Aqueous solubilities in the apatite solid-solution system Ca5[(P,V)O4]3(OH,F) with double co-substitution

To better understand the elevated vanadate concentrations in contaminated soils and aquifers, as well as the effects of this oxoanion on the human skeletal system, the solubility products of solid solutions between phosphate and vanadate phases in the apatite supergroup Ca 5 (PO 4 ,VO 4 ) 3 (OH,F) were investigated. Binary apatite solid solutions of varying compositions were synthesized hydrothermally and characterized by using X-ray diffraction and chemical analysis. The lattice parameters increase linearly with the degree of vanadate substitution. All binary subsystems follow Vegard's law, revealing no miscibility gaps in the solid-solution phases. The enthalpies of mixing were determined by acid solution calorimetry. The enthalpies were quite low, indicating nearly ideal solid solution behavior. Further theoretical interpretation of the data was carried out using linear equal-G curves on which the experimentally determined solubility product constants plot, indicating congruent stoichiometric saturation as metastable equilibrium. The results show that isomorphic substitution of phosphate by vanadate in hydroxylapatite or fluorapatite decreases vanadate solubility under short-term experimental conditions. However, over longer timescales allowing the system to approach thermodynamic equilibrium, Lippmann equations suggest that preferential release of vanadate may occur, potentially long after remediation activities, which may have important environmental implications for oxoanion immobilization in oxic groundwater environments. By analogous construction of a Lippmann diagram for the pyromorphite – vanadinite solid-solution aqueous-solution system based on previously published data it becomes clear that Pb-apatite is more suitable than Ca-apatite for scavenging and long-term disposal of vanadate pollutants.