Characterisation of metal forms in sewage sludge by chemical extraction and progressive acidification

Abstract

Sequential chemical extraction, acidification to pH 4, 2 and 0.5, and repetitive CaCl 2 (0.05 M) extractions were used to evaluate the forms of Cd, Cu, Ni, Pb and Zn in seven sewage sludges. There was more variation in the fractionation profiles of different metals in the same sludge than of the same metal in different sludges. Speciation in air-dried and liquid forms of sludges differed slightly, but the former gave more reproducible results. The sequential extraction scheme indicated that the predominant fractions of Pb and Zn were extractable in 0.1 M Na 4P 2O 7, corresponding to organic and some insoluble inorganic forms. The largest Cd and Ni fractions were extractable in 0.1 M sodium ethylenediaminetetraacetate (pH 6.5), corresponding to the carbonates. The major forms of Cu were probably the sulphide and organic phases, both extractable in 6 MHNO 3. Progressive acidification of liquid sludges mobilised significant quantities of Zn at pH 4, of Cd and Pb at pH 2 and of Ni at all pH values investigated. Copper was relatively immobile throughout. Application of the sequential extraction scheme to the residues obtained following acidification indicated that Cu, Cd and Pb forms were relatively stable, but that Ni and Zn speciation was likely to change with decreasing pH. Repeated CaCl 2 extractions resulted in a low degree of metal mobilisation and had little effect on the speciation of the metals remaining in the sludge.