Heavy metals are those with a specific gravity more than 5.0, and these metals at very low concentration damage living organisms and accumulate in the food chain, some of the examples for heavy metals are lead, mercury, chromium, cadmium and many more. Some of the main sources of heavy mental in the environment include sediment from waste water treatment plant, mining waste, municipal and industrial waste.
Plants can be used to remove heavy metal from the environment and hence will reduce the toxic effects of heavy metal on health of all living organisms, this is known as phytoremediation.
Types of Phytoremediation:
1. Phytotransformation, in this type the contaminants from water and soil are absorbed by the roots and they are transformed in other parts of the plant like leaves, stem
2. Rhizosphere Remediation, here transformation of contaminants occur in the root itself, hence it increases the organic carbon, bacterial and fungal population.
3. Phytostabilization, here movement of contaminants are restricted to the area of vegetation or immobilizes the contaminants.
4. Phytoextraction, here the plants are used to extract the metals from the soil and these metals gets accumulated in the parts of the plants, such as root, stem, leaves and then these metals are extracted from the plants.
5. Rhizofiltration, in this method of phytoremediation plants are used to absorb, accumulate and concentrate metals from the contaminated surface water or ground water.
Heavy Metal Phytoremediation:
Plants called as hyperaccumulators are used for the extraction of heavy metal from the contaminants. These plants that is hyperaccumulators, when compared to the usual plants they take up almost 100 times more of heavy metal from the contaminated water or soil. Heavy metals are extracted from the soil and accumulate in the root, stem or leaves of plants. The plant is then harvested and disposed and the contaminated site is again planted with new plant until all the contaminants are extracted from the soil.
Some of the examples of phytoremediation:
1. Indian mustard Brassica juncea are used to extract the heavy metal lead from contaminated soil. After harvesting Brassica juncea average concentration of lead from all the crops is more than 1,000 ppm.
2. Thlaspi caerulescens accumulates upto 26,000 ppm of cadmium or zinc from the contaminated soil, while a normal plant accumulates around 100ppm of heavy metal.
3. Ipomea alpina accumulates around 12,000 ppm of copper from contaminated soil.
4. Haumaniastrum robertii accumulates around 10,200 ppm of cobalt from the contaminated soil surface.
5. Astragalus racemosus is used to accumulate 10,000 ppm of heavy metal selenium from the contaminated soil
6. Sebertia acuminata is used to extract heavy metal nickel from the contaminated soil
Advantages of Phytoremediation:
1. Phytoremediation is conducted at the site of contamination, that is in situ
2. Low cost when compared to other chemical peocedures
3. Transfer id faster than natural attenuation
4. Well suited for large fields or large contaminated sites
5. Fewer or very less water and air emission
6. Soils are in place and they can be used for cultivation after phytoremediation
7. Compatible with engineered technology
8. Phytoremediation is environment friendly method
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