Environmental Engineering Reference
In-Depth Information
The metal concentrations in their dried foliage
for a specifi c 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
). Apart from metal tolerance, hyperac-
cumulation is thought to benefi t the plant by
means of allelopathy, defense against herbivores,
or general pathogen resistance (Boyd and Jaffré
2001
; Davis et al.
2001
). In the case of phytomin-
ing, the use of native fl ora (including local popu-
lations of hyperaccumulators) with limited
agronomic practices (extensive phytoextraction)
could be an alternative to intensively managed
crops. In situ phytoextraction of Ni by a native
population of
Alyssum murale
on an ultramafi c
site (Albania) has been reported by Bani et al.
(
2007
). Lasat et al. (
1998
) conducted a fi eld study
to investigate the potential of three plant species
for phytoremediation of a
137
Cs-contaminated
site. Approximately 40-fold more
137
Cs was
removed from the contaminated soil in shoots of
redroot pigweed than in those Indian mustard and
tepary bean. Among the plants,
Urtica dioica
found to be very effective due to its higher uptake
capacity for Cr.
Zea mays
has been showed that
high tolerance toward Cr with negligible concen-
tration in leaves. Due to its higher Cr uptake and
low biomass production of
U. dioica
, commonly
known as “stinging nettle,” it can be considered as
the right plant for remediation of Cr-contaminated
sites (Shams et al.
2010
).
The lack of success of phytoremediation is
largely related to the small biomass of most true
hyperaccumulator plants or to metal accumula-
tion by high-biomass (crop) plants being too low.
For example, while contaminant mixtures appear
to be the imperative rather than the exception at
polluted sites, metal tolerance, as well as an
effi cient metal accumulation by a given plant
species, is typically restricted to one or few ele-
ments. Moreover, high metal uptake rates in
plants as required for phytoextraction can only be
achieved if the metal activity in the rhizosphere
soil solution is sustained by rapid resupply from
the solid phase (Fitz et al.
2003
; Lehto et al.
2006
). The most studied approach is chelant-
assisted phytoextraction using ethylene diamine
tetraacetic acid (EDTA) and other artifi cial
chelants (Wang et al.
2006
).
3.2
Phytostabilization
In this technique, plants reduce the mobility and
migration of contaminated soil. Leachable con-
stituents are adsorbed and bound into the plant
structure so that they form a stable mass of plant
from which the contaminants will not reenter the
environment. Most of the organic chemical con-
taminants are lipophilic and are attracted to the
hydrophobic surfaces on organic matter, such as
humus and plant cell wall components or soil
particles (Rufyikiri et al.
2004
; Shahandeh and
Hossner
2002
). Also, plants are used to reduce
the bioavailability of environmental pollutants.
Inoculation with metal-resistant plant growth-
promoting bacteria (PGPB) can support the
establishment and improve vitality of the phyto-
stabilized crops, and detoxifi cation mechanisms
in the rhizosphere may be enhanced by inocula-
tion with microbial associates. Some plants and
microorganisms are able to precipitate metal
compounds in the rhizosphere (Cotter-Howells
and Caporn
1996
) and may provide an effective
means to reduce metal toxicity as well as metal
mobility which is termed as phytoimmobilization
(Cotter-Howells et al.
1999
). The design of phyto-
stabilization systems relates to combining differ-
ent approaches to ameliorate multiple constraints
(i.e., nutrient and water defi ciency, toxicity due to
mixed contamination) and to control their effi -
ciency in fi eld conditions (Roy et al.
2007
).
3.3
Phytodegradation
or Rhizodegradation
The breakdown of contaminants is through the
activity existing in the rhizosphere due to the
presence of proteins and enzymes produced by
the plants or by soil organisms such as bacteria,
yeast, and fungi. Rhizodegradation is a symbiotic
relationship where the plants provide nutrients
necessary for the microbes to thrive, while
Search WWH ::
Custom Search