Sabine Goldberg¹, Inmaculada Lebron², John C. Seaman³, and Donald L. Suarez¹

¹United States Department of Agriculture
²Centre for Ecology & Hydrology
³University of Georgia

Introduction: The importance of colloids in soil science has been appreciated for many years. However, recent understanding that organic and inorganic contaminants are often transported via colloidal particles has increased interest in colloid science. Essentially, all chemicals and individual species are to some extent reactive with soils, including species such as chloride ions, which undergo repulsion from negatively charged surfaces. With few exceptions, soil chemistry is primarily the chemistry of colloids and surfaces. The primary importance of colloids in soil science stems from their surface reactivity and charge characteristics. The overwhelming majority of surface area and electrostatic charge in a soil resides in the less than 1 µm size fraction with particles with radii between 20 and 1000 nm constituting the major part of the soil surface area (Borkovec et al., 1993). A significant fraction of reactive soil colloidal material falls within the <100 nm size range and thus is relevant to the growing interest in the properties and behavior of nanoparticles. Furthermore, soil is often the ultimate repository for anthropogenic nanomaterials of environmental concern (Hochella, 2008; Theng and Yuan, 2008; Waychunas and Zhang, 2008). The unique aspects of “nanoscience” as a discipline separate from colloid science reflect deviations in material properties in the nanoparticle size range, especially for materials <10 nm, and in many cases the lack of a natural bulk analog in the larger size fractions, for example, ferrihydrite (Hochella, 2008; Waychunas and Zhang, 2008).
Characterizations of size, shape, surface area, surface charge density, and changes in surface charge are required for understanding the processes of adsorption, flocculation, 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 effects 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. The charge associated with broken bonds is characterized as variable charge in as much as the solution influences the surface speciation. In addition to these chemical processes, colloids are mobile in soils and thus not only affect the chemical transport of otherwise immobile chemicals but also exert a strong influence on soil hydraulic properties.

SREL Reprint #3199