Environmental Engineering Reference
In-Depth Information
where the winters can be cold and harsh. The picture on the left shows the structure of
the soil obtained from a core sample to consist of small microstructural units apparently
uniformly distributed in the cross section. The right-hand picture shows the same soil
in the thawed state after 32 cycles of freezing and thawing (Yong et al., 1984). The dra-
matic increase in sizes of the microstructural units is obvious, testifying to the marked
decrease in surface areas presented to a permeating luid and also testifying to the signii-
cant increase in void spaces. Transmission or transport through the soil material shown on
the right-hand side will be considerably facilitated by the freeze-thaw effect. The lesson to
be learned from the pictures in Figure 10.7 is that environmental effects can alter the ini-
tial conditions to such an extent that design of mitigation and treatment procedures must
anticipate such events.
10.5.2 Chemical Properties
The chemical properties of signiicance include those that promote ion exchange, sorption,
and precipitation of solutes in the luid phase (including porewater) in the soil, and com-
plexation. These are properties that are more appropriately deined as soil-water system
properties. These have been discussed briely in Chapters 2 and 9 in respect to partitioning
processes involving heavy metals. To fully utilize soil as a resource material for manage-
ment of waste leachate streams and contaminants, a broader discussion of the important
chemical properties, and interactions between contaminants and soil particles or fractions
is needed.
10.5.2.1 Sorption
As discussed in the previous chapter, sorption processes involving molecular interactions
are (a) coulombic in nature, (b) interactions between nuclei and electrons, and (c) essen-
tially electrostatic in nature. The major types of interatomic bonds are ionic, covalent,
hydrogen, and van der Waals. Ionic forces hold together the atoms in a crystal. The various
types of bonds formed from various types of forces of attraction include (a) ionic bonds,
i.e., electron transfer between the atoms that are subsequently held together by the oppo-
site charge attraction of the ions formed, (b) covalent bonds developed as a result of elec-
tron sharing between two or more atomic nuclei, and (c) coulombic bonds developed from
ion-ion interaction.
For interactions between instantaneous dipoles, we have the three types of
van der Waals
forces: Keesom, Debye, and London dispersion forces. Bonding developed by van der
Waals forces is, by and large, the most common type of bonding between organic chemi-
cals and mineral soil fractions. Electrical bonds can be formed between negatively charged
organic acids and positively charged clay mineral edges. Sorption of organic anions can
occur if polyvalent exchangeable cations are present. The polyvalent bridges formed will
be due to (a) anion associated directly with cation or (b) anion associated with cation in the
form of a cation bridge (water bridge).
10.5.2.2 Cation Exchange
Cation exchange involves those cations associated with the negative charge sites on the
soil solids, largely through electrostatic forces. Ion-exchange reactions occur with the
various soil fractions, i.e., clay minerals and non-clay minerals. This process, which
has been discussed in detail in Chapter 9, is set in motion because of the need to satisfy
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