Graphdiyne (GDY) is a new type of two-dimensional carbon material with rich carbon chemical bonds, large conjugated system, multiple active sites, special triangular channel structure and excellent chemical stability. GDY has witnessed its applications in various fields such as energy storage, catalysis, solar energy, biological sensing or detection, and environment. The in-depth exploration on the structural characteristics and practical applications of GDY plays an important role in the development of the research area concerning GDY.
Separation between actinides and lanthanides is one of the hottest topics in the spent nuclear fuel cycle. In particular, due to the similar chemical behaviours between actinides and lanthanides, and because the concentrations of lanthanides are 20 to 50 times higher than those of minor actinides, the separation between trivalent actinides and lanthanides ions is very difficult. Moreover, as a fertile material, thorium has important application value in the field of nuclear energy. In both the ore and the thorium-base spent fuel, it often coexists with uranium. The separation between thorium and uranium is conducive to the efficient use of resources. 137Cs and 90Sr are the main radioactive and heat release nuclides in the spent fuel, so the extraction of 137Cs and 90Sr from high-level liquid wastes can effectively reduce the radiation and thermal effects of spent fuels. Since both 137Cs and 90Sr are raw materials of radiation sources, the further separation between Cs+ and Sr2+ is of great significance. For the achievement of the above-mentioned separations, it is demanding to explore a material with highly selective adsorption capability and excellent mechanical properties. Hopefully, the GDY is a good candidate for such purpose.
Figure 1 (A) Triangle structure of a GDY unit; (B) The changes of bond length and sides bending for the GDY triangle structure coordinating with Th4+ (Th4+ and nitrates omitted). Sides 1 and 2 are bended at the 1c and 2c carbon atoms, respectively; C, D, E, F, G illustrate local structures of GDY coordinating Th4+, Pu4+, Am3+, Cm3+, Cs+, respectively. There are no covalent bonds between Cs+ and GDY, because Cs+ is adsorbed through electrostatic interaction.
Recently, the research group of Prof. Xinghai Shen, from the College of Chemistry and Molecular Engineering (CCME) at Peking University, systematically studied the coordination of actinides, lanthanides, as well as strontium and cesium with GDY by experiments and theoretical calculations. On the basis of experimental results and/or theoretical calculations, it was suggested that Th4+, Pu4+, Am3+, Cm3+ and Cs+ exist in single ion states on the special triangle structure of GDY with various coordination patterns, in which GDY itself is deformed in different manners. Both experiments and theoretical calculations strongly supported that UO22+, La3+, Eu3+, Tm3+ and Sr2+ are not adsorbed by GDY at all. The effect of 5f electrons on the coordination was investigated particularly.
Figure 2 (A) The adsorption capacity of GDY coating copper foil towards Cs+, Sr2+, Th4+, UO22+, La3+, Eu3+ and Tm3+, respectively (100 mg/L). (B) The adsorption selectivity of copper foil itself and GDY coating copper foil in the mixed solution of Th4+ and UO22+. The concentrations of both Th4+ and UO22+ are 100 mg/L. (C) The XPS spectra of GDY after and before adsorption of Th4+. (D) The XPS spectrum of Th(NO3)4 and that of Th4+ adsorbed on GDY (Inset: the XPS spectrum and curve fitting of Th4+ adsorbed on GDY).
There have been some studies on the application of copper foams coated GDY membranes for ion adsorption. Therefore, based on the above research results combined with membrane separation technology, GDY is expected to achieve the separation between actinides and lanthanides, Th4+ and UO22+, Cs+ and Sr2+ in nuclear fuel cycle. The deformations in the structure of GDY adsorbing actinides are essential for deep understanding its physicochemical properties. Moreover, further investigation on the basis of this work would reveal that GDY is a suitable substrate for actinides single ions. It is believed that the stable existence of the actinides single ions will pave the way for their applications as single ion magnets and catalysts.
Figure 3 Schematic diagram of GDY membrane separation.
The related results were published online in Angew. Chem. Int. Ed. entitled "Coordination of Actinide Single Ions with Deformed Graphdiyne: Strategy on Essential Separation Processes in Nuclear Fuel Cycle" on July 2, 2020. This work was financially supported by the National Natural Science Foundation of China (Grant No. U1830202).