Optimizing covalent organic frameworks properties for uranium uptake: The role of alcohol in aqueous precipitation

Covalent Organic Frameworks (COFs) have emerged as promising adsorbents for uranium (UO₂²⁺) recovery, offering high stability under acidic conditions where conventional adsorbents typically degrade. However, their conventional solvothermal synthesis is hindered by hazardous solvents, prolonged high-temperature processing, and high energy demands, limiting scalability and sustainability. The aqueous dissolution–precipitation (DP) method, mediated by imidazole, provides a more environmentally benign alternative. Yet, systematic investigations into the role of alcohol precipitants in tailoring COF structural and functional properties remain scarce, particularly in the context of uranium adsorption. This study introduces a novel approach by systematically evaluating methanol (MeOH), ethanol (EtOH), isopropanol (IPA), and butanol (BuOH) as precipitants in DP synthesis. The results reveal that alcohol selection critically governs COF morphology, crystallinity, surface area, and sulfonate (-SO₃H) group distribution, thereby influencing uranium adsorption performance. Among the synthesized materials, isopropanol-derived COF-IPA exhibited superior characteristics, including a porous architecture (36.75 m²/g specific surface area), uniform sulfonate incorporation (11.27% sulphur), and a highly negative surface charge (−99.9 mV zeta potential). Structural analyses (NMR, FTIR, XRD) confirmed the formation of stable β-ketoenamine frameworks. COF-IPA achieved a maximum uranium adsorption capacity of 833 mg/g, fitting the Langmuir model (R² = 0.995), outperforming EtOH-precipitated (625 mg/g), BuOH-precipitated (714 mg/g), and solvothermal COFs (625 mg/g). The enhanced performance of COF-IPA is attributed to improved sulfonate site accessibility and monolayer chemisorption via coordination bonding. These findings establish alcohol precipitant selection as a decisive factor in optimizing DP-COF synthesis and position COF-IPA as a sustainable, high-performance adsorbent for uranium recovery and environmental remediation.