CoFe2O4@SiO2/HKUST-1, A Novel Three-Metallic Magnetic Metal-Organic Framework: Synthesis, Methylene Blue Removal, the Study of Adsorption Isotherms and Kinetic Models


 CoFe2O4@SiO2/HKUST-1 as a novel three-metallic magnetic metal-organic framework (MMOF) has been favorably synthesized via a simple self-assembly method. For this purpose, CoFe2O4@SiO2 was functionalized by Glutaric anhydride and 3-(triethoxysilyl)propylamine and then HKUST-1 was synthesized on the surface of CoFe2O4@SiO2. Powder X-ray diffractometry (XRD), Fourier transform infrared (FTIR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET), vibrating sample magnetometer (VSM), and simulated thermal analyzer (STA) were utilized to characterize the as-prepared samples. The methylene blue (MB) removal efficiency of CoFe2O4@SiO2/HKUST-1 has been set by examining factors such as the effect of pH, concentration of dye, amount of adsorbent, and components of a compound. Up to 100 percent dye removal in short reaction times and water media was reached in alkaline pH, by increasing the dose of adsorbent and in the presence of CoFe2O4@SiO2 and CoFe2O4@SiO2/HKUST-1 compounds. Freundlich adsorption isotherm that uses to describe the ongoing adsorption was found to be best fitted for the adsorption process.Based on pseudo-first-order, pseudo-second-order, and intraparticle diffusion kinetic equations, the pseudo-second-order kinetic model with rate constant (k2) 4 x10-3 g/mg.min is best fitted for adsorption methylene blue, indicating that removal of dye takes place dominantly through the chemisorption process.

framework composites (MMOF) are one that compound, which due to their inheriting the superiorities of magnetic nanoparticles and MOFs, provide a synergistic effect and expanded specific novel functionalities including drug delivery and imaging, environmental remediation and separation, and especially water purification [13][14][15][16].
Cobalt ferrite (CoFe 2 O 4 ) is one of the most appealing magnetic materials to make magnetic metal-organic frameworks due to its desired magnetic, mechanical and chemical properties. On the other hand, the silica matrix for ferrite nanoparticle dispersion is too important to improve magnetic properties, nanoparticle aggregation control, stability, and disperse in aqueous [17,18]. Several studies have reported applications of the same magnetic system in wastewater purification. For instance, L. Huang and coworkers reported Fe 3 O 4 @SiO 2 @HKUST-1 magnetic core-shell composite to enhanced removal of Hg 2+ from water [19], Ruiqi Zhang too reported magG@SiO 2 @ZIF-8 [20], and similar magnetic composites based on the magnetic nanoparticles coated with SiO 2 and MOF, Fe 3 O 4 @SiO 2 @MOF/TiO 2 [21], Fe 3 O 4 @SiO 2 @Zn-TDPAT [22] and Fe 3 O 4 @SiO 2 @Zr-MOF [23] were successfully synthesized in order to remove of organic contaminants from the environment.
United Nations General Assembly in 2015 approved the 17 Sustainable Development Goals, SDGs, as the "blueprint to achieve a better and more sustainable future for all" [24] and the 7 of the SDGs directly or indirectly emphasize environmental protection. Fortunately, a good deal of research is taking place as well in order to remove pollutants from the environment and protection of water, soil, air and the atmosphere [25][26][27].
Among water pollutants, textile dyeing and finishing procedures are the primary sources of pollution and cause a high amount of dye in wastewater [27]. Therefore, removing dyes from water is imperative due to its harmful effects like toxicity, carcinogenesis, suppressing photosynthetic activities, causing allergic reactions, mutagenesis, and inhibiting the growth of aquatic biota [28]. Methylene blue (MB) as a well-known cationic dye with diverse industrial applications was selected to serve as a contaminant model [29,30].
In this work, a novel magnetic CoFe 2 O 4 @SiO 2 /HKUST-1 nanocomposite was prepared via an in situ self-assembly method. This method includes modifying presynthesized magnetic CoFe 2 O 4 @SiO 2 particles and inducing them to copper nitrate and benzene-1,3,5-tricarboxylic acid (H 3 BTC) solution to self-assembly fabricate CoFe 2 O 4 @SiO 2 /HKUST-1 magnetic metal-organic framework. The newly synthesized compound needs to identify to understand it better and then apply it to remove methylene blue from water.

Methylene blue removal study
The following procedure performed removal experiments in aqueous solutions containing MB: To study the effect of pH, five samples of methylene blue solution with conc. 5 mgL -1 were prepared and the pH of the solutions was adjusted to 3, 5, 7,

Characterization
The thermal behaviors of the samples were evaluated using a simulated thermal analyzer (STA, Pyris Diamond). The XRD patterns were obtained on an X-ray diffractometer (PANalytical, Philips, the Netherlands). The FTIR spectra were recorded on a spectrophotometer (Spectrum One, Perkin Elmer, Shelton, CT, USA  with published literature reports [32][33]. The diffraction intensities of specific peaks of CoFe 2 O 4 @SiO 2 dramatically decreased, implying that the existence of HKUST-1 leads to partial decomposition of the crystalline form of CoFe 2 O 4 @SiO 2 and probably decreasing to a SiO 2 amorphous phase.

FTIR analysis
In Figure 4 and Table 1, the characteristic IR absorptions of CoFe 2 O 4 @SiO 2 and

.Morphology observations
As depicted in figure 3, SEM and TEM measurements were performed to evaluate the morphology and particle distribution for CoFe 2 O 4 @SiO 2 and CoFe 2 O 4 @SiO 2 /HKUST-1 samples. Figure 3a shows that the morphology of

Specific surface area measurement
Nitrogen adsorption-desorption isotherm, the specific surface area and pore structures of CoFe 2 O 4 @SiO 2 /HKUST-1 are shown in figure 5 and published report [35], this composite's specific surface area value in the absence of SiO 2 was 982.05 (m 2 g -1 ). of compounds is affected by particle size, particle shape, and synthesis method, so that this result can be attributed to reduced particle size and changes in the structure of the surface of the compounds. On the other hand, the coercive force, Hc, does not change after the combination of HKUST-1 and is less than 1000 A / m for both compounds, which indicates the soft magnetic behavior [36].  table 3). The results are summarized in Table 5.

Influence of pH
pH is one of the critical factors in determining the ability to remove pollutants because it directly affects the surface properties of composites and the ionization of the dye molecules [38]. According to Table 4 and Figure 8, removal efficiency has increased by increasing the pH of methylene blue solution from 3 to 10 to more than 95%. Therefore, the overall process efficiency at basic pH is significantly higher than at acidic pH so, the optimum pH for CoFe 2 O 4 @SiO 2 /Cu 3 (BTC) 2 Equal to 10 is considered. In alkaline environments, the surface of the compound is covered by negative OH ions, and as a result of more interactions with positive ions, the removal efficiency increases. The electrostatic attraction between the composite and methylene blue causes the substance to separate and the dye removed by the synthesized sample. On the other hand, the data showed that the amount of dye removal was not meaningfully different in the range of pH = 7-10; this could be due to the equilibrium between the positive charges of methylene blue dye and the negative charges on the composite surface, so for further experiments, a mild alkaline system with pH = 10 was used. As the dye concentration increases, the decolorization efficiency decreases, which is probably since at high concentrations, the active sites of the compound are covered by positively charged dye molecules.  Figure 11 and Table 4

Adsorption isotherms
To clarify the interaction of methylene blue with adsorbent Langmuir and Freundlich isotherm was studying .
In Langmuir theory, the basic premise is that adsorption occurs at a specific homogeneous site inside the adsorbent. This means that when the dye molecules attach to a location on the adsorbent surface, the molecule can no longer be located there. There is no interaction between the adsorbed molecules, and this theory is described by Equation (3).
(3) C e q e = 1 K L q max + C e q max Where q e (mg g ⁄ ) and C e (mg L ⁄ ) are the adsorption capacity and the equilibrium concentration of the dye in solution, respectively. q max (mg L ⁄ ) max is the maximum absorption capacity and K L (L mg ⁄ ) is the Langmuir constant.
One of the basic features of the Langmuir isotherm is the determination of the adsorption capacity for the separation of contaminants, which is defined by the separation factor R L and equation (4). Also, if 0 < R L < 1, the result is desirable.
Besides, the Langmuir isotherm is linear and irreversible when R L = 1 and R L = 0, respectively.

(5)
Ln q e = Ln K f + Ln C e n ⁄ Where q e (mg g ⁄ ) and C e (mg L ⁄ ) are adsorption capacity and equilibrium concentration of methylene blue.
The values of K and n are Freundlich constants. K f values and n are calculated from the slope and the width from the origin of the diagram of log q e versus log q e .
In the Freundlich isotherm, if the value of n is between 1 and 10, the adsorption process is acceptable, and also, when the values of n are less than 1, physical adsorption occurs.
Based on the data in Table 6 and Figure

Adsorption kinetics
When adsorption occurs by penetration from inside the layer or boundary, kinetics most often follow the first order. Equation (6) is used for the quasi-first order model.
In Equation 6, q t and q e are the amount of adsorbed material on the adsorbent at time t and at equilibrium time (mg/g) and K 1 is the pseudo-first-order adsorption rate constant (1 min ) ⁄ , respectively. Log(q e − q t ) is plotted against time to determine the constant value k and the coefficient R 2 .
Pseudo-second-order kinetics shows that chemical adsorption is a step-slowing process that controls surface adsorption processes, and equation (7) is used to describe it. In general, this model is exciting and valuable because research has shown that the kinetics of most adsorption systems at different concentrations of pollutants are well compatible with this system. Also, the adsorption capacity, the second-order velocity constant, and the initial adsorption rate can be determined from this equation without knowing any previous coefficient.
log(q e − q t ) = log q e − K 1 2.303 t (7) t q t = 1 K 2 ⁄ ⁄ q e 2 + t q e ⁄ In Equation 7, (g (mg min) ⁄ ) K 2 is a quasi-quadratic absorption rate constant. The graph of t q t ⁄ versus t is used to obtain velocity parameters and proposes the results of this kinetic model with experimental data. The values q e and K 2 are determined by calculating the slope and width from the origin of this graph.
The initial rate of adsorption rate h 0,2 (mg (g ⁄ min)) can be calculated based on equilibrium adsorption capacity using Equation (8).
The kinetic model influences the diffusion to investigate that the diffusion mechanism is absorbed in the porous adsorption and the phase shift that the velocity controller has and describes according to Equation 9.
(9) q t = K diff t 1 2 ⁄ + I K diff is a constant rate of intra-particle penetration in terms of (mg/g.min 0.5 ) and I width from the origin in terms of (mg/g) and indicates the thickness of the boundary layer that determines the increase or decrease of intra-particle penetration. The regression lineq t (R 2 ) versus t 1 2 ⁄ slope gives the K diff parameter.
The results of methylene blue adsorption kinetics on CoFe 2 O 4 @SiO 2 /Cu 3 (BTC) 2 are summarized in Table 7. The results of methylene blue adsorption kinetics on CoFe 2 O 4 @SiO 2 /Cu 3 (BTC) 2 are summarized in Table 7. The results showed that following the data from the reaction kinetics is intraparticle diffusion <pseudofirst-order < pseudo-second-order. It was also found that the calculated q e values correspond to the experimental q e .
Hence, the pseudo-second-order kinetic model better shows the adsorption kinetics. The function of quadratic equations also suggests that the adsorption process depends on the adsorption concentration because the pseudo-second-order equation is generally based on the absorption capacity (see Figure 13). According to the values of h 0,2 , the initial adsorption rate increases at higher dye concentrations.
The values of R 2 related to intraparticle diffusion are calculated and shown in Table ( 13). Due to the high regression coefficient close to 0.91, this system can follow this model. As can be seen in Figure (

Conclusion
In this research, the CoFe 2 O 4 @SiO 2 core was modified by glutaric anhydride and efficiency from aqueous media so that they can be considered as suitable decolorizing reagents in aqueous systems. As the amount of CoFe 2 O 4 @SiO 2 /HKUST-1 increases, the removal rate rises due to the increase in active sites, but the higher the initial dye concentration, the lower the decolorization efficiency.