Phosphonate-functionalized covalent organic frameworks-derived carbon aerogel for efficient separation of Zr4+ and Hf4+

The coexistence in nature, similar chemical properties, and completely different applications of zirconium (Zr) and hafnium (Hf) make their separation difficult but extremely important. Although solid-phase extraction has shown considerable promise in achieving effective separation of Zr 4+ and Hf 4+ ions, the critical challenge is to develop suitable extractant matrices. Herein, a carbon aerogel with high specific surface area and porosity, derived from covalent organic frameworks was prepared for the first time as an extractant matrix. Subsequently, a phosphonate ligand as the effective functional group was further used to modify the carbon aerogel to prepare the solid-phase extractant. The spectroscopic analysis revealed a tight host-guest interaction between the phosphonate ligand and the carbon aerogel matrix. Impressively, adsorption studies indicated that the phosphonate-functionalized carbon aerogel exhibits Langmuir-type adsorption, affording maximum capacities of 16.8 mg/g for Zr 4+ and 12.0 mg/g for Hf 4+ . Moreover, the Zr 4+ /Hf 4+ separation factor calculated from equilibrium data reached as high as 3.3. Further analysis and theoretical calculation exhibited that the phosphonate in the solid-phase extractant play a decisive role in the adsorption and separation of Zr 4+ and Hf 4+ . Beyond introducing an exceptional solid-phase extractant for Zr 4+ /Hf 4+ separation, this research diversifies the range of potential extractant matrices, thereby creating valuable methodological references for future research.