Sorption Kinetics - Transport and Fate of Chemicals in Soils: Principles and Applications - page 36

1 2

3

4

6 5

Liquid Film Solid

FIGURE 2.1
Transport processes in solid-liquid soil reactions. Nonactivated processes: (l) Transport in
the soil solution. (2) Transport across a liquid film at the solid-liquid interface. (3) Transport
in a liquid-filled micropore. Activated processes: (4) Diffusion of a sorbate at the surface of
the solid. (5) Diffusion of a sorbate occluded in a micropore. (6) Diffusion in the bulk of the
solid. (From C. Aharoni and Sparks, D. L. 1991. Pp. 1-18 in Rates of Soil Chemical Processes in
Soils , edited by D. L. Sparks and D. L. Suarez. Special publication 27. Madison, WI: Soil Science
Society of America. With permission.)

Soils and other geochemical systems are quite complex, and various sorp-
tion reactions are likely to occur. Such reactions are either consecutive or
concurrent. Amacher (1991) and Selim and Amacher (1997) provided a sche-
matic, shown in Figure 2.2, to illustrate several types of reactions that occur
in the geochemical media of soils and the range of time for these reactions
to reach equilibrium. The ion association, multivalent ion hydrolysis, and
mineral crystallization reactions are all homogeneous because they occur
within a single phase. The first two of these occur in the liquid phase while
the last occurs in the solid phase. The other reaction types are heteroge-
neous because they involve transfer of chemical species across interfaces
between phases. Ion association reactions refer to ion pairing, complexation
(inner- and outer-sphere), and chelation-type reactions in solution. Gas-
water reactions refer to the exchange of gases across the air-liquid interface.
Ion exchange reactions refer to electrostatic ion replacement reactions on
charged solid surfaces. Sorption reactions refer to simple physical adsorp-
tion, surface complexation (inner- and outer-sphere), and surface precipita-
tion reactions. Mineral-solution reactions refer to precipitation/dissolution
reactions involving discrete mineral phases and coprecipitation reactions by
which trace constituents can become incorporated into the structure of dis-
crete mineral phases.
Reactions in soil environments encompass a wide range of time scales as
Figure 2.2 shows. Furthermore, these reactions can occur concurrently and