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This research investigated the effectiveness of phosphate rocks in immobilizing Pb from aqueous solutions and contaminated soils. Different amounts of phosphate rocks were reacted with aqueous solutions containing Pb or with Pb-contaminated soils. The results show that phosphate rocks were effective in immobilizing Pb from aqueous solutions with a minimum Pb removal of 38.8-100%. The reaction time had less effect on Pb immobilization than the quantity of phosphate rocks used. Selected phosphate rocks reduced water-soluble Pb from a contaminated soil by 56.8-100%. The primary mechanism of Pb immobilization was via dissolution of phosphate rocks and precipitation of a fluoropyromorphite-like mineral. Different methods of mixing phosphate rocks and soil and incubation time had little effect on Pb immobilization. Our results strongly demonstrate that phosphate rocks may provide a cost-effective way to remediate Pb-contaminated water, soils, and wastes.

Remediation of Pb-contaminated wastes has received considerable attention recently. We have previously shown that hydroxyapatite [Ca 5 (PO 4 ) 3 OH] can reduce Pb 2+ concentrations below the EPA action level (72.4 nmol L -1 ) and, thus, has the potential for in situ Pb 2+ immobilization against leaching. This research investigated the effects of NO 3 - , Cl - , F - , SO 4 2- , and CO 3 2- on hydroxyapatite-Pb 2+ interactions. Solutions containing initial Pb 2+ concentrations of 24.1-482 μmol L -1 were reduced to below 72.4 nmol L -1 after reaction with hydroxyapatite, except in the presence of high levels of CO 3 2- and Pb 2+

The effects of aqueous Al, Cd, Cu, Fe(II), Ni, or Zn on Pb immobilization by hydroxyapatite [Ca 10 (PO 4 ) 6 (OH) 2 ] were studied. Lead was removed mainly via hydroxyapatite dissolution and hydroxypyromorphite [Pb 10 (PO 4 ) 6 (OH) 2 ] precipitation in the presence of these metals with a Pb removal efficiency of 37-100%. These metals inhibited Pb immobilization by hydroxyapatite in the order. Al>Cu>Fe(II)>Cd>Zn>Ni and Cu>Fe(II)>Cd>Zn>Al>Ni at high and low initial Pb concentrations, respectively. The inhibition was probably through the precipitation of amorphous to poorly crystalline metal phosphates, decreasingthe amountofdissolved P available for precipitation with dissolved Pb ions

Lead contamination is of environmental concern due to its effect on human health. The purpose of this study was to develop a technology to immobilize Pb in situ in contaminated soils and wastes using apatite. Hydroxyapatite [Ca 10 (PO 4 ) 6 (OH) 2 ] was reacted with aqueous Pb, resin-exchangeable Pb, and Pb-contaminated soil material. The effectiveness of natural apatite to attenuate Pb was also tested. Hydroxyapatite reduced initial Pb concentrations of 5-500 mg L -1 to 0.18-19.7 μg L -1 . Aqueous Pb in Pb-contaminated soil materials was reduced from 2273 to 36 μg L -1 after reaction with hydroxyapatite. We hypothesize that Pb was immobilized by dissolution of hydroxyapatite and precipitation of hydroxypyromorphite [Pb 10 (PO 4 ) 6 (OH) 2 ]