Alternatives for arsenic storage in waste forms - Thermodynamics of selected Fe,Al arsenates

砷是一种环境问题的非金属物质，对所有生物都有毒性。其中很大一部分通常通过矿石的开采和选矿进入环境。在这里，我建议研究铁、铝砷酸盐的热力学性质，它们似乎是在受污染的地点储存砷的有希望的替代品。药铁矿(K，Na，Ba0.5)Fe4(AsO4)3(OH)4·5H2O是一种常见于成熟、自然污染土壤中的矿物。与目前固定砷的最佳选择--蝎子石FeAsO4·2H2O相比，药用菱铁矿在中等酸性到近中性条件下也表现出稳定性。原砷酸铁FeAsO4·0.75H2O为高温(~200℃)相，形成于毒砂-黄铁矿的高压氧化过程中。因此，它以这类技术产生的废物形式存在，尽管数量很少。应该研究这一相的溶解程度；如果它的表现比其他含砷相更好，则可以调整技术，以有利于这一相的生产。蝎子石-芒硝石(AlAsO4·2H2O)固溶体由于Fe-Al混合降低了固溶体成员的吉布斯自由能(从而降低了溶解度)，因此可能是另一种有前途的候选化合物。拟议的工作将通过对合成样品进行量热技术的组合来完成，通过粉末X射线衍射、扫描电子显微镜和电子探针进行广泛的表征。对于芒硝石样品，其纯度将另外通过核磁共振(核磁共振)光谱进行检查。项目最后将计算一系列常温和高温(最高300摄氏度)条件下的相图，以显示和比较铁铝砷酸盐和硫代砷酸盐的溶解度和稳定性。

Arsenic is a metalloid of environmental concern, toxic to all living organisms. A substantial portion thereof enters the environment commonly by mining and beneficiation of the ores. Here, I propose to investigate the thermodynamic properties of Fe,Al arsenates which seem to be promising alternatives for the storage of arsenic at polluted sites. Pharmacosiderite, (K,Na,Ba0.5)Fe4(AsO4)3(OH)4·5H2O, was identified by our work and by work of others as a mineral commonly occurring in mature, naturally polluted soil profiles. In contrast to scorodite, FeAsO4·2H2O, currently the best choice for arsenic immobilization, pharmacosiderite appears to be stable also under mildly acidic to circumneutral conditions. The ferric orthoarsenate, FeAsO4·0.75H2O, is a high-temperature (~200 ºC) phase, forming in high-pressure oxidation of arsenopyrite-pyrite ores. It is therefore present, albeit in small amounts, in waste forms from this type of technology. It should be investigated how soluble this phase is; if it should perform better than the other arsenic-containing phases, the technologies could be adjusted to favor the production of this phase. The scorodite-mansfieldite (AlAsO4·2H2O) solid solution may be another promising candidate by the virtue of decreasing Gibbs free energy (hence, decreasing solubility) of the solid-solution members by Fe-Al mixing. The proposed work will be done by a combination of calorimetric techniques on synthetic samples, characterized extensively by powder X-ray diffraction, scanning electron microscopy, and electron microprobe. For the mansfieldite sample, its purity will be checked additionally by nuclear magnetic resonance (NMR) spectroscopy. The project will conclude by a calculation of a series of phase diagrams for ambient and high-temperature (up to 300 ºC) conditions, to display and compare the solubility and stability of Fe-Al arsenates and sulfoarsenates.