Site Complexation Model Triple-Layer

Triple-layer model Of limited value because of the complexity of adsorption sites, unpredictable... 

To be useful in modeling electrolyte sorption, a theory needs to describe hydrolysis and the mineral surface, account for electrical charge there, and provide for mass balance on the sorbing sites. In addition, an internally consistent and sufficiently broad database of sorption reactions should accompany the theory. Of the approaches available, a class known as surface complexation models (e.g., Adamson, 1976 Stumm, 1992) reflect such an ideal most closely. This class includes the double layer model (also known as the diffuse layer model) and the triple layer model (e.g., Westall and Hohl, 1980 Sverjensky, 1993). 

The TLM (Davis and Leckie, 1978) is the most complex model described in Figure 4. It is an example of an SCM. These models describe sorption within a framework similar to that used to describe reactions between metals and ligands in solutions (Kentef fll., 1988 Davis and Kent, 1990 Stumm, 1992). Reactions involving surface sites and solution species are postulated based on experimental data and theoretical principles. Mass balance, charge balance, and mass action laws are used to predict sorption as a function of solution chemistry. Different SCMs incorporate different assumptions about the nature of the solid - solution interface. These include the number of distinct surface planes where cations and anions can attach (double layer versus triple layer) and the relations between surface charge, electrical capacitance, and activity coefficients of surface species. 

Dove and Elston, this interfadal layer can be described by a triple layer snrface com-plexation model (TLM) as shown in Fig. 4.31. The interface consists of three electrostatically charged regions, each with an associated electric potential and snrface charge these are termed the o, p, and d planes. Hydrogen ions are permitted to coordinate with the nnsatnrated sites of the interface at the innermost o layer. Sodinm is positioned at the P layer or the d layer. The surface silicon-oxygen complex may have a different chemical character depending on the adsorbed species, hi a sodium chloride solution the surface complexes can be represented as sSiOHaCl, sSiOHj, =SiOH, =SiO-Na, and SiO". The concentration of each species depends on pH and salt concentration, and the sum of the fractions of these surface species equals 1 ... 

For this purpose it is possible to extend to a multiple oxide the one-site model of Johnson (1984), which provides a thermodynamic description of the double layer surrounding simple hydrous oxides. Briefly, in this model the double layer charge is divided into the charge inside the slip plane, slip plane a[d]. While occupied sites, a[tl] is obtained from the Poisson-Boltzmann equation. Note that unlike the triple-layer model (Davis et al., 1978) which allows ions to form surface complexes at two different planes (0 or / ) instead of al the slip plane only, this model does not distinguish between inner- and outer-sphere complexes. Expression of the... 

For modeling surface adsorption using the surface complexation theory, we need properties of the surfaces as well as complexation constants for the sorbant. Surface properties include site density, surface areas, and molecular formula weight. If we use the triple layer model, capacitance data are also needed see Chapter 7 for more details. 

Chemical adsorption mechanisms that are based on chemical interactions between the metal complexes and the solid support. In this case, all the ionic species present in the liquid solution compete for the active sites at the solid surface. This type of adsorption is usually described by surface ionization models, such as the triple-layer model and its extended versions (e.g., four-layer model) [1]. 

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. 

The siuface complexes formed between surface sites and protons or hydroxide ions formed at the IHP are referred to as inner surface complexes. Surface complexes formed between surface sites and solutes other than protons and hydroxide ions at the SP are referred to as outer surface complexes. While these adsorption layers might at first suggest that adsorbed layer stracture might be determined from adsorption experiments, upon careful reflection, the assumption that proton and hydroxide ions complex with the same surface sites as do other solutes, but at different locations relative to the interface, seems curious at best. As a result of the arbitrary assignment of two adsorption planes it would seem that any information from the triple layer model about adsorbed layer stracture must be equally arbitrary. 

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