Bioavailability of Sediment-bound Heavy Metals on the East African Coast

Abstract

Currently, environmental risk assessment of metals is based on comparisons of toxicity thresholds against environmental exposure levels measured as total metals, despite the fact that not all heavy metals in the sediments are in bioavailable form to the biota. Analysis was undertaken for nine elements (Al, Cd, Co, Cr, Cu, Fe, Mn, Ni, and Zn) in sediment samples collected at eight sites along the Eastern African Coast following aqua regia extraction (to determine the total quantity of metals) and three-step BCR (Community Bureau of Reference) sequential extraction (to obtain the metal fractionation patterns in the sediments in this region). The results revealed that heavy metal levels at some locations were higher than at others. Cd, Mn and Co were more concentrated in labile fractions compared to the other elements. These metals are easily liberated into overlying water, making them available for biological uptake. More than 62% of the total concentrations of Fe and Zn were present in the residual fraction, being immobile and less bioavailable.

Keywords: Metals, sediments, bioavailability, aqua regia extraction, sequential extraction.