- Email: [email protected]
Removal of acetic acid from aqueous solutions containing succinic acid and acetic acid by tri-n-octylamine
Separation and Purification Technology 42 (2005) 151–157
Removal of acetic acid from aqueous solutions containing succinic acid and acetic acid by tri-n-octylamine Yeon Ki Honga,∗ , Won Hi Hongb a
b
Department of Chemical Engineering, Chungju National University, 123 Geomdan-ri, Iryu-myeon, Chungju, Chungbuk 380-702, Korea Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Korea Received 4 December 2003; received in revised form 15 March 2004; accepted 17 March 2004
Abstract Extraction of succinic acid and acetic acid by tri-n-octylamine (TOA) was studied under various pHs of their aqueous solutions. For the extraction of both acids, the loading values decreased with increasing values of pH. In the extraction of acetic acid, the sharp decrease in loading values with increasing pH began at higher values than that of succinic acid extraction due to its higher pKA value. Based on the different extractability for succinic acid and acetic acid with pH, the selective removal of acetic acid from succinic acid and acetic acid aqueous mixture was carried out. After three successive extraction by 0.25 mol kg−1 TOA dissolved in 1-octanol at pH 5.1, the large amount of acetic acid from aqueous acid mixture could be removed. © 2004 Elsevier B.V. All rights reserved. Keywords: Succinic acid; Acetic acid; Reactive extraction; Tri-n-octylamine; pKA
1. Introduction Succinic acid is a dicarboxylic acid produced as an intermediate of the tricarboxylic acid cycle (TCA) and also as one of the fermentation products of anaerobic metabolism. Recently, succinic acid is of interested as the raw material of polysuccinate which is biodegradable polymer. As the importance of succinic acid for biodegradable polymer has been increased, the biological production by fermentation has been focused as the alternative to petrochemical-based process [1,2]. A downstream purification process is essential to remove impurities such as protein, carbon source, and acetic acid from culture broth. Normally, the downstream purification costs account for 60–70% of the production cost in fermentation based process. However, the recent development of new
∗
Corresponding author. Tel.: +82 43 841 5231; fax: +82 43 841 5220. E-mail address: [email protected] (Y.K. Hong).
1383-5866/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2004.03.015
separation technologies is enabling the recovery and purification of fermentation-based succinic acid from dilute broth and making the production of succinic acid based chemical from carbohydrates via fermentation practical and economically attractive [3]. Several possible alternatives for the recovery of succinic acid exist. Among these alternatives, it has been reported that the amine-based extraction is effective for the recovery of succinic acid [4,5]. However, there are few works on the selective separation of specific acid from multiple acid systems. In the fermentation process by Anaerobiospirillum succiniciproducens, the maximum yield was 0.99 g succinic acid g−1 glucose consumed and the molar ratio of succinic acid and acetic acid was 1.9 [6]. Lee et al. (1999) reported that acetic acid was also produced as a by-product with a gram ratio of succinic acid to acetic acid of 4:1 in the fermentation of Anaerobiospirillum succiniciproducens from glucose [7]. It is well known that the presence of by-product such as acetic acid can negatively affect the purification process and decrease the yield of fermentor. Furthermore, the byproduction of acetic acid reduces the yield of succinic acid and makes purification of succinic acid more difficult and
152
Y.K. Hong, W.H. Hong / Separation and Purification Technology 42 (2005) 151–157
Nomenclature A− B C D HA HA− H2 A K11 k1∗1 K21 KA Z
dissociated acid tertiary amine concentration (mol kg−1 ) distribution coefficient monocarboxylic acid (undissociated form) bicarboxylate anion dicarboxylic acid (undissociated form) reaction equilibrium constant for (1,1) acid-amine complex reaction equilibrium constant for (1,1) acid (HA− )-amine complex reaction equilibrium constant for (2,1) acid-amine complex apparent equilibrium constant for the acid dissociation reaction loading value
Greek symbol α separation factor i species Subscripts AA acetic acid SA succinic acid Superscript 0 initial (over bar) species in the organic phase
costly. Therefore, the removal of acetic acid from fermentation broth should be essential for economical purification of succinic acid as well as better fermentation performance. Jagirdar and Sharma recovered a variety of organic acids from aqueous acid mixtures by tri-n-octylamine in various water–immiscible solvents [8]. Siebold et al. studied on the separation of lactic acid from citric acid and acetic acids by various extractants [9]. When using Hostarex A337 dissolved in Cyanex 923/kerosene, it was possible to separate lactic acid from citric and acetic acids. They concluded that an optimized multistage extractor is required for the complete separation of these acids [9]. Kirsch and Maurer studied the extraction characteristics of binary mixtures of citric, acetic and oxalic acids in amine based extraction [10]. Husson and King investigated on the multiple-acid equilibria in ion-exchange adsorption and its equilibrium model [11]. Hong et al. (2000) selectively purified succinic acid from binary mixture of succinic acid and acetic acid in aqueous phase by using the different basicity with the variation of the chain length of tertiary amines [12]. An objective of this research was to remove the acetic acid from succinic and acetic acid aqueous mixtures by us-
ing different degree of dissociation of each acid with pH. In general, it has been reported that tertiary amines only extracted the undissociated form of carboxylic acid [4]. Therefore, the selective removal of specific acid from acid aqueous mixture will be possible at appropriate pH. In this study, tri-noctylamine(TOA) was selected for the extraction of succinic acid and acetic acid at various pH. TOA has been used in the extraction of carboxylic acids due to its high extractability. Finally, based on the extraction characteristics of each acid with pH, the selective removal of acetic acid from succinic acid and acetic acid aqueous mixture was carried out at various pHs.
2. Equilibrium model of the carboxylic acid–TOA complexes In the amine-based extraction of carboxylic acid, the reaction between carboxylic acid and amine can be described in various ways. Equilibrium data of amine-based extraction can be explained by the mass action law based on these reactions [13]. Some assumptions in the present description are required as follows: (1) The solubility of TOA in the aqueous phase is negligible. (2) TOA reacts only with the undissociated form of acid. It is well known that carboxylic acids may exist as dimers in the organic phase because of the intermolecular hydrogen bonding [14]. Based on these assumptions, the reactions of monocarboxylic acid by TOA can be described as follows: HA + B ↔ HA–B,
2HA + B ↔ (HA)2 –B,
K11 =
CHA–B CHA CB
K21 =
C(HA)2 −B 2 C CHA B
(1)
(2)
An apparent equilibrium constant KA for the acid dissociation reaction can be written in terms of species concentration for dilute solutions. HA ↔ H+ + A− ,
KA =
CH+ CA− CHA
(3)
The pKA value is pH at which CHA is half dissociated and its definition is as follows: pKA = −log KA
(4)
Eq. (4), combined with Eq. (3) and definition of pH, leads to the following relationship. pH = pKA + log
C A− CHA
(5)
Y.K. Hong, W.H. Hong / Separation and Purification Technology 42 (2005) 151–157
From Eq. (5), the concentration of undissociated monocarboxylic acid is given by CHA
(6)
CHA = CHA−B + 2C(HA)2 −B 2 = K11 CHA CB + 2K21 CHA CB
(7)
The concentration of free (unreacted) TOA is CB = CB0 − CHA−B − C(HA)2 −B 2 = CB0 − K11 CHA CB − K21 CHA CB
(8)
From Eqs. (7) and (8), the loading of TOA, Z can be written, CHA CBO
=
2 K11 CHA + 2K21 CHA
(9)
2 1 + K11 CHA + K21 CHA
where the concentration of the undissociated acid in the aqueous phase can be obtained from Eq. (6) and the measured pH values. For a dicarboxylic acid such as succinic acid, the reaction of undissociated acid and bicarboxylate anion with TOA must be considered as follows: H2 A + B ↔ H2 A − B, HA− + B ↔ HA− − B,
K11 =
CH2 A−B C H2 A C B
K1∗1 =
2H2 A + B ↔ (H2 A)2 − B,
CHA− −B CHA− CB
K21 =
C(H2 A)2 −B 2 C CH B 2A
(10)
(11)
(12)
Although succinic acid is a dicarboxylic acid, it is difficult for the (1,2) acid–amine complex to be formed. The stoichiometry of succinic acid and tri-n-octylamine has already reported in our previous work [15]. The concentrations of undissociated acid and bicarboxylic anion are expressed as follows: C H2 A = CHA− =
CH2 A,total
1 + 10pH−pKA1 + 102pH−pKA1 −pKA2 CH2 A,total × 10pH−pKA1
1 + 10pH−pKA1 + 102pH−pKA1 −pKA2
(13) (14)
By the same procedure that was used in the case of monocarboxylic acid, the loading value, Z, is given by Z=
3. Materials and methods 3.1. Materials
CHA,total = 1 + 10pH−pKA
The total equilibrium concentration of monocarboxylic acid in the organic phase is obtained as follows:
Z=
153
2 K11 CH2 A + K1∗1 CHA− + 2K21 CH 2A 2 1 + K11 CH2 A + K1∗1 CHA− + K21 CH 2A
(15)
Succinic acid (Sigma, 99.9%) and acetic acid (Junsei, Japan, 99.9%) were used as received to prepare their aqueous solutions of various pH values. The initial concentration of succinic acid was 50 g L−1 and that of acetic acid was 18.8 g L−1 . These concentrations were based on the concentration of practical fermentation broth produced by Anaerobiospirillum succiniciproducens [7]. The solution pH was adjusted by adding either NaOH or HCl solution. The solutions pH was measured using an Corning digital ion analyzer 255 equipped with an Orion Ross combination pH electrode. Trioctylamine (TOA) (Aldrich, 99.9%) was used as extractant. TOA is a straight-chain tertiary amine. 1-Octanol (Aldrich, 99.9%) was used as diluent in this work. 1-Octanol is a polar, water-insoluble alcohol. In general, an amine extractant must always be used in the form of a solution in organic diluents due to its high viscous and corrosive properties. The concentrations of TOA in organic phase were 0.25, 0.50, and 0.75 mol kg−1 as the basis of 1-octanol. 3.2. Amine based extraction experiments Equal volume (10 ml) of an organic solvent containing amine and diluent and an aqueous solution of organic acid were charged in 30 ml vials. And then, they were stirred by magnetic bar in water bath at 1000 rpm and 25 ◦ C for 2 h, followed by centrifuging at 4000 rpm for about 15 min to separate the two phases. The concentrations of organic acid were measured by HPLC with an ion exchange column (Aminex HPX-87H, 300 mm × 7.8 mm, Hercules, CA) using 0.01 N H2 PO3 as mobile phase.
4. Results and discussion 4.1. Extraction of succinic acid and acetic acid at various pH Aliphatic amines extract carboxylic acids from an aqueous phase by forming an acid–amine complex with the undissociated acid. Because the concentration of the undissociated acid is a function of pH as seen in Eqs. (6) and (13), and (14), the extraction of carboxylic acids is seen to be influenced with pH in aqueous phase. Fig. 1 represents the pH effect on the loading values for the succinic acid at 25 ◦ C by TOA in 1-octanol. For all amine concentrations, the loading values decreased with increasing values of pH. A decrease of the values of pH means that the concentration of undissociated acid increases. In general, the loading value increases with decreasing values of pH except at extremely high or low pHs, where the loading value did not change significantly with pH [14]. In particular,
154
Y.K. Hong, W.H. Hong / Separation and Purification Technology 42 (2005) 151–157
Fig. 1. Effect of pH on loading values for succinic acid at 25 ◦ C (TOA dissolved in 1-octanol, initial concentration of succinic acid in aqueous phase: 50 g L−1 ).
there are asymptotic points in each curve near the pKA1 value of the succinic acid. It was concluded that the extractability can be influenced by the concentration of undissociated acid. However, the concentration of bicarboxylate anion that exists above the pKA2 value of succinic acid has little influence on the extractability. The loading values for acetic acid at 25 ◦ C by TOA in 1octanol are illustrated in Fig. 2. The equilibrium curves for acetic acid were similar to those for succinic acid; loading values decreased with increasing pH and decreasing concentration of TOA. The sharp decrease in loading values with increasing pH began at larger pH value than that for succinic
acid. This result can be attributed to the larger pKA value for acetic acid (pKA = 4.76) relative to that for succinic acid (pKA1 = 4.207). In spite of a low concentration of acetic acid (18.8 g L−1 ), the loading values for acetic acid are almost similar to those for succinic acid. Generally, the more acidic the acid, as measured by the pKA value, the more it can be extracted, given the same solvent system [14]. The apparent equilibrium constants of the reactive extraction for succinic and acetic acids as a function of pH are given in Table 1. As seen in Table 1, the ratio of the apparent equilibrium constants for (2,1) to those for (1,1) complexes for succinic acid is small compared with the same ratio for
Fig. 2. Effect of pH on loaging values for acetic acid at 25 ◦ C (TOA dissolved in 1-octanol, initial concentration of acetic acid in aqueous phase: 18.8 g L−1 ).
Y.K. Hong, W.H. Hong / Separation and Purification Technology 42 (2005) 151–157
155
Table 1 Apparent equilibrium constants in the reactive extraction of each acid by pH Acid
TOA concentration in 1-octanol (mol kg−1 )
K11 (mol−1 kg)
K1∗1 (mol−1 kg)
K21 (mol−2 kg2 )
Acetic acid
0.25 0.50 0.75
2.50 1.39 1.60
– – –
6.39 2.36 0
Succinic acid
0.25 0.50 0.75
4.54 2.68 1.69
0 0.16 0.17
5.32 1.42 0.62
both acids decreased as the TOA concentration increased due to increasing of basic sites of TOA. 4.2. Removal of acetic acid from an aqueous mixture of acetic acid and succinic acid
Fig. 3. Two types of bissucinate anion.
acetic acid. This behavior can be attributed to the fact that the structure of succnic acid is bulkier than that of acetic acid and the stable bisuccinate anion can be formed due to the intramolecular hydrogen bonding between two carboxylic groups in succinic acid [15]. Because succinic acid is a dicarboxylic acid, the intramolecular hydrogen bonding between the two carboxylic groups is possible due to a rotation of the central C–C bond (Fig. 3). In addition, Table 1 shows that the K1∗1 values are small in comparing with other apparent equilibrium constants in the extraction of succinic acid. Therefore, it is concluded that the concentration of bisuccinate anion of succinic acid has little influence on the extractability. In the case of acetic acid, the K21 values are higher than those for succinic acid in spite of lower concentration of acetic acid. It is concluded that the dimer formation of acetic acid by means of the intermolecular hydrogen bonding is easy (Fig. 4). However, in the case of succinic acid, the degree of the formation of intermolecular hydrogen bonding is lower than that of acetic acid in spite of high concentration. The can be also explained by the formation of bisuccinate anion by intramolecular hydrogen bonding. In 0.75 mol kg−1 TOA dissolved in 1-octanol, the K21 values were 0.62 for succinic acid and 0 for acetic acid. Therefore, the dimer formation of
Considering the composition of succinic acid and acetic acid in practical fermentation the initial concentration of succinic acid in aqueous phase was 50 g L−1 and that of acetic acid was 18.8 g L−1 in binary acid aqueous solutions. Fig. 5 shows the experimental uptake isotherm for extraction of acetic acid and succinic acid onto TOA from aqueous acid mixture at 25 ◦ C. As the value of pH decreases, the concentration of dissociated acid increases. Therefore, the uptake curves asymptotically approach maximum values corresponding to TOA uptake capacity with decreasing the value of pH. It could be found that acetic acid is preferentially extracted from the mixture because it is the weaker acid, based on pKA values. And, the uptake isotherm for acetic acid remains relatively constant for pH values between 2 and 5. The higher pKA value of acetic acid than pKA1 value of succinic acid means that with increasing pH, the ratio of undissociated acetic acid to succinic acid concentration increases. Alkylammonium cation of TOA bound with succinic acid at low pH values are unoccupied by succinic acid between pKA1 of succinic acid and pKA of acetic acid and then, makes sites for acetic acid to be extracted more available [11]. Fig. 6 shows the separation factors for acetic acid with the values of pH. The separation factor for acetic acid is defined as follows: αAA/SA =
CAA CSA (CAA /CSA ) DAA = = (CAA /CSA ) CAA CSA DSA
Fig. 4. Acetic acid–amine complexes in amine-based extraction: (a) (1,1) acetic acid–TOA complex; (b) (2,1) acetic acid–TOA complex.
(16)
156
Y.K. Hong, W.H. Hong / Separation and Purification Technology 42 (2005) 151–157
Fig. 5. Effect of pH on the uptake isotherm for each acid in succinic acid and acetic acid aqueous solutions at 25 ◦ C (0.25 mol kg−1 TOA dissolved in 1-octanol).
where Ci and Ci are the equilibrium concentration of i component in aqueous phase and organic phase, respectively. The separation factor for acetic acid increases with increasing of pH and with decreasing of TOA concentration. Some of the basic sites of TOA which were occupied by succinic acid at low pH values are unoccupied by succinic acid at pH values near 5.0, making more sites available for acetic acid to be extracted. Based on these results, the multistage extraction was performed in order to remove the acetic acid from succinic acid and acetic acid aqueous mixture. The concentration of TOA in 1-octanol is 0.25 mol kg−1 and pHs are 5.1 and 6.5, respectively. As seen in Fig. 7, though the high selectivity was
obtained in pH 6.5, the removal of acetic acid from aqueous acid mixture do not well. It is due to the low distribution coefficient for acetic acid in this pH range. The concentration of succinic acid is near constant but the concentration of acetic acid in aqueous phase is decayed with each batch. The mole ratio of succinic acid to acetic acid before extraction was 1.35 and the final ratio after three successive batches is 6. If the amine-based extraction are used as pre-step of electrodialysis, the enhancement of the mole ratio of succinic to acetic acid makes the electrodialysis process be more economic. It is because that the selective separation of specific acid from succinic acid and acetic acid mixture is not possible in electrodialysis process.
Fig. 6. Effect of pH on the separation factors for acetic acid in succinic acid and acetic acid aqueous solutions at 25 ◦ C.
Y.K. Hong, W.H. Hong / Separation and Purification Technology 42 (2005) 151–157
157
Fig. 7. Successive batch extraction for the removal of acetic acid from succinic acid and acetic acid aqueous solutions at 25 ◦ C (0.25 mol kg−1 TOA dissolved in 1-octanol).
5. Conclusions
References
The extractability of TOA dissolved in 1-octanol decreases with increasing of pH in aqueous phase. And the extractabilities of TOA for succinic acid was lower than those for acetic acid. In binary aqueous mixture of succinic and acetic acid, the acetic acid was removed by TOA dissolved in 1-octanol at pH 5.1 based on the different extractability for both acids with pH values. If the amine-based extraction is to be used in an extractive fermentation process for succinic acid, the effect of pH on extractability for acid is important. For general fermentation process for succinic acid, which usually requires a pH value near 6.0, the selective extraction of acetic acid by using TOA dissolved in 1-octanol will be possible. Considering the composition of practical fermentation broth from succinic acid fermentation, the selective removal of other acids such as formic acid and lactic acid needs to be further studied.
[1] J.G. Zeikus, P. Elankovan, A. Grethlen, Chem. Proc. (1995) 71– 73. [2] M.K. Jain, R. Datta, J.G. Zeikus, High-value organic acids fermentation-emerging processes and products, in: T.K. Ghosh (Ed.), Bioprocess Engineering: The First Generation, Ellis Harwood Limited, Chichester, 1989, pp. 366–398. [3] J.G. Zeikus, M.K. Jain, P. Elankovan, Appl. Microbiol. Biotechnol. 51 (1999) 545–552. [4] J.A. Tamada, A.S. Kertes, C.J. King, Ind. Eng. Chem. Res. 29 (1990) 1319–1326. [5] Y.K. Hong, W.H. Hong, Biopro. Eng. 23 (2000) 535–538. [6] N.P. Nghiem, B.H. Davison, B.E. Suttle, G.R. Rechardson, Appl. Biochem. Biotechnol. 63–65 (1997) 565–576. [7] P.C. Lee, W.G. Lee, S. Kwon, S.Y. Lee, H.N. Chang, Enzyme Microbiol. Technol. 24 (1999) 549–554. [8] G.C. Jagirdar, M.M. Sharma, J. Sep. Proc. Technol. 1 (1980) 40– 43. [9] M. Siebold, P.V. Frieling, R. Joppien, D. Rindfleisch, K. Schugerl, H. Roper, Proc. Biochem. 30 (1995) 81–95. [10] T. Kirsch, G. Maurer, Fluid. Phase. Equilib. 131 (1997) 213–231. [11] S.M. Husson, C.J. King, Ind. Eng. Chem. Res. 38 (1999) 502– 511. [12] Y.K. Hong, W.H. Hong, H.N. Chang, Biotechnol. Lett. 22 (2000) 871–874. [13] Y.K. Hong, W.H. Hong, Y.K. Chang, Biotechnol. Biopro. Eng. 6 (2001) 347–351. [14] S.-T. Yang, S.A. White, S.-T. Hsu, Ind. Eng. Chem. Res. 30 (1991) 1335–1342. [15] Y.K. Hong, W.H. Hong, Korean J. Chem. Eng. 21 (2004) 488–493.
Acknowledgements The authors are grateful to Advanced Bioseparation Technology Research Center (BSEP, KOSEF) for the funding.
Removal of acetic acid from aqueous solutions containing succinic acid and acetic acid by tri-n-octylamine Yeon Ki Honga,∗ , Won Hi Hongb a
b
Department of Chemical Engineering, Chungju National University, 123 Geomdan-ri, Iryu-myeon, Chungju, Chungbuk 380-702, Korea Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Korea Received 4 December 2003; received in revised form 15 March 2004; accepted 17 March 2004
Abstract Extraction of succinic acid and acetic acid by tri-n-octylamine (TOA) was studied under various pHs of their aqueous solutions. For the extraction of both acids, the loading values decreased with increasing values of pH. In the extraction of acetic acid, the sharp decrease in loading values with increasing pH began at higher values than that of succinic acid extraction due to its higher pKA value. Based on the different extractability for succinic acid and acetic acid with pH, the selective removal of acetic acid from succinic acid and acetic acid aqueous mixture was carried out. After three successive extraction by 0.25 mol kg−1 TOA dissolved in 1-octanol at pH 5.1, the large amount of acetic acid from aqueous acid mixture could be removed. © 2004 Elsevier B.V. All rights reserved. Keywords: Succinic acid; Acetic acid; Reactive extraction; Tri-n-octylamine; pKA
1. Introduction Succinic acid is a dicarboxylic acid produced as an intermediate of the tricarboxylic acid cycle (TCA) and also as one of the fermentation products of anaerobic metabolism. Recently, succinic acid is of interested as the raw material of polysuccinate which is biodegradable polymer. As the importance of succinic acid for biodegradable polymer has been increased, the biological production by fermentation has been focused as the alternative to petrochemical-based process [1,2]. A downstream purification process is essential to remove impurities such as protein, carbon source, and acetic acid from culture broth. Normally, the downstream purification costs account for 60–70% of the production cost in fermentation based process. However, the recent development of new
∗
Corresponding author. Tel.: +82 43 841 5231; fax: +82 43 841 5220. E-mail address: [email protected] (Y.K. Hong).
1383-5866/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2004.03.015
separation technologies is enabling the recovery and purification of fermentation-based succinic acid from dilute broth and making the production of succinic acid based chemical from carbohydrates via fermentation practical and economically attractive [3]. Several possible alternatives for the recovery of succinic acid exist. Among these alternatives, it has been reported that the amine-based extraction is effective for the recovery of succinic acid [4,5]. However, there are few works on the selective separation of specific acid from multiple acid systems. In the fermentation process by Anaerobiospirillum succiniciproducens, the maximum yield was 0.99 g succinic acid g−1 glucose consumed and the molar ratio of succinic acid and acetic acid was 1.9 [6]. Lee et al. (1999) reported that acetic acid was also produced as a by-product with a gram ratio of succinic acid to acetic acid of 4:1 in the fermentation of Anaerobiospirillum succiniciproducens from glucose [7]. It is well known that the presence of by-product such as acetic acid can negatively affect the purification process and decrease the yield of fermentor. Furthermore, the byproduction of acetic acid reduces the yield of succinic acid and makes purification of succinic acid more difficult and
152
Y.K. Hong, W.H. Hong / Separation and Purification Technology 42 (2005) 151–157
Nomenclature A− B C D HA HA− H2 A K11 k1∗1 K21 KA Z
dissociated acid tertiary amine concentration (mol kg−1 ) distribution coefficient monocarboxylic acid (undissociated form) bicarboxylate anion dicarboxylic acid (undissociated form) reaction equilibrium constant for (1,1) acid-amine complex reaction equilibrium constant for (1,1) acid (HA− )-amine complex reaction equilibrium constant for (2,1) acid-amine complex apparent equilibrium constant for the acid dissociation reaction loading value
Greek symbol α separation factor i species Subscripts AA acetic acid SA succinic acid Superscript 0 initial (over bar) species in the organic phase
costly. Therefore, the removal of acetic acid from fermentation broth should be essential for economical purification of succinic acid as well as better fermentation performance. Jagirdar and Sharma recovered a variety of organic acids from aqueous acid mixtures by tri-n-octylamine in various water–immiscible solvents [8]. Siebold et al. studied on the separation of lactic acid from citric acid and acetic acids by various extractants [9]. When using Hostarex A337 dissolved in Cyanex 923/kerosene, it was possible to separate lactic acid from citric and acetic acids. They concluded that an optimized multistage extractor is required for the complete separation of these acids [9]. Kirsch and Maurer studied the extraction characteristics of binary mixtures of citric, acetic and oxalic acids in amine based extraction [10]. Husson and King investigated on the multiple-acid equilibria in ion-exchange adsorption and its equilibrium model [11]. Hong et al. (2000) selectively purified succinic acid from binary mixture of succinic acid and acetic acid in aqueous phase by using the different basicity with the variation of the chain length of tertiary amines [12]. An objective of this research was to remove the acetic acid from succinic and acetic acid aqueous mixtures by us-
ing different degree of dissociation of each acid with pH. In general, it has been reported that tertiary amines only extracted the undissociated form of carboxylic acid [4]. Therefore, the selective removal of specific acid from acid aqueous mixture will be possible at appropriate pH. In this study, tri-noctylamine(TOA) was selected for the extraction of succinic acid and acetic acid at various pH. TOA has been used in the extraction of carboxylic acids due to its high extractability. Finally, based on the extraction characteristics of each acid with pH, the selective removal of acetic acid from succinic acid and acetic acid aqueous mixture was carried out at various pHs.
2. Equilibrium model of the carboxylic acid–TOA complexes In the amine-based extraction of carboxylic acid, the reaction between carboxylic acid and amine can be described in various ways. Equilibrium data of amine-based extraction can be explained by the mass action law based on these reactions [13]. Some assumptions in the present description are required as follows: (1) The solubility of TOA in the aqueous phase is negligible. (2) TOA reacts only with the undissociated form of acid. It is well known that carboxylic acids may exist as dimers in the organic phase because of the intermolecular hydrogen bonding [14]. Based on these assumptions, the reactions of monocarboxylic acid by TOA can be described as follows: HA + B ↔ HA–B,
2HA + B ↔ (HA)2 –B,
K11 =
CHA–B CHA CB
K21 =
C(HA)2 −B 2 C CHA B
(1)
(2)
An apparent equilibrium constant KA for the acid dissociation reaction can be written in terms of species concentration for dilute solutions. HA ↔ H+ + A− ,
KA =
CH+ CA− CHA
(3)
The pKA value is pH at which CHA is half dissociated and its definition is as follows: pKA = −log KA
(4)
Eq. (4), combined with Eq. (3) and definition of pH, leads to the following relationship. pH = pKA + log
C A− CHA
(5)
Y.K. Hong, W.H. Hong / Separation and Purification Technology 42 (2005) 151–157
From Eq. (5), the concentration of undissociated monocarboxylic acid is given by CHA
(6)
CHA = CHA−B + 2C(HA)2 −B 2 = K11 CHA CB + 2K21 CHA CB
(7)
The concentration of free (unreacted) TOA is CB = CB0 − CHA−B − C(HA)2 −B 2 = CB0 − K11 CHA CB − K21 CHA CB
(8)
From Eqs. (7) and (8), the loading of TOA, Z can be written, CHA CBO
=
2 K11 CHA + 2K21 CHA
(9)
2 1 + K11 CHA + K21 CHA
where the concentration of the undissociated acid in the aqueous phase can be obtained from Eq. (6) and the measured pH values. For a dicarboxylic acid such as succinic acid, the reaction of undissociated acid and bicarboxylate anion with TOA must be considered as follows: H2 A + B ↔ H2 A − B, HA− + B ↔ HA− − B,
K11 =
CH2 A−B C H2 A C B
K1∗1 =
2H2 A + B ↔ (H2 A)2 − B,
CHA− −B CHA− CB
K21 =
C(H2 A)2 −B 2 C CH B 2A
(10)
(11)
(12)
Although succinic acid is a dicarboxylic acid, it is difficult for the (1,2) acid–amine complex to be formed. The stoichiometry of succinic acid and tri-n-octylamine has already reported in our previous work [15]. The concentrations of undissociated acid and bicarboxylic anion are expressed as follows: C H2 A = CHA− =
CH2 A,total
1 + 10pH−pKA1 + 102pH−pKA1 −pKA2 CH2 A,total × 10pH−pKA1
1 + 10pH−pKA1 + 102pH−pKA1 −pKA2
(13) (14)
By the same procedure that was used in the case of monocarboxylic acid, the loading value, Z, is given by Z=
3. Materials and methods 3.1. Materials
CHA,total = 1 + 10pH−pKA
The total equilibrium concentration of monocarboxylic acid in the organic phase is obtained as follows:
Z=
153
2 K11 CH2 A + K1∗1 CHA− + 2K21 CH 2A 2 1 + K11 CH2 A + K1∗1 CHA− + K21 CH 2A
(15)
Succinic acid (Sigma, 99.9%) and acetic acid (Junsei, Japan, 99.9%) were used as received to prepare their aqueous solutions of various pH values. The initial concentration of succinic acid was 50 g L−1 and that of acetic acid was 18.8 g L−1 . These concentrations were based on the concentration of practical fermentation broth produced by Anaerobiospirillum succiniciproducens [7]. The solution pH was adjusted by adding either NaOH or HCl solution. The solutions pH was measured using an Corning digital ion analyzer 255 equipped with an Orion Ross combination pH electrode. Trioctylamine (TOA) (Aldrich, 99.9%) was used as extractant. TOA is a straight-chain tertiary amine. 1-Octanol (Aldrich, 99.9%) was used as diluent in this work. 1-Octanol is a polar, water-insoluble alcohol. In general, an amine extractant must always be used in the form of a solution in organic diluents due to its high viscous and corrosive properties. The concentrations of TOA in organic phase were 0.25, 0.50, and 0.75 mol kg−1 as the basis of 1-octanol. 3.2. Amine based extraction experiments Equal volume (10 ml) of an organic solvent containing amine and diluent and an aqueous solution of organic acid were charged in 30 ml vials. And then, they were stirred by magnetic bar in water bath at 1000 rpm and 25 ◦ C for 2 h, followed by centrifuging at 4000 rpm for about 15 min to separate the two phases. The concentrations of organic acid were measured by HPLC with an ion exchange column (Aminex HPX-87H, 300 mm × 7.8 mm, Hercules, CA) using 0.01 N H2 PO3 as mobile phase.
4. Results and discussion 4.1. Extraction of succinic acid and acetic acid at various pH Aliphatic amines extract carboxylic acids from an aqueous phase by forming an acid–amine complex with the undissociated acid. Because the concentration of the undissociated acid is a function of pH as seen in Eqs. (6) and (13), and (14), the extraction of carboxylic acids is seen to be influenced with pH in aqueous phase. Fig. 1 represents the pH effect on the loading values for the succinic acid at 25 ◦ C by TOA in 1-octanol. For all amine concentrations, the loading values decreased with increasing values of pH. A decrease of the values of pH means that the concentration of undissociated acid increases. In general, the loading value increases with decreasing values of pH except at extremely high or low pHs, where the loading value did not change significantly with pH [14]. In particular,
154
Y.K. Hong, W.H. Hong / Separation and Purification Technology 42 (2005) 151–157
Fig. 1. Effect of pH on loading values for succinic acid at 25 ◦ C (TOA dissolved in 1-octanol, initial concentration of succinic acid in aqueous phase: 50 g L−1 ).
there are asymptotic points in each curve near the pKA1 value of the succinic acid. It was concluded that the extractability can be influenced by the concentration of undissociated acid. However, the concentration of bicarboxylate anion that exists above the pKA2 value of succinic acid has little influence on the extractability. The loading values for acetic acid at 25 ◦ C by TOA in 1octanol are illustrated in Fig. 2. The equilibrium curves for acetic acid were similar to those for succinic acid; loading values decreased with increasing pH and decreasing concentration of TOA. The sharp decrease in loading values with increasing pH began at larger pH value than that for succinic
acid. This result can be attributed to the larger pKA value for acetic acid (pKA = 4.76) relative to that for succinic acid (pKA1 = 4.207). In spite of a low concentration of acetic acid (18.8 g L−1 ), the loading values for acetic acid are almost similar to those for succinic acid. Generally, the more acidic the acid, as measured by the pKA value, the more it can be extracted, given the same solvent system [14]. The apparent equilibrium constants of the reactive extraction for succinic and acetic acids as a function of pH are given in Table 1. As seen in Table 1, the ratio of the apparent equilibrium constants for (2,1) to those for (1,1) complexes for succinic acid is small compared with the same ratio for
Fig. 2. Effect of pH on loaging values for acetic acid at 25 ◦ C (TOA dissolved in 1-octanol, initial concentration of acetic acid in aqueous phase: 18.8 g L−1 ).
Y.K. Hong, W.H. Hong / Separation and Purification Technology 42 (2005) 151–157
155
Table 1 Apparent equilibrium constants in the reactive extraction of each acid by pH Acid
TOA concentration in 1-octanol (mol kg−1 )
K11 (mol−1 kg)
K1∗1 (mol−1 kg)
K21 (mol−2 kg2 )
Acetic acid
0.25 0.50 0.75
2.50 1.39 1.60
– – –
6.39 2.36 0
Succinic acid
0.25 0.50 0.75
4.54 2.68 1.69
0 0.16 0.17
5.32 1.42 0.62
both acids decreased as the TOA concentration increased due to increasing of basic sites of TOA. 4.2. Removal of acetic acid from an aqueous mixture of acetic acid and succinic acid
Fig. 3. Two types of bissucinate anion.
acetic acid. This behavior can be attributed to the fact that the structure of succnic acid is bulkier than that of acetic acid and the stable bisuccinate anion can be formed due to the intramolecular hydrogen bonding between two carboxylic groups in succinic acid [15]. Because succinic acid is a dicarboxylic acid, the intramolecular hydrogen bonding between the two carboxylic groups is possible due to a rotation of the central C–C bond (Fig. 3). In addition, Table 1 shows that the K1∗1 values are small in comparing with other apparent equilibrium constants in the extraction of succinic acid. Therefore, it is concluded that the concentration of bisuccinate anion of succinic acid has little influence on the extractability. In the case of acetic acid, the K21 values are higher than those for succinic acid in spite of lower concentration of acetic acid. It is concluded that the dimer formation of acetic acid by means of the intermolecular hydrogen bonding is easy (Fig. 4). However, in the case of succinic acid, the degree of the formation of intermolecular hydrogen bonding is lower than that of acetic acid in spite of high concentration. The can be also explained by the formation of bisuccinate anion by intramolecular hydrogen bonding. In 0.75 mol kg−1 TOA dissolved in 1-octanol, the K21 values were 0.62 for succinic acid and 0 for acetic acid. Therefore, the dimer formation of
Considering the composition of succinic acid and acetic acid in practical fermentation the initial concentration of succinic acid in aqueous phase was 50 g L−1 and that of acetic acid was 18.8 g L−1 in binary acid aqueous solutions. Fig. 5 shows the experimental uptake isotherm for extraction of acetic acid and succinic acid onto TOA from aqueous acid mixture at 25 ◦ C. As the value of pH decreases, the concentration of dissociated acid increases. Therefore, the uptake curves asymptotically approach maximum values corresponding to TOA uptake capacity with decreasing the value of pH. It could be found that acetic acid is preferentially extracted from the mixture because it is the weaker acid, based on pKA values. And, the uptake isotherm for acetic acid remains relatively constant for pH values between 2 and 5. The higher pKA value of acetic acid than pKA1 value of succinic acid means that with increasing pH, the ratio of undissociated acetic acid to succinic acid concentration increases. Alkylammonium cation of TOA bound with succinic acid at low pH values are unoccupied by succinic acid between pKA1 of succinic acid and pKA of acetic acid and then, makes sites for acetic acid to be extracted more available [11]. Fig. 6 shows the separation factors for acetic acid with the values of pH. The separation factor for acetic acid is defined as follows: αAA/SA =
CAA CSA (CAA /CSA ) DAA = = (CAA /CSA ) CAA CSA DSA
Fig. 4. Acetic acid–amine complexes in amine-based extraction: (a) (1,1) acetic acid–TOA complex; (b) (2,1) acetic acid–TOA complex.
(16)
156
Y.K. Hong, W.H. Hong / Separation and Purification Technology 42 (2005) 151–157
Fig. 5. Effect of pH on the uptake isotherm for each acid in succinic acid and acetic acid aqueous solutions at 25 ◦ C (0.25 mol kg−1 TOA dissolved in 1-octanol).
where Ci and Ci are the equilibrium concentration of i component in aqueous phase and organic phase, respectively. The separation factor for acetic acid increases with increasing of pH and with decreasing of TOA concentration. Some of the basic sites of TOA which were occupied by succinic acid at low pH values are unoccupied by succinic acid at pH values near 5.0, making more sites available for acetic acid to be extracted. Based on these results, the multistage extraction was performed in order to remove the acetic acid from succinic acid and acetic acid aqueous mixture. The concentration of TOA in 1-octanol is 0.25 mol kg−1 and pHs are 5.1 and 6.5, respectively. As seen in Fig. 7, though the high selectivity was
obtained in pH 6.5, the removal of acetic acid from aqueous acid mixture do not well. It is due to the low distribution coefficient for acetic acid in this pH range. The concentration of succinic acid is near constant but the concentration of acetic acid in aqueous phase is decayed with each batch. The mole ratio of succinic acid to acetic acid before extraction was 1.35 and the final ratio after three successive batches is 6. If the amine-based extraction are used as pre-step of electrodialysis, the enhancement of the mole ratio of succinic to acetic acid makes the electrodialysis process be more economic. It is because that the selective separation of specific acid from succinic acid and acetic acid mixture is not possible in electrodialysis process.
Fig. 6. Effect of pH on the separation factors for acetic acid in succinic acid and acetic acid aqueous solutions at 25 ◦ C.
Y.K. Hong, W.H. Hong / Separation and Purification Technology 42 (2005) 151–157
157
Fig. 7. Successive batch extraction for the removal of acetic acid from succinic acid and acetic acid aqueous solutions at 25 ◦ C (0.25 mol kg−1 TOA dissolved in 1-octanol).
5. Conclusions
References
The extractability of TOA dissolved in 1-octanol decreases with increasing of pH in aqueous phase. And the extractabilities of TOA for succinic acid was lower than those for acetic acid. In binary aqueous mixture of succinic and acetic acid, the acetic acid was removed by TOA dissolved in 1-octanol at pH 5.1 based on the different extractability for both acids with pH values. If the amine-based extraction is to be used in an extractive fermentation process for succinic acid, the effect of pH on extractability for acid is important. For general fermentation process for succinic acid, which usually requires a pH value near 6.0, the selective extraction of acetic acid by using TOA dissolved in 1-octanol will be possible. Considering the composition of practical fermentation broth from succinic acid fermentation, the selective removal of other acids such as formic acid and lactic acid needs to be further studied.
[1] J.G. Zeikus, P. Elankovan, A. Grethlen, Chem. Proc. (1995) 71– 73. [2] M.K. Jain, R. Datta, J.G. Zeikus, High-value organic acids fermentation-emerging processes and products, in: T.K. Ghosh (Ed.), Bioprocess Engineering: The First Generation, Ellis Harwood Limited, Chichester, 1989, pp. 366–398. [3] J.G. Zeikus, M.K. Jain, P. Elankovan, Appl. Microbiol. Biotechnol. 51 (1999) 545–552. [4] J.A. Tamada, A.S. Kertes, C.J. King, Ind. Eng. Chem. Res. 29 (1990) 1319–1326. [5] Y.K. Hong, W.H. Hong, Biopro. Eng. 23 (2000) 535–538. [6] N.P. Nghiem, B.H. Davison, B.E. Suttle, G.R. Rechardson, Appl. Biochem. Biotechnol. 63–65 (1997) 565–576. [7] P.C. Lee, W.G. Lee, S. Kwon, S.Y. Lee, H.N. Chang, Enzyme Microbiol. Technol. 24 (1999) 549–554. [8] G.C. Jagirdar, M.M. Sharma, J. Sep. Proc. Technol. 1 (1980) 40– 43. [9] M. Siebold, P.V. Frieling, R. Joppien, D. Rindfleisch, K. Schugerl, H. Roper, Proc. Biochem. 30 (1995) 81–95. [10] T. Kirsch, G. Maurer, Fluid. Phase. Equilib. 131 (1997) 213–231. [11] S.M. Husson, C.J. King, Ind. Eng. Chem. Res. 38 (1999) 502– 511. [12] Y.K. Hong, W.H. Hong, H.N. Chang, Biotechnol. Lett. 22 (2000) 871–874. [13] Y.K. Hong, W.H. Hong, Y.K. Chang, Biotechnol. Biopro. Eng. 6 (2001) 347–351. [14] S.-T. Yang, S.A. White, S.-T. Hsu, Ind. Eng. Chem. Res. 30 (1991) 1335–1342. [15] Y.K. Hong, W.H. Hong, Korean J. Chem. Eng. 21 (2004) 488–493.
Acknowledgements The authors are grateful to Advanced Bioseparation Technology Research Center (BSEP, KOSEF) for the funding.