Ion Binding to Mineral Components

In previous chapters, the main modes of interaction between ions and soil mineral colloids have been discussed in Chapters 4 and 5, the principles of ion-surface interactions have been laid down, and in Chapters 8 and 9, the main features of adsorption onto silicate and oxide minerals have been reviewed. As introduced in Chapter 11, two main contributions should be considered, namely, the nonelectrostatic forces composed of physical (van der Waals forces) and/or chemical (specific bonding) interactions and the electrostatic forces arising from the charged nature of both the adsorbate and the surface this is reflected in two contributions (considered independent) to the Gibbs free energy, as in Equation 11.5  

The chemical part AG° corresponds in most cases to specific adsorption on reactive surface hydroxyl groups, whereas the electrical part AG is due to the cou-lombic interaction. In the first part, it arises quite naturally the concept of binding site as the place on the surface where the adsorbate binds these sites can be associated with individual hydroxyl groups, but not necessarily, because when bidentate adsorption takes place, as in the case of phosphate or arsenate adsorption onto iron oxides, two hydroxyls may be considered as forming a single site. 

These problems are almost always treated by chemical modeling, as described in Chapter 11, and reviewed by numerous authors (Mattigod and Zachara 1996 Zachara and Westall 1998 Goldberg 2005 Merdy, Koopal, and Huclier 2006 Van Riemsdijk et al. 2006 Gu, Evans, and Barabash 2010 and references therein). Albeit in most cases electrostatic interactions are considered, there are some authors proposing nonelectrostatic modeling for these phenomena (Pagnanelli et al. 2006). 

FIGURE 12.1 Transmission electron microscopy image of kaolinite particle showing surface precipitation of Al(OH)3. Mottled appearance resnlts from precipitation of the A1(0H)3 phase on basal surface. (Reprinted from Geochimica et Cosmochimica Acta, 63, Thompson et al., Dynamic interactions of dissolution, surface adsorption, and precipitation in an aging cobalt(II)-clay-water system, 1767-1779. Copyright 1999, with permission from Elsevier.) 

To treat the electrostatic effects in ion adsorption, a model of the interface is required, so as to relate the surface charge and potential with the system composition and conditions. Several models have been proposed in the literature, as we will develop in the following section. 

---