The competitive binding of lead by EDTA in soils and implications for heap leaching remediation

The effectiveness of removal of Pb from three contaminated alkaline soils and one acidic mine tailings sample by extraction with EDTA was determined. Experimental variables included EDTA concentration, time of shaking, and pH. The fraction of total Pb extracted by EDTA at pH values near that of the native soil or tailings ranged from less than 1% to nearly 90%. The fraction of EDTA which complexed Pb was decreased as higher concentrations of EDTA were added for all three soils, with only limited improvements in Pb extraction at EDTA:Pb molar ratios above 10.0. A high percentage of EDTA complexed with Ca, and to a lesser degree Zn, Mn, and Mg in the alkaline soils, and with Fe in the acidic tailings, consistent with model predictions. Decreasing the pH of the alkaline soils with HCl increased the removal of Pb in all cases. A significant proportion of the total lead in each of the contaminated soils was present in the adsorbed form, and the ability of EDTA to extract adsorbed lead from the three soils was variable.