Application of Spinel Ferrite-Based Advanced Oxidation Processes in Organic Wastewater Treatment

Ming Ge; Zheng Hu; Quanbao He

doi:10.7536/PC200851

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (9) : 1648-1664 DOI: 10.7536/PC200851

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Application of Spinel Ferrite-Based Advanced Oxidation Processes in Organic Wastewater Treatment

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Abstract

With the rapid development of China's economy and the acceleration of urbanization, the problem about organic pollutions in natural water bodies has become more and more serious. The advanced oxidation processes (AOPs) based on free radical reaction can efficiently degrade the non-biodegradable organic pollutants remaining in water environment. Under the action of the catalyst, the advanced oxidation processes can effectively generate the strong oxidizing free radicals to degrade organic pollutants. Spinel ferrite (MFe₂O₄(M=Zn, Ni, Co, Cu, Mn)) is widely used as a catalyst to promote the generation of free radicals in the advanced oxidation processes, and at the same time, its strong magnetism and high stability ensure that it is easy to recycle by an external magnetic field and further reused. This article mainly reviews the research progress of spinel ferrite-based heterogeneous Fenton-like technology, photocatalytic technology and persulfate advanced oxidation technology in organic wastewater treatment, and focuses on the catalytic degradation mechanism of organic pollutants in water bodies by the different ferrite magnetic nanomaterials in the above three advanced oxidation technologies, and the ways to enhance catalytic performances of ferrite magnetic catalysts. Finally, we point out some problems about the application of spinel ferrite in advanced oxidation processes, and the future research directions of spinel ferrite-based advanced oxidation processes are also prospected.

Contents

1 Introduction

2 Spinel ferrite-based heterogeneous Fenton-like oxidation technologies

2.1 ZnFe₂O₄

2.2 NiFe₂O₄

2.3 CoFe₂O₄

2.4 MnFe₂O₄

2.5 CuFe₂O₄

3 Spinel ferrite-based photocatalytic technologies

3.1 ZnFe₂O₄

3.2 MFe₂O₄(M=Ni,Co,Cu )

4 Persulfate oxidation technologies with spinel ferrite as the catalyst

4.1 The organic pollutants degradation in water by persulfate activated with CoFe₂O₄

4.2 The organic pollutants degradation in water by persulfate activated with CuFe₂O₄

4.3 The organic pollutants degradation in water by persulfate activated with MnFe₂O₄

5 Conclusion and prospect

Graphical abstract

Keywords

ferrite / Fenton-like oxidation / photocatalysis / persulfate / organic wastewater / catalytic mechanism

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Ming Ge, Zheng Hu, Quanbao He. Application of Spinel Ferrite-Based Advanced Oxidation Processes in Organic Wastewater Treatment. Progress in Chemistry, 2021, 33(9): 1648-1664 DOI:10.7536/PC200851

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1 Introduction

Nowadays, various organic pollutants (such as organic dyes, phenolic compounds, endocrine disruptors, drugs, etc.) Are inevitably produced in the process of rapid industrial development.It is not easy to be removed by sewage treatment process, on the one hand, it will endanger aquatic organisms when it is discharged into the water environment, on the other hand, it will enter the human body through the food chain through enrichment and accumulation in organisms and threaten human health. Therefore, it has become a research hotspot to seek efficient treatment technologies to remove the emerging organic pollutants which are difficult to biodegrade. The results showed that the advanced oxidation technology based on free radical reaction could effectively degrade the new persistent organic pollutants in water^[1,2]. Advanced oxidation technology refers to the generation of strong oxidizing free radicals (such as hydroxyl radicals, sulfate radicals, etc.) Under the conditions of light, electricity, ultrasound and catalysts.It is an oxidation technology that carries out a series of radical chain reactions with pollutants, thus destroying the molecular structure of pollutants and degrading them into harmless small molecular products (carbon dioxide, water and inorganic salts, etc.)^[2]. At present, Fenton oxidation technology, photocatalytic oxidation technology and persulfate oxidation technology have been widely studied, and these oxidation technologies can effectively produce free radicals only under the action of catalysts. Compared with the homogeneous catalytic oxidation process, the heterogeneous catalytic oxidation process based on solid catalyst has obvious advantages.

For example, the oxidation technology using ferrous ion (Fe²⁺) as the Fenton reagent faces the following problems: (1) hydroxyl radical (.OH) (Formula 1) can be effectively generated only under acidic conditions (pH ≈ 3), so a large amount of acid and alkali is consumed to adjust the pH, which increases the use cost; (2) Secondary pollution of iron-containing sludge. Heterogeneous Fenton-like oxidation technology can avoid these problems^[3⇓~5]. At present, most of the advanced oxidation processes (AOPs) use nano-sized solid powder as catalyst, which is not easy to precipitate ions and cause secondary pollution after being put into water, but the solid-liquid separation and recycling of powder catalyst after wastewater treatment is an urgent problem to be solved. In this context, the development of advanced oxidation technology driven by magnetic recyclable catalysts can effectively solve this problem^[6,7].

The spinel-type ferrite has the same crystal structure as the natural mineral spinel MgAl₂O₄,Its molecular formula is that MFe₂O₄,M is a divalent metal ion (such as Zn²⁺,Ni²⁺, Co²⁺, Cu²⁺, Mn²⁺, etc.),The schematic representation of the structural formula is shown in Figure 1. In the spinel structure, the O ions are arranged approximately in cubic close packing. In the unit cell containing 32 O ions, there are 32 octahedral sites and 64 tetrahedral sites, of which 16 octahedral sites (B sites) and 8 tetrahedral sites (A sites) are filled. If the divalent ions are all in the tetrahedral A-site, it is called the positive spinel; If the divalent ions are all in the octahedral B site, it is called inverse spinel; If both a and B sites are occupied by divalent ions, they are called intermediate spinels. Spinel ferrite is a kind of magnetic semiconductor material with stable crystal structure, strong thermal stability and wide optical absorption frequency, which has broad application prospects in the fields of electronic information, biomedicine and catalysis^[8⇓~10]. On the one hand, the spinel ferrite can be used as a catalyst to drive the generation of free radicals to oxidize organic pollutants in water, and on the other hand, the spinel ferrite can realize effective solid-liquid separation by means of an external magnetic field, which is beneficial to reuse. In recent years, the advanced oxidation processes (AOPs) using magnetic spinel ferrites as catalysts for wastewater treatment have attracted much attention^[11]. Our group has also carried out a series of work on the preparation and catalytic properties of magnetic ferrites.Comprises a magnetic Cu/CuFe₂O₂ for catalyzing H₂O₂ to remove methylene blue in water, a magnetic composite photocatalyst (Ag₃PO₄/ZnFe₂O₄, AgBr/NiFe₂O₄,AgBr/CoFe₂O₄ and AgBr/Cu/CuFe₂O₂) to degrade organic dyes in water and magnetic Cu/CuFe₂O₂ to activate peroxydisulfate to degrade tetracycline in water^{[12⇓⇓⇓⇓~17]}^[12]^{[13⇓⇓~16]}^[17]. In this paper, the application of different types of advanced oxidation technologies involving spinel ferrite in wastewater treatment is introduced, and the future development direction of this field is prospected.

(1)

F e 2 + + H 2 O 2 → F e 3 + + · OH + O H -

2 Heterogeneous Fenton-like Oxidation Technology Based on Spinel Ferrite

Heterogeneous Fenton-like oxidation technology based on magnetic ferrites (such as ZnFe₂O₄, NiFe₂O₄, CoFe₂O₄, MnFe₂O₄ and CuFe₂O₄) has achieved a series of research results in wastewater treatment, as shown in Table 1.

2.1 Sulfhydryl heterogeneous Fenton-like oxidation technology

ZnFe₂O₄ is a kind of soft magnetic material with strong light absorption, good thermal stability and non-toxicity, and the ferric iron (Fe (Ⅲ)) in its lattice can not directly activate the H₂O₂ to produce hydroxyl radicals, but its band gap energy is small (about 1.6 eV).The electrons generated under the irradiation of visible light can reduce Fe (Ⅲ) to Fe (Ⅱ) and H₂O₂ to generate · OH (Figure 2), thereby oxidizing and degrading different types of organic pollutants in water, which can be called heterogeneous photo-Fenton technology^[18]. He et al. Obtained mesoporous ZnFe₂O₄ nanocrystals by hydrothermal method.Under visible light irradiation, Fe (Ⅱ) generated by reduction can react with H₂O₂ to generate hydroxyl under the condition of pH = 6, thereby efficiently removing acid orange II in water.In addition, the mesoporous ZnFe₂O₄ catalyst has good stability and can be efficiently reused for five times^[19]. The narrow band gap of the ZnFe₂O₄ is beneficial to the absorption of visible light, but the photogenerated electrons are easy to recombine with holes, which reduces the performance of the photo-Fenton process in the degradation of pollutants to a certain extent. Some researchers combine ZnFe₂O₄ with carbon materials to suppress the recombination of electrons and holes in Figure 2 by using the excellent electron transport properties of carbon materials, thereby improving the efficiency. For example, Wang et al. Obtained ZnFe₂O₄- graphene composite materials by one-step solvothermal method, which can efficiently remove organic dyes (rhodamine B, methyl orange and methylene blue) in water by using visible light and H₂O₂. The existence of graphene can promote the separation of photogenerated electrons and holes in the ZnFe₂O₄, which is beneficial to the generation of Fe (II) to promote the photo-Fenton reaction^[20]. Similarly, the electron-hole pairs generated by the porous carbon /ZnFe₂O₄ composite catalyst under visible light irradiation are not easy to recombine, thereby promoting the photo-Fenton reaction to achieve the effective degradation of rhodamine B and phenol in water^[21]. It should be noted that the magnetism of the ZnFe₂O₄ is weak, and the introduction of non-magnetic carbon material will inevitably reduce the overall magnetic recycling performance of the material, so it is necessary to adjust the content ratio of the ZnFe₂O₄ and carbon material to enhance the photo-Fenton activity and ensure the magnetic recycling performance at the same time.

2.2 Sulfhydryl heterogeneous Fenton-like oxidation technology

Similar to ZnFe₂O₄, NiFe₂O₄ is also a kind of soft magnetic material, which has good stability and is non-toxic. According to the research of Zhang et al., the NiFe₂O₄ can activate the H₂O₂ to produce hydroxyl radicals to degrade phenol in water in acidic environment, which is attributed to the reaction between Ni (Ⅱ) and Fe (Ⅲ) in the structure of the NiFe₂O₄ to produce Fe (Ⅱ), which is conducive to promoting the Fenton reaction^[22]. However, the oxidation ability of the NiFe₂O₄/H₂O₂ system is weak even under acidic conditions, the reaction time for removing organic pollutants in water is long, and the dosage of both NiFe₂O₄ and H₂O₂ is relatively large. Researchers combine NiFe₂O₄ and carbon materials to drive H₂O₂ to produce more reactive oxygen radicals to achieve rapid removal of organic pollutants in water. Shi et al. prepared NiFe₂O₄/C composite nanospheres by a one-step calcination method, and the efficiency of driving H₂O₂ to degrade tetracycline hydrochloride in water under visible light irradiation was much higher than that of NiFe₂O₄ alone, because the presence of C improved the separation efficiency of photogenerated charges and promoted the photo-Fenton reaction, and the oxidation system could effectively remove tetracycline hydrochloride in a wide pH range (2.5 ~ 7.5)^[23]. Lee et al. pointed out that the existence of carbon nanotubes can effectively improve the separation of electron-hole pairs generated by NiFe₂O₄, and enhance the photo-Fenton reaction process through the synergistic effect between them, which greatly improves the effect of removing and mineralizing sulfamethoxazole. At the same time, the stability of carbon nanotubes /NiFe₂O₄ composite materials is good, and there is only a weak loss of activity after five times of recycling^[24]. In addition, Guo et al. introduced oxalic acid into the photo-Fenton reaction process driven by NiFe₂O₄, oxalate ions were adsorbed on the surface of the NiFe₂O₄ and reacted with Fe (Ⅲ) to form ferric oxalate, which could promote the formation of Fe (Ⅱ) and improve the performance of photo-Fenton degradation of methylene blue in water, and showed high removal efficiency of methylene blue in a wide pH range (2. 98-10. 09)^[25].

2.3 Sulfhydryl heterogeneous Fenton-like oxidation technology

CoFe₂O₄ is an excellent magnetic semiconductor material, which has been widely used in magnetic recording, ferrofluid, catalysis and other fields. Jiang et al. Obtained monodisperse CoFe₂O₄ nanoparticles by solvothermal method, which can activate H₂O₂ to effectively degrade methylene blue in water. At the same time, they confirmed that Co (II) in CoFe₂O₄ reacted with H₂O₂ to produce hydroxyl radical^[26]. Under visible light irradiation, CoFe₂O₄ nanoparticles prepared by co-precipitation method combined with H₂O₂ can achieve efficient removal of methylene blue in water, and show ideal reuse rate^[27]. Similar to ZnFe₂O₄ and NiFe₂O₄, the Fenton-like catalytic performance of CoFe₂O₄ can be improved with the help of carbon materials. For example, Hassani et al. deposited CoFe₂O₄ nanoparticles on the surface of reduced graphene oxide (rGO) by liquid phase self-assembly method to construct CoFe₂O₄-rGO composite nanomaterials, which can catalyze and activate H₂O₂ to produce hydroxyl radicals under ultrasonic irradiation to efficiently remove different types of organic dyes in water^[28]. Zhou et al. Synthesized rGO/CoFe₂O₄ composite catalyst by one-step solvothermal method, and introduced reducing agent hydroxylamine to promote the generation of Fe (II) while catalyzing the degradation of methylene blue in water, thereby accelerating the Fenton reaction. Meanwhile, they found that the synergistic effect of rGO and CoFe₂O₄ increased the generation of · OH^[29]. Polypyrrole (PPy) is a common conductive polymer. Deng et al. Wrapped PPy on the surface of CoFe₂O₄ nanospheres by in-situ oxidative polymerization to form a CoFe₂O₄@PPy core-shell structure. Under light irradiation, the photogenerated electrons on the conduction band of CoFe₂O₄ can be transferred to PPy, which promotes the effective separation of photogenerated charges, thereby enhancing the photo-Fenton activity of the composite material and realizing the efficient removal of Rhodamine B from water^[30].

2.4 Sulfhydryl heterogeneous Fenton-like oxidation technology

As a soft magnetic semiconductor material, MnFe₂O₄ has been widely used in the fields of magnetic storage, adsorption, biotechnology and heterogeneous catalysis. Like the three spinel ferrites introduced above, magnetic MnFe₂O₄ as a catalyst for heterogeneous Fenton-like technology has achieved remarkable results in the treatment of organic wastewater. Fang et al. Synthesized magnetic MnFe₂O₄ by sol-gel method, and the activated H₂O₂ could degrade norfloxacin in water. The mechanism study showed that Fe (Ⅱ)/Fe (Ⅲ) and Mn (Ⅱ)/Mn (Ⅲ) redox cycles on the surface of the MnFe₂O₄ promoted the generation of hydroxyl radicals to achieve the effective removal of norfloxacin^[31]. Qu et al. Combined MnFe₂O₄ with rGO to catalyze and activate H₂O₂ to remove methylene blue in water. Their research pointed out that rGO could not only enhance the adsorption of methylene blue, but also act as an electron transfer bridge to enhance the Fenton catalytic degradation reaction^[32]. Biochar has a rich pore structure, a large specific surface area and a variety of oxygen-containing functional groups on its surface, and its application in the field of catalysis has attracted wide attention recently. Lai et al. anchored the MnFe₂O₄ on the surface of biochar by co-precipitation method to obtain MnFe₂O₄/ biochar composite material, which can catalyze H₂O₂ to degrade tetracycline in water under visible light irradiation and achieve ideal results, and its performance is better than that of single MnFe₂O₄, because biochar hinders the aggregation of MnFe₂O₄ particles and improves the specific surface area, which is conducive to photo-Fenton reaction^[33]. In order to inhibit the recombination of photogenerated electrons and holes in the MnFe₂O₄, Wei et al. Constructed a MnFe₂O₄@SnS₂ nanoheterojunction. The study confirmed that the electrons generated by the composite material under photoexcitation promoted the formation of Mn (Ⅱ) and Fe (Ⅱ), which was conducive to the Fenton reaction and produced more hydroxyl radicals^[34]. The experiment of methylene blue removal in water with pH = 9 shows that the photo-Fenton performance of MnFe₂O₄@SnS₂ is better than that of single MnFe₂O₄.

2.5 Sulfhydryl heterogeneous Fenton-like oxidation technology

CuFe₂O₄ is a kind of functional material with excellent magnetic properties, which has been used in gas sensing, adsorption, catalysis and other fields. At present, magnetic CuFe₂O₄ can be used as a catalyst for Fenton-like reaction to effectively remove organic pollutants in water. Zhao et al. used porous silicon KIT-6 as a hard template to synthesize a CuFe₂O₄ catalyst with a large specific surface area and a rich pore structure by adopting a nano-casting strategy, and the catalytic activation H₂O₂ thereof can realize efficient degradation of imidacloprid pesticides in water, because the Cu (I)/Cu (II) and Fe (II)/Fe (III) redox cycles on the surface of the CuFe₂O₄ synergistically promote the generation of hydroxyl radicals^[35]. In order to further improve the Fenton catalytic performance of CuFe₂O₄, our group constructed Cu/CuFe₂O₄ composite materials, and confirmed that the synergistic effect of Cu and CuFe₂O₄ could greatly improve the Fenton catalytic activity. However, metal Cu is easy to be oxidized into Cu ions, which is not conducive to the recycling of Cu/CuFe₂O₄^[12]. Jiang et al. Modified the CuFe₂O₄ with Cu₂O to enhance the Fenton catalytic activity, and the reuse performance of the Cu₂O/CuFe₂O₄ catalyst was ideal^[36]. In view of the above analysis, carbon materials with excellent electrical conductivity can facilitate the heterogeneous Fenton catalytic reaction. For example, Guo et al. prepared CuFe₂O₄@ graphite-carbon composite catalyst by solvothermal method, whose photo-Fenton degradation performance of methylene blue in water is stronger than that of single CuFe₂O₄, and the composite material can effectively remove methylene blue in a wide pH range^[37]. Yao et al. Synthesized the CuFe₂O₄/rGO by hydrothermal method, and the activity of the composite material for the Fenton catalytic degradation of rhodamine B was stronger than that of the single CuFe₂O₄ under the action of microwave, which was attributed to the fact that rGO could enrich rhodamine B molecules to the surface of the catalyst, thus facilitating the degradation of rhodamin B by hydroxyl radicals^[38]. Ma et al. Used polydopamine (PDA) to wrap CuFe₂O₄. Similar to the role of rGO, PDA with large specific surface area enriched methylene blue on the surface of the catalyst through π-π interaction, which was beneficial to the Fenton catalytic degradation reaction^[39]. Carbon nitride (g-C₃N₄) is a visible light-responsive organic semiconductor and has been widely used as a catalyst in heterogeneous catalytic processes. Yao et al. designed and synthesized a CuFe₂O₄@g-C₃N₄ core-shell structure. Under visible light irradiation, the heterogeneous recombination of CuFe₂O₄ and g-C₃N₄ can improve the separation efficiency of photo-generated charges and contribute to the photo-Fenton reaction, thus achieving efficient removal of acid orange II in water^[40].

From the above, it can be seen that magnetic MFe₂O₄(M=Zn,Ni,Co,Mn,Cu) is widely used in the advanced oxidation technology of activated H₂O₂, and the performance of MFe₂O₄ in catalyzing H₂O₂ to produce hydroxyl radicals to remove organic pollutants in water can be effectively enhanced by modification. With the help of visible light, this technology can degrade organic pollutants in a wide range of pH values. It should be pointed out that the photo-Fenton process based on MFe₂O₄ mainly produces strong oxidizing hydroxyl radicals to degrade organic pollutants, while the contribution of photo-generated holes in the valence band of MFe₂O₄ to the degradation of organic pollutants should be paid attention to in future studies.

3 Photocatalytic Technology Based on Spinel Ferrite

Spinel ferrites (ZnFe₂O₄, NiFe₂O₄, CoFe₂O₄ and CuFe₂O₄, etc.) Are magnetic semiconductors with narrow band gaps. They have strong light absorption ability and can absorb and utilize most of the visible light (about 45%) in the solar spectrum. In addition, the ferrite with magnetism is easy to realize the solid-liquid separation by the external magnetic field. Therefore, the magnetic ferrite-based catalyst is able to achieve the degradation of typical organic pollutants in water using visible light, as shown in Table 2.

3.1 ZnFe₂O₄ based photocatalytic oxidation technology

Magnetic ZnFe₂O₄ have a small band gap and are easily excited by visible light. However, photogenerated electrons and holes are also easy to recombine, which leads to their poor photocatalytic performance under visible light. The photocatalytic activity of ZnFe₂O₄ was improved by a series of control methods, such as morphology adjustment, crystal face control, heterogeneous composite and carbon material modification. For example, Chen et al. prepared a hollow cubic ZnFe₂O₄ catalyst by using cubic Prussian blue as a template, and the performance of the hollow cubic ZnFe₂O₄ catalyst for photocatalytic degradation of tetracycline hydrochloride in water under visible light is superior to that of ZnFe₂O₄ particles, because the hollow cubic ZnFe₂O₄ enhances the adsorption of tetracycline hydrochloride and the absorption of visible light^[41]; Similarly, Xu et al. synthesized porous ZnFe₂O₄ hollow nanostructures using Fe-organic framework materials as templates, which can achieve rapid degradation of methylene blue in water by visible light^[42]. Adjusting the crystal plane is one of the effective ways to enhance the activity of semiconductor photocatalysts. Wang et al. Prepared high-energy (111) crystal face exposed ZnFe₂O₄ nanoparticles by sodium oleate-assisted hydrothermal method, and their photocatalytic activity was much higher than that of ZnFe₂O₄ without fixed crystal face exposure^[43]. Compared with the morphology and crystal plane control of single ZnFe₂O₄, the construction of ZnFe₂O₄ composite photocatalytic materials to obtain high activity and ideal recycling rate is more studied at this stage. Titanium dioxide (TiO₂) is a traditional excellent photocatalyst, which can not be excited by visible light. However, TiO₂ can often be used as an electronic platform to participate in the reaction process of visible light catalysis. For example, Huang et al. Obtained ZnFe₂O₄-TiO₂ composite nanoparticles by octanol-assisted reflux method, and electrons from the conduction band of ZnFe₂O₄ could be transferred to the conduction band of TiO₂ under visible light irradiation.It is beneficial to react with O₂ to generate active oxygen species-superoxide radical (

O 2 · -

), which also inhibits the recombination of electrons and holes of ZnFe₂O₄ (Fig. 3), thus improving the photocatalytic degradation performance of bisphenol A in water^[44]. Nada et al prepared ZnFe₂O₄@TiO₂ composite nanofibers by electrospinning method, and also confirmed that the photogenerated electrons on the ZnFe₂O₄ could migrate to the TiO₂ to achieve the improvement of photocatalytic activity^[45]. Zinc oxide (ZnO), which can not use visible light, can also be used as an electronic platform to promote the photocatalytic activity of ZnFe₂O₄. For example, Rameshbabu et al. synthesized ZnFe₂O₄-ZnO composite nanoparticles by co-precipitation method to enhance the performance of removing methylene blue in water, because the electrons generated by ZnFe₂O₄ excited by visible light can be transferred to the conduction band of ZnO to promote the separation of photogenerated charges (Figure 3)^[46]. In order to further improve the photocatalytic activity of visible light, Shen et al. Deposited Ag, which is easy to capture electrons, on the surface of ZnFe₂O₄ and ZnO to construct magnetic Ag/ZnO/ZnFe₂O₄ ternary nanostructures, which not only enhanced the absorption of visible light, but also further improved the separation efficiency of photogenerated charges, and the ternary materials were easy to recycle^[47]. Compared with the TiO₂ or ZnO which has no visible light response, it is more conducive to the absorption and utilization of visible light to select some photocatalysts which have visible light response to compound with ZnFe₂O₄. Bismuth oxyhalides (BiOX (X = Br, I)) are commonly used as visible-light-driven photocatalytic materials, and their photocatalytic activity can be improved by combining them with ZnFe₂O₄. For example, Kong et al. Obtained BiOBr-ZnFe₂O₄p-n heterogeneous composite catalysts by precipitation method, which showed excellent photocatalytic activity for the degradation of rhodamine B under visible light, significantly stronger than single BiOBr or ZnFe₂O₄^[48]. Dai et al. introduced biomass carbon into the composite system of BiOBr and ZnFe₂O₄ by solvothermal method. On the one hand, biomass carbon can improve the light absorption ability of the catalyst, and on the other hand, it can be used as an electron mediator to promote the establishment of Z-type photocatalytic system between BiOBr and ZnFe₂O₄ (Figure 4A), thus showing better visible light catalytic activity than BiOBr-ZnFe₂O₄^[49]. Compared with the single ZnFe₂O₄,Zhou et al and the BiOI-ZnFe₂O₄ composite material constructed by Yosefi et al, the composite material showed stronger performance of visible light photocatalytic degradation of dyes in water, which was attributed to the formation of type II heterojunction between BiOI and ZnFe₂O₄ to promote the separation and transport of photogenerated charges (Fig. 4B)^[50]^[51]. It should be pointed out that the energy band matching of the two catalysts in the Z-type photocatalytic system and the II-type heterojunction is consistent (Figure 4), and the indirect method (detecting the reactive species in the process,The next step is to develop in situ characterization methods to obtain direct interfacial charge transfer pathways to distinguish the two mechanisms. Silver semiconductor materials (such as Ag₃PO₄, AgBr, etc.) Have attracted much attention due to their high photocatalytic activity under visible light, but they are not easy to be recycled. On the one hand, the combination of ZnFe₂O₄ and silver-based catalyst can improve the photocatalytic activity, on the other hand, it can improve the reuse efficiency by using the magnetism of ZnFe₂O₄. The previous work of our group confirmed that the combination of Ag₃PO₄ and ZnFe₂O₄ can greatly improve the photocatalytic activity of ZnFe₂O₄ under visible light.Meanwhile, the ZnFe₂O₄ loaded on the surface of the Ag₃PO₄ can enhance the stability of the Ag₃PO₄ and realize higher recycling performance of the Ag₃PO₄/ZnFe₂O₄ composite material^[13]. In view of the excellent electron transport properties of carbon materials, Dai et al. constructed a carbon sphere @ZnFe₂O₄/Ag₃PO₄ core-shell structure, and confirmed that the conduction band electrons of ZnFe₂O₄ could be transported to the carbon sphere to react with O₂, and the ZnFe₂O₄ and Ag₃PO₄ formed a type II heterojunction to facilitate the transport of photogenerated carriers, thus improving the efficiency of visible photocatalytic degradation of 2,4-dichlorophenol in water^[52]. AgBr is a photosensitive material which is easy to prepare, and it is easy to be photoreduced to precipitate Ag, and Ag can often promote the photocatalytic degradation reaction. Li et al. Obtained Ag/AgBr/ZnFe₂O₄ nanocomposite catalyst by two-step liquid-phase synthesis method. Under visible light irradiation, the ternary catalyst showed better catalytic activity for the degradation of rhodamine B in water than the monocomponent and binary catalysts, which was attributed to: (1) the electrons generated by the plasmon vibration effect of Ag could be transferred to the conduction band of AgBr to promote the generation of

O 2 · -

^[53]; (2) Type Ⅱ heterojunction is formed between AgBr and ZnFe₂O₄, which is beneficial to the separation of photogenerated charges. Cheng et al. Prepared a ternary ZnFe₂O₄/AgBr/Ag composite photocatalyst with visible light catalytic activity stronger than that of ZnFe₂O₄ and AgBr/Ag by deposition method combined with photoreduction method. The study pointed out that a Z-shaped photocatalytic system with Ag as electron transport mediator was formed among ZnFe₂O₄, AgBr and Ag, which was conducive to promoting the production of reactive species^[54]. Compared with metal-containing semiconductor photocatalysts, the application of organic semiconductor materials g-C₃N₄ in the field of photocatalysis has attracted great attention of researchers^[55]. Wang et al. Synthesized g-C₃N₄-ZnFe₂O₄ composite magnetic catalyst by solution-thermal method. Under the excitation of visible light, the electric field in the interface of g-C₃N₄ and ZnFe₂O₄ is conducive to the separation of photogenerated charges, thus reflecting a better activity of visible light degradation of methyl orange in water than a single component^[56].

3.2 MFe₂O₄(M=Ni,Co,Cu) -based photocatalytic oxidation technology

MFe₂O₄(M=Ni,Co,Cu) has a small band gap energy, can be excited by visible light, has stronger magnetism than ZnFe₂O₄, and is easier to be separated from water. In order to overcome the disadvantage of easy recombination of photogenerated electron-hole pairs in MFe₂O₄(M=Ni,Co,Cu), magnetic MFe₂O₄(M=Ni,Co,Cu) (Fe3O4) is often heterogeneously compounded with other materials to enhance the photocatalytic activity under visible light.

When the noble metal (such as Ag, Au, Pd, etc.) Is in contact with the semiconductor photocatalyst, the electrons generated by photoexcitation will be transferred from the semiconductor catalyst with high Fermi energy level to the noble metal with low Fermi energy level, thus inhibiting the recombination of electron-hole pairs of the photocatalyst itself. Kim et al. prepared the magnetic Ag/NiFe₂O₄ catalyst by one-step combustion method, and the electrons generated by the photoexcited NiFe₂O₄ were transferred to the surface of Ag to inhibit the recombination of its own photogenerated charges, thus reflecting a better performance of visible photocatalytic removal of methylene blue in water than a single NiFe₂O₄^[57]. Similarly, Zhu et al. enhanced the activity of CuFe₂O₄ for photocatalytic degradation of 4-chlorophenol in water under visible light irradiation by Ag modification^[58]. Similar to the role of noble metals, the presence of rGO with high electron mobility can also promote the separation of photogenerated charges of the photocatalyst. He et al. Synthesized rGO-NiFe₂O₄ composite catalyst by ball milling method. Under visible light irradiation, the conduction band electrons of NiFe₂O₄ can be transferred to the surface of rGO and react with O₂ to form

O 2 · -

, which avoids the recombination of valence band holes to a certain extent, thus improving the efficiency of methylene blue degradation in water^[59]. Devi et al. Confirmed that rGO could also enhance the photocatalytic activity of CoFe₂O₄ under visible light^[60]. Shen et al. Used noble metals Pd and rGO to capture photogenerated electrons together, which greatly improved the visible light catalytic performance of the NiFe₂O₄, and the ternary magnetic material was easy to recycle, reflecting excellent recycling performance^[61]. It is also an effective means to enhance the photocatalytic performance of magnetic MFe₂O₄(M=Ni,Co,Cu) by heterogeneous recombination between semiconductors. BiOBr is usually used to modify the NiFe₂O₄ to improve its photocatalytic performance, because the BiOBr and the NiFe₂O₄ form a type II heterojunction to promote the separation and transport of photogenerated carriers^[62]. As mentioned above, the visible-light-responsive silver-based semiconductor catalyst has ideal photocatalytic performance, and the introduction of a magnetic MFe₂O₄(M=Ni,Co,Cu) can enhance the performance and improve the recycling rate. By adjusting the shape and size of the magnetic MFe₂O₄(M=Ni,Co,Cu), a matched energy band structure between the magnetic MFe₂O₄(M=Ni,Co,Cu) and the Ag₃PO₄ can be realized, thereby forming a type-II heterojunction or a type-Z photocatalytic system (Figure 4) to promote the separation and transmission of photogenerated carriers and enhance the photocatalytic activity of visible light^[63⇓~65]. In order to further enhance the photocatalytic performance, Zhou et al. And Chao et al. Loaded the Ag₃PO₄ and MFe₂O₄(M=Ni,Co) on the layered graphene oxide (GO) with excellent adsorption capacity and electron transport performance, which improved the separation efficiency of photogenerated charges and showed better activity and stability than Ag₃PO₄/MFe₂O₄(M=Ni,Co)^[66]^[67]. Compared with GO, biochar is easy to obtain and has lower cost. Dai et al. Constructed magnetic CoFe₂O₄/Ag₃PO₄@ biochar ternary catalyst by loading CoFe₂O₄ and biochar on Ag₃PO₄ microsphere by in-situ deposition method. Mechanism study shows that biochar can be used as electron transport mediator to form Z-shaped photocatalytic system (Fig. 4A), which enhances the activity of visible light photocatalytic degradation of BPA in water^[68]. AgBr can also be hetero-complexed with magnetic MFe₂O₄(M=Ni,Co,Cu) to synergistically enhance the photocatalytic activity. In our group, AgBr/MFe₂O₄(M=Ni,Co) composite type II heterostructures were constructed by hydrothermal method and direct precipitation method, which showed better photocatalytic activity than single MFe₂O₄(M=Ni,Co) or AgBr for the degradation of organic dyes in water under visible light^[14,15]. In addition, a magnetic Z-type AgBr-Cu-CuFe₂O₄ photocatalytic system was constructed by using Cu as electron transport mediator, which greatly improved the photocatalytic degradation efficiency of Rhodamine B in water, and the ternary catalyst was easy to recover and had good recycling performance^[16].

According to the above analysis, the visible light photocatalytic activity of the magnetic MFe₂O₄(M=Zn,Ni,Co,Mn,Cu) can be obviously improved by surface modification and heterogeneous compounding, and the existence of the magnetic MFe₂O₄ can ensure that the composite catalyst is easily separated from the water body and does not cause secondary pollution to the water body. It should be pointed out that although the heterogeneous composite of silver-based photocatalyst and magnetic MFe₂O₄ can synergistically enhance the photocatalytic activity, the silver-based photocatalyst (such as Ag₃PO₄, AgBr) itself has a certain solubility in water, resulting in the loss of Ag, which is not conducive to practical application^{[14,15,52,53,63⇓⇓~66]}. In this context, the wrapping of silver-based photocatalyst by magnetic MFe₂O₄ can be realized by reasonably designing the synthesis method, which will inevitably weaken the corrosion of silver-based photocatalyst in water and enhance its stability^[13].

4 Persulfate Oxidation Technology with Spinel Ferrite as Catalyst

Compared with Fenton and photocatalytic oxidation, persulfate oxidation is a new advanced oxidation technology, which produces sulfate radical (

SO 4 · -

) as the main active species. Compared with hydroxyl radical,

SO 4 · -

has higher redox potential, better selectivity and longer life, and its

SO 4 · -

is less affected by the acidity and alkalinity of water^[69]. Commonly used persulfates include peroxomonosulfate (PMS,

HSO 5 -

) and peroxodisulfate (PDS,S₂

O 8 2 -

) (Figure 5), both of which have some oxidation capacity, but are very limited and insufficient to degrade organic pollutants in water. Compared with PDS, the asymmetric O-O bond in PMS is easy to be activated and broken to produce

SO 4 · -

, but the use cost of PDS is low^[70]. At present, the activation methods of persulfate include thermal activation, ultraviolet activation, ultrasonic activation, alkali activation, carbon material catalytic activation and transition metal catalytic activation^[70]. Thermal activation, ultraviolet activation and ultrasonic activation consume a lot of energy, and alkali activation needs to consume a large amount of alkali. When carbon materials are used as activators, they can be destroyed by the generated

SO 4 · -

, resulting in poor reuse efficiency^[71]. Transition metals have a variety of valence States and are prone to oxidation-reduction reactions, thus facilitating the activation of persulfate to produce

SO 4 · -

(Formula 2 and Formula 3, where M represents a transition metal). For example, ferrous ions (Fe²⁺) can effectively activate PMS or PDS in water to produce

SO 4 · -

and oxidize and destroy organic pollutant molecules in water. It is undeniable that the transition metal ions injected into the water body are not easy to recover and reuse, which can cause secondary pollution of the water body. In this context, the heterogeneous activated persulfate oxidation technology using transition metal oxides as catalysts has attracted wide attention. Studies have shown that spinel ferrite MFe₂O₄(M=Co,Cu,Mn) are often used to activate persulfate to degrade pollutants in the water environment (Table 3), and because of their high stability and strong magnetism, they are easy to recycle.

(2)

HSO 5 -

+ Mⁿ→Mⁿ⁺¹+

SO 4 · -

+OH^-

(3)

S 2 O 8 2 -

+ Mⁿ→Mⁿ⁺¹+

S O 4 · -

S O 4 2 -

4.1 Degradation of organic pollutants in water by persulfate activated by CoFe₂O₄;

The magnetic CoFe₂O₄ is used as a catalyst for activating persulfate and has good performance of removing organic pollutants in water. Lai et al. Prepared CoFe₂O₄ nanoparticles by hydrothermal method, and the activated PMS could effectively degrade atrazine (ATZ) herbicide in water. Under the optimal experimental conditions, 99% of ATZ in water could be removed after 30 minutes of reaction^[72]. The surface of CoFe₂O₄ has hydroxyl (Co²⁺-OH^-), which can react with

HSO 5 -

to form

SO 4 · -

; In addition, the ≡Fe³⁺ on the surface of the CoFe₂O₄ first reacts with the

HSO 5 -

to generate ≡Fe²⁺,The

HSO 5 -

then reacts with the ≡Fe²⁺ to form a

S O 4 · -

(fig. 6). It can be seen that the

S O 4 · -

produced by redox reaction of Co, Fe and

HSO 5 -

are the main active species leading to the degradation of ATZ in water. Feng et al. Synthesized CoFe₂O₄ nanoparticles by sol-gel method, and used it to activate PMS to remove triphenyl phosphate (TPhP) in water. In water with pH value of 7, the oxidation system could degrade 99.5% of TPhP after 6 minutes^[73]. This work is consistent with the degradation mechanism proposed by Lai et al., and the main reactive oxygen species for the degradation of TPhP in water is

SO 4 · -

. Magnetic CoFe₂O₄ nanoparticles are easy to agglomerate, which will lead to the decrease of their surface area and is not conducive to the occurrence of interfacial catalytic reactions. Alumina (Al₂O₃) is a support material with a large specific surface area, which is often used as a catalyst support to increase the contact interface for catalytic reactions. Shao et al used sol-gel method to obtain CoFe₂O₄/Al₂O₃ composite materials to activate PMS to degrade sulfachloropyridazine (SCP) in water, and the experimental results showed that the efficiency of CoFe₂O₄/Al₂O₃ activating PMS was better than that of single CoFe₂O₄.It is only because the presence of Al₂O₃ increases the contact area for catalytic reaction, but the Al₂O₃ itself does not activate PMS to produce reactive species, and the

SO 4 · -

leading to SCP degradation comes from the activation of PMS by CoFe₂O₄^[74]. Like Al₂O₃, titanium dioxide (TiO₂) is a common catalyst support. Du et al. Synthesized CoFe₂O₄/TiO₂ binary composite by impregnation-calcination method, and its ability to activate PMS to remove Rhodamine B and phenol in water was stronger than that of CoFe₂O₄ alone, which was attributed to the introduction of TiO₂ nanotubes into the preparation process of CoFe₂O₄, which could reduce the size and improve its dispersion^[75]. More importantly, the TiO₂ reduces the precipitation of Co ions through ion exchange, which ensures that there are more reactive sites in the CoFe₂O₄ to activate PMS to produce

SO 4 · -

from two aspects. Based on the previous introduction, carbon materials can help Fenton and photocatalytic oxidation technology, and they can also help persulfate oxidation technology. Yao et al. Synthesized graphene oxide (GO) by Hummers method, and then obtained magnetic CoFe₂O₄-rGO composite catalyst by solvothermal method. The activated PMS can completely remove phenol in water within 30 min, and the efficiency is much higher than that of single CoFe₂O₄. The enhancement of activity may be due to: (1) rGO improves the electron transfer efficiency between CoFe₂O₄ and PMS, which will produce more reactive species^[76]; (2) rGO inhibits the agglomeration of magnetic CoFe₂O₄ particles; (3) The presence of rGO increases the specific surface area of the catalyst, providing more adsorption and catalytic sites. Ding et al. Synthesized CoFe₂O₄-GO binary catalyst by hydrothermal method, which activated PMS to degrade norfloxacin (NOR) in water for 20 min, and the activity was much higher than that of single CoFe₂O₄^[77]. At the same time, this work confirms that the stability of CoFe₂O₄-GO is stronger than that of CoFe₂O₄, because the existence of GO inhibits the precipitation of Co ions in CoFe₂O₄ during the degradation of NOR by activated PMS. The results showed that the mechanism of NOR degradation in the CoFe₂O₄-GO/PMS system was mainly a non-radical process, that is, NOR molecules were first adsorbed by GO, and at the same time, PMS molecules were degraded by NOR molecules through electron abstraction with the help of GO edge functional groups (such as — C = O). In addition, the

SO 4 · -

produced by the activation of PMS by the CoFe₂O₄ in the binary material also contributed to the degradation of NOR. Lai et al. Used coprecipitation method to load CoFe₂O₄ on exfoliated graphite (EG) to construct magnetic CoFe₂O₄-EG composite materials, which activated PMS to remove sulfamethoxazole (SMX) from water better than CoFe₂O₄ alone.At the same time, the study pointed out that the reaction process of SMX degradation in water by CoFe₂O₄-EG/PMS system involved free radical (

SO 4 · -

) and non-free radical (¹O₂) oxidation, and the non-free radical was the main one^[78]. After SMX molecules were adsorbed by CoFe₂O₄-EG, their electrons were transferred to PMS to promote the formation of ¹O₂. In addition, the CoFe₂O₄-EG can enhance the self-decomposition of the PMS molecule to generate more ¹O₂ (formula (4)); At the same time, Co and Fe on the surface of the CoFe₂O₄ produce

SO 4 · -

through redox and

HSO 5 -

, which together with ¹O₂ lead to the degradation of SMX. The non-free radical oxidation ability is weak, which can not cause the mineralization of organic pollutant molecules, but the selectivity is good, and the influence of water background components is small. The authors suggest that it is necessary to control the contribution of free radical oxidation and non-free radical oxidation pathways to the degradation of the target pollutants in the Hf-based catalyst/PMS system, so as to achieve efficient removal of the target pollutants.

PMS is easy to be activated by CoFe₂O₄ because of its asymmetric structure, while PDS with symmetric structure is not easy to be activated by CoFe₂O₄ to produce

SO 4 · -

^[72,73]. Wu et al. Obtained CoFe₂O_4-_x with oxygen vacancies by treating CoFe₂O₄ with H₂ at high temperature, which can efficiently activate PDS to degrade BPA in water because oxygen vacancies can accelerate electron transfer.O as to promote the oxidation-reduction cycle of the Co³⁺/Co²⁺ and the Fe³⁺/Fe²⁺ and ensure that enough Co²⁺ and Fe²⁺ activate the PDS to generate active free radicals (

SO 4 · -

and hydroxyl radicals)^[79]; In addition, oxygen vacancies, together with dissolved oxygen in water, can produce

O 2 · -

and ¹O₂, which can also lead to the degradation of BPA.

(4)

S O 5 2 - + HSO 5 - → S O 4 2 - + HS O 4 - + 1 O 2

4.2 Degradation of organic pollutants in water by persulfate activated by CuFe₂O₄;

Like CoFe₂O₄, magnetic CuFe₂O₄ are often used as activators for persulfates. Tang et al. Synthesized CuFe₂O₄ magnetic nanoparticles by sol-gel method, and used it to activate PMS to degrade TBBPA in water. After 30 minutes of catalytic reaction, 99% of TBBPA in water could be removed. In addition, the magnetic CuFe₂O₄ had high stability, and the efficiency of activating PMS to degrade TBBPA hardly changed after five times of recycling^[80]. The mechanism study points out the ≡Cu²⁺ and ≡Fe³⁺ of the surface of the CuFe₂O₄.First, it is reduced to ≡Cu⁺ and ≡Fe²⁺ by

HSO 5 -

.They then react with

HSO 5 -

to produce

SO 4 · -

; In addition, part of the

SO 4 · -

can react with the H₂O to generate hydroxyl radicals, which, together with the

SO 4 · -

, leads to the degradation of TBBPA in water. Crou Croué et al. Used CuFe₂O₄/PMS oxidation system to degrade iopromide (IPM) in water, and the removal effect was remarkable, and it was confirmed that the reuse efficiency of CuFe₂O₄ was high^[81]. It is worth noting that, unlike the mechanism proposed by Tang et al., this work points out that Fe in the CuFe₂O₄ does not participate in the activation of PMS, but the ≡Cu²⁺ in the CuFe₂O₄ forms a complex with

HSO 5 -

(Cu (Ⅱ)- (HO) O

SO 3 -

).It then loses an electron to produce a

SO 4 · -

and a Cu(Ⅲ)-OH^-,Finally, the Cu(Ⅲ)-OH^- reacts with the

HSO 5 -

and the H₂O again to produce the

SO 4 · -

. Zhang et al. Confirmed that the CuFe₂O₄/PMS system could efficiently degrade BPA in water, and the BPA in water could be completely removed after 60 minutes of catalytic degradation^[82]. Again, in contrast to the mechanisms given above by Tang et al. And Crou Croué et al., they point out that the ≡Cu⁺ and ≡Fe²⁺ and

HSO 5 -

produced by the reduction mainly produce · OH (Formula 5, 6), which is the main reactive oxygen species for BPA degradation. Wang et al. Obtained magnetic CuFe₂O₄ nanoparticles by sol-gel method, and the activated PMS could effectively degrade norfloxacin (NOR) in water, and more than 90% of NOR in water could be degraded after 120 minutes of reaction^[83]. The mechanism study shows that the ≡Cu²⁺ in the CuFe₂O₄ reacts with the

HSO 5 -

to form Cu (Ⅱ)- (HO) O

SO 3 -

complex, and then

SO 4 · -

is produced. At the same time, the reduced Fe²⁺ can also react with

HSO 5 -

to produce

SO 4 · -

to degrade NOR in water. When Fang et al used the CuFe₂O₄/PMS system to remove the refractory 2,4,4 '-trichlorobiphenyl (PCB28) in water, they pointed out that the oxygen vacancies (OVs) on the CuFe₂O₄ could transfer electrons to the O₂ and produce

O 2 · -

.Which can react with the ≡Cu²⁺ and the ≡Fe³⁺ on the surface of the CuFe₂O₄ to generate ≡Cu⁺ and ≡Fe²⁺,They then react with the

HSO 5 -

to produce

SO 4 · -

that oxidatively degrade the PCB28 (fig. 7)^[84]. In addition, some studies have been carried out to synergistically enhance the performance of PMS activated by CuFe₂O₄ through material compounding. Li et al. Compounded CuFe₂O₄ with manganese oxide octahedral molecular sieve (OMS-2) to further improve its performance of activating PMS. Degradation experiments showed that the activity of CuFe₂O₄-OMS-2/PMS system for degrading acid orange 7 (AO7) in water was stronger than that of CuFe₂O₄/PMS system^[85]. The enhanced activity of the CuFe₂O₄-OMS-2 was attributed to the synergistic effect of Mn in OMS-2 and Cu in CuFe₂O₄ on the production of

SO 4 · -

and hydroxyl radical. Oh et al. Obtained CuFe₂O₄-Fe₂O₃ composite materials by co-precipitation method to activate PMS to remove BPA in water, and its performance was better than that of single CuFe₂O₄.The ≡Fe³⁺ attributed to the Fe₂O₃ can reduce the consumption of ≡Cu⁺ in the CuFe₂O₄, thereby promoting the generation of

SO 4 · -

^[86]. Liu et al. Modified magnetic CuFe₂O₄ with multi-walled carbon nanotubes (MWCNTs) to enhance the performance of activated PMS. The study showed that the CuFe₂O₄/MWCNTs activated PMS could remove 90% of trimethoprim (TMP) in water within 24 min, and the effect was better than that of single CuFe₂O₄, because the high conductivity of MWCNTs was conducive to the electron transfer between Cu ions in the CuFe₂O₄, thus promoting the redox cycle of Cu and producing more active radicals (

SO 4 · -

and · OH)^[87]. Ma et al. Prepared nitrogen-doped graphene oxide (NG)-CuFe₂O₄ composites by hydrothermal method, and the activity of activated PMS in degrading orange II (OII) in water was higher than that of CuFe₂O₄^[88]. The mechanism study shows that the redox cycle between Fe (Ⅲ) and Fe (Ⅱ), Cu (Ⅱ) and Cu (Ⅰ), and

HSO 5 -

in the CuFe₂O₄ promotes the generation of

SO 4 · -

, and NG can assist the above formula (4) to generate ¹O₂, thereby improving the efficiency of OⅡ degradation in water. kaolinite is a natural mineral with a two-dimensional layered structure, which can be used as a catalyst carrier, and its aluminum hydroxyl is the active site for the activation of PMS. Sun et al. Obtained the CuFe₂O₄/kaolinite composite material by citric acid combustion method, and used it to activate PMS to remove BPA in water. The catalytic reaction lasted for 60 minutes and nearly 100% of BPA in water was degraded. The efficiency was higher than that of single CuFe₂O₄, which was attributed to: (1) the CuFe₂O₄/kaolinite with large specific surface area was easier to adsorb BPA molecules^[89]; (2) In addition to the abundant aluminum hydroxyl groups in the CuFe₂O₄,kaolinite, the activation of PMS can also be promoted.

(5)≡Fe(Ⅱ) +

HSO 5 -

→ ≡Fe(Ⅲ) +

SO 4 2 -

+ ·OH

(6)≡Cu(I) +

HSO 5 -

→ ≡Cu(Ⅱ) +

SO 4 2 -

+ ·OH

In addition to the catalytic activation of PMS, the CuFe₂O₄ also has a certain effect on PDS. Lai et al. Used CuFe₂O₄/PDS oxidation system to treat wastewater containing p-nitrophenol (PNP), and the degradation results showed that the system could remove PNP in water (the removal rate was 89% after 60 min reaction), but the amount of CuFe₂O₄ and PDS was large^[90]. The mechanism points out the low valence ≡Cu⁺ and ≡Fe²⁺ in the CuFe₂O₄.Can react with a S₂

O 8 2 -

to generate a

SO 4 · -

(formula 3),At the same time, part of the

SO 4 · -

reacts with the H₂O to produce hydroxyl radicals, and the two radicals degrade PNP in water together. In order to improve the performance of PDS activated by CuFe₂O₄, Zhang et al modified the CuFe₂O₄ with MWCNTs with large specific surface area and high conductivity^[91]. The degradation of diethyl phthalate (DEP) in water shows that the introduction of MWCNTs can cooperate with the CuFe₂O₄ to improve the

SO 4 · -

generation performance of the activated S₂

O 8 2 -

. In our research group, the efficiency of PDS activated by CuFe₂O₄ was improved by Cu modification, and tetracycline (TC) was effectively removed from water in a short time with a small amount of PDS, which was attributed to the fact that Cu could act as an electron bridge to promote the rapid electron transfer between PDS and CuFe₂O₄. In addition, the metal Cu is oxidized into Cu⁺ ions during the reaction, which can react with the S₂

O 8 2 -

to generate

SO 4 · -

. When sodium bicarbonate is added in the process of degrading the TC, on one hand, the efficiency of activating the S₂

O 8 2 -

to degrade the TC by the Cu-CuFe₂O₄ is not influenced; On the other hand, it improves the stability of the Cu-CuFe₂O₄^[17].

4.3 Degradation of pollutants in water by persulfate activated by MnFe₂O₄;

Magnetic MnFe₂O₄, like MFe₂O₄(M=Co,Cu), are often used to activate persulfate to degrade organic wastes in aqueous environments. Li et al. synthesized magnetic ordered mesoporous MnFe₂O₄ by nanocasting method using porous silicon KIT-6 as a template, and used it to activate PMS for the degradation of orange Ⅱ (OⅡ) in water.The reaction lasted for 30 min, and the complete removal of OⅡ was achieved^[92]. The performance of ordered mesoporous MnFe₂O₄ activated PMS is much better than that of MnFe₂O₄ nanoparticles, because the former has a larger specific surface area and exposes more active sites. In addition, the three-dimensional pore structure of the former reduces the mass transfer resistance and thus promotes the heterogeneous catalytic reaction. The mechanism study indicates that the Mn²⁺-OH on the surface of the MnFe₂O₄ can react with the

HSO 5 -

to form

SO 4 · -

. In addition, the ≡Fe³⁺ on the surface of the MnFe₂O₄ reacts with the

HSO 5 -

to generate ≡Fe²⁺,Which then reacts with the

HSO 5 -

to form a

SO 4 · -

. Deng et al. Used MnFe₂O₄-PMS system to oxidize and degrade BPA in water, which showed ideal performance. 90% of BPA in water could be removed after 30 minutes of reaction, and the recycling efficiency of MnFe₂O₄ was high^[93]. The redox cycles of Mn (Ⅱ) -Mn (Ⅲ) -Mn (Ⅱ) and Fe (Ⅲ) -Fe (Ⅱ) -Fe (Ⅲ) lead to the activation of PMS and the generation of

SO 4 · -

, which leads to the degradation of BPA. Chu et al., when using the MnFe₂O₄-PMS oxidation system to degrade triclosan (TCS) in water, pointed out that not only the

SO 4 · -

produced by the activation of PMS by the MnFe₂O₄ would lead to the removal of TCS, but also the MnO₂ formed in the reaction process itself could degrade TCS (formula (7, 8))^[94]. Similar to the MFe₂O₄(M=Co,Cu), the performance of PMS activated by MnFe₂O₄ can also be synergistically enhanced by the construction of composite catalysts. Yao et al. Obtained MnFe₂O₄-rGO composite catalyst by chemical deposition method, and its performance of activating PMS to degrade orange II (OII) in water is superior to that of single MnFe₂O₄, because the existence of rGO can improve the electron transfer ability in the reaction process^[95]. Ma et al. Constructed MnFe₂O₄/ biomass carbon (MX) composite by solvothermal method to activate the performance of PMS to remove Orange II (OII) in water. The experimental results showed that MX could enhance the activity of PMS activated by MnFe₂O₄, and the MnFe₂O₄/MX-PMS system could completely degrade OII in water after 6 min of reaction^[96]. The C = O functional group on the surface of MX can promote the self-decomposition of PMS into ¹O₂ (Formula 4), which, together with the free radicals (

SO 4 · -

and · OH) generated by the activation of PMS by MnFe₂O₄, can degrade OII in water. In addition, Cheng et al. Obtained magnetic MnO₂/MnFe₂O₄ by hydrothermal method, and used it to activate PMS to degrade Rhodamine B (RhB), the activity was significantly higher than that of MnFe₂O₄, and the stability was good, and the activity did not change after four cycles^[97]. Mechanism inquiry points out the Mn⁴⁺ in MnO₂ and the Mn²⁺ in MnFe₂O₄.And the oxidation-reduction cycle between the Fe³⁺ and the

HSO 5 -

cooperatively improves the output of the

SO 4 · -

(7)Mn³⁺ +

SO 4 · -

+ 2H₂O→MnO₂ +

SO 4 2 -

+ 4H⁺

(8)TCS + MnO₂ → [MnO ₂-TCS]_complex → [Mn ³⁺-TCS^·]→phenoxy radical + Mn²⁺

In addition to activating PMS, magnetic MnFe₂O₄ can also be used to activate PDS to produce active radicals. Muntean et al. Used magnetic MnFe₂O₄ nanoparticles prepared by thermal decomposition method to activate PDS to remove phenol in water, and showed that the system could effectively oxidize and degrade phenol (the removal rate reached more than 90% after 6 hours of reaction)^[98]. This work indicates that the Mn²⁺ on the surface of the MnFe₂O₄ reacts with the

O 8 2 -

of the S₂ to produce a

SO 4 · -

(Formula 2) leading to the degradation of phenol. Yang et al. Used solvothermal method to load MnFe₂O₄ on sludge activated carbon (AC) to enhance the performance of activated PDS^[99]. The experiment of degradation of Orange G (OG) in water shows that the performance of PDS activated by AC/MnFe₂O₄ is better than that of MnFe₂O₄ alone, because Mn²⁺ in MnFe₂O₄ can activate PDS to produce

SO 4 · -

.Some oxygen-containing functional groups (— OH and — COOH) on the AC surface can also react with the S₂

O 8 2 -

to form

SO 4 · -

(Fig. 8). Ma et al. Synthesized CuO/MnFe₂O₄ nanocomposite catalysts by impregnation method and used them to activate PDS to remove levofloxacin (LVF) from water^[100]. The results show that the performance of the CuO/MnFe₂O₄ is better than that of the single MnFe₂O₄, which is attributed to the fact that the redox reactions among Cu, Mn, Fe and S₂

O 8 2 -

in the composite synergistically promote the generation of active radicals (

SO 4 · -

and · OH).

5 Conclusion and prospect

Magnetic spinel ferrites are widely used in wastewater treatment as catalysts in advanced oxidation processes. By sorting out the research work in this field in recent years, the application of different magnetic ferrite catalysts in three advanced oxidation technologies, the mechanism of catalytic degradation of organic pollutants and the way of activity enhancement are compared in Table 4. For heterogeneous Fenton-like technology, the efficiency of magnetic MFe₂O₄(M=Zn,Ni,Co,Cu,Mn) to catalyze and activate H₂O₂ to generate hydroxyl radicals is low, and the performance of Fenton-like catalytic degradation of organic pollutants in water can be improved with the assistance of visible light and the construction of ferrite composite materials. The band gap of single magnetic MFe₂O₄(M=Zn,Ni,Co,Cu) is small (< 2.0 eV), which can absorb and utilize visible light. However, the electrons and holes generated by light excitation are easy to recombine. The composite modification (such as noble metal deposition, carbon material modification and composite heterojunction construction) can improve the separation and transmission of photogenerated charges and enhance the activity of visible light photocatalytic degradation of organic compounds. The multi-valence transition metal elements contained in the magnetic MFe₂O₄(M=Co,Cu,Mn) are easy to generate oxidation-reduction reaction, which is beneficial to activate persulfate to generate active oxygen species (

SO 4 · -

, hydroxyl and ¹O₂, etc.) To oxidize and degrade pollutants in water. In addition, the performance of magnetic ferrite for activating persulfate can be further improved by loading and surface modification. At the same time, it should be recognized that although a series of fruitful research results have been achieved in the treatment of organic wastewater by magnetic spinel ferrite catalysts, there are still some problems to be solved.

(1) The specific pathway of the activation of H₂O₂ to produce hydroxyl radical by irradiation needs to be further clarified. Current studies indicate that electrons generated by irradiation of ferrite reduce ≡Fe³⁺ to ≡Fe²⁺, which react with H₂O₂ to form hydroxyl radicals. In fact, theoretically speaking, photogenerated electrons can also directly react with H₂O₂ to produce hydroxyl radicals, and whether the hydroxyl radicals produced in this way contribute to the degradation of organic pollutants in the reaction system needs to be studied. Under the coexistence of ferrite and light, H₂O₂ can also be directly decomposed into O₂, which affects the utilization of H₂O₂, and this problem also needs to be considered. In addition, the transition element Fe in the magnetic ferrite lattice can drive the H₂O₂ to generate hydroxyl radicals, but whether the low valence state of another transition element (such as Ni, Co, Cu or Mn) can activate the H₂O₂ to generate hydroxyl radicals needs to be further studied.

(2) Most of the reported composite photocatalysts composed of ferrite and other semiconductor materials have enhanced photocatalytic activity under visible light, which is attributed to the effective separation of photogenerated electrons and holes and the formation of reactive species when ferrite is heterogeneously compounded with other photocatalysts. However, some studies have pointed out that the photogenerated electrons and holes of narrow bandgap ferrite itself do not participate in the generation of free radicals, but only play the role of electron transport or transfer when it is heterogeneously compounded with some photocatalysts^[101,102]. The above contradictions fully show that the synergistic catalytic mechanism between the components of ferrite composite photocatalytic materials needs to be further studied.

(3) The mechanism of persulfate activation catalyzed by some ferrites, such as CuFe₂O₄ and MnFe₂O₄, is different. Some studies show that only Cu or Mn participate in the activation, while Fe does not. However, some studies have shown that Fe is also involved in the activation of persulfate. Therefore, it is necessary to further reveal the valence transition of transition metals, the path of electron transfer and the interaction mechanism between ferrite surface and persulfate molecules in the process of catalytic activation of persulfate in the future, so as to obtain the exact mechanism of catalytic activation of persulfate.

(4) The introduction of carbon materials can improve the Fenton catalytic performance, photocatalytic activity and the efficiency of persulfate activation of magnetic ferrite. However, the reactive species (such as hydroxyl radicals and

SO 4 · -

) produced in these advanced oxidation processes can oxidize and destroy the carbon materials to some extent, thus reducing the reusability of the catalyst, which should be paid attention to in future studies.

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Funding

Natural Science Foundation of Hebei Province(B2019209373)

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1 Introduction

2 Heterogeneous Fenton-like Oxidation Technology Based on Spinel Ferrite

2.1 Sulfhydryl heterogeneous Fenton-like oxidation technology

2.2 Sulfhydryl heterogeneous Fenton-like oxidation technology

2.3 Sulfhydryl heterogeneous Fenton-like oxidation technology

2.4 Sulfhydryl heterogeneous Fenton-like oxidation technology

2.5 Sulfhydryl heterogeneous Fenton-like oxidation technology

3 Photocatalytic Technology Based on Spinel Ferrite

3.1 ZnFe2O4 based photocatalytic oxidation technology

3.2 MFe2O4(M=Ni,Co,Cu) -based photocatalytic oxidation technology

4 Persulfate Oxidation Technology with Spinel Ferrite as Catalyst

4.1 Degradation of organic pollutants in water by persulfate activated by CoFe2O4;

4.2 Degradation of organic pollutants in water by persulfate activated by CuFe2O4;

4.3 Degradation of pollutants in water by persulfate activated by MnFe2O4;

5 Conclusion and prospect

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RIGHTS & PERMISSIONS

3.1 ZnFe₂O₄ based photocatalytic oxidation technology

3.2 MFe₂O₄(M=Ni,Co,Cu) -based photocatalytic oxidation technology

4.1 Degradation of organic pollutants in water by persulfate activated by CoFe₂O₄;

4.2 Degradation of organic pollutants in water by persulfate activated by CuFe₂O₄;

4.3 Degradation of pollutants in water by persulfate activated by MnFe₂O₄;