“Boarding-Up”: Radiation Damage and Radionuclide Leaching

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“Boarding-Up”: Radiation Damage and Radionuclide Leaching Kinetics in Linker-Capped Metal−Organic Frameworks Anna A. Berseneva,§,† Corey R. Martin,§,† Vladimir A. Galitskiy,§,† Otega A. Ejegbavwo,† Gabrielle A. Leith,† Richard T. Ly,† Allison M. Rice,† Ekaterina A. Dolgopolova,† Mark D. Smith,† Hans-Conrad zur Loye,† David P. DiPrete,‡ Jake W. Amoroso,‡ and Natalia B. Shustova*,† †

Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States Savannah River National Laboratory, Aiken, South Carolina 29808, United States

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‡

S Supporting Information *

ABSTRACT: For the first time, we report the ability to control radionuclide species release kinetics in metal−organic frameworks (MOFs) as a function of postsynthetic capping linker installation, which is essential for understanding MOF potential as viable radionuclide wasteform materials or versatile platforms for sensing, leaching, and radionuclide sequestration. The radiation damage of prepared frameworks under γ radiation has also been studied. We envision that the presented studies are the first steps toward utilization of the reported scaffolds for more efficient nuclear waste administration.

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INTRODUCTION Fundamental knowledge about structural motifs and architectures, radionuclide leaching kinetics, and thermodynamics of wasteforms is crucial to address current concerns regarding efficient storage, separation, sensing, and selective sequestration of nuclear waste.1−10 Metal−organic frameworks (MOFs) have become a versatile tool to address some of these challenges due to their (1) unprecedented modularity, (2) high internal surface area, (3) selective binding affinities, and (4) “on-demand” topology.11−16 Another intriguing direction in this area is simultaneous detection/sequestration of radionuclides and the ability to seal them inside a porous scaffold using postsynthetic modification procedures. In other words, the sequestrated actinide components could be trapped inside the framework. There are a number of reports that provide guidelines for postsynthetic capping linker installation as a function of pore aperture and geometry.17,18 For instance, relatively robust zirconium-containing Zr6O4(OH)8(Me2BPDC)4 (Me2BPDC2− = 2,2′-dimethyl-4,4′-biphenyldicarboxylate)18

can be used, which will be referred to as Zr-MOF hereafter (Figure 1). The Zr-MOF possesses the required unsaturated metal nodes (i.e., less than 12 linkers per node) that provide the necessary sites for postsynthetic incorporation of capping linkers. Herein, we report the first studies of leaching kinetics of uranyl species as a function of installed capping linker, highlighting the possibility of controlling release rates upon postsynthetic linker installation. Using the example of H2BDC, H2NDC, and H2TPDC (BDC2− = benzene-1,4-dicarboxylate, NDC2− = naphthalene-2,6-dicarboxylate, and TPDC2− = 2′amino-[1,1′:4′,1″-terphenyl]-4,4″-dicarboxylate) linkers, we demonstrate that it is possible to “board-up” the different pores in a Zr-MOF affecting radionuclide release kinetics (Scheme 1). As an aspect of these studies, we also investigate the possible effect of γ radiation on framework integrity. A comprehensive analysis of the samples was performed using a combination of powder X-ray diffraction (PXRD), gas sorption analysis, inductively coupled plasma mass spectrometry (ICPMS), and thermogravimetric analysis (TGA) as well as nuclear magnetic resonance (NMR), photoluminescence (PL), UV− vis, and Fourier-transform infrared (FTIR) spectroscopies.

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Scheme 1. Radionuclide Leaching Kinetics Modulation in a Boarded-Up Zr-MOF

RESULTS AND DISCUSSION The Zr-MOF was prepared by heating ZrCl4 and H2Me2BPDC in N,N-dimethylformamide (DMF) at 120 °C for 72 h. The first attempts focused on installation of only one type of linker. Special Issue: Innovative f-Element Chelating Strategies Received: May 4, 2019

© XXXX American Chemical Society

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DOI: 10.1021/acs.inorgchem.9b01310 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (top) Schematic representation of capping linker installation in Zr-MOF. (bottom) Solvent-accessible space-fill models of isostructural Zr-MOFs different from each other by installed capping linkers along the crystallographic b- and c-axes, emphasizing the location of the installed capping linkers.

centered at 500 nm in the case of TPDC2−-containing ZrMOFs, which is characteristic of H2TPDC immobilization (Figures S19−S21). Furthermore, the samples were analyzed by FTIR spectroscopy (Figures S7−S15). For instance, in the case of Zr-MOF(TPDC), a new stretching mode at 3400 cm−1 appeared, which was attributed to the presence of a primary amine on the skeleton of the incorporated TPDC2− linker (Figure S7). After preparation and characterization of postsynthetically modified Zr-MOFs, we performed a similar capping linker installation in the presence of uranyl acetate. As a result, the following MOFs,UO2 @Zr-MOF, UO 2 @Zr-MOF(BDC), UO2@Zr-MOF(NDC), UO2@Zr-MOF(TPDC), UO2@ZrMOF(TPDC+BDC-s), UO 2 @Zr-MOF(TPDC+BDC-c), UO2@Zr-MOF(TPDC+NDC-s), and UO2@Zr-MOF(TPDC +NDC-c), were synthesized (experimental details are given in the Supporting Information). PXRD analysis was used to confirm the structural integrity of the prepared samples (Figures S1−S15). PL studies (λex = 350 nm) confirmed the presence of uranyl cations (UO22+) in the pores due to the appearance of characteristic bands between 450 and 550 nm (Figures S16−S18). TGA was utilized for studying thermal properties of the reported MOFs (Figures S22−S25). FTIR spectra further support UO22+ integration by the appearance of

For that, the prepared Zr-MOF was subjected to 30 mM DMF solutions of H2BDC, H2NDC, or H2TPDC and heated at 75 °C for 24 h. After single-linker immobilization, we focused on installation of two different linkers, H2BDC (or H2NDC) and H2TPDC, using two methods. The first method is a simultaneous procedure in which heating of Zr-MOF in the presence of H2BDC (or H2NDC) and H2TPDC at 75 °C for 24 h resulted in the formation of Zr-MOF(TPDC+BDC-s) or Zr-MOF(TPDC+NDC-s) (s = simultaneously installed linkers). The second method is a stepwise procedure in which H2TPDC was installed first after which Zr-MOF(TPDC) was washed, placed in a solution of H2BDC (or H2NDC) in DMF, and heated at 75 °C for 24 h, resulting in the formation of Zr-MOF(TPDC +BDC-c) or Zr-MOF(TPDC+NDC-c) (c = consecutively installed linkers). A schematic representation of linker installation is shown in Figure 1. The degree of linker installation was determined based on 1H NMR spectroscopic analysis of the digested samples (i.e., frameworks were destroyed in the presence of acid; Figures 2 and S2−S14). Analysis of PXRD patterns revealed consistent peak shifts related to linker installation previously reported for Zr-MOFs17 (Figures S1−S15). PL studies (λex = 350 nm) revealed the appearance of a new band B

DOI: 10.1021/acs.inorgchem.9b01310 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

There are two pockets available for installation of capping linkers, one of them is perpendicular to the crystallographic caxis (pockets A, Figure 2) and the other is parallel to the c-axis (pockets B, Figure 2). Upon installation of a linker into one pocket, the second pocket is reduced with geometric constraints.18 The three-phenyl ring linker TPDC2− (15.3 Å) can only occupy pockets A due to favorably matching in size (16.4 Å). Indeed, TPDC2− occupies ca. 100% available sites of pockets A according to 1H NMR spectroscopy. For BDC2− and NDC2− linkers, however, there is the opportunity for the linker to be immobilized in pockets B as well as A, i.e., they can be “occupied” by the linker through anchoring on one side. We observed 46 and 43% of linker installation (pockets A and B) for BDC2− (Zr-MOF(BDC)) and NDC2−(Zr-MOF(NDC)), respectively. In the case of consecutive linker installation, we chose to install TPDC2− first in an attempt to circumvent the anchoring of BDC2− or NDC2− to pockets A. The data are summarized in the table of Figure 2. Leaching kinetics experiments were performed on all uranylcontaining samples by taking 10−90 μL aliquots from the supernatant DMF solution during discrete time intervals. The collected aliquots were diluted with a complexing solution of arsenazo (III) as a coloring reagent to spectrophotometrically determine the UO22+ concentration (more experimental details are given in the Supporting Information).19 As expected, a significant increase of uranyl cation concentration in the supernatant solution was observed over time. As a first approximation for the kinetics data, we utilized previously explored models that were applied to a drug delivery process.20 We attempted to fit the experimental data using zero-, first-, and second-order models, including but not limited to the Higuchi, Korsemeyer−Peppas, and Baker−Lonsdale models (Table S2 and Figures S31−S32). However, none of these models provided a reasonable fit according to the estimated goodness of fit (R2) values (Table S2). Although in some cases the Higuchi and Korsemeyer−Peppas models resulted in a high R2 value, we did not apply these models due to the restrictions imposed by the shape of the considered matrix. As a result of the data analysis shown in Figures S31 and S32 and Table S2, we propose that UO22+ leaching from MOF matrixes occurs by two processes, specifically, an initial fast release, which is followed by a slow, kinetically hindered release. The former is associated with the so-called “burst effect”21 attributed to the adsorption of UO22+ on the MOF surface. We hypothesize that this initial burst upon beginning the leaching process is determined mostly by dissolution of UO22+ that is not trapped, but instead located primarily on the surface of the MOF. At the same time, the second process can be described by a zero-order model, i.e., a steady state release of trapped uranyl species.22,23 To describe these processes, we used a combination of first- and zero-order rate equations resulting in an R2 value of >0.95 (Table S2, Figures S33 and S34). The leaching rate during the first 24 h is described as a first-order process. The rate constant (k1) associated with this process was estimated to be approximately 4 × 10−5 s−1, which is independent of linker installation and similar for all studied MOF samples. This fact confirms our hypothesis that the first process is associated with the removal of uranyl cations from the framework surface. The zero-order model rate constants (k2) are listed in Table S2. In Figure 3, the release profiles related to Zr-MOF, Zr-MOF(TPDC), and Zr-MOF(TPDC +NDC-c) demonstrate the expected tendency: the capping linker installation results in a decrease of k2 due to the blockage

Figure 2. (top) Depiction of available pockets for capping linker installation and (bottom) the ratio of installed capping linkers inside the MOF.

a new vibrational mode at 915 cm−1 corresponding to the presence of UO (Figures S1−S15). ICP-MS analysis (Table S1) was utilized to quantify the amount of UO22+ inside the MOFs. In the case of UO2@Zr-MOF, the amount of UO22+ per metal node was found to be 2.1 cations. Due to the installation of capping linkers that affect the internal voids (Figure 1), the degree of uranyl incorporation decreases. Thus, we estimated the loading of 1.2 and 1.0 cations per metal node for UO2@Zr-MOF(NDC) and UO2@Zr-MOF(TPDC), respectively. In the case of two installed capping linkers, 0.63 and 1.1 cations per metal node were observed for UO2@ZrMOF(TPDC+NDC-s) and UO2@Zr-MOF(TPDC+NDC-c), respectively. Higher amounts of UO22+ inside the MOFs with consecutively installed linkers were caused by additional exposure to uranyl acetate in both steps of the synthesis. In support of the observed trend, gas sorption analysis was performed. The Brunauer−Emmett−Teller (BET) surface area of the parent Zr-MOF was estimated to be 1748 m2/g (Figure S26), which gradually decreased to 1415 m2/g in the case of Zr-MOF(TPDC) and 1323 m2/g for Zr-MOF(TPDC+NDCc) (Figures S27 and S28). Further reduction to 421 m2/g (UO2@Zr-MOF(TPDC+NDC-c)) occurred upon UO22+ incorporation (Figure S29). C

DOI: 10.1021/acs.inorgchem.9b01310 Inorg. Chem. XXXX, XXX, XXX−XXX

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irradiation, we performed a structural analysis using singlecrystal X-ray diffraction before and after irradiation, which determined that the structure of Zr-MOF remains intact under irradiation.

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CONCLUSION To conclude, the leaching kinetics of a series of Zr-based MOFs with postsynthetically installed capping linkers were evaluated. Using the example of Zr-MOFs, we demonstrate that the leaching rate constant decreases as the degree of installed linkers increases. Moreover, our studies of possible radiation damage demonstrate the structural stability of ZrMOF under γ irradiation (19.7 Mrad). The presented studies are the first example of leaching kinetics of uranium-containing species from MOFs, which are essential for understanding their potential as viable radionuclide wasteform materials or versatile platforms for selective actinide separation or sensing.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01310. Additional experimental details: PXRD patterns, TGA traces, N2 adsorption isotherms, FTIR, PL, and 1H NMR spectra (PDF)

Figure 3. (top, left) Plot of inverse structural density (1/d) and (top, right) zero-order rate constants, k2, as a function of the isostructural Zr-MOF with installed linkers. (bottom) Kinetics of uranyl leaching from Zr-MOF (blue), Zr-MOF(TPDC) (red), and Zr-MOF(TPDC +NDC-c) (green). 1 = Zr-MOF(NDC), 2 = Zr-MOF, 3 = ZrMOF(BDC), 4 = Zr-MOF(TPDC+NDC-s), 5 = Zr-MOF(TPDC), 6 = Zr-MOF(TPDC+NDC-c), (possible deviation due to a large discrepancy of linker installation percentage in this work and literature),17 7 = Zr-MOF(TPDC+BDC-c), and 8 = Zr-MOF(TPDC+BDC-s).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. of pores. For instance, installation of TPDC2− linkers block the largest pores (16.4 Å) perpendicular to the c-axis, while incorporation of BDC2− or NDC2− linkers occupy the pores parallel to the c-axis. The observed trend in k2 values agrees with changes in structural density, d. Indeed, capping linker installation results in an increase of d and a decrease of k2. For instance, Zr-MOF possesses a lower crystallographic density than that of Zr-MOF(TPDC+BDC-s), and therefore, the leaching rate (k2) for Zr-MOF (1.2 × 10−9 M·s−1) is three times higher than that of Zr-MOF(TPDC+BDC-s) (4.0 × 10−10 M·s−1). The established correlation between 1/d and k2 is shown in Figure 3. The exceptions from this trend are ZrMOF(NDC) and Zr-MOF(TPDC+NDC-c). In the former case, the deviation from the general tendency could be due to elongation of pockets B from 7.0 to 8.9 Å upon NDC2− installation.17 Analysis of the solvent-accessible voids also supports the observed deviation of Zr-MOF(NDC). The pore volume calculated for Zr-MOF was found to be 6073 Å3 versus 6371 Å3 estimated for Zr-MOF(NDC). The inconsistency for Zr-MOF(TPDC+NDC-c) was ascribed to a large deviation between the number of installed linkers found in this work and observed crystallographically, which can be due to an opposite sequence of linker installation.17 Upon completing leaching kinetics experiments, potential radiation24,25 damage of Zr-MOF was also studied using the Shepherd Model 109 Co-60 gamma irradiator equipped with Co-60 pencil sources. The Zr-MOF samples to be irradiated were placed as near to the center of the chamber as allowable to maximize dose uniformity. The dose rate has been decay corrected to 94.3 krad/h during the irradiation of Zr-MOF. The samples were irradiated for 209 h, corresponding to a total dose of 19.7 Mrad. To estimate possible damage due to

ORCID

Hans-Conrad zur Loye: 0000-0001-7351-9098 Natalia B. Shustova: 0000-0003-3952-1949 Author Contributions §

A.A.B., C.R.M., and V.A.G. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported as part of the Center for Hierarchical Wasteform Materials (CHWM), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science under Award DE-SC0016574. N.B.S. acknowledges the support from the Sloan Research Fellowship provided by Alfred P. Sloan Foundation and Camille Dreyfus Teacher-Scholar Award supported by the Camille and Henry Dreyfus Foundation. Work conducted at Savannah River National Laboratory was supported by the U.S. Department of Energy under contract DE-AC09-08SR22470.

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