Removal of elemental mercury from flue gas using microwave

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Removal of elemental mercury from flue gas using microwave/ultrasoundactivated Ce-Fe magnetic porous carbon derived from biomass straw Ye Shan, Wei Yang, Ying Li, Hui Chen, and Yangxian Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01940 • Publication Date (Web): 08 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019

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Energy & Fuels

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Removal of elemental mercury from flue gas using microwave/

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ultrasound-activated Ce-Fe magnetic porous carbon derived

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from biomass straw Ye Shan, Wei Yang, Ying Li, Hui Chen and Yangxian Liu*

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School of Energy and Power Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, China

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Abstract: Magnetic adsorbent shows good development prospects for separation of Hg0 from flue gas because it

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can be recycled. In this work, a novel magnetic biomass porous carbon adsorbent was developed by loading active

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ingredients (Ce and Fe mixed oxides) on renewable maize straw carbon with large specific surface area.

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Microwave activation and ultrasound treatment were applied to improve porous structure of maize straw carbon

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and distribution of active components. The influence of process parameters on Hg0 capture and removal

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mechanism were also investigated. The results reveal that CeFe11%(3/5)/MSWU700 possesses the optimal Hg0

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removing performance and adsorption capacity at 140 °C. The characterization results show that microwave

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activation can greatly increase the specific surface area of biomass carbon to form excellent porous structure, and

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ultrasound-assisted impregnation can facilitate the dispersion of active ingredients on the surface of adsorbent.

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The presence of Hg2+ on the surface of CeFe11%(3/5)/MSWU700 implies that chemisorption is occurred during

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the Hg0 removal process, which is also demonstrated by the well-matched pseudo-second-order reaction model. In

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the removal process of Hg0, the highly active chemisorbed oxygen was largely consumed, and the conversion of

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Ce4+/Fe3+ to Ce3+/Fe2+ was found. The magnetic adsorbent CeFe11%(3/5)/MSWU700 shows a large Hg0

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adsorption capacity (it is up to 7230.8 μg/g, which is far more than the common activated carbons and magnetic

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adsorbents), showing excellent application prospect.

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Keywords: Magnetism porous carbon; Maize straw; Hg0 removal; Cerium-iron metallic oxide; Flue gas

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*Corresponding

author phone: Tel.: +86 0511 89720178;Fax: +86 0511 89720178;E-mail: [email protected] (Y.X. Liu)

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ACS Paragon Plus Environment

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1. Introduction

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Mercury pollution has gradually became a fearful environment problem which threaten the human health

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owing to its persistence, volatility, toxicity and bio-accumulation1,2. On August 16th, 2017, ‘Minamata

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Convention on Mercury’ signed by 128 countries all over the world entered into force, which was conducted to

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control the global mercury emission3. It is well known that coal-fired power plants are the major source of

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anthropogenic mercury emissions, accounting for one-third of global mercury emissions4,5. It is reported that

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mercury in coal-fired flue gas is mainly existed in three forms: oxidized mercury (Hg2+), particle mercury (Hgp)

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and elemental mercury (Hg0)6. Most of the Hgp and water-soluble Hg2+ can be effectively removed based on

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existing dedusting equipments (e.g., ESP or FF) and wet flue gas desulfurization (WFGD) equipment,

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respectively7,8. However, the high volatility and low water solubility of Hg0 make it difficult to be captured by

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existing pollutant control equipment9.

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Current Hg0 removal technologies mainly concentrate on adsorption10-14 and oxidation15-19. Liu et al.20

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proposed the novel activated carbon based on the seaweed and found that the Hg0 removal performance of

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seaweed biomass-based activated carbons are much better than that of seaweed biomass-based the same pyrolysis

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chars. Shen et al.21 applied NTP treatment to introduce the activation gases (O2 and NO) onto the porous carbon,

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the obtained O2/NO co-doped porous carbon exhibited superior Hg0 adsorption ability. He et al.22 investigated the

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Hg0 capture performance of innovative Ce-Mn/Ti-PILC catalysts, and found that the Hg0 capture process

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combined adsorption and oxidation. Liu et al.23 developed a oxidation removal process of Hg0 using heat and

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Co2+/Fe2+ coactivated oxone oxidation system in a spraying reactor, they found that the produced free radicals

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(SO4•− and •OH) played a leading role in the oxidation of Hg0. Among these technologies, adsorption is

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recognized as the most promising technology for mercury capture because of simple process (e.g., the simplest

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process is based on simple flue injection & dust collector capture). However, after Hg0 capture, the used

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adsorbents/catalysts would be mixed with fly ash in flue gas and captured by dust removal equipment, which

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makes it quite difficult for the adsorbents/catalysts to be recovered from fly ash, greatly increasing the application

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cost and loss of the adsorbents/catalysts. It is reported that the separation of adsorbents/catalysts from fly ash can

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be achieved by introducing magnetic materials into adsorbents/catalysts under a additional magnetic field24,25.

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However, although magnetic separation between adsorbents/catalysts and fly ash can be achieved, the

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adsorbents/catalysts will be still lost to some extent in practical industrial applications, and thus the cost and loss

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of adsorbents/catalysts are still quite high based on the huge amount of flue gas treatment in coal-fired power

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plants (the flue gas flow of 300 MW unit is up to 106 m3/h).

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As a result, it is quite necessary to further reduce the amount of the active ingredients used to cut the cost of

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adsorbents/catalysts. Accordingly, the loading of the active ingredients on a cost-effective carrier that possesses a

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large specific surface area will be quite effective in reducing the amount of the active components used. Activated

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carbon is considered as the most common carrier due to its superior porous structure26, but its high-cost drawback

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seriously restricts its further development.

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Currently, agricultural straw wastes have attracted wide focus owing to its wide-source and low-cost. As a

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big agricultural country, the annual production of renewable maize straw in China reaches 200 million tons,

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accounting for about 25% of the total agricultural straw production. In addition, we compared the BET surface

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area of four common agricultural straws with microwave activation, the analysis results in the Supporting

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Information indicated that, under the same conditions, the maize straw obtained a much higher BET surface area,

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which is more suitable as a carrier material. However, the original agricultural straw biomass carbons usually

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exhibit relatively low specific surface area as compared to activated carbon. The results of Li et al.27 and Shen et

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al.28 had shown that microwave activation could effectively optimize the pore structure of biomass carbons, and

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increased the specific surface area. Accordingly, the microwave-activated maize straw biomass carbon possesses

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the great potential to replace the expensive activated carbon. In this regard, utilizing microwave-activated biomass

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carbons to support the active components may achieve the purpose of reducing the dosage of active ingredients.

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Recently, some materials (e.g., Fe3O4, γ-Fe2O3)29,30 have attracted much attention due to their good magnetic

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properties. However, single iron oxides usually exhibit poor Hg0 oxidation performance31. Many studies25,32 have

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reported that CeO2 plays a important role in the process of Hg0 oxidation because of its high oxygen storage

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ability and the special Ce3+/Ce4+ redox couple. Therefore, in this paper, Ce component was added into Fe-based

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magnetic catalyst to facilitate mercury oxidation performance of microwave-activated maize straw carbons.

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In this work, a novel magnetic biomass porous carbon adsorbent was proposed by loading active ingredients

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(Ce and Fe mixed oxides) on renewable maize straw carbon with large specific surface area. Microwave

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activation and ultrasound treatment were applied to improve the porous structure of the maize straw carbon and

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distribution of active components. The physical and chemical properties of the prepared adsorbents were

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determined by a variety of characterization methods (e.g., proximate and ultimate analysis, BET, SEM-EDS, XRD,

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VSM and XPS). The effects of process parameters (different preparation methods, loading value of Ce-Fe, molar

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ratio of Ce/Fe, calcining temperature and reaction temperature) on removing performance of Hg0 are investigated.

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Besides, the Hg0 removing mechanism, adsorption kinetics and regeneration performance are also studied. These

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results will provide the necessary reference for the development of low-cost and efficient magnetic Hg0 removing

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adsorbent and the utilization of biomass straw.

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2. Experimental

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2.1. Sample preparation

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The preparation of the adsorbent mainly consists of three steps: pyrolysis, microwave activation and

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chemical modification. The details of sample preparation are shown in Supporting Information. The sample after

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pyrolysis and microwave activation were tagged as MS and MSW, respectively. The final adsorbent was tagged as

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CeFeα(β)/MSWUγ, in which α represents the mass ratios of Ce-Fe elements to MSW (6%, 11% or 16%), β

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represents the molar ratios of Ce/Fe (2/5, 3/5 or 4/5), γ represents the calcining temperatures (300 °C, 500 °C or

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700 °C), U refers to the ultrasound treatment.

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2.2. Sample characterization

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The physical and chemical properties of the prepared adsorbents were determined by a variety of

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characterization methods, including proximate and ultimate analysis, BET, SEM, XRD, VSM and XPS. The

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details of various characterization methods are described in the Supporting Information.

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2.3. Experimental apparatus and procedure

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Figure S1 shows the schematic diagram of the fixed-bed system which is used to test the Hg0 removing

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performance of the samples. The system mainly contains the following sections: Coal-fired flue gas simulation

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system, the adsorbing and reacting system, the temperature control heating system and the analytical train and tail

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gas treatment system. The details of the experimental process are present in the Supporting Information.

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3. Results and discussion

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3.1. Sample characterization

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3.1.1. Proximate and ultimate analysis

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The results of the proximate and ultimate analysis of the raw maize straw are summarized in Table S2. The

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original maize straw mainly includes a majority of the volatile (66.15%) and fixed carbon (21.9%) as well as a

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small amount of moisture (6.33%) and ash (5.62%). The high content of volatile and low content of ash are

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reported to be favourable for the pore-creating during the the pyrolysis process33,34. The ultimate analysis results

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are list in Table S2.

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3.1.2. BET analysis

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The pore properties ( BET surface area, total pore volume and average pore diameter) of various samples are

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listed in Table 1. It can be found that MS shows a relatively poor BET surface area (SBET) (48.97 m2/g) and pore

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volume (0.061 cm3/g), while the sample with microwave activation (MSW) exhibits good pore structure with the

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relatively larger SBET (468.52 m2/g) and pore volume (0.343 cm3/g). The significant improvement in the pore

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structure of the microwave-activated sample may be due to the release of the volatile and ash ingredients in MS

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during the activation27. When 6% active Ce-Fe ingredients are introduced into the MSW, the SBET and pore

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volume have a further raise to 686.18 m2/g and 0.507 cm3/g, respectively. This indicates that some new pores

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might be formed result from the reaction between the Ce-Fe ingredients and MSW35. However, as the further

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increase of the Ce-Fe loading value, the SBET and pore volume of the sample have a evident drop. This might

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because some Ce-Fe mixed oxides block the partial pore36. When the molar ratio of Ce/Fe is 3/5, the SBET and

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pore volume of the modified sample reach the maximum compared with CeFe11%(2/5)/MSWU700 and

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CeFe11%(4/5)/MSWU700. Besides, the assistance of ultrasound during the impregnation can increase the SBET of

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sample (increasing by 35%) as compared with that without ultrasound (CeFe11%(3/5)/MSW700), indicating that

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ultrasound treatment facilitates the optimization of pore structure37.

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Table 1 The BET surface area, total pore volume and average pore diameter of the adsorbents.

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Samples

BET surface area (m2/g)

Total pore volume (cm3/g)

Average pore diameter (nm)

MS

48.97

0.061

4.950

MSW

468.52

0.343

2.930

CeFe6%(3/5)/MSWU700

686.18

0.507

2.954

CeFe11%(3/5)/MSWU700

557.14

0.426

3.056


435.04

0.343

3.150


439.64

0.364

3.314


435.93

0.365

3.345

CeFe11%(3/5)/MSW700

414.81

0.375

3.620

3.1.3. XRD analysis

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According to the XRD spectra of the samples with different impregnation methods shown in Figure 1(a),

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only two peaks (28.6° and 31.1°) belong to Ce and Fe oxides are detected in CeFe11%(3/5)/MSWU700 (the

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sample with ultrasound treatment), whereas two additional weak peaks (30.3° and 35.6°) assigned to Fe oxides

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were observed in CeFe11%(3/5)/MSW700 (the sample without ultrasound treatment). It suggests that the

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assistance of ultrasound treatment during impregnation could enhance the dispersion of the active ingredients over

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the surface of sample37.

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Figure 1. XRD spectra of the samples (a) with different impregnation methods, (b) with different molar ratios of Ce/Fe, (c) at different calcining temperatures.

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Figure 1(b) presents the XRD spectra of the sample with different molar ratios of Ce/Fe. It can be observed

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that both CeO2 (28.6°) and Fe2O3 (31.1°) are detected over the surface of the samples with three different molar

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ratios of Ce/Fe. For CeFe11%(2/5)/MSWU700 and CeFe11%(4/5)/MSWU700, there are two other diffraction

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peaks (30.3° and 35.6°) corresponding to the Fe oxides as compared to CeFe11%(3/5)/MSWU700. It suggests that

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CeFe11%(3/5)/MSWU700 possesses the better interaction and dispersion of the active ingredients35.

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Figure 1(c) exhibits the XRD spectra obtained for the sample at different calcining temperatures. As shown

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in Figure 1(c), for the sample whose calcining temperature is 300 °C, there are no obvious diffraction peaks

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belong to Ce and Fe oxides. When the calcining temperature rises to 500 °C, two weak characteristic peaks at 2θ

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of 28.6° and 31.1° (correspond to CeO2 and Fe2O3, respectively) appear. Compared with the weak peaks of the

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sample calcined at 500 °C, stronger peaks from CeO2 and Fe2O3 are detected over CeFe11%(3/5)/MSWU700 (the

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sample calcined at 700 °C). This results imply that the calcining temperature has a significant influence on the

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formation of active ingredients (Ce and Fe oxides) and crystalline phases on the surface of sample.

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3.1.4. SEM-EDS analysis

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Figure 2. SEM photographs of (a) MSW, (b) CeFe11%(3/5)/MSW700, (c) CeFe11%(3/5)/MSWU700.

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Figure 2 presents the SEM images of MSW, CeFe11%(3/5)/MSW700 and CeFe11%(3/5)/MSWU700. It can

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be seen from Figure 2(a) that the microwave-activated sample (MSW) exhibits a well-developed pore structure

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with a little particulate attached. However, the rich pores of MSW are blocked to somewhat by a large number of

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round particles with the impregnation of Ce-Fe mixed oxides, as shown in Figure 2(b). This phenomenon is

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consistent with the results of BET analysis show in Table 1 (the SBET of MSW is larger than that of

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CeFe11%(3/5)/MSW700). For CeFe11%(3/5)/MSWU700 (the sample with ultrasound treatment), the blocked

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particles are cleared by ultrasound treatment, which accounts for a better pore structure as compared to

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CeFe11%(3/5)/MSW700. It indicates that the ultrasound treatment can optimize the pore structure, and lead to the

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increase of the SBET of the sample. According to the EDS photograph shown in Figure S2, Ce and Fe are detected

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in CeFe11%(3/5)/MSWU700, indicating that Ce-Fe active ingredients have been successfully loaded into the

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sample. It can be intuitively seen from Figure 3 that the distribution of Fe and Ce in CeFe11%(3/5)/MSWU700

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(Figure 3(a)(b)) is more uniform than that in CeFe11%(3/5)/MSW700 (Figure 3(c)(d)), which indicates that

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ultrasound treatment can improve the dispersion of active ingredients on the surface of the sample.

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Figure 3. Element mapping photographs of (a) Fe, (b) Ce of CeFe11%(3/5)/MSWU700, (c) Fe, (d) Ce of

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CeFe11%(3/5)/MSW700.

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3.1.5. XPS analysis

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Figure 4(a) shows the XPS spectra of O 1s for the fresh and used samples. It can be seen that O 1s spectra

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can be divided into three peaks. The peaks with the lowest blinding energy (530.6/530.8 eV) are allocated to the

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the lattice oxygen ([O]) in metal oxides38, the main peaks around 531.4/531.7 eV correspond to chemisorbed

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oxygen (O*) [25], and the peaks center at 532.5/532.6 eV are regard as molecular water39. The introduction of

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chemisorbed oxygen, which is considered as the most active oxygen during the Hg0 oxidation40 , is mainly

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attributed to the charge imbalance, vacancies and chemical bonds generated by metal oxides41,42. According to the

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valence state analysis summarized in Table S3, the peak area ratio of O*/[O] has a significant decline from 2.94 to

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1.51 during the Hg0 removing, indicating that chemisorbed oxygen participates in the removal of Hg0.

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Ce 3d XPS spectra presented in Figure 4(b) consists of two groups, each of which contains four sub-peaks,

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corresponding to Ce 3d3/2 states (marked as U) and Ce 3d5/2 states (marked as V), respectively38,43. Among them,

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U1/V1 are regarded as Ce3+, while the other six peaks are assigned to Ce4+. This implies that Ce3+ and Ce4+ are

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coexisting in the sample. Moreover, Ce3+ could generate charge imbalance, vacancies and chemical bonds, which

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would result to the increase of chemisorbed oxygen on the surface of the samples44. Based on the peak area ratio

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shown in Table S3, the ratio of Ce4+/Ce3+ is 2.89 for the fresh sample, suggesting that Ce4+ is the major form and 10


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it is reported to be beneficial for the oxidation of Hg0 41. After Hg0 capture, the ratio of Ce4+/Ce3+ drops to 2.29,

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indicating that a part of Ce4+ is reduced to Ce3+ during the Hg0 removing.

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Figure 4(c) gives the Fe 2p XPS spectra for the fresh and used samples. Three sub-peaks (belong to Fe 2p3/2)

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concentrate on 710.4/710.6 eV, 711.5/711.8 eV and 713.0/713.7 eV correspond to Fe2+, Fe3+(octahedral) and

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Fe3+(tetrahedral), respectively31,45. As shown in Table S3, the ratio of Fe3+/Fe2+ has a drop from 4.03 to 3.65 after

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removing Hg0. The result implies that the conversion from Fe3+ to Fe2+ occurs during the Hg0 capturing.

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The XPS spectra of Hg 4f for the used sample is shown in Figure 4(d). As shown in Figure 4(d), the XPS

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spectra of Hg 4f contains two different peaks. The peak center at 102.6 eV is assigned to Si 2p46, while another

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peak with lower blinding energy of 101.8 eV refers to Hg2+ 35. No peaks correspond to Hg0 are detected, indicating

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that the Hg0 adsorbed on the surface of sample is oxidized to Hg2+. Therefore, it can be speculated that chemical

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adsorption dominates the process of mercury removal31.

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197

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Figure 4. (a) O 1s, (b) Ce 3d, (c) Fe 2p, (d) Hg 4f XPS spectra of fresh and used CeFe11%(3/5)/MSWU700

3.2. Influence of process parameters on mercury removal

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201

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Figure 5. Influence of (a) different preparation methods, (b) loading value of Ce-Fe, (c) molar ratio of Ce/Fe, (d)

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calcining temperature, (e) reaction temperature on Hg0 removal performance. Test conditions: 5% O2, 400 ppm

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NO, 600 ppm SO2, 4 vol% H2O, 60 μg/m3 Hg0 vapor.

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3.2.1. Influence of different preparation methods

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In order to determine the superior preparation method for subsequent research, the Hg0 removing efficiencies

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over MS, MSW, CeFe11%(3/5)/MSW700 and CeFe11%(3/5)/MSWU700 are depicted in Figure 5(a). It can be

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observed from Figure 5(a) that, the Hg0 removing performance of MSW (microwave-activated adsorbent) has a

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dramatic raise from 32.4% to 76.6% as compared to MS. According to the results of BET analysis in Table 1,

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MSW possesses the great higher SBET and pore volume than MS, which facilitates the Hg0 adsorption on the

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surface of MSW27,28. With the doping of Ce-Fe active ingredients, the average Hg0 capturing efficiency advances

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from 76.6% (MSW) to 92.4% (CeFe11%(3/5)/MSW700). Moreover, the Hg0 capturing efficiency has a further

214

increase to 95.6% after the ultrasound treatment. Based on the BET analysis in section 3.1.2, the chemical

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modification and ultrasound treatment can further boost the pore structure, which is beneficial for the the Hg0

216

adsorption47. Besides, the results of XRD analysis in section ‘3.1.3’ and SEM-EDS analysis in section ‘3.1.4’

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imply that the ultrasound treatment can optimize the dispersion of Ce-Fe active ingredients on the surface of the

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adsorbent, which further facilitates the capture of Hg0 37. These results suggest that both microwave activation and

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ultrasound treatment can promote the removal for Hg0.

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3.2.2. Influence of Ce-Fe loading value

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Figure 5(b) presents the influence of different Ce-Fe loading values (6%, 11% and 16%) on the performance

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of Hg0 removing. As the loading value of Ce-Fe increases from 0% to 6% and 11%, the average Hg0 removing

223

efficiency advances from 76.6% to 88.9% and 95.6%, respectively. This is ascribed to the fact that the more

224

loading value of active ingredients can generate more chemisorption sites on the surface of the adsorbent, which is

225

advantageous for the Hg0 removing27. However, continuing to increase the Ce-Fe loading value from 11% to 16%

226

leads to a significant decline (e.g., the average Hg0 removing efficiency droping from 95.6% to 76.2%). This

227

phenomenon may be result from the diminution of the pore structure of adsorbents caused by excessive loading

228

value (the SBET drops from 557.14 m2/g to 435.04 m2/g as the Ce-Fe loading value rises from 11% to 16%), which

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obstructs the physisorption of the Hg0

48,49.

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follow up study.

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3.2.3. Influence of the Ce/Fe molar ratio

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Therefore, 11% is selected to be the optimal Ce-Fe loading value for

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The performance of Hg0 removal over the adsorbents with different molar ratios of Ce/Fe (2/5, 3/5 and 4/5)

233

is illustrated in Figure 5(c). As depicted in Figure 5(c), with the molar ratio of Ce/Fe rises from 2/5 to 4/5, the

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average Hg0 removing efficiency advances first and then drops, reaching a maximum of 95.6% when the molar

235

ratio of Ce/Fe is 3/5. According to the results of BET analysis in Table 1, CeFe11%(3/5)/MSWU700 exhibits the

236

optimal pore structure among the three adsorbents with different Ce/Fe molar ratios. It indicates that more active

237

sites will be introduced on the surface of CeFe11%(3/5)/MSWU700, which can raise the Hg0 removing

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performance50. In addition, compared with CeFe11%(2/5)/MSWU700 and CeFe11%(4/5)/MSWU700,

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CeFe11%(3/5)/ MSWU700 has been proved to possess the better interaction and dispersion of the active

240

ingredients35 based on the XRD results in Figure 1(b), which could improve the oxidation capacity of

241

CeFe11%(3/5)/MSWU70051. Thus, 3/5 is the optimal molar ratio of Ce/Fe in this study.

242

3.2.4. Influence of calcining temperature

243

A series of tests are carried out to investigate the influence of different calcining temperatures (300 °C, 500

244

°C and 700 °C) on the Hg0 removing performance. According to the results displayed in Figure 5(d), the average

245

Hg0 removing efficiency advances from 75.4% to 84.0% and 95.6% as the calcining temperature raises from 300

246

°C to 500 °C and 700 °C, suggesting that the calcining temperature has an advantageous effect on the removal of

247

Hg0. As observed in the results of XRD analysis from Figure 1(c), two strong peaks belong to CeO2 and Fe2O3 are

248

detected in the adsorbent calcined at 700 °C, while no evident or only two weak peaks are detected in the

249

adsorbents calcined at 300 °C and 500 °C, confirming that the active ingredients (CeO2 and Fe2O3) are generally

250

formed at 700 °C. Reddy et al.52 found that CeO2 could boost the oxidation of Hg0 due to its special Ce4+/Ce3+

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redox couple. Wang et al.35 reported that the doping of CeO2 and Fe2O3 into the AC could provide enough

252

oxidation performance for Hg0. However, high calcination temperatures also cause high energy consumption.

253

Therefore, in this study, 700 °C is chosen to be the best calcining temperature for subsequent research.

254

3.2.5. Influence of reaction temperature

255

The position of the adsorbent is injected before the ESP or FF, where the temperature of the flue gas ranges

256

from about 100 to 160 °C, thus the range of reaction temperature in this study is set to 100-160 °C. Figure 5(e)

257

compares the the Hg0 removing performance over the adsorbents at different reacting temperatures (100 °C, 120

258

°C, 140 °C and 160 °C). It can be seen that the Hg0 removing efficiency increases with rising the reacting

259

temperature from 100 °C to 140 °C, and the maximum average Hg0 removing efficiency (95.6%) arrived at the

260

reacting temperature of 140 °C. This phenomenon reveals that the appropriate raise of reaction temperature is

261

advantageous for Hg0 removing. Many studies53,54 reported that the higher reaction temperature can accelerate the

262

chemical reaction rate between mercury and active sites on the adsorbents. However, when the reacting

263

temperature rises to 160 °C, the average Hg0 removing efficiency sharply falls by 33%, from 95.6% (140 °C) to

264

63.8% (160 °C). This may be due to the desorption of mercury on the adsorbent by the excessive high temperature,

265

which obstructs the catalytic removal of Hg0 48,55. Thus, 140 °C is determined as the optimal reacting temperature.

266

3.3. Mechanism discussion

267

Combining the above characterization analysis and experimental results, the mechanism and the process of

268

the Hg0 removal are elucidated preliminarily. (A) The gas phase Hg0 (Hg0(g)) is captured on the surface of the

269

adsorbent to form the adsorbed Hg0 (Hg0(ads))49, and the O2 in the flue gas is converted into the chemisorbed

270

oxygen ((O*)) by charge vacancies44. (B) The adsorbed Hg0 can either react with Ce and Fe oxides to form Hg2+ or

271

be oxidized into HgO by chemisorbed oxygen ((O*))25,31,56. (C) In the presence of gaseous O2, the reduced Ce3+

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and Fe2+ are reoxidized and the largely consumed chemisorbed oxygen is replenished during the process of

273

regeneration31,56. The relevant reactions can be described as follows:

274

Hg 0 ( g )  Hg 0 ( ads )

(1)

275

O2 ( g )  2O *( ads )

(2)

276

Hg 0 ( ads )  O *( ads )  HgO( ads )

(3)

277

2Ce 4  Hg 0 ( ads )  2Ce 3  Hg 2 ( ads )

(4)

278

2 Fe3  Hg 0 ( ads )  2 Fe 2  Hg 2 ( ads )

(5)

279 280

1 1 Ce 3  O2 ( g )  Ce 4  O 24 2 1 1 Fe 2  O2 ( g )  Fe3  O 24 2

(6) (7)

281

Furthermore, it is worth noting that the actual coal-fired flue gas or pyrolysis gas is very complex in

282

composition, usually containing nitrogen oxides, hydrogen sulfide, particulate matter, sulfur oxides, halides,

283

heavy metals, VOCs, etc.57-66, These components will have a competition with the Hg0 for the adsorption sites or

284

produce some other side reactions, which have certain effect on the removal of mercury12,49. Therefore, the future

285

work should focus on exploring the effect of mercury removal in actual flue gas components.

286

3.4. Magnetic separation and regeneration performance

287

In addition to good mercury removal efficiency and magnetic separation performance (Figure S3),

288

regeneration performance is also an important indicator of the potential adsorbent. In this study, the used

289

adsorbent is firstly heated at 400 °C for 1 h under N2 atmosphere to decompose mercury covered on the active

290

adsorption, and then cooled to room temperature. After that, it is further regenerated at 250 °C for 30 min in an air

291

atmosphere to replenish the consumed oxygen species. According to the regeneration performance under five

292

cycles exhibited in Figure S4, the Hg0 removing efficiency has a slight drop (about 4%) during each cycle, but it

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remains 82.5% after five cycles. This result suggests that CeFe11%(3/5)/MSWU700 presents a good performance

294

in regeneration.

295

3.5. Mercury adsorption model

296

In order to further investigate the the mechanism of the Hg0 adsorption process, adsorption model is

297

conducted in this work. The analysis results mentioned above reveal that chemisorption plays a significant role in

298

the process of Hg0 removal. Many studies27,64 suggested that the pseudo-second order model can be applied to

299

match the date of chemisorption process. In this regard, the Hg0 adsorption processes over MS, MSW and

300

CeFe11%(3/5)/MSWU700 were matched by the pseudo-second order model to verify the occurrence of

301

chemisorption. The introduction of adsorption model is shown in the Supporting Information.

302

According to the Hg0 adsorption capacity (qt) over various samples illustrated in Figure S5(a), the

303

corresponding matching curves of the pseudo-second-order kinetic model are exhibited in Figure S5(b)(c)(d),

304

respectively. It can be obviously observed that t/qt value of CeFe11%(3/5)/MSWU700 is in good agreement with

305

the pseudo-second order model, while those of MS and MSW are on the opposite. Based on the parameters

306

summarized in Table S4, the obtained R2 value of CeFe11%(3/5)/MSWU700 (0.9967) is quite higher than that of

307

MS (0.8937) and MSW (0.9014), proving that the Hg0 adsorption process over CeFe11%(3/5)/MSWU700 can be

308

properly described by the pseudo-second order model. Accordingly, it can be considered that the reaction of Hg0

309

capture over the modified adsorbent CeFe11%(3/5)/MSWU700 is mainly controlled by chemisorption whereas

310

MS and MSW do not65, which is consistent with the XPS analysis shown in Figure 4(d).

311

3.6. Comparison of mercury adsorption capacity

312

In order to evaluate the application of the prepared magnetic adsorbent, Table S5 summarizes the comparison

313

of the Hg0 adsorption capacity for CeFe11%(3/5)/MSWU700 with several other common activated carbons (ACs).

314

It can be observed that CeFe11%(3/5)/MSWU700 possesses a good adsorption capacity of 7230.8 μg/g, which is

17


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315

much higher than that of various modified ACs and magnetic adsorbents. This result suggests that

316

CeFe11%(3/5)/MSWU700 has the great potential to replace activated carbon.

317

4. Conclusion

318

In this study, the magnetic active ingredients (Ce and Fe mixed oxides) are supported on the microwave-

319

activated biomass porous carbon derived from renewable maize straw using ultrasonic impregnation method, and

320

the obtained novel adsorbents are used to remove Hg0 from flue gas. The experimental data shows that

321

CeFe11%(3/5)/MSWU700 possesses the good Hg0 removing efficiency (95.6%) and adsorption capacity (7230.8

322

μg/g) at 140 °C. The characterization results suggest that the microwave activation can greatly increase the

323

specific surface area of the maize straw carbon to form a porous structure, and ultrasound-assisted impregnation

324

can improve the dispersion of active ingredients on the surface of sample. The Hg0 adsorption process over

325

CeFe11%(3/5)/MSWU700 can be properly described by the pseudo-second order model, indicating chemisorption

326

occurs during the process of Hg0 removing, which is consistent with the XPS analysis results. In the removal

327

process of Hg0, the highly active chemisorbed oxygen was largely consumed, and the conversion of Ce4+/Fe3+ to

328

Ce3+/Fe2+ was found. The magnetic adsorbent CeFe11%(3/5)/MSWU700 shows a large Hg0 adsorption capacity

329

(it is up to 7230.8 μg/g, which is far more than the common activated carbons and magnetic adsorbents), showing

330

excellent application prospect.

331

Acknowledgement

332

This study was supported by National Natural Science Foundation of China (Nos.U1710108; 51576094).

333

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Graphical Abstract

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Removal of elemental mercury from flue gas using microwave

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