Chemical Speciation and the Potential Mobility of some toxic metals in Tropical Agricultural soils

T. M. Osobamiro, G. O. Adewuyi

Abstract


The knowledge of chemical forms of association of toxic metals is important in understanding metal mobility and the potential risk of metal contamination in the soil. This study determined distribution, availability and mobility of Mn, Fe, Ni, Zn and Pb in the soil samples from arable and plantation plots of a farm settlement in South-Western, Nigeria. Heavy metals in the representative soil samples collected were sequentially extracted into seven fractions and concentration of the extracted metals was determined using Atomic Absorption Spectrophotometer (AAS). The total level of Mn,  Pb,  Fe,  Zn  and  Ni  in the soil samples was  obtained  in  the  mean  range  of 30.0-48.6,  8.25-22.5, 1405-2243,  86.7-166.2 and 13.5-25.9 mg kg-1, respectively. The range of heavy metals extracted from each of the seven soil fractions in percentages are as follows; soluble exchangeable (0.50-1.80), surface adsorbed (0.80-5.10), organic matter (3.30-17.9), Mn oxide (6.00-27.9), poor crystalline Fe oxide (10.3-25.4), crystalline Fe oxide (14.6-28.5) and residual (21.3-53.1). Available metals in the studied soils ranges between 6.47-10.9, 241-481, 1.34-2.39, 12.9-24.1 and 1.04-2.37 for Mn, Fe, Ni, Zn and Pb, respectively, while mobility factors of all the metals studied were ≤ 0.257. In the two agro-ecosystems (oil palm and arable), levels of heavy metals in available forms do not differ significantly in both seasons. Though, majority of the extracted toxic metals are found in oxides and the residual fractions and as such, may not pose environmental risk considering their general relatively low availability and mobility factors. Nevertheless, soils from this study area should be effectively managed to prevent release of occluded heavy metals into the available form for plant uptake.

Keywords: Bio-availability; sequential extraction; mobility; tropical soil; Heavy metals


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Chemical Society of Nigeria