Multiform Sulfur Adsorption Centers and Copper-Terminated Active

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Multiform sulfur adsorption centers and copperterminated active sites of Nano-CuS for efficient elemental mercury capture from coal combustion flue gas Zequn Yang, Hailong Li, Shihao Feng, Pu Li, Chen Liao, Xi Liu, Jiexia Zhao, Jianping Yang, Po Heng Lee, and Kaimin Shih Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01181 • Publication Date (Web): 08 Jul 2018 Downloaded from http://pubs.acs.org on July 13, 2018

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Langmuir

Multiform Sulfur Adsorption Centers and Copper-terminated Active Sites of Nano-CuS for Efficient Elemental Mercury Capture from Coal Combustion Flue Gas Zequn Yanga, Hailong Lia,b*, Shihao Fengb, Pu Lia, Chen Liaob, Xi Liub, Jiexia Zhaob, Jianping Yangb, Po-Heng Leec, Kaimin Shiha** a. Department of Civil Engineering, The University of Hong Kong, Hong Kong, Hong Kong SAR, China b. School of Energy Science and Engineering, Central South University, Changsha, 410083, China c. Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong, Hong Kong SAR, China

Revision submitted to Langmuir

*To whom correspondence should be addressed: TEL: +86-18670016725 E-mail: [email protected]

**To whom correspondence should be addressed: TEL: +852-2859-2973 Email: [email protected]

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ACS Paragon Plus Environment

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ABSTRACT: Nanostructured copper sulfide synthesized with the assistance of surfactant with

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nano-scale particle size and high Brunauer-Emmett-Teller surface area was for the first time applied

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for the capture of elemental mercury (Hg0) from coal combustion flue gas. The optimal operation

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temperature of Nano-CuS for Hg0 adsorption is 75 °C, which indicates that injection of the sorbent

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between the wet flue gas desulfurization and the wet electrostatic precipitator systems is feasible.

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This assures that the sorbent is free of the adverse influence of nitrogen oxides. Oxygen (O2) and

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sulfur dioxide exerted a slight influence on Hg0 adsorption over the Nano-CuS. Water vapor was

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shown to moderately suppress Hg0 capture efficiency via competitive adsorption. The simulated

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adsorption capacities of Nano-CuS for Hg0 under pure nitrogen (N2), N2 plus 4% O2, and simulated

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flue gas reached 122.40, 112.06, and 89.43 mg Hg0/g Nano-CuS, respectively. Compared with that

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of traditional commercial activated carbons and metal sulfides, the simulated adsorption capacities

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of Hg0 over the Nano-CuS are at least an order of magnitude higher. Moreover, with only 5 mg

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loaded in a fixed-bed reactor, the Hg0 adsorption rate reached 11.93-13.56 mg/g·min over

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Nano-CuS. This extremely speedy rate makes Nano-CuS promising for a future sorbent injection

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technique. The anisotropic growth of Nano-CuS was confirmed by X-ray diffraction analysis and

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provided a fundamental aspect for Nano-CuS surface reconstruction and polysulfide formation.

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Further X-ray photoelectron spectroscopy and Hg0 temperature-programmed desorption tests

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showed that the active polysulfide, S-S dimers, and copper-terminated sites contributed primarily to

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the extremely high Hg0 adsorption capacity and rate. With these advantages, Nano-CuS appears to

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be a highly promising alternative to traditional sorbents for Hg0 capture from coal combustion flue

21

gas.

22 23 24

KEYWORDS: Elemental mercury; Nano-CuS; Coal combustion; Flue gas 2


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Introduction

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The Minamata Convention went into effect in August 2017 to limit mercury emissions among its

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128 signatories, including China and the United States, the two largest mercury-emitting countries

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worldwide.1 Coal combustion is one major source of anthropogenic mercury emission.2-3 Mercury

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emitted from coal-fired sources exists in three forms: elemental mercury (Hg0), particulate-bound

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mercury (Hgp), and oxidized mercury (Hg2+).4-6 Hgp and Hg2+ are easily removed by flue gas

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treatment facilities equipped with an electrostatic precipitator (ESP) system and a wet flue gas

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desulfurization (WFGD) system.7-8 In contrast, Hg0 is the form of mercury most difficult to be

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degraded in flue gas due to its high volatility and low solubility in water.9 Therefore, mercury

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emitted from coal combustion power plants endures mainly in the form of Hg0.10

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Activated carbon injection (ACI) is currently the most mature technique for control of Hg0

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emissions from coal combustion power plants.11 The injection of activated carbon between the

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selective catalytic reduction (SCR) set-up and the ESP system does not require the installation of an

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extra facility to balance the overall cost.12-15 However, the high operating cost, mediocre adsorption

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capacity, possible downstream mercury re-emission, and compromising effect on the use of fly ash

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impede the application of activated carbon.16-20 Thus, the development of new sorbents with a

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higher affinity toward Hg0 and a neutral effect on fly ash reclamation is urgently needed.

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Mineral sulfides were recently found to be promising alternatives to traditional activated carbons

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for Hg0 control because their surface is “entirely” covered by sulfur active sites with high affinity

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with mercury species.16, 21 After adsorption, Hg0 is transferred into the most stable mercury form,

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mercury sulfide (HgS), with extremely low solubility and high thermal stability, which ensures a

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minimal probability of Hg0 re-emission into the environment.22 The most common poisoning flue

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gas components, SO2 and water vapor (H2O), generally exert a slight hindrance to Hg0 adsorption 3


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over mineral sulfides.22-24 Moreover, the stable mineral sulfide not only showed a beneficial effect

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on the use of fly ash, it also inhibited mercury methylation.25-26 Based on these advantages, the

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mineral sulfides exhibit great potential for replacing activated carbon in future industrial Hg0

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removal from coal combustion flue gas.

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However, to the best of our knowledge, Hg0 adsorption over mineral sulfides has not yet been

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widely reported. The common issue with mineral sulfides is their very limited surface area.19, 22, 27-28

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Hence, even when mineral sulfides are fully covered with sulfur active sites, the Hg0 adsorption

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capacity was still similar to or only slightly higher than that of activated carbon.19 Nanostructured

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sphalerite (Nano-ZnS), which has a high surface area, was thus synthesized to overcome this

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intrinsic drawback.16 Nano-ZnS showed a better Hg0 adsorption capacity and a better adsorption

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rate than several commercial activated carbons. However, the surface and bulk stoichiometric

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analysis of ZnS revealed that the Zn/S ratio over the ZnS surface from 1:0.5 to 1:0.8 was much

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lower than its theoretical value of 1:1.29 In contrast, copper sulfide (CuS) was reported to show a

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high sulfur exposure ratio, with a Cu/S value exceeding 1:1. Moreover, CuS established sulfur sites

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enriched crystal surface under standard pressure and temperature below 100 °C.30-31 From the

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surface sulfur coverage aspect, nanostructured CuS (Nano-CuS) is superior to Nano-ZnS. It should

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also be noted that the most suitable temperature for Hg0 adsorption over the Nano-ZnS (180 °C)

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requires that it be injected between the SCR set-up and the ESP system. Considering the nitrogen

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(N2) selectivity of the SCR catalysts, a small amount of nitrogen dioxide (NO2) is present in the flue

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between the SCR set-up and the ESP system.32 Thus, with this scheme, the detrimental effect of

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NO2 on Hg0 adsorption over mineral sulfides is inevitable.25 One alternative strategy is to inject

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sorbents after the WFGD system and before the wet electrostatic precipitator (WESP) system, a

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facility equipped to collect fine particles immediately before the flue gas is discharged into the 4


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atmosphere.33 In this section, the water-soluble NO2 is fully removed by the WFGD system, but the

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reaction temperature is much lower, in the range of 40 °C to 100 °C.10, 22, 34 Coincidentally, Hg0

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interacts much more strongly with Cu-terminated active sites than Zn ones to form amalgam at low

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temperatures.35-36 Therefore, it is reasonable to believe that Nano-CuS would be preferable to

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Nano-ZnS as an excellent mineral sulfide sorbent for the removal of Hg0 from coal combustion flue

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gas within a lower temperature range.

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Based on the above hypothesis, Nano-CuS synthesized with a precipitation method with the

78

assistance of hexadecyl trimethyl ammonium bromide (CTAB) was for the first time applied for

79

Hg0 adsorption from coal combustion flue gas. The optimal CTAB addition on Nano-CuS

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preparation and the best-performing temperature were determined. The excellent adsorption

81

performance of the Nano-CuS was confirmed by comparison with some renowned sorbents. The

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possible application of Nano-CuS between the WFGD and WESP systems was discussed. The

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mechanism responsible for the extremely high adsorption rate and capacity of Nano-CuS for Hg0

84

removal was also investigated.

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Experimental

86

Sorbent preparation

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Nano-sized CuS was synthesized with a liquid-phase precipitation method. In a typical procedure, a

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1-M aqueous solution of copper chloride (CuCl2, anhydrous, 99.99%, Aladdin) and a 1-M aqueous

89

solution of ammonium sulfide ((NH4)2S, 20 wt%, Aladdin) were prepared in separate 100-mL

90

beakers. To adjust the particle size of the final products, a trace amount of CTAB (99.00%,

91

Sinopharm) was added to the beaker containing CuCl2 solution and stirred for 30 min until the CTAB

92

was fully dissolved. The (NH4)2S solution was then added dropwise into the CuCl2 solution to obtain

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a black precipitant, which was aged for another 1.5 h. The obtained precipitant was then centrifuged 5


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and the supernatant was poured off and washed with anhydrous ethanol (analytical grade, Sinopharm)

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five times and distilled water another five times. The purged samples were heated at 120 °C in a

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vacuum for 12 h before they were ground and sieved by 100 meshes to be the sorbents. Four groups

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of CuS samples were prepared by altering the amount of added CTAB from 1% to 10% versus the

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weight of the CuCl2 precursor. After the washing process, small amount of CTAB may be left in the

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as-prepared samples. However, based on our preliminary experiments, the left CTAB did not

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influence the Hg0 adsorption performance. Thus, The CuS sample with the highest surface area was

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simply denoted as the Nano-CuS instead of CTAB/CuS composite.

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For comparison, Nano-ZnS was prepared with the same method used in our previous work.16

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Activated carbon (AC, Kemiou Chemical Reagent Co., Ltd.) was also purchased and tested for its

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mercury adsorption efficiency.

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Sorbent Characterization

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The Brunauer-Emmett-Teller (BET) surface area of the sorbents was determined by the N2

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adsorption and desorption method with a BET analyzer (ASAP 2020, Micromeritics, USA). Before

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BET testing, the prepared sorbents were purged in pure N2 for 4 h to obtain a clean surface.

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Thermogravimetric testing was conducted with a thermogravimetric analyzer (SDT Q600, TA

110

Instruments, USA) to investigate the thermal stability of Nano-CuS. The sample was heated in 50

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mL/min Ar from 100 °C to 400 °C at a rate of 10 °C/min. Morphology of the nano-particles were

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recorded by a transmission electron microscopy (TEM, JEOL 2100F, Japan) at 200 kV. In the TEM

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characterizations, the fresh Nano-CuS was pretreated under pure N2 at 175 oC for 4 h to be the spent

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Nano-CuS. The crystallinity of the Nano-CuS was measured by X-ray diffraction (XRD, D8 Bruker

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AXS, Germany) with two theta from 10° to 80° in Cuα (λ = 0.15406 nm) radiation. With C 1s binding

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energy value of 284.8 eV as the reference, X-ray photoelectron spectroscopy (XPS) spectra were 6


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recorded for fresh and spent Nano-CuS (sorbent pretreated in the presence of 200 µg/m3 Hg0 carried

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by N2 for 12 h) with an X-ray photoelectron spectroscope (Thermo ESCALAB 250Xi).

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Adsorption Activity Test

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Hg0 removal over as-synthesized CuS was evaluated with a fixed-bed reaction system, as shown in

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Figure 1. Compressed gas cylinders containing N2, O2, and SO2 were used to provide the different gas

122

components. A washing bottle filled with 150 mL H2O was put in a thermostat water bath at 80 °C

123

and connected with a separated N2 gas cylinder to introduce water vapor into the reaction system. The

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total flow rate was controlled precisely at 1.0 L/min with a mass flow controller. A Dynacal Hg0

125

permeation device (VICI Metronics) heated in a water bath at an unchanged temperature was used to

126

provide a constant feed of gas-phase Hg0 (90 µg/m3 for testing and 200 µg/m3 for pretreatment). A

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reactor made of borosilicate glass with an inner diameter of 1 cm was put in a tubular furnace,

128

equipped with a temperature adjustment system, with temperature variation of less than 2.0 °C. The

129

Hg0 concentration was detected with a mercury analyzer (VM3000, Mercury Instrument, Inc.) and

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continuously recorded by a computer. The adsorption of Hg0 by an empty reactor was measured to

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rule out Hg0 adsorption by the borosilicate reactor. Before each test began, the gas flow bypassed

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the reactor loaded with sorbents until the detected Hg0 concentration was stable (with fluctuation of

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472.1

0.26

>497.8

0.28

H2S modified Fe-Ti spinel (5%) N2 Nano-ZnS

180

16

Air Pyrrohotite (4%)

SFG

60

220

0.26

22

[MoS4]2-/CoFe-LDH

N2 + O2

75

16390

5.04

23

CoMoS/γ-Al2O3

N2

50

18940

0.13

44

Calgon AC

Air

140

40-370

-

19

CarboChem AC

Air

140

400

-

19

Norit FGD AC

Air

70

81

1.01

45

AC Fiber

Air

25

~52.5

< 8.75×10-2

46

BPL AC

N2

140

~10

0.5

47

N2

140

2300

-

48, 49

Steam AC

N2

25

230

1.15

50

SIAC

N2

120

221

-

51

S-impregnated BPL AC

SFG represents simulated flue gas (4%/5% O2, 80/100 ppm SO2 and 8% H2O carried by N2). The percentage in the parentheses represents the breakthrough ratio of the sorbents. 25


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Table 3. Sulfide species analysis of fresh and spent Nano-CuS Sulfide (%)

S-S dimer (%)

Polysulfide (%)

Fresh Nano-CuS

52.9

14.6

32.5

Spent Nano-CuS

54.1

30.6

15.3

26


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List of figures: Figure 1. Schematic diagram of the reaction system. Figure 2. The influence of (a) surface area and (b) temperature on Hg0 adsorption over CuS. Figure 3. (a) Hg0 adsorption over Nano-CuS pretreated at different temperatures, and (b) TG curves of Nano-CuS. Figure 4. TEM images of (a) fresh Nano-CuS, and (b) Nano-CuS pretreated at 175 oC under pure N2. Figure 5. Hg0 adsorption over (a) Nano-CuS under N2 and SFG, (b) Nano-CuS pretreated by O2 at different temperatures, and (c) Nano-CuS with different concentrations of SO2 or H2O. Figure 6. (a) Half-breakthrough curves of Nano-CuS under N2, N2 plus 4% O2, and SFG. (b) Estimation of the adsorption capacity of Nano-CuS with kinetic simulation. Figure 7. Comparison of Hg0 adsorption capacity and rate for different sorbents.. Figure 8. XRD pattern of Nano-CuS. Figure 9. (a) Preferential growth of Nano-CuS; and (b) layered structure of Nano-CuS along direction. Figure 10. Hg-TPD profile of mercury loaded Nano-CuS. Figure 11. XPS patterns of (a) Hg 4f of spent Nano-CuS; and (b) S 2p and (c) Cu 2p of fresh and spent Nano-CuS.

27


Langmuir

Figure 1. Schematic diagram of the reaction system. Gas Mixing Chamber

MFC

MFC

MFC

3-way Valve

MFC

Hg Permeation Device

Catalyst

Tubular Furnace

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H2O Condenser

NaOH Vent

N2

O2

SO2

N2

Carbon Trap

N2

Temperature Controller

Water Bath Hg Analyzer

28


Terminal

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Normalized Mercury Concentration

Figure 2. Influence of (a) surface area; and (b) temperature on Hg0 adsorption over CuS. 1.0 0.9 0.8 0.7 0.6

1.0 0.9 0.8 0.7 0.6

(a)

28.90 m2/g 33.06 m2/g 20.95 m2/g 14.58 m2/g

0.04

0.04

0.02

0.02

0.00 0

30

60

90

120

150

180

210

0.00 240

Time (min)

1.0 Normalized Mercury Concentration

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(b)

25 oC 75 oC 125 oC 175 oC

0.8

1.0

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0 0

30

60

90

120 150 Time (min)

180

0.0 240

210

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Figure 3. (a) Hg0 adsorption over Nano-CuS pretreated at different temperatures; and (b) TG


curves of Nano-CuS. 1.0

(a)

Fresh Nano-CuS o Pretreated at 125 C o Pretreated at 175 C Pretreated at 250 oC Pretreated at 325 oC

0.8

1.0 0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 0

30

60

90

120 150 Time (min)

180

210

240

100

0.4 310.7

95

(b)

277.5 297.3

0.3

Deriv. Weight (%/oC)

Weight (%)

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

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90

0.2 85

80

75 100

0.1

157.0

150

200 250 300 Temperature (oC)

350

0.0 400

30


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Figure 4. TEM images of (a) fresh Nano-CuS, and (b) Nano-CuS pretreated at 175 oC under pure N2. (a)

(b)

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Figure 5. The Hg0 adsorption over (a) Nano-CuS under N2 and SFG; (b) Nano-CuS pretreated by O2 under different temperature; (c) Nano-CuS with the presence of different concentration


of SO2 or H2O. 1.0

(a)

N2

1.0

SFG

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0 0

30

60

90

120

150

180

0.0 240

210


Time (min) 1.0

(b)

0.9

1.0 0.9

Fresh Nano-CuS Pretreated at 125 oC with 4% O2 for 12 h Pretreated at 125 oC with 20% O2 for 12 h

0.1

0.1

0.0 0

30

60

90

120 150 Time (min)

180

0.0 240

210

20% H2O off

40 Mercury Concentration (µg/m3)

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(c)

With H2O addition

40

With SO2 addition

30

30

20% H2O or 1000 ppm SO2 on 15% H2O on

20

15% H2O off 8% H2O or 8% H2O on

20

1000 ppm SO2 off

10

10

0 0

30

60

90

120

150

0 210

180

Time (min)

32


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Figure 6. (a) Half-breakthrough curves of Nano-CuS under N2, N2 plus 4% O2 and SFG; (b) Estimation of the adsorption capacity of Nano-CuS with kinetic simulation.


1.0

1.0 (a)

N2 N2 + O2

0.8

0.8

SFG

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 0

120 Adsorption Capacity (mg/g)

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10

20

30

40 Time (h)

50

60

70

120

(b)

100

100

80

80

60

60 N2

40

N2 simulation

N2 + O2

N2 + O2 simulation

SFG

SFG simulation

20

40 20

0 0

100

200

300 Time (h)

400

500

0 600

∞

33


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Figure 7. Comparison of Hg0 adsorption capacity and rate for different sorbents. Nano-CuS 75 oC Nano-CuS 125 oC Nano-CuS 175 oC

1.2 Normalized outlet mercury concentration

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Commercialized AC 140 oC Nano-ZnS 180 oC

1.2

1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 0

2

4

6

8 10 12 14 16 18 20

50

100 Time (min)

34


150

200

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Figure 8. XRD pattern of Nano-CuS. (100)

(102)

Intensity (a.u)

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(103) & (006)

(101) (108)

10

20

30

40 50 Two theta (o)

(116)

60

70

80

35


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Figure 9. (a) Preferential growth of Nano-CuS; and (b) layered structure of Nano-CuS along direction.

(a)

(b)

36


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Figure 10. Hg-TPD profile of mercury loaded Nano-CuS. Hg-Cu HgS (metacinnabar)

Mercury signal (a.u)

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

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HgS (cinnanbar)

50

100

150

200

250

300

350

400

450

o

Temperature ( C)

37


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Figure 11. XPS patterns of (a) Hg 4f of spent Nano-CuS; and (b) S 2p and (c) Cu 2p of fresh and spent Nano-CuS. (a) Hg 4f

98.10 eV

Intensity (a.u)

100.67 eV

104.58 eV

97

99

101

103 105 Binding Energy (eV)

107

109

(b) S 2p Sulfide

Intensity (a.u)

S-S dimer Polysulfide Fresh

Spent

156 158 160 162 164 166 168 170 172 174 176 Binding Energy (eV) 932.38 eV

(c) Cu 2p

Cu 2p 3/2 Cu 2p 1/2

Intensity (a.u)

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

Satellite shakeup

Fresh 931.96 eV

Spent

925

930

935

940 945 950 Binding Energy (eV)

955

960

38


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Langmuir

Graphical Abstract

39


Multiform Sulfur Adsorption Centers and Copper-Terminated Active

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