Phytoremediation is an emerging technology that uses various plants to degrade, extract or immobilize contaminants from soil and water. Plants help in cleaning up many types of contaminants including metals, pesticides, explosives, and oil. Phytoremediation takes advantage of the natural ability of plants to concentrate elements and compounds from the environment and to metabolize various molecules in their tissues.
Some examples of plants used in Phytoremediation process are Water Hyacinth (Eichhorniacrassipes), Poplar Tress (Papulusspp), Forage Kochia (Kochiaspp), Alfalfa (Medicago sativa), Kentucky Bluegrass (Poapratensis), Sunflower (Helianthus Annuus L.), Typhalatifolia etc. Depending upon the type and nature of of contaminant phytoremediating plants are selected.
HOW DOES PHYTOREMEDIATION WORK?
The above mentioned hyper accumulating plants exhibit diverse mechanisms for the removal of environmental toxicants depending upon their inherent abilities. But in general, phytoremediation mechanisms can be classified into following important sub groups
1) Phytovolatilization :
Phytovolatilization is the uptake and transpiration of a contaminant by a plant to the atmosphere. Phytovolatilization occurs as growing trees and other plants take up water and the organic contaminants. Some of these contaminants can pass through the plants to the leaves and evaporate, or volatilize, into the atmosphere.
2) Phytoextraction :
It refers to the uptake of metals from soil by plant roots into above-ground portion of plant. Plantstake up contaminants and store them in their stems and leaves (harvestable regions). Pollutants accumulated in stems and leaves are harvested with accumulating plants and removed from the site. This technique is generally used for removal of metal contaminates like nickel, zinc, copper, lead, chromium and cadmium.
3) Phytodegradation :
Phytodegradation, also called as phyto-transformation, is the breakdown of contaminants taken up by plants through metabolic processes within the plant, or the breakdown of contaminants surrounding the plant through the effect of compounds (such as enzymes) produced by the plants. Complex organic pollutants are degraded into simpler molecules and are incorporated into the plant tissues to help the plant grow faster. Plants contain enzymes (complex chemical proteins) that catalyze and accelerate chemical reactions.
4) Phytostabilisation :
Phytostabilisation is the use of certain plant species to immobilize contaminants in the soil and groundwater through absorption and accumulation by roots, adsorption onto roots, or precipitation within the root zone of plants (rhizosphere). This process reduces the mobility of the contaminant and prevents migration to the groundwater or air, and also reduces bioavailability for entry into the food chain.
5) Rhizofiltration :
Rhizofiltration (‘Rhizo’ means ‘Root’) is the adsorption or precipitation onto plant roots (or absorption into the roots) of contaminants that are in solution surrounding the root zone. Rhizofiltration is similar to phytoextraction, but the plants are used to clean up contaminated groundwater rather than soil. The plants to be used for cleanup are raised in greenhouses with their roots in water. Contaminated water is either collected from a waste site or brought to the plants or the plants are planted in the contaminated area, where the roots then take up the water and the contaminants dissolved in it.
6) Rhizodegradation :
Rhizodegradation is the breakdown of contaminants in the rhizosphere (soil surrounding the roots of plants) through microbial activity that is enhanced by the presence of plant roots and is a much slower process than phytodegradation. Micro-organisms (yeast, fungi, or bacteria) consume and digest organic substances for nutrition and energy. Certain micro-organisms can digest organic substances such as fuels or solvents that are hazardous to humans and break them down into harmless products in a process called bio-degradation.
Advantages of Phytoremediation
- Phytoremediation is cost effective technology and it does not require procurement of huge equipment.
- The plants can be easily monitored
- It is potentially the least harmful method because it uses naturally occurring organisms and preserves the environment in a more natural state.