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Strategies, Research Progress and Enlightenment of Enhancing the Heterogeneous Fenton Catalytic Reactivity: A Critical Review

Wenyan Gao , Xuan Zhao , Xilin Zhou , Yaran Song , Qingrui Zhang

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (5) : 1191 -1202.

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Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (5) : 1191-1202 DOI: 10.7536/PC210728
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Strategies, Research Progress and Enlightenment of Enhancing the Heterogeneous Fenton Catalytic Reactivity: A Critical Review
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Abstract

Heterogeneous Fenton reactions have attracted tremendous attention for recalcitrant organic contaminants removal, as the reaction between solid Fenton catalysts and H2O2 can generate highly reactive hydroxyl radicals. Additionally, compared to the homogeneous Fenton reaction, it has the advantages of wide pH response range, good catalyst stability and reusability, as well as less-production of iron sludge. However, some defects of heterogeneous Fenton reaction, such as metal ion precipitation, low effective utilization of H2O2 and slow formation rate of Fe(Ⅱ), limiting its application in real wastewater treatment. Therefore, numbers of research have introduced numerous techniques to overcome these drawbacks. In the present review, we summarize the mechanism of homogeneous and heterogeneous Fenton reaction. We also elucidate the strategies for accelerating the formation of Fe(Ⅱ) and promoting the decomposition of H2O2. We believe this review will provide a new insight into the future direction research in the heterogeneous Fenton catalyst.

Contents

1 Introduction

2 Homogeneous and heterogeneous Fenton reaction mechanism

2.1 Homogeneous Fenton reaction mechanism

2.2 Heterogeneous Fenton reaction mechanism

3 Development of heterogeneous Fenton catalysts

3.1 Zero valent metal catalyst system

3.2 In situ doping catalysis system of metal ions

3.3 Cocatalyst

3.4 Two center catalytic system

4 Physical field assisted heterogeneous Fenton reaction

5 Conclusion and outlook

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Keywords

heterogeneous Fenton / Fe(Ⅲ)/Fe(Ⅱ) recycling / hydrogen peroxide / hydroxyl radicals

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Wenyan Gao, Xuan Zhao, Xilin Zhou, Yaran Song, Qingrui Zhang. Strategies, Research Progress and Enlightenment of Enhancing the Heterogeneous Fenton Catalytic Reactivity: A Critical Review. Progress in Chemistry, 2022, 34(5): 1191-1202 DOI:10.7536/PC210728

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1 Introduction

Water pollution control and deep purification has become one of the key and difficult points in environmental research[1⇓⇓~4]. Commonly used water treatment methods include membrane separation, adsorption, chemical precipitation and advanced oxidation[5,6][7~8][9][10]. Among them, advanced oxidation processes (AOPs) have shown outstanding application prospects in the field of water pollution, especially in the advanced treatment of organic wastewater, due to their advantages of simple operation, high purification depth and environmental friendliness[10,11]. The most representative ones, such as ozonation, Fenton and persulfate catalysis, are based on the generation of active species, such as hydroxyl radical (· OH), singlet oxygen (1O2) and sulfate radical ( SO 4 - ·), through free radical chain reactions, thus achieving efficient degradation of organic pollutants[12,13]. Throughout the existing advanced oxidation treatment methods, Fenton oxidation based on Fe (Ⅲ)/Fe (Ⅱ) has become one of the core technologies for large-scale application of modern water treatment because of its high reactivity, low price and mild conditions. The basic principle of Fenton reaction is that Fe (Ⅱ) reacts with H2O2 to produce a large amount of hydroxyl radical rapidly, so as to achieve efficient and broad-spectrum degradation of organic pollutants such as dyes, antibiotics, phenol and other organic pollutants in wastewater[1,14]. However, the traditional homogeneous Fenton oxidation has some technical defects, such as narrow application range of pH, large amount of iron sludge produced in the reaction process and low utilization rate of H2O2, which limit its large-scale application to a certain extent[15,16]. In order to solve the above technical bottlenecks, scientists have gradually developed a heterogeneous Fenton catalytic oxidation technology on the basis of homogeneous Fenton system, which is a new generation of Fenton oxidation technology that uses solid catalyst instead of conventional Fe (II) to achieve interfacial purification of pollutants[17]. Compared with the conventional Fenton process, heterogeneous Fenton reaction has the advantages of wide pH response range, less sludge production and recyclable catalyst, which has become one of the important research directions of Fenton technology in recent years[16]. However, there are still some challenges in the wide application of heterogeneous Fenton. Specifically, the heterogeneous Fenton reaction usually uses Fe (Ⅱ) as the catalyst reaction center to realize the Fe (Ⅱ)/Fe (Ⅲ) conversion at the catalyst interface by :(1)H2O2 oxidation, and produces active species such as · OH. The reduction of (2)H2O2 promotes the reverse reaction of Fe (Ⅲ)/Fe (Ⅱ) transformation and ensures the concentration of Fe (Ⅱ). However, it is generally believed that the conversion rate of Fe (Ⅱ)/Fe (Ⅲ) is fast and the rate constant is k=40~80 L·mol-1·s. In contrast, the reduction process of Fe (Ⅲ)/Fe (Ⅱ) is slow and the rate constant is k=0.001~0.01 L·mol-1·s, which determines that the overall efficiency of the whole Fenton reaction is not high, and the production of active oxygen radicals is limited, which greatly reduces the purification efficiency of pollutants in water-1[12]-1[18]. Therefore, how to accelerate the conversion efficiency of Fe (Ⅲ)/Fe (Ⅱ) has become the key to promote the development of heterogeneous Fenton technology[19].
In recent years, researchers have proposed a variety of effective strategies to accelerate the conversion efficiency of Fe (Ⅲ)/Fe (Ⅱ) in heterogeneous Fenton process, such as Fe (0) Co-catalysis, Co/Mn metal doping, organic and inorganic co-catalysis reactions, and double reaction center theory. In this paper, the mechanism of heterogeneous Fenton reaction is reviewed, and the related research on accelerating the redox cycle of Fe (Ⅲ)/Fe (Ⅱ) and improving the effective utilization of H2O2 is highlighted, hoping to provide an important scientific reference for promoting the further application of heterogeneous Fenton.

2 Homogeneous/heterogeneous Fenton reaction mechanism

2.1 Homogeneous Fenton reaction mechanism

In 1894, the French scientist H. J. H. Fenton, when degrading tartaric acid, found that the oxidation rate of tartaric acid was significantly accelerated when H. J. H. Fenton and H2O2 were added to the acidic solution at the same time. In 1964, Eisenhauer first used Fenton technology to treat phenol and alkylbenzene wastewater, and then Fenton technology began to be applied in the field of water treatment, especially in the treatment of organic wastewater, such as phenol-containing wastewater, dye wastewater, landfill leachate and comprehensive wastewater from industrial parks[20].
The mechanism of Fenton oxidation of organic matter is shown in Figure 1. The main reaction process is as follows: Fe (II) in the solution is oxidized to Fe (III) by H2O2, and · OH is formed, which has strong redox potential and high electronegativity (redox potential is 2.8 V,The electron affinity is 569.3 kJ), which can attack organic pollutants through oxidation-reduction reaction, dehydrogenation reaction and hydroxylation reaction, decompose them into organic free radical intermediates and hydroxylation products, and finally mineralize them into inorganic substances such as CO2 and H2O by · OH[21][16][22]. At the same time, the cyclic reaction that the Fe3+ generated by the reaction is reduced to Fe2+,Fe(Ⅱ)/Fe(Ⅲ) by H2O2 and HO2· promotes the further degradation of pollutants, and the specific reactions are shown in equations (1 – 7). Although Fenton oxidation can be understood with the help of these simple formulas, the process is complex and involves many side reactions. Generally speaking, these processes can be roughly divided into three stages: initiation, growth, and termination[23].
F e 2 + + H 2 O 2 → F e 3 + + · O H + O H -
· O H + R X → R X · + + O H -
R H + · O H → R · + H 2 O
R H X + · O H → R H X ( O H )
H 2 O 2 + · O H → H 2 O + H 2 O ·
F e 3 + + H 2 O 2 → F e 2 + + H + + H O 2 ·
F e 3 + + H O 2 · → F e 2 + + O 2 + H +

2.2 Heterogeneous Fenton reaction mechanism

Revealing the mechanism of homogeneous Fenton reaction is of great significance to explore the catalytic mechanism of heterogeneous Fenton reaction. However, compared with homogeneous Fenton reaction, the mechanism of heterogeneous Fenton reaction is more complex, which is due to the existence of solid-liquid interface between catalyst and reaction solution. The reaction rate is not only related to the concentration and proportion of Fenton reagent, the nature of pollutants and reaction conditions, but also closely related to the structural characteristics, surface properties and dosage of the catalyst itself[16]. In the traditional homogeneous Fenton system, a series of chain reactions occur between H2O2 and metal ions (Fe2+) in the solution, producing · OH to attack organic pollutants; In the heterogeneous Fenton catalytic system, two different reaction processes are involved, including the chemical reaction between · OH and organic pollutants and the physicochemical process on the surface of the heterogeneous catalyst, that is, H2O2 may react with leached iron ions in the solution or ferrous iron on the surface of the catalyst to produce · OH[24]. Some studies suggest that organic pollutants and · OH can be chemically adsorbed on the surface of the catalyst, which makes the heterogeneous Fenton reaction process more complex[25]. Because the heterogeneous Fenton reaction process is complex and changeable, many scholars can not reach a consensus on the reaction mechanism of heterogeneous Fenton. Even so, we can still draw some basic rules from it.
Most researchers believe that the main reaction of organic pollutants degradation in heterogeneous Fenton system occurs at the solid-liquid interface, and the reaction mechanism is still dominated by free radical generation mechanism. First, the redox reaction between H2O2 and metal ions occurs on the surface of the catalyst, which changes the valence state of the electron pair on the surface of the catalyst.However, H2O2 decomposes to form active species such as · OH, HO2· and O 2 - ·, which have strong oxidation ability and synergistic effect, and attack the refractory organic pollutants in water without selectivity, so that they are finally oxidized into green small molecular substances such as H2O and CO2. The reaction mechanism is shown in Equations (8 – 10)[26⇓~28].
≡ M n + + H 2 O 2 → ≡ M ( n + 1 ) + O H - + · O H
≡ M ( n + 1 ) + H 2 O 2 → ≡ M n + + H + + H 2 O ·
H 2 O · + ≡ M ( n + 1 ) → ≡ M n + + H + + O 2
He et al. Discussed the catalytic mechanism of heterogeneous Fenton reaction when iron-based materials are used as catalysts, as shown in Figure 2, which includes homogeneous Fenton reaction induced by surface leaching iron and heterogeneous Fenton reaction occurring on the surface of the catalyst, but so far, these two mechanisms are still difficult to distinguish and may even coexist[19]. However, the mechanism described does not include the interaction of organic pollutants and (or) · OH adsorbed on the surface of the catalyst[29,30].
Some researchers believe that the interaction (including physical and chemical effects) between organic pollutants and (or) · OH on the surface of the catalyst has a great influence on the heterogeneous Fenton reaction. Ji et al. Used density functional theory (DFT) to compare the Fenton reaction catalyzed by FeOCl and Fe2O3, and found that the catalytic efficiency of FeOCl was higher than that of Fe2O3, which was mainly due to the formation of [ Fe c u s 2 +- Fe c u s 3 +] units on the surface of FeOCl covered by H atoms, which promoted the cycle of catalytic reaction[31]. In order to confirm this conclusion, the study was elaborated from four aspects: (1) When the surface of FeOCl is not covered by H atoms, the redox reaction between Fe c u s 3 + and H2O2 on its surface produces OH (Formula 11),At the same time, the H2O2 is dehydrogenated with the assistance of surface Olatt to produce OOH and H+, which are adsorbed on Fe c u s 3 + and Olatt, respectively (Equation 12);(2) When the surface of FeOCl is completely covered by H atoms, the H atoms on the surface transfer electrons to the nearby Fe c u s 3 + and reduce them to Fe c u s 2 +, as shown in fig. 3A.Then Fe c u s 2 + catalyzes the decomposition of H2O2 to produce OH (formula 13); (3) when the surface of FeOCl is partially covered by H atoms, the H2O2 is dehydrogenated with the Olatt on the surface (formula 12),At the same time, H atom reduces the adjacent Fe c u s 3 + to Fe c u s 2 +,Both form [ Fe c u s 2 + − U Fe c u s 3 + units (Fig. UN 3b);(4)H2O2 UNK Fe c u s 2 + K1 Fe c u s 3 + to undergo dehydrogenation reaction,The generated · OOH is adsorbed on the Fe c u s 2 +, and the adsorbed · OOH is easily desorbed from the U Fe c u s 2 + point,That is to say, Fe c u s 2 + is easily released (Fig. 4A), and the O — O bond in the released U Fe c u s 2 + U N H2O2 is cleaved to form two — OH groups, and the — OH group attached to U N Fe c u s 3 + is decomposed into · OH.At the same time, the other — OH group receives an electron from the Fe c u s 2 +, becomes U OH-, and is finally neutralized by a proton (Fig. 4 B), allowing the entire catalytic process to be recycled. In addition, · OH reacts with other intermediates (Equations 14 – 17), reducing the utilization of H2O2.
H 2 O 2 + 2 F e c u s 3 + → 2 O H + 2 F e s u r f 3 +
H 2 O 2 + 2 F e c u s 3 + + [ O l a t t ] → O O H + 2 F e c u s 2 + + [ O l a t t ] H +
H 2 O 2 + 2 F e c u s 2 + → 2 · O H + 2 F e c u s 2 + ( 2 O H - + 2 F e s u r f 3 + )
· O H + · O H → ≡ H O 2 · + O ·
· O H + H · → H 2 O
· O H + H 2 O 2 → H 2 O + · O O H
· O H + · O O H → H 2 O + O 2
Some scholars believe that the interaction of organic pollutants and (or) · OH on the surface of the catalyst will inhibit the activity of the catalytic system and reduce the efficiency of the reaction. According to Pliego et al., in the heterogeneous Fenton reaction process, organic pollutants are first attached to the surface of the catalyst under the adsorption of the carrier, as shown in Figure 5[32]. Then the Fenton reaction between H2O2 and the surface iron-based active components occurs to produce · OH and HO2·/ O 2 - ·, which attacks the pollutants attached to the surface of the catalyst and degrades them. However, the degradation intermediates of aromatic ring pollutants are carboxylic acid organic molecules, which will quickly complex with Fe (Ⅲ) or Fe (Ⅱ) on the surface of the catalyst. This complexation will not only prevent the contact between the H2O2 and the active site,It reduces the activity of the catalyst, inhibits the activation of H2O2 to produce free radicals, and releases the active components on the carrier into the solution, thus destroying the stability of the catalyst.

3 Development of Heterogeneous Fenton Catalyst

At present, how to improve the catalytic efficiency of heterogeneous Fenton is the most concerned problem of most researchers, and in the heterogeneous Fenton system, the reaction of Fe (Ⅱ) and H2O2 to produce · OH is the most critical step to remove pollutants. Therefore, in order to accelerate the redox cycle of Fe (Ⅲ)/Fe (Ⅱ), improve the effective utilization of H2O2, and enhance the heterogeneous Fenton reaction activity. Many researchers have carried out various related studies, such as improving the catalyst (such as using zero-valence metal, doping metal, adding co-catalyst and constructing double reaction centers) to enhance the reaction activity of heterogeneous Fenton.

3.1 Zero-valence metal catalytic system

Zero-valent metals are often used as heterogeneous Fenton catalysts to degrade refractory pollutants in water because of their reducibility and catalysis. Nanometer zero-valent metal particles are excellent catalysts in Fenton oxidation because of their large specific surface area and high chemical reactivity. The Fe0 has reducibility, and can be used as a catalyst for Fenton oxidation reaction or a reductant for reduction reaction in the Fenton system. Under acidic conditions, Fe0 can generate Fe2+ in situ, which can react with H2O2 to generate Fe3+ and hydroxyl.The Fe3+ can reduce the Fe0 to the Fe2+ to ensure the circulation of the Fenton reaction; Fe0 can also react with O2 in water to produce H2O2, and the reaction process is shown in the formula (18 ~ 24).
F e 0 + H 2 O 2 + 2 H + → F e 2 + + 2 H 2 O
F e 2 + + H 2 O 2 → F e 3 + + · O H + O H -
2 F e 3 + + F e 0 → 3 F e 2 +
F e 3 + + H 2 O 2 → F e 2 + + H 2 O · + H +
F e 2 + + · O H + H + → F e 3 + + H 2 O
· O H + H 2 O 2 → H 2 O · + H 2 O
F e 0 + O 2 + 2 H + → F e 2 + + H 2 O 2
Because Fe0 directly transfers electrons from Fe0 to pollutants when removing pollutants, the degradation effect of pollutants is better, and some researchers have proposed to use Fe0 instead of Fe2+. Xia et al. Prepared three-dimensional dendritic zero-valent iron by electrodeposition and applied it to the heterogeneous Fenton degradation of phenol[33]. It was found that under the conditions of pH = 4. 0, H2O2 concentration of 6. 0 mmol/L and Fe0 of 100 mg/L, the removal rate of 35 ppm phenol could reach 90% within 15 min, and the catalyst had good stability and could be reused. Mechanistic studies have shown that Fe0 reacts under acidic conditions to form Fe (II) (Equation 18, 24), and then, Fe (II) converts the adsorbed H2O2 to · OH (Equation 19), and the produced · OH is used to degrade phenol. At the same time, Fe (Ⅲ) is converted into Fe (Ⅱ) through the formula (20, 21, 25) to realize the cyclic conversion of Fe (Ⅲ)/Fe (Ⅱ).
H 2 O · + ≡ F e I I I → ≡ F e I I + O 2 + H +
In recent years, some researchers have combined Fe0 with iron oxide to increase the redox cycling rate of Fe (Ⅲ)/Fe (Ⅱ), thus effectively utilizing the electrons of Fe0[34,36]. Ai et al. Used core-shell nano Fe@Fe2O3 as heterogeneous Fenton catalyst to degrade 4-chlorophenol, and found that the Fe@Fe2O3 could promote the degradation of 4-chlorophenol, and the Fe0 could transfer two electrons to the O2 to generate H2O2 (Formula 24).It reacts with Fe2+ at a pH of 4.3 to 7.9 to produce OH (Formula 19). In addition, Fe@Fe2O3 can activate molecular oxygen through a one-electron reduction pathway to produce a large amount of O 2 - (Formula 26-28) in the range[37]. However, O 2 - · and Fe0 can promote the Fe (Ⅲ)/Fe (Ⅱ) cycle and ensure the stable supply of ferrous iron (Formula 20, 29), so that H2O2 decomposes to produce more · OH, which degrades 4-chlorophenol together with O 2 - · (Fig. 6).
≡ F e I I I O H + F e ( Ⅱ ) + H 2 O → ≡ F e I I I O F e I I O H + 2 H +
≡ F e I I I O F e I I O H → ≡ F e I I O F e I I I O H
≡ F e I I I O F e I I O H / ≡ F e I I O F e I I I O H + O 2 → F e ( Ⅲ ) + O 2 - ·
O 2 - · + F e ( Ⅲ ) → F e ( Ⅱ ) + O 2
The research group also used core-shell nano-Fe@Fe2O3 to catalyze the removal of Cr (Ⅵ) in water[38]. The study showed that the removal of Cr (Ⅵ) could reach 177 mg·g-1 at pH = 6.5, which they believed was mainly due to the adsorption and reduction of Cr (Ⅵ) by Fe@Fe2O3. Under neutral conditions, Cr (Ⅵ) is adsorbed on the surface of Fe@Fe2O3 in the form of $Cr_{2}O^{2-}_{7}$ through electrostatic interaction. Since the reduction potential of iron oxide (0.260 V) is lower than that of $Cr_{2}O^{2-}_{7}$/Cr3+ (1.232 V), the electrons released from Fe0 can reduce Cr (Ⅵ) adsorbed on the surface of Fe@Fe2O3 to Cr (Ⅲ) (Equation 30), and also reduce Fe (Ⅲ) in the shell of Fe@Fe2O3 to Fe (Ⅱ) (Equation 20). At the same time, Fe (II) generated by the reduction and Fe (II) on the surface of the initial Fe@Fe2O3 reduce Cr (VI) adsorbed on the surface of the reduced Fe@Fe2O3 to Cr (III) (Formula 31). In addition, Cr (Ⅵ) can also be bound to these polymerized Cr (Ⅲ) hydroxides in the form of $Cr_{2}O^{2-}_{7}$ through Cr (Ⅲ) — O — Cr (Ⅵ) bonds, forming Fe (Ⅲ)/Cr (III)/Cr (VI) composite oxides on the surface of the Fe@Fe2O3 (Fig. 7) to improve the removal rate of Cr (VI).
C r O 4 2 - + F e 0 + 4 H 2 O → C r ( O H ) 3 + F e ( O H ) 3 + 2 O H -
C r O 4 ( s o l ) 2 - + 3 F e 2 + + 8 H + → C r 3 + + 3 F e 3 + + 4 H 2 O
Zero-valent iron can not only act as a catalyst for heterogeneous Fenton reaction, but also act as an electron donor in combination with iron oxide. When zero-valent iron is used alone as a catalyst, the circulation of Fe (Ⅲ)/Fe (Ⅱ) can be accelerated, but the oxidation of zero-valent iron to Fe (Ⅱ) requires additional H2O2, which reduces the effective utilization of H2O2. Combining with iron oxide can avoid this and improve the effective utilization of H2O2. At the same time, when zero-valent iron is used as an electron donor, the catalyst has a good degradation effect on organic matter and heavy metals in water. At present, the zero-valent metal-catalyzed Fenton reaction is mainly suitable for acidic and neutral conditions, and how to further expand the pH response range can be further studied in the future.

3.2 Metal ion in-situ doped catalytic system

Recently, many studies have shown that the introduction of doping metals (such as Co, Mn, Cu, Cr, Ti, Zn, etc.) into iron minerals (such as magnetite, hematite and goethite) can significantly improve the reaction activity of Fenton. Some polyvalent metals (such as Co, Mn and Cu) can produce HO2· and · OH with H2O2 through Haber-Weiss reaction during Fenton reaction (Equation 8, 9), which can promote the decomposition of H2O2[39,40]. In addition, these metals can also participate in the Fe (III)/Fe (II) redox cycle (formulas 32-34).
F e 2 + + C O 3 + → F e 3 + + C O 2 + , E 0 = 1.04 V
F e 2 + + M n 3 + → F e 3 + + M n 2 + , E 0 = 0.73 V
≡ F e ( Ⅲ ) + ≡ C u ( I ) → ≡ F e ( Ⅱ ) + ≡ C u ( Ⅱ )
Costa et al. Studied the effect of Co and Mn doping in magnetite on the heterogeneous Fenton reaction[39]. It was found that doping Co or Mn in magnetite could promote the decomposition of H2O2 and significantly improve the Fenton reaction activity of magnetite. On the one hand, the surface reaction initiated by M s u r f 2 + (Fe, Co or Mn) produces · OH radicals, which stimulate two reactions, namely, the decomposition of H2O2 and the oxidation of organic matter in aqueous medium; On the other hand, the redox potential of Fe3+/Fe2+ is 0.77 V, which is much lower than that of Co3+/Co2+(1.81 V) and Mn3+/Mn2+(1.51 V). Therefore, the reduction of Co3+ or Mn3+ by Fe (Ⅱ) is thermodynamically favorable, as shown in Eq. (32, 33).
Xie et al. Used a novel copper-doped Fe3O4@γ-Al2O3 as a catalyst in a heterogeneous Fenton system to introduce oxygen vacancies into the decomposition reaction of Ni-EDTA[40]. The experimental results show that the Cu-doped Fe3O4@γ-Al2O3 catalyst with oxygen vacancy group shows high activity and can effectively degrade Ni-EDTA complex. When they studied its decomposition mechanism, they found that the doping of Cu in the catalyst formed oxygen vacancies, which relaxed the catalyst structure and effectively elongated the O-O band of adsorbed H2O2, which meant that H2O2 was easily activated and decomposed into · OH to decompose Ni-EDTA even under neutral and alkaline conditions (Fig. 8). In addition, through the formation of stable layered double hydroxides on Ni-Al, they simultaneously achieved the synergistic effect of oxidation and adsorption on the catalyst, resulting in the reduction of organic pollutants and the recovery of Ni2+ released into the solution to reduce secondary pollution.
Metal doping in iron minerals can promote the decomposition of H2O2 and accelerate the redox cycle of Fe (Ⅲ)/Fe (Ⅱ), and the catalytic system has high activity and catalytic efficiency. However, the current metal ion in-situ doping catalytic system has the problem of metal ion dissolution, which may cause secondary pollution to the environment. In addition, at present, the doped metals are mainly Co, Cu, Mn and so on. In general, the metal ion in-situ doping catalytic system has a lot of room for improvement both in dealing with secondary pollution and exploring the types of metals that can be doped, and further in-depth study is needed.

3.3 Cocatalyst system

In recent years, it has been shown that the addition of organic ligands (such as carboxylic acid, hydroxylamine) and inorganic ligands (such as metal sulfides, carbon materials) in heterogeneous Fenton system can accelerate the redox cycle of Fe (Ⅲ)/Fe (Ⅱ), improve the utilization of H2O2, and enhance the activity of heterogeneous Fenton reaction.
There are many carboxylates in nature, such as oxalic acid, citric acid, humic acid and tartaric acid, which are natural chelating agents and reducing agents. Studies have shown that some carboxylates (such as ascorbic acid and humic acid) can accelerate the Fe (Ⅲ)/Fe (Ⅱ) cycle and promote the degradation of pollutants in Fenton system[41⇓~43]. For example, Huang et al. Combined ascorbic acid with hematite nanocrystals to degrade alachlor at pH = 5[41]. It was found that the dechlorination rate and denitrification rate of alachlor were 89. 0% and 30. 0%, respectively.More importantly, ascorbic acid could reduce hematite to Fe (Ⅱ), accelerate the Fe (Ⅲ)/Fe (Ⅱ) cycle, promote the decomposition of H2O2 to hydroxyl, and mineralize alachlor. In addition, the coupling effect of ascorbic acid and hematite inhibits the dissolution of iron ions on the surface of hematite into the solution and reduces the production of iron mud. In addition, Abida et al. Found that under UV irradiation, Fe (Ⅲ) -citrate complex (Fe (Ⅲ) -Cit) generated Fe (Ⅱ) and R-CO2· through the redox between the carboxylic acid group and Fe (Ⅱ), followed by the rapid decarboxylation of citrate to produce reactive oxygen species, such as HO2·/ O 2 - ·, H2O2, and · OH[44]. This study shows that the carboxylate can form a stable and strong complex with Fe (Ⅲ) due to the charge transfer between the ligand and the metal, which can promote the reduction of Fe (Ⅲ) and the oxidation of carboxylic acid.
Hydroxylamine (HA) has a high reducibility, which can accelerate the redox cycle of Fe3+/Fe2+ and the decomposition of H2O2, and promote the formation of · OH. Luo et al. Added cocatalyst HA in order to improve the catalytic activity of nano-sized Fe@Fe2O3 for Fenton degradation of rhodamine B (RhB) and p-nitrophenol (PNP)[45]. Studies have shown that HA exists mainly in the form of NH3OH+ under acidic conditions, and · OH is generated by a single electron transfer reaction between NH3OH+ and H2O2 (formula 35, 36). Increasing the concentration of NH3OH+ can accelerate the reduction of Fe3+ to Fe2+ (Equation 37), but excessive NH3OH+ can quench · OH (Equation 38). Therefore, in the Fe@Fe2O3/H2O2/HA system, a proper amount of HA can accelerate the redox cycle of Fe3+/Fe2+ and the decomposition of H2O2, and promote the generation of · OH to degrade pollutants (Fig. 9).
H 2 O 2 + N H 3 O H + → N H 3 + O · + · O H + H 2 O
H 2 O 2 + N H 3 O + · → N H O · + · O H + H 2 O + H +
N H 3 O H + + F e 3 + → F e 2 + + n i t r o g e n o u s p r o d u c t s
N H 3 O H + + · O H → O H - + n i t r o g e n o u s p r o d u c t s
Many studies have shown that metal sulfides (MoS2, WS2, Cr2S3, CoS2, PbS, ZnS, etc.) are excellent heterogeneous Fenton co-catalysts to accelerate the transformation of metal active sites Fe3+/Fe2+ and improve the reaction activity of heterogeneous Fenton. Liu et al. Used MoS2 to promote Fenton system to inactivate Escherichia coli and Staphylococcus aureus[46]. The results showed that the inactivation of Escherichia coli and Staphylococcus aureus was greatly promoted by MoS2, and the inactivation effect was 2. 5 times higher than that of conventional Fenton reaction. They suggested that in this system, the exposed active site Mo4+ on the surface of MoS2 could promote the conversion of Fe3+ to Fe2+, improve the decomposition efficiency of H2O2, and generate more · OH, which is the main reactive oxygen species in the process of bacterial inactivation (Fig. 10). In addition, other metal sulfates (CuSO4,NiSO4) and metal sulfides (WS2,ZnS2) were found to promote the inactivation of E. Coli.
Carbon materials are often used as catalysts or co-catalysts because of their diverse structures and good electrical and thermal conductivity, such as biochar, graphene and carbon nanotubes, which are heterogeneous Fenton co-catalysts of great concern. For example, Zhang et al. Prepared a novel Fenton catalyst CNT @ MIL-88B-Fe (C @ M) composed of a metal-organic framework (MIL-88BA-Fe) and carbon nanotubes (CNTs)[47]. The experimental results show that the degradation ability of C @ M for phenol is about 7 times that of MIL-88B-Fe, and C @ M has good catalytic performance in a wide pH range (4 ~ 9). They suggested that there may be electron transfer between carbon nanotubes and MIL-88 B-Fe, resulting in an increase in Fe (II) content. At the same time, the unsaturated coordination metal sites in MIL-88B-Fe behave as Lewis acid sites, which are more likely to adsorb H2O2 molecules (Lewis base), promoting the direct reaction of Fe (Ⅱ) with H2O2 to form · OH, and attacking pollutant molecules. In addition, the oxygen-rich functional groups on the carbon nanotubes transfer electrons to Fe (III), which promotes its reduction to Fe (II) and accelerates the Fe (III)/Fe (II) cycle, resulting in a relatively high concentration of Fe (Ⅱ) in C @ M (Fig. 11). Therefore, C @ M is more efficient in the degradation of organic pollutants.
In recent years, it has been shown that phosphate can improve the reaction activity of heterogeneous Fenton. Mu et al. Used phosphate ion to promote the degradation of 4-chlorophenol in Fenton system[48]. The results showed that phosphate (10.0 mmol·L-1) could promote the degradation of 4-chlorophenol, and the degradation efficiency of 4-chlorophenol in this system was 10 times higher than that in the conventional Fenton system, which may be due to the addition of phosphate to change the oxygen reduction pathway of Fe@Fe2O3 structure. The research group believes that when phosphate is not added, the oxygen reduction reaction on the surface of the Fe@Fe2O3 has four electron channels, and the O2 is directly converted into H2O, so that the generation rate of · OH in the air system of the Fe@Fe2O3/ is low; The addition of phosphate ion changes the oxygen reduction reaction on the surface of Fe@Fe2O3 into a continuous reduction channel with electrons, which promotes the reduction of molecular oxygen and the formation of · OH, thus greatly improving the degradation rate of 4-chlorophenol (Fig. 12).
The addition of co-catalyst in the heterogeneous Fenton system can promote the cycle of Fe (Ⅲ)/Fe (Ⅱ) and improve the activity of the reaction. However, most of the organic cocatalysts used at present have problems such as secondary pollution, poor stability and possible consumption of hydroxyl in the degradation process of organic compounds, while inorganic cocatalysts may have toxicity. In general, the cocatalyst needs further development in terms of catalyst reactivity, stability, and environmental impact.

3.4 Double reaction center catalytic system

In addition to Fe, other metals (such as Cu, Co, Mn, Ce, and Al) can also activate H2O2, persulfate, or peroxymonosulfate to produce · OH, and Cu-based catalysts have attracted increasing attention in heterogeneous Fenton-like reactions for their efficient H2O2 performance. In recent years, in order to improve the effective utilization of H2O2, researchers have devoted themselves to the study of double reaction centers (electron-poor centers and electron-rich centers). In a heterogeneous Fenton catalyst, all active atoms are no longer free metal ions, but are incorporated in the structure through connections with other atoms. Therefore, the electrons in their outer shell no longer belong to the metal itself, and the redistribution of these electrons may be an effective strategy to improve the activity of heterogeneous Fenton-like catalysts.
In recent years, Lyu et al. Have devoted themselves to the study of dual-reaction-center Fenton catalysts, showing that organic radical intermediates (· R) from pollutants can be used as electron donors by in situ construction of special bond bridges and metal-organic catalysts[49⇓⇓~52]. Lyu et Al. Synthesized a novel two-reaction center catalyst, d-TiCuAl-SiO2Ns, by doping Cu, Ti, and Al into dandelion-like nanosized SiO2 fiber spheres using a hydrothermal method[49]. In this Fenton-like reaction process, the pollutant and its radical · R adsorb in the anode electron-poor region near Al and Ti and provide electrons for them.Since Cu has a higher electronegativity, which prevents the nucleophilic H2O2 from binding to d-TiCuAl-SiO2Ns,Avoid the decomposition of H2O2 into HO2·/ O 2 - · and O2; However, H2O2 is adsorbed around the electron-rich Cu center in the cathode and reduced to · OH by electrons. Due to the special Fenton reaction process, almost all of the H2O2 is used to produce · OH to degrade pollutants, thus improving the utilization of H2O2 (Fig. 13).
In order to accelerate the reduction rate of M(n+m)+ to Mn+, the research group prepared a new type of Fenton catalyst (CN-Cu(Ⅱ)-CuAlO2) by surface complexation method to construct a more efficient and stable reaction center, as shown in Fig. 14. During the reaction process, the electron-rich center around Cu effectively reduces H2O2 to · OH, while the electron-poor center around C captures electrons from H2O2 or pollutants and transfers them to the electron-rich region through C — O — Cu bridge[50]. In addition, EPR analysis shows that the CN-Cu(Ⅱ)-CuAlO2 double reaction center has good electron recycling ability, and the O2 or H2O2 captures electrons from the electron-rich center, while the H2O or H2O2 donates electrons to the electron-poor center to compensate the electron of the double reaction center. Because of the particularity of the connection between Cu — O — C and the deprotonated hydroxyl group on C-g-C3N4, these donated electrons are quickly transferred from the electron-poor center to the electron-rich center. Xu et al. Also confirmed that the formation of dual reaction centers on Cu-Al2O3-g-C3N4 and Cu-Al2O3-C-dots by surface grafting of graphene π-domain could greatly improve the Fenton catalytic activity[53]. These studies demonstrate that the construction of specific metal – organic complexes on Fenton-like catalysts is the key to the efficient use of pollutant electrons and energy, and also to ensure the rapid electron transfer across the catalyst, pollutant, and H2O2 interfaces during the reaction.
The construction of double reaction centers breaks through the concept of classical Fenton reaction and realizes high selectivity of H2O2 and efficient degradation of pollutants. At present, the construction of double reaction centers is mainly Cu, and other metal systems can be studied in the future to find the best reaction system. In addition, metal-free dual-reaction center systems can also be explored to solve a series of problems caused by metals.

4 Heterogeneous Fenton reaction assisted by physical field

In the heterogeneous Fenton system, besides the improvement of the catalyst to provide electrons to reduce Fe (Ⅲ), external energy (such as light, electricity, ultrasound) can also be introduced to promote the redox cycle of Fe (Ⅲ)/Fe (Ⅱ).
The heterogeneous photo-Fenton reaction is powered by ultraviolet or visible light, which produces · OH through photoreduction of Fe (III) (Formula 39, 40) and photolysis of H2O2 (Formula 41), and is related to redox. In addition, because some iron-based heterogeneous Fenton catalysts are semiconductors, they will be excited to produce photogenerated electrons and holes under illumination, and the adsorbed pollutants will react with H2O2 to produce · OH (Formula 42 ~ 44)[54]. For example, Deng et al. Used TiO2/Fe2TiO5/Fe2O3 to degrade organic matter under visible light irradiation, the degradation rate of methyl orange (MO) and phenol could reach 87% within 10 min and 100% within 60 min, and the reaction activity remained basically unchanged after 10 cycles[55]. This is mainly due to the migration of photoexcited charges between different components (Fig. 15), MO molecules absorb visible light to produce electrons, and the electrons migrate from MO to Fe2O3step by step. At the same time, the band gap of Fe2TiO5 and Fe2O3 is very narrow, which can absorb visible light and excite electrons and holes. The photoexcited electrons gather on the surface of Fe2O3 and holes gather on the surface of Fe2TiO5. Phenol cannot absorb electrons generated by visible light. Therefore, most of the photoelectrons come from Fe2TiO2 and Fe2O3, as shown in Fig. 15. Under visible light, electrons transfer from the Fe2TiO5 to the Fe2O3, while holes transfer on the Fe2TiO5, and the efficient separation of charges increases the lifetime of electrons.The formation of Fe2+,TiO2/Fe2TiO5/Fe2O3 on the surface of the catalyst by Fe3+ is improved, and the improvement of the efficiency of the Fe3+/Fe2+ cycle reaction on the surface of the catalyst is the reason for the efficient degradation of phenol by photo-Fenton.
F e 3 + + H 2 O → F e ( O H ) 2 + + H +
F e ( O H ) 2 + + h v → F e 2 + + · O H ( λ < 580 n m )
H 2 O 2 + h v → 2 O H ( λ < 310 n m )
I r o n - b a s e d c a t a l y s t s + h v → e - + h +
h + + R H → R · + H +
e - + H 2 O 2 → · O H + O H -
Electro-Fenton generates Fe2+ (or) and H2O2 in situ by electrochemical reaction under the action of Fenton catalyst, and then generates · OH. There are two sources of H2O2 in electro-Fenton: (1) from dissolved oxygen (O2(g) generated by 2-electron redox reaction at the appropriate cathode interface (Equation 45); (2) from direct addition outside the system. There are three sources of Fe2+: (1) direct addition of ferrous salt from outside of the system; (2) the reduction of the Fe3+ catalyst added outside the system on the cathode (formula 46), and then the self-circulation of the Fenton reaction is realized; And (3) carrying out oxidation reaction by using the sacrificial iron anode to continuously release iron ions into the system[56]. In addition, as electro-Fenton proceeds, the H2O is also electrolyzed and produces O2,O2 at the anode that diffuses in the solution to the cathode to produce H2O2 (Equation 47), producing more Fenton reagent to increase the concentration of · OH. Mahmou et al. Degraded benzene, toluene and p-xylene in aqueous solution by electro-Fenton method[57]. The results showed that the TOC removal rate of the system was more than 91%, and the Fe (Ⅲ)/Fe (Ⅱ) cycle was promoted by the continuous release of iron ions into the system through the sacrificial anode.
O 2 + 2 H + + e - → H 2 O 2
F e 3 + + e - → F e 2 +
H 2 O → 1 / 2 O 2 + H + + e -
Ultrasound refers to the sound wave with a frequency higher than 20 kHz, which has been well used in many fields. For example, the combination of ultrasound technology and Fenton technology can improve the efficiency of Fenton reaction. The improvement of wastewater degradation efficiency by ultrasonic Fenton is mainly manifested in the following aspects: (1) The propagation of ultrasonic waves makes water molecules vibrate violently, accelerates the collision between molecules, and breaks the chemical bonds of pollutants[58,59]; (2) free radicals are generated by ultrasonic wave through cavitation; (3) Ultrasound and Fe2+ act together to accelerate the decomposition of H2O2 and produce more · OH; (4) Ultrasound can accelerate the mass transfer in the solution, promote the effective contact between free radicals and pollutants, and accelerate the reaction. Li et al. Used ultrasound to enhance the degradation of bisphenol A (BPA) by Fenton reaction, and found that the addition of ultrasound could promote the degradation of BPA (the removal rate was up to 98%)[59]. Its promoting effect is mainly manifested by the synergistic activation of H2O2 by cavitation and catalyst to produce free radicals. Reduce the activation energy of the reaction; Clean the catalyst surface. Jannatul et al. Also confirmed that ultrasound can promote the Fenton reaction[60]. On the one hand, ultrasound can promote the production of · OH and improve the removal of TOC; on the other hand, through cavitation, ultrasound can destroy the cell wall and release intracellular organic substances, so as to increase the destruction rate of extracellular polymeric substances, thereby improving the dehydration rate of sludge and the extraction rate of metal ions.
In order to further improve the utilization rate of the catalyst, in recent years, some researchers have combined two physical fields with Fenton reaction, such as photoelectric Fenton, microwave electro-Fenton and microwave photo-Fenton. Wang et al. Found that compared with the electro-Fenton system, the electro-Fenton system composed of Fe2+/H2O2, UV/H2O2 and UV degraded sulfamethazine faster and achieved effective mineralization at low current, which may be related to the high utilization rate of H2O2, the rapid regeneration of Fe2+ and the photolysis of intermediates[61].
The introduction of additional energy into the heterogeneous Fenton system can improve the efficiency of Fe (Ⅲ)/Fe (Ⅱ) cycle and enhance the reaction activity. At present, the heterogeneous Fenton reaction assisted by physical field has been widely used, but there are still some problems such as low catalyst efficiency and high energy consumption. Further research can be done to improve the efficiency of the catalyst in the future. In addition, we can also explore how to combine natural light with Fenton system to reduce energy consumption.

5 Conclusion and prospect

Heterogeneous Fenton reaction stands out among the advanced oxidation technologies because of its wide pH response range, high oxidation efficiency, good catalyst stability and reusability[15]. In this paper, the addition of zero-valence Fe, metal doping and co-catalyst to the heterogeneous Fenton catalyst can accelerate the formation of Fe (Ⅱ) and promote the decomposition of H2O2 by electron transfer and increasing active sites, and can broaden the pH of the reaction and reduce the formation of iron sludge. The construction of double reaction centers is mainly through the construction of special bond bridges, the establishment of rich and poor electron centers, the effective use of pollutant electrons and energy, thereby improving the effective utilization of H2O2 and the degradation rate of pollutants. In the Fenton system, light, electricity, ultrasonic wave and the like are combined to introduce extra energy which is used as an electron donor to promote the transfer of electrons and can generate Fe (II) and (or) H2O2 in situ with an iron-based catalyst, thereby improving the effective utilization rate of the H2O2 and promoting the Fe (III)/Fe (II) cycle. At present, the heterogeneous Fenton reaction is still in the laboratory research stage, and there is still much to be improved in order to make the heterogeneous Fenton catalytic technology truly applied and promoted. In the future, efforts can be made in the following aspects.
(1) Because there is a solid-liquid interface between the heterogeneous Fenton reaction catalyst and the solution, and the reaction includes a liquid phase reaction and an interface reaction, the heterogeneous Fenton reaction mechanism needs to consider not only the chemical reaction between · OH and organic pollutants, but also the physical and chemical reaction between · OH and pollutants on the surface of the heterogeneous catalyst. Although most researchers believe that the mechanism of heterogeneous Fenton reaction is mainly free radical mechanism, there is no consensus on the mechanism and kinetics of its degradation of organic wastewater.
(2) It is difficult to achieve multiple cycles due to the limited available electrons, whether it is to add zero-valence metal, doped metal, co-catalyst or to construct double reaction centers in the catalyst. In addition, the leaching of metal ions may occur when doping metals and constructing metal double reaction centers, so finding catalysts with strong activity, good stability and recyclability will become an important research direction of heterogeneous Fenton reaction. In addition, at present, the construction of double reaction centers is mainly Cu, so we can further study the double reaction center catalyst of other metal systems (such as Mn, Co, Ce, etc.) or metal-free double reaction center catalyst to explore the best heterogeneous Fenton reaction system.
(3) Heterogeneous Fenton reaction has not been applied in practical wastewater treatment due to the lack of economic evaluation, although it has been systematically studied in the laboratory. Therefore, the synergistic system of heterogeneous Fenton reaction process combined with other mature wastewater treatment technologies (such as adsorption, electrolysis, biofilter and other purification processes) will be a hot research direction in the future.

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Funding

National Natural Science Foundation of China(21876145)

Support Program for One Hundred Outstanding Innovative Talents in Higher Education Institutions in Hebei Province(SLRC2019041)

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