Comparison between sequential extraction procedures and single extractions for metal partitioning in sewage sludge samples

The conventional Tessier and BCR sequential extraction methods were applied to metal partitioning in sewage sludge samples collected from a waste water treatment plant. The results obtained by sequential extraction methods for Cu, Cr, Ni, Pb and Zn were compared with those estimated from single extractions using identical operating conditions applied in each individual Tessier and BCR fraction. The metal content in the extracts was measured by flame atomic absorption spectrometry. In all fractions of the BCR method, and in the three first fractions of the Tessier method, the results obtained from single extractions were in good agreement with those obtained using the sequential procedures. The oxidizable fraction of the Tessier method should be leached from the solid residue of the reducible extraction, since a very low extraction efficiency was obtained for this fraction when it was directly extracted. The total extractable metal contents obtained by both single and sequential extractions were similar for all metals in the BCR method and for Cr, Ni and Pb in the Tessier method. The recoveries obtained ranged from 93.52 to 105.8% in the two studied samples. For Cu and Zn the overall extraction efficiency of the proposed method was slightly lower than that obtained with the sequential procedure (recoveries around 90%). The precision of the proposed Tessier and BCR single extraction methods (expressed as RSD) was lower than 8% for all metals.