Ind. Eng. Chem. Res. 2000, 39, 1397-1408
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Ion-Exchange Equilibria of Amino Acids on Strong Anionic Resins Amel Zammouri,* Simone Chanel, Laurence Muhr, and Georges Grevillot Laboratoire des Sciences du Ge´ nie Chimique, CNRS, 1 rue Grandville, 54001 Nancy, France
In this paper we investigate the equilibrium uptake of neutral, basic, and acidic amino acids by strongly basic anion-exchange resins in the hydroxyl form. Because homogeneous and heterogeneous equilibria take place, both solution and ion-exchange equilibria were studied. A model that takes account of the variation of the separation factor with the ionic fraction in solution was used to correlate the experimental data. Introduction Amino acids are of considerable importance in the food and pharmaceutical industries. Their main production processes are extraction, chemical synthesis, enzymatic catalysis, and fermentation. For recovery and purification of amino acids from the products of these operations, ion-exchange resins are widely used. Because of their amphoteric nature, amino acid charges vary with pH solution, permitting thus their fixation or their elution from resins. To control and optimize separations on ion exchangers, the description and modelization of equilibrium sorption are needed. Several studies have been reported in this context.1-6 However, they supposed the constancy of the separation factor or selectivity coefficient. Saunders et al.12 studied the cation-exchange equilibria of amino acids. They developed a model, originally proposed by Myers and Byington,10 that takes account of the variation of the separation factor with the ionic fraction in solution. Because the separation factor is rarely constant, this model was used in this work to correlate equilibrium data of neutral, basic, and acidic amino acids on strong anion-exchange resins. The three types of amino acids are studied: neutral (phenylalanine, glycine, threonine), basic (histidine and lysine), and acidic (glutamic acid). Theory Equilibrium Models. (1) Dissociation Equilibria in Solution. Amino acids are weak electrolytes. In solution they dissociate into various ionic species. The proportion of these species depends on the solution pH and on the nature of the amino acid. (a) Neutral Amino Acids. The following dissociation equilibrium takes place in the case of neutral amino acids: +
K1
+
-
NH3 CHRCOOH 798 NH3 CHRCOO + H K2
+
NH3+CHRCOO- 798 NH2CHRCOO- + H+
(1) (2)
K1 and K2 are the dissociation constants of the amino acid. Some values of these constants are given in Table 1. * To whom correspondence should be addressed. Present address: ENIG, Route de Medenine, 6029 Gabe`s, Tunisia. E-mail:
[email protected].
Table 1. Equilibrium Dissociation Constants of Some Amino Acids9 amino acid
pK1
pK2
pK3
pI
Glu Gly His Lys Phe
2.19 2.34 1.82 2.18 1.83
9.67 9.60 9.17 8.95 9.13
4.25
3.22 5.97 7.59 9.74 5.48
6.00 10.53
In the presence of various amino acids, the total solution concentration of amino acid Ai is
CAi ) CA+i + CA-i + CA(i
(3)
where
CA i
CA+i ) 1+
CA-i )
K1i CH+
+
K1iK2i
(4)
CH+2
CA i CH+ CH+2 1+ + K2 i K1iK2i
(5)
The electroneutrality constraint in a solution containing sodium ions yields
∑CA
+ i
+ CH+ + CNa+ )
∑CA
i
+ Kw/CH+
(6)
Kw is the water dissociation constant.
Kw ) CH+COHThe ionic fraction of amino acid anions in solution is
XAi )
CA-i
∑CA
i
(7)
+ COH-
(b) Basic Amino Acids. Compared to neutral amino acids, basic amino acids have an additional amino group in their lateral chain. Though in addition to reactions (1) and (2) (with respectively the equilibrium constants K2 and K3) the following dissociation reaction occurs
NH3+CHR+COOH T NH3+CHRCOOH + H+ with the equilibrium constant K1
10.1021/ie990649g CCC: $19.00 © 2000 American Chemical Society Published on Web 03/28/2000
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Ind. Eng. Chem. Res., Vol. 39, No. 5, 2000
K1 ) CA+CH+/CA2+
|zA|R - OH + AzA T R|zA| - AzA + |zA|OH- (20)
the total solution concentration of amino acid Ai is
CAi ) CA2+ + CA+i + CA(i + CA-i i
(9)
CAi
CA2+ ) i 1+
K1i CH+
+
K1iK2i CH+2
+
1+
CH+3
∑CA
2+ i
+
K2i K2iK3i CH+ + + K1i CH+ C +2 CAi
(12)
CH+ CH+2 CH+3 1+ + + K3i K2iK3i K1iK2iK3i
∑CA
+ i
+ CH+ + CNa+ )
∑CA
i
+ Kw/CH+ (13)
(c) Acidic Amino Acids. Compared to neutral amino acids, acidic amino acids have an additional carboxylic group on the lateral chain, giving rise to the following additional reaction:
NH2CHRCOO- T NH2CHR-COO- + H+ (14)
K3 ) CA2-CH+/CAThe total solution concentration of amino acid Ai is
CAi ) CAi+ + CAi( + CAi- + CAi2-
(15)
CAi 1+
K1i C H+
+
K1iK2i CH+2
+
CH+ CH+ K3 + + K2i K1iK2i CH+ 2
1+
(18)
3
CH+ CH+ C H+ + + K3 i K2iK3i K1iK2iK3i
∑
∑
(23)
where E h j and σj are the mean and the standard deviation of the energy distribution and p is the skewness parameter. The adsorption of ion j on site i is represented by the Langmiur equation.
qij )
QCijXjN
∑j
(25)
CijXjN
The Cij constant is given by
(
)
Eij - E hj RT
(26)
Summing over sites and replacing the separation factor, we found in the case of two sites n ) 1 and for a binary system
{
f0+f1 f0 h 1,2 X1NS h 1,2W1,2 + X2N[(1 - p)W1,2 + S1,2 ) S
The electroneutrality constraint is
CA+i + CH+ + CNa+ )
0
