Evaluation of different phosphate amendments on availability of metals in contaminated soil

A pot trial was conducted to assess the efficiency of P-induced metal immobilization in soils. Natural hydroxyapatite (HA), phosphate rock (PR), triple-superphosphate (TSP), and diammonium phosphate (DAP) were evaluated for their ability to reduce cadmium (Cd), lead (Pb), and zinc (Zn) bioavailability in an artificially metal-contaminated soil with three addition levels: T0 (without metals added), 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 P2O5 kg−1 soil for each metal 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), and sequential extraction. The results showed that HA was superior to all other materials for reducing Pb, Zn, and Cd uptake in shoot with reductions 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 and the decreased Pb translocation from the plant root to shoot induced by added phosphate were identified with scanning electron microscope (SEM) equipped with energy dispersive X-ray elemental spectrometry (EDS). Sequential extraction results indicated that the phosphate amendments converted significant amounts of the soil Pb, Zn, and Cd from exchangeable (EX), organic bound (OC), carbonate bound (CB), amorphous Fe and Al oxides-bound (OX) (non-residual (RES) fractions) to RES fraction. 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 in contaminated soils.