**3. Mechanisms of phytoremediation**

Depending upon the process by which plants/microbes are removing or reducing the toxic effect of contaminants from the soil and water, phytoremediation technology can be broadly classified as follows:


Among the different methods of phytoremediation, phytoextraction by hyperaccumulators is the most efficient one as it helps in removal of the phytoextracted biomass from contaminated sites. But phytoremediation cannot be used as a primary treatment method for highly contaminated areas with heavy metals like Cd, Zn, Cr and Pb, because of the prolonged time taken for the complete clean up. The dominant families that include hyperaccumulators are Asteraceae, Brassicaceae, Caryophyllaceae, Cyperaceae, Cunouniaceae, Fabaceae, Flacourtiaceae, Lamiaceae, Poaceae, Violaceae, and Euphobiaceae. Brassicaceae has the largest number of taxa *viz*. 11 genera and 87 species. *Thlaspi* species are known to hyperaccumulate more than one metal *viz.,T. caerulescence* - Cd, Ni, Pb, and Zn; *T. goesingense* - Ni and Zn and *T. ochroleucum* - Ni and Zn and *T. rotundifolium* - Ni, Pb and Zn. Aquatic plants in freshwater, marine and estuarine systems act as receptacle for several metals. Several aquatic macrophyteslike*Eichhorneacrassipes, Hydrillaverticillata, Typhaangustata,* etc. can remove Zn, Cu, Pb, Ni and Cd from lakes and maintain water quality.
