A research team led by Xiangke Wang and Hui Yang at North China Electric Power University and Shenqian Ma at the University of North Texas, USA, recently developed a novel design strategy for constructing hydrazone-connected flexible COF photocatalysts. By introducing flexible connectors with varying bending capabilities, the researchers achieved precise control over the local pore curvature and photoelectric properties of COFs, promoting the separation and transport of photogenerated carriers. The constructed flexible COF photocatalyst, COF-3, exhibited excellent photocatalytic uranium removal performance in both contaminated groundwater and tap water. Mechanistic studies revealed that COF-3 can generate H₂O₂ in situ under visible light irradiation and convert dissolved uranyl ions into insoluble (UO₂)O₂ · 2H₂O, achieving efficient capture. This research provides a new design approach for the application of flexible COFs in the photocatalytic remediation of radioactive pollutants. The article was published as an open access Research Article in CCS Chemistry , the flagship journal of the Chinese Chemical Society.
Background information:
The rapid growth of global energy consumption and the gradual depletion of fossil fuels have triggered an urgent demand for renewable and clean energy. Nuclear energy, as a safe and environmentally friendly energy source, is considered one of the measures to address the global energy crisis. Uranium, as a strategic resource, is an indispensable fuel in nuclear energy applications. However, with the continuous mining of uranium and the ongoing expansion of the nuclear industry, uranium contamination has emerged in groundwater, tap water, and seawater, posing a significant potential threat to ecosystems and human health. Once it enters the human body through the air, water, or food chain, it can cause internal radiation damage and increase the risk of cancer. Uranium contamination is insidious and persistent in the environment, migrating through groundwater and accumulating in organisms, making remediation difficult and costly, and posing a long-term risk to ecosystems and public health. Therefore, developing efficient technologies and methods for extracting or removing uranium from contaminated groundwater and other water bodies has gradually become a focus of research for scholars.
Highlights of this article :
To address the above issues, the researchers synthesized a series of highly crystalline flexible COFs (COF-1, COF-2, and COF-3) using a solvothermal method by introducing ligands with different bending capabilities into the COF framework. These flexible COFs exhibit high specific surface area, abundant porosity, a wide light absorption range, and rapid photogenerated charge separation capabilities, demonstrating excellent photocatalytic uranium activity in various water bodies, including groundwater. COF-1, COF-2, and COF-3 were synthesized using 1,3,5-tris(formylphenyl)benzene (TFPB) with hydrazine hydrate (N₂H₄), carbohydrazine (CHYD), and oxaloyldihydrazine (ODH) under solvothermal conditions (Figure 1).
Structural and photoelectric characterization were used to investigate the structural features, physicochemical properties, and photoelectric properties of three COF frameworks with different bending angles. COF-3 exhibited the largest bending angle, demonstrating excellent photoelectric response; the photoelectric properties of COF-2 and COF-1 deteriorated with decreasing bending angles (Figure 2). The high crystallinity, large specific surface area, and stability of these COFs provide a foundation for efficient photocatalytic uranium extraction.
In photocatalytic uranium extraction experiments in groundwater, without the need for additional sacrificial reagents, COF-3 achieved a 92% removal rate within 4 hours in groundwater containing 20 ppm uranium, ultimately exhibiting a removal capacity of 403.6 mg/g. The removal rates of COF-1 and COF-2 were 34.6% and 85.6%, respectively (Figure 3). In photocatalytic uranium extraction experiments in tap water, COF-1, COF-2, and COF-3 achieved removal rates of 53.8%, 94.4%, and 96.2% within 10 hours, respectively. The experimental results indicate that the flexible unit plays a crucial role in enhancing photocatalytic uranium extraction, and the photocatalytic activity increases with the bending angle of the structure. Furthermore, COF-3 exhibits broad pH adaptability and photocatalytic selectivity under interference from competing ions ten times greater than uranium, demonstrating significant potential for practical applications in uranium extraction.
The products of the COF-3 photocatalytic uranium reactionwere analyzed using PXRD, FT-IR, XPS, and TEM. The results showed that soluble uranium in solution was converted to insoluble (UO₂)O₂ · 2H₂O under photocatalysis, thus achieving photocatalytic uranium capture. Photocatalytic H₂O₂ production performance and photocatalytic free radical quenching experiments in different solutions indicated that H₂O₂ is crucial in uranium removal. H₂O₂ can combine with uranyl ions to generate (UO₂)O₂ · 2H₂O, thereby enabling photocatalytic uranium extraction from groundwater and tap water (Figure 4). By adjustingthe flexible units in the COFs ligands and controlling the local curvature of the COFs pores, the bending angle of the flexible COFs structure was adjusted, promoting these paration and migration of charge carriers during photocatalysis, thereby improving the photocatalytic activity. The results indicate that flexible COF photocatalysts have great potential for uranium removal from polluted water sources and other applications.
Summary and Outlook:
In summary, this paper proposes a design strategy to enhance photocatalytic performance by modulating the flexible units of covalent organic frameworks (COFs). The flexible connectives exhibit gradually increasing bendability, allowing the COF frameworks to have varying degrees of flexibility (180°, 120°, and 60° dihedral angles). Studies show that COF-3 with high local pore curvature can promote carrier separation and reduce electron-hole recombination during the photocatalytic process. COF-3 demonstrates excellent uranium removal capabilities in contaminated groundwater (removal rate >96%) and tap water (removal rate >96%). Furthermore, COF-3 maintains good photocatalytic activity over a wide pH range and under high ionic strength. These results demonstrate the significant potential of flexible COF photocatalysts in uranium removal, providing theoretical and experimental support for the design of highly active COF photocatalysts.
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About the journal: CCS Chemistry is the Chinese Chemical Society’s flagship publication, established to serve as the preeminent international chemistry journal published in China. It is an English language journal that covers all areas of chemistry and the chemical sciences, including groundbreaking concepts, mechanisms, methods, materials, reactions, and applications. All articles are diamond open access, with no fees for authors or readers. More information can be found at https://www.chinesechemsoc.org/journal/ccschem .
About the Chinese Chemical Society: The Chinese Chemical Society (CCS) is an academic organization formed by Chinese chemists of their own accord with the purpose of uniting Chinese chemists at home and abroad to promote the development of chemistry in China. The CCS was founded during a meeting of preeminent chemists in Nanjing on August 4, 1932. It currently has more than 120,000 individual members and 184 organizational members. There are 7 Divisions covering the major areas of chemistry: physical, inorganic, organic, polymer, analytical, applied and chemical education, as well as 31 Commissions, including catalysis, computational chemistry, photochemistry, electrochemistry, organic solid chemistry, environmental chemistry, and many other sub-fields of the chemical sciences. The CCS also has 10 committees, including the Woman’s Chemists Committee and Young Chemists Committee. More information can be found at https://www.chinesechemsoc.org/ .
CCS Chemistry
10.31635/ccschem.025.202506940
Experimental study
Not applicable
Flexible Units in Covalent Organic Frameworks Promote Photocatalytic Uranium Extraction from Wastewater
5-Jan-2026
There is no conflict of interest to report.