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Sequential extraction by Tessier et al; 1979 was used to study the geochemical fractionation of the following heavy metals; cadmium, chromium, Nickel, lead and zinc in the sediments collected at the point of discharge of Iyalaro stream into Lagos Lagoon to understand their mobility and bioavailability. The highest percentage (%) of all heavy metals studied were observed in the oxidisible fraction; Cd (40.3) %, Cr (35.9) %, Pb (42.6) %, Zn (41.8) % except Ni (42.6) % which was observed at residual / lithogenous (crystal lattice or silicates bound) fraction of the sediment. High Cr (31.2) % contribution was also observed in the residual fraction, this is an indication that rock / soil weathering / formation is also playing meaningful contribution to the presence of Ni and Cr in the studied area. Sequential extraction for the speciation of elements among the geochemical phases in this study indicated that most of the metals studied were in exchangeable, carbonate bound, reducible (Fe-Mn oxides) and oxidisable (organic matter/sulphide bound) fractions which are potentially available fractions, and contributed by anthropogenic activities. The order of reducing availability of the studied heavy metals were as follow; Zn (93.2) % >Pb (81.6) % > Cd (73.1) % > Cr (68.8) % > Ni (57) %. Therefore, under favourable biogeochemical processes for remobilization of the studied metals from the sediment, the studied metals can get the water body heavily polluted, impact food chain and become an environmental risk.

Geochemical fractionation of Heavy metals in the sediment at the point of discharge of Iyalaro stream into Lagos Lagoon