Solubility and Stability of Calcium Arsenates at 25∘C

Abstract

The solubility and stability of calcium arsenates at 25 • C was determined by both precipitation and dissolution experiments. Ca 3 (AsO 4 ) 2 ·3H 2 O(c), Ca 3 (AsO 4 ) 2 ·2 1 / 4 H 2 O(c), Ca 5 (AsO 4 ) 3 (OH)(c) and Ca 4 (OH) 2 (AsO 4 ) 2 ·4H 2 O(c) were identified in our experiment over a wide range of pH and for Ca/As molar ratios between 1.25 and 4.0. The solids precipitated at pH = 3 ∼ 7 and Ca/As = 1.5 were phase-pure and well-crystallized Ca 3 (AsO 4 ) 2 ·xH 2 O(c) and had relatively larger grain size than those formed at pH > 7. Based on the analytical results and using the computer program PHREEQC, the solubility products for Ca 3 (AsO 4 ) 2 ·3H 2 O(c), Ca 3 (AsO 4 ) 2 ·2 1 / 4 H 2 O(c), Ca 5 (AsO 4 ) 3 (OH)(c) and Ca 4 (OH) 2 (AsO 4 ) 2 ·4H 2 O(c) were calculated as K sp of 10 −), respectively. Correspondingly, the free energies of forming ( G o f ) of these calcium arsenates were calculated to be −3787.87 kJ/mol, −3611.50 kJ/mol, −5096.47 kJ/mol and −4928.86 kJ/mol.

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

“…The mean K sp value of 10 À39.2 was calculated for Ca 5 (AsO 4 ) 3 (OH) from the results of the precipitation experiment or 10 À41.6 from the results of the dissolution experiment. [17] This value is ,2.08 log units lower than 10 À38.04 reported by Bothe and Brown, [27] but it is obviously higher than 10 À47. 25 .…”

“…[5] The thermodynamic solubility products for Ca 5 (AsO 4 ) 3 (OH) were 10 À45.20 at 358C, 10 À45.28 at 408C, 10 À45.34 at 458C and 10 À45.46 at 508C. [18] Therefore, the K sp values of 10 À57 for HAP [34] and 10 À41.68 for AsHAP [17] were used in the calculation using the program PHREEQC in the present study. The calculated saturation indices for Ca 5 (Fig.…”

“…It was constructed using thermodynamic solubility products for Ca 5 (PO 4 ) 3 (OH) of 10 À57 and Ca 5 (AsO 4 ) 3 (OH) of 10 À41.6 reported in the previous researches. [17,34] The diagrams contain the solidus (log SP AsPHA v. X HAP 1/3 or X HAP ) and the solutus (log SP AsPHA v. X PO 4 3À ,aq ) for the Ca 5 (P x As 1Àx O 4 ) 3 (OH) solid solution series, and the total solubility product curves at stoichiometric saturation for the nine different members of the arsenate-phosphate hydroxylapatite solid solution (SP SS v. X PO 4 3À ,aq ; x ¼ 0.12, 0.22, y, 0.92). In addition to the solutus and the solidus, we have plotted the saturation curves for pure endmembers HAP (x ¼ 1.00) and AsHAP (x ¼ 0.00).…”

“…[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.…”

Mobilization and speciation of arsenic from hydrothermally altered rock containing calcite and pyrite under anoxic conditions

“…PHREEQC (Parkhurst and Appelo, 1999) Bothe and Brown (1999) and Zhu et al (2006). In addition, PHREEQC was utilized to simulate an ideal system consisting only of pyrite and calcite.…”