Characterization, dissolution and solubility of synthetic svabite [Ca5(AsO4)3F] at 25?C45??C

Svabite is a secondary arsenate mineral, calcium fluoride arsenate Ca₅(AsO₄)₃F], in the apatite group of phosphates. Its dissolution and subsequent release of aqueous species play an important role in the cycling of arsenic and fluoride in the environment, but the thermodynamic and kinetic properties of svabite dissolution have never been investigated. In the present study, svabite was prepared by precipitation and characterized by various techniques, and then dissolution of synthetic svabite was studied at 25, 35 and 45°C in a series of batch experiments. In addition, the aqueous concentrations from the batch dissolution were used to calculate the solubility product and free energy of formation of svabite. The results of the X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy analyses indicated that the synthetic, microcrystalline svabite with apatite structure used in the experiments has not changed after dissolution. For the dissolution of svabite Ca₅(AsO₄)₃F] in ultrapure water, F⁻ ions were initially found to dissolve preferentially when compared with calcium and arsenate. Preferential dissolution of arsenate when compared with that of calcium was also observed. Dissolution of svabite in aqueous medium appeared to be always non-stoichiometric at the beginning, but when a dissolution equilibrium or steady state was reached at 25 and 35°C, the solid dissolved almost stoichiometrically. The release of calcium, arsenic and fluoride to solution increased with decreasing temperature. The mean K _sp value was calculated for Ca₅(AsO₄)₃F of 10^−39.21 (10^−39.18 ~ 10^−39.24) at 25°C; the free energy of formation ΔG _f ^o Ca₅(AsO₄)₃F] was −5210.46 kJ/mol.