Alkynyl-Based sp 2 Carbon-Conjugated Covalent Organic Frameworks with Enhanced Uranium Extraction from Seawater by Photoinduced Multiple Effects

Open AccessCCS ChemistryRESEARCH ARTICLE1 Nov 2021Alkynyl-Based sp2 Carbon-Conjugated Covalent Organic Frameworks with Enhanced Uranium Extraction from Seawater by Photoinduced Multiple Effects Cheng-Rong Zhang†, Wei-Rong Cui†, Rui-Han Xu, Xiao-Rong Chen, Wei Jiang, Yi-Di Wu, Run-Han Yan, Ru-Ping Liang and Jian-Ding Qiu Cheng-Rong Zhang† College of Chemistry, Nanchang University, Nanchang 330031 †C.-R. Zhang and W.-R. Cui contributed equally to this work.Google Scholar More articles by this author , Wei-Rong Cui† College of Chemistry, Nanchang University, Nanchang 330031 †C.-R. Zhang and W.-R. Cui contributed equally to this work.Google Scholar More articles by this author , Rui-Han Xu College of Chemistry, Nanchang University, Nanchang 330031 Google Scholar More articles by this author , Xiao-Rong Chen College of Chemistry, Nanchang University, Nanchang 330031 Google Scholar More articles by this author , Wei Jiang College of Chemistry, Nanchang University, Nanchang 330031 Google Scholar More articles by this author , Yi-Di Wu College of Chemistry, Nanchang University, Nanchang 330031 Google Scholar More articles by this author , Run-Han Yan College of Chemistry, Nanchang University, Nanchang 330031 Google Scholar More articles by this author , Ru-Ping Liang College of Chemistry, Nanchang University, Nanchang 330031 Google Scholar More articles by this author and Jian-Ding Qiu *Corresponding author: E-mail Address: [email protected] College of Chemistry, Nanchang University, Nanchang 330031 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000618 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Biofouling is a major obstacle to the efficient extraction of uranium from seawater due to the numerous marine microorganisms in the ocean. Herein, we report a novel amidoxime (AO) crystalline covalent organic framework (BD-TN-AO) by Knoevenagel condensation reaction of 2,2′,2″-(benzene-1,3,5-triyl)triacetonitrile (TN) and 4,4′-(buta-1,3-diyne-1,4-diyl)dibenzaldehyde (BD) that is highly conjugated and possesses excellent photocatalytic activity. The excellent photocatalytic activity endows the BD-TN-AO high anti-biofouling activity by producing biotoxic reactive oxygen species (ROS) and photogenerated electrons to efficiently reduce the loaded U(VI) to insoluble U(IV). Meanwhile, the surface-positive electric field has strong electrostatic attraction to the negative [UO2(CO3)3]4− in seawater, which can significantly enhance the extraction capacity of uranium. Benefiting from these outstanding photoinduced effects of BD-TN-AO, the adsorbent exhibits a high uranium adsorption capacity of 5.9 mg g−1 under simulated sunlight irradiation in microorganism-containing natural seawater, which is 1.48 times the adsorption capacity in darkness. Download figure Download PowerPoint Introduction Uranium is essential for the sustainable development of the nuclear industry,1–3 the uranium content in the ocean (about 4.5 billion tons) is almost 1000 times that of uranium on land.4,5 To achieve continuous development of nuclear energy and environmental protection, it is of considerable significance to develop effective technology for extracting uranium from natural seawater.6 Owing to economics and versatility, chemical adsorption is considered to be the most promising technology for extracting uranium from seawater.7,8 Recently, numerous porous materials have been developed for uranium extraction,9–13 such as porous aromatic frameworks (PAFs),14,15 metal–organic frameworks (MOFs),6,16 and porous organic polymers (POPs);17 all of them exhibit impressive uranium extraction performance. However, most adsorbents have poor stability in complex marine environments, leading to decomposition during long-term immersion in the ocean. Therefore, it is still a great challenge to construct a highly stable uranium adsorbent with good recyclability in complex marine environments.18,19 In the complex marine environment, severe marine biofouling is caused by the attachment of a mass of marine microorganisms, which is one of the most critical factors affecting the uranium extraction from the seawater.5,20 Recently, a few reports have synthesized antibacterial adsorbents with excellent performance by covalently cross-linking a broad-spectrum antibiotic into adsorbents18 or doping silver ions on chitosan–graphene oxide foam substrate.1 These methods can significantly improve the extraction ability of uranium from seawater. However, biotoxicity and leaching of precious metal nanoparticles may have a negative impact on the marine environment while sterilizing the adsorbent. Therefore, development of new functionalized adsorbents with high stability and antibacterial properties to extract uranium from seawater still remains a challenge.18 Covalent organic frameworks (COFs) represent a new class of light-weight crystalline porous material with regular pore structure, excellent chemical stability, and ultrahigh specific surface area.21–24 Their porous structural features and properties, versatility of framework structures, and high surface area make them promising materials for uranium extraction.25,26 However, most reported amidoxime-based COFs, such as COF-TpDb-AO11 and o-TDCOF,9 are linked by dynamically reversible imine bonds and may undergo hydrolysis under extreme conditions, which greatly impedes the real utilization of COFs.25 To overcome this challenge, a new class of carbon–carbon double-bond linkage sp2 carbon-conjugated COFs, including sp2c-COF,27 TP-COF,25 g-C34N6-COF,28 NDA-TN,29 and TFPT-BTAN COF,26 was reported and synthesized by the Knoevenagel condensation reaction. Compared with other imine-linked COFs, sp2 carbon-conjugated COFs, benefiting from the C=C linkages, have an excellent chemical stability even exposed to strong acidic, alkaline, and radiation conditions,25,26 which is a prerequisite for the extraction of uranium from seawater. Meanwhile, it has been proven that –C=C– is a key π-bridge unit, which can enhance the charge-carrier mobility or extend π-conjugation.30,31 Therefore, the COF linked by irreversible –C=C– has excellent photoelectric and charge-transfer properties.30,31 The alkynyl unit with a highly conjugated structure is conducive to improving the electron mobility and offering abundant active sites for photogenerated electrons.32,33 In the complex marine ecological environment, it is reasonable to construct alkynyl-functionalized COFs by integrating an alkynyl unit into the stable sp2 carbon-conjugated skeleton, which could generate reactive oxygen species (ROS) to achieve anti-biofouling activity, thus eventually enhancing the uranium extraction capacity from seawater. Herein, a novel amidoxime-functionalized sp2 carbon-conjugated COF (named BD-TN-AO), which consisted of diacetylene (–C≡C–C≡C–) moieties, was prepared by base-catalyzed Knoevenagel condensation reaction. Based on the highly conjugated features of the diacetylene (–C≡C–C≡C–) unit and the unique π-electron communication of the –C=C– linkage, BD-TN-AO exhibited excellent photoelectric and photocatalytic properties. Therefore, it could produce biotoxic ROS and photogenerated electrons, thereby resulting in extremely high anti-biofouling activity and efficient reduction of the soluble U(VI) to insoluble U(IV). Meanwhile, the surface-positive electric field, which was generated by the photoelectric effects, exhibited strong electrostatic attraction to the negative [UO2(CO3)3]4− in seawater. Therefore, the photoactivated BD-TN-AO exhibited a great uranium extraction capacity of 5.9 mg g−1, which was 1.48 times as that of the BD-TN-AO in bacteria-containing natural seawater under dark conditions. Experimental Section Details of synthesis and characterization of the target materials, as well as charge-carrier mobility measurement, 13C cross-polarization/magic-angle spinning (CP/MAS) solid-state NMR spectroscopy, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), N2 adsorption–desorption isotherms, adsorption capacity measurements, uranium extraction device, electron paramagnetic resonance (EPR) measurements, powder X-ray diffraction (PXRD) measurements, and Fourier transform infrared (FT-IR) measurements are presented in the Supporting Information. Results and Discussion The synthesis of BD-TN COF by the reaction between 2,2′,2″-(benzene-1,3,5-triyl)triacetonitrile (TN) and 4,4′-(buta-1,3-diyne-1,4-diyl)dibenzaldehyde (BD) was challenging. To overcome the low reversibility of the Knoevenagel reaction, we finally found an optimal experimental condition for the synthesis of highly crystalline BD-TN. In the presence of KOH as the catalyst, the solvent was a mixture of o-dichlorobenzene (o-DCB) and n-butyl alcohol (n-BuOH). The mixture was kept at 120 °C without interference for 4 days (Figure 1a and Supporting Information Figure S1 and Table S1). To highlight the advantages of COF containing diacetylene (–C≡C–C≡C–), another COF (ED-TN) was synthesized by condensing 4,4′-(ethyne-1,2-diyl)dibenzaldehyde (ED) and TN monomers under the same reaction conditions (Figure 1a and Supporting Information Figure S1 and Table S2). PXRD experiments were carried out to evaluate the crystal structure of ED-TN and BD-TN COFs (Figures 1b and 1c, black cross). Both spectra showed intense and sharp diffraction in the low-angle region at 2.9° and 2.5° for ED-TN and BD-TN, respectively, which can be assigned to the (100) reflections, indicating a highly crystalline structure.32 The diffraction peaks at 5°, 5.8°, and 7.6° were indexed as the (110), (200), and (210) reflections for ED-TN COF.34 Similarly, the PXRD pattern of BD-TN COF exhibited other distinguishable peaks at 4.4°, 5.1°, and 6.7°. After geometrical optimization of the models, their simulated PXRD patterns were calculated and compared with the respective experimentally measured patterns ( Supporting Information Tables S3 and S4). The PXRD profiles calculated for eclipsed (AA) stacking model matched well with the experimental PXRD profiles for both ED-TN and BD-TN COFs (Figures 1b and 1c, violet curves). According to the simulated structure of ED-TN and BD-TN, Pawley refinement was performed for the experimentally obtained PXRD profile (Figures 1b and 1c, red curves). The results showed excellent correlation between the experimental data and the eclipsed (AA) stacking model simulated structure with negligible difference for ED-TN and BD-TN (Figures 1b and 1c, blue curves). In contrast, the simulated pattern of the staggered (AB) layer stacking model did not match the experimental PXRD patterns of the ED-TN and BD-TN, and the relative intensity of diffraction peaks had a significant deviation from the experimental data ( Supporting Information Figure S2). Figure 1 | (a) Schematic diagram of the preparation of ED-TN and BD-TN COFs. PXRD profiles of (b) ED-TN and (c) BD-TN COFs: the Pawley-refined patterns (red curves), experimentally observed patterns (black cross), simulated patterns with AA-stacking mode (violet curves), Bragg positions (green bar), and the refinement differences (blue curves). Insets: structural models of ED-TN and BD-TN COFs assuming the eclipsed (AA) stacking mode. Download figure Download PowerPoint The porosities of the ED-TN and BD-TN COFs were measured by N2 adsorption isotherms at 77 K. The Brunauer–Emmett–Teller (BET) surface areas of ED-TN and BD-TN were calculated as 836 and 940 m2 g−1 (Figures 2a and 2b), respectively. The moderate surface area may be attributed to the low degree of π−π stacking among the adjacent COF layers in acetylene and diacetylene COFs, causing an offset stacking within the framework.32 A similar phenomenon has been previously found in acetylene and diacetylene functionalized β-ketoenamine COFs.32 The pore-size distribution of COFs was measured by the nonlocal density functional theory (NLDFT) model, revealing the pore-size distributions had one prominent peak at 3.07 and 3.58 nm for the ED-TN and BD-TN, respectively ( Supporting Information Figure S3), which correlated well with the eclipsed (AA) layer stacking models (3.2 and 3.7 nm). The pore volumes of ED-TN and BD-TN were calculated as 0.39 and 0.41 cm3 g−1, respectively ( Supporting Information Figure S3). FT-IR spectroscopy was used to evaluate the chemical composition of COFs. As shown in the FT-IR spectrum (Figures 2c and 2d), the characteristic signal peaks attributed to C=O and C–H stretching vibration of ED-TN and BD-TN at 1692 and 2848 cm−1 disappeared.27 Meanwhile, the –CN stretching vibration at 2246 cm−1 was observed,35 indicating a high degree of polymerization between TN and ED or BD. The structures of COFs were further confirmed via 13C CP/MAS NMR spectroscopy ( Supporting Information Figure S4). The 13C CP/MAS spectra of ED-TN and BD-TN revealed that the aldehyde carbon peak at ∼186 and ∼192 ppm of ED and BD disappeared.36 Meanwhile, the peak at ∼22 ppm for methylene carbon (–CH2CN) of TN disappeared, whereas the peak at ∼112 ppm (–CN) assigned to TN units was shifted to ∼105 ppm in ED-TN and BD-TN (–CN side group) and to ∼107 ppm in model compound ( Supporting Information Scheme S1).11 The peak at ∼87 ppm (ED-TN) was assigned to the acetylene (–C≡C–) functional group,32 and two signals at ∼71 and ∼79 ppm (BD-TN) belonged to diacetylene (–C≡C–C≡C–) functional groups,32 thereby validating the acetylene (–C≡C–) and diacetylene (–C≡C–C≡C–) as present in the as-synthesized COFs skeletons ( Supporting Information Figure S4). These results indicated that we had successfully synthesized alkynyl-functionalized COFs. The morphology of COFs was investigated through SEM. The images showed that both ED-TN and BD-TN exhibited similar porous network structures ( Supporting Information Figure S5). The thermal stability of the synthesized ED-TN and BD-TN was analyzed by TGA. The results showed that ED-TN and BD-TN COFs were stable up to 400 °C ( Supporting Information Figure S6) but lose some weight below 300 °C, which may be attributed to loss of residual solvent molecules. Figure 2 | Nitrogen adsorption–desorption isotherms measured at 77 K for (a) ED-TN and (b) BD-TN. (c) Infrared spectra comparison of the ED-TN, monomer ED, and monomer TN. (d) FT-IR spectra comparison of BD-TN, monomer BD, and monomer TN. PXRD patterns of (e) ED-TN-AO and (f) BD-TN-AO after immersion in different solutions. Download figure Download PowerPoint Amidoxime-functionalized COFs (ED-TN-AO and BD-TN-AO) were obtained by treating COFs with hydroxylamine hydrochloride ( Supporting Information Scheme S2). The PXRD pattern of amidoxime-functionalized COFs exhibited a diffraction comparable with the one of ED-TN and BD-TN with an intense reflection at the low-angle region (Figure 2a), suggesting that the synthesized materials maintain the excellent crystal structural after amidoximation ( Supporting Information Figure S7). The BET surface areas were measured as 508 and 639 m2 g−1, and exhibited a uniform pore size at 2.91 and 3.38 nm, with pore volumes of 0.36 and 0.37 cm3 g−1 for ED-TN-AO and BD-TN-AO, respectively ( Supporting Information Figure S3), suggesting the retention of porosity after the amidoximation process.11 To prove the successful conversion of cyano groups to amidoxime groups, FT-IR and solid-state 13C CP/MAS NMR spectroscopies were performed. From the FT-IR spectrum of amidoxime-functionalized COFs, new characteristic signal peaks appearing at 1657 and 956 cm−1 were attributed to C=N and N–O, respectively.25 Meanwhile, the stretching vibration peak of –CN at 2246 cm−1 disappeared ( Supporting Information Figure S8). Furthermore, solid-state 13C NMR analysis demonstrated this efficient conversion, as proved by the disappearance of the –CN peak at ∼105 ppm, and a newly formed peak at ∼169 ppm was attributed to C=N ( Supporting Information Figures S9 and S10).11 At the same time, the morphology of ED-TN-AO and BD-TN-AO did not change compared with ED-TN and BD-TN ( Supporting Information Figure S5), which implied structural integrity during chemical conversion. Besides, ED-TN-AO and BD-TN-AO exhibited an impressive chemical stability and can still maintain good crystal structure under strongly acidic conditions (1, 3, and 5 M HNO3), high radiation (γ-ray with dose of 200 kGy) (Figures 2e and 2f), and different basic conditions (1 M, 5 M KOH, 5 M Cs2CO3, and 5 M triethylamine) ( Supporting Information Figure S11). In addition, after exposure to the above conditions, the changes in the FT-IR spectrum were negligible ( Supporting Information Figures S12 and S13). Meanwhile, even upon immersion in complex seawater for 30 days, both amidoxime-functionalized COFs maintained good crystal structure (the pH of seawater is limited to the range of 7.5–8.4) ( Supporting Information Figures S11 and S12). Excellent stability is essential for the application of the adsorbent to extract uranium from actual seawater. To fully reflect the outstanding stability of ED-TN-AO and BD-TN-AO, two imine-based COFs 4,4',4''-(1,3,5-triazine-2,4,6-triyl)trianiline-benzene-1,3,5-tricarbaldehyde (TTA-Tb) and 4,4',4''-(1,3,5-triazine-2,4,6-triyl)trianiline-benzene-2,4,6-triformylphloroglucinol (TTA-Tp) were synthesized by using the reported method.37 After exposure to high concentrations of nitric acid (3.0 and 5.0 M), the crystallinity of TTA-Tb and TTA-Tp was completely destroyed ( Supporting Information Figure S14). The results showed that ED-TN-AO and BD-TN-AO had excellent stability. To further understand the photoelectric performance of ED-TN-AO and BD-TN-AO, photoelectrochemical (PEC) characterizations were performed. The UV–vis diffuse reflection spectroscopy of ED-TN-AO exhibited a narrow absorption edge around 500 nm, whereas the absorption tail of BD-TN-AO extended up to approximately 700 nm (Figure 3a). The optical band gaps of ED-TN-AO and BD-TN-AO estimated through the Kubelka–Munk equation were 2.51 and 2.47 eV, respectively ( Supporting Information Figure S15).38 According to the Mott–Schottky plot, the positive slopes of the plot suggest that amidoxime-functionalized COFs were n-type semiconductors.39 The conduction band of BD-TN-AO was −0.678 V versus the reversible hydrogen electrode (RHE) (Figure 3b), which was lower than ED-TN-AO (−0.5 V) ( Supporting Information Figure S16); both conduction bands of amidoxime-functionalized COFs were more negative than the redox reaction potential of U(VI) to U(IV), indicating that the energy levels of amidoxime-functionalized COFs were sufficient for reduction of U(VI).40 Figure 3 | (a) UV–vis–near-infrared diffuse reflectance spectra. (b) Mott−Schottky analysis of BD-TN-AO. (c) Steady-state PL emission spectra. (d) PL decay spectra. (e) EIS Nyquist plots. (f) Transient photocurrent responses for both amidoxime functionalized COFs. Download figure Download PowerPoint The steady-state photoluminescence (PL) spectra of BD-TN-AO showed obvious PL quenching compared with ED-TN-AO (Figure 3c). In addition, the PL decay curves of the ED-TN-AO and BD-TN-AO indicated that their average lifetimes were 1.38 and 3.39 ns, respectively (Figure 3d and Supporting Information Table S5). The above results showed that the recombination between photogenerated carriers and BD-TN-AO was greatly suppressed because of the conjugated diacetlylene moiety (–C≡C–C≡C–), which plays a vital role in improving photochemical activity. This can be further confirmed by the arc diameter of electrochemical impedance spectra (EIS) for BD-TN-AO, which was smaller than ED-TN-AO; compared with ED-TN-AO, BD-TN-AO showed better photocatalytic current response (Figures 3e and 3f).32 To evaluate the uranium extraction performance of two amidoxime-functionalized COFs, the effects of uranium extraction capacity were analyzed in simulated seawater under dark and visible light irradiation conditions. In dark, BD-TN-AO exhibited higher extraction capacity for uranium than ED-TN-AO (Figure 4a), which might be due to the large specific surface area of BD-TN-AO that provided more binding sites for uranium. Under visible light irradiation, the uranium extraction of BD-TN-AO increased from 450 (dark) to 562 mg g−1 (light). The irradiation of the BD-TN-AO by visible light irradiation caused a 20% increase in uranium adsorption capacity, which was higher than the enhanced capacity of ED-TN-AO (17.3%). The possible reason is a better PEC performance of the diacetlylene moiety (–C≡C–C≡C–). Moreover, BD-TN-AO had much higher uranium extraction capacity compared with other adsorbents ( Supporting Information Table S6). Figure 4 | (a) Adsorption isotherms of uranium. (b) High-resolution XPS spectra of U 4f under dark and light conditions. (c) Production of ROS. (d) Mechanism of BD-TN-AO enhanced uranium extraction. Download figure Download PowerPoint Since BD-TN-AO had extraordinary extraction capacity for uranium, XPS analysis was carried out to gain insights into the interaction mechanism between BD-TN-AO and uranium. In the dark condition, the XPS results indicated that the main mechanism of uranium extraction on the surface of BD-TN-AO was the complexation of UO22+ with amidoxime groups, which was consistent with our previous results ( Supporting Information Figures S17 and S18).25 Meanwhile, the high-resolution U 4f XPS spectra of uranium under dark and light irradiation conditions were compared (Figure 4b). The peaks for U(VI) (392.9 and 382.2 eV) and U(IV) (391.6 and 380.9 eV) were clearly observed after light irradiation, indicating that U(VI) and U(IV) coexisted on the surface of BD-TN-AO after the light irradiation.41 Compared with ED-TN-AO EPR spectra, the BD-TN-AO EPR spectra showed higher production of superoxide radicals (•O2−) and singlet oxygen (1O2) (Figure 4c). However, no •O2− or 1O2 was produced under the dark condition ( Supporting Information Figure S19). Meanwhile, these results also indicate that the utilization rate of the electrons was higher than those of holes under light irradiation conditions. The photoelectric effect produced the surface-positive electric field that exhibited electrostatic attraction to the negative [UO2(CO3)3]4−, which further increased the uranium extraction capacity of BD-TN-AO. Therefore, BD-TN-AO extracted uranium under light condition through the following mechanism (Figure 4d). First, the UO22+ was loaded onto the BD-TN-AO surface, which benefited from the unique open one-dimensional channel of COF, large specific surface area, and the affinity of amidoxime groups to UO22+.26 Second, the conduction band of BD-TN-AO was −0.678 V (vs RHE), suggesting it effectively photoreduced the loaded U(VI) to U(IV). Meanwhile, the photocatalytic reduction process could facilitate the regeneration of more functional sites for extraction of additional uranium. Because of its outstanding photoelectric effect, BD-TN-AO could generate the surface-positive electric field that exhibited electrostatic attraction to the negative [UO2(CO3)3]4−.5 Biofouling caused by marine microorganisms will inevitably affect the adsorbents and reduce the capability of uranium extraction from seawater. To study the effect of marine bacteria in seawater on the uranium adsorption performance of BD-TN-AO, we measured the capability of BD-TN-AO to extract uranium from filtered and unfiltered uranium spiked natural seawater. We used a 0.22 μm sterile filter to remove insoluble particles and bacteria from natural sea water. The results indicated that, under dark conditions, natural seawater filtered with a 0.22 μm sterile filter significantly improve the extraction capacity of uranium by 35% (Figure 5a), which showed the marine bacteria had a significant impact on the extraction of uranium from seawater. Meanwhile, under light irradiation conditions, the BD-TN-AO produced •O2− and 1O2 to sterilize bacteria effectively. The extraction capacity of uranium in filtered and unfiltered seawater was only slightly different (increased 7.5%), suggesting that the uranium extraction capacity of BD-TN-AO will not be affected by marine bacteria under light conditions. These results indicated that BD-TN-AO had excellent anti-biofouling properties and can be used in actual extraction of uranium from seawater. At the same time, the anti-biofouling activity of the two amidoxime-functionalized COFs was confirmed using the bacteria as targets.5 The result indicated that the BD-TN-AO exhibited excellent antibacterial activity to the growth of the tested bacterial strains (Figures 5b and 5c), including the Gram-negative bacteria Vibrio alginolyticus and Pseudomonas aeruginosa, and the Gram-positive bacteria Bacillus cereus and Bacillus subtilis. BD-TN-AO showed an inhibition rate > 90% for most strains. It can be observed from the SEM that the bacterial cell structure of V. alginolyticus was destroyed after irradiation and the content of bacterial cells was released from the cells, while the bacterial cells remained intact in the dark condition (Figure 5d). These results indicated that two amidoxime-functionalized COFs could produce biotoxic ROS through photocatalysis reaction (Figure 4c), which gives our COFs adsorbents broad anti-biofouling activity by damaging the organic components of biological entities thereby withstanding biofouling in complex marine bacteria.42 Figure 5 | (a) Influence of marine bacteria on uranium extraction capacity. (b) Antibacterial spectrum. (c) Antibacterial activity. (d) SEM images of BD-TN-AO-treated bacterial cells under light irradiation condition. Download figure Download PowerPoint The recyclability of adsorbents is also an important criterion for real application. Uranium-loaded BD-TN-AO samples can be regenerated by elution with 0.01 M HNO3 solution.6 The removal performance of uranium by BD-TN-AO decreased slightly with increasing number of cycles, with only a 12% reduction observed after six cycles (Figure 6a). At the same time, we used the elution efficiency to measure the quantity of recovered uranium after elution with 0.01 M HNO3. Even after six cycles, the quantity of recovered uranium was still 500 mg g−1 ( Supporting Information Figure S20). In addition, FT-IR spectroscopy confirmed its good recycle performance (Figure 6b). After extraction of UO22+, the appearance of a characteristic O=U=O stretch at 925 cm−1 indicates that UO22+ was adsorbed on the surface of BD-TN-AO. After treatment with 0.01 M HNO3 solution, the characteristic peak of O=U=O disappeared, suggesting that UO22+ was successfully eluted and the BD-TN-AO still retained its integral chemical structure after recycling, which was further confirmed by solid-state 13C NMR ( Supporting Information Figure S21). In addition, after six cycles, the crystallinity of ED-TN-AO and BD-TN-AO was maintained ( Supporting Information Figure S22). The morphology and porous structure of ED-TN-AO and BD-TN-AO were still well preserved ( Supporting Information Figure S23). The N2 adsorption measurements of ED-TN-AO and BD-TN-AO were performed to investigate changing porosity after recycling, and the results showed that isothermal curves were similar to the curves of ED-TN-AO and BD-TN-AO before elution. The BET surface areas were calculated as 451 and 571 m2 g−1. The pore size of 2.75 and 3.26 nm and the pore volumes of 0.30 and 0.32 cm3 g−1 for ED-TN-AO and BD-TN-AO, respectively, indicated retention of COFs’ porosity after the six sorption–desorption cycles ( Supporting Information Figures S24 and S25). Meanwhile, the BD-TN-AO adsorbent still exhibited excellent uranium adsorption performance under highly acidic media conditions. At pH 1.0, the BD-TN-AO still maintained a high extraction capacity of uranium (209 mg g−1) ( Supporting Information Figure S26). To avoid the reaction between the quartz tube and KOH, which generates traces of potassium silicate and causes COF containing impurities, we washed COFs with 0.1 M hydrofluoric acid to remove the generated silicate. Meanwhile, the results showed that after treatment with acid, the adsorption performance did not change significantly (531.6 and 562 mg g−1 for ED-TN-AO and BD-TN-AO) ( Supporting Information Figure S27), suggesting that even if there is a small amount of silicate impurities, its influence on the uranium extraction performance of COFs could be ignored. These result