Formation of chloropyromorphite in a lead-contaminated soil amended with hydroxyapatite

Conversion of soil Pb to pyromorphite [Pb-5(PO4)(3)Cl] was evaluated by rea cting a Pb contaminated soil collected adjacent to a historical smelter wit h hydroxyapatite [Ca-5(PO4)(3)OH]. In a dialysis experiment where the soil and hydroxyapatite solids were placed in separate dialysis bags suspended i n 0.01 M NaNO3 solution a crystalline precipitate, identified as chloropyro morphite, formed on the dialysis membrane containing the soil. The aqueous composition of the solution indicated that dissolution of solid-phase soil Pb was the rate-limiting step for pyromorphite formation. Addition of hydro xyapatite to the soil caused a decrease in each of the first four fractions of sequential extractable Pb and a 35% increase in the recalcitrant extrac tion residue, After a 240-d incubation at field-moisture content there was a further increase in the recalcitrant extraction residue fraction of the h ydroxyapatite-amended soil to 45% of the total soil Pb. The increase in the extraction residue fraction in the hydroxyapatite amended 0-d incubated so il as compared to the control soil illustrates that the chemical extraction procedure itself caused changes in extractability. Thus,the chemical extra ction procedure cannot easily be utilized to confirm changes occurring in a mended soils. The further increase after the 240-d incubation implies that the reaction also occurs in the soil during incubation. Extended X-ray abso rption fine structure (EXAFS) spectroscopy indicated that after the 240-d i ncubation the hydroxyapatite treatment caused a change in the average, loca l molecular bonding environment of soil Pb. Low-temperature EXAFS spectra ( chi data and radial structure functions - RSFs) showed a high degree of sim ilarity between the chemical extraction residue and synthetic pyromorphite, providing additional evidence that the change of soil Pb to pyromorphite i s possible by simple amendments of hydroxyapatite to soil.