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Activation Methods of Advanced Oxidation Processes Based on Sulfate Radical and Their Applications in The Degradation of Organic Pollutants

Wenliang Han , Linyang Dong

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (8) : 1426 -1439.

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Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (8) : 1426-1439 DOI: 10.7536/PC200771
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Activation Methods of Advanced Oxidation Processes Based on Sulfate Radical and Their Applications in The Degradation of Organic Pollutants
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Abstract

Advanced oxidation processes(AOPs) based on sulfate radical(SO4.-) have attracted more and more attention due to their high degradation ability and adaptability to new types of organic pollutants. Compared with hydroxyl radical(·OH), SO4.- has better selectivity, higher reduction potential, longer half-life, wider pH range and lower cost, so it can degrade pollutants more effectively. SO4.- can be produced by persulfate(PS) such as peroxymonosulfate(PMS), peroxydisulfate(PDS), etc. which are activated by thermal, mechanochemical, transition metal, carbonaceous materials, alkali, ultraviolet(UV), electrochemical methods, etc. Advantages and disadvantages of different activation methods and the research progress of their applications in the degradation of organic pollutants are analyzed. Three degradation mechanisms(addition, hydrogen abstraction and direct electron transfer) of pollutants with different functional groups by SO4.- are summarized. The degradation pathways, degradation products, and research progress of persistent organic pollutants(POPs), “pseudo-persistent organic pollutants”, i.e. pharmaceuticals and personal care products(PPCPs), and organic dyes by SO4.- are reviewed. Moreover, future research directions of this technique are prospected.

Contents

1 Introduction

2 Activation methods of persulfate(PS)

2.1 Thermal activation

2.2 Mechanochemical activation

2.3 Transition metal activation

2.4 Carbonaceous materials activation

2.5 Alkali activation

2.6 Ultraviolet(UV)activation

2.7 Electrochemical activation

3 Degradation of organic pollutants by sulfate radical

3.1 Persistent organic pollutants(POPs)

3.2 Pharmaceuticals and personal care products(PPCPs)

3.3 Organic dyes

4 Conclusion and outlook

Graphical abstract

Keywords

sulfate radical / advanced oxidation processes / peroxymonosulfate(PMS) / peroxydisulfate(PDS) / activation method / persistent organic pollutants(POPs) / organic dyes

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Wenliang Han, Linyang Dong. Activation Methods of Advanced Oxidation Processes Based on Sulfate Radical and Their Applications in The Degradation of Organic Pollutants. Progress in Chemistry, 2021, 33(8): 1426-1439 DOI:10.7536/PC200771

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1 Introduction

With the rapid development of modern industry, new environmental pollutants are emerging. organic pollutants such as persistent organic pollutants (POPs), pharmaceuticals and personal care products (PPCPs) and organic dyes have attracted wide attention because of their potential risks to the environment and human health[1⇓⇓⇓⇓~6]. These organic pollutants are not easy to degrade naturally, and need to be degraded artificially by physical, chemical or biological treatment methods, among which chemical treatment is the method with better treatment efficiency and removal effect.
Chemical oxidation technologies can destroy the molecular structure of organic pollutants and completely mineralize them. Among them, advanced oxidation processes (AOPs) or advanced oxidation technologies (AOTs) have attracted wide attention because of their good removal effect on refractory organic pollutants[7][8][9]. Among the common AOPs at present, the Fenton oxidation method based on hydroxyl radical (· OH) has been paid attention to earlier because the · OH produced by this method has strong oxidizing ability and can degrade and mineralize the refractory organic matter. However, this method has the disadvantages of large reagent consumption, no selectivity to the oxidized object, low oxidation efficiency and strong acid conditions (pH 3 ~ 4) required for the reaction[10][11]. Ozone oxidation has high energy consumption and low water solubility, and ozone is unstable and easy to decompose in the air[12]. Supercritical water oxidation (SCWO) has harsh operating conditions, high equipment requirements, and problems of corrosion and salt precipitation[13]. Although sulfate radical (SO4.-) based AOPs were reported as early as 1986, their application in the degradation of pollutants began in 1996[14]. In recent years, as a new type of AOPs, its oxidation mechanism, activation methods and application in the degradation of new organic pollutants have gradually become a research hotspot.
Compared with the ·OH,SO4.-, the invention has the following advantages: (1) the selectivity is better[15]; The reduction potential of (2)SO4.- (2.5 ~ 3.1 V) is higher than that of hydroxyl radical (1.8 ~ 2.7 V), so it has stronger oxidizing ability and higher degradation rate of pollutants[16]. The half-life of (3)SO4.- (30 ~ 40 μs) is much longer than that of hydroxyl radical (< 1 μs), so it is more stable and can adapt to the remediation of various water environments and soil pollution[15]. (4) It has a wider range of pH adaptation (2 ~ 10), while · OH acts only under neutral and alkaline conditions[17]; And (5) that cost is low[18]. Therefore, the research on the degradation of new organic pollutants by SO4.- has attracted wide attention in recent years[19,20].
The SO4.- is mainly produced by the activation of persulfate (PS) such as peroxymonosulfate (PMS) or peroxydisulfate (PDS). Some scholars also call persulfate (PS)[21,22]. PMS Is a white solid powder with an asymmetric structure and a solubility of 250 G/L. Its aqueous solution is acidic and stable when pH is less than 6 or equal to 12, but when pH is about 9, its stability is very poor and almost half of its HSO5- decomposes into SO52-.Commonly used PMS is potassium peroxomonosulfate complex salt, (2KHSO5·KHSO4·K2SO4);PDS is a colorless or white crystal with symmetrical structure, has high stability, is easily soluble in water, the solubility is 730 G/L, and the aqueous solution is also acidic. Commonly used PDS is Na2S2O8 and K2S2O8[17,23][15]. PDS has lower cost than PMS, higher water solubility and stability at room temperature, and higher degradation rate of pollutants due to the lower O-O bond energy (33. 5 kcal/mol) of PDS than PMS[19][24]. Shah et al. compared the degradation effects of endosulfan by UV/PMS, UV/PDS and UV/H2O2. Under the same conditions, the degradation rate of UV/PDS was the highest (91%), followed by UV/PMS (86%), and UV/H2O2 was the lowest (64%). This was because the bond energy of O-O bond of PDS (33.5 kcal/mol) was much lower than that of H2O2(51 kcal/mol), while PMS was between the above two[25].
PS itself is relatively stable at room temperature, the oxidation rate of organic matter is low, and the oxidation effect is not obvious, but the oxidation effect of SO4.- produced by activation is greatly improved. Chen et al.'S study on the degradation of cephalexin showed that when only PDS was added, there was almost no degradation, while when Cu was added to activate PDS to produce SO4.-, cephalexin was completely degraded[26].

2 Method for activating PS

The activation methods of PS include thermal activation, mechanochemical activation, transition metal activation, carbonaceous materials activation, alkali activation, ultraviolet (UV) activation and electrochemical activation. Different methods have different activation effects, and their applicability to pollutants with different properties and States is also different, so in practical application, different pollutants should choose appropriate activation methods according to their characteristics.

2.1 Thermal activation method

Thermal activation is the simplest activation method, which does not need to add foreign substances and avoids secondary pollution. Thermal activation is mainly used for the activation of PDS (easy for PMS), and the mechanism of thermal activation is:[27]
S2O82-+heat→2SO4.-
HSO5-+heat→SO4.-+·OH
Moderately increasing the thermal activation temperature can accelerate the rate of PS decomposition to produce SO4.-, thereby improving the degradation rate of organic pollutants, but the temperature is not the higher the better. Wang et al., the degradation rate of polycyclic aromatic hydrocarbons (PAHs) by thermally activated PDS increases with the increase of temperature below 60 ℃[28]; However, when the temperature was higher than 60 ℃, the degradation rate decreased due to the rapid consumption of oxidant.
Due to the energy consumption of thermal activation, waste heat such as industrial cooling water is used to activate PDS to degrade humic acid in water, and the degradation rate is more than 80%, but it is easily affected by pH[29]. The degradation rate was 98% at pH = 8, but decreased to ~ 50% at acidic conditions (2.5 ~ 7), which was mainly affected by the Cl- in the water. The Cl- competed with the organic pollutants for SO4.- under acidic conditions, which reduced the degradation rate. In general, this method can not only reduce the energy consumption, but also reduce the thermal pollution caused by waste heat emission, but the disadvantages are that the reaction rate is slow (120 H) and it will be affected by impurity ions such as Cl-, NO3- and CO32- in water.
Hu et al. Used microwave heating to activate PDS to degrade p-nitrophenol, which can activate PDS more effectively and reduce the activation energy of the reaction compared with the traditional heating method[30]. At 90 ℃, the degradation rate of microwave heating (93%) was much higher than that of traditional heating (35%). In addition, the degradation rate is not affected by pH (3 ~ 11) and impurity ions (Cl-, NO3-, etc.), indicating that microwave heating is a very promising thermal activation method.

2.2 Mechanochemical activation method

Mechanochemical activation is to put the material into a planetary ball mill, realize the conversion of electrons through high-speed collision between grinding balls, and use the high temperature conditions generated in the collision process to make PS produce a large number of SO4.- to degrade solid pollutants. The advantage of mechanochemical activation is that no other substances are added, which reduces the environmental risk caused by the by-products of additives, and is a green and efficient activation method. Different from other methods, mechanochemical activation is mainly used for the degradation of high concentration solid pollutants, but this activation method needs to select the appropriate abrasive, the operation is complex, the cost is high, and it is difficult to apply in engineering.
Because the O-O bond energy of PDS is lower than that of PMS, the degradation rate of PDS is higher than that of PMS. Huang et al. Added PDS particles and powdered decabromodiphenyl ether (BDE-209) into a ball mill tank, and the degradation rate of BDE-209 was 99.6% in 3 H, and the removal rate of total organic carbon (TOC) was 99.0%. The low-bromine intermediate was rapidly decomposed by total organic carbon, and no intermediate was detected[31]. When the grinding agent PDS was replaced by PMS, the degradation rate of BDE-209 decreased to 67. 8%, while the degradation rate of BDE-209 was only 51. 7% when calcium oxide was used as the dehalogenation agent. The method is not affected by pH, but the mass ratio of the grinding agent to the pollutant (MPDS/EDF-209=40), the number ratio of grinding balls of different sizes (N10mm/20mm=4∶1), and the ball-to-material ratio (M<sub> ball weight/BDE-209 </sub > = 50) affect the degradation rate, and too low or too high will affect the degradation rate because the surface contact is not optimal.
In order to eliminate the influence of thermal activation in the collision process, Wang et al used PDS and Al2O3 as abrasives to degrade perfluorooctanoic acid (PFOA) by mechanochemical activation of PDS, and the degradation rate reached 100%[32]. On the one hand, the Al2O3 is bound to PFOA through covalent bond, and on the other hand, the hydroxyl radical generated by the reaction of its surface hydroxyl group with SO4.- is also involved in the degradation. The control experiment showed that there was no obvious degradation of PFOA after heating PDS and Al2O3 to 100 ℃ for 2 H, which proved that only the collision could initiate the rotation, alignment and rebonding of molecules.

2.3 Transition metal activation method

When transition metals are used to activate PS, there are two ways: homogeneous and heterogeneous. For homogeneous metal activation, Co and Ru are the most effective activators for PMS and are Co2+>Ru3+>Fe2+>Ce3+>V3+>Mn2+>Fe3+>Ni2+, while Ag is the best activator for PDS, but Ag is expensive and harmful to human health[20][33]. V has been used to activate PS only in recent years, and its mechanism needs to be further studied[34]. In most cases, Cu plays a synergistic role in promoting the transition of other metals from high valence state to low valence state[35]. Mn is widely used because of its environmental friendliness, but its catalytic activity is low, so it is often used in combination with other metals[36]. Fe is the most widely studied, because it has a good activation effect on PMS and PDS, low cost and leaching is not as harmful to the environment and human health as other metals such as Co[20]. Heterogeneous metal activation can not only eliminate the negative effects of single metal, but also improve the degradation rate through the synergistic effect between ions[36]. In addition, the construction of some frameworks (carbonaceous materials or TiO2) can also avoid the environmental hazards caused by the leaching of metal ions[37]. The mechanism of metal-activated PMS or PDS is shown in Figure 1.
The nanometer metal has lar specific surface area, more sufficient contact with PS and higher degradation rate. Deng et al. Activated PDS with nano-zero-valent copper to degrade norfloxacin, and the degradation rate reached 96% in 5 min[38]. Wang et al. prepared nano-zero-valent iron (nZVI) by reducing FeSO4·7H2O with NaBH4 to activate PDS to degrade BDE-47, and the degradation rate was 64% in 48 H.The degradation rate increased to 83% when 2.5 mM of Cu2+ was added, because the high reducibility of Cu2+ reduced Fe3+ to Fe2+ to decompose more PDS.More SO4.- is generated to degrade the target pollutant, but sodium hydroxide is subsequently added to remove the Cu2+ in the solution, and the nZVI particles are easy to agglomerate due to strong magnetic force and van der Waals force, so that the contact points between the nZVI particles and PDS are reduced, the generation of the SO4.- is reduced, and the degradation rate is finally reduced[39]. Deng et al. Coated Ca(OH)2 on the surface of nZVI particles, which effectively avoided the above problems, and the Ca(OH)2 did not affect the contact between nZVI and PDS[40]. The modified catalyst activated PDS to degrade sulfamethazine, and the degradation rate reached 100% in 20 min, without the interference of low concentration of Cl-, HCO3- and NO3- in groundwater.
Table 1 summarizes the recent reports on the degradation of common organic pollutants such as POPs, PPCPs and organic dyes by PS activated by transition metals. Peng et al. activated PDS with Fe2+ to degrade BDE-209, and the degradation rate was 53% at pH = 3. By adding hydroxylamine (NH2OH), the oxidation product Fe3+ was reduced to Fe2+, which could continue to participate in the activation of PDS, thus producing more SO4.- and increasing the degradation rate to 66%[41]. For example, Zheng et al used Fe2O3 to activate PMS to degrade atrazine (ATZ), and improved the Fe2O3 by porous vulcanization, which greatly increased the degradation rate from 17% to 96%[20].
Except for POPs, the other three pollutants in Table 1 can reach a high degradation rate in a short time at room temperature, while the degradation time of POPs is long and the degradation rate is low. See 3.1 for the specific reasons. Due to the presence of the transition metal, the optimum range of pH varies with the nature of the activator. For example, the optimal pH for the degradation of BDE-47 by PDS activated by nZVI is 6.5 ~ 11.5, because the strong acid environment promotes the corrosion of nZVI and releases excessive Fe2+, which competes with BDE-47 for SO4.-, and causes the negatively charged oxygen atoms in PDS molecules to combine with H+, thus inhibiting its interaction with positively charged Fe ions and reducing the production of SO4.-[39]. Hydroxyl groups generated under alkaline conditions can be used as active centers for electron transfer to promote the decomposition of PDS, thereby improving the degradation rate[39]. Therefore, there is an urgent need to find transition metals with good activation effect, low price and adaptability to different environments.

2.4 Carbonaceous material activation method

Traditional carbon materials such as activated carbon (AC) and biochar (BC), as well as new nano-carbon materials such as ordered mesoporous carbon and graphene, can be used as metal carriers on the one hand, and can adsorb and activate PS to generate SO4.- on the other hand because of their large pore structure and abundant functional groups[49][50][51][52][53][54]. The physical and chemical properties of carbonaceous materials are different due to different raw materials and conditions, and the activation ability of PS is also different. Liang et al. prepared AC from sawdust at 800 ℃, and prepared BC400 and BC700 from peanut shells at 400 ℃ and 700 ℃, respectively. AC had the largest specific surface area and pore volume, followed by BC700, and BC400 had the smallest[55]. Under the same conditions, the degradation of sulfamethoxazde (SMX) by PDS activated by three kinds of carbonaceous materials was studied. Although the degradation rate of AC (91.2%) was higher than that of BC700 (88.7%) and BC400 (30.4%), the mineralization degree of BC700 was higher than that of AC, because AC mainly adsorbed pollutants on the surface (84% of the total degradation), while BC700 was more degraded in the liquid phase (53%). BC can be produced by anaerobic combustion of waste straw, which not only saves cost, but also solves the problem of straw recycling to a certain extent, and has a high degradation rate. For example, Wang et al prepared BC activated PDS for degrading norfloxacin by anaerobic combustion of corn straw, and the degradation rate was 99.57% in 50 min[56]. However, the stability of BC was poor, the degradation rate was only 70. 22% for the second reuse, and 40. 84% for the third reuse. Considering its green source and low price, it is still worth further study.
The surface chemical modification of carbonaceous materials can improve their adsorption and activation properties of PS[53]. Pi et al. Obtained oxidized BC (OBC) by treating BC with HNO3/H2SO4. BC and OBC were respectively combined with Fe3O4 to activate PDS. The adsorption (50%) and degradation (92.3%) of tetracycline on OBC were higher than those on BC (30% and 65.3%)[57]. BC can also promote the activation of PS synergistically with nZVI and other transition metals. However, the degradation of PS by BC was affected by pH, and the degradation rate was the best under acidic and neutral conditions, while the degradation rate was lower under alkaline conditions due to the formation of hydroxyl radicals. Because BC ash is rich in Na+, K+, Mg2+ and Ca2+, it will reduce the level of soil hydrogen ions and exchangeable aluminum ions in soil remediation, thus significantly increasing the soil pH value, and then reducing the degradation rate, so the engineering application of BC activation method needs to be further studied[58].
Carbonaceous materials can also be modified by heteroatom doping (nitrogen doping, sulfur doping, etc.) To improve their activation effect[59,60]. For example, the degradation rate of orange G by nitrogen-doped BC activated PDS prepared by Zhu et al increased from 39% to 100% in 120 min[61].
In addition to activating PS alone, carbonaceous materials can also be used as a carrier of transition metals to activate PS cooperatively, which can improve the activation effect and thus improve the degradation rate[62]. Li et al prepared a heterogeneous catalyst by loading BC on nZVI, which not only avoided the oxidation and agglomeration of nZVI, but also improved the stability of BC. The degradation rate of BDE-209 reached 87.5%, and BC could adsorb Br-, which was conducive to subsequent debromination[50]. Zhang et al. Used ordered mesoporous carbon to carry nZVI to activate PDS, and the degradation rate of sulfamethazine reached 70%, which was higher than that of nZVI/PDS (25%) and ordered mesoporous carbon/PDS (50%)[51].

2.5 Alkali activation method

The reaction of alkali-activated PS follows the following pathway:[63]
2S2O82-+2H2O→3SO42-+SO4.-+O2.- +4H+
SO4.- +OH-→SO42-+·OH
HSO5-+H2O→H2O2+HSO4-
H2O2+OH-→H2O+HO2-
HSO5-+HO2-→SO4.-+H2O+1O2
A large number of studies have shown that the influence of pH on the oxidative degradation of pollutants by SO4.- can not be ignored. Under acidic conditions, PS is mainly decomposed into SV, while under neutral and alkaline conditions, SO4.- and · OH coexist[64]. When PS is activated with alkali, a large amount of alkali needs to be added to adjust the pH, which increases the cost and corrodes the equipment.
Because the mechanism of alkali-activated PDS and PMS is still not clear, it is difficult to use them in engineering practice at present, but the combination of alkali and other activation methods can improve the degradation rate, for example, the degradation rate of BDE-209 by alkali-activated PDS increased from 30.3% to 46.6% after heat treatment; However, the degradation rate increased to 51.5% when a small amount of transition metal ions were added Fe2+ during alkali activation[65]. To sum up, although the degradation rate of the combination of alkali and other activation methods has a certain improvement, it is still lower than the effect of other activation methods, so it is mostly used to treat alkaline organic wastewater[66].

2.6 UV activation method

Under the irradiation of ultraviolet light, the O — O bond in PS was broken and the SO4.- was formed. There are two sources of free radicals produced by UV activation of PS, one is produced directly by UV irradiation, the other is produced indirectly by the reaction of PS with hydrogen radicals produced by the decomposition of water irradiated by UV. Recent reports on the degradation of organic pollutants by UV-activated PS, including anodyne, antihypertensive drugs, antibiotics, and endosulfan, are summarized in Table 2. Like transition metal activation, UV activation is also pH-dependent.
Serna-Galvis et al. Used UV/PDS to degrade five antibiotics, and the degradation rates were quite different under the same conditions (Table 2). The different chemical structures of antibiotics determined their different photodegradability[67]. Three fluoroquinolone antibiotics, ciprofloxacin, norfloxacin and levofloxacin, are not easily affected by photosubstitution because their benzene rings are not electrophilic, so their photodegradation coefficients are low and their degradation rates are low. On the other hand, the quantum yields of energy transfer from ciprofloxacin, norfloxacin and levofloxacin to molecular oxygen are 0.092, 0.081 and 0.076, respectively, which indicates that the formation of excited oxygen molecules induced by ciprofloxacin is easier than that of norfloxacin and levofloxacin[68]. Therefore, ciprofloxacin can be degraded more rapidly by the action of excited oxygen, and its degradation rate is relatively high.

2.7 Electrochemical activation method

Electrochemical activation is a new method for the activation of PS, which has attracted much attention because of its advantages of economy, environmental protection, controllable process and mild reaction conditions[72]. The mechanism of the degradation of pollutants by electrochemically activated PS is complex, which mainly depends on the direct and indirect oxidation reactions at the anode[73]. The oxidation mechanisms of different electrodes are also different. Song et al believe that the degradation mechanisms of pollutants on boron-doped diamond (BDD) anode include direct electrolysis, electrolysis of water and electro-activation of PS, in which electro-activation of PS can promote the generation of · OH in electrolysis water on the one hand, and convert PS into a transition state for non-radical degradation on the other hand[74]. Free radical quenching experiments showed that hydroxyl radical played a major role in the degradation of carbamazepine (CBZ), and its contribution rate was 82. 2% (PDS) and 90. 7% (PMS), which was also the reason why the degradation rate of electroactivated PMS was higher than that of PDS. However, it took Song et al. 120 min to completely degrade CBZ on Ti/Pt electrode, which was higher than that on BDD anode (20 min). Because Ti/Pt electrode did not produce hydroxyl radical, the degradation of CBZ on anode was mainly through non-radical pathway, while PDS was activated to produce SO4.- on cathode, but its contribution to the degradation rate was small (38% in 30 min), so the current research on electrochemical activation mainly focused on the anodic reaction[72][75].
Electrochemical activation can also be combined with transition metals. Cai et al. Added Fe-Co supported on SBA-15 (ordered mesoporous molecular sieve with SiO2 as the main body) to the electrolyte when they used electrochemical activation PDS to degrade acid orange.The degradation rate reached 60% in 5 min, while only 30% without Fe-Co, because Fe-Co activated PDS in addition to the electrochemical reaction, and the high valence Fe and Co ions were reduced to low valence at the cathode, which acted as catalysts to activate PDS again, thus producing more SO4.- to participate in the reaction[24].
In addition, the electrolyte can also choose sulfate, which is prevalent in surface water and wastewater, to replace PS, thereby reducing costs and avoiding secondary pollution[76]. When sulfate is used as the electrolyte, the anode generates hydroxyl radicals by electrolyzing water, and sulfate radicals also lose electrons and are converted into SO4.-, and the combination of the two SO4.- can be converted into PDS, and then the PDS is electrolyzed into a transition state to be activated by a non-free radical; and although the degradation rate is not as high as that when PS is used as the electrolyte, sulfate still has a wide application prospect due to low cost[77]. Pretreatment is required because natural organics and halide ions inhibit the reaction and produce harmful by-products[74].
To sum up, the application scope of each activation method is different, and the appropriate activation method should be selected according to the actual situation. Table 3 summarizes the advantages and disadvantages of different activation methods.

3 Degradation of organic pollutants by SO4.-;

There are three mechanisms for the degradation of organic pollutants by direct electron transfer: addition, hydrogen abstraction and direct electron transfer[22].
The addition reaction mainly occurs in the degradation reaction of pollutants containing unsaturated bonds such as carbon-carbon double bonds or carbon-carbon triple bonds, and the single electron of the SO4.- breaks the unsaturated bonds and reacts with the SO4.- to achieve the purpose of degrading the pollutants:[78]
SO4.- +H2C=CHR→-OSO2OCH2-CHR·
Hydrogen abstraction mainly occurs in the degradation of alcohols, ethers and organic acids. SO4.- directly abstracts hydrogen from the hydroxyl, ether and carboxyl groups of organic compounds, and finally degrades the organic compounds into CO2 and H2O:[78]
SO4.- +RH→HSO4-+R·
Direct electron transfer is the most common way in the degradation of organic pollutants by SO4.-, which is mainly found in the degradation of aromatic organic pollutants containing benzene rings. SO4.- directly transfers electrons from benzene rings to itself, resulting in changes in the structure of pollutants (Figure 2). This pathway may have the fastest reaction rate, but studies have shown that aromatic pollutants with electron-donating groups such as tertiary amine, secondary amine, amino, hydroxyl, ether and methoxy are more likely to react through this pathway, while aromatic pollutants with electron-withdrawing groups such as carbonyl, acyl and nitro are less likely to spontaneously react through this pathway[79]. The electronic properties of the substituents are closely related to the electron transfer. The sum of the Hammett constants of the ortho groups of organic pollutants almost determines whether the electron transfer can occur, while the para and meta groups have no significant effect[80].
The electrophilicity of the hydroxyl radical is weaker than that of the SO4.-, and the hydroxyl radical mainly reacts with pollutants through addition and hydrogen abstraction, and tends to attack substituents rather than the benzene ring itself in the degradation of aromatic compounds, so the selectivity for organic pollutants is poor, and both aromatic and aliphatic compounds can be degraded[81]; As an electrophilic radical, SO4.- is more likely to react with unsaturated pollutants and aromatic pollutants through direct electron transfer, that is, to extract electrons from organic pollutants and produce organic radical cations, so that the reaction rate with aromatic compounds is higher than that with aliphatic compounds, so its selectivity is better[82][81,83].
The order of selectivity of SO4.- to organic compounds is as follows: organic compounds with non-aromatic carbon-carbon double bonds > organic compounds with π-electrons on aromatic rings > organic compounds with α-H > organic substances with non-α-H[84]. Therefore, when different pollutants are degraded by SO4.-, their degradation mechanisms and capabilities are also different.

3.1 POPs

POPs are a class of global organic pollutants with high toxicity, strong bioaccumulation, difficult natural degradation and long-range transport, mainly including organochlorine pesticides (OCPs), polychlorinated biphenyls (PCBs), polychlorinated dibenzo-para-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs).PCDFs), as well as polybrominated diphenyl ethers (PBDEs), perfluorooctane sulfonic acid (PFOS) and its salts, perfluorooctane sulfonyl fluoride (PFOSF), PFOA and its salts and related compounds newly listed in the Stockholm Convention on Persistent Organic Pollutants in recent years. In addition, phthalic acid esters (PAEs) and PAHs have not been included in the POPs list of the Stockholm Convention so far, but many scholars regard them as POPs for research[85][86].
POPs are difficult to degrade, and traditional degradation methods are inefficient and produce harmful by-products. SO4.- is used to degrade POPs because of its high reduction potential (Table 4), in which OCPs without benzene rings are more easily degraded than PBDEs and PCBs with benzene rings, for example, Khan et al. Used UV/Fe2+ to activate PMS to degrade lindane, which was completely degraded in 40 min[87]; The reason for the high degradation rate of OCPs containing benzene rings such as DDT and PAHs such as phenanthrene may be that there are no biphenyl bonds, ether bonds and other chemical bonds that are difficult to break between benzene rings. Therefore, the benzene ring has a certain effect on the oxidative degradation rate of SO4.-. When the number of benzene rings is the same, the more the number of substituted atoms is, the more difficult it is to be degraded. For example, the apparent degradation rate constant of CB-28 kobs(0.49 h-1) is lower than that of CB-1(1.31 h-1).
As an oxidant and electrophilic free radical, SO4.- is easy to react with groups with strong electron-losing ability, while the substituent group Br of PBDEs is easier to lose electrons than Cl of PCBs, so the degradation rate of PCBs in Table 4 is faster than that of PBDEs, which may be affected by ether bonds and other factors[81][88][89]. Dominguez et al. thermally activated PDS degrades many chlorinated organic compounds, such as lindane and biphenyl, and all pollutants are nearly completely degraded within 25 ~ 72 H at 40 ℃, but the removed Cl- will react with SO4.- under certain conditions to form chlorine radicals with weak degradation ability, which inhibits the degradation of chlorinated organic compounds[90][91]. In recent years, the degradation of brominated organic PBDEs was mainly reported by nZVI or iron-containing complexes activated PS, but the degradation rate was low, and it was reported that the addition of reducing agents such as hydroxylamine and ascorbic acid and chelating agents could improve the degradation rate to a limited extent[39,50,92]. For example, Peng et al used PDS activated by Fe2+ to degrade BDE-209, and the degradation rate was only 51%. After adding chelating agents citrate and sodium gluconate, the degradation rate increased to 61% and 72%, respectively, because they could combine with Fe2+ and avoid the reaction between Fe2+ and SO4.-[93]. When ascorbic acid was added, on the one hand, it acted as an activator to activate PDS and make the system produce more SO4.-, on the other hand, it also acted as a reducing agent to reduce Fe3+ to Fe2+ and participate in the reaction again, but ascorbic acid could also consume SO4.-, so its degradation rate only increased to 61%. Like Cl-, Br- will compete with pollutants for SO4.-.Moreover, the reaction rate of Br- with SO4.- is faster (Cl-:2.7×108M-1·S-1[91],Br-:3.5×109M-1·S-1[27]), which may also be one of the reasons why brominated POPs are more difficult to degrade than chlorinated POPs.
The reason for the low degradation rate of PBDEs and PCBs may also be that their substituents are electron-withdrawing groups, so their degradation reaction is mainly through addition reaction, rather than through the faster direct electron transfer pathway[88]. However, p-chloroaniline, a synthetic intermediate of fine chemical products, contains only two substituents, one of which is an electron-donating amino group, so it is easier to be degraded by direct electron transfer. The degradation rate reaches 98% in 30 minutes, and the final degradation products are CO2, H2O and HCl. The system has a wide range of pH (3 ~ 9) (Table 4).
Perfluorinated compounds (PFASs) are difficult to degrade because they contain one of the strongest chemical bonds, the C-F bond, and require high energy for degradation. The study shows that the most effective oxidation method is the thermal activation PS method, but the energy consumption is high[94]. Lee et al. Used AC-Fe to activate PDS to degrade PFOA, and the activation energy (13.21 kJ/mol) required for the decomposition of PFOA was lower than that of PDS (66.8 kJ/mol), so that the degradation rate of 64% and the defluorination rate of 40% could be achieved at room temperature[95].
Superoxide Radicals (electron paramagnetic resonance) also have some effects on the degradation of CB-28, for example, Qin et al. Used CuFe2O4 to activate PMS to degrade CB-28, and confirmed the existence of electron paramagnetic resonance by electron paramagnetic resonance (EPR) detection.The degradation rate was reduced from 89% to 58% and 19% by BQ and superoxide dismutase, respectively. The reason for the lower degradation rate was that SOD decomposed the O2.- into H2O2,H2O2, which competed with PDS to consume Fe2+ and reduced the production of SO4.-, while BQ directly quenched the O2.-[96]. When the dissolved oxygen was increased, the degradation rate increased to 98%, which also confirmed that O2.- had a certain degradation effect.
A large number of studies have shown that most of the intermediates of POPs degradation by SO4.- can be subsequently degraded and mineralized. OCPs are easily degradable POPs and have a high degree of mineralization. Khan et al used UV/Fe2+ to activate PMS to degrade lindane, and the dechlorination rate and TOC removal rate were 96. 4% and 92. 2%, respectively, after 180 min[87]. Endosulfan ether is the main by-product of endosulfan degradation. Compared with direct UV photolysis, Shah et al. Found that the SO4.- produced by UV activation of PS can remove endosulfan ether more effectively, thus avoiding its adverse effects on the environment[25]. The final degradation products of DDT are the same as those of PCBs, which are mineralized into CO2, H2O and Cl-. The difference is that both carboxylation and hydroxylation on the benzene ring occur in the degradation process of DDT, while carboxylation is the main addition reaction on the benzene ring in the degradation of PCBs (Fig. 3,Summarized according to the degradation pathway reported by Qin et al. And Fang et al.), while in the degradation of PBDEs, the benzene ring is added by hydroxylation, and PBDEs is debrominated first, and then the ether bond is broken and ring-opened under the action of free radicals to complete the degradation (Fig. 4, summarized according to the degradation pathway reported by Peng et al., Li et al and Peng et al)[97][96][98][50][41][41][50][65]. Li et al. Used BC-nZVI to activate PDS to degrade BDE-209. GC-MS analysis showed that BDE-209 was finally mineralized into CO2, H2O, Br- and some short-chain acids after being converted into low-bromine intermediates[50]. The degradation results of BDE-209 by mechanochemically activated PDS by Huang et al were different from the former. BDE-209 directly reacted with SO4.-, and the products were CO2 and Br2. The degradation rate and debromination rate were close to 100%, and there was no low bromine intermediate detected by HPLC[31]. It can be seen that the degradation pathways of different activation methods may be different, and the activation method with good degradation effect should be selected according to the nature of pollutants.
Chen et al used V2O3 to activate PDS to degrade phenanthrene, and phenanthrene was degraded by direct electron transfer pathway. LC-MS detection showed that phenanthrene was converted into short-chain acids by ring opening, and finally degraded into CO2 and H2O. It can be seen that PAHs can also be degraded by SO4.-[42]. PFOAs are degraded by sequential removal of —CF2, thus producing harmful short-chain perfluoroalkyl species[94]. Bruton et al. Used thermally activated PDS to degrade PFOA, and the degradation rate reached 98% at 85 ℃, but 34% of them were not completely defluorinated and existed in the solution as C4-C7 short-chain perfluoroalkyl substances, so it was necessary to add PDS repeatedly to completely remove PFOA and its by-products[99]. The degradation of another common perfluoroalkyl PFOS by thermally activated PDS at 90 ℃ did not occur, probably because the C — F reduction potential (3. 6 eV) was higher than the SO4.-, so the SO4.- reacted with the C — S bond first, and then defluorinated in turn, so it was more difficult to degrade, which was also the reason why the degradation rate of PFOS was only 22% at higher temperature (350 ℃) and pressure (275. 8 kPa)[100][101]. However, Wang et al. degraded PFOA by mechanochemical activation of PDS, although short-chain perfluoroalkyl substances were also produced, GC-MS detection showed that they were finally completely degraded, the defluorination rate of PFOA reached 100%, and they were completely mineralized into CO2 and free F-, but this method only had 56% degradation rate and 9.7% defluorination rate for PFOS[32].

3.2 PPCPs

PPCPs include antibiotics, synthetic musks, analgesics, contraceptives, hypnotics, hair dyes and fungicides, most of which exist in trace concentrations in the environment and have attracted wide attention as new pollutants[105]. The use of veterinary drugs, pesticides, human drugs and cosmetics is the main way to introduce them into the environment[106]. PPCPs are also called "pseudo-persistent organic pollutants" because they are degraded and continuously introduced into the environment[107]. PPCPs entering the water body through the drinking water and food chain are harmful to the human body and increase bacterial resistance[107].
At present, the degradation technologies of PPCPs in water mainly include biological contact oxidation, ozone oxidation, photocatalytic oxidation and membrane separation technology, etc. These methods have low selectivity for PPCPs, while the SO4.- has high reduction potential and good selectivity, which is suitable for the deep degradation of drug-containing wastewater, such as the degradation of CBZ[106][8][108]. Lian et al. Used SO4.- to degrade 15 kinds of common PPCPs, the degradation rate of 8 kinds of PPCPs exceeded 60%, and 4 kinds of them were nearly 100%, while the highest degradation rate of 15 kinds of PPCPs was only about 65% when the hydroxyl radicals produced by traditional H2O2 degraded them[22]. Mahdi Ahmed degraded SMX and diclofenac sodium (DCF) in ultrapure water and wastewater (including activated sludge and Cl-, NO3-, HCO3-, Na+, etc.) With Co2+/PMS(SO4.-) and Fenton system (hydroxyl radical), respectively[109]. Under the condition of ultrapure water, the degradation rates of SO4.- and hydroxyl radical were similar, and the degradation rates of SMX and DCF reached 100% in 30 min. Under the condition of wastewater, the degradation rates of SMX and DCF by SO4.- were reduced to 81% and 71%, respectively, while the hydroxyl radical was reduced to 18% and 26%, respectively, due to the influence of impurities. The hydroxyl radical was more consumed by other substances due to its poor selectivity, and the degradation effect was far less than that of SO4.-. However, the degradation of PPCPs by SO4.- is also interfered by organic substances such as humic acid and anions such as Cl- and HCO3- in wastewater, so the impurity ions should be removed before degradation[110].
In recent years, reports on degradation of PPCPs by SO4.- mainly focused on antibiotics (Table 5), including fluoroquinolones (ciprofloxacin, norfloxacin, etc.), sulfonamides (sulfathiazole, sulfadimidine, etc.), tetracyclines (tetracycline, etc) and β-lactam antibiotics (cephalosporins, etc). The SO4.- played a major role in the degradation reaction, and hydroxyl radical and O2.- also participated in the reaction[111]. In general, the degradation of antibiotics by SO4.- can achieve high degradation rates through the direct electron transfer pathway, and the required activation method is simple. When bimetallic compounds such as CuCo2O4 are used, the synergistic effect of the two metals enables high degradation rates to be achieved over a wide pH range[67,112][111].
PPCPs have different structures, so the degradation pathways of PPCPs are different. For example, when the SO4.- degrades cephalexin and norfloxacin which belong to the same antibiotics, the primary amine, thioether on the side chain of phenylglycine and the double bond on the six-membered ring of cephalexin are attacked by the SO4.- first, and then react to the other side[26][44]; Norfloxacin first cleaves the piperazine ring, then partially oxidizes the quinolones, and finally achieves the purpose of degradation.
The intermediate products of PPCPs degraded by SO4.- can also be degraded by oxidation. Zhang et al. Activated PMS with nano-Co3MnFeO6 to degrade CBZ, and the degradation rate was 100% in 30 min. The results showed that CBZ was degraded into organic matter with smaller molecular weight by SO4.-, and then finally mineralized into CO2 and H2O[36]. The degradation rate of tetracycline was 92. 3% in 1 H, while the mineralization degree was only 40%, but the intermediate products were finally decomposed into CO2 and H2O[57]. Similar results were observed for sulfamethazine, norfloxacin, and cephalosporin[111][38][113]. Therefore, the degradation of PPCPs by AOPs based on SO4.- is an environmentally friendly method, in which the intermediate products can be mineralized under the ideal conditions of sufficient SO4.- and long enough time, and no secondary pollution is produced. However, due to the cost, activator and time, it is difficult to achieve the condition of complete mineralization in the actual engineering application. In addition, the unconsumed PS and excessive free radicals in the degradation reaction may also have a certain impact on the environment, so its engineering application needs further study[31].

3.3 Organic Dye

Organic dyes are widely used in textile and papermaking industries. Printing and dyeing wastewater is one of the most difficult industrial wastewater because of its strong toxicity, deep color and difficult biochemical degradation. Traditional treatment technologies, such as photocatalytic oxidation, biological treatment and adsorption, have the disadvantages of slow process, easy to cause secondary pollution and high cost[116].
In recent years, the application of AOPs based on SO4.- in the treatment of organic dye wastewater has attracted more and more attention. Jeon et al. Used Fe2+ to activate PDS to degrade orange, and the degradation rate reached 100%[117]. In order to reduce the SO4.- of excessive Fe2+ consumption, the Fe anode current method was introduced, which not only slowly increased the Fe2+, but also reduced the Fe3+ on the cathode to Fe2+ and further participated in the reaction. Li et al. Used reduced graphene oxide (rGO) supported Co-Mn layered double hydroxide (LDH) to activate PMS for the degradation of acid orange.The graphene layer can not only improve the dispersion of the metal oxide to reduce the leaching of the metal and enhance the synergistic effect between the two components, but also effectively prevent the agglomeration of oxide particles, so that more oxide particles can contact with PMS, thereby generating more SO4.- and achieving complete degradation[52]. Zhao et al. Used SO4.- to degrade several typical organic dye mixtures, such as acid orange, methylene blue, rhodamine B, methyl orange and methyl rosaniline chloride. The degradation rate of methyl rosaniline chloride was only 90%, and the others reached 100%, and the pH was wide (3 ~ 10)[118].
In addition, the crystal structure of the activator also has a great impact on the degradation rate, the specific surface area and crystal size of the same activator are different due to different preparation conditions, and the activator with good crystal structure has better degradation effect. Lyu et al. prepared FeOOH with different crystal morphologies. Among them, FeOOH prepared with FeCl3·6H2O and ethanol has the smallest crystal lattice size, the largest specific surface area and the highest surface hydroxyl density, and the degradation rate of acid orange reaches 100%, while the degradation rates of the other three FeOOHs prepared with NaOH/Fe(NO3)3, FeSO4 or FeSO4/NaOH are only 90%, 70% and 30% respectively[119]. Wei et al prepared MnO2 into octahedral molecular sieve activated PMS, which also improved the degradation rate by increasing the specific surface area of manganese oxide and reducing the crystal size[120].
Because of the particularity of wastewater environment, many impurity ions will affect the degradation rate. Ji et al. Found that in the process of degradation of chlorophenols by NO2- in water by SO4.-, not only the SO4.- was consumed to reduce the degradation rate, but also the generated nitrogen dioxide free radicals participated in the reaction to produce more toxic by-products of chloronitrophenol[121].
Similar to PPCPs, organic dyes have different structures, which lead to different degradation pathways. Hou et al used GC-MS to detect and analyze the reaction intermediates and products of the degradation of acid orange by SO4.-. The results showed that the SO4.- reacted with the azo bond first through addition, then attacked the C = N unsaturated bond, and then formed a series of intermediates. Because these intermediates did not contain biphenyl bond or ether bond and other chemical bonds that were difficult to break, they were easy to open the ring and convert into CO2, H2O and some small molecular organic compounds[45]. Yi et al. Detected the reaction intermediates and products of rhodamine B degradation by SO4.- through HPLC-MS, and found that SO4.- first removed the outermost ethyl group of rhodamine B through hydrogen abstraction, then the aromatic ring intermediates formed were ring-opened and decomposed into small fatty acids and short-chain olefins under the action of SO4.-, and finally mineralized into CO2, H2O and small organic compounds (alcohols, organic acids and amines)[122]. It can be seen that the final products of SO4.- degradation of organic dyes include organic acids and alcohols besides CO2 and H2O, but they are harmless to the environment.

4 Conclusion and prospect

AOPs based on SO4.- are superior to hydroxyl radicals in terms of degradation rate and adaptability in the degradation of new organic pollutants, and have broad research and application prospects. Although there are many reports on the degradation of organic pollutants by SO4.-, it is still in the experimental stage, and its engineering application needs to be further studied. The research on the degradation system of SO4.- should focus on the sufficient activation of persulfate, the reduction of cost and by-products.
(1) Select the best activation method according to the structure and physicochemical properties of different pollutants, and improve the degradation rate and mineralization degree of pollutants by improving the existing activation technology or combining multiple activation methods.
(2) Develop new natural activators with low cost, high efficiency and stability to reduce treatment cost and secondary pollution.
(3) Deeply study the mechanism of SO4.- degradation reaction in various mixture systems, reduce the interference of impurity ions on the degradation reaction as far as possible, and carry out the research on the degradation of new organic pollutants by SO4.- in practical environmental media such as wastewater and soil, so as to further promote the application of this technology.
(4) In practical application, due to the interference of impurities, the degradation of some organic pollutants by SO4.- may produce more difficult to degrade or even more toxic intermediates, by-products and residues, which need to be monitored and analyzed to eliminate their adverse effects.

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Funding

National Natural Science Foundation of China(41203077)

Natural Science Foundation of Fujian, China(2018J01065)

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