Lead immobilization by hydroxyapatite in aqueous solutions

Abstract

This study examines the possibilities for removing lead from water by reactions involving synthetic hydroxyapatite. batch experiments show that the rate of Pb²⁺ removal in these reactions is kinetically quite rapid. For example, ∼ 100 mg L⁻¹ Pb²⁺ in the starting solution are reduced from the solutions to < 0.5 μg L⁻¹ within several minutes. The reaction mechanisms are dominated by the dissolution of hydroxyapatite and the precipitation of lead apatites, namely hydroxypyromorphite and chloropyromorphite, both of which are very insoluble over the pH range of environmental concern. The results show that hydroxyapatite is capable of dissolving sufficient phosphate ions to promote lead precipitation and thus immobilized the lead from aqueous solutions. Further, evidence from the solid analysis shows how the morphology of the precipitates affects the reaction kinetics as well as the capacity of apatite to remove aqueous lead. For example, in the presence of chloride in the Pb²⁺-hydroxyapatite system. the formation of chloropyromorphite on the surface of hydroxyapatite provides an effective coating, which inhibits hydroxyapatite dissolution. While the potential for controlling lead exists, more work needs to be done to characterize the reactions fully. This study confirms the possibilities for using mineral apatite or natural phosphate rocks to treat lead-contaminated waste water or groundwater.