Lead carbonate–phosphate system: solid–dilute solution exchange reactions in aqueous systems

Abstract

The Pb²⁺—CO₃^2–—PO₄^3– system is described as an example of the exchange relationships that can be established between suspended solids of metal compounds of low solubility and ions in solution, which would also lead to the precipitation of low solubility compounds. Chemical analysis, X-ray diffraction powder data and FT–Raman spectroscopy confirm that contact between suspended PbCO₃ or Pb₃(CO₃)₂(OH)₂ and aqueous PO₄^3– leads to carbonate exchange with phosphate and to the formation of Pb₅(PO₄)₃OH. The films deposited on lead metal in contact with carbonate- and phosphate-containing solutions were characterized by X-ray diffraction, scanning electron microscopy with energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, electron diffraction and confocal Raman microscopy. In both systems, tetragonal PbO is identified as the phase formed in the immediate vicinity of the metal surface, although in the case of the carbonate system, it is present in combination with PbCO₃. These oxide layers are overlaid with Pb₃(CO₃)₂(OH)₂ and Pb₅(PO₄)₃OH in the carbonate and phosphate systems, respectively. The presence of calcium in solution leads to the formation of Pb_5–xCa_x(PO₄)₃OH, rather than Pb₅(PO₄)₃OH as a discrete phase.