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

砷是一种环境类金属，对所有生物体都有毒性。其中相当大的一部分通常通过采矿和矿石提纯进入环境。在这里，我建议调查的热力学性质的铁，铝砷酸盐，这似乎是有前途的替代品砷的存储在污染的网站。毒菱铁矿，（K，Na，Ba0.5）Fe_4（AsO_4）_3（OH）_4·5H_2O，是一种常见于自然污染土壤剖面中的矿物。与臭葱石、FeAsO 4·2 H2O（目前砷固定化的最佳选择）相比，毒菱铁矿在弱酸性至中性条件下也很稳定。原砷酸铁（FeAsO 4·0.75H2O）是一种高温（~200 ºC）相，在砷黄铁矿-黄铁矿矿石的高压氧化过程中形成。因此，尽管数量很小，但这类技术产生的废物形式存在。应该研究该相的可溶性;如果它比其他含砷相表现更好，则可以调整技术以有利于该相的生产。臭葱石-二水锰矿（AlAsO 4·2 H2O）固溶体由于Fe-Al混合降低了固溶体成员的吉布斯自由能（从而降低溶解度），可能是另一种有希望的候选者。拟议的工作将通过对合成样品的量热技术的组合来完成，其特征在于广泛的粉末X射线衍射，扫描电子显微镜和电子探针。对于锰硫铁矿样品，将通过核磁共振（NMR）光谱法额外检查其纯度。该项目最后将计算一系列环境和高温（高达300 ºC）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.