Electrolytic Membrane Extraction Enables Production of FineChemicals from Biorefinery SidestreamsStephen J. Andersen,†,⊥ Tom Hennebel,†,‡,⊥ Sylvia Gildemyn,† Marta Coma,† Joachim Desloover,†

Jan Berton,§ Junko Tsukamoto,§ Christian Stevens,§ and Korneel Rabaey*,†

†Laboratory of Microbial Ecology and Technology, Ghent University, Coupure Links 653, B-9000 Ghent, Belgium‡Department of Civil and Environmental Engineering, 407 O’Brien Hall, University of California, Berkeley, California 94720-1716,United States§SynBioC, Department of Sustainable Organic Chemistry and Technology, Ghent University, Coupure Links 653, B-9000 Ghent,Belgium

*S Supporting Information

ABSTRACT: Short-chain carboxylates such as acetate areeasily produced through mixed culture fermentation of manybiological waste streams, although routinely digested to biogasand combusted rather than harvested. We developed a pipelineto extract and upgrade short-chain carboxylates to esters viamembrane electrolysis and biphasic esterification. Carboxylate-rich broths are electrolyzed in a cathodic chamber from whichanions flux across an anion exchange membrane into an anodicchamber, resulting in a clean acid concentrate with neithersolids nor biomass. Next, the aqueous carboxylic acidconcentrate reacts with added alcohol in a water-excludingphase to generate volatile esters. In a batch extraction, 96 ±1.6% of the total acetate was extracted in 48 h from biorefinery thin stillage (5 g L−1 acetate) at 379 g m−2 d−1 (36% Coulombicefficiency). With continuously regenerated thin stillage, the anolyte was concentrated to 14 g/L acetic acid, and converted at 2.64g (acetate) L−1 h−1 in the first hour to ethyl acetate by the addition of excess ethanol and heating to 70 °C, with a final totalconversion of 58 ± 3%. This processing pipeline enables direct production of fine chemicals following undefined mixed culturefermentation, embedding carbon in industrial chemicals rather than returning them to the atmosphere as carbon dioxide.

■ INTRODUCTION

Both microbial fermentation and petrochemical production cangenerate industrially valuable building block chemicals. Thepetrochemical industry dominates chemical production becauseof the relatively low cost of the oil and gas substrates and well-entrenched separation and recovery technology, such as liquid−liquid extraction, adsorption, distillation, and more. So-calledbiorefineries can utilize microorganisms in fermentationprocesses to generate sustainable products from biologicalresources. Genetically modified organisms and single-straincultures can generate products with high specificity at relativelyhigh production rates, but such processes are cost-intensive andthereby limit the application to high-value products only. Mixedculture fermentations can be operated at lower costs on morecomplex feedstocks, including various bioindustrial wastes andsyngas, but generally without specificity at biologically con-strained rates.1−5 Example products include acetate, butyrate,caproate, and other short-chain carboxylates, commonintermediates in the anaerobic digestion of organic wastes tobiogas. These are industrially valuable building block chemicals,though often not valuable enough to overcome the cost ofrecovery from low titer broths. The separation and recovery of

fermentation products, even from highly specific pure cultures,can account for over 60% of the total plant costs, thus providingan incentive for low cost separation technologies.6 Alternatively,biogas can be generated from a broad range of organic waste,and though it is significantly less valuable, it is easily extractedand utilized, a key factor in the success of anaerobic digestion.Separation and recovery processes tailored to specific

compounds and low titer broths can improve the competitive-ness of biorefineres for both pure and mixed culturefermentation processes.7,8 We present a bioproductionalternative to digestion leading to nonfuel outcomes in abiorefinery context, a processing pipeline to recover andupgrade the ubiquitous short-chain carboxylate fermentationproducts as high value esters. The processing pipeline extractscarboxylates directly from a broth and upgrades SCFAs toesters in a second step. Membrane electrolysis (ME) generatesa concentrate stream that is delivered to a reactive extraction

Received: January 28, 2014Revised: May 12, 2014Accepted: May 21, 2014

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step, biphasic esterification (BE) (Figure 1). The ME process isa two-chamber electrochemical water treatment step in whichthe fermentation broth or carboxylate-rich stream is electro-lyzed by a cathode and the applied electrical potential drawscarboxylate ions across a single polymeric anion exchangemembrane (AEM) into a clean, highly saline, and low pHanolyte, without solids nor biomass. Ion exchange membranesare designed for selective transport of ions, and an AEM iscapable of excluding solids, microorganisms, and unchargedmolecules larger than the effective pore size of the membrane.A number of carboxylate ions have been demonstrated to crossAEMs: for example, synthetic lactate salts.9,10

ME stands apart from conventional electrodialysis andbipolar electrodialysis in which the treated streams are notengaged in the electrochemical reactions. In addition, only asingle membrane is required for ME, whereas electrodialysisrequires many.11 The ME process generates hydroxide ions andhydrogen gas in the broth (cathodic electrolysis) and protonsand oxygen gas in the extractant (anodic electrolysis). Thehydroxide ions can replace caustic soda management of afermentation broth. In the anode chamber, the extracted ionicfermentation product is protonated to its acidic conjugate byelectrolytically generated protons. This product is thenesterified in the BE step, increasing the volatility of the productfor separation. Conventional esterification is generallyperformed in the complete absence of water or in organicsolvents, as water kinetically constrains the esterificationreaction.12

The BE process consists of an unreactive water exclusionphase (e.g., solvent or ionic liquid) and an extractant (anolyte)phase. Esterification proceeds by excluding the watercomponent following the addition of excess alcohol and heat.In a fully realized process, the higher-value, volatile esters areextracted from a low-affinity, nonvolatile, water-exclusion layer,such as an ionic liquid, and the aqueous extractant is returnedto the anode to accrue more fermentation product.The objective of this research was to present and

demonstrate a viable processing pipeline for extracting andupgrading short-chain carboxylates. As presented, ME and BEare two interdependent processes that enable ester productiondirectly from fermenter broths. Acetate was used as the model

carboxylate for the ME and BE pipeline because acetate isamong the most common fermentation products. The MEprocess was demonstrated at low current density with a low-cost membrane and then with a higher cost, proton-excludingmembrane at a greater range of current densities. The anodesalinity was investigated as a means of maximizing carboxylateflux in broths in which the relatively low concentration ofproduct results in low Coulombic efficiency extraction. The MEand BE pipeline was ultimately demonstrated with an acidconcentrate extracted from biorefinery thin stillage, and withthis concentrate, BE was demonstrated with xylene and axylene/ionic liquid mixture. The ME and BE pipelinesuccessfully extracted acetate from biorefinery thin stillageand upgraded the product to methyl acetate and ethyl acetate.

■ MATERIALS AND METHODSElectrochemical Cells. Electrochemical cells (two cham-

bers separated by a membrane, each with internal dimensions80 × 80 × 20 mm) were used as previously described,13 withspacer material (ElectroCell A/S, Denmark) between thesurface of the electrode and the AEM. Membranes (AM-7001Anion Exchange Membranes, Membranes International Inc.,Ringwood, NJ; and fumasep FAB, FumaTech GmbH,Germany) were pretreated in accordance with manufacturerspecifications. The anode was an Ir MMO-coated titaniumelectrode (IrO2/TaO2; 0.65/0.35), 80 mm × 80 mm, with acentrally attached, perpendicular current collector (MagnetoSpecial Anodes BV, The Netherlands). The cathode was astainless steel wire mesh with a stainless steel sheet metalcurrent collector. For high-current, high-concentration experi-ments only (Supporting Information (SI) Figure S2), a low-volume two-chamber electrochemical cell was used (internaldimensions 70 × 10 × 20 mm with 50 × 10 mm effectivemembrane area) with a single pass of electrolytes at 1.54 L d−1

per chamber. The anode was an Ir MMO-coated titaniumelectrode (IrO2/TaO2; 0.65/0.35), 50 mm × 20 mm (MagnetoSpecial Anodes BV, The Netherlands), and the cathode wasstainless steel, 60 mm × 40 mm.The anolyte consisted of sodium sulfate, corrected to pH 2

with sulfuric acid. The synthetic catholyte consisted of a sodiumacetate, sodium butyrate, or sodium caproate solution (Sigma-

Figure 1. Schematic for carboxylate extraction from a fermenter by membrane electrolysis and biphasic esterification. Fermentation generates shortchain carboxylates that are driven across the anion exchange membrane by an applied current and are protonated. Alcohol and heat react with thecarboxylic acid in the water exclusion phase to generate esters.

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Aldrich, Belgium), corrected to pH 5.5 with sulfuric acid priorto operation. Fermenter broth from bioethanol production wassourced from the thin stillage of Alco Bio Fuel NV (Gent,Belgium). To avoid clogging within the laboratory scaleconnectors and to allow consistency in the reactor design,this broth was centrifuged at 10 000 rpm for 10 min, andsuspended solids were removed. This broth was dosed withsodium acetate to bring the concentration to 5 g L−1 acetate tosimulate conditions after carboxylate fermentation. For aconsistent concentration of acetate, active fermentation wasavoided by centrifugation of the thin stillage, a low residencetime in the cathode chamber, and daily replacement of the feed.The initial concentration of acetate in the broth was only 0.7 gL−1 because the fermentation process is focused on bioethanolproduction. In all tests, sodium sulfate salt was used in theelectrolyte rather than sodium chloride to prevent theformation of chlorine gas at the anode while increasing theconductivity.For continuous experiments, the electrochemical cells were

fed at a rate of 0.8 L d−1 to achieve a residence time of 5 h. Theanode and cathode batch experiments were performed withequal volumes of electrolyte at 1 L. Both compartments werecontinuously stirred at a recirculation rate of 6 L h−1.All electrochemical experiments were controlled with a VSP

multipotentiostat (Princeton Applied Research, France) and anAg/AgCl reference electrode (+0.197 V vs SHE, PrincetonApplied Research, France) in the cathode compartment. Theapplied current is reported as current density, defined as the setcurrent divided by the exposed surface area of the anionexchange membrane. The anode reaction was the oxidation ofwater (2 H2O → 4 H+ + O2 + 4e−), which replenishes thesulfuric acid/sulfate medium. The cathodic reaction was thereduction of water (2 H2O + 2e− → H2 + 2 OH−).Membrane Electrolysis. ME was tested with continuously

fed synthetic solution for flux characterization, as furtherdescribed below. Acetate, butyrate, or caproate in syntheticbroths were tested for flux through an AEM at various currentdensities. For two synthetic experiments with acetate, theanolyte was altered. In one experiment, the concentration ofsodium sulfate was increased in the anode to observe the trendof the flux of acetate relative to anode salinity. Similarly, theconcentration of sodium acetate at pH 2 was increased in theanode to observe the trend of the acetate flux relative to acetateaccumulation as acetic acid. For these experiments, the fluxeswere tested at 20 A m−2 only.Next, the process was tested with thin stillage from a

bioethanol plant to demonstrate the extraction process with areal matrix and native ions. By using this broth, we were able todemonstrate the full pipeline from a consistent and practicalsource fluid. Both the cathode compartment (broth) and theanode compartment (extractant) were first tested in a batchextraction. The cathode was then continuously fed while theanode compartment was run in batch to better mimic realisticoperating conditions (accumulation in the anode, excess brothrelative to extractant). For these experiments, the currentremained set at 20 A m−2.In all measurements of flux, the cell was set at 1.6 mA m−2 (a

galvanostatic setting of −0.1 mA controlled at the cathode) forat least 4 h prior to the run to allow for electrode andmembrane polarization, then was changed to the set currentdensity of interest. Hydroxide ions generated by waterreduction were mitigated by 1 M sulfuric acid by a pHcontroller (Consort, Belgium).

Carboxylate concentrations were measured by gas chroma-tography (GC), as described in the Supporting Information.All ME experiments were performed in triplicate, at a

minimum.Biphasic Esterification. Esterification experiments were

performed with both a synthetic anolyte and a real (i.e.,generated in a previous experiment) anolyte. The syntheticanolyte consisted of 20 g L−1 acetate (as acetic acid, pH 2) and0.5 M Na2SO4. In all cases, 20 mL of anolyte was used with axylene (Sigma-Aldrich, Belgium) solvent layer of 20 mL in a100 mL bottle for approximately 60 mL of headspace. Thelayered liquids were vacuumed to a slight under-pressure andheated to 70 °C. At t = 0 h, 5 mL of either ethanol, methanol,or distilled water (control) was injected into the biphasicsolutions. The reactors were then stirred and maintained at 70°C for 20 h. The aqueous fraction was sampled at t = 0 and t =20 h and analyzed for carboxylate concentration by GC. TheGC conditions are described in the Supporting Information.These BE experiments were performed in quadruplicate.Biphasic esterification was also performed with an ionic

liquid for water exclusion. The ionic liquid was trihexylte-tradecylphosphonium bis(trifluoromethylsulfonyl) amide(Sigma-Aldrich, Belgium). Real anolyte (6 mL), xylene (2mL), and ionic liquid (1 at 1.07 g/mL) were heated to 70 °Cand stirred in a high-pressure flask (15 mL). Ethanol (roomtemperature, 1 mL) was added at t = 0, and the xylene wastested for esters after t = 5 min, 30 min, 3 h, 18 h, and 24 h byGC/MS, as described in the Supporting Information.

■ RESULTSMembrane Electrolysis Extraction. In the first phase, we

demonstrated the basic ME concept on synthetic solutions withvarious concentrations of the model carboxylate, acetate(Figure 2a,b), and other carboxylates of interest, butyrate and

Figure 2. Extraction profile for acetate across a MembraneInternational AMI-7001 anion exchange membrane (MI) from astarting catholyte concentration of 1, 5, and 10 g/L acetate. (A) Massflux per area of membrane per day, (B) Coulombic efficiency for thesame data points. The error bars are occasionally obscured by themedallions.

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caproate (SI Figure S1), from 0 to 30 A m−2 projectedmembrane surface. For an initial catholyte concentration of 10g L−1, the flux of acetate was 1.05 kg m−2 d−1 at 20 A m−2,corresponding to a Coulombic efficiency of 99.4 ± 0.1%. At 30A m−2, the maximum flux of acetate was 1.38 ± 0.03 kg m−2

d−1, corresponding to a Coulombic efficiency of 87.0 ± 1.8%.The extraction efficiency was lower for 1 and 5 g L−1 acetatebroth, corresponding to less availability of the target anion inthe bulk (molar concentration), and similarly, butyrate andcaproate show a low flux rate and Coulombic efficiency (SIFigure S1). Transport limitations that manifest as membraneresistance and ion transport number have been previouslydemonstrated for low-concentration salt solutions.14 A targetmolecule at high concentration will flux efficiently at lowcurrent densities, as was seen with acetate in Figure 1, and athigh current densities for both acetate and caproate, as shownin SI Figure S2. High-current-density extraction (and therefore,higher flux) is preferable in a fully realized system. Acetate andcaproate were tested at equal molar concentrations (170 mM:10 g L−1 acetate and 19.6 g L−1 caproate) and display goodextraction for current densities up to 100 A m−2 (SI Figure S2),although caproate did not show efficiency equal to acetate at acurrent density of <30 A m−2. This demonstrates theimportance of the molecular availability for flux (molarconcentration), as opposed to the mass concentration in thebulk.The Coulombic efficiency in ME can be managed by

adjusting the salinity of the anolyte. An applied current must bebalanced by an equal transport of charge across the membrane.This is demonstrated experimentally in Figure 3 with twodifferent membranesthe Membrane International AMI-7001(MI) and the proton-blocking FumaTech fumasep FAB(FB)and represented in the illustration (Figure 3A,B,C). Thetargeted circuit for acetate extraction in ME is represented byFigure 3A, with the applied current balanced by the flux ofacetate anions. High anolyte salinity promotes diffusion acrossthe membrane against the potential gradient, as in Figure 3B.Diffusion of anionic species from the anolyte to the catholytewill be balanced by the transport of an equal charge in theopposite direction, such as acetate from the catholyte to theanolyte, or an opposite charge in the same direction, such asproton flux from the anolyte to the catholyte (Figure 3C). LowCoulombic efficiency is a result of acetate (Figure 3A)competing with other ions in the broth or proton diffusionfrom the anolyte to the catholyte (Figure 3C). With the FBmembrane, acetate flux across the membrane increases as thesalt gradient increases due to diffusion of sulfate from theanolyte to the catholyte to maintain the charge balance, as in

Figure 3B. Acetate flux does not increase for MI because thismembrane is more prone to proton flux and the chargeimbalance caused by sulfate diffusion drives protons across themembrane, as in Figure 3C, rather than the targeted acetate.This result highlights the importance of membrane selectionand management of the anolyte (salinity and pH) for greaterefficiency.Net acetate flux from the catholyte to the anolyte continues,

despite acetic acid accumulation in the anode (SI Figure S3).Accumulation of the organic product is possible by the low pHof the anolyte, driven by the local electrolytic production ofprotons. The acetate is protonated, and the uncharged acidicproduct does not pass back across the anion exchangemembrane if the system is sufficiently resistant to diffusion.However, the acetate/acetic acid equilibrium will result in someportion of the product remaining in ionic form and will impactthe net transfer of acetate. This can be seen in the inverserelationship between acetate flux and acetic acid concentrationin SI Figure S3. In addition to concentration gradient, this willalso be a function of membrane properties and applied current.Up to 1.07 kg d−1 of sodium hydroxide equivalent can be

generated electrolytically in the cathode per square meter ofmembrane by 30 A m−2 of applied current, assuming totalCoulombic efficiency. This decreases the chemicals require-ment for pH control in a fermentation process. However,inefficiencies in extraction can lead to an excess of hydroxideper molecule of product extracted, thus disproportionallyincreasing the pH. In a fully realized system, the pH correctionaspect must be carefully managed.

Extraction and Valorization from Thin Stillage. Wetested the ME on thin stillage of a bioethanol refinery, spiked to5 g L−1 sodium acetate (0.7 g L−1 of acetate native to thebroth). With equal volumes of anode and cathode compart-ments, 96 ± 12% of the acetate was transferred to the anolyteafter 48 h (Figure 4a). The flux through the AEM was 0.379kg/m2d at 20 A m−2, corresponding to a Coulombic efficiencyof 35.9%. Acetate was extracted from the catholyte andaccumulated in the anolyte as acetic acid at a linear rate. Thisresult exhibits a good transference from a real matrix to theclean anolyte. The relatively low Coulombic efficiency here isdue to the competitive flux of other native anions in the brothand proton backflux. To mimic a more practical operation ofME, the cathode was continuously fed and the acetic acidaccumulated in the anode in batch mode. Figure 4b presentsthe combined ME and BE experiment. The ME extraction hadaccumulated 14 g L−1 of acetic acid in the anolyte by theconclusion of the 6-day experiment, and was slowlyaccumulating other native anions. Over time, these anionic

Figure 3. Coulombic efficiency (molar flux of acetate/mol of applied current) can be improved with membrane selection and diffusion of anionsagainst the applied current. Experimental changes in efficiency of two different anion exchange membranes versus anolyte concentration of Na2SO4(left). Illustration of the mechanism (right).

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species would reach equilibrium across the membrane, enablingan optimum net flux of acetate from cathode to anode (as inFigure 3A,B). As the acetic acid concentration approached 12 gL−1, the net flux began to decrease as the acetic acid productaccumulated, demonstrating the need to remove the product.Aqueous acetic acid can be consumed and recovered as an

ester with the BE step. Water is a product of carboxylateesterification, as follows: R−CO2H + R′−OH ↔ R−CO2−R′ +H2O, in the presence of an acid catalyst. The high salinity andlow pH of the anolyte can support a phase transition of theacetic acid to the water-excluding phase, where it is esterifiedwith added alcohol and heat. This was tested with bothsynthetic and real anolyte using xylene as model solvent andvector for ester measurement. Gas chromatography and massspectrometry confirmed the presence of methyl acetate andethyl acetate and the absence of esters in the control test withdistilled water instead of alcohol. For the synthetic anolyte,acetic acid concentration fell by 26 ± 5% for methyl acetateproduction and 10 ± 3% for ethyl acetate, and no change inacetic acid was observed (0 ± 1%) where only water was added,after taking dilution and transference to the solvent layer intoconsideration.The same process was applied to the real anolyte collected at

t = 144 of the continuous cathode/batch anode experiment, asshown in Figure 4b. The drop in acetic acid concentration washigher for the true anolyte, at 58 ± 3% for the production ofethyl acetate and 65 ± 2% for methyl acetate. This is likely dueto improved catalysis by the lower pH in the real anolyte,measured at pH 0.94 compared with the synthetic anolyte’s pH2 because the batch was under electrolytic conditions for 144 hprior to the esterification. Acetic acid consumption proceededrapidly in the initial period, at 2.82 g L−1 h−1 over the first hourduring BE with ethanol (SI Figure S3A). This is equivalent to amaximum esterification rate calculated at 4.14 g L−1 h−1 in thefirst hour of the reaction (per gram of ethyl acetate per liter ofanolyte, assuming complete stoichiometric conversion). Thepresence of ethyl acetate and methyl acetate in the respectivereactions was confirmed by GC/MS but could not bequantified because of high decomposition in the xylenebetween sampling and the GC/MS measurement. With respectto the reaction of other organic species, butyrate was alsopresent in the broth, and butyric acid also accumulated in theanolyte up to 0.56 g L−1. This butyric acid showed 37 ± 1%conversion in the BE step after 6 h (SI Figure S3B). Althoughthe target species demonstrated a high rate of esterification, thereaction of side species may present a challenge in an appliedsystem when aiming to produce high-purity esters.

Effective recovery of esters ideally requires a water-excludingphase that has a low affinity for the product and low volatility toallow for a simple separation and regeneration of the water-excluding phase. Xylene was used as the experimental modelsolvent because it is water-excluding and relatively low cost andcan be analyzed for esters in a GC/MS; however, its physicalproperties would be nonideal in a fully realized system becauseof its relatively high volatility. The biphasic esterification can beenabled by an ionic liquid water-exclusion layer, and theresulting esters could be extracted from this layer by applyingan under-pressure.Ionic liquids have been previously demonstrated to enable

esterification,15 and therefore, an ionic liquid was introduced ina unique setup to demonstrate its capacity to enable BE withthe real anolyte. The production of ethyl acetate was observedat up to 2.05 g (ethyl acetate) L−1 h−1 and a final concentrationof 14 g L−1 ethyl acetate (SI Figure S5). This demonstrates thecapacity for the BE to be driven with an ionic liquid layer. Theproduction rate is lower than the pure xylene esterificationbecause this experiment was performed using differentequipment and with an adjusted method for rigorousquantification of the ethyl acetate in the ester-trapping xylene,rather than rate optimization.

■ DISCUSSIONEnergy and fuel production in a biorefinery context have beensuccessful because of fermentation processes that generateenergy-dense products such as ethanol and biogas productionby anaerobic digestion. Anaerobic digestion (AD) has beensuccessful as the first microbial technology dealing with wastebiomass and is applied worldwide in the treatment of organicrich streams. As a bioproduction technology, anaerobicdigestion is not an economically strong case in itself, asconsidering a typical power production of approximately 1 kWhper kg of biomass converted and a market price of 0.1 EUR,only 100 EUR is generated per ton of biomass converted tobiogas. The main benefit of AD in most cases is the reductionof waste, such as sludge and manure. In the current economicenvironment, electrical power remains a low value product, andfurther, one can argue that the final destination of carbonshould preferentially be embedded in industrial biochemicals,rather than returned to the atmosphere as carbon dioxide.However, bioproduction of chemicals is still in an over-whelmingly one-sided competition with petrochemical produc-tion and therefore often also dependent on subsidies. Thanks tonext-generation biofuels and bioplastics, the industry isbeginning to find its footing. Advances in mixed culturefermentation now enables successful production of organic

Figure 4. Extraction of acetate from a biorefinery thin stillage at 5 g L−1 acetate. (A) Batch extraction with equal volumes of catholyte and anolyte.(B) Continuously fed waste stream (catholyte) with batch operation of acid concentrate (anolyte), with biphasic esterification at 144 h. See SI FigureS4 for the acetic acid concentration in the first 6 h of the esterification.

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acids such as poly(hydroxybutyrate) and succinate.16 Theseproducts can be generated from renewable waste or low-costbiomass and have a higher value than biogas and ethanol, andyet, expensive processes for recovery of the final product stillcontribute to the rarity of economically sound biorefineries.1

Here, we have presented an alternate pipeline for asustainable bioproduction route that extracts and valorizesshort-chain carboxylates, although this valorization must besufficient to overcome the cost of extraction. At 10 g L−1, thepower cost of extraction is calculated at 6.2 MJ per kg of acetatebased on this study, which corresponds to 1.27 kWh per kg ofacetic acid produced. Considering an energy cost of 0.1 EURper kilowatt hour and an acetic acid value of 0.5 EUR perkilogram, there is a margin of 0.373 EUR per kilogram to dealwith other operation costs and write-off. At the highest currenttested for 10 g L−1 acetate, (100 A m−2, 65% CE) the powerinput is 24 MJ per kg of acetate per m2, achieving a flux 3 timeshigher for 4 times more power input. This demonstrates thatthe ME extraction would ideally fit a high production rate, highconcentration fermentation, which will be important whenoptimizing operation costs against unit size and investmentcost. For ethyl acetate in the BE step, for 1 kg of acetic acid at0.5 EUR per kg, the reaction requires 0.8 kg of ethanol at 0.4EUR per kg to produce 1.5 kg of ethyl acetate at 0.9 EUR perkg, stoichiometrically. From an input value of 0.82 EUR (0.5EUR, 1 kg of acetic acid and 0.32 EUR, 0.8 kg ethanol) thistranslates to a product value of 1.35 EUR. This is an attractivevalorization for an effective and cost-optimized BE step.Although only a superficial economic assessment is practicalat this stage, the presented case is sufficiently attractive tofurther explore the potential of this bioproduction pipeline.Projecting forward to a fully realized system, the ME and BE

pipeline is conceptually noninvasive and would require minimalchemical intervention. The electrolysis of the water replacescaustic and acid dosing in the broth and concentrate streams,respectively. After the ME and BE process, the organic fractionthat was unsuitable for fermentation could, in principle, bedigested and combusted to reduce the remaining solid fractionand help drive the applied current, and ethanol for esterificationcould be sourced from separate fermentations within anintegrated biorefinery. Ionic liquids, although currentlyexpensive, tend to be stable and can be regenerated, but thismust be assessed within the specific bounds of this process. Theanode (in this study, iridium oxide-coated titanium) is also anexpensive feature but is active in a clean, saline, acid mediumunder relatively mild electrical duress. The cathode thatinteracts with the broth is inexpensive stainless steel. Otherprocess engineering challenges of the ME and BE pipelineinclude power management, reactor design, heat management,biofouling, and possible degradation of the AEM by thealcohol; the water-excluding phase (or both); and, not least ofall, optimization of the fermentation process.The feed and the fermentation profile (e.g., target product,

production rate, concentration, specificity, and more) is integralto the performance of the ME and BE pipeline. On feedstocksuch as sludge and municipal solid waste, the carboxylateconcentrations typically do not exceed ∼17 g L−1 COD,17

which is in excess of the concentration range demonstrated inthis study. However, waste, residue, or other low-valuesidestreams from existing biorefineries, such as sugar orbioethanol production, still contain high amounts of organicsthat could be further processed for chemicals productionthrough the carboxylate platform at potentially high concen-

trations and rates. An ideal situation would see such streamsoptimized for high-rate production of valuable carboxylates forME extraction.Mixed culture fermentations can produce higher-value

products, such as caproate (C6) and caprylate (C8), bymicrobial chain elongation. In this process, acetate is elongatedto butyrate, and in a similar cycle, butyrate is elongated tocaproate.6,18 Although caproate is a higher-value product, thecommercial viability for biological caproate production remainsunproven because of the high cost of separation relative toproduct value. The extraction of such a product is a challengingcandidate for the ME and BE pipeline because the substrates(acetate, butyrate) also traverse the AEM, as demonstrated inthis study. However, if the chain elongation can be steered togenerate caproate at a greater concentration relative to thesubstrates, then the ME step becomes particularly attractivebecause the concentration of the target compound is linked tothe extraction efficiency and the relative concentration is linkedto the extraction selectivity.Alternatively, ME could generate an acetic and butyric acid

concentrate that could conceptually be used as a substrate in aseparate chain elongation, or other biological reaction, becausethe AEM should exclude nonionic inhibiting compounds, suchas furfural and phenolic compounds.19,20 Similarly, anotheropportunity is to use the technology to remove toxic acetatefrom ethanol fermentation, simultaneously solving a toxicityissue while recovering acetic acid. However, such processes canbe judged only with greater knowledge of economic viabilityand product selectivity.Targeted, selective extraction is a defining feature of a high-

performing separation system. Harvesting a specific productfrom a low-titer, mixed-species broth is a seminal challenge inbiorefinery separation technology, even with high-valueproducts.21 For a review of separation technologies inbiorefineries, see Huang et al., 2008.22 In its currentincarnation, the ME step has not been demonstrated toselectively extract specific molecules, although the flux profileand Coulombic efficiency clearly varies for acetate, butyrate,and caproate within the range of applied currents tested here(SI Figures S1 and S2). By extension, it is possible thatselectivity can be manipulated through membrane developmentand applied current scheme relative to, for example, the carbonchain length. However, a size exclusion model is highly unlikelyto represent the electrochemical transport of the carboxylatesbecause many other parameters come into play, includingdiffusion in the bulk electrolyte, the membrane surface interface(e.g., the molecular arrangement of the double layer), and theinteraction of the ion with the bulk membrane (both thepolymer backbone and charged group), among others. Indeed,it is known that the performance of electrodialysis is highlydependent on the intrinsic properties of the AEM.23 A deeperunderstanding of the electrochemical transport properties oforganic molecules with respect to the electrolytic membraneextraction process will reveal the potential for selectiveextraction of, for example, caproate rather than acetate orbutyrate.With respect to BE, opportunities for improved selectivity

and yield exist in a deeper knowledge of the reaction kinetics,operating conditions, and the ionic liquid’s inherent properties(solubility, stability, catalytic activity, etc.). Ionic liquids havebeen demonstrated to drive esterification and organic recoveryeven at room temperature,24 and their participation in thereaction also remains open to research and optimization. The

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ionic liquids used in this study are ideal candidates forseparating volatile organic compounds because ionic liquidshave a very low vapor pressure by definition and those used inthis study are not water-miscible. The ionic liquids have goodthermal stability and can be heated. In a fully realized system,applying a reduced pressure (vacuum) on the system lowers theboiling point of the esters so that only the esters volatilize, andthe esters are then passed through a condenser and captured asa liquid product.Other than upgrading waste streams, the ME process is also

interesting in cutting edge industrial biotechnology. Forexample, microbial electrosynthesis has been shown to generatecarboxylates from carbon dioxide and electricity.25,26 Theelectromotive force generated by delivering electricity to themicroorganisms can theoretically be used to drive products outof the biocathode to the anode in the ME step. Continuous insitu extraction directly from the site of the reaction couldpotentially increase the production rate by preventing productaccumulation. Microbial electrosynthesis at present generatesproducts in the more neutral pH range at relatively low titers, aswith mixed culture fermentation. For a product such as acetate,additional steps to recover the product would be required.Distillation, for example, would require an acidification step toconvert the acetate to acetic acid and a high energy input. TheME approach may lower the input chemical and energydemands for product recovery and improve production rates.Rigorous economic assessment, process development, and

selective extraction is required to bring the ME and BE pipelineforward. Although biorefineries may be uniquely poised to takeadvantage of this process as a result of chemical productioninfrastructure that traditional wastewater treatment facilitiesmay lack, this work has presented an attractive alternative forbiomass that would otherwise be destined for combustion,generating products instead of combusting waste.

■ ASSOCIATED CONTENT*S Supporting InformationDescription of the GC technique, two tables, and five figures.This material is available free of charge via the Internet athttp://pubs.acs.org

■ AUTHOR INFORMATIONCorresponding Author*Phone: +32 (0) 9/264 59 76; fax: +(0) 9/264 62 48; e-mail:korneel.rabaey@ugent.be.Author Contributions⊥S.J.A. and T.H. contributed equally.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSS.A. and T.H. are supported by the European UnionFramework Programme 7 project “ProEthanol 2G.” S.A.,T.H., M.C., and K.R. are supported by Ghent UniversityMultidisciplinary Research Partnership (MRP)Biotechnol-ogy for a sustainable economy (01 MRA 510W). T.H. is alsosupported by a postdoctoral fellowship from the ResearchFoundation Flanders (FWO Vlaanderen). S.G. is funded by theSpecial Research Fund (BOF) of the University of Ghent(Belgium). J.D. is supported by the Institute for the Promotionand Innovation through Science and Technology in Flanders(IWT-Vlaanderen, SB-091144). J.T. is supported by the IWT-

Vlaanderen. K.R. is supported by European Research CouncilStarter Grant ELECTROTALK. We acknowledge Alco BioFuel nv for generously providing thin stillage. We thank BramWuyts for preliminary esterification work and Michail Syrpasfor technical assistance with the GC/MS. Tim Lacoere iswarmly thanked for his graphical contribution.

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