Environmental Engineering Reference
In-Depth Information
Certain plants can hyperaccumulate metals with-
out any toxic effects. These plants are adapted
to naturally occurring, metalliferous soils. More
than 400 plant species can hyperaccumulate vari-
ous metals. However, most plants can only hy-
peraccumulate one specific metal.
Hyperaccumulating plants can contain more
than 1 % of a metal in their dry biomass. For ex-
ample, the hyperaccumulating plant
Berkheya
coddii
was found to contain as much as 3.8 % of
Ni in the dry, above-ground biomass, when grown
in contaminated soil. It is possible to extract
metals from the harvested biomass in a process
termed phytomining. The underlying mechanism
of hyper-accumulation of metals in plants is the
overexpression of genes that regulate cell mem-
brane transporters. These include the Cu-trans-
porter (COPT1) and Zn-transporter (ZNT1). The
main limitations on the use of hyperaccumulating
plants in phytoextraction are slow growth and low
biomass production. The effectiveness of phytoex-
traction is a function of a plant's biomass produc-
tion and the content of contaminants in the har-
vested biomass.
Therefore, fast-growing crops that accumu-
late metals have a great potential in phytoextrac-
tion. The use of crops in phytoextraction can be
improved by manipulation of their associated soil
microbes. Inoculation of plant growth-promot-
ing bacteria (PGPR) and arbuscular mycorrhizal
fungi (AMF) can increase plant biomass. The
AMF-plant symbiosis usually results in reduced
accumulation of metals in the above-ground
biomass of plants. Therefore, suppressing AMF
activity, by using specific soil fungicides, has re-
sulted in increased metal accumulation in plants.
The role of AMF in regulating metal uptake by
plants appears to vary depending on numerous
factors, such as AMF populations, plant species,
nutrient availability, and metal content in the
soil. Also, this regulation of AMF is usually met-
al-specific; where the uptake of essential metals
is generally increased, but the uptake of nones-
sential metals is inhibited. However, exceptions
have been found where AMF increases uptake of
Ni, Pb, and As in plants. Induced phytoextrac-
tion involves the use of fast-growing crops and
chemical manipulation of the soil. Low bioavail-
ability of metals in the soil is a limiting factor
in phytoextraction. The bioavailability of metals
can be increased by the use of synthetic chelates
such as ethylene diamine tetracetic acid (EDTA)
or acidifying chemicals (e.g., NH
4
SO
4
). The
use of synthetic chelates increases the absorp-
tion of metals to the root and the translocation
of metals from the roots to the foliage. The tim-
ing of chelate application is critical, and should
ideally take place at the peak of biomass pro-
duction. The effectiveness of using EDTA was
demonstrated by growing corn (
Zea mays
) in
Pb-contaminated soil treated with 10 mmol kg
−1
EDTA. This resulted in a high accumulation of
Pb (1.6 % of shoot dry weight), and facilitated
the translocation of Pb from the roots to the foli-
age. Some drawbacks of using synthetic chelates
in phytoremediation are the result of increased
solubility of the metals within the soil. In turn,
this increases the risk of metal migration through
the soil profile and into the groundwater. How-
ever, a possible solution is to treat contaminated
soil ex-situ in a confined site with an impervious
surface. Also, periodic application of low doses
of synthetic chelates reduces the risk of metal
migration.
1.9
Molecular Approach
of Bioremediation
Microbial removal of contaminants from the en-
vironment often takes place without human in-
tervention. This has been termed intrinsic biore-
mediation. Relying on intrinsic bioremediation is
increasingly the bioremediation option of choice
if it can be shown that the contamination does not
pose an immediate health threat and it remains
localized. If the rate of intrinsic bioremediation
is too slow, then environmental conditions can be
manipulated to stimulate the activity of microor-
ganisms that can degrade or immobilize the con-
taminants of concern. Engineered bioremediation
strategies include: the addition of electron donors
or acceptors that will stimulate the growth or
metabolism of microorganisms that are involved
in the bioremediation processes; the addition of
nutrients that limit the growth or activity of the
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