Metals, apatite-forming

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. 

In systems where all components are initially soluble, it is simple to form pyromorphites (Nriagu 1974, 1984 Ma 1996) or ternary metal apatites where Pb21, Cd2+, Cu2+, and Zn2+ iso-morphically substitute for Ca2+ and form solid... 

Kijima T and Tsutsumi M (1979) Preparation and thermal properties of dense poly crystalline oxy hydroxy lapatite. J Am Ceram Soc 62 455-460 Kim HM (2001) Bioactive ceramics challenges and perspectives. J Ceram Soc Japan 109 S49-S57 Kim HM, Miyaji F, Kokubo T, Nakamura T (1997) Apatite-forming ability of alkali-treated Ti metal in body enviromnent. J Ceram Soc Japan 105 111-116... 

Calcium phosphate ceramics are ceramics with varying calcium-to-phosphate ratios. Among them, the apatite ceramics, defined by the chemical formula M,o(X04)6Z2, have been studied most. The apatites form a range of solid solutions as a result of ionic substitution at the XO , or Z sites. In general, apatites are nonstoichiometric and contain less than 10 mol of ions, less than 2 mol of Z" ions, and exactly 6 mol of XOJ" ions (Van Raemdonck et al., 1984). The species is typically a bivalent metallic cation, such as Ca, Sr +, Ba ", Pb ", or Cd2+. The XO species is typically one of the following trivalent anions AsO , VO, CrO , or MnO. The monovalent 7r ions are usually F", OH , Br , or Cj (Van Raemdonck et al., 1984). 

FIGURE 14.1.17 Scanning electron micrograph and energy-dispersive X-ray microanalysis (a) and profile of Auger electron spectroscopy (b) of the cross-section of apatite formed on the NaOH- and heat-treated titanium metal in SBF. 

Tantalum metal also forms an amorphous sodium tantalate layer with a graded structure [45] on treatment with a 0.5 M NaOH solution at 60°C for 24h [46] and heat treatment at 300°C for 1 h [47]. The thus-treated tantalum metal also forms a bone-like apatite layer on its surface in an SBF as well as in the living body, and bonds to living bone [48]. 

Miyazaki, T., Kim, H.-M., Kokubo, T., Miyaji, F., Kato H., and Nakamura, T. (2001). Effect of thermal treatment on apatite-forming ability of NaOH-treated tantalum metal. J Mater. Sci Mater. Med. 12 683-687. 

Kim, HM Miyaji, F Kokubo, T Nakamura, T. Apatite-forming ability of alkali-treated Ti metal in body environment. J. Ceram. Soc. Japan, 1997,105,111-116. 

Calcium is a metallic element, fifth in abundance in the earth s crust, of which if forms more than 3%. It is an essential constituent of leaves, bones, teeth, and shells. Never found in nature uncombined, it occurs abundantly as limestone, gypsum, and fluorite. Apatite is the fluorophosphate or chlorophosphate of calcium. 

Calcium Phosphates. The alkaline-earth phosphates are generally much less soluble than those of the alkaH metals. Calcium phosphates include the most abundant natural form of phosphoms, ie, apatites, Ca2Q(P0 3X2, where X = OH, F, Cl, etc. Apatite ores are the predominant basic raw material for the production of phosphoms and its derivatives. Calcium phosphates are the main component of bones and teeth. After sodium phosphates, the calcium salts are the next largest volume technical- and food-grade phosphates. Many commercial appHcations of the calcium phosphates depend on thek low solubiHties. 

Based on che above reaction, the calcium overbased sulfonate detergent reacts with DAP to form a metallic hydroxyl apatite (Ca5(P04>30H), which is insoluble in aqueous and oil phase. This reaction mechanism is general for Ca, Ba, Mg, and Zn additives. Lead is present in a physically different form chan additive compounds in the used oil. The lead particles present in used oil are too large in size to react with DAP, except for surface reactions. 

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... 

The most common apatite is Ca5(P04)30H and is called hydroxyapatite. Other forms include chloroapatite (Ca5(P04)3Cl), fluoroapatite (Ca5(P04)3F) and carbonate apatite or dahllite (Ca5(P04)3C03). These minerals are in pure forms, but it is also possible to generate them by partial replacement of one anion by another or one cation by another. For example, Ca may be replaced by Pb by ionic substitution, yielding pyromorphites [Pb5(P04)3(0H,Cl,F)]. As we shall see in Chapter 16, this mineral is very important in stabilizing the hazardous metal Pb. Also as discussed in Chapter 2 and shall be seen in later chapters, Mg-based CBPCs have many applications, and hence minerals such as Mg5(P04)3(0H,Cl,F) are also very common. 

Fluorine occurs in nature in minerals such as fluorite, Cap2 fluor-apatite, Ca.(PO )3F, which is a constituent of bones and teeth and cryolite, Na3AlF<. and in small quantities in sea water. Its name fluorine, from Latin finere, to flow, refers to the use of fluorite as a flux (a material which forms a melt with metal oxides). 

Uranium(VI) readily precipitates in the presence of phosphate to form a number of sparingly soluble U-phosphate phases (U phases, such as saleeite, meta-autunite, and autunite) and also is removed by sorption and co-precipitation in apatite. Several studies have shown that hydroxyapatite is extremely effective at removing heavy metals, uranium, and other radionuclides from solution (Gauglitz et al., 1992 Arey and Seaman, 1999). 

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). 

Metal ions in biological systems tend to be distributed between four different states in vivo. First, there is the inert form of the metal complex whereby it is laid down into a solid matrix, just like the calcium in the apatite of bones and teeth (Scheme 1). Secondly, there are the three states of metal-protein having reversibly bound metal ions, known as high molar mass species, that are in turn in equilibrium with low molecular mass complexes and, for purely thermodynamic reasons, there must be the occasional aquated metal ion, although this is highly unlikely with a pH of 7.4 because of the threat of hydrolysis. 

Elemental phosphorus is not found free in nature, except in some meteorites. Phosphorus comprises 0.12% of the Earth s crust in the form of minerals (apatite, wave-lite, viviamte), and is always present in the form of a phosphate. Phosphoric acid combines with cations to form orthophosphate salts. If not all the hydrogen contained in phosphoric acid is exchanged with a metal. 

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