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H2S-modified Fe-Ti spinel: A recyclable magnetic sorbent for recovering gaseous elemental mercury from the flue gas as a co-benefit of the wet electrostatic precipitators Sijie Zou, Yong Liao, Shangchao Xiong, Nan Huang, Yang Geng, and Shijian Yang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05765 • Publication Date (Web): 22 Feb 2017 Downloaded from http://pubs.acs.org on February 24, 2017

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Environmental Science & Technology

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H2S-modified Fe-Ti spinel: A recyclable magnetic sorbent for

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recovering gaseous elemental mercury from the flue gas as a

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co-benefit of the wet electrostatic precipitators

4

Sijie Zou, Yong Liao, Shangchao Xiong, Nan Huang, Yang Geng, Shijian Yang *

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

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Environmental and Biological Engineering, Nanjing University of Science and Technology,

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Nanjing, 210094, P. R. China

8 9 10 11 12 13 14 15 16

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ACS Paragon Plus Environment


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Abstract:

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The non-recyclability of the sorbents used to capture Hg0 from the flue gas causes a high

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operation cost and the potential risk of exposure to Hg. The installment of wet electrostatic

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precipitators (WESPs) in coal-fired plants makes the recovery of spent sorbents for recycling and

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the centralized control of Hg pollution possible. In this work, H2S-modified Fe-Ti spinel was

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developed as a recyclable magnetic sorbent to recover Hg0 from the flue gas as a co-benefit of the

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WESP. Although Fe-Ti spinel exhibited poor Hg0 capture activity in the temperature range of the

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flue gas downstream of the flue gas desulfurization (FGD), H2S-modified Fe-Ti spinel exhibited

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excellent Hg0 capture performance with an average adsorption rate of 1.92 µg g-1 min-1 at 60 oC

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and the capacity of 0.69 mg g-1 (5% of the breakthrough threshold) due to the presence of S22- on

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its surface. The five cycles of Hg0 capture, Hg0 recovery, and sorbent regeneration demonstrated

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that the ability of modified Fe-Ti spinel to capture Hg0 did not degrade remarkably. Meanwhile,

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the ultra-low concentration of Hg0 in the flue gas was concentrated to a high concentration of Hg0,

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which facilitated the centralized control of Hg pollution.

31 32 33 34

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Table of Contents

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37 38

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1. Introduction

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As mercury emission from coal combustion is one of the most important sources of

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anthropogenic mercury emission,1-3 nations at the Minamata Convention agreed to control

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mercury emission from coal-fired plants.4 Hg species in the flue gas of coal-fired plants mainly

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present as particulate-bound mercury (Hgp), oxidized mercury (Hg2+) and elemental mercury

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(Hg0).5 Though Hg2+ and Hgp can be effectively removed by the flue gas desulfurization (FGD)

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and the electrostatic precipitator (ESP), the control devices in coal-fired plants cannot remove Hg0

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from the flue gas.6 Therefore, gaseous Hg0 is the main Hg species emitted by coal-fired plants to

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the atmosphere.7

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The strategy to control Hg0 emission from coal-fired plants now falls into the conversion of

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gaseous Hg0 to Hg2+ or Hgp.8, 9 The oxidation of Hg0 to Hg2+ can be achieved as a co-benefit of the

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selective catalytic reduction (SCR) catalyst,10, 11 which is installed to reduce NOx. Currently, the

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oxidation of Hg0 by the SCR catalyst is restricted for at least four reasons, including the lack of

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HCl in the flue gas, the interference of NH3 with Hg0 oxidation, Hg0 re-release in the FGD unit

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and the potential risk of exposure due to Hg2+ in the desulfurization gypsum.5, 8, 12, 13 The injection

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of sorbents upstream of the ESP for the conversion of gaseous Hg0 to Hgp is a potential

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commercial technology to control Hg0 emission from the flue gas.9, 14 As the ESP removes spent

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sorbents from the flue gas with a huge amount of fly ash particles, it is impossible to recover spent

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sorbents from the mixture collected by the ESP.15 Therefore, the injection of sorbents to control

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Hg0 emission from the flue gas is limited for at least three reasons, including the high operation

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cost because of the non-recyclability of the sorbent, the negative effect on the marketability of fly

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ash and the potential secondary emission of Hg during the utilization of the fly ash.16-19

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To recover spent sorbents for recycling and to achieve the centralized control of Hg pollution,

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the installation of a fabric filter (FF) downstream of the ESP was once suggested and in this setup,

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the sorbents would be injected between the ESP and the FF.15 As more than 99% of the fly ash

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particulates were collected by the ESP, the mixture collected by the FF consisted mainly of a

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small amount of ultrafine particulates and the sorbents which contained Hg. If the sorbents were

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magnetized, it was possible to recover spent sorbents for recycling and to achieve the centralized 4


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control of Hg pollution.17 Therefore, a large number of magnetic sorbents, including MagZ-Ag0,17,

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20

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and Co-MF 27 were developed to capture gaseous Hg0 in the flue gas. However, the installation of

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an FF merely to recover spent sorbents is particularly uneconomical.

MnOx/γ-Fe2O3,21 Fe-Ti-Mn spinel,15, 22 Mn-Fe spinel,16, 23, 24 Fe-Ti-V spinel,

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Fe-Ti spinel

26

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In China, wet electrostatic precipitators (WESPs) have been gradually installed downstream of

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the FGD to meet the stricter emission standards of coal-fired plants for ultrafine particulates.

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Magnetic sorbents can be injected downstream of the FGD for gaseous Hg0 capture. Then, the

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recovery of spent magnetic sorbents is a practical co-benefit of the WESP (the procedure is

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illustrated in Figure S1 in the Supporting Information).18 However, the temperature window of

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most reported magnetic sorbents (especially metal oxides) for gaseous Hg0 capture (100-200 oC)

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do not match the temperature range of the flue gas downstream of the FGD (40-100 oC).16, 22, 23, 26

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In our previous work, magnetic pyrrhotite (Fe1-xS) was developed as a recyclable sorbent for the

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recovery of gaseous Hg0 in the flue gas as a co-benefit of the WESP.

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showed excellent gaseous Hg0 capture performance at low temperatures, it was almost a

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permanent magnet, with a permanent magnetization of 3.7 emu g-1 (its saturation magnetization

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was only 7.0 emu g-1). This characteristic affected the re-dispersion of recovered pyrrhotite for the

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future recycling.

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18

Although pyrrhotite

In our previous work, super-paramagnetized Fe-Ti spinel with a saturation magnetization value

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higher than 30 emu g-1 was developed to capture gaseous Hg0 in the flue gas at 200-300 oC.

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However, this material showed a poor activity for gaseous Hg0 capture at low temperatures. To

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improve the performance of Fe-Ti spinel for Hg0 capture at low temperatures, it was modified by

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pretreatment with H2S. Modified Fe-Ti spinel showed an excellent performance for gaseous Hg0

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recovery and could be recycled; therefore, Hg0 recovery using the modified Fe-Ti spinel as a

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co-benefit of the WESP may be a promising and cost-effective technology for achieving the

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centralized control of Hg emission from the flue gas.

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2. Experimental Section

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2.1 Sample preparation

5


26


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Fe-Ti spinel (Fe/Ti=2) was prepared by co-precipitation followed by calcination at 400 oC for 3

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h.26, 28, 29 Then, Fe-Ti spinel was modified through treatment with 600 ppm of H2S/N2 at 300 oC

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for 1 h, with a gas hourly space velocity (GHSV) of 1.2×105 cm3 g-1 h-1.

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2.2 Characterization

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The BET surface area, magnetization, X-ray diffraction pattern (XRD) and X-ray photoelectron

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spectra (XPS) were determined using a nitrogen adsorption apparatus (Quantachrome,

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Autosorb-1), a vibrating sample magnetometer (VSM, LakeShore 735), an X-ray diffractometer

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(Bruker-AXS D8 Advance) and an X-ray photoelectron spectroscopy (Thermo, ESCALAB 250),

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respectively.

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2.3 Elemental mercury recovery

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Hg0 capture was conducted on a fixed-bed quartz tube microreactor at 40-100 oC. The GHSV

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used was generally 1.5×106 cm3 g-1 h-1 (i.e., approximately 1.5×106 h-1), 20 mg of the sorbent was

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used, and the gas flow was 500 mL min-1. The simulated flue gas contained approximately 110 µg

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m-3 of gaseous Hg0, 5% O2 (when used), 8% H2O (when used), 80 ppm of SO2 (when used) and a

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balanced amount of N2. The gaseous Hg0 concentration was determined online using a cold vapor

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atomic absorption spectrophotometer (CVAAS, Lumex R-915+). Meanwhile, the formation of

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SO2 due to the oxidation of modified Fe-Ti spinel during Hg0 capture was determined online using

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a Fourier transform infrared spectrometer (FTIR, Thermo SCIENTIFIC, ANTARIS, IGS

112

Analyzer). The temperature programmed desorption of Hg adsorbed on modified Fe-Ti spinel

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(Hg-TPD) under N2 and air atmospheres were both performed on the microreactor at a heating rate

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of 10 oC min-1 and a gas flow of 700 mL min-1. The leachability of Hg species from modified

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Fe-Ti spinel after Hg0 capture was determined using the toxicity characteristic leaching procedure

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(USA EPA) with 5.7 mL L-1 of glacial acetic acid (pH=2.88) as the extraction fluid.30

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The cycle of Hg0 capture, Hg0 recovery and sorbent regeneration was also performed on the

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fixed-bed microreactor. After the capture of Hg0 at 60 oC for 3 h with the GHSV of 3.0×105 cm3

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g-1 h-1 (i.e., 100 mg of modified Fe-Ti spinel and 500 mL min-1 of the gas flow), the Hg adsorbed

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on modified Fe-Ti spinel was recovered through thermal treatment at 300 oC for 40 min with a N2

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gas flow of 100 mL min-1. Next, the N2 gas flow was shifted to 600 ppm of H2S/N2 for 1 h to 6


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regenerate the spent modified Fe-Ti spinel. At last, the modified Fe-Ti spinel after the

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regeneration was recycled.

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3. Results and Discussion

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3.1 Characterization

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3.1.1

XRD and BET surface area

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The XRD pattern of Fe-Ti spinel corresponded notably well to the standard curve of cubic

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maghemite (γ-Fe2O3, JCPDS 39-1346), and the characteristic peaks corresponding to other iron

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oxides and TiO2 could not be observed (Figure 1a). This result suggests that Ti was incorporated

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into the spinel structure.26 After pretreatment with H2S, the slight peaks corresponding to cubic

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pyrite (FeS2, JCPDS 42-1340) were observed in Fe-Ti spinel, suggesting that the transformation of

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Fe-Ti spinel to iron sulfide occurred during the H2S pretreatment.31 The lattice parameter of

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modified Fe-Ti spinel (0.8388 nm) was much higher than that of Fe-Ti spinel (0.8335 nm),

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suggesting that some Fe3+ cations in Fe-Ti spinel were reduced to Fe2+ cations due to the H2S

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pretreatment. 32

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The BET surface areas of Fe-Ti spinel and modified Fe-Ti spinel were 87.5 and 32.7 m2 g-1,

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respectively.

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3.1.2

Magnetization

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Fe-Ti spinel exhibited super-paramagnetism with a minimized coercivity and a negligible

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magnetization hysteresis (Figure 1b). Meanwhile, Fe-Ti spinel showed an excellent magnetization

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with a saturation magnetization of 31.0 emu g-1. After pretreatment with H2S, the

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super-paramagnetism of Fe-Ti spinel was retained. However, the saturation magnetization of

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Fe-Ti spinel decreased to 24.6 emu g-1 due to the partial transformation of Fe-Ti spinel to

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non-magnetic pyrite. Due to the super-paramagnetism, the magnetization of modified Fe-Ti spinel

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simultaneously disappeared as the external magnetic field was removed. Therefore, spent

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modified Fe-Ti spinel, which was recovered by the magnetic separation, could be well

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re-dispersed without aggregation for recycling.

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3.1.3

XPS analysis

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The XPS spectra of Fe-Ti spinel, modified Fe-Ti spinel and denuded modified Fe-Ti spinel in

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the spectral regions of Fe 2p, Ti 2p, O 1s and S 2p are shown in Figure 2. The Fe 2p 3/2 binding

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energies on Fe-Ti spinel mainly appeared at 711.2 and 712.4 eV (Figure 2a), which were assigned

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to Fe3+ in the spinel structure and Fe3+ bonded with -OH, respectively.

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energies on Fe-Ti spinel mainly appeared at 458.7 and 464.4 eV (Figure 2b), which were

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attributed to Ti4+.

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531.7 eV (Figure 2c), which were assigned to O2- in transition metal oxides and O2- in -OH,

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respectively.29

28

26

The Ti 2p binding

The O 1s binding energies on Fe-Ti spinel mainly appeared at 530.2 and

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Two new binding energies appeared at 707.6 and 713.3 eV on modified Fe-Ti spinel in the

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spectral region of Fe 2p 3/2 (Figure 2d), which were attributed to Fe2+ bonded with S22- and Fe2+

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bonded with SO42-, respectively.26, 33, 34 The Ti 2p binding energies on modified Fe-Ti spinel still

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appeared at 464.6 and 458.9 eV (Figure 2e), which were consistent with those on Fe-Ti spinel

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(Figure 2b). A new binding energy at 532.2 eV appeared on modified Fe-Ti spinel in the O 1s

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spectral region (Figure 2f), which was assigned to SO42-.26 The S 2p binding energies on modified

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Fe-Ti spinel mainly appeared at 170.1, 168.9, 164.2 and 162.9 eV (Figure 2g), which were

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assigned to HSO4- , SO42-, polysulfur and S22-, respectively. 18, 26, 33, 35, 36

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Iron sulfide is easily oxidized after exposure to air. To exclude the effect of iron sulfide

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oxidation, modified Fe-Ti spinel was bombarded with Ar+ ions. The Fe 2p 3/2 binding energies on

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denuded modified Fe-Ti spinel mainly appeared at 707.2, 709.1, 711.1 and 712.4 eV (Figure 2h),

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which were assigned to Fe2+ bonded with S2- or S22-, Fe2+ and Fe3+ in the spinel structure and Fe3+

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bonded with -OH, respectively.18 The presence of Fe2+ in modified Fe-Ti spinel was also

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supported by the XRD analysis. The binding energies of S 2p on denuded modified Fe-Ti spinel

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mainly appeared at 161.7, 162.7 and 163.9 eV (Figure 2k), which were assigned to S2-, S22- and

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polysulfur, respectively.33, 34 However, the binding energy at 713.3 eV in the spectral region of Fe

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2p 3/2 corresponding to Fe2+ bonded with SO42-, the binding energy at 532.2 eV in the spectral

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region of O 1s corresponding to SO42-, and the binding energies at 168.9 and 170.1 eV in the

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spectral region of S 2p corresponding to SO42- and HSO4- could not be observed on denuded

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modified Fe-Ti spinel (Figure 2h, 2j and 2k). These results suggest that S2-, S22- and polysulfur 8


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directly resulted from the reaction between gaseous H2S and Fe-Ti spinel,31, 37-39 while SO42-

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indirectly resulted from the oxidation of S2- and S22- on the surface due to the exposure of

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modified Fe-Ti spinel to air. Therefore, the reaction between Fe-Ti spinel and H2S can be

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approximately described as: 37-39

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2H 2 S+2Fe 3+ =O → FeS 2 +Fe 2+ -O+H 2 O

(1)

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H 2 S+Fe 2+ =O → FeS+H 2 O

(2)

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FeS 2 → FeS+S 0

(3)

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The percentages of S species on modified Fe-Ti spinel and denuded modified Fe-Ti spinel,

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which were determined from the XPS analysis, are shown in Table 1. Table 1 shows that the

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percentages of S2- and S22- on modified Fe-Ti spinel were much lower than those on denuded

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modified Fe-Ti spinel, suggesting that most S2- and S22- on modified Fe-Ti spinel were oxidized to

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SO42- due to the exposure to air.40 However, the percentage of polysulfur on modified Fe-Ti spinel

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was only slightly less than that on denuded modified Fe-Ti spinel, suggesting that polysulfur was

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relatively stable on modified Fe-Ti spinel during exposure to air. This result was probably due to

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the strong binding energy of S-S.

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3.2 Elemental mercury capture

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The average rates of Hg0 capture by Fe-Ti spinel and modified Fe-Ti spinel at 40-100 oC were

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determined from the breakthrough curves (Figure 3) and are listed in Table 2. The average rates of

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Hg0 capture by Fe-Ti spinel were all less than 0.1 µg g-1 min-1 at 40-100 oC. However, the average

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rates of Hg0 capture by modified Fe-Ti spinel were in the range of 1.09-1.88 µg g-1 min-1, which

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were 12-169 times those of Fe-Ti spinel. This result suggests that the Hg0 capture performance of

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Fe-Ti spinel at low temperatures obviously improved after pretreatment with H2S.

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There is generally a low concentration of SO2 and a high concentration of water vapor in the

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flue gas downstream of the FGD, which often inhibit Hg0 capture by sorbents.16, 41 Therefore, the

201

effect of SO2 and H2O on Hg0 capture by modified Fe-Ti spinel was investigated (Figure 3c). The

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average rates of Hg0 capture by modified Fe-Ti spinel in the presence of 8% H2O and 80 ppm of

203

SO2 were generally close to those in the absence of H2O and SO2 (Table 2). This result suggests 9



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that the presence of H2O and SO2 did not show a remarkable inhibition on Hg0 capture by

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modified Fe-Ti spinel.

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As Hg0 capture is placed downstream of the FGD, the emission of SO2 due to the oxidation of

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modified Fe-Ti spinel is a serious concern. Figure S2 in the Supporting Information shows that the

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concentrations of gaseous SO2 formed during Hg0 capture by modified Fe-Ti spinel were

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generally less than 3 ppm at temperatures below 80 oC. This result suggests that the oxidation of

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modified Fe-Ti spinel to gaseous SO2 during Hg0 capture was negligible below 80 oC.

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As modified Fe-Ti spinel after Hg0 capture was collected with the fly ash from the WESP by

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water washing, the leachability of Hg species from modified Fe-Ti spinel was investigated.

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Although the initial amount of mercury adsorbed onto modified Fe-Ti spinel was approximately

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32.1 µg (100 mg of modified Fe-Ti spinel), neither Hg0 nor Hg2+ were observed in the extraction

215

fluid of the glacial acetic acid solution. This result suggests that the leaching of Hg species from

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modified Fe-Ti spinel was negligible.

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3.3 Mechanism for elemental mercury capture

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Figure 4 shows the XPS spectra of modified Fe-Ti spinel after Hg0 capture in the spectral regions

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of Fe 2p, Ti 2p, O 1s, S 2p and Hg 4f. After the adsorption of Hg0 onto modified Fe-Ti spinel,

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there were no obvious changes in the spectral regions of Fe 2p, Ti 2p and O 1s (Figures 4a, 4b and

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4c). However, a new, slight peak at 161.8 eV appeared in the spectral region of S 2p (Figure 4d),

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which was attributed to S2-.18 The binding energies of Hg 4f mainly appeared at 101.0 and 105.1

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eV (Figure 4e), which were assigned to HgO or HgS.22, 25, 33, 34

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The chemical adsorption of Hg0 onto Fe-Ti spinel was often attributed to the Mars-Maessen

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mechanism, which can be approximately described as follows: 14, 26

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Hg 0 (g) → Hg 0 ( ad )

(4)

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Hg 0 (ad) +2Fe 3+ +O 2- → HgO+2Fe 2+

(5)

228

4Fe 2+ +O 2 → 4Fe 3+ +2O 2-

(6 )

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Reaction 4 shows the physical adsorption of gaseous Hg0 on the surface. Then, physically

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adsorbed Hg0 is oxidized by Fe3+ on the surface to HgO (i.e., Reaction 5). Reaction 6 shows the 10


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regeneration of Fe3+ on the surface through oxidation by gaseous O2. As the oxidation ability of

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Fe3+ on Fe-Ti spinel was very poor at low temperatures, Fe-Ti spinel showed a poor performance

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for the chemical adsorption of Hg0 (Figure 3a). As the oxidation ability of Fe3+ on Fe-Ti spinel

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obviously increased at increasing reaction temperatures, Fe-Ti spinel showed a moderate activity

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for Hg0 adsorption at 200-350 oC (Figure S3).26

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The chemical adsorption of Hg0 onto modified Fe-Ti spinel can also be attributed to the

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Mars-Maessen mechanism, and the potential oxidants for the oxidation of physically adsorbed Hg0

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were Fe3+, polysulfur, S22- and gaseous O2. As Fe-Ti spinel showed a poor performance for Hg0

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capture at low temperatures, Fe3+ on modified Fe-Ti spinel was ruled out. Table 2 shows that the

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average rates of modified Fe-Ti spinel for Hg0 capture at 40-100 oC under N2 atmosphere were all

241

close to those in the presence of O2. Thus, gaseous O2 was ruled out. Elemental sulfur (i.e.,

242

polysulfur) showed a poor performance for Hg0 capture at low temperatures due to the strong

243

binding energy of S-S (Figure S4).18, 42 Thus, polysulfur was ruled out. Therefore, the oxidant on

244

modified Fe-Ti spinel for the oxidation of physically adsorbed Hg0 could only be S22-, and the

245

formed Hg species was HgS. Then, the chemical adsorption of Hg0 onto modified Fe-Ti spinel can

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be approximately described as follows:

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Hg 0 (g) → Hg 0 ( ad )

(4)

248

2Hg 0 (ad) +S 22 → HgS+S

(7)

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Table 1 demonstrates that the percentage of S22- on modified Fe-Ti spinel slightly decreased

250

after the chemical adsorption of Hg0 and that a small amount of S2- appeared. As the performance

251

of modified Fe-Ti spinel for the chemical adsorption of gaseous Hg0 was much better than that of

252

Fe-Ti spinel, the ability of S22- on the surface to oxidize the physically adsorbed Hg0 was much

253

better than that of Fe3+ on the surface.

254

3.4 Comparison of modified Fe-Ti spinel with other magnetic sorbents

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Table 3 compares the Hg0 capture performance of modified Fe-Ti spinel with those of other

256

reported magnetic sorbents. The capacity of modified Fe-Ti spinel for Hg0 capture was

257

approximately 0.69 mg g-1 at 60 oC with the breakthrough threshold of 5% (Figure S5 in the 11



258

Supporting Information), and the average rate of Hg0 capture by modified Fe-Ti spinel was 1.92

259

µg g-1 min-1 at 60 oC. This performance was much better compared to MagZ-Ag0,17, 20 Mn-Fe

260

spinel,16, 23 Fe-Ti-Mn spinel,15 Co-MF

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from modified Fe-Ti spinel was negligible because the formed Hg species was HgS, which was

262

much better than Mn-Fe spinel, 16 Fe-Ti-Mn spinel 15 and Co-MF

263

water soluble HgSO4 or HgCl2). Moreover, the magnetization of modified Fe-Ti spinel was much

264

higher than that of pyrrhotite,18 so the separation of modified Fe-Ti spinel from the mixture

265

collected by the WESP was much easier than when pyrrhotite was used. In summary, modified

266

Fe-Ti spinel showed the best performance for capturing Hg0 from the flue gas (Table 3).

267

3.5 Elemental mercury recovery and the recycle of sorbent

27

and pyrrhotite.18 Meanwhile, the leaching of Hg species

27

(the formed Hg species were

268

The simplest method to recover Hg from modified Fe-Ti spinel was thermal treatment. Figure 5

269

shows the Hg-TPD profiles of modified Fe-Ti spinel after Hg0 capture. The amount of Hg0

270

desorbed, which was determined from the integration of the Hg-TPD profile, was approximately

271

equal to the amount of Hg0 adsorbed, which was determined from the integration of the

272

breakthrough curve. These findings suggest that most of the Hg adsorbed on modified Fe-Ti spinel

273

could be thermally desorbed. The Hg-TPD profile under a N2 atmosphere showed a strong

274

desorption peak at 190 oC and a tailing desorption peak at 270 oC (Figure 5). However, the

275

Hg-TPD profile under an air atmosphere showed three desorption peaks at 190, 270 and 350 oC.

276

The desorption peak at 190 oC could be assigned to the decomposition of HgS.18, 43, 44 Figure 5

277

also shows that the intensity of the desorption peak at 190 oC under the air atmosphere was much

278

less than that under the N2 atmosphere. Figure S3 in the Supporting Information shows that Fe-Ti

279

spinel exhibited a moderate performance for gaseous Hg0 capture at 200-350 oC and the

280

adsorption of Hg0 onto Fe-Ti spinel in the presence of O2 was much better than that under the N2

281

atmosphere. This result suggests that during the Hg-TPD process, there was a re-adsorption of Hg0,

282

which resulted from the decomposition of HgS, onto Fe-Ti spinel. Moreover, the reduced intensity

283

of the desorption peak at 190 oC under air may be mainly related to the promotion of the

284

re-adsorption of Hg0 on Fe-Ti spinel in the presence of O2. Therefore, the desorption peak at 270

285

o

C was assigned to the decomposition of HgO,15, 16, 44 which resulted from the re-adsorption of 12


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Hg0. Furthermore, the desorption peak at 350 oC may be attributed to the decomposition of

287

HgSO4,44 which resulted from the oxidation of HgS. The complete desorption of adsorbed Hg

288

from modified Fe-Ti spinel shifted approximately 50 oC to high temperature in the presence of O2

289

(Figure 5). Thus, the recovery of Hg from modified Fe-Ti spinel was performed under a N2

290

atmosphere and the temperature was selected at 300 oC.

291

Figure 6 shows the breakthrough curves of Hg0 capture by modified Fe-Ti spinel during the five

292

cycles of Hg0 capture, Hg0 recovery and sorbent regeneration. The efficiencies of modified Fe-Ti

293

spinel for Hg0 capture during the 5 cycles were all close to 100%. These results suggest that the

294

ability of modified Fe-Ti spinel to capture Hg0 from the flue gas did not degrade remarkably.

295

Although the characteristic peaks corresponding to pyrite became more apparent in the XRD

296

pattern of modified Fe-Ti spinel after the 5 cycles due to the reduplicative pretreatment with H2S,

297

the characteristic peaks corresponding to spinel were still predominant in the XRD pattern (Figure

298

1a). This finding suggests that the spinel structure of modified Fe-Ti spinel did not change

299

remarkably after the five cycles and that modified Fe-Ti spinel still remained an excellent

300

super-paramagnetism with a saturation magnetization of 11.8 emu g-1 (Figure 1b). The inset

301

picture in Figure 1b indicates that modified Fe-Ti spinel after the 5 cycles can still be easily

302

magnetically separated from a mixture of 0.2 g of modified Fe-Ti spinel, 2.0 g of fly ash and 20

303

mL of deionized water. Furthermore, the concentration of gaseous Hg0 in the recovery process

304

(100 mg of modified Fe-Ti spinel, 100 mL of N2 and 300 oC) was much higher than 5000 µg m-3.

305

The high concentration of gaseous Hg0 in the recovery process may facilitate the centralized

306

control of Hg pollution. These results all suggest that modified Fe-Ti spinel can be recycled to

307

recover Hg0 from the flue gas.

308

4. Perspectives

309

The recovery of Hg0 by recyclable modified Fe-Ti spinel as a co-benefit of the WESP may be a

310

cost-effective technology for the centralized control of Hg0 emission from the flue gas. However,

311

the environmental benefit and cost-effect of this technology will become very low if most Hg

312

species present in the flue gas are Hgp and Hg2+, which are converted into fly ash and

313

desulfurization gypsum upstream of the WESP. The Hg species in the flue gas after coal 13



314

combustion primarily consist of gaseous Hg0, and the conversion of Hg0 to Hg2+ or Hgp in the flue

315

gas is mainly related to the presence of HCl in the flue gas.45 Meanwhile, HCl in the flue gas also

316

causes the extensive corrosion of metal components and the formation of organic chlorinated

317

compounds.46 Therefore, the removal of HCl in the flue gas upstream of the SCR unit by the

318

injection of sorbents is suggested.

319 320

14


Page 14 of 30

Page 15 of 30


321

322

Corresponding Author * School of Environmental and Biological Engineering, Nanjing University of Science and

323

Technology. Telephone: 86-18-066068302; E-mail: [email protected].

324

Acknowledgements

325

This study was financially supported by the National Natural Science Fund of China (Grant No.

326

41372044) and the Natural Science Fund of Jiangsu Province (Grant No. BK20150036).

327

Supporting Information

328

The Supporting Information is available free of charge on the ACS Publications Website (DIO:

329

XXXX), and includes the average rates of gaseous Hg0 capture by γ-Fe2O3 and modified γ-Fe2O3;

330

the procedure of Hg0 recovery from the flue gas by magnetic sorbents as a co-benefit of the WESP;

331

the concentration of SO2 formed during Hg0 capture by modified Fe-Ti spinel; and breakthrough

332

curves of gaseous Hg0 capture by γ-Fe2O3, modified γ-Fe2O3, Fe-Ti spinel, elemental sulfur,

333

Mn-Fe spinel, Fe-Ti-Mn spinel, pyrrhotite and modified Fe-Ti spinel.

334

15



335

References:

336

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337

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manocrystalline mackinawite (FeS). Environ. Sci. Technol. 2007, 41, 7699-7705.

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Manage. 2012, 101, 197-205.

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447

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448

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452

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453

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454

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455

1261-1273.

456 457 458

20


Page 20 of 30

Page 21 of 30


459 460

Table 1. The percentages of S species on modified Fe-Ti spinel, denuded modified Fe-Ti spinel

461

and

462

/%

modified

Fe-Ti

spinel

Hg0

after

capture

total S

S2-

S22-

polysulfur

SO42-/HSO4-

modified Fe-Ti spinel

6.2

0

0.8

1.9

3.5

denuded modified Fe-Ti spinel

9.9

3.4

4.4

2.2

0

modified Fe-Ti spinel after Hg0 capture

6.3

0.1

0.7

1.9

3.6

463 464 465

21



Page 22 of 30

466

Table 2. Average rates of gaseous Hg0 capture by Fe-Ti spinel and modified Fe-Ti spinel

467

/µg g-1 min-1 40 oC

60 oC

80 oC

100 oC

Fe-Ti spinel

0.01

0.02

0.06

0.09


1.69

1.88

1.57

1.09

modified Fe-Ti spinel under N2

1.61

1.84

1.60

1.07

modified Fe-Ti spinel with H2O and SO2

1.34

1.92

1.50

0.96

468 469

22


Page 23 of 30


470 471

Table 3. Comparison of the performance of modified Fe-Ti spinel for gaseous Hg0 capture with

472

those of other magnetic sorbents reaction rate

capacity for Hg0

reaction

Hg species

Magnetization

/µg g-1 min-1

capture/mg g-1

condition

formed

/emu g-1

0.74

0.013 (20%)

0.24

0.033 (25%)

MagZ-Ag0 Ar at 50-150 oC

amalgam

simulated flue

HgSO4 and

gas at 60 oC

HgO

simulated flue

HgSO4 and

gas at 60 oC

HgO

simulated flue

HgSO4, HgCl2

gas at 150 oC

and HgO

40.0

17, 20

Mn-Fe spinel 16, 23 Fe-Ti-Mn 0.25

29.6

0.075 (25%)

spinel 15 Co-MF 27

pyrrhotite 18

0.16

45.6

0.03 (35%)

24.4

simulated flue 0.28

0.22 (4%)

1.92

0.69 (5%)

gas at 60 oC

modified

HgS

7.0

HgS

24.6

simulated flue gas at 60 oC

Fe-Ti spinel 473 474 475

23



476

Figure captions

477

Figure 1. (a), XRD patterns of Fe-Ti spinel, modified Fe-Ti spinel and modified Fe-Ti spinel after

478

the 5 cycles of Hg0 capture, Hg0 recovery and sorbent regeneration; (b), magnetization

479

characteristics of Fe-Ti spinel, modified Fe-Ti spinel and modified Fe-Ti spinel after the 5 cycles.

480

Figure 2. XPS spectra of Fe-Ti spinel, modified Fe-Ti spinel and denuded modified Fe-Ti spinel in

481

the spectral regions of Fe 2p, Ti 2p, O1s and S 2p.

482

Figure 3. Breakthrough curves of gaseous Hg0 capture by: (a), Fe-Ti spinel; (b), modified Fe-Ti

483

spinel; (c), modified Fe-Ti spinel in the presence of H2O and SO2; (d): modified Fe-Ti spinel

484

under N2 atmosphere. Reaction conditions: [Hg0] =100-120 µg m-3, [O2] =5%, [H2O] =8% (when

485

used), [SO2] =80 ppm (when used), sorbent mass=20 mg for modified Fe-Ti spinel and 100 mg for

486

Fe-Ti spinel, total flow rate=500 mL min-1 and GHSV=300000 or 1500000 cm3 g-1 h-1.

487

Figure 4. XPS spectra of modified Fe-Ti spinel after Hg0 capture in the spectral regions of Fe 2p,

488

Ti 2p, O1s, S 2p and Hg 4f

489

Figure 5. Hg-TPD profiles of modified Fe-Ti spinel after Hg0 capture under N2 and air

490

atmospheres.

491

Figure 6. Breakthrough curves of Hg0 capture by modified Fe-Ti spinel during the five cycles of

492

Hg0 capture, Hg0 recovery and sorbent regeneration.

493 494 495

24


Page 24 of 30

Page 25 of 30


modified Fe-Ti spinel after the 5 cycles modified Fe-Ti spinel Fe-Ti spinel maghemite JCPDS 39-1346 pyrite JCPDS 42-1340 20

30

40

50

60

70

2θ/degree

a

b 496 497

Figure 1. (a), XRD patterns of Fe-Ti spinel, modified Fe-Ti spinel and modified Fe-Ti spinel after

498

the 5 cycles of Hg0 capture, Hg0 recovery and sorbent regeneration; (b), magnetization

499

characteristics of Fe-Ti spinel, modified Fe-Ti spinel and modified Fe-Ti spinel after the 5 cycles.

500

25



Fe-Ti spinel

Fe 2p

Fe-Ti spinel

Ti 2p

Page 26 of 30

O 1s

Fe-Ti spinel

458.7

711.2

724.8

530.2

712.4

719.3

464.4 531.7

730

725

720

715

710

705

468

466

464

462

460

458

456

536

534

532

530

Binding Energy/eV

Binding Energy/eV

Binding Energy/eV

a

b

c

Fe 2p


725.2


Ti 2p


O 1s

530.4

711.2

712.4

528

458.9

719.9 713.3

531.7 532.2

464.6 707.6

730

725

720

715

710

705

468

466

464

462

460

458

456

536

534

532

530

Binding Energy/eV

Binding Energy/eV

Binding Energy/eV

d

e

f

S 2p



Fe 2p

Ti 2p


168.9 722.0 720.2 724.1

164.2

170.1

714.2

162.9

174

172

170

168

166

164

162

160

730

725

720

464.6

715

710

705

468

466

464

462

460

Binding Energy/eV

Binding Energy/eV

g

h

i

162.7

532

456

161.7

163.9

531.5

534

458

S 2p


O 1s

530.4

536

458.9

709.1 711.1 707.2 712.4

Binding Energy/eV


528

530

528

174

172

170

168

166

164

Binding Energy/eV

Binding Energy/eV

j

k

162

160

501

Figure 2. XPS spectra of Fe-Ti spinel, modified Fe-Ti spinel and denuded modified Fe-Ti spinel in

502

the spectral regions of Fe 2p, Ti 2p, O1s and S 2p.

503 26


Page 27 of 30


-3

Hg concentration/µg m

100 80 o

o

40 C o 80 C

60 40

60 C o 100 C

0

0


-3

120

20

120 o

100 80

0

60 40 20

50

100

150

0

200

50

-3 o


o

40 C o 80 C

60 C o 100 C

60 40

0

-3

120

80

150

200

b

a

100

100

t/min

t/min

0

60 C o 100 C

0

0


o

40 C o 80 C

20

120 o

100

o

40 C o 80 C

80

60 C o 100 C

60 40 20 0

0 0

50

100

150

0

200

50

100

150

200

t/min

t/min

c

d

504 505

Figure 3. Breakthrough curves of gaseous Hg0 capture by: (a), Fe-Ti spinel; (b), modified Fe-Ti

506

spinel; (c), modified Fe-Ti spinel in the presence of H2O and SO2; (d): modified Fe-Ti spinel

507

under N2 atmosphere. Reaction conditions: [Hg0] =100-120 µg m-3, [O2] =5%, [H2O] =8% (when

508

used), [SO2] =80 ppm (when used), sorbent mass=20 mg for modified Fe-Ti spinel and 100 mg for

509

Fe-Ti spinel, total flow rate=500 mL min-1 and GHSV=300000 or 1500000 cm3 g-1 h-1.

510

27



Page 28 of 30

511 0

Fe 2p

modified Fe-Ti spinel after Hg capture

0


711.0

725.0

0

Ti 2p


458.8

712.4

530.2

713.3

719.8

O 1s

532.2

464.5

531.7

707.4

730

725

720

715

710

705

468

466

Binding Energy/eV

464

S 2p

170

168

166

164

534

532

530

c

528

Hg 4f

101.0

105.1 164.0

536

456

b modified Fe-Ti spinel after Hg capture

168.9

172

458

0

0


174

460

Binding Energy/eV

a

170.2

462

Binding Energy/eV

162.8 161.8

162

160

108

106

104

102

Binding Energy/eV

Binding Energy/eV

d

e

100

98

512 513

Figure 4. XPS spectra of modified Fe-Ti spinel after Hg0 capture in the spectral regions of Fe 2p,

514

Ti 2p, O1s, S 2p and Hg 4f.

515

28


Page 29 of 30


516

0

Hg concentration/ µg m

-3

517 1200 1000

under N2 under air

800 600 400 200 0 0

100

200

300

400

o

Temperature/ C

518 519

Figure 5. Hg-TPD profiles of modified Fe-Ti spinel after Hg0 capture under N2 and air

520

atmospheres.

29



Page 30 of 30

521

100 the first run the second run the third run the forth run the fifth run

80 60 40 20

0

Ηg concentration/µg m

-3

120

0 0

522

50

100

150

200

t/min

523

Figure 6. Breakthrough curves of Hg0 capture by modified Fe-Ti spinel during the five cycles of

524

Hg0 capture, Hg0 recovery and sorbent regeneration.

525 526 527

30


A Recyclable Magnetic Sorbent for Recovering ... - ACS Publications

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