Phytoremediation is defined as the use of green plants to remove pollutants from the environment or render them harmless. This cost-effective plant-based approach to remediation takes advantage of the remarkable ability of plants to concentrate elements and compounds from the environment and to metabolize various molecules in their tissues. Toxic heavy metals and organic pollutants are the major targets for phytoremediation. Phyto-(or green-plant based-) remediation is not a new concept. About 300 years ago, plants were proposed for use in the treatment of wastewater. Thlaspi caerulescens and Viola calaminaria were the first plant species documented to accumulate high levels of metals in leaves. In 1935, Byers reported the plants of the genus Astragalus were capable of accumulating up to 0.6% selenium in dry shoot biomass. One decade later, Minguzzi and Vergnano (1948) identified plants able to accumulate up to 1% nickel in shoots. In 1977, Rascio reported toluene and high zinc accumulated in the shoots of Thlaspi caerulescens. In recent years, knowledge of the physiological and molecular mechanisms of phytoremediation began to emerge together with biological and engineering strategies designed to optimizes and improve phytoremediation. Phytoremediation can be divided into the following areas.

I. Phytoextraction: In this process, pollutant-accumulating plants are used to remove metals or organics from soils by concentrating them in the harvestable parts. There are two basic strategies of phytoextraction being developed:

(a) Induced or Chelate-assisted phytoextraction: Synthetic metal chelates such as ethlenediaminetetraacetic acid (EDTA) or EGTA are applied to soils enhancing metal accumulation by plants. Metal-enriched plant residues can be disposed of as hazardous material or, if economically feasible used for metal recovery.

(b) Continuous phytoextraction: This relies on the ability to accumulate metals in their shoots, over extended periods. To achieve this, plants must possess efficient mechanisms for the detoxification of the accumulated metals. Nickle resistance in Thlaspi goesingense is a primary determinant of Ni hyperaccumulation when plants were grown hydroponically. Therefore, the ability to manipulate metal tolerance in plants will be key to the development of efficient phytoremediation crops. In order to develop hypertolerant plants capable of accumulating high concentrations of metals it will be vital to understand the existing molecular and biochemical strategies plants adopt to resist metal toxicity. Hg2+ - resistant Arabidopsis thaliana overexpressing bacterial mercury reductase was recently shown to remove Hg2+ efficiently from solution.

II. Phytodegradation: - This is the use of plats and associated microorganisms to degrade organic pollutants, including ammunition wastes (e.g. TNT and GTN), polychlorinated phenols (PCBs), and tricholoethylene (TCE). Successful phytodegradation requires organic contaminants to be biologically available for absorption to, or uptake and metabolism by, plant or plant-associated microbial systems. Bioavailability depends on the relative lipophilcity of the compound, the soil type, and the age of the contaminant.

The bioavailability of a pesticide is reduced in soils with high organic matter contents. Furthermore, the bioavailability of many other organic contaminants decrease with time; hence old "well-weathered" contaminates may be expected to be more difficult targets for phytoremediation. Accelerated degradation has been obtained for certain pesticides, trichloroethylene and petroleum hydrocarbons, but the overall rate and quantity of degradation has been relatively low.
Soil or rhizospheric microorganisms can play a major role in the decomposition of many organic contaminants. A nitroreductase enzyme of plant origin has trinitrotoluene (TNT) degradation potential. Similarly, a dehalogenase and a laccase enzyme can be used to degrade other containments.

III. Rhizofilteration: - This is the use of plant roots to absorb and adsorb pollutants, mainly metals, from water and aqueous waste streams. An ideal plant for rhizofilteration should have rapidly growing roots with the ability to remove toxic metals from solution over extended periods of time. A number of plants such as Indian mustard (Brassica juncea), rye, corn, sunflower have an intrinsic ability to absorb and precipitate having metals from solution.

Rhizofilteration is particularly effective and economically compelling when low concentrations of containments and large volume of waste are involved. Therefore, rhizofilteration may be particularly applicable to radionuclide contaminant water.

IV. Phytostabilization: - This is the use of plants to reduce the bioavailability of pollutants in the environment. Heavy metal polluted soils usually lack established vegetation cover due to the toxic effects of pollutants. Barren soils are more prone to erosion and leaching which spread pollutants in the environment. A simple solution to the stabilization of these wastes is re-vegetation with metal-tolerant plant species such as Agrotis tenuis (for acid Pb/Zn wastes), Festuca rubra cv. Merlin (for calcareous Pb/Zn wastes), Brassica juncea (for Cr wastes).
Plants may also reduce metal leaching by converting metals from a soluble oxidation state to an insoluble oxidation state. A good phytostabilizing plant should tolerate high levels of heavy metals and immobilize these metals in the soil via root uptake, precipitation or reduction. In addition, these plants should have low shoot accumulation of heavy metals to eliminate the necessity to treat harvested shoot residues as hazardous wastes.

V. Phytovolatilization: - This is the use of plants to volatilize pollutants. Phytovolatilization of metals may have unique advantages over phytoextraction, because it bypasses harvesting and disposal of metal rich biomass. Volatilization of arsenic as dimethyl arsenite has also been postulated as a resistance mechanism in marine algae. Volatilization of selenium from plant tissues may provide a mechanism of selenium detoxification. The volatile selenium compound released from the selenium accumulator Astragalus racemosus was identified as dimethyl diselenide. Selenium released from alfalfa, a selenium non-accumulator, was identified as dimethyl selenide.
More recently a modified bacterial mercuric ion reductase has been introduced into transgenic Arabidopsis thaliana, which converts Hg2+ into elemental mercury (Hg0). In addition to being more tolerant, these transgenic plants are very effective at volatilizing mercury.

Advantages of Phytoremediation: - Plants are solar-driven pumping and filtering systems that have measurable loading, degrading and fouling capacities. Similarly, root may be described as exploratory, liquid-phase extractors that can find, alter and/or translocate elements and compounds against large chemical gradients. Therefore, plants can also be a cost-effective alternative to physical remediation systems.

Limits of Phytoremediation: - As plants are alive, their root require oxygen, water and nutrients, soil texture, pH, salinity, pollutants concentrations and the presence of other toxins must be within the limits of plant tolerance. Contaminants that are highly water soluble may leach outside the root zone and require containment. Phytoremediation is also frequently slower than physio-chemical process, may need to be considered as long-term remediation process.
As we know that phytoremediation is a nascent technology that seeks to exploit the metabolic capabilities and growth habits of plants, this will require a multidisciplinary approach, before we can take full advantage of the genetic potential of the plants (Table1) used in this technology.

Table1. List of some plants used in phytoremediation

Plant -- Metal -- Method of Phytoremediation

Brassica juncea -- Lead -- Chelate-assisted phytoextraction
Thlaspi caerulescens -- Cadmium -- Continuous phytoextraction
Silense vulgaris -- Zinc -- Continuous phytoextraction
Brassica oleracea -- Zinc -- Continuous phytoextraction
Raphanus sativus -- Cadmium -- Continuous phytoextraction
Thlaspi caerulescens -- Nickel -- Continuous phytoextraction
Alyssum lesbiacum -- Copper -- Continuous phytoextraction
Alyssum murale -- Lead -- Continuous phytoextraction
Arabidopsis thaliana -- Chromium -- Continuous phytoextraction
Brassica juncea -- Selenium -- Continuous phytoextraction
Ipomea alpine -- Copper -- Continuous phytoextraction
Haumaniastrum robertii -- Cobalt -- Continuous phytoextraction
Sebertia acuminate -- Nickel -- Continuous phytoextraction
Agrotis tenuis -- Lead -- Phytostabilization
Festuca arundinacea -- Boron -- Phytovolatilization
Hibiscus cannibus -- Boron -- Phytovolatilization
Lotus corniculatus -- Boron -- Phytovolatilization
Astragalus racemosus -- Selenium -- Phytovolatilization
Helianthus annus -- Uranium -- Rhizofiltration

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