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Chelating Extraction of Heavy Metals from Contaminated Soils

Principal Investigators
A.P. Hong and R.W. Okey, University of Utah; S.K. Banerji, University of Missouri


Goal: The goal of this project is to assess the full potential of chelating extraction technology in removing and/or recovering heavy metals from contamin ated media, e.g., soils and mine tailings ponds. A large number of chelators will be examined, and those chelators which are suitable for selective removal or recovery of various heavy metals will be identified.

Rationale: Heavy metal conta mination of soil is a common problem encountered at many hazardous waste sites. Lead, chromium, cadmium, copper, zinc, and mercury are among the most frequently observed metal contaminants. They are present at elevated concentrations at many National Prio rity List sites, are toxic to people, and threaten ground water supplies. Once released into the soil matrix, most heavy metals are strongly retained and their adverse effects can last for a long time. Chelating extraction of heavy metals from contaminate d soils is a relatively new treatment method. There exists a need to assess the full potential of this technology in removing and/or recovering heavy metals from contaminated media. A methodology is needed to examine a large number of chelators and identi fy those chelators suitable for the selective removal or recovery of various heavy metals. Chelators that are identified, studied, and recommended as a result of this project could be used in on-site soil washing processes following excavation.

Approach: Investigators will assess the potential of extracting and recovering heavy metals from contaminated soils using chelators. Results of assessment of a large number of chelators will provide a guide to select suitable chelators for different metals. Equilibrium chemical modeling/calculation and connectivity index modeling will be performed to choose about ten chelators from the initial list for detailed experimental study. Study will be conducted on the extraction and recovery of the seven ta rget metals from collected contaminated soils using ten selected chelators. Investigators will also demonstrate that through a proper choice of functional groups in a chelator, the selectivity of the chelator can be greatly enhanced and that through a pro per choice of chelator, extracted mixed metals can be recovered separately through sequential treatment stages. Finally, stability of about six chelators will be evaluated with respect to biodegradation.

Status: Over 700 potential organic c helating agents have been screened and about two hundred selected for further evaluation. The complexation constants of about 170 chelators with lead and copper were used as a database to determine the quantitative structure-activity relationship. Results indicated that the complexation ability of the chelating agents are determined by factors including various molecular descriptors, numbers of hydroxyl, amino, carboxylic groups, as well as protons and heteroatoms. From modeling and computation results, t en chelators were selected for experiments. Results showed that (1) lead, copper, cadmium, and zinc can be extracted from contaminated soils using various chelating agents N-(2-acetamido)iminodiacetic acid (ADA), pyridine-2,6-dicarboxylic acid (PDA), and S-carboxymethyl-L-cysteine (SCMC); (2) the metals can be readily separated and recovered as metal precipitates by simply raising the solution pH; (3) the chelators that remained in solution after separation were successfully reused for further extraction during consecutive runs. Results suggest that ADA is particularly effective for extraction of lead while SCMC is especially effective for copper. Three additional chelators are currently being tested. Three organizations have expressed strong interest in the possible commercialization of these research results. This project is in its first year.

Clients/Users: This research will be of interest to public agencies such as U.S. Environmental Protection Agency, U.S. Department of Defense, and industry.

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