LEAD IMMOBILIZATION BY HYDROXYAPATITE IN AQUEOUS-SOLUTIONS

This study examines the possibilities for removing lead from water by reactions involving synthetic hydroxyapatite. Batch experiments show t hat the rate of Pb2+ removal in these reactions is kinetically quite r apid. For example, approximately 100 mg L-1 Pb2+ in the starting solut ion are reduced from the solutions to < 0.5 mug L-1 within several min utes. The reaction mechanisms are dominated by the dissolution of hydr oxyapatite and the precipitation of lead apatites, namely hydroxypyrom orphite and chloropyromorphite, both of which are very insoluble over the Ph range of environmental concern. The results show that hydroxyap atite is capable of dissolving sufficient phosphate ions to promote le ad precipitation and thus immobilize 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 ch loride in the Pb2+-hydroxyapatite system, the formation of chloropyrom orphite on the surface of hydroxyapatite provides an effective coating , which inhibits hydroxyapatite dissolution. While the potential for c ontrolling lead exists, more work needs to be done to characterize the reactions fully. This study confirms the possibilities for using mine ral apatite or natural phosphate rocks to treat lead-contaminated wast e water or groundwater.