Thermodynamic properties of tooeleite,Fe63+(As3+O3)4(SO4)(OH)4·4H2O |
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| Institution: | 1. Institutes of Geosciences, Friedrich-Schiller University, Burgweg 11, D-07749 Jena, Germany;2. Department of Material Research and Physics, Division Mineralogy, University of Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg, Austria;3. Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark;4. Karlsruhe Institute of Technology, ANKA Synchrotron Radiation Facility, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany;1. St. Kl. Ohridski University of Sofia, 15, Tsar Osvoboditel Blvd., 1504 Sofia, Bulgaria;2. Institute of Organic Chemistry, Bulgarian Academy of Sciences, 9 Bldg., Acad. G. Bonchev Str., 1113 Sofia, Bulgaria;3. Geological Institute, Bulgarian Academy of Sciences, 24 Bldg., Acad. Bonchev Str., 1113 Sofia, Bulgaria;1. Gmain Nr. 1, 84424 Isen, Germany;2. Hydroisotop GmbH, Woelkestrasse 9, 85301 Schweitenkirchen, Germany,;3. UMR GeoRessources 7359, Université de Lorraine. 54506 Vandoeuvre-lès-Nancy Cedex, France;4. Norges Geologiske Undersøkelse, 7491 Trondheim, Norway;5. Natural History Museum, Cromwell Road, London SW7 5 BD, United Kingdom;6. Institut für Mineralogie, TU Bergakademie Freiberg, Brennhausgasse 14, 09596 Freiberg, Germany;7. Secretaria de Mineria, Av. Hipólito Yrigoyen 401, 5000 Córdoba, Argentina;1. Geobiotec, LNEG: Geology, Hydrogeology and Coastal Geology Department, Estrada da Portela, Bairro do Zambujal, Ap. 7586-Alfragide, 2610-999 Amadora, Portugal;2. Geobiotec, Department of Earth Sciences, University of Coimbra, 3030-790 Coimbra, Portugal;3. Earth Sciences Department, Memorial University, St. John’s, NL A1B 3X5, Canada;1. Key Lab of Indoor Air Environment Quality Control, School of Environmental Science and Engineering, Tianjin University, Tianjin, PR China;2. School of Environmental and Chemical Engineering, Tianjin Polytechnic University, Tianjin, PR China;3. School of Chemical Engineering and Technology, Tianjin University, Tianjin, PR China;4. School of Textiles, Tianjin Polytechnic University, Tianjin, PR China;1. School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China;2. National Key Laboratory of Science and Technology on Remote Sensing Information and Image Analysis, Beijing Research Institute of Uranium Geology, China National Nuclear Corporation, Beijing 100029, China |
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| Abstract: | Tooeleite, nominally Fe63+(As3+O3)4(SO4)(OH)4·4H2O, is a relatively uncommon mineral of some acid-mine drainage systems. Yet, if it does occur, it does so in large quantities, indicating that some specific conditions favor the formation of this mineral in the system Fe-As-S-O-H. In this contribution, we report the thermodynamic properties of synthetic tooeleite. The sample was characterized by powder X-ray diffraction, scanning electron microscopy, extended X-ray absorption fine-structure spectroscopy, and Mössbauer spectroscopy. These methods confirmed that the sample is pure, devoid of amorphous impurities of iron oxides, and that the oxidation state of arsenic is 3+. Using acid-solution calorimetry, the enthalpy of formation of this mineral from the elements at the standard conditions was determined as ?6196.6 ± 8.6 kJ mol?1. The entropy of tooeleite, calculated from low-temperature heat capacity data measured by relaxation calorimetry, is 899.0 ± 10.8 J mol?1 K?1. The calculated standard Gibbs free energy of formation is ?5396.3 ± 9.3 kJ mol?1. The log Ksp value, calculated for the reaction Fe6(AsO3)4(SO4)(OH)4·4H2O + 16H+ = 6Fe3+ + 4H3AsO3 + SO42? + 8H2O, is ?17.25 ± 1.80. Tooeleite has stability field only at very high activities of aqueous sulfate and arsenate. As such, it does not appear to be a good candidate for arsenic immobilization at polluted sites. An inspection of speciation diagrams shows that the predominance field of Fe3+ and As3+ overlap only at strongly basic conditions. The formation of tooeleite, therefore, requires strictly selective oxidation of Fe2+ to Fe3+ and, at the same time, firm conservation of the trivalent oxidation state of arsenic. Such conditions can be realized only by biological systems (microorganisms) which can selectively oxidize one redox-active element but leave the other ones untouched. Hence, tooeleite is the first example of an “obligatory” biomineral under the conditions prevailing at or near the Earth's surface because its formation under these conditions necessitates the action of microorganisms. |
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| Keywords: | Tooeleite Thermodynamics Mine wastes Biomineralization |
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