Effects of phosphate rock on sequential chemical extraction of lead in contaminated soils

Lead contamination is of great concern because of its adverse effects on human health, especially children. This research evaluated the effects of phosphate rock on chemical associations of Pb in eight Pb-contaminated soils using a sequential extraction procedure. The chemical fractions are operationally defined by an extraction sequence in the order of increasing ability to dissolve Pb of lower solubilities. Additionally, more soluble forms of Pb are considered to be potentially more bioavailable than the less soluble forms. Lead in these soils was primarily associated with the carbonate and Fe-Mn oxide fractions (63-85%). Up to 21% of the Pb in these soils was associated with either the organic or the residual fraction and <11% was associated with the water-soluble and the exchangeable fractions. Phosphate reeks effectively converted Pb from the water soluble, exchangeable, carbonate, Fe-Mn oxide, and organic fractions (collectively the non-residual fraction) to the residual fraction, thus reducing Pb solubility and presumably bioavailability. Lead precipitation as a fluoropyromorphite-like mineral in these contaminated soils was suggested as the primary mechanism for reduced Pb solubility and Pb reduction in the nonresidual fraction. The effective conversion of Pb from potentially available fractions to the residual fraction suggests that phosphate rock has potential for in-situ immobilization in Pb contaminated soils.