Characterization, dissolution and solubility of synthetic svabite [Ca5(AsO4)3F] at 25–45°C

Abstract

Svabite is a secondary arsenate mineral, calcium fluoride arsenate [Ca 5 (AsO 4 ) 3 F], 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 [Ca 5 (AsO 4 ) 3 F] 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 Ca 5 (AsO 4 ) 3 F of 10 -39.21 (10 -39.18 * 10 -39.24 ) at 25°C; the free energy of formation DG f o [Ca 5 (AsO 4 ) 3 F] was -5210.46 kJ/mol.

Dissolution and solubility of the arsenate - phosphate hydroxylapatite solid solution [Ca5(PxAs1 - xO4)3(OH)] at 25°C

“…This result was consistent with the dissolution of the (Ba,Sr)SO 4 precipitates [33] and the Ca 5 (P x As 1Àx O 4 ) 3 F solid solution. [22] The calculated saturation indices for Ca 5 (PO 4 ) 3 (OH) show a distinctly different trend than those for Ca 5 (AsO 4 ) 3 (OH) (Fig. 7).…”

“…Therefore, from the point of view of the equilibrium thermodynamics, in solidification-stabilisation of As-containing hazardous wastes and As-contaminated soil using phosphates (apatite), large amount of phosphates (apatite) must be applied in the process to avoid the leaching of arsenic. [22] Although johnbaumite [Ca 5 (AsO 4 ) 3 OH] and other calcium arsenates have been considered for safe arsenic disposal, in treating industry and mineral processing wastes with lime, it has been assumed that these solids are extremely insoluble and stable. [11,14,23] The experimental results indicated that johnbaumite [Ca 5 (AsO 4 ) 3 OH] is stable under alkaline conditions, and in moderately soluble when compared with hydroxylapatite [Ca 5 (PO 4 ) 3 OH].…”

“…[15] Some synthesis experiments at ambient temperatures were conducted to evaluate the conditions of formation and solubilities of different calcium arsenates. [11,16,17] Hydroxylapatite (HAP) is one of the most stable forms of calcium phosphate and exhibits a low solubility (K sp ¼ 10 À53.02 -10 À53.51 ); however, the solubility of AsHAP [Ca 5 (AsO 4 ) 3 (OH)] is significantly greater than that of HAP, [11,[18][19][20][21][22] which suggested that phosphate substitution in the arsenate compound could significantly reduce their solubilities. [21][22][23][24] The thermodynamic properties of solid solutions have a vital influence on the transport and fate of toxic metals in the environment.…”

Dissolution and Solubility of the Erythrite/Annabergite Solid Solution [(CoxNi1-x)3(AsO4)2•8H2O] at 25 ºC

“…In the undistorted state, only the absorptions corresponding to ν3 and ν4 vibrations are observed. The two remaining fundamentals ν1 and ν2 become infrared active when the configuration of the AsO4 3ions is reduced to some lower symmetry 19 . The degenerate modes are split by distortion of the arsenate groups through lack of symmetry in the lattice sites.…”

“…These results suggest that the composition of these final equilibrated solutions may be controlled by a surface layer having a composition distinct from that of the erythrite/ annabergite solid solution. Because of the low solubility, they dissolution kinetics were controlled by chemical reactions occurring at the mineral surface 19,20 .…”