A magnetic hybrid Cu(I)-MOF@Fe3O4 with hierarchically engineered

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A magnetic hybrid Cu(I)-MOF@Fe3O4 with hierarchically engineered micropores for highly efficient removal of Cr(VI) from aqueous solution Huijun Li, Qingqing Li, Xinglei He, Ning Zhang, Zhouqing Xu, Yan Wang, and Yuan Wang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01053 • Publication Date (Web): 07 Sep 2018 Downloaded from http://pubs.acs.org on September 9, 2018

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Crystal Growth & Design

A magnetic hybrid Cu(I)-MOF@Fe3O4 with hierarchically engineered micropores for highly efficient removal of Cr(VI) from aqueous solution Huijun Li,a Qingqing Li,a Xinglei He,a Ning Zhang,a Zhouqing Xu,a* Yan Wang,b* Yuan Wanga* a

College of Chemistry and Chemical Engineering, Henan Polytechnic University,

Jiaozuo, Henan 454000, China, [email protected]; [email protected]. b

School of Materials Science and Engineering, School of Safety Science and

Engineering,

Henan

Polytechnic

University,

Jiaozuo

454000,

PR

China,

[email protected].

KEYWORDS: Cr(VI) removal; maximum capacity; adsorption mechanism; Cr(VI) reduction

ABSTRACT: It is a topic hot for public health and environmental protection to remove of Cr(VI) from wastewater effectively and conveniently. Herein, we fabricate a

novel

hybrid

magnetic

material

HPU-13@Fe3O4

{[Cu3(L)2]·(OH)·2(CH3CH2OH)·10(H2O)}n),

(HPU-13

=

(HL=2-(5-Pyridin-4-yl-2H-

[1,2,4]triazol-3-yl)-pyrimidine) which shows excellent Cr(VI) removal efficiency and

ACS Paragon Plus Environment

Crystal Growth & Design 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

is easy to be employed as reusable absorbents. The maximum capacities of Cr2O72and CrO42- adsorbed on HPU-13@Fe3O4 were 398.41 and 471.69 mg/g, respectively, larger than those of reported MOFs used in Cr2O72- and CrO42- adsorption. Adsorption mechanism studies confirmed that the synergistic reaction of Cr(VI) reduction and adsorption were responsible for the high removal efficiency. The above evidence proved that HPU-13@Fe3O4 has the potential functional ability to decontaminate Cr(VI) polluted water system.

INTRODUCTION

With the rapid development of industry, heavy metal pollution has been considered as a severe environmental pollution issue to be addressed because of its poor degradability, biologically related toxicity and easy accumulation.1-4 In particular, Cr(VI) is much more carcinogenic, mutagenic, and genotoxic on humans.5-7 Accordingly, various techniques applied in removing Cr(VI) such as adsorption, ion exchange, reduction or precipitation have been reported to effectively purify Cr(VI)contaminative water.8-9 Among the different purification methods, synergistic reaction of Cr(VI) reduction and adsorption is an attractive approach to remove Cr(VI) pollutants, which has the advantages of easy operation, high efficiency and is favourable for reducing toxicity of Cr(VI)-polluted water by the reduction of Cr(VI) to Cr(III).10-12 However, some common modified zeolites, activated carbon, sawdust, chitosan or nano-adsorbents always could not be completely satisfied by the abovementioned integrated approach because of their inherent drawbacks such as low


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functionality and characteristics, which obstruct their further development and application.13-14 So exploring the newly synthesized materials with large adsorption capacity, affinity and reactivity for Cr(VI) is particularly important.

As a new kind of porous materials, metal-organic frameworks (MOFs) possessing the favourable advantages of adjusting the properties of the pore surface and channel size, which would facilitate the synergistic reaction of Cr(VI) reduction, sizeconfinement effect, adsorption, or ion-exchange.15-16 However, powdered MOFs exists the defect that is difficult to be separated from water, which will restrict their practical application. With these criteria in mind, embedding ferrites into conducting MOFs to construct a hybrid material is expected as an effective approach, which is easy to achieve separation from water under the influence of an external magnetic and the water stability also could be enhanced moderately.17-19 Thus, we report a stable and recoverable magnetic hybrid material, which shows remarkable adsorption capability for Cr(VI) and elucidate the synergistic reaction of Cr(VI) reduction and adsorption to accomplish high removal efficiency (Scheme 1). Moreover, the effects of different experimental parameters such as initial Cr(VI) concentration, competing ions and contact time are also investigated on Cr(VI) adsorption. Large adsorption capacity and excellent reusability estimation make the hybrid material a highly effective adsorbent for Cr(VI).

Experimental Section



Materials and Physical Measurements. All chemical reagents were available commercially and used for purchase. IR data were collected on a BRUKER TENSOR 27 spectrophotometer with KBr pellets in the region of 400-4500 cm-1. Flash EA 1112 elemental analyzer was used for Elemental analyses (C, H and N). Powder X-ray diffraction (PXRD) patterns were examined by a PANalytical X’Pert PRO diffractometer with CuKα radiation. Netzsch STA 449C thermal analyzer with a heating rate of 10 °C min−1 in atmosphere was used to record Thermogravimetric analysis (TGA) in the temperature range of 30 and 800 °C. The UV spectra were characterized by a Purkinje General TU-1800 spectropho-tometer. The field-emission scanning electron microscopy (SEM) (S-4800, Hitachi, Japan) was introduced to characterize the microstructures and morphologies of the materials. The surface elements of the materials were characterized by electron dispersive spectroscopy (EDS). Thermo Scientific ESCALAB 250Xi X-ray photoelectron spectrometer (XPS) system was employed to obtain XPS spectra (Al K X-ray source was used).

Synthesis of {[Cu3(L)2]·(OH)·2(CH3CH2OH)·10(H2O)}n (HPU-13). A mixture of CuSO4 (24.9 mg) and HL (11 mg) in H2O and ethanol (8 : 2) were placed in a 15 mL of bottle and heated for 72 h at 160 °C. The final solution was cooled to room temperature and then yellow block crystals of HPU-13 were acquired, washed with acetone and dried in air. Yield: 78%. Anal. Calc. for C26H47Cu3N12O13: C, 33.71; H, 5.11; N, 18.14. Found: C, 33.64; H, 5.16; N, 18.22%.


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Preparation of Fe3O4 particles. The Fe3O4 particles were prepared according to the literature. 20

Synthesis of magnetic HPU-13@Fe3O4 particles. The mixture of CuSO4 (24.9 mg) and HL (11 mg) in H2O and ethanol (8 : 2) was firstly stirred for 30 min. Then, the sulution was added by 2 mg Fe3O4 and transferred to a 25 mL Teflon liner for 72 h at 160 °C. At last, the final products was centrifuged and washed several times with fresh water.

The adsorption experiment, sorption kinetics and sorption isotherm. The aqueous solutions of Cr2O72-, CrO42- or dyes were obtained by dissolving K2Cr2O7, K2CrO4 or dyes in deionized water. Equal amount of adsorbent (30 mg) was added to 10 mL of Cr2O72-, CrO42- (50, 100, 200, 300, 400, 500 ppm) or dyes (20, 50, 100, 200, 300, 400 ppm) in each adsorption experiment. The absorbance of UV spectra used to determine the Cr2O72-, CrO42- and dyes concentrations. The uptake (%) was calculated on the basis of the following formula:

q =

( )

× 100%

where qe (mg/g) stands for the amount of Cr2O72-, CrO42- and dyes adsorbed per gram of absorbents at equilibrium; C0 and Ce represent the initial and residual concentration, respectively (mg/L); V and m are the solution volume (mL) and the mass of the adsorbent, respectively. The solution was placed in a constant-temperature shaker (SHA-C, Guohua) under the speed of 150 r/min. 100 µL of the solution was picked



out during the shaking period and diluted to 2 mL, which was evaluated by UV absorbance.

The effects of the competing ions and recoverability of material. The effects of the competing ions was carried out under the disturbance of the most familiar competing anions, ie. F-, NO3-, SO42-, Cl-, H2PO4-, Br- (3-fold molar, 6-fold molar, 9-fold molar, 12-fold molar). The recoverability experiment was accomplished by soaking the Cr(VI)-loaded material in NaNO3 saturated aqueous solution for 2 h.

Crystal Data Collection and Refinement. The crystallographic diffraction data for HPU-13 was collected by using a Siemens Smart CCD single-crystal X-ray diffractometer with a graphite monochromatic MoKα radiation (λ= 0.71073 Å) at 293 K. Direct methods using SHELXS-2014 program of the SHELXTL package was employed to handle the structure which was then refined on F2 by full-matrix leastsquares techniques with SHELXL-2014. SADABS program is used to correct all empirical absorption. Anisotropic thermal parameters are used to refine all nonhydrogen atoms in the crystal structure. The crystallographic data and structural refinement parameters of HPU-13 are presented in Table S1.

RESULTS AND DISCUSSION

Structural description. HPU-13 crystalizes in Trigonal system with space group of p-3c1, which is clearly indicating by Single X-ray crystallographic analysis. It consists of hexagonal and triangular cationic micropores with a three-dimensional crystallographically-imposed molecule symmetry. As depicted in Figure 1a, the


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ligand is a µ4 bridged model with each of the ligands’ six nitrogen atoms linking with four Cu(I) ions by all of the nitrogen atoms. The two crystallographically independent Cu(I) atoms possess distorted tetrahedron geometries. Three independent ligands bridge six Cu(I) atoms forming a closed triangular unit, which link with another triangular unit in the face-to-face direction giving rise to a closed cage (Figure 1b and 1c). Four adjacent triangular cages are connected together forming a hexagonal mesropore. Investigation of the crystal packing indicates that HPU-13 shows porous structural character with two different channels: hexagonal and triangular micropores along the c axis due to the existence of the skeletons of ligands. The hexagonal cage has a large dimension of 20.947× 20.720 Å2 (Figure 1d and 1e). The triangular micropores have a pore size of 4 Å from the nitrogen atoms of HL groups. Finally, the whole structure might look as an expansion of the MIL-68 structure (Figure 1f).21 The effective free voids of HPU-13 are occupied as 36.0% of the crystal volume, which is calculated by PLATON analysis (1728.9 out of 4803.0 Å3 the unit cell volume).

Thermal and Water Stability test. The ecofriendliness, thermal and water stability of materials are the guarantees for application. Thermogravimetric analysis (TGA) and powder X-ray diffraction (PXRD) were examined for inspecting the thermal stability of HPU-13 (Figure S1a and b). The TGA analysis shows that HPU-13 loses weight from 70 to 102 °C, ascribed to the free CH3CH2OH and H2O molecules (calcd. 19.4% H2O and 9.9% CH3CH2OH) and the framework starts to collapse at 378 °C. The PXRD synthesized HPU-13 which is similar with the simulated result from the



single–crystal data reveals the purity of HPU-13. Besides, the PXRD patterns of the fresh sample of HPU-13 in H2O for 72 h also matches well with that of the original synthesized product, proving that this material has a good resistance against H2O. So such H2O stability makes HPU-13 more feasible in practical applications.

The hybrization of HPU-13 and Fe3O4. The SEM images clearly showed the Fe3O4 nanoparticle coated onto the long rod-like crystals of HPU-13 (Figure 2). The distribution of Fe, Cu, O contained in the products was also characterized by elemental mapping (Figure 3). The PXRD patterns of Fe3O4 and HPU-13@Fe3O4 are both presented in Figure 4a. The primary diffraction peaks of the magneticite-loaded HPU-13@Fe3O4 are analogous with those of HPU-13, which indicates that the crystal phases of HPU-13 maintained well after the hybridization with Fe3O4. Figure 4b provided the IR spectra of Fe3O4, HPU-13 and Fe3O4@HPU-13. Fe3O4@HPU-13 not only retained the IR spectrum of HPU-13, but also has the main IR peaks of Fe3O4. The above evidence provided supports the successfully synthesis of Fe3O4@HPU-13 hybrid material made up of Fe3O4 and HPU-13.

Comparison of adsorption performance. The performance of HPU-13@Fe3O4 for the removal of Cr(VI) from water was explored. The intensity of the characteristic adsorption peaks of Cr2O72- and CrO42- decreased largely within only 60 min as shown in Figure 5. The adsorption effect of Cr(VI) by Fe3O4 was also provided in Figure 5, which shows that Fe3O4 almost has no adsorption. It meant that HPU-


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13@Fe3O4 synthesized from the selective raw material could be used for the next research of Cr(VI) removal. Adsorptive Kinetics and Adsorptive Isotherms. The Cr2O72- and CrO42- adsorption capacities are a vital aspect of the sorbent’s performance metrics. The qe versus Ce plots at 25 °C are shown in Figures 6a and 7a. The experimental results were fitted by using Langmuir models, which are shown as the following equation:22-23

=

+

Where qe and qm represent the Cr(VI) adsorption capacity of material at equilibrium and the maximum amount of Cr(VI) occupying on the surface or pores of the material (mg/g), respectively. Ce and KL stands for the equilibrium Cr2O72- and CrO42concentration (mg /L) and Langmuir adsorption constant. The fitting results indicated that the experimental data matched well with the Langmuir model (R2 = 0.98643 and 0.99656). The maximum capacity of Cr2O72- and CrO42- adsorbed on HPU-13@Fe3O4 was 398.41 and 471.69 mg/g, respectively, which are larger than most of the reported cationic MOFs used in Cr2O72- and CrO42- adsorption (Tables S2 and S3).24-28 This suggests that HPU-13@Fe3O4 can be efficiently applied in purify Cr(VI)-polluted system. Besides, the maximum capacities of Cr2O72- and CrO42- adsorbed on HPU-13 were 431.65 and 495.12 mg/g, respectively, which are slightly larger than those of HPU-13@Fe3O4. The larger adsorption capacity is due to the larger BET surface area of HPU-13. When HPU-13 is hybrided by Fe3O4, the BET surface area of HPU-13 would be reduced to an extent.



Figures 6b and 7b showed the dependence of the amount of Cr(VI) adsorbing by HPU-13@Fe3O4 with increasing time. In the beginning, the adsorption capability of Cr(VI) by HPU-13@Fe3O4 increased rapidly. For obtaining the adsorption rate constants, pseudo-second-order model was introduced to fit the experimental data, the model was showed as below:29-30 t t 1 = + q q k q Where, qt, qe and k2 stand for the adsorption capacity at time, the adsorption capacity at equilibrium, the pseudo-second-order model rate constant, respectively. The pseudo-second-order kinetic curves were also shown in Figures 6b and 7b, which indicated that the experimental data could be evaluated well to the pseudo-secondorder model with appropriate correlation coefficients (R2>0.97) (Tables S4). Moreover, the qe values matches well with the experimental data. The experimental facts manifests that the adsorption behavior of Cr(VI) by HPU-13@Fe3O4 is chemical adsorption.

Effects of the Competing Ions. The selective research is also essential for evaluating the efficiency of HPU-13@Fe3O4 in Cr (VI) removal. Some anions, such as Cl-, H2PO4-, Br-, SO42- and NO3- exist in water largely and also have high affinity to sorbents. So the selectivity of HPU-13@Fe3O4 was carried out under the disturbance of the most familiar competing anions, ie. F-, NO3-, SO42-, Cl-, H2PO4-, Br-. As shown in Figure S2, Cr2O72- or CrO42- were almost totally removed even when 12-fold molar


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of disturbing anions (F-, NO3-, Cl-, H2PO4-, Br-, or SO42-) exist in the same environment, revealing the excellent selectivity of HPU-13@Fe3O4 towards Cr (VI).

Cycle Experiment. The reproducibility of HPU-13@Fe3O4 is also required for practical use (Figure 8). The cycle experiment was carried out by soaking the Cr(VI)loaded products in NaNO3 saturated aqueous solution. After 2 h, almost all of Cr(VI) was washed out. The adsorption and desorption process were performed for 6 cycles. The adsorption capacity was almost unchanged. It is notable that the PXRD pattern never changed from the first adsorption to the sixth desportion, indicating the recyclability and reversibility of HPU-13@Fe3O4 in purifying Cr(VI)-polluted water.

Plausible Adsorption Mechanism. As there are Cu(I) ions existing in HPU13@Fe3O4, redox reaction would be happen when HPU-13@Fe3O4 contact with Cr(VI). In our case, X-ray photoelectron spectroscopy (XPS) was performed to explain the adsorption mechanism. Figure 9a shows there is only Cu(I) in HPU13@Fe3O4. For HPU-13@Fe3O4∩Cr(VI) after 1 cycle, the peaks deconvoluted into two doublets belongs to Cu 2p1/2 and Cu 2p3/2 (Figure 9b). The one made up of two peaks has the binding energies of 942.42 and 962.23 eV ascribed to Cu(II); the other with binding energies of 932.17 and 951.98 eV is corresponding to that of the Cu(I) suggesting the partial oxidation of Cu(I) to Cu(II) on the HPU-13@Fe3O4 particles occurred during the adsorption. Besides, we also tested the chemical status of Cr on the surface of HPU-13@Fe3O4 (Figure 9b). From the results, the main presence of Cr(III) was supported by the high resolution XPS spectrum of the Cr 2p region. In this



situation, the adsorption for Cr(VI) on HPU-13@Fe3O4 is mainly attributed to the reducibility of Cu+. The low valent Cu(I) owns a high affinity towards Cr2O72- so as to working as efficient adsorptive sites for Cr(VI). XPS measurements were also performed on the HPU-13@Fe3O4∩Cr(VI) products after 5 cycles, which confirmed there are the main existence of Cu(II) (Figure 9c). Almost all of Cu(I) are oxidized to Cu(II). Besides, there are a large number of Cr(VI) and relative few Cr(III) in the products. Furthermore, the absorption capacity of Cr(VI) by HPU-13@Fe3O4 is almost unchanged. Therefore, the powerful hierarchical pore structure of HPU-13 permit the diffusion of Cr(VI) into the interior of the crystal, which have a dominant position for the high Cr (VI) removal in this cycle. Therefore, the adsorption of Cr(VI) by HPU-13@Fe3O4 is not just a single process,31-33 but contains two-steps as follows: 1 partial reduction of the adsorbed Cr(VI) by adsorbents because of the reducibility of Cu(I); 2 the large cationic channel could afford active sites for Cr (VI), which are the keys to achieve high Cr(VI) accessibility.

To further prove our presumption, the adsorption of anionic dyes from the mixed dyes was studied. From Figure S3, the absorption of MO (methyl orange) disappeared while the absorption of other cationic dyes was almost unchanged, which suggested that HPU-13@Fe3O4 could selectively adsorb anionic molecules MO. The selective adsorption of MO may result from the electrostatic interaction of the anionic MO molecules and cationic framework of HPU-13@Fe3O4. Besides, we also studied the separation of different anionic dyes (MO, NGB: Naphthol Green B or AYR: Alizarin yellow R) from AYR&MO, AYR&NGB, NGB&MO (Figure S4). The


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results indicated that the adsorption of AYR by HPU-13@Fe3O4 is better than other anionic dyes, which may be ascribed to the smaller size of AYR molecule. PXRD confirmed that the crystalline integrity of HPU-13@Fe3O4 retained during dye adsorption (Figure S5). The overall adsorption capacity of HPU-13@Fe3O4 for the MO and AYR reaches up to 76.336 mg g-1 and 163.399 mg g-1, respectively (Figures 10, S6 and S7, Tables S5, 6 and 7). Besides, the adsorption-release cycle experiments showed that this material still have the same high removal efficiency toward AYR (Figure S8). Furthermore, PXRD patterns of HPU-13@Fe3O4 after 4 cycles are consistent with those of HPU-13@Fe3O4, which indicate the framework integrity of HPU-13@Fe3O4 retains well (Figure S6). This demonstrates that the hierarchical pore structure is also a key to achieving Cr(VI) or dye removal.

CONCLUSION

In summary, an original magnetic HPU-13@Fe3O4 hybrid with excellent reusable Cr(VI) adsorption was introduced in our work. The maximum capacity of Cr2O72- and CrO42- adsorbed on HPU-13@Fe3O4 was 398.41 and 471.69 mg/g, respectively, which are larger than the reported cationic MOFs used in Cr2O72- and CrO4adsorption. Adsorption mechanism studies confirmed that the synergistic reaction of Cr(VI) reduction and adsorption gave rise to the high Cr(VI) removal efficiency. Our data presented a magnetic hybrid HPU-13@Fe3O4 which could be used as functional, practical and effective adsorbents for treating wastewater including Cr(VI).

ASSOCIATED CONTENT



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Supporting information The Supporting Information is available free of charge on the ACS Publications website. Accession Codes CCDC 1848579 includes the supplementary crystallographic data in this paper and can be acquired free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing

[email protected],

or

by

contacting

The

Cambridge

Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223336033.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Zhouqing Xu: 0000-0003-2600-0992 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21601050), NSFC - Henan region mutual funds (U1604124), the key scientific


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research project of Henan higher education (16A150010), Science and technology research project of Henan province (152102210314).

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Scheme 1 The schematics of adsorption of Cr(VI) by HPU-13@Fe3O4.



Figure 1 (a) View of the coordination environments of Cu1 and Cu2 in HPU-13; (b) The closed triangular unit; (c) the closed cage; (d) the hexagonal mesopores and triangular micropores; (e) the hexagonal cage; (f) the three-dimensional porous framework.


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Figure 2 The SEM images of HPU-13@Fe3O4.



Figure 3 The elemental mapping of HPU-13@Fe3O4.


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Figure 4 (a) The PXRD patterns of synthesized materials; (b) the IR spectrum of synthesized materials.



Figure 5 The adsorption of Cr2O72- and CrO42- by Fe3O4 and HPU-13@Fe3O4 (20 mg of adsorbents, 100 mg/L, 10 mL of Cr(VI)).


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Figure 6 (a) Equilibrium K2Cr2O7 sorption data for HPU-13@Fe3O4 material; (b) Langmuir plots of K2Cr2O7 adsorption onto HPU-13@Fe3O4; (c) the adsorption isotherms for K2Cr2O7 adsorption over HPU-13@Fe3O4; (d) plots of pseudo-secondorder kinetics for the adsorption of K2Cr2O7 on HPU-13@Fe3O4.



Figure 7 (a) Equilibrium K2CrO4 sorption data for HPU-13@Fe3O4 material; (b) Langmuir plots of K2CrO4 adsorption onto HPU-13@Fe3O4; (c) The adsorption isotherms for K2CrO4 adsorption over HPU-13@Fe3O4; (d) Plots of pseudo-secondorder kinetics for the adsorption of K2CrO4 on HPU-13@Fe3O4.


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Figure 8 The cycle experiment of Cr(VI) adsorption.



Cu2p Scan 4.00E+04

a

Cu2p 3.50E+04 Counts / s

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

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3.00E+04

2.50E+04

2.00E+04

1.50E+04 960

950

940

930

Binding Energy (eV)

Figure 9 XPS spectra of HPU-13@Fe3O4 before and after adsorption (a: Cu 2p before adsorption; b: Cu 2p and Cr 2p after adsorption for one cycle; (c): Cu 2p and Cr 2p after adsorption for 6 cycle).


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Figure 10 The adsorption isotherms for MO and AYR adsorption over HPU13@Fe3O4.



For Table of Contents Use Only：：

A magnetic hybrid Cu(I)-MOF@Fe3O4 with hierarchically engineered micropores for highly efficient removal of Cr(VI) from aqueous solution Huijun Li,a Qingqing Li,a Xinglei He,a Ning Zhang,a Zhouqing Xu,a* Yan Wang,b* Yuan Wanga*

A stable and recoverable hybrid magnetic material with hierarchically engineered micropores shows remarkable adsorption capability towards Cr(VI) removal through the synergistic reaction of Cr(VI) reduction and adsorption.


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A magnetic hybrid Cu(I)-MOF@Fe3O4 with hierarchically engineered

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