Thermodynamic properties of scorodite and parascorodite (FeAsOb>4b>路2Hb>2b>O), ka艌kite (FeAsOb>4b>路3.5Hb>2b>O), and FeAsOb>4b>

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• 作者：Juraj Majzlan^a ; ^{Juraj.Majzlan@uni-jena.de} ; Petr Drahota^b ; Michal Filippi^c ; Klaus-Dieter Grevel^a ; Wolf-Achim Kahl^d ; Jakub Plá ; &scaron ; il^e ; Juliana Boerio-Goates^f ; Brian F. Woodfield^f
• 关键词：Scorodite ; Ferric arsenates ; Formation enthalpy ; Entropy ; Solubility
• 刊名：Hydrometallurgy
• 出版年：2012
• 期刊代码：38_0304386x
• 类别：ce
• 出版时间：April, 2012
• 卷：117-118
• 期：Complete
• 页码：47-56
• 文件大小：722 K

摘要

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 (FeAsOb>4b>路2Hb>2b>O) 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 ka艌kite (FeAsOb>4b>路3.5Hb>2b>O), and the anhydrous FeAsOb>4b> 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, ka艌kite, and FeAsOb>4b> are 鈭?#xA0;1508.9 卤 2.9, 鈭?#xA0;1506.6 卤 2.9, 鈭?#xA0;1940.2 卤 2.8, and 鈭?#xA0;899.0 卤 3.0 kJ路mol^鈭?#xA0;1, respectively. The measured standard entropies for scorodite and ka艌kite are 188.0 卤 2.1 and 247.6 卤 2.8 J路mol^鈭?#xA0;1路K^鈭?#xA0;1, respectively; entropies of FeAsOb>4b> and parascorodite were estimated. The resulting Gibbs free energies of formation for scorodite, parascorodite, ka艌kite,\ and FeAsOb>4b> are 鈭?#xA0;1284.8 卤 2.9, 鈭?#xA0;1282.5, 鈭?#xA0;1629.6 卤 2.9, and 鈭?#xA0;786.7 kJ路mol^鈭?#xA0;1, respectively. The solubility product for scorodite of 鈭?#xA0;25.83 卤 0.52 is in an excellent agreement with a previously selected best value of 鈭?#xA0;25.83 卤 0.07 from . As expected, scorodite is stable under a wide range of conditions applicable to terrestrial surface environments. The anhydrous FeAsOb>4b>, parascorodite, and ka艌kite 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.