Fractionation of Heavy Metals by BCR Sequential Extraction in Coastal Marine Sediments around Colombo Harbour

Abstract

Marine environment around the Colombo harbour is prone to heavy coastal marine pollutiondue to heavy ship traffic, spilled oils, maintenance and repair of ships. Land based marinepollution is prominent near the coast of Kelani river estuary due to the fact that the rivercarries land run-offs, urban and domestic debris, industrial effluents, and agricultural wastesdumped directly into it. Lead (Pb), chromium (Cr), nickel (Ni), copper (Cu) and zinc (Zn)are some of the heavy metals that have a tendency to accumulate in food chains originatefrom the sea. However, the heavy metal should be present in bioavailable forms to be enteredin to biological systems. Thus the fractionation of heavy metals into four fractions (water andacid soluble, reducible, oxidisable, and residual) by the modified BCR (European CommunityBureau of Reference) sequential extraction procedure prior to the analysis is a better way ofinterpreting heavy metal pollution in marine sediments.

In this study, field sampling was carried out twice in six locations adjacent to Colombo portin October 2011 (batch 1) and in March 2012 (batch 2) at the depth of 9-20 m depending onthe location and the season. Metal fractionation was done according to modified BCRprocedure and five elements (Pb, Cr, Ni, Cu, and Zn) were analysed by flame atomicabsorption spectroscopy (FAAS).

The sum of three steps of BCR and residue analysis were in good agreement with the totalmetal content obtained by aqua regia digestion followed by the FAAS analysis via standardaddition method. The results showed that the levels of heavy metals vary widely by metal, bysampling site as well as by batch. Obtained results were compared with U.S.EPA-Region V,sediment quality criteria for concentration limits of heavy metal contaminants in sediments.The total metal contents in sediments of all metals except for Pb were above moderatelypolluted or polluted levels. More than 50% of total Cu and Cr contents were in residual andoxidisable fractions. Distribution of Ni and Zn did not have a clear pattern. Bioavailablemetal contents of all five metals under investigation were below the risk levels provided byUS EPA sediment quality criteria, except for Ni in one location and Zn in two locations.

Marine sediments, BCR Sequential extraction, Heavy metals, AAS