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Effluent and sediment samples were collected on seasonal basis, comprising of three months (August to October) in the wet season and three months (December to February) in the dry season from five points around a lubricating oil company in Osogbo - Nigeria. The sequential extraction method was used to respectively fractionate the sediment and effluent samples, then analyzed by Atomic Absorption Spectrophotometry (AAS) to determine concentrations of five heavy metals: Mn, Ni Co, Cd and Pb. Results showed that in wet season, higher concentration of Mn was found in exchangeable fraction, Ni and Pb were more pronounced in carbonate bound fraction, Cd in Fe-Mn oxide fraction, while Co was more in residual fraction. In dry season, Pb was more concentrated in exchangeable fraction, Coin residual fraction, Mn in Fe-Mn oxide fraction, Cd in organic matter bound, while Ni was more concentrated in the carbonate fraction. In both seasons for sediment samples, Cd concentrations were found above the set limit by WHO. The speciation study of the effluent showed that, Mn, Co and Ni concentration were found more in Organic substances fraction in both seasons, Pb was more concentrated in free metal ion fraction in wet season while it was high in organic fraction in the dry season. Concentrations of all metals in the effluent samples in both seasons were above the maximum permissible limit for effluent. The high mobility factors of heavy metals in the sediment indicated that the metals were labile and bioavailable thereby making the soil and the living organisms vulnerable to possibility of high heavy metal bioaccumulation.
Key Words: Heavy Metals, Speciation, Effluents, Sediment