Soil Colloidal Behavior
Characterizations of size, shape, surface area, surface charge density, and changes in surface charge are required for
understanding the processes of adsorption, ¥occulation, dispersion, and transport in soils and the resultant changes in soil hydraulic properties, as well as chemical migration. Since the major part of the surface area is in the colloidal fraction of the soil, almost all surface-controlled processes including adsorption reactions, nucleation, precipitation, and dissolution involve colloids. Colloids are reactive not only because of their total surface area but also because of enhanced reactivity related to rough surfaces and highly energetic sites, as well as the e¦ects of electrostatic charge. Colloid charge is associated with substitution of lower charge cations for those of higher charge in the mineral lattice (which results in a net permanent charge) as well as surface charge associated with broken bonds. ¤e charge associated with broken bonds is characterized as variable charge in as much as the solution in¥uences the surface speciation (Chapter 16). In addition to these chemical processes, colloids are mobile in soils and thus not only a¦ect the chemical transport of otherwise immobile chemicals but also exert a strong in¥uence on soil hydraulic properties.