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Heterogeneous Catalytic Persulfate Oxidation of Organic Pollutants in the Aquatic Environment: Nonradical Mechanism

Jia Liu , Jun Shi , Kun Fu , Chao Ding , Sicheng Gong , Huiping Deng

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (8) : 1311 -1322.

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Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (8) : 1311-1322 DOI: 10.7536/PC200750
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Heterogeneous Catalytic Persulfate Oxidation of Organic Pollutants in the Aquatic Environment: Nonradical Mechanism
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Abstract

Since the 1980s, the advanced oxidation process(AOP) in water treatment technology has been widely researched and applied. However, organic pollutants in water still plague scholars due to their wide variety and difficulty in degradation. Therefore, the mechanism of AOP needs to be clearly understood and get deeper into, which is conducive to technological application. As a typical AOP, the peroxymonosulfate & peroxydisulfate oxidation process has received a lot of attention in recent years, whereas there are still numerous disputes on the mechanism, lacking a unified understanding. The key reactive species of free radical process are generally ·OH and ·SO4-. Nonradical process contains 1O2 oxidation and peroxymonosulfate & peroxydisulfate oxidation(known as an electron transfer process), and high valence metal also participates in the oxidation process directly or indirectly. The specific mechanism of nonradical processes are still controversial, also the advantages and disadvantages. For the above reasons, this paper reviews the latest researches on the treatment of organic pollutants based on heterogeneous catalytic persulfate oxidation process in water, explains the reaction mechanism and the analysis methods, and points out the possible current research problems. Prospects for the future research direction and the application perspective of persulfate oxidation are proposed, with an emphasis on nonradical processes.

Contents

1 Introduction

2 Mechanism of singlet oxygen

2.1 Activated by ketone group

2.2 Produced by superoxide anion radical

2.3 Produced by PMS anion radical

2.4 Other ways for producing singlet oxygen

3 Mechanism of electron transfer

3.1 Catalyst-mediated electron transfer

3.2 Activated persulfate on the surface of catalyst

4 Mechanism of high valence metal

5 Analysis methods and catalyst deactivation

5.1 Analysis methods

5.2 Catalyst deactivation

6 Conclusion and outlook

Graphical abstract

Keywords

advanced oxidation process / persulfate / nonradical mechanism / singlet oxygen / electron transfer

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Jia Liu, Jun Shi, Kun Fu, Chao Ding, Sicheng Gong, Huiping Deng. Heterogeneous Catalytic Persulfate Oxidation of Organic Pollutants in the Aquatic Environment: Nonradical Mechanism. Progress in Chemistry, 2021, 33(8): 1311-1322 DOI:10.7536/PC200750

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1 Introduction

Advanced oxidation processes (AOPs) are powerful oxidation technologies that were developed in 1987 to generate strong oxidants in situ, such as hydroxyl radicals (· OH).Primary treatment processes aimed at the complete decomposition of organic pollutants in aqueous systems into carbon dioxide and water under ideal conditions, or the conversion of highly resistant and toxic compounds into less harmful compounds, combined with further biological treatment[1][2,3]. AOPs typically utilize photocatalytic, electrochemical, or highly reactive species produced by superoxide to oxidize and destroy organic molecules. It is generally believed that the free radicals produced by activated peroxides (ozone, persulfate and hydrogen peroxide, etc.), such as hydroxyl radical, sulfate radical (·SO4-) and superoxide ion (·O2-), are the main active species leading to organic degradation. Free radicals have a higher redox potential than their parent peroxides[4]. Fenton reaction and ozonation based on the oxidizing ability of hydroxyl radical (· OH), which can decompose various organic substances in aqueous systems almost non-selectively, have been widely studied and applied. ·SO4- based oxidation process (SR-AOP) is a Fenton-like oxidation system, which was first used in soil and groundwater remediation[5]. The ·SO4- are derived from peroxymonosulfate (PMS) and peroxydisulfate (PDS), which are believed to have higher redox potential (2.5~3.1 VNHE), longer lifetime (30-40 μs), wider pH range, and lower storage and transportation costs[6][4][7][8]. PMS and PDS are strong oxidants with redox potentials of 1. 82 and 2. 01 V, respectively, and their reaction rates are relatively low when they directly react with organic pollutants. In order to obtain the ·SO4- with the above advantages, PMS and PDS must be activated[9]. Searching with the keywords of "PMS/PDS" and "organic pollutant" (Web of Science), it can be found that the relevant research has been increasing year by year in the past six years (Figure 1), and the research has focused on effectively activating PMS or PDS to improve the degradation efficiency, such as heterogeneous metal/carbon-based material activation, homogeneous metal activation, energy (ultraviolet, heat, ultrasound, microwave) activation, electrochemical activation and alkali activation[10]. However, whether there are other oxidation pathways besides free radical oxidation pathways that contribute to the degradation of organic matter has become a hot topic[11]. Advanced oxidation processes that rely on strong oxidizing ·SO4- and hydroxyl radicals are generally called free radical processes. Recently, researchers have found that there are other active species and reaction pathways that dominate the removal of pollutants besides free radicals[12⇓~14]. In 2018, Duan et al. Classified non-radical processes into three types: catalyst-mediated electron transfer processes, direct oxidation processes of activated species on the surface of catalysts, and singlet oxygen processes[11]. The singlet oxygen (1O2) oxidation process can occur with or without the free radical process. Under appropriate pH conditions, unactivated PMS can also produce 1O2[15]. In fact, the direct oxidation process of activated species on the catalyst surface is also an electron transfer process, which refers to the electron transfer process from the target organic pollutant to the persulfate (PS) complexed on the catalyst surface.Among them, some researchers regard the catalyst-PS complex as an electron-accepting entity (the catalyst enhances electron transport), and some researchers regard the PS with elongated oxygen-oxygen bonds as a separate electron acceptor (the catalyst weakly activates PS)[16][17]. In addition to the above non-radical processes, a high valence metal oxidation mechanism based on the metal activation process has also been found in SR-AOP after the Fenton reaction[18].
The adverse reaction process caused by free radicals is shown in Formulas 1 to 14. Compared with the free radical process, the non-free radical process has unique advantages in the process of removing specific organic pollutants: the oxidation process is not obviously affected by the background substances in the water body because no secondary free radicals are generated[19]; Does not oxidize halide ions to produce ClO3- or BrO3-, where bromate is a carcinogen[20]; Persulfate (PS) and metal activators are not excessively consumed because the catalytic process is inhibited once the target organic pollutant is degraded[21][20][22]. The special advantages have attracted the attention of researchers. Figure 1 lists the number of studies on the non-radical oxidation mechanism of persulfate in recent years.
·SO4-+Cl-⇌SO42-+·Cl
HO·-+Cl-⇌HOCl·-
BrOH·-+H+→Br·+H2O
BrOH·-+Br-→·Br2-+OH-
Br·+Br·→Br2⇌HOBr+Br-+H+⇌OBr-+H+
OBr-+Br·+H2O→BrO3-+H+
·SO4-+HCO3-⇌SO42-+·HCO3-
HO·-+HCO3-⇌·CO3-
·SO4-+2Cl-⇌SO42-+·Cl2-
·SO4-+DOM⇌SO42-+reactiveintermediates
·SO4-+S2O82-→SO42-
S2O82-+·SO4-→SO42-+·S2O8-
·SO4-+·SO4-→S2O8-
·SO4-+Men+→SO42-+Me(n+1)+
In this paper, the non-radical mechanisms in the heterogeneous catalytic persulfate advanced oxidation process are summarized and divided into three mechanistic processes: the 1O2 oxidation process, the electron transfer process, and the high-valence metal oxidation process.The analysis methods and conclusions of the mechanism are described in detail. Based on the controversy of the non-radical process, this paper provides a positive outlook on its future research and application. Table 1 lists recent advances in the heterogeneous catalytic persulfate degradation of organic contaminants in water, based on 25 of these references, where the process follows or includes a non-radical mechanism.

2 Singlet oxygen mechanism

In 1938, Kautsky described the ability of oxygen to quench the fluorescence of a dye through an energy transfer process in which "activated" oxygen (O2) is produced[46]. The ground state oxygen can absorb energy from the triplet sensitizer and become singlet in an excited form (1O2). The 1O2 has the selective oxidation capability and has high reactivity to electron-rich organic molecules such as olefins, dienes, and polycyclic aromatic compounds due to its dienophile nature, while the 1O2 has a reduced reaction rate with the target containing an electron-withdrawing group, showing a lower oxidation efficiency[47]. In the persulfate oxidation process, 1O2 is one of the more common reactive oxygen species (ROS), such as unactivated PMS, which is formed by self-reaction at a lower reaction rate, and benzoquinone activated PMS can also be produced[15][48]. In the multiphase system, the sources of 1O2 are more abundant, and the precursors can be attributed to ketoid, superoxide anion (·O2-) and persulfate radical (·SO5-).

2.1 Activation proces by ketone group

Carbon-based catalysts have been widely studied due to their large specific surface area, special pore structure, abundant functional groups and defect sites[49,50]. It is generally believed that the defects and oxygen-containing functional groups at the edge of the structure are active sites, which play a role in catalyzing persulfate to produce free radicals[51,52]. In recent years, with the deepening of the research on the catalytic mechanism of carbon materials, non-radical processes have also been discovered. Guo et al studied the degradation of 2,4-dichlorophenol (2,4-DCP) by PDS catalyzed by carbon nanotubes (CNTs) and proposed the mechanism of oxidative 1O2[53]. In addition to the triplet signal of singlet oxygen detected by EPR, 9,10-diphenylanthracene (DPA) was used as a chemical scavenger for the 1O2 to form a stable endoperoxide DPAO2, which could be detected by LC-MS. In addition, 1O2 can be rapidly converted into 3O2, resulting in the increase of dissolved oxygen, which can also be used as a detection index to support the reaction mechanism. Researchers believe that CNTs contain carbonyl groups that make them follow a ketone-like activation mechanism, eventually producing 1O2[48]. Figs. 2 and 3 illustrate the generation of the 1O2 in this mechanism. It was found that the C = O groups on the end of CNT and benzoquinone were decomposed into cyclic dioxygen groups and sulfate groups after addition and dehydrogenation of PS, which continued to react with deprotonated PS to form 1O2, and the epoxy groups were converted into the original carbonyl groups to complete the catalytic cycle.

2.2 Generated by ·O2-.

The superoxide anion radical (·O2-) is often considered to be the precursor of the 1O2, and the conversion process is shown in Equation 15[54]. Taking the metal oxide catalyzed PS system as an example, Zhu et al. Considered that the MnO2 complexed PDS by the surface hydroxyl group, reacted with PDS to produce ·O2-, and further produced 1O2[25]. The fitting results of X-ray photoelectron spectroscopy (XPS) showed the changes of Mn — O — Mn and Mn — O — X bonds before and after the use of MnO2, and they believed that Mn — O-X bonds indicated metastable oxygen-containing bonds, and Mn participated in the reaction process with the change of redox valence.
2·O2-+2H2O→1O2+H2O2+2OH-
The existence of Mn (Ⅲ) was also confirmed by FTIR and Raman spectra. Figure 4 illustrates this process.
The crystal structure of metal oxides plays an important role in the reaction process. In the above systems, the first-order kinetic constants of phenol degradation corresponding to α-(β-)(γ-)MnO2 were 0.0264, 0.0441 and 0.0018 min-1, respectively. The (1 × 1) channel structure of the β-MnO2 is favorable for breaking the superoxide bond[55]. The researchers further analyzed the degradation ability of 1O2 to common organic pollutants, and found that it was closely related to the ionization potential (IP) of organic compounds. Benzoic acid (BA) and nitrobenzene (NB) with higher IP values were more difficult to be degraded. Different from the effect of electron-donating or electron-withdrawing groups, this explanation examines the selective oxidation ability of 1O2 to organic compounds from another angle.
In addition to single metal oxide activation, bimetallic oxide activation is also a hot research topic in recent years, such as cobalt-iron bimetallic oxides, copper-iron bimetallic oxides and copper-cobalt bimetalic oxides[56⇓⇓⇓~60]. The activation of bimetallic oxide takes advantage of that electron transfer between the two transition metal and between the transition metal and PS to generate redox reactions to generate free radicals. It is generally believed that the main metal reacts with PS to produce active species, and the auxiliary metal reduces the main metal to keep the main metal active[57]. In the latest research, besides free radicals, 1O2 are also considered to be active species. On the basis of CuO/Fe3O4, Jawad et al combined MgO to change the process of activating PMS from free radical pathway to non-free radical pathway[35]. The catalyst CuO-MgO/Fe3O4 showed stable performance over a wide pH range of 5.0 to 10.0, and the degradation kinetics was not inhibited by common radical scavengers, anions, or natural organic matter. The degradation rate of 4-chlorophenol (4-CP) was greatly increased to the 0.16 min-1, which was much higher than the 0.003 min-1 of the 0.008 min-1,Fe3O4 system of the 0.01 min-1,CuO system of the CuO/Fe3O4 system. They believe that the CuO-MgO/Fe3O4 forms a heterostructure, and the change of the density of States of the valence band (VB) and the conduction band (CB) after doping MgO causes Cu to form + 3 valence and accept electrons from PMS, resulting in the formation of ·O2-, which continues to form 1O2.
In the above two studies, the ·O2- originated from the oxidation of high valence metals (Mn (Ⅳ), Cu (Ⅲ)), and the oxygen in PS was oxidized into ·O2-. This process relies on the unique catalytic mechanism of metal oxides and is an indirect oxidation process of high-valence metals. However, it should be noted that not all metal oxide catalytic systems follow this mechanism.

2.3 Generated by ·SO5-.

Earlier, researchers proposed a series of responses to ·SO5-, as shown in Eqs. 16-18[61].
HSO5-→·SO5-+H++e
·SO5-+·SO5-→S2O82-+1O2
·SO5-+·SO5-→2SO4-+1O2
Wang et al. Developed carbon nitride (Fe-Co-O-C3N4) doped with iron, cobalt and oxygen, and found that the process of PMS degradation of antibiotics catalyzed by it was dominated by 1O2 oxidation, and proposed a more detailed production formula as shown in Formula 19[62].
·SO5-+·SO5-+H2O→1.51O2+2SO42-+2H+
They focused on analyzing the binding energy of each element before and after the use of the catalyst to infer the valence change of the metal elements, and found that oxygen-containing carbon nitride as an electron transport promoted the redox reaction between iron and PMS. Fe (Ⅲ) in high valence state oxidizes PMS to form ·SO5-, and Fe (Ⅱ) in low valence state reduces PMS to form ·SO4-. Analyzing the whole catalyst, they used electrochemical methods to conclude that the ability of the catalyst as an electron donor is greater than that as an electron acceptor, and that Fe (Ⅲ) and Co (Ⅱ) are the active sites of the catalyst.

2.4 Other modes of production

Bao et al. Found that the dominant active species was free radicals when they studied the degradation of antibiotic sulfamethoxazole (SMX) by PMS activated by cobalt-iron bimetallic oxides, but there were also 1O2 in the system, which were produced by the interaction of dissolved oxygen and free radicals[63].
Researchers believe that biological activated carbon (BC) is a low-cost, readily available and environmentally friendly carbon material with adjustable surface area and a large number of surface functional groups, which is also a good adsorbent itself and can be used for soil remediation and wastewater treatment[64][65]. Wang et al., in the study of degradation of typical azo dye Orange G (OG) by BC activated PDS, found that nitrogen doping of BC and high temperature treatment above 700 ℃ could not only improve the degradation efficiency, but also help to change the reaction from a free radical process to a non-free radical process[66]. After heat treatment at 400 ℃, the material generates sulfate radical through single electron transfer of persistent free radicals (PFRs) and further generates hydroxyl radical; Higher temperature heat treatment makes the material have large specific surface area and graphite nano-paper structure, such as higher degradation efficiency of target pollutants after heat treatment at 900 C.They believe that the dominant pathway at this time is the oxidation of 1O2 and carbon material-PDS composite. The determination of the active species was also based on a quenching experiment, in which potassium iodide (KI) and dimethyl sulfoxide (DMSO) were used to quench the active species bound to the surface of the catalyst after excluding the presence of free radicals.
In addition to carbon materials, some molecular sieves with regular pore structure, such as MCM, SBA and OMS, also have large specific surface area[67]. Researchers load transition metals on the surface of molecular sieves, and use molecular sieves to disperse metal ions to obtain more active sites[22]. In this study, a tetra-carboxy phthalocyanine cobalt-manganese octahedral molecular sieve (OMS-2) was synthesized and used as a catalyst for the degradation of diclofenac (DCF)[42]. DCF was degraded efficiently in the pH range of 5 ~ 9. It was found that Mn (Ⅲ) and Co (Ⅱ) reacted with HSO5- to form Mn (Ⅳ) and Co (Ⅲ). In addition, lattice oxygen is oxidized to 1O2, and oxygen vacancies formed on the surface ensure that the activity of the catalyst is maintained. Electron transfer from Mn to Co also occurred during the reaction. Figure 5 shows the various sources of 1O2.

3 Electron Transfer Mechanism

The electron transfer in the non-radical oxidation process means that the target organic compounds donate electrons to PMS or PDS to be degraded, and the catalyst in the system acts as an electron transfer medium or forms a complex with the oxidant PS to extract electrons from the organic compounds.

3.1 Catalyst-mediated electron transfer proces

The earliest SR-AOP study used Co2+ to activate PMS, but due to the toxicity of cobalt, researchers proposed to use other transition metals as activators to replace this activation method[7][68]. In heterogeneous processes, some inert metal oxides are often used as supports for metal-based activators. Lee et al. Supported a series of metal nanoparticles with Al2O3 and TiO2 respectively, and found a non-radical mechanism based on electron transfer in the activated PMS system[36]. In the study of Lee et al, the degradation of organic trichlorophenol (TCP) by activated PMS can be divided into three mechanisms: first, transition metals such as cobalt, copper, molybdenum, tungsten and nickel activate PMS to produce ·SO4-; Second, precious metals such as ruthenium, rhodium, iridium, platinum and gold can mediate electron transfer; Third, when manganese and palladium are activated, both free radical and non-free radical mechanisms may play a role. In particular, it should be noted that the PMS activated by the transition metal is oxidized and dissolved, and the transition metal does not play a catalytic role in this process.
Notably, they proposed that persulfates do not form active complexes with noble metals, thus ruling out a possible role for surface-complexed PMS in the catalyst-mediated electron transfer mechanism. In contrast, metal oxides containing surface hydroxyl groups and catalysts with strong adsorption capacity are more likely to complex persulfate.

3.2 Persulfate becomes an active species on the surface of the catalyst

Zhang et al. Proposed the surface process in 2014, which provides a crucial revelation[17]. Copper oxide (CuO) can effectively activate PDS under mild conditions without generating ·SO4-, and can effectively decompose 2,4-dichlorophenol (2,4-DCP). The researchers compared the catalytic ability of seven metal oxides (Fe2O3, MnO2, CuO, ZnO, TiO2, MoO3 and CeO2) and found that CuO stood out. According to the in situ characterization results of FT-IR and Raman spectra, it was found that there was no chemical bond formed between PDS and CuO, which could change the vibration frequency of PDS molecules adsorbed on the surface of CuO particles, that is, there was no dissociation of PDS on the surface of CuO particles. A new insight was proposed that PDS was first activated by CuO through outer-sphere interaction (rate-limiting step), and then reacted rapidly with 2,4-DCP in solution. In the presence of chloride ions in groundwater and industrial wastewater, this oxidation system has high degradation efficiency and can avoid the formation of highly chlorinated degradation products, showing significant advantages over ·SO4- oxidation. This study provides a new method for the removal of organic pollutants by persulfate. Based on the research work of Zhang et al., more organic pollutants have been verified to be effectively removed in this system (CuO/PDS). Du et al. conducted a study on p-chloroaniline (PCA) and found that the degradation rate decreased from 64.7% to 40.6% when the catalyst was reused three times (the adsorbed intermediates were washed with ethanol before the catalyst was reused), which they believed was related to the decrease of copper density on the CuO surface[39,40]. Xing et al. studied the antibiotic ciprofloxacin (CIP) and found the relationship between the reaction mechanism and pH on the basis of 100% degradation rate: they believed that (pH=4)1O2 and ·SO4- were the active species in the reaction under acidic conditions, while under alkaline conditions (pH = 8), the reaction process followed a non-radical mechanism, and CIP was degraded by activated PDS[43].
Carbon nanotubes themselves are good electron conducting materials. Lee et al. Used CNT as PDS activator to investigate the decomposition rate of PDS. Using linear sweep voltammetry and electron paramagnetic resonance spectroscopy, they proposed that PDS binds to the surface of CNT to form a complex, which decomposes immediately after reacting with organic pollutants[69][50]. This non-radical system is also selective, as shown in fig. 6, where phenyl derivatives substituted with electron-withdrawing groups, such as benzoic acid HBA and nitrobenzene NB, resist their degradation by the CNT/PDS system. Wang et al. Also carried out a series of studies on the system, using the characterization method of In-situ Raman spectroscopy, and detected the peaks marked as S2O82- and HSO5-. According to the change of the peaks when there is a catalyst In the system, it is inferred that the formation of surface complex CNT-PDS* and N-CNT-PMS*, and PS is easier to extract electrons from the electron donor (target organic matter) due to the elongation of the oxygen-oxygen bond[16,70,71]. It should be noted that N-CNTs can improve the adsorption capacity of CNTs on PMS, but not on PDS. Based on these studies, the persulfate system catalyzed by carbon nanotubes is expected to become a new treatment technology for the selective oxidation of organic pollutants in water.
Wang et al. Also compared the ability of various carbon materials such as multi-walled carbon nanotubes (MWCNT), mesoporous carbon CMK-3, diamond AND-900, AND reduced graphene oxide (rGO) to activate PMS for phenol degradation[19]. EPR AND chemical quenching experiments show that MWCNT and CMK-3 systems produce free radicals, while the edge defects of AND-900 and rGO promote the degradation of organic compounds but do not produce free radicals (the mode of action is to lengthen the oxygen-oxygen bond of PMS and transfer electrons from organic compounds to activated PMS). Further study on the effects of pH, common ions (Cl-,HCO3-,H2PO42-) and natural organic matter (NOM) in water on the degradation process showed that the degradation rate did not slow down when the concentration of anions increased to 20 mmol/L, which was attributed to the absence of secondary radicals with low oxidation. It is worth noting that bicarbonate plays a role in accelerating activation, which is different from the inhibition of bicarbonate in other non-radical process systems[19,39]. NOM and intermediates compete with phenol for PMS and hinder the interaction of the carbon material with PMS and phenol through π-π interactions.

4 Oxidation mechanism of high valence metal

Iron, manganese, copper, silver, and cobalt in high valence States have been identified as intermediate species in metal catalytic systems[18][72][73,74][75][76]. It has been proposed that some high-valence metal intermediates not only react with PS, but also oxidize the target organics.
After Jiang et al. Proposed in 2018 that Fe (Ⅳ) plays a dominant role in the homogeneous persulfate oxidation system, the role of Fe (Ⅳ) in the heterogeneous system has also been studied[18]. Compared with other transition metal-based catalysts, iron-based catalysts are less expensive and non-toxic. Zero-valent iron (Fe0) is the most widely used heterogeneous catalyst for PDS activation. The Fe0/PDS system has been proved to be effective in removing various organic matters from water. Li et al. Studied the process of removing atrazine (ATZ) by activating PMS with Fe0, and proposed that Fe (Ⅳ) played an oxidizing role in the system besides ·SO4- and hydroxyl. As shown in Formula 20, they used methyl phenyl sulfoxide (PMSO) as the indicator species of Fe (IV) to detect the amount of methyl phenyl sulfone (PMSO2) produced. The blank experiment confirmed that PMS could also oxidize PMSO to produce PMSO2 in the absence of Fe0, but the yield was only 45% of that in the activation system of Fe0 (dosage 0. 03 G/L). The ·SO4- and · OH in the system will compete with Fe (Ⅳ) to participate in the reaction with PMSO, but can not oxidize it to PMSO2 (as shown in formula 21 ~ 22), so the yield of PMSO2 in the system (η = mole ΔPMSO2/ mole ΔPMSO) is less than 100%. In addition, ·SO4- and hydroxyl radical can further oxidize PMSO2, and the reaction rate constants are in the order of 107 and 109, respectively, which also leads to the decrease of PMSO2 yield. Alcohol quenching experiments showed that methanol and tert-butanol could compete with Fe (Ⅳ) for the reaction with PMSO, thus inhibiting the formation of PMSO2. Hydroxide precipitates and sulfates were formed on the surface of the Fe0, and both ferrous and ferric iron were confirmed to be present.
Wang et al. studied the effect of pH on the Fe0/PS system[77]. Again using PMSO as the probe species, the yield of the PMSO2 was analyzed over a pH range of 3 to 7. When the pH value of the system increased from 3 to 7, both the decreasing value of PMSO and the increasing value of PMSO2 showed a downward trend. In addition, η increases from 80% to 91% when pH value increases from 3 to 4, and η decreases to 0 when pH value continues to increase to 7. They believe that the reason for the increase of η in strong acidic environment is that the reaction rate constant of Fe (Ⅳ) with small organic acids is much smaller than that of ·SO4- with acids. When pH increases, organic acids are mainly consumed by free radicals, resulting in more Fe (Ⅳ) reacting with PMSO. In the weak acid environment, the reasons for the decrease of η can be attributed to the following points: (1) the reduction of ferrous iron dissolved by acid corrosion; (2) precipitation of ferrous and ferric iron under near-neutral conditions; The ③Fe0 is passivated due to the formation of a (hydro) oxide shell on the surface. (4) The oxidation ability of Fe (Ⅳ) decreased with the increase of pH.
Fe2++S2O82-+H2O→FeIVO2++2SO42-+2H+
Fe2++HSO5-→FeIVO2++SO42-+H+
It can be judged from the above research discussion that although the researchers have confirmed the oxidizing effect of high-valent iron in the persulfate heterogeneous system, the direct production process of high-valent iron is still a homogeneous reaction, which is different from the singlet oxygen and electron transfer mechanism depending on the surface reaction of the catalyst, because its precursor is dissolved Fe (II) (such as formula 23, 24). Although in these systems, PS oxidizes Fe (Ⅱ) to produce high-valent iron, in other non-Fe0 systems, Fe (Ⅳ) or Fe (Ⅴ) is not detected, but Fe (Ⅲ) becomes the active center of PS oxidation, which shows that the role of high-valent metals can not be generalized[61]. In addition, it is not known whether the high-valence metals are directly involved in the oxidation of target pollutants or in other electron transfer processes, so the role of high-valence metals in heterogeneous persulfate systems remains to be studied.

5 Mechanism Analysis Method and Catalyst Deactivation

5.1 Mechanism analysis method

In non-radical processes, chemical quenching methods based on methanol (MeOH) and tert-butanol (TBA), and electron paramagnetic resonance (EPR) based on 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) were used to exclude the presence of ·SO4- and hydroxyl radicals. To further verify the existence of 1O2 in the system, sodium azide (NaN3) can be used as a quencher, furfuryl alcohol (FFA) and diphenyl anthracene (DPA) as chemical probes, which have been used to identify 1O2[37,78]. In addition, the characteristic triplet of the 1O2 was identified by spin trapping of 2,2,6,6-tetramethyl-4-piperidone (TMP) on electron paramagnetic resonance (EPR). Researchers have put forward different opinions on the reliability of the above active species identification methods.
It seems to be a general method to judge the existence of quenched ROS according to the inhibited degradation efficiency by adding alcohols to the reaction system. However, some studies have put forward different opinions, mainly focusing on whether the free radicals exist in the solution or on the surface of the catalyst. Sui et al. Suggested that hydroxyl radical may exist on the surface of the mesoporous material catalyst, and the quenching of tert-butyl alcohol is only effective for the free radical in the solution, so the quenching experiment can not simply determine that the reaction is a non-radical-dominated mechanism process[79]. Feng et al. Suggested that the ·SO4- might be bonded on the surface of the Pd/Al2O3, which explained the reason for the ineffective inhibition of the quenching experiment[80]. Other researchers have raised similar considerations[36,81]. As the heterogeneous system depends on the surface effect of the catalyst, it needs to be carefully verified, and more research methods should be used, such as EPR detection to obtain the response spectrum of ROS (TEMP/DMPO oxidation products), and the analysis of the oxidation products of the system can assist in judging the active species (based on the difference between free radical and non-free radical oxidation modes). If the multiphase persulfate oxidation system, which is actually a free radical process, is misjudged as a non-free radical process, it will be detrimental to the unique advantages of these two processes.
As an important intermediate active species in heterogeneous catalytic persulfate system, the analysis of the existence of ·O2- is particularly important to judge the reaction mechanism. Benzoquinone (BQ) is often used as a quencher of ·O2-, but the degradation efficiency of some systems is increased due to the presence of BQ, and it has been reported that BQ itself can activate PS[47,82]. NaN3, L-histidine, and FFA are often used as probes for 1O2. Although the second-order reaction rate constants of NaN3 and FFA with free radicals are in the order of 1010, they can still be distinguished from TBA and MeOH by controlling the concentration of quenchers in the system, so the existence of 1O2 can be inferred. It is worth noting that some researchers have proposed that NaN3 and L-histidine can accelerate the decomposition of PMS, but no one has proposed that the above two substances in PDS system are not suitable for probe research[15][13]. In conclusion, care must be taken to verify suitability with blank experiments before using chemical traps.
In order to determine the existence of 1O2, heavy water (D2O) was used to replace the solvent water (H2O).Because the lifetime of the 1O2 in the D2O is higher than that in the H2O, the reaction rate constant will increase[25,35][67]. Some researchers also found that there was no kinetic difference between solvent H2O and D2O on the oxidation of FFA by 1O2, so the effect of solvent may depend on the system[13].
To determine whether the oxidation process follows an electron transfer mechanism, the researchers used electrochemical methods to determine the increase in current values during the activation process to support the reaction mechanism. Voltammetry (including linear sweep voltammetry (LSV) and cyclic voltammetry (CV)), chronoamperometry, chronopotentiometry and electrochemical impedance spectroscopy can be used to analyze the electrochemical properties of the electrode[70]. However, voltammetry can also affect the analysis of electron transfer process for some reasons, such as the incomplete decomposition of persulfate and target organic matter in surface reaction, which affects the current signal[25]; The catalyst surface is oxidized[83]. Because the enhancement of electron transport may also lead to the generation of ROS, it is necessary to exclude ROS by quenching and EPR analysis before judging that the reaction only follows the electron transfer mechanism.
In the heterogeneous reaction system, catalyst characterization is an important means to analyze the structure, morphology, specific surface area, pore structure, element state and valence bond of the catalyst. The in-situ characterization method can monitor the change of the catalyst during the reaction[84]. Researchers have used infrared and Raman characterization methods to determine the complexation of the catalyst surface with PS, the valence of the metal, and the bonding between the catalyst support and the load[71][40].
As for how the oxidation process dominated by non-radical mechanism effectively decomposes the target pollutants to complete mineralization, the researchers use instrumental analysis and theoretical derivation to identify the intermediates, and sometimes the identified intermediates can also support the oxidation species in the system. Compared with radical oxidation, non-radical oxidation is difficult to form highly chlorinated products, for example, when trichlorophenol (TCP) is directly oxidized by PDS complexed on the surface of copper oxide (CuO) supported by reduced graphene oxide (rGO), only one chloride ion adjacent to the phenolic hydroxyl group is lost[40]. For example, sulfamethoxazole (SMX) contains benzene ring and isoxazole heterocycle, and the ring-opening product of isoxazole is not found in the system oxidized by ·SO4-, but is found in the system of carbon nanotubes (CNT)/PDS: SMX first undergoes an electron transfer with activated PDS to produce a free radical SMX·+, which is then hydroxylated and finally opens the isoxazole ring[85]. In the system of PDS catalyzed by Fe3C@NCNTs, the electron-rich aniline group is easily attacked by 1O2, which eventually leads to the opening of the benzene ring[33]. In this study, the frontier orbital energy calculation based on density functional theory provides theoretical support in the product analysis. SMX is the representative of sulfonamides antibiotics. Yin et al tried to analyze the oxidation process of sulfonamides (SA) drugs under non-free radical mechanism from the substituent group, and selected five kinds of SA as representative substances[45]. In the PDS system catalyzed by rGO, both 1O2 and electron transfer mechanisms exist. It was found that the energy gap (ΔE=ELUMO-EHOMO) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of SA with five-membered ring substituents was lower, and the degradation rate of SA with six-membered ring substituents should be higher, which was consistent with the experimental results. Researchers believe that the reason is that the SO2 in the -NH-SO2-R of SA with six-membered ring substituents are squeezed out of the molecule through a special rearrangement reaction pathway during degradation, and the rate of rearrangement reaction is lower than that of oxidation reaction of five-membered ring. Figure 7 shows the breakdown of five sulfonamides. Unlike the previous two systems, the open loop phenomenon was not traced in this system.
From the different conclusions of the above researchers, it can be found that the analysis of intermediates is difficult and controversial, and the combination of instrumental analysis and theoretical calculation is helpful to explore the degradation process of specific organic pollutants that need to be removed, which can be focused on in the future.

5.2 Catalyst deactivation

In the heterogeneous catalytic system, the reuse rate of the catalyst is an important indicator to judge the effectiveness of the catalyst.Although the latest research on the heterogeneous persulfate system shows that the degradation rate of the target pollutant remains at a high level after the catalyst is reused for five times, there are still problems of efficiency decline and metal ion dissolution. Catalyst deactivation occurs in both non-radical and radical processes.
The non-radical system of carbon catalysts depends on the adsorption of pollutants by the catalyst, but excessive adsorption and irreversible chemisorption of pollutants will block the active sites/pores of the catalyst, resulting in a decrease in the effective surface area of the catalytic reaction and affecting the reuse of the catalyst. Oh et al. Suggested that the adsorption contribution should be between 10% and 30%, and emphasized the physical adsorption depending on electrostatic and π-π orbital interactions[22]. For metal catalysts, even a small amount of dissolved metal ions can affect the reaction mechanism and the conversion frequency of the catalyst. It is difficult to analyze the contribution of high valence metals in the reaction system because they are unstable and difficult to be detected. Although researchers have made some achievements, more research methods are still needed to explore. In view of the characteristics of metal-based catalysts and carbon-based catalysts in the system, some researchers have developed composite catalysts in order to reduce the metal dissolution rate, increase the active sites and increase the dispersion.Although some composites have been shown to be highly repeatable and the reaction follows a non-radical process, the relationship between the reaction mechanism and the reusability of the catalyst has not been elucidated[24,61,86,87].
Some researchers used ultraviolet irradiation and heat treatment to restore the efficiency of the catalyst after the performance of the cycle experiment declined, and achieved certain results[37][88]. However, at present, the research based on different catalysts is too scattered, and it is more convenient and suitable for practical application to use catalysts with better repeatability than to reproduce the effectiveness of catalysts by means of post-reaction. From mechanism research to practical application, it is still necessary for researchers to systematically analyze and explore the heterogeneous persulfate non-radical system, and to develop low-cost and high-reuse catalysts.

6 Conclusion and prospect

Based on the three mechanistic processes of 1O2 oxidation process, electron transfer process and high valence metal oxidation process, the generation and mechanism analysis methods of non-radical processes in heterogeneous catalytic persulfate advanced oxidation process were summarized as far as possible. Some progress has been made in the study of non-radical processes, which has led scholars to explore the advanced oxidation processes more comprehensively and deeply. It must be admitted that both free radical and non-free radical processes have their own limitations, but non-free radical processes undoubtedly broaden the scope of research and practical applications.
The non-free radical process is less affected by the background substances in the water, and the reaction process is mild, so the non-free radical process has good application value for the water quality with complex background substances or the pretreatment process before biological treatment. However, as a selective oxidation process, the type of organic pollutants in water directly determines the applicability of non-free radical process, which is different from the process relying on the strong oxidation ability of free radicals. In addition, the mineralization rate is also a problem that needs to be solved directly in persulfate oxidation system dominated by non-free radical mechanism. The research on the non-radical mechanism is still in its infancy. There may be multiple reaction pathways in the heterogeneous catalytic persulfate oxidation system, and the species that play a direct or indirect role in the non-radical process are not yet fully understood, which is a challenge for future research. In view of the current research on the development of different catalysts and their mechanism analysis, a comprehensive evaluation is needed in order to promote the development of water treatment technology. In a word, future research can continue to focus on the unique advantages of non-radical processes and analyze the problems that need to be dealt with in practical applications.

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Funding

Major Science and Technology Program for Water Pollution Control and Treatment of China(2017ZX07201003-02)

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