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Electrochimica Acta 54 (2009) 4880–4887

Contents lists available at ScienceDirect

Electrochimica Acta

journa l homepage: www.e lsev ier .com/ locate /e lec tac ta

lectro-membrane reactor for separation and in situ ion substitution of glutamiccid from its sodium salt

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

lectro-Membrane Processes Division, Central Salt & Marine Chemicals Research Institute, Council of Scientific & Industrial Research (CSIR), G. B. Marg,havnagar 364002, Gujarat, India

r t i c l e i n f o

rticle history:eceived 19 February 2009eceived in revised form 8 April 2009ccepted 10 April 2009vailable online 21 April 2009

eywords:on-exchange membrane

a b s t r a c t

An electro-membrane reactor with four compartments (EMR-4) (anolyte, catholyte and comp. 1 and 2)based on in-house-prepared cation- and anion-exchange membrane (CEM and AEM, respectively) wasdeveloped to achieve separation and recovery of glutamic acid (GAH) from its sodium salt by in situion substitution and acidification. The physicochemical and electrochemical properties of CEM and AEMwere characterized and its suitability was assessed in operating environment. The separation of GA−

from the mixture of nonionic organic compounds and further ion substitution was achieved by EMR-4. But the higher energy consumption (5.75 kWh/kg of GAH produced), low current efficiency (50.5%)and recovery of GAH (57.2%) in this process were main obstacles for the industrial exploration of the

lectro-membrane reactor

lectro-electrodialysislutamic acidlectrode polarization

process. Latter, electro-membrane reactor with three compartments (EMR-3) (anolyte, catholyte andcentral compartment) was developed based on CEMs for only in situ ion substitution of GANa to achieveGAH, in which GA− was not allowed for electro-migration from its feed compartment. CE and recovery ofGAH were close to 73% and 96% that indicate the suitability of the EMR-3 process for industrial applicationover the EMR-4. It was concluded that EMR-3 was efficient as compared to EMR-4 for separation and

ment

recovery of GAH from fer

. Introduction

Glutamic acid has a wide range of application in the food,harmaceutical, biotechnology, biochemistry, and cosmetic indus-ries [1–3]. Its monosodium salt (monosodium glutamate, GANa),as been used commercially as a flavor enhancer. Glutamic acid

s produced on industrial scale by fermentation method usingicroorganisms such as cyanobacteria of Corneybacterium fam-

ly [4–7]. Usually, separation of GAH is carried out by isoelectricrystallization method with or without prior removal of biomassresent in the fermentation broth. However, the presence ofiomass reduces the crystallization process and favors the forma-ion of p-form of crystal [8]. The fermentation broth still contains–2% of GAH after separation in isoelectric supernatant [9]. The

ower solubility of GAH in water compared to its salt is also a seri-us problem for achieving high separation and recovery. In thiseparation process, a large amount of mineral acids and successive

ashing steps with water are required to remove the salts formed.

hus, it is important to investigate an alternative process for sepa-ation and recovery of GAH from GANa, to avoid use of various stepsnd chemicals.

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

013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2009.04.036

ation broth by in situ ion substitution in eco-friendly manner.© 2009 Elsevier Ltd. All rights reserved.

A number of processes for glutamic acid recovery from fermen-tation broth without precipitation have been studied such as ionexchange [10], nanofiltration [11], diafiltration [12], ultrafiltration[13], and electrodialysis [9]. The ion exchange process is disad-vantageous because large amount of acid and base is required toregenerate the ion-exchange resins, and thus enhance processingcost. In addition to this, wastewater produced from the process inreactivating and washing the ion-exchange resins will also causea serious environmental pollution. The separation with ultrafiltra-tion process requires a higher volume and concentration ratio todecrease the permeation flux and product recovery [13]. Electro-dialysis (ED) is an electro-membrane process in which ions aretransported through ion permeable membranes from one solu-tion to another under the influence of potential difference acrossthe electrodes. Therefore, ED can separate selectively ions on thebasis of their charge [14,15]. The main industrial application of thisprocess is found in chlor-alkali, water purification, and industrialeffluent treatments [16,17]. The most recent application of elec-trodialysis is in the separation and production of organic acidsfrom fermentation broths and enzymatically produced solutions

[18–20].

Recently, bipolar membrane electrodialysis (BMED) has beendeveloped for the conversion of salts into corresponding acids andbases [21–23]. The bipolar membranes are able to split water at highapplied potential, and generate the hydroxyl and hydrogen ions at

mica A

tBoteGbfrib

ntaoHNlawofeu

2

2

CgfCfcbscpaivbaabb

2

catPhctnpmm

(6 �m thickness), with 1.5 mm thickness and 8.0 × 10−3 m2 area.Distance between both electrodes and the effective membranearea was 1.10 × 10−2 m and 8.0 × 10−3 m2, respectively. Peristalticpumps were used to feed the solution (500 cm−3) in a recircula-tion mode into the respective compartments with constant flow

M. Kumar et al. / Electrochi

he anode and cathode side. Several studies have shown that, theMED process has economic potential for recovery of inorganic,rganic, and amino acids [24,25]. Thus in the light of above facts,wo routes of the membrane technologies can be used for the recov-ry of GAH from GANa: (i) the BMED process, to convert GANa intoAH and caustic soda and their recovery; (ii) acidification of GANay sulfuric acid and subsequent separation of GAH and sodium sul-ate by ED using ion-exchange membranes. Cong and coworkerseported GAH production by BMED [9] in a two-step process, whichnvolves an ED cell unit with several cell pairs of CEM, AEM andipolar membrane.

Herein, we are reporting membrane based two novel processesamely EMR-4 and EMR-3 for separation and in situ ion substitutiono recover the GAH from GANa. In EMR-4 process, ion substitutionnd separation of GAH and GANa occurred simultaneously, whilenly ion substitution phenomenon occurred in EMR-3 process. The+ generated by oxidative water splitting at the anode, replacesa+ of GANa and forms GAH of a low degree of dissociation. The

iberated Na+ further migrates towards the cathode through CEMnd formed NaOH as a byproduct with OH− produced by reductiveater splitting at the cathode. While in EMR-4 process, separationf GA− and further acidification was achieved by transfer of GA−

rom comp. 2 to 1. Thus total process efficiency depends on the prop-rties of ion-exchange membranes as well as feed concentrationsnder similar applied potential.

. Experimental

.1. Materials and membranes preparation

Poly(ether sulfone) (PES) was received from Sigma–Aldrichhemicals, and all other reagents such as sodium glutamate (GANa),lutamic acid (GAH), H2SO4, NaCl, (NH4)2SO4, glucose, dimethyl-ormamide (DMF), etc. of AR grade were obtained from S.D. finehemicals, India and used without any further purification. Sul-

onation of PES was carried out using conc. H2SO4 at 323 K underonstant stirring as reported earlier [26]. Cation-exchange mem-rane (CEM) was prepared by dissolving sulfonated poly(etherulfone) (SPES) in DMF (20%, w/v), and further casting onto alean glass plate. The membrane was allowed to dry at room tem-erature for 24 h and then in vacuum oven for 6 h at 60 ◦C. Thenion-exchange membrane (AEM) used in this work was preparedn our laboratory by a procedure reported earlier [27,28]. Variousersatile applications of electrodialysis with this membrane haveeen developed because of its high ionic conductivity, chemicalnd mechanical stabilities [29,30]. Thus obtained membranes (CEMnd AEM) were equilibrated by treatment with 1 M HCl and NaOHefore their use in process. The cleaned and equilibrated mem-ranes were stored in double distilled water.

.2. Investigations on membrane electrochemical properties

Membrane conductivity measurements for CEM and AEM werearried out after equilibration with sodium glutamate, glutamiccid and NaCl solutions of different concentrations using a poten-iostat/galvanostat frequency response analyzer (Auto Lab, ModelGSTAT 30). The membranes were sandwiched between two in-ouse made stainless steel circular electrodes (4 cm2). Directurrent (dc) and sinusoidal alternating currents (ac) were supplied

o the respective electrodes for recording the frequency at a scan-ing rate of 1 �A/s within a frequency range 106 to 10−3 Hz. Nyquistlots (Fig. S1 and S2; supporting information), were obtained andembrane resistance was determined using Fit and Simulationethod [31].

cta 54 (2009) 4880–4887 4881

The thickness of the membranes was measured by a digitalmicrometer up to 0.1 �m accuracy. The membrane water contentwas determined by taking the weight difference of membrane inwet and dry condition. For this purpose the membrane was driedin vacuum oven at 60 ◦C for 24 h and recorded its weight. Furtherthe dry membrane was kept in distilled water for same period oftime and their wet weight was recorded. The water content wasfinally calculated using the following equation:

Water content (%) = Ww − Wd

Wd× 100 (1)

where Ww and Wd are the weight of the wet and dry membrane,respectively.

For the estimation of ion exchange capacity (IEC), desired piecesof ion-exchange membranes were conditioned in 1 M HCl and NaOHsolution for overnight to convert them into H+ and OH− form. Theexcess HCl or NaOH was removed by washing with distilled water.The membranes were then equilibrated in 50 ml of 0.5 M NaCl solu-tion. The amount of H+ or OH− ions liberated from CEM and AEMwas determined by acid–base titration [30].

Counter-ion transport number (tmi

) across the membranes wasestimated by membrane potential measurement in 0.01 and 0.1 MNaCl solutions, according to Eq. (2) as reported previously usingTMS (Teorell, Meyer, and Sievers) approach [30].

Em =(

2tmi − 1

) RT

Fln

a1

a2(2)

where a1 and a2 are the activities of electrolyte solutions contactingtwo surfaces of the membrane, R is the gas constant, T is the absolutetemperature, and F is the Faraday constant.

2.3. Electro-membrane reactor with four compartments (EMR-4)for separation and in situ ion substitution of GANa

EMR-4 cell was made of polytetraflouroethylene (PTFE), in whichtwo pieces of CEM and one AEM were used to separate fourcompartments: catholyte, anolyte, comp. 1 and 2 as presentedin Fig. 1. Parallel-cum-series flow arrangement was used for theseparation of glutamate ion and in situ ion substitution fromisoelectric supernatant (IS) mixtures (12–20 g/l GANa, 3.0 g/l glu-cose). The electrodes, obtained from Titanium Tantalum Products(TITAN, Chennai, India) were made of expanded TiO2 sheet coatedwith a triple precious metal oxide (titanium–ruthenium–platinum)

Fig. 1. Schematic presentation of EMR-4 for separation and in situ ion substitutionof GAH from GANa and glucose mixture.

4882 M. Kumar et al. / Electrochimica Acta 54 (2009) 4880–4887

Fs

rpaatiLateot

2(

tpstriowtfaccTso

Table 1Physicochemical and electrochemical properties of CEM and AEM.

Properties CEM AEM

Thickness (�m)a 150 150Water content (%)b 13.6 9.8Ion-exchange capacity (mequiv./g

of dry membrane)c0.93 1.28

Counter-ion transport number(tm

i)d,e

0.95 0.91

Specific membrane conductivity(S cm−1)f ,g

2.02 × 10−2 1.2 × 10−2

a Uncertainty for measurements: 1.0 �m.b Uncertainty for measurements: 0.1%.c Uncertainty for measurements: 0.01 mequiv./g of dry membrane.d Measured by membrane potential in equilibration in with 0.01 and 0.1 M NaCl

solutions.e Uncertainty for measurements: 0.01.f

ig. 2. Schematic presentation of electrochemical principle for possible in situ ionubstitution of GANa in EMR-3.

ate (0.006 m3/h) to maintain the turbulence. The whole setup waslaced in ambient condition (303 K) without any additional temper-ture control. Distilled water was recirculated separately into anodend cathode wash stream. An IS mixture solution of known concen-ration was recirculated through comp. 2, while distilled water wasnitially feed into comp. 1. A dc power supply (Aplab India, model1285) was used to apply constant potential across the electrodesnd the resulting current variation was recorded as a function ofime using a digital multimeter in series. pH and conductivity ofach stream output were regularly monitored. The concentrationf GAH in comp. 1 and 2 was monitored using UV–visible spec-rophotometer (Shimadzu, Japan) at regular time of interval.

.4. Electro-membrane reactor with three compartmentsEMR-3) for in situ ion substitution of sodium glutamate

The EMR-3 cell was composed of three compartments, cen-ral compartment (CC), catholyte and anolyte, separated by twoieces of CEM as shown in Fig. 2. All other parts of the cell wasame as EMR-4. Three storage tanks and pumps were used to con-inuously feed CC and electrode streams (5.0 × 10−4 m3 each) inecirculation mode with 0.006 m3/h constant flow rate. The exper-ments were carried out in ambient condition. Under the influencef applied potential, H+ ions were produced at anode by oxidativeater splitting. The migration of H+ from anode compartment to

he CC through CEM leads to the in situ Na+ substitution and theormation of GAH. Substituted Na+ in CC migrated towards cathodend produced NaOH in catholyte. pH and conductivity of all theompartments were regularly monitored. Changes in NaOH con-entration in catholyte were determined by acid–base titration.

he concentration of GAH in CC was determined by UV–visiblepectrophotometer at �max 570 nm wavelengths at time intervalf 30 min.

Fig. 3. Membrane conductivity (�m) values for CEM and AEM in equilibration w

Measured in equilibration with 0.1 M NaCl solution.g Uncertainty for measurements: 0.01 × 10−3 S cm−1.

3. Results and discussions

3.1. Properties of ion-exchange membranes

The physicochemical and electrochemical properties of pre-pared CEM and AEM are given in Table 1. Both membranesexhibited good water content, ion exchange capacity and counter-ion transport numbers in the membrane phase and high membraneconductivity under the operating conditions. The properties ofused CEM and AEM were in good agreement with the best-knownion-exchange membranes [32]. The membrane conductivity wasrecorded in equilibration with NaCl, GANa and GAH solutions ofdifferent concentrations and specific membrane conductivity (�m)was estimated by equation given below:

�m = �x

ARm(3)

where �x is the thickness of membrane in wet condition, A is thearea, and Rm is the resistance (Ohm) of the membrane. The variationin �m values for both membranes in equilibration with GAH, GANaand NaCl solutions of different concentrations is presented in Fig. 3.It is evident from the figure that �m values were increased with theelectrolyte concentration, and depends upon the ionic strength ofmembrane/solution interface zone. The �m values for membranesin GAH were lower compared to GANa and NaCl solution of sameconcentration. This may be due to low degree of dissociation of GAH.The high membrane conductivity for the membranes was observedwith NaCl in comparison to GANa, may be due to high degree of dis-

sociation of NaCl. �m values of membranes in NaCl, GANa, and GAHsuggested their suitability for proposed electro-membrane appli-cation.

ith: (A) NaCl; (B) GANa; and (C) GAH solutions of varied concentrations.

mica Acta 54 (2009) 4880–4887 4883

3i

ittwEvItagiooHrGctaetnEcabw

1

H

ar

s

AfcattVsgate

where C0 and Ct is the initial and final concentration of GA in comp.1 (mol m−3), �t the time allowed for electro-membrane process (s),Va the total volume of solution in comp. 1 (0.50 × 10−3 m3), and Ais the effective membrane area (8.0 × 10−3 m2). Flux of GAH dur-ing electro-membrane processes in EMR-4 is presented in Fig. 6(A)

M. Kumar et al. / Electrochi

.2. Separation and in situ ion substitution of sodium glutamaten EMR-4

Separation and in situ ion substitution of GANa was carried outn EMR-4 under different applied potential (3–5 V), using the mix-ure of GANa and glucose (12.0–20.0 and 3.0 g/l, respectively) iso be treated as initial feed of comp. 2. Initially deionized wateras recirculated into the comp. 1, anode, and cathode chamber.

xperiments were performed at constant applied voltage, and theariation in current density was recorded as a function of time.ncrease in concentration of GAH in comp. 1 was determined withime. Schematic electrochemical process of EMR-4 for separationnd in situ ion substitution of GA− from the mixture of GANa andlucose is shown in Fig. 1. The single step separation and in situon substitution process occurred in three stages: (i) formationf OH− and H+ ion at the cathode and anode by reductive andxidative splitting of water, respectively; (ii) electro-transport of+ from anolyte and GA− from comp. 2 to 1 through CEM and AEM,

espectively, and in situ ion substitution by H+ and formation ofAH; and (iii) electro-transport of liberated Na+ from comp. 2 toatholyte and thus formation of NaOH. In this process, ion substitu-ion was achieved by the combined effect of electrode polarizationnd simultaneous migration of H+. This process may be defined aslectro-electrodialysis. Since GANa/GAH was separated from elec-rodes by CEMs, electro-migration of GA− towards cathode wouldot have happened because of strongly charged nature of CEM.lectro-transport of GA− towards cathode and anode was furtherhecked by regular monitoring of GA− concentration in catholytend anolyte using UV–visible spectrophotometer, and was found toe absent. Further, electrode reactions of the total process may beritten as given below:

Anode reaction

/2H2O → H+ + (1/4)O2↑ + e−

Cathode reaction

2O + e− → OH− + 1/2H2↑

One electron produced at the anode is consumed at the cathode,nd thus, this is a one electron process. The whole reaction can beegarded as a water splitting process.

Thus the overall electrochemical reaction for the in situ ionubstitution and separation of GAH from GANa can be written as:

When an electric field was applied, GA− was migrated throughEM from comp. 2 to 1 and also H + ions produced at anode migrated

rom anolyte to comp. 1 through CEM, thus formation of GAH inomp. 1 takes place. The different ionic forms of GAH existing inqueous solution at various pHs are given in supporting informa-ion Fig. S3. Simultaneously, Na+ migrated from comp. 2 to catholytehrough CEM followed by combination with OH− formed NaOH.ariation of current density with time during separation and in

itu ion substitution for producing GAH from mixture of GANa andlucose is depicted in Fig. 4, as a representative case. At constantpplied voltage, current density initially increased, after attaininghe maximum value, it decreased with time. At the beginning of thexperiments, distilled water was passed through comp. 1 and elec-

Fig. 4. Variation of current density with time at different applied voltage duringseparation and in situ ion substitution of GAH from GANa and glucose mixture inEMR-4.

trode compartments, which offers high electrical resistance. Butwith ion formation under applied voltage and electro-migrationof GA− from comp. 2 to 1 and Na+ from comp. 2 to catholyte,ionic concentrations build up in all compartments. As a result,the electrical resistance offered by EMR-4 is continuously beingdecreased in rapid manner with time, which caused an increasein current density under applied constant potential. After reachingmaximum current, electrolyte (GANa) concentration in the comp. 2was progressively lowered, which caused an increase in the overallelectrical resistance, and thus the current density decreased. Dur-ing the electrochemical process, applied voltage was responsiblefor the formation of OH−, H+ and migration of GA−. Variations ofsolution conductivity of comp. 1 and 2 in EMR-4 with electricitypassed (Coulombs) at 5 V constant applied voltage are depicted inFig. 5.

Ionic flux in electro-membrane processes (J) can be defined asrate of change of concentration (GA− in EMR-4). J values were esti-mated from GA− concentration in comp. 1, considering negligiblemass (water) transport through AEM, as in this case, J may bedefined as [30,33]:

J = Va

A

Ct − C0

�t(4)

−

Fig. 5. Variations of solution conductivity of comp. 1 and 2 in EMR-4 with electricitypassed (Coulombs) at 5 V constant applied voltage. Feed of comp. 2: GANa (12.0 g/l)and glucose (3 g/l).

4884 M. Kumar et al. / Electrochimica Acta 54 (2009) 4880–4887

F voltaG rent c

ataovGovG

ed

G

wssreorGctr

tp(o

W

weo

TCd

Av

345555

ig. 6. Variation of: (A) JGAH with electricity passed (Coulombs) at different appliedAH with electricity passed (Coulombs) at constant applied voltage (5 V) with diffe

s a function of electricity passed at different applied potential. Ini-ially J values increased linearly with the electricity passed and afterttaining maxima, it decreased because of reduced concentrationf GA− in comp. 2. The decline in flux after attaining the maximumalue may be due to the competitive transfer of H+ and OH− withA−, which eliminate predominance of applied voltage [34]. It wasbserved that J values were not only highly dependent on appliedoltage but also strongly depended on initial feed concentration ofANa in comp. 2.

Recovery of the product (GAH) is an important parameter toxamine the economic feasibility of any process, which may beefined as:

AH recovery (%) = CtpVtp

C0F V0F× 100 (5)

here V0F and Vtp are the initial and final volume of the producttream, C0F and Ctp are the initial concentration of GANa in feedtream and final concentration of GAH in the product stream. GAHecovery values at different feed concentration of GANa in the pres-nce of glucose solution are also presented in Fig. 6(B) as a functionf quantity of electricity passed (Coulombs). It can be seen thatecovery of GAH decreased with increase in feed concentration ofANa in comp. 2. The recovery of GAH decreased with increase inoncentration because it becomes independent of feed concentra-ion above a certain concentration [35]. Furthermore, about 52–57%ecovery of GAH was achieved in EMR-4 under operating conditions.

Energy consumption and current efficiency (CE) are also impor-ant parameters for assessing the suitability of any electrochemicalrocess for their practical applications. The energy consumptionW, kWh kg−1 of GAH produced) for EMR-4 or EMR-3 may bebtained as follows [26]:

( −1) ∫ t

VI dt

kWh kg =

0m

(6)

here V is the cell voltage, I the current, t the time allowed for thelectrochemical process, and m is the weight of GAH produced. Theverall current efficiency was defined as the fraction of Coulombs

able 2urrent efficiency, energy consumption and GAH recovery data of EMR-4 for the separatioifferent experimental conditions.

pplied Process occurring 103 × electricoltage (V) quantity passed (C)

Ion substitution + acidification + watersplitting + base formation

3.113.244.362.513.362.52

ge for GANa (12 g/l) and glucose (3 g/l) as feed of comp. 2 in EMR-4; (B) recovery ofoncentrations of GANa solution and glucose (3 g/l) as feed of comp. 2 in EMR-4.

utilized for the electro-migration and in situ ion substitution:

CE (%) = mnF

MQ× 100 (7)

where F is the Faraday constant, M the molecular weight of GAH,n the stoichiometric number (n = 1 in this case), and Q is theelectricity passed (Coulombs). An electrochemical process mustnot only be technically feasible, but should also be less expen-sive and green in nature. To evaluate the technical and economicfeasibility for the separation/ion substitution of GANa in EMR-4, energy consumption, CE (%) and recovery (%) of GAH dataunder different experimental conditions are presented in Table 2.Energy consumption, current efficiency and recovery of GAH wereincreased with increase in applied voltage, for initial feed of GANa(12.0 g/l) and glucose (3 g/l) mixed solution. In EMR-4, relativelyhigh energy consumption and low current efficiency may be dueto the following reasons: (i) water splitting by electrode reactionand electro-migration of different ions occurred simultaneously;(ii) at higher pH value in catholyte chamber a significant numberof OH− migrated towards the anode to form water molecule. Thuselectricity supplied to system was consumed for water splitting aswell as for ionic migration [35]. At relatively high applied voltage,electro-migration of GA− ions from comp. 2 to 1 was high, and thusenhanced the recovery of GAH. The current efficiency and recoveryof GAH was decreased, while energy consumption was increasedwith the increase in concentration of GANa in feed solution. Theflux of the GAH was limited because of the slow migration of bulkyGA− through AEM and becomes independent of feed concentrationabove a certain concentration. Under optimum operating condition(2.51 × 103 C), CE and W were found to be 48.5% and 5.15 kWh kg−1,respectively, corresponding to 56.8% recovery of GAH. Obtained val-ues of CE and GAH recovery were seem to be comparatively lower,while energy consumption was higher for EMR-4. It is important

to note here that loss in glucose concentration was observed fromcomp. 2 and pure GAH was received in this process. Also, after sev-eral experiments, electrochemical properties of CEM and AEM weremeasured, which were reproduced with ±1.0% deterioration. Thusany type of fouling on the ion-exchange membrane was completely

n and in situ ion substitution of GANa from mixed solution (GANa + glucose) under

GANa conc. in Current W (kWh kg−1) Recovery of GAHcomp. 2 (g/l) efficiency (%) of GAH produced (%) in EMR-4

12 42.7 2.71 45.612 44.9 5.04 47.812 48.4 5.15 56.812 50.5 5.75 57.215 39.4 6.14 47.220 31.5 9.37 23.3

M. Kumar et al. / Electrochimica A

Ffs

rpppre

3

iciofesffEe

dcDtpavrIc

anolyte to CC. Thus in CC, H+ has very less contact time to exchange

Ff

ig. 7. Variation of current density with time in EMR-3 at different applied potential,eed of catholyte and anolyte: water; feed of central compartment: GANa (12.0 g/l)olution.

uled out. Based on these observations, we can conclude that it isossible to separate GA− from the mixture of nonionic organic com-ounds to purify it and further ion substitution can be achieved byroposed EMR-4. Relatively higher energy consumption, low CE andecovery of GAH by this process are main obstacles for the industrialxploration of the process.

.3. In situ ion substitution of GANa in EMR-3

In EMR-3 only in situ ion substitution of GANa was occurred,n which GA− was not allowed for electro-migration from its feedompartment. Electrochemical principal of the EMR-3 is describedn Fig. 2. This process involved three stages: (i) generation of H+ byxidative water splitting at anode and its migration through CEMrom anolyte to CC; (ii) substitution of Na+ by H+ in the CC; (iii)lectro-migration of Na+ from CC to catholyte through CEM and sub-equent formation of NaOH. The current densities vs. time curvesor EMR-3 are presented in Fig. 7, which are similar to that obtainedor EMR-4 (Fig. 3). Thus EMR-3 was less resistive in comparison withMR-4, maybe because of less numbers of compartments betweenlectrodes.

Experiments for in situ ion substitution of GANa solutions ofifferent concentrations (12.0–20.0 g/l) as initial feed of CC, werearried out in an EMR-3 at different applied voltage ranging 3–5 V.istilled water was feed into the anolyte and catholyte in recircula-

ion mode. The variations of current density with time at differentotential and GANa solution of 12.0 g/l concentration as feed of CC

re presented in Fig. 7, as a representative case. At constant appliedoltage, initially current density was low because of high electricalesistance due to distilled water feed into electrode compartments.nitially, resistance offered by two compartments (anolyte andatholyte) was dominated, while resistance of the CC was low due

ig. 8. pH variations of different compartments with electricity passed (Coulombs) in EMeed of: (A) catholyte and anolyte; (B) CC, and (C) conductivity variations in CC under the

cta 54 (2009) 4880–4887 4885

to highly conducting medium (GANa solution). Progressively, cur-rent density increased due to increase in ionic mobility in electrodecompartments because of electrodes polarization and migration ofH+ and Na+. After certain interval of time, resistance offered bycatholyte and anolyte was reduced, while resistance offered by CCwas dominated due to the formation of GAH. Fig. 8(A–C) repre-sents the pH and conductivity change in the catholyte, anolyte, andCC compartments at constant applied voltage 5 V. Initially, approxi-mately pH 7 was recorded for all streams but latter it was decreasedin anolyte and CC due to the formation of H+ and GAH, respectively.The pH of catholyte was found to increase because of NaOH for-mation. These observations verified the electrode polarization andelectro-transport phenomena presented in Fig. 6.

In EMR-3, rate of in situ ion substitution of GANa (J) may beassessed by rate of migration of H+ from anolyte to CC and simulta-neous exchange of Na+ or formation rate of NaOH in the catholyte.J values were estimated from concentration change of GAH in theCC or concentration changes of NaOH in catholyte stream, usingEq. (4), where C0F and Ctp was taken as initial and final concentra-tion of GAH in the central compartment (mol m−3) or initial andfinal concentration of NaOH in the catholyte compartment and therespective data are presented in Fig. 9(A and B). Initially, J valuesincreased linearly with electricity passed and negligible back diffu-sion of Na+ from catholyte to CC was observed due to extremely lowconcentration of Na+ in the CC. It was also observed that ionic fluxeswere increased with the increase in applied voltage or concentra-tion of GANa in feed solution. In whole process electro-neutralityconditions were always maintained. J values were limited and fur-ther reduced progressively because of enhanced back diffusion ofNa+ from catholyte to CC. Furthermore, rate of ion substitution ofGANa was not only highly dependent on applied voltage but alsostrongly depended on initial feed concentration of GANa in CC. Alsorecovery of GAH was above than 80% in this case (Fig. 9(B)), which isquite higher compared to EMR-4. High recovery and rate of ion sub-stitution in EMR-3 indicates its economical feasibility for industrialexploitation.

To evaluate the technical and economic feasibility for in situion substitution of GANa in EMR-3, energy consumption, CE (%)and recovery (%) of GAH data under different experimental con-ditions are presented in Table 3. Energy consumption increased,while current efficiency and recovery of GAH were decreased withincrease in applied voltage for GANa solution (12.0 g/l) as initial feedof CC. Extent of electrode polarization enhanced at higher appliedpotential, and thus enhances the formation of H+/OH− that resultsa highly conductive medium for fast electro-migration of H+ from

with Na+, which lowered the recovery of GAH and CE along withhigh energy consumption. Also data presented in Table 3 revealedthat CE and GAH recovery values decreased while energy consump-tion increased with the increase in GANa concentration in the CC

R-3 at 5 V constant applied voltage with GANa solution of 12.0 g/l concentration assame conditions.

4886 M. Kumar et al. / Electrochimica Acta 54 (2009) 4880–4887

Fig. 9. Variation of: (A) JGAH at initial feed of 12.0 g/l GANa solution 5 V constant applied voltage; (B) recovery of GAH, in EMR-3 at various concentrations of GANa solution asinitial feed of central compartment.

cons

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TC

Av

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Fig. 10. Comparison of current efficiency, GAH recovery and energy

t constant applied voltage, 5 V. Reduction in CE and GAH recoveryay be due to the lower exchange of Na+ and migration of proton

rom CC to catholyte, due to the lack of selectivity of CEM for Na+ inomparison with H+ [31]. Earlier report [36] reveals the relationshipetween proton migration and water flux at high acidic conditionsnd found that proton flux increases while the water transport tohe anodic side decreases, which is in agreement with our findings.hus concentration of GANa, nature of the CEM and its counter ionsransport are also important parameters. Under highly acidic envi-onment, the proton migration from CC to catholyte was enhanced.hese data revealed that in situ ion substitution of GANa in EMR-is an efficient process under optimized conditions. For assessing

he suitability of developed ion-exchange membranes (CEM andEM), experiment for in situ ion substitution of GANa in EMR-3,as also preformed with the commercially available CEM based on

nterpolymer of polyethylene and styrene–divinylbenzene copoly-er with sulfonic acid functional groups [32] and relevant data are

ncluded in Table 3. The process efficiency and GAH recovery in

MR-3 was compared with prepared and commercially availableembrane, which showed suitability of CEM for electrochemical

rocess. Further to assess the suitability and process efficiency ofMR-3 and EMR-4 all discussed parameters were compared.

able 3urrent efficiency, energy consumption and GAH recovery data for in situ ion substitution

pplied Process occurring 103 × electric Goltage (V) quantity passed (C) i

Ion substitution and water splitting 4.51 16.10 17.29 17.07 17.29 18.91 2

a 6.10a 1

a Data for experiments performed in EMR-3 using IPC [32] as CEM.

umption in EMR-4 and EMR-3, under similar operating conditions.

3.4. Comparison of process parameters for EMR-4 and EMR-3

In EMR-4, separation and in situ ion substitution of GANa fromits mixture with nonionic organic (glucose) was achieved as modelcase, while in EMR-3 only in situ ion substitution of GANa wasachieved. For both processes same type of ion-exchange mem-branes were used. Under similar experimental conditions, CE, GAHrecovery and energy consumption of EMR-4 and EMR-3 are com-pared in Fig. 10, for 12.0 g/l feed of GANa solution under constantpotential, against electricity passed (Coulombs). It can be seen thatunder similar experimental conditions, CE and GAH recovery werehigh while energy consumption was low for EMR-3 in comparisonto EMR-4. In EMR-4, electro-migration of bulky GA− was allowedthrough AEM from comp. 2 to 1 towards anode. Thus EMR-4 processcoupled with electro-migration of GA− and further acidification,while EMR-3 process was meant only for acidification. Thus totalprocess efficiency depended on properties of ion-exchange mem-brane as well as feed concentration under similar applied potential.

Thus EMR-3 is a more efficient process in comparison to EMR-4,may be due to less number of compartments, less resistive natureand absence of electro-transport of bulky GA− for achieving its sep-aration from nonionic organics. CE and GAH recovery values were

of GANa in EMR-3 under different experimental conditions.

ANa feed Current W (kWh kg−1) Recoveryn CC (g/l) efficiency (%) in EMR-3 of GAH produced of GAH (%)

2 63.4 2.41 71.22 71.2 3.65 81.12 72.3 5.16 96.52 78.5 5.21 96.65 43.8 5.48 81.80 41.5 7.80 63.32a 70.9a 3.72a 80.5a

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lose to 72% and 96% indicating the suitability of the EMR-3 processor industrial application, while process efficiency for EMR-4 wasound to be 48%. In spite of comparatively high efficient EMR-3 pro-ess, its product purity (GAH; 96%) was less than the product puritybtained by EMR-4 (100%). This may be because of less number ofompartments, less resistive nature and thus low energy consump-ion. Also, absence of electro-transport of bulky GA− for achievingts separation from nonionic organics in EMR-3 also favors the pro-ess. However, high purity of GAH in EMR-4 was obtained that ituaranteed the separation of GAH from GANa, glucose and otheronionic compounds.

Furthermore, production of NaOH in the cathode stream is spinff of both electro-membrane processes. Thus, EMR-4 and EMR-3howed suitability for the production of GAH, but one has to opti-ize the process for their process efficiency and product purity,

efore its scale-up or industrial exploitation.

. Conclusions

In this work cation- and anion-exchange membranes with goodlectrochemical and physicochemical properties were preparednd employed for the developing electrochemical processes, EMR-

and EMR-3 based on the principle of electro-electrodialysisither for separation and in situ ion substitution of GANa or fornly in situ ion substitution of GANa for producing GAH. Processfficiency of EMR-4 for the separation and in situ ion substitu-ion was low (CE: 50.5%; energy consumption: 5.75 kWh kg−1 ofAH produced; and GAH recovery: 57.2% under optimized exper-

mental conditions) in comparison to EMR-3 for achieving onlyn situ ion substitution from GANa solution (CE: 72.3%; energyonsumption: 5.16 kWh kg−1 of GAH produced; and GAH recov-ry: 96.6% under optimized experimental conditions). Althoughhe purity of product in EMR-3 was less (96%) compared to thatf EMR-4 (100%), but it is a more efficient process compared toMR-4, may be due to the less number of compartments, lessesistive nature and thus low energy consumption. The absence oflectro-transport of bulky GA− for achieving its separation fromonionic organics in EMR-3 also favors the process. However, highurity of GAH in EMR-4 was obtained that it guaranteed theeparation of GAH from GANa, glucose and other nonionic com-ounds.

Furthermore, performances of processes were investigated inhe lab scale for the separation of GA− and its in situ ion substi-ution for producing GAH without the use any other chemicals, oreteriorating purity of the product. In the developed process, EMR-is a more efficient as compared to EMR-4, in view of recovery,

urrent efficiency, and energy consumption. These processes are

ompletely green in nature with further no generation of wasteasses. Furthermore, production of NaOH in the cathode stream is

pin off of the both processes. The developed process can also befficiently used for conversion, separation, and recovery of otherypes organic acids and bases.

[[[[[

cta 54 (2009) 4880–4887 4887

Acknowledgements

Authors are extremely thankful to the BRNS, DAE, Govt. ofIndia, for providing financial assistance by sanctioning projectno. 2007/35/35/BRNS/102. We acknowledge the Analytical ScienceDivision, CSMCRI, Bhavnagar for instrumental support.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.electacta.2009.04.036.

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