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Chemical Fractionation and Mobility of Heavy Metals in Soils in the Vicinity of Asphalt Plants in Delta State, Nigeria

Sequential chemical extractions were performed in order to determine the forms of metal contaminants such as cadmium (Cd), cobalt (Co), copper (Cu), chromium (Cr), iron (Fe), manganese (Mn), lead (Pb), nickel (Ni), and zinc (Zn) in the soils around the asphalt plants in Delta State, Nigeria. The results indicated that the reducible phase (consisting of Fe(III) and Mn(IV) oxides, and hydroxides) is the major host of Cd at the surface horizon, while at the sub-surface horizons, cadmium existed mainly in the residual form. Cobalt was predominant in the residual and Fe-Mn oxides fractions. Copper existed mainly in the residual, Fe-Mn oxides and organic bound fractions. Iron and manganese were predominantly in the residual, Fe-Mn oxides and exchangeable fractions. Higher amounts of Mn were extracted in the hydroxylamine hydrochloride extraction than Fe because of specific reduction of Mn oxide by hydroxylamine and specificity of dissolution with respect to ferric oxides. Fe-Mn oxides and residual fractions formed the major host of Pb in these soil profiles while the residual fraction formed the major host of nickel with significant amounts in the exchangeable fraction. Zinc was found predominantly in the Fe-Mn oxides fraction at the topsoils and the residual fractions at the sub-surface soils with significant amounts of zinc occurring in the organic matter bound fraction. The average mobility index of metals in the topsoils follow the order Ni > Cd > Co > Mn ≈ Zn > Fe > Cu > Pb > Cr. The results of the present study revealed that Fe-Mn oxides formed the major scavenging mechanism for metals in these soils. On average the amounts of these metals in the readily available form never exceeded 23% at all depths and sites, implying that these metals are not readily available at the prevailing soil physicochemical conditions. However, significant amounts of these metals existed in the Fe-Mn oxides fraction and may be available if the pH and redox conditions of the soil change, which could be induced by flooding. The results obtained provided information on the speciation, bioavailibity, and mobility of metals in soils in the vicinity of asphalt plants, which are useful in environmental quality management and environmental forensics.