Decoding molecular switches mediating uranium speciation transition in iron-based mineral dynamic phase evolution

The phase and structure of iron-based minerals play a crucial role in determining uranium speciation and its environmental migration potential. However, the molecular mechanisms of iron ion crystallization and phase evolution affecting uranium immobilization and release remain poorly understood. Capturing the dynamic effects of iron mineral phase transformations on uranium speciation in real-time is challenging. We developed a microcurrent-induced method to modulate iron ion release kinetics, creating a pathway from aqueous iron to mineral phases under mild conditions. By accurately capturing phase transformation intermediates and using stepwise extraction and density functional theory (DFT) calculations, we clarified uranium species evolution during iron mineralization. This approach revealed the microscopic driving forces responsible for the long-term immobilization of uranium, transitioning from surface adsorption to lattice incorporation. Uranium (VI) initially adsorbs onto ferrihydrite and lepidocrocite surfaces during magnetite crystallization. As lepidocrocite converts to magnetite, over 80% of uranium integrates into the magnetite lattice by substituting octahedral or tetrahedra iron sites, with partial reduction to uranium (IV) by structural, iron(II), which significantly enhances the stabilization of uranium within the crystal framework. This study offers a novel perspective on the environmental behavior of radionuclides, governed by the continuous evolution of multiphase iron minerals.