Approaches to soil remediation by complexometric extraction of metal contaminants with regeneration of reagents

Complexometric equilibrations were performed with six chelating reagents to mobilise Cu, Mn, Pb and Zn from a contaminated urban soil. The metal-laden aqueous extract was treated with sodium diethyldithiocarbamate (DEDTC) to precipitate the heavy metals from solution while liberating the chelating r eagent. The aqueous supernatant fraction was then re-combined with the soil particulates to extract more pollutants. A sparing quantity of EDTA (ethyl enediaminetetraacetic acid; 10 mmol) mobilised 32-54% of the 5 mmol of heav y metals from the soil with three cycles but only 0.1 and 1.0% of the iron and magnesium, respectively, was removed. Whereas DPTA (1,3-diamino-2-hydro xypropane-N,N,N',N'-tetraacetic acid) and citric acid also mobilised each o f the heavy metals to some extent and continued to extract these metals dur ing all three cycles, the DTPA (diethylenetriamine pentaacetic acid), altho ugh efficient initially, could not be recycled with these conditions. ADA [ N-(2-acetamido)iminodiacetate] and SCMC [(S)-carboxymethyl-L-cysteine] were selective for copper and zinc but mobilised only Cu when recycled. An alte rnate means of regenerating the chelating reagent involved treatment of the aqueous extract with magnesium (Mg-0) granules. Excess HEDC [bis(2-hydroxy ethyl)dithiocarbamate] mobilised appreciable quantities (19-57%) of heavy m etals from the soil and retained its complexing activity when recycled. An appreciable fraction of the mobilised Pb and Cu and a portion of the Zn wer e cemented to the surfaces of the excess magnesium whereas virtually all of the Fe and Mn was removed from solution as insoluble hydroxides.