Solid phase speciation and mobility of thorium in soil samples from a case study in Sri Lanka

Abstract Sri Lanka has high background radiation due to naturally occurring radionuclides like U-238, Th-232, and K-40 containing minerals. This study investigates the radiological characteristics of soil samples from the Matale District in central Sri Lanka, focusing on thorium (Th) and its potential mobility/bioavailability. Spectrometric data indicate that Th contributes most significantly to the elevated background radiation levels in this area. Thorium, present at approximately 0.2 wt.% in the bulk soil, was analyzed across various mineral phases, including oxides, silicates, and phosphates, using multiple extraction and characterization techniques. Light rare earth elements (REEs) were also examined due to their natural association with Th-bearing minerals. This study provides, for the first time, relevant information on Th minerals in central Sri Lankan soil, addressing a critical research gap in radiological assessments of inland soils in the country. Findings provide insights into radiation exposure risks and the environmental behavior of radionuclides, serving as an important starting point for future studies on radioactive risk assessment in central Sri Lanka. The results contribute to the understanding of soil properties and emphasize the importance of further comprehensive studies to fully assess health risks and develop potential environmental safety measures.