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Toxic Metals
and Phytoremediation
By: U. N.
Rai And Amit Pal
Phytoremediation, popularly known as 'green clean' is a novel strategy for the
removal of toxic contaminants from the environment by using plants. This
concept is increasingly being adopted as it is a cost effective and
user-friendly alternative to traditional methods of treatment. Toxic metal
pollution in water and soil is a major environmental problem and most
conventional remediation approaches do not provide acceptable solutions.
Rapid growth
in population and massive industrialisation in recent years has resulted in
pollution of the biosphere with toxic metals.
Plants
possess some characteristic features which enable them to absorb from soil and
water, such heavy metals which are essential for their growth and development.
These metals include iron (Fe), manganese (Mn), copper (Cu), molybdenum (Mo)
and nickel (Ni). Plants also accumulate toxic metals which may not have any
biological function, these include : silver (Ag), cadmium (Cd), chromium (Cr),
cobalt (Co), mercury (Hg), lead (Pb), and selenium (Se) etc.
However,
excessive accumulation of these metals can be toxic to most plants. The use of
terrestrial plants for environmental remediation through metal accumulation has
been gaining acceptance in recent years and accumulation of Ni, Co, Cu, Mn, Pb,
Zn, Se and Hg in high concentrations has been extensively reported.
Phytoremediation, which essentially involves the use of plants for
environmental cleanup can be divided into the following:
·
Phytoextraction
in which
metal accumulating plants are used to transport and concentrate metals from the
soil into the harvestable roots and above-ground shoots,
·
Rhizofiltration
in which
plant roots absorb, precipitate and concentrate toxic metals from polluted
effluents,
·
Phytostabilization
in which
heavy metal-tolerant plants are used to reduce the mobility of heavy metals,
thereby reducing the risk of further environmental degradation by leaching into
the ground water or by airborne spread.
History of
phytoremediation
The basic
idea that plants can be used for environmental remediation is quite old.
However, it was not untill 1948, that some Italian researchers first reported
nickel hyperaccumulation in the Italian serpentine plant Alyssum bertolonii.
This finding was all but forgotten untill 1977, when researcher Robert
Brooks, of Massey University in New Zealand, made similar observations. This
time, the concept caught on. Extensive researches on the use of semi-aquatic
plants (or ecosystems as a whole), for treating radionuclide contaminated
waters were initiated in Russia at the dawn of the nuclear era. The knowledge
that aquatic or semi-aquatic vascular plants such as, water hyacinth (Eichhornia
crassipes), pennywarth (Hydrocotyle umbellata), duckweed (Lemna
minor), and water velvet (Azolla pinnata), can take up Pb, Cu, Cd,
Fe and Hg from contaminated solutions existed for a long time. This ability is
currently utilized in many wetlands, which may be effective in removing some
heavy metals as well as organics from water.
While
initially researchers in the United Kingdom showed interest in these plants,
Chaney was the first US scientist to publish his report on 'hyperaccumulator
plants potential as toxic site cleaners'. Scott Cunningham a scientist at
DuPont began the study of lead removal. Raskin and his colleagues founded 'Phytotech'
a New Jersey remediation company, which filed two patents, one for
phytoextraction (the use of plants to remove heavy metals from soil) and
another for rhizofiltration, in which plants remove heavy metals, from streams.
Federal agencies have only recently begun funding phytoremediation projects.
Now, dozens of labs in academia and industry are undertaking phytoremediation
jobs on a large scale.
From
laboratory to market place
Phytoremediation has already been successfully implemented by the US Air Force
to clean trichloroethylene from ground water using poplar trees, and by the US
army to clean 2,4,6-trinitrotolune (TNT) and hexahydro
1,3,5-trinitro-1,3,5-triazine (RDX) from contaminated wetlands, using a variety
of plants.
In India,
aquatic vascular plants like Hydrilla verticillata, Spirodela polyrrhiza,
Bacopa monnierii, Phragmites karka and Scirpus lacustris have been
used to treat chromium contaminated effluent and sludge from leather tanning
industries.
Application
of phytoremediation for the clean up of industrial wastes contaminated with
toxic metals is another important area that has blossomed in recent years.
Certain plants translocate metals from their surroundings and accumulate them
in their above ground parts at high concentrations. Thus, they can be harvested
and removed from the site. Two routes are currently being explored to develop
metal accumulating plants: genetic engineering and, the selective breeding of
naturally occurring hyperaccumulator-plants.
Recently,
Richard Meagher, at the University of Georgia (Athens) by inserting an altered
mercuric ion reductase gene (mer A) into Arabidopsis thaliana, produced
mercury resistant transgenic merA plants which are soon to be tested in soil.
Results to date suggest that the cost of phytoremediation of mercury
contaminated soils will be one-tenth to one-hundredth the cost of other
traditional engineering methods, including land filling, thermal treatments and
chemical extraction.
Future
Currently
phytoremediation technology is being used to clear off various polluting
elements such as heavy metals, insecticides, petro products, explosives,
chlorinated solvents, and industrial by-products. Many of the plants used in
this programme, are slow growing and, although their metal accumulation ability
is high, they produced low biomass. Therefore, the clean-up. rates may prove
slower than conventional methods. This is particularly problematic as the
phytoextraction success is often based on the amount of contaminant
removed/hectare/year. In addition, frame limitations also apply. Despite these
short comings, many forms of phytoremediation have emerged from the
laboratories and are currently in practice and public acceptance is very
encouraging.
The
challenge facing phytoremediation in the market place should be faced jointly
by scientists, environmental engineers and regulators to prove the technology's
efficacy at pilot sites. It is incumbent upon phytoremediation researchers to
conduct basic laboratory work, before attempting field application. By
employing this technology as part of living cultural, ecological systems, we
can bring our understanding and appreciation of green clean into sharper focus
for increased societal benefits.
Dr. U.N.
Rai is a Scientist and Dr. Amit Pal is Research Associate in Environmental
Sciences Division at National Botanical Research Institute,
Lucknow,
India. |
