Robust Sulfonated Three-Dimensional Covalent Organic Frameworks for U(Ⅵ) Adsorption From Aqueous Solution

Abstract

With the continuous development of economy and industry, the demand for energy in modern society is ever-increasing. As a kind of high efficiency and low carbon energy, nuclear energy has attracted more and more attention. Developing nuclear energy, however, also brings some problems such as the release of radioactive metal ions that pose a threat to the environment and human health. Therefore, separation and removal of radioactive ions(e.g., U(Ⅵ)) from wastewater is of great significance to ensure the sustainable development of nuclear energy. Covalent organic frameworks(COFs) are a kind of periodic porous polymer materials composed of organic molecular units connected by covalent bonds. The emergence of COFs marks a new era in the field of materials. Because of the design ability of COFs and its unique advantages in structure and pore, it shows a good application prospect in catalysis, sensing and adsorption. Very recent efforts by some investigators have focused on the application of COFs in adsorption and separation of radionuclides, generally with encouraging results. According to the classification from the structural dimension, COFs can be divided into two-dimensional COFs materials(2D-COFs) and three-dimensional COFs materials(3D-COFs). Up to now, 2D COFs materials have been widely studied and applied. In contrast, the development of 3D-COF has been relatively slow, and the performance and application have been limited, a result of the limitations of molecular structural units and topological connections, as well as the difficulties of synthesis and structural analysis. To this end, in this paper, a combination of bond transformation and post-grafting was used to construct functional 3D-COFs materials for the efficient removal of U(Ⅵ) from wastewater. Firstly, the imine bond was reduced to an irreversible amine bond by reduction reaction, and then sulfonic acid group was introduced into the skeleton by reaction with 1, 3-propyl sulfonolactone to successfully synthesize COF-320-H2SO3. The structure and properties of COF-320-H2SO3 were characterized by various characterization methods, and the adsorption behavior of COF-320-H2SO3 on U(Ⅵ) was systematically studied. The results show that the chemical stability of COFs is significantly improved by the conversion of imine bond to amine bond, and the adsorption capacity of the material for U(Ⅵ) is clearly enhanced by the introduction of sulfonic acid group with hydrophilic and strong coordination ability. Even under the conditions of 3 mol/L HNO3, the adsorption capacity of U(Ⅵ) can reach more than 40 mg/g. This work further reveals the potential application value of COFs, especially 3D-COFs materials, in the separation of radioactive metal ions.

Keywords

• three-dimensional covalent organic frameworks
• post-synthetic modification
• transform
• adsorption
• u(ⅵ)