The microbial method of biodegradation is useful but has limited applications since the pollution contributing factors are on a steep rise compared to the agents which control such pollution. Environmental pollution is one of the biggest threats faced by all countries though the nature differs. Microbial biodegradation has not been much effective in metal and organic pollutants.
Plants contribute to biodegradation by their ability to fix metals due to the presence of proteins called metallothionins. This ability can be harnessed to extract heavy metals from polluted areas. Phytoremediation is the use of plants to combat the effects of environmental pollution.
There are four essential requirements for such plants
• The plant should be able to absorb the metal via roots.
• It should transfer the metal to the leaf canopy.
• The plant should be able to store and utilize the material
• Mobilization of the metal should be possible. I.e. it should be able to fix the metal in absorbable forms.
There are natural varieties of plants which do such fixation. A few are genetically engineered by transferring the gene for the trait from bioleaching microbes.
Hyper accumulators are those plants which can absorb and accumulate metals to such high levels which in other similar plant species will be hazardous. Common examples include the Alpine pennycross and bracken fern. The former absorbs Cadmium and Zinc while the fern is a hyper accumulator of Arsenic.
The plants take up the metal pollutant through its roots and process the metals either to be used or degraded. There are four different applications of Phytoremediation.
Plants use the technique to absorb and store metal pollutants in stems and leaves. It is the direct uptake and storage and the plants are subsequently removed from the site either by incineration or pulling out.
Usually hyper accumulator species are employed to clean sites which have higher levels of contaminants. The process is repeated to get greater benefits. Zinc, Nickel and copper have been extracted using the technique. In some cases, the metal is extracted from the plants before destroying it through a process called phytomining.
2. Phytotransformation or phytodegradation
Plants uptake the metal contaminant from surface or ground water and transform them into forms which are less mobile and toxic. These stable forms of the metals pose less risk to environment and do not cause caners.
Certain plant species are used to immobilize the contaminants in the soil. The pollutants are adsorbed and accumulated via the roots of the plants. The metal contaminants are adsorbed onto roots or accumulated in rhizosphere. The pollutants are therefore prevented from entering back into environment. It reduces the bioavailability of the metal pollutants. This prevents the transfer of the pollutant via the food chain also.
It is also known as the rhizodegradation. The process occur in the rhizosphere of plants and hence the name. Rhizospheres provide a symbiotic association with the microbes which fix the metal contaminants.
In some cases, the plants take up the organic contaminants and convert into other forms which can be utilized by the microbes. Rhizodegradation is in which the microbes breakdown the pollutants and make it available for the plants.
The process is used in natural wetlands and estuaries. It is similar to Phytoremediation except in the source of pollutant. In this process, the plant takes up the contaminants dissolved in ground water rather than the polluted soil. For the process, plants have to be selected and acclimatized to the pollutants before going for in situ remediation. First the plants are grown in clean water to develop extensive root system. The water is substituted with contaminated water. The plants are then transferred to the polluted area which absorb the contaminants and are then disposed off. One application of the process is the use of sunflower plants to remediate the Chernobyl site with radioactive contaminants.
In this process, the plants take up the organic contaminants and convert them to volatile components to be transpired into air. Evapotranspiration of selenium, volatile hydrocarbons and mercury has been found to be effective.
The processes are advantageous where other methods of remediation are costly and not practical. However, the choice of Phytoremediation should also consider the duration, potential effects on food chain, and high toxic levels of contaminants where it would be difficult to establish the vegetation. Genetic Modification approaches can be used to over express the genes involved in Phytoremediation.
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