ABSTRACT
Adsorption is arguably the most important of the physical-chemical processes responsible for the retention of inorganic and organic substances in the soil environment. The soil solution composition of inorganic and organic substances is controlled by surface or near-surface processes. With respect to trace elements, the activities of aqueous metal ions in equilibrium with an adsorbed phase are controlled to levels that can be orders of magnitude lower than levels controlled by even the most stable mineral phases. Strictly defined, adsorption is a surface process that results in the accumulation of a dissolved substance (an adsorbate) at the interface of a solid (the adsorbent) and the solution phase (Figure 7.1a). This interfacial region incorporates the volume of the soil solution that is under the direct influence of the surface, and is commonly referred to as the solid-solution interface. The process of adsorption can be contrasted with that of precipitation, in which the crystal structure of a mineral increases in volume as a result of the three-dimensional growth of the structure (Figure 7.1b). Inorganic and organic substances can also be retained through the process of absorption. In this process, a substance diffuses into the three-dimensional framework of a solid structure. Partitioning (absorption) is a mechanism that is frequently responsible for the retention of organic compounds by soil organic matter.
