Distribution of heavy metals in sewage sludges and sludge-amended soils as determined by sequential extractions

摘要

In the present work we studied the total content of Cd, Cr, Cu, Ni, Pb and Zn and the chemical forms in which these metals are retained in two soils amended with different doses of sewage sludges from two water treatment plants (WTP) in the province of Salamanca (NW-Spain). The sludge-soil mixtures were incubated and the distribution of metal fractions in different samples was determined periodically by a sequential extraction method. The metal total contents in the two samples of sludges was relatively low and sequential chemical fractionation pointed to a different distribution of the metals. The metal total contents in the samples of amended soil was related to the dose of sludge added. After one year of sample incubation, an increase in Cd concentration was observed in exchangeable (F1) and carbonate-bound (F2) fractions, Cu, Cr and Zn in oxide-bound fraction (F3) and organic matter-bound fraction (F4) and Ni and Pb in the F2 and F3 fractions. Mobility indices (MI) determined from the percentages of metals found in Fl and F2 fractions were very low for Cu and Cr (<5%) and high for Zn (>20%), and remained unchanged with the incubation time for these metals. These indices increased with the incubation time for Cd, Ni and Pb and followed an order of Cd>Pb>Ni. The influence of the type of sludge in the metal distribution in the different fractions and in their evolution as a function of time was found to be more important than the nature of the soil.