Amidoxime-functionalized hollow 2D-COFs for uranium separation

Constructing desirable morphology has been demonstrated to be an effective strategy for enhancing the capabilities of the covalent organic frameworks (COFs) for the processing of uranium. Traditional 3D-COFs only possess a limited number of structural nodes, not being the ideal structure for uranium adsorption, but 2D-COFs with rich structural nodes also suffer from reduced exposure of adsorption sites due to stacking during uranium treatment, which is not beneficial for achieving sustained and efficient adsorption. To this end, by polymerizing COFs on the silica carrier, followed by etching the silica carrier, and utilizing the conversion of nitrile to amidoxime groups as chelation sites, we designed and synthesized a hollow 3D-like structure of 2D-COFs (COF-TD-H-AO, similar to three-dimensional COFs) as adsorbents to capture the radionuclide uranium. Systematic studies were carried out for the adsorption characteristics of this material at different pH values and in the presence of interfering ions, as well as its adsorption thermodynamics and kinetics. Compared with COF-TD-AO (pure 2D-COFs) and SiO2@COF-TD-AO (unetched), COF-TD-H-AO exhibited a higher adsorption capacity as well as selective adsorption of uranium. This work provides an innovative perspective on material design and preparation by providing more adsorption sites in an effort to improve the capacity of adsorption and the selectivity of adsorption.