CTAB-surface-functionalized magnetic [email protected] composite adsorbent for Cr(VI) efficient removal from aqueous solution

Journal Pre-proof CTAB-surface-functionalized magnetic MOF@MOF composite adsorbent for Cr(VI) efficient removal from aqueous solution Lincheng Li, Yunlan Xu, Dengjie Zhong, Nianbing Zhong

PII:

S0927-7757(19)31250-6

DOI:

https://doi.org/10.1016/j.colsurfa.2019.124255

Reference:

COLSUA 124255

To appear in:

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Received Date:

7 October 2019

Revised Date:

19 November 2019

Accepted Date:

19 November 2019

Please cite this article as: Li L, Xu Y, Zhong D, Zhong N, CTAB-surface-functionalized magnetic MOF@MOF composite adsorbent for Cr(VI) efficient removal from aqueous solution, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2019), doi: https://doi.org/10.1016/j.colsurfa.2019.124255

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CTAB-surface-functionalized magnetic MOF@MOF composite adsorbent for Cr(VI) efficient removal from aqueous solution

Lincheng Li a, Yunlan Xu a,*, Dengjie Zhong a, Nianbing Zhong b

a

School of Chemical Engineering, Chongqing University of Technology, Chongqing 400054,

China b

School of Electrical and Electronic Engineering, Chongqing University of Technology,

*

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Chongqing 400054, China

Corresponding authors.

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E-mail address: [email protected] (Yunlan Xu)

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Graphical Abstract:

Abstract In

this

work,

a

surfactant-functionalized

magnetic

MOF@MOF

adsorbent

(Fe3O4@UiO-66@UiO-67/CTAB) was prepared by simple solvothermal method and applied to adsorb

Cr(VI) from aqueous solution. It exhibited excellent adsorption performance and recyclability mainly due to the tunable pores and plenty surface active sites of the MOF@MOF structure, and electrostatic attraction and reduction of CTAB modifier. It had excellent adsorption capacity in a wide pH range of 1.0-11.0, and the maximum adsorption capacity was 932.1 mg·g-1 at pH 2.0. The adsorption reached equilibrium at contact time of 240 min. Adsorption capacity increased with the increase of initial Cr(VI) concentration. High valence coexisting ions had negative effects on adsorption for both anions and cations. The adsorption of Cr(VI) by Fe3O4@UiO-66@UiO-67/CTAB is mainly chemisorption and monolayer adsorption, and the adsorption process is a spontaneous endothermic process. This study

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provided a valuable strategy and path for designing and developing effective MOF-based multifunctional adsorbents.

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Key words: Fe3O4@UiO-66@UiO-67/CTAB; MOF@MOF structure; Adsorption; Cr(VI)

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1. Introduction

Cr(VI) is commonly used in electroplating, printing, mining and metallurgy industries, usually

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presents in the aqueous environment as oxyanions (Cr2O72-, HCrO4- or CrO42-) [1-3]. Compared with Cr(VI), Cr(III) has lower mobility, solubility, non-biodegradability and toxicity in an acidic environment, and a stable hydroxide complex can be formed well in a wide pH range [4, 5].

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Furthermore, the World Health Organization (WHO) stipulates that the emission concentration of Cr(VI)-containing wastewater should not exceed 0.05 ppm. At present, the adsorption method has

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made good progress in wastewater treatment, which possesses the advantages of simple operation, high adsorption capacity and recyclability of the adsorbent [6].

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Metal-organic framework (MOF), also known as metal-organic coordination polymer, is a crystalline material in which an organic ligand is linked to an inorganic metal center by a coordination bond to form an infinitely extended network-like structure [7]. Because of its tunable pores, large surface area and good chemical stability, MOF shows great potential for metal ion adsorption [8-10]. Among the many MOF adsorbent materials, UiO-66 is one of the most attractive adsorbents with high stability and versatile modifiability [11]. Moreover, magnetic MOF composites inherit the excellent performance of MOF and the rapid separation and recovery of magnetic material. Thus, problems such as the

generation of secondary pollution or difficulty in separation can be avoided, and the recyclability of the adsorbent can be improved [12]. However, researches on new, purpose-designed MOF composites with versatile performance are still in progress. Inspired by the core-shell structured PBA@PBA crystals for the design of viable nanostructures with versatility and efficient phenol scavenging [13], we proposed to prepare core-shell MOF@MOF composite using [Zr6O4(OH)4(BDC)6] (UiO-66) MOF as core and [Zr6O4(OH)4(BPDC)6] (UiO-67) MOF as shell, aiming at obtaining excellent adsorption properties that a single MOF precursor could not obtain. Moreover, the core-shell structure of MOF@MOF could introduce new functions while

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maintaining the original properties of a single MOF crystal, since the strain field could be formed at the two-phase interface, which could affect the porosity of the core-shell material [14]. In addition, in order to reduce the separation difficulty of the core-shell material after adsorption, the core MOF was modified with Fe3O4 to obtain magnetic property. Furthermore, according to the literature, the

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adsorption performance of the adsorbent can be improved by surface functionalization. Therefore, in

order to enhance the ability of the core-shell material to remove heavy metal ions, the shell MOF was

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surface-functionalized with surfactant [15]. Thus, the cationic surfactant hexadecyltrimethylammonium

the MOF@MOF composite.

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bromide (CTAB) was selected to surface-modify the shell MOF to enhance the adsorption property of

In summary, in this work, for the first time, Fe3O4@UiO-66@UiO-67/CTAB nanocomposite was

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prepared through solvothermal reaction (Scheme 1) and applied to absorb Cr(VI) from water environment to evaluate its adsorption capability. The effects of adsorption parameters such as pH values, initial Cr(VI) concentration, coexisting ions and contact time on Cr(VI) adsorption were studied.

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Adsorption kinetics and thermodynamics were also investigated.

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2. Materials and methods

2.1 Materials

1,4-benzene dicarboxylic acid (H2BDC, purity ≥ 98%) , blphenyl-4-4'-dlcarboxylic acid (BPDC, purity ≥ 98%), zirconium chloride (ZrCl4, purity ≥ 98%), N,N-Dimethylformamide (DMF, purity ≥ 98%) and

hexadecyltrimethylammonium bromide (CTAB, purity ≥ 98%) were purchased from

Adamas Reagent Co., Ltd (Shanghai, China). All other reagents and materials met analytical grade

standards.

2.2 Preparation of Fe3O4@UiO-66@UiO-67/CTAB composite

Preparation of UiO-66：69.9 mg ZrCl4, 49.8 mg H2BDC and 20 mL DMF were mixed together and the solution was sonicated for 10 min. Later, 0.1 mL HCl and 0.5 mL HAc were sequentially added in the mixture solution, then, stirred and placed in a 100 mL Teflon-lined hydrothermal reaction kettle, and the hydrothermal reaction kettle was placed in an oven at 120 °C for 24 h, and then cooled down. After that, the resultant solid (UiO-66) was separated by centrifugation, washed alternately with

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methanol and DMF several times. Finally, it was dried overnight at 80 °C.

Preparation of UiO-67：The preparation procedure of UiO-67 was similar to that of UiO-66 described above. The difference was that 49.8 mg H2BDC was replaced by 72.7 mg BPDC. All the experimental steps were the same as those of UiO-66.

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Preparation of Fe3O4@UiO-66：The Fe3O4 nanoparticle was synthesized via a solvothermal

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reaction [16]. Then, the preparation procedure of Fe3O4@UiO-66 was the same as that of UiO-66 described above. The difference was that 100 mg Fe3O4 was added to the mixture of 69.9 mg ZrCl4,

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49.8 mg H2BDC and 20 mL DMF. All the experimental steps were the same as those of UiO-66. Preparation of Fe3O4@UiO-66@UiO-67：The preparation procedure of Fe3O4@UiO-66@UiO-67 was similar to that of UiO-66 described above. The difference was that 100 mg Fe3O4@UiO-66, 72.7

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mg BPDC and 20 mL DMF were mixed. All the experimental steps were the same as those of UiO-66. Preparation of Fe3O4@UiO-66@UiO-67/CTAB：100 mg Fe3O4@UiO-66@UiO-67 was dispersed

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in 20 mL methanol and the mixture was sonicated for 20 min, then, 1.5 mL ammonia solution and 60 mL CTAB solution were added in the mixture and stirred for 5 h to obtain a homogenous solution of

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Fe3O4@UiO-66@UiO-67/CTAB. Finally, the precipitate (Fe3O4@UiO-66@UiO-67/CTAB) was separated by centrifugation, washed with methanol several times and dried overnight at 80 °C.

2.3 Adsorption experiment

Different concentrations of Cr(VI) solution were obtained by diluting a 1000 ppm stock solution prepared with K2Cr2O7. The adsorbent (20 mg) was added in Cr(VI) solution of a designed concentration (5-500 ppm), then, the mixed solution was placed in a shaker and it was shaken at 200

rpm for 24 h at a constant temperature under various conditions of pH (1.0-11.0) (adjusted with 0.1 mol·L-1 HCl and NaOH), contact time (5-1440 min), temperature (298-318 K) and coexisting ions (without or with 10 mM of Cl-, NO3-, SO42-, PO43- , Na+, Ca2+ or Mg2+). After adsorption, the suspension was separated by a magnet and filtered. The concentration of filtrate was tested at a wavelength of 540 nm using a UV-Vis (UV-6100S, MRTASH, Shanghai, China) by the diphenylcarbazide method.

2.4 Characterization

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The morphology of the synthesized MOFs was observed with scanning electron microscope (SEM, ZEISS sigma500, Germany) and high-resolution transmission electron microscope (HRTEM, FEI F30, Germany). The X-ray diffraction patterns of the synthesized MOFs was measured by X-ray diffraction

(XRD, Shimadzu XRD-6100, Japan). Fourier transform infrared spectroscopy (FT-IR, Nicholet 6700,

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USA) of the adsorbents was recorded at 400-4000 cm-1. The Brunauer-Emmett-Teller (BET) results

were measured by a surface area and pore size analyzer (BET, JW-BK100C, Beijing, China). The

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thermal stability of samples was conducted on a thermogravimetric analyzer (TGA, NETZSCHSTA 2500, Germany) from 30 to 600 °C at a heating rate of 10 °C·min-1. The magnetic measurements of the

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loaded and unloaded Cr(VI) adsorbents were carried out with a vibrating sample magnetometer (VSM, Lake Shore 7307, USA). The oxidation state of zirconium and chromium was ascertained using X-ray

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photoelectron spectroscopy (XPS, FEI 250-xi, Germany).

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3. Results and discussion

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3.1 Characterization of adsorbent

Fig. 1 illustrated the XRD patterns of all samples. The spectrum of UiO-66 showed diffraction peaks

located at 7.32°, 8.57°, 12.03°, 14.27°, 17.11°, 22.32°, 25.78° and 33.21°, corresponding to (111), (002), (022), (113), (004), (115), (224) and (137) crystal faces, respectively. This result was consistent with the single crystal data of UiO-66 as reported [17]. Three sharp diffraction peaks of UiO-67 were observed around 5.72° (111), 6.61° (200) and 9.32° (220), indicating that its nanoparticles had high crystallinity [18]. Compared with the XRD patterns of UiO-66 and UiO-67, the intensity of the

characteristic diffraction peaks of Fe3O4@UiO-66@UiO-67 and Fe3O4@UiO-66@UiO-67/CTAB was significantly weakened, probably due to that the layer of UiO-66 and UiO-67 was not thick enough. Due to the limited content of Fe3O4 in Fe3O4@UiO-66@UiO-67 and Fe3O4@UiO-66@UiO-67/CTAB crystal powder, the diffraction peaks of Fe3O4 were only observed at 30.35° (220), 35.68° (311), 53.87° (422) and 62.91° (440) (JCPDS no. 74-0748). As the CTAB was coated on Fe3O4@UiO-66@UiO-67, the carbon chain enriched in CTAB increased the carbon content of the material, resulting in the peak intensity

of

Fe3O4@UiO-66@UiO-67/CTAB

was

slightly

different

from

that

of

Fe3O4@UiO-66@UiO-67.

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Figs. 2a and 2b depicted the morphology of UiO-66 and UiO-67 obtained by SEM. At low magnification, UiO-66 and UiO-67 samples presented agglomerated cubic morphology with a quite uniform size distribution. The diameters of UiO-66 and UiO-67 particles were approximately 60 ± 10

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nm and 130 ± 10 nm, respectively, which were smaller than those reported in the literatures [19, 20]. Obviously, Fe3O4@UiO-66@UiO-67 sample exhibited the different morphology compared to UiO-66

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and UiO-67, which presented as irregular weakly crystalline nanoparticles (Fig. 2c). In Fe3O4@UiO-66@UiO-67/CTAB sample, some of the crystallites were destroyed or changed to

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irregular due to the collapse of the UiO-66 and UiO-67 frameworks. It could be seen more clearly from the TEM image (Fig. 2g). With Fe3O4@UiO-66 as the core and UiO-67 as the shell, no obvious particles were found on the surface of Fe3O4@UiO-66@UiO-67/CTAB. The lattice fringe spacing of

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0.207 nm and 0.186 nm (Figs. 2e and 2f) marked in the HRTEM lattice image of a single Fe3O4@UiO-66@UiO-67/CTAB particle corresponding to the (137) plane of UiO-66 and (044) plane

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of UiO-67, respectively. The result indicated that the apparent interface was formed between UiO-66 and UiO-67 during the UiO-67 loading process. The diffraction ring in the selected area electron

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diffraction (SAED) pattern (Fig. 2h) was perfectly matched to (400), (440) planes of Fe3O4, (044) plane of UiO-66 and (244) plane of UiO-67. The results were consistent with these of XRD characterization. Fig. 3 exhibited the FT-IR spectra of UiO-66, UiO-67, Fe3O4@UiO-66@UiO-67 and

Fe3O4@UiO-66@UiO-67/CTAB samples. For all samples, the peaks at 1401 and 3442 cm-1 were corresponding to the Zr-OH groups and dicarboxylate linkers, respectively [36]. The peaks at around 1580 cm-1 were attributed to the vibration of O=C-O-, and the peaks at around 655 and 750 cm-1 were attributed to the vibration of Zr-O occurred, which demonstrated the existence of carboxylate in the

framework. The peak at 1159 cm-1 was the C-O stretching vibration peak of ester, ether or carboxyl groups, indicating that all samples were rich in oxygen-containing functional groups, which would play an important role during the adsorption and reduction process of Cr(VI). Moreover, the Fe-O characteristic stretching vibration peak was observed at 572 cm-1 for Fe3O4@UiO-66@UiO-67 and Fe3O4@UiO-66@UiO-67/CTAB, demonstrating that Fe3O4 nanoparticles were successfully embedded in the framework [24]. Furthermore, the characteristic peaks at 2857 and 2925 cm-1 were attributed to asymmetric and symmetric –CH2 stretching vibration of amine, indicating that CTAB was present in Fe3O4@UiO-66@UiO-67/CTAB [21]. All these results indicated that Fe3O4@UiO-66@UiO-67/CTAB

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composite was successfully synthesized. In order to investigate the change of the size and pore size of the adsorbent during the modification,

BET method was used to determine the specific surface area and pore volume of the prepared samples.

According to the IUPAC classification, Fig. 4a showed that the N2 adsorption-desorption curves of the

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prepared samples conform to the typical type I isotherms [22]. Furthermore, the pore size distribution

measured from BET illustrated in Fig. 4b showed that Fe3O4@UiO-66@UiO-67/CTAB possessed only

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one peak in the microporous region of approximately 1.76 nm [22]. As can be seen from Table 1, the CTAB modification and the incorporation of Fe3O4 had an effect on the pore structure and surface area

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of UiO-67 and UiO-66. The pore volume and the surface area of Fe3O4@UiO-66@UiO-67/CTAB calculated by isotherms were 0.02 cm3·g-1 and 115.94 m2·g-1, respectively, which were the minimum

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and could have a certain negative impact on adsorption performance. The result might be due to the incorporation of Fe3O4 NPs, the coating of UiO-67 and the surface deposition of CTAB [23]. However, the measured pore size of Fe3O4@UiO-66@UiO-67/CTAB, Fe3O4@UiO-66@UiO-67, UiO-66 and

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UiO-67 were 1.57, 1.46, 1.39 and 1.54 nm, respectively. The largest pore size of

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Fe3O4@UiO-66@UiO-67/CTAB might be more favorable for Cr(VI) adsorption. Table. 1. NEAR HERE

The thermal stability of samples was measured by thermogravimetric analysis (TGA) (Fig. 5). TGA

curve of Fe3O4 displayed a weight loss between 250 °C and 600 °C, which might be due to evaporation of unbound water resulting in the conversion of Fe3O4 to Fe2O3 [22, 24]. For UiO-66, a mass loss was gradually observed at around 200 °C, which may be due to the release of adsorbed water from the porous surface of the adsorbent. A gentle plateau appeared in the temperature range of 260-350 °C, indicating that the adsorbent possessed high thermal stability. However, the maximum weight loss

occurred at about 500 °C, and was finally completely decomposed at about 600 °C. The TGA curve of UiO-67 has observed two weight loss steps in the temperature range of 30-550 °C, which was consistent with the reported results [25]. The first weight loss occurred in the range of 30-350 °C, which was attributed to the evaporation of the absorbed water and the bound water of the framework surface. The second weight loss occurred at around 420 °C because of the decomposition of the skeleton. Compared to UiO-66 and UiO-67, Fe3O4@UiO-66@UiO-67/CTAB showed higher thermal stability because of the combination of Fe3O4. In the temperature range of 30-400 °C, a part of weight loss could be seen because of the direct decomposition of the CTAB moiety and the release of unbound

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water [26]. The results indicated that Fe3O4@UiO-66@UiO-67/CTAB exhibited exceptional high thermal stability.

The magnetic performance of adsorbent before (Fe3O4@UiO-66@UiO-67/CTAB) and after

(Fe3O4@UiO-66@UiO-67/CTAB-Cr) Cr(VI) adsorption was examined by vibrating sample

Fig.

6.

The

saturated

magnetization

values

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magnetometer (VSM) at room temperature, and the corresponding hysteresis loops were illustrated in of

Fe3O4@UiO-66@UiO-67/CTAB

and

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Fe3O4@UiO-66@UiO-67/CTAB-Cr were 36.05 and 22.87 emu·g-1, respectively. This indicated that the saturated magnetization of the adsorbent decreased obviously after adsorption due to the loss of Fe3O4.

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Moreover, the observed zero coercivity and reversible hysteresis behavior indicated that Fe3O4@UiO-66@UiO-67/CTAB was superparamagnetic [27]. When the external magnetic field was

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applied, Fe3O4@UiO-66@UiO-67/CTAB particles could be separated from the suspension system (Fig. 6 inset), which provided a simple and efficient separation route in adsorption applications. The adsorption behavior of Cr(VI) by Fe3O4@UiO-66@UiO-67/CTAB was further explored, the

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adsorbent before and after adsorption was analyzed by XPS. As seen from the XPS wide scan spectrum (Fig. 7a), a new characteristic peak of Cr was found after adsorption, proving that Cr was successfully

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absorbed by Fe3O4@UiO-66@UiO-67/CTAB. The spectrum of Cr 2p was illustrated in Fig. 7b. The Cr 2p peak was fitted to two components and showed two main peaks at 577.2 eV and 586.8 eV, corresponding to the binding energy of Cr(III) 2p3/2 and Cr(III) 2p1/2, respectively. For Cr(VI), the Cr 2p peak was deconvoluted into two main peaks at 579.3 eV and 588.9 eV, corresponding to the binding energy of Cr(VI) 2p3/2 and Cr(VI) 2p1/2, respectively [28]. This result demonstrated that Cr(VI) was absorbed by Fe3O4@UiO-66@UiO-67/CTAB, and partially reduced to Cr(III) [22]. Moreover, in the spectrum of Fe 2p (Fig. 7c), two characteristic peaks at 711.5 eV and 725.2 eV were observed in the

spectrum of Fe3O4@UiO-66@UiO-67/CTAB, corresponding to Fe 2p1/2 and Fe 2p3/2, respectively, which clearly consistent with the typical characteristics of Fe3O4 [29]. However, an obvious change in the

position

of

the

characteristic

peak

was

observed

in

the

spectrum

of

Fe3O4@UiO-66@UiO-67/CTAB-Cr, and the Fe 2p binding energy decreased by 0.7 eV (Fig. 7d), demonstrating that the Fe3O4 nanoparticles partially participated in the reduction reaction. The C 1s peak of absorbent before adsorption (Fig. 7e) was fitted to four major components corresponding to carbon in different groups: C-C (284.3 eV), C-OH (290.2 eV), C=C (286.3 eV) and C=O (287.7 eV) [30, 31]. Meanwhile, the C 1s binding energy of absorbent after adsorption corresponding to C-C,

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C-OH, C=C and C=O increased by 1.7 eV (Fig. 7f). The increase of carbon bond energy indicated that the density of C-electron clouds decreased, which indicated that CTAB could reduce Cr (VI) to Cr (III) as an electron donor [32].

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3.2 Feasibility analysis of Fe3O4@UiO-66@UiO-67/CTAB composite

Fig. 8 showed the adsorption performance tests on six adsorbents under the same conditions of initial

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Cr(VI) concentration 50 ppm (50 mL), adsorbent 20 mg, pH 2.0, contact time 24 h, and temperature 298 K. The results indicated that Fe3O4@UiO-66@UiO-67/CTAB had the best adsorption performance

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with adsorption capacity of 124.04 mg·g-1, indicating that it was feasible for the design of MOF@MOF structure with Fe3O4 magnetic core to simplify separation and CTAB surfactant shell to provide many

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positively charged groups, which may produce more positively charged binding sites and enhance its adsorption performance.

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3.3 Optimization of the adsorption influence factors of Fe3O4@UiO-66@UiO-67/CTAB

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3.3.1 Effect of initial pH values Fig.

9a

presented

the

effect

of

initial

pH

values

on

Cr(VI)

adsorption

with

Fe3O4@UiO-66@UiO-67/CTAB in the pH range of 1.0-11.0. Generally, the adsorption capacity exhibited a downward trend with the increase of pH. When the pH was lower than or equal to 5.0, the maximum adsorption capacity was almost the same. However, it was slightly higher at pH 2.0. Therefore, pH 2.0 was selected as the optimum pH value in this study. When the pH was higher than 5.0, the maximum adsorption capacity decreased slightly, which was consistent with the previous

report [33]. This result demonstrates that as the pH increases, the electrostatic repulsion between the deprotonated functional groups and the Cr-containing anion groups is enhanced. Generally, the form of Cr(VI) is depended on the pH of solution, and are commonly found in the form of Cr2O72-, HCrO4- and CrO42- ions (see Eqs. 3 and 4) [34, 35]. At strong acid condition, HCrO4- and Cr2O72- ions are the main forms. At this time, the electrostatic attraction of anisotropic charges in favor of adsorption is stronger since the surface of Fe3O4@UiO-66@UiO-67/CTAB is positively charged, which results in higher adsorption capacity. At weak acid or alkaline condition, CrO42- ion is the predominant form. At this time, the electrostatic interaction is weaker since the surface of Fe3O4@UiO-66@UiO-67/CTAB is

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weakly positively even negatively charged, which results in lower adsorption capacity. − Cr2 O2− 7 + H2 O → 2HCrO4

(1)

2− + HCrO− 4 → CrO4 + H

(2)

3.3.2 Effect of contact time

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As illustrated in Fig. 9b, at optimum pH 2.0, Fe3O4@UiO-66@UiO-67/CTAB was conducted to

Cr(VI) adsorption experiments at different contact times. Obviously, rapid adsorption was observed

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during the first 120 min and then the rate of adsorption was gradually slowed down. The initial rapid adsorption was related to a higher initial concentration of Cr(VI) and a large number of vacant

occupy

the

active

sites

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adsorption active sites on the surface of adsorbents, which made at the initial stage it easy for Cr(VI) to on

the

surface

of

the

adsorbent.

The

adsorption

of

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Fe3O4@UiO-66@UiO-67/CTAB reached adsorption equilibrium at 240 min, and the maximum adsorption capacity was 124.04 mg·g-1. For Fe3O4@UiO-66@UiO-67/CTAB, CTAB modification can provide many positively charged groups, which may produce more positively charged binding sites and

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enhance its adsorption performance. With the increase of time, the adsorption rate decreases significantly, and the adsorption eventually reaches equilibrium. This is due to that the positively

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charged groups and active sites on the surface of adsorbent reduces and a decrease in Cr(VI) concentration, which causes it is hard for Cr(VI) to be captured [36]. 3.3.3 Effect of initial Cr(VI) concentration Fig.

9c

showed

the

Cr(VI)

adsorption

at

different

initial

concentrations

by

Fe3O4@UiO-66@UiO-67/CTAB for 24 h. It was obvious that when Cr(VI) concentration increased, the adsorption capacity of Cr(VI) by Fe3O4@UiO-66@UiO-67/CTAB also increased. For a certain amount of Fe3O4@UiO-66@UiO-67/CTAB, several active sites are certain. As the initial Cr(VI) concentration

increased, the chance of contact between Cr(VI) and active sites increased, so the adsorption capacity increases. When a certain concentration was reached, the active sites will be fully occupied by Cr(VI), adsorption will reach saturation, and the adsorption capacity will remain stable. Within the concentration range (5-500 ppm) investigated in this experiment, the increase in Cr(VI) concentration causes an increase in the adsorption capacity, indicating the adsorption has not yet reached saturation and the adsorbent has a high adsorption capacity. 3.3.4 Effect of coexisting ions Generally, various soluble ions (e.g., Cl-, NO3-, SO42-, PO43-, Na+, Ca2+ and Mg2+) exist in actual

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industrial wastewaters and natural water. These coexisting ions can also occupy the active adsorption sites and ultimately affect the Cr(VI) adsorption. Therefore, it is necessary to carry out competitive

adsorption experiments to evaluate their effects on Cr(VI) removal by Fe3O4@UiO-66@UiO-67/CTAB. In this study, equal concentrations (10 mM, which was much higher than Cr(VI) concentration) of Cl-,

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NO3-, SO42-, PO43- , Na+, Ca2+ or Mg2+ were added to Cr(VI) solution, respectively, to investigate their effects on absorption. The result was displayed in Fig. 9d. The results of the influence of coexisting

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cations showed that Na+ has no obvious effect on the Cr(VI) adsorption, but Ca2+ or Mg2+ had negative effect. This is because the more the charge number of cation is, the more it can compete with

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Fe3O4@UiO-66@UiO-67/CTAB and adsorb Cr(VI), which makes Cr(VI) lose the chance of adsorption on Fe3O4@UiO-66@UiO-67/CTAB and be removed from the solution.The results of the influence of

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coexisting cations showed that Cl- and NO3- had almost no effect on Cr(VI) adsorption, but SO42- and PO43- had an obvious negative influence. In general, the anions with more negative charges (SO42- and PO43-) had higher affinity for adsorbents than monovalent anions (Cl- and NO3-) [37]. However, in the

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presence of coexisting ions, the adsorption capacity still retained more than 90% of that in the absence of coexisting ions. Fe3O4@UiO-66@UiO-67/CTAB possessed not only multiple active sites but also

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many positively charged groups on its surface, which made it more resistant to the interference of other coexisting ions [38, 39].

3.4 Adsorption kinetics

The pseudo-first-order model (Eq. 5), pseudo-second-order model (Eq. 6) and Elovich model (Eq. 7) [40] were applied to simulate the experimental kinetic data to investigate the adsorption behavior of

Cr(VI) on Fe3O4@UiO-66@UiO-67/CTAB. dqt dt dqt dt dqt dt

= k1 (q e − q t )

(3)

= k 2 (q e − q t )2

(4)

= α exp(−βq t )

(5)

Where qe and qt are the adsorption capacity at equilibrium and time t (mg·g-1); k1 (min-1) and k2 (g·mg-1·min-1) are the adsorption rate constants, respectively; α (mg·g-1) and β (g·mg-1) are the rate constant and desorption constant, respectively.

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The estimated values of the fitted curves and the three kinetic models were shown in Fig. 10 and Table S1. From the evaluation of the correlation coefficient of Fe3O4@UiO-66@UiO-67/CTAB, the

pseudo-second-order model showed the highest correlation coefficient (R2=0.9899) compared to pseudo-first-order model (R2=0.9789) and Elovich model (R2=0.9421), indicating the adsorption of

Cr(VI)

by

Fe3O4@UiO-66@UiO-67/CTAB

was

most

-p

behavior

consistent

with

the

pseudo-second-order model. Moreover, the experimental qt value (124.04 mg·g-1) was closest to the

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theoretical qe value (126.71 mg·g-1) calculated using the pseudo-second-order model. This could be further explained that chemisorption dominated the adsorption process of Cr(VI) with

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Fe3O4@UiO-66@UiO-67/CTAB and it was the rate-controlling step.

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Table. S1. NEAR HERE

3.5 Adsorption isotherms

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The behavior of Fe3O4@UiO-66@UiO-67/CTAB to remove Cr(VI) was investigated by four

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isothermal models: Langmuir, Freundlich, Redlich-Peterson and Temkin [41], which can be calculated by Eqs. (8), (9), (10) and (11), respectively: qe =

qm KL Ce 1+KL Ce

qe = K F Ce1/n qe = qe =

KP Ce

1+αCe β RT b

(ln K T Ce )

(6) (7) (8) (9)

Where qe and Ce are the equilibrium adsorption capacity (mg·g-1) and solution concentration (mg·L-1), respectively. KL (L·mg-1), KF (mg·g-1), KP (L·mg-1) and KT (L·mg-1) represent the correlation constants of four isotherm models. α and (RT/b) are the Redlich-Peterson and Tempkin constant, respectively. n is the heterogeneity factor. Fig. 11 and Table S2 displayed the fitting plots of different isotherm models and the values of isotherm correlation coefficients. Obviously, the data obtained by Fe3O4@UiO-66@UiO-67/CTAB at 298 K were fitted by four isotherm models, the results showed all the four isotherm models have a good fit. Compared with Freundlich, Redlich-Peterson and Tempkin isotherm models, Langmuir

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isotherm model had the highest correlation coefficient. In addition, the higher the temperature, the higher the R2 value, i.e., 298K (R2=0.9985), 308 K (R2=0.9987) and 318 K (R2=0.9989). This illustrated the adsorption process may be uniform monolayer adsorption. The maximum adsorption capacity (qm) of Fe3O4@UiO-66@UiO-67/CTAB fitted with Langmuir model at 298 K, 308 K and 318

MOF-based

adsorbents

(as

shown

in

Table

-p

K was 932.1, 1000.4 and 1089.0 mg·g-1, respectively. It was far superior to other reported magnetic or 2).

The

result

demonstrated

that

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Fe3O4@UiO-66@UiO-67/CTAB exhibited excellent adsorption performance. Table. 2. NEAR HERE

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Table. 3. NEAR HERE

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3.6 Adsorption thermodynamics

The thermodynamic of Cr(VI) adsorption on Fe3O4@UiO-66@UiO-67/CTAB was investigated at 298 K, 308 K and 318 K. As shown in Fig. 11, as the temperature increased, the adsorption capacity of

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Fe3O4@UiO-66@UiO-67/CTAB increased gradually, indicating that the adsorption process was

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spontaneously endothermic. To further study the adsorption behavior, thermodynamic studies were carried out [51, 52]. The thermodynamic parameters and Gibbs free energy can be calculated by Eqs. (12) and (13): ln

qe

Ce

=−

∆H RT

+

∆S R

∆G = ∆H − T∆S

(10) (11)

Where meanings qe and Ce are the same as Eqs. 8-11; ΔG is the Gibbs free energy change (kJ·mol−1), which can be calculated through Eq. 13; ΔH is the enthalpy change (kJ·mol−1); ΔS is the entropy

change (kJ·mol−1·K−1). The values of ΔH and ΔS were calculated from the slope and intercept of the linear fitting plot of ln (qe/Ce) against 1/T (Eq. 12, and result illustrated in Fig. 11d). The calculated thermodynamic parameters were listed in Table 3, the positive ΔH value indicated that Cr(VI) adsorption on Fe3O4@UiO-66@UiO-67/CTAB was an endothermic process, which could be calculated and used to explain the increase in temperature favoring the adsorption of Cr(VI). The positive ΔS value and the negative ΔG value illustrated the Cr(VI) removal process on magnetic Fe3O4@UiO-66@UiO-67/CTAB was spontaneous with high affinity.

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3.7 Regeneration studies

The reusability of Cr(VI) adsorption was investigated by adsorption-desorption cycle experiments, which

is

an

important

criterion

for

evaluating

the

commercial

application

value

of

Fe3O4@UiO-66@UiO-67/CTAB. Fe3O4@UiO-66@UiO-67/CTAB after absorption of Cr(VI) was

-p

collected and immersed in a NaOH solution, then magnetically separated, washed with DMF solution and sonicated. The regenerated Fe3O4@UiO-66@UiO-67/CTAB was washed and placed in an oven at

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80 °C to dry. Then, it was used to operate the next absorption. From Fig. 12, it still maintained the Cr(VI) removal efficiency of 90.14% with an adsorption capacity of 112.68 mg·g-1 even after five

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adsorption-desorption cycles, indicating Fe3O4@UiO-66@UiO-67/CTAB had excellent reusability in potential practical applications. Due to its broad pH applicable range, excellent adsorption properties

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and reusability, Fe3O4@UiO-66@UiO-67/CTAB will possess great potential for sustainable Cr(VI) removal.

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3.8 Adsorption mechanism

analysis

(Fig.

3)

demonstrates

that Fe3O4@UiO-66@UiO-67/CTAB

is

rich

in

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FT-IR

oxygen-containing functional groups. The results of pH effect (Fig. 9a) and coexisting ions (Fig. 9d) demonstrate that there exists electrostatic forces between Cr(VI) ions and positive group (CTA+) of the surface modified CTAB. Feasibility analysis (Fig. 8) demonstrates that there exists some forces such as van der Waals or hydrogen bond forces between Cr(VI) ions and active sites of UiO-66, UiO-67 and CTAB. XPS analysis (Fig. 7) proves that the adsorbed Cr(VI) ions is partially reduced to Cr(III). In summary, the adsorption mechanism of Fe3O4@UiO-66@UiO-67/CTAB can be summarized as an

adsorption coupled reduction mechanism. The adsorption of Cr(VI) ions on the adsorbent is carried out by electrostatic forces between Cr(VI) ions and positive group (CTA+) of the surface modified CTAB, and other forces (e.g. van der Waals forces and hydrogen bonding force) between Cr(VI) ions and the oxygen rich group of the shell-core UiO-66@UiO-67 structure. At the same time, the surface modified CTAB is used as an electron donor to provide electrons for Cr(VI) to be reduced to Cr(III) [32], while it itself becomes CTA+, further strengthening its electrostatic adsorption of chromium ions.

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4. Conclusions

A core-shell MOF@MOF (UiO-66@UiO-67) structure was proposed to prepare a novel adsorbent (Fe3O4@UiO-66@UiO-67/CTAB) by simple solvothermal method. Due to tunable pores, high surface

-p

area, high porosity and good chemical stability of UiO-66 and UiO-67, with the magnetic modified

Fe3O4 core and surfactant CTAB functionalized shell, Fe3O4@UiO-66@UiO-67/CTAB exhibited high

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efficiency, stability, reusability and easy separation properties for Cr(VI) adsorption from aqueous

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solutions. The adsorption capacity was the highest (qm：932.1 mg·g-1) at pH 2.0 but almost the same in a wide pH range of 1.0-5.0, and it increased with the increase of initial Cr(VI) concentration. High

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valence coexisting ions had negative effects on adsorption. Adsorption kinetics, thermodynamics and isotherms studies showed that the adsorption of Cr(VI) on Fe3O4@UiO-66@UiO-67/CTAB was an

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endothermic spontaneous process, and chemisorption and monolayer adsorption was dominant. The

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results of this work demonstrate Fe3O4@UiO-66@UiO-67/CTAB has high potential application prospect for Cr(VI) removal, and the MOF@MOF structure is a valuable strategy for developing effective MOF-based multifunctional adsorbents. .

Author Contributions Section Lincheng Li

The author's contribution is the provision and integration of the article data. Yunlan Xu The author's contribution is the research concept and designer of the article. Dengjie Zhong The author's contribution is the examiner of the article. Nianbing Zhong

The author's contribution is the writer of the article.

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Declaration of interest statement

We declare that we have no financial and personal relationships with

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other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in

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any product, service and/or company that could be construed as

entitled,

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influencing the position presented in, or the review of, the manuscript “CTAB-surface-functionalized

magnetic

MOF@MOF

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solution”

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composite adsorbent for Cr(VI) efficient removal from aqueous

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Acknowledgments

Financial support from the Natural Science Foundation of China (Project No.51876018) is gratefully

acknowledged.

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Figure captions

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Fig. 1. XRD patterns of UiO-66, UiO-67, Fe3O4@UiO-66@UiO-67 and

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Fe3O4@UiO-66@UiO-67/CTAB.

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Fig. 2. SEM images of UiO-66 (a), UiO-67 (b), Fe3O4@UiO-66@UiO-67 (c) and Fe3O4@UiO-66@UiO-67/CTAB (d). HRTEM images (e, f), TEM image (g) and SAED pattern (h) of

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Fe3O4@UiO-66@UiO-67/CTAB.

Fig. 3. FT-IR spectra of UiO-66, UiO-67, Fe3O4@UiO-66@UiO-67 and

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Fe3O4@UiO-66@UiO-67/CTAB.

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Fig. 4. N2 adsorption-desorption isotherm (a) and pore size distribution (b) of the prepared samples.

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Fig. 5. TGA curves of Fe3O4, UiO-66, UiO-67 and Fe3O4@UiO-66@UiO-67/CTAB.

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Fig. 6. Magnetization loops of absorbents before (Fe3O4@UiO-66@UiO-67/CTAB) and after

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(Fe3O4@UiO-66@UiO-67/CTAB-Cr) adsorption at room temperature (the inset photograph shows

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magnet separation after adsorption).

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Fig. 7. High-resolution XPS spectra of adsorbent before and after absorption in the wide scan (a), Cr

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2p (b), Fe 2p (c, d) and C 1s (e, f).

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Fig. 8. Adsorption properties of six adsorbents under the same conditions.

Fig. 9. Effect of (a) initial pH values, (b) contact time, (c) initial Cr(VI) concentration and (d) coexisting ions on Cr(VI) adsorption with Fe3O4@UiO-66@UiO-67/CTAB.

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Fig. 10. Adsorption kinetics of Cr(VI) adsorption on Fe3O4@UiO-66@UiO-67/CTAB fitting with

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pseudo-first-order model, pseudo-second-order model and Elovich models (Experiment conditions: initial Cr(VI) concentration 50 ppm (50 mL), adsorbent 20 mg, pH 2.0, contact time 24 h, and

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temperature 298 K).

Fig. 11. Adsorption isotherms of Cr(VI) adsorption on Fe3O4@UiO-66@UiO-67/CTAB at 298 K (a), 308 K (b) and 318 K (c) fitting with Langmuir, Freundlich, Redlich-Peterson and Temkin models and (d) the linear fitting plot of ln (qe/Ce) againsr 1/T (Experiment conditions: adsorbent 20 mg, pH 2.0 and

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contact time 24 h).

Fig. 12. The removal efficiencies of Cr(VI) by Fe3O4@UiO-66@UiO-67/CTAB over five cycles (Experiment conditions: initial Cr(VI) concentration 50 ppm (50 mL), adsorbent 20 mg, pH 2.0,

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contact time 24 h, and temperature 298 K).

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Scheme captions

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Scheme 1. Schematic diagram of the preparation of Fe3O4@UiO-66@UiO-67/CTAB.

Table

Table 1 Surface area, pore size and pore volume parameters of different adsorbents Pore volume

Adsorbent

Surface area (m2·g-1)

Pore size (nm)

UiO-66

571.87

1.39

0.30

UiO-67

206.92

1.54

0.09

Fe3O4@UiO-66@UiO-67

314.95

1.46

0.15

Fe3O4@UiO-66@UiO-67/CTAB

115.94

1.57

0.02

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(cm3·g-1)

Table 2 Comparison of maximum adsorption capacity of Fe3O4@UiO-66@UiO-67/CTAB for Cr(VI) removal with those literatures reported Adsorbent

Adsorbent

dosage (g·L-1)

Initial

Adsorption

concentration

capacity

Reference

-1

(ppm)

(mg·g )

0.15

21.6

129

[42]

Cu-BTC

0.5

10-40

48

[43]

ZIF-67 microcrystals

1

6-15

13.34

[44]

ZJU-101

0.5

50

245

[45]

0.4

20-1000

372.6

[45]

0.5

50

53.4

[46]

nanofibers MOF-867 PANI–magnetic mesoporous silica composite

-p

PAN/chitosan/UiO-66-NH2

0.8

100

193.85

[47]

200

238.09

[48]

48.4

293.3

[49]

0.2

50

156.3

[50]

0.4

5-500

932.1

This work

2

PPy–Fe3O4/rGO nanocomposite

0.25

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carbon microspheres

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Fe3O4@UiO-66@UiO-67/CTAB

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PPy/Fe3O4 nanocomposite

Mesoporous Fe3O4 loaded

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UiO-66-HA

Table 3 Thermodynamic parameters of Fe3O4@UiO-66@UiO-67/CTAB adsorbent for Cr(VI) adsorption ΔH (kJ·mol−1)

ΔS (kJ·mol−1·K−1)

14.143

0.060

ΔG (kJ·mol−1) 298 K

308 K

Ea (kJ·mol−1) 318 K 14.604

-4.337

-4.937

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-3.737