Tailored metal-organic frameworks facilitate the simultaneously high-efficient sorption of UO₂²⁺ and ReO₄^- in water

The simultaneously efficient extraction of radioactive metal cations and anions from radioactive waste is of great interest for the proper disposal of spent fuel and environmental protection. Modifying metal-organic frameworks (MOFs) into multifunctional materials with controllable and desired properties is an efficient strategy for broadening their practical applications. Herein, poly(ethyleneimine) (PEI) tailored MIL-101(Cr) (MILP) was obtained through an easy operation and low-cost strategy and was utilized to simultaneously extract uranium (UO₂²⁺) and rhenium (ReO₄^-) from water. The effects of PEI coating amounts, system pH, contact time, initial UO₂²⁺/ReO₄^- concentrations, ionic strength, as well as interfering ions were studied to evaluate the sorption performance of MILP composites. The maximum sorption capacity was 416.67 mg/g for UO₂²⁺ at pH 5.5 and 434.78 mg/g for ReO₄^- at pH 3.5, levels that are superior to those of most adsorbents. The sorption of UO₂²⁺/ReO₄^- occurred in a pH-dependent, spontaneous and endothermic manner, which showed preferable modeling by the pseudo-second-order (PSO) kinetic equation and Freundlich isotherm equation. The adsorption of ReO₄^- was inhibited by the coexistence of UO₂²⁺ and high ion strength. Batch experiments and X-ray photoelectron spectroscopy (XPS) results indicate that UO₂²⁺/ReO₄^- sorption was driven by the abundant amino groups and unsaturated metal sites in the MILP-3 composites. MILP-3 also showed excellent recycling performance and maintained high sorption capacities for UO₂²⁺/ReO₄^- in different simulated water samples. This study shows that MILP composites can effectively extract radioactive metal cations and anions from water, and lays a foundation for designing an excellent new category of candidates with versatile functions for wastewater management.