Strategies for enhancing peroxymonosulfate activation by heterogenous metal-based catalysis: A review

Jiahao Wei,Fan Li,Lina Zhou,Dandan Han*,Junbo Gong*

State Key Laboratory of Chemical Engineering,School of Chemical Engineering and Technology,Tianjin University,Tianjin 300072,China The Co-Innovation Center of Chemistry and Chemical Engineering of Tianjin,Tianjin 300072,China

Keywords:Peroxymonosulfate Heterogeneous catalyst Sulfate radical-advanced oxidation processes (SR-AOPs)Water treatment Enhancement strategies

ABSTRACT Sulfate radical-advanced oxidation processes(SR-AOPs)are promising technologies for organic pollutants elimination.Heterogeneous metal-based catalysis has been widely studied and applied to activate peroxymonosulfate (PMS) for producing sulfate radicals.Developing highly efficient catalysts is crucial for future extensive use.Importantly,the catalytic activity is mainly determined by mass and electron transfer.This paper aims to overview the recent enhancement strategies for developing heterogeneous metalbased catalysts as effective PMS activators.The main strategies,including surface engineering,structural engineering,electronic modulation,external energy assistance,and membrane filtration enhancement,are summarized.The potential mechanisms for improving catalytic activity are also introduced.Finally,the challenges and future research prospects of heterogenous metal-based catalysis in SR-AOPs are proposed.This work is hoped to guide the rational design of highly efficient heterogenous catalysts in SR-AOPs.

1.Introduction

Water crisis and water security have become the major environmental concerns due to the global population growth and industrial development[1,2].An ever-increasing variety of organic pollutants,such as pharmaceuticals and personal care products(PPCPs),per-and polyfluorinated chemicals (PFCs),microplastics,and endocrine-disrupting chemicals (EDCs) have been discharged into the environment [3,4].Many of them are characterized as high-toxic and non-biodegrade,posing a huge threat to environmental safety and human health.To this end,a lot of strategies and technologies have been applied for the elimination of organic pollutants,including membrane separation [5],adsorption [6,7],photocatalytic degradation [8],and advanced oxidation processes(AOPs) [9].

AOPs is an aqueous phase technology proposed by William H.Glazeet al.in 1987 and gradually developed as cutting-edge water treatment technology due to the high efficiency for organic pollutants degradation [10].In general,AOPs employ highly reactive oxygen species (ROS),such as hydroxyl radical (·OH),sulfate radicals (),and singlet oxygen (1O2),to degrade and mineralize organic pollutants into CO2,harmless inorganic salts,and water[11,12].Fenton and Fenton-like systems have prevailed for many years for generating·OH (redox potential=1.8-2.7 VvsNHE)[13,14].The traditional Fenton system employs iron-based catalysis to activate H2O2for the production of·OH and organic pollutants degradation [15].The typical reaction mechanism is described as Eq.(1)-(3).However,these systems suffer from some serious drawbacks,including a narrow working pH range (2-4),and insecurity of H2O2during transportation and storage [16].

In comparison to·OH,·shows higher redox potential of 2.5-3.1 V,longer half-time (30-40 μs for·vs20 ns for·OH),and a wider working pH range (2.0-8.0).Besides,· is a strong electrophilic reagent,which is easy to attack the electron-donating functional groups on the aromatic ring compound (such as-NH2,-OH,-OR) through electron transfer.Thus,· displays higher selectivity toward organic pollutants with electron-rich functional groups while·OH reacts with organics through hydrogen addition or extraction with C-H bonds without selective[17].In fact,when it comes to real industrial wastewater,their physicochemical properties are as wide as they come;the organics’load can reach thousands of mg·L-1,with wide resulting pH,high ionic charge,solids [17],etc.Thanks to the inherent advantages of·,the high efficacy of sulfate radical-based AOPs (SR-AOPs)in acidic or basic media,and versatility in application have led to attracting a keen interest in industrial wastewater treatment over the last years.However,can also react with the nontoxic (or low toxic) dissolved organic matter and inorganic ions (such as humic acid,Cl-,OH-)in industrial wastewater,leading to the waste of· and an increase in processing cost.Therefore,the main challenge is to improve the utilization of·.Usually,a pretreatment is required,such as biological (anaerobic/aerobic) or physicochemical treatment (such as precipitation and coagulation-flocculation),to decrease the impurity in wastewater.In addition,it is uneconomical for SR-AOPs to mineralize all organic matter in wastewater.The purpose of SR-AOPs application is to degrade partial organic pollutants and lower the toxicity of wastewater,so as to meet the requirements of subsequent water treatment processes (typically,biochemical treatment).

Various activation methods including heat,ultrasound,alkali,ultraviolet light,and catalysis have been applied for PMS activation[19,20].Among these approaches,activating PMS by catalysis is the most adopted due to low equipment requirements,simple and safe operation,and low energy consumption.As early as 2003,cobalt ion was first reported to homogeneous catalytic activate PMS for the production of· [21].After decades of years investigation,cobalt have been considered as the most efficient,due to the high standard reduction potential of Co3+/Co2+(E0=1.92 V) compared with Ce4+/Ce3+(E0=1.72 V),Ni4+/Ni2+(E0=1.59 V),Mn3+/Mn2+(E0=1.50 V),Fe3+/Fe2+(E0=0.771 V) and Cu2+/Cu+(E0=0.159 V)[22].Although the homogeneous catalytic activation can overcome the mass transfer limitation,the residual metal ions in water are prone to raise environmental risks.

Alternatively,heterogeneous catalysis has been developed and become the favorite for PMS activation,due to the following reasons [11]: (i) The separation of the spent catalysis from the reaction media can be achieved to maintain the reusability.(ii) The loss of metal ions can be effectively inhibited to prevent secondary contamination.(iii) The high ability to work under complex operational conditions and water parameters.

After years of development,various heterogeneous catalysts have been developed for PMS activation,mainly including transition metals (mainly Fe/Co/Mn/Cu/Ce/Ni/Zn) and their oxides or hydroxides [23,24],metal-carbon hybrid [25],and metal-organic frameworks(MOF)[26].Recently,metal-free carbonaceous materials(e.g.,graphene,carbon nanotubes,biochar,and nano/diamonds)have also emerged as heterogeneous catalysts,because of decent biocompatibility,large surface area,and great resistance toward acid and base [27,28].However,the carbonaceous catalysts often suffer from poor stability,because their surface-active group (e.g.,carbonyl groups and hydroxyl groups) are easily oxidized and deactivated during SR-AOPs.Furthermore,the undesirable oxidation of carbon network may release dissolved organic carbon into water.To this end,metal-based catalysts with better stability are preferable and widely researched in SR-AOPs.

Fig.1.(a)Activation of H2O2,PMS,and PDS through the electron-transferring process.(b)The number of papers searched by the database Web of Science for topic keywords‘‘peroxymonosulfate and degradation”.(c) Activation of PMS by heterogenous cobalt-based catalysis through electron-transferring processes.

Taking cobalt as an example,the one-electron transfer mechanism (Eqs.(4)-(6)) is generally accepted for heterogeneous catalytic activation of PMS (Fig.1(c)) [29].

However,the activity of heterogeneous catalyst system is restricted by mass transport and redox cycle reactions (Con+/Co(n+1)+)of active sites.Advanced in catalysis must rely on rational design and sophisticated tailoring.Generally,there are three principles to enhance the activity (or reaction rate) of heterogeneous catalytic systems: (i) Optimizing the structural configuration and spatial distribution,or applying process intensification to improve the mass transferability.(ii) Increasing the number of active sites for better contact with reactants and higher mass/electron transfer(e.g.,through increased loading or improved catalyst structuring to expose more active sites per gram).(iii) Increasing the intrinsic activity of each active site [30].These principles are not mutually exclusive and can ideally be addressed simultaneously,leading to the greatest improvements in activity.

Based on the above principles,this review aims to summarize the recent progress in enhancement strategies for efficient activating PMSviaheterogeneous metal-containing catalysis.These strategies include surface engineering,morphological and structural engineering,electronic modulation,external energy assistance,and membrane filtration enhancement.Meanwhile,the potential mechanisms for improving catalytic activity are also pointed out.Finally,the challenges and future research directions of heterogenous metal-based catalysis in SR-AOPs are discussed and proposed.This work is expected to advance the knowledge of the rational design of heterogeneous catalysis for environmental remediation.It is expected to provide new clues for the rational design of highly efficient catalysis.

2.Surface Engineering

An intimate surface interaction between catalysis and PMS molecules is a prerequisite for achieving the desired activity.Especially,surface physicochemical properties play important role in heterogenous catalytic activation of PMS.Because the process refers to two-phase(liquid and solid)reactions,in which the mass transfer and electron transfer are the key stages of PMS activation and ROS generation.

2.1.Surface modification

Surface modification by thein situformation of ligands using organic chelating agents can improve the interaction of reactant molecules with catalysis.For instance,LaMnO3catalysts werein situmodified by typical organic ligands,and their PMS activation performances were comparably investigated via ofloxacin (OFX)degradation.Obviously,the OFX degradation was increased from 32.9% (without modification) to 96.3% and 95.8% by ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA)modification,of which the rate constant increased by 5.2 and 21.8 times[31].However,the OFX removal efficiency decreased to 22.61%,8.47%,and 4.73% in the presence of glycine (GLY),citric acid (CA),and oxalic acid (OA).The authors concluded that the different modifiers have different impacts on the generation of surfacebonded Mn(II) and oxygen vacancy in LaMnO3,which determined the catalytic performance.Moreover,some inorganic anions (such asand H2) also show similar ligand effects [32-34].

The surface electronic structure could have an important impact on forming the surface-reactive complex of PMS.Modulating surface charge can benefit the adsorption of PMS through electrostatic attraction,therefore leading to faster electron transfer and better catalytic activity.Zuoet al.reported that the surface electronic structure of copper oxide/graphitic carbon nitride (CuOCN) was modified by glucose [35].Glucose modification enlarges the Cu-O covalency,resulting in an increased average valence of Cu,which facilitated the formation of surface reactive complexes with PMS.Therefore,modified CuO-CN exhibited a 14.8-fold higher kinetic reaction rate(0.10392 min-1)for PMS activation and pollutant degradation compared with the unmodified CuO-CN.Besides,surface modification by chemical etching and N2-annealing produced phase change and nano metal layer generation on stainless-steel mesh.The excellent electron conductivity and the favorable morphology with coexisting Fe,Ni,and Cr in modified stainless-steel mesh make an effective electron transfer during PMS activation [36].

2.2.Crystal facet tailoring

Exposure of specific active facetsviafacet engineering and morphology control can accelerate PMS activation to generate more ROS.Because the different distributions of unsaturated metal ions in various facets might result in varied areas of active sites and different adorability towards PMS,thus exhibiting the difference in catalytic performance [37,38].For example,a series of Co@N-C materials are prepared by selecting nitrates (LiNO3,NaNO3,and KNO3)as templates to control the preferential growth of the cobalt(2 0 0)facet(Fig.2(a)).Experiments showed that Co(2 0 0)was the most active facet for PMS activation compared with Co (2 2 0),Co(1 1 1),and Co (3 1 1),while nitrates promoted the preferential growth of the Co(2 0 0)facet.Characterization and theoretical calculations revealed the Co (2 0 0) facet is more active,which could accept or lose electrons more easily than other facets,thus leading to good activity and stability [39].

As for heterogeneous metallic catalysts,previous studies have shown that certain kinds of surface sites or defective facets can exhibit extraordinary improvement toward specific catalytic reactions [38,42].Planar defects (i.e.,defects of facet),including twin defects and grain boundaries,have been reported to exhibit excellent effects in enhancing the catalytic performance of a catalyst.Recently,particle attaching and merging,named attachment growth,have been proven to be an effective route for the construction of planar defects within nanomaterials.Yanget al.prepared surface defective Co/Fe-Co planar hybrid composite nanosheet by thermal treatment process under H2-N2atmosphere [43].During the thermal treatment of metallic precursors,abundant CoO werein situdissolved from the surface of Co(OH)2nanosheets,then attached together into nanocrystals,while defective surface holes were formed on the Fe-doped Co(OH)2nanosheets under a prolonged thermal procedure.As a result,the heterojunction phase CoO/Co(OH)2and defective surface holes on Co/Fe-Co planar hybrid obviously enhanced catalytic activity of PMS activation and methylene blue degradation(the rate constant increased from 0.3284 min-1to 0.9374 min-1).

Furthermore,introducing defective facets can improve the catalytic selectivity for the degradation of target pollutant.Zhanget al.constructed a defective (0 0 1)-TiO2-x viathermal treatment with the assistance of H2[44].The high-energy(0 0 1)polar facets were fully exposed and served as electron-rich centers for both persulfate activation and pollutants adsorption.As a result,the defective(0 0 1)-TiO2-xshowed low water matrix effects in the front of typical anionsnatural organic matters and real surface water for target pollutants degradation.

Fig.2.(a) Effect of different cobalt facets on PMS activation by Co@N-C [39].(b) PMS activation by 1D CoTiO3 nanofiber [40].(c) 2D CoNi3O4 nanoribbons/diatomite composite with abundant open diffusion channels [41].

3.Morphological and Structural Engineering

The diffusion of reactants from the bulk phase onto the surface of the catalyst is called an external diffusion process [45].The resistance of this process depends on the linear velocity of the fluid.Increasing the fluid flow rate and the dispersion of catalysts can eliminate the effect of the external diffusion process on the rate of reaction.Subsequently,the diffusion of reactants from the outer surface of the catalyst to the interior active sites of the catalyst through the pores is called the internal diffusion process.The resistance of internal diffusion depends on several morphological and structural factors,including the specific area,the number of pores,the pore diameter,the length of the pores,the degree of curvature,and the distance between the interior active sites and the catalyst surface[46].So far,many efforts have been paid to reduce diffusion resistance and enhance mass transferviamorphological and structural engineering.

3.1.Constructing porous structure

Enlarging specific surface area and porosity of catalysis would enhance the adsorption capability and expose more active sites.Moreover,the pore structures of catalysis play significant roles in PMS activationviareducing the internal diffusion resistance and modulating the transport rates of PMS and contaminants toward active sites.Nevertheless,small specific surface area and low pore volume are often encountered when synthesizing metal-based catalysis under high temperature.Hence,constructing porous catalysts with high specific surface area is attracting considerable critical attention.So far,serial methods have been adopted for constructing porous metal-based catalysis with high specific surface area can be summarized as pyrolysis of metal-organic framework[47-49],chemical etching[50],template method[51,52],and loading on porous support.

It is well known that MOFs have well-developed porosity.A certain high-temperature pyrolysis process can convert MOF into metal oxides or metal-carbon composites,which maintain decent porous structures.Yanget al.synthesized porous CoFe2O4nanocrystals (CoFe2O4NC,SBET=60.4 m2·g-1) nanocrystals by pyrolyzing the Co/Fe bimetal-organic frameworks at 400 °C [47].Compared with the hydrothermally fabricated CoFe2O4nanoparticles(SBET=12.6 m2·g-1),CoFe2O4NC displayed a 30%improvement in the catalytic activation of PMS for bisphenol A (BPA) degradation.The difference in catalytic capacity is attributable to the larger specific surface area and well-developed mesoporous structure(pore volume is 0.64 cm3·g-1) of the CoFe2O4NC.A threedimensional (3D) MOF-derived porous aerogels (Fe@NC-800/AG)was prepared by a freeze-drying and pyrolysis technique.The Fe@NC-800/AG possessed a hierarchical porous structure,which could significantly reduce diffusion resistance of reactants from bulk phase to surface active sites [53].Besides,the Fe@NC-800-0.15/AG can be easily separated from the aqueous solution due to its 3D compressible feature.

Template agents can be used to construct metal cluster precursors by intermolecular forces(hydrogen bond,chemical bond,electrostatic interaction) during the synthesis process.Porous metalbased catalysis can be obtained after removing the templating agent.For instance,hierarchically porous cobalt-iron oxide nanosheets catalysts were obtained by choosing NaBH4as a reductant and high concentration cetyltrimethylammonium bromide(CTAB) as a templating agent.A series of catalysis with different specific surface areas can be obtained by adjusting the additive dosage of CTAB and calcination temperature [51].As the specific surface area increases,the catalyst displayed better catalytic activity toward PMS activation.This phenomenon can be attributed to the hierarchical porous structure facilitating mass/electron transfer,while more active sites are exposed as the specific surface area increases.Highly porous and monodisperse manganese oxides(SBET=217 m2·g-1,pore volume is 0.609 cm3·g-1) were prepared by a one-pot hydration and calcination process using butyric acid andn-butanol as template agents [52].The abundant hierarchical porosity and a high surface area were favorable for PMS activation and phenol oxidation.

In addition,porous catalysis can also be prepared by posttreatment of chemical etching.Chenet al.prepared mesoporous CoFe2O4by nano-etching method and found that the increase in specific surface area and pore volume enhanced the catalytic activity of PMS activation for ciprofloxacin (CIP) degradation [50].

3.2.Composite with porous support

Loading the catalyst on porous support can not only further increase the pore structure and specific surface area,but also can alleviate the adverse brought by the aggregation of nano/micro scale catalysis.

Diatomite is considered attractive support with the advantages of well-developed porosity,high reusability,low cost,and excellent chemical stability [54].Diatomite-supported nano zerovalent iron (nZVI) catalyst (NDA) with complex network structure was prepared by precipitation method [55].The nano 3D network formed by nZVI and diatomite channels significantly increased the specific surface area and pore volume,and further exposed more active sites,which made NDA have better performance in activating PMS to degrade BPA than pure nZVI.

Kaolinite is another porous natural inorganic support,that possessed two-dimensional lamellar structure and abundant aluminum hydroxyl groups [56].Donget al.synthetized the CuFe2O4/kaolinite catalysis through a facile citrate combustion method[57].Owing to the higher specific surface area,larger pore volume,more exposed hydroxyl groups,and more accessible reactive sites,CuFe2O4/kaolinite exhibited better catalytic activity in PMS activation for BPA degradation compared to pristine CuFe2O4.

Similarly,zirconia-supported LaCoO3(LaCoO3/ZrO2) was prepared and applied as heterogeneous catalyst for Rhodamine B(RhB)degradation[58].Compared to the bulk LaCoO3,the LaCoO3/ZrO2performed an enlarged surface area (62.5 m2·g-1,≈10 times than bulk LaCoO3) and enhanced catalytic activity for activating PMS.

Furthermore,anchoring nano-catalysis on the porous support can not only facilitate the separation process but also prevent the catalysis from aggregation.For instance,nanoscale CoFe2O4(nCoFe2O4) is easy to aggregate due to its high surface energy and magnetic interaction,resulting in a catalytic activity decrease.To this end,organo-montmorillonite (OMt) was adopted as support fornCoFe2O4(nCoFe2O4/OMt) [59].nCoFe2O4/OMt displayed an increased specific surface area(136.1 m2·g-1)compared to that of barenCoFe2O4(80.6 m2·g-1) nanoparticles.Such combination solves the decrease of active sites and poor catalytic activity caused by the agglomeration ofnCoFe2O4nanoparticles,leading to better catalytic activation efficiency of PMS.

3.3.Nano-structuring

The development of nanotechnology brings many potential impacts in various fields.In the SR-AOPs,the main goal of nanotechnology is to improve the effective surface area of catalysis,reduce the distance between reactants and active sites,and alleviated the adverse effects of environmental factors (e.g.,natural organic matter,NOM).Some research on nanotechnology to improve SR-AOPs are introduced as follows.

Designing catalysis with special nanostructures can not only increase the specific surface area of the catalyst and expose more active sites due to the size effect,but also can reduce the diffusion distance,thereby improving the catalytic efficiency.Recently,advanced heterogeneous catalysis with nano-scaled architectures has been proven to be a competitive candidate for PMS activation.For instance,the catalysis with one-dimensional (1D) nanotube/-nanofiber structures displays decent catalytic performances due to the increase in the surface area,specific functional groups,stability,and electrical conductivity [60,61].As shown in Fig.2(b),CoTiO3nanofiber was prepared through the electrospinning method,and then served as a heterogeneous catalyst for PMS activation [40].CoTiO3nanofiber showed a much higher catalytic activity than the conventional bulk CoTiO3.This enhancement was ascribed to enlarged surface area and porosity.Besides,the CoTiO3nanofiber was assembled as a membrane reactor for SRAOPs,which showed advantages of convenient operation,easy regeneration,and low energy requirement.

Meanwhile,two-dimensional (2D) nano materials have attracted extensive attention in PMS activation due to the high exposure of catalytically active sites and short diffusion distance,thereby accelerating mass transport and electron transfer [62].As shown in Fig.2(c),Donget al.,constructed a novel 2D CoNi3O4/diatomite hybrid(CN@D)viavertically oriented growth of 2D CoNi3-O4nanoribbons on cost-effective diatomite template[41].Distinct from bulk CoNi3O4(CN) with a large proportion of ‘‘dead surface”,CN@D exhibited abundant sharp corners,exposed edges,and open diffusion channels.The open diffusion channels facilitate the migration of PMS and pollutants.These unique characteristics endowed CN@D with excellent PMS activation efficiency in real water.The reaction rate constant (k) of CN@D/PMS system was calculated to be 0.0842 min-1,which was 2.2 times that of CN/PMS system (0.0382 min-1) and 49.5 times that of diatomite/PMS system (0.0017 min-1).In another study,ultrathin 2D Fe3O4nanosheets with a monolayer thickness of about 1 nm were synthesized by coupling the confined interlayer growth method with melt infiltration under dry-chemical conditions (Fig.3(a)) [63].The 2D Fe3O4nanosheets could rapidly degrade various robust organic pollutants in the presence of PMS.The high efficiency of 2D Fe3O4could be ascribed to the following: (i) The ultrathin 2D structure remarkably lowered the energy barrier for binding organics,which could trigger the activation of PMS and facilitate the degradation of organic pollutants;(ii) the abundant oxygen vacancies provided abundant reactive electrons,thereby accelerating the redox cycle of Fe3+/Fe2+and flourishing the generation of ROS.

An unimpeded three-dimensional (3D) structure with open channels and hierarchical pores is attractive for enhancing mass transfer.Ferrum manganese oxide nanosheets(Fe-Mn-O NSs)werein situgrown on the microchannels of carbonized wood to construct a 3D wood-derived block for efficient PMS activation(Fig.3(b)).In such a 3D structure,the numerous open channels and highly hierarchical pores on channel walls contributed to unimpeded mass diffusion.Except for exposing sufficient active sites,several vital surface features (i.e.,surface defects,lowvalence metal concentration,and charge transfer) were also improved,resulting in greatly enhanced catalytic activity and stability [64].

Fig.3.(a)Proposed catalytic mechanism of PMS activation on ultrathin 2D Fe3O4 nanosheets[63].(b)Loading Ferrum Manganese oxide on 3D wood-derived block with open channels and hierarchical pores[64].(c)Proposed synergistic mechanisms of BPA degradation on Yolk-shell Co/C nanoreactors[65].(d)Ultrafast PMS activation and pollutant degradation on MoS2 membranes assisted by confined fluids [66].

Moreover,natural organic matter (NOM),such as humus acid(HA),is ubiquitous in nature water.In SR-AOPs,NOM may adhere to the surface of the catalyst and cover the active sites,which dramatically inhibits the catalytic efficiency.Besides,NOM can also react with the generated ROS from PMS,leading to the waste of ROS [65].Hence,it is appealing to develop novel catalysis with capability to insulate the NOM from target pollutants in two independent regions.The application of the nano yolk-shell structure is feasible,in which metal (oxides) nanocrystal insulated inside the porous shell can protect the metal (oxides) nanocrystal core from affecting by NOM through size-exclusion [65,67,68].As shown in Fig.3(c),Yolk-shell Co/C nanoreactors (YSCCNs) were preparedviapyrolysis of controllably etched ZIF-67 by tannic acid.The performance in activating PMS to degrade BPA in the presence of HA was investigated [65].For comparison,solid and hollow Co/C nanoparticles (referred to SCCNs and HCCNs) derived from ZIF-67,were also tested.Results showed that YSCCNs displayed the best BPA degradation rate of 0.32 min-1,which was 45.4% and 23.1% higher than that of SCCNs and HCCNs in the presence of HA (10 mg·L-1),respectively.The significant enhancement can be explained by the synergetic effects from shell layer (sizeexclusion of HA)and core/shell(confinement effect).Furthermore,another research has proven that the nano hollow structure combining functional shells and inner voids can also avoid catalyst agglomeration,provide higher surface area,shorten the mass transfer distance,and suppress metal leaching [69-72].

3.4.Confinement effect

Notably,when spatial size is confined to the order of only one or several nanometers,denoted as nanoconfinement,the behaviors of matter transfer and the energy diagram of a chemical reaction might be totally different from their analogs in bulk [73].

Typically,the half-life of free radicals in water is about 10-6-10-9s,and the ultimate mass transfer distance is about 90 nm under ideal conditions.For an efficient heterogeneous SR-AOPs system,the production and utilization of free radicals are equally important.Chenet al.prepared a MoS2catalytic membrane to reduce the mass transfer distance of free radicals to target pollutants by adjusting the interlayer confinement structure (Fig.3(d)),thereby enhancing the utilization of free radicals and achieving high-efficiency Fenton-like reaction efficiency [66].According to Fick’s law,the steep concentration gradient would drive the reactant molecules confined in the nanoscale interspacing to migrate from the center to the boundaries.The rapid mass transfer in the confined MoS2platform (～1.5 nm) guarantees ultrafast degradation of total degradation aqueous BPA (only about 100 ms was needed).Similarly,other research also suggested that the internal mass transfer in the confined space would enclose PMS molecules and target pollutant molecules,increasing PMS activation efficiency and the utilization of generated ROS [74,75].

Interestingly,ultrafast activation of PMS is achieved by employing Ti3C2MXene with abundant interlayer space(～0.9 nm)to confine Fe3+[76].The degradation rate of sulfamethoxazole in Fe3+/PMS/MXene process was enhanced by 72 times compared with that in the absence of Ti3C2MXene.Because the surface confinement effect of MXene inhibited the hydrolysis of iron ions.Simultaneously,the strong reducibility of MXene facilitated the redox cycle of Fe species.Likewise,Yuet al.stabilized the naturally prevalent undercoordinated iron(UCI)center on manganese oxidesviathe interface confinement effect between transition-metal oxides[77].The created heterostructure exhibited efficient activation of PMS,with aqueous organic pollutant oxidation efficacy increased several times that of reference metal oxides.Because such UCI canters not only facilitated thermodynamically PMS accumulation but also benefited surface-to-surface electron transfer across atomic interface-bonding channels.

4.Electronic Modulation

4.1.Heteroatom-doping

Because activation of PMS is triggered by withdrawing electrons from catalysis,the electron-rich active sites generally serve as a donor to drive the PMS decomposition and ROS generation.As such,the oxidized active sites during the catalytic process require a fast reduction in order to maintain the catalytic activity.That is to say,the redox cycle of the active site is the key factor to regulate the catalytic activity.Rationally electronic modulation of catalyst is a feasible strategy for accelerating the redox cycle and thus facilitating PMS activation.

Remarkably,doping heteroatoms with specific atomic radius,electronegativity,and electronic orbital structures can induce structural distortion,lattice defects,and redistribution of local charges [28].On the one hand,atom doping can regulate the local Lewis acidity and basicity for better interaction with reactants;On the other hand,the introduction of heteroatoms can induce surface/lattice defects as new positive active sites,which usually exhibit a synergistic effect on catalytic performance.Hence,doping heteroatoms can rationally regulate the physicochemical properties of catalysts at the atomic level to enhance the intrinsic catalytic performances [30,78,79].For instance,Fe was doped in ilmenite CoTiO3to modulate the electronic structure and surface chemistry for enhancing PMS activation[80].As the adjacent transition metal element to Co,Fe shows electron-acceptor and electron-donator properties due to the unoccupieddorbitals.The approximate radius of Fe (0.127 nm) and Co atoms (0.126 nm)guaranteed that substitutional doping can be achieved.Experiments and theoretical calculations demonstrated that doping Fe in CoTiO3lattice increased the electron density of Co atoms at active sites,thereby improving the redox cycles during PMS activation.Besides,the presence of Fe facilitated the formation of surface hydroxyl groups,which could be accepted by Co to form cobalt hydroxyl complex that was vital to the PMS activation [81].The enhanced PMS activation was also observed in Fe-doped Cu2O because the Fe dopant decreased the valence state of the surface Cu atom[37].The density of states and Bader charge analysis witnessed the charge transfer between the Fe atom and the surrounding O atoms and the covalent hybridization among Fe 3d,O 2p,and Cu 3d orbitals.Such elaborated modulation of the surface charge improved interaction between PMS and Cu2O surfaces.

Also,the substitution of partial Co by doping Cu in Co3O4(1 1 1)could modulate the valence states of adjacent Co atoms,which obviously improved the interactions between Co3O4(1 1 1) and PMS [82].The theoretical calculation revealed that the doping Cu accepted charges from surrounding Co and caused the empty antibonding states of Co,thus resulting in better PMS adsorption and dissociation.Gaoet al.reported that the reaction rate constant of PMS activation was increased by 8.3 times after doping Cu in LaMnO3[83].Electrochemical measurements showed that Cu doped LaMnO3had lower charge transfer resistance,greater reductive capability,and higher corrosion current compared to pure LaMnO3,indicating that Cu dopant improved the electron transfer process between catalyst and PMS.In other work,Liet al.unraveled that doping Cu in MnO2resulted in a higher-crystallized structure,more oxygen vacancies,and a higher ratio of Mn4+/Mn3+for a faster redox cycle to promote PMS activation [84].In addition,Li found that the Al dopant in Co3O4served as the promoter and stabilizer of Co sites,which not only enhanced the adsorption capability and electron transferability of Co to PMS but also limited the leaching Co ions (≤0.079 mg·L-1) [85].

4.2.Defect engineering

In general,solid defect means the distortion of the actual crystal structure relative to the ideal lattice structure [86].On the one hand,defects can make more unsaturated coordination sites in catalysis,which can facilitate the adsorption of reaction molecules;On the other hand,solid defects can regulate local charge density and improve electron transferability.Therefore,defectengineering is a maneuverable strategy for improving catalytic activity of heterogenous catalysts.

As the most common defect type in transition metal oxides,oxygen vacancy has been received extensive attention from researchers[87].The creation of cation defects or emigration of lattice oxygen could spontaneously generate oxygen vacancy for maintaining electric neutrality.Such defect feature facilitates the lattice oxygen migration,emission,and intercalation,thereby leading to an enhanced redox ability of metal active sites[88].Besides,oxygen vacancy formation could redistribute the local charge.The electron-deficient nature of oxygen vacancy is characterized as a Lewis acid site acting as an electron acceptor,which shows strong affinity toward electronegative PMS and organics[89].The oxygen vacancy-induced electron localization can accelerate electron transfer during PMS activation [90,91].

Yuet al.reported that oxygen vacancies endowed MnO2with strong affinity for PMS adsorption and promoted interfacial charge transfer [92].According to the theoretical calculation result,the value of adsorption energies (ΔEads) between pand perfect crystal MnO2(0 0 1) was positive (1.24 eV),suggesting that the adsorption process was undesirable.In contrast,the introduction of O vacancies on MnO2(0 0 1) gave rise to negative ΔEadsvalue(-1.53 eV),suggesting that the adsorption process was exothermic and spontaneous.Liet al.reported defect-engineered Co3O4with abundant oxygen vacancies (MS-VO-Co3O4) for significantly enhanced PMS activation (Fig.4(a)) [93].Theoretical calculations revealed that oxygen vacancies can regulate surface electronic state,thus improving binding energy and facilitating electron transfer for PMS activation.In the electron-transfer pathway,PMS molecules will be activatedviathe intense interaction with defective sites and then degraded target pollutants by electron extraction.Compared to commercial Co3O4,up to 46 times of catalytic enhancement was achieved MS-VO-Co3O4(0.186vs0.004 min-1).

Ndayiragijeet al.obtained oxygen vacancy-rich MnO2by ball milling method [89].The oxygen vacancy can not only enhance the mobility and exchange of oxygen but also improved the redox cycle of Mn3+/Mn4+,thus boosting the activation of PMS(～22 times increase in reaction rate constant).

Fig.4.(a) Catalytic activation of PMS by oxygen-vacancies-rich Co3O4 [93].(b) Possible PMS activation mechanism by carbon-encapsulated metal nanoparticles [94].(c)Summary of a variety of carbon-based SACs with possible coordination environments [95].(d) Spin-state-dependent PMS activation of single-atom M-N-CNTs [96].

As mentioned by Wanget al.,the content of oxygen vacancies has a good linear relationship with the reaction rate constant[38].The oxygen vacancies assisted fulfillment of the redox cycles of B site metal cations between Co3+/Co2+and Mn4+/Mn3+in perovskite La0.5Ba0.5CoxMn1-xO3-δ.The role of oxygen vacancies in PMS activation was explained by Eqs.(7)-(9),where M,andrepresented the B-site metal cations(Co/Mn),the active oxygen in oxygen site,and the doubly charged oxygen vacancy,respectively[97].

Besides,other research demonstrated that oxygen vacancy can enhance the mobility of surrounding O2-to produce reactive oxygen (),which subsequently reacts with PMS to generate singlet oxygen (1O2) as the major ROS (Eqs.(10) and (11)) [98,99].

In summary,doping of heteroatom or creation of vacancy is a feasible strategy to modulate the local charge distribution of catalyst and enhance the interaction with PMS.However,more dopants or vacancies do not mean higher catalytic activity.According to Sabatier’s theory,a possible principle is that the optimal interaction of reactive intermediate should be neither too strong nor too weak to achieve the best catalytic performance [100].Because too many dopants or vacancies would result in too strong adsorption of PMS on the catalyst,which then poisons the active site and hinders the desorption of generated ROS [37].

4.3.Composite with carbonaceous material

Recently,the advanced oxidation technology of carbonaceous materials to activate persulfate has attracted widespread attention in wastewater treatment [68,101,102].On the one hand,the carbon material itself has the advantages of low toxicity,stable structure,wide sources,and abundant pore structure;on the other hand,the carbon material is an excellent conductive carrier.The abundant π electrons on the carbon skeleton can not only act as a site for activating PMS but also act as a ‘‘bridge” to promote the interaction between the catalyst and the reactant,thereby enhancing the catalytic effect.Meanwhile,carbonaceous materials can provide additional active sites due to zigzag edges and ketonic carbonyl groups (C=O) on the edges [28].

Combining the advantages of carbon materials and metal catalysts,emerges as a feasible strategy for enhancing electronic conductivity and decreasing work function.Due to electronegativity differences,this combination tends to form electron-deficient region (metal site) and electron-rich region (carbon site),where the electron-rich region is favorable for PMS adsorption.For instance,Yaoet al.encapsulated cobalt nanocrystals in N-doped carbon nanotubes for catalytic activation of PMS [103].Results showed that the electrons on the cobalt nanocrystals can quickly transfer to the adsorbed PMS with the help of conjugated π system of the carbon matrix,thereby realizing efficient PMS activation.Duanet al.constructed different transition metal (TM)@carbon composites for PMS activation [104].Compared to CoP and Co3O4,the encapsulated metallic cobalt was proven to be more advantageous for electron transfer to adjacent carbon due to its low work function,high conductivity,and formation of multiple Co-C bonds for electron tunneling.

A synergistic effect was also found in the carbon substrates substance and nano zero-valent iron (nZVI) [105,106].On the one hand,carbon substrates could promote electron transfer between PMS and the surface iron species on nZVI.The carbon substrates also served as electron donors to facilitate PMS to generate active radicals.On the other hand,nZVI could selectively improve organics adsorption on carbon substrates.

Moreover,Yunet al.found that carbon encapsulation of metal nanoparticles switches the primary PMS activation route [94].As illustrated in Fig.4(b),the carbon shell impeded the reductive conversion of PMS to· by the metal core,while metal incorporation promoted electron delivery on the surface of carbon shells.Such synergy caused a switch in PMS activation route from radical-induced oxidation to non-radical activation (mediated electron transfer).It is worth mentioning that,the protective carbon shell not only reduced metal leaching but also alleviated catalyst deactivation and allowed facile regeneration.

4.4.Single-atom catalysis

In 2011,Zhang and co-workers firstly proposed that the atomically-dispersed Pt/FeOxcatalyst displayed a threefold improvement in CO oxidation compared to the bulk Pt counterpart[107].Since then,single-atom catalysis (SACs) has become a research hotspot in heterogeneous catalytic systems.

As for supported metal nanoparticles (NPs),only a small fraction of active metal sites is exposed during heterogeneous reactions due to geometric effects and aggregation.Once the anchored NPs shrink to an atomic level,where metal-metal bonds do not exist in supporting materials,the catalysis will achieve maximum atom utilization efficiency and exhibit extremely high catalytic activity [95,108].So far,well-designed carbon sported with different coordination environments has become the mainstream for anchoring metal atoms to produce SACs (Fig.4(c) [95].

In recent years,a series of carbon-based SACs have been used as PMS activators for the degradation of various organic pollutants.Since the catalytic performances of SACs mainly depend on their configuration and electronic structures,many researchers engaged to optimize the inherent characteristics of SACs (e.g.,spin state,charge density,and bandgap) for enhancing catalytic activity and selectively [68,96].

The spin state is a crucial electronic characteristic of SACs,which indeed affects the catalytic performance.In many cases,spin states of transition metals vary from high-spin and low-spin states.Miaoet al.developed a series of SACs with single transition metals anchored on carbon nanotubes (denoted M-N-CNTs,where M=Co,Fe,Mn,or Ni),to activate peroxymonosulfate for sulfamethoxazole oxidation [96].As shown in Fig.4(d),the catalytic activity correlated with the spin state of M-N-CNTs.That is,a large effective magnetic moment with a high spin state(i.e.,Co-N)benefited the overlap of d orbitals with oxygen-containing adsorbates(i.e.,peroxo species)on metal active sites and promoted the transfer of spin-oriented electrons[109-111],which enhanced PMS adsorption and promoted the oxidation capacity of the reactive species.

A parted from the metal atom center,the coordination environment is important to control the catalytic behaviors by effective modulation of the electronic properties of the metal sites.Zhuet al.anchored single-atom Co on hollow graphitized carbon polyhedrons to activate peroxymonosulfate.In the presence of the as-prepared catalyst and PMS,91.62% of BPA was removed within 15 s and the normalized kinetic rate constant was 92.92 L·min-1·g-1,which exceeded most reported heterogeneous catalysis by 1-2 orders of magnitude [68].The excellent catalytic performance could be ascribed to the strong adsorbability for PMS and high conductivity for electron transfer.Experiments and calculations showed that the planar potentials overall followed a rising trend and the macroscopic potentials increased from both sides towards the middle PMS-adsorbed single-atom Co sites.Such radiantly decreasing potentials around the single-atom Co center would excite a micro-electric field on the catalyst surface [112].Therefore,the electron in the BPA adsorbed on the low-potential region was extracted by the electric field force,and orientationally migrated towards the high-potential region(the single-atom CoN4or CoN3C1center),while the carbon substrate acted as the conductor.

Normally,the anchored single transition metal atoms can mediate electron configuration and optimize catalytic performance[113,114].Duanet al.found that anchoring of atomically dispersed Fe atom on g-C3N4(SAFe-CN) could significantly improve electron transfer of catalystviareducing band gap and electron redistribution[111].In g-C3N4,the delocalized π electrons tended to diverge,and were evenly distributed over the triazine ring,leading to low electron-donating efficiency to PMS molecules.Besides,the inherent poor electron transfer ability of g-C3N4resulted in negligible catalytic performance.In SAFe-CN,the diverging electron density in the triazine ring decreased because the delocalized π electrons were partially transferred to the Fe-O2bond,thus improving its electron receptivity from other species (e.g.,organic pollutants).Meanwhile,the band gap of SAFe-CN reduced to 1.46 eV from 1.94 eV of g-C3N4,which benefited separation and transfer of electrons.Consequently,PMS could be efficiently activated on SAFe-CN through mediated electron transfer mechanism.

4.5.Assistance by additional reductant

Generally,a reductant with low potential can facilitate the redox cycles of active sites in catalyst,and thus enhance the activation efficiency of PMS.The one-electron transfer is the dominant electron transfer pathway in the reduction reaction of the active site.An ideal reductant should have at least two criteria: (i) fast reduction rate(ii)minor side-reaction with PMS and the generated ROS [12].So far,the reported reductant includes inorganic homogeneous reductants (e.g.,NH2OH [115,116],and organic homogeneous reductants (e.g.,ascorbic acid [117]).

For instance,NH2OH was used to increase the activation of PMS by cobalt ferrite (CoFe2O4) to degrade sulfamethoxazole (SMX)[116].This system showed higher SMX-degradation efficiency than other cobalt oxides and iron oxides.This increased SMXdegradation efficiency was ascribed to the inhibition of selfdecomposed radicals and the increased Fe3+/Fe2+and Co2+/Co3+redox cycles of CoFe2O4due to the reducing power of NH2OH.

5.Enhancement by External Energy

In addition to the single heterogeneous catalyst,some external energy (i.e.,photo irradiation,microwave,ultrasound,and electric field)have been employed to enhance PMS activation through synergistic effect [118].

5.1.Photo-irradiation

As a usual energy input method,photo-irradiation(ultravioletvisible light,UV-vis)can also improve the catalytic activation efficiency of PMS[119-122].It should be mentioned that,PMS is photosensitive and the external light irradiation can directly lead to the photolysis to produce radicals through Eq.(12) [16].Hence,the application of light irradiation for enhancement of organic removal has aroused extensive interest in the hybrid activation processes.

Typically,the rapid exhaustion of low valance metal and poor redox cycle are the bottlenecks restricting PMS activation and radical generation.To this end,Liuet alreported a natural Ti-and Venriched magnetite (Ti-V-M) as an activator for PMS intergraded with UV irradiation to degraded bisphenol S (BPS) [123].The results showed that 100% of BPS was degraded in 20 min in the UV/PMS/Ti-V-M system.Its pseudo-first-order kinetic constant was 52.22 times,42.73 times,and 3.55 times higher than that for the UV/Ti-V-M,UV/PMS,and PMS/Ti-V-M systems,respectively.Distinct from the PMS/Ti-V-M system,the Fe(II)/Fe(III)redox cycle was significantly accelerated by the photo-induced electron form Ti in UV/Ti-V-M system.Meanwhile,the associated positive hole(h+)also responded to BPS degradation due to inherent oxidizability (Fig.5(a)).

Donget al.prepared hollow-structured Pt@CeO2@MoS2as a photoanode for photo-electrocatalysis-assisted PMS activation system(PEC/PMS)[128].The electrocatalytic(EC),photocatalytic(PC),and photoelectrocatalytic (PEC) systems with PMS for carbamazepine (CBZ) degradation were investigated,respectively.After adding PMS,the degradation efficiency of CBZ in EC/PMS and PC/PMS systems increased up to 28% and 19%,respectively.As for the PEC/PMS system,the degradation efficiency increased dramatically (100%),displaying an obvious synergistic effect between the electrocatalytic and photocatalytic processes.The reason was explained below.On the on hand,under the photo irradiation,the photo-generated electrons will accelerate the Mo6+/Mo4+and Ce4+/Ce3+redox cycle,as well as the involved(Eqs.(13) and(14)).Thecould transfer into non-radical1O2(Eq.(15)).On the other hand,the applied electric field could promote the photo-induced electrons and holes separation of CeO2and MoS2,which produced more available electrons to participate in PMS activation.Therefore,the synergistic effect of photo-irradiation and electric field significantly enhanced PMS activation and CBZ degradation.

However,UV irradiation often requires high input energy,limiting its application [129].In addition,UV light is only 3% of solar energy,and visible light,accounting for 44% of solar energy,is not effectively used [130].Therefore,efficient utilization of visible-light irradiation is more appealing for PMS activation.For instance,traditional TiO2is a well-known UV-driven photocatalyst.However,the practical photocatalytic applications of TiO2are challenged by its wide band gap(3.2 eV)that can hardly be excited by visible light.In another word,wide bandgap,insufficient chargecarriers separation,easy reorganization ability of photogenerated electron-hole pairs,and deteriorated recycling are the threshold that restring practical photocatalytic process.To address the bottleneck of poor visible light utilization,Weiet al.introduced narrow bandgap LaFeO3(2.0-2.7 eV) to TiO2to construct durable hollow core-shell TiO2@LaFeO3(TLFO) nanospheres as visiblelight-driven heterojunction photocatalyst [121].The built-in electric field originated from the three-dimensional heterojunction between TiO2and LaFeO3,acting as charge transfer driving force,and enhanced the charge separation rate.Meanwhile,PMS could function as electron acceptor to boost photogenerated charge separation,thus facilitating the generation of reactive oxidant species(e.g.,and1O2).

Fig.5.(a) Schematic diagram of PMS activation by ultraviolet -assisted Ti-V-M [123].(b) Microwave-assisted heterogeneous activation of PMS by MnFe2O4 [124].(c)Ultrasound-assisted zero valent iron corrosion for PMS activation [125].(d) Electric-field-assisted heterogeneous activation of PMS by Fe-supported bentonite [126].(e)Synergetic ‘‘trap-and-zap” process for PMS activation by MnOOH@nylon membrane [127].

Currently,iodine bismuth oxide (BiOI) has been considered to be one of the most promising,lowest cost,lowest toxicity photocatalyst due to its strong absorption ability of visible light [131].However,the photogenerated electron and hole pairs are easily to recombine due to the narrow band gap of BiOI (～1.7 eV)[132].To this end,Lvet al.constructed Z-scheme heterojunction BiOI/B4C (BOBC) as photocatalyst for PMS activation under vislight irradiation [133].The Z-scheme heterojunction can significantly optimize the band gap (from 1.69 eV to 1.89 eV) and improve the separation efficiency of photogenerated electron and hole pairs,thereby increasing the content of active free radicals during PMS activation and improving the degradation efficiency of bisphenol S (BPS).As a result,90.4% of BPS can be degraded in 30 min in the system.Carbon dots(CDs) is another photosensitive material.After photoexcitation,CDs can exhibit electron storage/-transfer capabilities,indicating great potentials in photocatalytic process.Liet al.synthesized N and Fe co-doped carbon dots (N,Fe-CDs) for visible-light-assisted PMS activation [130].Fe existed as Fe-N and Fe-OH complexes on N,Fe-CDs,and played a vital role in harvesting visible light.The Fe(II)could activate PMS to generatewith the oxidation of Fe(II) to Fe(III).

Although progress has been made in photo-irradiation-assisted PMS activation,the light may not be effectively transmitted through colored and turbid wastewater [134].

5.2.Microwave

Recently,the microwave-assisted heterogeneous activation for PMS by metal-based catalyst has been documented,because it is swift,highly efficient,and easy to operate [135].Previous studies showed that the non-thermal effect of the microwave can be selectively adsorbed by ferrites materials,leading to produce‘‘hot spots” through relaxation polarization and enhancing the generation of ROS[136,137].That is to say,the non-thermal effect of microwave results in the active motion of polar molecules,leading to a higher vibration and rotational energy levels of molecular excitation,causing the peroxy-bond to be easy to cleave by catalytic activation and accelerating the generation of free radicals [138].Besides,microwave-assisted heating is a kind of instantaneous heating mode,which can heat the reactant to a high temperature in a very short time temperature gradient[139].Benefiting from this heating pattern,the thermal effect makes it more efficiently and selectively transfer heat for enhancing polar molecular vibrations,thereby increasing the reactivity of chemical reactions [140].

Panget al.reported microwave irradiation acting as an assistant to promote the degradation efficiency of p-nitrophenol (PNP) in heterogenous MnFe2O4activated PMS catalytic oxidation (Fig.5(b))[124].On the one hand,under microwave irradiation,reaction temperature increased quickly (from 25 °C to 85 °C) due to the absorption of microwave by water molecular.Since activation of PMS was endothermic,the increased temperature promoted the forward reaction.On the other hand,MnFe2O4strongly absorbed microwave due to non-thermal effect,and then generated numerous hots pots on its surface.The hot spots,where temperature can reach over 1473 K,accelerated the movement of electrons.In addition,microwaves can induce violent motion of reactants molecules,enhancing collision possibility of reactants (PMS,MnFe2O4,and pollutants).

5.3.Ultrasound

Lately,ultrasound-irradiation has been studied and proposed as a promising technology for the degradation of organic pollutants[141,142].The most widely accepted mechanism of pollutants by ultrasound-irradiation is based on the‘‘hot spot”theory.The transmission of ultrasonic waves in a liquid medium leads to ultrasonic cavitation effects,which involve the formation,growth,and adiabatic collapse of bubbles.The collapse of bubbles is accompanied by the creation of‘‘hot spot”(＞5000°C,＞100 MPa),finally resulting in H2O homolysis and ROS generation (Eqs.(16) and (17)) [143].

The combination of ultrasound-irradiation and heterogenous catalytic activation of PMS and catalysis have been reported to show the good performance of pollutants degradation through synergistic effect [144].Firstly,ultrasound can be used as energy input to cause homolysis of PMS,thus generating ROS (Eq.(18))[145].

Besides,the localized cavitation bubbles in the water medium can act as microreactors,accelerating the transfer of free radicals,pollutant molecules,and catalysis between bulk solution and localized reactive zones (i.e.,the interior of cavitation bubble and gasliquid interface) [141].

Xuet al.reported that ultrasound irradiation significantly promoted acid red B degradation by Fe3O4-activated PMS system[146].The reaction rate constant of dye wastewater treated with ultrasound/Fe3O4/PMS came up to 0.02 min-1,which was 1.43 times and 2.86 times that of Fe3O4/PMS and ultrasound/PMS.They referred that the mass transfer between solid and liquid phases was enhanced with the increase of ultrasound power because the microjet effect was caused by cavitation bubble rupture.

Panget al.developed an ultrasonic-assisted zero-valent iron(ZVI) corrosion activation system for PMS (Fig.5(c)) [125].In this system,ultrasonic disturbance provided a strong driving force for mass transfer by reducing the mass transfer resistances of the solid-liquid interface.As a result,the collision probability of pollutants,PMS,and Fe0was significantly improved,leading to better catalytic performance.Furthermore,ultrasound irradiation accelerated the ZVI corrosion and cleaned the passive films on iron surface,which gave rise to rapid electron release from ZVI corrosion to Fe2+.The rapid electron transfer was another reason for efficient PMS activation.

Although ultrasound shows good performance in PMS activation,the total cost of ultrasonic energy input and PMS remains high.

5.4.Electric field

The catalytic activation efficiency of PMS can also be obviously promoted by the introduction of an electric field.Importantly,the introduction of the electric field has been reported to be highly efficient,multifunctional,and eco-friendly [118,147,148].A major advantage of the electric field is that high-valence state metals can be directly reduced to low-valence state metals on cathode so that maintained rapid redox cycle during PMS activation[16,149].Furthermore,the electrochemical process can directly provide electrons to PMS for producing·OH and· on the cathodeviaEqs.(19) and (20) [150].

Miaoet al.fabricated Fe-supported bentonite(Fe-B)catalyst for electro-assisted heterogeneous activation of PMS (Fig.5(d)) [126].The Fe-B catalyst exhibited good performance in the removal of Acid Orange 7 (AO7) due to the synergistic effect of electrolysis and the rapid redox cycle of Fe(III)/Fe(II) during PMS activation.The electrochemical system can directly provide electrons,which facilitated the continuous conversion of Fe(III)to Fe(II).Meanwhile,the direct anodic oxidation and surface-bound radicals also participated in AO7 degradation.

In summary,the introduction of an electric field promotes the redox cycle,and can achieve direct or indirect oxidation on the electrode surface.Such a combination provides new insight into improving catalytic activity,and may become the preferred process for complex wastewater treatment.In future studies,more effort needs to be devoted to evaluating the current efficiency and its economic viability.

6.Enhancement by Membrane Filtration

Although many efforts have been devoted to enhancing catalytic activity of heterogeneous powder catalysis,several drawbacks still need to be taken into account.(i) Undesirable agglomeration of nano/micro catalyst particles results in low mass transfer and limited active sites available [151-153].(ii) The suspended catalyst particles in water usually require an additional subsequent separation process (centrifugation or filtration) for the next usage.(iii) the inevitable loss of catalyst particles tends to cause secondary pollution[153,154].Coupling membrane filtration process and heterogeneous catalysis provide a feasible solution to solve the above problems [155].More importantly,the adsorption and oxidation kinetics in conventional batch reactors(e.g.,beaker in lab-scale) are typically limited by mass transfer,because of the slow diffusion of PMS to the catalyst surface and the generated ROS to react with target pollutants in the bulk solution [156].To this end,a flow-through filtration design is more advantageous because of convection-enhanced mass transfer,bringing about higher degradation efficiency of pollutants[157,158].For example,the sulfamethoxazole removal rate was increased by 9.2 folds under a flow-through filtration operation compared with the conventional batch-type operation [159].

In the mass transfer process,the concentration gradient is considered as the driving force.Therefore,a catalytic membrane that can concentrate reactant molecules and increase the concentration gradient,is highly anticipated.To this point,Zhanget al.constructed a novel catalytic filtration membrane by coating MnOOH nanoparticles on nylon membrane(MnOOH@nylon)for enhancing mass transferviaa synergetic ‘‘trap-and-zap” effect [127].In this hybrid membrane,the nylon substrate showed a strong adsorption affinity for target pollutants,2,4-dichlorophenol (2,4-DCP).During the catalytic process,the 2,4-DCP and PMS molecules were adsorbed on the catalytic membrane and formed a high concentration gradient under the action of forced filtration flow.Consequently,MnOOH@nylon membrane exhibited a much higher reaction rate (0.9575 mg·L-1·min-1) than that in the suspended MnOOH particle system (0.1493 mg·L-1·min-1) (Fig.5(e)).Meanwhile,the MnOOH@nylon membrane also showed enhanced resistibility of competitive anions,due to the much higher concentration ratio of the adsorbed target pollutants.

Significantly,the key to obtaining a highly active catalytic membrane is to fully expose the catalyst,enhance the contact possibility of reactants,and avoid the deactivation of catalysis embedded in membrane matrix [152,160,161].2D materials with large specific surface area,high density of surface-active sites,as well as highly exposed surfaces and edges,have emerged as a promising candidate for preparing robust catalytic membrane[162,163].Honeycomb-like holey membranes fabricated by 2D Co3O4nanosheets,was demonstrated as a decent activator of PMS for rapid removal of pollutants [164].During the filtration and catalytic reactions,the membrane pores and inter-/intralayer channels served as catalytic reactors,where numerous catalytic active sites were fully exposed to the reaction system to continuously generate ROS.Such configuration benefited ROS exposure and shortened the migration distance between ROS and the target pollutants,therefore improving mass transfer and maximizing the utilization of ROS.Consequently,the reaction first-order rate constant of the Co3O4membrane/PMS system can even reach 0.018 ms-1,which was 3-5 orders of magnitude higher than that of the conventional heterogeneous catalytic systems (0.0032-0.33 min-1).

Liuet al.also proposed that the membrane interception promoted the aggregation of PMS and pollutants towards the membrane surface under external pressure,which facilitated their mass transfer in N-MWCNTs modified membrane/PMS system[165].The removal of phenol by the N-MWCNTs-modified membrane was 83.67% in 2 min,which was much better than NMWCNT powder (41.42%).

Normally,SR-AOPs for multicomponent wastewater treatment suffer from low efficiency due to radical scavenging by competitive matters (such as NOM:k·OH/NOM,≈ 109L·mol-1·s-1[166],≈108L·mol-1·s-1[167].Pressure-driven membrane separation(including microfiltration and ultrafiltration)can efficiently reject large-size organic molecules through the size-excluded effect,and thus suppresses radical scavenging caused by them.Wuet al.anchored single atom cobalt on graphene oxide and then assemble it as 2D catalytic membrane (Co1-GO membrane).The observed 1,4-dioxane degradation kinetics in Co1-GO membrane was much faster (＞640 times) than the kinetics in the batch-type suspension model.In such a design,the target pollutant (1,4-dioxane) can permeate into the membrane pores and be degraded efficiently inside the pores.Meanwhile,Co1-GO membrane can significantly decrease the NOM scavenging effects by selective separating NOM from 1,4-D through a size-excluded effect [168].

7.Conclusions and Perspectives

In recent years,SR-AOPs have attracted much attention in environmental remediation due to their outstanding performance.Heterogeneous metal-based catalysts are promising and mainstream for PMS activation.In this paper,strategies for enhancing PMS activation efficiency are summarized in the following aspects:(i) Enhancing PMS affinity by surface modification,or increasing the number of active sites by exposing reactive crystal face for better contact and reacting with reactants.(ii) Optimizing the structural configuration and spatial distribution,increasing the specific surface area,or constructing a nano-reactor to enhance the mass transferability.(iii) Tunning surface chemistry and electronic configuration to increase the intrinsic activity of each active site by improving the redox cycle and enhancing electron transfer.(iv) Enhancing the activation efficiency of PMS by the synergistic effect of external energy assistance or process integration.

Although many studies focus on developing enhancement strategies,some efforts are still needed in future research and practical applications.

(1) To date,many researchers applied‘‘bottom-up”strategies to design efficient heterogenous catalysts,which may require a complex synthetic process.It would be more cost-effective and time-saving to develop general ‘‘top-down” strategies to convert bulk natural mineral crystals into the required catalysts.In addition,the influence of intrinsic crystal properties(such as polymorph and crystallinity)on catalytic performance is also worth studying.

(2) Most catalysts were applied for the removal of a single pollutant in current studies.More efforts are encouraged to target actual wastewater.It should be noted that some water parameters and constituents (e.g.,NOM,halide,phosphate,and carbonate)will cause adverse effects during PMS activation.For instance,NOM may adhere to the surface of the catalyst and cover the active sites,resulting in dramatically inhibiting the catalytic efficiency.Therefore,it is desirable to prepare catalysts that can selectively adsorb PMS and repel undesirable molecules.

(3) Simplified and continuous catalytic systems are necessity for industrial applications.Integrating membrane filtration and SR-AOPs into one continuous system is a frontier technology for organic pollutants removal by simultaneously physical separation and chemical oxidation.It still needs to be further explored in terms of membrane materials,experimental equipment,and process intensification for enhancing catalytic performance.

(4) In terms of the decreased catalytic activity of heterogeneous metal-based catalysts after multiple cycles,current studies suggest that this is due to the increase in surface metal valence and the loss of active sites covered by pollutants.Although some conventional experimental methods,including washing,heat treatment,and ultrasonic treatment,can restore catalyst activity to a certain extent,they are still inadequate and far from engineering applications.The grand challenge is to develop advanced catalyst regeneration methods that can achieve rapid regeneration without complicated and cumbersome operating procedures.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The study was financially supported by the National Natural Science Foundation of China (21938009).