Reduction-controlled silver nanoparticle decoration of MIL-101(Cr) metal organic frameworks for effective U(vi)...

This study presents the modification of azide-functionalized MIL-101(Cr) metal-organic framework (MOF) with silver nanoparticles to improve U(vi) sorption from aqueous media. Deposition of Ag nanoparticles (Ag NPs) onto the MOF surface was carried out by in situ reduction using different reducing agents: dimethylamine borane (DMAB), hydrazine hydrate (HH), ascorbic acid (Asc), and sodium borohydride (SBH), which enabled systematic variation in the size, distribution, and content of silver in the composites. The obtained materials were characterized by FTIR, XPS, XRD, SEM, TEM, and BET analyses, indicating retention of the main MIL-101(Cr)-based structural features together with the formation of metallic Ag NPs. The size and distribution of Ag nanoparticles depended strongly on the reducing agent and were associated with changes in U(vi) uptake behavior. Batch experiments showed that Ag modification increased U(vi) uptake compared with the parent MOF; the experimental equilibrium capacity increased from 2.7 mg g^-1 for MIL-101(Cr)-N₃ to 9.45-32.127 mg g^-1 for the Ag-modified composites, with the highest values observed for MOF@HH and MOF@SBH. Although MOF@HH and MOF@SBH showed comparable equilibrium capacities, MOF@HH reached equilibrium faster, indicating more favorable kinetic behavior. The Langmuir-fitted maximum sorption capacity was approximately 27 mg g^-1. Kinetic analysis indicated pseudo-second-order behavior, and diffusion modeling suggested mixed surface and intraparticle transport contributions. Selectivity experiments gave a U(vi) distribution coefficient of approximately 600 mL g^-1 under the tested competitive aqueous conditions, with MOF@HH showing the most favorable U(vi) selectivity among the investigated composites. Reusability tests showed that MOF@HH retained approximately 75% of its sorption capacity after five regeneration cycles, supporting its reusability under the tested conditions. Overall, the results suggest that reduction-controlled Ag nanoparticle formation affects site accessibility, pore/interfacial structure, and U(vi) adsorption performance. Among the investigated systems, MOF@HH emerged as the most favorable sample considering its high U(vi) uptake, faster equilibrium compared with MOF@SBH, and preferential U(vi) selectivity under the tested conditions. This study therefore provides insight into Ag nanoparticle incorporation as a complementary modification strategy for MOF-based radionuclide sorbents, provided that Ag loading, dispersion, and pore accessibility are properly controlled.