Review of elemental mercury(Hg0)removal by CuO-based materials

2022-07-20DongYEXiaoxiangWANGRunxianWANGXinLIUHuiLIUHainingWANG

Journal of Zhejiang University-Science A(Applied Physics & Engineering) 2022年7期

Dong YE,Xiao-xiang WANG,Run-xian WANG,Xin LIU,Hui LIU,Hai-ning WANG

1College of Quality&Safety Engineering,China Jiliang University,Hangzhou 310018,China

2Key Laboratory of Biomass Chemical Engineering of Ministry of Education,Institute of Industrial Ecology and Environment,College of Chemical and Biological Engineering,Zhejiang University,Hangzhou 310027,China

Abstract:Mercury emission has become a great environmental concern because of its high toxicity,bioaccumulation,and persistence.Adsorption is an effective method to remove Hg0 from coal-fired flue gas,with adsorbents playing a dominant role.Extensive investigations have been conducted on the use of CuO-based materials for Hg0 removal,and some fruitful results have been obtained.In this review,we summarize advances in the application of CuO-based materials for Hg0 capture.Firstly,the fundamentals of CuO,including its crystal information and synthesis methods,are introduced.Then,the Hg0 removal capability of some typical CuO-based adsorbents is discussed.Considering that coal-fired flue gas also contains a certain amount of NO,SO2,H2O,NH3,and HCl,the impacts of these species on adsorbent Hg0 removal efficiency are summarized next.By generalizing the mechanisms dominating the Hg0 removal process,the rate-determining step and the key intermediates can be discovered.Apart from Hg0,some other air pollutants,such as CO,NOx,and volatile organic compounds (VOCs),account for a certain portion of flue gas.In view of their similar abatement mechanisms,simultaneous removal of Hg0 and other air pollutants has become a hot topic in the environmental field.Considering the Hg0 re-emission phenomena in wet flue gas desulfurization (WFGD),mercury capture performance under different conditions in this device is discussed.Finally,we conclude that new adsorbents suitable for long-term operation in coal-fired flue gas should be developed to realize the effective reduction of mercury emissions.

Key words:Hg0 capture capability;CuO-based materials;Hg0 removal mechanisms;Gas components;Simultaneous removal of multiple pollutants

1 Introduction

As a global pollutant,mercury has drawn considerable concern in recent decades owing to its bioaccumulation,persistence,and high toxicity to human beings and ecosystems(Yang et al.,2019b;Zheng et al.,2021).Coal-fired power plants are responsible for about 30%of the mercury produced,and are well recognized as the major source of mercury emissions in China(Ye et al

,2021).In view of this serious problem,a standard GB13223-2011 was issued to reduce mercury emissions to less than 0.03 μg/m(Yang et al

,2019a).To meet this tough standard,it is essential to adopt effective mercury abatement techniques.In general,the emitted mercury species can be classified into three main categories:elemental mercury(Hg),oxidized mercury(Hg),and particulate-bonded mercury(Hg)(Mei et al

,2020;Ma et al

,2021).The high solubility of Hgensures its sufficient absorption by wet flue gas desulfurization(WFGD)devices,and Hgcan be effectively captured by dust removal systems because of its strong adherence to particulate surfaces (Wen et al

,2011;Jampaiah et al

,2019).However,low solubility and high volatility make Hgdifficult to remove by existing air pollutant abatement devices,and constitutes the main barrier to the reduction of mercury emissions(Liu H et al

,2020).Among the various air pollutant abatement techniques,injecting adsorbents into the flue gas is attracting increasing attention because it avoids the need to install additional air pollutant control devices.Generally,this process takes place in the upstream part of dust removal systems,in which the spent adsorbents can be separated from the flue gas (Mei et al

,2019).In this technology,the activity of the adsorbents plays a crucial role in the Hgremoval efficiency of the whole system (Wang C et al

,2020).Some materials,such as carbon (Liu DJ et al

,2020),calcium compounds (Zhang et al

,2016;Balasundaram and Sharma,2018),and fly ashes(Liu Z et al

,2020),are proven to be capable of capturing Hgin the flue gas.Owing to its wide availability,activated carbon is used commercially for Hgcapture in power stations.However,the narrow operating temperature window,adverse effects on fly ash products,and high cost constrain the further practical application of carbon-based materials(Ye et al.,2021).Similarly,despite the merits of availability,the relatively poor Hgcapture capability and stability of Ca-and fly ash-based adsorbents have driven researchers to develop alternative materials(Liu Z et al.,2020).Metal oxide is expected to become a potential Hgadsorbent material due to its merits of abundant natural reserves,wide operation operating temperature window,acceptable thermal stability,and ease of synthesis.When coupled with some other materials,an excellent Hgcapture performance can be obtained within a certain temperature range(Ye et al.,2021).CuO,a typical transition metal oxide,has some attractive physicochemical properties,such as tunable textural structures,abundant surface vacancies and reactive chemisorbed/lattice oxygen,and Lewis acidity,which make it suitable for Hgremoval reactions(Fang and Guo,2018).If diverse strategies,including chemical modification and substitution,are exploited,the functionalities of CuO could be improved (Chen et al

,2018;Yang et al

,2019c).Compared with carbon-,Ca-,and fly ash-based materials,CuO-based adsorbents are superior because of their wide operating temperature window,excellent stability,and satisfactory anti-poisoning ability (Fan et al

,2012;Chen et al.,2018;Galloway et al

,2018).Given these promising attributes,there has been a recent boom in research exploring the use of CuO-containing materials for Hgcapture at lab-scale.Many papers concerning aspects of Hgadsorption performance,mechanisms,and simultaneous removal of Hgwith other air pollutants have been published.The results demonstrate that CuO-based adsorbents have good prospects as potential adsorbents for the abatement of Hgin power plants.For these reasons,it is appropriate to review the advances in capturing Hgfrom coal-fired flue gas using CuO-based materials.Such knowledge could provide a guideline for effectively reducing mercury emissions.In this review,we cover progress in research on CuO-based materials for Hgremoval.First,the fundamentals of CuO,including crystal information and corresponding synthesis methods,are introduced.Based on the roles CuO plays in Hgremoval reactions,CuO-based materials can be divided into two categories,namely supported CuO adsorbents and CuO mixed oxide adsorbents.The performance of these two types of materials is illustrated next.Considering that there is a certain amount of NO,SO,HO,NH,and HCl in real coal-fired flue gas,the impacts of these gas species on adsorbent Hgremoval efficiency are summarized in Section 4.By generalizing the mechanisms dominating Hgremoval reactions,the ratedetermining step and key intermediates can be discovered.Aside from Hg,some other air pollutants,such as carbon monoxide,NO,and volatile organic compounds (VOCs),also account for a certain portion of emissions.In view of their similar abatement mechanisms,simultaneous removal of Hgwith other air pollutants has become a hot topic in the environmental field,and is introduced in Section 6.After flue gas enters the WFGD,re-emission of Hgalways occurs,which is detrimental to the effective reduction of mercury emissions.Thus,Hgabsorption performance under different conditions in this device is introduced in Section 7.Finally,we conclude that new adsorbents suitable for long-term operation in real coal-fired flue gas should be developed to realize the effective reduction of mercury emissions.

2 Fundamentals of CuO

2.1 Crystal information of CuO

CuO is a black metal oxide with a monoclinic structure (space group,

2/c).Each unit cell consists of four formula units (Fig.1).The Cucations and Oanions are located at the centers of inversion symmetry in a single fourfold site 4c(1/4,1/4,0)and site 4e(0,0.416(2),1/4)(Poizot et al

,2003).Lattice constants of CuO are presented in Table 1.No phase transition occurs when the pressure and temperature are lower than 70 MPa and 3000 K,respectively(Bourne et al

,1989).That is to say,in many cases CuO exhibits only a monoclinic phase,as confirmed by Radhakrishnan et al

(2014).Although CuO has a relatively high thermal stability,researchers have discovered that the particle size and lattice parameters of CuO continuously increase as the calcination temperature is elevated(Vidyasagar et al

,2012).

Table 1 Lattice constants of CuO(Poizot et al.,2003)

2.2 Preparation of CuO

Because CuO with other crystal structures,such as cubic and tetragonal phases,are rarely seen in catalysis and adsorption reactions,in this section we introduce the methods of synthesis of monoclinic CuO,the most common phase under normal conditions.It is well-recognized that mainly solution-based,solidstate,and electrochemical methods are adopted to prepare CuO materials.

2.2.1 Solution-based methods

A relatively low reaction temperature and pressure,together with a controllable morphology,composition and production,have led to increased attention on solution-based methods.Among the various solutionbased methods,hydrothermal/solvothermal,precipitation techniques are proven to be capable of synthesizing the target materials.

The preparation of CuO nanoparticles,also called 0D CuO materials,can be divided into two steps.First,cupric precursor reacts with a base solution,including ammonium hydroxide,NaOH and NaCO,to form Cu(OH).Then,the produced Cu(OH)is dehydrated at certain temperatures to obtain the final CuO product.Based on this concept,Neupane et al.(2009) and Chakraborty et al.(2011) used the inorganic precursor,Cu(NO),and the organic precursor,copper acetylacetonate,to successfully synthesize CuO nanoparticles with a size of several nanometers.For the synthesis of 1D,2D,and 3D CuO materials,in addition to a copper precursor and a precipitation agent,a surfactant/capping agent is involved.There are two different opinions regarding the mechanism of formation of CuO nanostructures with the use of a surfactant.The first proposes that the surfactant/capping agent is selectively adsorbed on certain facets of monoclinic CuO crystals,which then kinetically control the growth of the facets and hence the orientation of the crystals.The second proposes that the added surfactant/capping agent acts as a template for the formation of CuO materials with various morphologies.Besides,OHanions somehow influence the number of nuclei and the concentrations of growth units of the CuO crystals in reaction systems,which consequently determine the dimensions and morphologies of the final products.Li et al.(2010) used PEG(polyethylene glycol)200 as the capping agent to prepare CuO nanowires with an aspect ratio of several hundred.Given that PEG was present and the ratio of OH/Cuexceeded 4,ultra long CuO nanowires could be obtained.Otherwise,only CuO nanoleaves growing along the[111]direction were formed,confirming that the surfactant/capping agent and OHwere essential for the formation of CuO nanowires.Similarly,Yang et al.(2013) used CuSOas the copper precursor,PEG as the surfactant,NaOH and urea as the precipitants to prepare CuO materials with different morphologies(Fig.2).As the amount of PEG and CuSO,and the temperature in the precipitation stage were adjusted,the resulting CuO was transformed from a particle-shaped product to sheet-and rod-shaped products.This is because apart from the effect of the surfactant,ammonia coming from the decomposition of urea can facilitate the dissolution of CuO to a certain extent,and in turn have some impact on the recrystallization of CuO.This consequently contributes to the various dimensions and morphologies of the final products.

Fig.2 X-ray diffraction(XRD)patterns(a)and transmission electron microscope (TEM) images (b)-(e) of CuO particles with different shapes.Reprinted from (Yang et al.,2013),Copyright 2013,with permission from Elsevier

Generally,most cases related to the solutionbased synthesis of CuO materials are based on solutions containing Cusalt.But sometimes metallic Cu substrates have been used as the copper precursor to prepare CuO in the presence of some oxidative agents,such as (NH)SOand KSO.Zhang et al.(2008)fabricated CuO nanoflowers on Cu substrates using KSOas the oxidant under basic conditions at 70 ℃.When the reaction lasted for 15 min,materials with grass-like and flower-like structures coexisted.When the reaction process was ongoing,the relative concentration and size of nanoflowers increased.By extending the reaction time to 40 min,nanoflowers of 8-11 μm were obtained (Fig.3).It was claimed that Ostwald ripening could explain this morphology evolution process.Similarly,Liu et al.(2006) demonstrated fabrication of CuO nanostructures on Cu substrates using Oas the oxidative agent in ammonia-and NaOH-containing solutions.After reaction at 60 ℃for 2 h,CuO nanoflowers with assembled grass-like sheets were obtained.The longitude and transverse dimensions of the sheets were about 300 and 700 nm,respectively.

Fig.3 Scanning electron microscope (SEM) images of CuO nanostructures prepared after 25 min((a)and(b)),25 min ((c) and (d)),and 40 min ((e) and (f)).Reprinted from (Zhang et al.,2008),Copyright 2008,with permission from AIP Publishing

As mentioned above,thanks to the mild reaction conditions,mostly solution-based synthesis methods are used to prepare CuO materials with certain structures in a large scale.However,it always takes some time to separate the target product from the solution,which lowers the economy to some extent.

2.2.2 Solid-state methods

Compared with solution-based methods,separation of the final product and the supernatant can be avoided with solid-state methods,which saves time in obtaining the target materials.Solid state reaction and thermal oxidation of copper substrates are the main strategies used to successfully synthesize CuO nanostructures.Xu et al.(1999)used a one-step solid state reaction to prepare CuO particles.In this case,CuClwas adopted as the copper precursor and NaOH for precipitation.After grinding for 30 h,CuO was formed,as evidenced by the color change of the products from green to black.Similarly,Abboudi et al.(2011) prepared copper oxalate through the reaction between Cu(NO)and oxalic acid,and obtained CuO materials with calcination at high temperatures.In contrast,Balamurugan et al.(2001) used a conventional activated reactive evaporation(ARE)setup to prepare CuO particles.Briefly,copper vapor is passed through oxygen plasma with an oxidation reaction taking place;the produced CuO particles are then deposited in the chamber.Jiang et al.(2002) heated the cleared Cu substrates at between 400 and 700 ℃in air for 4 h and found that CuO nanowires grew perpendicular to the substrate surface.The diameter centered in the range of 30-100 nm and the length up to ＜15 μm.The vapor-solid (VS) mechanism,rather than the vapor-liquid-solid (VLS) mechanism,was dominant in the growth process of CuO nanowires.

2.2.3 Electrochemical methods

Apart from solution-based and solid-state methods,electrochemical techniques are also accepted to be an excellent way to prepare CuO nanomaterials,due to their merits of simplicity and a mild synthesis condition.Using these techniques it is also easy to control the morphology and size of the obtained products through tuning the experimental parameters,such as the current density and voltage.Yuan et al.(2007)developed an electrochemical route to prepare CuO materials with various morphologies.Here,NaNOsolution was taken as the electrolyte and metallic copper as the sacrificial anode.When at a low current density,uniform and mono-dispersed CuO rods or spindles were produced.When the current density was excessively high,irregular-shaped crystals were formed (Fig.4).This is because the growth of CuO followed three steps.First,Cu(OH)was generated,then Cu(OH)was dehydrated to tiny CuO particles.Finally,the formed CuO particles aggregated to form nanorods or nanospindles.As the current density was low,the rate of formation of CuO was at relatively low level,which resulted in a low concentration of CuO particles in the electrolyte,and consequently favored oriented aggregation with regular-shaped CuO materials produced.In contrast,a high current density would create a large amount of CuO particles and thus a random aggregation to form irregular-shaped crystals.Toboonsung and Singjai (2011) also used an electrochemical technique to synthesize CuO nanostructures.Stainless steel was adopted as the cathode,and copper plates as the anode.Deionized water was used as the electrolyte.Through tuning the voltage,electrode separation and deposition time,the production scale and physical structures of CuO nanorods and bundles could be well controlled.Similarly,Ulyankina et al.(2018)synthesized CuO powders via an electrochemical method,and by changing the average current density and the duty cycle,CuO octahedra,CuO polyhedral decorated with CuO,and CuO-CuO composites were successfully prepared.

3 Hg0 removal performance of CuO-based adsorbents

3.1 Supported CuO-based adsorbents

Here,CuO was used mainly as the active component and loaded on certain supports.To be the support of an adsorbent,a material should have the following characteristics.First,a relatively large specific surface area is essential so that the active species can be welldispersed.Second,the material should have satisfactory stability,to help stabilize the supported active components.Finally,anti-poisoning ability is needed to ensure the long-run of an adsorbent.Among the various materials,carbon,zeolites,and metal oxides are recognized as typical adsorbent supports.Table 2 summarizes the Hgremoval performance of some typical adsorbents.

Table 2 Hg removal performance of some typical adsorbents

represents the inlet concentration of Hg (μg/m); represents the Hg capture capacity(mg/g); represents the Hg removal efficiency(%).ppm:10;MOF:metal organic frameworks;WSWU:wheat straw char activated by water stream and microwave and ultrasonicated;MATP:magnetic attapulgite

3.1.1 CuO/Carbon-based adsorbents

As a commonly used porous material,activated carbon (AC) shows good potential for the abatementof Hgowing to its advantages of satisfactory mechanical strength and wide availability.Wang et al.(2012)prepared CuO/AC adsorbents using the wet impregnation method and tested their performance.They found that the capability of CuO/AC for Hgabatement was elevated by increasing the loading of CuO (1%-20%(in weight)),and at 180 ℃the Hgremoval capability reached a peak value of

ca.

38%(120-200 ℃).However,there are still some problems that need to be addressed:(1)Would the Hgremoval capability continue to increase with further increases in the loading of CuO? (2) Would the performance of CuO/AC exceed that of virgin CuO and AC? Based on CuO/ACbased adsorbents,Zhao et al.(2019) modified CuO/AC-H adsorbents with MnOand achieved more than 90% Hgremoval efficiency at 200 ℃as 5% MnOwas added.Here,AC-H was related to HNO-treated activated carbon.It was claimed that both lattice oxygen and reactive Mn-related intermediates had a positive effect on Hgabatement.Apart from AC,bio-char(BC) has also received considerable attention for Hgremoval because of its environmental protection and low cost.Tang et al.(2018)used Clactivated bio-char as the support and CuO-ZrOmixed oxides as the active species.When the loading amount of CuO-ZrOincreased to 10%,nearly 100%Hgremoval efficiency was reached and was maintained over 80% as the temperature was elevated to 240 ℃.It was proposed that interactions between CuO and ZrO,resulting in stronger oxidizability and improved texture properties,contributed to the superior performance of the Cu-Zr/Cl-BC adsorbent.Considering the separation problem of the spent adsorbents from fly ashes,Yang W et al.(2019) developed magnetic Fe-Cu oxide doped biochar adsorbents by microwave/ultrasound activation.When the molar ratio of Cu/Fe was fixed at 0.3 and the loading value of Fe-Cu oxide raised to 10%,the average Hgremoval efficiency reached 90.58% at 130 ℃.The microwave and steam activation optimized the pore structures of the adsorbents,thereby promoting Hgremoval.This adsorbent also has a certain magnetic property,suggesting that it could be

effectively separated from the flue gas using simple physical methods.

Graphitic carbon nitride (g-CN),a typical kind of 2D carbon material,is beginning to be used as a support for Hgremoval owing to its high chemical and thermal stability,and abundant feedstock.When CuO was introduced as the active species,nearly 100%Hgremoval efficiency was obtained at between 80 and 200 ℃,which was almost double that of raw g-CN(Liu et al

,2018a).During the Hgremoval process,Hgwas activated mainly by the chemisorbed oxygen,which originated from the O-C-N of g-CNand the lattice oxygen of CuO.Adding CuO can efficiently activate g-CNthrough the Mott-Schottky effect at the interface of CuO and g-CN,thereby significantly improving the Hgremoval capability(Liu et al.,2018b).

3.1.2 CuO/Zeolite-based adsorbents

Owing to their large specific surface area,abundant pore structures,and plentiful acid sites,zeolites are now accepted as another appropriate adsorbent support for Hgremoval.As in other pollutant removal reactions,ZSM-5 is used as the support with some metal oxides introduced as the active species and promoters.In the case of CuO/ZSM-5 adsorbent,about 70% Hgremoval efficiency was reached at 120 ℃.When CeOor LaO,the traditional promoters,were added,adsorbent Hgcapture performance was elevated to different extents (Liu DJ et al.,2017).The ability to remove Hgcomposites increased by about 60%after introducing 50%CeO,and by 18%after adding 10% LaO.This could be attributed to a cooperative effect between CuO and the doped promoter.

Apart from the properties of the promoters,the loading of CuO and the molar ratio of Si/Al also significantly affected the activity of the adsorbents (Fan et al.,2012).When the Cu loading value was elevated,adsorbent Hgremoval efficiency showed an upward trend,reaching a peak value of about 90% as the loading value of Cu reached 6% (in weight).Further increases in the loading value of Cu led to a decline in Hgremoval efficiency.This could be ascribed to the destruction of the tiny pores of the walls and blockage of internal porosity of the material by the excess doped copper.Unlike other researchers,Galloway et al.(2018) discovered that introducing copper species increased the Hgremoval efficiency of SSZ-13 only from 58.9% to 61.7%,demonstrating that SSZ-13 itself was primarily responsible for the abatement of Hg,and that CuO acted only as a promoter-like substance.As the molar ratio of Si/Al increased from 25 to 100,the adsorbent Hgremoval capability continuously decreased.This could be explained by a decline in the concentration of Cu,which was responsible for the high Hgremoval by the materials,as the Si content increased.

3.1.3 CuO/Metal oxide-based adsorbents

TiOand AlOare the main metal oxide supports used for Hgremoval because of their acceptable thermal stability and relatively large specific surface area.Xu et al.(2014) synthesized CuO/TiOusing a wet impregnation method.By changing the loading of CuO,7% CuO/TiOwas found to be the optimal material,with an Hgremoval efficiency of 98% at between 50 and 300 ℃.Li et al.(2017)observed that the Hgremoval efficiency of CuO/TiOincreased from 90% to 99% at 200 ℃with the modification of CeO.The elevated Hgremoval efficiency of CeO-CuO/TiOcould be explained mainly by the synergistic effect of copper oxide and cerium oxide promoting the formation of active chemisorbed oxygen,thereby accelerating the oxidation of Hg.Like TiO,adsorbents with AlOas the support also showed good prospects for application.Zhao et al.(2017) impregnated CuO and MnOonto an AlOsupport surface to form CuO-MnO/AlOadsorbent.The supported bimetallic adsorbent performed better than the monometal oxide adsorbents CuO/AlOand MnO/AlO.This is because although MnOwas a strong oxidant,its oxidation rate was quite low,and the added CuO accelerated the oxidation process,thereby achieving an elevated Hgpick capacity.As the content of CuO and MnOreached 20%,the adsorbent showed an Hgpick capacity of 12%(in weight),and the Hgconcentration could be reduced from more than 11600 to 10in a single pass.By modifying CuO-MnO/AlOwith FeO,over 70% removal efficiency was obtained in the first 10 h and no deactivation took place in a 3-d run,which suggests good potential for industrial applications(Wang et al.,2013).

3.1.4 Other CuO-supported adsorbents

Faced with the drawbacks of some traditional materials,such as their low-density of active sites and low adsorption capacity,porous metal organic frameworks (MOF),are attracting increasing attention in catalysis,adsorption and separation due to their open active sites,abundant functional groups and large specific surface area.Zhang et al.(2021b) prepared Cu-MOF materials using an ultrasonic-assisted hydrothermal method and found that the resulting adsorbent had an average Hgremoval efficiency of ＞90%and a equilibrium adsorption capacity of 123.5 mg/g,which is much higher than that of some commercial activated carbon materials.Later,Zhang et al.(2021a) used iron species to modify Cu-MOFs and obtained bimetallic iron-copper based MOFs.Compared with Cu-BTC and Fe-MIL-53,the Hgremoval efficiency of FeCu-MOFs was almost doubled.The synergistic effect of iron and copper species facilitated the electron transfer process.This helped to provide an increased number of active sites and hence contributed to a satisfactory Hgremoval performance.Apart from MOFs,attapulgite (ATP)could be an appropriate material for Hgremoval due to the advantages of satisfactory thermal stability and large specific surface area.When modified with FeO,MnO,and CuO,adsorbent Hgconversion showed an upward trend (Long et al.,2022).Here,FeOwas regarded as the magnetic source,while MnOand CuO functioned as the active components for the oxidation of Hg.As the CuO loading reached 5%,over 90%Hgremoval efficiency was obtained.Further increases in the CuO content led to a decline in Hgconversion,as excess CuO would cover the active sites of the adsorbents,thus impairing the Hgremoval process.

3.2 CuO mixed oxide-based adsorbents

Unlike the supported CuO adsorbents,CuO mixed oxides are always synthesized using co-precipitation or sol-gel methods.The homogeneous dispersion of the components ensures the sufficient occurrence of chemical interactions.The different valence states and radii of the constituent metal atoms promote the formation of surface oxygen vacancies to activate gaseous Oand the distortion of the lattice to bring about an improved texture structure.This somehow facilitates the capture of Hg.MnO,a typical metal oxide,is recognized as an appropriate dopant for the optimization of the adsorbent physicochemical properties because of its abundant surface vacancies and variable types of labile oxygen.Given that CuO is partially substituted by MnO,an increase in the adsorbent Hgremoval efficiency occurs.The adsorbent showed the best Hgcapture performance when 40% (in weight)CuO was replaced.About 70%Hgremoval efficiency could be maintained after 3 h.Further increasing the substitution of CuO led to a reduction in Hguptake.This could be ascribed to the formation of CuMnOphase in the Mn-modified sample.The high valence of Mn facilitates the oxidation of Hg,and CuO accelerates the transformation of Mn from a low to a high valence state (Fig.5).This was primarily responsible for the elevated Hgremoval performance of 40%MnO-modified adsorbent (He et al

,2018).In view of the superior Hgremoval performance of Mn-Cu mixed oxides,Yang R et al.(2019) synthesized a series of CuMnOmaterials using the sol-gel method.Hgcapture capability increased with increasing Cu content;over 95% Hgremoval efficiency was obtained between 50 and 350 ℃when the

value was equal to 1.The higher concentrations of surface copper species and chemisorbed oxygen,together with the larger specific surface area,contributed to the efficient abatement of Hgfrom the flue gas.Later,Wang Z et al.(2020)compared the Hgcapture performance of CuMnOwith that of NiMnOand ZnMnO.They found that CuMnOspinel still had a higher Hgadsorption capability:over 95%Hgremoval efficiency was observed at 200 ℃,about 30% higher than that of NiMnOand ZnMnO.

4 Effects of gas components

In general,apart from Nand Owhich account for about 80%of real coal-fired flue gas,there are certain amounts of NH,SO,NO,HCl,and HO.Ois the gas component typically involved in the evaluation of adsorbent Hgremoval performance under ideal conditions,and is proven to promote the capture of Hg.This is because Ocan replenish the consumed lattice or chemisorbed oxygen species and re-oxidize the reduced metal ions.This accelerates the oxidation of Hgand consequently facilitates the removal of Hg(Zhang et al.,2021a).Like O,some other species,including SOand NO,would also have some impact on the Hgcapture capability of adsorbents.This has been extensively studied by many researchers.Therefore,in this section,the effects of these other gas species on Hgremoval by some typical materials is summarized(Table 3).

Table 3 Effects of certain gas species on Hg removal performance by some typical materials

Table 3 (continued)

RSU:ultrasonicated rice straw chars;WSU:ultralsonicated wheat straw char

4.1 SO2

SO,a typical acid gas,comes from the combustion of sulfur-containing coals and accounts for several hundreds of ppm in coal-fired flue gas.Because of its relatively high reactivity,most adsorbents are quite sensitive to SO.As reported in the literature,both adverse and positive impacts of SOon the Hgremoval performance of the adsorbents have been detected.

Many papers have reported that SOexerts a certain inhibitory effect on Hgremoval.Yang W et al.(2019)observed that given an increase in the SOconcentration,adsorbent Hgremoval efficiency showed a downward trend (Fig.6).Xu et al.(2018) also discovered a negative impact of SOon Hgremoval.Moreover,this adverse effect was enhanced as SOwas at a high concentration.It is well-accepted that formation of inert metal sulfates (Eqs.(1) and (2))and competitive adsorption of Hgand SOon adsorbent active sites constitute the main reasons for the decline in Hgremoval efficiency in flue gas containing SO(Wang HN et al

,2020;Ye et al.,2021).

Faced with this serious problem,some researchers optimized the formula of the adsorbents to improve SOresistance.Mei et al.(2008) found that for CuCoOmixed oxide,the SOanti-poisoning ability was improved,with the

value continuously increasing from 0.75 to 2.25.Yang R et al.(2019) used the“redox-precipitation”method to prepare MnO-CuO mixed oxide adsorbent,which showed some sulfur tolerance.After exposure to flue gas containing 1200 ppm SOfor about 50 min,adsorbent Hgremoval efficiency decreased by 15%.It was reported that CuO was preferentially consumed by SOwhile MnOsurvived and compensated for the loss of the active sites.Similarly,multiple active sites (Cu and C sites) and multiple oxidizing media (Cl and O) gave Cu-MOFs good tolerance to SO.Increasing the SOcontent to 800 ppm reduced Hgremoval efficiency only slightly from 91.5% to 89.6% (Zhang et al.,2021b).Apart from the adsorbent formula,certain gas species can weaken the negative effect of SOon Hgremoval.Chen et al.(2018) found that when HCl and Oin the flue gas were at a high concentrations,the influence of SOcould be negligible.Zhang HC et al.(2020) discovered that the addition of 21 ppm SOwas capable of increasing the Hgremoval efficiency of Cu/ZSM-5 from 82% to 95% at 200 ℃.When the temperature was elevated to 400 ℃,the positive effect of SOon Hgremoval was enhanced.In this case,SOcould inhibit the adsorption of SOand function as a new site to capture Hgwith the formation of HgSO.

Apart from the above-mentioned inhibitory impact,sometimes SOcan facilitate the removal of Hgto some extent.Wang et al.(2013) observed that in the presence of 5% O,adding 1200 ppm SOsignificantly lowered the outlet concentration of Hgfrom about 63.0 to 47.1 μg/m.This is because CuO in CuO-MnO-FeO/AlOcatalyzes the oxidation SOto SO,which probably serves as an S-bonded chemisorption site for the abatement of Hg.

4.2 NO

NO constitutes an inherent component of coalfired flue gas.In view of its relatively high concentration and reactivity,NO was proven to be capable of influencing the Hgremoval performance of adsorbents to different extents.Like SO,both positive and negative effects on Hgremoval were observed.Long et al.(2022) found that NO could significantly promote the capture of Hgover MnCu-MATP adsorbent.Purging 500 ppm NO elevated the Hgremoval efficiency from 81.1% to 94.6%,which further increased to 98.4%with 6%O.It was claimed that the NO,NO,and NO,formed through reaction between NO and adsorbent oxygen species,were primarily responsible for the promoted oxidation of Hgto HgO,thereby facilitating the removal of Hg(Eqs.(3)-(6)).After increasing the NO concentration to 1000 ppm,no obvious variation in the adsorbent Hgremoval performance could be detected.Xu et al.(2018)also observed that as the NO concentration increased from 0 to 800 ppm,the Hgremoval efficiency of CeO-and CuO-modified rice straw chars increased from 44.79%to 94.74%.The formation of reactive nitrates or nitrites contributed to the improved Hgremoval performance of the adsorbents(Eqs.(3)-(6)).In contrast,Zhang et al.(2021b)discovered that when the concentration of NO increased from 50 to 600 ppm,the Hgremoval efficiency of Cu-MOF adsorbent declined slightly.The excessive NO would lead to competitive adsorption with Hg,which would limit any further facilitation of Hgremoval.

4.3 H2O

HO,accounting for around 10% of real coalfired flue gas,is another primary gas component with a crucial role to play in air pollutant abatement reactions.It is well recognized that in most cases HO has an adverse impact on the adsorption of Hg.For most MOF-,zeolite-,and carbon-based adsorbents,water vapor poisoning phenomena can always be observed(Chen et al.,2018;Xu et al.,2018;Long et al.,2022).Similar to the negative effect of SO,competition for the active sites over the adsorbents between Hgand HO mainly explains the decline in Hgremoval efficiency in the presence of HO.In contrast,Yang W et al.(2019) discovered that for iron-copper oxide modified porous char,introducing 4% HO had a certain promotional effect on Hgremoval.The hydroxyl radicals originating from the decomposition of water molecules would participate in the oxidation of Hgto HgO,which mainly contributes to the increased Hgremoval efficiency(Eqs.(7)and(8)).After increasing the HO content to 12%,similar phenomena could be obtained,showing a downtrend in the removal efficiency of Hg.Formation of a water film on the adsorbent surface could also explain the inhibitory effect of HO on Hgabatement.

4.4 HCl

HCl,considered a typical acid gas with high reactivity,accounts for about tens of ppm in real coalfired flue gas,and also affects the Hgremoval performance of the adsorbents to some extent.Most researchers observed that purging a certain amount of HCl into the reaction system could elevate the Hgremoval efficiency (Xu et al.,2014,2018;Du et al

,2015;Zhang et al.,2021a).It was claimed that HCl would interact with the adsorbent chemisorbed oxygen to form active chlorine species,which then accelerates the oxidation of Hgand enhances its removal (Eqs.(9)-(14)) (Yang YJ et al

,2017).Some researchers found that HCl could have an inhibitory effect on Hgremoval under certain conditions.Wang Z et al.(2020)observed that the Hgremoval efficiency of CuMnOspinel adsorbent obtained in the presence of 10 ppm HCl increased by 14.7% compared with that obtained in N.After further increasing the HCl concentration,a decline in the adsorbent Hgcapture capability occurred.When an appropriate amount of HCl was added,formation of reactive chlorine species mainly contributed to the facilitated oxidation of Hg.When the concentration of HCl was excessively high,it could not be efficiently converted to active chlorine species because of the limited chemisorbed oxygen available.Thus,the residual HCl would occupy the surface active sites and consequently constrain Hgcapture (Yang et al

,2014).Similar phenomena were obtained by Wang et al.(2013).In the presence of O,an enhancement in Hgremoval took place through adding HCl.In contrast,without O,HCl inhibited the abatement of Hg.Formation of reactive chlorine species and competitive adsorption of HCl and Hgcould well explain the effect of HCl on Hgremoval over CuO-MnO-FeO/AlO.

4.5 NH3

NH,a typical reducing component from the deNOsystems,is also proven to significantly affect the Hgremoval process over the adsorbents.Unlike SO,NO,HCl,and HO,no promotional effect of NHon Hgremoval has been reported.NHexerts only a small positive effect,or even a negative effect,on the abatement of Hg.Wang et al.(2013) discovered that given a purge of 200 ppm NHinto the reactor,Hgremoval efficiency declined by

.25%.Similarly,competition between Hgand NHfor the active sites and surface oxygen can mainly explain the inhibitory impact of NH.Li et al.(2021) found that NHhad little effect on Hgremoval over CuO/TiO(Fig.7).It was claimed that CuO is a borderline acid,which weakly interacts with NHleaving plenty of active sites available for Hgremoval.That enables CuO/TiOto achieve excellent Hgremoval performance in the presence of NHat lower temperatures.When the temperature increases,NHwould be oxidized to NO,which is the reactive species contributing to Hgremoval.Hence,a novel NH-tolerant ability is obtained.Over CuO-CeO/TiO,NHalso presented no obvious inhibitory effect on Hgremoval(Li et al.,2017).However,once NO and NHwere simultaneously added to the flue gas at a molar ratio of 1,deactivation of Hgconversion occurred.In this case,NO and NHcould react directly with HgO to form Hgand N.The reduced copper and ceria species in the selective catalytic reduction (SCR) reaction of NO would rob the oxygen species from HgO,resulting in a decline in Hgremoval efficiency.A similar adverse impact of SCR reactants (NO and NH) was also obtained over Cu-SSZ-13.Such knowledge is of fundamental importance for the simultaneous abatement of multi-air pollutants.

5 Hg0 removal mechanisms

Both physisorption and chemisorption are involved in Hgremoval reactions (Yang W et al.,2017;Yang Y et al.,2019).At relatively low-temperatures,physisorption sometimes took place,during which process adsorbent surface vacancies were mainly involved(Yang et al

,2011a,2011b).As the temperature rises,chemisorption becomes dominant.By tuning the components of the flue gas,different products can be obtained.Chen et al.(2018)found that in the absence of HCl,Hgwas removed mainly in the form of HgO.When HCl is present in the flue gas,Cu species in Cu-BTC adsorbent would interact with HCl to form an intermediate,namely CuCl or CuCl.Hgcould be oxidized with chemisorbed oxygen or activated chlorine species to form the final product,HgCl.Variation in the final product of the adsorbed mercury species indicated that purging HCl into the flue gas changed the intermediate in the Hgcapture process,which might have brought about the different removal mechanisms.Therefore,in this section,detailed Hgremoval routes over adsorbents in the presence or absence of HCl will be discussed.

5.1 Hg0 removal mechanisms in the absence of HCl

When HCl is absent from the flue gas,the capture of Hgis highly dependent on the reactivity of oxygen species and the kinds of functional groups on the adsorbent surfaces.As a consequence,the main products of the captured mercury species are always HgO and Hg-OM (M represents the carbon functional groups).Xiang et al.(2012) used a density functional theory (DFT) method to investigate the adsorption mechanism of Hgover CuO and observed that Hgcould physisorb onto the O-terminated surfaces,and chemisorb onto Cu-terminated CuO (110) surfaces(Fig.8).Compared with physisorption,chemisorption plays a dominant role in Hgcapture.Chemisorption always involves certain mechanisms,such as Deacon and Mars-Maessen mechanisms,and thus provides some guidelines for the investigation of the Hgcapture route on adsorbents.Among all the sites,Cutop was the most favorable for Hgadsorption,with an adsorption energy of-116.76 kJ/mol.Yang et al.(2019c)studied the active sites and the Hgadsorption mechanisms over CuMnO,a new kind of promising adsorbent.An excellent O-activation ability was found over CuMnO.The activation energy of the Odissociation reaction over CuMnOwas calculated to be 6.31 kJ/mol.Both chemisorbed oxygen molecules(O) and atoms (O) coming from the dissociation of Owere involved in the oxidation of Hg.In the whole mercury adsorption-oxidation-desorption process,HgO desorption constituted the rate-determining step (Fig.9).For CuFeOadsorbent,the oxidation of the adsorbed Hg(Hg(ad)) to the adsorbed HgO by chemisorbed oxygen was the rate-determining step in Hgabatement cycles,owing to the higher energy barrier of 116.94 kJ/mol (Yang et al

,2022).These results revealed that variation in the formula of the adsorbents can change the energy barrier of certain elementary reactions,hence leading to the different rate-determining steps.

Fig.8 Optimized geometries of Hg0 adsorption on Cu-terminated CuO (110) surface:(a) Cusuf top and bridge site;(b) Cusub top site;(c) hollow site.The salmon pink,red,and gray spheres denote Cu,O,and Hg atoms,respectively.References to color refer to the online version of this figure.Reprinted from (Xiang et al.,2012),Copyright 2012,with permission from Elsevier

Fig.9 Hg0 removal mechanisms over CuMn2O4.The reaction pathways and relative energies for mercury oxidation by chemisorbed oxygen atom O＊ (a) and oxygen molecule O2＊ (b) over CuMn2O4 surface. Ea:activation energy.Reprinted

Apart from DFT methods,some characterization experiments were also proven to be effective to uncover the Hgcapture mechanisms over the adsorbents.Liu DJ et al.(2017) explored variation in the valence states of the serial elements in CuO/ZSM-5 and observed that the Mars-Maessen mechanism dominated in the Hgcapture cycles.The physisorption of Hgon the adsorbent surface through the van de Waals force constituted the first step.Then lattice oxygen of the active component CuO was released and participated in the oxidation of Hgto HgO,accompanied by the reduction of Cuto Cu(Eqs.(15)-(17)),which was regarded as the chemisorption process.Zhang et al.(2019)also observed that Cuin CuO/montmorillonite acted as the active sites for the oxidation of Hg,and the adsorbed mercury species were present mainly in the form of HgO.Unlike virgin CuO or zeolite-based adsorbents,the complex physicochemical properties of carbon-based materials can make the adsorbed mercury species exist in various forms,thereby creating some different Hgcapture pathways.For copper-iron mixed oxide-modified biochar materials,some functional groups,namely carbonyl and carboxyl groups,could serve as the active sites to oxidize Hgto Hg-OM compounds.Lattice oxygen and chemisorbed oxygen of CuO and FeOparticipates in the direct oxidation of Hgto HgO(Eqs.(18)-(21))(Jia et al.,2018),giving Fe-Cu/BC a satisfactory Hgremoval performance.

5.2 Hg0 removal mechanisms in the presence of HCl

When HCl is present in the flue gas,the as-formed chlorine species with a relatively high reactivity can interact with Hg,forming HgCl.Variation in the final products of the captured mercury species must originate from the changed intermediates and the potentially different reaction routes.Zhang et al.(2021b)concluded that the Hgremoval process over Cu-MOFs followed the Langmuir-Hinshelwood mechanism (Fig.10).First,unsaturated active Cu or C sites were responsible for the physisorption of Hg,which was then oxidized to HgCl,with Cureduced to Cu.Finally,the reduced Cuwas re-oxidized by Oand HCl,forming a complete Hgoxidation cycle.When iron species are introduced to form bimetallic adsorbents,the Langmuir-Hinshelwood mechanism was still found to dominate during the whole Hgcapture process(Eqs.(22)-(30))(Zhang et al.,2021a).Zhang Q et al.(2020) used Hg balance,kinetics,and transient reactions to investigate the removal mechanisms of Hgover CuO/TiOin the presence of HCl.They discovered that HgO adsorbed on the adsorbent surface hardly reacted with HCl to form HgCl.Also,the reaction order of Hgbetween 300 and 400 ℃with respect to the concentration of gaseous Hgwas much lower than 1.This suggested that the Langmuir-Hinshelwood mechanism,rather than the Mars-Maessen,Deacon,or Eley-Rideal mechanisms,could explain the Hgremoval route on CuO/TiO,similar to the above-mentioned results.However,opposite conclusions were drawn by Du et al who suggested that the Mars-Maessen mechanism could explain the Hgremoval route over CuO-neutral AlO(Du et al.,2015).Active chlorine species (Cl) were formed via the dehydrogenation of HCl,which then oxidized Hgto HgCl.When the reaction temperature increased,the release step of Clwas accelerated,primarily contributing to an improved Hgcapture capability.In this process,CuOfunctioned as a catalyst through the redox shift of Cu↔Cu.

6 Simultaneous removal of multiple pollutants

Apart from Hg,other air pollutants,such as NO,VOCs,and carbon monoxide,are also present in real coal-fired flue gas.Considering the similar removal mechanisms of these pollutants,it has become popular to remove multiple pollutants simultaneously with the use of certain materials.The reduction of operating costs and space requirements further attracted attention to the simultaneous removal of multiple pollutants.

6.1 Simultaneous removal of Hg0 and NOx

Nitrogen oxides (NO) are another serious air pollutant emitted from power stations.Owing to the similar material properties,namely oxidizability and acidity,involved in the selective catalytic reduction of NHand oxidation of Hg,it is reasonable to attempt to remove Hgand NOsimultaneously from coalfired flue gas using an appropriate catalyst/adsorbent.Wang HY et al.(2019) modified the commercially used deNOcatalyst,VO-WO/TiO,with CuO.When the content of CuO increased,catalyst Hgremoval efficiency first increased to the peak value of

ca.

100%at temperatures of 280-360 ℃,and then slightly decreased.A slight variation in the deNOefficiency was noted,with Nselectivity maintaining

ca.

100%,indicating an excellent NOand Hgabatement capability.Sun et al.(2021)prepared CuO/Fe-Ti and investigated its deNOand Hgremoval performance.In this case,Fe-Ti mixed oxide was taken as the support and CuO was recognized as the active species.When the CuO loading was increased,material SCR deNOactivity always stayed at a satisfactory level.About 100% NOremoval efficiency was obtained between 300 and 450 ℃.The introduced CuO also facilitated the formation of Clradicals,which overpowered the negative effect of the slightly inhibited physisorption of Hg,thus contributing to a high Hgoxidation rate of 6.8-8.7 μg/(g·min).Similarly,Cu-Ce-Zr-O mixed oxide was deposited onto the AlOsupport surface to obtain a catalyst to simultaneously remove Hgand NO(Yue et al

,2019).With the molar ratio of Cu:Ce:Zr fixed to 1.40:0.55:0.25 and the loading amount of Cu-Ce-Zr-O mixed oxide set as 15%,deNOefficiency of 93% and Hgremoval efficiency of 85% were observed.Low crystallinity,optimized textural structures,strong acid properties,and oxidative ability mainly explained the excellent simultaneous Hgand NOremoval performance of the catalyst.Apart from the formula,the existing forms of the main active species,copper species,also significantly affected the performance of the materials.Wang Y et al.(2019)varied the content of Cu in Cu-SAPO-34 material and found that the Cu ions isolated inside the pores acted as the active sites for the SCR reactions,while crystallite CuO was primarily responsible for the oxidation of Hg.Through tuning the loading amount of Cu,both Cuand CuO increased,leading to an increased removal efficiency of NOand Hg(Fig.11).

6.2 Simultaneous removal of Hg0 and other air pollutants

Like Hg,CO is a toxic,persistent air pollutant that is of great harm to the environment.Considering that oxidative ability also plays a crucial role in the abatement of CO,it seems that the goal of simultaneous removal of Hgand CO should be achievable.Gao et al.(2021) deposited the binary mixed oxide CuO-CoOonto an HNO-pretreated activated carbon surface to form CuO-CoO/ACcatalyst.When the contents of CuOand CoOwere set to 2% and 10%,respectively,complex oxides,including CuO,CuO,CoO,CoO,and CoO phases,co-existed.In this catalyst,Co-species were responsible for the oxidation of CO,and Cu-species contributed to the removal of Hg.With increasing reaction temperature,CO removal efficiency exhibited an upward trend,which reached

ca.

94.7% at 200 ℃.The effect of reaction temperature on the material Hgremoval performance was negligible.

Formaldehyde (HCHO),a typical kind of VOC,also constitutes a hazardous substance discharged from coal-fired power plants.Yi et al.(2018) adopted an impregnation method to synthesize Cu-Mn mixed oxides,supported on biochar materials,for the simultaneous removal of Hgand HCHO.When the molar ratio of Cu/Mn was set to 1:1 and the mixed metal oxide loading value was fixed at 12%,the highest removal efficiencies of HCHO and Hg(89%and 83%,respectively) were obtained at 175 ℃.The strong synergistic effect between MnOand CuOplays an important role in the abatement reaction of HCHO and Hgbecause of the redox cycle of Mn+Cu↔Mn+Cu.

7 Hg0 removal in WFGD

Through the use of adsorbents,Hgin the flue gas can be oxidized to Hg.When flue gas passes through electrostatic precipitators,some Hgcan be captured,leaving the residual Hgto enter WFGD devices and finally be removed by the absorbents(Lim et al

,2020,2021,2022).When Hgis dissolved,the existing reductive ligands and compounds reduce Hgto Hg,resulting in the re-emission of Hg(Eqs.(31)-(34))(Hsu et al

,2021).

Among the various processing parameters,pH,temperature,ion concentration,and metal concentration have a significant impact on the amount of Hgreleased(Hsu et al.,2021).When the pH value exceeds 6,SOis favorable in solution and tends to react with Hgto form the unstable product HgSO,which will spontaneously decompose to Hg(Omine et al

,2012;Ma et al

,2014).With increasing SOconcentration,the formation of Hg(SO),which is less stable than HgSO,is facilitated,thus contributing to increased Hgre-emission (Blythe et al

,2010).These two sulfite formation reactions are proven to be exothermic and spontaneous.Therefore,the decomposition of Hg(SO)and HgSOwill be promoted at a high temperature point(van Loon et al

,2001).Chlorine compounds are a typical component of the absorbents and slurry ofWFGD systems(11-15 g/L).Cleasily complexes with Hgto form HgCl,HgCl,and HgCl,which have higher stability than Hg(SO)and HgSO.Clcan also compete with SOto react with Hgand thus interfere with the reaction between Hgand SO(Bessinger et al

,2012;Devarajan et al

,2018).Also,the formed ClHgSOelevates the stability of Hg,which can be regarded as an effective method to control the re-emission of Hgin WFGD devices(Peng et al

,2016).Like Cl,Br,F,and I are also capable of exerting an inhibitory effect on Hgre-emission.The effect of these halides follows the order I＞Br＞Cl＞＞F(Hsu et al.,2021).As a typical component of flue gas,Ocan indirectly affect the reduction of Hgand the re-emission of Hg.The mostly accepted influence mechanism of Ois the oxidation of SOin solution to SO.Hgfirst interacts with SOforming HgSO,and then complexes with SOto form a moderately stable product HgSO,which consequently leads to a decline in the re-emission of Hg(Chang et al

,2017;Hsu et al

,2019).Note that in the absence of SO,an unstable product,Hg(SO),will be produced through the complexation between two moles of SOand one mole of Hg,which in turn elevates the re-emission of Hg(Powell et al

,2005).Like O,As compounds also exert dual effects on Hgre-emission.When the concentration of As is in the range of 0.02-0.55 mg/L,HAsOfunctions as a reducing agent to reduce Hgto Hg.With the concentration of As shifting to 0.55-0.76 mg/L,a complexation reaction between two moles of HAsOand one mole of Hgoccurs with the formation of stable Hg(HAsO),making Hgstay in the liquid phase(Liu et al

,2017).Unlike Oand As,Fe and Cu,the main metal components in coals,play only the role of a catalyst to reduce Hgto Hg,which subsequently creates an increased Hgre-emission(Bogacki et al

,2018;Gingerich et al

,2018).Little information is available regarding economic analysis of the application of CuO-based materials for Hgremoval and effective absorption of Hgin WFGD,and extensive research should thus be conducted(Choi et al

,2020;Lee et al

,2021,2022).

8 Conclusions and perspectives

When the formula and preparation methods of adsorbents are properly selected,the corresponding physicochemical properties are greatly optimized,leading to an enhancement of Hgcapture capability.Considering that certain gas species somehow influence Hgremoval efficiency,appropriate modification should ensure the potential long-term operation of the adsorbents.Besides,by investigating Hgabatement mechanisms under different conditions,the ratedetermining step could be discovered,which can provide a guideline for the development of new adsorbents with excellent performance.In WFGD,once the reaction conditions,such as pH,temperature,ion concentration,and metal concentration,are optimized,the re-emission of Hgcan be inhibited,hence achieving a relatively high system Hgremoval efficiency.

At lab-scale,Hgcan be effectively captured,and even multiple air pollutants can be simultaneously removed by some reported materials.However,at pilotscale and full-scale,there is still some doubt as to whether Hgand other air pollutants can be thoroughly abated.The competitive adsorption mechanisms of multiple air pollutants onto the adsorbent surface active sites still needed to be explored.Knowledge of such effects would be beneficial for further increasing the corresponding pollutant removal efficiency.Apart from SO,NO,HO,NH,and HCl,there are other species in real coal-fired flue gas,such as arsenic,phosphorus,and lead compounds.The effects of these species on the adsorbent Hgcapture performance also needs investigation.Provided these species would deactivate the adsorbents,the related anti-poisoning and regeneration techniques should also developed to achieve the goal of the effective reduction of mercury emissions.Finally,a techno-economic assessment of the application of CuO-based materials for Hgremoval and effective absorption of Hgin WFGD should be carried out because knowledge of such effects lays a solid foundation for effective reduction of Hgemissions.

Acknowledgments

This work is supported by the Scientific Research Foundation of China Jiliang University and the Zhejiang Provincial Natural Science Foundation of China (Nos.LQ22E060003 and LY22E040001).

Author contributions

Dong YE wrote the first draft of the manuscript.Xin LIU and Run-xian WANG helped to organize the manuscript.Dong YE,Xiao-xiang WANG,Hai-ning WANG,and Hui LIU revised and edited the final version.

Conflict of interest

Dong YE,Xiao-xiang WANG,Run-xian WANG,Xin LIU,Hui LIU,and Hai-ning WANG declare that they have no conflict of interest.

Journal of Zhejiang University-Science A(Applied Physics & Engineering)

2022年7期