Water content-driven uranium fraction transformation and stability dynamics in contaminated soils

The environmental behavior of uranium (U) in soils is predominantly governed by its speciation, which is mediated by soil properties and environmental conditions. Soil water content is a critical driver of soil properties, but the specific mechanisms underlying its impacts on U speciation and stability remain poorly understood. Here, we investigated the mechanisms controlling U fraction and stability in U-contaminated soils under different water content conditions, by employing soil incubation experiments, chemical extraction, stirred-flow experiments, kinetic modeling, and statistical analyses. Results revealed that, relative to soils with a lower water content (e.g., 28 %), those with a higher water content (e.g., 58 %) significantly enhanced U stability, due to the prevalent reducing microenvironments (e.g., low redox potential). Such reducing conditions promoted the transformation of Fe/Mn oxide-U to organic matter-U during the microbial reductive dissolution of Fe minerals (e.g., hematite), and favored U(VI) reduction by Fe(III)-reducing bacteria (e.g., Pseudomonas and Anaeromyxobacter). Furthermore, dry-wet cycle process suppressed U release into soil solution by facilitating the formation of more stable U species (e.g., organic matter-U and residual U), a transformation mediated by Fe(III) reduction under wet conditions and Fe(II) oxidation/Fe mineral precipitation under dry conditions. Statistical analyses identified reactive Fe minerals and soil DOM as the two most critical drivers of U fraction. Additionally, the exchangeable U fraction was the most labile component governing release kinetics, while organic matter-U and carbonate-U fractions dominated U stabilization. Our results provide insights into the migration and transformation behavior of U in contaminated soils surrounding U tailings ponds.