Environmental Engineering Reference
In-Depth Information
the extent and the location of the contaminated
areas and assessment of the environmental risk
associated with the contamination would trigger
reaction and concerns in the local community
near or close the contaminated area. Ecosystem
disruption is not enough to trigger action by the
government. In the majority of cases when con-
tamination is 'mild' and does not cause evident
unsafe living conditions, the economic and social
responsibility of cleaning up the site or setting it
aside from food production is in the hands of the
landowner.
Most commercial soil remediation strategies
rely on the use of engineered methods such as
soil excavation (dig and haul), pump-and-treat
systems, soil washing, leaching, and soil
capping. These methods are effective and are
usually applied when the contaminated area has
commercial value (i.e. growth of urban areas)
and the soil needs to be remediated in short
time. However, these methods are prohibitively
expensive and destructive of the soil ecology
and fertility. They also generate high amounts
of waste that needs to be disposed of (Conesa
et al.
2012
).
Phytotechnologies represent a green alterna-
tive to conventional remediation methods because
they are based on the use of solar-driven biologi-
cal processes to remediate or reduce the risk of
contamination. Phytotechnologies are low cost
and gain wide public acceptance because of the
environmental benefi t they provide to the remedi-
ated areas, i.e. revegetation, decreased formation
of soil dust, reduced soil erosion, carbon seques-
tration, etc. They comprise a number of different
methods that aim at removing, extracting, trans-
forming, and immobilising contaminants using
plants and their associated root microorganism
(Pilon-Smits
2005
).
A number of authors have described in detail
the different phytoremediation technologies and
their advantages and disadvantages and their
applications (Chaney et al.
2007
; Dickinson et al.
2009
; Pilon-Smits
2005
). In this chapter, we
focus on cost-effective phytoextraction and on
the economic viability and environmental bene-
fi ts of phytoextraction as successful commercial
phytotechnology.
2
Limits of Phytoextraction
Phytoextraction is the removal of TEs from the
soil by growing plants that have the ability to take
up TEs in their above-ground biomass at high
concentrations. Harvest of the TE-rich biomass
and multiyear growth cycles of the plants may
allow the removal of the TEs from the soil to a
concentration level acceptable by the environ-
mental regulatory authority. The removed bio-
mass that has no value as bio-ore is usually
incinerated, composted, or digested to reduce the
volume and disposed in landfi lls or in hazardous
waste landfi lls. Chaney et al. (
2010
) note that
disposal of biomass represent only a disposal
cost rather than a problem in most cases except
for radionuclides.
The development of phytoextraction has
begun a few decades ago with the discovery of
hyperaccumulator plants by pioneering studies
by Robert Brooks (Brooks et al.
1977
), Alan
Baker (
1981
), and Rufus Chaney (
1983
). TE con-
centrations in the shoots of hyperaccumulator are
about 100-1,000 times higher than that found in
normal plants under most circumstances.
Specifi cally, the concentration values (in their
dried foliage) to defi ne a hyperaccumulator are as
follows: 100 mg kg
−1
of Cd, Se, and Tl; 300 mg
kg
−1
of Co, Cu, and Cr; 1,000 mg kg
−1
of Ni, Pb,
and As; 3,000 mg kg
−1
of Zn; and 10,000 mg kg
−1
of Mn when grown in its natural habitat (van der
Ent et al.
2013
). Based on this criteria, more than
500 plant taxa have been cited as 'hyperaccumu-
lators' of one or more elements including As, Co,
Cd, Cu, Mn, Ni, Pb, Se, Tl, and Zn. At present,
the approximate number of hyperaccumulators
for various elements is as follows: Ni (450), Cu
(32), Co (30), Se (20), Pb (14), Zn (12), Mn (12),
As (5), Cd (2), and Tl (2) (van der Ent et al.
2013
). Nearly 25 % of hyperaccumulators belong
to the family of Brassicaceae and the genera
Thlaspi
and
Alyssum
.
The benefi t and adaptive advantage of hyperac-
cumulators have not yet been explained, but a vari-
ety of hypotheses have been proposed. The most
popular one is the 'elemental defence' hypothesis
(Boyd
2007
) which suggests that the high amounts
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