Field Demonstration of Metal Immobilization in Contaminated Soils Using Phosphate Amendments
Lab tests and a field demonstration were conducted to evaluate the feasibility and effectiveness of immobilizing metals in contaminated soils using phosphate. Phosphate was more effective for Pb immobilization than for Zn, Cu, and Cd. The formation of insoluble pyromorphite-like minerals was responsible for Pb immobilization, whereas Zn, Cu, and Cd immobilization may be attributed to the coprecipitation and surface complexation mechanisms. The most efficient formation of pyromorphite-like minerals was found at pH 3 and at an application rate of 4 P/Pb. Soil acidification was necessary to dissolve soil Pb carbonates and to make them readily available for the formation of pyromorphite-like minerals. Therefore, a two-step phosphate amendment was applied at a contaminated site in which the soil was first acidified with H3PO4, and Ca(H2PO4)2 or phosphate rock was then added. Phosphate effectively induced transformation of soil Pb from the non-residual to the residual fraction, with residual Pb increase by up to 55%. Modeling indicated that lead phosphate minerals controlled Pb2+ activities in the P-treated soils. Phosphate treatments significantly reduced Pb uptake by Stenotaphrum secundatum. A mixture of H3PO4 and phosphate rock yielded the best overall results for in situ Pb immobilization, with less soil pH change and less phosphorus leaching.