Synergistically Consecutive Reduction and Oxidation of Per- and Poly-Halogenated Organic Pollutants

Nan Wang; Yuqi Zhou; Ziye Jiang; Tianyu Lv; Jin Lin; Zhou Song; Lihua Zhu

doi:10.7536/PC220403

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (12) : 2667-2685 DOI: 10.7536/PC220403

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Synergistically Consecutive Reduction and Oxidation of Per- and Poly-Halogenated Organic Pollutants

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Abstract

Most of per- and poly-halogenated organic pollutants (PHOPs)possess biodiversity and bioaccumulation, long persistency and global transport throughout the environment and pose adverse effects on humans. Due to the strong electron-withdrawing ability and a large number of halogen atoms, PHOPs usually possess low positive position for highest occupied molecular orbital level and consequently are resistant to the oxidative degradation. Reductive technologies can efficiently degrade PHOPs, but they suffer from a problem of the accumulation of highly toxic less halogenated products, because the lower-halogenated products tend to be more difficult to reduce. In contrast, these lowly-halogenated products can be easily oxidized. Thus, the consecutive reduction and oxidation method has been developed to degrade PHOPs, in which the pre-reduction of PHOPs and the consecutive oxidation of dehalogenated intermediates are combined to realize the complete dehalogation and mineralization. Herein, this review summarizes the latest efforts to develop consecutive reduction and oxidation processes that are categorized as five types: catalysis, electrochemistry, photocatalysis, photoelectrochemistry and mechanochemistry. Particular attention is focused on the chemical design principles of consecutive reduction and oxidation processes that can help develop treatment technologies to efficiently eliminate PHOPs.

Contents

1 Introduction

2 Reductive or oxidative degradation of PHOPs

2.1 Perfluorinated compounds

2.2 Chlorophenols

2.3 Polybrominated diphenyl ethers

3 Consecutive Reductive and Oxidative degradation of PHOPs

3.1 Traditional chemical reduction and Fenton like oxidation

3.2 Photochemical reduction and oxidation

3.3 Electrochemical reduction and oxidation

3.4 Photo-electrocatalytic reduction and oxidation

3.5 Mechanochemical reduction and oxidation

3.6 Hydrated electron-mediated reduction and persulfate-based catalytic oxidation

4 Conclusion and outlook

Graphical abstract

Keywords

halogenated organic pollutants / degradation / reductive dehalogenation / oxidation / reactive oxygen radical

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Nan Wang, Yuqi Zhou, Ziye Jiang, Tianyu Lv, Jin Lin, Zhou Song, Lihua Zhu. Synergistically Consecutive Reduction and Oxidation of Per- and Poly-Halogenated Organic Pollutants. Progress in Chemistry, 2022, 34(12): 2667-2685 DOI:10.7536/PC220403

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1 Introduction

Per- and polyhalogenated organic pollutants (PHOPs) refer to organic pollutants in which all or most of the hydrogens in the molecule are replaced by halogens, such as perfluorooctanoic acid/sulfonic acid (PFOA/PFOS), polychlorophenols, and polybrominated diphenyl ethers. Most of these pollutants have the characteristics of persistent organic pollutants, namely, high toxicity, environmental persistence, bioaccumulation, semi-volatility and long-distance transport^[1,2]. Because of its wide application in industrial and agricultural production, these substances are frequently detected in the global atmosphere, water, soil sediments and other environments, as well as human blood and body fluids^[3⇓⇓~6].

Because of their structural stability, PHOPs are difficult to be degraded by traditional biological and chemical methods. Advanced oxidation/reduction (AOR) technology combines oxidant/reductant with activation technology to degrade pollutants by producing highly active oxidative/reductive species (such as hydroxyl radicals (· OH), hydrogen atoms (H ·), etc.), which is more efficient than traditional chemical methods and has attracted wide attention in the degradation of PHOPs.

According to the types of degradation reaction, chemical methods can be divided into reduction method and oxidation method, both of which involve two main reaction processes: the production of oxidation/reduction active species and their reaction with pollutants. Different species of redox active species have different redox ability and reaction characteristics. The oxidative/reductive degradation rates of PHOPs vary with their molecular structures. In order to construct an efficient degradation method, it is necessary to understand the structural characteristics of pollutants and the characteristics of each reactive species, and select an appropriate reduction/oxidation degradation system, so as to enhance the dehalogenation efficiency by strengthening the production of reactive species and weakening the key structural units or chemical bonds of PHOPs. In the degradation process, with the change of the number of halogen atoms, the parent pollutants and their dehalogenated degradation products often show different or even opposite oxidation/reduction characteristics, which makes it difficult for them to be efficiently and completely dehalogenated and mineralized by a single reduction/oxidation method. Noting that "high-halogenated organic compounds are easy to reduce and low-halogenated organic compounds are easy to oxidize", some scholars have constructed a reduction-oxidation synergistic degradation system, which significantly improves the deep dehalogenation and mineralization of PHOPs. According to the structural characteristics of pollutants, this paper first introduces the current situation of single oxidation/reduction degradation of PHOPs and the enhanced dehalogenation strategy. Then, according to the types of chemical catalytic reactions, the reduction-oxidation synergistic degradation system was introduced in order to provide ideas and suggestions for the development of efficient disposal technology of PHOPs.

2 Degradation of PHOPs by single reduction/oxidation method

Different species of oxidation/reduction species have different oxidation/reduction ability and reaction characteristics. As shown in Table 1, · OH and sulfate radical ($\mathrm{SO}_{4}^{\cdot-}$) have strong oxidizing ability, while

e a q -

has the strongest reducing ability, and they are all able to degrade most of the PHOPs, but the degradation reaction rate varies with the structure of PHOPs. According to the different halogens, PHOPs are divided into three types: fluoro, chloro and bromo. Among them, fluorinated PHOPs mainly include PFOA, PFOS and their substitutes (such as perfluorohexyl sulfonic acid (PFHxS)), all of which are aliphatic compounds; Chlorinated and brominated PHOPs mainly include polychlorinated phenols (CPs) and polybrominated diphenyl ethers (PBDEs), most of which contain benzene ring structure and belong to aromatic compounds. In this section, perfluorinated compounds, chlorophenols and polybrominated diphenyl ethers are taken as typical examples to discuss the research status of PHOPs degradation.

2.1 Perfluorinated compound

PFOA and PFOS have been added to the list of persistent organic pollutants in 2009 and 2015, respectively. These two pollutants are perfluorinated saturated carboxylic/sulfonic acids, which lack suitable sites for electrophilic addition or hydrogen abstraction reactions, and C-F bonds are difficult to be broken by oxidation and reduction (reduction and oxidation potentials are 3.6 V and -2.6 V vs. NHE, respectively).Therefore, the advanced oxidation methods (such as traditional Fenton system and TiO₂ photocatalytic oxidation system) and zero-valent iron reduction method, which are mainly attacked by · OH, can not effectively degrade it. At present, the degradation methods studied include Fenton-like system assisted by external energy/enhanced by external field, persulfate catalytic oxidation, photogenerated hole mediated photocatalytic oxidation and hydrated electron reduction.

(1) Fenton-like oxidation enhanced by external field

The introduction of external energy assistance in Fenton and Fenton-like systems, such as ultraviolet irradiation (UV), ultrasound, microwave and bias electric field, is not only to increase the production of active oxygen species such as · OH, but also to promote the decarboxylation of PFOA and initiate its degradation reaction. Although ·OH/H₂O(OH^-) has a high redox potential (1.9 – 2.7 V), its three pathways of addition, hydrogen abstraction, or electron abstraction to attack PFOA are thermodynamically unfavorable reactions. Compared with the C — F bond, the terminal carboxyl/sulfonic acid group of PFOA has a lower dissociation energy, and can undergo pyrolytic decarboxylation under the action of ultrasonic cavitation bubbles or microwave. In addition, the oxidative decarboxylation of PFOA can be achieved by both ligand-to-metal (i.e., PFOA carboxyl -Fe³⁺) electron transfer initiated by UV illumination and electron-transfer reaction driven by electric field (i.e., PFOA carboxyl-electrode). The decarboxylation intermediates ^-C₇F₁₅ or ^·C₇F₁₅ are unstable and can undergo defluorination degradation by · OH, O₂, and/or H₂O, etc. Tang et al. Reported that the defluorination rate of PFOA was 53% in UV/Fe²⁺/H₂O₂ system for 5 H^[10]. Liu et al. Degraded PFOA by an electro-Fenton system composed of H₂O₂ generated by electrochemical reduction of dissolved oxygen with a polymer porous carbon cathode and Fe²⁺. The mineralization rate was 90% at a voltage of -0.4 V and pH = 2 for 4 H. The degradation process included oxidative decarboxylation of Pt anode and attack of · OH generated by electro-Fenton^[11]. Li et al. Studied the degradation of PFOA by microwave-enhanced heterogeneous Fenton-like system using the composite material Pb-BiFeO₃/rGO composed of Pb-BiFeO₃ and reduced graphene oxide (rGO) as catalyst, and the mineralization rate of PFOA reached more than 90% after 5 min of reaction^[12].

(2) persulfate catalytic oxidation method

Persulfates, including permonosulfate (PMS) and perdisulfate (PDS), can be activated to decompose into $\mathrm{SO}_{4}^{\cdot-}$, · OH, and/or ¹O₂. Among them, the redox potential of $\mathrm{SO}_{4}^{\cdot-}$/$\mathrm{SO}_{4}^{2-}$(2.3 V) is higher than that of ·OH/OH^-(1.9 V), and $\mathrm{SO}_{4}^{\cdot-}$ can attack the carboxyl group of PFOA by electron abstraction reaction to initiate oxidative decarboxylation, thus achieving effective degradation of PFOA. PDS activation produces $\mathrm{SO}_{4}^{\cdot-}$ more efficiently than PMS. Liu et al. Used heat-activated PDS to degrade PFOA(200 ng·L^-1). After 30 H of reaction, the degradation rate and defluorination rate of PFOA were 93.5% and 43.6%, respectively, and the degraded products were 3 – 6 carbon short-chain perfluorocarboxylic acids^[13]. Song et al. Used UV and Fe²⁺ to activate PDS to produce $\mathrm{SO}_{4}^{\cdot-}$ to degrade PFOA, and the defluorination rate was 53% after 5 H of reaction^[14]. Based on the product analysis, a degradation pathway was proposed: PFOA undergoes terminal decarboxylation (Kolbe decarboxylation process) under the attack of $\mathrm{SO}_{4}^{\cdot-}$, followed by hydrolysis and elimination reactions to lose CF₂ units and produce short-chain perfluorocarboxylic acids. Guo et al. Used porous carbon felt as a working cathode to activate sodium persulfate by electrochemical method. The initial solution pH was 3.29, and the cathode potential was controlled at -1.8 V. After potentiostatic electrolysis for 24 H, the mineralization rate of PFOA was about 62.5%. The possible degradation pathways included two Kolbe decarboxylation processes mediated by :$\mathrm{SO}_{4}^{\cdot-}$ and the gradual hydrogenation reduction of carboxylic acid on the cathode^[15]. Wang et al. Used mechanical force effect to activate PDS to degrade PFOA solid powder. After 2 H of reaction, the defluorination rate was less than 25%, and a large number of polymerization products of PFOA decarboxylation intermediates were detected^[16]. However, when γ-Al₂O₃ was introduced as a co-grinding agent, the defluorination rate and mineralization rate of PFOA were 100% and 98%, respectively, which were significantly higher than those of single γ-Al₂O₃ or PDS grinding agent system (less than 25%) under the same reaction conditions. Infrared spectroscopy, electron paramagnetic spectroscopy and ¹⁸O isotope labeling techniques were used.The mechanism of synergistic oxidative degradation of Al₂O₃ and PDS was clarified (Fig. 1): PFOA was adsorbed on Al³⁺ of Al₂O₃ surface through carboxyl monodentate coordination, while PDS was adsorbed on OH^- of Al₂O₃ surface through hydrogen bonding; On the one hand, the mechanical effect triggers the decarboxylation of PFOA to produce ^-C₇F₁₅, on the other hand, it promotes the decomposition of PDS to produce $\mathrm{SO}_{4}^{\cdot-}$,$\mathrm{SO}_{4}^{\cdot-}$, which oxidizes OH^- on the surface of Al₂O₃ to · OH. The decarboxylation intermediate ^-C₇F₁₅ abstracts a proton from water to form C₇F₁₅H, which is hydrogen-abstracted by · OH to form ^·C₇F₁₅, followed by the addition of · OH to ^·C₇F₁₅ to form an unstable fluoroalcohol, which is then converted to a short-chain perfluorocarboxylic acid with one less carbon via HF elimination and hydrolytic HF removal. The above results indicate that the organic combination of Brønsted base activation of terminal carboxyl group, $\mathrm{SO}_{4}^{\cdot-}$ oxidative decarboxylation and · OH coupling reaction can achieve efficient defluorination degradation of PFOA.

(3) Photo-generated hole oxidation of semiconductor

In the semiconductor photocatalytic oxidation method, the photogenerated hole (

h V B +

) can directly oxidize the electron donor adsorbed on the surface of the catalyst through a single electron transfer, and its oxidation ability depends on the valence band position of the semiconductor. It was found that some photocatalysts with higher valence band positions (such as TiO₂, In₂O₃, BiOCl, etc.) could degrade PFOA through the

h V B +

– mediated oxidation pathway^{[17⇓⇓~20]}. In order to inhibit the oxidation of OH^- into · OH by

h V B +

, Panchangam et al. Used TiO₂ photocatalytic oxidation to degrade PFOA under strong acidic conditions (pH < 3), and the degradation rate, defluorination rate and mineralization rate were 99%, 48% and 38%, respectively, after 7 H of reaction^[21]. Li et al. Used In₂O₃ to photocatalytically degrade PFOA, and found that the degradation rate was 8.4 times faster than that of TiO₂. The study found that PFOA was adsorbed on the surface of In₂O₃ in a bidentate coordination form, which was beneficial to the direct oxidation of

h V B +

^[22]. Song et al. Used oxygen vacancy-rich BiOCl to degrade PFOA by photocatalytic oxidation, and the defluorination rate and mineralization rate were 59.3% and 52.5%, respectively, and the defluorination rate was 16.53μmol·L^-1·h^-1, which was 1.7 and 14.6 times that of In₂O₃ and TiO₂, respectively^[23]. It was noticed that the defluorination rate of PFOA was positively correlated with the concentration of oxygen vacancies on the surface of BiOCl. Song et al. Regulated the concentration of oxygen vacancies on the surface of BiOCl by controlling the hydrolysis rate of bismuth salt, which further improved the defluorination rate of PFOA^[24]. Wang et al. Improved the photocatalytic activity of BiOF for PFOA degradation by adjusting the concentration of oxygen vacancies^[25]. In addition, some scholars have introduced persulfate into the photocatalytic oxidation system to degrade PFOA by co-oxidation of

h V B +

and $\mathrm{SO}_{4}^{\cdot-}$^[26].

(4) electrochemical oxidation method

According to the working principle, electrochemical oxidation methods are divided into two categories: direct oxidation and indirect oxidation. Direct oxidation means that when the applied anode potential is higher than the potential at which the organic matter is oxidized, the organic matter adsorbed on the electrode surface directly transfers electrons to the anode, resulting in oxidative degradation. Indirect oxidation refers to the degradation of organic matter by strong oxidizing active species produced by the oxidation of some media at the anode or the reduction of some media at the cathode, which is also called anode or cathode indirect oxidation. Among them, the cathodic indirect oxidation method includes the electro-Fenton method initiated by the H₂O₂ or Fe²⁺ generated at the cathode, and the electro-activated persulfate method in which the persulfate is reduced to $\mathrm{SO}_{4}^{\cdot-}$ at the cathode, which has been described in 2.1 (1) and (2) above. Therefore, only the anodizing method is described below.

Anodic direct oxidation induces decarboxylation or desulfonation by abstracting electrons from PFOA or PFOS, and then reacts with O₂ and/or H₂O, but the subsequent oxidative defluorination is difficult, so there are few reports on electrochemical direct oxidation. The anodic indirect oxidation method mainly uses the anode to oxidize the H₂O adsorbed on its surface to · OH. Although · OH can not directly degrade perfluorinated compounds, the energy barrier for · OH to attack the decarboxylation or desulfonation intermediates is small, which can quickly initiate the degradation and defluorination of decarboxylation or desulfonation intermediates. Therefore, the degradation of perfluorinated compounds is mainly due to anodic direct oxidative decarboxylation/desulfonation and · OH indirect attack oxidation, both of which are affected by the anode material. Niu Junfeng et al. Reviewed the degradation of perfluorinated compounds by various anode materials, and pointed out that for electrodes with low oxygen evolution potential, such as Ti/SnO₂-Sb/MnO₂, Pt and Ti/Ru-IrO₂, the oxygen in the adsorbed · OH will enter the lattice of the metal oxide of the electrode material or undergo oxygen evolution reaction on the electrode surface, which reduces the utilization of · OH, thus the degradation efficiency of PFOA is low or even ineffective^[27]^[28]^[29]. However, SnO₂, PbO₂ and boron-doped diamond film (BDD) have high oxygen evolution potential, and · OH does not react with the electrode materials. The "inactive electrodes" based on these materials, such as Ti/SnO₂-Sb, Ti/SnO₂-Sb/PbO₂, Ti/SnO₂-Sb/PbO₂-Ce and Ti/BDD, can mineralize PFOA by about 90%, and the defluorination rate is about 30% ~ 90%^[28]^[30,31]^[32,33]^[34]. Although these "inactive electrode" electrochemical oxidation methods can effectively degrade perfluorinated compounds, complete defluorination has not yet been achieved. In addition, these anode materials are expensive to produce and have a short service life.

(5) hydrated electron reduction method

Due to the extremely low reduction potential (-2.6 V) of saturated fluorinated hydrocarbons, the common zero-valent metal reduction method can not reduce and degrade perfluorinated compounds at normal temperature and pressure. Only by introducing high temperature and other conditions to overcome the reaction energy barrier, can the degradation of PFOA be achieved. For example, Hori et Al. Used zero-valent metals to degrade perfluorinated compounds in subcritical water, and the results showed that the order of metal reduction effect was: no metal = Al < Cu < Zn < < Fe^[35]; When zero-valent iron was used as reducing agent, the degradation rate and defluorination rate of PFOS were 98% and 46. 2% ~ 51.4%, respectively, at 350 ℃ for 6 H. Compared with zero-valent iron,

e a q -

has a strong reducing ability, and its standard redox potential (ϕ⁰) is as low as − 2.9 V, which can effectively degrade perfluorinated compounds. The methods of generating

e a q -

include ultraviolet (UV)/KI, UV/SO₃, UV/K₄Fe(CN)₆, and high energy electron beam^[36]^{2- [37]}^[38]^[39]. Under acidic conditions, H⁺ will consume

e a q -

and reduce the utilization rate of

e a q -

, so the

e a q -

system is mostly carried out under alkaline conditions. Song et al. Established the method of reductive degradation of PFOA by hydrated electrons generated from UV/S₃^2-, and the defluorination rate of PFOA was 88.5% at 24 H under N₂ atmosphere^[40]. Qu et al. Used a UV/KI system to produce

e a q -

for reductive degradation of PFOA, and the defluorination rate was 98% at room temperature, pH = 9, anaerobic and 254 nm irradiation for 14 H^[41]. The reaction rates of perfluorocarboxylates containing 2 ~ 10 carbon atoms C_nF_2n+1COO^-(n=2~10) and

e a q -

are similar and slower than that of perfluoroacetate CF₃COO^-. However, the reaction rate of its reduction product, polyfluoroalkyl carboxylate C_nF_2n+1CH₂CH₂COO^-(n=1~8), with

e a q -

becomes slower as the carbon chain becomes shorter. Compared with PFOA, PFOS has one more CF₂ group and a sulfonic group at the end, which is more stable than PFOA. Similar to polyfluoroalkyl carboxylates, the rate of reductive degradation of perfluorosulfonate C_nF_2n+1S₃^-(n=1~8) by

e a q -

is related to the length of the carbon chain, and the rate of defluorination decreases with the decrease of carbon number^[42,43]. Therefore, it is difficult for the

e a q -

reduction system to completely defluorinate and degrade PFOS. In addition, the

e a q -

reduction system needs to be used in the absence of oxygen, strong alkali and high-energy electron beam/vacuum UV irradiation, which consumes high energy and is rarely used in the treatment of actual wastewater.

2.2 Polychlorophenol

The basic structure of chlorophenols is that chlorine atoms partially or completely replace the hydrogen atoms on the phenol ring. According to the number and position of chlorine substitution, there are 19 kinds of chlorophenols. With the increase of the number of chlorine substitution, the lipophilicity of chlorophenols increases, which makes it easier to accumulate in organisms, and the toxicity increases gradually. Among them, pentachlorophenol (PCP) has been included in the list of persistent organic pollutants, and other chlorophenols have been listed as priority toxic pollutants by the Ministry of Ecological and Environmental Protection of China. Compared with the C-F bond of aliphatic perfluorinated compounds, the C-Cl bond on the aromatic ring has a lower bond energy and is more susceptible to reductive/oxidative dehalogenation. Chlorophenols can be degraded by common methods such as zero-valent metal reduction, photocatalytic reduction or oxidation, electrochemical reduction or oxidation, Fenton-like oxidation and persulfate catalytic oxidation.

The principle of reductive degradation of chlorophenols and the enhanced dechlorination strategy are similar to the reductive degradation of PBDEs in Section 2.3 below, so they are not described in detail here. The reductive degradation rate of chlorophenols becomes slower as the number of chlorine substitutions becomes smaller. However, even monochlorophenol can still be degraded by the usual zero-valent iron reduction method and TiO₂ photocatalytic reduction method. In addition, the reduction rate of chlorophenols is also related to the chlorine substitution position, for example, the order of the reduction rate of chlorophenols by zero-valent iron is 2-chlorophenol (2-CP) > 3-chlorophenol (3-CP) > 4-chlorphenol (4-CP). The above results show that chlorophenols can be finally reduced to phenol by reduction, but phenol still has ecological toxicity and needs to be further treated by other methods.

Among AOPs, · OH and/or $\mathrm{SO}_{4}^{\cdot-}$ based oxidation systems are more effective in degrading chlorophenols than non-radical systems such as singlet oxygen (¹O₂). · OH attacks the unsaturated bond on the benzene ring or removes the hydrogen atom on the benzene ring mainly through electrophilic addition; While $\mathrm{SO}_{4}^{\cdot-}$ mainly abstracts electrons from phenoxide ions in chlorophenols. Lim et al. Supported Co²⁺ on Black TiO₂ nanotubes, and the catalyst Co-Black TNT prepared activated PMS to degrade 4-chlorophenol.The pseudo-first order reaction rate constant is 1.6 ~ 2.5 times higher than that of Co₃O₄/rGO, CoFe₂O₄, Co₃O₄ and Co-TiO₂ catalytic systems^[44]. They believed that the reason for the high activity of Co-Black TNT was that the oxygen vacancies in black TiO₂ nanotubes not only enhanced the adsorption of PMS, but also promoted the Co³⁺/Co²⁺ cycle and accelerated the generation of $\mathrm{SO}_{4}^{\cdot-}$. Fukushima et al. Used Fe³⁺/H₂O₂ to degrade PCP under xenon lamp irradiation (λ > 370 nm), and the degradation rate of PCP was about 40% after 6 H of reaction^[45]. Tetrachlorocatechol, tetrachlorohydroquinone, octachlorodibenzodioxin and 8-hydroxyoctachlorodiphenyl ether were detected as degradation products. Tetrachlorinated and/or trichlorinated quinones, phenols and/or diphenols were also detected in the degradation of PCP by TiO₂ photocatalytic oxidation and microwave-assisted TiO₂ photocatalytic oxidation. Vallejo et al. Used BDD electrode to degrade 2-CP by electrochemical oxidation, and found that the total amount of polychlorinated dibenzodioxins/furans produced in NaCl electrolyte was 136 times that in Na₂SO₄ electrolyte, because :Cl^- was oxidized to Cl · at the anode, and then attacked dibenzodioxins and dibenzofurans, which were the degradation products of chlorophenols^[46]. It can be seen that it is difficult to completely mineralize and dechlorinate chlorophenols by advanced oxidation process, and the degradation products produced are even more toxic. Shen et al. Used 2,4-dinitrophenol, which has a similar structure to PCP, as a pseudo-template molecule, and o-phenylenediamine as a functional monomer and crosslinking agent to modify the molecular imprinting layer on the surface of TiO₂.Artificial antibody nanoscale TiO₂ photocatalyst was prepared, which could not only selectively and rapidly degrade PCP, but also inhibit the accumulation of degradation products and achieve complete dechlorination in the coexistence system of PCP and bisphenol A^[47]. However, this kind of catalyst has the problems of difficulty in large-scale preparation and poor long-term stability.

2.3 Polybrominated diphenyl ether

Polybrominated diphenyl ethers (PBDEs) are a class of compounds derived from the substitution of bromine atoms for hydrogen atoms in the aromatic ring of diphenyl ethers. Due to the number and position of substitutions, 209 congeners can be derived. Deca-, Octa- and PentaBDEs in its family have been included in the list of POPs. Polybrominated diphenyl ethers (PBDEs) have strong lipophilicity and hydrophobicity. If organic solvents or surfactants are used for solubilization, the organic components will compete for the consumption of oxidative active species and reduce the oxidation efficiency. Therefore, there are few reports on the degradation of PBDEs in liquid phase by oxidation. Huang et al. Found that the

h V B +

and · OH produced by the photocatalysis of nano-sized TiO₂ could oxidatively degrade decabromodiphenyl ether (BDE209) adsorbed on the surface of the catalyst in aqueous phase, but if organic solvents such as acetonitrile were used, or other small organic molecules that were more likely to capture

h V B +

and · OH existed in the system, the oxidative degradation of BDE209 would be severely inhibited^[48]. Considering that the mechanochemical method can directly treat solid pollutants, Huang et al. Used PDS as grinding agent to establish a mechanochemical oxidation method to degrade BDE209. The degradation rate and mineralization rate of BDE209 were 99. 6% and 99. 0% after 3 H of reaction^[49]. Although BDE209 can be degraded by · OH, $\mathrm{SO}_{4}^{\cdot-}$, etc., reductive debromination has a higher degradation efficiency, so chemical reduction methods, such as zero-valent metal reduction, photocatalytic reduction and electrochemical reduction, are still widely reported.

(1) Zero-valent metal reduction method

Zero-valent iron (Fe⁰) is widely used in the zero-valent metal reduction method^[50]. Keum et al. First used micron-sized Fe⁰ to treat PBDEs. After 40 days of reaction, 92% of BDE209 was degraded, but a large number of 3- to 6-BDE degradation products were accumulated^[51]. Three reductants, :Fe⁰, Fe²⁺ and H₂, may exist in the Fe⁰ system, among which, Fe⁰ has stronger reduction ability. This system mainly follows the electron transfer reductive dehalogenation mechanism due to the low activity of Fe⁰ to decompose H₂. In order to improve the efficiency of dehalogenation of Fe⁰, researchers have focused on the following aspects: (1) preparation of nano-or sub-nano-scale Fe⁰ to increase its specific surface area, that is, reaction sites^[52,53]; (2) surface modification of hydrophobic carbon materials or surfactants to inhibit the agglomeration of Fe⁰ and enhance the adsorption of polybrominated diphenyl ethers^[54,55]; (3) constructing a bimetallic system, such as Fe/Ag, Fe/Pd, Fe/Cu, Fe/Au, Fe-Ni, Fe-Pt and the like, strengthening the hydrogenation reaction and inhibiting the passivation of the surface of the Fe⁰^[56,57]; And (4) introducing microwave, ultrasound or high temperature to enhance mass transfer^[58]. Yu et al. Used montmorillonite as a template to prepare Fe⁰ with a particle size of about 0.5 nm, and degraded BDE209(2.5 mg·L^-1) in water/tetrahydrofuran (1 ∶ 1) mixed solvent for 90 min, BDE209 could be completely removed, while in the nanoscale Fe⁰ system, the degradation rate was only 25%^[52]. Compared with nano Fe⁰, the apparent reaction rate of nano bimetallic reduction dehalogenation can be increased by 1 – 2 orders of magnitude, and the synergistic effect of Pd and Ni is stronger, while that of Ag and Au is relatively low^[58]. In addition, Table 2 summarizes the active species for the degradation of PBDEs by typical zero-valent metal reduction systems. Fe/Pd, Fe/Ni and Fe/Pt systems are active H transfer mechanisms, Fe/Ag is electron transfer mechanism, while Fe/Cu and Fe/Au include both active H transfer and electron transfer mechanisms^[50,59].

Active hydrogen attack debrominates PBDEs to a greater extent than electron transfer pathways. In the process of active hydrogen transfer, first, hydrogen donors (such as water, H⁺, etc.) adsorb on the surface of the catalyst and get electrons to generate active hydrogen atoms (H^·); Next, H^· attacks the PHOPs adsorbed on the surface of the catalyst, initiating reductive dehalogenation. Compared with transition metals, noble metals have higher chemisorption and activation ability for hydrogen, lower hydrogen evolution potential, and can produce active hydrogen under mild conditions. If the hydrodehalogenation reaction rate is slower than the desorption rate of active hydrogen, the hydrogen evolution reaction will occur, reducing the dehalogenation efficiency^[26]. Taken together, Pd is considered to be the most effective hydrodehalogenation catalyst. From the point of view of the structure of pollutants, the reaction rate of active hydrogen attacking PBDEs is mainly affected by the space/three-dimensional environment around the bromine atom, and has little to do with the number of bromine atom substitution. The reaction is slow if the steric hindrance is large. Therefore, in the active hydrogen reduction debromination system, complete debromination degradation can mostly be achieved by optimizing the reaction conditions. However, the degradation process often faces problems such as poisoning or deactivation of precious metal catalysts. In addition, because Pd and Pt are precious metals, their atom utilization should be considered in the design of catalysts.

(2) Photocatalytic reduction

TiO₂ is the most commonly used catalyst for the photocatalytic reduction of PBDEs^[60]. In 2009, Sun et al. Used ultraviolet light to excite the conduction band photogenerated electrons generated by TiO₂ to reduce BDE209(20μmol·L^-1) in acetonitrile solution containing isopropanol with nitrogen introduced to remove oxygen. BDE209 was completely degraded after 8 min of reaction, but a large amount of hexa- to heptaBDEs were accumulated in the system after 24 H of reaction^[61]. In order to enhance the efficiency of photocatalytic reductive dehalogenation, researchers combined TiO₂ with other electron transport media, such as reduced graphene oxide (rGO) and noble metal nanomaterials. Lei et al. Prepared the rGO/TiO₂ composite catalyst, and the debromination rate of the catalyst for BDE209 was 59.4% under UV irradiation in deoxygenated water containing methanol for 12 H, which was four times higher than that of the TiO₂ system^[62]. Lei et al. Designed and prepared Ag/TiO₂ based on the weakening of C-Br bond by nano-Ag, aiming at the deficiency that TiO₂ can not photocatalytically reduce tetrabrominated diphenyl ether BDE47, and realized the efficient reductive debromination degradation of BDE47^[63]. In addition, they used CuO/TiO₂ to generate Cu₂O with higher conduction band potential in situ during photocatalytic reaction, which greatly enhanced the reduction ability of photogenerated electrons and rapidly reduced and degraded BDE47^[64]. However, these photocatalytic reduction systems still cannot achieve complete debromination. Li et al. Used Pd/TiO₂ as photocatalyst, and BDE209 was completely debrominated under UV irradiation in deoxygenated methanol for 1 H^[65]. Wang et al. Comparatively studied the degradation pathway and reaction mechanism of BDE47 in photocatalytic reduction systems such as Cu/TiO₂, Ag/TiO₂, Pd/TiO₂, and Pt/TiO₂, and found that the reductive debromination of BDE47 mainly occurred on the zero-valent metal deposited on the surface of TiO₂, and was degraded on the surface of Ag and Cu by electron transfer mechanism, with preferential removal of ortho-bromine atoms^[66]; On the surface of Pd and Pt, the para-bromine atom is preferentially lost, which follows the active hydrogen transfer mechanism (Table 2).

In the photocatalytic reduction system, the mechanism of active hydrogen transfer is similar to that of the zero-valent metal system described above, which will not be repeated here. The rate-determining step of the electron transfer initiated reductive debromination is the transfer of photogenerated electrons (

e c b -

) from the semiconductor to the LUMO orbital of the PBde. This process thermodynamically requires that the E_LUMO of the electron acceptor, PBDE, be lower than the electron energy level (E_donor) of the electron donor, that is, the greater the absolute value of ΔE=E_donor-E_LUMO<0,ΔE (ΔE), the greater the thermodynamic driving force for this reaction. Experiments and density functional theory (DFT) calculations show that the reductive degradation rate of PBDEs is related to their E_LUMO in the system of reductive debromination dominated by electron transfer, and the E_LUMO is mainly affected by the electron cloud density on the aromatic ring with more bromine atom substitutions.With the decrease of bromine atoms on the bromine-rich ring, the E_LUMO increases, resulting in the decrease of the reductive degradation efficiency of PBDEs^[67]. For example, when the number of bromine atoms on the bromine-rich ring is reduced from 5 to 1, the rate of electron reduction debromination decreases by 7 orders of magnitude (Fig. 2). Therefore, it is difficult to completely debrominate PBDEs in the reducing system dominated by electron attack.

(3) electrochemical reduction method

The degradation of PBDEs by electrochemical reduction was first reported by Bonin et al^[68]. They used a reticulated glassy carbon electrode as the cathode, IrO₂/Ti as the anode, and Pd/Al₂O₃ as the catalyst to perform the electrochemical degradation of PBDEs in a flow-through batch electrolytic cell. The degradation rate of dibromodiphenyl ether (BDE15;2.4×10^-3mol·L^-1) was 75% in the mixed solvent of 70% methanol and 30% water for 90 min, and the reduction products were monobromodiphenyl ether (BDE3;0.95×10^-3mol·L^-1) and diphenyl ether (0.83×10^-3mol·L^-1). Konstantinov et al. Studied the electrochemical debromination process of BDE209 on Pt electrode in water/tetrahydrofuran (2.5 ∶ 97.5) mixed solvent under the bias voltage of 20 ~ 26 V^[69]. Isotope labeling experiments confirmed that the H atom in the debromination product of BDE209 originated from water in the solvent, and then a two-step electron transfer debromination mechanism was proposed. Peverly et al. Used a glassy carbon electrode or a silver mesh electrode as a cathode to degrade BDE209(2.0×10^-3mol·L^-1) in a mixed solvent of dimethylformamide and dimethylsulfoxide. After 3 H of reaction, BDE209 could be completely degraded, and the degradation products included diphenyl ether, dibenzofuran, benzene and phenol, with the former two products as the main ones^[70]. Quan et al. Prepared a variety of cathodes, such as TiN electrode, nitrogen-doped nanodiamond/Si rod array electrode (VA-NDD/Si RA) and boron and nitrogen co-doped nanodiamond/Si rod array electrode, to degrade BDE47 with pseudo-first-order kinetic constants of 0.65 h^-1, 1.93 h^-1 and 0.029 min^-1, respectively^[71]; BDE47 can be completely degraded on VA-NDD/Si RA cathode after 3 H of reaction, but the debromination rate is still only 86. 5% even if the reaction is extended to 4 H.

3 Degradation of PHOPs by reduction-oxidation process

As mentioned above, due to the different number of halogen atoms, the reduction/oxidation properties of PHOPs parent pollutants and dehalogenation degradation products change, so that a single reduction or oxidation method can not achieve efficient degradation of both parent pollutants and dehalogenation products. For example, a single reduction method can only reduce chlorophenol to phenol, but can not further decompose and mineralize it. The degradation efficiency of perfluorinated compounds and decabromodiphenyl ether by single oxidation method is lower than that by reduction method. However, as the number of bromine atoms decreased, the reductive degradation rate constant k_red of BDE47 decreased by 5 orders of magnitude compared with that of BDE209; If the number of bromine atoms continues to decrease to 3, the k_red of BDE15 decreases by nearly three orders of magnitude compared with that of BDE47, and it is extremely difficult to be degraded by reduction (Fig. 3A – C)^[67]. In contrast, an increase in the number of bromine atoms led to a rise in E_HOMO, and BDE15 was more susceptible to oxidative degradation, with an oxidative degradation rate constant k_ox that was 9.3 times that of BDE47 (Fig. 3A – C)^[72]. If the high halogenated reductive dehalogenation is combined with the low halogenated oxidative degradation, the dehalogenation degree of PHOPs can be greatly improved, and even the complete dehalogenation and mineralization can be achieved. Therefore, researchers have tried to couple the reduction and oxidation methods to degrade PHOPs. As shown in Table 3, the combined reduction-oxidation method has been used to degrade perfluorinated compounds, chlorophenols, polybrominated diphenyl ethers, and Tetrabromobisphenol A (TBBPA), etc. According to the different types of active species, the reduction-oxidation combined method can be divided into chemical catalytic method, electrochemical method, photocatalytic method, Photoelectrochemical method and mechanochemical method.

3.1 Traditional chemical reduction-Fenton-like catalytic oxidation process

In the Fe⁰ chemical reduction dehalogenation system, the residual Fe⁰ and its oxidation product Fe²⁺ can activate H₂O₂ or persulfate to produce strong oxidizing OH or $\mathrm{SO}_{4}^{\cdot-}$ (formula 1 and 2),Therefore, some scholars combined the chemical reductive dehalogenation of Fe⁰ with Fenton-like catalytic oxidation, and established a reduction-oxidation two-step process to degrade PHOPs.

(1)Fe²⁺+H₂O₂➝Fe³⁺+·OH+OH^-

(2)$\mathrm{Fe}^{2+}+\mathrm{S}_{2} \mathrm{O}_{8}^{2-} \rightarrow \mathrm{Fe}^{3+}+\mathrm{SO}_{4}^{-\cdot}+\mathrm{SO}_{4}^{2-}$

Tan et al. Prepared magnetic Fe⁰@Fe₃O₄ nanocomposite and constructed a reduction-oxidation two-step degradation of BDE209^[73]. Firstly, the degradation rate of BDE209 by the reaction 36 h,Fe⁰@Fe₃O₄ with the assistance of ultrasound (US) is 80%, which is higher than that of the single nano Fe⁰ system, because the nano Fe₃O₄ improves the stability of the Fe⁰ and can accelerate the surface electron transfer, thereby enhancing the reduction performance of the Fe⁰; After that, H₂O₂ was added to continue the reaction for 12 H, and a variety of ring-opening degradation products were detected. In the second step, Fe⁰@Fe₃O₄ catalyzes the decomposition of H₂O₂ to produce · OH, which oxidizes and degrades the debrominated product of BDE209, resulting in its ring-opening degradation and further debromination. In addition to the H₂O₂, persulfate can also act as an oxidizing agent to initiate the second stage oxidation reaction. Moradi et al. Constructed a reduction-oxidation relay degradation system by using a Fe⁰ permeable reaction column as a reducing agent for the first stage reaction, and using the leached iron under UV irradiation to assist and additional PDS to initiate the second stage oxidation reaction^[74].

In order to improve the efficiency of reductive dehalogenation in the first stage, some studies have used bimetallic catalysts or external energy assistance such as ultrasound. Sun et al. Found that BDE47 could be reduced and debrominated by nano-Fe/Ag bimetallic catalyst under ultrasonic irradiation, but the acute toxicity of the reduced and debrominated products to photobacteria was higher^[75]. If H₂O₂ is added to the degradation solution after reduction to form an ultrasonic Fenton-like system to oxidize and degrade the reduction products, the toxicity of the reduction products can be almost completely removed. They also degraded TBBPA by the above ultrasound-assisted Fe/Ag reduction-Fenton-like oxidation two-step method^[76]. Yalfani et al. Prepared Al₂O₃ immobilized Pd/Fe bimetallic catalyst (Pd/Fe@Al₂O₃), and used Pd to catalyze the hydrodechlorination of PCP(c₀=14 mg/L) by H₂ produced from formic acid decomposition under anaerobic conditions. After 5 min of reaction, PCP was completely dechlorinated, and the main degradation products were phenol and cyclohexanone, in which the maximum cumulative amount of phenol was about 78% of the initial molar amount of PCP^[77]; Then, O₂ was introduced into the reaction solution at a flow rate of 20 mL/min, which was used to react with H₂ in situ to produce H₂O₂, which was then used to form a Fenton-like system with Fe⁰. After 3 H of reaction, the above dechlorination product phenol could be completely degraded, and after 6 H of reaction, almost no toxic degradation products could be detected. In addition, the recycled Pd/Fe@Al₂O₃ was reused for 2 times with almost unchanged performance. On the contrary, if PCP is exposed to air or oxygen-enriched conditions at the initial stage, the degradation of PCP will be hindered, and even stagnation will occur at the later stage of the reaction, because aerobic conditions mainly provide oxidative active species, and PCP is difficult to be degraded by oxidation. Wu Wen et al. Prepared a carbon-supported Pd/Fe bimetallic catalyst, which can catalyze the reduction of 4-CP or 2,4-dichlorophenol (2,4-DCP) to phenol in water within 20 min^[78]. Subsequently, the pH value of the solution was adjusted to 5, H₂O₂ was added, and the reaction was continued for 60 min, 97.5% of the phenol was oxidized and decomposed into H₂O and CO₂. Wei et al. Used zero-valent zinc (Zn⁰), which has a more negative redox potential than Fe⁰, as a reducing agent, and added surfactant cetyltrimethylammonium chloride to enhance the adsorption of BDE47 on the surface of Zn⁰. After 1 H of reaction, the degradation rate and debromination rate of BDE47 were 98.6% and 40.2%, respectively, and the main products were tribrominated diphenyl ether BDE28 and dibrominated diphenyl ether BDE17^[79]. Then, the pH value of the reaction solution was adjusted from 4 to 3, and Fe²⁺ and H₂O₂ were added to continue the reaction for 2 H. The low-brominated products were oxidized and decomposed into short-chain carboxylic acids and even CO₂ by the · OH produced by Fenton reagent, and the debromination rate reached 88. 6%.

3.2 Photocatalytic reduction-oxidation method

In the semiconductor photocatalytic process, the photogenerated electron

e c b -

in the conduction band and the photogenerated hole

h V B +

in the valence band have reducing and oxidizing abilities, respectively. Single photocatalytic reduction or oxidation has been confirmed to be able to degrade PHOPs, and the former is carried out under anaerobic conditions, while the latter is carried out under air-saturated conditions. By controlling the photocatalytic reaction atmosphere, Guo et al. Used Degussa P25 TiO₂ as photocatalyst to establish a two-step method of anaerobic photocatalytic reduction-aerobic photocatalytic oxidation for the degradation of TBBPA, and the debromination rate of TBBPA was 90% under UV irradiation for 4 H, which was higher than that of anaerobic photocatalytic reduction (60%) and aerobic photocatalytic oxidation system (40%)^[80]. Lei et al. Established a method for photocatalytic reduction-oxidation simultaneous degradation of BDE47 by using reduced graphene oxide modified P25 TiO₂(rGO/TiO₂) as photocatalyst and graphene to spatially separate photogenerated electrons and photogenerated holes on the surface of rGO and TiO₂ to improve the utilization of reduction and oxidation active species^[72]. It was found that anaerobic atmosphere and surface modification of rGO were the necessary conditions for the photocatalytic reduction of BDE47 by TiO₂, while the content of methanol in water was the key factor affecting the photocatalytic degradation of BDE47. Methanol, as a · OH radical scavenger, on the one hand, inhibits the recombination of photogenerated carriers and promotes the electron transfer reductive debromination process, while on the other hand, it directly or indirectly consumes · OH and

h V B +

and hinders the oxidative degradation process. In the optimized methanol aqueous solution without oxygen and containing 0.125 mol·L^-1, BDE47 can be completely degraded and debrominated under UV irradiation for 14 H, and the debromination rate is significantly higher than that of the single photocatalytic reduction (25%) or oxidation system (50.8%) (Fig. 4). Based on the analysis of reactive species and degradation products and kinetics, the degradation mechanism was elucidated that the :TiO₂ was excited by UV light to produce

e c b -

and

h V B +

, and the

e c b -

migrated and enriched on the surface of rGO, and then transferred to BDE47, which was reduced and debrominated to tribromodiphenyl ether. The tribromodiphenyl ether is then oxidatively decomposed by

h V B +

on the surface of the TiO₂.

TiO₂ can only absorb ultraviolet light, which accounts for only about 5% of sunlight. In order to improve the utilization of sunlight, some narrow band gap semiconductors (e.g., C₃N₄) have been used for photocatalytic degradation of PHOPs. However, in order to construct a photocatalytic reduction-oxidation system, the photocatalyst must have a high reduction and oxidation ability, that is, a large band gap, which is contradictory to its visible light responsiveness. In order to solve this contradiction, Weng et al compounded two kinds of narrow band gap semiconductor MoS₂ and SnIn₄S₈ with negative conduction band position and positive valence band position respectively to form a heterojunction, and prepared a visible light responsive MoS₂/SnIn₄S₈ composite catalyst, which was used for anaerobic photocatalytic reduction-aerobic photocatalytic oxidation relay degradation of TBBPA^[81]. TBBPA with completely 18.5μmol·L^-1 could be degraded by irradiation for 6 H under anaerobic conditions, and its debromination rate reached 93.1%, but the accumulation of its debromination product bisphenol A was as high as 15.9μmol·L^-1; The degradation rate of bisphenol A was more than 96. 5% and the mineralization rate was 60. 2% after 6 H of reaction with air. The results of acute toxicity test of Vibrio qinghaiensis Q67 showed that the inhibition rate of parent pollutant TBBPA on Q67 was 26.6%, the inhibition rate of parent pollutant TBBPA on Q67 was slightly reduced to about 25% by photocatalytic reduction alone, and the inhibition rate on Q67 increased to 41.8% by photocatalytic oxidation alone. However, after photocatalytic reduction-oxidation treatment, the degradation solution had no inhibitory effect on Q67. This further indicates that the combined photocatalytic reduction-oxidation method can be more efficient in reducing, or even completely eliminating, the toxicity of PHOPs.

3.3 Electrochemical reduction-oxidation method

The electrochemical reduction-oxidation method uses the electrochemically produced H₂ and/or H₂O₂ as the source of reactive species, which are then activated and decomposed into H · or · OH under the action of hydrodehalogenation catalyst and Fenton-like catalyst, respectively, to degrade the target pollutants of PHOPs by reduction-oxidation. Electrochemical hydrogen evolution (HER) is the electrolysis reaction of water at the cathode (Equation 3), and Pt, Ni, etc. have moderate free energy of hydrogen adsorption, which is beneficial to hydrogen atom adsorption and H₂ desorption, and are often used as electrocatalysts for hydrogen evolution. The 2-electron reduction reaction (2eORR) of dissolved oxygen on the cathode surface; Formula 4) is commonly used for electrochemical synthesis of H₂O₂, and the cathode materials widely used include metal materials such as Pt and Pd, and non-metallic carbon materials such as carbon nanotubes and graphite. In addition, the H₂ generated in situ by electrochemistry can also react with dissolved O₂ under the catalysis of Pd and the like to generate H₂O₂ (Formula 5). Dissolved oxygen can be added externally or by using the electrolytic reaction of water at the anode (i.e., electrochemical oxygen evolution (OER); Formula 6), where the latter involves 4-electron transfer and metal-oxygen bond cleavage, and Pt is often used as an electrocatalyst to reduce the reaction overpotential and accelerate the oxygen evolution reaction.

(3)2H⁺+e^-➝H₂

(4)O₂+2H⁺+2e^-➝H₂O₂

(5)H₂+O₂➝H₂O₂

(6)2H₂O+4H⁺-4e^-➝O₂

According to the type of electrode involved in the reaction, the electrochemical reduction-oxidation method can be divided into two types: single cathode type and combined cathode and anode type. According to the stage of reduction/oxidation reaction, the electrochemical reduction-oxidation method can be divided into two-step method and synchronous method. Among them, the electrochemical reduction-oxidation two-step method mediated by a single cathode is easy to realize. Hou et al. Used electrochemical deposition to load Pd and Fe bimetallic nanoparticles (Pd/Fe @ Ni) on nickel foam to prepare Pd/Fe @ Ni as a cathode, and established a reduction-oxidation two-step method to degrade TBBPA by controlling the reaction atmosphere^[82]. In the initial 30 min, the reaction was carried out in an oxygen-free environment with N₂, and the H⁺ was reduced at the cathode to produce H₂, which was then decomposed in situ to H · on the Pd surface. H · attack and Fe⁰ electron attack jointly initiated the reductive debromination of TBBPA. After that, the reaction atmosphere was adjusted to an oxygen-rich environment by air aeration, and the H₂ generated at the cathode reacted with dissolved oxygen to produce H₂O₂, while Fe⁰ and its oxidation product Fe²⁺ catalyzed the decomposition of H₂O₂ to produce · OH, which in turn oxidatively degraded the reductive debromination products of the first stage. Xu et al. First loaded Pd/Fe on graphene (Pd/Fe @ Gr), then loaded it on a stainless steel wire mesh as a cathode, and then placed two self-made cathodes and a commercial Ti/IrO₂/RuO₂ anode in three independent electrolysis chambers to form a three-electrode system, and comparatively studied the degradation of 4-bromophenol (4-BP) by cathodic electrochemical reduction-oxidation method and anodic oxidation method^[83]. In the two cathode chambers, the H₂ introduced at the initial stage of the reaction (0 ~ 30 min) was decomposed into H · on the surface of Pd/Fe @ graphene, and 4-BP was reduced and debrominated. In the later stage (30 ~ 60 min), the reaction atmosphere changed from H₂ to air, and the dissolved oxygen was first reduced to H₂O₂ and then decomposed to · OH on the surface of Pd/Fe @ Gr, and then the residual 4-BP and its intermediates were oxidized and degraded. In the anode chamber, Ti/IrO₂/RuO₂ electrocatalytically oxidizes water to · OH for oxidative degradation of pollutants. After 60 min of reaction, 4-BP was almost completely degraded and mineralized in both cathode chambers, and its toxicity to Photobacterium phosphoreum and Photobacterium phosphoreum gradually disappeared. However, the mineralization rate in the anode chamber was slightly lower, 93.4%, and the toxicity increased first and then decreased with the increase of reaction time, but it was still 85% of the initial toxicity. The above results show that the electrochemical reduction-oxidation method not only has higher mineralization ability, but also can achieve the ultimate goal of harmlessness.

On the same cathode, the key to constructing a simultaneous electrochemical reduction and oxidation method is to ensure that the hydrogenation reduction dehalogenation reaction occurs preferentially, that is, to control the relative formation rates of H₂ and H₂O₂. In order to reduce the consumption of the H₂ by the dissolved O₂ and to provide an appropriate amount of H₂O₂, a catalyst having both hydrogen evolution and 2e^- oxygen reduction activities can be used as the cathode. Carbon materials can electrocatalyze the reduction of O₂ to H₂O₂, and bimetallic catalysts supported on carbon materials can meet the above requirements. Carbon aerogel (CA) is characterized by large specific surface area, abundant porosity, high electrical conductivity, and tunable size. Shen et al. Prepared PdFe/CA by immobilizing Pd-Fe alloy on CA with carbon-rich defects, and used it as a cathode to simultaneously realize the electrocatalytic decomposition of H₂O for hydrogen evolution (i.e., the precursor of H ·) and the electrocatalytic reduction of O₂ for the generation of H₂O₂ (i.e., the precursor of · OH), and constructed an efficient reduction-oxidation simultaneous method for chlorophenol degradation system. After 6 H of reaction, the degradation, dechlorination and mineralization rates of 3-CP could reach 100%^[84]. The mechanism of cathode-induced electrochemical reduction-oxidation reaction was elucidated by in situ electrochemical-electron paramagnetic resonance spectroscopy. The PdFe alloy in the PdFe/CA cathode is the active site for the electrocatalytic hydrogen evolution reaction, and the produced H₂ is then decomposed into H ·. The carbon defects and their oxygen-containing functional groups in carbon aerogels significantly improve the selectivity and activity of 2e^- oxygen reduction and accelerate the generation of H₂O₂, while the H₂O₂ generated in situ electrochemically is decomposed into ^·OH by PdFe in real time. In addition, the system has the advantages of low metal dissolution amount, low electrochemical energy consumption, high electrode stability and high mechanical strength. Zhao et al. Loaded Fe, Cu bimetallic single-atom catalyst (FeCuSA-NPC) on nitrogen-doped porous carbon, and used it as a cathode for electrochemical degradation of chlorophenols, achieving effective degradation and mineralization of various chlorinated pollutants such as 3-CP, 4-CP, 2,4-CP, 2,4,6-trichlorophenol (2,4,6-TCP)^[85]. Based on DFT calculations and experimental results, the electrochemical reaction mechanism was clarified (Fig. 5): on the surface of ternary cathode material FeCuSA-NPC, highly unsaturated coordinated single-atom Cu is beneficial to the adsorption and electrochemical reduction dechlorination of chlorophenols; Nitrogen-doped porous carbon electrocatalytically reduces O₂ to H₂O₂, while the highly unsaturated coordinated single-atom Fe can quickly adsorb and activate H₂O₂ to decompose to produce a large amount of ^·OH, which can further oxidize and degrade the reductive dechlorination products of chlorophenols. In addition, the co-doping of bimetallic single-atom catalysts significantly improved the O₂ reduction activity, Fe utilization, and · OH production of the carbon materials.

The electrochemical reduction-oxidation process mediated by the anode and cathode is mostly synchronous, and the key is still to regulate the relative production rate of H · and · OH. Guo et al. Modified β-cyclodextrin on the surface of AgPd alloy nanoparticles to prepare AgPd @ CDs composite catalyst, and added it to the electrochemical system with Ti sheet as cathode (2.0 × 5.0 cm) and RuO₂/Ti sheet (2.0 × 5.0 cm) as anode to degrade 2,4,6-tribromophenol (2,4,6-TBP). Under constant current density 5.0 mA·cm^-2 for 90 min, the degradation rate of 2,4,6-TBP was 100%, which was higher than that of the single electrochemical system (i.e., without AgPd @^[86]. Moreover, the pseudo-first order apparent degradation rate constant of AgPd @ CDs for 2,4,6-TBP degradation is 0.0592 min^-1, which is higher than that of activated carbon, Al₂O₃ or polyvinyl alcohol modified AgPd and ferrocene modified AgPd @ CDs electrochemical systems (0.0128, 0.0087, 0.0188, 0.0393 min^-1, respectively). The above results indicate that the combination of AgPd @ CDs and electrochemistry is the key to the efficient degradation of 2,4,6-TBP. The degradation reaction mechanism of the system is that the electrochemical hydrogen evolution reaction at the anode of the :RuO₂/Ti and the electrochemical oxygen evolution reaction at the cathode of Ti provide H₂ and Pd in the O₂;AgPd@CDs has triple catalytic effects: the decomposition H₂ is H ·,Activating H₂ to reduce O₂ into H₂O₂, and Fenton-like catalyzing H₂O₂ to decompose and produce hydroxyl;Ag increases the electron cloud density of Pd surface and improves the catalytic performance of Pd for decomposition of H₂. The β-cyclodextrin on one hand enhances the adsorption of 2,4,6-TBP on the surface of the catalyst through hydrogen bonding and host-guest recognition, and on the other hand restricts the adsorption of O₂ on the surface of the catalyst and the entry and exit of cyclodextrin cavity through surface hydrophilicity and cavity confinement effect.Weakening the consumption of H₂ and H · by O₂ is beneficial to the attack of H · and · OH on pollutants, and finally the reduction and oxidation degradation reactions are carried out simultaneously. Xie et al. Prepared Pd-supported CeO₂ bifunctional nanocatalyst (Pd/CeO₂,Pd mass fraction of about 5%) by photodeposition method, and combined it with two platinum sheet electrodes (2.0 × 2.0 × 0.01 cm) and Fe²⁺ to establish a simultaneous electrochemical reduction-oxidation tandem coupling method for chlorophenol degradation^[87]. 2-CP, 3-CP and 4-CP can be completely dechlorinated when the dosage of Pd/CeO₂ and FeSO₄ is 0.1 G/L and 0.05 mM, respectively, under the conditions of pH = 3.0 and current density of 25 mA·cm^-2 for 1 H. After 6 H of reaction, the mineralization rate was about 70%. The reaction mechanism was elucidated by the analysis of the degradation products and the reactive species: the platinum cathode and the platinum anode electrochemically decompose water to produce H₂ and O₂, respectively, and the former is diffusively adsorbed on the Pd surface and catalyzed to decompose to H ·,However, O₂ diffusely adsorbs on the surface of CeO₂, and then reacts with H₂ adjacent to the Pd surface to form H₂O₂, which is then catalyzed by Fe²⁺ to decompose into · OH. The parent pollutant chlorophenols are preferentially dechlorinated by H · reduction, and the dechlorinated products are degraded by · OH oxidation.

3.4 Photoelectrochemical reduction-oxidation method

Photoelectrochemical method is a combination of photocatalytic technology and electrochemical method. Its principle is to use catalyst or photosensitizer with photoelectric response characteristics as photoelectrode.The photoelectrode is excited by light energy to generate photogenerated carriers, and then the applied electric field drives the photogenerated carriers to move and separate them in space, thereby improving the quantum efficiency of photocatalysis. According to the type of photoelectrode used, Photoelectrochemical methods can be divided into three categories: single photoanode system, single photocathode system and photoanode-photocathode dual photoelectrode system. The essence of constructing the Photoelectrochemical reduction-oxidation method is to use the Photoelectrochemical method to produce the reduction and oxidation active species, namely H · and · OH. Therefore, the conduction band of the photocathode catalyst should be more negative than the hydrogen evolution potential, while the valence band of the photoanode catalyst should be more positive than the ·OH/OH^- or ·OH/H₂O potential. The photocathode materials that meet the above conditions are mainly copper-based semiconductors such as Cu₂O, CuO, etc., while the photoanode materials are TiO₂, BiVO₄ and WO₃, etc.

Zhang et al. Constructed a cathodic Photoelectrochemical reduction-oxidation system for simultaneous degradation of trichloroacetic acid under a bias voltage of − 0.5 V by using a self-made Cu/Cu₂O/CuO nanowire array (Cu_xO@Pd) deposited with nano-Pd particles as the photocathode, a commercial Pt sheet as the anode, and a proton exchange membrane to separate the cathode and anode chambers, and by adjusting the reaction atmosphere in the cathode chamber^[88]. The reaction mechanism was elucidated by EPR experimental technique and DFT theoretical calculation: under the irradiation of xenon lamp, the Cu_xO@Pd absorbs light to produce electrons and holes, and the applied bias accelerates the separation of photogenerated carriers. The electrons on the photocathode reduce the H⁺ to H ·, and the supported Pd reduces the overpotential of H · formation and accelerates the reductive dehalogenation reaction. On the other hand, Pd can also catalyze the reduction of dissolved oxygen by H · to produce H₂O₂, which is then decomposed to produce · OH under the action of Cu⁺/Cu²⁺.

Zhang et al. Grew TiO₂ nanoarrays on FTO glass to prepare a photoanode, and established a Photoelectrochemical reduction-oxidation simultaneous degradation method of 2,4-dichlorophenol by using self-made Fe and P co-doped carbon aerogel (Fe-P-CA) as a cathode^[89]. The results showed that the degradation rate of 2,4-dichlorophenol was about 98% under the conditions of solution pH = 7.0, applied bias voltage = 1.2 V, oxygen exposure (300 mL·min^-1) and illumination reaction for 30 min. After 5 cycles, the degradation rate decreased, but the degradation rate still reached 90%. The structural identification of degradation products and the analysis of reactive species showed that the Fe-P-CA cathode and the TiO₂ photoanode provided H · and · OH, respectively, and the co-doping of P was beneficial to the production of H ·. Li et al. Prepared a WO₃ and Mo co-doped BiVO₄(WO₃/Mo@BiVO₄) composite photocatalyst, which was drop-coated on FTO glass as a photoanode, while Pd was deposited on foam Ni to prepare a Pd/foam Ni cathode, and then a cation exchange membrane Nafion-117 was used to separate the anode chamber from the cathode chamber to construct a two-step Photoelectrochemical system of reductive dehalogenation in the cathode chamber and relay oxidative degradation in the anode chamber^[90]. Under the conditions of pH = 4.3, overpotential = 1.2 V and illumination for 4 H, 2,4-dichlorophenol injected into the cathode chamber was completely dechlorinated and degraded, and the product was phenol. Then, the degradation solution in the cathode chamber was directly transferred to the anode chamber for Photoelectrochemical oxidation degradation, and 2,4-dichlorophenol was re-injected into the cathode chamber for Photoelectrochemical hydrodechlorination reaction. After 4 H of reaction, 2,4-dichlorophenol in the cathode chamber was completely degraded, while the degradation rate of phenol in the anode chamber was about 45%. Based on the quenching results of reactive species, a degradation mechanism was proposed: under light irradiation, the WO₃/Mo@BiVO₄ on the photoanode was excited by light, and the photogenerated electrons from the conduction band of the Mo@BiVO₄ were first transferred to the conduction band of the WO₃, and then transferred to the Pd/Ni foam cathode through the connecting wires of the anode and cathode, reducing the H⁺ adsorbed on the surface of the cathode to H ·, and then reducing 2,4-dichlorophenol to phenol. The photogenerated holes remaining in the valence band of the photoanode catalyst WO₃ are transferred to the surface of the Mo@BiVO₄, and directly attack phenol together with the photogenerated holes of the Mo@BiVO₄ to initiate the oxidative decomposition reaction.

In a single photoelectrode system, the electrochemical reaction on the other electrode tends to have a high overpotential, thus limiting the efficiency of the overall degradation reaction. If the photoanode and photocathode are used at the same time to convert light energy into electrical energy and chemical energy, driving the cathode and anode to produce reduction and oxidation active species, respectively, the applied bias can be reduced and the degradation efficiency can be improved. Zhang et al. Used self-made Pd deposited Cu/Cu₂O/CuO nanowire array (Cu_xO@Pd) and TiO₂ nanorod array deposited FTO glass as photocathode and photoanode, respectively, for Photoelectrochemical reduction-oxidation simultaneous degradation of chloroaniline^[91]. The apparent rate constant of p-chloroaniline degradation by the system was 5.6 and 4.5 times higher than that by the single Photoelectrochemical reduction or oxidation system, respectively, at a bias voltage of 0.3 V and in the absence of oxygen. In addition, the mineralization rate of the double photoelectrode system is 2.1 times higher than that of the photoanode oxidation system. The reaction mechanism of the system is shown in Figure 6: the H · produced by the photocathode first attacks the pollutant nucleophilically to form a hydrodechlorination intermediate, and then the · OH produced by the photoanode electrophilically attacks the intermediate dehalogenator and oxidizes it until it is completely mineralized.

3.5 Mechanochemical reduction-oxidation process

Some PHOPs, such as polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs), are hydrophobic and lipophilic, and are easily adsorbed or deposited on the surface of environmental solid particles. If they are treated by liquid phase reduction-oxidation method, organic solvents should be used to elute these pollutants from the surface of solid substrates first, and these coexisting organic solvents will seriously hinder the oxidative degradation process. If incineration is used, there is a risk of secondary pollution caused by dioxins. Ball milling mechanochemical method is considered to be an effective new method for the treatment of PHOPs in solid substrates, and the treatment effect is often improved by adding grinding agents. Zero-valent metals, metal oxides, alkali metal hydroxides, persulfates, etc. Have been reported^[92,93]^[94,95]^[96]^[49,95,97]. According to the type of reaction, it can be divided into three categories: (1) under the mechanical force of high energy ball milling, zero-valent metals and metal oxides lose oxygen and generate electrons in situ as reducing species to reduce and dehalogenate PHOPs; (2) PHOPs are degraded by lattice oxygen released from metal oxides and $\mathrm{SO}_{4}^{\cdot-}$ produced by persulfate decomposition, and (3) PHOPs are degraded by strong bases through nucleophilic substitution, elimination, and/or hydrolysis. A reduction-oxidation synergistic degradation system can be established by combining the first and second types of grinding agents, but if the reduction/oxidation ability of the grinding agent is too strong, the redox reaction between the binary grinding agents will occur, which will seriously weaken the degradation efficiency of PHOPs.

Zhang et al. Selected oxygen vacancy-rich neutral metal oxide Bi₂O₃ to provide oxidation active species, and combined it with reductive grinding agent iron powder (Fe⁰) to form a combined system, and constructed a reduction-oxidation relay mechanochemical degradation system^[98]. The degradation rate of BDE209 reached 96. 6% when the mixture of Fe⁰, Bi₂O₃ and BDE209 with Bi/Fe/Br molar ratio of 1 ∶ 1 ∶ 1 was ball-milled for 2 H.Higher than that of the individual Bi₂O₃(66.0%) and Fe⁰(24.0%) systems, suggesting a synergistic enhancement effect between Bi₂O₃ and Fe⁰ in the mechanochemical degradation of BDE209. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and thermogravimetric analysis (TGA) were used to test the materials before and after the reaction of the binary grinding agent system. The results showed that organic carbon in BDE209 molecules was converted into amorphous carbon, while organic bromine was completely converted into inorganic bromine and fixed in the form of BiOBr, which was due to the strong affinity of Bi₂O₃ for halogens. Grinding agent Fe⁰ is converted to Fe₃O₄, while Bi₂O₃ is gradually converted to BiOBr via substoichiometric Bi_xO_yBr_z. In the Bi₂O₃ alone system, the end product of the reaction was Bi₄O₅Br₂, indicating that the combination of Fe⁰ and Bi₂O₃ significantly improved the efficiency of debromination. The mechanochemical degradation mechanism of BDE209 by Fe⁰ and/or Bi₂O₃ is proposed as follows: under the action of mechanical force, electrons are released from the surface of Fe⁰, and the Bi-O bond in the Bi₂O₃ is activated and broken to release lattice oxygen and produce oxygen vacancies and electrons; Because of the strong electronegativity of bromine atom, BDE209 is difficult to be oxidized and degraded, and is easier to be debrominated by electron reduction. When Fe⁰ was used alone, a layer of iron oxide passivation layer was formed on the surface of Fe⁰ after a period of reaction, which hindered the reduction debromination reaction. When Bi₂O₃ was used alone, BDE209 was difficult to be degraded at the initial stage of the reaction. When Fe⁰ and Bi₂O₃ were used together, BDE209 was rapidly debrominated by Fe⁰, while Bi₂O₃ could oxidize the intermediate products of BDE209 degradation by releasing active lattice oxygen in situ. In addition, the Bi₂O₃ has a strong affinity for halogens, trapping the Br^- to form substoichiometric Bi_xO_yBr_z; Because of the large radius of Br^-, the lattice distortion of Bi₂O₃ is aggravated, which promotes the activation and cleavage of Bi-O bond and the generation of oxidized species. Therefore, Fe⁰ reduced -Bi₂O₃ relay oxidation, which synergistically promoted the degradation, debromination, and mineralization of BDE209.

3.6 Hydrated electron reduction-persulfate catalytic oxidation method

Fluorinated PHOPs have lower reduction potential and higher oxidation potential than chlorinated and brominated PHOPs, therefore, the above reduction-oxidation system cannot be used to degrade fluorinated PHOPs. As previously described in Section 2, among many chemical degradation methods, the hydrated electron reduction method has the strongest defluorination ability for fluorinated PHOPs, but the reductive defluorination rate of the degradation products, short-chain polyfluoroalkylcarboxylic/sulfonic acids, becomes slower as the carbon chain becomes shorter, making it extremely difficult to completely defluorinate. Liu et al. First used the hydrated electrons generated by UV/$\mathrm{SO}_{3}^{2-}$ at pH = 12 under anaerobic conditions to reduce and degrade perfluorocarboxylic acids (C_nF₂_n₊₁COO^-,n=1~7), and the defluorination rate was 73% ~ 93% after 8 H of reaction^[99]; After that, the reaction solution was stirred in air for 24 H to remove the residual $\mathrm{SO}_{3}^{2-}$, and then PDS and NaOH were added to initiate the oxidative degradation with the produced · OH at pH = 12 and 120 ℃ for 1 H, and almost quantitative complete defluorination was achieved. Noting that polyfluoroalkylcarboxylic/sulfonic acids and perfluorosulfonic acids can be degraded to short-chain perfluorocarboxylic acids by persulfate catalytic oxidation system, they used the above PDS thermal catalytic oxidation system to pre-oxidize polyfluoroalkylcarboxylic/sulfonic acids to perfluorocarboxylic acids, followed by UV/$\mathrm{SO}_{3}^{2-}$ reduction-PDS thermal catalytic post-oxidation treatment. Among the perfluoroalkyl acids with perfluorocarbon chain numbers from 1 to 8 studied, except for polyfluoroalkyl carboxylates with n = 7 and 8 and per-/polyfluoroalkyl sulfonates with n = 4 (the defluorination rate is 85% to 94%), complete defluorination can be achieved, which is higher than that of single reduction or oxidation and reduction-oxidation systems (35% to 95%). The above results indicate that according to the molecular structural characteristics of PHOPs and their degradation products, the design of appropriate reduction and oxidation combined system is expected to achieve complete dehalogenation, thus completely eliminating the ecotoxicity of parent pollutants and degradation products.

4 Conclusion and prospect

Most of the studies on the degradation of PHOPs are focused on the single reduction or oxidation system, while the studies on the reduction-oxidation combined degradation method are less. Due to the different number of halogen atoms, the parent pollutants of all/polyhalogenated PHOPs and their dehalogenation degradation products often show different or even opposite reduction/oxidation characteristics, so it is difficult to achieve deep dehalogenation by a single reduction or oxidation method. Comparatively speaking, the reduction-oxidation combined synergistic degradation method based on "all/multi-halogenated PHOPs are easy to reduce, and low-halogenated PHOPs are easy to oxidize" has the advantages of fast degradation rate, complete dehalogenation and complete mineralization.

At present, most of the reduction-oxidation combined systems adopt a two-step method, that is, the reduction method is used to dehalogenate the parent pollutants of PHOPs, and then the oxidation system is used to oxidize and decompose the dehalogenated products. Comparatively speaking, the one-step reduction-oxidation combined method is simpler to operate, and how to avoid the consumption of reducing and oxidizing active species by each other and ensure that they effectively attack the parent pollutant and the dehalogenation product respectively is the key to improve the efficiency of the whole degradation reaction. A multi-component catalyst with space separation capability is adopted to simultaneously activate and generate reducing species such as electrons or active hydrogen and oxidizing species such as active oxygen free radicals and the like on catalytic sites at different space positions,By adjusting the catalyst structure and reaction conditions to achieve a rate-matched reduction/oxidation half-reaction rate, the quenching of reactive species can be inhibited, and the degradation and dehalogenation efficiency of PHOPs can be improved, which is of great significance for the design of catalysts. In addition, the existing reduction-oxidation combined system is mainly aimed at the degradation of chlorinated and brominated PHOPs, and there are few reports on fluorinated PHOPs. According to the structural characteristics of fluorinated PHOPs, the construction of efficient reduction-oxidation combined system is one of the key points in future research. In addition, there is a complex matrix in the actual environment, and how to reduce or eliminate the matrix interference is also a key problem to be solved.

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Funding

National Natural Science Foundation of China(21976063)

National Natural Science Foundation of China(22076052)

Natural Science Foundation of Hubei Province of China(2019CFB432)

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1 Introduction

2 Degradation of PHOPs by single reduction/oxidation method

2.1 Perfluorinated compound

2.2 Polychlorophenol

2.3 Polybrominated diphenyl ether

3 Degradation of PHOPs by reduction-oxidation process

3.1 Traditional chemical reduction-Fenton-like catalytic oxidation process

3.2 Photocatalytic reduction-oxidation method

3.3 Electrochemical reduction-oxidation method

3.4 Photoelectrochemical reduction-oxidation method

3.5 Mechanochemical reduction-oxidation process

3.6 Hydrated electron reduction-persulfate catalytic oxidation method

4 Conclusion and prospect

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