吴王锁－sorptionandprecipita.ppt

,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 University E-mail: ,Objectives -Table of contents,Scope of this talk: overview on chemical models used to quantify radionuclide retention present a few selected results relevant to Performance Assessment (PA) show how data are integrated in PA models Table of contents: Defining radionuclide adsorption and (co)precipitation Adsorption: concepts, models and data (Co)precipitation: concepts, models and data Verifying the uptake mechanism Data integration in PA and their influence on calculations,Definitions from 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 mechanisms Precipitation: 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 recrystallization (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 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 balance 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 sites At low adsorbate concentrations, it is equivalent to a Kd model,s is the specific surface in m2 g-1 (e.g. 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 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 thus 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 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 equations 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 constants,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 model,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 (titration 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 depending on the solutes present in the aqueous solution Two 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 complexes,purely electrostatic interactions: low pH positive surface anion adsorption high 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 reactions (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 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 compensated 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 !,outer sphere surface cplx.,Classification of adsorption reactions,Example Eu 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. (2005) 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 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 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 between 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:,Applies 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: to distinguish between homogeneous or heterogeneous Ra incorporation to 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 pure 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: The 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 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 anion 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 trace 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 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 incorporation 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,important issues,How can we decide: whether adsorption or precipitation will be the relevant uptake mechanism which 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 problem) 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 resolve 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 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 integration in PA (1),How are data on radionuclide adsorption and (co)precipitation integrated in safety assessment calculation? Due to. complexity of calculations limited process understanding incomplete thermodynamic data . RN uptake is implemented in PA models through simple parameters that can be easily integrated into the transport eqs. adsorption distribution coefficients (Kd) (co)precipitation elemental 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,Retardation 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 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 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 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 so