Hydroxide apatite

In the geochemistry of fluorine, the close match in the ionic radii of fluoride (0.136 nm), hydroxide (0.140 nm), and oxide ion (0.140 nm) allows a sequential replacement of oxygen by fluorine in a wide variety of minerals. This accounts for the wide dissemination of the element in nature. The ready formation of volatile silicon tetrafluoride, the pyrohydrolysis of fluorides to hydrogen fluoride, and the low solubility of calcium fluoride and of calcium fluorophosphates, have provided a geochemical cycle in which fluorine may be stripped from solution by limestone and by apatite to form the deposits of fluorspar and of phosphate rock (fluoroapatite [1306-01 -0]) approximately CaF2 3Ca2(P0 2 which ate the world s main resources of fluorine (1). 

Other types of cements include anhydrite, barite, pyrite, iron hydroxides haematite, albite and apatite and are subordinate. 

The solvent extraction of rare-earth nitrates into solutions of TBP has been used commercially for the production of high-purity oxides of yttrium, lanthanum, praseodymium and neodymium from various mineral concentrates,39 as well as for the recovery of mixed rare-earth oxides as a byproduct in the manufacture of phosphoric acid from apatite ores.272 273 In both instances, extraction is carried out from concentrated nitrate solutions, and the loaded organic phases are stripped with water. The rare-earth metals are precipitated from the strip liquors in the form of hydroxides or oxalates, both of which can be calcined to the oxides. Since the distribution coefficients (D) for adjacent rare earths are closely similar, mixer—settler assemblies with 50 or more stages operated under conditions of total reflux are necessary to yield products of adequate purity.39... 

Pyrite formation (FeS2) is related to biologically mediated reduction of sulfate to sulfide and of Fe3+ to Fe2+ in anoxic zones. In the case of phosphate, this can be removed through precipitation reactions with Ca2+ and Fe3+ (by formation of apatite or iron phosphate, respectively), by co-precipitation, or by formation of surface complexes with Fe or Mn oxides or hydroxides. 

Arsenic in soilds has been fractionated by Jackson s T28) procedure for soil phosphorus (15. 27). In this laboratory, a modification of Jackson s procedure is being used on sediment solids. A series of 1 molar solutions of NH4CI, NH4OH, acid ammonium oxalate (29) and HCl are used in sequence. The chloride fraction, or exchangeable fraction, contains weakly adsorbed, coulombically bound arsenic. The hydroxide fraction, contains iron and aluminum arsenate precipitates and surface precipitates (i.e. adsorbed arsenic species having both chemical and coulombic bonding to oxide surfaces). The oxalate, or reductant soluble fraction, contains arsenic occluded in amorphous weathering products. The acid, or calcium, fraction contains arseno-apatites. 

Manganese nodules result from the precipitation of manganese and iron hydroxides from sea-water or pore-water around an offered nucleus. Nuclei available in the deep sea are pieces of volcanic material (balsalt, pumice) (Fig. 6) or apatitic fossils (shark teeth (Fig. 7), auditory canals of whales). 

Calcium hydroxy apatite contains the phosphate and hydroxyl groups. Using calcinm hydroxide as the precipitant, the chemical reaction is ... 

Tables 14.4 and 14.5 reveal very important information. The efficiency of removal of phosphorus increases as the concentration of calcium increases from 0 to 130 mg/L. Remember that we disallowed the use of the of Ca(OH)2 because it was too large, and we concluded that the hydroxide would not be precipitating alongside with the apatite. It is not, however, preventable to add more lime in order to increase the concentration of the calcium ion to the point of saturation and, thus, be able to use /fjp,ca(oe)2 calculations. In theory, before the hydroxide precipitation can happen, all the apatite particles would have already precipitated, resulting, indeed, in a very high efficiency of removal of phosphorus. Would this really happen The answer would be yes, but this could be a good topic for applied research.

Sinter phosphates Rhenania phosphates with a P2O5 content of ca. 29% are obtained by sintering apatite, silica and sodium carbonate or sodium hydroxide. Their annual production in the Federal Republic of Germany is over 300 10 t (corresponding to ca. 90 10- t/a P2O5). 

Bone ash, and synthetic apatite, which are essentially calcium phosphate (Ca3(P04)2), or a synthetic apatite, calcium hydroxide mixture, are also effective methods for fluoride removal because of their affinity for these phosphate salts. 

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). 

This chapter describes how individuals with severe enamel fluorosis (mottled tooth enamel) became associated with fluoride in the public water supply and protection from dental caries. A comparison of caries experience with the fluoride content of public water supplies and enamel fluorosis in adolescents indicated that 1 pg fluoride/mL (1 part/million) in the water provides caries protection with minimal enamel fluorosis (sect. 1). One mechanism is the spontaneous isomorphic replacement of apatite s hydroxide anions with fluoride, which reduces enamel solubility. A second is fluoride-mediated inhibition of enolase, which retards bacterial acid production at teeth surfaces. These findings led to the use of fluoride in toothpastes, which provides better protection from caries at tooth surfaces than water fluoridation alone (sect. 2). The chapter concludes with a discussion of potentially harmful effects of fluoride ingestion (sect. 3). 

Fig. 16.6 Hydroxyapatite and fluoroapatite formation and dissolution, (a) Hydroxyapatite is transformed to fluoroapatite by isomorphous replacement. Fluoride ions diffuse into a hydroxyapatite crystal where they replace the hydroxide ions, (b) Fluoroapatite cannot dissolve as easily as hydroxyapatite. Right to left shows the solid-state rearrangement of hydroxyapatite to calcium monohydrogen phosphate, free calcium ions, and monohydrogen phosphate. The latter becomes mostly dihydrogen phosphate above pH 6.2. Arrows between (b) and (a) indicate enhanced apatite formation or slower changes to the amorphous solid if fluoride is present. Left to right shows the precipitation of calcium monohydrogen phosphate and its change to hydroxyapatite if an acid solution is made alkaline.

The first and primary protective effect of fluoride is due to its strong, spontaneous reaction with metal ions. Biologically, the most important of these ions is the calcium ion, large amounts of which interact with phosphate to form bones and teeth. Studies show that fluoride reduces apatite solubility in acids by an isomorphic replacement of hydroxide ions with fluoride ions to form fluoro-hydroxyapatite and difluoro-apatite (Fig. 16.6a). 

Apatites must undergo a solid-state transition to amorphous calcium phosphate before they can dissolve and the spontaneous replacement of hydroxide with fluoride ions slows the rate at which this transition occurs (Fig. 16.6b). Conversely, as an acid environment becomes more alkaline, fluoride ions promote the precipitation and crystallization of amorphous calcium monohydrogen phosphate/calcium fluoride into fluoro- and difluoro-apatites faster than amorphous calcium phosphate would crystallize into hydroxyapatite. Thus, fluoride ions have two effects on enamel that protect from caries they slow enamel dissolution in lactic acid and promote its re-precipitation and crystallization when the lactic acid is neutralized. 

Section 1 considers the methods of synthesis and physico-chemical properties of new types of inorganic sorbents (complex carbon-mineral sorbents, co-precipitated hydroxides, functional polysiloxane sorbents, porous glasses with controlled porosity, colloidal silicas, aluminium oxyhydroxide colloids, apatites). These sorbents are widely used in scientific investigations, in chemical practice and are important from a technological point of view. The presented results provide additional possibilities for the preparation of inorganic sorbents possessing unique adsorption and catalytic properties. Moreover, Section 1 presents the possibilities of the computational studies on the design of synthetic materials for selective adsorption of different substances. 

Left ESEEM spectrum of a solution containing triphosphate and vanadyl ions in the ratio 3 1, pH = 5, c(VO +) = 0.7mM, recorded at the nij = — V2 EPR transition and v- are the Zeeman frequencies for and H, respectively. The doublets at 5.3 and 6.7 MHz (superhyperfine coupling constants 1-1.5 MHz) are indicative of direct bonding of phosphate to VO, as represented by the proposed structure (inset).The presence of water/hydroxide in the coordination sphere of vanadium is inferred from the respective HYSCORE spectrum. Reproduced from S. A. Dikanov et at, J. Am. Chem. Soc. 124, 2969-2978. Copyright (2002), with permission from the American Chemical Society. Right section of the structure of hydroxyapatite, with two phosphorus sites arbitrarily replaced by vanadium (full circles). The drawing of the apatite structure was provided by Barbara Albert, Technical University of Darmstadt, Germany. 

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. 

Osaka A, Mima Y, Takeuchi K, Asada M, Takahashi K (1991) Calcium apatite prepared from calcium hydroxide and orthophosphoric acid. J Mater Sci Mater in Med 2 51-55 Osaka A, Tsura K, lida H, Ohtsnki C, Hayakawa S, Miura Y (1997) Spray pyrolysis preparation of apatite-composite particles for biological application. J Sol-Gel Sci Technol 8 655-61 Otsuka M, Matsuda Y, Suwa Y, Fox JL, Higuchi W1 (1995) Effect of particle size of metastable calcium phosphates on mechanical strength of a novel self-setting bioactive calcium phosphate cement. J Biomed Mater Res 29 25-32... 

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