Fluoridated apatites substitution

Fluoride is the most unique halide chemically and is the most common halide in igneous rocks. The igneous minerals fluorspar (CaF2) and apatite (Cas(F,OH)(P04)3) are both insoluble in water. Fluoride can substitute for OH- to some extent in soil minerals. This mechanism is probably also responsible for F retention by aluminium and iron hydroxides in acid soils. Fluoride also associates strongly with H+. HF is a weak acid, pK = 3.45. 

There are multiple applications of fluoridated bioceramics, essentially as bone and tooth substitutes (Table 1), involving bulk ceramics, glasses, composite materials and coatings for medical devices and surface treatments. In some cases, fluoride ions can leach out of the material inducing a direct biological effect in a soluble form. However, considering the affinity of fluoride ions for apatite... 

The XRD pattern of FA is reported in Fig. 4. It is characteristic of the apatite structure. This pattern is very analogous to that of HA. Fluoride substitution for OH is related to a decrease in the a unit-cell dimension and a very slight increase in the c unit-cell parameter. 

Finally, other effects of fluoride addition to HA have been investigated. In particular, the electrical conductivity of HA was found to be modified with fluoride substitution, as an increase in conductivity was observed [82] for OH-F apatites where up to 50% of OH ions were replaced by F. Beyond this proportion, however, a sharp drop in conductivity was pointed out. It seems, however, difficult to... 

Control of the pH and temperature of the precipitating solution is important to provide optimised conditions for stoichiometric, homogeneous, fluorhydroxyapatite formation. Similar conditions and set-up can be used for the synthesis of fluoride-substituted apatite crystals with varying size, crystallinity and morphology depending on the preparation temperature [124] a purge of the synthesis system with nitrogen gas ensures the preparation of carbonate-free fluorhydroxyapatite at ambient temperature [125]. 

Nevertheless, fluoride does lead to a reduction in the solubility of hydroxyapatite in aqueous solution, even in the absence of trace levels of fluoride in solution, and hence can be seen to have an effect in the solid state as well [57], Apatites are complex and diverse materials which have the general formula Caio(P04)eX2 (X = F, Cl, OH) and they represent a crystallographic system, in which there can be considerable replacement of species. Thus, with little or no change in the dimensions of the crystal lattice, there can be exchanges of OH for F, Ca + for Sr +, and PO4 for CO and all of these are known to occur in biological systems. Natural hydroxyapatite, for example, is often partially carbonate substituted [58]. 

Apatite, a natural calcium fluoride phosphate, can adsorb low to moderate levels of dissolved metals from soils, groundwater, and waste streams. Metals naturally chemically bind to the apatite, forming extremely stable phosphate phases of metal-substituted apatite minerals. This natural process is used by UFA Ventures, Inc., and is called phosphate-induced metals stabilization (PIMS). The PIMS material can by used in a packed bed, mixed with the contaminated media, or used as a permeable barrier. The material may be left in place, disposed of, or reused. It requires no further treatment or stabilization. Research is currently being conducted on using apatite to remediate soil and groundwater contaminated with heavy metals, and the technology may also be applicable to radionuclides. The technology is not yet commercially available. 

The methods of synthesis of fluorapatite have been widely dis cussed (J ). It is for example possible to obtain fluorapatite by substituting the hydroxyl ion for the fluoride ion, either in a-queous solution at room temperature, or through a solid state reaction at 800°C. It can also be prepared by the action of 6-tricalcium phosphate on calcium fluoride at about 800°C. Its solubility and thermal stability have already been established. While much is known about fluorapatite, many questions still exist concerning the mechanism of their formation, their composition and the structure of some of them. Two of these problems are dealt with here. First, we discuss the formation mechanism of fluorapatite by a solid state reaction between calcium fluoride and apa-titic tricalcium phosphate. Then we present the preparation and the structure of a carbonated apatite rich in fluoride ions. 

LEHR and Me CLELLAN (3) demonstrated numerous natural apatites and a correlation between the amount of fluoride ions and that of the carbonate ions. This led them to propose that PO -ions can be replaced by COg - ions associated with F ions. Such a hypothesis could explain the abnormally high amount of fluoride in some FRANC0LITES. However this type of substitution was not proved by the authors. We studied some synthetic apatites where fluoride and carbonate ions were simultaneously introduced. Samples of B-type carbonated fluorapatite (COj - substituting PO -) were obtained as a powder from an aqueous medium rich in fluoride ions and also an aqueous medium poor in fluoride ions. 

The F content in recent bone or dentine apatite is normally less than 0.1 wt.%. For ancient specimen, F is known to diffuse during burial into bone material. Its enrichment is generally a part of many complex diagenetic changes of bone and tooth, which remains after their deposit. Fluorine can react with the bone and dentine mineral phase to form calcium fluoride compounds. It usually substitutes for hydroxyl ions in hydroxyapatite, leading to the less soluble fluorapatite compound (Ca10(PO4)6(F)2, FAP). 

The most common and widely distributed phosphate minerals are the apatite group, with the general formula Ca10(PO4)6(X)2. The apatite is designated as fluorapatite, hydroxyapatite, or chlorapatite, when X = F, OH, or Cl, respectively. The most abundant sedimentary apatite is carbonate fluorapatite (ffancolite). Relative to pure fluorapatite, francolite is characterized by the substitution of Na and Mg for Ca and of carbonate and fluoride for phosphate. An empirical formula for francolite... 

The value of i.r. spectroscopy in studying substituted apatites is stressed by Trombe and Montel,525 and the technique has been used to study calcium hydroxylapatite and its isotopically substituted species526 and the fluoride Ca5(P04)3 F.527528... 

The hydroxyapatite crystals in bone and teeth are imperfect due to other anions and cations, especially magnesium, chloride, carbonate, and fluoride ions. Carbonate (C032-) is the most important. At low carbonate contents (<4% by weight), a carbonate ion replaces a phosphate ion in the crystal ( A site substitution), but at higher contents (>4% by weight) it replaces a hydroxide ion ( B site substitution). Either substitution slightly shortens and fattens the crystal ( c or a axes increase) and increases solubility. In contrast, if hydroxide ions are present, they can be replaced by fluoride, which decreases apatite solubility (Sect. 16.2.1). Crystallographic analyses indicate that, in bone and dentin, phosphate is often replaced by carbonate, whereas in enamel it is more often replaced with chloride (Cl1-). Carbonated hydroxyapatite is critical for enamel development (see Sect. 9.5.3). 

Fluorine occurs exclusively as the fluoride anion, F , in soils, where it complexes strongly with metals such as and Fe ". It is found in structures of hydrous minerals, isomorphously substituting for structural OH . Thus, F can be found in micas, amphiboles, layer silicate clays, apatite (rock phosphate), and numerous other minerals. Because it is associated with clay structures, the natural concentrations of fluorine in fine-textured mineral soils and sedimentary rocks can be high. 

Zanetti D, Nassif N, Antonelli AR (2001) Surgical repair of bone defects of the ear eanal wall with flexible hydroxylapatite sheets A pilot study. OtolNeurotol 22 745-753 Zaremba CM, Morse DE, Maim S, Hansma PK, Stucky GD (1998) Aragonite-hydroxylapatite conversion in gastropod (abalone) naere. Chem Mater 10 3813-3824 Zerahn B, Kofoed H, Borgwardt A (2000) Increased bone mineral density adjacent to hydroxy-apatite-eoated ankle arthroplasty. Foot Ankle Inti 21 285-289 Zhang Y, Fu T, Xu K, An H (2001) Wet synthesis and eharacterization of fluoride-substituted hydroxylapatite. J Biomed Eng 18 173-176... 

Lambert SL, Kim DS (1994) Tank Waste Remediation System High-Level Waste Feed Processability Assessment Report. Westinghouse Hanford Company, C-SP-1143, UC-811 Lexa D (1997) Development of a substituted-fluorapatite waste form for the disposition of radioactive and toxic fluoride salt materials. Argoime National Laboratory Report ANL-NT-52, 21 p Lindberg ML, Ingram B (1964) Rare-earth silicate apatite from the Adirondack Mountains, New York. U S Geol Surv Prof Paper 501-B 64-65... 

The ideal Ca P ratio of hydroxyapatite is 10 6 and the calculated density is 3.219 g/cm. Substitution of OH with fluoride gives the apatite greater chemical stabihty due to the closer coordination of fluoride (symmetric shape) as compared to the hydroxyl (asymmetric, two atoms) by the nearest calcium. This is why fluoridation of drinking water helps in resisting caries of the teeth [Park and Lakes, 1992]. 

Elliott (1964) has stated that the frequency of the O—H band at 3570 cm in hydroxyapatite is lowered if fluorine partially substitutes for the hydroxyl ions. The shift depends on the amount of substitution, and is rather small a 50% substitution causes a lowering of only about 20cm However, he believed that the determination of the frequency of the O—H band to measure the amount of replacement of hydroxyl by fluoride ions in hydroxyapatite could be as sensitive as X-ray diffraction techniques and is not subject to the same limitation, namely, that the apatite must be well crystallized. 

Substitutions in the HA structure are possible. Substitutions for Ca, PO4, and OH groups result in changes in the lattice parameter as well as changes in some of the properties of the crystal, such as solubility. If the OH" groups in HA are replaced by F" the anions are closer to the neighboring Ca " ions. This substitution helps to further stabilize the structure and is proposed as one of the reasons that fluoridation helps reduce tooth decay as shown by the study of the incorporation of F into HA and its effect on solubility. Biological apatites, which are the mineral phases of bone, enamel, and dentin, are usually referred to as HA. Actually, they differ from pme HA in stoichiometry, composition, and crystallinity, as well as in other physical and mechanical properties, as shown in Table 35.7. Biological apatites are usually Ca deficient and are always carbonate substituted (COs) " for (P04). For... 

Dentine, p = 2.0-2.3 glan , like bone, contains about 72% apatite, 18% collagen some carbonate and small quantities of phospholipids, F, Na, Mg, and so on. The substitution of F for OH in dental apatite decreases the acid solubility and improves hardness and resistance to decay. For this reason, fluoride ions are sometimes added to toothpastes and drinking water supplies (Chapter 12.14) (Table 11.5). 

The ionic radii of the OH" lattice positions as located in the apatite structure, allow for substitution by fluoride ions (FAp Ca,o(P04) (OH)j. FJ (Lin et al. 1981). Incorporation of fluoride ions into the apatite structure results into stabilization of the lattice and correspondingly a decreased solubility (Little and Rowley 1961). Clinical studies on dental caries have revealed that fluoride inhibits acidic etching (Lin et al. 1981). FAp s resistance to acid etching is higher than all other CaP compounds (Budz et al. 1987). 

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