Decreasing Lead Bioaccessibility In Soils With Phosphate Amendments

Abstract

Ingestion of Pb-contaminated soil has been proposed as a primary exposure pathway for elevated blood Pb levels in young children (Dudka and Miller, 1999). Children are often exposed to soils contaminated with toxic metals such as lead (Pb) through hand-to-mouth activity. Therefore, the ingestion of Pb-contaminated soils by children is typically the risk driver at Pb-contaminated sites (Dudka and Miller, 1999). The subject of immobilizing Pb in Pb – contaminated soils has been frequently studied. In – situ stabilization using phosphorus (P) amendments, such as phosphate fertilizers and phosphate rock, were found to provide the most cost – effective and least disruptive alternative for stabilizing Pb in soils (Berti and Cunningham, 1997; Ma and Rao, 1999). However, few studies have examined the bioavailability of Pb – contaminated soils amended with P using the physiologically based extraction test (PBET). The primary objective of this research was to investigate the effects of aging time and P amendments on Pb bioaccessibility. Amendments were applied in situ to Pb – spiked (labile) soils and Pb – contaminated (non – labile) soils for comparison. Pb and Sb concentrations in small – arms firing range soils were also analyzed to determine whether the two are correlated. Analysis of phosphate concentrations in PBET supernatants revealed that the PBET samples were well undersaturated (SI<0) with respect to chloropyromorphite. Therefore, chloropyromorphite did not form as an experimental artifact in the PBET. The weathering and corrosion of Pb ammunition is a significant source of Pb in Pb - contaminated soils. The results from three small-arms firing ranges showed that Pb and Sb concentrations are linearly correlated (R2>0.90). The data was consistent with the general composition of Sb in Pb bullets and the co-mobility of Pb and Sb due to the weathering and corrosion of ammunition. Results from the two amendment studies revealed that large amounts (26.2% - 50.5% by weight) of P amendments must be applied in situ to achieve significant reductions in Pb bioaccessibility. Environmental implications for adding such large amounts of P to soil, such as increased leaching of oxyanions like Sb and As, should be considered. Thus, adding P amendments to soils may not be the most practical approach to reduce the bioaccessibility of Pb in Pb – contaminated soils.