Preparation and Ion Exchange Property of Needlelike Hydroxyapatite controlled Ca/P Ratios

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Needlelike HAps were synhthesized by a wet method for the present study. Aqueous solutions of CaCl_2 (A) and K_2HPO_4 (B) with a stoichiometric Ca/P molar ratio (1.67) were slowly and simultaneously dropped, with stirring, into water heated to -100ﾟC.The precipitated products were filtered with suction and dried at -100ﾟC for 24 to -48 h. After drying, the obtained particles were sintered at 1200ﾟC for 1 h.Longer, needlelike particles of HAp were obtained at a slower dropping rate and in the range of the dilute A and B solutions (<0.3mol/dm^3). The needlelike particles were only nonstoichiometric or Ca-deficient HAps, with a Ca/P ratio of -1.54, as determined by inductively coupled plasma emission spectrometry (ICP). No stoichiometric, needlelike particles of HAp have yet reported.A preparation of sintered, needle-like HAp filters for an ion-exchange was examined by a doctor blade method in order to facilitate handling in the strongly acidic region, with the following results :(1) The best aqueous slurry for tape casting was the combination of HAp 20 mass%, binder 15 mass%, plasticizer 15 mass%, dispersant 1 mass% and water including specific amounts of Ca (OH) _2 or Ca (NO_3) _2.(2) A shear thinning behavior appeared at all aqueous slurries.(3) The tapes without cracks were obtained by sintering at 1200ﾟC for 1h after freeze-dried.(4) The removal of Pb^<2+> ions was investigated by passing through the Pb^<2+> solutions into the sintered, needle-like HAp filters at pH2, and was -59% at the first run and reached to -100% after repeated 5 runs with the same procedure as before.