Environmental Engineering Reference
In-Depth Information
microbes provide an enhanced healthy soil
environment. Contaminated soils that have
undergone prolonged periods of ageing generally
appear to be much less responsive to rhizodegra-
dation than fresh soil (Chiapusio et al.
2007
;
Child et al.
2007
). Characterizing root exudation
in terms of chemical composition and quantity
and investigation of utilization pattern by micro-
bial strains competent to degrade the pollutants
are important requirements for this purpose. In
long-term fi eld-contaminated soil, enhancement
of bioavailability appears to be the key of suc-
cessful biodegradation.
wetlands and estuary areas. It is defi ned as the
use of plants, both terrestrial and aquatic, to
absorb, concentrate, and precipitate contaminants
from polluted aqueous sources in their roots.
It remediates metals like Pb, cadmium (Cd), Ni,
Cu, Cr, vanadium (V), and radionuclides [ura-
nium (U), cesium (Cs), strontium (Sr)]. The ideal
plants should produce signifi cant amounts of root
biomass or root surface area that could accumu-
late and tolerate signifi cant amounts of target
metals, involve easy handling and a low mainte-
nance cost, and have a minimum of secondary
wastes that require disposal (Dushenkov and
Kapulnik
2000
). Sunfl ower, Indian mustard,
tobacco, rye, spinach, and corn have been studied
for their ability to remove Pb from water, with
sunfl ower having the greatest ability. Indian mus-
tard has a bioaccumulation coeffi cient of 563 for
Pb and has also proven to be effective in removing
a wide concentration range of Pb (4-500 mg/L)
(Raskin and Ensley
2000
).
3.4
Phytovolatilization
It involves the use of plants to take up contami-
nants from the soil, transforming them into
volatile forms and transpiring them into the
atmosphere. Some metal(loid)s like arsenic
(As), mercury (Hg), and selenium (Se) may
exist as gaseous species in the environment. The
contaminant available in the water taken up by
the plant passes through the plant or is modifi ed
by the plant and is released to the atmosphere
(evaporates or vaporizes). Some naturally occur-
ring or genetically modifi ed plants, like
Brassica
juncea
(Indian mustard) and
Arabidopsis thali-
ana
, are reported to possess capability to absorb
heavy metals and convert them to gaseous spe-
cies within the plant and subsequently release
them into the atmosphere (Ghosh and Singh
2005
). Phytovolatilization has been used pri-
marily for the removal of Hg, where in mercuric
ion is transformed into the less toxic gaseous
elemental Hg (Ghosh and Singh
2005
). Some
plants growing in high Se media, e.g.,
A. thali-
ana
and
B. juncea
, produce volatile Se in the
form of dimethylselenide and dimethyldisele-
nide (Bañuelos
2000
).
4
Chelators
in Phytoremediation
The chemical amendments such as synthetic
organic chelates can enhance phytoremediation
effi ciency by increasing heavy metal bioavail-
ability in soil thus enhancing plant uptake and
translocation of metals from the roots to the aer-
ial parts of host plants (Purakayastha et al.
2008
).
Among the chelates, EDTA was often found to be
the most effective (Zaier et al.
2010
). Huang et al.
(
1997
) found that among different chelating
agent, EDTA is more effective in the accumula-
tion of Pb in corn and pea and also found that on
increasing the concentration of EDTA, accumu-
lation effi ciency of Pb in the shoot of corn and
pea was also increased. Therefore, the potential
risks of use of EDTA or other chelators for phy-
toremediation should be thoroughly evaluated
before taking steps toward further development
and commercialization of this remediation tech-
nology. The use of chelates as soil amendments
to increase the bioavailability of metals has raised
some concern over the potentially increased
mobility of the metal-chelate complex in the soil.
3.5
Rhizofi ltration
Rhizofi ltration is a water remediation technique
that involves the uptake of contaminants by plant
roots, used to reduce contamination in natural
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