Smelting process controls uranium leaching potential in historically disposed uranium-bearing wastes: Implications for risk assessment beyond total uranium content

The conventional risk assessment of naturally occurring radioactive material (NORM) wastes relies primarily on total uranium content or radiological dose, implicitly assuming that higher concentration implies greater environmental hazard. This study evaluates this assumption by systematically comparing the uranium leaching potential of historically disposed wastes from two distinct mineral origins and three waste categories. The results indicate that the smelting method exerts a strong apparent control on long-term uranium leaching potential, partly reshaping the traditional risk hierarchy. Pyrometallurgical slags undergo a “rapid stabilization” process. After an initial minor release of their total uranium, their leachability drops below 0.1% within a few years and remains negligible in decades-old samples. In stark contrast, hydrometallurgical residues and unsmelted mining wastes retain measurable leaching potential even after decades of disposal. Speciation analysis suggests that uranium in hydrometallurgical residues is predominantly associated with operationally defined exchangeable and carbonate-bound fractions, whereas in pyrometallurgical slags it is mainly associated with operationally defined residual fractions, implying stronger retention in refractory matrices. These findings indicate that higher total content does not necessarily equate to higher environmental hazard. Our work provides an evidence-based foundation for time-sensitive, process-aware waste management strategies, with important implications for environmental protection, groundwater safety, and public health in regions affected by historical NORM disposal.