Environmental Engineering Reference
In-Depth Information
Table 5.3 Cost advantage of phytoextraction for metals (Schnoor 1997 )
Type of treatment
Cost/m 3 ($)
Time required
(months)
Additional factors/expense
Safety issues
Fixation
90-200
6-9
Transport/excavation long-
term monitoring
Leaching
Landfilling
100-400
6-9
Long-term monitoring
Leaching
5000 m 3 minimum
Chemical recycle
Soil extraction,
leaching
250-500
8-12
Residue disposal
Phytoextraction
15-40
18-60
Time/land commitment
Residue disposal
5.3.1 Phytotransformation
Phytotransformation, also known as phytodegra-
dation, is the breakdown of organic and nutrient
contaminants present in soil and groundwater
after sequestration by plants via metabolic pro-
cesses within the plant and specific enzymes pro-
duced by the plant. The organic contaminants are
degraded into simpler compounds that are inte-
grated with plant tissue, which in turn, support
plant growth. Remediation of any site by phy-
totransformation is dependent on the efficiency
of direct uptake of contaminants from soil water
and the accumulation in form of metabolites in
plant tissue. The metabolites which are nontoxic
or significantly less toxic should be accumulated
in vegetation. Potential applications include phy-
totransformation of petrochemical sites and their
storage areas, ammunition wastes, fuel spills,
chlorinated solvents, landfill leachates and ag-
ricultural chemicals (pesticides and fertilizers).
Sometimes phytoremediation is used in combina-
tion with other approaches such as ex situ treat-
ment of highly contaminated wastes, or removal
actions or polishing treatment. Plants either di-
rectly uptake contaminants from the soil water
or release exudates that help to degrade organic
pollutants via cometabolism in the rhizosphere.
Direct uptake of organics by plants is general-
ly observed at shallow-depth contaminated sites
with moderately hydrophobic organic chemicals,
including benzene, toluene, ethylbenzene and xy-
lene (BTEX) chemicals, chlorinated solvents and
short-chain aliphatic chemicals. Hydrophobic
chemicals (log Kow > 3.5) are not easily translo-
cated within the plant due to strong bonding to
the surface of roots and soils. Chemicals which
are readily water-soluble (log Kow < 1.0) are not
Fig. 5.1 Different methods of phytoremediation are ex-
hibiting involved mechanism. (Source: http://www.per-
sonal.psu.edu/dgh5037/extEssay.html)
Conservation and Recovery Act (RCRA)-ap-
proved hazardous waste facility, and soil extrac-
tion. The only limitation of cheap and effective
phytoremediation technology is the requirement
of a long time period compared to competing
technologies (Table 5.3 ). Phytoremediation is
most comparable with in situ bioremediation
and natural attenuation.
5.3
Phytoremediation Methods
Phytoremediation consists of a collection of four
different plant-based technologies, each having a
different mechanism of action for the remedia-
tion of metal-polluted soil, sediment and water
(Fig. 5.1 ). The main processes that involve the
treatment of environmental problems by using
plants are:
 
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