1.1Introduction
Global industrialization, urbanization, and population in the last two decades have resulted in the generation of huge quantities of toxic waste. This hazardous waste includes a variety of organic and inorganic compounds which pose serious threats to ecosystems. Organic contaminants include different compounds such as petroleum hydrocarbons (e.g., benzopyrene), chlorinated solvents (i.e., polychlorinated benzenes, also known as PCBs), linear halogenated hydrocarbons (e.g., trichloroethylene), volatile organic carbons, and explosives such as trinitrotoluene. Inorganic compounds include nitrates, phosphates, metals and metalloids, such as mercury (Hg), arsenic (As), lead (Pb), cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), selenium (Se), silver (Ag), and zinc (Zn), and nonradioactive or radioactive nuclides, like uranium (U), strontium (Sr), and cesium (Cs). Despite requirements for pollution control measures, these wastes are generally dumped on land or discharged into water bodies (rivers, canals, lakes, etc.) without adequate treatment, and thus they become a large source of environmental pollution and health hazards. It is, therefore, urgent to adequately remove these pollutants from contaminated sites. Contamination of soil and water poses major environmental concerns in the present scenario. A wide range of methods based on not only physical and chemical but also biological means have been available for the remediation of soil and water for decades, but environmental preservation requires development of more sustainable approaches that promise thorough, economical, and environmentally friendly ways, compared to conventional methods (Salt et al. 1995).
In recent decades, phytoremediation has provided a cost-effective, long-lasting, and aesthetic solution for remediation of hazardous pollutants from contaminated sites (Kramer 2005, Suresh and Ravishankar 2004). The idea that plants can be used for environmental remediation is very old and cannot be traced to any particular source (Raskin et al. 1997). About 300 years ago, plants were proposed for use in the treatment of wastewater. The term phytoremediation (“phyto” meaning plants and the Latin suffix “remedium” meaning to clear or restore) refers to a diverse collection of plant-based green technologies that use either naturally occurring or genetically engineered plants to remove, transfer, stabilize, and/or degrade contaminants located in soil, sediment, and water (Cunningham et al. 1997, Newman and Reynolds 2004). Phytoremediation has also been called green remediation, botano-remediation, agroremediation, and vegetative remediation (Chaney et al. 1997). It is socially accepted by surrounding communities and regulatory agencies as a potentially effective and beautiful technology (Newman and Reynolds 2005). A number of green plants, including herbs, shrubs, and trees (both terrestrial and aquatic) have been reported to be endowed with magnificent abilities for restoration and reclamation of contaminated environments (Yoon et al. 2006, Gupta and Sinha 2007a, Qixing et al. 2011). Plants naturally provide roots, stems, and leaves as habitats for a wide array of microorganisms which can break down contaminants, enhancing the treatment process. Plants, through several natural biophysical and biochemical processes, such as adsor...
