Effect of electropolymerisation conditions on the permeability of polyphenol films deposited on a vitreous carbon electrode

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Effect of electropolymerisation conditions on the

permeability of polyphenol films deposited on a vitreous

carbon electrode

Noureddine Belhadj Tahar, André Savall

To cite this version:

Noureddine Belhadj Tahar, André Savall. Effect of electropolymerisation conditions on the

perme-ability of polyphenol films deposited on a vitreous carbon electrode. Electrochimica Acta, Elsevier,

2012, vol. 82, pp. 427-433. �10.1016/j.electacta.2012.06.080�. �hal-00878691�

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To cite this version : Belhadj Tahar, Noureddine and Savall, André

Effect of electropolymerisation conditions on the permeability of

polyphenol films deposited on a vitreous carbon electrode. (2012)

Electrochimica Acta, vol. 82 . pp. 427-433. ISSN 0013-4686

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Effect

of

electropolymerisation

conditions

on

the

permeability

of

polyphenol

films

deposited

on

a

vitreous

carbon

electrode

Noureddine

Belhadj

Tahar

_,

_André

_Savall

a_{InstitutSupérieurdesSciencesAppliquéesetTechnologiedeMahdia,UniversitédeMonastir,Tunisia}

b_{LaboratoiredeGénieChimique,CNRS,UniversitéPaulSabatier,118,RoutedeNarbonne,31062Toulousecedex,France}

Keywords: Rotating-diskelectrode Phenolelectropolymerisation Vitreouscarbon Permeability Wastewatertreatment

a

b

s

t

r

a

c

t

Polymericfilmswerepreparedfromalkaline(NaOH)phenolaqueoussolutionsonavitreouscarbon (VC)electrodebypotentiostaticorgalvanostaticelectro-oxidation.Permeationthroughsuchfilmswas studiedbyrotating-diskelectrodeusingtheferricyanideredoxcouple,andbycyclicvoltammetryusing phenateions.TheinfluenceoftheelectropolymerisationcontrollableparameterssuchasNaOHand phe-nolconcentrations,potentialorcurrentapplied,electrosynthesistime,temperatureandhydrodynamic conditions(electroderotation+solutionmagneticstirring)onthepermeabilityofthesepolymericfilms wasexamined.Conditionsfortheremovalofphenolbyelectropolymerisationarediscussedonthebasis ofthepermeabilityofpolyphenolfilmsobtainedbyelectrosynthesis.Permeablefilmswereformedfora concentrationoffreehydroxylanionlargerof0.1M.Anincreaseofthetemperatureto85◦_Cfavoursthe

formationofhighlypermeablefilms,thusavoidingelectrodefouling.

1. Introduction

In recent years there hasbeenan increasing interest inthe developmentofelectrochemicalpolymerisationfortheremoval ofphenolsfromaqueoussolutions.Themainadvantagein pro-ceedingbyelectropolymerisationisthelowconsumption ofthe electricchargecomparedwithelectrochemicalmineralisation[1]. Inthisway,theremovalofphenolsfromaqueoussolutionsby elec-trochemicalpolymerisationwasconductedongranularactivated carbon[2],carbonfibre[3],Ta/b–PbO2[1,4]andpolyaniline/glassy

carbonelectrodes[5].Nevertheless,electropolymerisationof phe-noliccompoundsleadstothedepositionofanon-conductivefilm that blocksthe electrode activityand preventsfurtherremoval of phenol [6–11]. Many works have shown that diffusion of electroactivespeciesthroughpolymericfilmsdependsonthe elec-tropolymerisationconditionssuchasthemonomerconcentration

[12],thenatureofthesolventused[13],thepHandbackground electrolyte[14],thedegreeofcross-linkingofthepolymerchains

[15],thenatureoftheelectrode[16],theelectrodepositionmodes (cyclicvoltammetryandpotentiostaticorgalvanostatic electroly-sis)[12,17],theappliedpotential[14,18]orcurrent[19]andthe electropolymerisationtime(filmthickness)[12].

Theaimofourpreviousworks[1,4,20,21]wastofindthe operat-ingconditionsthatallowelectropolymerisationtoproceedaslong

∗ Correspondingauthor.Tel.:+33561556110;fax:+33561556139. E-mailaddresses:nour.belhadj@fsm.rnu.tn(N.BelhadjTahar),

savall@chimie.ups-tlse.fr(A.Savall).

asacurrentissuppliedtotheelectrode;thus,thepolymericfilm inevitablydepositedontheelectrodesurfacemusthavethehighest permeabilityforphenolmolecules.Themainoperatingconditions weredescribedinRef.[1].Theelectrochemicalresponseof phen-ateiononbare VCandpolyphenolfilm-coated VCwasstudied in 1MNaOH aqueoussolutions [20]. Howeverusing suchhigh concentrationofNaOHintheprecursorphenolsolutionpenalizes anyeventualtreatmentbasedontheremovalofphenolsby elec-tropolymerisation.Theaimofthisworkwastomeasurebyusing arotating-diskelectrodethepermeabilityforferricyanideanionof polyphenolfilmsdepositedinalkalinemediumonVCundervarious electropolymerisationconditions.Inparticularwearelookingfor theminimalconcentrationofNaOHwhichallowsthecontinuous formingofafilmofsufficientpermeabilitynottoblockthe elec-tropolymerisation.Therelativepermeabilityforphenateanionwas alsoevaluatedbymonitoringchangesincyclicvoltammogramsof phenolsolutionrecordedonpolyphenolfilm-coatedVCas com-paredtobareVC.InordertooptimisetheconcentrationofNaOHin theprecursorphenolsolutionparticularattentionwaspaidtoits effectonthepermeabilityofthepolyphenolfilm.

2. Experimental 2.1. General

Cyclicand rotating-diskvoltammetryand chronoamperome-trymeasurementswereperformedusingacomputercontrolled Eco Chemie Autolab Model 30 (Utrecht, The Netherlands).The conventionalthree-electrodecellsystemconsistedofbareVCor

polyphenolfilm-covered VC(disk of3mmdiameter,Metrohm) working electrode, a platinum spiral counter electrode and an Hg/Hg2Cl2/Cl− (saturated) reference electrode connected tothe

electrolytic solutionby a long Luggin capillary placed nearthe workingelectrodeandfilledwith0.1MNa2SO4.Allthepotentials

arereportedversustheabovespecifiedreferenceelectrode.Before eachexperiment,theworkingelectrodewaspolishedtoamirror with1mmaluminaslurriesonpolishingsheet(3M262XIMPERIAL LappingFilm)andsubsequentlywashedwithdistilledwater.

2.2. Preparationofpolyphenolfilm-coatedvitreouscarbon electrode

Polyphenolfilmswerepreparedbyoxidative electropolymeri-sationperformedeithergalvanostaticallyorpotentiostaticallyonto thediskofVCelectrode surfacefromphenolaqueoussolutions containing0.1MNa2SO4andNaOHatvariousconcentrations.

Elec-tropolymerisationwasperformedonafixedelectrodeandwithout magneticstirringunlessotherwiseinstructed.Thepolyphenol film-coatedelectrodeswerecopiouslyrinsedwithdistilledwaterand usedimmediatelyinpermeationexperiments.

2.3. Polyphenolfilmpermeabilitymeasurement ThepermeabilityPfilm

ferr.ofthepolyphenolfilmforferricyanide

anionwasmeasuredwitha rotating-diskelectrode(RDE)inpH 7phosphate(0.1M)buffersolutioncontaining0.01Mpotassium ferricyanideat25◦_{C.ThelimitingcurrentI}

Lofthe

electrochemi-calreductionofferricyanideaniononthepolyphenolfilm-covered RDEcanbedescribedbythefollowingequation[22]:

I−1

L =(nFACsPferr.film) −1

+(0.62nFAC_sD2/3_s −1/6)

−1

ω−1/2 ₍₁₎

whereAistheelectrodearea(cm2_),C

sistheconcentrationof

fer-ricyanideanioninthesolution(molcm−3_),D

sistheferricyanide

diffusioncoefficientinthesolution(cm2_s−1_{),isthekinematic}

viscosityofthesolution(cm2_s−1_{),ωistheangularrotationrateof}

theelectroderotation(rads−1_{)andn(=1)andFhavetheirusual}

meanings.

Thepermeabilityofthepolyphenolfilmforferricyanideanion P_ferr.film wasobtainedfromtheinterceptsofIL−1 plotsversusω−1/2

atω−1/2_{=0.ThesolutionpermeabilityforferricyanideP}sol. ferr.was

obtainedfromtheslopesofplots Id versusω1/2 whereIdisthe

limitingcurrentatthebareVCelectrode asdescribed byLevich

[22]:

Id=0.62nFACsD2/3s −1/6ω1/2=nFACsP_ferr.sol. (2)

Therelativepermeability ofthepolyphenolfilmforphenate anionPfilm

Ph. wasassessedbysubjectingthepolyphenolfilm-coated

electrodetocyclinginthesamesolutionusedforthefilm prepa-ration.TherelativepermeabilityforphenateanionPfilm

Ph. ,expressed

asapercentage,isdefinedastheratiooftheoxidationpeak cur-rentintensityduringthe1stscanatthepolyphenolfilm-coated electrodeIfilmoverthatatthebareelectrodeIbare[23,24]:

PPh.film=

Ifilm

Ibare

×100 (3)

Inpractice,thepermeabilityofthefilmofpolyphenolfor fer-ricyanideanionswascalculatedfromvoltammogramsrecordedat 25◦_{Conbareglassycarbondisk(freshlypolished)andonthisdisk}

coveredwiththefilmevenwhenthefilmwaspreparedat85◦_C.

However,thepermeabilityofthefilmofpolyphenolforphenate anionswascalculatedfromvoltammogramsrecordedonbareVC andonpolyphenolfilm-coatedVCinthesamesolutionandatthe sametemperatureusedforthepreparationofthefilm.

-800 -600 -400 -200 0 200 400 200 0 -200 -400 -600 -800 E vs. SCE / mV I / µ A (1) (2)

Fig.1. VoltammogramsonaVCrotateddiskof0.01Mpotassiumferricyanidein pH7sodiumphosphate(0.1M)buffersolutionat25◦

C.Scanrate:10mVs−1_and

ω=5000rpm.(1)BareVC;(2)polyphenolfilmelectrosynthesizedonVC rotating-diskelectrode(ω=1000rpm)byanodicpolarisationat1800mVduring60minin amagneticallystirred0.01Mphenolsolutioncontaining0.1MNaOHand0.1M Na2SO4at85◦C.

3. Resultsanddiscussion

3.1. Generalbehaviourofthebarevitreouscarbonandthe polyphenolfilm

Fig. 1 shows typical rotating-disk voltammograms for the reductionof0.01Mpotassium ferricyanidesolutionatbareand polyphenol film-coated VC electrodes. The film did not affect thestartingferricyanidereductionpotentialbutthecurrentwas stronglyattenuated.

3.2. Polyphenolfilmspreparedbypotentiostaticoxidation

Polyphenolfilmswerepreparedbypotentiostaticoxidationin ordertohighlighttheinfluenceontheirpermeabilityofoxygen evolutionwhenitoccurssimultaneouslywith electropolymerisa-tion.

3.2.1. Influenceoftemperature

ValuesoffilmpermeabilityweredeterminedbyplottingI_L−1vs. ω−0.5_{andextrapolatingtoveryhighrotationrateatω}−0.5_=0,where

masstransportistotallyfilmlimited.Fig.2representsplotsofI_L−1vs. ω−0.5_{forbareandpolymermodifiedVCelectrodes.Table1shows}

30000 32000 34000 36000 VC/polyphenol (30 mM) VC/polyphenol (50 mM) (3) (4) 0 1000 2000 3000 4000 ϖ− 1/2 / (rad s-1)-1/2 ϖ− 1/2 _{/ (rad s}-1 )-1/2

(I

)

/ A

(I

)

/ A

Fig.2.ReciprocalLevichplotsoflimitingreductioncurrentsforpermeationof ferri-cyanideanionthroughfilmsofpolyphenolonaVCrotateddisk.Scanrate:10mVs−1_.

Polyphenolfilmswerepreparedat85◦

Conarotating-diskelectrode(ω=1000rpm) byanodicpolarisationat1800mVduring30mininamagneticallystirred0.1M NaOHand0.1MNa2SO4aqueoussolutionwithphenolconcentrations:0Mcurve

Table1

Relationshipsbetweentemperatureandthepermeabilityforferricyanideanion ofthepolyphenolfilmPfilm

ferr.electrosynthesizedonvitreouscarbonelectrode

dur-ing30minatvariousappliedpotentialsEfrom0.01Mphenolaqueoussolution containing1MNaOHand0.1MNa2SO4.

T/◦_{C 25 85} Evs.SCE/mV 500 900 1200 500 600 700 Pfilm ferr.×10 4 /cms−1 _{0 5 221 0 1961 3349}

therelationshipsbetweenthetemperatureandthepermeability ofthepolyphenolfilmselectrosynthesizedonVCduring30min atvariousappliedpotentialsfrom0.01Mphenolaqueoussolution containing1MNaOHand0.1MNa2SO4.Polyphenolfilmsprepared

atpotentialslessthan500mVwerenotatallpermeableatboth25 and85◦_{C.Above500mV,thepermeabilityincreasedsubstantially}

at85◦_{Cwhilepolyphenolfilmsremainedalmostimpermeableuntil}

900mVat25◦_C.

The raising of permeability with temperature and applied potential(Table1)mayresultfromanincrease inthe polyphe-nolfilmgrowthrate;thediameteranddensityoftheporesinthe filmareexpectedtostronglyincreasewiththefilmgrowthrate.On thecontrary,afilmgrownslowlyismorecompactanddensewith lessandnarrowerpores[15].Insummary,thelargerpermeability valueswereobtainedathightemperature,whichallowstheuseof lowerpotentials.

3.2.2. InfluenceofNaOHconcentration

Fig.3showscyclicvoltammogramsrecordedonbareVC elec-trodeat85◦_{Cin0.01Mphenolaqueoussolutioncontaining0.1M}

Na2SO4andNaOHatdifferentconcentrations.Decreasingthe

con-centrationofNaOHfrom1to0.01Mhadnoeffectonthepeak potentialbut noticeablydiminishedthe peakcurrent ofphenol oxidationindicatingthattheelectrodepassivationwasmoreand morepronounced.Notethatfor0.01MNaOHtheresponseof phe-nol(practicallypresentasphenate)isexpressedasawave(curve 3).

Table2showstherelationshipsbetweenthepermeabilityfor ferricyanide anion and theconcentration of NaOHused during thepolyphenolfilmpreparationatvariousappliedpotentials.For NaOHconcentrationof1M,polyphenolfilmspreparedatpotentials below500mVarenotatallpermeablewhiletheonesprepared atpotentialhigherthan600mVwerehighlypermeable.Inthese conditions,comparisonbetweenvaluesofPfilm

ferr.(Table2)andP sol. ferr.

(Fig.4)showsthatphenoloxidationratewaslimitedbythelow membranepermeabilityofpolyphenolfilmspreparedatpotentials lowerthan500mV.Onthecontrary,theoxidationratewaslimited bythesolutionforfilmspreparedatpotentialshigherthan600mV.

Table2showsthatpolyphenolfilmswithhighpermeabilityto fer-ricyanideanionwerepreparedatincreasingpotentialsastheNaOH concentrationdecreasedfrom1to0.01M.Ontheotherhand,very lowpermeabilityvalueswereobservedevenathighapplied poten-tialsforNaOHconcentrationslessthan0.1M.Theseresultsarein

0 50 100 150 200 250 900 700 500 300 100 -100 -300 E vs. SCE / mV I / µ A (1) (2) (3)

Fig.3.Effectofsodiumhydroxideconcentrationoncyclicvoltammograms(1st cycles).recordedonbareVCelectrodein0.01Mphenolaqueoussolution contain-ing0.1MNa2SO4andNaOHataconcentrationof1M(curve1),0.1M(curve2)and

0.01M(curve3).Scanrate:100mVs−1_andT=85◦

C.

Fig.4. Variationofthesolutionpermeabilityforferricyanideanionasafunctionof theelectroderotationrate.

agreementwithcyclicvoltammogramspresentedinFig.3ifwe considerthatthepeakcurrentofphenoloxidationdecreasedas thepermeabilityofthepolyphenolfilmdiminished.

CyclicvoltammogramspresentedinFig.5showthatnophenol oxidationpeakwasobservedforNaOHconcentrationslessthan 0.1M(curves1and2)indicatingthatundertheseconditionsthe polyphenolfilmswereimpermeabletophenateanion.Therelative permeabilityPfilm

Ph. forphenateanionofthepolymerfilmprepared

in0.1MNaOHwasevaluated,accordingtoEq.(3),fromcurve2 inFig.3and curve3inFig.5.Pfilm

Ph. isequal to184%;this value,

higherthan100%,suggeststhatthefilmpermeabilityfor phen-ateanionisgovernedbythepreferentialpartitioningoftheprobe intothepolyphenolfilmduetostrongerinteractions(electrostatic, hydrophobic,etc.)inthefilmthaninthesolution[24].

Phenolisaweakacid(pKa=9.9)andataninitialconcentration

of0.01MitsreactionwithNaOHinitiallyat1M,0.1M,0.05Mor 0.01MleadstoequilibriumconcentrationsoffreeOH−_respectively

equalto0.99M,0.09M,0.04Mand0.00M.Consequently,at rel-ativelyhighNaOHinitialconcentration(1Mand0.1M)thereis enoughfreeOH−_{toinstantaneouslyneutralizetheacidityarising}

attheelectrodesurfacefromwaterdischargeoccurringin paral-lelwithpolyphenolelectrosynthesis.Ontheotherhand,withlow NaOHconcentrations(0.05and0.01M)thefreeOH−

concentra-tionistooweaktoneutralizethetotalityofprotonsproducedat thesurfaceoftheanodewhentheappliedpotentialincreases. Fur-thermore,thelowvaluesofpermeabilityobservedwiththelower NaOHconcentrations(0.05and0.01M),evenforexperiments per-formedundermagneticstirringand withtherotatingelectrode (ω=1000rpm),indicatethathighacidityremainedattheelectrode surface(Table2).

From Table2,one can concludethat at 0.01Mphenol con-centrationandrelativelyhighNaOHconcentration(0.1and1M), thepermeability for ferricyanideanion increases as the poten-tialappliedincreases.However,atlowNaOHconcentration(0.01 and 0.05M), the permeability remains almost nil even at very highapplied potential. Therefore, we think that the key factor influencingthepermeabilityofthepolyphenolfilmsisthe concen-trationoffree OH−_{in thesolution:[OH}−_]

sol.=[NaOH]−[PhOH];

0 500 1000 1500 900 700 500 300 100 -100 E vs. SCE / mV I / µ A 0.01 M 0.05 M 0.1 M (3) (1) (2)

Fig.5.Measurementsofpolyphenolfilmpermeabilityforphenateion.Scanrate: 100mVs−1_{.Thepolyphenolfilmswerepreparedbyanodicpolarisationat1800mV}

for30minat85◦

Cin0.01Mphenolaqueoussolutionscontaining0.1MNa2SO4and

Table2

RelationshipsbetweentheNaOHconcentrationin0.01Mphenolaqueoussolutioncontaining0.1MNa2SO4at85◦CandthepermeabilityforferricyanideanionPfilm_ferr.ofthe

polyphenolfilmelectrosynthesizedduring30minonavitreouscarbonelectrodeatdifferentappliedpotentials.

Hydrodynamicconditions Fixedelectrodeandwithoutmagneticstirring Electroderotationrateof1000rpm

andundermagneticstirring

[NaOH]/M 1 0.1 0.05 0.01 Evs.SCE/mV 400 500 600 700 700 1300 1500 1800 1800 2500 1800 2500 Pfilm ferr.×10 4_/cms−1 _{0 0 1961 3349 0 0 9 925 5 5 3 3} 0 50 100 150 200 900 700 500 300 100 -100 -300 E vs. SCE / mV

I /

µ

A

(A)

I / mA

(B)

Fig.6. Cyclicvoltammograms(firstcycles)of0.05Mphenolsolutioncontaining0.1MNa2SO4andNaOHat0.1M(curve1)and0.15M(curve2)at85◦Con(A)bareVC(B)

VCelectrodescoveredbypolyphenolfilms.Scanrate:100mVs−1_{.Polyphenolfilmswereelectrosynthesizedat85}◦

Conarotateddisk(1000rpm)VCelectrodeatanapplied potentialof1800mVinamagneticallystirred0.05Mphenolsolutioncontaining0.1MNa2SO4andNaOHat0.1M(curve1)and0.15M(curve2).

the present results show that the concentration of free OH−

should be preferably higher than 0.1M to prepare polyphe-nol films with noteworthy permeability values to ferricyanide anion.Indeed,polyphenolfilmpreparedbyanodicpolarisationat 1800mV for30minin 0.05Mphenolsolution containing0.1M NaOH([OH−_]

sol.=0.05M)and0.1MNa2SO4 at85◦C wasnot at

allpermeableforferricyanideanionwhereastheonepreparedin thesameconditionsexceptthatNaOHconcentrationwas0.15M ([OH−_]

sol.=0.1M)hadapermeabilityequalto149×10−4cms−1.

Fig.6(AandB)showscyclicvoltammograms(firstcycles)recorded at85◦_{Crespectivelyonbareandpolyphenolfilm-coatedVC}

elec-trodesin0.05Mphenolsolutioncontaining0.1MNa2SO4and0.1M

NaOH(curve 1)or0.15MNaOH(curve 2).Curve 1in Fig.6(B) exhibitsa weakly defined peak for phenoloxidation indicating averylowpermeabilityofthecorrespondingpolyphenolfilmto phenateanion.Alarge,butbetterdefinedpeakwasobservedfor 0.15MNaOH(curve2inFig.6(B))indicatingthatunderthese condi-tionstheoxidationprocessoccurredwithoutinhibitionandthatthe polyphenolfilmwashighlypermeabletophenateanion.Itshould beemphasizedthatthephenoloxidationpeakonpolyphenol film-coated electrodes (Fig. 6(B)) is moved toward higher potential valuesascomparedtothatonbareVCelectrode(Fig.6(A)).

Theseresultshaveshownthattheremovalofphenolby elec-tropolymerisationispossiblebyapplyingahigherpotentialasthe NaOHconcentrationdecreases.

3.2.3. Influenceofphenolconcentration

The effect of phenol concentration on the permeability of polyphenol films prepared at 85◦_{C on a VC rotating disk}

(ω=1000rpm) was studied by potentiostatic polarisation at 1800mV during 30min in magnetically stirred 0.01, 0.03 and 0.05Mphenolaqueoussolutionscontaining0.1MNaOHand0.1M Na2SO4. Potentiostatic I–t curves (not shown) reveals that the

limiting current at the stationary state fell dramatically when phenolconcentrationincreasedfrom0.01to0.03M.Cyclic voltam-mograms (not shown) recorded on polyphenol film-coated VC electrodesinthesamesolutionsshowthat nophenoloxidation peakwasobservedfor0.03Mand0.05Mphenolconcentrations; theelectrochemicalresponseofphenolisexpressedasawavewhat preventstocalculatetherelativepermeabilityforphenateanion (almostimpermeable);for0.01Mphenolconcentration,the rela-tivepermeabilityisequalto143%.Theseresultsaswellasthose

presentedinSection3.2.2forfilmsrespectivelypreparedinNaOH 0.1Mand0.15Mmaybeexplainedintermofadecreaseinthe concentrationoffreeOH− _{astheconcentrationofphenolinthe}

solutionrises.Inparticular,thedecreaseoftheabilityofthe solu-tiontoopposechanges inpHwhen protonsare formedonthe electrodesurfaceduringwaterdischargecanexplaintheoriginof thepermeabilityfall[21](seeSection3.3.1).

3.2.4. Influenceofelectrolysistime

Polyphenolfilmsweresynthesizedonarotating(ω=1000rpm) VCelectrodebyanodicpolarisationat1800mVforvarious elec-tropolymerisationtimes (30–120min) in a magneticallystirred 0.01Mphenolsolutioncontaining0.1MNaOHand0.1MNa2SO4

at85◦_{C.ResultspresentedinTable3revealthatthepermeability}

decreased substantially with increasing the electropolymerisa-tion time; similar results were obtained by Wang et al. [12]. Thisbehaviourisinterpretedintermofanincreasein polyphe-nolfilmthicknesswithincreasingelectropolymerisationtime;the polyphenolfilmthicknessvariesbetween10and100nm depend-ingontheelectropolymerisationconditions[25].Fig.7(A)shows thatcyclicvoltammogramsoftheferricyanideanionare charac-terizedbyirreversiblewavesfor60and120minelectrosynthesis timeswhilemuchmorereversiblepeakswereobservedforonly 30min.Valuesoftherelativepermeabilityforphenateanion, cal-culatedfromcurves(2)and (3)inFig.7(B),wereequalto155% and 142% respectively for 60 and 120min indicating that the polyphenolfilm permeability for phenateanion isindependent of electrosynthesis times.Electropolymerisation can thus occur onpre-existingpolymerlayerobtainedbyelectrolysisofphenol 0.01Minalkaline(0.1MNaOH)solution.Thistechniquecouldbe usedasawaytoremovephenolsinceundertheseconditions elec-tropolymerisationproceedsaslongasacurrentisappliedtothe electrode.

Table3

Relationshipsbetween theelectropolymerisation time in magnetically stirred 0.01Mphenolaqueoussolutioncontaining0.1MNaOHand0.1MNa2SO4at85◦C

andthepermeabilityPfilm

ferr.oftheresultingpolyphenolfilmforferricyanideanion.

Electrosynthesistime/min 30 60 120

Pfilm ferr.×10

Fig.7.Measurementsofpolyphenolfilmpermeabilityfor(A)ferricyanideand(B)phenateions.Scanrate:(A)500mVs−1_and(B)100mVs−1_{.Thepolyphenolfilmswere}

preparedbyanodicpolarisationonarotating(ω=1000rpm)VCelectrodeat1800mVforvariouselectrolysistimesin0.01Mphenolaqueoussolutioncontaining0.1MNaOH and0.1MNa2SO4at85◦C.Electrolysistime:30min(curve1);60min(curve2)and120min(curve3).

3.3. Polyphenolfilmspreparedbygalvanostaticoxidation

In practice, a treatment of phenolic compounds based on electropolymerisationshouldbeconductedgalvanostatically;as a consequence an increase of the potential is expected during theelectropolymerisation.Therefore,theappliedcurrentmustbe determinedbytakingintoaccountthetransportpropertiesofthe polymericfilm.

3.3.1. Influenceofappliedcurrent

Fig. 8(A) exhibits galvanostatic E–t curves recorded during polyphenolfilmselectrosynthesisonarotating(ω=1000rpm)VC electrode at various applied currents in a magnetically stirred 0.05Mphenolaqueoussolutioncontaining0.15MNaOHand0.1M Na2SO4at85◦C;differentelectrosynthesistimeswerechosenin

suchawaythatthesameelectriccharge(Q=3600mC)waspassed inordertopreparepolyphenolfilmshavingmoreorlessthesame thickness.GalvanostaticE–t curves presentedin Fig.8(A)show thattheelectrodepotentialremainedconstantataround1300mV duringthefirststageofthepolyphenolfilmelectrosynthesis.The lengthofthisstepdecreasedwhenappliedcurrentincreased;then the potentialrose rapidly until it reacheda plateau at around 2000mV.Table4showsthatthepermeabilityofthefilmfor fer-ricyanideaniondropsfrom107×10−4cms−1 to2×10−4cms−1 byincreasingfrom1to3mAthecurrentappliedduringthefilm electrosynthesis.Thiscurrentdependencyofthepermeabilityis in agreementwithcyclic voltammogramsof ferricyanide anion recorded on polyphenol film-coated VC electrodes depicted in

Fig.8(B)inthatthereductionpeakcurrentdecreasedasthe cur-rentappliedincreasedintherange1–3mA.Fig.8(CandD)shows cyclicvoltammogramsof0.05Mphenolaqueoussolution contain-ing0.15MNaOHand0.1MNa2SO4recordedat85◦Conpolyphenol

film-coatedVCelectrodes.CyclicvoltammogramsinFig.8(D)show thatthephenoloxidationpeakrecordedonpolyphenolfilm-coated electrodesislargertothatonbareelectrodeanddisplacedtomore positivepotentials(curve2inFig.6(A)).

Table4 shows that when the appliedcurrent increases one observesadecreaseofthepermeabilityofthefilmtowardthe ferri-cyanideanionwhiletherelativepermeabilityofthesamefilmwith regardtothephenolincreases.Thecurrentintensityeffectonthe polyphenolfilmpermeabilitycanbeexplainedbythedischargeof

Table4

Relationshipsbetweenthe current Iapplied during theelectrosynthesis of a polyphenolfilmdepositedonarotating(ω=1000rpm)vitreouscarbonelectrode inamagneticallystirred0.05Mphenolaqueoussolutioncontaining0.15MNaOH and0.1MNa2SO4at85◦CandthepermeabilityP_ferr.filmforferricyanideanionandP_ph.film

forphenol. I/mA 1 1.5 2 3 Electrosynthesistime/min 60 40 30 20 Pfilm ferr.×10 4_/cms−1 _{107 45 29 2} Pfilm ph. (%) 176 233 399 1115

waterwhichissourceofprotons.ThecurrentIappliedduringfilms electrosynthesiscanbeexpressedasfollows:

I=I_Ph.+I_H

2O (4)

whereIPh.andIH2Oarethepartialcurrentsforrespectivelyphenol

andwaterelectro-oxidation.IPh.canbeevaluatedbythefollowing

equation: IPh.=nFAkPh[Ph]sol. considering thatthe concentration

ofphenolattheelectrodesurfacewasequaltozerobecausethe appliedcurrent Iwasmuchhigher thanthelimiting currentof phenoloxidation.Indeed,fortheexampleofFig.8,thelimiting cur-rentofphenoloxidation(duringelectrosynthesisofthefilms)was equaltoaround0.35mAforn=1,A=0.0707cm2_,k

Ph.=10−5ms−1

and[Ph.]_sol.=0.05M.Thus,asIincreased(from1to3mA),the par-tialcurrentofwaterdischargeIH2Orose,producinganincreasein

oxygenevolutionandtheoutputofprotons.Theconcentrationof protons[H+_]

ele.attheelectrodesurfacecanbecorrelatedtoIH₂O

by:IH2O=nFAkH+[H

−

]_ele.consideringthatthebulkconcentration ofprotons[H+

]sol.wasequaltozeroduetohydrodynamic

condi-tions(electroderotation,ω=1000rpmplusmagneticstirring)and therelativelyhighconcentrationoffreehydroxideioninthe solu-tion([OH−

]sol.=0.1M).Thus, concentrations [H+]_ele.,calculated forn=1 andkH+=10−5ms−1,wereequal toaround0.1Mand

0.4MrespectivelyforIequalto1or3mA.Theseresultsshowthat forI=3mA,theconcentrationofprotonsattheelectrodesurface ([H+

]ele.=0.4M)wasfourtimeshigherthanthatoffreehydroxide

ionin thesolution([OH−H+]_sol.=0.1M).Asa result,the elec-trode/solutioninterfaceremainedacidthoughelectrosynthesisof the polyphenolfilm was performed withan electrode rotation rateof1000rpmandundermagneticstirring.Ifweassumethat polyphenolfilmspreparedherehadthesamethickness,thedrop ofpermeabilityforferricyanideanion(Table4)canbeexplainedby astrongeracidityattheelectrodesurfaceduringpolyphenolfilm electrosynthesisasIincreased.

Thecontradictorybetweentheincreaseintheapparent per-meability forphenol(%) Pfilm

Ph. andthedramatic decreaseof the

permeabilityforferricyanideanionPfilm

ferr.whentheappliedcurrent

increases(Table4)canbeinterpretedintermofachangeinthe microstructureofthepolyphenolfilm.Wethinkthat increasing theappliedcurrentandthereforetheconcomitantoxygen evolu-tionincreasesthedensityofsmallpores(permeableforphenate andnotforferricyanideanion)tothedetrimentofthatoflargeones (permeableforbothphenateandferricyanideanions).Accordingto Rothwelletal.’ssuggestion[18]weintendtostudythesefilmsby scanningelectronmicroscopytounderstandtheseeffects.

Fortheremovalofphenolbyelectropolymerisationthe oper-atingconditionsshouldfavourahighpermeabilityofthefilmfor phenateanion.ComparisonbetweenvaluesofP_ferr.film(Table4)and Psol.

ferr.(P sol.

ferr.=45×10

−4_cms−1_{forω=1000rpm;Fig.4)showsthat}

therateofferricyanidereductionwaslimitedbythemembrane permeabilityforpolyphenolfilmspreparedatcurrentshigherthan 2mA whereas it wasunder solution transport control for cur-rentslowerthan1.5mA.Theaboveresultsshowthatelectrolysis

-200 -150 -100 -50 0 800 600 400 200 0 -200 -400 -600 E vs. SCE / mV 1 mA 3 mA 2 mA 1.5 mA (1) (4) (3) (2) (1) (2) (3) (4)

(B)

(C)

bare vitreous carbon

0 500 1000 1500 2000 900 700 500 300 100 -100 -300 E vs. SCE / mV I / μ A I / μ A I / μ A (4) (3)

(D)

(A)

Fig.8.(A)GalvanostaticE–tcurvesrecordedduringpolyphenolfilmselectrosynthesisonarotating(ω=1000rpm)VCelectrodeatvariousappliedcurrentsinamagnetically stirred0.05Mphenolaqueoussolutioncontaining0.15MNaOHand0.1MNa2SO4at85◦C.Appliedcurrent:(1)1mAfor60min,(2)1.5mAfor40min,(3)2mAfor30minand

(4)3mAfor20min.(B–D)Measurementsofpolyphenolfilmpermeabilityfor(B)ferricyanideand(CandD)phenateions.Scanrate:(B)500mVs−1_{and(CandD)100mVs}−1_.

performedatanappliedcurrentslightlyhigherthanthelimiting currentofphenoloxidationshouldallowtheremovalofphenol.

3.3.2. Influenceofelectroderotationandmagneticstirring

GalvanostaticE–tcurveswererecordedat85◦_ConaVC

elec-trodeat an applied current of 2mA in 0.05Mphenol aqueous solutioncontaining0.15MNaOHand0.1MNa2SO4;curveswere

registeredeitherinaquiescentsolutionorforanelectrode rota-tion rate of 1000rpm and a magnetic stirring. The E–t curves havethesameshapeexceptthatthepotentialplateauat2000mV wasreachedrightfromthebeginninginthecaseofpolyphenol filmelectrosynthesizedinaquietsolution.Thepermeability for ferricyanideanion dropped from29×10−4cms−1 (Table 4) for polyphenolfilm electrosynthesizedon a rotatingelectrode and undermagneticstirringtozerointhecaseofafixedelectrodeand unstirredphenolsolution.AsinSection3.2.1,theseresultsmay beinterpretedassuminganaccumulationofhighconcentrationof protonsH+_{attheelectrodesurfacewhichblocksthepermeability}

ofthepolyphenolfilmelectrosynthesizedonafixedelectrodein unstirredsolution[21].

4. Conclusions

Thisworkdemonstrates thatthepermeability ofpolyphenol filmsdepositedbyelectrochemicalpolymerisationonavitreous carbon electrode depends strongly on the composition of the startingphenol solution,temperature,appliedpotential or cur-rent,electrosynthesistimeandhydrodynamicconditions.Wehave shownthattheconcentrationoffreeOH−_{inthestartingphenol}

solution([OH−_]

sol.=[NaOH]−[PhOH])mustbehigherthanaround 0.1Mtopreparepolyphenolfilmspermeabletobothferricyanide andphenateanions;thiswasverifiedbyvaryingNaOH concen-trationforthesameconcentrationofphenolandviceversa;the relativelyhighfreeOH−_{concentrationwasnecessaryto}

instanta-neouslyneutralizetheacidity,attheelectrodesurface,arisingfrom theconcomitantoxygenevolutionoccurringduringthepolyphenol filmelectrosynthesis.Electrochemicalpolymerisationconductedat hightemperature(85◦_{C)yieldshighpermeablepolyphenolfilms}

atlowappliedpotentials.Nevertheless,polyphenolfilmsthathave suitablepermeability weresynthesized atlow temperature but

athighappliedpotential.Fortheremovalofphenolfrom waste-water by electropolymerisation,it would be bettertouse high temperatureandlowpotentialinordertominimizetheparasitic reactionofoxygenevolutionwhichaffectsthecurrentyield.We havealsoshownthatpolyphenolfilmspreparedat1800mVfrom 0.01Mphenoland0.1MNaOHhavelowpermeability,especially whenelectrolysistimeincreases;thiswasinterpretedintermof anincreaseinthepolyphenolfilmthickness.Theapparent perme-abilityofpolyphenolfilmspreparedgalvanostaticallyfrom0.05M phenoland0.15MNaOHat85◦_{Cincreasedforphenateanionas}

theappliedcurrentincreased.Theseresultscouldbeinterpreted interms of(1)theincreasein thelocalacidityattheelectrode surfaceand(2)anincreaseinthedensityofsmallpores (perme-ableforphenateandnotforferricyanideanion)tothedetrimentof largepores(permeableforbothphenateandferricyanideanions) withincreasingtheconcomitantoxygenevolution.Itwasshown thatpolyphenolfilmspreparedonarotatingelectrodeandastirred solutionweremorepermeablethanthosesynthesizedonafixed electrodeandanunstirredsolution.

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