THE POTENTIAL OF COWPEA (VIGNAUNGUICULATA) TO REMEDIATE LEAD IN SOIL CONTAMINATED WITH DIFFERENT CONCENTRATIONS OF LEAD
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CHAPTER ONE
1.1 INTRODUCTION
Different types of heavy metals such as Cu, Zn and Ni are necessary micronutrients compulsory for a variety of functions including electron transfer reactions and as cofactors in many proteins and enzymes, on the other hand other metals like Ar, Cd and Pb are considered non-necessary. Both types of metals are toxic above certain concentrations. They inactivate metal-sensitive enzymes consequential in growth retardation and the worst in case of death of the organism. The metals found in our environment come from natural weathering process of earth’s crust, soil erosion, mining, industrial discharge, urban runoff, sewage effluents, air pollution fall out, pest or disease control agents. These heavy metals also originate through human activities such as application of phosphate fertilizers, military activities, metal working industries, mining and smelting (Chowdhury et al., 2015).
In recent years, public concerns relating to ecological threats caused by heavy metal (HM) have led to intensive research of new economical plants based remediation technologies. Conventional methods used for reclamation of contaminated soils, namely chemical, physical and microbiological methods, are costly to install and operate (Danhet al., 2009). The rapid increase in population coupled with fast industrialization growth causes serious environmental problems, including the production and release of considerable amounts of toxic waste materials into environment (Zhuang et al., 2007). The concentrations of the contaminants can vary from highly toxic concentrations from an inadvertent spill to barely measurable concentrations that after long-term exposure can be injurious to human health (Alexander, 1999). Mainly, heavy metals are toxic because they cause DNA damage and their carcinogenic effects in animals and humans are probably caused by their mutagenic ability (Knasmulleret al., 1998). Heavy metals are not degradable, without intervention they stay in soil for centuries. Over recent decades, the annual worldwide release of heavy metals reached 22,000 t (metric ton) for cadmium, 939,000 t for copper, 783,000 t for lead and 1,350,000 t for zinc (Singh et al., 2003).
Phytoremediation is the use of green plants to remove pollutants from the environment or render them harmless. Phytoremediation is a new technology in which plants are used to remove pollutants from water and soil. The use of metal-accumulating plants to clean environment contaminated with heavy metals is the most rapidly developing component of this environmentally friendly and cost-effective technology. The ever-increasing environmental pollution in agricultural soil caused by heavy metals due to application of sewage sludge, city refuse, and heavy metals containing fertilizers or pesticides, is becoming a major problem in modern agriculture. Phytoremediation is based on the removal of contaminants from the soil by mechanisms such as phytoextraction, phytodegradation, rhizofiltration, phytostabilization and phytovolatilization (Salt et al., 1995). In phytoextraction, plants absorb metals from soil through the root system and translocate them to harvestable shoots where they accumulate. Hyperaccumulators mostly used this process to extract metals from the contaminated site. The recoveries of the extracted metals are also possible through harvesting the plants appropriately.
The mechanisms of phytoremediation involved in heavy metal remediation are limited to up-take, adsorption, transport and translocation, sequestration into vacuoles, hyperaccumulation and in some cases, volatilization (Meagher, 2000). When present at increased concentrations, both essential (Cu, Fe, Mn, Mo, Zn) and non-essential metals (e.g., Cd, Pd, Hg) are toxic. Heavy metals cannot be metabolized; only possible strategy to apply is their extraction from contaminated soil and transfer to the smaller volume of harvestable plants for their disposal (Padmavathiamma and Li, 2007).
A number of plant species have been recognized for the rationale of phytoremediation. Assured plant species known as hyper accumulators are able to accumulate potentially phytotoxic fundamentals to concentrations 50-500 times higher than average plants (Lasat, 2002). Transgenic rice plants have been used for phytoremediation (Chowdhury et al., 2015). However many of the hyper accumulators are slow growing and have reduced biomass pro-duction thus requiring several years for decontamination of the polluted sites. Trees appear as an attractive hyper accumulator due to their extensive root system, high water uptake, rapid growth, and large biomass production (Gullneret al., 2001).The hyperaccumulation of heavy metals in some plants has been recorded by many researchers during last few decades (Barman et al., 2000) and this has emphasized the importance of further advanced research in molecular basis of phytoremediation technology. The hyperaccumulation of heavy metals is depends on the plant species, soil condition (pH, organic matter content, cation exchange capacity etc.) and types of heavy metal (Barman et al., 2001; Spinoza-Quinones et al., 2005; Xian and Shokohifard, 1989; Otteet al., 1993). In metal biology, it is experimentally proved that even some metals that are essential for the normal plants growth (such as iron and copper) may become toxic, depending on the oxidation state, complex form, dose and mode of exposure (Beyersmann and Hartwig, 2008).
Vignaunguiculatais an herbaceous, annual plant in the pea family Fabaceae. The domestication of Vignaunguiculatawas originated in West Africa. It is one of the most widely used legumes in the semiarid tropics including Asia, Africa, southern Europe and Central and South America. Hundreds of experiments have been conducted on this legume to understand its morphology, physiology, as well as to understand the effect of different stresses on this legume. However, only a scanty number of experiments have been carried out to study the Zn phytoextracting ability of V. unguiculata(Tanee and Akonye, 2009). Basaket al. 2014 in their study reported that V. unguiculataaccumulated a considerable amount of the heavy metal Zn which makes it a potential candidate as a phytoremediation plant in the remediation of Zn from contaminated soil.
1.2 OBJECTIVES OF THE STUDY
The objectives of this study was to;
- Assess the potential of Cowpea (Vignaunguiculata) to remediate lead in soil contaminated with different concentrations of lead
- To examine the feasibility of Cowpea (Vignaunguiculata) as a hyperaccumulator plant for lead
- To determine the bioconcentration factor in Vignaunguiculata.
- To determine the translocation factor in Cowpea (Vignaunguiculata).