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

(1)

O

pen

A

rchive

T

OULOUSE

A

rchive

O

uverte (

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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

a

_,

_André

_Savall

b,∗

a_Institut_Supérieur_des_Sciences_Appliquées_et_Technologie_de_Mahdia,_Université_de_Monastir,_Tunisia

b_Laboratoire_de_Génie_Chimique,_CNRS,_Université_Paul_Sabatier,_118,_Route_de_Narbonne,₃₁₀₆₂_Toulouse_cedex,_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◦_C_favours_the

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:[email protected](N.BelhadjTahar),

[email protected](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

(3)

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._The_limiting_current_I

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_),_is_the_kinematic

viscosityofthesolution(cm2_s−1_),_ω_is_the_angular_rotation_rate_of

theelectroderotation(rads−1₎_and_n₍₌₁₎_and_F_have_their_usual

meanings.

Thepermeabilityofthepolyphenolfilmforferricyanideanion

P_ferr.film wasobtainedfromtheinterceptsofIL−1 plotsversusω−1/2

atω−1/2₌_0._The_solution_permeability_for_ferricyanide_Psol.

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◦_C_on_bare_glassy_carbon_disk_(freshly_polished)_and_on_this_disk

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_and_{extrapolating}_to_very_high_rotation_rate_at_ω−0.5₌_0,_where

masstransportistotallyfilmlimited.Fig.2representsplotsofI_L−1vs.

ω−0.5_for_bare_and_polymer_modified_VC_electrodes._Table₁_shows

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

li m .

)

-1

/ A

-1

(I

li m .

)

-1

/ A

-1 VC/polyphenol (10 mM) bare VC (2) (1) 0.00 0.02 0.04 0.06 0.08 0.10 0.00 0.02 0.04 0.06 0.08 0.10

Fig.2.ReciprocalLevichplotsoflimitingreductioncurrentsforpermeationof ferri-cyanideanionthroughfilmsofpolyphenolonaVCrotateddisk.Scanrate:10mVs−1_. Polyphenolfilmswerepreparedat85◦

Conarotating-diskelectrode(ω=1000rpm) byanodicpolarisationat1800mVduring30mininamagneticallystirred0.1M NaOHand0.1MNa2SO4aqueoussolutionwithphenolconcentrations:0Mcurve (1)(bareVC);0.01Mcurve(2);0.03Mcurve(3)and0.05Mcurve(4).

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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._Above₅₀₀_mV,_the_permeability_increased_{substantially}

at85◦_C_while_polyphenol_films_remained_almost_impermeable_until

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◦_C_in_0.01_M_phenol_aqueous_solution_containing_0.1_M

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_and_T₌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−_to_{instantaneously}_neutralize_the_acidity_arising

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 _the_solution:_[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_._The_polyphenol_films_were_prepared_by_anodic_polarisation_at₁₈₀₀_mV for30minat85◦

Cin0.01Mphenolaqueoussolutionscontaining0.1MNa2SO4and NaOHat0.01M(curve1),0.05M(curve2)and0.1M(curve3).

(5)

Table2

RelationshipsbetweentheNaOHconcentrationin0.01Mphenolaqueoussolutioncontaining0.1MNa2SO4at85◦_C_and_the_permeability_for_ferricyanide_anion_Pfilm 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_/cm_s−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

(2)

(A)

(1) 0 1 2 3 900 700 500 300 100 -100 -300 E vs. SCE / mV

I / mA

(2)

(B)

(1)

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

Con(A)bareVC(B) VCelectrodescoveredbypolyphenolfilms.Scanrate:100mVs−1_._Polyphenol_films_were_{electrosynthesized}_at₈₅◦

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◦_C_respectively_on_bare_and_polyphenol_film-coated_VC

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− _as_the_{concentration}_of_phenol_in_the

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._Results_presented_in_Table₃_reveal_that_the_permeability

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

(6)

Fig.7.Measurementsofpolyphenolfilmpermeabilityfor(A)ferricyanideand(B)phenateions.Scanrate:(A)500mVs−1_and_(B)₁₀₀_mV_s−1_._The_polyphenol_films_were 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◦

CandthepermeabilityPfilm

ferr.forferricyanideanionandP film ph. forphenol. I/mA 1 1.5 2 3 Electrosynthesistime/min 60 40 30 20 Pfilm ferr.×10 4_/cm_s−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_cm_s−1_for_ω₌₁₀₀₀_rpm;_Fig.₄₎_shows_that

therateofferricyanidereductionwaslimitedbythemembrane

permeabilityforpolyphenolfilmspreparedatcurrentshigherthan

2mA whereas it wasunder solution transport control for

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-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)

0 100 200 300 400 900 700 500 300 100 -100 -300 E vs. SCE / mV (1) (2)

(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)

0 500 1000 1500 2000 2500 70 60 50 40 30 20 10 0 Time / min E vs. SCE / mV I = 1 mA I = 1.5 mA I = 2 mA I = 3 mA

(A)

(4) (3) (2) (1)

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_(C_and_D)₁₀₀_mV_s−1_.

performedatanappliedcurrentslightlyhigherthanthelimiting

currentofphenoloxidationshouldallowtheremovalofphenol.

3.3.2. Influenceofelectroderotationandmagneticstirring

GalvanostaticE–tcurveswererecordedat85◦_C_on_a_VC

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+_at_the_electrode_surface_which_blocks_the_permeability

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−_in_the_starting_phenol

solution([OH−_]

sol.=[NaOH]−[PhOH])mustbehigherthanaround

0.1Mtopreparepolyphenolfilmspermeabletobothferricyanide

andphenateanions;thiswasverifiedbyvaryingNaOH

concen-trationforthesameconcentrationofphenolandviceversa;the

relativelyhighfreeOH−_{concentration}_was_necessary_to

instanta-neouslyneutralizetheacidity,attheelectrodesurface,arisingfrom

theconcomitantoxygenevolutionoccurringduringthepolyphenol

filmelectrosynthesis.Electrochemicalpolymerisationconductedat

hightemperature(85◦_C)_yields_high_permeable_polyphenol_films

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◦_C_increased_for_phenate_anion_as

theappliedcurrentincreased.Theseresultscouldbeinterpreted

interms of(1)theincreasein thelocalacidityattheelectrode

surfaceand(2)anincreaseinthedensityofsmallpores

(perme-ableforphenateandnotforferricyanideanion)tothedetrimentof

largepores(permeableforbothphenateandferricyanideanions)

withincreasingtheconcomitantoxygenevolution.Itwasshown

thatpolyphenolfilmspreparedonarotatingelectrodeandastirred

solutionweremorepermeablethanthosesynthesizedonafixed

electrodeandanunstirredsolution.

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