The effectiveness of municipal sewage sludge application on the stabilization of Pb, Zn, and Cd in a soil contaminated from mining activities

The effectiveness of municipal sewage sludge for the stabilisation of Pb, Z n and Cd in a heavily contaminated soil was evaluated by performing pot exp eriments on soil-sludge mixtures. The soil sample originated from the Monte vecchio mining district, Sardinia, Italy, and presented high Pb, Zn and Cd content, as well as US EPA TCLP solubility values for Pb and Cd, which exce eded the respective regulatory limits. Sewage sludge application increased the soil pH. Stabilisation experiments showed that 10% w/w sewage sludge ad dition effectively reduced Pb and Cd solubilities below the TCLP regulatory limits. At the same addition rate, the EDTA extractable fraction of Pb, Zn , Cd in the treated soil was reduced by 12, 47 and 50% respectively compare d with the untreated sample. The five-stage sequential extraction procedure applied on the untreated and treated soil samples, showed a remarkable shi ft of the metals towards more stable forms. The reducible fractions of Zn a nd Cd and the residual fraction of Pb were increased by 12, 20 and 18% resp ectively, while a corresponding decrease in the mobile fractions (exchangea ble and carbonate) occurred which accounted for 14, 23 and 25% respectively .