Advantages of Phytoremediation
The advantages of phytoremediation cannot be overemphasized. The cost of planting and maintenance is relatively low when compared to other cleanup options. The establishment of self - sustainable ecosystems and the improvement of aquatic and terrestrial habitats are some of the benefits (Dwyer, 1997). The extracted contaminant can be a resource (phytomining) for example, Bañuelos, Ajwa, Terry, and Downey (1997) reported that plant biomass containing essential nutrients such as Selenium may be used as feed for livestock. Contaminants could be transformed to less - toxic forms,
such as mercury and dimethyl selenite gas. These metabolites when released into the atmosphere may be subject to more effective or rapid natural degradation processes such as phytodegradation (Newman, Strand, Choe, Duffy, Ekuan, Ruszaj, Shurtleff, Wilmoth, Heilman and Gordon, 1997)Disadvantages of Phytoremediation
Some well-known hyperaccumulator plants may have a slow growth rate, a reduced biomass and poorly developed roots (USEPA, 2000). Species that accumulate high concentrations of heavy metals may have phytotoxic effects (Nanda’Kumar, Dushenkov, Motto and Raskin, 1995). For example, Reeves and Brooks (1993) explained that Thlaspi otundifolium (L.) Gaudin. accumulated over 0.8 % of lead and about 2 % zinc, while Armeria maritima var. halleri (Mill.) Wild. accumulated over 1000 g/g of lead. Phytoremediation is dependent on the volume and depth of roots because only the area reached by roots may undergo remediation (Reeves and Brooks, 1993). Edaphic factors such as soil structure and texture may limit the depth and spread of roots (USEPA, 2000). Raskin et al. (1997) suggested that metal immobilization and uptake results from laboratory and greenhouse studies may not be achievable in the field. This could be as a result of the fact that most phytoremediation designs are climate sensitive and may be effective only in specific environments. To ensure a sustainable plant cover system, special monitoring of the field may be required. Plants may undergo natural selection that may promote dominance of one species over many other sister plants that are part of the treatment (NandaKumar et al., 1995). In a phytoremediation cover, plants which have taken up contaminants may be used for human or animal consumption which may have adverse effects on the food chain as contaminants move up the chain (Sarma, 2011)
2.6 Vetiver Grass Technology
The use of Vetiver in waste water treatment started in 1995 in Australia. It spread to China in 1997 (Anon, 1997). Leachates from landfills were first treated with Vetiver and the result was successful (Truong and Stone 1996). In China, polluted river water was purified with Vetiver (Anon, 1997). The effectiveness of Vetiver in remedying wastewater has been linked to its high transpiration rate (6.86 L per day) that is either stored within the plant or evaporated through the leaves (Jayashree, Rathinamala, and Latshamaperumalsamy, 2011). Roongtanakiat and Chairoj (2001) noted that various contaminants like total suspended solids, dissolved solids (above 2000 mg/l), electrical conductivity (above 3.0 dS/m), hardness, biochemical oxygen demand (BOD), chemical oxygen demand (COD), dissolved oxygen, nitrogen, phosphorous, heavy metals, and other contaminants have been minimized below toxic levels in India using Vetiver grass.
Research has also shown that Vetiver has a high capacity for absorbing nutrients such as Nitrogen and Phosphorus in polluted water (Truong, 2000). In a hydroponic system using sewage effluent, Vetiver was able to remove over 90 % of both Nitrogen and Phosphorus from the effluent; it also reduced algal growth and feacal coliforms (Truong and Hart, 2001). Sixty- two (62 %) to ninety three (93 %) of dissolved phosphates was also removed during hydroponic treatments with Vetiver (Truong, Mason, and Waters, 2000). In Queensland, the Department of Natural Resources and Mines recorded a removal range of 36 – 100 % for ammonia, 26 – 97 % total nitrogen, and 38 – 62 % total phosphorus (Truong and Baker, 1998).
In China, it has been reported that Vetiver could remove nutrients from wastewater within two days under experimental conditions (Xia, Wang, and Gh, 1999). Jayashree et al. (2011) reported that when C. zizanioides was used to treat domestic effluents for 4 days in Australia, the removal of total Nitrogen was 94 % and 90 % for phosphorus. Similarly, a reduction of 83 – 92 % for Nitrogen and 74 % for Phosphorus was reported by Xia et al. (1999) in China.
Suresh and Ravishankar (2004) also reported a reduction in Nitrogen from 8.85 % to 0.53 % and phosphorus from 5.9 % to 0. 81 % in wastewater from textile industries treated in India. Jayashree et al. (2011) reported that Vetiver changed the pH of domestic effluent from 7.26 – 5.98 in India. This system is also known to have reduced pH from 8.6 to 7.8 in India (Suresh and Ravishankar, 2004). Also, dissolved oxygen increased from 0.21 to 8.81, while, Chemical Oxygen Demand (COD) was reduced from 248 – 79 mg/l (Zheng, Tu, and Chen, 1997). Cull, Hunter, Hunter, and Truong (2000) also reported a 30 % reduction of COD in hydroponic setups of C. zizanioides, while BOD was reportedly reduced by 63 % in the same study (Zhao, 2012).
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