Immobilization of lead with nanocrystalline carbonated apatite present in fish bone

Reactions of lead with phosphate can immobilize lead in situ in contaminated soils. We investigated reactions between dissolved lead and fish bones, which are an environmentally useful phosphate source for lead immobilization. Fish bones contain biogenic nanocrystalline apatite [Ca(5)(PO(4))(3)OH((s))] that is carbonated and hydroxyl-deficient. Experiments compared the lead removal upon reaction with unheated fish bones, fish bones that had been heated to char the organic phase, and a synthetic hydroxylapatite solid. The investigation combined analyses of the aqueous phase with direct characterization of the solid phase by laser Raman microprobe spectroscopy to determine the identity and location of any lead phosphate precipitates. Dissolved lead concentrations decreased by several orders of magnitude after reaction with all of the solid phosphates studied. The immobilization of lead had a slight but measurable effect on the continuing release of calcium and phosphate from the solid phase to solution. The fish bones were more soluble than the synthetic hydroxylapatite, and both phases were more soluble than was predicted from available equilibrium constants for hydroxylapatite. At high lead loading relative to the surface area of the phosphate solid, homogeneous nucleation of the lead phosphate pyromorphite [Pb(5)(PO(4))(3)(OH,Cl)((s))] was observed in suspension. At lower dissolved lead concentrations relative to surface area, immobilization may be dominated by adsorption of lead onto bone phases with high surface area rather than by precipitation of pyromorphite.