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The idea that plants can be used for environmental remediation is not new. Extensive research on using plants for treating radionuclide contamination was conducted in Russia in the early 1960s.
(Timofeev-Resovsky et al., 1962). Since then, there have been a number of reports that aquatic plants such as water hyacinth, duckweed and water velvet can accumulate heavy metals from contaminated water (Mo et al, 1989; Jackson et al, 1990).
Recently the value of terrestrial plants for environmental remediation has been recognized (Cunningham and Berti, 1993).
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Crop plants such as Indian mustard have been used to extract heavy metals from soil and translocate them to the leaves and stalks of plants (Raskin et al, 1994, Duchenkov et al 1995). A list of plants used in phytoremediation is given in Table 9.1. Aquatic plants can be used in bioremediation (Phytoremediation) especially to remediate sites contaminated with heavy metals.
For example, water hyacinth (Eichhornia crassipes) has been used for purifying not only domestic wastewater but also industrial wastewater. It can readily absorb, accumulate and concentrate heavy metals such as Pb, Fe, Cu, Cd, Hg and Ni. Other studies have evaluated the potential of water hyacinth to absorb various organic chemicals – phenols and toxaphene; and to remove even radioactive metals from effluents. Cell suspension cultures of Datura innoxia have been found to remove a wide variety of metal ions from solutions. 
Like animals, most plants can be poisoned by heavy metals. But certain plants can accumulate as much as 40% of their weight as heavy metals without harm to themselves. According to Ensley et al, (1997), further advances in phytoremediation require a multidisciplinary approach, covering fields as diverse as agricultural engineering, plant biology, agronomy, soil science, microbiology and genetic engineering.
A better understanding of the biochemical processes involved in plant heavy metal uptake, transport and accumulation will certainly improve phytoremediation using modem genetic approaches. One strategy for improving the phytoremediation potential of high biomass plant species is the introduction of genes responsible for metal accumulation and resistance from the wild metal accumulators. In the absence of known “phytoremediation genes” this may be accomplished via somatic and sexual hybridization followed by extensive screening and backcrossing of progeny.
Mutagenesis of selected high biomass plant species may also produce improved phytoremediating cultivars. Phytoremediation of heavy metals is designed to concentrate metals in plant tissues, thus minimizing the amount of hazardous waste, which needs to be treated and deposited at hazardous waste sites. But an economical method of reclaiming metals from plant residue should be developed.
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As present, methods for further concentration of metals in plant tissues include – sun, heat air drying, environmentally safe ashing or incineration, composting, pressing and. compacting, and acid leaching. These plants can then be harvested and burned to recover the metals. Plants can be useful in treating other chemicals also. One aquatic plant reduced the concentration of TNT from 128 ppm to 10 ppm. Submerged roots of sunflowers can concentrate radioactive wastes from the water. Because the radioactive chemicals accumulate only in the roots, only they require special disposal.
