Preparation of Fe-Mg MOFs and Its Application in Removal of RhB and MO ()
1. Introduction
The water pollution caused by organic dyes has attracted more attention, because dyes are widely applied in papermaking, fabric, lithography, leather and other industrial production (de Sá, Cunha, & Nunes, 2013). The water is toxic with complex composite, deep color, and concentrated refractory organic matters. And it will bring about a threat to agriculture and food chain, and consequently to human health which are carcinogenic, teratogenic, and genetic mutation (Shan et al., 2015). Wastewater treatment has become a major problem in global water treatment.
In recent years, researchers have adopted biodegradation (Shi et al., 2021; Srinivasan & Sadasivam, 2021), chemical oxidation (Liu, Ohko, Zhang, Yang, & Zhang, 2010), membrane treatment technology (Jin et al., 2021), coagulation (Hussein & Jasim, 2021; Mcyotto et al., 2021), photocatalysis (Le, Akhtar, Park, Lee, & Yang, 2012; Mahlambi et al., 2013) and other technologies in the treatment of printing and dyeing wastewater. By comparison, it has been found that the physical adsorption method has the following merits, like convenient operation, high removal efficiency, and low cost. Metal-organic frameworks (MOFs) which are a physical adsorption method of wastewater treatment are a kind of special three-dimensional crystal-like constituents built by multipurpose ligands and metallic ion classes, showing an advantage in removal of pollutants from solutions (Zhang et al., 2020). Single metal MOFs, like Ni-MOFs (Zhao et al., 2017), Zn-MOFs (Yang et al., 2018), Cu-MOFs (Hu et al., 2014), Fe-MOFs (Wei, Chen, Liang, & Zhao, 2018), Zr-MOFs (Chen, Feng, & Wei, 2019), Al-MOFs (Tehrani & Zare-Dorabei, 2016) as well as double metal MOFs, like Zn-Co MOFs (Noor, Raffi, Iqbal, Yaqoob, & Zaman, 2019), Ni-Cu MOFs (Hu et al., 2014) etc., have been extensively applied in elimination of dyes from wastewater. MOFs have received great attention owing to their great excellent properties, such as versatile porous structures and numerous potential applications. In general, the elastic and extremely porous structures of MOFs facilitate the diffusion of guest ions or molecules easily into the bulk structure. Also, both the size and shape of the formed pores help in increasing the selectivity of adsorbing definite ions or molecules. These distinct advantages make MOFs ideal sorbents in dye absorption. MOFs have been recognized as promising adsorbents for bulky dyes removal from wastewater due to their tunable organic functionalities and diverse metal compositions. MOFs have an important advantage in that various frameworks can be formed by the participation of several metal cations. MOFs can be synthesized with specific properties to improve their performances in reaching the desired targets by deliberately and systematically setting their functionalities and structures; for example, the surface area, pore size and/or shape can be controlled by varying the connectivity of the cations and the type of the organic ligands. MOFs have been used in different potential applications as drug delivery, catalytic reactions, sensing and gas adsorption/separation. In this paper, Fe-Mg MOFs was synthesized and used to remove organic pollutants rhodamine B (RhB) and methyl orange (MO) from wastewater after calcination and it showed an excellent performance. Furthermore, the effects of numerous elements such as adsorbent dosage, initial concentrations, solution pH, and contact time were evaluated. The adsorption kinetics and adsorption isotherms were also analyzed using theoretical models. In addition, the reusability of the adsorbent was tested.
2. Experimental
2.1. Materials
Mg(NO3)2∙6H2O, Fe(NO3)3∙9H2O, terephthalic acid and dimethylformamide (DMF) are purchased from Sigma Aldrich.
2.2. Apparatus
X-ray diffraction analysis (XRD) was performed on a DX-2700 X-ray diffractometer (Rigaku Corporation), with the Cu Kα radiation (λ = 0.15418 nm). N2 adsorption-desorption isotherms (BET) were measured on a QDS-30 physical adsorption instrument (Kanta, USA) at liquid nitrogen temperature (77 K). The specific surface area was calculated by BET (Brunauer-Emmett-Teller) method, and the pore volume and pore diameter were calculated by the BJH (Barret-Joyner-Halenda) method. Scanning electron microscopy (SEM) was performed on a Quanta 400 FEG field emission electron microscope with an accelerating voltage of 20 kV, and EDS-mapping was applied in analyzing the element distribution on the catalyst surface.
2.3. Preparation of the Absorbent
Fe-Mg bimetallic Organic Frameworks were synthesized according to the literature (Noor et al., 2019), 1 mmol Mg(NO3)2·6H2O, 1 mmol Fe(NO3)3·9H2O and 1 mmol terephthalic acid (H2BDC) were dissolved in 10 mL DMF solution at a continuous shaking. The mix was moved to a Teflon-lined autoclave and blended homogeneously and heated at 120˚C for 8 hours. Afterwards, cooling to a room temperature was done, the solid was recovered by centrifugation, cleaned with DMF for many times, and desiccated at 120˚C. The obtained solid was named as Fe-Mg MOFs.
2.4. Adsorption Experiments
In a typical run, certain quantity of Fe-Mg MOFs (50, 100, 150, 200 mg) was added into 50 mL of RhB/MO dye solution of different concentration and stirred for a certain time at room temperature (30, 60, 90, 120, 150 minutes…). Then, the suspensions were separated by centrifugation for analysis. The solution was tested by UV-vis spectrophotometer at 552 nm (RhB) and 464 nm (MO), respectively. The removal efficiency and adsorption capacity at equilibrium were computed with the formulas (1) and (2):
(1)
(2)
with C0 and Ce(mg/L) being the initial and equilibrium concentration of RhB/ MO mixture respectively, qe being the equilibrium adsorption capacity (mg/g), m (g) being the mass of the added adsorbents, and V (L) is the volume of the RhB/MO mixture.
3. Results and Discussion
3.1. Characterization
3.1.1. SEM
Figure 1 shows the SEM and outlook of the sample. From the inlet picture, it can be seen that the sample was brown powder. As shown in SEM diagram, the powder is composed by quadrangular with the width and length of 0.35 μm and 2.90 μm, respectively. It is interesting that the two sides of the quadrangular are truncated and become rectangular pyramid.
3.1.2. XRD and BET Analysis
The Brunauer-Emmett-Teller (BET) method is commonly applied to calculate the specific surface area on the basis of nitrogen desorption isotherm measurements at 77 k (8 - 10). Usually, data in the relative pressure range from 0.05 to 0.3 are used. As shown in Figure 2(a), compared with the simulated XRD as mentioned in literature (Noor et al., 2019), Fe-Mg MOFs was successfully prepared. Figures 2(b)-(d) show the N2-adsorption-desorption isotherms, pore size distribution curve and Horvath-Kawazoe Differential Pore Volume Plot of the Fe-Mg MOFs, which indicate the catalyst is composed of mesoporous and macroporous structure. In a typical BET analysis, NM surface is determined from the volume of N2 gas adsorbed on the NMs. N2 gas is assumed to have access to the entire NM surface. The surface area measurements are based on adsorption of gas molecules in infinite layers with no interlayer interaction. Under these conditions, the basic Langmuir theory can be applied to each layer to derive the BET surface area for NMs (SBET).
(3)
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Figure 1. Outlook and SEM of Fe-Mg MOFs.
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Figure 2. XRD pattern (a), N2-adsorption-desorption isotherms (b), pore size distribution curve (c) and Horvath-Kawazoe Differential Pore Volume Plot (d) of the Fe-Mg MOFs.
where, V0 is the volume of single monolayer of absorbed gas, Na is Avogadro’s number, Mv is molar volume of gas adsorbate and s is the surface area of a single gas molecule adsorbed on the solid. Table 1 lists the texture parameters of the Fe-Mg MOFs adsorbent. The figure shows that the specific surface area is 379.18 m2/g, which was examined by N2 adsorption/desorption analysis at −196˚C, the HK Median pore width is about 0.58 nm, which further confirms that the Fe-Mg MOFs has both mesopores and micropores.
3.1.3. TG and DTG Analysis
As shown in Figure 3, the MOFs material has poor thermal stability. The weight loss of 3.45% appearing at 26˚C - 118˚C is assigned to desorption of the adsorptive water on the surface and MOFs inner structures. 7.18% of weight loss at 118˚C - 236˚C is attributed to desorption of solvent DMF. From 236˚C the ligand of terephthalic acid in the MOFs structure begins to decompose to carbon species, it reaches a weigh loss of 14.11% until to 350˚C. The last weight loss of 42.82% takes place in 350˚C - 440˚C, which can be specified to the combustion of carbon.
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Table 1. Texture parameters of the Fe-Mg MOFs.
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Figure 3. TG and DTG curves of Mg-Fe MOFs adsorbent.
3.1.4. FT-IR Analysis
The FTIR spectrum of Figure 4 shows a primary specific top at about 3397 cm−1 attributed to the H-O-H stretching vibrations. The points that are observed at 1579 cm−1 for Fe-Mg MOFs could be allotted for the strong stretching shaking of C=O arising from -COOH, which can be recognized as the content of terephthalic acid in the MOF (Gu et al., 2019). The point at about 1386 cm−1 is because of the C-O stretching vibration. The three points at 1016, 1097 and 1153 cm−1 are specified for the twisting shaking of the hydroxyl group from M-OH (Zhang, Yao, Xiang, & Chen, 2014) and the replacement of the aromatic ring is appeared at 825 cm−1. These outcomes propose that the structure of Fe-Mg-MOF comprises of the elementary terephthalic acid skeleton and the carboxyl groups of terephthalic acid are deprotonated. Furthermore, the points at 759, 823 and 887 cm−1 were attributed to metal-oxygen vibrations (Li, Deng, Yu, Huang, & Lim, 2010).
3.1.5. EDX Analysis from SEM
The EDX analysis of SEM (Figure 5) shows that Fe, Mg, C and O elements were detected in the absorbent, confirming that terephthalic acid existing in the MOFs structure. According to the relative molar ratio of the elements, the ratio Mg: Fe is about 0.1:1, which could be further adjusted to enhance the stability of the MOFs structure.
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Figure 4. FT-IR spectrum of Mg-Fe MOFs adsorbent
3.2. Adsorptive Property
3.2.1. Effects of Adsorbent Dosage and Initial Concentration
The adsorbent dosage has some influence in the removal efficiency of the dyes. The experiments of water regeneration were done in a 50 mL of 50 mg/L dye solution with diverse amount of absorbent Mg-Fe MOFs (50 mg, 100 mg, and 150 mg). As shown in Figure 6(a), the results show that as the dosage of Mg-Fe MOFs (50 mg - 150 mg) increases, the removal efficiency of RhB and MO is almost stable, maintaining higher than 99.85%. The increase in removal efficiency is related to the sufficient adsorption sites of Mg-Fe MOFs. That might because that the initial concentration is not very high that the minimum dosage (50 mg) is enough for the dye. The changes caused by the initial concentration of RhB and MO solution (10 to 125 mg/L) on the removal efficiency was studied with 100 mg Mg-Fe MOFs as adsorbent (shown in Figure 6(b)). The removal efficiencies of RhB and MO dyes both increase firstly with the rise of the initial
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Figure 6. The effect of absorbent dosage and initial concentation on removal efficiency of RhB and MO.
concentration from 10 to 50 mg/L, but it shows much difference from the initial concentration of 75 mg/L. The removal efficiency for MO slightly goes up with the initial concentration increasing, while that for RhB dyes begin to decline when RhB initial concentration surpasses 50 mg/L, which suggests that it becomes more than its adsorption saturation value and cannot adsorb anymore. Otherwise the removal efficiency for RhB is still above 99.6%. The results also manifest that this absorbent shows good performance for both of MO and RhB, but even better for removal MO than RhB.
3.2.2. Effects of Contact Time and PH
As seen in Figure 7(a) and Figure 7(b), the figures show the consequence of contact time on the removal efficiency for MO (a) and RhB (b) with the initial concentrations of 125 mg/L, different amount of Mg-Fe MOFs were considered (50 mg, 100 mg, 150 mg). It is found that the equilibrium can be reached at about 50 minute. It suggests that Mg-Fe MOFs can absorb the dyes very quickly.
pH is one of the important factors affecting adsorption efficiency. In the adsorption experiments, the pH value was regulated with 0.10 mol/L HCL or NaOH solution. The test was conducted with pH of 1, 3, 5, 7, 9, 11 and 13. From Figure 7(c) it can be observed that pH has less influence on the removal efficiency, which is relatively stable, and has no major change within the pH test scope for both RhB and MO. Through research, it is proved that Mg-Fe MOFs does not need to adjust the pH value when removing RhB or MO, which can simplify the operation steps and be economical for application. It proves that Mg-Fe MOFs is expected to be a simple and economical adsorbent for removing RhB and MO.
3.2.3. Effect of Contact Time on Adsorption Kinetics
The adsorption kinetics of Mg-Fe MOFs for RhB and MO were studied by pseudo-first order kinetic model (Equation (3)), pseudo-second order kinetic model (Equation (4)), intra-particle diffusion model (Equation (5)).
(4)
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Figure 7. The effect of contact time (a for MO, b for RhB) and pH (c) on removal efficiency of MO and RhB dye with the initial concentration of 125 mg/L.
(5)
(6)
where k1 (min−1), k2 (g∙mg−1∙min−1), ki (mg∙g−1∙min−1/2) are the pseudo-first order, pseudo-second order and intraparticle diffusion model rate constant, qe and qt (g∙mg−1) are the adsorption capacity of the adsorbent for a certain dye molecular at equilibrium and at time t (min), respectively. The values of k1 and qe are calculated from the slope and intercept of plots of log(qe − qt) versus t. The values of k2 and qe can be obtained by plots of t/qt versus t (Figure 8(a), Figure 8(b)). ki and Ci are the intra-particle diffusion rate constant and intercept of stage i, respectively. The values of the kinetic parameters and correlation coefficients (R2) of the kinetic model are given in Table 2. The value of the correlation coefficient R2 of the pseudo-second order model was very close to 1, and the values of qe and qe,cal of theoretical adsorption capacity were basically consistent with the experimental adsorption capacity. This result shows that the adsorption process of RhB and MO follows the pseudo-second order model. Otherwise the adsorption capacity of MO was much higher than that of RhB. This indicates that the removal mechanism of RhB is the same with that for MO.
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Figure 8. Pseudo-Second order kinetic plots adsorption of MO (a), and RhB (b), on Mg-Fe MOF.
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Table 2. Comparison of pseudo-first order, pseudo-second order adsorption constants for dyes adsorption on Mg-Fe MOF.
3.2.4. Adsorption Isotherms
Adsorption isotherm studies related to adsorbent capacity and solute surface interaction. The Langmuir isotherm model that assumes monolayer sorption and Freundlich isotherm model that assumes a heterogeneous sorption are generally used isotherms to explain the solid-liquid adsorption system. The Langmuir (see Equation 7) and Freundlich (see Equation 8) isotherm models are applied in analyzing the experimental equilibrium adsorption results. The equations are as follows:
(7)
(8)
where Ce (mg·L−1) is the equilibrium concentration of the dye in solution, qe (mg·g−1) and qm (mg·g−1) are the equilibrium and theoretical maximum adsorption capacity, KL and KF are the Langmuir and Freundlich constants, n is the adsorption strength. If 2 < n < 10, the adsorption process is difficult to proceed. If n < 2, the adsorption is easy to proceed. The calculated parameters are listed in Table 3.
The adsorption capacity for RhB increased with the temperature increasing (shown in Figure 9(a)), which indicates that the adsorption process for RhB with Mg-Fe MOF is endothermic. The average R2 value of the Langmuir isotherm
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Table 3. Parameters of Langmuir and Freundlich isotherms for dyes adsorption on Mg-Fe MOF.
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Figure 9. Fitting curve of Langmuir (a, c) and Freundlich (b, d) adsorption isotherms of RhB/MO on Mg-Fe MOF.
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Figure 10. Reusability of Fe-Mg (0.1:1) MOFs for RhB and MO adsorption.
model is the closest to 1, indicating that Langmuir is most suitable for experimental data. According to the calculation, the fitted correlation coefficients was 0.9999 (318 K), which are larger than the fitting result of the Freundlich adsorption isotherm model (0.9986). The maximum adsorption capacity for RhB was 694.97 mg/g (318 K).
For MO, the Langmuir adsorption isotherm model was also better to describe the adsorption process with Mg-Fe MOF better than Freundlich adsorption isotherm model, and maximum adsorption capacity was 236.97 mg/g (318 K).
The experiment of regeneration and multiple cycles of Mg-Fe MOF are shown in Figure 10. The used Mg-Fe MOF was washed with ethanol after each use, which was followed by centrifugal separation and drying at 70˚C to obtain the regenerated adsorbent. After recovery, the adsorption experiment was repeated under the same conditions. The results showed that the adsorption of RhB/MO by Mg-Fe MOF can be used 4 times. The results showed that the adsorption of MO by Mg-Fe MOF still has a high adsorption capacity after four regenerations. After four cycles, the Removal efficiency for MO could maintain 99.716%. But after four regenerations, the adsorption capacity of Mg-Fe MOF for RhB was significantly reduced. After four cycles, the adsorption efficiency for RhB could maintain 99.46%.
4. Conclusion
Mg-Fe MOF was directly synthesized by the hydrothermal method, and the effects of adsorbent dosage, initial dye concentration, pH, and contact time on the adsorption of dye solution were systematically studied. It is found that the pH value has little effect on the adsorption. The adsorption of RhB and MO conforms to the pseudo-second-order kinetic model. The adsorption isotherm model of Fe-Mg MOF for RhB and MO removal obeyed Langmuir and Freundlich adsorption isotherm model, respectively. Fe-Mg MOF attracted RhB by an electrostatic force.
The large steric hindrance of RhB, did not prevent it from entering into the layers that were why the maximum adsorption capacity of Fe-Mg MOF for RhB is far higher than that for MO despite MO having linear molecule which makes it easy to get into the layers or pores of Fe-Mg MOF. From the perspective of the adsorption performance of the adsorbent, Fe-Mg MOF with a layered structure that is easy to synthesize and is a promising and efficient adsorbent for removing anions dyes (RhB) and cationic (MO).