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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2024 , Vol. 36 >Issue 5: 798 - 814

DOI: https://doi.org/10.7536/PC230707

Review

Sulfate Radicals: A New Tool for Enhancing Sludge Dewatering

  • Yue Lai 1 ,
  • Chao Wang 1 ,
  • Jie Zhang 1 ,
  • Shungui Zhou 1 ,
  • Changgeng Liu , 2, * ,
  • Jie Ye , 1, *
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  • 1 College of Resources and Environment, Key Lab of Soil Ecosystem Health and Regulation of Fujian Province, Fujian Agriculture and Forestry University, Fuzhou 350002, China
  • 2 College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610066, China
* e-mail: (Changgeng Liu)；
(Jie Ye)

Received date: 2023-07-14

  Revised date: 2024-01-28

  Online published: 2024-04-16

Supported by

Open Fund of Key Laboratory of Green Technology in Ecological Industry of Fujian Province(WYKF-EIGT2021-3)

Fold

Abstract

sludge is an inevitable by-product of the wastewater treatment process and due to its high water content,large volume,and inclusion of a large amount of toxic and hazardous substances,needs to be minimized and harmlessly treated.However,sludge possesses extracellular polymeric substances(EPS)formed by ionization of negatively charged functional groups,which maintain a stable hydrated colloidal structure and prevent the release of water.this is a key factor in the difficulty of sludge dewatering.in the last decade,sulfate radical-Based advanced oxidation processes(SR-AOPs)have received considerable attention due to their high efficiency for EPS disintegration,rapid reaction time and environmental friendliness,and thus sulfate radicals have become a new powerful tool for enhancing sludge dewaterability.in This paper,the development timeline and activation mechanisms of sulfate radicals are reviewed in detail,and the research advances of SR-AOPs for improving sludge dewaterability and removing micropollutants and heavy metals from sludge are systematically evaluated.based on the current scientific problems of SR-AOPs in sludge conditioning,the future research directions of SR-AOPs are proposed from the perspectives of mechanism research,cost-effectiveness,and experimental scale,in order to provide a solid theoretical reference for sludge conditioning in wastewater treatment plants in China。

Contents

1 Introduction

2 Timeline of sulfate radicals

3 Mechanism of sulfate radicals activation

3.1 Transition metal activation

3.2 Photo activation

3.3 Heat activation

3.4 Alkali activation

3.5 Metal-free catalyst activation

4 Sludge dewaterability improved by SR-AOPs

4.1 Iron activated methods

4.2 Heat activated methods

4.3 Electrochemically activated methods

4.4 Alkali activated methods

4.5 Other activation methods

5 Removal of micropollutants

6 Removal of heavy metals

7 Sludge dewatering mechanism

8 Conclusion and outlook

Key words： sulfate radicals; advanced oxidation process; sludge dewatering; conditioning mechanism; extracellular polymeric substances

Cite this article

Yue Lai , Chao Wang , Jie Zhang , Shungui Zhou , Changgeng Liu , Jie Ye . Sulfate Radicals: A New Tool for Enhancing Sludge Dewatering[J]. Progress in Chemistry, 2024 , 36(5) : 798 -814 . DOI: 10.7536/PC230707

1 Introduction

With the rapid development of industrialization and urbanization,the production of sewage and sludge is increasing year by year,and the cost of sludge treatment has become a huge burden for sewage treatment plants at home and abroad[1]。 Sludge,as a by-product of wastewater treatment,contains a large number of pathogenic microorganisms,refractory organic pollutants,heavy metal ions and other toxic and harmful substances,which may cause serious secondary environmental pollution if not properly treated[2,3]。 As a necessary step of sludge treatment,dewatering can minimize the volume of sludge and reduce its transportation cost,which is the key step to realize sludge reduction,harmlessness and resource utilization[4⇓~6]。 However,sludge has extracellular polymeric substances(extra cellular polymeric substances,EPS)formed by the ionization of negatively charged functional groups such as carboxyl,hydroxyl and amino groups,and its high negative surface charge density can prevent sludge instability and flocculation through electrostatic repulsion,and maintain a stable hydrated colloidal structure to prevent water release,which is the key factor causing sludge dewatering difficulties[1,4,7][7⇓⇓~10]。 Therefore,effectively destroying the structure of EPS and releasing the water bound to EPS is the key point to achieve deep sludge dewatering。
advanced oxidation processes(AOPs)are an effective method for sludge dewatering.Compared with traditional oxidation methods,AOPs have the advantages of strong oxidizability,rapid reaction and low selectivity[11]。 Fenton oxidation process is the most typical and widely studied AOPs,the principle of which is that the strong oxidizing hydroxyl radicals($\cdot \mathrm{OH}$ )produced by the reaction of hydrogen peroxide（H2O2）and Fe2+destroy the EPS colloid structure of sludge,and then release the internal bound water to improve the dewatering performance of sludge[12]。 the use of traditional Fenton reagents often requires acidic conditions(pH≈3),which greatly limits The further practical application of this method[13]。 In the past decade,researchers have tried to use sulfate radical-based AOPs(SR-AOPs)to improve sludge dewatering performance,and have achieved remarkable results[14]。 The common precursors of sulfate radical($\mathrm{SO}_{4}^{\centerdot -}$ )in SR-AOPs are peroxymonosulfate(PMS,$\mathrm{HSO}_{5}^{-}$ ),peroxydisulfate(PDS,${{\mathrm{S}}_{\mathrm{2}}}\mathrm{O}_{8}^{2-}$ ),and sulfite(SF,$\mathrm{HSO}_{3}^{-}$ and$\mathrm{SO}_{3}^{2-}$ ).Compared with$\cdot \mathrm{OH}$ ,$\mathrm{SO}_{4}^{\centerdot -}$ have comparable redox potential(2.5~3.1 V vs 2.8 V)and longer lifetime（t1/2:30~40μs vs 1μs）[15][16,17]。 in addition,$\mathrm{SO}_{4}^{\centerdot -}$ is able to selectively react with organic compounds containing unsaturated bonds or aromaticπelectrons through electron transfer,and thus is less affected by common interfering substances In wastewater[18]。 Based on the above advantages,$\mathrm{SO}_{4}^{\centerdot -}$ has been regarded as a new tool to enhance sludge dewatering。
At present,the published reviews on SR-AOPs summarize the activation generation methods of$\mathrm{SO}_{4}^{\centerdot -}$ and evaluate their applicability in the treatment of various practical wastewaters,removal of gaseous pollutants,and remediation of organic contaminated soils[19⇓⇓~22][23][24]。 However,the above review focuses on the activation of PMS and PDS,while ignoring the summary and comparison of another precursor SF,and the introduction of application areas focuses on the degradation of pollutants in different media,while there is a gap in the field of sludge dewatering.Based on This,this paper first summarizes the historical progress and activation mechanism of$\mathrm{SO}_{4}^{\centerdot -}$ activation in detail,and then comprehensively summarizes its application in the field of sludge dewatering and other sludge conditioning effects in the past decade.this paper focuses on the effects of different precursors and activation methods on sludge dewatering and the related mechanism analysis,in order to provide suggestions for the research direction of SR-AOPs enhanced sludge dewatering。

2 Development history of sulfate radical

in the 1960s and 1970s,researchers found that persulfate could be activated by light and metal ions(chromium and silver)to produce$\mathrm{SO}_{4}^{\centerdot -}$ ,which was applied to the in-situ degradation of organic pollutants in wastewater[25,26]。 in the 1980s,with the rise of sulfite radical oxidation kinetics,it was noticed that$\mathrm{SO}_{3}^{\centerdot -}$ could also produce$\mathrm{SO}_{4}^{\centerdot -}$ through a series of chain radical reactions,and the oxidative degradation of organic acids was realized In flue gas desulfurization[27]。 in the 1990s,the activation of sulfite and bisulfite In aqueous solution to produce$\mathrm{SO}_{4}^{\centerdot -}$ was further developed[28,29]。 Because of the strong stability of$\mathrm{SO}_{4}^{\centerdot -}$ ,researchers began to explore its application in in-situ remediation of soil[30]。 At the beginning of the 21st century,researchers have developed more effective$\mathrm{SO}_{4}^{\centerdot -}$ generation methods,including thermal activation and microwave activation[31,32]。 During this period,the methods of metal ion activation were no longer limited to heavy metal ions such as chromium,silver,cobalt and manganese,and iron activation(including Fe2+and zero-valent iron(ZVI))began to receive a lot of attention[33]。
With the improvement of environmental remediation technology,new progress has been made in the activation of persulfate oxidation by non-metallic activators to promote the degradation of pollutants[34⇓~36]。 Graphene,carbon nanotubes,carbon nanofibers and other nanostructured carbon materials are widely used in SR-AOPs[37⇓~39]。 These materials can act as direct activators or as carriers of transition metal activators to co-activate PMS/PDS to produce$\mathrm{SO}_{4}^{\centerdot -}$[36]。
In the past decade,SR-AOPs have been widely used to improve sludge dewatering performance[40⇓~42]。 On the basis of the original activation method,ultrasonic activation and alkali activation have been applied to sludge dewatering by persulfate activation to produce$\mathrm{SO}_{4}^{\centerdot -}$[43,44]。 In recent years,sulfite activated conditioned sludge has attracted the attention of researchers because of its more stable chemical properties and more affordable price than persulfate[45]。 The activation timeline of$\mathrm{SO}_{4}^{\centerdot -}$ between 1960 and 2020 is shown in Fig.1[25⇓⇓⇓~29,31⇓⇓~34,44,46⇓⇓⇓⇓⇓⇓⇓⇓⇓~56]。
图1 1960—2020年间硫酸根自由基的活化时间线

Fig. 1 Timeline of activation of sulfate radicals from 1960 to 2020

3 Activation mechanism of sulfate radical

Generation of$\mathrm{SO}_{4}^{\centerdot -}$ usually requires activation of PMS,PDS,or SF.PMS possesses an asymmetric molecular structure,whereas PDS possesses a symmetric molecular structure[57,58]。 Peroxy bond(O−O)in PMS and PDS molecules needs to be destroyed by certain activation methods to produce,while SF often cannot be directly activated to produce$\mathrm{SO}_{4}^{\centerdot -}$ ,which requires chemical transformation of other weakly oxidizing precursor radicals($\mathrm{SO}_{3}^{\centerdot -}$ and$\mathrm{SO}_{5}^{\centerdot -}$ ,etc.),as shown in Figure 2[59][15]。
图2 活化过一硫酸盐、过二硫酸盐和亚硫酸盐机理

Fig. 2 Mechanism of activation of PMS, PDS and sulfite

At present,activation methods mainly include transition metal activation,photoactivation,thermal activation,alkali activation and non-metallic activator activation.These activation mechanisms will be introduced and compared below(Table 1)。
表1 Mechanism of activation of different precursors to produce sulfate radical

Table 1 Activation of different precursors to produce sulfate radical mechanisms

Activation method Precursor Mechanism Predominant radical species Comments Ref
Transition metals PMS One electron transfer SO4 / 62
PDS One electron transfer SO4 / 62
SF Involves several transient short-lived oxysulfur radicals initiated by direct one-electron transfer from SF to metal ions $\mathrm{SO}_{4}^{\centerdot -}$/$\cdot \ \mathrm{OH}$ / 63
Photo PMS Breaking of O−O bond $\mathrm{SO}_{4}^{\centerdot -}$/$\cdot \ \mathrm{OH}$ / 64
PDS Breaking of O−O bond SO4 / 65
SF Photoelectrons and holes can be produced from photocatalysts, the generated holes can activate sulfite to form oxysulfur radicals $\mathrm{SO}_{4}^{\centerdot -}$/$\cdot \ \mathrm{OH}$/$\mathrm{O}_{2}^{\centerdot -}$ / 15
Heat PMS Breaking of O−O bond $\mathrm{SO}_{4}^{\centerdot -}$/$\cdot \ \mathrm{OH}$ The bond dissociation energy is higher, so a higher temperature is required to cleave O−O bond 66
PDS Breaking of O−O bond $\mathrm{SO}_{4}^{\centerdot -}$/$\cdot \ \mathrm{OH}$ The bond dissociation energy is lower, and increasing the temperature can effectively cleave O−O bond 67
SF SF autoxidation generates oxygen-sulfur radicals $\mathrm{SO}_{4}^{\centerdot -}$/$\cdot \ \mathrm{OH}$ / 68
Alkali PMS Alkali-catalyzed hydrolysis of PMS to hydrogen peroxide O2 / 69
PDS Alkali-catalyzed hydrolysis of PDS to hydroperoxides triggers free radical formation $\mathrm{SO}_{4}^{\centerdot -}$/$\cdot \ \mathrm{OH}$/$\mathrm{O}_{2}^{\centerdot -}$ pH > 11 70
SF / / / /
Metal-free catalyst PMS One electron transfer SO4 Graphene exhibits better catalytic properties than several other carbon isomers, including: activated carbon, graphite, graphene oxide, and carbon nanotubes 38
PDS Peroxide bond of PDS is weakened at the defective edge of the carbon catalyst and oxygen groups (of which the carbonyl group is the most active) OH Reduced mesoporous carbon, carbon nanotubes, and graphene oxide, displayed great catalytic properties, in contrary to nanodiamonds, fullerenes and graphitic carbon nitride 39
SF Bonding to carbon catalyst ketone groups to form complexes with internal electron transfer leading to the formation of oxygen-sulphur radicals SO4 / 71

3.1 Transition Metal Activation

The transition metal activation method is favored by researchers because it is carried out at room temperature,does not require additional energy and is simple to operate.It has been reported that PMS,PDS,and SF can all be activated by Fe2+,Mn2+,Cu2+,Co2+to produce $\mathrm{SO}_{4}^{\centerdot -}$[15,60]。 Iron is the most commonly used transition metal activator.Taking Fe2+as an example,the reaction equations of PMS,PDS and SF activated to produce $\mathrm{SO}_{4}^{\centerdot -}$ are shown in equations(1),(2)and(3~10),respectively[41,61]。 Fig.3A is a schematic diagram of the mechanism of activation of PMS,PDS,and SF by Fe2+ 。
$\mathrm{F}{{\mathrm{e}}^{\mathrm{2+}}}\mathrm{+HSO}_{2}^{-}\to \mathrm{F}{{\mathrm{e}}^{\mathrm{3+}}}\mathrm{+SO}_{4}^{\centerdot -}+\mathrm{O}{{\mathrm{H}}^{-}}$
$\mathrm{F}{{\mathrm{e}}^{\mathrm{2+}}}\mathrm{+}{{\mathrm{S}}_{\mathrm{2}}}\mathrm{O}_{8}^{2-}\to \mathrm{F}{{\mathrm{e}}^{\mathrm{3+}}}\mathrm{+SO}_{4}^{\centerdot -}+\mathrm{SO}_{4}^{2-}$
$\mathrm{F}{{\mathrm{e}}^{\mathrm{2+}}}\mathrm{+HSO}_{3}^{-}\to \mathrm{FeHSO}_{3}^{-}$
$\mathrm{FeHSO}_{3}^{+}\mathrm{+1/4}{{\mathrm{O}}_{\mathrm{2}}}\to \mathrm{FeSO}_{3}^{+}\mathrm{+1/2}{{\mathrm{H}}_{\mathrm{2}}}\mathrm{O}$
$\mathrm{FeSO}_{3}^{+}\to \mathrm{F}{{\mathrm{e}}^{\mathrm{2+}}}\mathrm{+SO}_{3}^{\centerdot -}$
$\mathrm{SO}_{3}^{\centerdot -}+{{\mathrm{O}}_{2}}\to \mathrm{SO}_{5}^{\centerdot -}$
$\mathrm{SO}_{3}^{\centerdot -}+\mathrm{HSO}_{3}^{-}\to \mathrm{SO}_{4}^{\centerdot -}+\mathrm{SO}_{4}^{2-}+{{\mathrm{H}}^{+}}$
$\mathrm{SO}_{5}^{\centerdot -}+\mathrm{SO}_{5}^{\centerdot -}\to 2\mathrm{SO}_{4}^{\centerdot -}+{{\mathrm{O}}_{\mathrm{2}}}$
$\mathrm{SO}_{5}^{\centerdot -}+\mathrm{HSO}_{3}^{-}\to \mathrm{SO}_{3}^{\centerdot -}+\mathrm{HSO}_{5}^{-}$
$\mathrm{SO}_{5}^{\centerdot -}+\mathrm{SO}_{5}^{\centerdot -}\to {{\mathrm{S}}_{\mathrm{2}}}\mathrm{O}_{8}^{2-}+{{\mathrm{O}}_{\mathrm{2}}}$

3.2 Photoactivation

The development of PMS,PDS,and SF activation by ultraviolet,visible,or solar irradiation has made tremendous progress in recent research[64]。 Fig.3 B and C are the mechanisms of photoactivation of PMS,PDS,and SF to produce active species.the mechanism of direct UV activation of PMS and PDS is that UV light(λ=200–400 nm)can provide excitation energy for the electrons in the precursor ions in the valence band,and then destroy the peroxo bond(O−O)of PMS and PDS to produce(formulas(11)and(12)),and the$\mathrm{SO}_{4}^{\centerdot -}$ further reacts with water or hydroxide anions to produce$\cdot \mathrm{OH}$ (formulas(13)and(14)),and a variety of strong oxidizing free radicals work together to complete the oxidation reaction[64,72]。 Different from PMS and PDS,SF generates a variety of reducing species under UV light excitation,including SF radical($\mathrm{SO}_{3}^{\centerdot -}$ ),hydrogen radical($\cdot \mathrm{H}$ )and hydrated electron($e_{aq}^{-}$ ),as shown in equations(15)and(16))[73]。
$\mathrm{HSO}_{5}^{-}\mathrm{+}hv\to \mathrm{SO}_{4}^{\centerdot -}\mathrm{+}\cdot \ \mathrm{OH}$
${{\mathrm{S}}_{\mathrm{2}}}\mathrm{O}_{8}^{2-}\mathrm{+}hv\to 2\mathrm{SO}_{4}^{\centerdot -}$
$\mathrm{SO}_{4}^{\centerdot -}\mathrm{+}{{\mathrm{H}}_{\mathrm{2}}}\mathrm{O}\to \mathrm{SO}_{4}^{2-}+\cdot \ \mathrm{OH+}{{\mathrm{H}}^{\mathrm{+}}}$
$\mathrm{SO}_{4}^{\centerdot -}\mathrm{+O}{{\mathrm{H}}^{-}}\to \mathrm{SO}_{4}^{2-}+\cdot \ \mathrm{OH}$
$\mathrm{SO}_{3}^{2-}\mathrm{+}hv\to \mathrm{SO}_{3}^{\centerdot -}+e_{aq}^{-}$
$\mathrm{HSO}_{3}^{-}\mathrm{+}hv\to \mathrm{SO}_{3}^{\centerdot -}\mathrm{+}\cdot \ \mathrm{H}$
图3 不同方法活化PMS、PDS和SF的机制（根据文献15,58,63,71,83绘制）

Fig. 3 Mechanisms of activation of PMS, PDS and SF by different methods (adapted from ref 15,58,63,71,83)

3.3 Thermal activation

Similar to the principle of photoactivation,the thermal activation method also produces$\mathrm{SO}_{4}^{\centerdot -}$ by breaking the peroxy bond(O−O)of PMS or PDS[74]。 Under the temperature of 40~99℃,the system temperature reaches the activation energy required for the reaction to break the O−O bond to produce$\mathrm{SO}_{4}^{\centerdot -}$ ,as shown in equations(17)and(18)[75]。 Compared with PDS,PMS has a shorter bond length(1.460 460Åvs 1.497 497Å),which also means that PMS possesses a higher O−O bond dissociation energy（377 kJ·mol-1vs 92 kJ·mol-1）[76]。 Theoretically,PMS requires more energy to cleave O−O to produce radicals than PDS,so the temperature required for thermal activation is correspondingly higher.Recently,related studies have pointed out that the thermally activated SF system can produce a precursor radical(formula(19)),and the presence of oxygen further converts the$\mathrm{SO}_{3}^{\centerdot -}$ into a$\mathrm{SO}_{5}^{\centerdot -}$ ,thereby producing a$\mathrm{SO}_{4}^{\centerdot -}$ ,which is similar to formulas(6–8)[68]。
$\mathrm{HSO}_{5}^{-}\xrightarrow{\Delta }\mathrm{SO}_{4}^{\centerdot -}\mathrm{+}\cdot \ \mathrm{OH}$
${{\mathrm{S}}_{\mathrm{2}}}\mathrm{O}_{8}^{2-}\xrightarrow{\Delta }\mathrm{2SO}_{4}^{\centerdot -}$
$\mathrm{SO}_{3}^{2-}\xrightarrow{\Delta }\mathrm{SO}_{3}^{\centerdot -}+e_{aq}^{-}$

3.4 Alkali activation

the mechanism of alkali activation of PDS mainly includes two steps(formula(20~22)):①PDS is rapidly decomposed into hydroperoxide($\mathrm{HO}_{2}^{-}$ )and$\mathrm{SO}_{4}^{2-}$ (formula(20))due to The fission of S−O bond under alkaline conditions[70]; ②formed by hydrolysis of PDS molecule reduces another PDS molecule to produce$\mathrm{SO}_{4}^{\centerdot -}$ ,while$\mathrm{HO}_{2}^{-}$ is oxidized to superoxide radical($\mathrm{O}_{2}^{\centerdot -}$ )are the main reactive oxygen species in the system,as shown in equations(23∼29)[77]。 Fig.3D is a schematic diagram of the mechanism of base activation of PMS and PDS。
${{\mathrm{S}}_{\mathrm{2}}}\mathrm{O}_{8}^{2-}+2{{\mathrm{H}}_{\mathrm{2}}}\mathrm{O}\xrightarrow{\mathrm{O}{{\mathrm{H}}^{-}}}\mathrm{HO}_{2}^{-}\mathrm{+2SO}_{4}^{2-}\mathrm{+3}{{\mathrm{H}}^{\mathrm{+}}}$
${{\mathrm{S}}_{\mathrm{2}}}\mathrm{O}_{8}^{2-}+2\mathrm{HO}_{2}^{-}\to \mathrm{SO}_{4}^{\centerdot -}+\mathrm{SO}_{4}^{2-}\mathrm{+}{{\mathrm{H}}^{\mathrm{+}}}+\mathrm{O}_{2}^{\centerdot -}$
$\mathrm{2}{{\mathrm{S}}_{\mathrm{2}}}\mathrm{O}_{8}^{2-}+{{\mathrm{H}}_{\mathrm{2}}}\mathrm{O}\xrightarrow{\mathrm{O}{{\mathrm{H}}^{-}}}\mathrm{SO}_{5}^{-}\mathrm{+SO}_{4}^{2-}\mathrm{+2}{{\mathrm{H}}^{\mathrm{+}}}$
$\mathrm{HSO}_{5}^{-}\to \mathrm{SO}_{5}^{2-}\mathrm{+}{{\mathrm{H}}^{\mathrm{+}}}$
$\mathrm{SO}_{5}^{2-}+{{\mathrm{H}}_{\mathrm{2}}}\mathrm{O}\to \mathrm{SO}_{4}^{2-}+{{\mathrm{H}}_{\mathrm{2}}}{{\mathrm{O}}_{\mathrm{2}}}$
${{\mathrm{H}}_{\mathrm{2}}}{{\mathrm{O}}_{\mathrm{2}}}\to \mathrm{HO}_{2}^{-}+{{\mathrm{H}}^{+}}$
${{\mathrm{H}}_{\mathrm{2}}}{{\mathrm{O}}_{\mathrm{2}}}\to 2\cdot \ \mathrm{OH}$
${{\mathrm{H}}_{\mathrm{2}}}{{\mathrm{O}}_{\mathrm{2}}}+\cdot \ \mathrm{OH}\to \mathrm{HO}_{2}^{\centerdot }+{{\mathrm{H}}_{\mathrm{2}}}\mathrm{O}$
$\mathrm{HO}_{2}^{\centerdot }\to {{\mathrm{H}}^{+}}+\mathrm{O}_{2}^{\centerdot -}$
$\mathrm{O}_{2}^{\centerdot }+\cdot \ \mathrm{OH}\to {}^{1}{{\mathrm{O}}_{\mathrm{2}}}+\mathrm{O}{{\mathrm{H}}^{-}}$

3.5 Nonmetallic activator

In recent years,carbon-driven metal-free activated persulfate AOPs have received a lot of attention because of their low cost,abundant resources,environmental friendliness,and good stability[78]。 Many carbon-based materials are used to activate PMS/PDS,including carbon nanotubes(formula(30,31)),reduced graphene oxide(formula(32,33)),etc[37⇓~39,79]。 There are two activation ways of carbon activator in the activation process of PMS/PDS,namely free radical pathway and non-free radical pathway.the radical pathway is mainly through radicals formed after cleavage of the O−O bond of PMS/PDS(PMS: and$\cdot \ \mathrm{OH}$ ;PDS:two$\mathrm{SO}_{4}^{\centerdot -}$ )for oxidation[80]。 The non-radical pathway involves electrophilic binding of electron-rich species(such as dyes,antibiotics,and phenols)through electron transfer or electron shuttle,including three mechanisms:formation of carbon-based catalyst-PDS/PMS complexes,direct electron transfer,and formation of singlet oxygen（1O2）,as shown in Figure 3 e[79]。 There are significantly more examples of PDS than PMS in the non-radical pathway through the formation of carbon-based catalyst-PDS/PMS complexes,which may be due to the asymmetric charge distribution of PMS compared with PDS,which are more easily activated during radical reactions to produce$\mathrm{SO}_{4}^{\centerdot -}$ /$\cdot \ \mathrm{OH}$ by one-electron reduction,or$\mathrm{SO}_{5}^{\centerdot -}$ by one-electron oxidation[58]。 In the direct electron transfer process,the carbon-based catalyst only acts as a fast electron transfer mediator from the organic pollutant(electron donor)to the PDS/PMS(electron acceptor)[81]。 The last non-radical reaction pathway is the formation of1O2,which includes two pathways.The former1O2is produced by the oxidation of PDS/PMS as an electron donor by carbon-based catalyst as an electron acceptor,and the latter is produced by the oxidation of C=O group in carbon-based catalyst by PDS/PMS(Fig.3e )[82]。
in recent years,researchers have found that SF can also be activated by carbon-based activators(based on glucose-derived carbon materials)to produce$\mathrm{SO}_{4}^{\centerdot -}$ .the excellent conductivity and abundant ketone functional groups of carbon-based activators are The key factors for activating SF,as shown In formulas(30~33)[71]。 Fig.3F is a schematic diagram of the free radical pathway for the activation of PMS,PDS,and SF by carbon materials[57][71]。
$\mathrm{CNT-C}$=$\mathrm{O+HSO}_{5}^{-}\to \mathrm{CNT-C}$—${{\mathrm{O}}^{\mathrm{*}}}+\mathrm{SO}_{4}^{\centerdot -}+\mathrm{O}{{\mathrm{H}}^{-}}$
$\mathrm{CNT-C}$=$\mathrm{O+HSO}_{5}^{-}\to \mathrm{CNT-C}$—${{\mathrm{O}}^{\mathrm{*}}}+\mathrm{SO}_{4}^{\centerdot -}+\mathrm{SO}_{4}^{2-}$
C=C=$\mathrm{O+HSO}_{5}^{-}\to \mathrm{C}$=C—${{\mathrm{O}}^{\mathrm{+}}}+\mathrm{SO}_{4}^{\centerdot -}+\mathrm{HO}_{{}}^{-}$
C=C—${{\mathrm{O}}^{\mathrm{+}}}\mathrm{+HSO}_{5}^{-}\to \mathrm{C}$=C=$\mathrm{O}+\mathrm{SO}_{5}^{\centerdot -}+\mathrm{HO}_{{}}^{+}$

4 Improvement of sludge dewaterability by SR-AOPs

4.1 Iron activation

iron is the most widely used transition metal activator in SR-AOPs,and has also been extensively studied in the field of sludge dewatering.SR-AOPs based on Iron activation can effectively produce strong oxidizing$\mathrm{SO}_{4}^{\centerdot -}$ ,which can effectively destroy sludge floc structure and break up sludge EPS and microbial cells,and then release EPS bound water and intracellular water to improve sludge dewatering performance[41,42]。 sludge EPS is a high molecular polymer formed by the ionization of negatively charged functional groups such as carboxyl,hydroxyl and amino.the negative surface charge can prevent Sludge instability and flocculation through electrostatic repulsion,and maintain a stable hydrated colloidal structure to prevent the release of water[1,4,7]。 Therefore,the effective disintegration of sludge EPS plays an important role in improving the dewatering performance of sludge.the study shows that the sludge conditioned by PMS/PDS and SF oxidation activated by capillary suction time and ZVI can achieve good sludge dewatering effect,and the capillary suction time(CST)of sludge is significantly reduced,as shown in Table 2[45,55,84⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~99]。 When SR-AOPs based on iron activation method are used to promote sludge dewatering,its dewatering efficiency will be affected by sludge type,initial CST of sludge,initial pH and reagent ratio,for example,some reports indicate that the optimal molar ratio of Fe2+/PDS for sludge dewatering is 0.6,while other studies show that the optimal molar ratio of Fe2+/PDS is 0.8 or 0.5[55,85,87,91,95⇓~97,100⇓⇓~103][95][104][83]。 In general,the best sludge dewatering performance can be achieved only under the best reagent ratio,and too small or too large ratio may not be conducive to dewatering.On the one hand,Fe2+can activate PMS/PDS to produce ,which accelerates the disintegration of sludge flocs and leads to the decrease of CST;On the other hand,excessive Fe2+may also consume $\mathrm{SO}_{4}^{\centerdot -}$ ($\mathrm{F}{{\mathrm{e}}^{\mathrm{2+}}}\mathrm{+SO}_{4}^{\centerdot -}\to \mathrm{F}{{\mathrm{e}}^{\mathrm{3+}}}\mathrm{+SO}_{4}^{2-}$ ),resulting in the reduction of system oxidation and the deterioration of sludge dewatering capacity[55,87,105]。 Similarly,For different types of sludge,the same conditioning method often achieves different dehydration effects.for example,Wang et al showed that the CST reduction rate of anaerobic digested sludge(ADS)was 49.3%under the condition that the molar ratio of anaerobic digested sludge to$\mathrm{HSO}_{5}^{-}$ was 1.2[106]; Under the same activation conditions,the CST reduction rate of waste activated sludge(WAS)can reach more than 90%[87]。 Therefore,when evaluating the sludge dewatering effect of a process,it is necessary to systematically compare a variety of factors that may affect the sludge dewatering efficiency。
表2 Effect of different iron activators on improvement of sludge dewaterability

Table 2 Effect of different iron activator on the improvement of sludge dewaterability

Catalyst Chemical Assisted activation method Solid content Dosage Sludge type CST（s） CST reduction（%） Ref
Fe2+ PMS / TSS = 12.3 g/L Fe2+ = 0.81 mmol/g VSS
PMS = 0.9 mmol/g VSS		 WAS 201.8 90 87
/ TSS = 20.4 g/L Fe2+ = 0.81 mmol/g VSS
PMS = 0.9 mmol/g VSS		 WAS 49.1 45.67 86
Chelator TS = 25.6 g/L Fe2+ = 0.6 mmol/g VS
PMS = 0.9 mmol/g VS
Chelator = 0.3 mmol/g VS		 ADS 130.8 88.3 107
Chelator TSS = 51.62 g/L Fe2+ = 1.2 mmol/g VSS
PMS = 1.0 mmol/g VSS
Chelator = 0.4 mmol/g VSS		 ADS 599.9 60 106
PDS / TSS = 11.9 g/L Fe2+ = 1.5 mmol/g VSS
PDS = 1.2 mmol/g VSS		 WAS 502.4 86.4 97
/ TS = 26.2 g/L Fe2+ = 2.0 mmol/g VS
PDS = 1.6 mmol/g VS		 WAS 681.0 88.0 96
/ TSS = 20.7 g/L Fe2+ = 1.5 mmol/g VSS
PDS = 1.2 mmol/g VSS		 WAS 573.4 94.2 94
Chelator TS = 20.14 g/L Fe2+ = 0.6 mmol/g TS
PDS = 0.4 mmol/g TS
Chelator = 0.15 mmol/g TS		 WAS 836.6 96.4 109
Chelator TS = 19.9 g/L Fe2+ = 0.3 mmol/g TS
PDS = 0.6 mmol/g TS
Chelator = 0.15 mmol/g TS		 WAS 720.4 61.5 83
Skeleton TSS = 35 g/L Fe2+ = 23.5 mg/g DS
PDS = 100 mg/g DS
Skeleton = 300 mg/g DS		 WAS 182 84.6 111
SF / VSS/TSS = 51.88% Fe2+ = 1.79 mmol/L TSS
SF = 1.43 mmol/L TSS		 WAS 139.7 68.9 45
ZVI PMS / TSS = 23.4 g/L ZVI = 0.25 g/g TSS
PMS = 0.1 g/g TSS		 WAS 48.6 ~50 88
PDS / TS = 28.5 g/L ZVI = 2 g/g TS
PDS = 0.5 g/g TS		 ADS 174.7 90 92
/ TS = 7.9 g/L ZVI = 15 g/L
PDS = 4 g/L		 WAS 20.5 50.2 84
/ TSS = 15.2 g/L ZVI = 2 g/g TSS
PDS = 0.5 g/g TSS		 ADS 144.6 42.6 89
/ TS = 26.1 g/L ZVI = 0.05 g/g TS
PDS = 0.1 g/g TS		 ADS 119.1 ~80 98
Chelator TSS = 56.37 g/L ZVI = 0.5 mmol/g TSS
PDS = 0.5 mmol/g TSS
Chelator = 0.05 mmol/g TSS		 WAS 254.9 51.7 108
SF / TS = 23.6 g/L ZVI = 0.9 mmol/g VS
SF = 1.2 mmol/g VS		 ADS 120.9 81.7 55
Without adjusting the pH value of the sludge,the added iron ions are usually easy to form precipitates,which can not be effectively used as an activator.In addition,the rapid conversion of Fe2+to Fe3+will lead to the rapid stop of the activation of PMS/PDS,and the excessive Fe2+will consume ,which will reduce the oxidation efficiency.Therefore,increasing the solubility of Fe2+and reducing the conversion rate of Fe2+are the key to improve sludge dewaterability by iron-activated SR-AOPs.Because ZVI can slowly release Fe2+in aqueous solution,thereby reducing the adverse consumption of Fe2+on $\mathrm{SO}_{4}^{\centerdot -}$ ,the research on the improvement of sludge dewaterability by activated PMS/PDS and SF oxidation has also received a lot of attention in recent years[55,84,88,89,91,92,98,99]。 the researchers further tried to use chelates to improve The sludge dewatering efficiency of SR-AOPs based on iron activation[83,106⇓⇓~109]。 The addition of chelating agents may promote the dissolution of iron and the formation of soluble chelated Fe2+species.These complexed species,once exposed to air,are immediately oxidized to soluble chelated Fe3+species via a Fenton-like reaction,and it can also be rapidly reduced to Fe2+via electron transfer from Fe0[110]。 This iron cycle ultimately promotes the decomposition of oxidants and the generation of highly reactive free radicals.It has been reported that the introduction of the chelating agent ascorbic acid into the ZVI-activated PDS system WAS able to improve the dehydration performance of WAS[108]。 Under the optimal conditions(0.5 mmol/G total suspended solids(TSS)ascorbic acid,0.5 mmol/G TSS ZVI and 0.5 mmol/gTSS PDS),the CST ratio total suspended solids of ZVI activated PDS with ascorbic acid before and after oxidative conditioning increased by 24.23%compared with the control group.In 2020,Liu et al.Used citrate as a Fe2+chelating agent.Under the conditions of 0.9 mmol/G volatile solids(VS)PMS,0.6 mmol/G VS Fe2+and 0.3 mmol/G VS citrate,the CST reduction rate of ADS was 88.3%,while the CST reduction rate of the system without citrate was 79.2%[107]。
the improvement of sludge dewaterability by Iron-activated SR-AOPs cannot be attributed solely To the oxidation of sludge by$\mathrm{SO}_{4}^{\centerdot -}$ ,and the flocculation of iron cannot be ignored.iron ions neutralize the negative charges of sludge colloidal particles through charge neutralization and compression of electric double layer,destroy the stability of sludge particles and promote their agglomeration and sinking,thus realizing sludge-water separation.In addition,adding skeleton additives(such as rice husk,phosphogypsum,etc.)to the sludge can effectively reduce the compressibility of the sludge[111,112]。 Generally,the particle size of sludge after oxidation conditioning will be reduced,resulting in the gap between sludge particles in the sludge cake becoming smaller or disappearing during high-pressure dewatering,which is not conducive to deep dewatering of sludge.the addition of the skeleton can provide favorable physical conditions,and a permeable rigid lattice structure can be formed in the sludge cake under high pressure to provide a flow channel for free water,which can effectively solve the above problems[113]。 For example,in the report that phosphogypsum is used as a skeleton to assist Fe2+to activate PDS to promote sludge dewatering,it is pointed out that the CST reduction rate is less than 15%when iron salt is added alone,and the CST reduction rate is increased to 84.6%when PDS is added,and the CST decrease rate is further increased to 88.5%when phosphogypsum is added[111]。 At the same time,61.1%of the bound water in the flocs can be effectively released after Fe2+/PDS treatment.With the addition of phosphogypsum skeleton,the released bound water content can be further increased to 68.9%.The study shows that phosphogypsum skeleton additives contain a large number of crystal modifiers and salts(such as Fe3+,Ca2+,SO42−,etc.),which can accelerate the nucleation rate by producing strong ionic attraction,thus promoting the formation of columnar dihydrate gypsum（CaSO4·2H2O）in sludge[114]。 On the one hand,the generated CaSO4·2H2O can build a more permeable and rigid cake lattice structure,which helps the release of bound water[111]; On the other hand,the cations（Na+,Fe2+,Fe3+,and Ca2+）in phosphogypsum skeleton additives themselves can also increase the ion concentration in the liquid phase of sludge,thereby reducing the chemical potential of water molecules between flocs and promoting sludge-water separation[115]。
Although as precursors of ,PMS,PDS and SF can be activated by transition metals,PDS has been shown to be more widely used than PMS and SF in sludge dewatering.This may be due to the fact that Compared with PMS and PDS,which can be directly activated to produce,SF needs to produce less oxidizing free radical species(such as$\mathrm{SO}_{3}^{\centerdot -}$ ,$\mathrm{SO}_{5}^{\centerdot -}$ ,etc.)Before further producing,and its$\mathrm{SO}_{4}^{\centerdot -}$ production efficiency is lower than that of PMS and PDS.compared with PMS,PDS is not only cheaper(PDS:~0.74 USD/kg,PMS:~2.2 USD/kg),but also much more stable[116]。 PDS is almost non-hygroscopic and has a long lifetime,and it has been reported that PDS anion can persist in soil systems for more than 5 months[117]。 PMS is easily affected by oxygen and temperature,and is unstable in water with high pH value(the stability is the lowest at pH 9)[118]。 Therefore,the$\mathrm{SO}_{4}^{\centerdot -}$ ,high production efficiency,economy and stability of PDS are the important reasons for its wide application in sludge dewatering。

4.2 Thermal activation

sludge dewatering promoted by thermally activated PMS/PDS usually involves the principle of solubilization/oxidation,that is,warming can effectively destroy the structure of sludge flocs and microbial cells,and release a large amount of EPS into the sludge liquid phase[119]; At the same time,heating accelerates the decomposition of PMS/PDS to form$\mathrm{SO}_{4}^{\centerdot -}$ oxidized EPS,which promotes the release of EPS bound water into free water and realizes the solid-liquid separation of sludge.As shown in Table 3,sludge dewaterability was significantly improved after thermally activated PMS/PDS oxidative conditioning.Guo et al.Reported a study on the dewatering of excess activated sludge by thermal activated PDS oxidation conditioning,and at the PDS dosage of 120 mg/G VS,the system temperature increased from room temperature to 70°C,and the CST0/CST increased from 1.43 to 2.91,which significantly improved the sludge dewatering performance[120]。 In order to further improve the dewatering effect of WAS,wheat straw biochar WAS used as a skeleton additive combined with thermal activated PDS to condition sludge dewatering.The biochar skeleton additive has a more rigid structure and high porosity,which can provide sufficient drainage channels for free water.Compared with the single treatment of 70°C heat-activated PDS(120 mg/G VS)and wheat straw biochar(150 mg/G VS),the CST0/CST increased from 2.91 to 5.03 after the combined treatment.Kim et al.Discussed in depth the different characteristics of heat-activated PMS and PDS oxidation on the improvement of WAS dehydration performance[121]。 Taking sludge filtration performance(CST)as the dewatering index,PDS is better than PMS.thermally activated PDS oxidation may be used as a pretreatment method when filtration is used as a sludge dewatering process in wastewater treatment plants.heat-activated PMS oxidation tended to decompose sludge flocs into fine colloidal particles,and oxidized EPS was broken down with the reaction,and the centrifuged weight reduction(CWR)value of heat-activated PMS treatment was lower than that of heat-activated PDS treatment.Therefore,Thermally activated PMS treatment may be an effective pretreatment method for centrifugal dewatering process。
表3 Sludge dewaterability after thermal activated PMS/PDS oxidation conditioning

Table 3 Sludge dewatering performance after treatment by thermally activated PMS/PDS oxidation

Temperature
(℃)		 Chemical Sludge type Solid content（g/L） Dosage Reaction time CST（s） CST reduction（%） Ref
120 PDS WAS TSS = 45.21 ZVI = 100 mg/g TSS
PDS = 200 mg/g TSS		 30 min 190.6 72.4 122
80 PDS WAS TSS = 16.03 PDS = 2.0 mmol/g VSS 60 min 88.1 76.6 121
PMS PMS = 0.5 mmol/g VSS 53.9
70 PDS WAS TS = 16.5 PDS = 120 mg/g VS
Skeleton = 150 mg/g VS		 10 min 163.5 65.5 120
80 PDS WAS TSS = 41.3 PDS = 2.0 mmol/g VSS 60 min 54.0 40.3 119
75 PMS WAS TS = 12.6 PMS = 150 mg/g VS
Skeleton = 100~400 mg/g VS		 40 min 158.5 63.9 126
55 PDS ADS TS = 29.3 PDS = 3 g/L 24 h 230 33.2 127
PMS PMS = 3 g/L 28.0
120 PDS WAS TSS = 24.49 PDS = 120 mg/g TSS 30 min 100.7 57.6 128
60 PMS WAS TSS = 13.6 Fe2+ = 0.6 mmol/g VSS
PMS = 1.0 mmol/g VSS		 20 min 206.3 92.9 124
60 PDS WAS TSS = 26.4 Fe2+ = 1.5 mmol/g VSS
PDS = 1.2 mmol/g VSS		 20 min 3006.1 96.6 123
the results show that mild hydrothermal conditions can improve The oxidation ability of ZVI activated PDS system[122]。 Under the condition of no hydrothermal treatment,the CST of sludge treated by ZVI activated PDS decreased from 190.6 s to 104.5 s;Under hydrothermal conditions,the CST continued to decrease to 52.6 s.At the same time,the study pointed out that the synergistic mechanism of hydrothermal treatment and ZVI activation of PDS was the cleavage of chemical bonds of C−N and O−P−O,which effectively cracked soluble proteins and humic-like acids,increased the hydrophobicity of sludge,and ultimately significantly improved the dewaterability of sludge.Similarly,Zhen et al.Pointed out that warming could assist volatile suspended solids to activate PDS to improve the dewatering performance of excess activated sludge.Under the conditions of Fe2+=1.5 mmol/g volatile suspended solids(VSS)and PDS=1.2 mmol/G VSS,the CST decreased from 174.2 s to 103.3 s when the temperature increased from 25℃to 60℃[123]。
Compared with the chelating-assisted activation method,the heat-assisted activation SR-AOPs have the following advantages:better dewatering effect can be obtained in a shorter reaction time,the reagent dosage is lower,the pH value of the sludge does not need to be adjusted,and some trace pollutants in the sludge can be released from the solid phase to the liquid phase and then degraded[124]。 Oncu et al.Showed that the sludge dewatering performance was significantly improved after heating(75℃)assisted Fe2+to activate PDS oxidation conditioning sludge,and the antibacterial agents in the sludge were also effectively degraded,and the degradation rates of oxytetracycline,ciprofloxacin and triclosan were as high as 99%,90%and 79%,respectively[125]。

4.3 Electroactivation

Compared with iron activation and thermal activation,electro-activated PMS/PDS oxidation has not been fully studied in sludge dewatering,but it has also received a lot of attention in recent years because of its advantages of high efficiency and low chemical consumption[105,129⇓⇓⇓⇓⇓⇓~136]。 It has been reported that PDS can be directly electrolytically activated to produce$\mathrm{SO}_{4}^{\centerdot -}$ ,as shown in Figure 4A[133]。 Under the conditions of 40 V electrolysis voltage,20 min electrolysis time and PDS dosage of 1.2 mmol/G TS,the CST of conditioned sludge decreased from 93.7 s to 9.7 s.In the electroactive SR-AOPs with the participation of iron-based materials,iron-based materials can act as both an activator and an electrode[137]。 For example,in an electro-activation system in which both the anode and cathode were Ti/RuO2,when Fe2+was additionally added to the system only as an activator to activate PDS,a moisture removal of 96.7%was achieved,and the final dry solids content of the filter cake exceeded 17.5 wt%[134]。
图4 电化学活化的SR-AOPs调理污泥脱水（根据文献105,133绘制）

Fig. 4 Electrochemically activated SR-AOPs for sludge dewatering (adapted from ref 105,133)

When iron plate was used as anode,the CST reduction rates of electrolytically activated PDS oxidation and electrolytically activated PMS oxidation were 49.1%and 67.6%,respectively,and the sludge dewaterability was significantly improved[136][130]。 The study shows that the iron anode can produce enough Fe2+by losing electrons,and the Fe3+/Fe2+cycle can be accelerated after the electrode provides electrons[138]。 The system slowly releases the Fe2+to achieve a stable Fe2+concentration,which avoids the adverse consumption of $\mathrm{SO}_{4}^{\centerdot -}$ by excessive Fe2+in the traditional PMS system activated by external Fe2+,thus effectively destroying the sludge floc structure and degrading EPS,and significantly improving the sludge dewatering performance[130]。 Recent studies have shown that electrochemically activated SF oxidation can effectively improve sludge dewaterability.Fig.4B shows the mechanism of the process[105]。 The reduction of CST was 87.9%at SF dosage of 40 mg/G DS and current density of 30 mA/cm2.Compared with the traditional SR-AOPs with additional iron source,the electrochemically activated SF oxidation has a higher sludge oxidation efficiency.At the same time,the electrochemical oxidation effect and thermal effect in the system have a synergistic effect on improving the sludge dewatering performance,which can effectively degrade macromolecular proteins and intracellular substances in EPS,and the produced hydrophobic amino acids can form drainage channels to effectively separate sludge from water 。
At present,there are still some problems in the electroactivation method,such as the practical application of the electroactivation method.Electroactivation is an energy-intensive activation method with high energy consumption and economic cost,so it is difficult for the electrochemical process to be scaled up and applied to sludge dewatering in actual sewage treatment plants;Secondly,SR-AOPs based on electrical activation method will lead to the residual SF and sulfate in the filtered water,and their potential impact on the ecological environment still needs to be systematically evaluated;In addition,the dewatered supernatant needs to be returned to the wastewater treatment plant for re-treatment,so that the sulfate returns to the wastewater treatment line and the sludge treatment line may produce H2S 。

4.4 Alkali activation

alkali-activated SR-AOPs are rarely reported in the field of sludge dewatering,and the only literature compares the improvement effect of alkali-activated PDS and PMS oxidation on sludge dewatering performance,and also compares the sludge dewatering effect of thermal activation and alkali-activated conditioning[44]。 the CWR of sludge conditioned by alkaline activation(pH 13.2~13.7)PDS system was 66%~76%,which was higher than that of sludge conditioned by thermal activation(50℃,80℃)PDS system(29%~68%).For Different activated objects,the CWR of heat-activated PMS after oxidation conditioning is higher than that of heat-activated PDS;However,in the alkali-activated system,even if PMS was completely decomposed during the treatment process,the alkali-activated PDS oxidation still showed better sludge dewatering performance than the alkaline-activated PMS oxidation.This is due to the fact that is hardly detected in the process of base activation of PMS,which mainly promotes the hydrolysis of PMS to produce weak oxidizing reactive oxygen species such as superoxide anion radical(formula(28))and singlet oxygen(formula(29)).different from PMS,PDS is prone to decompose under strong alkaline conditions,which will accelerate the formation of strong oxidizing active oxygen species such as$\mathrm{SO}_{4}^{\centerdot -}$ ,and then destroy the structure of EPS.Specifically,the first step of base activation of PMS is the hydrolysis of PMS to produce sulfate anions and hydrogen peroxide,and the hydroxyl radicals produced by hydrogen peroxide react with excess hydrogen peroxide to produce superoxide anion radicals(formulas(23-28)).the hydroxyl radical and superoxide anion formed in the previous stage can further react to form singlet oxygen and hydroxide ions(formula(29)).Therefore,the improvement effect of alkali-activated PDS on sludge dewatering performance was significantly better than that of alkali-activated PMS。
In terms of CST0/CST,the CST ratios of all alkaline treatments(with and without PMS/PDS)were significantly inhibited,which may be due to the increase of sludge viscosity caused by the release of small colloidal EPS,thus reducing the CST ratio.In the analysis of sludge EPS components,alkali-activated PMS/PDS treatment tended to increase S-EPS and LB-EPS concentrations more effectively than thermal activation treatment,which was due to the fact that sludge decomposition by alkali-activated PMS/PDS system depended on alkaline cell lysis rather than oxidation,thus minimizing the reduction of EPS content due to oxidation 。

4.5 Other activation methods

Researchers have also developed auxiliary activation methods including ultrasound,ultraviolet irradiation and microwave,which have been confirmed to improve sludge dewatering performance[104,105,130,135,136,139,140]。 For example,Liu et al.Used ultrasound-assisted ZVI to activate PMS.Under 50 W ultrasonic power,the passivation layer of ZVI could be effectively destroyed and the corrosion of ZVI was accelerated,and the CST reduction rate of sludge after conditioning reached 89.2%[140]; Compared with the system without ultrasonic assistance,the reduction rate of sludge CST increased by 12.8%.Recent studies have shown that UV light can also be used to assist ZVI to activate PDS to promote sludge dewatering.After 20 min of UV irradiation at 254 nm,the CST and specific resistance to filtration(SRF)of WAS were reduced by 64.0%and 78.2%,respectively[139]。 the effects of ultraviolet light mainly include two aspects:one is that ultraviolet light promotes PDS to produce more$\mathrm{SO}_{4}^{\centerdot -}$ ;Second,ultraviolet light can improve sludge dewatering capacity by attacking sludge cells and sludge EPS.Therefore,the sludge dewatering effect of PDS activated by ZVI assisted by UV is better than that of PDS activated by ZVI alone.At present,although the feasibility of photoassisted activation process in improving sludge dewaterability has been confirmed by relevant studies,most of the photoassisted SR-AOPs are mainly focused on the degradation of pollutants due to the high turbidity and colloidal characteristics of sludge,and the research on sludge dewaterability is still scarce[139,141,142][42]。
in addition to the above methods,Microwave irradiation is also a commonly used auxiliary method for sludge dewatering.microwave treatment decomposes sludge flocs by inducing the thermal effect caused by the rotation of dipole molecules and the non-thermal effect caused by the rapid change of dipole orientation In the side chain of cell membrane macromolecules,which has the characteristics of short reaction time,uniform reaction and low cost[143]。 The improvement of sludge dewaterability by microwave irradiation has been observed in previous studies.Zhen et al.Used microwave-assisted activation of PDS under the conditions of 0.4 mmol/G TS of Fe2+,0.5 mmol/G TS of PDS,and 500 W microwave power to achieve 94.6%CST reduction,and it was more cost-effective and saved oxidation reagent compared with microwave irradiation alone or Fe2+activation alone[104]。

5 Removal of trace pollutants

the produced by SR-AOPs not only contributes to the disintegration of EPS to promote sludge dewatering,but also plays a certain role in the removal of trace pollutants in sludge.Although sludge particles have a strong adsorption effect on organic pollutants in wastewater,these organic pollutants can not be effectively degraded by traditional sludge conditioning methods,resulting in the need for further advanced treatment of the water removed from sludge.This increases the burden of subsequent water quality treatment and is not conducive to the resource utilization of sludge cake.The produced by SR-AOPs not only contributes to the disintegration of EPS to promote sludge dewatering,but also plays a certain role in the removal of trace pollutants in sludge.Recent studies have shown that The$\mathrm{SO}_{4}^{\centerdot -}$ produced by SR-AOPs can effectively degrade even trace organic pollutants adsorbed by mineralized sludge,including polycyclic aromatic hydrocarbons,phenolic pollutants such as bisphenol A,antibiotics,etc.,due to their electrophilic properties[144,145][146][125,147]。 Table 4 summarizes the effectiveness of SR-AOPs in removing trace pollutants during sludge conditioning。
表4 Degradation of trace pollutants in sludge by different SR-AOPs

Table 4 Degradation of micropollutant in sludge by different SR-AOPs

Item Process Chemical Dosage Micropollutant Initial concentration Removal rate（%） Ref
Aromatic compounds poly(3-hydroxybutyrate)/PMS
Fe2+/S2O82-		 PMS
PDS		 PMS = 3.1 × 10-4 M
PHB = 3.3 g/L
Fe2+ = 2.0 mmol/g VS
PDS = 1.6 mmol/g VS		 PAHs 3099 ± 32 ng/g dw
2692.0 μg/kg		 79 149
96
Toluene 89.8
Biochar/S2O82- PDS PDS = 10 mmol/L
Catalyst = 0.5 g/L		 Phenol 200 mg/L 57.8 150
Phenolic compounds Biochar/S2O82- PDS PDS = 10 mmol/L
Catalyst = 0.5 g/L		 Phenol 200 mg/L 57.8 150
Antibiotics MW(160 ℃)/S2O82- PDS PDS = 0.87 g/g TS Oxytetracycline 2 mg/g TS 98.9 151
Ciprofloxacin 98.3
Modified sludge bio-hydrochar/S2O82- PDS PDS = 5 mmol/L
Catalyst = 0.2 g/L		 Tetracycline 60 mg/L 99.72 152
Fe2+/75 ℃/S2O82- PDS PDS = 22.7 mM Oxytetracycline 100 mg/kg TS 95 125
Ciprofloxacin 84
Triclosan >99
Fenton sludge-Cu/PMS PMS PMS = 20 mg/L
Catalyst = 0.2 g/L		 Tetracycline 10 mg/L 85.53 153
polycyclic aromatic hydrocarbons(polycyclic aromatic hydrocarbons,PAHs)are a class of persistent organic pollutants with strong carcinogenicity,and they are easily adsorbed on sludge particles.If the sludge containing PAHs is returned to the environment without proper treatment,it is bound to cause secondary pollution to the environment and pose a threat to human health[148]。 Hung et al.Used poly(3-hydroxybutyrate)/PMS oxidative conditioning to remove PAHs in sludge,including low molecular weight PAHs(number of benzene rings≤3)and high molecular weight PAHs(number of benzene rings≥4)[149]。 In the untreated sludge sample,4-ring PAHs accounted for 79%of the total PAHs and were the main PAHs.Oxidative conditioning could effectively promote the degradation of PAHs,and the maximum removal rates of 2-ring,3-ring,4-ring,5-ring and 6-ring PAHs were 64%,49%,71%,88%and 60%,respectively。
Phenolic pollutants are common pollutants in sludge,and SR-AOPs can effectively degrade these trace pollutants[149]。 In a study on the improvement of sludge dewaterability by activating PMS with Fe and Mn co-doped biochar activator,the synthesized activator showed good PMS activation performance and phenolic pollutant degradation performance[146]。 phenol,bisphenol A,and 2,4-dichlorophenol can all be completely removed In this system.in addition,it has been reported that the rapid degradation of Phenol was achieved by using biochar to activate PDS[150]。 Under the condition of 10 mmol/L PDS and 0.5 G/L biochar activator,the degradation rate of phenol was as high as 57.8%。
Given the persistence of antibiotics in the environment and the potential to accelerate the development of microbial resistance in sludge,SR-AOPs have been attempted to degrade antibiotics in sludge.Xiao et al pointed out that the process of sludge dewatering by Fe2+activated PMS conditioning could simultaneously realize the degradation of ciprofloxacin in the solid phase of sludge[147][147]。 Ciprofloxacin WAS mainly distributed in the sludge solid phase(99%in both the original ADS and WAS solid phases).Ciprofloxacin in the solid phase of sludge WAS degraded to some extent(15%for ADS and 27%for WAS)after conditioning with Fe2+activated PMS.In addition,Oncu et al.Promoted the degradation of oxytetracycline,ciprofloxacin and triclosan by heat-assisted ferrous iron activated PDS oxidation conditioning sludge[125]。 The average degradation rates of the three antibiotics were 95%,84%and>99%,respectively,when the molar ratio of Fe2+/S was 0.5 and the conditioning time was 120 min at 75℃ 。
However,most of the reports on the degradation of pollutants in sludge are limited to the laboratory level,that is,a certain concentration of pollutants needs to be added to the sludge,which causes the concentration of pollutants in the experiment to be much higher than that in the actual sludge system.in view of this,SR-AOPs should be able to effectively degrade trace pollutants in sludge to a certain extent,but the degradation characteristics of trace pollutants in actual sludge systems still need to be systematically studied。

6 Heavy metal removal

Due to the toxicity,easy accumulation and non-biodegradable characteristics of heavy metals,improving the removal rate and stability of heavy metals in sludge has become a key limiting factor for land use of sludge[154]。 the release of heavy metals from the sludge floc structure into the liquid phase is the key to improve the removal rate of heavy metals[131]。 Recent studies have shown that SR-AOPs can effectively dissolve heavy metals in sludge and promote the significant reduction of heavy metal content in the solid phase of sludge[40,155,156]。 Huang et al.systematically analyzed the chemical behavior of heavy metals in the solid-liquid phase of sludge after dehydration of thermally activated PDS oxidation conditioned sludge,and the study showed that the stability of heavy metals in the conditioned sludge was significantly improved and the leaching toxicity was significantly reduced,and the leaching toxicity of Pb,Zn,Cu and Cr was reduced by 100%,84%,83%and 33%,respectively[126]。 However,the effect of heat treatment alone on heavy metals in sludge is negligible.Guo et al.Found that Fe2+activated PMS oxidation could reduce the leaching toxicity of Cu,Zn,Cd and Cr by 86.3%,73.5%,59.1%and 60.9%,respectively[157]。 the leaching toxicity of heavy metals was further reduced by adding rice husk skeleton additive.These oxidative conditioning can effectively improve the degradation of organic matter and the release of heavy metals because oxidation destroys the binding sites between heavy metals and organic matter,and promotes the chelation reaction between active groups and soluble heavy metals[158]。
different heavy metals showed different migration and transformation behaviors during the conditioning process of SR-AOPs.the chemical species distribution of heavy metals in sludge can be divided into four components:soluble and exchangeable(F1),organic and sulfide bound(F2),iron and manganese oxide bound(F3)and residual(F4),and the instability of different heavy metal components is in the order of F1>F2>F3>F4[159]。 Recently,Bian et al pointed out that the contents of As,Cd,Cu,Ni,Pb and Zn in the sludge cake of ultrasonic activated PDS oxidation dewatered sludge decreased,but the concentrations of Cr and Pb increased by 40%and 20%,respectively,compared with those in the initial sludge cake[43]。 This may be because As,Cd,Cu,Ni,Pb and Zn changed from insoluble heavy metal state to soluble ionic state or complex state,while Cr and Pb formed chromium sulfate and lead sulfate precipitates after treatment.Zhang et al.and Hu et al.Also confirmed that the main forms of Cr and Pb were residual forms,and their forms did not change much after oxidation conditioning[158][131]。 Cu exists mainly in the organic form because Cu has a stronger affinity for oxygen,nitrogen,and sulfur ligands than other metals and can form very stable complexes[95,112,157,158]。 Ni showed a similar trend to Cu,showing good stability and low mobility.After SR-AOPs oxidation conditioning,the proportion of Cu and Ni in F3 decreased,while the proportion in F4 increased,showing good stability and low mobility,due to the effective degradation of organic matter by$\mathrm{SO}_{4}^{\centerdot -}$[159]。 The proportion of other heavy metals including Zn and Cd in F4 increased in different degrees,indicating that their toxicity decreased[159]。
Although the content of heavy metals in the solid phase of sludge was significantly reduced after SR-AOPs conditioning,the concentration of heavy metals in the liquid phase was significantly increased.If the mud cake leachate with high content of heavy metals is not properly treated,it will inevitably cause secondary environmental pollution,so the mud cake leachate needs further treatment。

7 Sludge dewatering mechanism

the mechanism of sludge dewatering conditioned by SR-AOPs mainly lies in The production of strong oxidative free radicals($\mathrm{SO}_{4}^{\centerdot -}$ ,$\cdot \ \mathrm{OH}$ ,$\mathrm{O}_{2}^{\centerdot -}$ ,etc.),which can effectively destroy the EPS structure of sludge and even microbial cells,and then promote the transformation of bound water into free water,and ultimately improve the sludge dewatering performance.In short,the mechanism includes degradation of organic compounds in each layer of EPS,neutralization of EPS surface charge,and promotion of microbial cell lysis in EPS,as shown in Fig.5.(1)Degradation of organic compounds in each layer of EPS.Polymers such as proteins and polysaccharides in EPS contain a large number of hydrophilic polar groups(such as—COOH,—OH,—NH4,—SH,etc.),which are not conducive to sludge dewatering[160]。 the results showed that the bonds in the macromolecular organic polymer skeleton in EPS were broken after the attack of$\mathrm{SO}_{4}^{\centerdot -}$ ,and the functional groups were transformed,so that the water bound to EPS was transformed into free water and removed[41]。 the results showed that$\mathrm{SO}_{4}^{\centerdot -}$ could effectively oxidize aromatic proteins in EPS,especially tryptophan-like proteins and tyrosine-like proteins,and could effectively improve The dewatering performance of sludge[97,119,136]。 Its mechanism of action is to change the protein structure and expose hydrophobic sites by interfering with hydrogen bonds and S—S bonds in aromatic proteins[161]。 (2)to neutralize The EPS surface charge.the degradation of EPS by$\mathrm{SO}_{4}^{\centerdot -}$ and the neutralization of negative surface charges by cations(e.g.,Fe2+and Fe3+）)together contribute to the increase in Zeta potential[41]。 When the sludge surface charge is close to the isoelectric point,the electrostatic repulsion between the bound water and the sludge decreases sharply,the sludge flocs become unstable,and the sludge dewaterability is improved[162]。 and(3)promote that lysis of microbial cells in EPS.Studies have shown that$\mathrm{SO}_{4}^{\centerdot -}$ can significantly reduce the activity of microbial cells,resulting in the effective release of intracellular substances(water And organic matter),thereby improving the sludge dewatering performance[106,130,163]。
图5 SR-AOPs降解污泥EPS和促进脱水机制

Fig. 5 Mechanisms of EPS degradation and enhanced dewaterability by SR-AOPs

8 Conclusion and prospect

in this paper,the research on the use of SR-AOPs to improve sludge dewaterability At home and abroad in the past decade was summarized,and the development history and activation mechanism of SR-AOPs were clarified.SR-AOPs can not only significantly improve the dewatering performance of sludge,but also effectively degrade trace pollutants and stabilize heavy metals in sludge.at present,transition metal activation has been widely used in sludge dewatering,and a variety of auxiliary means such as adding skeleton additives,chelating agents,ultrasound,microwave,light and so on have been combined to further improve sludge dewatering performance.These methods have confirmed that sulfate radical,as a new tool to enhance sludge dewatering,has a certain practical application prospect.However,despite the outstanding advantages of SR-AOPs in the field of sludge dewatering,there are still some scientific problems that need further systematic study.Based on this,the application prospect of SR-AOPs in the field of sludge dewatering is prospected as follows。
(1)Some energy-intensive activation methods,including electric,ultrasonic,microwave and other auxiliary activation methods,limit their application in practical sewage treatment plants economically,so it is necessary to develop more energy-saving and efficient activation methods to achieve sludge dewatering.in addition,it is necessary to systematically study the by-products(such as sulfate ions and heavy metals in the liquid phase)produced by SR-AOPs in the sludge conditioning process and evaluate their potential impact on the environment。
(2)at present,the research mechanism of SR-AOPs improving sludge dewatering is mainly focused on the free radical theory,including the strong oxidation characteristics of$\mathrm{SO}_{4}^{\centerdot -}$ and$\cdot \ \mathrm{OH}$ ,while the effect of some reducing free radicals on sludge dewaterability is not clear.in addition,the reasons for the different effects of various free radicals in SR-AOPs system need to be further explored,and the research of non-free radical pathway in the field of sludge dewatering needs to be clarified At this stage。
(3)although iron-based materials are more cost-effective than other transition metal materials,iron-based materials used For SR-AOPs are still very expensive at this stage,which seriously hinders their application in practical sludge conditioning.How to reduce the cost of activators and improve the activation efficiency is still the focus of future research.for non-metallic activators,there are only reports on promoting the degradation of organic pollutants in wastewater,and the research in the field of sludge dewatering is still very scarce,so it is necessary to carry out the research on the application of such activators in sludge dewatering.in addition,the precursors of$\mathrm{SO}_{4}^{\centerdot -}$ are mainly PMS and PDS,while there is a relative lack of research on SF,Although SF has stronger stability and lower cost.From the point of view of cost-effectiveness and dewatering efficiency,it is necessary to systematically carry out the research on SF activation and oxidation in sludge dewatering in the future。
(4)Although SR-AOPs can effectively improve sludge dewatering performance,the current research is basically in the laboratory stage,and more experimental optimization and pilot experiments need to be further carried out to lay a solid foundation for sludge dewatering in actual sewage treatment plants in the future.in addition,the degradation characteristics of trace pollutants in actual sludge system need to be systematically studied in the degradation of trace pollutants by SR-AOPs。

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