Ultrafast and Efficient Extraction of Uranium from Seawater Using an

Sep 14, 2017 - (1, 2) Alternatively, the oceans contain 4.5 billion tons of uranium in total, representing 99.9% of uranium inventory on earth, but in...
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Ultrafast and Efficient Extraction of Uranium from Seawater Using an Amidoxime Appended Metal−Organic Framework Long Chen,† Zhuanling Bai,† Lin Zhu,† Linjuan Zhang,‡ Yawen Cai,† Yuxiang Li,† Wei Liu,† Yanlong Wang,† Lanhua Chen,† Juan Diwu,† Jianqiang Wang,‡ Zhifang Chai,† and Shuao Wang*,† †

School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, 199 Ren’ai Road, Suzhou 215123, People’s Republic of China ‡ Shanghai Institute of Applied Physics and Key Laboratory of Nuclear Radiation and Nuclear Energy Technology, Chinese Academy of Sciences, 201800 Shanghai, People’s Republic of China S Supporting Information *

ABSTRACT: Enrichment of uranyl from seawater is crucial for the sustainable development of nuclear energy, but current uranium extraction technology suffers from multiple drawbacks of low sorption efficiency, slow uptake kinetics, or poor extraction selectivity. Herein, we prepared the first example of amidoxime appended metal−organic framework UiO-66-AO by a postsynthetic modification method for rapid and efficient extraction of uranium from seawater. UiO-66-AO can remove 94.8% of uranyl ion from Bohai seawater within 120 min and 99% of uranyl ion from Bohai seawater containing extra 500 ppb uranium within 10 min. The uranyl sorption capacity in a real seawater sample was determined to be 2.68 mg/g. In addition, the recyclability of the UiO-66-AO framework was demonstrated for at least three adsorption/desorption cycles. The origin for the superior sorption capability was further probed by extended X-ray absorption fine structure (EXAFS) analysis on the uranium-sorbed sample, suggesting multiple amidoxime ligands are able to chelate uranyl(VI) ions, forming a hexagonal bipyramid coordination geometry. KEYWORDS: metal−organic frameworks, amidoxime, uranyl, adsorption, seawater

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decades. Among these, amidoxime-modified polymers are the most promising materials. Dai et al. found that the amidoximated polymer shows a high uptake capacity of 1.99 mg U (g Ads)−1 after a contact time of ∼27 days in a real seawater sample test.11 However, one of the major issues for these modified polymers is the relatively slow adsorption kinetics, which likely originates from their irregular pore size and low surface area, further hindering effective transportation of uranium throughout the material. Because of their crystalline nature, tunable structural topology, pore size and functionality, large surface area, and enhanced chemical stability, metal−organic frameworks (MOFs) have recently been demonstrated to exhibit extensive applications as heavy metal sorbent materials with advantages in sorption kinetics, capacity, and/or selectivity.19−26 Specifically, several MOFs have been tested in the removal of uranium from aqueous solutions mostly aiming at contamination remediation or waste separation purposes.27−29 For example, Lin et al. initially found UiO-68 containing phosphorylurea groups can

ith the rapid increase of global demand for energy and air-pollution that partially originates from the fossil based energy production, nuclear power attracts much attention due to its nongreenhouse gas emission and extremely high energy density. Uranium is the key strategic resource for nuclear power and its reserves in terrestrial ores (about 6.3 million tons) would only last for less than 100 years at the current consumption rate if the used fuels are not recycled.1,2 Alternatively, the oceans contain 4.5 billion tons of uranium in total, representing 99.9% of uranium inventory on earth, but in extremely low concentration of 3.3 ppb along with a variety of coexisting metal cations in huge excess.3 Therefore, efficient and selective extraction of uranium from seawater is desirable for the sustainable development of nuclear energy but of great challenge.4 Amidoxime is the most widely used functional group in extracting uranium from seawater, owing to its high chelating affinity and selectivity toward uranyl ions in high salinity brine.5−10 Various amidoxime based adsorbents such as nanoporous polymers,11 nanostructured ceramic sorbents,12 amidoximated AI series adsorbents,13 electrospun nanofibrous adsorbents,14 graphene oxide based hydrogel,15 magnetic microspheres,16 and porous aromatic frameworks17,18 were developed to capture uranyl from seawater during the past few © XXXX American Chemical Society

Received: August 17, 2017 Accepted: September 14, 2017 Published: September 14, 2017 A

DOI: 10.1021/acsami.7b12396 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 1. (a) Synthetic route of UiO-66-AO: (i) CuCN, N-methyl pyrrolidone, microwave at 170 °C for 20 min; (ii) NH2OH·HCl, CH3CH2OH, refluxing for 24 h. (b) Powder X-ray diffraction (PXRD) patterns of UiO-66-Br, UiO-66-CN, UiO-66-AO, UiO-66-AO soaked in Bohai seawater for 24 h as well as the simulated one of UiO-66. (c) Fourier transform infrared (FT-IR) spectra of UiO-66-CN and UiO-66-AO. (d) Nitrogen adsorption isotherm curves of UiO-66-Br, UiO-66-CN, UiO-66-AO, and UiO-66-AO soaked in Bohai seawater for 24 h.

shown in Figure 1c. The new band at 2235 cm−1 was ascribed to the CN group.35 This band disappeared after amidoximation whereas two new peaks at 1655 and 919 cm−1 appeared in UiO-66-AO, which can be designated to CN and NO stretching vibration, respectively.37 Elemental analysis and energy-dispersive X-ray spectroscopy (EDS) experiments were performed to calculate the grafting ratio of UiO-66-CN and UiO-66-AO. From the results of elemental analysis (Table S2), the concentration of cyano group in UiO-66-CN is 2.2 mmol/g and the grafting ratios for cyano and amidoxime groups are 96% and 19.1% in UiO-66-CN and UiO-66-AO, respectively, according to the bromine contents in UiO-66-CN (Figure S5) and the nitrogen content in UiO-66-CN and UiO66-AO. To investigate the surface area of UiO-66-AO, the nitrogen adsorption/desorption experiment was conducted at 77 K. As shown in Figure 1d, all the adsorption/desorption isotherms are Type-I curves according to the IUPAC classification. The nitrogen adsorption isotherms begin with a rapidly increased step in low pressure region and end with the slow increase in relatively high pressure, indicating the microporous character of the MOFs.38 The surface area for UiO-66-Br is 1035 m2/g derived from the Brunauer−Emmett−Teller (BET) method, which is comparable with the reported one.39 On the other hand, the surface areas of UiO-66-CN and UiO-66-AO were determined to be 878 and 711 m2/g respectively after modification, which likely originates from partial blocking of the pores by larger functional groups. Moreover, the stability of UiO-66-AO in seawater and 0.01 M HNO3 was checked by PXRD and N2 adsorption measurements. After soaking in a real seawater sample from Bohai sea in the east of China for 24 h, the PXRD pattern of UiO-66-AO is almost identical with the original material. The surface area of UiO-66-AO after soaking in seawater is 697 m2/g, very close to that of the freshly synthesized UiO-66-AO (711 m2/g), suggesting UiO-66-AO

extract uranyl ion from aqueous solutions at pH 2.5 and 5.30 Sun et al. reported that luminescent MOF-76 can pre-enrich and detect uranyl ion at pH 3.31 However, very few examples show real utility of MOFs in uranium extraction from seawater likely because of two reasons.32 First, the long-term stability for the majority of MOFs in seawater sample with significantly high ionic strength remains questionable. Second, most investigated MOFs with uranium sorption capability does not possess qualified sorption selectivity that is certainly required for uranium extraction from seawater. Therefore, in this study, we selected one of the state of art MOFs UiO-66 that possesses great hydrolytical stability33 and prepared the first example of an amidoxime appended MOF material UiO-66-AO by a postsynthetic method. This material not only exhibits decent chemical stability in seawater samples but also possesses promising applications for rapid and effective recovery of uranyl from seawater. The target adsorbent UiO-66-AO was obtained according to the synthetic route shown in Figure 1a. The initial MOF UiO66-Br was synthesized from 2-bromoterephthalic acid and ZrCl4 in DMF by a solvothermal method.34 UiO-66-CN was prepared by microwave-assisted cyanation in the presence of 1.3 equiv of CuCN according to the method developed by Cohen.35 On the basis of general synthetic method, the final product UiO-66-AO was obtained by refluxing UiO-66-CN and hydroxylamine hydrochloride in ethanol for 24 h using triethylamine as the base.36 The powder X-ray diffraction (PXRD) patterns for the three above-mentioned UiO-66 MOFs are almost identical (Figure 1b), suggesting the major framework structure remains intact during the postsynthetic modification (PSM) process. This conclusion was further confirmed by the scanning electron microscopy (SEM) results (Supporting Information, Figures S1−S3). The conversion of cyano groups (CN) to amidoxime groups (C(NOH)NH2) was confirmed by Fourier transform infrared (FT-IR) spectra as B

DOI: 10.1021/acsami.7b12396 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

visualizes the presence of adsorbed uranium and its homogeneous distribution in UiO-66-AO.41 On the basis of the excellent performance of UiO-66-AO for uranium capture, we were further inspired to investigate the capability of this material for uranyl extraction from the real seawater sample from Bohai sea. The sorption experiment was conducted with the solid/liquid ratio of 1 g/L in seawater to measure the sorption kinetic of uranyl onto UiO-66-AO. Impressively as shown in Figure 2b, the sorption of uranium is extremely fast in the initial 25 min as more than 50% of uranyl ions were adsorbed in the first 30 s. With the decrease of the number of the active functional group, the sorption rate of uranium slowly gradually fell off and reached the sorption equilibrium at about 120 min. Finally, 98.7% of uranium was extracted and the Kd value reached up to 7.59 × 104 mL/g, confirming that UiO-66-AO is an excellent adsorption material for uranium from natural seawater.42 In comparison with other adsorption materials, the adsorption efficiencies for Fe−MnO2 nanoporous composite, 8 nm Mn−Fe3O4 magnetic nanoparticle, MnO2 nanoporous composite and 8 nm Fe3O4 magnetic nanoparticle in natural seawater are 80%, 70%, ∼40%, ∼10%, respectively, with the contact time of 240 min and liquid-to-solid ratio of 1000 mL/g.43 UiO-66-AO therefore exhibits a clear advance in the uranium sorption rate over these reported materials. More adsorption kinetics data for various adsorbents are listed in Table S4. We also investigated the uranium remediation capability of UiO-66-AO using real seawater samples contaminated with various concentrations of uranium (50 to 1000 ppb) and the solid/liquid ratio of 1 g/L. Notably, all of the adsorption percentages were larger than 93.7% (Figure 2c). For the case of Bohai seawater containing extra 500 ppb uranium, the sorption kinetics of uranyl by UiO-66-AO is extremely fast and more than 99% of uranyl can be removed within 10 min (Figure S11). After that, the uptake rate of uranyl for UiO-66-AO declined sharply and finally it reached sorption equilibrium at about 30 min. In order to expound the adsorption process of uranyl onto UiO-66-AO in Bohai seawater and the seawater containing extra 500 ppb uranium, the sorption kinetic data were analyzed by pseudo-first-order and pseudo-second-order kinetic models.43 The detailed fitting results are presented in the Supporting Information (Figure S12−S13). It is evident that the pseudo-first-order model poorly matches the experimental data (R2 = 0.920 and 0.624 for seawater and seawater containing extra 500 ppb uranium, respectively, Table S3), whereas the pseudo-second-order model provides a much better correlation coefficient (R2 ≥ 0.999 for both cases), indicating that chemical sorption is dominant in the sorption process. In order to check the adsorption capacity of uranium onto UiO-66-AO from a real seawater sample, 1 mg of UiO-66-AO encapsulated in a dialysis bag was soaked into 1 L of Bohai seawater sample and stirred for 3 days at ambient temperature. After the dissolution of the uranyl-adsorbed UiO-66-AO solid in 0.5 mL of HF (40% aqueous solution), the adsorption capacity was determined to be 2.68 mg/g according to the ICPMS analysis of the uranium content. Furthermore, the reutilization of UiO-66-AO was tested by equilibrating generated UiO-66-AO with Bohai seawater containing 500 ppb of uranium followed by multiple sorption/desorption cycles in order to validate the practical application of this material. As shown in Figure 2d, when 0.01

can fully retain its structure in seawater, which is required for the uranium extraction application. In addition, UiO-66-AO is also stable after soaking in 0.01 M HNO3 for at least 4 h according to the BET (789 m2/g) and PXRD results (Figures S8 and S9). To explore the uranyl adsorption behavior of UiO-66-AO, the sorption experiments with various solid-to-liquid ratios (abbreviated as Rs‑l), pH values, and contact time were systematically conducted. It is obvious that the uranium adsorption percentage increases with the increasing of the Rs‑l with the initial uranyl concentration of 9.8 mg/L. When Rs‑l is 0.5 g/L, the adsorption percentage of UiO-66-AO is 76.7% but only 1.6% for UiO-66-Br (Figure S10). The remarkable difference directly hints for the critical role of amidoxime groups in the uranyl uptake. The pH of the solution is an important factor in the sorption process, because the protons not only affect the speciation of U(VI) in solution but also affect the surface charge of the sorbent.40 Here we investigated the influence of pH for adsorbing uranyl in a wide range from 2 to 9. As shown in Figure 2a, UiO-66-AO shows negligible uptake of uranium

Figure 2. (a) Effect of pH in adsorption of uranyl by UiO-66-Br and UiO-66-AO ([U(VI)]initial = (9.55 ± 0.15) mg/L, msorbent/Vsolution = 0.4 mg/mL). (b) Effect of contact time on the sorption of Bohai seawater by UiO-66-AO (msorbent/Vsolution = 1 mg/mL). The solid line displays the pseduo-second-order fitting results. (c) Removal of uranyl by UiO66-AO from Bohai seawater containing various concentrations uranium (pH = 8.23 ± 0.02, msorbent/Vsolution = 1 mg/mL) in 2 h. (d) Adsorption and desorption percentage of UiO-66-AO for Bohai seawater containing extra 500 ppb uranium (pH = 8.28, msorbent/ Vsolution = 1 mg/mL).

when the pH is below 3, which is attributed to the positively charged surface of UiO-66-AO that repulses U(VI) hydrate cations.15 However, the adsorption percentage dramatically increased when the pH value was higher than 4. UiO-66-AO can even quantitatively remove uranyl from aqueous solution in the pH ranging from 7 to 9, which indicates that UiO-66-AO can be serve as an attractive material in adsorbing uranyl at neutral or weakly basic conditions. For comparison, the original MOF UiO-66-Br only adsorbed 44% of uranyl at pH 9. In addition, the sorption of uranium by UiO-66-AO was also confirmed by EDS mapping profiles as shown in Figure S7. Elemental mapping of uranium-loaded UiO-66-AO directly C

DOI: 10.1021/acsami.7b12396 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

for complexation of uranyl ion by UiO-66-AO because there are only five equatorial donor atoms for uranyl hydrate compounds. In addition, the equatorial bond distances in the sample are similar to those in uranyl hydrate compound, further indicating that the hydroxyl groups play an important role in the inner-sphere complexation mechanism because oxime oxygen atoms have a stronger binding ability to uranium than the nitrogen atoms.49 The almost identical of equatorial U−O bond distances between those in 6-coordiante uranyl in uranium-adsorbed UiO-66-AO and 5-coordiante uranyl in the hydrate compound suggests the extremely tight binding of uranyl by UiO-66-AO. The fit is in line with experiential observation and indicates that the strong binding model by a η2 coordination motif does not exist although it was found in small molecules and theoretic investigations.50−52 The foregoing results demonstrate the capability of the amidoxime-modified MOF material in the rapid and efficient extraction of uranium from real seawater samples. The good acidic and alkaline stability provide prerequisites for UiO-66AO to extract uranyl for at least three adsorption/desorption cycles. The uptake mechanism was demonstrated by EXAFS analysis on uranyl coordination environment and bonding. This work calls for further investigations of searching for more amidoxime-modified MOF materials with different pore structures and topologies as well as improved extraction capabilities.

M HNO3 solution was used as the eluent, the uranium desorption percentage was higher than 80% for at least three cycles, which suggests not only the decent stability of the material but also its facile regeneration procedure. The synchrotron radiation extended X-ray absorption fine structure (EXAFS) analyses were performed to investigate the coordination environment of uranium after extracted by UiO66-AO (Figure 3). The sample was prepared by soaking UiO-

Figure 3. Direct comparison of uranium EXAFS spectra for (a) uranyl hydrate and (b) UiO-66-AO adsorbed uranyl (solid lines = experimental data, open circles = fitting results). Inset: EXAFS data and fitting result displayed in k space. EXAFS data were fitted with kweight of 3 from 3 to 11 Å−1.



The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b12396. SEM images, elemental analysis results and EDS spectra for UiO-66 MOFs, adsorption kinetics of UiO-66-AO for seawater containing extra 500 ppb uranium, pseudo-firstorder and pseudo-second-order kinetic model fitting results, and parameters obtained from EXAFS data fitting (PDF)

66-AO in uranyl solution overnight. The EXAFS data were collected at the uranium LIII-edge (17.166 keV) and analyzed by the Demeter software suite of the IFEFFIT 1.2.10.44−47 According to the analysis result, the best fitting on uranium coordination combines with two axial uranyl oxygen atoms at distances of 1.76 ± 0.02 Å and six equatorial donor atoms (nitrogen or oxygen) at distances of (2.41 ± 0.02) Å (Table 1),



Table 1. Parameters Obtained from EXAFS Data Fitting for UO22+@UiO-66-AOa sample Uranyl hydrate

UO22+@ UiO-66AO

bond type

N

R (Å)

σ2×10−3 (Å2)

R factor 0.01

U− Oax U− O/ Neq U− Oax

2

1.77 ± 0.02

1.5 ± 0.4

5 ± 0.2

2.41 ± 0.02

5.9 ± 0.8

2.0 ± 0.4

1.76 ± 0.02

1.8 ± 0.6

U− O/ Neq

5.7 ± 0.5

2.41 ± 0.02

10.5 ± 0.9

ASSOCIATED CONTENT

S Supporting Information *

AUTHOR INFORMATION

Corresponding Author

*S. Wang. E-mail: [email protected]. ORCID

Shuao Wang: 0000-0002-1526-1102 Author Contributions

The paper was written through contributions of all authors. All authors have given approval to the final version of the paper.

0.016

Notes

The authors declare no competing financial interest.



a

Fitting procedure was performed on the k3-weighted FT-EXAFS from 3.0 to 11.0 Å−1. An R window of 1−3 Å was used for the fitting.

ACKNOWLEDGMENTS This work was supported by grants from the National Natural Science Foundation of China (21422704, 21601131), the Science Foundation of Jiangsu Province (BK20140007, BK20150313), the State Key Laboratory of Pollution Control and Resource Reuse Foundation (PCRRF16003), a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and Jiangsu Provincial Key Laboratory of Radiation Medicine and Protection, and “Young Thousand Talented Program” in China.

which is very close to the coordination environment of uranium in simulated seawater after contacting with amidoxime based polymer fibers.48 These coordination numbers and bond distances are in agreement with the average local coordination environment of uranyl containing 2 or 3 amidoximes binding in a chelating fashion.48 The six donor atoms in the second shell, on one hand, afford hexagonal bipyramid coordination configuration.38 On the other hand, it provides robust evidence D

DOI: 10.1021/acsami.7b12396 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces



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DOI: 10.1021/acsami.7b12396 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX