Adsorption of polymers onto iron oxides: Equilibrium isotherms

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isotherms

C.H. Veloso, L.O. Filippov, I.V. Filippova, S. Ouvrard, A.C. Araujo

To cite this version:

C.H. Veloso, L.O. Filippov, I.V. Filippova, S. Ouvrard, A.C. Araujo. Adsorption of polymers onto

iron oxides: Equilibrium isotherms. Journal of Materials Research and Technology, Elsevier, 2020, 9

(1), pp.779-788. �10.1016/j.jmrt.2019.11.018�. �hal-03252456�

j materres technol.2020;9(1):779–788

w w w . j m r t . c o m . b r

Availableonlineatwww.sciencedirect.com

Original

Article

Adsorption

of

polymers

onto

iron

oxides:

Equilibrium

isotherms

C.H.

Veloso

_,

_L.O.

_Filippov

_,

_I.V.

_Filippova

_,

_S.

_Ouvrard

_,

_A.C.

_Araujo

a_{UniversitédeLorraine,CNRS,GeoRessouces,F-54000,Nancy,France}

b_{ArcelorMittalGlobalResearchandDevelopment,VoieRomaine,BP3032057283Maizières-lès-Metz,France} c_{UniversitédeLorraine,CNRS,LSE,F-54000,Nancy,France}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received28June2019 Accepted10November2019 Availableonline12December2019

Keywords: Hematite Magnetite Depressant Adsorption Isotherm

a

b

s

t

r

a

c

t

Theinteractionsofpolymers(cornstarch,dextrinfrommaizestarch,humicacidsodium saltandsodiumcarboxymethylcellulose)withironoxides(hematiteandmagnetite)have beeninvestigatedbymeasuringadsorptionisothermsandbyelectrophoresis.Accordingto theelectrophoresismeasurementsatpH7bothironoxidespresentnegativesurfacecharge andpositiveatpH5.TheequilibriumadsorptionisothermswerethendeterminedatpH7 foralltheadsorbatesexceptforhumicacidwhichwasstudiedatpH5,duetoitsanionic characteristics.TheequilibriumdataofbothironoxideswerestudiedusingFreundlichand LangmuirmodelsanditwasfoundtobestfittotheFreundlichone.ThevaluesforFreundlich constantsindicatethatthemechanismthatcontributesmosttotheadsorptionprocessin all experimentswasthehydrogenbonding.However,thecoexistenceofmorethanone adsorptionmechanismiswhatbestexplainstheprocessitself,inadditiontoexplaining thedifferencesfoundamongsttheoriesovertheyears.

©2019TheAuthors.PublishedbyElsevierB.V.Thisisanopenaccessarticleunderthe CCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1.

Introduction

Innature,ironoccursboundtomorethe1200different min-eralsbutonlyfromveryfewitcanbeeconomicallyextracted. Amongalltheseminerals,hematite(␣-Fe2O3)andmagnetite

(Fe3O4)arethemainironoxidesfoundinironoresfromwhich

metallicironcanbeeconomicallyextracted.Priortothe pro-ductionofmetalliciron,separationprocesses arerequired. Theseprocessesalwaysexploresomeintrinsicpropertiesof themineralstogenerateacontrastbetweenthemandhave

∗ _{Correspondingauthors.}

E-mails:carlos.veloso@arcelormittal.com(C.Veloso),lev.filippov@univ-lorraine.fr(L.Filippov).

adifferentiatingability:shape,specificgravity,magnetic sus-ceptibilityandsurfacereactivity.

Amongthe techniquescurrentlyappliedtoprocessiron ores, magnetic separation is the most used one [1], how-ever facedtothe challenges thatthe mineralindustry has beenexperiencingwiththeemergenceofrawmaterials con-taininglessmineralsofinterestandexhibitinglesscontrast between them, this technique hasoften proved inefficient and/orinsufficient.Theexplorationoflow-gradeironoresis veryoftenassociatedwithmorecomplexmineralogical com-position [2], requiringfiner grinding toachieve the desired

https://doi.org/10.1016/j.jmrt.2019.11.018

2238-7854/©2019 The Authors. Publishedby Elsevier B.V. This isan open access articleunder the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

Nomenclature

Ci initialconcentationofsolution(mgL-1)

Ce concentration atadsorption equilibrium (mg L-1₎

KF Freundlich constant related to adsorption

capacity(mg1-1/n_L1/n_g-1₎

KL Langmuirconstant(Lmg-1)

n Freundlich isotherm constant related to adsorptionintensity

qe equilibrium adsorptioncapacityofadsorbent (mgm-2₎

qec equilibriumcapacityobtainedfrommodel(mg

m-2₎

qee equilibrium capacity obtained from experi-ment(mgm-2₎

qm monolayer adsorption capacity of adsorbent

(mgm-2₎

r2 _{coefficientofdetermination}

R dimensionlessseparationfactor 2 _{chi-squareerrorfunction}

WL wavelenght

mineralliberation.Flotationprocesswhichinvolvestheuseof thephysical-chemistrypropertiesofthemineralstoseparate them,appearsinthiscaseasasolutiontoproducehighgrade concentrates,beingabletobeappliedtoveryfineparticlesand complexessilicatesganguesystems[3,4].Itsfirstindustrial applicationdebutedin1954[5]andsincethenalotofresearch hasbeenconductedinflotationofironores[1,2,4,6–19].The surfacechemistryofironoxideshasbeeninvestigatedover theyearsaimingtofindsolutionsmostlyforenvironmental issuesrelatedtothepresenceofthesechemicalspecies.The abilityofironoxidestoadsorbmetalions[20–22],inorganic anions[20]and organiccompounds[23–25]were theobject ofseveralstudiestohaveabetterunderstandingofthe sur-faceandthecollectoranddepressantadsorptionbehaviour ofthese minerals. However,an in-depth study ofhow the maindepressantsusedinthisprocessadsorbontheminerals surfacehasneverbeenconductedinasystemicway.Several studiespresentedtheadsorptionofnaturalpolysaccharides (dextrin,starch)onmineralsurfacesthroughchemical inter-actionswithsuperficialmetalhydroxides.KhoslaandBisiyaw havesuggestedastarchadsorptionmechanismoncalciteand hematitesurfacesaschemicalcomplexformations[26].This facthasbeensupportedbystudiesofstarch adsorptionon the hematite and Filippov et al. studies on the Fe-bearing amphiboles[27].Eventhechemicalinteractionwasproposed asthemainmechanismofpolysaccharideadsorptionon min-eralsurfacesthetotallydifferentmechanismswereobserved bydifferentresearchersforthesamemineral-polysaccharide system.However,fortheflotationsystemitiscrucialto under-standhowtheadsorptionofeachreagenttakesplaceonthe mineralssurfacesandwhatistheadsorptioncapacityofeach onetohavemoreinformationabouttherateofovercoatingof themineralsandwhichadsorptionprocessesareinvolved.

The aims of this study were to determine extents of adsorptionofpolymersontoironoxides, andtoinvestigate

Table1–Chemicalanalysesofpuremineralsamples.

Sample Elementalanalyses(%ofoxides)

FeO Fe2O3 SiO2 Al2O3 CaO MgO TiO2

Hematite 0.43 98.20 0.28 0.10 0.12 0.08 0.01

Magnetite 29.90 68.91 0.44 0.45 0.02 0.04 0.08

Table2–SpecificsurfaceareaofpuremineralsbyBET method.

Mineral BETsurfacearea(m2_g-1₎

Hematite 0.9±0.2

Magnetite 1.4±0.2

the mechanismsinvolvedineach process.The

experimen-taldataweretreatedandmodelledaccordingtoadsorption

theories. Despite the Langmuiradsorption isotherms have

beenoriginallydevelopedtodescribegas-solid-phase

adsorp-tion, it havebeen widely usedfor other phasesystems as

well[25,28–35].Thistheoryreferstoahomogeneous adsorp-tion and the empiricalmodel assumesthat theadsorption occursonlyinamonolayerhavingnointeractionsbetweenthe adsorbed molecules.Freundlich adsorptionisotherms were alsoinvestigatedtocomparewiththatLangmuiradsorption isotherms.Freundlichmodeldoesnotlimittheadsorptionto theformationofmonolayeranditiswidelyusedtodescribe heterogeneoussurfaces,asexpectedforthecaseofadsorption ofpolymersonironoxides.

2.

Materials

and

methods

2.1. Materials

2.1.1. Ironoxides

Hematite(␣-Fe2O3)andmagnetite(Fe3O4)samplesare from

Brazilandpresentahighdegreeofpurity.Theywereprepared inthesamewayasthesilicatesamplesdescribedby[1],being the fractionbelow38␮musedfortheadsorptiontestsand thefractionbelow5␮mfortheelectrophoreticmeasurements. TheidentitiesoftheironoxideswereconfirmedbyXRF(X-ray fluorescence)(Table1)andbyXRD(X-raydiffraction)analyses (Fig.1).

ThechemicalanalysesshowthatFeOandFe2O3valuesare

very closetothe theoretical valuesforhematite and mag-netite,indicatinganexcellentlevelofpurityofthesamples used.TheXRDdiffractograms(Fig.1)showonlythe charac-teristicpeaksofthemagnetiteandhematite,supportingthe resultsoftheXRFanalysis.

2.1.2. Reagents

The reagents –corn starch (C6H10O5)_n, dextrinfrom maize

starch(C6H10O5)_n,humicacidsodiumsalt(C9H8Na2O4)_n and

sodium carboxymethyl cellulose (CMC) (C8H16NaO8)_n, used

during the adsorption tests were purchased from Sigma Aldrich, Table 2 presents the average molecular weight informedbythesupplier.

j mater res technol.2020;9(1):779–788

781

Fig.1–X-raydiffractionspectrumofhematite(a)andmagnetite(b).

Polymer Averagemolecularweight Cornstarch 500,000

Dextrinfrommaizestarch 20,000 Humicacidsodiumsalt 500,000 Sodiumcarboxymethylcellulose 700,000

Cornstarchpreparationwascarriedout withits gelatin-isationbyadding NaOHata 5:1ratio anddeionised water whileaqueoussolutionsofallotherreagentswereprepared withdeionisedwater.Astocksolutionat1000mgL-1_ofeach

depressantwasprepareddaily,to avoiddegradationofthe compoundsasdiscussedbyBalajeeandIwasaki[9]. Analyt-icalgradeHClandNaOHwereusedaspHmodifiersforthe adsorptiontests.

2.2. Specificsurfaceareameasurements

Thespecificsurfacearea(areaperunitmassorvolume)of themineralswasdeterminedbyamulti-pointBET method withN2adsorption,usingBelsorpmini-IIequipmentfromBel

JapanInc.Thismethodconsistsofmodellingtheportionof

theisothermthatcorrespondstotheendoftheadsorptionof thefirstlayerofthegas.Finally,thesurfaceareaisdetermined usingt-plotmethodbyconsideringthevolumeofN2adsorbed

forthetotalcoatingofthesolid.TheresultsfortheBETsurface areaofeachmineralarepresentedinTable2.

2.3. Electrophoreticmobilitymeasurements

Measurements were performed with a Zetaphoremetre IV, modelZ3000apparatus.Anelectricfieldof80±1Vcm-1_was

employedduringthezetapotentialdeterminations.Theywere carriedoutinanindifferentelectrolytesolution(KCl)of con-centration10-2_{MatpHfrom2to12.Thepuremineralpowder}

ofsizefractionbelow5␮mwasaddedtodeionisedwaterto prepareadilutesuspensionusedforthemeasurements.The suspensionwas thenpassedthroughanultrasonicbathto bewell disaggregated and thenkeptconstantly under stir-ring. The pH value was adjusted through the addition of analyticalgradeHClorKOHsolutions.ForeachpHvaluethe electrophoretic mobility was determined from atleast 100 particlesandtheZetaPotentialwascalculatedbythe Smolu-chowskiequation.Theexperimentswererepeatedtwice,and

theresultswereaveraged.Thestandarddeviationwas calcu-latedusingtheaverageofallvalues.

2.4. Adsorptionstudies

The adsorption determination was made by a TOC (Total Organic Carbon) analysis using a Shimadzu analyser TOC-VCSHand itsvalueisthedifferencebetweentheTC (Total Carbon)andIC(InorganicCarbon).Forthetests,1gofeach mineralwasplacedina100mLvolumeflaskwithtotal depres-santsconcentrationsrangingfrom25to300mgL-1_.After1h

of conditioning on a digital horizontal shaker at23±2◦C, thesolid-liquidseparationwasperformedinacentrifugeat 10,000Gandtheliquidphasewasthenpassedthroughthe TOC analyser.The quantity of reagent adsorbed was then determinedfromthe differencebetweentheinitial(Ci)and

residual (Ce)concentrations. All theadsorption testswere

madeinduplicateandthestandarddeviationpresentedwere calculated from the average value of adsorption quantity results.

2.5. Theory

2.5.1. Adsorptionisothermsmodels

Twowell-known adsorptionisothermsmodels, namely Fre-undlich [36] and Langmuir [37], were used to explain the adsorptionofthereagentsontothesurfaceoftheironoxides.

2.5.2. Freundlichisotherm

qe=KFC1/ne (1)

Eq.1canberearrangedintolinearformas:

logqe=logKF+1_nlogCe (2)

whereqeistheamountofreagent(mg)adsorbedperunitarea ofironoxide(m2_{)atequilibrium;C}

e istheequilibrium con-centration(mgL-1_)ofreagent;K

FistheFreundlichadsorption isothermconstant





adsorp-tioncapacity ofthe adsorbentandnisalsothe Freundlich adsorptionisothermconstantwhichrepresentsanindication ofthedeviationfromlinearity.AplotlogqeagainstlogCegives astraightlineofslope1/n andinterceptKFbymeansofwhich theconstantscanbedetermined.

2.5.2.1. Langmuirisotherm.

qe=₁q₊mK_KLCe

LCe (3)

Eq.3canalsobelinearizedinto: 1 qe = 1 qm+ 1 KLqm 1 Ce (4)

whereqmisthevalueofqeatsaturationand,KLisLangmuir’s adsorptionisothermconstant





energyofadsorption.Byplotting1/qe against1/Ce,the

con-stantsqmandKLcanbedeterminedfrominterceptandslope

ofthestraightline,respectively.

Fig.2–ZetaPotentialofironoxidesasfunctionofpH.

Table3–IEPsreportedbyotherauthors.

Mineral pHIEP Hematite 5.4a_;5.7a_;6.0b_;6.6a_;6.2-6.5e_;4.8g Magnetite 4.8c_;5.0d_;6.5a_;4.4f a _[39]. b _[40]. c _[41]. d _[42]. e _[43]. f _[44]. g _[45]. 2.5.3. Erroranalysis

Thebest-fittingisotherm,whenlinearizationisused,hasbeen veryoftendeterminedwiththeuseofthecoefficientof deter-mination(r2_{).Hoprovedthatthiscoefficientisnotappropriate}

forcomparingwhichmodelfitsbestthe experimentaldata andsuggestedtheChi-squaretest(2_{)asamethodthatdoes}

notpresentsignificantlyimpactwiththedifferentequations’ forms[38]. Chi-squaretest considersthe experimentaland modelcalculateddataandcanbedeterminedby:

2₌

 

qec



(5)

whereqeeisthesamevalueofqecomingfromtheexperiments andqecisthesameequilibriumcapacityoftheadsorbent

cal-culated accordingtothemodel. Thelowestthe valueof2

-test,thebest,provingthatexperimentaldatafitwellintothe model.

3.

Results

and

discussion

3.1. Electrophoreticmobilitymeasurements

Fig.2showsthezetapotentialoftheironoxidesasafunction ofpH.Fromthesamefigureonecanseethattheisoelectric points(IEPs)ofhematiteandmagnetitewereachievedatpH valuesof6.2and5.4,respectively.Table3showsreported val-uesobtainedbyotherresearchersforthoseminerals.

j mater res technol.2020;9(1):779–788

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Fig.3–AmountadsorbedasfunctionofpHforeachdepressantstudied.

TheIEPofbothhematiteandmagnetitesamplesarequite well aligned with the reported values published by other authorsasshowninTable3.Thevaluesvariationismostofthe timerelatedtothenaturalvariationinthecrystalstructureor evenwiththemethodsofsamplepreparationand measure-ments,aswellasdifferencesinsupportingelectrolytes.Also, theuseofnaturalandsyntheticmineralshasbeenreportedas animportantfactorthatjustifiesthedifferentvaluesachieved. Thezetapotentialvaluesobtainedareintotalagreement with the theory ofpH dependence, since Fig. 2 shows an importantvariationofthesevalueswiththepH.Inaqueous medium,iron oxides are hydroxylated and the addition of acids(H+_{)willleadtoapositivevalueofZetaPotentialwhile}

theadditionofbases(OH-)willleadtoanegativevalueofZeta Potential.

3.2. EffectofpHonadsorption

TheeffectofpHintherange5–11ontheadsorptionofthe reagentsontheironoxidessurfaceswasinvestigated(Fig.3). AlmostnoadsorptionwasobservedforCMCintohematite andmagnetitesurfaces.Humicacidanddextrinshowed bet-teraffinityfortheseminerals,butcornstarchisbyfarthebest performer. According to the electrophoreticmeasurements (Fig.2)atpH7bothironoxidespresentnegativesurfacecharge andpositiveatpH5.SincehumicacidandCMCareanionic polymersandatlowerpHvalues,thecarboxylicacidicgroups becomeprotonatedandlessnegativelycharged[46],ahigher adsorptionwasexpectedatapHvaluebelowtheirIEP.This behaviourwasslightlyobservedforhumicacid,howeverCMC showedalmostnoadsorptionwiththeinitialconcentration usedforthepHtests.BasedonthepHvaluethatshowedthe highestadsorptionforeachpolymer,theequilibrium adsorp-tionisothermspresentedlaterweredeterminedatpH7forall theadsorbatesexceptforhumicacidwhichwasobtainedat pH5,thisisexactlythesamebehaviourobservedinaprevious studyusingthesamepolymersontosilicatesminerals[1].

3.3. Adsorptionisotherms

Adsorptionisothermsareapowerfultooltodeterminehow adsorbatesmoleculesinteractwithadsorbentssurfaces.The understandingoftherelationshipbetweenexperimentaldata

Fig.4–FreundlichmodellinearizationforCMCadsorption onhematiteandmagnetite.

andproposedmodelscanbeextremelyusefulforthese inter-pretations.

Theexperimentaldatawerefittedintwodifferentmodels: theFreundlichoneusedtodescribeheterogeneoussystems, assumingthatsitesofadsorptionwithdifferentadsorption freeenergyoccurinparallel,notrestrictingtheformationof multiplelayers[36]and the Langmuirone,whichare used tohomogeneoussystemsinwhichconstantadsorptionfree energiesareobserved[37].Fromthelinearizationofthe exper-imental data it is possible to determine all the constants foreachmodelandevaluatethefittingqualityofeachone (Figs.4–11).

Freundlichconstant KF hasalinearcorrelation withthe adsorptioncapacityoftheadsorbent,whichmeansthatthe greater this constant, the greater adsorption capacity. Fre-undlichconstantncanalsoindicatesthetypeofadsorption thattakesplace.Whenn=1,theisothermislinear,andthe adsorption sites are homogenous (as in Langmuir model); whenn<1,thepresenceofmoreadsorbateintheabsorbent enhancesthefreeenergiesoffurtheradsorption;finally,when n>1,theaddedadsorbatesareboundwithweakfree ener-gies.

Table4–R-valuesisothermclassification. Rvalue Isotherm R=0 Irreversible 0<R<1 Favourable R=1 Linear R>1 Unfavourable

Fig.5–Freundlichmodellinearizationforcornstarch adsorptiononhematiteandmagnetite.

TheLangmuirmodelinotherhandisusedmostofthetime

topredictifanadsorptionsystemisfavourableornotunder

specificexperimentalconditions.Tryingtoclassifytheshape

oftheisotherms bygroups,Langmuir,1918,introducedthe

conceptofadimensionlessequilibriumparameter (R):

R=₁₊1_K

LCi (6)

FromthevaluesobtainedbytheEq.6,theclassificationcan

bedoneaspresentedinTable4

AlltheobtainedvaluesofFreundlichandLangmuirmodel’s parametersaredepictedinTable5forbothminerals investi-gated.

Bothn andR valuesindicate that theadsorptionofthe reagentsonthesurfaceofthesetwomineralsisfavourable. Consideringonlythevaluesofr2_{,therewouldbe}

experimen-taldatathatwouldfitbetterintotheFreundlichmodeland othersthatwouldfitbetterintotheLangmuirone.However, asstatedinsection2.5.2,2_{-testisabettermethodoferror}

thanr2_{analysisandfromthevaluesshownonTable5,itis}

clearthatFreundlichmodelfitsbestinalltheexperimental data.

The experimental equilibrium adsorption isotherms of CMC,corn starch,dextrinandhumicacidonhematiteand magnetitearepresentedinFigs.12–15.Thecirclesandsquares representtheexperimentaldataandthelinesthemodelthat bestfitthisdata–Freundlichmodel.

ThevaluesoftheFreundlichconstantKFarehigherforcorn

starch,showingthatthispolymerhasmoreadsorption capac-itythan theothers.Corn starchhashigh affinitywithiron oxides,throughinitialhydrogenbonds[47]leadingtothe

for-Fig.6–Freundlichmodellinearizationfordextrin adsorptiononhematiteandmagnetite.

Fig.7–Freundlichmodellinearizationforhumicacid adsorptiononhematiteandmagnetite.

Fig.8–LangmuirmodellinearizationforCMCadsorption onhematiteandmagnetite.

j mater res technol.2020;9(1):779–788

785

Table5–AdsorptionparametersofFreundlichandLangmuirisothermsmodels.

Material Adsorbate Freundlich Langmuir

KF n r2 _2 _{KL R qm r}2 _2

Hematite Cornstarch 0.38 1.44 0.9372 0.7872 0.006 0.86 28.08 0.938 1.975

CMC 0.01 1.39 0.8478 0.2695 0.001 0.96 4.12 0.962 0.379

Dextrin 0.03 1.28 0.9252 0.1987 0.006 0.98 23.14 0.922 0.622

Humicacid 0.07 1.51 0.9284 0.2239 0.005 0.88 5.30 0.944 0.267

Magnetite Cornstarch 0.10 1.27 0.9372 0.6575 0.001 0.95 26.88 0.985 1.149

CMC 0.006 1.02 0.9961 0.0149 0.0005 0.98 11.31 0.999 0.019

Dextrin 0.08 2.21 0.9069 0.1005 0.019 0.67 1.17 0.949 0.154

Humicacid 0.09 1.71 0.9505 0.1609 0.013 0.74 2.56 0.976 0.183

mationofchemicalcomplexes[6,27].Thishigh affinitydue

totheformationofcomplexeswasnotobservedinthisstudy fortheother polymers.Humicacid adsorptionisrelatedto anionicinteractionbetweenmineralsandreagentsrendering theadsorptionquitestable.Dextrinresultswereunexpected sincethispolymershouldhavethesametrendofcornstarch, thisshowsthatchainlengthplaysanimportantroleinthe adsorptionmechanismsofpolymers.CMCadsorptionwaslow onbothironoxidessurfacesmainlyduetotheinfluenceof thesubstitutiondegreethatwaslowwhichleadstoalow sol-ubilityofthispolymer.Thedifferenceinmagnitudebetween theadsorptionofcornstarch ontohematiteand magnetite seemstoberelatedtothedifferencesbetweentheircrystalline structures.Whileinhematitetheoxygensionsarearranged inahexagonalwithFe3+ _{ionsoccupyingoctahedralsites,in}

magnetitetheoxygenionsarearrangedinacubicwithFe3+

ionsdistributedbetweenoctahedralandtetrahedralsitesand Fe2+_{ionsinoctahedralsites.Weissenbornetal.[27]}

demon-stratedthattheamylopectinhasastrongcomplexingability forFe3+_{inthesolutionforamolarratioof5.6:1and100:1and}

suggestedtotranslatethe complexationofiron(III)in solu-tiontothecomplexationofiron(III)atomsonthesurfaceof hematite.Thus, the presenceofiron(II) inoctahedral posi-tioninthe magnetitestructuremayimpactthe adsorption behaviourofthestarchonthemagnetitesurface.The con-formationaleffectsbetweenthehydroxylgroupsofpolymers andtheinteratomicdistanceonthemagnetiteneedtobe con-sideredandmayrestrictalsocomplexationonthemagnetite surface.

Themechanism ofpolysaccharidesadsorptionhasbeen the object of study of several researchers over the years. Hydrogenbondingstandsoutbetweentheothers forbeing reportedbyseveralresearchers[9,48,49].Hydrophobic interac-tions[32,50,51]andchemicalcomplexation[6,26,27,52]were also proposed. Nakatani [53] found that glucose adsorbed muchmorestronglyonabasicaluminasurfacethanonan acid one, this fact associated with other researchers find-ings [54], made possible the proposition of an acid-base interaction between polysaccharides and minerals, mean-ing thatthese interactions are responsible fordetermining theadsorptionmechanism ashydrogenbonding or chemi-calcomplexation,dependingonthebasicityoftheminerals surface.

Inthe present study,humic acid showed a slightly dif-ferenceinthe amountadsorbed whenpHisvariated..The samebehaviourwasreportedbyseveralstudiesleadingwith humicsubstances[24,25,33,55–57].Theincreaseordecrease

Fig.9–Langmuirmodellinearizationforcornstarch adsorptiononhematiteandmagnetite.

Fig.10–Langmuirmodellinearizationfordextrin adsorptiononhematiteandmagnetite.

in theamount adsorbed relatedtothe pH valueis consis-tentwiththepropositionofacomplexation-ligandexchange mechanism for humic substances [57] and also with the possibilityofaprotonationofsurfacehydroxylstoform com-plexes[33].Ingeneral,ananionadsorptionatoxidessurfaces involves ligandexchangewithsurfaceFe-OH+

2 andFe−OH

Fig.11–Langmuirmodellinearizationforhumicacid adsorptiononhematiteandmagnetite.

Fig.12–AdsorptionisothermforCMCadsorptionon hematiteandmagnetite(initialpH7.0±0.3).

Fig.13–Adsorptionisothermforcornstarchadsorptionon hematiteandmagnetite(initialpH7.0±0.3).

hydroxylandH2O.Analysingallpossibleexplanationsforthe

adsorptionmechanismsofhumicsubstances,whatisclear isthat different mechanisms can coexist inan adsorption process. Some authors have even quantified the contribu-tionofthosemechanisms(e.g.,vanderWaals interactions, ligandexchangeandcationbridging)forthewholeprocess

[46,58].

Fig.14–Adsorptionisothermfordextrinadsorptionon hematiteandmagnetite(initialpH7.0±0.3).

Fig.15–Adsorptionisothermforhumicacidadsorptionon hematiteandmagnetite(initialpH5.0±0.2).

It isalso worth noting that the constant n has avalue greaterthan1forallstudiedcases,indicatingthatall equilib-riumadsorptionisothermsareassociatedwithaweakbound process,eventhose thatmayhavehigher adsorption ener-gies.

Theobservedvariationsintheadsorptionmechanismsof polymersreportedbyseveralauthorsovertheyearscanbe explainedbythedifferencesintermsofheterogeneityof sam-plesandintheintrinsicvariationsoftheconditionsofeach experiment.

From this study it was possible to prove by the mod-ellingofexperimentalisothermsthatFreundlichmodelfits better the adsorption of these polymers on the surface of hematite and magnetite. Freundlich model constant n indicatesthathydrogenbondingisthemainadsorption mech-anismpresentintheseinteractions.However,aspreviously discussed, the coexistence ofadsorption mechanisms is a widely studied subject and our experimental results com-binedwiththeliteraturereviewallowsustostatethatinthe polysaccharides (corn starch,CMC and dextrin)and humic substance(humic acid)adsorptionbyironoxides(hematite and magnetite),thecoexistenceofadsorptionmechanisms is clearly what happens at different degrees and intensi-ties.

j mater res technol.2020;9(1):779–788

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4.

Conclusions

Theequilibriumadsorptiondatawerefittedtotwoisotherm modelsandtheresultsshowedthatforallexperimentsthe Freundlichmodelfitsbettertheexperimentaldatathanthe Langmuirone.Itisprovedherethatthe2_{-testisabettertool}

toanalysethefitofmodelswhencomparedtor2_.Thevalueof

KFindicatesthatcornstarchhasahigheradsorptioncapacity

inbothironoxides,whichisexpectedgiventhewidelyuseof thispolymerinthemineralprocessingindustry.

Theadsorptionmechanismthatmostcontributestothe adsorptionprocess in all experiments waslikely hydrogen bonding,provedbythevalueoftheconstantnbeinggreater than1inallcases,indicatingthattheadsorbatesarebound withweakfreeenergies.However,thecoexistenceof adsorp-tion mechanisms is forus whatbest explainsthe process itself,inadditiontoexplainingthedifferencesfoundbetween theoriesovertheyears.

r

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