Objective: To determine whether demineralisation rates in vitro of compacted apatite pellets depend on their initial porosity.
Methods: Pellets (8.8 mm diameter) were pressed uniaxially (at 200, 300, 400, 500, 600, 700, 800 MPa) from a single batch of well-characterised carbonate-apatite powder (prepared by hydrolysis of CaHPO4 in aqueous ammonium carbonate at 70°C, then dried at 100°C). Leaving an exposed flat surface unvarnished, each pellet was mounted on a polymethylmethacrylate disk, then in a scanning microradiography cell. At 4 points/pellet, mineral mass/unit area was measured during demineralisation by pumped (5 ml/min) acetic acid buffer (0.1 mol/l, pH 4.5) for 200 h. X-ray microtomographic (XMT) images were recorded before and after demineralisation. A range of porosities was examined in each of three experiments, using 12 pellets in total.
Results: Porosity decreased as compaction pressure increased (e.g. 41% at 200 MPa; 28% at 700 MPa). Variation among pellets in rate of mineral loss and depth of partly-demineralised near-surface material did not correspond to their porosities. In each experiment, ranking of pellets by rate of mineral loss persisted over 200 h, though the rates decreased so that they became similar for all pellets. XMT showed that the pattern of demineralisation varied among pellets, a few suggesting patchy subsurface demineralisation, but most erosion.
Conclusion: Rate of demineralisation did not depend on the initial porosity of compacted apatite pellets under these experimental conditions. Variation among pellets in pattern of demineralisation suggests that the system is sensitive to very small differences in local conditions. These might pertain to the pellet or the solution conditions. The persistence of ranking of rates of mineral loss would also be consistent with a long-term influence of differences between pellets in early local patterns of demineralisation.
We thank St Bartholomew's and The Royal London Hospital Charitable Foundation (Grant No. RAB 03/PJ/18).