Selective removal of lead from calcareous polluted soils using the Ca-EDTA salt

EDTA is a powerful chelating reagent which has been often proposed for the decontamination of lead polluted soils. Despite the pronounced selectivity of this reagent for Pb, a low degree of utilisation is observed when treating calcareous soils, due to the co-dissolution of calcite. This study demonstrates that it is possible to suppress calcite dissolution and optimise the degree of EDTA utilisation, using the calcium salt of EDTA, instead of the common sodium salts. Initial experiments, which were carried out mixing pure cerrusite (PbCO3) with a Na(2)CaEDTA solution, have shown that Pb is quantitatively solubilised, while calcium precipitates in the form of aragonite. The performance of this reagent in the decontamination of soils has been tested on a soil sample from Lavrion (Greece) containing 7.3-8.8% of Ca and heavily polluted with Pb at levels up to 3.5%. The parameters which were tested include the reaction time, the stoichiometric excess of Na(2)CaEDTA with respect to Pb and the effect of successive treatment stages with fresh Na(2)CaEDTA solutions. The experimental results have shown that long reaction times, exceeding 24 hr, are required in order to obtain sufficient Pb removal and preserve the calcium content of the soil. Lead extraction increases from 27 to 40% prolonging the reaction time from 1 to 24 hr at Na(2)CaEDTA/Pb = 1 mol/mol. Increasing the Na(2)CaEDTA/Pb molar ratio from 1 to 3.5 mol/mol, enhances the dissolution of Pb from 40 to 53%, which is not proportional to the stoichiometric excess supplied. The efficiency of Ph removal is maximized, up to 75%, applying three successive leaching stages. The major benefit of Na(2)CaEDTA in comparison with the disodium EDTA salt is the preservation of soil calcite. The initial calcium content of the soil was found to increase by 4% using the Ca salt; on the contrary, the treatment with Na(2)H(2)EDTA under similar experimental conditions has resulted in dissolving 27% of soil Ca.