Fractionation of Carboxylic Acids Mixture Obtained by P ...

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Fractionation of Carboxylic Acids Mixture Obtained by P. acidipropionici Fermentation Using Pertraction with tri‑n‑Octylamine and 1‑Octanol Dan Caşcaval,† Madalina Poştaru,† Anca-Irina Galaction,‡,* Lenuta Kloetzer,† and Alexandra Cristina Blaga† †

Gheorghe Asachi Technical University of Iasi, Faculty of Chemical Engineering and Environmental Protection, Dept. of Biochemical Engineering, D. Mangeron 73, 700050 Iasi, Romania ‡ Gr.T. Popa University of Medicine and Pharmacy of Iasi, Faculty of Medical Bioengineering, Dept. of Biomedical Science, M. Kogalniceanu 9-13, 700454 Iasi, Romania ABSTRACT: Propionic, acetic, and succinic acids have been selectively separated from their mixture obtained by P. acidipropionici fermentation using facilitated pertraction with tri-n-octylamine (TOA). This technique allows the recovery of acetic and succinic acids from the mixture, the feed phase raffinate containing only propionic acid. The pH-gradient between the feed and stripping phases, the carrier concentration in the liquid membrane, and the addition of 1-octanol control the selectivity of acids pertraction, with TOA concentration exhibiting the most important influence. In the absence of 1-octanol, at a pH-value of feed phase = 2, pH-value of stripping phase = 10, the maximum selectivity factor (S = 25) was reached for the carrier concentration 70 g L−1. By 1-octanol addition, at the same pH-values of the aqueous phases, the maximum selectivity factor (S = 19) was reached for lower carrier concentration, namely 50 g L−1. The reduction of the selectivity factor for the pertraction system containing 1-octanol is compensated by diminishing the material consumption required for separation.

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INTRODUCTION

calcium salts, ionic exchange followed by elution and crystallization, but with high energy and materials costs. Although the liquid−liquid extraction represents an accessible and efficient alternative for many downstream processes in biotechnology, its application for ionizable compounds, particularly the carboxylic acids, is less efficient due to their low solubility in the usual hydrophobic organic solvents. The extraction yields of propionic and succinic acids in hydrophobic organic solvents are less than 10%, while the maximum extraction degree for acetic acid is reached for aliphatic alcohols with over four carbon atoms (30−37%).8 As it was previously concluded, the efficiency of liquid−liquid extraction of these acids can be improved by adding tri-n-octylamine into the organic phase, the process being called reactive extraction.9,10 The physical or reactive extraction represents the basis of the development of a rather new separation technique, namely pertraction or permeation through liquid membranes. Pertraction consists in the transfer of a solute between two phases separated by a solvent layer, the driving force being the gradient of property (pH, concentration, etc.) between the feed and stripping phases.11,12 By comparison to the liquid−liquid extraction, the use of pertraction diminishes the loss of solvent during the separation cycle, requires a small quantity of solvent and carrier because of their continuous regeneration, and offers the possibility of solute transport against its concentration gradient, as long as the pH-gradient between the two aqueous

Propionic acid is a monocarboxylic acid with numerous applications in chemical industry (reagents, plasticizers, solvents, emulsifying agents, monomers, resins, paints, electroplating solutions), agriculture (herbicides, mold preventing), pharmaceutical (antiarthritic drugs), and food industries (preservatives, acid or salts, antifungal agent, fruits artificial flavors), as well as for perfumes production.1,2 Propionic acid is commercially produced using liquefied petroleum gas, namely propane or ethylene, by chemical synthesis via propionaldehyde.2−4 The cost of this technology increased in the last years, due to the increasing price of the liquefied petroleum gas, and depends on the acid desired purity. Moreover, the separation of propionic acid at industrial scale requires high consumption of lime and sulfuric acid and produces important amounts of acidic wastewaters and solid wastes of calcium sulfate sludge.5 Due to the difficulties of the chemical synthesis and to the demands for implementing at larger-scale of the new ecofriendly technologies, the interest in producing propionic acid by low-cost processes of fermentation has increased in the last years. The bacteria of genuses Propionibacterium (P. acidipropionici, P. acnes, P. arabinosum, P. shermanii), Clostridium (C. propionicum), Veillonella, and Selenomonas species have been tested, but only the strain P. acidipropionici has been considered promising for industrial applications.1,6,7 The final fermentation broth is a mixture of carboxylic acids containing propionic acid, as the main product, and secondary acids (especially, succinic and acetic acids).2,6,7 The selective separation of these acids from the biosynthetic mixture is achieved by precipitation as © 2013 American Chemical Society

Received: Revised: Accepted: Published: 2685

August 31, 2012 January 22, 2013 January 25, 2013 January 25, 2013 dx.doi.org/10.1021/ie302339z | Ind. Eng. Chem. Res. 2013, 52, 2685−2692

Industrial & Engineering Chemistry Research

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phases is maintained.11,12 The addition of a carrier in the liquid membrane, such as organophosphoric compounds, long chain amines or crown-ethers, etc., could lead to the significant improvement of the pertraction efficiency and selectivity. In the mentioned context, in this work, the possibility to separate selectively propionic, succinic, and acetic acids from the aqueous solution containing their mixture obtained by fermentation with P. acidipropionici using facilitated pertraction with tri-n-octylamine (TOA) in presence of 1-octanol has been investigated. In this purpose, the influences of the pH-gradient between the aqueous phases, the carrier concentration in the liquid membrane, and the addition of 1-octanol on the efficiency and selectivity of pertraction have been analyzed.

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RESULTS AND DISCUSSION The pertraction efficiency is strongly influenced by the pHgradient between the two aqueous phases, carrier concentration in liquid membrane, and phase mixing intensity. For the separation of carboxylic acids obtained by propionic acid fermentation, the influence of the pH-gradient between the phases is amplified by the ionization-protonation of these acids in the aqueous solution, these processes controlling the efficiency of extraction and re-extraction, as well as the rate of transport through the liquid membrane. As can be observed from Figure 1, plotted for pertraction using liquid membrane without 1-octanol, the increase of pH-

MATERIALS AND METHODS

The experiments have been carried out using the pertraction equipment that allows obtaining and maintaining easily the solvent layer between the two aqueous phases. The pertraction cell and operating parameters have been described in previous papers.13 The experiments have been carried out in a pseudo-steadystate regime, at the steady-state conditions related to the aqueous phases and unsteady-state mode related to the membrane phase. The aqueous solutions have been separately fed with a volumetric flow of 2.5 L h−1. The liquid membrane phase consisted of dichloromethane in which the carrier TOA has been dissolved, its concentration varying between 5 and 300 g L−1 (0.014−0.85 M). 1-Octanol (dielectric constant of 10.3 at 25 °C14) has been added into the membrane phase, its concentration being 10% vol. The feed phases were aqueous solutions with composition similar to that obtained by fermentation with P. acidipropionici: 32 g L−1 (0.43 M) propionic acid, 7 g L−1 (0.06 M) succinic acid, and 5 g L−1 (0.08 M) acetic acid.15 The pH-value of the feed phase varied between 1 and 7, the pH adjustment being made with solution of 3% sulfuric acid or 3% sodium hydroxide, based on the prescribed pH-value. The stripping phases consisted of solutions of sodium hydroxide with pH = 7−12. The pH-values of both aqueous phases were determined using a digital pH-meter of Consort C836 type and have been recorded throughout each experiment. Any pH change was recorded during the extraction experiments. The pertraction process was analyzed based on the initial and final mass flows of the carboxylic acids and permeability and selectivity factors, previously defined.13 For calculating these parameters, the acids concentrations in the feed and stripping phases have been measured and the mass balance for the pertraction system has been used. Propionic, succinic, and acetic acid concentrations have been determined by high performance liquid chromatography technique (HPLC, Star Varian Chromatography Workstation) with a PL Hi-Plex H column (7.7 mm diameter, 300 mm length, 8 μm porous particle), provided with UV Prostar 330 PDA detector.10 The mobile phase was a solution of 0.1% trifluoroacetic acid with a flow rate of 0.6 mL/min. The analysis has been carried out at 60 °C. Each experiment has been performed in triplicate, the average value of the considered parameters being used. The maximum experimental error was of ±6.11%.

Figure 1. Influence of pH-value of feed phase on acetic, succinic, and propionic acid mass flows (carrier concentration = 200 g L−1, pH of stripping phase = 10).

value of feed phase, pHF, induces the decrease of acids initial mass flows, due to the reduction of the efficiency of reactive extraction with TOA at the interface between the feed phase and liquid membrane. The decreasing of extraction efficiency with the pHF increase is the consequence of the carboxylic group dissociation, process that is slower for pHF values less than 3. This variation of extraction efficiency, and, implicitly, of the initial mass flows is the result of the dissociation of propionic and acetic acids to their single carboxylic group and to the partial dissociation of succinic acid to one carboxylic group (at 25 °C, Ka = 1.38 × 10−5 for propionic acid, Ka = 1.78 × 10−5 for acetic acid, Ka1 = 6.92 × 10−5 for succinic acid14). Although the dependence between the initial mass flow and pHF is similar for all three carboxylic acids, the values of the interfacial mass transfer rate are correlated with the solutes acidity, because the acidity controls the rate of interfacial reaction between solute and carrier. However, the efficiency of reactive extraction is also controlled by the structure of the interfacial compounds formed between solute and carrier, respectively, by the number of carrier molecules stoichiometrically needed. According to the previous results on reactive extraction in dichloromethane,9,10 the interfacial interactions between the acid and the carrier could be of hydrogen bonding type with the undissociated carboxylic groups, or of ionic type, if the acid dissociates in the aqueous solution: R(COOH)m(aq) + pQ (o) ⇄ R(COOH)m ·Q p(o)

where m = 1 for acetic and propionic acid and m = 2 for succinic acid. Thus, at pHF < 4, the studies on reactive extraction of these acids with TOA in dichloromethane without 1-octanol 2686

dx.doi.org/10.1021/ie302339z | Ind. Eng. Chem. Res. 2013, 52, 2685−2692


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For pHF > 4, due to the partial dissociation of succinic acid, the structure of the extracted compound is modified and becomes R(COOH)2·Q, similar to that of the product formed by interfacial reaction between acetic acid and TOA. Moreover, the acidity of the second carboxylic group of succinic acid is the lowest one compared to the other two acids (Ka2 = 2.45 × 10−6 at 25 °C14). In these circumstances, the relative amplification of the final mass flow becomes more relevant for succinic acid, its permeability factor exceeding those recorded for acetic and propionic acids. To quantify the effect of 1-octanol addition inside the liquid membrane on the initial and final mass flows of carboxylic acids, as well as on the membrane permeability, the factors FN and FP have been considered.16 The dependence of factor FN on the feed phase pH, plotted in Figure 3, suggests that the addition of 1-octanol exhibits a

indicated that the structures of the interfacial compound are RCOOH·Q2 for propionic acid, RCOOH·Q for acetic acid, and R(COOH)2·Q2 for succinic acid (Q symbolizes the carrier).9,10 The highest rates of mass transfer from feed phase to membrane have been recorded for acetic acid, the initial mass flows decreasing then to succinic and propionic acids, respectively. In this case, the corresponding order of the acids initial mass flows is the consequence of the cumulated effects of the decreasing of acidity of carboxylic groups from succinic to propionic acid (considering the first step of succinic acid dissociation) and of the increased complexity of acid−carrier compound structure in the same sequence. The acids pertraction becomes not possible for pHF values over 7, as a result of the total dissociation of all acids and to the corresponding sodium salts formation in the feed phase. The variations of acids final mass flows are similar with those of the initial mass flows, due to their direct dependence to the extracted acids amount in the organic layer. Contrary to the influence on initial mass flows, the increase of pHF exhibits a positive effect on the permeability factor for all three carboxylic acids, the yields of acids extraction and reextraction yields becoming closer for neutral values of pHF, due to the low amounts of acids transferred into the membrane phase (Figure 2). Moreover, Figure 2 suggests two domains of

Figure 3. Influence of pH-value of feed phase on factor FN (carrier concentration = 200 g L−1, pH of stripping phase = 10).

positive effect on acids mass flows. Although for all studied acids the factor FN, calculated either for the initial mass flows or for the final ones, is greater than the unit and increases for the entire considered pHF domain, the magnitude of pHF influence is different, the highest values of factor FN being reached for propionic acid. Thus, for pHF variation from 1 to 7, FN calculated for the initial mass flows increased for about 1.6 times for acetic acid, 1.9 times for succinic acid, and over 2.6 times for propionic acid, respectively. For the same pHF variation, the values of FN related to the final mass flows increased for about 1.4 times for acetic acid, 1.7 times for succinic acid, and 2.1 times for propionic acid. These results are the consequence of the favorable effect of 1-octanol on the solubilization of acids molecules, free or bounded to the carrier molecules, on the membrane phase. The increase of pHF induces the dissociation of carboxylic acids in the feed phase, the presence of 1-octanol improving the solubilization also of the dissociated molecules of acid. Compared to acetic and succinic acids, the more important influence of 1-octanol addition on propionic acid mass flows is due especially to the modification of the structure of the extracted compound. Therefore, the structure of the compound resulted from the interfacial reaction between propionic acid and TOA becomes RCOOH·Q, less complex than in absence of this alcohol.10 Consequently, the extraction rate of this acid is strongly enhanced.

Figure 2. Influence of pH-value of feed phase on acetic, succinic, and propionic acid permeability factors (carrier concentration = 200 g L−1, pH of stripping phase = 10).

permeability factors variation. Thus, for pHF below 4, the plotted dependences indicate that the transport capacity of liquid membrane is positively influenced by the acidity of transferred solute but affected by the complexity of extracted compound structure. Both variations of permeability factors are controlled by the rate of reaction between acid−carrier compound and sodium hydroxide at the interface separating the membrane and stripping phases. Obviously, the increase of solute acidity and complexity of extracted compound structure lead to the appearance of a kinetic resistance to the re-extraction process. Therefore, at pHF < 4, in absence of 1-octanol in liquid membrane, the simplest structure of the extracted compound corresponds to acetic acid, RCOOH·Q.9 In this case, the corresponding order of the acid permeability factors is the consequence of the cumulated effects of the increased complexity of chemical structure of the extracted compound and of acidity of carboxylic groups from propionic to succinic acid. 2687



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intensified. However, as it can be seen from Figure 5, the initial and final mass flows are significantly increased only in the pHS domain varying from 8 to 11. At lower values of pHS, both fluxes tend to 0; at higher pHS-values, they remain at a rather constant level. The order of the mass flows is similar to that recorded by increasing the values of pHF. For all three acids, the increase of pHS leads to the continuously increase of the permeability factors (Figure 6).

The addition of 1-octanol also influences rather significantly the mass flows of succinic acid, as the result of the solubilization of acid molecules partially dissociated at one carboxylic group. For all three acids, the relative magnitude of the positive effect of alcohol addition is superior in the case of the initial mass flows, due to the supplementary kinetic resistance to the acid re-extraction process from the membrane phase to the stripping solution. Contrary, the values of factor FP are lower than 1 for the entire experimented domain of the feed phase pH, the increase of pHF inducing the reduction of this factor (Figure 4). In all

Figure 6. Influence of pH-value of stripping phase on acetic, succinic, and propionic acid permeability factors (carrier concentration = 200 g L−1, pH of feed phase = 2). Figure 4. Influence of pH-value of feed phase on factor FP (carrier concentration = 200 g L−1, pH of stripping phase = 10).

This variation suggests that the acceleration of final mass flow becomes more important as compared to that of the initial mass flow, due to the more important increase of the acids reextraction rate at higher pH value of stripping phase. However, depending on the pHS variation domain, Figure 6 indicates that the magnitude of this positive effect differs from one acid to another, the highest permeability factor being reached for propionic acid. As it was above-discussed, this sequence is the result of the increment of kinetic resistance to the re-extraction process from propionic acid to succinic acid, due to the increase of acidity, and, therefore, of the strength of the acid−carrier bond. This differentiation is more important at lower concentrations of sodium hydroxide in the stripping phase. At higher pHS-values, respectively at higher concentration of the re-extraction agent, the magnitude of the effects of higher acidity or complexity of extracted compound structure is significantly diminished. In this domain of pHS, the values of permeability factors of acetic and succinic acids exceed that of propionic acid, as the consequence of the superior increase of the first two acids final mass flows related to that of propionic acid. This phenomenon is the result both of the higher amounts of acetic acid−TOA and succinic acid−TOA compounds extracted in the liquid membrane and of the superior acidity of these two acids as compared to propionic acid. In the presence of 1-octanol, Figure 7 indicates the similar influence of pHS on the acid factor FN as compared to that of pHF. These factors are superior to 1 for all experimented pHvalues of stripping phase, with greater values being recorded for propionic acid. The factor FN related to the initial mass flows increased for about 1.4 times for acetic acid, 1.5 times for succinic acid, and 1.7 times for propionic acid. The values of FN corresponding to the final mass flows increased for about 1.5 times for acetic acid, 1.6 times for succinic acid, and 1.8 times for propionic acid. The magnitude of the effect induced by 1-

cases, the increase of the initial mass flows due to the addition of 1-octanol inside the liquid membrane exceeds the membrane capacity to transport the acids and to release them into the stripping phase. Obviously, for the above-discussed reasons, this effect is more important for propionic acid; at pHF = 6, its permeability factor in presence of 1-octanol is 2.3 times lower than that corresponding to the pertraction without alcohol. For a pertraction system without 1-octanol, the increase of pH of the stripping phase, pHS, leads to the increase of the rate of sodium salts formation and, implicitly, of acids re-extraction from the membrane phase. Therefore, the final mass flows of the three carboxylic acids are accelerated (Figure 5). Due to the amplification of their concentration gradient between the feed and stripping phases, the acid initial mass flows are also

Figure 5. Influence of pH-value of stripping phase on acetic, succinic, and propionic acid mass flows (carrier concentration = 200 g L−1, pH of feed phase = 2). 2688



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acid extraction mechanisms, as well as to the difference of solutes acidity and hydrophobicity of the extracted compounds.9,10 The increase of TOA concentration exhibits a favorable effect on the mass transfer of all three acids, due, on the one hand, to the increase of the concentration of one reactant (carrier) to the interface between the feed and membrane phases and, on the other hand, to the accumulation of the interfacial compound inside the liquid membrane (Figure 9). However,

Figure 7. Influence of pH-value of stripping phase on factor FN (carrier concentration = 200 g L−1, pH of feed phase = 2).

octanol addition into the liquid membrane is more important for the final mass flows, because the influence of pHS on the acid re-extraction step from the membrane phase is stronger. However, for pHS values over 10, FN calculated for the final mass flows remains at a rather constant level, a phenomenon that could be associated with the achievement of the maximum capacity of liquid membrane to transport the acids from the feed phase to the stripping one. Therefore, for pHS values over 10, the acceleration of re-extraction rates does not sustain the higher value of the extraction rates induced by 1-octanol addition. Although its values are below 1 for the entire studied domain of stripping phase pH, the variation of FP, plotted in Figure 8,

Figure 9. Influence of carrier concentration on acetic, succinic, and propionic acid mass flows (pH of feed phase = 2, pH of stripping phase = 10).

by increasing the carrier concentration in the membrane phase, it can be observed that the carboxylic acids are extracted from the feed phase in the following succession: acetic acid, succinic acid, and propionic acid, respectively. Therefore, for TOA concentration below 30 g L−1 (0.084 M), only acetic acid is transferred from the feed phase to the membrane one because the carrier reacts first with the solute with higher acidity and that forms the simplest interfacial compound. In the absence of 1-octanol, this level of TOA concentration corresponds to the stoichiometric need for the formation of interfacial compound RCOOH·Q with acetic acid,9 the influence of the supplementary increase of carrier concentration on this acid initial mass flow becoming insignificant. Practically, the pertraction of succinic acid becomes possible for carrier concentration over 30 g L−1, this acid initial mass flow increasing strongly for TOA concentration variation from 30 to 70 g L−1 (0.20 M). Because without 1-octanol the structure of the compound formed by the interfacial reaction between succinic acid and TOA at pHF = 2 is R(COOH)2·Q2, the superior limit of this domain of carrier concentration represents the amount stoichiometrically needed for reacting first with acetic acid and then with succinic acid, both acids from the feed phase. As in the case of acetic acid pertraction, higher values of TOA concentration do not induce important effect on succinic acid mass flow from the feed phase to the liquid membrane. In these circumstances, due to its lower acidity and superior complexity of the formed compound with the carrier, propionic acid is extracted from the feed phase only after the TOA concentration exceeds the sum of those stoichiometrically required for reacting with the other two acids. Because in absence of 1-octanol in the membrane phase, the structure of the interfacial product between propionic acid and TOA is

Figure 8. Influence of pH-value of stripping phase on factor FP (carrier concentration = 200 g L−1, pH of feed phase = 2).

suggests the positive influence of pHS increases. This effect is more pronounced for the solutes with higher acidity, namely, acetic and succinic acids. Moreover, due to the limited transport capacity of liquid membrane, for all three acids, FP reaches the maximum value at pHS = 11 and then decreases for higher pHS. The maximum of FP is more evident for acetic and succinic acids. The carrier concentration inside of the liquid membrane induces different influences on the pertraction efficiency of these carboxylic acids. According to the previous results on reactive extraction of acetic, succinic, and propionic acids with TOA, the difference on carrier influence is due to the different 2689



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RCOOH·Q2;10 the stoichiometric amount of the carrier corresponds to more than 300 g L−1 (more than 0.85 M). Below the carrier concentrations that allow transferring the acids from the feed phase to membrane one, their pertraction is possible only by physical solubilization in dichloromethane, but in this case, the acid mass flows are very low. These results suggest the major influence of the TOA concentration inside the liquid membrane on pertraction selectivity. Being in direct correlation with the amount of acids extracted into the membrane phase, the final mass flows vary similarly to the initial ones with the increase of carrier concentration. The acid permeability factors have a particular evolution with TOA concentration increases. These parameters initially decrease from a value corresponding to the absence of TOA in the membrane phase to a minimum value, reached at the concentration of 10 g L−1 TOA for succinic acid and 20 g L−1 for acetic and propionic acids, finally increasing concomitantly with the carrier concentration (Figure 10). This variation could

The factor FN values, corresponding to 1-octanol addition in the liquid membrane, are over 1 for the initial and final mass flows of all studied acids. However, according to Figure 11, the

Figure 11. Influence of carrier concentration on factor FN (pH of feed phase = 2, pH of stripping phase = 10).

increase of carrier concentration into the liquid membrane exhibits a negative influence on this factor, explained by the diminution of the magnitude of carrier concentration influence in presence of 1-octanol. The highest values of FN have been reached for propionic acid, especially due to the reduction of carrier molecules number stoichiometrically needed to react with this acid in presence of 1-octanol. For the free pertraction of all carboxylic acids, the factor FP is higher than 1, this underlining the positive effect of alcohol addition on the acids free pertraction, respectively, on their solubilization into the membrane phase and, implicitly, on their amount re-extracted into the stripping solution (Figure 12). Figure 10. Influence of carrier concentration on acetic, succinic, and propionic acid permeability factors (pH of feed phase = 2, pH of stripping phase = 10, rotation speed = 500 rpm).

be the result of the changes in the relative rate of the chemical reactions at the separation interface between the liquid membrane and stripping phase. In the absence of the carrier, the extraction and transport of the solute through the liquid membrane occurs only by the physical process of solubilization, the limiting steps of the overall separation process being of diffusional type. The addition of TOA in dichloromethane leads to the change of separation mechanism. Due to the chemical reactions between the acids and the carrier at the feed phase− liquid membrane interface, as well to the chemical reactions between acids−carrier compounds and sodium hydroxide at the liquid membrane−stripping phase interface, the additional limiting steps of the kinetic type appear. Moreover, because the acids do not participate in free acid form to the re-extraction process (they are combined with the carrier), the rate of sodium salt formation is diminished. Consequently, the final mass flows will be initially smaller for the facilitated pertraction process as compared to the free pertraction. Because the yields of physical extraction of acetic and propionic acids are higher than that of succinic acid, the amounts of free acids extracted in the membrane phase are superior for the two monocarboxylic acids. For this reason, the values of TOA concentrations corresponding to the minimum permeability factors for monocarboxylic acids are higher than the value recorded for succinic acid.

Figure 12. Influence of carrier concentration on factor FP (pH of feed phase = 2, pH of stripping phase = 10).

However, by adding TOA and increasing its concentration, FP decreases and becomes lower than the unit. Similar to the above-discussed effects, the increasing of the relative contribution of the physical coextraction of these acids to their transport from the feed phase by 1-octanol addition leads to the overflow of the transport capacity of liquid membrane and, consequently, to the acids accumulation inside the membrane phase. The effect is more important for acetic acid, due to its superior extractability into membrane phase compared to the other two acids. 2690



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Figure 13. Influence of pH-values of feed and stripping phases on selectivity factor (carrier concentration = 200 g L−1).

The presented experimental data suggest that these three acids can be selectively separated by facilitated pertraction with TOA from their biosynthetic mixture. Therefore, acetic and succinic acids can be selectively removed from the feed phase by pertraction, while propionic acid remains in this phase. The pertraction efficiency and selectivity can be modified by adding 1-octanol inside the liquid membrane. To establish the required conditions for reaching high selectivity of separation, the influences of pH-gradient on the aqueous phases, carrier concentration, and mixing intensity on pertraction selectivity have been studied in direct relation with 1-octanol presence in the membrane phase. The selectivity of pertraction was described by means of the selectivity factor, S, defined as the ratio between the cumulated final mass flows of acetic and succinic acids, and the final mass flow of propionic acid: S=

Figure 14. Influence of carrier concentration on selectivity factor (pH of feed phase = 2, pH of stripping phase = 10).

na f.acetic.acid + na f.succinic.acid na f.propionic.acid

(1)

Due to the solubilization of supplementary amounts of acids or of their interfacial compounds, as well as to the reducing of the structure complexity of the extracted compounds, phenomena that are more important for propionic acid, the pertraction selectivity is affected by 1-octanol addition in the membrane phase. Moreover, the carrier concentration related to the maximum value of selectivity factor (S = 19) is reduced to 50 g L−1, as the consequence of the decreasing of the carrier molecules number participating to the interfacial reaction with acetic and succinic acids. Therefore, more important increases of the selectivity factor can be achieved by optimization of the carrier concentration compared to the modification of the aqueous phases pH-values.

As it can be observed from Figure 13, for the pertraction system without 1-octanol, the increase of the pH-gradient between the feed and stripping phases leads to the increase of selectivity factor. According to the discussed effects of pH of feed and stripping phases on acids mass transfer, this variation is the result of the more important positive influence of lower pHFvalues on extraction rate and of higher pHS-values on reextraction rate of acetic and succinic acids. Although the addition of 1-octanol does not change the dependence between the selectivity and the pH of feed phase, the values of selectivity factor for each pHF are lower than those recorded for the pertraction system without alcohol. However, the influence of the stripping phase pH on the pertraction selectivity becomes contrary to that plotted in absence of 1octanol. The variations of selectivity factor with pHF and pHS increases are the result of the more important positive effect of 1-octanol addition on propionic acid pertraction, from the reasons discussed. The decisive influence of carrier concentration on pertraction selectivity is underlined by the dependence between the selectivity factors, and this parameter is plotted in Figure 14. Thus, in absence of 1-octanol, the experimental results indicate that the maximum selectivity (S = 25) is reached for 70 g L−1 TOA in liquid membrane. This concentration value corresponds to the stoichiometry of the reaction with acetic and succinic acids.

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CONCLUSIONS The study on the facilitated pertraction with TOA of acetic, succinic, and propionic acids from their mixture obtained by P. acidipropionici fermentation indicated that it is possible to separate selectively these acids from their biosynthetic mixture. Thus, acetic and succinic acids can be transferred from the feed phase through liquid membrane to the stripping phase, while propionic acid remains in the feed phase. In the absence of 1-octanol in liquid membrane, the increase of the pH-gradient between the feed and stripping phases induced positive effects on separation selectivity. The carrier concentration inside the liquid membrane exhibited the most important influence on the pertraction selectivity, the maximum 2691



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selectivity factor being reached for 70 g L−1 TOA (S = 25, pHF = 2, pHS = 10). The addition of 1-octanol led to the enhancement of mass transfer rates from the feed phase to the stripping one for all considered acids. Because this effect is more important for propionic acid, the selectivity factor was reduced compared to the pertraction without the alcohol. In this case, the highest selectivity factors were reached at lower pHF and pHS values. Moreover, the maximum selectivity corresponded to 50 g L−1 TOA in the liquid membrane (S = 19). By comparing the pertraction systems without and with 1octanol, it can be concluded that the use of 1-octanol allowed to reaching the maximum selectivity with lower material consumption (pHS close to the neutral domain, lower amounts of carrier). This advantage could compensate the inferior values of selectivity factors recorded in presence of 1-octanol. By combining the effects of pH-gradient among the aqueous phases and carrier and 1-octanol concentrations on pertraction of carboxylic acids obtained by propionic acid fermentation, greater values of selectivity factors can be obtained. In this purpose, on the basis of these results, the aim of the future work is to establish a mathematical model describing the influences of the considered parameters on the pertraction selectivity and to optimize it for finding the operating conditions corresponding to the maximum selectivity factor. Moreover, these results will be verified for the real fermentation broths, because the presence of some cellular or biosynthetic compounds (proteins, amino acids, etc.) could affect the pertraction efficiency or change some of the separation conditions.

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(4) Bertleff, W.; Roeper, M.; Sava, X. Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH: Weinheim, 2005. (5) Wasewar, K. L.; Pangakar, V. G. Intensification of Propionic Acid Production by Reactive Extraction: Effects of Diluents on Equilibrium. Chem. Biochem. Eng. Q. 2006, 20, 325. (6) Goswami, V.; Srivastava, A. K. Propionic Acid Production in Situ Cell Retention Bioreactor. Appl. Microbiol. Biotechnol. 2001, 56, 676. (7) Posada, J. A.; Cardona, C. A. Propionic Acid Production from Raw Glycerol Using Commercial and Engineered Strains. Ind. Eng. Chem. Res. 2012, 51, 2354. (8) Schuegerl, K. Solvent Extraction in Biotechnology; Springer-Verlag: Berlin, 1994. (9) Caşcaval, D.; Kloetzer, L.; Galaction, A. I. Influence of Organic Phase Polarity on Interfacial Mechanism and Efficiency of Reactive Extraction of Acetic Acid with tri-n-Octylamine. J. Chem. Eng. Data 2011, 56, 2521. (10) Poştaru, M.; Cârlescu, A.; Galaction, A. I.; Caşcaval, D. Direct Separation of Propionic Acid from Propionibacterium acidipropionici Broths by Reactive Extraction 1. Interfacial Mechanism and Influencing Factors. Env. Eng. Manage. J. 2012, 11, 709. (11) Noble, R. D.; Stern, S. A. Membrane Separations Technology. Principles and Applications; Elsevier: London, 1995. (12) Belafi-Bako, K.; Gubicza, L.; Mulder, M. Integration of Membrane Processes into Bioconversions; Kluwer Academic: New York, 2000. (13) Galaction, A. I.; Caşcaval, D.; Nicuta, N. Selective Removal of Gentamicin C1 from Biosynthetic Gentamicins by Facilitated Pertraction for Increasing the Antibiotic Activity. Biochem. Eng. J. 2008, 42, 28. (14) Weast, R. C. Handbook of Chemistry and Physics, 54th ed.; CRC Press: Cleveland, OH, 1974. (15) Zhu, Y.; Li, J.; Tan, M.; Liu, L.; Jiang, L.; Sun, J.; Lee, P.; Du, G.; Chen, J. Optimization and Scale-Up of Propionic Acid Production by Propionic Acid-Tolerant Propionibacterium acidipropionici with Glycerol as the Carbon Source. Biores. Technol. 2010, 101, 8902. (16) Galaction, A. I.; Kloetzer, L.; Caşcaval, D. Separation of pAminobenzoic Acid by Reactive Extraction in the Presence of 1Octanol as Phase Modifier. Chem. Biochem. Eng. Q. 2010, 24, 149.

AUTHOR INFORMATION

Corresponding Author

*Fax: 00232271311. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Grant No. PN-II-ID-PCE2011-3-0088 authorized by The National Council for Scientific Research−Executive Unit for Financing Higher Education, Research, Development, and Innovation (CNCS-UEFISCDI)

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NOMENCLATURE n = mass flow of carboxylic acid (mol m−2 h−1) ni = initial mass flow of carboxylic acid (mol m−2 h−1) nf = final (overall) mass flow of carboxylic acid (mol m−2 h−1) P = permeability factor S = selectivity factor

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REFERENCES

(1) Barbirato, F.; Chedaille, D.; Bories, A. Propionic Acid Fermentation from Glycerol: Comparison with Conventional Substrates. Appl. Microbiol. Biotechnol. 1997, 47, 441. (2) Uslu, H.; Inci, I. Liquid + Liquid Equilibria of the (Water + Propionic Acid + Aliquat 336 + Organic Solvents) at T = 298.15 K. J. Chem. Thermodyn. 2007, 39, 804. (3) Zhu, L.; Wei, P.; Cai, J.; Zhu, X.; Wang, Z.; Huang, L.; Xu, Z. Improving the Productivity of Propionic Acid with FBB-Immobilized Cells of an Adapted Acid-Tolerant Propionibacterium acidipropionici. Bioresour. Technol. 2012, 112, 248. 2692
