Physical bonding, soil adsorption

A full understanding of adsorption requires that the interaction of a solute with a surface be characterized in terms of the fundamental physical and chemical properties of the solute, the sorbent and the solvent (water) (Westall, 1987). The adsorption reactions of importance in waters, sediments and soils are listed in terms of intermolecular reactions in Table. 4.1. The fundamental chemical interactions of solutes with the surfaces by formation of coordinative bonds were already discussed in Chapter 2. The electrostatic interactions and the electric double layer were considered in Chapter 3. 

Three types of adsorption occur between the macromolecule of the soil and pesticide compounds chemical, physical, and hydrogen bonding. 

Moderate amounts of acidic pesticides were adsorbed to organic soil colloids, such as are present in muck soils, (51,143,147,175, 179) and to charcoal (53, 57,147,182,183). For both adsorption depended upon pH, being greater under acid conditions where the pesticides were adsorbed in the molecular form. The compounds were readily desorbed from the adsorbents with water (51, 57). Adsorption probably occurred through hydrogen bonding or weak physical adsorption. 

The adsorbing of bromacil by soil organic matter relative to several other herbicides is shown in Figure 11. The compound was adsorbed in low amounts, probably as a result of its high solubility. The pH of the system was approximately 5.7, so bromacil was present predominantly in the molecular form and was probably adsorbed through hydrogen bonding or other physical adsorption forces. 

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 ... 

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