Desalination 228 (2008) 84–96

Lactic acid recovery from whey ultrafiltrate fermentation brothsand artificial solutions by nanofiltration

M. Isabel Gonzáleza, Silvia Alvarezb, Francisco A. Rieraa*, Ricardo Álvareza

aDepartment of Chemical Engineering and Environmental Technology, University of Oviedo,Julián Clavería, 8, 33006 Oviedo, Spain

Tel. +34 (98) 510-3436; Fax: +34 (98) 510-3434; email: far@uniovi.esbDepartment of Chemical and Nuclear Engineering, Polytechnic University of Valencia,

Camino de Vera s/n, 46022 Valencia, Spain

Received 12 December 2005; Accepted 14 August 2007

Abstract

In this work, lactic acid recovery from clarified fermentation broths by nanofiltration was studied. The effect offeed concentration, flow rate, transmembrane pressure and pH on flux and rejection were analysed. The separationefficiency of nanofiltration membranes in the treatment of ionic solutions may be explained by the combination ofsize and charge effects. In the case of lactic acid solutions, the ratio of lactate ions to non-ionized lactic acid affectsthe rate at which both pass through nanofiltration membranes. At pH values around 5.0, which are typical values inindustrial fermentations, lactic acid is mainly found in the dissociated form. Two polyamide nanofiltrationmembranes were used: DK2540C® (Filtration Engineering) and AFC80® (PCI Membrane Systems). A stronginfluence of pH on lactic acid transport through these membranes was observed. The pH value affected both rejectionand permeate flux. Rejection increased with pH while flux decreased with this variable. However, at high feedconcentrations, solute retention was much lower as the Donnan exclusion effect was attenuated. At the acidic pHrequired to convert lactate into undissociated lactic acid (pH 3.0), the recovery of lactic acid must be improved inorder to reduce lactic acid losses and increase purity.

Keywords: Lactic acid; Lactate; Nanofiltration; Fermentation broth

1. Introduction

Lactic acid is a versatile chemical used in foodand chemical industries. It can be manufactured

*Corresponding author.

either by chemical synthesis or by carbohydratefermentation. The conventional process for fer-mentative production of lactic acid is a batchprocess with low productivity and high capitaland operating costs. The highest cost of the

0011-9164/08/$– See front matter © 2008 Published by Elsevier B.V.doi:10.1016/j.desal.2007.08.009

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traditional process for lactic acid production bylactose fermentation corresponds to the separationsteps that are necessary to achieve the qualityrequirements for food grade lactic acid (Table 1).Alternatives to this manufacturing process arebeing studied [1]. In order to reduce costs,numerous studies on lactic acid separation havebeen conducted using different separation tech-niques such as reactive extraction, membranetechnology, ion exchange, electrodialysis anddistillation [1–7].

Lactic acid fermentation is carried out at pHvalues between 4.0 and 6.0. Some authors haveused nanofiltration (NF) and/or reverse osmosis(RO) to remove lactic acid from the fermentationbroths, improving the fermentation yield using amembrane [8,9]. However, most of the reportedworks on lactic acid separation have onlyconsidered synthetic diluted solutions.

The separation efficiency of nanofiltrationmembranes in the treatment of ionic solutionsmay be explained by the combination of size andcharge effects. The membrane charge depends onthe pH of the solution and on its isoelectric point(IEP). For many commercial nanofiltration mem-branes, the isoelectric point lies in the pH rangebetween 3 and 6. Thus, nanofiltration membranes

Table 1Quality requirements for food-grade lactic acid

Parameter Lactic acidfermentationbroth

Lactic acidFCCa 50

Process Fermentation Fermentationand purification

Lactic acid (wt %)

5.55 49.5–50.5

Density at 20ºC (g/mL)

1.04 1.12–1.14

Ash (wt %) 2.8 max. 0.06Calcium (ppm) 495 max. 20Chloride (ppm) 2900 max. 10

aFCC: food grade, complies with Food Chemical Codex.

are usually negatively charged in neutral oralkaline conditions and positively charged inhighly acidic conditions [10]. However, mem-brane IEP depends on the type and concentrationof electrolyte [11,12]. Then, similar membranescan show slightly different IEP depending on thesolution composition.

When a NF membrane is negatively charged,solutes with high anionic charge densities aremore effectively rejected. For that reason divalentco-ions usually show higher rejection than mono-valent ones. On the other hand, the rejection ofsolutes without electric charge is less effective.As a result, the charged membrane may rejections much smaller than the membrane pore size,meanwhile uncharged solutes only are rejected bysteric reasons [13].

In case of an organic electrolyte the disso-ciation equilibrium is of great importance. Theamount of undissociated lactic acid and lactateanions present in the broth is based on lactic acidequilibrium at the operating conditions [14]. AtpH values in the range of 5.5–6.0, which aretypical values for industrial fermentations, lacticacid is mainly found in the dissociated form [14],but if lactic acid is the desired product pH around3 is necessary.

The fixed charge of organic membranesdepends greatly on their affinity for ions. Thuspredicting the ionic retention of an organiccompound by a nanofiltration membrane becomesvery specific and complex [10,15,16].

The separation of ions according to theirvalence by electric repulsion force (Donnanexclusion) can be described by equilibrium ther-modynamics, which allow the chemical potentialof the ionic component in the two phases presentto be calculated when an ionic solution is inequilibrium with a charged membrane. If theconcentration in the feed is low and the mem-brane charge density is high, the Donnan exclu-sion effect is very important. However, withincreasing feed concentration, this exclusionbecomes less effective [17].

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Childress and Elimelech reported that thecharge of membrane pores might have a signi-ficant effect on membrane flux and rejection. Thepore size of the membrane was found to besignificantly reduced at higher pH values, sincecharged carboxylic groups adopt an extendedchain conformation due to electrostatic repulsion.This expanded conformation reduces the poresize (or pore volume) of the membrane andthereby causes decreased flux and increased saltrejection [18].

They have also attributed permeate flux differ-ences with pH to the electroviscous effect. Theelectroviscous effect is a physical phenomenonthat occurs when an electrolyte solution ispressed through a narrow capillary or pore with acharged surface. This effect refers to the back-flow of counter-ions and water in the double layeradjacent to the capillary pore surface due to astreaming potential that develops between thecapillary ends. The electroviscous effect is leastpronounced at the pore surface point of zerocharge (or isoelectric point), where double-layereffects are negligible. At low pore surfacecharges, the permeating solution appears to exhi-bit reduced viscosity when its flow rate is com-pared with the flow at a high pore surface charge[18].

In this work, the recovery of lactic acid fromfermentation broths using nanofiltration mem-branes is studied. Firstly, the clarified fermen-tation broth was acidified and nanofiltered toseparate lactic acid from the salts present in thebroth (mainly sodium chloride). Taking intoaccount the larger molecular weight of lactic acidwhen compared to the inorganic salts present inthe broth, it was expected that lactic acid wasretained, while the salts permeated the membrane.However, the results obtained show that a sievingmechanism is not enough to explain the rejectionof lactate/lactic acid at all conditions studied.Then, the mechanism of lactic acid transportthrough nanofiltration membranes was studiedwith synthetic solutions at different pH and feed

concentration values. Two different membraneswere tested, which were different to those used inprevious works [8,9,19–21].

2. Materials and methods

2.1. Feed preparation

Two types of experiments were carried out.Firstly, lactic acid fermentation broth was nano-filtered to separate lactic acid from the saltspresent in the broth. The fermentation broth wasprepared by fermentation of sweet whey permeatesupplemented with 2.5 g/L yeast extract at 42ºCand pH 5.6. The fermentation was carried out bythe microorganism Lactobacillus Helveticus(Rhodia, France). The fermentation broth wasthen clarified by ultrafiltration using an inorganicmembrane of 15 kDa molecular weight cut-off(MWCO) (Kerasep, 482/1T model, Rhodia,France). The feed was acidified to pH 3 to con-vert lactate into undissociated lactic acid by theaddition of hydrochloric acid. The composition ofthe clarified fermentation broth used as feed isshown in Table 1.

Secondly, the mechanism of lactic acid trans-port through nanofiltration membranes wasstudied using artificial mixtures of lactic acid atdifferent concentrations and pH values. Feed wasprepared with food grade lactic acid 80% (Purac,Spain) at two different concentrations: the con-centration of the fermentation broth (40 g/L) anda solution two times more concentrated (80 g/L).The pH was adjusted using NaOH 40%. Theamount of undissociated lactic acid and lactateanions in the solution is based on lactic acidequilibrium at the operating temperature and canbe calculated by the Henderson–Hasselbalchequation:

CH3CHOHCOOH ↔ CH3CHOHCOO! + H+

[ ]

-3

a3

CH CHOHCOOpH pK log

CH CHOHCOOH

⎡ ⎤⎣ ⎦= +

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The pKa for lactic acid at 25ºC is 3.86. AtpH 2.7 only 6.47% of the lactic acid is disso-ciated, whereas 93.25% of the lactic acid is dis-sociated at pH 5 and 99.28% at pH 6. The pKa oflactic acid decreases with temperature. At 40ºCthe pKa was estimated to be 3.67.

2.2. Experimental set-up

The experimental set-up used is shown inFig. 1. The equipment consists of a membranemodule, a 70 L feed tank, two pumps and pres-sure and temperature sensors. The first pump wasa piston pump (Cat Model 661, 7.5 kW, USA)and the second one (Inoxpa A-20C, 1.5 kW,Spain) a centrifugal pump that was used toachieve a sufficient recirculation flow. Twodifferent membrane modules were used: a mem-brane module FE 2540 SS (Filtration Engineer-ing, USA) that contained a spiral-wound mem-brane with a MWCO of 300 Da (FE-700-002DK2540C UO), and a module that had 18 tubularmembranes model AFC80 (PCI MembraneSystems Ltd., UK) inside. Table 2 shows theproperties of the membranes used. The AFC80

membrane is a tight membrane (see average poresize and water permeability in Table 2) comparedto the DK membrane.

2.3. Operating mode

Concentration was kept constant in all theexperiments; therefore retentate and permeatewere recycled back to the feed tank. Two dif-ferent flow rates were considered (1.2 m/s and2.7 m/s), while transmembrane pressure variedwithin the range of 10–40 bar. The experimentswere performed at 40ºC as this was the tempera-ture of the fermentation broth. Several samples ofpermeate and retentate were taken at eachtransmembrane pressure. The experiments per-formed at each operating condition lasted about2 h, but no significant flux decline with time wasobserved. After each run the membranes werecleaned. When the membranes were used to treatthe fermentation broth, they were first cleanedwith a 1% Divos 110 (Divos System, DiverseyJohnson, Barcelona, Spain) alkaline solution at50ºC for 40 min and afterwards with a 0.25%Divos 2 (Divos System, Diversey Johnson) acid

Table 2Characteristics of the membranes used

FE-700-002 DK2540C UO(Filtration Engineering)

AFC 80(PCI Membrane System Ltd.)

Geometry Spiral wound TubularMaterial Polyamide PolyamideDimensions (mm) Length: 1016

Diameter: 63.5Spacer thickness: 1.27

Length: 1200Internal diameter: 12External diameter: 12.5

Membrane area (m2) 1.8 0.72Membrane pore diameter (nm) 0.92 0.68Rejection, % R (CaCl2): 30

R (Lactose): 96–100R (NaCl): 80

pH range 2.3–11 1.5–10.5Maximum temperature (EC) 57 70Maximum pressure (bar) 41 60Water permeability (L m!2 h!1 bar!1) 6.14 2.9

I.M. G

onzález et al. / Desalination 228 (2008) 84–96

Fig. 1. Nanofiltration experimental set-up (T, tank; P, pump; F, flowmeter; E, heat exchanger; M, membrane module; V, valves; P, pressure transmitter;T, temperature transmitter).

88

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solution at 50ºC for 30 min. Rinsing with micro-filtered water was performed before and aftereach cleaning step till neutral pH was reached.For those experiments performed with artificialsolutions, the membranes were enzymaticallycleaned with a 0.5% Ultrasil 53 (Henkel Ecolab,Germany) solution for 30 min. The membraneswere rinsed with microfiltered water after andbefore the enzymatic cleaning. For some of theexperiments the membrane water permeabilitywas recovered after the first rinsing step andtherefore the enzymatic cleaning was not neces-sary. After each cleaning cycle the membranewater permeability was tested at 25ºC and attransmembrane pressures between 1.0 and4.0 MPa. It was observed that the membranewater permeability was fully recovered for all theruns and for both membranes.

The cross flow velocity for the spiral-woundmembrane module was calculated according tomethod described by Schock and Miquel [22].

2.4. Analytical methods

The concentration of lactic acid and lactatewas determined by ion chromatography (Dionex-120, Dionex, USA) and also using an enzymaticmethod (Test 1112821, Boehringer Mannheim,Germany, ISO 8069:2005). The amount of mine-rals was determined by measuring ash contentand total solids content was measured followingthe standard methods of analysis (FIL-IDF 21:1962). Chloride concentration was determined bypotentiometric titration (Crison Compact Titrator,Crison Instruments S.A., Spain).

3. Results and discussion

First of all, AFC80 nanofiltration membraneswere used to recover lactic acid from clarifiedfermentation broths. In order to convert lactateinto undissociated lactic acid, the feed solutionwas acidified to pH 3.0. Fig. 2 shows the results

Fig. 2. Rejection of lactic acid and salts by the AFC80membrane when clarified lactic acid fermentation brothwas treated at pH 3, 40ºC and 1.1 m/s.

obtained in terms of lactic acid and mineral saltrejection. Lactic acid rejection was observed to beslightly lower than inorganic salt rejection. Lacticacid rejection increased from 35% to 58% whentransmembrane pressure increased from 10 to40 bar, whereas inorganic salt rejection rangedbetween 45 and 76% in the pressure interval of10–40 bar. Therefore, the recovery and the demi-neralization of lactic acid were not successful.Chloride rejection at 30 bar is similar to thevalues supplied by the membrane manufacturer.The lower lactic acid rejection compared withchloride cannot be explained by the differentsolutes sizes. Electrostatic repulsion betweenmembrane and anions present in the feed hasbeen by other authors using diluted syntheticsolutions to explain the lactate/lactic rejection ina NF membrane [23]. At pH 3 the organic soluteis practically undissociated and lactic acid isretained by size effects. Chloride rejection ishowever explained by electrostatic repulsionbetween negative charges (membrane-solute).

Fig. 3 shows the values of permeate flux atdifferent transmembrane pressures when theclarified fermentation broth was nanofiltered with

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Fig. 3. Permeate flux through the AFC80 membranewhen clarified lactic acid fermentation broth was treatedat pH 3, 40ºC and 1.1 m/s.

a AFC80 membrane. It can be observed thatpermeate flux increased linearly with pressure.No substantial fouling problems were detected, aspermeate flux did not significantly decreased withtime. The increasing permeability to the solventwith transmembrane pressure explains the higherrejection of solute with this variable (rejection foreach solute was calculated with the expression

Ri = 1![Cip/Cir)].

where Ri represents solute rejection and Cip andCir represent solute concentration in the permeateand and in the retentate, respectively.

The results obtained with clarified fermen-tation broths were compared to those obtainedwith synthetic solutions of pure lactic acid underthe same operating conditions. No significantdifferences were observed with respect to lacticacid rejection, although permeate flux was higherwhen the artificial solutions were used. Permeateflux increased linearly with pressure in both casesand it did not appreciably decrease with time,thus indicating that there were no significantfouling problems.

From Fig. 2 it can be seen that AFC80 mem-branes do not show enough rejection differencesbetween lactic acid and salts to get an efficient

Fig. 4. Permeate flux of AFC80 and DK2540C mem-branes as a function of transmembrane pressure at twodifferent flow rates, 2.7 m/s (full symbols) and 1.2 m/s(empty symbols), when a solution of 40g/L lactic acidwas used as feed (pH: 2.7; temperature: 40ºC).

purification process. The aim of the next part ofthis work was to investigate the transport mecha-nism of lactic acid across nanofiltration mem-branes. Synthetic solutions were used in order tostudy the operating conditions at which lacticacid rejection are maximum.

Studying water flux and solute rejection as afunction of pH is of great interest because, as pHchanges, so do several characteristics of thesystem. Information about lactic acid rejection atacidic pH will allow to know about the possibilityof its purification. At pH values lower than itspKa, lactic acid is found in the undissociatedform, whereas for higher pH values, lactic acid iscompletely dissociated what affects to itsrejection by charged membranes [18].

Several experiments were carried out withlactic acid solutions at different concentration andpH values. The experiments were performed attransmembrane pressures in the range of 10–40 bar and at different feed flow rates (1.2 m/sand 2.7 m/s). Cross flow velocity had no effect onpermeate flux, as shown in Fig. 4. As can be seenin this figure, concentration polarization does notseem to play a noticeable role, because therelationship between permeate flux and pressure

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was linear and independent on the flow rate forthe two flow rates studied. The same conclusionswere obtained for all the experiments performed.This is particularly unexpected for the tubularAFC 80 membrane, whose channel dimensionsare large. The different permeabilities of bothmembranes agree with their pore size (seeTable 2).

A solution of 40 g/L lactic acid was nano-filtered using AFC 80 membranes at different pHvalues adjusted by the addition of sodiumhydroxide and hydrochloric acid. Temperaturewas fixed at 40ºC. A linear dependence betweenpermeate flux and pressure was found in all thecases, as shown in Fig. 5. Permeate fluxdecreased with increasing pH due to the higherionic concentration (sodium hydroxide added toadjust the pH) thus resulting in a higher osmoticpressure difference across the membrane. In thefigure, the experimental data were fitted to astraight line by linear regression. It can beobserved that the y-axis intercept was negative atthe higher pH values, due to the increase in theosmotic pressure difference. However, for thelower pH value (pH 2.7) the y-axis intercept waspositive. That can be due to the experimentalerror made. However, some authors have pointedout the possibility of a change in pore size withpH. Oak et al. and Childress and Elimelech testedseveral membranes having carboxylic acidgroups. They demonstrated that at pH valueshigher than the membrane isoelectric point, thecharged COO! groups adopt an extended chainconformation due to electrostatic repulsion be-tween them. This expanded conformation reducesthe pore size of the membrane and thereby causesdecreased permeate flux [18,24]. However, othernegatively charged membranes, such as sulfo-nated polysulfone membranes, show a differentperformance. In this case water permeabilityincreased with pH and it was attributed byBraghetta et al. to an increase in the pore size asthe charged functional groups in the membranematrix force the adjacent polymers apart [25].

Fig. 5. Permeate flux of the AFC80 membrane as afunction of transmembrane pressure and pH when asolution of 40 g/L lactic acid was used as feed(temperature: 40ºC).

Fig. 6 shows lactic acid/lactate rejections as afunction of permeate flux. Rejection increaseswith increasing permeate flux (and transmem-brane pressure) and pH. At the operatingconditions considered water flux is more affectedby pressure than lactate flux through the mem-brane, then lactate concentration decreases in thepermeate when membrane permeability increases.With respect to the influence of pH on lactaterejection, it is known that commercial polyamidemembranes have an isoelectric point in the pHrange of 3–6 [10]. Thus, at pH values higher thanthe isoelectric point, the membrane is negativelycharged, while for pH values lower than the iso-electric point it is positively charged. The nega-tive charge above their isoelectric point is a result

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Fig. 6. Lactic acid/Lactate rejection of the AFC80membrane as a function of permeate flux and pH at 40ºCusing a solution of 40 g/L lactic acid as feed.

of deprotonation of the carboxylic groups [19].Furthermore, as the pKa of lactic acid is 3.67 (at40ºC), at pH values between 5.0 and 6.0, lacticacid is found in the dissociated form, whereas atpH 2.7, lactic acid is mainly in undissociatedform. The higher lactate rejection with increasingpH may be due to the electrostatic repulsionbetween lactate ions and the membrane surface(both negatively charged), as the membranecharge becomes more negative when the pH rises.Moreover, several authors demonstrated that thesize of polyamide membrane pores is reducedwith increasing pH due to conformational chan-ges as a result of repulsion between chargedCOO! groups. Therefore, as pore size decreasessolute rejection increases [18,24].

Similar results were observed with theDK2540C membrane. The results obtained withthis membrane are shown in Figs. 7–10. As ob-served for the AFC80 membrane, permeate fluxincreased linearly with pressure and decreasedwith the pH of the feed solution (Fig. 7). Fig. 8shows lactic acid/lactate rejection at differentpermeate flux and pH values. It can be observedthat the differences in rejection with increasingpH were greater than those obtained when the

Fig. 7. Permeate flux of the DK2540C membrane as afunction of transmembrane pressure and pH at 40ºC usinga solution of 40 g/L lactic acid as feed.

AFC 80 membrane was used. Permeabilities andlactic/lactate rejections are quite different for bothmembranes. Higher water flux of the DK mem-branes can be explained by its higher pore size(Table 2).

For both membranes lactic/lactate rejectionincreases with pH due to the electrostatic repul-sion between the ions and the membrane surface,but rejection of lactate/lactic acid at the higher pHvalues increases is greater for the DK2540membrane, probably due to a higher charge den-sity of this membrane. The IEPs of both mem-branes were not measured in this work. However,the IEP of DK2540 membranes has been mea-sured in different works and values between 2.9and 4.2 have been reported [11,16]. Other authorshave written that IEP values are strongly depen-dent of the electrolyte concentration. Martin etal. [12] demonstrated that the IEP of the

M.I. González et al. / Desalination 228 (2008) 84–96 93

Fig. 8. Lactic acid/Lactate rejection of the DK2540Cmembrane as a function of permeate flux and pH at 40ºCusing a solution of 40 g/L lactic acid as feed.

membranes decreases when increasing the elec-trolyte concentration rises. In this work chlorideconcentration (80 mM) was several times higherthan the values used by other authors.

At acidic pH (2.7) (organic solute in lacticacid form) both membranes show the lowestsolute permeabilities. When comparing lacticrejection for both membranes at this pH, thelower rejection for the DK membrane can beexplained by a sieving effect.

Finally, a number of experiments were per-formed to study the effect of concentration onlactate rejection. This point is important whenpurified concentrated lactic acid is desired. Theresults obtained are shown in Figs. 9 and 10 (DKmembrane). In Fig. 9 it can be observed that,when lactate concentration was 80 g/L, permeateflux decreased with increasing pH and it waslinearly dependent on pressure as well. Higherosmotic pressures at these conditions explain thisbehaviour. In Fig. 10 lactic/lactate rejection isplotted vs. permeate flux at different pH valueswhen lactic acid concentration is 80 g/L. Soluterejection increased with permeability due to thereasons explained before, but the differences

Fig. 9. Permeate flux of the DK2540C membrane as afunction of transmembrane pressure and pH at 40ºC usinga solution of 80 g/L lactic acid as feed.

Fig. 10. Lactic acid/Lactate rejection of the DK2540Cmembrane as a function of permeate flux and pH at 40ºCusing a solution of 80 g/L lactic acid as feed.

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Fig. 11. Comparative lactic acid/lactate rejection and permeate flux for the two membranes considered as a function of pHat 30 bar and 40ºC using a solution of 40 g/L lactic acid as feed.

found at different pH were greater for highpermeate flux values. When the results are com-pared to those shown in Fig. 8 (40 g/L), soluterejection was significantly lower.

The lower rejection for the same membraneand at similar conditions must be attributed to thereduction in the Donnan effect at higher soluteconcentration [17]. At acidic conditions (pH = 3.0in Fig. 8 and pH = 2.7 in Fig. 10) lactic acidrejection is slightly higher at 80 g/L, but differ-ences are not significant.

In Fig. 11, the effect of pH on lactic acid/lactate rejection and permeate flux is comparedfor the two membranes considered. The resultsshown were obtained at a contant transmembranepressure of 30 bar and lactic concentration of40 g/L. It can be observed that rejection increaseswith pH for both membranes. For higher pHvalues, both membranes are negatively chargedand thus the repulsion between lactate and themembrane surface is important due to the Donnanexclusion effect. Although the DK2540C mem-brane has a higher molecular weight cut-off thanthe AFC80 membrane, lactate rejection is higherfor the former membrane than for the AFC80membrane at high pH values. One of the reasonsmay be the higher permeability of the DK2540Cmembrane. Another reason can be the charge

density of the DK2540C, which is possiblygreater than that of the AFC 80 membrane.

Similar results were reported by other authorsusing different membranes. Jeantet et al. workedwith the spiral-wound membrane module R76A(Millipore, France) with a 400 Da MWCO. Theyobserved that lactic acid rejection increased withincreasing pressure and pH, from 12–35% atpH 3.3 to 36–72% at pH 6.0 [26]. Timmer et al.also reported an increase in lactate rejection withpH for different nanofiltration and reverseosmosis membranes made of cellulose acetate andpolyamide [19,20]. Schlicher and Cheryan ob-tained permeate flux and rejection data for lacticacid aqueous solutions at pH 2.38, 3.86, 5.6, 6.25and 6.8 with a tubular reverse osmosis membrane(ZF-99, PCI Membrane Systems, UK) [27]. Theyreported that rejection increased with transmem-brane pressure and pH. Gyo et al. also studied theeffect of pH on lactate rejection using the NTR-729 membrane (Nitto-Denko, Japan). They ob-served as well that rejection increased with pHfor lactic acid concentrations below 50 g/L [21].

With respect to the water flux of both mem-branes (Fig. 11) pH shows a limited influence thatcan be explained by an increase in osmoticpressure with the pH.

M.I. González et al. / Desalination 228 (2008) 84–96 95

Fig. 12. Comparative lactic acid/lactate rejection and permeate flux of the DK2540C membrane as a function of pH fortwo different feed concentrations when an artificial solution of lactic acid was used as feed (TMP: 30 bar; temperature:40ºC).

Fig. 12 shows how lactic acid/lactate rejectionand water permeability depend on the pH whenconcentrated lactic acid (80 g/L) is nanofiltered.At higher pH values, the necessary addition ofsodium hydroxide leads to an increase in osmoticpressure and ionic strength, thus reducing themembrane permeability and the Donnan exclu-sion effect. In these cases lactic/lactate rejectionis explained by sieving effect.

Similar results were obtained by Gyo et al.,who studied the effect of feed concentration in ananofiltration process. At concentrations higherthan 50 g/L, they found that rejection decreasedwith increasing pH. For a solution of 10 g/L lacticacid rejection increased from 18% to 90% whenthe pH changed from 2.5 to 6.5. However, whenthe feed concentration was 100 g/L, rejectiondecreased from 15% to 10% when the pH wasraised [12].

Permeate flux was lower at higher concen-trations as the osmotic pressure difference acrossthe membrane increased with concentration. Inboth cases (40 g/L and 80 g/L lactic acid), per-meate flux decreased with increasing pH.

Therefore, from the afore-mentioned results,it can be concluded that the selection of theadequate pH can improve the separation of lacticacid due to the Donnan effect. The choice of a

membrane with the adequate charge density canalso improve lactic acid separation. However, aslactic acid concentration increases, the Donnanexclusion effect becomes less significant andlactic acid rejection decreases regardless the pHand the type of membrane used. Other separationtechniques, such as ion exchange, may offerbetter results in the purification of lactic acidfermentation broths. In a previous work by theauthors, lactic acid concentrations higher than99% were reached using firstly a strong cationresin and afterwards a weak anion exchanger[28].

4. Conclusions

When the clarified fermentation broth wastreated by NF at the acidic pH value required toconvert lactate into undissociated lactic acid,lactic acid rejection was low (35–58%) and simi-lar to that of the inorganic salts present in thefermentation broth (45–76%). The electrostaticeffect is a limiting factor in the recovery of lacticacid by means of NF. Depending on the charge ofthe membrane, retention can be higher or lowerthan expected on the basis of molecular weight orany other size parameter. Lactic acid transportthrough AFC80 and DK2540C membranes was

M.I. González et al. / Desalination 228 (2008) 84–9696

strongly affected by pH. The pH of the feed solu-tion affects both lactate rejection and permeateflux. Permeate flux increased with pressure anddecreased with pH. The feed flow rate had noeffect on flux and rejection, which indicates thatconcentration polarization was not a limitingphenomenon. Lactate rejection increased withpressure and pH. Lactate rejection of theDK2540C membrane at 60 L/h m2 was 10–91%in the pH range of 2.7–6, whereas lactate rejec-tion of the AFC80 membrane was 45–82%.Although the DK2540C membrane has a highermolecular weight cut-off than the AFC80 mem-brane, lactate rejection was higher for the formermembrane at high pH values. With increasingfeed concentration, solute rejection was lower asthe Donnan exclusion effect was attenuated.

Acknowledgements

This research work has been supported by thedairy industry ILAS S.A. (Reny Picot) and theSpanish Ministry of Science and Technology.

References

[1] B.C. Saha and J. Woodward, ACS Symp. Series 666,Amer. Chem. Soc., Washington, 1997.

[2] A.M. Baniel, A.M. Eyal, J. Mizarahi, B. Hazan andR. Fisher, US Pat. No. 6,187,951, 2001.

[3] R. Datta, US Pat. No. 4,885,257, 1989.[4] A.M. Eyal, J.N. Starr, R. Fisher, B. Hazan, R.

Canari, D.R. Witze, P.R. Gruber and J.J. Kolstad, USPat. No. 6,229,046, 2001.

[5] S. Kulprathipanja, US Pat. No. 5,068,418, 1991.[6] L.J. Russo and H. Kim, US Pat. No. 5,503,750, 1996.[7] J.G.M. Van Nispen and R. Jonker, US Pat. No.

5,002,881, 1991.[8] J. Fitzpatrik, C. Engler and J. Rosenau, J. Food Eng.,

26 (1995) 97–111.

[9] S. Milcent and H. Carrere, Sep. Purif. Technology,22 (2001) 393–401.

[10] J. Tanninen and M. Nyström, Desalination, 147(2002) 295–299.

[11] A.I. Cavaco, A.M. Brites and M. Diná, J. Membr.Sci., 281 (2006) 417–428.

[12] A. Martin, F. Martinez, J. Malfeito, L. Palacio,P. Prádanos and A. Hernandez, J. Membr. Sci., 213(2003) 225–230.

[13] Y. Xu and R.E. Lebrun, J. Membr. Sci., 158 (1999)93–104.

[14] J. Russo and J. Lawrence, US Patent No 5,503,750,1996.

[15] C. Labbez, P. Fievet, A. Szymezyck, A. Vidonne,A. Foisy and J. Pagetti, J. Sep. Purif. Technology, 30(2003) 47–55.

[16] G. Hagmeyer and R. Gimbel, Desalination, 117(1998) 247–256.

[17] M. Mulder, ed., Basic Principles of Membrane Tech-nology, Kluwer Academic, Dordrecht, 1996.

[18] A.E. Childress and M. Elimelech, Environ. Sci.Technol., 34 (2000) 3170–3716.

[19] J.M.K. Timmer, Properties of nanofiltration mem-branes; model development and industrial appli-cation. PhD Thesis, University of Eindhoven, 2001.

[20] J.M.K. Timmer, H.C. van der Horst and T.Robbertsen, J. Membr. Sci., 85 (1993) 205–216.

[21] E.L. Gyo, S.H. Kang and Y.K. Chang, Kor. J. Appl.Microbiol. Biotechnol., 27 (1999) 320–326.

[22] G. Schock and A. Miquel, Desalination, 64 (1987)339–352.

[23] A. Bouchoux, H. Roux-de Balmann and F. Lutin,Sep. Purif. Technol., 52 (2006) 266–273.

[24] M.S. Oak, T. Kobayashi, H.Y. Wang, T. Fukaya andN. Fujii, J. Membr. Sci., 123 (1997) 185–195.

[25] A. Braghetta, F.A. DiGiano and W.P. Ball, J.Environ. Eng., 123 (1997) 628–641.

[26] R. Jeanet, J.L. Maubois and P. Boyaval, EnzymeMicrob. Technol., 19 (1996) 614–619.

[27] L.R. Schlicher and M.J. Cheryan, J. Chem. Technol.Biot., 49 (1990) 129–140.

[28] M.I. González, S. Álvarez, F.A. Riera and R.Álvarez, Ind. Eng. Chem. Res., 45 (2006) 3243–3247.