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

Abstract

Addition of phosphate to lead [Pb(ll) Pb from anglesite to'chloropyromorphite. Increasing the P/Pb ratio increased the transformation rate, but anglesite was still present after a' 120-min reaction time. in the dynamic pH system, the added apatite was rapidly dissolved at the initial low pH, and complete transformation of anglesite to chloropyromorphite was obtained within 25 min., The soluble Pb level was controlled by the solubility of chloropyromorphite during the entire reaction process. These results demonstrate the effect of reaction kinetics on the formation rate of chloropyromorphite and the mechanisms controlling the solubilization of Pb in the anglesite-apatite system. Furthermore, they illustrate that a complete transformation of ingested anglesite to chloropyromorphite can be achieved. under gastrointestinal tract pH conditions if sufficient phosphate is provided.

Formation of Chloropyromorphite in a Lead-Contaminated Soil Amended with Hydroxyapatite

“…The dialysis systems used in this study were designed to study the dissolution of phosphate and solid-phase Pb, dissociation of Pb, and formation of Pb-P compounds (20). Although it has been demonstrated that hydroxyapatite is able to provide sufficient phosphate to immobilize Pb in solutions as pyromorphite (9,13), Pb adsorbed on goethite (20), and Pb primary minerals (21)(22)(23), this process has not been demonstrated in a -complex soil system. In addition, the dialysis system used in this study helps isolate the reaction product(s) for a direct observation with proper instruments as illustrated in related studies in our laboratory (20).…”

“…The high ratio of water to solid, relatively low pH, and continuous agitation in the extraction process created a favorable environment for both soil Pb and apatite P release into the extraction solution. Reported data (9,(20)(21)(22)30) illustrate that the precipitation of pyromorphite is rapid when both Pb and P are in the solution. Thus, pyromorphite formation during extraction offers a plausible explanation for the differences observed by the two measurements and raises concern about the use of extractions to determine if amendments have altered the species of Pb present in the sample.…”

“…However, as an increase in the residual fraction in both the O d and 240 d incubated soil treated with hydroxyapatite occurs, it is apparent that the extraction processes formed pyromorphite. A rapid reaction between soluble Pb and hydroxyapatite to precipitate solid pyromorphite has previously been demonstrated (9,(20)(21)(22)(23). The increase in Pb in the residual fraction of the 240 d incubated soil treated with hydroxyapatite as compared to the O d incubated soil treated with hydroxyapatite implies that a conversion of soil Pb to pyromorphite happens during soil incubation.…”

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.…”

Treatment and Management of Closed or Inactive Small Arms Firing Ranges

“…For this reason, the rate of formation of pyromorphite from phosphate materials such as hydroxyapatite can be limited by the dissolution rates of the lead contaminants. Under neutral pH conditions, the observed solubility of lead and phosphate remain low, causing a major limitation for an efficient lead/phosphate reaction (Zhang and Ryan 1998). Dissolution of the initial lead phase has been reported to be the limiting factor in the formation of hydroxypyromorphite (Pb 5 (PO 4 ) 3 OH) at pH values between 5 and 8 (Laperche et al 1996).…”