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The dissolution rates of natural fluorapatite (FAP), Ca 10 (PO 4 ) 6 F 2 , were measured at 25°C in mixed-flow reactors as a function of pH from 3.0 to 11.7, and aqueous calcium, phosphorus, and fluoride concentration. After an initial preferential Ca and/or F release, stoichiometric Ca, P, and F release was observed. Measured FAP dissolution rates decrease with increasing pH at 3 6 pH 6 7, FAP dissolution rates are pH independent at 7 6 pH 6 10, and FAP dissolution rates again decrease with increasing pH at pH P 10. Measured FAP dissolution rates are independent of aqueous Ca, P, and F concentration at pH % 3 and pH % 10.Apatite dissolution appears to be initiated by the relatively rapid removal from the near surface of F and the Ca located in the M1 sites, via proton for Ca exchange reactions. Dissolution rates are controlled by the destruction of this F and Ca depleted surface layer. The destruction of this layer is facilitated by the adsorption/penetration of protons into the surface at acidic conditions, and by surface hydration at neutral and basic conditions. Taking into account these two parallel mechanisms, measured fluorapatite forward dissolution rates can be accurately described using2þ a 1:4 F À a 0:6 OH À a 6 H þ Kex þ 3:69 Â 10 À8 ½BCaOH þ 2 0:6AE0:1where a i refers to the activity of the ith aqueous species, ½BCaOH þ 2 denotes the concentration of hydrated calcium sites at the surface of the leached layer (mol m À2 ), and K ex and K ads stand for the apparent stability constants of the Ca 2+ /H + exchange and adsorption/penetration reactions, respectively.

International audienceThe surface chemistry of fluorapatite in aqueous solution was investigated using electrokinetic techniques, potentiometric titrations, solubility measurements, and attenuated total reflection infrared spectroscopy. All methods indicate the formation of Ca/F depleted, P enriched altered layer via exchange reactions between H+ and Ca2+, and OH? and F? at the fluorapatite (FAP) surface. Observations suggest that this leached layer has a di-calcium phosphate (CaHPO4) composition and that it controls the apparent solubility of FAP. Electrokinetic measurements yield an iso-electric point value of 1 ± 0.5 consistent with a negatively charged FAP surface at pH > 1. In contrast, surface titrations give an apparent pH of point of zero charge of ?7.7, consistent with a positively charged surface at pH < 7.7. These differences are shown to stem from proton consumption by both proton exchange and dissolution reactions at the FAP surface. After taking account for these effects, FAP surface charge is shown to be negative to at least pH 4 by surface titration analysis

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