In many cases the industrious processes of man have left behind contaminants in the soil. These contaminants pose a significant threat to the health of not just humans but to all natural life in the surrounding environment. This problem is made worse when the contaminants enter the food chain or the underlying water table where they spread and become hard to manage.
The most common contaminants present in the soil are; heavy metals, industrial chemicals, pesticides and crude oil all of which have been shown to be toxic to wildlife and humans. There have been substantial changes by governments and industries to limit and even remove the need for these harsh chemicals (most notably the ban on polychlorinated biphenyls), however much of the damage has already been done as so emphasis is on the removal of these toxins. The traditional methods for removing contaminants are based around man-made technologies and processes to either degrade or extract the toxins in the soil. In the past methods such as soil removal, soil oxidation and land fill have been used to decontaminate sites. These methods are; labor intensive, costly and in many cases they can be relatively damaging to the environment. This has caused the social and political push towards cleaner and environmentally sustainable methods. Plants and trees have the natural ability to degrade and absorb metals and compounds from the soil, it is in fact the way in which they obtain much of their essential nutrients. This capability can be exploited and utilised to extend to contaminants, and was conceptualised by Dr. IlyaRaskin as “Phyto-remeditaion”.
The term phytoremediation (phyto meaning plant and the Latin suffix remedium meaning to clean or restore) refers to a diverse collection of plant-based technologies that use either naturally occurring, or genetically engineered, plants to clean contaminated environments (Flathman and Lanza, 1998). Phytoremediation is clean, simple, cost effective, non-environmentally disruptive (Wei et al., 2004) green technology and most importantly, its by-products can find a range of other uses (Truong, 2003).
Phytoremediation is an eco friendly approach for remediation of contaminated soil and water using plants comprised of two components, one by the root colonizing microbes and the other by plants themselves, which accumulates the toxic compounds to further non toxic metabolites. Various compounds viz., organic synthetic compounds, xenobiotics, pesticides, hydrocarbon and heavy metals, are among the contaminants that can be effectively remediated by plants (Suresh and Ravishankar, 2004).
TYPES OF PHYTOREMEDIATION TECHNOLOGY
The four different plant-based technologies of phytoremediation, each having a different mechanism of action for remediating metal-polluted soil, sediment or water:.
1. Phytoextraction: Plants absorb metals from soil through the root system and translocate them to harvestable shoots where they accumulate. Hyperaccumulators mostly used this process to extract metals from the contaminated site. The recoveries of the extracted metals are also possible through harvesting the plants appropriatel.
2. Phytovolatilization: Plants used to extract certain metals from soil and then release them into the atmosphere by volatilization
3. Phytostabilization: In this process, the plant roots and microbial interactions can immobilized organic and some inorganic contaminants by binding them to soil particles and as a result reduce migration of contaminants to grown water
4. Phytofiltration:Phytofiltration is the use of plants roots (rhizofiltration) or seedlings (blastofiltration) to absorb or adsorb pollutants, mainly metals, from water and aqueous waste Streams (Prasad and Freitas, 2003).
Limitations: The application of phytoremediation for pollution control has several limitations that require further intensive research on plants and site-specific soil conditions (Danhet al., 2009). It is generally slower than most other treatment viz., chemical, physical and microbiological plants with low biomass yields and reduced root systems do not support efficient phytoremediation and most likely do not prevent the leaching of contaminants into aquatic system. Environmental conditions also determine the efficiency of phytoremediation as the survival and growth of plants are adversely affected by extreme environmental conditions, toxicity and the general conditions of soil in contaminated lands (Danhet al., 2009). In phytoremediation technology, multiple metals contaminated soil and water requires specific metal hyperaccumulator species and therefore requires a wide range of research prior to the applications. Though the phytoremediation is cost effective, environment friendly, ability to reclaim heavy metals contaminated site, several limitations also create trouble in implementing the strategy, e.g., metal must be in bio-available form to plants; if metals is tightly bound to the organic portions of the soil, some time it may not be available to plants. Furthermore, if the metals are water soluble, in nature it will pass by the root system without accumulation.
The phytoremediation of mixed heavy metals contaminated soil have conformant with some problem e.g., The cadmium/zinc model hyperaccumulatorThlaspicaerulescens is sensitive toward copper (Cu) toxicity, which is a problem in remediation of Cd/Zn from soils in the presence of Cu by application of this species. In T. caerulescens Cu induced inhibition of photosynthesis followed the sun reaction type of damage, with inhibition of the photosystem II reaction center charge separation and the water-splitting complex (Mijovilovichet al., 2009). Despite some limitations, present day phytoremediation technology are using worldwide and various research laboratories are at present engaged to overcome the limitations.
Plants Selections Criteria For Phytoremediation
Plant species selection is a critical management decision for phytoremediation. Grasses are thought to be excellent candidates, because their fibrous rooting systems can stabilize soil and provide a large surface area for root-soil contact (Kulakowet al., 2000). The selection of plants is possibly the single most important factor for fruitful phytoremediation strategy. The application of indigenous plant species for phytoremediation is often favoured as it requires less management and acclimatizes successfully in native climate conditions and seasonal cycle. However, some exotic plant species may perform better in remediation of specific metals and can be safely used where the possibility of invasive behavior has been eliminated (USEPA, 2000). Some important criteria in selecting plant species for phytoremediation are:
• The levels of tolerance with respect to metal known to exist at the site
• The level of adequate accumulation, translocation and uptake potential of metals
• High growth rate and biomass yield
• Tolerance to water logging and extreme drought conditions
• Availability, habitat preference e.g., terrestrial, aquatic, semi-aquatic etc.
• Tolerance to high pH and salinity
• Root characteristic and depth of the root zone
Source: Flathman, P.E. and G.R. Lanza, 1998. Phytoremediation: Current views on an emerging green technology. Soil Sediment Contamin. Int. J., 71: 415-432.. Truong, P., 2003. Vetiver system for water quality improvement. Proceedings of 3rd International Vetiver Conference, Oct. 6-9, Guangzhou, China, pp: 61-74.. Wei, S.H., Q.X. Zhou, X. Wang, W. Cao, L.P. Ren and Y.F. Song, 2004. Potential of weed species applied to remediation of soils contaminated with heavy metals. J. Environ. Sci., 16: 868-873. Suresh, B. and G.A. Ravishankar, 2004. Phytoremediation-A novel and promising approach for environmental clean-up. Crit. Rev. Biotechnol., 24: 97-124. Danh, L.T., P. Truong, R. Mammucari, T. Tran and N. Foster, 2009. Vetiver grass, Vetiveriazizanioides: A choice plant for phytoremediation of heavy metals and organic wastes. Int. J. Phytorem., 11: 664-691. Mijovilovich, A., B. Leitenmaier, W. Meyer-Klaucke, P.M. Kroneck, B. Gotz and H. Kupper, 2009. Complexation and toxicity of copper in higher plants. II. Different mechanisms for copper versus cadmium detoxification in the copper-sensitive cadmium/zinc hyperaccumulator Thlaspicaerulescens (Ganges Ecotype). Plant Physiol., 151: 715-731. Kulakow, P.A., A.P. Schwab and M.K. Banks, 2000. Screening plant species for growth on weathered, petroleum hydrocarbon-contaminated sediments. Int. J. Phytorem., 2: 297-317. USEPA, 2000. Introduction to Phytoremediation.United States Environmental Protection Agency, Washington, DC., USA.