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Study on cellulose acetate membranes for reverse osmosis and polyethersulfone membranes for ultrafiltration by electron spin resonance technique
DESALINATION Desalination
ELSEVIER
148 (2002) 329-332 www.elsevier.com/locate/desal
Study on cellulose acetate membranes for reverse osmosis and polyethersulfone membranes for ultrafiltration by electron spin resonance technique K.C. Khulbe*, T. Matsuura, C.Y. Feng Industrial Membrane Research Institute, Department of Chemical Engineering, University of Ottawa, Ottawa, Ontario KIN 6N5, Canada Fax +I (613) 562-5172; email: [email protected] Received
1 February 2002; accepted 7 March 2002
Abstract Electron spin resonance (ESR) technique was used to study cellulose acetate (CA) membranes for reverse osmosis (RO) and polyethersulfone (PES) membranes for ultrafiltration. TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy free radical) was used as a spin probe that was brought into the membranes by immersing the membranes into solutions involving TEMPO, or by blending TEMPO into membrane casting solutions. It was found that the ESR signal from TEMPO in the membrane is useful for the choice of polymeric materials to prepare RO membranes. From ESR results it seems that for CA membranes, which are more hydrophilic and swollen by water, channels through the swollen polymer matrix contribute to the transport of water. On the other hand, the transport of water in PES membranes is primarily through pores and the contribution of the little swollen polymer matrix is insignificant. Keywords:
Polyethersulfone (PES) membrane; Cellulose acetate (CA) membrane; Reverse osmosis (RO); Ultrafiltration (UF)
Electron spin resonance
(ESR);
1. Introduction
as candidate
Since the announcement of the asymmetric CA membrane for seawater desalination in 1960, a number of polymeric materials have been tested
some polymers, it is possible to prepare membranes that exhibit high separations for sodium chloride and organic molecules of low molecular weights, while for others it is not possible. On the other
*Corresponding
OOII-9164/02/$-
hand, UF membranes
author.
Presented at the International July 7-12, 2002.
materials
Congress on Membranes
and Membrane
Processes
See front matter 0 2002 Elsevier Science B.V. All rights reserved
PII: SO0 I I-9 164(02)00725-7
for RO membranes.
can be prepared (ICOM),
For
from most
Toulouse, France,
330
K.C. Khulbe et al. /Desalination
of polymeric materials. Methods to screen polymers without doing RO experiments, which are preceded by a tedious process of membrane preparation, have long been sought for. In one of such attempts, it was found that the polymers, from which RO membranes of high salt selectivities could be prepared, were isolated in a limited range when two dimensional plots were made for hydrogen bonding and dispersion force components of solubility parameters of polymers [ 11. An attempt is made in the paper to find criteria for the choice of polymeric materials for RO membranes. For this purpose CA and PES membranes were studied to represent RO and UF membranes, respectively, and a novel membrane characterization technique based on ESR was used. 2. Experimental CA solution (CA, Eastman 393-3, 17 wt.%; Mg (ClO,),, 1.45 wt.%; water, 12.35 wt.%; acetone, 83 wt.%) was cast on a glass plate to a thickness of 0.33 mm. The solvent was evaporated at room temperature for 1 min before the cast film together with the glass plate was immersed in icecold water for 2 h. The membrane was shrunk in a hot water bath at different temperatures (60,70, 75, 80, 85 and 9O’C) for 10 min prior to RO experiments. The membranes so prepared were asymmetric in structure and did not contain TEMPO. A dense CA film was prepared from the above solution into which TEMPO (0.01 wt.%) was blended. The solvent was evaporated at room temperature for three days. PES solution (PES, 15 wt.%; polyvinyl pyrrolidone, 15 wt.%; N-methyl pyrrolidone, 70 wt.%) was cast on a glass plate. The cast solution film was placed in an oven with air circulation at 90°C for a predetermined period (O-6 min) before it was immersed in ice-cold water for 1 h. These membranes are asymmetric in structure and did not contain TEMPO. A dense PES film was also prepared from the above solution into which
148 (2002) 329-332
TEMPO (0.01 wt.%) was blended. The solvent was evaporated in the oven for 48 h. RO, UF and ESR experiments were carried out by the methods described elsewhere in detail ~2~31.
3. Results and discussion Fig. 1 shows the ESR spectra of an aqueous solution of TEMPO (0.01 wt.%). The spectra consist of three symmetric peaks (isotropic spectra), since the NO’ radical of TEMPO is freely mobile. The two outermost peaks of the nitroxide radicals are caused by the parallel transitions of the nuclear spin state, M, = +l [4,5]. Three hfs lines are almost equal to height; ratio of height of ‘A’ and ‘C’ with respect to middle line ‘B’ is almost unity. If the ratio ‘b/a’(b = height of the middle line; a = height of the outer most line in the lower field) becomes more than unity (anisotropic), it suggests that the radical is less mobile. Thus, the ratio could be used as an arbitrary measure for the degree of orientation or mobility of the radical [6,7].
-----i-.
Fig. 1. ESR spectra of aqueous solution of TEMPO (0.01 wt.%).
K.C. Khulbe et al. /Desalination
Fig. 2 shows the ESR spectra of the dense PES membrane prepared from the casting solution blended with TEMPO. The ESR spectra were taken when the membrane was in wet state. Unlike Fig. 1, the spectra are anisotropic and similar to the one reported by Tormala et a1.[4] and also to the one reported by Griffith and Waggnor [8] when TEMPO was in glass or in a highly viscous liquid. The spectra in Fig. 2 suggest that the radical is very immobile in the polymer. On keeping the membrane in water for 24 h, no appreciable change either in shape or in peak intensity was noticed. It seems that the radicals are diffused in the polymer matrix where water cannot affect them. Similar anisotropic spectra were observed for the dense CA film that contained TEMPO. However, the spectra disappeared after the membrane was soaked in water for less than 1 h, suggesting that the radicals were leached out from the swollen polymer matrix of CA. When the asymmetric PES membranes, which did not contain TEMPO, were immersed in 0.01 wt.% aqueous TEMPO solution for 1 h almost isotropic ESR spectra similar to those shown in Fig. 1 were obtained, indicating that the radicals were almost freely mobile. The ‘b/a’ ratios in all ESR spectra were almost one. Upon immersing the membranes in the TEMPO solution for 3 h, the spectra changed and the overlap of those from the radicals of low mobility (Fig. 2) and those from the radicals of high mobility(Fig. 1) was observed. Fig. 3 shows the ESR spectra of PES membrane (evaporation period 6 min) when it was soaked in aqueous solution of 0.01 wt.% TEMPO for 3 h. From Fig. 3, it seems that there are two types of radicals. One, which has hfs D, B and E and less mobile and the other, which has hfs A, B and C and more mobile. Both spectra share the middle line B. On keeping this membrane in water for 24 h, the more mobile radical was leached out. Only the less mobile radical remained inside the membrane and the ESR spectra became similar to those shown in Fig. 2. It indicates that the less mobile radical diffused into the polymer matrix,
331
148 (2002) 329-332
-
Fig. 2. ESR spectra of a dense PES membrane prepared from the casting solution blended with TEMPO (0.01 wt.% of PES).
Fig. 3. ESR spectra of PES membrane (evaporation period 6 min, without TEMPO) when it was soaked in 0.01 wt.% TEMPO solution for 3 h.
while the more mobile radical was in the pore. Similar results were observed with other PES membranes, when they were soaked in the TEMPO solution for 3 h. The ‘b/a’ ratios of the PES membranes were in a narrow range of 1.3-1.5. The separations of polyethylene oxide (MW = 100,000) by the PES membranes ranged from 72-92%. Asymmetric CA membranes without TEMPO were immersed in the aqueous TEMPO solution for 1 h before being subjected to ESR experiments. The membranes exhibited ESR spectra of NO. radical similar to Fig. 3, but the variation in ‘b/a’ ratio was larger than that for the PES membranes. The ‘b/a’ ratio of the NO. hfs lines varied from 1.3-l .9 as the shrinkage temperature increased from 60-90°C. Fig. 4 shows ‘b/a’ ratio vs. shrinkage temperature for the asymmetric
K.C. Khulbe et al. /Desalination
332
2 1.9 16 .z :! :m 2 2
1.7 1.6
70
7
60
g
50
$ 0 0 u g
40
1.5
30
1.4
too large to reject salt but can reject macromolecules. These results have been supported by the data from polyamide RO membranes as well [9].
4. Conclusions
20
13
10
1.2
0
60
148 (2002) 329-332
65
70
75
80
65
90
95
Shrinkage temperature(°C)
Fig. 4. ‘b/a’ ratio and NaCl separation vs. shrinkage temperature for asymmetric CA membranes. cellulose acetate membranes along with NaCl separation obtained from RO experiments using 0.1 wt.% NaCl solution as feed. Both b/a ratio and NaCl separation increased with the shrinkage temperature. The entire spectra disappeared by soaking the CA membranes in distilled water for 2 h, indicating that radicals diffuse into and out of the polymer matrix much faster than the PES membranes. It should be noted that the dense CA membrane that was cast from a solution into which TEMPO was blended showed a “b/a” ratio of 1.9 when it was wet. It is thought that the CA and PES membranes behaved differently since hydrophilic/ hydrophobic characters of both polymers are different. Water flows through pores of both CA and PES membranes. In CA membranes water may also flow through continuous channels that are formed in the water swollen polymer matrix. The latter water channels contribute to salt rejection while pores do not. It may be rather superfluous to discuss whether the water channels in the polymer matrix should also be called “pores”. But the presence of two distinct water passages was recognized. In PES membranes, on the other hand, water can flow only through pores. There is practically no water channel in polymer matrix since it is little swollen by water. The pores are
From the above study following conclusions can be drawn: 1. ESR technique can be used to study the structure and the transport of RO and UF membranes. 2. Water may flow through the pores of PES membranes. The sizes of pores are those of UF membranes. Unlike CA, the polymer matrix of PES membrane is little swollen or not at all swollen by water, and continuous channels through which water flows cannot be formed. In CA, spaces in water swollen polymer matrix were the primary provider of continuous flow channels that contribute to the separation of salt and small organic molecules. In absence of such water channels, PES membranes cannot act as RO membranes.
References S. Sourirajan and T. Matsuura, Reverse Osmosis/ Ultrafiltration Process Principles, National Research Council Canada, No. 24188.1985. 121 K.C. Khulbe, T. Matsuura, G Lamarche, A.-M. Lamarche, C. Choi and S.H. Noh, Polymer, 42 (2000) 6479484. [31 K.C. Khulbe, T. Matsuura, C.Y. Feng, G Lamarche and A.-M. Lamarche, Characterization of ultrafiltration membrane prepared from polyethersulfone by using electron resonance technique, Sep. Purif. Sci. and Tech., in press. t41 PT&rnalill, K. Silvennoinen and J.J. Lindberg, Acta Chemica Stand., 25 (1971) 2659-2665. t51 N. Edelstein, A. Kwok and A.H. Maki, J. Chem. Phys., 41 (1964) 179-183. [‘51 S.P Verma, H. Schneider and I.C.P. Smith, FEBS Lett., 25 (1972) 197-200. I71 S.P. Verma, H. Schneider and I.C.P. Smith, Arch. Biochem. Biophys., 162 (1974) 48-55. WI O.H. Griffith and A.S. Waggoner, ACC. Chem. Res., 2 (1969) 17-24. [91 K.C. Khulbe, C. Feng andT. Matsuura, Study of the structure and transport of asymmetric polyamide membranes for reverse osmosis using electron spin resonance (ESR) method, communicated for publication.
[II
ELSEVIER
148 (2002) 329-332 www.elsevier.com/locate/desal
Study on cellulose acetate membranes for reverse osmosis and polyethersulfone membranes for ultrafiltration by electron spin resonance technique K.C. Khulbe*, T. Matsuura, C.Y. Feng Industrial Membrane Research Institute, Department of Chemical Engineering, University of Ottawa, Ottawa, Ontario KIN 6N5, Canada Fax +I (613) 562-5172; email: [email protected] Received
1 February 2002; accepted 7 March 2002
Abstract Electron spin resonance (ESR) technique was used to study cellulose acetate (CA) membranes for reverse osmosis (RO) and polyethersulfone (PES) membranes for ultrafiltration. TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy free radical) was used as a spin probe that was brought into the membranes by immersing the membranes into solutions involving TEMPO, or by blending TEMPO into membrane casting solutions. It was found that the ESR signal from TEMPO in the membrane is useful for the choice of polymeric materials to prepare RO membranes. From ESR results it seems that for CA membranes, which are more hydrophilic and swollen by water, channels through the swollen polymer matrix contribute to the transport of water. On the other hand, the transport of water in PES membranes is primarily through pores and the contribution of the little swollen polymer matrix is insignificant. Keywords:
Polyethersulfone (PES) membrane; Cellulose acetate (CA) membrane; Reverse osmosis (RO); Ultrafiltration (UF)
Electron spin resonance
(ESR);
1. Introduction
as candidate
Since the announcement of the asymmetric CA membrane for seawater desalination in 1960, a number of polymeric materials have been tested
some polymers, it is possible to prepare membranes that exhibit high separations for sodium chloride and organic molecules of low molecular weights, while for others it is not possible. On the other
*Corresponding
OOII-9164/02/$-
hand, UF membranes
author.
Presented at the International July 7-12, 2002.
materials
Congress on Membranes
and Membrane
Processes
See front matter 0 2002 Elsevier Science B.V. All rights reserved
PII: SO0 I I-9 164(02)00725-7
for RO membranes.
can be prepared (ICOM),
For
from most
Toulouse, France,
330
K.C. Khulbe et al. /Desalination
of polymeric materials. Methods to screen polymers without doing RO experiments, which are preceded by a tedious process of membrane preparation, have long been sought for. In one of such attempts, it was found that the polymers, from which RO membranes of high salt selectivities could be prepared, were isolated in a limited range when two dimensional plots were made for hydrogen bonding and dispersion force components of solubility parameters of polymers [ 11. An attempt is made in the paper to find criteria for the choice of polymeric materials for RO membranes. For this purpose CA and PES membranes were studied to represent RO and UF membranes, respectively, and a novel membrane characterization technique based on ESR was used. 2. Experimental CA solution (CA, Eastman 393-3, 17 wt.%; Mg (ClO,),, 1.45 wt.%; water, 12.35 wt.%; acetone, 83 wt.%) was cast on a glass plate to a thickness of 0.33 mm. The solvent was evaporated at room temperature for 1 min before the cast film together with the glass plate was immersed in icecold water for 2 h. The membrane was shrunk in a hot water bath at different temperatures (60,70, 75, 80, 85 and 9O’C) for 10 min prior to RO experiments. The membranes so prepared were asymmetric in structure and did not contain TEMPO. A dense CA film was prepared from the above solution into which TEMPO (0.01 wt.%) was blended. The solvent was evaporated at room temperature for three days. PES solution (PES, 15 wt.%; polyvinyl pyrrolidone, 15 wt.%; N-methyl pyrrolidone, 70 wt.%) was cast on a glass plate. The cast solution film was placed in an oven with air circulation at 90°C for a predetermined period (O-6 min) before it was immersed in ice-cold water for 1 h. These membranes are asymmetric in structure and did not contain TEMPO. A dense PES film was also prepared from the above solution into which
148 (2002) 329-332
TEMPO (0.01 wt.%) was blended. The solvent was evaporated in the oven for 48 h. RO, UF and ESR experiments were carried out by the methods described elsewhere in detail ~2~31.
3. Results and discussion Fig. 1 shows the ESR spectra of an aqueous solution of TEMPO (0.01 wt.%). The spectra consist of three symmetric peaks (isotropic spectra), since the NO’ radical of TEMPO is freely mobile. The two outermost peaks of the nitroxide radicals are caused by the parallel transitions of the nuclear spin state, M, = +l [4,5]. Three hfs lines are almost equal to height; ratio of height of ‘A’ and ‘C’ with respect to middle line ‘B’ is almost unity. If the ratio ‘b/a’(b = height of the middle line; a = height of the outer most line in the lower field) becomes more than unity (anisotropic), it suggests that the radical is less mobile. Thus, the ratio could be used as an arbitrary measure for the degree of orientation or mobility of the radical [6,7].
-----i-.
Fig. 1. ESR spectra of aqueous solution of TEMPO (0.01 wt.%).
K.C. Khulbe et al. /Desalination
Fig. 2 shows the ESR spectra of the dense PES membrane prepared from the casting solution blended with TEMPO. The ESR spectra were taken when the membrane was in wet state. Unlike Fig. 1, the spectra are anisotropic and similar to the one reported by Tormala et a1.[4] and also to the one reported by Griffith and Waggnor [8] when TEMPO was in glass or in a highly viscous liquid. The spectra in Fig. 2 suggest that the radical is very immobile in the polymer. On keeping the membrane in water for 24 h, no appreciable change either in shape or in peak intensity was noticed. It seems that the radicals are diffused in the polymer matrix where water cannot affect them. Similar anisotropic spectra were observed for the dense CA film that contained TEMPO. However, the spectra disappeared after the membrane was soaked in water for less than 1 h, suggesting that the radicals were leached out from the swollen polymer matrix of CA. When the asymmetric PES membranes, which did not contain TEMPO, were immersed in 0.01 wt.% aqueous TEMPO solution for 1 h almost isotropic ESR spectra similar to those shown in Fig. 1 were obtained, indicating that the radicals were almost freely mobile. The ‘b/a’ ratios in all ESR spectra were almost one. Upon immersing the membranes in the TEMPO solution for 3 h, the spectra changed and the overlap of those from the radicals of low mobility (Fig. 2) and those from the radicals of high mobility(Fig. 1) was observed. Fig. 3 shows the ESR spectra of PES membrane (evaporation period 6 min) when it was soaked in aqueous solution of 0.01 wt.% TEMPO for 3 h. From Fig. 3, it seems that there are two types of radicals. One, which has hfs D, B and E and less mobile and the other, which has hfs A, B and C and more mobile. Both spectra share the middle line B. On keeping this membrane in water for 24 h, the more mobile radical was leached out. Only the less mobile radical remained inside the membrane and the ESR spectra became similar to those shown in Fig. 2. It indicates that the less mobile radical diffused into the polymer matrix,
331
148 (2002) 329-332
-
Fig. 2. ESR spectra of a dense PES membrane prepared from the casting solution blended with TEMPO (0.01 wt.% of PES).
Fig. 3. ESR spectra of PES membrane (evaporation period 6 min, without TEMPO) when it was soaked in 0.01 wt.% TEMPO solution for 3 h.
while the more mobile radical was in the pore. Similar results were observed with other PES membranes, when they were soaked in the TEMPO solution for 3 h. The ‘b/a’ ratios of the PES membranes were in a narrow range of 1.3-1.5. The separations of polyethylene oxide (MW = 100,000) by the PES membranes ranged from 72-92%. Asymmetric CA membranes without TEMPO were immersed in the aqueous TEMPO solution for 1 h before being subjected to ESR experiments. The membranes exhibited ESR spectra of NO. radical similar to Fig. 3, but the variation in ‘b/a’ ratio was larger than that for the PES membranes. The ‘b/a’ ratio of the NO. hfs lines varied from 1.3-l .9 as the shrinkage temperature increased from 60-90°C. Fig. 4 shows ‘b/a’ ratio vs. shrinkage temperature for the asymmetric
K.C. Khulbe et al. /Desalination
332
2 1.9 16 .z :! :m 2 2
1.7 1.6
70
7
60
g
50
$ 0 0 u g
40
1.5
30
1.4
too large to reject salt but can reject macromolecules. These results have been supported by the data from polyamide RO membranes as well [9].
4. Conclusions
20
13
10
1.2
0
60
148 (2002) 329-332
65
70
75
80
65
90
95
Shrinkage temperature(°C)
Fig. 4. ‘b/a’ ratio and NaCl separation vs. shrinkage temperature for asymmetric CA membranes. cellulose acetate membranes along with NaCl separation obtained from RO experiments using 0.1 wt.% NaCl solution as feed. Both b/a ratio and NaCl separation increased with the shrinkage temperature. The entire spectra disappeared by soaking the CA membranes in distilled water for 2 h, indicating that radicals diffuse into and out of the polymer matrix much faster than the PES membranes. It should be noted that the dense CA membrane that was cast from a solution into which TEMPO was blended showed a “b/a” ratio of 1.9 when it was wet. It is thought that the CA and PES membranes behaved differently since hydrophilic/ hydrophobic characters of both polymers are different. Water flows through pores of both CA and PES membranes. In CA membranes water may also flow through continuous channels that are formed in the water swollen polymer matrix. The latter water channels contribute to salt rejection while pores do not. It may be rather superfluous to discuss whether the water channels in the polymer matrix should also be called “pores”. But the presence of two distinct water passages was recognized. In PES membranes, on the other hand, water can flow only through pores. There is practically no water channel in polymer matrix since it is little swollen by water. The pores are
From the above study following conclusions can be drawn: 1. ESR technique can be used to study the structure and the transport of RO and UF membranes. 2. Water may flow through the pores of PES membranes. The sizes of pores are those of UF membranes. Unlike CA, the polymer matrix of PES membrane is little swollen or not at all swollen by water, and continuous channels through which water flows cannot be formed. In CA, spaces in water swollen polymer matrix were the primary provider of continuous flow channels that contribute to the separation of salt and small organic molecules. In absence of such water channels, PES membranes cannot act as RO membranes.
References S. Sourirajan and T. Matsuura, Reverse Osmosis/ Ultrafiltration Process Principles, National Research Council Canada, No. 24188.1985. 121 K.C. Khulbe, T. Matsuura, G Lamarche, A.-M. Lamarche, C. Choi and S.H. Noh, Polymer, 42 (2000) 6479484. [31 K.C. Khulbe, T. Matsuura, C.Y. Feng, G Lamarche and A.-M. Lamarche, Characterization of ultrafiltration membrane prepared from polyethersulfone by using electron resonance technique, Sep. Purif. Sci. and Tech., in press. t41 PT&rnalill, K. Silvennoinen and J.J. Lindberg, Acta Chemica Stand., 25 (1971) 2659-2665. t51 N. Edelstein, A. Kwok and A.H. Maki, J. Chem. Phys., 41 (1964) 179-183. [‘51 S.P Verma, H. Schneider and I.C.P. Smith, FEBS Lett., 25 (1972) 197-200. I71 S.P. Verma, H. Schneider and I.C.P. Smith, Arch. Biochem. Biophys., 162 (1974) 48-55. WI O.H. Griffith and A.S. Waggoner, ACC. Chem. Res., 2 (1969) 17-24. [91 K.C. Khulbe, C. Feng andT. Matsuura, Study of the structure and transport of asymmetric polyamide membranes for reverse osmosis using electron spin resonance (ESR) method, communicated for publication.
[II