Multiple-Acid Equilibria in Adsorption of Carboxylic Acids from Dilute

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Ind. Eng. Chem. Res. 1999, 38, 502-511

Multiple-Acid Equilibria in Adsorption of Carboxylic Acids from Dilute Aqueous Solution Scott M. Husson† and C. Judson King* Department of Chemical Engineering and Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720

Equilibria were measured for adsorption of carboxylic acids from aqueous, binary-acid mixtures of lactic and succinic acids and acetic and formic acids onto basic polymeric sorbents. The experimentally determined adsorption isotherms compared well with model predictions, confirming that simple extensions from adsorption of individual acids apply. Fixed-bed studies were carried out that establish the efficacy of chromatographic fractionation of lactic and succinic acids using basic polymeric sorbents. Finally, sequential thermal and solvent regeneration of lactic and acetic acid-laden sorbents was investigated as a method to fractionate among coadsorbed volatile and nonvolatile acids. Essentially complete removal of the acetic acid from the acid-laden sorbent was achieved by vaporization under the conditions used; a small amount of loss of lactic acid (about 11%) was observed. Introduction Previous researchers1-4 have worked with basic polymeric sorbents that sustain the uptake capacity for carboxylic acids at pH > pKa of the acid and have defined equilibria for the uptake of individual acids. In many instances, multiple acids are present in solution. To recover the acids from such solutions and subsequently separate them, understanding equilibria for adsorption from aqueous acid mixtures is essential. To reduce experimental work, it would be desirable to be able to predict the adsorption equilibria for aqueous acid mixtures using only experimental single-acid adsorption data. Several separation processes have been developed to recover and to fractionate carboxylic acids from aqueous acid mixtures. Jagirdar and Sharma5 employed dissociation extraction with organic solutions of tri-noctylamine in various water-immiscible solvents to recover and to separate a variety of organic acids from aqueous acid mixtures. Wo´dski and Nowaczyk6 described the extraction and separation of propionic and acetic acids using a hybrid membrane system comprised of a liquid membrane bordered by two anion-exchange membranes. Kirsch and Maurer7 studied the distribution of binary mixtures of citric, acetic, and oxalic acids between water and organic solutions of tri-n-octylamine. An objective of this research was to provide information and insight concerning multiple-acid equilibria in adsorption of carboxylic acids from dilute aqueous solutions. Three models were examined for their ability to predict multiple-acid equilibria. Binary-Acid Mixtures of Interest. Lactic and Succinic Acids. Interest in this acid mixture stems from the production of ethanol by fermentation of cornstarch, where lactic and succinic acids are coproduced, along * To whom correspondence should be addressed at the Department of Chemical Engineering. Telephone: (510) 6421534. Fax: (510) 642-4778. E-mail: [email protected]. † Present address: Department of Chemical Engineering, Clemson University, Clemson, SC 29634. Telephone: (864) 656-4502. Fax: (864) 656-0784. E-mail: [email protected].

with glycerol.8 An incentive for economical production of lactic acid is coming from the development of new, large-volume uses of lactic acid, particularly as feedstocks for biodegradable polymers9 and oxygenated chemicals.10 Increasing demand for naturally derived food acidulants and chemicals such as 1,4-butanediol, γ-butyrolactone, tetrahydrofuran, and adipic acid provides an increasing market for succinic acid.11 Acetic and Formic Acids. This acid mixture is one of many that result during aerobic fermentation. The bioconversion of glucose performed by the glycolytic Embden-Meyerhof pathway produces pyruvic acid as the key metabolic intermediate.12 Pyruvic acid is oxidized in a cyclic manner known as the tricarboxylic pathway to yield a number of di- and tricarboxylic aliphatic acids of four to six carbons. One special modification of this cycle involves cleavage of pyruvic acid to form acetic and formic acids as the major end products.13 Acetic acid is one of the largest chemical intermediates, with a total annual worldwide production of about 4 million tons for 1991.14 Formic acid is a medium-volume commodity chemical, with a worldwide capacity of around 330 000 tons/year as of 1994.15 Experimental Materials and Methods Materials. Chemical Reagents. Reagents and sources are tabulated elsewhere.16 All aqueous solutions were prepared from distilled water that had been passed through a Milli-Q water purification system (Millipore Corp.). Lactic acid (85+ wt %) was diluted with water to approximately 15 wt % and boiled under constant reflux for at least 12 h to hydrolyze any lactic acid polymers. Complete hydrolysis of the esters was confirmed by high-performance liquid chromatography (HPLC). Sorbents. The polymeric sorbents utilized were Dowex MWA-1 (Dow Chemical Co.), Amberlite IRA-35 (Rohm and Haas Co.), and Reillex 425 (Reilly Industries, Inc.). The first two incorporate tertiary amine functionalities, while the third has pyridyl groups. All three sorbents are commercially available, and all are macroreticular. Tung and King3 provide a detailed discussion of the

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Ind. Eng. Chem. Res., Vol. 38, No. 2, 1999 503

chemical structures, measured capacities, and basicities of these and several other polymeric sorbents. Prior to use, the sorbents were washed repeatedly with aqueous hydrochloric acid, aqueous sodium hydroxide, water, and methanol; further purified by Soxhlet extraction with methanol for at least 24 h; and dried to constant weight in a vacuum oven at 60 °C and 15-25 kPa. Methods. Multiple-Acid Adsorption Isotherms. Equimolar, aqueous mixtures of lactic and succinic acids (0.15 M each) and acetic and formic acids (0.5 M each) were contacted with known weights of Dowex MWA-1, Amberlite IRA-35, or Reillex 425 sorbent in 20-mL scintillation vials sealed with Teflon-lined caps in a constant-temperature, reciprocating shaker bath at 25 °C and 120 rpm for at least 24 h. Previous studies of sorption of lactic acid on Dowex MWA-1 showed that equilibrium was reached within experimental error in 1 h.3 The initial solution pH was adjusted to enable investigation of the dependence of acid uptake on pH. The solution pH was adjusted by mixing different ratios of the carboxylic acids and their corresponding sodium carboxylate salts and diluting with distilled water to the desired concentration. Initial and final aqueous-phase acid concentrations were determined by HPLC using a Bio-Rad Aminex HPX-87H strong cation-exchange column or a Bio-Rad fast-acid-analysis column maintained at 60 °C, a 0.01 N H2SO4 mobile phase, and an ultraviolet detector operating at 210 nm. Final pH values were measured using an Orion 601A digital ionalyzer equipped with an Orion Ross combination pH electrode. Multiple-Acid Fixed-Bed Adsorption. A known dry mass of Dowex MWA-1 sorbent was prewet with methanol and fed as a slurry to a 1 cm i.d. × 30 cm length glass chromatography column. Pure water was fed to the column via poly(tetrafluoroethylene) tubing connected to an adjustable plunger at the top of the column to displace the methanol from the sorbent. The adjustable plunger was positioned to minimize the mixing volume above the sorbent bed. All solutions were delivered to the top of the column with a peristaltic pump. The feed solutions included aqueous mixtures of lactic and succinic acids in a 3:1 lactic-to-succinic acid molar ratio (0.39 and 0.13 M or 0.18 and 0.06 M, respectively). Initial feed pH values were 2.0 and 4.0. The feed solutions were introduced to the top of the column at flow rates of 12.0-14.0 bed volumes/h. Effluent samples were collected from the column over specified time intervals in glass scintillation vials using a fraction collector. Effluent acid concentrations were measured by HPLC, and effluent pH values were measured as previously described. Fractionation among Volatile and Nonvolatile Acids. A separation scheme involving adsorption, vacuum distillation, and solvent leaching was investigated for fractionating lactic and acetic acids. Known weights of Dowex MWA-1 (typically 1 g) were contacted with equimolar, aqueous mixtures of lactic and acetic acids (0.23 M each). Initial and final acid concentrations were measured by HPLC. Total solution uptakes were determined by weighing the sorbent samples following centrifugation for 8 min at 2000 rpm in a fritted-glass funnel enclosed within a plastic centrifuge tube. These conditions are sufficient to remove nearly all of the interstitial and adhering bulk liquid.17 The acid-laden sorbent samples were then placed in a 16 dm3 vacuum

oven at 80 °C and 20-25 kPa with an ambient purge stream of 8.3-9.8 dm3/h. Samples were withdrawn from the oven at specified time intervals and weighed to determine the mass of sorbed fluid that had been vaporized. To determine the amounts of acid remaining on the sorbent following heating, the sorbent was leached with an aqueous sodium hydroxide solution (3 wt %) using a solution-to-sorbent phase ratio of 8-9 g/g. Acid concentrations in the strip solution were measured by HPLC. Additional details on the experiments, including schematic representations of the apparatuses, are available elsewhere.16 Multiple-Component Isotherms. Competitive Langmuir Model. Adsorption of carboxylic acids onto basic polymeric sorbents occurs by the formation of acid-base complexes between undissociated acid molecules and basic sites on the sorbent. Assuming that only (1,1) stoichiometric acid-base complexes exist, the relevant complexation reaction is

HA + S h T HA-S

(1)

where HA is the carboxylic acid, S is a basic site, and overbars represent adsorbed-phase species. The complexation reaction can be described by an equilibrium constant, Ktrue:

Ktrue )

aHA-S

(2)

aHAaS

Assuming that the ratio of activity coefficients is constant over the concentration range of interest, an apparent equilibrium constant, K, can be written in terms of species concentrations:

K)

xHA-S

(3)

xHAxS A particularly simple and useful model to describe adsorption of individual carboxylic acids onto basic polymeric sorbents1,3 is the Langmuir isotherm model.18 The Langmuir model assumes that the surface consists of a fixed number of energetically equivalent adsorption sites, that only one solute molecule can adsorb per adsorption site (i.e., no more than monolayer coverage), and that there are no lateral interactions between adsorbed solute molecules.19 The assumptions are equivalent to independent, 1:1 complexation. The Langmuir isotherm is described mathematically as

q Kx ) qm 1 + Kx

(4)

where q represents the uptake of acid (mmol/g), qm represents the acid adsorption capacity (mmol/g) of the sorbent for monolayer coverage, and x is the aqueousphase undissociated acid concentration (mol/L). When more than one acid is present in solution, the resulting adsorbed phase will consist of free basic sites and acid-base complexes for each acid. The relative amounts of each of these species will depend on the apparent equilibrium constants for adsorption of each acid, with competition between the acids for the basic sites. Assuming independent 1:1 complexation, a competitive Langmuir adsorption isotherm for each acid results:

504 Ind. Eng. Chem. Res., Vol. 38, No. 2, 1999

qi

Kixi

)

qm

1+

∑j Kjxj

(5)

ln KAB ) xln KAA ln KBB

where the summation is over all adsorbing species. Values of qm and Kj for each acid can be obtained from a nonlinear least-squares fit of the experimental data for single-acid adsorption to eq 4. The Kj values obtained from the single-acid adsorption data can then be used in eq 5 to predict adsorption equilibria for multiple-acid adsorption. Thus, the model provides a simple and logical method to predict multiple-acid equilibria using only single-acid adsorption data. In some situations, the Langmuir model might well describe the uptake isotherm of an acid at low uptake values, while deviating considerably from the isotherm at high uptake values. Deviations from ideal Langmuir behavior at high uptake values might stem from lateral interactions among adsorbed acid molecules and/or stoichiometric overloading of the basic sites on the sorbent, i.e., two or more acid molecules adsorbed on each basic site. The next model accounts for stoichiometric overloading of the basic sites on the sorbent. A model that accounts for lateral interactions among adsorbed acid molecules is given elsewhere.16 Overloading Model. A model based on the Law of Mass Action can be used to describe overloading of the basic sites on the sorbent. In this formulation, formation of multiple stoichiometric complexes of acid and basic sites is postulated. An equilibrium description of the system consists of a set of reactions of p acid, A, molecules and r basic sites, S, to form the corresponding set of (p, r) complexes, described by their apparent equilibrium constants, Kpr. The uptake of acid A is then given as

q qm

)

∑r ∑p 1+

(6) rKprxApxSr-1

For the case that r ) 1 and only (1, 1) and (2, 1) complexes are formed, the resulting adsorption isotherm is given by eq 7. Here, KA and KAA describe 1:1 and 2:1

KAxA + 2KAAxA2

q ) qm 1 + K x + K x 2 A A AA A

(7)

complexation, respectively. Values of KA and KAA can be obtained from a nonlinear least-squares fit to eq 7 of the experimental data for single-acid adsorption. For adsorption from a binary-acid solution of acids A and B, the corresponding adsorption isotherm for acid A is given by eq 8.

qA KAxA + 2KAAxA2 + KABxAxB ) qm 1 + K x + K x + K x 2 + K x 2 + K x x A A B B AA A BB B AB A B (8) An analogous expression can be written for the uptake of acid B. All parameters in eq 8 except KAB, the equilibrium constant describing the formation of a ternary complex of A, B, and a basic site, are known from the single-acid isotherms. A value for KAB can be

(9)

Equations 8 and 9 can be used to predict uptake values for adsorption of each acid during adsorption from a mixed-acid solution using only single-acid experimental data. An expression analogous to eq 8 results for the previously referenced model that accounts for lateral interactions among adsorbed acid molecules.16 It is, therefore, not possible to distinguish between the two modeling approaches, or between lateral interactions and site overloading, mathematically. Ideal Dilute Solution Theory. In an effort to minimize experimental effort, Radke and Prausnitz20 established a method that employs experimental data from only single-solute adsorption to predict multiple-solute adsorption from dilute liquid solution. This method, extended from the work of Myers and Prausnitz21 for mixed-gas adsorption and referred to as the Ideal Dilute Solution Theory (IDST), is based on an exact formulation of the thermodynamics of adsorption from dilute solution and is not restricted to a specific theoretical adsorption model. Several papers have explored using IDST for dissociating compounds adsorbed onto activated carbon.22-24 The adsorption isotherm is given as a function of the concentration of undissociated acid. Modeling of the adsorption-pH isotherms must therefore account for aqueous-phase acid dissociation. For a monocarboxylic acid, HA (e.g., lactic, formic, and acetic acids), there is only one dissociation reaction and corresponding equilibrium constant, Ka:

HA T H+ + A-

pKprxApxSr-1

∑r ∑p

estimated using a familiar geometric-mean combining rule:

Ka )

xH+xAxHA

(10)

The expression for the concentration of undissociated acid is given as

xHA )

xHAT

(11)

1 + 10pH-pKa

where superscript T represents total. For a dicarboxylic acid (e.g., succinic acid), two reactions and their corresponding apparent equilibrium constants, Ka1 and Ka2, are needed to describe the acid dissociation:

H2A T H+ + HA-

Ka1 )

xH+xHAxH2A

(12)

HA- T H+ + A2-

Ka2 )

xH+xA2xHA-

(13)

The resulting expressions for the concentrations of undissociated acid and bicarboxylate anion are

xH2AT

)

xHA- )

xH2AT 1 + 10pH-pKa1 × 102pH-pKa1-pKa2 xH2AT × 10pH-pKa1 1 + 10pH-pKa1 + 102pH-pKa1-pKa2

(14)

(15)

Ind. Eng. Chem. Res., Vol. 38, No. 2, 1999 505 Table 1. IDST Equations Used To Describe Multiple-Acid Adsorption equivalence of spreading pressures definition of spreading pressures liquid-adsorbed-phase equilibria undissociated acid concentrations definition of mole fraction total acid uptake

π ) π 1 ) π2 π1 ) f(x1,uo) π2 ) f(x2,uo) x1,um ) x1,uoz1s x2,um ) x2,uoz2s x1,um ) f1(pH,x1m) x2,um ) f2(pH,x2m) z1s + z2s ) 1

(T1-1,2) (T1-3) (T1-4) (T1-5) (T1-6) (T1-7) (T1-8) (T1-9)

z1s z2s 1 ) o+ o qT q q

(T1-10)

1

2

The basis of the IDST model is the thermodynamic equivalence of the spreading pressure, πi, of each solute at equilibrium. To use the IDST model to predict multiple-acid equilibria, spreading pressures for each of the singly adsorbing acids in the mixture must be determined. These spreading pressures can be evaluated using the thermodynamic relationship derived by integration of the Gibbs adsorption isotherm:21

πi(xi,uo) )

o

xi,uoqi

∫o

RT A

(xi,uo)

xi,uo

dxi,uo (constant T)

(16)

where superscript o represents a single-solute value and subscript u represents undissociated species. Single-acid adsorption isotherms of qio/xio versus xio are generated; the area under the isotherms corresponds to the value of πi(xio). Computing Multiple-Acid Equilibria. To predict the isothermal adsorption equilibria as a function of pH for an aqueous binary-acid mixture, 12 unknown quantities must be determined: π, π1, π2, z1s, z2s, pH, x1,uo, x2,uo, x1m, x2m, x1,um, and x2,um, where superscript m denotes a liquid-phase mixture value and zis corresponds to the adsorbed-phase mole fraction of species i. These variables are computed using the relations derived from IDST and summarized in Table 1. Equations T1-1-9 provide nine independent equations. To solve this system of equations, a value of π was specified and the initial values of x1m and x2m were used. The remaining unknowns were determined from eqs T1-1-9. The total acid uptake was determined from eq T1-10. The individual uptakes of each acid were calculated as the multiplication products of the total acid uptake and the mole fractions of the acids in the adsorbed phase. On the basis of the amounts of each acid adsorbed, new values of x1m and x2m were determined. The equations were iterated until the calculated values of x1m and x2m matched those from the previous iteration.

Figure 1. Adsorption isotherm for lactic acid onto Dowex MWA-1 at 25 °C.

Results and Discussion Single-Acid Adsorption Isotherms. To predict multiple-acid equilibria using data from single-acid adsorption, experimental data were needed for singleacid adsorption. Adsorption isotherms for lactic, succinic, acetic, and formic acids onto Dowex MWA-1 were measured and are presented in Figures 1-4. The single-acid adsorption isotherms were fit to the Langmuir model (eq 4) using a nonlinear least-squares regression to determine best-fit values of qm and K. For lactic acid, the experimental data are well described by this model; the dashed line in Figure 1 shows the fit to the Langmuir model. Succinic, acetic, and formic acids show considerable deviations from ideal Langmuir

Figure 2. Adsorption isotherm for succinic acid onto Dowex MWA-1 at 25 °C.

behavior at the high aqueous-phase acid concentrations studied (Figures 2-4). Therefore, only data for xi < 0.22 M were used to determine the fitted constants for the Langmuir model. For succinic and formic acids, deviations from ideal Langmuir behavior might also occur for concentrations < 0.22 M, as suggested by the imperfect fits to the data using model parameters

506 Ind. Eng. Chem. Res., Vol. 38, No. 2, 1999

Figure 3. Adsorption isotherm for acetic acid onto Dowex MWA-1 at 25 °C.

trations, uptake was assumed to occur primarily by (1, 1) complexation. Thus, the value of the apparent equilibrium constant for (1, 1) complexation was set to the fitted low-concentration value obtained with the Langmuir model. Inclusion of (2, 1) complexes accounted for deviations from ideal Langmuir behavior. Table 2 lists the fitted values of the maximum uptake capacities and the apparent equilibrium constants for adsorption of the four acids onto Dowex MWA-1 at 25 °C. Values obtained by other researchers are also listed in Table 2 for comparison. Multiple-Acid Adsorption Isotherms. Lactic and Succinic Acids. Figure 5 shows the experimental uptakepH isotherms for adsorption of lactic and succinic acids onto Dowex MWA-1 from an equimolar (0.15 M each), aqueous acid mixture at 25 °C. The solution-to-sorbent ratio was 20 mL/g. With increasing undissociated acid concentration (i.e., decreasing pH), the uptake curves asymptotically approach maximum values corresponding to the sorbent uptake capacity. An interesting result of this system is that succinic acid is preferentially adsorbed from the mixture, despite being the weaker acid based on pKa values (pKa1 ) 4.16 for succinic acid, compared to 3.86 for lactic acid). This observation can be rationalized by considering the relative hydrophilicities of the two acids. Lactic acid, being the more hydrophilic acid, has a lower aqueous-phase activity coefficient than succinic acid.16 Additionally, succinic acid might have a lower adsorbedphase activity coefficient than lactic acid. Upon examination of eq 17, a higher aqueous-phase activity coefficient and/or a lower adsorbed-phase activity coefficient for succinic acid would result in a higher apparent equilibrium constant for adsorption, defined as

K ) Ktrue

Figure 4. Adsorption isotherm for formic acid onto Dowex MWA-1 at 25 °C.

obtained by fitting data with xi < 0.22 M. The experimental isotherms for these acids show no plateau with increasing aqueous-phase acid concentration. This result suggests stoichiometric overloading of the amine sites on Dowex MWA-1; overloading would occur by hydrogen bonding of a second acid to the carbonyl oxygen of the first acid. Reisinger and King4 found similar results in their experimental data for acetic acid on Dowex MWA-1. The absence of overloading in the lactic acid system might be attributed to intramolecular hydrogen bonding of the R-OH group of lactic acid with the carboxylate group, thereby preventing hydrogen bonding of a second acid at this site. An internally bonded structure for R-hydroxyisobutyric acid has been found by NMR experiments.25 To describe the data in Figures 2-4, (1, 1) and (2, 1) stoichiometric complexes were postulated and the data were fit to eq 7; the dotted lines represent the best fit of the data to eq 7. At low aqueous-phase acid concen-

γHAγB γHAB

(17)

Therefore, adsorption from an aqueous mixture to a nonaqueous surface “phase” would favor the less hydrophilic succinic acid. A second interesting result for this system is that the uptake isotherm for succinic acid remains relatively constant for pH values between 2 and about 5. This result may be attributable to either or both of two effects, namely, the diacid nature of succinic acid and its higher pKa1 value relative to that of lactic acid. The higher pKa1 value of succinic acid means that with increasing pH the ratio of undissociated succinic-tolactic acid concentrations increases. Additionally, some of the basic sites on the adsorbent which were occupied by lactic acid at low pH values are unoccupied by lactic acid at pH values above the pKa of lactic acid and below the pKa1 of succinic acid, making more sites available for succinic acid (or bisuccinate anion) to adsorb. Furthermore, the diacid nature of succinic acid sustains adsorption capacity at pH > pKa1 of the acid. Figure 5 also compares the predicted uptake-pH isotherms for the competitive Langmuir model with the experimental data for uptake of lactic and succinic acids onto Dowex MWA-1. The predictions by IAST and the overloading model are almost exactly the same as the predictions by the competitive Langmuir model for this acid mixture under the conditions used. The highly similar predictions are likely due to the final aqueousphase undissociated acid concentrations achieved in the batch experiments; these concentrations were pKa1. 1. Factors Governing Equilibria. Ind. Eng. Chem. Res. 1994, 33, 3217-3223. (4) 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, 845-852.

(5) Jagirdar, G. C.; Sharma, M. M. Recovery and Separation of Mixtures of Organic Acids from Dilute Aqueous Solutions. J. Sep. Proc. Technol. 1980, 1, 40-43. (6) Wo´dzki, R.; Nowaczyk, J. Extraction and Separation of Propionic and Acetic Acid by Permeation in a Hybrid Membrane System Composed of Liquid and Ion-exchange Polymer Membranes. Solvent Extr. Ion Exch. 1997, 15, 1085-1106. (7) Kirsch, T.; Maurer, G. Distribution of Binary Mixtures of Citric, Acetic and Oxalic Acids between Water and Organic Solutions of Tri-n-octylamine. Part I. Organic Solvent Toluene. Fluid Phase Equilib. 1997, 131, 213-231. (8) Cygnarowicz-Provost, M.; Shapouri, H. Opportunities for New Coproducts from Ethanol Production. Industrial Uses; Report IUS-3,33-38; U.S. Department of Agriculture: Washington, DC, June 1994. (9) Kim, I.; Ondrey, G.; Kamiya, T. Betting Big on Biopolymers. Chem. Eng. 1998, July, 43-47. (10) Lipinsky, E. S.; Sinclair, R. G. Is Lactic Acid a Commodity Chemical? Chem. Eng. Prog. 1986, 82 (8), 26-32. (11) Jain, M. K.; Datta, R.; Zeikus, J. G. High-value Organic Acids Fermentation- -Emerging Processes and Products. In Bioprocess Engineering: the First Generation; Ghose, T. K., Ed.; Ellis Horwood Ltd.: Chichester, U.K., 1989; pp 366-389. (12) Atkinson, B.; Mavituna, F. Biochemical Engineering and Biotechnology Handbook; MacMillan: New York, 1983. (13) Stanier, R. Y.; Adelberg, E. H.; Ingraham, J. L. The Microbial World, 4th ed.; Prentice-Hall: Englewood Cliffs: NJ, 1976. (14) Wagner, F. S., Jr. Acetic Acid and Derivatives. In KirkOthmer Encyclopedia of Chemical Technology; Kroschwitz, I., Howe-Grant, M., Eds.; John Wiley & Sons: New York, 1991; Vol. 1, pp 121-139. (15) Drury, D. J. Formic Acid and Derivatives. In Kirk-Othmer Encyclopedia of Chemical Technology; Kroschwitz, I., Howe-Grant, M., Eds.; John Wiley & Sons: New York, 1994; Vol. 11, pp 951958. (16) Husson, S. M.; King, C. J. Regeneration of Basic Adsorbents in the Recovery of Carboxylic Acids from Dilute Aqueous Solution and Multiple-acid Equilibria in the Recovery of Carboxylic Acids from Dilute Aqueous Solution; Report 41885; Lawrence Berkeley National Laboratory: Berkeley, CA, June 1998. (17) Frierman, M. The Use of Solid Adsorbents for the Recovery of Acetic Acid from Aqueous Solutions. M.S. Thesis, University of California at Berkeley, Berkeley, CA, 1983. (18) Langmuir, I. The Adsorption of Gases on Plane Surfaces of Glass, Mica and Platinum. J. Am. Chem. Soc. 1918, 40, 13611403. (19) Adamson, A. W. Physical Chemistry of Surfaces; Interscience: New York, 1967. (20) Radke, C. J.; Prausnitz, J. M. Thermodynamics of MultiSolute Adsorption from Dilute Aqueous Solutions. AIChE J. 1972, 18, 761-768. (21) Myers, A. L.; Prausnitz, J. M. Thermodynamics of MixedGas Adsorption. AIChE J. 1965, 11, 121-127. (22) Mu¨ller, G.; Radke, C. J.; Prausnitz, J. M. Adsorption of Weak Organic Electrolytes from Dilute Aqueous Solution onto Activated Carbon. Effect of pH. J. Phys. Chem. 1980, 84, 369376. (23) Mu¨ller, G.; Radke, C. J.; Prausnitz, J. M. Adsorption of Weak Organic Electrolytes from Dilute Aqueous Solution onto Activated Carbon. I. Single-Solute Systems. J. Colloid Interface Sci. 1985, 103, 466-483. (24) Mu¨ller, G.; Radke, C. J.; Prausnitz, J. M. Adsorption of Weak Organic Electrolytes from Dilute Aqueous Solution onto Activated Carbon. II. Multisolute Systems. J. Colloid Interface Sci. 1985, 103, 484-492. (25) Mori, N.; Asano, Y.; Irie, T.; Tsuzuki, Y. Intramolecular Hydrogen Bonds. XIII. The Preferable Conformation of R-hydroxycarboxylic and o-hydroxybenzoic Acids. Bull. Chem. Soc. Jpn. 1969, 42, 482-487. (26) Evangelista, R. L.; Mangold, A. J.; Nikolov, Z. L. Recovery of Lactic Acid by Sorption. Appl. Biochem. Biotechnol. 1994, 4546, 131-144. (27) Ng, M. Regeneration of Basic Sorbents Used in the Recovery of Acetic Acid from Dilute Aqueous Solutions. M.S. Thesis, University of California at Berkeley, Berkeley, CA, 1988. (28) Tung, L. A. Recovery of Carboxylic Acids at pH > pKa1. Ph.D. Thesis, University of California at Berkeley, Berkeley, CA, 1993.

Ind. Eng. Chem. Res., Vol. 38, No. 2, 1999 511 (29) Gustafson, R. L.; Fillius, H. F.; Kunin, R. Basicities of Weak Base Ion Exchange Resins. Ind. Eng. Chem. Fundam. 1970, 9, 221-229.

Base in an Organic Solvent. Ind. Eng. Chem. Res. 1998, 37, 29963005.

(30) Husson, S. M.; King, C. J. Regeneration of Lactic and Succinic Acid-Laden Basic Sorbents by Leaching with a Volatile Base in an Organic Solvent. U.S. Patent pending, 1997.

Received for review July 8, 1998 Revised manuscript received November 9, 1998 Accepted November 11, 1998

(31) Husson, S. M.; King, C. J. Regeneration of Lactic and Succinic Acid-Laden Basic Sorbents by Leaching with a Volatile

IE9804430