Transport of glycine by neutralization dialysis

j o u r n a l of MEMBRANE SCIENCE ELSEVIER

Journal of Membrane Science 106 ( 1995 ) 207-211

Transport of glycine by neutralization dialysis Guoqiang Wang 1, Hideo Tanabe, Manabu Igawa Faculty of Engineering, Kanagawa University, Rokkakubashi, Kanagawa-Ku, Yokohama221, Japan Received 20 January 1995; accepted 31 March 1995

Abstract The transport behaviors of glycine in neutralization dialysis were investigated. Glycine was effectively transported and the transport rate was the largest at the pH of the isoelectric point (pH = 6.0). The dependence of the transport rate on the solution pH corresponded well to the calculated concentration of glycine exchanged to the cation-exchange membrane and the anionexchange membrane as a function of the solution pH.

Keywords: Neutralizationdialysis; Ion-exchange membrane; Glycine; Isoelectric point

1. Introduction Neutralization dialysis shows very interesting transport phenomena, which makes cation- and anionexchange membranes very promising separators in deionization [ 1,2] and the separation of weak acids and bases [ 3 ]. It has found industrial applications such as in the desalination of the aqueous solutions of carbohydrates and milk whey [4]. The driving force is the concentration gradient of solutes and the mass transfer in the neutralization dialysis is similar to the transport behaviors in living systems [5]. The purpose of this work is to examine the transport behaviors of glycine in neutralization dialysis, because amino acids are materials of great importance in food and medication.

2. Experimental 2.1. Permeation experiment The permeation experiment was carried out in a neutralization dialysis cell [2], which is shown in Fig. 1. t Present address: Department of Chemistry, Shanghai Institute of BuildingMaterials, 100,Wudong Road, Shanghai,200434, China. 0376-7388/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI0376-7388(95)00094-1

NaOH GlycineI HC G-.q --G - J - ~ G ~ OHB

H"

DIA H lJ

Fig. 1. Neutralization-dialysiscell and the schematic ion transport. a, anion-exchangemembrane;c, cation-exchangemembrane;B, base solutioncompartment;D, desalinationcompartment;A, acid solution compartment. The cell is composed of three compartments divided by two membranes, an anion-exchange membrane and a cation-exchange membrane (Selemion A M V and CMV, Asahi Glass Co. Ltd.). The compartments were named as compartments A, B, and D and an acidic solution, an alkaline solution, and a solution of glycine

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were pumped by tubing pumps from each reservoir into compartments A, B, and D, respectively. The solution in compartment A was 10 mM hydrochloric acid, the solution in compartment B was 10 mM sodium hydroxide, and the solution in compartment D was 10 mM glycine. The pH of the solution of compartment D was adjusted by the addition of hydrochloric acid or sodium hydroxide. The change of pH in the compartment D solution was small (less than __+0.3) in the dialysis experiment for 2 h. The volume of each solution was 100 ml and the pumping rate was 10 ml/min. The membrane area was 11 cm 2 and the compartment was 3 mm thick. In the permeation experiment, an aliquot of the solution in each compartment was collected at an interval and the concentration of glycine was measured.

2.2. Ion-exchange sorption experiment The selectivity coefficient of ion-exchange membranes for glycine were measured by a general procedure [ 6 ]. The cation-exchange membrane was soaked into 1 M HC1 solution for 1 h to obtain the H + form of the membrane and the anion-exchange membrane was soaked in 1 M NaOH solution for 1 h to obtain the O H form of the membrane. The membrane was rinsed with pure water and then, soaked in a glycine solution, whose pH was adjusted with HC1 or NaOH, for 4 h with mechanical shaking. The glycine concentration in the solution was 1 raM, because the amount of ion sorbed from the solution to the membrane should not exceed the ion-exchange capacity of the membrane to obtain a selectivity coefficient exactly. In each experiment, the membrane area was 18 cm 2 and the solution volume was 50 ml. The glycine concentration of the solution equilibrated with the membrane and its pH value were measured. The membrane phase concentration of the glycine was obtained by the concentration change of the solution during this adsorption experiment.

2.3. Analysis

2.4. Reagents The glycine, HC1, and NaOH were of the reagent grade and used without further purification. Reagent grade alkali metal chloride salts were used as received. The water used in this work was the super purified water (Milli-Q II, Millipore Co. ).

3. Results and discussion In the neutralization dialysis, glycine was transported across the cation- and anion-exchange membranes, simultaneously. The concentration changes of glycine in compartments A, B, and D are shown in Fig. 2. Since amino acids are amphoteric electrolytes, they are dissociated into some ion species in solution. The glycinium anion and cation were transported through the anion- and cation-exchange membranes on the basis of an ion exchange reaction with the countertransport of hydroxide ion and hydrogen ion in neutralization dialysis. Hirahara et al. reported that amino acids were selectively transported across a mosaic membrane depending on their degree of dissociation by the adjustment of the solution pH. They found that the glycine did not permeate across a charge-mosaic membrane at the pH of the isoelectric point (pH = 6.0 ) but permeable in the pH ranges below and above the point [7]. It was also reported that an amino acid did not permeate across ion-exchange membranes at its isoelectric point in electrodialysis [8]. The transport behaviors of glycine in the neutralization dialysis was opposite from them. The concentrations of glycine in

¢¢..) E O

o

i

The concentration of glycine was determined by a total organic carbon analyzer (Shimadzu TOC-5000). The pH of solutions was determined by a pH meter (HM-60S, TOA Denpa Ind., Co. Ltd.).

0

1

2

3

4

5

Time (hour) Fig. 2. Concentration change of glycine with time. The solution pH in compartment D was equal to the isoelectric point and the concentration of glycine was 10 raM. Compartment: O, D; A, B; D, A.

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G. Wang et al. / Journal of Membrane Science 106 (1995) 207-211

10

The bars over the chemical formula indicate the species in the membrane. The selectivity coefficients corresponding to the cation- and anion-exchange reactions are shown as Kex(H +) and Kex(OH-), respectively. The electroneutrality in the membrane phase may be expressed as follows.

g tO

E e-

0

3

5

7 pH

9

11

Fig. 3. Concentrationof glycinein the solutionafter2 hoursdialysis in differentpH conditions.The concentrationof glycinein compartment D was 10 mM. Compartment:(3, D; A B; [-1,A. compartments A, B, and D after dialysis for 2 h were plotted against pH of the solution in compartment D in Fig. 3. The permeated amount of glycine at the isoelectric point was larger than those in the ranges below and above the point. This behavior may be accounted for by the calculated concentration of glycine exchanged to the membrane phase in different pH conditions. The dissociation of glycine in the solution may be indicated as bellow. G+=G+H +

K~=[G].[H+]/[G

+]

(1)

G=G- +H +

K2= [ G - ] • [ H + ] / [ G ]

(2)

In these equations, G is HzNCHzCOOH, G + is + H3NCH2COOH, G - is H2NCHzCOO-, and K1 and K2 are the dissociation constants. The values of K~ and K2 are 10 -z35 and 10 -9'78, respectively [9]. The concentration of the total glycine, [G]T, in the solution is shown as a function of the terms of its ionic species by the following equation. [G]T = [G] + [G +] + [ G - ]

(3)

The fraction of each form of glycine in the solution can be calculated as the function of pH from Eqs. ( 1 ) to (3) and is illustrated in Fig. 4. The ion-exchange sorption of glycine from the solution to the cation- and the anion-exchange membranes can be described as follows. G++W+--=~--+--+H +

Kex(H +)

= [~-+--] • [H+ ] / ( [~+-] • [G+ ] ) G- +OH~=G = =OH-

(6)

[ O H - ] + [ G - ] = C~x

(7)

In these equations, Qx and ~ x are the ion-exchange capacity of the cation- and anion-exchange membrane, respectively. The cation-exchange membrane, Selemion-CMV, has a capacity of 2.15 mol/1 membrane and the anion-exchange membrane, Selemion-AMV, has a capacity of 2.35 mol/l in the wet form [ 10]. The selectivity coefficients of glycine to the ion-exchange membranes were measured by the adsorption experiment and Kex(H + ) and Kex(OH- ) were determined to be 1.06 and 0.96, respectively. Fig. 4 shows that glycine has a monovalent cationic form below pH 3 and monovalent anionic form above pH 9 and the predominant form of glycine was zwitteflon at pH values from 3 to 9. In the pH range, glycine does not behave as a cation nor as an anion in the bulk solution but the glycine reacts with proton or hydroxide ion in the membranes. Then, the glycine is dissociated to be transported across the ion-exchange membranes as follows. G + ~-+--=~--+-

(8)

G + O H - = G - +H20

(9)

The concentration of glycine exchanged to each membrane phase can be expressed as follows from the equations described above and the equation of the ion product of water. 1.0 !

f

0.8

Gj \

e-

o

0.6

u_ 0.4

\

0.2 j (4)

2

Kex(OH-)

= [G- ] • [OH- ]/([OH- ] - [G-])

[~v-] + [~-+--] = C~x

4

6

8

10

12

14

pH

(5)

Fig. 4. Fractionof someformsof glycinedependenton solutionpH.

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G. Wang et al. / Journal of Membrane Science 106 (1995) 207-211 ,

,

,

,

,

Anion-exchange membrane E

2

~tion-e_xchan_ge memb2

8

61 ==

E ®

0

3

5

7

9

11

13

pH

Fig. 5. Calculated concentration of glycine in the cation- and anionexchange membranes in different pH conditions. The concentration of glycine was 10 mM.

[6-+-] m

K ~ ( H +) . C ~ - [ G ] T Kex(H +). [G]T + K1 + K~. 1(2/[H ÷ ] + [H + ] (lO)

[G-] Kex(OH-) "C~x'K2" [G]T -Kex(OH-)-/(2" [G]T+Kw+ [H + ] . K J K , + K 2 . K , / [ H + ] (11) The glycine concentration in the r e c e i v i n g phase was v e r y low at the initial stage o f the e x p e r i m e n t and the flux was proportional to the concentration at the m e m b r a n e surface o f the source phase side. Then, the glycine concentration at the m e m b r a n e adjacent to c o m p a r t m e n t D was calculated in different p H conditions and are shown in Fig. 5, where glycine concentration in c o m p a r t m e n t D is supposed to be I0 m M . I

~" 10

g t..-

.o_ E

5

c O

L) 1

2

3

4

5

Time (hour)

Fig. 6. Concentration change of glycine affected by coexisting salt. The solution pH in compartment D was equal to the iso-electric point and the solution was the mixed solution of 10 mM glycine and 10 mM NaC1. Compartment: O, D; A, B; D, A.

The results in Fig. 3 agreed well with the calculated values in Fig. 5, although the interference of coexisting ions such as Na t and C1- occurred in the low and high pH regions. Glycine in the zwitterion form is much distributed from the solution to the membrane phase and results in larger fluxes at the pH around the isoelectric point than at the lower and higher pH ranges. Fig. 6 shows the interference of the coexisting inorganic substance on the permeation of glycine, which was examined by using a mixed solution of 10 mM glycine and 10 mM NaC1. In comparison with the results shown in Fig. 2, the transport rate of glycine across the anion-exchange membrane in the mixed solution was less than that in the single solute solution but that across the cation-exchange membrane was not influenced by the coexisting ion in this condition. This is the reason why the pH dependence of glycine concentration change in compartment A was different from that in compartment B and the pH dependence of the calculated values in Fig. 5 was a little different from that of the measured values in Fig. 3. In conclusion, an amino acid, glycine, can be effectively transported through the anion-exchange membrane and the cation-exchange membrane in neutralization dialysis. The transport rate of glycine was the largest at the pH of its isoelectric point.

References [ 1] M. Igawa, K. Echizenya, T. Hayashita and M. Seno, Donnan dialysis desalination, Chem. Lett., (1986) 237-238. [2] M. Igawa, K. Echizenya, T. Hayashita and M. Seno, Neutralization dialysis for deionization, Bull. Chem. Soc. Jpn., 60 (1987) 381-383. [3] M. Igawa, H. Tanabe, T. Ida, F. Yamamoto and H. Okochi, Separation of weak acids and bases by neutralization dialysis, Chem. Lett., (1993) 1591-1594. [4] M. Bleha and G.A. Tishchenko, Neutralization dialysis for desalination, J. Membrane Sci., 73 (1992) 305-311. [ 5 ] E.N. Lightfoot, Transport phenomena in living systems, Wileylnterscience, New York, 1979. [6] H.L. Yeager and A. Steck, Ion-exchange selectivity and metal ion separations with a perfluorinated cation-exchange polymer, Anal. Chem., 51 (1979) 862-865. [7] K. Hirahara, S. Takahashi, M. Iwata, T. Fujimoto and Y. Miyaki, Artificial membranes from multiblock copolymers (IV). Transport behaviors of organic and inorganic solutes through a charge-mosaic membrane, Ind. Eng. Chem. Prod. Res. Dev., 25 (1986) 305-313.

G. Wang et al. / Journal of Membrane Science 106 (1995) 207-211 8] T. Yamabe, M. Seno and N. Takai, Permeability of amino acid across ion exchange membrane, Bull. Chem. Soc. Jpn., 32 (1959) 1383-1384.

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[9] Handbook of Chemistry and Physics, 66th ed., CRC Press Inc., Boca Raton, FL, 1985. [ 10] S. Itoi, Properties evaluating method of the ion-exchange membrane, Membrane, 6 ( 1981 ) 185-196.