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This explanation does not hold in the extraction of strong acids such as hydrochloric or dichloroacetic acid by a relatively strong base extractant (p...
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Ind. Eng. Chem. Res. 2003, 42, 1315-1320

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Acids Extraction by Amine-Based Extractants: An Analysis of the Effect of Anion Concentration in the Aqueous Phase Using the Donnan Model Riki Canari and Aharon M. Eyal* Casali Institute of Applied Chemistry, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel

The concentration of an acid’s anion in the aqueous phase affects that acid’s extraction efficiency by amine-based extractants. The anion effect is explained by using the acid-dissociation equation in the case that the acid is mainly extracted though H-bonding or solvation. This explanation does not hold in the extraction of strong acids such as hydrochloric or dichloroacetic acid by a relatively strong base extractant (pKaB . pKaA). To explain the anion effect in these systems, we reexamined our theory and modified it with the Gibbs-Donnan model. The modified theory shows that the proton activity in the organic phase could differ significantly from that in the aqueous phase, depending on the relative activities of the anions in those two phases. Thus, in turn, the relative activity of the anion affects the protonation of the amine in the organic phase, pKaB. Accordingly, the modified theory successfully explains the effect of the anion concentration on the extraction efficiency. 1. Introduction The fermentation of most carboxylic acids is productinhibited and slows dramatically when the pH is lowered to values below the pKa of the acid.1-5 A base is usually added to the fermenter to maintain the pH above the pKa of the fermented acid.6-10 In some cases, the broth contains a mixture of the acid and its salt. The free acid is extracted, and the salt is recycled to the fermenter in an extractive mode of operation. In other cases, a neutral broth is partially acidulated, for example, by CO2, to form a mixture of the free acid and the salt. What is the effect of the anion (salt) concentration on the extraction of the acid in the downstream process? This question is important, especially in light of the fact that the anion concentration can be controlled by a number of means such as salt recycling to the fermenter. In a previous article,1 we investigated the effect of pH on the extraction of carboxylic and mineral acids by amine-based extractants. In more recent publications, we have developed a theory that successfully explains the effects of pH on the extractions of monocarboxyic11 and dicarboxylic12 acids and on the selectivity of carboxylic acids extraction from their mixture solutions.13 How does the extraction depend on the concentration of the anion in the aqueous phase? Is the effect of the anion concentration in the case of weak acid extraction by a weak base extractant similar to that in the extraction of strong acid by a strong base extractant? These questions and others are investigated in the present study. 2. Experimental Section The amines used were a C18-C22 primary amine, Primene JMT (Rohm & Haas, technical grade); the secondary amine diisononylamine (DINA; Hoechst, tech* To whom correspondence should be addressed. Tel.: 9722-6585843. Fax: 972-2-6584533. E-mail: [email protected].

Figure 1. Propionic acid extraction by 0.5 mol/kg TOA in kerosene.

nical grade); and the straight-chain tertiary amine trin-octylamine (TOA; Hoechst, technical grade). The diluents used were 1-octanol (Merck, 99%) and the lowaromatics kerosene Parasol (Paz). Methods. Organic solutions were prepared containing 0.5 mol/kg amine in diluent, loaded with various concentrations of the extracted acid. The organic solutions were then equilibrated with an aqueous solution of the corresponding sodium salt, which resulted in the redistribution of the extracted acid. The pH of the aqueous phase and the acid concentration in the organic phase were determined by a pH meter and 0.1 N NaOH titration, respectively. The pH at which the extraction capacity drops dramatically was calculated by determining the torsion points in the curve of the extraction vs pH. This pH is referred to as the drop-off pH. 3. Results Figure 1 presents the effects of the anion concentration and pH on the extraction of propionic acid by 0.5 mol/kg TOA in kerosene in two experiments. The aqueous phase in the first initially contained water and

10.1021/ie010581g CCC: $25.00 © 2003 American Chemical Society Published on Web 03/08/2003

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and 6.65 for NaCl concentrations of 0.01, 0.1, and 1 M, respectively. Figure 4 presents the extraction of dichloroacetic acid by 0.5 mol/kg TOA + 20%1-octanol in kerosene from aqueous solutions of two different sodium dichloroacetate concentrations. This figure shows that the pH values of the drop-offs are 5.4 and 6.0 for sodium dichloroacetate concentrations of 0.01 and 0.1 M, respectively. 4. Discussion Figure 2. Extraction of HCl by 0.5 mol/kg TOA + 20% 1-octanol in kerosene from aqueous solutions containing 0.01 and 0.1 M NaCl.

Extraction Mechanisms. In a previous article,1 we concluded that the extraction mechanism is strongly dependent on the acid-base properties of both the extractant and the extracted acid. To explain this dependence, we used the dissociation equation of the acid (eq 1) and of the protonated amine (eq 2)

HAaq T H+aq + A-aq KaA ) [H+]aq[A-]aq/[HA]aq R3NH+org T R3Norg + H+aq KaB ) [H+]aq[R3N]org/[R3NH+]org Figure 3. Extraction of HCl by 0.5 mol/kg DINA + 20%octanol in kerosene from aqueous solutions containing 0.01, 0.1, and 1 M NaCl.

(1)

(2)

log([R3NH+]org/[R3N]org) ) log([HA]aq/[A-]aq) + pKaB - pKaA (3) where R3N, KaA, and KaB represent the amine and the dissociation constants of the acid and the ammonium, respectively. Equation 3 is a combination of eqs 1 and 2, assuming that the activity ratios are proportional to the concentration ratios. According to eq 3, if pKaB , pKaA, extraction through amine protonation to form ion pairs is small (as shown by the ratio between the concentration of the protonated amine and the free base amine). In that case, extraction of the undissociated acid molecules is mainly by Hbonding (eq 4) or solvation through no specific interaction with the amine (eq 5)

R3Norg + HAaq T R3N‚‚‚HAorg

Figure 4. Extraction of dichloroacetic acid by 0.5 mol/kg TOA + 20%octanol in kerosene from aqueous solutions containing 0.01 and 0.1 M sodium dichloroacetate.

propionic acid, whereas in the second experiment, it contained also 0.35-0.25 M sodium propionate. Both curves show high loadings in the low pH range and a drop-off in extraction at higher pH. This figure shows that the anion concentration affects the pH of the dropoff. For the first experiment, the drop-off was at about pH ) 3, whereas for the second, it was at about pH ) 5. Figure 2 presents the extraction of HCl by 0.5 mol/ kg TOA + 20% 1-octanol in kerosene from aqueous solutions containing 0.01 and 0.1 M NaCl. This figure shows that the extraction efficiency is highly sensitive to the NaCl concentration in the aqueous phase. In the case of 0.01 M NaCl, the drop-off pH is about 3.2, whereas in the case of 0.1 M NaCl, it is about 4.2. Figure 3 presents the extraction of HCl by 0.5 mol/ kg DINA + 20% 1-octanol in kerosene from aqueous solutions of various NaCl concentrations. This figure shows that the pH values of the drop-offs are 5.75, 6.2,

KH ) [R3N‚‚‚HA]org/([R3N]org[HA]aq) HAaq T HAorg KHS ) [HA]org/[HA]aq

(4)

(5)

where KH and KHS are the mass-action constants of H-bond formation and solvation, respectively. The effect of the anion concentration on the extraction, in these cases, can be seen by combining eqs 4 and 5 with the equation for the dissociation of the acid (eq 1); hence

log([R3N‚‚‚HA]org/[R3N]org) ) log [A-]aq + log KH + pKaA - pH (6) and

log[HA]org ) log [A-]aq + log KHS + pKaA - pH (7) respectively.

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On the other hand, in systems where pKaB . pKaA, the contribution of ion-pair formation to the extraction mechanism dominates. In such cases, the pH dependence of the extraction can be calculated according to

log([R3NH+]org/[R3N]org) ) pKaB - pH

(8)

Judging from this equation, we did not expect the extraction of acid by ion-pair formation to be affected by the acid’s anion concentration in the aqueous phase. Hence, the pKa of the protonated amine, pKaB, plays an important role in the extraction mechanism. Grinstead14 used extraction efficiency to the determine apparent basicities of water-immiscible amines. He formed water-immiscible amine hydrochlorides by equilibrating amines with aqueous solutions containing equivalent amounts of hydrochloric acid. He then added NaOH to the aqueous phase at half the equivalent amount to achieve equal concentrations of the protonated and unprotonated amine in the organic phase, that is, [R3NH+]org ) [R3N]org. According to Grinstead’s research, the pH of the aqueous phase at that point, i.e., the pH of half-neutralization (pHhn), is a measure of the amine basicity. HCl is suitable as the reference acid because its extraction through other mechanisms is negligible as long as the aqueous phase is dilute.15 Grinstead determined the pHhn’s of many alkylamines in toluene and found values of 2-6, 4-7, and 6.5-8.5 for tertiary, secondary, and primary amines, respectively. He concluded that steric hindrance decreases the basicity. In a subsequent article, Grinstead and Davis16 concluded that the aggregation of ammonium chloride in the organic phase has an important effect on pHhn. They estimated the aggregation value by determining the pHhn in very dilute ammonium chloride organic phases. This article also referred to an anion effect, stating that the chloride concentration in the aqueous phase should be 1 M for pHhn to represent the amine basicity according to the equation of hydrochloric acid extraction

KaB ) {Cl-}aq{H+}aq{R3N}org/{R3NH+Cl-}org where the braces indicate the activity of the component. These two articles are fundamentally important for this field. However, the authors chose to use the pH in the aqueous phase to determine the amine basicity in the organic phase. The question of whether such a choice is appropriate was not discussed by Grinstead and his co-workers. This important aspect is considered in the following discussion. Anion Effect in Cases of Extraction Dominated by H-Bonding or Solvation Interactions. The effect of the anion in the extraction of propionic acid can be explained on the basis of the extraction mechanism. In a previous article, we examined the IR spectrum of 0.5 mol/kg TOA in kerosene loaded with 0.25 mol/kg propionic acid.11 The spectrum showed a peak at 1720 cm-1, but no peak at about 1588 cm-1. These peaks represent the undissociated and dissociated carboxylic groups, respectively. Thus, this spectrum indicated that propionic acid was present in the extractant mainly in its undissociated form. In such cases of extraction dominated by H-bonding or solvation interactions, the pH of the equilibrium aqueous phase in the absence of salt is lower than the pKa of the acid. Adding the salt to the aqueous phase in increasing concentration decreases the dissociation

Figure 5. Propionic acid extraction by 0.5 mol/kg TOA in kerosene.

of the acid and increases the pH according to eq 1. The effect of the salt, however, will occur in the presence or absence of the extractant. Figure 5 represents the extraction data of Figure 1 as a function of the free acid concentration in the aqueous phase (titrated by NaOH solution). The extraction curve for the salt-containing system almost overlaps that of the salt-free system. Hence, the added salt in this system increases the pH in the aqueous phase and, therefore, shifts the drop-off toward higher pH (see Figure 1), but the added salt does not change the distribution of the free acid (see Figure 5). The mass-action constants, KH and KHS, in these experiments were calculated using eqs 4 and 5, respectively. The calculated value of KH was about 146, but with a high deviation (>200), whereas the calculated KHS value was 0.75 with a very low deviation (∼0.07). On the basis of these calculations, we concluded that the extraction of propionic acid is mainly through a mechanism that involves no specific interaction with the amine. This conclusion is also supported by Yang et al.’s10 results, which show that the efficiencies of propionic acid extraction by an amine and by an alkanol are similar. Anion Effect in the Case of Extraction by IonPair Formation. In the extraction of strong acids such as hydrochloric and dichloroacetic acids (pKa’s of -7 and 1.5, respectively) by relatively strong base extractants (pHhn of about 3.5-6, Figures 3-5) ion-pair formation makes an important contribution to the extraction mechanism. The pH of the drop-off in the extraction is, therefore, controlled by the pKa of the protonated amine, which is at least two pH units above the pKa of the acid. In these cases, the acids are completely dissociated in the aqueous phase, and addition of the corresponding salt to the aqueous phase does not increase the pH. Still, the results show an increase in the pH as the anion concentration increases. What is the explanation of this phenomenon? A key for understanding this effect seems to be eq 2, which is reconsidered in the following section. This equation evaluates the amine basicity in the organic phase by determining the pH of the aqueous phase when the amine is half-loaded. Can one use the proton activity in the aqueous phase to determine the proton activity in the organic phase? Alternatively, are the proton activities in the two phases equal at equilibrium? In the following discussion, we use the Gibbs-Donnan model to answer this question. Gibbs-Donnan Model. The Gibbs-Donnan model is an important tool in analyzing systems in which ions transfer from one phase to another. Many books and articles have employed this tool to study the properties

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of both ion-exchange membranes and solid ion-exchange sorbents. For example, this model was used to analyze the thermodynamics in ion-exchange systems,17-20 to analyze the selectivity in ion-exchange reactions,18 to calculate the water absorption in solid ion-exchange sorbents,18 and to determine the acidity or basicity of the active components in ion exchangers.21,22 In addition, researchers21,22 have used this model to analyze the effect of the concentration of the counterion in the aqueous phase on the ion-exchange reaction. Despite the structural differences between extractants and solid ion exchangers, the acid-base interactions play a major role in both cases, and many phenomena are similar, e.g., the effect of polar groups23-25 and temperature.24,27-29 Thus, knowledge can be applied from one area to the other. In the following discussion, we modify our theory for acid extraction systems1 according to the Gibbs-Donnan model on the basis of Marinsky and Gustavson’s analyses.21,22 The chemical potential, µ, of an electroneutral component, a, is defined as the Gibbs free energy per mole of substance as follows

µa ) (δG/δna)P,T,nj,nk

(9)

where G is the free energy; na is the number of moles of component a; and P, T, j, and k are the pressure, the absolute temperature, and the other electroneutral components in the system, respectively. In most extraction systems, both the temperature and pressure are constant (assuming that the surface tension is also constant because no emulsion forms). Therefore, the chemical potential can be defined as follows

µa ) µa° + RT ln {a}

(10)

where the braces indicate the activity of the component and µa° represents the Gibbs free energy per mole of pure substance at unit pressure and at the same temperature as the mixture under discussion. The activity of the electroneutral component can be expressed by the molar fraction, χa (or by the molality), and by its activity parameter, γa, to give

µa ) µa° + RT ln(χaγa)

(11)

This Gibbs analysis describes systems containing only electroneutral components where the sum of the potentials of a component is given by the chemical potential, G. In systems of interest here, in which ion-exchange reactions take place, the sum of the potentials of an ion is described by the electrochemical potential, E. In the extraction of acids by amine-based extractants, the ions are transferred from one phase to the other; thus, the electrochemical potentials of an ion in the phases can be described by the Donnan model. The Donnan model30 assumes that, at equilibrium, the electrochemical potentials of each ionic species, Ei, are equal in the two phases. This electrochemical potential is given by

Ei ) µi,aq + ziFψaq ) µi,org + ziFψorg

(12)

where zi, F, ψaq, and ψorg represent the valence of the ion; the Faraday constant; and the electrical potentials in the aqueous phase and in the organic phase, respectively. The difference between the electric potentials,

ψorg -ψ

aq,

is defined as the Donnan potential

EDonnan ) ψ org - ψ aq ) (µi,aq - µi,org)/ziF

(13)

This potential is the same for all ions in the system (such as i and k). Hence

EDonnan ) (µi,aq - µi,org)/ziF ) (µk,aq - µk,org)/zkF (14) or

(µi,aq - µi,org)/zi ) (µk,aq - µk,org)/zk In the extraction of a monovalent acid, the transferred species are the proton, H+, and the anion of the acid, A-. Thus

EDonnan ) µA,aq - µA,org ) µH,org - µH,aq

(15)

or

µA,aq + µH,aq ) µA,org + µH,org Equation 16 is a combination of eqs 10 and 15

(µA,org° + µH,org° - µA,aq° - µH,aq°)/RT2.3 + log({A-org}{H+org}) ) log({A-}{H+}) (16) As can be seen, this equation includes a term (on the left-hand side) including the differences between the values of the Gibbs free energies per mole of the pure substances, µ°, in the two phases. Is this term equal to 0? According to fundamental thermodynamic analysis,18,31,32 the standard chemical potential of the distribution can be chosen to be the same in the two phases, µi,aq° ) µi,org°. Such a choice, however, automatically defines the activity coefficient ratio γi,aq/γi,org. In other cases, it is more convenient to choose values for the activity coefficients (e.g., approaching unity at infinite dilution, γi f 1 as χi f 0, or approaching unity as the mole fraction approaches unity, γi f 1 as χi f 1) that automatically result in definitions for the standard chemical potentials µi,aq° and µi,org°. In our case, we adopt the first convention, setting the Gibbs free energies per mole of pure substance in the two phases equal (µA,org° ) µA,aq° ) µA° and µH,org° ) µH,aq° ) µH° ). Hence

pHorg ) pHaq + log({A-org}/{A-aq})

(17)

In this case, the interactions of A- and H+ in both the phases are determined by their concentrations and their activity coefficients, γ. Gibbs-Donnan Model in Acid Extraction by an Amine-Based Extractant. We begin with the question: Are the proton activities in the two phases at equilibrium equal? To answer this question, we used the Gibbs-Donnan model and concluded that the proton potential in the organic phase, {H+org}, will be equal to that in the aqueous phase if the ratio between the activities of the anion in the two phases approaches unity. As a result, eq 2 should be written as

log({R3NH+}org/{R3N}org) ) pKaB - pHorg (18) or by also using eq 17, it should be

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log({R3NH+}org/{R3N}org) + log({A-org}/{A-aq}) ) pKaB - pHaq (19) The activity values of the substances can be expressed as the products of their concentrations and activity coefficients, giving

log([R3NH+]org/[R3N]org) + log([A-org]/[A-aq]) + log(γR3NH+/γR3N) + log(γA,org/γA,aq) ) pKaB - pHaq (20) Equation 20 shows that the extraction of acid through ion-pair formation (as shown by the ratio between the organic-phase concentrations of the protonated and free amine) is a function of the amine basicity, the pH, the interactions of the amine and anion in both phases (represented by their activity coefficients), and the anion concentrations in both phases. According to eq 20, the value of pHhn is also a function of the anion concentration in both phases; that is

log([A-org]/[A-aq]) + log(γR3NH+/γR3N) + log(γA,org/γA,aq) ) pKaB - pHhn (21) According to eq 21, the slope of a plot of the dependence of pHhn on the logarithm of the anion concentration in the aqueous phase (log[A-aq]) is expected to be unity if γR3NH+ ) γR3N and γA,org ) γA,aq). Figure 6 presents that dependence for our acid extraction results and for results presented in the literature for solid anion and cation exchangers.21,22 Table 1 lists the absorbent/ extractant compositions, the salts present, and the slopes of the curves in Figure 6, assuming linearity. These results show that the absorption and extraction are very sensitive to the anion/cation concentration in the aqueous phase, as predicted by eq 21. The apparent acidity/basicity of the anion/cation exchanger increases with the concentration of exchanged ion in the aqueous phase (the pHhn increases in the case of base extractant/ sorbent and decreases in the case of acid sorbent). Moreover, the results show that the slopes of the curves are close to unity, as expected. Deviations from unity were explained by Gustavson et al.21 as the effect of “the electrostatic interactions of the neighboring functional group”. This explanation support our results, which show that the slope is closer to unity in the case of solid ion-exchange systems. Higher mobility of the molecules leads to stronger electrostatic interactions of the neighboring functional groups in the extraction systems. In a pervious article,1 we expressed the apparent basicity of the amine by semiquantifiable parameters as

pKaB ) pHhnda

(22)

According to this equation, the amine’s apparent basicity is a function of its specific (intrinsic) basicity (pK′aB), which is determined by pHhn for the amine in kerosene in equilibrium with HCl (pHhn(HCl)), multiplied by coefficients for the diluent’s solvation properties (d) and for the properties of the anion of the extracted acid (a). The term d can be represented by parameters such as the nucleophilic/electrophilic properties, the Hildebrand solubility parameters, etc. a can be derived from the

Figure 6. Apparent basicity/acidity (pHhn) of extractants and sorbents as a function of the anion/cation concentration in the aqueous phase. Table 1. Absorbent/Extractant Compositions, Salts Present, and Slopes of the Curves in Figure 6, Assuming Linearity extractant/ sorbent

composition

anion/cation

slope

TOA + 20% octanol NaCl 1.2 DINA + 20% octanol NaCl 0.43 TOA + 20% octanol sodium 0.56 dichloroacetate anion-exchange DMAPA NaCl 1 resina cation-exchange poly(acrylic acid) NaCl -0.7 resinb cation-exchange Sephadex CM-25c NaCl -1.1 resinb

extractant extractant extractant

a From Gustavson et al.21 carboxymethyldextran

b

From Marinsky.22

c

Polysaccharide

properties of the anion in nonacidic media (hydrophilic/ hydrophobic properties and steric hindrance). In the present article, we modified our theory using the Gibbs-Donnan model. On the basis of the modified theory, we concluded that the amine basicity is a function of the interactions of the amine and the anion in the two media (represented by their activity coefficients). The amine basicity is also a function of the anion concentrations in the two phases (eq 21). In light of this analysis, two adjustments should be made to eq 22. A term that includes the anion concentrations of the extracted acid in the two phases should be add, giving

pKaB ) pHhn(HCl)da + log([A-org]/[A-aq])

(23)

HCl extraction is also a function of the anion (chloride) ratio in the two phases; therefore, the pHhn(HCl) values of the amines as described in Grinstead’s work14,16 should be determined at equal concentrations of the anion in the two phases. In summary, the effect on the extraction of the concentration of the acid’s anion in the aqueous phase depends on the acid-extraction mechanism. In the cases of acid extracted though H-bonding or solvation, increasing the anion concentration increases the pH in the aqueous phase according to the dissociation equation of the acid (eq 1), but does not increase the extraction when presented as a function of the free acid in the

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aqueous phase. This explanation does not hold in cases of acids extracted through ion-pair formation like extracting hydrochloric or dichloroacetic acid by relatively strong base extractants (pKaB . pKaA). To explain the anion effect in these systems, we modified our theory1 with the Gibbs-Donnan model (typically used for solid anion exchangers). On the basis of this modified theory, we concluded that increasing the anion (salt) concentration in the aqueous phase increases the extraction. Moreover, we concluded that the proton activity in the organic phase cannot be equal to that in the aqueous one, unless the activity values of the anions in the two phases are equal. These conclusions led to a readjustment of the equation for the apparent basicity of the amine. Acknowledgment We thank Tali Reuveni for her important experimental help. Literature Cited (1) Eyal, A. M.; Canari, R. pH Dependence of Carboxylic and Mineral Acid Extraction by Amine-Based Extractants: Effect of pKa, Amine Basicity, and Diluent Properties. Ind. Eng. Chem. Res. 1995, 34, 1789. (2) Amrane, A.; Prigent, Y. Differentiation of pH and Free Lactic Acid Effects on the Various Growth and Production Phases of Lactobacillus helveticus. J. Chem. Technol. Biotechnol. 1999, 74, 33. (3) Gu, Z.; Glatz, B. A.; Glatz, C. E. Propionic Acid Production by Extractive Fermentation. I. Solvent Considerations. Biotechnol. Bioeng. 1998, 57 (4) 454. (4) Yabannavar, V. M.; Wang, D. I. C. Extractive Fermentation for Lactic Acid Production. Biotechnol. Bioeng. 1991, 37, 1095. (5) Ye, K.; Jin, S.; Shimizu, K. Performance Improvement of Lactic Acid Fermentation by Multistage Extractive Fermentation. J. Ferment. Bioeng. 1996, 3, 240. (6) Chen, R.; Lee, Y. Y. Membrane-Mediated Extractive Fermentation for Lactic Acid Production from Cellulosic Biomess. Appl. Biochem. Biotechnol. 1997, 63-65, 435. (7) Davison, B. H.; Scott, C. D. A Proposed Biparticle Fluidized Bed for Lactic Acid Fermentation and Simultaneous Adsorption. Biotechnol. Bioeng. 1992, 39, 365. (8) Hongo, M.; Normura, Y.; Iwahara, M. Novel Method of Lactic Acid Production by Electrodialysis Fermentation. Appl. Environ. Microbiol. 1986, 52 (2), 314. (9) Tsao, G. T.; Lee, S. J.; Tsai, G. J.; Seo, J. H.; McQuigg, D. W.; Vorhies, S. L.; Iyer, G. Process for Producing and Recovering Lactic Acid. U.S. Patent 5,786,185, 1998. (10) Yang, S. T.; White, S. A.; Hsu, S. T. Extraction of Carboxylic Acids with Tertiary and Quaternary Amines: Effect of pH. Ind. Eng. Chem. Res. 1991, 30, 1335. (11) Canari, R.; Eyal, A. M. Extraction of Carboxylic Acids by Amine-Based Extractants: Apparent Extractant Basicity According to the pH of Half-Neutralization. Ind. Eng. Chem. Res. 2003, 42, 1285-1292. (12) Canari, R.; Eyal. A. M. Effect of pH on Dicarboxylic Acids Extraction by Amine-Based Extractants. Ind. Eng. Chem. Res. 2003, 42, 1293-1300. (13) Canari, R.; Eyal, A. M. Selectivity in Monocarboxylic Acids Extraction from Their Mixture Solutions Using an Amine-Based

Extractant: Effect of pH. Ind. Eng. Chem. Res. 2003, 42, 13011307. (14) Grinstead, R. R. Base Strengths of Amine in Liquid-Liquid Extraction Systems. In Proceedings of the International Solvent Extraction Conference (ISEC); Dryssen, D., Liljenzin, J. O., Rydberg, J., Eds.; North Holland Publishing Company: Amsterdam, 1966; p 427. (15) Eyal, A. M.; Arbel-Hadad, M.; Hadi, S.; Canari, R.; Haringman, A.; Hazan, B. Extraction of Acids, Water and Hydrophilic Molecules by Amine and Amine Salts. In Proceedings of International Solvent Extraction Conference (ISEC); Logsdail, D. H., Slater, M. J., Eds.; Elsevier Science: New York, 1993; Vol. 2, p 723. (16) Grinstead, R. R.; Davis, J. C. Base Strengths of AmineAmine Hydrochloride Systems in Toluene. J. Phys. Chem. 1968, 72 (5), 1630. (17) Kesting, R. E. Synthetic Polymeric Membranes; McGrawHill: New York, 1971, pp 181-195. (18) Marcus, Y.; Kertes, A. S. Ion Exchange and Solvent Extraction of Metal Complexes; Wiley: New York, 1969; pp 267274, 868-869. (19) Rousseau, R. W. Handbook of Separation Process Technology; Wiley: New York, 1987; p 969. (20) Winston, Ho W. S.; Sirkar, K. K. Membrane Handbook; Van Nostrand Reinhold: New York, 1992; p 274. (21) Gustafson, R. L.; Fillius, H. F.; Kunin, R. Basicities of Weak Base Ion Exchange Resins. Ind. Eng. Chem. Fundam. 1970, 9 (2), 221. (22) Marinsky, J. A. A Gibbs-Donnan Based Analysis of Ion Exchange and Related Phenomena. In Ion Exchange and Solvent Extraction. A Series of Advances; Marinsky, J. A., Marcus, Y., Eds.; Marcel Dekker: New York, 1993; Vol. 11, p 237. (23) Bizek, V.; Horacek, J.; Kousova, M. Amine Extraction of Citric Acid: Effect of Diluent. Chem. Eng. Sci. 1993, 48 (8), 1447. (24) Bolto, B. A.; Weiss, D. E. The Thermal Regeneration of Ion Exchange Resins. In Ion Exchange and Solvent Extraction. A Series of Advances; Marinsky, J. A., Marcus, Y., Eds.; Marcel Dekker: New York, 1977; Vol. 7, p 221. (25) Tung, L. A.; King, C. J. Sorption and Extraction of Lactic and Succinic Acid at pH > pKa1 1. Factors Governing Equilibria. Ind. Eng. Chem. Res. 1994, 33, 3217. (26) Reisinger, H.; King, C. J. Extraction and Sorption of Acetic Acid at pH above pKa To Form Calcium Magnesium Acetate. Ind. Eng. Chem. Res. 1995, 34, 845. (27) Baniel, A. M.; Blumberg, R.; Hajdu, K. Recovery of Acids from Solutions. British Patent 1,426,018, 1973. (28) Tamada, J. A.; King, C. J. Extraction of Carboxylic Acids with Amine Extractants. 3. Effect of Temperature, Water Coextraction, and Process Considerations. Ind. Eng. Chem. Res. 1990, 29, 1333. (29) Wennersten, R. A. The Extraction of Citric Acid from Fermentation Broth Using a Solution of a Tertiary Amine. J. Chem. Technol. Biotechnol. 1983, 33B, 85. (30) Donnan, F. G. Die Genaue Thermodynamik der Membrangleichgewichte II. Z. Phys. Chem. (Leipzig) Abt A. Bd. 168, Heft 5/6. 1934, 24, 369-380. (31) Denbigh, K. The Principles of Chemical Equilibrium, 4th ed.; Cambridge University Press: New York, 1981. (32) Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. The Properties of Gases and Liquids, 3rd ed.; McGraw-Hill Book Company: New York, 1977.

Received for review July 5, 2001 Revised manuscript received October 31, 2002 Accepted November 3, 2002 IE010581G