- Email: [email protected]
Effect of heat treatment on performance of chlorine-resistant polyamide reverse osmosis membranes
Desalination 247 (2009) 370–377
Effect of heat treatment on performance of chlorine-resistant polyamide reverse osmosis membranes Takuji Shintania,b, Hideto Matsuyamaa*, Naoki Kuratab a
Department of Chemical Science and Engineering, Kobe University, Rokkodai, Nada-ku, Kobe 657-8501, Japan b Core Technology Center, Nitto Denko Corporation, Shimohozumi, Ibaraki, Osaka 567-8680, Japan Tel: +81-78-803-6180; Fax: +81-78-803-6206; email: [email protected] Received 12 September 2006; accepted 29 September 2008
Abstract In our series of studies for the development of a reverse osmosis polyamide membrane with high resistance to chlorine, N,N ’-dimethyl-m-phenylenediamine (N,N ’-DMMPD) was selected in the preparation of the polyamide membrane after the screening of various diamines. High resistance to chlorine was confirmed for the prepared membrane in our previous work. However, the heat treatment conditions were not yet optimized. In this work, we focused on the effects of heat treatment conditions on the membrane properties. The heat treatment temperature and time remarkably influenced salt rejection and flux. Maximum rejection was found with respect to the temperature. Both rejection and flux showed trends to become constant with longer treatment times. However, rejection reached the constant value in less time than flux. Thus, to obtain both high rejection and flux, an optimum treatment time existed. The newly prepared polyamide membrane showed higher salt rejection than the previously prepared membrane while maintaining high resistance to chlorine. Keywords: Reverse osmosis membrane; Heat treatment; Chlorine resistance; Polyamide; Interfacial polymerization
1. Introduction Reverse osmosis (RO) membranes are presently being used and are playing an important role in a wide range of applications such as water treatment and water pollution control. Thin-film-composite (TFC) membranes have been widely commercialized for use in RO separations [1–3]. The TFC membrane consists of a supporting layer coated with an ultra-thin barrier *Corresponding author.
layer on its top surface. The performance of a TFC membrane depends on the barrier layer, which is typically formed by an interfacial polymerization. The most successful commercial RO membranes have aromatic polyamide layers. The TFC membranes have reached a relatively high level in regard to salt rejection and water permeability. However, a major concern with polyamide TFC membranes is their sensitivity to chlorine disinfectants, which are commonly used in the field of seawater desalination and food processing. The membrane performances were
0011-9164/09/$– See front matter Ó 2009 Publish e d by Elsevier B.V. doi:10.1016/j.desal.2008.09.003
T. Shintani et al. / Desalination 247 (2009) 370–377
sometimes lost when they were exposed to an aqueous chlorine solution. A detailed understanding of the membrane degradation during chlorine exposure and the guidelines for development of TFC membranes with high resistance to chlorine are undoubtedly quite important to overcome the disadvantage of present RO membranes. The correlation between the chemical structure of polyamides and the reactivity to hypochlorous acid was investigated to evaluate their chlorine resistance [4]. In the interaction of polyamides with hypochlorous acid, three modes were classified: no reaction, reversible chlorination and irreversible chlorination. Wu et al. investigated the effects of chlorine on three kinds of aromatic polyamides and clarified that the chlorination depends on not only their chemical structures but also chlorination conditions such as pH and reaction time [5]. Soice et al. correlated the chlorine concentration, pH and exposure time with chemical changes in the model compounds. From the observed reactivity trends, the mechanism of RO membrane performance loss upon chlorine exposure was considered [6]. Konagaya and colleagues prepared various polyamides from isophthaloyl dichloride and aliphatic, cycloaliphatic and aromatic diamines, and also prepared copolyamides from mixed diamine components [7–9]. They found that some chemical structures of diamines were important for higher chlorine resistance. In our previous paper [10], various polyamides were obtained from isophthaloyl dichloride (IPC) or 1,3,5-benzenetricarbonyl trichloride (TMC) and 17 kinds of diamines. For polyamides obtained from IPC and the diamines, the molecular weights could be measured because they were dissolved in a solvent. To evaluate the chlorine resistance after immersion in chlorine solution, the decrease of molecular weight was used. The polymer weight loss was measured for the evaluation of chlorine resistance for polyamides obtained from TMC and diamines. Based on this screening of diamines, the RO membrane
371
was prepared from N,N’-dimethyl-m-phenylenediamine (N,N’-DMMPD) and TMC. The newly prepared polyamide membrane showed better chlorine resistance, compared with a commercial polyamide RO membrane and also cellulose acetate RO membrane. In preparing membranes from N,N’-DMMPD and TMC and IPC, the process conditions have not yet been optimized. In this work, the effects of heat treatment were investigated on the newly prepared polyamide membrane in order to improve the membrane performance. 2. Experimental 2.1. Materials Polyamide was prepared from N,N’-DMMPD and a mixture of isophthaloyl dichloride (IPC) and 1,3,5-benzenetricarbonyl trichloride (TMC). IPC and TMC were purchased from Wako Pure Chemical Industries and Tokyo Kasei Kogyo Co., Ltd., respectively. N,N’-DMMPD was synthesized according to the patent [11]. The monomer structures and the polymerization scheme are shown in Fig. 1. 2.2. Membrane preparation and heat treatment The microporous support substrate was first prepared for the thin-film-composite (TFC) membranes. A solution of 18 wt% polysulfone (PSf, Udel P-3500, Solvay Advanced Polymers Co.) in N,N-dimethylformamide was cast on a glass plate using an applicator with a 254 mm gap. The glass plate was immediately immersed in a water bath at room temperature. The PSf membrane was rinsed with water and stored in deionized water. The substrate structures heattreated for 3 min at different temperatures were examined with a scanning electron microscope (Hitachi Co., Semedx type N). The square PSF membrane of 15 cm 15 cm was used for the following interfacial polymerization.
372
T. Shintani et al. / Desalination 247 (2009) 370–377
Fig. 1. Polymerization scheme.
The polyamide barrier layer on the top surface was prepared from N,N’-DMMPD and a mixture of TMC and IPC. The diamine solution used for the interfacial reaction was a solution of N,N’-DMMPD (3 wt%), sodium lauryl sulfate (0.15 wt%), triethylamine (2.5 wt%), camphorsulfonic acid (5 wt%) and ethylene glycol (89.35 wt%). The organic solution was n-hexane containing TMC (0.2 wt%) and IPC (0.4 wt%). In this work, the diamine solution was the ethylene glycol solution, while 2-propanol was added to the aqueous phase in the previous work [10]. The support PSf membrane was immersed in diamine solution for 30 sec and pulled up slowly. Excess diamine solution was removed from the surface of the PSf support membrane. The PSf substrate was then covered with a solution of TMC and IPC for 10 sec to deposit the polymeric thin layer on the substrate by the interfacial reaction. Then, the TMC and IPC mixture solution was immediately drained from the substrate. After the interfacial polymerization, the membrane was heat-treated at various
temperatures. In addition, the heat treatment time was changed at a constant temperature of 1208C. 2.3. Membrane performance Membrane performances were measured by the same method as described in the previous paper [10]. Salt rejection and water permeability were measured for 1500 ppm NaCl solution with pH = 6.5–7 at 258C using the common continuous pump-type RO apparatus under an applied pressure of 1.5 MPa. For the measurement of the chlorine resistance, the membrane was immersed in aqueous sodium hypochlorite solution (200 ppm) including calcium chloride (500 ppm) at pH = 7.0 at 408C for 96 h. Calcium chloride was used as the oxidation accelerator [12]. The membrane performance was checked every 24 h. For comparison, a commercial polyamide RO membrane (Nitto Denko Co., NTR-759HR) was also used for the membrane performance measurement in addition to the prepared membrane.
T. Shintani et al. / Desalination 247 (2009) 370–377
98 NaCl Rejection [%]
For the analysis of the thin barrier layer, FTIR and TGA measurements were carried out. The diamine solution used for the interfacial reaction was an aqueous solution of N,N’DMMPD (3 wt%), sodium lauryl sulfate (0.15 wt%), triethylamine (2.5 wt%), camphorsulfonic acid (5 wt%) and 2-propanol (30 wt%). The organic solution containing TMC and IPC was the same as described above. The interfacial polymerization was carried out using the PSf support. The membrane was heat-treated at 408C or 1208C for 3 min. The ATR method was used for FT-IR analysis (JASCO FT/IR-470). For the TGA analysis (SEIKO Instruments Inc. TG/DTA300), only the barrier layer was recovered from the composite membrane. The membrane with PSf was immersed in cyclohexane to dissolve the PSf and the solution was stirred for one night. After the filtration, the barrier layer was recovered. The weight loss of the polyamide layer was measured by the heating rate of 158C/min under nitrogen atmosphere.
100
96
94
92
90 1.5
Flux [m3/m2/d]
2.4. Analysis of thin barrier layer
373
1.0
0.5
3. Results and discussion Figure 2 shows the effect of heat treatment temperature on salt rejection and membrane flux. With increases in temperature, the flux was initially almost constant and decreased monotonically. The rejection showed a strange trend. That is, it increased first and after reaching the maximum, then decreased. When the temperature is increased, the un-reacted site of TMC and IPC can react with the un-reacted site of N,N’-DMMPD to form the amide group and the structure of the barrier layer becomes denser and more crosslinked, resulting in a decrease of flux and an increase of rejection. These tendencies were observed in the temperature region up to 1208C in Fig. 2. The further temperature increase probably brings about the
0.0 0
50 100 150 Heat treatment temperature [°C]
200
Fig. 2. Relation between heat treatment temperature and RO performance. Heat treatment time: 30 s.
shrinkage of the pore structure of the support PSf membrane. This leads to the decrease of the water flux. Of course, the support membrane has no rejection for the salt. Thus, the flux of salt is hardly influenced by this shrinkage of the support membrane because the permeation resistance in the barrier layer is dominant. By the heat treatment at more than 1208C, both water and salt fluxes decreased due to the further densification of the barrier layer. However, the
374
T. Shintani et al. / Desalination 247 (2009) 370–377
50°C
130°C
150nm
150nm
Fig. 3. SEM results for surfaces of support membranes heat-treated at different temperatures.
The peaks at 1245 cm 1 attributed to C-O stretching of the PSf support are almost the same for the two membranes. However, the peak at 1645 cm 1 attributed to C=O stretching of the amide group formed by the reaction between diamine and acid chloride is clearly larger for the membrane heat-treated at 1208C than for the membrane heat-treated at 408C. Actually, the ratio of the peak intensity at 1645 cm 1 to that at 1245 cm 1 was 0.95 for the membrane heat-treated at 408C, while that ratio was 1.11 for the membrane heat-treated at 0.05 1645 C=O( ) 1590
1490 C=C( ) C=C( )
0.04 Absorbance [a.u.]
decrease of the flux was more pronounced for water than for salt because of the support membrane shrinkage. This leads to the decrease of the salt rejection, as shown in the higher temperature region in Fig. 2. The flux in Fig. 2 is the total flux including water and salt. However, the salt rejection is high enough so that it almost corresponds to the water flux. The flux decreased more abruptly from 1208C, at which the salt rejection showed the maximum. This seemed to be evidence for the shrinkage of the support membrane. To confirm this support membrane shrinkage, the support membrane alone was heat-treated for 3 min at different temperatures. The SEM results for support membrane surfaces are shown in Fig. 3. Image analysis (Nippon Roper Co., Image Pro Ver.5) of SEM data was carried out for quantitative analysis by twovalued treatment method. The measurements were repeated 8 times for both membranes heat-treated at 508C and 1308C. The surface porosity of the membrane heat-treated at 508C was 0.155 ± 0.020, while that of the membrane heat-treated at 1308C was 0.115 ± 0.023. The heat treatment at higher temperature brought about the decrease of the surface porosity due to the shrinkage of the PSf support membrane. Figure 4 shows the FT-IR results for two membranes heat-treated at 408C and 1208C.
1245 C=O( ) 1360 C-H( )
0.03
0.02 0.01
0
1800
1700
1600
1500
1400
1300
1200
Wavenumber [cm⫺1]
Fig. 4. FT-IR results of barrier layers. (- - -) Membrane heat-treated at 408C, (—) Membrane heat-treated at 1208C.
T. Shintani et al. / Desalination 247 (2009) 370–377
However, the times at which the constant values were obtained are different for the salt rejection and the flux. At about 20 s, the salt rejection approached the constant value, while more than 60 s were necessary to obtain the constant flux. The salt rejection reaches the highest level when the densification occurs at a certain zone in the barrier layer, and further densification of the other zones of the barrier layer brings about no pronounced effect on the salt rejection because the dense structures are not so different 100
98 NaCl Rejection [%]
1208C. Although the spectrographic results were not replicated, it demonstrates that the amide group is likely to form with heat treatment at the higher temperature, which brings about the formation of a denser barrier layer. Figure 5 shows the TGA results. For the membrane heat-treated at 408C, the weight decreased from about 4008C. On the other hand, the membrane heat-treated at 1208C showed better resistance to weight loss. The data shown in Fig. 5 probably indicate that heat treatment at higher temperature leads to a more highly crosslinked structure due to accelerated amide group formation. Generally, a more highly crosslinked structure would have improved resistance to weight loss via degradation. The effect of the heat treatment time is shown in Fig. 6. As the time was increased, the salt rejection first increased and then became constant. On the other hand, the flux approached the constant value after the abrupt decrease. These trends can be understood from the densification of the barrier layer due to the higher crosslinking for the longer heat treatment time. The decrease of the flux at longer treatment time was already reported in the literature [1].
375
96
94
92
90 2.0
120 a)
110 100
1.5 Flux [m3/m2/d]
Weight residue [%]
90 80 70 60 120 b)
110 100
1.0
0.5
90 80 70 60
0.0 0
100
200
300
400
500
Temperature [°C]
Fig. 5. TGA curves of barrier layers. (a) Membrane heattreated at 408C. (b) Membrane heat-treated at 1208C.
0
50 100 150 Heat treatment time at 120°C [sec]
200
Fig. 6. Effect of heat treatment time at 1208C on RO performance. Data (D) for the polyamide membrane prepared in our previous work [10].
T. Shintani et al. / Desalination 247 (2009) 370–377 99.9
99.0
NaCl Rejection [%]
at each zone. However, the flux is much influenced by the thickness of the dense zones. Thus, more time is necessary to achieve a constant flux. In addition, the shrinkage of the support membrane described above may be another reason why the flux still continued to decrease after the rejection reached a constant value. From this experimental result, a heat treatment of about 30 s was found to be suitable to obtain both high rejection and high flux. The data (D) for the polyamide membrane prepared in our previous work [10] were also added in Fig. 6. These data showed very low rejection although the flux was high. In the previous work, 2-propanol was used as an additive into the aqueous solution. In this case, the interface between the aqueous phase and organic phase was not clear, and thus the interfacial polymerization could not proceed satisfactorily. On the other hand, a clear interface could be formed between the ethylene glycol solution used in this work and the organic phase. The concentrations of the diamine and acid chlorides in this membrane preparation were higher than those used previously [10]. This higher concentration may be another reason for the higher solute rejection and the lower flux. Figure 7 showed the effects of immersion time in the chlorine solution on the membrane rejection and membrane flux. Membrane performances were compared for three membranes. Polyamide-1 and -2 are the commercial membrane and the membrane prepared from N,N’DMMPD and mixture of TMC and IPC in the previous work [10], respectively. Polyamide-3 is the membrane newly prepared in this work with the heat treatment at 1208C for 30 s. For polyamide-1, the decomposition of polymer material by chlorine resulted in the drastic reduction of the rejection accompanying the abrupt increase in the flux. In contrast, polyamide-2 and -3 showed superior chlorine resistance due to the better choice of the diamine. The higher salt rejection was obtained
90.0
0.0 100
Flux [m3/m2/d]
376
10
1
0.1
0
10 20 30 40 50 60 70 Immersion time in chlorine solution [hr]
80
Fig. 7. Effect of immersion time in chlorine solution on membrane rejection and membrane flux.*: polyamide-1 Ñ: polyamide-2 à: polyamide-3.
for the polyamide-3 membrane compared with the polyamine-2 membrane. This is owing to both the optimization of the heat treatment conditions and the selection of ethylene glycol solution in the membrane preparation. 4. Conclusions For the development of a RO polyamide membrane with high resistance to chlorine, the suitable diamine was selected in the previous paper. In this paper, the effect of heat treatment
T. Shintani et al. / Desalination 247 (2009) 370–377
conditions were investigated. With temperature increase, the salt rejection displayed a maximum value, while the flux was initially almost constant and decreased monotonically. When the treatment time increased, the rejection increased first and then reached a constant value. The flux also approached a constant value after the reduction. However, the rejection approached a constant value in a shorter time than the flux, which means that the optimum time existed to obtain both high rejection and high flux. The membrane heat-treated in optimum conditions showed better salt rejection as well as very high chlorine resistance. References [1] J. E. Cadotte :U.S. Patent 4,277,344 (1981). [2] J. E. Cadotte: D. R. Lloyd, (Ed.), Materials Science of Synthetic Membrane, American Chemical Society, Washington, D.C., 1984, pp. 273–294.
377
[3] R. J. Petersen, J. Membr. Sci., 83 (1993) 81–150. [4] T. Kawaguchi and H. Tamura, J. Appl. Polym. Sci., 29 (1984) 3359–3367. [5] S. Wu, J. Xing, G. Zheng, H. Lian and L. Shen, J. Appl. Polym. Sci., 61 (1996) 1305–1314. [6] N. P. Soice, A. C. Maladono, D. Y. Takigawa, A. D. Norman, W. B. Krantz and A. R. Greenberg, J. Appl. Polym. Sci., 90 (2003) 1173–1184. [7] S. Konagaya and O. Watanabe, J. Appl. Polym. Sci., 76 (2000) 201–207. [8] S. Konagaya, H. Kuzumoto and O. Watanabe, J. Appl. Polym. Sci., 75 (2000) 1357–1364. [9] S. Konagaya and M. Tokai, J. Appl. Polym. Sci., 76 (2000) 913–920. [10] T. Shintani, H. Matsuyama and N. Kurata, Desalination, 207 (2007) 340–348. [11] T. Shintani, M. Maeda and T. Ohara, Jpn Kokai Tokkyo Koho JP2003-257933 (2003). [12] C. J. Gabelich, J. C. Frankin, F. W. Gerringer, K. P. Ishida and I. H. Saffet, J. Membr. Sci., 258 (2005) 64–70.
Effect of heat treatment on performance of chlorine-resistant polyamide reverse osmosis membranes Takuji Shintania,b, Hideto Matsuyamaa*, Naoki Kuratab a
Department of Chemical Science and Engineering, Kobe University, Rokkodai, Nada-ku, Kobe 657-8501, Japan b Core Technology Center, Nitto Denko Corporation, Shimohozumi, Ibaraki, Osaka 567-8680, Japan Tel: +81-78-803-6180; Fax: +81-78-803-6206; email: [email protected] Received 12 September 2006; accepted 29 September 2008
Abstract In our series of studies for the development of a reverse osmosis polyamide membrane with high resistance to chlorine, N,N ’-dimethyl-m-phenylenediamine (N,N ’-DMMPD) was selected in the preparation of the polyamide membrane after the screening of various diamines. High resistance to chlorine was confirmed for the prepared membrane in our previous work. However, the heat treatment conditions were not yet optimized. In this work, we focused on the effects of heat treatment conditions on the membrane properties. The heat treatment temperature and time remarkably influenced salt rejection and flux. Maximum rejection was found with respect to the temperature. Both rejection and flux showed trends to become constant with longer treatment times. However, rejection reached the constant value in less time than flux. Thus, to obtain both high rejection and flux, an optimum treatment time existed. The newly prepared polyamide membrane showed higher salt rejection than the previously prepared membrane while maintaining high resistance to chlorine. Keywords: Reverse osmosis membrane; Heat treatment; Chlorine resistance; Polyamide; Interfacial polymerization
1. Introduction Reverse osmosis (RO) membranes are presently being used and are playing an important role in a wide range of applications such as water treatment and water pollution control. Thin-film-composite (TFC) membranes have been widely commercialized for use in RO separations [1–3]. The TFC membrane consists of a supporting layer coated with an ultra-thin barrier *Corresponding author.
layer on its top surface. The performance of a TFC membrane depends on the barrier layer, which is typically formed by an interfacial polymerization. The most successful commercial RO membranes have aromatic polyamide layers. The TFC membranes have reached a relatively high level in regard to salt rejection and water permeability. However, a major concern with polyamide TFC membranes is their sensitivity to chlorine disinfectants, which are commonly used in the field of seawater desalination and food processing. The membrane performances were
0011-9164/09/$– See front matter Ó 2009 Publish e d by Elsevier B.V. doi:10.1016/j.desal.2008.09.003
T. Shintani et al. / Desalination 247 (2009) 370–377
sometimes lost when they were exposed to an aqueous chlorine solution. A detailed understanding of the membrane degradation during chlorine exposure and the guidelines for development of TFC membranes with high resistance to chlorine are undoubtedly quite important to overcome the disadvantage of present RO membranes. The correlation between the chemical structure of polyamides and the reactivity to hypochlorous acid was investigated to evaluate their chlorine resistance [4]. In the interaction of polyamides with hypochlorous acid, three modes were classified: no reaction, reversible chlorination and irreversible chlorination. Wu et al. investigated the effects of chlorine on three kinds of aromatic polyamides and clarified that the chlorination depends on not only their chemical structures but also chlorination conditions such as pH and reaction time [5]. Soice et al. correlated the chlorine concentration, pH and exposure time with chemical changes in the model compounds. From the observed reactivity trends, the mechanism of RO membrane performance loss upon chlorine exposure was considered [6]. Konagaya and colleagues prepared various polyamides from isophthaloyl dichloride and aliphatic, cycloaliphatic and aromatic diamines, and also prepared copolyamides from mixed diamine components [7–9]. They found that some chemical structures of diamines were important for higher chlorine resistance. In our previous paper [10], various polyamides were obtained from isophthaloyl dichloride (IPC) or 1,3,5-benzenetricarbonyl trichloride (TMC) and 17 kinds of diamines. For polyamides obtained from IPC and the diamines, the molecular weights could be measured because they were dissolved in a solvent. To evaluate the chlorine resistance after immersion in chlorine solution, the decrease of molecular weight was used. The polymer weight loss was measured for the evaluation of chlorine resistance for polyamides obtained from TMC and diamines. Based on this screening of diamines, the RO membrane
371
was prepared from N,N’-dimethyl-m-phenylenediamine (N,N’-DMMPD) and TMC. The newly prepared polyamide membrane showed better chlorine resistance, compared with a commercial polyamide RO membrane and also cellulose acetate RO membrane. In preparing membranes from N,N’-DMMPD and TMC and IPC, the process conditions have not yet been optimized. In this work, the effects of heat treatment were investigated on the newly prepared polyamide membrane in order to improve the membrane performance. 2. Experimental 2.1. Materials Polyamide was prepared from N,N’-DMMPD and a mixture of isophthaloyl dichloride (IPC) and 1,3,5-benzenetricarbonyl trichloride (TMC). IPC and TMC were purchased from Wako Pure Chemical Industries and Tokyo Kasei Kogyo Co., Ltd., respectively. N,N’-DMMPD was synthesized according to the patent [11]. The monomer structures and the polymerization scheme are shown in Fig. 1. 2.2. Membrane preparation and heat treatment The microporous support substrate was first prepared for the thin-film-composite (TFC) membranes. A solution of 18 wt% polysulfone (PSf, Udel P-3500, Solvay Advanced Polymers Co.) in N,N-dimethylformamide was cast on a glass plate using an applicator with a 254 mm gap. The glass plate was immediately immersed in a water bath at room temperature. The PSf membrane was rinsed with water and stored in deionized water. The substrate structures heattreated for 3 min at different temperatures were examined with a scanning electron microscope (Hitachi Co., Semedx type N). The square PSF membrane of 15 cm 15 cm was used for the following interfacial polymerization.
372
T. Shintani et al. / Desalination 247 (2009) 370–377
Fig. 1. Polymerization scheme.
The polyamide barrier layer on the top surface was prepared from N,N’-DMMPD and a mixture of TMC and IPC. The diamine solution used for the interfacial reaction was a solution of N,N’-DMMPD (3 wt%), sodium lauryl sulfate (0.15 wt%), triethylamine (2.5 wt%), camphorsulfonic acid (5 wt%) and ethylene glycol (89.35 wt%). The organic solution was n-hexane containing TMC (0.2 wt%) and IPC (0.4 wt%). In this work, the diamine solution was the ethylene glycol solution, while 2-propanol was added to the aqueous phase in the previous work [10]. The support PSf membrane was immersed in diamine solution for 30 sec and pulled up slowly. Excess diamine solution was removed from the surface of the PSf support membrane. The PSf substrate was then covered with a solution of TMC and IPC for 10 sec to deposit the polymeric thin layer on the substrate by the interfacial reaction. Then, the TMC and IPC mixture solution was immediately drained from the substrate. After the interfacial polymerization, the membrane was heat-treated at various
temperatures. In addition, the heat treatment time was changed at a constant temperature of 1208C. 2.3. Membrane performance Membrane performances were measured by the same method as described in the previous paper [10]. Salt rejection and water permeability were measured for 1500 ppm NaCl solution with pH = 6.5–7 at 258C using the common continuous pump-type RO apparatus under an applied pressure of 1.5 MPa. For the measurement of the chlorine resistance, the membrane was immersed in aqueous sodium hypochlorite solution (200 ppm) including calcium chloride (500 ppm) at pH = 7.0 at 408C for 96 h. Calcium chloride was used as the oxidation accelerator [12]. The membrane performance was checked every 24 h. For comparison, a commercial polyamide RO membrane (Nitto Denko Co., NTR-759HR) was also used for the membrane performance measurement in addition to the prepared membrane.
T. Shintani et al. / Desalination 247 (2009) 370–377
98 NaCl Rejection [%]
For the analysis of the thin barrier layer, FTIR and TGA measurements were carried out. The diamine solution used for the interfacial reaction was an aqueous solution of N,N’DMMPD (3 wt%), sodium lauryl sulfate (0.15 wt%), triethylamine (2.5 wt%), camphorsulfonic acid (5 wt%) and 2-propanol (30 wt%). The organic solution containing TMC and IPC was the same as described above. The interfacial polymerization was carried out using the PSf support. The membrane was heat-treated at 408C or 1208C for 3 min. The ATR method was used for FT-IR analysis (JASCO FT/IR-470). For the TGA analysis (SEIKO Instruments Inc. TG/DTA300), only the barrier layer was recovered from the composite membrane. The membrane with PSf was immersed in cyclohexane to dissolve the PSf and the solution was stirred for one night. After the filtration, the barrier layer was recovered. The weight loss of the polyamide layer was measured by the heating rate of 158C/min under nitrogen atmosphere.
100
96
94
92
90 1.5
Flux [m3/m2/d]
2.4. Analysis of thin barrier layer
373
1.0
0.5
3. Results and discussion Figure 2 shows the effect of heat treatment temperature on salt rejection and membrane flux. With increases in temperature, the flux was initially almost constant and decreased monotonically. The rejection showed a strange trend. That is, it increased first and after reaching the maximum, then decreased. When the temperature is increased, the un-reacted site of TMC and IPC can react with the un-reacted site of N,N’-DMMPD to form the amide group and the structure of the barrier layer becomes denser and more crosslinked, resulting in a decrease of flux and an increase of rejection. These tendencies were observed in the temperature region up to 1208C in Fig. 2. The further temperature increase probably brings about the
0.0 0
50 100 150 Heat treatment temperature [°C]
200
Fig. 2. Relation between heat treatment temperature and RO performance. Heat treatment time: 30 s.
shrinkage of the pore structure of the support PSf membrane. This leads to the decrease of the water flux. Of course, the support membrane has no rejection for the salt. Thus, the flux of salt is hardly influenced by this shrinkage of the support membrane because the permeation resistance in the barrier layer is dominant. By the heat treatment at more than 1208C, both water and salt fluxes decreased due to the further densification of the barrier layer. However, the
374
T. Shintani et al. / Desalination 247 (2009) 370–377
50°C
130°C
150nm
150nm
Fig. 3. SEM results for surfaces of support membranes heat-treated at different temperatures.
The peaks at 1245 cm 1 attributed to C-O stretching of the PSf support are almost the same for the two membranes. However, the peak at 1645 cm 1 attributed to C=O stretching of the amide group formed by the reaction between diamine and acid chloride is clearly larger for the membrane heat-treated at 1208C than for the membrane heat-treated at 408C. Actually, the ratio of the peak intensity at 1645 cm 1 to that at 1245 cm 1 was 0.95 for the membrane heat-treated at 408C, while that ratio was 1.11 for the membrane heat-treated at 0.05 1645 C=O( ) 1590
1490 C=C( ) C=C( )
0.04 Absorbance [a.u.]
decrease of the flux was more pronounced for water than for salt because of the support membrane shrinkage. This leads to the decrease of the salt rejection, as shown in the higher temperature region in Fig. 2. The flux in Fig. 2 is the total flux including water and salt. However, the salt rejection is high enough so that it almost corresponds to the water flux. The flux decreased more abruptly from 1208C, at which the salt rejection showed the maximum. This seemed to be evidence for the shrinkage of the support membrane. To confirm this support membrane shrinkage, the support membrane alone was heat-treated for 3 min at different temperatures. The SEM results for support membrane surfaces are shown in Fig. 3. Image analysis (Nippon Roper Co., Image Pro Ver.5) of SEM data was carried out for quantitative analysis by twovalued treatment method. The measurements were repeated 8 times for both membranes heat-treated at 508C and 1308C. The surface porosity of the membrane heat-treated at 508C was 0.155 ± 0.020, while that of the membrane heat-treated at 1308C was 0.115 ± 0.023. The heat treatment at higher temperature brought about the decrease of the surface porosity due to the shrinkage of the PSf support membrane. Figure 4 shows the FT-IR results for two membranes heat-treated at 408C and 1208C.
1245 C=O( ) 1360 C-H( )
0.03
0.02 0.01
0
1800
1700
1600
1500
1400
1300
1200
Wavenumber [cm⫺1]
Fig. 4. FT-IR results of barrier layers. (- - -) Membrane heat-treated at 408C, (—) Membrane heat-treated at 1208C.
T. Shintani et al. / Desalination 247 (2009) 370–377
However, the times at which the constant values were obtained are different for the salt rejection and the flux. At about 20 s, the salt rejection approached the constant value, while more than 60 s were necessary to obtain the constant flux. The salt rejection reaches the highest level when the densification occurs at a certain zone in the barrier layer, and further densification of the other zones of the barrier layer brings about no pronounced effect on the salt rejection because the dense structures are not so different 100
98 NaCl Rejection [%]
1208C. Although the spectrographic results were not replicated, it demonstrates that the amide group is likely to form with heat treatment at the higher temperature, which brings about the formation of a denser barrier layer. Figure 5 shows the TGA results. For the membrane heat-treated at 408C, the weight decreased from about 4008C. On the other hand, the membrane heat-treated at 1208C showed better resistance to weight loss. The data shown in Fig. 5 probably indicate that heat treatment at higher temperature leads to a more highly crosslinked structure due to accelerated amide group formation. Generally, a more highly crosslinked structure would have improved resistance to weight loss via degradation. The effect of the heat treatment time is shown in Fig. 6. As the time was increased, the salt rejection first increased and then became constant. On the other hand, the flux approached the constant value after the abrupt decrease. These trends can be understood from the densification of the barrier layer due to the higher crosslinking for the longer heat treatment time. The decrease of the flux at longer treatment time was already reported in the literature [1].
375
96
94
92
90 2.0
120 a)
110 100
1.5 Flux [m3/m2/d]
Weight residue [%]
90 80 70 60 120 b)
110 100
1.0
0.5
90 80 70 60
0.0 0
100
200
300
400
500
Temperature [°C]
Fig. 5. TGA curves of barrier layers. (a) Membrane heattreated at 408C. (b) Membrane heat-treated at 1208C.
0
50 100 150 Heat treatment time at 120°C [sec]
200
Fig. 6. Effect of heat treatment time at 1208C on RO performance. Data (D) for the polyamide membrane prepared in our previous work [10].
T. Shintani et al. / Desalination 247 (2009) 370–377 99.9
99.0
NaCl Rejection [%]
at each zone. However, the flux is much influenced by the thickness of the dense zones. Thus, more time is necessary to achieve a constant flux. In addition, the shrinkage of the support membrane described above may be another reason why the flux still continued to decrease after the rejection reached a constant value. From this experimental result, a heat treatment of about 30 s was found to be suitable to obtain both high rejection and high flux. The data (D) for the polyamide membrane prepared in our previous work [10] were also added in Fig. 6. These data showed very low rejection although the flux was high. In the previous work, 2-propanol was used as an additive into the aqueous solution. In this case, the interface between the aqueous phase and organic phase was not clear, and thus the interfacial polymerization could not proceed satisfactorily. On the other hand, a clear interface could be formed between the ethylene glycol solution used in this work and the organic phase. The concentrations of the diamine and acid chlorides in this membrane preparation were higher than those used previously [10]. This higher concentration may be another reason for the higher solute rejection and the lower flux. Figure 7 showed the effects of immersion time in the chlorine solution on the membrane rejection and membrane flux. Membrane performances were compared for three membranes. Polyamide-1 and -2 are the commercial membrane and the membrane prepared from N,N’DMMPD and mixture of TMC and IPC in the previous work [10], respectively. Polyamide-3 is the membrane newly prepared in this work with the heat treatment at 1208C for 30 s. For polyamide-1, the decomposition of polymer material by chlorine resulted in the drastic reduction of the rejection accompanying the abrupt increase in the flux. In contrast, polyamide-2 and -3 showed superior chlorine resistance due to the better choice of the diamine. The higher salt rejection was obtained
90.0
0.0 100
Flux [m3/m2/d]
376
10
1
0.1
0
10 20 30 40 50 60 70 Immersion time in chlorine solution [hr]
80
Fig. 7. Effect of immersion time in chlorine solution on membrane rejection and membrane flux.*: polyamide-1 Ñ: polyamide-2 à: polyamide-3.
for the polyamide-3 membrane compared with the polyamine-2 membrane. This is owing to both the optimization of the heat treatment conditions and the selection of ethylene glycol solution in the membrane preparation. 4. Conclusions For the development of a RO polyamide membrane with high resistance to chlorine, the suitable diamine was selected in the previous paper. In this paper, the effect of heat treatment
T. Shintani et al. / Desalination 247 (2009) 370–377
conditions were investigated. With temperature increase, the salt rejection displayed a maximum value, while the flux was initially almost constant and decreased monotonically. When the treatment time increased, the rejection increased first and then reached a constant value. The flux also approached a constant value after the reduction. However, the rejection approached a constant value in a shorter time than the flux, which means that the optimum time existed to obtain both high rejection and high flux. The membrane heat-treated in optimum conditions showed better salt rejection as well as very high chlorine resistance. References [1] J. E. Cadotte :U.S. Patent 4,277,344 (1981). [2] J. E. Cadotte: D. R. Lloyd, (Ed.), Materials Science of Synthetic Membrane, American Chemical Society, Washington, D.C., 1984, pp. 273–294.
377
[3] R. J. Petersen, J. Membr. Sci., 83 (1993) 81–150. [4] T. Kawaguchi and H. Tamura, J. Appl. Polym. Sci., 29 (1984) 3359–3367. [5] S. Wu, J. Xing, G. Zheng, H. Lian and L. Shen, J. Appl. Polym. Sci., 61 (1996) 1305–1314. [6] N. P. Soice, A. C. Maladono, D. Y. Takigawa, A. D. Norman, W. B. Krantz and A. R. Greenberg, J. Appl. Polym. Sci., 90 (2003) 1173–1184. [7] S. Konagaya and O. Watanabe, J. Appl. Polym. Sci., 76 (2000) 201–207. [8] S. Konagaya, H. Kuzumoto and O. Watanabe, J. Appl. Polym. Sci., 75 (2000) 1357–1364. [9] S. Konagaya and M. Tokai, J. Appl. Polym. Sci., 76 (2000) 913–920. [10] T. Shintani, H. Matsuyama and N. Kurata, Desalination, 207 (2007) 340–348. [11] T. Shintani, M. Maeda and T. Ohara, Jpn Kokai Tokkyo Koho JP2003-257933 (2003). [12] C. J. Gabelich, J. C. Frankin, F. W. Gerringer, K. P. Ishida and I. H. Saffet, J. Membr. Sci., 258 (2005) 64–70.