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Selective Extraction of Thorium from Rare Earth Elements using Wrinkled Mesoporous Carbon Zijie Wang, Alexander Brown, Kui Tan, Yves J. Chabal, and Kenneth J. Balkus J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b07610 • Publication Date (Web): 13 Oct 2018 Downloaded from http://pubs.acs.org on October 14, 2018
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Journal of the American Chemical Society
Selective Extraction of Thorium from Rare Earth Elements using Wrinkled Mesoporous Carbon Zijie Wang†, Alexander T. Brown†, Kui Tan‡, Yves J. Chabal‡, Kenneth J. Balkus Jr*,† †Department of Chemistry and Biochemistry, 800 West Campbell Rd, University of Texas at Dallas, Richardson, Texas 75080, United States ‡Laboratory for Surface and Nanostructure Modification, Department of Material Science and Engineering, 800 West Campbell Rd, University of Texas Dallas, Richardson, Texas 75080, United States. Supporting Information Placeholder ABSTRACT: Liquid Fluoride Thorium Reactors (LFTR) have been considered as replacements for Uranium based nuclear reactors having many economic and environmental advantages. The production of Thorium is usually accompanied by the separation of thorium from rare earth elements since the major thorium production mineral, monazite, contains other rare earth elements. The conventional manufacturing process involves a liquid-liquid extraction with organic ligands. There is a need to develop solid state absorbents with good reusability for metal ion separation processes. Porous carbon is particularly interesting due to acid/base resistance. A new absorbent, surface oxidized Wrinkled Mesoporous Carbon (WMC-O), has been prepared for the selective extraction of thorium ions from rare earth ions. The WMC-O shows high selectivity for thorium adsorption due to the 4+ oxidation state of thorium. The distribution coefficient (Kd) of the WMC-O for Thorium from all rare earth elements is two orders of magnitude larger than that of surface oxidized activated carbon (13104 vs 35102 at pH of 2.15). The WMC-O also shows a high adsorption capacity for pure rare earth ions (Kd> 3105). These features make the WMC-O a promising absorbent for Thorium extraction and rare earth ion recovery.
1 INTRODUCTION Thorium may be the next generation nuclear fuel as the development of Liquid-Fluoride Thorium Reactors (LFTR) gets closer to commercialization1. Compared to Uranium-fueled Light Water Reactors, the LFTR produces ~250 times more energy using the same weight of nuclear fuel with much less long-lived radiotoxic nuclear waste. The LFTR also improves safety since it is based on Molten Salt Reactors, which has a very low chance of meltdown or explosion like the Fukushima accident in Japan1–3. In addition, thorium is much more abundant than uranium and mildly radioactive but can be turned into uranium in a nuclear reactor. The estimated thorium reserve could afford hundreds of years of CO2 emission-free energy3. More importantly, energy cost is about 3 cents/kWh compared to 5 cents/kWh for energy from coal burning1. Oak Ridge National Lab has invested considerable effort on LFTR research and successfully solved many major technical problems. China is also actively developing LFTR technology to reduce the energy contribution from coal4. Bill Gates and TerraPower are also investigating thorium as a nuclear fuel because of the environmental, safety and economic advantages of thorium over uranium5,6. Typically, thorium is produced from mining of monazite together with some other rare earth elements using liquid-liquid extraction with tri-n-butylphosphate7–9. However, this process consumes large quantities of acids and solvents, and usually requires multiple steps. Solid state absorbents have attracted considerable attention for rare earth and actinide element recovery due to the ease of operation, recovery of elements and reuse of absorbent, which have a positive impact on the economics of the
production process10. There has been numerous reports on solid state absorbents using polymers11–15, porous silicas16,17, MOFs18 and porous carbon19–25. Usually, the surfaces of these materials are functionalized with organic ligands to serve as binding sites. Mesoporous silica and zeolites are particularly interesting materials as absorbents since they have well defined pore structures and high surface areas as well as the ability to be functionalized with organic ligands. The major problem with silica based absorbents is the weak stability in strong acidic media. In contrast, carbon based materials are resistant to both acid and base environments. Recently, a surface functionalized Ordered Mesoporous Carbon (OMC), CMK-8, was reported for separation of rare earth elements21. CMK-8 combines the advantages of both porous silica and carbon, containing ordered mesopores as well as acid/base resistance. The other CMK series carbon, CMK-3, was also reported with excellent performance in extraction of single thorium ions25. The pore structure and surface area of mesoporous carbon directly affects the performance as either an absorbent or catalyst support. Usually, the structure of mesoporous carbons replicates the pore structure of the template, mesoporous silicas. Representative mesoporous carbon like CMK-3 and CMK-8 are synthesized using SBA-1526 and KIT-627 as the templates, respectively. In recently years, Wrinkled Mesoporous Silica (WMS) has been widely studied for catalysis28,29 and biomedical applications30,31. The WMS particles possess unique conical shaped wrinkled pores that radiate from the center to the outer surface of each particle of the WMS, providing special mass transport pathways. The WMS based materials with radial pores have an advantage over traditional mesoporous materials, such as MCM-41 with one-dimensional pore systems28. The WMS can be employed as a hard template to make carbon, Wrinkled Mesoporous Carbon (WMC). The WMC was used as a catalyst support for Pd nanoparticles with superior catalytic performance compared to commercial Pd on activated carbon32–34. It is discerned that functionalization at the surface of the WMC with oxygen groups (-OH, -COOH) by a simple acid treatment results in a carbon absorbent for thorium and rare earth ions. The adsorption capacity and selectivity arises from the unique wrinkled structure and high surface area of functionalized WMCO. 2 EXPERIMENTAL SECTION The WMC and the WMS template were prepared by reported procedures32. Typically, 1 g of the WMS powder was dispersed in a solution of 1.25 g of sucrose, 75 μL of sulfuric acid and 5 mL of deionized water. The mixture was dried at 100 °C overnight and then heated to 160 °C for 6 h. To the same mixture, a solution containing 0.75 g of sucrose, 43 μL of sulfuric acid and 5ml of water was added. The resulting mixture was dried again at 100 °C overnight and then heated to 160 °C for 6h. The solid product was carbonized at 900 °C for 3 h in a vacuum. Finally, the WMC was obtained after template removal by washing the product with a hot
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1 M NaOH solution (1:1 ethanol-water) twice. The template-free WMC (1g) was then treated with conc. nitric acid (30 mL) at 90 °C for 1 h followed by centrifugation, washing with water and drying. The WMC after acid treatment is denoted as WMC-O. In the extraction tests, the batch solutions containing rare earth ions or rare earth ions plus thorium and uranium were prepared from ICP (Inductively Coupled Plasma) standard solutions with a final concentration of 300 ppb for each element in nitric acid environment. In general tests, 5 mg of carbon absorbent was mixed with 5 mL of extraction batch solution which was placed on a shaker for 4 h. The mixture was then centrifuged and the solution was separated with syringe filter before analysis by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). All the tests are repeated trice to get standard deviations. To recover the elements from the absorbent (5 mg), 5 mL of a 0.1 M ammonium oxalate solution was used to strip all the ions for further ICP-MS analysis.
increase for the C=O. The AC is apparently more difficult to oxidize by this simple oxidation process than the WMC. It might due to the fact that the WMC is prepared from sucrose, which is an oxygen rich carbon precursor. This leaves more defective oxygen groups on the surface of the WMC after carbonization making the WMC more readily oxidized. The WMC also has larger area of accessible surface for nitric acid to attack than the nonporous AC and there might be more oxygen atoms buried inside the AC particles.
3 RESULTS AND DISCUSSION
Figure 2. XPS spectra of (a) WMC and (b) WMC-O with binding energies of C1s core electrons. Figure 1. TEM (a, b) and SEM (c,d) images of (a,c) WMC and (b,d) WMC-O. The morphology of the WMC and WMC-O materials was examined by scanning electron microscopy (SEM) and transmission election microscopy (TEM) as shown in Figure 1. The WMC are spherical particles with an average size of 400 nm and some partial spheres are also observed due to the incomplete coating of carbon precursor on the WMS template (Figure 1a, c). The wrinkled pores radiate from the center to the surface of each particle with gradually increased pore sizes. After surface oxidation, the resulting material, WMC-O, retained the same wrinkled pore structure (Figure 1b, d). Oxidation of the surface of the WMC does not destroy the structure of these particles. The reference material, commercial activated carbon (AC), underwent the same oxidation process and the resulting product, AC-O, also maintained the same morphology in nonporous nanoparticles with an average size of 30 nm (Figure S1). The status of oxygen functional groups on both WMC-O and AC-O were investigated by X-ray Photoelectron Spectroscopy (XPS) shown in Figure 2 and Figure S2. Before surface oxidation, the amount of oxygen functional groups on WMC and AC are similar (52% and 53%). For the WMC-O, there is an obvious increase in the portion of C=O on the sample compared to the WMC (31% vs 11%), indicating the introduction of more COOH groups on the surface. However, the AC-O only shows a 3%
Figure 3. N2 adsorption-desorption isotherms of WMC, WMC-O, AC and AC-O. The BET surface area of each sample is indicated following each sample tag. In order to evaluate the accessible surface of WMC and AC, N2 adsorption-desorption isotherms were recorded (Figure 3). Both the WMC and the AC show a small change in surface area after oxidation, indicating the introduction of surface oxygen groups does not largely affect the surface area of these carbon materials. The WMC and the WMC-O show much larger surface area than the AC and the AC-O because of the wrinkled mesoporous structure. Thus, during the oxidation process with nitric acid, the
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Journal of the American Chemical Society WMC has more accessible area for nitric acid to attack, resulting in higher density of oxygen functional groups on the surface of
Figure 4. Extraction distribution coefficients (Kd) on different carbon absorbents for (a) rare earth elements plus Thorium and Uranium and (b) four lanthanide elements. The extraction batch solution has a concentration of 300 ppb for all elements and a pH of 2.15. the WMC-O compared to the AC-O. In addition, the WMC and the WMC-O show type IV isotherms with H3 type hysteresis, which is characteristic of mesopores. The four carbon materials, WMC, WMC-O, AC and AC-O, were first tested as absorbents in the batch extraction of rare earth elements plus uranium and thorium. The adsorption capacity for the absorbents is represented using distribution coefficient Kd: (𝐶𝑖 ― 𝐶𝑓) 𝑉 𝐾𝑑 = × 𝐶𝑓 𝑚 Where Ci is the initial concentration of the elements in the extraction solution, Cf is the final concentration after the extraction process using carbon absorbents, V is the volume (mL) of the extraction solution and m is the mass (g) of the absorbents. As shown in Figure 4a, all the carbon absorbents exhibit selectivity for thorium ions. Even though the amount of oxygen functional groups on the absorbents varies, the thorium ions still bind most strongly. There is a huge difference in the adsorption capacities of the four carbon materials in the order of WMCO>WMC>AC-O>AC. One of the reasons is that the WMC and WMC-O have larger surface areas than the AC and the AC-O, providing more accessible binding sites. The other reason is that the WMC and the WMC-O have a higher density of oxygen functional groups on the surface as noted above. As a result, the WMC-O shows the highest adsorption capacity. It was not expected that the average adsorption capacity of the WMC is even higher than that of the AC-O, which might due to the combination
of the higher surface area and abundance of surface oxygen functional groups. Since a Kd larger than 5104 is considered outstanding35, the WMC-O shows an average Kd of ~13104 (Kd of ~3500 for AC-O) for thorium ions in the presence of all the other rare earth elements. This WMC-O is an effective and superior absorbent for thorium from rare earth elements. The capacity of the WMC-O for thorium from all rare earth elements is two orders of magnitude larger than that of the surface oxidized activated carbon. These results are promising for thorium separation process associated with the monazite mining industry in order to get high purity thorium as a nuclear fuel. The WMC-O also shows excellent adsorption capacity for only rare earth elements (Figure 4b), achieving a Kd range of 20104 to 50104 for La, Eu, Gd and Yb when the batch solution only contains these four elements. The performance of the WMC-O on only rare earth elements exceed the reported results on other carbon based absorbents attributed to the unique wrinkled pore structure, high surface area and the abundance of surface oxygen functional groups of the WMC-O. Because of the large mesopores in the WMC-O, it is unlikely the selectivity for Thorium from rare earth elements is due to the special pore structure and the AC-O also shows similar selectivity for thorium. It is assumed that the reason for this selectivity is more related to the oxidation state of these metal ions. In the batch solution of rare earth elements plus thorium and uranium, only thorium ions are 4+, therefore, a stable 4+ ion, Zr4+, was added to
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the batch solution for extraction. As a result (Figure S3), Zr4+ was also selectively absorbed on the WMC-O as expected. In this case, the Kd for Thorium decreases compared to previous result, which does not have Zr4+, because part of the binding sites on the WMCO are occupied by Zr4+. Although the Zr ion has a large charge to radius ratio, it forms more complicated polynuclear species in solution that is not 4+ charged. The percentage of hydrolysis species as well as other non 4+ species of both Zr and Th changes under different pH, but Th has larger portion of 4+ ions than Zr at a pH of 2.15, which makes Th binding better than Zr36,37. Besides, Th forms very stable metal complex with carboxylic groups as well as thermal stable MOFs38. All these factors interpreted the high selectivity for Th on carbon-based absorbents with oxygen functional groups on the surface. The selectivity and adsorption capacity of carbon absorbents generally exhibit a dependence on the pH of the extraction solution. The WMC-O was also tested in a series of batch solutions containing rare earth elements plus thorium and uranium ions with different pH values. As shown in Figure S4a, the WMCO selectively absorbs thorium under a pH of 2.78 while rare earth elements are largely absorbed in the WMC-O in a higher pH range of 3-5. Figure S4b shows the Kd of the adsorption of Th on the WMC-O as a function of pH. The highest adsorption capacity is achieved at a pH of 2.1. Both increasing and decreasing the pH lowers down the adsorption capacity for Th, which makes the pH a controllable parameter to adjust the adsorption selectivity of the WMC-O for either Th or rare earth elements. Again, Th ions may undergo hydrolysis at higher pH values as mentioned above. The lanthanides start to seriously hydrolyze above pH 5 which also contributes to the selectivity difference between Th and the lanthanides21. At extremely low pH (
