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Distribution Coefficients of Th(IV) Onto Granite and Biotite Gneiss in Gyeongju Groundwater
p.186
Th(IV) is a stable actinide that can act as a chemical analogue of U(IV) and Pu(IV), which are important radionuclides in safety assessments of deep geological repositories (DGR). Therefore, to understand the geochemical behaviour of U(IV) and Pu(IV), batch sorption of Th(IV) onto crystalline rocks were performed in oxidising conditions. The distribution coefficients (Kd) of Th(IV) were of particular interest. Gyeongju fresh groundwater (GF) and Gyeongju brackish groundwater (GB) were obtained at the Gyeongju Low and Intermediate Level Radioactive Waste (LILW) Disposal Facility. Crystalline granite (gr) and biotite gneiss (bg) were collected in Gyeongju and Gwacheon respectively and were grounded to a particle size smaller than 150 μm. Sorption samples were continuously shaken for 7 days under 200 rpm at 25°C. The liquid-to-solid ratio (V/m) was 200 L·kg-1. Th(IV) concentrations of the sorption samples were determined by UV-Vis-NIR absorption colorimetry from the formation of Th(IV)-arsenazo III complexes. Although the method allowed the initial Th(IV) concentrations to be determined, the final Th(IV) concentrations fell below the limit of detection (LOD), 6.27×10-9 mol·L-1. Taking the LOD as the final concentrations, conservative Kd were calculated to be 4,410 L·kg-1 for GF-gr and GF-bg, and 7,830 L·kg-1 for GB-gr and GB-bg. The result indicates a strong sorption affinity of Th(IV) onto granite and biotite gneiss within Gyeongju groundwater, suggesting a similar behaviour for U(IV) and Pu(IV). Furthermore, comparison of the conservative Kd obtained from the experiment were compared with existing Kd values of Th(IV). Such analysis and comparison of Th(IV) Kd in various types of groundwater could help locate the optimal site for a DGR in South Korea.
