Field assessment of lead immobilization in a contaminated soil after phosphate application

Abstract

A pilot-scale field demonstration was conducted at a Pb-contaminated site to assess the effectiveness of Pb immobilization using P amendments. The test site was contaminated by past battery recycling activities, with average soil Pb concentration of 1.16%. Phosphate amendments were applied at a 4.0 molar ratio of P/Pb with three treatments: T1, 100% P from H 3PO 4; T2, 50% from H 3PO 4+50% from Ca(H 2PO 4) 2; and T3, 50% from H 3PO 4+5% phosphate rock. Soil samples were collected and characterized 220 days after P application. Surface soil pH was reduced from 6.45 to 5.05 in T1, to 5.22 in T2, and to 5.71 in T3. Phosphate treatments effectively transformed up to 60% of total soil Pb from the non-residual fraction (sum of water soluble and exchangeable, carbonate, Fe–Mn oxide, and organic fractions) to the residual fraction relative to the control. In addition, P treatments reduced Toxicity Characteristic Leaching Procedure (TCLP) Pb from 82 mg l −1 to below EPA's regulatory level of 5 mg l −1 in the surface soil. Scanning electron microscopy-energy dispersive X-ray elemental analysis and X-ray diffraction analysis indicated formation of insoluble chloropyromorphite [Pb 5(PO 4) 3Cl] mineral in the P-treated soils. Although H 3PO 4 is necessary to dissolve meta-stable Pb in soil for further lead immobilization, it should be used with caution due to its potential secondary contamination. A mixture of H 3PO 4 and Ca(H 2PO 4) 2 or phosphate rock was effective in immobilizing Pb with minimum adverse impacts associated with pH reduction.