Enhancement of Heavy Metal Immobilization in a Lead-Smelting Polluted Soil with Soluble Phosphate and Calcium

It has been well established that lead (Pb) in polluted soil can be immobilized with phosphate. Many kinds of phosphates and phosphorus-containing materials have been applied in Pb-polluted soil to reduce the availability and mobility of Pb. These investigations successfully improved the growth of plants and reduced the extractable Pb in polluted soils.

Research has found that, in a Pb-spiked soil, when calcium was added shortly after or before soluble phosphate was added, the calcium amendment reduced the DTPA-extractable Pb concentrations, and improved plant growth.

In this work, a Pb-smelting polluted soil (pH=7.93, total Pb=2337 mg/kg, total Cd=21.4 mg/kg and total Zn=486 mg/kg) from Henan Province, China was first spiked with Monopotassium phosphate (MPP) or monocalcium phosphate (MCP) at 22.6 mmolP/kg, then calcium nitrate was added at 10 mmolCa/kg. The soil water content was kept at 20% for 80 days. For both phosphates, calcium amendment decreased soil pH by about 0.25 pH unit; calcium amendment also decreased Olsen-P contents. The calcium amendment decreased the DTPA-Pb concentration from 492 mg/kg to 464 mg/kg (p<0.05) for the Ca and MPP amended soil than the MPP-only amended soil, for the MCP amended soil, the DTPA-Pb concentration decreased by 11.7 mg/kg (p>0.05). The Ca amendment also reduced the DTPA-Cd and Zn concentrations in soil, but the effect was not statistically significant. The results suggested that Pb immobilization with phosphate can be enhanced with amendment of Ca in Pb-smelting polluted soils.