Extraction of lead using EDTA: factors affecting extraction, effects of amorphous iron and recycling of used EDTA

Abstract

<p>The effects of major cations present in soils, soil:extractant ratio, and EDTA:lead stoichiometric ratio on the extraction efficiency of lead using EDTA were studied for three different Superfund soils, one rifle range soil, and one artificially lead-contaminated soil. Extraction of lead from the lead-contaminated soils was not affected by a soil:extractant ratio as low as 1:3 but instead was dependent on the quantity of EDTA present. The extraction efficiency for each soil was different. The differences in extraction efficiencies may be due to the major cations present in soils. Among the cations present, ferric ions probably competed most strongly with lead for EDTA ligand sites for pH values less than 6. In addition, Cu and Zn may have a potential to compete with Pb for EDTA ligand sites. At high pH values, Ca may have an effect on lead extraction. For some of the soils tested, the extraction efficiency of lead may be affected by being occluded in the Fe and Mn oxides present in the soil. While major cations present in the soil may be one of the factors affecting lead extraction efficiency, the type of lead species present may also play a role;Bench-scale experiments were conducted with three different artificially prepared lead-contaminated soils (lead sulfate, lead carbonate and lead phosphate) with and without addition of amorphous iron to investigate the amorphous iron effects on the lead extraction. The experimental results indicated that amorphous iron effects on lead extraction was dependent on the equilibrium pH and lead species present;A method to recycle used Pb-EDTA was proposed. The proposed method consists of substituting the Pb ions with Fe ions, uncomplexed Pb ions precipitation with phosphate or sulfate ions and with/without Fe precipitation using pH control. According to the experimental results, the used EDTA wastewater can be recycled for further treatment. In addition, experimental results showed that recycled used EDTA with phosphate precipitation was slightly more superior than recycled used EDTA with sulfate precipitation. Both recycled EDTA solutions (phosphate and sulfate precipitation) showed similar or slightly lower extractive abilities when compared to fresh EDTA solution.</p>