H2S-modified natural ilmenite: A recyclable magnetic sorbent for

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H2S-modified natural ilmenite: A recyclable magnetic sorbent for recovering gaseous elemental mercury from flue gas Sijie Zou, Yong Liao, Wei Tan, Xiaoliang Liang, Shangchao Xiong, Nan Huang, Yang Geng, Hongping He, and Shijian Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02447 • Publication Date (Web): 17 Aug 2017 Downloaded from http://pubs.acs.org on August 23, 2017

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Industrial & Engineering Chemistry Research

H2S-modified natural ilmenite: A recyclable magnetic sorbent for recovering gaseous elemental mercury from flue gas Sijie Zou, ┼ Yong Liao, ┼ Wei Tan, ╪, § Xiaoliang Liang, ╪, § Shangchao Xiong, ┼ Nan Huang, ┼ Yang Geng, ┼ Hongping He, ╪, § Shijian Yang *, ┼ ┼

Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School of

Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing, 210094, P. R. China ╪

CAS Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry,

Chinese Academy of Sciences, Guangzhou, 510640, P. R. China §

Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou, 510640,

P. R. China * Corresponding Author School of Environmental and Biological Engineering, Nanjing University of Science and Technology. Telephone: 86-18-066068302; E-mail: [email protected].

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Abstract: H2S-modified natural ilmenite (i.e., ilmenite-C-S) was developed as a recyclable magnetic sorbent to recover Hg0 in the flue gas as a co-benefit of the wet electrostatic precipitators (WESPs). Ilmenite-C-S showed an excellent Hg0 capture performance at 40-100 oC and the chemical adsorption of Hg0 on ilmenite-C-S was hardly inhibited by H2O and SO2. The chemical adsorption of Hg0 on ilmenite-C-S mainly followed the Mars-Maessen mechanism (i.e., gaseous Hg0 was firstly adsorbed and it was then oxidized to HgS by S22- on ilmenite-C-S). As the formed Hg species was HgS, the leaching of Hg species during the wet dust collection of the WESP was negligible. Ilmenite-C-S after the five cycles of Hg0 capture, Hg0 recovery and sorbent regeneration still had an excellent super-paramagnetism with the saturation magnetization of 14.1 emu g-1 and a superior performance for Hg0 capture with Hg0 removal efficiency of approximately 100%. Meanwhile, the ultralow concentration of Hg0 in the flue gas was recovered as a high concentration of gaseous Hg0 (>10 mg m-3), which can be condensed to liquid Hg0. Therefore, Hg0 recovery by ilmenite-C-S as a co-benefit of the WESP was a cost-effective technology for the centralized control of Hg0 emission from coal-fired plants.

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Table of Contents

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1. Introduction As mercury emission from coal combustion is considered as the largest source of anthropogenic mercury emission,

1, 2

nations at the Minamata Convention agreed to control mercury emission

from coal-fired plants.3 Therefore, the limit of Hg emission from coal-fired plants was setted in the emission standard of air pollutants for thermal power plants of China (GB 13223-2011). Hg species exists in the flue gas mainly as elemental mercury (Hg0), particulate-bound mercury (Hgp) and oxidized mercury (Hg2+).4 Hgp can be effectively removed from the flue gas with the fly ash by the electrostatic precipitator (ESP), and Hg2+ can be removed from the flue gas by the flue gas desulfurization (FGD) due to its water solubility. 5 However, Hg0 cannot be removed from the flue gas by currently available control devices in coal-fired plants, so it is the main Hg species emitted to the atmosphere. 6 The control of Hg0 emission from coal-fired plants is preferred as a co-benefit of currently available control devices in coal-fired plants.

7, 8

Now, it mainly falls into one of the two routes.

One is the catalytic oxidation of Hg0 by the selective catalytic reduction (SCR) catalyst with HCl in the flue gas as the oxidant to Hg2+,4, 9-11 which is then removed from the flue gas in the FGD unit. The other is the conversion of Hg0 to Hgp by the capture of sorbents (for example brominated activated carbon),5, 12, 13 which is then removed from the flue gas with the fly ash in the ESP unit. However, there is a major disadvantage for the two methods that gaseous Hg0 is not captured for the centralized control or converted to low toxic Hg species. 14 Hg0 in the flue gas is converted to HgCl2 in the desulfurization gypsum or to HgBr2 in the fly ash, whose toxicities are much higher than that of Hg0.

15

Therefore, the control of Hg0 emission from coal-fired plants by the two

methods may increase the exposure risk of local residents to mercury. 16 Now, many researchers focus on the centralized control of Hg0 emission from the flue gas. Because magnetic sorbents may be magnetically separated from the fly ash for the recycle of sorbents and the centralized control of Hg pollution, many magnetic sorbents such as MagZ-Ag0, 17, 18

Fe-based spinels, 19-24 magnetic biochar, 25 and Co-MF 26 were developed to capture gaseous

Hg0 from the flue gas. Magnetic sorbents will be injected into the flue gas upstream of the ESP to capture gaseous Hg0. Then, they are removed from the flue gas by the ESP with a large amount of fly ash. However, it is still very difficult to separate magnetic sorbents from the mixture collected 4


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by the ESP because the amount of fly ash in the mixture is much higher than that of the spent magnetic sorbents. 14 To meet the stricter emission standards for the ultra-low emission, wet electrostatic precipitators (WESPs) are gradually installed downstream of the FGD in coal-fired plants of China. Magnetic sorbents may be injected into the flue gas downstream of the FGD to capture gaseous Hg0. Then, they are removed from the flue gas by the WESP. As more than 99% of the fly ash has been removed from the flue gas by the ESP, the mixture collected by the WESP mainly contained spent magnetic sorbents, small amounts of ultrafine particulates and desulfurization gypsum. Therefore, it is practical to recover spent magnetic sorbents from the mixture collected by the WESP for the recycling and achieving the centralized control of Hg pollution. The sorbents, which are used to recover gaseous Hg0 in the flue gas as a co-benefit of the WESP, should possess the following five characteristics: (1) The sorbents need have excellent super-paramagnetism with a large saturation magnetization and a negligible coercivity. The large saturation magnetization makes it easy to magnetically separate spent sorbents from the mixture collected by the WESP. The magnetization of sorbents will simultaneously disappear due to the negligible coercivity as the external magnetic field is removed, which will facilitate the re-dispersion of recovered magnetic sorbents for future recycling. (2) The temperature window of the sorbents for gaseous Hg0 capture need match the temperature range of the flue gas downstream of the FGD (i.e., 40-100 oC). Furthermore, the adsorption of Hg0 on the sorbents is hardly affected by H2O and residual SO2. (3) As the mixture collected is removed from the WESP by water spraying, the leaching of Hg species need be avoided. (4) Hg species on sorbents can be thermally desorbed rapidly for the centralized control of Hg pollution. (5) The sorbents are low cost and natural minerals are preferred. In our previous study, magnetic Mn-Fe spinel was developed to recover gaseous Hg0 in the flue gas as a co-benefit of the WESP.

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However, Mn-Fe spinel showed a poor activity for gaseous

Hg0 capture at 40-100 oC. Furthermore, approximately 50% of Hg species formed were HgSO4, which would be leached during the dust collection process. Furthermore, natural derived magnetic pyrrhotite (Fe1-xS) was once developed to recover gaseous Hg0 in the flue gas as a co-benefit of the WESP.

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Pyrrhotite showed excellent gaseous

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Hg0 capture performance at 40-100 oC, and the adsorption of Hg0 on pyrrhotite was hardly inhibited by the presence of H2O and SO2. Meanwhile, the formed Hg species on pyrrhotite was HgS, which is insoluble in water and acid. However, the saturation magnetization of pyrrhotite was very low (only 7.0 emu g-1), so it is relatively difficult to magnetically separate pyrrhotite from the mixture collected by the WESP. Furthermore, pyrrhotite had a permanent magnetization of 3.7 emu g-1, which would affect the re-dispersion of recovered pyrrhotite for future recycling. In this work, natural ilmenite (Fe2-xTixO3) was developed as a recyclable magnetic sorbent to recover gaseous Hg0 from the flue gas as a co-benefit of the WESP. As natural ilmenite exhibited a poor activity for gaseous Hg0 capture at low temperatures, it was modified with the pretreatment of H2S. H2S-modified natural ilmenite conformed very well to the five characteristics of the sorbents used as a co-benefit of the WESP for the centralized control of Hg pollution. Therefore, Hg0 recovery by H2S-modified ilmenite as a co-benefit of the WESP may be a cost-effective technology for the centralized control of Hg0 emission from coal-fired plants.

2. Experimental 2.1 Sample preparation

Natural ilmenite used in this study was collected from the Baima ore cluster area in Panzhihua City, Sichuan Province, China. The chemical composition of natural ilmenite, which was determined using an X-ray fluorescence analyzer (ThermoFisher, ARL perform), is shown in Table S1 of the Supporting Information. Natural ilmenite was calcined under air atmosphere at 300 oC for 3 h, which was named as ilmenite-C. Then, natural ilmenite and ilmenite-C were both modified with H2S at 300 oC for 2 h (600 ppm of H2S/N2, 500 mL of the total flow rate and 500 mg of the sorbent), which were named as ilmenite-S and ilmenite-C-S, respectively. 2.2 Characterization

BET surface area, crystal structure, magnetization and surface analysis were determined by a nitrogen adsorption apparatus (Quantachrome, Autosorb-1), an X-ray diffractionmeter (XRD, Bruker-AXS D8 Advance), a vibrating sample magnetometer (VSM, LakeShore 735) and an X-ray photoelectron spectroscopy (XPS, Thermo, ESCALAB 250), respectively. The thermal stability of Hg species adsorbed on the sorbents under N2 and air atmospheres was studied using temperature programmed desorption of Hg (Hg-TPD) at a heating rate of 10 oC min-1 and a gas 6


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flow of 700 mL min-1. The leachability of Hg species from ilmenite-C-S after Hg0 capture was determined using the toxicity characteristic leaching procedure (USA EPA) with 5.7 mL L-1 of glacial acetic acid (pH=2.88) as the extraction fluid. 27 2.3 Elemental mercury capture and recovery

Hg0 capture, Hg0 recovery and sorbent regeneration were all performed on a fixed-bed quartz tube microreactor. Hg0 capture was conducted at 40-100 oC with the sorbent mass of 100-300 mg, the total flow rate of 500 mL min-1 and the gas hourly space velocity (GHSV) of 1.0×105-3.0×105 cm3 g-1 h-1 (i.e., approximately 1.4×105-4.2×105 h-1). The sorbent after Hg0 capture for 3 h was thermally treated at 300 oC for 40 min with a N2 gas flow of 100 mL min-1 to recover Hg0. Then, the spent sorbent was regenerated with the calcination and the treatment of H2S. At last, the regenerated sorbent was recycled to capture gaseous Hg0. As Hg0 capture would be placed downstream of the FGD, the simulated flue gas for Hg0 capture contained approximately 110 µg m-3 of gaseous Hg0, 5% O2 (when used), 8% H2O (when used), 80 ppm of SO2 (when used) and balance of N2.28-30 The concentrations of gaseous Hg0 and SO2 were determined online using a cold vapor atomic absorption spectrophotometer (CVAAS, Lumex R-915+) and a Fourier transform infrared spectrometer (FTIR, Thermo SCIENTIFIC, ANTARIS, IGS Analyzer), respectively.

3. Results and Discussion 3.1 Performance for elemental mercury capture

Figure 1 shows the breakthrough curves of the adsorption of gaseous Hg0 on ilmenite, ilmenite-C, ilmenite-S and ilmenite-C-S at 40-100 oC. The amounts of Hg0 captured by ilmenite, ilmenite-C, ilmenite-S and ilmenite-C-S in 180 min, which resulted from the integration of the breakthrough curves, are listed in Table 1. Although ilmenite and ilmenite-C both showed poor Hg0 capture performance (Figures 1a and 1b), the amounts of Hg0 captured by ilmenite were 2.8-8.1 times that by ilmenite-C. It suggests that the adsorption of Hg0 on ilmenite was obviously restrained after the thermal treatment at 300 oC. The amounts of Hg0 captured by ilmenite-S were 3.0-5.6 times those by ilmenite. Meanwhile, the amounts of Hg0 captured by ilmenite-C-S were 11.1-33.8 times those by ilmenite-C. They suggest that the adsorption of Hg0 on ilmenite and that on ilmenite-C were both promoted remarkably after the treatment with H2S. Table 1 also shows 7



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that the performance of ilmenite-C-S for gaseous Hg0 capture was much better than that of ilmenite-S. There is a high concentration of water vapor and a low concentration of SO2 in the flue gas downstream of the FGD. Therefore, the effect of H2O and SO2 on Hg0 capture by ilmenite-C-S was investigated (Figure 1e). Table 1 shows that the amounts of Hg0 captured by ilmenite-C-S did not decrease obviously after the introduction of 8% H2O and 80 ppm of SO2, suggesting that H2O and SO2 did not showed a notable inhibition on Hg0 capture by ilmenite-C-S. 3.2 Characterization 3.2.1 XRD and BET surface

XRD pattern of natural ilmenite (Figure 2A) mainly corresponded to the standard cards of pure phase of ilmenite (JCPDS 29-0733). Meanwhile, some slight characteristic peaks corresponding to titanomagnetite (Fe3-xTixO4) can be observed in the XRD pattern of natural ilmenite.

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XRD

analysis indicates that the content of titanomagnetite in natural ilmenite was approximately 14%. XRD patterns of ilmenite-C, ilmenite-S and ilmenite-C-S were consistent with that of natural ilmenite. It suggests that the crystal structure of natural ilmenite did not change remarkably after the thermal treatment and the treatment with H2S. The BET surface areas of ilmenite, ilmenite-C, ilmenite-S and ilmenite-C-S were approximately 1.2, 0.2, 4.3 and 7.9 m2 g-1, respectively. 3.2.2 Magnetization

Figure 2B shows the magnetic characteristics of ilmenite, ilmenite-C, ilmenite-S and ilmenite-C-S. Natural ilmenite showed an excellent magnetization with the saturation magnetization of 23.9 emu g-1. However, it exhibited an obvious magnetization hysteresis with a coercivity of 13000 A m-1 and a permanent magnetization of 3.0 emu g-1. After the thermal treatment at 300 oC, the saturation magnetization and the permanent magnetization of natural ilmenite both slightly decreased (shown in Figure 2B). After the treatment with H2S, the saturation magnetizations of ilmenite and ilmenite-C both slightly decreased and the magnetization hysteresis almost disappeared. Ilmenite-C-S showed an excellent super-paramagnetism with the saturation magnetization of 18.8 emu g-1. The inset photograph in Figure 2B indicates that the

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magnetic separation of spent ilmenite-C-S from the simulated mixture collected by the WESP (0.5 g of ilmenite-C-S, 2.0 g of fly ash and 20 mL of deionized water) was practicable. 3.2.3 XPS

XPS spectra of ilmenite, ilmenite-C, ilmenite-S and ilmenite-C-S in the spectral regions of Fe 2p, Ti 2p, O1s and S 2p are shown in Figure 3 and Figure S1 of the Supporting Information. Fe 2p 3/2 binding energies on ilmenite and ilmenite-C mainly appeared at 709.6, 711.0 and 712.4 eV (Figures 3a and 3b), which were assigned to Fe2+, Fe3+ and Fe3+ bonded with -OH, respectively.

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Ti 2p binding energies on ilmenite and ilmenite-C mainly appeared at

approximately 458.3 and 464.0 eV (Figures S1a and S1c of the Supporting Information), which were attributed to Ti4+.22, 32 O 1s binding energies on ilmenite and ilmenite-C mainly appeared at 529.9 and 531.6 eV (Figures S1b and S1d of the Supporting Information), which were assigned to O2- in metal oxides and O2- in -OH, respectively. 33 Furthermore, the binding energy corresponding to SiO2, which is the impurity of natural ilmenite, can be clearly observed at 533.3 eV in the O 1s spectral region. 15 After the treatment with H2S, two new binding energies at approximately 707.1 and 713.4 eV appeared on ilmenite and ilmenite-C in the spectral region of Fe 2p (Figures 3c and 3e), which were attributed to Fe2+ bonded with S2-/S22- and Fe2+ bonded with SO42-, respectively.

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S 2p

binding energies on ilmenite-S mainly appeared at 161.8, 162.6, 163.8, 168.7 and 170.0 eV (Figure 3d), which were attributed to S2-, S22-, polysulfur, SO42- and HSO4-, respectively.

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However, the binding energy at 161.8 eV corresponding to S2- cannot be observed on ilmenite-C-S (Figure 3f). The percentages of Fe and S species on ilmenite, ilmenite-C, ilmenite-S and ilmenite-C-S, which resulted from the XPS analysis, are listed in Table 2. Table 2 shows that the percentage of Fe3+ on ilmenite obviously increased after the calcination under air. The reaction between gaseous H2S and Fe3+ on ilmenite and ilmenite-C can be approximately described as: 37-39

2H 2S+2Fe3+ =O → FeS2 +Fe 2+ +2H 2 O

(1)

As the percentage of Fe3+ on ilmenite was much less than that on ilmenite-C, the percentage of S22- on ilmenite-S was much less than that on ilmenite-C-S (Table 2). 3.3 Mechanism of Hg0 adsorption on ilmenite-S and ilmenite-C-S 9



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Figure 3 and Figure S1 of the Supporting Information show the XPS spectra of ilmenite-C-S after Hg0 capture in the spectral regions of Fe 2p, Ti 2p, O 1s, S 2p and Hg 4f. The Fe 2p, Ti 2p and O 1s binding energies on ilmenite-C-S after Hg0 capture was similar to those of ilmenite-C-S. However, a slight binding energy at 161.7 eV corresponding to S2- appeared on ilmenite-C-S after Hg0 capture (Figure 3h). Meanwhile, the Hg 4f binding energies on ilmenite-C-S after Hg0 capture mainly appeared at 100.7 and 104.7 eV (Figure 3i), which were attributed to Hg2+ (i.e., HgS or HgO). 15 Furthermore, the binding energy at 102.8 eV corresponding to Si in SiO2 can be observed in Figure 3i.19 The chemical adsorption of Hg0 on sorbents is often attributed to the Mars-Maessen mechanism (i.e., gaseous Hg0 was firstly physically adsorbed on the sorbents, and then physically adsorbed Hg0 was oxidized to HgO or HgS).

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The potential oxidants for the oxidation of Hg0

physically adsorbed on ilmenite-S and ilmenite-C-S were Fe3+, polysulfur, S22- and gaseous O2. As both ilmenite and ilmenite-C showed poor Hg0 capture performance at low temperatures, Fe3+ was not the oxidant for the oxidation of Hg0 physically adsorbed on ilmenite-S and ilmenite-C-S. To investigate the role of gaseous O2 on Hg0 oxidation, gaseous Hg0 capture by ilmenite-C-S under N2 atmosphere was investigated (Figure 1f). Table 1 shows that the amounts of Hg0 captured by ilmenite-C-S under N2 atmosphere were close to those under 5% O2/N2. It suggests that gaseous O2 did not play a key role on the chemical adsorption of gaseous Hg0 on ilmenite-C-S. As there was a strong binding energy of S-S in polysulfur, the activation energy of the oxidation of physically adsorbed Hg0 by polysulfur is relatively high.42 Figure S2 of the Supporting Information demonstrates that powder S8 (i.e., one kind of polysulfur) showed poor Hg0 capture performance at 60 oC. They suggest that polysulfur may not be the oxidant for the oxidation of Hg0 physically adsorbed on ilmenite-C-S. Therefore, the oxidant for the oxidation of Hg0 physically adsorbed on ilmenite-C-S was S22- and the formed Hg species was HgS. Therefore, the chemical adsorption of Hg0 on ilmenite-S and ilmenite-C-S can be approximately described as follows:

Hg 0 (g) → Hg 0 (ad )

(2)

Hg 0 (ad) +S22 − → HgS+S2 −

(3) 10


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Reaction 2 was the physical adsorption of gaseous Hg0 on ilmenite-S and ilmenite-C-S. Then, physically adsorbed Hg0 was oxidized by S22- on the surface to HgS (i.e., Reaction 3). According to Reaction 3, the kinetic equation of the chemical adsorption of gaseous Hg0 on ilmenite-S and ilmenite-C-S can be approximately described as:

−

d[Hg 0 (g) ] dt

=−

d[Hg 0 (ad) ] dt

= k1[Hg 0 (ad) ][S22 − ]

(4)

Where, k1, [Hg0(g)], [Hg0(ad)] and [S22-] were the reaction kinetic constant of Reaction 3, the concentration of gaseous Hg0, the concentration of Hg0 physically adsorbed on the surface and the concentration of S22- on the surface, respectively. The oxidation ability of Fe3+ on ilmenite and ilmenite-C was very low, which can hardly oxidize physically adsorbed Hg0 at low temperatures. Therefore, the adsorption of Hg0 on ilmenite and ilmenite-C at 40-100 oC can be approximately assigned to the physical adsorption of gaseous Hg0. Table 1 shows that the amounts of Hg0 adsorbed on ilmenite were 2.8-8.1 times those of ilmenite-C. It suggests that the concentration of Hg0 physically adsorbed on ilmenite-S may be much higher than that on ilmenite-C-S. However, the percentage of S22- on ilmenite-C-S was approximately 8.4 times that on ilmenite-S as the percentage of Fe3+ on ilmenite-C was much higher than that on ilmenite-C (hinted by Reaction 1). Therefore, [S22-][Hg0(ad)] of ilmenite-C-S was higher than that of ilmenite-S. Hinted by Equation 4, the chemical adsorption of gaseous Hg0 on ilmenite-C-S was much better than that on ilmenite-S, which was demonstrated in Table 1. As the physical adsorption was an exothermic reaction, [Hg0(ad)] obviously decreased with the increase of the reaction temperature. However, k1 increased with the increase of the reaction temperature. As a result, there was an optimal temperature (i.e., 60 oC) for the chemical adsorption of gaseous Hg0 on ilmenite-S and ilmenite-C-S. 3.4 Cycle test of Hg0 capture, Hg0 recovery and sorbent regeneration

The simplest method to recover Hg adsorbed on ilmenite-C-S was the thermal treatment. Hg-TPD profiles of ilmenite-C-S after Hg0 capture under N2 and air atmospheres were performed (Figure 4). The amount of Hg0 captured which resulted from the integration of the breakthrough curve was approximately close to the amount of Hg0 desorbed which resulted from the integration of the Hg-TPD profile. It suggests that most Hg adsorbed on ilmenite-C-S can be thermally 11



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desorbed as gaseous Hg0. Hg-TPD under N2 atmosphere showed a strong desorption at approximately 220 oC, which was assigned to the decomposition of HgS adsorbed.

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However,

Hg-TPD under air atmosphere showed two desorption peaks at approximately 200 and 360 oC. Figure S3 of the Supporting Information shows that little Hg0 can be adsorbed on ilmenite-C at 250 oC. It suggests that the decrease of the first desorption peak under air atmosphere was not related to the re-desorption of Hg0 (resulted from the decomposition of HgS) on ilmenite-C. Therefore, the decrease of the first desorption peak under air atmosphere was mainly related to the oxidation of HgS to HgSO4, and the desorption peak at approximately 360 oC was attributed to the decomposition of HgSO4.

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As the desorption temperature under air atmosphere was

approximately 140 oC higher than that under N2 atmosphere, the thermal treatment for recovering Hg0 was performed at 300 oC under N2 atmosphere. As the method for dust collection of the WESP is water spraying, the leachability of Hg species on ilmenite-C-S was investigated. The amount of Hg species in the leachate of glacial acetic acid was hardly observed, suggesting that the leaching of Hg species from ilmenite-C-S after Hg0 capture was negligible. As ilmenite-C-S would be injected into the flue gas to capture Hg0 downstream of the FGD, the oxidation of ilmenite-C-S to SO2 was a serious concern. Figure S4 of the Supporting Information shows that the concentration of SO2 resulted from the oxidation of ilmenite-C-S were all less than 2 ppm, suggesting that the oxidation of ilmenite-C-S to gaseous SO2 during Hg0 capture was negligible. Figure 5 shows Hg0 concentration in the outlets of Hg0 capture and Hg0 recovery during the five cycles of Hg0 capture, Hg0 recovery and sorbent regeneration. The removal efficiencies of Hg0 in the simulated flue gas during the 5 cycles were all close to 100% (Figure 5). Meanwhile, XRD pattern of ilmenite-C-S did not change remarkably after the five cycles (Figure 2A). Furthermore, ilmenite-C-S after the 5 cycles still showed an excellent super-paramagnetism with the saturation magnetization of 14.1 emu g-1 (Figure 2B). They all suggest that the key properties of ilmenite-C-S for gaseous Hg0 capture did not degrade obviously during the five cycles. Figure 5 also shows that the concentration of gaseous Hg0 in the outlet during Hg0 recovery can reach 15 mg m-3, which can be cooled and condense to liquid Hg0 for the safety disposal. 3.5 Comparison of ilmenite-C-S with other magnetic sorbents 12


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The comparison of the performance of ilmenite-C-S for recovering Hg0 in the flue gas with those of other magnetic sorbents is shown in Table 3. The capacity of ilmenite-C-S for Hg0 capture was approximately 0.17 mg g-1 at 60 oC with the breakthrough threshold of 6% (shown in Figure S5 of the Supporting Information), which was much higher than those of MagZ-Ag0, Mn-Fe spinel, Fe-Ti-Mn spinel and Co-MF. Ilmenite-C-S was obtained from the modification of natural ilmenite, which is widely dispersed with a huge reserve.

44

Therefore, the cost of ilmenite-C-S was much

lower than that of artificial magnetic biochar although the capacity of ilmenite-C-S for Hg0 capture was less than that of magnetic biochar. Furthermore, Hg species formed on magnetic biochar contained some water-soluble HgSO4 and gaseous HgCl2, which would cause a low efficiency of Hg0 recovery. Although the capacity of ilmenite-C-S for Hg0 capture was less than that of pyrrhotite, ilmenite-C-S showed an excellent super-paramagnetism with the saturation magnetization of 18.8 emu g-1. Therefore, the magnetic separation of ilmenite-C-S from the mixture collected by the WESP was much easier than that of pyrrhotite. Meanwhile, the re-dispersion of recovered ilmenite-C-S for future recycling was much easier than that of pyrrhotite due to its super-paramagnetism.

4. Conclusion Elemental mercury recovery by magnetic sorbents as a co-benefit of the WESP was a promising and cost-effective technology for the centralized control of Hg emission from coal-fired plants. In this work, natural ilmenite was developed as a recyclable magnetic sorbent to recover Hg0 as a co-benefit of the WESP. Although natural ilmenite showed a poor activity for gaseous Hg0 capture at low temperatures (40-100 oC), its performance for gaseous Hg0 capture was obviously enhanced after H2S pretreatment. Meanwhile, Hg0 adsorbed on ilmenite-C-S can be recovered as a high concentration of gaseous Hg0, which can be condensed to liquid Hg0. Therefore, natural ilmenite was a low-cost and recyclable magnetic sorbent for the recovering of gaseous Hg0 from the flue gas as a co-benefit of the WESP. Furthermore, the possibility of using the sorbent for capturing mercury in coal-derived syngas could also be an interesting area for future research attrition resistance of the sorbent will be examined.

13


45-47

and the


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Acknowledgements This study was financially supported by the National Natural Science Fund of China (Grant No. 41372044) and the Natural Science Fund of Jiangsu Province (Grant No. BK20150036).

Supporting Information The Supporting Information includes the chemical components of natural ilmenite, XPS spectra of ilmenite, ilmenite-C, ilmenite-S, ilmenite-C-S and ilmenite-C-S after Hg0 capture in the spectral regions of Ti 2p and O 1s, the concentration of SO2 formed due to the oxidation of ilmenite-C-S, and the breakthrough curves of gaseous Hg0 capture by powder S8, ilmenite-C, Mn-Fe spinel, Fe-Ti-Mn spinel, pyrrhotite and ilmenite-C-S.

14


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19



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Page 20 of 28

Table 1. Amounts of Hg0 captured by ilmenite, ilmenite-C, ilmenite-S and ilmenite-C-S in 180 min

/µg 40oC

60oC

80oC

100oC

ilmenite

1.28

1.08

1.30

0.88

ilmenite-C

0.24

0.38

0.16

0.28

ilmenite-S

4.3

6.0

4.5

2.6

ilmenite-C-S

8.0

8.1

5.4

3.1

ilmenite-C-S under N2

7.3

7.7

6.4

3.8

ilmenite-C-S with H2O and SO2

7.6

8.2

6.4

3.4

20


Page 21 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Percentages of Fe and S species on ilmenite, ilmenite-C, ilmenite-S, ilmenite-C-S and ilmenite-C-S after gaseous Hg0 capture

/%

Fe3+

Fe2+

Fe3+-OH

Fe2+-SO42-

Fe2+-S2-/S22-

S2-

S22-

S0

SO42-

ilmenite

9.1

11.1

8.2

-

-

-

-

-

-

ilmenite-C

17.6

5.0

10.8

-

-

-

-

-

-

ilmenite-S

4.8

7.1

9.9

1.7

0.7

0.6

0.7

7.1

1.8

ilmenite-C-S

7.1

5.7

4.4

4.3

4.1

-

5.9

5.0

4.2

6.9

5.6

4.3

4.2

4.0

0.2

5.7

4.8

4.1

ilmenite-C-S after Hg0 capture

21



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Page 22 of 28

Table 3. Comparison of the performance of ilmenite-C-S for gaseous Hg0 capture with those of other magnetic sorbents source

BET surface 2

-1

capacity for Hg0 -1

reaction

Hg species

super-paramagnetism

no

yes

40.0

yes

yes

45.6

yes

yes

29.6

yes

yes

24.4

yes

yes

38.0

HgS

no

no

7.0

HgS

no

yes

18.8

area/m g

capture/mg g

condition

on sorbent

artificial

164

0.013 (20%a)

Ar at 50-150 oC

amalgam

artificial

128

0.033 (25%)

simulated flue

HgSO4 and

artificial

53.7

0.075 (25%)

artificial

6.2

0.03 (35%)

artificial

121

0.95 (30%)

pyrrhotite 15

natural

19.7

0.22 (4%)

ilmenite-C-S

natural

7.9

0.17 (6%)

MagZ-Ag0 17, 18 Mn-Fe spinel

16,

19, 24

Fe-Ti-Mn spinel

14, 21

Co-MF 26 magnetic biochar

a

25

o

gas at 60 C

HgO

simulated flue

HgSO4 and

o

gas at 60 C

HgO

simulated flue

HgSO4, HgCl2

o

gas at 150 C

and HgO

simulated flue

HgSO4, HgCl2

o

gas at 120 C

and HgO

simulated flue gas at 60 oC simulated flue gas at 60 oC

saturation

leaching

The data in the brackets were the breakthrough points for Hg0 adsorption.

22


magnetization/emu g-1

Page 23 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure Captions Figure 1. Breakthrough curves of gaseous Hg0 capture by: (a), ilmenite; (b), ilmenite-C; (c), ilmenite-S; (d), ilmenite-C-S; (e), ilmenite-C-S in the presence of H2O and SO2; (f), ilmenite-C-S under N2 atmosphere. Reaction conditions: [Hg0] =100-120 µg m-3, [O2] =5%, [H2O] =8% (when used), [SO2] =80 ppm (when used), sorbent mass=100 mg, total flow rate=500 mL min-1 and GHSV=300000 cm3 g-1 h-1. Figure 2. (A), XRD patterns of ilmenite, ilmenite-C, ilmenite-S, ilmenite-C-S and ilmenite-C-S after the 5 cycles of Hg0 capture, Hg0 recovery and sorbent regeneration; (B), Magnetization characteristic of ilmenite (a), ilmenite-S (b), ilmenite-C (c), ilmenite-C-S (d) and ilmenite-C-S after the 5 cycles (e). Figure 3. XPS spectra of ilmenite, ilmenite-C, ilmenite-S, ilmenite-C-S and ilmenite-C-S after Hg0 capture over the spectral regions of Fe 2p, S 2p and Hg 4f. Figure 4. Hg-TPD profiles under N2 and air atmospheres of ilmenite-C-S after Hg0 capture. Figure 5 Hg0 concentrations in the outlets of Hg0 capture and Hg0 recovery during the five cycles of Hg0 capture, Hg0 recovery and sorbent regeneration. Reaction conditions of Hg0 capture: reaction temperature=60 oC, [Hg0] =110 µg m-3, [O2] =5%, [H2O] =8%, [SO2] =80 ppm, sorbent mass=300 mg, total flow rate=500 mL min-1 and GHSV=100000 cm3 g-1 h-1; Reaction conditions of Hg0 recovery: reaction temperature=300 oC, sorbent mass=300 mg, total flow rate of N2=100 mL min-1 and GHSV=20000 cm3 g-1 h-1.

23


-3

Hg concentration/µg m

100 80 60 o

o

40 C o 80 C

40 20 0

50

60 C o 100 C

100

t/min

150

0

0


120

0

100 80 60 o

20 0

50

100

o

-3

60 C o 100 C

80 60

0

40 20 0

0

50

100

t/min

150

120 100

-3

o

60 C o 100 C


60 40

0

-3 0

o

80

20 0

0

150

200

o

60 C o 100 C

60 40 20 0

50

100

t/min

d

40 C o 80 C

100

200

80

0

200

o

40 C o 80 C

c 120

150

t/min


-3 0


o

100

60 C o 100 C

b

40 C o 80 C

120

o

40 C o 80 C

40

0

200

Page 24 of 28

120

a


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-3


50

100

150

120 100

o

60 C o 100 C

80 60 40 20 0

200

o

40 C o 80 C

0

50

100

t/min

t/min

e

150

200

f

Figure 1. Breakthrough curves of gaseous Hg0 capture by: (a), ilmenite; (b), ilmenite-C; (c), ilmenite-S; (d), ilmenite-C-S; (e), ilmenite-C-S in the presence of H2O and SO2; (f), ilmenite-C-S under N2 atmosphere. Reaction conditions: [Hg0] =100-120 µg m-3, [O2] =5%, [H2O] =8% (when used), [SO2] =80 ppm (when used), sorbent mass=100 mg, total flow rate=500 mL min-1 and GHSV=300000 cm3 g-1 h-1.

24


Page 25 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ilmenite-C-S after 5 cycles ilmenite-C-S ilmenite-S ilmenite-C ilmenite

ilmentite JCPDS 29-0733 20

30

40 50 2θ/degree

60

70

magnetite JCPDS 19-0629

A

B

Figure 2. (A), XRD patterns of ilmenite, ilmenite-C, ilmenite-S, ilmenite-C-S and ilmenite-C-S after the 5 cycles of Hg0 capture, Hg0 recovery and sorbent regeneration; (B), Magnetization characteristic of ilmenite (a), ilmenite-S (b), ilmenite-C (c), ilmenite-C-S (d) and ilmenite-C-S after the 5 cycles (e).

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ilmenite

Fe 2p

ilmenite-C

Fe 2p

724.4

710.9

711.0

723.9 712.4

719.1

709.6

712.4

719.2

Page 26 of 28

ilmenite-S

Fe 2p

724.2

712.4 719.8 713.4

711.0 709.6

709.6

707.1

730

725

720

715

710

705

730

725

720

715

725

720

715

710

a

b

c

S 2p

ilmenite-C-S

724.5

168

709.6

168.8

169.9

166

164

162

160

730

725

720

715

710

174

705

172

Binding Energy/eV

Binding Energy/eV

d

170

168

0

Fe 2p

712.4

713.4

711.0

164

162

160

f

0

S 2p

ilmenite-C-S after Hg capture

707.2

0


162.6

709.6

166

Binding Energy/eV

e


719.9

713.3

163.9

161.8

170.0

170

162.7 707.2

162.6

168.7

S 2p

711.0

712.4

705

ilmenite-C-S

Fe 2p

719.9

724.5

730

Binding Energy/eV

163.8

172

705

Binding Energy/eV

ilmenite-S

174

710

Binding Energy/eV

104.7

102.8

Hg 4f

100.7

163.9 169.9

168.9 161.7

730

725

720

715

710

705

174

172

170

168

166

164

162

160

108

106

104

102

Binding Energy/eV

Binding Energy/eV

Binding Energy/eV

g

h

i

100

98

Figure 3. XPS spectra of ilmenite, ilmenite-C, ilmenite-S, ilmenite-C-S and ilmenite-C-S after Hg0 capture over the spectral regions of Fe 2p, S 2p and Hg 4f.

26


-3 0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 27 of 28

under N2

1000

under air

800 600 400 200 0

0

100

200

300

o

Temperature/ C

400

500

Figure 4. Hg-TPD profiles under N2 and air atmospheres of ilmenite-C-S after Hg0 capture.

27



R1-R5: Hg0 recovery stage

A1-A5: Hg0 capture stage A1

16000

Hg0 concentration/µg m-3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

R1

A2

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A3

R3

A4

R4

A5

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12000 8000

inlet Hg0

100 50 0 0

200

400

600

800

1000

1200

t/min

Figure 5. Hg0 concentrations in the outlets of Hg0 capture and Hg0 recovery during the five cycles of Hg0 capture, Hg0 recovery and sorbent regeneration. Reaction conditions of Hg0 capture: reaction temperature=60 oC, [Hg0] =110 µg m-3, [O2] =5%, [H2O] =8%, [SO2] =80 ppm, sorbent mass=300 mg, total flow rate=500 mL min-1 and GHSV=100000 cm3 g-1 h-1; Reaction conditions of Hg0 recovery: reaction temperature=300 oC, sorbent mass=300 mg, total flow rate of N2=100 mL min-1 and GHSV=20000 cm3 g-1 h-1.

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H2S-modified natural ilmenite: A recyclable magnetic sorbent for

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