Precipitation as the main mechanism for Cd(II), Pb(II) and Zn(II) removal from aqueous solutions using natural and activated forms of red mud

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Citation for this paper:

da Conceição, F. T., da Silva, M. S. G., Menegário, A. A., Antunes, M. L. P., Navarro, G. R.

B., Dorea, C., … Moruzzi, R. B. (2021). Precipitation as the main mechanism for Cd(II),

Pb(II) and Zn(II) removal from aqueous solutions using natural and activated forms of red

mud. Environmental Advances, 4, 1-10. https://doi.org/10.1016/j.envadv.2021.100056.

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Precipitation as the main mechanism for Cd(II), Pb(II) and Zn(II) removal from

aqueous solutions using natural and activated forms of red mud

Fabiano Tomazini da Conceição, Mariana Scicia Gabriel da Silva, Amauri Antonio

Menegário, Maria Lucia Pereira Antunes, Guillermo Rafael Beltran Navarro, Caetano

Dorea, … & Rodrigo Braga Moruzzi

July 2021

© 2021 Fabiano Tomazini da Conceição et al. This is an open access article distributed under the

terms of the Creative Commons Attribution License.

https://creativecommons.org/licenses/by/4.0/

This article was originally published at:

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EnvironmentalAdvances4(2021)100056

ContentslistsavailableatScienceDirect

Environmental

Advances

journalhomepage:www.elsevier.com/locate/envadv

Precipitation

as

the

main

mechanism

for

Cd(II),

Pb(II)

and

Zn(II)

removal

from

aqueous

solutions

using

natural

and

activated

forms

of

red

mud

Fabiano

Tomazini

da

Conceição

a,∗

_,

_Mariana

_Scicia

_Gabriel

_da

_Silva

a

_,

Amauri

Antonio

Menegário

b

_,

_Maria

_Lucia

_Pereira

_Antunes

c

_,

_Guillermo

_Rafael

_Beltran

_Navarro

a

_,

Alexandre

Martins

Fernandes

a

_,

_Caetano

_Dorea

d

_,

_Rodrigo

_Braga

_Moruzzi

a

a Instituto de Geociências e Ciências Exatas, UNESP - Universidade Estadual Paulista, 1 – Avenida 24-A, n° 1515, C. P. 178, CEP 13506-900, Bela Vista, Rio Claro, São

Paulo, Brazil

b Centro de Estudos Ambientais, UNESP - Universidade Estadual Paulista, Rio Claro, Brazil c Instituto de Ciência e Tecnologia, UNESP - Universidade Estadual Paulista, Sorocaba, Brazil d Department of Civil Engineering, University of Victoria, Victoria, Canada

a r t i c l e

i n f o

Keywords:

Brazilian red mud Trace elements Sequential extraction Kinetics modelling

a b s t r a c t

Theredmud(RM)hasbeenusedasanalternativelow-costadsorbenttoremovetraceelements,withthe ad-sorptionontosodalitesurfacedescribedasthemainremovalmechanismfortraceelements.However,recent studieshaveshownthatprecipitationmightbeofgreatimportanceforsometracemetalsremovalusingnatural andthermalactivatedRM.Therefore,theaimofthisstudywastoidentifythemainmechanismresponsiblefor Cd(II),Pb(II)andZn(II)removalfromaqueoussolutionsusingnaturalandactivatedformsofRM,basedon se-quentialextractionsandaprecipitationkineticmodelwasdeveloped.Resultsshowedthatthecarbonatefraction wasresponsibleforthehighesttraceelementsremoval(ca.85%),withthemineralsassemblagesprecipitated: otavite– CdCO3,cerussite-PbCO3,smithsonite-ZnCO3andanglesite-PbSO4.Thekineticmodelshowedthatthe

mineralprecipitationwaslimitduetotheHCO₃−_consumption_during_the_formation_of_new_minerals._Hence,_this

studyshowedthatprecipitationwasthecentralmechanismontraceelementsremoval,regardlessthenaturalor activatedformsofRM.Thisﬁndingraisedoubtabouttheeﬀectivenessofthetraditionaladsorptionisothermsand kineticsmodelstodescribetracemetalsremovalusingRM,contributingwithnewinsightsforfutureresearches involvingthesehazardousmaterials.

1. Introduction

Brazilian mining activitiescontributesignificantly toglobal min-eralproduction,includingthethird-largestglobalproductionofbauxite (Brasil,2018).Brazilbenefitedmorethan35megatonsofbauxitevia theBayerProcess®,generatingaresidueknownasbauxiteresidueor redmud– RM(Hindetal.,1999).AccordingtoFortesetal.(2016), about10–25milliontons/yearofRMaregeneratedinBrazil.The Brazil-ianRM can be considered ahazardous material due to presence of different oxidesandtoxictrace elementsmixed in a highlyalkaline matrix(Antunesetal.,2012;Souzaetal.,2013).Thedisposalofthis residueusuallyoccursintailingdams,producingahighfinancialand environmentalcost, leadingtoproblemsrelatedtocontaminationof soil,groundwaterand surfacewateranddamage tofloraandfauna (SilvaFilhoetal.,2007;JonesandHaynes,2011).Themainaccident involvingtheruptureoftheRMtailingdamwasinOctober2010inAjka (Hungary),causing10deathsandmorethan100injuries(Huaetal.,

∗_{Corresponding}_author.

E-mailaddress:fabiano.tomazini@unesp.br(F.T.daConceição).

2017).InBrazil,anenvironmentaldisastercausedbyhighrainfall oc-curredinaRMtailingdamlocatedinBarbacena(Pará State)inFebruary 2018,aﬀectingthirteenriversidecommunities,whichdependsonthe naturalresourcesofthePará Riverbasininthismunicipality (Amazô-niaReal,2018).

Cadmium – Cd(II),lead - Pb(II) andzinc– Zn(II) arecommonly used in several human activities, such asmining, smelting, electro-plating, dyes, ceramics,among others.These trace elements arenot compatiblewithbiologicaltreatmentprocesses,andadsorptionisthe maintechniqueforthese traceelementsremovalinthetreatmentof industrialeﬄuents (Nadarogluet al.,2010). TheCd(II)causes vom-iting and lung andkidneydiseases, withthe Pb(II) aﬀectingalmost all organs,withthecentralnervoussystembeingthemostsensitive, while theZn(II)can causesvomiting,anaemiaandkidneyandliver damage (SãoPaulo,2012). RMhasbeen used forCd(II), Pb(II) and Zn(II)removal,withapplicationofthenatural(Vaclavikovaetal.2005;

Santona et al. 2006; Pichinelli et al., 2017; Ayala and Fernández 2019; Silva et al., 2019) or activated (Apak et al. 1998a 1998b;

https://doi.org/10.1016/j.envadv.2021.100056

Received4March2021;Receivedinrevisedform2April2021;Accepted12April2021

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Table 1

StudiesofCd(II),Pb(II)andZn(II)removal(mmolg−1₎_from_aqueous_solutions_using

naturalredmud(RM)andwithdiﬀerentactivationsprocedures.

Metal Natural and activated forms Removal Reference

Cd(II) RM 0.26 Ayala and Fernández (2019)

RM 0.87 Silva et al. (2019)

RM 1.41 Vaclavikova et al. (2005)

RM 1.35 Santona et al. (2006)

RM - heated 1.04 Silva et al. (2019) RM - heated 0.27 Gupta and Sharma (2002) RM - heated 0.38 Yang et al. (2020)

RM - HCl 0.95 Santona et al. (2006)

RM –HCl 0.14 Silva et al. (2019)

RM - HCl 2.24 Apak et al. (1998a)

RM - CaSO 4 0.22 Lopez et al. (1998) RM - Ca(NO 3 ) 2 0.68 Silva et al. (2019)

Pb(II) RM 2.13 Pichinelli et al. (2017)

RM - heated and H 2 O 2 0.35 Gupta et al. (2001) RM - carbonised 0.45 Pulford et al. (2012)

RM - HCl 0.84 Apak et al. (1998a)

RM - Ca(NO 3 ) 2 2.23 Pichinelli et al. (2017) RM – colloidal silica and NaOH 2.66 Lyu et al. (2020)

RM 0.18 Ayala and Fernández (2019)

Zn(II) RM 1.14 Pichinelli et al. (2017)

RM 2.05 Vaclavikova et al. (2005)

RM - heated 0.22 Gupta and Sharma (2002)

RM - CaSO 4 0.19 Lopez et al. (1998) RM - Ca(NO 3 ) 2 0.96 Pichinelli et al. (2017)

RM - CO 2 0.23 Sahu et al. (2011)

Lopez et al. 1998; Gupta et al. 2001; Gupta and Sharma 2002;

Santonaetal.2006;Silvaetal.,2019;Sahuetal.2011;Pulfordetal., 2012;Pichinellietal.,2017;Silvaetal.,2019;Lyuetal.,2020)forms ofRM.AllstudiespresentinTable1havebeenassociatedtothe ad-sorptionofthesetraceelementsontotheRMsurface,andthe adsorp-tionwasmodelledusingLangmuirandFreundlichisotherms.Sodalite pointedoutasthemainresponsiblefortheCd(II),Pb(II)andZn(II) ad-sorptionin naturalandactivated formsofRM(Santonaetal., 2006;

Pichinellietal.,2017;Silvaetal.,2019).Inaddition,differentkinetics modelswereappliedtodescribetheadsorptionprocesses,suchasthe pseudo-first-order,thepseudo-second-order,theElovichandthe intra-particlediffusionmodels.

Thetraceelementsadsorptionontosodaliteisassociatedtoion ex-changeableand,consequently,theuseofsequentialextractionscanbe ausefultooltoconfirmthismechanism.Tessieretal.(1979)proposed a methodfor sequentialextraction toidentify thegeochemical frac-tions.Thismethodassessesthepotentialmobilityfortraceelements, showing the labile (ion exchangeable, bound to carbonates, bound toFe-Mn oxidesand bound toorganic matter/sulfide) and residual phases.Recentstudiesusing sequentialextraction havereportedthat the Cu(II) (Qi et al., 2018) and Cr(II) (Qi et al., 2020) removal in RM werepreferably associated with bound tocarbonate andbound toFe-Mn oxides,respectively,insteadofadsorptionin ion exchange-able. Yang et al. (2020) proposed the Cd(II) removal is associated to adsorption in bound to Fe-Mn oxides rather thanadsorption (in ionexchangeable) whenthermaltreated RM wasstudied. Lyu etal. (2020)applied adsorptionisotherms todescribeadsorption ofPb(II) ontoRMmodifiedbycolloidalsilicaandsodiumhydroxide;however, theauthors have concluded thatthe precipitationprocesses was re-sponsiblefor78%ofPb(II)removal,asPb-carbonates,evennospecific analysiswasperformed.

Takingintoaccounttherecentﬁndingsontraceelementsremoval mechanismsusingRM,theroleofprecipitationshouldbethoroughly developedandtheuseofadsorptionmodelstodescribetraceelements removalreconsidered.Thefewstudiesusingsequentialextractionsare

limitedtoCu(II) andCr(II) removalbynatural RM(Qiet al.,2018,

2020,respectively)andCd(II)removalbythermalactivatedformsof RM(Yangetal.,2020),andtheyhaveshowntheremovalprecipitations productsonly,withnoprecipitationkineticsmodelproposed.Thus,the mainaimofthisstudywastodeterminethecentralmechanism respon-sibleforCd(II),Pb(II)andZn(II)removalusingnaturalandactivated formsofRM(heatedat400°C-RM400andwithchemicaltreatments

usingHCl-RMHClandCa(NO3)2-RMCa),applyingthesequential

ex-tractionmethod.Secondary,akineticmodelwasperformedtodescribe thetimeeﬀecton theCd(II), Pb(II)andZn(II)removalandto deter-minetheprecipitationkineticconstantsandthereactionorder. There-fore,thispaperexpandstheunderstandingandprovidenewinsightinto theinteractionsmechanismsamongCd(II),Pb(II)andZn(II)andthese hazardousmaterials,whichcanbeusedaslow-costmaterialintheﬁeld ofenvironmentalremediationandwaterindustry.

2. Materialsandmethods

2.1. Samplingandactivationprocedures

ThemunicipalityofAlumínio(Fig.1a),SãoPauloState,Brazil,hosts themainaluminiumplantinBrazil,wherethenaturalRMwassampled (June– 2017)inatailingdam(Fig.1b).TheRMwasdriedfor12h at50°C.Antunesetal.(2012)studiedthethermalbehaviorand phys-icalpropertiesofRMfromBrazil(from400to800°C)andconcluded thatthebesttemperaturetoproduceRMwithalargesurfaceareais 400°Cduetophasetransitionofgoethitetohematiteandgibbsiteto alumina.Thus,theRMwasheatedinamuﬄefurnaceovenat400°Cfor twohours(RM400)toincreasetheremovalcapacityinrelationto

nat-uralRM(Antunesetal.,2012).TheRMwithchemicaltreatments(HCl -RMHClandCa(NO3)2-RMCa)wereperformedtopromotethe

extrac-tionoftheexchangeablephasebymeansofthedesorptionontheRM surface(Santonaetal.,2006;Pichinellietal.,2017).Forthechemical activation,RMwasmixedeitherwith0.05molL−1_HCl_or_with_0.1_mol

L−1_Ca(NO

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re-F.T. da Conceição, M.S.G. da Silva, A.A. Menegário et al. Environmental Advances 4 (2021) 100056

Fig. 1. LocationofAlumíniointheSãoPaulo

State (a). The aluminium plant and tailing dam,withtheimagefromGoogleEarthPro -04/10/2020(b).

movedandtheRMHClandRMCasampleswerewashedthreetimes,using

ultrapurewaterwithelectricalconductivitylowerthan0.02μScm−1

and,then,driedat50°Cfor12h.

2.2. CharacterizationofnaturalandactivatedformsofRM

ThepHvalues forRM,RM₄₀₀, RM_HCl andRM_Ca in solutionwere characterizedusing1g:25mLofultrapurewater,usingYSI556 Multi-ProbeSystemcalibratedwithpurestandardsatpH4and7.Thespeciﬁc surfacearea(SSA)fortheRM,RM400,RMHClandRMCasampleswere

determinedbyBETmethod,usingaMicromeriticsASAPTristar3000 analyseroperatedat-196°Ccalibratedwithnitrogenadsorptioncurves. ThepHPCZ is anotherimportantissueon Cd(II),Pb(II) andZn(II)

removal by RM, once it determines whether electrostatic attraction orrespulsion betweenthesorbents andsorbates(Orfãoetal., 2006;

Jesusetal.,2015).Thepointofzerocharge(pHPZC)valueofRM,RM400,

RM_HCl andRM_Ca wascharacterized using amixed of 0.1g of these materialswith20mLof0.1molL−1 _NaCl_at_initial_pH_varying_from

1to11 (Jesusetal.,2015).Thesolutionwasshakenat2501rpmat 25°Cfor24h,andthentheﬁnalpHwasmeasured.InitialandﬁnalpH valueswereplottedandthepHPZCvaluewasdeterminedaccordingto

Orfãoetal.(2006).

InordertoidentifythemineralsintheRM,RM400,RMHCl,RMCaand

controlsamples,theX-raydiffractometry(XRD– PANalyticalEmpyrean Instrument)wasusedonpowderedsamplesfrom2° to90° with0.02° step-sizes,operatingat40kVand40mA,withCuK𝛼 radiation.The min-eralogicalidentificationwasperformedbythesoftwareX’PertHighscore Plus®,usingICDDPDF2database.Themorphologyofallsampleswas identifiedusingaScanningElectronMicroscopewithanEnergy Disper-siveX-raySpectrometer(SEM-EDS,JEOLJSM-6010LA).

2.3. Removalexperiment

TheCd(II), Pb(II)andZn(II)aqueous solutions(25mL), withthe initial concentrationof80mmol L−1_,_were_mixed _with_1.0_g _of_RM,

RM₄₀₀,RM_HClandRM_Ca.ThesolutionsofCd(II),Pb(II)andZn(II)were preparedusinganalyticalgradenitratesalts:Cd(NO3)2.4H2O,Pb(NO3)2

andZn(NO3)2.6H2O.Thesampleswerestirredat145rpmat25°Cfor

12 handthencentrifuged for25 minat3000 rpm.Afterwards,the RM,RM400, RMHCl andRMCa samplesweredriedfor12 hat50 °C.

Silvaetal.(2019)andPichinelli,etal.(2017)studiedtheinﬂuenceof pH(2,4,7,10and12)ontheCd(II)andPb(II)andZn(II)removal.The authorsshowedthepH7asthebestvalueforremovalofthesetrace el-ements,althoughthepHragingbetween5.0and5.5havebeenwidely used(Santonaetal.,2006;Nadarogluetal.,2010,Smiljamicetal.,2010,

Smiciklasetal.,2014;Conceiçãoetal.,2016).Thus,alltheexperiments fortraceelementsremovalanalysiswereperformedatinitialpHof7, withtheﬁnalpHvaluescharacterizedat5.

2.4. Sequentialextraction

The sequential extraction in RM, RM400, RMHCl and RMCa was

applied as described by Tessier et al. (1979) and Leleyter and Probst(1999).Thedetailedsequentialextractionprocedureispresented inTable2,withtwodifferentgeochemicalfractions:labile(F1– ion ex-changeable,F2– boundtocarbonate,F3– boundtoFe-MnoxidesandF4 – boundtoorganicmatter/sulfide)andresidual(F5).Aftereach extrac-tionstep,thesampleswerecentrifugedat3000rpmfor25min.at25°C. Oncefinished,theresidualRM,RM400,RMHClandRMCaweredriedat

25°Candappliedforthenextsteps.Thepercentage(P)ofCd(II),Pb(II) andZn(II)duetosequentialextractioninRM,RM₄₀₀,RM_HClandRM_Ca

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Table 2 Sequential extraction protocols. Step Fraction Protocols F1 Ion e x ch ang e able 10 mL of MgCl 2 0.5 M fo r 2 h at 25 °C and pH 5.5 F2 Bound to Carbonat e 10 mL of CH 3 C OON a 1 M fo r 5 h at 25 °C and pH 4.5 F3 Bound to Fe -M n ox id e s 10 mL of NH 2 OH.HCl 0.04 M in 25% (v/v) ace tic acid fo r 5 h at 85 °C and pH 2.5–3.0 F4 Bound to or g a nic matt er/sulﬁde 3 mL of HNO 3 0.02 M + 8 mL of H2 O 2 35% fo r 5 h at 85 °C and pH 2.0. Af te r cooling to 25 °C, it wa s adde d 20 mL of ammonium ace tat e 0.85 M in 5% (v/v) HNO 3 fo r 30 min F5 R e sidual d ige st io n pr oce d ur e fo llo wing the EP A 3010A ( USEP A , 1990 )

wascalculatedusingtheEq.(1).

𝑃𝑗 (%)= [ 𝐹𝑗 ] ∑5 𝑖 =1[𝐹𝑖] .100 for𝑗 = 1,2..5 (1) where:

P= percentageof eachgeochemical fractionfor Cd(II),Pb(II) or Zn(II);[F]=Cd(II),Pb(II)andZn(II)concentrationineachgeochemical fraction(mmolg−1_);_Indexes_j_and_i₌_geochemical_fraction_in_which_P

iscalculatedoverallextractedforms,respectively.

2.5. Kineticsstudies

Thekineticsstudieswerecarriedoutusing1g(m):25mL(V)ofan aqueoussolution,withtheCd(II),Pb(II)andZn(II)initialconcentrations of80mmolL−1₍_C

0 ).Thesampleswerestirredat145rpm,removed

af-ter15,30,60,120,420,660and1440min,centrifugedat3000rpm for25minat25°C,withthesupernatantseparatedandtheresidual Cd(II),Pb(II)andZn(II)measured(C_f).TheinitialpH(t=0min)was adjustedto7asexplainedabove,withtheﬁnalpHcharacterizedat5 after1440min.TheEqs.(2)and(3)representtheamountofCd(II), Pb(II)andZn(II)retained(A_S-mmolg−1₎_and_the_removal_eﬃciency

in allexperiments(RE-%),respectively.Afterconﬁrmingthe domi-nantmechanismofCd(II),Pb(II)andZn(II)removal,thekineticmodel fortracemetalsprecipitationwasperformedasdescribedindetailat

Section3.4. 𝐴𝑠 =(𝐶0−𝐶𝑓 ).𝑉_𝑚 (2) 𝑅𝐸 = 𝐶0−𝐶𝑓 𝐶0 .100 (3) 2.6. Analysis

The supernatants associated tosequentialextraction and kinetics studieswerethentransferredtoaTeﬂontubeof50mL,madeupto volumewithultrapurewater,foranalysisoftheCd(II),Pb(II)andZn(II) concentrations byinductively coupledplasma optical emission spec-trometry(ICPOES),iCAP6000SERIESmachineThermoScientiﬁc.The detectionlimitwas0.006mgL−1_for_all_trace_elements._All_experiments

werecarriedoutintriplicate.

3. Resultsanddiscussion

3.1. CharacterisationofRM,RM₄₀₀,RM_HClandRM_Ca

ThepHvaluesfortheRM,RM400,RMHClandRMCainsolutionwere

10.5, 10.7,8.3and7.5,respectively. Thethermal treatmentdidnot changethepHvaluesinrelationtoRM.However,thepHvaluesafter thechemicaltreatmentwerelowerthantheRMduetoCO32−

consump-tionduringthereactionsofCO₃2−_present_in_the_RM,_water_and_HCl_or

Ca(NO3)2(Santonaetal.,2006;Pichinellietal.,2017).ThepHPCZ

val-ues werefor8.8,8.3,9.1and9.9forRM,RM400,RMHClandRMCa,

respectively.Thethermaltreatmentincreasedthespeciﬁcsurfacearea (SSA)inRM400 inrelationtoRM(from33to61m2 g−1),whilethe

chemical treatmentdecreased theSSAvalues (25and27 m2 _g−1 _to

RM_HClandRM_Ca,respectively).

Fig. 2 shows the XRD patterns with the minerals found in the RM,RM400,RMHClandRMCasamples.TheRMiscomposedof

kaoli-nite(Al2Si2O5(OH)4),gibbsite(Al(OH)3),sodalite(Na8Al6Si6O24Cl2),

goethite(FeO(OH)),quartz(SiO2)andcalcite(CaCO3).However,after

thethermaltreatment,thepeakscausedbyaluminumandiron hydrox-ideswerenotdetectedfromtheRM400XRDpatterns.Thiscanbe

ex-plainedbytheconversionofgoethitetohematiteat243°Candalsoby thefactthatthegibbsiteistransformedtotransitionaluminas(𝜒Al2O3)

at272°C(Antunesetal.,2012).TheRMHClandRMCapresentsthesame

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F.T. da Conceição, M.S.G. da Silva, A.A. Menegário et al. Environmental Advances 4 (2021) 100056

Fig. 2. XRDpatternsofRM,RM400,RMHClandRMCa.

andRMCa particleswas observed bySEM-EDS. Particles ofdiﬀerent

size,shapeandtexturewereobservedinRMsample,asillustratedin

Fig.3.Heterogeneousmaterialswithparticlediametersbetweenfrom

<1μmto>10μmcanbeseen.Itcanbeobservedthatthechemical orthermaltreatmentdidnot alterthemineralmorphology,withthe smallestparticlescorrespondingtoironoxidesandthelargestonesto silicon.

3.2. Sequentialextractions

The percentages of labile and residual geochemical fractions of Cd(II),Pb(II)andZn(II)intheRM,RM400,RMHClandRMCasamples

arepresentinFig.4.Bytheanalysisoftraceelementsinthe geochemi-calfractions,itispossibletonotethatthelabilegeochemicalfractions wereresponsibleforca.95%ofthesetraceelements,as6.0±0.3%for Cd(II);3.5±0.3%forPb(II)and7.3±0.4%forZn(II)areassociatedto residualfraction.ThelowestconcentrationsforCd(II),Pb(II)andZn(II) weremeasuredintheionexchangeablefraction(<0.4%foralltrace elements)incomparisontootherlabilegeochemicalfractions,witha maximumremovalof0.04,0.06and0.04mmolg−1_for_Cd(II),_Pb(II)

andZn(II),receptively.

Sodaliteisatectosilicateconsideredaszeolite-typeandithasbeen consideredthemainresponsiblefortheCd(II),Pb(II)andZn(II) adsorp-tioninnaturalandactivatedRM(Santonaetal.,2006;Silvaetal.,2019;

Fig. 3. SEMimagesofRM.

Yangetal.,2020).Unfortunately,thepH_PCZofsodalitehasnotbeen characterisedyet,butithasbeenadvisedthatthenegatively-charged surfacecanbeneutralizedbytheadsorptionofCd(II),Pb(II)andZn(II) withintheouter-spherebondsandinthecagesandchannelsofits frame-work(Whittingtonetal.,1998;Monetal.,2005).However,thelow Cd(II), Pb(II)andZn(II)percentages intheionexchangeablefraction clearlyindicatethatthesetraceelementsremovalbynaturaland acti-vatedformsofRMcannotbeonlyexplainedbythetracemetals adsorp-tionontosodalite.

TheCd(II),Pb(II)andZn(II)boundtoFe-Mnoxidesandboundto organicmatter/sulﬁdewere7.9±0.3%,4.3±0.2%and0.4±0.1%, 2.8±0.2%and0.6±0.2%,respectively.Qietal.(2020),showedthat themainCr(II)removalprocessesintheRMcollectedfromShanxiin ChinawastheboundtoFe-Mnoxides.Yangetal.(2020)suggestedthat theFe-MnoxideswereresponsibleforCd(II)removalfromaqueous so-lution,insteadcarbonateprecipitation,duetolowcontentoftotal inor-ganiccarbonpresentintheRMwithheattreatmentrangingfrom200 to900°CsampledinthenorthofChina.TheCd(II), Pb(II)andZn(II) removalin theFe-Mn oxidesisdue toadsorptionontogoethitesand hematites presentinthenaturalandactivatedRM,whichhavelarge speciﬁcsurfaceareaandreactive-OHand-OH2functionalgroups

ex-posedontheirsurface(LiuandHuang,2003).Inaddition,thesetrace el-ementsalsocanbeadsorbedontooxidesandhydroxidesofAl3+_through

theformationofinner-spherebounds(Santonaetal.,2006).

Thecarbonate fractionwas thelabilefractionresponsiblefor the higherCd(II),Pb(II)andZn(II)removalpercentagesintheRM,RM400,

RM_HClandRM_Ca,withaverageof85.4±0.6%forCd(II),88.0±0.9%for

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Fig. 4. PercentagesoflabileandresidualgeochemicalfractionsoftheCd(II) (a),Pb(II)(b)andZn(II)(c),usingRM,RM400,RMHClandRMCa(C0=2mmol 25mL−1_)._The_experiment_was_performed_in_triplicate;_with_the_bars_indicate

standarddeviation.

Pb(II)and87.7±0.4%forZn(II).Thus,themechanismrelatedtoCd(II), Pb(II)andZn(II)removalfromaqueoussolutioninthenaturaland ac-tivatedRMcanbetrulyassociatedwiththesetraceelementsboundto carbonate.Similarresultshavealsoshownthatcarbonateisthemain labilefractionresponsibleforCu(II)(Qietal.,2018)removalfrom aque-oussolutionbynaturalRMcollectedfromShanxiinChina,respectively. Evenwithoutasequentialextractionstudy,Lyuetal.(2020)showed thattheprecipitationprocesses,asPb-carbonates,wasresponsiblefor 78%ofPb(II)removalfromaqueoussolutionbymodiﬁedRM(colloidal silicaandsodiumhydroxide).

3.3. MechanismsofCd(II),Pb(II)andZn(II)removalbymineral precipitation

Duringthesequentialextractions,thelowerremovalpercentagesfor Cd(II),Pb(II)andZn(II)weredetectedintheionexchangeablefractions (<0.4%foralltraceelements),whereasthehigherpercentageswere boundtocarbonatefraction.Thissuggestthemineralprecipitationas themainmechanismsofCd(II),Pb(II)andZn(II)removalinsteadof ad-sorptionontosodalite.Therewith,adsorptionLangmuirandFreundlich

Fig. 5. TheRM400XDRpatternsafterCd(II)(a),Pb(II)(b)andZn(II)(c)removal

experiments.

isothermsarenotvalidtomodeltheCd(II),Pb(II)andZn(II)removal usingthesehazardousmaterials.

Mann and Deutscher (1980) studied the Pb(II) and Zn(II) mo-bility in water containing carbonate, sulphate and chloride ions.

SangameshwarandBarnes(1983)assessedthermodynamicallythe dis-tributionandstabilitiesofmineralassemblagesformedinsystemwith Cd(II),Pb(II)andZn(II)+CO2+S+H2Oat25°Cand1atm,withEh-pH

diagramsillustratingclearlythatthemineralassemblagesdependson theEhandpHconditions.AtthepHvaluesusedintheexperimental procedures(initialof7andﬁnalof5),thenaturalandactivatedforms ofRMinsolutionswithpHvalueslowerthanthepHPCZ developeda

positivechargeontheirsurface,whenpHvalueswerelowerthanthe pHPCZ.Thisresultintheelestrostaticrepulsionexistsbetweenthe

pos-itivelychargedsurfaceoftheRMandthecationicions,suchasCd(II), Pb(II)andZn(II).

ConsideringtheinitialandﬁnalpHvalues,theEh-pHdiagrams pro-posedbySangameshwarandBarnes (1983)andthepHPCZvalues in

thenaturalandactivatedformsofRM,themainmechanismsofCd(II), Pb(II) andZn(II)removal by mineral precipitation, formingotavite, cerussite, smithsoniteandanglesite.Fig.5illustratestheRM400 XRD

patternsafterCd(II),Pb(II)andZn(II)removalexperiment,conﬁrming themineralassemblagesproposed.Fig.6presentsthereactionproducts duringtheinteractionbettwenCd(II),Pb(II)andZn(II)andRMCainthe

aqueoussolutions.Themineralprecipitationprocessescanbedescribed asfollowing:

(a) WhenRM,RM400,RMHClandRMCa areaddedintheaqueous

so-lutionwithCd(II),Pb(II)andZn(II)atpH7,these traceelements reactwithHCO3− availablein thenaturalandactivatedformsof

RM,producingCd(II),Pb(II)andZn(II)precipitates,suchasotavite (Eq.(4)),cerussite(Eq.(5))andsmithsonite(Eq.(6));

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Fig. 6. ReactionproductsduringtheinteractionbettwenCd(II)(a),Pb(II)(b)

andZn(II)(c)intheaqueoussolutionsandRM.

Cd 2+_{+ HCO} 3−→ CdCO 3(otavite) + H + (4) Pb 2+_{+ HCO} 3−→ PbCO 3(cerussite) + H + (5) Zn 2+_{+ HCO} 3−→ ZnCO 3(smithsonite) + H + (6)

(b) Fornatural andactivated forms of RM,withthe total consump-tionofHCO3−andproductionofH+,thepHvaluesdecreasedand

Pb(II) precipitatesas anglesite (Eq. (7))at pHvalues below 5.4 (Sangameshwar andBarnes, 1983; Marani etal., 1995). The

an-Fig. 7. Removaleﬃciency(RE)oftheCd(II)(a),Pb(II)(b)andZn(II)(c)versus

time,usingRM,RM400,RMHClandRMCa(C0=80mmolL−1).Theexperiment wasperformedintriplicate;withthebarsindicatestandarddeviation.

glesiteprecipitationexplainsthePb(II)removalpercentagesinthe boundtoorganicmatter/sulﬁde(2.8±0.2%);

Pb 2+₊_S2−_{+ 4H}

2O → PbSO 4(anglesite) + 8H + (7)

(c) Greenockiteorhawleyite(CdS),galena(PbS)andsphalerite(ZnS) arenotprecipitatedduetooxidationconditionsduringtheCd(II), Pb(II)andZn(II)removalexperiments.

3.4. KineticsstudyofCd(II),Pb(II)andZn(II)removal

Fig.7showstheA_svalues(Eq.(2))overtimefortheCd(II)Pb(II)and Zn(II)removalontoRM,RM₄₀₀,RM_HClandRM_C.TheCd(II),Pb(II)and Zn(II)removaldependsonthereactiontimeandeithernaturaland acti-vatedRMisused.Intheﬁrst15min,ca.70%ofCd(II),Pb(II)andZn(II) wasremoved.Inaddition,Cd(II),Pb(II)andZn(II)removaltrendtotheir maximumafter120min(2h)foralltestedRMvariants.Themaximum amountoftraceelementsremovedusingRM,RM400,RMHClandRMCa

were0.95,0.99,0.37and0.35andmmolg−1_for_Cd(II),_1.27,_1.39,_0.51

and0.50mmolg−1_for_Pb(II)_and_0.90,_0.94,_0.28_and_0.30_mmol_g−1

forZn(II),respectively,after1440min(24h).DiﬀerentRMremoval ca-pacitieshasbeendescribedintheliteratureforCd(II),Pb(II)andZn(II) (Table1),asaconsequenceofnotonlythelargemineralsvariability, butalsothespeciﬁcsurfacearea,chemicalcompositionandactivation proceduresaswell(Wangetal.2008).

Thethermaltreatmentpromotedthetransformationofgoethiteand gibbsite intohematiteandalumina andconsequently,promotingthe incrementintheSSAintheRM400(61m2g−1)whencomparedtoRM

(33m2_g−1_)._Antunes_et_al.₍₂₀₁₂₎_proposed_that_the_thermal_treatment

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at400–500°CwouldbethebesttemperaturetoincreasetheSSAand, consequently,theadsorptionassociatedwithnewmineralphases gen-erated(hematiteandalumina).Inaddition,thepHvalueswere practi-callythesamefortheRM(10.5)andRM400(10.7),indicatingthatthe

thermaltreatmentdidnotremovetheHCO₃−_available_in_these

mate-rials.Thus,theincrementintheSSAand,consequently,inthereactive -OHand-OH2functionalgroupsexposedontheRM400surface(Liuand

Huang,2003),increasestheCd(II),Pb(II)andZn(II)removalontothe Fe-MnoxidescomparedtoRM.

Ontheotherhand,thechemicaltreatmentwithHClandCa(NO3)2

decreasedtheCd(II),Pb(II)andZn(II)removaleﬃcacyinca.60%in relationtoRM.ThisfactwasalsodescribedbySantonaetal.(2006),

Pichinellietal.(2017)andSilvaetal.(2019),whosuggestedthatthe chemicaltreatmentsdissolvedaportionofthezeolite-typesminerals, re-ducingthetraceelementsremovalcapacity.However,ourstudyshowed clearlytheCd(II),Pb(II)andZn(II)removalontheactivatedformsof RMisassociatedtochemicaltreatmentusedtoactivateRMHClandRMCa

samples.ThiscausethereductionoftheamountHCO₃−_available_in

so-lutionandtheconsequentlimitationtoaminimumresidualfractionsin solutionforCd(II),Pb(II)andZn(II).

Kineticsmodelsweremadetodescribethesorptionof pollutants on solid surfaces for liquid-solid phase sorption systems (Ho and McKay,1998),suchasthepseudo-ﬁrst-orderLagergren, pseudo-second-order,Elovichandintraparticlediﬀusionmodels.Thesetraditional ki-neticsmodelshavebeenusedtostudytheremovalofseveraltrace el-ementsusingnaturaloractivatedRM(Lópezetal.,1998;Guptaetal., 2001;GuptaandSharma,2002;Sahuetal.,2011;Pichinellietal.,2017;

Silvaetal.,2019;Yangetal.,2020;Lyuetal.,2020).However, con-sideringthecentralroleofmineralprecipitationmechanismontrace elementsremoval,thetraditionalkineticsmodelscommonlyassociated totraceelementsadsorptionbyRMshouldappliedcarefully,oncemay notrepresentthemineralprecipitationphenomenon.

3.5. ModellingCd(II),Pb(II)andZn(II)removalbycarbonate precipitation

Precipitationwasthemainresponsibleforremovalofca.85%for Cd(II),Pb(II)andZn)II),asseeninsequentialextractionsexperiment. ConsideringGreenbergandTomson(1992),weassumedthekinetic re-actionofnth_order_as_plausible_to_explain_the_behaviour_of_metals_decay.

Toperformthemodel,weusednofthenth_order_reaction_as_one_of_the

adjustingparameters,alongwiththekineticconstant.Accordingtothe pHvaluesusingduringthekineticsstudies,theprecipitationkineticwas consideredtodependuponthetraceelementsandRMconcentrations, andtheavailabilityofHCO₃−_in_solution,_for_a_given_pH_and_PCO

2 .Eq.

(8)showsageneralderivativeformforasecondorderreaction.

𝑑[𝐴]

𝑑𝑡 =−𝑘[𝐴].[𝐵].𝑃𝐶𝑂2 (8)

where:

k=precipitationrateconstantforpartialpressureofCO₂at25°C (mmolL−1 _min−1₎_(atm)−1_;_[_A_]₌_{concentration}_of_the_trace_element

insolutionattime(t)(mmolL−1_);_[_B_]₌_{concentration}_of_HCO 3−

de-rivedofthenaturaloractivatedRMinsolutionattime(t)(mmolL−1_);

PCO₂=partialpressureofCO2duetopHat25°C(atm).

Assuming[B]≈[A]andintegratingfrominitialconcentrationtothe concentrationattimet,thenEq.(8)yieldsEq.(9).

[𝐴]=([𝐴0 ]−𝑛 +1 +(𝑛−1).𝑘′_.𝑡)− 1 𝑛−1 +𝑘_𝐶 (9) where:

[A₀]=concentrationof[A]insolutionattime(t)zero(mmolL−1_);

n=nth_general_reaction_order;_k’ ₌_{precipitation}_rate_constant_(mmol_L−1

min−1_),_k

c =constantappliedtotakeintoaccountthelimitedcapacity ofHCO3−transferfromnaturaloractivatedRMtosolution(mmolL−1),

describingtheasymptoticlimitforthetraceelementsprecipitationdue toHCO₃−_availability_in_solution.

Fig. 8. KineticsofCd(II)(a),Pb(II)(b)andZn(II)(c)precipitation,usingRM,

RM400,RMHClandRMCa,consideringC0=80mmmolL−1andCd(II),Pb(II)and Zn(II)removalbycarbonateprecipitationinca.85%.Symbolsareexperimental data.

Table 3

ParametersobtainedforthemodellingPb(II)andZn(II) removalbyprecipitation.

Element k’ (mmol L −1 min −1 ) k

c (mmol L −1 ) R 2 RM Cd(II) 0.32 35.6 0.99 Pb(II) 0.44 24.8 0.98 Zn(II) 0.32 37.6 0.98 RM400 Cd(II) 0.32 34.4 0.99 Pb(II) 0.48 20.4 0.96 Zn(II) 0.32 32.0 0.99 RMHCl Cd(II) 0.12 55.6 0.99 Pb(II) 0.16 50.8 0.99 Zn(II) 0.08 58.4 0.99 RMCa Cd(II) 0.12 56.0 0.99 Pb(II) 0.16 51.2 0.98 Zn(II) 0.12 57.6 0.99

TheCd(II),Pb(II)andZn(II)precipitationmechanismsusingnatural oractivatedformsofRMwasmodelledusingEq.(9)(Fig.8).The pa-rameters(k’ ,kcandn)ofEq.9werecalculatedbyminimisingthesum ofthesquareddiﬀerencebetweenexperimentalandmodelleddata, us-ingnon-lineargeneralizedreducedgradientalgorithmofSOLVER-MS Excell®.Table3showsresultsobtainedforEq.(9)parametersandR2

aswell.Theresultsshowedthesecondorderreactionasthebesttrend lineforCd(II),Pb(II)andZn(II)precipitation,withR2 higherthan0.96 foralltraceelements.Thesecondorderreactionareinaccordancewith

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datareportedinliterature(e.g.,Gilmouretal.,1977;Kazmierczaketal., 1981,GreenbergandTomson,1992).Theproposedconstantk_climit precipitationaccordingly, asHCO3− concentrationin solutiondecay

overtime.

TheaverageCd(II),Pb(II) andZn(II)precipitationratesconstants were0.36,0.40,0.12and0.13(mmolL−1_min−1₎_for_RM,_RM

400,RMHCl

andRMCa,respectively.Thekineticsofthesetraceelements

precipita-tionshowedfastremovalwithintheﬁrst15min,withlittleincrement oftheCd(II),Pb(II)andZn(II)removalbyprecipitationascarbonates byHCO3− andH2CO3equilibrium.Inaddition,theCd(II),Pb(II)and

Zn(II)removalbymineralprecipitationisatypicalbiphasic precipita-tionkineticsforallnaturalandactivatedformsRM,withafasttrace elementsprecipitationwithintheﬁrst120min,followedbysteadyof thesetraceelementsremoval.Inthefastprecipitationkinetics,mainly duetoCd(II),Pb(II)andZn(II)precipitationwithHCO₃−_available_in

theaqueoussolution,providesarapiddecreaseintheHCO3−

concen-tration.ThisfactlimitstheCd(II),Pb(II)andZn(II)removalbymineral precipitationfrom120to1440minbecausethelessHCO₃−_available

fortheformationofotavite,cerussite,smithsoniteandanglesiteinthe naturalandactivatedformsofRM.

Summarizing,undertheexperimentalconditionshereinpresented, bothselectiveextractionandkineticsstudyhaveshownthatcarbonate precipitationisthemostrelevantmechanismforCd(II),Pb(II)andZn(II) removalusingRM,RM400,RMHCl andRMCa fromaqueoussolutions,

regardlessthenaturalandactivatedformsofRM,andthattheofHCO₃−

playsacrucialroleinkineticsbylimitingtheprecipitateformation.

4. Conclusions

The Cd(II), Pb(II) and Zn(II) removal mechanismsfrom aqueous solutionusing naturalanddiﬀerent chemical(HCl0.05molL−1_and

Ca(NO3)20.1molL−1)andthermal(400°C)RMwerestudiedusing

se-quentialextraction.Inaddition,thekineticsofCd(II),Pb(II)andZn(II) removal bycarbonate precipitations weremodelled. Results showed thecarbonatefractionwasthemainlabilefractionresponsibleforthe higher Cd(II), Pb(II) andZn(II) removal percentages (ca. 85%), fol-lowedbyboundFe-Mnoxides,boundtoorganicmatter/sulphideand ionexchangeablefractions. Irrespectiveofwhethernatural and acti-vatedformsofRM,themineralprecipitationwasthemainmechanism forCd(II),Pb(II)andZn(II)removal,formingotavite– CdCO3,

cerus-site-PbCO3,smithsonite-ZnCO3andanglesite-PbSO4.Kinetics

stud-iesshowedamaximumamountoftraceelementsremovedusingRM, RM400,RMHClandRMCaof0.95,0.99,0.37and0.35andmmolg−1for

Cd(II),1.27,1.39,0.51and0.50mmolg−1_for_Pb(II)_and_0.90,_0.94,

0.28and0.30mmolg−1_for_Zn(II)._These_results_clearly_indicated_that

thechemicaltreatmentsdecreasedtheCd(II),Pb(II)andZn(II)removal whilstthethermaltreatmentsincreasedupto10%thetraceelements re-movalinrelationtoRM.Therefore,chemicalandthermaltreatmentsdo notbringarelevantimprovementfortraceelementsremovalandthey canbeavoided,loweringthecostsassociatedtotheseprocedures.Afast traceelementsprecipitationoccurredwithintheﬁrst120min,with lim-itationinthesetraceelementsremovalduetoHCO₃−_consumption

dur-ingthemineralprecipitation.Consequently,thetraditionaladsorption isothermsandkineticsmodelscommonlyassociatedtotraceelements adsorptionbyRMshouldbeconsideredcarefullyinfuturestudies.Asa whole,theresultsprovidenewinsightsintotherelativeimportanceof theCd(II),Pb(II)andZn(II)removalmechanismsfromaqueous solu-tionusingnaturalandactivatedformsofRM.Furthermore,theresults pointedouttotheuseofthesehazardousmaterialsonenvironmental systemsintheframeworkofenvironmentalremediationandwater in-dustryinareasheavilyimpactedbyanthropogenicactivities.Itshould beunderscoredthattheresidualRMshouldbeproperlystored,in or-dertopreventthesolubilizationofthelabilefractionoftraceelements totheenvironment,especiallyforthelabiletraceelementsfractions. FuturestudiesapplicabletouseofRMonalargerscaleandforreuse

ofCd(II),Pb(II)andZn(II)precipitatedintheindustrialactivitiesare advised.

DeclarationofCompetingInterest

Theauthorsdeclarethattheyhavenoknowncompetingﬁnancial interestsorpersonalrelationshipsthatcouldhaveappearedtoinﬂuence theworkreportedinthispaper.

Acknowledgment

TheauthorsthanktheFundaçãodeAmparoà PesquisadoEstado deSãoPaulo(FAPESP-ProcessesNo.2009/02374-0and 2013/00994-6)andConselhoNacionaldeDesenvolvimentoCientíﬁcoeTecnológico (CNPq-ProcessNo.480555/2009-5)forﬁnancialsupport.Dr.Moruzzi isalsogratefultoCNPqforgrantawarded301210/2018-7.Wethankall therefereesfortheirdetailedandinsightfulreview’scomments,whom helpedtoimprovethemanuscript.

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