THE DISSOLUTION OF APATITE IN THE PRESENCE OF AQUEOUS METAL-CATIONS AT PH 2-7

Apatite dissolution was studied at 25 degrees C in a series of batch e xperiments carried out within the pH range of 2-7 with or without the presence of aqueous Pb2+ or Cd2+. The synthetic, microcrystalline hydr oxylapatite used in the majority of the experiments was found to have a significantly higher solubility than natural fluorapatite, but a low er dissolution rate. The dissolution rates of both phases increased wi th decreasing pH. When Pb2+ was present in solution in contact with sy nthetic hydroxylapatite its concentration decreased over a time interv al ranging from several days to several weeks, to a steady state minim um. The rate of Pb2+ loss from solution was sensitive to acidity, and progressed faster at lower pH, but maximum loss was independent of pH. Calcium release to solution matched aqueous lead loss on a mole for m ole basis. By the end of each experiment mass calculations suggest tha t all apatite had been consumed regardless of reaction rate and pH. Th e solid residue was newly crystallised Pb-hydroxylapatite. This reacti on was also observed in situ using Atomic Force Microscopy (AFM) and w as found to take place epitaxially onto apatite surfaces. The concentr ation of aqueous Cd2+ in solution was also reduced in the presence of hydroxylapatite. Cadmium losses were, however, substantially lower. Un like Pb2+, the maximum amount of Cd2+ lost from solution was a functio n of pH, and was higher as solution composition approached neutral pH. Cadmium was present in the solid residue at the end of these experime nts, probably as a Ca-Cd phosphate solid solution. This work suggests that the interaction between apatite and metals in solution is control led by apatite dissolution and results in the precipitation of new met al phosphates. The new phosphates nucleate heterogeneously onto the hy droxylapatite surfaces, which acts as a catalyst for the reaction. (C) 1998 Elsevier Science B.V. All rights reserved.