Formation of chloropyromorphite from galena (PbS) in the presence of hydroxyapatite

Transformation of unstable lead [Pb(ll)] forms into sparingly soluble pyromorphite [Pb-5(PO4)(3)(OH, Cl, F, ...)] by the addition of phosphate to Pb-contaminated soil has been proposed as a remediation technology to reduce the mobility and bioavailability of Pb. Galena, an insoluble lead sulfide mineral (PbS) frequently found in mining wastes, becomes unstable upon exposure to oxidizing conditions causing it to become a source of liable Pb forms in soils. Thus, a galena ore was reacted with synthetic hydroxyapatite [Ca-5(PO4)(3)OH] under various pH conditions to determine the formation rate of pyromorphite and the solubility of galena under the ambient condition. In a 6-day reaction period, the dissolution rate of galena increased with pH due to the oxidation of dissolved sulfide. Hence, formation of chloropyromorphite became apparent in the galena-apatite suspensions with increasing pH. The insignificant effect of mineral P/Pb molar ratio on the formation of chloropyromorphite implied that dissolution/oxidation of galena was the rate-limiting step.