Effects of Incubation and Phosphate Rock on Lead Extractability and Speciation in Contaminated Soils

Abstract

Lead distribution in 11 contaminated soils among five physical size fractions was studied. Lead was present in elevated concentrations in these soils, ranging from 198 to 12 523 mg kg−1. Although Pb in these soils was randomly distributed among the five size fractions studied, it was slightly concentrated in the silt and clay (< 53 µm) fraction with some exceptions. Several single chemical extractants were used to evaluate Pb availability in the contaminated soils. The effects of incubation and phosphate rock on Pb availability were also investigated. Significant amounts of Pb were extracted with all the extractants. The extractability increased in the order: H2O < Ca(NO3)2 < HOAc < EDTA-NH4OAc. The percentage of Pb extracted generally decreased with increasing incubation time as the soils gradually lost moisture during incubation. Net extractable Pb reduction with incubation time was not correlated with soil pH, organic matter, or total Fe and Mn contents in soils. Reduced Pb extractability with incubation may be attributed to precipitation/adsorption of Pb, rendering Pb less available. Addition of phosphate rock reduced Ca(NO3)2 and HOAc extractable Pb from Pb-contaminated soils, but had little effect on EDTA-NH4OAc extractable Pb. Net extractable Pb reduction by phosphate rock was not correlated with soil pH, organic matter, or other metals such as Zn, Cu, Ni, and Cd in soils, but it was highly correlated with total Pb (r = 0.7). Our results indicate that physical-chemical speciation of Pb-contaminated soils can be a useful tool in evaluating Pb availability.