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 Pykg soil based on a PyPb molar ration of 4.0, with four treatments: T , no P application; T , H PO only; T , 1y2 H PO q1y2 CaH PO ; and T , 0 1 3 4 2 3 4 2 4 3 1y2 H PO q1y2 phosphate rock. Soil samples were collected and analyzed 220 days after the P applications. For all 3 4 P treatments, toxicity characteristic leaching procedure (TCLP) extractable Pb levels in surface soils (0–10 cm) were reduced to below 5.0 mg Pbyl, with T also effective in reducing the TCLP Pb level in subsurface soils (10–30 cm, 2 -4.63 mgyl). 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 was the most effective in reducing Pb mobility and minimizing P and Pb leaching 3 in the soil. Our research clearly demonstrates that P amendments were effective in immobilizing Pb in contaminated soil. 2002 Elsevier Science Ltd. All rights reserved.