[Stabilization of heavy metals in sewage sludge by using a ferrous iron]

The stabilization of heavy metals in sewage sludge using a ferrous iron is investigated through leaching test and fractionation analysis. The results showed that FeSO4 x 7H2O was not helpful to stabilize the heavy metals in the sludge when used individually since it reduced the pH value of sludge. The pH value dropped significantly from 6.5 to 4.4, and leaching coefficients of Zinc (Zn) and Nickel (Ni) raised, respectively, to 363.3% and 118.3% when FeSO4 x 7H2O was added individually at a ratio of 5%. However, it maintained the pH value within a slight alkaline range and the leaching coefficients of heavy metals reduced significantly when combined with Ca(OH)2 as an auxiliary agent. The pH value of sludge kept to 8.4 and leaching coefficients of Zn and Ni were reduced to 11.5% and 24.1%, for example, when 15% FeSO4 x 7H2O and 7.5% Ca(OH)2 were added together into the sludge. Besides, when Ca(OH)2 was used individually, the pH value of the sludge raised highly although the heavy metals were stabilized effectively. Under same pH value, combined addition of FeSO4 x 7H2O and Ca(OH)2 promoted the stabilization of heavy metals had a better effect than individual use of Ca(OH)2. The fractionation analysis revealed that the stable species of Zn, Cu and Pb were increased, and the instable species of Ni, Cr and Cd were also raised.