Reduction of Pb, Zn and Cd availability from tailings and contaminated soils by the application of lignite fly ash

The effectiveness of lignite fly ash for stabilization of Pb, Zn and Cd in tailings and contaminated soils was examined. Fly ash was mixed with the contaminated soil or oxidic tailings samples at various doses and pot experiments were performed. The effectiveness of stabilization was mainly evaluated by the standard US EPA TCLP toxicity test. The 5-stage sequential extraction procedure was also applied to determine the form of contaminants in the fly ash amended soil or tailings samples. Complementary EDTA extraction tests were also carried out. The fly ash treatment resulted in the reduction of TCLP solubility of Pb, Zn and Cd to below the respective regulatory limits at 5 and 10% w/w fly ash addition rates in tailings and soil, respectively. Speciation of lead, in the treated soil and tailings samples indicated that there was a significant transition of the heavy metals form from the exchangeable and carbonate fractions to reducible and residual, suggesting that the potential mechanisms of heavy metals retention are, apart from increase of pH, sorption on the oxides and hydroxides surfaces and binding with the hydrated fly ash compounds.