Apatite dissolution

Apatite is used to remediate Pb contaminated soils because apatite dissolution releases phosphate, which combines with Pb to form highly insoluble Pb-phosphate minerals. Apatites follow linear (zero-order) dissolution kinetics (Manecki et al., 2000) with rates of Pb uptake by the apatites decreasing in the same order as the apparent dissolution rate... 

Welch, S. A., Taunton, A. E. Banfield, J. F. (2002). Effect of microorganisms and microbial metabolites on apatite dissolution. Geomicrobiology Journal, 19, 343-67. 

Blum et al. (2002) and Probst et al. (2000) have found, in areas similarly cation-depleted by acid deposition, that a considerable proportion of calcium released by weathering came from apatite dissolution. This apatite was utilized directly by ectomycorrhizal tree species (spruce and fir) bypassing the soil exchange complex. In the Blum et al. (2002) study, ectomycorrhizal fungi associated with the roots of conifers provided 95% of the calcium found in the foliage of the trees and 35% of the Ca leaving the mixed conifer hardwood watershed in stream water. 

Guidry M. W. and Mackenzie F. T. (2003) Igneous and sedimentary apatite dissolution and the long-term phosphorus cycle. Geochim. Cosmochim. Acta 67(16), 2949-2963. 

Voegel JC, Frank RM The influence of salivary glycoproteins and fluoride on synthetic apatite dissolution, in Leach SA (ed) Dental Plaque and Surface Interactions in the Oral Cavity. Ixmdon, IRL Press, 1980, pp 301-311. 

In contrast to apatite, the solubilities of monazite and xenotime are much more limited in peraluminous melts compared with apatite solubility, with maximum solubilities of <0.05 wt % P2O5 (Wolf and London 1995). Indeed, Wolf and London (1995) report precipitation of monazite at sites of apatite dissolution in melting experiments. REE solubilities, in contrast, are higher in mafic compositions relative to granitic compositions (Ryerson and Hess 1978, 1980), and monazite solubility is greater in peralkaline melts than in meta or peraluminous melts (Montel 1986, Ellison and Hess 1988). Rapp and Watson (1986) also report a strong temperature dependence of monazite solubility. 

Wolf MB, London D (1994) Apatite dissolution into peralnminous haplogranitic melts An experimental stndy of solubilities and mechanisms. Geochim Cosmochim Acta 58 4127-4145... 

Apatite dissolution phosphate rock and synthetic hydroxyapatite (HAP). The... 

Several workers have attempted to develop dissolution rate equations to model apatite dissolution (Olsen 1975, Smith et al. 1977, Christoffersen et al. 1978, Fox et al. 1978, Chien et al. 1980, Onken and Metheson 1982, Hull and Lerman 1985, Hull and Hull 1987, Chin and Nancollas 1991). Rate equations from these models include zero order, first order, parabolic diffusion, mixed order, and other forms. The most current model (Hull and Hull 1987) focuses on surface dissolution geometry, which the authors argue fit the experimental results better than previous dissolution models. These experiments and the dissolution rate equations derived from them are missing the experimental conditions that replicate the natural dissolution processes and agents in soils, as they do not include the range of apatite mineralogies likely to be naturally weathering in soils. 

Hull AB, Lerman A (1985) The kinetics of apatite dissolution Application to natural aqueous systems. Natl Mtg Am Chem Soc Div Environ Chem 25 421-424... 

Fig. 5. Treatment effects on percent apatite dissolution in large and fine-mesh bags at the Flakaledin (plots F 12BI F7A IF) and the Skogaby (plots S 24C, S 251 6, S 22 IF) sites. Mean Values followed by the same letter (a, b or c) are not significantly different at the 5% level.

Inorganic reactions in the soil interstitial waters also influence dissolved P concentrations. These reactions include the dissolution or precipitation of P-containing minerals or the adsorption and desorption of P onto and from mineral surfaces. As discussed above, the inorganic reactivity of phosphate is strongly dependent on pH. In alkaline systems, apatite solubility should limit groundwater phosphate whereas in acidic soils, aluminum phosphates should dominate. Adsorption of phosphate onto mineral surfaces, such as iron or aluminum oxyhydroxides and clays, is favored by low solution pH and may influence soil interstitial water concentrations. Phosphorus will be exchanged between organic materials, soil inter-... 

In addition to Au, a variety of Au-Sb, Au-Ag and Bi-Te alloys accompanied post-D2 arsenopyrite precipitation. Au and Bi-Te alloys are also locally concentrated in the biotite-rich margins of tonalite dykes that were contaminated by the host sediments. These alloys also appear to be concentrated within the HGA and are locally associated with disseminated scheelite and F-apatite. The latter also occur in late-stage veins that cross-cut weakly foliated granodiorite stocks that lie immediately beneath the HGA. A combination of BSE and CL imaging reveals that precipitation of these post-D2 sulfides and alloys occurred within a micro-porosity network that records dissolution-precipitation reactions... 

Fluoride ions considerably change the physico-chemical properties of apatites and particularly their dissolution properties. 

Low-temperature exchange reactions have been described forfluorhydroxyapatite solid solutions [115,130,131], They generally occur in aqueous media and in most instances involve a dissolution-reprecipitation mechanism. Such reactions may be used to partly or totally modify the surface composition of ceramics or coatings. In order to observe such reactions, the resulting apatites should be less soluble than the starting compounds in the solution conditions [132], This is the case, for example, with fluoride uptake by HA. 

In the presence of fluoride, calcium ions have been found to be more firmly anchored than in pure hydroxyapatite [67]. This enhances the overall resistance to dissolution. Thus, the presence of a thin stable film of fluorapatite on the surface of hydroxyapatite crystals has two effects, namely (i) resistance to diffusion and dissolution of the anion and (ii) firmer binding of calcium ions into the surface. Both of these make the resulting apatite structure more resistant to dissolution, regardless of the pH of the external medium, and they thereby increase the resistance of the mineral phase to the onset of caries. 

The use of high-concentration gels and varnishes has been practised clinically for many years by dentists and dental hygienists [180]. When originally formulated, they were designed to be used in application procedures based on the concept that fluoride becomes incorporated into the crystalline phase of the enamel and leads to the development of a more acid-resistant form of apatite. They were not expected to make any difference to the levels of fluoride in saliva, or to influence the demineralisation/dissolution phase of the behaviour of tooth mineral. 

In summary, the study of the basalt dykes shows that the REE of the primary basalt have been mobilized during basalt alteration, most probably by dissolution of primary apatite. This mobilization was strongest in the intermediate samples R 3958 and R 3960 (Fig. 1) and the mobilized REE were subsequently immobilized by precipitation of secondary apatite. 

The study of the basaltic dykes in evaporites demonstrates that dissolution and precipitation of phosphate minerals is a key process for the control of REE mobility and REE fractionation. In the present case, all REE found in secondary apatite in the basalt and in the salt are derived from the dissolution of primary magmatic apatite during basalt corrosion. This loss of REE from the basalt to the salt was not sufficient to lower significantly the REE concentrations of the basalt and it could only be detected by the analysis of the salt. The absolute quantity of REE transferred from the basalt into the salt, however, cannot be quantified because we have no three-dimensional control on the REE concentrations around the basalt apophy sis. 

The presence of the metal cation and an appropriate anion (OH-, Cl-, F-, etc.) in contact with the surface of the apatite is sufficient to cause partial/complete dissolution of the apatite and precipitation of the more stable pyromorphite. A typical set of stoichiometric dissolution and precipitation reactions would be (Lower et al. 1998b) ... 

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