Plant Control on the Fate of Common Groundwater Contaminants - Introduction to Phytoremediation of Contaminated Groundwater - page 325

mandate for fuels, alternative additives such as ethanol have
begun to replace the MTBE in fuels. One of the more
popular choices is ethanol derived from the fermentation of
plant material such as corn. Little information exists as to the
interaction of ethanol with plants, but ethanol was found to
be transpired by willows ( Salix babylonica ) under laboratory
hydroponic conditions, as well as removed from solution
after root accumulation (Corseuil and Moreno 2001).
Similarly, the interaction of EDB with plants is not well
known. This is unfortunate, considering the extremely low
maximum contaminant level for EDB in water of 0.05

are needed for other purposes such as their solvent
action or their heat-absorbing qualities. The synthesis of
chlorinated hydrocarbons provided a chemical whose
qualities make it useful for meeting these needs. The
characteristics of chlorinated hydrocarbons are derived
from its synthesis, from the substitution of Cl atoms for the
H in the C H of saturated hydrocarbons. This process par-

tially oxidized the originally reduced compound, which
renders it greater stability in the presence of oxygen. This
is why chlorinated hydrocarbons are used for purposes
where a low-flammability product is needed, such as the
polychlorinated biphenyls (PCBs) used in electrical
transformers and as fire suppressors.
The polychlorinated biphenyls contain chlorine atoms
substituted on biphenyl rings in numerous combinations
called congeners. Isomers of PCB exist when the same
number of chlorine atoms exist on a ring but are located in
different positions. Their synthesis in the early 1930s was
heralded as a chemical breakthrough, because they resisted
oxidation and could be used as heat removal solutions in
electrical transformers and flame retardants. Their ability to
resist combustion is due to the many chlorine atoms added to
the organic molecule, as previously described. This substitu-
tion renders them already partially oxidized, so that they are
more prevalent to be reduced and accept electrons rather
than donate electrons through oxidative reactions. These
compounds also were found to bioaccumulate and were
later detected throughout the environment. In the lab,
PCBs have been shown to cause mutations. In response,
their use was banned in 1979 in the United States under the
Toxic Substances Control Act (TSCA).
Another quality of chlorinated hydrocarbons is their abil-
ity to be used as a solvent and this is perhaps the most widely
used means for these compounds, as degreasers and
cleaners. The dry-cleaning industry uses these compounds
rather than use soap and water to remove organic-based
stains from materials. These compounds also are used by
the tool-and-die, semiconductor, and commercial print
industries to remove organic compounds from metal
surfaces. Due to its widespread use and chemical stability
in the presence of atmospheric levels of oxygen, it is not
surprising that it is one of the most common pollutants of
soil, groundwater, and surface water at hazardous waste
sites.
Some of the most common chlorinated solvents found in
the environment are perchloroethylene (PCE) and trichloro-
ethylene (TCE; Moran et al. 2007).They may be readily
detected in shallow groundwater because they are stable
in the presence of oxygen, and dissolved oxygen con-
centrations are typically higher in shallower groundwater.
Because they also have a specific gravity greater than water,
for example TCE is 1.46 at 20 C, the release of pure-phase
free product will tend to move through groundwater in

g/L.
However, some of its physical and chemical characteristics
suggest that a plant-based remediation strategy may be fea-
sible (Table 13.3 ). A review paper by Davis and Erickson
(2002) sheds some light on important plant and contaminant
interactions that may occur at sites characterized by EDB-
contaminated groundwater, either from use as a fuel additive
or as a soil fumigant. One of the rate-limiting steps of EDB
transformation is its slow diffusivity in water relative to air.
Trees that can transpire groundwater or vadose zone water
may accelerate EDB loss by increasing the percent air space
in the contaminated soil or aquifer. However, the high solu-
bility of EDB in water often rapidly removes it from near the
water-table surface and, therefore, extensive plumes of EDB
often are found in contaminated areas also characterized by
permeable sediments, oxic conditions, and high recharge
rates.

m

13.4.2 Plant Transformation Reactions

The fate of MTBE that is not transpired in plant cells is not
entirely known (Newman and Arnold 2003). Typically,
the transformation of ethers like MTBE would follow an
oxidation pathway of Phase I detoxification. For example,
MTBE can undergo o -dealkylation reactions and can be
hydroxylated to tert -butoxymethanol and then tert -butyl
alcohol and formaldehyde. This transformation is problematic
because of the increased toxicity of these MTBE-transformation
intermediates, also formed anaerobically by bacteria in
groundwater (Bradley et al. 2001). Newman et al. (1998)
reported that MTBE could be metabolized by cell cultures of
hybrid poplars, albeit at a low percentage of total MTBE
added.

13.5 Plant Interactions with Chlorinated

Hydrocarbons and Solvents

Many of the petroleum-derived compounds are high-energy
compounds—they are saturated hydrocarbons, in which car-
bon is reduced with hydrogen ( H). This molecular

quality makes them excellent fuel stock. But compounds

C