FACILITATED LEACHING OF LEAD FROM AN URBAN SOIL: OPTIONS FOR SUSTAINABLE REMEDIATION

Lead in urban soils is a common contaminant and prevalent human health risk. Remediation of these soils, especially those in residential areas is problematic. To investigate options, a study was conducted on soils from a single family home. A composite soil sample was collected from the top 10 cm of soil. Total Pb in the soil averaged 1400 mg kg^-1. Small flow-through columns were prepared using a mixture of 1 part soil to 4 parts acid washed sand to facilitate flow. Each column contained 3.2 g of soil, with a total lead content of 4.48 mg. Four solutions were selected to complete 20 pore volume facilitated leaching experiments. The four solutions selected were 0.05 M sodium citrate, 0.01 M CaCl₂ at pH 9, vinegar and 0.1 M EDTA. The EDTA solution was chosen as it has been commonly used as a soil washing agent. The remaining solutions were selected as being more environmentally benign. Results show the EDTA to be the most effective, removing 25.8% of the total Pb. The other solutions removed less Pb with the citrate, CaCl₂ and vinegar removing 6.1, 0.3 and 8.1%, respectively. Plots of total Pb removed show the EDTA beginning to show a curvilinear trend, indicating that the removal of Pb was slowing. The other solutions show linear trends, suggesting that the reactions are not near completion.

In addition to the leaching experiments, a modified Tessier sequential extraction procedure was performed on the original soil sample, and on the leached soils to evaluate the distribution of Pb in the soil and how each leaching solution might be interacting with the soil Pb. Results for the untreated soil show that approximately 63% of the Pb is associated with easily reducible phases, 24% associated with organic matter and 12.5% associated other reducible phases. Although total concentrations were reduced, for leached soils, the relative percentage of Pb in exchangeable, carbonate and easily reducible phases increased an average of approximately 500, 400 and 130%, respectively. This is likely due to readsorption of Pb from solutions during leaching. Lead associated with organic matter and other reducible phases decreased by approximately 60 and 40%, respectively. Given the removal rates for Pb and the similarity of sequential extraction results, common vinegar might be considered for use as an in situ soil washing solution for urban residential soils.