Aminoacid Recovery

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Research Article

Received: 9 August 2009 Revised: 5 December 2009 Accepted: 5 December 2009 Published onlinein Wiley Interscience: 25 January 2010

(www.interscience.wiley.com) DOI 10.1002/jctb.2348

Electro-membrane process for the separation

of amino acids by iso-electric focusingMahendra Kumar, Bijay P. Tripathi and Vinod K. Shahi

Abstract

BACKGROUND: Amino acids (AAs) are usually produced commercially using chemical, biochemical and microbiologicalfermentation methods. The product obtained from these methods undergoes various treatments involving extraction andelectrodialysis (ED) for salt removal and AA recovery. This paper describes an electro-membrane process (EMP) for the chargebased separation of amino acids.

RESULTS: Iso-electric separation of AAs (GLULYS) from their mixture, using ion- exchange membranes (IEMs) has beenachieved by an efficient and indigenous EMP. It was observed that electro-transport rate (flux) of glutamic acid (GLU) at pH 8.0(above its pI) was extremelyhigh, while that for lysine (LYS) (pH9.6) acrossthe anion-exchange membrane (AEM) was very low,under similar experimental conditions. Under optimum experimental conditions, separation of GLU from GLULYS mixturewasachievedwith moderate energyconsumption(12.9 kWhkg1), high current efficiency (CE) (65%) and 85% recovery of GLU.

CONCLUSIONS: On the basis of the electro-transport rate of AA and membrane selectivity, it was concluded that the separationof GLULYS mixture was possible at pH 8.0, because of the oppositely charged nature of the two amino acids due to theirdifferent pIvalues. Moreover, any type of membrane fouling and deterioration in membrane conductivity was ruled out underexperimental conditions. This work clearly demonstrates the great potential of EMP for industrial applications.c 2010 Society of Chemical Industry

Keywords: amino acid separation; iso-electric focusing; ion-exchange membrane; electro-membrane process

NOTATION

CE Current efficiencytmi Counter-ion transport number

m Specific membrane conductivity

Wd Weight of dry membrane

Ww Weight of wet membrane

R Gas constant (8.314 J mol1 K1)

a1 and a2 Activities of electrolyte solutions

T Absolute temperature (K)

F Faraday constant (96500 coulombs)

x, area Thickness of the wet membrane (cm)

A Electrode area (cm2)

V0F and Vtp Initial and final volume of the FC (cm3)

C0F and CtF Initial and final concentration of AA in PC(M)

W Energy consumption (kWh kg1)

V Cell voltage (V)

I Current (A)

t Time (s)

m Weight of GLU or LYS (g)

M Molecular weight of GLU or LYS, n t

n Stoichiometric number (n = 1 in this case)

Q Electric quantity passed (Coulombs; A s)

(J)GLU Flux of GLU (mol m2 s1)

(J)LYS Flux of LYS (mol m2 s1)

INTRODUCTION

Withthe advance in lifescience research,numbersof biomoleculeswith pharmaceutical characteristics have been discovered. In the

down-stream process, separation of amino acids (AA) (amphoteric

in nature with more than one ionizable group, fundamental

constituents of all proteins) was stimulated by an increasing

demand forhigh purity in orderto reduce the side effects induced

by impurities.1 5 During recent years, a noticeable growth of AA

production from molasses and raw sugar by a fermentation pro-

cess, andtheir utilization as an additive in food products, chemical

and pharmaceutical industries, has occurred.610 In particular,

glutamic acid (GLU) and lysine (LYS) have found a wide range

of applications in the food, pharmaceutical, biotechnology, bio-

chemistry and cosmetic industries.1113 Various membrane based

processes, such as ion-exchange, nanofiltration (NF), diafiltration,

ultrafiltration(UF), and electrodialysis (ED), were usedfor the sepa-ration of AAs fromfermentationbroths withoutprecipitation.1418

Use of acid and base to regenerate resins in the ion-exchange

process is a disadvantage because it increases the operational

cost. In addition, wastewater produced during resin regeneration

Correspondence to: Vinod K. Shahi, Electro-Membrane Processes Division,

Central Salt & Marine Chemicals Research Institute, Council of Scientific &

Industrial Research (CSIR), G. B. Marg, Bhavnagar-364002 (Gujarat) India.

E-mail: [email protected]; [email protected]

Electro-MembraneProcessesDivision,CentralSalt&MarineChemicalsResearch

Institute,CouncilofScientific&IndustrialResearch(CSIR),G.B.Marg,Bhavnagar-

364002 (Gujarat) India

J Chem Technol Biotechnol2010; 85: 648657 www.soci.org c 2010 Society of Chemical Industry


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EMPfor the separation of amino acids by iso-electric focusing www.soci.org

Electrode Wash

0.1M Na2SO4

GLU + LYS

in NaAC Buffer

NaAC Buffer

Electrode WashGLULYS

CEM CEMAEM

GLU-

LYS

FeedComp.(FC)

PermeateComp.(PC)

A

node

Ca

thode

Figure 1. Schematic of the EMP cell for separation of amino acids.

and washing causes serious environmental pollution. Membrane

processes such as NF and UF, are size based separation processes

andtheir separation performance is nothigh unless the molecular

weight (MW) ratio of the two components is larger than 10.17,19

In ED, ions are transported through ion-exchange membranes

(IEMs) under the influence of an applied potential to separate

ionic and nonionic constituents.20,21 Recently, bipolar membrane

electodialysis (BMED) processes have been developed for the

conversion of salts into corresponding acid and base.2224 BMED

processeshavealsobeenfoundtobesuitableandcosteffectiveforrecovering organic acid or AAs from their respective salt.2527 Salt

diffusion (co-ion leakage) through the bipolar membrane in BMED

is a serious problem, which affects the purity of the product and

enhances the process cost. There is no ion-exchange membrane

with100% selectivity. Thereis no electro-membrane process (EMP)

based on the principles of ED, used for the efficient separation of

amino acids, with close MWs and different iso-electric points.2831

Working principle of the electro-membrane process (EMP)

The iso-electric focusing technology has been used extensively in

separation of protein or AAs from fermentation broths with great

success. The performance is mainly determined by the net charge

of molecules at different pH and nature of the membranes.15,3234

A schematicdiagramof theEMP cell used forthe separation of AAs

from their mixture is illustrated in Fig. 1. The feed and permeate

chambers were partitioned by an anion-exchange membrane

(AEM) which only allows the passage of anions. Both electrode

chambers (catholyte, anolyte) were separated by cation-exchangemembranes (CEMs), which prevent the approach of AA molecules

to the electrode and further denaturation by electrode reaction.

Thus, at pHpI.

+H2N+H3NCC

(CH2)2

O

OH

COOH

H

CC

(CH2)2

O

O-

COOH

H

H2N CC

(CH2)2

O

O-

COOH

H

Cationic (pI)

C COO-H2N

H

CH2)4

NH3+

C COO-H2N

H

CH2)4

NH2

C COO-+H3N

H

CH2)4

NH3+

Cationic (pI)

J Chem Technol Biotechnol2010; 85: 648657 c 2010 Society of Chemical Industry www.interscience.wiley.com/jctb


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www.soci.org M Kumar, BP Tripathi, VK Shahi

At pH 8.0, GLU existed in the GLU state and LYS in the LYS+

state. There was the possibility for migration of LYS+ towards

the catholyte through CEM. But, at pH 8.0 (near to its pI), due to

the small net positive charge on LYS, its electro-migration from

the feed compartment (FC) to the catholyte was not feasible.

Further, the presence of LYS+ in the catholyte was checked and

found to be negligible. Also, LYS+ remained in the FC due to

its electrical polarity and repulsion between positively charged

AEM. Also, this observation was validated by the mass balance

of LYS in FC before and after the experiments: mass balance was

found to be about 95%. GLU passed through the AEM from

the FC to the permeate compartment (PC), under the applied

electric potential. Thus, high-purity GLU may be obtained in the

PC. For high resolution AA separation, a stable and continuous

pH gradient with relatively constant conductivity and high buffer

capacity is required. Ampholyte was commonly used to provide a

stable pH gradient in the traditional iso-electric focusing system

due to various advantages, such as high buffer capacity, high

solubility, goodconductivity at pIofAAs, and absence of biological

effects.

Therefore, the purpose of this work is to investigate an EMP,

based on the principles of ED to achieve the separation of AAsabove or below the pI of one component in spite of their close

MW and thus molecular size. Herein, we report an EMP for the

separation of AAs from their mixture by iso-electricfocusing using

CEM and AEM as a separation media. A mixture of GLU and LYS in

solution was studied as a model case under similar experimental

conditions.

MATERIALS AND METHODSMaterials

Poly (ether sulfone) (PES) was obtained from Sigma-Aldrich

Chemicals (Mumbai, India) and all other reagents such as GLU,

LYS, H2SO4, NaCl, CdCl2.H2O, DMF, acetic acid, CH3COONa(NaAc), ninhydrine, of Analytical grade reagents (AR) grade were

obtained from S.D. Fine Chemicals, India,and used without further

purification.

Membranes preparation

Sulfonation of PES was carried out as reported earlier.35 CEM

was prepared using sulfonated poly (ether sulfone) (SPES) in

dimethylformamide (DMF) (20%, w/v) and further by casting onto

a cleaned glass plate. The prepared membrane was dried in

ambient condition for24 h, then at 60 C for 6 h ina vacuumoven.

The AEM used in this work was prepared in the laboratory by a

procedure reported earlier and used for various processes.

2831,36

Thus, the membranes obtained (CEM and AEM) were equilibrated

with 1.0 mol L1 HCl and 1.0 mol L1 NaOH solution before use.

The cleaned and equilibrated membranes were stored in double

distilled water.

Electrochemical properties of membranes

The thickness of the wet membrane was measured by micrometer

with 0.10 m accuracy. The membrane water content was

determinedfromtheweightofdryandwetmembrane.Theweight

of dry membrane was recorded after 24 h drying at 60 C, and the

weight of the wet membrane was determined after equilibrating

themembranein water for24 h, with surface water removed using

Table 1. Physicochemical and electrochemicalpropertiesof the CEMandAEM

Properties CEM AEM

Thickness1 (m) 150 150

Water content2 (%) 14.6 10.8

Ion-exchange capacity3

(mequiv./g of drymembrane)

1.03 1.28

Counter-ion transportnumbera,4 (tm )

0.96 0.91

Specific membraneconductivityb,5

(S cm1)

2.05 102 1.12 102

a Measuredby membrane potentialin equilibrationin with0.01 mol L1

and0.1 mol L1 NaCl solutions.b Measured in equilibration with 0.1 mol L1 NaCl solution.Uncertainty for measurements: 1: 1.0 m; 2: 0.1%; 3: 0.01 mequiv g1

of dry membrane; 4: 0.01; and 5: 0.01 103 S cm.

tissue paper. The water content was estimated from:

Water content(%) =WwWd

Wd 100 (1)

For estimation of the ion-exchange capacity (IEC), a piece of

the membrane was equilibrated in 1.0 mol L1 HCl or 1.0 mol L1

NaOH solution overnight to convert them into H+ or OH form.

The excess acid and base was removed by washing with distilled

water. The washed membranes were then equilibrated in 50 mL

of 0.50 mol L1 NaCl solutions to exchange the H+/OH by

Na+/Cl. The amount of H+ or OH ions liberated in solution

was determined by acidbase titration.37

tmi acrossthemembraneswasestimatedbymembranepotential

measurements in equilibration with 0.01 and 0.10 mol L1 NaClsolutions, according to following equation:31

Em = (2tm 1)RT

Fln

a1

a2(2)

The physicochemical and electrochemical properties of CEM

and AEM are presented in Table 1. Both membranes exhibited

good water content, IEC and counter-ion transport number in

the membrane phase under operating conditions. Properties of

these membranes are comparable with the best-known IEMs.38

The knowledge on membrane conductivity in an actual operating

environment is an essential parameter to assess the suitability of

membranes for an EMP.Membrane conductivity measurements for CEM and AEM were

carried out in equilibration with GLU, LYS and NaCl solution

of different concentrations using a potentiostat/galvanostat

frequency response analyzer (Auto Lab, Model PGSTAT 30,

EcoChemie, B.V. Utrecht, The Netherlands). The membrane was

sandwiched between two stainless steel circular electrodes

(4 cm2). Direct current (dc) and sinusoidal alternating currents

(ac) were supplied to the respective electrodes to record the

frequency at a scanning rate of 1.0 A s1 within a frequency

range 106 to103 Hz.39The membrane resistances were obtained

from Nyquist plots using a fit and simulation method. Membrane

conductivity was recorded in equilibration with NaCl, GLU and

LYS solutions of different concentrations. The specific membrane

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conductivity (m) was estimated by the given equation:

m =x

ARm(3)

The variation ofm for both type membranes (CEM and AEM)

in equilibration with NaCl, GLU, and LYS solutions of different

concentrations are shown in Fig. 2(A, B and C), respectively. m

values for both types of IEMs increased with concentration,and were highly dependent on the nature of equilibrating

environment and ionic concentration in the membrane/solution

interfacial zone.Both membranes exhibitedexcellentconductivity

in equilibration with NaCl solution, but m values were relatively

low in equilibration with GLU or LYS solutions of similar

concentration. This may be explained due to extremely low

dissociation constant of AAs and thus lower ionic concentration.40

Furthermore, conductivities values exhibited by CEM and AEM,

suggested their suitability for use in EMP under AAs environment.

Experimental procedure for separation of amino acids byiso-electric focusing

An experimental cell used for EMP was made up of poly vinylchloride (PVC), in which two pieces of CEM and one AEM

were used to separate the four compartments: two electrode

wash chambers (EW) (anolyte and catholyte), FC and PC as

depicted in Fig. 1. The parallel-cum-series flow arrangement

was used for the separation of AAs (GLU and LYS) from their

mixture. Expanded TiO2 sheets coated with a triple precious

metal oxide (titaniumrutheniumplatinum; 6.0 m thickness)

of 1.5 mm thickness and 8.0 103 m2 area, received from

Titanium Tantalum Products (TITAN, Chennai, India) were used

as cathode and anode. Space between each electrode and the

effective membrane area was 1.10 102 m and 8.0 103 m2,

respectively. Peristaltic pumps were used to feed the AA solution

(500 cm3) in a recirculationmode intothe respective compartment

with a constant flowrate (60 cm3 h1) to maintain the turbulence.The whole setup was operated in ambient conditions (303 K)

without any additional temperature control. The 0.1 mol L1

Na2SO4 solution was recirculated into the EW chambers. The

AA solution (GLU or LYS, separately or their mixture) of known

concentration (0.01 0.05 mol L1) in NaAc buffer (0.01 mol L1)

at known pH was fed into the FC, while NaAc buffer (0.01 mol L1)

wasfed into thePC. A dc power supply(Aplab, India, model L1285)

was used to apply constant potential across the electrodes and

the resulting current variations was recorded as a function of time

using a multimeter.

Under the influence of applied potential, amino acid with

negative net charge (AA ; depending on pH) was electro-

transported from FC to PC, across AEM. Solution conductivity and

pHchangesofeachcompartmentwereregularlymonitoredduring

all the experiments. For individual AA solution, concentration

of amino acid (GLU or LYS) was determined by UV-visible

spectrophotometer (Shimadzu, Japan) at max 504 nm fixed

wavelength.41 In a mixture case, AAconcentrationwasanalyzedby

high performance liquid chromatography (HPLC) after precolumn

derivitization using dansyl chloride. A mixture of acetonitrile and

acetate buffer (pH= 4.5; 0.045 mol L1) was used as the mobile

phase flowing through a C18 reverse phase column. The solvent

gradient was 20% to 60% acetonotrile over 30 min. In all cases

an equal volume of AAs solution (separately or their mixture) was

taken for simplicity and to study the feasibility of their separation,

above the iso-electric point of the AAs.

RESULTS AND DISCUSSIONElectro-transport of GLU or LYS

Electro-transport of GLU or LYS (individually) of 0.01 mol L1

concentration in acetate buffer at known pH was carried outin the EMP cell (Fig. 1) at different applied potential (4.06.0 V).

AA (GLU or LYS) solution of different concentrations in acetate

buffer was initially fed into the FC, while only acetate buffer was

fed to the PC. Both EW streams were reconnected and 0.1 mol L1

Na2SO4 solution was recirculated. Variation in current density with

time during electro-transport of AA from FC to PC across AEM

is presented in Fig. 3(A) and (B) for GLU and LYS, respectively, as

a representative case. Experiments were performed at constant

applied potential (4.06.0 V) and resulting variations in current

and concentrations of AA in both compartments (FC and PC)were

recorded as a function of time. At constant applied potential,

current density initially increased and then decreased with time

for both AA solutions. Initial feed concentration (0.01 mol L1 AA

+ 0.01 mol L1 NaAc) of FC and the electrode rinsing solution(0.1 mol L1 Na2SO4) were higher than the concentration of

permeate (0.01 mol L1 NaAc). In this case, buffer solution (NaAc)

and electrode rinsing solution (0.1 mol L1 Na2SO4) did not

influence the electro-migration of AA because the concentration

of NaAc was the same in FC and PC. Migration of Na2SO4was prohibited due to the electro-neutral conditions. Thus the

observed current of the EMP cell varied due to variation in the

electro-migration of AA. A schematic of the EMP for separation of

GLUorLYSor theirmixtureis shownin Fig. 1.Single step separation

was achieved by electro-transport of AA from FC to PC through

0

5

10

15

20

25

30

0 0.02 0.04 0.06

Conc. (M)

0 0.02 0.04 0.06

Conc. (M)

0 0.02 0.04 0.06

Conc. (M)

km

1

0-3

(Scm-1)

AEM

CEM

A

0

2

4

6

8

10

12

AEM

CEM

C

0

2

4

6

8

AEM

CEMB

Figure 2. Membrane conductivity (m) values for CEM and AEM in equilibration with: (A) NaCl; (B) GLU; and (C) LYS solutions of different concentrations.



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5

6

7

8

9

10

11

30 90 150 210

Time (min)

30 90 150 210

Currentdensity(mAcm-2)

4 V

5 V

6 V

A

2

6

10

14

18

4 V

5 V6 V

B

Figure 3. Variation of current density with time at different applied potential during electro-transport of: (A) GLU (0.01 mol L1; pH 8.0) (B) LYS(0.01 mol L1; pH 12) solutions, across the AEM.

2

4

6

8

10

12

14

FC

PC

B

2

4

6

8

10

12

0 50 100 150 200 0 50 100 150 200

Time (min) Time (min)

pH

FC

PC

A

Figure 4. pH variations of FC and PC with time in the EMP at 5.0V for (A) GLU (0.01 mol L1) (B) LYS (0.01 mol L1) solutions as feed for FC.

AEM under the influence of the applied potential. Since, AA was

separated from electrodes by CEMs; electro-migration of AA atspecific pH would not occur because of the strongly charged

nature of CEM. Electro-transport of AA at a particular pH was

further checked by regularly monitoring AA concentration in the

catholyte and anolyte using UV-visible spectrophotometer, and it

was found to be absent.

Under the influence of the applied potential (below limiting

current density), electro-transport of AA from FC to PC was also

verified by the variation of pH and solution conductivity for both

streams (FC and PC) with time, and is presented in Figs 4 and 5 forGLUandLYS,respectively.ThepHofPCdecreasedduetomigration

of AA from FC to PC. Increase in the pH of FC may be attributed

to depletion of GLU andLYS+ inFC.ThepIvalues of GLU and LYS

are3.22 and9.59,respectively. At pH 8.0; GLUexistedas negatively

charged ion (GLU), while at pH 12; LYS existed in the LYS state.

GLU or LYS was electro-transported from FC to PC across AEM

under the applied potential. Simultaneously, AA concentration

0

3

6

9

12

0 100 200 300

Time (min)

0 50 100 150 200

Conductivity(mS

)

FC

PC

A

3

8

13

18

23

FC

PC

B

Figure 5. Variation of conductivity in FC and PC with time at 5.0 V during electro-transport of: (A) GLU(0.01 mol L1 ; pH 8.0) (B) LYS (0.01 mol L1; pH12)solutions.



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in FC decreased, and increased in PC. This phenomenon resulted

in an increase in conductivity and AA concentration in PC. The

conductivity of FC decreased with the time due to depletion

of conducting species (AA). These observations confirmed the

electro-transport of AA across AEM from FC to PC in the EMP.

The electro-transport rate can be assessed on the basis of

GLU or LYS flux (J) in EMP. J values were estimated from the

change of GLU or LYS concentration in FC or PC, considering

negligiblemass(water)transportthroughAEM,usingthefollowing

equation:30,31

J=Va

A

Ct C0

t(4)

where C0 and Ct is the initial andfinal concentration of GLU/LYS

in compartment 1 (mol m3), t is the time allowed for EMP (s),

Va the total volume of solution in the permeate compartment

(0.50 103 m3), and A is the effective membrane area. GLU

and LYS flux during electro-transport are presented in Fig. 6(A)

and Fig. 7(A), respectively, as a function of electricity passed (i.e.

coulombs) at differentappliedpotentials. In bothcases at constant

applied potential, flux initially increased linearly with electricity

passed and then decreased, similarly to the variation of current

density (Fig. 3(A)). Initially, concentration of AA in the PC wasalmostzero. At the start of the process, the concentration of GLU

or LYS reduced in FC, and increased in PC with the amount of

electricity passed. Over time the concentration of AA in the FC

approached a minimum, while in the PC it was enhanced, which

further decreased flux value, similarly to the variation of current

density. Thus we can say that the amount of current (charge)

passed in the EMP at constant applied potential is also a measure

ofAA flux acrossthe AEM in the EMP.Here, itis important tonote

that GLU flux was extremely high, whereas LYS flux across the

AEM was very low under similar experimental conditions, in spite

of their almost equal molecular weight.

Recovery, current efficiency (CE) andenergyconsumption (W)

Recovery of product (GLU or LYS) is an important parameter

to examine the economic feasibility of any process, and may be

defined as:

Glu re covery(%) =CtpVtp

C0FV0F 100 (5)

GLU and LYS recovery (%) at different feed concentrations of

AA into PC is presented in Fig. 6(B) and Fig. 7(B), respectively, as a

function of electricity passed (coulombs). For both cases, recovery

increased with quantity of electricity passed, and decreased

with increasing concentration of AA in FC. Recovery of GLU

was greater than that of LYS, maybe due to the extremely

low migration velocity of LYS across the AEM because of itsbulkier size. It was found that at constant applied potential,

recovery of GLU decreased with increased concentration in FC

because flux becomes independent of feed concentrationabove a

certain minimum concentration.26 Under optimized experimental

0

2

4

6

8

10

12

800 2400 4000 5600 800 2400 4000 5600

Electricity passed (Coulombs)

J1

0-5

(molm-2

s-1)

4 V

5 V

6 V

5

25

45

65

85

Rec

overy(%) 0.01M

0.02M

0.05M

BA

Figure 6. Variation of: (A) flux (J) for GLU (0.01 mol L1; pH 8.0) through the AEM (from FC to PC) with electricity passed (coulombs) at different appliedpotentials; (B) recovery of GLU with electricity passed (coulombs) at 5.0V constant applied potential for different concentrations at pH 8.0; as feed of FC.

6

9

12

15

18

Recovery(%)

0.01M

0.02M

0.05M

B

2

4

6

8

10

800 1800 2800 3800

Electricity passed (Coulombs)

800 1800 2800 3800 4800

J1

0-6

(molm-2

s-1)

4 V

5 V

6 VA

Figure 7. Variation of (A) flux (J) for LYS solution (0.01 mol L1; pH 12) across the AEM with electricity passed (coulombs) at different applied potentials;(B) recovery of LYS with electricity passed (coulombs) at 5.0V constant applied potential for different concentrations at pH 12; as feed of FC.



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which increased steeply beyond pH 9.59 (pI of LYS) due to the

electro-transport of LYS across AEM in the EMP. On the basis

of the above observations, it was concluded that separation of a

GLULYS mixture was possible at pH 8.0, because of the existence

ofoppositely charged statesdue tothe difference in theirpIvalues.

Separation of amino acid (GLU-LYS) from their mixture in EMP

The separation of GLU and LYS (0.01 mol L1, each) from theirmixture was carried out using the EMP cell (Fig. 1) at constant

applied potential (5.0 V) and pH 8.0. Flux values for GLU and

LYS are presented in Fig. 9(A) as a function of electricity passed

(coulombs).GLU fluxwas high, while LYSfluxacross theAEM in the

EMP was extremely low (less than 1.0 mol m2 s1). The separation

factor (SF) can be defined as:

SF= (J)GLU

(J)LYS (8)

Initially the SF value was low, and reached about 7.5 after

passage of an appreciable amount of electricity (Fig. 9(B)). The

positively charged membrane (AEM) allowed GLU flux under

constant applied potential. Thus relatively high SF values reveal

the feasibility of separating GLU and LYS at pH 8.0. For AAs GLU

and LYS, transmission was strongly dependent on the nature of

the charge on the transmitting species (pH), the charge nature of

membranes and the electric gradient. Diffusion of LYS+ andGLU

from PC to anolyte and catholyte was also monitored and both

were found to be negligible.

The selective separation of GLU and LYS from their equi-molar

mixed solution (0.01 mol L1), was carried out at different pH

values from 2.011.0 at 5.0 V. The resulting flux variations are

presented as a function of pH in Fig. 10(A). Below pH 3.22, both

GLU and LYS existed in a positively charged state, and thus their

fluxes were very low and similar across the AEM under given a

electrical polarity. At pH 3.229.59, GLU existed in a negativelycharged (GLU) state, while LYS at pH 8.0 was in a positively

charged state (LYS+). Thus electro-transport and flux of GLU

across the AEM increased progressively, while the flux of LYS+

remained very low due to the diffusion through AEM. At pH 8.0

the difference in the fluxes of GLU and LYS+ was high. Beyond

pH 9.59, both AAsexisted in a negatively charged state (GLU and

LYS), and their flux across the AEM was high but very similar.

The SF values are presented as a function of pH in Fig. 10(B),

showing the optimum pH 8.0 for selective separation of the AA

mixture. The W, CE(%)andrecovery(%)data when separating GLU

at pH 8.0 from the equi-molar mixture of GLULYS at different

feed concentrations are presented in Table 4. Under optimized

experimental conditions (i.e. at 5.0 V applied potential, pH 8.0for 0.01 mol L1 GLU+LYS mixture), 12.9 kWh kg1 energy was

consumed for the separation of GLU from the equi-molar mixture

of GLULYS, CE(65%) and recovery (85%) of the product, showed

the economicfeasibility of the process for industrial exploitation. It

is clearly evident that separation of GLU and LYS can be efficiently

0

2

4

6

8

1000 2000 3000 4000

Electricity passed (Cuolombs)

1000 2000 3000 4000

J1

0-5(

molm-2

s-1)

Glu

Lys

A

3

4

5

6

7

8

Separationfactor(SF)

B

Figure 9. Variation of (A) J; (B) separation factor, with electricity passed (coulombs) at constant applied potential (5.0V) for GLU and LYS mixed solution(0.01 mol L1 each) at pH 8.0 as feed of FC.

0

1

2

3

4

5

6

7

0 2 4 6 8 10 12

J1

0-5

(molm-2

s-1)

pH

0 2 4 6 8 10 12

pH

GLU

LYS

A

0

2

4

6

8

10

Separationfactor(S

F)

B

Figure 10. Variation of (A) J; (B) separation factor, with pH of GLU and LYS mixed solution (0.01 mol L1 each) as feed into FC, after passage of 4000coulombs electricity at constant applied potential (5.0 V).



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Table 4. Energy consumption (W), current efficiency (CE), andrecovery data for electro-transport of GLU from equi-molar GLU+LYSmixed solution (pH 8.0) across the AEM under different experimentalconditions

GLU+LYSconc. of each(mol L1)

Appliedvoltage

(V)W(kWh kg1GLU

separated) CE(%)Recovery of

GLU (%)

0.01 5.0 12.9 65.5 850.02 5.0 16.2 40.9 59

0.05 5.0 20.5 30.5 44

achieved using AEM under a constant applied potential in EMP

due to the difference in their pI values, in spite of very close

molecular weights and sizes. Thus the proposed EMP using IEM is

an importanttool forseparating AAswith close molecularweights

by focusing on their iso-electric values.

In this process no appreciable membrane fouling or AAs

denaturation was observed, while by focusing one component

at itsiso-electric point,and theothercomponent in theoppositely

charged state (depending on pH), they may be effectivelyseparated. In this particular case the difference in the p Ivalues of

GLU and LYS was quite high, but the method could be applied

in cases where pI values are very similar, by very careful control

of pH.

CONCLUSIONSIndigenously prepared CEM and AEM with good physicochemical

and electrochemical properties and stabilities were used to

develop an EMP for the separation of AAs with different pI

values but similar molecular weights. Relatively good membrane

conductivityvaluesforbothtypesofIEMinequilibrationwithNaCl,

GLU or LYS solutions suggested their suitability for application inanEMPundertheexperimentalenvironment.AnEMPcellwithfour

compartments (2-EWs, FC and PC)was fabricated, in whichelectro-

transport of GLU and LYS across the AEM towards the anode

side was studied to see the feasibility of their electro-transport

under given experimental conditions. This is due to the fact that

both the pH of AAs solutions and the electrostatic interaction

between the zwitterionic AA molecules and membrane surface

charge density play important roles in their electro-transport;

thus, demonstrating the importance of employing IEMs in the

EMP system to obtain AA separation by focusing their iso-electric

points with high throughput (flux), purity, recovery and CE. It was

observed that during electro-transport, GLU flux was extremely

high, whereas LYS

fluxes across the AEM were very low undersimilar experimental conditions, in spite of their almost equal

molecular weight and similar charged condition. Also, due to the

relatively high W and low CE and recovery, electro-transport of

LYS is not economically feasible, for the separation of GLULYS

mixture. It was concluded that separation of GLUfromthe mixture

of GLU LYS is possible at pH 8.0, because of the existence

of negatively charged state for both due to the difference in

their pI values. Under optimized experimental conditions (i.e. at

5.0V constant applied potential, pH 10 for 0.01 mol L1 GLU+LYS

mixture), 12.9 kWh kg1 energy was consumed for the separation

of GLU from an equi-molar mixture of GLULYS, CE (65%) and

recovery (85%) of the product, showed the economic feasibility of

the process for industrial exploitation.

Also from data it is clearly evident that separation of GLU

and LYS can be efficiently achieved by using AEM under applied

potential in EMPdue to difference in their pIvalues, in spite of very

close molecular weights. Membrane fouling and deterioration in

membrane conductivitywas notobservedunder the experimental

conditions tested. This work clearly demonstrates the great

potential of EMP for industrial applications.

ACKNOWLEDGEMENTSThe authors thank the Department of Atomic Energy, Government

of India for providing financial assistance by sanctioning project

no. 2007/35/35/BRNS. We also acknowledge the services of the

Analytical Science Division, CSMCRI, Bhavnagar for instrumental

support.

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