Mobility of heavy metals in soils amended with sewage sludge

Sewage sludges added to arable land can improve soil fertility and physical properties. However, the concentrations of heavy metals commonly found in sludges limits their application to soil. The purpose of this paper is to evaluate the mobility of heavy metals (Cd, Cu, Ni, Pb and Zn) in four soils amended with different rates (0, 80, 60 and 320 t ha-1) of anaerobically stabilized urban sewage sludge. Total metal content in the sewage sludge was Zn much greater than Cu > Pb > Ni much greater than Cd. Sludge, soils and sludge-soil mixtures were fractionated by the Tessier sequential extraction procedure. The fractions extracted by H2O2/HNO3 and NH2OH.HCl were the most abundant pools for metals under study. The apparent mobility of metals in the sludge was Zn approximate to Cd approximate to Ni > Pb > Cu. The addition of sewage sludge in soils increased the percentages of metal extracted in non-residual fractions. ANOVA showed that the most significant increases were those of Zn, followed by Cu and Pb; there were no statistical differences (P < 0.05) for Ni and Cd. Exchangeable Zn from sludge was immobilized in basic soils. The other trace metals showed no fraction redistribution. The soils and sludge-treated soil samples were also extracted with EDTA and DTPA. Extraction with EDTA was more sensitive to soil type, whereas extraction with DTPA showed wider variation with metals. Both chelates seemed to be more effective to assess the mobility of metals added with the sludges at low concentrations than the Tessier's chemical partitioning.This work was supported by a project of Instituto Nacional de Investigación y Tecnología Agraria y Alimentación (INIA SC94-026)