PRECURSOR EVOLUTION AND METAL UPTAKE IN APATITE FORMATION FROM AQUEOUS SOLUTIONS

Use of apatite formation in contaminated sediments has been proposed as a method for heavy metal and radionuclide sequestration and environmental remediation. One mechanism of sequestration is coprecipitation of metals into apatite. After incorporation, the metals are far less dispersible and bioavailable, and thus pose a substantially reduced heath risk. Immobilization is then controlled by apatite solubility. Studies of simultaneous uptake of Pb and As in contaminated sediments from a former Pb smelting site in Florida show that both Ca and PO₄ addition is necessary for significant reduction of releasable Pb and As in TCLP solutions. TCLP-releasable As concentrations were reduced from about 300 ppb to 6-28 ppb when Ca concentration was at least 0.36 g/L and P concentration at least 0.03 g/L. TCLP- releasable Pb concentrations decreased from about 200 ppb to 17 ppb or lower in every experiment where P was added (0.01 g/L was the lowest used P concentration).

The formation of hydroxylapatite from aqueous solutions under conditions similar to those found in most sediments and soils, including those in the Florida site, has been shown to involve the precipitation and transformation of precursor phases. Yet this fundamental aspect of apatite formation under earth-surface conditions has not been addressed in the context of metal sequestration and fate.

Low-temperature aqueous precipitation experiments in the Ca(OH)₂-H₃PO₄-H₂O system, free of heavy metals, were conducted and analyzed ex situ and in situ via time-resolved synchrotron X-ray diffraction. At near neutral pH initially formed amorphous calcium phosphate and/or crystalline brushite transformed to octacalcium phosphate (OCP). Subsequently OCP transforms to hydroxylapatite. This is accompanied or followed by the transformation of the remaining brushite to monetite. Precipitation experiments in the presence of heavy metals including Pb²⁺, AsO₄^-3, Zn²⁺, Sr²⁺, Cd²⁺, (UO₂)²⁺, and Th⁴⁺ show that these can affect precursor formation and evolution, and in the case of Pb and As are dependent on initial speciation.

Precursor development during apatite formation may have a critical bearing on the effective use of apatite for metal immobilization and our understanding of the role of phosphate in the global geochemical cycling of heavy metals.