Plant Control on the Fate of Common Groundwater Contaminants - Introduction to Phytoremediation of Contaminated Groundwater - page 332

contamination. Other chlorinated solvents may be present, as
well as co-contaminants such as petroleum hydrocarbons.
For many laboratory studies of the interaction between
plants and groundwater contaminants, such as TCE, the
contaminant interaction with the plant is studied by itself,
rather than as a mixture, such as has been done to examine
the TSCF of a particular contaminant (see Chap. 12). Sorp-
tion of VOCs such as TCE has been modeled as being a
linear response. However, nonlinear responses have been
observed in the laboratory for a mixture of TCE and TCA
(Graber et al. 2007). TCE added to seedlings and to wood
from a mature Eucalyptus camaldulensis was observed to
follow a Langmuir isotherm. In all cases, less TCE was taken
up into the tree tissue at a given TCE concentration if
another contaminant was present. This study also pointed
out that TSCFs derived from a single solute may not be
representative of sites where a mixture of contaminants has
been released (Graber et al. 2007).
For PCE, James et al. (2009) investigated the fate of PCE
in water added to hybrid poplars growing in a controlled
field-scale experiment. At the end of the treatment up to 99%
of the added PCE was reduced and free chloride recovered in
near an equal amount.

suggested was due to abiotic binding to plant cell material.
They reported that this TCE residue has important
implications for the phytoremediation of TCE, because the
bound residue might be perceived as being more desirable
than gaseous TCE release to the atmosphere through leaves.
This may not necessarily be the case, however, due to the
rapid destruction reactions that can affect TCE in the atmo-
sphere. Storage of TCE metabolites in plant tissue has a
minimal effect on overall TCE taken up by plants, however.
Shang et al. (2001) reported that the TCE-intermediate
trichloroethanol is glycosylated in poplar trees, and if the
TCE source is removed, the accumulation of trichlor-
oethanol does not occur, suggestive of additional plant met-
abolic capability.
Strand et al. (1998) continued the investigation of the
plant-mediated mineralization of TCE. They report that for
axenic tissue cultures of poplar-tree cells exposed to TCE,
carbon tetrachloride (CCl 4 ), and PCE, the complete mineral-
ization of the contaminants to CO 2 occurred. At the field
scale, TCE and CCl 4 were added to containers that held soil
and poplar trees. The removal of TCE and CCl 4 approached
95% of that added relative to no loss from control containers
with no plants.
Experiments to trace the fate of TCE also were performed
using whole plants in PVC pipes grown in the greenhouse.
Each pipe contained of a plant, soil at the top, and a sand
layer at the bottom to which water was added through a
small-diameter pipe. To some of the plant-pipe setups was
added a solution of 50 ppm TCE in water. Poplar trees have
been show to survive when grown in water that contained
50 ppm TCE (Gordon et al. 1997). The plants exposed to
TCE did not suffer toxic effects but were somewhat affected
morphologically. First, the plants exposed to TCE grew to
85% of the height of plants not exposed to TCE. Second,
there was a demonstrable decrease in the amount of fine root
material in the sand layer to which TCE-laden water was
added relative to that in the plants that received water only.
TCE was found in the stems to a greater extent than the
leaves, and root material also contained TCE. The TCE
concentration in leaf tissue was lower than that found in
the stems, in part because TCE was released into the atmo-
sphere near the leaves, as monitored using polyethylene bags
that contained a charcoal filter placed around leaves.
Amounts of TCE detected in the leaf-bag charcoal after
desorption with pentane ranged from 0.053 to 0.811

13.5.2 Plant Transformation Reactions

At the Aberdeen Proving Ground in Maryland described in
Chap. 8, tree-tissue samples collected from trees growing
above TCE-contaminated groundwater indicated the detec-
tion of trichloroacetic acid (TCAA), a breakdown product of
TCE. TCAA also was detected in a study of axenic cell
cultures dosed with TCE (Newman et al. 1997). Axenic
cultures are sterilized cell cultures that do not contain bacte-
ria and are the conventional way to observe the function of
plant cells without the interference of bacterial cells. Nodule
cell cultures, or spherical photosynthetic cell aggregates,
also accomplish the same goal.
In that study, they took axenic cell cultures of a Populus
trichocarpa P. deltoides clone and added TCE. Cells

were viable in the presence of TCE at 260 ppm TCE. The
cell cultures were used to determine the fate of the TCE, and
the intermediate byproducts of TCE mineralization, such as
dichloroacetic acid, trichloroacetic acid, and trichlor-
oethanol, were produced, similar to the fate of TCE in
mammalian livers.
To determine if complete mineralization of TCE
occurred, in addition to the production of the intermediate
compounds, 14 C-TCE was added to the cell cultures. The
production of 14 CO 2 was monitored, and between 1% and
2% of the TCE was detected as CO 2 by the end of the
experiment. Newman et al. (1997) reported that some of
the label remained as an insoluble residue, which they

g TCE.
The importance of this paper in the study of
phytoremediation of chlorinated solvents is that the experi-
mental data show that the effect on TCE concentrations was
caused by the plant directly , through the formation of TCE
metabolites in the sterilized axenic cell cultures, not indi-
rectly by microbial processes through a rhizospheric effect.
Although poplar trees contain cytochrome P-450 enzymes
under natural conditions, accelerated transformation of TCE

m