Desalination 229 (2008) 68–81

*Corresponding author.

sodium alginate membranes

Swayampakula Kalyania, Biduru Smithab, Sundergopal Sridharb*,Abburi Krishnaiaha

aBiopolymers and Thermo physical Laboratory, Department of Chemistry, Sri Venkateswara University,Tirupati 517 502, India

bMembrane Separations Group, Chemical Engineering Division, Indian Institute of Chemical Technology,Hyderabad 500 007, India

Tel. +91 (40) 27193142; Fax +91 (40) 27193626; email: sridhar11in@yahoo.com

Received 3 March 2006; accepted revised 16 July 2007

Abstract

A sodium alginate (SA) dense membrane was prepared by the casting and drying of sodium alginate solutionson an acrylic plate, followed by crosslinked with phosphoric acid for the separation of ethanol–water mixtures at30°C by the pervaporation method. The membrane containing 3 wt.% of sodium alginate shows the highest separationselectivity of 2182 with a substantial flux of 0.035 kg m–2 h–1. Permeation flux increased with an increase in weightpercentage of water in the feed mixture, but separation selectivity decreased. The effect of experimental factors,such as the concentration of the feed solutions, membrane thickness and the operating permeate pressure on SAmembrane performance were evaluated. The membranes were also subjected to sorption studies to evaluate theextent of interaction and degree of swelling in pure as well as binary feed mixtures. Further, ion exchange capacity(IEC) studies were carried out for all the crosslinked and uncrosslinked membranes to determine the total numberof interacting groups present in the membranes.

Keywords: Pervaporation; Sodium alginate membrane; Ethanol/water azeotrope; Phosphoric acid

1. Introduction

Presently, the dehydration of organic mixturesis the most important application for pervapora-tion. Research efforts have been directed to the

selection of proper membrane materials. A goodpervaporation membrane material should havehigh permeation flux and separation factor for thepervaporation dehydration of alcohol. It has longbeen recognized that hydrophilic polymers are se-lected as membrane material for the dehydrationof various solvents because the water molecule is

doi:10.1016/j.desal.2007.07.0270011-9164/08/$– See front matter © 2008 Published by Elsevier B.V.

Pervaporation separation of ethanol–water mixtures through

S. Kalyani et al. / Desalination 229 (2008) 68–81 69

easily sorbed by the hydrophilic polymeric mem-brane. According to the solution-diffusion mecha-nism, separation in the pervaporation processtakes place in two steps: selective sorption of liq-uid molecules into the membrane surface on thefeed side and selective diffusion of them throughthe membrane due to differences in both the solu-bilities and diffusivities of permeating constitu-ents, respectively. From these points of view, natu-ral high polymers, polysaccharides, are worthyof consideration as a membrane materials for de-hydration. They possess good affinity towardwater molecules as shown in several studies deal-ing with the dehydration of alcohols throughpolysaccharide membranes. Alginic acid mem-brane [1] and generated cellulose membrane [2]are reported to show a good selectivity in the de-hydration of water–alcohol mixture. Chitosanmembrane [3], one of the cationic polysaccharidemembranes, also yielded high selectivity when itsamino group was neutralized by poly basic acidsor when it was cross-linked using H2SO4 [4].

Among the hydrophilic polysaccharide typepolymers, alginate membrane has gained specialinterest because it showed the highest flux andseparation factor among the hydrophilic materi-als tested for the pervaporation dehydration [5,6].However, a very high hydrophilicity of sodiumalginate resulting from both of its carboxyl andhydroxyl groups, leads to a significant swellingof membrane in aqueous solution, followed by aremarkable decline of selectivity and mechanicalstrength. To overcome these drawbacks, severalresearchers have modified the alginate membranesfor the effective dehydration performance. Yeomand Lee [7], crosslinked the sodium alginate mem-brane with glutaraldehyde for the separation ofwater–isopropanol mixture. Haung and co-work-ers [8] prepared a novel two-ply dense compositemembranes using successive casting of sodiumalginate and chitosan for the dehydration of iso-propanol and ethanol. However, the performanceof a pure sodium alginate membrane was still notsatisfactory because of a large free volume be-

tween the molecular chains [9]. Its membrane per-formance has been improved by modifying algi-nate with different methods such as blending [10],grafting [11] and cross-linking [12].

In this paper, we prepared a novel compositemembrane from sodium alginate (SA) cross-linked in a novel fashion with phosphoric acid,for water/ethanol mixtures separation bypervaporation. The work also explores the effectof varying water concentration in the binary feedmixture on membrane flux and selectivity. Fur-thermore, the diffusion coefficient andpervaporation results have been explained on thebasis of solution-diffusion principles.

2. Experimental

2.1. Materials

Sodium alginate, having an average molecu-lar weight of 5,00,000, was purchased fromAldrich Chemicals, Mumbai, India. Ethanol ofpurity 99.9%, isopropanol and phosphric acidwere purchased from Loba Chemicals, Mumbai.Demineralised water (conductivity = 0.02 S cm–1),which was used for the preparation of feed solu-tion, was generated in the laboratory itself.

2.2. Preparation of membranes

Sodium alginate membranes were prepared bysolution casting and solvent evaporation method.The 3 wt.% solution of sodium alginate in aque-ous medium was prepared, stirred and filtered toremove the undissolved matter. A bubble-free so-lution was cast on a clean acrylic plate/Petri dishto the desired thickness and dried in atmosphericconditions at room temperature followed byvacuum drying for a period of 5 h at the elevatedtemperature (50°C) in an oven to remove the lasttraces of solvent. The so formed membrane wascrosslinked for 3 h in an isopropanol–water bath(90/10 vol.%) containing 3.5 vol.% of phospho-ric acid. After removing the membrane from thecross-linking bath, it was washed with water re-

70 S. Kalyani et al. / Desalination 229 (2008) 68–81

peatedly and dried in an oven at 80°C to elimi-nate the presence of residual acid, if any. Themembrane thickness as measured by a microme-ter screw gauge was in the range of 35–40 μm.The dried membranes were utilized in PV experi-ments. The stability of the membrane was ana-lyzed by bending the membrane before and afterPV studies. The experiment performed for theduration of 3 months during which the membranewas found to be stable. After this period, the mem-brane was bent to ensure its mechanical stabilityand it was noted that despite bending completelythe membrane did not break. Hence, the mem-brane durability and stability appears to be rea-sonably good [13].

2.3. Membrane characterization

2.3.1. Fourier transform infra red (FTIR)studies

The FTIR spectra of unmodified and phospho-ric acid crosslinked NaAlg membranes werescanned in the range 400–4000 cm–1 using aNicolet–740, Perkin–Elmer 283B FTIR spectrophotometer by KBr pellet method.

2.3.2. XRD analysis

A Siemens D 5000 powder X-ray diffracto-meter was used to assess the solid-state morphol-ogy of phosphorylated sodium alginate (P-NaAlg)in powdered form. X-rays of 1.54 Å wavelengthswere generated by a CuK source.

2.3.3. Thermal gravimetric analysis (TGA)

Thermal stability of the polymer membraneswas examined (Seiko 220TG/DTA analyzer) inthe temperature range of 25–700°C at a heatingrate of 10°C min–1 with continuous flushing us-ing pure nitrogen gas at 200 ml min–1. The sampleswere subjected to TGA both before and after phos-phorylation to determine the thermal stability anddecomposition characteristics.

2.4. Sorption characteristics

In order to evaluate membrane-liquid affini-ties, weighed samples of circular pieces of thepolymer membranes (3 cm dia) were soaked inpure water and ethanol as well as their binarymixtures of various concentrations. The mem-branes were taken out after different soaking pe-riods and quickly weighed, after carefully wip-ing off excess liquid, to estimate the amount ab-sorbed at the particular time “t”. The membranewas then quickly placed back in the solvent. Theprocess was repeated until the membranes attainedsteady state as indicated by constant weight aftera certain period of soaking time. Degree of swell-ing was calculated using Eq. (1):

DS s

d

MM

= (1)

The percentage sorption was calculated usingEq. (2):

Sorption [%] = 100s d

d

M MM−

× (2)

2.5. Determination of the ion exchange capacity(IEC)

The ion exchange capacity (IEC) indicates thenumber of milliequivalents of ions in 1 g of thedry polymer. To determine the degree of substitu-tion by acid groups, the phosphorylated mem-branes and unphosphorylated specimens of simi-lar weight were soaked in 50 ml of 0.01 N NaOHsolution for 12 h at ambient temperature. Then,10 ml of the solution was titrated with 0.01 NH2SO4. The sample was regenerated with 1 MHCl, washed free of acid with water and dried toa constant weight. The IEC was calculated accord-ing to Eq. (3) [14]

NaOH 5IEC = B P Nm

− × × (3)

S. Kalyani et al. / Desalination 229 (2008) 68–81 71

where IEC is the ion exchange capacity (meq g–1),B the amount of 0.01 M sulfuric acid used to neu-tralize unmodified sodium alginate, P the amountof 0.01 M sulfuric acid used to neutralize thecrosslinked membrane, 5 the factor correspond-ing to the ratio of the amount of NaOH taken todissolve the polymer to the amount used for titra-tion, and m the sample mass in g.

2.6. Pervaporation procedure

Experiments were carried out on a 100 mlbatch level with an indigenously constructedpervaporation manifold (Fig. 1) operated at avacuum as low as 0.05 mm Hg in the permeateline. The membrane area in the pervaporation cellassembly was approximately 20 cm2. The experi-

Fig. 1. Schematic of laboratory vacuum pervaporation set-up.

ATM

VP

VGV1V2

V3C1

C2

PV Cell

M

SM

DF

FC

SM - Stirrer MotorM - MembraneFC - Feed ChamberC1& C2 - CondensersDF- Dewar FlaskV1, V2, V3- Control valvesVG- Mcleod GuageVP- Vacuum PumpATM - Atmosphere

ATM

mental procedure is described in detail elsewhere[15]. Permeate was collected for a duration of 8–10 h. Tests were carried out at room temperature(30±2°C) and repeated twice using fresh feed so-lution to check for reproducibility. The collectedpermeate was weighed after allowing it to attainroom temperature in a sartorius electronic balance(accuracy: 10–4 g) to determine the flux and thenanalyzed by gas chromatography to evaluate themembrane selectivity.

2.6.1. Flux and selectivity equations

In pervaporation the flux J of a given species,say faster permeating component i of a binary liq-uid mixture comprising of i (water) and J (etha-nol) is given by Eq. (4)

72 S. Kalyani et al. / Desalination 229 (2008) 68–81

ii

WJAt

= (4)

In the present study, though different mem-brane thicknesses were utilized, the flux has beencalculated for the thickness of the membrane to10 μm (normalized flux) and is given by Eq. (5).

Normalized flux flux membrane thickness=

10 m×

μ

(5)

The membrane selectivity is the ratio of per-meability coefficients of water and ethanol andcan be calculated from their respective concen-trations in feed and permeate as given in Eq. (6)

( )( )11

y xx y

−α =

− (6)

Pervaporation separation index (PSI) is a mea-sure of the separation capability of a membraneand is expressed as a product of selectivity andflux [16], which was given in Eq. (7).

PSI = J ×α (7)

2.7. Analytical procedure

The feed and permeate samples were analyzedusing a Nucon Gas Chromatograph (Model 5765)installed with Thermal Conductivity Detector(TCD) and packed column of 10% DEGS on80/100 Supelcoport of 1/8″ ID and 2 m length.The oven temperature was maintained at 70°C(isothermal) while the injector and detector tem-peratures were maintained at 150°C each. Thesample injection size was 1 μl and pure hydrogenwas used as a carrier gas at a pressure of 1 kg cm–2.The GC response was calibrated for this particu-lar column and conditions with known composi-tions of ethanol–water mixtures and the calibra-tion factors were fed into the software to obtaincorrect analysis for unknown samples.

3. Results and discussion

3.1. Ion exchange capacity (IEC)

The amount of residual acetate and hydroxylgroups present after crosslinking was estimatedfrom IEC studies. It was noted that unmodifiedsodium alginate showed an IEC of 1.26 meq g–1

whereas phosphoric acid crosslinked polymer ex-hibited an IEC of 0.75 meq g–1. The IEC is equiva-lent to the total number of free acetate groups (con-sidering the fact that acetate groups are more in-teractive than hydroxyl groups), R-COONa pre-sent in the membrane, which decreased uponcrosslinking [17]. This result shown that almost50% of the acetate groups present in the unmodi-fied sodium alginate has now formed the cross-links with phosphoric acid. A model of the pos-sible interaction is shown in Fig. 2. IEC and FTIRstudies prove the occurrence of crosslinking.Hence, phosphoric acid will establish a linkagewith NaAlg through ester formation as confirmedby FTIR (Fig. 3). To the best of our knowledge, itis the first kind of study wherein phosphoric acidis employed as a crosslinking agent and the mem-brane could withstand the solvent environmentand pervaporation employed in this study.

3.2. FTIR studies

The FTIR spectrum of the pure and the cross-linked NaAlg are shown in Fig. 3. The spectrumof NaAlg shows the prominent peaks of C=Ostretching of the carboxylic group at 1651 cm–1

and the characteristic peak of free hydroxyl groupis observed in the range of 3188 cm–1 to 3583 cm–1

are shown in Fig. 3a. The spectrum of crosslinkedNaAlg showed a shift in the C=O peak from1651 cm–1 in the uncrosslinked membrane to1759 cm–1 for the modified membrane. This shiftmay be attributed to the formation of –OC–O–Pbond due to the interaction of hydroxyl group ofphosphoric acid with acetate group of NaAlg. Itcan also be noted that the P=O remains intact anddoes not participate in the reaction. This can beconfirmed by the presence of a peak of 960 cm–1

S. Kalyani et al. / Desalination 229 (2008) 68–81 73

H3PO4

O

O OHO

OH

COONa

C

O

O

O

O

OH

HO

OH

C OO

OO

HO

O

O O

OH

COO

P OO

O

HO

Sodium alginate Phosphoric acid

P Na Alg

which corresponds to P=O. The formation of anew peak (Fig. 3b) at 1419 cm–1 corresponds tothe interaction of hydroxyl group of phosphoricacid with carboxyl group present in the NaAlg

Fig. 2. Structural representation of Sodium alginate membrane crosslinked with phosphoric acid.

Fig. 3. FTIR spectra of uncrosslinked NaAlg (a) and Crosslinked NaAlg (b).

resulting in the formation of –P–O–C bond. Sucha changes in the spectrum confirmed the success-ful crosslinking of membrane.

74 S. Kalyani et al. / Desalination 229 (2008) 68–81

3.3. XRD studies

The XRD spectra of uncrosslinked and cross-linked NaAlg are shown in Fig. 4. From the spec-tra of unmodified (Fig. 4a) and crosslinked(Fig. 4b) sodium alginate it can be noted that theXRD pattern of uncrosslinked sodium alginatemembrane appears to be amorphous in nature,whereas the XRD pattern of crosslinked mem-brane appears to be semi crystalline in nature.From Fig. 4a it can also be seen that there are twodistinct peaks at 9° and 15° of 2 θ. The peak at15° of 2 θ indicates the presence of crystallinityas reported by Yang et al. [18]. These two peaksare related to two types of crystals: crystal 1 andcrystal 2. Crystal 1, is responsible for the separa-tion, since it corresponds to the hydroxyl func-tional group. A reduction in d–spacing value from7.2 Å for un-crosslinked to 6.88 Å in crosslinked

Fig. 4. X-RD diffractograms of uncrosslinked NaAlg (a) and crosslinked NaAlg (b).

polymer gives an indication of reaction betweenthe carboxylic groups of sodium alginate with hy-droxyl groups of phosphoric acid, which wouldimprove the selective permeation of the membrane.

3.4. TGA studies

The TGA curves of the uncrosslinked andcrosslinked NAALG membranes are shown inFig. 5. The TGA curve of NaAlg (Fig. 5a) showsthe beginning of weight loss at 220°C followedby a final decomposition at 260°C.

Fig. 5b shows that P–NaAlg begins to undergoa weight loss starting at 180°C followed by thefinal decomposition at 250°C. This observationindicates that there is no considerable differencein thermal stability before and after crosslinking.Hence the crosslinked membranes could be usedfor PV studies even at high temperatures.

S. Kalyani et al. / Desalination 229 (2008) 68–81 75

3.5. Sorption studies

The effect of equilibrium sorption percentageand degree of swelling data of the membrane inaqueous ethanol of different compositions isshown in Table 1. The crosslinked membraneshowed a high degree of sorption (42.6%) in purewater, with a relatively negligible sorption for pureethanol. From Fig. 6, it can be seen that the weightpercentage water uptake of the binary feed mix-tures increased from 12.02 to 42.63% with increasein feed water concentration from 5.2 to 38.6 wt.%

Fig. 5. TGA curves for uncrosslinked NaAlg (a) and crosslinked NaAlg (b).

Table 1Effect of feed concentration on degree of swelling, permeate water concentration and water flux

No. Feed water concentration (wt.%)

Degree of swelling

% of sorption

Permeate water concentration (wt.%)

Water flux (kg/m2.h)

PSI

1. 5.22 1.12 12.03 99.16 0.035 535 2. 9.74 1.20 19.78 98.76 0.048 242 3. 15.65 1.26 25.80 96.03 0.051 47 4. 20.52 1.32 32.30 93.71 0.065 26 5. 31.57 1.38 38.53 88.30 0.071 7 6. 38.6 1.43 42.63 80.50 0.080 4

signifying the possibility of attaining enhancedflux using these membranes. However, absorp-tion of large amounts of water at higher feed con-centrations could cause increased swelling andsubsequent fall in membrane selectivity due toplasticization of the polymer chains.

3.6. Influence of operating conditions

It is well know that separation characteristicsof a membrane depend upon the interaction be-tween solvent to be separated and the membrane

76 S. Kalyani et al. / Desalination 229 (2008) 68–81

matrix. Hydrophilic membrane like chitosan andsodium alginate can develop hydrogen bond in-teraction with water leading to preferential sorp-tion and diffusion of water through the barriermembrane [7]. The influence of feed composi-tion, membrane thickness and permeate pressureon membrane performance has been examined indetail.

3.6.1. Effect of feed composition

The relation between liquid feed compositionand pervaporation performance over a wide rangeof feed mixtures at 30°C was investigated usingcrosslinked membranes. For this study, the mem-brane thickness and permeate pressure were keptconstant at 70 mm and 0.5 mm Hg respectively.The pervaporation performance of P-sodium al-ginate membrane was investigated for varyingfeed water compositions comprising of 5.2–38.6 wt.% water keeping other operating param-eters such as permeate pressure and membranethickness constant. Expectedly, a rise in the feedconcentration of water produced an increase in

Fig. 6. Effect of feed water concentration on % sorption.

5 10 15 20 25 30 35 40

10

15

20

25

30

35

40

45

% S

orpt

ion

Feed water concentration (wt%)

the water-normalized flux from 0.245 to0.553 kg m–2 h–1 (10 μm)–1 (Fig. 7). At higher feedwater concentrations, the membrane swells ap-preciably because of the availability of more wa-ter molecules for sorption and diffusion. The pref-erential interaction with water molecules causesthe membrane to swell, leading to plasticizationand unrestricted and quicker transport of bothvolatile components through the barrier. On swell-ing, the polymer chains become more flexible andhence the transport through the membrane be-comes easier for both the feed components result-ing in high flux [19]. Hence the selectivity de-creased from 2182 at 5.2 wt.% feed water con-centration to 6.58 at 38.6 wt.%. Higher water con-centrations render greater swelling of the mem-branes that enables permeation of both compo-nents into the downstream side yielding higherflux but a drop in selectivity. Thus the swellingincreases with increasing water concentrationleading to reduction in membrane selectivity.However, it is worth mentioning that the mem-brane showed promising results for dehydratingfeeds having 5–25% water. Moreover, the azeo-

S. Kalyani et al. / Desalination 229 (2008) 68–81 77

tropic composition of 96 wt.% ethanol was easilybroken by pervaporation with >99% water con-centration in the permeate.

3.6.2. Effect of membrane thickness

The effects of varying membrane thickness onseparation performance was studied at constantfeed composition (azeotropic) and permeate pres-sure (0.5 mm Hg) by synthesizing membranes ofthickness ranging from 30 μm to 120 μm. With anincrease in the membrane thickness a gradual re-duction in the flux from 0.102 to 0.016 kg m–2 h–1

can be clearly evidenced from Fig. 8. Though theavailability of polar groups enhances with an in-crease in the thickness, flux decreases since dif-fusion becomes increasingly retarded as the feedmolecules have to travel a greater distance to reachthe permeate side. The permeate concentration ofwater varied from 96.2 to 99.4 wt.% which meantthat the selectivity increased from 469 to 3182.In pervaporation process, the upstream layer ofthe membrane is swollen and plasticized due to

Fig. 7. Effect of feed water composition on PV performance of NaAlg (membrane thickness 70 μm and permeate pressureat 0.5 mm Hg).

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42

0

500

1000

1500

2000

2500

Feed water concentration (Wt%)

Mem

bran

e se

lect

ivity

0.2

0.3

0.4

0.5

0.6

Norm

alized fulx (Kg/m

2. hr. 10 μm)

absorption of feed liquid and allows unrestrictedtransport of feed components. In contrast, thedownstream layer is virtually dry due to continu-ous evacuation in the permeate side and thereforethis layer forms the restrictive barrier which al-lows only interacting and smaller sized moleculessuch as water to pass through. It is expected thatthe thickness of the dry layer would increase withan increase in the overall membrane thickness re-sulting in improved selectivity as observed in thepresent case.

3.6.3. Effect of permeate pressure

The permeate pressure was varied from 0.5 to8 mm Hg, to study the permeation characteristicsat a constant thickness of 70 μm and azeotropicfeed composition. At lower pressures (highvacuum) the influence of the driving force on thediffusing molecules in the membrane is high, andwill result in the components being swept outimmediately from the permeate side resulting inhigh mass transfer rates. Fig. 9 shows that the

78 S. Kalyani et al. / Desalination 229 (2008) 68–81

Fig. 8. Effect of membrane thickness on PV performance (azeotropic feed composition 4 wt.% of water and permeatepressure at 0.5 mm Hg).

20 40 60 80 100 120

500

1000

1500

2000

2500

3000

3500

Membrane thickness (μm)

Mem

bran

e Se

lect

ivity

0.00

0.02

0.04

0.06

0.08

0.10

Flux (Kg/m

2. hr)

Fig. 9. Effect of permeate pressure on PV performance (azeotropic feed composition 4 wt.% of water and membranethickness 70 μm).

0 2 4 6 8

500

1000

1500

2000

2500

Permeate Pressure (mm Hg)

Mem

bran

e Se

lect

ivity

0.05

0.10

0.15

0.20

0.25

0.30

Norm

alized flux (Kg/m

2.hr. 10 μm)

S. Kalyani et al. / Desalination 229 (2008) 68–81 79

membrane exhibits considerable lowering of nor-malized flux from 0.245 to 0.081 kg m–2 h–1.(10 μm)–1

as well as a reduction in selectivity from2187.57 to 469, with an increase in permeate pres-sure from 0.5 mm Hg to 8 mm Hg. Under highvacuum conditions (lower pressures) diffusionthrough the membrane is the rate determining stepof the pervaporation process and the diffusing wa-ter molecules experience larger driving force,which enhances the desorption rate at the down-stream side. Lower vacuums reduce the drivingforce, thus slowing desorption of molecules. Insuch cases the relative volatilities of the two com-ponents of the mixture govern the separation fac-tor of the membrane.

4. Comparison of present work with literature

Data from literature on selectivity and flux ofdifferent membranes for pervaporation separationof ethanol/water system are included in Table 2[20–24] along with those obtained in the presentstudy. The data in the table indicate that the fluxand selectivity of P–SA membranes are compa-rable with those of other membranes. Further, P–SA has optimum values for selectivity and flux,whereas in the case of other membranes maxi-

Table 2Comparison of flux and selectivity of PVA–SA blend membrane with values reported in literature

CS/SA — chitosan/sodium alginate; CS/HEC — chitosan/hydroxyethylcellulose; SA — sodium alginate; poly(vinylalcohol)/poly (styrene sulfuric acid); P–SA — phosphorylated sodium alginate

Feed composition No. Membrane Water (%) Ethanol (%)

Selectivity Flux (kg m–2 h–1) Reference

1 Novel two ply composite CS/SA

5 95 1110 0.07 8

2 CS/HEC 10 90 10491 0.11 20 3 CS/SA 13.5 86.4 436 0.22 21 4 Chitosan 10 90 8000 0.26 22 5 SA 10 90 120 0.29 23 6 PVA/PVS 6.2 93.8 700 0.50 24 7 P–SA 5.2 94.8 2182 0.24 Present work

mum flux is associated with lower values of se-lectivity and vice-versa except in case of chitosanmembrane. Furthermore, the ease in fabricationof these membranes associated with low cost ren-der them more attractive for pervaporation ofaqueous alcohols.

5. Conclusions

Sodium alginate membranes were prepared forpervaporation-based dehydration of ethanol wa-ter mixture. The efficient water permselectivecharacteristic of alginate could be appropriatelyutilized to yield good flux, separation factors, andbetter thermal properties. Sorption studies re-vealed that the membrane had greater affinity to-wards water than ethanol. With increasing feedwater concentration, the membrane performancewas found to be affected substantially by increasein the extent of swelling of the polymer, whichresulted in a rise in flux but a reduction in selec-tivity. The crosslinked membranes were found toshow promising performance for dehydration ofethanol containing smaller amounts of water. Theazeotropic composition of ethanol water mixturewas easily broken by pervaporation. Increasingmembrane thickness decreased the flux but had a

80 S. Kalyani et al. / Desalination 229 (2008) 68–81

less profound effect on the separation factor.Higher permeate pressure caused a reduction inboth flux and selectivity.

Symbols

A — Membrane areaB — Amount of sulfuric acid used to neutral-

ize blind sample soaked in NaOHJi — Flux of water component im — Mass of the membraneMd — Mass of the dry polymerMs — Mass of the swollen polymerP — Amount of sulfuric acid used to neutral-

ize the sulfonated membrane soaked inNaOH

t — Evaluation timeWi — Mass of water in permeatex,y — Feed and permeate water concentrationα — Membrane selectivity(1 – x), (1 – y) — Feed and permeate ethanol

concentration

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