Plant Control on the Fate of Common Groundwater Contaminants - Introduction to Phytoremediation of Contaminated Groundwater - page 309

done in glass flasks filled with water and air and a test plant
to which radiolabeled pesticides were added and the fate
tracked over time.
Perhaps the first interface between the interactions of
plants used for phytoremediation, or hybrid poplars, with
chemicals that were agricultural in use but are similar to
those released to groundwater can be traced to a study by
Burken and Schnoor (1997). They presented one of the
first reports of the uptake and metabolism by plants of
2-chloro-4-(ethylamine)-6-(isopropylamine)- s -triazine, more
commonly known as atrazine. Plant uptake of atrazine was
hypothesized due to its log K ow of 2.56.
What was interesting about their study, however, was the
fate of the atrazine in the plants after uptake. To evaluate the
fate of the translocated atrazine in the hybrid poplar trees, it
was added as 14 C-atrazine. Atrazine was taken up near 30%
over 80 days for poplar trees grown in soil, and 71% in 13
days for poplar trees grown in sand alone. The amount of
atrazine radiolabel present as nonextractable, unbioavailable
residue was 8.4% for cuttings in sand and near 16% for
cuttings in soil. These bound residues are most likely less
toxic than atrazine itself, but more importantly from the
perspective of contaminant risk exposure, are rendered
unbioavailable. Extraction and analysis of the poplar
cuttings revealed that the balance of the atrazine underwent
transformation to various metabolites from Phase I detoxifi-
cation reactions such as dealkylation and hydrolysis.

to decrease to half its original amount. Mathematically, it is
expressed as

t 1 = 2 ¼ = l

ln 2 (13.2)

The half-life of a compound is controlled by many
factors. One factor is the ability for the contaminant to be
degraded in the presence of various enzymes in the plant or
to be excreted or stored in the plant. These enzymatic
reactions are the most important aspect of the detoxification
and degradation of a xenobiotic with respect to phyto-
remediation. These enzymes are usually not consumed in
the reaction, but aid to catalyze the reaction to completion,
usually at much lower temperatures and at higher rates than
would be available without the enzyme. Degradation by
these processes often results in the conversion of the con-
taminant back to the original photosynthetic reactants of
CO 2 and water. In some cases, however, intermediate
compounds are formed that are more resistant to further
degradation than the parent compound.
The fact that some herbicides need to be applied to
different parts of a plant implies that the uptake of
xenobiotics occurs in different parts of the plant and suggests
different contaminant detoxification pathways. Chemicals
applied to the leaves are taken up by the plant through
absorption and translocated within the plant by the phloem.
Conversely, chemicals applied to the soil are taken up by the
roots and translocated by the xylem. Others are applied to
either area but are moved throughout the entire plant by the
symplast, such as leaf-applied chemicals, or the apoplast,
such as the soil-applied chemicals or the systemic
herbicides. Herbicides that move through the plant through
the phloem also can move to the roots, however, if applied
on the leaves or shoots. On the other hand, herbicides that
move primarily in the xylem move to the leaves if applied on
the leaf, or if applied to the roots move throughout the plant
after entry into the cortical tissues. These differences in the
uptake and translocation of herbicides provide an important
analogy into the potential for environmental pollutants
released to groundwater to be taken up by plants during
phytoremediation.

13.1.1 Contaminant Half-Life Concept

A characteristic that can be used to describe the relative
degree that a herbicide, or any other xenobiotic, may
bioaccumulate in plants is the concept of a half-life.
Contaminants present in plant tissues are exposed to the
living processes of the plant and, therefore, can undergo
the Phase I, II, and III detoxification reactions described in
Chap. 12. Generally, these reactions result in a decreased
contaminant concentration in planta , whose kinetics follow
first-order, concentration-dependent kinetics, where the rate
of detoxification is directly proportional to the contaminant
concentration. Such first-order kinetics are analogous to the
variable flow rate of water from a pipe stuck into the bottom
of a water tank relative to the amount of water in the tank,
where the rate of flow is faster when the water level is high
and slower when the water level is low. These kinetics can
be shown as

13.1.2 Contaminant Bioavailability

For most contaminants, once they are released to the envi-
ronment, they are no longer considered to be in a stable
system. In the case of petroleum hydrocarbons, an increase
in time since release will result in a decrease in contaminant
bioavailability. This is because the contaminants are
exposed to various biotic and abiotic processes that act to
remove the more soluble and volatile fractions from the

d C = d t ¼ l C (13.1)

where d C /d t is the change of concentration over time, and l

is the reaction rate constant. The half-life, t 1/2 , in general
terms, is the time required for a compound added to a system