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Synthesis and characterization of thin film composite reverse osmosis membranes via novel interfacial polymerization approach
Separation and Purification Technology 72 (2010) 256–262
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Synthesis and characterization of thin film composite reverse osmosis membranes via novel interfacial polymerization approach Hao Zou a,b , Yan Jin b,∗ , Jun Yang c , Hongjun Dai a , Xiaolan Yu a , Jian Xu a,∗ a b c
National Laboratory of Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (ICCAS), Beijing 100190, PR China Vontron Technology Co., Ltd., Beijing 102249, PR China Zhuzhou Times New Material Technology Co., Ltd., Zhuzhou 412007, PR China
a r t i c l e
i n f o
Article history: Received 24 November 2009 Received in revised form 25 January 2010 Accepted 28 January 2010 Keywords: Thin film composite RO membranes Polyamide Interfacial polymerization approach Reverse osmosis membrane
a b s t r a c t A novel interfacial polymerization approach was used in this study and applied to prepare the thin film composite reverse osmosis (RO) membranes with some excellent properties. The surface of the membranes was characterized by scanning electronic microscopy (SEM), attenuated total reflectance infrared (ATR-IR) and X-ray photoelectronic spectroscopy (XPS). The results indicated that the membranes prepared by novel interfacial polymerization approach have smoother surface than the membranes obtained by traditional interfacial polymerization approach. There is a large amount of amino group (–NH2 ) on the topmost surface of the active skin layer of the membrane yielded by novel interfacial polymerization approach, comparing to carboxylic acid groups (–COOH) synthesized by traditional interfacial polymerization approach. Moreover, the membrane made by novel interfacial polymerization approach exhibited better antifouling property than that prepared by traditional interfacial polymerization approach. A unique and simple method to prepare thin film composite RO membranes with good membrane performance property was demonstrated. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Reverse osmosis (RO) membrane has become one of the most efficient approaches for water purification, especially in successful applications for desalination of sea and brackish water, waste treatment and various separations in chemical, food, pharmaceutical and other industries [1–4]. The concepts of ‘osmosis’ and ‘reverse osmosis’ have been known as early as 1750s. However, the use of reverse osmosis (RO) as a feasible separation process is a relatively young technology since Loeb [5] developed a method for making asymmetric cellulose acetate membranes with relatively high water flux and separation. Moreover, the invention of thin film composite RO membrane was a milestone in the development of reverse osmosis membrane [6,7]. The active skin layer of polyamide plays the key role in thin film composite RO membranes, which controls mainly the separation property of the membrane, while the support layer gives the membrane necessary mechanical property [8–10]. Traditionally, the active skin layers synthesized via interfacial polymerization have varying physicochemical property and mechanical strength depending on the types of monomers. Whereas more recently, the production of thin film composite RO membrane by in situ
∗ Corresponding authors. Tel.: +86 01082619667; fax: +86 01082619667. E-mail addresses: [email protected] (Y. Jin), [email protected] (J. Xu).
polycondensation of polyfunction amides [11–14] such as aliphatic or aromatic diamine, poly(aminostyrene), poly(m-aminostyreneco-vinyl alcohol), m-phenylenediamine-5-sulfonic acid and the polyfunction acid chlorides [12,15–17] as trimesoyl chloride (TMC), isophthaloyl chloride, terephthaloyl chloride, monomer-5isocyanato-isophthaloyl chloride and 3,3 ,5,5 -biphenyl tetraacyl chloride had been investigated. Among these membranes, the thin film composite of RO membrane, which is produced by the interfacial polymerization of m-phenylenediamine (MPD) and trimesoyl chloride (TMC), has become the most successful commercial product [2,8]. However, the use of reverse osmosis membrane in many applications is limited by membrane fouling [18–24]. In order to enhance membrane performance, surface modification methods ranged from simple physical adsorption [18–20] to chemical bond formation [21–24] have been used. These modifications result in thin film composite RO membranes with enhanced water flux but simultaneously accompanying a considerable loss of salt rejection or enhanced salt rejection but simultaneously accompanying a considerable loss of water flux. Moreover, some methods showed the promise of the increase in antifouling property, but most of them needed special instruments or modification conditions which increased the preparation cost and restricted their application. In order to form the active skin layer of thin film composite RO membrane, both of polyfunctional amine dissolving in aqueous phase and polyfunctional acid chloride dissolving in organic phase partition across the liquid–liquid interface and react to form
1383-5866/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2010.01.019
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the thin polyamide film. Because the growing thin polyamide film behaves as a barrier for the diffusion of the two monomers, there would be fewer and fewer amine monomers on the organic phase side to react with the polyfunctional acid chloride monomers. Thus, there must exist the excessive unreacted polyfunctional acid chloride groups or polyfunctional acid chloride monomers on the surface of the active skin layer. Since the membrane performance is deeply related to the membrane surface properties [18–24], it is probably to improve the membrane performance by using monomers with amino groups to react with these unreacted acyl chloride groups and/or acyl chloride monomers (ACG–TMC) before their hydrolysis. Cao and co-workers [25] used aminopolyethylene glycol monomethylether with an amino group as grafting monomer to react with the unreacted acyl chloride groups and obtained interesting results. However, the aminopolyethylene glycol monomethylether with one amino group is a macromolecular monomer having comparative lower activity. Thus the modified membrane surface was not completely covered by aminopolyethylene glycol monomethylether and had larger roughness which was disadvantageous to the membrane antifouling ability [25]. Since ordinary polyfunctional amine monomers with double amino groups have high activity reacting with acyl chloride group, what is about using the ordinary polyfunctional amine monomers to modify the membrane surface? When the residual unreacted acyl chloride groups have reacted with the ordinary polyfunctional amine monomers, it is possible to obtain a membrane with comparatively smooth surface and have more hydrophilic groups of amino groups of –NH2 instead of carboxylic acid groups on the membrane surface. Smooth surface and the existence of hydrophilic groups of –NH2 were benefited to the antifouling ability [26]. However, to the best of our knowledge, there have been few reports on modifying the surface of thin film composite RO membrane by using ordinary polyfunctional amine monomers with double amino groups to react with the residual ACG–TMC on the membrane surface. In this paper, we present the results of thin film composite RO membranes by means of a novel interfacial polymerization approach in which ordinary polyfunctional amine monomers with double amino groups of MPD reacted with the unreacted ACG–TMC existing on the surface of the active skin layer before their hydrolysis. The microstructure and properties of the resultant thin film composite RO membranes were characterized by SEM, XPS and ATR-IR. Moreover, the antifouling ability of membrane was studied by using dodecyltrimethylammonium bromide and humic acids. Using novel interfacial polymerization approach different from those in previous reports, we prepared polyamide thin film composite RO membranes with unique morphology features and good membrane performance as described below. 2. Experimental 2.1. Materials Solvents and reagents in analytical grade were purchased from commercial sources. Polysulfone supported membranes were provided by Vontron Technology Co., Ltd., Bei Jing of China. 1,3,5-Benzenetricarbonyl trichloride (TMC) (>98%) and m-phenylenediamine (MPD) (>99%) were purchased from Sigma–Aldrich. 2.2. Membrane formation The membranes were prepared based on the traditional interfacial polymerization approach described elsewhere [6,7]. Firstly, polysulfone supported membranes taped to stainless steel plates were placed in an aqueous solution, with 3.0 wt.% MPD in the
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mixing aqueous solutions of triethylamine (TEA) (approximately 2.0 wt.%) and camphor sulfonic acid (approximately 4.0 wt.%) and different contents of dimethyl sulfoxide (DMSO), for 20 s. Subsequently, MPD soaked supported membranes were placed on a rubber sheet and rolled with a rubber roller to remove the excess solution. Then the MPD saturated polysulfone supported membranes were immersed in a solution of 0.15 wt.% TMC in hexane about 10 s. After that, the membranes were placed in an aqueous solution of 3.0 wt.% MPD in the mixing aqueous solutions of triethylamine (TEA) (approximately 2.0 wt.%) and camphor sulfonic acid (approximately 4.0 wt.%) solution for 0–60 s. After removing the excess aqueous solution, the resulting composite membranes were heated to 80 ◦ C in an air dryer about 5 min for further polymerization, leading to the formation of the active skin layer. Finally, the obtained membranes were washed in deionized (DI) water and stored in it. 2.3. Performance test All tests for thin film composite RO membranes performance were conducted at 225 psi using a 2000 ppm NaCl solution at room temperature by using a cross-flow type apparatus. Both permeate and retentate were recycled back to the feed tank during the tests. Circular membrane samples were placed in the test apparatus with the active skin layer facing the feed water. The effective membrane area was around 19 cm2 . All membrane samples were prepared and tested at least twice with a total of 3 membranes tests for RO performance, results of which have been averaged. The permeating volume collected for 1 h was used to describe flux in terms of gallons per square feet per day (gfd). A standardized conductivity meter was used to measure the salt (NaCl) concentrations in the feed and product water for determining membrane selectivity as given below: salt rejection (%) =
1−
Cp Cf
× 100
in which Cf and Cp are feed and permeating concentration, respectively. 2.4. Characterization of membranes The membranes used for the chemical structure and morphology analysis of the active skin layer were rinsed with DI water for several times. Then, the membranes were dried under vacuum at 80 ◦ C for 6 h. Attenuated total reflectance infrared (ATR-IR) characterization of the thin film composite RO membrane surface was made with a Nicolet avator 230 spectrometer at room temperature. For ATRIR analysis of membrane samples, Irtran crystal at 45◦ angle of incidence was employed. Surface chemical characterization was carried out by X-ray photoelectron spectra (XPS) of ESCALab220IXL with Al/K␣ (h = 1486.6 eV) anode mono X-ray source. Surface microstructures were observed by a field emission scanning electron microscope with SEM of Hitachi S-4300. Magnifications up to 20,000 were obtained at 5 kV. 2.5. Fouling experiments Membrane fouling experiments were performed with a crossflow type apparatus. Each membrane was compacted with deionized water until the permeate flux became almost constant. Subsequently the dodecyltrimethylammonium bromide (DTAB) or humic acids as surfactant were added to the reservoir to 50 ppm and the permeating experiment was performed for a due course time. Then the membrane was rinsed with deionized water to investigate
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Fig. 1. The schematic preparation procedure for thin film composite RO membrane using novel interfacial polymerization approach.
the flux recovery property. Both permeate and retentate were recycled back to the feed tank during the tests. The test was conducted at 225 psi and 25 ◦ C. Evaluation of the fouling extent of membranes was based on the value of relative flux, which was calculated by comparing the water flux before and after fouling or cleaning.
with double amino groups such as MPD for a few seconds to construct the second aqueous phase. It was supposed that the ordinary polyfunctional amines will react with the unreacted ACG–TMC to form the second polyamide skin layer on the top surface of previous TFC membrane.
3. Results and discussion
3.2. Permeation properties of the thin film composite RO membranes
3.1. Feasibility analysis of novel interfacial polymerization approach As previously reported [2], the surface of thin film composite RO membrane contained carboxylic acid groups coming from the hydrolysis of unreacted ACG–TMC. Meanwhile, since the hydrolysis reaction of acyl chloride is a relatively slow step, the nascent membrane would contain numerous unreacted ACG–TMC on the surface of the active skin layer. As a result, it is possible to modify the nascent thin film composite RO membrane surface with these unreacted ACG–TMC by chemical coupling with simple ordinary polyfunctional amines. The schematic illustration of the novel interfacial polymerization approach process, based on the reaction of simple ordinary polyfunctional amine with unreacted ACG–TMC existing on the surface of the active skin layer of thin film composite RO membrane, is demonstrated in Fig. 1 and detailed as follows: firstly, the polysulfone supported membrane was placed in an aqueous solution of MPD for some time to construct an aqueous phase on the surface, and the MPD soaked membrane was then placed on a rubber sheet and rolled with a rubber roller to remove excess solution. Secondly, the MPD saturated polysulfone supported membrane was then immersed in a solution of TMC in hexane to build the organic phase. The MPD dissolved in aqueous phase and TMC dissolved in organic phase contacted and reacted to form the active skin layer of thin film composite RO membrane. Since the growing thin polyamide film behaves as a barrier for diffusion of the MPD to the organic phase, there still existed lots of unreacted ACG–TMC on the surface of the active skin layer when the reaction stopped. Finally, after the excess hexane solution was removed, the membrane was then immersed in an aqueous solution of ordinary polyfunctional amines
Using novel interfacial polymerization approach with MPD as the monomer dissolving in the second aqueous phase, a series of thin film composite RO membranes were synthesized. The membrane performance was tested by cross-flow type apparatus and the results were listed in Table 1. Keeping the contact time with the first aqueous phase for 20 s and with the organic phase for 10 s, the contact time with the second aqueous phase was varied from 0 to 60 s. As shown in Table 1, the contact time with the second aqueous phase influenced the membrane performance deeply. The membranes produced by novel interfacial polymerization approach have a lower flux than tradition membrane. A possible reason is that the membranes synthesized by novel interfacial polymerization approach have thicker polyamide films than tradition membrane which increased the resistance for water to come through the membrane. With the contact time increasing from 0 to 20 s, the water flux decreased while the salt rejection increased greatly (Run Nos. 1–3 and Run Nos. 5–8 in Table 1). According to analysis above, there must existed lots of unreacted acyl chloride groups and TMC monomers on the surface of thin film skin layer after the formation of the first thin active skin layer of polyamide. When the polysulfone supported membrane with nascent thin film skin layer dipped into the third phase, the MPD dissolving in the second aqueous phase reacted with the residual ACG–TMC to form the second thin active skin layer of polyamide. It is reasonable to believe that the thickness increase of the thin active skin layer on the membrane would decrease the water flux and increase the salt rejection at the same time. However, as the contact time with the second aqueous phase further increased from 20 to 60 s, the membranes have higher water flux. It may because that the longer immersion time with the second aqueous phase is apt to produce a membrane
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Table 1 Formation and transport characteristics of thin film composite RO membranes prepared by novel interfacial polymerization approacha . Run No.
1 2 3 4 5 6 7 8 9 10 11
The first aqueous phase
The organic phase
The second aqueous phase
RO performanceb
DMSO (wt.%)
Contact time (s)
Contact time (s)
Contact time (s)
Flux (gfd)
Salt rejection (%)
0 0 0 0 1 1 1 1 1 0.5 0.25
20 20 20 20 20 20 20 20 20 20 20
10 10 10 10 10 10 10 10 10 10 10
0 10 20 60 0 10 20 30 60 20 20
20.50 15.60 15.30 19.10 30.62 25.18 29.26 32.67 33.35 24.97 19.43
98.50 99.08 99.34 99.12 97.65 98.87 99.04 98.78 98.80 98.76 98.74
a Preparation conditions: MPD = 3.0 wt.% in the first phase and the third phase; TMC = 0.15 wt.% in the second phase; curing temperature = 80 ◦ C; curing time = 10 min; aqueous pH = 8.3. b Test conditions: feed: 2000 ppm of NaCl aqueous solution, pressure: 225 psi, temperature: 25 ◦ C and pH 7.0.
Fig. 2. SEM images of thin film composite RO membranes from run 1 (a), run 2 (b), run 3 (c), run 4 (d), run 5 (e) and run 7 (f) with the contact time to the second aqueous phase with 0 s (a), 10 s (b), 20 s (c), 60 s (d), 0 s (e) and 20 s (f) in Table 1.
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surface with larger area which was benefit to improve the water flux. The dimethyl sulfoxide (DMSO) was used as additive into the first aqueous phase and the results were listed in Table 1 (Run Nos. 5–12 in Table 1), since the addition of DMSO in the aqueous amine solution can increase the water flux of thin film composite RO membrane [27]. As shown in Table 1, thin film composite RO membrane with DMSO as additive have much better water flux than thin film composite RO membrane with no DMSO as additive. Interestingly, as the content of DMSO decreased, the increased rate of water flux decreased sharply (Run Nos. 8, 11 and 12 in Table 1). Moreover, the thin film composite RO membrane with DMSO as additive had the similar rules as the thin film composite RO membrane with no DMSO as additive about the relationship between the contact time and the membrane performance. 3.3. Membrane surface morphologies SEM was used to characterize the surface of the active skin layer of thin film composite RO membrane as shown in Fig. 2. The membrane produced by traditional interfacial polymerization approach with no DMSO has a rough surface with lots of “leaf-like” folds on it (a in Fig. 2), typical for polyamide composite membranes prepared by traditional interfacial polymerization of aromatic acid chloride and aromatic amides [27]. The addition of DMSO in the first aqueous phase changed the quantity and shape of “leaf-like” folds and obtained more and bigger “leaf-like” folds on the membrane surface (e in Fig. 2). Obviously, the novel interfacial polymerization offered membrane with littler “leaf-like” folds, when compared with the typical thin film composite RO membranes. Moreover, the contact time with the second aqueous phase affected the surface morphology of membrane greatly. With the contact time increasing from 0 to 20 s, the “leaf-like” folds on the membrane surface decreased. Surprisingly, when the contact time further increased from 20 to 60 s, the “leaf-like” folds on the membrane surface increased also (c and d in Fig. 2). The longer immersion time with the second aqueous phase is apt to produce more “leaf-like” folds with larger surface area. As a result, the larger surface area is favorable to improve the water flux, which was in agreement with the results as shown in Table 1. 3.4. Characterization of the active layer In order to grasp the chemical structure of the active skin layer of thin film composite RO membranes, ATR-IR and XPS measurements were carried out. As seen, the major characteristic of aromatic polyamide at 1660 cm−1 (amide I, C O stretching), 1554 cm−1 (amide II, N–H bending) and 1608 cm−1 (aromatic ring breathing) is present in the ATR-IR spectrum of Fig. 3. According to the illustration of Fig. 1, the surface of the active skin layer of thin film composite RO membranes prepared by the novel interfacial polymerization approach was supposed to present excessive amino groups. However, the peak of amino groups is hardly obviously observed in the spectra of the membranes, which are possibly overwhelmed by other peaks. XPS is a highly sensitive technique for surface analysis, with the ability to measure elemental composition and chemical binding information for the 1–9 nm depth of the surface. Therefore, the membranes were analyzed for the chemical composition of the active skin layer through XPS and the results were shown in Table 2 and Fig. 4. It is necessary to point out that the elements detected by XPS were not only oxygen (O), nitrogen (N), and carbon (C), but also chlorine (Cl) in the active skin layer when the thin film composite RO membranes were produced by novel interfacial polymerization approach. The existence of chlorine (Cl) in the active skin layer might be due to the hindered hydrolysis of the unreacted
Fig. 3. ATR-FTIR spectra of thin film composite RO membranes from run 5 (a), run 6 (b), run 7 (c) and run 9 (d) with the contact time to the second aqueous phase with 0 s (a), 10 s (b), 20 s (c) and 60 s (d) in Table 1.
Fig. 4. XPS spectra of thin film composite RO membranes from run 5 (a), run 7 (b) and run 9 (c) with the contact time to the second aqueous phase with 0 s (a), 20 s (b) and 60 s (c) in Table 1.
acyl chloride groups and TMC monomers in the second compact polyamide layer. Obviously, the thin film composite RO membranes produced by novel interfacial polymerization approach have the higher contents of nitrogen (N) and the lower ratio of oxygen (O) and nitrogen (N) when comparing to the traditional membranes. With the increase of contact time with the third phase, the con-
Table 2 Surface composition by XPS analysis of various RO membranes.
Run No.b 5 6 7 8 9 Theoretical valuesc Fully cross-linked Fully linear a b c
Contact time (s)a
O%
N%
C%
Cl%
O/N
0 10 20 30 60
16.9 17.7 15.5 12.5 13.7
9.6 9.9 11.0 12.9 10.4
73.5 71.4 73.4 74.3 75.7
0 1.0 0.1 0.3 0.2
1.8 1.8 1.4 0.9 1.3
0 0
12.5 19.1
12.5 9.5
75.0 71.4
0 0
1.0 2.0
Contact time with the second aqueous phase. The Run No. in Table 2 is the same Run No. in Table 1. The numerical value comes from Ref. [8].
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interfacial polymerization approach showed a relatively better antifouling property. The phenomenon could be interpreted as follows: firstly, several literatures have stated that membrane fouling is directly related to membrane surface roughness [20,28]. The smoother surface of membrane prepared by the novel interfacial polymerization approach would contribute to the membrane antifouling performance. Secondly, some researchers indicated that the membrane charge also affected the membrane fouling performance [26]. The presence of amino groups of –NH2 in the active skin layer of the membrane prepared by the novel interfacial polymerization approach makes the membrane surface more close to neutral charge than traditional membrane and probably benefits to membrane antifouling performance. 4. Conclusions
Fig. 5. Fouling experiment with DTAB as pollutants for thin film composite RO membranes prepared by traditional interfacial polymerization approach (a) from run 5 in Table 1 and novel interfacial polymerization approach (b) from run 8 with the contact time to the second aqueous phase with 0 s (a) and 30 s (b) in Table 1.
A series of thin film composite RO membranes were successfully synthesized by using a novel interfacial polymerization approach here. The membrane performance of water flux and salt rejection varied greatly with the contact time with the second aqueous phase. The results of ATR-IR and XPS revealed that the active skin layer of thin film composite RO membrane is composed of aromatic polyamide with the functional bonds of amide (–CONH–) and the functional groups such as amino group (–NH2 ), chlorine (Cl). The SEM images indicated that the novel interfacial polymerization approach offered smoother surface membrane. Fouling experiments results indicated the membrane prepared by the novel interfacial polymerization approach showed a relatively better antifouling property than traditional membrane. Using the novel interfacial polymerization approach with different polyfunctional amine monomers with double amino groups in the third phase, it is possible to prepare multifunctional thin film composite RO membranes. Acknowledgments
Fig. 6. Fouling experiment with Humic acid as pollutants for thin film composite RO membranes prepared by traditional interfacial polymerization approach (a) from run 5 in Table 1 and novel interfacial polymerization approach (b) from run 8 with the contact time to the second aqueous phase with 0 s (a) and 30 s (b) in Table 1.
tents of nitrogen (N) increased while the ratios of oxygen (O) and nitrogen (N) decreased. When the contact time with the third phase is 30 s, surprisingly, the content of nitrogen (N) is much higher than the theoretical value (>12.5%) and the ratio of oxygen (O) and nitrogen (N) is much lower than the theoretical value (<1.0) [8]. Since the residual unreacted acyl chloride groups have reacted with MPD and there existed excessive unreacted amino groups of –NH2 on the surface (as shown in Fig. 1) which led to the decrease of the contents of oxygen (O) while the contents of nitrogen (N) increased at the same time, it is reasonable to believe that the thin film composite RO membranes produced by the novel interfacial polymerization approach will have higher contents of nitrogen (N) and lower ratios of oxygen (O) and nitrogen (N) than the traditional membranes as shown in Table 2 and Fig. 4. 3.5. Fouling experiments A 50 ppm dodecyltrimethylammonium bromide (DTAB, a cationic surfactant) which was known to have high fouling potential [25] and 50 ppm humic acids were used for fouling experiments. The preliminary results are summarized in Figs. 5 and 6. As seen from the data provided, the membrane prepared by the novel
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Synthesis and characterization of thin film composite reverse osmosis membranes via novel interfacial polymerization approach Hao Zou a,b , Yan Jin b,∗ , Jun Yang c , Hongjun Dai a , Xiaolan Yu a , Jian Xu a,∗ a b c
National Laboratory of Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (ICCAS), Beijing 100190, PR China Vontron Technology Co., Ltd., Beijing 102249, PR China Zhuzhou Times New Material Technology Co., Ltd., Zhuzhou 412007, PR China
a r t i c l e
i n f o
Article history: Received 24 November 2009 Received in revised form 25 January 2010 Accepted 28 January 2010 Keywords: Thin film composite RO membranes Polyamide Interfacial polymerization approach Reverse osmosis membrane
a b s t r a c t A novel interfacial polymerization approach was used in this study and applied to prepare the thin film composite reverse osmosis (RO) membranes with some excellent properties. The surface of the membranes was characterized by scanning electronic microscopy (SEM), attenuated total reflectance infrared (ATR-IR) and X-ray photoelectronic spectroscopy (XPS). The results indicated that the membranes prepared by novel interfacial polymerization approach have smoother surface than the membranes obtained by traditional interfacial polymerization approach. There is a large amount of amino group (–NH2 ) on the topmost surface of the active skin layer of the membrane yielded by novel interfacial polymerization approach, comparing to carboxylic acid groups (–COOH) synthesized by traditional interfacial polymerization approach. Moreover, the membrane made by novel interfacial polymerization approach exhibited better antifouling property than that prepared by traditional interfacial polymerization approach. A unique and simple method to prepare thin film composite RO membranes with good membrane performance property was demonstrated. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Reverse osmosis (RO) membrane has become one of the most efficient approaches for water purification, especially in successful applications for desalination of sea and brackish water, waste treatment and various separations in chemical, food, pharmaceutical and other industries [1–4]. The concepts of ‘osmosis’ and ‘reverse osmosis’ have been known as early as 1750s. However, the use of reverse osmosis (RO) as a feasible separation process is a relatively young technology since Loeb [5] developed a method for making asymmetric cellulose acetate membranes with relatively high water flux and separation. Moreover, the invention of thin film composite RO membrane was a milestone in the development of reverse osmosis membrane [6,7]. The active skin layer of polyamide plays the key role in thin film composite RO membranes, which controls mainly the separation property of the membrane, while the support layer gives the membrane necessary mechanical property [8–10]. Traditionally, the active skin layers synthesized via interfacial polymerization have varying physicochemical property and mechanical strength depending on the types of monomers. Whereas more recently, the production of thin film composite RO membrane by in situ
∗ Corresponding authors. Tel.: +86 01082619667; fax: +86 01082619667. E-mail addresses: [email protected] (Y. Jin), [email protected] (J. Xu).
polycondensation of polyfunction amides [11–14] such as aliphatic or aromatic diamine, poly(aminostyrene), poly(m-aminostyreneco-vinyl alcohol), m-phenylenediamine-5-sulfonic acid and the polyfunction acid chlorides [12,15–17] as trimesoyl chloride (TMC), isophthaloyl chloride, terephthaloyl chloride, monomer-5isocyanato-isophthaloyl chloride and 3,3 ,5,5 -biphenyl tetraacyl chloride had been investigated. Among these membranes, the thin film composite of RO membrane, which is produced by the interfacial polymerization of m-phenylenediamine (MPD) and trimesoyl chloride (TMC), has become the most successful commercial product [2,8]. However, the use of reverse osmosis membrane in many applications is limited by membrane fouling [18–24]. In order to enhance membrane performance, surface modification methods ranged from simple physical adsorption [18–20] to chemical bond formation [21–24] have been used. These modifications result in thin film composite RO membranes with enhanced water flux but simultaneously accompanying a considerable loss of salt rejection or enhanced salt rejection but simultaneously accompanying a considerable loss of water flux. Moreover, some methods showed the promise of the increase in antifouling property, but most of them needed special instruments or modification conditions which increased the preparation cost and restricted their application. In order to form the active skin layer of thin film composite RO membrane, both of polyfunctional amine dissolving in aqueous phase and polyfunctional acid chloride dissolving in organic phase partition across the liquid–liquid interface and react to form
1383-5866/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2010.01.019
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the thin polyamide film. Because the growing thin polyamide film behaves as a barrier for the diffusion of the two monomers, there would be fewer and fewer amine monomers on the organic phase side to react with the polyfunctional acid chloride monomers. Thus, there must exist the excessive unreacted polyfunctional acid chloride groups or polyfunctional acid chloride monomers on the surface of the active skin layer. Since the membrane performance is deeply related to the membrane surface properties [18–24], it is probably to improve the membrane performance by using monomers with amino groups to react with these unreacted acyl chloride groups and/or acyl chloride monomers (ACG–TMC) before their hydrolysis. Cao and co-workers [25] used aminopolyethylene glycol monomethylether with an amino group as grafting monomer to react with the unreacted acyl chloride groups and obtained interesting results. However, the aminopolyethylene glycol monomethylether with one amino group is a macromolecular monomer having comparative lower activity. Thus the modified membrane surface was not completely covered by aminopolyethylene glycol monomethylether and had larger roughness which was disadvantageous to the membrane antifouling ability [25]. Since ordinary polyfunctional amine monomers with double amino groups have high activity reacting with acyl chloride group, what is about using the ordinary polyfunctional amine monomers to modify the membrane surface? When the residual unreacted acyl chloride groups have reacted with the ordinary polyfunctional amine monomers, it is possible to obtain a membrane with comparatively smooth surface and have more hydrophilic groups of amino groups of –NH2 instead of carboxylic acid groups on the membrane surface. Smooth surface and the existence of hydrophilic groups of –NH2 were benefited to the antifouling ability [26]. However, to the best of our knowledge, there have been few reports on modifying the surface of thin film composite RO membrane by using ordinary polyfunctional amine monomers with double amino groups to react with the residual ACG–TMC on the membrane surface. In this paper, we present the results of thin film composite RO membranes by means of a novel interfacial polymerization approach in which ordinary polyfunctional amine monomers with double amino groups of MPD reacted with the unreacted ACG–TMC existing on the surface of the active skin layer before their hydrolysis. The microstructure and properties of the resultant thin film composite RO membranes were characterized by SEM, XPS and ATR-IR. Moreover, the antifouling ability of membrane was studied by using dodecyltrimethylammonium bromide and humic acids. Using novel interfacial polymerization approach different from those in previous reports, we prepared polyamide thin film composite RO membranes with unique morphology features and good membrane performance as described below. 2. Experimental 2.1. Materials Solvents and reagents in analytical grade were purchased from commercial sources. Polysulfone supported membranes were provided by Vontron Technology Co., Ltd., Bei Jing of China. 1,3,5-Benzenetricarbonyl trichloride (TMC) (>98%) and m-phenylenediamine (MPD) (>99%) were purchased from Sigma–Aldrich. 2.2. Membrane formation The membranes were prepared based on the traditional interfacial polymerization approach described elsewhere [6,7]. Firstly, polysulfone supported membranes taped to stainless steel plates were placed in an aqueous solution, with 3.0 wt.% MPD in the
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mixing aqueous solutions of triethylamine (TEA) (approximately 2.0 wt.%) and camphor sulfonic acid (approximately 4.0 wt.%) and different contents of dimethyl sulfoxide (DMSO), for 20 s. Subsequently, MPD soaked supported membranes were placed on a rubber sheet and rolled with a rubber roller to remove the excess solution. Then the MPD saturated polysulfone supported membranes were immersed in a solution of 0.15 wt.% TMC in hexane about 10 s. After that, the membranes were placed in an aqueous solution of 3.0 wt.% MPD in the mixing aqueous solutions of triethylamine (TEA) (approximately 2.0 wt.%) and camphor sulfonic acid (approximately 4.0 wt.%) solution for 0–60 s. After removing the excess aqueous solution, the resulting composite membranes were heated to 80 ◦ C in an air dryer about 5 min for further polymerization, leading to the formation of the active skin layer. Finally, the obtained membranes were washed in deionized (DI) water and stored in it. 2.3. Performance test All tests for thin film composite RO membranes performance were conducted at 225 psi using a 2000 ppm NaCl solution at room temperature by using a cross-flow type apparatus. Both permeate and retentate were recycled back to the feed tank during the tests. Circular membrane samples were placed in the test apparatus with the active skin layer facing the feed water. The effective membrane area was around 19 cm2 . All membrane samples were prepared and tested at least twice with a total of 3 membranes tests for RO performance, results of which have been averaged. The permeating volume collected for 1 h was used to describe flux in terms of gallons per square feet per day (gfd). A standardized conductivity meter was used to measure the salt (NaCl) concentrations in the feed and product water for determining membrane selectivity as given below: salt rejection (%) =
1−
Cp Cf
× 100
in which Cf and Cp are feed and permeating concentration, respectively. 2.4. Characterization of membranes The membranes used for the chemical structure and morphology analysis of the active skin layer were rinsed with DI water for several times. Then, the membranes were dried under vacuum at 80 ◦ C for 6 h. Attenuated total reflectance infrared (ATR-IR) characterization of the thin film composite RO membrane surface was made with a Nicolet avator 230 spectrometer at room temperature. For ATRIR analysis of membrane samples, Irtran crystal at 45◦ angle of incidence was employed. Surface chemical characterization was carried out by X-ray photoelectron spectra (XPS) of ESCALab220IXL with Al/K␣ (h = 1486.6 eV) anode mono X-ray source. Surface microstructures were observed by a field emission scanning electron microscope with SEM of Hitachi S-4300. Magnifications up to 20,000 were obtained at 5 kV. 2.5. Fouling experiments Membrane fouling experiments were performed with a crossflow type apparatus. Each membrane was compacted with deionized water until the permeate flux became almost constant. Subsequently the dodecyltrimethylammonium bromide (DTAB) or humic acids as surfactant were added to the reservoir to 50 ppm and the permeating experiment was performed for a due course time. Then the membrane was rinsed with deionized water to investigate
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Fig. 1. The schematic preparation procedure for thin film composite RO membrane using novel interfacial polymerization approach.
the flux recovery property. Both permeate and retentate were recycled back to the feed tank during the tests. The test was conducted at 225 psi and 25 ◦ C. Evaluation of the fouling extent of membranes was based on the value of relative flux, which was calculated by comparing the water flux before and after fouling or cleaning.
with double amino groups such as MPD for a few seconds to construct the second aqueous phase. It was supposed that the ordinary polyfunctional amines will react with the unreacted ACG–TMC to form the second polyamide skin layer on the top surface of previous TFC membrane.
3. Results and discussion
3.2. Permeation properties of the thin film composite RO membranes
3.1. Feasibility analysis of novel interfacial polymerization approach As previously reported [2], the surface of thin film composite RO membrane contained carboxylic acid groups coming from the hydrolysis of unreacted ACG–TMC. Meanwhile, since the hydrolysis reaction of acyl chloride is a relatively slow step, the nascent membrane would contain numerous unreacted ACG–TMC on the surface of the active skin layer. As a result, it is possible to modify the nascent thin film composite RO membrane surface with these unreacted ACG–TMC by chemical coupling with simple ordinary polyfunctional amines. The schematic illustration of the novel interfacial polymerization approach process, based on the reaction of simple ordinary polyfunctional amine with unreacted ACG–TMC existing on the surface of the active skin layer of thin film composite RO membrane, is demonstrated in Fig. 1 and detailed as follows: firstly, the polysulfone supported membrane was placed in an aqueous solution of MPD for some time to construct an aqueous phase on the surface, and the MPD soaked membrane was then placed on a rubber sheet and rolled with a rubber roller to remove excess solution. Secondly, the MPD saturated polysulfone supported membrane was then immersed in a solution of TMC in hexane to build the organic phase. The MPD dissolved in aqueous phase and TMC dissolved in organic phase contacted and reacted to form the active skin layer of thin film composite RO membrane. Since the growing thin polyamide film behaves as a barrier for diffusion of the MPD to the organic phase, there still existed lots of unreacted ACG–TMC on the surface of the active skin layer when the reaction stopped. Finally, after the excess hexane solution was removed, the membrane was then immersed in an aqueous solution of ordinary polyfunctional amines
Using novel interfacial polymerization approach with MPD as the monomer dissolving in the second aqueous phase, a series of thin film composite RO membranes were synthesized. The membrane performance was tested by cross-flow type apparatus and the results were listed in Table 1. Keeping the contact time with the first aqueous phase for 20 s and with the organic phase for 10 s, the contact time with the second aqueous phase was varied from 0 to 60 s. As shown in Table 1, the contact time with the second aqueous phase influenced the membrane performance deeply. The membranes produced by novel interfacial polymerization approach have a lower flux than tradition membrane. A possible reason is that the membranes synthesized by novel interfacial polymerization approach have thicker polyamide films than tradition membrane which increased the resistance for water to come through the membrane. With the contact time increasing from 0 to 20 s, the water flux decreased while the salt rejection increased greatly (Run Nos. 1–3 and Run Nos. 5–8 in Table 1). According to analysis above, there must existed lots of unreacted acyl chloride groups and TMC monomers on the surface of thin film skin layer after the formation of the first thin active skin layer of polyamide. When the polysulfone supported membrane with nascent thin film skin layer dipped into the third phase, the MPD dissolving in the second aqueous phase reacted with the residual ACG–TMC to form the second thin active skin layer of polyamide. It is reasonable to believe that the thickness increase of the thin active skin layer on the membrane would decrease the water flux and increase the salt rejection at the same time. However, as the contact time with the second aqueous phase further increased from 20 to 60 s, the membranes have higher water flux. It may because that the longer immersion time with the second aqueous phase is apt to produce a membrane
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Table 1 Formation and transport characteristics of thin film composite RO membranes prepared by novel interfacial polymerization approacha . Run No.
1 2 3 4 5 6 7 8 9 10 11
The first aqueous phase
The organic phase
The second aqueous phase
RO performanceb
DMSO (wt.%)
Contact time (s)
Contact time (s)
Contact time (s)
Flux (gfd)
Salt rejection (%)
0 0 0 0 1 1 1 1 1 0.5 0.25
20 20 20 20 20 20 20 20 20 20 20
10 10 10 10 10 10 10 10 10 10 10
0 10 20 60 0 10 20 30 60 20 20
20.50 15.60 15.30 19.10 30.62 25.18 29.26 32.67 33.35 24.97 19.43
98.50 99.08 99.34 99.12 97.65 98.87 99.04 98.78 98.80 98.76 98.74
a Preparation conditions: MPD = 3.0 wt.% in the first phase and the third phase; TMC = 0.15 wt.% in the second phase; curing temperature = 80 ◦ C; curing time = 10 min; aqueous pH = 8.3. b Test conditions: feed: 2000 ppm of NaCl aqueous solution, pressure: 225 psi, temperature: 25 ◦ C and pH 7.0.
Fig. 2. SEM images of thin film composite RO membranes from run 1 (a), run 2 (b), run 3 (c), run 4 (d), run 5 (e) and run 7 (f) with the contact time to the second aqueous phase with 0 s (a), 10 s (b), 20 s (c), 60 s (d), 0 s (e) and 20 s (f) in Table 1.
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surface with larger area which was benefit to improve the water flux. The dimethyl sulfoxide (DMSO) was used as additive into the first aqueous phase and the results were listed in Table 1 (Run Nos. 5–12 in Table 1), since the addition of DMSO in the aqueous amine solution can increase the water flux of thin film composite RO membrane [27]. As shown in Table 1, thin film composite RO membrane with DMSO as additive have much better water flux than thin film composite RO membrane with no DMSO as additive. Interestingly, as the content of DMSO decreased, the increased rate of water flux decreased sharply (Run Nos. 8, 11 and 12 in Table 1). Moreover, the thin film composite RO membrane with DMSO as additive had the similar rules as the thin film composite RO membrane with no DMSO as additive about the relationship between the contact time and the membrane performance. 3.3. Membrane surface morphologies SEM was used to characterize the surface of the active skin layer of thin film composite RO membrane as shown in Fig. 2. The membrane produced by traditional interfacial polymerization approach with no DMSO has a rough surface with lots of “leaf-like” folds on it (a in Fig. 2), typical for polyamide composite membranes prepared by traditional interfacial polymerization of aromatic acid chloride and aromatic amides [27]. The addition of DMSO in the first aqueous phase changed the quantity and shape of “leaf-like” folds and obtained more and bigger “leaf-like” folds on the membrane surface (e in Fig. 2). Obviously, the novel interfacial polymerization offered membrane with littler “leaf-like” folds, when compared with the typical thin film composite RO membranes. Moreover, the contact time with the second aqueous phase affected the surface morphology of membrane greatly. With the contact time increasing from 0 to 20 s, the “leaf-like” folds on the membrane surface decreased. Surprisingly, when the contact time further increased from 20 to 60 s, the “leaf-like” folds on the membrane surface increased also (c and d in Fig. 2). The longer immersion time with the second aqueous phase is apt to produce more “leaf-like” folds with larger surface area. As a result, the larger surface area is favorable to improve the water flux, which was in agreement with the results as shown in Table 1. 3.4. Characterization of the active layer In order to grasp the chemical structure of the active skin layer of thin film composite RO membranes, ATR-IR and XPS measurements were carried out. As seen, the major characteristic of aromatic polyamide at 1660 cm−1 (amide I, C O stretching), 1554 cm−1 (amide II, N–H bending) and 1608 cm−1 (aromatic ring breathing) is present in the ATR-IR spectrum of Fig. 3. According to the illustration of Fig. 1, the surface of the active skin layer of thin film composite RO membranes prepared by the novel interfacial polymerization approach was supposed to present excessive amino groups. However, the peak of amino groups is hardly obviously observed in the spectra of the membranes, which are possibly overwhelmed by other peaks. XPS is a highly sensitive technique for surface analysis, with the ability to measure elemental composition and chemical binding information for the 1–9 nm depth of the surface. Therefore, the membranes were analyzed for the chemical composition of the active skin layer through XPS and the results were shown in Table 2 and Fig. 4. It is necessary to point out that the elements detected by XPS were not only oxygen (O), nitrogen (N), and carbon (C), but also chlorine (Cl) in the active skin layer when the thin film composite RO membranes were produced by novel interfacial polymerization approach. The existence of chlorine (Cl) in the active skin layer might be due to the hindered hydrolysis of the unreacted
Fig. 3. ATR-FTIR spectra of thin film composite RO membranes from run 5 (a), run 6 (b), run 7 (c) and run 9 (d) with the contact time to the second aqueous phase with 0 s (a), 10 s (b), 20 s (c) and 60 s (d) in Table 1.
Fig. 4. XPS spectra of thin film composite RO membranes from run 5 (a), run 7 (b) and run 9 (c) with the contact time to the second aqueous phase with 0 s (a), 20 s (b) and 60 s (c) in Table 1.
acyl chloride groups and TMC monomers in the second compact polyamide layer. Obviously, the thin film composite RO membranes produced by novel interfacial polymerization approach have the higher contents of nitrogen (N) and the lower ratio of oxygen (O) and nitrogen (N) when comparing to the traditional membranes. With the increase of contact time with the third phase, the con-
Table 2 Surface composition by XPS analysis of various RO membranes.
Run No.b 5 6 7 8 9 Theoretical valuesc Fully cross-linked Fully linear a b c
Contact time (s)a
O%
N%
C%
Cl%
O/N
0 10 20 30 60
16.9 17.7 15.5 12.5 13.7
9.6 9.9 11.0 12.9 10.4
73.5 71.4 73.4 74.3 75.7
0 1.0 0.1 0.3 0.2
1.8 1.8 1.4 0.9 1.3
0 0
12.5 19.1
12.5 9.5
75.0 71.4
0 0
1.0 2.0
Contact time with the second aqueous phase. The Run No. in Table 2 is the same Run No. in Table 1. The numerical value comes from Ref. [8].
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interfacial polymerization approach showed a relatively better antifouling property. The phenomenon could be interpreted as follows: firstly, several literatures have stated that membrane fouling is directly related to membrane surface roughness [20,28]. The smoother surface of membrane prepared by the novel interfacial polymerization approach would contribute to the membrane antifouling performance. Secondly, some researchers indicated that the membrane charge also affected the membrane fouling performance [26]. The presence of amino groups of –NH2 in the active skin layer of the membrane prepared by the novel interfacial polymerization approach makes the membrane surface more close to neutral charge than traditional membrane and probably benefits to membrane antifouling performance. 4. Conclusions
Fig. 5. Fouling experiment with DTAB as pollutants for thin film composite RO membranes prepared by traditional interfacial polymerization approach (a) from run 5 in Table 1 and novel interfacial polymerization approach (b) from run 8 with the contact time to the second aqueous phase with 0 s (a) and 30 s (b) in Table 1.
A series of thin film composite RO membranes were successfully synthesized by using a novel interfacial polymerization approach here. The membrane performance of water flux and salt rejection varied greatly with the contact time with the second aqueous phase. The results of ATR-IR and XPS revealed that the active skin layer of thin film composite RO membrane is composed of aromatic polyamide with the functional bonds of amide (–CONH–) and the functional groups such as amino group (–NH2 ), chlorine (Cl). The SEM images indicated that the novel interfacial polymerization approach offered smoother surface membrane. Fouling experiments results indicated the membrane prepared by the novel interfacial polymerization approach showed a relatively better antifouling property than traditional membrane. Using the novel interfacial polymerization approach with different polyfunctional amine monomers with double amino groups in the third phase, it is possible to prepare multifunctional thin film composite RO membranes. Acknowledgments
Fig. 6. Fouling experiment with Humic acid as pollutants for thin film composite RO membranes prepared by traditional interfacial polymerization approach (a) from run 5 in Table 1 and novel interfacial polymerization approach (b) from run 8 with the contact time to the second aqueous phase with 0 s (a) and 30 s (b) in Table 1.
tents of nitrogen (N) increased while the ratios of oxygen (O) and nitrogen (N) decreased. When the contact time with the third phase is 30 s, surprisingly, the content of nitrogen (N) is much higher than the theoretical value (>12.5%) and the ratio of oxygen (O) and nitrogen (N) is much lower than the theoretical value (<1.0) [8]. Since the residual unreacted acyl chloride groups have reacted with MPD and there existed excessive unreacted amino groups of –NH2 on the surface (as shown in Fig. 1) which led to the decrease of the contents of oxygen (O) while the contents of nitrogen (N) increased at the same time, it is reasonable to believe that the thin film composite RO membranes produced by the novel interfacial polymerization approach will have higher contents of nitrogen (N) and lower ratios of oxygen (O) and nitrogen (N) than the traditional membranes as shown in Table 2 and Fig. 4. 3.5. Fouling experiments A 50 ppm dodecyltrimethylammonium bromide (DTAB, a cationic surfactant) which was known to have high fouling potential [25] and 50 ppm humic acids were used for fouling experiments. The preliminary results are summarized in Figs. 5 and 6. As seen from the data provided, the membrane prepared by the novel
The financial supports from the National Natural Science Foundation of China (contract grant number: 20804050), the National High-tech R&D Program (863 Program, No. 2007A030301), the National Basic Research Program of China (973 Program, No. 2009CB623400) and China Postdoctoral Science Foundation (20080440067) are gratefully acknowledged. References [1] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Marin, A.M. Mayes, Science and technology for water purification in the coming decades, Nature 452 (2008) 301–310. [2] R.J. Petersen, Composite reverse osmosis and nanofiltration, J. Membr. Sci. 83 (1993) 81–150. [3] D. Bhattacharyya, M.J. Evttch, J.K. Ghosal, J. Kozminsky, Reverse-osmosis membrane for treating coal-liquefaction wastewater, Environ. Prog. 3 (1984) 95–102. [4] S. Lee, M. Elimelech, Relating organic fouling of reverse osmosis membrane to intermolecular adhesion forces, Environ. Sci. Technol. 40 (2006) 980–987. [5] S. Loeb, The Loeb-Sourirajan membrane: how it came about, in: A. Turbak (Ed.), Synthetic Membranes, vol. I, ACS Symposium Series 153, Washington, DC, 1981. [6] J.E. Cadotte, Interfacially synthesized reverse osmosis membrane, US Patent 4,277,344 (1981). [7] J.E. Cadotte, R.J. Petersen, R.E. Larson, E.E. Erickson, New thin-film composite seawater reverse-osmosis membrane, Desalination 32 (1980) 25–31. [8] Ch.Y. Tang, Y.N. Kwon, J.O. Leckie, Probing the nano- and micro-scales of reverse osmosis membranes—a comprehensive characterization of physiochemical properties of uncoated and coated membranes by XPS, TEM, ATR-FTIR, and streaming potential measurements, J. Membr. Sci. 287 (2007) 146–156. [9] J.R. McCutcheona, M. Elimelech, Influence of membrane support layer hydrophobicity on water flux in osmotically driven membrane processes, J. Membr. Sci. 318 (2008) 458–466. [10] P.S. Singh, S.V. Joshi, J.J. Trivedi, C.V. Devmurari, A. Prakash Rao, P.K. Ghosh, Probing the structural variations of thin film composite RO membranes obtained by coating polyamide over polysulfone membranes of different pore dimensions, J. Membr. Sci. 278 (2006) 19–25.
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