Preparation of metal – organic frameworks by microwave - assisted ball milling for the removal of CR from wastewater

: Metal – organic frameworks ( Sm - MOFs ) were pre pared using a microwave - assisted ball milling method with a water solution. The structure was characterized by Fourier transform infrared spectroscopy ( FTIR ) , X - ray di ﬀ raction ( XRD ) , and SEM, and the thermal stability of the Sm - MOFs was tested by Thermogravimetry ( TGA ) . The results showed that the Sm - MOF material exhibited a favorable e ﬀ ect on removing the organic dye Congo red ( CR ) . When the concentration of CR was 80 ppm, adding 50 mg of Sm - MOF material achieved an adsorption capacity of 396.8 mg · g − 1 . The experimental data were analyzed theoretically through dynamics, and the experi mental results were consistent with the second dynamics model, with correlation coe ﬃ cients ( R 2 ) all above 0.99. Comprehensive data analysis revealed that the Sm - MOF materials had great potential for future application in was tewater treatment.


Introduction
The rapid pace of industrial development and continual increase in urbanization have led to escalating threat of pollution to the water environment. Industrial waste can contain herbicides/pesticides, dyes, hexavalent chromium, and aromatics/organics, and these inorganics and organics should be removed from wastewater. The processes of printing and dyeing produce substantial wastewater with high chromaticity that can cause serious pollution to the environment. Methylene blue, Congo red (CR), and orange II are typical organic dyes that have been widely used in the production of textiles, cosmetics, papers, carpets, and plastics. If these dyes are discharged without treatment, they can cause serious environmental pollution problems, and their toxicity can harm human health. The global textile industry uses more than 10,000 tons of dye each year, and about 5,000 tons of dye and 3,600 tons of different dye-containing waste are discharged into rivers [1,2]. Most dyes in industrial wastewater are toxic, teratogenic, and carcinogenic. Therefore, effective treatment of wastewater from dyeing processes is imperative for the green and sustainable development of the textile industry.
At present, the adsorption method is a simple, direct, cheap, and highly efficient method [3][4][5] used in the treatment of wastewater, in addition to coagulation [6], photodegradation [7,8], filtration [9,10], among others. As potential effective adsorbents, porous materials have drawn more and more interest in recent years, one of which is the metal-organic frameworks (MOFs).
Microwave-assisted ball milling is based on a solidliquid ball milling approach with a ball milling machine placed in a microwave oven [45]. Microwave heating can speed up the chemical reaction due to the specific microwave effects and high-energy ball milling can effectively suppress the grain growth and refine crystalline particles. Water is used as the solvent. And the use of clean deionized water clear will have the least impact on the environment. Therefore, the method can not only decrease the reaction time from several days to a couple of hours or even minutes, but also avoid the use of a large amount of organic solvent. Thus, the production cost of the adsorbent can obviously decrease.
In this study, Sm(CH 3 COO) 4 was used as the metal ion and terephthalic acid was used as the organic ligand to prepare Sm-MOF materials under the condition of microwave-assisted ball milling. The prepared materials were then used to remove CR.

Experimental
The ligands p-phthalic acid and samarium(III) acetate hydrate were purchased from Aladdin Biological Technology Co., Ltd, Shanghai, China. CR was supplied by Beike New Material Technology Co. Ltd, Beijing, China.
The structure and morphology of the Sm-MOFs were tested by FTIR, XRD, field emission scanning electron microscopy, and TGA.
Briefly, terephthalic acid (0.026 mol), hydrated samarium acetate (0.017 mol), deionized water (700 mL), and stainless-steel balls (480 g) were placed in a Teflon jar. After sealing the gap with insulating tape, the microwave was turned on, and samples were observed at 10 min. The color of the solution gradually turned blue, accompanied by a significant sour taste, until the color no longer changed. The reactant was then filtered, cooled to room temperature, and extracted. Next, the solid part was placed into a beaker containing 100 mL of ethanol, stirred with a magnetic stirrer for 3 h, followed by extraction, filtering, and drying. The light red solids of Sm-MOFs were obtained in 81.57% yield based on hydrated samarium acetate.
To evaluate the removal capacity of the Sm-MOFs, CR was chosen as a model contaminant and dissolved in a 250 mL beaker at indoor temperature (∼15°C). The removal of CR was tested in a beaker containing 30, 40, 50, 80, and 100 ppm of CR solution with 40, 50, 80, and 100 mg Sm-MOFs placed under the action of natural light on a magnetic stirrer. Every 0.5 h, the absorbance at 496 nm [40] from the reaction suspension was monitored by UV-visible spectrophotometer. In this way, the removal rate of CR could be obtained at different time intervals. The quantity of CR removal was calculated according to the following equation: where C 0 and C e are the initial concentration and equilibrium concentration of the solution (ppm), respectively; V is the volume of the CR solution (L); and m is the amount of the Sm-MOFs (g).

Structural characterization
As shown in Figure 1a, the antisymmetric and symmetric stretching vibrations of carboxylate were 1,559 and 1,372 cm −1 , respectively. This was mainly due to the formation of large bonds after the delocalization of the carboxylic group, resulting in tendency of the two oxygen atoms to be equivalent, while the electron cloud tended to be average. Strong absorption peaks appeared at 1,610-1,550 and 1,420-1,300 cm −1 . The stretching vibrations of aromatic ring skeletons are 1,662 and 1,436 cm −1 , 1,112 cm −1 is the deformation vibration of C-H bond of aromatic hydrocarbon, and 762 cm −1 is the displacement characteristic peak of aromatic ring. XRD measurement of the Sm-MOFs is shown in Figure 1b, indicating that the material has no wide absorption peak, suggesting that the material has good crystallinity. As it can be seen from the SEM figure shown in Figure 1c and d the Sm-MOF particle morphology had different structures and good dispersion. This was mainly because the intermolecular interactions of organic ligands were weakened or even disappeared, and the deprotonation of organic ligands was enhanced, which promoted the growth of crystal in the aqueous solution.
The stability of the material is one of the key factors for its performance. The stability of the Sm-MOFs was tested by the thermogravimetric analyzer and the results are shown in Figure 2, which can be divided into three stages. The MOF material is porous, and the first stage showed that a small amount of solvent (6.3%) was still in the material. The second stage mainly involved the oxidation of unreacted metal ions which accounted for 18.7% of the loss [43,46] between 178°C and 349°C, while the third stage was due to the destruction of the whole frame structure. After reaching 610°C, the whole molecule was completely destroyed. The quality of this loss is stable at 452°C and then completed at 800°C. The residue was 33.9% of the initial mass. Most often the reaction of organic ligands and metal ions via a common synthesis procedure yields MOFs with more stable structures [47]. Figure 3 shows that the Brunauer-Emmett-Teller surface area was 7.89 m²·g −1 . The average particle size was 759.6 nm, the adsorption average pore width was 9.6 nm, Barret-Joyner-Halenda (BJH) adsorption average pore width was 16.2 nm, and BJH desorption average pore width was 18.5 nm, indicating the mesoporous nature of the material. As can be seen from Figure 3, the Sm-MOFs have four types of isotherms with H3 hysteresis rings, which represent the mesoporous properties of the sample [48,49].

Removal of organic dye
To evaluate the quality of Sm-MOF, CR was used as the target molecule to test Sm-MOF's ability to remove  organic pollutants. Sm-MOFs of different masses were assessed for removal of CR. The results showed that the CR removal rate increased with increasing mass of Sm-MOFs, while decreased with increasing concentrations of CR. The CR removal rate could reach 100% within 30 min for low concentration of CR. When the CR concentration was 80 ppm and the mass of Sm-MOFs was 50 mg, the adsorption capacity was the largest, reaching 396.8 mg·g −1 , but when the CR concentration was 100 ppm and the mass of Sm-MOFs was 50 mg, the removal rate was the worst, only 55.7%. The CR adsorption capacity was mainly influenced by the specific area and pore size of Sm-MOFs. First, because the pore size of Sm-MOFs is equivalent to the volume of CR molecules, the CR molecules flow into the pores of Sm-MOFs and adsorb onto the material. Second, Sm-MOFs and CR both have benzene rings and π electrons, which are adsorbed together by π-π bonds. Finally, Sm-MOFs have metal active sites that can adsorb CR; however, the large mass of Sm-MOFs can lead to coverage of active sites, thereby decreasing the adsorption capacity. In addition, there may also be electric charges in the solution. In the system, the presence of π-π interaction, hydrogen bonding, and electrostatic attraction can result in a highly efficient removal of CR by MOFs [50,51]. Therefore, Sm-MOFs have a good CR removal capacity that is due to both physical and chemical adsorption. To test the reusability of Sm-MOFs, the used Sm-MOFs were filtered and washed three times with water and dried. The consequence indicated that the removal rate of Sm-MOFs remained above 80% after four cycles (as shown in Figure 4), indicating a reasonable reusability. The CR adsorptions with other adsorbents are shown in Table 1.
A kinetic model was used to verify the consistency between the experiment and theory, and the formula used is presented below.  The pseudo-second-order kinetic model: where k is the kinetics reaction rate constant (min −1 ), q t (mg·g −1 ) is the adsorbing capacity of the Sm-MOFs at time t, and q e is the adsorbing capacity of the MOFs at equilibrium. It can be seen from Figure 5 and Table 2 that all R 2 of the second kinetic model were greater than 0.99, suggesting the theory that Sm-MOF's ability to remove CR is consistent with the results obtained in practice.
At a certain temperature, the Langmuir isotherm, Freundlich isotherm, and Temkin isotherm models were applied to describe the relationship between the adsorption capacity of the Sm-MOFs and the equilibrium concentration of CR. To determine the efficiency of the adsorbent to remove CR from the environment, the following equations were used:   (5) Figure 6 shows the Temkin isotherm of CR adsorption by MOFs, and Table 2 shows the parameters. The correlation coefficient was 0.982, suggesting that the Temkin isotherm was in good agreement with the experimental data. The q max of MOFs to CR was 304.9 mg·g −1 . We also tested the Freundlich isotherm model. As shown in Figure 6 and Table 3, it can be seen that the adsorption of CR by MOFs is a monolayer adsorption. There was no interaction between the adsorbed molecules, and the adsorption equilibrium process was dynamic equilibrium [49].
To further understand the CR adsorption mechanism, the thermodynamic of CR removal was studied. The thermodynamic equilibrium constant and Gibbs free energy change were evaluated by the following formulas:  where K 0 is the Langmuir adsorption constant (L·mol −1 ) and R is the gas constant. As it can be seen from Figure 7 that ΔG 0 , ΔH 0 , and ΔS 0 are negative, suggesting that the adsorption of CR by Sm-MOFs is spontaneous, and the adsorption process is exothermic. Therefore, the driving force for CR adsorption by Sm-MOFs is due to both enthalpy effect and entropy effect.

Conclusion
In summary, the Sm-MOFs were prepared by microwaveassisted ball milling, and the structure was tested using XRD, SEM, TG, and nitrogen adsorption and desorption. The Sm-MOFs material was tested for removal of CR. The experimental results were in accordance with the second dynamic model, and the correlation coefficients were all above 0.99. Langmuir, Freundlich, and Temkin isotherm analyses were performed to further investigate the adsorption of CR by Sm-MOFs, and the results showed that the correlation coefficients had good coincidence.
These results indicate that the theory is consistent with practice. Therefore, Sm-MOFs can be well applied for CR removal in the future.