For several decades, it has been observed that sorption and desorption of various chemicals on matrix surfaces are kinetic or time dependent. Numerous studies on the kinetic behavior of solutes in soils are available in the literature. Recent reviews on kinetics include Sparks and Suarez (1991), Sparks (2003), and Carrillo-Gonzalez et al. (2006). The extent of kinetics varied extensively among the different solute species and soils considered. Generally, trace elements and heavy metal species exhibit strong sorption, as well as extensive kinetic behavior during sorption and release or desorption. In contrast, weak sorption and less extensive kinetic behavior are often observed for organic chemicals in soils and porous media. According to Aharoni and Sparks (1991) and Sparks (2003), a number of transport and chemical reaction processes affect the rate of soil chemical reactions. The slowest of these will limit the rate of a particular reaction. The actual chemical reaction at the surface, for example, adsorption, is usually very rapid and not rate limiting. Transport processes (see Figure 2.1) include (1) transport in the solution phase, which is rapid and, in the laboratory, can be eliminated by rapid mixing; (2) transport across a liquid film at the particle/liquid interface (film diffusion); (3) transport in liquid-filled macropores (>2 nm), all of which are nonactivated diffusion processes and occur in mobile regions; (4) diffusion of a sorbate along pore wall surfaces (surface diffusion); (5) diffusion of sorbate occluded in micropores (<2 nm-pore diffusion); and (6) diffusion processes in the bulk of the solid, all of which are activated diffusion processes. Pore and surface diffusion can be referred to as interparticle diffusion, whereas diffusion in the solid is intraparticle diffusion.