The Greek word "phyto" means plant and the Latin word "retmedium" means to cure or restore. Phyto-remediation is defined as "the process of restoration of quality of environment by the application of plants." i.e. use of green plants based systems to detoxify or remove toxic, substances from contaminated air, water, soil and sediments, also called as green clean. Hence Phyto-remediation is the direct use of living green plants for in situ, or in place, removal, degradation, or containment of contaminants in soils, sludges, sediments, surface water and groundwater.
The plants exhibiting phyto-remediation should have following characteristics:
a) They should be fast growing plants.
b) They should have high biomass potential to accumulate large amount of toxicants. They normally show a property of binding the metal into their root tissues, transporting upward from their roots.
c) They should be locally growing tress to be able to adapt to the climatic conditions.
Types of Phyto-remediation
Following forms of phyto-remediation are applicable in different technologies:
Phyto-volatilization is the uptake and transpiration of a contaminant by a plant, with release of the contaminant or a modified form of the contaminant to the atmosphere from the plant through contaminant uptake, plant metabolism, and plant transpiration.
Certain transgenic plants reduce hazardous ionic and methylated forms of mercury to elemental mercury, which is then volatilized. Selenium phyto-remediation is also done by phyto-volatilization.
Phyto-stabilization is defined as (1) immobilization of a contaminant in soil through absorption and accumulation by roots, adsorption onto roots, or precipitation within the root zone of plants, and (2) the use of plants and plant roots to prevent contaminant migration via wind and water erosion, leaching, and soil dispersion.
In this form of phyto-remediation, agronomic techniques are used to stabilize contaminated sites (in-situ) i.e. checking the spread of contaminant in the different segments of environment, with following steps: Vegetation is grown on the site to reduce the movement of contaminants with water-runoff and soil erosion or Plants are grown to maximize the root uptake of contaminants.
It is the most important form of phytoremediation in which toxic and non-biodegradable (persistant) contaminants e.g. Heavy metals are extracted from the environment by growing 'green plants on the contaminated soil.
The toxic compounds are captured by the special compounds present in the plants, by the mechanism of chelating known as phytochelatins.
Removal of toxic compounds in the contaminated water environment, by the filtration process of the roots of the plants is known as rhizofiltration. Roots of the plants act as biofilters i.e. roots help absorb toxic compound or metals from contaminated water and accumulate them. Contaminants are removed from the plants by harvesting (collecting from) the root biomass.
Example of Phytoremediation
Water hyacinth and salvinia are fast growing, weeds. Their higher productivity and resilience i.e. ability to accumulate very large amount of toxic substances (viz. heavy metals) make them ideal macrophytes for wastewater treatment (secondary and tertiary waste water treatment). The biomass is harvested- frequently to assure maximum productivity and removal of toxic substances.