Effects of Amorphous Iron on Extraction of Lead-Contaminated Soil with EDTA

Abstract

Ethylenediaminetetraacetic acid (EDTA) is one of the more common chelating agents used in the extraction of heavy metals from soil. For soil extraction processes to be economically competitive with other remedial technologies, the amount of EDTA applied for extraction should be optimized; that is, extraction of metals should be maximized with a minimum amount of EDTA used. Generally, the molar amount of EDTA used is several times higher than the molar amount of the target metal present in the soil. This is to ensure there will be sufficient EDTA to extract the target heavy metals from the soil, since a host of non-target metals such as iron and calcium may compete for EDTA ligand sites. Bench-scale experiments using artificially prepared lead-contaminated soils were conducted to investigate the effects of amorphous iron and the different types of lead species on lead extraction. Three different lead species—lead sulfate, lead carbonate, and lead phosphate—were used. The extraction efficiency of lead was found to be a function of EDTA:Pb molar ratio, pH, lead species, and amorphous iron concentration. Experimental results showed that for a pH less than 6.0, amorphous iron in soil was found to strongly compete with lead for EDTA ligand sites. The extent of competition was dependent not only on the solution pH but also on the lead species present and the EDTA:Pb molar ratio. Amorphous iron was found to affect lead sulfate-contaminated soil the most, while lead carbonate-contaminated soil was the least affected. For EDTA:Pb molar ratios of less than 2, a solution pH between 6.5 and 8.5 must be maintained to optimize extraction of lead from lead sulfate- and lead phosphate-contaminated soil, if amorphous iron is present. The degree of difficulty of extraction of various lead species using EDTA:Pb molar ratios of less than 2 and in the presence of amorphous iron was: lead sulfate > lead phosphate > lead carbonate.