Department of Civil Engineering, University of Utah, Salt Lake City, UT, 84112, 801-581-7232
Heavy metal contamination 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 Priority List sites. Heavy metals are toxic to people and pose a great risk for safe ground water supply. 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 contaminated soils has recently been seen as a treatment method. However, results from our laboratories show that, in order to recover the heavy metals and reclaim the chelating agents, only suitable chelating agents should be used for this application.
The structure and electronic configuration of a chelating agent are important to its metal-complexing ability, speciation, selectivity, biodegradability, toxicity and transportability. The molecular connectivity index model which can encode the structural and electronic information of a chelate is seen as a useful tool to predict the said properties and aid in the selection of useful chelating agents for extractive removal of heavy metals from contaminated soils. The previous findings of various researchers have provided a simple method of encoding the essential information concerning a molecule into terms which can be employed in a linear equation; the latter is employed to validate or reject any or all variables. The approach is based on the assumption that there is, within the structural formula, sufficient information such that a useful index based upon non-empirical counts of atoms can be calculated. The technique has been found useful for correlating molecular structures with properties such as molar refraction, polarizability, water solubility, chromatographic retention data, orientation of ring substitution, phenol toxicity to fathead minnows, antimicrobial action of phenyl-propylethers, flavor and taste threshold, and many others. Relevant properties of the metals can be included in the LFER providing a basis for developing a "designer" chelator. New methods for selection of variables and regression diagnostics have been employed.
In its final form, this paper will present the results of using the molecular connectivity index model to correlate the metal-complexing ability of a large number of chelating agents with their molecular structure. The developed model will be useful for predicting metal complexation for other agents for which the complexing constants are unavailable. The limitations of this model will be identified. This paper will also present the experimental results of heavy metal extraction from soil using a few selected chelating agents. The removal of metals including cadmium, copper, lead and zinc has been studied under different pH, soil suspension, total chelator concentration, total carbonate concentration, and age conditions and will be presented. In particular, the selectivity and subsequent recovery of heavy metals will be emphasized.
structure-activity, metal, soil, chelator, contamination, remediation
This paper is from the Proceedings of the 10th Annual Conference on Hazardous Waste Research 1995, published in hard copy and on the Web by the Great Plains/Rocky Mountain Hazardous Substance Research Center.