Fractionation of copper, nickel, and zinc in metal-spiked sewage sludge

The enrichment or spiking of sewage sludge with heavy metals for experimental purposes is a fairly widely used, although sometimes controversial, technique. A study was undertaken, using a sequential fractionation scheme, to assess the degree of incorporation of Cu, Ni, and Zn into sewage sludge samples spiked with these metals and incubated for 6 mo. For all three metals, substantial proportions of the metals were incorporated into the sludge matrix, particularly as evidenced by their occurrence in the oxide-bound, organic-bound, and residual fractions. In particular, for Cu very little of the added copper remained in the sludge supernatant solution after 6 mo of incubation and, apart from at the highest level of Cu addition, there was very little difference in the fractional distribution of Cu between the Cu-spiked and non-Cu-spiked sludges. For Ni and Zn, however, although there was substantial incorporation of these metals into the sludge, the higher levels of Ni and Zn addition resulted in greater proportions of the metals in the most soluble fractions (soluble, exchangeable, specifically sorbed) compared with nonspiked sludges. The fractionation data for Ni also showed that large additions of Cu and Zn can affect the fractional distribution of Ni in the sludge. The drying of sewage sludge prior to analysis was shown to increase metal solubility in the sludge samples, and the potential implications of this finding for the prediction of sludge metal bioavailability are discussed.