Progress in Photocatalytic Assisted Uranium Extraction in Air Atmosphere

Photocatalytic assisted uranium extraction technology is a rapidly developed technology for uranium extraction from wastewater and seawater in recent years, which can significantly improve the extraction capacity and extraction rate of uranium. However, the activity of most photocatalysts in air atmosphere is inhibited, which hinders the practical application of this technology. Based on this, this paper discusses the latest progress of photocatalytic assisted uranium extraction technology in air atmosphere, focusing on the analysis of different mechanisms and product characteristics of photocatalytic assisted uranium extraction in air atmosphere, especially the photoreaction and chemical reaction mechanism in the absence of catalyst. There are mainly two mechanisms for the photocatalysis assist uranium extraction. The first one is based on the photocatalytic reduction of U(Ⅵ) to U(Ⅳ). Under illumination, electrons and holes pairs are generated on photocatalysts, and the electrons could reduce the soluble \begin{document}${\mathrm{UO}}_2^{2+} $\end{document} to insoluble uranium oxide(UO2). However, under air atmosphere, at the presence of oxygen, the newly formed UO2 could be oxidated back to \begin{document}${\mathrm{UO}}_2^{2+} $\end{document} and dissolved into solution, resulting in the decrease of uranium extraction performance. Therefore, this mechanism is usually applied under inert atmosphere. Although with proper design of photocatalyst, the oxygen could be covert to \begin{document}${ · {\mathrm{O}}_2^- } $\end{document} to further reduce \begin{document}${\mathrm{UO}}_2^{2+} $\end{document}, the performance is still not comparable with that under inert atmosphere. The other recently developed photocatalysis assisted extraction mechanism is based on the convert of soluble \begin{document}${\mathrm{UO}}_2^{2+} $\end{document} to uranium peroxide((UO2)O2•xH2O), which can work well under air atmosphere. Under light illumination, hydrogen peroxide(H2O2) can be first produced with the assist of efficient photocatalysts or the photoactivity of \begin{document}${\mathrm{UO}}_2^{2+} $\end{document} itself. Then H2O can react with to \begin{document}${\mathrm{UO}}_2^{2+} $\end{document} form insoluble uranium peroxide. As uranium peroxide is stable under air and oxygen may contribute to the formation of H2O2, this mechanism can obtain excellent performance under air. Researchers have developed composite materials such as carbon nitride, carbon dots, graphene aerogels, and metal-organic frameworks(MOFs), which exhibit superior uranium removal performance under aerobic conditions. Besides, research on uranium extraction via photocatalysis under air is still in its infancy, and the specific reaction mechanisms may vary under different environments, necessitating further investigation. Based on the mechanism of uranium extraction via the photo-assisted transformation to uranium peroxides, the design of catalysts and uranium extraction materials will be greatly broadened. Combined with the discussion of the reaction mechanism of uranium extraction under light conditions and the design of catalyst materials, we hope to provide inspiration for researchers to develop highly efficient uranium extraction catalyst and mechanism research under air atmosphere.