Mobilization of lead from highly weathered porous material by extracting agents

Batch and dynamic leaching experiments were conducted to evaluate the influ ence of clay mineralogy and soil organic matter (SOM) present in two highly weathered, surface and subsurface soil materials on Pb sorption and subseq uent extraction using ethylenediaminetetraacetic acid (EDTA) and carboxymet hyl beta -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 retardati on was observed within the surface soil compared to the subsurface soil mat erial, and a significant increase in effluent turbidity was observed coinci dent with the Pb breakthrough in the subsurface soil material. Electrophore tic, thermo-gravimetric, and X-ray diffraction analysis of the resulting su spension suggested that the presence of strongly adsorbing Pb2+ and excess H+ increased the positive charge on amphoteric mineral surfaces and reduced the negative charge on clay minerals, which induced selective dispersion a nd transport of Fe oxides. In contrast, no increase in turbidity was observ ed 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 Ph was extracted from surface soil when Pb-contaminated soil co lumns were flushed with AGW. Following leaching with AGW, extraction soluti ons 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 Ph was extracted from the surface soil by EDTA as compared to about 56% removal by CMCD unde r similar conditions. About 80% of residual Ph was removed by EDTA from the Pb-contaminated subsurface material, but less than 2% of residual Pb was e xtracted by CMCD under similar conditions.