The bioaccumulation of some selected heavy metals in an old ESWAMA central dumpsite was studied. This has become important as the knowledge of it will assist one in choosing an appropriate remediation method. The sequential extraction procedure developed by Tessier was used to partition six most common heavy metals into different fractions. The result showed that the concentration of zn and cd in the dumpsite measured by their Potential Mobile Fraction (PMF) are at high risk level 30mg/kg > PMF < 50mg/kg while Cu and Pb are at medium risk level 10mg/kg < PMF < 30mg/kg. As and Cr are at low risk levels. 1mg/kg < PMF < 10mg/kg. Therefore the consumption of crops planted around the dumpsite or consumption of animals that graze on plants that grow on the site is at high risk and integrated pollution management and prevention measures and regular monitoring should be enforced on those industries producing the waste.
Heavy metals are those elements or substances with an atomic density greater than 5 gcm-3 and atomic number greater than 20 (Adriano, 2001; Alloway, & Ayres, 1997; Jadia, & Fulekar, 2008; Ismail et al., 2013). They are natural components of the soil and are broadly divided into two groups; Essential Heavy Metal and Non-Essential Heavy Metals (Subhashini and Swamy, 2013). Essential heavy metals are Iron (Fe), Manganese(Mn), Copper(Cu), Zinc(Zn) and Nickel(Ni) because they are micronutrients necessary for vital physiological and biochemical functions of plant growth (Cempel, & Nikel, 2006; Gohre, & Pasckowski, 2006). They are constituents of many enzymes and other proteins and all plants have the ability to accumulate them from soil solution (Djingova, & Kuleff 2000). The non-essential heavy metals Cadium (Cd), Lead (Pb), Chromium (Cr), Arsenic (As) & Mecury (HG) have unknown biological or physiological function and consequently are non-essential for plant growth (Gaur and Adholeya, 2004). Both groups are toxic to plants, animals and humans above certain concentrations, specific to each element (Adriano 2001; Alloway 1995).
There are two main sources of heavy metal in the environment viz: natural sources and anthropogenic sources (Ali et al., 2013). Natural source is as a result of pedogenetic processes of weathering of parents materials at levels that are regarded as trace (< 1000 mg. kg -1) and the heavy metals that occur by this process is rarely toxic (Wuana, & Okieimen, 2011). Anthropogenic sources are human activities such as mining, smelting, electroplating energy and fuel production, power transmission, intensive agriculture, sludge dumping and melting operations (Ali et al., 2013). Heavy metals in the soil from anthropogenic sources tend to be more mobile, hence bio available than pedogenic or lithogenic ones (Wuana, & Okieimen, 2011; Kaasalainen, & Yli-Halla, 2003;). Heavy metals in the soil cannot be biodegraded but can be bioaccumulated or biotransformed by plants and pose toxicity to plants beyond certain limits (Rajesh et al., 2007).
Several studies have suggested that the toxicity and mobility of heavy metals depend not only on their total amounts but also on their chemical fractionation in soil (Gupta and Sinha, 2006). The most mobile elements are Cadmium (Cd) and Zn while the least mobile are Chromium (Cr), Ni and Pb (Fijalkowski, et al., 2012).
The chemical speciation of heavy metals determines their bioavailability. It is related to the different natures of the metals, their bounding strength and either in free ionic form or complexed by organic matter, or incorporated in the mineral fraction of the sample; speciation analysis can be therefore be used to highlight the relationship between soil and metals to gain a better understanding of the different behaviour and mechanism (Singh and Kalamdhad, 2013). Speciation can be defined as the identification and gratification of the different species or forms of phases in which the elements occur (Ogundiran and Osibanjo 2009). Sequential extraction is the usual method of metal speciation analysis. It involves successive extraction steps using chemical reagents of different binding strengths and metal specificity. In principle, each step is supposed to destroy the bond between the metals and inorganic specific fractions of the soil. Therefore, metal species determined by chemical reactions are operationally defined, and may reflect the exact nature of the existing species. Tesssier’s procedure is the most commonly used to determine both the actual and potential mobility of trace elements in soils (Tessier et al., 1979).
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