CHAPTER ONE BACKGROUND OF THE STUDY 1.0 INTRODUCTION

Although heavy metals are naturally present in the soil, geologic and anthropogenic activities increase the concentration of these elements to amount that are harmful to both plants and animals. Some of these activities include; Mining and smelting of metals, burning of fossil fuels, use of fertilizers and pesticides in agriculture, production of batteries and other metal products in industries, sewage sludge, municipal waste disposal. (Alloway, 1990; Raskin etal., 1994; Shen et al., 2002). Metals are found naturally in the earths crust and their compositions vary among different localities, resulting in spatial variations of surrounding concentrations. The metal distribution in the atmosphere is monitored by the properties of the given metals and by various environmental factors (Khalifi and Hamza-Chaffai, 2010).These metals are quintessential to maintain various biochemical and physiological functions in living organisms when in very low concentrations, however they become noxious when they exceed certain threshold concentrations. Although it is acknowledge that heavy metals have many adverse heaths effects and last for a long period of time, heavy metals exposure continues and is increasing in many parts of the world. Heavy metals are significant environmental pollutants and their toxicity is a problem of increasing significance for ecological, evolutionary, nutritional and environmental reasons (Jaishankar et al., 2013; Nagajyoti et al., 2010). Some heavy metals such as Co, Cu, Fe, Mo, Ni, V and Zn are required in minute quantities by organisms. However, excessive amount of these elements can become harmful to organism. Other heavy metals such as Pb, Cd, Hg and As (a metalloid but generally referred to as heavy metal) do not haveany beneficial effect on organisms and are thus regarded as the “main threats” since they are very harmful to both plants and animals. Metals exist either as separate entities or in combination with other components. These components may include exchangeable ions sorbed on the surfaces of inorganic solids, non-exchangeable ions and insoluble inorganic metal compound or free metal ions in the soil solution, metal complex of organic materials and metals attached to silicate minerals. (Marques et al., 2009). Heavy metals may have significant toxic and hazardous effects on human health, especially cadmium and lead which are contained in crude oil. Heavy metal contamination affects the biosphere in many places worldwide (Cunningham and Lee, 1997; Raskin and Ensley, 2000; Meagher, 2000). Heavy metals enter the environment by natural and anthropogenic means, such include; natural weathering of the earth’s crust, mining, soil erosion, industrial discharge, urban runoff, sewage effluents, pest or disease control agents applied to plants, air pollution fallout and a number of other (Ming-Ho, 2005). For most people the main route of exposure to these toxic elements is through the diet (food and water). The contamination chain of heavy metals almost always follows a cyclic order, industrial, atmosphere, soil, water, food and human (Castro et al., 2008). Anthropogenic activities have represented a growing environmental problem affecting food quality and human health in the Niger Delta region of Nigeria. Nigeriaas a major producer and exporter of crude petroleum oil continues to experience oil spills and this exposes the environmental hazards and its attendant affect on agricultural lands as well as on plants growth and development (Agbogidi et al., 2005). Growth reduction as a result of changes in physiological and biological process in plants growing on heavy metal polluted soils has been recorded (Chatterjee et al., 2000;Oncel et al., 2000; Oancea et al., 2005). Continued decline in plant growth reduces yield which eventually leads to food insecurity. Therefore the remediation of heavy metal polluted soils cannot be overemphasized.Various methods of remediation metal polluted soils exist; they range from physical and chemical methods (such as encapsulation, solidification, stabilization, electro-kinetics, vitrification, vapour extraction and soil washing and flushing) are expensive and do not make the soil suitable for plant growth (margque et al., 2009). Biological approach (bioremediation) on the other hand encourages the establishment/re-establishment of plants on polluted soils. Its is an environmentally friendly approach because it is achieved via natural processes. Bioremediation is also an economical remediation technique compared with other remediation techniques. This paper discusses the nature and properties of soils polluted with heavy metals. Plant growth and performance on these soils were examined. Biological approaches employed for the remediationof heavy metal polluted soil were equally highlighted. Toxic metal pollution of waters and soils is a major environmental problem and most convential remediation approach to not provide acceptable solutions (Salt et al.,1995). Medical plants are known to grow in various adverse environmental condition including crude oil contaminated soils. Heavy metals are serious pollutants in natural environments due to their toxicity. Persistence and bioaccumulation problem. The accumulation of heavy metal contaminations in the environment has become a concern due to the health risk to humans and animals. The problems is not restricted to soils with high metal levels such as mining areas but also includes those with moderate to low contamination of metals. These toxicity elements, such as Cd, Cu and Zn are present at elevated levels mainly through human activities, as smelting, refining of non-ferrous metals, electroplating, Agriculture practice (Ross, 1994). Certain plants don’t only accumulate metals in their roots but also translocation from roots to the leaves or shoots (Baker et al., 2000). Although these heavy metals have crucial biological functions in plants and animals, sometimes their chemical coordination and oxidation reduction properties have given them an additional benefit so that they can escape control mechanisms such as homeostatic transport, compartmentalization and binding to required cell constituents. Thesemetals bind with protein sites which are not made for them by displacing, original metals from their natural binding site causing malfunctioning of cells and ultimately toxicity. Previous research had found that oxidative deterioration of biological macro molecules is primarily due to binding of heavy metals to the DNA and nuclear protein (Flora et al., 2008). The presence of one heavy metal may affect the availability of another in the soil and hence plant. In other words, antagonistic and synergistic behaviours exist among heavy metals,Salgare and Archearkar reported that the inhibitory effect of Mn on the total amount of mineralized carbon was antagonized by the presence of Cd (Salgare and Acharekar 1992). Among wild xerophytic plant, Calotropisprocera and Citrulluscolocynth’s as a dominant and a common desert plants that grow widely in warm and urbanizing regions. These species have a high capacity for taking heavy metals into it’s tissue due to their ability to absorb and tolerate heavy metals without serious physiological damage as reported by Hashem and Al-ferra (1997) and Tulyan and Al-farraj (2002) in Saudi Arabia. The plants growing in metalliferous soil can be grouped into the following three categories according to Bake (1981) (a) Excluders; in which metal concentration in the shoots are maintained at low level up to a critical value across a wide range of soil concentrations (b) Accumulators; in which metals are concentrated in above ground plant parts from low to great soil concentrations; and (c) Indicator’s in which the internal concentration reflects external levels. Moreover, the bioavailability of trace elements for plants is dependent on many environmental factors; concentrations in the environment, biotic factors, exposure time, and growth form of the plant, type of absorption sites and element specification (Mazeij and Germ, 2009). Bioremediation is a non disruptive method of soil remediation. It is usually time consuming and its use for the treatment of heavy metal polluted soils is sometimes affected by the climatic and geological conditions of site to be remediated (Schmoger et al., 2000). Heavy metals cannot be degraded during bioremediation but can only be transformed from one organic complex or oxidation state to another. Due to a change in their oxidation state, heavy metals can be transformed to be come either less toxic, easily volatilized, more water soluble (which allows them to precipitate and become easily removed from the environment) or less bioavailable. (Garbisu and Alkorta, 1997; Garbisu and Alkorta, 2003). Bioremediation can also occur indirectly via bioprecipitation by sulphate reducing bacteria (Desulpfovibrio desulfricans) which converts sulphate to hydrogen sulphate which subsequently reacts with heavy metals such as Cd and Zn to insoluble forms of metal sulphate (white et al., 1998). The objective of the study was to access heavy metal contents in melon (Colocynthis citrullus) growing in a contaminated soil.