Soil surface speciation, models

Al-Hamdan AZ, Reddy KR. (2005). Surface speciation modeling of heavy metals in kaolin Imphcations for electrokinetic soil remediation processes. Adsorption 11 529-546. 

Hie parameters ois, oos and at, reflect the disposition of ions in the soil solution after they have become incorporated into the interfacial region. Therefore, these surface charge densities represent the net charging effects of the surface speciation of the ions. By analogy with the use of speciation models (ion-association models) to estimate the distribution of ionic charge in aqueous phases like soil. solutions, surface speciation models (surface... 

A deeper perception of the mechanistic implications of equation (9.2) can be had if the rational activity coefficients are described on the molecular level using the methods of statistical mechanics. This approach is the analogue of the statistical mechanical theory of activity coefficients for species in aqueous solution (Sposito, 1983). Fundamental to it is the prescription of surface speciation and the dependence of the rational activity coefficient on surface characteristics. Three representative molecular models of adsorption following this paradigm are summarised in Table 9.8. Each has been applied with success to describe the surface reactions of soil colloids (Goldberg, 1992). 

Over the last three decades, two general approaches have been proposed in the literature for describing the interactions of sulfate in soils. The first approach is that of a chemical nature where thermodynamic interrelationships with speciation of cations and anions present in soil solution and the interaction with the soil surface are the major mechanisms. These models may be referred to as chemical models. Examples of such models include that of Cosby et al. (1986), Reuss and Johnson (1986), De Vries et al. (1994), among others. A common feature of these models is that both ion exchange and aluminum hydrolysis reactions are similar. Their capability of quantifying these processes varies according to whether the interactions are... 

The purpose of this chapter is to present several of the most common models used to describe metal and metalloid ion adsorption by soil components. Empirical models used in soil chemistry are described and their limitations discussed. Common chemical models used to describe metal adsorption on soil minerals are described and their advantages over empirical approaches discussed. Methods for obtaining model parameters are provided. Methods for establishing adsorption mechanisms and surface speciation are addressed. Limitations and approximations in the application of chemical models to natural systems are presented. 

The CA and GC modeling approaches can differ in the selection of surface species to fit experimental data. Surface speciation in the CA modeling approach is normally fixed by a previous fit of an SCM to a reference mineral phase, such as ferrihydiite (Waite et al., 1994 Dzombak Morel, 1990). Ideally, spectroscopic data (XAS, FTIR, and others) are used to constrain the selection of surface species in CA models for reference minerals, so that molecular scale details of the bonding are included within the model as well as EDL terms however, as is illustrated in the CA model calculations presented here and elsewhere (Waite et al., 2000 Davis et al., 1998), application of CA models to soils and sediments is not straightforward. Without very detailed characterization of the physical and chemical characteristics of the surface, several assumptions need to be made to apply a CA model. 

Zachara et al. [8] described sorption of divalent metals (Ba, Sr, Cd, Mn, Zn, Co and Mo) on calcite with a model that included aqueous speciation and Me + - Ca + exchange on cation specific surface sites. Engesgaard and Traberg [40] included ion exchange in the modeling of contaminant transport at a waste residue deposit. They found that ion exchange was the dominant process, with Na, K+ and NH4 from the leachate exchanging with an initial soil population of Ca + and Mg +. 

Surface waters, soils, and sediments are often divided into "parcels" that are modeled as open qrstems (16). Inputs and outputs of water, water-borne solutes, and water-borne particles change over time. As a consequence, chemical conditions within the parcel change. If metal ion exchange and ligand exchange reactions take place quickly, equilibrium descriptions of chemical speciation are appropriate. If these reactions take place slowly, however, then an appraisal of speciation requires knowledge of reaction kinetics (17). 

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