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

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 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 _0.8Na _0.2TiO ₃ 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.