Crystallization of some heavy-metal phosphates alone and in the presence of calcium ion

Crystallization of Cd, Pb and Cu phosphates at 5 and 37 degrees C has been studied. Cadmium phosphate is found exclusively as Cd5H2(PO4)(4), 4H(2)O. With increasing pH the morphology changes from regular prismatic crystals to twins, aggregates and dendrites. Mixed crystals are formed with Ca substitution up to 75%. Solubility decreases with increasing Ca substitution. Lead phosphate is found mainly as PbHPO4, but at high pH Pb-5 OH(PO4)(3) is dominating as very small crystals. Those of PbHPO4 are elongated tabular and rather irregular. Aggregates are frequent at low pH and highly irregular crystals otherwise. Calcium reduces irregularities and aggregation. The degree of Ca substitution is lower than in the other systems studied. No effect of Ca on solubility or IR spectra was observed. The copper system yielded Cu-3(OH)(3)PO4 and CuNH4PO4 at 5 degrees C, but Cu-2 OHPO4 at 37 degrees C. Ca increases crystal size markedly. Ca substitution at 37 degrees C ranges from 14 to 55%. (C) 2000 Elsevier Science B.V. All rights reserved.