Article pubs.acs.org/cm
Separate or Simultaneous Removal of Radioactive Cations and Anions from Water by Layered Sodium Vanadate-Based Sorbents Sarina Sarina,† Arixin Bo,† Dejun Liu,‡ Hongwei Liu,§ Dongjiang Yang,† Cuifeng Zhou,§ Norbert Maes,∥ Sridhar Komarneni,⊥ and Huaiyong Zhu*,† †
School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, QLD 4001, Australia State Nuclear Power Research Institute, Beijing 100029, P. R. China § Australian Centre for Microscopy and Microanalysis, The University of Sydney, Sydney, NSW 2006, Australia ∥ SCK-CEN, Belgian Nuclear Research Center, Mol 2002400, Belgium ⊥ Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡
ABSTRACT: Nanofibers of sodium vanadate, consisting of very thin negatively charged layers and exchangeable sodium ions between the layers, are efficient sorbents for the removal of radioactive 137Cs+ and 85Sr2+ cations from water. The exchange of 137Cs+ or 85Sr2+ ions with the interlayer Na+ ions eventually triggered structural deformation of the thin layers, trapping the 137 Cs+ and 85Sr2+ ions in the nanofibers. Furthermore, when the nanofibers were dispersed in a AgNO3 solution at pH >7, well-dispersed Ag2O nanocrystals formed by firmly anchoring themselves on the fiber surfaces along planes of crystallographic similarity with those of Ag2O. These nanocrystals can efficiently capture I− anions by forming a AgI precipitate, which was firmly attached to the substrates. We also designed sorbents that can remove 137Cs+ and 125I− ions simultaneously for safe disposal by optimizing the Ag2O loading and sodium content of the vanadate. This study confirms that sorbent features such as fibril morphology, negatively charged thin layers and readily exchangeable Na+ ions between the layers, and the crystal planes for the formation of a coherent interface with Ag2O nanocrystals on the fiber surface are very important for the simultaneous uptake of cations and anions.
1. INTRODUCTION The increasing use of radioisotopes and nuclear power worldwide makes processing and managing radioactive wastes a critical part of the nuclear energy option. Uranium fission and nuclear reactor accidents usually lead to the significant release of radioactive ions into the water and soil around the world. The released radioactive species include iodine-131 (131I) and cesium-137 (137Cs) from accidents at Fukushima, Japan, recently, Chernobyl in 1986, and Three Mile Island in Pennsylvania in 1979. In 2009, leaks of radioactive materials such as 137Cs and 131I isotopes also occurred during minor accidents at nuclear power stations in Britain, Germany, and the United States. Radioactive iodine is also used in the treatment of thyroid cancer, and as a result, radioactive wastewater is discharged by a large number of medical research institutions.1 However, methods currently deployed to manage radioactive waste can be complex and extremely costly.2 Inorganic cation exchangers, such as zeolites, clay minerals, layered zirconium phosphates, and layered sulfide frameworks, are used for the removal of radioactive cations because of their ability to withstand intense radiation and elevated temperatures in addition to their high ion exchange capacity.3−21 However, ion exchange in materials is usually a reversible process, except in swelling micas,4 and thus, there is a potential risk that the © 2014 American Chemical Society
radioactive ions captured by the exchanger may be released to water again, resulting in secondary pollution. In our previous study, we found that titanate nanofibers and nanotubes (with a chemical formula Na2Ti3O7) are highly effective for the removal and safe disposal of radioactive cations, such as Cs+, Sr2+, Ra2+, and Ba2+.18−20 Safe disposal is feasible because of a structural deformation of the titanates, which eventually trapped the cations in the titanate structures. The resulting titanate structures are refractory solids that not only are very stable to radiation, chemical, and thermal changes but also can be easily synthesized at low cost.21−23 However, there has been limited success in developing efficient sorbents for the removal of the dangerous anions with extremely long half-lives, such as 129I−, 79SeO32−, and 99TcO4−, from aqueous environments and immobilizing them for safe disposal because of the poor sorption abilities of most geological materials for anions.24−33 The titanates were also successfully used for radioactive iodine removal after grafting with Ag2O nanocrystals on the surface.21 The direct use of Ag2O or another silver compound to precipitate iodide species Received: May 22, 2014 Revised: July 28, 2014 Published: July 29, 2014 4788
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2.2. Sorption Test. Briefly, 5.0 mg of adsorbent (vanadate, titanates, and vanadate anchored with Ag2O) was dispersed into 5.0 mL of an aqueous mixture solution with a required concentration of CsCl, SrCl2, and NaI. The Cs+ solution contained 15 kBq of radioactive 137Cs+ activity as a tracer in the required concentration of CsCl. Similarly, 15 kBq of radioactive 85Sr2+ activity was present in the required concentration of SrCl2. The NaI solution containing 25 kBq of 125I− was prepared for the 125I− adsorption test. The suspension was then mixed on a rotary shaker at ambient temperature. After each appropriate time interval, the sorbent was separated from the solution by centrifugation. Aliquots of supernatants were sampled to determine the radioactivity of 137Cs+ and 125I− using a Packard COBRA 5003 gamma counter, and the radioactivity of 85Sr2+ in solution was determined with a Wallac Wizard 1480 γ-counter. 2.3. Characterization of the Sorbent. A transmission electron microscopy (TEM) study conducted on a Philips CM200 transmission electron microscope with an accelerating voltage of 200 kV was used to characterize the catalysts. The chemical compositions of the sorbents were determined by the energy dispersion X-ray spectrum (EDX) technique using the attachment to a FEI Quanta 200 Environmental scanning electron microscope. X-ray diffraction (XRD) patterns of the sample powders were collected using a Philips PANalytical X’pert Pro diffractometer. Cu Kα radiation (λ = 1.5418 Å) and a fixed power source (40 kV and 40 mA) were used. Diffuse reflectance ultraviolet and visible (DR-UV−vis) spectra of the sample powders were examined with a Varian Cary 5000 spectrometer.
is impractical. When large particles (with a small specific surface area) are used, the removal ability becomes poor. If very fine particles with a large specific surface area are used, the removal ability is improved, but the separation of the used sorbents (loaded with the radioactive species) from water will be extremely difficult and costly. Ag2O nanocrystals have been shown by us21 to be firmly bonded to the surface of sodium titanate nanofibers through coherent interfaces, at which oxygen atoms are shared by the Ag2O nanocrystals, and such fibers could be separated after decontamination. The nanofibril morphology also brings about significant advantages. First, the nanofibers have large specific surface areas on which numerous Ag2O nanocrystals can be grafted by innovative surface modification approaches to target particular species with high selectivity. Second, the fibers have large interfiber voids (from tens of nanometers to micrometers).22,23 Therefore, large liquid fluxes can pass through the adsorption beds of the fibril adsorbents even after Ag2O grafting and adequately access the created adsorption sites on the nanofiber surface. An interesting and important question is whether the structural features found in the sorbents from titanate fibers are generally applicable for developing a new generation of efficient sorbents for the removal and safe disposal of radioactive ions. The useful features are summarized as follows: (1) one-dimensional (1D) structures consisting of negatively charged thin layers, whose deformation could trap the target cations permanently, (2) readily exchangeable Na+ ions that need to be located between the layers, and (3) the ability to firmly bind Ag2O nanocrystals on the surface of the 1D structures. This is useful knowledge for the design of efficient sorbents. For instance, the titanate nanostructures are not stable in acidic media and can convert to anatase TiO2.22,23 This limits their application in acidic wastewater. Therefore, a search for other materials that are stable under acidic conditions is warranted. Another challenge is to develop bifunctional sorbents for the removal of both 137Cs+ cations and 131I− anions at the same time from water because the nuclear wastewater usually contains both these radioactive cations and anions as in the case of the Fukushima nuclear accident. In this study, nanofibers of a layered vanadate are investigated to verify the general applicability of the sorbent features in solving radioactive ion decontamination issues.
3. RESULTS AND DISCUSSION The theoretical cation exchange capacity (CEC) of the vanadate nanofibers is calculated from its formula to be 3.29 mmol g−1 for monovalent ions (137Cs+) and 1.65 mmol g−1 for divalent ions (85Sr2+). The isotherms (Figure 1) approach plateaus that are the experimental saturated exchange capacities for 137Cs+ and 85Sr2+ ions, which are 2.15 and 1.10 mmol g−1, respectively. These capacities are ∼65% of the theoretical CEC of 137Cs+ and 67% of the theoretical CEC of 85Sr2+. Generally, the uptake capacities for conventional cation exchangers, natural or synthetic clays, are 60% of its sorption capacity. Thus, the vanadate is a much better sorbent with respect to 137Cs+ removal in an acidic solution (Figure 3a). A dramatic change was also found in 85Sr2+ adsorption; i.e., vanadate showed sorption ability from neutral solution weaker than that of titanate nanotubes and fibers, but the vanadate nanofibers showed uptake ability that was better than that of titanates (Figure 3b) in acidic solutions at low pH values. These results indicate that the vanadate nanofibers are more suitable for removing radioactive ions from acidic solutions than the titanate nanostructures. To capture I− anions from water, we functionalized vanadate nanofibers with Ag2O nanocrystals for use as an efficient sorbent. The Ag2O crystals can readily precipitate I− ions.21 Nonetheless, when the Ag2O is loaded on the sodium vanadate upon being dispersed in a solution of Ag+ ions, the Ag+ ions could also exchange with the interlayer Na+ ions. Such Ag+ exchange can affect the capacity of the sorbents to remove Cs+ ions by the vanadate sorbent via exchange with Na+ ions. In this study, one of our goals is to develop sorbents that can simultaneously capture I− anions from water by reaction with Ag2O on the surface and Cs+ cations via ion exchange with the Na+ ions between the layers. Hence, we optimized the amount of Na+ ions remaining between the layers in the sorbent loaded with Ag2O nanocrystals. When the vanadate was dispersed into a neutral or basic aqueous solution of silver nitrate (pH ≥7), most of the silver existed in the Ag2O nanoparticles on the substrate surface and the rest as Ag+ ions in the interlayer region because of the exchange with sodium ions. From the XRD patterns (Figure 4e), we found that magnitudes of 001 diffraction peaks in vanadate decreased because of the uptake of Ag+ ions, but the diffraction peaks of Ag2O nanoparticles in the sample were not found, possibly because of the well-dispersed fine particle size (∼10 nm) and low content of the nanocrystals. In this study, because of the larger radiation dose of 131I−, we used 125I− anions along with radioactive 125I− to monitor the behavior of the 131I isotope. The two isotopes, however, have the same chemical reaction properties. Furthermore, the distribution of the Ag2O nanocrystals can be readily controlled by adjusting
Figure 1. Removal of radioactive 137Cs+ and 85Sr2+ cations from aqueous solution by sodium vanadate nanofibers (SV) through cation exchange. (a) Removal of 137Cs+ cations and 85Sr2+ cations as a function of the cation concentration in water. (b) Isotherms of 137Cs+ uptake and 85Sr2+ uptake by 5 mg of the vanadate dispersed in 5 mL of 137 Cs+ or 85Sr2+ solutions at various concentrations over 2 h. For the adsorption experiments depicted above, all of the data points represent the average of triplicate runs with a mean variation of less than ±3%.
the structural deformation of the vanadate sorbent occurs when a relatively large amount of the interlayer Na+ ions has been exchanged with Cs+ ions. After the structural deformation, the sorption ceased. This fact implies that the vanadate layers have better mechanical strength (and thus weaker deformation ability) than the titanate layers, requiring more exchanged Cs+ ions to induce a structural change in the former. This is supported by the XRD results: the change in the XRD patterns of the titanates,21 due to the adsorption of Cs+ ions, is more significant than that of the vanadate. Nonetheless, the weaker deformation ability may also be responsible for a shortcoming: the release of the trapped Cs+ ions to pure water is 7.8% by vanadate when the used vanadate sorbent was dispersed in pure water, while it is only 0.3% by titanates. In the case of Sr2+ ion sorption, the structural transformation is more complicated. As shown in Figure 2b, a new diffraction peak at 2θ = 9.74° arose in the XRD pattern of parent vanadate sample, which is assigned to the diffraction peak of the (101) plane. On the basis of the XRD pattern change and sorption capacity (∼50% of the theoretical CEC), we propose a mechanism for the structural transformation caused by the sorption of Sr2+ ions (Figure 2d). The captured Sr2+ ions are distributed in an orderly manner in the vanadate nanofibers via the replacement of the Na+ cation sites in every other (101) plane. The pronounced structural deformation or trans4790
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Figure 2. (a and b) XRD patterns of the vanadate nanofibers before and after sorption of 137Cs+ and 85Sr2+, respectively. (c and d) Schematic diagrams of the structures of vanadate nanofibers before and after the absorption of 137Cs+ and 85Sr2+ ions, respectively, derived on the basis of the information from the literature and the XRD patterns. TEM images of vanadate nanofibers after (e) Cs+ sorption and (f) Sr2+ sorption. (g and h) EDS spectra of vanadate nanofibers after Cs+ and Sr2+ sorption, derived via scanning electron microscopy.
Figure 4 shows that the nanocomposite of sodium vanadate with Ag2O nanocrystals anchored on its surface with different ratios (Ag:Na molar ratios of 1:1 and 1:2) was able to remove >96% of 125I− and >90% of 137Cs+ ions from a solution containing a mixture of 100 ppm of Cs+ ions and 100 ppm of I− anions (panels a and c). Both ions were completely removed when the concentration was below 100 ppm (Figure 4a,c). The pure sodium vanadate (without Ag2O, VB of 1:0) showed negligible adsorption of I− ions but a higher level of removal of Cs+ ions (>85% of 137Cs+ ions from a solution containing 350 ppm of 137Cs+ and 350 ppm of 125I−), compared with those of the two Ag2O-anchored sodium vanadate nanocomposites (40−45%), as shown in Figure 4c. The maximal capacities of Ag2O-anchored sodium vanadate for 137Cs+ ion removal were found to be 1.15 mmol g−1 (VB1) and 0.9 mmol g−1 (VB2), while that of pure sodium vanadate was ∼2.1 mmol g−1. For the removal of 125I− ions, the maximal capacities were 1.21 mmol g−1 for VB1 and 1.70 mmol g−1 for VB2. Element mapping (Figure 4g) and EDS analysis (Figure 4f) of the particles in the scanning electron microscopy image of Ag2O-anchored sodium vanadate after Cs+ and I− sorption clearly show the coexistence of Cs+ and I− in the vanadate after sorption. This result confirmed the simultaneous sorption of cations and anions by one adsorbent material. The results also indicate that the sorption capacity of Ag2Oanchored vanadate depends on the Ag:Na ratio, as well. Thus, we can adjust the properties of the sorbents to meet the different needs of variously contaminated types of water. This flexibility in designing sorbents is amenable for the highest rate of in situ removal of cations and anions from one solution and the highest efficiency of utilizing the active components of the materials (Ag+ and Na+).
Figure 3. Removal of radioactive 137Cs+ (a) and 85Sr2+ (b) by vanadate nanofibers, titanate nanotubes, and nanofibers from solutions at different pH values.
the concentration of the Ag+ cations in the aqueous silver nitrate solution. 4791
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Figure 4. Removal of radioactive 137Cs+ cations and 125I− anions from one solution by vanadate sorbents with different Ag:Na ratios. Panels a and c show the removal from solution using various initial concentrations of the target ions. Panels b and d are the isotherms of 137Cs+ uptake and 125I− sorption by Ag2O-doped vanadate adsorbents, respectively, The XRD patterns of the adsorbents before and after adsorption of Cs+ ions and I− anion are shown in panel e. Panel f shows EDS of the selected area in a scanning electron microscopy image of the sorbent particles used for sorption of Cs+ cations and I− anions. Element mapping of the particles in the scanning electron microscopy image is shown in panel g.
For the nanocomposite sorbents, the key issue is that the Ag2O nanoparticles must be firmly attached to the vanadate nanostructures. If they readily detach from the substrates, it will be extremely difficult and costly to recover the fine nanoparticles from a solution. Also, the small Ag2O nanoparticles may aggregate if they are not well dispersed on a substrate, forming a solid with a small surface area and thus poor ability to precipitate I− anions. On the vanadate nanofibers, Ag2O nanocrystals are welldispersed with particle sizes between 5 and 10 nm, as shown in Figure 5a, but after sorption of I−, slightly larger AgI nanocrystals formed [10−15 nm (Figure 5b)]. The highresolution transmission electron microscopy (HRTEM) image of a Ag2O nanocrystal on a vanadate nanofiber (Figure 6A) and the inverse fast Fourier transform (IFFT) image of the selected area (Figure 6G) indicated that the Ag2O nanocrystals are anchored on the surface of vanadate nanofibers by the coherent interface. The interplane distance of d(013) planes of the Ag2O crystals is 0.15 nm and approximately three-quarters of that (0.18 nm) of d(020) planes of the vanadates, with a difference of ∼0.1%. If every four (013) of Ag2O matches with every three (020) of vanadates when the Ag2O nanocrystal and the vanadate substrate join at these surfaces, the number of oxygen atoms at the interface between the two phases, which are shared by the two phases, is maximized and full coordination can be achieved forming well-matched interfaces (coherent interface).
Figure 5. TEM images of the vanadate nanofibers anchored with Ag2O nanocrystals and the vanadate nanofibers coated with AgI nanocrystals formed during the removal of I− ions. (a) Typical TEM image showing the abundant Ag2O crystals (5−10 nm) dispersed on vanadate nanofibers. (b) TEM image of numerous AgI crystals (10−15 nm) formed on single vanadate nanofibers.
Such coherence between the two phases reduces the overall energy by minimizing the surface energy and thereby anchoring the Ag2O nanocrystals firmly to the surface of the 1D vanadate structures.37 Even after reacting with I−, the AgI crystals could be firmly attached. The HRTEM image of AgI nanocrystals anchored on the vanadate is shown in Figure 7A. From the image, an image equivalent to the selected area electron diffraction pattern was 4792
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Figure 7. TEM characterization of a Na2V6O16 nanofiber doped with Ag2O and adsorbed with Cs+ and I−. (A) HRTEM image showing nanosized AgI particles on the broad surface of a Na2V6O16 nanofiber. (B) Corresponding FFT image that could be indexed as AgI (D and E) and Na2V6O16 nanofiber (G and H). (C) Mutual orienting relationship between the two phases. It is [0001]AgI//[100]Na2V6O16 and (12̅10)AgI//(013)Na2V6O16. (D and G) Fast Fourier transform (FFT) images corresponding to the blue square areas in panel A that could be indexed with the lattice parameter of a Na2V6O16 nanofiber (monoclinic, panel H) and AgI (hexagonal, panel E), respectively. (F) Inverse FFT image of panel B providing lattice fringes of planes (006) and (013) of Na2V6O2 and (112̅0), (1̅21̅0), and (2̅110) of AgI. Accordingly, the (013)Na2V6O16//(112̅0)AgI interface is deduced from panel F as shown in panel I. Bin panels C, E, and H denotes the electron beam incident direction.
Figure 6. TEM characterization of a Na2V6O16 nanofiber doped with silver. (A) Bright field TEM image showing nanosized Ag2O particles on the broad surface of a Na2V6O16 nanofiber. (B and D) Fast Fourier tansform (FFT) images corresponding to the red square areas that could be indexed with lattice parameters of a Na2V6O16 nanofiber (monoclinic, panel C) and Ag2O (monoclinic, panel E), respectively. B in panels C and E denotes the electron beam incident direction. Panel F is a scheme of the composite electron diffraction patterns of both phases, revealing that the orientation relationship between the two phase is expressed as [100]Na2V6O16//[100]Ag2O and (020)Na2V6O16//(013)Ag2O. (G) Inverse FFT images corresponding to panel B providing lattice fringes of planes (006) and (020) of Na2V6O16 and (020) and (002) of Ag2O. Accordingly, the interface between Na2V6O16 and Ag2O deduced from panel F could be plotted as shown in panel H for the interface of (020)Na2V6O16//(013)Ag2O. Note that the oxygen atoms show a good matching at the interface area as highlighted by semitransparent blue circles in panel H.
interface between the two phases to bond the newly deposited AgI nanocrystals firmly to the vanadate substrates. The selective uptake of Cs+ ions (50 ppm) by sodium vanadate in the presence of a high concentration of Na+ (100 or 250 ppm) was investigated (the molar ratio of Cs+ ions to Na+ ions was as high as 1:10 or 1:32, respectively). The adsorption capacity of Cs+ ions decreased slightly at a 10-fold higher Na+ ion concentration (as shown in Figure 8a, 94% of adsorption capacity in a pure Cs+ solution), and the time to reach equilibrium capacity increased from 10 min (without competitive ions) to 30 min (in the presence of a high concentration Na + ions). The higher competitive ion concentration (32 times) shows a stronger influence on the uptake of Cs+ ions but still remains ∼67% of uptake. Those data indicate the sodium vanadates are highly selective for the adsorption of Cs+ ions.
obtained by fast Fourier transformation (Figure 7B). Indexing the FFT pattern, we obtained the results shown in Figure 7C, which describes the mutual orienting relationship between the AgI and vanadate phases. The (12̅10) plane of the AgI nanocrystal was observed to be parallel to the (013) plane of vanadate fibers. Thus, the crystallographic registry between the two phases should be that [0001] and (12̅10) of the AgI nanocrystal are parallel to [100] and (013) of vanadate nanofibers, respectively. Given that the interplanar distance of the (12̅10) planes of the AgI nanocrystal is 0.2290 nm, which is close to that of the (013) planes of vanadate nanofibers (0.2156 nm), the two planes can join together to form a well-matched 4793
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ACKNOWLEDGMENTS
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REFERENCES
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We gratefully acknowledge financial support from QUT, SCKCEN, and the National Natural Science Foundation of China (NSFC, Grant 21207073).
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Figure 8. (a) Cs+ adsorption kinetic curve of sodium vanadate, in the presence and absence of high concentrations of competitive Na+ ions. (b) I− adsorption kinetic curve of Ag2O-anchored vanadate, in the presence and absence of high concentrations of competitive Cl− ions.
Similarly, the selective uptake of I− ions (50 ppm) by Ag2Oanchored vanadate was determined. As shown in Figure 8b, the I− adsorption capacity decreased by 25% in the presence of a high concentration of competitive Cl− ions (150 ppm) with respect to that in the absence of competitive ions. The result of I− and Cl− selective adsorption can be explained by the similar theory for Gibbs energies of the reactions of Ag2O with NaI and with NaCl, which has been discussed previously.21,38
4. CONCLUSIONS This study of vanadate nanofibers confirms that the three structural features mentioned in the Introduction are generally applicable for superior, efficient, and irreversible sorbents for the removal of radioactive ions from water. The fibril nanostructure not only assures a high capacity for uptake of cations and anions but also guarantees trouble-free separation of the adsorbents from solutions for safe disposal ultimately. This investigation also demonstrates that both radioactive Cs+ and I− ions can be removed simultaneously from water with a nanocomposite sorbent. The vanadate nanofibers as a sorbent exhibited performance in acidic media much better than that of the Na titanate nanostructures. The knowledge acquired in this study is useful for the further development other efficient sorbents for a variety of ions of concern.
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[email protected]. Notes
The authors declare no competing financial interest. 4794
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(36) New developments and improvements in processing of ‘problematic’ radioactive waste, results of a coordinated research project, 2003−2007, IAEA-TECDOC-1579, 11. (37) Liu, H. W.; Zheng, Z. Z.; Yang, D. J.; Ke, X. B.; Jaatinen, E.; Zhao, J. C.; Zhu, H. Y. ACS Nano 2010, 4, 6219. (38) Bo, A.; Sarina, S.; Zheng, Z.; Yang, D. J.; Liu, H. W.; Zhu, H. Y. J. Hazard. Mater. 2013, 246−247, 199.
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