Summary:
The main purpose of the present PhD Thesis was to combine diffraction (PXRD),
analytical in bulk and microscale ((SF)-ICP-MS/OES, LA-ICP-MS), thermal
(TGA/DTG, DSC), spectroscopic in bulk and microscale (FTIR, 57Fe Mossbauer,
high-resolution gamma-ray, Laser μ-Raman, SR μ-XRF, (μ)-XANES/(μ)-EXAFS),
microscopic (optical, SEM-EDS/WDS), and nanoscopic (FEG TEM-EDS & EELS/
STEM-HAADF) techniques, together with magnetic susceptibility measurements, for
the detailed mineralogical and geochemical characterization of karst-type
bauxites and their metallurgical residues (red mud) from active mines of the
Parnassos-Ghiona area (central Greece), for the first time in the literature.
For the sake of the goal of this dissertation, the investigation of mineral
nanoparticles and nanominerals in both typical Fe-rich and Fe-depleted bauxite
samples, as well as in red mud was also concerned.
Among all trace elements that were studied (e.g., HFSE, REE, actinides etc.),
Th is stated to be important related to mining, metallurgical and environmental
issues. SEM-EDS indicated the existence of Ti-Fe-containing phases, chromites,
LREE-minerals, and zircon hosting part of the bulk Th. Besides, the presence of
Th in diaspore and in Ti-containing phases, into distinct pisoliths of
Fe-depleted bauxite, was found out using μ-XRF and μ-XAFS. Accordingly, it was
revealed -for the first time in the literature- that Ti-phases, and
particularly anatase, host significant amounts of Th. This novel conclusion was
complementary supported by LA-ICP-MS analyses indicated significant Th in
anatase grains together with abundant Nb, Ta and U. Th LIII-edge XAFS study
give, also, evidence that Th4+ may not replace Ti4+ in distorted [TiO6]
fundamental octahedral units of anatase and ilmenite lattice, but occupies
either extraframework sites of higher coordination or defected/vacant sites.
Further investigation of mineral nanoparticles and nanominerals in Fe-depleted
samples, using SEM-EDS/WDS, proved the presence of Fe-Cr-Ti-containing
diaspore. The subsequent study by means of 57Fe Mossbauer, in correlation with
magnetic susceptibility measurements, and complementary Synchrotron-based
spectroscopic techniques in microscale (μ-XRF and μ-XANES/-EXAFS), indicated
that Fe3+, in contrast to [6]Cr3+, is not exclusively a component of the
structure of α-AlOOH (diaspore). The nanoscopic study, using FEG TEM-EDS &
EELS, revealed TiO2 polymorph mineral nanoparticles, particularly rounded
anatase nanocrystals dispersed into the diaspore matrix (in areas appearing
phase-homogeneous in microscale), as well as individual needle-shaped rutile.
Additionally, it was proved that, except Fe3+ substituting Al3+ in the
structure of diaspore, a percentage of the metal exists also in the form of
peculiar Fe3+ nanominerals.
Finally, Greek red mud was characterized using a combination of diffraction,
microscopic, analytical, and spectroscopic techniques (PXRD, SEM-EDS,
STEM-EDS/EELS, XRF, ICP-OES/MS, HR γ-ray Spectrometry, and XANES/EXAFS). Th
LIII-edge EXAFS spectra showed, for first time in the literature, that Th4+ is
hosted in novel nano-perovskite. It is, therefore, herein stated that the above
nanophase is the reason of low Th release in acid medium, and subsequently that
explains the Th immobility in Greek red mud as exposed in Mediterranean
seawater.
