Remediation of hexavalent chromium from

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ContentslistsavailableatScienceDirect

Applied

Surface

Science

j o u r n a l ho me p ag e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Remediation

of

hexavalent

chromium

from

aqueous

solution

using

clay

mineral

Fe(II)–montmorillonite:

Encompassing

anion

exclusion

impact

Mirle

Vinuth

a

_,

_Halehatty

_Seethya

_Bhojya

_Naik

a,∗

_,

_Jayappa

_Manjanna

b

a_Department_of_Industrial_Chemistry,_Kuvempu_University,_{Shankaraghatta}_577451,_India b_Department_of_Chemistry,_Rani_Channamma_University,_PB_NH-4,_Belagavi_591156,_India

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received16May2015 Receivedinrevisedform 14September2015 Accepted19September2015 Availableonline25September2015

Keywords:

Hexavalentchromium Remediationbyreduction Fe(II)–montmorillonite Anionexclusionimpact

a

b

s

t

r

a

c

t

Wehaveexploredthehighlyefficientandenvironmentallybenignclaymineral,Fe(II)–montmorillonite, forthereductionofCr(VI)inaqueoussolution.Fe(II)–MtwastreatedwithK2Cr2O7solutionatdifferent

pH,temperatureandsolid-to-liquidratio.The[Cr2O7]2−wasestimatedbyUV–visspectrawitha

correc-tionforanionexclusionimpact.Ingeneral,theCr(VI)reductionwasrapidatacidicpHandincreasedwith temperatureupto50◦_C._A_complete_reduction_occurred_in_about₅_min_at_pH_3–5._The_time_taken_for

completereductionat0◦_C,_RT₍₃₀◦_C)_and₄₀◦_C_are₁₂_min,₈_min_and₅_min,_{respectively.}_The

reduc-tionfollowedbyimmobilizationofCr(III)onthespentclaymineralwaswellcharacterizedbyEDX andchemicalextractionanalysis.Thisremediationprocesscouldbeeasilyscaled-upforrealsystem applications.

1. Introduction

Chromiumisanextensivelyusedinvariousindustriessuchas steel,paint,leatherand ceramics.It exitsat highconcentration in theeffluents ofelectroplating, chromiumtanning and paper industries[1].Thehexavalentstateofchromiumisawellknown carcinogenicelementwhichishighlytoxic,solubleandmobile;this wascommonlyfoundinsoilandwastewaterreleasedfromvarious industries.

The Cr(VI) is highly toxic to humans, animals, plants and microorganismsandis associatedwiththedevelopmentof var-iouschronichealthdiseasesincludingorgandamage,dermatitis andrespiratoryimpairment[2].ItiswellknownthatCr(VI)ismore toxicthanCr(III)asitleadstocancerandkidneydamagebecause ofitshighoxidizingpotential,anditcaneasilypenetrate biologi-calmembranes[3].Giventhepotentialmagnitudeoftheproblem, itisobviousthatCr(VI)contaminationofsurfaceorgroundwater possessasignificantthreattohumanhealthandtheenvironment

[4].

Theremediationbyreduction,Cr(VI)→Cr(III),isthepotentially

usefulprocesstocleanupthecontaminatedsitesbecauseCr(III)is lesstoxicandcanbeimmobilizedwithsolidphase[5]andbecame

∗Correspondingauthor.

E-mailaddress:hsbnaik@rediffmail.com(H.S.BhojyaNaik).

bioavailabletomicroorganismsandplants.Accordingly,Fe(II) con-tainingoxidesurfaceslikeFe3O4,mixedferrites,etc.arecommonly

usedforthereduction/immobilization.Recently,the biogeochem-icaltransformationi.e.,Cr(VI)reductionbythenaturallyoccurring bacteria,is alsobeing explored[6–9]. In mostof thereduction processes,thekineticsofCr(VI)reductionwasnotonlyslowbut stoichiometricallyinefficientduetothelackoffreshreactivesites and/ordiffusioncontrolledpathwaysforreactants[10–15].Inthe literature,H2S[15,16],SO2[11],H2O2[17],ferrousiron[13,18]are

reportedforthechemicalreductionofCr(VI)→Cr(III).TheSO2and

H2S,themselvesshowtoxicityandcreateadditional

environmen-talproblems.Ontheotherhand,ferrousironandglycerolarenot effectiveinbasicmedium[2].

Carbonaceousadsorbentssuchasactivatedcarbonsand oxi-dized activatedcarbons arealsoused fortheremoval ofCr(VI) ions[19,20]becausetheyarecheap,corrosionresistantandhave shownenhancedadsorptioncapacityforheavymetals.However, theprobleminutilizingthesematerialsistheirseparationby con-ventionalmethodssuchasfiltrationandcentrifugation,whichare timeconsumingandlikelytoloseadsorbentsinsmallamounts[20]. Cr(VI) reduction is also reported by using magnetite, Fe3O4

[21], green rust [22,23], ferrous sulfate–sodium dithionite[24], granularzero-valentiron,ZVI[25–28]andnanoscaleZVI[29,30]. Although these heterogeneous reductants provide high surface areaforadsorptionandprecipitation,theseareeffectiveonlyin lowerpH[31]andmaynotbesuitableforrealsystemapplications

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Fig.1.Schematicdiagramof2:1dioctahedralsmectiteclaymineral,whereMn+ indicatestheinterlayer/exchangeablecation,fore.g.,Fe2+_ions_in_Fe(II)–Mt.

suchaswaterandsoiltreatmentswhereinlargeamountsofthese adsorbents/reductantsarerequired.Therefore,itisimperiousto lookforeffectivereductantand/oradsorbentforCr(VI)inthewide rangeofpHandtemperature.Accordingly,wehaveexploredthe useofFe-basedclaymineralforreductionandimmobilizationof Cr(VI),andwasfoundtobehighlyeffectiveandfeasibleintermsof stoichiometryandreactionkinetics.

Claymineralsareimportantclassofmaterialswhicharereadily availableinnature.Theseare usedasverygood adsorbentsfor toxicelementssuchasarsenate[32,33],decolourationagents,ion exchangers,molecularsieves,catalystsandalsousedinbrick man-ufacturingindustries[34,35].Therearefewreportsonnaturaland modifiedclaymineralsusedforCr(VI)reduction[36–41]. More-overTunisianclay,ElHariaclayandraw/aluminumpillaredclays areusedforremovaltoxicelementslikePb2+_,_As(III),_Cu2+_and_Hg2+

ionsinaqueoussolutionthrough adsorptionprocesses[42–45]. Eloussaiefetal.haveinvestigatedtheefficiencyofthreedifferent claymaterialssuchasraw,acid-activatedandaluminum-pillared Tunisiansmectite(RSM, ASM, andAl-SM)for theadsorptionof Pb(II),Zn(II)andCd(II)insingleandmulti-elementsystems[46]. Fromthe aboveexperimentalresults revealedthat natural and modifiedclaymineralsactaseffectiveadsorbentforremovalof toxicelementsinaqueoussolution.

Montmorillonite(Mt)isa2:1dioctahedralsmectitegroupclay mineralhavingalayeredstructure,Fig.1.Theoctahedralalumina sheetissandwichedbetweentetrahedralsilicatesheets.The neg-ativechargeiscreatedontheclaymineralduetotheisomorphic substitutionintheoctahedralsites(byMg,Fe,andTi)and tetra-hedralsites(byAl,Fe).Suchapermanentnegativelayerchargeis balancedbyexchangeablecationslikeCa2+_,_Na+_,_etc._at_the

inter-layer.Thus, thecation exchange capacity (CEC)of clay mineral dependsonthenetelementalcomposition,whichvarieswiththe geographicalavailabilityintheenvironment.

Thepropertiesand usesofthis claymineralcanbemodified notonlybyalteringthestructuralFe(II)/Fe(III)ratio[36],butalso byreplacingtheinterlayercationswithavarietyofinorganicand organic cations [32,33]. Further, the availability as wellas the amountand/oraccessofstructuralFe(II)forredoxreactionsisvery limited.ThereforeitisrationaltomakeuseofFe(II)–Mt,i.e.,the redoxsensitiveFe(II)ionsplacedintheinterlayerofclaymineral toaugmenttherealfieldapplications.Hence,itispossibletomake useofsuchanimportantredoxsensitiveclaymineralforthe reduc-tionofCr(VI).Thus,inthepresentstudy,Fe(II)–Mtisusedasan effectivereductantforCr(VI)inaqueoussolutionfollowedbyits immobilization.Thereductionreactionwascarriedoutat differ-entpHandtemperaturebyvaryingtheamountofFe(II)–Mt.Anion exclusionimpactencounteredinthisstudyisalsoinvestigated.

2. Materialsandmethods

Na-montmorillonite (Kunipia F, Japan) with a CEC of about 113meq/100g having approximate chemical com-position, (Na0.431K0.002Ca0.002) (Al1.56Mg0.305Fe0.099Ti0.007)oct

(Si3.949Al0.051)tetO10(OH)2nH2Oisusedhere[47].Aqueous

solu-tionofK2Cr2O7waschosenasthemodelhexavalentchromium

contaminant. Double distilled water was used throughout this study.TheconcentrationofCr(VI)wasestimatedfromitsoptical densityatmax=350nmusingUV-Visspectrophotometer.

2.1. PreparationofFe(II)–Mt

Inthefirststep,Fe(III)–Mtwasobtainedbythecationexchange ofaboverawclaymineralin0.4MFeCl3solution.SuchanFe(III)–Mt

wastreatedwithascorbicacidtoreduceinterlayerFe(III)toFe(II) ions[47].Forcomparison,Na(I)–andCa(II)–Mtwerealsoprepared bytheconventionalcationexchangemethodwith1Msolutionsof NaClandCaCl2,respectively.

In order to estimate theinterlayer iron, Fe(II)–Mt was sub-jectedforcationexchangewith0.05MH2SO4forabout24h.The

ratioofferroustoferricions(Fe2+_/Fe

totalwhereFetotal=Fe2++Fe3+)

released was determined by 1,10-phenanthroline method [48]

usingUV-Visspectrophotometer(max=510nm).

The X-ray diffraction pattern (XRD) of the samples were recordedusingD2phaserXRD(BrukerAXSGmbH,Germany)with Ni-filteredCuK␣radiation,=1.5417nm.Infraredspectraofthe

sampleswererecorded byKBrpelletmethodusingIRanalyzer (FT-IR8600PC,ShimadzuCorporation,Japan).Themicrographsof freshly prepared and spent Fe(II)–Mt was recorded using field emissionscanningelectronmicroscope(NovaNanoSEM600,FEI Company,Netherlands)alongwithenergydispersiveX-ray(EDX) analysisforelementalcomposition.

2.2. ReductionofCr(VI)→Cr(III)byFe(II)–Mtinaqueoussolution

In a typical stoichiometric case, freshly prepared Fe(II)–Mt (0.35g)wasaddedtotheK2Cr2O7solution(1mM,100mL)andkept

stirringmagnetically.Thereactionswerealsocarriedoutat differ-entsolid-to-liquidratio,pH(adjustedwithdil.HClandNaOH)and temperature.Thereactionmixturewaswithdrawnperiodicallyby usingsyringetubeandthenfilteredthrough0.2␮mmembrane

fil-tertoremovedispersedclayparticles.Thedecreaseinthe[Cr(VI)] concentrationwasestimatedfromitsopticaldensity.Itis impor-tanttonotethattheabsorbancevaluesherewerecorrectedfor anionexclusionimpacti.e.,equivalenttothatobservedwithtypical divalentclaymineral,Ca(II)–Mt(obtainedinaseparateexperiment withidenticalconditions).ThedecreaseinK2Cr2O7concentration,

duetoreductionofFe(II)–Mt,wasexpressedhereintermsof% reductionasfunctionoftime.

ThespentoroxidizedFe(II)–Mtwasseparatedby centrifuga-tion,washedthoroughlywithwater andvacuumdried atroom temperature for further characterization using FESEM/EDX and FT-IR.Theadsorbed(immobilized)chromiumonspent/oxidized Fe(II)–Mtwasextractedusingdifferentreagentsviz.,0.05MH2SO4,

0.5M(NH)4C2O4,1MNaCl,0.05MNa2EDTA.Ineachcase,aknown

amount(≈0.2g)ofthespent/oxidizedFe(II)–Mtwasdispersedin

50mLofreagentsfor24h.Aftertheextraction, thesupernatant liquid,filteredthrough0.2␮mmembranefilter,wasanalyzedfor

chromiumbyinductivelycoupledplasmaopticalemission spec-troscopy(ICP-OES;PerkinElmer,Optima-7000DV,USA).

3. Resultsanddiscussion

3.1. FormationofFe(II)–Mt

The ratio of ferrous to ferric ions (Fe2+_/Fe

total where

Fetotal=Fe2++Fe3+)releasedfromthefreshlypreparedFe(II)–Mt

oncationexchangewith0.05MH2SO4 wasfoundtobecloseto

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10 20 30

Fig.2. PowderXRDpatternsofFe(II)–MtincomparisonwithCa(II)–andNa(I)–Mt atrelativehumidityof40%.

500 1000 1500 2000 2500 3000 3500 4000

Fig.3.FT-IRspectraoffreshlypreparedFe(II)–MtcomparedwithNa(I)–Mtaswell asspent/oxidizedFe(II)–Mt.

tobe97meq/100g.InadditiontotheXRDandFT-IRresultsalso confirmedtheformationofFe(II)–Mt.

TheXRDpatternsofFe(II)–MtincomparisonwithCa(II)–Mtand Na(I)–Mtatrelativehumidity(RH)of40%isshowninFig.2.The basalspacing(d001)areasfollows: Fe(II)–Mt,15.2 ˚A;Ca(II)–Mt,

14.9 ˚A and Na–Mt, 12.1 ˚A. It is clear that the divalent cation exchangedclaymineralsshowedhigherd001 duetolargerlayer

ofhydrationwhencomparedtomonovalentcationexchangedclay mineral.Thesevaluesareingoodagreementwiththepreviously reportedvalues[49,50].

AsshowninFig.3,theFT-IRspectraoffreshlypreparedFe(II)–Mt issimilartothatofNa(I)–Mt.Thebasicstructureofclaymineral hasnotundergoneanychanges.Forinstance,thebendingvibration bandsat∼520cm−1_for_Si _O _Al,_and₉₂₀_cm−1_for_Al₂_OH_are_intact.

HoweverthestretchingvibrationsofSi Ogroup∼1046cm−1are

slightlybroadened.Thevibrationbandsat1628cm−1_corresponds

500

Fig.4.UV–visabsorptionspectraofK2Cr2O7solutionwithandwithoutCa(II)–Mt andFe(II)–Mt[theanionexclusionimpactisclearlyseenwithCa(II)–Mt].

toadsorbedwaterand3429cm−1_for_water_present_at_the

inter-layer.

3.2. Cr(VI)reductionbyFe(II)–Mt

3.2.1. Anionexclusionimpact(AEI)

Itiswell-establishedthatthepermanentnegativelycharged lay-eredclaymineralsuchasmontmorillonite exhibitstronganion exclusion impact(AEI).Theseeffects have animportantimpact on the adsorption and diffusion of anion. The diffusion of Cl−

ionsin compactedmontmorillonite hasbeenstudiedfor better understandingandmodelingofengineeredbarriersystemforthe geologicaldisposalofnuclearwaste[51].Inviewofthis,ithasbeen suggestedtocorrecttheAEIforCr2O72−ionstoarriveattheproper

reductionlevelofhexavalentchromiumbyFe(II)–Mt.Hence,we treatedatypicaldivalentcation-exchangedclaymineral,Ca(II)–Mt withK2Cr2O7solutionunderidenticalconditionbeforesubjecting

theactualclaymineral,Fe(II)–Mt.AsshowninFig.4,theoverall absorptionvalueofK2Cr2O7solutionhasincreasedsignificantlyin

presenceofCa(II)–Mt(OD:1)whencomparedtoabsorption spec-traofK2Cr2O7solutionalone(OD:0).InthepresenceofFe(II)–Mt,

althoughasimilarAEIis applicable,wecouldseea decreasein absorbancevalue(OD:2)ofK2Cr2O7 solutionduetoredox

reac-tion(Cr6+₊_3Fe2+

→Cr3++3Fe3+).However,theabsorbancevalue

herehasbeeninfluencedbytheAEI.Inordertoobtaintheactual decreaseinabsorbancevaluewithFe(II)–Mt,wemustsubtractby avaluewhich isequaltothatenhanced valueobservedin case ofCa(II)–Mt.ItisimportanttonotethattheAEIvariedwith dif-ferentparametersviz.,contacttime,pH,solid-to-liquidratioand temperature.Thereforehere,alltheabsorptionvalueshavebeen correctedusingthecorrespondingOD:1andOD:2valueswhilethe OD:0remainedalmostthesame.

3.2.2. Effectofstoichiometryonredoxreaction

Fig.5(a–c)showsthe%reductionof[Cr2O7]2−atdifferentpHas

afunctionoftimefordifferentsolid-to-liquidratioviz.,(a)oxidant andreductantareinstoichiometricamounts,(b) reductantisin excessand(c)oxidantinexcess.Ingeneral,[Cr2O7]2−reduction

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0 5 10 15 20 25 120 150 180

Reductant & oxidant are in stoichiometric amounts

i.e. [Fe(II)-mont] : [Cr2O72-]

Fig.5. ReductionofCr(VI)by(a)stoichiometricamountofFe(II)–Mtatdifferent pH.(b)ExcessamountofFe(II)–MtatpH5and6.(c)Sub-stoichiometricamountof Fe(II)–MtatpH5and6.

Itiswell-knownthatthepHhasasignificanteffectontheCr(VI) reduction.Forinstance,Xiang-Rongetal.[2]haveshownthatthe reductionofCr(VI)byascorbicacidunderacidicpHisfasterthanin neutralpHandslowerinalkalinepH.Asimilarlyobservationwas madeforCr(VI)reductionusingmagnetite[52].Therewas>90% removalofCr(VI)bymagneticnanoparticlesatpH2–4whereasit was55%atpH4–7andonly40%atpH7–10[53].Usingnanoscale zerovalentironsupportedonmesoporoussilica(nZVI@MCM-41)

[54],acompletereductionwasachievedatpH3inabout9hand itwasdecreased to50%atpH5.Butatneutral andhigher pH,

0 2 4 6 8 10 12 14 16

Stoichiometric amounts of reductant & oxidant @ pH = 5

i.e. [Fe(II)-mont] _{: [Cr2O7}2-]

Fig.6.ReductionofCr(VI)atpH5by(a)stoichiometricamountofFe(II)–Mtat differenttemperatures.(b)ExcessamountofFe(II)–Mtat0◦_C_and_RT._(c)

Sub-stoichiometricamountofFe(II)–montat0◦_C_and_RT.

therewasnoreductionofCr(VI)usingnZVI@MCM-41. Bentonite-supportednZVIisalsousedforremovalCr(VI)fromwastewater.At pH2,almostcompletereductionoccurredwithinonemin,butat pH8only27%reductionwasobservedevenafter20min[40]. How-ever,inthepresentstudy,wecouldobtainasignificantreduction ofCr(VI)evenatpH8.

Kaduetal.[55]havereportedtheremediationofCr(VI)from simulated water streams using Fe–Ni bimetallic nanoparticles (Fe–NiNPs)andtheirnanocompositespreparedwith montmoril-loniteclay.Batchexperimentswitha25mgL−1_Cr(VI)_solution_and

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10minthatfollowedfirstorderreactionkinetics. Amongst25%, 50%,75%in situ andloadednanocomposites, 75%compositions showedbetteractivitywithenhancedreductioncapacitybelowpH 4duetothegenerationofreactiveH•_species.

Among the clay minerals used for Cr(VI) reduction, Fe(II)-bearingphyllosilicatessuchasiron-richmontmorillonite,chlorite andaregularlyinterstratifiedchlorite-smectite(corrensite)have beenstudiedatacidicpH3[37].Chloriteandcorrensite,owingto thetheirhighFe(II)/Fe(III)ratio,showedrapidreductionofCr(VI). TheoxidationofstructuralFe(II)toFe(III)wasconfirmedbyFe K-edgechangesintheX-rayabsorptionspectra.Similarly,iron-rich claymineral(ferroussaponitefromDeccanregionofIndia)was showntoreduceCr(VI)gradually[38].Therearenotmanyreports onnaturalormodifiedclaymineralsforCr(VI)reduction.Further, theavailabilityaswellastheamountand/oraccessofstructural Fe(II)forredoxreactionsisverylimited.Hence,thisstudyishaving agreatsignificancebecauseFe(II)–Mtcouldbepreparedinlarge amountstoaugmenttherealfieldapplications.

AlthoughtherewasstoichiometricallylessamountofFe(II)–Mt (0.2g≈65%)inFig.5c,itisinterestingtoseeacomplete

reduc-tionofCr(VI)inabout30min.Thismustbeduetothedifference intheabsorptionvaluewhilecorrectingtheAEIbecausetheentire Fe(II)–Mtisconsumed(oxidized)within30min,therebyceasing theAEIvalueclosetozero.Therearesomereports onthe sol-ventextractionofCr(VI)withtetrabutylammoniumbromidefrom aqueoussolution[56]whichshowedthattheefficiencydecreased considerablywithincreasingpHandceasedtozeroatpH∼6.

How-ever,inthepresentstudyweseetheefficientreductionofCr(VI) evenatnearneutralpH.

3.2.3. EffectoftemperatureonCr(VI)reductionbyFe(II)–Mt

ThereductionofCr(VI)byFe(II)–Mtwascarriedoutindifferent temperatures(0–50◦_C)_at_pH_5,_Fig.₆_(a–c)._In_general,_the_Cr(VI)

reductionincreasedwithtemperatureupto40◦_C._The_time_taken

forcompletereductionat0◦_C,_RT₍₃₀◦_C)_and₄₀◦_C_are₁₂_min,₈_min

and5min,respectively.Whentherewasanexcessof Fe(II)–Mt (Fig.6b),ittookjust3minforcompleteCr(VI)reductionat0◦_C_and

RT.However,whenFe(II)–Mt(Fig.6c)wasstoichiometricallyless, atRTittookabout40minforcompleteCr(VI)reductionwhereas at0◦_C,_the_reduction_was_about_95%_even_after₁_h.

Xiang-Rong et al. [2] have reported that the temperature dependentreduction of Cr(VI) byascorbic acidin the range of 5–40◦_C_at_pH₇_took₃₀_min_for_completion._A_significant_effect_on

thereductionofCr(VI)wasobservedwhenthetemperaturewas 5–25◦_C._In_vitro_studies_of_Cr(VI)_reduction_by_cell_free_extracts

ofchromate-reducingbacteriahaveshownthemaximum reduc-tionatambienttemperature,28◦_C_[57]_._{Bentonite-supported}_nZVI

Table1

AmountsofFeandCrreleasedfromspentoroxidizedFe(II)–Mtindifferentreagents.

Extractionreagents Fepresentin(mM) Crpresentin(mM)

0.05MH2SO4 3.45 3.65

0.5M(NH)4C2O4 2.42 3.39

1MNaCl 1.29 2.0

0.05MNa2EDTA 1.22 8.51

usedfortheremovalCr(VI)fromwastewater[40]showedabout 74%reductionat25◦_C_and_82%_reduction₄₀◦_C._On_the_contrary,

whenFe3O4-stabilizedFe0nanoparticleswereusedforreduction

ofCr(VI)fromaqueoussolution[41],therateofreductionwashigh atlowertemperature,forinstance,about90%reductionwasseenat 25◦_C_while_it_is_79%_at₄₀◦_C._However,_in_the_present_study,_we_see

theefficientreductionofCr(VI)byFe(II)–Mtinallthetemperatures from0to50◦_C.

3.3. ExaminationofspentoroxidizedFe(II)–Mt

InordertounderstandthereductionofCr(VI)and immobiliza-tionofCr(III)ontheclaymineral,itisimportanttoexaminethe spentoroxidizedFe(II)–Mt.Fig.7showsthephotographofadry claymineralFe(II)–Mtbeforeandaftertreatingwith1mMK2Cr2O7

solutionatpH5.ThespentFe(II)–MtwasanalyzedbyFESEM/EDX spectratoobserveanymorphologicalchangesuponCr(VI) reduc-tion.AsshowninFig.8therewasnoappreciablechangeinthe microstructureof Fe(II)–Mt.However,EDX confirmedthe pres-enceofimmobilizedCrpresentinthespent/oxidizedFe(II)–Mt.As revealedbyFTIRspectra(Fig.3),thespentclaymineralisintact whencomparedtofreshFe(II)–Mtinallrespects.

Thefollowing reactionsmaybe writtenfor theoxidation of interlayerFe(II)ionsandtheirprecipitationtoneutralspecies(if any)andtheCr(VI)reductionfollowedbyimmobilizationasCr(III) species: 6 (Fe2+

→Fe3++e−_); _3Fe3+₊_9H₂_O_→_3Fe(OH)₃₊_9H+_;

(Cr2O7)2−+8H++6e−→2Cr(OH)3+H2O.

Also, thereis noindicationaboutthepresence ofadditional Fe–Croxidephaseprobablyduetotheirsmallfraction.Inorderto quantitativelyestimatetheadsorbedCrandironinthespentclay mineral,Fe(III)–Mt,thechemicalextractionwasdonewith differ-entreagentsviz.,0.05MH2SO4,0.5M(NH)4C2O4,1MNaCl,0.05M

Na2EDTA.Ineachcase,aknownamount(0.2g)ofthespentclay

mineralwasdispersedin50mLofreagentsfor24handthe sam-pleswereanalyzedbyICP-OES.AsshowninTable1,Na2EDTAwas

foundtobetheeffectivereagenttoextractthesemetalsdueto itschelatingability.TheFeandCrcontentwereslightlysmaller thantheexpectedvalues.Thisisprobablyduetotheirexistenceas

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Fig.8. FESEMwithEDXof(a)freshFe(II)–Mtand(b)oxidized/spentFe(II)–Mt.

oxyhydroxides(FeOOHorCrOOH)whichrequirerepeated treat-mentpreferablyatelevatedtemperatureforcompleteextraction.

AlthoughsomeFe-containingclaymineralslikeferroussaponite arefoundinthenaturalenvironment[38],theavailabilityaswellas theamountand/oraccessofstructuralFe(II)isverylimitedforlarge scalerealsystemapplications.Ontheotherhand,fewreduction processesdevelopedseemedtoberestrictedonlyforlaboratoryuse

[58]andnotforrealsystemapplications.Ifferroussulfateorsodium sulfiteisusedasreductantsforCr(VI)→Cr(III),ferrichydroxideand

sulfurdioxide(toxicandhighlyvolatile)willbeformed, respec-tively,asbyproductswhicharedifficulttohandle[59].Henceitis essentialtoproposeasuitablereductant/adsorbentforfield appli-cation,especiallytotreatwaterandsoil.WebelievethatFe(II)–Mt couldbetheefficientandsuitable materialtoaugmentthereal filedapplications.Forthis,aslurryofFe(II)–Mtfilledindialysis bagscouldbesuspendedinthecontaminatedbodiessuchas flow-ingorstagnantwatersand/orindustrialeffluents.Inthecaseof soilcontamination,itmustbesufficientlymoisturized(wet)before dispersingtheFe(II)–Mt.

4. Conclusions

WehaveconfirmedoneofthepotentialapplicationsofFe(II)–Mt totreatthehazardousCr(VI)contaminationinaqueoussolution.

Theanionexclusionimpactof[Cr2O7]2−ionwiththenegatively

chargedclaymineralherewasconsideredinallourestimations ofCr(VI)reductions.Ingeneral,[Cr2O7]2−reductionbyFe(II)–Mt

isarapidprocess,especiallyunderstoichiometricconditions.For instance,acompletereductionoccurredinabout5minatpH3–5. AtneutralpHandabove,thereductionwasrelativelyslow.The Cr(VI)reductionincreasedwithtemperatureupto40◦_C._The_time

takenforcompletereductionat0◦_C,_RT₍₃₀◦_C),₄₀◦_C_are₁₂_min,

8minand5min,respectively.Theimmobilizationofthereduced Cr(III)wasconfirmedfromtheEDXspectraofspentclaymineral andchemicalextractions,especiallyinNa2EDTA.

Acknowledgements

TheleadauthorM.VinuthwishestothankMr.K.Chandrasekhar forhishelpwithXRDanalysis,PavanS.atTuv-SudSouthAsiaPvt. Ltd.BangaloreforassistancewithICP-OESanalysisandProf.G.U. KulkarniatJNCASRforprovidingFESEMfacility.

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