Environmental Engineering Reference
In-Depth Information
hazard themselves. For instance, animals can eat
the metal rich hyperaccumulators and cause the
toxins to enter the food chain. If the concentra-
tion of metals in the plants is thought to be high
enough to cause toxicity, there must be a way to
segregate the plants from humans and wildlife,
which may not be an easy task. Additionally,
phytoremediation is in its infancy, and its effec-
tiveness in cleaning up various toxins compared
to conventional means of treatment is not always
known. However, with more research and prac-
tice, the practicality of using phytoremediation
should increase.
Phytostabilization aims to retain contaminants
in the soil and prevent further dispersal. Con-
taminants can be stabilized in the roots or within
the rhizosphere. Revegetation of mine tailings is
a common practice to prevent further dispersal
of contaminants. Mine tailings have been stabi-
lized using commercially available varieties of
metal tolerant grasses such as
Agrostis tenuis
cv.
Goginan
Phytodegradation involves the degradation
of organic contaminants directly, through the
release of enzymes from roots, or through meta-
bolic activities within plant tissues (Fig.
1.5
). In
phytodegradation organic contaminants are taken
up by roots and metabolized in plant tissues to
less toxic substances. Phytodegradation of hy-
drophobic organic contaminants have been par-
ticularly successful. Poplar trees (
Populus
sp.)
have been used successfully in phytodegradation
of toxic and recalcitrant organic compounds.
Phytovolatilization involves the uptake of
contaminants by plant roots and its conversion to
a gaseous state, and release into the atmosphere.
This process is driven by the evapotranspiration
of plants. Plants that have high evapotranspira-
tion rate are sought after in phytovolatilization
(Fig.
1.5
). Organic contaminants, especially vol-
atile organic compounds (VOCs) are passively
volatilized by plants. For example, hybrid poplar
trees have been used to volatilize trichloroethyl-
ene (TCE) by converting it to chlorinated acetates
and CO
2
. Metals such as Se can be volatilized by
plants through conversion into dimethylselenide
[Se(CH
3
)
2
]. Genetic engineering has been used
to allow plants to volatilize specific contami-
Fig. 1.5
Schematic model of different phytoremediation
technologies involving removal and containment of con-
taminants. (Source: Greipsson
2011
)
nants. For example, the ability of the tulip tree
(
Liriodendron tulipifera
) to volatilize methyl-Hg
from the soil into the atmosphere (as Hg
0
) was
improved by inserting genes of modified
Esch-
erichia coli
that encode the enzyme mercuric ion
reductase (merA).
Phytoextraction uses the ability of plants to ac-
cumulate contaminants in the above-ground, har-
vestable biomass. This process involves repeated
harvesting of the biomass in order to lower the
concentration of contaminants in the soil. Phy-
toextraction is either a continuous process (using
metal-hyperaccumulating plants, or fast growing
plants), or an induced process (using chemicals
to increase the bioavailability of metals in the
soil). Continuous phytoextraction is based on the
ability of certain plants to gradually accumulate
contaminants (mainly metals) into their biomass.
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