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1、,Radionuclide adsorption and precipitation:models,data and applications to performance assessment（放射性核素的吸附与沉淀：模型、数据及其在性能评价方面的应用）WU Wangsuo(吴王锁)Radiochemistry Laboratory,Lanzhou UniversityE-mail:,Objectives-Table of contents,Scope of this talk:overview on chemical models used to quantify radionuclide
2、 retention present a few selected results relevant to Performance Assessment(PA)show how data are integrated in PA modelsTable of contents:Defining radionuclide adsorption and(co)precipitation Adsorption:concepts,models and data(Co)precipitation:concepts,models and data Verifying the uptake mechanis
3、m Data integration in PA and their influence on calculations,Definitionsfrom the point of view of radionuclides(RNs),Adsorption:Electrostatic or chemical binding of RNs to a stable mineral surface Usually a reversible,kinetically fast process there are different types of adsorption mechanismsPrecipi
4、tation:Incorporation of RNs in the lattice of a growing mineral Usually irreversible,as long as the solid remains stable“Precipitation”is a generic concept which includes:formation of pure RN phases(i.e.RN is a major constituent of precipitate)formation of solid solutions via coprecipitation or recr
5、ystallization(RN is a minor component of precipitate),Adsorption Models(1)Empirical partitioning models the distribution coefficient(Kd),The distribution coefficient is the simplest(and most inadequate)adsorption model:,Equilibrium solute conc.depends only on solid/liquid ratio V/m In reality,Kd is
6、a conditional constant:strong variations as f(pH,I,composition),Adsorption Models(2)Empirical partitioning models Isotherms,Isotherm models rely on law of mass action equation similar to a complex formation constant.,density of occupied sites,density of unoccupied sites,mol m-2,.and on a mass balanc
7、e for available sites:,(Langmuir isotherm eq.),Adsorption Models(3)Empirical partitioning models Langmuir isotherm,GT,tot.available sites,Langmuir isotherm accounts for saturation of available sitesAt low adsorbate concentrations,it is equivalent to a Kd model,s is the specific surface in m2 g-1(e.g
8、.BET),Adsorption Models(4)Empirical partitioning models failures,Poinssot et al.(1999)GCA,63,32173227,Problems with Kd and Langmuir isotherm models:distribution ratios(Rd)are strongly dependent on pH and solution composition isotherm non linearity starts much earlier than site saturation,Why?reality
9、 is more complex:multiple sites many competing surface species,Eu on Ca-montmorillonite,pH 6.9,pH 6.0,Cs on illite,Adsorption Models(5)Empirical partitioning models Conclusions,Kd and KL are conditional constants.Their values may greatly vary with pH,I and solution composition.Empirical models are t
10、hus inadequate to describe adsorption in a realistic way.Nevertheless,Kd and KL are acceptable parameters in PA calculations as long as they refer to the chemical conditions relevant to the specific repository system.This means that Kd values used to calculate nuclide retardation in PA must rely on
11、accurate and realistic determination of the relevant pore water compositions.,Surface complexation models(6)basic assumptions,-oxide surfaces in water develop amphoteric(acid or basic)surface sites(=SOH2+,=SOH,=SO-)-solute species bind to such functional sites according to law of mass action equatio
12、ns similar to complexation reactions in solution-such surfaces are charged(+or-).Charge may strongly depend on pH and the type/amount of surface complexes formed-in turn,surface charge directly affects the formation of surface complexes a charge dependent term is required in law of mass action const
13、ants,oxide mineral surfaces,e.g.Al2O3,FeOOH,SiO2,oxygen,metal,Surface protonation/deprotonation leads to a non-permanent pH-dependent surface charge(+or-),Surface complexation models(7)the amphoteric oxide surface a microscopic view,Surface complexation models(8)the amphoteric oxide surface DDL mode
14、l,in the diffuse double layer(DDL)model,the electrical potential decays asymptotically due to a characteristic distribution of counterions,Surface complexation models(9)the amphoteric oxide surface effect of electrostatic term,DpH 1,The charge-dependent term does matter,as shown by this example(titr
15、ation of montmorillonite suspension):the adsorption of hydroxyl ions is damped by the negative charge on the surface developed by the excess of=SO-complexes,Surface complexation models(10)additional surface complexes,In addition to=SO-,=SOH,=SOH2+,a large variety of surface complexes may form depend
16、ing on the solutes present in the aqueous solutionTwo main categories of complexes:inner sphere complexes:directly bound to surface functional groups(strong chemical bonds)outer sphere complexes:hydrated ions attracted to the surface by purely electrostatic or Van der Waal forces(weak),Outer sphere
17、complexes,purely electrostatic interactions:low pH positive surface anion adsorptionhigh pH negative surface cation adsorption,Inner sphere complexes,bidentate cationic,monodentate cationic,monodentate anionic,surface complexation reactions can be considered as surface proton/hydroxyl replacement re
18、actions(Me-O-H bonds are broken,either H+replaced by metal or OH-replaced by anion),=SOH2+Cu2+(=SO)2Cu+2H+,Ion exchange(Clay minerals),pyrophyllite,Al2Si4O10(OH)2,has neutral(uncharged)structural units no exchangeable ions,neutral,neutral,Note:the uncharged TOT packets are held together by weak Van
19、der Waal forces,Ion exchange(Clay minerals),Li+for Mg2+,Mg2+for Al3+(octahedral charge),Al3+for Si4+(tetrahedral charge),negatively charged TOT layer,exchangeable interlayer cations,ionic substitution in the tetrahedral/octahedral structural layer leads to a permanent negative charge,which is compen
20、sated through exchangeable cations,e.g.in montmorillonite,Combined adsorption mechanisms in expandable clay minerals,inner sphere surface cplx.,exchangeable interlayer cations,in expandable clay minerals like montmorillonite/illite,at least 3 different adsorption mechanisms operate simultaneously!,o
21、uter sphere surface cplx.,Classification of adsorption reactions,ExampleEu sorption measurements on Ca-montmorillonite,Contributions of single complexes:curve 1:Exchanged-Eu 3+curve 2:=SSO-Eu 2+curve 3:=SSO-EuOH+curve 4:=SSOEu(OH)2 o curve 5:=SW1O-Eu 2+curve 6:total sorption,from:Bradbury et al.(200
22、5)GCA 69,54035412,Note:the distribution coeff.Rd varies by 5 orders of magnitude!,Precipitation,Precipitation is the separation of solute species as a new solid phase,occurring when the thermodynamic solubility product is exceeded:,In this example,a pure RN phase precipitates following accumulation
23、of dissolved Ra2+and SO42-in solution,Coprecipitation,As a rule,even in HLW repository environments,RNs are present in trace amounts compared to major elements.In most cases,RN accumulation in solution following release from the waste will not be sufficient to reach the solubility product of a pure
24、RN solid.Solids of major elements(e.g.calcite)will precipitate,carrying with them the RNs:,If Ni2+(trace ion)replaces Ca2+sites in the calcite lattice,the mixture is thermodynamically a true solid solution（固体溶液，或混晶）.,The homogeneous partition coefficient D describes partitioning of a trace element b
25、etween liquid and solid(analogous to Kd),(Trace:Carrier)concentration ratio in solid,(Trace:Carrier)total element concentration ratio in aqueous solution,partition coefficient,Partition coefficients(1),Heterogeneous partition law describes partitioning between liquid and an inhomogeneous solid:,Appl
26、ies when the precipitated solid fails to equilibrate internally zoned crystals,frequent at low T,Partition coefficients(2),No full thermodynamic equilibrium in this case!,Partition coefficients(3),The 2 models applied to Ra-barite coprecipitation data(Germann,1921;Doerner and Hoskins,1925)allow us:t
27、o distinguish between homogeneous or heterogeneous Ra incorporationto determine the partition coefficient,Solid solution thermodynamics(1),For every pure or impure solid the correct thermodynamic solubility product has the form:,ion activity product,activity of end-member in the solid solution(for p
28、ure phases a=1),Simplest case,ideal solid solution,For a constant ionic strength solution with buffered sulfate activity:,Solid solution thermodynamics(2),xRaSO4,0,1,mRa2+,solubility of pure RaSO4,solubility of dilute(Ba,Ra)SO4 solid solution,The beneficial effect of solid solutions in a nutshell:Th
29、e solubility limit of a dilute SS is orders of magnitude smaller than for the pure solid!,orders of magnitude,Solid solution thermodynamics(3),-Only thermodynamic quantities appear on the right-side!-For chemically similar cations:,Combining the definitions of K0sp and D leads to:,activity coeff.of
30、solid sol.end-members,fraction of dissolved metal as free ion(1 means NO complexation),ratio of thermodynamic solubility products,activities of free ions in aqueous solution,How do empirical partition coefficients relate to thermodynamics?,Consider a binary mixture of BL(s)and CL(s),with a common an
31、ion L,Correlation of trace metal partition coefficients in calcite with solubility products of pure carbonates(Fajans rule),Equilibrium activity of the free metal Mz+in eq.with pure Mx(CO3)y at a(CO32-)=10-5 a(M z+)=Kosp/a(CO32-)1/y 1/x,Partition coefficients(4),Published partition coefficients of t
32、race metals in calcite were compiled as a function the pure phase solubility product:the more insoluble the pure metal carbonate,the higher the partition coefficient,The correlation between D and Ksp works well and can be used to estimate unknown partition coefficients for radionuclides:,Summary of
33、estimated and measured partition coefficients for trace radionuclides in calcite(Curti,1997),Partition coefficients(5),Ternary ideal SS:EuO(OH)EuH(CO3)2 CaCO3 G*(kJ/mol)=-955-1775-1129,cEu,pH=8,pH=6,pH=13,With the Gibbs Energy Minimization(GEM)technique,it was possible to model data on Eu incorporat
34、ion in calcite.A ternary solid solution model,with EuO(OH)and EuH(CO3)2 as end-member stoichiometries for Eu,successfully reproduces data obtained at widely different pH-pCO2 conditions,Solid solution thermodynamics(4),from:Curti et al.(2005),GCA 69,1721-1737,Thermodynamic SS models:an example,impor
35、tant issues,How can we decide:whether adsorption or precipitation will be the relevant uptake mechanismwhich solids will be the relevant RN host phases in the repository,Problems for application of adsorption/precipitation reactions to PA:metastability:thermodynamics sometimes“fails”(e.g.dolomite pr
36、oblem)kinetics of precipitation reactions may be too slow for laboratory scale multitude of host phases with complex chemistry(pick out the right ones)stability of the sorbent surface(recrystallization,phase transformations),Laboratory experiments and natural analogue studies should be combined to r
37、esolve these issues,Verifying the RN uptake mechanism(1),4,2,100,m,m,m,100,mm,Swiss nuclear waste reference glass MW corroded during 13 y Ni is trapped in Mg-rich secondary clays(green in XRF map)EXAFS consistent with incorporation as dilute SS in hectorite(Mg-Li-clay)but adsorption cannot be ruled
38、out(low CN for Ni-Mg),m-XRF Na,Mg map,Verifying the RN uptake mechanism(2),Uptake of 45Ca during Eu3+-calcite interaction in cement pore water indicates that-after initial adsorption Eu3+is incorporated into recrystallized calcite.Two steps kinetics still unexplained,mmol 45Ca exchanged,Data integra
39、tion in PA(1),How are data on radionuclide adsorption and(co)precipitation integrated in safety assessment calculation?Due plexity of calculationslimited process understandingincomplete thermodynamic data.RN uptake is implemented in PA models through simple parameters that can be easily integrated i
40、nto the transport eqs.adsorption distribution coefficients(Kd)(co)precipitationelemental solubility limits(Cs),Data integration in PA(2),Distribution coefficients(Kd)plotted as a function of half-life for cationic RNs,used in PA calculations of“Project Opalinus Clay”from Nagra(2002),NTB 02-05,Retard
41、ation is sufficient for most RNs to decay almost completely within the barrier system(near-field+geosphere),Magenta points indicate Reference Case values of the sorption coefficient(Kd).Green points correspond to pessimistic values.Blue points are used where the Reference Case and pessimisticvalues
42、are the same.,Data integration in PA(3),Distribution coefficients(Kd)plotted as a function of half-life for anionic RNs used in PA calculations of“Project Opalinus Clay”from Nagra(2002),NTB 02-05,Due to weak sorption,a considerable fraction of long-lived anionic radionuclides(129I,79Se,36Cl)will not
43、 decay within the barrier system,Near field solubility limits for“project Opalinus Clay”safety assessment calculations from Nagra(2002),NTB 02-05,application of solid solution theory(Ra,Ba)SO4,pure RaSO4 solubility,Data integration in PA(4),solubility limits,effect of adsorption on bentonite,effect
44、of Ra incorporation in barite,In the“Opalinus Clay project”,the Ra solubility limit was determined from a simple Ra inventory mass balance model,taking into account solid solution formation with minor amounts of barite in bentonite,Assuming instantaneous dissolution of the total Ra inventory,the dec
45、rease in Ra dissolved concentration due to sorption+solid solution formation is 5 orders of magnitude,resulting in Cs(Ra)10-11 M,Data integration in PA(5),Data integration in PA(6),Effect of chemical retention parameters in PA calculations examples from sensitivity analysis(Kristallin-I),STRENG code
46、,decrease of Kd earlier break-throughincrease of Cs higher release rates,Effect of Ni solubility:above a threshold of 10-8 M,release rates to geosphere become insensitive to Cs.Below this limit,release rate maxima are very sensitive to Cs.,Conclusions,Adequate thermodynamic models exist to treat rad
47、ionuclide adsorption/precipitation reactions.Such models are used to derive sorption coefficients and solubility limits in PA calculations.In PAs chemistry and transport are not fully coupledExtreme care must be taken when using such empirical parameters,as they must“fit”to the relevant repository c
48、onditions(pH,pCO2,I,solution composition,interacting solids)A full coupling of chemistry and transport in PA is still out of reach,because of(1)complexity of the calculations and(2)lack of a complete thermodynamic databaseNevertheless,“full coupling”should be the ultimate goal of performance assessment,in order to increase realism and hence confidence from public and scientific community-in the predictions,Thank you!,
