Effect of Nitrogen Oxides on Elemental Mercury Removal by

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Effect of Nitrogen Oxides on Elemental Mercury Removal by Nanosized Mineral Sulfide Hailong Li, Lei Zhu, Jun Wang, Liqing Li, Po Heng Lee, Yong Feng, and Kaimin Shih Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00224 • Publication Date (Web): 29 Jun 2017 Downloaded from http://pubs.acs.org on July 2, 2017

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Environmental Science & Technology

TOC Art

ACS Paragon Plus Environment


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Effect of Nitrogen Oxides on Elemental Mercury

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Removal by Nanosized Mineral Sulfide

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Hailong Li1,4, Lei Zhu1, Jun Wang2, Liqing Li1, Po-Heng Lee3, Yong Feng4, Kaimin Shih4*

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

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Department of Occupational and Environmental Health, College of Public Health, University of

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Oklahoma Health Sciences Center, Oklahoma City, OK 73126 3

Department of Civil & Environmental Engineering, The Hong Kong Polytechnic University, Hong

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Kong SAR, China 4

Department of Civil Engineering, The University of Hong Kong, Hong Kong SAR, China

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Revision Submitted to Environmental

Science & Technology

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*To whom correspondence should be addressed:

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TEL: 86-18670016725

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FAX: 86-731-88879863

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Email: [email protected]

17 ACS Paragon Plus Environment

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ABSTRACT: Because of its large surface area, nanosized zinc sulfide (Nano-ZnS) has been

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demonstrated in a previous study to be efficient for removal of elemental mercury (Hg0) from coal

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combustion flue gas. The excellent mercury adsorption performance of Nano-ZnS was found to be

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insusceptible to water vapor, sulfur dioxide, and hydrogen chloride. However, nitrogen oxides (NOx)

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apparently inhibited mercury removal by Nano-ZnS; this finding was unlike those of many studies on

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the promotional effect of NOx on Hg0 removal by other sorbents. The negative effect of NOx on Hg0

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adsorption over Nano-ZnS was systematically investigated in this study. Two mechanisms were

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identified as primarily responsible for the inhibitive effect of NOx on Hg0 adsorption over Nano-ZnS: (1)

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active sulfur sites on Nano-ZnS were oxidized to inactive sulfate by NOx; and (2) the chemisorbed

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mercury, i.e., HgS, was reduced to Hg0 by NOx. This new insight into of the role of NOx in Hg0

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adsorption over Nano-ZnS can help to optimize operating conditions, maximize Hg0 adsorption, and

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facilitate the application of Nano-ZnS as a superior alternative to activated carbon for Hg0 removal

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using existing particulate matter control devices in power plants.

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KEYWORDS: Mercury, Nitrogen oxides, Mineral sulfide, Adsorption

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INTRODUCTION

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The 50-ratification milestone for the Minamata Convention on Mercury was reached on 18 May 2017,

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as a result of which, the Minamata Convention aims at protecting human health and the environment

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from anthropogenic mercury emissions will take into effect for all its parties from 16 August 2017. To

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plan ahead, national standards to restrict mercury emission from coal-fired power plants have come into

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force prior to September 2016 in both China and the United States.1, 2 In addition to federal efforts,

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many US states had adopted their own emission regulations which are generally more strict to limit

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mercury emission from coal-fired power plants.3 To meet the requirements of both global and regional

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mercury regulations, various technologies such as activated carbon injection (ACI), have been

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thoroughly researched and developed for the removal of mercury, especially elemental mercury (Hg0),

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from coal-fired power plants.4-6

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Among these technologies, ACI is recognized as the maximum achievable control technology for

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mercury capture from coal combustion flue gas.7 Either electrostatic precipitators or baghouses are

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installed in most coal-fired power plants to control the emission of particulate matters. In these existing

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particulate matter control devices, the injected activated carbons loaded with mercury can be efficiently

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collected. However, ACI still has several drawbacks associated with carbon materials, such as its low

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operating temperature and prevention of the beneficial reuse of coal fly ashes. Therefore, the injection

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of non-carbon sorbents is considered to be a better technology for mercury emission control.8 Grafting

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of sulfur species has been used to efficiently enhance the performance of non-carbon adsorbents for

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mercury removal from both aqueous solution and air.9,

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believed to be the key factor that affects the performance of sulfur-impregnated materials.8, 11, 12 From

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A large surface coverage of sulfur was


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the perspective of surface sulfur coverage, mineral sulfides would be promising for mercury removal,

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because they comprise entirely of “active” sulfur sites. Fabrication of sulfide-based sorbents requires

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fewer and safer raw materials and can be even achieved through recycling mining wastes that contain

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metal sulfides.13 Moreover, mineral sulfides such as zinc sulfide (ZnS), pyrite (FeS2), and mercuric

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sulfide (HgS) are very stable14 and able to restrain mercury methylation.15 Furthermore, a small amount

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of mineral sulfides in concrete does not reduce the performance of concrete, more than that, they could

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limit the leaching of heavy metals.16 With these advantages, it is probably a scientifically sound and

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economically feasible method to use mineral sulfides for the removal of mercury from coal combustion

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flue gas. Hg0 capture performance and the involved mechanism for a mineral sulfide, i.e., sphalerite,

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were preliminarily investigated in our previous study.17 Under relatively simple atmospheres, the

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developed nano-sphalerite (Nano-ZnS) was found to be preferable in both Hg0 adsorption capacity and

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adsorption rate compared to several commercial activated carbons. However, more in-depth studies

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under real flue gas conditions are urgently needed before the technology can be commercialized.

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Coal combustion flue gas generally contains variable amounts of carbon dioxide (CO2), water (H2O),

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sulfur dioxide (SO2), nitrogen oxides (NOx), and hydrogen chloride (HCl), the concentrations of which

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depend on the coal types, the firing systems, and the combustion conditions.18 For carbon-based

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sorbents, the impacts of different flue gas components on mercury removal have been widely

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investigated. CO2 neither reacts with Hg0 nor interferes with the reactions between carbon sorbents and

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mercury.19 HCl has significant benefits for Hg0 oxidation and subsequent adsorption.20, 21 In contrast,

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H2O is generally believed to decrease the mercury adsorption capacity due to competitive adsorption

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and/or additional internal mass transfer resistance.19, 22, 23 The role of SO2 in mercury adsorption is still


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in controversial.24 Favorable,20 adverse,25, 26 and negligible27 effects have all been reported, depending

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on the materials and flue gas conditions. Mixtures of nitrogen monoxide (NO) and nitrogen dioxide

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(NO2), also known as NOx, always show a promotional effect on mercury adsorption by carbon

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materials.20, 21, 26, 28 The effects of flue gas constitutes on mercury adsorption have not yet been studied

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for the recently developed Nano-ZnS sorbent.

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In this study, the mercury removal performance of Nano-ZnS synthesized via a liquid-phase

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precipitation method was evaluated under different flue gas atmospheres. We found that the Nano-ZnS

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performed well in Hg0 adsorption under simulated coal combustion flue gas at temperatures below 180

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°C. More importantly, the excellent mercury adsorption performance of Nano-ZnS was insusceptible to

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H2O, HCl, and SO2 (as shown in Figure S1-S3 in the Supporting Information). With these merits, it is

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probably promising to use Nano-ZnS for the removal of mercury from real coal-fired power plants,

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especially those that burn low-chlorine or high-sulfur coals. However, NO, especially in the presence of

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oxygen (O2), was observed to be detrimental for mercury capture by Nano-ZnS, which is contrary to the

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findings of studies regarding the effects of NOx on other mercury adsorbents. Therefore, the focus of

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this study was to examine the effects of NOx on mercury adsorption over Nano-ZnS and to clarify the

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involved mechanisms. An in-depth understanding of the interactions between mercury, NOx, and Nano-

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ZnS is critical to the design of a novel mineral sulfide sorbent for the abatement of mercury pollution

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from coal combustion.

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EXPERIMENTAL SECTION

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Material Preparation. Nano-ZnS was synthesized using a liquid-phase precipitation method

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reported in our previous study 17, the details of which are also presented in the Supporting Information.


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The prepared Nano-ZnS exhibited a specific surface area of 196.1 m2⋅g−1. The mean diameter of the

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ZnS nanocrystal was 6 nm, indicating that the prepared material was in the nanometer range.

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Experimental System and Method. The Hg0 adsorption performance of Nano-ZnS was evaluated on

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an identical bench-scale experimental system following the same experimental protocol described in our

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previous study.17 Six sets of experiments with details summarized in Table S1 were conducted in this

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study. Set I experiments were carried out to examine the effects of NOx on Hg0 adsorption over Nano-

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ZnS. Set II experiments conducted on Nano-ZnS loaded with Hg0 (fresh Nano-ZnS adsorbed mercury in

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nitrogen (N2) gas flow containing 300 µg⋅m−3 Hg0 at 180 °C for 1 h) were designed to determine

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whether competitive adsorption occurred between NOx and Hg0 on Nano-ZnS. In Set III experiments, a

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relatively long (72 h) mercury adsorption process was used to accumulate sufficient HgS on the Nano-

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ZnS surface; Nano-ZnS loaded with HgS was then heated to eliminate a small amount of Hg0, and NO

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and NOx were subsequently added to explore their respective reactions with HgS after stabilization of

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the outlet Hg0 concentration. The experiments in Set IV aimed to determine the detrimental effects of

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NOx on Hg0 adsorption via alteration of the Nano-ZnS surface using O2, NO, or NOx pretreatments

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(fresh Nano-ZnS treated by 4% O2 balanced in N2, 1000 ppm NO balanced in N2, and 1000 ppm NO

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plus 4% O2 balanced in N2 at 180°C for 2 h; the Nano-ZnS was then flushed by pure N2 gas flow at the

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same temperature for 30 min). Pure HgS was used in the Set V experiments to further identify the

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induced HgS reduction by NOx. In Set VI, temperature-programmed desorption (TPD) experiments

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were conducted to further verify our inference regarding the mechanisms responsible for the detrimental

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effects of NOx on Hg0 removal by Nano-ZnS. The heating rate from room temperature to 600 °C was

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nominally 5 °C ⋅ min−1. Mercuric compounds on the Nano-ZnS decomposed to Hg0 during the TPD


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experiments. A pure N2 gas low with flow rate of 250 ml⋅min−1 was adopted to extract Hg0 from the

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reactor.

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For each Hg0 adsorption experiment, the gas stream upstream the reactor inlet was monitored until the

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desired inlet Hg0 concentration (Cin) stabilized for at least 30 min. The gas flow was then passed

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through the Nano-ZnS sorbent in the reactor, and was online measured at the exit of the reactor as the

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outlet Hg0 concentration (Cout). Each experiment was conducted at least in duplicate, and the mean

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values are reported. The relative errors were smaller than 5% for most experiments.

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Characterization of Sorbent. The surface changes of the Nano-ZnS after Hg0 adsorption in the

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presence of NOx were analyzed by X-ray photoelectron spectroscopy (XPS) and Fourier transform-

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infrared (FTIR) spectrum. XPS analysis was conducted using an Escalab 250Xi (Thermo Fisher

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Scientific, USA), in which a vacuum was maintained at 10−6 P, and a monochromatized Al Kα radiation

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(hυ = 1486.6 eV) was adopted as the excitation source. The sorbent was cooled and dried in a desiccator

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before XPS analysis. The binding energy values were calibrated by the carbonaceous C1s line of 284.8

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eV. Mixtures of 0.5% finely ground samples and KBr were fabricated to be KBr disks which were then

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detected by an FTIR spectrometer (Nicolet, 6700) to get the samples’ FTIR spectra. The spectra were

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recorded from 4000 to 400 cm−1 at a resolution of 4 cm−1.

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RESULTS AND DISCUSSION

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NOx Exhibited Obvious Detrimental Effects on Hg0 Adsorption by Nano-ZnS. NO has usually

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been reported to facilitate Hg0 oxidation and/or subsequent adsorption with the aid of fly ashes,29

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sorbents,26 or catalysts.18, 30 However, in this study, NO was found to have inhibitory effects on Hg0

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adsorption over Nano-ZnS. As shown in Figure 1, a slightly prohibitive effect was observed for the gas


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flow containing 300 ppm NO balanced in N2, resulting in the detection of more Hg0 at the reactor outlet.

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A further increase in the NO concentration to 1000 ppm led to a relatively lower mercury adsorption

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capacity, further identifying the insignificant inhibitive effect of NO on Hg0 adsorption over Nano-ZnS.

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However, NO with the aid of O2 exhibited a pronounced inhibitive effect on Hg0 adsorption. The

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normalized outlet Hg0 concentration at the end of a 4-h test increased from 0.08 to 0.15 with the

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introduction of 4% O2 to the gas flow containing 300 ppm NO. For 1000 ppm NO plus 4% O2, the

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normalized outlet Hg0 concentration sharply increased to 0.45 at the end of a 4-h test. As illustrated in

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Figure 1, O2 was almost inert in Hg0 adsorption on the Nano-ZnS. However, with the aid of O2, NO

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exhibited evident inhibitory effect on Hg0 adsorption, and more NO led to a greater inhibitory effect on

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Hg0 adsorption. In the presence of O2, the NO was partially oxidized into NO2, particularly at the low

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flue gas temperatures.31 It is therefore reasonable to conclude that NOx exhibited a noticeable

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detrimental effect on Hg0 adsorption by Nano-ZnS.

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It should be noted that Nano-ZnS is likely to be used as an alternative to activated carbons, which are

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usually injected downstream of a selective catalytic reduction device, where the NOx concentration is

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relatively low. Moreover, the adsorption rate within 30 min was barely affected by NOx. The 30-min

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timespan is the most important parameter because the residence time of the injected sorbent that

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interacts with mercury is about 3 to 5 s in the case of an electrostatic precipitator and approximately 25

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min in the case of a fabric filter.10 Even though NOx could limit Hg0 adsorption by Nano-ZnS, the

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results still imply that Nano-ZnS is a promising sorbent candidate for Hg0 removal in coal-fired power

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plants, because the combination of the high Hg0 adsorption rate17 and the low NOx concentration at the

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injection site give Nano-ZnS excellent Hg0 adsorption performance.


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NOx Did Not Repel Hg0 From Nano-ZnS Surface. Competitive adsorption between Hg0 and other

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flue gas components on a sorbent usually resulted in the deactivation of the sorbent for Hg0 removal.

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For example, the competitive adsorption between water vapor and Hg0 reduced the capacity for Hg0

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uptake of sulfur-impregnated activated carbon by as much as 25%.19 Sulfur oxides (SOx) limited Hg0

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adsorption on a carbon surface because the Hg0 and the SOx were competing for the same binding

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sites.27 Although the primary mechanism responsible for Hg0 removal over Nano-ZnS was identified as

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chemisorption, physical adsorption of Hg0 also occurred on the Nano-ZnS surface.17 The physical

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adsorption was probably a result of the Nano-ZnS’s large surface area. In addition, weakly adsorbed

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Hg0 is generally believed to be crucial for the subsequent Hg0 oxidation/chemisorption process.32 If

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competitive adsorption occurred between Hg0 and NOx on the Nano-ZnS surface, both physical

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adsorption and the subsequent chemisorption of Hg0 will be negatively affected by the presence of NOX.

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This mechanism should be responsible for the deactivation of Hg0 adsorption as shown in Figure 1.

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However, the introduction of NOx did not cause any Hg0 discharge from the used Nano-ZnS as

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illustrated in Figure 2. Neither the addition of 1000 ppm NO nor the addition of 1000 ppm NO plus 4%

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O2 repelled Hg0 from the Nano-ZnS loaded with Hg0. In contrast to the results reported in the literature,

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our results indicate that competitive adsorption between Hg0 and NOx was not responsible for the

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deactivation of Hg0 adsorption.

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NOX Deactivated Nano-ZnS Surface for Hg0 Adsorption. The inhibition of mercury capture by flue

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gas components such as SO2 was generally attributed to the depletion of surface active sites by reactions

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between the sorbent and flue gas components.27 Oxidative NOx would probably react with reduced

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sulfur species on Nano-ZnS surface, which has a high binding affinity for mercury8, 11 and hence leads


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to the inactivation of Nano-ZnS for Hg0 capture. Pretreatment of Nano-ZnS with either NO or NOx was

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adopted to identify the possible surface deactivation for Hg0 adsorption. As shown in Figure 3, O2

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pretreatment exhibited negligible effect on Hg0 adsorption. This is in agreement with the results shown

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in Figure 1 that O2 was almost inert in Hg0 adsorption on the Nano-ZnS sorbent. NO pretreatment had

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an inhibitory effect on Hg0 adsorption in a pure N2 atmosphere. After passing through the Nano-ZnS

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pretreated by NO, the normalized outlet Hg0 concentration gradually increased to 0.36 at the end of the

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4-h test, which was much higher than that observed over fresh Nano-ZnS, which indicates that the active

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surface Hg0 adsorption sites were at least partially destroyed or decreased during the NO pretreatment

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process. The Hg0 adsorption performance of NOx-pretreated Nano-ZnS was significantly lower than that

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for the NO-pretreated Nano-ZnS, indicating that NOx was more reactive than NO for consumption of

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the active Hg0 adsorption sites on the Nano-ZnS. The findings also imply that the active Hg0 adsorption

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sites on the Nano-ZnS were probably oxidized, because NOx is more oxidative than NO alone.

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NOx Induced HgS Reduction on Nano-ZnS. It is also likely that flue gas components induced the

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reduction of oxidized mercury to generate Hg0,33, 34 which then left the sorbent’s surface and resulted in

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a lower Hg0 removal rate, as observed in this study. NO was capable of promoting the reduction of

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oxidized mercury to Hg0 at elevated temperatures.35 If NOx can react with HgS to form Hg0 and then

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discharge the Hg0 from the Nano-ZnS surface at the experimental temperature, the detrimental effects of

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NOx on Hg0 adsorption will be somewhat clarified, because the observed Hg0 removal rate reflected the

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net sum of Hg0 adsorption and desorption from the sorbent. As shown in Figure 4, Nano-ZnS loaded

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with HgS was first heated at 180°C in a pure N2 gas flow to establish equilibrium, as marked by

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fluctuation of the Hg0 concentration at the reactor outlet within 5% for more than 30 min. Relative to the


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equilibrium state, more Hg0 was detected at the reactor outlet at the initial few minutes, probably due to

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desorption of the physically adsorbed Hg0. At the equilibrium stage, a small amount of Hg0 at the

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reactor outlet was attributed to the slow decomposition of HgS. More Hg0 emission was observed after

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the addition of 1000 ppm NO. This is in accordance with Figure 1 that NO slightly inhibited Hg0

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adsorption by Nano-ZnS. A sharp increase in Hg0 was observed after the simultaneous addition of 1000

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ppm NO and 4% O2 to an N2 gas flow passing through Nano-ZnS loaded with mercury. As HgS was

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identified to be the primary mercury species on the Nano-ZnS,17 it is very likely that HgS was reduced

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by NOx to form Hg0.

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Identification of Mechanisms Responsible for Detrimental Effect of NOx on Hg0 Adsorption. As

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stated above, two hypotheses were considered to be accountable for the inhibitive effects of NOx on Hg0

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adsorption over Nano-ZnS: (1) the active Hg0 adsorption sites on Nano-ZnS were oxidized by NOx; and

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(2) the chemisorbed mercury, i.e., HgS, was reduced to Hg0 by NOx.

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To identify the possible oxidation of a sorbent surface by NOx, the surface changes of Nano-ZnS after

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72 h of Hg0 adsorption in the presence of NOx were analyzed by FTIR spectrum and XPS, and the

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results are shown in Figures 5 and 6, respectively. The absorption peaks at 3430 cm−1 and 1621 cm−1 on

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the FTIR spectra were attributed to water molecules and hydrogen bonded hydroxyl groups,

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respectively.36 The insignificant peak at 1388 cm−1 can be ascribed to the N-O stretching mode after

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NOx adsorption.37 The bands at 1117 cm−1 and 618 cm−1 corresponded to sulfate.38, 39 The sulfate peak

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intensity for the Nano-ZnS with NO was greater than that for the Nano-ZnS without NO, which

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demonstrates that the NO oxidized the low-valence sulfur on the Nano-ZnS surface to form sulfate.

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Low-valence sulfur compounds including elemental sulfur (S0) and sulfide (HS−, S2−) generally have a


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high binding affinity for gas phase Hg0.8, 11 Once the low-valence sulfur on the Nano-ZnS was oxidized

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into sulfate, its affinity to Hg0 disappeared due to the inhibition of Hg0 adsorption from sulfate.27, 40

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Therefore, the Hg0 adsorption performance in the presence of NO was inferior to that under pure N2

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atmosphere. The Nano-ZnS under an NOx atmosphere exhibited two stronger sulfate peaks at 1117 cm−1

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and 618 cm−1 compared to the Nano-ZnS used in the presence of NO, which indicates that NOx is more

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active than NO in the oxidation of reduced sulfur to sulfate. This finding is in accordance with the

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results of the Hg0 adsorption experiments in Set I, in which Nano-ZnS exhibited an inferior Hg0 removal

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performance in the presence of NOx compared to that in the presence of NO alone.

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The S 2p spectra of fresh Nano-ZnS and Nano-ZnS used under NO or NOx are presented in Figure 6.

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As shown, the S 2p spectra of the different Nano-ZnS have two peaks at 161.9 and 168.9 eV,

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respectively. The significant peak at 161.9 eV was ascribed to S2−,39, 41 and the other small peak around

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168.9 eV corresponded to sulfur at oxidative states such as sulfate.42 For fresh Nano-ZnS, a trace of

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sulfate seemingly came from the raw material, i.e., zinc sulfate. Compared to fresh Nano-ZnS, the

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intensity of the peak around 168.9 eV showed an apparent increase when Nano-ZnS was used for Hg0

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removal in the presence of NO. The increased intensity indicates that NO facilitated sulfate formation

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via oxidation of low-valence sulfur on the Nano-ZnS surface, because no other sulfur source other than

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that on the Nano-ZnS existed in the experimental system. This finding is in line with the FTIR results,

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which show that NO oxidized the low-valence sulfur on the Nano-ZnS surface into sulfate. NOx with a

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greater affinity for oxidation further promoted sulfate formation and hence led to a much greater sulfate

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peak on the S 2p XPS spectrum. A similar phenomenon has been reported in the literature in which NO2

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was observed to be more reactive than NO for sulfate accumulation on a mercury-sorbent surface.43 The


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O 1s peaks also rose from fresh Nano-ZnS to Nano-ZnS used under NO, and further to Nano-ZnS used

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under NOx. This finding is in agreement with the sulfate formation trend on the Nano-ZnS surface.

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For the second mechanism, NO or NOx reacted with HgS on the Nano-ZnS surface to form Hg0, which was then released from the Nano-ZnS surface. The reaction processes are presumably as follows: HgS(s) + 2NO(g) → Hg0(g) + SO2(g) + N2(g)

(1)

∆Hθm = -357.81 kJ/mol; ∆Gθm = -390.87 kJ/mol; Kθ (180 ̊ C) = 1.14×1045 3HgS(s) + 2NO(g) + 2NO2(g)→ 3Hg0(g) + 3SO2(g) + 2N2(g)

(2)

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∆Hθm = -778.78 kJ/mol; ∆Gθm = -929.03 kJ/mol; Kθ (180 ̊ C) = 1.23×10107

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According to thermodynamics, both the Gibbs free energy and the enthalpy variations of reactions (1)

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and (2) are far less than 0, indicating that reactions (1) and (2) are spontaneous and favorable from the

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thermodynamic perspective. Moreover, Hg2+ conversion experiments over pure HgS was conducted at

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180 °C to further verify the induced Hg2+ reduction by NOx. As shown in Figure 7, pure HgS was

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reduced by 1000 ppm NO, indicating that reaction (1) is practical and feasible. More HgS was reduced

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when 1000 ppm NO and 4% O2 were added simultaneously. These results conform to the

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thermodynamic data of equations (1) and (2), that is, the equilibrium constant of equation (2) is much

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higher than that of equation (1). It should be noted that the phenomenon observed in the Set III

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experiments (Figure 4) was similar to that in the Set V experiments (Figure 7), which demonstrates that

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Hg0 was chemisorbed on the Nano-ZnS to form HgS during the 72-h pretreatment process. This finding

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is consistent with our previous finding that physically adsorbed Hg0 reacted with the adjacent surface

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sulfur on the Nano-ZnS to form the most stable HgS.17 When Nano-ZnS accumulates sufficient HgS on

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the surface, Hg0 from the reduction of HgS by NOx would be released from the sorbent and could hence


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lead to the deactivation of Nano-ZnS for Hg0 removal. However, the small amount of Hg0 from the

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reduction of HgS in Equations (1) and (2) would be re-adsorbed by Nano-ZnS with a large surface area,

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assuming that insufficient HgS is present on the Nano-ZnS surface.

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In short, NOx could deactivate the Hg0 removal performance of Nano-ZnS via sulfation of the Nano-

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ZnS surface and the induced reduction of HgS. Both reactions induced by NOx could result in less HgS,

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which is the immobilized form of Hg0 on the Nano-ZnS. This hypothesis was further validated by a

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TPD experiment, the results of which are shown in Figure 8. No Hg0 peak was observed for fresh Nano-

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ZnS that was free of mercury. Decomposition of mercuric compounds occurred from 180 °C to 420 °C,

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with a peak decomposition rate at approximately 310 °C, indicating that the primary mercury species on

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the used Nano-ZnS was HgS.44, 45 The Nano-ZnS used for Hg0 removal under pure N2 atmosphere

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accumulated maximum HgS on its surface, which then decomposed to Hg0 during the TPD experiment.

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A quantity of 1000 ppm NO in N2 inhibited HgS accumulation on the Nano-ZnS, and hence less Hg0

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was detected in the TPD experiment. Minimal Hg0 discharge was observed over the Nano-ZnS used for

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Hg0 adsorption in the presence of NO and O2. These results coincide fully with those shown in Figure 1

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that NO slightly inhibited Hg0 adsorption over Nano-ZnS, whereas NOx evidently limited Hg0

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adsorption by Nano-ZnS.

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ACKNOWLEDGMENTS

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This project was supported by the National Natural Science Foundation of China (No. 51476189), the

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Environmental Research Funds of Hunan Province (NO.2016QNZT032), and the Research Grants

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


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ASSOCIATED CONTENT

Material preparation, one table, and three figures. This material is available free of charge at

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http://pubs.acs.org.

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REFERENCES

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(1) Ministry of Environmental Protection The people's Republic of China, Emission standard of air

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pollutants

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http://www.zhb.gov.cn/gkml/hbb/bgg/201109/W020120113518288046763.pdf (accessed Jan, 2015).

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(2)

292

http://www.epa.gov/airquality/powerplanttoxics/actions.html (accessed July, 2010).

293

(3) Milford, J. B.; Pienciak, A. After the Clean Air Mercury Rule: Prospects for Reducing Mercury

294

Emissions from Coal-Fired Power Plants. Environ. Sci. Technol. 2009, 43 (8), 2669-2673.

295

(4) Jones, A. P.; Hoffmann, J. W.; Smith, D. N.; Feeley, T. J.; Murphy, J. T. DOE/NETL's Phase II

296

Mercury Control Technology Field Testing Program: Preliminary Economic Analysis of Activated

297

Carbon Injection. Environ. Sci. Technol. 2007, 41 (4), 1365-1371.

298

(5) Qu, Z.; Yan, N.; Liu, P.; Guo, Y.; Jia, J. Oxidation and Stabilization of Elemental Mercury from

299

Coal-Fired Flue Gas by Sulfur Monobromide. Environ. Sci. Technol. 2010, 44 (10), 3889-3894.

300

(6) Liu, Y.; Wang, Q. Removal of Elemental Mercury from Flue Gas by Thermally Activated

301

Ammonium Persulfate in A Bubble Column Reactor. Environ. Sci. Technol. 2014, 8 (20), 12181-12189.

302

(7) Pavlish, J. H.; Sondreal, E. A.; Mann, M. D.; Olson, E. S.; Galbreath, K. C.; Laudal, D. L.; Benson,

303

S. A. Status Review of Mercury Control Options for Coal-fired Power Plants. Fuel Process. Technol.

U.S.

for

Environmental

thermal

Protection

power

Agency,

plants

Air

Toxics


(GB

Standards

13223-2011);

for

Utilities;

15

Page 17 of 30


304

2003, 82 (2-3), 89-165.

305

(8) Lee, J. Y.; Ju, Y. H.; Keener, T. C.; Varma, R. S. Development of Cost-effective Noncarbon

306

Sorbents for Hg0 Removal from Coal-fired Power Plants. Environ. Sci. Technol. 2006, 40 (8), 2714-

307

2720.

308

(9) Li, B. Y.; Zhang, Y. M.; Ma, D. X.; Shi, Z.; Ma, S. Q. Mercury Nano-trap for Effective and Efficient

309

Removal of Mercury(II) from Aqueous Solution. Nat. Commun. 2014, 5, 5537.

310

(10) Sun, Q.; Aguila, B.; Perman, J.; Earl, L. D.; Abney, C. W.; Cheng, Y. C.; Wei, H.; Nguyen, N.;

311

Wojtas, L.; Ma, S. Q. Postsynthetically Modified Covalent Organic Frameworks for Efficient and

312

Effective Mercury Removal. J. Am. Chem. Soc. 2017, 139 (7), 2786-2793.

313

(11) Otani, Y.; Kanaoka, C.; Emi, H.; Uchijima, I.; Nishino, H. Removal of Mercury Vapor from Air

314

with Sulfur-impregnated Adsorbents. Environ. Sci. Technol. 1988, 22 (6), 708-711.

315

(12) Makkuni, A.; Varma, R.; Sikdar, S.; Bhattacharyya, D. Vapor Phase Mercury Sorption by Organic

316

Sulfide Modified Bimetallic Iron-copper Nanoparticle Aggregates. Ind. Eng. Chem. Res. 2007, 46 (4),

317

1305-1315.

318

(13) Martellaro, P.; Moore, G.; Peterson, E.; Abbott, E.; Gorenbain, A. Environmental Application of

319

Mineral Sulfides for Removal of Gas-phase Hg(0) and Aqueous Hg2+. Sep. Sci. Technol. 2001, 36 (5-6),

320

1183-1196.

321

(14) Lau, B. L.; Hsu-Kim, H. Precipitation and Growth of Zinc Sulfide Nanoparticles in the Presence of

322

Thiol-containing Natural Organic Ligands. Environ. Sci. Technol. 2008, 42 (19), 7236-7241.

323

(15) Benoit, J. M.; Gilmour, C. C.; Mason, R. P.; Heyes, A. Sulfide Controls on Mercury Speciation and

324

Bioavailability to Methylating Bacteria in Sediment Pore Waters. Environ. Sci. Technol. 1999, 33 (10),


16


Page 18 of 30

325

1780-1780.

326

(16) Shi, H. S.; S., H. C. Study of Effect of Sulfide on Zinc Ion in Cement-based Materials. Cement

327

2006, (02), 15-18.

328

(17) Li, H.; Zhu, L.; Wang, J.; Li, L.; Shih, K. Development of Nano-Sulfide Sorbent for Efficient

329

Removal of Elemental Mercury from Coal Combustion Fuel Gas. Environ. Sci. Technol. 2016, 50 (17),

330

9551-9557.

331

(18) Li, H.; Wu, C. Y.; Li, Y.; Li, L.; Zhao, Y.; Zhang, J. Role of Flue Gas Components in Mercury

332

Oxidation over TiO2 Supported MnOx-CeO2 Mixed-oxide at Low Temperature. J. Hazard. Mater. 2012,

333

243 (0), 117-123.

334

(19) Liu, W.; Vidic, R. D.; Brown, T. D. Impact of Flue Gas Conditions on Mercury Uptake by Sulfur-

335

impregnated Activated Carbon. Environ. Sci. Technol. 2000, 34 (1), 154-159.

336

(20) Miller, S. J.; Dunham, G. E.; Olson, E. S.; Brown, T. D. Flue Gas Effects on A Carbon-based

337

Mercury Sorbent. Fuel Process. Technol. 2000, 65, 343-363.

338

(21) Tong, L.; Xu, W. Q.; Zhou, X.; Liu, R. H.; Zhu, T. Y. Effects of Multi-component Flue Gases on

339

Hg0 Removal over HNO3-Modified Activated Carbon. Energy Fuels 2015, 29 (8), 5231-5236.

340

(22) Hsi, H. C.; Tsai, C. Y.; Lin, K. J. Impact of Surface Functional Groups, Water Vapor, and Flue Gas

341

Components on Mercury Adsorption and Oxidation by Sulfur-Impregnated Activated Carbons. Energy

342

Fuels 2014, 28 (5), 3300-3309.

343

(23) Lopez-Anton, M. A.; Rumayor, M.; Diaz-Somoano, M.; Martinez-Tarazona, M. R. Influence of A

344

CO2-enriched Flue Gas on Mercury Capture by Activated Carbons. Chem. Eng. J. 2015, 262, 1237-1243.

345

(24) Liu, J.; Qu, W. Q.; Joo, S. W.; Zheng, C. G. Effect of SO2 on Mercury Binding on Carbonaceous


17

Page 19 of 30


346

Surfaces. Chem. Eng. J. 2012, 184, 163-167.

347

(25) Ochiai, R.; Uddin, M. A.; Sasaoka, E.; Wu, S. J. Effects of HCl and SO2 Concentration on Mercury

348

Removal by Activated Carbon Sorbents in Coal-Derived Flue Gas. Energy Fuels 2009, 23, 4734-4739.

349

(26) Hsi, H. C.; Chen, C. T. Influences of Acidic/Oxidizing Gases on Elemental Mercury Adsorption

350

Equilibrium and Kinetics of Sulfur-impregnated Activated Carbon. Fuel 2012, 98, 229-235.

351

(27) Presto, A. A.; Granite, E. J. Impact of Sulfur Oxides on Mercury Capture by Activated Carbon.

352

Environ. Sci. Technol. 2007, 41 (18), 6579-6584.

353

(28) Eswaran, S.; Stenger, H. G.; Fan, Z. Gas-phase Mercury Adsorption Rate Studies. Energy Fuels

354

2007, 21 (2), 852-857.

355

(29) Laudal, D. L.; Brown, T. D.; Nott, B. R. Effects of Flue Gas Constituents on Mercury Speciation.

356

Fuel Process. Technol. 2000, 65, 157-165.

357

(30) Li, Y.; Murphy, P. D.; Wu, C. Y.; Powers, K. W.; Bonzongo, J. C. J. Development of

358

Silica/Vanadia/Titania Catalysts for Removal of Elemental Mercury from Coal-Combustion Flue Gas.

359

Environ. Sci. Technol. 2008, 42 (14), 5304-5309.

360

(31) Garin, F. Mechanism of NOx Decomposition. Appl. Catal. A- Gen. 2001, 222 (1-2), 183-219.

361

(32) Li, H.; Wu, C. Y.; Li, Y.; Zhang, J. Superior Activity of MnOx-CeO2/TiO2 Catalyst for Catalytic

362

Oxidation of Elemental Mercury at Low Flue Gas Temperatures. Appl. Catal. B- Environ.2012, 111-112

363

(0), 381-388.

364

(33) Stolle, R.; Koeser, H.; Gutberlet, H. Oxidation and Reduction of Mercury by SCR DeNOx

365

Catalysts under Flue Gas Conditions in Coal Fired Power Plants. Appl. Catal. B- Environ. 2014, 144 (0),

366

486-497.


18


Page 20 of 30

367

(34) Li, H.; Wu, S.; Wu, C. Y.; Wang, J.; Li, L.; Shih, K., SCR Atmosphere Induced Reduction of

368

Oxidized Mercury over CuO-CeO2/TiO2 Catalyst. Environ. Sci. Technol. 2015, 49 (12), 7373-7379.

369

(35) Zhao, Y. X.; Mann, M. D.; Olson, E. S.; Pavlish, J. H.; Dunham, G. E. Effects of Sulfur Dioxide

370

and Nitric Oxide on Mercury Oxidation and Reduction under Homogeneous Conditions. J. Air Waste

371

Manage. Assoc. 2006, 56 (5), 628-635.

372

(36) Naskar, M. K.; Patra, A.; Chatterjee, M. Understanding the Role of Surfactants on the Preparation

373

of ZnS Nanocrystals. J. Colloid Interface Sci. 2006, 297 (1), 271-275.

374

(37) Hadjiivanov, K.; Bushev, V.; Kantcheva, M.; Klissurski, D. Infrared Spectroscopy Study of the

375

Species Arising During Nitrogen Dioxide Adsorption on Titania (Anatase). Langmuir 1994, 10 (2), 464-

376

471.

377

(38) Siriwardane, R. V.; Woodruff, S. FTIR Characterization of the Interaction of Oxygen with Zinc

378

Sulfide. Ind. Eng. Chem. Res. 1995, 34 (2), 699-702.

379

(39) Konishi, M.; Isobe, T.; Senna, M. Enhancement of Photoluminescence of ZnS : Mn Nanocrystals

380

by Hybridizing with Polymerized Acrylic Acid. J. Lumin. 2001, 93 (1), 1-8.

381

(40) Olson, E. S.; Crocker, C. R.; Benson, S. A.; Pavlish, J. H.; Holmes, M. J. Surface Compositions of

382

Carbon Sorbents Exposed to Simulated Low-Rank Coal Flue Gases. J. Air Waste Manage. Assoc. 2005,

383

55 (6), 747-754.

384

(41) Barreca, D.; Gasparotto, A.; Maragno, C.; Tondello, E.; Spalding, T. R. Analysis of

385

Nanocrystalline ZnS Thin Films by XPS. Surf. Sci. Spectra 2002, 9 (1), 54-61.

386

(42) Saha, A.; Abram, D. N.; Kuhl, K. P.; Paradis, J.; Crawford, J. L.; Sasmaz, E.; Chang, R.; Jaramillo,

387

T. F.; Wilcox, J. An X-ray Photoelectron Spectroscopy Study of Surface Changes on Brominated and


19

Page 21 of 30


388

Sulfur-Treated Activated Carbon Sorbents during Mercury Capture: Performance of Pellet versus Fiber

389

Sorbents. Environ. Sci. Technol. 2013, 47 (23), 13695-13701.

390

(43) Laumb, J. D.; Benson, S. A.; Olson, E. A. X-ray Photoelectron Spectroscopy Analysis of Mercury

391

Sorbent Surface Chemistry. Fuel Process. Technol. 2004, 85 (6-7), 577-585.

392

(44) Rumayor, M.; Fernandez-Miranda, N.; Lopez-Anton, M. A.; Diaz-Somoano, M.; Martinez-

393

Tarazona, M. R. Application of Mercury Temperature Programmed Desorption (HgTPD) to Ascertain

394

Mercury/Char Interactions. Fuel Process. Technol. 2015, 132, 9-14.

395

(45) Rumayor, M.; Lopez-Anton, M. A.; Díaz-Somoano, M.; Martínez-Tarazona, M. R. A New

396

Approach to Mercury Speciation in Solids Using A Thermal Desorption Technique. Fuel 2015, 160,

397

525-530.

398


20


Page 22 of 30

399

List of Figures

400

Figure 1. Effect of NOx on Hg0 adsorption on Nano-ZnS at 180°C.

401

Figure 2. Desorption of Hg0 from Nano-ZnS loaded with Hg0 by NOx.

402

Figure 3. Hg0 adsorption over NO- and NOx-pretreated Nano-ZnS.

403

Figure 4. Desorption of Hg0 from Nano-ZnS loaded with HgS by NOx.

404

Figure 5. FTIR spectra of Nano-ZnS used for Hg0 adsorption in presence of NOx: (a) fresh Nano-ZnS;

405

(b) used Nano-ZnS (under NO); (c) used Nano-ZnS (under NO + O2).

406

Figure 6. S2p (a) and O1s (b) XPS spectra of Nano-ZnS used for Hg0 adsorption in presence of NOx.

407

Figure 7. Induced reduction of pure HgS by NOx.

408

Figure 8. TPD spectra of Nano-ZnS used for Hg0 adsorption in presence of NOx.

409


21

Page 23 of 30

410


Figure 1. Effect of NOX on Hg0 adsorption on the Nano-ZnS at 180 °C

0

Normalized outlet Hg concentration

1.0 N2 N2 + 4% O2 N2 + 300 ppm NO N2 + 1000 ppm NO N2 + 300 ppm NO + 4% O2 N2 + 1000 ppm NO + 4% O2

0.8

0.6

0.4

0.2

0.0 0

411

1

2 Time (h)

3

4


22


412

Page 24 of 30

Figure 2. Desorption of Hg0 from Nano-ZnS loaded with mercury by NOX 40 Outlet Hg0 concentration (µg/m3)

With Hg feed

Without Hg feed

Pure N2

Add different flue gas 1000 ppm NO + 4% O 2

30

1000 ppm NO pure N2

20

10

0 0

413

20

40

60

80

100

120

Time (min)

414


23

Page 25 of 30

415


Figure 3. Hg0 adsorption over NO and NOX pretreated Nano-ZnS

0

Normalized outlet Hg concentration

1.0 Fresh ZnS Pretreated ZnS (by O2) Pretreated ZnS (by NO) Pretreated ZnS (by NO + O2)

.8

.6

.4

.2

0.0 0

416

1

2

3

4

Time (h)

417 418


24


419

Page 26 of 30

Figure 4. Desorption of Hg0 from Nano-ZnS loaded with HgS by NOX

Outlet Hg0 concentration (¦Ìg/m3)

Pure N2

With different gases added

40 1000 ppm NO 1000 ppm NO + 4% O2

30

20

10

0 0

420

20

40

60 80 Time (min)

100

120

140


25

Page 27 of 30

421


Figure 5. FTIR spectra of Nano-ZnS used for Hg0 adsorption in the presence of NOX: (a) Fresh Nano-

422

ZnS; (b) Used Nano-ZnS (under NO); (c) Used Nano-ZnS (under NO + O2)

Transmittance (%)

(a)

1621 1388 1117 796 618

(b)

(c)

4000

423

3430

3500

3000

2500

2000

1500

1000

500

-1

Wavenumbers (cm )


26


424

Page 28 of 30

Figure 6. XPS spectra of S2p, O1s of Nano-ZnS used for Hg0 adsorption in the presence of NOX (a)

S 2p (S2-)

Intensity (a.u.)

Fresh Nano-ZnS Used Nano-ZnS (under NO) Used Nano-ZnS (under NO + O2)

S 2p (SO42-)

174

172

170

425

168 166 164 162 Binding energy (eV)

158

156

Fresh Nano-ZnS Used Nano-ZnS (under NO) Used Nano-ZnS (under NO + O2)

O 1s

Intensity (a.u.)

(b)

160

526

426

528

530

532

534

536

538

Binding energy (eV)

427


27

Page 29 of 30

428


Figure 7. Induced reduction of pure HgS by NOX Pure N2

Outlet Hg0 concentration (µg/m3)

60

With different gases added

50 40 30 20 1000 ppm NO 1000 ppm NO + 4% O2

10 0

0

429

20

40

60 Time (min)

80

100

120

430


28


431

Page 30 of 30

Figure 8. TPD spectra of Nano-ZnS used for Hg0 adsorption in the presence of NOX

0

Used Nano-ZnS (under NO + O2+ Hg ) 0

Used Nano-ZnS (under NO + Hg ) Used Nano-ZnS (under N2+ Hg0)

Mercury Signal

Fresh Nano-ZnS

0

432

100

200

300

400

500

600

Temperature (oC)

433


29

Effect of Nitrogen Oxides on Elemental Mercury Removal by

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