Leaching behavior of heavy metals from sewage sludge solidified by cement-based binders

Sewage sludge was stabilized and solidified (S/S) with cement-based binders prior to landfill in order to reduce the potential hazard of heavy metals. The leaching behavior and chemical speciation of heavy metals in the solidified sludge were studied by semi-dynamic leaching test, toxicity characteristic leaching procedure (TCLP) and sequential extraction procedures. The effectiveness of S/S treatment was evaluated by determining diffusion coefficients (De) calculated from the data achieved in semi-dynamic leaching test. High De values under low pH (4.0 compared to 7.0 and 10.0) and high temperature (35°C compared to 10°C and 25°C) conditions were always recorded. Acid-assisted S/S product showed lower De values and leaching tendency from TCLP test results for all the selected heavy metals (Ni, Cu, Zn and Pb) compared to traditional S/S product using the same binders. More heavy metals were converted into stable fractions by acid-assisted S/S than usual S/S according to sequential extraction test.