Surface complexation models SCMs

The main, currently used, surface complexation models (SCMs) are the constant capacitance, the diffuse double layer (DDL) or two layer, the triple layer, the four layer and the CD-MUSIC models. These models differ mainly in their descriptions of the electrical double layer at the oxide/solution interface and, in particular, in the locations of the various adsorbing species. As a result, the electrostatic equations which are used to relate surface potential to surface charge, i. e. the way the free energy of adsorption is divided into its chemical and electrostatic components, are different for each model. A further difference is the method by which the weakly bound (non specifically adsorbing see below) ions are treated. The CD-MUSIC model differs from all the others in that it attempts to take into account the nature and arrangement of the surface functional groups of the adsorbent. These models, which are fully described in a number of reviews (Westall and Hohl, 1980 Westall, 1986, 1987 James and Parks, 1982 Sparks, 1986 Schindler and Stumm, 1987 Davis and Kent, 1990 Hiemstra and Van Riemsdijk, 1996 Venema et al., 1996) are summarised here. 

Surface complex models (SCMs) are now finding widespread application in the fields of pollutant retention behaviour (Zachara etal., 1989,1992), the soil chemistry of plant nutrient retention (Goldberg and Sposito, 1984 Goldberg and Glaubig, 1986 Goldberg and Traina, 1987) and the retention of radionuclides by sediments and transport of pollutants by colloids (Davis and Kent, 1990 Dzombakand Morel, 1990 Goldberg, 1992). 

The site binding model based on reactions (1), (2), (14) and (15), often called surface complexation model (SCM), was, beside the simple site binding models (for example two layer model or constant capacitance model) readily applied to a description of the edl on the metal oxide-electrolyte solution interface. Reactions (14) and (15) describe the adsorption of so-called back-... 

Surface complexation models (SCM s) provide a rational interpretation of the physical and chemical processes of adsorption and are able to simulate adsorption in complex geochemical systems. Chemical reactions at the solid-solution interface are treated as surface complexation reactions analogous to the formation of complexes in solution. Each reaction is defined in terms of a mass action equation and an equilibrium constant. The activities of adsorbing ions are modified by a coulombic term to account for the energy required to penetrate the electrostatic-potential field extending away from the surface. Detailed information on surface complexation theory and the models that have been developed, can be found in (Stumm et al., 1976 ... 

In the area of interfacial charging at the solid/liquid interface of metal oxide aqueous suspensions, the "surface complexation or site binding concept is commonly used [3-20]. This concept is characterised by consideration of specific ionic reactions with surface groups, rather than assuming simple binding of ions to the surface or their accumulation at the interface (adsorption). In the past decade several different models were introduced on the basis of the surface complexation model (SCM) they differ in the assumed structure of the electrical interfacial layer (EIL) and in the proposed mechanisms and stoichiometries of surface reactions leading to surface charge. 

The surface complexation approach discussed in this section is suitable for adsorption, which dominates at relatively short equilibration times and relatively low concentrations of the adsorbate (up to a few hours, and up to 10 mol dm respectively, for typical experimental conditions). In principle this model is not suitable for long equilibration times or high adsorbate concentrations. Successful applications of surface complexation model SCM to uptake data obtained by coprecipitation (e.g.. Ref. [80]) and other sorption experiments which can be hardly described as adsorption has been reported. In such instances, however, SCM should be rather regarded as a data fitting model than as a mechanistic model. 

Surface complexation models (SCM) are surface chemical equilibrium models that originate from work with metal oxides. The basic premise of SCM... 

To date, potentiometric titration is still a main approach to study the surface acid base chemistry of clay minerals. Only some papers deal with the dissolution of a solid matrix resulting in various hydrolyzed aluminum species, silicic acid and their product hydrous aluminosilicates, though their interaction with a clay surface should be considered in the modeling description. The surface complexation model (SCM) was successfully applied in a recent paper [6] to interpret surface acid-base reactions involving the dissolution of illite clays during prolonged titration. Voluminous literature on ion adsorption and surface complexation... 

The pronounced effects of aqueous chemistry on actinide sorption behavior suggest that sorption modeling should account for changing physicochemical conditions. A number of different modeling approaches of varying complexity can be used to incorporate the effects of chemistry on radionuclide sorption. A class of models that has been used with success in modeling pH-dependent sorption for actinides and other metals is the electrostatic surface complexation model (SCM). These models are equilibrium representations of sorption at the mineral-water interface and are discussed in detail elsewhere (Davis Kent, 1990 Dzombak Morel, 1990 Hayes etal., 1991 Seme etal., 1990 Turner, 1995), with only a brief overview presented here. 

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