Chemical and Mineralogical Characterizations of Pb in a Contaminated Soil:  Reactions with Synthetic Apatite

Abstract

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 Pbcontaminated soil with apatite. Specimen-grade PbO and PbCO 3 were reacted separately with hydroxylapatite (HA) in controlled pH reactors. Hydroxypyromorphite (HP) formed at the expense of HA, PbO, and PbCO 3 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 PbCO 3 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.

Formation of Pyromorphite in Anglesite-Hydroxyapatite Suspensions under Varying pH Conditions

“…The pH range of [2][3][4][5][6][7] was selected to simulate the pH values in the GI tract that has been diseussed elsewhere (19). Reactions were carried out under static pH condition (pH-constant) and in suspensions in which the pH was adjusted'from 2 to 7 within 30 min (pH-dynamic).…”

Peer Reviewed: Reducing Children's Risk from Lead in Soil

“…The NRMRL research program's initial goal was to demonstrate the feasibility of altering the mineralogy of lead in soil in a laboratory setting by using thermodynamic and kinetic studies. Orthophosphate (aqueous phosphorus, hydroxyapatite, or phosphate rock) rapidly and effectively precipitates lead from solution to form a series of lead phosphates of low aqueous solubilities (27,(33)(34)(35)(36)(37)(38)(39)(40)(41)(42)(43). The final product of lead immobilization is primarily pyromorphite [Pb 5 (PO 4 ) 3 X, where X is OH, Cl, or F], which is stable under normal soil environmental conditions.…”

“…Further, environmentally relevant concentrations of other metals had no significant effect on lead immobilization by hydroxyapatite; in fact, hydroxyapatite may remove the metals (38). Also, pyromorphite formation from lead adsorbed on goethite (Fe(OH) 3 ) (39) and primary lead minerals, including cerrusite (PbCO 3 ) (40, 41), anglesite (PbSO 4 ) (42), litharge (PbO) (40), massicot (PbO) (40), and galena (PbS) (43), by addition of apatite have been reported. The completeness and kinetics of this transformation depend on the mineralogy of lead, the amount of apatite added, and the pH of the system.…”

Effect of bone char application on Pb bioavailability in a Pb-contaminated soil

“…Similarly, Ma et al (1995) and Laperche et al (1997) observed a peak at 2.96 Å upon P addition to a Pb-contaminated soil. Thermodynamic considerations predict that Pb phosphate minerals are the most stable among Pb compounds (Laperche et al, 1996;Seaman et al, 2003). Thus, they can be readily formed in the presence of adequate Pb and P. Ruby et al (1994) also demonstrated that the weathering of galena (PbS) to insoluble lead phosphates occurred in soils at a port facility historically used for shipment of phosphate.…”