gap of 550 or 880 mm, finally the whole membrane structure was completed by the inner coagulant before the fiber entered the outer coagulation water bath. Furthermore, Figs. 3c and 4c also reveal that the inner skin became denser and thicker with increasing air gap, which could be attributed to the enhanced gravity-induced elongational stress and
Table 2 Effect of air gap on properties of CA hollow fiber UF membranes Membrane ID
Air gap (mm)
Outer diameter (m)
Inner diameter (m)
Wall thickness (m)
Pure water flux (×10−5 l m−2 h−1 Pa−1 )
Retention (%) PVP 10 K
PVP 24 K
BSA 66 K
M1 M2
550 880
960 940
700 700
260 240
87.1 68.2
0.9 5.2
36.1 55.4
98 100
J.-J. Qin et al. / Journal of Membrane Science 218 (2003) 173–183
179
Fig. 4. SEM images of CA hollow fiber membrane spun from an air gap of 880 mm. (a) Overall, magnification: 75×; (b) partial cross-section, magnification: 750×; (c) inner edge, magnification: 5000×; (d) outer edge, magnification: 5000×.
orientation on the inner skin in an increased air gap [15,17,18]. The fiber with a denser and thicker skin would result in a lower water flux and a higher solute retention, which was supported by the latter results. 3.2.2. Effect of hypochlorite treatment Figs. 5a and b show SEM images of inner and outer surfaces of the fiber M2, respectively and Figs. 5c and d show the corresponding inner and outer surfaces after the treatment with a hypochlorite solution of 200 mg l−1 over 24 h. Comparing Fig. 5a and c, the inner surface of the fiber appeared to become smooth and the pores in the inner surface appeared to be larger after the hypochlorite treatment. The porosity on the outer surface of the treated fiber in Fig. 5d appeared to be higher than that of the untreated fiber in Fig. 5b. These changes in the membrane surface morphology could be attributed to the removal of PVP contents in
the blend membrane. As a result, the treated fiber with larger pore size would show a higher flux and a lower retention and the treated fiber with less PVP contents would show an increased fouling tendency, which was also subsequently confirmed. 3.3. Fluxes and retentions of hollow fiber membranes 3.3.1. Effect of air gap Measurements of fluxes and retentions of hollow fiber UF membranes spun from different air gap are also summarized in Table 2. It can be seen that water flux of a membrane decreased while retention increased with increasing air gap due to the thicker and denser inner skin layer of the fiber spun from higher air gap as observed by SEM. Table 2 also shows that the solute retention increased with an increase in
180
J.-J. Qin et al. / Journal of Membrane Science 218 (2003) 173–183
Fig. 5. Effect of hypochlorite treatment on surface morphology of CA hollow fiber membrane. (a) Untreated inner surface, magnification: 7500×; (b) untreated outer surface, magnification: 7500×; (c) treated inner surface, magnification: 7500×; (d) treated outer surface, magnification: 7500×.
molecular weight of the solute used. Both membranes gave high separations of albumin (MW 66 kg mol−1 ) or larger macromolecules from other bio-products with lower molecular mass. 3.3.2. Effect of hypochlorite treatment At first, the as-spun fibers were soaked in a 4000 mg l−1 hypochlorite solution for trial testing. It was observed that the fibers were very weak and fragile after being soaked in the hypochlorite solution over 6 h, which confirmed that CA could not tolerate high concentration of hypochlorite. According to Roesink’s work [21], a hypochlorite solution of 200 mg l−1 could cause a significant reaction between hypochlorite and PVP 360 K. Therefore, a hypochlorite solution of 200 mg l−1 was then selected to treat the hollow fiber membranes over differ-
ent durations in this study. Measurements of fluxes and retentions of M2 membranes, untreated and treated over different durations are summarized in Table 3. From the data, it could be seen that fluxes of the treated membranes were about three times higher than that of the untreated membrane while retentions of the treated membranes were much lower than the untreated one, which indicated that the pore size increased due to the hypochlorite treatment as shown in Fig. 5. This could be attributed to the reaction between hypochlorite and PVP in which the pyrrolidone ring of PVP was mainly involved, causing chain scission of PVP molecules and the eventual leaching of PVP from the membrane after the treated fibers were rinsed in water [19]. The results in this study were also in agreement with the previous work [23–25] but differed from Wienk et al. [22] who reported that the membrane pore
J.-J. Qin et al. / Journal of Membrane Science 218 (2003) 173–183
181
Table 3 Effect of hypochlorite treatment time on pure water flux and solute retention of CA UF membranes Membrane ID
Treatment time (h)
M2 M2a M2b
0 (untreated) 6 24
Pure water flux (×10−5 l m−2 h−1 Pa−1 )
Retention (%) PVP 24 K
BSA 66 K
68.2 221 225
55.4 1.9 0
100 79 78
structure was not altered substantially by hypochlorite treatment because membrane retentions for BSA were found to be constant although the membrane water flux increased after hypochlorite treatment. The difference in observation may be due to the fact that the pore size of their membranes after hypochlorite treatment was possibly still too small for BSA passing through, resulting in no significant change in retentions but the pore size of the membranes in this study was significantly increased after hypochlorite treatment, resulting in an obvious change in retentions for BSA. It could also be seen that there was no significant change in both flux and retention for the membranes treated over 6 and 24 h, which suggested that there was an optimum treatment time of less than 6 h to obtain a high flux membrane. It is emphasized that both M2a and M2b showed the same physical appearance in flexibility as the untreated fiber M2, which was used as an indication that the CA membrane tolerated these hypochlorite treatments. However, when the treatment time was 48 h, the CA membranes were attacked by hypochlorite because the fibers became fragile. 3.4. Fouling tendency Table 4 shows the experimental measurements of Jvw , Jv , and Jva and the calculated Rm , Ra , σ Π and Jrt of the membranes, untreated and treated with
hypochlorite. In the calculation, µ was assumed to be 0.89 cPa of water viscosity at 25 ◦ C as the solution of 100 mg l−1 was very dilute. Rm was directly calculated from Eq. (3). Ra was then obtained from Eq. (5). And then σ Π was gained by substituting Rm and Ra into Eq. (4). Finally, Jrt was obtained from Eqs. (6) and (7) by substituting Rm , Ra and σ Π into Eq. (6). For the untreated membrane M2, it could be observed that the hydraulic resistance of the absorbed layer (Ra ) was much smaller (around 200 times) than that of the membrane (Rm ) and the osmotic pressure (σ Π) was around 20 times lower than the operating pressure (P). The total relative flux reduction (Jrt ) was low of 5.1% that was mainly caused by Π (or the osmotic pressure σ Π) as Ra was small according to Eqs. (6) and (7). For the treated membrane M2a , however, Ra was only 22 times smaller than Rm and σ Π was 18 times lower than P. Therefore, the total relative flux reduction Jrt was contributed by both Ra and Π as they were close. As a consequence, Jrt was high of 9.4% that was around twice of the untreated membrane. These results indicated that the treated membrane showed higher fouling tendency due to its higher resistance of the absorbed layer than the untreated membrane, which may be partially caused by the facts that the treated membrane had lower hydrophilicity due to the removal of PVP contents in the membrane matrix after hypochlorite treatment and
Table 4 Rm , Ra , σ Π and Jrt of CA UF membranes with hypochlorite treatment Membrane ID
M2 M2a
Solute
BSA BSA
Flux (×10−5 m s−1 ) Jvw
Jv
Jva
1.493 5.512
1.416 4.995
1.485 5.361
Rm (m−1 )
Ra (m−1 )
σ Π (Pa)
Jrt
7.81E+11 2.15E+11
4.00E+09 9.63E+09
4820 5613
0.051 0.094
Note: P in the measurements of Jvw , Jv , and Jva was 104 kPa for the membrane M2 and was 105, 105 and 107 kPa, respectively, for the membrane M2a .
182
J.-J. Qin et al. / Journal of Membrane Science 218 (2003) 173–183
had higher porosity than the untreated membrane as shown in Fig. 5.
4. Conclusions CA hollow fiber UF membranes were developed from a new dope solution containing CA/PVP 360K/NMP/water with a mass ratio of 19.0/5/74.8/1.2 using a dry-jet wet spinning process. The experimental results showed that water flux of a membrane decreased while retention increased with increasing air gap. Fluxes of treated membranes with hypochlorite solution were about three times greater than that of an untreated membrane and retentions of the treated membranes were much lower. A 6 h treatment with 200 mg l−1 hypochlorite solution could achieve a CA membrane with a high pure water flux of 220 × 10−5 l m−2 h−1 Pa−1 . The experimental results also showed that the treated CA membrane experienced higher fouling tendency than the untreated membrane. SEM images revealed that PVP additive in the dope would favor the suppression of macrovoids and the thickness of inner skin increased with increasing air gap. PVP contents in the blend membrane could be significantly removed by hypochlorite treatment and the pore size of the treated membrane was increased. The latter observation differed from published results and an attempt at explaining the difference was given. References [1] S. Loeb, S. Sourirajan, Sea water demineralization by means of an osmotic membrane, Adv. Chem. Ser., ACS 38 (1963) 117. [2] OSMONICS, 2 , 4 and 8 RO Cellulose Acetate, Catalogue of Products, 2001. [3] Millipore Corporation, MF-MilliporeTM Membrane Filters, Catalogue of Products, 2001. [4] W.J. Schell, Cellulose acetate membranes for CO2 /CH4 separation, ACS Div. Fuel Chem. Preprints 20 (1975) 253. [5] M.-A. Chaudry, Water and ions transport mechanism in hyperfiltration with symmetric cellulose acetate membranes, J. Membr. Sci. 206 (2002) 319. [6] C. Combe, E. Molis, P. Lucas, R. Riley, M.-M. Clark, The effect of CA membrane properties on adsorptive fouling by humic acid, J. Membr. Sci. 154 (1999) 73. [7] W. Byrne (Ed.), Reverse Osmosis, Tall Oaks Publishing, Inc., USA, 1995.
[8] Y. Chen, X.-P. Xiong, G.-A. Yang, L.-N. Zhang, S.-L. Lei, H. Liang, Characterization of regenerated cellulose membranes hydrolyzed from cellulose acetate, Chin. J. Polym. Sci. 20 (2002) 369. [9] J.-H. Hao, S. Wang, Influence of quench medium on the structure and gas permeation properties of cellulose acetate membranes, J. Appl. Polym. Sci. 68 (1998) 1269. [10] J.-H. Hao, H. Dai, P. Yang, J. Wei, Z. Wang, Cellulose acetate hollow fiber performance for ultra low pressure RO, Desalination 107 (1996) 217. [11] M.J. Kell, R.D. Mahoney, Cellulose acetate hollow fiber and method for making same, US Patent 4,276,173 (1981). [12] M. Suzuki, M. Ohno, I. Yamamoto, Cellulose ester hollow fiber membrane for plasma separation, US Patent 4,886,631 (1989). [13] G. Duweg, L. Steinfeld, W. Ansorge, Process for manufacturing cellulose acetate membranes, US Patent 5,505,890 (1996). [14] G.-H. Zhu, T.-S. Chung, K.-C. Loh, Activated carbon-filled cellulose acetate hollow-fiber membrane for cell immobilization and phenol degradation, J. Appl. Polym. Sci. 76 (2000) 695. [15] T.S. Chung, X. Hu, Effect of air-gap distance on the morphology and thermal properties of polyethersulfone hollow fibers, J. Appl. Polym. Sci. 66 (1997) 1067. [16] T.S. Chung, Z.L. Xu, W.H. Lin, Fundamental understanding of the effect of air gap distance on the fabrication of hollow fiber membranes, J. Appl. Polym. Sci. 72 (1999) 379. [17] M.C. Porter (Ed.), Handbook of Industrial Membrane Technology, Noyes Publications, New Jersey, 1990, pp. 61– 305. [18] T. Liu, D. Zhang, S. Xu, S. Sourirajan, Solution-spin hollow fiber polysulfone and polyethersulfone ultrafiltration membranes, Sep. Sci. Technol. 27 (1992) 161. [19] I. Wienk, Ultrafiltration membranes from a polymer blend, Ph.D. Thesis, University of Twente, The Netherlands, 1993 (Chapter 2). [20] T. Miyano, T. Matsuura, S. Sourirajan, Effect of polyvinylpyrrolidone additive on the pore size and pore size distribution of polyethersulfone (Victrex) membranes, Chem. Eng. Commun. 119 (1993) 23. [21] H.D.K. Roesink, Microfiltration, membrane development and module design, Ph.D. Thesis, University of Twente, 1989. [22] I.M. Wienk, E.E.B. Meuleman, Z. Borneman, A. vanden Boomgaard, C.A. Smolders, Chemical treatment of membranes of a polymer blend: mechanism of the reaction of hypochlorite with poly(vinyl pyrrolidone), J. Polym. Sci.: Part A: Polym. Chem. 33 (1995) 49. [23] 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. [24] J.J. Qin, F.S. Wong, Hypochlorite treatment of hydrophilic hollow fiber ultrafiltration membranes for high fluxes, Desalination 146 (2002) 307.
J.-J. Qin et al. / Journal of Membrane Science 218 (2003) 173–183 [25] J.J. Qin, F.S. Wong, Y. Li, Y.T. Liu, A high flux ultrafiltration membrane spun from PSU/PVP (K90)/DMF/1,2-propanediol, J. Membr. Sci. 211 (2003) 139. [26] J.J. Qin, J. Gu, T.S. Chung, Effect of wet and dry-jet wet spinning on the shear-induced orientation during the formation of ultrafiltration hollow fiber membranes, J. Membr. Sci. 182 (2001) 57.
183
[27] M.K. Ko, J.J. Pellegrino, Determination of osmotic pressure and fouling resistances and their effects on performance of ultrafiltration membranes, J. Membr. Sci. 74 (1992) 141. [28] J.J. Qin, T.S. Chung, Effect of dope flow rate on morphology, separation performance, thermal and mechanical properties of ultrafiltration hollow fiber membranes, J. Membr. Sci. 157 (1999) 35.
Cellulose acetate hollow fiber ultrafiltration membranes made from CA/PVP 360 K/NMP/water Jian-Jun Qin a,∗ , Ying Li b , Leng-Siang Lee c , Hsiaowan Lee a a
c
Centre for Advanced Water Technology, Singapore Utilities International Pte Ltd., Blk 2, #241, Innovation Centre (NTU), 18 Nanyang Drive, Singapore 637723, Singapore b Institute of Environmental Science and Engineering, Blk 2, #237, Innovation Centre (NTU), 18 Nanyang Drive, Singapore 637723, Singapore Department of Civil Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore Received 1 March 2003; accepted 4 April 2003
Abstract Hydrophilic hollow fiber ultrafiltration (UF) membranes have been prepared from a new dope solution containing cellulose acetate (CA)/poly(vinyl pyrrolidone) (PVP 360 K)/N-methyl-2-pyrrolidone (NMP)/water with a mass ratio of 19.0/5.0/74.8/1.2 by using a dry-jet wet spinning process. The effect of air-gap length was studied. The as-spun fibers were post-treated by means of a hypochlorite solution of 200 mg l−1 (200 ppm) over different duration. The morphology of the membranes was investigated by Scanning Electron Microscopy (SEM). Pure water fluxes and solute retentions of the membranes were measured and different fouling resistances were analyzed. The experimental results showed that water flux of a membrane decreased while retention increased with increasing air gap. Fluxes of hypochlorite treated membranes were three times higher than that of an untreated membrane and retentions of the treated membranes were much lower. A 6 h treatment with 200 mg l−1 hypochlorite solution could achieve a CA UF membrane with a high pure water flux of 220 × 10−5 l m−2 h−1 Pa−1 . SEM images revealed that, PVP additive would favor the suppression of macrovoids and the thickness of inner skin increased with increasing air gap. PVP contents in the blend membrane could be significantly removed with the hypochlorite treatment. As a result, the treated CA membrane experienced higher fouling tendency than the untreated membrane. It was concluded that hypochlorite treatment altered the pore size of hollow fiber UF membranes made from the blend of CA and PVP 360 K. This is an extended conclusion to the published results and an attempt at explaining some differences were provided. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Cellulose acetate; Poly(vinyl pyrrolidone); Hollow fiber spinning; Hydrophilic ultrafiltration membranes; Hypochlorite treatment
1. Introduction The cellulose acetate (CA) membrane was the first high performance asymmetric membrane [1]. It has been widely used for reverse osmosis (RO), microfil∗ Corresponding author. Tel.: +65-67941507; fax: +65-67942791. E-mail address: [email protected] (J.-J. Qin).
tration (MF) and gas separation [2–4]. CA membranes have excellent hydrophilicity that is very important in minimizing fouling, good resistance to chlorine and solvent [5–7]. A regenerated CA membrane that was hydrolyzed from cellulose acetate has significantly improved solvent-resistance and thermostability [8]. Commercial CA membranes are either flat sheet or spiral-wound modules [2,3]. The hollow fiber configuration has become a favorite choice because the
0376-7388/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0376-7388(03)00170-4
174
J.-J. Qin et al. / Journal of Membrane Science 218 (2003) 173–183
hollow fiber membrane has three major advantages: (1) hollow fiber modules have much larger ratio of membrane area to unit volume (several thousand M2 /M3 ) compared to flat and spiral-wound modules (a few hundred M2 /M3 ), and hence higher productivity per unit volume of membrane module; (2) they are self-supporting which can be back-washed to recover the flux; and (3) they have good flexibility in the mode of operation. Hao and coworkers [9,10] have studied CA hollow fiber membrane preparation by the dry-jet wet spinning for gas separation and for ultra low pressure RO application. There are a number of patents on the fabrication of CA hollow fiber MF membranes [11–13]. An activated carbon-filled cellulose acetate hollow fiber membrane for cell immobilization and phenol degradation has been studied recently [14]. The air-gap length during spinning affects the performance of final fibers and this has been studied by a number of researchers [15–19]. Chung and Hu [15] reported that an increase in air-gap distance resulted in a significant decrease in permeation of the membrane, most likely due to a higher gravity-induced elongational stress on the fibers spun from a higher air-gap distance, resulting in a greater orientation and tighter molecular packing. Porter [17] stated that an increase in the air-gap length would result in tighter fibers with lower permeability and increased retentivity because of an increase in the degree of orientation due to the added weight of the fiber below and the extended polymer chains. Liu et al. [18] demonstrated that the average size of pores on the inner surface of polysulfone/polyethersulfone membranes decreased when the air-gap length increased due to the nascent fiber being stretched and elongated by its own weight, resulting in the polymer aggregates moving closer together and rearranging themselves into a state of greater stability. However, very few studies on CA hollow fiber ultrafiltration (UF) membranes have been published. PVP as an additive is often blended into the dope solution to increase hydrophilicity and flux of membranes [19–23]. However, the addition of PVP with high molecular mass results in a low flux membrane due to swelling of the residual PVP at the surface of pore walls when water passes through the membrane pores [19,21]. Therefore, hypochlorite treatment has been applied to increase flux of MF/UF membranes made from a blend of PVP and hydrophobic poly-
mers [21–25]. Wienk et al. [22] investigated the effect of hypochlorite treatment on a hollow fiber UF membrane made from a blend of PES and PVP 360 K and concluded that hypochlorite treatment reduced the swelling of PVP in the pores of the membranes without substantially changing the pore structure. Recently, Xu et al. [23] studied the effect of hypochlorite treatment on PEI/PVP hollow fiber UF membranes for oil/water separation. They reported that the treated membranes showed an obvious lower solute retention or larger pores than that of untreated membranes. They also found that the addition of PVP with high molecular mass would suppress the formation of macrovoids. More recently, the authors [24,25] have reported an optimum treatment time with hypochlorite and an optimum hypochlorite concentration to achieve high flux membranes spun from a blend of polysulfone and PVP. To our best knowledge, it does not appear to report how hypochlorite treatment impacts on a hollow fiber UF membrane fabricated from blends of CA and PVP of high molecular mass. This paper explores the effects of air-gap length and hypochlorite treatment on properties and morphology of CA hollow fiber UF membranes prepared from a new dope solution containing CA/PVP 360 K/NMP/water. 2. Experimental 2.1. Materials The membrane material, cellulose acetate (CA398-30) with acetyl content of 39.7 wt.% was purchased from Eastman Chemical Company (Kingsport, USA). Poly(vinyl pyrrolidone) (PVP 360 K, average MW 360,000) polymer was supplied by Sigma. The solvent, N-methyl-2-pyrrolidone (NMP, >99%) was supplied by Merck (Darmstadt, Germany). Poly(vinyl pyrrolidone) [PVP 10 K (average MW 10,000) and PVP 24 K (average MW 24,000)] from Sigma–Aldrich and bovine serum albumin (BSA, MW 66,000) from Calbiochem® Biosciences, Inc. (La Jolla, CA, USA) were used to characterize the separation performance of the hollow fiber UF membranes. Sodium hypochlorite solution (10–12%) from SINO Chemical Co. (Singapore) was used in the post-treatment of the UF hollow fiber membranes.
J.-J. Qin et al. / Journal of Membrane Science 218 (2003) 173–183
2.2. Fabrication of hollow fiber UF CA membranes 2.2.1. Preparation of a spinning dope CA and PVP 360 K were dried in an oven at 65 ◦ C to remove the moisture in the respective polymer samples. The weight loss of the polymers was monitored for 11 days until constant weight was achieved. The amount of water content in both CA and PVP 360 K could be determined from the final weight loss and was used to design the formulation of a spinning dope containing water. The spinning dope of CA/PVP 360 K/NMP/water was prepared by the following procedure. First, the solvent, NMP and the additive, undried PVP 360 K were well mixed in a glass bottle. The undried membrane polymer CA was then added into the mixture with the amount of water in the undried CA and PVP taken into consideration. After that, the mixture was stirred for at least 48 h to ensure the dope was a true solution, which was supported by the observation that the dope was still transparent after a period of 3-month storage in a 50 ml triangle flask. Finally, the formulated dope in a solution tank and the bore fluid in a metal canister were degassed to remove any gas bubbles before spinning.
175
different periods of time up to 48 h. Before the treatment, pH of the hypochlorite solution had to be adjusted from 11.4 to 7.0 by using H2 SO4 as CA membranes could not tolerate high pH. The treated fibers were then rinsed again in flowing town water. After that, the same procedure as described above was followed to characterize the treated fibers. 2.4. Characterization of hollow fiber UF membranes
2.3. Post-treatment of hollow fiber UF membranes
2.4.1. Measurements of flux and retention The experiments to measure flux and retention of hollow fiber UF membranes were carried out at temperature 25 ± 0.5 ◦ C in a cross-flow filtration setup as shown in Fig. 1. Six fibers with the length of around 30 cm were assembled into a test module. Since the hollow fibers had an inner dense skin, the feed was pumped into the lumen side of the fibers. The permeate was then collected at the bottom of the U type modules. In the tests, ultra pure water was first used to characterize the pure water flux of a fresh membrane. Feed solutions containing 100 mg l−1 of different solutes in ultra pure water were then used to test the separation performance of the membrane. After that, the fouled membrane was flushed with ultra pure water and the pure water flux of the fouled membrane was measured again. The average transmembrane pressure was around 100 kPa and the feed flow rate was adjusted to allow a feed velocity of 1 m s−1 . The solute concentration in the feed or in the permeate was determined by a TOC-5000A Analyser (Shimadzu). The permeate flux for an inner skin membrane was calculated by Eq. (1). The retention was calculated by Eq. (2). Q Q = (1) Flux = A P Nπdi P
The as-spun fibers were rinsed in flowing town water at room temperature for 16 h. After rinsing, the fibers were immersed in a 50% aqueous glycerol solution for 48 h and then dried in air at room temperature before used for making test modules. The pure water fluxes and solute retentions of the modules were then measured. To investigate the effect of hypochlorite treatment on membrane properties, the rinsed fibers were immersed in a hypochlorite solution of 200 mg l−1 over
in which, Cf (mg l−1 ) and Cp (mg l−1 ) represent the solute concentrations in the feed and permeate, respectively.
2.2.2. Spinning of hollow fibers Hollow fiber UF membranes were spun using the well-known dry-jet wet spinning process [15,25,26]. The dope solution and the bore fluid were fed into the spinneret by means of pressure from a nitrogen gas cylinder. The flow rates of the dope solution and the bore fluid could be regulated using valves that were provided. A flow meter was attached to the bore fluid setup to measure the flow rate.
where Q is the volume flow rate of permeate (l h−1 ), A the effective membrane area (m2 ), P the transmembrane pressure (Pa), N the number of fibers, di the inner diameter of fiber (m), and the effective length of fiber (m). Cp × 100% (2) Retention = 1 − Cf
176
J.-J. Qin et al. / Journal of Membrane Science 218 (2003) 173–183
Fig. 1. Experimental setup in testing of hollow fiber UF membranes.
2.4.2. Study of fouling tendency of hollow fiber membranes A UF membrane would show a reduction in flux with fouling. One of the most significant flux reduction mechanisms is concentration–polarization at the membrane surface. This phenomenon would lead to an increase in the osmotic pressure. Membrane fouling due to adsorption of solute molecules on the membrane surface and in the membrane pores would then occur. The osmotic pressure-adsorption model [27] was used in this study. Under normal operating conditions, the flux would decrease due to two main mechanisms: (i) reduction in hydrodynamic driving force by osmotic pressure (σ Π) and (ii) increase in fouling resistance (Ra ) from surface adsorption and pore plugging. The pure water flux through a fresh membrane (Jvw ) can be described by Darcy’s law (3). The solution flux (Jv ) and pure water flux through a fouled membrane (Jva ) can be represented as in Eqs. (4) and (5). Jvw = Jv =
P µRm
P − σ Π µ(Rm + Ra )
(3)
(4)
Jva =
P µ(Rm + Ra )
(5)
where P is the transmembrane pressure, Rm the hydraulic membrane resistance, µ the viscosity of the solution, Ra the hydraulic resistance of the adsorbed layer and σ Π the osmotic pressure. For comparison, the normalization of fluxes and flux reductions are usually computed with respect to the individual membrane resistances of the individual membrane. The relative flux (Jr ) is obtained by normalizing Jv by Jvw and represented as Eq. (6): Jr =
1 − Π 1 + Ra
(6)
where the normalized resistance parameters are Ra =
Ra Rm
and
Π =
σ Π P
Thus, the total flux reduction (Jrt ) can be represented as Π + Ra Jrt = 1 − Jr = (7) 1 + Ra In the case where no fouling occurs, then Jrt = 0. This implies that no flux reduction relative to pure water is observed. As Jrt → 1, the flux reduction is very large and it would mean that serious fouling has occurred.
J.-J. Qin et al. / Journal of Membrane Science 218 (2003) 173–183
177
2.4.3. Morphology study by SEM For Scanning Electron Microscopy (SEM) study, the as-spun fiber samples were immersed in liquid nitrogen and fractured to obtain tidy cross-section of fibers, followed by an ethanol–hexane–air drying procedure [28]. The fibers were then dried in a vacuum oven for at least 3 h at room temperature. Following which, they were immediately sputtered with gold using JEOL® JEE-400 Ion Sputtering Device. To investigate the structure and morphology of the fibers, these samples were studied using a JEOL® JSM 5310 LV scanning microscope. Fig. 2. Weight loss of polymers vs. drying time.
3. Results and discussion
3.2. Morphology of hollow fiber membranes
3.1. Formulation design of a spinning dope containing water
3.2.1. Effect of air gap Figs. 3 and 4 show SEM images of hollow fiber membranes spun from an air gap of 550 and 880 mm, respectively. The fiber diameters measured from the SEM images are shown in Table 2. It can be seen that the outer diameter or wall thickness of a fiber decreased with increasing air gap due to the enhanced elongation of a nascent fiber in an increased air gap while the inner diameter remained the same. Figs. 3a and b and 4a and b show that the cross-sections of the as-spun fibers regardless the air gap had sponge-like structures. The observed morphology confirmed that addition of high MW PVP in the spinning dope would favor the suppression of macrovoids [21,23,25]. Figs. 3c and 4c indicate that
CA and PVP 360 K polymers at room temperature absorb moisture due to their hydrophilic properties. Normally, the moisture has to be removed before making a spinning dope. The step to remove moisture may be omitted if the dope is purposely designed to contain water and the moisture contents in the polymers remain constant in the environment of almost unchanged temperature and humidity, which is the case in this study. Two advantages are apparent. One is savings on process time and energy (normally the moisture is removed by heating). Another is the ease in mixing polymers with the non-solvent water and the solvent. However, the quantity of the moisture contents in CA and PVP 360 K polymers have to be determined before designing the dope formulation. Fig. 2 shows the loss in weight of the polymers over time during heating. The weight loss could be attributed to the removal of moisture. It could be observed that weight loss of the polymers would tend to stabilize after the 8th day. The moisture content of CA and PVP 360 K was found to be 5.51 and 2.22%, respectively. As a consequence, 60.60 g CA, 225 g NMP and 15.30 g PVP 360 K could be mixed in practice to obtain a designed dope formulation of CA/NMP/PVP 360 K/water in a ratio of 19.0/74.8/5.0/1.2. Experimental parameters used in the spinning of hollow fiber membrane are summarized in Table 1.
Table 1 Experimental parameters used in spinning of CA hollow fiber UF membranes Parameters
Range of variables
Dope solution composition
CA/NMP/PVP 360 K/water (19/74.8/5/1.2) 0.29 Water 24 0.85 550 or 880 0.71 30 Water 1.0/0.6 (o.d./i.d.) 24 60–62
Dope flow rate (g min−1 ) Bore fluid composition Bore fluid temperature (◦ C) Bore fluid flow rate (cm3 min−1 ) Air-gap distance (mm) Take-up speed (m min−1 ) Coagulant temperature (◦ C) External coagulant Dimensions of spinneret (mm) Spinneret temperature (◦ C) Room relative humidity (%)
178
J.-J. Qin et al. / Journal of Membrane Science 218 (2003) 173–183
Fig. 3. SEM images of CA hollow fiber membrane spun from an air gap of 550 mm. (a) Overall, magnification: 75×; (b) partial cross-section, magnification: 1000×; (c) inner edge, magnification: 5000×; (d) outer edge, magnification: 10,000×.
the inner edges of the fibers were dense but Fig. 3d (where the outer layer faces the top-right) and Fig. 4d show that the outer edges were fully microporous, which suggested that the hollow fiber membranes had an inner dense skin. The as-spun fibers with an inner dense skin and outer porous surface were formed due to the fact that an instantaneous phase separation started from the inner surface of the nascent fiber immediately after the spinning dope exited from the
spinneret and the demixing of the dope solution continued toward to the outer surface as the fiber fell through an air gap of 550 or 880 mm, finally the whole membrane structure was completed by the inner coagulant before the fiber entered the outer coagulation water bath. Furthermore, Figs. 3c and 4c also reveal that the inner skin became denser and thicker with increasing air gap, which could be attributed to the enhanced gravity-induced elongational stress and
Table 2 Effect of air gap on properties of CA hollow fiber UF membranes Membrane ID
Air gap (mm)
Outer diameter (m)
Inner diameter (m)
Wall thickness (m)
Pure water flux (×10−5 l m−2 h−1 Pa−1 )
Retention (%) PVP 10 K
PVP 24 K
BSA 66 K
M1 M2
550 880
960 940
700 700
260 240
87.1 68.2
0.9 5.2
36.1 55.4
98 100
J.-J. Qin et al. / Journal of Membrane Science 218 (2003) 173–183
179
Fig. 4. SEM images of CA hollow fiber membrane spun from an air gap of 880 mm. (a) Overall, magnification: 75×; (b) partial cross-section, magnification: 750×; (c) inner edge, magnification: 5000×; (d) outer edge, magnification: 5000×.
orientation on the inner skin in an increased air gap [15,17,18]. The fiber with a denser and thicker skin would result in a lower water flux and a higher solute retention, which was supported by the latter results. 3.2.2. Effect of hypochlorite treatment Figs. 5a and b show SEM images of inner and outer surfaces of the fiber M2, respectively and Figs. 5c and d show the corresponding inner and outer surfaces after the treatment with a hypochlorite solution of 200 mg l−1 over 24 h. Comparing Fig. 5a and c, the inner surface of the fiber appeared to become smooth and the pores in the inner surface appeared to be larger after the hypochlorite treatment. The porosity on the outer surface of the treated fiber in Fig. 5d appeared to be higher than that of the untreated fiber in Fig. 5b. These changes in the membrane surface morphology could be attributed to the removal of PVP contents in
the blend membrane. As a result, the treated fiber with larger pore size would show a higher flux and a lower retention and the treated fiber with less PVP contents would show an increased fouling tendency, which was also subsequently confirmed. 3.3. Fluxes and retentions of hollow fiber membranes 3.3.1. Effect of air gap Measurements of fluxes and retentions of hollow fiber UF membranes spun from different air gap are also summarized in Table 2. It can be seen that water flux of a membrane decreased while retention increased with increasing air gap due to the thicker and denser inner skin layer of the fiber spun from higher air gap as observed by SEM. Table 2 also shows that the solute retention increased with an increase in
180
J.-J. Qin et al. / Journal of Membrane Science 218 (2003) 173–183
Fig. 5. Effect of hypochlorite treatment on surface morphology of CA hollow fiber membrane. (a) Untreated inner surface, magnification: 7500×; (b) untreated outer surface, magnification: 7500×; (c) treated inner surface, magnification: 7500×; (d) treated outer surface, magnification: 7500×.
molecular weight of the solute used. Both membranes gave high separations of albumin (MW 66 kg mol−1 ) or larger macromolecules from other bio-products with lower molecular mass. 3.3.2. Effect of hypochlorite treatment At first, the as-spun fibers were soaked in a 4000 mg l−1 hypochlorite solution for trial testing. It was observed that the fibers were very weak and fragile after being soaked in the hypochlorite solution over 6 h, which confirmed that CA could not tolerate high concentration of hypochlorite. According to Roesink’s work [21], a hypochlorite solution of 200 mg l−1 could cause a significant reaction between hypochlorite and PVP 360 K. Therefore, a hypochlorite solution of 200 mg l−1 was then selected to treat the hollow fiber membranes over differ-
ent durations in this study. Measurements of fluxes and retentions of M2 membranes, untreated and treated over different durations are summarized in Table 3. From the data, it could be seen that fluxes of the treated membranes were about three times higher than that of the untreated membrane while retentions of the treated membranes were much lower than the untreated one, which indicated that the pore size increased due to the hypochlorite treatment as shown in Fig. 5. This could be attributed to the reaction between hypochlorite and PVP in which the pyrrolidone ring of PVP was mainly involved, causing chain scission of PVP molecules and the eventual leaching of PVP from the membrane after the treated fibers were rinsed in water [19]. The results in this study were also in agreement with the previous work [23–25] but differed from Wienk et al. [22] who reported that the membrane pore
J.-J. Qin et al. / Journal of Membrane Science 218 (2003) 173–183
181
Table 3 Effect of hypochlorite treatment time on pure water flux and solute retention of CA UF membranes Membrane ID
Treatment time (h)
M2 M2a M2b
0 (untreated) 6 24
Pure water flux (×10−5 l m−2 h−1 Pa−1 )
Retention (%) PVP 24 K
BSA 66 K
68.2 221 225
55.4 1.9 0
100 79 78
structure was not altered substantially by hypochlorite treatment because membrane retentions for BSA were found to be constant although the membrane water flux increased after hypochlorite treatment. The difference in observation may be due to the fact that the pore size of their membranes after hypochlorite treatment was possibly still too small for BSA passing through, resulting in no significant change in retentions but the pore size of the membranes in this study was significantly increased after hypochlorite treatment, resulting in an obvious change in retentions for BSA. It could also be seen that there was no significant change in both flux and retention for the membranes treated over 6 and 24 h, which suggested that there was an optimum treatment time of less than 6 h to obtain a high flux membrane. It is emphasized that both M2a and M2b showed the same physical appearance in flexibility as the untreated fiber M2, which was used as an indication that the CA membrane tolerated these hypochlorite treatments. However, when the treatment time was 48 h, the CA membranes were attacked by hypochlorite because the fibers became fragile. 3.4. Fouling tendency Table 4 shows the experimental measurements of Jvw , Jv , and Jva and the calculated Rm , Ra , σ Π and Jrt of the membranes, untreated and treated with
hypochlorite. In the calculation, µ was assumed to be 0.89 cPa of water viscosity at 25 ◦ C as the solution of 100 mg l−1 was very dilute. Rm was directly calculated from Eq. (3). Ra was then obtained from Eq. (5). And then σ Π was gained by substituting Rm and Ra into Eq. (4). Finally, Jrt was obtained from Eqs. (6) and (7) by substituting Rm , Ra and σ Π into Eq. (6). For the untreated membrane M2, it could be observed that the hydraulic resistance of the absorbed layer (Ra ) was much smaller (around 200 times) than that of the membrane (Rm ) and the osmotic pressure (σ Π) was around 20 times lower than the operating pressure (P). The total relative flux reduction (Jrt ) was low of 5.1% that was mainly caused by Π (or the osmotic pressure σ Π) as Ra was small according to Eqs. (6) and (7). For the treated membrane M2a , however, Ra was only 22 times smaller than Rm and σ Π was 18 times lower than P. Therefore, the total relative flux reduction Jrt was contributed by both Ra and Π as they were close. As a consequence, Jrt was high of 9.4% that was around twice of the untreated membrane. These results indicated that the treated membrane showed higher fouling tendency due to its higher resistance of the absorbed layer than the untreated membrane, which may be partially caused by the facts that the treated membrane had lower hydrophilicity due to the removal of PVP contents in the membrane matrix after hypochlorite treatment and
Table 4 Rm , Ra , σ Π and Jrt of CA UF membranes with hypochlorite treatment Membrane ID
M2 M2a
Solute
BSA BSA
Flux (×10−5 m s−1 ) Jvw
Jv
Jva
1.493 5.512
1.416 4.995
1.485 5.361
Rm (m−1 )
Ra (m−1 )
σ Π (Pa)
Jrt
7.81E+11 2.15E+11
4.00E+09 9.63E+09
4820 5613
0.051 0.094
Note: P in the measurements of Jvw , Jv , and Jva was 104 kPa for the membrane M2 and was 105, 105 and 107 kPa, respectively, for the membrane M2a .
182
J.-J. Qin et al. / Journal of Membrane Science 218 (2003) 173–183
had higher porosity than the untreated membrane as shown in Fig. 5.
4. Conclusions CA hollow fiber UF membranes were developed from a new dope solution containing CA/PVP 360K/NMP/water with a mass ratio of 19.0/5/74.8/1.2 using a dry-jet wet spinning process. The experimental results showed that water flux of a membrane decreased while retention increased with increasing air gap. Fluxes of treated membranes with hypochlorite solution were about three times greater than that of an untreated membrane and retentions of the treated membranes were much lower. A 6 h treatment with 200 mg l−1 hypochlorite solution could achieve a CA membrane with a high pure water flux of 220 × 10−5 l m−2 h−1 Pa−1 . The experimental results also showed that the treated CA membrane experienced higher fouling tendency than the untreated membrane. SEM images revealed that PVP additive in the dope would favor the suppression of macrovoids and the thickness of inner skin increased with increasing air gap. PVP contents in the blend membrane could be significantly removed by hypochlorite treatment and the pore size of the treated membrane was increased. The latter observation differed from published results and an attempt at explaining the difference was given. References [1] S. Loeb, S. Sourirajan, Sea water demineralization by means of an osmotic membrane, Adv. Chem. Ser., ACS 38 (1963) 117. [2] OSMONICS, 2 , 4 and 8 RO Cellulose Acetate, Catalogue of Products, 2001. [3] Millipore Corporation, MF-MilliporeTM Membrane Filters, Catalogue of Products, 2001. [4] W.J. Schell, Cellulose acetate membranes for CO2 /CH4 separation, ACS Div. Fuel Chem. Preprints 20 (1975) 253. [5] M.-A. Chaudry, Water and ions transport mechanism in hyperfiltration with symmetric cellulose acetate membranes, J. Membr. Sci. 206 (2002) 319. [6] C. Combe, E. Molis, P. Lucas, R. Riley, M.-M. Clark, The effect of CA membrane properties on adsorptive fouling by humic acid, J. Membr. Sci. 154 (1999) 73. [7] W. Byrne (Ed.), Reverse Osmosis, Tall Oaks Publishing, Inc., USA, 1995.
[8] Y. Chen, X.-P. Xiong, G.-A. Yang, L.-N. Zhang, S.-L. Lei, H. Liang, Characterization of regenerated cellulose membranes hydrolyzed from cellulose acetate, Chin. J. Polym. Sci. 20 (2002) 369. [9] J.-H. Hao, S. Wang, Influence of quench medium on the structure and gas permeation properties of cellulose acetate membranes, J. Appl. Polym. Sci. 68 (1998) 1269. [10] J.-H. Hao, H. Dai, P. Yang, J. Wei, Z. Wang, Cellulose acetate hollow fiber performance for ultra low pressure RO, Desalination 107 (1996) 217. [11] M.J. Kell, R.D. Mahoney, Cellulose acetate hollow fiber and method for making same, US Patent 4,276,173 (1981). [12] M. Suzuki, M. Ohno, I. Yamamoto, Cellulose ester hollow fiber membrane for plasma separation, US Patent 4,886,631 (1989). [13] G. Duweg, L. Steinfeld, W. Ansorge, Process for manufacturing cellulose acetate membranes, US Patent 5,505,890 (1996). [14] G.-H. Zhu, T.-S. Chung, K.-C. Loh, Activated carbon-filled cellulose acetate hollow-fiber membrane for cell immobilization and phenol degradation, J. Appl. Polym. Sci. 76 (2000) 695. [15] T.S. Chung, X. Hu, Effect of air-gap distance on the morphology and thermal properties of polyethersulfone hollow fibers, J. Appl. Polym. Sci. 66 (1997) 1067. [16] T.S. Chung, Z.L. Xu, W.H. Lin, Fundamental understanding of the effect of air gap distance on the fabrication of hollow fiber membranes, J. Appl. Polym. Sci. 72 (1999) 379. [17] M.C. Porter (Ed.), Handbook of Industrial Membrane Technology, Noyes Publications, New Jersey, 1990, pp. 61– 305. [18] T. Liu, D. Zhang, S. Xu, S. Sourirajan, Solution-spin hollow fiber polysulfone and polyethersulfone ultrafiltration membranes, Sep. Sci. Technol. 27 (1992) 161. [19] I. Wienk, Ultrafiltration membranes from a polymer blend, Ph.D. Thesis, University of Twente, The Netherlands, 1993 (Chapter 2). [20] T. Miyano, T. Matsuura, S. Sourirajan, Effect of polyvinylpyrrolidone additive on the pore size and pore size distribution of polyethersulfone (Victrex) membranes, Chem. Eng. Commun. 119 (1993) 23. [21] H.D.K. Roesink, Microfiltration, membrane development and module design, Ph.D. Thesis, University of Twente, 1989. [22] I.M. Wienk, E.E.B. Meuleman, Z. Borneman, A. vanden Boomgaard, C.A. Smolders, Chemical treatment of membranes of a polymer blend: mechanism of the reaction of hypochlorite with poly(vinyl pyrrolidone), J. Polym. Sci.: Part A: Polym. Chem. 33 (1995) 49. [23] 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. [24] J.J. Qin, F.S. Wong, Hypochlorite treatment of hydrophilic hollow fiber ultrafiltration membranes for high fluxes, Desalination 146 (2002) 307.
J.-J. Qin et al. / Journal of Membrane Science 218 (2003) 173–183 [25] J.J. Qin, F.S. Wong, Y. Li, Y.T. Liu, A high flux ultrafiltration membrane spun from PSU/PVP (K90)/DMF/1,2-propanediol, J. Membr. Sci. 211 (2003) 139. [26] J.J. Qin, J. Gu, T.S. Chung, Effect of wet and dry-jet wet spinning on the shear-induced orientation during the formation of ultrafiltration hollow fiber membranes, J. Membr. Sci. 182 (2001) 57.
183
[27] M.K. Ko, J.J. Pellegrino, Determination of osmotic pressure and fouling resistances and their effects on performance of ultrafiltration membranes, J. Membr. Sci. 74 (1992) 141. [28] J.J. Qin, T.S. Chung, Effect of dope flow rate on morphology, separation performance, thermal and mechanical properties of ultrafiltration hollow fiber membranes, J. Membr. Sci. 157 (1999) 35.