Phosphonate-Decorated Covalent Organic Frameworks for Actinide Extraction: A Breakthrough Under Highly Acidic Conditions

Open AccessCCS ChemistryRESEARCH ARTICLE1 Aug 2019Phosphonate-Decorated Covalent Organic Frameworks for Actinide Extraction: A Breakthrough Under Highly Acidic Conditions Jipan Yu, Liyong Yuan, Shuai Wang, Jianhui Lan, Lirong Zheng, Chao Xu, Jing Chen, Lin Wang, Zhiwei Huang, Wuqing Tao, Zhirong Liu, Zhifang Chai, John K. Gibson and Weiqun Shi Jipan Yu Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 (China) , Liyong Yuan Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 (China) , Shuai Wang Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 (China) College of Nuclear Science and Engineering, East China University of Science and Technology, Nanchang 330013 (China) , Jianhui Lan Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 (China) , Lirong Zheng Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 (China) , Chao Xu Collaborative Innovation Center of Advanced Nuclear Energy Technology, Institute of Nuclear and New Energy Technology (INET), Tsinghua University, Beijing 100084 (China) , Jing Chen Collaborative Innovation Center of Advanced Nuclear Energy Technology, Institute of Nuclear and New Energy Technology (INET), Tsinghua University, Beijing 100084 (China) , Lin Wang Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 (China) , Zhiwei Huang Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 (China) , Wuqing Tao Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 (China) , Zhirong Liu College of Nuclear Science and Engineering, East China University of Science and Technology, Nanchang 330013 (China) , Zhifang Chai Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 (China) , John K. Gibson Chemical Sciences Division, Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA 94720 (USA) and Weiqun Shi *Corresponding author: E-mail Address: [email protected] Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 (China) https://doi.org/10.31635/ccschem.019.20190005 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Although solid-phase extraction is a useful approach for metal ion separation from aqueous solutions, existing sorbents suffer from low extraction efficiencies and/or instability when in contact with strong acidic media. We report here the first study on rational design and fabrication of phosphonate-decorated covalent organic frameworks, COF-IHEP1 and COF-IHEP2, for efficient and selective extraction of of uranium (VI) [U(VI)] and plutonium(IV) [Pu(IV)] from highly acidic solutions. We found that the negatively charged frameworks with excellent stability under harsh conditions and strong chelating ability from incorporated phosphonate moieties make the COFs superb U(VI) and Pu(IV) sorbents. In 1 M HNO3 solution, COF-IHEP1 achieved recorded U(VI) uptake of 112 mg·g−1, with extremely high selectivity toward Pu(IV) even in the presence of large excess of competing metal cations. Our mechanistic study confirms a sandwich-type microstructure of hydrated U(VI) and Pu(IV) cations bound by the oxygen sites of phosphonate groups. These newly fabricated tailored hybrid porous materials could afford new opportunities to achieve highly efficient actinide extraction from acidic nuclear fuel effluents. Download figure Download PowerPoint Introduction The recent rapid expansion of the nuclear industry presents significant challenges, and thus, opportunities for developing novel materials to improve the nuclear fuel cycle. In particular, advances in the fabrication of new materials and techniques are critically needed to extract uranium [U(VI)] and plutonium [Pu(IV)] from acidic wastewater and mine drainage.1 Although conventional liquid–liquid extractions (e.g., plutonium–uranium recovery by extraction [PUREX]), have been well documented and widely used for processes relative to U(VI) and Pu(IV) separation, practical issues such as the use of toxic or flammable solvents, formation of emulsions, and generation of large volumes of secondary hazardous organic wastes are becoming increasingly tricky. Nonetheless, solid-phase extraction, based on the utilization of solid sorbents with high sorption capacities, has proven advantages of simplicity, reliability, and low solvent consumption, as well as potentially high enrichment factors.2 Traditional ion-exchange resins, for example, are commercial products that exhibit efficient uptake of U(VI) and Pu(IV),3 but their radiation resistances and chemical stabilities under harsh conditions require attention.4 Furthermore, relatively slow sorption kinetics and low ion capacity greatly limit their practical applications.2 Many neutral sorbents based on mesoporous silicas,5 carbons,6 polymers,7 and others8 have also been evaluated for uranium extraction in the last decades, and a current review of this topic is available.9 Under highly acidic conditions, such as what is observed with nuclear fuel cycle-related wastewater treatment, none of the reported materials used were effective, which is most likely due to sorbent disassembly and/or surface protonation.10,11 Thus far, there are no solid materials that fulfill all the requirements for real actinide separations under highly acidic conditions. In recent years, a new family of organic porous and crystalline materials that incorporate strong covalent linkages known as covalent organic frameworks (COFs) has emerged.12,13 Owing to the diversity of skeletons and porous structures, COFs have been exploited in applications such as gas storage and separation,14,15 energy conversion and storage,16 catalysis,17 semiconduction,18 photoemissions,19 proton conduction,20,21 optoelectronics,22,23 and so on.24,25 COFs also offer a platform for designing versatile materials for addressing environmental issues.26–33 Meanwhile, there have been insufficient promising results on this important topic. COFs can be readily prepared and modified by designing suitable precursors and reactions. The strong covalent bonds in COFs endow them with desirable stability under various harsh conditions and are made entirely from light elements (H, B, C, N, and O), thus, have a low density, which offers a clear benefit for improving adsorption capacities. All of these merits make COFs excellent candidates for actinide sorbents under highly acidic conditions, although no clear data on this topic have been disclosed. Herein, we rationally designed and constructed two new COFs, COF-IHEP1 and COF-IHEP2, in which structural units are linked through hydrazone bonds to form extended two-dimensional (2D) nanosheets. Hydrazone linkage was chosen because of readily available phosphonate precursors and mild reaction conditions, as well as a suitable hydrogen-bond system to ensure desirable stability under highly acidic conditions. More importantly, the weaker alkalinity of hydrazone linkage versus amide and amine hinders proton combination and ensures a negatively charged or neutral surface at a low pH, which, undoubtedly, facilitates metal cation trapping from an acidic solution. Phosphonate groups were decorated into the skeletons of COFs to provide active chelating sites, given that phosphoryl groups show desirable selectivity toward U(VI) and Pu(IV) as evidenced in the PUREX process. The prepared COFs exhibit exceptional stability under various harsh conditions, including gamma irradiation, and negatively charged surfaces over a wide pH range. Moreover, the side arm of COF phosphonate units exhibit favorable and selective coordinating properties for U(VI) and Pu(IV), achieving a record of high U(VI) uptake and unprecedented selectivity toward Pu(IV) in 1 M HNO3 solution. To the best of our knowledge, this is the first report on substantial actinide uptake by solid-phase extraction under highly acidic conditions, including the first Pu(IV) trapping in metal–organic frameworks/COFs and also, the first evaluation of radiation stability of COFs. Results and Discussion Material design, synthesis, and characterization The key intermediate, 2,5-bis[2-(diethoxy-phosphoryl)ethoxy]-terephthalohydrazide ( 1), was prepared following four steps with 63% yield (see for details). COF-IHEP1 and COF-IHEP2, which appeared as yellow powders in 84% and 88% yields, respectively, were synthesized via condensation reactions between ( 1) and 1,3,5-Triformylbeneze ( 2) or 1,3,5-Tris(4-formylphenyl)benzene ( 3) in flame-sealed tubes at 120 °C for 3 days; 3∶1 mesitylene/dioxane was used as the solvent, and 6 M acetic acid was the catalyst (Figure 1 and details in ). The resultant two COF-bearing phosphonate units in an ordered porous structure were expected to be promising candidates for U(VI) and Pu(IV) chelation. Scanning electron microscopy images of the prepared COFs were recorded (), and only one morphologically unique crystallite could be found for each COF, suggesting the desirable phase purity. Fourier-transform infrared spectra of the two COFs were compared with those of the precursors ( and ). The characteristic vibrational peaks for C=N bonds at 1618 cm−1 34 for COF-IHEP1 and 1668 cm−1 for COF-IHEP2 were indications of condensation reactions. The blueshift for the C=N bond in COF-IHEP2 was related to the extended π-conjugation in the framework. The thermal stabilities of COF-IHEP1 and COF-IHEP2 were evaluated by thermogravimetric analysis under air flow, which demonstrated that both COFs are thermally stable for up to 240 °C when partial degradation of the phosphate side chain occurred ( and ). Crystalline structures of COF-IHEP1 and COF-IHEP2 were analyzed using powder X-ray diffraction (PXRD) with Cu Kα radiation in conjunction with multiscale computational simulations and Pawley refinement (Figure 2a,b and ). COF-IHEP1 exhibited a strong PXRD peak at 3.55°, weaker peaks at 6.20° and 6.97°, and low-intensity broad features around 12.23°, 15.38°, and 26.56°, corresponding to the (100), (110), (200), (220), (310), and (001) facets, respectively (Figure 2a). Peak broadening was observable at > 5°, which could be ascribed to the disturbing effect of the phosphate side chain, given that the counterpart of COF-IHEP1 without phosphate sidechains (COF-IHEP3) was readily prepared with high crystallinity (). To elucidate the detailed structures of these two COFs, we constructed a series of structural models with AA binarized neural network (bnn) topology and AB grade (gra) topology stacking modes based on molecular mechanics simulations. Our results revealed that for each structural topology, there were two conformations, with cis and trans orientations of the phosphonate side chains (details in ). On the whole, four possible structures of COF-IHEP1 were derived from the simulations: bnn with cis, bnn with trans, gra with cis, and gra with trans. Periodic density functional theory (DFT) calculations were then performed to obtain theoretically accurate lattice parameters and atomic coordinates. Pawley refinements with the PXRD data were carried out to optimize the crystal structures with the AA and AB packing modes. The results suggest that the crystal structure in the AA stacking mode provided the best fit to the PXRD pattern of COF-IHEP1 (Figure 2a, red plot). A hexagonal unit cell (P3) with refined parameters a = b = 28.574 Å, c = 3.598 Å, α = β = 90°, and γ = 120° was deduced for COF-IHEP1 in eclipsed bnn packing mode with a cis orientation of the phosphonate side chains. Changing the orientation of the phosphonate side chains only minimally affected the framework structure and PXRD pattern. The (001) facet corresponded to a π–π stacking distance of about 3.6 Å along the stacking direction perpendicular to the 2D layers ( and and and ). In contrast, the AB stacking mode resulted in small pores covered by neighboring layers and a predicted PXRD pattern that poorly matched the experimental pattern ( and ). At the present resolution, either topology bnn or gra could be assigned. Figure 1 | Synthetic procedures for COF-IHEP1 and COF-IHEP2. Construction of COF-IHEP1 was accomplished by condensation of 2,5-bis[2-(diethoxyphosphoryl)ethoxy]terephthalohydrazide (1) and 1,3,5-Triformylbeneze (2). Formation of COF-IHEP2 was accomplished by treating (1) with 1,3,5-Tris(4-formylphenyl)benzene (3) under solvothermal conditions. Download figure Download PowerPoint Figure 2 | Characterizations of the COFs. Experimental (black) and predicted (red) PXRD patterns of COF-IHEP1 (a) and COF-IHEP2 (b) in bnn packing mode. Inset: AA stacking mode of the COFs. C, gray; N, blue; P, purple; O, red. H atoms are omitted for clarity. N2 adsorption and desorption isotherms of COF-IHEP1 (c) and COF-IHEP2 (d) measured at 77 K. (e) PXRD patterns of COF-IHEP1 before and after the treatment in various solvents. (f) pH-Dependent zeta potentials of COF-IHEP1. Download figure Download PowerPoint COF-IHEP2 exhibits a strong PXRD peak at 2.50°, a weaker peak at 4.83°, and broad features around 15.17° and 25.99°, which were assigned to the (100), (200), (600), and (001) facets of the P3 space group, respectively (Figure 2b). The corresponding π–π stacking distances between neighboring layers were deduced as 3.6 Å. Following the same procedure above, we obtained three possible structures of COF-IHEP2: bnn with trans, gra with cis, and gra with trans. The experimental pattern matches reasonably well with the simulated pattern (Figure 2b, red curve) for the AA stacking structure ( and ); Despite the absence of the predicted peak around 7°, the match is much better than for alternative structures. The refined lattice parameters for COF-IHEP2 in eclipsed bnn packing mode are a = b = 42.912 Å, c = 3.629 Å, α = β = 90°, and γ = 120°. The porosity of both COFs was determined by measuring N2 adsorption–desorption isotherms at 77 K (Figure 2c,d). The Brunauer–Emmett–Teller (BET) surface areas of COF-IHEP1 and COF-IHEP2 were estimated as 110 and 330 m2·g−1, respectively. These values are much smaller than those for COF-IHEP3 (without phosphate side chain, ), which likely originated from the existing phosphate side chain in the pore disturbing transport of the gas molecules. A nonlocal density functional theory model was fitted to the isotherms of COF-IHEP1 and COF-IHEP2 to estimate their pore size distributions (). The average pore diameters of 1.0 nm for COF-IHEP1 and 2.3 nm for COF-IHEP2 were in good agreement with the expected pore sizes from the crystal simulations based on the bnn topology. Chemical stability of the COFs was assessed by soaking COF-IHEP1 in a variety of organic solvents, as well as in 3 M aqueous HNO3 and 1 M NaOH. Comparison of PXRD patterns suggested that COF-IHEP1 is stable in common organic solvents, such as dimethylformamide, tetrahydrofuran, acetone, and hexane, as well as in boiling water. In addition, COF-IHEP1 retains its crystalline structure without discernible changes in the dominant (100) PXRD peak position and intensity even after 24 h treatment with 3 M HNO3 or 1 M NaOH, revealing excellent chemical stability (Figure 2e). In contrast, COF-IHEP2 exhibits relatively poor chemical stability as evidenced by dramatic changes in PXRD features after equal treatment (), likely resulting from its larger pore size. Zeta potentials of the COF surfaces as a function of solution pH were estimated to elucidate the COF surface properties, especially under low pH and highly acidic conditions. Figure 2f shows the zeta potentials of COF-IHEP1 at various pH levels. The value of the zeta potential was found to be zero at pH 1.2 and revealed a negatively charged surface of COF-IHEP1 over a pH range of 1.2–7, undoubtedly facilitating metal cations trapping. U(VI) and Pu(IV) sorption studies After confirming the porosity, structural integrity, and high density of chelating sites of COF-IHEP1 and COF-IHEP2, we examined their abilities to capture U(VI) and Pu(IV) from aqueous solutions. The sorption kinetics data at pH 1.0 ( and ) suggested that U(VI) sorption into both COFs is fast during the initial 20 min, followed by a slower process before reaching equilibrium at ∼ 3 h, and the kinetics was independent of the initial U(VI) concentration (50–188 ppm). The pseudo-second-order model fitted well into the experimental data with a correlation coefficient > 0.999 and rate constants (k) of ca. 1.0 × 10−3 (), revealing fast chemical sorption processes.35 Sorption isotherms for U(VI) into the two COFs were also determined at pH 1.0 by varying the initial U(VI) concentrations from 5 to 200 mg·L−1 (). Both COF-IHEP1 and COF-IHEP2 exhibited very steep sorption profiles for U(VI) with an increase of U(VI) concentration at equilibrium (Ce, positively correlated to initial U(VI) concentrations). At an initial U(VI) concentration over 120 mg/L corresponding to a Ce of > 60 mg/L, U(VI) sorption into the two COFs reached equilibrium from which the derived saturated sorption capacities are 160 and 140 mg·g−1 for COF-IHEP1 and COF-IHEP2, respectively. The new value of 160 mg·g−1 represents the highest U(VI) capture by solid extraction at such a low pH. Both sorption isotherms are well fitted by the Langmuir model with a correlation coefficient of more than 0.99 (), indicating a monolayer uniform sorption mode.36 To further assess the effect of solution acidity on U(VI) removal by the two COFs, U(VI) sorption into COF-IHEP1 from highly acidic HNO3 media and aqueous solutions at various pH was studied. The results (Figure 3a) revealed a strong influence of solution acidity on sorption. In particular, at pH 5, the U(VI) uptake into COF-IHEP1 reached 257 mg·g−1, corresponding to almost complete U(VI) uptake for a 10 mL of 100 mg·L−1 U(VI) solution using only 4 mg of the sorbent. As the solution acidity increased from pH 3.0 to pH 1.0, the U(VI) uptake into COF-IHEP1 decreased to 239 and 155 mg·g−1, respectively. These values, however, were much higher under the same conditions than for most benchmark materials, such as UiO-68-P(O)(OEt)2,1 bio-inspired nanotraps,37 SZ-3,38 NP10,39 KIT-6-80-P,2 MIPAF-11 c40, and MXene.41 More significantly, when highly acidic media were used as is typical for nuclear fuel reprocessing and wastewater treatment, high U(VI) uptake of 112 mg·g−1 in 1 M HNO3 and 70 mg·g−1 in 2 M HNO3 were obtained. These values set a new record for U(VI) uptake under highly acidic conditions. This remarkable performance of U(VI) could be rationalized based on accessible phosphonate active sites well dispersed in the COF channels and zeta potentials of the COF surfaces. Although the surface charge of the COF sorbents changed from negative to positive under acidic conditions (pH < 1.0), the zeta potential was not high enough to repel all the positively charged U cations. In this case, the strong coordination between active phosphonate groups and U cations played a key role, thus leading to a considerable U(VI) uptake. To test adsorbability of the COFs toward other actinides, 239Pu(IV) and 241Am(III) sorption into COF-IHEP1 and COF-IHEP2 from highly acidic HNO3 media and aqueous solutions at various pH was studied. Both COFs achieve Pu(IV) removal as high as ∼ 90% regardless of the solution acidity (Figure 3b), whereas almost no Am(III) sorption occurs in all the solutions. This result suggested that, in addition to U(VI), COF-IHEP1 and COF-IHEP2 could as superb sorbents for Pu(IV) in acidic solutions, which is given that these COFs phosphonate groups that are known to both U(VI) and Pu(IV) in the PUREX process. Figure 3 | U(VI) and Pu(IV) sorption studies with and radiation stability (a) U(VI) sorption into COF-IHEP1 for a wide range of acidity and with that of other benchmark the value in this (b) of Pu(IV) with COF-IHEP1 and COF-IHEP2 for a wide range of (c) use of COF-IHEP1 and COF-IHEP2 for U(VI) uptake at pH (d) sorption of U(VI) and Pu(IV) with COF-IHEP1 and COF-IHEP2 from a solution competing metal at pH (e) PXRD patterns of COF-IHEP1 before and after gamma at (f) U(VI) sorption by COF-IHEP1 before and after and Download figure Download PowerPoint and radiation stability far, our results suggest the of utilization of the two COFs, especially for U(VI) and Pu(IV) capture from acid media. To such applications of the sorbent be stable during the sorption and the metal be with the sorbent surface areas and of COF-IHEP1 after U(VI) uptake from 1 M HNO3 solution were and the results were compared with those for synthesized COFs ( and ). for the of the of at there were no changes in the of COF-IHEP1 after harsh conditions, confirming its excellent chemical The surface of COF-IHEP1 decreased from 110 to m2·g−1, which could be to the of U(VI) into the pores of the COFs. the COF-IHEP1 was readily on with saturated solution, followed by a which resulted in a high rate of The COF-IHEP1 was in and to the of U(VI) sorption. the sorption capacity of COF-IHEP1 toward U(VI) only very after the with of the sorption capacity after four (Figure thus After COF-IHEP1 was by and surface , and ). spectra no whereas the and peaks in the PXRD suggested a decreased crystallinity of the COFs. the porous structure of the COFs was with a surface of The sorbent also show substantial selectivity toward metal over competing metal to efficient Herein, the selectivity test was performed at pH 1.0 using a solution. The results in Figure and show that the two COFs, especially and U(VI) and Pu(IV) in the presence of competing metal with an equal concentration of The coefficient for Pu(IV) is as high as × which is two of higher than that for indicating an unprecedented selectivity of this toward Pu(IV) in acidic solutions. A key is that the sorbents be to high radiation such as be in nuclear fuel To assess the radiation stability of the two COFs, and in were to gamma using a rate of up to a of 200 were no changes in PXRD patterns (Figure indicating of the COF framework during irradiation, whereas and thermogravimetric analysis results suggested that there was no significant of the phosphonate groups ( and , More importantly, no significant in U(VI) sorption (Figure was observed for of the test even at 200 irradiation, which excellent radiation This is in that the COFs exhibit extended π-conjugation frameworks and π–π stacking structures. the extended π-conjugation framework energy to be well dispersed the COF whereas the π–π stacking energy between Both of these provided to the of bonds in the COFs, thus achieving good toward gamma The suggested that opportunities for COFs as efficient and practical U(VI) and Pu(IV) sorbents. In particular, the stable and performance of COF-IHEP1 in HNO3 the of this for U(VI) and Pu(IV) separation in used nuclear fuel Sorption To the sorption was used to assess the between U(VI) and COF-IHEP1. Figure shows the of spectra of COF-IHEP1 before and after U(VI) After U(VI) the at due to atoms of with the peak and to indicating a strong between phosphonate groups and U(VI) cations. We prepared the COF-IHEP3 as a to assess the of the structure to U(VI) uptake. COF-IHEP3 was obtained as a yellow powder in yield and using and (details in ). The saturated capacity of this was only 5 mg·g−1 at pH 1.0, which revealed a of the structure of the COFs to U(VI) uptake and further the of phosphonate groups. Figure 4 | Sorption (a) spectra of COF-IHEP1 curve) and COF-IHEP1 The modes of (b) and (c) in COF-IHEP1 as predicted by C, P, purple; O, (d) U spectra of COF-IHEP1 including the best and as a corresponding for phase Download figure Download PowerPoint our is that U(VI) sorption into COF-IHEP1 occurs through to phosphonate groups in the COF To further elucidate the detailed coordination structure at the modes of to COF-IHEP1 were predicted from calculations (details in ). The electron and correlation energy were using the the of and the electron with = were all the U were To elucidate the influence of the and model the pore a large × 1 × 2) with atoms was in all Figure shows the modes of in COF-IHEP1. is that the uranium to two oxygen atoms from neighboring layers and two oxygen atoms from molecules. The average bond distance is ∼ Å. The mode of in COF-IHEP1 was also predicted using the same The results in Figure suggest a coordination structure of in COF-IHEP1 that the plutonium to achieve coordination to two oxygen atoms from neighboring layers and oxygen atoms from and exhibited bond distances of ∼ Å. X-ray structure spectra of U(VI) into COF-IHEP1 at pH 1 were recorded to the simulated mode of with COF-IHEP1 (