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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2023 , Vol. 35 >Issue 3: 458 - 474

DOI: https://doi.org/10.7536/PC220822

Review

MIL-101(Fe) and Its Composites for Catalytic Removal of Pollutants: Synthesis Strategies, Performances and Mechanisms

  • Lan Mingyan ,
  • Zhang Xiuwu ,
  • Chu Hongyu ,
  • Wang Chongchen , *
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  • Key Laboratory of Functional Materials for Building Structure and Environment Remediation, Beijing University of Civil Engineering and Architecture,Beijing 100044, China
* Corresponding author e-mail:

Received date: 2022-08-26

  Revised date: 2023-01-08

  Online published: 2023-02-15

Supported by

National Natural Science Foundation of China(22176012)

Beijing Natural Science Foundation(8202016)

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Abstract

MIL-101(Fe) is a typical Fe-based metal-organic framework (Fe-MOF), which demonstrates the advantages of flexible structure, large specific surface area, large porosity, and adjustable pore size. In recent years, MIL-101(Fe) and its composites have been extensively studied in the field of water pollution remediation, especially in the hexavalent chromium (Cr(Ⅵ)) reduction and advanced oxidation processes for removing organic pollutants in water. The water stability, light absorption activity and the carrier separation efficiency can be significantly improved by functional modification with specific functional materials. In this review, the preparation strategies of MIL-101(Fe) and its composites, as well as their application as heterogeneous catalysts for photocatalysis, H2O2 activation, and persulfate activation were introduced. The future development of MIL-101(Fe) and its composites as catalysts for water purification is prospected.

Key words： MIL-101(Fe); catalysis; hexavalent chromium; organic pollutants; wastewater treatment

Cite this article

Lan Mingyan , Zhang Xiuwu , Chu Hongyu , Wang Chongchen . MIL-101(Fe) and Its Composites for Catalytic Removal of Pollutants: Synthesis Strategies, Performances and Mechanisms[J]. Progress in Chemistry, 2023 , 35(3) : 458 -474 . DOI: 10.7536/PC220822

Contents

1 Introduction
2 Preparation of MIL-101(Fe)and its composites
2.1 MIL-101(Fe)
2.2 MIL-101(Fe)composites
3 MIL-101(Fe)and its composites for reduction of Cr(Ⅵ)
4 Advanced oxidative degradation of organic pollutants in wastewater by MIL-101(Fe)and their composites
4.1 Photocatalysis
4.2 Activation of H2O2
4.3 Activation of persulfate
5 Water stability and biotoxicity of MIL-101(Fe)
6 Conclusions and prospective

1 Introduction

Metal-organic frameworks (MOFs) are porous coordination polymers formed by bridging self-assembly of Metal ions or metal clusters and organic ligand units[1]. MOFs have many excellent characteristics, such as large specific surface area, tunable pore size and structure, and abundant active sites, which have excellent performance and great potential application value in the fields of adsorption, sensing, catalysis, sterilization, ion conduction, drug delivery, fluorescence detection and energy storage[2][3][4][5][6][7][8][9][10]. Since the concept of MOFs was first proposed by Yaghi's team in 1995, the synthesis and application of MOFs materials have sprung up like mushrooms after a spring rain, which has aroused widespread interest and in-depth research among researchers[11].
MIL-101 (MIL for Materials of Institute Lavoisier) is a three-dimensional MOF material with two pore-cage structures, which was first synthesized by F FÉrey's team in 2005 using terephthalic acid (Materials of Institute Lavoisier) ligand and chromium nitrate as templates[12]. Other researchers have successfully prepared MOFs such as MIL-101 (Fe), MIL-101 (Al), ML-101 (Ti) and ML-101 (V) through the replacement of metal elements in subsequent research work[13]. Most of the metal ions in this kind of materials exist in the form of + 3 valence and have a quasi-octahedral coordination environment. In which each metal ion is linked to an oxygen atom located in the middle of the metal trimer and to four oxygen atoms associated with the carboxylic acid of terephthalic acid, occupying the vertices of the octahedron. Finally, the coordination of each metal ion with the surrounding is completed by a water molecule, a fluorine atom, or a hydroxyl group[14]. MIL-101 (Fe) has potential applications in the field of water environment remediation, such as removal of water pollutants and oil-water separation, due to its huge unit cell volume, large specific surface area, high porosity, excellent thermal stability and many unsaturated active sites[15]. In particular, the abundant Fe-O clusters in MIL-101 (Fe) can broaden the light absorption range to the visible region, which is an excellent photocatalytic material[16]. The active valence transformation of Fe (Ⅱ)/Fe (Ⅲ) and the excellent ligand-metal electron transfer ability of MIL-101 (Fe) in the catalytic process make it also have potential application prospects in the field of Fenton reaction and activated persulfate[17]. As shown in Figure 1, based on the Web of Science search, the number of articles related to MIL-101 (Fe) is increasing year by year, and it has also received extensive attention from researchers in the field of catalysis, especially in the field of catalytic removal of pollutants in water, showing great potential for development. In this paper, the synthesis methods of MIL-101 (Fe) and its composites are summarized, and the latest research progress of MIL-101 (Fe) as a new catalytic material for photocatalytic reduction of Cr (Ⅵ) and advanced oxidation of organic pollutants in water is reviewed, and suggestions and prospects for future research work are put forward.
图1 近八年发表的MIL-101(Fe)相关文章的数量(来源:Web of Science,日期:2023年1月6日,关键词:MIL-101(Fe)和catalysis)

Fig. 1 Number of publications of MIL-101(Fe) during the past eight years (source: Web of Science, date: 6th January 2023, keywords: MIL-101(Fe) and catalysis)

2 Preparation of MIL-101 (Fe) and Its Composite

2.1 Preparation of MIL-101 (Fe)

(1) Solvothermal method. Solvothermal method is a method to synthesize MOFs by the coordination reaction of organic ligands and metal salts in solvent at a certain temperature and pressure, which is also one of the most commonly used methods to synthesize MIL-101 (Fe). Li et al. Ultrasonically dissolved FeCl3·6H2O and terephthalic acid in a molar ratio of 2 ∶ 1 in N, N-dimethylformamide (DMF) solution, and the mixed solution was added to a Teflon-lined stainless steel autoclave and heated at 110 ° C for 20 H[18]. After cooling to room temperature, the obtained product was filtered out, washed and purified with DMF and hot ethanol, and then dried in vacuum to obtain MIL-101 (Fe) with octahedral morphology (Fig. 2a), with a particle size of about 2 ~ 3 μm. The specific surface area and pore volume of MIL-101 (Fe) obtained by adding acetic acid as a monocarboxylic acid regulator in the synthesis process by Yang et al. Were 2670 m2·g-1 and 0.75 cm3·g-1, respectively[19].
图2 (a) 溶剂热法[18];(b) 微波辅助法[21];(c) 电化学法[23]和(d) 室温搅拌法[24]合成的MIL-101(Fe)形貌图;(e) 原位合成法合成的MIL-101(Fe)/g-C3N4复合材料[27];(f) 一步合成法合成的MIL-101(Fe)/CuS复合材料[30]和(g) 室温浸渍法合成的Ag/AgCl/MIL-101(Fe)复合材料的形貌图[32]

Fig. 2 The morphologies of MIL-101(Fe) synthesized via (a) solvothermal method[18]; (b) microwave-assisted method[21]; (c) electrochemical method[23] and (d) room temperature method; the morphologies of (e) MIL-101(Fe)/g-C3N4 composite[27]; (f) MIL-101(Fe)/CuS composite[30] and (g) Ag/AgCl/MIL-101(Fe) composite[32] synthesized via in-situ synthesis, one-step synthesis and room temperature impregnation, respectively

(2) Microwave-assisted method. Microwave-assisted method is to use microwave radiation to heat the reaction system[20]. Taylor-Pashow et al. Dissolved FeCl3·H2O and terephthalic acid with the same molar ratio in DMF solution, reacted under microwave irradiation at 150 ℃ for 10 min, cooled to room temperature, centrifuged the product and washed with DMF and ethanol to remove unreacted components[21]. MIL-101 (Fe) particles with uniform structure, small particle size (about 200 nm) and octahedral shape were obtained (Figure 2b), and their specific surface area was in the range of 3700~4535 m2·g-1. Dong et al. Reported that MIL-101 (Fe) was synthesized by microwave-assisted method as an adsorbent to remove tetracycline (TC) from wastewater. The results showed that the synthesized MIL-101 (Fe) had smaller particle size and stronger adsorption capacity for TC than that synthesized by solvothermal method[22].
(3) Electrochemical method. The electrochemical method has the advantages of mild reaction conditions, low cost of raw materials, simple process and environmental friendliness, and has good research potential and industrial prospects. Wu et al added FeCl2, terephthalic acid, tetrabutylammonium hexafluorophosphate (TBAPF6) and DMF solution as reaction raw materials into the corresponding reactors of the electrolytic cell according to a certain ratio, and the working electrode, the reference electrode and the counter electrode were carboxyl modified ITO (indium tin oxide) electrode, Ag wire and Pt mesh, respectively.MIL-101 (Fe) was synthesized by electrochemical method at room temperature by oxidizing the dissolved Fe2+ on the surface of the electrode, and the BET (Langmuir) specific surface area was measured to be 2368(3333)m2·g-1,MIL-101(Fe), as shown in Fig. 2C[23].
(4) Room temperature stirring method. The room temperature stirring method is to stir the reaction solution at room temperature. Compared with other methods, it has the advantages of normal temperature, normal pressure and high efficiency. Hang et al. First dissolved different molar ratios (0 ∶ 1, 1 ∶ 1, 1 ∶ 2, 1 ∶ 3 and 1 ∶ 0) of H2BDC and NH2-H2BDC(3.95 mmol) in NaOH aqueous solution and kept stirring for 5 min to obtain solution A[24]. Subsequently, the 2.135 g FeCl3·4H2O was dissolved in 48.6 mL of pure water to obtain solution B. Add solution A to solution B while stirring, and stir at room temperature for 24 H. The product was filtered and washed several times with distilled water and ethanol to remove the residual organic ligand. The yellow powder (Fe-BDC) and brown powder (Fe-(BDC)x(BDC)-NH2)(1-x)(x=1,0.5,0.33,0.25,0) were obtained by drying, and the target products MIL-101 (Fe) and NH2-MIL-101(Fe) were obtained by activation in a vacuum drying oven at 60 ° C. Where Fe-BDC (MIL-101 (Fe)) consists of irregularly shaped fine particles and regular polyhedral particles (Fig. 2D).
MIL-101 (Fe) synthesized by solvothermal method, microwave-assisted method, electrochemical method and room temperature stirring method have their own advantages and disadvantages. Although the solvothermal synthesis is slower than other methods, it can obtain high-quality MIL-101 (Fe) with typical octahedral morphology, which is a widely used synthesis method at present. Compared with the traditional solvothermal method, the microwave-assisted method can greatly accelerate the nucleation and growth of MIL-101 (Fe) particles due to its high-energy rapid heating process, so the prepared MIL-101 (Fe) has smaller size and better dispersion. The stirring method at room temperature has mild reaction conditions, but it faces the problems of long time, irregular product morphology and more waste liquid. Electrochemical method has simple reaction conditions, short reaction time and can control the particle size and morphology, but there are few studies at present. Because the raw materials of MOFs involve metal sources, organic ligands, solvents and acid/base modifiers, in recent years, some studies have been devoted to the conversion of industrial metal scrap into MOFs with high added value, which can not only save costs but also realize resource utilization[25,26].

2.2 Preparation of MIL-101 (Fe) complex

(1) In-situ synthesis. In-situ synthesis of MIL-101 (Fe) composites refers to the addition of pre-synthesized active materials into the precursor reaction solution of MIL-101 (Fe). Under certain reaction conditions, MIL-101 (Fe) is uniformly dispersed and grown on the surface of the active material or the active material is wrapped on the surface of MIL-101 (Fe), and the MIL-101/Fe composite material prepared by the method not only has high purity, but also has high heterogeneous interface bonding degree. Zhao et al. Used the in-situ synthesis method to disperse the pre-prepared g-C3N4 in the precursor reaction solution of MIL-101 (Fe) to obtain the MIL-101(Fe)/g-C3N4 composite[27]. The results showed that the octahedral morphology of MIL-101 (Fe) was tightly anchored on the surface of g-C3N4 (Fig. 2e). Huo et al. Added CeO2 during the synthesis of MIL-101 (Fe) to prepare CeO2/MIL-101(Fe) composite for photocatalytic oxidative desulfurization[28].
(2) One-step synthesis. The one-step synthesis of MIL-101 (Fe) composites means that the metal salt, organic ligand and active material required for the synthesis are placed in the same synthesis system and synthesized in one step. The method has the advantages of low cost, simple operation steps and easy control of reaction conditions. Vu et al. Replaced 25 wt% of Cr with Fe in the synthesis of MIL-101, and synthesized Fe-Cr-MIL-101 composite in one step for photo-Fenton degradation of dyes[29]. Jiang et al. Added commercial CuS powder to the precursor reaction solution for the synthesis of MIL-101 (Fe), and synthesized the CuS-modified MIL-101 (Fe) composite in one step[30]. As shown in Figure 2 f, CuS was uniformly dispersed on the surface of MIL-101 (Fe) and showed stronger photocatalytic activation of persulfate to inactivate bacteria than CuS and MIL-101 (Fe) alone.
(3) Room temperature impregnation method. Room temperature impregnation synthesis of MIL-101 (Fe) composites refers to the method of synthesizing MIL-101 (Fe) composites by physical or chemical reactions without the need for high temperature heating by impregnating MIL-101 in a solution containing active materials. Xu et al. Dispersed TiO2 and prepared MIL-101 (Fe) in methanol solution at room temperature for 2 H to obtain TiO2@MIL-101(Fe) composite for photocatalytic activation of persulfate for degradation of methyl orange and nitrobenzene[31]. Gong et al. Synthesized Ag nanoparticles and MIL-101 (Fe) respectively, and stirred them in DMF solution at room temperature for 2 H to obtain Ag/AgCl/MIL-101 (Fe) composite as shown in Fig. 2g[32].

3 Reduction of Cr (Ⅵ) by MIL-101 (Fe) and its complex

Hexavalent chromium (Cr (Ⅵ)) mainly comes from printing and dyeing wastewater, tannery wastewater and metallurgical wastewater, etc. It has strong biological toxicity and carcinogenicity, and is listed as a first-class carcinogen by WTO[33]. Another form of chromium in aqueous solution is trivalent chromium (Cr (Ⅲ)) with low toxicity. Cr (Ⅲ) is not only easy to form Cr(OH)3 precipitation under alkaline and neutral conditions, but also one of the trace elements needed by organisms[34][35]. Photocatalytic technology can reduce Cr (Ⅵ) to Cr (Ⅲ), which is an effective and low-cost water treatment technology to remove Cr (Ⅵ).Our group has prepared a series of novel MOFs (e.g., BUC-21, (OH)2-UiO-66-X%), MOFs composites (e.g., WO3/MIL-100(Fe),Ag/Ag3PO4/MIL-125- NH 2 , etc.) as well as immobilized MOFs (such as UiO-66-NH2(Zr/Hf) membrane and MIL-101(Fe)-NH2@Al2 O 3 ) as excellent photocatalysts for the reduction of Cr (Ⅵ) to Cr (Ⅲ)[36][37][38][39][40][41][42].
MIL-101 (Fe) is a kind of photocatalyst, which can be excited under visible light. However, due to the easy recombination of photogenerated electrons and holes, single MIL-101 (Fe) does not show excellent Cr (Ⅵ) reduction performance under visible light[43]. At present, there have been many studies on the photocatalytic reduction of Cr (Ⅵ) by MIL-101 (Fe) and its complexes (Table 1), and three solutions to improve the photocatalytic reduction of Cr (Ⅵ) by MIL-101 (Fe) have been summarized: (1) ligand functionalization; (2) compounding with semiconductor materials; (3) Composite with conductor materials.
表1 MIL-101(Fe)及其复合物用于光催化还原Cr(Ⅵ)

Table 1 MIL-101(Fe) and its composites for photocatalytic Cr(Ⅵ) reduction

Catalyst/Dosage (g·L-1) Volume (mL)/
Concentration (mg·L-1)/pH		 Light source Reaction time (min) Degradation efficiency (%) ref
NH2-MIL-101(Fe)/0.5 40/80/2 visible light 60 100 44
150-g-C3N4/NH2-MIL-101(Fe)/0.5 40/10/2 300 W Xe lamp (λ≥ 400 nm) 60 100 45
MIL-101(Fe)/g-C3N4/0.5 40/20/5 150 W halogen cold light source (λ≥ 420 nm) 60 92.6 27
1%Ag/AgCl/MIL-101(Fe/)1 50/10/6 300 W Xe lamp (λ≥ 420 nm) 75 100 32
Cellulose/NH2-MIL-101(Fe) hybrid
foams/1		 40/20/5 light intensity: 100 mW/cm2(λ≥ 420 nm) 180 100 46
Sand-Cl@NH2-MIL-101(Fe)-50%/0.5 100/10/ 1000 W halogen lamp 20 97.3 47
g-C3N4 (150 mg)/ NH2-MIL-101(Fe)/1 30/20/2 solar light (60,000 lux) 90 91 48
MIL-101(Fe)-NH2@Al2O3/0.3 50/5/3.4 300 W Xe lamp 180 100 42
TmErNd@Nd(x)@NFM/0.5 40/20/2 300 W Xe lamp 50 91 49
Ligand-functionalized MIL-101 (Fe) can be synthesized by introducing functional groups such as amino (—NH2), sulfosulfur (—SO3H), and nitro (—NO2). Ligand functionalization can regulate the surface properties of MIL-101 (Fe) without changing its topology and broaden its application. Especially, the presence of —NH2 can effectively regulate the energy level of HOMO-LUMO of MIL-101 (Fe), enhance its visible light absorption range, and reduce the recombination rate of hole-electron pair (h+-e-) to improve the photocatalytic performance of MIL-101 (Fe)[50]. Shi et al. Prepared NH2-MIL-101(Fe) using an amino terephthalic acid ligand[44]. Compared with MIL-101(Fe),NH2-MIL-101(Fe), it showed better photocatalytic reduction of Cr (Ⅵ) under visible light irradiation (Fig. 3A). This is mainly due to the fact that both the organic ligand of NH2-MIL-101(Fe) (aminoterephthalic acid) and the Fe3-μ3-oxo cluster can be excited under the irradiation of visible light[51]. Cr (Ⅵ) can be reduced to Cr (Ⅲ) by the electrons generated by the excited Fe3-μ3-oxo cluster, and the photogenerated electrons in the organic ligand can be transferred to the Fe3-μ3-oxo cluster to complete the reduction of Cr (Ⅵ). Therefore, the introduction of amino groups can promote electron transfer and reduce electron-hole recombination, thus improving the photocatalytic performance. In order to overcome the problem that the powder catalyst is easy to agglomerate in aqueous solution and difficult to recover, Zhao et al. Successfully immobilized NH2-MIL-101(Fe) on α-Al2O3 carrier by secondary seed method to prepare immobilized catalyst NH2-MIL-101(Fe)@Al2O3(MA)[42]. Immobilized catalyst MA can completely reduce Cr (Ⅵ) with an initial concentration of 5 mg·L-1 to Cr (Ⅲ) within 8 min under white light irradiation with the addition of small molecule organic acid (oxalic acid) as a hole trap. In addition to the reduction of Cr (Ⅵ) by photogenerated e-, oxalic acid reacts with holes to form CO 2 · - and participates in the reduction of Cr (Ⅵ) (Fig. 3B). In addition, the immobilization greatly improved the stability of the material, and the photocatalytic reduction efficiency of Cr (Ⅵ) by MA was still maintained at 100% after 20 batch cycles (400 min). On this basis, a fixed-bed reactor was prepared for the continuous reduction of Cr (Ⅵ), and the reduction efficiency of Cr (Ⅵ) was stabilized at 100% within 30 H, which provided a new strategy for the large-scale reduction of chromium (Ⅵ) by NH2-MIL-101(Fe).
图3 (a) 不同材料光催化还原Cr(Ⅵ)的性能图[44];(b) 在草酸存在下MA光催化还原Cr(Ⅵ)的机理图[42];(c) g-C3N4/NH2-MIL-101(Fe)光催化还原Cr(Ⅵ)的机理图[45];(d) Ag/AgCl/MIL-101(Fe)光催化还原Cr(Ⅵ)的机理图[32]

Fig. 3 (a) Performances of photocatalytic Cr(Ⅵ) reduction over different materials[44]; schematic illustration of photocatalytic Cr(Ⅵ) reduction mechanism of (b) MA[42]; (c) g-C3N4/NH2-MIL-101(Fe)[45]; (d) Ag/AgCl/MIL-101(Fe)[32]

In recent years, many semiconductor catalysts such as WO 3 , TiO 2 and ZnS have been used for photocatalytic reduction of Cr (Ⅵ)[39][52][53]. However, these catalysts have some disadvantages, such as poor light absorption efficiency, low utilization of visible light, and fast recombination of photogenerated electrons and holes, which limit the reduction efficiency of Cr (Ⅵ). MIL-101 (Fe) has the properties of n-type semiconductor and can be excited under visible light, so a single semiconductor material can be combined with MIL-101 (Fe) to construct a heterojunction catalyst to improve the photocatalytic reduction efficiency and enhance the stability of the catalyst[21]. Graphite carbon nitride (g-C3N4) is a typical polymer semiconductor with a band gap of about 2.7 eV, which is a photocatalyst material responding to visible light and is regarded as one of the most promising photocatalysts because of its convenient synthesis, non-toxicity and high chemical stability[54][55]. Zhao et al. Formed a Z-shaped MIL-101(Fe)/g-C3N4 heterojunction by in-situ growth of MIL-101 (Fe) on the surface of g-C3N4, which has excellent light absorption characteristics and can effectively inhibit the recombination of photogenerated electrons and holes[27]. Under the condition of using ammonium oxalate as hole trap, 92.6% of Cr (Ⅵ) (initial concentration of 20 mg·L-1) can be reduced by MIL-101(Fe)/g-C3N4 in 60 min, which is superior to MIL-101 (Fe) (53.8%) and C3N4(48.1%) alone. Liu et al. Prepared g-C3N4/NH2-MIL-101(Fe) composite by hydrothermal method[45]. The UV-Vis diffuse reflectance spectra showed that the synthesized composites could be excited under visible light. The results show that pH = 2 has the best photocatalytic reduction efficiency of Cr (Ⅵ). Under visible light irradiation, 100% of Cr (Ⅵ) (initial concentration of 20 mg·L-1) can be reduced to Cr (Ⅲ) in 60 min. As shown in fig. 3C, photogenerated electrons are still the main active species for the reduction of Cr (Ⅵ). The main reason for the enhanced photocatalytic reduction of Cr (Ⅵ) is that the conduction band (CB) edge potential of g-C3N4 is more negative than the LUMO edge potential of NH2-MIL-101(Fe), so the photogenerated electrons in the CB of g-C3N4 can be easily transferred to the LUMO of NH2-MIL-101(Fe)), which promotes the photogenerated electrons and separation, thus obtaining excellent photocatalytic reduction performance[56]. The above results show that MIL-101 (Fe) can be combined with narrow band gap semiconductor catalysts to prepare a variety of visible-light-excited catalysts for efficient reduction of Cr (Ⅵ).
Metal nanoparticles (M-NPs), as a typical conductor material, have many highly active catalytic sites, and have been widely studied in the field of heterogeneous catalysis[57]. However, the disadvantage of easy agglomeration in the reaction process restricts the further application and development of M-NPs. Loading M-NPs on MOFs with high porosity and large specific surface area is an effective solution to overcome this problem. Ag/AgCl/MIL-101 (Fe) ternary composites were prepared by encapsulating Ag-NPs into MIL-101 (Fe) framework at room temperature[32]. In the mixed solution of Cr (Ⅵ) and phenol, Ag/AgCl/MIL-101 (Fe) can not only achieve the effective degradation of phenol, but also achieve 100% reduction of Cr (Ⅵ) (initial concentration of 10 mg·L-1), and the reduction efficiency is increased by 82% compared with the single Cr (Ⅳ) solution. The mechanism study shows that there is a synergistic effect between the oxidation of phenol and the reduction of Cr (Ⅵ), as shown in Figure 3D. Both MIL-101 (Fe) and Ag-NPs can be excited to generate electrons, and the construction of the ternary heterojunction system accelerates the transfer of photogenerated carriers; however, the generated photogenerated electrons are not enough for the effective reduction of Cr (Ⅵ), and hydroquinone (HQ) produced by the oxidation of phenol by singlet oxygen (1O2) in the reaction system plays a role in promoting the reduction of Cr (Ⅵ)[58]. This research idea provides a new way for the photocatalytic reduction of Cr (Ⅵ) in the mixed system.

4 Advanced Oxidation Degradation of Organic Pollutants in Water by MIL-101 (Fe) and Its Composites

With the development of industry and modernization, water pollution caused by organic pollutants such as dyes, pesticides, antibiotics and personal care products has become one of the most serious threats to the ecological environment and human health. However, these organic pollutants have high chemical stability and are difficult to be degraded in the natural environment, and traditional biological treatment processes are also difficult to effectively remove them. In recent years, reactive oxygen species (ROS) such as Hydroxyl radical (· OH), sulfate radical (Advanced oxidation processes) and Superoxide radical (· O 2 -) produced by Advanced oxidation processes (AOPs) can effectively attack the molecular structure of these organic pollutants, degrade them into low-toxic or biodegradable small molecules, and even mineralize them into CO2 and H2O, which is the most promising water. In this section, the latest research progress of MIL-101 (Fe) and its composites for the degradation of organic pollutants in water by advanced oxidation technologies such as photocatalysis, activated H2O2 and activated persulfate (PS) is reviewed.

4.1 Photocatalysis

Photocatalysis is considered to be an environmentally friendly technology for the removal of organic pollutants because it can use the inexhaustible sunlight to cooperate with catalysts to produce active substances such as photogenerated h+, · OH and · O 2 - with strong oxidation ability. Traditional semiconductor catalytic materials (such as TiO2, ZnS and ZnO) are prone to agglomeration or photocorrosion in aqueous solution, which makes it difficult to treat trace pollutants, so it is urgent to develop new photocatalytic materials. At present, there are many reports about the photocatalytic degradation of organic pollutants by MIL-101 (Fe) and its composites. Wang et al. Compared the performance of three Fe-based MOFs, MIL-100 (Fe), MIL-53 (Fe) and MIL-101 (Fe), for the photocatalytic degradation of Tetracycline (TC) under visible light irradiation[59]. Among them, MIL-100 (Fe) consists of [Fe3O(X)(H2O)2]6+(X=OH- or F-) clusters and trimesic acid, MIL-53 (Fe) consists of one-dimensional chain-Fe-O-O-Fe-O-Fe- and terephthalic acid, while MIL-101 (Fe) is a crystal material composed of Fe3-μ3-oxo clusters and terephthalic acid. Among the three Fe-MOFs, MIL-101 (Fe) has the best photocatalytic performance for TC degradation (180 min, 96.6%). This is due to the large specific surface area, pore size, and pore volume of MIL-101 (Fe), with · O 2 -, · OH, and photogenerated h+ as the main active species (Figure 4A). In order to further improve the carrier separation efficiency of MIL-101 (Fe) in the photocatalytic process and produce more active radicals, researchers have modified MIL-101 (Fe) or compounded it with other active substances in recent years (Table 2) to enhance its photocatalytic degradation of organic pollutants.
图4 (a) 可见光照射下MIL-101(Fe)中光生电子-空穴对的分离和转移示意图[59];(b) m-MIL-101-1.0的制备过程示意图[64];(c) m-MIL-101-1.0中e-和h+的转移过程和光催化机理图[64];(d) MIL-101(Fe)/WO3光催化降解TCH机理图[74];(e) TiO2/MIL-101(Fe)的透射电子显微镜图[63];(f) CFs/TiO2/MIL-101(Fe)的吸附及光催化示意图[63]

Fig. 4 (a) Schematic diagram of the separation and transfer of photo-generated electron-hole pairs in MIL-101(Fe) under visible light irradiation[59]; (b) schematic diagram of the preparation process of m-MIL-101-1.0[64]; (c) schematic diagram of the transfer process and photocatalytic mechanism of e- and h+ in m-MIL-101-1.0[64]; (d) proposed charge separation process and catalytic mechanism for TCH photodegradation over MIL-101(Fe)/WO3 hybrid system[74]; (e) TEM image of TiO2/MIL-101(Fe)[63]; (f) schematic diagram of adsorption and photocatalytic mechanism of CFs/TiO2/MIL-101(Fe)[63]

表2 MIL-101(Fe)及其复合物用于光催化降解有机污染物

Table 2 MIL-101(Fe) and its composites for photocatalytic organic pollutants degradation

Catalyst/dosage (g·L-1) Polluant/Volume (mL)/
Concentration (mg·L-1)/pH		 Light source Reaction time (min) Degradation efficiency (%) ref
MIL-101(Fe)/0.5 tetracycline/100/50/- 300 W Xe lamp (λ≥ 420 nm) 180 96.6 59
V2O5/NH2-MIL-101(Fe)-10/0.5 tetracycline/100/-/- ultraviolet-visible light from a 300 W xenon lamp 120 88.3 60
NH2-MIL-101(Fe)/Cu2O-2/1 rhodamine B/100/4.8/- 300 W Xe lamp (λ≥ 420 nm) 90 92 61
Electrospun graphene oxide/MIL-101(Fe)/poly (acrylonitrile-co-maleic
acid) nanofiber/2		 rhodamine B/20/-/- ultraviolet lamp (16 W) 20 93.7 62
carbon fibers/TiO2/MIL-101(Fe)/2 17β-estradiol/100/3/-;tetracycline/100/20/- visible light 60 87.4 (17β-estradiol)/94.2 (tetracycline) 63
m-MIL-101-1.0/0.5 tetracycline/20/20/- 300 W Xe lamp (λ≥ 420 nm) 60 85.41 64
Magnetic MIL-101(Fe)/TiO2/1 tetracycline/50/20/7 solar light 10 92.76 65
5-Bi2MoO6/MIL-101(Fe)/0.3 rhodamine B/100/15/6.5 blue light LED 83.2 90 66
MIL-101(Fe)/gC3N4/0.5 bisphenol A/40/10/6.8 150 W halogen cold light source (λ≥ 420 nm) 240 94.8 27
1%Ag/AgCl/MIL-101(Fe/)1 phenol/50/10/6 300 W Xe lamp (λ≥ 420 nm) 30 70 32
g-C3N4/NH2-MIL-101(Fe)/1 2,6-dichlorophen/30/10/-
2,4,5-trichlorophenol/30/10/-		 300 W Xe-lamp 180 98.7 (2,6-
dichlorophen)/
97.3 (2,4,5-
trichlorophenol)		 67
Cu2O/Fe3O4/MIL-101(Fe)/0.5 ciprofloxacin//20/7 500 W Xe lamp 105 99.2 43
NCQDs/MIL-101(Fe)/0.5 tetracycline/100/10/- 500 W Xe lamp (λ≥ 420 nm) 180 100 68
g-C3N4@NiO/Ni-3@MIL-101/0.01 ibuprofen/30/30/- 500W Xenon (λ>400 nm) 120 95.6 69
Tm@Yb@Y/NMF/0.03 tetracycline/levofloxacin/ rhodamine B/60/20/- 500 W Xe lamp 50 47 (tetracycline)/
70 (levofloxacin)/
77 (rhodamine B)		 70
NH2-MIL-101(Fe)/Ti3C2Tx/1 phenol/chlorophenol/100/23.5/- 300 W Xe lamp (λ≥ 420 nm) 60 99.36 (phenol)/
99.83 (chlorophenol)		 71
Reasonable control of the structure of MOFs can improve the utilization efficiency of photogenerated carriers[72]. Based on the defect engineering strategy, Xie et al. First used cetyltrimethylammonium bromide (CTAB) to introduce defect sites inside MIL-101 (Fe)[64]. Subsequently, sodium borohydride (NaBH4) was used to regulate the metal valence, and Fe (Ⅱ) was introduced into MIL-101 (Fe), and a novel MOFs material (M-MIL-101-1.0) with both internal ligand and Fe element valence regulation was successfully prepared (Fig. 4B). Compared with single MIL-101 (Fe), the photocatalytic degradation rate of TC by m-MIL-101-1.0 was increased by 3. 48 times. This is mainly due to its unique structure (Fig. 4C). The redox reaction of Fe (Ⅲ)/Fe (Ⅱ) caused by internal and external charge transport effectively consumes part of the photogenerated e-, promotes the separation of carriers and produces active species such as · OH, which accelerates the oxidative decomposition of TC.
In addition to structural defects, combining MIL-101 (Fe) with semiconductor materials to form heterojunction photocatalysts has unique advantages in effectively promoting the separation of photogenerated carriers and the generation of more active oxygen species, which is another feasible method to achieve efficient degradation of organic pollutants. Tungsten oxide (WO3) is a common semiconductor material that can be well matched with other semiconductors to form heterojunction complexes due to its ease of synthesis, non-toxicity, low cost, narrow band gap, and strong photo-generated hole oxidation ability[73].
Yang et al. Designed and prepared S-type heterojunction MIL-101(Fe)/WO3 by tightly fixing MIL-101 (Fe) nanoparticles on the surface of WO3 nanoplates via solvothermal reaction[74]. This ingenious structural design endows the synthesized MIL-101(Fe)/WO3 materials with efficient photocatalytic activity and good photostability. The MIL-101(Fe)/WO3(1∶1) material with the highest photocatalytic efficiency was obtained by adjusting the molar ratio between MIL-101 (Fe) and WO3, and the degradation efficiency of Tetracycline hydrochloride (TC-HCl) reached 93. 8% in 6 H. The enhancement of photocatalytic activity is attributed to the S-type heterojunction promoting fast separation of photogenerated carriers and maintaining high oxidation/redox ability of photogenerated holes/electrons.
Most of the powder catalysts are difficult to be recovered from aqueous solution, and are easy to cause secondary pollution of water quality. The prepared immobilized catalyst can effectively solve the above problems. Carbon fibers (CFs), as a flexible material, have excellent electrical conductivity and mechanical strength, and can be used as a good carrier for many semiconductor catalysts[75]. Zhang et al. Used the impregnation-hydrothermal-solvent method to immobilize TiO2 and MIL-101 (Fe) on easily recyclable CFs to synthesize CFs/TiO2/MIL-101(Fe) composite[63]. The prepared MIL-101 (Fe) has an ultra-small size (5 ~ 10 nm) (Fig. 4E), which greatly improves the BET specific surface area and total pore volume of the composite, and enhances the adsorption capacity for 17β-estradiol (E2) and TC. Under visible light irradiation, the photocatalytic degradation efficiency of E2 and TC can reach 87. 4% and 94. 2% within 60 min, which is 6. 0-13. 1 times higher than that of CFs/TiO2. As shown in Figure 4F, the main reason for the improvement of photocatalytic removal efficiency is that the CFs/TiO2/MIL-101(Fe) composite not only broadens the light absorption range, but also the photogenerated e- is efficiently transferred from MIL-101 (Fe) to TiO2 and then to CFs, and the generated · O 2 - and photogenerated h+ are the main active species. More importantly, the large-sized (4 cm × 4 cm) CFs/TiO2/MIL-101(Fe) catalyst has convenient recyclability and efficient utilization. Huang et al. Immobilized MIL-101 (Fe) on electrospun graphene oxide (E-GO) and poly (acrylonitrile-maleic acid) nanofibers (PANCMA NFs) by electrospinning technology, and prepared E-spun GO/MIL-101 (Fe)/PANCMA NFs composite for efficient degradation of rhodamine B (RhB), which achieved at least 20 times of recycling[62]. The preparation and design of these immobilized catalysts provide a new strategy for the construction of efficient, stable and easily recyclable photocatalytic platforms.

4.2 Activated H2O2

As a classical Fenton reaction, activated H2O2 can produce highly active · OH (formula 1 and 2) through the cyclic reaction of Fe (Ⅱ) and Fe (Ⅲ), which has the characteristics of simple operation, easy availability of raw materials and low energy consumption, and is one of the most important advanced oxidation technologies[76]. The traditional homogeneous Fenton reaction produces a large amount of iron sludge and can only be carried out at a narrow pH value (2 ~ 4)[77]. Heterogeneous Fenton system can solve the above problems well. As an excellent heterogeneous catalyst, MOFs have shown great potential in the field of activated H2O2 treatment of organic wastewater. In recent years, our group has prepared a series of Fe-based MOFs and their composites, such as MIL-88A, BUC-21 (Fe), immobilized NH2-MIL-101(Fe), WO3/MIL-100(Fe), MIL-100 (Fe)/CoS and PANI/MIL-88A (Fe), for the degradation of organic pollutants by activated H2O2[78][79][80][39][81][82]. MIL-101 (Fe), as a classical Fe-MOFs, has numerous unsaturated Lewis acid active iron sites, which can adsorb Lewis base H2O2 and generate Fe-H2O2 complexes, thus reacting to produce · OH[83][84]. At present, there have been numerous relevant reports on the use of MIL-101 (Fe) and its composites in the activation of H2O2 for the degradation of organic pollutants (Table 3).
表3 MIL-101(Fe)及其复合物用于活化H2O2降解有机污染物

Table 3 MIL-101(Fe) and its composites for organic pollutants degradation via activation of H2O2

Catalyst/dosage (g·L-1) Polluant/Volume (mL)/ Concentration (mg·L-1)/pH H2O2 dosage Light source Reaction time (min) Degradation efficiency (%) ref
MIL-101(Fe)/0.1 phenol/150/50/4 15 mM in dark 30 62 83
Fe3O4/MIL-101(Fe)/0.5 rhodamine B/100/10/7 20 mM in dark 30 100 85
NH2-MIL-101(Fe)/0.1 rhodamine B/50/0.025 mM/7.22 0.5 mL in dark 4 100 86
GA/MIL-101(Fe)/0.1 phenol/50/0.1 mM/5 6 mM in dark 40 99 84
MIL-101(Fe,Cu)/0.1 ciprofloxacin/100/20/7 3 mM in dark 30 100 87
NH2-MIL-101(Fe) -EPU/0.5 tetrabromobisphenol A/20/1.84 mM/3 165 mM light-emitting diodes (λ≥ 400 nm) 120 120 88
MIL-101(Fe,Co)/0.2 ciprofloxacin/100/20/5 5 mM in dark 30 97.8 89
NH2-MIL-101(Fe)/0.2 bisphenol A/50/50/6 10 mM in dark 30 100 24
MIL-101 (Fe)/PANI/Pd/0.05 methylene Blue/-/25/7 1 M - 34 92 90
MoS2@NH2-MIL-101(Fe)/0.2 rhodamine B/50/50/-
bisphenol A/50/20/-		 1.76 mM 300 W Xe lamp 10 97.4 (RhB)
99.9 (BPA)		 91
Fe/Ce-MIL-101/0.3 norfloxacin/-/10/7 20 mM in dark 60 94.8 92
TiO2@17%NH2-MIL-101(Fe)/1 methylene Blue/100/50/- - 300 W Xe lamp (λ≥ 420 nm) 30 96 93
CNT@MIL-101(Fe)/0.5 ciprofloxacin/100/3.02 μM/3 165 mM white light LEDs, 360-830 nm 45 90 94
GO@MIL-101(Fe)/0.5 tris(2-chloroethyl) phosphate/-/3.51μM/3 165 mM multiple wavelength LEDs 30 95 95
AFG@30MIL-101(Fe)/0.4 diazinon/50/30/9
atrazine/50/30/2		 1.5 mL high-pressure mercury-
vapor lamp (400 W and λ = 546.8 nm)		 120 100 (diazinon)
81 (atrazine)		 96
MIL/Co/(3%)GO/0.2 direct Red 23/-/100/3
reactive Red 198/-/100/3		 50 μL 100 W LED projector 70 99.93
(Direct Red 23)
99.65
(Reactive Red 198)		 97
MIL-101(Fe)@Zn/Co-ZIFs/0.2 rhodamine B/50/100/5 90 mM 350 W Xe lamp (λ≥ 420 nm) 180 98 98
MIL-101(Fe)/Bi2WO6/Fe(Ⅲ)/
0.5		 methylene Blue/100/20/- 500 μL 200 W incandescent lamp 75 86.7 99
MIL-101(Fe)-NH2@Al2O3/0.3 norfloxacin/50/10/- 15 μL 350 W Xe lamp 97.3 100 80
Fe(Ⅱ) + H2O2 → Fe(Ⅲ) + ·OH + OH
Fe(Ⅲ) + H2O2 → Fe(Ⅱ) + ·HO2 + H+
Gao et al. Experimentally verified that MIL-101 (Fe) could catalytically activate H2O2 to achieve 62% degradation of phenol within 30 min (initial concentration was 50 mg·L-1)[83]. Zhang et al. Grown MIL-101 (Fe) in situ in Graphene aerogel (GA) by nano-confinement strategy, and obtained GA/MIL-101 (Fe)[84]. The experiment of catalytic activation of H2O2 showed that the degradation efficiency of phenol by GA/MIL-101 (Fe) was 3. 7 times higher than that by powder MIL-101 (Fe). The nano-confinement effect makes the MIL-101 (Fe) in GA have smaller particle size and more abundant Fe (Ⅱ) content, and the abundant carbon defects of GA also contribute to the improvement of catalytic performance. It is worth mentioning that GA/MIL-101 (Fe) has a higher hydrophobicity, which provides a natural protective barrier for the catalyst (Figure 5A), avoiding the deactivation of the catalyst due to the occupation of the active site by water molecules in a humid environment. Compared with powder MIL-101 (Fe), GA/MIL-101 (Fe) has a broader practical application prospect, which also provides a new idea and perspective for the storage of MOFs catalysts and the improvement of the stability of MOFs catalysts. Molding microcrystalline MOFs materials into macroscopic bulk water treatment materials is also a feasible strategy[100]. Zheng et al. Designed and constructed a MIL-101-Fe-NH2@ Melamine sponge (MS) with hierarchical porous composites[101]. The MIL-101-Fe-NH2 particles were singly attached to the MS surface by the adhesion of polydopamine (PDA). Compared with the powder catalyst, the composite material exposes more catalytic sites, which avoids the deactivation of the catalyst due to aggregation. The composite MIL-101-Fe-NH2@MS showed the highest degradation efficiency of tetracycline hydrochloride in weak acid environment, which reached 77. 24% in 30 min. Due to the synergistic effect of the layered porous structure and monodispersed nanoparticles, the composite exhibited faster reaction rate and longer lasting degradation ability compared to the powder MIL-101-Fe-NH2. The good stability, easy recovery and less ion leaching make the composite MIL-101-Fe-NH2@MS a great advantage for future practical applications.
图5 (a) GA/MIL-101(Fe)的制备策略示意图[84];(b) Fe-BDC-NH2/H2O2系统催化降解BPA的机理图[24];(c) CUMSs/MIL-101(Fe, Cu)/H2O2系统催化降解CIP的机理图[87];(d) MIL-101(Fe)/H2O2/vis系统催化降解TC—HCl的机理图[103]

Fig. 5 (a) Schematic diagram showing the design strategy of GA/MIL-101(Fe)[84]; (b) schematic diagram of the proposed mechanisms involved for BPA degradation in Fe-BDC-NH2/H2O2 system[84]; (c) schematic diagrams of the proposed mechanism involved in CIP degradation by CUMSs/MIL-101(Fe, Cu)/H2 O 2 [87]; (d) illustration of the proposed reaction mechanism for TC-HCl removal in MIL-101(Fe)/H2O2/visible light system[103]

In Fenton system, the consumption rate of Fe (Ⅱ) is 103 times of its production rate, and the low conversion rate of Fe (Ⅲ) to Fe (Ⅱ) greatly limits the reaction activity of heterogeneous catalyst[102]. Therefore, it is very important to increase the reduction rate of Fe (Ⅱ) in MIL-101 (Fe) for the production of · OH. Huang et al. Proved by theoretical calculation that the introduction of —NH2 could make the catalyst have a higher H2O2 adsorption energy (Eabs:-1.58 eV), a higher Fermi level (− 4.88 eV) and a lower activation energy (0.32 eV) in the reaction, which improved the conductivity and accelerated the Fe (Ⅲ)/Fe (Ⅱ) cycle[24]. In the presence of H2O2, the degradation performance of Bisphenol A (BPA) was improved with the increase of the content of —NH2. The catalyst using pure aminated terephthalic acid as the ligand (Fe-BDC-NH2) had the highest catalytic activity, and the complete removal of BPA could be achieved in 10 min. The detailed reaction mechanism is shown in Fig. 5B. The introduction of —NH2 plays an important role. The —NH2 with higher electron density of States can not only accelerate the decomposition of H2O2, but also adjust the nodes of Fe — O cluster by increasing the electron density of Fe (Ⅲ), so as to generate Fe (Ⅱ) in situ in MOFs. More importantly, —NH2, as an electron-donating group, accelerates the electron transfer and accelerates the reaction rate of Fe (Ⅲ) → Fe (Ⅱ) and the formation rate of · OH.
MIL-101 (Fe) complexed with other guest molecules to construct Coordinatively unsaturated metal sites (CUMS), which is another way to promote the conversion of Fe (Ⅲ) to Fe (Ⅱ) and increase the rate of Fenton reaction. Liang et al. Doped transition metal Cu in MIL-101 (Fe) and constructed MIL-101 (Fe) with Cu (II)/Cu (I) and Fe (III)/Fe (II) coordinatively unsaturated metal sites, namely CUMSs/MIL-102 (Fe, Cu)[87]. Due to the synergistic effect of Fe and Cu, the removal rate of Ciprofloxacin (CIP) by CUMSs/MIL-101 (Fe, Cu) was 5. 9 times and 20. 0 times higher than that of CUMSs/MIL-101 (Fe) and ML-101 (Fe), respectively, after the introduction of H2O2. As shown in Fig. 5C, both Fe (II) CUMSs and Cu (I) CUMSs can produce · OH by activating the H2O2 (Equations 3 and 4); Cu (I) CUMS promotes the conversion of Fe (III) CUMSs to Fe (II) CUMSs (Equation 5), increases the conversion rate of Fe (III) → Fe (Ⅱ), and promotes the continuous production of · OH; On the other hand, the introduction of CUMS enhances the polarization distribution of electrons and accelerates the conversion of high-valence Fe (III) and Cu (II) to low-valence Fe (II) and Cu (I), respectively (Equations 6 and 7). Liang et al. Also constructed MIL-101 (Fe, Co) bimetallic MOFs[89]. The degradation efficiency of MIL-101 (Fe, Co) for CIP is 7. 5 times higher than that of MIL-101 (Fe) due to the synergistic effect of Fe and Co dual active sites, which enhances the activation performance of H2O2. Li et al. Combined MoS2 with NH2-MIL-101(Fe) for efficient degradation of BPA[91]. As a classical cocatalyst, MoS2 can accelerate the conversion of Fe (Ⅲ) to Fe (Ⅱ) and improve the reaction efficiency of activated H2O2.
Fe(Ⅱ)CUMSs + H2O2 → Fe(Ⅲ)CUMSs + ·OH + OH-
Cu(Ⅰ)CUMSs + H2O2 → Cu(Ⅱ)CUMSs + ·OH + OH-
Cu(Ⅰ)CUMSs + Fe(Ⅲ)CUMSs → Cu(Ⅱ)CUMSs + Fe(Ⅱ)CUMSs
Fe(Ⅲ)CUMSs + π → Fe(Ⅱ)CUMSs + π*
Cu(Ⅱ)CUMSs + π → Cu(Ⅰ) + π*
The reaction of H2O2 activation by MIL-101 (Fe) combined with photocatalysis can construct a photo-Fenton system. Photocatalysis and Fenton reaction have a synergistic effect. On the one hand, H2O2, as an electron acceptor, can effectively react with photogenerated electrons generated by photocatalysis to generate · OH and inhibit the recombination of photogenerated electrons and holes (Formula 8). On the other hand, the photogenerated electrons have strong reduction ability and can reduce Fe (III) to Fe (II) (Formula 9). Wu et al. Synthesized and characterized a series of typical Fe-MOFs with the same organic ligand (terephthalic acid) but different topologies, namely MIL-101, MIL-53, and MIL-88 B,Their photo-Fenton properties were evaluated by comparing the degradation of tetracycline hydrochloride by activated H2O2 under visible light irradiation, and the reasons for the differences were analyzed according to the structural, optical, and redox properties, as well as the number of coordinatively unsaturated iron sites[103].
First, the Fe-O clusters of the three Fe-MOFs are different, with corner-sharing FeO4(OH)2,MIL-101 in MIL-53 and Fe3-μ3-oxo in MIL-88B[104]. The catalytic performance of MIL-53 is relatively poor because the μ3-O atom in the catalyst can promote the formation of electron delocalized States in the μ3-O cluster unit, which helps the electron transfer from the metal ion to the oxidant to form active species. Secondly, by comparing the specific surface area, pore volume and the number of coordination unsaturated sites of the three, it is concluded that MIL-101 > MIL-88 B > MIL-53. MIL-101 has the largest specific surface area, pore volume and the largest number of coordinatively unsaturated iron sites, thus showing the best photo-Fenton performance. By exploring the mechanism of MIL-101 activating H2O2 to degrade TC-HCl under visible light irradiation, it can be concluded that there is a significant synergistic effect between photocatalysis and Fenton process in the :MIL-101/H2O2/vis system, and visible light can accelerate the cycle of Fe (Ⅲ)/Fe (Ⅱ) in the system, which helps to increase the amount of · OH and improve the oxidation ability of the whole system (Figure 5D).
In order to further improve the optical absorption characteristics of MIL-101 (Fe) and its stability in the photo-Fenton system, many researchers have used MIL-101 (Fe) and other functional materials for the photo-Fenton degradation of organic pollutants. In order to overcome the problem that the powder material is easy to run off and difficult to recycle, Zhao et al. Fixed the NH2-MIL-101(Fe) on a α-Al2O3 to improve the stability and recycling of the catalyst in the photo-Fenton degradation of Norfloxacin (NOR)[80]. The fabrication of MIL-101 (Fe) -based magnetic composites, such as MIL-101(Fe)/CoFe2O4/GO composites and MIL-101(Fe)/Fe3O4/GO composites, not only enhances the performance of photo-Fenton synergistic degradation of organic pollutants, but also the magnetic composites improve the catalyst stability and recyclability[97][96].
H2O2 + e- → ·OH + OH-
Fe(Ⅲ) + e- → Fe(Ⅱ)

4.3 Activated persulfate

Sulfate radical (SO4·-) -based advanced oxidation process (SR-AOP) is an emerging technology for the degradation of organic pollutants, which has attracted more and more researchers' attention due to its obvious advantages in the degradation of emerging pollutants[105]. Compared with · OH-based AOPs, SR-AOP has many advantages. The redox potential of :(1)SO4·- (2.5 ~ 3.1 V) is higher than that of · OH, which can degrade pollutants that · OH can not treat[106]. (2) The half-life of hydroxyl radical is 10~30μs,SO4·-, which has a longer half-life (30 ~ 40 μs), increasing the probability of free radical contacting with reactants and improving the degradation efficiency of pollutants[107]; The (3)SO4·- has excellent degradation effect on pollutants in a wide pH range, and has better selectivity for specific pollutants; (4) Persulfate (PS) usually exists in solid form and has good stability, which is convenient for transportation and storage. MIL-101 (Fe) has dispersed metal sites in the framework and is chemically stable, which can compensate for the uncontrollability of free radical generation and the shortcomings of iron leaching, and is regarded as a promising heterogeneous catalyst in the field of SR-AOP. The relevant studies and properties of activated persulfates of MIL-101 (Fe) and its complexes are shown in Table 4.
表4 MIL-101(Fe)及其复合物用于活化过硫酸盐降解有机污染物

Table 4 MIL-101(Fe) and its composites for organic pollutants degradation via activation of persulfate

Catalyst/Dosage (g·L-1) Polluant/Volume (mL)/ Concentration (mg·L-1)/pH PS dosage Light source Reaction time (min) Degradation efficiency (%) ref
MIL-101(Fe)/0.625 acid orange 7/25/80/6.16 15 mM in dark 120 95 108
Fe3O4@MIL-101/1 acid orange 7/10/25/3.58 25 mM in dark 60 98.1 109
Quinone-modified NH2-MIL-101(Fe)/0.2 bisphenol A/25/60/5.76 10 mM in dark 120 97.7 110
6 wt% Co-MIL-101(Fe)/0.2
6 wt% Cu-MIL-101(Fe)/0.2		 acid orange 7/100/0.1 mM/- 8 mM in dark 180 92 (6 wt% Co-MIL-101(Fe))
98 (6 wt% Co-MIL-101(Fe))		 17
g-C3N4/MIL-101(Fe)/0.5 bisphenol A/-/10/- 1 mM 350 W Xe lamp (λ≥ 400 nm) 60 98 111
MIL-101(Fe) via vacuum thermal treatment/0.1 X-3B/100/100/- 15 mM in dark 180 95.7 112
MIL-101(Fe)/0.5 tris(2-chloroethyl)
phosphate/20/3.51 μM/-		 500 mg·L-1 light-emitting diodes (LEDs) with emission peaks 180 > 90 113
MIL-101(Fe)/TiO2/1 tetracycline/-/80/7 1 g·L-1 500 W Xe lamp 30 93.02 114
MIL-101(Fe)-NH2/1 amaranth/200/50/7 200 mg·L-1 150 W visible light 30 100 115
NH2-MIL-101(Fe)/0.02 bisphenol F/200/20/5 1 mM in dark 120 100 116
MIL-101(Fe)/1 methylene Blue/20/10/7 500 mg·L-1 in dark 25 > 90 117
N,S:CQD/MIL-101(Fe)/0.4 bisphenol A/100/20/- 3 mM 350 W Xe lamp (λ≥ 400 nm) 60 100 118
CuS-modified MIL-101(Fe)/0.1 E. coli/100/ 107.5 cfu·
mL-1/6.5		 50 μM white LED lamps (11,000 Lux, 400~700 nm) 40 100 30
TiO2@MIL-101(Fe)/1.052 nitrobenzene/28.5/800 μM/- 1.6 mM Xe lamp (λ≥ 420 nm) 240 66.53 31
RGO/MIL-101(Fe)/0.5 trichlorophenol/-/20/3 20 mM in dark 180 92 119
MIL-101(Fe)/0.1 orange G/50/15/3 0.05 mM in dark 40 74 120
MIL-101(Fe)/g-C3N4/0.08 tetracycline hydrochloride/
50/-/3.5		 0.85 mM 30-W LED lamp (λ=410~760 nm) 40 99 121
NH2-MIL-101(Fe)-ferrocene/0.2 bisphenol A/25/60/5.76 10 mM in dark 40 100 122
NH2-MIL-101(FeCo)-2/0.005 orange G/99/0.2 nM/7 2 mM in dark 45 100 123
M/Z2/0.01 2-chlorophenol/100/100/9 300 mg·L-1 in dark 10 90.3 124
Li et al. Used MIL-101 (Fe), MIL-100 (Fe), MIL-53 (Fe) and MIL-88B (Fe) as catalysts to degrade Acid orange 7 (AO7)[108]. The results show that MIL-101 (Fe) has the highest adsorption performance and catalytic activity for persulfate, and shows excellent catalytic activity in a wide pH operating range and good chemical stability after continuous recycling. Mechanism exploration is shown in Fig. 6a. First, the unique pore structure of MIL-101 (Fe) can adsorb PS, and PS reacts with metal site Fe (Ⅲ) to generate a large amount of S2O8·- and Fe (Ⅱ) (Formula 10), and the generated Fe (Ⅱ) is oxidized by S2 O 8 2 - to generate SO4·- (Formula 11). · OH can be generated by Equations 12 and 13. In addition, Fe (Ⅱ) can activate dissolved oxygen to form · O 2 -. The generated active species such as SO4·-, · OH, and · O 2 - can degrade AO7 (Equation 14).
图6 (a) MIL-101(Fe)活化PS催化降解AO7的机理图[108];(b) 分别使用HBC、HAC、OA和CA(Fe,深橙色;C,黑色;O,红色;H,白色)调控,合成缺陷MIL-101(Fe)催化剂的制备策略[128];(c) RGO/MIL-101(Fe)活化PS的反应机理图[119];(d) N, S: CQDs /MIL-101(Fe)/PS/vis体系中的光催化降解机理图[118]

Fig. 6 (a) The possible elimination mechanism of AO7 by MIL-101(Fe)[108]; (b) scheme of the strategy for the syntheses of defective MIL-101(Fe) by modulating synthesis using HBC, HAC, OA, and CA, respectively (Fe, dark orange; C, black; O, red; H, white)[128]; (c) schematic diagram of the reaction mechanism of the PS activation by RGO/MIL101(Fe)[119]; (d) possible photocatalytic degradation mechanism in the N, S: CQD/MIL-101(Fe)/PS/vis system[118]

Fe(Ⅲ) + S2 O 8 2 -→ Fe(Ⅱ) + S2O8·-
Fe(Ⅱ) + S2 O 8 2 -→ Fe(Ⅲ) + SO4·-+ S O 4 2 -
SO4·-+ H2O → ·OH + H ++ S O 4 2 -
SO4·-+ OH- → S O 4 2 -+ ·OH

AO7 + SO4·- / ·OH /· O 2 - →···→

CO2 + H2O + ···

Compared with other heterogeneous catalytic materials such as zero-valent iron or Fe3 O 4 , the content of unsaturated iron active sites in MIL-101 (Fe) is low, and only the less active Fe (Ⅲ) sites exist[125][126][109]. Compared with carbon materials such as activated carbon and graphene, MIL-101 (Fe) faces the limitations of single channel structure and easy agglomeration, which leads to problems in the transport and utilization of active oxygen species[127]. Therefore, the key issues to be addressed are to increase the active sites of MIL-101 (Fe) to generate free radicals and to improve the utilization of free radicals to accelerate the catalytic degradation process. Defect engineering can help overcome diffusion constraints and create active sites. Guo et al. Successfully prepared four novel defective MIL-101 (Fe) catalysts with coordinatively unsaturated sites by different regulators (benzoic acid (HBC), acetic acid (HAC), citric acid (CA), and oxalic acid (OA)), and the preparation strategy is shown in Figure 6B[128]. Compared with the single MIL-101 (Fe), the degradation efficiency of RhB by activated PS was increased from 58.70% to 94.05% (MIL-101 (Fe) -HBC), 86.11% (MIL-102 (Fe) -HAC), 82.62% (ML-102 (Fe. The introduction of defects is helpful to increase the content of Fe (Ⅱ) and improve the charge transfer efficiency, which is of great significance for accelerating electron transfer and enhancing catalytic performance. In addition, the catalyst has good stability and reusability, and can reduce the dissolution of iron ions to a certain extent. The construction of defective MIL-101 (Fe) with coordinatively unsaturated sites has great potential for the degradation of environmental pollutants.
Xu et al. First successfully prepared redox graphene (Reduced graphene oxide, RGO)/MIL-101 (Fe) as a heterogeneous catalyst to activate PS, and the reaction mechanism is shown in Figure 6C[119]. Benefiting from the rich active sites of the composite and the good electronic conductivity generated by the repaired large π-conjugated planar structure, RGO/MIL-101 (Fe) is superior to RGO and MIL-101 (Fe) alone in the activation of PS and the catalytic degradation of Trichlorophenol (TCP). RGO/MIL-101 (Fe) can degrade 92% of TCP (initial concentration of 20 mg·L-1) in 180 min. In addition, chemical and thermal reduction processes play a key role in regulating defect levels and electron transfer channels. The changes and physicochemical properties of RGO before and after complexing with MIL-101 (Fe) were characterized by various methods. The results show that the introduction of an appropriate amount of RGO increases the specific surface area and pore volume without changing the crystal structure of MIL-101 (Fe), which is beneficial to the contact between the activation site and PS.
Carbon quantum dots (CQDs) are a new type of quasi-zero-dimensional Carbon nanomaterials, which have become a hot research topic in recent years due to their excellent electron transport and efficient light harvesting ability[129][130]. The modification of MOFs materials with CQDs can greatly improve the photocatalytic efficiency and help the regeneration of materials[131][132]. In addition, N, S, P and B atom doping can effectively promote charge delocalization, reduce the work function of carbon, and improve the electron transfer ability of CQDs[133]. Jiang et al. Prepared N, S: CQDs/MIL-101 (Fe) composites by encapsulating N, S: CQDs in the framework of MIL-101 (Fe) through a two-step solvothermal method[118]. The degradation efficiency of BPA by N, S: CQDs/MIL-101 (Fe)/PS/vis system reached 100% within 60 min, and the reaction rate was 3. 6 times that of MIL-101 (Fe)/PS/vis system. The detailed reaction mechanism diagram is shown in Fig. 6d. Firstly, under the irradiation of visible light, the N, S: CQDs composite catalyst produces a large number of photogenerated electrons and holes, in which the electrons in MIL-101 (Fe) are transferred from the valence band to the conduction band to produce Fe (II), thus activating PS to produce SO4·-. At the same time, N, S: CQDs are excited by visible light to generate photogenerated electrons, which are then transported from N, S: CQDs to the conduction band of MIL-101 (Fe) through the interface channel, thus promoting the conversion of active metal centers from Fe (Ⅲ) to Fe (Ⅱ) and accelerating the activation of PS. In addition, part of the SO4·- produced by the activation of PS reacted with OH- in solution to form · OH. Some of the electrons transferred to the surface of N, S: CQDs/MIL-101 (Fe) composite directly act on PS to break its O — O bond to form SO4·-. In addition, Hu et al. Directly used MIL-101 (Fe) to activate PS to degrade organophosphorus flame retardants under different wavelength irradiation[113]. Bi et al. Constructed a MIL-101(Fe)/g-C3N4 system to activate PS for efficient degradation of tetracycline hydrochloride[121]. Inactivation of E. Coli was achieved by Jiang et al. Using CuS-modified MIL-101 (Fe) to produce singlet oxygen under visible light[30]. The introduction of light provides new ideas and unlimited possibilities for PS activation by MIL-101 (Fe).

5 Water Stability and Biological Toxicity of MIL-101 (Fe)

As a potential water treatment material, it is necessary to explore the water stability of MIL-101 (Fe) in order to improve its practical application value. According to the HSAB theory, MOFs constructed by hard Lewis acid (metal salt) and hard Lewis base (organic ligand) or soft Lewis acid (metal salt) and soft Lewis base (organic ligand) usually have stronger chemical stability[134]. Fe (Ⅲ) and terephthalic acid, which constitute MIL-101 (Fe), belong to hard Lewis acid and hard Lewis base, respectively. In addition to this, MOFs synthesized as the rigid ligand terephthalic acid are more water stable than other flexible ligands[135]. Therefore, MIL-101 (Fe) is also referred to as water-stable MOFs. MOFs synthesized from hard Lewis acids and hard Lewis bases are more water-stable under acidic conditions than under alkaline conditions, so MIL-101 (Fe) is also more suitable for the treatment of pollutants in acidic wastewater. However, long-term exposure of MIL-101 (Fe) to water may result in the dissolution of ligands or the loss of metal clusters, especially in practical applications, the stability of MIL-101 (Fe) is not only affected by water, but also by other complex environmental factors (such as pH, inorganic anions and cations, organic pollutants and reactive oxygen species). In recent years, many researchers have also improved the water stability of MIL-101 (Fe) by enhancing the strength of the coordination bond, improving the synthesis method, and constructing and immobilizing hydrophobic MIL-101 (Fe). Fu et al. Prepared MIL-101 (Fe) -1- (4- (ethyl) phenyl) urea (named MIL-101 (Fe) -EPU) for photocatalytic activation of H2O2 for degradation of Tetrabromobisphenol A by grafting phenylethyl side chain with low surface energy on MIL-101(Fe)-NH2 by post-synthesis modification[88]. MIL-101 (Fe) -EPU not only has higher hydrophobicity, but also remains intact after exposure to water for 72 H, showing stronger water stability than single MIL-101(Fe)-NH2. Kuznicki et al. Used amide-functionalized ligands as modifiers during the synthesis of MIL-101 (Fe) to improve its water stability[136]. In addition, Zhao et al. And Zhang et al. Immobilized MIL-101(Fe)-NH2 and MIL-101 (Fe) on α-Al2O3 substrate and graphitized aerogel, respectively, which enhanced the water stability while improving its recycling[42,80][84].
In addition to water stability, its biological toxicity and possible environmental problems need to be explored. Based on the toxicity data of IC50, the toxicity of Fe nanoparticles is low, next to Ca, Bi, Eu and Ti, etc[137]. Tamames-Tabar et al. analyzed and evaluated the cytotoxicity of 14 MOFs (including MIL-101(Fe)-NH2 and MIL-101(Fe)-CH3) on human epithelial cells (HeLa) and murine macrophage cell line (J774) of fetal cervical cancer.Among them, the toxicity of Fe-based MOFs (MIL-101(Fe)-NH2:IC50(HeLa)=1.00 mg·mL-1, IC50(J774)=0.07 mg·mL-1;MIL-101(Fe)-CH3:IC50(HeLa)=2.50 mg·mL-1, IC50(J774)=0.17 mg·mL-1) is much lower than that of Zn-based MOFs represented by ZIF-8 (ZIF-8:IC50(HeLa)=0.100 mg·mL-1;IC50(J774)=0.025 mg·mL-1)[138]. Ruyra et al. Studied the extracellular toxicity (in vitro cytotoxicity of HepG2 and MCF7 cells) and intracellular toxicity (effects on zebrafish embryos) of a variety of typical MOFs, and the intracellular toxicity of MIL-101 (Fe) was greater than the extracellular toxicity[139]. In general, the biological toxicity of MIL-101 (Fe) is higher than that of Mg-based MOFs and Zr-based MOFs, but lower than that of Zn-based MOFs and Cu-based MOFs.

6 Conclusion and prospect

MIL-101 (Fe) is one of the most important Fe-MOFs with abundant Fe-O clusters and excellent water stability, which has broad application prospects in catalytic removal of pollutants in water. Under the irradiation of visible light, MIL-101 (Fe) and its complexes can reduce Cr (Ⅵ) to Cr (Ⅲ), and photocatalytically degrade organic pollutants in water. MIL-101 (Fe) and its complexes can also efficiently activate H2O2 and persulfate to produce active substances such as · OH and SO4·-, thus achieving efficient degradation of organic pollutants. In recent years, MIL-101 (Fe) and its composites have made remarkable progress as efficient catalysts in the field of water treatment through structural defects, functional group modification, and the construction of binary or multiple heterojunctions, but there are still some key issues, challenges, and opportunities to be considered in the future.
(1) The traditional synthesis method of MIL-101 (Fe) usually requires expensive chemicals and high-purity solvents, as well as complex and harsh synthesis conditions and operation steps. Therefore, a novel high-throughput, low-cost, and green method for the efficient synthesis of MIL-101 (Fe) is urgently needed. (2) It is a feasible synthesis method to obtain metal ions and organic ligands by recycling various wastes and convert them into MIL-101 (Fe) with high added value, which is in line with the low-carbon development strategy. However, the current hydrolysis process of waste polymer and the synthesis process of MIL-101 (Fe) consume acid/alkali and some toxic organic solvents, and the purity of MOF obtained is not ideal due to the influence of coexisting substances in various wastes. Therefore, the green and efficient production of MIL-101 (Fe) without introducing additional toxic chemicals is the future development direction. (3) Tremendous research progress has been made regarding the application of MIL-101 (Fe), but the use of this MOF in practical and industrial water treatment applications is still limited. From a sustainable point of view, more research is needed on the "long-term stability" of MIL-101 (Fe) and its composites under harsh physical/chemical conditions. For example, stability under harsh conditions such as pharmaceutical wastewater, aquaculture wastewater, and industrial wastewater. (4) At present, the structure and morphology of MIL-101 (Fe) and its composites are mainly characterized by XRD, FTIR, XPS, SEM and TEM. In order to further explore the growth and degradation mechanisms of MIL-101 (Fe) and its complexes, it is necessary to use more sophisticated characterization methods, such as Mossbauer spectroscopy, synchrotron radiation and transmission electron microscopy with spherical aberration correction, to analyze the structure and morphology of MIL-101 (Fe) and its complexes in future studies. (5) Although MIL-101 (Fe) and its complexes can photocatalytically reduce Cr (Ⅵ) to Cr (Ⅲ), the residue of Cr (Ⅲ) in water is still a major problem to be solved. It is necessary to explore the technology of simultaneous removal of Cr (Ⅲ) in water by photocatalytic reduction of Cr (Ⅵ) with MIL-101 (Fe) and its complexes. (6) MIL-101 (Fe) and its composites can degrade organic pollutants in water through advanced oxidation technologies such as photocatalysis, activated hydrogen peroxide and persulfate. However, in the advanced oxidation system of MIL-101 (Fe) and its complexes, the degradation pathways of many organic pollutants are not clear, or the removal rate of total organic carbon is low. Therefore, it is necessary to further explore the biological toxicity of the degradation intermediates of organic pollutants, or to achieve zero emission of organic pollutants by coupling other advanced oxidation technologies and biotechnology. (7) The removal of trace or trace pollutants in water is still a difficult problem in the field of advanced oxidation. MIL-101 (Fe) and its complexes can adsorb many micro-pollutants, which can be used to pre-enrich low-concentration pollutants, thus achieving effective removal and providing direction for selective oxidation of pollutants.

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