Importance of Phytoremediation
Phytoremediation is special application of bioremediation. It is a natural biological process of degradation of xenobiotic and recalcitrant compounds responsible for environmental pollution. The word phyto stands for 'plant' hence the remediation mediated by plant system. The mechanisms of phytoremediation are similar to biological remediation employing microbial agents. Microbes have been utilized to remove, destroy toxic substances like thiocarbamates, herbicides, insecticides, organophosphorus compounds, chlorinated aliphatic and aromatic chemicals, aromatic amines, sulfonates and heavy metals; they also convert lignin, cellulases, hemicellulases into fuels, solvents, SCPs and other useful products. Phytoremediation is used for similar purpose but have many advantages as compared to microbial remediation. Plant system solely is capable of remediation. It does not require any support from external agency or labor and uses its own natural processes to clean up the site. It not only decontaminates pollutants but also inhibits spreading of pollutant/s from one site to other. Therefore the pollution remains localized, confined to particular area. Here the root system plays an important role; soil particles remain bound to roots even during rainy season. If the soil is polluted it is not washed away by rainwater to aquatic bodies or carried away by winds to far locations. Plant system is the only living agency that prevents spread of pollution. The most important advantage is that plants can directly be planted on contaminated sites.
Selection of plant for remediation:
Depending upon location of contaminated sites such as air, water or land; the plants growing in respective habitats can be chosen. Thus aquatic plants or algae will be used for water and arboreal plants or tall tree species will be suitable for remediating air pollution. Generally, the plants selected are the trees. Because tree roots are tap roots and reach deep underground in search of water and nutrients. It is assumed that plants can clean up the pollution as deep as their roots. Most of the tree species grow without dormancy period and are annual. Trees can also bear harsh environmental conditions like heavy rain, snow fall, heat or winds. Tree species like poplar, mulberry, birch, pine and maple have been recommended for phytoremediation. If grasses and bushes are selected for remediation, then they should have short life cycle. So that they can be harvested, destroyed or recycled for further use. New crop again can be planted for continual remediation. Grasses and bushes/shrubs like rye, wheat, Bermuda, buffalo and hydrilla, water velvet and indigo bush have been used for short term bioremediation program. Grasses are selected as plant agents for bioremediation when soil contamination is superficial and recent. Recently, the attempts were made to incorporate bacterial biodegradation genes into plants to enhance their biodegradation potential. Plant breeding programmes to develop plant verities hypertolerant to toxic concentrations of heavy metals and pesticides are also fruitful and encouraging.
Precautions and mechanism:
Precautions: Before on field application of phytoremediation, some precautionary measures needs to be taken. The plant should not release harmful gases into air during the remediation. Number of plants and types to be used, disposal sites and plantation time should be determined. Weather like frost or snow fall should be avoided for plantation. If this is not considered it may create trouble to clean the site of destroyed plants for remediation. The plant agent should be tested for its insect pests if any. Because these insects after feeding on plant host may carry toxic traces of pollutants and eaten by frogs (assume so) of particular food chain. This can lead to bioconcentration of toxic chemical into food chain and affecting its every component.
Mechanisms: Phytoremediation involves many processes which are carried out by plant during their growth on contaminated site. A contaminant is treated by plants using all or some of these reactions like phytoextraction, phytostabilization, phytotransformation, phytostimulation and phytovolatization. Of these, phytoextraction that is uptake of pollutants into plant biomass via roots is first step of phytoremediation. Sometimes, plants do not absorb but immobilize and stabilize pollutants in the soil (Phytostabilization). Plant then secretes root exudates that attract and stimulate rhizobacteria (Phytostimulation). Immobilized pollutants are later on degraded by stimulated rhizobacteria of that plant. Sometimes, only roots are involved in remediation. They remove toxic compounds by filtering them along with water absorbed via dense root hairs (Rhizofiltration). During phytotransformation, toxic chemicals are converted to inactivated form by plant metabolism; inactivated substances are further degraded by soil bacteria or released into air (Phytovolatization).
To the world facing increasing environmental pollution, currently available approaches like incineration and storage to solve pollution problems are expensive, incomplete and again create additional problems of disposal. As compared to this, phytoremediation is the most promising approach for environmental clean-up.
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