TiO2 to SO2 for capturing

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Outstanding resistance of H2S-modified Cu/TiO2 to SO2 for capturing gaseous Hg0 from non-ferrous metal smelting flue gas: Performance and reaction mechanism Lingnan Kong, Sijie Zou, Jian Mei, Yang Geng, Hui Zhao, and Shijian Yang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03484 • Publication Date (Web): 09 Aug 2018 Downloaded from http://pubs.acs.org on August 16, 2018

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Environmental Science & Technology

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Outstanding resistance of H2S-modified Cu/TiO2 to SO2 for

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capturing gaseous Hg0 from non-ferrous metal smelting flue gas:

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Performance and reaction mechanism

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Lingnan Kong, ┼, ╪ Sijie Zou, ╪ Jian Mei, ┼ Yang Geng, ╪ Hui Zhao, ┼ Shijian Yang ┼, *

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┼

Engineering, Jiangnan University, Wuxi 214122, P. R. China

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Jiangsu Key Laboratory of Anaerobic Biotechnology, School of Environment and Civil

╪

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

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ACS Paragon Plus Environment


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Abstract:

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The utilization of H2SO4, produced using SO2 from non-ferrous metal smelting flue gas as a

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source of S, is extremely restricted due to Hg contamination; therefore, there is great demand to

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remove Hg0 from smelting flue gas. Although the ability of Cu/TiO2 to capture Hg0 is excellent, its

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resistance to H2O and SO2 is very poor. In this study, Cu/TiO2 was treated with H2S to improve its

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resistance to H2O and SO2 for capturing Hg0. The chemical adsorption of Hg0 on Cu/TiO2 was

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primarily through the HgO route, which was almost suppressed by H2O and SO2 due to the

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transformation of CuO into CuSO4. Besides the HgO route, the HgS route also contributed to the

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chemical adsorption of Hg0 on modified Cu/TiO2. As the CuS on modified Cu/TiO2 was inert to

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H2O and SO2, the chemical adsorption of Hg0 on modified Cu/TiO2 through the HgS route was

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barely inhibited. Meanwhile, the HgS route was predominant in the chemical adsorption of Hg0 on

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modified Cu/TiO2. Therefore, modified Cu/TiO2 exhibited an excellent resistance to H2O and SO2,

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and its Hg0 capture capacity from simulated flue gas was up to 12.7 mg g-1 at 100 oC.

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Graphic Abstract

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1. Introduction

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The Minamata Convention on mercury was recently implemented to prevent the emission of Hg

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due to its toxicity and ability to bioaccumulate.1-3 Non-ferrous metal smelting, which is an

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important source of Hg emissions, is ranked as second to coal-fired power plants by the Minamata

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Convention.1, 4-8 However, the Hg concentration in smelting flue gas is at least 100 times higher

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than that in coal-fired flue gas as the Hg content of metal sulfide ores is much higher than that of

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coal.7, 9, 10 Similar to coal-fired flue gas,11 smelting flue gas contains gaseous Hg0, gaseous Hg2+,

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and particle-bound mercury (Hgp).9

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Modern smelters are equipped with the air pollution control devices, such as electrostatic

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precipitators (ESPs), flue gas scrubbers, and acid plants with double conversion and double

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adsorption (DCA) (see Figure S1 in the Supporting Information).12, 13 Hg2+ can be efficiently

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captured by scrubbers due to its solubility, and it then enters the acid waste water, while Hgp can

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be efficiently captured by the ESPs along with particulate matter.9 Due to the high insolubility and

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volatility of Hg0, it cannot be removed using either method. The SO2 concentration in the smelting

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flue gas usually exceeds 2%, so SO2 emissions from smelters are controlled by using this sulfur

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source to produce sulfuric acid (H2SO4).12 However, the Hg0 in smelting flue gas will be

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catalytically oxidized into soluble Hg2+ in the DCA unit, which would eventually enter the H2SO4

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product. The use of Hg-polluted H2SO4 is extremely restricted by the Minamata Convention.12

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Therefore, a specific device should be installed upstream of the DCA to remove high

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concentrations of Hg0.

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As the Hg0 concentration in smelting flue gas is relatively high (~30 mg m-3),13 recovering

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gaseous Hg0 is the preferred method of removing it from smelting flue gas. Smelters are often

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equipped with condensers to recover gaseous Hg0 (see Figure S1 in the Supporting Information).

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However, the Hg0 concentration downstream of the condenser (~5 mg m-3) cannot meet the strict

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limitations due to the high saturated vapor pressure of Hg0. Therefore, a Boliden-Norzink soak-up

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unit is installed downstream of the condenser to further recover gaseous Hg0 from the smelting

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flue gas.9 Although the strict emission limitations can be met by the Boliden-Norzink technology,

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the technology is very dangerous and complicated due to the use of hypertoxic HgCl2 as the

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absorptive solution. Therefore, there is great demand to develop a more effective and 4


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environmentally sustainable technology for recovering gaseous Hg0 from smelting flue gas.

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The adsorption of gaseous Hg0 using monolithic TiO2-based sorbents to recover Hg0 from

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smelting flue gas was recently proposed (see Figure S1 in the Supporting Information). An

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adsorption tower containing monolithic TiO2-based sorbents can be installed downstream of the

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condenser to further remove Hg0 from the smelting flue gas. Once the Hg0 concentration

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downstream of the adsorption tower exceeds the limitations, the smelting flue gas will be shifted

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to another adsorption tower. The spent sorbents will then be thermally treated in air to desorb the

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adsorbed Hg0. Subsequently, the desorption gas containing ultrahigh concentrations of Hg0 will be

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introduced into the smelting flue gas upstream of the condenser for Hg0 recovery (see Figure S1 in

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the Supporting Information). Finally, the Hg0 in the smelting flue gas will be completely collected

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as liquid Hg in the condenser, and the sorbents will be recycled after regeneration. Among the

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first-row transition metal oxides supported on TiO2 (i.e., Cu/TiO2, Fe/TiO2, Co/TiO2, and Ni/TiO2),

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only Cu/TiO2 exhibited excellent Hg0 capture capacity at 40-100 °C. However, the resistance of

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Cu/TiO2 for capturing Hg0 to high concentrations of H2O and SO2 is very poor, resulting in a poor

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ability to capture Hg0 from actual smelting flue gas.

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To improve the resistance of Cu/TiO2 to H2O and SO2 when capturing Hg0, Cu/TiO2 was

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modified by treatment with H2S. The H2S-modified Cu/TiO2 not only exhibited an excellent

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ability to capture Hg0 at 40-100 oC but also exhibited an excellent resistance to high

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concentrations of H2O and SO2. Therefore, H2S-modified Cu/TiO2 may be a promising and

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cost-effective sorbent for recovering Hg0 from smelting flue gas.

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2. Experimental section

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2.1 Sample preparation and characterization

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Cu/TiO2 with a CuO loading of 5 wt.% was prepared following the wet impregnation method

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using Degussa P25 TiO2 as a support and cupric nitrate as a precursor.14 After drying at 110 oC for

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12 h, the sample was calcined at 500 °C for 3 h in air. It was then treated with 600 ppm of H2S/N2

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in a fixed-bed quartz tube microreactor at 300 °C for 60 min with a gas hourly space velocity

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(GHSV) of 1.2×105 cm3 g-1 h-1.15, 16 The X-ray photoelectron spectra (XPS) of the Cu/TiO2 and

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modified Cu/TiO2 were measured using a Thermo ESCALAB 250 X-ray photoelectron

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spectroscope. 5



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2.2 Hg0 recovery

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Hg0 capture and desorption were both conducted in the fixed-bed microreactor.17-19 The total gas

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flow for capturing Hg0 was 300 mL min-1, and the sorbent mass was generally 25 mg, resulting in

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a GHSV of 7.2×105 cm3 g-1 h-1. The reaction temperature for capturing Hg0 was within the range

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of 40-100 °C, which was close to that of the smelting flue gas downstream of the condenser. The

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simulated smelting flue gas contained 1% SO2 (when used), 8% H2O (when used), 7% O2 (when

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used), approximately 4200-4300 µg m-3 of gaseous Hg0, and was balanced with N2. The

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temperature programmed desorption of Hg (Hg-TPD) in N2 and air was conducted at a heating

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rate of 10 oC min-1 with a gas flow of 700 mL min-1. The Hg0 concentration was determined online

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using a Lumex R-915+ cold vapor atomic absorption spectrophotometer (CVAAS).

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The cycle of Hg0 capture, Hg0 desorption, and sorbent regeneration was also conducted in the

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fixed-bed microreactor. Hg0 capture was conducted at 100 °C for 3 h with a GHSV of 1.8×105 cm3

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g-1 h-1. The spent modified Cu/TiO2 was then thermally treated in air at 400 oC for 40 min with a

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GHSV of 1.5×104 h-1 to desorb the adsorbed Hg0. Subsequently, the spent modified Cu/TiO2 was

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regenerated with 600 ppm of H2S/N2 at 300 °C for 1 h with a GHSV of 1.8×105 cm3 g-1 h-1. Finally,

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the regenerated sorbent was recycled.

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3.

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3.1 Performance for Hg0 capture

Results and discussion

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Figure S2 in the Supporting Information shows the breakthrough curves of Hg0 capture using

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Cu/TiO2 and modified Cu/TiO2. The amounts of Hg0 captured per gram of sorbent within 3 h,

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which were obtained from the integration of the breakthrough curves, are listed in Table 1.

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Cu/TiO2 exhibited poor Hg0 capture ability at 40 oC (Figure S2a in the Supporting Information).

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Its ability to capture Hg0 was notably improved by an increase in the reaction temperature from 40

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to 100 oC (Figure S2a in the Supporting Information), and the amount of Hg0 captured per gram of

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Cu/TiO2 could reach approximately 6.0 mg g-1 at 100 °C (see Table 1). However, high

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concentrations of SO2 and H2O, which are inevitable in smelting flue gas, remarkably inhibited the

114

adsorption of Hg0 on Cu/TiO2 (see Figure S2b in the Supporting Information). The amounts of

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Hg0 captured per gram of Cu/TiO2 within 3 h decreased by at least 87% at 60-100 oC after the

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introduction of 8% of H2O and 1% of SO2 (see Table 1). 6


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The modified Cu/TiO2 exhibited an excellent ability for capturing Hg0 at 40-100 oC, and the

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amounts of Hg captured per gram of modified Cu/TiO2 exceeded 6.0 mg g-1 (see Table 1). This

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suggests that the performance of modified Cu/TiO2 for capturing Hg0 was superior to that of

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Cu/TiO2. Meanwhile, the ability of modified Cu/TiO2 to capture Hg0 only slightly decreased after

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the introduction of 8% H2O and 1% SO2 (see Figure S2e in the Supporting Information). Table 1

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shows that the amounts of Hg0 captured per gram of modified Cu/TiO2 were 7.5-53 times higher

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than those of Cu/TiO2. This suggests that the resistance of Cu/TiO2 to H2O and SO2 when

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capturing Hg0 was improved notably by H2S modification.

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3.2 Modification of Cu/TiO2 by the H2S treatment

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The XPS spectra of Cu 2p and S 2p for the Cu/TiO2 and modified Cu/TiO2 are shown in Figures

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1a-1c. The binding energies of Cu 2p 3/2 for Cu/TiO2 appeared at approximately 933.3 and 932.4

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eV (see Figure 1a), and were assigned to Cu2+ and Cu+, respectively.

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non-ignorable satellite peaks were observed at 939-945 eV between Cu 2p 3/2 and Cu 2p 1/2,

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indicating the presence of Cu2+ on Cu/TiO2. 21

20, 21

Meanwhile,

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Although the binding energies of Cu 2p 3/2 for modified Cu/TiO2 still appeared at

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approximately 933.4 and 932.2 eV (see Figure 1b), the ratio of the intensity of the peak at 933.4

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eV to that at 932.4 eV decreased below that of Cu/TiO2. Meanwhile, the satellite peaks at 939-945

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eV indicating the presence of Cu2+ almost disappeared. These suggest that the amount of Cu2+ on

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Cu/TiO2 decreased notably after H2S treatment. The binding energies of S 2p for modified

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Cu/TiO2 peaked at 170.0, 168.8, 166.7, 162.8, and 161.8 eV (see Figure 1c). The binding energies

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at 170.0, 168.8, and 166.7 eV were attributed to HSO4-, SO42-, and SO32-, respectively.17-19 The

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binding energy at 162.8 eV was assigned to the disulfide (S22-) in CuS (CuISI), while that at 161.8

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eV was assigned to the S2- in Cu2S.21-23 Although Figure 1c suggests the presence of SO42- on the

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modified Cu/TiO2, there was little binding energy of Cu 2p 3/2 corresponding to CuSO4 at 934.9

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eV in Figure 1b. 24 This suggests that the SO42- on modified Cu/TiO2 did not bond with the Cu2+

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on the surface.

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The percentages of Cu and S species on Cu/TiO2 and modified Cu/TiO2 obtained from the XPS

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analysis are shown in Table 2, and they suggest that a large amount of the CuO loaded on TiO2

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was transformed into copper sulfide (including CuS and Cu2S) after H2S treatment. 7



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3.3 Mechanism for Hg0 capture

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3.3.1 Cu/TiO2

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The XPS spectra of Cu 2p and Hg 4f for Cu/TiO2 after capturing Hg0 at 100 oC in N2+O2 are

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presented in Figures 1d and 1e. The binding energies of Cu 2p 3/2 for Cu/TiO2 after capturing Hg0

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still appeared at 933.4 and 932.4 eV (see Figure 1d). The binding energies of Hg 4f for Cu/TiO2

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after capturing Hg0 mainly appeared at 101.1 and 105.2 eV (see Figure 1e), and were attributed to

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Hg2+.18, 25 As the decomposition temperature of Hg species adsorbed on Cu/TiO2 was centered at

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approximately 200 °C (see the Hg-TPD profile in Figure S3 in the Supporting Information), the

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Hg species formed on Cu/TiO2 was attributed to HgO.

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gaseous Hg0 on Cu/TiO2 was mainly attributed to chemical adsorption, which involved the

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oxidation of Hg0.

26

This suggests that the adsorption of

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The potential oxidants contained on Cu/TiO2 for the oxidation of the physically adsorbed Hg0

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may include gaseous O2 and CuO. To investigate their roles in capturing Hg0, Hg0 was captured

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using Cu/TiO2 in N2 (see Figure S2c in the Supporting Information). Table 1 shows that Cu/TiO2

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can still capture a small amount of Hg0 in the absence of O2 at temperatures above 60 °C.

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Meanwhile, the binding energies of Hg 4f for Cu/TiO2 after capturing Hg0 at 100 °C in N2 still

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appeared at 101.1 and 105.2 eV (see Figure 1g), suggesting that the Hg species adsorbed on

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Cu/TiO2 was still HgO. These suggest that the CuO on Cu/TiO2 could participate in the chemical

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adsorption of Hg0 on Cu/TiO2. Table 1 shows that the ability of Cu/TiO2 to capture Hg0 in N2+O2

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was much better than that in N2, suggesting that gaseous O2 also participated in the chemical

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adsorption of Hg0 on Cu/TiO2. Table 2 shows that the percentage of CuO on Cu/TiO2 decreased

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after capturing Hg0 at 100 °C in N2. However, the percentage of CuO on Cu/TiO2 after capturing

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Hg0 in N2+O2 was much higher than that in N2. These suggest that the role of gaseous O2 may be

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attributed to the regeneration of CuO. Therefore, the chemical adsorption of gaseous Hg0 on

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Cu/TiO2 can be described as follows:

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Hg 0 (g ) → Hg 0 (ad )

(1)

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2Cu II O+Hg 0 (ad ) → Cu I 2 O+HgO

(2)

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1 Cu I 2 O+ O 2 → 2Cu II O 2

(3) 8


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3.3.2 Modified Cu/TiO2

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The XPS spectra of Cu 2p, S 2p, and Hg 4f for modified Cu/TiO2 after capturing Hg0 at 100 °C

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in N2 are presented in Figures 1h-1j. The binding energy of Cu 2p 3/2 at 933.4 eV corresponding

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to Cu2+ almost disappeared for modified Cu/TiO2 after Hg0 was captured in N2 (see Figure 1h).

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Meanwhile, the binding energies of Hg 4f at 105.2 and 101.1 eV corresponding to HgO were still

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present (see Figure 1j), suggesting that the HgO route (i.e., Reactions 1-2) contributed to the

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chemical adsorption of Hg0 on modified Cu/TiO2.

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As well as CuO and gaseous O2, the CuS on modified Cu/TiO2 may participate in the chemical

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adsorption of Hg0 on modified Cu/TiO2.27 To investigate the role of CuS in the capture of Hg0,

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Hg0 was captured using modified Cu/TiO2 in N2. The modified Cu/TiO2 exhibited an excellent

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ability for Hg0 capture in N2 (see Figure S2f in the Supporting Information), and the amount of

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Hg0 captured per gram of modified Cu/TiO2 was 3.1-22.9 times higher than that of Cu/TiO2 (see

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Table 1). This suggests that the CuS on modified Cu/TiO2 participated in the chemical adsorption

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of Hg0 on modified Cu/TiO2. A new binding energy of S 2p appeared at 161.2 eV for modified

188

Cu/TiO2 after capturing Hg0 in N2 (see Figure 1i), which was assigned to the S2- in HgS.15, 28 As

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well as the binding energies of Hg 4f at 105.2 and 101.1 eV corresponding to HgO, two binding

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energies of Hg 4f appeared at 104.2 and 100.2 eV for modified Cu/TiO2 after capturing Hg0 in N2

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(see Figure 1j), and were attributed to HgS.23, 29, 30 Table 2 shows that the percentage of CuS (i.e.,

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disulfide) on modified Cu/TiO2 decreased notably after capturing Hg0 in N2. These suggest that

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another reaction route (the HgS route, Reaction 4) contributed to the chemical adsorption of Hg0

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on modified Cu/TiO2.

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2Cu ISI +Hg 0 ( ad ) → Cu I 2SII +Hg IISII

(4)

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The Hg-TPD profiles under N2 atmosphere of modified Cu/TiO2 after capturing Hg0 in N2 and

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N2+O2 are shown in Figures 2a and 2b, respectively. After the peak-fit, the Hg-TPD profiles of

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modified Cu/TiO2 after capturing Hg0 in N2 and N2+O2 both exhibited two Hg species

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decomposition peaks at 200 and 250 °C, which were attributed to the decomposition of HgO and

200

HgS, respectively.26 The amounts of Hg species adsorbed (i.e., HgO and HgS), which were

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obtained from the integration of the desorption peaks, are listed in Table 3. The amount of HgO

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adsorbed on modified Cu/TiO2 after capturing Hg0 in N2+O2 was approximately 2.4 times higher 9



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than that in N2. This suggests that the chemical adsorption of Hg0 on modified Cu/TiO2 through

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the HgO route was promoted by O2, similar to that on Cu/TiO2. However, the amount of HgS

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adsorbed on modified Cu/TiO2 after capturing Hg0 in N2+O2 was close to that after capturing Hg0

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in N2 (see Table 3). This suggests that the chemical adsorption of Hg0 on modified Cu/TiO2

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through the HgS route was barely influenced by O2. HgS accounted for approximately 91% and 76%

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of the Hg species adsorbed on modified Cu/TiO2 after capturing Hg0 in N2 and N2+O2,

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respectively. Meanwhile, the binding energies of Hg 4f at 100.2 and 104.2 eV, corresponding to

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HgS on modified Cu/TiO2, were approximately 4.6 times more intense than those at 101.1 and

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105.2 eV corresponding to HgO (see Figure 1j). These both suggest that the HgS route was

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predominant in the chemical adsorption of Hg0 on modified Cu/TiO2.

213

3.4 Mechanism of the improvement of SO2 and H2O resistance

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To investigate the mechanism of inhibition by H2O and SO2, Hg0 was captured in N2+O2 at

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100 °C using Cu/TiO2 after treatment with H2O and SO2. The performance of Cu/TiO2 after

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treatment with H2O and SO2 for capturing Hg0 in N2+O2 was close to that of Cu/TiO2 in the

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presence of H2O and SO2 (see Figure 3a), suggesting that the inhibition of the chemical adsorption

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of Hg0 on Cu/TiO2 by H2O and SO2 was mainly related to the chemical reaction between Cu/TiO2

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and SO2/H2O.31, 32 The XPS spectra of Cu 2p and S 2p for Cu/TiO2 after treatment with H2O and

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SO2 are shown in Figures S4a and S4b. Figure S4a in the Supporting Information shows that a

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new binding energy of Cu 2p 3/2 at 934.9 eV corresponding to CuSO4 appeared for Cu/TiO2 after

222

treatment with H2O and SO2. Meanwhile, two new binding energies of S 2p appeared at 169.8 and

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168.6 eV, corresponding to HSO4- and SO42-, respectively (see Figure S4b in the Supporting

224

Information). Furthermore, Table 2 shows that the percentage of CuO on Cu/TiO2 decreased from

225

3.6% to 0.5% after treatment with H2O and SO2. These suggest that the CuO on Cu/TiO2 was

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transformed into CuSO4 in the presence of H2O and SO2, which accounted for the deactivation of

227

Cu/TiO2’s ability to capture Hg0 by H2O and SO2.

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The Hg-TPD profile in N2 of modified Cu/TiO2 after capturing Hg0 in the presence of H2O and

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SO2 is shown in Figure 2c. After the peak-fit, the Hg-TPD profile exhibited three Hg species

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decomposition peaks at 200, 250, and 290 °C, which were attributed to the decomposition of HgO,

231

HgS, and HgSO4, respectively.15, 26 The HgSO4 on modified Cu/TiO2 may result from the reaction 10


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of HgO with H2O and SO2.33, 34 Table 3 shows that the total amount of HgO and HgSO4 adsorbed

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on the modified Cu/TiO2 in the presence H2O and SO2 was much less that in N2+O2. This suggests

234

that the chemical adsorption of Hg0 on modified Cu/TiO2 through the HgO route was restrained by

235

H2O and SO2, similar to that on Cu/TiO2. However, the amount of HgS adsorbed on modified

236

Cu/TiO2 in the presence of H2O and SO2 was close to that in N2+O2 (see Table 3). This suggests

237

that the chemical adsorption of Hg0 on modified Cu/TiO2 through the HgS route was barely

238

restrained by H2O and SO2.

239

Meanwhile, the adsorption of Hg0 on modified Cu/TiO2 after treatment with SO2 and H2O at

240

100 °C in N2+O2 was conducted for comparison. Figure 3b shows that modified Cu/TiO2 after the

241

treatment of SO2 and H2O exhibited an excellent ability to capture Hg0, unlike Cu/TiO2 after the

242

treatment of SO2 and H2O. However, this ability was worse than that of modified Cu/TiO2 without

243

SO2 and H2O treatment (Figure 3b). The XPS spectra of Cu 2p and S 2p for modified Cu/TiO2

244

after treatment with SO2 and H2O are shown in Figures S4c and S4d, respectively. After treatment

245

with H2O and SO2, the binding energy of Cu 2p 3/2 at 933.4 eV, corresponding to CuO, almost

246

disappeared and a new binding energy at 934.9 eV, corresponding to CuSO4, appeared (see Figure

247

S4c in the Supporting Information). This suggests that the CuO on modified Cu/TiO2 was almost

248

transformed into CuSO4 after treatment with SO2 and H2O. Therefore, the deactivation of the

249

ability of modified Cu/TiO2 to capture Hg0 by treatment with SO2 and H2O was attributed to the

250

cut-off of the HgO route, similar to Cu/TiO2. However, Table 2 shows that there was still a large

251

amount of CuS on modified Cu/TiO2 after treatment with SO2 and H2O, and the slight decrease of

252

CuS percentage on modified Cu/TiO2 was mainly attributed to the notable increase in the number

253

of atoms on modified Cu/TiO2 due to the incorporation of a large amount of SO42-. This suggests

254

that little of the CuS on modified Cu/TiO2 was destroyed by H2O and SO2, resulting in an

255

excellent ability to capture Hg0. These results also indicate that H2O and SO2 did not restrain the

256

chemical adsorption of Hg0 on modified Cu/TiO2 through the HgS route considerably, although

257

the chemical adsorption through the HgO route was remarkably inhibited. Meanwhile, the HgS

258

route was predominant in the chemical adsorption of Hg0 on modified Cu/TiO2. Therefore, the

259

modified Cu/TiO2 for capturing Hg0 had a higher resistance to SO2 and H2O than Cu/TiO2.

260

3.5 Recycle of modified Cu/TiO2 for Hg0 capture 11



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The simplest method of desorbing the Hg species adsorbed on the sorbents is thermal

262

treatment.16 Although the temperature required for the complete desorption of the Hg adsorbed on

263

the modified Cu/TiO2 increased by approximately 50 oC in the presence of O2 due to the oxidation

264

of HgS to HgSO4 (see Figure S5 in the Supporting Information),15 thermal desorption was much

265

more feasible in air than in N2. Therefore, the Hg adsorbed on modified Cu/TiO2 was thermally

266

desorbed in air at 400 oC.

267

Figure 4 shows the Hg0 concentrations in the exhaust gases of Hg0 capture and desorption

268

during the five cycles of Hg0 capture, Hg0 desorption, and sorbent regeneration (i.e., H2S

269

treatment). The Hg0 removal efficiencies from the simulated smelting flue gas were all close to

270

100% during the five cycles. This suggests that multiple operations of Hg0 capture, Hg0 desorption,

271

and H2S treatment may not decrease the ability of modified Cu/TiO2 to capture Hg0 much. The

272

Hg0 concentration in the desorption gas could reach 50 mg m-3, which facilitated its recovery by

273

the condenser in the smelters (see Figure S1 in the Supporting Information).

274

Table S1 in the Supporting Information compares the ability of modified Cu/TiO2 to capture

275

Hg0 with those of other reported sorbents. The capacity of modified Cu/TiO2 to capture Hg0 from

276

the simulated smelting flue gas was approximately 12.7 mg g-1 at 100 °C, with a breakthrough

277

threshold of 0.1% (see Figure S6 in the Supporting Information). This performance was much

278

better than those of most carbon-based sorbents (including I-AC, S-AC, AC, and Darco AC),35

279

MagZ-Ag0,36,

280

modified Fe-Ti spinel,15 and pyrrhotite.28 Meanwhile, the Hg captured by modified Cu/TiO2 can

281

be desorbed as an ultrahigh concentration of Hg0 (~50 mg m-3), which was ultimately collected as

282

liquid Hg by the condenser in the smelters. Furthermore, the spent modified Cu/TiO2 can be

283

regenerated through H2S treatment with negligible degeneration of its ability to capture Hg0. In

284

summary, modified Cu/TiO2 could be a cost-effective and promising sorbent for recovering Hg0

285

from smelting flue gas.

37

Pt/wool,35 Mn-Fe spinel,17,

34

Fe-Ti-Mn spinel,33 MnO2/Al2O3,35 Co-MF,

286

12


38

Page 13 of 25


287 288

Corresponding Author * School of Environment and Civil Engineering, Jiangnan University. Telephone:

289

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

290

Acknowledgements

291

This study was financially supported by the National Natural Science Fund of China (Grant Nos.

292

21777070 and 41372044) and the Natural Science Fund of Jiangsu Province (Grant No.

293

BK20150036).

294

Supporting Information

295

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

296

XXXX), and includes illustration of Hg0 recovery from smelting flue gas using monolithic

297

TiO2-based sorbents, the breakthrough curves of gaseous Hg0 capture by Cu/TiO2 and modified

298

Cu/TiO2, Hg-TPD profiles of Cu/TiO2 and modified Cu/TiO2 after Hg0 capture, XPS spectra for

299

Cu/TiO2 and modified Cu/TiO2 after H2O and SO2 treatment, XRD patterns and BET surface areas

300

of Cu/TiO2 and modified Cu/TiO2, TPO profile of modified Cu/TiO2, and the comparison of the

301

performance of modified Cu/TiO2 for capturing Hg0 with those of other reported sorbents.

302 303 304

13



305

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413 414

17



Page 18 of 25

415

Table 1 Amounts of Hg0 captured by Cu/TiO2 and modified Cu/TiO2 within 3 h and their

416

breakthrough points at 3 h

Cu/TiO2

Modified Cu/TiO2

/mg g-1 40 oC

60 oC

80 oC

100 oC

N2+O2

0.16 (99%)

3.2 (79%)

3.8 (60%)

6.0 (55%)

In the presence of SO2 and H2O

0.12 (99%)

0.2 (99%)

0.48 (96%)

0.8 (97%)

N2

0.28 (98%)

0.64 (98%)

0.80 (98%)

1.4 (97%)

N2+O2

6.7 (34%)

6.9 (34%)

6.9 (35%)

7.0 (32%)

In the presence of SO2 and H2O

6.5 (44%)

5.2 (60%)

4.8 (62%)

6.8 (37%)

N2

6.7 (41%)

6.6 (41%)

5.9 (48%)

5.8 (55%)

417 418 419 420

18


Page 19 of 25


421 422

Table 2 Percentages of Cu, S, and Hg species on Cu/TiO2 and modified Cu/TiO2 Cu species

/%

S species

CuO Cu2O CuS Cu2S CuSO4 SO42- SO32- S22-

Hg species S2-

HgO HgS

Cu/TiO2

3.6

0.8

-

-

-

-

-

-

-

-

-

modified Cu/TiO2

0.8

1.7

1.6

0.8

-

1.9

0.5

1.5

0.7

-

-

0.5

1.0

-

-

1.9

-

-

-

-

-

-

-

0.3

1.1

0.5

1.4

4.0

-

1.1

0.2

-

-

3.1

1.2

-

-

-

-

-

-

-

0.1

-

3.4

1.0

-

-

-

-

-

-

-

0.2

-

-

1.6

0.8

1.6

-

1.2

0.3

0.8

1.9

0.1

0.6

H2O and SO2 pretreated Cu/TiO2 H2O and SO2 pretreated modified Cu/TiO2 Cu/TiO2 after Hg0 capture in N2 Cu/TiO2 after Hg0 capture in N2+O2 modified Cu/TiO2 after Hg0 capture in N2 423 424

19



Page 20 of 25

425 426

Table 3 Amounts of Hg species adsorbed on modified Cu/TiO2 Amount of Hg species adsorbed

The total amount of Hg

(obtained from the Hg-TPD profiles)

species adsorbed

HgO o

0

Hg adsorption in N2

/µg

HgS o

HgSO4 o

(obtained from the

(200 C)

(250 C)

(290 C)

breakthrough curves)

8

82

-

93

27

85

-

115

4

83

9

101

0

Hg adsorption in N2+O2 Hg0 adsorption in the presence of H2O and SO2 427

20


Page 21 of 25


428

Figure captions

429

Figure 1 XPS spectra of Cu 2p, S 2p, and Hg 4f for Cu/TiO2, modified Cu/TiO2, and those after

430

Hg0 capture.

431

Figure 2 Hg-TPD profiles under a N2 atmosphere of: (a), modified Cu/TiO2 after Hg0 capture in

432

N2; (b), modified Cu/TiO2 after Hg0 capture in N2+O2; (c), modified Cu/TiO2 after Hg0 capture in

433

the presence of H2O and SO2. Reaction conditions of Hg0 capture: adsorption temperature=100 oC,

434

[Hg0] =4200-4300 µg m-3, [O2]=7% (when used), [H2O]=8% (when used), [SO2]=1% (when used),

435

sorbent mass=15 mg, total flow rate=300 mL min-1, adsorption time=180 min.

436

Figure 3 (a) Comparison of Hg0 capture using Cu/TiO2 at 100 °C: ▲, Cu/TiO2 in N2+O2; ●,

437

Cu/TiO2 in the presence of H2O and SO2; ■, Cu/TiO2 after the treatment of H2O and SO2 in N2+O2.

438

(b) Comparison of Hg0 capture using modified Cu/TiO2 at 100 °C: ▲, modified Cu/TiO2 in N2+O2;

439

●, modified Cu/TiO2 in the presence of H2O and SO2; ■, modified Cu/TiO2 after the treatment of

440

H2O and SO2 in N2+O2. Reaction conditions: [Hg0]=4200-4300 µg m-3, [O2]=7%, [H2O]=8%

441

(when used), [SO2]=1% (when used), sorbent mass=25 mg, total flow rate=300 mL min-1,

442

GHSV=7.2×105 cm3 g-1 h-1.

443

Figure 4 Hg0 concentrations in the exhaust gases from Hg0 capture and desorption during the five

444

cycles of Hg0 capture, Hg0 desorption, and sorbent regeneration. Reaction conditions of Hg0

445

capture: adsorption temperature=100

446

[SO2]=1%, sorbent mass=100 mg, total flow rate=300 mL min-1, GHSV=1.8×105 cm3 g-1 h-1;

447

Reaction conditions of Hg0 desorption: desorption temperature=400 oC, sorbent mass=100 mg,

448

total air flow rate=100 mL min-1, GHSV=6.0×104 cm3 g-1 h-1.

o

C, [Hg0]=4200-4300 µg m-3, [O2]=7%, [H2O]=8%,

21



Page 22 of 25

449 Cu/TiO2

Cu 2p

932.2

933.3

941.4

943.9

960

955

950

945

940

162.8

935

930

960

955

950

945

a

b

955

941.2

945

940

950

Binding Energy/eV

930

172

170

935

162

160

Cu 2p

Cu/TiO2 after Hg capture in N2

Hg 4f

932.4

101.1

953.7

952.1

933.4

930

108

106

104

102

100

98

960

955

950

Hg 4f

945

940

935

930

Binding Energy/eV

e

f

0

Cu 2p

modified Cu/TiO2 after Hg capture in N2

0

S 2p


931.6

101.1 105.2

932.2

951.8

100

98

960

955

950

945

940

935

161.2 161.9

168.7 170.1

102

164

c

Binding Energy/eV

Cu/TiO2 after Hg capture in N2

104

166

0

105.2

d

106

168

943.8 941.3

0

108

161.8

166.7

Binding Energy/eV

0

932.4

943.7

935

Cu/TiO2 after Hg capture in N2+O2

933.4

953.6 952.1

960

940

Binding Energy/eV

Cu 2p

170.0

944.4

Binding Energy/eV

Cu/TiO2 after Hg capture in N2+O2

168.8

933.4

953.7

0

S 2p

952.0

932.4

952.1

953.7

modified Cu/TiO2

Cu 2p

modified Cu/TiO2

930

172

170

162.8

166.8

168

166

164

Binding Energy/eV

Binding Energy/eV

Binding Energy/eV

g

h

i

0


162

160

Hg 4f

100.2 104.2

101.1

105.2

108

106

104

102

100

98

Binding Energy/eV

j 450

Figure 1 XPS spectra of Cu 2p, S 2p, and Hg 4f for Cu/TiO2, modified Cu/TiO2, and those after

451

Hg0 capture.

22


Page 23 of 25


−3

12000

o

Hg concentration/µg m

250 C

8000 o

200 C

4000

20000 250 C ο

15000 200 C

10000

ο

5000

0

0


−3

452

0 0

100

200

300

400

500

0 0

−3

a

0


200

300

400

500

ο

Temperature/ C

15000

100

Temperature/ C

ο

b

250 C ο

10000 290 C ο

200 C ο

5000

0 0

100

200

300

400

500

Temperature/ C ο

453

c Figure 2 Hg-TPD profiles under a N2 atmosphere of: (a), modified Cu/TiO2 after Hg0 capture in

454

N2; (b), modified Cu/TiO2 after Hg0 capture in N2+O2; (c), modified Cu/TiO2 after Hg0 capture in

455

the presence of H2O and SO2. Reaction conditions of Hg0 capture: adsorption temperature=100 oC,

456

[Hg0] =4200-4300 µg m-3, [O2]=7% (when used), [H2O]=8% (when used), [SO2]=1% (when used),

457

sorbent mass=15 mg, total flow rate=300 mL min-1, adsorption time=180 min.

458

23



Page 24 of 25

5000 4000 3000 2000 1000

0


−3

459

0 0

50

100

150

200

150

200

t/min

5000 4000 3000 2000 1000

0


−3

a

0 0

50

100

t/min

b 460 461

Figure 3 (a) Comparison of Hg0 capture using Cu/TiO2 at 100 °C: ▲, Cu/TiO2 in N2+O2; ●,

462

Cu/TiO2 in the presence of H2O and SO2; ■, Cu/TiO2 after the treatment of H2O and SO2 in N2+O2.

463

(b) Comparison of Hg0 capture using modified Cu/TiO2 at 100 °C: ▲, modified Cu/TiO2 in N2+O2;

464

●, modified Cu/TiO2 in the presence of H2O and SO2; ■, modified Cu/TiO2 after the treatment of

465

H2O and SO2 in N2+O2. Reaction conditions: [Hg0]=4200-4300 µg m-3, [O2]=7%, [H2O]=8%

466

(when used), [SO2]=1% (when used), sorbent mass=25 mg, total flow rate=300 mL min-1,

467

GHSV=7.2×105 cm3 g-1 h-1.

468

24


Page 25 of 25


60000

st

nd

2 desorption

1 desorption

th

rd

th

4 desorption 5 desorption

3 desorption

40000 20000 4500

0


−3

469

st

nd

1 capture

2 capture

rd

3 capture

th

4 capture

th

5 capture

0 0

200

400

600

800

1000

1200

t/min

470 471

Figure 4 Hg0 concentrations in the exhaust gases from Hg0 capture and desorption during the five

472

cycles of Hg0 capture, Hg0 desorption, and sorbent regeneration. Reaction conditions of Hg0

473

capture: adsorption temperature=100

474

[SO2]=1%, sorbent mass=100 mg, total flow rate=300 mL min-1, GHSV=1.8×105 cm3 g-1 h-1;

475

Reaction conditions of Hg0 desorption: desorption temperature=400 oC, sorbent mass=100 mg,

476

total air flow rate=100 mL min-1, GHSV=6.0×104 cm3 g-1 h-1.

o

C, [Hg0]=4200-4300 µg m-3, [O2]=7%, [H2O]=8%,

477 478

25


TiO2 to SO2 for capturing

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