Vetiver (Chrysopogon spp) belongs to the family Poaceae, subfamily Panicoideae, tribe Andropogonae and subtribe Sorghinae (APG, 2003; Marcacci, 2004). The genus Chrysopogon is related to Sorghum (Hengchaovanich, 1999). The African indigenous Vetiver (C. nigritanus) is a tall tufted perennial grass, about 150 – 300 cm in length. Its inflorescence is a large panicle, (up to 40 cm long) with 8 – 10 whorls made up of 15 slender branches and sessile linear-lanceolate spikelets. It grows on river banks, occasionally on roadsides with flowers arranged in panicles (Goudiaby and Diatt, 2003).
In Nigeria, Vetiver is known as Chor’dor’de, Ngongonari, So’dornde, So’mayo, Zemako (Fulani) and Jema (Hausa) (Chomchalow, 2003; Juliard, 2003).2.2 Phytoremediation Technology
Phytoremediation is a group of technologies that uses plants to reduce, remove, degrade or immobilize environmental toxins, primarily those of anthropogenic origin, with the aim of restoring sites to a condition suitable for private or public uses/applications (Mukhopadhyay and Maiti, 2010). Plants which accumulate high concentrations of pollutants are known as hyperaccumulators. They include plants such as Thlaspi carulescens (L.) Czern. (Alpine pennycress), Brassica juncea (L.) Czern (Indian mustard), and Pteris vittata L. (Chinese brake) (Sarma, 2011). The efficiency of these plants can however be limited as a result of their low biomass output, intolerance to extreme conditions, slow growth rate and shallow root system. The characteristic high root volume of Vetiver has been noted as a feature responsible for its potentials in absorption, translocation and accumulation of contaminants (Roongtanakiat, Nirunrach, Chanyotha, and Hengchaovanich, 2003).
2.3 Types of Phytoremediation
There are different types of phytoremediation technologies, these depend on the mechanism employed by plants in solving environmental problems. An effective cleanup may involve one or more of the following technologies: phytoextraction, rhyzofilteration, phytostabilization, rhizodegradation, phytodegradation, phytovolatilization and hydraulic control (Raskin, Smith, and Salt, 1997).
2.3.1 Phytoextraction
Phytoextraction, is the process of planting species of a crop that is known to accumulate contaminants in the shoots and leaves of the plants, and then harvesting the crop to remove the contaminants from the site. With this technique, only a small amount of contaminant is actually thrown away. It is more reliable than excavation, land-filling and other methods. Its use is popular in soil treatments, sediment and sludge reduction and wastewater treatment (USEPA, 2000).
2.3.2 Rhizofiltration
Rhizofiltration is the absorption of contaminants into the roots in solution. This technology can be used in treating extracted ground water, surface water and wastewater. It depends on the nature of contaminants which are often removed by physically uprooting the plant (USEPA, 2000).
2.3.3 Rhizodegradation
Rhizodegradation is a phytoremediation technology which breaks down organic pollutants in a medium through microbial activities which are often influenced by root secretions (exudates). The contaminant may not be translocated to other parts of the plant because degredation takes place within the root zone (Raskin et al., 1997). Plant roots release exudates which contain enzymes, flavonones, amino acids and other compounds necessary for phytoremediation. (Shimp, Tracy, Davis, Lee, Huang, Erikson, and Schnoor, 1993).
2.3.4 Phytodegradation
Phytodegradation is the breakdown of contaminants during metabolic process within the plant. This breakdown occurs when plants release certain compounds (enzymes) on an absorbed contaminant (Raskin and Ensley, 2000; USEPA, 2006).
2.3.5 Phytovolatilization
Phytovolatilization involves the uptake and transpiration of a contaminant by a plant, with the release of the contaminant or a modified form of the contaminant to the atmosphere through plant metabolism (Dwyer, 1997; USEPA, 2000).
2.3.6 Hydraulic Control
Hydraulic control with plants is a type of technology which uses plants to reduce and contain ground water contaminants by enhancing speedy uptake by plants. This process is also known as phytohydraulics. It can be used in treating ground, surface and soil water (Raskin and Ensley, 2000; USEPA, 2000)
2.3.7 Vegetative Cover Systems
These are systems which grow plants over environmentally harmful materials for a long period of time. This process requires minimal maintenance and reduces environmental risk to an acceptable limit (Schnoor, Licht, McCutcheon, Wolfe, and Carreira, 1995).
2.3.8 Riparian Corridors/Buffer Strips
Riparian corridors/buffer strips are used to control and remediate surface runoff and groundwater contamination moving into a river. Riparian corridors/buffer strips installed along water bodies are active against down – gradient movement of a polluted plume (Dwyer, 1997). These systems degrade contaminants through uptake and metabolism (Schnoor et al., 1995). They can be used in the effective remediation of surface and ground water by combining different phytoremediation technologies such as phytodegradation, rhizodegradation, phytovolatilization, and phytoextraction (USEPA, 2000).
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