Environmental Engineering Reference
In-Depth Information
species because of the lower hydration energies and higher positive charges and stronger
interactive electrostatic forces.
The unsymmetrically shared electrons in the double bond endow carbonyl compounds
with dipole moments, thus allowing for hydrogen bonding between the OH group of the
adsorbent (soil particles) and the carbonyl group of the ketone or through a water bridge.
Sorption onto soil particles, especially clays, for the carbonyl group or organic acids (e.g.,
benzoic, acetic acids) occurs directly with the interlayer of cation or by the formation of
hydrogen bonds with the water molecules (water bridging) coordinated to the exchange-
able cation of the clay complex.
2.6.3 Partitioning of Contaminants and Partition Coefficients
Section 9.5 in Chapter 9 provides a more detailed discussion of contaminant partitioning
in transport through a soil. The introduction to this subject in this section provides an
overall appreciation of “what happens” when waste leachate streams enter the subsurface
soil that constitutes the land environment. The partitioning of contaminants refers to the
transfer of contaminants from the porewater in the soil to the soil solids by processes that
include all of those described in Sections 2.6.1 and 2.6.2. It is important to determine the
partitioning of target contaminants because this will tell us something about the distri-
bution of contaminants, i.e., contaminants sorbed by the soil particles and contaminants
remaining in the porewater. This can be interpreted in terms of the quantity or proportion
of contaminants likely to move from one location to another. Determination of partition-
ing of inorganic contaminants is generally conducted using batch equilibrium tests. The
conventional procedure shown in Figure 2.14 uses soil solutions. The candidate soil is used
with an aqueous solution consisting of the contaminant of interest to form a soil solu-
tion. Figure 2.14 shows the various steps and analyses required. Results obtained from the
tests and graphically plotted as shown in the diagram are called adsorption isotherms.
Graphical representation of these results show sorbed concentration (of contaminants or
contaminants) on the ordinate and equilibrium concentration of contaminants (contami-
nants) on the abscissa, as seen at the bottom right-hand part of Figure 2.14. The three
common types of adsorption isotherms (Freundlich, Langmuir, and Constant) found in
reported literature on sorption characteristics of inorganic contaminants are shown in
Figure 2.15. The parameter k n in the equations shown with the various curves denotes the
slope of the curves.
In the case of organic chemicals, partitioning is denoted by an equilibrium partition coef-
icient k ow (i.e., coeficient describing the ratio of the concentration of a speciic organic con-
taminant in other solvents to that in water). This coeficient k ow , which relates the water
solubility of an organic chemical with its n -octanol solubility is more correctly referred
to as the n -octanol-water partition coeficient. It has also been found to be suficiently
correlated to soil sorption coeficients. The relationship for the n -octanol-water partition
coeficient k ow has been given in terms of the solubility S by Chiou et al. (1982) as
log k ow = 4.5 − 0.75 log S (ppm).
Organic chemicals with k ow < 10 are generally considered to be relatively hydrophilic.
They tend to have high water solubilities and small soil adsorption coeficients. Organic
chemicals with k ow > 10 4 have low water solubilities and are considered to be very hydro-
phobic. The greater the hydrophobicity, the greater is its bioaccumulation potential (Section
9.5 in Chapter 9).
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