PHYTOREMEDIATIONBioremediation through the use of plants whichmitigate the environmental problem withoutthe need to excavate the contaminant materialand dispose of it elsewhere. phyto = plant, andremedium = restoring balance.
Advantages• the cost of the phytoremediation is lower than that of traditional processes both in situ and ex situ• the plants can be easily monitored• the possibility of the recovery and re-use of valuable metals (by companies specializing in “phyto mining”)• it is potentially the least harmful method because it uses naturally occurring organisms and preserves the environment in a more natural state.
Limitations Phytoremediation is limited to the surface area and depth occupied by the roots. slow growth and low biomass require a long-term commitment with plant-based systems of remediation, it is not possible to completely prevent the leaching of contaminants into the groundwater the survival of the plants is affected by the toxicity of the contaminated land and the general condition of the soil. bio-accumulation of contaminants, especially metals, into the plants which then pass into the food chain, from primary level consumers upwards and/or requires the safe disposal of the affected plant material.
Various phytoremediation processes• Phytoextraction• Phytostabilization• Phytotransformation• Phytostimulation• Phytovolatilization• Rhizofiltration
PhytoextractionPhytoextraction (or phytoaccumulation) uses plants or algae to removecontaminants from soils, sediments or water into harvestable plant biomass.Advantages:• The main advantage of phytoextraction is environmental friendliness. Traditional methods which are used for cleaning up heavy metal contaminated soil disrupt soil structure and reduce soil productivity, whereas phytoextraction can clean up the soil without causing any kind of harm to soil quality.• Another benefit of phytoextraction is that it is less expensive than any other clean up process.Disadvantages: As this process is controlled by plants, it takes more time thantraditional soil clean up methods.
Examples of phytoextraction from soils• Arsenic, using the Sunflower, or the Chinese Brake fern , a hyperaccumulator. Chinese Brake fern stores arsenic in its leaves.• Cadmium, using Willow. As willow has some specific characteristics like high transport capacity of heavy metals from root to shoot,huge amount of biomass production, can use also for production of bio energy in the biomass energy power plant.• Cadmium and zinc, using Alpine pennycress, a hyperaccumulator of these metals at levels that would be toxic to many plants. On the other hand, the presence of copper seems to impair its growth.• Lead, using Indian Mustard, Ragweed, Hemp Dogbane, or Poplar trees, which sequester lead in its biomass.• Uranium, using sunflowers, as used after the Chernobyl accident.• Mercury, selenium and organic pollutants such as polychlorinated biphenyls (PCBs) have been removed from soils by transgenic plants .
PhytostabilizationPhytostabilization focuses on long-termstabilization and containment of the pollutant.Unlike phytoextraction, phytostabilizationmainly focuses on sequestering pollutants insoil near the roots but not in plant tissues.Pollutants become less bioavailable andlivestock, wildlife, and human exposure isreduced.
PhytotransformationChemical modification of environmental Substances as a directresult of plant metabolism.After uptake of the contaminats, plant enzymes increase thepolarity of the contaminants by adding functional groups such ashydroxyl groups (-OH). This is known as Phase I metabolism.In the second stage of phytotransformation, known as Phase IImetabolism, plant biomolecules such as glucose and amino acidsare added to the polarized xenobiotic to further increase thepolarity (known as conjugation). In the final stage ofphytotransformation (Phase III metabolism), a sequestration ofthe xenobiotic occurs within the plant.e.g: Trinitrotoluene phytotransformation
• Phytostimulation - enhancement of soil microbial activity for the degradation of contaminants, typically by organisms that associate with roots. This process is also known asrhizosphere degradation.• Phytovolatilization - removal of substances from soil or water with release into the air, sometimes as a result of phytotransformation to more volatile and / or less polluting substances.• Rhizofiltration - filtering water through a mass of roots to remove toxic substances or excess nutrients. The pollutants remain absorbed in or adsorbed to the roots.
The role of geneticsGenetic engineering is a powerful method forenhancing natural phytoremediationcapabilities, or for introducing new capabilitiesinto plants. For example, genes encoding anitroreductase from a bacterium were insertedinto tobacco and showed faster removal of TNTand enhanced resistance to the toxic effects ofTNT