The Sorption Behaviors Of Th(Ⅳ) And Eu(Ⅲ) In Metal-organic Framework Materials UiO-66 And Its Carboxyl Derivatives

With the fast development of economy, the contraction among tremendous energy demand, fast fossil fuel consumption, severe climate change and environmental problems baffles individuals especially in recent decades. Finding environmentally friendly new energy is thus becoming an urgent need. Nuclear energy is believed to be a kind of“clean energy" in view of the fact that it doesn’t cause lots of greenhouse gas and its energy density is higher than fossil fuel, therefore our country has put lots of vigor into the development of nuclear energy. Besides, the development of nuclear energy is inseparable from the nuclear fuel reprocessing, which involves the separation/extraction of radionuclides, the lanthanide/actinide separation and other key scientific issues, however, the related basic research is in the ascendant. Metal-Organic Framework materials have been widely applied in the fields of gas adsorption, catalysis and separation, owing to their advantages of ultrahigh surface area and porosity.This study contributes to assessment of the feasibility of MOFs applied in actinides separation, which mainly includes two sections, that is the preparation of metal organic framework materials UiO-66 and its carboxyl derivatives and the adsorption behaviors of actinide Th （IV） and lanthanide Eu（III）. In our work, we systematically researched a series of factors in sorption process, including solution pH, contact time, the initial concentration and ionic strength and so on. At the same time, we made detailed mechanism interpret according to fourier transformation infrared spectroscopy （FTIR）, extended X-ray absorption fine spectroscopy （EXAFS） and positron annihilation lifetime spectroscopy （PALS）. Thus the main conclusions we made are as follows: we successfully synthesized UiO-66 and its carboxyl derivatives,which have uniform structure and excellent thermal stability and hydrothermal stability, characterized by Power x-ray diffraction, N₂ adsorption/desorption isotherms, thermal gravity analysis and fourier transformation infrared spectroscopy. From the adsorption data, we can conclude that the UiO-66 and its carboxyl derivatives have excellent removal efficiency towards Th（IV） and Eu（III）, and the sorption process is apparently dependent on the solution pH,that is the sorption capacity obviously increases along with the augument of solution pH. At the same time, the introduction of carboxyl group can incite the adsorption capacity of Th（IV） and Eu（III） to some extent and accelerate the adsorption rate. In addition, we can draw some conclusion through the fitting analysis of the experimental data: UiO-66-COOH and UiO-66-2COOH remove Th（IV） and Eu（III） from aqueous solution is dominated chemical sorption and independent of ionic strength, UiO-66, however, the sorption process is physical sorption and the sorption capacity increases with the ionic concentration augument. Possible mechanism for the adsorption behaviors of Th（IV） and Eu（III） in UiO-66 and its carbocyl derivatives were explored based on extended X-ray absorption fine spectroscopy （EXAFS） and FTIR analysis.It is concluded that UiO-66-COOH and UiO-66-2COOH sorb Th（Ⅳ） and Eu（III） through the coordination of free carboxyl groups in the pores of the MOFs, while in the case of UiO-66, both the precipitation and the exchange with organic solvent contribute to the Th（IV） and Eu（III） uptake. The findings of this work further highlight the vast opportunities of MOFs applied in the separation/extraction of radionuclides and the lanthanide/actinide separation.