![]() This radioisotope contributes to the background radiation levels worldwide (i.e., Th-232 content ~ 30 Bq kg -1 3), even though several regions on the planet have high background radiation areas (HBRAs). Naturally occurring Th is dominated by the primordial radioisotope Th-232 (half-life: 1.40 × 10 10 years). In fact, the average concentration of Th in the upper continental crust is ~ 10 – 15 mg kg -1 2. Thorium (Th) is a naturally occurring radioactive element, which is more abundant in the Earth’s crust (i.e., ~ 8 × 10 –4 wt.% ) than uranium (i.e., ~ 4 × 10 –4 wt.%) and other radioactive elements 1. This is the first study providing quantitative and XAS based speciation information on Th-mineral phases in soil samples from Sri Lanka. Further studies on selected individual particles using micro-focus X-ray Fluorescence (μ-XRF) and micro-X-ray Absorption Spectroscopy (μ-XAS) along with SEM–EDX elemental mapping provided information about the nature of Th-bearing mineral particles regarding mixed phases. ![]() Linear combination fitting (LCF) of the Extended X-ray Absorption Fine Structure (EXAFS) data involving reference Th-monazite (phosphate) and thorianite (oxide) compounds suggested that Th is mostly present as Th-phosphate (76 ± 2%) and Th-oxide (24 ± 2%), even though minor amounts of thorite (silicate) were also detected by SEM–EDX. X-ray Absorption Near Edge Structure (XANES) spectra are applied as a fingerprint indication for Th existing in different chemical environments. Four different soil samples were characterized using X-ray Absorption Spectroscopy (XAS) at the Th L 3-edge (16.3 keV), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) spectroscopy. In the present context, we focus on naturally occurring Th-232 at a location in central Sri Lanka presenting high background radiation levels. Mobility and bioavailability of radionuclides in the environment strongly depend on their aqueous speciation, adsorption behavior and the solubility of relevant solid phases.
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