What is phytoremediation:

The technique of employing green plants and their associated microorganisms, soil amendments and agronomic practices to remove, inhibit or neutralize hazardous environmental contaminants is termed as "phytoremediation". This technique was for the first time used in Germany around 300 years ago. It was used for the treatment of sewage. Phytoremediation also is used to remove organic and inorganic wastes from soil. One of the best yet simple examples of this technique is use of carrots. Carrots have the ability to absorb DDT(dichloro-diphenyl-trichloroethylene).
So they are are cultivated in soils contaminated with DDT and after harvesting they are dried and incinerated to destroy the absorbed DDT.

Phytoremediaton of organic environmental waste:
Plant roots are lypopyhylic in nature and the rate at which they absorb organic contaminants from soils is almost proportional to their relative lypophilicity. Phytoremediation takes place due to the following mechanisms:
1. Absorption of organic matter and accumulation of the same in plant tissues.
2. Translocation of the absorbed organic matter to leaves from where it is volatized through leaf surface.
3. Metabolization of the organic matter inside the plant tissues or in the rhizosphere by the action of plant enzymes.
4. Microorganisms living in association with the plants breakdown the organic matter and degrade them.

Phytoremediation of Inorganic environmental contaminants:
Inorganic contaminants are removed by plant by the following methods:

1. Most plants have the ability to accumulate large amounts of metal contaminants from the environment. These plants are called hyperaccumulators. After the metal is accumulated, the biomass of such plants is decreased in volume by heat, microbial activity, physical or chemical methods. Then the biomass is appropriately disposed. In case of large scale operations, the metal accumulated by the plants in can be recovered from the biomass.
Examples of some hyperaccumulators are:

Name of the Plant | Metal accumulated
Thlaspi caerulenscens : Cd
Thlaspi rotundifolium : Pb
Sebertia acuminate : Ni
Ipomoea alpine : Cu

2. Some plants volatilize the inorganic contaminants. This method is very useful in the remediation of mercury. Arabidopsis thaliana is transgenic plant to which a bacterial mercuric ion reductase enzyme has been transferred. This plant absorbs mercury and reduces it to volatile hg(o), by the action of the enzyme. Then the hg(o) is volatilized through leaf surfaces.

Applications of phytoremediation
Tata Energy Research Institute India, applies Phytoremedietion to control problems caused by Flyash. Flyash is a mixture of various heavy metals.It is generated in huge quantities by thermal power and coal energy driven industries. This flyash then becomes airborne and spreads everywhere around the production area, with the heavy metals present in it contaminating the ground water. To overcome this, seedlings of Eucalyptus and Melia are grown in abandoned flyash ponds.

Appropriate amount of compost and arbuscicular myccorhizal fungi(AFM) cultures are added to the soil which enhances establishment of the plants. AFM increases the effective root zones of plants and helps them to absorb large quantities of water and nutrients leading to better survival and growth of plants.

As the plants grow, their roots hold the flyash making it less prone to become airborne. Also the AFM, which is in symbiotic association with the plant roots accumulate the heavy metal from Flyash into their mycelial network. This way, it decreases the risk of the heavy metal contaminating the ground water.

Advantages of Phytoremediation:
Phytoremediation is much cost effective as it doesn't require procurement of huge equipments.
Planting trees on the remediation sites make these sites aesthetic and appealing to the eyes.
Plants can be easily grown without much effort and also can be monitored easily.

Limitations of phytoremediation
Though Phytoremediation is easily applicable and cost effective it does have some inherent technical constraints.
1.The contaminating material should be present within the root zone to be accessible to the roots.
2. The phytoremediation site should be must be large enough to grow plants.
3. This process is a slower process.

However this approach is being researched to make it more refined and improvised by applying the tools of biotechnology. One example is the development of genetically engineered transgenic Arabidopsis thaliana which carries bacterial mercuric reductase enzyme and is widely applied in the phytoremediation of mercury.

Trees having deep root penetration and high rates of transpiration are being analysed for application in phytoremediation of landfill leachates, pesticide contaminants etc. Use of plants in remediation of air contaminants is also being analy

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