Covalent organic framework with excessive hydroxyl groups, a platform ready for phosphorylation then involved in uranium adsorption under high acidity

The uranium recovery under high acidities by solid phase extraction is constrained by the adsorbents. Covalent organic frameworks (COFs) have been promising adsorbents. However, the majority of COF adsorbents suffer from low extraction efficiencies or structural decomposition when put into use under highly acidic environments. Here we present COF-Tris-P as an excellent adsorbent for uranium recovery under high acidities. The remarkable performance of COF-Tris-P can be attributed to the high stability inherited from the precursor COF-Tris and considerable amount of phosphate groups within its pores after phosphorylation of COF-Tris. Adsorption experiments showed the adsorption amounts of COF-Tris-P for uranyl ions decrease gradually with the increase of acidities, but slightly rebound when the acidity further increased. Encouragingly, the saturated adsorption capacity still reaches 112.8 mg/g in 2 mol/L HNO3 solutions. Meanwhile, COF-EAA-P was synthesized as the analogue for COF-Tris-P employing a consistent protocol. COF-EAA-P showed the same trend as COF-Tris-P in adsorption amounts at different acidities, only the adsorption amounts of COF-EAA-P were lower overall. By comparison, COF-Tris-P outperforms because COF-Tris carries massive hydroxyl groups, which enhance the post-modification for increased amount/density of specific functional groups on COF-Tris-P. This work offers an innovative approach to the structural design and applied research of COFs.