A Combined Experimental and Theoretical Study on the Extraction of

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A Combined Experimental and Theoretical Study on the Extraction of Uranium by Amino-Derived Metal-Organic Frameworks through Post-Synthetic Strategy Linnan Li, Wen Ma, Sensen Shen, Hexiang Huang, Yu Bai, and Huwei Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11332 • Publication Date (Web): 27 Oct 2016 Downloaded from http://pubs.acs.org on November 1, 2016

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ACS Applied Materials & Interfaces

A Combined Experimental and Theoretical Study on the Extraction of Uranium by Amino-Derived MetalOrganic Frameworks through Post-Synthetic Strategy Linnan Li, † Wen Ma, † Sensen Shen, † Hexiang Huang, § Yu Bai, † and Huwei Liu *,† † Beijing National Laboratory for Molecular Sciences, Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, Institute of Analytical Chemistry, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China. §Institute of Materials, China Academy of Engineering Physics, Mianyang, 621900, P. R. China.

KEYWORDS: metal-organic frameworks, uranium extraction, post-synthetic strategy, radioactive water treatment, molecular dynamics simulation, DFT calculation.

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ABSTRACT

A novel carboxyl-functionalized metal-organic framework for highly efficient uranium sorption was prepared through generic post-synthetic strategy, and this MOF’s saturation sorption capacity is found to be as high as 314 mg·g-1. The preliminary application illustrated that the grafted free-standing carboxyl groups have notably enhanced the sorption of uranyl ions on MIL-101. In addition, we have performed molecular dynamics simulation combined with density functional theory calculations to investigate the molecular insights of uranyl ions binding on MOFs. The high selectivity and easy separation of as-prepared material have shown tremendous potential for practical applications in nuclear industry or radioactive water treatment, and the functionalized MOF can readily be extended upon the versatility of click chemistry. This work provides a facile and purposeful approach for developing MOFs toward a highly efficient and selective extraction of uranium (VI) in aqueous solution, and further facilitates the structurebased design of nanomaterials for radionuclides-containing mediums pretreatment.

Introduction With the shortage of traditional energy sources in the near future and the growing energy security issues around the world, the long-term sustainable energy supply is one of the most serious challenges facing human society1-2. Considering various environmental concerns that associated with burning of fossil fuels, such as global warming, pollution, and biodiversity loss, the demand for alternative energy sources including hydroelectric, nuclear, geothermal, solar, and wind energy has taken on enormous importance both in science and technology3-4. Among these energy sources, nuclear power has been considered as a cost-effective technology and the


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requirement for uranium raw materials is increased in nuclear industry5-6. Meanwhile, as the heaviest of the stable, naturally occurring element, a certain amount of uranium is dissolved in the oceans, which is estimated to have 1,000 times abundant than that land contains7. This have provided momentous opportunities for developing novel versatile materials to separate and recover uranyl cation, the predominant form of uranium (VI) in aqueous medium. As a result, the development of novel sorbents including various porous materials for uranium (VI) extraction have attracted extensive attentions in recent years8-12. These materials could be utilized not only for reprocessing of uranium resources dissolved in aqueous solution, but also for minimizing the adverse impacts on the environment or human health through removing the radionuclides in the waste disposals or streams produced from the nuclear fuel cycle. Metal-organic frameworks (MOFs) are a family of novel porous materials, which composed of metal ions or metal ion clusters bridged by various organic ligands13. Benefits from the structural diversity, high porosity, large surface area, tunable pore sizes and functionalities, they have shown promising applications in a variety of fields, such as gas storage14, drug delivery15, chemical sensing16, molecular separation17, heterogeneous catalysis18, and so on. MOFs had been used as solid-phase sorbents for adsorption and removal of environmental contaminants or toxic metal ions from the aqueous solution19-22. Recently, a MOF of UiO-68 network topology containing phosphorylurea groups was reported for uranium (VI) extraction, which opened up an encouraging trend for enrichment and separation of actinide elements23. After that, there had been several kinds of amino functionalized chromium-based MOFs (MIL-101 series) developed for efficient sorption of uranium (VI) in aqueous solution24-25. These materials have fully utilized the plenty of amino groups that anchored through coordinately unsaturated metal centers to enhance the sorption capacity of uranium (VI). It has been indicated that MOFs with well-


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ordered tunable porous channels facilitate the effective extraction of actinide elements. Thus they could become one of the finest candidates of functional nanomaterials for uranium (VI) selective recovery and radioactive water treatment26-29. On the other hand, uranyl cation is classified as a strong Lewis acid that could exhibit high affinity for hard oxygen donors involving hydroxide and carboxylate30. Therefore, the preferred sequences of artificially designed peptides or proteins for uranyl binding mostly have involved carboxylate donors from aspartates, glutamates, or the carboxyl terminus of the chains31-33. Moreover, the introduced carboxyl units in MOFs channels have notably improved the rapid separation and selective recovery of uranium (VI) in artificial seawater34. Nonetheless, the exploring utilization of MOFs materials for metal ions removal from aqueous solution is still required. And it is meaningful that MOFs possess adequate stability and adaptability in rigorous sorption conditions including nearly neutral solution pH and coexist ions. In addition, the molecular insights of binding process and coordination modes of the uranium (VI) on MOFs remains largely elusive as well. In this study, we proposed an expandable post-synthetic strategy based on amino-derived MIL101 for the effective extraction of uranium (VI) from aqueous solution (Scheme 1). The MOFs were prepared via a generic post-synthetic strategy starting from amino functionalized MIL-101 (MIL-101(Cr)-NH2, MOF-1)35-37. This chromium-based material was selected as a potential substrate due to its outstanding porosity and stability38-39, and the promising applications in glycopeptides enrichment and hydrogen storage40-42. Then the desired functionalized MOF (MIL-101(Cr)-triazole-COOH, MOF-3) was facilely obtained by “click chemistry” using Cu(I)catalyzed on the corresponding intermediate azide material (MIL-101(Cr)-N3, MOF-2)37. During this stage, a variety of preferred ligands with both the favored binding sites for uranyl binding


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like carboxyl oxygens and terminal alkyne moiety could be chosen as candidates for improving uranium extraction43-44. Thereupon the functionalized MIL-101 grafted with free-standing carboxylic acid units was synthetized for effective extraction of uranium (VI) in aqueous solution. The experimental results suggest the carboxyl functionalized MIL-101 as an efficient sorbent for uranium both in water and artificial seawater. Furthermore, the binding behaviors and coordination modes of the uranium (VI) were described in details through theoretical investigations. The mechanism of uranyl ions adsorption on MOF was explored to reveal the important features that make it a promising sorbent for practical applications.

Results and discussion Characterization of materials (MOF 1-3). Figure 1 shows the characterization results of amino functionalized MIL-101 (MOF-1) and its derivatives (MOF 2-3). As shown in the FT-IR spectrum (Figure 1a), the stretching vibration absorption of the uncoordinated carboxyl groups in the framework skeleton appeared in the range of 3200-3500 cm-1 while the peaks at 1498 cm-1 and 1430 cm-1 illustrated the -(O-C-O)stretching vibration. The peak at 1582 cm-1 was ascribed to the N-H bending vibration . The formation of azide intermediate material and the subsequent cycloaddition were clearly illustrated by a sharp peak at 2124 cm-1 that corresponded to the characteristic asymmetric stretching vibration of N3 group. The intensity of peak at 2124 cm-1 decreased dramatically, even almost disappeared after post-grafting process, showing the transformation of the azide groups. Furthermore, successful modification of MOF-1 is also evidenced by liquid 1H-NMR after digestion, as depicted in Fig S1. The results confirmed the formation of the amino functionalized material (MOF-1) by the appearance of new aromatic signals (7.13 ppm, d, J=8 Hz, 1H; 6.94


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ppm, s, 1H; 7.38 ppm, d, J=8 Hz, 1H). The consistency with the complete disappearance of the aromatic signals of the amine, thus indicating full conversion to the azide form compound (6.87 ppm, d, J=8 Hz, 1H; 7.29 ppm, d, J=8 Hz, 1H; 7.37 ppm, s, 1H). In addition, the corresponding triazole derivative (MOF-3) was formed as indicated from new assigned aromatic shifts of the compound (7.51 ppm, d, J=8 Hz, 1H; 7.78 ppm, s, 1H; 7.85 ppm, d, J=8 Hz, 1H). Powder XRD patterns (Figure 1b) indicated that the resulting powders were typical chromium based MIL-101 structures with the patterns similar to the simulated. The results also confirmed the maintenance of the MIL-101 topology with no loss of crystallinity during the synthesis process. The rather broad Bragg reflections were consequence of the small particle sizes, as supported by TEM images (Figure S1a), which revealed the uniform and narrow size distribution of particles with the diameter of about 25 nm. N2 adsorption experiment illustrated the BET surface area of 1907.5, 1616.4 and 890.5 m²·g-1 for MOFs 1-3, respectively (Figure 1c). The surface area for MOF-3 was observably smaller than the azide precursor MOF-2, and both were lower than that of substrate MOF-1. It is likely that the post-synthetic process caused minor framework distortion, thus reducing the surface area from the expected value. The sharp uptake under low pressure (P/P0=10-5 to 0.1) in N2 adsorption/desorption isotherms demonstrated the microporous feature of the material while another uptake occurred near P/P0=1.0 attributed to the textural pores created by nanoparticles aggregation. Additionally, thermogravimetric analysis showed that the nanoparticles were stable up to 260 ℃ (Figure S1c), suggesting that MOF-3 possessed adequate stability for potential applications in uranium (VI) extraction. Uranium (VI) sorption studies.


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To evaluate the performance of uranium (VI) sorption properties of as-prepared MOF-3, a series of batch experiments were carried out at various conditions of pH, contact time, initial uranium concentration, different elution solutions and presence of competing metal ions. Effect of solution pH on uranium (VI) sorption. The pH value of solution plays a key role for metal ion sorption, because it notably affects the speciation of metal ions as well as the surface charge distribution of the binding sites across the sorbent. Thus, the influence of pH on the sorption were investigated at different pH ranging from 1 to 12. The results presented in Figure 2a revealed that the sorption of uranium (VI) is strongly pH-dependent: At a pH below 3.0, for example, very few uranyl ions absorbed on the MOF-3, whereas the sorption capacity of sorbent clearly increased to maximum at a neutral pH range and then the steep decrease in sorption occurred at pH value above 10. The influence of solution pH might be reasonably explained with the pH induced protonation and deprotonation of grafted carboxyl groups on MOFs. Besides, the multi-nuclear hydroxide species of uranium at a higher pH such as (UO2)2(OH)22+, (UO2)3(OH)5+, (UO2)4(OH)7+, and (UO2)(OH)7- may be more favored by the asprepared MOF sorbent. To achieve higher sorption capacity and avoid severe hydrolysis and precipitation of uranyl ion from the solution, pH 7 was selected for subsequent sorption experiments. Adsorption kinetic. The sorption rate of uranium from aqueous solution is associated with assessing the sorbents for practical applications. To investigate the effect of different contact time, the sorption experiments covering from 15 minutes to several hours were performed at the initial uranyl ion concentration (C0) of 100 mg·L-1. As shown in Figure 2b, the uranium sorption rate for MOF-3 was rapid especially in the initial 30 minutes, then the sorption process reached equilibrium at around 2 h. The results indicate significant affinity interaction between uranyl ions


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and the MOF material, which is possibly due to intense coordination interactions with uranyl ions of the grafted plenty of free-standing carboxyl groups on the surface and channels of material. This explanation could be further supported by the investigation of the adsorption kinetic. The adsorption isotherm can be well fitted by the pseudo-second-order rate model. The model details and parameters, as well as the obtained correlation coefficient are listed in the SI. The results implied a dominating chemical chelating rather than physical adsorption in uranium sorption process. Adsorption isotherm. To evaluate the maximum sorption capacity of MOF-3, the adsorption isotherm was measured. The amounts of uranyl ions adsorbed by the sorbent as a function of uranium concentration in supernatant at the equilibrium state were determined at a constant pH of 7.0 by varying the initial uranyl concentrations from 5 mg·mL-1 to 500 mg·mL-1. A continuous increase of uranium (VI) sorption with augmentation of the initial uranyl concentrations was observed from Figure 2c. When it reached equilibrium, the saturated sorption capacity was obtained under the experimental conditions as 304 mg·g-1. It is well known that Langmuir and Freundlich models are the most frequently used for describing adsorption isotherms. As shown in Figure 2d, the Langmuir model is more appropriate than the Freundlich model to depict the adsorption of uranium with a higher correlation coefficient (R2=0.994), indicating that the adsorption is localized in a monolayer. In terms of the Langmuir equation, the maximum sorption capacity qe is estimated to be 314 mg·g-1, which is close to the experimental value of 304 mg·g-1. The details of the two models and the fitting results are provided in SI. Compared with other previous reported MOFs for uranium extraction, our results clearly suggest that as-prepared carboxyl functionalized MIL-101 is more efficient in uranyl ions


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capture compared to bare MIL-101 and amino functionalized substrate24-25. As for amino functionalized MIL-101, the amino groups were directly anchored to the aromatic rings in the MIL-101 framework. Although it has a larger coverage of functional groups and larger surface area, the availability of amino groups is much lower because of the steric hindrance of the aromatic rings and formation of intermolecular hydrogen bonds between the carboxyl groups in the MOF and the grafted amino groups. Selective extraction of uranium (VI). As a desired property in many case, the selectivity is significant for the practical applications of the materials. Herein, the competitive sorption tests containing 10 coexistent ions including Co2+, Ni2+, Zn2+, Sr2+, La3+, Ce3+, Sm3+, Gd3+ and Yb3+ were carried out to evaluate the selectivity of MOF-3. Figure 3a depicted the distribution coefficient (Kd) among the uranyl ion with other competing ions. And the selectivity coefficient (SU) for uranium (VI) over other ions is listed in Table S4. The results exhibit an excellent selectivity of MOF-3 toward uranium (VI) over a range of competing metal ions, especially for Co2+, Ni2+, Zn2+, Sr2+. The extraordinary high value of Kd (nearly 18000 mL·g-1) means that the MOF-3 possesses an outstanding binding ability for selective separation of uranium (VI) from the aqueous solution. Considering the uranyl ion is a strong Lewis acid, the carboxylate donors from MOF-3 channels possess a better affinity toward uranium (VI) over other tested metal ions. Furthermore, the hydrolysis effect of uranium (VI) occurred by the transformation of free uranyl ions to multi-nuclear hydroxide species likes (UO2)2(OH)22+.These complexes may be more favored by the as-prepared MOF sorbent at current sorption conditions. The high selectivity for uranium adsorption over other competing ions illustrates the potential feasibility applied in uranium (VI) recovery from nuclear waste or seawater with high concentration salts.


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Desorption experiments. Figure 3b demonstrated the desorption tests of uranium (VI) loaded on MOF-3 with different elution solutions. Notably, the amount of uranyl ions remaining in the sorbent is approximately unchanged after washing with water, while more than 98% of uranium (VI) is eluted by washing with 1 M Na2CO3 (aq). Meanwhile, after washing with 1 M HNO3 (aq), most of the sorbed uranium (VI) can be eluted as well (Table S5). However, after three cycles, there is a dramatic reduction of sorption capacity (ca. 50 %) compared to the fresh material. This probably due to the partly destruction of functionality under the desorption process. Though, the regeneration efficiency of the adsorbent is advantageous in uranium (VI) separation for practical applications. Uptake of uranium (VI) in artificial seawater. The uranium (VI) adsorption on MOF-3 in artificial seawater was also performed with similar procedures. The details of the composition of artificial seawater are listed in Table S145. The initial concentration of uranyl ions in artificial seawater was kept consistent with batch experiment ([UO2]2+ = 100 mg·L-1). The solution pH of artificial seawater was determined as 7.7 by pH-meter, which is close to the neutral sorption condition in water. As shown in Figure S4, although the sorption capacity of MOF-3 in artificial seawater is decreased compared with that in water, the sorption percentage is still more than 75% due to its high affinity and selectivity toward uranium. Thus the high selectivity, easy separation and performance in artificial seawater for uranium (VI) extraction by MOF-3 were confirmed, showing great potential for practical applications. Molecular binding process of uranyl ions. The underlying microscopic mechanisms about uranyl ions adsorption on MOF-3 were further investigated by molecular dynamics (MD) study with Gromacs 5.146. Figure 4 shows the representative snapshots of binding process in the simulation. Initially, as shown in Figure 4a,


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when t=0 (prior to simulation), all uranyl ions are located in solution compartment and MOF-3 has been assumed as rigid and fixed at the right side of the periodic box 47. When the simulation starts, as shown in Figure 4b, uranyl ions are rapidly adsorbed on the surface of MOF-3. And once adsorbed, the uranyl ions prefer staying in the frameworks rather than moving back to solution. When t=0.5 ns, as shown in Figure 4c, a large number of uranyl ions have moved onto MOF-3, particularly close to the solution-MOF interface. Meanwhile, part of uranyl ions located at the solution-MOF interface have moved into the interior of MOF-3. At this moment, there are only very few uranyl ions left in solution compartment. As shown in Figure 4d-f, all uranyl ions are completely extracted from the solution in less than 5 ns, and the snapshots at the simulation time beyond 5 ns remain essentially unchanged. It could imply that an equilibrium state is reached rapidly. The numbers of uranyl ions adsorbed in MOFs as a function of simulation duration are shown in Figure 5a. Consistent with the tendency in Figure 4, the initial adsorbed numbers of uranyl ions rapidly increase within a few picoseconds. After approximately 5 ns, the numbers of adsorbed uranyl ions in MOF-3 is nearly constant, and all uranyl ions were adsorbed on the MOF. Besides, to further evaluate the binding process of amino functionalized substrate (MOF-1) with uranyl ions and the effect between protonated and deprotonated binding motifs on the MOF, additional MD simulations were also investigated and the representative snapshots were shown in Figure S6-7, respectively. The results suggest the introduced carboxylic acids exhibit more intense binding affinity with positively charged uranium (VI) on carboxyl oxygen than on the amino nitrogen. The snapshots also demonstrate that the protonation of carboxyl groups could notably reduce the binding capacity of the material and indicate that strongly acidic medium is


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not conducive for the extraction of uranium, which is in accordance with the batch experiments as discussed above. Density profiles. Figure 5b quantitatively depicts the density profiles of uranyl ions at different simulation time, corresponding to the snapshots shown in Figure 4. The density profiles were calculated by dividing the simulation box into a series of small slices along the z axis with Δz = 0.2 nm, then the partial densities in each slice were estimated. When t=0 ns, all uranyl ions are in solution, leading to a broad peak. After binding occurs, at t=0.1 ns, uranyl ions begin to move from solution onto MOFs. Thus, the density curve decreases in solution region but increases on MOFs. Similar variations are observed until a dynamic equilibrium is reached and the density of uranyl ions decreases to zero in solution region. Besides, as exhibited in density profiles, the preferential location of uranyl ions is solution-MOF interface. Parallel density profiles of different materials are presented in Figure S9, which indicate the same trends with molecular binding process as discussed above. Coordination environment. To elucidate the structural features and dynamics nature of uranyl ions in MOFs, radial distribution function (RDF) and mean squared displacement (MSD) were calculated by equations in SI. The RDF result of central uranium with carboxyl oxygens was depicted in Figure 5c, which reveals that grafted free-standing carboxyl groups on MOF-3 are intensively bounded to uranyl ions. The deduction is clearly evident from the pronounced first peak of RDF curve at r= 2.25 Å. Such strong interaction with carboxyl groups remarkably immobilize the uranyl ions on MOFs, whose MSD is remarkably smaller than those in substrate MOF-1 or unbound state in aqueous solution (Figure 5d).


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The self-diffusion coefficient is an important transport property for describing a molecule spontaneously moving in material. The low value indicates better adsorption behavior of the metal ion on the MOFs48-49. Hence, the diffusion coefficient of uranyl ions on MOF-3 is determined to be 1.13×10-7 cm2·s-1, whereas the value in MOF-1 is 2.04×10-6 cm2·s-1. Also, the two values are both significantly lower than that of unbound state in aqueous solution, in which the value is 9.77×10-6 cm2·s-1. The order of these MSDs reflected the dynamics difference in variant coordination environments. The RDF of coordinated oxygens around the uranium was then calculated to determine the average interatomic distances and coordination numbers. As shown in Figure S10, the average distance between the uranium atoms and coordinated oxygens is 2.24 Å, and there are five coordinated oxygen atoms in the first coordination shell. The electrostatic attraction with uranium in coordinated carboxyl oxygens behave more intense than that in surrounding water molecules, as indicted by the splitting of peaks in the first solvation shell. And then the g(r) is close to zero after the maximum of the first solvation shell peak. The average bond lengths and coordination numbers were consistent with previous studies of various uranyl complexes, suggesting the stable coordination interaction and the formation of complexes50-52. Binding energy change. After the detailed description of the binding process and coordination environment, the potential of mean force (PMF) is computed to understand the free energy change of uranyl ions48-49. Figure 6 shows the PMF for a specific uranyl ion moving from MOF3 into aqueous solution. The reference position is at the interior of the frameworks, where the uranyl ion is tightly bonded with binding motifs grafted on the channels of MOFs. As the uranyl ion is forced to deviate from the frameworks into solution, the PMF is dramatically uplifted until the attractions of binding motifs with the uranyl ion are becoming negligible. The net change of


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PMF values indicates favorable adsorption of the uranyl ion on MOF-3. And the value is calculated to be -22.6 kcal·mol-1 which also means that the binding process of uranium extraction is spontaneous in aqueous solution. It should be noted that the performance of simulation is limited to accuracy of theoretical models that are required to further improvement53-56. Besides, the effects of post-synthetic efficiency and framework flexibility of functionalized materials are not involved in the simulation. And the stirring exerted on solvent molecules in actual experiments would affect the results as well. DFT calculations of uranyl-sorbent interactions. Furthermore, the structure-based design of uranyl extraction materials requires essential knowledge of the binding modes for the uranyl ion with sorbent moiety. Thus density functional theory (DFT) calculations were performed to assess optimized geometries and relative stabilities for a series of uranyl complexes. Considering the MOFs are too big for modeling, the simplified binding motifs are used in this work. The representative models greatly reduce the cost of our computing resources and allow for a good description of the first shell around the uranyl ion. The details of calculations are presented in SI. Uranyl ion is known to form a pentahydrate complex when solvated, and all coordination sites on uranyl not interacting with a sorbent were filled with aqua ligands. Inspired by the obtained results from simulation study, we have proposed three different possible coordination modes for binding uranyl ions on MOF-3. Figure 7 displays the investigated representative structures with optimized binding geometries of different coordination modes. And the structural features of the first coordination sphere of the uranyl ion are summarized in Table 1. Specifically, for mode I and mode II, one of carbonyl oxygens is coordinated to central uranium in a monodentate fashion


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and there are four water molecules in the first solvation shell. The bond lengths of U-Ocab are 2.229 and 2.226 Å, respectively, which are smaller than 2.472-2.552 Å of U-Ow, and these are implying the incense binding affinity between uranyl and sorbent. Alternatively, for mode III, both oxygen atoms could coordinate in a bidentate fashion and the central uranium was located at 2.439 Å and 2.451 Å from the two coordinating carboxyl oxygens. Thus the electrostatic attraction of U-Ocab in mode III is less intensive than mode I and II. The distances of uranium with surrounding water oxygens among three modes remain basically unchanged. Then the Natural Bond Order (NBO) analysis was performed for all coordination modes. The Wiberg bond indices for all structures gives Ocab-U (N-U in amino motif) bond order: mode I is 0.6996, mode II is 0.7877, mode III are 0.4754, 0.4545 and amino motif is 0.3513. The results indicate the bonding of Ocab-U and uranium in mode I-II are partially ionic in character and thus exhibit the high affinity and selectivity between the uranyl and sorbent. In addition, the reaction enthalpies were calculated from the difference between the heat of formation for products and reactants, obtained from frequency calculations of optimized structures. All thermodynamic data are summarized in Table 1, where ΔHaq refers to the reaction enthalpy in the aqueous phase. As a result, the most enthalpically favored binding mode was type 2, and the difference between mode I and mode II were only slightly (

A Combined Experimental and Theoretical Study on the Extraction of

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