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Sorption of di- and tricarboxylic acids by an anion-exchange membrane
Journal of Membrane Science 222 (2003) 103–111
Sorption of di- and tricarboxylic acids by an anion-exchange membrane Hiroshi Takahashi∗ , Kazuya Ohba, Ken-ichi Kikuchi Department of Materials-Process Engineering and Applied Chemistry for Environment, Faculty of Engineering and Resources Science, Akita University, 1-1 Tegatagakuencho, Akita City, Akita 010-8502, Japan Received 20 December 2002; received in revised form 22 December 2002; accepted 7 June 2003
Abstract Sorption equilibria of the dicarboxylic acids, oxalic, succinic, adipic, and malic acids, and the tricarboxylic acid, citric acid, by the strongly basic anion-exchange membrane (SELEMION AMV) were studied to obtain the fundamental data for the development of a model of electrodialysis process to separate those acids. These acids were increasingly sorbed with increasing pH and solution concentration, while the sorption of citric acid reached a maximum at pH about 4, and gradually decreases. Non-dimensional selectivities of ionic species of these acids by the membrane were determined by an ion-exchange model that considered the dissociation of carboxylic acid in the solution, and electro-neutrality in the solution and in the membrane. The selectivities of mono-, di-, and tricarboxylic acids were correlated with their molecular weight as well as those of eight mono carboxylic acids. These results are useful for the general estimation of selectivity of carboxylic acids. © 2003 Elsevier B.V. All rights reserved. Keywords: Ion-exchange; Carboxylic acids; Sorption; Selectivity
1. Introduction Carboxylic acids, which are used as acidulant compounds in soft drinks, and an additive in medicines, cosmetics, etc. are mainly produced by chemical or fermentation processes. The production by fermentation processes [1] will gradually increase in future in view of energy efficiency. However, fermentation processes have many problems which need solution, especially in downstream processing. Separation of carboxylic acids from the fermentation broth is composed of many complicated processes because the carboxylic acids are generated ∗ Corresponding author. Tel.: +81-889-2743; fax: +81-837-0404. E-mail address: [email protected] (H. Takahashi).
as a metabolite and mixed with various compounds such as proteins, enzymes, and inorganic salts in the culture broth. The first operation in the separation is often crystallisation by lime neutralisation. As the next step, solvent extraction [2] or ion-exchange [3] is used for purification. Ion-exchange with resin beads is a simpler treatment than solvent extraction, for it is operated in a solid–liquid system. Moreover, ion-exchange membranes, which have the same function of ion-exchange as resin beads, enable a continuous separation. Dialysis or electrodialysis with ion-exchange membranes is one of promising processes for carboxylic acid separation not only from culture broth but also from wine and juice. Some works on the application of electrodialysis to carboxylic acid separation have been published by Novalic et al. [4], Andres et al. [5],
0376-7388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0376-7388(03)00259-X
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H. Takahashi et al. / Journal of Membrane Science 222 (2003) 103–111
and Tronc et al. [6]. However, few works have dealt with carboxylic acid fluxes on the basis of the permeation mechanism across ion-exchange membranes. The flux of electrolytes across ion-exchange membranes has been generally evaluated in terms of permeability. However, it is difficult to explain the complicated pH-dependency of flux of carboxylic acids across ion-exchange membranes in terms of their permeabilities only [7]. Carboxylic acids are pHdependently dissociated into various ionic species. For example, citric acid, a tricarboxylic acid, is dissociated into monocharged, dicharged, and tricharged anions. Each of them is sorbed by an ion-exchange membrane, transported in it, and desorbed from it. They permeate competitively with each other and with inorganic ions. The total flux of a citric acid results from the fluxes of its ionic species. Consequently, for accurately simulating the separation of carboxylic acids using electrodialysis from multi-component bio-products, a model should be established in consideration of the dissociation, sorption equilibrium, and transport in the membranes. Construction of the model, as the first step, requires the sorption equilibrium data of carboxylic acids with ion-exchange membranes. We have studied the sorption characteristics of eight monocarboxylic acids by an anion-exchange membrane and determined the selectivity of carboxylic acids between ionic species in the solution and fixed charge in the membrane by using an ion-exchange model [8]. In this work, we examine the sorption characteristics of four dicarboxylic acids and one tricarboxylic acid by an anion-exchange membrane under the various experimental conditions, and evaluate the selectivity of those acids. In addition, we discuss the relationship between selectivity and physical properties of the acids.
2. Theory In the derivation of selectivity, we assume that dissociated carboxylic acids in the solution are exchanged with fixed ion in the membrane. On this assumption, we extend the ion-exchange model [8] for monocarboxylic acids to di- and tricarboxylic acids, in consideration of the electro-neutrality in the solution and in an anion-exchange membrane.
Table 1 z Species no. (i) and valence no. (j) for ionic species Ai j
j zj zj Acids Oxalic acid Succinic acid Adipic acid Malic acid Citric acid
Species no. (i)
Valence no. 0 0 0
1 −1 –
2 −2 2−
3 −3 3−
1 2 3 4 5
Oxa0 Suc0 Adi0 Mal0 Cit0
Oxa− Suc− Adi− Mal− Cit−
Oxa2− Suc2− Adi2− Mal2− Cit2−
Cit3−
2.1. Dissociation of carboxylic acids and electro-neutrality in the solution The dissociation equilibrium of i-carboxylic acid having zj valence proceeds as in Eq. (1). The relationship between valence and ionic species are summarised in Table 1 z
Ki,j
z
Ai j Ai j+1 + H+
(i = 1, 2, . . . ; j = 0, 1–3)
(1)
z
where Ai j denotes an ionic species of i-carboxylic acid having zj valence, and the dissociation constant of the ionic species, Ki,j , is defined as in Eq. (2). Ki,j =
Ci,j+1 CH Ci,j
(2)
The electro-neutrality condition in the solution including a carboxylic acid or more is given in Eq. (3). CNa + CH = |zj |Ci,j + CCl + COH (3) i=1 j=1
The material balances of chemical species are shown in Eqs. (4)–(7). CNa = CNaOH + CNaCl
(4)
CCl = CHCl + CNaCl
(5)
COH CH = Kw Ci,T = Ci,j
(6) (7)
j=0
2.2. Ion-exchange equilibrium When dissociated carboxylic acids and chloride ions are sorbed by an anion-exchange membrane,
H. Takahashi et al. / Journal of Membrane Science 222 (2003) 103–111
ion-exchange schemes can be defined as in Eqs. (8) (j1) and (9). z
|zj |R–OH + Ai j R|zj | –Ai + |zj |OH−
(8)
R–OH + Cl− R–Cl + OH−
(9)
These ion-exchanges can be expressed as the following equations by using the law of mass action. j−1 |zj |Ci,j + COH + CCl A i=1 j=1 SOHi,j QA
=
|zj | C¯ Ai,j COH
(10)
|zj | CAi,j C¯ OH
Cl SOH =
C¯ Cl COH CCl C¯ OH
(11)
In the above equations, the dimensionless selectivA ity, SOHi,j , of monocharged, dicharged, and tricharged z carboxylic anions, Ai j , is derived from ion-exchange capacity and total-anion concentration in the solution. The values of selectivities were determined by the Marquardt optimising method as an objective function of electro-neutrality in the membrane as shown
105
in Eq. (12). |zj |C¯ i,j + C¯ OH + C¯ Cl = QA
(12)
i=1 j=1
3. Materials and methods The acids selected in this experiment were five carboxylic acids: four dicarboxylic acids, oxalic, succinic, adipic, and malic acids; and a tricarboxylic acid, citric acid. All the acids were analytical grade. Table 2 shows the physical properties of these acids. The ion-exchange membrane was a strongly basic anion-exchange membrane, SELEMION AMV (Asahi Glass Co. Ltd.), and the physical properties were shown in our previous paper [9]. All the sorption equilibrium experiments were carried out in the presence of chloride, hydroxide, and carboxylic acid at constant temperature of 298 K. A piece of the anion-exchange membrane was soaked into 1 kmol/m3 sodium chloride aqueous solution to prepare the chloride form of membrane. After two repeats of the preparation, the membrane was immersed in the pH-adjusted mixed solution of carboxylic acid and chloride ions for 12 h, and this operation was repeated four times to attain equilibrium. The equilibrated membrane was treated three times for 12 h each with fresh 1 kmol/m3 sodium hydroxide solution in order to determine the
Table 2 Physical properties of carboxylic acids Acids
Molecular weight
Dissociation constants (298 K) pK1
pK2
90.04 118.09
1.271 4.207
4.266 5.638
Adipic acid
146.15
4.430
5.277
Malic acid
134.09
3.460
5.050
Citric acid
192.12
3.128
4.761
Oxalic acid Succinic acid
(COOH)2
pK3
6.396
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concentrations of carboxylic acid and chloride in the membrane. The concentration of chloride ion was determined by the Mohr’s method, and that of oxalic acid was measured by indole colorimetric determination [10]. The concentrations of the other carboxylic acids were analysed by reversed phase HPLC with phosphoric acid buffer solution as eluent. The details of the operation were described elsewhere [8].
4. Results and discussion 4.1. Dissociation of carboxylic acids Fig. 1 shows the calculated concentration distributions of carboxylic acid species including their ions produced by dissociation at various pHs in the solution. When the solution pH is lower than pK1 , the un-dissociated species is dominant, whereas at neutral and alkaline pHs, the fractions of negatively charged species increase. 4.2. Sorption of dicarboxylic acids Figs. 2–5 show the results of sorption equilibrium experiments at various solution pH for oxalic acid, succinic acid, adipic acid, and malic acid, respectively, for the anion-exchange membrane. These results for
Fig. 2. Sorption of oxalic acid (i = 1) from the solution including NaCl by anion-exchange membrane. Keys: (䊊) C¯ 1,T ; () C¯ Cl . Lines: (—) calculated with model.
dicarboxylic acids show similar tendency with each other. To take an example, malic acid shown in Fig. 5 is increasingly sorbed by the anion-exchange membrane with increasing concentration in the solution,
Fig. 1. Calculated concentration distributions of carboxylic acid species at various pHs in the solution. Lines: (—) Ci,0 ; (---) Ci,1 ; (– – –) Ci,2 ; (– - – - –) Ci,3 .
H. Takahashi et al. / Journal of Membrane Science 222 (2003) 103–111
107
Fig. 3. Sorption of succinic acid (i = 2) from the solution including NaCl by anion-exchange membrane. Keys: (䊊) C¯ 2,T ; () C¯ Cl . Lines: (—) calculated with model.
Fig. 5. Sorption of malic acid (i = 4) from the solution including NaCl by anion-exchange membrane. Keys: (䊊) C¯ 4,T ; () C¯ Cl . Lines: (—) calculated with model.
Fig. 4. Sorption of adipic acid (i = 3) from the solution including NaCl by anion-exchange membrane. Keys: (䊊) C¯ 3,T ; () C¯ Cl . Lines: (—) calculated with model.
but chloride ion is decreasingly sorbed. The sorption of malic acid is also affected by the solution pH: Malic acid is more sorbed at pH 4 than at pH 3, but over the range of pH 4–6 seems to show similar sorption with each other, while chloride ion shows decreasing sorption with increasing pH. This fact would result from the following. At pH 3, malic acid is sorbed as a monocharged anion of malic acid, because the total concentration of malic acid and chloride ion nearly agrees with the ion-exchange capacity of anion-exchange membrane. In the range of pH 5–6, malic acid is sorbed as a dicharged anion in addition to a monocharged anion; thus the total concentration of malic acid and chloride ion is lower than the ion-exchange capacity. This reasoning can be applied to the carboxylic acids, oxalic acid, succinic acid, and adipic acid. The lines in the figures were calculated by the ion-exchange model described above. The calculated values almost agree with the experimental data, which supports the validity of the ion-exchange model for the mechanism of carboxylic acid sorption by an anion-exchange membrane. The pH dependence of sorption equilibrium at a given solution concentration is shown for malic acid
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decrease with increasing pH, because dicharged malic acid anions also exist in the membrane and contribute to the transport of electric current. 4.3. Sorption of tricarboxylic acid
Fig. 6. Concentration profiles of malic acid (i = 4) in an anion-exchange membrane (a) and solution (b) at various pHs. Conditions: CNaCl = 10 mol/m3 ; C4,T = 10 mol/m3 .
The sorption equilibrium experiments were also carried out for citric acid. Fig. 7 shows the sorption behaviour of citric acid under various conditions of citric acid concentration and pH in solution. The sorption of citric acid by the membrane increases with increase in citric acid concentration and pH as well as that of dicarboxylic acids. However, the amount of citric acid in the membrane is less than half the ion-exchange capacity. This suggests that citric acid sorbed by the membrane will be a mixture of monocharged, dicharged, and tricharged anions of citric acid. The pH dependencies of concentration profiles of citric acid species in the solution and membrane are shown
as an example, with the concentration profiles of all anions in the solution in Fig. 6. The sorption of malic acid increases with increasing pH in the solution, while that of chloride ions rapidly decreases. The lines shown in the figure are results calculated from the model. At pH about 3, most malic acid is present as monocharged anions in the solution, so that it is sorbed in the ionic form by the membrane. However, at pH over 3, dicharged anions of malic acid also exist in the solution, so both monocharged and dicharged anions are exchanged with chloride ions in the membrane. At pH greater than 6, most malic acid sorbed by the membrane is as dicharged anions. The results calculated from the model show that the concentration of monocharged anions in the membrane reaches a maximum at pH 4.0, while that in the solution at pH 4.3, which is a little higher than the former. Therefore, dicharged anions of malic acid are expected to be preferably sorbed by the membrane than monocharged anions. This fact provides valuable information to malic acid separation using dialysis or electrodialysis: When malic acid solution is dialysed at the pH higher than the pH where a fraction of monocharged malic acid anions in the solution shows maximum, that is, at the pH corresponding to weak acidic or neutral region, the malic acid flux through the membrane at a constant current is estimated to
Fig. 7. Sorption of citric acid (i = 5) from the solution including NaCl by anion-exchange membrane. Keys: (䊊) C¯ 5,T ; () C¯ Cl . Lines: (—) C¯ 5,T , C¯ Cl ; (---) C¯ 5,1 ; (– – –) C¯ 5,2 ; (– - – - –) C¯ 5,3 .
H. Takahashi et al. / Journal of Membrane Science 222 (2003) 103–111
109
the model explains these tendencies as follows. The sorbed species of citric acid are mainly monocharged anion at pH lower than 3, and change into di- and tricharged anions at pH higher than 4. 4.4. Relationship between molecular weight and selectivities of mono-, di-, and tricarboxylic acids
Fig. 8. Concentration profiles of citric acid (i = 5) in an anion-exchange membrane (a) and solution (b) at various pHs. Conditions: CNaCl = 10 mol/m3 ; C5,T = 10 mol/m3 .
in Fig. 8. The amount of citric acid in the membrane increases with an increase in pH, reaches a maximum about pH 4, and gradually decreases while chloride ion concentration decreases. The calculation with
Table 3 shows the non-dimensional selectivities of di- and tricarboxylic acids in this work, and monocarboxylic acids in the previous work [8]. Monocharged anions of di- and tricarboxylic acids have selectivities higher than hydroxide ion, lower than chloride ion Cl = 9.0) except that of oxalic acid, and higher by (SOH one order of magnitude than monocharged anions of monocarboxylic acids. Dicharged and tricharged anions of the di- and tricarboxylic acid show selectivities larger than their monocharged anions by two or three orders of magnitude. Now, we have determined the selectivities of eight monocarboxylic, four dicarboxylic, and one tricarboxylic acids. It is convenient if these of selectivity can be used for deriving a general principle of estimating the selectivities. Kawakita et al. [11] have correlated the selectivities of amino acids with molecular
Table 3 Selectivities of mono-, di-, and tricarboxylic acids Acids
Monocarboxylic acids[8] Formic acid Acetic acid Propionic acid Butyric acid Valeric acid Hexanoic acid Heptanoic acid Lactic acid
Monocharged anions
Dicharged anions
Tricharged anion
A SOHi,1
A SOHi,2
Keysb
SOHi,3
Keyb
3840
䊒
Keysb
0.94 0.74 0.67 0.70 0.68 1.61 2.06 0.71
䊊 䊊 䊊 䊊 䊊 䊊 䊊
Dicarboxylic acidsa Oxalic acid Succinic acid Adipic acid Malic acid
20.3 5.77 3.84 4.87
130 129 109 113
䉱 䉱 䉱
Tricarboxylic acida Citric acid
6.23
䊐
843
䉱 䊏
a b
This work. In Fig. 9.
A
110
H. Takahashi et al. / Journal of Membrane Science 222 (2003) 103–111
acids by the anion-exchange membrane increases with an increase in the hydrophobicity. However, note that the selectivities of acids lacking methylene groups such as formic and oxalic acids considerably deviate from the trend of the carboxylic acids having methylene groups. This indicates that it is necessary to study the effect of chemical interaction in the selective sorption by ion-exchange membrane. Consequently the selectivities of carboxylic acids cannot be precisely estimated with their molecular weights only, but the parameter of molecular weight could be useful for the general estimation.
5. Conclusions
Fig. 9. Relationship between selectivities and molecular weights of carboxylic acids. Species: A− –M, monocharged anion having methylene groups; A2− –M, dicharged anion having methylene groups; A3− –M, tricharged anion having methylene groups; A− –ML, monocharged anion lacking methylene groups. Keys are shown in Table 3.
weight, and indicated that the correlations are divided into three classes. Thus we will attempt to correlate the selectivities with the molecular weights of carboxylic acids. Fig. 9 shows the correlation of selectivities with molecular weights for mono-, di-, and tricarboxylic acids listed in Table 3. The selectivities increase with increasing molecular weight. The observed values in the figure seem to be divided in four classes: monocharged anions of carboxylic acids having methylene groups (A− –M); dicharged anions of carboxylic acids having methylene groups (A2− –M); tricharged anions of carboxylic acids having methylene groups (A3− –M); and monocharged anions of carboxylic acids lacking methylene groups (A− –ML). It is interesting that in each class selectivities are correlated with molecular weight irrespective of the number of carboxylic groups. An increase in molecular weight corresponds to an increase in the number of methylene groups, that is, an increase in hydrophobicity. Therefore, the selective sorption of carboxylic
The sorption equilibria of the dicarboxylic acids, oxalic, succinic, adipic and malic acids, and the tricarboxylic acid, citric acid, by an anion-exchange membrane were studied, which led to the following conclusions. The sorption of dicarboxylic and tricarboxylic acids by the membrane increased with increase in the solution concentration and pH, and depended on the solution concentrations of their ionic species. The sorption characteristics were successfully explained by the ion-exchange model considering the dissociation of carboxylic acids in solution, and electro-neutrality in the solution and in the membrane. This supports that the sorption of carboxylic acids by anion-exchange membranes proceeds by the proposed ion-exchange mechanism. The model simulation yielded the ion-exchange selectivities of the five carboxylic acids. Their selectivities were correlated with the molecular weights as well as those of eight monocarboxylic acids. The correlation parameter of molecular weight was useful for the general estimation of selectivities. Nomenclature z
Ai j C Ci,j K
species of i-carboxylic acid having zj valence concentration (mol/m3 ) concentration of species of i-carboxylic acid having zj valence (mol/m3 ) dissociation constant (mol/m3 )
H. Takahashi et al. / Journal of Membrane Science 222 (2003) 103–111
Kw QA S zj
ion product constant of water (mol2 /m6 ) ion-exchange capacity (mol/m3 ) selectivity valence
Subscripts H hydrogen ion HCl hydrogen chloride i number of carboxylic acid defined in Table 2 j number of valence defined in Table 2 Na sodium ion NaCl sodium chloride NaOH sodium hydroxide
[3]
[4]
[5]
[6]
[7]
Superscript − membrane phase
References [1] F.A. Roddick, M.L. Britz, Production of hexanoic acid by free and immobilised cells of Megasphaera elsdenii, influence of in-situ product removal using ion exchange resin, J. Chem. Technol. Biotechnol. 69 (1997) 383. [2] M. Siebold, P.V. Frieling, R. Joppien, D. Rindfleisch, K. Schugerl, H. Ropper, Comparison of the production oflactic
[8]
[9]
[10] [11]
111
acid by three different Lactobacilli and its recovery by extraction and electrodialysis, Process Biochem. 30 (1995) 81. J. Rincon, J. Fuertes, J.F. Rodriguez, L. Dodriguez, J.M. Monteagudo, Selection of a cation exchange resin to produce lactic acid solutions from whey fermentation broths, Solvent Extr. Ion Exch. 15 (1997) 329. S. Novalic, F. Jagsxhits, J. Okwor, K.D. Kulbe, Behavior of citric acid during electrodialysis, J. Membr. Sci. 108 (1995) 201. L.J. Andres, F.A. Riera, R. Alvarez, Recovery and concentration by electrodialysis of tartaric acid from fruit juice industries waste waters, J. Chem. Technol. Biotechnol. 70 (1997) 247. J.-S. Tronc, F. Lamarche, J. Makhlouf, Enzymatic browning inhibition in cloudy apple juice by electrodialysis, J. Food Sci. 62 (1997) 75. H. Takahashi, T. Higashi, K. Kikuchi, Permeation behavior of mono-, di-, and tricarboxylic acids in electrodialysis, J. Ion Exch. 14 (suppl.) (2003) 197. H. Takahashi, K. Ohba, K. Kikuchi, Sorption of monocarboxylic acids by an anion-exchange membrane, Biochem. Eng. J., in press, corrected proof, Available online 23 April 2003. K. Kikuchi, S. Miyata, H. Takahashi, T. Gotoh, T. Sugawara, J.S. Dranoff, Sorption of amino acids by ion exchange membranes, J. Chem. Eng. Japan 27 (1994) 391. J. Bergerman, J.S. Elliot, Method for direct colorimetric determination of oxalic acid, Anal. Chem. 27 (1954) 1014. T. Kawakita, T. Ogura, M. Saeki, H. Hayasaki, Selectivity coefficients of amino acids for the ammonium ion on a strong cation exchange resin, Agric. Biol. Chem. 54 (1990) 1.
Sorption of di- and tricarboxylic acids by an anion-exchange membrane Hiroshi Takahashi∗ , Kazuya Ohba, Ken-ichi Kikuchi Department of Materials-Process Engineering and Applied Chemistry for Environment, Faculty of Engineering and Resources Science, Akita University, 1-1 Tegatagakuencho, Akita City, Akita 010-8502, Japan Received 20 December 2002; received in revised form 22 December 2002; accepted 7 June 2003
Abstract Sorption equilibria of the dicarboxylic acids, oxalic, succinic, adipic, and malic acids, and the tricarboxylic acid, citric acid, by the strongly basic anion-exchange membrane (SELEMION AMV) were studied to obtain the fundamental data for the development of a model of electrodialysis process to separate those acids. These acids were increasingly sorbed with increasing pH and solution concentration, while the sorption of citric acid reached a maximum at pH about 4, and gradually decreases. Non-dimensional selectivities of ionic species of these acids by the membrane were determined by an ion-exchange model that considered the dissociation of carboxylic acid in the solution, and electro-neutrality in the solution and in the membrane. The selectivities of mono-, di-, and tricarboxylic acids were correlated with their molecular weight as well as those of eight mono carboxylic acids. These results are useful for the general estimation of selectivity of carboxylic acids. © 2003 Elsevier B.V. All rights reserved. Keywords: Ion-exchange; Carboxylic acids; Sorption; Selectivity
1. Introduction Carboxylic acids, which are used as acidulant compounds in soft drinks, and an additive in medicines, cosmetics, etc. are mainly produced by chemical or fermentation processes. The production by fermentation processes [1] will gradually increase in future in view of energy efficiency. However, fermentation processes have many problems which need solution, especially in downstream processing. Separation of carboxylic acids from the fermentation broth is composed of many complicated processes because the carboxylic acids are generated ∗ Corresponding author. Tel.: +81-889-2743; fax: +81-837-0404. E-mail address: [email protected] (H. Takahashi).
as a metabolite and mixed with various compounds such as proteins, enzymes, and inorganic salts in the culture broth. The first operation in the separation is often crystallisation by lime neutralisation. As the next step, solvent extraction [2] or ion-exchange [3] is used for purification. Ion-exchange with resin beads is a simpler treatment than solvent extraction, for it is operated in a solid–liquid system. Moreover, ion-exchange membranes, which have the same function of ion-exchange as resin beads, enable a continuous separation. Dialysis or electrodialysis with ion-exchange membranes is one of promising processes for carboxylic acid separation not only from culture broth but also from wine and juice. Some works on the application of electrodialysis to carboxylic acid separation have been published by Novalic et al. [4], Andres et al. [5],
0376-7388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0376-7388(03)00259-X
104
H. Takahashi et al. / Journal of Membrane Science 222 (2003) 103–111
and Tronc et al. [6]. However, few works have dealt with carboxylic acid fluxes on the basis of the permeation mechanism across ion-exchange membranes. The flux of electrolytes across ion-exchange membranes has been generally evaluated in terms of permeability. However, it is difficult to explain the complicated pH-dependency of flux of carboxylic acids across ion-exchange membranes in terms of their permeabilities only [7]. Carboxylic acids are pHdependently dissociated into various ionic species. For example, citric acid, a tricarboxylic acid, is dissociated into monocharged, dicharged, and tricharged anions. Each of them is sorbed by an ion-exchange membrane, transported in it, and desorbed from it. They permeate competitively with each other and with inorganic ions. The total flux of a citric acid results from the fluxes of its ionic species. Consequently, for accurately simulating the separation of carboxylic acids using electrodialysis from multi-component bio-products, a model should be established in consideration of the dissociation, sorption equilibrium, and transport in the membranes. Construction of the model, as the first step, requires the sorption equilibrium data of carboxylic acids with ion-exchange membranes. We have studied the sorption characteristics of eight monocarboxylic acids by an anion-exchange membrane and determined the selectivity of carboxylic acids between ionic species in the solution and fixed charge in the membrane by using an ion-exchange model [8]. In this work, we examine the sorption characteristics of four dicarboxylic acids and one tricarboxylic acid by an anion-exchange membrane under the various experimental conditions, and evaluate the selectivity of those acids. In addition, we discuss the relationship between selectivity and physical properties of the acids.
2. Theory In the derivation of selectivity, we assume that dissociated carboxylic acids in the solution are exchanged with fixed ion in the membrane. On this assumption, we extend the ion-exchange model [8] for monocarboxylic acids to di- and tricarboxylic acids, in consideration of the electro-neutrality in the solution and in an anion-exchange membrane.
Table 1 z Species no. (i) and valence no. (j) for ionic species Ai j
j zj zj Acids Oxalic acid Succinic acid Adipic acid Malic acid Citric acid
Species no. (i)
Valence no. 0 0 0
1 −1 –
2 −2 2−
3 −3 3−
1 2 3 4 5
Oxa0 Suc0 Adi0 Mal0 Cit0
Oxa− Suc− Adi− Mal− Cit−
Oxa2− Suc2− Adi2− Mal2− Cit2−
Cit3−
2.1. Dissociation of carboxylic acids and electro-neutrality in the solution The dissociation equilibrium of i-carboxylic acid having zj valence proceeds as in Eq. (1). The relationship between valence and ionic species are summarised in Table 1 z
Ki,j
z
Ai j Ai j+1 + H+
(i = 1, 2, . . . ; j = 0, 1–3)
(1)
z
where Ai j denotes an ionic species of i-carboxylic acid having zj valence, and the dissociation constant of the ionic species, Ki,j , is defined as in Eq. (2). Ki,j =
Ci,j+1 CH Ci,j
(2)
The electro-neutrality condition in the solution including a carboxylic acid or more is given in Eq. (3). CNa + CH = |zj |Ci,j + CCl + COH (3) i=1 j=1
The material balances of chemical species are shown in Eqs. (4)–(7). CNa = CNaOH + CNaCl
(4)
CCl = CHCl + CNaCl
(5)
COH CH = Kw Ci,T = Ci,j
(6) (7)
j=0
2.2. Ion-exchange equilibrium When dissociated carboxylic acids and chloride ions are sorbed by an anion-exchange membrane,
H. Takahashi et al. / Journal of Membrane Science 222 (2003) 103–111
ion-exchange schemes can be defined as in Eqs. (8) (j1) and (9). z
|zj |R–OH + Ai j R|zj | –Ai + |zj |OH−
(8)
R–OH + Cl− R–Cl + OH−
(9)
These ion-exchanges can be expressed as the following equations by using the law of mass action. j−1 |zj |Ci,j + COH + CCl A i=1 j=1 SOHi,j QA
=
|zj | C¯ Ai,j COH
(10)
|zj | CAi,j C¯ OH
Cl SOH =
C¯ Cl COH CCl C¯ OH
(11)
In the above equations, the dimensionless selectivA ity, SOHi,j , of monocharged, dicharged, and tricharged z carboxylic anions, Ai j , is derived from ion-exchange capacity and total-anion concentration in the solution. The values of selectivities were determined by the Marquardt optimising method as an objective function of electro-neutrality in the membrane as shown
105
in Eq. (12). |zj |C¯ i,j + C¯ OH + C¯ Cl = QA
(12)
i=1 j=1
3. Materials and methods The acids selected in this experiment were five carboxylic acids: four dicarboxylic acids, oxalic, succinic, adipic, and malic acids; and a tricarboxylic acid, citric acid. All the acids were analytical grade. Table 2 shows the physical properties of these acids. The ion-exchange membrane was a strongly basic anion-exchange membrane, SELEMION AMV (Asahi Glass Co. Ltd.), and the physical properties were shown in our previous paper [9]. All the sorption equilibrium experiments were carried out in the presence of chloride, hydroxide, and carboxylic acid at constant temperature of 298 K. A piece of the anion-exchange membrane was soaked into 1 kmol/m3 sodium chloride aqueous solution to prepare the chloride form of membrane. After two repeats of the preparation, the membrane was immersed in the pH-adjusted mixed solution of carboxylic acid and chloride ions for 12 h, and this operation was repeated four times to attain equilibrium. The equilibrated membrane was treated three times for 12 h each with fresh 1 kmol/m3 sodium hydroxide solution in order to determine the
Table 2 Physical properties of carboxylic acids Acids
Molecular weight
Dissociation constants (298 K) pK1
pK2
90.04 118.09
1.271 4.207
4.266 5.638
Adipic acid
146.15
4.430
5.277
Malic acid
134.09
3.460
5.050
Citric acid
192.12
3.128
4.761
Oxalic acid Succinic acid
(COOH)2
pK3
6.396
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concentrations of carboxylic acid and chloride in the membrane. The concentration of chloride ion was determined by the Mohr’s method, and that of oxalic acid was measured by indole colorimetric determination [10]. The concentrations of the other carboxylic acids were analysed by reversed phase HPLC with phosphoric acid buffer solution as eluent. The details of the operation were described elsewhere [8].
4. Results and discussion 4.1. Dissociation of carboxylic acids Fig. 1 shows the calculated concentration distributions of carboxylic acid species including their ions produced by dissociation at various pHs in the solution. When the solution pH is lower than pK1 , the un-dissociated species is dominant, whereas at neutral and alkaline pHs, the fractions of negatively charged species increase. 4.2. Sorption of dicarboxylic acids Figs. 2–5 show the results of sorption equilibrium experiments at various solution pH for oxalic acid, succinic acid, adipic acid, and malic acid, respectively, for the anion-exchange membrane. These results for
Fig. 2. Sorption of oxalic acid (i = 1) from the solution including NaCl by anion-exchange membrane. Keys: (䊊) C¯ 1,T ; () C¯ Cl . Lines: (—) calculated with model.
dicarboxylic acids show similar tendency with each other. To take an example, malic acid shown in Fig. 5 is increasingly sorbed by the anion-exchange membrane with increasing concentration in the solution,
Fig. 1. Calculated concentration distributions of carboxylic acid species at various pHs in the solution. Lines: (—) Ci,0 ; (---) Ci,1 ; (– – –) Ci,2 ; (– - – - –) Ci,3 .
H. Takahashi et al. / Journal of Membrane Science 222 (2003) 103–111
107
Fig. 3. Sorption of succinic acid (i = 2) from the solution including NaCl by anion-exchange membrane. Keys: (䊊) C¯ 2,T ; () C¯ Cl . Lines: (—) calculated with model.
Fig. 5. Sorption of malic acid (i = 4) from the solution including NaCl by anion-exchange membrane. Keys: (䊊) C¯ 4,T ; () C¯ Cl . Lines: (—) calculated with model.
Fig. 4. Sorption of adipic acid (i = 3) from the solution including NaCl by anion-exchange membrane. Keys: (䊊) C¯ 3,T ; () C¯ Cl . Lines: (—) calculated with model.
but chloride ion is decreasingly sorbed. The sorption of malic acid is also affected by the solution pH: Malic acid is more sorbed at pH 4 than at pH 3, but over the range of pH 4–6 seems to show similar sorption with each other, while chloride ion shows decreasing sorption with increasing pH. This fact would result from the following. At pH 3, malic acid is sorbed as a monocharged anion of malic acid, because the total concentration of malic acid and chloride ion nearly agrees with the ion-exchange capacity of anion-exchange membrane. In the range of pH 5–6, malic acid is sorbed as a dicharged anion in addition to a monocharged anion; thus the total concentration of malic acid and chloride ion is lower than the ion-exchange capacity. This reasoning can be applied to the carboxylic acids, oxalic acid, succinic acid, and adipic acid. The lines in the figures were calculated by the ion-exchange model described above. The calculated values almost agree with the experimental data, which supports the validity of the ion-exchange model for the mechanism of carboxylic acid sorption by an anion-exchange membrane. The pH dependence of sorption equilibrium at a given solution concentration is shown for malic acid
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decrease with increasing pH, because dicharged malic acid anions also exist in the membrane and contribute to the transport of electric current. 4.3. Sorption of tricarboxylic acid
Fig. 6. Concentration profiles of malic acid (i = 4) in an anion-exchange membrane (a) and solution (b) at various pHs. Conditions: CNaCl = 10 mol/m3 ; C4,T = 10 mol/m3 .
The sorption equilibrium experiments were also carried out for citric acid. Fig. 7 shows the sorption behaviour of citric acid under various conditions of citric acid concentration and pH in solution. The sorption of citric acid by the membrane increases with increase in citric acid concentration and pH as well as that of dicarboxylic acids. However, the amount of citric acid in the membrane is less than half the ion-exchange capacity. This suggests that citric acid sorbed by the membrane will be a mixture of monocharged, dicharged, and tricharged anions of citric acid. The pH dependencies of concentration profiles of citric acid species in the solution and membrane are shown
as an example, with the concentration profiles of all anions in the solution in Fig. 6. The sorption of malic acid increases with increasing pH in the solution, while that of chloride ions rapidly decreases. The lines shown in the figure are results calculated from the model. At pH about 3, most malic acid is present as monocharged anions in the solution, so that it is sorbed in the ionic form by the membrane. However, at pH over 3, dicharged anions of malic acid also exist in the solution, so both monocharged and dicharged anions are exchanged with chloride ions in the membrane. At pH greater than 6, most malic acid sorbed by the membrane is as dicharged anions. The results calculated from the model show that the concentration of monocharged anions in the membrane reaches a maximum at pH 4.0, while that in the solution at pH 4.3, which is a little higher than the former. Therefore, dicharged anions of malic acid are expected to be preferably sorbed by the membrane than monocharged anions. This fact provides valuable information to malic acid separation using dialysis or electrodialysis: When malic acid solution is dialysed at the pH higher than the pH where a fraction of monocharged malic acid anions in the solution shows maximum, that is, at the pH corresponding to weak acidic or neutral region, the malic acid flux through the membrane at a constant current is estimated to
Fig. 7. Sorption of citric acid (i = 5) from the solution including NaCl by anion-exchange membrane. Keys: (䊊) C¯ 5,T ; () C¯ Cl . Lines: (—) C¯ 5,T , C¯ Cl ; (---) C¯ 5,1 ; (– – –) C¯ 5,2 ; (– - – - –) C¯ 5,3 .
H. Takahashi et al. / Journal of Membrane Science 222 (2003) 103–111
109
the model explains these tendencies as follows. The sorbed species of citric acid are mainly monocharged anion at pH lower than 3, and change into di- and tricharged anions at pH higher than 4. 4.4. Relationship between molecular weight and selectivities of mono-, di-, and tricarboxylic acids
Fig. 8. Concentration profiles of citric acid (i = 5) in an anion-exchange membrane (a) and solution (b) at various pHs. Conditions: CNaCl = 10 mol/m3 ; C5,T = 10 mol/m3 .
in Fig. 8. The amount of citric acid in the membrane increases with an increase in pH, reaches a maximum about pH 4, and gradually decreases while chloride ion concentration decreases. The calculation with
Table 3 shows the non-dimensional selectivities of di- and tricarboxylic acids in this work, and monocarboxylic acids in the previous work [8]. Monocharged anions of di- and tricarboxylic acids have selectivities higher than hydroxide ion, lower than chloride ion Cl = 9.0) except that of oxalic acid, and higher by (SOH one order of magnitude than monocharged anions of monocarboxylic acids. Dicharged and tricharged anions of the di- and tricarboxylic acid show selectivities larger than their monocharged anions by two or three orders of magnitude. Now, we have determined the selectivities of eight monocarboxylic, four dicarboxylic, and one tricarboxylic acids. It is convenient if these of selectivity can be used for deriving a general principle of estimating the selectivities. Kawakita et al. [11] have correlated the selectivities of amino acids with molecular
Table 3 Selectivities of mono-, di-, and tricarboxylic acids Acids
Monocarboxylic acids[8] Formic acid Acetic acid Propionic acid Butyric acid Valeric acid Hexanoic acid Heptanoic acid Lactic acid
Monocharged anions
Dicharged anions
Tricharged anion
A SOHi,1
A SOHi,2
Keysb
SOHi,3
Keyb
3840
䊒
Keysb
0.94 0.74 0.67 0.70 0.68 1.61 2.06 0.71
䊊 䊊 䊊 䊊 䊊 䊊 䊊
Dicarboxylic acidsa Oxalic acid Succinic acid Adipic acid Malic acid
20.3 5.77 3.84 4.87
130 129 109 113
䉱 䉱 䉱
Tricarboxylic acida Citric acid
6.23
䊐
843
䉱 䊏
a b
This work. In Fig. 9.
A
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H. Takahashi et al. / Journal of Membrane Science 222 (2003) 103–111
acids by the anion-exchange membrane increases with an increase in the hydrophobicity. However, note that the selectivities of acids lacking methylene groups such as formic and oxalic acids considerably deviate from the trend of the carboxylic acids having methylene groups. This indicates that it is necessary to study the effect of chemical interaction in the selective sorption by ion-exchange membrane. Consequently the selectivities of carboxylic acids cannot be precisely estimated with their molecular weights only, but the parameter of molecular weight could be useful for the general estimation.
5. Conclusions
Fig. 9. Relationship between selectivities and molecular weights of carboxylic acids. Species: A− –M, monocharged anion having methylene groups; A2− –M, dicharged anion having methylene groups; A3− –M, tricharged anion having methylene groups; A− –ML, monocharged anion lacking methylene groups. Keys are shown in Table 3.
weight, and indicated that the correlations are divided into three classes. Thus we will attempt to correlate the selectivities with the molecular weights of carboxylic acids. Fig. 9 shows the correlation of selectivities with molecular weights for mono-, di-, and tricarboxylic acids listed in Table 3. The selectivities increase with increasing molecular weight. The observed values in the figure seem to be divided in four classes: monocharged anions of carboxylic acids having methylene groups (A− –M); dicharged anions of carboxylic acids having methylene groups (A2− –M); tricharged anions of carboxylic acids having methylene groups (A3− –M); and monocharged anions of carboxylic acids lacking methylene groups (A− –ML). It is interesting that in each class selectivities are correlated with molecular weight irrespective of the number of carboxylic groups. An increase in molecular weight corresponds to an increase in the number of methylene groups, that is, an increase in hydrophobicity. Therefore, the selective sorption of carboxylic
The sorption equilibria of the dicarboxylic acids, oxalic, succinic, adipic and malic acids, and the tricarboxylic acid, citric acid, by an anion-exchange membrane were studied, which led to the following conclusions. The sorption of dicarboxylic and tricarboxylic acids by the membrane increased with increase in the solution concentration and pH, and depended on the solution concentrations of their ionic species. The sorption characteristics were successfully explained by the ion-exchange model considering the dissociation of carboxylic acids in solution, and electro-neutrality in the solution and in the membrane. This supports that the sorption of carboxylic acids by anion-exchange membranes proceeds by the proposed ion-exchange mechanism. The model simulation yielded the ion-exchange selectivities of the five carboxylic acids. Their selectivities were correlated with the molecular weights as well as those of eight monocarboxylic acids. The correlation parameter of molecular weight was useful for the general estimation of selectivities. Nomenclature z
Ai j C Ci,j K
species of i-carboxylic acid having zj valence concentration (mol/m3 ) concentration of species of i-carboxylic acid having zj valence (mol/m3 ) dissociation constant (mol/m3 )
H. Takahashi et al. / Journal of Membrane Science 222 (2003) 103–111
Kw QA S zj
ion product constant of water (mol2 /m6 ) ion-exchange capacity (mol/m3 ) selectivity valence
Subscripts H hydrogen ion HCl hydrogen chloride i number of carboxylic acid defined in Table 2 j number of valence defined in Table 2 Na sodium ion NaCl sodium chloride NaOH sodium hydroxide
[3]
[4]
[5]
[6]
[7]
Superscript − membrane phase
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