Chemical Speciation and Potential Mobility of Heavy Metals in the Soils of Onitsha South Local Government Area Anambra Nigeria

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.