Effects of Phosphorus Additions on Lead, Cadmium, and Zinc Bioavailabilities in a Metal-Contaminated Soil

Abstract

The use of phosphorus (P) to reduce lead (Pb)bioavailability is being proposed as an alternative to excavationand disposal as a remedial technology for Pb-contaminated soilsin residential areas. The objective of this study was todetermine the influence of P sources and rates andCaCO3additions on the bioavailabilities of Pb, cadmium (Cd), and zinc(Zn) in a contaminated soil material using plants, a sequentialextraction procedure, and ion activities in equilibrium solutionas indicators. A contaminated soil containing 370 mg kg-1 Cd, 2800 mg kg-1 Pb and 29100 mg kg-1 Zn was amended ina factorial arrangement of CaCO3 (0 or 2000 mg kg-1) and P as rock phosphate or KH2PO4 at 0:1, 2:1 or 4:1P: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 anddecreased with increasing amounts of P from rock phosphate. Sequential extraction results suggested a much greater reduction in Pb bioavailability from treatment withKH2PO4 than with rock phosphate and that P influencedthe fractionations of Cd and Zn. Activities of Cd2+,Pb2+, and Zn2+ in equilibrium solutions generally weredecreased by rock phosphate and increased by KH2PO4. Saturation indices suggested the addition ofKH2PO4shifted the soil equilibrium from octavite to hydroxypyromorphite, whereas solid-phase control of Cd2+ andZn2+ was not influenced by soil amendments. A soluble Psource was more effective in reducing Pb bioavailability thanrock phosphate but had variable effects on Cd and Znbioavailabilities.