Kinetics and mechanism of natural fluorapatite dissolution at 25°C and pH from 3 to 12

Abstract

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.

Submarine groundwater discharge of rare earth elements to a tidally-mixed estuary in Southern Rhode Island

“…Moreover, apatite is a common accessory mineral in both the Esmond Igneous Suite and the Narragansett Pier Granite (Hermes et al, 1994), and is expected to be present as a trace mineral in the local glacial deposits. The solubility of apatite increases with decreasing pH, and becomes substantial at pH less than 7 (Chaïrat et al, 2007). Consequently, apatite in contact with groundwaters A, B, and D, all of which have pH b 7, is expected to be susceptible to dissolution reactions.…”

Dissolution rates of actinolite and chlorite from a whole-rock experimental study of metabasalt dissolution from 2 ≤ pH ≤ 12 at 25 °C

“…Chou and Wollast, 1984;Pokrovsky and Schott, 2000;Chaïrat et al, 2007). The reaction temperature was held constant by immersing the reactor in a thermostated water bath.…”

Natural phosphate as an efficient and green catalyst for synthesis of tetraketone and xanthene derivatives

“…The chemical composition was determined as: pH measurement of the catalyst A suspension of NP in deionized water was stirred at room temperature for 2 h and kept in closed vessel for 24 h without stirring. The supernatant pH at room temperature was measured to be 8.2-8.5 [30][31][32].…”