Thermodynamic properties of mansfieldite (AlAsO4·2H2O), angelellite (Fe4(AsO4)2O3) and kamarizaite (Fe3(AsO4)2(OH)3·3H2O)
Thermodynamic properties of mansfieldite (AlAsO4·2H2O), angelellite (Fe4(AsO4)2O3) and kamarizaite (Fe3(AsO4)2(OH)3·3H2O)
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DOI：
10.1180/mgm.2018.107
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发表时间：
2018-05
影响因子：
2.7
通讯作者：
J. Majzlan;U. Nielsen;E. Dachs;A. Benisek;P. Drahota;U. Kolitsch;J. Herrmann;Ralph M. Bolanz;M. Števko
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文献类型：
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作者：
J. Majzlan;U. Nielsen;E. Dachs;A. Benisek;P. Drahota;U. Kolitsch;J. Herrmann;Ralph M. Bolanz;M. Števko
各种金属的砷酸盐的热力学数据对于计算它们的溶解度和评价它们作为砷存储介质的潜力是必要的。如果一些不太常见的砷酸盐矿物已被证明比目前使用的砷处理方案（特别是臭葱石和含砷氧化铁）的溶解度低，则应进一步研究它们作为有前途的储存介质。此外，与砷矿物质相关的健康风险是其溶解度和生物利用度的函数，而不仅仅是它们的存在。为了所有这些目的，需要知道这些矿物的溶解度。在这项工作中，一套完整的热力学数据已被确定为mansfieldite，AlAsO 4·2 H2O; angelellite，Fe 4（AsO 4）2 O3;和kamarizaite，Fe 3（AsO 4）2（OH）3·3 H2O，使用高温氧化物熔体量热法，松弛量热法，溶解度测量的组合，并在可能和适当的情况下估计。评估了用于高温氧化物熔融量热法的As参比化合物的几种选择。臭葱石被选为最好的一种。计算得到的生成吉布斯自由能（均以kJ·mol-1计）为：菱锰矿-1733.4 ± 3.5，当归石-2319.2 ± 7.9，铁橄榄石-3056.8 ± 8.5。溶解反应的溶度积分别为：锰钠石-21.4 ± 0.5，当归石-43.4 ± 1.5，铁橄榄石-50.8 ± 1.6。可用的，但有限的，天然臭葱石-mansfieldite固溶体系列的化学数据暗示在一个兼容性差距，因此该系列的非理想性质。然而，由于需要更多的数据，因此没有推导出混合参数。钙镁矾的溶解度比臭葱石高几个数量级。另一方面，kamarizaite的溶解度与臭葱石相当，并且kamarizaite在pH-pε图中甚至具有小的稳定域。据预测，它是在酸性排水系统中的弱酸性条件下形成的，该系统不受快速中和和突然的强过饱和的影响。白英石的溶解度高，矿物明显地局限于不寻常的环境，如水。它的结晶可以通过其与更常见的赤铁矿的外延关系来增强。使用臭葱石-锰矾固溶体处理砷，无论固溶体是否理想，都是不实际的。两种端元（臭葱石和锰钡石）的溶度积的差异如此之大，以至于几乎任何系统都将驱动基本上纯的臭葱石沉淀，将铝留在水相中。
ABSTRACT Thermodynamic data for the arsenates of various metals are necessary to calculate their solubilities and to evaluate their potential as arsenic storage media. If some of the less common arsenate minerals have been shown to be less soluble than the currently used options for arsenic disposal (especially scorodite and arsenical iron oxides), they should be further investigated as promising storage media. Furthermore, the health risk associated with arsenic minerals is a function of their solubility and bioavailability, not merely their presence. For all these purposes, solubilities of such minerals need to be known. In this work, a complete set of thermodynamic data has been determined for mansfieldite, AlAsO4·2H2O; angelellite, Fe4(AsO4)2O3; and kamarizaite, Fe3(AsO4)2(OH)3·3H2O, using a combination of high-temperature oxide-melt calorimetry, relaxation calorimetry, solubility measurements, and estimates where possible and appropriate. Several choices for the reference compounds for As for the high-temperature oxide-melt calorimetry were assessed. Scorodite was selected as the best one. The calculated Gibbs free energy of formation (all data in kJ·mol–1) is –1733.4 ± 3.5 for mansfieldite, –2319.2 ± 7.9 for angelellite and –3056.8 ± 8.5 for kamarizaite. The solubility products for the dissolution reactions are –21.4 ± 0.5 for mansfieldite, –43.4 ± 1.5 for angelellite and –50.8 ± 1.6 for kamarizaite. Available, but limited, chemical data for the natural scorodite–mansfieldite solid-solution series hint at a miscibility gap; hence the non-ideal nature of the series. However, no mixing parameters were derived because more data are needed. The solubility of mansfieldite is several orders of magnitude higher than that of scorodite. The solubility of kamarizaite, on the other hand, is comparable to that of scorodite, and kamarizaite even has a small stability field in a pH-pε diagram. It is predicted to form under mildly acidic conditions in acid drainage systems that are not subject to rapid neutralization and sudden strong supersaturation. The solubility of angelellite is high, and the mineral is obviously restricted to unusual environments, such as fumaroles. Its crystallization may be enhanced via its epitaxial relationship with the much more common hematite. The use of the scorodite–mansfieldite solid solution for arsenic disposal, whether the solid solution is ideal or not, is not practical. The difference in solubility products of the two end-members (scorodite and mansfieldite) is so large that almost any system will drive the precipitation of essentially pure scorodite, leaving the aluminium in the aqueous phase.