Evolutional solid phase and solid-liquid interface uranium immobilization mechanisms by nanoscale zero-valent iron and enhanced uranium stability control strategy

Uranium is a carbon dioxide free nuclear energy, and uranium-contained wastewater poses serious chronic health effects toward human. Nanoscale zero-valent iron (nZVI) separates uranium from wastewater with ultrafast kinetic, high capacity and selectivity. However, the dynamic interfacial uranium binding mechanisms and its stability, controlled by the phase transformation of nZVI, is important but poorly understood. After 120 h reaction, the fresh nZVI was oxidized and transformed to ferrihydrite and then lepidocrocite and hematite. Analysis for the structures and valence states of U, Fe species indicated that the dominate U(VI) uptake mechanism changed from Fe0 induced reduction to adsorption as nZVI transformed to iron (oxyhydr)oxides. Density functional theory calculation revealed that uranyl formed corner-sharing configurations on the surface of iron nanoparticles, and some uranium ions prefer to incorporate into the structure of lepidocrocite than hematite during the crystallization processes of ferrihydrite. Meanwhile, 100 ∼ 10000 μg/L uranyl ions were quickly captured, the residual uranium could be maintained at ∼9.31 μg/L and the content of uranium in reacted iron nanoparticles reached ∼24.16 wt%. Interestingly, the oxidization reduction potential (Eh) was a potential parameter to control uranium immobilization in the CSTR system, and the Eh control strategy could be used to increase the fraction of U(IV)/U(V), sparingly soluble and highly stable, higher than 90 %. The findings augment our understanding of U-nZVI reactions and guide nZVI technology to remedy real uranium-contained wastewater.