English

• 1. Introduction

  Due to the continuous increase in industrial production and living demands, some emerging contaminants of pharmaceuticals and personal care products (PPCPs), endocrine disrupting compounds (EDCs), polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyl (PCBs), are continuously accumulated in environment. Those emerging pollution caused potential threats and direct harm to the ecological environment and human health [1-4], such as affecting human reproduction, metabolism and neurodevelopment, even causing cancer, deformity, mutation and death [5, 6]. Hence, academics have begun to develop new methods to control and reduce the contaminants in aqueous environment. The treatment methods include anode degradation, biodegradation, inorganic heterogeneous catalysis and activated carbon adsorption, etc. [7-9]. However, these technologies have diverse limitations of pollutants transfer, incomplete degradation and high energy requirements. For example, anode degradation needs high operating costs and challenging problems with fouling. Adsorption techniques have problems with the recovery and disposal of irreversible adsorbents.

  Advanced oxidation progresses (AOPs) could degrade organic pollutants by generating hydroxyl radicals (^•OH), superoxide radicals (O₂^•−), and sulfate radicals (SO₄^•−) that with strong oxidizing. A variety of AOPs, such as electrochemical oxidation, Fenton, ozone oxidation and photocatalysis, have been widely developed and applied [10, 11]. The central issue of AOPs is developing new catalysts that act as absolute domination for activity oxide species and their catalytic performances.

  In 1995, Omar M. Yaghi [12] synthesized a ligand compound with a two-dimensional structure composed of rigid organic ligand 1, 3, 5-benzenetricarboxylate (BTC) and transition metal Co, and named as metal-organic framework (MOF). However, MOFs did not attract much attention due to the materials were not stable at that time. By 1999, the Yaghi's research team [13] reported on a three-dimensional metal organic framework material MOF-5 with a simple cubic structure made of rigid organic ligand terephthalic acid and transition metal Zn. And the skeleton of MOF-5 remained intact after removing the guest molecules in the pores. This phenomenon attracted the attention to scholars and made MOFs develop rapidly. As shown in Fig. 1, the number of papers about MOFs is increasing during the past decade. The tunability of MOFs originated from the fact that various metal sites and organic ligands can constantly change and combine with each other to form another structure [14]. Tens of thousands of MOFs have been prepared and applied for catalytic synthesis [15, 16], drug delivery [17], separation and storage of gas [18, 19], electrode material [20] and fuel cells [21], etc.

  Figure 1

  

  Figure 1.  The number of publications in the area of MOFs from 2011 to 2020 (research Web of Science on Sep. 10, 2021, MOFs research volume growth trend).

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  Recently, MOFs are used to deal with various environmental issues in adsorption of heavy irons, degradation of organic pollutants in wastewater [22-24], or separation and adsorption of carbon dioxide and sulfur dioxide [25-27]. As compared to heterogeneous catalysts, MOFs have adjustable structure and designable catalytic center, which is easy to recycle than homogeneous catalysts [28, 29]. Therefore, AOPs, which need new catalysts to improve the effect of wastewater treatment, combined with a new functional environmental remediation materials MOFs, have been used by scholars to remove organic pollutants from water [24]. You et al. [30] used NiZn@doped graphite activated peroxymonosulfate to degrade bisphenol A in water. Hu et al. [31] carbonized Co/NH₂−MIL-88B(Fe) at high temperature in N₂ to obtain MOF derived nitrogen doped porous carbon rod (Fe₂Co₁/NPC), which removed 91% of 20 mg/L tetracycline within 60 min in electro-Fenton progress.

  This paper summarizes the latest advances in MOFs as catalyst in AOPs and their catalytic degradation mechanisms for various pollutants. This carefully prepared review is hoped to promote the development of this kind of materials and their practical applications for water treatment.

  2. Materials composition and function of MOFs

  Both metal center and organic ligand are two important components of MOF materials. The catalytic process of MOFs in AOPs is originated from the adsorption of pollutants by the pores of the material and then oxidation by active group or free radicals generated from active sites. Therefore, both the metal center and organic structure will affect the catalytic performance of MOFs.

  2.1 Catalytic core of metal center

  Generally, metals serve as central role in MOFs can be divided into two categories of single and multiple metal centers. The change of the metal center will affect its surface morphology, and further affect the catalytic activity of the catalyst [32]. For instance, Ahmad et al. [33] synthesized CUCs-MIL-88B-Fe with a coordinated unsaturated metal center (CUC) to degrade phenol and sulfamethoxazole (SMX) in photo-Fenton process. From the scanning electronic microscopy (SEM) results, MIL-88B-Fe showed length of around 1 mm with long smooth surfaces. Surface of CUCs-MIL-88B-Fe was a little bit rough and course with evenly distributed pores. The former transform infrared spectroscopy and Barrett-Joyner-Halenda results showed that water molecules and hydroxyl groups were eliminated, and formated mix-valence Fe^II/Fe^III which led CUCs-MIL-88B-Fe to presented micro and mesopores. These results manifested that the introduction to coordinated unsaturated iron centers could create ^•OH by mesopores in CUCs-MIL-88B-Fe along with inherent micropores. Taha et al. [34] respectively synthesized NH₂−MIL-101(Fe) and NH₂−MIL-101(Al) with Fe and Al as metal centers, which were applied for Fenton-like oxidation system. The surface area of NH₂−MIL-101(Fe) was 2572.5 m²/g, 1.24 times of NH₂−MIL-101(Al). Within 5 min, the degradation rate of Rhodamin B (RhB) could be exceeded to 99% by NH₂−MIL-101(Fe), while the degradation rate of RhB by Al-based NH₂−MIL-101 was only 76.51% [34]. This study indicated that catalytic activity in NH₂−MIL-101 could be greatly affected by catalytic center in same MOF.

  Moreover, the degradation capacity for organic contaminants of MOFs could be enhanced by using other transition metals to instead. Kirchon et al. [35] synthesized PCN-250(Fe₂Mn) with Mn instead of Fe (Fig. 2). Compare with the 1460 m²/g BET area and 711.2 eV binding energy of PCN-250(Fe₃), PCN-250(Fe₂Mn) had 1472 m²/g of surface area and lower binding energy (710.9 eV) displacement, which indicated that the isomorphic substitution of Mn for Fe could bring faster electron transfer rate and higher oxidation capacity for methylene blue (MB) degradation in photo-Fenton.

  Figure 2

  

  Figure 2.  The PCN-250(Fe₃) building unit composed of Fe₃-μ3-oxo clusters and 3, 3′, 5, 5′-azobenzenetetracarboxylate (ABTC). Reproduced with permission [35]. Copyright 2020, American Chemical Society.

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  The identity of metal oxidation state and rations of MOFs are also the key issue for their high degradation efficiency. Tang et al. [36] reported Fe_0.75Cu_0.25(BDC) could degrade SMX completely within 120 min in Fenton-like system. Different from octahedral Fe(BDC) and cubic layered Cu(BDC), Fe_0.75Cu_0.25(BDC) was formed in an irregular flake-like structure. The incorporation of copper enhanced pore size and Fe(II) of Fe_0.75Cu_0.25(BDC). Moreover, [Fe, Cu]-BDC was utilized as precursor to obtain a three-dimensional flower-like FeCu@C by pyrolysis [37]. FeCu@C had a higher particle size (100–300 nm width and 1–2.5 µm length), less agglomerated structure, more dispersed active center and better permeability than Fe/C (its particle size was 100–200 nm). Under oxidation progress of H₂O₂ catalyzed by FeCu@C, sulfamethazine (SMT) could be completely removed in 90 min. Xie et al. [38] synthesized Ag₃PO₄/MIL-53(Fe) by tightly anchoring Ag₃PO₄ nanoparticles on the surface of MIL-53(Fe). Ag₃PO₄/MIL-53(Fe) exhibited more active sites and larger surface area of 15.525 m²/g, 1.82 times than pure MIL-53(Fe). In addition to directly synthesizing MOFs with bimetallic centers, catalytic efficiency of the MOF can also be enhanced by introducing additional active sites. Liu et al. [39] introduced Co into Zn-MOF and prepared a bimetallic site catalyst Zn/Co-MOF. Du et al. [40] doped Zn²⁺ and Cu²⁺ ions into Mn²⁺-based MOF (STU-2) to obtain a robust hetero metallic MOF, of that material had high heterogeneous catalytic performance for the cyano-silylation of aromatic aldehydes and high stability.

  Therefore, changes in metal sites, coordination unsaturated metal sites or metal complexes will affect the shape and size of the pores, even change the structure of the material and thereby affecting its catalytic performance.

  2.2 Organic ligands of MOFs

  MOFs catalytically degrade pollutants in two steps of pollutants adsorption and then reacting with the catalytic sites or active groups of MOFs. Therefore, both pore diameter and pore volume of organic linkers play significant roles in the progress of contaminants removal. As an essential part of organic linker is mainly used as a frame structure to connect various metal centers to form channels with various pore diameters, which can promote highly dispersed reactant molecules to enter the inner surface to independently react with each metal site.

  Generally, the chain length of the organic ligand is long to obtain a larger size and specific surface area. Wang et al. [41] synthesized NH₂−MIL-88B(Fe) with 2-aminoterephthalic acid, and then synthesized various MOFs (the corresponding MOFs were named Bac-MIL88(Fe), Py-MIL88(Fe) and Pca-MIL88(Fe), respectively) by doping with different organic ligands (benzoic acid, pyrrole and pyrrole-2-carboxylic acid) used for photo-Fenton to degrade acetamide. The adsorption efficiency of these materials was consistent with the surface area and pore volume increased in the order of MIL88(Fe) < Bac-MIL88(Fe) < PyMIL88(Fe) < Pca-MIL88(Fe).

  Similarly, Fig. 3 clearly showed that the structure of Zr-MOF (UiO-66, UiO-67 and UiO-68) changed with the length of the linkers [42]. The surface area of UiO-66 with only one benzene ring as the organic linker was 1187 m²/g. When linkers were expanded to two and three benzene ring dicarboxylic acids, the surface area of UiO-67 and UiO-68 could increase to 3000 and 4170 m²/g, respectively [42]. With the increase of the number of benzene rings, their specific surface areas increased by 1.5 times and 2.5 times, respectively. Meantime, according to the research of Duren et al. [43], the van der Waals surface area per volume of chain structure is about twice that of nonchain structure. Therefore, MOFs obtained by changing the organic ligands with same metal site can adjust the specific surface area, diameter and pore size. However, a major disadvantage of MOFs with abnormally high surface area is weak stability. When the organic frame of the material is too long, it is easy to cause the collapse of the cavity. Therefore, the synthesis of an economical and efficient MOF is the key research direction of MOFs.

  Figure 3

  

  Figure 3.  Effect of organic linker on surface area and pollutant removal. Reproduced with permission [42, 46]. Copyright 2008, American Chemical Society; Copyright 2018, Elsevier.

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  When the pore diameter of MOFs is not large enough, the reaction rate will be limited by the diffusion of reagents and products, due to channels are too small to capture contaminants. Yu et al. [44] used terephthalic acid and 1, 3, 5-benzenetricarboxylic acid to synthesize MIL-53(Fe) and MIL-100(Fe) for catalytic ozonation. The pore size of MIL-100(Fe) was 4.25 nm, while MIL-53(Fe) was only 2.96 nm. The adsorption experiment showed that MIL-100(Fe) removed 74.3% of RhB within 10 min, 1.13 times that of MIL-53(Fe). The high catalytic performance comes from its abundant active sites and large specific surface area. Taha et al. [34] synthesized MIL-101(Fe) and NH₂−MIL-101(Fe) with BET surface areas of 2122.6 m²/g and 2572.5 m²/g, respectively. In contrast to MIL-101(Fe), NH₂−MIL-101(Fe) had larger surface area and higher catalytic performance groups (-NH₂). Therefore, the degradation rate of RhB by NH₂−MIL-101(Fe) exceeded 99% in 5 min, which was much higher than MIL-101(Fe) of 61.2% [34]. In addition, different organic linkers can be mixed to prepare MOFs with large specific surface area and used to selectively remove pollutants [45]. As shown in Fig. 3, UiO-66-n(COOH)₂ (n = 0, 0.25, 0.5, 0.75 and 1) was prepared by mixing 1, 4-benzendicarboxylic acid (BDC) and 1, 2, 4, 5-benzentetracarboxylic acid (BTC) ligands in different proportions for the selective removal and separation of RhB and MB [46]. With the increase in BTC content, the pore diameter and surface area decreased. This was attributed to the fact that the carboxylate group of BTC occupied the pores, reduce the surface area and the crystallinity of catalyst. Therefore, UiO-66–0.25(COOH)₂ showed the highest adsorption capacity for those samples of the RhB which is cationic dye with large size. To further examine the selective removal ability of these multivariate MOFs of UiO-66–0.25(COOH)₂ and UiO-66–0.75(COOH)₂, cationic dye MB and anionic dye methyl orange were tested. In the mixed dye wastewater, MB was quickly removed, which proved that UiO-66–0.25(COOH)₂ and UiO-66–0.75(COOH)₂ had the selective removal capacity of MB. The mechanism of this phenomenon was that the uncoordinated carboxylate groups on the TBC linkers made a negatively charged skeleton and negative surface potential of the MOF, which contribute to remove the cationic dye. Meanwhile, UiO-66–0.75(COOH)₂ had small pores and only allowed MB with small molecules to enter its skeleton.

  The stability of the material is also closely related to the organic linking agent. PCN-134, which with two different topological linkers (4, 4′, 4′′-benzene-1, 3, 5-triyl-tris(benzoic acid) (H₃BTB) and tetrakis (4-carboxyphenyl) porphyrin (TCPP)), was synthetically achieved by Gao et al. [47]. As a mixed-linker Zr-MOFs, PCN-134 exhibits much improved stability compared to the 2D Zr-BTB, because the mixed-linker 3D structure increases the rigidity of the entire framework and further affects the stability of the material. Therefore, by designing MOFs with suitable organic linkers, the performance of catalysts can be further improved. And as shown in Table 1 [33, 34,48-55], the difference of organic linkers will lead to great differences in the pore size, pore volume and specific surface area of the catalyst.

  Table 1

  

  Table 1.  Comparison of diameters, pore sizes and specific surface areas of MOFs with different organic ligands.

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  3. Synthesis methods

  To prepare MOFs with excellent physical and chemical characteristics and improve the catalytic performance by changing their composition, structure and size, various methods such as hydrothermal method [45, 52, 56], sol-gel method [57-59], electrochemical method [60], microwave [61-63and ultrasonic assisted synthesis method [47, 64, 65] and other emerging synthesis method have been developed and widely used.

  3.1 Hydrothermal method

  Hydrothermal method is the most popularly used for the synthesis of MOFs which react with soluble pre-products under high temperature and high pressure in a sealed space. The processed material is put into a pressurized autoclave, heated to cause polymerization reaction and obtain a precipitated material. The advantage of this method is easy operation and general applicability.

  MIL-53(Fe) is usually obtained by heating at 150 ℃ for 15 h [38, 48]. In order to improve the performance of the material, usually the prepared MIL-53(Fe) is mixed with other materials and then heated under high temperature and high pressure to obtain a derivative of MIL-53(Fe) or a modified catalyst. For instance, MIL-53(Fe) and Bi(NO₃)₃•5H₂O were heated at 160 ℃ for 24 h to prepare MIL-53(Fe)/BiOCl composite [66]. However, MIL-53(Al) was synthesized by heating at 150 ℃ for 24 h [67]. Usually, the preparation pressure in MIL-101(Fe) was about 110 ℃, and the reaction time was relatively short (20–24 h) [34, 68, 69]. However, for other MOFs, this method even takes a few days. Li et al. [24] synthesized NH₂−MIL-53(Al) by mixing the dissolved AlCl₃•6H₂O and 2-aminoterephthalic acid into a reaction kettle, holding it at 130 ℃ for 72 h. MIL-100(Fe) also was heated at 130 ℃ for 72 h to be prepared [70]. Guo et al. [71] used 2-sulfo-terephthalic acid monosodium salt as the organic ligand and chromium oxide as the metal center, heated in a reactor at 180 ℃ for 6 days, and repeatedly washed with water and methanol to remove impurities, finally dried to obtain MIL-101(Cr)-SO₃H. Meanwhile, it can be clearly seen from Table 2 that the hydrothermal reaction takes a long time [72-76].

  Table 2

  

  Table 2.  Examples of MOFs synthesized by hydrothermal method.

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  It is precisely because of the long preparation time (several days to several weeks), high energy consumption, high equipment requirements and poor safety performance of hydrothermal method that researchers have begun to develop some technologies of electrochemical, microwave and ultrasonic-assisted methods to shorten preparation time.

  3.2 Electrochemical method

  The advantages of electrochemical method include mild reaction conditions, special physical and chemical properties and relatively fast synthesis process. Electrochemical synthesis of MOFs is to produce metal ions or hydroxide anions through oxidation or reduction reaction by electrolysis [77]. These ions combine with ligands to deposit MOFs on conductive substrates. Therefore, electrolyte, organic ligands and electrochemical parameters affect the structure and composition of MOFs, which further affects the degradation of pollutants. Meantime, this method can improve the yield of MOFs. Kumar et al. [78] synthesized Cu₃(BTC)₃−MOF under constant voltage electrolysis (with keeping current varying to make voltage constant) for more than 2.5 h, and the Cu content of MOF (54%) was higher than the previously reported value (22%). Ji et al. [79] prepared Cu-MOF by electrochemical method, found and demonstrated that using different organic ligands were expected to have varied morphology. Besides, Cu-MOF synthesized by 1, 3, 5-benzenetricarboxylic acid owned better electrochemistry than that using 1, 4-benzenedicarboxylic acid or 1, 2, 4, 5-benzenetetracarboxylic acid. Arul et al. [80] made improvement on the basis of Ji's study [79], and obtained Cu-BTC MOF-carbon nanotubes with high conductivity by BTC. The above materials were all prepared by electrochemical deposition method, and materials were deposited on the surface of the electrolytic cell, which is of low efficiency.

  Microplasma could also be used to synthesize MOFs efficiently. Wei et al. [60] prepared HKUST-1 using N, N-dimethyllformamide to dissolve Cu(NO₃)₂ and BTC as precursor solution, treated with microplasma cathode for 15 min. MOFs were also synthesized by dielectric barrier discharge plasma-assisted method. The high temperature in the channel and the partial superheating of the solution led to the rapid Ce-MOFs formation. This method needed a voltage generator, which generated non-thermal plasma between electrodes [81]. Therefore, relatively expensive equipment and complicated operation are shortcomings of this electrochemical method.

  3.3 Sol-gel method

  The raw material in sol-gel method is first treated for a liquid state, and highly active components that act as precursors are fully mixed in the liquid phase; then, the solution mixed with a metal salt solution and undergo chemical reactions such as hydrolysis or condensation to form sol system; the sol particles of the system polymerized slowly to form a gel with network structure; the gels were dried and sintered to obtain gelatin porous materials [57, 58, 82, 83].

  Mehta et al. [61] dissolved Zn(NO₃)₂•6H₂O and 2-methylimidazole (C₄H₆N₂) in ethanol, respectively. After the two liquids were mixed, stirred at room temperature for 15 min, and centrifugation, washing, drying, and grinding, NP@_monoZIF-8 was obtained. Cu/SiO₂−MOF was synthesized by two steps. Cu-MOF as precursor was heated to synthesize, then further fixed in situ in the silica carrier by sol-gel method [83]. However, this indirect synthesis method is complicated to operate and takes a long time.

  The advantages of sol-gel method are low cost and relatively mild reaction conditions. However, sol and gel must be produced in the composite solution during the synthesis process. Because some solutions can not form sol-gel state (the size of dispersed particles in solution should be between 1 nm and 1000 nm, and the sol-gel solution also needs to aggregate to form solid state gel with network structure), therefore, there are limitations in the preparation of MOF materials by this method.

  3.4 Emerging synthesis method

  Emerging synthesis methods include microwave-assisted method, ultrasound-assisted method and mechanochemistry method, etc.

  Both microwave-assisted method and ultrasound-assisted method are utilized to save the experimental time, make the crystals nucleate rapidly. For example, both MIL-140A and MIL-140B were synthesized at temperature of 220 ℃ by solvothermal method, which took 16 and 6 h, respectively. However, microwave heating only spent 17 min [84]. Also, using hydrothermal method to prepare CPM-5 needed 5 days, and only 10 min was needed by microwave-assisted heating [61]. In addition, due to the simultaneous nucleation of all crystals in the solution, the microwave method also makes the formation of nano-products with more uniform and smaller particle size [85]. In the C—NH₂−MIL-101 sample which was synthesized by traditional solvothermal, the number of particles with a size diversity of 200–600 nm was observed. Compared with the samples prepared by the traditional solvothermal method, the MW-NH₂−MIL-101 sample obtained by the microwave-assisted method had a smaller particle size of 150–250 nm [75]. Moreover, through manipulating reaction condition, the size and shape of the crystals can be varied [86]. The reaction time can be found from Table 3. The size and distribution of crystals increased from the irradiation time, eventually allowing particles with high surface area to be separated out. Meanwhile, reaction times affect the particle size of MOFs. As can be seen from Table 3, the microwave heating synthesis method has greatly prospered the design and preparation of MOFs. Meanwhile, this technology also has several outstanding advantages: It accelerates the reaction speed and greatly shortens the reaction time; the material with a high absorption rate has a significant purity and phase selectivity in some cases; and from microcrystals to nanoparticles, the real possibility of controlling crystal size is achieved; a compact synthesis device with low energy consumption can be used, while generating a small amount of chemical waste [87-89].

  Table 3

  

  Table 3.  Microwave-assisted and other method preparation of MOFs.

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  Ultrasound assistance can shorten reactions time that has been increasingly used in the last decades [90]. Besides, the high yields and mild conditions are also its advantages [47]. The high frequency of ultrasonic waves is used to make liquid molecules violently vibrate to generate holes, and then collide with synthetic materials. In the continuous collision and extrusion, chemical reactions occur to the raw materials, thereby synthesizing various MOFs [91]. However, this method is rarely used in the synthesis of materials, and is usually used as a raw material processing method, rather than a separate synthetic material. Therefore, the ultrasonic method is usually used in combination with other methods. For example, to ensure uniform solution, Wan et al. [64] sonicated the mixed solution of FeSO₄•7H₂O and H₃BDC for 20 min, and synthesized MIL-100(Fe)-MW after 90 min under microwave radiation. Burgaz et al. [62] utilized consecutive combination of ultrasound and MW method to synthesize MOF-5. The SEM images of MOF-5 that was synthesized by using MW method with no ultrasound treatment resulted in micron-sized MOF-5 crystals (3–10 µm) with some unidentified impurities. Using ultrasound and MW method had 20–80 nm of sizes and 95% of yields.

  Mechanochemical synthesis is a method in which reagents produce chemical reactions through mechanical forces to synthesize materials [92]. Compared with other methods, this method is a greener synthesis process, due to synthesize MOFs with little or no solvent. Ali-Moussa et al. [93] synthesized two Zr-based MOFs, UiO-67 and its dipyridyl analogue (UiO-67-bpy) by ball milling. However, due to the low crystallinity of the material, its specific surface areas were 750 m²/L and 650 m²/L respectively, far lower than the expected value of 3000 m²/L. Moreover, Kong et al. [58] prepared ZIF-8 by solvothermal, stirring and ball milling methods, respectively. Through characterization, the BET surface area of ZIF-8 synthesized by hydrothermal method (1390 m²/g) and stirring method (1220 m²/g) was similar, and that of ZIF-8 prepared by ball milling method (916 m²/g) was significantly lower than that of other methods. Therefore, the mechanochemical method produces less waste and environmental protection, but the disadvantages of low crystallinity of the prepared materials and the need for special synthesis equipment make the application scope of this method lower than that of hydrothermal method.

  4. Application of MOFs in AOPs and catalytic mechanism

  4.1 Fenton-like oxidation

  Fenton progress is that Fe²⁺ catalyzes H₂O₂ to produce ^•OH to oxidize organic pollutants into small molecular substances. Although this method has advantages of simple equipment, convenient operation and low cost, it has obvious shortcomings of narrow pH range and secondary pollution to water which caused by easily leached iron ions [94-97]. Therefore, Fenton-like process is developed on the basis of Fenton progress. The mechanism of Fenton-like method to expand the reaction pH range is that substances such as light, electricity and persulfate activate the regeneration of hydroxide complex (iron sludge) to form Fe²⁺, and the generation of some acidic substances in the reaction process reduces the pH of the solution [98].

  Among the various types of MOFs applied to Fenton-like, Fe-based MOFs as heterogeneous Fenton-like catalysts have great potential [99, 100]. NH₂−MIL-101(Fe) synthesized by Taha et al. [34], which completely removed 0.25 mmol/L RhB in the pH range of 3.1–11 within 10 min, and its performance was much better than the Fenton progress removing pollutants in the pH range of 4–6. Moreover, although MIL-100(Fe) is also successfully used as a Fenton-like catalyst, pure MIL-100(Fe) has low catalytic activity and lack of coordination, it still needs improvement. Due to synergistic catalytic action of Fe(II) and Fe(III) ions accelerated the production of ^•OH, adding Fe(II) to MIL-100(Fe) to synthesize Fe^II@MIL-100(Fe) could improve the catalytic efficiency and mineralization ability [101]. However, in Fenton-like progress, a noteworthy issue is that the leaching of metal ions. The leaching experiments found that Fe(II) ions supported on MIL-100(Fe) could be dissolved into the solution, and the concentration of elemental iron leached from Fe^II@MIL-100(Fe) could reach to 7.1 mg/L, which was far higher than the environmental standard (2 mg/L, established by the European Union) [101]. In addition, the leaching concentration of metal ions in MOF materials with bimetallic centers is also serious. It was detected in the solution that the leaching concentration of Fe in iron-copper bimetallic organic framework material (Fe_xCu_1-x(BDC)) reached 7.23 mg/L, and the leaching concentration of Cu was 0.11 mg/L [36]. The reason for the leaching of metal ions may be that the metal active sites are loaded on the surface of the catalyst, and the combination with the organic ligands is not tight, which causes the metal ions to fall off easily into solution, and bimetallic interweaving can easily cause the pores of MOF material to be collapsed. Therefore, further researches have been conducted about MOFs modification to improve their catalytic activity and reduce the leaching rate of metal ions. Tang et al. [37] obtained three-dimensional flower-shaped MOF FeCu@C as Fenton-like catalyst based on the iron-copper bimetallic organic framework material after calcination under high temperature, and the copper leaching rate reached to 3.34 mg/L.

  In contrast, the metal leaching rate of MOFs catalysts with single metal centers is much low. For example, the maximum concentration of iron ions leached in phenol degradation by MIL-88B(Fe) was only 0.68 mg/L, which was much lower than that of other MOF materials which with bimetallic centers [102]. Tang et al. [50] coordinated unsaturated iron center metal organic framework material CUS-MIL-100(Fe) with mixed Fe^II/Fe^III could degrade SMT almost 100% within 180 min. Though the activity of the catalyst decreased slightly after repeated five times, the highest leaching concentration of iron ions in the solution was 0.41 mg/L, that was much lower than using Fe^II loaded on MIL-100(Fe) (Fe^II@MIL-100(Fe)) [101] and bimetal MOFs [36, 37]. And the morphology, crystal structure, functional group and catalytic activity of activated CUS-MIL-100(Fe) under vacuum conditions were similar as the fresh catalyst. The degradation mechanism of such Fenton-like MOFs catalyst is that the adsorbed H₂O₂ on the surface of the catalyst was rapidly decomposed by the change and conversion of iron valence, and thereby enhancing the catalytic activity and degradation efficiency. Therefore, enhancing the tight bonding between organic ligands and metals is an effective method to improve the stability of materials and reduce the leaching rate of metal ions.

  In addition to iron-based catalysts, other redox states of chromium, cerium, copper, cobalt, manganese, and ruthenium can also be selected as Fenton-like catalysts. And the Fenton-like systems which with Cu(II) catalysts have attracted increasing attentions. Comparing with the Fe(III)-based catalysts, the Cu(II) in H₂O₂ system exhibited the advantages of high efficiency [103, 104], wide adaptability to temperature and pH [105, 106]. Using Cu-MOF as templates and precursors, Xiong et al. [107] prepared Cu_xO/C composite materials with CuO and Cu₂O nanoparticles to degrade RhB through two chemical reactions. After adding H₂O₂, the degradation rate of RhB reached above 98%. Wu and coworkers constructed MIL-100(Fe)/CoS to degrade 99.2% 10 mg/L bisphenol A [36]. The study found that the Fe-S bond formed at the interface of the two components promote Fe³⁺/Fe²⁺ cycle by increasing the electron mobility (Co to Fe and S to Fe), this is the first time that the Fe³⁺/Fe²⁺ cycle in Fe-MOF has been explained by studying Fe-MOF/MSx interface interaction in Fenton-like progress.

  Photo-Fenton degrades pollutants by reducing Fe³⁺ and photolysing H₂O₂ to generate ^•OH. Tang et al. [108] synthesized a series of iron-based metal organic frameworks (Fe-MOF) to degrade hydrochloride tetracycline. To increase the visible light absorption of the iron-based MOF, Nguyen et al. [109] introduced another metal to prepare bimetallic organic framework of Al/Fe-MOF and MIL-88B(Fe) as a photo-Fenton catalyst. In the characterization test, it was found that the specific surface area of Al/Fe-MOF and MIL-88B(Fe) were 285 m²/g and 47 m²/g, and the pore sizes were 1.5 nm and 1.9 nm, respectively. Within 120 min, the degradation rate of RhB which using MIL-88B(Fe) was 83%, while that using Al/Fe-MOF could exceed 96%. This study demonstrated that the larger specific surface area allows MOFs have more active sites, coupled with the transition of the bimetallic charge, and therefore the catalytic efficiency could be significantly enhanced.

  In addition, electro-Fenton process also could generate a large amount of ^•OH and other strong oxidants in bulk solution. Compared with the decomposition of H₂O₂ to generate ^•OH, the reduction of O₂ in the cathode reaction to generate ^•OH is safer, cheaper and more environmentally friendly [110]. Ye et al. [111] obtained nano-ZVI@CN by calcining NH₂−MIL(Fe) under N₂, and used nano-ZVI@CN as a catalyst to treat the gemfibrozil in the electric-Fenton process with a high removal rate of 100%. The H₂O₂ produced at the cathode undergoes a heterogeneous Fenton reaction to Fe(II) on the catalyst surface, producing ^•OH and iron ions. In addition, the active chlorine and ^•OH generated by the anode reaction will also degrade pollutants. And the nano-ZVI and Fe₃O₄ nanoparticles present in the rod ensure the supply of Fe(II)/Fe(III), and nano-ZVI was the main element responsible for the continuous regeneration of Fe(II). Meantime, the presence of N-doped porous carbon promoted the electron transfer. The method to increase the active oxide species in the solution is to increase the catalytic center of MOFs. Zhou et al. [112] used Mn-doped MIL-53(Fe) as a precursor to successfully synthesize Mn/Fe@PC catalyst after carbonization of MOF, which was used as catalyst in electro-Fenton process to degrade triclosan Mn/Fe@PC—CP showed high catalytic performance and stability, triclosan could be completely degraded within 120 min, and TOC removal reached 56.9% ± 2.0% within 240 min. The reason was that dispersion of the active sites of Fe^Ⅱ/≡Mn^Ⅱ/Ⅲ obviously promoted the formation of H₂O₂ and ultimately promote the degradation of triclosan, and the conversion between ions reduced the ion leaching rate [112].

  4.2 Photocatalysis

  As a low-cost and environmentally friendly technology, photocatalysis has shown significant potential for water pollution purification. MIL-53(Fe) composed of Fe(III) and 1, 4-benzenedicarboxylic acid is also commonly used in photocatalyst due to its high response to visible light response area. Nevertheless, owing to the rapid recombination of electron-hole pairs, scholars have made many attempts to overcome this drawback. In the research by Chen et al. [113], the MIL-53(Fe) with mixed valence Fe metal centers formed under vacuum inert gas exhibited considerable photocatalytic performance. It could degrade 96.28% and 95.01% of 20 mg/L RhB and hydrochloride tetracycline, which was 1.13 and 1.33 times higher than that of MIL-air53(Fe) prepared in air, because the charge migration in the mixed valence Fe^II/Fe^III chains reduced h⁺-e⁻ pairs. Jiang et al. [114] prepared photocatalysts with core-shell structure ZnO@ZIF-8. Its catalytic performance was 40% higher than that of ZnO, because the core-shell structure increased the active sites. Zhang et al. [115] synthesized a new type of absorptive photocatalysis TiO₂/mag-MIL-101(Cr) with tetrabutyl titanate and magnetic MIL-101(Cr). 0.5 g/L TiO₂/mag-MIL-101(Cr) could completely remove 20 mg/L bisphenol F, and the removal rate of acid red 1 was 90%. In addition, Miao et al. [66] used BiOCl to anchor the surface of MOF-53(Fe) for synthesizing MIL-53(Fe)/BiOCl, in that the photoelectrons were accumulated on the BiOCl with lower conduction band (CB) while the holes stored on the valence band (VB) of MIL-53(Fe). Therefore, the key of improving photocatalytic efficiency of MOFs is to reduce the rapid recombination rate of electron-hole pairs.

  Manufacturing heterojunction is also a method to realize the effective separation and transfer of light-excited electron and hole charge. The composite of UiO-66-NH₂ and Cu₂O to obtain UiO-66-NH₂/Cu₂O with heterojunction was superior to pure Cu₂O and UiO-66-NH₂ in the photocatalytic degradation of methyl orange [116]. The composite material obtained by coupling carbonitride carbon (g-C₃N₄) with unique crystal structure and visible light responsiveness to MOFs will also form a heterojunction that facilitates the separation of electron-hole pairs to improve photocatalytic activity [117]. For example, Zhang et al. [53] used g-C₃N₄/UiO-66 nanohybrids CNUO as photocatalyst, and found that the addition of g-C₃N₄ improved the pore size of CNUO, and O₂^•− was formed by photo-generated electron of UiO-66 which improved the degradation rate of RhB. Moreover, Huang et al. [118] fabricated the protonated g-C₃N₄ coated with MIL-100(Fe), which denitrified and oxidized the pyridine of RhB by molecular oxygen. He et al. [68] used MIL-101(Fe) and TiO₂ to synthesize the magnetic MIL-101(Fe)/TiO₂ composite material, which removed 92.76% of tetracycline in 10 min under visible light, which was 2.3 times than that of pure MIL-101(Fe). Hejazi et al. [70] used TiO₂ and MIL-100(Fe) to construct a heterojunction TiO₂@MIL-100(Fe) to degrade MB with a high degradation rate of 100%, in which that the mineralization rate could reach 87%. The heterojunction between semiconductors and MOFs in the composite has a synergistic effect, and the effective interface charge transfer improved the separation effect of photo-induced electron-hole pairs, and improve the absorption of visible light by reducing band gap.

  The energy-saving advantages of photocatalytic reaction make it widely concerned in the field of wastewater treatment. However, its shortcomings are also very obvious. Under normal circumstances, the photocatalytic reaction takes a long time of 1–3 h, and large number of impurities in the water will greatly affect the catalytic efficiency. Therefore, the practical application of photocatalyst needs further research.

  4.3 Ozone catalysis oxidation

  The disadvantages of ozone catalytic oxidation are low ozone utilization and TOC removal rate. Therefore, using MOFs to overcome those shortcomings are necessary. Yu et al. [44] studied and compared four MOFs (MIL-53(Fe), MIL-88B(Fe), MIL-100(Fe) and MIL-101(Fe)) to catalyze ozone for RhB degradation. MIL-53(Fe) showed the highest removal efficiency in catalytic ozonation process, that could shorten the degradation times from 10 min (only ozone treatment) to 2.5 min to achieve complete degradation, and a TOC removal rate was nearly 40% at 30 min. Capture experiments and active site detection showed that the Lewis acid sites (LAS) on the surface of MOFs were active sites for catalytic ozonation, which could promote the decomposition of ozone into reactive oxide species (ROS). Besides, the mechanism of photocatalytic ozone oxidation technology was also proposed to degrade 4-nitrophenol [119]. MIL-88A(Fe) produced conduction band e⁻ and h⁺ in the CB and VB under UV (254 nm) radiation. And the CB electrons were transferred to ozone to reduce charge carrier recombination and catalytically decompose O₃ to generate more ROS on the LAS of MIL-88A(Fe) for 4-nitrophenol degradation and mineralization. Therefore, the TOC removal rate (75%) of 4-nitrophenol in this system was much higher than that of photocatalysis (17.6%) and catalytic ozonation (38.7%) [119].

  Material modification is an effective way to enhance O₃ decomposition to produce ^•OH. MOFs of ZIF-67-derived Co₃O₄—C@FeOOH composite was used as an efficient catalyst for norfloxacin degradation by ozonation [120]. ZIF-67 was used as precursor and heated treatment to obtain Co₃O₄-carbon. Through modification by FOOH, which was wrapped on Co₃O₄-carbon to prepare MOF-derived Co₃O₄-carbon composite material. The specific surface area of the formed composite was about 2.5 times that of Co₃O₄-C, which provided active sites for catalytic O₃ decomposition, enhanced the generation of ^•OH and TOC removal rate. The coexistence of Fe and Co in various valence states in catalyst improved the conversion of Co^III/Co^II and Fe^III/Fe^II, which would increase the catalytic activity in catalytic ozonation process.

  4.4 Persulfate catalysis

  SO₄^•− has a strong oxidation potential (2.6 V) with wider pH range than ^•OH, meanwhile generate less disinfection by-products in oxidation process [69]. Heating, UV irradiation, homogeneous transition metal catalysis and heterogeneous photocatalysis are common activating methods for sulfate radical AOPs [121].

  Wan et al. [64] added the regulator CH₃COOH to obtain MIL-100, used to activate persulfate (PS) to degrade orange G. In addition, through the predicting the reaction site by frontier molecular orbital theory and dual descriptor method, it was found that the degradation pathway of orange G was attacking N₂ ═ N₂ of aromatic ring hydroxyl groups by SO₄^•− and O₂^•− to form hydrolyzed phenol substitutes and finally mineralize the pollutant [64]. Wu et al. [122] encapsulated ZIF-67 with Fe₃O₄ as the core in ZIF-8. The electron transfer between Fe₃O₄ and cobalt active center activated PS to produce SO₄^•−, which can completely remove 5 mg/L carbamazepine within 30 min.

  Yi and colleagues prepared PDINH/MIL-88A(Fe) with perylene-3, 4, 9, 10-tetracarboxylic diimide as raw materials to remove 95.7% of chloroquine phosphate by activating PS under low power LED visible light [123]. Studies had shown that the efficient removal rate was mainly attributed to the production of different active species (SO₄^•−, ^•OH, O₂^•−, h⁺ and ¹O₂) through direct and indirect activation of activating PS. Moreover, Wang et al. [124] fixed MIL-88A(Fe) on cotton fiber to prepare MIL-88A(Fe)/cotton fibers activated PS to produce SO₄^•− and ^•OH under UV radiation. Through quantitative structure-activity analysis, it was found that tetracycline antibiotics were transformed into corresponding intermediates with low toxicity within 8 min. Liao et al. [125] synthesized surface-oriented MIL-88B-Fe with rod-like structure based on benzoic acid functionalized g-C₃N₄ and MIL-88B-Fe, which removed 95% RhB in 60 min, with PS as oxidant. Gong et al. respectively designed MOFs based on nitrogen-doped carbon, using MIL-101(Fe) and ZIF-CN (ZIF-CN was obtained by carbonizing ZIF-8) as template to obtain g-C₃N₄/MIL-101(Fe) and ZIF-CN/g-C₃N₄ at high temperature [69]. The SEM image showed that the shape of ZIF-NC was rhombic dodecahedron, about 200 nm of the size. ZIF-CN/g-C₃N₄ activated PMS under visible light (Vis) irradiation, the degradation rate of BPA (20 mg/L) reached 97% within 60 min. The g-C₃N₄/MIL-101(Fe) had an octahedral structure with a particle size range of 0.5–2 µm. In g-C₃N₄/MIL-101(Fe)/Vis/PS system, BPA (10 mg/L) could be removed 98% within 60 min. In both experiments, ethylenediaminetetraacetic acid disodium salt, p-benzoquinone, n-butanol and methanol were used to quench active radicals. It was found that two free radicals of O₂^•− and h⁺ played major roles in BPA degradation by ZIF-NC/g-C₃N₄. In g-C₃N₄/MIL-101(Fe)/Vis/PS system, SO₄^•−, h⁺ and O₂^•− were major ROS. Moreover, Lv et al. [1] used MOF and COF to form a new type of MOF@COFs hybrid material, as an effective photocatalyst, combined with advanced sulfate-based oxidation process, which removed BPA about 99%.

  Hu et al. [126] prepared MIL-101(Fe) to degrade tris(2-chloroethyl) phosphate (TCEP). The mechanism of photocatalysis reaction on MIL-101(Fe) was transformation of Fe(III) to Fe(II) under visible light which further induced transformation of S₂O₈²⁻ to SO₄^•− and produced ^•OH to oxide TCEP. Zhang et al. [127] utilized FeCl₃•6H₂O and 2-aminoterephthalic acid to synthesize MIL-101-NH₂ to activate oxone and degrade acid azo dye amaranth completely. In this process, amine groups absorbed visible light and transferred electrons to the Fe-O clusters on MIL-101-NH₂ to generate ^•OH and SO₄^•−, and thereby greatly improving the degradation efficiency of azo dye amaranth. Moreover, Lu et al. [72] synthesized a new type of composite material g-C₃N₄@CoFe₂O₄/Fe₂O₃ by hydrothermal method and calcination, and then used the resulting hybrid photocatalyst as a high-efficiency mediator to activate PS under visible light. The heterojunction formed between the MOF-derived CoFe₂O₄/Fe₂O₃ and g-C₃N₄ could effectively inhibit the recombination of photo-induced electrons and holes. Meanwhile, doping a small amount of MOF-derived CoFe₂O₄/Fe₂O₃ significantly increased visible light absorption capacity of g-C₃N₄ and reduced the band gap of g-C₃N₄, effectively improving the catalytic efficiency. Within 80 min, tetracycline of 99.7%, BPA of 98.1%, SMX of 94.8%, diclofenac sodium of 97.0%, ibuprofen of 96.1% and ofloxacin of 96.5% were removed [72].

  In addition, it is also possible to synthesize a core-shell structured MOFs for increasing the reactive sites of the material to improve the catalytic performance. Zhu et al. [128] utilized ZIF-67@GN and thioacetamide to synthesize Co₃S₄@GN. After calcination, a reactive shell CoS@GN with a large number of exposed active sites was obtained, which activated peroxymonosulfate to degrade 100% of BPA within 8 min.

  4.5 Application of MOFs in other methods

  In addition to the oxidation techniques summarized above, MOFs have also been applied in some new developed physical field assisted AOPs. Sisi et al. [129] synthesized MOF-2 nanomaterials, which was used to compare the degradation rate of dye acid blue 7 in six water treatment processes including sonolysis, adsorption, peroxydisulfate (S₂O₈²⁻), sono-activated peroxydisulfate (US/S₂O₈²⁻), sonocatalytic process using MOF-2 (US/MOF-2) and sonocatalytic process using MOF-2 in the presence of peroxydisulfate (US/MOF-2/S₂O₈²⁻). After 60 min of each individual process, the results showed that US/MOF-2/S₂O₈²⁻ had the highest degradation efficiency, which can remove 76.44% of the dye acid blue 7. The reason was that ultrasound promoted the activation of PS by the catalyst and lead to accelerating the production of various free radicals (^•OH, SO₄^•−, O₂^•− and ¹O₂) [129].

  Moreover, electrochemical AOPs under gamma ray radiation also accelerate iron-based MOFs to produce ROS to decompose antibiotics. In the study of ionizing radiation-induced catalytic degradation of antibiotic wastewater, the TOC removal rate of cephalosporin C and SMT were 20.2% and 4.5% by γ-irradiation alone (3.6 × 10¹⁴ Bq of radioactivity). After adding Fe/C nanomaterials (DMOFs) carbonized with MIL-100(Fe) as the precursor, the TOC removal rate of cephalosporin C and SMT increased to around 42% and 51%, and degradation rate of them were raised by 1.3 times and 1.8 times [130].

  5. Conclusions and outlook

  In this review, the recent progresses of MOFs applied in AOPs for emerging organic contaminants degradation were summarized. The multifunctional sites introduced into MOFs through changing the metal center and organic framework, could realize selective removal of structurally diverse pollutants. Besides, another pathway is to dope other materials from MOF precursor to offer more opportunities for catalysis reaction.

  Though MOFs are competitive for degradation of emerging pollutants, some challenges of reusability and stability should be solved to achieve multicycle and large-scale application, due to leaching of metal ions and recovery of brittleness powdered catalysts are difficult to fully maintain their functions and critical structure. Fixing it on a stent or plating it on a biofilm is a potential solution, despite that metal leaching is still unavoidable. Lack of treating actual wastewater is another challenge, as of most studies are conducted under simulated wastewater. Therefore, it is urgent to study the catalytic performance and stability in the actual wastewater which the situation will be more complicated. Another issue is selection of economically viable and environmental-friendly organic ligands to decrease the cost of MOFs preparation. With the continuous efforts on these worthy improvements, more high-performance MOFs would be provided and generate a large impact on the wastewater treatment.

  Declaration of competing interest

  All authors declared that they do not have any commercial and associative interests that represent a conflict of interest in connection with the other work submitted.

  Acknowledgments

  The work was supported by the National Natural Science Foundation of China (Nos. 21906088, 51902169, 52170039), the National Science Foundation for Post-doctoral Scientists of China (No. 2021T140165), the Heilongjiang Provincial Natural Science Foundation of China (No. LH2020B023), Department of Education Heilongjiang Province (Nos. 135309338, 135309351), University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province (Nos. UNPYSCT-2020068, UNPYSCT-2020067).

  
  
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  Figure 1  The number of publications in the area of MOFs from 2011 to 2020 (research Web of Science on Sep. 10, 2021, MOFs research volume growth trend).

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  Figure 2  The PCN-250(Fe₃) building unit composed of Fe₃-μ3-oxo clusters and 3, 3′, 5, 5′-azobenzenetetracarboxylate (ABTC). Reproduced with permission [35]. Copyright 2020, American Chemical Society.

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  Figure 3  Effect of organic linker on surface area and pollutant removal. Reproduced with permission [42, 46]. Copyright 2008, American Chemical Society; Copyright 2018, Elsevier.

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  Table 1.  Comparison of diameters, pore sizes and specific surface areas of MOFs with different organic ligands.

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  Table 2.  Examples of MOFs synthesized by hydrothermal method.

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  Table 3.  Microwave-assisted and other method preparation of MOFs.

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•