Today we have an environmental concern of soil and water pollution due to the dispersal of industrial and urban wastes. Controlled and uncontrolled disposal of waste, accidental and process spillage, mining and smelting of metalliferous ores, sewage sludge application to agricultural soils are responsible for the migration of contaminants into non-contaminated sites as dust or leachate and contribute towards contamination of our valuable agriculture land, water and destroy of natural ecosystems. A wide range of inorganic and organic compounds causes contamination including heavy metals, combustible and putriscible substances, hazardous wastes, explosives and petroleum products. Major components of inorganic contaminates are heavy metals. Soil micro-organisms can degrade organic contaminants, while metals need immobilization or physical removal. Although many metals are essential but all metals are toxic at their higher concentrations. These metals render the land unsuitable for plant growth and destroy the biodiversity. Heavy metals are elements having atomic weight from 63.54 to 200.59, and a specific gravity greater than 4. Trace amount of some heavy metals is required by living organisms, however, any excess can be detrimental to the organisms. Nonessential heavy metals include arsenic, antimony, cadmium, chromium, mercury, lead, etc; are of particular concern to water and soil pollution.
Plant-based bioremediation technologies are collectively termed as phytoremediation, this refers to the use of green plants and their associated micro biota for the treatment of contaminated soil and ground water. The idea of using metal accumulating plants to remove heavy metals and other compounds was first introduced in 1983. The conventional methods of remediation of the contaminated soil and ground water are very costly, but phytoremediation is a cheap and eco-friendly way to solve this issue. Phytoremediation consists of five main processes as;
Rhizofiltration
It is defined as the use of plants, both terrestrial and aquatic; to absorb, concentrate, and precipitate contaminants from polluted aqueous sources with low contaminant concentration in their roots. Rhizofiltration can partially treat industrial discharge, agricultural runoff, or acid mine drainage. It can be used for lead, cadmium, copper, nickel, zinc and chromium, which are primarily retained with in the roots. Plants like sunflower, Indian mustard, tobacco, rye, spinach and corn have been studied for their ability to remove lead from effluent, with sunflower having the greatest ability. Indian mustard has proven to be effective in removing a wide concentration range of lead.
Phytostabilisation
It is mostly used for the remediation of soil, sediment and sludges and depends on ability of roots to limit contaminant mobility and bioavailability in the soil. Phytostabilisation can occur through the sorption, precipitation, complexaction, or metal valence reduction. The plants primary purpose is to decrease the amount of water percolating through the soil matrix, which may result in the formation of hazardous leachate and prevent soil erosion and distribution of the toxic metal to other areas. A dense root system stabilizes the soil and prevents erosion. It is very effective when rapid immobilization is needed to preserve ground and surface water and disposal of biomass is not required.
Phytoextraction
It is the best approach to remove the contamination primarily from soil and isolate it, without destroying the soil structure and fertility. Plant absorb, concentrate and precipitate toxic metals and radionuclide from contaminated soils into the biomass. It is best suited for the remediation of diffusely polluted areas, where pollutants occur only at relatively low concentration and superficially. Several approaches have been used but the two basic strategies of phytoextraction, are; i) Chelate assisted phytoextraction in which artificial chelates are added to increase the mobility and uptake of metal contaminant. ii) Continuous phytoextraction in which removal of metal depends on the natural ability of the plant to remediate; Discovery of hyperaccumulator species has further boosted this technology. The removed heavy metal can be recycled from the contaminated plant biomass.
Phytovolatilization
Phytovolatilization involves the use of plants to take up contaminants from the soil, transforming them into volatile form and transpiring them into the atmosphere. Phytovolatilization occurs as growing trees and other plants take up water and the organic and inorganic contaminants. Phytovolatilization has been primarily used for the removal of mercury, the mercuric ion is transformed into less toxic elemental mercury. Some plants grow in high Selenium media and produce volatile selenium in the form of dimethylselenide and dimethyldiselenide reducing its concentration in soil.
Phytodegradation
Phytodegradation is the breakdown of organics taken up by the plant to simpler molecules that are incorporated into the plant tissues. Plants contain enzymes that can breakdown and convert ammunition wastes, chlorinated solvents such as trichloroethylene and other herbicides. The enzymes are usually dehalogenases, oxygenases and reductases. Rhizodegradation is the breakdown of organics in the soil through microbial activity of the root zone and is a much slower process than phytodegradation. Yeast, fungi, bacteria and other microrganisms consume and digest organic substances like fuels and solvents. All phytoremediation technologies are not exclusive and may be used simultaneously.