Environmental Engineering Reference
In-Depth Information
quantity among the four plant species, irrespec-
tive of the clipping or soil type. Calculation of
recovery percentage based on Cr removed from
the soil after cultivation ranged between 3.7 and
40.6 % of total initial Cr. The highest values were
noticed in case of clover and canola. Diwan et al.
(
2008
) also reported the ability of Pusa Jai Kisan
genotype of Indian mustard to grow in the pres-
ence of high Cr(VI) levels in the hydroponic as
well as natural environmental conditions, and
the amounts of Cr concentrated in the aerial part
of this plant indicate that there is great potential
for its use in the remediation of Cr-contaminated
sites. On the other hand
Azolla
(an aquatic water
fern) biosystem has already been proven to be a
potent tool for biofiltration of various toxic met-
als. In a report by Rai (
2008
), high increase of
the metal content in the biomass suggests that
Azolla pinnata
has tremendous potential to take
up Cr(III) and Cr(VI) (70-88 %) and may be
used as a bioaccumulator to polish heavy met-
als in ash slurry, coal mines, and tannery efflu-
ent. The concentration of metals in the
A. pin-
nata
biomass was directly related to that of the
solution. Aquatic macrophytes can also be a good
remediation option, and duckweeds have proven
to be promising prospective scavengers of heavy
metals from polluted waters (Zurayk et al.
2001
;
Zhang et al.
2007
).
Lemna gibba
and
Lemna
minor
L. are the most studied species for phy-
toremediation (Mkandawire and Dudel
2005
).
They exhibit relatively high tolerance to Cr tox-
icity and are capable of active uptake and accu-
mulation of this element against the concentra-
tion gradient (Staves and Knaus
1985
). Chandra
and Kulshreshta (
2004
) revealed that
Spirodela
polyrrhiza
is a potential accumulator of Cr(VI).
Since phytoremediation generally removes only
a small percentage of heavy metals from con-
taminated sites and can only be used in situations
with low-level contamination, for extremely
contaminated sites other approaches must be ap-
plied (Lasat
2002
). Peterson and Girling (
1981
)
reported other plants for phytoextraction, such
as
Sutera fodina
,
Dicoma niccolifera
and
Lep-
tospermum scoparium,
which accumulate Cr to
high concentrations in their tissues. Hyperaccu-
mulators are generally metal-specific and yield
a low annual biomass production, thus limiting
the overall amount of heavy metals that can be
extracted per harvest (Meers et al.
2004
). In most
cases, limited translocation of Cr following up-
take by the roots is the bottleneck limiting the
overall efficiency of phytoextraction from the
environment.
6.7
Other Techniques for Cr
Remediation
Overall, the cleanup goals discussed so far are
based on the Cr(VI) concentration in the soils
and the volume and physical-chemical proper-
ties of the Cr-containing soils. Therefore, most
of the available treatment technologies consist
of following these three mechanistic approaches,
i.e., (1) removing the Cr(VI)-containing soils
from the site; (2) immobilizing the Cr so that it
will not leach after treatment under field condi-
tions; or (3) reducing the Cr(VI) in the soils to
the Cr(III) state (Abdel-Sabour
2007
). As an ex-
ample of first technology, “
soil washing and in
situ flushing
” involves the addition of water with
or without additives, including organic and inor-
ganic acids, sodium hydroxide, methanol, EDTA,
acids in combination with complexation agents
or oxidizing/reducing agents as well as biosur-
factants, which enhance removal of metals from
contaminated soils and sediments (Mulligan
et al.
2001
). According to United States Environ-
mental Protection Agency (USEPA), efficiency
of metal removal by soil washing ranges from 75
to 99 % (USEPA
1992
), depending on a number
of factors, including the length of time the soil
has been exposed to the metals of concern, the
amount of fines in the soil, and the affinity of the
contaminants for the washing solution. It is be-
lieved that if a soil has greater than 20-30 % fines
(with particle sizes less than 0.06 mm in diam-
eter), soil washing may not be the most effective
technology (Oravetz et al.
1992
).
The second technology for alleviation includes
“
in situ immobilization
” of the pollutants has the
advantage of minimizing the exposure of site
works and local residents to airborne pollutants
as well as minimizing disruption to or demolition
Search WWH ::
Custom Search