Effects of phosphorus additions on lead, cadmium, and zinc bioavailabilities in a metal-contaminated soil

The use of phosphorus (P) to reduce lead (Pb) bioavailability is being proposed as an alternative to excavation and disposal as a remedial technology for Pb-contaminated soils in residential areas. The objective of this study was to determine the influence of P sources and rates and CaCO3 additions on the bioavailabilities of Pb, cadmium (Cd), and zinc (Zn) in a contaminated soil material using plants, a sequential extraction procedure, and ion activities in equilibrium solution as indicators. A contaminated soil containing 370 mg kg(-1) Cd, 2800 mg kg(-1) Pb and 29100 mg kg(-1) Zn was amended in a factorial arrangement of CaCO3 (0 or 2000 mg kg(-1)) and P as rock phosphate or KH2PO4 at 0:1, 2:1 or 4:1 P:Pb mole ratios. A pot study was conducted using sorghum-sudangrass (Sorghum bicolor L. Moench). The addition of P did not influence Pb concentrations in plant tissue and had little effect on Cd concentrations. An interaction between P source and level of P addition was found for Zn concentrations in plant tissue; concentrations increased with increasing amounts of P from KH2PO4 and decreased with increasing amounts of P from rock phosphate. Sequential extraction results suggested a much greater reduction in Pb bioavailability from treatment with KH2PO4 than with rock phosphate and that P influenced the fractionations of Cd and Zn. Activities of Cd2+, Pb2+, and Zn2+ in equilibrium solutions generally were decreased by rock phosphate and increased by KH2PO4. Saturation indices suggested the addition of KH2PO4 shifted the soil equilibrium from octavite to hydroxypyromorphite, whereas solid-phase control of Cd2+ and Zn2+ was not influenced by soil amendments. A soluble P source was more effective in reducing Pb bioavailability than rock phosphate but had variable effects on Cd and Zn bioavailabilities.