Photocatalytic Reduction of U(Ⅵ) on Y3+ Doped Carbon Nitride

Abstract

The photocatalytic conversion of soluble U(Ⅵ) to insoluble U(Ⅳ) is regarded as an effective method for the separation of solution uranium. During the photocatalytic reactions, the design of photocatalyst plays a role. As a metal free polymer semiconductor, carbon nitride(CN) has attracted increasing attentions in photo-catalytic reduction of U(Ⅵ) under visible light irradiation due to its low cost, easy preparation, excellent physicochemical stability and adjustable structure. However, the application of carbon nitride in photocatalytic U(Ⅵ) reduction is limited by its low photocatalytic reduction efficiency, which is caused by the poor reactive sites low separation rate of charge carriers, and slow transport efficiency of electrons. This study aimed to evaluate the transformation of U(Ⅵ) in aqueous solution over Y-doped CN(YCN). YCN photocatalysts with different amount of Y3+ were prepared by in situ doping to modify the energy band structure and physicochemical properties of carbon nitride. It was revealed that the presence of Y3+ changed the in-plane periodic topology and inter-layer stacking structure of the tri-s-triazine unit of carbon nitride, resulting in the incomplete polymerization of the tri-s-triazine unit. The doping of Y3+ lead to the formation of porous structure in YCN, which increases the specific surface area and the active sites. Comparing with pristine CN, YCN showes enhanced absorbance for light and increases separation rate of charge carriers. The average fluorescence life is subsequently increased under light irradiation, which is 1.8 times for YCN2 than the pristine CN. The EPR spectra demonstrates that, compared to CN, significantly more \begin{document}$ · {\mathrm{O}}_2^- $\end{document} radicals and less \begin{document}$· {\mathrm{OH}} $\end{document} radicals are generated on YCN2 under light irradiation. The free radical trapping experiments verifies that \begin{document}$ · {\mathrm{O}}_2^- $\end{document} radicals act as the dominant reactive species for the reduction of U(Ⅵ) and the small amount of other radicals do not bring obvious influence. The enhanced generation of \begin{document}$ · {\mathrm{O}}_2^- $\end{document} radicals on YCN2 therefore provides favorable conditions for the photocatalytic reduction of U(Ⅵ). When using methanol as a sacrificial agent, it can remove about 95% of U(Ⅵ) within 20 min for the YCN2-containing system under light. The rate of the photocatalytic reduction of U(Ⅵ) on YCN2(0.112 min−1) is 5.3 times higher than CN(0.021 min−1). Analysis of the products reveals that water-soluble U(Ⅵ) is reduced in the photocatalytic reaction, with approximately 80% U(Ⅳ) and 20% U(Ⅵ). The soluble U(Ⅵ) is mainly reduced to insoluble UO2+x deposited on the surface of YCN by \begin{document}$ { · {\mathrm{O}}_2^- } $\end{document} radicals. This study improves the photocatalytic performance of CN for the reduction of U(Ⅵ) by Y3+ doping.

Keywords

• photocatalysis
• carbon nitride
• u(ⅵ) reduction
• y3+
• doping