RECOVERY OF CARBOXYLIC ACIDS FROM AQUEOUS SOLUTIONS BY LIQUID-LIQUID EXTRACTION WITH A TRIISOOCTYLAMINE DILUENT SYSTEM

RECOVERY OF CARBOXYLIC ACIDS FROM AQUEOUS SOLUTIONS BY LIQUID-LIQUID EXTRACTION WITH A TRIISOOCTYLAMINE DILUENT SYSTEM

G.Malmary, J.Albet, A.M.H.Putranto and J.Molinier

Ecole Nationale Supérieure des Ingénieurs en Arts Chimiques Et Technologiques.

Equipe Génie Chimique. Laboratoire de Chimie-Agro-Industrielle, UMR 1010,

INRA, 118, Route de Narbonne, 31077 Toulouse Cedex 4, France.

E-mail: [e-mail]

(Received: April 27, 2001 ; Accepted: October 21, 2001)

Abstract - Tertiary alkylamines in solution with organic diluents are attractive extractants for the recovery of carboxylic acids from dilute aqueous phases. The aim of this study was to investigate the mechanism for extraction of organic acids from water by a long-chain aliphatic tertiary amine. In order to attain this objective, we studied the liquid-liquid equilibria between the triisooctylamine + 1-octanol + n-heptane system as solvent and an aqueous solution of an individual carboxylic acid such as citric, lactic and malic acids. The experiments showed that the partition coefficient for a particular organic acid depends on the kind of solute, notably when the acid concentration in the aqueous phase is low. A mathematical model, where both chemical association and physical distribution are taken into consideration, is proposed. The model suggests that the various complexes obtained between amine and organic acids contribute to the distribution of the solute between the coexisting phases in equilibrium.

Keywords : liquid-liquid extraction, triisooctylamine, citric acid, lactic acid, malic acid, complexation model.

INTRODUCTION

In recent years, the recovery of carboxylic acids by liquid-liquid extraction with aliphatic tertiary amines dissolved in organic diluents has been studied by several authors (Kertes and King, 1986; Tamada et al., 1990; Bizek et al., 1992; Prochazka et al., 1994; Juang and Huang, 1997; Kirsch and Maurer, 1997).

Traditionally, diluents were used to modify physical properties of solvents, such as viscosity, specific gravity and surface tension of the mixtures. However, the chemical structure of a diluent may have various effects connected with the formation of acid-amine complexes in the organic phase. Indeed, a change in polarity of an organic diluent in the extractant can cause changes in complexation. In the case of extraction of lactic acid with a tertiary alkylamine mixed with a specific diluent, such as chloroform, xylene or methylisobutylketone, it has been explicitly shown that liquid-liquid equilibrium was significantly affected by the nature of the diluent (King, 1993). Chloroform favors the formation of hydrogen bonds with the oxygen in the complex. Methylisobutylketone provides a more favorable polar medium for complex formation than does xylene. Consequently, complexation can be manipulated by an appropriate choice of diluent and extractant concentration. In the present work, the solvent consisted of a synergistic mixture of an extractant (triisooctylamine), an active modifier (1-octanol) and an inert diluent (n-heptane). The pure diluent does not extract the solute, while the modifier influences the extracting power of the tertiary amine. Since the amine salts with carboxylic acids are slightly soluble in the aqueous phase, a pertinent role of the modifier was to improve the solubility of the salts in the extract phase (Yankov et al., 1999). Furthermore, it was obviously interesting to verify that the 1-octanol concentration of the solvent is also of great importance for extraction efficiency. This paper discusses the mechanism of the extraction of citric, lactic and malic acids from dilute aqueous solutions with triisooctylamine (tris-(6-methylheptyl)-amine) in 1-octanol/n-heptane mixtures. Mathematical models of a common general form have been developed to allow correlation of the experimental data. These features of complexation afford the wherewithal for designing the liquid-liquid equilibria to achieve particular objectives in relation to the extraction equipment.

EXTRACTION MECHANISM

In the present work it is assumed that the reactions between amine and acid molecules occur in the organic phase and that only undissociated acid molecules take part in them. The reaction of complex formation is

where H_kA represents the undissociated mono, di and tri acids and the bar over the symbols, the species in organic phase. represents triisooctylamine (TIOA).

The extraction constant is

In the model, the dissociation equilibrium of acid in the aqueous phase has been taken into account:

for k=3

H₃A = H₂A^- + H⁺

H₂A^- = HA^2- + H⁺

HA^2- = A^3- + H⁺

for k=2

H₂A = HA^- + H⁺

HA^- = A^2- + H⁺

for k=1

HA = A^- + H⁺

Hence, the dissociation constants are

From the preceding equations, we obtain

The total concentration of the acid [TA] in aqueous phase is

with

Consequently,

The expression of the extraction constant becomes

Furthermore, the experimentally accessible partition coefficient (D), defined as the ratio of the total molar concentration of the acid in the organic phase and the acid in all its possible forms in the aqueous phase, is given by

Finally, we can write :

Owing to the low dissociation constant of the acids in the study (Table 1) and a pH for the initial aqueous solutions (pH<2.5) below the pK_a of acids, the solutes were mainly in molecular form in an aqueous medium. The ratios K_a1/[H⁺], K_a1 K_a2/[H⁺]² and K_a1 K_a2 K_a3/[H⁺]³ have negligible values, finally b_k can be neglected.

Thus, the relation between log D and [] can be formulated as

where [] represents the molar concentration of free amine at equilibrium in the organic phase and depends on the value for order of association (n).

The plot of the log D function of log [] yields a straight line; the slope corresponds to the number of triisooctylamine molecules reacting with one molecule of a particular acid and the intercept to the extraction constant (K_e).

The quantities (n) and (K_e) remain constant over the range of extractant concentrations.

EXPERIMENTAL SECTION

Chemicals

Triisooctylamine, 1-octanol, n-heptane, citric acid, malic acid and lactic acid with a purity close to 99% were supplied by Aldrich Chemical Co. Distilled water that had been passed through a Milli-Q purification system (Millipore Corp.) was used to dilute the carboxylic acids.

Procedures

Batch extraction experiments were performed in glass-stoppered separatory funnels shaken at a temperature of 25°C in a thermostated chamber. Based on the average acid concentrations of various agroindustrial effluents, the synthetic aqueous solutions of carboxylic acid in this work contain 5 g/L of separate solute. With regard to the recovery of carboxylic acids from aqueous media using triisooctylamine dissolved in 1-octanol, the extractability of the solute depends on the composition of the organic phase. Previous work (Malmary et al., 1998) showed that the partition coefficient for a specific acid reaches a maximum for a concentration of tertiary amine in a binary mixture of about 25% (vol/vol) amine + 1-octanol. Hence, in the present work, the concentration of tertiary amine in the triisooctylamine + 1-octanol + n-heptane ternary system was changed by modifying the n-heptane composition for a constant volume ratio of amine to 1-octanol, fixed at 0.25. The amount of n-heptane was adjusted in each case.

Thus, 100 cm³ of solvent was mixed with 100 cm³ of acidic aqueous solution. Both phases were shaken for 3 hours, which is sufficient to achieve the equilibrium distribution of the solute. After they had settled for over a period of 2 hours, the aqueous and organic phases were analyzed. In all cases, a negligible change in volume of the phases present was observed. Since the mutual solubility of solvent (triisooctylamine + 1-octanol + n-heptane) and aqueous phase containing 5 g/L of solute is negligible, only the equilibrium distribution of acid between the organic and aqueous phases was taken into consideration. Analyzes were performed by High Performance Liquid Chromatography (HPLC). In the case of the organic phase, a complete stripping of carboxylic acid with a sulfuric acid solution (0.5 M) was necessary before injecting samples into the apparatus. In all cases, a material balance, closed within 2%, was used to verify the corresponding organic-phase acid concentrations. The HPLC method consisted of a pump (Spectra-Physics P100), an integrator (Chromjet SP4400), a UV spectrophotometer (Spectra UV-Vis detector: wavelength = 210 nm) and an organic acid column (Bio-Rad Aminex ion exclusion HPX-87H) operated at 41°C. The mobile phase was a sulfuric acid solution (0.005 M) and its flowrate was 0.6 cm³ min^-1. In comparison with a conventional titration method, the percent standard deviation of the partition coefficients, which are defined as the ratio of molar concentration of acid in the organic phase to molar concentration of acid in the aqueous phase, was estimated at 2% for all batch experiments (Table 2).

RESULTS AND DISCUSSION

The liquid-liquid equilibrium in the aqueous solution of carboxylic acid-triisooctylamine in 1-octanol/n-heptane mixture system was measured at a temperature of 25°C for a wide range of solvent compositions (Table 2). The mathematical model developed for this study allows determination of the number of molecules of triisooctylamine reacting with one molecule of the carboxylic acid in the study. In the cases of citric, lactic and malic acids, the (1-1.8), (1-2.5) and (1-2.8) acid-amine complexes, respectively, were proposed (Fig. 1, 2 and 3). Furthermore, for citric, lactic and malic acids, the extraction constants (K_e) were found to be 4.6 10², 1.2 10³ and 2.5 10³, respectively. A relatively good fit between the model and the experimental data was found The correlation coefficients were mentioned in Fig. 1, 2 and 3. The standard deviation between the extraction constant (K_e) obtained from the mathematical model and its experimental value was found close to 3%, 9% and 6% for citric, lactic and malic acids, respectively. In most studies of the reactive extraction of carboxylic acids by a long-chain tertiary amine, the significant effect of amine concentration on the organic phase and hence the synergistic effect of the modifier were clearly shown (Bizek et al., 1992; Prochazka et al., 1994; Yankov et al., 1999). Indeed, for low solute concentrations in the aqueous phase, the equilibrium distribution ratios of organic acids essentially depend on the composition of the solvent. The difference between the conventional solvents such as alcohols, ethers or ketones (oxygen-bearing solvents) and tertiary amine (nitrogen-bearing basic extractants) used in the extraction of carboxylic acids is the behavior of the acid proton during the transfer of solutes from the aqueous phase into an organic solution. In a system with oxygen-bearing solvents, whether carbon, phosphorus or sulfur bound, the strength of the acid in the aqueous phase and that of the hydrogen bond in the organic extract allows quantification of the extractability of the solvent. Moreover, these solvents are relatively soluble in the aqueous phase. Consequently, regeneration of these components is costly. When using a tertiary amine as extractant, the extent of ion pair association between the alkylammonium cation and the acid radical is the measure of the stability of the organic phase species. The pairing of carboxylic acid with tertiary alkylamine has a substantially ionic character. Although water co-extraction is relatively negligible, it is sufficient for clusters of water molecules to be associated with the complexation sites, supporting ionization (King, 1993). Furthermore, it is of paramount importance in process of extraction of carboxylic acid from the aqueous solution to take into account the pH of the medium, which must be lower than the pK_a of the acid. The role of an active diluent such as 1-octanol, known as the modifier is to stabilize the acid-amine complex because its hydrogen bonds with the oxygen accessible on the complexed carboxylic acid in the organic phase. Thus, for the extraction process a diluent which has an acid-interacting functional group (1-octanol) is preferable to nonpolar aliphatic hydrocarbon (n-heptane). Furthermore, we observed that the partition coefficients of individual acids go through a maximum as the amine concentration in the mixture tertiary amine + 1-octanol is increased, which can be explained as follows. Since the undissociated form of carboxylic acids was the only one extracted by the solvent, the acidic pH of the initial aqueous solution was thus favorable to the transfer of solute into the organic phase. However, at lower carboxylic acid concentrations in the aqueous phase, the amine has a substantial effect on the aqueous phase, hence the pH of the latter is increasing. Consequently, the dissociation of carboxylic acid becomes predominant and a decreasing amount of solute was extracted by the solvent. Generally, at pH values lower than pK_a for the acids under study (Table 1), the distribution coefficient for separate acid is nearly independent of the pH value for the aqueous solution at equilibrium with the organic phase. Assuming that water in the organic phase has no significant influence on the association between amine and organic acid, the simplified model log D function of log [] can be accurately applied to determine the number of molecules of triisooctylamine associated with one molecule of the solute and the extraction constant.

CONCLUSIONS

The extraction of proton-bearing organic compounds such as carboxylic acids from dilute aqueous media by long-chain aliphatic tertiary amines dissolved in water-immiscible organic diluents is one of the recent developments in separation technology. The solutes in this study citric, lactic and malic acids, are representative of hydrocarboxylic acids present in a variety of aqueous effluents generated by pharmaceutical, food and biotechnological industries. From measurements of the liquid-liquid equilibria between preceding organic acids in aqueous solution and a solvent which consisted of triisooctylamine in a 1-octanol/n-heptane mixture, it was found that the partition coefficients of individual carboxylic acids depend on the kind of solute. The experimental data can be explained using an extraction model involving both chemical association and physical distribution. Even though the extraction capacity of tertiary amine was sometimes limited by the stoichiometry of complexation, in view of its quasi-insolubility in water, its easy recovery by back-extraction with an aqueous base and its slight toxicity, this class of solvent tends to be a complexant of choice. Indeed, the model developed in this work suggests that the complexes obtained between the trialkylamine and various organic acids contribute to the distribution ratio of the solute in the region of low-acid concentration. For optimum design of an extraction process, it is necessary to clarify the mechanism for extraction of organic acids with the mixed solvent system that gives the highest partition coefficient. Finally, the reversible chemical complexation implemented through the process of liquid-liquid extraction was a suitable route for removing low carboxylic acids from dilute aqueous effluents from various industries.

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Publication Dates

• Publication in this collection
  17 Apr 2002
• Date of issue
  Dec 2001

History

• Received
  27 Apr 2001
• Accepted
  21 Oct 2001