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

Abstract

Despite the extremely favorable properties of metal–organic frameworks (MOFs) for multifunctional applications, the challenge to synthesize MOFs with high chemical, radiation, and mechanical stability still remains a major challenge. To circumvent this issue, we have developed a chemically, thermally, and radiation-stable silicon-shielded amidoxime-based nanocrystalline MOF, Si/UiO-66-AO, using UiO-66-NH2 and employed for the removal of U(VI) from both acidic and aqueous media (pH). The developed Si/UiO-66-AO MOF demonstrated a very facile, rapid, and efficient recovery of U(VI). The MOF exhibited an excellent sorption capacity from both aqueous (∼217 mg/g) and acidic media (∼65 mg/g) with a sorption efficiency of ∼70–75% (acidic and aqueous media), and capacities were fairly higher than that for other reported sorption materials. The synthesized MOF showed very rapid extraction ability (∼5 min) toward U(VI) from both acidic and aqueous media. Si/UiO-66-AO also showed an elevated extraction efficiency of 31.6 mg/g from simulated seawater, suggesting its significant potential for the extraction of U(VI) from seawater. The bonding between U(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 understand the nature of bonding of the functional moiety (amidoxime group) of the MOF with U(VI). Such high adsorption efficiency and efficient recovery of U(VI) by Si/UiO-66-AO is facilitated by complex formation through monodentate coordination of the oxime nitrogen atom of the amidoxime group with U(VI); hydrogen bonding plays an important role in stabilizing the same. Si/UiO-66-AO also exhibited green fluorescence with a quantum yield of ∼6%, highlighting its multifunctional potential. The present study provides a successful path for the development of acid-stable MOFs for fulfilling the needs of commercial applications for U(VI) recovery, especially in the plant-scale applications of nuclear field, seawater, as well as from environmental samples.