Environmental Engineering Reference
In-Depth Information
contaminant functional group following Phase I reactions
with the
The presence of natural toxins such as cyanide in plants is
widespread, as was introduced in Chap. 11. For example,
more than 1,000 plant species contain CN, including com-
mon plants, such as apples, whose seeds contain cyanide.
These toxins probably developed as a selective advantage to
predation by herbivores. The various tissues of cherry trees,
as well as peach trees in the same genus, contain amygdalin.
Similarly, oaks (
Quercus spp.
) contain phenolic
compounds, often collectively called tannins. Concentrations
of these compounds are highest in green seeds (acorns) and
young leaves. As discussed previously, black walnut (
Juglans
nigra
) contains the phenol juglone in the bark, wood, nuts,
and roots. The common landscape plant privet (
ligustrum
spp
.) contains glycosides. Essentially, a rule of thumb is that
if a plant doesn't seem to have any blemishes, such as holes or
rough edges, it probably is a species that contains defensive
toxic compounds, such as glycosides.
Although these compounds can be found in most parts of
plants, the predominant location of storage in seeds seems to
do more with inhibiting seed germination until conditions
are right rather than thwarting ingestion. Other parts of the
plants, once dead and fallen, also can release these toxins to
inhibit the germination of other plants in an allelopathic
manner.
The interaction of trees with cyanide also can occur when
cyanide has been released to the environment as a contami-
nant. Sources of cyanide include manufacturing activities,
such as electroplating. Blacksmiths, for example, use ferricy-
anide to harden iron. The surface soils and unsaturated zone
sediments at many former MGP sites often contain cyanide.
Trees have been shown to take up cyanide into their tissues.
Following uptake, the cyanide is either stored or metabolized.
At a former MGP site in South Carolina, hybrid poplar trees
installed as part of a phytoremediation system and were
growing over a plume of PAH-contaminated groundwater
that also had CN were observed by the author to contain
blue annual growth rings after being cut down, blue being
associated with many compounds that contain CN.
SH group of the glutathione cysteine. In fact,
evidence indicates significant DNA homologies of these
enzymes from bacteria to mammals, including man. Essen-
tially, many xenobiotics in the oxidized form tend to react
with genetic materials, like DNA and RNA. Glutathione is
hydrophilic, and conjugations of xenobiotic compounds that
are more hydrophobic render these compounds more soluble
in water. This is a protective mechanism that allows less
exposure time of the xenobiotic to the animal or plant.
These processes are not inducible, and remain in effect
continuously.
It is possible for additional cell metabolism of the
transformed xenobiotic to occur, or transfer to the external
plant environment, such as the rhizosphere or atmosphere
(Schr
oder and Collins 2002). In essence, in some cases the
conjugated xenobiotic can be re-released, almost as an alle-
lopathic agent. This fact of conjugated xenobiotic release
has some interesting consequences. In the early summer of
2001, for example, in the horse country of Kentucky, more
than 500 foals were stillborn or died after delivery, and many
of the foals born alive had respiratory problems. Researchers
initially thought that fungal spores in the grass were the
causative agent. That year, however, also was a year of
high numbers of eastern tent caterpillars, which just happen
to like to forage on the leaves of local cherry trees (
Prunus
spp.
). Cherry trees, like many other plants, contain toxic
substances, including cyanide (CN) in their leaves. Cyanide
is extremely toxic; between 50 and 70 mg (0.0025 oz) in
air
has a 50% chance of causing death to an average man, and it
has no odor to warn of exposure. In humans, ingestion of
1 mg CN/kg/day will result in death. Cyanide poisoning
occurs through blocking the binding of oxygen to hemoglo-
bin in red blood cells during the initial point of electron
transport. No electron transport means no ATP production
and, therefore, no energy for growth.
The cyanide is not necessarily harmful to the plants,
because it is conjugated with sugars to form a cyanogenic
glycoside, compartmentalized in vacuoles or seeds, as amyg-
dalin. In some plants, such as willows, death will occur only
after exposure to 200 mg CN/kg/day. One of the
explanations is that plants can take the CN ion and make
asparagines from it. This can occur as long as the uptake rate
is
less
than the rate of CN metabolism. Once the uptake rate
is
greater
than the metabolism rate, however, accumulation
of CN occurs and toxicity results. When these plant parts
that contain the cyanogenic glycosides are eaten and burst
open amygdalin will hydrolyze to hydrocyanic acid. This
process effectively renders animals that eat such leaves, such
as eastern tent caterpillars, a potent source of cyanide poi-
soning, and for the horses which came into contact when the
leaves entered water troughs, etc., and became ingested by
the pregnant horses.
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12.4.2.2 Volatilization
Some compounds released to groundwater have the physical
and chemical properties to move unattenuated though plants
after uptake and be volatilized to the atmosphere. This
movement of a contaminant unaffected through a plant is a
form of phytoremediation, because the half-life of the con-
taminant once in the atmosphere will be considerably short-
ened by photooxidation, increased oxygen concentration,
etc., relative to a longer half-life in anoxic groundwater.
Volatilization of contaminant compounds from groundwater
through plants is a logical extension of the rationale behind
Phase I and II processes: production of a byproduct that has
an increased solubility, in this case air, for elimination. In
fact,
this process of contaminant volatilization may be
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