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Interactions between cadmium and lead with acidic soils: Experimental evidence of similar adsorption patterns for a wide range of metal concentrations and the implications of metal migration

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To link to this article:

DOI:10.1016/j.jhazmat.2011.11.027

http://dx.doi.org/10.1016/j.jhazmat.2011.11.027

This is an author-deposited version published in:

http://oatao.univ-toulouse.fr/

Eprints ID: 5591

To cite this version: Pokrovsky, O.S and Probst, Anne and Leviel, E. and Liao,

Bo-han Interactions between cadmium and lead with acidic soils: Experimental

evidence of similar adsorption patterns for a wide range of metal concentrations

and the implications of metal migration. (2012) Journal of Hazardous Materials,

vol. 199-200 . pp. 358-366. ISSN 0304-3894

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rchive

T

oulouse

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rchive

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Interactions

between

cadmium

and

lead

with

acidic

soils:

Experimental

evidence

of

similar

adsorption

patterns

for

a

wide

range

of

metal

concentrations

and

the

implications

of

metal

migration

O.S.

Pokrovsky

a

,

A.

Probst

b,c,∗

,

E.

Leviel

a

,

B.

Liao

d aGET-CNRS-UPS-IRD-UMR5563,14,AvenueEdouardBelin,31400Toulouse,France

bUniversitédeToulouse;INPT,UPS;EcoLab(LaboratoireEcologiefonctionnelleetEnvironnement);ENSAT,Avenuedel‘Agrobiopôle,31326Castanet-Tolosan,France cCNRS;EcoLab;31326Castanet-Tolosan,France

dInternationalCollege,CentralSouthUniversityofForestryandTechnology,Changsha410004,China

a

r

t

i

c

l

e

i

n

f

o

Keywords: Cadmium Lead Acidsoils Adsorption Isotherm Competition

a

b

s

t

r

a

c

t

Theimportanceofhigh-andlow-affinitysurfacesitesforcadmiumandleadadsorptionintypical Euro-peanandAsiansoilswasinvestigated.Adsorptionexperimentsonsurfaceanddeephorizonsofacidic brown(Vosges,France)andredloesssoils(Hunan,China)wereperformedat25◦Casafunctionofthe

pH(3.5–8)andalargerangeofmetalconcentrationsinsolution(10−9–10−4moll−1).Westudiedthe

adsorptionkineticsusingaCd2+-selectiveelectrodeanddesorptionexperimentsasafunctionofthe

solid/solutionratioandpH.AtaconstantsolutionpH,allsamplesexhibitedsimilarmaximaladsorption capacities(4.0±0.5␮mol/gCdand20±2␮mol/gPb).Aconstantslopeofadsorbed–dissolved concentra-tiondependencewasvalidover5ordersofmagnitudeofmetalconcentrations.UniversalLangmuirand FreundlichequationsandtheSCMformalismdescribedtheadsorptionisothermsandthepH-dependent adsorptionedgeoververybroadrangesofmetalconcentrations,indicatingnohigh-orlow-affinitysites formetalbindingatthesoilsurfaceundertheseexperimentalconditions.AtpH5,CdandPbdidnot compete,inaccordancewiththeSCM.Themetaladsorptionabilityexceededthevalueforsoilprotection bytwoordersofmagnitude,butonlycriticalloadguaranteessoilprotectionsincemetaltoxicitydepends onmetalavailability.

1. Introduction

Sorption onto soil particles is knownto control the migra-tion of metals through soils, plants and river systems and to determinemetalavailability and toxicity tothe componentsof trophicchains,includinganimalsandhumans.Consequently,the adsorptionofimportantpollutantssuchascadmiumandleadonto minerals[1–8]andorganicmatter[9–11]surfacesandthe inter-actionsbetweenheavymetalsandsoilcolloids[12–17]havebeen extensivelystudiedwithaparticularlyfocusontherigorous char-acterizationofmetaladsorptionontonaturalsorbentssuchassoils [18–34]andsediments[35].Atthebeginningofstudiesofmetal adsorptionontomineralsubstratesthetypicalapproach wasto investigatethepH-dependentadsorptionedgeataconstant con-centration of theadsorbent and a rather high concentrationof dissolved metals, in the order of mmoll−1. Many investigators

∗ Correspondingauthorat:UniversitédeToulouse;INPT,UPS;EcoLab (Labora-toireEcologiefonctionnelleetEnvironnement);ENSAT,Avenuedel‘Agrobiopôle, 31326Castanet-Tolosan,France.Tel.:+33534323949;fax:+33534323955.

E-mailaddress:anne.probst@ensat.fr(A.Probst).

proposedthatthereismorethanonetypeofsiteonthesurface of thesubstrate: dependingonthemetaltosurface concentra-tionratio,alowabundanceofsiteswithahighaffinityformetals canbemore importantthan anabundanceoflow-affinitysites [3,36].Asaresult,theadsorptionconstantitselfcandependon the metalconcentration in solution.For example, theconstant measuredata high(mmolar)metalconcentrationmightnotbe applicabletonaturalsettingsofthenanomolartomicromolar con-centrationrange. Incontrasttothelargeeffortsthathave been undertakentotestthishypothesisfornaturalorganicmatter(OM) or mineralsurfaces[37–40]for natural multicomponent adsor-bentssuchassoils,thisissuehasrarelybeenaddressedbyrigorous experimentalapproachesundercontrolledlaboratoryconditions [41].

Moststudiesofmetaladsorptiononvariousmineralsurfaces wererestrictedtomeasuringtheadsorptionedgeinaverynarrow rangeofmetalconcentrationsandwereaimedatpredicting sur-faceadsorptionconstantsandthepercentageofadsorbedmetalas afunctionofpHundergivenconditions(i.e.,[21,42–45]).However, thesestudiesdidnotalwaysallowquantificationofthemaximal numberofavailablesurfacesites,acrucialparameterforthe envi-ronmentalmodellingofmetalmigration.Forthis,experimentson

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boththepH-dependent adsorptionedgeand the“Langmuirian” adsorptionataconstantpHshouldbeconducted.

Thisstudyaimedtoprovidenewinsightsintotoxicmetal inter-actionswithsoilsbyperformingathoroughinvestigationofCd andPbadsorptionontovarioustypesofacidicsoilsinaverywide rangeofmetalconcentrations.Indeed,CdandPbareamongthe mostcommontoxicmetalsinsoilsintheworldandtheirsoil trans-portandbioavailabilitystronglydependonsoilparameters(pHand organicmattercontent,amongothers)(cf.[46,47]).

Acidsoilsaremostvulnerabletoheavymetalloadsduetotheir rather low adsorptioncapacities at acidicor circumneutral soil solutionpHlevels.Aspartofageneralquantitativeassessmentof acidsoilinteractionswithmetaltoxicantsallovertheworld,this studycomparedmetaladsorptionparametersintwo representa-tiveEuropeanandAsianacidsoils.Thespecificobjectivesofthis workwere:(1)tomeasureandcomparetheadsorptionparameters ofCdandPbonsurfaceanddeepsoilhorizonsasafunctionofpH; and(2)toassesstheadsorptionyieldinthewidestpossible exper-imentalrangeofmetalconcentrationsinsolution(severalorders ofmagnitude).Theresultsshouldallowabetterquantificationof theCdandPb-adsorptioncapacityofthesesoilsandshouldhelpto constrainthemodellingofcriticalloads[46,47]inEuropeandAsia (whereincreasinglevelsofmetaldepositionarebeingfound)and, moregenerally,toimproveourunderstandingofsoilprotection regardingacidandmetalpollutionaroundtheworld.

2. Materialsandmethods

2.1. Soilsamples

Theexperimentswereperformedonsoilsthatoriginatedfrom FranceandChinainordertorepresentavarietyofacidsoilsfrom regions that receivea wide range ofacidic atmospheric inputs [27,47–49].Aredacidicsoil(labelledCS,AlliticUdicFerrisols,FAO system,ChangshaRegion,NorthofHunanProvince,China) devel-opedonQuaternaryredclayand anacidicbrownsoil(labelled PP,DystricCambisol,FAOsystem)fromtheStrengbachcatchment (Vosges,France),developedongranite,wereconsidered.Samples fromdifferentdepthswerecollectedusingaplasticshovel,sieved througha2mmmesh,driedat105◦Candkeptatambient temper-atureinthedark.Thechemicalandphysicalcompositions,specific surfaceareasandselectedparametersofthesoilsstudiedare pro-videdinTable1.Twomineralhorizonsfromeachsoil:thesurface (CS1:0–30cmandPPAB:5–30cm)andbottom(CS4:90–120cm andPPC:±180cm),wereselectedforthisstudy.

ThePPsoilhadasandyloamtexture;thepHofthewaterextract variedfrompH4.1onthesurfacetopH4.6atdepth.Themain mineralsinthecompositionofthissoilwerequartzandmicawith plagioclaseandtracesofapatite[50].TheOMcontentdecreased from2.6to0.1%fromthesurfacetothedeephorizon,respectively. Illiteandsmectitewerethedominantclaymineralswithinthissoil type.Thespecificsurfaceareas(SSA)ofthesurfaceanddeep hori-zonsofthissoilwere6.2and4.8m2g−1,respectively,asmeasured

bytheB.E.T.(Brunauer–Emmett–Teller)N2multi-pointadsorption

technique.Thealuminiumcontent(mainly asAloxides)ranged between15and20%.TheCSsoilhadaclaytexture(∼60%offine particles<0.01mm),withlowaluminiumcontent.ThepHwaterwas

aroundpH5andtheOMcontentwasclosetothatofthePPsoil. Theclaymineralogyofthesoilswasstudiedusingsmall-angle X-raydiffractionoforientedsamplespreparedbydepositingaclay suspensiononaglassslideandsubjectingittoconventional treat-ment(heatingto500◦C, ethyleneglycolexpansion).Thespectra wererecordedusingadiffractometerwithaCuKaanti-cathode. Theclaymineralsillite andchlorite, andinterlayeredI–C, were thedominantconstituentsintheCSsoil.TheSSAsrangedfrom

14.8m2g−1to19.7m2g−1fromthesurfacetodepth,respectively.

TherangeofSSAsforsoils<2mminthisstudy(5–20m2g−1)isin

goodagreementwithdataintheliteraturefordifferentsoilsaround theworld[51].

2.2. Adsorptionexperiments

Batchadsorptionexperimentswereperformedinpre-cleaned 30mlpolypropylenevialsat25±0.5◦C in0.01MNaNO3 witha

very wide range of metalconcentrations, from2–4nmoll−1 to 0.1–0.2mmoll−1. Theconcentrationsof Cdand Pbrangedfrom 10−9 to10−4M.Alladsorptionexperimentswereperformedata constantsoil/solutionratioequalto8gl−1.Desorptionexperiments wereperformedwitheachtypeofsoilinMilliQwatersuspensions atconcentrationsof4,8,20and40gl−1andpH5.0±0.5.The expo-suretimevariedfrom1to2weekstoensuretheequilibriumof metaldistributionbetweenthesoilandthesolutions,asevidenced frompreliminaryadsorptionexperimentswhichshowedthat1–2 dayswassufficienttoachievestable(±10%)metalconcentrations insolutionin contactwiththesoils.Aftertheadsorption equi-libriumwasachievedthepHofthesuspensionsweremeasured andthesampleswerecentrifugedfor15minat2000×gand fil-teredthroughapre-washed0.22␮mNylonfilter.Thefiltratewas acidifiedwithbidistilledHNO3andstoredat4◦Cbeforeanalysis.

Foralloftheexperimentsdescribedbelow,CO2-freeMilliQwater

saturatedwithhigh-purityN2gaswasused.

2.2.1. AdsorptionofCdandPbataconstantpH

All of the experiments were performed at a constant soil/solutionratioequalto8gl−1in0.01MNaNO3asabackground

electrolytesolution.TheinitialCdand Pbconcentrationsvaried from4nMto100␮M(Cd)andfrom2.4nMto240␮M(Pb).The amountofmetalsaddedalwaysexceededtheamountthatcould bepotentiallydesorbedfromthesoilsurface(seeSection2.4)by aminimumfactorof5.Thenecessarycorrectionsforthedesorbed metalswerealwaysmade,especiallyatlowconcentrations.The sampleswerecontinuouslyshakenat25±0.5◦Cin thedarkfor 1–2weekstoensureadsorptionequilibriumwasachieved.Sucha longexposuretimeisalsoconsistentwiththeratherlong(daysto weeks)residencetimeofinterstitialsoilsolutions.Blanksamples werepreparedinMilliQwateror0.01MNaO3solutionwiththe

samemetalconcentrationsbutwithouttheadditionofsoilinorder toaccountfortheadsorptionofmetalsontocontainerwallsunder typicalexperimentalconditions.Correctionsfortheblank adsorp-tionrangedfrom10%oftheadsorbedamountofmetalatanMol concentrationrangeto,typically,<1%atthe␮Molconcentration range.

2.2.2. EffectofpHonadsorption

InordertostudytheeffectofpHonadsorptionequilibria,the soilswereplacedincontactwith0.01MNaNO3,with8gsoill−1and

aninitialCdorPbconcentrationof0.9or0.48␮M,respectively. ThepHwasvariedbyaddingHNO3 orNaOHtocovertherange

betweenpH2and8.Alladsorptionexperimentswereperformed insolutionsundersaturatedwithrespecttometalhydroxidesand carbonates,asverifiedbyMINTEQA2calculations[53]oftheinitial solutioncompositions.

2.2.3. AdsorptioncompetitionbetweenCdandPb

Competitiveadsorptionexperimentswereperformedwitheach type of soil at pH∼ 5 in 0.01M NaNO3, with 8gsoill−1 and

threetypesofinitialsolutions:0.44␮MCd+0.12␮MPb,0.22␮M Cd+0.36␮M Pb, and 0.66␮M Cd+0.06␮M Pb. After an expo-sureperiod of 48hat 25◦C in the dark,thepHwas measured andthesolutionwascentrifuged(15min,speed2500×g),filtered

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Table1

Propertiesofthetestedsoils.

Property PPAB PPC CSS1 CSS4

Samplingsite Strengbachcatchment– Vosges,France

Strengbachcatchment– Vosges,France

Changsha,Hunan,China ChangshaHunan,China

Soiltype Acidbrownsoil(Dystic Cambisol)

Acidbrownsoil(Dystric Cambisol)

AlliticUdicFerrisols AlliticUdicFerrisols

Bedrock Granite Granite Quaternaryredclay Quaternaryredclay

Depth(cm) 5–30 ±180 0–30 90–120

Density(gcm−3) 1.60 2.0 1.15 1.33

Specificsurfacearea(m2g−1) 6.2 4.8 14.8 19.7

pH(H2O) 4.10 4.60 5.0 5.2 pH(KCl) 3.20 3.90 4.0 4.1 CEC(cmolkg−1) 9.10 5.10 3.6 3.3 BS(%) 4.90 7.10 14.40 12.47 Corg(gkg−1) 13.10 7.70 13.23 4.74 Organicmatter(gkg−1) 22.5 13.2 22.8 8.2

Contentofmajormetal iron(gkg−1)

Al2O3 15.60 18.20 2.25 2.14

Fe2O3 2.10 2.20 3.44 3.63

Contentofsoilparticles (mm,%) 2–0.20 60.70 62.10 7.07 10.18 0.20–0.05 7.40 8.10 10.44 8.2 0.05–0.02 1.60 7.40 6.27 8.16 0.02–0.002 12.50 8.10 43.86 40.81 <0.002 17.80 15.90 32.37 32.65 Mainconstituent mineralsa(%) Quartz 35–8 26–7 x x Orthose 15–1 10–1 x x Muscovite 14–0.4 33–2 x x Plagioclase 5–3 11–4 x x Apatite 0.06–0.05 0.21–0.09 x x Kaolinite 1–3 1–3 x x Illite 22–64 9–57 ++ +++ Vermiculite – – – – Chlorite – – +++ +++ Smectite 4–14 4–15 – – InterlayeredI–V ++ + – – InterlayeredC–S – – ++ +

Totalcontentoftrace metals(mgkg−1) Cd 0.03 0.03 0.18 0.10 Cu 2.0 0.50 23.47 27.34 Pb 56.6 9.9 24.1 22.17 Zn 25.0 24.0 99.19 75.32 Cr 9.0 19.0 124.71 128.12

aFortheVosgessamples(PPABandPPC):afterFichteretal.[50],thefirstvalueisbasedonweightedmineralogy,thesecondoneonmineralsurfacearea.Interlayered I–V:InterlayeredclayIllite–Vermiculite;InterlayeredC–S:InterlayeredclayChlorite–Smectite.+++:abundant;++,mediumabundant;+detected;–:notdetected;x:not measured.

(0.22␮macetatecellulosefilter),acidified(ultrapureHNO3)and

storedat4◦Cbeforeanalysis.

2.3. Adsorptionkinetics

Theadsorption kineticswerestudied in 0.01MNaNO3 with

8gl−1 of solid concentrationand aninitialCd concentrationof 88␮M.InordertomonitorCdactivity,aCd2+-selectiveelectrode

wasused.Solid-contactCd-sulphideelectrodes(NIKOAnalit®,

Rus-sia) coupled with an Ag/AgCl reference electrode in 3.5M KCl connected withthesolution viaa 1M NaNO3 salt bridgewere

calibratedat3<pH<9and4<pH<7instandardCd(NO3)2

solu-tions.Potentiometricmeasurementswereperformedat25±0.5◦C inaworkingconcentrationrange of10−6–10−2Mwitha detec-tionlimitof10−7Mandanuncertainty oflevel5%;aNernstian slope of29.1±0.5mV/pCd was obtained. Priortoand afterthe measurementstheelectrodewascalibratedinacellsupernatant solutionwiththesameconcentrationofbackgroundelectrolytes astheexperimentalsolution.Measurementswerealsoperformed in10mgl−1Cdsolutionin0.01MNaNO3withoutaddedsoil.The

electrode potentialwasrecordedevery10sand converted toa freeCd2+concentrationusingastandardcalibrationprocedure.The

typicalresponsetimeofthiselectrodetovariationsintheCd con-centrationwas2s(see[52]fordetails).Theseexperimentswere typicallyrunfor30min,untilasteady-stateCd2+(aq)concentration

wasachieved.

AlthoughtheadsorptionkineticsofPb2+werenotstudied,a

comparisonoftheadsorptionconstantvaluesderivedfortheCd andPbadsorptionexperimentsonorganicandmineralsurfaces, performedatthesamepH,showedthattheadsorptionkinetic con-stantofPbwasabout20–25timeshigherthanthecorresponding valueforCd,aratiothatwasalmostthesameasthatbetweenthe rateconstantsofwaterlossfromaqueousPbandCdions(i.e.,[52]). Assuch,theadsorptionequilibrium ofPb2+ shouldbeachieved

significantlyfastercomparedtothatofCd2+.

2.4. Desorption

Metaldesorptionexperimentsweredesigned tomeasurethe amountof metal releasedfrom theoriginal soilsasa resultof mineraldissolutionorsurfacedesorptionphenomena.Desorption experimentswereperformedwitheachtypeofsoil,insuspensions ofMilliQwateratasoilconcentrationof4,8,20and40gl−1at pH=5.0±0.5.ThereasonwhyMilliQwaterwasusedinsteadofan electrolytesolutionwastosimulatetheinteractionbetweensoil andpenetratinglow-levelmineralizedrainwater,andtoavoidthe releaseofexchangedmetalcomplexes.Anotherseriesof experi-mentswasperformedinasoilsuspensionof20gl−1,0.01MNaCl andpH4–8,asadjustedbytheadditionofHClandNaOH.Inallof theadsorptionexperimentsdescribedbelowtheamountof des-orbedCdandPbateachgivenpHwassubtractedfromthemetal concentrationmeasured.

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2.5. Analyses

ThepHwasmeasured,withanaccuracyof±0.01pHunits,using a combined Schott–Geräte electrode calibrated against N.I.S.T. (NationalInstituteofStandardsandTechnology)buffersolutions (pH=4.00and6.86at25◦C).Dissolvedorganiccarbon(DOC)was analysedusinga TotalOrganic CarbonAnalyzer(Shimadzu TOC 5000,Paris,France, uncertainty betterthan 3% [47]).Cadmium andleadweremeasuredbyInductivelyCoupledPlasmaMass Spec-trometry(ICP-MS7500ce, AgilentTechnologiesSantaClara,CA, USA,uncertaintyof 2%,detectionlimitsof0.001␮gl−1).Indium andrheniumwereusedasinternalstandards.Theinternational geostandardSLRS-4 wasused to check thevalidity and repro-ducibilityoftheanalyses.Themainsoilpropertieswereanalysed attheInstitutNationaldelaRechercheAgronomique(INRA, Lab-oratoryofSoilScience,Arras,France),followingstandardmethods (http://www.arras.inra.fr/).

2.6. Adsorptionmodels

Thepartitioncoefficient(Kd,lg−1)isdefinedastheratioofthe

concentrationofmetalinthesoilandintheequilibriumsolution, respectively:

Kd=

[Me]soil

[Me]solution (1)

Theadsorptionreactiononasolidsurface,includingthe inter-actionofoneionononesurfacesite,canbewrittenas:

S−+Me2+= S–Me+ (2)

where S−isthesurfacesite,Me2+isCd2+orPb2+ionsinsolution

and S–Me+istheadsorbedcomplexes.Theconventionalstability

constantofthisreaction(KL)isdefinedas:

KL= {

S–Me+}

{ S−}·[Me2+] (3)

where{}isthesurfaceconcentrationand[]standsfortheaqueous metalconcentration.Theconcentrationofadsorbedmetal(ads)is

representedbytheLangmuirequation: ads={ S–Me+}= KL·max·

[Me2+] 1+KL·[Me2+]

(4) wheremaxisthemaximaladsorbedconcentrationcorresponding

totheoccupationofallsurfacesitesavailableforagivenmetalata givenpH.Anotherwaytodescribetheadsorptionphenomenonis viatheFreundlichequation:

S−+(n)Me2+= S–Me+ (5)

Thestabilityconstantisdefinedas: KF={ S–Me

+}

[Me2+]n (6)

andtheadsorbedmetalconcentrationcanbecalculated bythe Freundlichequation(5):

ads=KF·[Me2+] n

(7) whereKFistheFreundlichadsorptionconstantandncorresponds

tothenumberofmetalionsadsorbedontoonesurfacesite.The values of n inEq. (7) usually vary between0.9 and 1.4. When n=1,KF=ads/[Me2+]=Kd,the so-called distributioncoefficient,

whichistheslopeoftheisothermattheorigin[43].Eqs.(4)and (7) areonly validfor a given pHand ionicstrength. Neverthe-less,theLangmuirandFreundlichmodelsarethemostextensively usedformodellingsolid(soil,sediments)sorptionaffinities.The useofboth Langmuir andFreundlich equations waswarranted

0,0 0,5 1,0 1,5 2,0 2,5 3,0 0 500 1000 1500 2000 Elapsed time, s [C d ]ad s , µ m o l/g

Fig.1. AdsorbedCdconcentrationasafunctionoftimefortheacidloesssoil(CS-S1). Experimentalconditions:0.01MNaNO3,8gsoill−1,[Cd]0=10mgl−1,pH=7.0±0.2. because:(1)mostexperimentsinthisstudywereperformedata constantpHandionicstrength;(2)bothequationswerewidely usedto describesoil adsorptionphenomena, and(3) they cov-eredthefullrangeofthemetalconcentrationsinvestigatedand alloweda straightforward comparison oftheadsorption capac-itybetweenthedifferentsoils.Incontrast,a differentsetofKL

andKFwasnecessaryforeachsolutioncompositionincaseofpH

variationsinsolution.Thesedifficultiesareovercomeinsurface complexationmodels(SCM),whichaccountforchemicalreactions thatoccuratthesolid/solutioninterfaceintermsofrealsurface species,andpredictthepH-dependenceofadsorptioninawide rangeofsolutioncompositionsusingauniquesetofsurface stabil-ityconstants[36].Typically,arigorousSCMapproachrequiresan explicitidentificationofproton/hydroxylandmetal-activesurface sites(concentrationandstoichiometryofsurfacebindinggroups suchas Si–OH, Al–OH, FeOH, R–COOH,different exchange-ablesites).Thethoroughcharacterizationofthesesurfacemoieties impliesamultidisciplinaryapproachcomprisingsurfacetitration and electrophoresis supportedbystate-of-the-art spectroscopic techniquessuchasFT-IR(FourierTransformed-InfraRed)andXPS (X-RayPhotoelectronSpectroscopy),forexample(see[53]asan exampleofmultisitesurfacecharacterization).Becauseofthe com-plexityofmineralandorganicsurfacesinsoils,weusedasimplified SCMapproach implyingmetal adsorptiononto onesurface site accordingtothereaction:

S–H◦+Me2+= S–Me++H+ (8)

Theintrinsicsurfacestabilityconstant(Kads◦)isdefinedas:

Kads◦=



aH+·{ S–Me+} aMe2+·{ S–H◦}



·exp



− 0 RT



(9) whereaiistheactivityofithspeciesinsolution,{ S-i}isthe

sur-faceconcentrationofithspecies,0istheelectricpotentialatthe

surface,RisthegasconstantandTisthetemperature.Thesurface andsolutionequilibriawerecalculatedusingtheMINTEQA2 pro-gram[54]withintheconceptofconstantcapacitanceoftheelectric doublelayer.

3. Results

3.1. Kinetics

Theadsorptionkineticswereassessedin ordertodetermine theminimalcontacttime necessarytoachieveadsorption equi-librium, corresponding to the constant concentration of metal adsorbedduringthefollowingsorption/desorptionexperiments. TheadsorbedCdconcentrationisshowninFig.1asafunctionof

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-11 -10,5 -10 -9,5 -9 -8,5 -8 -7,5 -7 -6,5 -6 40 30 20 10 0 [Soil], g/L log [Pb], M PPAB PPC CS S1 CS S4

A

-11,5 -11 -10,5 -10 -9,5 -9 40 30 20 10 0 [Soil], g/L log [Cd], M PPAB PPC CS S1 CS S4

B

Fig.2.Concentrationsoflead(A)andcadmium(B)inasolutionsaturatedwith non-contaminatedsoilsasafunctionofsoilconcentrationinsolution.Experimental conditions:pH=4.43±0.22(PPAB),5.48±0.3(PPC),5.58±0.23(CSS1),5.47±0.18 (CSS4);0.01MNaCl,1weekexposureat25◦C.Thesymbolsrepresentthe measure-mentsandtheconnectinglinesareforguidingpurposes.

time.Similartomostadsorptionprocesses(i.e.,[7]andreferences therein),theconstantconcentrationofmetalinsolutionincontact withthesuspensionwasachievedwithin10–20minanddidnot changeforatleast1week(datanotshown).Asaresult,aperiod of24hwasacceptedasbeingsufficienttoachievetheadsorption equilibrium.

3.2. Metaldesorptionfromnon-contaminatedsoils

Theseexperiments,whichaimedtodeterminetheamountof metalthatcanbereleasedfromsoildueto(i)thedissolutionof somemineral/organicphasesbearingthismetaland(ii) “desorp-tion”fromthesurfaceofsoilparticles,wereperformedwithvarious soiltosolutionmassratios(Fig.2).ForthesurfacehorizonofthePP soilwiththehighestPbandCdcontents,theamountofdesorbed metalsincreasedwiththemassofthesolid.Fortheother sam-ples,thereleaseofPbdidnotchangewithanincreaseinsoilmass (Fig.2A)whereasanincreaseinCdreleasewithsoilmass(Fig.2B) wasfound.TheseresultsenabledthelowestlimitsoftheCdandPb concentrationsthatcouldbeusedfortheadsorptionexperiments tobedetermined.

TheeffectofpHonmetaldesorptionfromsoilisshowninFig.3A andB.TheconcentrationofCdandPbinsolutiondecreasedwith thepH,similartothesituationfoundfortheadsorptionequilibria (seebelow).Theweakincreaseinthedesorbedlead concentra-tionatpH>6(Fig.3A)canbepartlyexplainedbyitsmobilization byorganicmatter,whichismoresolubleatthesepHlevels[47]. Indeed,theconcentration ofDOC inthe experimentswith sur-facehorizonsinbothsoilsincreasedfrom2–3mgl−1at4≤pH≤6 to8–10mgl−1 atpH∼7(datanotshown).Itwasalsopreviously

-11 -10 -9 -8 -7 7 6 5 4 pH log [Pb], M PPC CS S1 CS S4

A

-11 -10 -9 -8 7 6 5 4 pH log [Cd], M PPC CS S1 CS S4

B

Fig. 3.Concentrations of desorbed lead (A) and cadmium (B) from non-contaminatedsoils asafunctionofpH.Experimentalconditions:0.01MNaCl, 20gsoill−1,1weekexposureat25C.Thesymbolsrepresentthemeasurements andthelinesareforguidingpurposes.

Table2

ParametersoftheLangmuirequation(4)forthefourstudiedsoilsamples.

Soil KL(l/mol) max(␮mol/g)

Cadmium PPAB (0.8±0.2)×105 4.8±0.3 PPC (0.6±0.1)×105 4.0±0.2 CSS1 (2.2±0.1)×105 5.8±0.2 CSS4 (1.0±0.1)×105 4.2±0.2 Lead PPAB 4000±500 22±2 PPC 6000±100 14±2 CSS1 4500±500 25±2 CSS4 8000±500 14±1

shownthattheproportionofdissolvedPbpresentintheformof organiccomplexesinsoilsolutionsstronglyincreasesata near-neutralpHcomparedtoalowpH[55].

3.3. Effectofmetalconcentrationinsolutiononadsorptionata constantpH

Theaimoftheseexperimentswastoquantifytheamountof adsorbedmetalataconstantpHinordertoallowastraightforward comparisonoftheadsorptioncapacityofdifferentsoilsunder oth-erwiseidenticalenvironmentalconditions. Theconcentrationof adsorbedCdisplottedasafunctionofitsequilibrium concentra-tioninsolution(Fig.4)andtheparametersofEq.(4)areshownin Table2.Ofthefourtypesofsoilsamplesstudied,only the sur-facehorizon oftheChineseredsoil (CSS1sample)exhibiteda highercadmiumadsorptioncapacityandadsorptionconstant,bya factorof2to3,whereasthedeephorizonsofthetwosoils demon-stratedaloweradsorptioncapacitythanthesurfacehorizons.The

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1,E-04 1,E-03 1,E-02 1,E-01 1,E+00 1,E+01 1,E+02 1,E+01 1,E+00 1,E-01 1,E-02 1,E-03 1,E-04 [Cd], µmol/L [Cd] ads , µmol/g PPAB PPC CS S1 CS S4 model Langmuir model Frendlich

Fig.4.ConcentrationsofadsorbedCdasafunctionoftheequilibratedCd con-centrationsinsolutionforthefourtypesofsolid.Experimentalconditions:25◦C, 0.01MNaNO3,1weekexposureunderdarkness.ThepHvalueswerepH4.47±0.09; 4.16±0.30;5.5±0.2and5.4±0.1forthePPAB,PPC,CSS1andCSS4soils, respec-tively.Whennotvisible,theexperimentaluncertaintiesarewithinthesizeof thesymbols.Thesolidanddashedlinescorrespondtomodelcalculationsusing theLangmuirandFreundlichequations,respectively.ParametersoftheLangmuir equation:max=4.0±0.5␮mol/g,KL=(1.3±0.3)×105l/mol.Parametersofthe Fre-undlichequation:KF=5×105;n=1.

maximaladsorptioncapacitywasestimatedfromfittinga Lang-muirisothermequationandrangedbetween3and5␮mol/gor0.2 and0.8␮mol/m2forthedifferentsoils,whichiscomparabletothe

valuesofgoethite,amorphoussilicaandclays[36].Weattempted totreatthedataobtainedforthewholeconcentrationrange(i.e., from10−9to10−4M)usingLangmuir(Eq.(4))andFreundlich(Eq. (7))isotherms.Weusedanon-linearleastsquaresfittingwith10% residualdisagreementbetweenthemodelandtheexperimental data.Incontrasttothetraditionallinearfittingofadsorption equa-tions,thissimultaneouslyaccountedforthewiderangeofhighand lowCdandPbconcentrationsusedinthisstudy.Thesemi-linear dependenciesobtainedonalog–logscale(Fig.4)werevalidover alargeconcentrationrangeand couldbesuccessfully modelled usingthesameequations((4)and(7))parametersforalldata col-lectedfromthefourdifferentsoilsamples:max=4.0±0.5␮mol/g,

KL=(1.3±0.3)×105,KF=5×105;n=1.

TheconcentrationofadsorbedPbwasplottedasafunctionofits equilibriumconcentrationinsolution(Fig.5),andtheparameters ofEq.(4)arelistedinTable2.Similartotheresultsforcadmium,the surfacehorizonofChineseredsoilexhibitedthehighestadsorption capacityfor lead,particularlyfor highcadmiumconcentrations. However,withinthelevel of experimentaluncertainty, thefull setof data could beapproximated using the same parameters oftheLangmuirandFreundlichequations:max=20±2␮mol/g,

KL=6000±500;KF=1.25×105;n=1.The1:1slopebetweenthe

adsorbedand aqueous Pb concentration was preserved over 5 ordersofmagnitude,suggestingthepresenceofonlyonetypeof surfacesiteforallofthesoiltypes,orthatthedifferentsiteswere notdistinguishablewithintheresolutionofourmeasurements. 3.4. EffectofpHonmetaladsorption

TheexperimentsonthepH-dependentadsorptionedgeallowed theeffectofsolutionpHonthedegreeofmetaladsorptiontobe assessedandthusanevaluationoftheimpactofamphoteric prop-ertiesof soilsurface sites onmetalbindingin a wide range of solutionpHlevels.ThepercentageofadsorbedCdisplottedasa functionofpHinFig.6.Thelevelofadsorptionincreasedwiththe pHleveltoachieveamaximumadsorptionatpH≥6foreachof thefoursamples.Theadsorptionedgewassimilarforbothsoils

1,E-04 1,E-03 1,E-02 1,E-01 1,E+00 1,E+01 1,E+02 1,E+03 1,E+02 1,E+01 1,E+00 1,E-01 1,E-02 1,E-03 [Pb], µmol/L [Pb] ads , µmol/g PPAB PPC CS-S1 CS-S4 model Langmuir model Frendlich

Fig.5.ConcentrationsofadsorbedPbasafunctionoftheequilibratedPb con-centrationsinsolutionforthefourtypesofsolid.Experimentalconditions:25◦C, 0.01MNaNO3,exposure1weekinthedarkness.ThepHvalueswerepH4.4±0.2; 5.0±0.3;5.3±0.4and5.2±0.3forthePPAB,PPC,CSS1andCSS4soils, respec-tively.Whennotvisible,theexperimentaluncertaintiesarewithinthesizeofthe symbols.Thesolidanddashedlinescorrespondtothemodelcalculationsusingthe LangmuirandFreundlichequations,respectively.ParametersoftheLangmuir equa-tion:max=20±2␮mol/g,KL=6000l/mol.ParametersoftheFreundlichequation: KF=1.25× 105;n=1. 0 20 40 60 80 100 6 5 4 3 2 1 % Pb adsorbed PPAB PPC CS S1 CS S4 (b) 0 20 40 60 80 100 8 7 6 5 4 3 pH % Cd adsorbed PPAB PPC CS S1 CS S4 (a) pH

Fig.6. PercentagesofadsorbedCd(a)andPb(b)asafunctionofpHforthefourtypes ofsoils.Experimentalconditions:[Cd]o=0.9␮M,0.01MNaNO3,[Pb]o=0.48␮M, 0.01MNaNO3,8gsoill−1.Thesolidlinerepresentsthesurfacecomplexation mod-elling(SCM)resultsusingaconstantcapacitanceoftheelectricdoublelayerequal to1F/m2,surfacesitesdensityof4␮mol/g(Cd,(a))and20␮mol/g(Pb,(b))forsoils withaspecificsurfaceareaof15m2g−1andsurfacestabilityconstantsof reac-tions(10)and(11)equalto−4.0and−0.1±0.6(Cd)and−4.0and0.7±0.3(Pb), respectively.TheaverageuncertaintyoftheSCMpredictionofadsorbedCdwas ±10%.

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butthesurfacehorizonsexhibitedastrongeraffinityformetalas theadsorptionbegan,andachievedamaximumatlowerpH lev-els.ThedecreaseinCdadsorptionatpH>6.5forthePPABsoilwas mostlikelyrelatedtotheincreaseintheDOCconcentrationand thereleaseofmetalintheformoforganiccomplexes,asdiscussed earlierforthedesorptionexperiments(Section3.2).

WeattemptedtodescribethepH-dependenceofCdadsorption insoilswithintheframeworkofthesurfacecomplexationmodel (SCM).Theexactproportionsofvariousmineralandorganic com-posites ofthestudiedsoilsand theirmetal andprotonbinding propertiescouldnotbedetermined.Therefore,weusedthe gen-eralizedcompositeapproach,whichassumesthattheadsorptive reactivityofthesurfacecanbedescribedbysurfacecomplexation equilibriawiththestoichiometryandformationconstants deter-minedbyfittingtheexperimentaldata[56].Forsimplicity,two surfacereactionswerepostulatedtocontrolthemetaladsorption onsoils:

SO–H◦= SO−+H+, logKa=−4.0 (10)

SO–H◦+Cd2+= SO–Cd2++H+, logKCd=−0.1 (11)

Thefirstconstant(Ka)wasfixedwithintherangereportedfor

acid–baseparametersofChineseloesssoils[57]andsoilclay min-erals[1]andthesecondconstantwasallowedtovaryinorderto reproducetheexperimentaldatausingtheMINTEQA2code.The surfacesitedensitywasfixedat4␮mol/g(0.3–0.7␮mol/m2),as

measuredinourexperiments usingtheLangmuirian modelling approach(Fig.4),andtheconstantcapacitanceoftheelectric dou-blelayer(EDL)waspostulatedas1F/m2,whichisthemostcommon

valueusedinsurfacecomplexationmodelling.Anexample ofa modelcalculationoftheexperimentaldataisshowninFig.6.We usedanumericalapproachtofitthepercentageofadsorbedmetal asafunctionofpH.A≤10%differencebetweenthemeasuredand modelledpercentages of adsorbedmetalat each pHvaluewas takenasacriterionofgoodnessoffit.

TheuncertaintyinexperimentaldatadoesnotallowKaandKCd

constantstoberesolvedwithanaccuracygreaterthan±0.5log units.SimilaruncertaintylevelscanbeattributedtologKCd

vari-ationsbetweenthesurfaceandbottom horizonsofvarioussoil types.However,wewouldliketonotethesimilarityofthe pH-dependentadsorptionedgesforthefourtypesofsoilinthepresent study.ThisresultagreeswiththepreviousfindingsofMullerand Duffek[58],whofoundthatthesurface-normalizedvalueofthe adsorptionconstantforCdonmineraloxides,sedimentsand sus-pendedmattercanbeconsideredasaconstantparameterwithin ±0.4logarithmicunits.Thisfindingsuggests,inaccordancewith thesurfacecoordinationtheory[36],thattheterminaloxygensites S–O− responsiblefor metalbindingonsolidsurfacesare simi-larbetweenvarioussoilmineralsandorganicmatter.Asaresult, onecanuseauniversalsurfaceadsorptionconstanttodescribethe interactionbetweenmetalpollutantsandvarioussoils.

Thepercentage ofadsorbedPbisplottedasafunctionofpH inFig.6b.Similartocadmium,theadsorptionincreasedwithpH, achievingamaximumadsorptionatpH≥4forthefourtypesof soils.Theadsorptionedgewassimilarforthetwosoils.Theresults ofthe surfacecomplexationmodellingare presentedas a solid linecorrespondingtotheuncertaintiesattributedtothereaction (Eq.(8))stability constant(Eq.(9)).For thePbadsorption mod-ellingthesurfacesitedensityforallsoilswasfixedat20␮mol/g (1–3␮mol/m2),whichwasfivetimeshigherthanforcadmium,as

measuredusingtheadsorptionisotherms(Fig.5).Thisdifference betweenCdandPbinmaximalsurfaceadsorptiondensitiesisoften reportedfortheadsorptionofthesemetalsoncommoninorganic andorganicsorbents[31,52,59,60].Arecentstudy[61]showedthat Pbretentionon FeOHsoilsitesoccurredatalowerpHthanCd retention,suggestingthatPbadsorbstosurfacehydroxylgroupsat

Fig.7.PercentagesofadsorbedCd(A)andPb(B)forfourtypesofsoilsamples anddifferentinitialCd/Pbratios.Experimentalconditions:[Pb]o,[Cd]o=25,50and 75␮gl−1(0.06,0.12and0.36␮MforPband0.22,0.44and0.66␮MforCd),0.01M NaNO3,8gsoill−1.ThepHvalueswerepH4.12,4.61,4.86and4.98forPPAB,PPC, CSS1andCSS4,respectively.SeethecommentswritteninFig.6.

pHvalueswhereCdonlyinteractswithexchangesites.Inthisstudy, consistentwiththeresultsfor Cd,similar Pbadsorption edges, within±10%,canberecognizedinthefoursoilsamples,in agree-mentwithpreviousfindings[11,58].Thedisagreementbetween themeasuredandcalculatedadsorptionyieldsat4≤pH≤5could be related to Pb complexation with dissolved organic matter, which would not have been taken into account by the model calculations.

3.5. Competitionexperiments

Themetaladsorptioncompetitionexperimentsquantifiedthe adsorptionreactionsunderthemostrelevantenvironmental pollu-tionconditions,i.e.,whenbothmetalswerepresentininterstitial soilsolutions.TheresultsofsimultaneousCdandPbadsorption ontofoursoilsamplesarereportedasstackdiagrams(Fig.7AandB). Withinexperimentaluncertainty,thepercentageofeachadsorbed metaldidnotdependonthepresenceofthesecondmetaloverthe concentrationrange,whichcorrespondedtorealistic environmen-talpollution(0.1–1␮M).Indeed,withinthisconcentrationrange at circumneutral pH, the surface sites were undersaturated by 100–1000timeswithrespecttothemaximalamountofadsorbed metal.Thiswasfurthersupportedbysurfacecomplexation mod-ellingperformedusingthemodelparametersassessedforCdand PbinSection3.4:thepercentagesofadsorbedleadandcadmium did not vary withthe initial Cd/Pb ratio for each soil sample. Althoughhighlevelsofexperimentaluncertaintyassociatedwith thereaction(Eq.(8))stabilityconstantdonotallowthepercentage ofadsorbedmetalsateachgivenpHtobepreciselyreproduced,the generaltrendcanbedetermined.

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4. Discussionandconclusions

Thefirstmainresultofthis studyis thatovertheverywide metalconcentrationrange studied,fromthenMoltothemMol scaleinsolution,theconcentrationofadsorbedCdandPbfollowed asimilarpatterninalog–logscaleforallfour,rathercontrasting, soilsamples(Figs.4and5).Thisindicatesthesimilarity,within anorderof magnitude,ofthesurfaceadsorptionconstants and thenumber ofbindingsitesover theconcentrationrange stud-ied,fromextremelypollutedenvironmentswithmMol-levelmetal loads[49]topristineinterstitialsoilsolutions(nMollevel[47,62]). Thisfindingdisagreeswiththenotionofhavingtoconsider low-andhigh-affinitysitesinordertodescribeCdorPbadsorptionin thesoilsamplesstudied.Assuch,itcontradictsthecommonview ofhighsurfaceheterogeneityofnaturalsorbentsbutgreatly facili-tatesthepredictivemodellingofmetalmigrationinbothpolluted andpristinesoilsettings.

Thesurfacehorizonsofacidicbrownsoil(France)andredsoils (China)exhibitgreatcapacitiesformetaladsorption.Leadexhibits ahigheradsorptionaffinitythanCdwithrespecttothesoil sur-facesstudied,whichisconsistentwiththedifferentaffinitiesof metalstoclay[1]andsoilorganicmatter[41].Itisknownthat anthropogenicleadmigrateswithinthesoilprofileintheformof organicmatterandFeoxy(hydr)oxides[63].TheaffinityofPbto podzolsoiliscontrolledbyexchangeableclayfractionsinthe sur-facezone andbyiron oxidesthroughoutthewholeprofile[64]. However,arigorouscharacterizationofthechemicalstatusofPb, whichcanbeassociatedbothwithorganicandmineral compo-nents,wouldrequirein situspectroscopicobservations [65,66]; forexample,themaximumsurfaceadsorptioncapacitiesforthese soils(i.e.,0.3–0.7and1–3␮mol/m2forCdandPb,respectively)are

comparablewiththoseofthemosteffectivesorbents,suchasiron oxides,claysandalgalbiomass[5,6,58].Themaxvaluesobtained

arewithintherangereportedformajorIndiansoils(0.3–0.6␮mol/g forPb,0.5–0.9␮mol/gforCd[18])buttheyaremuchlowerthan therange reportedforhumicacidsoils(∼770␮mol/g)[15] and organic-richtopsoils(3–10␮mol/g)[31].Apparently,thepresence ofsoilorganicmatterintheformofhumusislikelytoincrease theadsorptionofmetalsatpH∼5[19,32]:theformationofternary surfacecomplexesbetweenmetalions,oxidesurfacecentresand organicligands[67]canbeevoked.Moreover,ahumic-acid coat-ingonmontmorillonitemightsignificantlyenhancethesorptionof cadmiumandlead[4,68],andthepresenceofsolubleorganic mat-teramplifiestheinteractionofcopperandzincwithamorphous allophanes[69].

Theadsorptionresultsindicatethat thesoilsamplesstudied werenotstrongly polluted.Comparedtotheamountof metals thatcanberetainedinthesolidphase(i.e.,∼450mgkg−1 forCd and∼3000mgkg−1 forPb, Figs.4and5),theactualmetal con-centrationswerelow(0.03–0.2mgkg−1forCdand10–60mgkg−1 for Pb). This is also supported by results of the simultaneous adsorptionofCdandPbatenvironmentallyrelevantconcentration levels:nocompetitionwasobservedbetweenthesetwo metals (Fig. 7)as thesurface sitesremained farfrombeing saturated. Thesesoilsarethuscapableofadsorbing100–1000timesmore metalsthan levelsauthorizedbytheEnvironmental Policy.The actualmetalconcentrationsdidnotexceedthemaximum accept-ablelimitof2mgkg−1forCdor70mgkg−1forPb,asreportedby [70]forsoilorganismprotection.However,themaximal adsorp-tion capacities were very different from the critical limits for soil damage. Although the soil samples studied had very high total metal adsorption capacities, the toxicity of metals to soil organismsismainlyrelatedtometalavailabilityandspeciationin soils[47,48,71].Therefore,increasingknowledgeaboutthemetal sorptioncapacity ofsoilsisofgreatimportancefor understand-ingaccumulatedanthropogenicinputsbecausetheseconstitutea

futurechallengeforecosystemprotectionbasedonthecriticalload concept[72].

Acknowledgements

Wearegratefultotwoanonymousreviewersfortheir construc-tivecommentsthatenabledustoimproveourpresentationand discussion.Thisworkispartoftheproject“Effectsofacid depo-sitiononchemicalprocessesofsoilheavymetals”(PRAE00-04), whichisacooperativeprojectbetweenChineseandFrench scien-tiststhatispartlyfinanciallysupportedbytheChineseMinistry ofScienceandTechnology,theChineseMinistryofEducation,the FrenchMinistryofResearchand theFrenchMinistryofForeign Affairs.

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Figure

Fig. 1. Adsorbed Cd concentration as a function of time for the acid loess soil (CS-S1).
Fig. 2. Concentrations of lead (A) and cadmium (B) in a solution saturated with non-contaminated soils as a function of soil concentration in solution
Fig. 6. Percentages of adsorbed Cd (a) and Pb (b) as a function of pH for the four types of soils
Fig. 7. Percentages of adsorbed Cd (A) and Pb (B) for four types of soil samples and different initial Cd/Pb ratios

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