Mobilization of Lead from Highly Weathered Porous Material by Extracting Agents

Abstract

Batch and dynamic leaching experiments were conducted to evaluate the influence of clay mineralogy and soil organic matter (SOM) present in two highly weathered, surface and subsurface soil materials on Pb sorption and subsequent extraction using ethylenediaminetetraacetic acid (EDTA) and carboxymethyl ‚-cyclodextrin (CMCD). Saturated soil columns were leached with an acidic Pb solution to simulate a contamination event, followed by injection of artificial groundwater (AGW) at constant velocity. Greater Pb retardation was observed within the surface soil compared to the subsurface soil material, and a significant increase in effluent turbidity was observed coincident with the Pb breakthrough in the subsurface soil material. Electrophoretic, thermo-gravimetric, and X-ray diffraction analysis of the resulting suspension suggested that the presence of strongly adsorbing Pb 2+ and excess H + increased the positive charge on amphoteric mineral surfaces and reduced the negative charge on clay minerals, which induced selective dispersion and transport of Fe oxides. In contrast, no increase in turbidity was observed for the surface soil following Pb injection. While only 30% of adsorbed Pb was mobilized from contaminated subsurface soil material, less than 10% of adsorbed Pb was extracted from surface soil when Pb-contaminated soil columns were flushed with AGW. Following leaching with AGW, extraction solutions containing either 0.37 g L -1 (1 mM) EDTA or 10 g L -1 CMCD were applied to extract the residual bound Pb. Nearly 100% of residual Pb was extracted from the surface soil by EDTA as compared to about 56% removal by CMCD under similar conditions. About 80% of residual Pb was removed by EDTA from the Pbcontaminated subsurface material, but less than 2% of residual Pb was extracted by CMCD under similar conditions.