Chemical and mineralogical characterizations of Pb in a contaminated soil: Reactions with synthetic apatite

Previous studies have shown that the interactions of apatite with dissolved Pb are caused by the dissolution of apatite grains concomitant with the precipitation of lead orthophosphates (pyromorphites). The present study extends this work by examining the interactions of selected Pb minerals and a Pb-contaminated soil with apatite. Specimen-grade PbO and PbCO3 were reacted separately with hydroxylapatite (HA) in controlled pH reactors. Hydroxypyromorphite (HP) formed at the expense of HA, PbO, and PbCO3 after a reaction period of 2 days, causing significant decreases in aqueous Pb concentrations. The extent of reaction was pH dependent, with more HP formation at pH 5 than at pH 6 or pH 7. Equilibrium modeling with MINEQL(+) indicated the stoichiometric conversion of the native Pb solids to HP at all pH values examined in laboratory experiments. In companion experiments, particle size and density separation techniques were used to obtain Pb-enriched fractions from a contaminated soil. These were identified as PbO and PbCO3 with X-ray diffraction (XRD) and scanning electron microscopy (SEM). The Pb-enriched fractions were reacted with HA, and the formation of HP (at the expense of ''native'' Pb solids) was observed by XRD and SEM. Clearly, apatite amendments to Pb-contaminated soil materials can induce the formation of pyromorphites.