Long-term Behaviour of Uranium in Soil System: Understanding the Processes Controlling the Bioavailability

Radiological and environmental risk assessments of uranium require knowledge of its immediate and long-term behaviour in the soil system. To better understand U(VI) behaviour in contaminated soil, a mesocosm experiment was conducted from 2018 to 2021 by amending soil with U(VI). Study revealed that short-term behaviour of U(VI) is controlled by soil Fe-oxide (hydroxide) content, as approximately 55% of total U binds to the soil reducible fraction within a month of contamination. However, the stability of U-Fe oxide (hydroxide) complexes decreases with aging, possibly due to bio-geochemically induced transformation of Fe-oxide/hydroxide minerals. For this reason, long-term performance testing of the Fe-based permeable reactive barriers for U immobilization is recommended. Study shows that, about 50% of total U in top soil leached out in the first year. Meanwhile, progressive incorporation of U in different soil fractions was also taking place with ageing. X-ray absorption near edge spectra and carbonate leaching studies confirm that, in the long-term reductive transformation of U (VI) to U (IV) and enrichment of U in the residual fractions are important processes that lead to U immobilization. The study also highlights the challenges of long-term study in an open system due to its dynamic nature. The study suggests that for reliable radiological environmental impact assessment, a long-term study is required and the findings of this study could be valuable in hazard management and remediation strategy development.