Chemical and Physical Properties That Affect the Interaction Between Plants and Contaminated Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 277

12.1.2 Water-Soil-Air-Contaminant,
and Henry's Law

The octanol-water partition coefficient is such a widely
used concept in chemistry as well as geochemistry that a
little history is warranted. In the 1970s, the correlation
between a chemical's K ow and its toxicity and potential for
bioaccumulation in crop plants was studied and showed to
correlate strongly with changes in biological activity if plot-
ted against the log transform of K ow (Leo et al. 1969; Hansch
and Leo 1979)— K ow can range up to values over 5,000, so
the log transform lowers the values and makes it easier to
compare contaminant-environment interactions. With
respect to the uptake of chemicals by plants, the parameter
of log K ow is considered the “gold standard” for researchers
tasked with contaminant fate studies in various aquatic and
terrestrial ecosystems. As might be anticipated, much of this
early work was done on pesticides.
The concept of the partitioning of an organic contaminant
compound between the organic phase, the solid phase, and
the water phase has been used to determine the extent of
bioaccumulation. Bioaccumulation is a measure of the rela-
tive accumulation of the compound present in water into
environmental media. The solid phase can be the organism
in question, such as fish. In fact, much of what is known
today about the interaction between various chemicals
dissolved in water and organic matter is derived from the
early work into the bioaccumulation potential of these
compounds in fish.
The removal of a contaminant solute from the aqueous
phase onto a solid immobile phase is called sorption. Sorp-
tion, the transfer of mass from a liquid phase to a solid phase,
can occur by adsorption, absorption, and ion exchange,
where adsorption is the interaction with the surface chemis-
try, absorption is the interaction with the bulk chemistry, and
absorption that is exchangeable is called ion exchange.
Sorption in colloidal material can result in a mobile phase,
however.
The partition coefficient, or soil-water distribution coeffi-
cient, K d , is where

The above partition coefficients are important to consider
when the contaminant solute is present in the dissolved
phase, such as occurs during many groundwater contamina-
tion events. Many organic contaminants also can exist in the
vapor phase, and this section describes the partitioning for
such solutes into a vapor phase.
The extent of this phase change is determined in part
by the chemical's vapor pressure, the concentration gradient
between water and soil and air that often is driven by
diffusion, its water solubility, and its potential to be sorbed
onto aquifer media. There will be an equilibrium established
between the phases of contaminant-water-soil-air, such as is
the case with the first three discussed above. The equilibrium
concept is important, because it reveals that as the gases are
evolving, they also can reenter the solution at an equal rate
once equilibrium concentrations are reached.
Under conditions of equilibrium, the partitioning of a
particular contaminant compound between itself in the
liquid phase and gas phase can be thought of as

H ¼ G = A

(12.6)

where H is the Henry's Law partition coefficient (Pa m 3 /
mol), G is the contaminant concentration (or partial pres-
sure) in the vapor phase, and A is the concentration, or
solubility, of the contaminant in the aqueous phase. The
Henry's Law partition coefficient

is dimensionless. The

air-water partition coefficient, K aw is

K aw ¼ H = ðÞ¼ RT C a = C w (12.7)

where C a is the contaminant's equilibrium concentration in
air, C w is the equilibrium concentration in water, R is the
universal gas constant (8.314 J/mol/K), and T is the tempera-
ture (K). Studies have shown that if the K aw is greater than
10 4 (dimensionless), the chemical is more apt to be found in
the gas phase rather than in the soil or water, K aw between
10 4 and 10 6 , the chemical can be present both in the air
and water, and for K aw less than 10 6 , uptake would be by the
water phase. The degree of volatilization is dependent upon
the vapor pressure as well as the concentration in water.
Plant roots that reach the water table must do so through
at least some thickness of unsaturated zone. The plant roots
share the pore spaces with water and air. Because VOCs
dissolved in water also can have high vapor pressures and,
therefore, exist in the vapor phase in near-surface
environments, the possibility exists that the vapor phase
will be taken up by plant roots. This uptake could occur
with the unsaturated zone or the water table being the source
of

K d ¼ C s = C w (12.3)

where C s is the concentration in the soil and C w is the
concentration in the water. This partition coefficient, K d ,
also is related to the amount of soil organic matter (as %
SOM) by

K d ¼ K om % ð SOM = 100 Þ (12.4)

where K om is determined from the log K ow of the particular
contaminant because K d tends to be proportional to the
lipophilicity of the contaminant. The soil sorption constant,
K oc , is defined as

K oc ¼ ð K d =% organic carbon Þ 100 (12.5)

the VOCs (Lahvis et al. 1999). We have already