Acid-Resistant Luminescent Si/UiO-66-Amidoxime (AO) Nanostructures for Rapid and Efficient Recovery of U(VI): Experimental and Theoretical Studies

Despite the extremely favorable properties of metal-organicframeworks (MOFs) for multifunctional applications, the challengeto synthesize MOFs with high chemical, radiation, and mechanical stabilitystill remains a major challenge. To circumvent this issue, we havedeveloped a chemically, thermally, and radiation-stable silicon-shieldedamidoxime-based nanocrystalline MOF, Si/UiO-66-AO, using UiO-66-NH2 and employed for the removal of U-(VI) from both acidic andaqueous media (pH). The developed Si/UiO-66-AO MOF demonstrated avery facile, rapid, and efficient recovery of U-(VI). The MOF exhibitedan excellent sorption capacity from both aqueous (similar to 217 mg/g)and acidic media (similar to 65 mg/g) with a sorption efficiency of similar to 70-75%(acidic and aqueous media), and capacities were fairly higher thanthat for other reported sorption materials. The synthesized MOF showedvery rapid extraction ability (similar to 5 min) toward U-(VI) from bothacidic and aqueous media. Si/UiO-66-AO also showed an elevated extractionefficiency of 31.6 mg/g from simulated seawater, suggesting its significantpotential for the extraction of U-(VI) from seawater. The bonding betweenU-(VI) and Si/UiO-66-AO was confirmed by X-ray photoelectron spectroscopy(XPS) and extended X-ray absorption fine structure (EXAFS) analysis.Density functional theory (DFT) calculations were performed to understandthe nature of bonding of the functional moiety (amidoxime group) ofthe MOF with U-(VI). Such high adsorption efficiency and efficientrecovery of U-(VI) by Si/UiO-66-AO is facilitated by complex formationthrough monodentate coordination of the oxime nitrogen atom of theamidoxime group with U-(VI); hydrogen bonding plays an important rolein stabilizing the same. Si/UiO-66-AO also exhibited green fluorescencewith a quantum yield of similar to 6%, highlighting its multifunctionalpotential. The present study provides a successful path for the developmentof acid-stable MOFs for fulfilling the needs of commercial applicationsfor U-(VI) recovery, especially in the plant-scale applications ofnuclear field, seawater, as well as from environmental samples.

Automated summary

A chemically, thermally, and radiation-stable Si/UiO-66-AO nanocrystalline MOF has been synthesized for the efficient removal of U-(VI) from acidic and aqueous media. The MOF demonstrated a high sorption capacity and rapid extraction ability, with a potential for U-(VI) extraction from seawater. The bonding between U-(VI) and Si/UiO-66-AO was confirmed, and the study provides a successful path for the development of acid-stable MOFs for commercial U-(VI) recovery applications.