Characterization of mineral composition using PIXE and EXAFS analyses to elucidate the Barium adsorption mechanism

Ningyo-toge is a uranium (U) mine that was operative in Japan. Currently, the mining area retains a dump of mill tailings. Groundwater is contaminated with U ore, and mill tailings might contain radioactive elements, such as U and radium (Ra), which can negatively impact the human body. This study determined the soil composition at the tailings dumping site using particle-induced X-ray emission, scanning electron microscopy, and X-ray diffraction and observed the adsorption structure of barium (Ba), an analog of Ra, using X-ray absorption fine structure analysis. The results revealed the existence of silicate minerals, such as quartz and mica, and metal oxides, such as Fe and Mn, in the soil. Ningyo-toge soil is similar to montmorillonite and bentonite. In particular, it was proposed that the adsorption of Ba on Mn oxide is an inner-sphere complex that immobilizes Ba. These findings will provide valuable information for estimating the behavior of Ra in the environment. The results of this study will be helpful for safety assessment and evaluating the impacts on the surrounding environment.