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Radionuclideadsorptionandprecipitation:models,dataandapplicationstoperformanceassessment(放射性核素旳吸附与沉淀:模型、数据及其在性能评价方面旳应用)WUWangsuo(吴王锁)RadiochemistryLaboratory,LanzhouUniversityObjectives-TableofcontentsScopeofthistalk:overviewonchemicalmodelsusedtoquantifyradionuclideretentionpresentafewselectedresultsrelevanttoPerformanceAssessment(PA)showhowdataareintegratedinPAmodelsTableofcontents:Definingradionuclideadsorptionand(co)precipitationAdsorption:concepts,modelsanddata(Co)precipitation:concepts,modelsanddataVerifyingtheuptakemechanismDataintegrationinPAandtheirinfluenceoncalculationsDefinitions
fromthepointofviewofradionuclides(RN’s)Adsorption:ElectrostaticorchemicalbindingofRN’stoastablemineralsurfaceUsuallyareversible,kineticallyfastprocesstherearedifferenttypesofadsorptionmechanismsPrecipitation:IncorporationofRN’sinthelatticeofagrowingmineralUsuallyirreversible,aslongasthesolidremainsstable“Precipitation”isagenericconceptwhichincludes: formationofpureRNphases(i.e.RNisamajorconstituentofprecipitate)formationofsolidsolutionsviacoprecipitationorrecrystallization(RNisaminorcomponentofprecipitate)AdsorptionModels(1)
Empiricalpartitioningmodels–thedistributioncoefficient
(Kd)
Thedistributioncoefficientisthesimplest(andmostinadequate)adsorptionmodel:A =SAEquilibriumsoluteconc.dependsonlyonsolid/liquidratioV/mInreality,Kdisaconditionalconstant:strongvariationsasf(pH,I,composition)AdsorptionModels(2)
Empiricalpartitioningmodels–Isotherms
A+=S =SAIsothermmodelsrelyonlawofmassactionequationsimilartoacomplexformationconstant...densityofoccupiedsitesdensityofunoccupiedsitesmolm-2...andonamassbalanceforavailablesites:(Langmuirisothermeq.)AdsorptionModels(3)
Empiricalpartitioningmodels–Langmuirisotherm
GT,tot.availablesitesLangmuirisothermaccountsforsaturationofavailablesitesAtlowadsorbateconcentrations,itisequivalenttoaKdmodelsisthespecificsurfaceinm2g-1(e.g.BET)AdsorptionModels(4)
Empiricalpartitioningmodels–failures
Poinssotetal.(1999)GCA,63,3217–3227ProblemswithKdandLangmuirisothermmodels:distributionratios(Rd)arestronglydependentonpHandsolutioncompositionisothermnonlinearitystartsmuchearlierthansitesaturationslope=1datapH6.9Why?realityismorecomplex:multiplesitesmanycompetingsurfacespeciesEuonCa-montmorillonitepH6.9pH6.0Csonillite
AdsorptionModels(5)
Empiricalpartitioningmodels–ConclusionsKdandKLareconditionalconstants.TheirvaluesmaygreatlyvarywithpH,Iandsolutioncomposition.Empiricalmodelsarethusinadequatetodescribeadsorptioninarealisticway.Nevertheless,KdandKLareacceptableparametersinPAcalculationsaslongastheyrefertothechemicalconditionsrelevanttothespecificrepositorysystem.ThismeansthatKdvaluesusedtocalculatenuclideretardationinPAmustrelyonaccurateandrealisticdeterminationoftherelevantporewatercompositions.Surfacecomplexationmodels(6)
basicassumptions-oxidesurfacesinwaterdevelopamphoteric(acidorbasic)surfacesites(=SOH2+,=SOH,=SO-)-solutespeciesbindtosuchfunctionalsitesaccordingtolawofmassactionequationssimilartocomplexationreactionsinsolution-suchsurfacesarecharged(+or-).ChargemaystronglydependonpHandthetype/amountofsurfacecomplexesformed-inturn,surfacechargedirectlyaffectstheformationofsurfacecomplexes>achargedependenttermisrequiredinlawofmassactionconstantsoxidemineralsurfaces,e.g.Al2O3,FeOOH,SiO2=SOH+H+ =SOH2+
=SOH
=SO-+H+oxygenmetalSurfaceprotonation/deprotonationleadstoanon-permanentpH-dependentsurfacecharge(+or-)
Surfacecomplexationmodels(7)
theamphotericoxidesurface–amicroscopicviewSurfacecomplexationmodels(8)
theamphotericoxidesurface–DDLmodel=SOH+H+ =SOH2+
chargedependenttermfromprotonsreleasedatsurfaceprotonsinsolution=pHapparentconstant
Kappinthediffusedoublelayer(DDL)model,theelectricalpotentialdecaysasymptoticallyduetoacharacteristicdistributionofcounterions=SOH+H+ =SOH2+
=SOH
=SO-+H+Surfacecomplexationmodels(9)
theamphotericoxidesurface–effectofelectrostatictermDpH~1Thecharge-dependenttermdoesmatter,asshownbythisexample(titrationofmontmorillonitesuspension):theadsorptionofhydroxylionsisdampedbythenegativechargeonthesurfacedevelopedbytheexcessof=SO-complexesSurfacecomplexationmodels(10)
additionalsurfacecomplexesInadditionto=SO-,=SOH,=SOH2+,alargevarietyofsurfacecomplexesmayformdependingonthesolutespresentintheaqueoussolutionTwomaincategoriesofcomplexes:innerspherecomplexes:directlyboundtosurfacefunctionalgroups(strongchemicalbonds)outerspherecomplexes:hydratedionsattractedtothesurfacebypurelyelectrostaticorVanderWaalforces(weak)Outerspherecomplexes
purelyelectrostaticinteractions:lowpH>positivesurface>anionadsorptionhighpH>negativesurface>cationadsorptionInnerspherecomplexes
bidentatecationicmonodentatecationicmonodentateanionicsurfacecomplexationreactionscanbeconsideredassurfaceproton/hydroxylreplacementreactions(Me-O-Hbondsarebroken,eitherH+replacedbymetalorOH-replacedbyanion){=SOH}2+Cu2+ {(=SO)2Cu}+2H+
=SOH+Pb2+ =SOPb++H+
=SOH+F- =SF+OH-
Ionexchange(Clayminerals)
pyrophyllite,Al2[Si4O10](OH)2,hasneutral(uncharged)structuralunits>noexchangeableionsneutralneutralNote:theunchargedTOTpacketsareheldtogetherbyweakVanderWaalforcesIonexchange(Clayminerals)
Li+forMg2+,Mg2+forAl3+(octahedralcharge)Al3+forSi4+(tetrahedralcharge)negativelychargedTOTlayerexchangeableinterlayercationsionicsubstitutioninthetetrahedral/octahedralstructurallayerleadstoapermanentnegativecharge,whichiscompensatedthroughexchangeablecations,e.g.inmontmorillonite
Combinedadsorptionmechanisms
inexpandableclayminerals
innerspheresurfacecplx.exchangeableinterlayercationsinexpandableclaymineralslikemontmorillonite/illite,atleast3differentadsorptionmechanismsoperatesimultaneously!outerspheresurfacecplx.Classificationofadsorptionreactions
SurfacecomplexProcessForcesinvolvedouterspherephysiosorption(e.g.gasadsorption)VanderWaalsionexchangeadsorptionofhydratedionspurelyelectrostaticinnerspherechemisorption(surfacecomplexationsensustrictu)chemicalbond(ionicorcovalent)Example
EusorptionmeasurementsonCa-montmorillonite
Contributionsofsinglecomplexes:curve1:Exchanged-Eu3+
curve2:=SSO-Eu2+
curve3:=SSO-EuOH+
curve4:=SSOEu(OH)2o
curve5:=SW1O-Eu2+
curve6:totalsorptionfrom:Bradburyetal.(2023)GCA69,5403–5412Note:thedistributioncoeff.Rdvariesby5ordersofmagnitude!Precipitation
Precipitationistheseparationofsolutespeciesasanewsolidphase,occurringwhenthethermodynamicsolubilityproductisexceeded:Inthisexample,apureRNphaseprecipitatesfollowingaccumulationofdissolvedRa2+andSO42-insolutionCoprecipitationAsarule,eveninHLWrepositoryenvironments,RN’sarepresentintraceamountscomparedtomajorelements.Inmostcases,RNaccumulationinsolutionfollowingreleasefromthewastewillnotbesufficienttoreachthesolubilityproductofapureRNsolid.Solidsofmajorelements(e.g.calcite)willprecipitate,carryingwiththemtheRN’s:IfNi2+(traceion)replacesCa2+sitesinthecalcitelattice,themixtureisthermodynamicallyatruesolidsolution(固体溶液,或混晶).
ThehomogeneouspartitioncoefficientDdescribespartitioningofatraceelementbetweenliquidandsolid(analogoustoKd)(Trace:Carrier)concentrationratioinsolid(Trace:Carrier)totalelementconcentrationratioinaqueoussolutionpartitioncoefficientPartitioncoefficients(1)
Heterogeneouspartitionlaw
describespartitioningbetweenliquidandaninhomogeneous
solid:Applieswhentheprecipitatedsolidfailstoequilibrateinternally>zonedcrystals,frequentatlowTPartitioncoefficients(2)
Nofullthermodynamicequilibriuminthiscase!Partitioncoefficients(3)
The2modelsappliedtoRa-baritecoprecipitationdata(Germann,1921;DoernerandHoskins,1925)allowus:todistinguishbetweenhomogeneousorheterogeneousRaincorporationtodeterminethepartitioncoefficientSolidsolutionthermodynamics(1)
Foreverypureorimpuresolidthecorrectthermodynamicsolubilityproducthastheform:ionactivityproductactivityofend-memberinthesolidsolution(forpurephasesa
=1)Simplestcase,idealsolidsolutionForaconstantionicstrengthsolutionwithbufferedsulfateactivity:
Solidsolutionthermodynamics(2)xRaSO401mRa2+solubilityofpureRaSO4
solubilityofdilute(Ba,Ra)SO4
solidsolutionThebeneficialeffectofsolidsolutionsinanutshell:ThesolubilitylimitofadiluteSSisordersofmagnitudesmallerthanforthepuresolid!ordersofmagnitude
Solidsolutionthermodynamics(3)-Onlythermodynamicquantitiesappearontheright-side!-Forchemicallysimilarcations:CombiningthedefinitionsofK0sp
andD
leadsto:activitycoeff.ofsolidsol.end-members
fractionofdissolvedmetalasfreeion(1meansNOcomplexation)
ratioofthermodynamicsolubilityproductsactivitiesoffreeionsinaqueoussolutionHowdoempiricalpartitioncoefficientsrelatetothermodynamics?ConsiderabinarymixtureofBL(s)andCL(s),withacommonanionL
Correlationoftracemetalpartitioncoefficientsincalcitewithsolubilityproductsofpurecarbonates(Fajan’srule)EquilibriumactivityofthefreemetalMz+ineq.withpure
Mx(CO3)y
ata(CO32-)=10-5a(Mz+)=[Kosp/a(CO32-)1/y
]1/xalkalialkalineearthstransitionmetalsREE(III)Partitioncoefficients(4)
Publishedpartitioncoefficientsoftracemetalsincalcitewerecompiledasafunctionthepurephasesolubilityproduct:themoreinsolublethepuremetalcarbonate,thehigherthepartitioncoefficient
ThecorrelationbetweenDandKspworkswellandcanbeusedtoestimateunknownpartitioncoefficientsforradionuclides:Summaryofestimatedandmeasuredpartitioncoefficientsfortraceradionuclidesincalcite(Curti,1997)
Chem.AnalogonRadionuclides
DBasedon...REE(III)Cm,Am,Pu(III)100-5000Fajan’sruleCo(Zn,Mn,Fe)Ni1-10Fajan’srule-Sr0.03–0.2directmeasurementsBaRa0.01–0.1Fajan’sruleLi,NaCs<0.01Fajan’srule-U(VI)<0.2directmeasurements-Se(VI)~10-5directmeasurementsPartitioncoefficients(5)
TernaryidealSS:EuO(OH)EuH(CO3)2CaCO3
G*(kJ/mol)=-955-1775-1129cEupH=8pH=6pH=13WiththeGibbsEnergyMinimization(GEM)technique,itwaspossibletomodeldataonEuincorporationincalcite.
Aternarysolidsolutionmodel,withEuO(OH)andEuH(CO3)2asend-memberstoichiometriesforEu,successfullyreproducesdataobtainedatwidelydifferentpH-pCO2conditionsSolidsolutionthermodynamics(4)from:Curtietal.(2023),GCA69,1721-1737ThermodynamicSSmodels:anexampleimportantissues
Howcanwedecide:whetheradsorptionorprecipitationwillbetherelevantuptakemechanismwhichsolidswillbetherelevantRNhostphasesintherepositoryProblemsforapplicationofadsorption/precipitationreactionstoPA:
metastability:thermodynamicssometimes“fails”(e.g.dolomiteproblem)
kineticsofprecipitationreactionsmaybetooslowforlaboratoryscale
multitudeofhostphaseswithcomplexchemistry(pickouttherightones)
stabilityofthesorbentsurface(recrystallization,phasetransformations)>>LaboratoryexperimentsandnaturalanaloguestudiesshouldbecombinedtoresolvetheseissuesVerifyingtheRNuptakemechanism(1)
42100mmm100
mmSwissnuclearwastereferenceglassMWcorrodedduring13y
NiistrappedinMg-richsecondaryclays(greeninXRFmap)
EXAFSconsistentwithincorporationasdiluteSSinhectorite(Mg-Li-clay)butadsorptioncannotberuledout(lowCNforNi-Mg)m-XRFNa,Mgmap
VerifyingtheRNuptakemechanism(2)Uptakeof45CaduringEu3+-calciteinteractionincementporewaterindicatesthat-afterinitialadsorption–Eu3+isincorporatedintorecrystallizedcalcite.Twostepskineticsstillunexplainedmmol45CaexchangedDataintegrationinPA(1)
Howaredataonradionuclideadsorptionand(co)precipitationintegratedinsafetyassessmentcalculation?Dplexityofcalculationslimitedprocessunderstandingincompletethermodynamicdata...RNuptakeisimplementedinPAmodelsthroughsimpleparametersthatcanbeeasilyintegratedintothetransporteqs.adsorption > distributioncoefficients(Kd)(co)precipitation > elementalsolubilitylimits(Cs)
DataintegrationinPA(2)Distributioncoefficients(Kd)plottedasafunctionofhalf-lifeforcationicRN’s,usedinPAcalculationsof“ProjectOpalinusClay”fromNagra(2023),NTB02-05RetardationissufficientformostRN’stodecayalmostcompletelywithinthebarriersystem(near-field+geosphere)MagentapointsindicateReferenceCasevaluesofthesorptioncoefficient(Kd).Greenpointscorrespondtopessimisticvalues.BluepointsareusedwheretheReferenceCaseandpessimisticvaluesarethesame.
DataintegrationinPA(3)Distributioncoefficients(Kd)plottedasafunctionofhalf-lifeforanionicRN’susedinPAcalculationsof“ProjectOpalinusClay”fromNagra(2023),NTB02-05Duetoweaksorption,aconsiderablefractionoflong-livedanionicradionuclides(129I,79Se,36Cl)willnotdecaywithinthebarriersystem
lowerlimitreferencecasehigherlimitNi1x10-53x10-58x10-5Se2x10-115x10-91x10-5Sr3x10-62x10-51x10-4Zr3x10-112x10-92x10-9IhighhighhighCshighhighhighRa4x10-122x10-115x10-8Pu3x10-95x10-81x10-6Am5x10-81x10-63x10-5Nearfieldsolubilitylimitsfor“projectOpalinusClay”safetyassessmentcalculationsfromNagra(2023),NTB02-05applicationofsolidsolutiontheory(Ra,Ba)SO4pureRaSO4solubilityDataintegrationinPA(4)solubilitylimitseffectofadsorptiononbentoniteeffectofRaincorporationinbariteInthe“OpalinusClayproj
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