Selective Removal of Lead from Calcareous Polluted Soils using the Ca-EDTA Salt

EDTA is a powerful chelating reagentwhich has been often proposed for the decontaminationof lead polluted soils. Despite the pronouncedselectivity of this reagent for Pb, a low degree ofutilisation is observed when treating calcareoussoils, due to the co-dissolution of calcite. Thisstudy demonstrates that it is possible to suppresscalcite dissolution and optimise the degree of EDTAutilisation, using the calcium salt of EDTA, insteadof the common sodium salts. Initial experiments, whichwere carried out mixing pure cerrusite (PbCO₃)with a Na₂CaEDTA solution, have shown that Pb isquantitatively solubilised, while calcium precipitatesin the form of aragonite. The performance of thisreagent in the decontamination of soils has beentested on a soil sample from Lavrion (Greece)containing 7.3–8.8% of Ca and heavily polluted withPb at levels up to 3.5%. The parameters which weretested include the reaction time, the stoichiometricexcess of Na₂CaEDTA with respect to Pb and theeffect of successive treatment stages with freshNa₂CaEDTA solutions. The experimental resultshave shown that long reaction times, exceeding 24 hr, are required in order to obtain sufficient Pbremoval and preserve the calcium content of the soil.Lead extraction increases from 27 to 40% prolongingthe reaction time from 1 to 24 hr atNa₂CaEDTA/Pb = 1 mol/mol. Increasing theNa₂CaEDTA/Pb molar ratio from 1 to 3.5 mol/mol,enhances the dissolution of Pb from 40 to 53%, whichis not proportional to the stoichiometric excesssupplied. The efficiency of Pb removal is maximized,up to 75%, applying three successive leaching stages.The major benefit of Na₂CaEDTA in comparison withthe disodium EDTA salt is the preservation of soilcalcite. The initial calcium content of the soil wasfound to increase by 4% using the Ca salt; on thecontrary, the treatment with Na₂H₂EDTA undersimilar experimental conditions has resulted indissolving 27% of soil Ca.