Colloids and Surfaces A: Physicochemical and Engineering Aspects
Reaction of calcium hydroxyapatite with Cd2+ and Pb2+ ions
Introduction
Calcium hydroxyapatite (noted as CaHap) with a formula of Ca10(PO4)6(OH)2 is a primary constituent of biological hard tissues in animal organisms. Synthetic CaHap is nowadays widely used as artificial bones and tooth roots, adsorbents for chromatography to separate protein, catalysts for dehydration, dehydrogenation of alcohols, powders for tooth paste and so on [1]. The apatite (Hap) structure is so tolerant to ionic substitutions that Ca2+ in the crystals can be replaced by various divalent cations including Mg2+, Cd2+, Sr2+, Pb2+ and Ba2+ [1], [2], [3], [4]. We have synthesized and characterized CaHap [3] and various Hap solid solutions such as hydroxyapatites of Mg–Ca [4], Cd–Ca [5], Sr–Ca [6], Pb–Ca [7] and Ba–Ca [8] by different wet methods. To clarify the formation mechanism of these solid solutions, the ion-exchange of Mg2+ in MgCaHap particles with Ca2+ has been investigated and it was revealed that the ion-exchange takes place by surface dissolution of the particles and recrystallization of a new layer on the particles [9]. The prominent ion-exchangeability of Hap is available for removal of noxious metal ions from industrial liquid wastes and concerns to the accumulation of metal ions in hard tissues. Thus, the exchange of Ca2+ ions in CaHap crystals with harmful ions such as Cd2+and Pb2+ is an important subject in medical and environmental sciences. There have been many investigations on the incorporation of Pb2+ [10], [11], [12] and Cd2+ [13], [14], [15], [16], [17] in CaHap. Several sorption mechanisms were proposed as follows; exchange with Ca2+, dissolution and recrystallization, and formation of new phases, that depend on the reaction conditions such as concentration of cations, solution pH, temperature, nonstoichiometry of CaHap, and so on. Unfortunately, in these studies the sorption of Cd2+ and Pb2+ was examined separately under different conditions. To compare the sorption mechanism of Cd2+ and Pb2+, the experiments should be done under the same condition. In the present study we investigated the reaction of CaHap particles with Cd2+ and Pb2+ in the same experimental condition in order to make clear the difference between the sorption mechanism of these harmful metal ions. The influence of nonstoichiometry of CaHap on the ion-exchange was also discussed.
Section snippets
Materials
CaHap particles with different Ca/P atomic ratios were synthesized by the following precipitation method [3]. Twenty cubic decimeters of solution containing 0.4 mol Ca(OH)2 were prepared using deionized and distilled water free from CO2 under an N2 atmosphere and stirred for 1 h. To the Ca(OH)2 solutions were added the solutions containing 0.240–0.269 mol H3PO4. The resulting suspensions were stirred at room temperature for 1 h and then aged in a 20 dm3 screw-capped Teflon vessel at 100 °C for 48 h
Crystal structure
Fig. 1 shows XRD patterns of CaHap particles (D) with Ca/P atomic ratio of 1.64 before and after the reaction with Cd2+ or Pb2+. The material before the reaction is characteristic to CaHap (JCPDS 9-432) and the crystallinity is not influenced by the reaction with Cd2+. The unit cell dimensions a and c obtained from the XRD pattern of the CaHap particles before the reaction were 0.944 and 0.687 nm, respectively. The a and c dimensions after the reaction were 0.945 and 0.686 nm, respectively, and
Conclusions
The reaction of CaHap particles with Cd2+ in the solution accompanied no change of crystal structure and particle morphology. The reaction mechanism is dissolution of the CaHap surface and recrystallization of a new CdCaHap layer on the particles. The amount of Cd2+ incorporated per unit surface area and the atomic (Ca + Cd)/P ratio of the particles after the reaction were independent of the nonstochiometry of CaHap. The cation/P ratio of the particle surface was lower than that of the whole
Acknowledgements
The authors thank Mr. Masao Fukusumi of Osaka Municipal Technical Research Institute for help with the TEM observation and Dr. Takenori Nakayama of Kobe Steel Ltd. with the XPS measurement. This study was supported in part by the Grant-in-Aid for Science Research Funds (B) and (C) from the Ministry of Education, Science, Sports and Culture, Japanese Government.
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