Article Index

1.1.2. Phytoremediation of Heavy Metals

Phytoremediation is an eco-friendly in situ remediation technology driven by solar energy. Plants and associated microorganisms can be used for removal of heavy metals partially or completely remediate selected contaminants from soil, sludge, sediments, wastewater and ground water. In the phytoremediation of heavy metals, the initial step is phytoextraction, the uptake of heavy metal contaminants from soil or water by plant roots and their translocation to and accumulation in biomass. Translocation of metals to shoots is an important biochemical process and is desirable in an effective phytoextraction. The next important process of phytoremediation is phytofiltration, which includes rhizofiltration, blastofiltration or caulofiltration. In this, the metals are absorbed or adsorbed and thus their movement in soil and underground water is minimized. In addition to the above process, phytostabilization or phytoimmobilization reduces the mobility and bioavailability of metals in the environment. Plants perform the immobilization of heavy metals in soils by sorption through roots, precipitation, complex formation or metal valence reduction in the rhizosphere. Some of the heavy metals such as Hg and Se, absorbed by plants from polluted soils, get converted into volatile forms and subsequently released into the atmosphere by phytovolatilization process. This process does not remove the metals completely but rather transfers them from one medium (soil or water) to another (atmosphere) from which they can reenter soil and water.

Removal of heavy metals through phytoremediation, especially hyperaccumulators to degrade and detoxify contaminants receives wide attention due to its efficacy and cost efficiency. The criteria used for hyperaccumulation varies according the metal. Hyperaccumulator plants exhibit higher heavy metal tolerance and accumulating abilities compared to other plants. Difficulty in finding heavy metal hyperaccumulators, slow growth and lower biomass yield limit the use of hyperaccumulators. This makes the process time-consuming and therefore not feasible for rapidly contaminated sites or sewage treatments. Rhizospheric microorganisms such as Arbuscular mycorrhizal fungi and plant growth-promoting rhizobacteria, playing important roles in plant growth and/or metal tolerance via different mechanisms, are beneficial for the design of a phytoremediation plan to select appropriate multifunctional microbial combinations from the rhizosphere. It is likely that remediation role of rhizosphere is the main part of phytoremediation and removal of contaminants is achieved by the combined activity of plants and microorganisms. The main reason for the enhanced removal of metals in the rhizosphere is likely the increase in the number and metabolic activities of microorganisms. In the rhizospheric degradation process, the metal toxicity to plants can be reduced by the use of plant growth-promoting bacteria, free-living soil microorganisms that exert beneficial effects on plant growth. In this process, plants can stimulate microbial activity about 10–100 times by the secretion of exudates which contain carbohydrates, amino acids, flavonoids etc. In return, the rhizosphere bacteria facilitate the generation of larger roots helping to enhance plant survival.