English

• 0.   Introduction

  Adsorption is playing an increasingly vital role in wastewater treatment, which can greatly improve the natural environment in this industrial era. Therefore, it is urgent to develop high - performance and costeffective adsorbents for removing pollutants from wastewater^[1-3].

  Metal-organic frameworks (MOFs), a class of highly porous hybrid materials self - assembled from metal nodes and organic ligands, possess lots of advantages, such as flexible structure, easy functionalization, large specific surface area, and abundant active sites, which show great potential in the fields of water purification, gas adsorption, and photo - luminescence^[4-6]. Compared with other mature traditional adsorbents, MOF materials are still in the initial stage of development. To reach the point of practical application, there are still many problems to be solved. Especially, on the one hand, the stability and dispersion of MOF material in an aqueous solution are far from ideal, and on the other hand, the removal rate in the solvent of pure MOF material is poor^[7-9]. Therefore, it is high time to take some measures to improve its stability, dispersion, and adsorption performance.

  Graphene has been considered an efficient adsorbent for various adsorption applications due to its porous structure, high stability, and environmentfriendly properties. Different from graphene, graphene oxide (GO) can be easily synthesized from natural graphite and dispersed in water and other polar solvents because of its hydrophilic character. In addition, the abundant functional groups of GO sheets, like epoxy, carbonyl, and hydroxy, provide a favorable basis for the chelation of metal ions in MOF, which is conducive to the formation of MOFs/GO hybrids^[10-16]. MOF/ GO composites often exhibit superior properties than pristine MOFs due to their rich active sites, strong hydrothermal stability, and excellent adsorption proper- ties^[15].

  In this work, we have prepared two new types of GO - functionalized Cd-MOF materials with high structural stability and enhanced adsorption performance. These two Cd-MOF/GO composites with different morphologies were further explored for the adsorption of rhodamine B (RhB). Compared with pristine Cd - MOF, the introduction of GO enhanced the stability of Cd - MOF in water and improved the adsorption capacity of Cd - MOF. In addition, under acidic conditions, MOF/ GO composite exhibited optimal adsorption capacity for RhB, and the adsorption rate can reach ca. 95%.

  1.   Experimental

  1.1   General procedures

  1.1.1   Synthesis of Cd-MOF

  Cd - MOF was synthesized according to the previously reported method^[17]. Firstly, 3CdSO₄·8H₂O (0.033 mmol), 2, 1, 3 - benzothiadiazole - 4, 7 - dicarboxylic acid (H₂BTDC, 0.05 mmol), EtOH (3 mL), and H₂O (2 mL) were placed in a 25 mL Teflon - lined stainless - steel container. Then, the mixture was sealed and heated at 140 ℃ for 72 h. After the mixture was cooled to ambient temperature at a rate of 5℃·h^-1, the red - brown crystals of Cd-MOF were obtained with a yield of 51%.

  1.1.2   Synthesis of Cd-MOF/GO composites

  A certain amount of GO powders were dispersed into an ethanol - water mixed solution with ultrasonication. The Cd - MOF/GO composites with different GO contents (the mass ratios of GO and Cd - MOF initial raw material were 2.5%, 5%, and 10% respectively) were synthesized under the same condition. To be specific, 0.1 mmol H ₂BTDC, 0.05 mmol 3CdSO₄·8H₂O, 1 mL ethanol, 1 mL water, 1 mL alkali (NH₃·H₂O, triethylamine (TEA), DMF, or 0.5 mol·L^-1 NaOH) and 4 mL GO suspension were subjected to solvothermal conditions in a Teflon - lined stainless - steel container. The resulting solution was treated by ultrasonic for 30 min to obtain a uniform dissolution of the reactants and then kept at 140 ℃ for 72 h. After the reaction, the resultant precipitate was separated by centrifugation, and washed repeatedly with water and ethanol, respectively. Finally, the obtained composites were dried at 45 ℃ under a vacuum for 24 h. Remarkably, the optimized condition with 5% of GO was selected for further adsorption study.

  1.2   Characterization

  Morphology of the samples was characterized by scanning electron microscopy (SEM) on a SIGMA 500/ VP (ZEISS, 3.00 kV) and HITACHI TM4000 at an accelerating voltage of 15 kV. UV - Vis spectra were measured with a JASCO V - 750 UV - Vis spectrometer (200-900 nm). The Fourier transform infrared (FT -IR) spectra were acquired on a Bruker Tensor 27 spectrophotometer in the range of 400 to 4 000 cm^-1. Powder X-ray diffraction (PXRD) patterns were performed on a Bruker - AXS D8 Advance X - ray diffractometer with CuKα radiation (λ=0.154 18 nm, 40 kV, 40 mA) in a range of 5°-60° (2 θ). The measurements of steady-state emission spectra were conducted on a JASCO FP-8300 fluorescence spectrophotometer at room temperature.

  1.3   Adsorption experiments

  The adsorption performance of the materials was measured in RhB solutions (100 mL, 20 mg·L^-1). Briefly, Cd-MOF or Cd-MOF/GO powders (20 mg) were added to the RhB solution under continuous stirring. Then, a small amount of the RhB solution was taken out at regular time intervals and centrifuged to wipe Cd - MOF or Cd - MOF/GO powders out. Finally, the obtained RhB supernatant was detected by a UV-Vis spectrometer to acquire the concentration of the remaining RhB. The absorbance of RhB was measured at the maximum absorption wavelength, and the adsorption rate (R) was calculated by

  $ R=\frac{A_0-A_t}{A_0} \times 100 \% $

  where A₀ represents the absorbance of the initial solution, and A_t represents the absorbance of the solution after adsorption.

  A series of RhB solutions with concentrations ranging from 2 to 100 mg·L^-1 was prepared beforehand for the establishment of the standard curve. Besides, the real-time concentration of RhB was determined by the maximum absorbance at 553 nm in the UV - Vis spectrum. Then, the adsorption amount was calculated by the following equation:

  $ Q=\frac{V\left(\rho_0-\rho_t\right)}{m} $

  where Q (mg·g^-1) is the adsorption amount, ρ₀ (mg·L^-1) and ρ _t (mg·L^-1) are the initial and final mass concentrations of RhB, respectively, V (L) is the volume of RhB solution, and m (g) is the mass of the adsorbent.

  To investigate the effect of pH on the adsorption capacities of Cd-MOF/GO composites, the pH of the 20 mg·L^-1 RhB solution was adjusted with 1 mol·L^-1 NaOH and 1 mol·L^-1 HCl solutions. The pH values of the 20 mg·L^-1 RhB solution was controlled to 3.5, 7.0, and 9.6.

  2.   Results and discussion

  As shown in Fig. 1, firstly, Cd - MOF/GO composites were synthesized by a solvothermal method. Then, Cd - MOF/GO composites with different morphologies were obtained by adjusting the species of alkaline reagent (with the same amount of 1 mL) in the hydrothermal process (Fig. S1, Supporting information). Finally, two as - prepared composites (Cd - MOF/GO - 1 and Cd - MOF/GO - 2 obtained with NH ₃·H₂O and TEA respectively, the mass ratios of GO and Cd-MOF initial raw material were all 5%) were used for effective adsorption of RhB.

  Figure 1

  

  Figure 1.  Schematic diagram of the preparation procedure of Cd⁃MOF/GO composite and its adsorption process

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  2.1   SEM

  The morphologies of pure Cd-MOF, Cd -MOF/GO- 1, and Cd - MOF/GO - 2 were observed by SEM. The pure Cd-MOF showed smooth rod-like morphology (Fig. S2). Different from the pure Cd - MOF, Cd -MOF/GO -1 presented a rod structure with a superimposed layer (Fig. 2a and 2b) whereas Cd-MOF/GO-2 (Fig. 2c and 2d) exhibited porous cotton - shaped network morphology. This result showed that the introduction of GO can affect the crystallinity of a self - assembling MOF. This was due to the organic ligands and metal ions cannot be completely coordinated, increasing the disorder of the crystal structure.

  Figure 2

  

  Figure 2.  SEM images of Cd-MOF/GO-1 (a, b) and Cd-MOF/GO-2 (c, d)

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  Moreover, Fig. S1 exhibited the SEM images of different samples obtained under different alkaline environments (NH ₃·H₂O, TEA, DMF, and NaOH conditions). It can be seen that all samples displayed the sheet structure of GO. Additionally, the samples obtained under NH₃·H₂O, TEA, and DMF showed distinctly Cd - MOF structures. However, no MOF was observed under NaOH conditions. Besides, the GO sheets were stacked seriously under NaOH conditions. These results preliminarily indicated that the different types of alkali may influence the dispersion ability of GO, and further affect the binding force of MOF material to the GO, which can control the morphology of the composite material.

  2.2   XRD

  Firstly, the effect of an alkaline environment on the crystallinity of Cd-MOF has been verified by XRD, as shown in Fig. 3a. Obviously, the characteristic peaks of Cd-MOF were the same as those reported in the literature ^[17] and can be observed in the Cd - MOFs obtained under these different alkaline reagent conditions. In addition, in the NH₃·H₂ O and TEA environment, the MOF crystallinities were fine, while in DMF, the MOF crystallinity was relatively poor, and even in NaOH, no crystalline material was obtained.

  Figure 3

  

  Figure 3.  (a) XRD patterns of Cd-MOF under different alkaline reagent conditions; XRD patterns of GO, Cd-MOF, Cd-MOF/GO composites under (b) NH₃·H₂O and (c) TEA conditions with different GO contents

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  Besides, XRD studies were also carried out on the Cd - MOF/GO composites, as shown in Fig. 3b and 3c. Notably, the GO and Cd- MOF peaks both appeared in all of the XRD patterns of the Cd-MOF/GO composites. In addition, a well-defined peak ascribed to the oxygencontaining functional groups of GO at 2θ =10.2° was observed in the XRD pattern of GO, which was consis- tent with the reported literature^[18-19]. With the introduction of GO, the characteristic peak of GO (at ca. 10.2°) appeared. And with the increase of the GO content, the peak of GO became more obvious. The Cd - MOF/GO composites showed sharp and narrowed peaks which are similar to the pristine MOF, indicating the products were high - quality crystalline. These features indicated that the Cd - MOF crystals were successfully formed with the presence of the GO layers. However, the intensities of the XRD peaks for Cd-MOF/GO became weaker, revealing that a higher content of GO could influence the formation of the crystal. When the GO content increased to 10%, the characteristic peaks of Cd-MOF in the composites became quite weak. Therefore, the optimized content of 5% of GO was selected for further study.

  2.3   FT-IR

  The FT - IR spectra of pristine GO, Cd -MOF, and Cd - MOF/GO composites were given in Fig. 4. The typical peaks were found to correspond to the —OH stretching vibrations at 3 431 cm^-1, the carbonyl or carboxylic moiety of C=O stretching frequency at 1 790 cm^-1, the aromatic stretching frequency of C=C at 1 551 cm^-1, and the stretching vibration of C—O epoxy at 1 403 cm^-1, the C—O stretching vibration at 1 101 cm^-1, and the epoxy functional group at 848 cm^-1 in the FT - IR spectrum of GO^[20]. The pure Cd - MOF showed two strong bands at 1 564 and 1 458 cm^-1 corresponding to the C=O and C—O stretching vibration of the aromatic ring of benzimidazole - 4, 7 - dicarboxylic acid. The peaks at 1 394 cm^-1 can be attributed to the C—H bending vibration of a —CH₃ group of Cd - MOF. The several bands that can be observed in the region of 1 200 -600 cm^-1 are related to the out - of - plane vibrations of the aromatic benzene ring of the Cd-MOF^[21-22]. FT-IR spectra of Cd-MOF/GO composites showed that the majority of peaks for GO and Cd-MOF were retained and there was no significant change. It was also proven that GO did not affect the formation of Cd-MOF.

  Figure 4

  

  Figure 4.  FT-IR spectra of GO, Cd-MOF, and Cd-MOF/GO composites

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  2.4   XPS

  XPS was conducted to further study the chemical state and surface chemical composition of the Cd- MOF/ GO composites. In the survey spectra (Fig. 5a), the characteristic peaks of Cd, C, and O can be observed, demonstrating the coexistence of these elements. All the spectra of Cd-MOF/GO composites contained four clear strong peaks corresponding to C1s (ca. 284.8 eV), Cd3d (Cd - MOF/GO - 1: ca. 405.3/412.2 eV; Cd - MOF/ GO -2: ca. 405.6/412.2 eV), and O1s (Cd - MOF/GO -1: ca. 531.8 eV; Cd - MOF/GO - 2: ca. 531.7 eV), respectively. The high-resolution spectrum of Cd3d (Cd-MOF/ GO-1) with featured peaks at 412.2 (Cd3d_3/2), 405.3 eV (Cd3d_5/2), and spin - orbit separation of 6.9 eV reveals the Cd element with +2 oxidation state (Cd²⁺). For Cd - MOF/GO - 2, the two peaks at 405.6 and 412.2 eV for Cd3d_5/2 and Cd3d_3/2, respectively, with 6.6 eV splitting between the two peaks.

  Figure 5

  

  Figure 5.  (a) Survey XPS spectra of Cd-MOF/GO-1 and Cd-MOF/GO-2; (b) C1s and (c) O1s XPS spectra of Cd-MOF/GO-1; (d) C1s and (e) O1s XPS spectra of Cd-MOF/GO-2

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  To further analyze the state of each element, the high - resolution spectra of each element were studied, as shown in Fig. 5b-5e. The high-resolution C1s spectra can be deconvoluted into three peaks related to C=C/ C—C (Cd - MOF/GO - 1: 284.6 eV, Cd - MOF/GO - 2: 284.7 eV), C—O (Cd-MOF/GO-1: 286.1 eV, Cd-MOF/ GO - 2: 285.7 eV), and C=O (Cd - MOF/GO - 1: 288.3 eV, Cd - MOF/GO - 2: 288.3 eV). The high - resolution O1s spectra can be deconvoluted into three peaks corresponding to O—C=O (Cd-MOF/GO-1: 531.2 eV, Cd - MOF/GO - 2: 531.6 eV), C=O/C—OH (Cd - MOF/ GO-1: 532.1 eV, Cd-MOF/GO-2: 531.7 eV), and C— O—C (Cd - MOF/GO - 1: 533.1 eV, Cd - MOF/GO - 2: 532.5 eV).

  As shown in Table 1, around 42.27% of C and 42.80% of O were measured in Cd-MOF/GO-1. Howev- er, the C content in Cd - MOF/GO - 2 increased to 52.65% while the O content decreased to 31.07%. Fur- ther information can be gained by taking the ratio between C to O atoms. Here, the C/O ratio (n_C/n_O) of Cd- MOF/GO-2 (1.69) was larger than that of Cd-MOF/ GO - 1 (0.99). A larger C/O ratio hinted at a greater degree of reduction of GO. Therefore, during the ad- sorption process, Cd - MOF/GO - 2 with a higher degree of reduction can provide more π-π interactions, there- by obtaining a higher adsorption capacity^[23-24].

  Table 1

  

  Table 1.  Atomic fractions obtained by XPS for Cd-MOF/GO-1 and Cd-MOF/GO-2

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  Sample	Atomic fraction / %			C/O ratio
  	C1s	O1s	Cd3d
  Cd-MOF/GO-1	42.27	42.80	7.61	0.99
  Cd-MOF/GO-2	52.65	31.07	9.92	1.69

  2.5   Photoluminescence spectra

  The solid - state photoluminescent properties of Cd - MOF, Cd - MOF/GO - 1, and Cd - MOF/GO - 2 were investigated at room temperature under 340 nm excitation. Cd-MOF showed a significant fluorescence enhancement, one strong emission band at 415 nm, and one shoulder emission peak around 540 nm (Fig. 6a)^[17]. The result showed that GO can change the fluorescence properties of the material.

  Figure 6

  

  Figure 6.  (a) Photoluminescence spectra and (b) UV⁃Vis spectra of Cd-MOF, Cd-MOF/GO-1, and Cd-MOF/GO-2 in solid state at room temperature

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  The emission peak of Cd - MOF/GO composites were red-shifted (426 and 429 nm). The reason for this red -shift was supposed to originate from the interaction of the electron-rich GO providing electrons for the ligand. The energy from the radiation of the excitation state of GO can be transferred to the ligand to promote it to be excited. Additionally, strong π - π interactions between molecules can lead to red-shift and quenching of photoluminescence^[25]. The red-shift of Cd-MOF/ GO- 2 was larger and its photoluminescence quenching was more obvious, which means faster charge transfer speed, higher reduction degree of GO, and stronger π-π interactions between molecules. The above results were consistent with XPS analysis.

  2.6   UV-Vis spectra

  The UV-Vis spectra of Cd-MOF, Cd-MOF/GO-1, and Cd - MOF/GO - 2 were obtained in the wavelength range of 200-800 nm and all the trends of spectra were similar (Fig. 6b). Cd - MOF showed absorption peaks at 250, 325, and 495 nm. Cd - MOF/GO - 1 showed strong absorption bands around 210 and 336 nm and a relatively weak absorption at around 448 nm. Cd - MOF/ GO-2 had an obvious absorption peak at 215, 330, and 450 nm. The maximum absorption peak of Cd - MOF/ GO-1 (448 nm) and Cd-MOF/GO-2 (450 nm) was blueshifted relative to Cd-MOF (495 nm).

  According to literature reports, the absorption peak of GO is usually in the range of 230-300 nm^[26-27]. Therefore, the introduction of GO resulted in changes in the absorption peak positions of Cd - MOF/GO. In addition, the absorption peak of Cd - MOF/GO - 2 was slightly stronger than that of Cd - MOF/GO - 1 because the reduction degree of GO in Cd - MOF/GO - 2 was higher, and then more π-π interactions of reduced-GO brought about higher absorbance^[28-29]. This is consistent with XPS and photoluminescence results.

  2.7   Adsorption studies

  The adsorption capacity of Cd-MOF and Cd-MOF/ GO composites towards RhB was studied in 60 min. It was worth mentioning that the adsorption effect of Cd-MOF/GO-2 was slightly stronger than that of Cd-MOF/GO-1. Combining multiple characterization results, Cd-MOF/GO -2 was confirmed to possess a higher degree of GO reduction. During the adsorption process, Cd- MOF/GO-2 with a higher reduction degree can provide more π - π interactions, thereby obtaining higher adsorption capacity^[23-24]. In addition, the changes in absorbance spectra caused by the adsorption of RhB are shown in Fig.S3. Besides, the adsorption rates for RhB on different materials are shown in Fig. 7a. In addition, the adsorption capacities of Cd-MOF and Cd - MOF/GO composites for RhB at pH values of 3.5, 7.0, and 9.6 are exhibited in Fig. 7b and Fig.S4, respectively. In the pH range of 3.5-9.6, the adsorption capacity decreased with the increase in pH. The pH of 3.5 was the optimized condition. Moreover, these composite adsorbents showed desirable pH stability in different environments. Fig. S5 exhibited the XRD patterns of Cd-MOF/GO-1 and Cd-MOF/GO -2 after the adsorp- tion experiments in different pH environments. The peak positions in all XRD patterns were almost identical. These results show that the obtained composites were stable for different pH environments. Therefore, this composite material has wide versatility in practical adsorption applications.

  Figure 7

  

  Figure 7.  (a) Bar chart of specific adsorption rates of Cd-MOF and Cd-MOF/GO composites for RhB in different pH values; (b) Change of absorbance of RhB in time for Cd-MOF and Cd-MOF/GO composites at pH=3.5

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  Furthermore, the addition of GO resulted in a significant increase in the adsorption capacity of Cd-MOF/ GO- 1 and Cd-MOF/GO-2. Remarkably, the adsorption rate of Cd-MOF/GO -2 can reach 95.04% within 60 min at pH=3.5, and the maximum adsorption capacity was 100.09 mg·g^-1 (Table S1). This implies that the introduction of GO in the composites surely plays a vital role in increasing RhB adsorption compared with pristine Cd -MOF. Based on these data, Cd-MOF/GO composites seem a suitable material for wastewater treatment.

  2.8   Adsorption mechanism of RhB on Cd - MOF/ GO composites

  First, the pore size of Cd - MOF in this work was about 2.00 nm (Fig.S6), and the size of the RhB molecule is about 1.59 nm×1.18 nm×0.56 nm, which provides the basis for the RhB molecule to enter the pores of Cd-MOF. Meanwhile, RhB is a kind of cationic dye, the main groups involved in the adsorption process are acidic groups and aromatic groups. The RhB adsorption of Cd - MOF/GO composites can be attributed to several different factors, like electrostatic/ionic interactions, hydrogen bonding, and π-π interactions^{[22, 30]}. The specific mechanism illustration image is shown in Fig. 8. Remarkably, on the one hand, electrostatic/ionic interactions occur among unsaturated bonds in Cd-MOF, negatively charged GO surface, and positively charged RhB. On the other hand, the aromatic groups in RhB can make strong π - π interactions with the benzene rings in Cd-MOF and GO.

  Figure 8

  

  Figure 8.  Specific mechanism illustration of RhB adsorption of Cd-MOF/GO composites

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  3.   Conclusions

  To effectively remove dye from wastewater, MOF/ GO composites were prepared by a one-step hydrothermal method using 3CdSO₄·8H₂O, ligand H₂ BTDC, and GO as raw materials in the different types of alkaline environments. Cd-MOF/GO -1 presented a rod structure with a superimposed layer whereas the morphology of the Cd - MOF/GO - 2 exhibited a porous cotton - shaped network structure. These results preliminarily indicate that the different types of alkali may be influenced the dispersion ability of GO, and further improve the binding force of MOF to the GO, which can control the morphology and adsorption performance of the composite material. The influence factors of Cd - MOF/GO on the adsorption process of RhB solution were discussed. Cd - MOF/GO composites were stable for different pH environments, exhibiting wide versatility in practical adsorption applications. Under acidity conditions, the adsorption effect was the best, the adsorption rate can reach ca. 95%.

  Acknowledgments: This work is financially funded by the Science and Technology Project of the Hebei Education Department (Grant No. ZD2019309, BJ2020209), the Handan Science and Technology Research and Development Program of China (Grant No. 19422111001 - 10), and the Key Projects of Handan College (Grant No.XZ2019102).

  Supporting information is available at http://www.wjhxxb.cn

  
  
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  Figure 1  Schematic diagram of the preparation procedure of Cd⁃MOF/GO composite and its adsorption process

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  Figure 2  SEM images of Cd-MOF/GO-1 (a, b) and Cd-MOF/GO-2 (c, d)

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  Figure 3  (a) XRD patterns of Cd-MOF under different alkaline reagent conditions; XRD patterns of GO, Cd-MOF, Cd-MOF/GO composites under (b) NH₃·H₂O and (c) TEA conditions with different GO contents

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  Figure 4  FT-IR spectra of GO, Cd-MOF, and Cd-MOF/GO composites

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  Figure 5  (a) Survey XPS spectra of Cd-MOF/GO-1 and Cd-MOF/GO-2; (b) C1s and (c) O1s XPS spectra of Cd-MOF/GO-1; (d) C1s and (e) O1s XPS spectra of Cd-MOF/GO-2

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  Figure 6  (a) Photoluminescence spectra and (b) UV⁃Vis spectra of Cd-MOF, Cd-MOF/GO-1, and Cd-MOF/GO-2 in solid state at room temperature

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  Figure 7  (a) Bar chart of specific adsorption rates of Cd-MOF and Cd-MOF/GO composites for RhB in different pH values; (b) Change of absorbance of RhB in time for Cd-MOF and Cd-MOF/GO composites at pH=3.5

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  Figure 8  Specific mechanism illustration of RhB adsorption of Cd-MOF/GO composites

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  Table 1.  Atomic fractions obtained by XPS for Cd-MOF/GO-1 and Cd-MOF/GO-2

  Sample	Atomic fraction / %			C/O ratio
  	C1s	O1s	Cd3d
  Cd-MOF/GO-1	42.27	42.80	7.61	0.99
  Cd-MOF/GO-2	52.65	31.07	9.92	1.69

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