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

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 treati ng calcareous soils, due to the co-dissolution of calcite. This study demon strates that it is possible to suppress calcite dissolution and optimise th e degree of EDTA utilisation, using the calcium salt of EDTA, instead of th e common sodium salts. Initial experiments, which were carried out mixing p ure cerrusite (PbCO3) with a Na(2)CaEDTA solution, have shown that Pb is qu antitatively solubilised, while calcium precipitates in the form of aragoni te. The performance of this reagent in the decontamination of soils has bee n tested on a soil sample from Lavrion (Greece) containing 7.3-8.8% of Ca a nd 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 fres h Na(2)CaEDTA solutions. The experimental results have shown that long reac tion times, exceeding 24 hr, are required in order to obtain sufficient Pb removal and preserve the calcium content of the soil. Lead extraction incre ases from 27 to 40% prolonging the reaction time from 1 to 24 hr at Na(2)Ca EDTA/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 prop ortional to the stoichiometric excess supplied. The efficiency of Ph remova l is maximized, up to 75%, applying three successive leaching stages. The m ajor benefit of Na(2)CaEDTA in comparison with the disodium EDTA salt is th e 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 di ssolving 27% of soil Ca.