Effect of Apatite Amendments on Plant Uptake of Lead from Contaminated Soil

Phosphate compounds of Pb [e.g., pyromorphite Pb5(PO4)3(X) where X = OH, F, or Cl] are comparatively insoluble, and inducing their formation in contaminated soils may be a means of reducing the bioavailability and chemical lability of Pb in soil. Previous research has documented the formation of pyromorphite subsequent to the addition of phosphates, as soluble phosphate (Cotter-Howells, J.; Caporn, S. Appl. Geochem. 1996, 11, 335) and as apatite (Laperche et al. Environ. Sci. Technol. 1996, 30, 3321), to Pb-contaminated soils. In the present study, the effect of apatite amendments on the bioavailability of Pb in contaminated soil and the stability of pyromorphite were examined. A Pb-contaminated soil was treated with natural and synthetic apatites, and the bioavailability of Pb was determined in plant uptake studies with sudax (Sorghum bicolor L. Moench). The Pb content in shoot tissue decreased as the quantity of added apatite increased. However, Pb and P contents in the plant roots increased as the quantity of added apatite increased. In the absence of apatite amendments, Pb content in the shoot was 170 mg of Pb/kg dry weight; apatite decreased the shoot Pb concentration to 3 mg/kg. XRD and SEM analysis indicated that apatite reacted with Pb in the contaminated soil to form pyromorphite, in situ. However, accumulation of Pb in the roots and formation of pyromorphite on root surfaces was also noted. This study indicates that apatite amendments to contaminated soils can lower the bioavailability and increase the geochemical stability of soil Pb.