Chemical and Physical Properties That Affect the Interaction Between Plants and Contaminated Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 276

12.1.1 Water-Soil-Contaminant and K ow

from the perspective of the physical and chemical
characteristics of the compounds and the phase(s) in ques-
tion. This relation can be quantified using the concept of
the degree of partitioning of a particular species between its
original phase and other phases present. The extent by which
this interaction occurs can be quantified with a partition
coefficient.
The interaction of xenobiotic solutes in groundwater
will be discussed in terms of plant and groundwater
interactions. For example, what is the degree of parti-
tioning between soil-water, roots-water, wood-water, and
leaves-air for various types of xenobiotics? Answers to
these questions and others can be examined in the frame-
work of previous work done on chemical partitioning,
such as the partitioning of a chemical between a non-
polar organic solvent and polar water, between the
dissolved phase and a gaseous phase, or between the
dissolved phase and the solid phase.
In general, processes that result in a particular species
being partitioned into at least two phases can be described in
terms of the following ratio

The potential that an organic contaminant solute will partition
into soil organic matter can be described in terms of a partition
coefficient. This coefficient is determined experimentally and
can be approximated by the extent that a particular solute is
hydrophobic and will tend to dissolve into a surrogate lipid-
phase relative to being hydrophilic and, therefore, not dissolve
into an organic phase. To determine this partition coefficient,
the organic solvent widely used as the surrogate for nonionic
(or net nonpolar) organic matter, such as lipids found in
plants, is the hydrocarbon n -octanol, also known as 1-octanol
or simply octanol. These values are determined experimen-
tally in the laboratory under controlled conditions over a
range of solute concentrations.
Most unsaturated subsurface sediments consist predomi-
nately of about 50% mineral matter, about 25% water and
air, and less than 2% organic matter, by weight. An organic
contaminant released to the subsurface will partition into the
water, air, mineral, or organic phases that are present. The
resulting partition coefficient is an estimate of the tendency
for the solute, or compound, C , to dissolve into the organic
lipid, or solvent, C o , or water, C w

K d ¼ C a = C b (12.1)

where K d is the partition coefficient (or constant) for a
compound, C , present in two phases, C a and C b . A com-
pound will have many coefficients to describe the chemical's
partitioning from its original phase into solution, into the
gas phase, into a lipid phase, and so on. Most partition
coefficients typically are determined experimentally under
controlled laboratory conditions. As with any simplification
of a complex interaction, such as in using a partition coeffi-
cient, the concept of equilibrium needs to be fulfilled by
assumptions that may not always be defensible under field
situations.
Perhaps the partition coefficient that has received the most
attention, in terms of scientific and layperson recognition, is
that related to xenobiotic bioaccumulation. Since the 1970s,
contaminants that had the potential to bioaccumulate, or
strongly partition, into the food chain were studied. Today,
the bioaccumulation of certain chemicals, such as heavy
metals, by non food crop plants is specifically engineered,
and the interaction between organic chemicals and plants that
can take up but not bioaccumulate these chemicals also is
engineered. As we will see later in this chapter, plants possess
the ability to decrease the threat of chemical bioaccumulation
in their tissues similar to the manner in which the mammalian
liver provides chemical detoxification.
For these processes to occur, the chemicals must first
enter the plant from the surrounding environmental media.
The various partitioning that occurs in the subsurface to
control this uptake are described first.

K ow ¼ C o = C w (12.2)

where K ow is the partition coefficient (or constant) that
describes the magnitude of the chemical's affinity for
partitioning into either the nonpolar octanol phase, as C o is
the solute concentration in the octanol phase (kg/m 3 ), or water
phase, as C w is the solute concentration in the water phase (kg/
m 3 ). K ow is a surrogate for the water solubility of a particular
chemical; a higher value of K ow implies that the solute is more
likely to be dissolved in the octanol or lipid phase and, there-
fore, have a lower water solubility, and a lower value of K ow
indicates the solute is less likely to be in the octanol phase and
more likely to be dissolved in the water phase.
The contaminant solubility in water is proportional to
the contaminant's mass and can be approximated by the
partition coefficient between the organic phase and water
phase, K ow . By definition, a K ow of 1 indicates that a solute
will partition equally between the octanol phase and the
water phase. A K ow greater than 1 indicates that the solute
will partition more into octanol phase. For example, a K ow
of 50 indicates that the compound is 50 times more likely
to partition into octanol than water. For contaminants
released to the environment, the greater extent that a con-
taminant dissolves into a contaminant mixture, the higher
the K ow and the lower its solubility in water. How this
partition coefficient can be used in the phytoremediation
of contaminated groundwater is discussed in a following
section.