Chemical and Physical Properties That Affect the Interaction Between Plants and Contaminated Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 278

established that the necessary plant structures exist to carry
atmospheric gases to the root zone. Even then, gaseous
uptake into the cortex is one of two pathways for VOCs to
partition, with the other being into the transpiration stream of
the xylem. Moreover, some pesticides, such as the fumigants
ethylene dibromide (EDB) and dibromochloropropane
(DBCP), are designed to slowly volatilize over time after
placement in the soil in order to most efficiently decrease the
number of pests.
This partitioning also can be used to explain the transfer
of organics from the leaf water surface to the atmosphere
from inside the stomata after translocation within the plant.
As was discussed earlier, gas exchange at the stomata occurs
by diffusion and can occur in both directions depending
upon the gas concentration gradients. The gain or loss
of gases in the stomata is not simple, however, as the flow
must first overcome resistances in gas conduction, as was
discussed in Chap. 3.

C pt ¼ kC w f pom K pom þ f pw (12.8)

where f pw is the fraction of water in the plant, K pom is the
contaminant partition coefficient between plant organic mat-
ter and water, and f pom + f pw ¼ 1, and f pom is the fraction of

organic matter (OM) in the plant. As such, if k 1, then the

chemical in the external solution will be in equilibrium with
that in the plant, under conditions of passive uptake; if
k

¼

>

1, then active uptake has to be assumed (Chiou 2002).
The contaminant concentration in water can be deter-
mined if the concentration of that released is known, through
the relation

C s ¼ K d C w (12.9)

where C s equals the concentration of the contaminant in soil,
C w is the concentration in the water, and K d is the partition
factor of a compound between soil and water. But the con-
centration has to be normalized to the amount of SOM
present by

12.1.3 Water-Soil-Air-Plant-Contaminant

C som ¼ C s = f som (12.10)

The partition coefficients described above also can be
applied to understanding the interaction of subsurface
contaminants with plants. For example, the distribution of
a solute between the water and lipid phases previously
discussed provides a direct analogy with the potential fate
of the water and lipids that comprise a plant, such as lipid-
rich plant cell membranes or water in the xylem. Moreover,
the movement of solutes in the transpiration stream also will
partition onto plant membranes according to the passive
equilibrium-based distribution of the solute between water
and lipids.

where C som is the SOM-normalized contaminant concentra-
tion in soil and f som is the fraction of SOM in soil. The
contaminant uptake by plants can now be reduced to the
following

C pt ¼ kC som = ð K som Þ f pom K pom þ f pw

(12.11)

One observation of earlier researchers was that the results
presented by Briggs et al. (1982) could not be reproduced
accurately. This observation in part is explained by the
relative difference in percent composition of the water,
carbohydrates, and lipids in various plants and in different
parts of the same plant. Li et al. (2005) set up a series of
laboratory experiments to evaluate the effect of plant-lipid
content on contaminant uptake.
In an extension of this uptake research, Briggs et al.
(1982) found that the log K ow was positively related to the
uptake into and translocation throughout non-woody plants.
They looked at the uptake of different classes of organic
compounds that had different degrees of lipophilicity with
respect to the test plant barley. A useful surrogate for lipids
in water and plants is 1-octanol and is used to determine the
potential for uptake relative to water. Essentially, a chemical
that is hydrophobic will partition into the organic octanol.
Chemicals that have low to intermediate log K ow from
between 1 and 3.5 are more likely to be taken up and
translocated through a plant. Briggs et al. (1982) indicated
that peak uptake would be for those compounds with log K ow
of 1.8. These chemicals tend to have moderate water

12.1.3.1 Molecular Mass
The molecular mass of a contaminant compound also will
influence its potential for diffusional uptake by plant roots.
This is because entry to the xylem must be through
membranes, either the cellular membrane and wall after
symplastic uptake or the Casparian strip by way of
apoplastic uptake. If an organic compound has a molecular
mass less than 1,000, it can cross both of these boundaries,
assuming an osmotic or diffusion gradient is present.
Another chemical property that may be used to determine
if a chemical could be taken up by plants is the molecular
weight. Chemicals with a lower molecular weight tend to be
taken up by plants at a greater rate than chemicals with a
higher molecular weight.

12.1.3.2 Log K ow

Because natural SOM can act like octanol in natural systems,
the potential partitioning of organic contaminants onto SOM
was derived by Chiou (2002) as