Elimination of Emerging Organic Contaminants in Wastewater by Advanced Oxidation Process Over Iron-Based MOFs and Their Composites

Xiaohong Yi; Chongchen Wang

doi:10.7536/PC200562

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (3) : 471-489 DOI: 10.7536/PC200562

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Elimination of Emerging Organic Contaminants in Wastewater by Advanced Oxidation Process Over Iron-Based MOFs and Their Composites

Xiaohong Yi ¹^,²
, Chongchen Wang ¹^,²^,^*

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Abstract

The emergence of emerging organic contaminants(EOCs) has attracted increasing attention due to their wide distribution and persistence in aquatic ecosystems, as well as the potential threat to the health and safety of aquatic organisms. Traditional water treatment processes represented by activated sludge processes are generally insufficient to eliminate these persistent pollutants. For efficient removal of EOCs, advanced oxidation technology based on new materials is one of the most important advanced treatment technologies. Fe-MOFs and their composites have been widely used in many fields, especially in the catalytic oxidation of pollutants in wastewater. With the aid of the improvement of synthesis methods, post-synthetic modification and being composited with specific functional materials, Fe-MOFs can be used to effectively improve the adsorption performance, enhance the light absorption characteristics, and promote the effective separation of charge carriers. The review focuses on the progress of advanced oxidation processes(photocatalysis, Fenton-like reaction and sulfate radical($SO_{4}^{-·}$) mediated oxidation) of Fe-MOFs and their composites to remove emerging organic contaminants in wastewater. As well, the opportunities and challenges of Fe-MOFs in the field of EOCs removal are proposed.

Contents

1 Introduction

2 Preparation of Fe⁃MOFs and their composites for oxidative degradation of EOCs

2.1 MIL⁃100(Fe)and its composites

2.2 MIL⁃101(Fe)and its composites

2.3 MIL⁃53(Fe)and its composites

2.4 MIL⁃88(Fe)and its composites

3 The application of Fe⁃MOFs and their composites in advanced oxidation degradation of EOCs

3.1 Advanced oxidative degradation of drugs in wastewater by Fe⁃MOFs and their composites

3.2 Advanced oxidative degradation of environmental hormone in wastewater by Fe⁃MOFs and their composites

3.3 Advanced oxidative degradation of pesticide in wastewater by Fe⁃MOFs and their composites

3.4 Advanced oxidative degradation of multiple EOCs in wastewater by Fe⁃MOFs and their composites

4 The influencing factors of advanced oxidation degradation of EOCs by Fe⁃MOFs and their composites

4.1 The influence of physical properties of materials

4.2 The influence of operation conditions

4.3 The influence of active substances

5 Conclusions and prospects

Graphical abstract

Keywords

emerging organic contaminants / metal-organic frameworks(MOFs) / advanced oxidation process / catalysis / degradation

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Xiaohong Yi, Chongchen Wang. Elimination of Emerging Organic Contaminants in Wastewater by Advanced Oxidation Process Over Iron-Based MOFs and Their Composites. Progress in Chemistry, 2021, 33(3): 471-489 DOI:10.7536/PC200562

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1 Introduction

As one of the most necessary natural resources, water is an indispensable substance for life activities. However, with the intensification of human activities and the rapid development of modern industry, water pollution has become a serious worldwide problem. To date, the main sources of water pollution include industrial and domestic sewage discharges, oil and nuclear waste spills, and pesticide use, all of which have the potential to leave harmful residues in water^[1⇓~3]. In recent years, emerging organic pollutants (EOCs) have attracted increasing attention, including not only newly developed compounds, but also newly discovered compounds in the environment (usually compounds classified as pollutants with the development of analytical testing technology).Including but not limited to endocrine disrupting chemicals (EDCs), pharmaceutical and personal care products (PPCPs), carcinogenic polycyclic aromatic hydrocarbons (PAHs), brominated flame retardants, perfluorinated compounds (PFOS, PFOA) and so on^[4,5].

The traditional wastewater treatment technology represented by activated sludge process is difficult to degrade and remove the new recalcitrant pollutants with complex and stable structure^[6⇓~8]. Advanced oxidation processes (AOPs), including ozonation, photocatalysis, (photo) Fenton reaction and their combination, are expected to be used for the removal of hazardous or recalcitrant emerging organic pollutants in wastewater with the increasing discharge of wastewater containing various emerging organic pollutants^[1,9]. Generally, AOPs oxidize and decompose organic pollutants efficiently through highly active free radicals such as Hydroxyl radical (· OH), superoxide radical (Superoxide radical,$O_{2}^{-}$), and sulfate radical (Sulfate radical,$SO_{4}^{-·}$)^{[1,10⇓ ~12]}. Among them, photocatalytic and Fenton-like reactions are widely used because of their simple operation, mild reaction conditions and high rate of free radical formation^[9]. Up to now, various iron-based catalysts (such as zero-valent iron, iron-oxygen compounds and iron-based metallic glasses) have been proved to have high catalytic activation ability and become the leader of many advanced oxidation catalysts^[13]^[14,15]^[16]. Taking Fenton reaction as an example, compared with homogeneous Fenton reaction, heterogeneous Fenton catalyst usually shows good catalytic activity in a wider pH range (acidic or neutral). However, in the actual operation process, the general heterogeneous Fenton catalysts (such as zero-valent iron, iron oxide compounds, etc.) Can not completely avoid the dissolution of iron ions, and iron sludge may still be produced with the reaction, which may lead to the destruction of catalysts due to the dissolution of iron ions, the limitation of recovery and the possible secondary pollution^{[13⇓~15,17,18]}. Therefore, the development of more efficient and stable iron-based catalysts for advanced oxidation reactions has become a research hotspot in this field.

Metal-organic frameworks (MOFs) are porous inorganic-organic functional materials formed by coordination and self-assembly of multidentate organic ligands containing O or N atoms with transition Metal ions, which have ordered structures and can extend indefinitely in space^[19]. MOFs have unique structures and excellent properties, such as ultra-large specific surface area, abundant nanocavity, tunable porosity, open pore channels and unlimited possibilities for post-modification. Therefore, in recent years, MOFs materials have become the darling of the new materials field, showing great potential applications in the fields of gas adsorption/separation, drug delivery, fluorescence sensing and (photo) catalysis^{[1,20⇓⇓⇓ ~24]}. In particular, the abundant nanoscale cavities and open channels in the framework of MOFs can provide favorable channels for the entry of the target to be treated and the discharge of the products after treatment, which is conducive to promoting the mass transfer process of catalytic reactions. In addition to these remarkable properties, well-dispersed metal components in MOFs can also provide sufficient active sites for catalytic reactions, thus achieving high catalytic activity^[25]. Of course, MOFs also face some bottlenecks that need to be overcome. For example, when used as photocatalysts, most MOFs do not respond to visible light due to their wide band gap, which limits the use of sunlight in these materials. However, Fe-MOFs formed by oxygen-containing organic ligands and iron ions show intrinsic absorption characteristics of visible light due to the formation of Fe-O clusters inorganic nodes, which has aroused great interest of researchers^[26]. In addition, Fe-MOFs also show excellent thermal stability and chemical stability, and the iron metal element is abundant in the earth's crust, cheap and easy to obtain, and shows environmental friendliness^[27]. In 2013, Roeffaers et al. First evaluated the efficiency of five Fe-MOFs, including MIL-88B (Fe), NH₂-MIL-88B(Fe), MIL-100 (Fe), NH₂-MIL-101(Fe) and commercial Basolite F300 (FeBTC), as visible-light-active catalysts for the degradation of rhodamine 6G (Rh6G), which aroused the interest and enthusiasm of researchers in Fe-MOFs as catalysts for advanced oxidation reactions^[26]. Among them, some researchers have proposed that the main photocatalytic mechanism of Fe-MOFs materials is that Fe-O clusters are excited by light radiation to produce active substances, and even organic ligands can be excited by light radiation to produce ligand-metal charge transfer (LMCT) mechanism, which further enhances their photocatalytic activity^[28⇓~30]. In addition, in the presence of H₂O₂, Fe-MOFs cooperate with photocatalysis in the Fenton process to form a photo-Fenton reaction system: (1) The interaction between Fe-MOFs and the surface of H₂O₂ will trigger Fenton reaction to generate hydroxyl radicals (formula 1,2);(2)H₂O₂ will capture the photogenerated electrons in MOFs to generate hydroxyl radicals, thereby inhibiting the recombination of electron-holes and enhancing the degradation efficiency of pollutants (formulas 3 and 4)^[31,32]. Similar to the existence of H₂O₂ in the system, the abundant coordination unsaturated sites on the surface of Fe-MOFs can also activate persulfate (PS), which can react with photogenerated electrons to inhibit the recombination of photogenerated electron-hole pairs and produce $SO_{4}^{-·}$, thus accelerating the degradation of organic matter^[33,34].

(1)Fe(Ⅲ)species+H₂O ₂→Fe(Ⅱ)species+H

O 2 ·

+H ⁺

(2)Fe(Ⅱ)species+H₂O ₂→Fe(Ⅲ)species+·OH+OH^-

(3)Fe-MOFs+ hυ→h ⁺+ e ^-

(4)H₂O ₂+ e ^-(Fe-MOFs)→·OH+OH^-

So far, Fe-MOFs have shown excellent performance in advanced oxidation, but researchers have gradually realized that there are some shortcomings in using them as single-component catalysts. For example, in the photocatalytic process, the rapid recombination of photogenerated electron-holes limits its photocatalytic activity. In order to overcome the bottleneck of Fe-MOFs as a single catalyst, it is an effective strategy to improve the catalytic activity of MOFs by introducing some semiconductors, conducting polymers or noble metal atoms^{[22,23,35⇓ ~37]}. In this paper, several studies on the removal of EOCs in water by AOPs (including photocatalytic, Fenton-like and $SO_{4}^{-·}$ oxidation) using Fe-MOFs and their complexes are reviewed (Table 1), the current status of the use of advanced oxidation processes for the removal of EOCs is summarized, and a series of problems to be solved and future development trends are proposed.

2 Preparation of Fe-MOFs and their composites for oxidative degradation of EOCs

Fe-MOFs constructed by hard acid Fe³⁺ and organic carboxylic acid ligands have strong coordination bonds, very stable skeleton, and the existence of Fe-O clusters makes them have a narrow band gap and can be excited by visible light, so this kind of Fe-MOFs can become a candidate photocatalyst for efficient oxidative degradation of organic compounds^[38,39]. Fe-MOFs are available in a wide variety of Materials, including the MIL series (Materials from Institut Lavoisier, such as MIL-53 (Fe), MIL-88 (Fe), MIL-100 (Fe) and MIL-101 (Fe)), MOF-74 (Iron), MOF-525 (Iron), PCN-222 (iron), PCN-223 (iron) and others. This section will focus on Fe-MOFs and their composites for advanced oxidation degradation of EOCs. At present, MIL-series Fe-MOFs are mainly focused on, including MIL-53 (Fe), MIL-88B (Fe) and MIL-101 (Fe) with terephthalic acid as the ligand, ML-100 (Fe) with trimesic acid as the ligand and ML-88a (F e) with fumaric acid as ligand. Wherein, terephthalic acid can also be replaced by amino terephthalic acid to form amino Fe-MOFs with enhanced visible light absorption performance. Fig. 1 lists the structure diagrams of several carboxylic acid ligands for the construction of Fe-MOFs and the structure diagrams of common Fe-MOFs^{[40⇓⇓~43]}.

2.1 MIL-100 (Fe) and its composites

MIL-100 (Fe) is composed of iron (Ⅲ) octahedra and hypertetrahedral units constructed by trimers of trimesic acid ligands with oxygen as the center, and has mesopores and micropores in its three-dimensional cubic structure^[65]. Meanwhile, the physical and chemical properties of metal nanoparticles (M-NPs) are different from those of Bulk metal particles. For example, the thermal and electrical conductivities of M-NPs are superior to those of bulk metal particles due to the delocalization of their free electrons. However, due to the large surface area and high surface energy of M-NPs, their thermodynamic stability is low, which hinders highly uniform control of size and shape. In order to control the nucleation and growth of M-NPs, it is an effective strategy to encapsulate M-NPs in materials^[66]. Wu et al. First applied M-NPs/MOFs materials to advanced oxidative degradation of pharmaceuticals and personal care products (PPCPs)^[54]. They successfully prepared a new highly dispersed Pd @ MIL-100 (Fe) nanocomposite by a simple alcohol reduction method (Fig. 2a).

The addition of a surfactant (PVP) during the synthesis of the above Pd/MIL-100 (Fe) catalyst may result in insufficient interfacial contact between the noble metal Pd and MIL-100 (Fe). Therefore, Wu et al. Synthesized phosphotungstic acid (PTA) -MIL-100 (Fe) nanocomposites modified by palladium nanoparticles (Pd NPs) by in-situ reduction method in a follow-up study Pd-H₃PW₁₂O₄₀-MIL-100(Fe)(Pd-PTA-MIL-100(Fe)) (Fig. 2b)^[55]. Negatively charged polyoxometalates (POMs),Pecially Keggin-type [XM₁₂O₄₀]^N- ions (such as X=P⁵⁺, Si⁴⁺,Al³⁺;M=Nb⁵⁺, Mo⁶⁺, W⁶⁺) are metal oxide clusters with a variety of topologies,The stepwise multi-electron redox reaction can be carried out without undergoing a structural change. The moderate size of POMs enables them to be encapsulated in the cavities of MOFs, forming highly dispersed molecules that can serve as "coordination sites" for the in situ anchoring of M-NPs, and the surface of negatively charged POMs is conducive to attaching to positively charged metal ions, thus stabilizing M-NPs and inhibiting their aggregation^[67]. In addition, the encapsulated phosphotungstic acid (PTA) acts not only as a UV-excited reducing agent, but also as a stabilizer to modify Pd NPs in MIL-100 (Fe) matrix.

In most MOFs, the metal nodes are often completely occupied by the coordination atoms of the organic ligands, which limits the activity of the metal center and makes it difficult to activate the H₂O₂. To this end, an unsaturated coordination metal center (CUS) can be fabricated in the MOFs to expose the active metal sites therein, thereby catalytically activating the H₂O₂ to produce · OH. Wang et al. Synthesized a new Fenton-like catalyst CUS-MIL-100 (Fe) (MIL-100 (Fe) containing Fe^II/Fe^III mixed-valence unsaturated coordination iron centers) for the catalytic activation of H₂O₂ for the degradation of sulfamethazine (SMT)^[17]. As shown in the inset of Fig. 2C, the particle size of the prepared MIL-100 (Fe) is 250 ~ 500 nm, while the CUS-MIL-100 (Fe) generated after introducing CUS by vacuum thermal activation in Fig. 2C still retains the morphology and size of MIL-100, but its surface becomes rougher. The addition of CUS induces the formation of mesopores that facilitate the diffusion of reactants into the internal structure and allows the reaction medium to be in full contact with both the internal and external active centers. In addition, due to the Lewis acidity of the iron CUS in the channels, it has regioselectivity for H₂O₂, thus improving the chance of contact between H₂O₂ and active centers.

In recent years, many natural biomasses derived from animals and plants have been converted into biochar materials, which are promising potential substitutes for traditional catalysts due to their low cost and high yield^[68]. Zhao et al. embedded Fe-doped nano-spongy porous biochar (Fe-SPC) into MIL-100 (Fe) crystal by in-situ growth method, and formed a conductive heterojunction MIL-100 (Fe)/Fe-SPC with enhanced interface through Fe bridging effect (the material was named MIL (Fe)/Fe-SPC in the original text)^[57]. MIL (Fe)/Fe-SPC was used as a Fenton-like catalyst for the efficient degradation of thiamethoxam (THIA). As shown in fig. 2D, the heterojunction is a coral-like structure with abundant channels, that is, multiple interfaces are formed between Fe-SPC and MIL-100 (Fe). However, only an appropriate amount of Fe-SPC can effectively improve the crystallinity of the final product MIL (Fe)/Fe-SPC. To this end, they suggest that porous biochar be pre-doped with Fe prior to MIL-100 (Fe) growth^[57]. Similarly, in order to strengthen the interface between inorganic materials and organic materials, Zhao et al. Used Cu (II) as an ideal medium and pre-doped MIL-100 (Fe) through metal coordination^[58]. Then, the Cu (Ⅱ) -doped MIL-100 (Fe) was compounded with the Cu₂O to form a novel Cu₂O/MIL-100(Fe/Cu) composite material (named Cu₂O/MIL(Fe/Cu) in the original text) formed through the Cu bridging effect. As shown in the figure, the morphology of 2e,Cu₂O/MIL(Fe/Cu) is similar to that of MIL (Fe/Cu), but the surface is rougher than that of pure MIL-100 (Fe), while the morphology of Cu₂O/MIL(Fe) without Cu bridge in Figure 2F is completely different from that of Cu₂O/MIL(Fe/Cu), and some flaky particles are embedded in the surface of MIL (Fe). In addition, these lamellar particles are only partially connected with MIL-100 (Fe), indicating that the composite interface structure between in-situ grown Cu₂O and MIL-100 (Fe) is loose, and no Cu (Ⅱ) participates in the connection with MOFs. This also proves that the strategy of pre-loading Cu (Ⅱ) into MOFs can effectively improve the dispersion of Cu₂O on MIL (Fe/Cu) surface. In addition, the existence of MOFs in the precursor solution may affect the formation of Cu₂O, which leads to a huge difference in crystal morphology.

TiO₂ is the most classical semiconductor photocatalyst. Wang et al. prepared a MIL-100(Fe)/TiO₂ composite (M/P) by loading MIL-100 (Fe) crystals on a commercially available P25 (TiO₂ with a particle size of less than 25 nm) coated with FeOOH as a precursor^[59]. As shown in Fig. 2g, MIL-100 (Fe) is wrapped on the surface of P25 to form a core-shell structure.

Mesoporous structure contributes to the mass transfer process in the photocatalytic degradation of pollutants, but the formation of mesoporous structure is difficult due to the instability of some MOFs. Quan et al. Constructed a stable mesoporous material M. MIL 100 (Fe) by in-situ self-assembly using unstable metal-organic assembly (MOAS) as a template^[61]. In order to promote the effective electron-hole separation of catalytic materials, Quan et al. Synthesized M. MIL MIL-100 (Fe) @ ZnO by referring to Wu et al.'S alcohol reduction method to synthesize M @ MIL-100 (Fe) (M = Pd, Au and Pt)^[61]^[54]. As shown in fig. 2h, MIL-100 (Fe) is a smooth octahedron, while the M. MIL 100 (Fe) in fig. 2h is a rod-like structure, and the smaller spherical ZnO nanoparticles are loaded on the surface of the M. MIL (Fe).

How to separate photocatalyst effectively is one of the bottlenecks that limit the use of powder photocatalyst. The magnetic material Fe₃O₄ is introduced to be compounded with the photocatalyst, so that the separation and recovery of the catalyst can be realized by using the magnet, and the catalytic activity of the photocatalyst can be kept^[60]. In recent years, carbon aerogels (CA) have attracted great interest because of their large specific surface area, low density, thermal stability and non-toxicity, and they are helpful for the adsorption of pollutants^[69]. Lv et al. Synthesized MIL-100 (Fe) at low temperature, and compounded it with Fe₃O₄ and CA to obtain MIL-100(Fe)@Fe₃O₄/CA magnetic nano-photocatalysts (MNPCs) (Figure 2 I), which improved the specific surface area and porosity of the material^[60].

Another bright light catalyst graphite-like C₃N₄(g-C₃N₄) is a metal-free semiconductor material. Due to its π-π conjugated electronic structure, narrow band gap (~ 2.7 eV), excellent chemical stability, good thermal stability and low cost, it has attracted more and more attention^[22]. Wang et al. prepared MIL-100(Fe)/g-C₃N₄ heterojunction by simple ball milling and quenching^[21]. As shown in fig. 3A, a g-C₃N₄ with a smooth layered structure is wrapped around the edge of the polyhedral MIL-100 (Fe) structure.

Wang et al. also used simple mechanical ball milling method to compound MIL-100 (Fe) with conducting polyaniline (PANI) with π-π conjugated structure, n-type semiconductor tungsten trioxide (WO₃) capable of effectively absorbing visible light and oxygen-rich Bi₁₂O₁₇C_l2 with sufficiently negative conduction band position (about -0.56 eV), respectively, and prepared Z-type MIL-100 (Fe)/PANI, Z-type WO₃/MIL-100(Fe) and Bi₁₂O₁₇Cl₂/MIL-100(Fe) heterojunction composite materials (Figure 3B ~ 3D)^[36]^[56]^[64].

2.2 MIL-101 (Fe) and its composites

Similar to MIL-100 (Fe), MIL-101 (Fe) is also a zeolite-type crystalline material composed of two kinds of cage structures. Zhao et al. Used a simple hydrothermal method to prepare a heterostructure composite composed of g-C₃N₄ and MIL-101 (Fe)^[34]. As shown in the inset of fig. 4A and the SEM image shown in fig. 4A, the surface of MIL-101 (Fe) with a smooth surface becomes rough after being introduced into the g-C₃N₄, and is tightly wrapped by dense lamellar nanosheets. Similar to POMs, quinones can also accelerate electron transfer during reduction or oxidation^[70]. Among them, 2-anthraquinone sulfonate (AQS), as a redox mediator, can act as an electron shuttle in the process of pollutant degradation^[35]. Guo et al. Synthesized a new type of AQS modified NH₂-MIL-101(Fe) (named AQS-NH-MIL-101 (Fe)) by a simple chemical method under mild conditions^[35]. As shown in Fig. 4B, AQS-NH-MIL-101 (Fe) appears as a smooth octahedron with a size of 500-800 nm, indicating that the modification of the quinone has no significant effect on the structure of the NH₂-MIL-101(Fe). The AQS used to modify the NH₂-MIL-101(Fe) acts as a redox mediator, which promotes the activation of persulfate, thus improving the degradation efficiency of BPA.

2.3 MIL-53 (Fe) and its composites

MIL-53 (Fe) is a three-dimensional porous material composed of an infinite one-dimensional chain-Fe-o-O-Fe-o-Fe-and 1,4-terephthalic acid, which has the advantages of stable structure and can be excited by visible light^[71]. In order to combine the photocatalytic properties of MIL-53 (Fe) and WO₃ and produce a synergistic effect, Oladipo successfully distributed the cluster macroaggregate WO₃ uniformly on the surface of MIL-53 (Fe) with a corrugated plate-like structure (Fig. 5A)^[45]. Considering that 1T-MoS₂ with abundant active centers and excellent metal conductivity can improve the photocatalytic activity under visible light, Zhang et al. Constructed 1T-MoS₂@MIL-53(Fe)(TSMF) heterojunction materials by combining 1T-MoS₂ with MIL-53 (Fe) by one-pot solvothermal method, and used them for photocatalytic degradation of organic pollutants^[72]^[50]. The TSMF heterojunction has a large specific surface area (337 m²·g^-1), which is about 16 times higher than that of MIL-53 (Fe) alone, and can provide more adsorption and reaction sites, which is conducive to rapid charge transfer at the interface. The structure of MIL-53 (Fe) in the TSMF-5 composite (in which the mass fraction of the 1T-MoS₂ is 5%) changes from an irregular shape to a microrod-like needle shape after the addition of the 1T-MoS₂ with a flower-like structure (fig. 5B inset). This may be due to the introduction of 1T-MoS₂, which changes the balance between organic ligands and metal ions in MIL-53 (Fe), affects its nucleation and growth process, and finally leads to the change of MIL-53 (Fe) morphology.

Among various semiconductor photocatalysts, CdS, as a narrow band gap semiconductor (2. 2 ~ 2.4 eV), has been proved to be a promising visible-light photocatalyst, which can be used to degrade pollutants, decompose water for hydrogen evolution and convert organic compounds^[73]. However, the photogenerated electron-hole pairs are easy to recombine, which inhibits the photocatalytic activity. In order to overcome the shortcomings of CdS as a photocatalyst, Kansal et al. Prepared CdS/MIL-53 (Fe) by two-step ultrasound-assisted solvothermal technology^[46]. As can be seen from fig. 5C, after the two materials are compounded, a layer of CdS nanoparticles is coated on the MIL-53 (Fe) microrod.

In recent years, metal iodates have been widely studied because the lone pair electrons in the IO₃^- are beneficial to the formation of layered structures and polarize them^[74]. The layered structure and polarity may play an important role in reducing the recombination of photoinduced electrons and holes, thus improving the photocatalytic activity. Oladipo et al. Successfully synthesized a AgIO₃/MIL-53(Fe) nano-hybrid composite (NC) driven by real sunlight^[37]. The NC exhibits a rough woody surface morphology in which AgIO₃ nanoparticles are highly dispersed and attached to the MIL-53 (Fe) surface, forming a robust hybrid heterostructure (Fig. 5d).

In addition, the g-C₃N₄ doped with pyromellitic diimide (PDI) can make the energy band structure of the material adapt to specific photoreactions and effectively inhibit the recombination of photogenerated electrons and holes^[75,76]. Lu et al. Added the g-C₃N₄/PDI prepared in advance during the solvothermal synthesis of the NH₂-MIL-53(Fe), and finally obtained the g-C₃N₄/PDI@NH₂-MIL-53(Fe)(CPM) composite^[51]. SEM showed that the surface of the spindle-shaped NH₂-MIL-53(Fe) was packed with laminate-like g-C₃N₄/PDI (fig. 5e).

2.4 MIL-88 (Fe) and its composites

At present, most MOFs are synthesized by solvothermal method^[77]. However, the synthesis of MOFs at room temperature can not only reduce energy consumption, but also facilitate the realization of large-scale production of MOFs under mild conditions, so it has attracted more and more attention of researchers^[78]. Wang et al. Reported the preparation of MIL-88a (Fe) at room temperature, using water to promote the deprotonation of fumaric acid and the hydrolysis of iron salts, thus accelerating the nucleation of crystals^[52]. The size of the resulting product MIL-88a (Fe) can be controlled by varying the amount of reactants. The reactant concentration has no effect on the morphology, but a high concentration of reactant helps to reduce the particle size of the product (figs. 6a and 6B). Wang et al. Also prepared PANI/MIL-88a (Fe) composites using a simple one-pot solvothermal method for photo-Fenton degradation of BPA under visible light irradiation from LEDs (Fig. 6C)^[53]. Zhang et al. Synthesized MIL-88A/g-C₃N₄ (M88/g-C₃N₄) heterojunction photocatalyst by a simple in-situ hydrothermal method (Fig. 6d)^[47]. Under visible light irradiation, the MIL-88A/g-C₃N₄ photocatalyst has good photocatalytic activity on the typical antibiotic tetracycline (TC).

Molecular imprinting (MIP) is to add target molecules as template molecules (imprinting molecules) in the process of preparing materials. After removing the template molecules, holes similar to the template molecules will be left in the polymer, which will show a high degree of recognition of the target molecules^[79]. Guan et al. modified MIL-88a with dibutyl phthalate (DBP) as the target imprinting molecule for the first time, and obtained MIL-88a @ MIP and MIL-88A @ NIP without template molecule DBP (as shown in Figure 6e and its inlay)^[62]. The surface of MIL-88a @ NIP is relatively smooth, while the surface of MIL-88a @ MIP forms a number of holes of different sizes, which serve to enhance the adsorption capacity of target pollutants.

Generally speaking, Ag/AgCl is also a typical composite photocatalyst, which has a high surface plasmon resonance (SPR) effect and can significantly enhance the absorption of visible light^[80]. However, the photocatalytic effect of Ag/AgCl under visible light irradiation is limited due to its weak adsorption. Liu et al. Synthesized Ag/AgCl @ MIL-88a (Fe) (ACMA) nanocomposites from plasma Ag/AgCl and metal-organic framework MIL-88a (Fe) by one-pot solvothermal method^[48]. As shown in Figure 6 f and its inset, the surface of MIL-88A (Fe) is smooth, while the surface of ACMA becomes rough.

MIL-88B (Fe) is a water-stable MOF material constructed from 1,4-terephthalic acid and trimeric iron octahedral (Fe₃-μ₃-O) clusters^[26]. Hu et al. synthesized a typical spindle-shaped MIL-88 B (Fe) (Fig. 6G) and constructed a MIL-88 B (Fe)/persulfate/visible light (M88/PS/Vis) system for photocatalytic degradation of BPA in aqueous solution^[63]. Considering that BiOX has a layered tetragonal structure, the interaction between the [Bi₂O₂]²⁺ layer and the dihalogen atom layer will produce an internal electrostatic field perpendicular to each layer, which will enhance the separation of photogenerated electron-hole pairs^[81]. Kim et al. Successfully prepared Z-type heterojunction BiOI/MIL-88B (Fe) by in-situ synthesis method, in which BiOI has a narrow band gap, which is conducive to the use of visible light^[49]. As shown in Fig. 6h, MIL-88 B (Fe) has an irregular spindle shape, while BiOI in the inset of Fig. 6h has a three-dimensional hierarchical flower-like microsphere structure. In the BiOI/MIL-88B (Fe) composite, the BiOI is very tightly packed in the composite and the individual layers are connected by MIL-88B (Fe) (Fig. 6i).

3 Application of Fe-MOFs and Their Composites in Advanced Oxidation Degradation of EOCs

Worldwide, with population growth, urban development and climate change, water shortage has become a huge problem. Today, emerging organic pollutants (EOCs) are ubiquitous in landfill leachate, sewer leakage, storm runoff, and wastewater treatment plant discharges^[82]. Among them, drugs are considered to be the most important class of EOCs because they are widely used by humans and livestock, so many researchers have used Fe-MOFs and their complexes to degrade drugs in water by advanced oxidation^[82]^{[17,46,47,50]}. At the same time, some researchers have used it to degrade environmental hormones and pesticide residues in the environment and remove a variety of EOCs in wastewater at the same time^{[52,54,61,63]}.

3.1 Advanced Oxidation Degradation of Drugs in Water by Fe-MOFs and Their Complex

In the past few decades, the presence of antibiotics in water has attracted considerable attention. Tetracyclines (TC) and sulfonamides (SAs) are broad-spectrum antibiotics that have been widely used in humans, animal husbandry, and aquaculture. The intensive use or even abuse of various antibiotics leads to the existence of more antibiotic residues in the water environment, resulting in extremely high antibiotic selection pressure, and eventually forming a certain scale of antibiotic resistant bacteria (ARBs) and resistant genes (ARGs), which pose a serious threat to the ecosystem and human health^[83,84]. Zhang et al. Used -type heterojunction as a photocatalyst to achieve photocatalytic degradation of TC under visible light irradiation^[47]. Among them, M88/g-C₃N₄-10(MIL-88A with a mass fraction of 10% in the composite showed the best photocatalytic activity, and the degradation of TC (initial concentration of 10 mg/L) was completed within 30 min. Wang et al. explored the activity of Z-type MIL-100 (Fe)/PANI heterojunction for photocatalytic degradation of TC under white light irradiation^[36]. The results showed that MIL-100 (Fe)/PANI could degrade 84% of TC (initial concentration was 10 mg/L) in 120 min. Wang et al. explored the activity of MIL-100(Fe)/TiO₂(M/P-x) composites in degrading TC in the presence of H₂O₂^[59]. Among them, M/P-10 (the number 10 indicates that the initial molar concentration of Fe(NO₃)·9H₂O added in the synthesis process is 10 mmol/L) has the best performance, which can degrade 85. 8% of TC (initial concentration is 100 mg/L) in 60 min. Lv et al. Used MIL-100(Fe)@Fe₃O₄/CA to catalyze the decomposition of H₂O₂ to produce hydroxyl radicals in the process of photo-Fenton reaction, and the degradation efficiency of TC (initial concentration of 10 mg/L) was 85% in 3. 5 H, which was much higher than that of MIL-100(Fe)@Fe₃O₄(c.a.42%)^[60]. In addition, the addition of the magnetic Fe₃O₄ improves the stability and recyclability of the material and promotes an environment-friendly water purification process. Wang et al. Used a new Fenton-like catalyst CUS-MIL-100 (Fe) to degrade the sulfonamide antibiotic sulfamethazine (SMT)^[17]. The results showed that the degradation efficiency of CUS-MIL-100 (Fe) was much higher than that of MIL-100 (Fe) alone. Ciprofloxacin (CIP) is a broad-spectrum antimicrobial agent, which has caused serious environmental problems due to its high stability in the environment^[80]. Kim et al. Used Z-type heterojunction BiOI/MIL-88B (Fe) for photocatalytic degradation of CIP^[49]. BiOI/MIL-88B (Fe) could remove more than 80% of CIP (initial concentration of 10 mg/L) in 4. 5 H, while BiOI alone could only remove less than 30% of CIP.

Ketorolac tromethamine (KTC), ibuprofen (IBP) and diclofenac sodium (DCF) are commonly used non-steroidal anti-inflammatory drugs (NSAIDs), and their residues have genotoxicity, aquatic environmental toxicity, and can easily cause endocrine disorders in organisms^{[85⇓⇓~88]}. Kansal et al used CdS/MIL-53 (Fe) to degrade KTC (initial concentration of 10 mg/L) under the optimal conditions, and the efficiency reached 80%, which was significantly better than that of MIL-53 (Fe) (degradation rate was only 36%)^[46]. The Ag/AgCl @ MIL-88a (Fe) (ACMA) nanocomposite synthesized by Liu et al. Has excellent photocatalytic degradation effect on IBP under visible light irradiation^[48]. Specifically, under visible light irradiation, ACMA-2 (Fe ∶ Ag = 2 ∶ 1) exhibited the highest photocatalytic degradation efficiency for IBP (initial concentration of 10 mg/L), which was 10. 8 times and 40. 4 times higher than that of MIL-88a (Fe) and Ag/AgCl alone, respectively. Under the catalysis of ACMA-2, the removal rate of total organic carbon (TOC) was as high as 91%. Zhang et al. Used 1T-MoS₂@MIL-53(Fe)(TSMF) heterostructures to degrade IBP (initial concentration of 10 mg/L) under the combined action of visible light and H₂O₂^[50]. Compared with MIL-53 (Fe) and 1T-MoS₂ alone, the TSMF nanocomposite with the optimal proportion had the best effect on the Fenton-like oxidation of IBP, and its degradation rate constant was 1.183 h^-1, which was 2. 7 and 8 times of that of pure 1T-MoS₂(0.126 h^-1) and MIL-53(Fe)(0.158 h^-1), respectively. In addition, the removal rate of TOC can reach more than 70% after 1 H of visible light irradiation. When the illumination time was extended to 2 H, the mineralization efficiency reached 95%. This indicates that TSMF can effectively mineralize IBP through catalysis. This also indicates that IBP can not be completely mineralized into CO₂ and H₂O directly because of the production of intermediates during the degradation of IBP under short illumination time. Wang et al. Also tested the activity of the MIL-100/g-C₃N₄(MG-x) heterojunction system to catalyze the decomposition of H₂O₂ to produce hydroxyl radicals to degrade DCF under simulated sunlight irradiation, and the results showed that 100% of DCF (initial concentration of 0.1 mM) was degraded within 50 min^[21].

3.2 Degradation of Environmental Hormones in Water by Fe-MOFs and Their Composite

Bisphenol A (BPA), a plastic monomer and plasticizer, is one of the most produced chemicals in the world. BPA can leach from food and beverage containers and some dental sealants and composites under normal use conditions^[89]. The results also show that BPA can increase the activity of macrophages, reduce antibodies and cause immune system disorders^[90]. Wang et al. Used MIL-88A-1 and MIL-88A-2 synthesized at room temperature to carry out photo-Fenton catalytic degradation of BPA under visible light irradiation and in the presence of H₂O₂, and the smaller MIL-88A-2 showed better photo-Fenton catalytic activity than the slightly larger ML-88A-1^[52]. PANI/MIL-88a (Fe) and WO₃/MIL-100(Fe) also showed excellent photo-Fenton activity for BPA degradation under visible light irradiation^[53]. As shown in Fig. 7, Wang et al. explored the degradation pathway of BPA during the experiment to understand the mechanism of the advanced oxidation process. In the three systems, BPA was hydroxylated by hydroxyl radical, and then intermediates such as phenols, quinones and ketones were produced, and finally the intermediates were completely oxidized to CO₂ and H₂O by ring-opening reaction.

Hu et al. constructed MIL-88B (Fe)/persulfate/visible light (M88/PS/Vis) system for the catalytic degradation of BPA in aqueous solution^[63]. There was a synergistic effect in M88/PS/Vis system, and BPA with an initial concentration of 10 mg/I could be completely removed in 25 min, while the removal rates of BPA by M88/Vis or M88/PS were only 26. 0% and 34. 1%, respectively. In addition, the degradation rate (0.107 min^-1) of BPA in M88/PS/Vis was 4. 7 times higher than that in M88/PS(0.023 min^-1) and 8. 9 times higher than that in M88/Vis(0.012 min^-1), respectively. In this reaction system, BPA was not completely mineralized, but some intermediate products were formed. As shown in fig. 8A, first, the C — C bond between the benzene ring and the isopropyl group of BPA (labeled A) is cleaved by radical attack to produce an isopropylphenol radical. On the one hand, it is attacked by photogenerated Desaturation and unsaturation progress, forming 4-isopropylphenol (labeled C1) through Desaturation and unsaturation progress (green arrow). On the other hand, 4,4 '- ( (4-hydroxy-1,3-phenyl) bis (propane-2,2-diyl)) phenol (denoted by A1) having a molecular weight larger than that of BPA was produced by the combined action of · OH and SO₄^-·, and then further decomposed into phenol (denoted by A2) and 4-tert-butyl-2- (.alpha.alpha.-dimethylbenzyl) phenol (denoted as B1). Subsequently, the phenol formed is gradually oxidized to hydroquinone (labeled A3) and p-benzoquinone (labeled A4). At that same time, 4-tert-butyl-2- (. Alpha. Alpha. -dimethylbenzyl) phenol is convert to phenol and 2, 4-bis (1, 1-dimethylethyl) (label B2) (red arrow). The third transformation pathway is the hydroxylation of BPA, i.e. A series of polyhydroxylation reactions (D1 and D2) occur when the aromatic ring is attacked by water molecules through electrophilic effects (black arrows). The hydroxylated BPA is then dehydrated to form a quinone (D3) and a carboxyl compound (E1). Finally, the aromatics are converted to ring-opening intermediates that are ultimately mineralized to CO₂ and H₂O.

In order to clarify the potential environmental impacts of BPA and its degradation intermediates, the risk assessment of BPA and its degradation intermediates was carried out using the ECOSAR model, and the acute toxicity and transformation by-products of BPA on fish, Daphnia and green algae were predicted. As shown in Fig. 8B, although intermediates A1, B1 and B2 with higher biotoxicity than BPA are produced, the toxicity of most intermediates is lower than that of BPA, which indicates that the toxicity of degradation products of BPA in M88/PS/Vis is lower than that of BPA itself, that is, BPA in M88/PS/Vis system is in a detoxification process. Therefore, Hu et al. Used the principle of "the amount of formazan dye produced by dehydrogenase activity in cells is proportional to the number of viable cells" in CCK-8 method to carry out cell proliferation toxicity test (Fig. 8C)^[63]. The results showed that the relative absorption intensity of CCK-8 in the system decreased after 10 min reaction, indicating that the number of living cells decreased, that is, the intermediate product had higher toxicity than BPA.

However, the toxicity of the further degradation products was lower than that of BPA with the extension of reaction time, which indicated that M88/PS/Vis could degrade BPA effectively and achieve detoxification gradually. BPA was completely removed after 30 min of reaction in this system (as shown in Fig. 8d), while TOC removal efficiency was only 67.5%, indicating that BPA could not be completely mineralized within 30 min; When the reaction time was extended to 75 min, BPA was completely mineralized, indicating that the toxicity of BPA could be completely eliminated by prolonging the reaction time.

Wang et al. Used the Bi₁₂O₁₇Cl₂/MIL-100(Fe)(BMx) composite prepared by simple ball milling method as a catalyst to stimulate PS to produce sulfate radicals for BPA degradation under the action of white light^[64]. In the BM200/white light/PS system (BM200 represents 200 mg of Bi₁₂O₁₇Cl₂ and MIL-100 (Fe) during ball milling), BPA (initial concentration of 10 mg/L) can be completely eliminated within 60 min. In order to explore the possible degradation pathway of BPA, they first analyzed the Fukui index of each reaction site of BPA molecule ($f^-$ and $f^+$ represent electrophilic and nucleophilic attack values, respectively), and understood the exact degradation pathway of BPA through DFT calculation.

Previous studies have shown that HOMO and LUMO orbitals can describe the sites where electrons are easily lost or gained in BPA molecules during advanced oxidation processes, respectively. As shown in Fig. 9B, purple represents the electron-rich region, and light blue represents the electron-poor region. Therefore, the symmetrical benzene ring is vulnerable to the attack of the electrophilic radicals Sh, ·

O 2 -

and · OH; At the same time, the carbon atom in the para position of the phenolic hydroxyl group tends to be attacked by the electrophilic radical h⁺. The shaded intensities of the red boxes in Figure 9c represent different levels of

f -

and

f +

, where C (1), C (3), C (10) and C (14) in BPA (highlighted in red) would be the active sites most likely to be attacked by free radicals. Finally, based on the results of UHPLC-MS and DFT calculation, they determined the preferential attack position of free radicals on BPA in the reaction process, and proposed that the possible degradation pathways of BPA were mainly divided into three: (1) ·

O 2 -

/· OH addition (blue line); (2)h⁺ attack C (7) -C (3) or C (7) -C (10) cleavage (red line); (3) S

O 4 - ·

hydroxylation process (green line)^[64]. Among them, hydroxylation leads to the production of dihydroxylated BPA (B, m/Z = 259), which is a common process when BPA is degraded. At the same time, a large number of ·

O 2 -

tend to attack the aromatic ring, which is also the reason for a series of polyhydroxylation processes. Subsequently, the C-C bond of BPA between the benzene ring and isopropanol was broken by · OH to form the intermediate C (m/Z = 151), and then BPA was rapidly converted to 2-isopropylxylobionic acid (D, m/Z = 183) through a further ring-opening process. Secondly, the highest 09524 of C (3) and C (10) is 0.09524, which indicates that the two C sites are vulnerable to nucleophilic attack. Therefore, C (3) and C (10) are easily attacked and cleaved to form 4-isopropylphenol radical (compound E) and phenol (F, m/Z = 93). The 4-isopropylphenol radical tends to form a stable 4-isopropylphenol molecule (G, m/Z = 133), which is further oxidized to form 4 '-hydroxy-acetophenone (H, m/Z = 135). In addition, the attack of · OH on compounds E and F may also produce the same intermediate C; When the S

O 4 - ·

attacks the BPA molecule, an electron is transferred from the BPA molecule to form a C-centered BPA cation radical. This cationic radical undergoes a hydrolysis process to form an OH-adduct (OH)(BPA^·+) or (OH)₂(BPA^·+), which is further oxidized to a hydroxyl BPA radical, such as compound I (m/Z = 261). In general, the intermediates of BPA degradation can be further degraded into low-molecular-weight compounds (such as short-chain fatty acids), and finally completely mineralized into CO₂ and H₂O.

Under visible light irradiation, the removal rate and mineralization rate of BPA by g-C₃N₄/MIL-101(Fe) activated PS were 98% and 82% within 60 min, respectively, which were better than those of g-C₃N₄ and MIL-101 (Fe) alone^[34]. Guo et al. Also found that AQS-NH-MIL-101 (Fe) had better catalytic degradation performance of BPA than free NH₂-MIL-101(Fe)/AQS and NH₂-MIL-101(Fe)^[35]. The degradation rate constant of BPA catalyzed by AQS-NH-MIL-101 (Fe) was 9 times higher than that of NH₂-MIL-101(Fe) and 7 times higher than that of NH₂-MIL-101(Fe)/AQS.

3.3 Degradation of Pesticides in Water by Fe-MOFs and Their Composite

Organochlorine herbicides, which can control the growth of weeds, have been widely used in agricultural production worldwide. Among them, 2,4-dichlorophenoxyacetic acid (2,4-D) is difficult to be decomposed because of its biological stability and high fluidity. The International Institute on Cancer (IARC) lists 2,4-D as a possible mutagen, teratogen, and carcinogen in biological systems^[91]. The Z-shaped WO₃/MIL-53(Fe) heterojunction can achieve complete photocatalytic degradation of 2,4-D under real sunlight irradiation for 240 min, and its performance is significantly better than that of WO₃ and MIL-53 (Fe) alone^[45]. In this process, the mineralization rate of 2,4-D by WO₃/MIL-53(Fe) was as high as 96. 5%, while that by WO₃ and MIL-53 (Fe) alone was only 63. 8% and 51. 9%. It is worth mentioning that the WO₃/MIL-53(Fe) composite material can simultaneously oxidize 2,4-D and reduce Cr (Ⅵ).

Neonicotinoid pesticides have strong insecticidal activity, while thiamethoxam (THIA) and thiacloprid (TCL) belong to the second and third generation of neonicotinoid pesticides, respectively, which are toxic to mammals through oral, dermal and inhalation exposure^[92,93]. Zhao et al. Used conducting heterojunction MIL (Fe)/Fe-SPC to carry out ultrasonic-Fenton catalytic degradation of THIA^[57]. Interestingly, during the experiment, they found that the Fe-SPC material alone removed nearly 100% of THIA in 40 min, 1 H faster than the composite material, and to find out why, they further extracted samples from the catalyst.The calculated THIA content was almost the same as that extracted from the reaction system, indicating that Fe-SPC had no degradation ability to THIA, but only adsorption led to the decrease of THIA content^[57]^[57]. On the contrary, THIA was not detected in the extracts of MIL (Fe)/Fe-SPC reaction system, indicating that they had high degradation activity, and the TOC removal rate of MIL(Fe)/Fe-SPC₆(Fe-SPC (6% of the mass fraction of MIL-100 (Fe)) reached 95.4% within 3 H. Zhao et al. Used Cu₂O/MIL-100(Fe/Cu) composite materials for photo-Fenton degradation of TCL, in which the catalytic rate (0.0279 min^-1) of Cu₂O/MIL-100(Fe/Cu)) composite materials was 2.5 times that of pure MIL-100(Fe)(0.025 min^-1)^[58].

3.4 Simultaneous degradation of multiple EOCs in water by Fe-MOFs and their complex advanced oxidation

Theophylline (TP) is a xanthine used to treat lung conditions such as asthma, shortness of breath and wheezing^[94]. In nature, the accumulation of TP may cause adverse health effects in humans and various animals, for example, TP has been found to be moderately toxic to mammals (LD50>200 mg·kg^-1)^[95]. Wu et al. Used Pd @ MIL-100 (Fe) for photo-Fenton degradation of three PPCPs (TP, IBP and BPA) under visible light irradiation, which had higher photocatalytic activity than the raw material MIL-100 (Fe)^[54]. To ensure sufficient interfacial contact between noble metals and MOFs, in a follow-up study, they combined MOFs with palladium nanoparticles (Pd NPs) modified phosphotungstic acid (PTA) (Pd-PTA-MIL-100 (Fe))^[55]. Under visible light irradiation, the degradation efficiency of TP and IBP was significantly improved. This work demonstrates a convenient method for the controlled preparation of Pd-PTA-MIL-100 (Fe) complexes and highlights the promise of using heteropolyacids as multifunctional reducing agents to design more efficient noble metal/MOFs composite photocatalysts for solar energy capture and conversion.

The herbicide atrazine alters plant growth, enzymatic processes, and photosynthesis, causing errors in lipid synthesis, hormonal imbalances, and even mutagenicity and genotoxicity in aquatic and non-target animals^[96]. Quan et al. evaluated the performance of the prepared M. MIL 100 (Fe) @ ZnO NS for the removal of organic pollutants BPA and atrazine by photo-Fenton degradation^[61]. The results showed that the M. MIL -100 (Fe) @ ZnO NS had the highest photocatalytic degradation rate for BPA and atrazine. The TOC removal rate of MIL-100 (Fe) under visible light irradiation for 120 min was 43% ~ 49%, which was higher than that of MIL-100 (Fe) alone (< 30%).

Carbamazepine (CBZ) is a widely used antiepileptic drug, which is resistant to biodegradation and cannot be effectively removed by traditional biological treatment processes^[97]. Lu et al. Used g-C₃N₄/PDI@NH₂-MIL-53(Fe)(CPM) to cooperate with heterojunction to remove several water-soluble and toxic organic pollutants (TC, CBZ and BPA) under visible light irradiation, showing excellent photo-Fenton catalytic degradation performance. The degradation efficiency of TC can reach 90% (50 ppm, 1 H), and the degradation efficiency of CBZ can also reach 78% (50 ppm, 2.5 H), while the degradation of BPA with the same concentration can reach 100% in 10 min^[51]. In addition, low concentration of BPA (2 ppm) can also be rapidly degraded within 10 min, which is of great significance for the removal of trace pollutants in the environment.

In recent years, organophosphorus pesticides (OPPs), such as malathion methyl (MP) and pyrithione (CP), have been frequently used because of their effectiveness against a wide range of pests. These OPPs have irreversible inhibitory effects on important enzymes in the human body, such as acetylcholinesterase (AChE), and can cause long-term damage to the liver even at low concentrations^[98]. The AgIO₃/MIL-53(Fe) nano-hybrid composite (NC) prepared by Oladipo et al. Can activate PS to degrade MP and CP under the drive of real sunlight^[37]. After 120 min of reaction, the AgIO₃/MIL-53(Fe) showed high activity in the photodegradation of MP and CP, and the conversion rate reached 93% ~ 97%. Meanwhile, in the simultaneous oxidation of the binary mixture (MP + CP), nearly 70% mineralization was achieved within 180 min. This study also compared the experimental data with the prediction data of artificial neural network (ANN) classification algorithm to understand the details of the mechanism of photodegradation process and provide more basis for the subsequent purification of water environment.

Phthalic acid esters (PAEs) are typical endocrine disruptors, among which dibutyl phthalate (DBP) has estrogen-like effects, which can cause teratogenesis, carcinogenesis and mutagenesis^[99]. Guan et al. explored the ability of MIL-88a @ MIP catalyst to activate PS for oxidative degradation of DBP^[62]. Compared with pure MIL-88a, the catalytic activity of MIL-88a @ MIP was effectively improved by targeted modification. At the same time, the catalytic activity of MIL-88a @ NIP without DBP was much lower than that of MIL-88a @ MIP. The results showed that the catalytic activity of MIL-88a could be greatly improved by molecular imprinting. At the same time, Guan et al. Also explored the degradation effect of MIL-88a @ MIP on PAEs^[62]. The degradation efficiency of MIL-88a @ MIP for dimethyl phthalate (DMP), diethyl phthalate (DEP) and dibutyl phthalate (DBP) was in the order of DEP > DBP > DMP, and there was little difference in the degradation efficiency, which indicated that MIL-88a @ MIP could catalyze and activate PS to degrade PAEs, so the modified material was of great significance for the removal of endocrine disruptors.

4 Factors affecting the degradation of EOCs by Fe-MOFs and their composites

4.1 Influence of the physical properties of the material itself

The catalytic efficiency (η) of photocatalytic materials is generally affected by the light absorption efficiency (η_abs), carrier separation efficiency (η_sep) and active surface efficiency (η_cat) of the materials, which can be roughly described as η=η_abs×η_sep×η_cat^[100]. Among them, many studies have been devoted to improving the catalytic activity of catalysts by enhancing their light absorption and carrier separation efficiency. Through UV-Vis DRS scanning, the researchers found that these two ways significantly expanded the absorption of visible light by the catalyst, indicating that it could use more solar energy^{[45⇓⇓⇓⇓⇓~51]}. At the same time, in addition to the enhancement of light absorption, many researchers have also explored the lifetime of photogenerated carriers in the photocatalytic process of composite materials through photoluminescence (PL), transient photocurrent response and AC impedance scanning. The results show that the PL intensity of the composite material is significantly reduced, the photocurrent response is significantly enhanced, and the EIS radius is significantly reduced, which confirms that the composite material has a lower electron-hole pair recombination rate^{[34⇓~36,47⇓⇓⇓ ~51]}. However, Kim et al. Found that the PL intensity of the composite material BiOI/MIL-88B (Fe) was contrary to other research results, showing a significant increase trend, which may be due to the fact that the BiOI/MIL-88B (Fe) nanocomposite contains surface defects or oxygen vacancies, resulting in the best photocatalytic performance for pollutants^[49]. Another possible reason is that the BiOI/MIL-88B (Fe) composite utilizes a Z-type heterojunction system with higher oxidation and reduction capabilities. At the same time, Hu et al. Found that the M88/PS/Vis system has a lower photocurrent response than M88/Vis, which may be due to the consumption of photogenerated electrons by PS to generate sulfate radicals^[63]. This change directly reveals that MIL-88B (Fe) can scavenge electrons in the presence of persulfate and visible light, prevent the recombination of electrons and holes, and realize the effective separation of carriers.

In addition, the high specific surface area and porosity of the catalyst will provide more active centers, which is conducive to the adsorption and diffusion of the catalytic substrate, and provide faster carrier transport capacity, thus improving the catalytic performance. At the same time, the crystallinity and coordination unsaturation center of the composite material also affect its catalytic activity. For example, in the MIL(Fe)/Fe-SPC_x composite material, the MIL(Fe)/Fe-SPC₆ has the highest crystallinity and the highest activity for degrading THIA^[57]; In CUS-MIL-100 (Fe), due to the existence of reductive Fe^II/Fe^III mixed-valence coordination unsaturated center CUS, the interaction between CUS and H₂O₂ was promoted, and the decomposition of H₂O₂ to hydroxyl was further promoted, thus improving its catalytic performance for the degradation of SMT^[17]. In addition, the in-situ Cu doping in the Cu₂O/MIL-100(Fe/Cu) can significantly enhance the interface connection between the Cu₂O and the MOFs, thereby shortening the reaction time between the photogenerated electrons and the free radicals and improving the catalytic activity^[58]

4.2 Effect of process operating conditions

4.2.1 Effect of pH

The pH of the solution has a great influence on the catalytic reaction, which mainly affects the catalytic activity of the materials through the following aspects: (1) The pH affects the surface electronegativity of the catalyst, thus affecting its adsorption performance for pollutants, and ultimately leading to different catalytic efficiencies^{[45,46,54,55,64]}. For example, the surface of the WO₃/MIL-53(Fe) has a negative charge when the pH value is higher than 4.2 and has a positive charge when the pH value is lower than 4.2. When the initial pH value of the solution is lower than 4.2, 2,4-D is adsorbed on the surface of the WO₃/MIL-53(Fe) through electrostatic interaction,2,4-D was adsorbed on the active sites of the photocatalyst, which improved the photocatalytic degradation efficiency. However, when the pH value was higher than 4.2, the degradation rate of 2,4-D decreased gradually due to the repulsion between 2,4-D anions and negatively charged WO₃/MIL-53(Fe)) surface^[45]. (2) pH affects the catalytic activity by affecting the stability of the catalyst^[50,57]. For example, Liu et al. Investigated the performance of TSMF-5 in the degradation of IBP in the pH range of 1 to 11^[50]. The results showed that TSMF-5 had high catalytic activity in the pH range of 1 ~ 7. However, the removal efficiency of TSMF-5 is poor in strongly alkaline solutions (pH > 9). Through XRD pattern analysis, the researchers found that TSMF-5 was unstable under alkaline conditions, thus its catalytic activity decreased. (3) pH affects the catalytic activity by affecting the self-decomposition of the H₂O₂^[61]. It has been proved that H₂O₂ tends to be decomposed into molecular oxygen and H₂O in alkaline environment, and the catalytic activity may also decrease under peracidic conditions (pH = 2), mainly because the formation of H₃

O 2 +

can inhibit the formation of hydroxyl radicals, thus affecting the catalytic activity^[61]. (4) pH affects the oxidation potential of hydroxyl radical (E₀=+2.8 V at pH = 0); E₀=+2.0 V at pH = 14)^[17,61]; (5) pH affects the catalytic activity by affecting the existing form of pollutants. For example, TC has three acid dissociation constants (3.3, 7.68 and 9.68). Under different pH conditions, TC exists as cation, zwitterion and anion, which affects the adsorption of TC on the catalyst and thus affects the final catalytic activity^[60]. (6) pH affects the catalytic activity by affecting the formation of active species. In general, S

O 4 - ·

tend to be produced in acidic environments. However, S

O 4 - ·

can also be converted to S

O 4 2 -

ion and · OH in alkaline environment, but S

O 4 - ·

has a higher oxidation potential (S

O 4 - ·

: 2.6 V ~ 3.1 V vs · OH: 1.8 V ~ 2.7 V) and a longer half-life (S

O 4 - ·

: 30 ~ 40 μs vs · OH: < 1 μs), so the catalytic reaction rate is relatively faster in acidic environment^[64].

4.2.2 Influence of foreign ions

Inorganic cations and anions are well known to be common in surface water, groundwater, and wastewater, often in combination with organic contaminants. In order to explore the practical application of photocatalysis in water remediation, Lu et al. Evaluated the effect of inorganic salts on the performance of photocatalyst g-C₃N₄/PDI@NH₂-MIL-53(Fe) in photoFenton system to remove target pollutants^[51]. They found that 2.5 mmol/L cations (Ca²⁺, K⁺, Mg²⁺, Na⁺) had no significant effect on the degradation rate of TC, while 2.5 ~ 10 mmol/L anion N

O 3 -

had almost no effect on the degradation of TC. However, it is interesting that the addition of S

O 4 2 -

can produce S

O 4 - ·

, which can promote the degradation of TC synergistically with hydroxyl radical. However, when the concentration of anionic Cl^- and HC

O 3 -

in the solution gradually increases, the degradation rate decreases.This may be due to the fact that Cl^- and HC

O 3 -

can compete for the consumption of · OH to produce Cl · and HC

O 3 ·

O 3 - ·

,The HC

O 3 ·

O 3 - ·

mainly reacts with electron-rich compounds, and its activity is lower than that of · OH and S

O 4 - ·

. Hu et al. Discussed the effect of inorganic anions on the degradation of BPA by activated PS^[63]. Similarly, HC

O 3 -

can inhibit the degradation of BPA, mainly because HC

O 3 -

can compete for the consumption of hydroxyl and sulfur

O 4 - ·

in the system and form a weak oxidizing HC

O 3 ·

O 3 - ·

. Similarly, N

O 3 -

is easy to react with S

O 4 - ·

to form N

O 3 - ·

, but the degradation efficiency of BPA in the system with N

O 3 -

is higher than that in the system with C

O 3 -

.This may be due to the fact that C

O 3 -

is more likely to react with hydroxyl radicals and S

O 4 - ·

, thus consuming more highly active radicals. It is reported that H₂P

O 4 -

can be attached to the surface of photocatalysts as chelating agents and have quenching effect on free radicals, thus affecting the catalytic activity. In general, the addition of Cl^- will compete for the consumption of hydroxyl radical and sulfur

O 4 - ·

, which will inhibit the catalytic activity of the material. However, in this study, the addition of Cl^- will enhance the catalytic activity of the material. The main reason is that the addition of :Cl^- will increase the hydroxyl radical in the system. The Cl^- can not compete with the photocatalyst to absorb visible light; The active chlorine species HClO and Cl₂ produced by the Cl^- of PS oxidation will participate in the removal of BPA in the system.

4.2.3 Effect of other process operating conditions

Because the amount of catalyst can affect the active center of the reaction, it plays a vital role in the catalytic reaction. For example, with the increase of catalyst dosage, the efficiency of catalytic degradation of pollutants is gradually improved, which may be due to the gradual increase of active sites in the catalytic reaction. With the further increase of catalyst dosage, the degradation efficiency will decrease, which may be due to the aggregation of too much catalyst in the solution, affecting the penetration of light in the solution, thus hindering the light absorption of the catalyst^{[17,34,46,50]}. At the same time, Lu et al also found that excessive catalyst may also lead to competitive reactions, thus reducing the number of available radicals^[51].

The initial concentration of pollutants will also have an impact on the reaction process. When the initial concentration of pollutants is low, the degradation rate will increase with the increase of the concentration of pollutants due to the relatively more active centers of the catalytic material, but when the initial concentration reaches a certain limit, the degradation rate will decrease with the increase of the concentration of pollutants.The main reason may be that the active center on the catalyst is covered, which prevents it from continuing to participate in the reaction. In addition, higher concentrations of pollutants will produce more intermediates, which may lead to their competitive consumption of free radicals, thus reducing the degradation rate^[45]^{[17,34,37,46]}^[17]. At the same time, the addition of appropriate amount of H₂O₂ and PS can enhance the catalytic activity, but excessive dosage will compete with pollutants to consume free radicals, resulting in the reduction of activity^{[61⇓⇓~64]}. Kim et al explored the degradation of pollutants by Z-heterojunction BiOI/MIL-88B (Fe) under visible light (λ ≥ 425 nm), ultraviolet light (λ ≤ 425 nm) and simulated sunlight irradiation, respectively. The results showed that the photocatalytic efficiency under simulated sunlight irradiation was higher than that under other light sources, indicating that the choice of light source also affected the activity of the catalyst^[49].

4.3 Effect of active substances

4.3.1 Active Substance in Photocatalytic Reaction

AOPs rely on radical reactions, in which photocatalytic technology can convert light energy (including solar energy) into chemical energy by means of photocatalysts, with mild reaction conditions, low energy consumption and high reaction efficiency^[101]. Photocatalytic technology has been widely used in the fields of photodegradation of water to produce hydrogen and oxygen, reduction of carbon dioxide and removal of environmental pollutants^{[1,6,20,22,47]}. So far, various materials, including metal oxides, graphene, graphite-like carbon nitride, metal-organic frameworks and their composites, have been widely studied and ^[102~104] as efficient photocatalysts. Since Garc García et al first demonstrated the photocatalytic degradation of phenol by MOF-5 under UV irradiation in 2008, more and more researchers have systematically studied the photocatalytic activity of various MOFs and their composites^[104]^[1,20,23].

Photocatalytic process is often due to the separation of electron-hole pairs caused by photoexcitation of catalysts, thus producing hydroxyl and

O 2 -

, and the generated hydroxyl and

O 2 -

may cooperate with e^- and h⁺ to catalyze the degradation of pollutants, but the participation of the same active substance in different systems is different. For example, in the WO₃/MIL-53(Fe) photocatalytic system, h⁺ and hydroxyl radical are the main active species in the degradation of 2,4-D, while the

O 2 -

radical has little effect on the degradation of 2,4-D^[45]. In the system of CdS-MIL-53 (Fe), hydroxyl radical, h⁺ and

O 2 -

promoted the decomposition and removal of KTC, but hydroxyl radical played a leading role^[46]. In the Ag/AgCl @ MIL-88a (Fe) system, ·

O 2 -

, h⁺ and e^- were the main species for IBP removal. In addition, Cl^o atoms are also reactive species, which can oxidize IBP and be reduced to Cl^-, thus keeping the ACMA system stable^[48]. In the MIL-88A/g-C₃N₄ system, hydroxyl radical was the main active species, and the h⁺ and

O 2 -

had a slight effect on the photocatalytic reaction^[47]. In the photocatalytic degradation of TC by Z-heterojunction MIL-100 (Fe)/PANI, the main reaction species involved in the photocatalytic process were hydroxyl radical and ·O₂^- detected by ESR^[36]. In the photocatalytic degradation of CIP on BiOI/MIL-88B (Fe), h⁺ was the main active species, hydroxyl radical was also involved in part of the reaction, and

O 2 -

had a slight effect on the catalytic reaction^[49]

4.3.2 Active Substance in Fenton-like Reaction

Homogeneous Fenton reaction is a very typical AOPs, and its reaction principle is relatively simple. As shown in formula (5, 6), under acidic conditions, the Fe²⁺/Fe³⁺ reacts with the H₂O₂ to generate free radicals that can react with the pollutants, thereby finally achieving the purpose of removing the pollutants^[105,106]. Later, researchers found a way to improve the efficiency of the reaction, that is, to add some energy in the reaction process, such as ultraviolet light, sunlight, etc., which is called homogeneous photo-Fenton method (as shown in formulas 7 and 8)^[107]. However, several inherent shortcomings of homogeneous Fenton reaction, such as the narrow pH range required for operation, the generation of iron-containing sludge and the non-renewable catalyst, have significantly hindered its further large-scale application^[108]. In order to overcome the problems of homogeneous Fenton reaction, the insoluble solid catalyst was used in heterogeneous Fenton reaction, which has the advantages of high stability, wide pH range and easy recovery^[17]. Up to now, various iron-based catalysts, such as zero-valent iron, iron-oxygen compounds, Fe-MOFs, have been used for the catalytic oxidation of H₂O₂^[109]. Among them, (I) the interaction between Fe-MOFs and the surface of the H₂O₂ will trigger the Fenton reaction to generate · OH (Formula 1,2);(ii)H₂O₂ will capture the photogenerated electrons in the MOFs to generate · OH, thereby inhibiting electron-hole recombination to enhance the degradation of pollutants (Formulas 3 and 4).

(5) Fe²⁺+H₂O ₂→Fe³⁺+·OH+OH^-

(6) Fe ³⁺+H ₂O ₂→Fe²⁺+HO ₂·+H⁺

(7) Fe ³⁺+H ₂O+ hυ→Fe²⁺+·OH+H⁺

(8) H₂O₂+ hυ→Fe²⁺+HO ₂·+H⁺

Unlike photocatalytic reaction, Fenton-like reaction mainly produces hydroxyl radicals to degrade pollutants. Wang et al. Found that the degradation of SMT by CUS-MIL-100 (Fe) was mainly attributed to the excellent oxidation ability of · OH in the Fenton-like system^[17]; The degradation of organic compounds by MIL (Fe)/Fe-SPC, M. MIL and M. MIL-100 (Fe) @ ZnO NS was also attributed to hydroxyl radical^[51,57,61]. The photo-Fenton degradation of TP by Pd @ MIL-100 (Fe) synthesized by Wu et al. Is mainly due to the degradation of PPCPs by e^- and hydroxyl radical, and they also found that Pd-PTA-MIL-100 (Fe) has a strong catalytic degradation of PPCPs by the generation of hydroxyl radical^[54]^[55]. The Z-shaped WO₃/MIL-100(Fe) composite material prepared by Wang et al shows excellent photo-Fenton degradation activity of BPA under visible light irradiation of LEDs^[53]. This is mainly due to the excitation of H₂O₂ (the primary step) and the reaction of h⁺ with water molecules (the secondary step) to produce · OH, while ·

O 2 -

has no contribution to the degradation of BPA. As a photocatalyst, the h⁺ in the system of MIL-100(Fe)@Fe₃O₄/CA had a slight effect on the degradation of TC, but ·

O 2 -

and · OH were more active species in the degradation of TC by MIL-100(Fe)@Fe₃O₄/CA^[60]. Zhang et al. Found that ·

O 2 -

, · OH and e^- were involved in the photo-Fenton process of IBP degradation by 1T-MoS₂@MIL-53(Fe)^[50].

4.3.3 Active Substance in Activated Persulfate Reaction

Recently, persulfate (PS) has received increasing attention as an emerging oxidant, but its direct reaction with most pollutants is so slow that it needs to be activated to produce reactive species for the degradation of toxic and refractory pollutants. As shown in Eqs. (9-12), PS can be excited to produce S

O 4 - ·

, which behaves like · OH with a non-selective oxidation mode, and also produces ·

O 2 -

(Eq. 12) and · OH (Eq. 13 and 14)^[110~115]. The · OH and ·

O 2 -

formed in the previous stage can react to form singlet oxygen (¹O₂) and hydroxide ions (OH^-) (Eq. 15)^[116]. In addition, the

O 2 -

can react with itself to form H₂O₂ and ¹O₂ (Formula 16)^[117]. At the same time, the reactions shown in Formulas 17 and 18 can synthesize more reaction intermediates, and peroxomonosulfate (HSO) may be excited to form S

O 4 - ·

^[110]. In situ remediation by activated PS oxidation may be preferable to the · OH process because the S

O 4 - ·

is more stable and reacts with a wide range of contaminants over a wider pH range^[110]. There are many ways to activate PS, including UV irradiation and thermal radiation, or using transition metal ions (such as Fe²⁺, CO²⁺, and Mn²⁺)^[118]^[111]. Similar to the existence of H₂O₂ in the system, the abundant coordination unsaturated sites on the surface of Fe-MOFs can also activate PS, which can react with photogenerated electrons to inhibit carrier recombination and produce S

O 4 - ·

, thus accelerating the degradation of organic matter^[33,34].

(9)S ₂

O 8 2 -

+energy in put→2S

O 4 - ·

(10)S ₂

O 8 2 -

+ e ^-→S

O 4 2 -

O 4 - ·

(11)S ₂

O 8 2 -

+M ⁿ ⁺→M ⁿ ⁺¹+S

O 4 2 -

O 4 - ·

, 其中M表示金属

(12)S ₂

O 8 2 -

+2H ₂O

O 4 -

O 4 - ·

+·

O 2 -

+4H ⁺

(13)S

O 4 - ·

+H ₂O→S

O 4 2 -

+·OH+H⁺

(14)S

O 4 - ·

+OH ^-→S

O 4 2 -

+·OH

(15)·

O 2 -

+·OH→¹O ₂+OH ^-

(16)2·

O 2 -

+2H ⁺→H₂O ₂+ ¹O ₂

(17)S

O 4 - ·

+·OH→HS

O 5 -

(18)2·OH→H₂O ₂

Guan et al. Found that the main active species in the degradation of DBP by MIL-88a @ MIP was S

O 4 - ·

, and · OH also contributed partly, but it was negligible compared with S

O 4 - ·

^[62]. In the M88/PS/Vis system, Hu et al. Found that the

O 2 -

and S

O 4 - ·

play a leading role through the active material capture experiment^[63]; Wang et al. Found that the degradation of BPA in Bi₁₂O₁₇Cl₂/MIL-100(Fe) composites was mainly through S

O 4 - ·

and ·

O 2 -

, and h⁺ also participated in the degradation of BPA in BM200/light/PS^[64]. g-C₃N₄/MIL-101(Fe) The main active species in the degradation of BPA were S

O 4 - ·

, ·

O 2 -

and · OH, and the · OH radical was from S

O 4 - · [34]

; At the same time, Guo et al found that S

O 4 - ·

, · OH and ·

O 2 -

were all active species for the degradation of BPA by AQS-NH-MIL-101 (Fe), but S

O 4 - ·

was the main free radical species, which was the result of the synergistic effect of AQS, Fe (Ⅱ) and Fe (Ⅲ) in the activation process of persulfate^[35].

5 Conclusion and prospect

Emerging organic pollutants (EOCs), due to their persistence in the environment, can produce strong toxicity even at trace levels, and may lead to bioaccumulation, thus posing environmental risks to ecosystems. Therefore, the removal of these emerging organic pollutants is an important task and goal in the current water treatment process. This paper introduces not only the AOPs technology for EOCs degradation, but also the advanced materials Fe-MOFs and their composites for complete removal and mineralization of EOCs. At present, the research mainly focuses on enhancing the effective absorption of sunlight, reducing the recombination of carriers and adding more reactive materials. However, in order to broaden the application of MOFs in the catalytic water purification process, it is necessary to further understand and control the reaction process. The main results are as follows: (1) The conventional synthesis methods of MOFs are time-consuming and have low yields, the use of organic solvents may lead to environmental pollution, the non-stoichiometric input of raw materials leads to low utilization of raw materials, and the use of reagent-grade raw materials leads to high costs. Therefore, low-cost and large-scale production of MOFs is one of the main problems to solve the bottleneck of their large-scale use, and is also the focus of future research; (2) MOFs have abundant pore structure, functional groups and active sites, and the significant adsorption and catalytic properties of MOFs should be fully utilized to realize the enrichment of low concentration pollutants and the advanced oxidation treatment; (3) In order to enhance the advanced oxidation performance, MOFs can be combined with other materials to construct heterojunctions, and the mechanism of catalyst enhancement and advanced oxidation treatment of EOCs can be explored by various characterization, noble metal deposition experiments, LC-MS and DFT calculations.The detoxification process and mechanism in the process of advanced oxidation were studied by mathematical model evaluation and biological toxicity detection. (4) couple with other processes such as photochemical and/or electrochemical technology and even biological treatment to achieve sustainable wastewater treatment; (5) At present, there is little research on the effective separation of catalysts, so it is necessary to increase the magnetism of materials or process them into supported catalysts for easy separation and application in practice. In addition, how to combine good degradability, recyclability and low cost is also a key problem that must be solved. Although there are still many problems and challenges in this field, MOFs and their composites have a promising future in the removal of pollutants from wastewater. It is reasonable to believe that through the continuous efforts of researchers in many fields, MOFs and their composites are expected to be applied in water purification.

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Funding

the National Natural Science Foundation of China(51878023)

the Beijing Natural Science Foundation(8202016)

the Great Wall Scholars Training Program Project of Beijing Municipality Universities(CIT&TCD20180323)

the Beijing Talent Project(2019A22)

the BUCEA Post Graduate Innovation Project(PG2020038)

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1 Introduction

2 Preparation of Fe-MOFs and their composites for oxidative degradation of EOCs

2.1 MIL-100 (Fe) and its composites

2.2 MIL-101 (Fe) and its composites

2.3 MIL-53 (Fe) and its composites

2.4 MIL-88 (Fe) and its composites

3 Application of Fe-MOFs and Their Composites in Advanced Oxidation Degradation of EOCs

3.1 Advanced Oxidation Degradation of Drugs in Water by Fe-MOFs and Their Complex

3.2 Degradation of Environmental Hormones in Water by Fe-MOFs and Their Composite

3.3 Degradation of Pesticides in Water by Fe-MOFs and Their Composite

3.4 Simultaneous degradation of multiple EOCs in water by Fe-MOFs and their complex advanced oxidation

4 Factors affecting the degradation of EOCs by Fe-MOFs and their composites

4.1 Influence of the physical properties of the material itself

4.2 Effect of process operating conditions

4.2.1 Effect of pH

4.2.2 Influence of foreign ions

4.2.3 Effect of other process operating conditions

4.3 Effect of active substances

4.3.1 Active Substance in Photocatalytic Reaction

4.3.2 Active Substance in Fenton-like Reaction

4.3.3 Active Substance in Activated Persulfate Reaction

5 Conclusion and prospect

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