Geochemical Controls on Uranium Behavior During Water–Rock Interactions at a Natural Analogue Site in Korea

Uranium behavior during water–rock interactions is strongly influenced by geochemical conditions relevant to geological disposal environments. This study investigated how variations in pH, redox conditions, carbonate availability, and temperature regulate uranium behavior through a series of batch experiments. Uranium-bearing coaly slate was collected from a natural analogue site in Boeun-gun within the Okcheon Metamorphic Belt, Korea. The coaly slate contains approximately 99.6 ppm of uranium, primarily hosted in uranium-bearing minerals such as uraninite and ekanite. Five batch experiments were conducted using artificial groundwater designed to represent the groundwater chemistry of the study site. The experimental design isolated the effects of pH, carbonate buffering, temperature, and uranium spiking. Batch 1 and Batch 2 were conducted under initially acidic (pH 5) and alkaline (pH 9) conditions, respectively. Batch 3 involved uranium-spiked artificial groundwater (2 mg L^-1), while Batch 4 and Batch 5 were conducted under carbonate-buffered, near-neutral pH conditions at 15 °C and 30 °C, respectively. In Batches 1–3, pH decreased rapidly immediately after the reaction began, resulting in acidic and high Eh conditions driven by pyrite oxidation in the coaly slate. This process promoted the formation of secondary Fe(III) (oxyhydr)oxides and Fe-bearing secondary phases. In Batch 1 and Batch 2, uranium concentrations increased rapidly, reaching approximately 60 and 30 µg L^-1 within 72 hours, respectively, and approached near-equilibrium, indicating limited uranium release under acidic conditions. In contrast, despite similarly acidic conditions, Batch 3 exhibited a gradual decrease in aqueous uranium concentration over time, suggesting uranium removal through adsorption or surface complexation onto newly formed Fe(III) (oxyhydr)oxides. In carbonate-buffered systems (Batch 4 and Batch 5), pH remained near neutral throughout the experiments, and uranium concentrations increased continuously with time, reaching levels of up to ~20 µg L⁻¹, which were lower than those observed under acidic conditions. Uranium speciation was dominated by aqueous carbonate complexes, with Ca₂UO₂(CO₃)₃ prevailing at 15 °C and UO₂(CO₃)₂²⁻ dominating at 30 °C. This sustained increase under neutral conditions contrasts with the rapid but limited uranium release observed in acidic systems, highlighting the role of carbonate complexation in regulating uranium mobility in groundwater.