Desalination 275 (2011) 252–259

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Desalination

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Surface modification of a commercial thin film composite polyamide reverse osmosismembrane by carbodiimide-induced grafting with poly(ethylene glycol) derivatives

Guodong Kang a, Haijun Yu a, Zhongnan Liu a,b, Yiming Cao a,⁎a Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian 116023, Chinab Graduate University of Chinese Academy of Science, Beijing 100049, China

⁎ Corresponding author. Tel.: +86 411 84379053; faE-mail address: ymcao@dicp.ac.cn (Y. Cao).

0011-9164/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.desal.2011.03.007

a b s t r a c t

a r t i c l e i n f o

Article history:Received 24 January 2011Received in revised form 1 March 2011Accepted 2 March 2011Available online 4 May 2011

Keywords:Polyamide reverse osmosis membraneSurface modificationCarboxylic acid groupsCarbodiimide-induced graftingAntifouling

Based on the existing carboxylic acid groups on surface of used commercial thin film composite polyamidereverse osmosis membrane, two kinds of poly(ethylene glycol) derivatives with different chains were graftedby carbodiimide-induced method to improve the antifouling property. The membranes before and aftermodification were characterized by attenuated total reflectance Fourier transform infrared spectroscopy,scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy and static contactangle measurement. Moreover, the water flux, NaCl rejection and fouling experiments were also conducted.The changes of chemical composition and morphology on membrane surfaces indicated the successfulgrafting of poly(ethylene glycol) derivatives. Compared to unmodified membrane, the modified membraneswere more resistant to fouling in protein and cationic surfactant feeding solutions. The method in this studyprovided a new way for surface modification of commercial polyamide reverse osmosis membranes.

x: +86 411 84379906.

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, reverse osmosis (RO) membrane technology hasbeen widely used in industrial water treatment and desalination [1,2].At present, most commercially available RO membranes are of thinfilm composite type, where a dense aromatic polyamide barrier layeris formed on a microporous support such as polysulfone by aninterfacial polymerization process [3]. These membranes exhibitexcellent performance and superior economics. However, one of theissues limiting their use is the proneness of fouling [4,5]. Therefore, anumber of studies have been conducted to mitigate this problemincluding pretreatment processes, designing special modules anddevelopment of antifouling membranes. Meanwhile, the surfacemodification of commercial RO membrane is considered as aneffective and relatively economic route [6,7]. So far, the reportsabout the surface modification methods of RO membranes rangedfrom physical adsorption [8], coating [9,10], in-situ redox initiatedgrafting [11,12], free-radical graft polymerization [13], plasmapolymerization [14] and chemical coupling [15]. In general, thepurpose of these methods is to introduce hydrophilic layer orhydrophilic molecular chains onto RO membrane surface to improvethe antifouling properties.

Among the hydrophilic monomers applied for surface modification,poly(ethylene glycol) (PEG) and its derivates have already been

comprehensively studieddue to their extraordinary antifoulingabilities.PEG is an unchargedwater-soluble polymer having good hydrophilicity,flexible long chains, large exclusion volume and unique coordinationwith surrounding water molecules in an aqueous medium. It is foundthat the surface-bounded PEG molecules can be very effective inpreventing adsorption of hydrophobic or large molecules frommembrane surface, which was advantageous to reducing membranefouling [16]. In recent years, the surface modification of membranes bythe immobilization of PEG chains or protective layer has been used inultrafiltration [17,18], nanofiltration [19] and RO [11,12,15,20–23]. Forexample, Belfer and Freger et al. studied redox-initiated radical graftingmethod of poly(ethylene glycol) methacrylate (PEGMA) onto commer-cial polyamide RO membrane surfaces to improve fouling resistance[11,12]. Van Wagner et al. modified polyamide RO (XLE) andnanofiltration (NF90) membranes by grafting poly(ethylene glycol)diglycidyl ether (PEGDE) to their top surfaces from aqueous solution[20]. In the previous study, we also developed a novel surfacemodification method of nascent polyamide RO membrane by graftingaminopoly(ethylene glycol) monomethylether [15]. In general, themodified membranes exhibited improved fouling resistance and animproved ability to be cleaned after fouling compared to unmodifiedmembranes.

As we know, some commercial thin film composite polyamide ROmembranes prepared from 1,3,5-benze-netricarbonyl trichloride(TMC) and m-phenylene-diamine (MPD) such as FT-30 containcarboxylic acid groups on the surface [24]. These reactive groupsprovide the possibility of surfacemodification of these ROmembranesto improve the antifouling properties. On the other hand, theactivation of carboxylic acid groups by coupling reagent such as 1-

253G. Kang et al. / Desalination 275 (2011) 252–259

ethy-3-(3-dimethyl amidopropyl) carbodiimide (EDC) has been usedin organic synthesis [25] and chemical grafting [26,27]. Combiningwith these two aspects, a surface modification method of commercialthin film composite polyamide RO membrane by EDC-inducedgrafting with hydrophilic PEG derivatives was developed in thisstudy. The chemical composition and morphology of membranesurface before and after modification were investigated usingattenuated total reflectance Fourier transform infrared spectroscopy(ATR-FTIR), X-ray photoelectron spectroscopy (XPS), scanning elec-tron microscopy (SEM) and atomic force microscopy (AFM). Theperformance and fouling resistance of modified and unmodifiedmembranes were also compared.

2. Experimental

2.1. Membranes and reagents

The RO membranes used in this work were kindly supplied byHangzhou Beidouxing Membrane Co., Ltd., China, which werefabricated by interfacial polymerization using TMC and MPD onpolysulfone support. 1-Ethy-3-(3-dimethyl amidopropyl) carbodii-mide (EDC) was purchased from Shanghai Medpep Co., Ltd. The PEGderivates with end amine groups called Jeffamine ED600 andJeffamine ED2001 (abbreviated to ED600 and ED2001) were pur-chased from Hunstman Company Limited and used for surfacemodification. The chemical structures of EDC, ED600 and ED2001were illustrated in Fig. 1. All reagents used in this work were ofanalytical grade.

2.2. Surface modification of polyamide RO membrane

200 mg of EDC was dissolved in 200 mL of sodium citrate buffersolution (pH 4.7) to prepare a 0.1 wt.% EDC aqueous solution. Thethoroughly washed ROmembrane (6 cm×6 cm)was immersed in theEDC solution at 4 °C for 3 h to activate the carboxylic acid groups onmembrane surface. The membrane was gently washed twice withdeionized water and then incubated in an aqueous solution contain-ing an excessive amount of ED600 or ED2001 at 4 °C for 24 h. Finally,the modified membrane was thoroughly washed with deionizedwater to remove the residual adsorbed PEG derivative, and thenstored in deionized water for use. The schematic diagram showingsurface modification of polyamide RO membrane by EDC-inducedgrafting with PEG derivates was represented in Fig. 2.

2.3. Surface characterization

2.3.1. ATR-FTIR analysisThe Fourier transform infrared spectroscopy (Equinox 55) with an

ATR unit (ZnSe crystal, 45°) was employed to investigate the changeof chemical composition between unmodified and modified mem-

N

H3C

H3C

CH2CH

2CH

2 N NCH2CH

3 . HClC

(a) EDC

O

O

Ox y z

(b) ED600 and ED2001

CH3

CH3

CH3

NH2

H2N

Fig. 1. Chemical structure of (a) 1-ethy-3-(3-dimethyl amidopropyl) carbodiimide(EDC); (b) ED600 (x+z=3.6, y=9.0) and ED2001 (x+z=6.0, y=39.0).

branes and confirm the grafting of PEG derivatives onto ROmembranesurface. The membrane samples were dried in vacuum oven beforeanalysis. IR spectra of the membranes were recorded in the range ofwavenumber 800–4000 cm−1 at 25 °C.

2.3.2. SEM and AFM analysesThe surface morphology of unmodified and modified RO mem-

branes was observed by SEM (Philips XL30E) and AFM (DigitalInstruments Inc., Santa Barbara) using a Nanoscope III equipped with1553D scanner. The tapping mode in air was preferable and used totake AFM images [12]. The AFM images of 10 μm scans were acquiredby scanning the sample in air under ambient laboratory conditions ata scan rate of 2 Hz. The roughness of the membrane surface wasassessed by measuring roughness parameters.

2.3.3. XPS analysisXPS spectra were recorded on a Leybold LHS-12 ESCA unit

employing Mg Kα excitation radiation (1253.6 eV). The X-ray sourcewas run at a power of 200 W (10.0 kV, 20 mA). The surface elementalstoichiometries were determined from peak area ratios after correct-ing with experimentally determined instrumental sensitivity factors.

2.3.4. Static contact angle measurementThe static contact angle of membranes was measured using a

contact angle goniometer (JC2000C Contact Angle Meter, PowereachCo., Shanghai, China) by the sessile drop method as described inliteratures [28]. An average value of five measurements was taken.

2.4. Performance measurement and fouling experiment

The performance measurement and fouling experiment of un-modified and modified ROmembranes were tested with a laboratory-scale crossflow test unit. The salt rejection (R) and permeation flux (F)of the membrane were calculated as Eqs. (1) and (2):

R = 1−Cp = Cf

� �× 100% ð1Þ

F = V = At ð2Þ

where Cp and Cf were the salt concentration of permeate solution andfeed solution, V was the volume of permeate solution during the testtime, A was the effective membrane area, and t was the test time,respectively.

In fouling experiment, each membrane was compacted withdeionized water until the permeate flux became constant. The purewater flux was recorded. Then, the reservoir was emptied and thecontaminant solution was poured. The time-varying permeate fluxwas measured. After 160 min, the membrane was rinsed withdeionized water and the flux recovery was measured. Data in foulingexperiment were normalized to make the comparison more straight-forward. The value of relative flux (%) was calculated by comparingthe water flux before and after fouling or cleaning.

3. Results and discussion

3.1. ATR-FTIR spectroscopy

ATR-FTIR spectroscopy can provide a convenient and effective wayto determine the composition of outmost part in a thin film compositeRO membrane. It was performed in this study to confirm the successfulgrafting reaction of PEG chains onto membrane surface. Since thethickness of polyamide barrier layer was about 0.2–0.3 μm [24], whichwas thinner than the penetration depth of ATR-FTIR (about 0.5–1.0 μm)[12,29], the spectra of composite membranes showed characteristicpeaks of bothpolyamide layer (suchas1660 cm−1 and1544 cm−1) and

Fig. 2. Schematic diagram showing surface modification of polyamide RO membrane by EDC-induced grafting with PEG derivates.

254 G. Kang et al. / Desalination 275 (2011) 252–259

polysulfone support membrane (such as 1584 cm−1 and 1243 cm−1).By comparing the IR spectrum between unmodified and modifiedmembranes in Fig. 3, the latter showed some new or intensity increasedpeaks at 943 cm−1, 1080 cm−1 and 2875 cm−1,whichwere ascribed toCH2 rock and C―C stretch, C―O and C―C stretch, and the CH2

symmetric stretchof PEG, respectively [30]. These results suggested thatthe PEG derivatives were successfully immobilized onto the surface ofcommercial RO membrane by carbodiimide-induced grafting method.

3.2. SEM and AFM analyses

In order to examine the morphological changes of RO membranesbefore and after surface modification, the SEM observations wereconducted with a magnification of 2500 and 10,000. The images inFig. 4(a)–(f) presented the surface structure of unmodified mem-brane, ED600 modified membrane and ED2001 modified membranewith different magnifications, respectively. It could be seen that somenew regular nodules appeared on modified membranes compared tothe control membrane. Moreover, this phenomenon was moredistinct for long-chain ED2001 than that of short-chain ED600. Thiswas probably due to the connection or aggregation of grafted PEGchains in dry state. In order to more intuitively observe the stereostructure of membrane surface, the AFM images were also performed.

Fig. 5 showed the images of three-dimensional 10 μm scans forunmodified and PEG-modified RO membranes. As shown in Fig. 5(a),the unmodified membrane exhibited a unique and characteristicridge-and-valley structure consistent with that in Ref. [24]. Aftersurface grafting with PEG chains, some regions were covered withnew nodules. The similar morphological changes were also observedwhen PEG-like monomers were grafted onto RO membranes byplasma polymerization [14]. Meanwhile, the results showed anincrease in the roughness of membrane surface after modification,

1080cm-1

943cm-12873cm-1

Ab

sorb

ance

Wavenumber / cm-1

ED2001 modifiedED600 modifiedUnmodified

3600 3200 2800 1600 1400 1200 1000 800

Fig. 3. ATR-FTIR spectra of unmodified and modified polyamide RO membranes.

from 78 nm to 100 nm and 111 nm, respectively. The results were inagreement with those conducted by some other researchers. Forexample, Van Wagner et al. observed a 19–33% increase in surfaceroughness upon modification with PEGDE [20]. Zou et al. grafted aPEG-like hydrophilic polymer (trimethylene glycol dimethyl ether)onto RO membrane by plasma polymerization and found that, theroughness increased to 89.3 nm after 60 s treatment from 61.9 nm ofthe unmodified membrane [14].

The analytical results of SEM and AFM confirmed again that thePEG chains were successfully grafted onto the surface of commercialthin film polyamide RO membrane.

3.3. XPS analysis

XPS is particularly well suited for examining the chemicalcomposition of skin layer in RO composite membranes, and thequantitative elemental composition of the top-most layer of mem-brane sample can be calculated from XPS scanning spectrum. Table 1presented the atomic concentration data of unmodified membrane,ED600, ED600 modified membrane, ED2001 and ED2001 modifiedmembrane. Since ED600 and ED2001 contained higher oxygen andlower nitrogen, the increases in oxygen content and decreases innitrogen content after modification were observed. Meanwhile, thecarbon content in ED600 and ED2001 was lower than that inunmodified membrane, so the carbon content in modified mem-branes decreased. Consequently, the ratio of O/C also increased. Thesetrends have also been observed in our previous study [15] and by VanWagner et al. [20].

According to the membrane manufactures, the RO membranesample used in this study was fully aromatic polyamide compositemembrane. The polyamide was formed by an interfacial polymeriza-tion process based on TMC and MPD as shown in Fig. 6. Where, whenn=1, the resulting polymer is fully crosslinked, giving a molecularformula of C18H12N3O3. When n=0, the resulting polymer is linearwith pendant COOH, giving a molecular formula of C15H10N2O4 [3,29].Therefore, the value of n in unmodified RO membrane could beobtained based on the data in Table 1 using Eqs. (3), (4) and (5):

O %ð ÞC %ð Þ =

3n + 4 1−nð Þ18n + 15 1−nð Þ =

17:072:5

ð3Þ

O %ð ÞN %ð Þ =

3n + 4 1−nð Þ3n + 2 1−nð Þ =

17:010:5

ð4Þ

N %ð ÞC %ð Þ =

3n + 2 1−nð Þ18n + 15 1−nð Þ =

10:572:5

: ð5Þ

It could be calculated that n1=0.283, n2=0.291, and n3=0.305.Therefore, the average n=0.293 was obtained. In other words, thetop-most polyamide layer in used composite ROmembrane containedabout 70.7% of linear structure with pendant COOH, which wasaccordant with that mentioned above and Ref. [24].

Fig. 4. SEM micrographs of membranes before and after surface grafting.Unmodified: (a) ×2500, (b) ×10,000; ED600 modified: (c) ×2500, (d) ×10,000; ED2001 modified:(e) ×2500, (f) ×10,000.

255G. Kang et al. / Desalination 275 (2011) 252–259

Meanwhile, the grafting rate of carboxylic acid (i.e., the proportionof carboxylic acid reactedwith PEG derivates) inmodifiedmembranescould be approximately estimated. The molecular formula of ED600and ED2001 were C31.8H67.6N2O12.6 and C99H202N2O45, respectively. Ifxwas the grafting rate of carboxylic acid, then 1−xwas the unreactedcarboxylic acid groups. Based on the measured chemical compositionof modified membranes, the following equations could be obtained:ED600 modified membrane:

O %ð ÞC %ð Þ =

3 × 0:293 + 4 × 0:707 × 1−xð Þ + 3 + 12:6ð Þ × 0:707 × x18 × 0:293 + 15 × 0:707 × 1−xð Þ + 31:8 + 15ð Þ × 0:707 × x

=21:270:5

ð6Þ

ED2001 modified membrane:

O %ð ÞC %ð Þ =

3 × 0:293 + 4 × 0:707 × 1−xð Þ + 3 + 45ð Þ × 0:707 × x18 × 0:293 + 15 × 0:707 × 1−xð Þ + 99 + 15ð Þ × 0:707 × x

=22:070:3

:

ð7Þ

It could be calculated that x1≈0.74 and x2≈0.14. In other words,about 74% of carboxylic acid groups on RO membrane surface reactedfor ED600 modified membrane, but only about 14% for ED2001modified membrane. The significant difference of grafting rate forED600 and ED2001 was probably due to their different chain lengths.Since the repulsion effect of initially grafted PEG chains in liquidsurroundings would hinder the further grafting reaction around, theED2001 with longer molecular chain had more distinct steric effect,resulting in lower grafting rate.

3.4. Performance test

The pure water flux and NaCl rejection of unmodified and PEG-modified membranes were presented in Table 2. Similar to theresearch results conducted by Belfer et al. [11] and Zou et al. [14], thesurface modification by grafting in this study also caused decreases inwater flux (decreased by about 35% in this study, and 36% in Ref. [11]and 18% in Ref. [14] after modification for 60 min using methacrylic

Fig. 5. Three-dimensional AFM images of unmodified and modified RO membranes: (a) unmodified; (b) ED600 modified; and (c) ED2001 modified. The size of images is10 μm×10 μm. The average roughnesses are: (a) 78 nm; (b) 100 nm; and (c) 111 nm.

256 G. Kang et al. / Desalination 275 (2011) 252–259

Table 1Chemical composition of grafting PEG derivatives, unmodifiedmembrane andmodifiedmembranes (mol%).

Sample C (%) O (%) N (%) O/C

ED600 (theoretical value) 68.5 27.2 4.3 0.399ED2001 (theoretical value) 67.8 30.8 1.4 0.454Unmodified membrane 72.5 17.0 10.5 0.234ED600 modified membrane 70.5 21.2 8.3 0.302ED2001 modified membrane 70.3 22.0 7.7 0.312Totally crosslinked 75.0 12.5 12.5 0.167Linear with pendant COOH 71.5 19.0 9.5 0.266

Table 2Pure water flux, salt rejection and static contact angle of membranes before and aftermodification.

Membrane Pure water fluxa,(L/m2h)

Salt rejectionb,(%)

Static contactangle (°)

Unmodified membrane 50±4 96.7±0.4 53±2ED600 modified membrane 32±3 96.4±0.2 42±2ED2001 modified membrane 36±3 96.5±0.4 38±3

a Testing condition of pure water flux: deionized water, 1.05 MPa, 25 °C.b Testing condition of salt rejection: 1500 ppm of NaCl solution, 1.05 MPa, 25 °C.

257G. Kang et al. / Desalination 275 (2011) 252–259

acid and triglyme, respectively). The decrease of permeation flux wasdue to the increased resistance of grafted layer which affected thepressure drop of membrane. However, compared to some coatingprocesses [9,10], the surface grafting method often showed relativelylower decrease in water flux.

On the other hand, since the grafting method in this study wasconducted in a very mild aqueous environment without any dryingprocess or damaging effect, the NaCl rejection of ROmembrane had nosignificant change after surface modification, which was also reportedby many researchers [11,14,23]. For example, NaCl rejection of ROmembrane changed from 98.5% to 98.0% in Ref. [14], and from 99.0±0.3% to 99.3±0.5% in Ref. [23] after surface modification usingtriglyme and PEGDA, respectively. However, unlike some othergrafting methods such as UV-initiated polymerization or plasmapolymerization, this method needed no special equipments. More-over, it was very simple and easily operated, providing a new way forsurface modification of commercial polyamide RO membranes.

3.5. Fouling experiments

In this study, 100 ppm of milk solution (for simulating proteinsolution) and 100 ppm of dodecyltrimethylammonium bromide(DTAB, a cationic surfactant) solution were used for fouling experi-ments. Protein solution and DTAB solution were commonly used as

O

C

O

C O

HN NH C

nNH

NH

COCl

ClOC COCl

+

TMC

Fig. 6. Interfacial polymerization of commercial polyamid

contaminants in antifouling tests of RO membranes [15,22,23,31].According to the experimental method, the fouling and cleaningresults were summarized in Fig. 7(a) and (b).

It could be clearly seen that, the modified membranes showedrelatively better antifouling properties than those of unmodifiedmembrane. Through surface modification, the membranes exhibitedsmaller flux decrease after fouling and higher flux recovery aftercleaning. It could be explained as follows: (1) The grafting ofhydrophilic PEG derivatives enhanced the hydrophilicity of mem-brane surface, which was in good agreement with the static contactangle measurement in Table 2. The enhancement of surface hydro-philicity was advantageous to improvement of antifouling perfor-mance [17,18,32]. (2) The grafted PEG chains presented a good stericrepulsion effect in liquid surroundings, which was helpful to restrainand prevent the adsorption of protein onto membrane surface. Thiswas also reported by Che et al. [26] and Wang et al. [33]. (3) Themodification process eliminated a portion of carboxylic acid groups byreaction with ED600 and ED2001, thus the surface charge of modifiedmembrane was less negative, reducing the electrostatic attractionwith DTAB having positive charge. Similarly, Hachisuka et al. coatedelectric neutral polyvinyl alcohol onto polyamide RO membrane tocreate a neutral membrane surface, and the treated membraneshowed better tolerance to water containing a surfactant [9].

However, it should bementionedhere that themodifiedmembranesexhibited larger surface roughness, which may be disadvantageous to

HN NH C

O

C

O

C O

OH 1-n

H2N NH2

MPD

e RO membrane dense layer based on TMC and MPD.

20

40

60

80

100

120

Milk solution Cleaning WaterWater

Rel

ativ

e F

lux

(%)

0

20

40

60

80

100

120

Rel

ativ

e F

lux

(%)

Time (min)

ED2001 modifedED600 modifedUnmodified

b

0

DTAB solution Cleaning WaterWater

ED2001 modifiedED600 modifiedUnmodified

50 100 150 200 250 300

Time (min)0 50 100 150 200 250 300

a

Fig. 7. Fouling and cleaning experiments of membranes before and after modification:(a) milk solution; and (b) DTAB solution.

258 G. Kang et al. / Desalination 275 (2011) 252–259

the improvement of colloidal fouling according to the research resultsby Elimelech et al. [34].

4. Conclusion

A novel surface modification method of commercial thin filmcomposite polyamide RO membrane by EDC-induced grafting withhydrophilic PEG derivatives was developed in this study. The FTIR-ATR,SEM, AFM and XPS analyses confirmed the successful immobilization ofPEG chains onto polyamide RO membrane surface. The hydrophilicitywas relatively enhanced after modification. Surface modification in thisstudy caused acceptable decreases in pure water flux but no significantchange in NaCl rejection. Compared to unmodified membrane, themodified membranes were more resistant to fouling in protein andcationic surfactant feeding solutions. The surface modification methodin this studywas conducted in amild aqueous environmentwithout anydrying process or damaging effect. Moreover, it was simple, easilyoperated and needed no special equipments, which were all veryadvantageous to industrial applications.

Acknowledgement

Financial support from the National Natural Science Foundation ofChina (Grant No. 20906086) is gratefully acknowledged.

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