Evaluation of sequential extraction procedures for metal speciation in model sediments

The three-step sequential extraction protocol designed by the BCR (Community Bureau of Reference) was evaluated with regard to total recovery, reproducibility, selectivity of extractants and extent of phase exchanges or redistribution of metals during the extraction. Model sediments of known composition were developed consisting of humic acid and natural minerals like kaolin, quartzite and ochre. Synthetic compounds like calcium carbonate (calcite) and iron oxide (goethite) which could be used to simulate typical components of sediments, were used for comparison. Our results indicate that the procedure gives excellent recoveries for all 6 metals (Cu, Cr, Cd, Zn, Ni and Pb) studied. Reproducibility of 3% was achieved. Redistribution of Pb and Cu in the presence of humic acid was demonstrated. The selectivity of the reagents was insufficient to warrant interpretation of results in terms of a specific origin of a metal in a particular phase. It was shown that the chemistry of a metal can be a more important parameter than the actual phase location of the metal in the sediment in determining its response to the extractants. This was demonstrated in the case of Cr which reported to Step 3 irrespective of its location.