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

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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 Pb5–xCax(PO4)3OH, rather than Pb5(PO4)3OH as a discrete phase.