Topotactic Transformations of MetalâOrganic Frameworks to Highly

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Topotactic Transformations of Metal−Organic Frameworks to Highly Porous and Stable Inorganic Sorbents for Efficient Radionuclide Sequestration Carter W. Abney,† Kathryn M. L. Taylor-Pashow,*,‡ Shane R. Russell,§ Yuan Chen,∥ Raghabendra Samantaray,∥ Jenny V. Lockard,∥ and Wenbin Lin*,† †

Department of Chemistry, University of Chicago, 929 East 57th Street, Chicago, Illinois 60637, United States Savannah River National Laboratory, Aiken, South Carolina 29808, United States § Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599, United States ∥ Department of Chemistry, Rutgers University, 73 Warren Street, Newark, New Jersey 07102, United States ‡

S Supporting Information *

ABSTRACT: Innovative solid-phase sorbent technologies are needed to extract radionuclides from harsh media for environmental remediation and in order to close the nuclear fuel cycle. Highly porous inorganic materials with remarkable sorptive properties have been prepared by topotactic transformations of metal−organic frameworks (MOFs) using both basic and acidic solutions. Treatment of Ti and Zr nanoMOFs with NaOH, Na3PO4, and H3PO4 yields Ti and Zr oxides, oxyphosphates, and phosphates via sacrificial removal of the organic ligands. This controlled ligand extraction process results in porous inorganic materials, which preserve the original MOF morphologies and impart useful surface functionalities, but are devoid of organic linkers. Structural investigation by X-ray absorption spectroscopy reveals preservation of the coordination environment of the scattering metal. Changing the MOF template introduces different metal and structural possibilities, while application of different digest solutions allows preparation of metal oxides, metal oxyphosphates, and metal phosphates. The high stability and porosity of these novel materials makes them ideally suited as nanosorbents in severe environments. Their potential for several radionuclide separations is demonstrated, including decontamination of high level nuclear waste, extraction of lanthanides, and remediation of radionuclide-contaminated seawater.

1. INTRODUCTION

been explored as sacrificial templates for the preparation of porous carbon32−37 or cross-linked cube-shaped polymer gels,38,39 as the fragile metal−ligand bond allows removal of the metal from the MOF. Conversely, by removing the organic bridging ligand, MOFs can also be used as templates for the synthesis of inorganic materials. MIL-88 was used to prepare mesoporous α-Fe2O3 and Fe3O4 materials by calcination, and the resulting mesoporous α-Fe2O3 was investigated as an anode for lithium batteries.40,41 Prussian blue was used to prepare mixed β- and γFe2O3 materials by selective etching with CTAB followed by thermal decomposition,42 whereas treatment of Prussian blue under basic conditions was used to form hollow metal oxide and hydroxide nanoboxes.43 Porous CuO,44 MgO, and CeO245 were also prepared from MOF precursors through an annealing process, and it was demonstrated that varying gas flow and

Metal−organic frameworks (MOFs) are highly porous coordination polymers composed of organic bridging ligands and inorganic secondary building units (SBUs), which can be either individual metals or metal clusters.1−10 Their vast potential is largely due to the structural control afforded by tuning the bridging ligand with respect to length, symmetry, and functionality, lending them to investigation for use in gas storage and separations,11−19 catalysis,20−22 molecular sensing,23−25 and many other applications.26−29 The relatively weak coordination bond strength between the SBU and the bridging ligand, while a bottleneck for many proposed applications of MOFs, can be exploited to afford new materials that benefit from the modest chemical stability and modifiable structure. Several promising applications leverage this intrinsic characteristic to great advantage. For instance, the relative weakness of the metal−ligand bond makes nanoMOFs an ideal delivery vehicle for imaging contrast agents and chemotherapeutics, as degradation in biological systems allows release of cargoes in a timely fashion.30,31 Alternatively, MOFs have © 2014 American Chemical Society

Received: May 24, 2014 Revised: July 8, 2014 Published: August 29, 2014 5231

dx.doi.org/10.1021/cm501894h | Chem. Mater. 2014, 26, 5231−5243

Chemistry of Materials

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for environmental remediation in the event that radioactive material is released. To demonstrate the utility and flexibility of these novel MOF-templated materials, we investigate their potential for decontamination of high-level nuclear waste (HLW), lanthanide extraction, and remediation of radioactive seawater simulating the contaminated cooling water from the Fukushima Daiichi disaster. In all instances, these new inorganic sorbents were superior to the current state-of-the-art materials employed for these applications, surpassing their sorption properties in both capacity and kinetics.

temperature allowed tuning of pore dimensions in the resulting oxide materials.45 Very recently, MOFs prepared with ionic liquid-water templates were used to prepare porous crystalline Mn3O4.46 Mn MOF precursors were formed by combining terephthalic acid and MnCl2 in solutions of methylimidazolium hexafluorophosphate and water, while subsequent calcining under N2 at 400 °C yielded the Mn3O4 materials. Varying the size of the molecule composing the ionic liquid allowed for systematic tuning of the surface area and pore diameter of the final material. An iron MOF, MIL-101, was used as a template for coating with amorphous TiO2. Subsequent calcination to an Fe2O3@TiO2 nanoparticle, followed by deposition of Pt particles, allowed its use for photocatalytic hydrogen production.47 A microporous TiO2 material was also prepared by infiltration of HKUST-1 with titanium isopropoxide, heating, and etching of the MOF template.48 The resulting structure replicated the topology of the MOF precursor and yielded X-ray diffraction patterns suggestive of phase pure brookite. In this work, we report the use of MOFs as templates to prepare six new stable and porous inorganic materials by a novel, controlled ligand extraction process.49 Similar to postsynthetic functionalization,50,51 postsynthetic exchange,52 and metal-ion exchange,53 common MOF precursors can form different functional materials by extracting the ligands and substituting with inorganic moieties dissolved in the extraction solution. Unlike thermally prepared porous metal oxides, changing the ligand extraction solution allows for varying the composition of the final material while retaining surface area, porosity, and imparting delicate surface functionalities. The resulting robust inorganic materials also preserve the morphologies of the original MOF templates. We broadly use the term “topotactic” to highlight the retention of morphology, porosity, and metal coordination environment from the original MOF template,54 rather than the strict definition of topotactic from the organic solid state literature.55,56 Leveraging the high surface areas and stabilities of these new inorganic materials, we demonstrate their applicability as sorbents for several nuclear power-related processes. As the global consumption of energy exceeds 13 TW and population continues to grow, sustainable production of energy is one of the most pressing scientific and technological challenges.57 While great progress has been made with renewable energy, nuclear power remains the only scalable, carbon neutral energy source capable of replacing fossil fuels in the near term. However, the potential for environmental contamination and the release of radioactive material remains a significant hazard; in the United States alone, more than 340 million liters of legacy nuclear waste currently await a long-term disposal solution.58 The complex media from which these radionuclides must be extracted often varies dramatically in pH, ionic strength, and temperature, presenting significant challenges for conventional sorbents.59 Additionally, recovering actinides from spent nuclear fuel while minimizing the volume of nuclear waste is an important topic for closing the fuel cycle. One of the greatest challenges pertains to separating trivalent actinides from lanthanides (Ln), due to numerous similarities in their physical properties.60−62 While various liquid−liquid extraction processes have been developed for this purpose, an advanced solid phase sorbent could dramatically increase the efficiency of these processes.63 Finally, the ability to rapidly and efficiently decontaminate large quantities of water is essential

2. EXPERIMENTAL SECTION Synthesis of MOF precursors was performed as reported in the literature,64−66 with additional synthetic details provided in the Supporting Information. 2.1. Base-Treatment of MOF Templates (TiOx, TiOxyPhos, ZrOx, and ZrOxyPhos). Up to 100 mg of MOF was collected from the storage solution by centrifugation (10,000 rpm for 10 min). The MOF was then sonicated to full suspension in 5 mL of water and transferred to an HDPE bottle. An equal volume of 10 M NaOH solution (TiOx, ZrOx) or 210 mM Na3PO4 solution (TiOxyPhos, ZrOxyPhos) was added, creating a suspension of MOF with mass/vol ratio less than 10. The bottle was capped and agitated at 300 rpm on a plate shaker overnight. The resulting white solid was collected by centrifugation and washed with water three times. The inorganic material was stored in water until use. Material was obtained in up to 90% yield (TiOx), 75% yield (TiOxyPhos), 72% yield (ZrOx), and 57% yield (ZrOxyPhos). 2.2. H3PO4-Treatment of MOF Templates (TiPhos and ZrPhos). Fifty milligrams of MOF was collected from the storage solution by centrifugation (10,000 rpm for 10 min). The MOF was then sonicated to full suspension in 6.66 mL of water (MIL-125) or 7.50 mL of water (UiO-66) and transferred to an HDPE bottle. Then 3.40 mL (MIL-125) or 2.50 mL (UiO-66) of 1 M H3PO4 solution was added to the MOF-template solution. The bottle was capped and agitated at 300 rpm on a plate shaker overnight. The resulting white solid was collected by centrifugation and washed with water three times, followed by three washes with N,N-dimethylformamide (DMF) and three additional washes with water. The inorganic material was stored in water until use. Material was obtained in up to 39% yield (TiPhos) and 61% yield (ZrPhos). 2.3. X-ray Absorption Spectroscopy. The X-ray absorption data were collected at Beamline X18A at the National Synchrotron Light Source (NSLS). Spectra were collected at the titanium K-edge (4966 eV) or zirconium K-edge (17998 eV) in transmission mode. The X-ray white beam was monochromatized by a Si(111) monochromator and detuned by 25% to minimize the harmonic content of the beam. A Ti foil was used as the reference for energy calibration. The incident beam intensity (I0) was measured by a 15 cm ionization chamber with 25% N2 and 75% He gas composition. The transmitted beam intensity (It) and reference (Ir) were both measured by 30 cm ionization chambers with 90% N2 and 10% Ar gas composition. X-ray absorption spectroscopy (XAS) spectra (three scans) were collected at room temperature (∼25 °C) for each sample, which was mixed with boron nitride to achieve approximately one absorption length. The data were processed and analyzed using the Athena and Artemis programs of the IFEFFIT package based on FEFF 6.67,68 2.4. High Level Waste (HLW) Decontamination. Strontium and actinide removal testing with the HLW simulant occurred at 25 °C with sorbent concentrations targeted to provide a Ti concentration of approximately half of the concentration used for the current baseline material at Savannah River Site (i.e., 0.4 g/L of monosodium titanate (MST) or 0.192 g Ti/L, 4.04 mM Ti). Zr materials were added at approximately one-quarter of the molar concentration (∼1 mM Zr). Samples of MST were also run in these tests to provide a direct comparison. Test bottles were shaken at 175 rpm in a shaker-oven during sorption testing. Sampling of the test bottles occurred at 5232



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Figure 1. Schematic depicting the ligand extraction process. The UiO-66 MOF is treated with H3PO4 solution, resulting in the extraction of the BDC bridging ligand to form the porous amorphous material, ZrPhos.

Figure 2. Representative characterization data for MOF templates and MOF-derived porous inorganic materials. (a) TGA of Ti materials show no significant weight loss, while the MIL-125 template shows a distinct drop in weight around 400 °C. (b) PXRD reveals long-range ordering for the MIL-125 template beyond 2θ of 50°, while the treated materials show no significant ordering. (c) Nitrogen isotherm for surface area characterization of the MIL-125 templated materials reveal preservation of porosity and surface areas. (d) TGA, (e) PXRD, and (f) nitrogen isotherms of Zr materials possess characteristics similar to those derived from MIL-125. varying times of contact. Prior to sampling the test bottles, the bottles were manually agitated to obtain a representative aliquot of both the solids and solutions. The samples were filtered through 0.10 μm polyvinylidene fluoride (PVDF) membrane syringe filters to remove the solids. A measured amount of the filtrate was then acidified with an equal volume of 5 M nitric acid solution, mixed well, and allowed to stand with occasional mixing for a minimum of 2 h before radiochemical analyses. Gamma spectroscopy measured the 85Sr activity. The 239,240Pu content was analyzed by radiochemical separation of the plutonium followed by alpha counting of the extracted plutonium. Selected isotopes including 86,88Sr, 237Np, 239,240 Pu, and 235,238U were also measured by inductively coupled plasma−mass spectrometry (ICP−MS). 2.5. Lanthanide (Ln) Separation. Fifty milligrams of each sorbent was suspended in aqueous solution at either pH = 3 or pH = 6, with pH adjusted using 1 M HNO3 or 1 M NaOH. The pH for each sorbent, as well as the Ln solutions, were measured every 24 h and readjusted to the appropriate pH. Samples were not used until the pH did not change by more than 0.1 pH units over 24 h. The samples

were collected by centrifugation and suspended in 1 mL of Ln solution to obtain the proper phase ratio of 20 mL/g. Full suspension was obtained by sonication, followed by agitation for 24 h on a plate shaker at 300 rpm. Sorbents were then extracted by centrifugation and supernatant analyzed by ICP−MS in 5% aqueous HNO3 solution. Ln concentrations were obtained by measuring against a negative control. For sorbent recycling experiments, a similar protocol was used as the one described above. After a contact time of 24 h, the supernatant was removed and the sorbents suspended and sonicated in 1 M HNO3 for 1 h. Sorbents were collected by centrifugation and the supernatant retained for analysis. The sorbents were washed two times with water adjusted to pH 6 before being reused for Ln solution, as discussed above. Aliquots of supernatant following sorption and elution were analyzed by ICP−MS for Ln concentration and compared against a control sample. 2.6. Fukushima Seawater Remediation. Sorption testing was performed using simulated seawater containing 8 ppm of Sr (composition provided in Supporting Information). The seawater was also spiked with 85Sr radiotracer to allow for quantification of the 5233



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Figure 3. TEM images of (a) MIL-125, (b) UiO-66, (c) TiOx, (d) ZrOx, (e) TiOxyPhos, (f) ZrOxyPhos, (g) TiPhos, and (h) ZrPhos. The scale bars are 500 nm for all images. Sr removal via gamma spectroscopy. Each sorbent was added at a metal (Ti or Zr) concentration of 2.4 g/L. MST and SrTreat were included in the test set for comparison. Test bottles were shaken at 175 rpm in a shaker-oven at 25 °C. Test bottles were sampled after 1 or 24 h of contact. At the sampling time the test bottle was filtered through a 0.10 μm polyvinylidene fluoride (PVDF) membrane syringe filter to remove the solids. Aliquouts of the filtrate were then analyzed on a Packard Cobra II Gamma Counter. Comparison to a blank control was used for quantifying the amount of Sr removed in each test. Similar experiments were also performed using a 10× diluted sample of simulated seawater. For these experiments, sorbents were added at a metal concentration (Ti or Zr) of 0.19 g/L.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization. The MOF templates MIL-125 and UiO-66, with framework formulas Ti8O8(OH)4(BDC)6 and Zr6O4(OH)4(BDC)6 (BDC = 1,4benzenedicarboxylate), respectively, were prepared by solvothermal procedures that were modified from the literature syntheses.64−66 These MOFs were selected as precursors because they share a common bridging ligand and possess stable SBUs formed from metal-oxo clusters. Characterization by transmission electron microscopy (TEM) and scanning 5234



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unattainable by pyrolytic methods (Table S1, Supporting Information). Additional characterization was performed by FT-IR spectroscopy to confirm the functionalities incorporated by the treatment processes (Figure S11−S12, Supporting Information). The materials prepared from the MIL-125 template, TiOx and TiOxyPhos, possess a distinct peak around 3421 cm−1, attributable to Ti−OH.70,71 This differs noticeably from the broad band due to coordinated water molecules at 3500− 3000 cm−1, which can be observed in TiPhos, the MIL-125 precursor, TiO2 obtained by calcining the MOF template, and literature spectra of neat TiO2.70,71 The band at 1616 cm−1 is due to the bending mode of absorbed water molecules71 as well as from Ti−OH,70 despite extensive drying of all materials under vacuum at 95 °C for 48 h. In TiPhos, the peak at 1248 cm−1 can be assigned to an assymmetric stretching mode of P− O−H,72,73 while in TiPhos and TiOxyPhos, the peak at 1018 cm−1 is due to symmetric P−O stretching.72 Similar characterization of UiO-66 templated materials reveal a significant Zr− OH stretching peak around 3390 cm−1 for ZrOx and ZrOxyPhos, and a shoulder at 3417 cm−1 for PO−H stretching in ZrPhos.72 A strong band is clearly visible from 1072 to 1051 cm−1 in both ZrPhos and ZrOxyPhos, attributable to the combination of symmetric P−O stretching and both symmetric and asymmetric P−OH stretching. The band for asymmetric P−O stretching is present as a shoulder at approximately 1140 cm−1 for both materials.72 It is worth noting no bands attributable to hydroxyl groups are visible in the materials prepared by calcining the MOF or in the literature spectra for TiO2 or ZrO2.70−72 As the presence of surface hydroxyl or phosphoryl groups is often necessary for effective sorption, this novel wet templating method has great potential for developing new functional materials. Nitrogen-uptake isotherms were used to investigate surface areas and pore sizes of the materials. Brunauer−Emmett−Teller (BET) analyses of the materials revealed that porosity was preserved by this novel ligand extraction method. Surface areas of 329, 184, 366, 379, 281, and 401 m2 g−1 were obtained for TiOx, TiOxyPhos, TiPhos, ZrOx, ZrOxyPhos, and ZrPhos, respectively. These surface areas are comparable with pillared Zr-phosphonates obtained by Clearfield and colleagues (250− 440 m2/g)74 and zirconium oxophosphate materials reported by Um et al. (468 m2/g),75 as well as other titanium and zirconium phosphate materials prepared by various means.76 Pore size distribution was initially investigated for all materials using the method of Barrett−Joyner−Hallenda (BJH);77 however, all significant pore features were observed to be smaller than 20 nm where the method is inaccurate. Thus, we applied the Horvath−Kawazoe (HK) method78 in an effort to examine the fine pore structure. Plots of pore size distribution with both BJH and HK methods are included for all materials in Figure S13−S18, Supporting Information. TiOx had significant pore features of 13.8, 30, and 100 Å. TiOxyPhos also had pores of 13.8 Å with a second significant pore distribution around 80 Å. TiPhos only had one feature with an average pore size of 5.6 Å. In comparison, ZrOx and ZrPhos had pores approximately 5.0 Å, with a second lesser feature around 60 or 50 Å, respectively, while ZrOxyPhos had pores closer to 12.8 Å. All materials possess Type IV isotherms, with hysteresis loops resulting from capillary condensation within pores, indicating the presence of mesopores of diameters of 3 to 10 nm. These mesopores should facilitate rapid transport of adsorbates into the MOF-templated materials and are essential

electron microscopy (SEM) revealed distinct crystals for both MOFs, with MIL-125 having disc-shaped morphology and UiO-66 having octahedral morphology. The identity of these MOF precursors was confirmed through powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA), which are consistent with those previously reported.64,65 BET surface areas similar to those reported in the literature64,65 were obtained for the materials using a previously published freezedrying technique.69 We surmised that the organic linkers could be removed by introduction of an appropriate digestion solution. Thermodynamics would drive a ligand exchange process where metal atoms from the SBUs link together via inorganic bridging groups present in the reaction solution, depicted in Figure 1. To investigate this hypothesis, the MOF templates were suspended overnight in aqueous solutions of NaOH, Na3PO4, or H3PO4, followed by collection via centrifugation and multiple washes with water to yield porous oxide (MOx), oxyphosphate (MOxyPhos), and phosphate (MPhos) materials, where M = Ti or Zr. MOFs treated with H3PO4 were also washed with DMF to remove the H2BDC liberated from the MOF during the ligand extraction process. In contrast to that of the MOFs, TGA measurements of the products revealed no weight loss apart from residual solvent evaporation (Figure 2a,d). The absence of any distinct weight loss from decomposition of organic material indicates the bridging ligands were fully removed during the treatment process, further supported by CHN analysis (Table S1, Supporting Information). Investigation by PXRD revealed the materials were amorphous (Figure 2b,e). Observation by electron microscopy revealed the inorganic materials retained the morphology of the original MOF precursor (Figure 3). SEM imaging shows the surface of the materials to be irregular compared to the original framework, which can be attributed to the formation of pores during the decomposition process (Figure S1, Supporting Information).42 TEM images revealed electron permeability was markedly increased following treatment with the digest solution, in some instances to the point of near transparency. The dimensions of the MOF-templated materials were observed to be smaller than those of the MOF precursors, which follows as a consequence of removing the bridging ligand. The diameters for all materials were investigated by dynamic light scattering (DLS) measurements, which revealed MIL-125 templated materials decreased by 53−61%, while UiO-66 templated materials decreased by 35−57% (Figures S2−S9 and Table S2, Supporting Information). These data are consistent with the reduction of the particle dimensions following the selective extraction of BDC ligands. The composition of these materials was assessed through inductively coupled plasma-mass spectrometry (ICP−MS). Because of the stability of the sorbents, microwave digestion was performed at 180 °C in concentrated H2SO4. ICP−MS data reveal distinctly different metal composition than that of nonporous metal-oxides, metal-phosphates, or MOF precursors (Table S3, Supporting Information). Energy dispersive spectroscopy (EDS) revealed the incorporation of Na when MOF precursors were treated with solutions derived from Na salts (Figure S10, Supporting Information), but otherwise supported the ICP−MS data. Microelemental analysis revealed trace amounts of carbon and nitrogen, comprising less than 0.5 wt % of any final material, while large values of hydrogen composition attest to the presence of surface functionalities 5235



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Figure 4. X-ray absorption data for materials obtained by transformation of MOF templates. (a) Experimental EXAFS spectrum for Ti K-edge for MIL-125 and following treatment with NaOH and H3PO4. (b) XANES spectrum for the Ti K-edge of materials prepared from MIL-125. The preedge feature (inset) contains three overlapping peaks, indicative of six-coordinate Ti. (c) Experimental EXAFS spectrum for Zr K-edge for UiO-66 and following treatment with NaOH and H3PO4. (d) XANES spectrum for Zr K-edge for UiO-66 and following treatment with NaOH and H3PO4. The pre-edge shounder region is enlarged in the inset plot, showing a change of intensity with different treatment solutions. Experimental EXAFS spectra for Ti and Zr materials treated with Na3PO4 are available in the Supporting Information, as are EXAFS spectra and fits for UiO-66 and MIL125. Data are plotted without phase correction.

increased over the MOF precursors. Importantly, these materials were prepared from only two MOF templates, demonstrating that pore size, structure, and composition can be modified by choice of treatment solution. This provides a unique means for influencing the structure of an inorganic material and allows for systematic tuning to impart and optimize desired characteristics for a variety of applications. 3.2. X-ray Absorption Spectroscopy. XAS studies were performed to investigate the local coordination environments of the Ti or Zr sites in the amorphous inorganic materials compared to those of the original crystalline MOF structures (Figure 4). The X-ray absorption near edge structure (XANES) region of Ti K-edge spectra typically contains well resolved preedge features whose relative intensities are indicative of the coordination geometry of the absorbing Ti atoms. Prior XAS analysis of TiO279 and amorphous titanium phosphates80 revealed the presence of three small peaks in the pre-edge region, which unambiguously indicate Ti in a six-coordinate environment with approximately octahedral geometry. A similar pre-edge pattern was also observed in the XAS data for the MOF precursor. This observation indicates a comparable sixcoordinate Ti environment, which is consistent with the known

to their functionality as radionuclide sorbents. Interestingly, the observed hysteresis appears to be amplified for both materials prepared through treatment with H3PO4, though it is also increased for the Ti material prepared with Na3PO4. While the rationale for this observation is not immediately evident, the variation in isotherms further reveal the differences obtained through choice of treatment solution. Though the weight-specific surface areas of the materials are significantly smaller than those of their MOF precursors (1550 m2 g−1 for MIL-125 and 1521 m2 g−1 for UiO-66), this decrease is attributable to removal of the organic bridging ligands, which are significantly less dense than the inorganic SBUs. Volumespecific surface areas allow for a more meaningful comparison, as these values are not skewed by the material density. Multiplying the mass-based surface area by the material density reveals MIL-125 and UiO-66 have volumetric surface areas of 1124 and 1115 m2 cm−3, respectively. In contrast, using densities for TiO2 (4.23 g cm−3) and ZrO2 (5.68 g cm−3) gives approximate volumetric surface areas of 1392 m2 cm−3 for TiOx and 2153 m2 cm−3 for ZrOx. These measurements indicate volumetric surface areas were not only preserved during this treatment process but, in both instances, were significantly 5236



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Figure 5. Separation of radionuclides from HLW using MOF-derived inorganic materials over 24 h. MOF-templated materials were added at different concentrations of metal, as indicated in the legend for each plot. Plots display decontamination factor as a function of contact time (h) for (a) strontium, (b) neptunium, (c) plutonium, and (d) uranium. Open symbols signify analyses surpassing the limits of detection. Lines are a guide for the eye.

structure derived from X-ray diffraction studies. Comparison of these XANES data with those obtained for the amorphous Ti materials shows minimal change in pre-edge features for all three inorganic materials, suggesting the preservation of a sixcoordinate Ti environment follows treatment regardless of solution. Notably, the pre-edge region is significantly different in spectra obtained for materials containing Ti atoms with 4fold, tetrahedral coordination environments,79 demonstrating a derivative of Ti(OH)4 has not been formed. These data indicate hydroxyl groups observed by IR must necessarily be located on the surface of the pores, rather than comprising the bulk material. EXAFS data collected for the Ti materials provide insight into the short-range structure around X-ray absorbing atoms. A reasonable fit to these data were obtained for the MIL-125 MOF precursor by relying on the known crystal structure as the model for generating theoretical scattering path data (Figure S19 and Table S4, Supporting Information). The porous sorbent materials produce EXAFS spectra that are significantly different from those of the MOF precursor, indicating some change in the local coordination environment of the metal sites. Without a priori knowledge on the type of coordination changes, the overlapping and multiple scattering paths coupled with the inhomogeneity of the local structure, typical of amorphous materials, prevent accurate fitting of the EXAFS

data and therefore a quantitative assessment of the local structure changes. This is not uncommon. For example, αTi(HPO4)2·H2O is not fit beyond the first coordination shell.80 Qualitative assessment of the EXAFS data, however, which show changes across the entire measurable distance range, reveals that the amorphous MOF-derived Ti materials experience not just altered coordination environment at second and third shell scattering distances but in the immediate coordination sphere as well. These XAS data suggest that the six coordinate environment of the Ti scattering atom is preserved throughout ligand extraction. Similar conclusions for the Zr materials can be drawn through their XAS analysis. The type of XANES analysis that was discussed for the Ti materials is not possible for the Zrbased systems since the pre-edge features are not resolved, as is usually the case for heavier (second and third row) elements. However, an increased intensity in the pre-edge shoulder region may be attributed to increased 1s to 4d transitions resulting from reduced symmetry of Zr sites. EXAFS analysis revealed changes in the coordination environment for the metals upon treatment with digestion solutions. As in the case of the Ti MOF precursor, a good fit to the EXAFS data was obtained for the Zr-MOF UiO-66 using the known structure as the model (Figure S19 and Table S5, Supporting Information). The parameters obtained from this fit are consistent with those of 5237



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recently reported EXAFS data for this MOF.81 The first shell peak, attributed to the Zr−O scattering paths, reflects two distinct Zr−O coordination distances separated by ∼0.2 Å. The feature at slightly longer distance is mostly attributed to the Zr−Zr scattering paths and arises due to the unique atomic arrangement in the SBU. In the amorphous Zr-based materials, two distinct qualitative changes are observed: the first shell peak coalesces into one average Zr−O scattering distance, and the intensity ratio between the first and second shell feature is altered. Again, unique statistically significant fits to these data were precluded by the combination of unknown coordination number and geometry changes, as well as the unknown amount of inhomogeneity introduced to the local structure in creating the amorphous material. 3.3. Radionuclide Sequestration. 3.3.1. Decontamination of High Level Waste (HLW). The wet processing conditions and high porosity of the MOF-derived materials prompted us to examine their applications as novel sorbents. We evaluated the utility of these stable materials in several radionuclide separation processes. Initial experiments used NaOH- and Na3PO4-treated materials for decontamination of HLW, the byproduct of dissolving spent fuel rods for extraction of enriched uranium and weapons-grade plutonium (Pu). The Savannah River Site (SRS) near Aiken, SC currently houses approximately 144 million liters of HLW that must be treated and disposed.58 The current disposal path involves separation of the highly radioactive species using both an inorganic sorbent and a solvent extraction process. The decontaminated stream is then disposed of on-site in a cement-like waste form, while the separated radionuclides are vitrified along with the sludge portion of the waste, forming a highly stable glass waste form suitable for geological disposal. The inorganic sorbent currently used at SRS is monosodium titanate (MST). This material selectively removes 90Sr and actinides (U, Np, Pu) from the highly alkaline, high ionic strength HLW supernatant.82−84 Ti3(PO4)4 and Zr3(PO4)4 are unstable at high pH so TiPhos and ZrPhos were not tested for this particular application. Sorption testing was performed using a simulated waste solution based on the composition of typical HLW at SRS (Table S6, Supporting Information).59,85 MST, the current state-of-the-art sorbent used for HLW decontamination, was included in the test set for comparison. Porous titanate materials were added at one-half of the Ti concentration of MST, while Zr materials were added at approximately onequarter molar equivalent to ensure sufficient Sr remained in solution for detection. Select results are displayed in Figure 5 as decontamination factors (DFs), which is defined as DF =

C0 Ct

by uniquely distorted Ti octahedra. As this mechanism requires specific binding sites, we expect that the porous inorganic materials extract Sr more rapidly due to their increased surface area allowing facile access to the entire surface of the material and thus more potential binding locations. The Zr materials, in particular the ZrOxyPhos, had dramatically higher DFs than the MST or the other materials tested, further amplified by being added at approximately one-quarter molar equivalent to MST. Deploying this material in place of MST could increase throughput of HLW decontamination by more than an order of magnitude, mitigating a critical problem in the long term storage of nuclear waste. The porous inorganic materials also showed excellent actinide removal performance in the HLW simulant. When compared to MST, the ZrOxyPhos material removed more Pu at a faster rate when added at a one-half molar equivalent concentration. The titanate materials, TiOx and TiOxyPhos, showed comparable performance to MST when added at onehalf the concentration with a similar trend observed for U removal as well. When looking at Np removal performance, the MST, TiOx, and ZrOxyPhos all reached the same level of decontamination after 24 h of contact, with the TiOx and ZrOxyPhos being added at one-half the molar concentration of MST. Previously reported XAFS studies on actinides sorbed by MST indicate Np(IV) is bound by electrostatic sorption, while Pu(IV), Pu(VI), and Np(V) exhibit specific adsorption. Sorption was observed to be site specific, occurring on distorted Ti octahedra, making the coordination environment of Ti essential for good sorption.87 It is possible the structural change induced by using Na3PO4 as digest solution promotes formation of metal sites uniquely oriented for enhanced actinide bonding. Furthermore, these wet processing conditions yield surface hydroxyl and phosphoryl groups, which are known to be favorable for sorption processes. The strong sorptive properties of these materials would not likely be obtained if prepared through pyrolytic techniques, as surface hydroxyl groups would be cleaved and metal coordination geometry altered by crystallization. Overall, these results show that the new porous inorganic sorbents presented here can be used for effective decontamination of HLW, often using less material than the current standard. Since these sorbents are nonelutable, they would be incorporated directly into a glass waste form for disposal; therefore, a large DF value (i.e., use of less sorbent) is the most important performance metric as there is a limit to the amount of Ti (or Zr) that can be incorporated while still retaining the necessary stability properties of the glass. 3.3.2. Lanthanide Separations. The second application investigated was minor actinide and lanthanide separation, which is a key area of research for closing the nuclear fuel cycle. The consumption of uranium oxide fuel in a nuclear power reactor leads to the generation of many radioactive species including fission products such as Cs, Sr, and lanthanides (Ln), as well as Pu and minor actinides. The minor actinides are a large contributor to the long-term radiotoxicity of the waste generated from used nuclear fuel; therefore, their separation is desired. The current separation chemistry required is complex, often involving liquid−liquid extractions and requiring multistep schemes to accomplish adequate removal of actinides, Ln, Cs, and Sr.60−62 Development of selective sorbents would significantly expedite waste reprocessing, potentially allowing

(1)

where C0 is the initial sorbate concentration and Ct is the sorbate concentration at time t. All porous inorganic materials had greatly enhanced Sr removal, surpassing the DF obtained with MST, even at lower concentrations, with the exception of TiOx. TiOx still removed 97.8% of the Sr after 24 h when added at one-half the concentration of MST. Detailed studies indicate both U and Sr sorption occurs on the fibrous surface of MST, binding Sr2+ by a specific adsorption mechanism rather than by ion exchange.86 It was proposed that two Ti atoms at different radial distances contributed to the bonding of Sr by MST, while U was bound 5238



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Figure 6. Extraction of lanthanides (Ln) using MOF-templated materials. (a) Percent Ln removed at pH 3. ZrOx had no detectable extraction of any Ln and is not included. (b) Percent Ln removed at pH 6 for less effective sorbents and controls (TiO2 and ZrO2). TiOx, TiOxyPhos, and ZrPhos all removed greater than 99% of the Ln in solution and are not displayed in the graph for the sake of clarity. (c) Distribution coefficient (Kd) values for Ln extraction at pH 6 for select materials demonstrating particularly remarkable extraction properties. (d) Percent Ln removed by ZrPhos at pH 6 over four cycles of sorption followed by elution in 1 M HNO3.

for recovery of fissionable materials and increased energy utilization.63 Experiments were performed to examine the affinity of the MOF-derived materials for Ln under acidic conditions relevant to nuclear fuel reprocessing. Experiments were performed at both pH 3 and 6 using a Ln stock solution with the composition provided in Table S7, Supporting Information, and MOF-templated sorbents added at a phase ratio of 20 mL/ g. Both the sorbents and the Ln stock solution were equilibrated at the target pH prior to testing, with regular pH adjustments until the change was less than 0.1 pH units over 24 h. The pH was also measured at the end of the experiment. The materials performed better at higher pH where both the TiOx and ZrPhos showed excellent performance (Figure 6). At pH 3, ZrPhos removed between 72% and 92% of all Ln, while both TiOxyPhos and ZrOxyPhos removed approximately 50%. At pH 6, the TiOx, TiOxyPhos, and ZrPhos materials each removed approximately 99% of all Ln present. Under both conditions, leaching of Ti and Zr was negligible, with the detected concentrations consistent with the sample blank. A decrease in pH values measured after sorption suggests different sorption mechanisms for different materials. At pH 3, ZrPhos had a postsorption pH 1.22 units lower than the control solution, indicative of an exchange of Ln cations with surface-bound hydrogen. In contrast, TiOxyPhos was only 0.46 pH units lower than the control, and the pH of ZrOxyPhos was

unchanged after sorption. Additionally, TiPhos, which absorbed less than 42% of each Ln, had the second largest deviation in pH, 0.92 pH units lower than the control. As pH was equilibrated over 96 h until consistently stable readings were obtained, ion exchange with Ln cations is the only explanation for the change in pH. The pH readings before and after sorption are available in Table S8, Supporting Information. At pH 6, significant deviations in postsorption pH were observed for all materials. TiOx had the smallest deviation, lowering the pH by 0.87 units relative to the control. It was also one of the most effective sorbents, removing more than 99% of Ln in solution. TiOxyPhos and ZrPhos both removed more than 99% of Ln, but decreased the final pH by 1.06 and 2.50 units, respectively. TiPhos decreased the pH by the greatest amount, 3.21 pH units, but was one of the worst sorbents in this application. Under both conditions, the largest decrease from the control occurred for the materials treated with H3PO4 (TiPhos and ZrPhos). Even after thorough washing and stabilizing the solution pH prior to sorption, the presence of additional surface H atoms on these materials is expected. The materials treated with NaOH or Na3PO4 were observed to contain small quantities of Na. It is possible this cation modulated the change in pH through exchange of Na+ for H+. On the basis of these postsorption pH measurements, no correlation between pH and Ln extraction is apparent. It is evident that ion exchange 5239



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specific binding sites may not be a function of surface area. Unique binding sites have been reported for MST as well.86 Interestingly, the selectivity of ZrPhos is altered following elution with HNO3. While the percent of each Ln extracted is decreased, repeated use shows the extraction of Eu, Gd, and Tb, three Ln with lower initial Kd values, are less affected than La, Pr, and Nd. The ability of these nonoptimized materials to discriminate between different Ln of similar size and identical charge suggested great potential for separation of Ln from actinides. TiOx had similar affinity for all Ln, with a slight decrease observed for Tb and Er. In contrast, TiOxyPhos absorbed significantly more Eu and less Gd. ZrPhos was selective for Sm but also had particularly strong affinity for Pr, Nd, and Er. TiOx and ZrPhos were investigated at a phase ratio of 100 mL/g for selective extraction of Ln from a solution spiked with Am and Pu. The pH of the solutions containing the MOF-templated materials were equilibrated at pH 6 and observed to be stable over 72 h prior to the addition to the Ln solution. ICP−MS analysis was performed after 24 h of contact. Separation factors were obtained by dividing Kd values for each Ln by the Kd value for either Am or Pu. Even at a decreased concentration of sorbent, the MOFtemplated materials extracted greater than 98% of all Ln in solution, often surpassing the limits of detection by ICP−MS. Both ZrPhos and TiOx had better separation factors for the early lanthanides, La, Ce, Pr, and Nd. Notably, TiOx was over 16× more selective for Nd and almost 21× more selective for La as compared to Am. These separation factors are of the same relative magnitude as recently reported for Zr(IV) and Sn(IV) phosphate materials, though without the requirement of oxidizing Am to AmO22+.89 More detailed experiments are needed to further optimize and fully investigate the capability of these novel inorganic materials for selective separation of lanthanides and actinides. The difference in performance between MOF-templated materials clearly demonstrates the influence of structure and composition upon Ln separations. Similar behavior has been observed for other porous Zr and Sn pillard phosphonates, where changing the phosphate source resulted in large differences for Ln and An extraction efficiencies.74,89 By systematically tuning the treatment conditions, we anticipate the optimized structures will have great potential for these challenging separations. 3.3.3. Simulated Fukushima Seawater Remediation. The third and final application investigated was removal of radioactive strontium from simulated seawater. This application is relevant to the cleanup of the Fukushima Daiichi disaster site where large amounts of contaminated seawater that was used for cooling is currently being stored and treated to reduce the radioactivity. One of the major contaminants needing removal is 90Sr. Experiments were performed using the oxide, oxyphosphate, and phosphate materials in simulated seawater90 containing Sr. MST and SrTreat, a state-of-the-art Sr sorbent, were tested under the same conditions as a basis for comparison. Two different simulants were tested using varying amounts of sorbent. The as-prepared simulated seawater containing 8 ppm of Sr was spiked with 85Sr to enable analysis by gamma spectroscopy. Ti-based sorbents were added at a concentration of approximately 50 mM Ti, which is equivalent to 5 g/L MST. The Zr sorbents were added at an equal mass of metal ion and therefore approximately half of the molar concentration (∼26 mM Zr). A 10× dilution of the simulated seawater was also

cannot be the sole mechanism for the observed lanthanide extraction, suggesting specific binding to be occurring. Additional studies are ongoing to investigate the coordination environment of the extracted lanthanides in an effort to elucidate the sorption mechanism. Distribution coefficients (Kd), values expressing a sorbent’s capability at a given concentration, were determined for the materials using the following formula: Kd =

C0 − Ce v × Ce m

(2)

where C0 (μg/mL) and Ce (μg/mL) are the initial and equilibrium concentration of each Ln, V (mL) is the volume of testing solution, and m (g) is the sorbent dose. High Kd values demonstrate that the sorbent is effective at extracting the species of interest, with values above 50,000 considered outstanding.88 Kd values were modest at pH 3, with ZrPhos demonstrating the best properties and Kd values ranging from 75 to 275 mL/g (Figure S21, Supporting Information). Sorption significantly improved at pH 6, possibly due to passing the isoelectric point of the MOF-templated sorbents (Figure 6c). ZrPhos, TiOxyPhos, and TiOx all yielded impressive Kd values, with many above 1 × 105 mL/g. Notably, the porous inorganic materials prepared by calcining the MOF templates were capable of extracting less than 46% of any lanthanide, even at pH 6, with all corresponding Kd values below 20 mL/g. This four order of magnitude difference clearly indicates the importance of incorporating surface functionalities to achieve efficient sorption, made possible through this novel wet treatment process. The top performing materials for Ln separations were investigated for recyclability by eluting in aqueous HNO3 solution. Preliminary elution studies revealed approximately 75% of all sorbed Ln could be stripped from TiOx and TiOxyPhos by washing with 0.1 M HNO3, though similar conditions eluted less than 25% of Ln sorbed by ZrPhos. Subsequent attempts for further elution using 1 M HNO3 improved Ln stripping from ZrPhos to 75%, but negligible improvements were obtained with TiOx and TiOxyPhos. These materials were investigated for recyclability over four sorption/elution cycles. Ti materials showed significant decreases after the initial Ln sorption, extracting less than 20% of all Ln after the first elution with HNO3 despite more than 75% of sorbed Ln being successfully eluted. Sorption by ZrPhos was also diminshed following each elution, though to a much lesser extent than the Ti materials. After four sorption/ elution cycles, ZrPhos was extracting between 50−75% of the Ln in solution. For ZrPhos, the initial elution with 1 M HNO3 stripped approximately 75% of all Ln, while subsequent treatments removed all Ln extracted during the preceding sorption cycle (Figure S22, Supporting Information). The inability to elute all Ln in conjunction with the decreased sorption capacity of the materials suggests the presence of at least two types of binding sites. Surface hydroxyl and phosphoryl groups extract cations by nonspecific binding, allowing sorbed metals to be readily eluted by washing with HNO3. In contrast, specific binding sites irreversibly extract Ln from solution. It appears subsequent sorption/elution cycles result in the accumulation of firmly bound Ln, decreasing sorbent efficiency. These observations are consistent with the minimal influence of surface area on sorption, as formation of 5240



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allowed the preparation of the first well-defined porous Ti and Zr materials as well as new porous metal phosphates that are prepared from MOF precursors. By varying the digest solution, we successfully altered the compositions, surface areas, and pore sizes of the resulting materials. The wet processing techniques result in the formation of surface hydroxyl and phosphoryl groups, which are not accessible through thermal preparations. ZrOxyPhos was superior in decontaminating HLW simulant, removing Sr, Pu, Np, and U to a significantly greater extent than the current state-of-the-art sorbent and with a lower quantity of metal. ZrPhos and TiOx extracted almost all Ln from slightly acidic aqueous solution, with TiOx showing up to 21× more selectivity for Am over early lanthanides. TiOxyPhos showed significant affinity for Sr in seawater, removing as much Sr in 1 h as SrTreat removes in 24 h. The rapid uptake of radionuclides in these experiments surpassed the state-of-the-art sorbents due to the high porosity, accessibility of coordinating metal sites, presence of surface hydroxyl groups, and well-defined morphologies of these novel materials. This work clearly demonstrates the potential of using this selective ligand extraction technique for preparing porous inorganic materials for use in radionuclide extractions and other diverse applications.

prepared and spiked with 85Sr, with sorbents added at concentrations of 4 mM and 2 mM Ti and Zr, respectively, and analyzed after 1 and 24 h of contact. The results are summarized in Figures 7 and S23, Supporting Information.

Figure 7. Decontamination of Fukushima seawater simulant following 1 h contact time with MOF-templated materials, MST, and SrTreat. Plots display percent Sr removed in undiluted seawater (gray) and 10× diluted seawater (red).

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ASSOCIATED CONTENT

S Supporting Information *

Results generally indicated that the Sr sorption is rapid and, for most sorbents, is essentially complete within 1 h of contact. SrTreat is the notable exception to this trend as it appears to have a slower uptake of Sr. Of all of the newly prepared materials tested, TiOxyPhos appears the most promising, removing greater than 70% of the Sr in seawater simulant within 1 h. This is comparable to the total amount of Sr removed by SrTreat after a 24 h contact. A similar trend is seen in the diluted seawater where TiOxyPhos appears the most promising as well. After 1 h of contact, the TiOxyPhos has removed a greater percentage of Sr than all of the materials tested. However, as seen in the undiluted seawater, SrTreat continues to sorb Sr over time ultimately reaching a higher percentage of removal after 24 h. The enhanced rate of sorption observed in the MOFtemplated materials can be attributed to the extremely porous nature of TiOxyPhos facilitating rapid transport of radionuclides throughout the material, while uniform distribution of Ti provides abundant sites for Sr sorption. While the Ti materials outperform the Zr materials in general, because of the difference in atomic masses, the Zr materials are present at approximately half of the molar concentration and may ultimately prove more effective. Additional work in this area is needed to optimize the amount of sorbent and contact times needed to achieve the necessary decontamination. The overwhelming volume of water in need of treatment by December, 2015, projected to be between 620−871 million liters, requires implementation of materials with exceptionally rapid sorptive properties. The unoptimized MOF-templated inorganic materials are capable of extracting more Sr in 1 h than the current state-of-the art sorbent over 24 h, making them ideal candidates for the remediation of radioactive seawater from the Fukushima Daiichi disaster.

Detailed synthetic procedures for the MOFs and MOFtemplated materials, instrumentation, SEM images, DLS data and plots, ICP-MS data, EDS data, microelemental analysis, FT-IR spectra, pore size distributions, EXAFS plots for TiOx, TiOxyPhos, ZrOx, and ZrPhos, EXAFS plots, fits, and fitting parameters for MIL-125 and UiO-66, composition of simulated HLW solution, composition of lanthanide solution, measured pH change following lanthanide extraction, distribution coefficients (Kd) for lanthanide sorption at pH 3, lanthanide elution data from ZrPhos, 24 h seawater simulant decontamination data, and composition of seawater simulant. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(K.M.L.T.-P.) E-mail: [email protected]. *(W.L.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the SRNL-LDRD program and the DoE Office of Nuclear Energy’s Nuclear Energy University Program (SubContract 20 #120427, Project #3151) for funding support. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. DOE Office of Science under Contract No. DE-AC02-98CH10886.

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REFERENCES

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4. CONCLUSIONS We have prepared a series of porous inorganic materials from Group IV MOF templates. This novel ligand extraction process 5241



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