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Micro-Interface Electron Transfer Oxidation Based on Persulfate Activation

Yong Feng , Yu Li , Guangguo Ying

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (11) : 2138 -2149.

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Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (11) : 2138-2149 DOI: 10.7536/PC201219
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Micro-Interface Electron Transfer Oxidation Based on Persulfate Activation
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Abstract

Advanced oxidation processes based on persulfate activation have attracted increasing attention in the field of environmental remediation. However, the ubiquitous presence of radical scavengers in the environment limits the practical application of this technology. The latest publications show that the persulfate, under the activation of certain catalysts, can oxidize contaminants through the non-radical mechanism of electron transfer, and the catalyst mainly serves as an electron shuttle. This technology is not easily affected by common anions, such as chloride ions and bicarbonate ions, and natural organic matters and has high selectivity for the oxidation of target contaminants. Meanwhile, this technology may degrade pollutants without the need of direct contact between oxidants and pollutants, thereby avoiding the generation of toxic halogenated products due to the direct interaction between halogen ions and persulfate. This review mainly summarizes the types of common electron shuttles, the characterization methods of electron transfer processes, and the effects of common water components. Finally, the problems and application potential of this technology are proposed.

Contents:

1 Introduction

2 Mechanism and categories of interfacial electron transfer oxidation

2.1 Development and mechanism of interfacial electron transfer

2.2 Types of electron shuttles

2.3 Advantages and disadvantages of different electron shuttles

3 Identification of electron transfer mechanism

3.1 Radical measurements

3.2 Involvement of nonradical species

3.3 Electrochemical tests

3.4 Decomposition of persulfates

3.5 Theoretical calculation

4 Influencing factors of interfacial electron transfer

4.1 Factors influencing the catalytic reactivity

4.2 Factors influencing the adsorption reactivity

5 Effects of common water components

5.1 Chloride ions

5.2 Carbonate and bicarbonate ions

5.3 Natural organic matters

5.4 Treatment of wastewate by electron transfer

6 Applications of interfacial electron transfer

7 Conclusions and outlook

Graphical abstract

Keywords

persulfate / electron transfer / microcontaminants / oxidative degradation / selective oxidation / matrix effects

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Yong Feng, Yu Li, Guangguo Ying. Micro-Interface Electron Transfer Oxidation Based on Persulfate Activation. Progress in Chemistry, 2021, 33(11): 2138-2149 DOI:10.7536/PC201219

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1 Introduction

Since Anipsitakis and Dionysiou first applied the combination of peroxomonosulfate and Co2+ to the oxidative degradation of pollutants in water, the oxidation reaction activated by peroxomonosulfate has attracted wide attention of researchers at home and abroad in the fields of environmental remediation, such as wastewater treatment, soil remediation and groundwater purification, and has become a hot research topic in the international environmental field[1][2⇓~4]. According to the difference of molecular structure, persulfate can be divided into persulfate (PMS,HSO5-) and persulfate (PDS,S2O82-). The physicochemical properties of PMS and PDS are shown in Table 1. Under the action of a catalyst and an external energy source such as ultraviolet rays, the O — O bond in the persulfate is broken through single electron transfer or energy transfer to generate a sulfate radical (SO4·-) and a hydroxyl radical (· OH). The redox potentials of · OH and SO4·- are as high as 1.8 ~ 2.7 V and 2.5 ~ 3.1 V, respectively, and they have strong oxidation properties, which can theoretically oxidize most organic pollutants into CO2 and H2O[5][6][7,8]. However, due to its high oxidation activity and poor selectivity, chloride ion, carbonate ion and natural organic matter, which are widely present in water, often interfere with the removal of pollutants, especially micro-pollutants, in the application of advanced oxidation processes based on free radicals[9⇓~11]. Therefore, improving the selectivity of active species is one of the important research directions in the field of activated persulfate.
In recent years, researchers have found that persulfate can degrade a variety of pollutants by electron transfer non-free radical mechanism under the action of some materials[12,13]. Compared with the free radical-dominated advanced oxidation reactions, this electron transfer process has higher selective oxidation, which can largely avoid the adverse effects of common matrices in water. At the same time, the galvanic oxidation process based on the electron transfer mechanism can avoid the direct interaction between persulfate and nucleophiles such as halogen anions, thus greatly reducing the formation of toxic halogenated degradation products[13,14]. In this paper, the common types of electron shuttles in persulfate electron transfer systems are summarized, the characterization methods of electron transfer mechanisms are introduced, and the effects of common matrices in water and the purification effects of electron transfer reactions on actual wastewater are reviewed. On this basis, the advantages and disadvantages of electron transfer technology are reviewed, and the application prospects of this technology in the field of advanced treatment of wastewater and purification of micro-pollutants in surface water are prospected.

2 Mechanism and classification of interfacial electron transfer oxidation

2.1 Development and Mechanism of Interfacial Electron Transfer Technology

When common transition metal ions such as Fe2+ and Co2+ react with persulfate, single electron transfer reactions occur to produce free radicals such as · OH and SO4·-[20,23,24]. In the electron transfer non-radical system, the catalyst is not an electron donor but acts as an electron shuttle medium (electron shuttle), mediating the transfer of electrons from the organic substance to the persulfate. The treatment effect of electron transfer reaction depends not only on the electron transfer activity of the catalyst, but also on the electron transfer performance of the organic pollutants. The ionization potential (ψ) and the half-wave potential (E1/2) can be used to describe the activity of organic compounds in gaining and losing electrons. The higher the ionization potential is, the poorer the ability of organic compounds to provide electrons is. Hu et al. Have shown that there is a critical value for the ionization potential of organic compounds, and only the organic compounds below this critical value can be oxidized by the electron transfer mechanism[25]. Half-wave potential reflects the ability of organic compounds to carry out an electron transfer oxidation reaction, and the higher the value, the more difficult it is for organic compounds to carry out an electron transfer oxidation reaction[26]. Ren et al. investigated the degradation of pollutants in carbon nanotube (CNT)/PDS system, and found that the degradation kinetics of pollutants had a significant negative correlation with the half-wave potential[27]. Therefore, the half-wave potential of pollutants can be used to evaluate the difficulty of electron transfer.
The process of persulfate microinterface electron transfer mediated by electron shuttle to degrade pollutants can be roughly summarized as follows: persulfate and pollutants are first adsorbed on the surface of electron shuttle, and persulfate and electron shuttle form a metastable complex through outer electron transfer (Fig. 1). The electronic layout of PDS or PMS in the metastable complex is changed, the oxidation performance is improved, electrons are extracted from pollutants adsorbed on the surface of the electron shuttle, and then the oxidative degradation of pollutants and the decomposition of persulfate are realized[28]. The mechanism and development process of catalytic persulfate electron transfer degradation of pollutants are shown in Figure 1.

2.2 Type of electron shuttle

Unlike the long history of free radical oxidation, the electron transfer oxidation reaction based on persulfate activation was proposed by Zhang et al. And Duan et al. In 2014[29][30]. Since then, this technology has developed rapidly, and a variety of electron shuttles have been reported in academia. The electron transfer oxidation technology has high oxidation activity for various pollutants including phenol, 2,4-dichlorophenol (2,4-DCP), 4-chlorophenol, bisphenol A (BPA) and sulfonamide antibiotics (Table 2). In addition, some studies have found that this non-radical technology can effectively inactivate Escherichia coli. According to the different main components of materials, the common electron shuttles can be divided into three categories: carbonaceous electron shuttles, transition metal electron shuttles and metal-porous carbon composite electron shuttles. Among them, carbonaceous electron shuttles mainly include CNT, biochar, and nitrogen-doped porous carbon (Table 2).

2.2.1 Carbon electron shuttle

Carbon materials can not only be used as adsorbents to remove pollutants, but also as electronic mediators to promote the degradation and transformation of pollutants. This recognition comes from the research of Zee et al., who found that activated carbon can be used as an electron donor for the reductive degradation of azo dyes[43]. With the development of the concepts of carbon nanomaterials and biochar, more and more attention has been paid to this role of carbon materials[44⇓-46]. Recent studies have found that carbon materials with sp2 hybrid structure (CNT, reduced graphene, nanodiamond, graphitized porous carbon, etc.) Can also play a role of electron transfer mediator in the field of catalytic persulfate. Duan et al. First found that there was a non-radical-dominated oxidation process in the system in the study of N-doped CNT catalyzed PMS for phenol removal[30]. Lee et al. Found that the oxidation process was not dominated by common free radicals when using graphitized nanodiamonds to catalyze PDS[47]. On the basis of studying the catalytic process by using linear sweep voltammetry, chronoamperometry and various spectral characterization techniques, they proposed that PDS and pollutant phenol first form a charge transfer complex with the catalyst, and then the catalyst mediates the transfer of electrons from the pollutant to PDS to achieve oxidative degradation. Subsequently, Yun et al. Also confirmed the important role of CNT in the electron transfer process by using similar electrochemical means, and proposed that the CNT-mediated electron transfer from organic matter to PMS was the main mechanism of non-radical degradation of pollutants[12,31]. In addition to PMS, Ren et al. Found that CNT has a similar mechanism of action with PDS[27]. On the basis of electrochemical characterization and in-situ Raman spectroscopy results, Ren et al. proposed that CNT and PDS first form a CNT surface-confined and activated PDS complex (CNT-PDS*)[27]. This complex has a high oxidation potential and can selectively extract electrons from the organic matter adsorbed by the catalyst. Hu et al. Used N-doped carbon nanospheres as PMS catalyst to degrade phenol, and proposed that the degradation of pollutants was carried out in two steps: 1) electrons were transferred from graphitized N in the catalyst to PMS, and PMS itself was decomposed after receiving electrons[25]; 2) The catalyst that loses electrons is in a metastable oxidation state, and this metastable state seizes electrons from the organic matter, thus realizing the oxidative degradation of pollutants.
According to the mechanism of electron transfer degradation, the adsorption performance of electron shuttles on pollutants is very important. To this end, researchers have used porous carbon materials with high specific surface area to catalyze persulfate. Tang et al. employed CMK-3 mesoporous carbon to catalyze the removal of 2,4-DCP by PDS, and found that the removal of the target pollutants was dominated by the electron transfer mechanism in addition to the participation of various radicals (SO4·-,·OH, O 2 · -)[40]. Chu et al. synthesized N-doped hierarchically ordered porous carbon (N-HOPC,702 m2/g) with high specific surface area by polymerizing 2,6-diaminopyridine using SiO2 as a template[38]. The results of XRD and XPS characterization show that C and N in N-HOPC are in sp2 hybrid state. When it was used to remove organic pollutants, it was found that the porous carbon had excellent adsorption and catalytic properties. N-HOPC can adsorb more than 95% of BPA in 1 min, and the adsorption equilibrium time of pollutants on its surface is much lower than that on the surface of activated carbon. Under the same reaction conditions, N-HOPC can adsorb and remove 85% of BPA, while CNT and reduced graphene oxide (rGO) can only remove 21% and 22% of BPA, respectively. The high adsorption activity and extremely short adsorption equilibrium time of N-HOPC can be attributed to the π-π electron donor-acceptor interaction and hydrophobic interaction between N-HOPC and pollutants[48]. The catalytic performance of N-HOPC was much higher than that of CNT and rGO in the removal of various organic pollutants (BPA, SMX, 4-CP, etc.) by PMS. It was found that the electron transfer from organic compounds to PMS mediated by N-HOPC was the main mechanism of pollutant degradation, and the unique electronic structure of N-HOPC caused by N doping played a key role in the process of induced electron transfer.
On the basis of previous research results on low-dimensional nanomaterials, researchers found that a variety of N-doped graphitized biochars can also induce persulfate to oxidize and degrade organic pollutants by electron transfer mechanism (Table 2). Zhu et al. Synthesized N-doped biomass porous carbon by pyrolysis of reed as carbon source[34]. With Orange G and PDS as the target pollutant and oxidant, respectively, the catalytic activity of this biochar was found to be much higher than that of common catalysts such as CNT, Co3O4, ZVI, and Fe3O4. Ho et al. Prepared N-doped biochar by pyrolysis of Spirulina platensis and used it to catalyze PDS to remove pollutants[35]. The results showed that this non-radical system could not only remove sulfamethoxazole, but also inactivate Escherichia coli efficiently. Biochar is an important development direction of carbonaceous electron shuttles because of its wide source and low cost.

2.2.2 Metal oxides

The oxidative degradation of organic pollutants by transition metal-catalyzed persulfate electron transfer was initiated by Zhang et al[29]. Zhang et al. used CuO to catalyze the oxidative removal of 2,4-DCP by PDS, and found that the removal of 2,4-DCP was hardly affected by high concentrations of ethanol ( [ethanol]/ [2,4-DCPs] = 4000) and chloride ions ([Cl-]/[2,4-DCP]=1000). On the basis of kinetic studies and material characterization results, Zhang et al. Proposed that PDS was activated on the surface of CuO through an outer-shell complex mechanism, and the activated PDS received electrons from 2,4-DCP, thus realizing the oxidative degradation of pollutants and the decomposition of PDS. In recent years, Huang and Zhang found that iron-doped δ-MnO2(FexMn1-xO2) bimetallic oxides can also catalyze PMS oxidation to remove BPA by electron transfer[13]. Interestingly, when MnFe2O4 and Mn1.8Fe1.2O4 were used as catalysts to activate PMS to degrade pollutants (dyes, BPA), the degradation of pollutants was a radical (SO4·-,·OH) dominated process[49,50]. This difference may be caused by the oxidation state of manganese in the catalyst and the crystallinity of the catalyst. The catalytic activity of MnO2 for PMS is mainly controlled by its crystallinity[51]. With the transformation of crystal form, the main active species produced by PMS will change from free radical to non-free radical[52].

2.2.3 Metal-carbon composite material

In addition to using single metal oxides and carbon materials as electron shuttles, metal-porous carbon composites mediated electron transfer oxidation of pollutants have also been reported. Luo et al. Studied the activation of PDS by CuO/biochar, Fe3O4/ biochar and ZnO/biochar, and found that CuO/biochar mainly acted as an electron shuttle, while the latter two activated PDS to produce free radicals[53]. Meanwhile, the removal and mineralization efficiency of CuO/biochar-PDS for the target pollutant BPA was much better than that of Fe3O4/ biochar-PDS and ZnO/biochar- PDS.

2.3 Advantage and disadvantages of different electron shuttle

Although the electron shuttles of non-radical-activated persulfates can be divided into three categories, the current research focus is still on carbonaceous materials. Compared with metal oxides, carbonaceous materials often have larger specific surface area, and their surface functional groups are easier to control. At the same time, pure carbonaceous materials do not have the problem of metal ion dissolution. However, the stability of carbonaceous catalysts tends to be relatively poor. Due to the diversity of surface microstructure, the catalytic mechanism of carbonaceous materials is more complex. At the same time, some carbonaceous electron shuttles, such as CNT, have great potential toxicity, and their residues in water may cause more serious secondary pollution[54,55]. Compared with CNT and other two-dimensional carbon nanomaterials which have high preparation cost and potential toxicity, porous carbon represented by biochar is more environmentally friendly and has a wider range of sources. Biochar has shown great application prospects in wastewater treatment, soil remediation and other fields, and the proposal and development of this material as an electron shuttle has greatly promoted the application of carbonaceous electron shuttles in the field of micro-interface electron transfer oxidation and degradation of pollutants[56,57]. However, due to the wide source of synthetic raw materials, the surface physical and chemical properties of biochar are easily affected by the composition of raw materials and pretreatment methods, and the regulation of its catalytic mechanism and active sites is more difficult than that of low-dimensional carbon nanomaterials.

3 Characterization of Electron Transfer at Catalytic Microinterfaces

Due to the diversity of microstructure of carbon materials, the characterization of active species and degradation mechanism is a major difficulty in the field of persulfate activation, especially the situation involving non-free radical oxidation degradation mechanism is extremely complex. Common transition metals such as iron, copper and cobalt can not only catalyze persulfate to produce · OH, SO4·- and O 2 · - radicals, but also produce high-valence metal oxides such as FeIV=O, CuIII=O and CoIV=O[58,59][60⇓-62][63]. The characterization of free radicals is relatively simple, and the electron paramagnetic resonance (EPR) spectroscopy combined with the classical radical trapping agent test can be used to study them qualitatively. Furthermore, the production of free radicals can be indirectly quantified by using benzoic acid as a molecular probe. In addition to free radicals, complex radicals have also been proposed[64]. However, the physical and chemical characteristics of complex free radicals are not very clear, and the situation of the production of such free radicals is more complicated. At present, the characterization of high-valence metal oxides in catalytic oxidation systems is often based on the directional oxidation of sulfoxide compounds (dimethyl sulfoxide and methyl phenyl sulfoxide). For example, high-valence metal oxide FeIV=O will convert dimethyl sulfoxide to dimethyl sulfone through oxygen transfer reaction, while hydroxyl radicals will oxidize it to methanesulfinic acid and ethane[65][66,67]. Before studying the degradation of organic pollutants by electron transfer mechanism, it is generally necessary to use various means to exclude the role of other reaction mechanisms. Like the electron transfer process, singlet oxygen and high-valence metal oxides can oxidize pollutants by the electron transfer mechanism. Therefore, when these different mechanisms coexist, how to quantitatively study the role of these mechanisms in the degradation process of pollutants is still a difficult point for future research. The identification process of a typical electron transfer system is shown in Figure 2.

3.1 Free radical determination

3.1.1 Electron paramagnetic resonance

EPR induces the hyperfine splitting of EPR line, which promotes the interaction between unpaired electrons in free radicals. EPR spectroscopy detects free radicals by detecting the unpaired electrons in free radicals to obtain structural information. However, the lifetime of common free radicals in aqueous solution is very short, so they can not be directly analyzed. Typically by the addition of a radical scavenger, the scavenger forms stable radical addition products with the radicals, and these products have specific EPR signals. Therefore, by detecting these addition products, the type of free radical can be determined[68]. At present, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) is generally used as a scavenger for common free radicals including hydroxyl, SO4·- and O 2 · - ,, and tetramethylpiperidinol (TMP) is generally used as a scavenger for singlet oxygen (1O2)[69,70][71].

3.1.2 Free radical trapping experiment

The signal of free radicals detected by EPR could not show the role of free radicals in the degradation of pollutants, and it was difficult to distinguish the relative content of different kinds of free radicals (such as · OH and S O 4 · -). In the vast majority of advanced oxidation technology studies, researchers often investigate the role of free radicals based on the role of common alcohol scavengers (methanol, ethanol, isopropanol, etc.). The reaction rate of chloroform with O 2 · - is as high as k<sub> chloroform </sub>=3.0×1010M-1·s-1[72], which is higher than that of most organic substances with O 2 · -. Therefore, when these alcohols or chloroform can completely inhibit the degradation of the target pollutant in the persulfate system, it is generally considered that this system is a free radical-dominated reaction. On the contrary, when there is no obvious inhibitory effect, it is generally considered that this is a non-radical-dominated system or the participation of surface complex radicals[64,73].

3.2 Nonradical identification

In the non-radical oxidation system, although there have been many reports on the use of high-valence metal oxygenates in catalytic persulfate systems, there is still no direct experimental means to prove the role of these high-valence transition metals in the degradation of pollutants. As mentioned above, the identified high-valence metal oxides widely used in current research are often based on the directional oxidation reactions of sulfoxide compounds (dimethyl sulfoxide and methyl phenyl sulfoxide)[66]. In addition, some studies have shown that high-valence metal FeIV, unlike free radicals, can not oxidize benzoic acid to p-hydroxybenzoic acid[66]. Therefore, the use of benzoic acid as a molecular probe may be helpful for the identification of high-valence metal oxides.
1O2 are more common as non-radical species in activated persulfate systems than high-valence metal oxides (complexes)[74,75]. The identification of the 1O2 is similar to that of free radicals, and EPR spectroscopy is used to characterize and investigate the effects of NaN3 and furfuryl alcohol on the degradation of pollutants. NaN3(4.5×108M-1·s-1) and furfuryl alcohol (1.2×108M-1·s-1) react rapidly with 1O2 and are therefore frequently used as trapping agents for 1O2[76]. However, both NaN3 and furfuryl alcohol react with persulfates such as PMS ( k Na N 3 =0.66 M-1·s-1,k<sub> furfuryl alcohol </sub>=0.059±0.004 M-1·s-1)[77,78]. Although 1O2 are believed to be produced in a variety of activated persulfate systems, based on the strong capture of 1O2 by water molecules (2.5×105s-1), it is believed that trace contaminants in aqueous solutions are unlikely to have the opportunity to react with 1O2[77].

3.3 Electrochemical test

Since PMS is also able to oxidize many electron-rich organic pollutants through electron transfer (mainly double electron transfer), electrochemical characterization plays a crucial role in revealing the mechanism of electron transfer reactions at microinterfaces[79]. At present, the commonly used electrochemical characterization methods are linear sweep voltammetry, chronoamperometry and chronopotentiometry[80]. By using linear sweep voltammetry and chronoamperometry, it can be found whether the presence of pollutants will seriously affect the efficiency of electron transfer at the micro-interface. When pollutants and persulfate coexist on the surface of the electron shuttle, they can greatly promote the generation of current in the system[28]. Electrochemical impedance spectroscopy (Eis) can characterize the conductivity of electron shuttles, linear sweep voltammetry (LSV) can measure the half-wave potential of pollutants, and the structure-activity relationship between conductivity and catalytic activity of electron shuttles can be constructed by combining Eis with LSV or chronoamperometry.

3.4 Decomposition of persulfate

In the electron transfer system, the decomposition of PDS/PMS depends on the electron transfer of organic compounds. In the reported electron rotator system, although the existence of other catalytic mechanisms can lead to the decomposition of PMS/PDS, the addition of pollutants has a significant effect on the decomposition of persulfate[13,38,81]. Nie et al. Observed that in the PDS electron transfer system, the rate of sulfate ion formation was significantly accelerated in the presence of contaminant phenol[82]. In contrast, in the radical-dominated advanced oxidation system, the report showed that the pollutants had a significant inhibitory effect on the decomposition of persulfate[11,64]. Therefore, the effect of pollutants on the decomposition rate of persulfate can be used to investigate the role of electron transfer mechanism.

3.5 Theoretical calculation

In recent years, catalytic reaction processes have increasingly relied on a combination of experimental phenomena and theoretical calculations (simulations). Theoretical calculation can overcome the limitations of current characterization methods and reveal the reaction mechanism at the molecular and atomic levels. In the field of persulfate activation, Ao Zhimin's team of Guangdong University of Technology has done a lot of work using density functional theory (DFT) calculation, which has greatly promoted the understanding of the micro-interface effect of pollutant-persulfate-carbon-based activator by researchers in the industry[82⇓⇓~85]. Recently, Ao et al. Used DFT to study the active sites of PDS activated by B and N doped carbon nanomaterials. On the basis of traditional evaluation indexes (adsorption energy, O — O bond length and S — O bond length), they proposed to add local electrophilic index (ω) to evaluate the active sites, which provided a reliable standard for the identification of active sites in electron transfer technology[82].

4 Factors affecting the activity of electron shuttles

There are many factors affecting the activity of electron shuttles. For example, reaction conditions, water quality parameters and the microstructure of the catalyst itself will affect the catalytic activity. In this section, the influencing factors are summarized and reviewed from the perspective of catalyst activity regulation.

4.1 Factors affecting the catalytic activity

The electron transfer kinetics between pollutants and persulfate is affected by the electronic structure of the electron shuttle and the surface characteristics of the material, such as surface functional groups, surface charge and specific surface area[80]. It was found that the PMS-mediated electron transfer oxidation of pollutants by carbon materials depends heavily on the electron density in the carbon lattice, which is affected by the degree of graphitization, electron-withdrawing groups and electron-donating groups of the catalyst[86]. The higher the degree of graphitization, the less the electron-withdrawing groups and the more the electron-donating groups, the easier the carbon catalyst acts as an electron shuttle. Therefore, one of the main directions for the modification of carbon materials is around the regulation of electron density. For example, doping with N heteroatoms with higher electronegativity can make neighboring carbon atoms have stronger electron-withdrawing ability[87]. At the same time, the doping process is also expected to produce more structural defects, which can activate PMS or PDS[88].

4.2 Influencing factors of adsorption activity

It was found that graphitized N had a strong adsorption effect on PMS, and doping with N could significantly promote the adsorption of PMS on the surface of N-CNT, thereby increasing the oxidation potential of the metastable complex of N-CNT-PMS*[89]. The higher the oxidation potential of the N-CNT-PMS*, the stronger its ability to oxidize pollutants.
In addition, there is a positive correlation between the adsorption effect of porous carbon materials on pollutants and the total removal rate of pollutants in the catalytic system[34,90]. Therefore, the adsorption performance of electron shuttles on pollutants plays an important role in the removal efficiency of pollutants. It was found that pyrrole N could adsorb BPA through a donor-acceptor complex mechanism, so N-doping could improve the degradation effect by promoting the adsorption of pollutants on the surface of the catalyst[74]. Increasing the specific surface area of the catalyst can significantly improve the adsorption efficiency of pollutants and persulfate oxidants under the condition of the same number of adsorption sites per unit.

5 Influence of common substrates in water environment

As shown in Figure 3, various matrices widely present in the water environment will compete with pollutants for free radicals and active sites, which is one of the biggest challenges for the practical application of advanced oxidation technology, and is also the basis for the development of selective oxidation technology. In this paper, the effects of high concentrations of chloride (Cl-), carbonate/bicarbonate ( C O 3 2 -/ HC O 3 -) and natural organic matter (NOM) on persulfate activation systems (including free radical systems and electron transfer systems) were summarized and compared.

5.1 Chloride ion

Cl- is widely distributed in the aquatic environment, with concentrations up to 3 mmol·L-1 and 100 mmol·L-1 in secondary treated effluent and industrial wastewater, respectively, and between 0.09~0.14 mmol·L-1 and 0.28~0.42 mmol·L-1 in surface water and groundwater, respectively[91~93][94]. This concentration of Cl- is much higher than the concentration of common micro-pollutants in the effluent of sewage treatment plants and surface water (ng/L ~ μg/L). At the same time,The rate constants of Cl- with · OH and S O 4 · - are up to k C l - , · OH, respectively.=4.3×109M-1·s-1 and k C l - , S O 4 · - =1.9×108M-1·s-1[95][22]. Therefore, the Cl- in the polluted water, whose concentration is much higher than that of trace pollutants, will capture hydroxyl radicals and S O 4 · - to generate chlorine radicals (· Cl). Cl is rapidly convert to C l 2 · - in that presence of excess Cl-. Although the oxidation ability of C l 2 · - is weaker than that of hydroxyl radical and S O 4 · -, its instantaneous concentration is much higher than that of the latter two, and it can also oxidize pollutants through H abstraction, addition and an electron transfer mechanism. Therefore, the removal efficiency of pollutants in the presence of a large number of Cl- is related to the activity of pollutants and C l 2 · -. In addition, PMS having an asymmetric structure is more likely to react with a nucleophile such as a halogen anion (X-) to generate HOX than PDS having a symmetric structure[20,96]. HOX will react with organic matter to produce halogenated toxic by-products[97]. In the electron transfer system, the Cl- is not converted into the C l 2 · - by the free radical. At the same time, the galvanic cell oxidation system developed based on electron transfer can separate persulfate and X- contaminated water into different reactors, thus avoiding the generation and accumulation of toxic halogenated degradation products by eliminating the direct interaction between PMS and X-[13,14]. As shown in Table 3, the Cl- as high as 1000 times the concentration of the pollutant had almost no effect on the degradation efficiency of the pollutant in the electron transfer process.

5.2 Carbonate/bicarbonate ion

Like Cl-, C O 3 2 -/ HC O 3 - ions are common components in contaminated water, with concentrations up to 2.7 mmol·L-1 in effluent from wastewater treatment plants[93,98]. High concentrations of C O 3 2 - ( k · OH =8.5×106M-1·s-1; k S O 4 · - =1.6×106M-1·s-1) and HC O 3 - ( k · OH =3.9×108M-1·s-1; k S O 4 · - =6.1×106M-1·s-1) will S O 4 · - · OH and U to the carbonate radical (UN C O 3 · -[5,99] The oxidation ability of C O 3 · - (E0=1.63 V) is significantly lower than that of · OH and S O 4 · -, but it has higher selective oxidation activity for electron-rich compounds[100]. Therefore, studies on advanced oxidation processes promoted by C O 3 2 -/ HC O 3 - have also been reported. The persulfate activation technique based on the electron transfer mechanism does not involve the role of free radicals and thus is able to avoid the interference of C O 3 2 -/ HC O 3 - (Table 3). Ho et al. studied the effect on N-doped graphitized biochar/PDS and found that the C O 3 2 -/ HC O 3 - of the 7.2 mmol·L-1 had no significant effect on the removal of the target pollutant sulfamethoxazole[35].

5.3 Natural Organic Matter

NOM is a complex mixture of aliphatic and aromatic compounds, which is widely found in surface water, groundwater, drinking water and sewage. NOM is mainly composed of humic substances (such as humic acids, HA), fulvic acids (FA), Humins and non-humic substances (such as carbohydrates, proteins, etc.). The reaction rates of humic acid with · OH and S O 4 · - were up to (1.4±0.2)×104L·mgC-1·s-1 and (6.8±0.3)×103L·mgC-1·s-1[101], respectively. Therefore, humic acid will compete with pollutants for free radicals, which will seriously affect the actual treatment effect of advanced oxidation technology (especially the oxidation reaction dominated by · OH). Feng et al. studied the effect of humic acid on the degradation of 1,4-dioxane by FeS2/PMS advanced oxidation system (· OH as the main active species), and found that the presence of 5 mg/L humic acid could increase the degradation efficiency of 1,4-dioxane from 92% to 53%[11]. Xie et al. Examined the treatment effect of UV/PDS in the presence of different concentrations of NOM, and found that when 1 mg/L NOM was present, the degradation rate constant of pollutants dominated by · OH decreased sharply[102]. Therefore, the presence of NOM in water has become one of the major challenges for the practical application of radical-based advanced oxidation technologies[21,103]. Although there are few studies on the effect of NOM on the persulfate electron transfer reaction, the reported studies show that the effect of NOM on this mechanism is not obvious (Table 3). For example, Chu et al. Found that N-doped porous carbon/PMS could efficiently remove a variety of pollutants including BPA, while Suwannee NOM up to 50 mg/L had no significant effect on the removal of pollutants[38]. Therefore, the electron transfer oxidation technology based on persulfate activation is expected to solve the competition between NOM and pollutants in advanced oxidation systems. However, NOM contains quinone groups and is also a good electron shuttle (Fig. 3), which has active electrochemical characteristics and can accelerate the transformation of some active sites between reduced and oxidized States[104,105]. At the same time, some functional groups in NOM, such as p-benzoquinone, can catalyze the decomposition of PMS to produce 1O2[71]. Therefore, the interaction between NOM and electron shuttles, pollutants and their potential activation of PMS in the electron transfer system need to be further studied.

5.4 Treatment of real wastewater by electron transfer

The effect of substrate in the laboratory is often based on the effect of different concentrations of individual substances on the removal efficiency or removal kinetics of pollutants. In the actual polluted water, there are many kinds of common substrates such as Cl-, HC O 3 - and NOM. Table 4 summarizes the effect of the electron transfer reaction based on persulfate activation on the purification of (simulated) real wastewater. It can be seen that even in the case of coexistence of multiple substrates, the target pollutants still have a good removal effect, indicating that this non-radical oxidation technology has a high selective oxidation.

6 Application of Interfacial Electron Transfer Technology

Although the effects of common substrates on the persulfate electron transfer oxidation system need to be further studied, it has been reported that this electron transfer technology has higher selectivity than the traditional radical oxidation process. Due to the existence of Cl-, C O 3 2 - ( HC O 3 -) and NOM, the selective and efficient removal of trace organic pollutants in water is a worldwide problem, and it is a great challenge to find an efficient, economical and green pollution control method[106]. The persulfate-based electron transfer system can selectively oxidize and remove pollutants under the background of high concentration of substrates, which provides a new solution for the removal of trace organic pollutants in surface water and the deep purification of sewage. At the same time, based on this principle, a variety of new reaction equipment can be developed according to local conditions. Fig. 4A shows the CNT membrane catalytic reactor developed by Yun et al.: the contaminant TCP and the oxidant PMS are placed in the A and B reaction chambers, respectively, and separated by a CNT membrane[12]. Lectron can pass through that CNT film, while wat molecules and ions cannot. During the reaction, TCP in chamber a loses electrons on the surface of CNT, and the electrons pass through the CNT membrane and react with PMS in chamber a, thus realizing the oxidative degradation of pollutants and the decomposition of PMS. Fig. 4B shows a galvanic oxidative degradation system developed by Huang and Zhang, in which the electrode material is a Fe0.15Mn0.85O2 supported on a graphite sheet[14]. Pollutants lose electrons at the anode, and the electrons interact with PMS through the copper wire and the anode, thus realizing the synchronous decomposition of pollutants and PMS. The oxidant PMS can be separated from the pollutants either in the CNT-membrane-based reactor or in the galvanic cell reactor, and the pollutants can be oxidized and degraded by transferring electrons through the electron shuttle medium. Because the PMS does not directly contact with the pollutant solution, the direct interaction of the PMS with the matrix in the water body, such as Cl- and Br-, is avoided. At the same time, the decomposition speed of PMS depends on the amount of electrons transferred from organic pollutants. Therefore, the utilization efficiency of the oxidizing agent is improved.

7 Conclusion and prospect

Although the development of activated persulfate micro-interface electron transfer oxidation technology is not long, due to its high selectivity and strong oxidation activity, this technology has shown great application prospects in the field of removing micro-pollutants coexisting with common substrates. However, from the perspective of micro-research, there are still some problems in this field. (1) Most of the pollutants reported in the literature are phenolic compounds (BPA, 2,4-DCP, etc.), and future studies need to focus more on micro-pollutants with different physical and chemical characteristics and strong environmental relevance. Different from phenolic compounds such as BPA(pKa=9.9~11.3), which are often studied as model pollutants, sulfonamides have multiple ionizable polar groups (pKa1=1.39~2.72;pKa2=5.0~10.67), and the interfacial adsorption behavior of these compounds is easily affected by the surface charge of electron shuttles. (2) In addition to the electron transfer reaction, the reported activation systems are often accompanied by the action of free radicals and other non-free radical mechanisms (such as 1O2). The key active sites of electron transfer induced by persulfate on the surface of the electron shuttle are not very clear, and the mechanism of electron conduction on the shuttle is not clear, so there is still a lack of regulatory mechanism and strategy for the active sites of the electron shuttle. The reported electron shuttles, such as CNT(pHzpc<3) and Fe-δ-MnO2(pHzpc=2.2~2.4), have negative surface charges in the environmentally relevant pH range (3 ~ 9), which is not conducive to the interaction with some equally negatively charged pollutants under neutral conditions. Therefore, how to control and design suitable electron shuttles according to the target pollutants is the focus of future research in this field. (3) When multiple non-radical mechanisms coexist, there is no clear quantitative method to determine the contribution of electron transfer mechanism in the removal of organic pollutants. (4) The mineralization effect of micro-interfacial electron transfer oxidation technology on organic pollutants is still not clear, and the transformation mechanism of pollutants needs to be further studied. The reported electron transfer processes are often accompanied by the generation of other non-radical active species (such as singlet oxygen), so the pollutant degradation pathways obtained from the studies are not necessarily entirely due to the contribution of the electron transfer mechanism at the solid-liquid micro-interface. This technology has strong selectivity for organic pollutants, so there is a risk of producing and accumulating degradation products in the process of pollutant degradation and transformation. (5) The interaction of common matrix in water with persulfate and pollutants on the surface of electron shuttle, the potential activation of NOM on persulfate and its impact on pollutant degradation are still unclear, which need to be further investigated in future research. At the same time, at the macro practical application level, there are also the following problems to be solved. (1) In view of the advantages of biotechnology in operating costs, the current mainstream water treatment technology is still dominated by biological methods, and how to couple this catalytic oxidation technology with traditional biotechnology is a key issue to be considered. (2) The long-term stability of the electron shuttle, the regeneration strategy and the operating cost of the process. (3) Potential secondary pollution of catalyst (electron shuttle) dissolved matter and its own residues.

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Funding

National Natural Science Foundation of China(42077340)

National Natural Science Foundation of China(52000080)

Guangdong Basic and Applied Basic Research Foundation(2019A1515110988)

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