Abstract
The understanding of chemical forms of heavy metals and how they associate with themselves is very essential in studying the potential mobility and risk assessment of heavy metals in soils. This study determined the spatial distribution and mobility of Mn, Ni, Zn, Cr, Cu, Pb and Fe in the soil samples of Onitsha South Local Government Area in South East Nigeria. The soil samples collected were examined for the heavy metal contamination using five-step sequential extraction method. The concentrations of the extracted metals were determined using Atomic Absorption Spectrophotometer (AAS). The range of heavy metals extracted fractions for each of five fractions in percentages were as follows; exchangeables (0.0-3.5%), bound to carbonates (0.0-4.49%), bound to Fe-Mn oxides (0.03-4.14%), bound to organic matter (0.03-3.38%) and residual (0.0-86.70%) in wet season; exchangeables (0.0-3.94%), bound to carbonates (0.0-4.80%), bound to Fe-Mn oxides (0.09-5.19%), bound to organic matter (0.09-3.53%) and residual (0.0-126.00%) in dry season. Available heavy metals in the soil studied had mean values 0.52, 0.76, 1.21, 1.24, 1.40, 9.08 and 96.37 all in mgkg-1 for Pb, Cu, Zn, Ni, Mn, Cr, and Fe respectively for wet season while the mean values in dry season were 0.36, 1.21, 1.60, 1.82, 2.45, 11.54 and 143.54 all in mgkg-1 for Pb, Cu, Mn, Ni, Zn, Cr and Fe respectively. Mobility factor were 0.22-47.90% in the two seasons, levels of heavy metals in available forms differ significantly though majority of the extracted toxic metals are found in oxides and the residual fractions and as such may not pose environmental risk due to their relatively low availability and mobility factor. That notwithstanding, soils from studied area should be carefully monitored to prevent the release of these metals due to redox reactions which may make them available to plants through absorption.
Highlights
Soils serve as the most important sink for heavy metal pollutants in terrestrial ecosystems [1] and soil heavy metal pollution is a worldwide problem [2]
Results of total metal concentration for the following; Mn, Fe, Zn, Pb and Ni in Woliwo, Odoakpu and Fegge soils in Onitsha South Local Government Area are presented in Tables 2 and 3, while that of sequential extraction of these heavy metals using modified Tessier et al method are presented in percentages in Tables 4 and 5
The results obtained from speciation studies showed that some of the metals considered had the highest abundance in the residual fraction
Summary
Soils serve as the most important sink for heavy metal pollutants in terrestrial ecosystems [1] and soil heavy metal pollution is a worldwide problem [2]. Sequential extraction is the simplest method of identifying the chemical forms in which these heavy metals exist in the soil. Ochiagha Kate Ekwutosi et al.: Chemical Speciation and Potential Mobility of Heavy Metals in the Soils of Onitsha. Many research works have been carried out on the Sampling: Sampling sites were from eleven streets in speciation of heavy metals in soils in different parts of Odoakpu, Fegge and Woliwo areas in Onitsha south local. With reference to heavy metal pollutants of such highly Fifty five soil samples were collected from top soil using industrialized and commercialized area as Onitsha South hand trowel and Geographical Position System (GPS) into local government, part of industrial and commercial city of well labeled polythene bags. 1 mol/l MgCl2 (pH 7), 20°C 1 mol/l NaOAc + HOAc (pH 5), 20°C 0.04 mol/l NH2OH.HCl + 25% HOAc, 95°C 30% H2O2 + 0.02 mol/l HNO3 (pH 2), 85°C; 2 mol/l NH4OAc + 20% HNO3 HF/HClO4
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