Sorption Kinetics - Transport and Fate of Chemicals in Soils: Principles and Applications - page 35

2
Sorption Ki netics
For several decades, it has been observed that sorption and desorption of var-
ious 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 ele-
ments 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) trans-
port in liquid-filled macropores (>2 nm), all of which are nonactivated diffu-
sion 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.
The form of chemical retention reactions in soils and geological porous
media must be clearly identified if predictions of their potential mobility,
toxicity, and impact on the environment are sought. In general, chemical
retention processes with matrix surfaces have been quantified by scien-
tists using a number of empirically based approaches. One approach rep-
resents equilibrium-type reactions such as those discussed in Chapter 1.1
Equilibrium models are those where sorption reactions are assumed fast or
instantaneous in nature. Under such conditions, “apparent equilibrium” may
be observed in a relatively short reaction time (minutes or hours). Langmuir
and Freundlich models are perhaps the most commonly used equilibrium
models for the description of fertilizer chemicals, especially phosphorus,
heavy metals, and pesticides. These equilibrium models include the linear
and Freundlich (nonlinear) and the one- and two-site Langmuir type.
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