- Email: [email protected]
Hydrolysis differences of polyacrylonitrile support membrane and its influences on polyacrylonitrile-based membrane performance
Desalination 242 (2009) 313–324
Hydrolysis differences of polyacrylonitrile support membrane and its influences on polyacrylonitrile-based membrane performance Guojun Zhanga*, Hong Mengb, Shulan Jia a
Center for Membrane Technology, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing, 100022, P.R. China Tel. +86 (10) 6739-2393; Fax +86 (10) 6739-1983; email: [email protected] b College of Chemical Engineering, Beijing University of Chemical Technology, Beijing, 100029, P.R. China
Received 7 January 2008; accepted revised 19 May 2008
Abstract As polyacrylonitrile (PAN)-based membrane plays an important role in the processes of pervaporation, bioproduct purification and water treatment, this study aims at better understanding the hydrolysis differences by using different alkaline species, and how the hydrolysis degree affect the subsequent modification of hydrolyzed PAN membranes. The ATR-FTIR spectra and the measurements of ion exchange capacity (IEC) demonstrated the differences of hydrolysis degree among the KOH, NaOH and LiOH hydrolyzed PAN membranes. The microtopographical changes on membrane surfaces were characterized by atomic force microscopy. It was found that the membrane surface roughness decreased with the increase in the hydrolysis degree. The molecular weight cut-off (MWCO) of the hydrolyzed membranes was determined by filtration studies using poly(ethylene glycol) and Cytochrome C solutions. It was noted that the MWCO of the PAN membranes decreased from 20,000 to 6000 after hydrolysis with 2 N NaOH or 2 N KOH for 1 h at 65°C. Subsequently, the polyethyleneimine (PEI) was deposited on the top surface of the hydrolyzed PAN membrane to form a composite membrane. The pervaporation performance and ions rejection property of the PAN/PEI composite membrane clearly demonstrated that the hydrolysis difference had a great influence on subsequent modification of the PAN-based membrane. Keywords: Polyacrylonitrile (PAN); Hydrolysis degree; Composite membrane; Pervaporation; Ion rejection
1. Introduction Polyacrylonitrile (PAN) membranes have attracted much attention due to their excellent char*Corresponding author.
acteristics of thermal stability, tolerance to most solvents and commercial availability [1]. Till now, PAN ultrafiltration (UF) has been widely used in protein filtration and membrane bioreactor [2,3]. However, the relative poor hydrophilicity and
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biocompatibility limited its applications. To overcome these problems, the antifouling property could be improved by modifying the membrane surface characteristics. Meanwhile, the rejection property of PAN UF membranes could also be controlled by surface modification. For instance, Qiao et al. [4] recently used the PAN UF membrane with hydrophilic modification to purify Chinese herbs. It was found that the content of danshensu and protocatechuic aldehyde in the permeate was different when PAN membranes with different hydrophilicities were employed for filtration. Additionally, there is other increasing application of PAN UF membranes as the support membranes of the pervaporation and nanofiltration composite membranes due to their relatively active surface [5–12]. For examples, Wang et al. [5] prepared asymmetric PAN nanofiltration (NF) membranes with controlled highly dense pore surface functional groups by hydrolysis of the nitrile groups with NaOH. They also investigated the combined effects of heat treatment and the presence of ZnCl2 on the formation of nanofiltration membranes. In our previous studies [9,10], the polyelectrolyte multilayer pervaporation membranes have already been successfully assembled on a hydrolyzed PAN UF membrane. The modifications of PAN UF membranes usually include plasma treatment [6], plasma-initiated graft polymerization [7,8], photoinduced grafting [12] and hydrolysis [9–11]. Among these methods, hydrolysis under alkaline condition is one of the most important and most frequently used methods for PAN-based membranes, due to the convenience and favorableness for further modification. The hydrolysis mechanism has already been extensively discussed by
~ CH2—CH CN
many researchers [1,4,5,11,13]. As shown in reaction (1), the hydrolysis with alkaline solution is based on the conversion of –CN groups on the PAN membrane surface firstly into –CONH2, then into –COO- groups. In the past studies, sodium hydroxide was commonly used as hydrolysis agent for PAN membrane. Few works have dealt with the effects of alkaline species on hydrolysis degree and the subsequent modification process. Lohokare et al. [14] recently treated PAN UF membranes with organic and inorganic bases including ethanolamine, triethylamine, NaOH and KOH to improve the UF membrane flux. However, they only considered the UF process and did not provide the hydrolysis degree differences among the bases used. This paper therefore intended to examine three inorganic alkaline species such as KOH, NaOH and LiOH for hydrolysis of PAN membranes and to observe if the hydrolysis degree would affect the subsequent modifications. The hydrolysis differences with different alkaline species were characterized by ATR-FTIR spectra and IEC measurements. The microtopographical and mean roughness changes on membrane surfaces were analyzed by AFM. Furthermore, the molecular weight cut-off (MWCO) of the hydrolyzed PAN membrane was determined. Subsequently, one of the application examples was conducted to understand the effects of hydrolysis degree on subsequent modification. The polyethyleneimine (PEI) was deposited on the hydrolyzed PAN membranes to form a composite membrane based on the electrostatic adsorption between PEI and the hydrolyzed PAN support. The pervaporation performances and ions separation capacity of PANbased membranes were discussed.
OH~ CH2—CH H2O C= O NH2
OHH2O
~ CH2—CH + NH3 C=O O-
(1)
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2. Experimental 2.1. Materials The flat-sheet PAN UF membrane was provided by the Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. The molecular weight cut-off of the fresh PAN UF membrane was 20,000. Lithium hydroxide, sodium hydroxide potassium hydroxide, hydrochloric acid, lithium chloride, calcium chloride and magnesium sulphate were of analytical grade. A series of poly(ethylene glycol)s (PEG) having the molecular weight of 400, 2000, 4000 and 6000 were supplied by Beijing Chemical Factory. Cytochrome C (CytC) with the molecular weight of 12,384 was obtained from Aldrich. Linear poly (ethyleneimine) (PEI) with a molecular weight of 60,000 was purchased from Acros. 2.2 Hydrolysis of the PAN UF membrane The PAN UF membrane was hydrolyzed by immersing into the aqueous solutions of lithium hydroxide, sodium hydroxide and potassium hydroxide, respectively. The hydrolysis tests were conducted in a water bath so that the temperature could be manipulated to predetermined levels. After hydrolyzing for a pre-determined period, the membranes were taken out and rinsed with deionized (DI) water until the pH values of the rinsed water reached about 7.0. 2.3. Characterization of hydrolyzed membranes Attenuated total reflectance FTIR spectra were obtained using a Vertex-70 spectrophotometer (Bruker, Germany). A scanning electron microscopy (SEM) (Hitachi-4700, Japan) was used to observe the top surface of the membranes before and after hydrolysis. All membrane samples were dried under vacuum, fractured in liquid nitrogen and gold-coated before observation. Atomic force microscopy (AFM) surface topography was performed in tapping mode by a Digital Nanoscope III AFM (Digital Instruments, Buffalo, NY). Be-
315
fore AFM tests, the membrane samples were cut with a knife and adhered onto a specimen dais for AFM observation without any modification. The AFM was operated in air. The data obtained were processed by a microcomputer equipped on this microscope. The ion exchange capacity (IEC) of the hydrolyzed PAN membrane was determined by immersing the hydrolyzed membranes into 0.1 N HCl solution. The metal ions, which were exchanged from the hydrolyzed membranes, were detected by ion chromatogram (ICS-1000, Dionex) after sufficient contact. The corresponding IEC could be calculated as mmol/(g dry membrane) based on the ion concentration, solution volume and the weight of dry membrane. In addition, the MWCO was determined by filtrating the PEG and CytC solutions. A total organic carbon (TOC) analyzer (TOC-VCPH, Shimadzu) was used to determine the TOC values of PEG solutions before and after ultrafiltration. The CytC concentrations were measured on a UV-2550 UV– visible spectrophotometer Shimadzu at the wavelength of 280 nm. The rejection ratios of PEG and CytC were calculated based on the concentration values in the feed and permeate solutions. 2.4. Applications of hydrolyzed PAN UF membranes In order to identify the effects of hydrolysis degree on the subsequent modification of composite membranes, the hydrolyzed PAN UF membranes were loaded in a dead-ended filtration cell and the polyethyleneimine (PEI) was then deposited on the hydrolyzed PAN membranes by filtrating 0.25 wt% PEI ethanol solution under a pressure of 0.1 MPa. Since PEI has its own positive charge while the hydrolyzed PAN is negatively charged, a polyelectrolyte composite membrane is easily formed based on the electrostatic adsorption. The PAN/PEI composite membranes were then characterized by the pervaporation of alcohol/water mixtures (95:5 g/g) using a selfmade pervaporation apparatus as previously re-
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ported [9,10]. The composition of the pervoparation permeate was determined by gas chromatography (GC-14C, Shimadzu). Meanwhile, the ions rejections were evaluated by filtrating the solutions of lithium chloride, calcium chloride and magnesium sulphate in a stirred dead-end filtration cell with approximately 79 cm2 membrane area [9,13]. The feed concentration was typically 1000 mg/l in DI water. The ions contained in the feed and permeate were determined by ion chromatogram. 3. Results and discussion 3.1. Effects of alkaline species on hydrolysis degree As the effects of alkaline concentration, hydrolysis time and hydrolysis temperature have been extensively discussed using sodium hydroxide solution, the appropriate hydrolysis conditions have been selected in the previous study [13]. The appropriate conditions were: alkaline concentration 2 N, hydrolysis time 60 min and temperature 65°C. This study focused on the effects of alkaline species on the hydrolysis degree. The ATRFTIR spectra of PAN membrane after hydrolysis with different alkaline species are shown in Fig. 1. It was noted that the shoulders at around 2243 cm–1 and 1451 cm–1, which were due to the –CN group, decreased sharply after hydrolysis in all cases. Taken together, the bands at 1568 cm–1 and 1405 cm–1 in the spectra of hydrolyzed PAN membranes provided strong evidence that carboxyl groups occurred primarily via hydrolysis of alkaline solutions. It was clearly observed that the peak at 1568 cm–1 with the KOH hydrolysis were much stronger than those with NaOH and LiOH hydrolysis, The similar results occurred at the band of 1405 cm–1. This suggested that the contribution of alkali species on the hydrolysis degree were in the order KOH > NaOH > LiOH. Since the carboxyl functionality would remain in the forms of –COO-K+, –COO-Na+ and –COO-Li+, IEC could also be used as an important value to characterize
the difference of the hydrolysis degree. In this study, using the hydrolysis of 2 N LiOH, 2 N NaOH and 2 N KOH for a period of 60 min under the temperature of 65°C, the IEC was determined as 0.618 mmol/g, 1.77 mmol/g and 2.74 mmol/g, respectively. Although the ion diameters of K+ was larger than those of Na+ and Li+, the IEC obtained from –COO-K+ structure was still much higher than those obtained from –COO-Na+ and –COOLi+ structures. These results further confirmed that different hydrolysis degrees were due to the attack strength differences of the hydroxyl group. Obviously, the hydrolysis degree mainly accorded with the order of basicity intensity. 3.2. Effects of the hydrolysis degree on the surface microtopograph The resulting PAN membranes before and after hydrolysis were characterized by SEM. The top surface morphologies are shown in Fig. 2. Having compared Figs. 2a, 2b and 2c, it could be observed that the membrane became a little compact after hydrolyzing with LiOH and NaOH solutions expect for some large pores that remained. In comparison with Fig. 2a and 2d, the top layer obtained from KOH solution was much more uniform than those obtained from LiOH and NaOH hydrolyses. In order to clearly observe the microtopographical changes on membrane surfaces, the AFM images of the membrane surfaces before and after hydrolysis were taken and are shown in Fig. 3. It could be observed that the microtopographs of the hydrolyzed membrane with NaOH and KOH were very similar while these two membranes were quite different from LiOH hydrolyzed membrane. The values of mean roughness (Ra) were obtained based on 10.0 μm × 10.0 μm scan area. It was noted that the top surfaces became smoother after hydrolysis with alkaline solution. For example, the Ra of the PAN membranes decreased from 16.718 nm to 6.895 nm after 2 N KOH hydrolysis for 1 h. This is because PAN with a certain content of carboxyl group is easily swollen when exposed to aqueous
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Fig. 1. IR spectra of PAN membranes without and with hydrolysis by different alkaline species (a) without hydrolysis, (b) LiOH hydrolysis, (c) NaOH hydrolysis, (d) KOH hydrolysis. Hydrolysis conditions: alkaline concentration 2 N, hydrolysis time 60 min, hydrolysis temperature 65°C.
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(a)
(c)
(b)
(d)
Fig. 2. SEM pictures of the top surface of membranes before and after hydrolysis. (a) SEM of the top surface before hydrolysis (×50.0 k) (Hydrolysis conditions for PAN support membrane: alkaline solution 2 N NaOH, hydrolysis temperature 60°C, hydrolysis time 15 min). (b) SEM of the top surface after LiOH hydrolysis (×50.0 k) (Hydrolysis conditions: alkaline concentration 2 N, hydrolysis time 60 min, hydrolysis temperature 65°C.) (c) SEM of the top surface after NaOH hydrolysis (×50.0 k) (Hydrolysis conditions: alkaline concentration 2 N, hydrolysis time 60 min, hydrolysis temperature 65°C). (d) SEM of the top surface after KOH hydrolysis (×50.0 k) (Hydrolysis conditions: alkaline concentration 2 N, hydrolysis time 60 min, hydrolysis temperature 65°C.)
medium. The swollen macromolecules then become more mobile to move towards the pores. This process decreases the pore size and makes the membrane surface smoother [1]. It was also noted that the membrane surface roughness decreased with the increase in the hydrolysis degree. In the cases of hydrolysis with 2 N LiOH, 2 N NaOH, 2 N KOH, the Ra values of the hydrolyzed membranes were 12.324 nm, 7.186 nm and 6.895 nm, respectively.
3.3. MWCO of hydrolyzed PAN UF membrane Table 1 gives rejection ratios of PEG and CytC using the hydrolyzed PAN membranes with different alkaline solutions. These PEG solutions were prepared from poly(ethylene glycol)s having molar mass from 400 to 6,000 g/mol at a concentration of 0.5 wt.% in deionized water. It was determined that the pure water flux was quite small after hydrolysis. In our experiments, the
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(a)
(b)
Fig. 3. AFM images of membrane surfaces with and without hydrolysis (10.0 μm × 10.0 μm area). (a) PAN membrane without hydrolysis (Ra = 16.718 nm); (b) PAN membrane with 2 N LiOH hydrolysis (Ra = 12.324nm).
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G. Zhang et al. / Desalination 242 (2009) 313–324 (c)
(d)
Fig. 3 (continued). AFM images of membrane surfaces with and without hydrolysis (10.0 μm × 10.0 μm area). (c) PAN membrane with 2 N NaOH hydrolysis (Ra = 7.186 nm); (d) PAN membrane with 2 N KOH hydrolysis (Ra = 6.895 nm).
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Table 1 MWCO measurements of hydrolyzed PAN UF Substances
PEG-400 PEG-2000 PEG-4000 PEG-6000 CytC
Molecular weight
400 2000 4000 6000 12384
Rejection ratio（%） Hydrolyzed by 2 N LiOH
Hydrolyzed by 2 N NaOH
Hydrolyzed by 2 N KOH
2.40 20.12 49.34 80.50 91.35
3.55 28.28 56.70 92.83 95.70
8.89 34.06 60.21 93.03 96.13
Hydrolysis conditions: hydrolysis time 1 h, hydrolysis temperature 65°C Filtration conditions: feed temperature 25°C, transmembrane pressure 0.10 MPa
pure water flux usually decreased from around 200 l/m2.h to 6–10 l/m2.h (20°C, ΔP = 0.1 MPa). As the MWCO value is defined as the solute molar mass at which the rejection is 90%, the NaOH and KOH hydrolyzed membranes have MWCO values of 6000 while the corresponding value is around 12,000 for LiOH hydrolyzed membranes. These results indicated that the higher hydrolysis degree would lead to much smaller membrane pores. 3.4. Polyacrylonitrile-based membrane The hydrolyzed PAN membrane was widely used as a support for the assembly of a composite membrane. The following experiments were intended to examine the effects of the hydrolysis degree on the polyacrylonitrile-based membrane performance. In this study, the PAN UF membranes were first hydrolyzed by 2 N LiOH, 2 N NaOH and 2 N KOH for a period of 60 min under the temperature of 65°C. The PEI was subsequently deposited on the hydrolyzed PAN membrane to form a composite membrane. The pervaporation separation of water–ethanol mixture was conducted and the results are listed in Table 2. In all cases, a preferential transport of water is observed. However, the performances of polyacrylonitrile-based membranes were strongly
dependent on the hydrolysis degree of the support membrane. It is well known that the pervaporation membrane performance attributed to the integrity and regularity of the top separation layer. With the increase in the basicity intensity, more and more –CN groups on the PAN support membrane surface and pore will be converted into carboxyl groups. The more –COO- groups converted, the more compact PEI layer formed on the hydrolyzed PAN support. Considering that the hydrolysis degree with KOH was relatively high, the water content in permeate could reach 85.13wt% and the corresponding separation factor (α) was 109. As a comparison, the water content in permeate obtained from NaOH was 75.92 wt% (α = 60) while the corresponding value in the case of LiOH hydrolysis was only 40.23 wt% (α = 13). Table 3 shows the differences with regard to the rejections of Ca2+, Mg2+ and Li+. It can be noted from Table 3 that the rejections of three cations were negligible for the hydrolyzed PAN UF membranes without the PEI deposition. However, the PAN/PEI composite membranes show a certain ion rejection capacity. This capacity due to the top separation layer was formed on the hydrolyzed PAN membranes. Obviously, all rejection ratios obtained from KOH and NaOH hydrolysis were much higher than those obtained from LiOH hy-
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Table 2 Pervaporation performance of polyacrylonitrile-based membranes Alkaline solution used for hydrolysis of the Item support
Value
2 N LiOH
40.23 1021 13 75.92 429 60 85.13 415 109
2 N NaOH
2 N KOH
Water content in permeate (%) Permeate flux (g/m2.h) Separation factor Water content in permeate (%) Permeate flux (g/m2.h) Separation factor Water content in permeate (%) Permeate flux (g/m2.h) Separation factor
Hydrolysis conditions for PAN support membrane: hydrolysis temperature 65°C, hydrolysis time 60 min Preparative conditions for polyacrylonitrile-based membrane: one PEI layer deposited, dynamic pressure: 0.1 MPa, 20 min filtration time, 0.25 wt% PEI ethanol solution, 25°C. Pervaporation conditions: feed temperature 40°C, down-stream pressure 100 Pa, EtOH content in feed solution 95 wt%
Table 3 Ion rejection using polyacrylonitrile-based membrane Membrane type
Rejection ratio (%)
Hydrolyzed PAN membrane with 2 N NaOH (without PEI) PAN/PEI membrane (PAN hydrolyzed membrane with 2 N 2N LiOH) PAN/PEI membrane (PAN hydrolyzed membrane with 2 N 2N NaOH) PAN/PEI membrane (PAN hydrolyzed membrane with 2 N 2N KOH)
Ca2+
Mg2+
Li+
0.76 25.31 56.43 62.12
0.75 24.54 54.87 61.32
0.21 19.8 34.7 43.0
Hydrolysis conditions for PAN support membrane: hydrolysis temperature 65°C, hydrolysis time 60 min Preparative conditions for polyacrylonitrile-based membrane: one PEI layer deposited, dynamic pressure: 0.1 MPa, 20 mins filtration time, 0.25 wt% PEI ethanol solution, 25°C. Filtration conditions: feed temperature 25°C, transmembrane pressure 0.25 MPa
drolysis. For example, the rejection of Ca2+ using the composite membrane based on KOH hydrolyzed PAN membrane was 62.12% while this value was only 43% for LiOH hydrolyzed PAN based composite membrane. This is because the top PEI separation layer became much denser and more uniform due to the higher charge density in the case of KOH and NaOH hydrolysis. These results further confirmed that the differences of the hydrolysis degree caused by the different al-
kaline species had a great influence on the final separation performances of the PAN-based membranes. 4. Conclusions This study aims at better understanding the hydrolysis difference of PAN UF membranes by using different hydrolysis species. The ATR-FTIR spectra clearly shows much stronger carboxyl
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bands at 1568 cm–1 and 1405 cm–1 obtained from KOH hydrolysis than those obtained from NaOH and LiOH hydrolysis. It was found that the hydrolysis degree was strongly dependent on the alkali species. Basically, the contributions to the hydrolysis degree accorded with the order of basicity intensity. Subsequently, the surface roughness was analyzed by AFM, which suggested that the higher hydrolysis degree would possibly lead to a smoother surface. Different MWCO were also determined as about 12000, 6000 and 6000, respectively, in the cases of LiOH, NaOH and KOH hydrolysis. Finally, the polyethyleneimine (PEI) was deposited on the top surface of the hydrolyzed PAN membrane to form a composite membrane and in turn to verify how the hydrolysis degree affects subsequent modification. The pervaporation experiments suggested that the pervaporation performance of PAN/PEI composite membranes varied with the PAN hydrolysis degree. The separation factor could reach 109 with the KOH hydrolyzed PAN support. As a comparison, the corresponding value was only 13 in the case of LiOH hydrolysis. Further investigations on ion rejection with PAN/PEI membranes confirmed that the PAN support membrane obtained from different hydrolysis solutions would strongly affect the separation performances of the composite membranes. Therefore, to obtain a good performance of a polyacrylonitrile-based membrane, it is very important to select suitable alkaline species and hydrolysis conditions.
Acknowledgements This work was supported by the Natural Science Foundation of Beijing (No. 8073022 and No. 8071001), the Beijing NOVA Programme (No. 2006B13 and No.2007A020), the National Basic Research Program of China (No. 2003CB615701), and the Special Funding Project of the Beijing Municipal Commission of Education, P.R, ChinaNo. 05005999200601).
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193 (2001) 85–95. [12] M. Ulbricht, A. Oechel, C. Lehmann, G. Tomaschewski and H.G. Hicke, Gas-phase photoinduced graft polymerization of acrylic acid onto polyacrylonitrile ultrafiltration membranes, J. Appl. Polym. Sci., 55 (1995) 1707–1723. [13] H. Yan, G. Zhang, S. Ji, W. Gu and Z. Liu, Study on
hydrolysis modification of polyacrylonitrile ultrafiltration membranes, Membr. Sci. Technol., 27(5) (2007) 56–61 (in Chinese). [14] H.R. Lohokare, S.C. Kumbarkar, Y.S. Bhole and U.K. Kharul, Surface modification of polyacrylonitrile based ultrafiltration membrane, J. Appl. Polym. Sci., 101 (2006) 4378–4385.
Hydrolysis differences of polyacrylonitrile support membrane and its influences on polyacrylonitrile-based membrane performance Guojun Zhanga*, Hong Mengb, Shulan Jia a
Center for Membrane Technology, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing, 100022, P.R. China Tel. +86 (10) 6739-2393; Fax +86 (10) 6739-1983; email: [email protected] b College of Chemical Engineering, Beijing University of Chemical Technology, Beijing, 100029, P.R. China
Received 7 January 2008; accepted revised 19 May 2008
Abstract As polyacrylonitrile (PAN)-based membrane plays an important role in the processes of pervaporation, bioproduct purification and water treatment, this study aims at better understanding the hydrolysis differences by using different alkaline species, and how the hydrolysis degree affect the subsequent modification of hydrolyzed PAN membranes. The ATR-FTIR spectra and the measurements of ion exchange capacity (IEC) demonstrated the differences of hydrolysis degree among the KOH, NaOH and LiOH hydrolyzed PAN membranes. The microtopographical changes on membrane surfaces were characterized by atomic force microscopy. It was found that the membrane surface roughness decreased with the increase in the hydrolysis degree. The molecular weight cut-off (MWCO) of the hydrolyzed membranes was determined by filtration studies using poly(ethylene glycol) and Cytochrome C solutions. It was noted that the MWCO of the PAN membranes decreased from 20,000 to 6000 after hydrolysis with 2 N NaOH or 2 N KOH for 1 h at 65°C. Subsequently, the polyethyleneimine (PEI) was deposited on the top surface of the hydrolyzed PAN membrane to form a composite membrane. The pervaporation performance and ions rejection property of the PAN/PEI composite membrane clearly demonstrated that the hydrolysis difference had a great influence on subsequent modification of the PAN-based membrane. Keywords: Polyacrylonitrile (PAN); Hydrolysis degree; Composite membrane; Pervaporation; Ion rejection
1. Introduction Polyacrylonitrile (PAN) membranes have attracted much attention due to their excellent char*Corresponding author.
acteristics of thermal stability, tolerance to most solvents and commercial availability [1]. Till now, PAN ultrafiltration (UF) has been widely used in protein filtration and membrane bioreactor [2,3]. However, the relative poor hydrophilicity and
0011-9164/09/$– See front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2008.05\.010
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G. Zhang et al. / Desalination 242 (2009) 313–324
biocompatibility limited its applications. To overcome these problems, the antifouling property could be improved by modifying the membrane surface characteristics. Meanwhile, the rejection property of PAN UF membranes could also be controlled by surface modification. For instance, Qiao et al. [4] recently used the PAN UF membrane with hydrophilic modification to purify Chinese herbs. It was found that the content of danshensu and protocatechuic aldehyde in the permeate was different when PAN membranes with different hydrophilicities were employed for filtration. Additionally, there is other increasing application of PAN UF membranes as the support membranes of the pervaporation and nanofiltration composite membranes due to their relatively active surface [5–12]. For examples, Wang et al. [5] prepared asymmetric PAN nanofiltration (NF) membranes with controlled highly dense pore surface functional groups by hydrolysis of the nitrile groups with NaOH. They also investigated the combined effects of heat treatment and the presence of ZnCl2 on the formation of nanofiltration membranes. In our previous studies [9,10], the polyelectrolyte multilayer pervaporation membranes have already been successfully assembled on a hydrolyzed PAN UF membrane. The modifications of PAN UF membranes usually include plasma treatment [6], plasma-initiated graft polymerization [7,8], photoinduced grafting [12] and hydrolysis [9–11]. Among these methods, hydrolysis under alkaline condition is one of the most important and most frequently used methods for PAN-based membranes, due to the convenience and favorableness for further modification. The hydrolysis mechanism has already been extensively discussed by
~ CH2—CH CN
many researchers [1,4,5,11,13]. As shown in reaction (1), the hydrolysis with alkaline solution is based on the conversion of –CN groups on the PAN membrane surface firstly into –CONH2, then into –COO- groups. In the past studies, sodium hydroxide was commonly used as hydrolysis agent for PAN membrane. Few works have dealt with the effects of alkaline species on hydrolysis degree and the subsequent modification process. Lohokare et al. [14] recently treated PAN UF membranes with organic and inorganic bases including ethanolamine, triethylamine, NaOH and KOH to improve the UF membrane flux. However, they only considered the UF process and did not provide the hydrolysis degree differences among the bases used. This paper therefore intended to examine three inorganic alkaline species such as KOH, NaOH and LiOH for hydrolysis of PAN membranes and to observe if the hydrolysis degree would affect the subsequent modifications. The hydrolysis differences with different alkaline species were characterized by ATR-FTIR spectra and IEC measurements. The microtopographical and mean roughness changes on membrane surfaces were analyzed by AFM. Furthermore, the molecular weight cut-off (MWCO) of the hydrolyzed PAN membrane was determined. Subsequently, one of the application examples was conducted to understand the effects of hydrolysis degree on subsequent modification. The polyethyleneimine (PEI) was deposited on the hydrolyzed PAN membranes to form a composite membrane based on the electrostatic adsorption between PEI and the hydrolyzed PAN support. The pervaporation performances and ions separation capacity of PANbased membranes were discussed.
OH~ CH2—CH H2O C= O NH2
OHH2O
~ CH2—CH + NH3 C=O O-
(1)
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2. Experimental 2.1. Materials The flat-sheet PAN UF membrane was provided by the Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. The molecular weight cut-off of the fresh PAN UF membrane was 20,000. Lithium hydroxide, sodium hydroxide potassium hydroxide, hydrochloric acid, lithium chloride, calcium chloride and magnesium sulphate were of analytical grade. A series of poly(ethylene glycol)s (PEG) having the molecular weight of 400, 2000, 4000 and 6000 were supplied by Beijing Chemical Factory. Cytochrome C (CytC) with the molecular weight of 12,384 was obtained from Aldrich. Linear poly (ethyleneimine) (PEI) with a molecular weight of 60,000 was purchased from Acros. 2.2 Hydrolysis of the PAN UF membrane The PAN UF membrane was hydrolyzed by immersing into the aqueous solutions of lithium hydroxide, sodium hydroxide and potassium hydroxide, respectively. The hydrolysis tests were conducted in a water bath so that the temperature could be manipulated to predetermined levels. After hydrolyzing for a pre-determined period, the membranes were taken out and rinsed with deionized (DI) water until the pH values of the rinsed water reached about 7.0. 2.3. Characterization of hydrolyzed membranes Attenuated total reflectance FTIR spectra were obtained using a Vertex-70 spectrophotometer (Bruker, Germany). A scanning electron microscopy (SEM) (Hitachi-4700, Japan) was used to observe the top surface of the membranes before and after hydrolysis. All membrane samples were dried under vacuum, fractured in liquid nitrogen and gold-coated before observation. Atomic force microscopy (AFM) surface topography was performed in tapping mode by a Digital Nanoscope III AFM (Digital Instruments, Buffalo, NY). Be-
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fore AFM tests, the membrane samples were cut with a knife and adhered onto a specimen dais for AFM observation without any modification. The AFM was operated in air. The data obtained were processed by a microcomputer equipped on this microscope. The ion exchange capacity (IEC) of the hydrolyzed PAN membrane was determined by immersing the hydrolyzed membranes into 0.1 N HCl solution. The metal ions, which were exchanged from the hydrolyzed membranes, were detected by ion chromatogram (ICS-1000, Dionex) after sufficient contact. The corresponding IEC could be calculated as mmol/(g dry membrane) based on the ion concentration, solution volume and the weight of dry membrane. In addition, the MWCO was determined by filtrating the PEG and CytC solutions. A total organic carbon (TOC) analyzer (TOC-VCPH, Shimadzu) was used to determine the TOC values of PEG solutions before and after ultrafiltration. The CytC concentrations were measured on a UV-2550 UV– visible spectrophotometer Shimadzu at the wavelength of 280 nm. The rejection ratios of PEG and CytC were calculated based on the concentration values in the feed and permeate solutions. 2.4. Applications of hydrolyzed PAN UF membranes In order to identify the effects of hydrolysis degree on the subsequent modification of composite membranes, the hydrolyzed PAN UF membranes were loaded in a dead-ended filtration cell and the polyethyleneimine (PEI) was then deposited on the hydrolyzed PAN membranes by filtrating 0.25 wt% PEI ethanol solution under a pressure of 0.1 MPa. Since PEI has its own positive charge while the hydrolyzed PAN is negatively charged, a polyelectrolyte composite membrane is easily formed based on the electrostatic adsorption. The PAN/PEI composite membranes were then characterized by the pervaporation of alcohol/water mixtures (95:5 g/g) using a selfmade pervaporation apparatus as previously re-
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ported [9,10]. The composition of the pervoparation permeate was determined by gas chromatography (GC-14C, Shimadzu). Meanwhile, the ions rejections were evaluated by filtrating the solutions of lithium chloride, calcium chloride and magnesium sulphate in a stirred dead-end filtration cell with approximately 79 cm2 membrane area [9,13]. The feed concentration was typically 1000 mg/l in DI water. The ions contained in the feed and permeate were determined by ion chromatogram. 3. Results and discussion 3.1. Effects of alkaline species on hydrolysis degree As the effects of alkaline concentration, hydrolysis time and hydrolysis temperature have been extensively discussed using sodium hydroxide solution, the appropriate hydrolysis conditions have been selected in the previous study [13]. The appropriate conditions were: alkaline concentration 2 N, hydrolysis time 60 min and temperature 65°C. This study focused on the effects of alkaline species on the hydrolysis degree. The ATRFTIR spectra of PAN membrane after hydrolysis with different alkaline species are shown in Fig. 1. It was noted that the shoulders at around 2243 cm–1 and 1451 cm–1, which were due to the –CN group, decreased sharply after hydrolysis in all cases. Taken together, the bands at 1568 cm–1 and 1405 cm–1 in the spectra of hydrolyzed PAN membranes provided strong evidence that carboxyl groups occurred primarily via hydrolysis of alkaline solutions. It was clearly observed that the peak at 1568 cm–1 with the KOH hydrolysis were much stronger than those with NaOH and LiOH hydrolysis, The similar results occurred at the band of 1405 cm–1. This suggested that the contribution of alkali species on the hydrolysis degree were in the order KOH > NaOH > LiOH. Since the carboxyl functionality would remain in the forms of –COO-K+, –COO-Na+ and –COO-Li+, IEC could also be used as an important value to characterize
the difference of the hydrolysis degree. In this study, using the hydrolysis of 2 N LiOH, 2 N NaOH and 2 N KOH for a period of 60 min under the temperature of 65°C, the IEC was determined as 0.618 mmol/g, 1.77 mmol/g and 2.74 mmol/g, respectively. Although the ion diameters of K+ was larger than those of Na+ and Li+, the IEC obtained from –COO-K+ structure was still much higher than those obtained from –COO-Na+ and –COOLi+ structures. These results further confirmed that different hydrolysis degrees were due to the attack strength differences of the hydroxyl group. Obviously, the hydrolysis degree mainly accorded with the order of basicity intensity. 3.2. Effects of the hydrolysis degree on the surface microtopograph The resulting PAN membranes before and after hydrolysis were characterized by SEM. The top surface morphologies are shown in Fig. 2. Having compared Figs. 2a, 2b and 2c, it could be observed that the membrane became a little compact after hydrolyzing with LiOH and NaOH solutions expect for some large pores that remained. In comparison with Fig. 2a and 2d, the top layer obtained from KOH solution was much more uniform than those obtained from LiOH and NaOH hydrolyses. In order to clearly observe the microtopographical changes on membrane surfaces, the AFM images of the membrane surfaces before and after hydrolysis were taken and are shown in Fig. 3. It could be observed that the microtopographs of the hydrolyzed membrane with NaOH and KOH were very similar while these two membranes were quite different from LiOH hydrolyzed membrane. The values of mean roughness (Ra) were obtained based on 10.0 μm × 10.0 μm scan area. It was noted that the top surfaces became smoother after hydrolysis with alkaline solution. For example, the Ra of the PAN membranes decreased from 16.718 nm to 6.895 nm after 2 N KOH hydrolysis for 1 h. This is because PAN with a certain content of carboxyl group is easily swollen when exposed to aqueous
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Fig. 1. IR spectra of PAN membranes without and with hydrolysis by different alkaline species (a) without hydrolysis, (b) LiOH hydrolysis, (c) NaOH hydrolysis, (d) KOH hydrolysis. Hydrolysis conditions: alkaline concentration 2 N, hydrolysis time 60 min, hydrolysis temperature 65°C.
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(a)
(c)
(b)
(d)
Fig. 2. SEM pictures of the top surface of membranes before and after hydrolysis. (a) SEM of the top surface before hydrolysis (×50.0 k) (Hydrolysis conditions for PAN support membrane: alkaline solution 2 N NaOH, hydrolysis temperature 60°C, hydrolysis time 15 min). (b) SEM of the top surface after LiOH hydrolysis (×50.0 k) (Hydrolysis conditions: alkaline concentration 2 N, hydrolysis time 60 min, hydrolysis temperature 65°C.) (c) SEM of the top surface after NaOH hydrolysis (×50.0 k) (Hydrolysis conditions: alkaline concentration 2 N, hydrolysis time 60 min, hydrolysis temperature 65°C). (d) SEM of the top surface after KOH hydrolysis (×50.0 k) (Hydrolysis conditions: alkaline concentration 2 N, hydrolysis time 60 min, hydrolysis temperature 65°C.)
medium. The swollen macromolecules then become more mobile to move towards the pores. This process decreases the pore size and makes the membrane surface smoother [1]. It was also noted that the membrane surface roughness decreased with the increase in the hydrolysis degree. In the cases of hydrolysis with 2 N LiOH, 2 N NaOH, 2 N KOH, the Ra values of the hydrolyzed membranes were 12.324 nm, 7.186 nm and 6.895 nm, respectively.
3.3. MWCO of hydrolyzed PAN UF membrane Table 1 gives rejection ratios of PEG and CytC using the hydrolyzed PAN membranes with different alkaline solutions. These PEG solutions were prepared from poly(ethylene glycol)s having molar mass from 400 to 6,000 g/mol at a concentration of 0.5 wt.% in deionized water. It was determined that the pure water flux was quite small after hydrolysis. In our experiments, the
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(a)
(b)
Fig. 3. AFM images of membrane surfaces with and without hydrolysis (10.0 μm × 10.0 μm area). (a) PAN membrane without hydrolysis (Ra = 16.718 nm); (b) PAN membrane with 2 N LiOH hydrolysis (Ra = 12.324nm).
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G. Zhang et al. / Desalination 242 (2009) 313–324 (c)
(d)
Fig. 3 (continued). AFM images of membrane surfaces with and without hydrolysis (10.0 μm × 10.0 μm area). (c) PAN membrane with 2 N NaOH hydrolysis (Ra = 7.186 nm); (d) PAN membrane with 2 N KOH hydrolysis (Ra = 6.895 nm).
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Table 1 MWCO measurements of hydrolyzed PAN UF Substances
PEG-400 PEG-2000 PEG-4000 PEG-6000 CytC
Molecular weight
400 2000 4000 6000 12384
Rejection ratio（%） Hydrolyzed by 2 N LiOH
Hydrolyzed by 2 N NaOH
Hydrolyzed by 2 N KOH
2.40 20.12 49.34 80.50 91.35
3.55 28.28 56.70 92.83 95.70
8.89 34.06 60.21 93.03 96.13
Hydrolysis conditions: hydrolysis time 1 h, hydrolysis temperature 65°C Filtration conditions: feed temperature 25°C, transmembrane pressure 0.10 MPa
pure water flux usually decreased from around 200 l/m2.h to 6–10 l/m2.h (20°C, ΔP = 0.1 MPa). As the MWCO value is defined as the solute molar mass at which the rejection is 90%, the NaOH and KOH hydrolyzed membranes have MWCO values of 6000 while the corresponding value is around 12,000 for LiOH hydrolyzed membranes. These results indicated that the higher hydrolysis degree would lead to much smaller membrane pores. 3.4. Polyacrylonitrile-based membrane The hydrolyzed PAN membrane was widely used as a support for the assembly of a composite membrane. The following experiments were intended to examine the effects of the hydrolysis degree on the polyacrylonitrile-based membrane performance. In this study, the PAN UF membranes were first hydrolyzed by 2 N LiOH, 2 N NaOH and 2 N KOH for a period of 60 min under the temperature of 65°C. The PEI was subsequently deposited on the hydrolyzed PAN membrane to form a composite membrane. The pervaporation separation of water–ethanol mixture was conducted and the results are listed in Table 2. In all cases, a preferential transport of water is observed. However, the performances of polyacrylonitrile-based membranes were strongly
dependent on the hydrolysis degree of the support membrane. It is well known that the pervaporation membrane performance attributed to the integrity and regularity of the top separation layer. With the increase in the basicity intensity, more and more –CN groups on the PAN support membrane surface and pore will be converted into carboxyl groups. The more –COO- groups converted, the more compact PEI layer formed on the hydrolyzed PAN support. Considering that the hydrolysis degree with KOH was relatively high, the water content in permeate could reach 85.13wt% and the corresponding separation factor (α) was 109. As a comparison, the water content in permeate obtained from NaOH was 75.92 wt% (α = 60) while the corresponding value in the case of LiOH hydrolysis was only 40.23 wt% (α = 13). Table 3 shows the differences with regard to the rejections of Ca2+, Mg2+ and Li+. It can be noted from Table 3 that the rejections of three cations were negligible for the hydrolyzed PAN UF membranes without the PEI deposition. However, the PAN/PEI composite membranes show a certain ion rejection capacity. This capacity due to the top separation layer was formed on the hydrolyzed PAN membranes. Obviously, all rejection ratios obtained from KOH and NaOH hydrolysis were much higher than those obtained from LiOH hy-
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Table 2 Pervaporation performance of polyacrylonitrile-based membranes Alkaline solution used for hydrolysis of the Item support
Value
2 N LiOH
40.23 1021 13 75.92 429 60 85.13 415 109
2 N NaOH
2 N KOH
Water content in permeate (%) Permeate flux (g/m2.h) Separation factor Water content in permeate (%) Permeate flux (g/m2.h) Separation factor Water content in permeate (%) Permeate flux (g/m2.h) Separation factor
Hydrolysis conditions for PAN support membrane: hydrolysis temperature 65°C, hydrolysis time 60 min Preparative conditions for polyacrylonitrile-based membrane: one PEI layer deposited, dynamic pressure: 0.1 MPa, 20 min filtration time, 0.25 wt% PEI ethanol solution, 25°C. Pervaporation conditions: feed temperature 40°C, down-stream pressure 100 Pa, EtOH content in feed solution 95 wt%
Table 3 Ion rejection using polyacrylonitrile-based membrane Membrane type
Rejection ratio (%)
Hydrolyzed PAN membrane with 2 N NaOH (without PEI) PAN/PEI membrane (PAN hydrolyzed membrane with 2 N 2N LiOH) PAN/PEI membrane (PAN hydrolyzed membrane with 2 N 2N NaOH) PAN/PEI membrane (PAN hydrolyzed membrane with 2 N 2N KOH)
Ca2+
Mg2+
Li+
0.76 25.31 56.43 62.12
0.75 24.54 54.87 61.32
0.21 19.8 34.7 43.0
Hydrolysis conditions for PAN support membrane: hydrolysis temperature 65°C, hydrolysis time 60 min Preparative conditions for polyacrylonitrile-based membrane: one PEI layer deposited, dynamic pressure: 0.1 MPa, 20 mins filtration time, 0.25 wt% PEI ethanol solution, 25°C. Filtration conditions: feed temperature 25°C, transmembrane pressure 0.25 MPa
drolysis. For example, the rejection of Ca2+ using the composite membrane based on KOH hydrolyzed PAN membrane was 62.12% while this value was only 43% for LiOH hydrolyzed PAN based composite membrane. This is because the top PEI separation layer became much denser and more uniform due to the higher charge density in the case of KOH and NaOH hydrolysis. These results further confirmed that the differences of the hydrolysis degree caused by the different al-
kaline species had a great influence on the final separation performances of the PAN-based membranes. 4. Conclusions This study aims at better understanding the hydrolysis difference of PAN UF membranes by using different hydrolysis species. The ATR-FTIR spectra clearly shows much stronger carboxyl
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bands at 1568 cm–1 and 1405 cm–1 obtained from KOH hydrolysis than those obtained from NaOH and LiOH hydrolysis. It was found that the hydrolysis degree was strongly dependent on the alkali species. Basically, the contributions to the hydrolysis degree accorded with the order of basicity intensity. Subsequently, the surface roughness was analyzed by AFM, which suggested that the higher hydrolysis degree would possibly lead to a smoother surface. Different MWCO were also determined as about 12000, 6000 and 6000, respectively, in the cases of LiOH, NaOH and KOH hydrolysis. Finally, the polyethyleneimine (PEI) was deposited on the top surface of the hydrolyzed PAN membrane to form a composite membrane and in turn to verify how the hydrolysis degree affects subsequent modification. The pervaporation experiments suggested that the pervaporation performance of PAN/PEI composite membranes varied with the PAN hydrolysis degree. The separation factor could reach 109 with the KOH hydrolyzed PAN support. As a comparison, the corresponding value was only 13 in the case of LiOH hydrolysis. Further investigations on ion rejection with PAN/PEI membranes confirmed that the PAN support membrane obtained from different hydrolysis solutions would strongly affect the separation performances of the composite membranes. Therefore, to obtain a good performance of a polyacrylonitrile-based membrane, it is very important to select suitable alkaline species and hydrolysis conditions.
Acknowledgements This work was supported by the Natural Science Foundation of Beijing (No. 8073022 and No. 8071001), the Beijing NOVA Programme (No. 2006B13 and No.2007A020), the National Basic Research Program of China (No. 2003CB615701), and the Special Funding Project of the Beijing Municipal Commission of Education, P.R, ChinaNo. 05005999200601).
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