Assessment of heavy metal speciation in soils impacted with crude oil in the Niger Delta, Nigeria

Abstract

Chemical fractionation of Cd, Cu, Pb, Ni, Cr, Zn and Mn in soils that had received a significant impact of crude oil spillage in the Niger Delta, Nigeria was studied using a sequential chemical extraction method. The results revealed that significant amounts of the metals (Cd, Cu, Pb, Ni, Cr, Zn and Mn) were found in the most labile fractions as compared to the control site. On average, Ni, Cr and Zn were mostly associated with the residual fraction; Pb and Mn with the Fe-Mn oxides; Cu with the organic fraction and Cd with the exchangeable fraction. Among the non-residual fractions, metals were mostly associated with the Fe-Mn oxides fraction. The mobility factors of these metals were higher at the impacted sites as compared with the control site. High mobility of the metals in the impacted soils was associated with the oil spillage, since exogenous metals are usually weakly bound to soil particles. The mobility factors for the metals follow the order: Cd > Zn > Pb > Ni > Mn > Cu > Cr.