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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 University E-mail: ,Objectives -Table of contents,Scope of this talk: overview on chemical models used to quantify ra
2、dionuclide 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
3、 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 adso
4、rption 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 solu
5、tions 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
6、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
7、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
8、 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 n
9、on 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
10、 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 me
11、ans 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-)
12、 - 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 t
13、he 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 amph
14、oteric 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 ox
15、ide 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 mod
16、els (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
