solution the polymer removal for of phenanthrene fromaqueous Surface of modification electrospun polyester nanofibers withcyclodextrin Journal of Hazardous Materials

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Journal of Hazardous Materials

j o ur na l h o me p a g e :w w w . e l s e v i e r . c o m / l o c a t e / j h a z m a t

Surface modification of electrospun polyester nanofibers with cyclodextrin polymer for the removal of phenanthrene from aqueous solution

Fatma Kayaci, Zeynep Aytac, Tamer Uyar

UNAM-InstituteofMaterialsScience&Nanotechnology,BilkentUniversity,Ankara06800,Turkey

h i g h l i g h t s

•Electrospun PET nanofibers were surface modified with cyclodextrin polymer(CDP).

•ThreedifferenttypesofnativeCD(␣- CD, ␤-CD and ␥-CD)were used to formCDP.

•Nanofibrous structure of PET mats was preserved after CDP surface modification.

•PET/CDP nanofibers have shown enhanced mechanical and thermal properties.

•PET/CDP nanofibers efficiently remove PAH (e.g. phenanthrene) fromaqueoussolution.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Articlehistory:

Received4March2013

Receivedinrevisedform5July2013 Accepted18July2013

Available online 25 July 2013

Keywords:

Electrospinning Cyclodextrinpolymer Nanofibers Polyester Phenathrene

a b s t r a c t

Surfacemodifiedelectrospunpolyester(PET)nanofiberswithcyclodextrinpolymer(CDP)wereproduced (PET/CDP).CDPformationontoelectrospunPETnanofiberswasachievedbypolymerizationbetween citricacid(CTR,crosslinkingagent)andcyclodextrin(CD).ThreedifferenttypesofnativeCD(␣-CD,

␤-CDand␥-CD)wereusedtoformCDP.Water-insolublecrosslinkedCDPcoatingwaspermanently adheredontothePETnanofibers.SEMimagingindicatedthatthenanofibrousstructureofPETmatswas preservedafterCDPsurfacemodificationprocess.PET/CDPnanofibershaveshownrougher/irregular surfaceandlargerfiberdiameterwhencomparedtountreatedPETnanofibers.Thesurfaceanalysesof PET/CDPnanofibersbyXPSelucidatedthatCDPwaspresentonthefibersurface.DMAanalysesrevealed theenhancedmechanicalpropertiesforPET/CDPwherePET/CDPnanofibershaveshownhigherstorage modulusandhigherglasstransitiontemperaturecomparedtountreatedPETnanofibers.Thesurface areaofthePET/CDPnanofibersinvestigatedbyBETmeasurementsshowedslightdecreaseduetothe presenceofCDPcoatingcomparedtopristinePETnanofibers.Yet,itwasobservedthatPET/CDPnanofibers weremoreefficientfortheremovalofphenanthreneasamodelpolycyclicaromatichydrocarbon(PAH) fromaqueoussolutionwhencomparedtopristinePETnanofibers.OurfindingssuggestedthatPET/CDP nanofiberscanbeaverygoodcandidateasafiltermaterialforwaterpurificationandwastetreatment owingtotheirverylargesurfaceareaaswellasinclusioncomplexationcapabilityofsurfaceassociated CDP.

© 2013 Elsevier B.V. All rights reserved.

∗ Correspondingauthor.Tel.:+903122903571;fax:+903122664365.

E-mailaddresses:tamer@unam.bilkent.edu.tr,uyar@unam.bilkent.edu.tr, tameruyar@gmail.com(T.Uyar).

1. Introduction

Electrospun nanofibers and their nanofibrous mats have demonstratedhugepotentialforfiltrationapplicationsduetotheir 0304-3894/$–seefrontmatter © 2013 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.jhazmat.2013.07.041

highsurface-to-volumeratioand nanoporousstructure[1–3].It hasbeenreported that electrospunnanofibrousmats are quite effective for particulate separation [2,3], liquid filtration [1–3], wastevaportreatment[4,5]aswellasdesalination[6].Electro- spinninghasadvantageoverconventionalmembraneproduction techniques,sincevarietyoffunctionalnanofibrousmaterialscan beeasilyobtainedintheformofnonwovenmembraneswhichcan bereadilyusedasafilteringmaterial[1–3].Inaddition,thedesign flexibilityofelectrospunnanofibersforspecificsurfacefunction- alitycanyieldbetteradsorptivecapacityandselectiveseparation performance[7,8].

Cyclodextrins(CD)haveanoutstandingcapabilitytoforminclu- sioncomplexeswithvarietyofmoleculesthroughnon-covalent host–guestinteractionsduetotheirtoroid-shapedmolecularstruc- ture[9].CD arequite applicablein pharmacy, cosmetics, food, textiles, since CD might enhance the solubility, stability, and bioavailabilityoftheguestmolecules[9–11].Inaddition,CDhave alsopotentialtobeusedasafilteringmaterialduetotheirabilityto selectivelyforminclusioncomplexeswithorganicwastemolecules [12,13].CDarenontoxicandnaturalcyclicoligosaccharidesderived fromstarch.ThemostcommonCDtypesarenamedas␣-CD,␤-CD and␥-CDhavingsix,sevenandeightglucopyranoseunits,respec- tively.TheseCDhavethesamecavitydepthwhichis∼7.8 ˚A,while thediameterofthecavityfor␣-CD,␤-CD,␥-CDare∼6,8,and10 ˚A, respectively[9].Hence,␣-CD,␤-CDand␥-CDshowdifferentcapa- bilitiesfortheinclusioncomplexformationwiththesameguest molecule,becausetheformationofinclusion complexprimarily dependsonthesizematchandbindingforcesbetweenCDcavity andguestmolecule[14].

CDarewatersoluble,therefore,theycannotbeuseddirectly asafilteringmaterialfortheremovaloforganicpollutantsfrom water and wastewater. So, crosslinked and water-insoluble CD basedpolymersweresynthesizedforcapturingorganicpollutants fromthesurroundings[13].Alternatively,CDmoleculescouldbe permanentlyimmobilizedbychemicallygraftingontopolymeric fibers[15–18],orthesurfaceofthefiberscouldbemodifiedby crosslinkedCDpolymer[19–21]forfiltrationpurposesordelivery ofadditives.Moreover,inordertocombinethecomplexforma- tion capability of CD along with very highsurface areaof the electrospunnanofibrousmat,surfacefunctionalizationofelectro- spunnanofiberswithCDwouldbequiteinterestingfordesigning efficientfilteringmaterials.Infact,inourrecentstudieswehave incorporatedCDintonanofibersbyelectrospinningofphysicalmix- tureofpolymer/CDsolution[4,22,23].AlthoughmostoftheCD moleculeswereburiedinsidethefibermatrix,weobservedthat someCDmoleculeswerepresentonthefibersurface,andthese surfaceassociated CD moleculeswereeffectivefor theremoval oforganicmoleculesfromliquidmedia[22,23]andvaporphase [4].However,CDmoleculeswerephysicallyattachedtothefiber surface;so,theleaching ofCDmoleculesfromthefibersurface duringfiltrationespeciallyintheliquidmediawasinevitable.Con- sequently,permanentCDmodificationontoelectrospunnanofibers wouldbeidealfor designing novelfilteringmaterialsfor water purificationandwastewatertreatment.Eventhoughsurfacemod- ificationsoffibersandnonwovenfabricsbyCDgrafting[15–17]

orcoatingwithcrosslinkedCDpolymer[19–21,24]werereported, tothebestofourknowledge,thesurfacemodificationofelectro- spunpolymericnanofiberswithcrosslinkedCDpolymerwasnot reportedpreviously.

In this study, we have achieved thesurface modification of theelectrospunpolyester(PET)nanofiberswithcyclodextrinpoly- mer(CDP).Water-insolubleandcrosslinkedCDPcoatingontoPET nanofiberswasformedbythepolymerizationreaction between CDandcrosslinkingagent(citricacid).Foracomparativestudy, threetypesofCD(␣-CD,␤-CDand␥-CD)wereusedinorderto formCDPcoatingontoPETnanofibers.Themorphological,surface,

thermalandmechanicalpropertiesofsurfacemodifiedelectrospun PETnanofiberswithCDP(PET/CDP)wereexamined.Thefiltration performance ofthePET/CDPnanofibrousmatswasinvestigated byremovalofamodelpolycyclicaromatichydrocarbon(phenan- threne)fromaqueoussolution.

2. Materialsandmethods 2.1. Materials

Polyethyleneterephthalate(PET)chipsweregiftsfromKorteks (Bursa, Turkey). Dichloromethane (DCM, Sigma Aldrich, extra pure),trifluoroaceticacid(TFA,AlfaAesar,99%),acetonitrilechro- masol V (Sigma Aldrich, 99.9%), citric acid monohydrate-gritty puriss(CTR,SigmaAldrich,99.5–100.5%),sodiumhypophosphite hydrate(SHPI,SigmaAldrich),phenanthrene(SigmaAldrich,98%), andcyclodextrins(␣-CD,␤-CDand␥-CD,WackerChemieAG)were purchasedandusedas-receivedwithoutanypurification.Distilled waterwasfromMilliporeMilli-Qultrapurewatersystem.

2.2. Preparationofpolymersolutionandelectrospinningof nanofibers

First,differentpolymerconcentrationswereusedfortheelec- trospinningofPETsolutioninordertoobtainuniformandbead-free PETnanofibers,and22.5%(w/v)polymerconcentrationwasfound tobe theoptimal. Therefore, 22.5% (w/v)PET wasdissolvedin TFA/DCM(50/50,v/v),andtheresultingsolutionwasloadedinto 5mLsyringefittedwithametallicneedlehavinganinnerdiameter of0.8mm.Then,thesyringewasplacedhorizontallyonthesyringe pump(KDScientific,KDS101).Thepolymersolutionwaspumped with1mL/hflowrateduringtheelectrospinning,andthedistance wassetto12cmbetweenneedletipandgroundedstationarycylin- dricalmetalcollector(height:15cm,diameter:9cm)coveredwith apieceofaluminumfoil.Avoltageof15kVwasappliedforthe electrospinningbyusinghighvoltagepowersupply(Matsusada, AUSeries).Theelectrospinningprocesswascarriedoutat24.5^◦C and17%relativehumidityinanenclosedPlexiglasbox.

2.3. Formationofcyclodextrinpolymer(CDP)ontoPETnanofibers

10%(w/v)of␣-CD,␤-CDand␥-CDwasmixedindividuallyin 150mLaqueoussolutionat50^◦C, andthen,10%(w/v)CTRasa crosslinkingagentand1.2%(w/v)SHPIasacatalystwereaddedto eachCDsolutionseparately,andstirredfor30minat50^◦C.After allreactantsweredissolvedinaqueoussolution,threerectangular shaped(about12cm×11cm,0.4g)electrospunPETnanofibrous matswereimmersedintotheeachresultingsolutionandkeptfor 3hat50^◦C.Thenthesenanofibrousmatsweredriedat105^◦Cfor 10min,andthencuredat180^◦Cfor7minfortheCDPformation ontoPETnanofibers.Finallytheresultingnanofibrousmatswere washedtwotimeswithwarmwater(40^◦C)fortheremovalofunre- actedCDandCTRifanypresent,andthendriedat105^◦Cfor7min.

Inordertomakeclearidentification,CDPmodifiedPETnanofibers arenamedasPET/␣-CDP,PET/␤-CDPandPET/␥-CDPaccordingthe typeofCDused(␣-CD,␤-CDand␥-CD).

2.4. Characterizationsandmeasurements

The morphology and the diameter of the PET and PET/CDP nanofiberswereexaminedbyusingscanningelectronmicroscope (SEM,FEI-Quanta200FEG).Thenanofiberswerecoatedwith5nm Au/PdpriortoSEManalysis.Toreporttheaveragefiberdiameter (AFD)ofthenanofibers,around100fibersofeachsample were measured.

The chemical surface analyses of the PET and PET/CDP nanofiberswerecarriedoutbymeansofhigh-performanceX-ray photoelectronspectroscopy(XPS,ThermoScientific).XPSdatawere takenbyafloodgunchargeneutralizersystemequippedwitha monochromatedAlK-␣X-raysource(hv=1486.6eV).Inorderto determinethesurfaceelementalcompositionswideenergysurvey scansofthenanofiberswereacquiredoverthe0–1360eVbinding energyrange,atpassenergyof150eVwithenergystepsizeof1eV from400␮mdiametercircularspotinnanofibers.Thehighresolu- tionspectrawererecordedforO1sregionatpassenergyof30eV andwithenergystepsof0.1eVinordertoanalyzethebonding states.

Thethermalanalysesofthesampleswereinvestigatedbyusing thermogravimetricanalyzer(TGA,TAQ500).TGAmeasurements werecarriedout underthenitrogenatmosphere, andthesam- pleswereheatedfromroomtemperatureto600^◦C(nanofibers) or500^◦C(CTRandCD)ataconstantheatingrateof20^◦C/min.

Thedynamicthermo mechanical performanceof thenanofi- brousmatswasdeterminedusingadynamicmechanicalanalyzer (DMA, TA Q800) in tension film clamp at a constant fre- quencyof1Hz.Thesampleshavingsizeof10mm(gap)×∼3mm (width)×∼0.12mm(thickness)weremeasured.Theamplitudeof 20␮mwasapplied.Thestoragemodulusandlosstangent(tanı) ofthenanofibrousmatswererecordedintherangeof50–150^◦Cat aheatingrateof3^◦C/min.

Thesurfacearea,averageporediameter(mesopore)andcumu- lativeporevolumeoftheelectrospunPETandPET/CDPnanofibers were examined using Brunauer–Emmett–Teller (BET) surface areaanalyzer(Quantachrome,IQ-Cmodel)withlow-temperature (77.35K) nitrogenadsorption isotherms measured over a wide rangeofrelativepressuresfrom0.00to1.00.Priortomeasurement, theeachsamplewasplacedina9mmcellanddegassedat323.15K for12hinthedegaspotoftheadsorptionanalyzer.Thesurfacearea ofthesampleswasdeterminedwithmultipointBETmethod.Onthe otherhand,densityfunctionaltheory(DFT)wasusedtodetermine cumulativeporevolume.

Themolecularfiltrationperformanceoftheresultingnanofi- brousmatsforwaterpurificationwastestedusingphenanthrene asamodelpolycyclicaromatichydrocarbon(PAH).First,phenan- threnewasdissolvedinacetonitrile,andthen10␮Lofthissolution was dropped in 50mL pure water in order to obtain 1.8ppm phenanthreneaqueoussolution.SquareshapedofPET,PET/␣-CDP, PET/␤-CDPand PET/␥-CDPnanofibrousmats(6cm×6cm)were immersedindividuallyinthe1.8ppmphenanthreneaqueoussolu- tion(50mL).Wekeptthesizeofthematsidentical;however,the weightofeachPET/CDPmatwasabout0.38g,whilethatofPET nanofibrousmatwasabout0.63gduetodifferenceinthethickness ofthemats,sincethenanofiberswerecollectedindifferenttimes foreachsample.Itisquitedifficulttokeepthethicknessofthe electrospunmatsevenidenticaltime.Forfiltrationmeasurements, 0.5mLofeachsolutionwaswithdrawntomeasurephenanthrene concentrationinthesolutionandreplenishedwithsameamount ofwateratpre-determinedtimeintervals.Thephenanthrenefil- trationperformancefromaqueoussolutionbyPETandPET/CDP nanofibrousmats was investigated by highperformance liquid chromatography(HPLC,Agilient1200series)equippedwithVWD UVdetector.ThecolumnwasAgilientC18,150mm×4.6mm(5␮m pores) and thedetection was accomplished at 254nm. Mobile phase,flowrate,injectionvolumeandtotal runtimewereace- tonitrile(100%), 0.6mL/min,10␮Land5min,respectively. Asa result,theamountofphenanthreneremaininginthesolutionwas determinedfromtheareaofphenanthrenepeakobservedinHPLC chromatograms.Thenthecalibrationcurvewaspreparedbyusing phenanthrene solutions (1.8ppm, 0.9ppm, 0.45ppm, 0.23ppm, 0.12ppm)and R² wascalculatedas0.985.Thepeakareaunder curves wasconverted toconcentration (ppm)according tothe

calibrationcurve.Thisexperiment wasrepeatedthreetimesfor eachsample.Theresultswerereportedastheaverage±standard deviationofphenanthreneconcentrationremaininginthesolution.

3. Resultsanddiscussion

3.1. TheCDPformationontoelectrospunPETnanofibers

In this study, polyester (PET) nanofibers were obtained by electrospinningof22.5% (w/v)PETsolutionin TFA/DCM(50/50, v/v), as it is schematicallygiven in Fig. 1a. The chemical reac- tioncannotoccurbetweencyclodextrin(CD)/citricacid(CTR)and PETnanofibers directly,since PET, a polymerbased ontereph- talic acid and ethylene glycol, does not contain free reactive groups.Therefore,wemodifiedthesurfaceoftheelectrospunPET nanofibersthroughthepolymerizationreactionbetweenCTRand CD[21,24,25].Water-insoluble cyclodextrinpolymer (CDP)net- workwasformedbythecrosslinkingreactionbetweenCD and CTR[26].ThreedifferenttypesofnativeCD(␣-CD,␤-CDand␥-CD) wereusedtoform␣-CDP,␤-CDPand␥-CDP.Initially,electrospun PETnanofibrousmatswereimpregnatedinasolutionofCD,CTR, andsodiumhypophosphite(SHPI, catalyst),and thendried,fol- lowedbycuringat180^◦Cfor7min.CTRturnintoacyclicanhydride intermediatebythermaldehydrationatelevatedtemperature,and thenhydroxylgroupsofCDreactedwiththecarboxylgroupsof citricacid[25].ThemechanismoftheCDPformationisschemati- callydescribedinFig.1b.CDPwasformedasathree-dimensional networkstructureontoPETnanofibers.Duetocrosslinkedstruc- ture,theCDPisstableandwater-insoluble[27,28].Thereby,surface modificationof CDPonto PETnanofibersis permanent andcan resisttoleachingorwashingprocess[21,24,25,29].Theresulting CDPhavingtheessentialstructuralcharacteristicsofCDwasnot covalentlyfixedtothePETnanofibers,butitwasphysicallyadhered orwasentangledontoPETfibermatrix[21,24].CDPmodifiedPET nanofibersarecalledasPET/CDP.Therepresentativephotograph oftheeasilyhandledfree-standingPET/CDPnanofibrousmatand theschematicrepresentationofPET/CDPnanofibersaregivenin Fig.1c.

3.2. Morphologicalcharacterizationofthenanofibers

Scanningelectronmicroscope(SEM)analysiswasperformedto investigateanymorphologicalchangesafterthesurfacemodifica- tionofPETnanofiberswithCDP.Fig.2showstherepresentative SEM images and average fiber diameter (AFD) of unmodified PET,PET/␣-CDP,PET/␤-CDPandPET/␥-CDPnanofibers.Asclearly seen from SEM images, the surface morphologies of all three PET/CDPnanofiberswereobviously differentfromtheunmodi- fiedPETnanofibers.ThesurfaceoftheunmodifiedPETnanofibers wassmooth anduniform, whereas thesurfacesofthePET/CDP nanofibersappearroughpossiblyduetoCDPlayerontonanofibers.

The rough surface has also been reported for cotton fabrics graftedwithglycidylmethacrylate/␤-CD[17]andhydroxypropyl- CD grafted woven PET vascular prosthesis [20,21]. Moreover, surface irregularities at certain points were also observed in the SEM images of PET/CDP nanofibers. Similar morphological observations were also reported for cotton fabricgrafted with monochlorotriazinyl-␤-CD/butylacrylate [30].Inbrief,therough andirregularsurfaceofmodifiedPETnanofiberssuggestedthesuc- cessfulattachmentofCDPontoPETnanofibers.Moreimportantly, CDPsurfacemodificationprocessdidnotdeformthefibrousstruc- tureofPETasclearlyseenfromtheSEMimages.Theunmodified PETnanofibershave870±260nmofAFD,whiletheAFDofPET/␣- CDP, PET/␤-CDP and PET/␥-CDP weremeasured as 1200±350, 1290±490and950±270nm,respectively.TheincreaseintheAFD

Fig.1. Schematicrepresentationsof(a)electrospinningofPETnanofibers,(b)formationmechanismofCDPand(c)therepresentativephotographofPET/CDPnanofibrous matanditsSEMimageandschematicrepresentationofPET/CDPnanofibers.

of PET/CDPnanofiberscompared to unmodified PETnanofibers could be due to the coating of the CDP onto PET nanofibers.

Additionally,slightswellingofnanofibersduringthemodification processmightalsohaveresultedinfiberdiameterincrease.

3.3. Surfacechemicalcharacterizationofthenanofibers

Thesurface chemicalcharacterization ofPET/CDPnanofibers wasperformedbyusingX-rayphotoelectronspectroscopy(XPS) in order to further demonstrate the coating of CDP onto PET nanofibers. Table 1 shows elementary compositions based on wideenergysurveyspectraoftheunmodifiedPETnanofibersand PET/CDPnanofibers.Thesurveyspectracomprisingtwopeaks:C

Table1

AtomicconcentrationsgeneratedfromXPSwideenergysurveyscans.

Samples C(%) O(%)

PET 72.21 27.79

PET/␣-CDP 64.92 35.08

PET/␤-CDP 61.29 38.71

PET/␥-CDP 67.89 34.31

1sandO1sareconsistentwiththemolecularstructureofPETand CDP.TheXPSdatashowedthattheunmodifiedPETnanofibershave C1s:O1s=72.21:27.79(%)whichisinfullagreementwiththelit- erature[31].Oxygencontentonthesurfaceofthesampleswas increasedwiththemodificationofCDPontoPETnanofibers.Thus, theappearanceofhigheroxygencontentprovidesanevidenceof thepresenceofCDPonthePETfibersurfaces.

High-energyresolutionO1sXPSspectrawerealsorecordedto getmoredetailedchemicalstateinformationaboutsurfacechem- istryofthePET/CDPnanofibers.Fig.3showsthenormalizedO1s spectraofPETandPET/␥-CDPnanofibers.Theassigneddifferent componentswithinthesespectraandtheirindividualizedfitting parameters(peakbindingenergyand%arearatio)arealsogiven inTable2.SincetheO1sspectraofallPET/CDPnanofibers(PET/␣- CDP,PET/␤-CDPandPET/␥-CDP)aresimilartoeachother,those XPSdataacquiredforPET/␣-CDPandPET/␤-CDPnanofiberswere notgiven.TheO1sspectrumofunmodifiedPETnanofibersclearly representthetwotypesofoxygenatomswithintheestergroups;

␲-bondedoxygen(C O^*)and␴-bondedoxygen(C O^*)atbinding energiesof531.54and533.12eV,respectively[32–35].Theratioof thesepeaksis56.2:42.1,whichisinreasonableagreementwiththe theoreticalratioof50:50[36].InadditiontotheseexpectedO1s

Fig.2. RepresentativeSEMimagesandAFDof(a)PET,(b)PET/␣-CDP,(c)PET/␤-CDPand(d)PET/␥-CDPnanofibers.Theinsetsshowhighermagnificationimages.

peaks,PETnanofibershaveaverysmallpeaksituatedat534.52eV assignedtoadsorbedwater[32].AftertheCDPmodificationonthe surfaceofPETnanofibers,thecontributionofadditionalO1sfit- tingpeakat532.35relatedtoaliphaticC O^*Hcameintoview.The appearanceofC O^*Hcomponentbelongstohydroxylgroupsand carboxylgroupsofCDPelucidatedthesuccessfulsurfacemodifica- tionofPETnanofiberswithCDP.Moreover,asitwasexpected,CDP

modificationontoPETnanofibersresultedinsignificantincreaseof relativeXPSsignalintensityintheO1speaksituatedat533.06eV assigned to␴-bonded oxygen(C O^*)compared with␲-bonded oxygen(C O^*)locatedat532.35eV.Inbrief,therearethreediffer- entcomponents(C O^*,C O^*andC O^*H)forO1shigh-resolution spectraofthePET/CDPnanofibers.Theincreaseinoxygencontent ofPET/CDPnanofiberscomparedtounmodifiedPETnanofiberswas

Fig.3.HighresolutionO1sXPSspectraofPETandPET/␥-CDPnanofibers.

Fig.4.(a)TGAthermogramsofCTRandthreeCDtypes(␣-CD,␤-CDand␥-CD),(b)TGAandderivativeTGA(inset)thermogramsofnanofibers.

especiallyduetotheappearanceofC O^*Hforthesamples.The presenceofCDPonthefibersurfaceisquiteimportantintermsof thefiltrationapplicationofPET/CDPnanofibrousmats[4,22,23].

3.4. Thermalcharacterizationofthenanofibers

ThethermalcharacteristicsofthePET/CDPsampleswereinves- tigatedbyusingthermogravimetricanalyzer(TGA).InFig.4,the TGAthermogramsofCTRandCD(␣-CD,␤-CDand␥-CD)(Fig.4a), andunmodifiedPETandPET/CDPnanofibers(Fig.4b)aregiven.

Moreover,thederivativeTGAthermogramsofnanofibersarealso shownasinsetinFig.4b.TheweightlossforCTRstartedataround 130^◦C,andCTRcompletelydegradedbefore250^◦C.TGAthermo- gramsofCD(␣-CD,␤-CDand␥-CD)presentedaninitialweight lossbelow100^◦Candamajorweightlossbetween300and350^◦C whichcorrespondtothewaterlossandmaindegradationofCD, respectively[37].ThemaindegradationofPETnanofibersoccurred between375and475^◦C. ForthePET/CDPnanofiberstwomajor weightlosseswererecordedbetween200–350^◦Cand375–475^◦C whichcorrespondtomainthermaldegradationofCDPandPET, respectively. The% weightloss between200and 350^◦C corre- spondingtoCDPinthePET/CDPnanofiberswas23%,44%and32%

forPET/␣-CDP,PET/␤-CDPandPET/␥-CDPnanofibers,respectively, suggestingthattheamountofCDPcoatingontoPETnanofiberswas ontheorderof␤-CDP>␥-CDP>␣-CDP.Whenthederivativeweight

%losswasanalyzed(Fig.4b),itwasobservedthatthepeakpoint fortheunmodifiedPET(∼437^◦C)shiftedslightlytohighertemper- ature(∼445^◦C)forthePET/CDPnanofibers.Thisindicatedthatthe modificationofPETnanofiberswithCDPresultedinslightlyhigher thermalstabilityduetomoreenergyrequirementfordecomposi- tionofthesesampleshavingcrosslinkedstructure.Theincreased thermalstabilityhasbeenalsoobservedforCDgraftedpolyamide 6fabrics[38].Moreover,thecharyieldwashigherforPET/CDP nanofiberswhencomparedtounmodifiedPETnanofiberspossibly owingtothecrosslinkedCDPstructure providinghighercarbon residueuponburning.

Table2

FittingparametersoftheO1sXPSspectraofPETandPET/␥-CDPnanofibers.

Samples Fittingpeaks Bonds Peakbinding energy

Arearatio (%)

PET O1s#1 C O^* 533.12 56.2

O1s#2 C O^* 531.54 42.1

O1s#3 AdsorbedH2O 534.52 1.7

PET/␥-CDP O1s#1 C O^* 533.06 42.5

O1s#2 C O^* 531.5 27.7

O1s#3 C O^*H 532.35 29.8

3.5. Mechanicalcharacterizationofthenanofibers

Dynamicmechanicalanalyzer(DMA)wasusedtoinvestigate theeffectofCDPmodificationonthethermomechanicalproper- tiesofthePETnanofibers.Thestoragemodulusandlosstangent (tanı)oftheunmodifiedPETandPET/CDPnanofibrousmatswere recordedupto150^◦C(Fig.5).Thestoragemodulusofthesam- plesdecreasedwithincreasingtemperatureduetothetransition fromglassystatetorubbery state.Itwasobservedthatstorage modulusofthePET/CDPnanofibrousmatswasmuchhigherthan theunmodified PET nanofiberspossibly dueto stiffeningeffect of crosslinkedCDP coating. Sincethe transferredstress forPET nanofiberswas sharedby CDP coating,the storagemodulus of PET nanofibersenhanced withCDP modification. Moreover,for CDPmodifiednanofibers,tanıpeakshiftedtothehighertempera- tureregionindicatingthattheglasstransitiontemperature(T_g)for thesenanofiberswashigherwhencomparedtounmodifiedPET nanofibers.TheTgvalueofPETnanofiberswas92^◦C,whiletheTg

valuesofPET/CDPnanofiberswererecordedas109,112and113^◦C forPET/␣-CDP,PET/␤-CDPandPET/␥-CDPnanofibers,respectively.

ThisresultsuggestedthatthemobilizationofPETmacromolecular chainswereaffectedandthesegmentalmotionofPETchainswere hinderedbyCDPmodification.Furthermore,broadertan␦peaks observedforPET/CDPnanofiberswhichcanbeoriginatedfromtwo TgvaluescorrespondtonotonlyPET,butalsoCDP[39].

3.6. Surfaceareaofthenanofibers

Thesurfacearea,averageporediameterandcumulativepore volumeofthePETandPET/CDPnanofiberswereinvestigatedby BET measurementsandthedatais summarized inTable3.The resultsindicatedthatthemultipointBETsurfaceareaofelectro- spunPETnanofibersis6.03m²/g.Thesurfaceareadecreased to 1.56,0.57and0.72m²/gforPET/␣-CDP,PET/␤-CDPandPET/␥-CDP nanofibers,respectively.AsmentionedinSEMcharacterization,the surfacemodificationofthePETnanofiberswithCDPresultedinthe irregularitiesonthefibersurfacesandAFDforthesesampleswere

Table3

Surfacearea,averageporediameterandcumulative porevolumedataofthe nanofibers.

Samples MultipointBET surfacearea(m²/g)

Averagepore diameter(nm)

DFTcumulative porevolume(cc/g)

PET 6.03 15.3 1.03×10⁻²

PET/␣-CDP 1.56 12.4 3.22×10⁻³

PET/␤-CDP 0.57 13.6 1.28×10⁻³

PET/␥-CDP 0.72 14.0 1.05×10⁻³

Fig.5.DMAthermogramsofnanofibrousmats(a)storagemodulusand(b)losstangent(tanı).

increasedaswell,andtherefore,thesurfaceareaofthePET/CDP nanofibersweredecreased.Thesurfaceirregularitiesofnanofibers suchascrosslinkedCDP coating areclearly observed especially intheSEMimagesof thePET/␤-CDPandPET/␥-CDPnanofibers (Fig.2candd).Hence,thesurfaceareaofPET/␤-CDPandPET/␥- CDPnanofiberswaslessthanthatofPET/␣-CDP.Moreover,since PET/␤-CDPhasthelargestAFDamongthesamples(Fig.2c),thesur- faceareaofPET/␤-CDPnanofiberswasslightlylessthanPET/␥-CDP nanofibers.ItiswellknownthattheAFDhavegreateffectonthe surfaceareaoffibers[40].Wehavealsocalculatedthemesopore structure(averageporediameterandcumulativeporevolume)of thePETandPET/CDPnanofibers.Itwasobservedthat,theaverage porediameterandcumulativeporevolumedeterminedbydensity functionaltheory(DFT)alsodecreasedaftersurfacemodificationof thePETnanofiberspossiblyduetothecrosslinkedCDPcoatingonto nanofibersurfacewhichresultedinsurfaceirregularities.Inshort, thesurfaceareaofthePET/CDPnanofiberswasdecreasedduetothe presenceofCDPcoatingcomparedtopristinePETnanofibers,nev- ertheless,asdiscussedinthefollowingsection,PET/CDPnanofibers weremore efficientfor the removalof the phenanthrenefrom aqueoussolutionwhencomparedtopristinePETnanofibers.

3.7. Molecularfiltrationperformanceofthenanofibersforwater purification

ThemolecularfiltrationcapabilityofPETandPET/CDPnanofi- brous mats has been tested using a phenanthrene as a model polycyclicaromatichydrocarbon(PAH).Phenanthreneisacommon

pollutantandcanforminclusioncomplexeswithCD[12,41,42].

Fig.6summarizesthecumulative%decreaseofphenanthrenecon- centrationovertimewhenPETandPET/CDPmatshavebeenkeptin aqueoussolutionofphenanthrene.AsseeninFig.6,theconcentra- tionofphenanthreneintheaqueoussolutiondecreasedwithinthe contacttime.TheadsorptionofphenanthrenebyPETnanofibersfor thefirst2hwasobserved,andthentheconcentrationofphenan- threneslightlydecreasedovertime.Ontheotherhand,thedecrease ofphenanthreneconcentrationforPET/CDPmatswasmoresig- nificant.AlthoughlessamountofPET/CDPnanofiberswereused comparedtoPETnanofibersfor filtrationtest, theremoval effi- ciencyofthephenanthrenefromitsaqueoussolutionwasbetter whenPET/CDPnanofiberswereused.Water-insolubleCDPcanbe veryeffectiveinremovalofmanyorganicpollutantsfromaqueous media,sinceCDcavityiscapableofforminginclusioncomplexes withawidevarietyoforganicmolecules[9–11,13,18,22,28].There- fore,thesurfacemodificationofelectrospunPETnanofiberswith CDPincreasedtheefficiencyoffiltrationbyfacilitatingcomplex formationwithphenanthrenecompounds.Here,allthreePET/CDP nanofibersdemonstratedtheabilitytofunctionasamolecularfilter forwaterpurificationthroughcomplexationofthephenanthrene withCDP.Asit mentioned in theprevioussection itis notable thatthesurfacearea,averageporediameterandcumulativepore volumeofnanofibersweredecreasedafterCDPmodification.How- ever,themolecularfiltrationefficiencywasstillfurtherimproved forPET/CDPnanofiberscomparedtopristinePETnanofibersdue totheCDPstructure ontonanofibers,whichplaysa crucialrole inmolecularcapturingofphenanthrene.WhentheCDtypeswere

Fig.6.Time-dependentdecreaseofphenanthreneconcentrationintheaqueoussolutioncontainingnanofibrousmats.

Fig.7.RepresentativeSEMimagesof(a)PET,(b)PET/␣-CDP,(c)PET/␤-CDPand(d)PET/␥-CDPnanofibersafterthefiltrationtest.Theinsetsshowhighermagnification images.

compared,allthreePET/CDPsamplesshowedapproximatelysame filtrationefficiencyfortheremovalofphenanthreneattheendof filtrationtest.AlthoughTGAsuggestedthattheamountofCDPcoat- ingontoPETnanofiberswasontheorderof␤-CDP>␥-CDP>␣-CDP, itislikelythatnotalltheCDmoleculesareavailableforcomplexa- tion.So,thethreePET/CDPsamplesmayhavecomparableamount ofCDcavityavailableforcomplexation.Evenso,theaverageper- centageremovalofphenanthrenewithrespecttoinitialtimewas slightlybetterforPET/␣-CDPand thisispossiblybecauseofthe highersurfaceareaofPET/␣-CDPnanowebcomparedtoPET/␤-CDP and PET/␥-CDP nanowebs. We have alsoinspected thedimen- sionstabilityofthePETandPET/CDPnanofibers,andweobserved thatthematskepttheirnanofibrousstructureafterthefiltration test(Fig.7).Inshort,thesurfacemodificationofelectrospunPET nanofiberswithCDPenhancedtheefficiencyofitsfiltrationperfor- mancebyfacilitatingcomplexformationwithorganiccompounds suchasphenanthrene.

4. Conclusion

In this study, we have achieved thesurface modification of electrospunPETnanofiberswithCDP.First,PETnanofiberswere obtained via electrospinning, then, water-insoluble crosslinked CDPcoatingwasformedontoPETnanofibersbypolymerization reaction betweenCD and crosslinking agent (citric acid).For a

comparativestudy,threedifferenttypesofCD:␣-CD,␤-CDand␥- CDwereusedtoformCDPontoelectrospunPETnanofibers.The imaging analysisbySEMrevealedthat nanofibrousstructureof thePETnanofiberswaspreservedaftersurfacemodificationwith CDP.Yet,thesurfaceofthePET/CDPnanofiberswasrough/irregular, whereasthatofunmodifiedPETnanofiberswassmooth.Moreover thediameterofthePETnanofibersincreasedafterCDPmodification possiblyduetothepresenceofCDPlayerontonanofibersand/or swellingofthenanofibersduringmodificationprocess.BETmea- surementsindicatedthatthesurfaceareaofthePET/CDPnanofibers wasdecreasedduetothepresenceofCDPcoatingcomparedtopris- tinePETnanofibers.ThepresenceofCDPcoatingonthesurfaceof PETnanofiberswassupportedbyXPSanalyses.Thethermalanal- ysisofPET/CDPnanofiberscarriedoutbyTGAshowedtwomain thermaldegradationstepscorrespondingtoCDPandPETdegra- dation.ThemodificationofPETnanofiberswithCDPresultedin slightlyhigherthermalstability,andthecharyieldwashigherfor PET/CDPnanofiberscomparedtounmodifiedPETnanofibers.The TGAdataalsoindicatedthattheamountofCDPcoatingontoPET nanofiberswasontheorderof␤-CDP>␥-CDP>␣-CDP.DMAresults elucidatedtheimprovementofmechanicalpropertiesforPET/CDP nanofibers,thatis,PET/CDPnanofibershaveshownhigherstor- agemodulusand higherglasstransitiontemperaturecompared tounmodifiedPETnanofibers.Thefiltrationperformance ofthe CDPsurfacemodifiedPETnanofiberswastestedbyremovalofthe

polycyclicaromatichydrocarbonwastemolecule(phenanthrene) fromitsaqueoussolution.AlthoughthesurfaceareaofthePET/CDP wereless,weobservedthatPET/CDPnanofibershaveshownbetter filtrationefficiencywhencomparedtothepristinePETnanofibers due to the inclusion complexation capability of CDP onto PET nanofibers.Initially,theaveragepercentageremovalofphenan- threnewithrespecttotimewasslightlybetterforPET/␣-CDP,but attheendoffiltrationtesttheallPET/CDPsamplesshowedmoreor lesssamefiltrationefficiencyfortheremovalofphenanthrenefrom theaqueoussolution.ItwasalsoobservedthatPET/CDPmatshave kepttheirnanofibrousstructureafterthefiltrationtest.Inbrief,our resultsindicatedthatPET/CDPnanofibershaveshownthepoten- tialstobeusedasafilter/membraneforwaterpurificationowing toveryhighsurfaceareaofelectrospunnanofibersand surface associatedCDP,sinceCDmoleculeshaveinclusioncomplexation capabilitywithpolycyclicaromatichydrocarbonsandothertypes oforganicwastemolecules.

Acknowledgements

StatePlanningOrganization(DPT)ofTurkeyisacknowledged forthesupportofUNAM-InstituteofMaterialsScience&Nano- technology.Dr.T.UyaracknowledgesTUBITAK-TheScientificand Technological Research Council of Turkey for funding project

#110M612andEUFP7-PEOPLE-2009-RGMarieCurie-IRGforfund- ingNANOWEB(PIRG06-GA-2009-256428).F.Kayaciacknowledges TUBITAK-BIDEBforthenationalPh.D.studyscholarship.

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