Fractionation of Carboxylic Acids Mixture Obtained by P. acidipropionici Fermentation Using...

Fractionation of Carboxylic Acids Mixture Obtained by P.acidipropionici Fermentation Using Pertraction with tri‑n‑Octylamineand 1‑OctanolDan Cascaval,† Madalina Postaru,† Anca-Irina Galaction,‡,* Lenuta Kloetzer,†

and Alexandra Cristina Blaga†

†Gheorghe Asachi Technical University of Iasi, Faculty of Chemical Engineering and Environmental Protection, Dept. of BiochemicalEngineering, 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 ofacetic and succinic acids from the mixture, the feed phase raffinate containing only propionic acid. The pH-gradient between thefeed and stripping phases, the carrier concentration in the liquid membrane, and the addition of 1-octanol control the selectivityof acids pertraction, with TOA concentration exhibiting the most important influence. In the absence of 1-octanol, at a pH-valueof feed phase = 2, pH-value of stripping phase = 10, the maximum selectivity factor (S = 25) was reached for the carrierconcentration 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 pertractionsystem containing 1-octanol is compensated by diminishing the material consumption required for separation.

■ INTRODUCTION

Propionic acid is a monocarboxylic acid with numerousapplications in chemical industry (reagents, plasticizers,solvents, emulsifying agents, monomers, resins, paints, electro-plating solutions), agriculture (herbicides, mold preventing),pharmaceutical (antiarthritic drugs), and food industries(preservatives, acid or salts, antifungal agent, fruits artificialflavors), as well as for perfumes production.1,2

Propionic acid is commercially produced using liquefiedpetroleum gas, namely propane or ethylene, by chemicalsynthesis via propionaldehyde.2−4 The cost of this technologyincreased in the last years, due to the increasing price of theliquefied petroleum gas, and depends on the acid desired purity.Moreover, the separation of propionic acid at industrial scalerequires high consumption of lime and sulfuric acid andproduces important amounts of acidic wastewaters and solidwastes of calcium sulfate sludge.5

Due to the difficulties of the chemical synthesis and to thedemands for implementing at larger-scale of the new eco-friendly technologies, the interest in producing propionic acidby low-cost processes of fermentation has increased in the lastyears. The bacteria of genuses Propionibacterium (P. acid-ipropionici, P. acnes, P. arabinosum, P. shermanii), Clostridium (C.propionicum), Veillonella, and Selenomonas species have beentested, but only the strain P. acidipropionici has been consideredpromising for industrial applications.1,6,7 The final fermentationbroth is a mixture of carboxylic acids containing propionic acid,as the main product, and secondary acids (especially, succinicand acetic acids).2,6,7 The selective separation of these acidsfrom the biosynthetic mixture is achieved by precipitation as

calcium salts, ionic exchange followed by elution andcrystallization, but with high energy and materials costs.Although the liquid−liquid extraction represents an acces-

sible and efficient alternative for many downstream processes inbiotechnology, its application for ionizable compounds,particularly the carboxylic acids, is less efficient due to theirlow solubility in the usual hydrophobic organic solvents. Theextraction yields of propionic and succinic acids in hydrophobicorganic solvents are less than 10%, while the maximumextraction degree for acetic acid is reached for aliphatic alcoholswith over four carbon atoms (30−37%).8 As it was previouslyconcluded, the efficiency of liquid−liquid extraction of theseacids can be improved by adding tri-n-octylamine into theorganic phase, the process being called reactive extraction.9,10

The physical or reactive extraction represents the basis of thedevelopment of a rather new separation technique, namelypertraction or permeation through liquid membranes. Per-traction consists in the transfer of a solute between two phasesseparated by a solvent layer, the driving force being the gradientof property (pH, concentration, etc.) between the feed andstripping phases.11,12 By comparison to the liquid−liquidextraction, the use of pertraction diminishes the loss of solventduring the separation cycle, requires a small quantity of solventand carrier because of their continuous regeneration, and offersthe possibility of solute transport against its concentrationgradient, as long as the pH-gradient between the two aqueous

Received: August 31, 2012Revised: January 22, 2013Accepted: January 25, 2013Published: January 25, 2013

Article

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

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phases is maintained.11,12 The addition of a carrier in the liquidmembrane, such as organophosphoric compounds, long chainamines or crown-ethers, etc., could lead to the significantimprovement of the pertraction efficiency and selectivity.In the mentioned context, in this work, the possibility to

separate selectively propionic, succinic, and acetic acids fromthe aqueous solution containing their mixture obtained byfermentation with P. acidipropionici using facilitated pertractionwith tri-n-octylamine (TOA) in presence of 1-octanol has beeninvestigated. In this purpose, the influences of the pH-gradientbetween the aqueous phases, the carrier concentration in theliquid membrane, and the addition of 1-octanol on theefficiency and selectivity of pertraction have been analyzed.

■ MATERIALS AND METHODS

The experiments have been carried out using the pertractionequipment that allows obtaining and maintaining easily thesolvent layer between the two aqueous phases. The pertractioncell and operating parameters have been described in previouspapers.13

The experiments have been carried out in a pseudo-steady-state regime, at the steady-state conditions related to theaqueous phases and unsteady-state mode related to themembrane phase. The aqueous solutions have been separatelyfed 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 concentrationvarying 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 themembrane 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) succinicacid, and 5 g L−1 (0.08 M) acetic acid.15 The pH-value of thefeed phase varied between 1 and 7, the pH adjustment beingmade with solution of 3% sulfuric acid or 3% sodium hydroxide,based on the prescribed pH-value. The stripping phasesconsisted of solutions of sodium hydroxide with pH = 7−12.The pH-values of both aqueous phases were determined usinga digital pH-meter of Consort C836 type and have beenrecorded throughout each experiment. Any pH change wasrecorded during the extraction experiments.The pertraction process was analyzed based on the initial and

final mass flows of the carboxylic acids and permeability andselectivity factors, previously defined.13 For calculating theseparameters, the acids concentrations in the feed and strippingphases have been measured and the mass balance for thepertraction system has been used. Propionic, succinic, andacetic acid concentrations have been determined by highperformance liquid chromatography technique (HPLC, StarVarian Chromatography Workstation) with a PL Hi-Plex Hcolumn (7.7 mm diameter, 300 mm length, 8 μm porousparticle), provided with UV Prostar 330 PDA detector.10 Themobile phase was a solution of 0.1% trifluoroacetic acid with aflow rate of 0.6 mL/min. The analysis has been carried out at60 °C.Each experiment has been performed in triplicate, the

average value of the considered parameters being used. Themaximum experimental error was of ±6.11%.

■ RESULTS AND DISCUSSIONThe pertraction efficiency is strongly influenced by the pH-gradient between the two aqueous phases, carrier concentrationin liquid membrane, and phase mixing intensity. For theseparation of carboxylic acids obtained by propionic acidfermentation, the influence of the pH-gradient between thephases is amplified by the ionization-protonation of these acidsin the aqueous solution, these processes controlling theefficiency of extraction and re-extraction, as well as the rateof 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-

value of feed phase, pHF, induces the decrease of acids initialmass flows, due to the reduction of the efficiency of reactiveextraction with TOA at the interface between the feed phaseand liquid membrane. The decreasing of extraction efficiencywith the pHF increase is the consequence of the carboxylicgroup dissociation, process that is slower for pHF values lessthan 3. This variation of extraction efficiency, and, implicitly, ofthe initial mass flows is the result of the dissociation ofpropionic and acetic acids to their single carboxylic group andto the partial dissociation of succinic acid to one carboxylicgroup (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 theinterfacial mass transfer rate are correlated with the solutesacidity, because the acidity controls the rate of interfacialreaction between solute and carrier. However, the efficiency ofreactive extraction is also controlled by the structure of theinterfacial compounds formed between solute and carrier,respectively, by the number of carrier molecules stoichiometri-cally needed. According to the previous results on reactiveextraction in dichloromethane,9,10 the interfacial interactionsbetween the acid and the carrier could be of hydrogen bondingtype with the undissociated carboxylic groups, or of ionic type,if the acid dissociates in the aqueous solution:

+ ⇄ ·R(COOH) pQ R(COOH) Qm(aq) (o) m p(o)

where m = 1 for acetic and propionic acid and m = 2 forsuccinic acid.Thus, at pHF < 4, the studies on reactive extraction of these

acids with TOA in dichloromethane without 1-octanol

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

Industrial & Engineering Chemistry Research Article

dx.doi.org/10.1021/ie302339z | Ind. Eng. Chem. Res. 2013, 52, 2685−26922686

indicated that the structures of the interfacial compound areRCOOH·Q2 for propionic acid, RCOOH·Q for acetic acid, andR(COOH)2·Q2 for succinic acid (Q symbolizes the carrier).9,10

The highest rates of mass transfer from feed phase tomembrane have been recorded for acetic acid, the initial massflows decreasing then to succinic and propionic acids,respectively. In this case, the corresponding order of the acidsinitial mass flows is the consequence of the cumulated effects ofthe decreasing of acidity of carboxylic groups from succinic topropionic acid (considering the first step of succinic aciddissociation) and of the increased complexity of acid−carriercompound 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 thecorresponding 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 theextracted 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 forall three carboxylic acids, the yields of acids extraction and re-extraction yields becoming closer for neutral values of pHF, dueto the low amounts of acids transferred into the membranephase (Figure 2). Moreover, Figure 2 suggests two domains of

permeability factors variation. Thus, for pHF below 4, theplotted dependences indicate that the transport capacity ofliquid membrane is positively influenced by the acidity oftransferred solute but affected by the complexity of extractedcompound structure.Both variations of permeability factors are controlled by the

rate of reaction between acid−carrier compound and sodiumhydroxide at the interface separating the membrane andstripping phases. Obviously, the increase of solute acidity andcomplexity of extracted compound structure lead to theappearance of a kinetic resistance to the re-extraction process.Therefore, at pHF < 4, in absence of 1-octanol in liquidmembrane, the simplest structure of the extracted compoundcorresponds to acetic acid, RCOOH·Q.9 In this case, thecorresponding order of the acid permeability factors is theconsequence of the cumulated effects of the increasedcomplexity of chemical structure of the extracted compoundand of acidity of carboxylic groups from propionic to succinicacid.

For pHF > 4, due to the partial dissociation of succinic acid,the structure of the extracted compound is modified andbecomes R(COOH)2·Q, similar to that of the product formedby interfacial reaction between acetic acid and TOA. Moreover,the acidity of the second carboxylic group of succinic acid is thelowest one compared to the other two acids (Ka2 = 2.45 × 10−6

at 25 °C14). In these circumstances, the relative amplification ofthe final mass flow becomes more relevant for succinic acid, itspermeability factor exceeding those recorded for acetic andpropionic 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 FPhave been considered.16

The dependence of factor FN on the feed phase pH, plottedin Figure 3, suggests that the addition of 1-octanol exhibits a

positive effect on acids mass flows. Although for all studiedacids the factor FN, calculated either for the initial mass flows orfor the final ones, is greater than the unit and increases for theentire considered pHF domain, the magnitude of pHF influenceis different, the highest values of factor FN being reached forpropionic acid. Thus, for pHF variation from 1 to 7, FNcalculated for the initial mass flows increased for about 1.6times for acetic acid, 1.9 times for succinic acid, and over 2.6times for propionic acid, respectively. For the same pHFvariation, the values of FN related to the final mass flowsincreased for about 1.4 times for acetic acid, 1.7 times forsuccinic acid, and 2.1 times for propionic acid. These results arethe consequence of the favorable effect of 1-octanol on thesolubilization of acids molecules, free or bounded to the carriermolecules, on the membrane phase. The increase of pHFinduces the dissociation of carboxylic acids in the feed phase,the presence of 1-octanol improving the solubilization also ofthe dissociated molecules of acid. Compared to acetic andsuccinic acids, the more important influence of 1-octanoladdition on propionic acid mass flows is due especially to themodification of the structure of the extracted compound.Therefore, the structure of the compound resulted from theinterfacial reaction between propionic acid and TOA becomesRCOOH·Q, less complex than in absence of this alcohol.10

Consequently, the extraction rate of this acid is stronglyenhanced.

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

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



The addition of 1-octanol also influences rather significantlythe mass flows of succinic acid, as the result of the solubilizationof 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 initialmass flows, due to the supplementary kinetic resistance to theacid re-extraction process from the membrane phase to thestripping solution.Contrary, the values of factor FP are lower than 1 for the

entire experimented domain of the feed phase pH, the increaseof pHF inducing the reduction of this factor (Figure 4). In all

cases, the increase of the initial mass flows due to the additionof 1-octanol inside the liquid membrane exceeds the membranecapacity to transport the acids and to release them into thestripping phase. Obviously, for the above-discussed reasons, thiseffect is more important for propionic acid; at pHF = 6, itspermeability factor in presence of 1-octanol is 2.3 times lowerthan 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 rateof sodium salts formation and, implicitly, of acids re-extractionfrom the membrane phase. Therefore, the final mass flows ofthe three carboxylic acids are accelerated (Figure 5). Due to theamplification of their concentration gradient between the feedand stripping phases, the acid initial mass flows are also

intensified. However, as it can be seen from Figure 5, the initialand final mass flows are significantly increased only in the pHSdomain varying from 8 to 11. At lower values of pHS, bothfluxes tend to 0; at higher pHS-values, they remain at a ratherconstant level. The order of the mass flows is similar to thatrecorded 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).

This variation suggests that the acceleration of final mass flowbecomes more important as compared to that of the initial massflow, due to the more important increase of the acids re-extraction 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 fromone acid to another, the highest permeability factor beingreached for propionic acid.As it was above-discussed, this sequence is the result of the

increment of kinetic resistance to the re-extraction process frompropionic acid to succinic acid, due to the increase of acidity,and, therefore, of the strength of the acid−carrier bond. Thisdifferentiation is more important at lower concentrations ofsodium hydroxide in the stripping phase.At higher pHS-values, respectively at higher concentration of

the re-extraction agent, the magnitude of the effects of higheracidity or complexity of extracted compound structure issignificantly diminished. In this domain of pHS, the values ofpermeability factors of acetic and succinic acids exceed that ofpropionic acid, as the consequence of the superior increase ofthe first two acids final mass flows related to that of propionicacid. This phenomenon is the result both of the higher amountsof acetic acid−TOA and succinic acid−TOA compoundsextracted in the liquid membrane and of the superior acidityof 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 ofpHF. These factors are superior to 1 for all experimented pH-values of stripping phase, with greater values being recorded forpropionic acid. The factor FN related to the initial mass flowsincreased for about 1.4 times for acetic acid, 1.5 times forsuccinic acid, and 1.7 times for propionic acid. The values of FNcorresponding to the final mass flows increased for about 1.5times for acetic acid, 1.6 times for succinic acid, and 1.8 timesfor propionic acid. The magnitude of the effect induced by 1-

Figure 4. Influence of pH-value of feed phase on factor FP (carrierconcentration = 200 g L−1, pH of stripping phase = 10).

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

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



octanol addition into the liquid membrane is more importantfor the final mass flows, because the influence of pHS on theacid re-extraction step from the membrane phase is stronger.However, for pHS values over 10, FN calculated for the finalmass flows remains at a rather constant level, a phenomenonthat could be associated with the achievement of the maximumcapacity of liquid membrane to transport the acids from thefeed phase to the stripping one. Therefore, for pHS values over10, the acceleration of re-extraction rates does not sustain thehigher value of the extraction rates induced by 1-octanoladdition.Although its values are below 1 for the entire studied domain

of stripping phase pH, the variation of FP, plotted in Figure 8,

suggests the positive influence of pHS increases. This effect ismore pronounced for the solutes with higher acidity, namely,acetic and succinic acids. Moreover, due to the limited transportcapacity of liquid membrane, for all three acids, FP reaches themaximum value at pHS = 11 and then decreases for higher pHS.The maximum of FP is more evident for acetic and succinicacids.The carrier concentration inside of the liquid membrane

induces different influences on the pertraction efficiency ofthese carboxylic acids. According to the previous results onreactive extraction of acetic, succinic, and propionic acids withTOA, the difference on carrier influence is due to the different

acid extraction mechanisms, as well as to the difference ofsolutes acidity and hydrophobicity of the extracted com-pounds.9,10

The increase of TOA concentration exhibits a favorable effecton the mass transfer of all three acids, due, on the one hand, tothe increase of the concentration of one reactant (carrier) tothe interface between the feed and membrane phases and, onthe other hand, to the accumulation of the interfacialcompound inside the liquid membrane (Figure 9). However,

by increasing the carrier concentration in the membrane phase,it can be observed that the carboxylic acids are extracted fromthe feed phase in the following succession: acetic acid, succinicacid, and propionic acid, respectively. Therefore, for TOAconcentration below 30 g L−1 (0.084 M), only acetic acid istransferred from the feed phase to the membrane one becausethe carrier reacts first with the solute with higher acidity andthat forms the simplest interfacial compound. In the absence of1-octanol, this level of TOA concentration corresponds to thestoichiometric need for the formation of interfacial compoundRCOOH·Q with acetic acid,9 the influence of the supple-mentary increase of carrier concentration on this acid initialmass flow becoming insignificant.Practically, the pertraction of succinic acid becomes possible

for carrier concentration over 30 g L−1, this acid initial massflow increasing strongly for TOA concentration variation from30 to 70 g L−1 (0.20 M). Because without 1-octanol thestructure of the compound formed by the interfacial reactionbetween succinic acid and TOA at pHF = 2 is R(COOH)2·Q2,the superior limit of this domain of carrier concentrationrepresents the amount stoichiometrically needed for reactingfirst with acetic acid and then with succinic acid, both acidsfrom the feed phase. As in the case of acetic acid pertraction,higher values of TOA concentration do not induce importanteffect on succinic acid mass flow from the feed phase to theliquid membrane.In these circumstances, due to its lower acidity and superior

complexity of the formed compound with the carrier, propionicacid is extracted from the feed phase only after the TOAconcentration exceeds the sum of those stoichiometricallyrequired for reacting with the other two acids. Because inabsence of 1-octanol in the membrane phase, the structure ofthe interfacial product between propionic acid and TOA is

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

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

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



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 ispossible only by physical solubilization in dichloromethane, butin this case, the acid mass flows are very low. These resultssuggest the major influence of the TOA concentration insidethe 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 tothe initial ones with the increase of carrier concentration.The acid permeability factors have a particular evolution with

TOA concentration increases. These parameters initiallydecrease from a value corresponding to the absence of TOAin the membrane phase to a minimum value, reached at theconcentration of 10 g L−1 TOA for succinic acid and 20 g L−1

for acetic and propionic acids, finally increasing concomitantlywith the carrier concentration (Figure 10). This variation could

be the result of the changes in the relative rate of the chemicalreactions at the separation interface between the liquidmembrane and stripping phase. In the absence of the carrier,the extraction and transport of the solute through the liquidmembrane occurs only by the physical process of solubilization,the limiting steps of the overall separation process being ofdiffusional type. The addition of TOA in dichloromethane leadsto the change of separation mechanism. Due to the chemicalreactions between the acids and the carrier at the feed phase−liquid membrane interface, as well to the chemical reactionsbetween acids−carrier compounds and sodium hydroxide at theliquid membrane−stripping phase interface, the additionallimiting steps of the kinetic type appear. Moreover, because theacids do not participate in free acid form to the re-extractionprocess (they are combined with the carrier), the rate ofsodium salt formation is diminished. Consequently, the finalmass flows will be initially smaller for the facilitated pertractionprocess as compared to the free pertraction.Because the yields of physical extraction of acetic and

propionic acids are higher than that of succinic acid, theamounts of free acids extracted in the membrane phase aresuperior for the two monocarboxylic acids. For this reason, thevalues of TOA concentrations corresponding to the minimumpermeability factors for monocarboxylic acids are higher thanthe value recorded for succinic acid.

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

increase of carrier concentration into the liquid membraneexhibits a negative influence on this factor, explained by thediminution of the magnitude of carrier concentration influencein presence of 1-octanol. The highest values of FN have beenreached for propionic acid, especially due to the reduction ofcarrier molecules number stoichiometrically needed to reactwith 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 alcoholaddition on the acids free pertraction, respectively, on theirsolubilization into the membrane phase and, implicitly, on theiramount re-extracted into the stripping solution (Figure 12).

However, by adding TOA and increasing its concentration, FPdecreases and becomes lower than the unit. Similar to theabove-discussed effects, the increasing of the relative con-tribution of the physical coextraction of these acids to theirtransport from the feed phase by 1-octanol addition leads to theoverflow of the transport capacity of liquid membrane and,consequently, to the acids accumulation inside the membranephase. The effect is more important for acetic acid, due to itssuperior extractability into membrane phase compared to theother two acids.

Figure 10. Influence of carrier concentration on acetic, succinic, andpropionic acid permeability factors (pH of feed phase = 2, pH ofstripping phase = 10, rotation speed = 500 rpm).

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

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



The presented experimental data suggest that these threeacids can be selectively separated by facilitated pertraction withTOA from their biosynthetic mixture. Therefore, acetic andsuccinic acids can be selectively removed from the feed phaseby pertraction, while propionic acid remains in this phase. Thepertraction efficiency and selectivity can be modified by adding1-octanol inside the liquid membrane.To establish the required conditions for reaching high

selectivity of separation, the influences of pH-gradient on theaqueous phases, carrier concentration, and mixing intensity onpertraction selectivity have been studied in direct relation with1-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 cumulatedfinal mass flows of acetic and succinic acids, and the final massflow of propionic acid:

=+

Sn n

na a

a

f.acetic.acid f.succinic.acid

f.propionic.acid (1)

As it can be observed from Figure 13, for the pertraction systemwithout 1-octanol, the increase of the pH-gradient between thefeed and stripping phases leads to the increase of selectivityfactor. According to the discussed effects of pH of feed andstripping phases on acids mass transfer, this variation is theresult of the more important positive influence of lower pHF-values on extraction rate and of higher pHS-values on re-extraction 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 thoserecorded for the pertraction system without alcohol. However,the influence of the stripping phase pH on the pertractionselectivity becomes contrary to that plotted in absence of 1-octanol. The variations of selectivity factor with pHF and pHSincreases are the result of the more important positive effect of1-octanol addition on propionic acid pertraction, from thereasons discussed.The decisive influence of carrier concentration on pertraction

selectivity is underlined by the dependence between theselectivity factors, and this parameter is plotted in Figure 14.Thus, in absence of 1-octanol, the experimental results indicatethat the maximum selectivity (S = 25) is reached for 70 g L−1

TOA in liquid membrane. This concentration value corre-sponds to the stoichiometry of the reaction with acetic andsuccinic acids.

Due to the solubilization of supplementary amounts of acidsor of their interfacial compounds, as well as to the reducing ofthe structure complexity of the extracted compounds,phenomena that are more important for propionic acid, thepertraction selectivity is affected by 1-octanol addition in themembrane phase. Moreover, the carrier concentration relatedto the maximum value of selectivity factor (S = 19) is reducedto 50 g L−1, as the consequence of the decreasing of the carriermolecules number participating to the interfacial reaction withacetic and succinic acids.Therefore, more important increases of the selectivity factor

can be achieved by optimization of the carrier concentrationcompared to the modification of the aqueous phases pH-values.

■ CONCLUSIONSThe 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 toseparate selectively these acids from their biosynthetic mixture.Thus, acetic and succinic acids can be transferred from the feedphase through liquid membrane to the stripping phase, whilepropionic 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 phasesinduced positive effects on separation selectivity. The carrierconcentration inside the liquid membrane exhibited the mostimportant influence on the pertraction selectivity, the maximum

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

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



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 allconsidered acids. Because this effect is more important forpropionic acid, the selectivity factor was reduced compared tothe pertraction without the alcohol. In this case, the highestselectivity 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 1-

octanol, it can be concluded that the use of 1-octanol allowedto reaching the maximum selectivity with lower materialconsumption (pHS close to the neutral domain, lower amountsof carrier). This advantage could compensate the inferior valuesof 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 pertractionof carboxylic acids obtained by propionic acid fermentation,greater values of selectivity factors can be obtained. In thispurpose, on the basis of these results, the aim of the futurework is to establish a mathematical model describing theinfluences of the considered parameters on the pertractionselectivity and to optimize it for finding the operatingconditions corresponding to the maximum selectivity factor.Moreover, these results will be verified for the real fermentationbroths, because the presence of some cellular or biosyntheticcompounds (proteins, amino acids, etc.) could affect thepertraction efficiency or change some of the separationconditions.

■ AUTHOR INFORMATION

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

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the Grant No. PN-II-ID-PCE-2011-3-0088 authorized by The National Council for ScientificResearch−Executive Unit for Financing Higher Education,Research, Development, and Innovation (CNCS-UEFISCDI)

■ NOMENCLATUREn = 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 factorS = selectivity factor

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Fractionation of Carboxylic Acids Mixture Obtained by P. acidipropionici Fermentation Using...

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