Levels and speciation of heavy metals in soils of industrial southern Nigeria

A knowledge of the total content of trace metals is not enough to fully assess the environmental impact of polluted soils. For this reason, the determination of metal species in solution is important to evaluate their behaviour in the environment and their mobilization capacity. Sequential extraction procedure was used to speciate five heavy metals (Cd, Pb, Cu, Ni and Zn) from four contaminated soils of Southern Nigeria into six operationally defined geochemical species: water soluble, enchangeable, carbonates, Fe-Mn oxide, organic and residual. Metal recoveries were within +/- 10% of the independently determined total Cd, Pb, Cu, Ni and Zn concentrations. The highest amount of Cd (avg. 30%) in the nonresidual fractions was found in the exchangeable fraction, while Cu and Zn were significantly associated with the organic fraction. The carbonate fraction contained on average 14, 18.6, 12.6, 13 and 11% and the residual fraction contained on average 47, 18, 33, 50 and 25% of Cd, Pb, Cu, Ni and Zn respectively. Assuming that mobility and bioavailability of these metals are related to the solubility of the geochemical form of the metals, and that they decrease in the order of extraction sequence, the apparent mobility and potential bioavailability for these five metals in the soil were: Pb > Zn > Cu > Ni > Cd. The mobility indexes of copper and nickel correlated positively and significantly with the total content of metals, while mobility indexes of cadmium and zinc correlated negatively and significantly with the total content of metals.