Chemical Engineering Science 87 (2013) 152–159

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Chemical Engineering Science

0009-25

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A novel kind of thin film composite nanofiltration membrane with sulfatedchitosan as the active layer material

Jing Miao a, Lai-Chang Zhang a,n, Hechun Lin b

a School of Engineering, Edith Cowan University, 270 Joondalup Drive, Joondalup WA 6027, Australiab Key Laboratory of Polar Materials and Devices, Ministry of Education, East China Normal University, Shanghai 200241, China

H I G H L I G H T S

c Thin-film amphoteric composite (TFC) NF membranes.c Cross-linking of sulfated chitosan (SCS) and epichlorohydrin (ECH).c Structure and morphology of SCS composite with epichlorohydrin (ECH) NF membranes.c Separation of mono/divalent inorganic electrolytes from low-molecular-weight organics.

a r t i c l e i n f o

Article history:

Received 2 August 2012

Received in revised form

6 October 2012

Accepted 8 October 2012Available online 12 October 2012

Keywords:

Desalination

Composite nanofiltration membranes

Sulfated chitosan

Epichlorohydrin

Morphology

Separation

09/$ - see front matter & 2012 Elsevier Ltd. A

x.doi.org/10.1016/j.ces.2012.10.015

esponding author. Tel: þ61 8 6304 2322; fax

ail address: l.zhang@ecu.edu.au (L.-C. Zhang).

a b s t r a c t

Nanofiltration (NF) has been used as pretreatment to reverse osmosis (RO) desalination processes for

lowering the required pressure. In this work, a novel kind of amphoteric thin-film composite (TFC) NF

membranes were prepared through a method of coating and cross-linking using sulfated chitosan (SCS),

polysulfone (PS) ultrafiltration (UF) membrane, and epichlorohydrin (ECH) as the active layer material,

the base membrane, and the cross-linking agent, respectively. At 0.40 MPa and ambient temperature,

the rejections of the resultant membrane to Na2SO4 and NaCl solutions (1000 mg L�1) were 90.8% and

32.5%, while the permeate fluxes were 22.9 and 58.4 kg m�2 h�1, respectively. The structure and

morphology of the resultant SCS/PS composite NF membranes were characterized with attenuated total

reflection infrared spectroscopy (ATR-IR), environmental scanning electron microscopy (ESEM), and

atomic force microscopy (AFM). The rejection performances suggested that SCS/PS composite NF

membranes cross-linked by ECH have a potential for the separation of mono/divalent inorganic

electrolytes from low-molecular-weight organics.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Separation technology is a fundamental aspect of chemical anddairy industries. Membrane is a permselective barrier between twophases that facilitates separation of components on application of adriving force (Mulder, 1996). Compared with conventional techni-ques, membrane technology can offer a simple, easy-to-operate,low-maintenance process option, and have been employed widelyfor the manufacturing of basic products (Rektor and Vatai, 2004;Vanderhorst et al., 1995), the solving of complex separationprocesses (Christy and Vermant, 2002; Zhu et al., 2003), andwastewater treatment (Lopes et al., 2005; Scholzy and Fuchs,2000; Yang and Li, 2008). Although the inorganic membranes areunder fast development recently, most of the membranes today are

ll rights reserved.

: þ61 86304 5811.

still prepared from polymers. The design and synthesis of functionalpolymeric membranes for liquid or gas separations, electrodialysis,water treatment, etc., is one of the important fields of modernpolymer materials and engineering.

Nanofiltration (NF), emerging in the mid-1980s, is a kind ofrelatively new membrane liquid separation technology that fallsbetween reverse osmosis (RO) and ultrafiltration (UF). In contrastwith RO membrane, NF membrane exhibits higher permeate flux,even under high salinity and low operating pressure. Typically, NFinvolves separation of monovalent and divalent salts, or organicsolutes with molecular weight in the range of 200 to 1000 g mol�1

(Vandezande et al., 2008, Zhong et al., 2012). The typicallyexcellent performance of NF membrane, such as high permeateflux, small investment, and low operation cost, brings it wider andlarger applications in water softening (Anim-Mensah et al., 2008;Ghizellaoui et al., 2005), food and pharmaceutical applications(Zhu et al., 2003; Baumgarten et al., 2004), desalination of dyestuffs(Huang and Zhang, 2011; Koyuncu et al., 2004; Yu et al., 2001), acid

Fig. 1. Diagram of the cross-flow UF membrane evaluation apparatus.

J. Miao et al. / Chemical Engineering Science 87 (2013) 152–159 153

and caustic recovery (Visser et al., 2001), and color removal (Mutluet al., 2002).

Chitosan is the deacylated derivative of chitin, the second mostabundant natural polymer just after cellulose. There are manyactive groups in its molecular chains, such as –OH, –NH2, etc. Sochitosan is easily modified, and many derivatives have beenprepared to meet different needs. Sulfated chitosan (SCS) is thesulfation product of chitosan, having sulfate groups on some ofboth the amino and primary hydroxyl sites of the glucosamineunits. SCS is a typical kind of hydrophilic amphoteric polyelec-trolyte, and an excellent potential candidate for gel and mem-brane materials because of its film forming characteristics, havingbeen used to prepare NF membranes cross-linked with glutar-aldehyde (Miao et al., 2005), microcapsules (Baumann et al.,2003), composite gel beads (Markvicheva et al., 2000).

Recently NF membranes based on natural polysaccharidesgained more and more attention due to their abundance andenvironmental benignity. The problems related to the membranesprepared from natural polysaccharide and their derivatives arewater solubility and excessive swelling, both of which could besolved by chemical cross-linking. Epichlorohydrin (ECH) is a kindof widely used cross-linker. It is well known that the polysacchar-ides including chitosan could be easily cross-linked with ECH. Theearlier works investigated the mechanisms of the reactionsbetween ECH and starch/cellulose, and physicochemical proper-ties of the modified carbohydrate polymers (Roberts, 1965;Kartha and Srivastava, 1985). The porous structure, forming fromchitosan macromolecules cross-linked with ECH, forces theimmobilization of oxygen and nitrogen groups of the polymermatrix. Zheng et al. (2000) fabricated a kind of cross-linkedchitosan films using ECH as the cross-linking agent, investigatedtheir mechanical properties, and characterized them with Fouriertransform infrared spectroscopy (FTIR), X-ray diffraction (XRD),and scanning electron microscope (SEM). Gumusderelioglu andAgi (2004) prepared macroporous chitosan membranes throughthe method of solvent evaporation under alkaline conditions,using ECH as the cross-linking agent, and investigated theadsorption and desorption properties of concanavalin A (Con A.)on the membranes by fluorescence spectroscopy.

In this work, a novel kind of TFC composite NF membranes basedon SCS, were prepared via the method of coating and cross-linkingwith ECH. The preparation conditions on the rejection performance,the structure and morphology, and the rejection performance of SCScomposite NF membranes to inorganic electrolytes and mono/oligo-saccharides were investigated and characterized, respectively. It wasfound that this kind of amphoteric membranes show the similarrejection performance to the negatively-charged NF membranes.To explore the feasibility of the developed membranes for low-molecular-weight (LMW) dyes separation from textile effluents, threenegatively-charged dyes and one positively charged dye, were furtherselected as the solutes for NF permeation test. The objective of thisstudy was to develop a novel kind of amphoteric composite NFmembrane from chitosan with the flexibility of tuning the membranerejection performance, and investigate systematically the effectsof preparation and operation variables on the NF performances.The results may provide a basis for its applications in water andwastewater treatment, including desalination, separation of LMWdyes from salts, etc., and further study on it.

Scheme 1. Schematic representation of the cross-linking of SCS with ECH.

2. Materials and methods

2.1. Materials and apparatus

Chitosan (MWE5.4�105 Da, degree of deacetylation (D.D.)¼91%) was purchased from Haihui Bioengineering Co., Qingdao

(China). All the other reagents and solvents were of analyticalgrade and used without further purification. Deionized (DI) waterwith a conductivity of 2�10�6 S cm�1 was used for membranepreparation and permeation experiments. The polysulfone (PS) UFmembrane with 10,000 Da molecular weight cut-off (MWCO) andthe UF membrane evaluation apparatus were provided by theDevelopment Center of Water Treatment Technology, State Ocea-nic Administration, Hangzhou (China). Fig. 1 shows the diagramof the cross-flow UF membrane evaluation apparatus.

2.2. Synthesis of SCS and its cross-linking with ECH

Sulfated chitosan (SCS) was synthesized through a homoge-neous method described in the literature (Jiang, 2003). The sulfategroup content in SCS was determined to be 38.14 wt% by thegravimetric analysis of hydrochloric and barium sulfate precipita-tion (Zhang et al., 2002). Scheme 1 shows the cross-linking of SCSwith ECH.

Fig. 2. Streaming potential (E)-operating pressure (DP) figure of SCS/PS composite

NF membrane.

J. Miao et al. / Chemical Engineering Science 87 (2013) 152–159154

2.3. Preparation of SCS/PS composite NF membranes

PS UF membrane with490% rejection for PEG 10000, had purewater permeate flux of 246 kg m�2 h�1 at ambient temperatureand 0.10 MPa. Before coating the casting solution of the activelayer, PS UF membranes were immersed in water overnight andrinsed with DI water thoroughly.

SCS solutions were prepared by dissolving SCS into DI waterat different weight ratios and were filtered with G4 sand filter.SCS/PS composite membranes were fabricated by coating the SCSsolutions onto the PS UF membrane with a finely polished glassrod, followed by curing in an oven at 50 1C for 1 h, then immer-sing the membranes in the ECH/EtOH 95% (0.067 M NaOH)solution, and cross-linking at 50 1C for a certain period of time.The 0.08 mm-diameter brass wires attached to the two ends ofthe glass rod determined the thickness of the active layer. Aftercross-linking, the membranes were washed with ethanol andacetone extensively, and immersed in DI water before thepermeation tests.

2.4. Characterization of the resulting composite NF membrane

The surface charge characteristic, the chemical structure, andthe morphology of SCS/PS composite NF membrane, were char-acterized with tangential steaming potential (TSP), attenuatedtotal reflection infrared spectroscopy (ATR-IR), environmentalscanning electron microscopy (ESEM), and atomic force micro-scopy (AFM), respectively. The sample for characterization wasprepared under the following conditions: SCS concentration2.2 wt%, curing time 1 h at 50 1C, ECH concentration 2.0 wt%,and cross-linking time 2 h at 50 1C.

TSP is the potential difference at zero current caused by theconvective flow of charge due to a pressure gradient through acharged membrane. Surface charge characteristic of a membranecan be determined by the TSP, and the slope of a DE–DP plot (b)could reflect the membrane charge properties qualitatively andreliably. Here, SurPASS Electrokinetic Analyzer equipped withclamping cell (Anton Paar GmbH, Graz, Austria) was used tomeasure the TSP at ambient temperature and various differentialpressures ranging from 0 to 0.4 MPa in 0.01 mol/L KCl aq. solution

b¼� @E=@DP� �

ð1Þ

ATR-IR characterization of the membrane surface was madewith an ATR accessory of Nicolet Avatar 360 IR Spectrometer.ZnSe crystal was used in the ATR accessory. Membrane sampleswere mounted flush to the ATR crystal.

Surface and cross-section morphologies were performed witha Philips environmental scanning electron microscopy (ESEM) XL-30 instrument operating at 15 kV. The sample for cross-sectionalviewing was prepared as described in reference Bartels (1989).

Contact mode AFM images in air were taken on an SN-AF01AFM system (Seiko Instruments Inc., Japan), using the OLYMPUSMicro Cantilever with a specified spring constant of 0.09 N m�1.The tip on the end of the micro cantilever employed a sharpenedpyramidal tip of silicon nitride (SiN).

2.5. Permeation experiments

To test the rejection properties of the resulting compositemembranes, Na2SO4, NaCl, MgSO4, and MgCl2 solutions wereemployed as the inorganic electrolytes, and low-moleculra-weight (LMW) organics as the organic solutes, including mono/oligosaccharides, and polyethylene glycols (PEG), and dyes. Theconcentrations of the single inorganic electrolytes were measuredwith the conductivity meter. As for LMW organics, the concentra-tions of mono/oligosaccharides and polyethylene glycols (PEG)

with different Mw, were determined using Shimadzu TOC-VCPN

Total Organic Carbon (TOC) analyzer, while those of dyes weremeasured with a Cary 50 UV–vis spectrophotometer. The per-meation flux of the membrane was determined by weighing thepermeate penetrating through the membrane during a certainperiod of time.

The permeation tests were carried out at ambient temperatureusing a flat sheet membrane cell of 19.6 cm2 effective area at apressure, concentration of inorganic electrolyte or LMW organic,and cycling flow rate of the feed solution of 0.40 MPa,1000 mg L�1 or 500 mg L�1, and 1.1 m s�1 respectively. Themembranes were immersed in DI water for 36 h and then pre-pressurized under 0.45 MPa for 0.5 h before testing. Both theretentate and the permeate were recirculated to the feed tank tokeep the feed concentration constant during the permeationexperiments. The observed rejection was calculated with thefollowing equation:

R¼ 1�CP

Cfð2Þ

where Cp and Cf are the concentrations of the permeate and thefeed solution, respectively. The presented data were the averagesof three measurements conducted on two pieces of membranescut from a single sheet. The standard deviations were r5%.

2.6. Molecular weight cut-off

The molecular weight cut-off (MWCO) is commonly defined asthe molecule weight of the smallest molecule, which is retainedby the membrane with 90% rejection. The MWCO of the resultantSCS/PS membrane was determined through the rejections to500 mg L�1 aq. solutions of PEG (MW 200–1000 g mol�1).

3. Results and discussion

3.1. Membrane characterization

3.1.1. Surface charge characteristic of SCS/PS composite NF

membrane

The plot about TPS against operating pressure (DP) is shown inFig. 2. It could be seen that the absolute value of TPS increasedlinearly with increasing DP, and all TPS are below zero, suggesting

Fig. 3. ATR-IR spectra of SCS/PS composite NF membrane.

J. Miao et al. / Chemical Engineering Science 87 (2013) 152–159 155

this kind of SCS TFC NF membrane is negatively surface charged inthe inorganic electrolyte solution, might be due to the intrinsicsulfate groups in the membrane surface and the adsorption of theanions in the aq. solution (Afonso and de Pinho, 2000). Thepressure osmotic coefficient is attained at �8.78 mV MPa�1 forthis TFC NF membrane by the linear fit of the experimental data.

3.1.2. ATR-IR spectra of SCS/PS composite NF membrane

Fig. 3 shows the ATR-IR spectra of a virgin PS UF membranewith just the SCS coating (PS–SCS), and with the cross-linked SCSactive layer (PS–SCS–ECH). As for PS–SCS–ECH, there is a distinctband at 1585 cm�1 attributed to the bending vibration of alipha-tic secondary amine, which was synthesized by the reactionbetween –NH2 and ECH. The sharper band at 1486 cm�1 wasattributed to the bending vibration of the chain aliphatic CH. Theband at 1325 cm�1 was corresponding to C–O–C stretchingvibration of the ether bonds, confirming the etherization betweenECH and –OH. Another new band at 731 cm�1 represented theformation of the methylene present in the straight-chain aliphaticalcohol, which also confirmed the occurrence of the cross-linkingreaction between SCS and ECH.

Fig. 4. Morphologies of SCS/PS composite NF membrane: (a) and (b): cross,

3.1.3. Surface and cross-sectional morphology of SCS/PS composite

NF membrane

Fig. 4 shows the surface and cross-sectional morphologies ofSCS/PS composite NF membrane. No visible pores can be observedon the membrane surface in Fig. 4(c). The SCS/PS compositemembranes were composed of nonwoven fiber, PS UF membrane,and the thin, dense active layer with about 120 nm in thickness.Below the active layer, there is a sponge-like support layer withabout 30 mm in thickness.

(c) surface.

3.1.4. AFM image of SCS/PS composite NF membrane

Fig. 5 shows the typical 2-D and 3-D AFM images of SCS/PScomposite NF membrane in the scale of 2 mm. It could be seenthat the regular fluctuations exist on the membrane surface. Thecolor density shows the vertical profiles, where the bright regionsare the high peaks, and the dark regions are valleys. The rough-ness can be defined as Ra (the mean roughness of the surface),RMS (the root-mean squared roughness), and P–V (the peak-to-valley distance). Here, RMS, Ra, and P–V are 87 nm, 65 nm, and199 nm, respectively.

3.2. Permeation characteristics

3.2.1. MWCO of SCS/PS TFC NF membrane

Fig. 6 shows the rejections of the resultant TFC NF membraneto polyethylene glycol (PEG) with different molecular weights. Asshown in Fig. 6, the observed rejections to PEG increased with thegrowth of their Mws under the operating pressure of 0.4 MPa. Therejections to PEG 200, PEG 400, PEG 600, PEG 800, and PEG 1000,are 24.1, 44.9, 73.4, 87.9, and 95.5%, respectively. Hence, the

Fig. 5. AFM images of SCS/PS composite NF membrane: (a) two-dimensional

image, and (b) three-dimensional image.

Fig. 6. Rejections of SCS/PS composite NF membrane to PEG with different

molecular weights.

Table 1Rejections to different inorganic electrolytes by SCS/PS

composite NF membranes. F: permeate flux; R: rejection.

Feed solution

(1000 mg L�1)

F (kg m�2 h�1) R (%)

Na2SO4 22.9 90.8

NaCl 58.4 32.5

MgSO4 21.3 27.9

MgCl2 58.2 8.6

J. Miao et al. / Chemical Engineering Science 87 (2013) 152–159156

MWCO of this SCS/PS composite NF membrane was approximate925 g mol�1, corresponding to a rejection of 90%.

3.2.2. Rejection performances for different inorganic electrolyte

solutions

The rejection performances of SCS/PS composite NF membranefor different inorganic electrolyte solutions are shown in Table 1.It suggests that the rejection of this kind of composite NFmembranes to the inorganic electrolyte solutions decrease inthe order of Na2SO4, NaCl, MgSO4, and MgCl2, which agreesqualitatively with the ‘‘Donnan’’ exclusion principle. With aMWCO of 925 g mol�1, it is obviously that sieving effects willbe of no importance in the case of single inorganic electrolytes.Additionally, it is well known that the charge effect of electrolyte-ions should be considered as a dominant factor for NF of inorganicelectrolytes of low concentration (Wang et al., 2002). It is also knownfrom the experimental results of the TPS, the active layer of SCS/PS

composite NF membrane could acquire a negative surface chargedistribution due to the intrinsic sulfate groups in the membranesurface, and the adsorption of anions from the solution, and thischarge distribution mainly determines the NF performance (Afonsoand de Pinho, 2000). As for RNaCl4RMgSO4

, it might result from thecombination between Mg2þ and the anions on the membranesurface, which would decrease the effective surface charge ofmembranes, and then reduced the rejection performance.

3.2.3. Rejections for mono/oligosaccharides

NF membranes have been applied to the separation of mono/divalent inorganic electrolytes from LMW organics. In this work,SCS/PS composite NF membrane was also characterized by therejections of LMW organics, where mono/oligosaccharides wereemployed as the solutes. Table 2 shows the rejection perfor-mances to mono/oligosaccharides by SCS/PS composite NF mem-brane. For the neutral solutes, the transport through the NFmembrane is being controlled by the pore size of the membraneand the size of the solute. Mono/oligosaccharides being neutralsolutes, the rejection performance to them is only determined bythe sieving effect, not by the ‘‘Donnan’’ effect. The rejections ofthis kind of membrane to such neutral solutions decreased in theorder of raffinose, sucrose, and glucose. A distinctive differencecould be seen clearly in the rejections for mono/divalentinorganic electrolytes and LMW organics by SCS/PS membranes.There is a potential for application of such NF membranes to theseparation of mono/divalent inorganic electrolytes from LMWorganics.

Table 2Rejections of mono/oligosaccharides by SCS/PS composite NF membrane. Mw:

molecular weight; F: permeate flux; R: rejection.

Feed solution

(500 mg L�1)

Mw (g mol�1) F (kg m�2 h�1) R (%)

Glucose 180.16 28.3 32.3

Sucrose 342.30 27.0 36.0

Raffinose 504.42 26.9 50.3

Operating condition: 0.40 MPa and ambient temperature.

Table 3Dye rejection performances of SCS/PS composite NF membrane. Mw: molecular

weight; F: permeate flux; R: rejection.

Dyestuff Mw (g mol�1) Charge R (%)

Reactive black KN-B 975 � 99.9

Xylend orange 761 � 99.2

Nitroso-R-salt 377 � 97.5

Methlene Green 609 þ 97.3

Operating condition: 0.40 MPa and ambient temperature.

Fig. 7. Effect of SCS concentration on the rejection performance of SCS/PS

composite membranes.

Fig. 8. Effect of ECH concentration on the rejection performance of SCS/PS

composite membranes.

J. Miao et al. / Chemical Engineering Science 87 (2013) 152–159 157

3.2.4. Rejections for different LMW dyes

The Mws of Some LMW dyes with negative or positive chargeare in the range of a few hundred to 1000 g mol�1. To explore thefeasibility of the developed membranes for LMW dyes separationfrom salts, three negatively-charged dyes with different Mw andone positively-charged dye were selected for NF permeation test.Table 3 shows the rejections to the dyes. Reactive black KN-B andXylend orange being negatively charged in aqueous solution withrelatively higher Mw, the rejections to these two dyes could beabove 99%. Although Mw of Methlene Green is higher than that ofNitroso-R-salt, the rejection to Methlene Green was even a bitlower due to its positive charge, which also confirms that thesurface charge characteristic has effect on NF selectivity of NFmembranes (Zhong et al., 2012).

3.3. Preparing conditions on the rejection performance

3.3.1. Effect of SCS concentration

A series of SCS/PS composite membranes were prepared fromSCS solutions with different concentrations in the range of 1.0–2.5 wt% following the same preparation technique as mentionedin 2.4. Fig. 7 shows the effect of SCS concentration on the rejectionperformance of the composite membranes to 1000 mg L�1

Na2SO4 solution. The Na2SO4 rejection increased, and the perme-ate flux decreased with the increase of the SCS concentration untilit was 2.2 wt%, which was likely to be resulted from the followingreasons. The amount of amino and hydroxyl groups increasedwith the increase of SCS concentration, which led to the increaseof the amount of the cross-linking points, the increase of thecross-linking density, and the decrease of the dimension of thecross-linking network. The rejection to Na2SO4 solution began todecrease as SCS concentration was 2.5 wt%, which might be dueto the blemished active layer, resulting from the partial precipita-tion of SCS during the curing period. The membrane preparedfrom SCS solution concentration of 2.2 wt% has high rejection(90.8%) and moderate permeate flux (22.9 kg m�2 h�1). There-fore, an SCS solution concentration of 2.2 wt% was selected, whilethe effects of the other preparation conditions on the rejectionperformance were investigated.

3.3.2. Effect of the cross-linking solution concentration

Cross-linked with ECH solutions of different concentrations inthe range of 1.0–2.5 wt%, a series of SCS/PS composite membranes

were prepared from 2.2 wt% SCS solutions following the samepreparation technique as mentioned in Section 2.4. Fig. 8 showsthe effect of ECH concentration on the rejection performance for1000 mg L�1 Na2SO4 solution. It can be seen clearly that thepermeate flux decreased and the rejection to the electrolytesolution increased, with the increase of ECH concentration untilit was42.0 wt%, which could be explained by the sieving effect.The increase of the concentration of the cross-linking solutionwould result in the reduction in pore size and water absorption,the increase in the hydrophobicity and the pore tortuosity, whichwould cause the decrease of permeate flux and the increase of therejection to inorganic electrolyte. The similar trend was alsoobserved in the literature (Musale and Kumar, 2000). However,the rejection began to decrease as ECH concentration42.0 wt%,which could not be explained plausibly yet.

3.3.3. Effect of the cross-linking time

Cross-linked with 2.0 wt% ECH solution for a period rangingfrom 1–5 h, a series of SCS/PS composite membranes wereprepared from 2.2 wt% SCS solutions following the same prepara-tion technique as mentioned in Section 2.4. Fig. 9 shows the effectof the cross-linking time on the rejection performance of SCS/PS

Fig. 9. Effect of the cross-linking time on the rejection performance of SCS/PS

composite membranes.

Fig. 10. Effect of the operating pressure on the rejection performance of SCS/PS

composite NF membranes.

Fig. 11. Effect of the feed concentration on the rejection performance of SCS/PS

composite NF membranes.

J. Miao et al. / Chemical Engineering Science 87 (2013) 152–159158

composite membranes to 1000 mg L�1 Na2SO4 solution. It showsthat the cross-linking time had a significant effect on the rejectionperformance while it wasr2 h: the rejection increased from79.9% to 90.8%, and the permeate flux decreased from54.2 kg m�2 h�1 to 22.9 kg m�2 h�1 with the increase of thecross-linking time. However, the rejection began to decrease afterca. 2 h, for which there is no plausible explanation yet.

3.4. Operating conditions on the rejection performance

3.4.1. Operating pressure

The effect of operating pressure on the rejection performance isshown in Fig. 10. With the operating pressure increasing in therange of 0.20–0.40 MPa, both the permeate flux and the rejectionincreased, and the curve of R became smooth after 0.35 MPa. Thepermeate flux presented a nearly linear relation with the operatingpressure, which could be partly explained by the solution-diffusionmodel. The permeate flux is in direct proportion to the trans-membrane pressure, the difference of the operating pressure andthe osmotic pressure. Compared with the operating pressure, theosmotic pressure could be neglected for NF of inorganic electrolytesolution at low concentration. Thus the operating pressure isapproximately equal to the trans-membrane pressure.

3.4.2. Feed concentration

The effect of feed solution concentration on the permeate fluxand the rejection to Na2SO4 solution by SCS/PS is shown in Fig. 11.Obviously the feed solution concentration had a marked effect onthe rejection. The rejection decreased with the increase of thefeed solution concentration, and it was in the range of 86.3–98.0%as the feed solution concentration ranged from 500 to2000 mg L�1, which could be explained by ‘‘Donnan’’ exclusiontheory. The cations (Naþ) shield effect on the membrane nega-tively charged groups became stronger because of the increase offeed concentration, leading to the decrease of the membranecharge density and repulsion forces on the anions. On the otherhand, the trans-membrane pressure decreased because the osmo-tic pressure increased with the increase of the feed concentration,which resulted in the decrease of the permeate flux.

4. Conclusions

Novel amphoteric TFC NF membranes were prepared througha method of coating and cross-linking using SCS, PS UF mem-branes, and ECH as the active layer material, the base membranes,and the cross-linking agent, respectively. Under the certainpreparing conditions, the resultant NF membrane had moderateflux and excellent rejection to Na2SO4 solution (1000 mg L�1).Based on the data of MWCO, the rejections for different inorganicelectrolytes, and the rejections for mono/oligosaccharides andLMW dyes with negative and positive charge, it could be con-cluded that the SCS/PS composite NF membranes cross-linked byECH have the potentials for the separation of mono/divalentinorganic electrolytes from LMW organics, and the separation ofLMW dyes from salts.

The preparation conditions, including SCS concentration, ECHconcentration, and the cross-linking time, had effects on therejection performance of the resultant composite membranes.Both of rejection and permeate flux increased with the increase ofthe operating pressure in a certain range. The permeate fluxpresented a nearly linear increase, and the rejection curve becamesmooth after the operating pressure was higher than 0.35 MPa.

The rejection of this kind of membrane to inorganic electrolytesolutions decreased in the order of Na2SO4, NaCl, MgSO4, andMgCl2. It suggested that this kind of amphoteric TFC NF mem-branes showed similar rejection performances to negativelycharged composite NF membranes, might due to the intrinsic

J. Miao et al. / Chemical Engineering Science 87 (2013) 152–159 159

sulfate group and the adsorption of anions from the inorganicelectrolyte solution. This surface charge characteristic mainlydetermined the membrane rejection performance.

Acknowledgments

Financial support from the National Center Excellence inDesalination, Australia (NCEDA) (Project no. 23634) is gratefullyacknowledged.

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