Ion-exchange driving preparation of HPO42- balanced cationic metal-organic framework for efficient U(VI) removal

Metal-organic framework (MOF)-based functionalized composites with enhanced properties are usually made by pre-functionalization and post-synthetic strategies. However, some reported modification methods still suffer from low modification degree and high energy consumption. Herein, a γ-irradiation-induced cationic MOF (NiBTAB-γ) is selected to be the host to encapsulate HPO 4 2- by an ion-exchange method as a novel strategy for MOF functionalization and for the first time applied to the uranium adsorption. The combination of the irradiation method and ion-exchange makes synthetic process much milder and more energy-efficient. The HPO 4 2- can substituted Cl - counterions in the void spaces, which creates abundant phosphate functional sites and maintains the structural integrity. Consequently, the resulting HPO 4 2- balanced MOF (PN-0.50) exhibits excellent adsorption performance for radioactive uranium with high adsorption capacity ( q max =342.0 mg/g at pH=5.0 and T  = 298 K) and excellent selectivity. Electrostatic potential analyses and DFT calculations reveal that the introduction of HPO₄²⁻ not only optimizes the initial capture of uranium ions by enhancing the local negative electrostatic potential but also achieves stronger binding via strong chemical coordination bonds. This work provides new insight into the functionalized modification of MOFs for U(VI) removal from wastewater.