Thorium partitioning in Greek industrial bauxite investigated by synchrotron radiation and laser-ablation techniques


Gamaletsos P, Godelitsas A, Mertzimekis TJ, Göttlicher J, Steininger R, Xanthos S, Berndt J, Klemme S, Kuzmin A, Bárdossy G. Thorium partitioning in Greek industrial bauxite investigated by synchrotron radiation and laser-ablation techniques. Nucl. Instrum. Meth. Phys. Res. B. 2011;269:3067 – 3073.


Typical red–brown (Fe-rich) and high-quality white–grey (Fe-depleted) bauxite samples from active mines of the Parnassos-Ghiona area, central Greece, were investigated. According to \{XRF\} and ICP-MS analyses their actinide content, and particularly of Th, is relatively increased. Fe-depleted samples contain up to 62.75 ppm Th corresponding to 220 Bq/kg due to 228Ac (232Th-series), whereas Fe-rich samples are less Th-radioactive (up to 58.25 ppm Th, 180 Bq/kg due to 228Ac). Powder-XRD patterns showed that Th-enriched (Fe-depleted) bauxite consists mostly of diaspore (AlOOH polymorph), anatase and rutile (TiO2 polymorphs). SEM-EDS indicated the presence of Ti–Fe–containing phases (e.g. ilmenite, FeTiO3), chromite (Cr-spinel) and besides LREE-minerals (mostly bastnäsite/parisite-group) and zircon (ZrSiO4) hosting a part of the bulk Th. The presence of Th in diaspore and in Ti-containing phases (not detected by SEM-EDS as in the case of REE-minerals and zircon) was investigated, into distinct pisoliths of Fe-depleted bauxite, using μ-XRF and μ-XAFS in the SUL-X beamline of the \{ANKA\} Synchrotron facility (KIT, Germany). \{XAFS\} spectra of Th salts and Th-containing reference materials were obtained as well. 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 an average of 73 ppm Th in anatase grains together with abundant Nb (3356 ppm), Ta (247 ppm) and U (33 ppm). The Th LIII-edge \{XAFS\} spectra as compared to reference materials, give also evidence that Th4+ may not replace Ti4+ in distorted [TiO6] fundamental octahedral units of anatase and ilmenite lattice (CN = 6). The occupation of either extraframework sites of higher coordination (CN = 6.9 or even CN = 7.4), according to \{EXAFS\} signals evaluation, or of defected/vacant (**) sites is more probable. This is likely explained by the difficulty of Th4+ to replace directly Ti4+ in [6]-coordinated (octahedral) sites due to the large difference in the relevant ionic radii (0.940 and 0.605 Å respectively).