Tailored covalent organic framework with phosphorylation for boosting typical actinide adsorption under acidic conditions

Uranium and plutonium are typical actinides tightly associated with nuclear fuel cycle. In this work we rationally design and fabricate a phosphonic group-decorated covalent organic framework, TzDa-Phos, for highly selective removal of uranium [U(VI)] and plutonium [Pu(IV)]. The saturation adsorption capacity for U(VI) can reach 394 mg/g at pH 4.0, and even maintains 40 mg/g in 1-4 M HNO3 solutions. The distribution coefficient of Pu(IV) with TzDa-Phos is recorded up to 9.7 x 103 mL/g at 3 M HNO3. The excellent adsorption performance of TzDa-Phos for U(VI) and Pu(IV) is reasonably attributed to phosphonic groups decorated in TzDa. Specifically, the strong complexation of P--O with uranium nitrate species (e.g., UO2(NO3)+, UO2(NO3)20) is indicated at high acidities, in contrary to the coordination of P -O- with uranium at pH 4.0 as evidenced by FT-IR, XPS and EXAFS analyses. This work provides a promising adsorbent to recover uranium and plutonium from various wastewaters via a solid-phase extraction approach.