Abstract:
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 232Th and 238U,
respectively) with a total dose rate of 295 nGy h−1. Leaching experiments, in conjunction with SFICP-
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-STEMHAADF
study of the leached BR at the nanoscale revealed that the significant immobility of Th4+ is due
to its incorporation into an insoluble perovskite-type phase with major composition of Ca0.8Na0.2TiO3
and crystallites observed in nanoscale. The Th LIII-edge EXAFS spectra demonstrated that Th4+ ions,
which are hosted in this novel nano-perovskite of BR, occupy Ca2+ sites, rather than Ti4+ sites. That is
most likely the reason of no Th release in Mediterranean seawater.
Keywords:
bauxite, thorium, red-mud, titanium, perovskite, nano, synchrotron, TEM
