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Sci Rep. 2016 Feb 22;6:21737. doi: 10.1038/srep21737.

The role of nano-perovskite in the negligible thorium release in seawater from Greek bauxite residue (red mud).

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Center for Electron Nanoscopy, Technical University of Denmark, 2800 Kongens Lyngby, Denmark.
Faculty of Geology &Geoenvironment, National and Kapodistrian University of Athens, Zografou Campus, 15784 Athens, Greece.
Karlsruhe Institute of Technology, ANKA Synchrotron Radiation Facility, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.
Institute of Solid State Physics, University of Latvia, Kengaraga str. 8, 1063 Riga, Latvia.
Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.
Faculty of Physics, National and Kapodistrian University of Athens, Zografou Campus, 15784 Athens, Greece.
Department of Electrical and Computer Engineering, Nuclear Technology Laboratory, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece.
Department of Automation Engineering, Alexander Technological Educational Institute of Thessaloniki, 57400 Thessaloniki, Greece.
KU Leuven, Department of Materials Engineering, Kasteelpark Arenberg 44, 3001 Leuven, Belgium.
University of Patras, Department of Chemical Engineering, 26500 Rio, Greece.
PANalytical B.V., 7600 AA Almelo, The Netherlands.
School of Geology, Aristotle University of Thessaloniki, 54124, Thessaloniki, Greece.


We present new data about the chemical and structural characteristics of bauxite residue (BR) from Greek Al industry, using a combination of microscopic, analytical, and spectroscopic techniques. SEM-EDS indicated a homogeneous dominant "Al-Fe-Ca-Ti-Si-Na-Cr matrix", appearing at the microscale. The bulk chemical analyses showed considerable levels of Th (111 μg g(-1)), along with minor U (15 μg g(-1)), which are responsible for radioactivity (355 and 133 Bq kg(-1) for (232)Th and (238)U, respectively) with a total dose rate of 295 nGy h(-1). Leaching experiments, in conjunction with SF-ICP-MS, using Mediterranean seawater from Greece, indicated significant release of V, depending on S/L ratio, and negligible release of Th at least after 12 months leaching. STEM-EDS/EELS &HR-STEM-HAADF study of the leached BR at the nanoscale revealed that the significant immobility of Th(4+) is due to its incorporation into an insoluble perovskite-type phase with major composition of Ca(0.8)Na(0.2)TiO3 and crystallites observed in nanoscale. The Th L(III)-edge EXAFS spectra demonstrated that Th(4+) ions, which are hosted in this novel nano-perovskite of BR, occupy Ca(2+) sites, rather than Ti(4+) sites. That is most likely the reason of no Th release in Mediterranean seawater.

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