Metal–organic frameworks for radionuclide sequestration from aqueous solution: a brief overview and outlook

Chengliang Xiao, Mark A. Silver and Shuao Wang*
Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, School for Radiological and Interdisciplinary Sciences (RAD-X), Soochow University, Suzhou 215123, China. E-mail: shuaowang@suda.edu.cn

Received 29th September 2017 , Accepted 19th October 2017

First published on 19th October 2017


In this Frontier article, we pursue the sequestration of radionuclides from aqueous solution by using recently emerging metal–organic framework (MOF) materials. The design of MOF materials and their corresponding sorption properties towards radionuclides (137Cs, 90Sr, 238U, 79Se, and 99Tc) as well as their interaction mechanisms are highlighted. The present challenges and future prospects of removing radionulides with MOFs as sorbents are also demonstrated.


1. Introduction

The nuclear accident in 2011 at Fukushima's Daiichi Nuclear Power Plant stirred immediate caution among the worldwide nuclear community, especially with regard to further developing nuclear power; however, restraint from nuclear operations has relaxed over the past six years, especially in China and the United Kingdom.1 A large majority of countries using nuclear power have employed a strategy to recycle used nuclear fuel with the intent of minimizing the volume of radioactive waste, which is of the greatest concern, and maximizing the use of fissile materials.2 Optimizing the performance of these fuel cycles for actinides and fission products is still a great challenge, because interest in either of these isotopes drives new separations to be investigated.3 Most commonly used is the Plutonium Uranium Redox EXtraction (PUREX) separation process which recovers uranium and plutonium from the used fuel for further utilization as fissile materials. Minor actinides, such as americium and curium, possess long-term radiotoxicity and are separated and transmuted into lowly radiotoxic or stable metals.4–7 Fission products, such as 137Cs and 90Sr, are either removed by solvent extraction-based macrocyclic supramolecular hosts or by inorganic ion-exchange processes, while 79Se and 99Tc are separated using anion-exchange resins.8–11 These separation methods have been beneficial to harnessing as much nuclear energy as can be provided by available used fuel, but this disguises a considerable amount of improvement that can be made to achieve conservation of nuclear materials. The solvent extraction technique uses volatile and toxic organic diluents and modifiers, while still generating a large volume of secondary waste. Polymer-based anion-exchange resins exhibit slow uptake kinetics, as well as low stability towards alkaline solutions and high radiation doses. Lastly, purely inorganic ion-exchangers do not possess large capacity for uptake at high acidity, nor selectivity at high salinity.

Metal–organic frameworks (MOFs) are an emerging class of porous materials, comprising organic linkers that join metal ions/clusters.12 MOFs are known to have high specific surface areas, tunable pore size and shape, and facile functionalization that endow them with superior properties compared with traditional porous materials in applications of catalysis, gas storage, separation, sensing, and biomedicine.12–19 In this Frontier article, we summarize the recent development of MOF materials that sequester radionuclides, both cationic and anionic, from aqueous solution. The present challenges and further perspectives concerning this area are strongly underlined.

2. Cationic radionulides sequestration

Cs+ and Sr2+, which are largely produced during nuclear fuel decay, possess strong solvation energies and are difficult to desolvate to form complexes with organic functional groups. Therefore, ion exchange becomes one of the most efficient and applicable methods to separate them from aqueous solution. The removal of 137Cs and 90Sr has been considerately reported by using traditional inorganic ion exchangers,20,21 since both of them have stable isotopes that can be well simulated and operated without the need for any radiation protection facilities. However, few MOF materials have been investigated to sequester 137Cs and 90Sr, because the design of anionic metal–organic frameworks with weak-complexing cations in the pores is challenging. One of the more efficient strategies is to take advantage of the hydrolysis of organic solvents, such as N,N-dimethylformamide (DMF) or N,N-diethylformamide (DEF), to form [NH2(CH3)2]+ or [NH2(CH2CH3)2]+ cationic complexes under relatively high temperature conditions. The appropriate selection of ligands and secondary building units are also the key factors to construct anionic frameworks, especially regarding framework stability. Recently, a rare example of a 3D uranyl organic framework material (Fig. 1) was synthesized by the solvothermal reaction of 3,5-di(4′-carboxylphenyl) benzoic acid with UO2(NO3)2·6H2O in the presence of DMF and water.22 The structure was built through polycatenating three sets of graphene-like layers with exchangeable [NH2(CH3)2]+ cations within available pores. Batch experiments showed that the sorption kinetics of Cs+ was fast (20 min) and the maximum capacity achieved was as high as 145 mg g−1, which was comparable to the most efficient cesium ion-exchanger material reported. Even in the presence of 20 mass equivalences of competing Li+, Na+, K+, Rb+, Mg2+, or Ca2+ cations, the removal of Cs+ could reach as high as 72–94%. Even after 200 kGy 60Co γ irradiation and 200 kGy β irradiation, there was no structural or crystal degradation observed in this material, indicative of excellent resistance to radiation. Aguila et al.23 have functionalized the ultra-stable MIL-101 with sulfonic acid groups to obtain excellent cation exchange towards Cs+ and Sr2+. This MOF material exhibited high removal capabilities for Cs+ and Sr2+ at varied pH levels and in the presence of competing ions. At a molar ratio of 4[thin space (1/6-em)]:[thin space (1/6-em)]1 sorbent to Cs+/Sr2+ solution, 99.9% and 98.92% of available Cs+ and Sr2+, respectively, had been removed, yielding particularly high distribution coefficient (Kd) values. However, the removal kinetics was relatively slow, requiring 24 h to reach equilibrium. MOF materials provide an abundance of pores in tandem with high specific surface area to accommodate a variety of functional groups. Naeimi et al.24 prepared an efficient Cs+-sorbent from modified HKUST-1 with potassium nickel hexacyanoferrate. This functionalized MOF exhibited fast adsorption kinetics (30 min), high adsorption affinity (Kd = 1.5 × 103 mL g−1), high adsorption capacity (153 mg g−1), and excellent selectivity towards Cs+.
image file: c7dt03670a-f1.tif
Fig. 1 Structures of a rare case of polycatenated anionic uranyl organic framework material.

Uranium, normally existing as UO22+ in radioactive water, is believed to be sequestered from the viewpoint of nuclear fuel cycle and environmental protection. Recently, the extraction of U from resources like seawater has also necessitated the development of U-selective sorbents.25,26 Until now, about twenty MOF materials have been investigated and characterized for their abilities to sequester UO22+. The first application of MOF materials as novel sorbents to separate UO22+ from aqueous solution was reported by Lin's group.27 Two different phosphorylurea groups were covalently grafted onto UiO-68 MOFs and the resulting materials were shown to be efficient in extracting UO22+ with the maximum sorption capacity as high as 217 mg g−1. Density Functional Theory (DFT) calculations revealed that two phosphorylurea ligands bind to one uranyl ion. Taking advantage of chemical grafting, UO22+-selective groups can be facilely modified onto porous MOF surfaces. Two additional cases of functional MOF materials, carboxyl-derived MIL-10128 and CMPO-MIL-101,29 were also designed to remove UO22+ from aqueous solution. Another similar strategy involves binding organic functional groups to the open metal sites in MOF materials to selectively capture UO22+. Amine-functionalized MIL-101 MOFs are typical examples of this case.30,31 Several organic amines with different chain lengths were used to modify MIL-101 MOFs to investigate the sorption properties of UO22+. MIL-101-NH2, MIL-101-ED, and MIL-101-DETA showed maximum sorption capacities of 90, 200, and 350 mg UO22+ per g, respectively, at pH 5.5, the latter two being much higher when compared to that of their parent MOF, MIL-101 (90 mg g−1). Zhang et al.32 grafted coumarin onto the unsaturated Zn sites in Zn-MOF-74 to achieve an even greater sorption capacity of 360 mg UO22+ per g at pH 4. Despite these advances, functionalization of –NH2 groups onto UiO-66 did not greatly improve its sorption properties.33 UO22+-selective functional groups could also be incorporated into MOFs by a one-pot method during synthesis. Luo's group34,35 prepared two such MOFs, Zn(HBTC)(L)·(H2O)2 and Zn(ADC)(4,4′-BPE)0.5, each displaying efficient UO22+ extraction. As for those MOFs without post-modification, they still exhibited good sorption towards UO22+, but that may be attributed to the defects in MOF crystals containing bare non-coordinated carboxylic groups as well as the matched pore sizes. Both MOF-7636 with the formula of [Ln(BTC)(H2O)·DMF (Ln = Y, Tb, BTC = 1,3,5-benzenetricarboxylate)] and HKUST-137 could remove UO22+ from aqueous solution with remarkably high capacities (298 and 787.4 mg g−1, respectively).

MOF sorbents are not quite stable in aqueous solution, especially in highly acidic solutions, which limits the efficacy and reusability of these materials. Therefore, the development of MOF sorbents to sequester UO22+ from strong acidic solutions is very challenging but will result in highly impactful materials. Recently, our group synthesized three highly crystalline 3D microporous zirconium(IV) phosphonate MOFs (Fig. 2) from the ionothermal reactions of ZrCl4 with tetrakis[4-(dihyroxyphosphoryl)phenyl]methane or 1,3,5,7-tetrakis(4-phosphonophenyl)adamantine in ionic liquids.38 These structures were composed of zirconium phosphate clusters as the secondary building unit and displayed superior stability in acidic solutions, including aqua regia (Fig. 2). SZ-2 and SZ-3 exhibited potential applications in the removal of UO22+ ions from aqueous solution over a wide range of pH values. Common ion exchange materials, including most of the MOFs, often lose their UO22+ uptake capability below pH 2. Remarkably, UO22+ removal could reach 62.4% using SZ-2 at pH 1.0. Extended X-ray absorption fine structure (EXAFS) spectra and molecular dynamics (MD) simulations indicated that the strong electrostatic interaction between UO22+ and SZ-2 effectively drives UO22+ into the framework of SZ-2, while the formation of a much denser hydrogen bond network results in UO22+ being sufficiently trapped.


image file: c7dt03670a-f2.tif
Fig. 2 Crystal structures of (a) SZ-1, (b) SZ-2, and (c) SZ-3; stability of (d) SZ-1 and (e) SZ-3 towards different strong acids (SZ = Suzhou).

Fortuitously, lanthanide MOFs can be used to detect UO22+ at low concentrations in water, because of their high sorption capabilities and their unique fluorescent fingerprint. MOF-76 exhibited not only high sorption capacity, but also high sensitivity for the detection of UO22+.36 However, the fluorescent sensing of UO22+ in MOF-76 was not cautiously designed and the detection limit is still unknown. Recently, we reported a hydrolytically stable mesoporous Tb-based MOF with a channel as large as 27 Å × 23 Å with an abundance of open Lewis basic sites.39 The luminescence intensity of this MOF could be selectively and sensitively quenched by UO22+ ions even in the presence of other competing metal ions. The detection limit in deionized water reached as low as 0.9 μg L−1, well below the permissible level of 30 μg L−1 in drinking water (EPA). Similarly, Ye et al.40 reported a stable 3D Tb-based MOF, [Tb(BPDC)2]·(CH3)2NH2 (DUT-101, BPDC = biphenyl-4,4′-dicarboxylate) for UO22+ sensing, which was constructed by an anionic framework and protonated dimethylamine molecules in the channels. The lower limit of detection of this MOF for UO22+ was 8.34 μg L−1.

Although the sequestration of UO22+ by MOF materials has been investigated thoroughly recently, there is no report on the application of MOFs on the removal of other actinide cations (NpO2+, Pu4+, Am3+, and Cm3+).

3. Anionic radionuclide sequestration

In radioactive wastewater, common anionic contaminants include 129I, 79SeO32−, 79SeO42−, and 99TcO4. The non-complexing nature, high water solubility, and great stability of these anions lead to extremely high mobility in the environment. 131I is usually isolated by Ag-based sorbents,41 but the removal of anionic 79Se and 99Tc species from radioactive wastes remains a significant challenge. Purely inorganic cationic materials are very rare in the literature and their anion removal qualities are quite ineffective for practical use.42–44 Although anion exchange resins exhibit good removal efficiency in some cases, their slow uptake kinetics and poor resistance towards radiation limit their applications.10 These factors provide MOF materials the opportunity to compete with and surpass resins in this regard.

Until now, there have been very few examples of applying MOFs in the removal of 79Se, this only coming from Hupp and Farha's group45 and Wang's group.46 A series of stable Zr-based MOFs [UiO-66, UiO-66-NH2, UiO-66-(NH2)2, UiO-66-(OH)2, UiO-67, NU-1000 (Fig. 3a), and NU-1000BA] were investigated to remove SeO32− and SeO42− anions from aqueous solution. NU-1000 was shown to have the highest sorption capacity and fastest uptake rates for both SeO32− and SeO42− among all MOFs studied. The maximum sorption capacity of NU-1000 for SeO32− is 95 mg g−1 and for SeO42− is 85 mg g−1. Pair distribution function (PDF) analysis clearly showed that SeO32− and SeO42− anions were bound to the zirconium cluster nodes in a bridging (η2μ2) fashion where one anion bridges two zirconium metal centres (Fig. 3b).


image file: c7dt03670a-f3.tif
Fig. 3 (a) Crystal structures of NU-1000 and (b) potential sorption mechanisms between NU-1000 and SeO32−/SeO42−.

Although large amounts of cationic MOFs have been constructed for capturing anionic contaminants,47–51 few investigations were directly conducted with radioactive 99TcO4. Due to the similar ionic size and chemical properties between ReO4 and TcO4, ReO4 is usually considered as a good surrogate for TcO4 in the study of its sorption process. Fei et al.52 reported a cationic layered [Ag2(4,4′-bipy)2(O3SCH2CH2SO3)·4H2O] (SLUG-21) MOF material for the efficient removal of ReO4. The sorption capacity reached as high as 602 mg g−1, but the ethanedisulfonate anions would be exchanged out, which may be problematic in the waste vitrification process. Furthermore, this MOF material has relatively high solubility in aqueous solution. Banerjee et al.53 prepared a stable cationic Zr-based MOF for the removal of ReO4 from aqueous solution. However, there is still huge room for improvement in sorption kinetics, capacity, and selectivity.

Recently, we designed a new 3D cationic MOF material, SCU-100 (SCU = Soochow University), constructed using a tetradentate neutral nitrogen-donor ligand (tetrakis[4-(1-imidazol-yl)phenyl]methane) and Ag+ cations.54 SCU-100 contains 1D channels filled with exchangeable nitrate anions. This material could retain its crystallinity in aqueous solution over a wide pH range from 1 to 13 and exhibited excellent resistance towards high β and γ radiation (Fig. 4a). Sorption experiments involving SCU-100 with pertechnetate represent the first example of an MOF material being directly tested with radioactive TcO4. The results showed that all available TcO4 could be removed in 30 min (Fig. 4b), a remarkable finding for MOF sorbents. The maximum sorption capacity of ReO4 was 553 mg g−1 with the highest distribution coefficient of 1.9 × 105 mL g−1. It should be noted that large amounts of competing anions (NO3, SO42−, CO32−, and PO43−) showed nearly no influence on the removal of TcO4. SCU-100 could also selectively sequester 87% of TcO4 from simulated Hanford low-level waste. The interaction mechanism between SCU-100 and TcO4 was disclosed by single crystal X-ray diffraction. The exchange process occurs in a single-crystal-to-single-crystal transformation from an 8-fold interpenetrated framework with disordered nitrate anions to a 4-fold interpenetrated framework with ordered ReO4 anions. After sorption, the ReO4 anions were bound to open Ag+ sites forming Ag–O–Re bonds (Fig. 4c). A much denser hydrogen bonding network and the highly hydrophobic nature of the ligand also contributed to the high sorption capacity and excellent selectivity for TcO4.


image file: c7dt03670a-f4.tif
Fig. 4 (a) Stability of SCU-100 towards pH and radiation, (b) sorption properties of SCU-100 towards TcO4 as a function of contact time, and (c) single-crystal-to-single-crystal transformation during the sorption process.

Another cationic MOF material, Ag(4,4′-bipyridine)NO3, was also noted to effectively remove ReO4 with a high sorption capacity of 786 mg g−1,55 setting it apart from other reported scavenger materials. More importantly, upon capture, ReO4 was stabilized and immobilized in the solid phase, forbidden to exchange even in the molar excess of 1000 times NO3. After irreversible single-crystal-to-single-crystal transformation, the resulting material was the least soluble perrhenate/pertechnetate salt known to date, which might be considered as a potential technetium waste form to directly immobilize TcO4. DFT calculations unravelled that open soft Ag+ sites played a more critical role in the selective removal of ReO4/TcO4 than hydrogen bonding, which gave an important hint to designing more efficient cationic MOFs for TcO4 capture.

4. Summary and outlook

MOFs are porous crystalline materials constructed by the coordination of organic linkers with inorganic metal nodes. Bearing high surface area, abundant porosity, and tunable chemical properties, MOFs have been considered as a particularly promising class of sorbent materials that display unique performance when sequestering radionuclides. Compared to traditional sorbents, MOFs exhibit higher sorption capacity and faster kinetics towards cationic radionuclides. Methods used to prepare anionic MOF materials are only limited to the hydrolysis of organic solvents or sulfonation; so new concepts to construct a diverse catalogue of anionic MOFs are encouraged. For example, an anionic MOF may be synthesized by utilizing a negative ligand as the organic linker to complex metal ions or clusters. In addition, the pore size of MOFs can be easily altered to precisely match with certain cations (i.e. Cs+) by mimicking supramolecular crown-ether chemistry.

Due to the fragile nature of MOFs in aqueous solution, the sequestration of UO22+ from highly acidic solution is still challenging. It should also be noted that the application of MOF materials in capturing transuranic radionuclides (e.g. Np, Pu, Am, and Cm) is still unknown. As for the sequestration of radioactive anions by MOFs, an increasing trend of experiments in the literature illustrates their capability in this regard. Several cationic MOFs reported for the removal of anionic contaminants (i.e. AsO43−, CrO42−, and ClO4) provide excellent direction for designing appropriate MOFs for capturing anionic 79Se and 99Tc species. Those cationic MOFs constructed from neutral soft-donor ligands and transition metals offer great promise for anionic radionuclide separation and will be a mainstay in further research. Four rules provide guidance to design more exceptional sorbents: (1) ClO4 and SO42− may be used as templates for forming perfect pores, but they are problematic species during spent fuel reprocessing. During the synthesis of cationic MOFs, the use of metal chlorides or nitrates is highly recommended, instead. (2) Metals or clusters play a critical role in determining the sorption properties of the MOF. Cationic MOFs holding open metal sites will prefer to bind strongly with anions. (3) The hydrophobic nature of the pore surface in MOFs will be beneficial for selectively trapping TcO4 rather than SO42− and NO3. (4) The hydrogen bond network is also an important factor for the recognized sequestration of certain anions.56 However, the synthesis of hydrolytically and radiolytically stable cationic MOFs is still the most important aspect in this area of research.

From the view of real applications, it is best to prepare large single crystals (>100 μm) of MOF materials that are suitable for column chromatographic separation. Fortunately, powdered MOFs can still be combined with exchange resins, chitosan, and cellulose to prepare granular or fibre composites appropriate for actual field situations. In addition, magnetic57 and membrane MOFs for radionuclide sequestration are areas of research whose practicality is likely certain.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledged the financial supports from the National Natural Science Foundation of China (21422704, 11605118, U1732112), the Natural Science Foundation of Jiangsu Province (BK20140007, BK20150313), the State Key Laboratory of Pollution Control and Resource Reuse Foundation (PCRRF16003), the “Young Thousand Talented Program” in China, and the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions.

Notes and references

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