Adsorption toward Trivalent Rare Earth Element from Aqueous

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Adsorption toward Trivalent Rare Earth Element from Aqueous Solution by Zeolitic Imidazolate Frameworks LU JIANG, Wei Zhang, Congguang Luo, Daojian Cheng, and Jiqin Zhu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00422 • Publication Date (Web): 19 May 2016 Downloaded from http://pubs.acs.org on May 24, 2016

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Industrial & Engineering Chemistry Research

Adsorption toward Trivalent Rare Earth Element from Aqueous Solution by Zeolitic Imidazolate Frameworks Lu Jiang1, Wei Zhang1,2, Congguang Luo1, Daojian Cheng2, and Jiqin Zhu1* 1

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China

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International Research Center for Soft Matter, State Key Laboratory of OrganicInorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China E-mail: [email protected] Abstract:

Zeolite imidazolate frameworks (ZIFs) have been applied to gas

adsorption, but there remain no reports detailing the adsorption-desorption process for lanthanum metal ion (LMI). This work firstly testifies ZIF-8 has a better adsorption property than ZIF-90 for remediation of LMI from water. Adsorption results demonstrated ZIF-8 to remove 100.0 % metals ions, whose concentration is below 20 mg·L-1. And the saturated adsorption capacity is 385 mg·g-1. Characterization by PXRD, ICP and FT-IR indicates that the main structure of ZIF-8 remained unchanged to a large extent during the adsorption process. Density functional theory (DFT) calculations illustrated the adsorption process is spontaneous and the adsorption site is in the center of two imidazole rings. La-ZIF-8 was subjected to two polar desorbents, acetonitrile and methyl alcohol, possessing a desorption ratio of 94.4 % and 61.0 %, respectively. Recycling the adsorption/desorption process using acetonitrile resulted in significant crystal structure collapse after three recycles reducing the adsorption capacity to 150 mg·g-1. However, acetonitrile still maintains a desorption ratio of over 90.0%. Our work shows ZIF-8 can be used as an efficient adsorbent in the removal of aquatic LMI, especially at trace concentration levels. 1

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1. INTRODUCTION Zeolite imidazolate frameworks (ZIFs), as a sub-class of metal-organic frameworks (MOFs)1, are typical coordination polymers composed of metal ions and organic ligands2. The properties of ZIFs feature high chemical and thermal stability, large accessible surface areas and abundant active surface sites. Additionally, it is feasible to control the size, shape and functionality of the porous structures by varying the organic ligands. Therefore, the inherent physico-chemical and textural properties of ZIFs form the basis of a promising transition into real-world applications. ZIFs have been utilized as gas adsorption/desorption/separation vehicles3-5, catalyst6 and molecular recognition materials7. Differing from conventional inorganic crystalline materials, the flexible structure of ZIFs allow for dramatic changes in the adsorption and diffusion properties, and accordingly are themselves applicable to alternative applications, such as the remediation of aquatic ions. The porous structure of ZIF-8 is composed of zinc metal ions and the organic ligand, 2-methylimidazole. ZIFs have been reported to possess properties termed the ‘rotary door effect’ (or the ‘respiratory effect’), essentially acting as a molecular sieve8. Specifically related to ZIF-8, there are abundant alkaline and acid functional groups residing on the external surface, such as alkaline hydroxyl groups offered by OH-1 and N-1, Lewis acid sites from low coordinated Zn and Brønsted acid sites related to NH. Thus, ZIFs can be selective in the uptake of guest species allowing ingress to varying degrees9. For the capture of gases, such as CO2 and CxHy, ZIF-8 has been regarded as a reliable adsorbent. Notably, previous studies have reported the appearance of an unanticipated phase transition when ZIF-8 is exposed to humid gas10. However, there has been significantly less systematic work regarding the mechanism and outcomes in aquatic ion remediation adsorption applications. 2


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Even though the structural walls are composed of different organic ligands, both ZIF-8 and ZIF-90 show the same (SOD) topology structure. The density of metal atoms per unit volume of ZIF-8 and ZIF-90 are 2.45 and 2.33 T/nm3, respectively. The largest diameter of a sphere given ingress to the pores (dag) and the diameter of the largest sphere that can access the cages without contacting the framework atoms (dph) are 3.4 and 11.6 Å for ZIF-8, and 3.5 and 11.2 Å for ZIF-90, respectively11. Zhou et al.12 proposed the reversible adsorption of HgCl2 into a porous coordination polymer (TMBD) and the uptake of metal species provides a springboard for further functionalizing the coordination materials. The TATAB ligand composed of PCN-100 was employed to capture heavy metal ions (Cd(II) and Hg(II)) by constructing complexes within the pores with a possible coordination mode similar to that which was found in aminopyridinato complexes13. The adsorption effect varies as a function of the organic ligand. The functional groups of ZIF-90 direct the attachment of intermediate Au species resulting in the stabilization of substantially smaller monodispersed Au nanoparticles in contrast to the parent, non-functionalized ZIF-814. Trace (ppb, 10-9) arsenate in water was directly treated by ZIF-8, and consequently the ion concentration decreased to under 9.8 µg·L−1, which conform to WHO and US EPA limits: 10 µg·L−1 15. In recent years, researchers proposed that MOFs containing rare earth elements—which allow a diverse array of coordination chemistry and functionality— may be preferred to other classes of porous materials for a variety of applications, such as catalysts, chemo-sensory materials, light-emitting materials, or proton conductors16-18. With more attention was paid to rare earth associated materials, significant quantities of rare earth elements enter the environment and the nest 3



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problem is removal and recover of rare earth elements. The traditional methods include ion exchange, extraction, precipitation, reverse osmosis, and adsorption.19-21 Among them, adsorption (including biosorption, chemisorption and physical adsorption) is a promising technique because of the facile and economical operation. There are a variety of natural or synthetic inorganic materials with different chemical, structure and superficial characteristics that have been used as adsorbent materials in elimination process of rare earth elements, such as sargassum fluitans22 (pH = 5, 0.72 mmol·g-1), iron oxide loaded calcium alginate beads23 (298 K, pH = 5, 123.5 mg·g-1), magnetic alginate-chitosan gel beads24 (97.1 m·g-1), activated carbon prepared from rice husk25 (175.4 mg·g-1), oxidized multiwalled carbon nanotubes26 ( 99.01 mg·g-1 for La(III)) and 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester-grafted magnetic silica nanocomposites27 (pH=5.5, 55.9 mg·g-1). For the unsatisfactory outcomes and irreversible adsorption, it is urgent to explore an efficient method to adsorb and recover rare earth elements. In this research the emphasis relates to the adsorptive application of ZIFs in the uptake LMI together with the reversibility of the process, which is the first report of such studies to the best of our knowledge. The LMI concentration ranges from 25 to 1000 mg·L-1. Two polar desorbents, acetonitrile and methyl alcohol, are selected to desorb the LMI from ZIF-8. Furthermore, in the studies pertaining to the reuse of ZIFs, the crystal structure and the adsorption and desorption ratio are studied by means of a single factor experiment method. Analysis by inductively coupled plasma optical emission spectrometry (ICP-OES), powder X-ray diffraction (P-XRD) and Fourier Transform Infrared spectroscopy (FT-IR) reveal the efficiency and reversibility of the ZIFs. The process simulated by density functional theory (DFT)

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calculations indicate the adsorption sites in the structure of ZIF-8 for LMI and free energies of the molecular before and after adsorption. 2. MATERIALS AND METHODS 2.1. Materials and preparation N,N-dimethylformamide (DMF) was purchased from Aladdin Chemistry Co., Ltd.; 2-methylimidazole and zinc nitrate hexahydrate (Zn(NO3)2·6H2O) were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. For the source of metal ions, lanthanum nitrate hexahydrate (La(NO3)3·6H2O) was chosen because of its solubility properties and purchased from Beijing HWRK Chem Co., Ltd. All salts were of reagent grade, while other reagents were of analytical grade. All chemicals were used without further purification, and all solutions were prepared with deionized water. ZIF-8 and ZIF-90 were obtained by the modified processes of previous reports28,29. Regarding ZIF-8, 2-methylimidazole (0.096 mmol) and Zn(NO3)2·6H2O (0.032 mmol) were dissolved into DMF (75 mL), and thereafter transferred to a Teflon-lined Parr autoclave and heated to 413 K for 24 h. After cooling, the product was filtered and washed with methanol to obtain a yellowish powder. For ZIF-90, Zn(NO3)2·6H2O (1.25 mmol) and imidazole-2-carboxaldehyde (5 mmol) were added to DMF (12.5 mL) in a round bottom flask (50 mL). The mixture was stirred at 355 K for 4 h. After the solution cooled to room temperature, the sample was washed with copious amounts of deionized water followed by 12.5 ml methanol for several times. LMI stock solutions (1.0 g·L-1) was prepared by dissolving solid lanthanum nitrate hexahydrate (La(NO3)3·6H2O) into deionized water. Thereafter, a series of LMI 5



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solutions varying in concentration (i.e., 25, 50, 100, 150, 200, 250, 300, 400 and 500 mg·L-1) were prepared by diluting the stock solution. Diluted HNO3 and NaOH solutions (0.01 M) were introduced into the solutions for pH adjustment. 2.2. Adsorption experiments ZIFs are well known for their outstanding hydrothermal stability, however, after activation at 250 °C in the presence of air, a degree of Zn and N atoms were dissociated and converted to ZnOH and NOH, respectively, resulting in the partial structural integrity loss of the ZIFs30. The stability of ZIF-8 in water was studied by subjecting ZIF-8 to the presence of water for four days. Fig.1 shows the X-Ray diffraction powder patterns of ZIF-8 before and after treatment with distilled water. After water treatment at room temperature, some new diffraction peaks were observed at 2θ of 12. The main diffraction peaks all retained. This is consistent with the result Cheng and Hu presented31. Therefore, ZIF-8 could be used in aqueous environments for extended periods of time. Adsorption experiments for LMI from aqueous solutions were conducted using a thermostatic shaker. A desired dosage of ZIF-8 and ZIF-90 was added into 100 mL LMI solution at a given concentration. The mixtures were oscillated at a constant speed of 150 rpm at 298 ± 1 K. The LMI concentration was determined by ICP- OES. The adsorption capacity of LMI onto ZIF-8 at time t, qt (mg·g-1), was calculated by

(1)

where C0 and Ct (mg·L-1) are the initial LMI concentration and residual concentration at time t in the solution, respectively; V (L) is the volume of LMI solution; and m (g) is the mass of dry ZIF-8. 6


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The removal ratio (R) of LMI using ZIF-8 was calculated by

(2)

2.3. Characterization The powdery X-ray diffraction (XRD) patterns of fresh ZIF-8 and the corresponding ZIF after LMI adsorption (La-ZIF-8) were recorded on a Bruker D8ADVANCE X-ray diffractometer (40 kV, 40 mA) using Cu-Kα (λ = 0.15418 nm) in the 2θ range from 5° to 50°. Fourier transform infrared (FT-IR) spectra of the samples were measured on a Bruker Equinox 55 infrared spectrum apparatus using the KBr sheeting method in the range of 4000–400 cm-1. The concentration data of metal ions prior to adsorption and after desorption was determined by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES, Thermo, 6300). 3. RESULTS AND DISCUSSION 3.1. Adsorption capacity and removal ratio of LMI Aqueous metal ion solutions varying in their concentration were treated with a fixed mass of ZIF-8 and ZIF-90. The concentrations of cations were measured by ICP and adsorptive results are presented in Fig. 2. There is a significant difference between the adsorption performances of the two ZIF materials (Table.1). The specific value of the maximum adsorption capacity (qm) of ZIF-8 are several times that of ZIF90. The structure of ZIF-8 contains methyl groups, which are regarded as a poor 7



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electron donor to the imidazole ring. Conversely, for ZIF-90, the aldehyde groups are regarded as an electron acceptor weakening the electron cloud density of the imidazole ring. Therefore, ZIF-8 possesses a stronger binding force with the metal cation. For La3+ of the high concentration, full and dashed lines coincide. This indicates that 24 h is enough for adsorption processes to reach equilibrium. With regard to trace metal ions, ZIF-8 and ZIF-90 can both attain a removal ratio of 100.0 %. As shown in Fig.3, for initial trace metal ion concentrations of < 20 mg·L-1, ZIF8 has almost 100.0 % rate of remediation efficiency. Increasing the concentration range from 20 to 50 mg·L-1, the adsorption capacity increases to 200 mg·g-1 and removal ratio of ZIF-8 decreases to below 90%. This indicates that ZIF-8 displays high uptake efficiency at low concentration and high adsorption quantity at high concentration for lanthanum metal ions. 3.2. Characterization of metal ion adsorption. 3.2.1. Analysis of X-Ray diffraction powder patterns. The outcomes of XRD characterization display the Crystallographic structural of ZIF-8 after adsorbing metal ions changed a little. Figure 4 shows that ZIF-8 material has diffraction peaks at 2θ values of 7.5°, 12.5° and 18°. La-ZIF-8 material presents a weaker diffraction pattern at 2θ values of 7.5° and an additional peak reflection at 2θ around 9° and 14°. At the magnified part, 2θ value changing from 20° to 30°, there is few change between ZIF-8 and La-ZIF-8 except for a new little peak at 2θ around 21° which can be overlooked. 3.2.2. Analysis of FT-IR spectra. 8


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FT-IR data (see Fig. 5), shows satisfactory matching of the main peaks to those previously reported36,37. The relatively sharp peaks at approx. 3136, 2931, 1685, 1587, 1460 and 1425 cm-1 indicate infrared absorption characteristic of imidazole ring. The narrow peak in the region of 3600 cm-1 is correlated to the presence of free hydroxyl bonds in the structure of La-ZIF-8. The peak ~420 cm-1 indicates the presence of the Zn-O stretching vibration. The similarity in the spectra of ZIF-8 before and after adsorption was in good agreement with that in the results obtained from P-XRD except for the addition some new peaks close to 520 cm-1 for the La-ZIF-8 material. The formation of additional peak can be ascribed to the La-O stretching vibrations. In water, due to the dissociative adsorption of water, the bonds of Zn-OH and N-H on the surface of ZIF-8 ware formed. Adding the lanthanum ion into the system, the bond Zn-O-La bond was formed. Previous studies have reported the As-O stretching vibration peak to appear at 844 cm-1 during the uptake of As into ZIF-815. 3.3. Adsorption site of La in ZIF-8 and Interaction energy For the theoretical calculations, the spin-polarized density function theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP).32,33 To improve the calculation efficiency, core electrons were replaced by the projector augmented wave (PAW) pseudopotentials,34 and the generalized gradient approximation (GGA) of Perdew, Burke, and Ernzernh of (PBE)35 was used to describe the exchange and correlation. The cutoff energy for the plane-wave basis set was set to 500 eV in all calculations. The Monkhorst-Pack mesh k-points (4×4×4) were used for the convergence criteria for the electronic self-consistent iteration and the ionic relaxation loop were set to 10−4 eV and 10−2 eV/Å, respectively.

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The adsorption energy (Eads) of metal atom(s) (A, representing lanthanum ion) on a substrate (B, representing the unit of ZIF-8) is calculated using the expression

Eads = E AB − E A − E B

(3)

where EA(B) represents the total energies of A(B), and EAB represents the total energy of the adsorbed system. By this definition, the more positive the energy, the stronger the interaction. The outcomes of simulation indicate the value of EA, EB and EAB are -0.41, 289.35 and -291.70 eV, respectively. The calculated value of Eads is -1.94 eV, indicating this adsorption process is spontaneous. Fig. 6 shows the structural parameters of ZIF-8 and La-ZIF-8. From (a) and (b) part, after adsorbing La, the position of Zn remains unchanged, and the C1 and C3 draw close to each other because of La added in the whole structure. From (c) part, it is obvious that the adsorption site is in the center of C1 and C3 imidazole rings. Meanwhile, methyl group shows the repulsive force to La. 3.4. Characterization of adsorption reversibility. 3.4.1. Analysis of X-Ray diffraction powder patterns. The X-Ray powder diffraction patterns of ZIF-8, La-ZIF-8, La-ZIF-8-(CH3CN) and La-ZIF-8-(CH3OH) were measured to determine the crystal structure change before and after the adsorption and desorption of La, and the results are shown in Fig. 6. It can be observed that the sample before adsorption exhibited a typical crystal structure of ZIF-8, in which all diffraction peaks are consistent with those reported by Yaghi et al.36. However, there is the presence of an additional diffraction peak appearing close to 2θ of 13.9° for the La-ZIF-8 material formed after the uptake of the 10


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lanthanum ion onto ZIF-8. Furthermore, the intensity of main (011) diffraction peak at a 2θ value of 7.5° decreased. Interestingly, the lanthanum ion in the porous material can be removed by polar solvents, such as acetonitrile and methyl alcohol at 60 °C for 24 h, while maintaining the structural integrity of the cubic ZIF-8. ICP measurements show that the concentration of lanthanum decreased by 61.0 % and 94.4 % after the ZIF-8 sample was immersed in the polar solvent including methyl alcohol and acetonitrile, respectively. In Fig. 7, the intensity ratio of the main diffraction peak to the peak at 13.9° (2θ) is found to be increase to the point where the unindexed peak (13.9° (2θ)) is significantly reduced (but not absent) after polar solvent treatment. Due to the ion exchange in the adsorption process, a small amount of Zn ions were replaced by La ions. Therefore, the polar solvent removes the lanthanum ion varying degrees and the resulting crystallographic structure reverts back to that of the pristine ZIF-8 sample partly. Combining the experiments and DFT calculations, during the adsorption, ionexchange happens, and La-O bonds and force between La and imidazle rings are formed. After the process of adsorption, the fresh ZIF-8 and ZIF-8-La were treated and detected by ICP-ES and results show nitric acid. And the results of ICP-ES show that 11.25% Zn2+ lost in the structure of ZIF-8, which is the part of ion exchange. Due to the dissociative adsorption of water, the bonds of Zn-OH and N-H on the surface of ZIF-8 ware formed. Adding the lanthanum ion into the system, the bond Zn-O-La bond was formed. Density functional theory (DFT) calculations show there is action force between La and imidazle ring. Therefore, La-ZIF-8 material presents other diffraction pattern than ZIF-8 not only due to the ion exchange in the adsorption process, but also the La-O bonds and force between La and imidazle rings.

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The results of ICP element analyses and powder XRD revealed the coordinating ability with the lanthanum ion to be: N of acetonitrile > O of alcohol > N of imidazole. Similar phenomenon was presented by Xiao-Ping Zhou et al. They found the acetonitrile solvent can remove the HgCl2 component from PbTMBD-HgCl2 because of the different coordination ability for thioether and acetonitrile with the Hg ion37. 3.4.2. Analysis of FT-IR spectra. The FT-IR spectra of La-ZIF-8 treated by polar solvents, methyl alcohol and acetonitrile, are shown in Fig. 8. After the desorption process, the main imidazole structure is retained and the La-O stretching vibration peak significantly reduced, although did not disappear completely, indicating that desorption of lanthanum was not entirely complete. 3.5. Adsorption ability change after recycling use. Desorption experiments show acetonitrile to be a highly efficiency desorbent, which provides a route to the possibility of recycling ZIF-8 in lanthanum readsorption. Fig.9 shows that the desorption ratio maintains a high value, above 90.0 % after recycling three times. It is found that the adsorption capacity significantly decreases from 385 to 150 mg·g-1 after recycling ZIF-8 three times at 60 °C, due to the treatment with the aqueous metal solution and acetonitrile. XRD diffractograms of the ZIF-8 material recycled three times and the pristine ZIF-8 material at 60 °C is shown as Fig.10. It is found that the ZIF-8 crystal structure increasingly collapsed significantly as a function of recycling at 60 °C, due to the high temperature. In comparison, we tested the reversible adsorption after recycling ZIF-8 three times at room temperature in Fig. 11. It is very interesting to find that the ZIF-8 remains its

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structure after recycling ZIF-8 three times. It means that the temperature is of a great effect on the reversible adsorption of ZIF-8. 4. CONCLUSION In conclusion, a series of sorption experiments, the adsorption capacity and removal ratio of two adsorbents, ZIF-8 and ZIF-90, with different organic groups, have been investigated. ZIF-8 demonstrated to be a highly effective adsorbent for lanthanum ion. For trace metal ion concentration, the removal ratio of ZIF-8 can reach 100%. Increasing the metal ion concentration, the adsorption quantity increase to 385 mg·g-1, which is the saturated adsorption capacity. Characterization by P-XRD, ICP and FT-IR indicates that the main structure of ZIF-8 remained unchanged to a large extent during the adsorption process. Density functional theory (DFT) calculations illustrated the adsorption process is spontaneous and the adsorption site is in the center of two imidazole rings. Combining the experiments and DFT calculations, during the adsorption, ion-exchange happens, and La-O bonds and force between La and imidazle rings are formed. Furthermore, the two polar desorbents, acetonitrile and methyl alcohol, were studied to determine their effectiveness to remove lanthanum from ZIF-8 and the desorption ratios were 94.4 % and 61.0 %, respectively. Subjecting the ZIF-8 material to re-adsorption/desorption resulted in significant loss of structural integrity after three recycles. Additionally, the adsorption capacity reduced to 150 mg·g-1, however, acetonitrile maintains a high desorption ratio >90 %. The studies herein indicate the rare metal ion and lanthanum metal ion uptake ability of ZIF-8 and ZIF-90, providing a platform for further functionalizing the coordination materials.

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Acknowledgements This work is supported by the National Natural Science Foundation of China (21176010, 21576008, 91334203), BUCT Fund for Disciplines Construction and Development (Project No. XK1501), Fundamental Research Funds for the Central Universities (Project No. buctrc201530), and “Chemical Grid Project” of BUCT. REFERENCES (1) Khan, N. A.; Hasan, Z.; Jhung, S. H.; Adsorptive removal of hazardous materials using

metal-organic frameworks (MOFs): A review. J. Hazard. Mater. 2013, 244, 444–456. (2) Banerjee, R.; Furukawa, H.; Britt, D.; Knobler, C.; O’Keeffe, M.; Yaghi, O. M.; Control of pore size and functionality in isoreticular zeolitic imidazolate frameworks and their carbon dioxide selective capture properties. J. Am. Chem. Soc. 2009, 131, 3875–3877. (3) Furukawa, H.; Yaghi, O. M,; Storage of hydrogen, methane, and carbon dioxide in highly porous covalent organic frameworks for clean energy applications. J. Am. Chem. Soc. 2009, 131, 8875–8883. (4) Millward, A. R.; Yaghi, O. M.; Metal-organic frameworks with exceptionally high capacity for storage of carbon dioxide at room temperature. J. Am. Chem. Soc. 2005, 127, 17998–17999. (5) Zhang, X.; Liu, Y.; Li, S.; Kong, L.; Liu, H.; Li, Y.; Han, W.; Yeung, K. L.; Zhu, W.; Yang, W.; Qiu, J.; New membrane architecture with high performance: ZIF-8 membrane supported on vertically aligned ZnO nanorods for gas permeation and separation. Chem. Mater. 2014, 26, 1975–1981. (6) Li, Z.; Zeng, H. C.; Surface and bulk integrations of single-layered Au or Ag nanoparticles onto designated crystal planes {110} or {100} of ZIF-8, Chem. Mater. 2013, 25, 1761– 1768. (7) Li, H.; Feng, X.; Guo, Y.; Chen, D.; Li, R.; Ren, X.; Jiang, X.; Dong, Y.; Wang. B.; A malonitrile-functionalized metal-organic framework for hydrogen sulfide detection and selective amino acid molecular recognition. Scientific Reports. 2014, 4, 4366. (8) Gücüyener, C.; Bergh, J.; Gascon, J.; Kapteijn, F.; Ethane/ethene separation turned on its head: selective ethane adsorption on the metal-organic framework ZIF-7 through a gate-opening mechanism. J. Am. Chem. Soc. 2010, 132, 17704–17706. (9) Chizallet, C.; Lazare, S.; Bazer-Bachi, D.; Bonnier, F.; Lecocq, V.; Soyer, E.; Quoineaud, A.A.; Bats, N. Catalysis of transesterification by a nonfunctionalized metal − organic framework: acido-basicity at the external surface of ZIF-8 probed by FTIR and ab initio calculations. J. Am. Chem. Soc. 2010, 132, 12365–12377. (10) Mottillo, C.; Friscić, T.; Carbon dioxide sensitivity of zeolitic imidazolate frameworks. Angew. Chem. Int. Ed. 2014, 53, 7471−7474. (11) Phan, A.; Doonan, C. J.; Uribe-Romo, F. J.; Knobler, C. B.; O’Keeffe, M.; Yaghi, O. M. Synthesis, structure, and carbon dioxide capture properties of zeolitic imidazolate frameworks. Accounts Chem. Res. 2010, 43, 58–67. 14


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Page 17 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. The maximum adsorption capacity of lanthanum ion onto ZIFs

qm onto ZIF-8 Metal ions La3+

mg/g 385

qm onto ZIF-90

mmol/g 2.77

mg/g 168

17


mmol/g 1.21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

List of Figure Captions Fig.1. X-Ray diffraction powder patterns of ZIF-8 before and after treatment of water for 1 to 4 days under room temperature.

Fig.2. Adsorption capacity and removal ratio of metal ions onto ZIFs. Full and dashed lines represent the adsorption time of 24 and 48 h, respectively. Square and circle symbols represent the adsorbents ZIF-8 and ZIF-90, respectively.

Fig.3. Adsorption of trace lanthanum ion using ZIF-8.

Fig.4. X-Ray diffraction powder patterns of ZIF-8 and La-ZIF-8. In left part, 2θ range from 5o to 40o. In right part, 2θ range from 20 o to 30 o.

Fig.5. FTIR spectra of ZIF-8 before and after lanthanum ion adsorption. The new peak was marked out with arrow.

Fig.6. Simulated structure parameter of (a) ZIF-8, (b) La-ZIF-8 and (c) side view of La-ZIF-8 by VASP.

Fig.7. X-Ray powder diffraction patterns of adsorption and desorption of lanthanum ion onto ZIF-8. In left part, 2θ range from 5o to 40o. In right part, 2θ range from 20 o to 30 o.

Fig.8. FTIR spectra of adsorption and desorption of lanthanum ion onto ZIF-8.

Fig.9. Recycling use of ZIF-8 to adsob and desorb lanthanum ion at 60 °C.

Fig.10. X-Ray powder diffraction patterns of ZIF-8 before and after recycling use at 60 °C.

Fig.11. X-Ray powder diffraction patterns of ZIF-8 before and after recycling use at room temperature. 18


Page 18 of 29

(134)

(114)

(233)

(222)

(013)

(112)

(022)

(002)

Normalized Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(011)

Page 19 of 29

ZIF-8

ZIF-8-1d

ZIF-8-2d

ZIF-8-3d

ZIF-8-4d 5

10

15

20

25

30

35

40

45

50

2θ ( degree)

Fig.1. X-Ray diffraction powder patterns of ZIF-8 before and after treatment of water for 1 to 4 days under room temperature.

19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

A d sorp tion cap a city (m g/g)R em oval ratio (% )


100 80 60 40 20

0 400 300 200 100 0

0

200

400

600

800

1000

Initial concentration of La3+ (mg/L)

Fig.2. Adsorption capacity and removal ratio of metal ions onto ZIFs. Full and dashed lines represent the adsorption time of 24 and 48 h, respectively. Square and circle symbols represent the adsorbents ZIF-8 and ZIF-90, respectively.

20


Page 20 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Adsorption capacity (mg/g) Removal ratio (% )

Page 21 of 29

100

90

80 200

150

100

50

0 0

10

20

30

40

Initial concentration (mg/L)

Fig.3. Adsorption of trace lanthanum ion using ZIF-8.

21


50

ZIF-8

(134)

(223)

(114)

(222)

(013)

(112) (022)

(002)

Normalized Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 29

(011)


La-ZIF-8

5

10

15

20

25

30

35

40

20

22

24

26

28

30

2θ (degree)

Fig.4. X-Ray diffraction powder patterns of ZIF-8 and La-ZIF-8. In left part, 2θ range from 5o to 40o. In right part, 2θ range from 20 o to 30 o.

22


Page 23 of 29

3136

ZIF-8

2931

1685 1587 694

1180 995

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1311 1460 1425

760 420

1147

520

La-ZIF-8 3639

4000

3500

3000

2500

2000

1500

1000

500

Wavenumber (cm-1)

Fig.5. FTIR spectra of ZIF-8 before and after lanthanum ion adsorption. The new peak was marked out with arrow.

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

(b )

(c)

Fig.6. Simulated structure parameter of (a) ZIF-8, (b) La-ZIF-8 and (c) side view of La-ZIF-8 by VASP.

24


Page 24 of 29

ZIF-8

(134)

(233)

(114)

(222)

(013)

(112)

(022)

(002)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(011)

Page 25 of 29

La-ZIF-8

La-ZIF-8 (CH3CN)

La-ZIF-8 (CH3OH) 5

10

15

20

25

30

35

40 20

22

24

26

28

30

2θ (degree)

Fig.7. X-Ray powder diffraction patterns of adsorption and desorption of lanthanum ion onto ZIF-8. In left part, 2θ range from 5o to 40o. In right part, 2θ range from 20 o to 30 o.

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

La-ZIF-8

La-ZIF-8-CH3OH

La-ZIF-8-CH3CN

4000

3000

2000

1000

Wavenumber (cm-1)

Fig.8. FTIR spectra of adsorption and desorption of lanthanum ion onto ZIF-8.

26


Page 26 of 29

100

100 400

80

60

40

20

0

Adsorption capacity (mg/g)

80

Desorption ratio(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Collapse ratio (%)

Page 27 of 29

60

40

20

1

2

3

300

200

100

0

0 1

2

3

1

2

3

Recycle time

Fig.9. Recycling use of ZIF-8 to adsob and desorb lanthanum ion at 60 °C.

27


Page 28 of 29

ZIF-8

La-ZIF-8

Re-ZIF-8 5

10

15

20

25

30

35

40

2θ (degree)

Fig.10. X-Ray powder diffraction patterns of ZIF-8 before and after recycling use at 60 °C.

ZIF-8


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



ZIF-8-CH3CN-1

ZIF-8-CH3CN-2

ZIF-8-CH3CN-3 0

10

20

30

40

50

2θ ( degree)

Fig.11. X-Ray powder diffraction patterns of ZIF-8 before and after recycling use at room temperature.

28


Page 29 of 29

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Table of Contents (TOC) Graphic

29


Adsorption toward Trivalent Rare Earth Element from Aqueous

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