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Adsorption of glutamic acid on weakly basic ion exchanger: Equilibria
Pergamon
Chemical Engineering Science, Vol. 50, No. 14, pp. 2203 2210, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0009 2509/95 $9.50 + 0.00
0009-2509(95)00077-1
ADSORPTION OF GLUTAMIC ACID ON WEAKLY BASIC ION EXCHANGER: EQUILIBRIA HIROYUKI YOSHIDA t and N O B O R U K I S H I M O T O Department of Chemical Engineering, University of Osaka Prefecture, 1-1, Gakuen-Cho, Sakai 593, Japan (Received 25 October 1994; accepted in revised form 20 January 1995)
Abstract Equilibria for adsorption of L-glutamic acid on a general weakly basic ion exchanger which has only one kind of fixed ammonium group are investigated theoretically and experimentally. The experimental equilibrium isotherm for adsorption of L-glutamic acid on the commercial weakly basic ion exchanger was independent of the initial concentration of L-glutamic acid but depended on pH of the solution significantly. The significant effect of pH in the experimental equilibrium isotherm disappeared in the plots of equilibrium amount of L-glutamic acid in the resin phase vs the concentration of the neutral L-glutamic acid in the liquid phase. This suggested that the adsorption of L-glutamic acid was controlled by the acid/base neutralization reaction between neutral L-glutamic acid and fixed ammonium group of the weakly basic ion exchanger. Theoretical equations for equilibrium isotherm for adsorption of L-glutamic acid on the weakly basic resin was derived by assuming acid/base neutralization reaction between carboxylic group of the neutral L-glutamic acid and the fixed ammonium group of resin. The experimental equilibrium isotherms for any constant pH and the effect of pH on the equilibrium amount of L-glutamic acid adsorbed on the weakly basic resin were determined well from the theoretical equations.
1. INTRODUCTION The separation, purification, and recovery of amino acids have been done by using ion exchangers. Ion exchange resins are particularly effective for those processes since the net charge on these molecules may vary in magnitude and sign when the pH of the solution changes. So it is important to study the representation of uptake equilibria of amino acids on ion exchange resins as a function of solution composition. Seno and Yamabe (1960, 1961) reported the way in which pH of the solution affects the uptake equilibria of amino acids. They assumed that the amino acids were adsorbed on strongly acidic or basic ion exchange resins by ion exchange reaction and presented a theoretical equation for the equilibrium. They showed theoretically that the amount of the amino acid adsorbed was the largest at the isoelectric point of amino acid. Dye et al. (1990) investigated the equilibria for adsorption of amino acids on a strong-acid cation-exchange resin. They showed that the uptake of an amino acid by the hydrogen form of the resin occurred primarily as the stoichiometric exchange of amino acid cations for hydrogen ion. DeCarli et al. (1990) reported the operation of a continuous displacement chromatograph for the separations of dilute mixtures of amino acids by displacement development using a strong-acid cation-exchange resin. Helfferich (1990) derived the equations for the equilibrium uptake of neutral, acidic, and basic amino acids by strong-acid cation-exchangers as a function of pH, concentrations of amino acid, and electrolyte or buffer added in the solution.
tAuthor to whom correspondence should be addressed.
The above investigations are mainly adsorption of amino acids on strongly acidic or strongly basic ion exchangers. The equilibrium isotherms for adsorption of an amino acid on weakly basic or weakly acidic ion exchangers especially in the theoretical investigations have not been reported. Yoshida et al. (1995) investigated the possibility of using the polyaminated highly porous chitosan PEI-CH (Kawamura et al., 1993) for adsorption of L-glutamic acid. PEI-CH is a kind of weakly basic ion exchangers which has four different amino groups. They assumed that the adsorption of L-glutamic acid was controlled by the acid/base neutralization reaction between neutral Lglutamic acid and four different ammonium groups of PEI-CH, and presented theoretical equations of equilibrium isotherm for adsorption of L-glutamic acid, which correlated the experimental data reasonably well. PEI-CH is a special weakly basic ion exchanger. Almost commercial weakly basic ion exchangers have only one kind of functional group in the resin phase. In the present work, the equilibria for adsorption of L-glutamic acid on such a general weakly basic ion exchanger are investigated theoretically and experimentally. D I A I O N WA30 was used as such a general commercial ion exchanger in this experimental study. Equilibrium isotherm and titration curve for adsorption of HC1 are presented to show the resin has only one kind of functional group and to give the concentration of the functional group in the resin phase. Theoretical equations for equilibrium isotherm and effect of pH on the equilibrium amount of L-glutamic acid adsorbed on a weakly basic ion exchanger (hereafter called qA-pH curve) are derived by assuming neutralization reaction between neutral L-glutamic
2203
2204
HIROYUKI YOSHIDA a n d NOBORU KISHIMOTO
acid and fixed ammonium group of the weakly basic ion exchange resin. They are compared with the experimental equilibrium isotherms and qA-pH curve. 2. ADSORBENT
We used D I A I O N WA30 in this experimental study. It is a commercial weakly basic ion exchanger, which has been produced by Mitsubishi Kagaku Co. in Japan. Its network is styrene-divinylbenzene and the functional group is a tertiary amine, -CH2N(CH3)2. The experimental physical properties of DIAION WA30 are given in Table 1. a. EXPERIMENTAL PROCEDURES
Before measuring the equilibrium isotherms, the resin particles were conditioned by the same way as our previous paper (Yoshida et al., 1994). L-Glutamic acid was the guaranteed reagent (Tokyo Kasei Co.). The equilibrium isotherms for adsorption of HCI and L-glutamic acid were measured by the batch method. The equilibrium was fully reached in 4 days. In addition, four days were sufficiently enough to reach the equilibrium for the resin because of our breakthrough curve experiments which will be reported. The pH of the L-glutamic acid solution was adjusted using HCI or NaOH. The solution for L-glutamic acid was analyzed with a SHIMADZU LIQUID CHROMATOGRAPH Model LC-3A and a SHIMADZU FLUORESCENCE HPLC M O N I T O R Model RF-535. The pH of the equilibrium solution of L-glutamic acid was analyzed with a Horiba pH meter Model F-16. In addition, in the measurement of the equilibrium isotherm of HCI, when the concentration of HCI in the liquid phase was higher than 5 molm -3, HC1 was analyzed by neutralization titration, and when it was lower than 5 mol m - 3, HCI was analyzed with the pH meter. The resin-phase concentration was calculated according to eq. (1). (Co.i - Ci) V qi W
the experimental study on the equilibrium, the value of V/W (m 3 of solution kg -1 of dry resin) was 0.0625-1. Since the volume of water which soaked in the dry resin particles was smaller than 1.9% of the volume of the solution, the resin-phase concentration was calculated according to eq. (1) without correcting the value of V. Subscript A denotes L-glutamic acid. All experiments were carried out at 298 K. 4. RESULTS 4.1. Equilibrium isotherm of HCl We presented the experimental equilibrium isotherm for adsorption of HC1 on D I A I O N WA30 and showed that the data were correlated by the Langmuir equation, eq. (3), which was caused by the acid/base neutralization reaction eq. (2) (Yoshida et al., 1994). KHcI R-N+HCI
,
qHCl
KHo
"E' 4
3.12 0.541 1063 471 0.557 0.421
-~- Qao
CHC I
(4)
-q.ct
i
~-zr-~- zxo ~
Concentration of amino groups fixed in resin phase, Q (mol kg-1 dry resin) Water content (kg water kg- 1 wet resin) Density true (kg m- 3) apparent (kg dry resin m -3 wet resin) Porosity Diameter (mm) (in water)
(3)
The data measured for three different CO,HOare correlated well by the straight line without scattering (the
o
,
0
o-
o
2
E "r O"
o---o
C0[kmol/m a] 0.02 0.05 o 0.1 WA30 + HCI 298K
"0
Table 1. Experimental physical properties of DIAION WA30
Qncl Krlcl Cnci I + Kac, CHCI
1
(I)
where C0,i and Ci are the initial concentration and equilibrium concentration in the liquid phase (kmol m-3), respectively, qi denotes the equilibrium concentration in the resin-phase (molkg-1 of dry resin). V and W are the volume of the solution (m 3) and the weight of the dry resin (kg), respectively. In
(2)
where R - N denotes tertiary amine fixed in the resin phase. Figure 1 shows the experimental equilibrium isotherm. Since the number of the data was small in the previous paper (Yoshida et al., 1994), we increased the equilibrium data in this study. The isotherm is very favorable. The experimental equilibrium isotherm is independent of the initial concentration of HCI (C0,no). This result supports eqs (2) and (3). The solid line shows the Langmuir isotherm, eq. (3), and it correlates the data well. The Langmuir equilibrium constant Krlo (m 3 kmo1-1) and saturation capacity Qao (mol k g - 1 of dry resin) are listed in Table 2. The value of Qao gives the concentration of the fixed ammonium group in the resin phase. The values of K . o and Q , o were determined using the following equation to which eq. (3) was transformed. CHCI
(i = A, HCI)
~ R NH + C I -
i
i
,
{
i
,
0.05
h
i
0.1
CHCl [kmol/m 3] Fig. 1. Equilibrium isotherms for adsorption of HCI on DIAION WA30: (El) Co =0.02kmolm-3; (A) Co = 0.05 kmol m-3; ((3) Co = 0.10 kmol m- 3; ( ) eq. (3).
Adsorption of glutamic acid on weakly basic ion exchanger Table 2. Experimental Langmuir coefficients for adsorption of HCI and L-glutamicacid on DIAION WA30 i HCI A A +-
Qi (mol kg- 1 of dry resin) Ki (m3kmol- l) 3.14t 3.12 (Qno) 2.20: 2.2&
1.51 x 104t 2.29 x 104 3.27 × 1 0 2~: 4.98 × 1 0 2 §
'
2205
I
•
i
'
I
QHcl=3.12mol/kg dry resin KA=2.29 × 104m3/kmol
QJQHo
.-
3 "
,e
O
L'-
--0.705 0.705
"t3 C~ 2 o u
t Yoshida et al. (1994). : Equation (5); the data for the case that there existed no electrolyte except for L-glutamicacid in the solution. ~Equation (19); the data with NaOH system.
..r ID'-
WA30 + HCI q~ 298K
correlation coefficient was 0.9998). The values of KHCI and Qno were determined from the intercept and slope of the straight line, respectively. 4.2. Titration curve In order to understand the adsorption mechanism of L-glutamic acid on the weakly basic ion exchanger clearly, especially when HCI coexists in the solution, the reliable equilibrium data for adsorption of HCI is necessary. The values of Kncl and Qno were checked out by measuring the titration curve of HCI over a wide range of pH. In Fig. 2, the equilibrium amount of HCI adsorbed is plotted vs the equilibrium value of pH in the solution (open circle). The data shown in Fig. 1 were also plotted using closed circles. A part of the data in Fig. 2 was presented in our previous paper (Yoshida et al., 1994). Although the data in Fig. 2 were not obtained by the titration method but by the batch method for many different initial concentrations of HCI, these types of plots may be called as titration curves and have been considered as an excellent means for studying ion exchange or adsorption characteristics (Kunin, 1958; Helfferich, 1962). The solid line shows the Langmuir isotherm calculated from eq. (3) using the Langmuir coefficients determined in this study (Table 2) and it correlates the data well over the wide range of pH. 4.3. Equilibrium isotherm of L-glutamic acid Figure 3 shows the experimental equilibrium isotherms for adsorption of L-glutamic acid on DIAION WA30 for the case that there existed no electrolyte except for L-glutamic acid in the solution. The isotherms were measured for three different initial concentrations of L-glutamic acid (Co,A). Since the experimental equilibrium isotherms are independent of CO.A, t-glutamic acid may be adsorbed by chemisorption and the equilibrium isotherm may be expressed by the Langmuir equation: qA
QA KACA I + KA CA
(5)
where QA is the saturation capacity of L-glutamic acid (mol kg 1 of dry resin) and KA shows the equilibrium constant (m 3 kmol-~). The solid line in Fig. 3 shows Langmuir isotherm. The data are correlated well by
0
2
4
6
pH Fig. 2. Titration curve for adsorption of HC1 on DIAION WA30: ( ) eq. (3).
.
.
.
.
i
.
.
.
.
i
.
.
.
.
WA30 + L-Glu re
"5
y
298K
< t3.o ........
0.bl .......
6.b , .......
0.03
CA [kmol/m 3] Fig. 3. Equilibrium isotherms for adsorption of L-glutamic acid on DIAION WA30 for the case that there existed no electrolyte except for L-glutamic acid in the ,solution: ((3) Co = 0.01 kmolm-a; (A) Co = 0.02 kmolm-3; (V1) Co = 0.03 kmolm-3; ( ) eq. (5).
eq. (5). The Langmuir coefficients KA and QA are listed in Table 2. They were determined using the following equation as the same way as above: CA --
1
KA
C, -I- QA '~. qA
(6)
The data measured for three different CO,A were plotted based on eq. (6) and were correlated well by the straight line without scattering (the correlation coefficient was 0.964). The values of K A and QA were determined from the intercept and slope of the straight line, respectively. Saturation capacity QA is smaller than the total concentration of fixed amino groups of resin Qno as shown in Table 2. This will be discussed in the theoretical section in detail. Figure 4 shows the effect of pH on the equilibrium amount of L-glutamic acid in the resin phase. The data were obtained by the batch method. The volume of the solution V was 1 x 10 -5 m 3, the weight of the
2206
HIROYUKIYOSHIDAand NOBORUKISHIMOTO .
% i
WA30 + L-Glu
'E"
W=2 x 10 -5 kg dry resin. V=l x 10 -5 m 3 Co=10mol/m 3
.
.
.
.
.
.
.
.
i
.
.
.
.
.
.
.
.
.
I
.
.
.
.
.
.
.
.
.
w,, o + , - G , u 298K
-o E~ m
o < o-
E
o •
<
0
298K
0.01
0.03
CA[km01/m 3]
oi O,' .OOi" J
0.02
7
14
pH Fig. 4. Effect ofpH on the equilibrium amount of L-glutamic acid adsorbed on DIAION WA30: (©) with HC1 system; (0) with NaOH system; (,t) without inorganic electrolyte; (--) theoretical line calculated from eqs (21), (24) and (26); (- - - -) theoretical line calculated from eqs (19) and
Fig. 5. Equilibrium isotherms for adsorption of L-glutamic acid on DIAION WA30 at constant pH which was adjusted using NaOH: (©, A, [], O, ~)Co = 0.0l kmolm-3;(~, /t, I!, ¢,, ~') Co=0.02kmolm-3; (Q, &, II, O, T) Co=0.03kmolm-3; (©, ~, Q) pH=3.7; (&, A, II) pH = 4.1; (D, I!, II) pH = 4.5; (©, *, O) pH = 4.9; (V, ~', ~') pH = 5.3;( ) eq. (5).
(24).
10
~
'
,
'
,
'
PEI-CH + L-Glu, 2 9 8 K resin W was 2 x 10-Skg of dry resin, and the initial concentration of L-glutamic acid CO.A was I x 10 -2 kmol m - 3 for each datum. The initial value of the pH of each solution was adjusted using HCI or N a O H (see the keys in Fig. 4). The pH value of each datum in Fig. 4 was the final equilibrium value. Only when 2 < pH < 7, L-glutamic acid is adsorbed, qA-pH curve shows a peak when the pH of the solution is about the isoelectric point of L-glutamic acid rpi = 3.22 (Yamakawa and Yamabe, 1979)]. As the qA-pH curve in D I A I O N WA30 is sharp, the separation process that L-glutamic acid is adsorbed on it at pH ~ pI and is desorbed at pH < 2 or pH > 7 may be technically feasible. In order to make clear the effect of the pH on the adsorption of L-glutamic acid, the equilibrium isotherms were measured at different constant pH values (within + 0.2) and the results are given in Fig. 5. The pH values were adjusted by using NaOH. The isotherm is independent of the initial concentration of L-glutamic acid but depends on the pH value significantly. The amount of L-glutamic acid adsorbed is the largest at pH ~ 3.7 and it decreases with increasing pH. In addition, since the qA-pH curve for pH < 3.7 is very sharp as shown in Fig. 4, it was impossible to maintain pH constant in pH < 3.7 and we could not obtain the equilibrium isotherms for constant pH in the pH region. The solid lines in Fig. 5 show the Langmuir isotherms [eq. (5)] and they correlate the data reasonably well. The correlation coefficients for pH = 3.7, 4.1, 4.5, 4.9, and 5.3 are 0.995, 0.991, 0.979, 0.969, and 0.987, respectively. The saturation capacity QA and equilibrium constant K^ were determined according to the plots based on eq. (6) as mentioned earlier. Figure 6 shows that Q^ and KA decrease with
o "o
¢o
-6 101
:~
E O
--A..
..~. ~- A - .
10 c
,
4~
,
5i
,
6
pH Fig. 6. Effect of pH on saturation capacity Q, and equilibrium constant KA: ( ) eq. (7); (. . . . . ) eq. (8).
increasing pH. The data give following estimating equations for KA and QA: IOgKA = -- 7.93 x 10 -2 pH + 2.65
(7)
IogQA = - 1.61 x 10 -1 pH + 1.03.
(8)
5. EQUILIBRIUM THEORY
The following conclusions were obtained from the above experimental study: (i) When only L-glutamic acid dissolved in water, the equilibrium isotherm was correlated by the Langmuir equation. The saturation capacity was about 70% of the total concentration of the fixed amino groups of D I A I O N WA30 (Table 2). (ii) The qA-pH curve showed that L-glutamic acid was adsorbed only in the region 2 < pH < 7 and the peak appeared around pH = pI. (iii) When the pH of the solution was constant, the isotherm was independent of the initial concentration of L-glutamic acid but depended on the pH significantly. The isotherm for each constant pH value was correlated by the Langmuir equation.
Adsorption of glutamic acid on weakly basic ion exchanger In order to understand the above complicated results and to estimate equilibrium isotherms in any conditions, a theoretical analysis is necessary. DIAION WA30 is a weakly basic ion exchanger, because the fixed functional group in the resin is weakly basic. Since the fixed functional group does not have O H - in acid region, the maximum of the uptake in Fig. 4 may not be explained by the ion exchange between O H - and negatively charged glutamic acid. L-Glutamic acid dissociates as follows: K1
L-glutamic acid is adsorbed on a weakly basic resin according to the following acid/base neutralization: COOI R-N + Ra-NH~ , I COOH
Equation (15) is simply written by KA±
(9)
K2 Ra-NH~ (-COO-)(-COOH)
.
Ra-NH~ (-COO-)2 + H +
" R - N H + A -.
CA
CA± --
(10)
Ca +
K2
I +21-
K2 K3
R a - N H 2 ( - C O O - ) 2 + H +.
(11)
The equilibrium relations for eqs (9)-(11) are given by eqs (12)-(14), respectively. CA± CH+ K1 - - CA*
K3
(17)
+ c --T
x
CA=CA++CA~ +CA +CA'
Kz
(16)
The concentration of A -+ in the liquid phase, CA, (kmolm -3) is given by eq. (17).
Ka -
I COO -
R-N + A ±
Ra-NH~ (-COO-)(-COOH) + H ÷
(-COO-)2
COOl R - N H ÷" R a - N H ~
(15)
R a - N H ~ (-COOH)2 ,
Ra-NH~
2207
(12)
C A CH+
(13)
CA±
CA2 Ca+
(14)
CA-
where A +, A ± , A - , and A 2- are the L-glutamic acid of R a - N H ~ ( - C O O H ) 2 , R a - N H ~ - ( - C O O - ) (-COOH), R a - N H ~ ( - C O O - ) 2 , and R a - N H 2 ( - C O O - ) 2 , respectively. Figure 7 shows theoretical concentration distributions of A +, A +, A - , and A 2of L-glutamic acid in the liquid phase calculated from eqs (12)-(14) by using K1 = 6.46x l0 -3 k m o l m -a, K 2 = 5.62 x 10- 5 kmol m - 3, and Ka = 2.14 x 1 0 - 1 ° k m o l m -a (Yamakawa and Yamabe, 1979). The distribution curve of A ± is similar to the experimental qA-pH curve in Fig. 4. This may suggest that
•
(18)
Equations (15) or (16) imply that the equilibrium isotherm for adsorption of L-glutamic acid on a weakly basic resin is given by an equation in which qA is expressed by only one independent variable CA+ • 5.1. Without HC1 system
In Fig. 8, experimental equilibrium data without HC1 system (the data with N a O H shown by closed keys in Fig. 4 and all data in Fig. 5) are plotted in the relation between qA--CA t . The significant effect of pH in the qA--CA plots in Fig. 5 disappears in the qA-CA + plots in Fig. 8. In addition, we also plotted qA VS CA', qA VS CA , and qA VS CA2 using the same data as the ones in Fig. 8, respectively, but the plots were very scattered. These results suggest that the reaction mechanism eq. (16), which is proposed here, may be acceptable. From the above discussion, eq. (19) which is derived from eq. (16) is reasonable. QA KA + CA + 1 + KA Ca
qA -
(19)
+
.
.
.
.
.
.
.
.
.
i
.
.
.
.
.
.
.
.
WA30 + L - G l u
.
298K
_ .
"o t:l::z~" o
o 3.7
o
\ 0
7
.4..,,9., []
< .
14
pH Fig. 7. Theoretical concentration distributions of L-glutamic acid in the liquid phase,
0
.
.
.
.
.
.
.
.
i
.
.
.
.
.
0.01
.
.
4.5
.
.
0.02
C A + [ k m o l / m 3]
Fig. 8. Relation between qA and CA_+ for DIAION WA30 without HCI system: ( ) eq. (19).
2208
H1ROYUKI YOSHIDA a n d N O B O R U K I S H I M O T O
Equation (19) is transformed to eq. (20). 1 CA+ CA. . . .ga+ + QA - -qA
(20)
Figure 9 shows the plots of the same data in Fig. 8 based on eq. (20). The data are correlated well by the straight line without scattering (correlation coefficient was 0.991). The values of QA and KA± were determined from the intercept and slope of the straight line, respectively, and they are listed in Table 2. The solid line in Fig. 8 was calculated from eq. (19) using the equilibrium coefficients listed in Table 2. The data are correlated reasonably well by the solid line. Figure 10 shows the same experimental equilibrium isotherms as Fig. 5. The solid lines represent the theoretical lines calculated from eqs (17) and (19) using the equilibrium coefficients QA and K^± given in Table 2. The experimental isotherm is correlated well by the solid line for each constant pH.
0.03
I
WA30 + L-Glu
298K
~-- 0.02
5.2. With HC! system When HCI exists in aqueous solution of L-glutamic acid, L-glutamic acid and HC1 are adsorbed on the tertiary ammonium group of DIAION WA30 by an acid/base neutralization reaction simultaneously. The reaction for adsorption of L-glutamic acid on the fixed ammonium group is expressed by eqs (15) or (16). The reaction for adsorption of HCl on it is given by eq. (2). The value of QA was about 70% of Qno (see Table 2). We assumed that (i) HC1 is adsorbed on all fixed ammonium groups (QHo) and (ii) L-glutamic acid is adsorbed on the ammonium groups fixed on the network with large size (QA) but cannot be adsorbed on the ammonium groups fixed on the network with small size (Qnm - QA). In addition, Jones and Carta (1993) investigated ion exchange of some amino acids (alanine, leucine, and phenylalanine) and dipeptides (phenylalanylalanine and phenylalanyl phenylalanine) on sulfonated polystyrene-divinyibenzene cation ion exchange resins with degree of cross-linking from 4 to 10%. They reported that the maximum uptake of amino acid was considerably lower than the resin ion exchange capacity as the solute size and the degree of cross-linking of the resin were increased. Applying the mass action law to eqs (2) and (16) based on the above assumptions, the equilibrium solidphase concentrations of L-glutamic acid and HCI are given by eqs (21) and (22), respectively,
E
KA + QA CA +
O
E
pH
x . 0.01 +1 <
O
1 + g k + C A ~. + K n o
Kno
" 4.1 [] 4.5
_.,t:l;tt~g~
qHCl =
* 4.9 v 5.3
0 0
i
i
h
h
I
~
i
h
h
5
10
CA+/qA × 103[kg/m 3] Fig. 9. Relation between CA+ and CA± /qA for DIAION WA30 without HC1 system: f ) eq. (20).
.........
i .........
i .........
WA30 + L-Glu 298K
"~ >,
2
"~
1
pH=3.7 \ 4.1 \
o
o
4.9
*
0
v
0.01 0.02 CA[kmol/m 3]
(21)
qA =
i
Cno
QA Cncl
1 + K A ± C A +~ + K H c I C H C 1
+
Krlcl(Qncl
-- QA)C~icl
1 + Kncl Cnct
(22)
As all equilibrium coefficients KAy, K , o , Qno, and QA are known now, theoretical equilibrium isotherms can be calculated from eq. (21), when HCI exists in the solution. Equation (21) should be tested by comparing experimental equilibrium isotherms for constant pH. However, the experimental isotherms at constant pH for adsorption of L-glutamic acid with HCI could not be obtained, because it was difficult to maintain pH of the solution constant. Therefore, the theoretical qA-pH curve was calculated using eq. (21) and it was compared with the experimental qA-pH curve with HCI system (Fig. 4). Equation (19) was also tested by comparing with the experimental qA-pH curve without HC1 system (Fig. 4).
.
0.03
Fig. 10. Equilibrium isotherms for adsorption of L-glutamic acid on DIAION WA30 at constant pH which was adjusted using NaOH: (O) pH = 3.7, (£) pH = 4.1, ([El) pH = 4.5, (~) pH = 4.9, (~7) pH = 5.3 ( ) eqs (17) and (19).
5.3. Theoretical qA-pH curve Theoretical qA-pH curves with HC1 system and without HCI system were calculated using eqs (21) and (19), respectively. The experimental resin-phase concentrations of Lglutamic acid were calculated according to eq. (1). Equation (1) is transformed to eq. (23). W C A = C O , A - - q A --~ "
(23)
Adsorption of glutamic acid on weakly basic ion exchanger Substituting eq. (23) into eq. (17), eq. (24) is obtained. W Co.A -- qA --~ CA+ --
CH+ K2 K2 K3 l + - - ~ - x +C-~n++ C~+
CH+ "[- CA+ = CA- -]- 2CA 2- q- COIl "4- Ccl.
(25)
When Ccl- < Ca+, Cncl is calculated from eq. (26) which is derived from eqs (12)-(14) and (25). Cacl=Co
+
=Ca*--CH+ ~
Kw Ca.
K2 Ca*
2K2 K3"~C ~ ,} A±-
well with the experimental qA-pH curve without HC1 system in pH > 3.7.
(24)
When HCi exists in the solution, it is necessary to calculate the concentration of HCI because of eq. (21). The condition of electroneutrality is given by eq. (25).
(26)
When Ccl > Ca+, Cao is equal to Ca*. The dashed line in Fig. 4 was the theoretical line (with HC1) calculated from eqs (21), (24), and (26). Equilibrium coefficients QA, KA ±, and Kucl in eq. (21) are given in Table 2. CO.A, W, and V in eq. (24) are 1 x 10 -2 k m o l m -3, 2 × 10 -5 kg, and 1 x 10 - s m 3, respectively. Dissociation constants of L-glutamic acid, K1, K2, and K 3 were 6.46× 10 -3, 5.62x 10 -s, and 2.14 x 10-lo kmol m -3, respectively (Yamakawa and Yamabe, 1979). The value of qA was assumed for a given value of Ca*. The value of CA + was calculated from eq. (24) using the C~+ value and the assumed qAThereafter, the value of Cncl was calculated from eq. (26) when C c i < C . . . When Co > Ca+, it was equal to Ca+. Substituting the value of CA~ and CHcl into eq. (21), the new value of qk was obtained. When the relative error of the value of qk for the assumed value of qA was smaller than 10 -6, the value of qk gave the solution of eqs (21), (24), and (26). If the relative error was larger than 10 -6, a new value of qA was set as (qA + qk)/2, and the above calculation was repeated until the relative error became within 10 -6. The dashed line agrees reasonably well with the experimental qA-pH curve with HC1 system in pH < 3.7. When the pH of the solution is adjusted using N a O H or no inorganic electrolytes exist in the solution, only L-glutamic acid is adsorbed on D I A I O N WA30. The solid line in Fig. 4 was a theoretical line (without HCI system) calculated from eqs (19) and (24). Equilibrium coefficients QA and KA± in eq. (19) are given in Table 2. The value of qA was assumed for a given value of CH+. The value of CA + was calculated from eq. (24) using the Cw value and the assumed qA. Substituting the value of CA± into eq. (19), the new value of qk was obtained. When the relative error of the value ofqk for the assumed value ofqA was smaller than 10- 6, the value of qk gave the solution of eqs (19) and (24). If the relative error was larger than 10-6, the value of qA was set as (qA + qk)/2, and the above calculation was repeated until the relative error became within 10 -6 . The solid line agrees reasonably
2209
6. CONCLUSION
Equilibrium theory for adsorption of L-glutamic acid on a weakly basic ion exchanger which has only one kind of functional group was proposed. The experimental equilibrium isotherms and qA-pH curve for adsorption of L-glutamic acid on D I A I O N WA30 were presented and the following conclusions were obtained: (1) The experimental equilibrium isotherm for adsorption of L-glutamic acid on D I A I O N WA30 was independent of the initial concentration of L-glutamic acid but depended on pH of the solution significantly. (2) The significant effect of pH in the qA--CA plots disappeared in qA--CA~ plots. This suggested that L-glutamic acid was not adsorbed by the stoichiometric ion exchange but by the acid/base neutralization reaction between A ± type of L-glutamic acid and the weakly basic functional groups in the ion exchanger [eq. (15)]. (3) Theoretical equations for equilibrium isotherm and qA-pH curve were derived by assuming that A + type of L-glutamic acid was adsorbed on the weakly basic resin by the acid/base neutralization reaction, eq. (15). When HCI did not exist in the solution, the theoretical isotherm was given by eq. (19). When HC1 existed in the solution, the theoretical equation was expressed by eq. (21). The isotherms (qA--CA curve) for any constant pH value were determined reasonably well by eq. (17), and eq. (19) or (21). The qA-pH curves were obtained by the theoretical equations [eq. (19) or (21) and eq. (24)] reasonably well. NOTATION A+
A± AA 2C0.A C0,HCI CHCI
CA CA+ CA
CA CA2-
K1
L-glutamic acid of R a - N H ~ (-COOH)2 L-glutamic acid of Ra-NH~- ( - C O O - ) ( - C O O H ) L-glutamic acid of Ra-NH~- ( - C O O - ) 2 L-glutamic acid of R a - N H 2 ( - C O O - ) 2 initial concentration of L-glutamic acid in liquid phase, kmol m 3 initial concentration of HC1 in liquid phase, kmol m - 3 equilibrium concentration of HCI in liquid phase, kmol m - 3 equilibrium concentration of L-glutamic acid in liquid phase, kmol m - 3 equilibrium concentration of A + type of Lglutamic acid in liquid phase, kmol m - 3 equilibrium concentration of A ± type of L-glutamic acid in liquid phase, kmol m - 3 equilibrium concentration of A - type of Lglutamic acid in liquid phase, kmol m - 3 equilibrium concentration of A 2- type of L-glutamic acid in liquid phase, kmol m - 3 first dissociation constant of L-glutamic acid, kmol m 3
2210
K~ K3 KA±
KHCI
Q^ OHCI
q^ qNo R-N
V W
HIROYUKIYOSHIDAand NOBORUKISHIMOTO
second dissociation constant of L-glutamic acid, kmol m - 3 third dissociation constant of L-glutamic acid, kmol m - 3 equilibrium constant for adsorption of A ± type of L-glutamic acid on resin [eq. (19)], m 3kmol- 1 equilibrium constant for adsorption of HC1 on resin [eq. (3)], m 3 k m o l saturation capacity of L-glutamic acid, mol k g - 1 concentration of fixed a m m o n i u m group in resin, mol k g equilibrium concentration of L-glutamic acid in resin phase, mol k g equilibrium concentration of HCI adsorbed in resin phase, mol kg-1 a m m o n i u m group of resin, R-CH2N(CH3)2 volume of solutions, m 3 weight of dry resin particles, kg
REFERENCES
DeCarli, J. P., II, Carta, G. and Byers, C. H., 1990, Displacement separations by continuous annular chromatography. A.I.Ch.E. J. 36, 1220-1228.
Dye, S. R., DeCarli, J. P., II and Carta, G., 1990, Equilibrium sorption of amino acids by a cation-exchange resin. Ind. Engng Chem. Res. 29, 849-857. Helfferich, F. G., 1962, Ion Exchange, p. 81. McGraw-Hill, New York. Helfferich, F. G., 1990, Ion exchange equilibria of amino acids on strong-acid resins. Reactive Polymers 12, 95-100. Jones, I. L. and Carta, G., 1993, Ion exchange of amino acids and dipeptides on cation resins with varying degree of cross-linking. 1. Equilibrium. Ind. Engng Chem. Res. 32, 107 117. Kawamura, Y., Mitsuhashi, M., Tanibe, H. and Yoshida, H., 1993, Adsorption metal ions on polyaminated highly porous chitosan chelateing resin. Ind. Engn# Chem. Res. 32, 386-391. Kunin, K., 1958, Ion Exchange Resins, p. 55. Wiley, New York. Seno, M. and Yamabe, T., 1960, Ion-exchange behavior of neutral amino acids. Bull. Chem. Soc. Japan 33, 1532-1536. Seno, M. and Yamabe, T., 1961, Ion-exchange behavior of acidic and basic amino acids. Bull. Chem. Soc. Japan 34, 1021-1026. Yamakawa, M. and Yamabe, T., 1979, Seikagaku Data Book, Vol. I, Chap. 2. Tokyo Kagaku Dojin, Tokyo. Yoshida, H., Kishimoto, N. and Kataoka, T., 1994, Adsorption of strong acid on polyaminated highly porous chitosan: equilibria. Ind. Engng Chem. Res. 33, 854-859. Yoshida, H., Kishimoto, N. and Kataoka, T., 1995, Adsorption of glutamic acid on polyaminated highly porous chitosan: equilibria. Ind. Engng Chem. Res. 34, 347-355.
Chemical Engineering Science, Vol. 50, No. 14, pp. 2203 2210, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0009 2509/95 $9.50 + 0.00
0009-2509(95)00077-1
ADSORPTION OF GLUTAMIC ACID ON WEAKLY BASIC ION EXCHANGER: EQUILIBRIA HIROYUKI YOSHIDA t and N O B O R U K I S H I M O T O Department of Chemical Engineering, University of Osaka Prefecture, 1-1, Gakuen-Cho, Sakai 593, Japan (Received 25 October 1994; accepted in revised form 20 January 1995)
Abstract Equilibria for adsorption of L-glutamic acid on a general weakly basic ion exchanger which has only one kind of fixed ammonium group are investigated theoretically and experimentally. The experimental equilibrium isotherm for adsorption of L-glutamic acid on the commercial weakly basic ion exchanger was independent of the initial concentration of L-glutamic acid but depended on pH of the solution significantly. The significant effect of pH in the experimental equilibrium isotherm disappeared in the plots of equilibrium amount of L-glutamic acid in the resin phase vs the concentration of the neutral L-glutamic acid in the liquid phase. This suggested that the adsorption of L-glutamic acid was controlled by the acid/base neutralization reaction between neutral L-glutamic acid and fixed ammonium group of the weakly basic ion exchanger. Theoretical equations for equilibrium isotherm for adsorption of L-glutamic acid on the weakly basic resin was derived by assuming acid/base neutralization reaction between carboxylic group of the neutral L-glutamic acid and the fixed ammonium group of resin. The experimental equilibrium isotherms for any constant pH and the effect of pH on the equilibrium amount of L-glutamic acid adsorbed on the weakly basic resin were determined well from the theoretical equations.
1. INTRODUCTION The separation, purification, and recovery of amino acids have been done by using ion exchangers. Ion exchange resins are particularly effective for those processes since the net charge on these molecules may vary in magnitude and sign when the pH of the solution changes. So it is important to study the representation of uptake equilibria of amino acids on ion exchange resins as a function of solution composition. Seno and Yamabe (1960, 1961) reported the way in which pH of the solution affects the uptake equilibria of amino acids. They assumed that the amino acids were adsorbed on strongly acidic or basic ion exchange resins by ion exchange reaction and presented a theoretical equation for the equilibrium. They showed theoretically that the amount of the amino acid adsorbed was the largest at the isoelectric point of amino acid. Dye et al. (1990) investigated the equilibria for adsorption of amino acids on a strong-acid cation-exchange resin. They showed that the uptake of an amino acid by the hydrogen form of the resin occurred primarily as the stoichiometric exchange of amino acid cations for hydrogen ion. DeCarli et al. (1990) reported the operation of a continuous displacement chromatograph for the separations of dilute mixtures of amino acids by displacement development using a strong-acid cation-exchange resin. Helfferich (1990) derived the equations for the equilibrium uptake of neutral, acidic, and basic amino acids by strong-acid cation-exchangers as a function of pH, concentrations of amino acid, and electrolyte or buffer added in the solution.
tAuthor to whom correspondence should be addressed.
The above investigations are mainly adsorption of amino acids on strongly acidic or strongly basic ion exchangers. The equilibrium isotherms for adsorption of an amino acid on weakly basic or weakly acidic ion exchangers especially in the theoretical investigations have not been reported. Yoshida et al. (1995) investigated the possibility of using the polyaminated highly porous chitosan PEI-CH (Kawamura et al., 1993) for adsorption of L-glutamic acid. PEI-CH is a kind of weakly basic ion exchangers which has four different amino groups. They assumed that the adsorption of L-glutamic acid was controlled by the acid/base neutralization reaction between neutral Lglutamic acid and four different ammonium groups of PEI-CH, and presented theoretical equations of equilibrium isotherm for adsorption of L-glutamic acid, which correlated the experimental data reasonably well. PEI-CH is a special weakly basic ion exchanger. Almost commercial weakly basic ion exchangers have only one kind of functional group in the resin phase. In the present work, the equilibria for adsorption of L-glutamic acid on such a general weakly basic ion exchanger are investigated theoretically and experimentally. D I A I O N WA30 was used as such a general commercial ion exchanger in this experimental study. Equilibrium isotherm and titration curve for adsorption of HC1 are presented to show the resin has only one kind of functional group and to give the concentration of the functional group in the resin phase. Theoretical equations for equilibrium isotherm and effect of pH on the equilibrium amount of L-glutamic acid adsorbed on a weakly basic ion exchanger (hereafter called qA-pH curve) are derived by assuming neutralization reaction between neutral L-glutamic
2203
2204
HIROYUKI YOSHIDA a n d NOBORU KISHIMOTO
acid and fixed ammonium group of the weakly basic ion exchange resin. They are compared with the experimental equilibrium isotherms and qA-pH curve. 2. ADSORBENT
We used D I A I O N WA30 in this experimental study. It is a commercial weakly basic ion exchanger, which has been produced by Mitsubishi Kagaku Co. in Japan. Its network is styrene-divinylbenzene and the functional group is a tertiary amine, -CH2N(CH3)2. The experimental physical properties of DIAION WA30 are given in Table 1. a. EXPERIMENTAL PROCEDURES
Before measuring the equilibrium isotherms, the resin particles were conditioned by the same way as our previous paper (Yoshida et al., 1994). L-Glutamic acid was the guaranteed reagent (Tokyo Kasei Co.). The equilibrium isotherms for adsorption of HCI and L-glutamic acid were measured by the batch method. The equilibrium was fully reached in 4 days. In addition, four days were sufficiently enough to reach the equilibrium for the resin because of our breakthrough curve experiments which will be reported. The pH of the L-glutamic acid solution was adjusted using HCI or NaOH. The solution for L-glutamic acid was analyzed with a SHIMADZU LIQUID CHROMATOGRAPH Model LC-3A and a SHIMADZU FLUORESCENCE HPLC M O N I T O R Model RF-535. The pH of the equilibrium solution of L-glutamic acid was analyzed with a Horiba pH meter Model F-16. In addition, in the measurement of the equilibrium isotherm of HCI, when the concentration of HCI in the liquid phase was higher than 5 molm -3, HC1 was analyzed by neutralization titration, and when it was lower than 5 mol m - 3, HCI was analyzed with the pH meter. The resin-phase concentration was calculated according to eq. (1). (Co.i - Ci) V qi W
the experimental study on the equilibrium, the value of V/W (m 3 of solution kg -1 of dry resin) was 0.0625-1. Since the volume of water which soaked in the dry resin particles was smaller than 1.9% of the volume of the solution, the resin-phase concentration was calculated according to eq. (1) without correcting the value of V. Subscript A denotes L-glutamic acid. All experiments were carried out at 298 K. 4. RESULTS 4.1. Equilibrium isotherm of HCl We presented the experimental equilibrium isotherm for adsorption of HC1 on D I A I O N WA30 and showed that the data were correlated by the Langmuir equation, eq. (3), which was caused by the acid/base neutralization reaction eq. (2) (Yoshida et al., 1994). KHcI R-N+HCI
,
qHCl
KHo
"E' 4
3.12 0.541 1063 471 0.557 0.421
-~- Qao
CHC I
(4)
-q.ct
i
~-zr-~- zxo ~
Concentration of amino groups fixed in resin phase, Q (mol kg-1 dry resin) Water content (kg water kg- 1 wet resin) Density true (kg m- 3) apparent (kg dry resin m -3 wet resin) Porosity Diameter (mm) (in water)
(3)
The data measured for three different CO,HOare correlated well by the straight line without scattering (the
o
,
0
o-
o
2
E "r O"
o---o
C0[kmol/m a] 0.02 0.05 o 0.1 WA30 + HCI 298K
"0
Table 1. Experimental physical properties of DIAION WA30
Qncl Krlcl Cnci I + Kac, CHCI
1
(I)
where C0,i and Ci are the initial concentration and equilibrium concentration in the liquid phase (kmol m-3), respectively, qi denotes the equilibrium concentration in the resin-phase (molkg-1 of dry resin). V and W are the volume of the solution (m 3) and the weight of the dry resin (kg), respectively. In
(2)
where R - N denotes tertiary amine fixed in the resin phase. Figure 1 shows the experimental equilibrium isotherm. Since the number of the data was small in the previous paper (Yoshida et al., 1994), we increased the equilibrium data in this study. The isotherm is very favorable. The experimental equilibrium isotherm is independent of the initial concentration of HCI (C0,no). This result supports eqs (2) and (3). The solid line shows the Langmuir isotherm, eq. (3), and it correlates the data well. The Langmuir equilibrium constant Krlo (m 3 kmo1-1) and saturation capacity Qao (mol k g - 1 of dry resin) are listed in Table 2. The value of Qao gives the concentration of the fixed ammonium group in the resin phase. The values of K . o and Q , o were determined using the following equation to which eq. (3) was transformed. CHCI
(i = A, HCI)
~ R NH + C I -
i
i
,
{
i
,
0.05
h
i
0.1
CHCl [kmol/m 3] Fig. 1. Equilibrium isotherms for adsorption of HCI on DIAION WA30: (El) Co =0.02kmolm-3; (A) Co = 0.05 kmol m-3; ((3) Co = 0.10 kmol m- 3; ( ) eq. (3).
Adsorption of glutamic acid on weakly basic ion exchanger Table 2. Experimental Langmuir coefficients for adsorption of HCI and L-glutamicacid on DIAION WA30 i HCI A A +-
Qi (mol kg- 1 of dry resin) Ki (m3kmol- l) 3.14t 3.12 (Qno) 2.20: 2.2&
1.51 x 104t 2.29 x 104 3.27 × 1 0 2~: 4.98 × 1 0 2 §
'
2205
I
•
i
'
I
QHcl=3.12mol/kg dry resin KA=2.29 × 104m3/kmol
QJQHo
.-
3 "
,e
O
L'-
--0.705 0.705
"t3 C~ 2 o u
t Yoshida et al. (1994). : Equation (5); the data for the case that there existed no electrolyte except for L-glutamicacid in the solution. ~Equation (19); the data with NaOH system.
..r ID'-
WA30 + HCI q~ 298K
correlation coefficient was 0.9998). The values of KHCI and Qno were determined from the intercept and slope of the straight line, respectively. 4.2. Titration curve In order to understand the adsorption mechanism of L-glutamic acid on the weakly basic ion exchanger clearly, especially when HCI coexists in the solution, the reliable equilibrium data for adsorption of HCI is necessary. The values of Kncl and Qno were checked out by measuring the titration curve of HCI over a wide range of pH. In Fig. 2, the equilibrium amount of HCI adsorbed is plotted vs the equilibrium value of pH in the solution (open circle). The data shown in Fig. 1 were also plotted using closed circles. A part of the data in Fig. 2 was presented in our previous paper (Yoshida et al., 1994). Although the data in Fig. 2 were not obtained by the titration method but by the batch method for many different initial concentrations of HCI, these types of plots may be called as titration curves and have been considered as an excellent means for studying ion exchange or adsorption characteristics (Kunin, 1958; Helfferich, 1962). The solid line shows the Langmuir isotherm calculated from eq. (3) using the Langmuir coefficients determined in this study (Table 2) and it correlates the data well over the wide range of pH. 4.3. Equilibrium isotherm of L-glutamic acid Figure 3 shows the experimental equilibrium isotherms for adsorption of L-glutamic acid on DIAION WA30 for the case that there existed no electrolyte except for L-glutamic acid in the solution. The isotherms were measured for three different initial concentrations of L-glutamic acid (Co,A). Since the experimental equilibrium isotherms are independent of CO.A, t-glutamic acid may be adsorbed by chemisorption and the equilibrium isotherm may be expressed by the Langmuir equation: qA
QA KACA I + KA CA
(5)
where QA is the saturation capacity of L-glutamic acid (mol kg 1 of dry resin) and KA shows the equilibrium constant (m 3 kmol-~). The solid line in Fig. 3 shows Langmuir isotherm. The data are correlated well by
0
2
4
6
pH Fig. 2. Titration curve for adsorption of HC1 on DIAION WA30: ( ) eq. (3).
.
.
.
.
i
.
.
.
.
i
.
.
.
.
WA30 + L-Glu re
"5
y
298K
< t3.o ........
0.bl .......
6.b , .......
0.03
CA [kmol/m 3] Fig. 3. Equilibrium isotherms for adsorption of L-glutamic acid on DIAION WA30 for the case that there existed no electrolyte except for L-glutamic acid in the ,solution: ((3) Co = 0.01 kmolm-a; (A) Co = 0.02 kmolm-3; (V1) Co = 0.03 kmolm-3; ( ) eq. (5).
eq. (5). The Langmuir coefficients KA and QA are listed in Table 2. They were determined using the following equation as the same way as above: CA --
1
KA
C, -I- QA '~. qA
(6)
The data measured for three different CO,A were plotted based on eq. (6) and were correlated well by the straight line without scattering (the correlation coefficient was 0.964). The values of K A and QA were determined from the intercept and slope of the straight line, respectively. Saturation capacity QA is smaller than the total concentration of fixed amino groups of resin Qno as shown in Table 2. This will be discussed in the theoretical section in detail. Figure 4 shows the effect of pH on the equilibrium amount of L-glutamic acid in the resin phase. The data were obtained by the batch method. The volume of the solution V was 1 x 10 -5 m 3, the weight of the
2206
HIROYUKIYOSHIDAand NOBORUKISHIMOTO .
% i
WA30 + L-Glu
'E"
W=2 x 10 -5 kg dry resin. V=l x 10 -5 m 3 Co=10mol/m 3
.
.
.
.
.
.
.
.
i
.
.
.
.
.
.
.
.
.
I
.
.
.
.
.
.
.
.
.
w,, o + , - G , u 298K
-o E~ m
o < o-
E
o •
<
0
298K
0.01
0.03
CA[km01/m 3]
oi O,' .OOi" J
0.02
7
14
pH Fig. 4. Effect ofpH on the equilibrium amount of L-glutamic acid adsorbed on DIAION WA30: (©) with HC1 system; (0) with NaOH system; (,t) without inorganic electrolyte; (--) theoretical line calculated from eqs (21), (24) and (26); (- - - -) theoretical line calculated from eqs (19) and
Fig. 5. Equilibrium isotherms for adsorption of L-glutamic acid on DIAION WA30 at constant pH which was adjusted using NaOH: (©, A, [], O, ~)Co = 0.0l kmolm-3;(~, /t, I!, ¢,, ~') Co=0.02kmolm-3; (Q, &, II, O, T) Co=0.03kmolm-3; (©, ~, Q) pH=3.7; (&, A, II) pH = 4.1; (D, I!, II) pH = 4.5; (©, *, O) pH = 4.9; (V, ~', ~') pH = 5.3;( ) eq. (5).
(24).
10
~
'
,
'
,
'
PEI-CH + L-Glu, 2 9 8 K resin W was 2 x 10-Skg of dry resin, and the initial concentration of L-glutamic acid CO.A was I x 10 -2 kmol m - 3 for each datum. The initial value of the pH of each solution was adjusted using HCI or N a O H (see the keys in Fig. 4). The pH value of each datum in Fig. 4 was the final equilibrium value. Only when 2 < pH < 7, L-glutamic acid is adsorbed, qA-pH curve shows a peak when the pH of the solution is about the isoelectric point of L-glutamic acid rpi = 3.22 (Yamakawa and Yamabe, 1979)]. As the qA-pH curve in D I A I O N WA30 is sharp, the separation process that L-glutamic acid is adsorbed on it at pH ~ pI and is desorbed at pH < 2 or pH > 7 may be technically feasible. In order to make clear the effect of the pH on the adsorption of L-glutamic acid, the equilibrium isotherms were measured at different constant pH values (within + 0.2) and the results are given in Fig. 5. The pH values were adjusted by using NaOH. The isotherm is independent of the initial concentration of L-glutamic acid but depends on the pH value significantly. The amount of L-glutamic acid adsorbed is the largest at pH ~ 3.7 and it decreases with increasing pH. In addition, since the qA-pH curve for pH < 3.7 is very sharp as shown in Fig. 4, it was impossible to maintain pH constant in pH < 3.7 and we could not obtain the equilibrium isotherms for constant pH in the pH region. The solid lines in Fig. 5 show the Langmuir isotherms [eq. (5)] and they correlate the data reasonably well. The correlation coefficients for pH = 3.7, 4.1, 4.5, 4.9, and 5.3 are 0.995, 0.991, 0.979, 0.969, and 0.987, respectively. The saturation capacity QA and equilibrium constant K^ were determined according to the plots based on eq. (6) as mentioned earlier. Figure 6 shows that Q^ and KA decrease with
o "o
¢o
-6 101
:~
E O
--A..
..~. ~- A - .
10 c
,
4~
,
5i
,
6
pH Fig. 6. Effect of pH on saturation capacity Q, and equilibrium constant KA: ( ) eq. (7); (. . . . . ) eq. (8).
increasing pH. The data give following estimating equations for KA and QA: IOgKA = -- 7.93 x 10 -2 pH + 2.65
(7)
IogQA = - 1.61 x 10 -1 pH + 1.03.
(8)
5. EQUILIBRIUM THEORY
The following conclusions were obtained from the above experimental study: (i) When only L-glutamic acid dissolved in water, the equilibrium isotherm was correlated by the Langmuir equation. The saturation capacity was about 70% of the total concentration of the fixed amino groups of D I A I O N WA30 (Table 2). (ii) The qA-pH curve showed that L-glutamic acid was adsorbed only in the region 2 < pH < 7 and the peak appeared around pH = pI. (iii) When the pH of the solution was constant, the isotherm was independent of the initial concentration of L-glutamic acid but depended on the pH significantly. The isotherm for each constant pH value was correlated by the Langmuir equation.
Adsorption of glutamic acid on weakly basic ion exchanger In order to understand the above complicated results and to estimate equilibrium isotherms in any conditions, a theoretical analysis is necessary. DIAION WA30 is a weakly basic ion exchanger, because the fixed functional group in the resin is weakly basic. Since the fixed functional group does not have O H - in acid region, the maximum of the uptake in Fig. 4 may not be explained by the ion exchange between O H - and negatively charged glutamic acid. L-Glutamic acid dissociates as follows: K1
L-glutamic acid is adsorbed on a weakly basic resin according to the following acid/base neutralization: COOI R-N + Ra-NH~ , I COOH
Equation (15) is simply written by KA±
(9)
K2 Ra-NH~ (-COO-)(-COOH)
.
Ra-NH~ (-COO-)2 + H +
" R - N H + A -.
CA
CA± --
(10)
Ca +
K2
I +21-
K2 K3
R a - N H 2 ( - C O O - ) 2 + H +.
(11)
The equilibrium relations for eqs (9)-(11) are given by eqs (12)-(14), respectively. CA± CH+ K1 - - CA*
K3
(17)
+ c --T
x
CA=CA++CA~ +CA +CA'
Kz
(16)
The concentration of A -+ in the liquid phase, CA, (kmolm -3) is given by eq. (17).
Ka -
I COO -
R-N + A ±
Ra-NH~ (-COO-)(-COOH) + H ÷
(-COO-)2
COOl R - N H ÷" R a - N H ~
(15)
R a - N H ~ (-COOH)2 ,
Ra-NH~
2207
(12)
C A CH+
(13)
CA±
CA2 Ca+
(14)
CA-
where A +, A ± , A - , and A 2- are the L-glutamic acid of R a - N H ~ ( - C O O H ) 2 , R a - N H ~ - ( - C O O - ) (-COOH), R a - N H ~ ( - C O O - ) 2 , and R a - N H 2 ( - C O O - ) 2 , respectively. Figure 7 shows theoretical concentration distributions of A +, A +, A - , and A 2of L-glutamic acid in the liquid phase calculated from eqs (12)-(14) by using K1 = 6.46x l0 -3 k m o l m -a, K 2 = 5.62 x 10- 5 kmol m - 3, and Ka = 2.14 x 1 0 - 1 ° k m o l m -a (Yamakawa and Yamabe, 1979). The distribution curve of A ± is similar to the experimental qA-pH curve in Fig. 4. This may suggest that
•
(18)
Equations (15) or (16) imply that the equilibrium isotherm for adsorption of L-glutamic acid on a weakly basic resin is given by an equation in which qA is expressed by only one independent variable CA+ • 5.1. Without HC1 system
In Fig. 8, experimental equilibrium data without HC1 system (the data with N a O H shown by closed keys in Fig. 4 and all data in Fig. 5) are plotted in the relation between qA--CA t . The significant effect of pH in the qA--CA plots in Fig. 5 disappears in the qA-CA + plots in Fig. 8. In addition, we also plotted qA VS CA', qA VS CA , and qA VS CA2 using the same data as the ones in Fig. 8, respectively, but the plots were very scattered. These results suggest that the reaction mechanism eq. (16), which is proposed here, may be acceptable. From the above discussion, eq. (19) which is derived from eq. (16) is reasonable. QA KA + CA + 1 + KA Ca
qA -
(19)
+
.
.
.
.
.
.
.
.
.
i
.
.
.
.
.
.
.
.
WA30 + L - G l u
.
298K
_ .
"o t:l::z~" o
o 3.7
o
\ 0
7
.4..,,9., []
< .
14
pH Fig. 7. Theoretical concentration distributions of L-glutamic acid in the liquid phase,
0
.
.
.
.
.
.
.
.
i
.
.
.
.
.
0.01
.
.
4.5
.
.
0.02
C A + [ k m o l / m 3]
Fig. 8. Relation between qA and CA_+ for DIAION WA30 without HCI system: ( ) eq. (19).
2208
H1ROYUKI YOSHIDA a n d N O B O R U K I S H I M O T O
Equation (19) is transformed to eq. (20). 1 CA+ CA. . . .ga+ + QA - -qA
(20)
Figure 9 shows the plots of the same data in Fig. 8 based on eq. (20). The data are correlated well by the straight line without scattering (correlation coefficient was 0.991). The values of QA and KA± were determined from the intercept and slope of the straight line, respectively, and they are listed in Table 2. The solid line in Fig. 8 was calculated from eq. (19) using the equilibrium coefficients listed in Table 2. The data are correlated reasonably well by the solid line. Figure 10 shows the same experimental equilibrium isotherms as Fig. 5. The solid lines represent the theoretical lines calculated from eqs (17) and (19) using the equilibrium coefficients QA and K^± given in Table 2. The experimental isotherm is correlated well by the solid line for each constant pH.
0.03
I
WA30 + L-Glu
298K
~-- 0.02
5.2. With HC! system When HCI exists in aqueous solution of L-glutamic acid, L-glutamic acid and HC1 are adsorbed on the tertiary ammonium group of DIAION WA30 by an acid/base neutralization reaction simultaneously. The reaction for adsorption of L-glutamic acid on the fixed ammonium group is expressed by eqs (15) or (16). The reaction for adsorption of HCl on it is given by eq. (2). The value of QA was about 70% of Qno (see Table 2). We assumed that (i) HC1 is adsorbed on all fixed ammonium groups (QHo) and (ii) L-glutamic acid is adsorbed on the ammonium groups fixed on the network with large size (QA) but cannot be adsorbed on the ammonium groups fixed on the network with small size (Qnm - QA). In addition, Jones and Carta (1993) investigated ion exchange of some amino acids (alanine, leucine, and phenylalanine) and dipeptides (phenylalanylalanine and phenylalanyl phenylalanine) on sulfonated polystyrene-divinyibenzene cation ion exchange resins with degree of cross-linking from 4 to 10%. They reported that the maximum uptake of amino acid was considerably lower than the resin ion exchange capacity as the solute size and the degree of cross-linking of the resin were increased. Applying the mass action law to eqs (2) and (16) based on the above assumptions, the equilibrium solidphase concentrations of L-glutamic acid and HCI are given by eqs (21) and (22), respectively,
E
KA + QA CA +
O
E
pH
x . 0.01 +1 <
O
1 + g k + C A ~. + K n o
Kno
" 4.1 [] 4.5
_.,t:l;tt~g~
qHCl =
* 4.9 v 5.3
0 0
i
i
h
h
I
~
i
h
h
5
10
CA+/qA × 103[kg/m 3] Fig. 9. Relation between CA+ and CA± /qA for DIAION WA30 without HC1 system: f ) eq. (20).
.........
i .........
i .........
WA30 + L-Glu 298K
"~ >,
2
"~
1
pH=3.7 \ 4.1 \
o
o
4.9
*
0
v
0.01 0.02 CA[kmol/m 3]
(21)
qA =
i
Cno
QA Cncl
1 + K A ± C A +~ + K H c I C H C 1
+
Krlcl(Qncl
-- QA)C~icl
1 + Kncl Cnct
(22)
As all equilibrium coefficients KAy, K , o , Qno, and QA are known now, theoretical equilibrium isotherms can be calculated from eq. (21), when HCI exists in the solution. Equation (21) should be tested by comparing experimental equilibrium isotherms for constant pH. However, the experimental isotherms at constant pH for adsorption of L-glutamic acid with HCI could not be obtained, because it was difficult to maintain pH of the solution constant. Therefore, the theoretical qA-pH curve was calculated using eq. (21) and it was compared with the experimental qA-pH curve with HCI system (Fig. 4). Equation (19) was also tested by comparing with the experimental qA-pH curve without HC1 system (Fig. 4).
.
0.03
Fig. 10. Equilibrium isotherms for adsorption of L-glutamic acid on DIAION WA30 at constant pH which was adjusted using NaOH: (O) pH = 3.7, (£) pH = 4.1, ([El) pH = 4.5, (~) pH = 4.9, (~7) pH = 5.3 ( ) eqs (17) and (19).
5.3. Theoretical qA-pH curve Theoretical qA-pH curves with HC1 system and without HCI system were calculated using eqs (21) and (19), respectively. The experimental resin-phase concentrations of Lglutamic acid were calculated according to eq. (1). Equation (1) is transformed to eq. (23). W C A = C O , A - - q A --~ "
(23)
Adsorption of glutamic acid on weakly basic ion exchanger Substituting eq. (23) into eq. (17), eq. (24) is obtained. W Co.A -- qA --~ CA+ --
CH+ K2 K2 K3 l + - - ~ - x +C-~n++ C~+
CH+ "[- CA+ = CA- -]- 2CA 2- q- COIl "4- Ccl.
(25)
When Ccl- < Ca+, Cncl is calculated from eq. (26) which is derived from eqs (12)-(14) and (25). Cacl=Co
+
=Ca*--CH+ ~
Kw Ca.
K2 Ca*
2K2 K3"~C ~ ,} A±-
well with the experimental qA-pH curve without HC1 system in pH > 3.7.
(24)
When HCi exists in the solution, it is necessary to calculate the concentration of HCI because of eq. (21). The condition of electroneutrality is given by eq. (25).
(26)
When Ccl > Ca+, Cao is equal to Ca*. The dashed line in Fig. 4 was the theoretical line (with HC1) calculated from eqs (21), (24), and (26). Equilibrium coefficients QA, KA ±, and Kucl in eq. (21) are given in Table 2. CO.A, W, and V in eq. (24) are 1 x 10 -2 k m o l m -3, 2 × 10 -5 kg, and 1 x 10 - s m 3, respectively. Dissociation constants of L-glutamic acid, K1, K2, and K 3 were 6.46× 10 -3, 5.62x 10 -s, and 2.14 x 10-lo kmol m -3, respectively (Yamakawa and Yamabe, 1979). The value of qA was assumed for a given value of Ca*. The value of CA + was calculated from eq. (24) using the C~+ value and the assumed qAThereafter, the value of Cncl was calculated from eq. (26) when C c i < C . . . When Co > Ca+, it was equal to Ca+. Substituting the value of CA~ and CHcl into eq. (21), the new value of qk was obtained. When the relative error of the value of qk for the assumed value of qA was smaller than 10 -6, the value of qk gave the solution of eqs (21), (24), and (26). If the relative error was larger than 10 -6, a new value of qA was set as (qA + qk)/2, and the above calculation was repeated until the relative error became within 10 -6. The dashed line agrees reasonably well with the experimental qA-pH curve with HC1 system in pH < 3.7. When the pH of the solution is adjusted using N a O H or no inorganic electrolytes exist in the solution, only L-glutamic acid is adsorbed on D I A I O N WA30. The solid line in Fig. 4 was a theoretical line (without HCI system) calculated from eqs (19) and (24). Equilibrium coefficients QA and KA± in eq. (19) are given in Table 2. The value of qA was assumed for a given value of CH+. The value of CA + was calculated from eq. (24) using the Cw value and the assumed qA. Substituting the value of CA± into eq. (19), the new value of qk was obtained. When the relative error of the value ofqk for the assumed value ofqA was smaller than 10- 6, the value of qk gave the solution of eqs (19) and (24). If the relative error was larger than 10-6, the value of qA was set as (qA + qk)/2, and the above calculation was repeated until the relative error became within 10 -6 . The solid line agrees reasonably
2209
6. CONCLUSION
Equilibrium theory for adsorption of L-glutamic acid on a weakly basic ion exchanger which has only one kind of functional group was proposed. The experimental equilibrium isotherms and qA-pH curve for adsorption of L-glutamic acid on D I A I O N WA30 were presented and the following conclusions were obtained: (1) The experimental equilibrium isotherm for adsorption of L-glutamic acid on D I A I O N WA30 was independent of the initial concentration of L-glutamic acid but depended on pH of the solution significantly. (2) The significant effect of pH in the qA--CA plots disappeared in qA--CA~ plots. This suggested that L-glutamic acid was not adsorbed by the stoichiometric ion exchange but by the acid/base neutralization reaction between A ± type of L-glutamic acid and the weakly basic functional groups in the ion exchanger [eq. (15)]. (3) Theoretical equations for equilibrium isotherm and qA-pH curve were derived by assuming that A + type of L-glutamic acid was adsorbed on the weakly basic resin by the acid/base neutralization reaction, eq. (15). When HCI did not exist in the solution, the theoretical isotherm was given by eq. (19). When HC1 existed in the solution, the theoretical equation was expressed by eq. (21). The isotherms (qA--CA curve) for any constant pH value were determined reasonably well by eq. (17), and eq. (19) or (21). The qA-pH curves were obtained by the theoretical equations [eq. (19) or (21) and eq. (24)] reasonably well. NOTATION A+
A± AA 2C0.A C0,HCI CHCI
CA CA+ CA
CA CA2-
K1
L-glutamic acid of R a - N H ~ (-COOH)2 L-glutamic acid of Ra-NH~- ( - C O O - ) ( - C O O H ) L-glutamic acid of Ra-NH~- ( - C O O - ) 2 L-glutamic acid of R a - N H 2 ( - C O O - ) 2 initial concentration of L-glutamic acid in liquid phase, kmol m 3 initial concentration of HC1 in liquid phase, kmol m - 3 equilibrium concentration of HCI in liquid phase, kmol m - 3 equilibrium concentration of L-glutamic acid in liquid phase, kmol m - 3 equilibrium concentration of A + type of Lglutamic acid in liquid phase, kmol m - 3 equilibrium concentration of A ± type of L-glutamic acid in liquid phase, kmol m - 3 equilibrium concentration of A - type of Lglutamic acid in liquid phase, kmol m - 3 equilibrium concentration of A 2- type of L-glutamic acid in liquid phase, kmol m - 3 first dissociation constant of L-glutamic acid, kmol m 3
2210
K~ K3 KA±
KHCI
Q^ OHCI
q^ qNo R-N
V W
HIROYUKIYOSHIDAand NOBORUKISHIMOTO
second dissociation constant of L-glutamic acid, kmol m - 3 third dissociation constant of L-glutamic acid, kmol m - 3 equilibrium constant for adsorption of A ± type of L-glutamic acid on resin [eq. (19)], m 3kmol- 1 equilibrium constant for adsorption of HC1 on resin [eq. (3)], m 3 k m o l saturation capacity of L-glutamic acid, mol k g - 1 concentration of fixed a m m o n i u m group in resin, mol k g equilibrium concentration of L-glutamic acid in resin phase, mol k g equilibrium concentration of HCI adsorbed in resin phase, mol kg-1 a m m o n i u m group of resin, R-CH2N(CH3)2 volume of solutions, m 3 weight of dry resin particles, kg
REFERENCES
DeCarli, J. P., II, Carta, G. and Byers, C. H., 1990, Displacement separations by continuous annular chromatography. A.I.Ch.E. J. 36, 1220-1228.
Dye, S. R., DeCarli, J. P., II and Carta, G., 1990, Equilibrium sorption of amino acids by a cation-exchange resin. Ind. Engng Chem. Res. 29, 849-857. Helfferich, F. G., 1962, Ion Exchange, p. 81. McGraw-Hill, New York. Helfferich, F. G., 1990, Ion exchange equilibria of amino acids on strong-acid resins. Reactive Polymers 12, 95-100. Jones, I. L. and Carta, G., 1993, Ion exchange of amino acids and dipeptides on cation resins with varying degree of cross-linking. 1. Equilibrium. Ind. Engng Chem. Res. 32, 107 117. Kawamura, Y., Mitsuhashi, M., Tanibe, H. and Yoshida, H., 1993, Adsorption metal ions on polyaminated highly porous chitosan chelateing resin. Ind. Engn# Chem. Res. 32, 386-391. Kunin, K., 1958, Ion Exchange Resins, p. 55. Wiley, New York. Seno, M. and Yamabe, T., 1960, Ion-exchange behavior of neutral amino acids. Bull. Chem. Soc. Japan 33, 1532-1536. Seno, M. and Yamabe, T., 1961, Ion-exchange behavior of acidic and basic amino acids. Bull. Chem. Soc. Japan 34, 1021-1026. Yamakawa, M. and Yamabe, T., 1979, Seikagaku Data Book, Vol. I, Chap. 2. Tokyo Kagaku Dojin, Tokyo. Yoshida, H., Kishimoto, N. and Kataoka, T., 1994, Adsorption of strong acid on polyaminated highly porous chitosan: equilibria. Ind. Engng Chem. Res. 33, 854-859. Yoshida, H., Kishimoto, N. and Kataoka, T., 1995, Adsorption of glutamic acid on polyaminated highly porous chitosan: equilibria. Ind. Engng Chem. Res. 34, 347-355.