Field demonstration of in situ immobilization of soil Pb using P amendments

A long-term field-scale demonstration project on the feasibility of using P to immobilize Pb in a Pb-contaminated urban soil was conducted. Phosphate was applied at an average rate of 3040 mg P/kg soil based on a P/Pb molar ration of 4.0, with four treatments: T-0, no P application; T-1, H3PO4 only; T-2, 1/2 H3PO4 + 1/2 CaH2PO4; and T-3, 1/2 H3PO4 + 1/2 phosphate rock. Soil samples were collected and analyzed 220 days after the P applications. For all P treatments, toxicity characteristic leaching procedure (TCLP) extractable Pb levels in surface soils (0-10 cm) were reduced to below 5.0 mg Pb/l, with T-2 also effective in reducing the TCLP Pb level in subsurface soils (10-30 cm, <4.63 mg/l). Sequential extraction analysis indicates that P was effective in transforming soil Pb from non-residual fractions to a residual fraction. Such a transformation was mainly through dissolution of Pb associated with carbonate fraction and precipitation of pyromorphite-like minerals, which were confirmed using X-ray diffraction (XRD). Among these three treatments, T-3 was the most effective in reducing Pb mobility and minimizing P and Pb leaching in the soil. Our research clearly demonstrates that P amendments were effective in immobilizing Pb in contaminated soil. (C) 2002 Elsevier Science Ltd. All rights reserved.