Current status of applying microwave-associated catalysis ...

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Review

Current status of applying microwave-associatedcatalysis for the degradation of organics in aqueousphase – A review

Chao Xue1, Yanpeng Mao1,*, Wenlong Wang1, Zhanlong Song1, Xiqiang Zhao1, Jing Sun1,Yanxiang Wang2

1. School of Energy and Power Engineering, Shandong University, Jinan 250100, China2. School of Material Science & Engineering, Shandong University, Jinan 250100, China

A R T I C L E I N F O

⁎ Corresponding author. E-mail: maoyanpeng

https://doi.org/10.1016/j.jes.2019.01.0191001-0742 © 2018 The Research Center for Ec

A B S T R A C T

Article history:Received 8 July 2018Revised 14 January 2019Accepted 15 January 2019Available online 22 February 2019

Interactions between microwaves and certain catalysts can lead to efficient, energy-directedconvergence of a relatively dispersed microwave field onto the reactive sites of the catalyst,which produces thermal or discharge effects around the catalyst. These interactions form“high-energy sites” (HeS) that promote energy efficient utilization and enhanced in situdegradation of organic pollutants. This article focuses on the processes occurring betweenmicrowaves and absorbing catalysts, and presents a critical review of microwave-absorbingmechanisms. This article also discusses aqueous phase applications of relevant catalysts (iron-based, carbon-based, soft magnetic, rare earth, and other types) and microwaves, specialeffects caused by the dimensions and structures of catalytic materials, and the optimizationand design of relevant reactors for microwave-assisted catalysis of wastewater. The results ofthis study demonstrate that microwave-assisted catalysis can effectively enhance thedegradation rate of organic compounds in an aqueous phase and has potential applicationsto a variety of engineering fields such as microwave-assisted pyrolysis, pollutant removal,material synthesis, and water treatment.© 2018 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences.

Published by Elsevier B.V.

Keywords:MicrowavesCatalystsHigh energy sitesOrganics degradationAqueous phase

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1201. Mechanism of microwave absorption by catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

1.1. Principles of wave absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1211.2. Dielectric loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1211.3. Magnetic loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

2. Classification and applications of catalysts in aqueous phase microwave reactions . . . . . . . . . . . . . . . . . . 1222.1. Brief introduction to catalysts in aqueous phase reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

[email protected]. (Yanpeng Mao).

o-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.

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2.2. Iron-based catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1232.2.1. Nanocrystallization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1232.2.2. Improved structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1232.2.3. Composite. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

2.3. Carbon-based catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1262.4. Soft magnetic catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1262.5. Rare earth catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1272.6. Other catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

3. Dimensions and structure of catalysts used in aqueous phase. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1273.1. Conventional size and nanometer size of catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1273.2. Different structures of catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128

4. Relevant reactors for applications of microwave-assisted catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1284.1. Improvements to traditional microwave reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1284.2. Continuous microwave catalytic reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128

4.2.1. Continuously rotating device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1294.2.2. Continuous four-stage device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129

4.3. Intermittent microwave catalytic reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1295. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132

Introduction

Microwave technology has been rapidly developed in recentyears because it has unique characteristics that enable interac-tions with chemical media and produce various effects that canbe used in several applications (Sun et al., 2016a). Duringmicrowave–material interactions, the electromagnetic field ofmicrowaves generated heat at the atomic level, but thematerial's properties ultimately determine the effect that theelectromagnetic field has on the material's energy absorptioncharacteristics. Consequently, microwave processing primarilydepends on the physics of the microwave-material interactionphenomena and the relationships between the material'sproperties and the electromagnetic characteristics of micro-waves (Mishra and Sharma, 2016). When materials are irradi-ated by microwaves, three types of expression, absorption,projection, and reflection will appear (Li et al., 2018). Chemicalmedia composed of non-polar materials allow microwaveradiation to pass and can be used to create microwave reactionkettles. Conversely, when a medium is a polar material, amicrowave field can cause friction between the polar moleculesand produce amacroscopic thermal effect, which can be used inmany technological and scientific fields, including food pro-cessing, activated carbon regeneration, sintering of metals andceramics, plasma processing, solution treatment, polymerprocessing, preparation of functional materials, pollution con-trol, pyrolysis reactions (Bhattacharya and Basak, 2016; Ekezie etal., 2017; Colombini et al., 2017; Fu et al., 2017; Garcia et al., 2017;Lourenco et al., 2017; Ng et al., 2017; Oliveira et al., 2017;Pawelczyk and Zaprutko, 2006; Remya and Lin, 2011; Tian et al.,2012). When amedium is ametalmaterial, microwave radiationwill be reflected; thus, metal materials can be used to createmicrowave shielding devices. Additionally, when a metal orsemiconductor is irradiated by microwaves, plasma will beproduced by a strong discharge phenomenon that can increasethe material's temperature significantly (Wang et al., 2017). Hot

spots and plasmas arising from this discharge phenomenon canreduce the chemical reaction times of contaminants present onthe material and increase their degradation rates. Sun et al.(2016b) demonstrated that hot-spot, plasma, and photocatalyticeffects can arise during the discharge process whenmetals withsharp edges, sharp tips, or submicroscopic irregularities areirradiated bymicrowaves and these effects have been employedinmicrowave-assisted pyrolysis, removal of pollutants,materialsynthesis, and many other applications. Wang et al. (2016b)used silicon carbide as a model material to investigate the hot-spot effect with numerical simulations and observed that hotspots can occur via microwave heating when the heatedmaterials have different absorption wavelengths in the micro-wave spectral region, or via the discharge phenomenon owingto microwave–metal interactions. Zhou et al. (2017) researchedthe different variables that influence the thermal effect ofmicrowave–metal discharge and the factors that influence thedensity of microwave–metal discharge; the type, mass, and sizeof the metal and the microwave power were also studied andcharacterized. These results demonstrated that the dischargeinduced by microwave–metal interactions could significantlyincrease the temperature of liquid paraffin.

As mentioned above, the interactions between micro-waves and medium can effectively promote chemical reac-tions and it has been applied successfully in many fields(Antonetti et al., 2016; Bohdziewicz et al., 2014; Bousbia et al.,2009). Among the successful applications, a microwave-assisted traditional water treatment technology that degradescontaminants in wastewater and generates usable water is apromising technology (Mao et al., 2015). In sewage treatment,microwave technology mainly uses the thermal and non-thermal effects of microwaves (Bhattacharjee and Delsol,2014; Hu et al., 2018; Liu and Ma, 2016; Zheng et al., 2017).The thermal effect of microwaves is mainly the product ofmicrowave absorption by catalysts, which can produce a largenumber of hot spots that can accelerate the reaction rate of

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organic compounds. The non-thermal effect is mainly causedby the coupling effect of microwaves and a catalyst with astrong wave absorption capacity, which can produce hydroxylradicals with high oxidation potentials that can increase theoxidation rate of organic compounds (Garcia-Costa et al.,2017). Therefore, microwave technology can significantlyreduce the activation energy and reaction time of organicreactions (Zhang et al., 2017a).

Existing water treatment technologies that degrade or-ganic matter have occupied a large market; for example,membrane bioreactor technology combines membrane sepa-ration and biodegradation technology to treat wastewater,and photocatalytic technology initiates redox reactionsthrough photocatalysis. However, the reactions that thesetechnologies promote are all relatively slow and exhibit poortreatment efficiency for refractory organic matter. Microwavetechnology assisted by catalysts does not have these limita-tions and demonstrates three main advantages in wastewatertreatment as compared to existing treatment methods. First,microwave heating can penetrate into solutions and directlyaffect organicmatter or catalysts, consequently improving thedegradation efficiency. Second, microwave technology hascharacteristic selective heating that increases the degradationrates of organic pollutants in sewage but has a minimal effecton inorganic pollutants, which results in a high energyutilization rate. Furthermore, microwave technology exhibitsa high treatment efficiency for refractory organic matter (Duet al., 2013). The unique advantages of microwave technologymake it highly valuable for research and mean that it hasextensive application prospects throughout the chemicalindustry. However, there are still challenges to be addressedby current research; the craft and technology of microwavewastewater treatment are still in development and furtherresearch is needed to design reliable and stable processsystems (Anuj K. Rathia et al., 2015). The selection of catalystsis particularly crucial because catalysts are coupled withmicrowaves in the solution and can drastically influenceexperimental results. Therefore, research on microwave-assisted water treatment technology should focus on theoptimization and design of reactors, the preparation andapplication of high-performance catalysts, and the optimiza-tion of catalytic processes (Mei, 2004; Tan et al., 2013).

This article surveys state-of-the-art research and applica-tions of interactions between microwaves and chemicalmedia in order to present a method for better utilization ofmicrowave-assisted water treatment. As research on thespecific effects of coupling microwaves and catalysts has notbeen clearly summarized, the mechanism of microwaveabsorption by catalysts is reviewed first in Section 2. Follow-ing this section, a brief overview of the classification andrelevant applications of microwaves and catalysts in aqueousphase conditions is provided in Section 3, the effects ofthe dimensions and structures of catalysts on microwavetreatments are discussed in Section 4, to optimize the reactorbetter, the advantages of various reactors relevant tomicrowave-assisted catalysis of wastewater are reviewed inSection 5. Finally, ideas for further research and potentialapplications of microwave-assisted catalysis are suggested.As microwave technology and water treatment technologycontinue to be developed, it is reasonable to believe that

microwave technology will play a greater role in the field ofwater treatment in the future.

1. Mechanism of microwave absorption by catalysts

1.1. Principles of wave absorption

As is well known, microwaves are electromagnetic waves thathave the same characteristics as conventional electromagneticwaves (Mishra and Sharma, 2016). Materials that absorbmicrowaves, called microwave-absorbing materials, can beused as catalysts to assist reactions energized by microwaves(Bhongale et al., 2017; Du et al., 2013; Gregory et al., 2016; Huanget al., 2015; Jiang et al., 2015; Jiang et al., 2017). The essentialaspect of microwave absorption by catalysts is the absorbedelectromagnetic wave energy that radiates onto the catalystsurface. Microwave-absorbing materials need to meet twoconditions: these materials need to absorb maximal electro-magnetic energy without reflecting energy and electromag-netic waves that enter the material need to decay quickly(Meng et al., 2018; Mishra and Sharma, 2016). Electromagneticmatching characteristics should be considered during thedesign of microwave-absorbing materials in order to ensurethat they meet the above requirements to the maximumextent possible (Huang et al., 2015). In pursuit of theserequirements, relevant measures can be taken, such asincreasing the magnetic loss and electric loss of thesematerials and impedance matching in order to affect theabsorbing performance of the materials. Impedance matchingallows incident electromagnetic waves to enter absorbingmaterials with maximum penetration and is achieved whenthe material impedance and the free space wave impedancematch or nearly match each other (Mishra and Sharma, 2016).When impedance matching is achieved, microwave energy isabsorbed and dissipated as heat or other forms of energy. Themechanism of microwave energy conversion can generally bedivided into two main processes: dielectric loss and magneticloss, which depend on the type of loss favored by the catalyst(Meng et al., 2018; Mishra and Sharma, 2016; Sarkar et al., 2014).

1.2. Dielectric loss

Dielectric loss can be described as the conversion of electro-magnetic energy in an alternating electric field into heatenergy (Du et al., 2013). Fig. 1 lists the charge distribution ofthe matter that resonates at different wavelengths of electro-magnetic radiation and illustrates that electric field compo-nent of an electromagnetic field can cause polarization ofmedium and opposing movements of positive and negativecharges (Ju et al., 2016; Mishra and Sharma, 2016). The specificmovements of microscopic particles induced by the micro-wave frequency band will be described below.

When a non-conductive material is irradiated by micro-waves, the electric field of the microwaves instantaneouslyrotates the electric dipoles of the molecules so that they arealigned with the electric field (Mishra and Sharma, 2016; Sarkaret al., 2014). When the direction of the electric field changes, themolecules will rotate again to maintain alignment with theelectric field (Ju et al., 2016). As the molecules rotate, heat is

Electron

X-ray Ultraviolet Infrared Microwave High frequency Ultrasonic waveWave length (m)

Frequency (Hz)

Orientation of particles

•

UV IR

Alternating E field

Alternating E fieldIonicpolarization

Dielectricpolarization

Alternating E field

Atom species Molecular species Molecule assemblies

3×1018

10–10 10–8 10–6 10–4 10–2 102 1041

3×1016 3×1014 3×1012 3×1010 3×108 3×106 3×104

Proton

Fig. 1 – Resonance of absorbing materials across the electromagnetic spectrum.

Fig. 2 – Interaction between microwaves and a microwave-absorbing medium.


efficiently produced by the friction between the molecules andthe alternation of the electromagnetic field. Conductive mate-rials have another loss mechanism that can produce heatenergy, in addition to the mechanism described above. Accord-ing to Joule's law, the electric field of microwave radiationproduces an electric current when a conductive material isirradiated by microwave. In this case, heat energy is producedby the emergence of the electric current, which is known as aresistance loss (Li et al., 2014; Mishra and Sharma, 2016).

1.3. Magnetic loss

Magnetic catalysts such as ferrites have another loss mode,called magnetic loss. Magnetic loss refers to the phenomenonwherein work done outside of the material is converted toheat energy when the magnetic material is magnetized orback-magnetized (Kaur et al., 2016; Li et al., 2014; Liu et al.,2018; Meng et al., 2018; Sarkar et al., 2014). Magnetic loss canbe divided into three categories based on the different heatingmechanisms: eddy current loss, hysteresis loss, and magneticresonance loss. Eddy current loss occurs when a conductivematerial and external magnetic field are between the relativemovement and an eddy current is produced; the resultingheat is called eddy current loss (Jiang et al., 2017; Ju et al.,2016). Hysteresis loss is caused by an irreversible magnetiza-tion process under an alternating magnetic field; aftermagnetization, the magnetic dipole moment has a frictionthermal effect caused by the magnetic field vibrations(Anzulevich et al., 2017; Du et al., 2013; Gregory et al., 2016;Huang et al., 2015; Jiang et al., 2015; Jiang et al., 2017; Kauret al., 2016; Li et al., 2014). Magnetic resonance losses aremainly caused by domain wall resonance and electron spinresonance, which contribute significantly to the microwaveheating of some metal oxides, such as ferrite, and othermagnetic materials. Under the combined action of dielectricloss and magnetic loss, microwave-absorbing materials willproduce a large amount of heat and can be regarded as highenergy sites (HeS). Fig. 2 illustrates that catalysts become HeSunder microwave irradiation and the contaminants aroundHeS can be effectively degraded; thus, the properties of highenergy sites established the foundation for microwave tech-nology applications in the field of energy and the chemicalindustry.

2. Classification and applications of catalysts inaqueous phase microwave reactions

2.1. Brief introduction to catalysts in aqueous phase reactions

As catalysts can interact with microwaves to improve the rateof a chemical reaction, catalysts can be used to intensify theeffects of microwaves in various scenarios, such as aqueousreactions. In aqueous phase reactions, the interactions be-tween microwaves and catalysts can form HeS, which provideenergy efficient utilization and increase the degradation ratesof organic pollutants. The behavior of the catalyst determinesthe effects of the microwave–catalyst interaction and whetherthe reaction rate is enhanced; thus, a large number of domesticand foreign studies have investigated the development of high-performance microwave-assisted catalysts.

The catalysts used to assist microwave reactions can beclassified in many ways. For instance, catalysts can beclassified as resistance type, dielectric type, and magneticmedium type according to their loss mechanism, or they canbe classified as coating type and structural type based on theirforming process. Catalysts can also be classified according totheir main elemental components, which is the classificationmethod that will be used in this article. Previous studies haveconcluded that a large proportion of catalysts used inmicrowave chemical reactions are either carbon-based oriron-based because these elements are abundant in natureand have good absorbing properties (Ania et al., 2007; Augustaand Kalaichelvi, 2015; Chang et al., 2010; Chen et al., 2011; Chenet al., 2016a; Chen et al., 2016b; Dalal et al., 2016; Li et al., 2016b;Liao et al., 2010; Liu et al., 2013). However, carbon-based and


iron-based catalysts have different absorbing mechanisms; forexample, ferrite mainly absorbs microwaves through magneticloss, while carbon nanotubes mainly absorb microwavesthrough dielectric loss. The different absorbing mechanismsof these catalysts have determined the field and mode of theirapplications. Furthermore, there are many catalysts composedof other elements that are not widely used to enhancemicrowave reactions, such as soft magnetic catalysts, rareearth catalysts, and some special types of catalysts like ionicliquids. These more rarely used catalysts have differentabsorbing mechanisms; thus, they are often used in specialreactions. Table 1 lists relevant applications of catalysts andtheir effects on aqueous phase reactions exposed tomicrowaveradiation. Different types of catalysts and their applications inmicrowave reactions are described in the following sections.

2.2. Iron-based catalysts

Iron-based catalysts are composed of iron or iron-containingcompounds. Because of the wide range of iron contents innatural materials, iron-based catalysts are used widely inindustrial production to increase the rates of reactions (Chenet al., 2011; Chen et al., 2016b; Li et al., 2016b; Liao et al., 2010;Liu et al., 2013; Liu et al., 2016; Mao et al., 2015; Ramasahayamet al., 2012; Roy and Bhattacharya, 2012; Wang et al., 2015;Wang et al., 2016a; Wang et al., 2013; Wang et al., 2016c).Moreover, most iron-based catalysts can interact with micro-waves to produce thermal effects; which gives iron-basedcatalysts great potential for applications in certain reactions,such as the degradation of aqueous phase organic matter.Iron-based catalysts can be classified according to theircomponent: for example, iron, iron oxides, and iron-basedcomposites. Iron and iron oxides are prevalent in nature andhave beenwidely used in production because of their low cost.

Undri et al. (2014) investigated the use of ferrite particles inthe microwave-assisted treatment of bio-plastic bags. Micro-wave treatment is a green way to handle bio-plastic bags asthe bags produce carbon dioxide, water, and some residues,which can be used as a source for new materials or for theproduction of fuels. However, simple iron-based catalystscannot achieve the desired objective in some applicationsbecause they have several deficiencies that are detrimental topractical applications, such as insufficient absorption bands,weak absorptive capacity, and single structure. There isongoing research studying different ways to improve theadaptability of iron-based catalysts so that they can be usedfor production in a variety of different situations. Thefollowing three methods are the primary options for improv-ing the adaptability of iron-based catalysts in order to achievethe desired production objectives.

2.2.1. NanocrystallizationRecent research has demonstrated that nanosizing iron-basedcatalysts can reduce the density of catalyst materials, improvethe microwave-absorbing properties, and generate catalyststhat have wide applications in practical aqueous phasereactions (Anzulevich et al., 2017; Bhattacharjee et al., 2016;Bodale et al., 2016; Elmachliy et al., 2010; Mao et al., 2015). Maoet al. (2015) applied nanocatalysts to treat the wastewater oftextile mills by using nanoscale, zero-valent iron as a catalyst

in the microwave-assisted degradation of dyes. Mao and theircolleagues determined that if the pH of the aqueous phase iskept constant, the MW-nZVI method for dye removal cansignificantly enhance the efficiency of dye degradation. Royand Bhattacharya (2012) studied the removal of aqueous heavymetal ions (Cu2+, Zn2+, and Pb2+) by magnetic microcrystallinenanotubes (c-Fe2O3), which were synthesized by microwaveirradiation. The results showed that this iron-based catalystcan remove aqueous heavy metal ions through adsorption.Compared with conventional iron-based catalysts such asferric oxide, nanomaterial catalysts can enhance the degrada-tion rate of organic matter because they can create more hotspots and generate more free radicals with high oxidationpotential. Fig. 3 illustrates the formation process of active freeradicals and it is clear that more active free radicals will begenerated when more hot spots are present. In summary,nanocatalysts are thin and light weight, and they have goodabsorption frequency bandwidth and compatibility properties,all of which give them the potential to be widely applicable.

2.2.2. Improved structureAccording to the principles of microwave-absorbing mate-rials, particle shape can effectively improve the microwaveabsorption capacity of materials and adapt the materials tocertain reactions where there is high demand for a catalyst.Cheng and Ren (2016) prepared barium hexaferrite by sol–gelcombustion and determined that the microwave absorptioncapacity of the hexagonal ferrite improved significantlybecause of the increased permittivity and permeability. Assome reactions require the use of liquid catalysts, Robertoresearched the use of ferrous ions as catalytic agentssupplemented by microwaves to degrade amoxicillin. Thismethod was able to achieve a high amoxicillin degradationrate with a low concentration iron ion solution, despite thetraditional Fentonmethod requiring a high concentration ironion solution (Homem et al., 2013).

The performance of catalysts is enhanced by improve-ments to the catalyst structure, which manifest in twoprimary ways: (1) improvements to the catalyst structure canenhance the degree of coupling between microwaves and thecatalyst, consequently enhancing the catalyst's absorptioncapacity; and (2) catalysts can effectively match the chemicalreactions related to the application modes used to developthem; for instance, some catalysts can be used as a matrix toload reactants. Therefore, the development of multi-type,multi-structure, and even multi-phase catalysts can enablethe application of microwave technology to conventionalindustrial fields and enhance the efficiency of reactions.

2.2.3. CompositeThe combination of different ferrites or ferrites and othermaterials, such as metal powders or conductive polymercomposites, is the most common and promising method tooptimize the performance of iron-based catalysts. Thismethod can adjust the electromagnetic parameters of iron-based catalysts, reduce their density, and broaden theirabsorption band. Thus, many studies in recent years haveconcentrated on the use of iron matrix composites in aqueousphase reactions. Liu and their colleagues (Liu et al., 2016)prepared a Cu–Zn ferrite composite material and used it to

Table 1 – Relevant applications of catalysts in aqueous phase reactions.

Catalyst Type Pollutant Results Ref

Nanoscale zero-valent iron

Iron-based Dyes Compared to the removal of Solvent Blue 36 and ReactiveYellow K-RN using only nZVI, more rapid and efficient dyeremoval and total organic carbon removal were achieved usingMW–nZVI, it was also concluded that microwave heating of thedye solutions as well as acceleration of corrosion of nZVI andconsumption of Fe2+ were possible reasons behind theenhanced dye degradation

Mao et al. (2015)

Magneticmicrocrystallinenanotubes

Iron-based Cu2+; Zn2+; Pb2+ Adsorption experiments were carried out systematically bybatch experiments to investigate the influence of differentfactors, such as contact time, initial concentration of metal ions,and pH of the solutions, the results show that the catalyst canadsorb and degrade heavy metal ions in water to achieve theremoval effect

Roy andBhattacharya (2012)

Cu–Znferrite composite

Iron-based Three methylphenol

This project investigate the oxidation reaction of TMP greenchemistry process design for magnetic nano-catalysts bymicrowave induced combustion process, and the kinetics andmechanism of two phase medium are also studied in Cu–Znferrite composite, it is shown that the degradation rate has beengreatly improved

Liu et al. (2016)

Iron oxidenanocomposites

Iron-based Phosphorus It provides a fast, easy, and economical way to produce reducediron oxide nanocomposites without requiring the need forhydrogen or inert gas during the transformation, and theprepared media were highly effective and efficient in removingPhosphorus

Ramasahayamet al. (2012)

Co–Fe binary oxideloaded adsorbent

Iron-based Cr(VI) The results showed that a cobalt and iron binary oxide (CoFe2O4)was uniformly formed on the BC through redox reactions, andthe adsorption of Cr(VI) can be strongly promoted by thiscompound material

Wang et al. (2013)

Cu2+–Fe2+–H2O2 Iron-based 3-nitro aniline The experimental conditions were studied by investigating theremoval rates and reaction rate constants of 3-NA at differentlevels of influence factors, the removal rates and reaction rateconstants both increased with the increase of Cu2+, Fe2+ andH2O2 dosages when they were at reasonable concentrationranges, it is conclude that Cu2+–Fe2+–H2O2 process can promotethe degradation rate of 3 nitro aniline effectively compared withFenton process

Wang et al. (2015)

Activated carbon Carbon-based Industrialwastewater

The prepared activated carbons were optimized for lead (Pb2+)and cadmium (Cd2+) uptake from aqueous solution and furtherthe same was used to treat industrial effluent, and the increasegranular activated carbon can significantly increase theremoval rate of industrial wastewater

Isaac et al. (2013)

Carbon nanotubes Carbon-based Dyes The methyl orange (MO), methyl parathion (MP), sodiumdodecyl benzene sulfonate (SDBS), bisphenol A (BPA) andmethylene blue (MB) in aqueous solution can be removedeffectively

Chen et al. (2016a)

Multi-walledcarbonnanotube

Carbon-based Cu2+ In order to investigate the dynamic behavior of MW-CNTs as anadsorbent, the kinetic data were modeled using the pseudo-second-order and second-order, and the statistical analysisreveals that the optimum conditions for the highest removal(99.9%) of Cu2+ are at pH 5.5, MW-CNTs dosage 0.1 g, contacttime 35 min and agitation speed of 160 rpm. Our results provethat microwave-assisted MW-CNTs can be used as an effectiveCu2+ adsorbent due to the high adsorption capacity as well asthe short adsorption time needed to achieve an equilibrium

Mubarak et al. (2015)

Activated carbonsupported onrutile/anatase TiO2

Carbon-based SulfurPhosphorus

The results showed that the anatase and rutile-TiO2 supportedon AC both significantly improved the MW/AC degradationeffect and the supported A-TiO2/AC showed higher MWcatalytic activity than R-TiO2/AC. Also, the supported A-TiO2 onAC can be excited leading to the production of more hydroxylradicals (–OH) than the supported R-TiO2, the supported A-TiO2/AC/MW showed to be a promising technology for the removal ofparathion in wastewater treatment applications on AC inaqueous solution under MW irradiation

Zhang et al. (2013)


Table 1(continued)

Catalyst Type Pollutant Results Ref

Graphitecarbonitride

Carbon-based Rhodamine B Graphite carbon nitride (g-C3N4)/Ag2SO4 nanocomposites wereprepared by a fast microwave-assisted method. Effect ofmicrowave irradiation time, calcination temperature, andscavengers of the reactive species on the degradation reactionwas also evaluated. The enhanced photo catalytic activity wasmainly ascribed to the matching band energies of g-C3N4 andAg2SO4 which leads to an improved separation of photogenerated electron hole pairs, the carbon-based catalyst afterthe composite has a more powerful catalytic ability, greatlyreducing the difficulty of the reaction and improve theefficiency

Akhundi and Habibi-Yangjeh (2016)

Nano-TiO2-supportedactivated carbon

Carbon-based Methyl orange Nano-TiO2-supported activated carbon (TiO2/AC) was developedfor the microwave (MW) degradation of an azo dye, methylorange (MO), selected as a model contaminant in aqueoussolution, the results showed that the supported TiO2 on ACcould be excited resulting in the production of hydroxyl radical(-OH) in aqueous solution under MW irradiation, whichsignificantly enhanced the performance of AC/MW process forthe degradation of MO

Zhang et al., (2012)

Activated carbon Carbon-based Reactive Black5

GAC of 20 g L−1 effectively removed COD and toxicity ofelectrocoagulation-treated solution within 4 hr. Microwaveirradiation effectively regenerated intermediate-loaded GACwithin 30 s at power of 800 W, GAC/water ratio of 20 g L−1, andpH of 7.8

Chang et al. (2010)

Carbonaceousadsorbents

Carbon-based Pb2+ The best performing adsorbent rendered an outstanding Pb2+

sorption capacity (261 mg/g), in spite of its rather low porosity.Such a high capacity was explained in terms of the materialssurface chemistry, because the obtained carbons have a highamount of acidic and oxygenated groups, as well as calciumdeposits

Durán-Jiménezet al. (2015)

Zeolite Soft magnetic Ammonia–nitrogen

The results show that the modified zeolite by microwave-sodium acetate (SMMZ) has a high sorption efficiency andremoval performance, and the adsorption kinetics of ammonia-nitrogen follows the pseudo-second-order kinetic model

Dong and Lin (2016)

Ceric ammoniumnitrate

Rare earth Polyacrylamide Synthesis of polyacrylamide (PAM) grafted soya peptone wasperformed by the microwave assisted method using cericammonium nitrate (CAN) as free radical initiator and find thesynthesis efficiency is greatly improved

Hasret et al. (2011)

Mn2+ Others Bisphenol A In this study, Mn2+ ion was introduced into a microwave-enhanced Fenton system for removing BPA in wastewater. Theresults showed that the BPA removal rate in Microwaveenhanced Mn-Fenton (MW-Mn-Fenton) process was higherthan that of in Fenton process or in other Fenton process withinclusion of metal under microwave irradiation

Li et al. (2016b)


catalyze the degradation of 3-methylphenol, which effectivelyreduced the amount that would be released as an environ-mental pollutant. Aphesteguy et al. (2012) combined apolyaniline composite and Fe3O4 particles mixed with epoxyresin composite to create composite particles that have an

Fig. 3 – Free radical formation process.

increased absorption capacity and can be used to absorb thespecific electromagnetic wave band that is incident on thecoating surface.

Composite catalysts are advantageous because of theirimproved microwave absorption properties and the full playof their respective characteristics. Similarly, carbon nanotubesloaded with titanium dioxide have the excellent microwaveabsorption capacity of carbon nanotubes and use the catalyticability of titanium dioxide to enhance reaction rates, whichenables more reactive radicals to be activated in aqueous phasereactions and degrade pollutants effectively. The aforemen-tioned studies demonstrate that composite materials canpossess the desirable properties of the individual materialsand synergistically produce additional outstanding propertiesthat assist microwave reactions and contribute to theirpotential for development.


2.3. Carbon-based catalysts

Carbon-based catalysts are composed of a carbon compoundor carbon composite. Carbon-based catalysts, such as graph-ite, carbon fiber, activated carbon, and silicon carbide, aremore commonly used than iron-based catalysts because ofthe universality of carbon materials (Wang and Wang, 2016).Lin et al. (2010) used granular activated carbon in a microwavereaction to treat chloramphenicol-contaminated soil andfound that the addition of granular activated carbon effec-tively increased the temperature of the soil and promoted thedegradation of chloramphenicol. Liu et al. (2013) also usedgranular activated carbon to treat industrial wastewater fromchemical plants and study the effects of pH, granularactivated carbon dosage, and microwave irradiation time.They found that by increasing the concentration of granularactivated carbon while other conditions were kept constant,they could significantly increase the contaminant removalrate from industrial wastewater and achieve the goal of savingenergy.

In recent years, research has focused on improving basiccarbonmaterials and twomainmethods have been developed(Ania et al., 2007; Augusta and Kalaichelvi, 2015; Chang et al.,2010; Chen et al., 2016a; Dalal et al., 2016; Durán-Jiménezet al., 2016; Durán-Jiménez et al., 2015; Isaac et al., 2013; Liet al., 2016a; Makeswari and Santhi, 2013; Maldhure and Ekhe,2011; Mubarak et al., 2015; Rathour et al., 2015; Sun et al.,2016a; Wang et al., 2012; Zhang et al., 2007; Zhang et al., 2012).One method is the nanocrystallization of carbon materials, aswas discussed for iron-based catalysts. Nanometer-sizedcarbon-based catalysts have reduced specific gravity andimproved microwave absorption capacity compared to thebulk material, and they have great application potential inpractical production (Roy and Bhattacharya, 2012; Varisli etal., 2017). Chen et al. (2016a) used microwave-induced carbonnanotubes (MW-CNT) to propose a new catalytic degradationtechnology and the results of their study demonstrated thatmicrowave irradiation can effectively remove methyl orange(MO), methyl parathion (MP), sodium dodecyl benzene sulfo-nate (SDBS), bisphenol A (BPA), and methylene blue (MB) fromaqueous solutions. Moreover, Mubarak et al. (2015) synthe-sized a microwave-induced, multi-walled carbon nanotube(MW-CNT), studied its ability to adsorb Cu(II) undermicrowave-assisted conditions and optimized microwave-assisted Cu(II) adsorption parameters, such as pH, MW-CNTdosage, agitation speed, and adsorption time. The adsorptiondata followed both the Freundlich and Langmuir isothermsand demonstrated that microwave-induced MW-CNTs can beused as an effective Cu(II) adsorbent because of their highadsorption capacity and short adsorption time.

Carbon-based catalysts can also be prepared from com-posites of carbon or other materials that have been loadedwith carbon. These composites have the microwave absorp-tion capacity of the original carbon-based catalyst and can beused in different circumstances because of their differentefficacy. Zhang et al. (2013) developed activated carbonsupported on rutile (TiO2) for the microwave-induced degra-dation of sulfur and phosphorus pollutants in water andstudied the effects of selected process parameters in detail;for example, they studied the anatase or rutile TiO2 content,

heat-treatment temperature and time, microwave irradiationtime, initial parathion concentration, catalyst dose, solutionpH, and microwave power on the degradation. The results ofthis study showed that supporting activated carbon (AC) onanatase and rutile-TiO2 significantly improved the MW/ACdegradation, and the anatase-TiO2/AC showed higher MWcatalytic activity than the rutile-TiO2/AC. Akhundi and Habibi-Yangjeh (2016) prepared a graphite carbonatite (g-C3N4)/Ag2SO4 nanocomposite to degrade rhodamine B; they foundthat the nanocomposite exhibited significantly improvedstability and could be recycled five times without a significantloss of photocatalytic activity.

It is clear that the carbon-based composite catalysts havegreater catalytic ability, which reduces the reaction energyand improves efficiency. Carbon-based catalysts have strongmicrowave absorption capacities and good catalytic perfor-mance, and they also have very good performance uponrepeated use. Most carbon-based catalysts have very strongthermal stability and will not lose catalytic activity as thereaction temperature increases, which prevents problemscaused by local high temperatures in microwave reactionsthat result from a large number of hot spots. Furthermore,carbon-based catalysts will not be oxidized during thereaction, unlike iron-based catalysts or some other metaloxide catalysts. These advantageous properties make carbon-based catalysts good candidates for research and microwavetechnology developments.

2.4. Soft magnetic catalysts

Soft magnetic materials are ferromagnetic materials that havelow remanence and coercivity. These materials are rarely usedcommercially and can be classified as ferrite soft magneticmaterials, amorphous soft magnetic alloys, and soft magneticcomposite materials that consist of metals or alloys (Mishra andSharma, 2016). The soft magnetic nanocrystalline alloys thathave been developed in recent years are a new form of softmagnetic materials; soft magnetic materials exhibit easymagnetization and demagnetization, which make them usefulin electrical and electronic equipment (Anton et al., 2016; Gu etal., 2017; Horikoshi et al., 2014; Sharma and Rajesh, 2017;Shoushtari et al., 2016).

In industrial production, the most widely used soft magneticmaterials are silicon steel, which is an iron–silicon alloy, and avariety of soft ferrites that can increase the efficiency of catalyticdegradation by acting as either a catalyst or an adsorbent insome experimental reactions (Ramasahayam et al., 2012; Shahet al., 2016). Zeolites are an example of soft magnetic materialsthat are used in a variety of reactions because of their goodinteractions with microwaves. Dong and Lin (2016) prepared azeolite that was modified by a microwave-assisted sodiumacetate method and studied its ability to absorb ammonia-nitrogen from simulated water samples. The results showedthat the microwave-sodium acetate (SMMZ) modified zeolitehave a high absorption efficiency and good removal perfor-mance. Manganese oxides are other soft magnetic materialsthat have excellent microwave absorption properties anddielectric properties that make them a good compound agent.Huifang Pang et al. developed a low-temperature method todeposit crystalline MnO2 over carbon nanotubes (CNTs). This


facile, low-cost, scalable, high-yield method produces anenhanced microwave-absorbing nanocomposite because coat-ing CNTs with manganese oxide increases both their dielectricand magnetic losses and improves the impedance matchbetween free space and the absorber (Pang et al., 2018). Donget al. have studied the effects of four parameters on themicrowave-induced catalytic oxidation of PCB77 in diatomite:microwave radiating time, addition of different metal oxidenanoparticles, concentration and type of acids, and dosage ofMnO2. The results of this study indicated that microwave-induced degradation of PCB77 using MnO2 nanoparticles as thecatalyst had up to a 90% removal rate after 1 min and the MnO2

nanoparticles exhibited good absorbing properties (Dong, 2009).Previous studies have demonstrated that the development

of nanotechnology has brought great opportunities andchallenges to the traditional magnetics industry. Nanotech-nology also provides a practical opportunity for improving theperformance of traditional soft ferrite materials, so the futureof soft magnetic materials is expected to develop in thedirection of the nanoscale.

2.5. Rare earth catalysts

Rare earth metals, also known as rare earth elements and raremetals are typically considered to be the 15 elements in thelanthanide period of the periodic table plus scandium andyttrium. Catalysts doped with rare earth metals can effec-tively improve microwave absorption performance (Cybinskaet al., 2017; He et al., 2015; Polychronopoulou et al., 2017;Stergiou, 2017; Yang et al., 2016). As applications of rare earthelements have becomemore extensive and have been studiedin greater detail, applications of rare earth elements ascatalysts have attracted progressively more attention fromchemical workers; a large number of experiments havedemonstrated that rare earth catalysts are viable as theyhave good catalytic activity. The technique is mainly used tocatalyze the degradation of reactants or the auxiliary synthe-sis of ammonia, rubber, and some organic materials. Recently,microwave co-catalysis with a rare earth metal catalyst hasbecome a new and exciting field of research. Mahto et al.(2014) performed a synthesis of polyacrylamide grafted soyapeptone by using ceric ammonium nitrate (CAN) as a radicalinitiator in the presence of microwave radiation. The resultsshowed that the efficiency of the synthesis was optimized byvarying the concentrations of acrylamide and CAN.

2.6. Other catalysts

In addition to the four types of catalysts described above, manyother types of catalysts can interact with microwaves andaccelerate reactions, suchas somemetal elements,metal oxides,and some ionic liquids (Barik et al., 2016; Bartoli et al., 2016;Bhattacharya and Basak, 2016; Boccaccini et al., 2007; Caudillo-Flores et al., 2016; Dong and Lin, 2016; Fuerhacker et al., 2005;Hasret et al., 2011; He and Cheng, 2016; Kuo and Lee, 2010; Lei etal., 2008; Lu andPei, 2016; Tajik et al., 2014; Tsintzou andAchilias,2012; Tsintzou et al., 2012; Zhang et al., 2005; Zhang et al., 2013;Zheng et al., 2016). Currently, many scholars are studyingadaptations of these unconventional catalysts for specificchemical reactions. For example, Li et al. (2016b) introduced

Mn2+ ions into a microwave-enhanced Fenton system forremoving BPA from wastewater. The results showed that theBPA removal rate was higher in the microwave-enhanced Mn-Fenton (MW-Mn-Fenton) process than in the normal Fentonprocess or in other types of Fenton processes that includemicrowave irradiation of metals, and that a greater reagentutilization efficiency was achieved in the MW-Mn-Fentonprocess than in the normal Fenton process. Unconventionalcatalysts can be used in various special circumstances in whichspecific catalysts are required because of their unique features.

3. Dimensions and structure of catalysts used inaqueous phase

3.1. Conventional size and nanometer size of catalysts

As catalysts can interact withmicrowaves to produce HeS, thedimensions of a catalyst have a significant influence on thecatalyst's performance during actual industrial production.Generally, the dimensions of catalysts can be divided intoconventional size and nanometer size (Bagha et al., 2017). Asdescribed previously, conventional catalysts are now themostwidely used catalysts in reactions and they play an importantcatalytic role in the synergistic reaction with microwaves(Klinbun et al., 2011). For instance, conventional iron-based,carbon-based, and soft-magnetic-based catalysts, as well asconventional catalysts of other materials, represent a largeproportion of the catalysts used in current applications;however, nanoscale catalysts are becoming more common inapplications because their small size and large specificsurface area make them good adsorbents in addition to theirbeing good catalysts (Anzulevich et al., 2017; Bhattacharjee etal., 2016; Bodale et al., 2016; Elmachliy et al., 2010; Fowsiya etal., 2016; Guo et al., 2016; Meng et al., 2016; Ramasahayam etal., 2012; Roy and Bhattacharya, 2012). With the emergenceand development of nanotechnology, the unique structure ofnanomaterials has gained attention for its surface effect,quantum size effect, macroscopic quantum tunneling effect,and small size effect. Nanomaterials exhibit uniform electro-magnetic absorption in a wide frequency range because oftheir unique structure (Varisli et al., 2017). Explanations ofnanomaterials' excellent electromagnetic wave absorptionproperties are described below.

(1) Nanoparticles are small in size and thus have largerspecific surface areas that promote interfacial polarizationand multiple scattering during absorption. The quantumsize effect of nanomaterials causes the energy level ofelectrons to split, and the energy level of the split is in therange corresponding to microwaves, which creates a newabsorption channel for nanomaterials (Anzulevich et al.,2017; Bhattacharjee et al., 2016; Bodale et al., 2016).

(2) Nanomaterials have a large number of dangling bonds,a higher proportion of interfacial components, and ahigher proportion of atoms on their surface, whichcause interface polarization and absorption bandbroadening (Elmachliy et al., 2010; Fowsiya et al., 2016;Guo et al., 2016).


(3) The motion of atoms and electrons in nanomaterials aremore intense than that in bulk materials and magneti-zation can be transformed into heat energy when theatoms and electrons in nanomaterials are irradiated in amicrowave field. These phenomena can increase theabsorption of electromagnetic energy and increase theconversion efficiency of electromagnetic energy intoheat energy, which consequently improves the absorp-tion of electromagnetic energy (Meng et al., 2016;Ramasahayam et al., 2012; Roy and Bhattacharya, 2012).

(4) Nanomaterials are thin and light weight, and they havegood absorption frequency bandwidth and compatibil-ity properties (Ju et al., 2016).

3.2. Different structures of catalysts

The structures of catalysts can be divided into macro- andmicro-structures. Macro-structure catalysts used in auxiliarymicrowave applications are further divided into three maintypes: powdery, granular, and bed (He and Cheng, 2016).

(1) Powdery type. Powdery catalysts have been used widelybecause of their small particle size and large specificsurface area. Moreover, powdery catalysts have a goodadsorption capacity because of their large specificsurface area, in addition to their catalytic and wave-absorbing abilities. Major examples of powdery cata-lysts are CNTs and powdered activated carbon(Tsintzou and Achilias, 2012).

(2) Granular type. Compared to powdery-type catalysts,granular catalysts are widely used in aqueous phasereactions because they are easily separated from thesolution. These catalysts can assist microwave reac-tions and produce many HeS because of their micro-wave absorption capacity and dimensionalcharacteristics. The main representatives of this groupare granular activated carbon and molecular sieves(Zhang et al., 2005).

(3) Bed type. As opposed to the two structures describedabove, bed catalysts have no fixed structure and theirstructure can be changed according to the demand. Inproduction, bed catalysts are mainly used with large-scale experimental equipment.

The main micro-structures of catalysts are rod-shape,hexagonal-crystal shape, and tubular and cystic types. Thesestructures are not discussed in this manuscript and thus havenot been described here.

4. Relevant reactors for applications of microwave-assisted catalysts

4.1. Improvements to traditional microwave reactors

In microwave-assisted catalysis used for water treatment, thereactor has a significant influence on the performance of thecatalyst and the final reaction effects. A suitable reactor needs tohave good wave permeability, temperature and corrosionresistance, and be a suitable location for catalysts. Compared to

the great progress that has beenmade inmicrowave technology,research on microwave generators is still relatively limited.Currently, most microwave reactors are modeled after domesticmicrowave ovens because these are easily handled and areconvenient for researchers (Benmoussa et al., 2016; Cydzik-Kwiatkowska et al., 2012; De la Fuente et al., 2016, De la Fuente etal., 2017; Gao et al., 2017; Mitani et al., 2016; Ohtsu et al., 2010;Pianroj et al., 2016; Qu et al., 2014; Zhang et al., 2017b; Zhang etal., 2017c; Zhu et al., 2017; Zielinski and Krzemieniewski, 2007).However, reactors modeled after household microwave ovenshave many disadvantages: (1) the power density is low andcannot meet the requirements of high-field-strength experi-ments; (2) it is difficult to heat the oven uniformly because theregion of heating is concentrated mainly on the tray at thebottom of the furnace cavity; and (3) the experimental results donot match the results of large reactors because of the limitationsof the equipment (Zhu et al., 2017).

The disadvantages listed above have prompted re-searchers to develop new microwave reactors that can beused under different conditions and provide reasonablereaction vessels for catalysts. Gao et al. (2017) investigatedthe use of a spiral reactor to treat printing and dyeing sludgewith microwave radiation; microwave power, temperature,auger speed, gas velocity, and the addition of the catalystwere studied and the results demonstrated that this type ofspiral reactor can effectively improve degradation efficiency.Mitani et al. (2016) designed and fabricated a widebandmicrowave reactor with output at 915 MHz and 2.45 GHz ISM(Industrial, Scientific, and Medical). The reactor structure wasbased on a coaxial cable and a liquid sample was placed in thespace between the inner and outer conductors. A truncated,cone-shaped polytetrafluoroethylene (PTFE) device wasinserted to reduce microwave reflection over a wide fre-quency range. The reactor had a volume of 360 mL and wasdesigned using a 3D electromagnetic simulation. The reflec-tion ratio and heating data demonstrated that this reactorfunctioned over a wide frequency range: 800 MHz and2.7 GHz. Ohtsu et al. (2010) designed a waste managementsystem without oxygen injection, which was achieved byhybrid heating using microwave energy and an electricheater. The microwave reactor had a rectangular parallelepi-ped shape and a volume of approximately 0.1 m3. Microwaveenergy (2.45 GHz, about 800 W) was introduced through thetop of the reactor, and the heater (approximately 1 kW) waslocated at the bottom. The results showed that this reactorhad a favorable effect on the microwave reaction.

The new types of microwave reactors described abovehave positively affected applications of microwave-assistedcatalysis; however, most of these reactors are used for solidwaste treatment and rarely involve water treatment. Reactorsused in water treatment applications can be divided into twomain categories: continuous microwave catalytic reactors andintermittent microwave catalytic reactors. The followingsections provide brief descriptions of these reactors, whichhave their own functions in the field of wastewater treatment.

4.2. Continuous microwave catalytic reactor

When treating organic wastewater by microwave-assistedcatalysis, continuous reactors have always been used to


house the reaction because they have the advantages of largethroughput, high recovery rate, and no secondary pollution.Fixed bed catalysts are often used in this type of reactor asthey allow the catalyst to interact with microwaves andgenerate HeS in a fixed position, which enables degradationwhen wastewater passes through this position. Tang et al.(2018) designed a continuous, constant-temperature micro-wave device for the degradation of 4-nitrophenol, in whichthe microwave power, reaction time, and temperaturecould be controlled. An appropriate amount of catalyst wassuspended in the solution and there were continuous air andwastewater inputs during the run. In this microwave device,catalytic degradation enhanced by the microwave effect wasobserved. Mao et al. (2016) built a continuousmicrowave/nZVItreatment system for the removal of malachite green (MG).The analysis of the system's shock resistance indicated thatthis system stabilized the effluent quality and MG wasefficiently degraded over a short hydraulic retention time.

Because catalysts play a crucial role enhancing microwave-assisted reactions, the catalysts in reactors should be replacedconstantly to ensure good absorption and catalytic capability;however, this results in a huge waste of resources. To addressthis problem, our laboratory has been working to design andupdate reactors based on the traditional continuous reactor inorder to achieve catalyst regeneration and thus attain sustain-able utilization of resources. The specific form of our reactorsand the mode of work are described below.

4.2.1. Continuously rotating deviceTo enable regeneration and reuse of catalysts after a micro-wave reaction, we designed a reactor that rotates continuouslyduring wastewater treatment in order to reduce the high costand low efficiency of catalyst replacement. Fig. 4 presents thedetailed structure of this reactor and shows that the reactorincludes a shell that can be divided into two symmetrical parts:a reaction chamber and a microwave generator attached to theside of the reaction chamber. The interior of the shell includesthe adsorption chamber and the degradation and regenerationchamber. The upper and lower ends of the shell are equippedwith an air inlet, an air outlet, a liquid inlet, and a liquid outlet;the air inlet and outlet are positioned on the same side ofthe clapboard, and the liquid inlet and outlet are positioned onthe other side of the clapboard. The reaction chamber is in themiddle of the shell so that the catalyst layer rotates frequently,

Fig. 4 – An experimental, continuously rotating, microwavecatalytic system.

which contributes to both the degradation of organic com-pounds and the regeneration of the catalyst. The rotation of thecatalyst plate promotes continuous adsorption, catalytic deg-radation, and regeneration processes, which saves the volumeof the reactor. The catalysts are regenerated in one region ofthe reactor while the reaction occurs in another; thus, theentire reaction process is an approximately continuous pro-cess, which improves treatment efficiency.

When using this reactor in an organic wastewater treat-ment experiment, the organic wastewater is first adsorbedonto the catalyst layer and then oxidized in the degradationchamber. After the reaction, the catalysts involved in thedegradation reaction are regenerated by carbonization at hightemperature. Our experimental results showed that thisreactor has a positive effect on the treatment of organicwastewater when it is operated correctly.

4.2.2. Continuous four-stage deviceSimilar to the continuously rotating reactor described above,this reactor can continuously rotate while the reaction is inprogress; however, unlike the continuously rotating reactor,this reactor has one stage of adsorption, two stages ofdegradation, and one stage of regeneration. This reactorstructure provides suitable locations for the adsorption ofcontaminants onto the carbon-based catalyst, degradation ofthewastewater contaminants, and regeneration of the catalyst.As shown in Fig. 5a, the reactor is divided into four chamberswith specific functions: adsorption, degradation on low power,degradation on high power, and regeneration. In the firstchamber, the organic wastewater is adsorbed onto activatedcarbon as a result of its strong adsorption capacity. In thesecond chamber, thewastewater is rotated into a heated regionfor a pretreatment that evaporates water from the wastewater.After the pretreatment, the organic compounds in the waste-water are decomposed by the oxidizing gas and the hightemperature, which is produced by the high-power microwavegenerator. Finally, the activated carbon catalyst is regeneratedin the fourth, regeneration chamber by the heat from the high-power microwave generator and the steam produced in thesecond chamber. The effective residence time of wastewater inthe device is regulated by the microwave controller, which weuse to control the start and stop the microwave generator tomeet our requirements.

The effects of different variables on this method's degrada-tion of highly concentrated organic wastewater were deter-mined by controlling the retention time of the wastewater indifferent reaction zones. The results demonstrated that thismethod can efficiently degrade organic wastewater. This devicehas good application prospects in organic wastewater treat-ment because it regenerates the catalyst, enables a continuousreaction, includes post-treatment of the whole reaction, andallows the valuable gas that remains after the microwave-assisted decomposition of organic matter to be collected.

4.3. Intermittent microwave catalytic reactor

Compared to the continuous microwave catalytic reactors,the intermittent microwave catalytic reactor is able tosuccessfully degrade high-concentration and refractory or-ganic wastewater because it has the advantages of high

Microwave generator

High concentration organic wastewater

Low concentration organic wastewater

Low power

Water Low calorific value gas Catalyst regeneration

High poweroxidizing gas

High power Nitrogenprotective gas

Zone 1 Zone 2 Zone 3 Zone 4

Microwave generator

a

b

Microwave generator

Zone4

Zone 1Zone2

Zone3

Fig. 5 – (a) Overhead view of the continuous four-stage device. (b) Flow chart of the continuous four-stage device.


reaction temperature, long residence time of wastewater inthe reactor, and high dispersion of the catalyst. Thus,intermittent microwave catalytic reactors have been used todeal with small quantities of wastewater that is difficult todegrade. Most intermittent reactor designs were based ontraditional microwave ovens, but some special structureshave been added in order to meet process requirements.Wang et al. (2015) designed a reaction vessel based on amicrowave oven to study the Cu(II)–Fe(II)–H2O2 degradationprocess of aqueous 3-nitroaniline (3-NA) exposed to micro-wave radiation; this process removed the 3-NA (100 mg/L)in8.5 min and the reaction rate constant reached 0.310 min−1.Patil and Shukla (2015) investigated the degradation ofReactive Yellow 145 dye by persulfate using both microwaveheating and conventional heating. The microwave experi-ments were performed in a stoppered, 200 mL Erlenmeyerflask and a modified domestic microwave oven with ratedoutput power of 800 W was used as the source of microwaveradiation. The setup was fitted with a programmer for settingthe temperature, time, and rate of heating, and a Teflon stirrerdriven by amotor was used to ensure slow agitation of the dyesolution during microwave heating.

Based on the existing research, intermittent microwavereactors are primarily designed for reactions under atmo-spheric pressure and are rarely used for high pressurereactions, which have higher degradation efficiency forwastewater. In order to study the effects of pressure on thedegradation rate of wastewater, Garcia-Costa et al. (2017)designed an autoclave for microwave-assisted wastewatertreatment. In the experiments with this autoclave, phenol

(100 mg/L) was used as the target pollutant and the followingoperating conditions were used: pH 3, 120°C, catalyst concen-tration of 100 mg/L, and addition of the theoretical stoichio-metric amount of H2O2 (500 mg/L). The results demonstratedthat wastewater is more completely degraded at high pres-sure. Compared to conventional microwave treatment tech-nology for wastewater, high pressure treatment provides theadvantages of high boiling point, high dissolved oxygenconcentration, and reduced evaporation of solution. Degrada-tion reactions under high pressure can be described asmicrowave catalytic wet oxidation and they are characterizedby a high degradation rate; therefore, our laboratory hasdesigned an industrial high-pressure microwave reactor tostudy its effects on wastewater treatment.

The reactor that we designed is capable of withstanding ahigh pressure and constantly monitoring the pressure; thus, itcan be used to study the effect of pressure on the catalyticoxidation of wastewater. Fig. 6 shows a photograph of thesystem, which consists of a microwave control system (tocontrol the radiation power), a water load (to avoid damage tothe instrument caused by reflected microwave radiation), awaveguide (to transmit themicrowave radiation), cylinders, anda microwave reaction cavity. To allow the system to withstandand maintain high pressure conditions, Teflon was selected asthe material for the reactor. The pressure of the reaction vesselis constantly controlled by controlling the pressure beingdelivered from the gas cylinder according to our requirements.The top cover of the reaction vessel is connected to the gascylinder by a thread in order to seal the reactor and multipleholes were designed to connect the reactor with the valves and

Microwave power sourceWater load

Dual directional coupler

Three screw adapter

E–wave guide

Single mold cavity

Pressure gauge

Cylinders

Short–circuited piston

N2

Reaction vessel

Fig. 6 – Microwave catalytic wet oxidation system under high pressure.


pressure gauges, which can instantly display the pressure insidethe reaction vessel. The pressure in the reaction vessel can beeasily controlled and set between 0 to 1 MPa, and the organicwastewater can be highly oxidized under high pressure.

The system was tested using nitrophenol (100 mg/L) as amodel wastewater contaminant and carbon nanotubes with astrong adsorption capacity were used as the catalyst. Theexperimental results showed that the system has gooddegradation efficiency for organic wastewater and that thedegradation rate can reach 80% after 5 min under 0.3 MPa. Thissystem allows the factors that can increase the degradationrate of wastewater, such as high pressure, to be controlled andmodified in order to accelerate the chemical reactions in thereaction vessel. Preliminary results from the use of highpressure in both wastewater processing and the oxidation ofvolatile organic compounds (VOCs), as demonstrated bymicrowave wet oxidation technology, are encouraging.

5. Conclusions

The mechanisms, main influencing factors, and relevantapplications of microwave–catalyst interactions in the aque-ous phase were reviewed through several experimental andtheoretical studies presented in this article. The interactionsbetween microwaves and certain catalysts can form “high-energy sites” (HeS), which lead to energy efficient utilizationand enhanced in situ degradation of organic pollutants. Theproperties of the catalyst ultimately determine the effect thatthe electromagnetic field has on the energy absorptioncharacteristics of the catalyst. This article presents themicrowave absorption mechanism, the classification of cata-lysts, and applications of microwave-assisted catalysis in theaqueous phase. The dimensions and structure of materialsused as catalysts were also surveyed as different forms of acatalyst were applied to different scenarios in order to achievemaximal energy efficiency and product quality.

Relevant reactors for applications of microwave-catalystinteractions were also reviewed in order to determine theoptimal chemical reaction process and identify the idealreaction conditions; microwaves can exhibit positive feedbackwhen paired with the appropriate catalysts in applications.Microwave technology can not only be used in water treatment,but also has great potential applications in a variety of scientific

and engineering fields, such as microwave-assisted pyrolysis,pollutants removal,material synthesis, andmicrowave-assistedsintering. However, most studies on microwave technology forwastewater treatment are performed in laboratory becausethere have been some problems during practical industrialapplications. Firstly, the interactions between microwaves andcatalysts used in wastewater treatment can lead to wastedenergy because water absorbs a large amount of the energy anda significant amount of heat is lost when water evaporates.Moreover, the throughput of wastewater treated by microwave-assisted catalysis is relatively small, so it is difficult formicrowave-assisted catalysis to be widely used in sewagetreatment plants, and the recycling of catalysts exposed tomicrowaves is a problem that must be considered. Therefore,intensive studies on these issues are urgently required in orderto solve the critical problems preventing implementation ofmicrowave-assisted catalysis in wastewater treatment plants.Since reflux heating can use the additional heat from hightemperature water to save energy, developing a circulatingreactor is a main research task for the near future.

Microwave catalytic wet oxidation technology is also afocus of on-going research as it combines microwaves withwet oxidation technology to save energy and achieve highefficiency degradation of refractory organic matter. Highlyefficient and recyclable catalysts are also needed in order todevelop microwave-assisted catalysis technology that isviable for industrial applications. Microwave technology canalso be combined with traditional water treatment technol-ogy, such as membrane bioreactors, electrocatalysis, andphotocatalysis, to design effective sewage treatment plants.Microwave technology has many unique advantages that arewaiting to be exploited. With the development of optimizedcatalysts and reactors, microwave–catalyst interactions areexpected to start being incorporated into more commercialapplications and to deliver good benefits.

Acknowledgments

The authors thank the support of the Natural Science Founda-tion of Shandong Province (No. ZR2018MEE030), the NationalNatural Science Foundation of China (Nos. 51506116,51576118,51376112), the Young Scholars Program of Shandong University(No. 2016WLJH37), and the Fundamental Research Funds ofShandong University (No. 2016JC004).


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