Synthesis and characterization of zero valent iron nanoparticles supported on sba 15

Iron nanoparticles are another source of iron for this homogeneous reaction, during the oxidationof elementar iron.. Silicaismoreadvantageouswhencomparedtoother mate-rials, since it aggl

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Original Article

Synthesis and characterization of zero-valent iron

nanoparticles supported on SBA-15

Felipe Sombra dos Santos∗, Fernanda Rodrigues Lago, Lídia Yokoyama,

Fabiana Valéria Fonseca

a r t i c l e i n f o

Article history:

Received21June2016

Accepted16November2016

Availableonlinexxx

Keywords:

Zero-valentiron

NanosilicaSBA-15

Synthesisandcharacterization

Nanomaterial

a b s t r a c t Thispaperaimstosynthesizezero-valentironnanoparticles(nZVI)supportedonSBA-15 nanosilica.Thenanosilicagenerateinthesystembypolymerreactionwithhydrochloric acidundercontrolledtemperature.After,theironnanomaterialwasobtainedbysodium borohydridereductionasdescribedinthiswork.Afterwardthesynthesisofthe nanopar-ticlescontainedironsupportedonsilicaSBA-15,thematerialwascharacterizedbyX-ray diffraction,transmissionelectronmicroscopy,scanningelectronmicroscopy,zetapotential andX-rayfluorescencespectroscopy.Theresultsindicatedthatnanomaterialobtainedwas

innanometricscale,byTEMresults,andshowingcharacteristicpeaksatEDSresults,with 11.9%ironand14.0%siliconcontent,respectively,andcontaining73.0%and27.0%oftheir respectiveoxidesthroughX-rayfluorescencespectroscopy.Theisoelectricpotentialofthe samplewasaround2.0,closetothevaluereportedforsilica,duetothehigherpercentage

ofsilicainthesamplewhencomparedtoiron.Theobtainedmaterialcanbeused,forsome cases,asanpossiblealternative,totheFentonreactionforthedegradationofxenobiotic compoundsorotherapplicationsinthegroundwaterandwastewatertreatments

©2016BrazilianMetallurgical,MaterialsandMiningAssociation.PublishedbyElsevier EditoraLtda.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://

creativecommons.org/licenses/by-nc-nd/4.0/)

Over the years various xenobiotic substances resistant to

biodegradationhavebeensynthesizedbymanforapplications

invarioussectorssuchastheagriculture,oil,petrochemical

andtextilesegments,amongothers.Howeveraftertheiruse,

thesesubstances,beingrefractorytobiologicaldegradation,

are discardedinnatureand havethe potentialtopromote

severalenvironmentalimpactsandcompromisethequality

ofwatersupplysystems[1,2]

∗ Corresponding author.

E-mail:fpsombra@ig.com.br(F.S.Santos)

The nanotechnology is the engineering and art of manipulating matter at the nanoscale between 1 and

100nm[3,4] Theuseofironnanoparticles,comparedthemicrometric particlesisbecausetogreaterefficiencyinreductionreactions, highreactivity,duetothehighsurfacearea,mobilityand fil-trationefficiencywhenusedintechnologiesforremedyinga certainenvironment.Theparticlesbeinginnanosizeremain

insuspensionforalongperiodoftime,thereby facilitating thevariousknownapplicationssuchaswatertreatmentand wastewater[5]

http://dx.doi.org/10.1016/j.jmrt.2016.11.004

2238-7854/©2016BrazilianMetallurgical,MaterialsandMiningAssociation.PublishedbyElsevierEditoraLtda.Thisisanopenaccess articleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/)

Zero-valentironions(ZVI),hematiteandmagnetite,among

others,maybeusedintheFentonreaction[6–12]orinthe

wastewatertreatmentfromexplosivemanufacturing[13],as

anpossiblealternativetothisreactionmentioned.Wheniron

initszero-valentform,Fe0isusedtheFentonreactionmay

occurvia ahomogeneousreaction afterdissolutiontoFe2+

or on the particle surface Iron nanoparticles are another

source of iron for this homogeneous reaction, during the

oxidationof elementar iron The advantagesof using iron

nanoparticlescomparedtomicrometerparticlesincludetheir

higherefficiencyinthedegradationreactions,high

reactiv-ityduetohighsurfacearea,highmobilityandhighfiltration

efficiency.Beingnanosized,theyalsoremaininsuspension

longer,therebyfacilitatingtheirvariousknownapplications

[8,9,14,15]

Ironnanoparticles can alsobeusedsupportedon other

materials,suchassilica,carbon,resinsornylonmembranes

Silicaismoreadvantageouswhencomparedtoother

mate-rials, since it agglomerates ZVI nanoparticles, better than

othermaterials[16–18].Thisincreasedagglomeration

drasti-callyreducesreactivityandparticlemobilityduring“in situ”

treatments.Sincesilicaisinertbiocompatible,non-toxic,and

showsgoodchemicalandthermalstability,thestabilizediron

nanoparticlestendtodisperseonthesilicasurface[16–19]

Theuse of highlyorganized nanostructured SBA-15

sil-icaiswellknown,withapplicationsinseveralareas,suchas

catalysis,drugdelivery[19]andasasupporttoimmobilize

well-dispersedZVI nanoparticles [20,21] Thistypeofsilica

isabletointeractwithatoms,ionsandmolecules,notonly

thesurface,butalsoinsideitsapproximately10nmdiameter

nanopores

So, this paper aimsto synthesize and characterize ZVI

nanoparticlessupportedonSBA-15nanosilica.This

synthe-sizedmaterialcanbeusedinotherstudiesinvolvingadvanced

oxidationprocessesreactions,whichmaypromotethe

degra-dationofxenobioticcompoundsorinotherapplicationsfor

groundwatertreatment

2.1 Materials

IronIIInitratenonahydrate(Fe(NO3)3·9H2O),sodium

borohy-dride (NaBH4) and hexane, used for the synthesis of iron

nanoparticles, were all PA grade (VETEC, Rio de Janeiro,

Brazil).Theco-TriblockPolymerreagent(PluronicP123,5800,

(C3H6O·C2H4O)x)andTetraethyl orthosilicate(TEOS)wereused

fortheSBA-15silicasynthesis(Sigma–Aldrich,St.Louis,USA)

2.2 Sample preparation

2.2.1 SBA-15 silica synthesis

Differentmethodsarereportedintheliteratureforthe

synthe-sisofSBA-15silica[19,21,22].Theprocedureadoptedherein

isanadaptationofsomereportedmethodologies[21,22].In

thepresent study,2.0gofP123wereaddedtoamixtureof

15mLofwaterand60mLofanaqueous2.0molL−1HCl

solu-tionandstirredfor2hat308K.Ifthemixturewascompletely

solubilizedbeforethe2-hperiod,waitingwasnotnecessary

Subsequently, 4.25g of TEOS were added to the solution undermoderatestirring(150rpmfor10min).Afewminutes (5–10min,nearly)aftertheadditionofTEOStothesystemthe formationofawhite-coloredprecipitatewasobserved,which should remain in the system during the mentioned time-frame.Afterstirring,themixturewasmaintainedat308Kfor

20hinafume-hoodtodrytheacidpresentinthesolution, andthenmaintainedat373Kfor24h

Thesolids were thenwashed withdeionized waterand collectedbyfiltrationatroomtemperaturetoremoveexcess unreacted material The washing was carried out several times,untilnomorefoamingwasobservedinthepermeate Thesamplewasthendriedinalaboratoryovenat373Kfor

24handcalcinedat823Kfor5h

2.2.2 Iron deposition on the silica

One gram of silica nanoparticles obtained by the above methodwassolubilizedin30mLofhexanewithrapid stir-ring (500rpm)for30min.AsmallamountofironIIInitrate 2.0molL−1solutionwasthenaddeddropwisetothesystem Thevolumeofthe ferricnitratesolutionisthreetimesthe massofsilicausedinthesystem.Aftermixing,thehexane wasdrainedfromthesystemandstoredfortheotherstages

oftheprocess.Thesolidphasewasthendriedonahotplate insideafume-hoodat318K

2.2.3 Reduction of the iron-deposited silica

Onegramofsilicapermeatedwithironwassolubilizedwith

30mLofhexane.TofacilitatesolubilizationaHClsolutionat

pH2.0wasused.Thereductionreactionwithsodium borohy-drideisreportedintheliterature[3,10,14,23–26].An8.0molL−1 sodiumborohydridesolutionpreparedina50%alcohol solu-tionwasaddeddropwisetothesystemunderstrongstirring, andavacuumof–500mmHg.Theboron/ironrationusedwas from 4:1 Thesystemheats up duringthe reaction, dueto

aspontaneousexothermicreaction, howevercoolingisnot necessary.Thesystemremainedunderstirringandvacuum untilallthegaseousbyproductswereremovedbythevacuum ThefinalpHofthesystemwasaround9.0.Thesamplewas thenwashedseveraltimeswithPAalcohol,untilthesolution remainedclearwithnomoregasformation.Thezero-valent ironnanoparticlessupportedontheSBA-15silicawerethen storedinvialscontainingpurealcoholinadesiccator

2.3 Characterization

2.3.1 X-ray diffraction

Thisassayallowsforthecharacterizationofthe crystalline structureofthematerial.Byusingasetofinformationfrom thecrystallographicplanesofknownmaterialsitispossible

toidentifythechemicalcompositionofunknownmaterials TheX-raydiffractionexperimentswereperformedonaRigaku MiniflexIIapparatusat30kVand15mA,rangingfrom5to

90◦,withavariationof0.5◦.Thecharacterizationofindividual peakswasperformedbytheMaterialsDateJade5(5.0.37)XRD PatternProcessingsoftwarepackage

2.3.2 Transmission electron microscopy (TEM)

ThemicroscopyimageswereobtainedonaFEIMorgagni268 electronmicroscopeoperatedat80kV.0.047mmcoppergrids wereused(#300and63␮m)forsupportingthematerial.The

grids were preparedusing a dilute 0.3% solution of

form-varin1,2-dichloroethane,and the samplesthenreceived a

graphitelayerinthe vacuumrecipient Asmall amount of

thesamplewasthendilutedinPAalcoholuntilalmostfull

transparency.Twoorthreedrops,atmost,ofthisdiluted

sam-plewerethenplacedonthegrid,subjectedtovacuumuntil

completealcoholevaporation,andthenexaminedunderthe

microscope

2.3.3 Scanning electron microscopy (SEM)

AJEOL 6460LVscanning electron microscopecoupledto a

NoranSystemSixEDSoperatedatlowvacuumunder20kV

was used A small amount ofnon-metallized sample was

placedinthevesselandintroducedintothemicroscope

2.3.4 X-ray fluorescence spectrometry (XRF)

ThistestwascarriedoutonPriminiRigakufluorescentX-ray

spectrometerandthedry samplewas analyzedbytheZSX

softwarepackage

2.3.5 Zeta potential ( )

Thisassaywasconductedona+3.0Zetameterbythe

elec-trophoreticmobility/velocitytechnique, withan indifferent

NaCl electrolytesolution(0.01molL−1).ThepH adjustment

wasconductedwithNaOHandHCl,rangingbetween0.01and

0.1molL−1.Theconcentrationoftheironsamplewithsilica

wasof50mgL−1

2θ(degrees)

70

c

b

a

90

Fig 1 – Diffractogram (a) zero-valent iron, (b) SBA-15 and (c) Fe–Si.

ThecharacterizationbyX-raydiffractionresultsaredisplayed

inFig.1,inwhichthe bottomfigure(a) correspondstothe ZVIdiffractogram,inwhichthecharacteristicpeakof high-estintensityoccursat44.75◦.Themiddleline(b),associated withpureSBA-15silica,isaconstantlinesincethisisan amor-phoussubstance.Finally,thetopline(c)displaysthegraphfor

Fig 2 – Micrographs of the ZVI nanoparticles (a), SBA-15 (b), ZVI supported on SBA-15 (c) and (d).

Fig 3 – Micrographs of silica SBA-15 (a) and the nZVI supported on SBA-15 (b).

zero-valentironsupportedonsilica.Alargerphaseis

associ-atedwithFe3Siand asmallerphaseisassociatedwiththe

zero-valentiron

Theimagesobtainedbytransmissionelectronmicroscopy

revealthatmostoftheZVIparticleshavediametersofless

than 100nm, asdisplayedinFig 2(a).Fig 2(b)displaysthe

SBA-15silicastructurewithnoironparticles.Fig.2(c)and(d)

displaysthesphericalnZVIsupportedontheSBA-15silica.As

observedinlattertwofigures,thesupportedamountofiron

isassociatedwiththe volumeofthe solutionaddedtothe

systemduringreductionwithsodiumborohydride

The photomicrograph of the SBA-15 silica is displayed

Fig.3(a).Acluster ofthestructuresis observedduetothe

2000×resolution.ThephotomicrographoftheSBA-15silica

withnZVIisdisplayedinFig.3(b)ata5000×resolution.The

agglomeratedironparticlesarenoticeablymuchsmaller

com-paredtotheSBA-15silica

Fig.4(a)and(b)illustratestheEDSoftheSBA-15silicaand

ZVIsupportedontheSBA-15silica,respectively.Peaks

belong-ingtocarbonaredisplayedinbothfigures,duetothepresence

Fig 4 – EDS spectra of SBA-15 silica (a) and the nZVI

ofthetapeusedtosecurethesampleinthecontainer,that containscarbon.Thepeaksfoundduringthescanforsodium (Fig.4b)areduetothesodiumborohydrideusedduringthe ironreductionprocess.However,thesepeaksarequiteminor comparedtootherpeaks,forexample,siliconandiron.These elements,alsodisplayedinFig.4(b),representapercentageby sampleweightof11.86%siliconand14.01%iron,respectively The non-destructive X-ray fluorescence assay indicated 26.96%Fe2O3and73.04%SiO2inthesample,aspredictedand confirmedbyothercharacterizationtests

Thesurfaceofthehydratedsamplemayfavorincreased charges.ThepHvalueatwhichthereisachargeneutralityof theliquidsurfaceofthesampleistheisoelectricpoint(IEP),

afunctionofpH.Fig.5displaysthisbehavioratdifferentpH values.TheIEPobtainedfortheZVInanoparticleswasclose

toapHvalueof2.0.ThisvalueisclosetotheIEPofsilica,

as described inthe literature[27,28], whilethe IEPfor ZVI nanoparticles reportedintheliterature[3,21]isaround8.3 Thiscorroboratesthegreaterpresenceofsilicainthesample,

asverifiedbytheX-rayfluorescenceresults

TheEh-pHdiagramisdisplayedinFig.6,obtainedat298K and at a ratio of 4:1 (B: Fe) and 0.15molL−1 of silica At

pH 2.0 the silica is present in its anionic form, while the metalisinferricionform.Ironisattractedtothesilicaand remainsinsolution,subsequentlyundergoingreductionwith

0

-20 -15 -10 -5 0 5

pH

Fig 5 –␨potential as a function of pH.

Fig 6 – Eh-pH diagram of the Fe-B-Si-H 2 O system at 298 K.

borohydridewithinthesilicapores,thusgeneratingtheZVI

nanoparticles

ThepermeationoftheSBA-15silicawithaferricnitrate

solu-tionandreductionwithsodiumborohydrideallowedforthe

synthesisofthezero-valentironnanoparticlessupportedon

SBA-15,confirmedbyXRD,TEM,SEM,EDSandXRFanalyses

Thesampleswerefoundtooxidizeeasily,requiring

hand-ling and preparation in an alcoholic solution A nitrogen

stream,however,wasnotnecessary

Thecharacterizationtechniquesconfirmedthe presence

ofsiliconandironnanoparticlesinthesample,inweight

per-centageof11.86%and14.01%fortheEDS,respectively,and

73.04%and26.96%fortheirrespectiveoxides

TheIEPofthesamplewascloseto2.0,duetothesilica

presentinhigheramountsasconfirmedbythequantitative

results,attractingtheironintoitspores.Therefore,thefirst

layerofthesampleispredominantlyfilledbythesilica.At

thisIEPvalue,theironispresentasferricions,being

electro-staticallyattractedtothesilicaandsubsequentlyreducedto

pH2.0toZVInanoparticles

Overtime the nanoparticles tend toagglomerate,

espe-ciallyinwaterduetoexistingcharges, thusformingoxides

andhydroxidesinthemicrometerrange

ItissuggestedthatZVInanoparticlescanbeusedto

sup-port,forexample,onSBA-15,canbeagoodalternative for

applicationinFenton-typereactions,sincetheironoxidation

tendstooccurinthesilicaitself,canthusminimizingthe

for-mationofundesiredsludgeasaresultoftheclassicalFenton

reaction

Conflicts of interest

Theauthorsdeclarenoconflictsofinterest

Acknowledgments

The authors would like to thank the IMPG Electronic MicroscopySectorandtheHydrogenTechnologyLaboratory

atUFRJ

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