Journal of Membrane Science 340 (2009) 52–61

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Journal of Membrane Science

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Ionic transport phenomenon across sol–gel derived organic–inorganiccomposite mono-valent cation selective membranes

Mahendra Kumar, Bijay P. Tripathi, Vinod K. Shahi ∗

Electro-Membrane Processes Division, Central Salt and Marine Chemicals Research Institute, Council of Scientific & Industrial Research (CSIR),G.B. Marg, Bhavnagar 364021, Gujarat, India

a r t i c l e i n f o

Article history:Received 2 June 2008Received in revised form 5 May 2009Accepted 7 May 2009Available online 15 May 2009

Keywords:Ionic-transportMono-valent cation selective membranesWater transport numberSol–gel

a b s t r a c t

Organic–inorganic composite mono-valent cation selective membranes (MCSMs) were prepared bysol–gel under acidic conditions, in which sulfonic acid groups were introduced at the inorganic seg-ment. Studies on physicochemical and electrochemical properties revealed their excellent mechanical,thermal, and oxidative stabilities, high conductivity, ion-exchange capacity, permselectivity for mono-valent cations, ionic diffusion and water transport number. These properties suggested the suitability ofMCSMs, especially Si-65%, for electro-separation of Na+ from Ca2+, Mg2+, and Fe3+. The effect of electrolytesolution on the characteristics of the current–voltage (i–v) curve in MCSM was studied based on the con-centration polarization. Electro-transport of different ions in terms of plateau length and concentrationprofiles for different ions in the solution phase, diffusion boundary layer and membrane phase werepresented. Information obtained from i–v curve analysis were validated by electrodialysis (ED) experi-

Organic–inorganic composite ments for individual or mixed electrolyte solutions. Electro-transport efficiency and separation factor ofdifferent ions for MCSM and Nafion117 (N117) membranes were compared, which suggested suitability

cation

1

aicaitaoftnItIcfma

0d

of MCSMs for separating

. Introduction

In the past few year membrane technologies have gained muchttention in the industry and in research centers because of anncreasing necessity of versatile and economical separation pro-esses [1–3]. The ion-exchange membrane (IEM) is one of the mostdvanced separation membranes, which have been widely usedn the solutions containing multi-ions: electrodialytic concentra-ion of seawater or brackish water to produce sodium chloridend potable water, separation of inorganic ions from unchargedrganics, alkali and transition metal ion recovery, as a separatoror electrolysis such as chlor-alkali production, etc. [4–11]. For allhese applications, IEMs should exhibit high ionic conductivity, buto electronic conductivity, and high counter-ion permselectivity.

n addition, membranes should display good mechanical proper-ies and high chemical stability in unfriendly working conditions.n particular, requirements for IEMs having selectivity for spe-

ific ions have been increasing for the removal of valuable ionsrom waste solutions with high efficiency, separation of mono- and

ulti-valent cations from mixed electrolyte solutions. Thus, sep-ration of ions with same charge using ion-exchange membranes

∗ Corresponding author. Tel.: +91 278 2569445; fax: +91 278 2567562/2566970.E-mail addresses: vkshahi@csmcri.org, vinodshahi1@yahoo.com (V.K. Shahi).

376-7388/$ – see front matter © 2009 Published by Elsevier B.V.oi:10.1016/j.memsci.2009.05.010

s.© 2009 Published by Elsevier B.V.

is important and challenging from both industrial and academicviewpoint.

Several efforts have been made to prepare mono-valent cationselective membrane (MCSM) by adsorption or ion exchange acationic polyelectrolyte such as polyethyleneimine PEI on the sur-face of the cation-exchange membranes (CEMs) [12,13]. However,the mono-valent cation permselectivity of the membranes grad-ually deteriorates during ED because the PEI was desorbed fromthe membrane surface. Thus, fixation of the cationic polyelec-trolyte on the membrane surface by covalent bonding has beenactively studied. On the other hand, conducting polymers suchas polyaniline, and polypyrrole, were used to modify the surfacesof CEMs from the viewpoint of the selective permeation of spe-cific cations [14–17]. Polypyrrole/polyaniline is highly rigid andtight polymers with weakly basic anion-exchange groups, andtherefore restricted their permeation into the interior of the mem-brane. On other hand, modification of CEMs surfaces by polyrroleor polyaniline with basic nature led to an increase in membraneresistivity due to decline in surface charge density. Thus, thesetypes of composite membranes were not very suitable for electro-separation of cations. For achieving ion specific selectivity, it is

necessary to have controllable membrane surface charge density,its hydrophilic/hydrophobic nature and porosity.

Organic–inorganic nanostructured composites constitute anemerging research field, which opened the possibility to con-trol the hydrophobic/hydrophilic micro-domains, surface charges

embra

aoai[rcetuihtdbdoaogcpwbcsttisfda

2

2

rsmfiafdmppmcwsc

2c

mddtp

M. Kumar et al. / Journal of M

nd porosity for designing MCSM of new generation. This typef materials showed the attractive properties of a mechanicallynd thermally stable inorganic backbone and the specific chem-cal reactivity and flexibility of the organo-functional groups10,18–23]. Organic–inorganic nanocomposite membranes wereecently proposed as an alternative of Nafion membrane for fuelell applications because of their high water retention capacity atlevated temperature and comparable physicochemical and elec-rochemical properties to the Nafion [10,24–28]. Nafion is mainlysed for fuel cell and electro-membrane processes because of

ts excellent conductivity and stabilities [29–31]. Aggregations ofydrophilic pendent SO3

− groups form isolated ionic clusters inhe hydrophobic perfluorinated polymer matrix resulted randomistribution of spherical ionic clusters [32,33]. This problem cane cleverly solved by well-organized hydrophobic and hydrophilicomains with controllable functionalization and porosity of therganic–inorganic nanocomposite membrane while tailoring suit-ble MCSM. In the following text, we describe the preparationf organic–inorganic nanocomposite MCSMs, in which functionalroups (–SO3H) were grafted on the inorganic segment (silica), withontrollable charged nature, ionic selectivity, ionic and water trans-ort across it. Ionic transport properties of the developed MCSMsere compared with N117 (commonly used cation selective mem-

rane). Although several types of cation selective membranes areommercially available but N117 was considered due to it is welltudied and stable nature [2,32]. The current–voltage characteris-ics of these membranes were used to assess their suitability forhe fractionation of cations with different charge and hydratedonic radius. Concentration profiles of the counter-cations were pre-ented under polarized conditions. Latter informations obtainedrom i–v curves were validated by elelctrodialytic experiments ofifferent electrolyte mixed solutions, which showed the potentialpplicability of MCSMs.

. Experimental

.1. Materials

Poly(vinyl alcohol) (PVA) (Mw: 125,000 g mol−1), hydrochlo-ic acid, ammonia (sp gr 0.98), hydrogen peroxide (37–40% w/v)olution, sodium hydroxide, sodium chloride, calcium chloride,agnesium chloride, ferric chloride, etc. of AR grade were obtained

rom S. d-fine Chemicals, Mumbai, India. Tetraethyl orthosil-cate (TEOS), mercaptopropylmethyldimethoxysilane (MPDMS),nd Nafion 117 (N117, perfluorinated) membrane were obtainedrom Sigma–Aldrich Chemicals and used as obtained. Double-istilled water was used in all experiments. Anion-exchangeembrane (AEM) used in these investigations was based on inter-

olymer of polyethylene and styrene–divinyl benzene copolymer,repared by the procedures reported earlier [2,34]. To thin poly-er films, quaternary ammonium groups were introduced by

hloromethylation and subsequent ammination. These membranesere used for versatile industrial applications of the electrodialy-

is process because of their high chemical stability and high ioniconductivity due to their strong ionic character [2,35,36].

.2. Preparation of organic–inorganic composite mono-valentation selective membranes by sol–gel

Preparation methodology of organic–inorganic composite

embrane was reported earlier [20]. PVA was dissolved in hot

eionized water to obtain a homogeneous solution (10% w/v). Thenesired amount of TEOS and MPDMS were added and pH was main-ained at 2.0 using 1 M HCl, in the case of the acid-catalyzed sol–gelrocess. Amounts of TEOS and MPDMS were taken in such way that

ne Science 340 (2009) 52–61 53

total silica content was varied between 50 and 65% (w/w) with PVA.The resulting gel was cast on a cleaned glass plate to a desired thick-ness and dried at room temperature to obtain a film. These filmswere then immersed in a solution containing formaldehyde (54.1 g),sodium sulfate (150.0 g), sulfuric acid (125.0 g), and water (470.0 g)for 2 h at 60 ◦C for effecting cross-linkage in the membrane by for-mal reaction. The membranes obtained were subjected to oxidationfor the conversion of the mercapto group into the sulfonic acidwith hydrogen peroxide. The resulting membranes before beingsubjected to electrochemical studies were conditioned in 0.10 MHCl and 0.10 M NaOH solutions alternately several times and thenequilibrated with the experimental solution for further characteri-zation. Thus prepared MCSMs were designated as Si-X%, where X isthe silicon content (%) in the membrane phase.

2.3. Membrane characterization

FTIR spectra were obtained using spectrum GX series 49387.The thermal stability and degradation process of the membraneswere investigated by thermogravimetric analyzer (TGA/SDTA851c

with starc software, Mettler Toledo). The TGA thermograms wererecorded between 50 and 600 ◦C with 10 ◦C/min heating rate underthe nitrogen atmosphere. Mechanical strength of the membraneswas analyzed by dynamic mechanical analyzer (DMA861c withstarc software, Mettler Toledo) under isothermal conditions. Forscanning electron microscopy (SEM), gold sputter coatings werecarried out on the desired membrane samples at pressure rangingin between 0.1 and 1 Pa. Sample was loaded in the machine, whichwas operated at 10−2–10−3 Pa with EHT 15.00 kV with 300 V collec-tor bias using Leo microscope and SEM images were recorded andgiven in supporting information.

MCSM membranes were characterized by measurements oftheir ion-exchange capacity (IEC) in equilibration with differentions, water uptake, counter-ion transport numbers (tm

i) and equiv-

alent membrane conductivity (�m) in equilibration with differentelectrolytic solutions and their method of determinations is given insupporting information. The dimensional change (�Lx) of the mem-branes were investigated by refluxing square pieces of the samplesin water at 70 ◦C for 12 h. Samples were dried at 80 ◦C for 12 h andchange in the dimensions of the swollen and dried membranes wererecorded for estimating �L by Eq. (1):

�Lx = Lx − Lxo

Lxo(1)

where Lx and Lxo may be the length/width/thickness of the swollenand dried membrane, respectively. Uncertainties for the mea-surements of IEC, (tm

i), �m and �Lx were 0.01 mequiv/g, 0.01,

0.01 S cm2 equiv−1, and 0.1%, respectively.

2.4. Cation diffusion coefficients through membranes

In order to study interactions between different cations andMCSM matrix, cation permeability was measured using two com-partments diffusion cell. Both compartments were separated bycircular piece of MCSM of 20.0 cm2 effective area and were stirredby magnetic stirrers. Two peristaltic pumps were used for recircula-tion of both streams. Diffusion of ions was monitored by measuringconductivity of both compartments with respect to time. The geo-metric characteristics of the cell can be grouped into a constant ˇ,may be called as cell constant, and estimated by

A(

1 1)

ˇ =L V1

+V2

(2)

where A is effective membrane area (20.00 cm2), L the membranethickness (0.015 cm), V1 and V2 are the volume of solutions used incompartments 1 and 2 (100 cm3).

5 embrane Science 340 (2009) 52–61

∫

l

wcTc(to

2

cscmcteecMdkcrpcb

�

weit

t

w�a0

2e

cDsams

� = zimF

MQ(9)

4 M. Kumar et al. / Journal of M

From Fick’s second law for diffusion can be written as:

∂C2

(C1 − C2)= ˇD∂t (3)

Under the conditions, t = 0 to t = f, integrated the Eq. (3) is,

t=f

0

∂C2

(C1 − C2)= ˇD

∫∂t (4)

Finally get the equation as:

n

(C1

f− C2

f

C10 − C2

0

)= −ˇDt (5)

here C is the final concentration or initial concentration in theompartment 1 or 2, D the diffusion coefficient, t the diffusion time.he super and subscripts are 0, initial; f, final; 1, compartment 1; 2,ompartment 2. By Eq. (2), diffusion coefficients of different cationsNa+, Ca2+, Mg2+ and Fe3+) were estimated. 0.01–0.10 M initial elec-rolyte concentrations were taken. Uncertainty for the estimationf D was 0.01 × 10−7 cm2 s−1.

.5. Electro-osmotic permeability and water transport numbers

An electro-osmotic permeability measuring cell contained twohambers with a volume of 100 cm3, made of acrylic glass andeparated by the MCSM of 20.0 cm2 cross-sectional areas. Bothhambers were kept in a state of constant agitation by means ofagnetic and mechanical stirrers. A known amount of constant

urrent was imposed across the membrane with the help of poten-iostat/galvanostat (Auto Lab, Model PGSTAT 30), using Ag/AgCllectrodes fixed in both chambers, while resultant potential differ-nce across the membrane was measured with the help of saturatedalomel electrodes. The water dragged by the cations crossing theCSM from the anode to the cathode was determined from the

isplacement of the liquid in the horizontal fixed capillary tube ofnown radius in the cathodic compartment of the electro-osmoticell. Several measurements were performed in order to obtaineproducible values. The change in volume in the cathodic com-artment, �V, arising from the transport of Q coulombs of electricalurrent across the membranes is related to observed volume, �V0,y [37,38]:

V = �V0 + Q

F(V̄AgCl − VAg − t+V̄MCl) (6)

here V̄MCl and t+ are, respectively, the partial molar volume of thelectrolyte and the cation transport number in the solution, and Fs Faraday’s constant (96480 C/mol). The water transport number,0w , in moles of water per Faraday is given by

0w = F

Q

�V

Vw

= F�

18(7)

here Vw is the partial molar volume of water (18 cm3/mol) and, the volume of water flowing per coulomb (�V/Q) from the

node to the cathode. Uncertainty for the estimation of t0w was

.1 mol/Faraday.

.6. Current–voltage polarization curves and electro-transportfficiency of different cations

Current–voltage polarization curves were recorded in a four-ompartment cell with four-electrode arrangement (Fig. 1) under

C using 0.01 M different electrolytic solutions. Constant current in

teps was applied through the anode and cathode, which were sep-rated by AEM. Whereas, the resultant potential across MCSM waseasured with the help of salt bridges, placed near the membrane

urfaces, and saturated calomel electrodes using digital multimeter.

Fig. 1. Four-compartment cell for recording current–voltage polarization curves andelectro-transport efficiency of different cations across MCSM.

The membrane was equilibrated in the measuring solution for morethan 24 h Na2SO4 solution (0.1 M) was used in both electrode com-partments to minimize the influence of electrode reactions, whileelectrolytic solutions of desired concentration was passed throughother two compartments divided by MCSM. All the compartmentswere feed by recirculation of the different streams, separately usingperistaltic pumps with 100 cm3/min flow rate. Total volume of eachcompartment was 8.0 cm3.

Current–voltage curves were recorded in steady-state mea-surements, by applying a stepwise increase in current densitythrough the cell and recording the potential drop across the MCSMmembrane. Flow of current would result electro-migration ofcounter-ions across MCSM from compartment 1 to 2. Whole phe-nomena were governed by Faraday’s law, and current density, I, canbe given by [1,38]:

I = ziDiF(Cb − Cm)ı(tm

i− ts

i)

(8)

where tmi

and tsi

are the counter-ion transport numbers acrossmembrane and solution phase, respectively; zi is electro-valenceof the counter-ion; Di is the diffusion coefficient of counter-ion;F is the Faraday constant; Cb is bulk concentration of elec-trolyte solution, Cm is the electrolyte concentration in solution atmembrane–solution interface, and ı is the thickness of the bound-ary layer. No variation in pH change in compartments 1 and 2 (Fig. 1)was occurred. Change in the concentrations of counter-ions acrossMCSM due to their electro-migration from compartment 1 to 2 wasmeasured as a function of coulombs passed, and used for the esti-mation of electro-transport efficiency (�) for different counter-ionsby [1,39]:

where M is the molecular weight of electrolyte passed through com-partment 1; m is the weight of electrolyte electro-transported fromcompartment 1; and Q is the number of coulombs (A s) passed.Uncertainty for the estimation of � was 0.01.

M. Kumar et al. / Journal of Membra

F

3

3

cpbTfmotcbtcmRicf

i81Toura–gbhw

bi

Water uptake at equilibrium is tabulated in Table 1 for differentMCSMs. It can be observed that an increase in the silica content inthe membrane phase leads to a substantial increment in the wateruptake of the membrane; this may be attributed to the increase inmolality of the functional groups because it is proportional to the

ig. 2. Schematic structure of sol–gel derived organic–inorganic composite MCSM.

. Results and discussion

.1. Membrane preparation and characterizations

For the preparation of organic–inorganic composite MCSMs,ondensation polymerization of the silica precursor (mercapto-ropylmethyldimethoxysilane) was carried out in aqueous mediay acid-catalyzed sol–gel process in presence of poly(vinyl alcohol).he resulting membranes were cross-linked with formaldehyde,ollowed by the oxidation with hydrogen peroxide to convert the

ercapto group into the sulfonic acid group. Schematic structuref the membrane material is shown in Fig. 2. We observed thathe nature of the catalyst affects the properties of the resultingharged membranes. Acid-catalyzed sol–gel process was precededy rapid protonation of the OR or OH substituents bonded directlyo the silicon atom, and formed linear polymers, which were weaklyross-linked due to steric crowding. While base catalysis formsore highly branched clusters due to more rapid hydrolysis [20].

esulting acid-catalyzed sol–gel derived membranes were flexiblen nature, while base catalyzed membranes were brittle. Thus, acid-atalyzed organic–inorganic composite MCSMs were prepared forurther investigation.

FTIR spectra of MCSMs were recorded and given in support-ng information, which showed characteristic absorption bands at00 cm−1 (characteristic of the symmetric Si–O–Si stretch) and050–1200 cm−1 (characteristic of the asymmetric Si–O–Si stretch).hese observed bands confirmed that hybridization betweenrganic and inorganic parts was successfully achieved at the molec-lar level [40]. Presence of the broad band in the 1030–1090 cm−1

egion confirmed –SO3H group. The broad peak absorption atround 3400 cm−1 indicated that there were a significant number ofOH groups due to the noncondensed SiOH and/or unreacted –OHroups of the PVA or –OH groups of water contained in the mem-ranes. We suppose that these –OH groups provide the sites forydrogen bonding between polymer and water, and provide high

ater retention capacity at elevated temperatures [20,41].

Membranes thermal and mechanical stabilities were evaluatedy recording TGA and DMA curves and are presented in supporting

nformation. TGA curve was fitted using three main degrada-

ne Science 340 (2009) 52–61 55

tion stages, arising from the processes of thermal desolvation(<100 ◦C), thermal desulfonation (300–400 ◦C), and thermal oxi-dation/degradation of the polymer (>500 ◦C). It was also observedthat an increase in the silica content in the MCSM matrix delayedthe weight loss and thus improved the membrane thermal stabil-ity. MCSMs were also exhibited good mechanical strengths underconstant applied force (15 N) and frequency (10 Hz), and withoutany breaking and/or elongation in the DMA curves. Boil watertest for 6 h of MCSMs was performed by recording the membraneIEC and conductivity values, before and after treatment, but nomeasurable change was noticed. Oxidative stability of these mem-branes was also studied by recording their weight loss after Fenton’sreagent (3% H2O2 containing 2 ppm FeSO4) at 60 ◦C for 1 h. About4–6% weight loss was recorded for MCSM with different silica con-tent, while N117 lost 5% weight under same oxidative conditions.These studies indicated excellent stable nature of sol–gel derivedorganic–inorganic composite MSCM.

Ion-exchange capacity indicates the density of exchangeablefunctional groups in the membrane matrix, which are responsi-ble for the ionic conductivity of the ion-exchange membrane. Ingeneral, membranes having the same degree of cross-linking andcomposition absorb the same amount of water, where the densityof ionizable groups is the same throughout the membrane matrix[42,43]. IEC values for MCSMs were measured in equilibration withdifferent electrolyte solutions and results are presented in Fig. 3 asfunction of silica content in the membrane matrix. For MCSM withdefinite composition, IEC values for different exchangeable cationsfollow the trend: Na+ > Ca2+ > Mg2+ > Fe3+, which is similar to theirhydrated ionic radius. It seems, exchange of bulkier cations is rel-atively low due to their difficult accessibility to the exchangeablefunctional (–SO3H) groups in the membrane matrix. IEC arouseddue to the presence of the –SO3H group after the oxidation of –SHgroup present in MPDMS and thus was proportional to the silicacontent. Also, relatively high IEC values of MCSMs in equilibra-tion with NaCl, in comparison to N117 (0.90 mequiv/g) [44] maybe explained on the basis of polymer structure. For linear poly-mer clusters (acid-catalyzed sol–gel), the availability of functionalgroups for exchange will always be higher in comparison to highlybranched polymer clusters [20].

Fig. 3. Variation of IEC with silica content in the MCSM matrix for differentexchangeable cations.

56 M. Kumar et al. / Journal of Membrane Science 340 (2009) 52–61

Table 1Water uptake, percentage dimensional change and cation transport numbers in themembrane phase (tm

i) values.

Membrane Wateruptake1 (%)

Dimensionalchange2 (%)

(tmi

)a

Na+ Ca2+ Mg2+ Fe3+

Si-50% 35.6 11.2 0.84 0.58 0.38 0.32Si-55% 40.1 10.3 0.88 0.63 0.41 0.36Si-60% 46.7 9.6 0.91 0.65 0.45 0.40Si-65% 50.5 8.9 0.93 0.69 0.49 0.41Si-70% 56.1 8.6 0.93 0.70 0.52 0.44N117 41.6 7.2 0.92 0.84 0.79 0.55

b1

sawotwittah

tmTctdtc1Tt(tacwFiocbmmiIawft

3

ascwsi

a (tmi

) values were estimated from membrane potential measurements in equili-ration with 0.01/0.001 M electrolyte solutions. Uncertainty for measurements of:: 0.1%; 2: 0.1%; 3: 0.01.

ilica content on which –SO3H groups were introduced. Addition-lly, the greater content of silica in the membrane forming materialill result higher absorption of water due to water retentive nature

f silica. Water uptake values of these membranes were very closeo that for N117 membrane (41.6%). MCSM shows slightly higherater uptake values in spite of hydrophobic nature of introduced

norganic segments (silica) in the PVA matrix. Silica forms very rigidhree-dimensional network [45], which might be inhibited the crys-al growth of PVA [46] as results of relative higher salvation of –OHnd exchangeable functional groups were responsible for relativelyigh water uptake of MCSM.

When the membrane separates solutions of unequal concentra-ions of an electrolyte, an electrical potential develops across the

embrane due to the different mobility of counter-ion and co-ion.he magnitude of membrane potential depends on the electricalharacteristic of the membrane along with the nature and concen-ration of electrolyte solution used [47]. The membrane potentialata for different MCSMs were obtained in equilibration with elec-rolyte solutions of unequal concentration (the ratio of electrolyteoncentration of the higher to the lower side, was kept constant at0.0 while mean electrolyte concentration was fixed at 0.055 M).his was used for the estimation of cations transport numbers inhe membrane phase (tm

i) presented in Table 1, according to TMS

Teorell, Meyer, and Sievers) approach [37]. For an ideal ion selec-ive membrane tm

ivalue should be close to one, all MCSMs behaved

s highly Na+ selective membranes. Selectivity of MCSMs for otherounter-ions (Ca2+, Mg2+ and Fe3+) was quite low in comparisonith Na+. The hydrated ionic radius values of Na+, Ca2+, Mg2+ and

e3+ are 450, 600, 800 and 900 pm, respectively [48]. Thus, selectiv-ty of MCSM was decreased with increase in hydrated ionic radiusf counter-ions. Due to the inclusion of silica in the PVA matrix,ompactness of MCSMs was enhanced, and easy accessibility ofulkier counter-ions to the fixed ionogenic sites on the membraneatrix became more and more difficult for more compact/rigidembrane matrix. Further, for all counter-ions, membrane selectiv-

ty was increased along with silica content in the membrane matrix.n the case of MCSM, functional groups (–SO3H) were introducedt inorganic part, and their concentration in the membrane matrixas proportional to the silica content. Thus, membranes with high

unctional group concentration exhibited more selectivity due tohe enhanced Donnan-exclusion of co-ions [37].

.2. Membrane conductivities

Membrane conductivity is provided an important information tosses the suitability of given membrane achieving selective electro-

eparation under desired electrolytic environment. The equivalentonductivities (�m) of different MCSMs (1.50 × 10−4 m thickness)ere measured in equilibration with 0.01 M different electrolytic

olutions, and data are presented in Fig. 4 as function of sil-ca content in the membrane matrix. With the increase in silica

Fig. 4. Equivalent membrane conductivity (�m) values as function of silica contentin the MCSM matrix in equilibration with different electrolyte solutions on 0.01 Mconcentration.

content, �m values, initially increased slowly up to 65% (w/w)and than decreased rapidly. Incorporation of silica more than 65%(w/w) was not possible, because membrane could not withstandas thin film due to its low mechanical stability. Initial incrementin �m may be attributed to: (i) increase in ion-exchange capac-ity value for membranes with silica incorporation, (ii) reductionin swelling of the membrane matrix with the increase in silicacontent because of highly cross-linked or compact MCSM matrix.Factor (i) is expected to increase concentration of the functionalgroups in the membrane matrix, which plays vital role for mem-brane conductivity and/or selectivity. While factor (ii) on the otherhand will results enhancement in membrane void volume due tohigher degree of cross-linking and consolidation of polymer clus-ters, expected to increase water uptake values of the membrane (asin this case). It seems that the both factors together were respon-sible for the observed initial increment in �m values with silicacontent. Relatively high �m values for MCSMs in equilibrium withNaCl solution may be explained on the basis of high affinity betweenNa+ and membrane matrix, its lower hydrated ionic radius and thushigh mobility. Thus conductivity of MCSM in sodium form washigher than in comparison with other metal ions forms. Thoughthe conductivity mechanism across charged polymer electrolytemembranes is not a well-understood phenomenon [38,49], thisprocess presumably involves dissociation of H+ from fixed acidicsite (–SO3H) in the membrane matrix, subsequent exchange of H+

with other counter-ion and finally diffusion of the counter-ion inthe confined water within polymer matrix [49]. Further, �m valuefor Nafion 117 membrane (13.25 mS cm−1 equiv−1) in equilibrationwith 0.01 M NaCl solution is quite high in compare with preparedmembranes. Thus, we will have further idea about water and saltdiffusion process for assessing the specific selectivity of MCSMs.

3.3. Cation diffusion coefficients and water transport numbers

The Na+ diffusion coefficient (Di) values for different MCSMscan be seen in Fig. 5(A) as a function of its concentration. As inusual cases, Di values for Na+ decreased slowly with concentration

because of lower counter-ion mobility at higher equilibrating con-centration. But remarkable changes in Di values were observed forthe MCSMs with different silica content. As seen from Fig. 3, con-centration of exchangeable ionic sites (IEC) were increased withincrease in the silica content in the membrane matrix, because

M. Kumar et al. / Journal of Membrane Science 340 (2009) 52–61 57

oncentrations of NaCl; (B) different MCSMs as function of hydrated cation radius.

fdbtfd(pvtanwvbrpmNMgobobFva

swtcr6ictop

P

wmlb

of an electrical field should not be ruled out. Furthermore, it isnecessary to optimize the silica content in the membrane matrixfor achieving an efficient membrane with high ionic migration andleast water transport.

Table 2�V, �i and Ilim values for different membranes derived from Fig. 8(A).

1 2 −2 3 −2

Fig. 5. Cation diffusion coefficient values for: (A) Si-65% MCSM at different c

unctional groups were introduced at the inorganic part. Diffusionalata indicate some interaction between –SO3H groups of the mem-rane matrix and Na+ occur in chemical way rather than by size ofhe counter-ions. Di values for different ions are also presented asunction of their hydrated ionic radius in Fig. 5(B), in which a clearifference in the Di values for Na+ (450 pm), Ca2+ (600 pm), Mg2+

800 pm) and Fe3+ (900 pm) (hydrated ionic radius are given in thearentheses) ions can be seen. In the case of Na+, variation in Dialues with silica content or exchangeable charge concentration inhe membrane matrix was large, while for other cations no clearlteration was observed for different silica content. If MCSMs didot show any interaction with Na+, and size of the diffusing ionsould be the only parameter for defining their diffusion coefficient

alues through membrane. Then multi-valent ions would have noteen shown almost equal Di values for different MCSMs. Incorpo-ation of silica in the PVA matrix and functionalization of inorganicart formed well-organized hydrophilic channels in the membraneatrix, through which ionic diffusion occurred. Among all cations,a+ has high interaction within these hydrophilic channels and thusCSMs behaved as strong mono-valent selective. This study sug-

ested that membrane selectivity could be altered by the formationf hydrophilic channels by varying inorganic content in the mem-rane matrix. These results show, a different reality and diffusionf these ions is not only a phenomenon of electrostatic interactionetween counter-ions and fixed charges in the membrane matrix.urthermore, observations obtained from the measurement of tm

ialues also verify the trend of diffusional migration of these ionscross MCSMs.

Water dragged by ionic migration through MCSMs was mea-ured by electro-osmotic permeation, and expressed in terms ofater transport number (t0

w) plotted as function of silica content inhe membrane matrix for different electrolyte solutions of 0.01 Moncentration in Fig. 6. For NaCl solution, electro-osmotic flow wasather high, with t0

w reaching a value of 174 mol/Faraday for Si-5% MCSM. The value of t0

w initially undergoes sharp increase withncrease in silica content in the membrane matrix and at high silicaontent, it turned towards limiting. The electro-osmosis arises fromhe motion of ions and thus associated water molecules in the poresf membranes under the action of an electric field. Electro-osmoticermeability (Pe) was expressed by [4]:

e = ωXF

�0m(10)

here X is the concentration of fixed ionic charge in the membraneatrix, �0 is the specific friction coefficient between the moving

iquid and walls of the membrane pores, m is the specific mem-rane conductivity, and ω = −1 and +1, for negatively or positively

Fig. 6. Variation of water transport number across MCSM with silica content in themembrane matrix for different electrolyte solutions. t0

w values were derived from theelectro-osmotic experiments for 0.01 M electrolyte solutions at 1.0 mA cm−2 appliedcurrent density.

charged membranes. Accordingly, the electro-osmotic permeabil-ity and thus t0

w are directly proportional to the concentration fixedcharges (X). With the increase in silica content in the membranematrix, IEC and thus concentration of fixed charges increased, andwas responsible of high water transport numbers. Furthermore,relatively high t0

w for Na+ than other multi-valent in spite of itsleast hydration numbers suggested the selective electro-migrationof Na+ across MCSM. Also, the possibility of the existence of sodiumion selective clusters in the hydrophilic micro-domains that con-tribute to drag an anomalous amount of water under the action

Membranes �V (V) �i (mA cm ) Ilim (mA cm )

Si-50% 1.19 1.00 2.52Si-70% 1.59 1.34 2.84N117 1.68 1.55 3.15

Uncertainty for measurements of: 1: 0.01 V; 2: 0.01 mA cm−2; 3: 0.01 mA cm−2.

58 M. Kumar et al. / Journal of Membra

F

3o

tcattcpp(miirctdt

Fd

ig. 7. Schematic presentation of current–voltage curve for ion selective membrane.

.4. Concentration polarization and electro-transport efficiencyf different cations

Transport of charged species towards respective electrodeshrough ion-selective membranes leads a decrease of counter-ionsoncentration in the membrane–solution interfacial zone (bound-ry layer) facing the diluate cell (compartment 2 in Fig. 1). Thisendency for concentration and depletion of counter-ions, respec-ively, at both membrane-solution interfacial zones is referred asoncentration polarization in the electrodialysis or other relatedrocesses and caused by differences between counter-ion trans-ort numbers in the membrane phase (tm

i) and solution phase

ti) [1,50,51]. As result i–v profile during the course of electro-igration of counter-ions through ISM is changing with increase

n applied voltage. A typical i–v curve of an ISM can be dividednto three regions (Fig. 7): ohmic region (linear part giving ohmic

esistance); plateau region (limiting current density caused byounter-ion depletion in membrane-solution interfacial zone andhus increase in resistance); and overlimiting region (inflectionue ionic electro-migration and water splitting at electrodes andhus generation of acid and base) [51]. Fig. 8(A) shows i–v curves

ig. 8. Current–voltage polarization curves for: (A) MCSM and N117 membranes in equilifferent electrolytic solutions of 0.01 M concentration.

ne Science 340 (2009) 52–61

obtained for Si-50%, Si-65% MCSMs and N117 in equilibration with0.01 M NaCl solution. As in standard case for ISMs, these curvesshow three characteristics regions. Properties of i–v curve suchas �V (plateau length), �i, and Ilim values for Si-50%, Si-65% andN117 membranes in equilibration with 0.01 M NaCl solution werederived from Fig. 8(A) and presented in Table 2. For MCSMs, �Vand �I values increased with the increase in silica content in themembrane matrix. It was also observed that Si-65% membrane hasthe highest permselectivity and surface charge density because ofhigh density of functional charge groups. The plateau region of thei–v curves is distinguished because development of a weak spacecharges within local electro-neutral diffusion layer. The interactionof the self-consistent electrical field with its space charge inducesa non-potential volume that eventually grows strong enough toset the fluid in motion. The overlimiting current modes of EDusing IEMs are very important because of enhancement in masstransfer (ions). The secondary effects of concentration polarizationcausing current growth over its limiting value are: (i) contributionof ions (H+ and OH−) generated at membrane-solution interfacesin the total current; (ii) convective liquid flows in the vicinity ofmembrane–solution interface (coupled convection). Overlimitingcurrent aroused as a result of combined effect of different physicalphenomena: such as electro-convection, gravitational convection,sometimes water splitting, etc. [38,52–55]. Therefore, the plateaulength can be regarded as a transition region in which the maintransport mechanism changed from diffusion to electroconvection.The electroconvection in a membrane aroused due to the followingreasons: (i) presence of a sufficiently large space charge; (ii)localization of the charge in the solution at a distance long enoughfrom the membrane surface and (iii) a non-uniform space chargedistribution [55,56]. Balster et al. [38] studied the effect of mem-brane surface morphology on the plateau length and overlimitingcurrent and reported large plateau length obtained due to themembrane structural heterogeneity. It seems due to incorporationof silica matrix heterogeneity of the MCSM was increased andresulted delayed occurrence of overlimiting region. Thus withSi-65% membrane one can get higher window for electro-transportwithout any polarization (because overlimiting current appears athigher potential), which is evident from Ilim values.

Electro-transport properties for different electrolytes acrossMCSMs were also studied from i–v curves analyses and derived

values for �V, �i, and Ilim from Fig. 8(B) are presented inTable 3. All the derived properties increased rapidly with theincrease in hydrated radius of counter-cations and follow the trend:NaCl > CaCl2 > MgCl2 > FeCl3. Relatively higher �V values for bi-or tri-valent cations indicate their lower electro-transport from

ibration with 0.01 M NaCl solution; and (B) for Si-65% MCSM in equilibration with

M. Kumar et al. / Journal of Membrane Science 340 (2009) 52–61 59

Table 3�V, �i and Ilim values for Si-65% membrane in equilibration with different elec-trolytic solutions derived from Fig. 8(B).

Electrolyte �V1 (V) �i2 (mA cm−2) Ilim3 (mA cm−2)

NaCl 1.59 1.34 2.84CMF

U

dcstsi2itadta(dfst(dptlmams

MpnrpimNt

Fbc

aCl2 1.93 1.46 4.30gCl2 2.23 2.31 5.22

eCl3 2.31 3.36 5.56

ncertainty for measurements of: 1: 0.01 V; 2: 0.01 mA cm−2; 3: 0.01 mA cm−2.

iffusive boundary layer to membrane matrix and thus results con-entration polarization at higher potential. The transport of chargedpecies through a set of IEMs leads to a decrease in concentra-ion of counter-ions in the diffusive boundary layer at the MCSMurface facing the diluate cell (compartment 1 in Fig. 1) and anncrease at the surface facing the concentrated cell (compartment

in Fig. 1). Since (tmi

) > (tsi), i.e. their flux in the membrane (Jm

i)

s always larger than in the boundary layer because tmi

is propor-ional to the Jm

i(Jm

i= tm

iI/Fzi) [1,16]. If the current is raised above

certain value, the concentration at membrane surface facing theiluate solution will approach zero [1]. The schematic diagram illus-rating concentration polarization in the diffusive boundary layert the surface of MCSM is presented in Fig. 9. In the diluate cellcompartment 1 in Fig. 1), initial electrolyte concentration (Cd

s )ecreased sharply to (dCm

Na+ ) in the diffusive boundary layer, whileor multi-valent counter-cations (Mn+) their rate of depletion waslower (dCm

Mn+ > dCmNa+ ). A sharp depletion of Na+ concentration in

he diffusive boundary layer may be explained by its higher fluxJmNa+ > Jm

Mn+ ) because tmNa+ > tm

Mn+ . Also for Mn+, due to very slowepletion of counter-ion concentration, relatively higher appliedotential was need for attending zero counter-ion concentration inhe membrane interface. Thus, Ilim obtained for Na+ was relativelyower than that for Mn+. Thus prepared MCSMs especially Si-65%

embrane showed excellent electro-transport properties for Na+

nd may be used for the practical separation of cations. Further-ore, studies on i–v curves are quite useful for the prediction of

uitability of given IEM for achieving desired separation.Electro-transport efficiency (�) for different cations across

CSM was derived from the change of their concentration in com-artment 1and/or as shown in Fig. 1 with the passage of desiredumber coulombs and estimated by equation 7. � values sepa-ately for different cations across MCSM and N117 membranes are

+

resented in Fig. 10. � values for Na increased with the increasen silica content in the MCSM matrix, and for Si-65% and N117

embrane it was 0.87 and 0.84, respectively. Thus Si-65% and117, both membranes were almost equaled efficient for electro-

ransport of Na+. All three membranes showed quite low � values

ig. 9. Schematic diagram illustrating concentration polarization in the diffusiveoundary layer at the surfaces of MCSM during operation. (1) dCm

Mn+ ; (2) dCmNa+ ; (3)

CmNa+ ; (4) cCm

Mn+ .

Fig. 10. Electro-transport efficiency (�) for various membranes in equilibration withdifferent electrolyte solutions of 0.01 M concentration.

for Mn+ ions. Interestingly, N117 membrane showed higher electro-transport efficiency for Mn+ in comparison to MCSMs, and Si-70%showed least � values Mn+. Thus, Si-65% membrane was efficient forelectro-transport of Na+, while its efficiency for Mn+ was low. Actualseparation of Na+ and Mn+ was studied by electrodialytic processusing equal concentration of mixed electrolyte solution. Separationfactor was estimated by the ratio of �Na+ and �Mn+ . Relevant datafor Si-65% and N117 membrane is presented in Fig. 11. Data indi-cated that separation factors of Si-65% membrane for Na+/Mn+ werehigh; especially for Na+/Fe3+ it was close to ten. While separationfactors for N117 membrane were very low. Thus among the MCSMs,Si-65% membrane was efficient for the separation of cations withdifferent valences or hydrated ionic radius, while N117 membranewas not suitable for separating these charged species. MCSMs hadthe special structural characteristics, in which Na+ were electro-transported through the hydrophilic micro-domains of functionalgroups attached with silica, while it was not suitable for electro-migration of relatively bulky Mn+. The MCSMs are not suitable forelectro-transport of the bi- or tri-valent ions may be because of(i) membrane charge density (it seems lower charge density haveincreased selectivity for mono-valent ions); (ii) the electrostatic

repulsion from the positively charged layer of this membrane couldcome at higher current densities. At higher current densities, adepletion of H+ ions occurs next to the membrane, leading to theaccumulation of multi-valent ions [57]. These observations suggest

Fig. 11. Separation factor values of Si-65% and N117 membranes for different mixedelectrolyte solution (0.01 M each).

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he suitability of MCSMs for electro-separation of cations, where atresent N117 or any other cation-exchange membrane are used forxtracting inorganic and its fractionation such as sea water, brackishater, waste water of hydrometallurgy or electroplating industries.

. Conclusions

Organic–inorganic composite cation selective membranes basedn PVA–SiO2 composite, in which –SO3H groups were introducedy oxidation of the existing –SH group in MPDMS at inorganicart, were prepared using a sol–gel process under acidic condi-ions, and their physicochemical and electrochemical propertiesave been studied. These membranes with high water uptake, IECalues, low resistivity, and high permselectivity offer a new dimen-ion for the preparation of organic–inorganic composite chargedembranes, in which the membrane porosity and density of

unctional groups could be controlled for introducing specific selec-ivity for electrodialytic separations of cations of practical interest.haracterization by TGA, DMA testing, and oxidative/dimensionaltabilities revealed an adequate stable nature of these membranes,hich is essential for practical applications of MCSMs. Ionic diffu-

ion coefficients and water transport numbers (derived from thelectro-osmotic drag of water across membrane) values suggestedheir mono-valent selective nature. i–v curves were used as efficientools for studying electro-transport process of mono- or multi-alent cations across these membranes. Concentration profiles forifferent types of cations across membrane and diffusive bound-ry layer was presented suggesting higher applied potential forchieving concentration polarization across MCSMs and thus rela-ively slower electro-migration of Mn+. Information obtained fromlectro-transport efficiencies validated observations of i–v curvesroved that it is an efficient tool for assessing the suitability of

on-exchange membranes for desired separation. Separation factorata obtained from the mixed electrolyte systems suggested thatSCMs, especially Si-65% membrane, were efficient for separatinga+ from Ca2+, Mg2+ and Fe3+. While under similar experimentalonditions, electro-transport efficiency and separation factor for117 membrane were very low.

MCSMs possessed special structural feature, in which functionalroups were introduced at silica part and their controllable chargeensity, selectivity, and porosity by varying silica content. This typef structure formed two types of micro-domains, which can alter forchieving desired selectivity. Due to its high mono-valent selectiveature, MSCMs can only successfully replace N117 membrane if the

on process recovery focuses on mono-valent cations.

cknowledgements

One of the authors Mahendra Kumar (MK) is thankful to Councilf Scientific and Industrial Research (CSIR), New Delhi, for provid-

ng Senior Research Fellowship. We also acknowledge the servicesf analytical science division, CSMCRI, Bhavnagar for instrumentalupport.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.memsci.2009.05.010.

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