Natural clays (i.e., montmorillonite and diatomite) modified by iron and manganese oxide were used as immobilization agents for the remediation of Pb contaminated soil. Batch sorption experiments were conducted to investigate the effect of surface modification on Pb sorption. Pb sorption capacity in modified clays was higher than that in natural clays. Pb sorption capacity in modified and unmodified clays was in the order of Mn-diatomite (1.967 mmol/g) > Mn-montmorillonite (0.718 mmol/g) > Fe-diatomite (0.457 mmol/g) > Fe-montmorillonite (0.330 mmol/g) > montmorillonite (0.233 mmol/g) > diatomite (0.025 mmol/g). Effects of modified clay amendment and ageing (or contact time) on immobilization of Pb were evaluated by a toxicity characteristic leaching procedure (TCLP) and sequential extraction. An artificially polluted natural soil (APNS) with Pb (59.72 mmol Pb/kg soil) was used in immobilization experiment. Lead immobilization efficiency in APNS with modified clay was in the order of Mn-montmorillonite (93%) > Mn-diatomite (86%) > Fe-montmorillonite (81%) > Fe-diatomite (80%). Among the tested modified clays, Mn-montmorillonite was the most efficient in Pb immobilization. Results of sequential extraction of Pb from APNS after immobilization with modified clays indicate that mobile fraction (i.e., exchangeable and carbonate fraction) decreased as contact time increased, while less mobile (i.e., reducible) fraction increased from 27% to 60% of the total amount extracted. This indicates that addition of modified clay to APNS transforms Pb in mobile fraction into reducible fraction.

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