Chemical fractionation of metals in core sediments of Orogodo River,southern Nigeria

Sequential chemical extraction of metals (Cu, Ni, Mn, Fe, Pb, Cr, and Zn) from core sediments of the Orogodo River was carried out with a view of providing information on the phase distribution of these metals with respect to depths and seasons. The results indicate that copper was speciated into organic matter/sulfide, Fe–Mn oxides, and residual forms at 0–5?cm depth while at deeper sections copper was predominantly in the residual form. Nickel was speciated mainly into Fe–Mn oxides and residual forms. The average of lead in various particulate phases of the sediment are 0–100% for residual; 0–8% for carbonate; 0–16.2% for exchangeable; 0–10% for organic matter/sulfide form and 22–56% for Fe–Mn oxides bound metals. Manganese was speciated into exchangeable (10–36%) and Fe–Mn oxides bound (10–48%) at the surface layers of 0–20?cm depths and predominantly in the residual form in deeper sections implying partitioning into trioctahydral clay and/or well-defined crystalline oxides. Iron was predominantly in the Fe–Mn oxides and residual fractions, less than 8% in the organic fraction, and 24.4–37.3% in the exchangeable and carbonates fractions at the surface. The percentage contributions of iron in the exchangeable and carbonate fractions decreased to 3.2% at 55–60?cm depth. The amount of chromium in the exchangeable fraction ranged from 0% to 7% for exchangeable; 0–26.5% for organic matter/sulfide bound; 3.8–17.9% for Fe–Mn oxides; and 46.8–100% for residual fraction. The mobility factors of the metals indicated that these metals were relatively mobile at top sections as compared with deeper sections. The overall picture of metal mobility and availability indicates potential contamination risk by Fe, Zn, Mn, and Pb in the Orogodo River sediment system.