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Journal of Membrane Science 370 (2011) 116–123

Contents lists available at ScienceDirect

Journal of Membrane Science

journa l homepage: www.e lsev ier .com/ locate /memsci

urface modification of thin-film-composite polyamide membranes for improvedeverse osmosis performance

ia Xua, Xianshe Fengb, Congjie Gaoa,∗

Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education; College of Chemistry and Chemical Engineering, Ocean University of China,38 Songling Road, Qingdao, Shandong 266100, ChinaDepartment of Chemical Engineering, University of Waterloo, Waterloo, ON, Canada N2L 3G1

r t i c l e i n f o

rticle history:eceived 14 August 2010eceived in revised form9 November 2010ccepted 3 January 2011vailable online 12 January 2011

eywords:urface modification

a b s t r a c t

In this study, a novel process to modify thin-film-composite polyamide membrane was developed. Itinvolved surface treatment of the polyamide membrane by chlorine, followed by supramolecular assem-bly of chitosan on the membrane surface. While the original polyamide membrane was negativelycharged, the surface modification led to a charge reversal due to skin layer of chitosanium in the resultingpolyamide/chitosan composite membrane. The polyamide/chitosan (PA/CS) composite membrane wasshown to perform better than the original polyamide membrane. The parameters involved in the chlo-rination of polyamide (e.g., chlorination time and chlorine concentration) and supramolecular assemblyof chitosan (e.g., concentration and deposition time) on the membrane surface were investigated. The

hlorinationhitosanolyamideeverse osmosis

following conditions for membrane modification were found to be appropriate: NaClO concentration200 mg/L, chlorination time 2–5 min, and chitosan concentration 1000 mg/L; the PA/CS membrane soformed exhibited a permeation flux of 57.7 L/(m2 h) and a salt rejection of 95.4% for a feed NaCl con-centration of 1500 mg/L at 0.8 MPa. The PA/CS composite membrane also exhibited good performancefor rejection of divalent salts (99.8% for MgCl2 and 98.5% for Na2SO4) at the same concentration. Thismodification technique is simple and practical because dilute solutions are used for surface treatment

ne un

and commercial membra

. Introduction

Composite membranes generally prepared by interface poly-erization or surface functionalization (e.g., grafting, coating and

rosslinking [1]) have been widely used for reverse osmosis,anofiltration, gas separation and pervaporation [2–7]. In spite offforts in developing composite membranes based on pore fillingr surface functionalization of the pores, thin-film composite (TFC)embranes are currently still the primary membranes for various

pplications, especially for reverse osmosis and nanofiltration [8].ypically, a TFC membrane is comprised of an active skin layer,hich essentially determines the performance of the membrane in

erms of permeability and selectivity, and a microporous substratehat provides the mechanical strength to the membrane. Obviously,esearch focus on suitable polymer materials and appropriate man-

facturing techniques for producing an ultrathin, defect-free skin

ayer on a compatible, robust substrate is essential to advancementf TFC membranes.

∗ Corresponding author. Tel.: +86 532 66781872; fax: +86 532 66781872.E-mail addresses: qdxujia2007@gmail.com (J. Xu), gaocjie@mail.hz.zj.cn (C. Gao).

376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2011.01.001

its can be modified in their original modules.© 2011 Elsevier B.V. All rights reserved.

Supramolecular assembly of polyelectrolytes based on electro-static layer-by-layer (LBL) deposition is a convenient and promisingapproach for fabricating TFC membranes. This technique not onlycreates a charged skin layer but also allows for a better controlof the thickness, charge density and hydrophilicity of the activeskin layer [9]. As a result, the LBL polyelectrolyte membranes haverecently attracted significant attention for use in pervaporation[10–12], reverse osmosis [2,13] and nanofiltration [14–16] pro-cesses. There is, however, a technical challenge that limits theindustrial acceptance of such membranes due to the large numberof alternating depositions of oppositely charged polyelectrolytes ona porous substrate in order for the membrane to become perms-elective enough [10,11], which makes the membrane fabricationprocess very tedious and time consuming. One potential solution tothis issue is to combine the LBL self-assembly with interlayer cross-linking. For example, Sullivan and Bruening [17] used poly(amicacids) and polyamines as polyelectrolytes for LBL self-assembly,followed by crosslinking and imidization, and the resulting mem-

brane consisting of 12.5 bilayers showed a permeation flux of2 kg/(m2 h) and a selectivity of 6100 for pervaporation dehydra-tion of 90% isopropanol at 50 ◦C. Here a bilayer is referred to asa supramolecular layer from a pair of cationic and cationic poly-electrolytes. Another solution is to facilitate the polyelectrolyte

J. Xu et al. / Journal of Membrane Science 370 (2011) 116–123 117

OC CO

HN

HNCO

COOH

CO

HN

NaClO

OC

CO

CO

HNCO

COOH

CO

NHNCl

NaClO

OC

COOH

COOH

+ +

Cl

HN

O

HN

O

Cl

CO

OC

COOH

CO

+

Excessive

of ar

dgoofao[

ppiotunptatmmto

fAciatbaommfwiHdaswacttp

chlorination

Fig. 1. Chlorination

eposition onto the substrate under an electric field or pressureradient rather than under the sole electrostatic forces betweenppositely charged polyelectrolytes [18–23]. A separation factorf 1075 and a permeation flux of 4.05 kg/(m2 h) were reportedor separation of an isopropanol–water mixture (90%/10%, w/w)t 70 ◦C by pervaporation using a membrane having 4.5 bilayersf polyethyleneimine/poly(acrylic acid) assembled in electric field22].

In spite of the above, membranes with more than two bilayers ofolyelectrolytes are generally still required because of the micro-ores in the substrates. While many efforts have been devoted to

mprove the membrane fabrication process as well as screeningf polyelectrolyes with good performance, little attention is paido the properties of the substrates used. As one may expect, these of nonporous substrates allows for dramatic reduction in theumber of polyelectrolyte depositions needed, but the membraneermeability will be compromised because of their large resis-ance to mass transport [24]. Zhou et al. [25] showed that usingn interfacially polymerized polyamide membrane as a substrate,he deposition of only a single layer of polyethyleneimine on the

embrane surface enhanced salt rejection without lowering theembrane permeability significantly. Especially, the fouling resis-

ance of the membrane to cationic foulants was increased becausef charge reversal on the membrane surface.

In this study, a polyamide/chitosan (PA/CS) TFC membrane wasabricated and investigated for desalination by reverse osmosis.

commercial polyamide TFC membrane was surface modified byhlorine treatment, followed by surface deposition of chitosan. Its well known that the amide bonds are vulnerable to chlorinettack, and controlled chlorination with a dilute hypochlorite solu-ion will not only make the membrane surface more hydrophilicut also lead to a more negative zeta potential [26–29]. This char-cteristics is favorable for electrostatic deposition of a polycationn the membrane surface. Although excessive exposure to chlorineay deteriorate the thin polyamide layer of the TFC membrane, aoderate degree of chlorination can improve the membrane per-

ormance [30]. In the present study, the polyamide TFC membraneas treated with dilute solutions of sodium hypochlorite in antic-

pation to enhance the performance of the resulting membranes.igh molecular weight chitosan was selected as a polycation foreposition onto the polyamide membrane because of its moder-te charge density, high hydrophilicity, mechanical and chemicaltabilities as well as good film forming properties. The membranesere investigated for salt rejection using NaCl, MgCl2 and Na2SO4

s model solutes. It was shown that after surface modification (i.e.,hlorination with hypochlorite and electrostatic deposition of chi-osanium), the membrane performance had improved as comparedo the original polyamide membrane. From an application stand-oint, this approach is practical because both surface chlorination

O O

omatic polyamide.

and polyelectrolyte deposition are done with dilute solutions sothat existing commercial thin-film composite membranes can betreated in their original modules.

2. Experimental

2.1. Materials

A commercially available membrane produced by interfacialpolymerization of m-phenylenediamine and trimesoyl chloride ona microporous polysulfone substrate was used as representativethin-film composite polyamide membrane. It was manufacturedby the Development Center of Water Treatment Technology(Hangzhou, China), primarily for nanofiltration and low-pressurereverse osmosis applications. Chitosan (MW = 540 kDa, degree ofdeacetylation = 90%) was purchased from Haihui Bioengineering(Qingdao, China). Deionized water with a conductivity of less than5 �S/cm was used in membrane preparation as well as in salt rejec-tion experiments. All other chemicals, i.e., hydrochloric acid (HCl),acetic acid, sodium hypochlorite (NaClO, effective chlorine content15%), sodium chloride (NaCl), magnesium chloride (MgCl2·6H2O),and magnesium sulfate (MgSO4·7H2O), were of analytical grade andused as received.

2.2. Chlorination of polyamide membrane

It is well known that aromatic polyamide membranes are sen-sitive to chlorine attack and can be degraded because of thecleavage of the amide-bonds and Orton transition [31,32]. Fig. 1is a schematic diagram illustrating the mechanism of chlorination.In order to improve the long-term stability of the resulting modi-fied membrane, the virgin polyamide membrane was treated withdilute hypochlorite solution in a controlled manner. To this end,sodium hypochlorite works effectively by producing a more neg-atively charged membrane surface, which enhances subsequentelectrostatic adsorption of polycations. Through controlled chlori-nation, while the polysulfone support layer is unaffected, the activepolyamide skin layer becomes thinner and less compact, which issupported by the work of Soice et al. [33] who observed a lossof the barrier layer by extensive chlorination. This helps compen-sate the additional permeation resistance from the polycation layer.The chlorination treatment was carried out by immersing the vir-gin polyamide thin-film composite membrane into dilute aqueousNaClO solutions at predetermined concentrations (200–1000 mg/L)

for a given period of time (0–20 min) at 19 ◦C, and the effect of thedegree of chlorination on the membrane performance was eval-uated. Throughout the study, the NaClO solutions maintained aconstant pH of 5.5, and hydrochloric acid was used to adjust thesolution pH. After chlorination treatment, the membrane was taken

118 J. Xu et al. / Journal of Membrane Science 370 (2011) 116–123

Permeate

Retentate

Feed

Membrane module

Rotameter

P

P

oo

2

actwpwotctadAwpa

2

msstpawtMviwc

J

wpa

R

0 400 800 1200 160040

50

60

70

80

90

100

NaClO concentration (mg/L)

J (L

/(m

2 h))

R (

%)

0

20

40

60

80

100

Feed pump

Feed tank

Fig. 2. Schematic diagram of experimental setup for membrane testing.

ut and rinsed with deionized water thoroughly until the pH valuesf the rinsed water reached 7.0.

.3. Chitosanium deposition on polyamide membrane

Chitosan was dissolved in dilute acetic acid solution undergitation at 70 ◦C to obtain homogeneous solutions at variousoncentrations ranging from 0.1 to 0.5 wt%. In the acidic solu-ion, the amino groups in chitosan were protonated and chitosanas present in the form of chitosanium. The chlorination-treatedolyamide membrane was mounted in the capping assembly of aide-mouth bottle containing chitosan solution and the surface

f the polyamide membrane was brought in contact with the chi-osan solution by placing the bottle upside down. This allows forhitosan deposition onto the polyamide surface. It was observedhat at room temperature (19 ◦C), no additional chitosan would bebsorbed on the polyamide surface after 30 min, and a chitosaneposition time of 30 min was used throughout the experiments.fter chitosan deposition, the membrane was thoroughly rinsedith deionized water, and the polyamide/chitosan (PA/CS) com-osite membrane so obtained was subjected to water permeabilitynd salt rejection evaluations.

.4. Membrane performance

The experimental setup for evaluating the membrane perfor-ance is shown in Fig. 2. It consisted of three identical stainless

teel RO test cells arranged in parallel. The feed solution was pres-urized and admitted to the test cells using a hydraulic pump, andhe retentate was recirculated to the feed tank. The PA/CS com-osite membrane was subjected to pressurization with pure watert a pressure of 0.5 MPa for 1 h prior to permeation experimentsith aqueous salt solutions of different concentrations at room

emperature (19 ◦C) at pressures ranging from 0.8 to 3.0 MPa. NaCl,gCl2 and MgSO4 were chosen as representative solutes for con-

enience of evaluating the membrane performance to reject an ionn the presence of different counter ions. The permeation flux (J)

as determined gravimetrically by weighing the permeate sampleollected over a given period of time,

= Q

A × �t(1)

here Q is the quantity of the permeate sample collected over a

eriod of time (t), and A is the effective membrane area for perme-tion. The salt rejection (R) was evaluated from,

(%) =(

CF − CP

CF

)× 100 (2)

Fig. 3. Effects of chlorine concentration on the permeation flux and solute rejectionof chlorinated polyamide membrane. Chlorination time 2 min, temperature 19 ◦C.RO operating conditions: pressure 0.8 MPa, temperature 19 ◦C, feed NaCl concentra-tion 1500 mg/L.

where CF and CP are the salt concentrations in the feed and per-meate, respectively. They were measured by a conductivity meter(Thermos Orion 145A+). This study was focused on steady-statepermeation, and the permeation was considered to have reachedsteady state when the permeation flux and permeate concentrationbecame constant. During any experimental runs, a substantiallylarge quantity of feed was used, while the quantity of the permeatesamples collected was negligibly small; consequently, a constantfeed concentration was maintained. All experiments were repeatedat least twice, and the data presented in this study were averagevalues with a batch-to-batch variation of within 5%.

3. Results and discussion

3.1. Controlled chlorination of polyamide membrane

Since the chitosan is deposited onto the polyamide mem-brane by supramolecular assembly under electrostatic forces, thepolyamide membrane should preferably possess a high chargedensity. In addition, because the chitosan layer will contributeto an additional resistance to permeation, it is desirable for thepolyamide substrate to be less compact. Thus, effects of the extentof chlorination (e.g., NaClO concentration and chlorination time)on the RO performance of the polyamide substrate were investi-gated to establish suitable conditions for the chlorine treatment.Fig. 3 shows the RO performance of the resulting polyamide sub-strate membrane for 1500 mg/L NaCl solution after treatmentwith NaClO at various concentrations for a fixed period of chlo-rination (i.e., 2 min). For the purpose of comparison, the originalpolyamide membrane without any chlorine treatment was alsotested. The original polyamide membrane exhibited a permeationflux of 50.3 L/(m2 h) and a salt rejection of 91.0% for a feed solutioncontaining 1500 mg/L NaCl at 0.8 MPa. Such a polyamide mem-brane is not considered to be an ideal substrate for subsequentdeposition of polyelectrolytes due to its relatively high perme-ation resistance. However, after chlorine treatment with 200 mg/Lof NaClO for 2 min, the permeation flux was increased by 16% whilethe salt rejection was lowered by about 2% under the same operat-ing conditions. This indicates that the skin layer of the polyamidemembrane is loosened by the chlorine treatment due to cleavage

of macromolecule chains, but the chlorine attack on the skin layerdid not go far inside the membrane to open up the pores. Thisis different from hydrolysis of a poly(acrylonitrile) ultrafiltrationmembrane [34], where the hydrolyzed membrane was shown to be

rane Science 370 (2011) 116–123 119

ms1w8bhcsicu

epbpf2api

0 4 8 12 16 2040

50

60

70

80

90

100

Chlorination time (min)

J (L

/(m

2 h))

0

20

40

60

80

100

R (

%)

Fig. 4. Effects of chlorination time on the permeation flux and solute rejection

F

J. Xu et al. / Journal of Memb

ore compact, resulting in a lower permeation flux and a higheralt rejection. As the NaClO concentration increased from 200 to500 mg/L, a further enhancement in the permeation flux of 25%as observed, but the salt rejection dropped considerably (from

9% to 67%), suggesting that the interior of the skin layer woulde attacked significantly as the chlorine dosage was sufficientlyigh. This was confirmed that a further increase in the NaClO con-entration would lead to excessive degradation in the polyamideubstrate membrane, causing a significant reduction in its mechan-cal strength and integrity. In consideration of the above, a NaClOoncentration of 200 mg/L was considered appropriate and it wassed for polyamide chlorination in subsequent studies.

The duration of chlorination is also an important factor to influ-nce the membrane. The effects of chlorination time on the ROerformance of the chlorine-treated polyamide substrate mem-rane are shown in Fig. 4, where the performance of the originalolyamide membrane (i.e., chlorination time zero) is also shown

or comparison. When the chlorination time was increased from

to 7 min, the permeation flux increased rapidly to 75.2 L/(m2 h)nd the salt rejection slightly decreased to 86.3%. As expected,rolonging the chlorination time will lower the salt rejection and

ncrease the permeation flux because the chlorine molecules grad-

ig. 5. AFM images of the polyamide membrane surface (a) before chlorination (i.e., origi

of chlorinated polyamide membrane. NaClO concentration 200 mg/L, temperature19 ◦C. Operating conditions for RO tests are the same as shown in Fig. 3.

nal PA membrane), (b) after chlorination, and (c) chitosan modified membrane.

120 J. Xu et al. / Journal of Membrane S

0 4 8 12 16 2040

50

60

70

80

75

80

85

90

95

100

Chlorination time (min)

J (L

/(m

2 h))

R (

%)

Fig. 6. Effects of chlorination time of polyamide membrane on the RO performanceoct

umlttegatcnricomc

micFtamsmnasna((0dFbflteesf

on the PA membrane surface provides additional mass transfer

f the resulting PA/CS composite membranes. Other chlorination conditions: NaClOoncentration 200 mg/L, temperature 19 ◦C. Chitosan concentration 1000 mg/L. ROest conditions are the same as shown in Fig. 3.

ally reach and attack the interior of the skin layer of the polyamideembrane. However, when the chlorination time was sufficiently

ong (>13 min), a further increase in the chlorination time had lit-le impact on both the permeation flux and the salt rejection. Athis point, the surface of the polyamide skin layer was partiallyxfoliated by chlorine. This can be shown by the atomic force micro-raphs (AFM) of the surface of the polyamide membrane before andfter chlorine treatment (Fig. 5). These results are consistent withhe work of Soice et al. [33], who observed a significant morphologyhange that led to loss of barrier layer during extensive chlori-ation. Fig. 5 shows that the membrane surface becomes slightlyougher when chlorinated with hypochlorite. In a study of chem-cal/morphological changes, Kwon and Leckie [28] observed thathlorination replaced hydrogen with chlorine on the amide groupf the membrane, resulting in an increased flexibility of the poly-er chains and thus a flux shifting with time. The AFM image of

hitosan modified membrane is also illustrated in Fig. 5.To investigate the effects of chlorination degree of PA substrate

embrane on the performance of the resulting PA/CS compos-te membranes, a thin layer of chitosan was deposited onto thehlorinated PA membranes, and their RO performance was tested.ig. 6 shows the experimental data obtained. For chitosan deposi-ion onto the PA membrane, the chitosan solution was maintainedt a constant value of 1000 mg/L. If the original un-chlorinated PAembrane was used as substrate, the PA/CS composite membrane

howed a permeation flux of 5.3% lower than that of the original PAembrane, while the salt rejection was increased by 2.8%. This is

ot surprising as the top skin layer of chitosanium will contributen additional resistance to permeation and thus help enhance theize sieving effect in spite of the higher hydrophilicity of chitosa-ium than polyamide. Zhou et al. [25] have also reported that whenvirgin PA membrane was surface modified by polyethyleneimine

concentration 2000 mg/L), a slight increase in the salt rejection4.3%) was observed for desalinating a 90 mg/L NaCl solution at.8 MPa, which was, however, compromised by a more obviousecline in the permeation flux (36.2%). Interestingly, the data inig. 6 show that when the PA membrane was treated by NaClOefore being surface coated with chitosan, both the permeationux and salt rejection were enhanced. This demonstrates the effec-iveness of chlorine treatment of the PA membrane for surface

tching followed by supramolecular assembly of a polycation tonhance the RO performance. For example, when the PA sub-trate was chlorinated with NaClO at a concentration of 200 mg/Lor a period of 2 min, the resulting PA/CS composite membrane

cience 370 (2011) 116–123

showed a permeation flux of 54.3 L/(m2 h) and a salt rejection of94.5%. This represents a 7.9% increase in permeability and a 3.8%increase in salt rejection as compared to the original PA membrane.The enhanced performance may be attributed to the followingaspects: (1) the top surface layer of chitosanium is more hydrophilicthan polyamide, which favors water permeation. (2) The surfacedeposition of chitosan on the PA substrate is accomplished bysupramolecular assembly, and the surface layer so obtained can beas thin as 0.5–3 nm [35]. The added resistance due to the chitosanlayer can be estimated to be 5.3% of the original PA membrane basedon the permeation flux. On the other hand, the surface of the PA sub-strate is ablated by chlorination, which will reduce the membraneresistance. As shown earlier, the resistance of the PA membranewill decrease by 14% after chlorination with 200 mg/L of NaClO for2 min. Apparently, the reduction in the membrane resistance due toPA chlorination is more significant than the resistance caused by thechitosan layer. Note that because of the positively charged surfacelayer of the PA/CS composite membrane, the membrane is expectedto perform better for rejecting polyvalent cations than the originalnegatively charged PA membrane, and this will be discussed laterin more details.

The data in Fig. 6 also show that both the permeation flux andsalt rejection began to decrease after a certain period of chlorina-tion time and eventually became constant when the chlorinationtime was sufficiently long. While the decline in salt rejection is easyto understand in consideration of the surface exfoliation of the PAmembrane, the decrease in permeation flux can be explained as fol-lows: (1) the PA membrane surface is more negatively charged aschlorination continues, which causes more chitosan to be absorbedon the membrane surface due to stronger electrostatic interactionsbetween chitosan macromolecules and the PA membrane surface,and (2) the surface exfoliation of the PA membrane by chlorinationcauses a rougher peak-and-valley structure, which also facilitateschitosan deposition. Nonetheless, the membrane permeability andselectivity are still slightly better than original PA membrane inthe experimental range investigated. It appears that relatively mildconditions (e.g., 200 mg/L NaClO for a duration of 2–7 min) shouldbe used for chlorine treatment of the PA membrane. Unless pointedout otherwise, the PA membranes were chlorinated under suchconditions in fabrication of PA/CS membranes in the following stud-ies.

3.2. Concentration of chitosan deposition solution

To investigate the effectiveness of polyelectrolyte depositionon the surface of the PA membrane substrate to improve theRO performance, a series of PA/CS membranes were fabricatedusing aqueous chitosan solutions at concentrations in the range of1000–5000 mg/L while the chlorination conditions for the PA mem-brane remained the same. The permeation flux and salt rejection ofthe resulting PA/CS membranes are presented in Fig. 7. It is shownthat as the chitosan concentration increases from 0 to 5000 mg/L,the salt rejection increase from 91.0% to 95.3%, whereas the per-meation flux increases initially and then gradually decreases whenthe chitosan concentration is above 1000 mg/L.

The improvement in the salt rejection can be attributed to theincreased size sieving effect due to the thicker and more com-pact packing of chitosan on the membrane surface. As far as thepermeation flux is concerned, the following two opposing effectscan be considered. On the one hand, the chitosan layer deposited

resistance, thereby reducing the permeation flux. On the otherhand, cationic chitosanium is more hydrophilic than PA and theenhanced hydrophilicity of the skin layer favors the sorption ofwater molecules. In dilute chitosan solutions, the macromolecules

J. Xu et al. / Journal of Membrane S

0 1000 2000 3000 4000 500040

45

50

55

60

65

70

Chitosan concentration (mg/L)

J (L

/m2 h)

75

80

85

90

95

100

R (

%)

Fig. 7. Effects of chitosan concentration in the depositing solution on the ROpct

aatbarAwfcomtdpfaFac

solutions. This is due to the increased osmotic pressure of the feed

Fd

erformance of PA/CS composite membranes. PA chlorination conditions: NaClOoncentration 200 mg/L, temperature 19 ◦C, duration 2 min. RO test conditions arehe same as shown in Fig. 3.

re well stretched and dispersed, and thus these molecules formrelatively loose skin layer on the PA membrane substrate. As

he chitosan concentration increases, the chitosan molecules wille less stretched due to the charge balance of counter-ions [21],nd consequently the sealing effect as well as the mass transferesistance will decrease, which yield a higher permeation flux.nd, the surface hydrophilicity of the membrane is enhancedith more chitosan molecules deposited on the membrane sur-

ace. However, with a further increase in the concentration ofhitosan solutions, molecular entanglement and aggregation willccur, leading to a more compact and thicker chitosan layer. In theeanwhile, the surface hydrophilicity of the membrane will essen-

ially be constant when more and more chitosan molecules areeposited on the membrane so that the membrane surface is com-letely covered by chitosan [25]. Similar observations have beenound for self-assembly of poly (styrenesulfonic acid-co-maleic

cid) sodium salt on hydrolyzed polyacrylonitrile membranes [21].or the system studied here, a maximum permeation flux due to theforementioned two opposing effects appears to occur at a chitosanoncentration of around 1000 mg/L.

0.8 1.2 1.6 2.0 2.4 2.8 3.20

40

80

120

160

200

240

Pressure

J (L

/(m

2 h))

(a) Chitosan 1000 mg/L80

84

88

92

96

100

R (

%)

ig. 8. RO performance of the PA/CS composite membranes at different operating pressuuration 2 min. Other RO operating conditions are the same as in Fig. 3.

cience 370 (2011) 116–123 121

3.3. RO performance of PA/CS composite membranes

Operating pressure is an important parameter in RO processbecause of its effect on salt rejection and water flux as well asthe energy consumption of the process. The permeation flux andsalt rejection of a PA/CS membrane prepared using a 1000 mg/Lchitosan solution deposited on the chlorinated PA membrane sub-strate were plotted in Fig. 8(a) as a function of the operatingpressure. The permeation flux appears to be correlated with theoperating pressure in a linear relationship, which agrees with thesolution-diffusion model. The permeability of the composite mem-brane was evaluated from the slope to be 68.1 L/(m2 h MPa). Acomparison with the original PA membrane, whose permeabil-ity was determined to be 62.8 L/(m2 hMPa), indicates that thepermeability is increased by 8.4% with the chlorine treatment sur-face deposition of chitosan. Similar trends were also found to betrue with the PA/CS membrane fabricated using a higher chitosanconcentration (i.e., 3000 mg/L), as shown in Fig. 8(b), though theincrease in the permeability is a little less significant. With regardto the salt rejection, both PA/CS membranes showed similar per-formance. The rejection rate for NaCl increased from 93.3 to 96.5%when the operating pressure increased from 0.8 to 1.4 MPa, anda further increase in the operating pressure did not lead to a sig-nificant change in the salt rejection (97.0–97.5%). It is shown thatan operating pressure of around 1.2–1.4 MPa appears to be appro-priate to achieve a good separation with the PA/CS compositemembranes when a low pressure is preferred from the standpointsof energy consumption and equipment requirement, and such pres-sures can be delivered readily with the commonly used low-costcentrifugal pumps.

The RO performance of the PA/CS composite membranes weretested at 0.8 MPa over a feed NaCl concentration of 750–5000 mg/L,a concentration range typical of brackish water. Figs. 9 and 10show, respectively, the permeation flux and salt rejection of themembranes produced at different durations of PA chlorination andchitosan concentrations in the depositing solution. The permeationflux is shown to decline at higher concentrations of NaCl in the feed

that needs to be overcome for water permeation. At a given operat-ing pressure, the net driving force for water permeation decreasesas the osmotic pressure increases, and so is the permeation flux.For instance, the permeation flux of the PA/CS membrane prepared

J (L

/(m

2 h))

(b) Chitosan 3000 mg/L

(MPa)

0.8 1.2 1.6 2.0 2.4 2.8 3.20

40

80

120

160

200

240

R (

%)

80

84

88

92

96

100

res. PA chlorination conditions: NaClO concentration 200 mg/L, temperature 19 ◦C,

122 J. Xu et al. / Journal of Membrane S

0 1000 2000 3000 4000 500030

40

50

60

70 Original PA membrane substrate NaClO 2min+Chitosan 1000 mg/L NaClO 2min+Chitosan 2000 mg/L NaClO 5min+Chitosan 1000 mg/L NaClO 5min+Chitosan 2000 mg/L

J (L

/(m

2 h))

NaCl concentration (mg/L)

Fig. 9. Permeation flux of the PA/CS composite membranes at different feed NaClconcentrations. PA chlorination time and chitosan concentration are shown in thelegend; other chlorination conditions: NaClO concentration 200 mg/L, temperature19 ◦C. RO operating conditions are the same as in Fig. 3.

1000 2000 3000 4000 500088

90

92

94

96

98

100

NaCl concentration (mg/L)

Original PA membrane substrate NaClO 2min+Chitosan 1000 mg/L NaClO 2min+Chitosan 2000 mg/L NaClO 5min+Chitosan 1000 mg/L NaClO 5min+Chitosan 2000 mg/L

Sal

t rej

ectio

n (%

)

Ft

aewcHtt

permeation flux while the salt rejection remains the same. This

TP

ig. 10. Solute rejection of the PA/CS membranes at different feed NaCl concentra-ions. Membrane modification and operating conditions same as in Fig. 9.

t a chlorination time of 5 min and chitosan solution of 1000 mg/Lxperienced a 35.7% reduction when the feed NaCl concentrationas increased from 750 to 5000 mg/L. An increase in feed NaCl con-

entration also decreased the salt rejection, as shown in Fig. 10.owever, the decrease in the salt rejection was less drastic as

he permeation flux, and in general the decrease in the salt rejec-ion rate was within 2%. Throughout the feed concentration range

able 1ermeation flux (J) and rejection rate (R) of PA/CS membranes for different solutesa.

Membrane fabrication conditionsb NaCl

J (L/(m2 h)) R (%)

Original PA membrane 50.3 91.0Chlorination 2 min 58.2 88.7Chlorination 2 min + chitosan 1000 mg/L 54.3 94.5Chlorination 5 min + chitosan 1000 mg/L 57.7 95.4Chlorination 5 min + chitosan 2000 mg/L 56.4 95.7

a Operating conditions: pressure, 0.8 MPa; temperature, 19 ◦C; solute concentration, 15b NaClO concentration: 200 mg/L.

cience 370 (2011) 116–123

studied, all these PA/CS composite membranes exhibited a higherpermeation flux and salt rejection than the original PA membrane.

3.4. RO performance for different salt solutions

The RO performance of the membranes for different soluteswas tested, and the results are presented in Table 1, where theperformance of the original PA membrane was also included forcomparison. The following observations can be made: (1) For allthree solutes tested, the PA/CS membranes showed a better perfor-mance than the original PA membrane in terms of permeation fluxand salt rejection. (2) While the negatively charged PA membranehas a better rejection to NaCl than MgCl2, the PA/CS membraneswith reversed charges (i.e., positively charged) on the membranesurface showed a better rejection to MgCl2. (3) The PA/CS mem-branes also showed a better rejection to Na2SO4 than NaCl, in spiteof the divalent SO4

2− anions, although the improvement in therejection is at a lesser extent compared to MgCl2.

These observations are in good agreement with physical rea-soning. Let us have a close look at the rejection of solutes NaCl andMgCl2 that carry the same anions. The positively charged PA/CSmembrane tends to exclude ions of same charges (e.g., Na+ andMg2+) due to Donnan effect, resulting in lower concentrations ofthese ions on the membrane surface than in the bulk solution.Because of the higher valence of Mg2+ than Na+, there is a strongerrepulsive force between the membrane and Mg2+ ions than thatbetween the membrane and Na+ ions, and thus Mg2+ ions areexcluded more significantly by the membrane than Na+ ions inthe presence of the same anions in the solutions. In addition, thegreater size of Mg2+ ions (0.345 nm) than that of Na+ ions (0.183 nm)favors the retention of Mg2+ on the basis of size sieving. There-fore, a higher rejection to MgCl2 than to NaCl is obtained withthe use of PA/CS membranes. The situation is quite different forthe membrane rejection to solutes Na2SO4 and NaCl that carry thesame cations. In spite of the larger attractive force between divalentSO4

2− ions and the membrane surface than the attraction betweenmonovalent Cl− ions and the membrane surface, which favors thepermeation of SO4

2−, the PA/CS membranes showed a higher rejec-tion to Na2SO4 than to NaCl. This is because the size of hydratedSO4

2− ions is about 50% bigger than the size of hydrated Cl− ions[36], and the size sieving effects due to steric hindrance appears tobe dominant over the electrostatic interactions.

It is interesting to note from the data in Table 1 that regardlessof the monovalance or divalence of the ionic solutes, increasingthe chlorination time from 2 to 5 min during membrane fabri-cation tends to enhance both permeation flux and salt rejection,and increasing the chitosan solution concentration will lower the

observation is based on results from the one-variable-at-a-timeexperiments, and a factorial design experiment may be used toidentify the most suitable parameters involved in the membranefabrication.

MgCl2 Na2SO4

J (L/(m2 h)) R (%) J (L/(m2 h)) R (%)

50.1 89.2 50.9 91.858.0 86.5 59.1 90.252.5 99.4 52.6 98.253.4 99.8 56.0 98.552.2 99.8 53.7 98.6

00 mg/L.

rane S

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J. Xu et al. / Journal of Memb

. Conclusions

In this study, a thin-film-composite polyamide membrane foreverse osmosis was modified by chlorine treatment followedy electrostatic depositing of chitosan on the membrane surface,esulting in a novel polyamide/chitosan membrane with a posi-ively charged surface. The parameters involved in the chlorinationf polyamide (e.g., chlorination time and chlorine concentration)nd electrostatic supramolecular assembly of chitosan (e.g., con-entration and deposition time) on the membrane surface werenvestigated. The RO performance of the membrane for salt rejec-ion was tested using NaCl, MgCl2 and Na2SO4 as model solutes. Itas shown that the PA/CS composite membrane performed better

han the original polyamide membrane. The following conditionsor membrane modification were found to be appropriate: NaClOoncentration 200 mg/L, chlorination time 2–5 min, and chitosanoncentration 1000 mg/L; and the PA/CS membrane exhibited aermeation flux of 57.7 L/(m2 h) and a salt rejection of 95.4% forfeed NaCl concentration of 1500 mg/L at 0.8 MPa. The PA/CS com-osite membrane also exhibited good performance for rejection ofivalent salts (99.8% for MgCl2 and 98.5% for Na2SO4) at the sameoncentration. This new technique to improve the performance ofhin-film-composite polyamide membrane is practical in that theurface treatment is all done with dilute solutions and commercialembrane units can be modified in their original modules.

cknowledgements

This work was supported by the New Teachers Fund (no.0090132120007). The polyamide membrane used in this studyas generously supplied by the Development Center of Water

reatment Technology (Hangzhou, China).

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