Chemical forms of heavy metals in soils around municipal waste dumpsites in Asaba Metropolis, Delta State, Nigeria

Abstract

The chemical forms of copper, lead, manganese and zinc in soils around five selected municipal waste dumpsites in Asaba metropolis, Delta State, Nigeria, were studied, using the Ma and Rao six-step sequential chemical extraction procedure, with a view to providing information on the bioavailability and mobility of the metals in the soil matrix. The results showed that lead and manganese were speciated into residual fractions with averages of 60.32 and 82.24% respectively, while copper and zinc were associated with organic and Fe–Mn oxide fractions respectively with respective averages of 47.91 and 63.21%. The residual fraction also contained relatively high percentages of copper (33.18%) and zinc (20.81%). Relatively very low levels of the metals were found in the water soluble, exchangeable and carbonate fractions. The mobility factors of all the metals in all the soil profiles were generally low ranging from 1.97 to 39.99 indicating generally low availability as well as mobility. Lead and manganese associated with the residual fraction were strongly bound to soil matrix and were not bioavailable since they were occluded within the crystal lattice layer of silicates and well crystallised oxide minerals. Our results show that the metals do not pose any environmental risk or hazard. However, the soils stand the risk of being contaminated by copper and zinc since metals associated with organic and Fe–Mn oxide fractions could be bioavailable and mobilisable when the soil conditions such as pH and redox potentials change.