Fractionation, mobility and multivariate statistical evaluation of metals in marine sediments of Cape Town harbour, South Africa

Abstract

Distribution of possible chemical forms of Al, Si, Sn, Pb, Zn, Fe, Hg, Cd and Cu in marine sediments of Cape Town harbour was investigated using a modified Tessier's sequential extraction procedure and ICP-MS and ICP-AES for heavy metals determination. The mean fractions for all metals at all locations were: 1.5-7196mg kg-1 for Si, 7.79-7266mg kg-1 for Al, 161-639mg kg-1 for Cu, 19-41978mg kg-1 for Fe, 2.83-5864mg kg-1 for Zn, 1.45-13.26mg kg-1 for Cd, 9.87-223mg kg-1 for Sn, 11.98-979mg kg-1 for Pb and 0.13-5.93mg kg-1 for Hg. Si, Al and Zn were mostly associated with Fe-Mn oxides, whereas Sn and Hg were mainly bound to residual and organic matter. Pb existed mainly in the residual and iron/manganese oxide phases while Cd was evenly distributed in all the five phases. The loading plots of heavy metals bound to the various chemical forms, as well as Pearson correlation coefficients, enabled the determination binding relationship. Pb, Sn and Hg exhibited similar binding behaviour which indicated an anthropogenic point source from wastes from the ship maintenance workshop, and the presence of Sn in the organic phase can be identified with the use of anti-fouling paints at the harbour, whereas Al, Fe, Si, Cu and Zn would probably be of natural origin. Lastly Cd probably came from a diffuse pollution sources in the harbour due to its unique binding characteristic. The mobility of heavy metals varied depending on location and the heavy metal type. The mobility of metals followed the order: Si > Zn > Fe > Cu> Al> Cd> Pb > Sn > Hg. The high percentage of Cd and Pb in the bioavailable forms suggested the need to keep close surveillance on these metals because of their high toxicity.