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Separation and Purification Technology 76 (2010) 814

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Separation and Purification Technology

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / s e p p u r

Transport of salicylic acid through supported liquid membrane based onionic liquids

Noura kouki, Rafik Tayeb, Ramzi Zarrougui, Mahmoud Dhahbi

Laboratoire Eau et Technologies Membranaires, BP 273, 8020 Soliman, Tunisia

a r t i c l e i n f o

Article history:

Received 25 May 2010

Received in revised form 7 September 2010Accepted 10 September 2010

Keywords:

Supported liquid membraneImidazolium-based ionic liquidEthylammonium nitrateSalicylic acidFacilitated transportTransport mechanism

a b s t r a c t

Transport of salicylic acid (SA) through flat-sheet supported liquid membrane (SLM) was investi-gated using as liquid membrane the ionic liquids 1-hexyl-3-methylimidazolium hexafluorophosphate([C6mim][PF6]) or ethylammonium nitrate (EAN). Using [C6mim][PF6], it has been observed that thetransport efficiencydecreaseswith increasingpH, indicatingthat theun-dissociated form of SA is mainlyextracted. On the other hand, the ionic dissociated form of salicylic acid is mostly extracted via theanion exchange mechanism between nitrateand salicylateanionswhen EANis used as liquidmembrane.Parameters such as nature and concentration of the strippant in the receiving phase and concentrationof the SA in the feed phase were studied. By comparing the SLM transport efficiency of SA (initial flux) ofthe two used ionic liquids, EAN appears to be slight efficient than [C6mim][PF6].

Despite the use of different stripping solutions (NaCl, NaOH and Na 2CO3) and even with pH mainte-nance around initial values, uphill transport driven by pH difference was not observed using both ionicliquids. The absence of uphill transport has been attributed to the formation, along the course of theexperiment, of water microenvironments (aggregates)insidethe ionic liquid. SA transport throughthesewater microenvironments inside the liquid membrane becomes the main mechanism. The main featureof SLMs based on ionic liquids is their higher stabilitycompared to classical SLMs. In fact, ourSLM systemretained its stability and initial performance during the 9 days long experiment.

2010 Elsevier B.V. All rights reserved.

1. Introduction

Several investigations have shown evidence that some pharma-ceuticals and personal care products (PPCPs) are not completelyremoved during sewage treatment [17]. On thisbasis,the demandfor the development of efficient systems for removing these sub-stances from water has assumed recently a great research interest.SincePPCPs aresuspiciousenvironmental contaminants as theyarebiologically active and often have a low biodegradability[8,9].

Membrane operations are increasingly employed in manyindustrial sectors as important alternative technologies to the clas-sical processes of separation such as distillation, crystallisation,

solvent extraction and precipitation.Among membrane-based sep-aration processes, the use of supported liquid membranes (SLMs)has received growing attention during recent years[1014]. SLMsconsist of poroussupportswhoseporesare filled with a liquid. Theyhave been widely applied to extract metal ions species[10,12]andsome organic compounds[11,14]from aqueous solutions. More-over, these systems offer numerous process advantages such as

Corresponding author. Tel.: +216 79325798; fax: +216 71430934.E-mail addresses:[email protected],[email protected]

(M. Dhahbi).

low capital investment and operating cost, low energy consump-tion, low liquid membrane requirement and simple to operate andeasy to scale up[10,15]. Nevertheless, their industrial applicationis still limited, mainly due to concerns about SLM stability andlong-term performance[16,17]. SLMs with conventional liquidseventually deteriorate duo to liquid vaporisation, dissolution intoa contacting phase, and displacement from the porous structureunder low-pressure gradient (


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N. kouki et al. / Separation and Purification Technology 76 (2010) 814 9

3-methylimidazolium hexafluorophosphate ([C6mim][PF6]) andethylammoniumnitrate(EAN).Theincidenceofseveralparameterssuch as feed phase pH, nature and concentration of the strippingagent in the receiving phase and concentration of the salicylic acidin the source phase were studied.

2. Experimental

2.1. Reagents and membranes

The imidazolium-based ionic liquid used in this study, 1-hexyl-3-methylimidazolium hexafluorophosphate [C6mim][PF6], withnominal purity of greater then 97.0 mass %, was purchased fromFluka. Ethylammonium nitrate EAN was synthesized in our labora-tory according to Evans et al.by neutralization of ethylamine withnitric acid[24].

A hydrophobic polyvinylidene fluoride membrane (Durapore

GVSP, Millipore) was used as supporting membrane. The mem-brane nominal pore size, porosity and thickness were 0.2m, 65%and 120m, respectively.

Feedsolutionswerepreparedbydissolvingsalicylicacidsodiumsalt (MW = 160.11 g/mol, purity99.5%, Fluka) in ultrapure water(Milli Q Plus Colum, Millipore). The pHs of these solutions were

maintained at different values ranging from 1 to 6. Mixtures ofsolutions of potassium chloride (Fluka) and hydrochloric acid, cit-ric acid (Fluka) and sodium citrate (Fluka), acetic acid and sodiumacetate (Fluka) and 2-(N-morpholino)-ethane sulphonic acid (MES,Sigma) andsodium hydroxide(Carlo Erba) were prepared to obtainrespectivelythepHsof1,3,4.7and6.Reagentsofanalyticalgradeorhigher were used without further purification. Chemical structuresof SA, [C6mim][PF6] and EAN are given inFig. 1.

2.2. Analytical methods

SA concentration measurements in both aqueous source andreceiving solutions were carried out using a UVvisible Perkin-Elmer double beam spectrophotometer type Lambda 20 at a

wavelength of 305.4 nm. A pH meter (C861 Consort) was used forpH measurements.

2.3. SLM transport experiments

The flat-sheet supported liquid membrane was prepared atroom temperature by impregnating the porous film with ionic liq-uidforatleast24h.Then,itwaswipedwithasoftpapertoeliminatethe excess of ionic liquid. Obtained SLM was placed in the mid-dle of a two-compartment permeation cell described elsewhere[12,25]. The exposed membrane area was 3.1cm2. The feed andthe stripping solutions (50mL each) were placed in each compart-

ment of the cell. Both aqueous feed and stripping solutions weremagnetically stirred at 600 rpm at 251 C to avoid concentrationpolarization conditions at the membrane interfaces andin the bulkof the solutions. 1 mL sample of each feed and stripping solutionswas periodically taken to determine SA concentration. All experi-ments were carried out in duplicate and standard deviations wereless than 5%.

The efficiency of SLM system can be evaluated using the extrac-tion percentageE(%), the recovery percentage R(%) and the initialfluxJof SA.

The extraction percentage,E(%), was calculated by Eq.(1):

E(%) =[SA]feed.0 [SA]feed,t

[SA]feed,0 100 (1)

where [SA]feed,0is the initial SA concentration in the feed solutionand [SA]feed,tis the SA concentration in the feed solution at time t.

The recovery percentage,R(%), was calculated by Eq.(2):

R(%) =[SA]strip,t[SA]feed,0

100 (2)

where [SA]strip,tis the SA concentration in the stripping solution attimet.

The initial flux, J, for the SA can be expressed as follows[12,13,25]:

J=

V

A

.

d [SA]dt

(3)

where Vis the volume of the aqueous stripping solution (L), Sis theeffective exposed surface area of the membrane (m2), and [SA]stripistheconcentrationofSAinthestrippingphase(molL1)atelapsedtime (s).

3. Results and discussion

3.1. Effect of feed solution pH

In order to assess the role of the feed phase pH in the SA trans-

port process, feed solution was buffered at different pHs: 1, 3,4.7 and 6.Fig. 2shows the increase of the extraction percentageas a function of time using [C6mim][PF6] and EAN as ionic liq-uids. It can be observed that when using imidazolium-based ionicliquid [C6mim][PF6], the amounts of SA extracted decrease withincreasing pH. In fact, after 8 h of transport process, the extrac-tion percentage decreases from 46.3% (pH = 1) to 32.1% (pH = 6). Itis trivial that themolecular(un-dissociated) form of SA is extracted(pKa of SA is 2.97). However, the amount of decrease does notnecessarily correspond to fraction of un-dissociated form may bedue to the relative solubility of the SA salt into the imidazolium-based ionic liquid phase. The same behaviour was observed in the

CHa

c

b3N+ N

CH3

P-

F

F

F

F

F

F

N+

HCH3

H

HNO3

-

OH

CO

OH

Fig. 1. Chemical structures of (a) [C6mim] [PF6], (b) EAN and (c) SA.


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10 N. kouki et al. / Separation and Purification Technology 76 (2010) 814

1086420

0

20

40

60

E%

Time (h)

a b

pH=1

pH=3

pH=4.7

pH=6

1086420

0

20

40

60

E%

Time (h)

pH=1

pH=3

pH=4.7

pH=6

Fig. 2. Effect of feed pH on the SA extraction percentage. Feed solution: [SA] = 103 M; polymeric support: PVDF. (a) Stripping solution: NaOH 102 M; organic phase:[C6mimPF6] and (b) stripping solution: NaCl 102 M; organic phase: EAN.

recovery of phenol and lactic acid into some imidazolium-basedroom-temperature ionic liquids (RTILs)[26,27].

In the case of EAN, the SA extraction percentage increases from28.4% to 36.8% (8h of transport process) when the initial pH of the

feed phase increases from pH 1 to pH 6. Taking into considerationthat the maximum value of the extraction percentages is at pH 6,SA exists as its anion state, suggesting that extraction proceeds bythe anion exchange mechanism between the nitrate and salicylateanions [2830]. In fact, at the source solution-membrane interface,salicylate (SA) forms a neutral ion-pair complex with ethylam-monium nitrate (RNH3+, NO3). Due to its concentration gradient,theion-paircomplex diffuses acrossthe membrane porosity. At thereceiving solution-membrane interface, salicylate is released in thepresence of anaqueous NaCl (alkaline) solution.The driving force toachieve uphill transport of SA is the difference in chloride concen-tration between both aqueous solutions. A simplified mechanismto describe the permeation of SA across an EAN-based SLM can besummarized using the following reactions:

SA+RNH3+,NO3 SA,RNH3++NO3 (4)

SA+RNH3++Cl SA+RNH3

++NO3

(5)

3.2. Effect of different strippants in the stripping solution

To compare [C6mim][PF6] and EAN salicylic acid recovery effi-ciencies, the feed solution was buffered in both cases at pH 3throughout the investigation.

The effect of the strippant nature in the receiving phase onthe SA transport efficiency was checked using several aqueous

strippants such as NaOH, NaCl and Na2CO3. The feed phase was,in all cases, a 103 M SA aqueous solution buffered at pH 3.In the case of [C6mim][PF6], the time courses of the concen-tration changes of SA in both phases using NaOH as stripping

agent are shown in Fig. 3(a). It can be noted that up to 20h oftransport, SA concentration decreases sharply in the feed phaseand, in contrast, increases in the receiving phase. Beyond 20 h oftransport, the concentration profile in both solutions reaches aplateau. Similar time courses trends were registered using NaCland Na2CO3 as strippants. The plateau appearance is probablydue to the disappearance of the pH difference between aqueousphases subsequent to proton permeation, as shown in Fig. 3(b).This pH difference disappearance has been also observed by Mat-sumoto et al. [29] studying the SLM permeation of penicillin Gusing [C4mim][PF6], [C6mim][PF6] and [C8mim][PF6] as ionic liq-uids. Hence, pH was manually maintained during the run aroundinitial valuesby adding concentratedNaOH solution to theaqueousreceiving phase, and time courses, when NaCl is used as strip-

pant, are shown in Fig. 4. We can see a slight improvement onthe recovery percentage and almost 53% was transported to thereceiving phase after 27 h of transport process. As seen in this fig-ure, uphill transportdriven by pH difference was notobserved. Thesame behaviour (absence of uphill transport) was observed whenusing the other strippants as well as by Matsumoto et al. [29] whenstudying the extraction of penicillin G through a SLM impregnatedwith different imidazolium-based ionic liquids. Indeed, despite themaintenance of the pH difference between the source and receiv-ing phases, uphill transport of SA and of the antibiotic was notobserved.

100806040200

0,0

0,2

0,4

0,6

0,8

1,0 Feed phasea b

Stripping phase

[SA](10-3mol.L-1)

Time (h)100806040200

0

2

4

6

8

10

12Feed phase

Stripping phase

pH

Time (h)

Fig.3. Time courses ofSA concentration (a)andpH (b)in feed andstripping phases.Feed solution:[SA]= 103 M bufferedat pH3; strippingsolution: NaOH 102 M; polymeric

support: PVDF; organic phase: [C6mim][PF6].


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100806040200

0,0

0,2

0,4

0,6

0,8

1,0

[SA](10-3mol.L-1

)

Time (h)

Feed phase

Stripping phase

Fig. 4. Time courses of SA concentration in feed and stripping phases with pH con-trol. Feed solution: [SA]= 103 M buffered at pH 3; stripping solution: NaCl 102 M;polymeric support: PVDF; organic phase: [C6mim][PF6].

Table 1

Effect of the strippant nature on the SA initial flux. Feed solution: [SA]=103 Mbuffered at pH 3; polymeric support: PVDF; transport time: 8h.

Strippant (102 M) Initial flux (106 molm2 s1)

[C6mim][PF6] EAN

NaOH 2.1 2.7Na2CO3 1.7 3.1NaCl 1.5 3.2

The experimental results concerning the transport of SA acrossthe EAN-SLM using NaCl as strippant are shown inFig. 5(a). It isworthy to note that after 24 h of transport process, around 48% ofSAwas transportedto thereceivingphaseandafterward a plateau isreached. As in the case of[C6mim][PF6], the reachedconstant value

is may be in relation with the disappearance of the pH difference,as shown inFig. 5(b). Comparable time courses trends were alsoobtained using NaOH and Na2CO3as strippants.

Forbothusedionicliquids, theSA initial flux, calculated from thefirst 8 h of transport experiment, has been estimated using NaOH,NaCl and Na2CO3as strippants and results are reported in Table 1.As it can be seen, comparable initial fluxes were obtained. Eventhough, NaOH and NaCl seem to be the most efficient strippants inthe case of [C6mim][PF6] and EAN, respectively.

Despite the lower extraction efficiencies using [C6mim][PF6] orEAN (around 50%), the elaborated SLM system can be promisingly

applied using more efficient ionic liquids. Actually, this systemrecommends low capital investment and operating cost and lowenergy consumption compared to other membrane processes.Kumar et al. [31]reported a SA recovery with current efficiencyclose to 90% but the energy consumption was around 10 kW h/kgof the SA produced. On the other hand, Payan et al.[32]suggestedthe application of hollow fiber-based liquid-phase microextraction(HF-LPME) using a polypropylene membrane supporting dihexylether for the determination of acidic pharmaceuticals (ibuprofen,diclofenac and salicylic acid) in wastewaters. These authors men-tioned thatthe extractioneffectiveness for the analyzed substancesis unrelated to the type of wastewater, remaining practically con-stants (about 100% SA, 71% diclofenac and 52% ibuprofen). In spiteof this SA higher extraction efficiency, no data about the dihexylether-SLM system stability have been provided.

3.3. Effect of strippant concentration

The effect of NaOH and NaCl concentrations in the receivingphase on the SA recovery percentage using [C6mim][PF6] and EANareshownin Fig.6(a) and(b), respectively. As observed in these fig-ures, therecovery efficiencyappears tobe more orless independentof the strippant concentration. A concentration of 102 M of strip-pant can be considered as adequate to allow the dissociation of thecomplex formedbetween SA andthe ionic liquidat the membrane-receiving phase interface andthus the release of SA in the receivingaqueous phase. Similar phenomena were observed in the recoveryof phenol from aqueous solution by a SLM system using vegetableoils as liquid membrane and NaOH as stripping agent[33]and thefacilitated transport of citric acid using tri-n-octylamine as carrierand Na2CO3and NaHCO3as strip pants[34].

3.4. Effect of salicylic acid concentration

The influence of the initial SA source concentration on the SLMextraction efficiency was also investigated. This study was carried

out using source solutions containing different SA concentrationsranging from 103 to 101 M, and results are shown in Fig. 7as the SA initial flux against the SA concentration in the sourcesolution. This shows that under the experimental conditions andfor both used ionic liquids the flux is influenced by the initial SAconcentration, being enhanced when the SA concentration in thefeed solution increases. Moreover, in the studied concentrationrange, we did not observe the typical plateau of the flux. Simi-lar observations were reported in SLM studies of urea transport bymacro-cyclic carriers [35], for phenol transport [36]and facilitatedtransport of lactic acid and its ethyl ester [37]. An increase in flux

1008060402000

1

2

3

4

5

6

7

Feed phase

Stripping phase

pH

Time (h)

100806040200

0,0

0,2

0,4

0,6

0,8

1,0Feed phase

Stripping phase

[SA](10-3mol.L-1)

Time (h)

a b

Fig. 5. Time courses of SA concentration (a)and pH (b)in feed andstripping phases. Feed solution:[SA]= 103 M buffered at pH 3; strippingsolution: NaCl102 M; polymeric

support: PVDF; organic phase: EAN.


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0,50,40,30,20,10,00

20

40

60

R%

[NaOH](mol.L

a b

-1)

1,00,80,60,40,20,00

20

40

60

R%

[NaCl](mol.L-1

)

Fig. 6. Effect of the strippant concentration on the SA recovery percentage. Feed solution: [SA]= 103 M buffered at pH 3; polymeric support: PVDF; transport time: 24 h. (a)[C6mim][PF6] and (b) EAN.

at low feed concentrations is expected, followed by an approachto a plateau value at high feed concentration when the carrier isfully loaded [35]. As the flux increases almost linearly through-out the studied concentration rangein this particular investigation,the liquid membranes have probably not reached saturation. Thisphenomenon can be attributed to the permeation process beingcontrolled by diffusion of solute species in the range of concentra-tion studied[38].

It can be also noticed that throughout the studied concentra-tion range, NEA seems to be slight efficient than [C 6mim][PF6] forthe transport of SA (initial flux) across a SLM based on these ionicliquids.

3.5. Transport mechanism

Despite theslight improvement on therecoveryefficiency whenpH is manually maintained during the run around initial values,

uphill transport driven by pH difference was not observed using[C6mim][PF6] or NEA as membrane liquid. The absence of uphilltransport can be attributed to the formation, in the ionic liq-uid membrane phase, of water microenvironments as reversedmicelles. Accordingly, solute (salicylic acid) transport throughthese microenvironments becomes the dominant mechanism aspointed out by Fortunato et al.[39]. The back transport of waterthrough a SLM containing an imidazolium or a phosphonium-based ionic liquids was identified experimentally by some authors[40,41]. Martak et al.[41]suggest that the transport of lactic acid

is closely related to the back transport of water in the reversemicelles which are formed on the stripping interface. It is impor-tant to mention that Cammarata et al.[42]reported the presenceof water aggregates (clusters), using IR spectroscopy, in the ionicliquid [C4mim][PF6], when this RTIL is saturated with water. Theformation of reverse micelles or water aggregates in some ionicliquids was also observed experimentally in molecular simulationstudies[43,44].

Thus, it is possible to conclude that, before the formation ofwater microenvironments, transport is mainly regulated by ionicliquid selectivity towards salicylic acid. However, along the courseof the experiment, a complete loss of selectivity is observed andSA transport through water microenvironments inside the ionicliquid becomes the main mechanism[39]. The formation of watermicroenvironments, inside the ionic liquid, is shown to be respon-sible for the deterioration of the liquid membranes performance,due not to a displacement of ionic liquid from the porous structureof the membrane, but to a marked loss of selectivity. A verification

and deeper understanding of this mechanism will require futureworks devoted to the study of the present system by advancedphysicochemical methods and by molecular modeling.

3.6. SLM stability

To study the long-term membrane stability, the SA trans-port efficiency for a period of 9 days on continuous run modeunder the optimum conditions without re-impregnation of the

0,100,080,060,040,020,00

0

5

10

15

20

25

Flux(10-5mol.m-2.s

-1)

[SA](mol.L-1) [SA](mol.L

-1)

0,100,080,060,040,020,00

0

5

10

15

20

25

30

Flux(10-5mol.m-2.s

-1)

a b

Fig. 7. Effect of the SA concentration on the initial flux. Feed solution: SA solution buffered at pH 3; polymeric support: PVDF; transport time: 8 h. (a) Stripping solution:

NaOH 102

M; organic phase: [C6mimPF6] and (b) stripping solution: NaCl 102

M; organic phase: EAN.


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1098765432100

20

40

60

R%

Day

1098765432100

20

40

60

R%

a

b

Fig. 8. Stability behaviour of the SLM system: SA recovery percentage as a functionof time. Feed solution: [SA]= 103 M buffered at pH 3; polymeric support: PVDF.(a) Stripping solution: NaOH 102 M; organic phase: [C6mimPF6] and (b) strippingsolution: NaCl 102 M; organic phase: EAN.

membrane was examined. The SA recovery percentage was mea-sured throughout 24 h, and after this period of time, depletedsource and enriched strip solutions were replaced with fresh ones.From the data reported in Fig. 8, it can be observed that bothionic liquids ([C6mim][PF6] and NEA) show only marginal differ-ences and do not exhibit a time dependent negative tendency.It can be concluded that after 9 days the membrane retainedits initial performance. This suggests a good expectancy for thelifetime of SLM with C6mim][PF6] or NEA, which is usually a crit-ical weakness of classical SLMs [16,17]. Likewise, Martak et al.[41]observed that the SLM with tetradecyl(trihexyl)phosphoniumbis(2,4,4-trimethylpentyl)phosphinate (IL-104) retained its initial

performance for lactic acid transport until 5.3 days.

4. Conclusion

Supported liquid membranes based on the ionic liquids 1-hexyl-3-methylimidazolium hexafluorophosphate ([C6mim][PF6])and ethylammonium nitrate (NEA) effectively transport salicylicacid from aqueous solutions. If the un-dissociated form of SA ismainly extracted in case of the imidazolium-based ionic liquid,an anion exchange mechanism is mentioned in the case of NEA.Under optimum operating conditions, an extraction efficiency of48% and 48.7%, a recoveryefficiency of47.2% and 48.5% and a flux of2.1106 and3.2106 molm2 s1 using [C6mim][PF6]andNEAareachieved,respectively. The back transportof water in thetrans-

port of SA through SLM with both ionic liquids was proposed asresponsible for the absence of uphill transport and hence, the dete-rioration of the liquid membranes performance. A stable recoveryefficiency of SA through SLM based on studied ionic liquids wasfound during the 9 days long experiment, which is promising.

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