Stressors and Soil Contamination - Sustainable Practices in Geoenvironmental Engineering - page 61

The soil fractions that possess signiicant reactive surfaces include layer silicates (clay
minerals), soil organics, hydrous oxides, carbonates, and sulfates. The surface hydroxyls
(OH group) are the most common surface functional group in inorganic soil fractions
(soil solids) such as clay minerals with disrupted layers (e.g., broken crystallites), hydrous
oxides, and amorphous silicate minerals. The common functional groups for SOM
include the hydroxyls, carboxyls, phenolic groups, and amines. More detailed explana-
tions concerning the nature of these functional groups and their manner of interaction
with the functional groups associated with contaminants can be found in soil science
and geoenvironmental engineering textbooks (e.g., Sposito, 1984; Greenland and Hayes,
1981; Huang et al., 1995a,b; Knox et al., 1993; Yong, 2001; Yong and Mulligan, 2004; Yong
et al., 2012).
2.6 Contaminant Transport and Land Contamination
The transport of contaminants in soils refers to the movement of contaminants through
the pore spaces in soils. Liquid contaminants such as organic chemical compounds and
inorganic/organic contaminants carried in waste streams and leachate streams that pass
through the soil pore spaces will interact with the exposed surfaces of the soil fractions.
The reactions arising from the interactions between contaminants and soil fractions will
dictate the nature of the transport of contaminants, and indirectly or directly, the fate
of the contaminants. It is useful to remember that except for liquid chemicals, water is
the primary carrier or transport agent for contaminants. The liquid phase of a soil-water
system—i.e., the porewater—consists of water and dissolved substances such as free salts,
solutes, colloidal material, and/or organic solutes. All dissolved ions, and probably all dis-
solved molecules are to some extent, surrounded by water molecules.
The questions relating to what happens to contaminants in the ground are perhaps the most
critical concerns at hand. Will they eventually disappear? How long will they likely stay
in the ground? Will they move to other locations or be transported? Will they be harmful
to human health and the environment? To address the questions, it is necessary to obtain
an understanding of the partitioning mechanisms (i.e., the chemical mass transfer of con-
taminants from the porewater to the surfaces of the soil solids), and how these are related
to the soil and contaminant properties. A detailed consideration of these will be found
in Chapters 9 and 10. For this section, we will highlight some of the main elements of the
interactions as they relate to the mass transfer of dissolved solutes in the porewater (i.e.,
partitioning of dissolved solutes).
2.6.1 Mechanisms of Interaction of Heavy Metal Contaminants in Soil
The processes of transfer of metal cations from the soil porewater can be grouped as follows:
• Sorption : This includes physical adsorption (physisorption), occurring principally
as a result of ion-exchange reactions and van der Waals forces, and chemical
adsorption (chemisorption), which involves short-range chemical valence bonds.
The general term sorption is used to indicate the process in which the solutes (ions,
molecules, and compounds) are partitioned between the liquid phase and the soil