Assessment of the fate and behavior of naturally occurring thorium and uranium in the environment of central Sri Lanka

Although Sri Lankan coastal areas are well-known for their high background radiation levels, mostly due to Th-bearing minerals, higher natural background radiation levels inland were discovered in 2015. The radioactivity data collected at this location raised concern regarding the risk of public radiation exposure since the screened area comprised a school playground.
The present study reports on-site radioactivity measurements and geochemical analyses of environmental samples, such as four soil samples, two grass samples and six water samples from around the area of study. Subsequent series of batch and column experiments with the soil samples were carried out to better understand the environmental risk and potential mobility of dose-dominating NRs in the area. Since the mobility and bioavailability of NRs in the environment strongly depend on their aqueous speciation, adsorption behavior and the solubility of relevant solid phases, classical sequential and single extraction methods were applied and modern spectroscopic techniques were used to gain insight.
Gamma spectrometry from the soil samples clearly showed the dominant contribution of Th-232 and progenies to total activity ranging between 4440.0 ± 364.7 and 7037.9 ± 647.0 Bq kg-1 followed by K-40 in the range from 339.6 ± 16.9 to 538.9 ± 55.8 Bq kg-1 and U-238 including progenies in 318.7 ± 88.7 – 510.9 ± 106.3 Bq kg-1. ... mehrIn-situ measurements yielded background radiation levels at one-meter height above the ground of 2.5 ± 1.2 µGv h-1 (maximum of 21.6 ± 10.9 mSv yr-1). The calculated absorbed dose rates in air, 3 – 4.6 µGy h-1, while the calculated radiation hazard indices yielded mean values of 9300 ± 1800 Bq kg-1, 33 ± 6, 25 ± 5, and 20 × 10-3 ± 4 × 10-3 for Radium Equivalent Activity (Req), Gamma Index (Iγ), External Hazard Index (Hex) and Excess Life Time Cancer Risk (ELCR), respectively. All these indices exceed their corresponding world averages. Activity concentrations of Th-232, U-238 and K-40 in grass sample dry masses are in the range of 770 – 975, 21 – 30, and 540 – 574 Bq kg-1, and the estimated soil-grass transfer factors (TFs) are 0.12 – 0.16, 0.05 – 0 08, and 0.94 – 1.40, respectively. Thoron gas releases from soil samples in exhalation experiments using a closed system result in activities 35 – 76 kBq m-3. Conversion to annual effective doses via the inhalation pathway yields values of 2.49 to 5.46 mSv. Note that those values do not reflect the real situation, where extensive dilution by air will reduce thoron concentrations by orders of magnitude even in closed rooms.
All soil samples were characterized by several spectroscopic and microscopic techniques. The basic physicochemical characterization evidenced acidic properties and moderate total organic carbon content in the soils. X-ray Diffraction (XRD) data reveal that the major mineralogy of the site is dominated by kaolinite and quartz with minor fractions of Fe crystalline phases like hematite and goethite. Target trace elements were found to be present in different mineral phases, i.e., as oxides, silicates and phosphates, and in mixed phases as identified by Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray (EDX) spectroscopy. Linear Combination Fitting (LCF) of X-ray Absorption Spectroscopy (XAS) data involving Th-monazite (phosphate), thorite (silicate), and thorianite (oxide) provided evidence that Th in the samples is present as Th-monazite (61 ± 7%), thorite (ThSiO4, 24 ± 7%), and thorianite (ThO2, 16 ± 7%). Further study of single particles selected from the sediment samples using micro-focus X-ray Fluorescence (μ-XRF) and micro-X-ray Absorption Spectroscopy (μ-XAS) confirmed the nature of the mineral phases and the chemical forms of Th.
Element mobility was tentatively addressed by applying several batch and column experiments. Selective chemical extractions showed that ~8 and 16 wt.% of total Th and U could be dissolved/extracted from the operationally defined soil non-residual fractions and >85 wt.% of Th and U are associated with the major residual fraction of NR-containing mineral phases (oxide, phosphate and silicate phases). Mass balance calculations confirmed full recovery of 100 ± 15% Th while for U it was 100 ± 40% as compared to the amount quantified by X-ray Florescence (XRF) spectroscopy.
Batch extractions and column leaching experiments with simulated rainwater (SRW), silica nanoparticles (Si NPs) and humic acid (HA), show clearly lower leachability of Th and U when compared to the chemical extractions. Overall potential mobilization of the trace elements with the above solutions in both batch and column experiments are <1% under the experimental conditions. Column experiments with SRW show relatively higher release of particulate species at the beginning likely due to erosion and eventually reach the measured on-site groundwater concentration levels. Added Si NPs and HA did not cause any significant effect on metal leaching and seemed strongly retained by the soil. Scoping calculations involving geochemical models qualitatively retrieved the trend of the target elemental mobilities in the experiments.
The obtained results suggest important public, on-site exposure to radioactive NR-containing particles. Complementary results on associated light rare earth elements (REEs) are also presented given current socio-economic interest and in view of potential future environmental impact.
The present study provides for the first time relevant information about NR minerals in Sri Lankan soil, marking the onset for future studies concerning radioactive risk assessment in Sri Lanka.