Preparation of Fe-Mg MOFs and Its Application in Removal of RhB and MO

The world wide application of dyes in papermaking, fabric, lithography, leather and other industrial production, has attracted more attention, due to water pollution caused by these organic dyes. 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 because of its unique properties are convenient for operation, high removal efficiency, and low cost. In this study, we investigated Fe-Mg based metal organic framework, Fe-Mg MOFs which was directly synthesized by the hydrothermal method. The obtained materials were analyzed with XRD, FT-IR, TG-DTG, SEM etc. and used for the treatment of printing and dyeing wastewater. The results showed that it has good adsorption performance for cation dye rhodamine B (RhB) and anion dye methyl orange (MO) in a wide pH range. The Fe-Mg MOF even after the 4 th run, the Fe-Mg MOF catalyst still maintained nearly the initial catalytic activities. The kinetic studies revealed the adsorption process of the both contaminants obeys a pseudo-second order model. In addition, the equilibrium adsorption data of RhB and MO are in good agreement with Langmuir models. The maximum adsorption capacities are 694.44 and 236.97 mg/g at 308 K respectively. This work synthesizes a promising dual-functional adsorbent that can remove cationic and anionic dyes, which provide potential applications for actual wastewater treatment.


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.
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

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). N 2 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.

Preparation of the Absorbent
Fe-Mg bimetallic Organic Frameworks were synthesized according to the literature (Noor et al., 2019), 1 mmol Mg(NO 3 ) 2 ·6H 2 O, 1 mmol Fe(NO 3 ) 3 ·9H 2 O and 1 mmol terephthalic acid (H 2 BDC) 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.

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): with C 0 and C e (mg/L) being the initial and equilibrium concentration of RhB/ MO mixture respectively, q e 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. 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.

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    where, V 0 is the volume of single monolayer of absorbed gas, N a is Avogadro's number, M v 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 m 2 /g, which was examined by N 2 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.

TG and DTG Analysis
As shown in Figure 3, the MOFs material has poor thermal stability.

FT-IR Analysis
The FTIR spectrum of Figure 4 shows

Effects of Adsorbent Dosage and Initial Concentration
The adsorbent dosage has some influence in the removal efficiency of the dyes.  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.

Effects of Contact Time and PH
As seen in Figure 7(a) and Figure 7 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.

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)).
where k 1 (min −1 ), k 2 (g•mg −1 •min −1 ), k i (mg•g −1 •min −1/2 ) are the pseudo-first order, pseudo-second order and intraparticle diffusion model rate constant, q e and q t (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 k 1 and q e are calculated from the slope and intercept of plots of log(q e − q t ) versus t. The values of k 2 and q e can be obtained by plots of t/q t versus t (Figure 8(a), Figure 8(b)). k i and C i are the intra-particle diffusion rate constant and intercept of stage i, respectively. The values of the kinetic parameters and correlation coefficients (R 2 ) of the kinetic model are given in Table 2. The value of the correlation coefficient R 2 of the pseudo-second order model was very close to 1, and the values of q e and q e,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. Journal of Geoscience and Environment Protection where C e (mg·L −1 ) is the equilibrium concentration of the dye in solution, q e (mg·g −1 ) and q m (mg·g −1 ) are the equilibrium and theoretical maximum adsorption capacity, K L and K F are the Langmuir and Freundlich constants, n is the adsorption strength. If 2 < n < 10, the adsorption process is difficult to proceed. If O. S. Ojinna et al. 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 R 2 value of the Langmuir isotherm 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%.

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. that is easy to synthesize and is a promising and efficient adsorbent for removing anions dyes (RhB) and cationic (MO).