Abstract
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.
Xinde Cao, Lena Q. Ma, Satya P. Singh, Ming Chen, Willie G. Harris, and Peter Kizza, University of Florida. January 2003.