Role of Sulfur Trioxide (SO3) in Gas-Phase Elemental Mercury

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Remediation and Control Technologies

Role of Sulfur Trioxide (SO3) in Gas-Phase Elemental Mercury Immobilization by Mineral Sulfide Zequn Yang, Hailong Li, Wenqi Qu, Mingguang Zhang, Yong Feng, Jiexia Zhao, Jianping Yang, and Kaimin Shih Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b07317 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on March 4, 2019

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Environmental Science & Technology

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Role of Sulfur Trioxide (SO3) in Gas-Phase Elemental Mercury Immobilization by Mineral Sulfide

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Zequn Yanga, Hailong Lib*, Wenqi Qub, Mingguang Zhangb, Yong Fenga, Jiexia Zhaob, Jianping Yangb,

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Kaimin Shiha**

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a. Department of Civil Engineering, The University of Hong Kong, Hong Kong, Hong Kong SAR,

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China

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b. School of Energy Science and Engineering, Central South University, Changsha, 410083, China

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Revision Submitted to

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*To whom correspondence should be addressed:

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TEL: +86-18670016725

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E-mail: [email protected]

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**To whom correspondence should be addressed:

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TEL: +852-2859-2973

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Email: [email protected]

ACS Paragon Plus Environment


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ABSTRACT: Mineral sulfide based sorbents were superior alternatives to traditional activated carbons

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for elemental mercury (Hg0) immobilization in industrial flue gas. A systematical study concerning the

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influence of sulfur trioxide (SO3) on Hg0 adsorption over a nano-sized copper sulfide (Nano-CuS) was

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for the first time conducted. SO3 was found to significantly inhibit the Hg0 removal over Nano-CuS

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partially because SO3 oxidized the reduced sulfur species (sulfide) with high affinity to mercury to its

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oxidized sulfur species (sulfate). Moreover, a brand new “oxidation-reduction” mechanism that led to a

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simultaneous oxidation of sulfide and reduction of mercury on the immobilized mercury sulfide (HgS)

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was responsible for the inhibitory effect. Even though the released Hg0 from the reduction of mercury in

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HgS could be oxidized by SO3 into its sulfate form (HgSO4) and re-captured by the sorbent, the

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“oxidation-reduction” mechanism still compromised the Hg0 capture performance of the Nano-CuS

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because HgSO4 deposited on the sorbent surface could be easily leached out when environmentally

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exposed. These new insight into the role of SO3 in Hg0 capture over Nano-CuS can help to determine

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possible solutions and facilitate the application of mineral sulfide sorbents as outstanding alternatives to

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activated carbons for Hg0 immobilization in industrial flue gas.

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Keywords: Mercury; Mineral sulfide; Sulfur trioxide; Adsorption; Flue gas


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INTRODUCTION

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Mercury is a notorious environmental hazard that attracts worldwide attention for decades owing to its

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toxicity, persistence, transportability and bioaccumulation.1-5 On August 2017, the Minamata

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Convention came into force to urge the implementation of ever rigorous standards among its 128

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signatories for limiting mercury emission from anthropogenic sources.2, 6 Generally, mercury enriched

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in fuels and raw materials emits in three forms from boiler/furnace after combustion/smelting, i.e.,

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elemental mercury (Hg0), oxidized mercury (Hg2+) and particulate-bound mercury (Hgp).7-10 The Hg2+

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and Hgp are easily to be removed from flue gas co-benefitted by the equipment of air pollution control

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devices (APCDs) including desulfurization and particulate-control systems.11-13 On the contrary, Hg0 is

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extremely difficult to be degraded due to its high volatility and insolubility in water. Thus, Hg0 persists

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in flue gas and acts as the main mercury species discharged from stack to immediate environment.14-16

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Reduced sulfur species (S0/S2-) is an ideal active candidate to treat Hg0 polluted flue gas attributed to

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the relatively high affinity between S0/S2- and Hg0.17 Activated carbon (AC) impregnated by element

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sulfur exhibited significantly enhanced Hg0 capture capacity (2.3 mg/g) compared to that of bare AC.18,

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19

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renowned for its extreme stability when exposed to complex environment.20,

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sulfur still showed relatively limited adsorption capacity to Hg0, which caused exceedingly high

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operation cost of the activated carbon injection (ACI) strategy.22-24 The adoption of carbonaceous

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material also introduced overwhelming carbon content in fly ash that impeded its reusability as raw

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concrete materials.17 To overcome these drawbacks, nano-sized mineral sulfides were recently

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developed and demonstrated to establish outstanding performance for Hg0 immobilization in flue gas,25-

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with Hg0 adsorption capacity reaching as high as 122.4 mg/g. Moreover, the mineral sulfides exhibited

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neutral even beneficial effect on the reusability of fly ash as concrete feed.29 With these advantages,

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mineral sulfides were recognized as promising alternatives to traditional ACs for Hg0 pollution control

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in various industrial processes.

Moreover, the existence of sulfur species warrants the conversion of Hg0 into mercury sulfide (HgS), 21

However, elemental

among which the nano-sized copper sulfide (Nano-CuS) exhibited the best adsorption performance,



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Industrial flue gas contains complicated gas compositions like hydrogen chloride (HCl), water vapor

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(H2O), nitrogen oxides (NOx) and sulfur oxides (SOx). They may exert different impacts on Hg0

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adsorption over mineral sulfides with mechanisms complex and varied. For instance, NOx inhibited the

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Hg0 adsorption over nano-sized zinc sulfide mainly because NOx oxidized the sorbent surface.25 Gas-

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phase H2O suppressed the adsorption performance of Nano-CuS due to both of the active sites

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prevention and competitive adsorption effects.22 SO2 was found to have neutral effect on Hg0 adsorption

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over mineral sulfides,29 which further justified the candidature of the mineral sulfides as efficient Hg0

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trap, since SO2 widely exists in combustion/smelting flue gas,8, 31 and its concentration is relatively high

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for many cases. In non-ferrous smelting flue gas, the content of SO2 generally exceeds 2.0 vt%

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attributed to the burning of sulfide ores.32 In oxy-fuel combustion technique, which was regarded as a

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promising alternative to traditional air combustion strategy due to its significance in reducing carbon

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dioxide (CO2) emission, high-concentration of SO2 (2-2.5 vt%) accumulates in flue gas due to the flue

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gas recycle.33

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Besides SO2, depending on the burning factors and feed materials, sulfur trioxide (SO3) is another

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form of sulfur species partitioned from fuels and raw materials after combustion/smelting and

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concentrated in flue gas.8,

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smelting results in both high concentration of SO2 and SO3 in flue gas.8 On the other hand, for coal

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combustion, the selective catalytic reduction (SCR) system partially oxidized SO2 into SO3.34 This

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makes the SO3 concentration at the inlet of the electrostatic precipitator (ESP), where is an ideal site for

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sorbent injection, doubled compared to that at the inlet of SCR system.36 Oxy-fuel combustion flue gas

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even carries pronouncedly high content of catalyzed SO3 attributed to the abundance of SO2.33, 37, 38 SO3

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was found to suppress the Hg0 removal over bare AC due to the competitive adsorption and depletion of

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surface active sites even at very low concentration (20 ppm).39, 40 Moreover, the SO3, with extremely

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strong oxidizability,41 is highly probable to oxidize the reduced sulfur species to its oxidized species that

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exhibits less affinity to Hg0. As suggested by these perspectives, it is of considerable importance to

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conduct a systematical research to investigate the effect of SO3 on Hg0 removal over sulfur based

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High content of sulfur species in sulfide ores used for non-ferrous


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materials, especially for mineral sulfides, where the sulfur species in sorbent and adsorbate are both in

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their reduced forms.

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Herein, a systematical study concerning with the influence of SO3 on Hg0 adsorption over mineral

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sulfide based materials is for the first time reported in this work with the mechanisms discussed in detail.

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Nano-CuS with the best performance among various mineral sulfides was adopted to represent the

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mineral sulfide sorbents. This work provides an in-depth comprehension of the interaction between the

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SO3, mineral sulfide sorbent and the product after Hg0 adsorption, which is critical for designing novel

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gas phase Hg0 sorbents and developing optimal sorbent applications in a real-world condition.

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EXPERIMENTAL AND COMPUTATIONAL METHODS

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Sorbent Preparation. Please refer to the Supporting Information, the Sorbent preparation section.

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Experimental setup and methods. The Hg0 adsorption over Nano-CuS was tested in a fixed-bed

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reaction system (shown in Figure S1). Compressed gas cylinders containing nitrogen (N2), oxygen (O2)

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and SO2 were adopted to generate desired gas atmospheres. SO3 was supplied by oxidizing SO2 over a

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vanadium-titanium oxides based catalyst (VTBC) put in a quartz tube prior to the Hg0 removal reactor,

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for which the concentration was adjusted by the inlet SO2 concentration with the conversion efficiency

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of SO2 kept at 100%. Mass flow controllers was adopted to precisely control the total flow rate of 1 L

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min-1. A constant feed of Hg0 was provided by a Dynacal Hg0 permeation device (VICI Metronics)

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heated to an unchanged temperature. The Hg0 removal was conducted in a borosilicate glass reactor

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with 10 mm inner diameter that was placed in a tubular furnace to control the reaction temperature. A

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mercury analyzer (VM3000, Mercury Instruments, Inc.) was used to record the mercury concentration.

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Before each test, the Hg0 carried by different flue gas atmospheres bypassed the reactor to record the

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inlet Hg0 concentration (Cin). Then, the Hg0 feed passed through the reactor and the as-obtained Hg0

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concentration was denoted as outlet Hg0 concentration (Cout). Hence, the Hg0 adsorption capacity (C, μg

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Hg0/g sorbent) was calculated by equation (1):

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C

1 t2 (Cin  Cout )  f  dt m t1


(1)


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in which m is the mass of the sorbent (g), f is the total gas flow rate (m3/h), and t is the duration time of

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the adsorption processes (h).

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Four sets of experiments were conducted with parameters detailed in Table S1. Set I experiments

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were designed to investigate the Hg0 removal over Nano-CuS under different gas atmospheres. In set II

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experiments, the competitive adsorption between SO3 and Hg0 for active sites was studied. The fresh

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Nano-CuS was pretreated under 500 μg/m3 Hg0 for 1 h to be the Hg-laden Nano-CuS. The pure HgS

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was directly purchased from Aladdin Co. Ltd without any purification. In set III experiments, the Nano-

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CuS pretreated under 4% O2 plus 1000 ppm SO2 in the presence or absence of 500 ppm SO3 carried by

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pure N2 for 2 h was used for Hg0 removal to investigate the possible surface active sites damage effects

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by SO3. Set IV experiments was designed to explore the possible reduction of HgS by the mixture of

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SO2 and SO3. The fresh Nano-CuS was pretreated by 500 μg/m3 until equilibrium to be the Hg-saturated

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Nano-CuS. In all five sets of the experiments, the adsorption temperature was fixed at 75 oC, the optimal

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operation temperature of Nano-CuS,29 with the inlet Hg0 concentration kept as 100 μg/m3. The dosage

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of Nano-CuS was 5 mg, much lower to that in a real-world condition, to magnify the influence of

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different gas components.

130 131 132 133

Sorbent characterizations. Please refer to the Supporting Information, Sorbent characterization section. Temperature programmed desorption/decomposition (TPD). Please refer to the Supporting Information, the Temperature programmed desorption/decomposition section.

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First principle calculation. The CuS model was in hexagonal structure with space group P63/mmc

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(a=b=3.796 Å, c=16.360 Å, α=β =90°, γ=120°). An 8-layer slab with a (3×3) unit cell based on a

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(3×3×1) primitive cell was used to model the structure of CuS(001) surface. A 15 Å vacuum region

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between the slabs was constructed to avoid the spurious interactions. During the geometric optimization,

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the bottom 3 layers were fixed and the rest were allowed for relaxation. All calculations were conducted

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with the quantum mechanics based Dmol3 program package in Materials Studio 8.0. The exchange

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correlation potential was determined by Perdew-Burke-Ernzerhoff (PBE) approximation in the place of ACS Paragon Plus Environment

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the generalized gradient approximation (GGA) scheme.42 The interaction between valance electrons,

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inner electrons and atomic nucleus was performed with the double numerical basis sets plus polarization

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functional (DNP). The Monkhorst-Pack scheme k-points grid of 2×2×1 was used to simplify the

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Brillouin zone in the cell of CuS (001) and the real space basis set functions are adjusted to be 4.0 Å.

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The criteria for the tolerances of energy, force, displacement, and SCF convergence criteria are set as

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2×10-5 Ha, 4×10-3 Ha Å-1, 5×10-3 Å, and 10-5, respectively. A Methfessel-Paxton smearing of 0.005 Ha

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was used to improve calculation performance. Gas-phase species (Hg0 and SO3) molecules were also

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optimized separately in a large crystal cell of 10×10×10 Å.

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Mercury leaching test. Please refer to the Supporting Information, the Mercury leaching test section. RESULTS AND DISCUSSIONS

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SO3 inhibited the Hg0 adsorption over Nano-CuS. As shown in Figure 1(a), the as-prepared Nano-

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CuS established outstanding performance for Hg0 capture under pure N2, showing almost 100% Hg0

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removal efficiency within a 4-h experiment. The adsorption capacity reaching 28.3 mg/g is the highest

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value among various mineral sulfides developed yet for Hg0 decontamination from industrial flue gas.17,

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26-28

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over Nano-CuS, which exhibited 4-h uptake capacities of 28.4 and 28.1 mg/g, respectively. The

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excellent resistance of Nano-CuS towards SO2 poisoning makes it a promising candidate for Hg0

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adsorption from industrial flue gas containing high concentration of SO2. However, SO3 showed adverse

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impact on Hg0 adsorption over Nano-CuS (shown in Figure 1(a)). The outlet Hg0 concentration climbed

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to 17.6 μg/m3 at the end of a 4-h experiments when 100 ppm SO3 was added to the pure N2 gas flow.

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The as-derived adsorption capacity was slightly suppressed from 28.3 mg/g under pure N2 to 26.1 mg/g

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under 100 ppm SO3. When the addition of SO3 was increased from 100 to 500 ppm, a significant

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inhibitory effect was observed with the outlet Hg0 concentration sharply increased within 4 h and

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reached 86.7 μg/m3 at the end. This indicates that the Nano-CuS almost lost its activity for Hg0 capture

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after in contact with 500 ppm SO3 for 4 h. Also, under this condition, the Hg0 adsorption capacity of

The additions of 4% O2 and 4% O2 plus 1000 ppm SO2 showed negligible effect on Hg0 removal



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Nano-CuS within 4 h pronouncedly decreased to 16.8 mg/g compared to that of 28.3 mg/g under pure

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N2.

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It should be noticed that in a coal combustion case, Nano-CuS is a promising alternative to traditional

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AC to be injected through the pipeline during the flue gas treatment process, which means that the

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contacting time between the sorbent and flue gas is short. The 30 min timespan is a very important

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parameter to evaluate the performance of the sorbent in a real-world condition because the residence

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time of the injected sorbent that interacts with mercury is about 3-5 s in the case of an electrostatic

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precipitator and approximately 25 min in the case of a fabric ﬁlter.25 As shown in Figure 1(a), the

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adsorption performance of Nano-CuS was just negligibly influenced within 30 min when 100 ppm SO3

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was present. The concentration of 100 ppm was even higher than the highest SO3 concentration in a real

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coal combustion flue gas.36 Thus, even though Hg0 capture over Nano-CuS was limited by the presence

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of SO3, the results still suggest that Nano-CuS is a promising candidate to be used in an injection

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strategy for Hg0 immobilization in coal combustion flue gas. However, for a fix-bed strategy that was

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generally applied to realize a better economical friendliness, the long-exposure to SO3 may cause

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accumulating poisoning effect on the sorbent. To avoid the sorbent deactivation and discuss the possible

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solutions in such cases, investigating the involved mechanisms for the inhibitive effect of SO3 is

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indispensable.

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SO3 repelled negligible Hg0 from the sorbent surface. Competitive adsorption is a renowned reason

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accounted for the prohibitive influence of flue gas components on the performance of Hg0 traps. For

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example, SO2 was found to deactivate metal oxide for Hg0 adsorption by repelling Hg0 from the metal

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oxide surface.43 Gas-phase H2O suppressed the Hg0 uptake capacity of sulfur-impregnated AC by as

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much as 25% due to the occupation of surface active sites.44 Moreover, SO3 was also proven to compete

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for the adsorption sites with Hg0 on traditional AC.40 For mineral sulfides, although SO2 was

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demonstrated to show no competitive effect with Hg0,29 the role of SO3 for this perspective remains

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unknown. Previous study reported that a very low concentration of SO3 showed significant adverse

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influence on Hg0 removal over AC due to competitive adsorption,40 which indicates that SO3 may ACS Paragon Plus Environment

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exhibit stronger capability for competition than SO2. If this is the same case for Nano-CuS, the weakly

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adsorbed Hg0 on Hg-laden Nano-CuS will be repelled out by the addition of SO3. However, similar to

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that for SO2, 500 ppm SO3 repelled no Hg0 from the Hg-laden Nano-CuS surface (shown in Figure S2).

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This evidences that the competitive adsorption negligibly accounted for the deactivation of Nano-CuS

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for Hg0 capture in the presence of SO3, and suggests that the mercury over mineral sulfides surface was

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primarily anchored by sulfide species to form HgS chemisorbed on the sorbent surface with negligible

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weakly adsorbed mercury exists. On the contrary, for traditional AC, considerable amount of mercury

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was physiosorbed on the sorbent surface and easily to be repelled out when SO3 was added.45

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SO3 oxidized sulfide on Nano-CuS into sulfate. As shown in Figure 2, the Nano-CuS pretreated by

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4% O2 plus 1000 ppm SO2 showed unchanged Hg0 removal performance in a 4-h test compared with the

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fresh sample. However, when pretreated in the extra presence of 500 ppm SO3, the Hg0 removal

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performance of the pretreated Nano-CuS was significantly suppressed. The outlet Hg0 concentration

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rapidly increased during the experiments and reached 50.6 μg/m3 at the end of 4 h. This gives us strong

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hints that the sorbent surface was permanently deactivated by SO3. Considering the oxidizability of SO3

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and the reduced nature of sulfur species on Nano-CuS, it is highly probable that the sulfide was oxidized

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to high-valence sulfur species that exhibits mediocre affinity towards Hg0.

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To further identify the as-formed sulfur species over Nano-CuS after pretreated by SO3, X-ray

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photoelectron spectroscopy (XPS) analysis was conducted and the results are shown in Figure 3. The S

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2p peaks for fresh and spent Nano-CuS centering at 162.2, 163.5 and 164.4 eV were assigned to the

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characteristic peaks of monosulfide, S-S dimer and polysulfide, respectively (Figure 3(a)).29 The spike

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locating at 168-172 eV was due to the presence of sulfite (SO32-) and/or sulfate (SO42-) species.46 For the

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fresh sample, sulfur primarily exists as sulfide with no impurity like elemental sulfur detected, which

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indicates that the as-prepared Nano-CuS was almost in its pure phase. The existence of minority of

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oxidized sulfur species was probably due to the residual of CuSO4 precursor. For the fresh Nano-CuS,

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the surface ratio between the reduced sulfurs species and oxidized sulfur species was derived from the

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peak integration to be around 9:1. However, after pretreated by SO3, the amount of SO42- and/or SO32ACS Paragon Plus Environment


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significantly increased compared with that on fresh Nano-CuS and accompanied by the corresponding

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peak intensity decline of the sulfide species. For the pretreated sample, the surface ratio between the

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sulfide species and SO42-/SO32- decreased to 5:4. These results suggest that SO3 efficiently converted the

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reduced sulfur forms on Nano-CuS to oxidized sulfur forms (SO42-/SO32-) that is inert for Hg0 removal.

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The oxidation of the Nano-CuS sorbent was also evidenced by the O 1s spectrum shown in Figure 3(b).

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The O 1s peak for the pretreated Nano-CuS was notably intensified compared to that for the fresh Nano-

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CuS, which is in agreements with SO42- and/or SO32- formation on the sorbent surface. The binding

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energy difference occurred between different copper species may be resulted from the coordination and

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charge changes when copper was bonded to different functional groups.47, 48 In the Cu 2p pattern (shown

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in Figure 3(c)), the peak centering at 932.4 eV was attributed to the Cu2+ on CuS while the peak locating

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from 934-936 eV was due to the presence of CuSO4.49 For the fresh Nano-CuS, the ratio between the

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CuS and CuSO4 was 9:1. For the pretreated CuS, the ratio between CuS and CuSO4 decreased to 5:4.

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These results are in line with the S 2p spectra and indicate that sulfide species in Nano-CuS has been

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oxidized to its highest valence, i.e., SO42-, instead of SO32-, by SO3.

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SO2-TPD is an effective method to identify the as-formed sulfur compounds by their characteristic

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decomposition temperature. As shown in Figure 4, the Nano-CuS pretreated by 500 ppm SO3 only

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exhibited one decomposition peak to release SO2 from 50 to 750 oC, which centers around 680 oC. The

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characteristic peak indexes to the decomposition of CuSO4,7 which further proves that CuSO4 is the

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final destination of the sulfide species on Nano-CuS after pretreated by SO3. The inserted Fourier

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transform-infrared (FTIR) spectra show two spikes in the wavenumber range from 800-2400 cm-1, of

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which the peak locating at 1150 cm-1 was resulted from the existence of SO42- while the peak from

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1600-1800 cm-1 was assigned to the surface water molecule.25, 50 No separate peak centering at 950 cm-1

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and belonging to SO32- was detected. These results demonstrate that SO3 with strong oxidizability

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directly oxidized the Nano-CuS sorbent into CuSO4 with no intermediate observed. The as-formed

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sulfate irreversibly damaged the Hg0 removal performance of the Nano-CuS due to the inhibition of

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Hg0 adsorption by sulfate.51 ACS Paragon Plus Environment

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SO3 compromised the stability of adsorbate (HgS) over Nano-CuS. As shown by Equation (1),

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SO3 has the potential to oxidize sulfide in HgS and simultaneously reduce oxidized mercury into its

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elemental form. HgS + 2SO3 (g) → Hg0 (g) + 3SO2 (g) ΔG (75 oC) = -90.2 kJ/mol

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(1)

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The negative Gibbs free energy indicates that the reaction is thermodynamically spontaneous under 75

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oC.

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when SO3 passed through. This seems “contradictory” with the thermodynamic results that suggests

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reaction (1) definitely processes to form Hg0. To reveal the microcosmic mechanisms behind the

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“contradiction”, the first principle calculation based on density functional theory (DFT) was conducted

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to illustrate the microcosmic role of SO3 in reducing HgS into Hg0 and compromising the adsorption

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capacity.

However, as shown in Figure S3, no Hg0 was detected downstream of the Hg-saturated Nano-CuS

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Figure 5(a) shows a stable structure of Hg0 immobilized by a monosulfide over CuS surface in the

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absence of SO3 molecule. The bond length of the as-formed HgS was 2.47 Å, close to the experimental

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value of the bond length of HgS (2.58 Å).52 The Mulliken charge of the adsorbed mercury was derived

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to be 0.15 e, indicating the electron loss of Hg0 after captured by CuS. The partial density of states

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(PDOS) analysis of mercury and the monosulfide (shown in Figure S4(a)) further evidenced the

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interaction between them. The overlap of the peaks between -5 to -6 eV indicates the strong interaction

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between the d orbital of mercury and the s and p orbitals of the monosulfide. Moreover, the interaction

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also introduced a weak orbital hybridization between -10 to -12 eV. These results demonstrate that

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mercury was stably immobilized by the monosulfide to form HgS. Then, a SO3 molecule was used to

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attack the stable structure, and the final products after geometric optimization was shown in Figure 5(b).

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After the geometric optimization, one O atom obviously dissociated from SO3 and recombined with the

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monosulfide dissociated from the sorbent surface to form an O-S-Hg intermediate. The left two O atoms

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and one S atom was repelled away to form an O-S-O structure with the bond angle of 117.7o and bond

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length of 1.48 and 1.47 Å, respectively. These parameters are in line with the experimental results

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showed that the bond length of SO2 is 1.43 Å and the bond angle between O-S-O is 118o.53 The bond ACS Paragon Plus Environment


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length of Hg-S in the O-S-Hg intermediate was slightly prolonged to 2.49 Å and Mulliken charge of the

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adsorbed mercury was decreased by 0.01 e to be 0.14 e, which suggests a non-obvious reduction of the

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mercury after attacked by one SO3 molecule. The reaction process after one HgS was attacked by one

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SO3 can be derived as followed: HgS + SO3 (g) → Hg-S-O + SO2 (g)

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(2)

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However, after the HgS was attacked by two SO3 molecules (shown in Figure 5(c)), the distance

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between the mercury and the initial monosulfide was increased obviously to 3.27 Å. The PDOS analysis

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of mercury and the initial monosulfide dissociated from the CuS surface shows that there was no

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interaction between them (Figure S4(b)). These results indicate a full dissociation of the HgS and the

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formation of Hg0. The monosulfide is fully oxidized by SO3 into a SO2 molecule and the adsorbed

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mercury was reduced to its elemental form. The overall reaction after the HgS was attacked by another

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SO3 can be expressed as:

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Hg-S-O + SO3 (g) → Hg0 + 2SO2 (g)

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Reaction (1) is the sum of reaction (2) and (3). The initial monosulfide expected to react with Hg0 was

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fully oxidized into SO2 and co-existed with another two SO2 molecules due to the dissociation of SO3

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(shown in Figure 5(d)), which molecularly prove that the adsorbed mercury can be successfully

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transformed by SO3 to Hg0 over CuS surface. However, these results still disagree with our

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experimental results that shows no Hg0 was detected when the Hg-saturated Nano-CuS was attacked by

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SO3.

(3)

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As SO3 can oxidize Hg0 into HgSO4,41 it is highly probable that, even though the released Hg0

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successfully escaped from the sorbent, the abundant SO3 will oxidize it into its sulfate salt and deposit

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on the sorbent surface. The Hg-TPD results of Nano-Cus pretreated under different conditions

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evidenced this point (shown in Figure 6(a)). An extra characteristic peak locating at 550 oC appeared

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after pretreated in the presence of SO3, which indexed to the decomposition peak of HgSO4.54 The

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extremely high decomposition temperature of mercury sulfate (HgSO4) makes it stable in an air

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condition. However, the solubility of HgSO4 would result in its extensively environmental leaching as ACS Paragon Plus Environment

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compared to HgS. As shown in Figure 6(b), the Nano-CuS pretreated in the co-presence of Hg0 and SO3

297

showed more than 200 folds higher leachability in simulated environmental condition as compared to

298

that pretreated in the absence of SO3. The presence of SO3 in gas flow inhibits the re-emission of the

299

reduced Hg0 into air, but its re-emission into aqueous environment is inevitable. The non-observation of

300

Hg0 when the Hg-saturated Nano-CuS was attacked by SO3 was mainly attributed to the existence of

301

SO3 transformed the escaped Hg0 into a water-soluble HgSO4.

302

In sum, SO3 in high concentration was found to significantly inhibited the Hg0 adsorption over Nano-

303

CuS because the sulfation of Nano-CuS surface and the transformation of immobilized adsorbate. This

304

would not compromise the candidature of mineral sulfides as alternative to activated carbon for Hg0

305

removal from coal combustion process because the SO3 concentration is relatively low and the contact

306

time is extremely short. However, for fixed-bed Hg0 removal applications in other scenarios like

307

nonferrous industry flue gas containing relatively high-concentration of both Hg0 and SO3, a feasible

308

solution is worthy to be discussed further to abate the detrimental effect of SO3, like adding sacrifice

309

agent to react with SO3 and protect the sorbents. The results obtained in this study will provide valuable

310

reference for optimizing the design of mineral sulfide based sorbents in the future that can be applied for

311

Hg0 decontamination from various industrial processes.

312

ASSOCIATED CONTENT

313

Supporting

Information.

Experimental

details

regarding

sorbent

preparation,

sorbent

314

characterization, mercury temperature programmed desorption/decomposition, mercury leaching test,

315

Table S1, Figure S1-S4. This material is available free of charge via the Internet at http://pubs.acs.org.

316

AUTHOR INFORMATION

317

Corresponding Author

318

*TEL: +86-18670016725; E-mail: [email protected]

319

**TEL: +852-2859-2973; Email: [email protected]

320

ORCID

321

Hailong Li: 0000-0003-0652-6655 ACS Paragon Plus Environment


Page 14 of 24

322

Kaimin Shih: 0000-0002-6461-3207

323

ACKNOWLEDGEMENTS

324

This project was supported by the National Natural Science Foundation of China (51776227), Natural

325

Science Foundation of Hunan Province, China (2018JJ1039, 2018JJ3675), and the Research Council of

326

Hong Kong (17257616, C7044-14G, and T21-771/16R).

327

References

328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372

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442

List of Figures:

443

Figure 1. (a) breakthrough curves and (b) derived adsorption capacity of Nano-CuS for Hg0 removal

444

under different conditions.

445

Figure 2. 4-h Hg0 breakthrough curves of fresh and spent Nano-CuS pretreated under different

446

conditions.

447

Figure 3. XPS spectra of (a) S 2p, (b) O 1s and (c) Cu 2p of fresh and spent Nano-CuS.

448

Figure 4. SO2-TPD profile of fresh and pretreated Nano-CuS (inserted with the FTIR spectra of fresh

449

and pretreated Nano-CuS).

450

Figure 5. Reduction mechanism of HgS by SO3 on a molecular level: (a) Hg0 immobilized by a

451

monosulfide; (b) surface HgS attacked by a SO3; (c) surface HgS attacked by another SO3; and (d) as-

452

formed three SO2 in panel (c).

453

Figure 6. (a) Hg-TPD of spent Nano-CuS pretreated under different conditions; and (b) leaching of

454

mercury from different samples.



456

Figure 1. (a) 4-h breakthrough curves and (b) derived adsorption capacity of Nano-CuS for Hg0

457

removal under different conditions.

Outlet Hg0 concentration (g/m3)

100

80

60

40

20

0

0

60

120 Time (min)

458 30

180

240

(b)

Pure N2 N2 + 4% O2 N2 + 4% O2 + 1000 ppm SO2 N2 + 4% O2 + 1000 ppm SO2 + 100 ppm SO3 N2 + 4% O2 + 1000 ppm SO2 + 500 ppm SO3

25 Hg0 uptaken by CuS (mg/g)

(a)

Pure N2 N2 + 4% O2 N2 + 4% O2 + 1000 ppm SO2 N2 + 4% O2 + 1000 ppm SO2 + 100 ppm SO3 N2 + 4% O2 + 1000 ppm SO2 + 500 ppm SO3

20 15 10 5 0

459

0

40

80

120 160 Time (min)

200

240


Page 18 of 24

Page 19 of 24


461

Figure 2. 4-h Hg0 breakthrough curves of fresh and spent Nano-CuS pretreated under different

462

conditions.

Outlet Hg0 concentration (g/m3)

100

80

60

40

20

0

463

Fresh Pretreated by 4% O2 + 1000 ppm SO2 Pretreated by 4% O2 + 1000 ppm SO2 + 500 ppm SO3

0

60

120 Time (min)

180

240



465

Figure 3. XPS spectra of (a) S 2p, (b) O 1s and (c) Cu 2p of fresh and spent Nano-CuS (a) S 2p S4+/S6+

S2-

Sx2- S22-

Intensity (a.u.)

Spent Nano-CuS

Fresh Nano-CuS

176

172

168 164 Binding energy (eV)

466

160

156

(b) O 1s

Intensity (a.u.)

Spent Nano-CuS

Fresh Nano-CuS

540

535 530 Binding energy (eV)

467 (c) Cu 2p

525

Cu2+ in CuS

Cu2+ in CuSO4

Intensity (a.u.)

Spent Nano-CuS

Fresh Nano-CuS

940

468

938

936 934 932 Binding energy (eV)

930

928


Page 20 of 24

Page 21 of 24


470

Figure 4. SO2-TPD profile of fresh and pretreated Nano-CuS (inserted with the FTIR spectra of

471

fresh and pretreated Nano-CuS).

Absorbance (a.u.)

Intensity (a.u.)

Fresh Pretreated with N2 + 4% O2 + 1000 ppm SO2 + 500 ppm SO3

Pretreated Nano-CuS Fresh Nano-CuS

800

100

472

CuSO4

200

1200 1600 2000 Wavenumber (cm-1)

300

2400

400 500 600 Temperature (oC)

700

800

900



Page 22 of 24

474

Figure 5. Reduction mechanism of HgS by SO3 on a molecular level: (a) Hg0 immobilized by a

475

monosulfide; (b) surface HgS attacked by a SO3; (c) surface HgS attacked by another SO3; and (d)

476

as-formed three SO2 in panel (c).

477


Page 23 of 24


479

Figure 6. (a) Hg-TPD of spent Nano-CuS pretreated under different conditions; and (b) leaching

480

of mercury from different samples. (a)

HgS

CuS pretreated by Hg0 with SO3 Mercury signal (a.u.)

CuS pretreated by Hg0 without SO3

HgSO4

481

50

150

250

350 Temperature (oC)

450

550

650

Leaching mercury concentration (g/L)

160

482

140

(b)

120 100 80 60 40 4 3 2 1 0

CuS pretreated by Hg0 without SO3

CuS pretreated by Hg0 with SO3



484

Table of contents

485


Page 24 of 24

Role of Sulfur Trioxide (SO3) in Gas-Phase Elemental Mercury

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