Synthetic Hydroxyapatites as Inorganic Cation-Exchangers; Exchange Characteristics for Pb2+ and Sn2+ Ions in Acidic Solution

Abstract

Abstract The cation-exchange characteristics between Pb2+ ions of aqueous solutions containing counter-anions (F−, C1−) and Ca2+ ions of synthetic hydroxyapatite samples have been investigated in detail under the conditions of low pH values (3.0, 4.0 and 5.0) by a normal batch method. Even at the low pH value of 3.0 the apatite structure in a solution containing F− or C1− ions was maintained via a concurrent ion-exchange effect of Pb2+ ions together with F− or C1−ions, which are known to be exchangeable with OH- ions of the apatite. Moreover, it was found that Ca2+ ions in the apatite sample can easily be exchanged for Pb2+ ions almost without distinction between MI and M2 sites, assisted by the loosening effect of protons even at room temperature. Next, it was found that the hydroxyapatite samples are transformed into amorphous states by the reactions between Ca 2+ ions in the samples and Sn 2+ ions in the SnC12 acidic aqueous solutions with pH of 3.0 or below with a molar ratio of Sn2+/Ca2+ -1.0. The existence of hydroxyapatite as amorphism in acidic aqueous solutions such as SnC12 is quite interesting, because in general the hydroxyapatite has been found to be dissolved in acidic aqueous solutions. Moreover, the obtained amorphism are found to be stable up to at least 500 to 6OO°C but to be unstable in alkaline solutions. The characteristics of Sn2+ ions are found to have been found to form crystalline Pb 2+ apatite even in be quite different from those of homologous Pb2+ ions which have been found to form crystalline pb2+ apatite even in such an acidic atmosphere.