Elemental mercury removal from flue gas by CoFe2O4 catalyzed peroxymonosulfate

Accepted Manuscript Title: Elemental mercury removal from flue gas by CoFe2 O4 catalyzed peroxymonosulfate Authors: Yi Zhao, Xiaoying Ma, Peiyao Xu, Han Wang, Yongchun Liu, Anen He PII: DOI: Reference:

S0304-3894(17)30553-8 http://dx.doi.org/doi:10.1016/j.jhazmat.2017.07.047 HAZMAT 18739

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

27-5-2017 18-7-2017 20-7-2017

Please cite this article as: Yi Zhao, Xiaoying Ma, Peiyao Xu, Han Wang, Yongchun Liu, Anen He, Elemental mercury removal from flue gas by CoFe2O4 catalyzed peroxymonosulfate, Journal of Hazardous Materialshttp://dx.doi.org/10.1016/j.jhazmat.2017.07.047 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Elemental mercury removal from flue gas by CoFe2O4 catalyzed peroxymonosulfate

Yi Zhao*, Xiaoying Ma, Peiyao Xu, Han Wang, Yongchun Liu, Anen He

School of Environmental Science & Engineering, North China Electric Power University, Beijing 102206, P. R. China

*Corresponding author. Tel: +86-0312-7525513; fax: +86-0312-755513. E-mail address: [email protected] (Y. Zhao)

Graphic abstract Hg0 ·OH

Hg2+

CoFe2o4

O

.

PMS

O

s

O

O

H 2O

SO4 OH

·OH Hg0

-

CoFe2o4

OH OH O O

-·

s O

Hg0

Hg2+

1

O O

.

Highlights   

A magnetic CoFe2O4 catalyst was firstly used in air pollutant control; A new method of removing Hg0 was developed based the catalytic oxidation process. Removal mechanism was proposed according to the characterizations and literatures.

ABSTRACT A magnetic cobalt ferrite (CoFe2O4) catalyst was prepared by sol-gel method, and characterized by a X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM), Brunauer-Emmett-Teller (BET) and hysteresis loop method. The chemical states on surface of the fresh and spent catalysts were analyzed by a X-ray photoelectron spectroscopy (XPS). The experiments of elemental mercury(Hg0) removal from flue gas were conducted in a laboratory scale using activated peroxymonosulfate (PMS) catalyzed by CoFe2O4, and the effects of the dosage of catalyst, the concentration of PMS, initial solution pH and reaction temperature on mercury removal efficiency were investigated. The average removal efficiency of Hg0 could maintain steady at 85% in 45 min when the concentrations of CoFe2O4 and PMS were 0.288g/L and 3.5mmol/L respectively, solution pH was 7 and reaction temperature was 55℃. In order to speculate the reaction mechanism, ethyl alcohol and isopropyl alcohol were used as the quenching agents to indirectly prove the existence of SO4•− and ·OH.

Keywords: Hg0 removal; peroxymonosulfate; CoFe2O4 catalyst

2

1. Introduction Mercury is a kind of toxic trace metal, it can do serious harm to humans and other advanced evolved creatures. The concentration of mercury in the environment has been increasing continually, which has drawn great attention of national governments

and

environmental protection organizations

and has

become a

worldwide environmental issue. It was reported that coal-fired power plants were the most important mercury emission resource [1], so aiming to that issue, the emission concentration of mercury was first limited in Power Plant Air Pollutants Emission Standards (GB 13223-2011) released in July of 2011, which regulated the concentration of mercury and mercury compounds discharged by the coal-fired power plant could not higher than 0.03mg/m3. It is generally acknowledged that there are three forms of mercury in coal-fired flue gas, which are elementary mercury (Hg0), oxidation state mercury (Hg2+) and particulate mercury (HgP) [2], and using flue gas purification device to synergistically remove mercury is considered as the most potential mercury removal method. Dust removal devices such as electrostatic precipitators and fiber filters can remove HgP; as a flue gas denitration system, selective catalytic reduction (SCR) make some Hg0 oxidize to water-soluble Hg2+ that can be absorbed in wet flue gas desulfurization (WFGD) device. In general，the wet flue gas desulfurization device can absorb all kinds of mercury compounds except for Hg0, so how to oxidize Hg0 into Hg2+ in flue gas is the key issue to realize mercury synergetic removal by wet flue gas desulfurization (WFGD) device. Several methods used to remove Hg0 such as adsorption, traditional chemical oxidation, advanced oxidation, and catalytic oxidation were widely investigated [3]. 3

Among of them, the traditional chemical oxidation methods, including gaseous phase oxidizing technologies using O3 [4] and Cl2 [5], liquid phase oxidizing technologies employing ferrate(VI) solution [6-7] and gas-like phase oxidizing technologies [8-10] were commonly attempted. However, some reported processes have disadvantages of higher reagent prices and releasing secondary environmental pollutants, such as O3 and Cl2. In recent years, the catalytic oxidation methods based on the ·OH [11] have been concerned. To enhance the production of ·OH and Hg0 oxidation efficiency, Liu [12] conducted the experiment to remove Hg0 in flue gas using classical Fenton reagent and results suggested that Hg0 removal was affected by the solution pH, NO, SO2 and CO32- concentrations, Zhou [13] established Fe2.45Ti0.55O4/H2O2 advanced oxidation processes to improve Hg0 removal. Compared to OH, SO4•- had the advantages of strong oxidation ability, well stability and high utilization rate of oxidizing agent and innocuous, thus it was widely applied in the field of water environmental pollution control. Recently, SO4•− resulting from the thermally activated (NH4)2S2O8 [14] and H2O2/Na2S2O8 [15] system have been used to efficiently remove Hg0 from flue gas. Meanwhile, SO4•− can also produced by catalytic activating potassium peroxymonosulfate (PMS) in the presence of catalysts containing cobalt, and can induce a series of free radical chain reaction which attacks pollutants in solution until oxidizes the them to the highest valence state oxides. It was reported that the catalysts containing cobalt such as Co2+ [16], cobalt oxide [17], cobalt oxide supported catalyst [18], CoFe2O4 [19], copper cobaltate [20] and so on were the most widely studied, in which, the CoFe2O4 had the advantages of high surface catalytic activity and lower prepared cost, and was usually used to catalyze PMS to degrade organics in industrial wastewater [21-23]. However, so far, 4

researches on Hg0 removal from flue gas by using CoFe2O4 have not been report. The goal of our research is to use the prepared magnetic CoFe2O4 to activate PMS and investigate the effects of various factors such as the dosage of catalyst, concentration of PMS, solution pH and reaction temperature on Hg0 removal, and propose a new process of Hg0 removal to avoid the disadvantages of high operation cost and disposal of hazardous discarded activated carbon. Meanwhile, to speculate the Hg0 removal mechanism, CoFe2O4 will be characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), Brunauer-Emmett-Teller (BET) and hysteresis loop method, and the experiments of quenching free radicals and the analyses of removal products will also be carried out.

2 Experimental Apparatus and Methods 2.1. Reagents All the chemicals were analytical grade and were used as received (Fuchen and Guangfu Reagent Manufactory, Tianjin), including ferric nitrate（Fe(NO3)3.9H2O）, cobalt nitrate（Co(NO3)2.6H2O）, peroxymonosulfate （KHSO5.0.5KHSO4.0.5K2SO4）， nitric acid（HNO3），sulfuric acid (H2SO4)，sodium hydroxide（NaOH），potassium permanganate（KMnO4），and anhydrous ethanol. 2.2. Preparation and characterization of catalyst During preparation, 60 ml egg white was put in a 250ml beaker with string for 30 min to be butyrous. Ferric nitrate (Fe(NO3)3.9H2O) of 8.0800g and cobalt nitrate (Co(NO3)2.6H2O) of 2.9103g were dissolved in water and then mixed with the butyrous egg white and stirred 2 h, and then dried by distillation in a water bath at 80 ℃, so that the reddish brown gel was formed. The gel was calcined at 500 ℃ for 5 h in muffle furnace, then quenched by the mixture of ice and water, and grinded. Hence, a black power, CoFe2O4 catalyst was obtained. A X ray diffractometer (XRD, Bruker D8 Advance) equipped with a graphite 5

monochromator filter, with the scanning area of 5-90°, the measurement accuracy of ±0.0001 °, the monitoring wavelength of 0.154nm and scanning speed of 0.02°/S was used to analyze the phase of the prepared catalyst. The magnetic property of catalyst was detected by a magnetic property measurement system (MPMS, SQUID VSM， Quantum

Design).

As

N2 was

an

adsorbate,

the

isothermal

curve

of

adsorption/desorption for CoFe2O4 catalyst was measured by a surface area and pore size analyzer (Beckman Coulter SA 3100, USA) at different pressure, in which each sample was out-gassed under vacuum for 2 h at 150 ℃ before the measurement. The Brunauer-Emmett-Teller (BET) method and the Barrett-Joyner-Halenda (BJH) model of the adsorption isotherm were used to calculate the surface area, pore size distribution, and pore volume of the catalyst. The surface morphology of catalyst was observed using a scanning electron microscope (SEM, Hitachi S-4800, Japan) and the chemical states on surface of the fresh and spent catalysts were analyzed by a X-ray photoelectron spectroscopy (XPS, ESCALAB250 spectrometer). The dissolved quantity of CO2+ in the absorption liquid was tested by inductively coupled plasma source mass spectrometer (ICP-MS Agilent 7700，USA), and Hg2+ in the spent absorption liquid was determined by an atomic fluorescence spectrophotometer (AFS-933, Beijing Jitian). 2.3. Experiments of mercury removal The experimental devices are shown in Figure 1, which have four main parts: flue gas simulation and gas flow control system, mercury vapor generator system, catalytic oxidation reaction system and mercury detection system. In the experiments, one of gas circuit was N2 that carried mercury vapor into the catalytic oxidation reaction device, and the other was also N2 that balanced the total gas flow to 6

1 L/min. The mercury vapor generator used in the experiments was a mercury penetration tube (permeability of 101ng/m3, VICI, USA) immersed in a constant temperature oil bath, it could provide stable mercury vapor concentration under the constant pressure and flow rate. The catalytic oxidation reaction of Hg0 occurred when mercury vapor passed a bubbling reactor containing catalyst and PMS solution (Fig.1, 9) with the volume of 500 mL. during the reaction, The temperature was controlled by a heat-collected constant temperature heating magnetic stirrer (DF-101S, Henanyuhuan, China) and the pH absorption liquid was adjusted by HNO3 and NaOH. Then the tail gas was induced to a coal-fired flue gas mercury measurement instrument (QM201H, Suzhou Qingan, China) with the accuracy of 1µg /m3± 0.1µg/m3 and variation coefficient ≤ 8% to measure the remained Hg0. In the measurement, the continuous measuring mode was adopted at the gas flow of 0.6 L/min. The removal efficiencies were calculated according to the concentrations of Hg0 before and after reaction (Eq. 1). η= ( 1-C out / C in )×100%

(1)

where, η is the removal efficiency of Hg0, %;

C

in

and C

out

are the

concentration of Hg0 at the inlet and outlet of the reactor respectively, mg/m3.

3. Results and discussion 3.1. Characterization of the magnetic catalyst 3.1.1 . Analysis of XRD The XRD patterns of CoFe2O4 crystal shown in Figure 2 suggest that the prepared CoFe2O4 catalyst has the classical spinel structure and the classical diffraction peaks appear at 2  of 18.28°, 30.14°, 35.52°, 37.08°, 43.16°, 53.44°, 56.973°, and 62.58°, respectively, and the corresponding prague crystals are (111), 7

(220), (311), (222) ,(400), (422), (511), and (440), and the diffraction peaks in the patterns agree with those of standard XRD patterns (JCPDS 22-1086), in which no diffraction peaks of Fe2O3, CO2O3 and other oxides are discovered. The data also show that CoFe2O4 has Fd3m space group (227) that belongs to the ferrite cubic crystal system. The results are consistent with the data in the paper of Lee [24]. According to Scherrer formula (β = 0. 89λ /Dcosθ, where λ is the x-ray wavelength, D is the average crystal size.) and the peak width at half height of (311) diffraction peak ( 2θ = 35. 52°), the average crystal size of CoFe2O4, D was calculated as 35 nm， which was similar to that of Ren [25]. 3.1.2. Analysis of magnetism Figure 3 shows the magnetic properties of CoFe2O4 at the room temperature, in which the saturation intensity of magnetization (Ms), residue intensity of magnetization and coercivity of the catalyst are 41.62, 17.92 emu/g and 4.2×104Am-1, respectively, indicating that the catalyst has the obvious hysteresis behavior. It was reported [26] that the smaller the coercivity was, the catalyst was easier to magnetize in applied magnetic field and to loss magnetism out of applied magnetic field, thus the phenomenon of large particles magnetic reunion could be avoided. It is found that from the experimental phenomena that after attracting for less than 1 min, the solution contained the spent catalyst become clarification that suggests that the catalyst is almost recycled completely, which means that it is very convenient to the magnetic recovery of the catalyst to make the recycle use of the catalyst. 3.1.3 .Analysis of morphology of the catalyst Though the average crystal size of CoFe2O4 calculated by Scherrer is 35 nm, the average crystal size of the CoFe2O4 catalyst prepared in reality is about 1um, which 8

may be due to the agglomeration between the minicrystals during the preparation, as shown in Figure 4. Figure 4 also shows that, the surface of the catalyst is irregularly granulated which presents porous structure that can increase the contact area between CoFe2O4 and reactant to improve the catalytic performance. By measuring, the BET surface area of CoFe2O4 is 10.0630 m²/g, it is proximate to the data in the issue of Du [21]. The N2 adsorption and desorption curves of CoFe2O4 are shown in Figure 5 that conforms to the curve of type II according to the adsorption materials classification of IUPAC. Figure 5 shows that the adsorption amount increases slowly with an increase of partial pressure (P/P0) at the relative pressure between 0.2 and 0.9, and at this area the hysteresis loop appears but not very obviously, which means that the mesoporous structure is in few and the porous distribution has a wide range. The adsorption amount increases sharply in relative pressure range of 0.9 to 1.0, which may be because that the pilled pores are formed between the particles. 3.2. Mercury removal experiments 3.2.1. The effect of CoFe2O4 dosage As shown in Figure 6, the Hg0 removal efficiency is only 28.9% when there is no catalyst in solution containing PMS. However, the Hg0 removal efficiencies increased from 28.9% to 68.1% rapidly in the CoFe2O4 concentration range of 0 to 0.048 g/L, which was because that the surface of CoFe2O4 existed a lot of active cobalt (≡Co2+) that could catalytic activate PMS to produce a kind of strong oxidizing species, SO4•− (equation 2)[19]. From the viewpoint of electrochemistry, the redox potential of SO4•− (E0 =2.6V vs NHE) is higher than that of •OH (E0 =1.8VvsNHE) obviously, which suggests that Hg0 can preferentially be oxidized to water-soluble Hg2+ by SO4•− . ≡Co2+ +HSO5−→≡Co3+ +SO4•− +OH− 9

(2)

FeOH2+ formed from the iron atom in the structure of CoFe2O4 contributed to the formation of CoOH+ from Co2+ (eqs. 3-5) [19], and CoOH+ was the essential specie in the process of catalytic activation of PMS to form SO4•−, thus the eqs. 4-5 were the control step of mercury removing [27]. Fe3++ H2O → FeOH2++ H+

(3)

Co2++ FeOH2+→ CoOH++ Fe3+

(4)

CoOH++HSO5-→COO++SO4-.+H2O

(5)

The Hg0 removal efficiency reached the highest level (about 85%) in the CoFe2O4 concentration range of 0.048 to 0.288g/L, and the Hg0 removal efficiency began to decrease slowly when the addition of CoFe2O4 was larger than 0.288g/L, which may be by that with the increase of the addition of CoFe2O4, the active point of ≡Co2+ in reaction system and the rate of producing SO4•− by active oxidation of PMS all increased. Hence, the mercury removal efficiency increased. However, the excess catalyst might decrease the effective specific surface area of solid-liquid contraction so that decrease the rate of the formation of SO4•−. Moreover, a part of SO4•− might also reduced to SO5•− of lower activity [28], as shown in equation (6). The optimal dosage of the catalyst was set as 0.288g/l that was lower than those in the other similar works [12-13, 29]. HSO5−+SO4•−→ SO5•−+SO42−+H+

(6)

3.2.2. The effect of PMS concentration PMS is the precursor of producing SO4•− that affects the oxidation and removal of Hg0 directly. The effect of the concentration of PMS to the removal of Hg0 is shown in Figure 7, which suggests that the efficiency of Hg0 removal increases first and then decreases with the PMS concentration increasing. In the neutral medium, 10

increasing the concentration of PMS meant the concentration of formed SO4•− by CoFe2O4 catalyzing on the surface of catalyst also increased, thus the mercury removal efficiency increased from 73 to 85% in the PMS concentration range of 1.0 to 3.5 mmol/l. However, when the concentrations of PMS was between 3.5 and 6.5 mmol/l, the removal efficiency of Hg0 had a little decrease, which may attribute to that when the dosage of catalyst was constant, the production of SO4•− was restricted at the fixed active point number. And more importantly, according to previous work [28], the excess HSO5- might react with SO4•− to form SO5•− that had lower reactivity (equation 7), so the mercury removal efficiency decreased with PMS concentration increasing. The optimal PMS concentration was determined as 3.5 mmol/l accordingly. 3.2.3. The effect of reaction temperature As shown in Figure 8, the Hg0 removal efficiency reaches 70% when the temperature is 25 ℃, which is because that PMS can be decomposed to form a small amount of SO4•− at the present of catalyst. When the temperature was between 20 and 55 ℃ the Hg0 removal efficiency increased obviously from 70 to 85%. According to the reference, the bond length of O-O in PMS molecule is 0.1453nm [30], and the bond energy is between that of H2O2 (213.3kJ/mol) [31] and that of persulfate (140kJ/mol) [32]. In general, increasing temperature is conducive to the break of chemical bond. In this experiment, the variation tendency of the removal efficiency proved indirectly increasing the temperature promoted the breakage of O-O bond to produce more SO4•− in the present of catalyst (equation 7). The mercury removal efficiency decreased from 85 to 70% in the temperature range of 55 to 85 ℃, which might be by that Hg0 had the solubility coefficient of 2.7×10-7mol/(Pa·l) at 55℃, and 11

that was 0.999×10-7mol/(Pa·l) at 80℃[33], suggesting that the higher the temperature the smaller solubility of Hg0 was. Besides, increasing the temperature could improve the recombination probability of free radical, which made the Hg0 removal efficiency decrease. Thus the optimal reaction temperature was set as 55℃ that was closed the flue gas temperature discharged from WFGD without gas-gas heater (GGH). For practical application, the reaction reactor of Hg0 removal can be installed after WFGD in the coal-fired power plants accordingly. HSO5   heat  SO4  OH

(7)

3.2.4. The effect of solution pH The protonation level of the solute will change with the solution pH and also affects the activity of catalyst. Hence, the effects of solution pH ranging 2.0 to 8.0 on the Hg0 removal were investigated. Figure 9 shows that the Hg0 removal efficiencies increase from 62% to 85% when the solution pH is between 2.0 and 7.0, which can be explained by that PMS is a binary weak acid with pKa of 9.4, its protonation level will be enhanced as the solution pH increases according to ionization balance, namely, H2SO5 can gradually be transformed to HSO5- that is in favor for the formation of SO4•− with an increase of the solution pH. Meanwhile, the generation of key intermediate product, (Co(OH)+) in the catalytic reactions will be decreased at low pH, which leads the decrease of catalyst activity and the production of SO4•− [34]. It was reported that SO4•− or HSO5− could change OH−/H2O into •OH [35] when the solution pH was 7.0, hence, Hg0 could be oxidized by SO4•− and •OH simultaneously, from which the highest removal efficiency of Hg0 was obtained. However, the removal efficiencies decreased from 85 to 75% rapidly when the solution pH increased from 7.0 to 8.0, which may be contributed to the masking effect of •OH to SO4•− and the 12

oxidizability of the former is lower than that of the latter, thus the Hg0 removal efficiency decreased. So the solution pH was selected as 7.0. 3.2.5. The effects of coexistence gases in flue gas It can be seen from Fig. 10 (a) and from Fig. 10 (b) that in the CO2 concentration range of 0 to 16% and the O2 concentration range of 0 to 8%, the removal efficiencies of Hg0 is almost unaffected as CO2 and O2 concentrations increasing, which may be due to the nonreactivity of CO2 and low oxidizability of O2. As shown in Fig. 10(C), the removal efficiency of Hg0 is basically maintained at 85.23 when SO2 concentration is between 0 and 300mg/m3. After that, the efficiency decrease slightly, which might be due to the competing reaction between Hg0 and SO2. Fig. 10 (d) shows that the removal efficiencies of Hg0 increase from 85.23 to 87.46% with NO concentrations increasing from 0 to 252 mg/m3. The reason for improved Hg0 removal was likely that NO could be oxidized into NO2, nitrite, and nitrate, in which, the formed NO2 would oxidize Hg0 as Hg(NO)3[10]. But the efficiencies decreased from 87.46 to 75.40 % when NO concentration increased from 252 to 800 mg/m 3, It meant that higher NO concentration seemed to go against the removal of arsenic, mainly because of the competing reaction between Hg0 and NO. The last two years, power plants in China is requires to reach the ultra-low emission limits of 35mg/m3 for SO2 and 50mg/m3 for NOx, respectively. Hence, such low SO2 and NOx concentrations after flue gas desulfuration and denitration will not dramatically affect Hg0 removal. 3.2.6. Parallel experiments The parallel experiments were conducted under the determined optimal conditions in which the CoFe2O4 dosage was 0.288g/l, PMS concentration was 13

3.5mmol/l, solution pH was 7.0, reaction temperature was 55 ℃ and Hg0 concentration of was 101 μg/m3. The better reproducibility of Hg0 removal efficiencies indicates that this method has the stable performance and can provide a reference for industrial application, as shown in Table 1. 3.2.7. The recycle of catalyst and ion dissolving out The recycle experiments of CoFe2O4 were carried out to investigate the stability and retrievability of CoFe2O4, and the process of which was that CoFe2O4 was separated at applied magnetic field from the spent solution and dried at 80℃ after washing to remove impurities. To investigate the catalytic activity of recycle catalyst, the Hg0 removal experiments were conducted three times under the optimal experimental conditions. As shown in Figure 11, the Hg0 removal efficiencies of the three times are 85.39%，84.40% and 83.39% respectively, suggesting that the catalyst has the possibility of recycling utilization. In order to assess the stability of the catalyst, an inductively coupled plasma (ICP) was used to determine the Co2+ concentration in the spent solution, the results showed that the Co2+ concentration was 0.1mg/l and the ion dissolving out rate was 0.03%, indicating that the catalyst had the better stability. 3.3. Hg0 removal mechanism According to the previous work [20], there may be three kinds of free radicals such as SO4•−, •OH and HSO5•- in the reaction process of degrading organic matter in wastewater. Hence, the experiments of quenching free radicals were preformed to verify the existence of these free radicals in order to explain the reaction mechanism. As is well-known, the reaction rate between methyl alcohol (MeOH) and •OH is 1.2-2.8×109 m-1S-1, and that between methyl alcohol and SO4•− is 1.6-7.8×106 m-1S-1. 14

Thus MeOH was used as quenching reagent of •OH and SO4•− in the experiments. Because the reaction rate between tert butyl alcohol (TBA) and •OH was high to 3.8-7.6×108 m-1S-1, and that between TBA and SO4•− was only 4-9.1×105 m-1S-1, so TBA was also used as quenching reagent of •OH [36-37]. In view of the weak oxidizability of HSO5•- according to the relative low potential (E(SO5•−/SO42−)=1.1EV) [38], the effect of HSO5•- on the Hg0 oxidation was neglected and also did not be tested here. As seen in Figure 12, the Hg0 removal efficiencies decreases inconspicuously and the decreasing rate is only 3% when the concentrations of TBA and MeOH are all 1 mmol/l in the solutions, however, the Hg0 removal efficiencies decrease from 85% to 60% for 3 mmol/l MeOH and 85% to 79% for 3 mmol/l TBA, which shows that MeOH has a higher quenching effect to SO4•− and •OH than TBA and demonstrates indirectly that PMS has been catalyzed by CoFe2O4 to generate SO4•− and •OH. For the generation of SO4•−, It could be considered that ≡CoII and ≡FeIII on the surface of catalyst captured H2O2 molecules to form the key active species, such as ≡CoOH+ and ≡FeOH2+ [25] (eqs. 8 and 3), moreover, the generated FeOH2+ could further react with ≡CoII to form CoOH+ (equation 4). When PMS existed in the solution, its dissociative product, HSO5− combined with the formed active species by a hydrogen bond on the surface of the catalyst to produce a kind of new species, CoFe2O4-O-H-HSO5− and CoOH+ in the compound could activate HSO5− to form SO4•− and ≡CoIIIO+ (eqs 9-10), while ≡CoIIIO+ could reduce HSO5− to ≡CoIIOH+, forming a benefit circle of producing ≡CoIIOH+ repeatedly. In addition, ≡FeIII on the surface of the catalyst could also activate HSO5− to form SO4•−[39-40], as shown in eqs. 11 -12. 15

≡CoII+H2O →≡CoIIOH+ + H+ ≡ CoIIOH++HSO5− → ≡CoIIIO+

(8)

+SO4•−+ H2O

(9)

≡ CoIIIO++HSO5− → ≡CoIIOH+ + SO5•−

(10)

≡FeIII+HSO5− → ≡FeII+ SO5•−+ H+

(11)

≡ FeII + HSO5− → ≡Fe III+SO4•−+ OH−

(12)

It can be seen from eqs.13-14 that the side reactions between PMS and ≡ CoII , and ≡ FeII can generate HO• [41]. Meanwhile, it also may be resulting from the reactions between SO4•− and water molecule [42-43] or hydroxyl (eqs. 15-16). ≡ CoII +HSO5− →≡CoIII +SO4 2−+HO•

(13)

≡ FeII +HSO5− →≡FeIII +SO42− +HO•

(14)

SO4•−+ OH−→ •OH + SO42−

k = (6.5 ± 1.0) × 107M−1 s−1

(15)

SO4•−+ H2O → •OH + SO42−+ H+

k[H2O]< 1.0 × 103M−1 s−1

(16)

In order to verify the chemical states on surface of the fresh and spent catalysts, and the functions of the Co and Fe species and hydroxyl species in the activation of PMS, XPS spectra of the CoFe2O4 before and after reaction were analyzed. It can be seen from Fig.12(a) that for the fresh catalyst, three peaks at 779.3, 780.4, and 781.53eV represented Co(II) in octahedral sites, tetrahedral sites, and Co(III) in octahedral sites [44] appear in the XPS spectra of the Co 2p3/2, respectively, with relative contributions to the total Co intensity of 50.32%, 31.23% and 18.45%, respectively. For the spent catalyst, the binding energy values of Co 2p3/2 peak slightly decrease to 779.0 eV for Co(II) in octahedral sites, 780.1 eV for Co(II) in tetrahedral sites and 781.53 eV for Co(III) in octahedral sites, with relative contributions to the total Co intensity of 22.01 %, 50.20% and 27.79%, which indirectly shows that Co(II) of 9.34% is oxidized into Co(III). In Fig.12(b), two peaks at 710.1 and 711.8V 16

attributed to Fe(II) and Fe(III) oxidation states are observed in the fresh catalyst, with a ratio of 45.37%:54.63%, respectively[13]. For the spent catalyst, the Fe 2p 3/2 peak is also composed of two peaks at 710.1 and 711.8V with a ratio of 56.14%:43.86%, respectively, indicating indirectly that Fe(II) of 10.77% is oxidized into Fe(III) in the activation of PMS. In Fig.12(c), the O1s spectra show two peaks at 529.7 and 531.1 eV, which can be assigned to the lattice oxide oxygen and adsorbed oxygen or surface hydroxyl species [20]. In the fresh CoFe2O4 surface, the relative intensity of lattice oxide oxygen and surface hydroxyl species are 71.87%, 28.13%. for the spent CoFe2O4, it is found out that the relative intensity of hydroxyl species increase from 28.13% to 31.7%. The increase of hydroxyl species concentration on the catalyst surface may be due to the formations of Co-OH and Fe–OH, which may play crucial role in removal mercury. For further speculating the reaction mechanism, the removal products were determined and the average Hg2+ concentration of 8.61μg/l was found in the spent solutions, which proved that Hg0 was oxidized by SO4•− and •OH. According to our previous work, this solution containing high-concentration Hg2+ could be precipitated by the organic sulfides from the solution, then the sediment containing HgS might be recycled or landfilled. Meanwhile, the qualified spent solution was discharged to municipal sewage treatment system. Based on the characterizations of CoFe2O4, experiments of quenching free radicals, XPS analyses of the fresh and spent catalysts, analyses of removal products and references, the Hg0 removal mechanism was proposed, as shown in equation 17. SO4−•（or •OH）+Hg0→Hg2++SO42− 4. Conclusions 17

(17)

CoFe2O4 catalyst was prepared by sol-gel method and the characterizations suggested that the catalyst had the typical structure of spinel and the surface of the catalyst was uneven with the average particle diameter of about 1 μm. By measuring the magnetic characteristic, the saturation magnetization of the catalyst was 41.62emu/g, which could provide the basis of recycling the spent catalyst from adsorption liquid. The optimal experimental condition was set as the dosage of CoFe2O4 was 0.288g/l, the concentration of PMS was 3.5mmol/l, the initial pH was 7.0 and the reaction temperature was 55℃, and under that condition the average Hg0 removal efficiency could come to 85% in a reaction circle of 54 min, thus a new kind of Hg0 removal method from flue gas was put forward. According to the characterizations of CoFe2O4, experiments of quenching free radicals, XPS analyses of the fresh and spent catalysts, analyses of removal products and references, the Hg0 removal mechanism was proposed, which was that in the removal process, Hg0 was oxidized into Hg2+ by SO4•−·and •OH together.

Acknowledgement The authors appreciate the financial support by a grant from the key project of the

National major

research

and

development

Program

of

China

(No.

2016YFC0203700 and No. 2017YFC0210600), National Science-technology Support Plan of China (No. 2014BAC23B04-06, Beijing Major Scientific and Technological Achievement

Transformation

Special Scientific

Project

Research Fund of Public

18

of

China

(No.Z151100002815012),

Welfare Profession of

China.

(No.

201309018) and Fundamental Research Funds for the Central Universities (No. 2014ZD41).

19

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25

Figure Captions Figure 1. Schematic diagram of experimental system. 1-N2 cylinder; 2-reducing valve; 3-rotor flow meter; 4-Hg0 generator; 5-oil bath pan; 6-buffer bottle; 7-three-way valve; 8- water bath ; 9-bubble reactor 10-dryer; 11-coal-fired flue gas mercury measurement instrument;12-absorbent solution. Figure 2. XRD pattern of CoFe2O4 crystal Figure 3.The hysteresis loop of CoFe2O4 at room temperature Figure 4. SEM photo of CoFe2O4 Figure 5. N2 adsorption-desorption curves of CoFe2O4 Figure 6. Effect of catalyst dosage on Hg0 removal. PMS concentration,3.5mmol/L； solution pH，7; catalyst dosage, 0.288 g·L-1; reaction temperature,55℃; total gas flow, 1L·min-1; Hg0 concentration,101μg/m3；reaction time, 54 min. Figure 7. Effect of PMS concentration on Hg0 removal. Solution pH，7; catalyst dosage, 0.288 g·L-1; reaction temperature,55℃; total gas flow, 1L·min-1; Hg0 concentration,101μg/m3；reaction time, 54 min. Figure 8. Effect of temperature on Hg0 removal. PMS concentration,3.5mmol/L； solution pH ， 7; catalyst dosage, 0.288 g·L-1; total gas flow, 1L·min-1; Hg0 concentration,101μg/m3；reaction time, 54 min. Figure 9. Effect of pH on Hg0 removal. PMS concentration,3.5mmol/L; catalyst dosage, 0.288 g·L-1; reaction temperature,55℃; total gas flow, 1L·min-1; Hg0 concentration,101μg/m3；reaction

time, 54 min.

Figure 10. Effect of coexistence gases in flue gas on Hg0 removal. PMS concentration,3.5mmol/L；solution pH，7; catalyst dosage, 0.288 g·L-1; reaction temperature,55℃; total gas flow, 1L·min-1; Hg0 concentration,101μg/m3；reaction time,

26

54 min. Figure 11. The experiments of catalyst recycle. PMS concentration,3.5mmol/L； solution pH，7; catalyst dosage, 0.288 g·L-1; reaction temperature,55℃; total gas flow, 1L·min-1; Hg0 concentration,101μg/m3；reaction time, 54 min. Figure 12. The variation of Hg0 removal efficiency after adding inhibitor. PMS concentration,3.5mmol/L；solution pH，7; catalyst dosage, 0.288 g·L-1; reaction temperature,55℃; total gas flow, 1L·min-1; Hg0 concentration,101μg/m3；reaction time, 54 min. Figure 13. XPS spectra of the fresh and spent catalysts

27

Figures Figure 1

Figure 2

28

Figure 3

Figure 4

29

Figure 5

Figure 6

30

Figure 7

Figure 8

31

Figure 9

Figure 10

32

Figure 11

Figure 12

33

Figure 13

34

Tables Table 1. The results of parallel test for Hg0 removing Numbers

1

2

3

4

5

Average

Standard error

Oxidation efficiencies，%

85.3

84.9

85.0

85.5

85.4

85.2

0.25

35