PPES hollow fiber composite nanofiltration membrane with high temperature resistance

Desalination 350 (2014) 95–101

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Preparation and performance of SPPES/PPES hollow fiber composite nanofiltration membrane with high temperature resistance Junnan Han a,b, Daling Yang b, Shouhai Zhang a, Lijiu Wang b, Xigao Jian a,⁎ a b

College of Chemical Engineering, Dalian University of Technology, Dalian 116024, PR China Institute of Building Materials, Dalian University of Technology, Dalian 116024, PR China

H I G H L I G H T S • A novel SPPES/PPES hollow fiber composite nanofiltration membrane was prepared. • The stability test of SPPES/PPES composite membranes had been evaluated for about 200 h. • The back-washing operation proved that the SPPES coating layer had good stability.

a r t i c l e

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Article history: Received 8 January 2014 Received in revised form 27 June 2014 Accepted 28 June 2014 Available online xxxx Keywords: Nanofiltration Hollow fiber composite membrane Sulfonated poly(phthalazinone ether sulfone) High temperature resistance

a b s t r a c t Sulfonated poly(phthalazinone ether sulfone)/poly(phthalazinone ether sulfone) (SPPES/PPES) hollow fiber composite nanofiltration membrane was prepared by coating a SPPES layer on top of the PPES hollow fibers via the dip-coating method. Effects of the composition of coating solution and preparation conditions as well as the operating conditions on the performance of the SPPES/PPES composite membrane (CM) were investigated. It was found that an optimum rejection ratio of CM1 for 1000 mg/L Na2SO4 solution was 93.6%, and the pure water flux was 8.7 L/(m2·h) under an operating pressure of 0.35 MPa at 20 °C. The flux of CM1 can be up to 58 L/(m2·h) with the operating temperature increased from 20 °C to 90 °C, while the rejection for 1000 mg/L Na2SO4 solution only had a slight change at the pressure of 1.0 MPa. Furthermore, a short time stability measurement for about 200 h proved that the SPPES/PPES composite membrane had much better performance at a high operating temperature. Finally, a back-washing method was used to clean the pollution substances after the stability measurement, and performance of the composite membrane almost reached the initial value with the hydraulic washing time of 20 min at 50 °C under the pressure of 0.3 MPa. © 2014 Published by Elsevier B.V.

1. Introduction Nanofiltration (NF) is a pressure-driven membrane separation technology, which exhibits a typical separation characteristic in the intermediate range between ultrafiltration (UF) and reverse osmosis (RO) [1]. Based on its unique pore size and charge character, NF membrane has a good separation of mono and bivalent ions as well as a high rejection for organic compounds with a molecular weight from 100 to 500 Da [2]. NF has been widely used for the real applications such as groundwater and seawater softening [3,4], wastewater reclamation [5,6] and dye and biochemical substance separation and purification [7,8]. Generally, a NF membrane consists of an active layer, determining the separation properties, and a support structure, giving mechanical strength. The active selective layer of the most successful commercial NF membranes is made from polyamide, which is usually prepared by ⁎ Corresponding author. Tel.: +86 411 83653426/84986107; fax: +86 411 83659223. E-mail address: [email protected] (X. Jian).

http://dx.doi.org/10.1016/j.desal.2014.06.029 0011-9164/© 2014 Published by Elsevier B.V.

the interfacial polymerization [9,10]. However, the main drawback of polyamide membrane is its susceptibility against free chlorine and alkaline which causes degradation of the amide group [11]. The second most successful commercial NF product is based on the sulfonated polymer materials, which could be modified simply by concentrated sulfuric acid without any toxic chemicals. Sulfonated aromatic polymers are reported as superior alternatives to standard reverse osmosis polyamide membranes due to their excellent chlorine resistance [12]. A series of materials such as sulfonated polysulfone [13], sulfonated polyethersulfone [14], sulfonated poly(phenylene oxide) [15], sulfonated poly(2,6dimethyl-1,4-phenylene oxide) [16] and sulfonated poly(ether ether ketone) [17,18] were also reported as the active layer for the composite NF membranes. Moreover, it has been shown that these poly(arylether) s materials may have a promising potential application for the high temperature-resistance membrane because of their favorable characteristics of perfect thermal stability[19]. The membranes usually show higher flux at higher temperatures, which allows the reduction of the operating pressure to save operating costs. The high-temperature

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resistant membranes can be used directly in the hot stream without using heat exchangers, thus the working efficiency will be greatly improved and energies will be saved. Poly(phthalazinone ether sulfone ketones) (PPESKs) with different ratios of sulfone and ketone (S/K) units, having high glass transition temperatures of 263–305 °C, possessed excellent comprehensive properties including an outstanding thermal stability[20], and it had been widely used as the high-temperature resistance membrane material. Y. Dai et al. [21] studied the separation of a variety of dyes of 600–900 Da by the PPESK membranes under different operating temperatures. It was shown that the permeation flux increased greatly by raising operation temperature without a significant change of rejection. Zhu et al. [22] investigated the thermal tolerance of PPESK hollow fiber membranes for permeation under different operating conditions, and experimental results showed that pure water flux increased several folds when the operating temperature increased from 20 to 80 °C under different operating pressures, whereas only a slight decrease in solute rejections was observed. Yang et al. [23] had evaluated the thermal stability of PPESK hollow fiber ultrafiltration membranes at different operating temperatures, and the result showed that when the temperature of the feed solution was raised from 15 to 100 °C, the permeation flux increased more than three times without a significant change of rejection. Sulfonated poly(phthalazinone ether sulfone ketone) (SPPESK) was also used to prepare thin film composite(TFC) membrane by coating sulfonated poly(phthalazinone ether sulfone ketone) (SPPESK) as a top layer onto poly(phthalazinone ether sulfone ketone) (PPESK) ultrafiltration (UF) flat support membranes [24]. A similar result was found that when the TFC membranes were operated at higher temperatures (80 °C), the rejection rates declined slightly and pure water flux was increased more than two-fold compared with the corresponding properties in the room temperature. Similar to PPESK in structure, poly(phthalazinone ether sulfone) (PPES) contains only diphenylsulfone moieties, resulting in a higher glass transition temperature of 305 °C. Due to lack of the ketone unit, PPES has a lower price and a much better solubility than PPESK, and the development of new applications can be achieved with low cost and high performance [25,26]. In the present work, a new type of hollow fiber nanofiltration membrane is reported based on coating a sulfonated poly(phthalazinone ether sulfone) (SPPES) layer on top of the poly(phthalazinone ether sulfone) hollow fibers via the dipcoating method. Since the hollow fiber configuration has an advantage of self-support and provides a much higher ratio of membrane area over module volume, it may have a potential advantage in the industries application. Additionally, the stability of SPPES/PPES composite NF membranes under high temperature was also investigated in this study.

O

O

O

N N

S O

PPES

(HO3S)X O

O N N

O S O

SPPES Fig. 1. The chemical structure of poly(phthalazinone ether sulfone) (PPES) sulfonated poly(phthalazinone ether sulfone) (SPPES).

casting solution; N-methyl-2-pyrrolidone (NMP)/tetrahydrofuran (THF) (8:2) was used as the mixed solvent; diethylene glycol (DegOH) with an approaching ratio of 0.85 combined with 1.5 wt.% LiCl was used as the additive. During the fabrication process, the takeup velocity was 1.5–2.0 ml/min and the internal coagulant water flow rate was in the range of 1.5–2.0 ml/min, and the air-gap of hollow fibers processing was 50 mm while the relative humidity of the working environment was about 85%. The support membranes were firstly pretreated with 30% glycerol–water solution and then packaged with epoxy resin as the “Cage Type”(as shown in Fig. 2), which has a dead end in the bottom and an open structure on the top. The packaged module was then immersed into the SPPES solution with ethylene glycol monomethyl ether (EGME) as the solvent, and a vacuum pump was used to provide a sucking force of 0.01 MPa in the inner of the packaged module. The SPPES solution was allowed to adsorb outside the fibers for about 30 s. Finally, the packaged module was drained from the bottom of the fibers by gravity. Immediately after the drainage, the coated fiber was dried at ambient temperature, and then cured at 20–80 °C for a certain time. Morphologies of the hollow fibers were studied with a scanning electron microscope (SEM; JSM-5600 L, JEOL, Japan). And the hollow fiber membrane samples were cryogenically fractured in liquid nitrogen before examination. As seen from Fig. 3 that there is a spongeous structure in the PPES support membrane. It also demonstrates that thin and

2. Experimental

4

2.1. Materials PPES and SPPES with a DS(Degree of Sulfonation) of 80% (as shown in Fig. 1) were provided by Dalian New Polymer Co. Ltd.(PR China); ethylene glycol monomethyl ether (EGME), N,Ndimethylacetamide(DMAc), N,N-dimethylform-amide (DMF), glycerol, sodium dodecyl sulfate, polysorbate-80, sodium chloride and sodium sulfate were obtained commercially as reagent grade chemicals and used as received. 2.2. Preparation of the SPPES/PPES nanofiltration composite membrane PPES hollow fiber support-membrane was prepared by the dry/wet phase inversion method, and a membrane molecule weight cut-off (MWCO) of 30,000 Da was optimized as the experiment membrane, while the water pure flux was about 636 L/(m2·h) in the room temperature under the operating pressure of 0.1 MPa. For preparation of the PPES support membrane, a PPES content of 17 wt.% was used in the

6

5 1 2

3

Fig. 2. Schematic diagram of the coating setup. (1) coating solution; (2) peristaltic pump; (3) vacuum pump; (4) vacuum gauge; (5) membrane module; (6) hollow fibers.

J. Han et al. / Desalination 350 (2014) 95–101

A

B

C

D

97

Fig. 3. Morphologies of the hollow fibers. A, cross-section of PPES support membrane; B, outer surface of PPES support membrane; C, cross-section of SPPES coating layer; D, surface of SPPES coating layer.

dense SPPES layer was successfully formed on the PPES support fiber (SPPES content of 5 wt.% used in the coating solution with EGME as solvent and no additive). And both of the PPES support layer and the SPPES active layer have a smooth surface morphology. 2.3. Properties of the SPPES/PPES nanofiltration composite membrane Nanofiltration experiments were carried out in a laboratory-scale test unit described as reported previously [10]. The SPPES/PPES composite hollow fiber membranes were characterized after pretreatment with pure water under 0.3 MPa pressure for 30 min. The pure water flux and the rejection of Na2SO4 and NaCl feed solutions with a concentration of 1000 mg/L were measured under the operating pressure of 0.35 MPa in room temperature. The permeation flux(F) is calculated according to F = V/At, where V is the total volume of the water or solution permeated during the experiment; A represents the membrane area; and t denotes the operation time. The membrane solute rejection (R) is calculated according to R = 1 − Cp/Cf, in which Cp and Cf are the permeate and feed concentration in the bulk, respectively. Furthermore, the measurement deviation of permeation flux (F) is about 3% and the measurement deviation of membrane solution rejection (R) is about 1% in this study. 2.4. Back-washing efficiency The fouled membranes were cleaned according to the following protocol: before fouling, the water permeation flux of the membrane was measured by passing distilled water through the membrane (initial water flux = Jwi). The fouled membrane was washed with distilled water for 10 min to remove unbound substances from the membrane

surface, and the water flux was measured after washing (Jww). Then the back-washing with distilled water for 20 min under the pressure of 0.3 MPa with different operating temperatures was took to remove bound substances from the membrane surface. The water flux was measured after back-washing (Jwc) and the flux recovery (FR) is defined as [27]: FR (%) = [(Jwc − Jww) / (Jwi − Jww)] ∗ 100. 3. Results and discussion 3.1. Effect of SPPES concentration in the coating solution The effect of SPPES concentration in the coating solution on the performance of SPPES/PPES composite NF membrane was shown in Fig. 4. The nascent composite membrane prepared with a different SPPES concentration in the coating solution was dried at ambient temperature, and then cured at 60 °C for 50 min. The coating qffiffiffiffi process may follow the Navier–Stokes equations [28,29]: h∞ ¼ 23 ην ρg , where h∞ is the final coating thickness; ν is the pulling speed during the coating process; η is the viscosity of the coating solution and g is the acceleration of gravity. It was shown that the final coating thickness is proportional to the viscosity of the coating solution. And as polymer concentration in the coating solution increased, the thickness of the coating layer increased, leading to a lower flux and higher rejection. It can be seen that the pure water flux decreased while the rejection for Na2SO4 increased with an increase of the SPPES concentration in the coating solution. As the SPPES concentration in the coating solution changed from 2 wt.% to 7 wt.%, the flux of pure water decreased from 45.5 L/(m2·h) to 10 L/ (m2·h) and the flux of 1000 mg/L Na2SO4 solution decreased from 40.8 L/(m2·h) to 8.7 L/(m2·h), while the rejection for 1000 mg/L Na2SO4 solution increased from 61.1% to 92.1% accordingly.

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90 80

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Flux of Na2SO4 solution 70 Rejection of Na2SO4

30

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20 50

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3

4

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6

7

40

Polymer concentration, wt%

Flux of pure water

50

Flux of Na2SO4 solution Rejection of Na2SO4

2

Flux, L/(m · h)

Flux of pure water

Rejection, %

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2

Flux, L/(m · h)

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Rejection, %

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Time, min Fig. 4. Effect of SPPES concentration on the performance of SPPES/PPES composite membrane.

Comprehensively, composite membranes prepared from the optimum SPPES solution concentration of 4–6 wt.% can have high Na2SO4 rejection and moderate flux by using the coating procedure, and a SPPES content of 5 wt.% in the coating solution was selected for the further study. 3.2. Effect of membrane curing treatment temperature and curing treatment time During the curing process of the nascent composite membrane, the active layer formed with the solvent evaporated in the coating solution. Increasing the curing treatment temperature and the curing treatment time can both affect the evaporation process of organic solvent. It can be seen from table 1 that rejection of the SPPES/PPES composite membrane for 1000 mg/L Na2SO4 solution increased from 66.4% to 87.6% with the curing treatment temperature rising from 20 °C to 60 °C when the curing treatment time was 50 min. When the curing treatment temperature was further raised to 80 °C, however, the rejection decreased to 83.7%. The rate of solvent evaporation in the nascent coating layer increases with the rise of curing treatment temperature, which may be benefit of enhancing the compactness of the SPPES coating layer [30], but the increased solvent evaporation rate might also damage the uniformity of the coating layer and results in the composite membrane having a high pure water flux and a low Na2SO4 solution flux. Correspondingly, the effect of the curing treatment time at 60 °C on the performance of the SPPES/PPES composite membrane was shown in Fig. 5. And with the curing time increased from 20 to 60 min, the rejection of the composite membrane generally increased from 47.2% to 87.3%. Moreover, the pure water flux of the SPPES/PPES composite membrane decreased from 35.0 L/(m2·h) to 27.4 L/(m2·h) with the curing temperature raised from 20 to 60 °C, while the flux decreased from 41.3 L/ (m2·h) to 15.8 L/(m2·h) with the curing time increased from 20 to 80 min. Taking both the salt rejection and permeation flux into account, curing at 60 °C for about 50–60 min is thought to be satisfactory. It also should be noticed that the curing treatment process might have an important effect on the performance of the PPES support

Fig. 5. Effect of curing treatment time on the performance of SPPES/PPES composite membrane.

membrane. For the PPES support membranes, a severe shrink may occur during the curing process even in moderate condition. It can be seen from Fig. 6 that the pure water flux of the PPES support membrane with non-pretreatment was almost reduced to zero when the membrane was dried in room temperature for about 24 h. As compared to the other different pretreatment conditions for the support membrane, pre-immersing in a 30% glycerol–water solution for about 24 h is reasonable.

3.3. Effect of glycerol and polar aprotic solvents It has been proved that multi-alcohol is an effective additive for the sulfonated coating material. In this work, glycerol was used as the additive and the effect of the concentration of glycerol on the performance of composite membranes was evaluated with the SPPES content kept constant as 5 wt.% in the coating solution. As shown in Table 2 that with the introduction of glycerol in the coating solution, the performance of the composite membranes was improved. And when the content of glycerol was 15 wt.% in the coating solution, the pure water flux of the SPPES/ PPES composite membrane was 18.4 L/(m2·h) and the rejection for 1000 mg/L Na2SO4 solution was 87.8%. There might be three effects of

Table 1 Effect of curing treatment temperatures on the performance of SPPES/PPES composite membrane. Temperature, °C

Flux of pure water, L/(m2·h)

Flux of Na2SO4 solution, L/(m2·h)

R Na2SO4 %

20 40 60 80

35.0 30.8 27.4 28.3

34.6 29.3 26.0 19.3

66.4 80.4 87.6 83.7

Fig. 6. Effect of different pretreatments on the properties of PPES support-membranes. 1,2,3,4 represent the without any pretreatment, with 30% glycerol pretreatment, with 30% polysorbate-80 pretreatment and with 30% sodium dodecyl sulfate pretreatment, respectively. The pretreatment time at 20 °C was 24 h and at 70 °C was 2 h, respectively.

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Table 2 Effect of glycerol content on the performance of the composite membrane. Glycerol concentration, %

Flux of pure water, L/(m2·h)

Flux of Na2SO4 solution, L/(m2·h)

Rejection of Na2SO4, %

5 10 15 20 25

20.8 19.8 18.4 22.8 28.3

17.2 16.0 15.1 21.2 26.0

81.6 83.0 87.8 84.3 82.9

glycerol introduced in the coating solution: (1) it increased the solubility of SPPES in the casting solution because of a higher hydrogen-bond component contribution in the solubility parameters; (2) increased the viscosity of the coating solution and a thicker coating layer could be obtained during the coating process; (3) improved the stability of the coating layer as a potential cross-link agent for the SPPES within the curing treatment and enhanced the properties of the selective layer [31]. However, when the content of glycerol in the coating solution was above 15 wt.%, the increased residual volume of water soluble glycerol might reduce the rejection of the composite membrane. Further increasing the glycerol concentration to 25 wt.%, it can be seen that the rejection for 1000 mg/L Na2SO4 solution was 82.9% and the flux of the pure water increased to 26.0 L/(m2·h). Additionally, aprotic solvents in terms of DMF and DMAc were also studied with a content of 15 wt.% glycerol used in the coating solution. DMAc and DMF both are good solvents for PPES and SPPES and there might be benefit of lowering the surface strain between the SPPES coating solution and the surface of the PPES support layer in the presence of DMAc or DMF, which was advantageous for preparing TFC membranes with high rejection and high flux [24]. As shown in Fig. 7 that with the concentration of DMAc increased in the coating solution, the flux of the composite membrane firstly decreased and then increased, and the rejection for 1000 mg/L Na2SO4 solution was reversed. When the DMAc content was 6 wt.% in the coating solution, an optimum rejection of the composite membrane (defined as CM1) was 93.6%, while the pure water flux was 8.7 L/(m2·h) under the pressure of 0.35 MPa in room temperature. The increased aprotic solvents in the coating solution also had a swell effect on the PPES support layer. Compared with DMAc present in the coating solution, DMF has stronger hydrogen bonding capabilities in the solubility parameters, which may provide a worse compatible solvency for PPES based on the solubility parameters theory [25,32]. It can be seen from Fig. 8 that with the concentration of DMF increased in the coating solution, rejection of the composite membrane firstly increased and then decreased but the flux of the SPPES/PPES composite membrane generally decreased. Similar to glycerol introduced in the coating

solution, the excess aprotic solvents also increased residual volume and resulted in a lower rejection of the composite membrane. When the content of DMF was 6 wt.% in the coating solution, the pure water flux of the SPPES/PPES composite membrane was 11.4 L/(m2·h) and the rejection for 1000 mg/L Na2SO4 solution was 87.4% under the pressure of 0.35 MPa.

3.4. Effect of operating conditions According to the solution-diffusion model, the rejection and flux of NF membrane both increased along with the growing transmembrane pressure; in the condition of a certain trans-membrane pressure, the rejection decreased with the feed concentration increased [33]. A dead-end filtration method was used in this study due to the restriction of equipment and the concentration of the feed solution was continuously increased in the permeability experimental process. It can be seen from Fig. 9 that the flux of the composite membrane generally increased with the operation pressure while the rejection decreased because of an increased salt concentration resulted in a much higher osmosis pressure (a stable pretreatment process for about 30 min is necessary before testing for the composite membranes at each raised pressure). And when the operation pressure was 1.0 MPa, the flux of CM1 was 22.4 L/(m2·h) and the rejection for 1000 mg/L Na2SO4 solution was 93.2%. The effect of operating temperature on the performance of the composite membrane was shown in Fig. 10. For the water solution, the viscosity coefficients decreased while the diffusion coefficients increased with the increasing temperature [34,35]. And with the operation temperature changed from 20 to 90 °C, the flux of CM1 increased from 22.4 L/(m2·h) to 58 L/(m2 h) under the operation pressure of 1.0 MPa, while the rejection for 1000 mg/L Na2SO4 solution only has a slight decreased and the rejections were above 92.0%. It was found that the SPPES/PPES composite membrane had a good high-temperature resistance.

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Flux of pure water Flux of Na2SO4 solution Rejection of Na2SO4 solution

9

2

Flux of pure water

8

7

60

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Flux of Na2SO4 solution Rejection of Na2SO4 solution 2

4

6

8

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The concentration of DMAc in the coating solution, %

Fig. 7. Effect of DMAc content on the performance of the composite membrane.

0

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The concentation of DMF in the coating solution, % Fig. 8. Effect of DMF content on the performance of the composite membrane.

Rejection, %

9

Flux, L/(m · h)

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Rejection, %

2

Flux, L/(m · h)

80 10

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Flux (CM1) Rejection (CM1)

0.4

0.6

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1.0

Operating pressure, MPa

1.2

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0 0.2

Rejection of Na2SO4 (5000 mg/L) Flux of NaCl (5000 mg/L) Rejection of NaCl (5000 mg/L)

15

2

Flux, L/(m .h)

20

Rejection, %

2

Flux, L/(m · h)

80

80

Flux of Na2SO4 (5000 mg/L)

30

Rejection, %

100

0

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0 300

Operating time, h

50

1.4

150

Fig. 11. The stability of SPPES/PPES NF membrane with continuous operation test under 20 °C.

Fig. 9. Effect of operating pressure on the performance of the composite membrane of CM1.

3.5. Thermal stability of the SPPES/PPES HF composite membrane The optimized SPPES/PPES composite membrane of CM1 was selected for the short time stability measurement under the pressure of 1.0 MPa and an improved feed storage with high-capacity was used to reduce the affection of the increased feed solution during the deadend filtration process. It was found that when the salt feed solution was 1000 mg/L, CM1 composite membrane showed a good stability during 200 h at 20 °C with the operation pressure of 1.0 MPa. However, when the salt feed solution was 5000 mg/L, the performance of the composite membrane had a significant reduction. It can be seen from Fig. 11 that the initial salt solution fluxes of the SPPES/PPES composite membrane for the NaCl and Na2SO4 solutions were 7.8 L/(m2·h) and 8.3 L/(m2·h), while the according rejections were 38.3% and 91.2%, respectively. After 200 h operating measurement, the fluxes for NaCl and Na2SO4 solutions were 2.1 L/(m2·h) and 5.4 L/(m2·h), while the according rejections were 31.8% and 84.6%, respectively. Further increasing the operating temperature to 60 °C (a much higher temperature for about 200 h is hard to maintain the stability during the measurement process because of heat dissipation of the laboratory-scale test unit), the efficiency of SPPES/PPES membrane 80

was improved. It can be seen from Fig. 12 that after 200 h testing operation, the fluxes for 5000 mg/L NaCl and Na2SO4 solutions were 12.3 L/(m2·h) and 13.4 L/(m2·h), while the according rejections were 33.7% and 87.3%, respectively, and the result proved that the SPPES/ PPES composite membrane had a good high temperature resistance performance. 3.6. Back-washing of the SPPES/PPES composite membrane The membrane pollution caused by the concentration polarization during the membrane operating period is inevitable. For cleaning of the membrane pollution after the short time stability measurement, a back-washing operation of hydraulic washing was used to recover the properties of the SPPES/PPES composite membrane. As shown in Fig. 13 that hydraulic washing had a good recovery rate with the hydraulic washing time of 20 min at 50 °C under the pressure of 0.3 MPa, the performance of the composite membrane almost reached the initial value. It also certified that the SPPES coating layer had good stability. 4. Conclusions The SPPES/PPES HF composite membrane was prepared by coating a sulfonated poly(phthalazinone ether sulfone) layer on top of the

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Fig. 10. Effect of operating temperature on the performance of the composite membrane of CM1.

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Flux, L/(m .h)

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Operating time, h Fig. 12. The stability of SPPES/PPES NF membrane with continuous operation test under 60 °C.

J. Han et al. / Desalination 350 (2014) 95–101

The flux recovery rate of NF membrane, %

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Fig. 13. The flux recovery rate of NF membrane by hydraulic washing with varied temperature.

poly(phthalazinone ether sulfone) hollow fibers via the dip-coating method. It was shown that the SPPES concentration in the coating solution was the dominant factor for the rejection and permeation flux, and the composite membranes prepared from the optimum SPPES solution content of 4–6 wt.% could have high rejection for Na2SO4 solution and moderate flux. The curing treatment process not only affected the properties of the SPPES coating layer, but also played an important role in the performance of the PPES support membrane. For the PPES support membranes, a severe shrink may occur during the curing process even within moderate conditions, and the PPES support membrane pretreated with a 30 wt.% glycerol–water solution for about 24 h was reasonable. The presence of additives in the coating solution was beneficial for improving the properties of the SPPES/PPES composite membrane but the increased aprotic solvent concentration in the coating solution might also have a swell effect on the PPES support layer. Additionally, the short time stability measurement proved that the SPPES/ PPES composite membrane had good high temperature resistance. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 20776025, No. 21276037) and the National ScienceTechnology Support Plan Projects (No. 2013BAC05B03). References [1] L.P. Raman, M. Cheryan, N. Rajagopalan, Consider nanofiltration for membrane separations, Chem. Eng. Prog. 90 (1994) 68–74. [2] A.I. Schafer, A.G. Fane, T.D. Waite, Nanofiltration: Principles and Applications, Elsevier, 2005. [3] C.M. Galanakis, G. Fountoulis, V. Gekas, Nanofiltration of brackish groundwater by using a polypiperazine membrane, Desalination 286 (2012) 277–284. [4] D. Hu, Z. Xu, Y. Wei, Y. Liu, Poly(styrene sulfonic acid) sodium modified nanofiltration membranes with improved permeability for the softening of highly concentrated seawater, Desalination 336 (2014) 179–186. [5] A. Bes-Piá, B. Cuartas-Uribe, J.A. Mendoza-Roca, et al., Pickling wastewater reclamation by means of nanofiltration, Desalination 221 (2008) 225–233. [6] L. Shu, T.D. Waite, P.J. Bliss, A. Fane, V. Jegatheesan, Nanofiltration for the possible reuse of water and recovery of sodium chloride salt from textile effluent, Desalination 172 (2005) 235–243. [7] I. Koyuncu, Reactive dye removal in dye/salt mixtures by nanofiltration membranes containing vinylsulphone dyes: effects of feed concentration and cross flow velocity, Desalination 143 (2002) 243–253.

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