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Bioremediation is one of nature's prudent ways to purify the polluted environment and that degraded by the anthropogenic activities. Although the term 'bioremediation' may be recent, but the process in not new as its origin relates to the origin of life when the first organism was stressed by certain compounds and it evolved the process to convert such compounds in less harmful forms by adopting certain detoxifying mechanisms in order to overcome the stress. The present day bioremediation technologies are based on the processes and potentials of almost all types of life forms, viz., plants (phytoremediation), microorganisms (microbial remediation) and animals (zooremediation). The approach under demand and discussion now can be termed as 'ecoremediation' to express the wider range of objectives and scope. However, for optimizing the maximal benefit sustainability of such technologies, requires an effective policy directives supported by social perception and must necessarily incorporate the application of environmental impact assessments, intellectual property rights and cost-benefit analysis for commercial viability.
PHYTOREMEDIATION
Phytoremediation is a cost-effective, simple and sustainable beneficiary technique to remove pollutants from the environmental components - air, water or soil, using plants. Phytoremdiation targets include contaminant metals, metalloids, petroleum hydrocarbons, pesticides, explosive or toxic gases, chlorinated solvents and a range of industrial by-products. Commercially viable phytoremediation systems for clean-up of shallow aquifers and water-borne contaminant are now well in practice. Phytoremediation may be defined as 'the process of restoration of the quality of environment by the application of plants'. It is the use of 'green plant based systems' to remediate contaminated soils, sediments, water or air. Sometimes it is also referred as 'greenclean'. Certain techniques that end with the volatilization are used for the remediation of volatile forms of contaminants. These techniques are collectively called ` as 'phytovolatilization'. Recently, certain transgenic plants have been shown to reduce mercury from the more hazardous ionic and methylated forms to elemental mercury that is later volatilized. Similar technique is also applicable for selenium. 'Under the scope of 'phytostabilization', agronomic techniques are used to stabilize contaminated sites in-situ. It mainly serves to check the spread of contaminant in the environment to reduce further damage.
Phytoremediation may be defined as 'the process of restoration of the quality of environment by the application of plants'. It is the use of 'green plant based systems' to remediate contaminated soils, sediments, water or air. Sometimes it is also referred as 'greenclean'. Certain techniques that end with the volatilization are used for the remediation of volatile forms of contaminants. These techniques are collectively called ` as 'phytovolatilization'. Recently, certain transgenic plants have been shown to reduce mercury from the more hazardous ionic and methylated forms to elemental mercury that is later volatilized. Similar technique is also applicable for selenium. 'Under the scope of 'phytostabilization', agronomic techniques are used to stabilize contaminated sites in-situ. It mainly serves to check the spread of contaminant in the environment to reduce further damage.
'Phytoextraction' involves the use of plants to extract contaminants from the environment. During this process, metals are transported from media to the plant parts by absorption through roots. It can be used for rernediation targeting at lead, radionuclides with chromium, arsenic and mercury. A technique for extraction of precious metals with high economic importance such as nickel, copper, etc., is evolved and called 'phytomining'. Plants selected for bioremediaiton purpose are expected to possess one or more of the following properties::
Such plants must not only accumulate metals but should also be fast growing in different conditions and lend themselves to easy harvesting. The binding of metals in plant tissue involves certain proteins known as metallothionins. Metal hyperaccumulating species adopt different strategies to avoid metal toxicity, the most important being the vacuolar accumulation of the heavy metal. Amongst various endogenously produced molecules that help in this process are the phytochelatins known for sequestering heavy metals in the vacuoles. Members of the family Brassicaceae have shown tremendous potential for cleaning up of polluting metals. The pine tree has been noted for absorbing Beryllium whereas Acedium plants were demonstrated for vanadium accumulation.
Plants that can trap or filter out the dust content or can absorb the gaseous pollutants of the atmosphere to detoxify them or precipitate the metals are used for the purpose of atmospheric phytoremediation or air pollution attenuation. For application to dust attenuation, plants are expected to possess the following characteristic features:
The biochemical features of the resistant plants are mainly based on following terms:
Thus, plants on the basis of their atmospheric phytoremdiation capacity can be grouped in three categories:
Less absorption + strong tolerance = strongest resistance.
Less absorption + weak tolerance = fair resistance.
More absorption + strong tolerance = ideal plants for use as mitigator species.
MICROBIAL BIOREMEDIATION
Microbial bioremediation is defined as the process by which microorganisms are stimulated to rapidly degrade hazardous organic contaminants to environmentally safe levels in soils, subsurface materials, water, sludge, and residues. Microbes deal with poisonous chemicals by applying enzymes to convert one chemical into another form and taking energy or utilizable matter from this process. The chemical transformations generally involve breaking of large molecules into several small molecules in simpler form. In some cases the by-products of bacterial bioremediation are not only harmless but may prove useful. For example, methane is derived by the bacterial degradation of sulphite liquor, a waste product in paper manufacturing.
ZOOREMIDIATION
When the decontamination of environment or waste treatment is performed through the activities of animals, the process is known as Zooremediation. However, earthworm based technology has proved commercial potentials as the role of earthworms in the conversion of organic materials and improvement of soil has been observed and appreciated, called as vermicomposting.