Thermodynamic properties of scorodite and parascorodite (FeAsO4·2H2O), kaňkite (FeAsO4·3.5H2O), and FeAsO4

Iron arsenates, either well or poorly crystalline, are the usual phases of choice for arsenic immobilization in waste forms of variable origin. Among these phases, scorodite (FeAsO4·2H2O) is used very often because of its favorable properties. The thermodynamic properties of this phase, necessary for the prediction of its dissolution or precipitation, have been usually constrained by solubility experiments. Here, we measured the thermodynamic properties of scorodite, its polymorph parascorodite, the mineral kaňkite (FeAsO4·3.5H2O), and the anhydrous FeAsO4 by a combination of calorimetric techniques, thus avoiding the inherent uncertainties of the solubility experiments. The enthalpies of formation from elements at standard temperature and pressure for scorodite, parascorodite, kaňkite, and FeAsO4 are − 1508.9 ± 2.9, − 1506.6 ± 2.9, − 1940.2 ± 2.8, and − 899.0 ± 3.0 kJ·mol− 1, respectively. The measured standard entropies for scorodite and kaňkite are 188.0 ± 2.1 and 247.6 ± 2.8 J·mol− 1·K− 1, respectively; entropies of FeAsO4 and parascorodite were estimated. The resulting Gibbs free energies of formation for scorodite, parascorodite, kaňkite,\ and FeAsO4 are − 1284.8 ± 2.9, − 1282.5, − 1629.6 ± 2.9, and − 786.7 kJ·mol− 1, respectively. The solubility product for scorodite of − 25.83 ± 0.52 is in an excellent agreement with a previously selected best value of − 25.83 ± 0.07 from Langmuir et al. (2006). As expected, scorodite is stable under a wide range of conditions applicable to terrestrial surface environments. The anhydrous FeAsO4, parascorodite, and kaňkite are either metastable or stable under restricted conditions which are unlikely for the terrestrial surface environments. Using the thermodynamic data for scorodite and for a suite of ferric oxides, we can predict the aqueous As concentrations in systems in which scorodite dissolves and ferric oxides precipitate. These models show that the As concentration can vary widely as function of the nature, chemical composition, and crystallinity of these ferric oxides.