Chemical and Biological Stability of Phosphate-Stabilized Lead in Contaminated Urban Soil and Mine Wastes.

Environmental lead (Pb) contamination resulting from mining activities is a threat to human health and ecosystem.  In situ immobilization using phosphate amendments is being evaluated as an effective remedial alternative for reducing the human exposure and ecological risks through induced transformation of labile Pb to  insoluble species.  To assess the stability of phosphate-stabilized Pb in soil, urban soil and mill waste containing about 4000 ppm Pb were treated with 1% phosphoric acid and packed into the PVC columns of 30-cm long, 4-cm diameter.  The soil columns were subjected to leaching procedures using the Toxicity Characteristic Leaching Procedure (TCLP) and Simulated Precipitation Leaching Procedures (SPLP) with or without presence of plant growth.  The soluble P-treatment effectively immobilized soil Pb and reduced leachable Pb in contaminated urban soil and mill waste, regardless of the leaching procedures and plant growth, resulting in aqueous Pb levels below the EPA water quality criteria.  The leaching by TCLP had slightly higher leachable Pb than that by SPLP.  Metal uptake by plants was significantly reduced by the treatment, and presence of plant growth had no adverse impacts on metal leabability.  This study demonstrated that the Pb species formed after treatment were chemically and biologically stable in the surface soil conditions and the health and ecological risk reduction by the treatment would be long-term.