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** Capacitance complexation model **

** Constant-capacitance surface complexation **

** Surface complexation models reactions **

** Surface complexation reactions **

The triple layer modeP offers a molecular description of surface complexation reactions that differs from the constant capacitance model in several fundamental respects. These differences can be brought into clear relief through a comparative listing of the principal chemical assumptions that underlie the triple layer model [Pg.177]

The characteristic features of parameter estimation in a molecular model of adsorption are illustrated in Table 9.9, taking the simple example of the constant-capacitance model as applied to the acid-base reactions on a hydroxylated mineral surface. (It is instructive to work out the correspondence between equation (9.2) and the two reactions in Table 9.9.) Given the assumption of an average surface hydroxyl, there are just two chemical reactions involved (the background electrolyte is not considered). The constraint equations prescribe mass and charge balance (in terms of mole fractions, x) and two complex stability constants. Parameter estimation then requires the determination of the two equilibrium constants and the capacitance density simultaneously from experimental data on the species mole fractions as functions of pH. [Pg.252]

METAL CATION ADSORPTION. The formation of inner-sphere surface complexes involving metal cations is typically described in the constant capacitance model with the chemical reactions [Pg.174]

Surface complexation reactions are assumed on surface sites, S—OH. The total site density (Ns, mol/m ), has to be defined for the given system. In the constant-capacitance and diffuse-layer models, all surface species are supposed to be inner-sphere complexes, whereas in the triple-layer model, both inner- and outer-sphere complexes are assumed. [Pg.727]

Most studies of metal ion complexation rely on the two-pKH model. Schindler and Stumm [59, 69, 78, 82, 87] combine the two-pKH constant capacitance model with stoichiometric reactions of the metal ions with surface hydroxyls. Huang et al. [62] and Dzombak and Morel [63] tabulate ion affinity constants on the basis of the two-pKH GC model. Leckie and co-workers [88-90] combine the model cation and anion adsorption with the two-pKH TL model. Hayes makes a distinction between strongly and weakly adsorbing ions [89, 90]. A series of reviews on s.a. using the two-pKH model can be found in [91]. [Pg.784]

The electrostatic models discussed in Sections 2.9.2.1 through 2.9.2.5 apply to a simple chemical model involving one reaction (Reaction 2.14) of the transfer of one species (proton) from the solution to the surface. More complex chemical models involving the transfer of two or more species from the solution to the surface and/or allowing various distances of the adsorbed solution species from the surface require more complex electrostatic models. Usually, three or more capacitors in series are considered. All the capacitors but one have constant capacitances, and the relationship between and Vd in one capacitor (diffuse layer) is expressed by Equation 2.18. [Pg.95]

Several models have been developed to describe reactions between aqueous ions and solid surfaces. These models tend to fall into two categories (1) empirical partitioning models, such as distribution coefficients and isotherms (e.g., Langmuir and Freundlich isotherms), and (2) surface-complexation models (e.g., constant-capacitance, diffuse-layer, or triple-layer model) that are analogous to solution complexation with corrections for the electrostatic effects at the solid-solution interface (Davis and Kent, 1990). These models have been described in numerous articles (Westall and Hohl, 1980 Morel, Yeasted, and Westall, 1981 James and Parks, 1982 Barrow, 1983 Westall, 1986 Davis and Kent, 1990 Dzombak and Morel, 1990). Travis and Etnier (1981) provided a comprehensive review of the partitioning and kinetic models typically used to define sorption of ions by soils. The reader is referred to the cited articles for details of the models. [Pg.35]

** Capacitance complexation model **

** Constant-capacitance surface complexation **

** Surface complexation models reactions **

** Surface complexation reactions **

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