Efficient removal of Sb(III) from aqueous solution using recyclable magnetic metal-organic frameworks Fe3O4@MIL-100 and Fe3O4@MIL-101

doi:10.21203/rs.3.rs-4800741/v1

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Efficient removal of Sb(III) from aqueous solution using recyclable magnetic metal-organic frameworks Fe₃O₄@MIL-100 and Fe₃O₄@MIL-101

https://doi.org/10.21203/rs.3.rs-4800741/v1

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Antimony (Sb) pollution is becoming increasingly serious. The pollution of water bodies containing antimony poses a great threat to the ecological environment and human health. In this study, two types of magnetic metal-organic frameworks(MOFs), namely Fe₃O₄@MIL-100(Fe) and Fe₃O₄@MIL-101(Fe), were prepared to investigate the removal effect on antimony (Sb). The influence of factors such as pH value and initial concentration on the adsorption effect was studied in detail. Compared to Fe₃O₄@MIL-100(Fe), the study found that the adsorption reaction rate of Fe₃O₄@MIL-101(Fe) was much higher than the former, reaching equilibrium after 10 minutes of addition. The fitting results conformed to the chemical adsorption behavior of the pseudo-second-order kinetic model. Mechanism characteristic analysis revealed that the Fe-O-Sb formed by Fe ions in the metal clusters and Sb (III) was the main adsorption mechanism. In addition, vibrating sample magnetometer (VSM) and adsorption tests showed that the material still possesses magnetism after adsorption, and the adsorption capacity remains basically unchanged after three cycles. These results indicated that the two magnetic Fe-based MOFs were effective and recyclable adsorbents, providing a new approach to the design of regenerative adsorbent materials.

Metal-organic framework

Fe3O4@MIL-100

Fe3O4@MIL-101

Antimony

Adsorption

Recyclable adsorbent

Chemisorption

The amount of antimony used in industrial processing in various countries has been increasing in recent years, so the widespread use of antimony has brought about endless pollution problems. It not only brings great damage to the environment but also threatens human health (Ungureanu et al., 2015). Antimony is a quasi-metal, which is metallic and non-metallic, belongs to the VA group in the periodic table of elements. The inorganic state of antimony in the environment generally includes pentavalent antimony and trivalent antimony, the form Sb(Ⅴ) is present in water in an aerobic environment (Wilson et al., 2010). Sb (III) usually exists in water as Sb(OH)₃ when pH = 2–10. In addition, compared with Sb(III) and Sb(Ⅴ), the former is more toxic (Filella et al., 2002). Therefore, the removal of Sb(Ⅲ) from wastewater is of great significance. The sources of antimony pollution include two kinds of pollution caused by nature and human activities. Pollution caused by human activities includes the mining of minerals, use of pesticides, burning of fuels, and use of plastics, rubber and other products as flame retardants in research and development. These activities caused serious antimony pollution of soil, air and water, which directly led to the destruction of the ecological environment and thus affected human survival. Therefore, in the 1970s, antimony pollution attracted the attention of the United States Environmental Protection Agency (Callahan et al., 1979), and antimony was included in the list of priority pollutants. Later, the United States Environmental Protection Agency issued relevant regulations, stipulating that the limit concentration of antimony is 6 µg L^− 1. In 1998, under the strong advocacy of the European Community (EC), new regulations on antimony were issued, requiring that the concentration of Sb in drinking water should be less than 5µg L^− 1 (Directive, 1976). The concentration limits set by China for antimony in drinking water were in line with those set by the European Union (He et al., 2012).

At present, there are various technologies to deal with antimony pollution in water, including adsorption, electrochemical, coagulation/flocculation, membrane technology and so on (Guo et al., 2021; Singh et al., 2015). Among them, the adsorption technology has the characteristics of high treatment efficiency, low operation technical requirements and recyclability (Wan et al., 2021). Different types of adsorbents determine the removal efficiency of heavy metals in wastewater to a certain extent. In previous studies on antimony adsorption, graphene, biochar and some traditional metal oxides have been widely used (Long et al., 2020). However, due to their small surface area, these materials have relatively low adsorption capacity and low recycling value. Some inorganic adsorbents have high adsorption capacity, but they can only maintain excellent adsorption performance under acidic conditions, which often brings the risk of secondary pollution. Metal-organic frameworks (MOFs) are a type of porous coordination polymer, made up of polyhedral or bimetallic organic ligands and clusters of ions or transition metals (Wang et al., 2023). In addition to being an adsorbent, MOF is also used in other fields, such as gas storage (Gutov et al., 2014), catalysis (Konnerth et al., 2020), sensing (Diamantis et al., 2018) and so on (Lorzing et al., 2017). As adsorbent, MOFs have adjustable pore size, gap and shape. So their structure can be flexibly adjusted according to the properties of heavy metals adsorbed to achieve better adsorption effect (Zhu et al., 2015). In addition, MOFs have advantages over other materials in terms of cost and structure (Zhang et al., 2021). For one, the synthesis cost of MOFs is low and the synthesis method is simple, so it is conducive to commercial use after synthesis. In addition, MOFs have a much higher specific surface area than conventional materials. (Rubio-Martinez et al., 2017). These advantages indicate that MOFs, as a frontier adsorbent, have incomparable advantages and potential (Wang et al., 2022).

The research on the adsorption of antimony by MOF materials has increased year by year. In 2017, Li et al. (Li et al., 2017) used seven aperture Zr-MOFs to adsorb Sb. The results showed that NU-1000 has a good adsorption effect on Sb. The adsorption capacity of Sb(V) reached 287.88 mg g^− 1. In 2020, Cheng et al. studied and synthesized Fe-MIL-88B, and used it to adsorb Sb (V) and Sb (Ⅲ), with the maximum adsorption capacity reaching 318.9 mg g^− 1 and 566.1 mg g^− 1 respectively (Cheng et al., 2020). These kinds of MOF materials have been successfully used to remove antimony from wastewater, which shows that iron-based MOF materials have great potential in antimony adsorption. The bimetallic MOF (ZrxFe(1-x)-MOF-808) was successfully created by Sun et al. in 2021. Its adsorption capacities for antimonate are 524 mg g^− 1 and 310 mg g^− 1, respectively.(Sun et al., 2022). As with other adsorbents, recovery of nonmagnetic MOF materials after undergoing adsorption is quite difficult. Therefore many scholars explore synthesizing of magnetic MOF materials for the removal of Sb(Ⅲ). In 2019, Qi et al. synthesized magnetic MOF microspheres Fe₃O₄@TA@UiO-66 can remove Sb(Ⅲ) and As(Ⅲ) from water, and it can be quickly separated from water in two minutes (Qi et al., 2019). Zhu et al. prepared recyclable materials in proportion to remove Sb (Ⅲ) in 2021 using magnetic MOF (UIO-66-2COOH@Fe₃O₄) and biochar made from mushroom waste (Zhu et al., 2021). In 2022, Wang et al. successfully synthesized MnFe₂O₄@SiO₂ @MIL-101(Cr)-NH₂(MSM) to study its adsorption properties for Sb(Ⅲ) in water and test its recyclability (Wang et al., 2022). However, on the one hand, there are few types of research on iron-based MOF to remove Sb. On the other hand, there are relatively few studies on the recovery of magnetic MOF for Sb adsorption.

In order to remove Sb(Ⅲ) from the wastewater, two magnetic iron-based MOFs, MIL-100(Fe)@Fe3O4 and MIL-101(Fe)@Fe3O4, were effectively synthesized in this work. At the end of the adsorption process, magnetic MOFs were separated from the solution by external magnet pairs. Then the properties of MIL-101(Fe)@Fe3O4 and MIL-101(Fe)@Fe3O4 with different proportions were compared. Moreover, the effects of the initial Sb(III) solution concentration, adsorbent dose, reaction temperature, time, competitive anion and cation were investigated. The study of the adsorption mechanism and the adsorbent's recoverability is more significant.

2.1 Chemicals

Potassium antimony tartrate(C₈H₄K₂O₁₂Sb₂), terephthalic acid(H₂BDC), N,N-dimethylformamide(DMF), Ferric nitrate hydrate (Fe(NO₃)₃·9H₂O), Ferrous chloride tetrahydrate(FeCl₂·4H₂O), ammonium hydroxide (NH₄OH), chloride hexahydrate(FeCl₃·6H₂O), sodium chloride(NaCl), trisodium phosphate(Na₃PO₄), sodium sulphate(Na₂SO₄), sodium nitrate(NaNO₃), potassium chloride(KCl), calcium chloride(CaCl₂), magnesium chloride(MgCl₂), methanol(CH₃OH), ethanol (C₂H₅OH), hydrochloric acid(HCl), hydrofluoric acid (HF), sodium hydroxide(NaOH). Sb(Ⅲ) solution with different concentration was prepared by adding potassium antimony tartrate(C₈H₄K₂O₁₂Sb₂) into water. Shanghai Macklin Biochemical Technology Co., LTD. supplies the chemicals, which don't need to be further purified.

2.2 Synthesis of MIL-100(Fe) and MIL-101(Fe)

2.2.1 MIL-100(Fe)

MIL-100(Fe) was made via hydrothermal synthesis. 3.03 g of ferric nitrate and 1.05 g of homobenzoic acid were weighed and combined with 36 ml of water. The mixture was stirred for 30 minutes until the solid dissolved. Added 0.27 ml of hydrofluoric acid to the beaker, stirred, and transferred to autoclave. Turned off the oven, set temp to 150°C, and waited 12 hours. After cooling, the liquid and precipitate were moved to a beaker and cleaned with ethanol and water. Immersed in water at 80°C for 3 hours, then ethanol at 60°C for 3 hours. Dry at 60°C, then ground.

2.2.2 MIL-101(Fe)

MIL-101(Fe) was prepared by the solvothermal method according to previous research (Maksimchuk et al., 2012). Dissolve 4.96 mmol(0.824 g) of H₂BDC and 9.8 mmol(2.648 g) of FeCl₃·6H₂O in 60 mL of DMF by ultrasonic waving for 30 minutes and stirring well. Then poured the mixed solution into the polytetrafluoroethylene lining of the stainless steel autoclave, put it into the oven and keep it at 110 ℃ for heating for 20 hours. After 20 hours, the autoclave was removed from the oven for cooling. Then, in order to get rid of impurities, the material was cleaned three times using two different types of organic liquid DMF and methanol, respectively. The synthesized MIL-101(Fe) was poured into a beaker and secured with a sealing film. MIL-101(Fe) was dried entirely to a clear powder in about 30 hours by keeping it in a drying oven set at 70°C.

2.3 Preparation of Fe₃O₄

Fe₃O₄ nanoparticles were also prepared based on earlier studies, and their synthesis used the traditional chemical coprecipitation process with slight modification (Hachemaoui et al., 2021). First, added 200 mL of water to the beaker, then added 5.451 g FeCl₃·6H₂O and 3.340 g FeCl₂·4H₂O to the beaker. Thereafter, slowly added 20 ml 25 wt% of ammonium hydroxide (NH₄OH) solution under stirring. The solution immediately changed from orange to black. The Fe₃O₄ nanoparticles were repeatedly rinsed with deionized water and ethanol in order to eliminate any chloride ions that might have remained on the surface. After that, they were kept in a vacuum drying oven set at 70°C for 30 hours to obtain black solid Fe₃O₄.

2.4 Preparation of Composite Fe₃O₄@MIL-100(Fe) and Fe₃O₄@MIL-101(Fe)

2.4.1 Fe₃O₄@MIL-100(Fe)

Preparation of Fe₃O₄@MIL-100(Fe) composites, due to the change in the proportion of precursor and its effect on the properties of MOF such as adsorption and magnetic properties. A certain amount of Fe₃O₄, 36 ml H₂O, 3.03 g ferric nitrate hydrate, and 1.05 g homotrimellitic acid mixed, sonicated for 30 minutes, placed in a 100 mL steel autoclave with a PTFE liner and heated for 20 hours at 110°C. (Xi et al., 2013). It was cooled to ambient temperature, cleaned three times with ethanol and purified water, and then dried for 30 hours at 70°C. Orange-colored Fe₃O₄@MIL-100(Fe) particles were synthesized and ground for use. The prepared composites were dubbed Fe₃O₄@MIL-100(Fe)-0.05, Fe₃O₄@MIL-100(Fe)-0.10, Fe₃O₄@MIL-100(Fe)-0.15 and Fe₃O₄@MIL-100(Fe)-0.30. All of the aforementioned stages were performed for varying amounts of Fe₃O₄ (0.05 g, 0. 10 g, 0.15 g and 0.30 g).

2.4.2 Fe₃O₄@MIL-101(Fe)

Adjusting the Fe₃O₄ amount alters composite properties due to precursor ratio changes in Fe₃O₄@MIL-101(Fe) synthesis. Dispersed 0.15 g Fe₃O₄ in 20 mL DMF, then added 4.96 mmol H₂BDC, 9.8 mmol FeCl₃·6H₂O to 40 mL DMF. Treated the mixture with ultrasound for 30 minutes, then poured the solution containing Fe₃O₄ nanoparticles into the solution for preparing MIL-101(Fe) and treated it with ultrasound for 30 minutes. Transferred the mixed suspension into a 100 mL polytetrafluoroethylene lining of a stainless steel autoclave and heated it. The temperature of the oven was set to 110 ℃ for 20 hours. When the material had cooled to ambient temperature, it was dried for 30 hours at 70°C after using DMF and methanol washing three times apiece, and synthesized orange-red Fe₃O₄@MIL-101(Fe) particles, used after grinding. Using the above synthesis method, different quality Fe₃O₄ (0.05 g, 0.1 g, 0.15 g and 0.3 g) were mixed with MIL-101 to synthesize materials with different proportions. And the prepared composite was named Fe₃O₄@MIL-101(Fe)-0.05, Fe₃O₄@MIL-101(Fe)-0.1, Fe₃O₄@MIL-101(Fe)-0.15 and Fe₃O₄@MIL-101(Fe)-0.3. In addition, the synthesis of the two magnetic iron-based MOFs as well as the recycling process were shown more intuitively in Fig. 1.

2.5 Batch experiments

To investigate the influence of different environmental factors on the adsorption degree of Sb(Ⅲ) by Fe₃O₄@MIL-100(Fe) and Fe₃O₄@MIL-101(Fe), the adsorption status of absorbent materials on Sb(Ⅲ) was observed by batch experiment at pH of 2 ~ 10. Prepare 0.01 mol L^− 1, 0.1 mol L^− 1 HCl and 0.01 mol L^− 1, 0.1 mol L ^− 1 NaOH respectively. Then, 50 ml 50 mg L^− 1 Sb (III) solution were adjusted to pH 2 ~ 10. 50 mg of adsorbent material was added to 50 ml of Sb solution, and then the mixture was kept at room temperature (25 ℃) for 12 hours at 120 r min^− 1 in a shaker. The formula for adsorption quantity is as follows:

$$\:{\text{Q}}_{\text{e}}=\frac{({\text{C}}_{0}-{\text{C}}_{\text{e}})\times\:\text{V}}{\text{W}}$$

C₀(mg L^− 1) is the initial concentration of antimony solution; C_e(mg L^− 1) is the concentration of heavy metals in solution when adsorption reaches the equilibrium state; Q_e(mg g^− 1) is the adsorption capacity at the adsorption equilibrium concentration after adding the adsorbent; V(L) is the volume of the solution; W(g) is the mass of the adsorbent. According to batch experiment observation Fe₃O₄@MIL-100(Fe) and Fe₃O₄@MIL-101(Fe) adsorption efficiency under different influence factors. In 50 mg L^− 1 Sb(Ⅲ) solution, added 20 mg, 40 mg, 50 mg, 70 mg and 100 mg of adsorbent to 50 mL Sb solution respectively. Then placed in a solution at room temperature (25 ℃), pH = 7, and shaken in a shaker for 12 hours.

2.6 Instruments for experiment

XRD images could clearly distinguish whether Fe₃O₄@MIL-100(Fe), Fe₃O₄@MIL-101(Fe) and MIL-101(Fe) have been successfully synthesized. Using the Japanese Rigaku SmartLab SEX X-ray diffractometer, the range of 2θ was set at 5°-70° and X-ray diffraction (XRD) patterns were drawn. The surface area of the material was measured at 77K using a Micromeritics ASAP 2460 surface analyzer. To investigate the morphology of adsorbents, images were captured using the Czech TESCAN MIRA LMSS scanning electron microscope (SEM) as well as using the CTF20 transmission electron microscope (TEM). Fourier transform infrared spectroscopy (FTIR) of the ThermoferNiolet iN10 model was used to analyze the changes in materials before and after adsorption. At room temperature, LakeShore7404 vibrating sample magnetometer (VSM) was used to measure the magnetic properties of the material before and after adsorption. In order to conduct the full spectrum and fine spectrum of the materials before and after adsorption, the changes before and after the material reaction were characterized by Thermo Scientific K-Alpha X-ray photoelectron spectroscopy.

3.1 Effect of different Fe₃O₄ loadings on adsorption

The effect of the addition of magnetic Fe₃O₄ nanoparticles to the structure of MIL-100(Fe) and MIL-101(Fe) was studied. In the synthesis of MIL-100(Fe) 0.05 g, 0.1 g, 0.15 g and 0.3 g of Fe₃O₄ were added and screened for the best ratio of adsorption efficiency. The adsorbents were tested for uptake of 50 mg L^− 1 of Sb(III) from solution at pH = 7 and the results were shown in Table 1. Among all the nanoparticles in the synthesized ratios, the highest adsorption efficiency of 0.15g Fe₃O₄@MIL-100(Fe) could reach 41%, which was a 5% increase in adsorption as compared to the composite with the addition of 0.05 g Fe₃O₄.

Table 1

Adsorption efficiency of Fe₃O₄@MIL-100(Fe) with different ratios.
Adsorbent material	Adsorption rate (%)
0.05 g Fe₃O₄@MIL-100(Fe)	36%
0.10 g Fe₃O₄@MIL-100(Fe)	35%
0.15 g Fe₃O₄@MIL-100(Fe)	41%
0.30 g Fe₃O₄@MIL-100(Fe)	36%

Similarly, in the synthesis of MIL-101(Fe) similarly 0.05 g, 0.1 g, 0.15 g, 0.3 g of Fe₃O₄ were added and screened for the best ratio of adsorption efficiency. The results were shown in Table 2. Among all the nanoparticles in the synthesis ratios, the highest adsorption efficiency of 0.15 g Fe₃O₄@MIL-101(Fe) could reach 73%, which was an 8% increase in adsorption compared to the composite material with 0.05 g Fe₃O₄ adding. It was experimentally tested that the material with this ratio was more magnetic, and easy to separate from the solution by an external magnetic field.

Table 2

Adsorption efficiency of Fe₃O₄@MIL-101(Fe) with different ratios.
Adsorbent material	Adsorption rate (%)
0.05 g Fe₃O₄@MIL-101(Fe)	65%
0.10 g Fe₃O₄@MIL-101(Fe)	70%
0.15 g Fe₃O₄@MIL-101(Fe)	73%
0.30 g Fe₃O₄@MIL-101(Fe)	64%

3.2 Characterization

The XRD spectra of the two MOF samples were shown in Fig. 2(a)(b). The locations of the main diffraction peaks in the MIL-100(Fe) sample located at 2.14°, 4.28°, 4.92°, 6.11°, 10.35°, 11.48°, 12.62°, 17.96°, 18.96°, 19.25°, 20.11° ( Song et al., 2014; Ke et al., 2012). The XRD pattern of MIL-101(Fe) showed characteristic peaks of MIL-101(Fe) at 2θ = 5.18°, 8.78°, 9.36°, 18.98°, 23.7°. It was the same as previously reported (Liu et al., 2021), so it could be inferred that the successful synthesis of MIL-101(Fe) and MIL-100(Fe). The synthesized Fe₃O₄ comparison with a standard diffraction card (PDF#65-3107) showed matching peaks. The significant peaks of Fe₃O₄ are located at 2θ = 30°, 35°, 43°, 58° and 62°, corresponding to (220), (311), (400), (511) and (440), respectively. Moreover, the significant peak positions of Fe₃O₄ observable in Fe₃O₄@MIL-100(Fe), located at 2θ = 30°, 35°, 43°, 58° and 62°, respectively. The result proved the successful loading of Fe₃O₄ onto the MOF, which indicated that the sample was successfully prepared and had a good crystallinity (Ozkaya et al., 2009). Finally, the distinctive characteristic peaks of Fe₃O₄@MIL-101(Fe) were observed in the XRD image of the composite material, confirming the successful synthesis of the composite.

Using the N₂ adsorption/desorption method, the specific surface area and pore size of two types of MOF materials were calculated using the isothermal adsorption model. The N₂ adsorption/desorption curve of Fe₃O₄@MIL-100(Fe) rapidly rose in the P/P₀ range of 0–0.2 and approached saturation, indicating a Type I adsorption isotherm. In most cases, this type of isotherm could prove that the material is a microporous material with a pore size of less than 2 nm. By using the corresponding formulas, the specific surface area of Fe₃O₄@MIL-100(Fe) was calculated to be 729.991 m² g^− 1, and the total pore volume was 0.323 cm³ g^− 1. Both of the results reflected the high specific surface area and large porosity characteristics of Fe₃O₄@MIL-100(Fe). As shown in Fig. 2(d), the total pore volume and surface area of immobilized Fe₃O₄@MIL-101(Fe) were and 0.1 cm³ g^− 1 and 141.3 m² g^− 1, respectively. A type IV isotherm with H1 hysteresis loop behavior is seen in the adsorption/desorption curve, which is typical of mesoporous materials. Mesoporous materials possessed a larger specific surface area than macroporous materials. Their larger pore size than microporous materials allowed them to adsorb large molecules without pore blockage, facilitating the transport of reactants or solvents. Therefore, they had greater selectivity for adsorbing antimony in different forms.

Our previous study found that under hydrothermal conditions, MIL-100 (Fe) particles had obvious angles, but their morphology was not regular. It has been shown that the addition of hydrofluoric acid (HF) during preparation could improve its crystallinity, obtain grains with larger morphology and more regularity. In Fig. 2(e), the dispersed particles of MIL-100(Fe) prepared in the experiment had a particle size of about 100–300 nm, and the particles were in a cluster-like morphology. Additionally, Fe₃O₄ nanoparticles loaded on top of the crystalline MIL-100(Fe) could be observed. The SEM of Fe₃O₄@MIL-101(Fe) showed that they formed a segregated and uniform octagonal structure. It was clearly visible that Fe₃O₄ was attached to the surface of MIL-101(Fe), with a crystal diameter of 1.6 ± 0.26 µm for MIL-101(Fe) and 63 ± 35 nm for Fe₃O₄. In addition, the structure and morphology of Fe₃O₄@MIL-101(Fe) were further explored using TEM analysis, as can be observed in Fig. 2(g). The individual crystals of MIL-101(Fe) and Fe₃O₄ were clearly visible, which proved that the magnetic Fe₃O₄ nanoparticles were successfully immobilized in MIL-101(Fe). Figure 2(h) showed that the synthesis was well done, with individual particles being uniform in size and regular in shape.

Figure 3(a)(b) showed the magnetic adsorption curves of Fe₃O₄@MIL-101(Fe) and Fe₃O₄@MIL-100(Fe) before and after adsorbing Sb(III). From the graph, it could be observed that the materials have been endowed with magnetism. Further indicating that the magnetic nanoparticles Fe₃O₄ have been successfully fixed into the structure of MIL-100(Fe) and MIL-101(Fe). By imparting magnetic properties to the composite material, with an external magnet, it may be effectively and simply removed from the test solution. Furthermore, the study found that the materials used in the adsorption experiment still retained their magnetism, albeit slightly decreased, proving their structural stability. Especially for Fe₃O₄@MIL-101(Fe), it maintained good magnetism before and after adsorption, making it easy to separate from water. In comparison, the magnetic properties of Fe₃O₄@MIL-100(Fe) were weaker, as it requires perfect occupation of Fe²⁺ and Fe³⁺ in the corresponding sites to exhibit good magnetism. It could be inferred that the effect of loading Fe₃O₄ on MIL-101(Fe) was superior to that on MIL-100(Fe).

3.3 Effect of pH on adsorption

Fe₃O₄@MIL-100(Fe) and Fe₃O₄@MIL-101(Fe) showed different adsorption effects on Sb(Ⅲ) as shown in Fig. 3(c) and (d). In the pH range of 1 to 2, Fe₃O₄@MIL-100(Fe) material had a greater capacity to adsorb Sb(Ⅲ), reaching the highest value of 58.9% at pH = 2. The adsorption rate at pH = 1 was slightly lower than at pH = 2. Because for Sb(Ⅲ), at around pH = 2, antimony existed mainly as Sb(OH)₂⁺ and had a negative zeta potential, resulting in electrostatic attraction and increased adsorption capacity at pH = 2. Between pH = 3 and 10, the adsorption rate of Fe₃O₄@MIL-100(Fe) for Sb(Ⅲ) decreased. With an isoelectric point of approximately 7.1, the zeta potential of Fe₃O₄@MIL-101(Fe) decreased as the pH value went from 4 to 9. Under extremely acidic conditions, the electrostatic repulsion between positively charged Sb(OH)₂⁺ and Sb(Ⅲ) made it difficult for Sb(Ⅲ) to be efficiently adsorbed. As the pH value increased, the proportion of Sb(OH)₃ in water increased, weakening the electrostatic repulsion. Consequently, across the pH range of 3 to 10, the removal rate of Sb(Ⅲ) by Fe₃O₄@MIL-101(Fe) stayed nearly constant at roughly 72%. The results indicated that unless under strong acidic conditions, changes in pH did not significantly affect the adsorption of MOF materials (Liu et al., 2017). This was due to the fact that in the pH range of 2 to 11.8, Sb(Ⅲ) primarily occurs as neutral Sb(OH)₃, so electrostatic attraction had almost no effect on the adsorption rate within this range (Wilson et al., 2010). In conclusion, compared to Fe₃O₄@MIL-101(Fe), Fe₃O₄@MIL-100(Fe) had a lower adsorption rate for Sb(Ⅲ) and a narrower optimal pH range. Chemical adsorption predominated the adsorption process at pH = 2, while there was also a physical electrostatic attraction. The adsorption mechanism was nearly totally reliant on chemical adsorption above pH = 2.

3.4 Kinetics of adsorption

Investigated was the impact of the adsorption period on the clearance rate of Sb(III) adsorbed by Fe₃O₄@MIL-100(Fe) and Fe₃O₄@MIL-101(Fe). In this experiment, experimental data at various time points were fitted using pseudo-first-order model (2) and pseudo-second-order model (3). The relevant calculation formula is as follows (Rahmani et al., 2010):

$$\:{\text{Q}}_{\text{t}}={\text{Q}}_{\text{e}}(1-{\text{e}}^{{-\text{k}}_{1}\text{t}})$$

$$\:{\text{Q}}_{\text{t}}=\frac{{\text{k}}_{2}{\text{Q}}_{\text{e}}^{2}\text{t}}{1+{\text{k}}_{2}{\text{Q}}_{\text{e}}\text{t}}$$

Parameter k₁(h^− 1) is the first-order kinetic rate constant, and parameter k₂(g mg^− 1 h^− 1)is the second-order kinetic rate constant. Q_t(mg g^− 1) is the adsorption amount at each time point, and Q_e(mg g^− 1) is the adsorption amount when the adsorption reaches equilibrium.

Fe₃O₄@MIL-100(Fe) reached adsorption equilibrium after approximately 12 hours. In comparison, the adsorption of Sb(III) by Fe₃O₄@MIL-101(Fe) was much faster, reaching maximum adsorption capacity in just 10 minutes. The kinetic constants shown in Table 3 indicate that both materials exhibit high correlation coefficients for the adsorption of Sb(III). The adsorption behavior was more accurately characterized by the pseudo-second-order model, which also fitted the adsorption capacity to the experimental data with good accuracy. Demonstrating once more that the primary mechanism involved in the adsorption process was chemical adsorption. It was preliminarily suggested that the main adsorption mechanism involves chemical adsorption, where Sb(III) formed Fe-O-Sb bonded with incompletely coordinated Fe ions within the metal clusters (Ho and Mckay, 2000).

Table 3

Kinetic fitting results for the adsorption of Fe₃O₄@MIL-100(Fe) and Fe₃O₄@MIL-101(Fe) on Sb(Ⅲ).
		Parameter/unit	Sb(Ⅲ)
Fe₃O₄@MIL-100(Fe)	Pseudo-first-order model	Q_e/mg g^− 1	29.8909
		K₁/h^− 1	0.0155
		R²	0.9085
	Pseudo-second-order model	Q_e/mg g^− 1	31.9603
		K₂/g mg^− 1h^− 1	7.29×10^− 4
		R²	0.9747
Fe₃O₄@MIL-101(Fe)	Pseudo-first-order model	Q_e/mg g^− 1	27.2251
		K₁/h^− 1	3.4763
		R²	0.9046
	Pseudo-second-order model	Q_e/mg g^− 1	28.0464
		K₂/g mg^− 1h^− 1	0.2376
		R²	0.9847

3.5 Thermodynamic study

The relevant experimental data were substituted into Langmuir(Eq. (4)) and Freundlich(Eq. (5)) isotherm models for fitting. The fitting relevant formulas are as follows (Ozdemir et al., 2004):

$$\:\frac{{C}_{e}}{{q}_{e}}=\frac{1}{{\text{q}}_{\text{m}}{\text{K}}_{\text{L}}}+\frac{{\text{C}}_{\text{e}}}{{\text{q}}_{\text{m}}}$$

$$\:\text{l}\text{o}\text{g}{\text{q}}_{\text{e}}=\frac{1}{\text{n}}\text{l}\text{o}\text{g}{\text{C}}_{\text{e}}+\text{l}\text{o}\text{g}{\text{K}}_{\text{F}}$$

Q_e (mg g^− 1) is the equilibrium adsorption capacity of Sb; Q_max (mg g^− 1) is the maximum adsorption capacity of the material for Sb(Ⅲ) under different concentration conditions; C_e (mg L^− 1) is the equilibrium concentration; In the formula of Freundlich isotherm, n represents the adsorption strength and K_F represents the adsorption capacity. In the equation of Langmuir isotherm, K_L represents the adsorption correlation constant. These two groups of fitting curves represent the isotherm and data fitting results respectively, reflecting the adsorption law of adsorbent on Sb(III). In the designed experiment, the concentration of heavy metals in the solution was used as a variable and was gradually increased from 5 mg L^− 1. The amount of adsorption grew progressively as the concentration rose. The maximum adsorption capacity of Fe₃O₄@MIL-100(Fe) and Fe₃O₄@MIL-101(Fe) for Sb(III) was achieved at concentrations of 50 mg L^− 1 and 60 mg L^− 1 respectively.

In order to gain a better understanding of the adsorption behavior of Fe₃O₄@MIL-100(Fe) and Fe₃O₄@MIL-101(Fe) on antimony, Freundlich and Langmuir isotherm models were used to match the experimental results. The fitting results for the adsorption of Sb(III) by Fe₃O₄@MIL-100(Fe) were shown in Fig. 4(c), the adsorption capacity data was well-fit by both the Freundlich and Langmuir isotherm models. The relevant fitted parameters showed the Freundlich model yielded a correlation coefficient (R²) of 0.9813, while the Langmuir isotherm model yielded an R² of 0.9838. For the adsorption of Sb(III) by Fe₃O₄@MIL-101(Fe), the Freundlich and Langmuir isotherm models resulted in R² values of 0.9961 and 0.9997, respectively. Both adsorption isotherm models showed good fitting, in contrast, the adsorption of Sb(III) by Fe₃O₄@MIL-100(Fe) and Fe₃O₄@MIL-101(Fe) was more consistent with the Langmuir isotherm model. It indicated that the adsorbent existed as monolayer adsorption on the adsorbent, showed obvious homogeneity in the adsorption sites, and was generally dominated by chemisorption.

According to the principles associated with the Langmuir model, the model reflects the number of adsorption sites on the surface of the material. Under these conditions, the excellent result of monolayer adsorption is to reach the maximum adsorption capacity. This occurs when the initial concentration of antimony solution is high enough to bind to all surface sites. It is clear that when the starting concentration range is large enough to occupy all surface sites, the Langmuir model performs better. The highest adsorption capacities for antimony(III) of Fe₃O₄@MIL-100(Fe) and Fe₃O₄@MIL-101(Fe) were 32.91 mg g^− 1 and 28.56 mg g^− 1, respectively, based on the results of the Langmuir isotherm model fitting.

Table 4

Isotherm fitting results for the adsorption of Fe₃O₄@MIL-100(Fe) and Fe₃O₄@MIL-101(Fe) on Sb(III).
		Parameter/unit	Sb(Ⅲ)
Fe₃O₄@MIL-100(Fe)	Freundlich	Q_m/mg g^− 1	33.1833
		K_F/mg g^− 1	73.5485
		R²	0.9813
	Langmuir	Q_m/mg g^− 1	32.9101
		K_L/L mg^− 1	7.47×10^− 4
		R²	0.9838
Fe₃O₄@MIL-101(Fe)	Freundlich	Q_m/mg g^− 1	28.7673
		K_F/mg g^− 1	35.0169
		R²	0.9961
	Langmuir	Q_m/mg g^− 1	28.5552
		K_L/L mg^− 1	0.0058
		R²	0.9997

3.6 Ion coexistence experiment

In practical applications, some other substances in the adsorption system would interfere with the adsorption of the target adsorbate (including heavy metals, salts, etc.), competing for adsorption sites on the adsorbent and thus affecting the adsorption performance to some extent. Therefore, it is necessary to explore the selectivity of Fe₃O₄@MIL-100(Fe) and Fe₃O₄@MIL-101(Fe) for the target adsorption. Firstly, the impact of coexisting cations on the adsorption efficiency of Fe₃O₄@MIL-100(Fe) was investigated. The experimental design involved introducing the adsorbent into Sb(III) solution containing 0.01 M cations Na⁺, K⁺, Mg²⁺and Ca²⁺. The results showed that the impact on adsorption followed the order Mg²⁺ > Ca²⁺ > Na⁺ > K⁺. With Mg²⁺ having the greatest inhibitory effect on adsorption, leading to a decrease in Sb(III) adsorption rate of over 15%. The main reason for this was the different ionic radii and electronegativities of the cations. The ionic radius of Mg²⁺, K⁺, Na⁺, and Ca²⁺ are 0.072, 0.138, 0.102, and 0.1, respectively. When the ionic radius is smaller, it is conducive to free diffusion in the pores of MOFs and interaction with the adsorption sites. At the same time, the performance of Fe₃O₄@MIL-101(Fe) in removing Sb(III) in different coexisting ion water environments was also explored. By introducing the adsorbent into Sb(III) solution containing 0.01 M anions Cl⁻, F⁻, NO₃⁻, SO₄²⁻, and cations Na⁺, Mg⁺, Ca²⁺, and K⁺. The adsorption efficiency fell by 17.60%, 44.68%, 13.69%, and 24.92%, respectively, as shown in Fig. S3, after the addition of Cl⁻, PO₄³⁻, NO₃⁻ and SO₄²⁻ions. It could be concluded that Cl⁻, NO₃⁻ and SO₄²⁻ had competitive adsorption with Sb(Ⅲ) but had little influence, while PO₄³⁻ had a great influence. Antimony and phosphorus were thought to form a similar inner sphere complex because of their similar electron orbitals, shared main group membership, and chemically similar structures. As a result, the competition for adsorption sites would be what was causing the decrease in Sb(III) removal efficiency (Wang et al., 2021). However, there was almost no competition for Sb(Ⅲ) adsorption by cations Na⁺、Mg⁺、Ca²⁺、K⁺, and the adsorption efficiency of Sb(Ⅲ) decreased by less than 10%.

3.7 Adsorption regeneration studies

The ability to be reused is an important factor in assessing the use of adsorbents. In order to evaluate the reusability of Fe₃O₄@MIL-100(Fe) as an adsorbent, the used adsorbent material was separated using a magnet. Then soaked in a 30 ml 0.01 M HCl solution for 6 hours, and reused in a 50 mg L^− 1 antimony solution 2 more times. As shown in Fig. S4, the adsorption of Sb(Ⅲ) by the adsorbent basically did not change after 3 uses, with the removal efficiency only decreasing by about 14%. After separating the Fe₃O₄@MIL-101(Fe) material, it was soaked in a 30 ml 0.1 M sodium hydroxide solution for 6 hours. Then reused in a 60 mg L^− 1 antimony solution 2 more times, the result was shown in Fig. S5. After 3 complete adsorption-desorption tests, the adsorption capacity basically did not change, with the removal efficiency only decreasing by about 8%. The results indicated that Fe₃O₄@MIL-100(Fe) and Fe₃O₄@MIL-101(Fe) not only had recyclability as adsorbents but also had a high potential for repeated use. In addition, the reusability stability of Fe₃O₄@MIL-101(Fe) was slightly higher than that of Fe₃O₄@MIL-100(Fe).

3.8 Study on adsorption mechanism

First of all, according to the infrared spectrum in Fig. 5(a), it could be observed that there were slight changes in the position and intensity of the characteristic peaks before and after adsorption. The absorption peak at 1382 cm^− 1 decreased in intensity and shifted slightly, corresponding to the changes in the carboxylic functional groups of Fe₃O₄@MIL-100(Fe) after adsorbing Sb. Indicating that the adsorption process was related to the unsaturated sites on the material surface. The weaker absorption peak at 748 cm^− 1 for Fe₃O₄@MIL-100(Fe) slightly increased, attributed to the out-of-plane deformation vibration of -OH in Fe-OH. After the adsorption of antimony, the absorption band at 545 cm^− 1 underwent a slight shift, indicating the possible formation of Fe-O-Sb bonds between Fe₃O₄@MIL-100(Fe) and Sb(III) molecules. The FTIR changes before and after the adsorption of Sb(III) by Fe₃O₄@MIL-101(Fe) were shown in Fig. 5(e). The peak at 552 cm^− 1 before adsorption was due to the coordination bond formed by carboxyl groups and Fe. After adsorption, the peak shifted to 540 cm^− 1, possibly due to the formation of Fe-O-Sb complexes, which was the main adsorption mechanism (Li et al., 2019). The peak at 752 cm^− 1 corresponded to the vibration of the C-H bond in the benzene ring. After adsorption, its intensity decreased, indicating a decrease in the content of benzene ring C-H groups in the reaction process. Thereby causing BDC²⁻ to become free, suggesting that part of the crystal shape has been disrupted. The strong peak (Li et al., 2017) at 1300 cm^− 1 was caused by the stretching vibration of the carboxyl group on the ligand H₂BDC (Xie et al., 2017).

The coordination effect is an important mechanism for antimony adsorption in Fe-based MOF materials. Taking MIL-101(Fe) as an example, in the structure of MIL-101(Fe) with the chemical formula Fe₃OCl(H₂O)₂(BDC)₃. It could be inferred that each node contains three BDC²⁻ and the remaining three coordination sites were connected to Cl⁻ and H₂O (Taylor-Pashow et al., 2009). When MIL-101(Fe) was immersed in an aqueous solution, Fe-OH was formed, due to the coordination of unsaturated sites (CUS) containing Lewis acid in its three metal clusters (Hong et al., 2009). Sb(Ⅲ) would exchange with the hydroxyl groups in the metal clusters, thus forming Fe-O-Sb coordination to remove Sb(Ⅲ) from the water. To gain a deeper understanding of the adsorption mechanism of Fe₃O₄@MIL-100(Fe)/MIL-101(Fe) for Sb(Ⅲ), XPS characterization of the materials before and after adsorption was carried out.

First, the changes in the elemental composition of Fe₃O₄@MIL-100(Fe) before and after adsorption were analyzed. The results were shown in Fig. 5(b). XPS full spectrum analysis showed that the material not only contains Fe, C, and O elements but also F elements, which was due to the addition of hydrofluoric acid during the synthesis process. The increased Sb element in the spectrum after adsorption proved the effective binding of Sb to the material. Additionally, in the XPS spectrum of Fe 2p, as shown in Fig. 5(c), the peak positions of Fe elements were at 729.9 eV, 726.5 eV, 716.5 eV and 712.7 eV corresponding to the characteristic peaks of Fe 2p3/2 and Fe 2p1/2 (Li et al., 2020). After the reaction, the characteristic peaks shifted to the right to 727.8 eV, 724.9 eV, 715.7 eV and 711.6 eV, respectively. The shift in binding energy positions demonstrated a chemical reaction, indicating the formation of Fe-Sb coordination bonds at unsaturated sites of the material. In the O 1s XPS spectrum, a characteristic peak appeared at 530.9 eV, which corresponded to Fe-O-C. After adsorbing Sb(III), an additional peak appeared at 540.2 eV in the Fe₃O₄@MIL-100(Fe) material, corresponding to Sb(III) 3d3/2, while the Fe-O-C peak significantly weakened with a slight shift to 531.9 eV. The peak corresponding to the Sb 3d5/2 position is at 533.3 eV. Therefore, it can be inferred that Sb(III) effectively adsorbs on Fe₃O₄@MIL-100(Fe). In conclusion, the unsaturated Lewis acid sites of the metal, forming Fe-O-Sb bonds, and electrostatic forces collectively contributed to the adsorption process of Sb(III) on Fe₃O₄@MIL-100(Fe).

Observing the XPS results of Fe₃O₄@MIL-101(Fe), it could be clearly seen that the three peaks corresponding to O 1s, C 1s, and Fe 2p were at binding energies of 530.70 eV, 283.33 eV, and 709.55 eV, respectively. After adsorption, an Sb 3d peak appeared, with binding energies at 538.06 eV and 530.25 eV, corresponding to Sb 3d3/2 and Sb 3d5/2, respectively. It could be also proven that Fe₃O₄@MIL-101(Fe) successfully adsorbed Sb(III). As shown in Fig. 5(g), in the Fe 2p spectrum, two peaks were convoluted with binding energies of 723.36 eV and 710.01 eV, corresponding to Fe 2p1/2 and Fe 2p3/2, respectively. After adsorption, they shifted to binding energies of 722.52 eV and 709.55 eV, respectively, due to the formation of Fe-Sb coordination. In addition, a small amount of unreacted Fe³⁺ was attached to the surface of the Fe₃O₄@MIL-101(Fe) material during the synthesis process, which was released from the pores of Fe₃O₄@MIL-101(Fe). When it encountered water and reacted to form hydrated iron oxide (HFO), corresponding to two peaks at binding energies of 725.66 eV and 712.20 eV. The formation of HFO generated more active sites and improved the adsorption efficiency of the material for Sb(III). It could be inferred that the Fe-O-Sb coordination formed by the incomplete coordination of Fe ions in the Fe₃O₄@MIL-101(Fe) metal cluster with Sb(III) and the in-situ formation of HFO were the main adsorption mechanisms. As shown in Fig. 5(h), the O 1s spectrum was deconvoluted into three peaks before adsorption, corresponding to the metal bond Fe-O, the C-O bond in the carboxyl group of terephthalic acid, and the hydroxyl group-OH at binding energies of 530.86 eV, 532.91 eV, and 533.85 eV, respectively. After adsorption, a new Sb-O bond appeared at a binding energy of 532.62 eV, further proving the successful adsorption of Sb(III). The peak area of OH decreased by 18.8% after adsorption, indicating that the terminal -OH of Fe₃O₄@MIL-101(Fe) was the key functional group for removing Sb(III). In addition, the peak area of C-O decreased after adsorption, which was related to the C-O connected to terephthalic acid. It corresponded to the FTIR spectrum analysis results, where some BDC²⁻ turned into a free state. It could be concluded from the O1s spectrum analysis that the Fe-O-Sb formed by the hydroxyl group was the key to adsorption. Therefore, it was known that the main mechanism for Fe₃O₄@MIL-101(Fe) to remove Sb(III) is based on coordination and the active sites provided by the in-situ formation of HFO.

Based on the above analysis, it could be concluded that the main mechanism of the two magnetic iron-based MOFs, namely Fe₃O₄@MIL-100(Fe) and Fe₃O₄@MIL-101(Fe). The adsorption of Sb(III) was due to the coordination of Fe-O-Sb formed by the incomplete coordination of Fe ions in the metal clusters with Sb(III), as shown in Fig. 6.

In summary, this experiment synthesized two magnetic Fe-based MOFs with the optimal synthesis ratio for removing Sb(III). In a wide pH range, both materials showed good adsorption performance for Sb(III). Fe₃O₄@MIL-101(Fe) reached adsorption saturation in just 10 minutes, while Fe₃O₄@MIL-100(Fe) took 12 hours to reach adsorption equilibrium, with maximum adsorption capacities of 28.56 mg g^− 1 and 32.91 mg g^− 1, respectively. The kinetic fitting results indicated that both materials were suitable for the pseudo-second-order model, further demonstrating the driving mechanism of chemical adsorption. FTIR and XPS analysis revealed that both materials remove Sb(III) through the formation of Fe-O-Sb via incomplete coordination of iron ions in the metal clusters. Additionally, Fe₃O₄@MIL-100(Fe) involved electrostatic attraction, while Fe₃O₄@MIL-101(Fe) utilizes active sites provided by in-situ formed HFO for synergistic binding with Sb(III). Furthermore, VSM tests before and after adsorption showed that the magnetic properties of the adsorbent remained consistent at a solid level after three cycles, allowing for easy separation from the solution. Detection from coexisting ions and adsorption regeneration experiments demonstrated that the two synthesized magnetic Fe-based MOFs could effectively and recyclable remove Sb(III) from complex water bodies.

Author Contributions:Zhen Zeng: Manuscript draft, Drawing pictures, Design. Xinyu Song: Manuscript draft, Drawing pictures. Lei Huang: Guidance, Framework proposed. Shuo Wang: English revision. Yingqi Liang: English summary. Wenhao Rao: Data statistics. Kun Li: English summary. Zhen-xing Wang: Guidance, Editing review.

Acknowledgements:This work was supported by the Science and Technology Program of Guangdong Forestry Administration (2020-KYXM-08), the Major Science and Technology Program for Water Pollution Control and Treatment (2017ZX07101003), Youth Foundation of SCIE(PM-zx097-202304-147), National Key Research and Development Project (2019YFC1804800), and Pearl River S&T Nova Program of Guangzhou, China (No. 201710010065).

Funding: The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Ethical Approval Not applicable.

Consent to Participate Not applicable.

Consent to Publication Not applicable.

Competing Interests The authors have no relevant financial or non‑financial interests to disclose.

Data Availability Statements:The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Efficient removal of Sb(III) from aqueous solution using recyclable magnetic metal-organic frameworks Fe₃O₄@MIL-100 and Fe₃O₄@MIL-101

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Abstract

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1 Introduction

2 Chemicals and methods

2.1 Chemicals

2.2 Synthesis of MIL-100(Fe) and MIL-101(Fe)

2.2.1 MIL-100(Fe)

2.2.2 MIL-101(Fe)

2.3 Preparation of Fe₃O₄

2.4 Preparation of Composite Fe₃O₄@MIL-100(Fe) and Fe₃O₄@MIL-101(Fe)

2.4.1 Fe₃O₄@MIL-100(Fe)

2.4.2 Fe₃O₄@MIL-101(Fe)

2.5 Batch experiments

2.6 Instruments for experiment

3 Results and discussion

3.1 Effect of different Fe₃O₄ loadings on adsorption

3.2 Characterization

3.3 Effect of pH on adsorption

3.4 Kinetics of adsorption

3.5 Thermodynamic study

3.6 Ion coexistence experiment

3.7 Adsorption regeneration studies

3.8 Study on adsorption mechanism

4 Conclusion

Declarations

References

Supplementary Files

Status:

Version 1

Efficient removal of Sb(III) from aqueous solution using recyclable magnetic metal-organic frameworks Fe3O4@MIL-100 and Fe3O4@MIL-101

Status:

Version 1

Abstract

Figures

1 Introduction

2 Chemicals and methods

2.1 Chemicals

2.2 Synthesis of MIL-100(Fe) and MIL-101(Fe)

2.2.1 MIL-100(Fe)

2.2.2 MIL-101(Fe)

2.3 Preparation of Fe3O4

2.4 Preparation of Composite Fe3O4@MIL-100(Fe) and Fe3O4@MIL-101(Fe)

2.4.1 Fe3O4@MIL-100(Fe)

2.4.2 Fe3O4@MIL-101(Fe)

2.5 Batch experiments

2.6 Instruments for experiment

3 Results and discussion

3.1 Effect of different Fe3O4 loadings on adsorption

3.2 Characterization

3.3 Effect of pH on adsorption

3.4 Kinetics of adsorption

3.5 Thermodynamic study

3.6 Ion coexistence experiment

3.7 Adsorption regeneration studies

3.8 Study on adsorption mechanism

4 Conclusion

Declarations

References

Supplementary Files

Status:

Version 1

Efficient removal of Sb(III) from aqueous solution using recyclable magnetic metal-organic frameworks Fe₃O₄@MIL-100 and Fe₃O₄@MIL-101

2.3 Preparation of Fe₃O₄

2.4 Preparation of Composite Fe₃O₄@MIL-100(Fe) and Fe₃O₄@MIL-101(Fe)

2.4.1 Fe₃O₄@MIL-100(Fe)

2.4.2 Fe₃O₄@MIL-101(Fe)

3.1 Effect of different Fe₃O₄ loadings on adsorption