Contamination of soils with toxic metals has often resulted from human activities, especially those related to mining, industrial emissions, disposal or leakage of industrial wastes, application of sewage sludge to agricultural soils, manure, fer­tilizer, and pesticide use. Excessive metal concentration in soil poses significant hazard to human, animal and plant health, and to the environment. The aim of phytoextraction is to reduce the concentration of metals in contaminated soils to regulatory levels within a reasonable time frame. This extraction process depends on the ability of selected plants to grow and accumulate metals under the specific climatic and soil conditions of the site being remediated.

It uses plants to remove metals from soils and to transport and concentrate them in above-ground biomass [4]. In this process, plant roots sorb the contaminants along with other nutrients and water. The contaminant mass is not destroyed but ends up in the plant’s shoots and leaves. This method is used primarily for wastes containing metals where water-soluble metals are taken up by plant species selected for their ability to take up large quantities of metals. The metals stored in the plant’s aerial shoots are harvested and smelted for potential metal recycling/ recovery which were earlier disposed off as a hazardous waste. As a general rule, readily bio-available metals for plant uptake include cadmium, nickel, zinc, arsenic, selenium, and copper. Moderately bio-available metals are cobalt, manganese, and iron. They can be made much more bio-available by the addition of chelating agents to soils.

Phytoextraction has been growing rapidly in popularity worldwide for the last 20 years or so. In general, this process has been tried more often for extracting heavy metals than for organics. The plants absorb contaminants through the root system and store them in the root biomass and/or transport them up into the stems and/or leaves. A living plant may continue to absorb contaminants until it is harvested. After harvest, a lower level of the contaminant will remain in the soil, so the growth/harvest cycle must usually be repeated through several crops to achieve a significant cleanup. After the process, the cleaned soil can support other vegetation.

There are two main categories of plants to clean up toxic metals from soil:

• Metal hyper-accumulator plants

• High biomass plants