Characterization of an Iron-Copper Bimetallic Metal-Organic Framework for Adsorption of Methyl Orange in Aqueous Solution

Iron-based organic frame material MIL-53 (Fe) was synthesized by the hydrothermal method with Cu2+ incorporated to obtain bimetallic composite MIL-53 (Fe, Cu). The structure and morphology of the material were characterized by SEM, XRD, BET, FTIR, XPS, and zeta potential. The adsorption performance of MIL-53 (Fe, Cu) on methyl orange was tested under a variety of conditions, including the effects of pH and material dosage, by the static adsorption test. The results show that under the condition of pH = 7, a temperature of 30°C, and an adsorbent dosage of 20 mg, the removal rate of MIL-53 (Fe, Cu) for methyl orange can reach more than 96% within 4 h, and the maximum adsorption capacity after fitting by the thermodynamic model can reach 294.43 mg/g, showing the excellent adsorption performance of MIL-53 (Fe, Cu) on methyl orange. In addition, by exploring the adsorption mechanism of MIL-53 (Fe, Cu) on methyl orange, it is found that the adsorption of MIL-53 (Fe, Cu) on methyl orange depends on chemical adsorption, as evidenced by combining Fe3+ and Cu2+ in the material with methyl orange molecules to form complexes to achieve adsorption. While the specific surface area of the material had no obvious effect on adsorption, the effects of pH, temperature, and concentration were explored. At a pH of 6.5, greater adsorption occurred at higher temperatures, as determined by thermodynamic model fitting, as well as with higher initial methyl orange molecule concentration.


Introduction
Te rapid development of textile, leather, paper, food, printing, and paint industries has led to water pollution becoming a major threat to many living organisms [1].Dye wastewater produced by these industries increases the chroma of water, decreases irradiation of water, and poses a serious threat to aquatic life and potentially human health as well [2].Methyl orange, an azo dye commonly used in printing and textile industries, is a known carcinogen [3], thus making an efcient and economical means of removal from waste water attractive.Adsorption of methyl orange as a possible means to a simple, practical, and renewable [4] remediation method represents an important area of research.
Metal-organic framework materials (MOFs) act as a crystalline porous material with a repeating network structure composed of inorganic metal ions and organic ligands connected through self-assembly [5][6][7].MOFs are not only important as research materials in inorganic, organic, and crystal chemistry, but they also display excellent performance in drug delivery [8], gas storage [9], and catalysis due to their unique properties as adsorbents [10].Compared with ordinary adsorption materials, MOFs have the advantages of a porous structure, large specifc surface area, and facile adjustment of pore size [11][12][13].Tese features make the surface binding ability of MOFs robust, resulting in higher adsorption afnity and greater adsorption capacity.Furthermore, MOF surfaces have a high concentration of carboxylic functional groups, which make them ideal for contaminant removal.Pollutants are adsorbed on MOFs through both physical [14] and chemical adsorption [15] through the force of attraction between liquid molecules and the backbone atoms of MOFs as well as the interactions with surface functional groups.
Among the variety of MOFs, MIL-53 (Fe) has the advantages of good structural stability and high density of unsaturated metal nodes [14].Pollutants in water can be directly absorbed by electrostatically to organic ligands through π-π interactions and hydrogen bonding [4].Furthermore, the incorporation of Cu 2+ in MIL-53 (Fe) can reduce the electron density of the Fe center, reduce the band gap, accelerate the Fe 3+ and Fe 2+ cycles, and improve degradation efciency.Despite these benefts, there are few reports on the efectiveness of Cu 2+ doping in MIL-53 (Fe).
In this study, Cu 2+ was successfully incorporated in MIL-53(Fe) to make bimetallic material MIL-53 (Fe, Cu).MIL-53 (Fe, Cu) composites were analyzed by a variety of methods to determine the efects of Cu 2+ incorporation on morphological characteristics, crystal structure, and functional groups.In addition, the adsorption isotherm model and the adsorption kinetic model were used to determine the adsorption performance of the composite material and explore the mechanism of methyl orange adsorption, which, when combined with the characterization analysis, informs a new direction for adsorption-based remediation of methyl orange.

Experiment
2.1.Reagents and Instruments.Te main experimental reagents of this experiment are shown in Table 1.
Te main experimental instruments in this experiment are shown in Table 2.

Preparation of Materials
49 g), and DMF (40 mL) were added sequentially to the beaker.After sonication, the contents of each beaker were combined, mixed for 10 min, and transferred to a reactor at 150 °C for 17 h.Upon cooling to ambient temperature, the product was washed with deionized water and hot ethanol solution until the supernatant was clear so as to wash away the incomplete material and then collect by centrifugation.Te fnal product was dried under vacuum at 80 °C to obtain an orange product, MIL-53 (Fe, Cu).

Adsorption Experiment.
Te efects of the dosage and pH on methyl orange adsorption by MIL-53 (Fe, Cu) were investigated.

Infuence of the Dosage. Solutions were prepared as follows:
To each of 8 test tubes were added 20 ml of 200 mg/L methyl orange solution and 5, 10, 15, 20, 25, 30, 40, and 50 mg MIL-53 (Fe, Cu), respectively.pH was adjusted to 7 with NaOH and HCl and then placed in a thermostatic shaker at 30 °C and 150 r/min.After 4 h, samples were taken after fltration using a 0.45 μm membrane, the concentration of remaining methyl orange was measured at 464 nm using a UV/Vis spectrophotometer.
Te efect of pH.Each of 10 test tubes containing 20 ml of 200 mg/L methyl orange solution and 20 mg MIL-53 (Fe, Cu) were adjusted to a fnal pH of 2, 3, 4, 5, 6, 7, 8, 9, 10 and 11 and placed in a thermostatic shaker at 30 °C and 150 r/ min.After 4 h, samples were taken after fltration using a 0.45 μm membrane, and the remaining methyl orange concentration was measured at 464 nm using a UV/Vis spectrophotometer.
Te adsorption efciency was evaluated as the removal rate R (%) and the adsorption capacity as q t (mg/g), calculated from (1) and ( 2), respectively: where q t is the adsorption amount t min of adsorbent methyl orange, mg/g, C 0 is the initial concentration of methyl orange, mg/L, C t is the methyl orange concentration after t min , mg/L, m is the dosage of the adsorbent, g, and V is the volume of the solution and L.
2.4.Model Fitting.Adsorption isotherms were ftted to the Langmuir model and the Freundlich model with the following equations: where q e is the adsorption amount of methyl orange at adsorption equilibrium, mg/g, Q max is the maximum adsorption capacity, mg/g, K L is the Langmuir constant, L/mg, K F is the Freundlich constant, L/mg, 1/n is the constant of the afnity of the adsorption reaction, and C e is the concentration of methyl orange in the solution at adsorption equilibrium, mg/L.Tree groups of methyl orange solution with the concentration gradient (100-500 mg/L) were prepared.pH was adjusted to 6.5, and 20 mg of MIL-53 (Fe, Cu) was added to each group.Te groups were then placed into a constant temperature oscillation reactor at 25 °C, 35 °C, and 45 °C for 4 hours, respectively, to ensure that the material was fully adsorbed with methyl orange.Finally, the supernatant was used to measure the remaining methyl orange concentration at a wavelength of 464 nm using a UV/Vis spectrophotometer.Te adsorption capacity was calculated, and the Langmuir and Freundlich models were used to ft the experimental data.
Te adsorption process data were ftted to quasi-frstorder and quasi-second-order kinetic equations as follows: where q e is the adsorption amount of methyl orange at adsorption equilibrium, mg/g, q t is the adsorption amount of methyl orange at t min , mg/g, k 1 and k 2 are the quasi-frstorder and quasi-second-order model rate constants, g/ (mg•min), and t is the adsorption time, min.

Journal of Analytical Methods in Chemistry
To each of ten 20 mL methyl orange solutions at 200 mg/L was added 20 mg MIL-53 (Fe, Cu).Te solutions were then reacted in a constant temperature (25 °C) oscillation reactor for 5, 10, 15, 20, 30, 60, 90, 120, 180, 240, and 300 min.Te remaining methyl orange concentration in the supernatant was determined at a wavelength of 464 nm by using a UV/Vis spectrophotometer.Te results, shown in Figure 1, indicate that within 0-5 min, the adsorption capacity of the adsorbent increased rapidly from 5 to 300 min and the adsorption capacity of the adsorbent increased slowly, with the growth rate gradually decreasing before reaching equilibrium.2(a) and 2(b).Te sample size is relatively uniform, the surface is relatively smooth, and the shape presents a crystalline bar octahedral structure [15,16].As shown in Figures 2(c) and 2(d), the morphological characteristics of MIL-53 (Fe, Cu) are indicated after the adsorption of methyl orange.It can be seen that after adsorption of methyl orange, the material is no longer an angular crystalline rod polyhedral structure with multiple bonding, but its morphology has become more disordered and irregular.Tis indicates that methyl orange has been successfully adsorbed on the surface of the MOF crystals, causing signifcant morphological changes.showed difraction peaks at 2θ � 9.2 °, 12.59 °, 17.42 °, and 25.31 °, which ft very well with the previously reported MIL-53 (Fe) spectra, indicating the successful synthesis of MIL-53 (Fe) [17].Te diference in the intensity of the peaks and the position of the weak peaks may be attributed to the diferent reaction times of synthetic MIL-53 (Fe).

Results and Discussion
In addition, MIL-53 (Fe, Cu) shows two difraction peaks near 2θ � 8.8 °compared to MIL-53 (Fe) with three peaks originally at 2θ � 17.42 °, 18.91 °, and 22.1 °shifted to 16.57 °, 18.48 °, and 21.14 °, respectively.Tis may be due to the addition of Cu 2+ in precursors for MOF synthesis competing for coordination with Fe ions, thus promoting the growth of some crystal surfaces, resulting in a partial change of the crystal structure.At the same time, the peak shape of MIL-53 (Fe, Cu) is smoother with fewer peaks.Tis may be due to the disordered crystal structure caused by the doping of Cu 2+ during material synthesis.[18] However, the peak of MIL-53 (Fe, Cu) remained sharp, indicating MIL-53 (Fe, Cu) has a high degree of crystallinity [19].showing the shape of the type IV isotherm with a H 3 lag loop, indicating that MIL-53 (Fe, Cu) has slit pores [20][21][22].Te adsorption material displays higher absorption at higher relative pressure (P/P 0 ), indicating that MIL-53 (Fe, Cu) has a mesoporous structure and belongs to mesoporous material [20][21][22][23].At a relative pressure of 0-0.9, the hysteresis ring corresponds to the slot hole formed by the accumulation of MIL-53 (Fe, Cu) fake particles and the small mesoporous hole formed by the contact between the edge and surface.At a relative pressure of 0.9-1.0, the hysteresis ring corresponds to the larger mesoporous pores formed by the edge-surface        characteristic peaks appeared at 1116 and 1316 cm −1 , which indicated that methyl orange had been successfully adsorbed to MIL-53(Fe, Cu).Furthermore, the disappearance of the absorption peak of C�O at 1654 cm −1 and the Fe-O vibration peak at 532 cm −1 displayed a blue shift (+7 cm −1 ) after the addition of methyl orange, indicating that C�O and Fe-O are involved in the adsorption process, and fnally, the peaks at 800-1200 cm −1 became stronger, which may be caused by the complex of Fe 3+ , Cu 2+ , and methyl orange molecules in MIL-53 (Fe, Cu) [4,37].

N 2 Adsorption-Desorption.
3.1.5.XPS.Te element composition and chemical states of the MIL-53 (Fe, Cu) surface before and after adsorption were analyzed by XPS.As seen from the measurement curve in Figure 6(a), C, N, O, Fe, and Cu are present in MIL-53 (Fe, Cu), while the characteristic spectrum of S appears after adsorption of methyl orange.In addition, the percentage of N increases from 0.35% to 0.96%, consistent with successful adsorption of methyl orange on MIL-53 (Fe, Cu) [38,39].
As shown in Figure 6(b), the C 1 s spectrum of MIL-53 (Fe, Cu) corresponds to the C-C, C-N, and O-C�O bonds at 284.54 eV, 284.80 eV, and 288.59 eV, respectively [39,40].After the adsorption of methyl orange, the peaks of C-C and O-C�O shift to 284.54 eV and 288.54 eV, respectively, which is possibly due to the electronic interaction between the elements in the adsorption process [41].Te N 1 s spectrum in Figure 6(c) has only one peak at a binding energy of 400.17 eV, corresponding to the C-N bond [42].After adsorption of methyl orange, a new N�N bond was observed at a binding energy of 402.08 eV and the C-N peak was also shifted to 399.58 eV [15,17,38,43].At the same time, the content of the C-N bond also increased relative to before methyl orange adsorption.Te above observations may be explained by the introduction of dye molecules [44].Te O 1 s spectrum is decomposed into three located at 529.83 eV, 531.56 eV, and 533.18 eV, corresponding to C�O, C-O, and Fe-O bonds, respectively [17,45].During methyl orange adsorption, the C�O, C-O, and Fe-O peaks all shifted toward higher binding energies, indicating a decrease in the outer electron density of oxygen in the organic ligand [39].Moreover, the area of C�O and Fe-O binding peaks changed signifcantly after methyl orange adsorption, indicating that C�O and Fe-O functional groups are involved in the adsorption process.
3.1.6.Zeta Potential.Te zeta potentials of MIL-53 (Fe, Cu), recorded at diferent pH values, are shown in Figure 7.As seen in the fgure, the pH PZC (zero charge point) of the material is 5.3.When solution pH is greater than 5.3, the adsorbent surface is negatively charged, and when solution pH is less than 5.3, the adsorbent surface is positively charged.Methyl orange, an anionic azo dye, exists as a quinoid structure under acidic conditions and an azo structure under basic conditions.Regardless of whether the solution is acidic or alkaline, methyl orange is anionic and negatively charged in an aqueous solution.
Taken together, this means that the electrostatic attraction between negatively charged methyl orange and the cation on the adsorbent surface leads to an increased adsorption capacity when the pH of the solution is less than 5.3.8 shows that when all other factors are constant, the removal rate of methyl orange by MIL-53 (Fe, Cu) increases with an increasing dosage, while at the same time, the adsorption   and 146.22 mg/g, respectively.With the pH increasing from 3 to 11, the removal rate and adsorption capacity decreased to 84.40% and 126.74 mg/g, respectively.Te efect of pH on the adsorption performance is due to the electrostatic interaction between the adsorbent and dye molecules.Given the pH PZC of MIL-53 (Fe, Cu) of 5.3, when solution pH is greater than 5.3, the adsorbent is negatively charged, which is not conducive to the adsorption of negatively charged methyl orange.When solution pH is less than 5.3, the adsorbent is positively charged and the negatively charged methyl orange is attracted electrostatically.At higher pH values, hydroxide also hindered the adsorption of methyl orange on the surface of MIL-53 (Fe, Cu), resulting in a decreased removal rate with increasing pH.However, when solution pH was 2, the removal rate and adsorption capacity were only 72.60% and 109.03mg/g, respectively.Tis may be because methyl orange is largely in the form of R-SO 4 H when solution pH is 2. Te presence of high concentration of H + not only inhibits the formation of molecular aggregates but also weakens the interaction between molecules and adsorbent particles, thereby increasing the distance between molecules and adsorbent particles, resulting in a lower adsorption efciency.

Adsorption Isotherms.
Te ftted nonlinear adsorption isotherms and calculated parameters are shown in Figure 10 and Table 5, respectively.
As can be seen from Table 5, the correlation coefcient (R 2 ) of the Freundlich model is signifcantly higher than that of the Langmuir model under the three temperature gradients, indicating that the Freundlich model is more consistent with the experimental data than the Langmuir isotherm model.Meanwhile, the maximum adsorption capacity calculated by the Freundlich model is 294.43 mg/g, which is close to the experimentally determined value of 293.25 mg/g.Te good agreement of the experimental data with the Freundlich isotherm model indicates the nonuniform distribution of surface active sites and the nonsingle interaction among adsorbents.

Termodynamic Parameters.
In this study, the thermodynamic data were ftted, and the Gibbs free energy (ΔG), enthalpy (ΔH), and entropy changes (ΔS) of the adsorption reaction were calculated as follows: with ΔH and ΔS derived from the following equation: where q e is the amount of methyl orange adsorbed at adsorption equilibrium, mg/g, C e is the residual concentration of methyl orange at adsorption equilibrium, mg/L, K D is the temperature equilibrium constant, R is the ideal gas constant, generally 8.314 J•mol −1 •K −1 , and T is the Kelvin temperature, K.
According to the results in Figure 11, the correlation coefcient (R 2 ) of adsorption for methyl orange is 0.9853, showing a linear relationship.Values for ΔS and ΔH were calculated according to (7), from the intercept and the slope of the line, and ΔG, at the corresponding temperature, was calculated by (6).Te calculated results, from Table 6, are as follows: ΔG °�°− 2.32∼−1.00°<°0 , ΔS °>°0 , and ΔH °>°0 under the three temperature gradients in this study.Tis indicates that the adsorption reaction is a spontaneous, endothermic process with increased disorder in the solid-liquid system [49][50][51].Second, the ΔH of complexation is between 8 and 60 kJ/mol, while ΔH of MIL-53 (Fe, Cu) is 18.52 kJ/mol, which indicates that adsorption is driven by complexation, with the main route occurring by chemoadsorption rather than physical adsorption [13].

Adsorption Dynamics.
To understand the adsorption mechanism and possible rate-limiting steps, kinetic analysis was performed.Te ftted nonlinear adsorption kinetics and calculation parameters are shown in Figure 1 and Table 7, respectively.
Te results show that for MIL-53 (Fe, Cu), R 2 (0.998) of the quasi-second-order kinetic model is greater than that of the quasi-frst-order kinetic model (0.990).Tis indicates that the adsorption of methyl orange is more consistent with a pseudo-second-order kinetic model and that methyl orange is adsorbed on the surface of MIL-53 (Fe, Cu) mainly through chemical interaction [52,53].

Adsorption Mechanism.
Te efect of pH on methyl orange adsorption along with zeta potential characterization, which determined pH PZC of MIL-53 (Fe, Cu) to be 5.3, is consistent with a positively charged adsorbent surface interacting electrostatically with anionic methyl orange in the pH range of 2-5.3.Te adsorption process is greatly afected by the adsorbent and the adsorbate surface potential, supporting electrostatic interaction as one of the adsorption mechanisms.Furthermore, the results of FTIR and XPS spectra before and after adsorption show that C�O and Fe-O in MIL-53 (Fe, Cu) are involved in the adsorption process and that the protonation/deprotonation of oxygen containing functional groups is implicated in the adsorption of methyl orange.Fe 3+ and Cu 2+ in the adsorbent material combine with methyl orange to form a complex.BET characterization analysis showed that although the incorporation of Cu 2+ resulted in a decrease in a specifc surface area, making it lower than other types of MIL-53.However, the incorporation of Cu 2+ improves the electron  Journal of Analytical Methods in Chemistry utilization efciency and codecomposition of MO from H 2 O 2 and indirectly increases the adsorption capacity of MO.Second, from the characterization analysis of SEM and FITR, it can be seen that the incorporation of Cu 2+ does not change the structure of MIL-53 (Fe) and that the fexible structure of MIL-53 (Fe) is conducive to the adsorption of methyl orange.Finally, the adsorption data are consistent with the Freundlich equation, as well as the pseudo-secondorder kinetic model.Tis lends support to adsorption which occurs mainly through chemisorption means on multilayer heterogeneous surfaces, and physical adsorption is less.Terefore, although the specifc surface area of MIL-53 (Fe, Cu) obtained in this study is low, it still has good adsorption performance for methyl orange.

Reusability of Sorbents
A good sorbent should be able to be recycled in practical applications.After the adsorption experiment, MIL-53 (Fe, Cu) after adsorbing methyl orange was soaked in absolute ethanol to elute methyl orange and then collected by centrifugation and dried in a vacuum drying oven.Te obtained product was subjected to the adsorption experiment again, the temperature was 30 °C, the initial concentration of methyl orange was 200 mg/L, the pH was adjusted to 3, the adsorbent dosage was 20 mg, the adsorption time was 4 h, the adsorption-desorption cycle experiment was performed 5 times, and the experiment was repeated three times.Figure 12 shows the removal performance of MIL-53 (Fe, Cu) on    Journal of Analytical Methods in Chemistry methyl orange after 5 cycles.Te results showed that the removal rate of MIL-53 (Fe, Cu) was more than 70% after 5 adsorption-desorption cycles, indicating that MIL-53 (Fe, Cu) had repeatability and broad application prospects in the removal of methyl orange wastewater.

Conclusion
Cu 2+ incorporation partially changed the crystal structure of MIL-53 (Fe) but had no obvious efect on the functional groups of MIL-53 (Fe).Newly prepared MIL-53 (Fe, Cu) is a mesoporous material with slit pores.
Although the specifc surface area and pore volume of MIL-53 (Fe, Cu) are not high, it has a high adsorption capacity for methyl orange due to its fexible structure, which may be due to the small physical adsorption efect of the material on methyl orange, and adsorption is mainly chemical adsorption.C�O and Fe-O in MIL-53 (Fe, Cu) participated in the adsorption process, and Fe 3+ and Cu 2+ in MIL-53 (Fe, Cu) combined with methyl orange molecules to form a complex, leading to adsorption.Te pH PZC of MIL-53 (Fe, Cu) is 5.3.When pH is less than 5.3, the adsorption capacity is increased by electrostatic attraction of negatively charged methyl orange molecules.Adsorption kinetic models indicate that within 0-5 min, the adsorption capacity of the adsorbent increased rapidly from 5 to 300 min and the adsorption capacity of the adsorbent increased slowly, with the growth rate gradually decreasing before reaching equilibrium.Furthermore, the adsorption process conforms to the pseudo-second-order kinetic model (R 2 ≥ 0.995) and the Freundlich equation (R 2 ≥ 0.99), indicating that adsorption is mainly chemical adsorption on a multilayer heterogeneous surface.Termodynamic parameter analysis shows that adsorption is a spontaneous, endothermic, and entropy-increasing process, and increasing the temperature is benefcial to the adsorption of methyl orange.

Figure 4 (
a) depicts the N 2 adsorption-desorption isotherm of MIL-53 (Fe, Cu), Figure 4(b) depicts the pore size distribution curve of MIL-53 (Fe, Cu).As shown in the fgure, the pore diameter distribution of MIL-53 (Fe, Cu) is roughly distributed between 3.23 and 10.25 nm, indicating that this material has a highly uniform pore structure [24].Te pore structure parameters of MIL-53 (Fe, Cu) in Pseudo-first-order dynamics Pseudo-second-order dynamics

Figure 1 :
Figure 1: Fitting curves of the pseudo-frst-order and pseudo-second-order dynamic models.

Figure 6 :
Figure 6: XPS spectra of MIL-53(Fe, Cu) before and after adsorption of methyl orange.(a) Full spectrum map before and after adsorption.(b) C before and after adsorption.(c) N before adsorption.(d) O after adsorption.(e) Fe before and after adsorption.(f) Cu before and after adsorption.

Figure 10 :
Figure 10: Sorption isotherm ftting curve: (a) Fitting curve of the Langmuir model.(b) Fitting curve of the Freundlich model.

Table 1 :
List of the main raw materials and chemical reagents.

Table 2 :
List of laboratory equipment.

Table 3
show that the specifc surface area of newly prepared MIL-53 (Fe, Cu) is only 12.34 m 2 /g, while the maximum adsorption amount of the material at 45 °C was 294.43 mg/g.Tis indicates that the specifc surface area has very little efect on the adsorption of methyl orange, suggesting instead absorbance may be dominated by chemical adsorption.Te comparison of diferent types of MIL-53 and MIL-53 (Fe, Cu) surface area is shown in Table 4. Te specifc surface area of MIL-53 (Fe, Cu) prepared in this study is lower than that of other types of MIL-53, which may

Table 4 :
Compare surface area of diferent samples.

Table 5 :
Parameters of the isothermal ftting models.

Table 6 :
Termodynamic parameters of the methyl orange adsorption process.

Table 7 :
Dynamic model parameter values.