Modification to the polyamide TFC RO membranes for improvement of chlorine-resistance

Journal of Membrane Science 376 (2011) 302–311

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Modification to the polyamide TFC RO membranes for improvement of chlorine-resistance Dong Ho Shin a , Nowon Kim b , Yong Taek Lee a,∗ a b

Department of Chemical Engineering, College of Engineering, Kyung Hee University, Gyeonggi-do, Republic of Korea Department of Environmental Engineering, College of Engineering, Dong-Eui University, Busan, Republic of Korea

a r t i c l e

i n f o

Article history: Received 7 February 2011 Received in revised form 19 April 2011 Accepted 21 April 2011 Available online 30 April 2011 Keywords: Chlorine resistance Polyamide RO membrane Alkyl-silane Phenyl-silane Surface modification

a b s t r a c t Most current high-performance composite membranes comprise aliphatic or aromatic amines condensed with acyl chlorides or other reactive groups that usually contain substituted amide linkages. Aromatic rings bonded to the N–H group of amide linkages are sensitive to attack by chlorine radicals because of their high electron density. Consequently, the N–H group is converted to an N–Cl group by N-halogenation. This causes the failure of the polyamide reverse osmosis (RO) membrane resulting in decreased salt rejection and increased water flux after exposure to chlorine. In this study, the performance of a silane-coated RO membrane was investigated and its surface analyzed using field emission-scanning electron microscopy (FE-SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The stability of the coated membranes in the presence of chorine was also studied in order to ascertain the effect of incorporating different silane compounds with alkyl, aryl and vinyl substituents. In the uncoated membrane, degradation of the polyamide network of the RO membrane by free chlorine in the feed solution resulted in decreased salt rejection and increased water flux after 15,000 ppm h. However, the silane-coated membrane maintained a salt rejection of above 99.0% even after 25,000 ppm h. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Thin-film-composite membranes (TFCs) that contain aromatic polyamides are widely used in commercial high-performance reverse osmosis (RO) membranes and thus have been used for the desalination of sea and brackish water and in the production of ultrapure water [1,2]. Most current high-performance composite membranes incorporate aliphatic or aromatic amines condensed with acyl chlorides, isocyanates, or other reactive groups. The resulting membranes usually contain amide linkages with various degrees of substitution, which are susceptible to degradation in the presence of aqueous chlorine [3]. The ability of membranes to withstand exposure to chlorine or chlorine-induced oxidants is an important indicator of membrane performance because chlorine is commonly added to water as a disinfectant and bactericide [4]. Thus, significant research efforts have focused on finding effective ways to prevent this membrane degradation. Chlorine sensitivity is enhanced by nitrogen-based functional groups such as amines and secondary amide linkages. Polyamides containing alkyl or aryl groups bonded to the amide nitrogen or tertiary amides are less

∗ Corresponding author. Tel.: +82 31 201 2577, fax: +82 31 204 8114. E-mail address: [email protected] (Y.T. Lee). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.04.045

sensitive to chlorine [5]. Additionally, in a previous paper we reported that a silane-coated membrane with long-chain alkyl and rigid phenyl groups enhances NaCl rejection [6]. The purpose of this study is to investigate the chemical structures and performance of TFC-RO membranes before and after hypochlorite exposure. Moreover, the performance of a silane-coated RO membrane was investigated and its surface analyzed using field emission-scanning electron microscopy (FE-SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The chlorine-resistance of the coated membranes was also studied to ascertain the effect of the incorporation of silane compounds substituted with alkyl, aryl and vinyl groups with differing chain lengths.

2. Background 2.1. Mechanism of chlorine degradation of RO membrane The degradation mechanism of TFC RO membranes is represented in Fig. 1. The polyamide network is degraded by the attack of aqueous chlorine induced by hyperchlorite (–OCl). The chemical structure of polyamide has numerous amide linkages, which are random copolymers consisting of repeating units of the amide (–O C–N–H–) group. Aromatic rings bonded to the N–H group of

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303

Fig. 1. Mechanism of degradation of polyamide RO membrane by aqueous chlorine.

amide linkages are sensitive to attack by chlorine radicals because of their high electron density. Consequently, the N–H group is converted to an N–Cl group by N-halogenation. The resulting Nchloroamide then undergoes an intermolecular rearrangement to form various aromatic substitution products. It also induces a chain reaction to produce chloro-substituted benzene rings as the quinone derivatives or oxidation groups [4,5]. This causes the failure of the polyamide reverse osmosis (RO) membrane resulting in decreased salt rejection and increased water flux after chlorination [7–9]. 2.2. Deactivating the chemical group to suppress aqueous chlorine attack Polyamides with nitrogen atom substituted with different alkyl and aryl groups were reported not to react with HOCl [5]. In order to compare the effect of the substitution pattern of the amide on chlorine-resistance, tertiary amides of N-methyl and N-phenyl benzanilide were incorporated into the polyamide. The results of the chlorine-resistance test demonstrated that tertiary amides are unreactive towards chlorine. However, secondary amides that contain aliphatic nitrogen substituents formed N-chloro derivatives, which were easily dechlorinated. The chemical structures of the groups that are unreactive towards chlorine attack are shown in Fig. 2 [5,10]. The amide generally used to generate polyamide RO membranes is similar to benzanilide. In this study, we use alkyl groups and aryl groups to protect the polyamide linkage from chlorine attack.

Fig. 2. The chemical groups that are unreactive towards chlorine attack; (a) Nmethyl benzanilide and (b) N-phenyl benzanilide.

2.3. Silane compounds and the membrane-coating process The silane compounds used have the general structure of X3 SiRY, where X is the alkoxy group and Y is a functional group, i.e., alkyl, phenyl, or vinyl. The chemical bond is formed by a two-step reaction on the active layer of the membrane surface. Fig. 3 shows the mechanism of sol–gel condensation using the silane compound. If electron-withdrawing groups are present on the polyamide chain, a chemical reaction would occur on the alkoxy group of silane compound by redox. Assuming that this occurs, the alkoxy group may act as a bridge to bond the silane compound to the polyamide through a chain of primary bonds that would lead to the strongest interfacial bond. This reaction has two steps: (1) X is hydrolyzed to form a reactive silanol group and (2) a condensation reaction occurs between the silanol and the polyamide [11]. Ultimately, it produces a stable chemical structure with Si–O–N or Si–O–C bonds, which form from the amide linkages or unreacted COOH groups. In this experiment, we used 5 types of silane compounds for chlorine resistance with the base structure of X3 SiRY, where X is a –OC2 H5 alkoxy group and Y is an alkyl (CH3 , C8 H17 , orC18 H37 ), vinyl, or phenyl group. 3. Experimental 3.1. Materials We used a commercial polyamide RO membrane (SWC1, Hydranautics Co., spiral-wound type) as the virgin membrane for silane coating. Methyltriethoxysilane (MeTES, Sigma–Aldrich, 98%), octyltriethoxysilane (OcTES, Sigma–Aldrich, 96%), octadecyltrimethoxysilane (OdTMS, Sigma–Aldrich, 98%), phenyltriethoxysilane (PhTES, Sigma–Aldrich, 98%) and vinyltriethoxysilane (VTES, Sigma–Aldrich, 98%) were used without further purification as coating reagents. Absolute ethanol (Carloerba, 99.9%) was used to dissolve the silane coupling agent. Propylene glycol (PG; Duksan Chemical Co., 99.8%) was used as a wetting agent. Potassium metabisulfite (Sigma–Aldrich, 99%) and potassium persulfate (Sigma–Aldrich, 99%) were used to etch the polyamide RO membrane surface as the redox initiator [12]. Sodium chloride (NaCl, Duksan Chemical Co., 98%) and sodium hypochlorite (NaOCl, Duk-

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Fig. 3. Coating mechanism using sol–gel method with silane compounds.

san Chemical Co., 12%) were used to test the performance of the modified RO membranes.

coated with the appropriate 1.5 wt% silane solution. The modified membranes were then cured at 70 ◦ C for 10 min.

3.2. Preparation of coated RO membrane

3.3. Surface analysis

Fig. 4 shows the flow diagram for the preparation of the silanecoated membranes. The membranes were soaked in a 5% (w/v) PG solution for 30 min to prevent the membrane surface from drying out, and then dried at 70 ◦ C in a vacuum oven. The membrane was packed in a plastic container so that the coating solution contacted only the polyamide surface of the membrane. 0.01 M solutions of potassium metabisulfite and potassium persulfate were used as the initiators. The membranes were dipped for 30 min and then

The surface properties of the membranes are important for many applications and are dependent on the structure and composition of the outermost molecular layer. Therefore, understanding the relationships between the surface structure and membrane properties requires analytical techniques that have a high degree of sensitivity. The surface morphology of the skin layer of the RO membranes was analyzed via FE-SEM using a LEO SUPRA (Carl Zeiss, Germany) scanning electron microscope with a magnification of 100,000. The surface topology of the membranes was structurally characterized by AFM using a D3100 (Tecsco, U.S.A.). To minimize the sample damage and maximize the resolution, AFM was operated in the non-contact mode and the scan size was 10 ␮m × 10 ␮m. The changes in the chemical structures of the membrane surfaces were determined using X-ray photoelectron spectroscopy (K-Alpha, Thermo Electron, U.K.). The spot size was 400 ␮m, which are 3 times that of the survey and 20 times that of the narrow mode with a sample angle of 54.7◦ and a high resolution per step of 0.1 eV. The spectrum was recorded using a monochromatic Al K␣ X-ray source. The quantitative chemical information of the top layer of the sample can be calculated from the curve fitting results of the XPS spectrum for C 1s, O 1s, N 1s, Si 2p3/2 and Cl 2p3/2 . 3.4. Performance

Fig. 4. Procedure for the preparation of silane-coated membranes.

The test for chlorine-resistance was done using NaCl and NaOCl solutions at concentrations of 35 g/L and 2 g/L, respectively, at a pressure of 55 kgf /cm2 at 25 ◦ C. The test cell has an effective membrane area of 27 cm2 . Fig. 5 shows the experimental apparatus, including a schematic diagram of the RO membrane cell-testing apparatus and a cross-section of the membrane test cell. The testing apparatus comprises a high pressure pump (323, CAT pump, USA), feed tank, cooling system and flat-type permeate cell (stain steel SUS316L). The membrane sample was supported on a porous spacer net with a silicon o-ring around it to ensure leak-free operation. The modified membranes were evaluated for permeate water

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Fig. 5. Schematic diagram of RO membrane cell-testing apparatus and the cross-section of the membrane test cell.

flux (MMD) and salt rejection (%) on RO test equipment. A digital conductivity meter (460CP, Istek electronics, Korea) was used to measure the conductivities of the feed solution and permeate sample. The salt rejection and water flux were calculated using the following equations:

water flux (MMD) =

1−

 conductivity of permeate

conductivity of feed solution

permeate volume (m3 ) membrane area (m2 ) × time (days)

× 100 (1) (2)

3.5. Evaluation of chlorine-resistance

98

Salt Rejection [%]

salt rejection (%) =



100

Chlorine exposure comprised a NaOCl solution (2000 ppm) that contacted the surface of the membrane during the experiment. The concentration of NaOCl and contact time were expressed as ppm h, which indicates the total exposure of the membrane to chlorine. After measuring the initial performance of the membrane, the NaOCl solution was introduced to the feed tank resulting in a pH value of 7–8.

96 94

Un-coated MeTES OcTES OdTMS PhTES VTES

92 90 88 0

10,000

15,000

20,000

25,000

20,000

25,000

ExposureTime [ppm·hr]

(a) Salt Rejection 2.0

4. Results and discussion

Un-coated MeTES

4.1. Chlorine-resistance of silane-coated membranes

OcTES

1.5

OdTMS PhTES

2

Water Flux [m /m ·day]

VTES

3

The performance results of the uncoated and coated membranes shown in Fig. 6 revealed that the salt rejection of the uncoated membrane is reduced and the water flux is increased as the exposure goes beyond 15,000 ppm h, due to the degradation of the polyamide network of the RO membrane by free chlorine in the feed solution [13]. However, the silane-coated membrane maintained a salt rejection of above 99.0% even after an exposure of 25,000 ppm h. The combined concentration and exposure-time effect (ppm h) was determined using the product of the concentration of aqueous chlorine and hours of exposure. Table 1 shows the differences in the performance of the uncoated and coated membranes. The OcTES- and OdTMS-coated membranes have a similar salt rejection performance, but the water flux of the OdTMS-coated membrane was lower than that of the OcTES-coated membrane. The membranes coated with long alkyl chain-substituted silanes showed a relatively poor RO performance. The initial performance of the uncoated membrane was 0.883 MMD and 99.3% for water flux and salt rejection, respectively. In contrast, the OcTES-coated mem-

5,000

1.0

0.5

0.0 0

5,000

10,000

15,000

ExposureTime [ppm·hr]

(b) Water Flux Fig. 6. Performance result of the uncoated and coated membranes; (a) salt rejection and (b) water flux.

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Fig. 7. FE-SEM images at a magnification of 100,000 and AFM surface images of uncoated and coated TFC membranes; (a) uncoated, (b) MeTES-coated, (c) OcTES-coated, (d) OdTMS-coated, (e) PhTES-coated and (f) VTES-coated.

brane, which showed the best results of the coated membranes, had an initial water flux of 0.658 MMD and salt rejection of 99.5%. After 12 h of chlorine exposure, the performance of the uncoated membrane changed to 1.329 MMD and 88.3% for the water flux and salt rejection, respectively, and the OcTES-coated membrane

had a water flux of 0.553 MMD and a salt rejection of 99.5%. Therefore, although there is a difference in the initial performance of the coated membranes, they display an increased chlorine-resistance because the alkyl and aryl groups protect the amide moieties from chlorine attack.

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307

Fig. 7. (Continued )

4.2. Analysis of surface structure by FE-SEM and AFM FE-SEM and AFM images show the change in morphology of the surfaces of the uncoated and coated membranes (Fig. 7). The high-resolution FE-SEM images display the enlarged skin layers, which are magnified 30,000 times more than previously reported

SEM images [6]. FE-SEM and AFM images are complementary techniques to assess the surfaces of the uncoated and coated membranes. The surface of the uncoated membrane appears to be composed of smooth granular particles; the peak-and-valley and nodular structures are caused by the aromatic polyamide, as shown in Fig. 7(a) [14]. MeTES, OcTES and OdTMS have similar

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Table 1 The performance results of uncoated and coated membranes after 12 h. Condition

Uncoated

MeTES

OcTES

OdTMS

PhTES

VTES

Initial water flux (MMD) Water flux (MMD) after 12 h Initial salt rejection (%) Salt rejection (%) after 12 h

0.833 1.329 99.30 88.35

0.534 1.114 98.99 96.00

0.658 0.553 99.57 99.52

0.308 0.255 99.55 99.46

0.739 0.426 99.53 98.88

0.599 0.755 97.98 97.21

5000

Number of elements

MeTES 3000

OcTES OdTMS

2000

1000

0

0

100

200

300

400

500

600

700

800

Height range [nm]

(a) Different chain length 5000

Un-coated

4000

VTES 3000

PhTES

2000

1000

4.3. Chemical structure of the silane-coated membranes by XPS study The change in the chemical structure of the coated membrane was investigated using XPS, which has been previously used to characterize uncoated and silane-coated membranes. Our previous study provided direct evidence on the success of the sol–gel method for surface modification [6]. This report mentioned that the change in the atomic content provided evidence of the chemical reaction. In this paper, we have expanded our investigation into the fitting peaks of the XPS spectrum, which contain information on the identity of the chemical species. The range of standard values of binding energies is summarized in Table 2 [15]. These values contain the information on the identity of the chemical compound. If there is an XPS peak in that range, it demonstrates the existence of a silane coating on the polyamide. The value of the binding energy usually varies because the chemical bonds change. The chemical states are assessed by the shift of the peak, which is referred to as the chemical shift [16]. These peaks offer information about the chemistry of the surface. The XPS spectrum of each element is a fitted curve based on Guassian and Lorentzian functions [17]. The results of the peak fitting of the line-shapes are shown in Figs. 9–13 and summarized in Table 3. As mentioned earlier, chloro-substitution to form the quinone derivatives or oxidation groups causes failure of the

Un-coated

4000

Number of elements

chemical structures, altering only in the length of the aliphatic chain. The different aliphatic chain lengths affect the surface of the coated membrane, i.e., the image of the MeTES-coated membrane shows a well-defined peak-and-valley structure, while that of the OcTES-coated membrane shows a more obscured peak-andvalley structure and the OdTMS-coated membrane shows an even small peak-valley structure. Thus, the skin layers of the various silane-coated membranes differ in their smoothness, as shown in the AFM images in Fig. 7(b)–(d), with longer alkyl chains on the silane moiety leading to a less-defined valley structure. Also, the excessive length of the aliphatic carbon chains may interrupt the surface of the OdTMS-coated membrane. The PhTES- and VTEScoated membranes, containing aryl and vinyl groups, respectively, show a smaller and denser peak-and-valley structure, as displayed in Fig. 7(e) and (f). This is because the aryl and vinyl groups are relatively small and rigid moieties. The substitution pattern on the silane affects not only the surface structure, but also the surface height of the coated membrane. Fig. 8 shows the change in the surface height histograms of the silane-coated RO membranes from a specific projection area using an AFM histogram, which displays the height distribution of the membrane surface. The height histograms of the MeTES-, PhTES- and VTES-coated membranes do not shift to the right because they contain either short aliphatic alkyl chains or a rigid group. On the other hand, the height histograms of OcTES- and OdTMS-coated membranes shift to the right because of the increase in the surface height due to the long alkyl chains on the silane moieties. These results suggest that the surface properties can be tuned by altering the silane compound. In particular, if these groups could be incorporated onto the surface of membrane, the membrane performance could be improved due to increased chlorine-resistance.

0

0

100

200

300

400

500

600

700

800

Height range [nm]

(b) Functional group Fig. 8. The comparison of histograms of height distribution of uncoated and coated membranes as determined by AFM; (a) different chain lengths and (b) functional groups. Table 2 Standard values of binding energy (eV) for the core-level electrons of atoms. Core-level electrons

Binding energy, eV

Compound type

Si 2p3/2

100.5 ± 0.5 101.8 ± 0.5 102.4 ± 0.5 200.5 ± 0.5 208.5 ± 0.5 282.0 ± 1.0 284.5 286.5 ± 0.5 397.0 ± 1.0 399.0 ± 1.0 400.0 ± 1.0 529.5 ± 1.0 531.0 ± 0.5 533.0 ± 0.5

C with Si N with Si Silicones (silane) C6 H5 Cl Perchlorate C with SiO –COOH/C with O (alcohols) C as graphite N with SiO N with amide linkage Polyamide/Organic Matrix –COOH C O with amide linkage SiO2 /O with Si

Cl 2p3/2 C 1s

N 1s

O 1s

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309

8500

(a)

O1s spectrum -C=O- of PA (531.3) -COOH (530.0)

Count / s

531.3 75% of area ratio

8000

N1s spectrum C6H5C(O)NH (400.10)

7500

C6H4-CN (399.10)

7000

NH of PA (398.41) N-SiO (397.77)

399.10

398.41

6500 6000

400.10

397.77

5500

530.0

5000

25% of area ratio

4500 411

408

405

402

399

396

393

390

Binding Energy [eV]

(a) N1s spectrum (b) 30000

Un-coated OcTES SiO2 (532.92)

25000

530.0

C=O of PA (531.98)

20000

38% of area ratio

Count /s

531.3 62% of area ratio

531.98

15000 10000

532.92

5000 0 543 546

543

540

537

534

531

528

540

525

Binding Energy [eV]

199.62

5000

Count / s

528

525

4000

Si2p3/2 spectrum OSi-C6H4-Si (101.80)

3500

Si-O-Si or Si-O-N (101.45)

101.45

3000

Count / s

tetrahydeoquinone (196.9)

531

4500

Cl2p3/2 specturm C6H5Cl (199.62)

6000

534

(b) O1s spectrum

Fig. 9. The O 1s curve fitting of XPS spectrum data of the uncoated membrane (a) before and (b) after chlorine exposure.

7000

537

Binging Energy [eV]

4000

2500 2000 1500

101.80

3000

196.9 2000

1000 500

1000

0

114

112

110

108

106

104

102

100

98

96

94

Binding Energy [eV]

0 210

207

204

201

198

195

Binding Energy [eV] Fig. 10. The Cl 2p3/2 curve fitting of XPS spectrum data of the uncoated membrane after 12 h of chlorine exposure.

polyamide RO membrane and a decline in salt rejection. Fig. 9 shows the O 1s curve fitting of the XPS spectrum data concerning the uncoated membrane before and after 12 h of chlorine exposure. As shown in the graph, a comparison of Fig. 9(a) and (b) reveals that the amount of carbon and oxygen double bonds and carboxyl groups changes from 75% and 25% to 62% and 38% of the

(c) Si2p3/2 spectrum Fig. 11. The (a) N 1s, (b) O 1s, and (c) Si 2p3/2 curve fitting of the XPS spectrum on the surface of the OcTES-coated membrane.

area ratio, respectively, after chlorine exposure [18,19]. The electron counts of the vertical axes do not correspond to the absolute value of the element’s concentration; rather, they refer to the difference in the oxygen content on the membrane before and after chlorine exposure. The carbon and oxygen double bond content of the uncoated membrane decreased after 12 h of chlorine exposure,

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35000

C1s spectrum C of PA (286.15) COOH of PA (284.24) Si-C of phenyl (283.30)

35000 30000

283.30

25000

284.24

Count / s

Count / s

25000 20000 15000 286.15

10000

C1s spectrum C of PA (286.10) COOH of PA (284.30) Si-C (283.37)

30000

20000

283.37 284.30

15000 10000

5000

286.10

5000

0

0

300

297

294

291

288

285

282

279 300

297

294

Binding Energy [eV]

(a) C1s spectrum

282

279

Si2p3/2 spectrum Si-O (102.22) Si-N (101.40)

3000 101.22

101.40

2500

Count / s

2000

Count / s

285

3500

Si2p3/2 spectrum Si-N (101.71) Si-C6H5 (101.22)

1500

1000

288

(a) C1s spectrum

3000

2500

291

Binding Energy [eV]

2000 1500

101.71

1000

102.22

500 500 111

108

105

102

99

96

93

Binding Energy [eV]

while the carboxyl or oxidation group content increased because the amide groups of the polyamide RO membrane were broken after chlorine attack. Fig. 10 displays the Cl 2p3/2 curve fitting of the XPS spectrum data of the uncoated membrane after 12 h of chlorine exposure. The peaks at 196.9 eV and 199.62 eV indicate the formation of the newly generated tetrahydroquinone and Table 3 The results of curve fitting from XPS spectra. Binding energy, eV

Uncoated

–

–

–

Uncoatedtreateda OcTES

–

196.90 199.62 –

–

283.30 284.24 286.15 283.37 284.30 286.10

101.45 101.80

PhTES

101.22 101.71

–

VTES

101.40 102.22

–

C 1s

–

108

106

104

102

100

98

96

94

(b) Si2p3/2 spectrum

Fig. 12. The (a) C 1s and (b) Si 2p3/2 curve fitting of the XPS spectra on the surface of the PhTES-coated membrane.

Cl 2p3/2

110

Binding Energy [eV]

(b) Si2p3/2 spectrum

Si 2p3/2

112

N 1s

O 1s

398.65 399.61 400.87

530.00 531.30 530.00 531.30 531.98 532.92

397.77 398.41 399.10 400.10 –

–

–

–

a Uncoated membrane is treated with 2000 ppm NaOCl for chlorine exposure during 12 h.

Fig. 13. The (a) C 1s and (b) Si 2p3/2 curve fitting of the XPS spectra on the surface of the VTES-coated membrane.

chloro-substituted phenyl rings from broken polyamide chains due to chlorine attack [15,20]. N-chlorination is followed by oxidation of the phenyl ring to form substituted quinine derivatives. These reactions result in the decreased performance of RO membrane. Fig. 11 displays the O 1s, N 1s and Si 2p3/2 curve fitting of the XPS spectrum of the surface of the OcTES-coated membrane. The N 1s spectrum, shown in Fig. 11(a), indicates that nitrogen has different energy values in some bonding environments. This spectrum shows a change in the amine group in the PA network because it reacts easily with siloxane in the presence of an OH radical provided by the initiator reagents. The binding energy of N 1s much lower than the N–Si, N–phenyl, and N–amide bonds in that order. However, the presence of the N–SiO chemical group indicates the reaction between the amine and the siloxane [21–25]. For the example of SiO2 , if the same chemical structure is found in both the oxygen and silicon spectra, it proves the presence of the SiO2 group. Fig. 11(b) shows the O 1s curve fitting data for the uncoated and OcTEScoated membranes. The energy shift can provide clear evidence of a reaction with OcTES in the PA network. First, the OcTES-coated membrane peak is shifted to the right, which indicates an increased binding energy and changes to the chemical environment of oxygen. Second, the Si 2p3/2 spectrum in Fig. 11(c) shows that the fitting curve indicates the presence of Si–O–Si or Si–O–N moieties [26]. Therefore, the chemical composition of the amide or carboxyl group incorporates silane in the coating of the membrane.

D.H. Shin et al. / Journal of Membrane Science 376 (2011) 302–311

Fig. 12 shows the C 1s and Si 2p3/2 curve fitting of the PhTEScoated membrane. The Si 2p3/2 spectrum also evidences the presence of a Si–N group [22]. Both the C 1s and Si 2p3/2 spectra show a chemical bond between the Si and C of phenyl [16] from the PhTES group. Similar to the previous results, there is evidence of Si–N bonds. Fig. 13 shows the evidence of the Si–C bond of VTES. These results confirm the coating of the RO membrane with silane compounds [16,27]. 5. Conclusion This study clearly shows the chemical structures and performance of the TFC-RO membranes before and after hypochlorite exposure. The performance results of the uncoated membranes show that the salt rejection is reduced and the water flux is increased after chlorine exposure past 15,000 ppm h. However, the silane-coated membrane maintains a salt rejection of above 99% after 25,000 ppm h of chlorine exposure. The surface-analysis results by FE-SEM and AFM suggest that the surface properties can be tuned by altering the silane compound. In particular, if these groups could be incorporated onto the surface of membrane, the membrane performance could be improved due to increase chlorine-resistance. In addition, the results of XPS spectra analysis show that the chlorine-sensitive features of membranes comprise the nitrogen functional groups, aromatic rings and the active layer of the polyamide. These results indicate that coated membranes have an increased resistance to chlorine because the alkyl and aryl groups protect the polyamide skeleton from chlorine attack. Acknowledgments This work was supported by the Industrial Strategic Technology Development Program (No. 10035373) funded by the Ministry of Knowledge Economy (MKE, Korea). References [1] M. Hirose, Y. Minamizaki, Y. Kamiyama, The relationship between polymer molecular structure of RO membrane skin layers and their RO performances, J. Membr. Sci. 123 (1997) 151. [2] N.P. Soice, A.R. Greenberg, W.B. Krantz, A.D. Norman, Studies of oxidative degradation in polyamide RO membrane barrier layers using pendant drop mechanical analysis, J. Membr. Sci. 243 (2004) 345. [3] G. Kang, C. Gao, W. Chen, X. Jie, Y. Cao, Q. Yuan, Study on hypochlorite degradation of aromatic polyamide reverse osmosis membrane, J. Membr. Sci. 300 (2007) 165. [4] R. Singh, Characteristics of a chlorine-resistant reverse osmosis membrane, Desalination 95 (1994) 27.

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