Transformation of cerussite into lead apatites upon reaction with phosphate ions

Abstract

During natural reactions and in-situ immobilization of lead thermodynamically stable phases like pyromorphite ($Pb_{5}(PO_{4})_{3}Cl$) can form by replacing cerussite. In experiments in which lead carbonate have been reacted with phosphate solutions, cerussite is partially or completely replaced by polycrystalline pyromorphite and hydroxypyromorphite. The study was aimed to investigate the influence of pH and presence or absence of chloride ion on the transformation reaction. Cerussite crystals were put into 2M ($NH_{4})_{2}HPO_{4}$ solution or into 0,7M $NH_{4}Cl$ and 2M ($NH_{4})_{2}HPO_{4}$ solution, the pH was set to initial values of 3, 5 and 7. The products were identified with X-Ray powder diffraction. The microtextural relationships between parent and product phases, determined by scanning electron microscopy and electron microprobe analysis, are consistent with coupled dissolution-precipitation mechanism. In presence of chloride ions, regardless of the initial pH conditions, the dissolution of cerussite is followed by crystallization of pyromorphite in form of nonporous cover that stops diffusion of ions and progress of the reaction. When chloride ions are not available hydroxypyromorphite and, at low pH conditions, other phases such as $Pb_{3}(PO_{4})_{2}$ and $PbHPO_{4}$ are formed.