Effects of phosphate amendments on the bioavailability of lead, cadmium and zinc in metal-contaminated soil

摘要

Phosphate compounds of metis (e.g., pyromorphite M_5 (PO_4)_3-(X), where M= Pb, Cd and Zn; X= OH, F, or Cl) are comparatively insoluble and their formation in contaminated soils may be a cost-effective way to reduce contaminant bioavailability in soils To assess the efficiency of P-induced metals immobilization in soils, four chemical materials, natural hydroxyapatite (HA), mineral phosphate rock (PR), triple-superphosphate (TSP) and diammonium phosphate (DAP) were evaluated using pot experiments to determine their ability to reduce cadmium (Cd), lead (Pb), and zinc (Zn) bioavailabihty in an artificially metal-contaminated soil with three addition levels: control (CK), T1 (Cd/Pb/Zn:0.6/100/66 mg kg~(-1)) and T2 (Cd/Pb/Zn:1.5/300/200 mg kg~(-1)). Phosphate compounds were applied at 2500 mg P kg~(-1) soil for each metals addition level with five treatments: CK (control), TSP, DAP, PR and HA, respectively. The immobilization and bioavailability of Cd, Pb, and Zn were determined by plant (Brassica campestris L) uptake, scanning electron microscope (SEM), transmission electron microscope (TEM) and sequential extraction. The results showed that HA was superior to all other materials for reducing Pb, Zn and Cd uptake in shoot with reduction of 34.6-53.3% for Pb, 31.2-47.3% for Zn and 39.1-42.4% for Cd, respectively, as compared with the control treatment. The mechanisms of Pb immobilization in the soil by P-decreased Pb translocation from the plant root to shoot were identified with SEM-EDS and TEM-EDS micrographs, whereas SEM and TEM analysis identified no phosphate mineral Cd or Zn. Triple super phosphate was generally less effective for reducing Cd, Pb and Zn in plant tissues. Sequential extraction results indicated that the addition of phosphate amendments convert soil Pb, Zn and Cd from non-residual fractions to residual fraction substantially. In general, the effect of different phosphates on plant uptake of Pb, Zn and Cd followed the order: HA > PR > DAP > TSP. The results suggested that HA and PR amendments could significantly reduce the bioavailability and increase the geochemical stability of soil Pb, Zn and Cd, so there is a potential to use them for in-situ remediation in metal-contaminated soils.