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

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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.