Page 36 - Soil and water contamination, 2nd edition

Basic environmental chemistry 23

-3
where C and C refer to the concentration in two different phases [M L ] and K =
phase1 phase2
the partitioning coefficient [-]. The partition coefficient is also known as the distribution
coefficient . For partitioning among the aqueous and gas phases, the equilibrium Equation
(2.9) is known as Henry’s law and K as Henry’s law constant (K ).
H
A special case of the partition coefficient is the octanol–water partition coefficient K ,

ow
which is commonly used for organic compounds. The octanol–water partition coefficient
K is defined as the ratio between the concentration of a chemical in octanol (C H OH)
ow 8 17
and in water at equilibrium and at a specified temperature and is a measure of the tendency
of a chemical to partition itself between the aqueous phase and organic phase. Octanol is
an organic solvent used as a surrogate for natural organic matter . Non-polar, hydrophobic
organic compounds prefer octanol (large value of K ) but polar, hydrophilic organic
ow
compounds prefer water (small value of K ). Values of K for many chemicals are available
ow ow
from the literature or chemical fact sheets (e.g. EPA, 2013; ATSDR, 2013) and are useful in
the estimation of other parameters, such as the aqueous solubility and distribution coefficient
for adsorption of organic compounds to organic matter. The distribution coefficient
approach for sorption of substances to solids is discussed in more detail in the next section.
In addition, partition coefficient s are also often used to describe the partitioning of
chemicals between the various trophic level s of the food chain. The partition coefficient is
then referred to as a concentration factor for water–biota transfer or a transfer factor for soil
biota, or a food–biota transfer (Blust, 2001).
2.5.4 Partitioning between dissolved phase and adsorbed phase
When an aqueous solution of a chemical is mixed with a suspension of solids, the total

mass of the chemical equilibrates between the dissolved phase and the adsorbed phase . If

this experiment of mixing a solution with a solid medium is repeated for different initial
concentrations C at the same temperature (a so-called batch experiment), the values of
i
the concentration of adsorbed chemical on the solids C plotted against the equilibrium
s

concentration C form an adsorption isotherm . Isotherms can be linear, convex, or
w
concave, or a complex combination of these shapes. The most commonly adopted empirical
relationships are the Freundlich isotherm :
n
C K C (2.10)
s w
and the Langmuir isotherm :
Q 0 KC
C w (2.11)
s
1 KC w
-1
where C = the concentration of the chemical adsorbed to the solid [M M ], C = the
s w
-3
equilibrium concentration of the chemical in solution [M L ], K = a partition coefficient
reflecting the extent of sorption , n = an exponent usually ranging between 0.7 and 1.2, and
0
Q = the maximum sorptive capacity of the solids. Figure 13.2 shows some examples of
Freundlich and Langmuir isotherms .
A Freundlich isotherm with an exponent n = 1 is a special case, since the isotherm

becomes linear. The resulting equation is analogous to Equation (2.9) and relates the
concentration of the solids to the solute concentration, using a distribution coefficient :
C
K s (2.12)
d
C
w

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