Removal of gaseous elemental mercury using seaweed chars impregnated by NH4Cl and NH4Br

Journal of Cleaner Production 216 (2019) 277e287

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Removal of gaseous elemental mercury using seaweed chars impregnated by NH4Cl and NH4Br Wen Xu, Jianfeng Pan, Baowei Fan, Yangxian Liu* School of Energy and Power Engineering, Jiangsu University, Zhenjiang, Jiangsu, 212013, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 February 2018 Received in revised form 27 November 2018 Accepted 18 January 2019 Available online 18 January 2019

Enteromorpha is a kind of waste algae that can cause marine ecological disaster. Actively exploring some effective ways for resource utilization of Enteromorpha has important economic and environmental significance. In this article, NH4Cl and NH4Br modified Enteromorpha chars (E8C5 and E8B5) were prepared by pyrolysis and simple impregnation method for capturing gas phase elemental mercury (Hg0) for the first time. The effects of pyrolysis temperature, loading value, reaction temperature and concentrations of main flue gas components (e.g. O2, NO, SO2, H2O) on Hg0 capture were studied. Mercury removal mechanism and kinetics were also investigated. The results show that Hg0 removal efficiency increases with the increase of pyrolysis temperature, and the optimal pyrolysis temperature is 800
C. Hg0 removal is significantly enhanced by NH4Cl or NH4Br modification, and the chars modified by NH4Br show better performance than those modified by NH4Cl. Optimal loading values and reaction temperature for E8C5 and E8B5 are 5 wt.% and 130
C, respectively. Presence of O2 and NO promotes Hg0 removal, whereas presence of SO2 inhibits Hg0 removal. Lower concentration of H2O is beneficial to Hg0 removal, but higher concentration of H2O shows an obvious inhibitory effect on Hg0 capture. Hg0 adsorption over E8C5 and E8B5 are endothermic processes. Chemisorption plays a key role in Hg0 removal. Both Hg0 removal processes over E8C5 and E8B5 meet pseudo-second-order kinetic model. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Elemental mercury Seaweed chars NH4Cl and NH4Br modified Flue gas

1. Introduction

prepared the nanostructured CeO2-MnOx catalyst and Ce0.7catalysts for Hg0 removal by a coprecipitation method, respectively, and found that the CeO2-MnOx mixed oxide developed catalysts exhibited superior Hg0 oxidation activity than pure CeO2 and/or MnOx. Wan et al. (2011) synthesized the CeO2WO3/TiO2 catalysts for Hg0 removal and found that about 95% of the Hg0 could be removed by hydrogen chloride over the CeO2-WO3/ TiO2 catalysts in the presence of O2. Li et al. investigated the modified Mn/a-Al2O3 catalyst to remove elemental mercury, and indicated that Mo doped catalyst achieved the best Hg0 removal performance. However, the preparation processes of these catalysts are relatively complicated and may lead to high cost. Activated carbon (AC) adsorption is one of the most mature mercury removal methods, but high adsorbent loss and operation cost have limited the further development of this technology (Yao et al., 2014; Tan et al., 2012). In order to develop more economic and effective alternative adsorbents, many researchers have focused on developing new carbon-based adsorbents from cheaper precursors (Xu et al., 2016, 2018a, 2018b; Shen et al., 2015; Shu et al., 2013; Yang et al., 2018a, 2018b, 2018c; Liu et al., 2016). Xu et al. (2016), Yang et al. (2017) and Liu et al. (2018c) studied mercury adsorption of xMn0.3FexO2-d

Due to its toxicity, persistence and bioaccumulation, mercury has received sustained attention (Tang et al., 2016; Lu et al., 2013; Dranga et al., 2012; Liu and Adewuyi, 2016). Coal combustion is considered as the biggest source of anthropogenic mercury emissions to the atmosphere (Pirrone et al., 2010; Liu et al., 2016; Pacyna et al., 2010). Mercury from coal-fired flue gas often presents as three forms: oxidized mercury (Hg2þ), elemental mercury (Hg0) and particulate-bound mercury (Hgp) (Zhao et al., 2016; Liu and Wang, 2014). Among them, Hg2þ can be removed via wet flue gas desulfurization (WFGD) devices, and Hgp can be captured through dust control units such as electrostatic precipitators (ESP) or fabric filters (FF) (Du et al., 2015). Nevertheless, Hg0 from flue gas is difficult to be captured due to its high volatility and low solubility in water (Liu and Wang, 2018). Therefore, developing cost-effective Hg0 removal technologies has been one of the hot topics in the field of flue gas purification. Jampaiah et al. (2015a and 2015b)

* Corresponding author. E-mail address: [email protected] (Y. Liu). https://doi.org/10.1016/j.jclepro.2019.01.195 0959-6526/© 2019 Elsevier Ltd. All rights reserved.

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W. Xu et al. / Journal of Cleaner Production 216 (2019) 277e287

modified bamboo chars, and the results showed that bamboo char modified by HNO3 had good performance for Hg0 adsorption. Shu et al. (2013) studied the feasibility of mercury removal by modified mulberry twig chars, and they found that the Hg0 removal capabilities of mulberry twig chars were improved after modification. Due to industrial pollution and climate change, Enteromorpha, a kind of waste algae, has grown explosively on the coast of China, and its production even reached tens of millions of tons (Yao, 2011). The overgrowth of Enteromorpha has caused large-scale “green tide”, which causes serious ecological disasters and hinders people's daily activities (Yao, 2011). Hence, actively exploring some effective ways for resource utilization of Enteromorpha has important economic and environmental significance. In recent years, the resource utilization of Enteromorpha in several areas, including agriculture, food, energy, environmental protection, medicine and other industries, has been carried out (Shan et al., 2016; Jalali et al., 2002; Sun et al., 2011). In China, Enteromorpha has been used to successfully prepare chemicals and fuels by pyrolysis, and as a biomass fuel for direct combustion in biomass burning boilers for power generation or heating (Jalali et al., 2002; Sun et al., 2011; Wang, 2010; Wang et al., 2013). In the pyrolysis process of Enteromorpha preparing bio-oil, a large amount of Enteromorpha chars were produced. To further utilize these pyrolysis byproducts, and avoid solid waste pollution, the authors propose to use Enteromorpha chars as low-cost adsorbents to adsorb Hg0 from simulated flue gas. In order to improve the Hg0 adsorption performance of adsorbents, halides such as chloride, bromide and iodide were widely used to modify raw adsorbents, which have achieved remarkable results (Ghorishi et al., 2002; Zhou et al., 2015a; De et al., 2013; Cai et al., 2014; Yang et al., 2016a, 2016b). Liu, Z.Y. et al. (2018c) modified Enteromorpha chars by potassium iodide impregnation method, and found that the Enteromorpha chars modified by potassium iodide exhibited good Hg0 removal capability. Although the activity of iodine is often higher than that of chloride and bromine, chloride and bromine are much more inexpensive, which are more easily available for large-scale applications for Hg0 capture (Ghorishi et al., 2002; Lee et al., 2004). Li et al. and Shen et al. (Shen et al., 2015; Li et al., 2017a,b) used NH4Cl to modify various biomass pyrolysis chars for Hg0 removal from flue gas, and they found that Hg0 removal performance of these adsorbents had been greatly improved after the modification by NH4Cl. The results of Zhou et al.

Table 1 Proximate and ultimate analysis of Enteromorpha. Sample

Enteromorpha a b c d e f g h i

Air-dried Air-dried Air-dried Air-dried Air-dried Air-dried Air-dried Air-dried Air-dried

Proximate analysis (wt.%)

Ultimate analysis (wt.%)

Vada

FCadb

Madc

Aadd

Cade

Hadf

Oadg

Nadh

Sadi

71.71

12.35

7.01

8.93

50.88

1.26

22.03

6.08

3.81

Volatiles. Fixed Carbon. Moisture. Ash. Carbon. Hydrogen. Oxygen. Nitrogen. Sulfur.

(2015b) indicated that compared with raw activated carbon, Hg0 capture rate of activated carbon modified by NH4Br was improved significantly. The above situation drives us to try to develop the NH4Cl and NH4Br modified Enteromorpha chars adsorbents for capturing Hg0 from flue gas. In this article, NH4Cl and NH4Br modified Enteromorpha chars (E8C5 and E8B5) were prepared by pyrolysis and simple impregnation method. The physicochemical properties of the adsorbents before and after reaction were characterized by a variety of methods (Thermogravimetric analysis (TGA), BrunauerEmmett-Teller (BET), Scanning electron microscopy (SEM), X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS)). The effects of pyrolysis temperature, loading values, reaction temperature and concentrations of main flue gas components (e.g. O2, NO, SO2 and H2O) on Hg0 capture were studied. Mercury removal mechanism and kinetics were also investigated based on experimental results, characterization analysis and kinetic models. These results will provide some useful information for the future application and development of the adsorbents. 2. Experimental section 2.1. Preparation of adsorbents Enteromorpha, obtained from Weihai of Shandong Province, China, was washed by deionized water, dried at air naturally and then dried for 3 h in a thermostatic drying oven at 105
C. The dried Enteromorpha was crushed and sieved in a 50 Chinese mesh (<300 mm). The proximate and ultimate analysis of Enteromorpha was shown in Table 1. According to the data in Table 1, Enteromorpha includes high volatile (71.71 wt.%) and fixed carbon (12.35 wt.%), and low ash (8.93 wt.%). C and O are the main element components, and H, S and N are the complementary element components in Enteromorpha. The Enteromorpha ash was prepared for 20 min at a temperature-controlled tube furnace (at 815
C) under the air atmosphere. The results of ash analysis by XRF and XRD were shown in Table 2 and Fig. 1, respectively. According to Table 2, the main components of Enteromorpha ash include SiO2, Na2O, SO3, CaO, MgO, K2O, Al2O3, Fe2O3, Cl and P2O5. These main components can also be observed from the results of XRD in Fig. 1 (Li et al., 2017a,b). It is reported that these metal oxides in ash can adsorb Hg0 and promote oxidation of Hg0, thus might be also beneficial to Hg0 removal (Cai et al., 2014; Fang et al., 2010; Shen et al., 2015; Xing et al., 2012). 5.0 g of Enteromorpha was pyrolyzed for 20 min at a temperature-controlled tube furnace (at 400, 600 and 800
C) under the protection of N2 (350 mL/min). Then the furnace was cooled to room temperature under a N2 atmosphere. The chars were ground and sieved to 50 mesh. The chars pyrolyzed at 400, 600 and 800
C were denoted as E4, E6 and E8, respectively. The modified chars were synthesized by impregnation method using NH4Cl or NH4Br solutions. Fig. 2 shows the schematic picture of the modified chars preparation processes. NH4Cl or NH4Br was dissolved in deionized water to form the corresponding solutions. The concentrations of NH4Cl or NH4Br were controlled at 1, 5 and 9 wt.%, respectively. Then 5 g E8 were dispersed to the NH4Cl or NH4Br solutions. The ratio of NH4Cl or NH4Br solution to E8 char

Table 2 The XRF analysis of Enteromorpha ash. Sample (wt.%)

SiO2

Na2O

SO3

CaO

MgO

K2O

Al2O3

Fe2O3

Cl

P2O5

Others

Ash

11.24

9.46

7.24

6.75

6.16

5.66

5.09

3.64

1.31

1.16

0.51

W. Xu et al. / Journal of Cleaner Production 216 (2019) 277e287

SiO2

Na2O

Samples were prepared by compressing a well-mixed adsorbents powder with potassium bromide (KBr). A K-Alpha X-ray photoelectron spectrometer (Thermo Fisher, USA) with an Al Ka X-ray source were used to measure the element valence state on the surface of the adsorbents by X-ray photoelectron spectroscopy (XPS). The binding energies were calibrated by the C1s binding energy value at 284.6 eV (Cai et al., 2014).

CaO Fe2O3

Intencity (a.u.)

MgO Al2O3

279

2.3. Experimental setup and procedure

10

20

30

40

50

60

70

80

90

2 (degree) Fig. 1. XRD of Enteromorpha ash.

was 20 mL/g. The mixtures were stirred by a magnetic stirrer for 1 h, the temperature was controlled at 50
C to accelerate the impregnation between the chars and NH4Cl/NH4Br solutions. The NH4Cl or NH4Br-impergnated Enteromorpha chars was dried in a thermostat drying oven at 110
C for 8 h after stir. The E8 chars treated with different NH4Cl concentrations are denoted as E8C1, E8C5 and E8C9, respectively. The E8 chars treated with different NH4Br concentrations are denoted as E8B1, E8B5 and E8B9, respectively. 2.2. Characterization of adsorbents The proximate analysis of Enteromorpha was measured according to Chinese National Standards (GB/T 212e2008). The ultimate analysis of Enteromorpha was determined by Flash 2000 (Thermo Fisher, USA). Thermogravimetric analysis (TGA) was carried out by STA6000 (PerkinElmer, USA). X-ray fluorescence (XRF) spectrometry (Axios PW4400, Netherlands) and X-ray diffraction (XRD) measurements (Bruker D8, Germany) were used to detect the main components of Enteromorpha ash using Cu Ka radiation in the range of 10e90
(2q) with a step size of 0.02
. BrunauerEmmett-Teller (BET) surface area of the Enteromorpha chars was determined by N2 adsorption/desorption with ASAP 2020 automated gas adsorption system (Micromeritics, USA). Each sample was tested in triplicated and their mean values were reported. The surface morphology of Enteromorpha chars was analyzed by Scanning Electron Microsc ope (SEM) JSM-7001F (JEOL, Japan). Fourier transform infrared spectroscopy (FTIR) spectra was applied by a TENSOR 27 infrared spectroscopy (Thermo Fisher, USA).

Fig. 3 shows the experimental setup, which is used to evaluate the performance of Enteromorpha chars on Hg0 capture. The experimental setup is composed of four parts: a simulated flue gas system, a fixed-bed reactor, a temperature control system and an analytical system. The simulated flue gas system mainly consists of four gas cylinders N2/O2/SO2/NO (purity, 99.99%), five flow meters, a water vapor generator, a Hg0 vapor permeation device and a gas mixing box. The fixed-bed reactor includes a quartz fixed bed reactor (inner diameter of 35 mm; length of 50 mm, quartz glass) and a silicone cover. The temperature control system contains two thermostat water baths, a thermostat drying oven and a heating tape. The analytical system includes a flue gas mercury analyzer (QM201H, Suzhou Qingan Instrument Co., Ltd, China), a flue gas analyzer (MRU-VARIOPLUS, Germany), a hygrometer (DT-625, Shenzhen Huashengchang Machinery Industry Co., Ltd) and a waste gas absorption. 300 mg samples (<50 mesh) mixing with 4 g quartz sand (20e50 mesh) were loaded in a fixed-bed reactor. The fixed-bed reactor was placed vertically in a thermostatic drying oven, which was kept at reaction temperature. The main flue gas components (N2þ5% O2þ600 ppm SO2þ400 ppm NOþ1.5 vol% H2O (g)) from cylinders were mixed in a gas mixing box (Quartz glass). The inlet and outlet gaseous streams were heated by wrapping the heating tape to avoid Hg0 and H2O (g) condensation. The flow rate was controlled by flowmeters and the total flow rate was set at 800 mL/min, which corresponds to a gas hourly space velocity (GHSV) of 10,000 h1. The reaction temperature was set as 130
C. 50 mg m3 Hg0 vapor was produced from a Hg0 permeation tube (VICI Metronics, USA) with N2 carrying. Hg0 concentrations in the inlet and outlet of the reactor were monitored by the Hg0 analyzer.

Fig. 3. Schematic diagram of the experimental system.

E4 pyrolysis 5g Entermoprha

E6 E8 stir 1h

NH4Cl/NH4Br Deionized water

Mixture

Drying 110 8h

NH4Cl solutions /NH4Br solutions

Fig. 2. The schematic picture of the modified chars preparation processes.

E8C1 E8B1 E8C5 E8B5 E8C9 E8B9

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Each adsorption experiment was repeated three times and their mean values were reported. Before these experiments, a blank test in fixed-bed reactor with 4.3 g quartz sand was conducted, and the results showed that the mercury adsorbed on reactor and pipelines was negligible. The concentration of Hg0 measured by bypass line was considered as inlet concentration of Hg0 (Hg0in). The concentration measured by reactor outlet was used as outlet concentration of Hg0 (Hg0out). The definition of Hg0 removal efficiency (h) was given below:

DHg0 Hg0in

¼

Hg0in  Hg0out Hg 0in

 100%

(1)

where h represents the Hg0 removal efficiency; Hg0in and Hg0out represent the inlet and outlet concentrations of Hg0 in the flue gas, respectively, mg/m3. (1e4) SO2/NO/O2/N2 Cylinders; (5e9) Flowmeters; (10) Gas Mixing Box; (11) Thermostatic Waterbath; (12) Mercury Vapor Generator; (13) Constant Temperature Magnetic Stirrer; (14) Conical Flask; (15) Heating Tape; (16) Thermostatic Drying Oven; (17) Quartz Fixed-bed Reactor; (18) Hg0 Analyzer; (19) Flue Gas Analyzer; (20) Waste Gas Absorption;

Fig. 4. SEM photographs of (a) E4, (b) E6, (c) E8 and (d) E8B5.

3.2. Effects of pyrolysis temperature on Hg0 removal

3.1. Characterization 3.1.1. BET The BET specific surface area, pore volume and average pore diameter are given in Table 3. It can be seen that with the increase of pyrolysis temperature, the BET surface area and total pore volume increase, while average pore size decreases, showing that increasing pyrolysis temperature improves the pore structure of Enteromorpha char. When NH4Cl or NH4Br is loaded on the surface of the E8, the specific surface areas of adsorbents decrease in different degrees, which may be due to the blockage or collapse of the pore structure caused by NH4Cl or NH4Br. 3.1.2. SEM Fig. 4 shows the SEM micrographs of E4, E6, E8 and E8B5. From Fig. 4, it can be seen that both E4 and E6 have relatively smooth surfaces, and large particles and gaps. Compared with E4 and E6, E8 has more small pores and wrinkles, which are beneficial for the increase of the specific surface area of the sample. The results are consistent with the BET analysis (in Table 3). Besides, it can be seen from Fig. 4(c) and (d) that the a large number of small pores disappear and some slight particles attach to the surface of E8 after modification. The results indicate that NH4Cl/NH4Br modification result in the blockage or collapse of the pore structure, which are consistent with the BET analysis.

The effects of pyrolysis temperature on Hg0 removal efficiency over the chars are displayed in Fig. 5(a). As displayed in Fig. 5(a), compared with E4 and E6, the average Hg0 removal efficiency of E8 reached 29.65%, exhibiting better performance for Hg0 removal. The initial Hg0 removal efficiency (18.65%) of E4 was close to E6 (16.35%), whereas it sharply decreased to 4.09% at 90 min. The results indicate that 800
C is the best pyrolysis temperature. It is known that the Hg0 adsorption on unmodified bio-chars is often physisorption (Zhu et al., 2013). Therefore, the specific surface area will play a key role in the physisorption process (Zhang et al., 2008). The BET analysis in Table 3 shows that with the increase of pyrolysis temperature, the BET surface area and total pore volume increase, which are beneficial to Hg0 removal. Therefore, E8 is chosen in the subsequent study due to its better performance.

100

40

(a) 0 Hg removal efficiency (%)

3. Results and discussion

30

E8 E6 E4

20

10

0

0

10 20

30 40

50 60

70 80

(b)

80

Hg0 removal efficiency (%)

h¼

60

Loading value (wt%)

40 20 0

90 100

0

10

20

30

Time (min)

E8B1 E8B5 E8B9

E8C1 E8C5 E8C9

E8 40

50

60

70

80

90 100

Time (min) 100

100

(d)

Sample E4 E6 E8 E8C1 E8C5 E8C9 E8B1 E8B5 E8B9

BET surface area (m2/g) 4.02 ± 1.24 14.41 ± 0.84 21.15 ± 0.71 11.95 ± 1.12 16.55 ± 0.93 13.98 ± 1.06 15.03 ± 0.79 19.67 ± 0.92 18.56 ± 0.69

BJH total pore volume (cm3/g) 0.018 ± 0.007 0.092 ± 0.021 0.466 ± 0.134 0.249 ± 0.086 0.198 ± 0.091 0.137 ± 0.045 0.201 ± 0.098 0.363 ± 0.151 0.356 ± 0.126

Average pore size (nm) 32.254 ± 1.682 25.002 ± 1.039 16.724 ± 0.968 26.325 ± 0.879 19.568 ± 1.364 24.268 ± 1.075 25.978 ± 0.947 22.442 ± 0.667 23.893 ± 1.003

Hg0 removal efficiency (%)

Table 3 The BET surface area, pore volume and pore diameter of the samples.

Hg0 removal efficiency (%)

(c) 80 60 40

70 90 110 130

20 0

0

20

40

E8C5 Reaction temperature (oC) 60

80

Time (min)

100

120

140

160

80

E8B5

60

70 90 110 130

Reaction temperature (oC)

40

0

20

40

60

80

100

120

140

160

Time (min)

Fig. 5. The effects of (a) pyrolysis temperatures, (b) loading value, (c) reaction temperatures of E8C5 and (d) reaction temperatures of E8B5 on Hg0 removal efficiency. Conditions: O2 concentration, 5%; NO concentration, 400 ppm; SO2 concentration, 600 ppm; H2O concentration, 1.5 vol%; Hg0 concentration, 50 mg/m3; Reaction temperature, 130
C.

W. Xu et al. / Journal of Cleaner Production 216 (2019) 277e287

281

3.3. Effects of loading value on Hg0 removal

3.5. Effects of individual flue gas components on Hg0 removal

Fig. 5(b) shows the effects of loading value on Hg0 removal efficiency. From Fig. 5(b), it can be seen that Hg0 removal efficiency increased obviously after the modification by NH4Cl or NH4Br solution. With the NH4Cl loading value increasing from 1 wt.% to 5 wt.%, the average Hg0 removal efficiency increased from 58.62% to 79.16%. When the NH4Cl loading value further increased to 9 wt.%, the average Hg0 removal efficiency slightly reduced to 78.81%. The char modified by NH4Br has the similar results. The average Hg0 removal efficiency increased from 75.40% to 89.83% when the NH4Br loading value increased from 1 wt.% to 5 wt.%. However, the average Hg0 removal efficiency slightly reduced to 89.05% when the NH4Br loading value further increased to 9 wt.%. The reasons may attribute to the fact that the increase of NH4Cl or NH4Br impregnation concentration promotes the introduction of more chemical adsorption sites on the surface of the samples (Li et al., 2017a,b). However, when the impregnation concentration exceeds a certain amount, excessive NH4Cl or NH4Br will cause the blockage or collapse of the pores in the chars, resulting in the decrease of BET surface area (it has been confirmed by the data in Table 3), which further results in a decrease in Hg0 removal efficiency (Zhu et al., 2013). Considering the removal efficiency and reagent costs, 5 wt.% is selected as the optimized loading value in the following studies. In addition, Fig. 5(b) also shows that the Hg0 removal performance over the chars modified by NH4Br is better than those over the chars modified by NH4Cl. This is likely to be explained by two reasons. The BET analysis in Table 3 shows that compared with the chars modified by NH4Br, the BET surface areas of the chars modified by NH4Cl were smaller. This shows that NH4Cl impregnation is more likely to block the pore structure of the char surfaces, which is detrimental to Hg0 adsorption. Besides, Li et al. (2017a,b) indicated that because of the highly reducibility and larger size, Br is easier to produce covalent halide groups on the adsorbent surface than Cl. The produced covalent halide groups are the major chemical active sites that can oxidize Hg0 to mercury halide during the Hg0 removal process, which has been verified in Section 3.7 “Discussion of Hg0 removal mechanism”. Hence, the chars modified by NH4Br show better Hg0 removal performance than those modified by NH4Cl. The similar conclusions can also be found in other literature (De et al., 2013).

3.5.1. Effects of O2 on Hg0 removal The experiments about the effects of O2 concentration on Hg0 adsorption using the E8C5 and E8B5 were carried out at 130
C under different O2 concentrations. The results are shown in Fig. 6(a) and (b). It can be seen that both E8C5 and E8B5 exhibit low removal efficiency for Hg0 in the absence of O2. The average Hg0 removal efficiency of E8C5 greatly increased from 55.26% to 93.94% as the O2 concentration increased from 0% to 15%. Similarly, with the O2 concentration increased from 0% to 15%, the average Hg0 removal efficiency of E8B5 greatly increased from 61.95% to 95.33%. It is observed that increasing O2 concentration from 0% to 5% greatly promoted Hg0 removal. While the promotion tendency gradually weakened when the O2 concentration increased from 5% to 15%. The results can be explained by the following possible reasons. Some results (Tao et al., 2012; Zhao et al., 2016) have showed that gas-phase O2 can regenerate the consumed chemisorbed and lattice oxygen on adsorbent surface, which favors the Hg0 removal. Besides, gas-phase O2 can also generate oxygen functional groups on bio-chars, which act as effective chemical active sites, thereby promoting the Hg0 removal (Skodras et al., 2007).

3.4. Effects of reaction temperature on Hg0 removal The effects of reaction temperature on Hg0 removal efficiency of E8C5 and E8B5 are shown in Fig. 5(c) and (d), respectively. As shown in Fig. 5(c), the average Hg0 removal efficiency of E8C5 increased from 38.91% to 84.34% with the increase of temperature from 70
C to 130
C. E8B5 has the similar results, which is shown in Fig. 5(d). As shown in Fig. 5(d), with the reaction temperature increased from 70
C to 130
C, the average Hg0 removal efficiency increased from 74.50% to 91.41%. The results indicate that the increase of reaction temperature promotes Hg0 removal. Some results (Zhao et al., 2016; Xie et al., 2015) indicated that the chemical reaction between Hg0 and active sites on adsorbent will be promoted with the increase of reaction temperature, which is beneficial to Hg0 removal. In lower temperature range, Hg0 removal mainly depends on the physisorption because of van de Waals forces. While in higher temperature range, the reaction activation energy is met, leading to the activation of more chemisorption sites, and then results in the increase of Hg0 removal efficiency. The similar results can also be found in other literature (Xu et al., 2013). Therefore, 130
C is chosen in the following study.

3.5.2. Effects of NO on Hg0 removal Fig. 6(c) and (d) show the Hg0 removal efficiency over E8C5 and E8B5 under four NO concentrations (0 ppm, 400 ppm, 600 ppm and 800 ppm). The results show that with the NO concentration increased from 0 ppm to 800 ppm, the average Hg0 removal efficiency of E8C5 and E8B5 significantly increased from 61.13% to 95.19% and from 78.89% to 97.40%, respectively. The results indicate that the presence of NO greatly promotes Hg0 removal. It may be ascribed to the formation of NO2 on the surface of adsorbent. The produced NO2 is an effective Hg0 oxidant, and can react with Hg0 to generate HgO and Hg(NO3)2 (Zhao et al., 2016; Ma et al., 2015). 3.5.3. Effects of SO2 on Hg0 removal The effects of SO2 on Hg0 removal efficiency are shown in Fig. 6(e) and (f). It is observed that the introduction of SO2 inhibits the Hg0 removal over the adsorbents at 130
C. For E8C5, the average Hg0 removal efficiency decreased from 89.02% to 47.29% with the SO2 concentration increasing from 0 ppm to 1200 ppm. For E8B5, when SO2 concentration increased from 0 ppm to 1200 ppm, the average Hg0 removal efficiency decreased from 94.59% to 76.56%. The main factors for inhibiting Hg0 removal is the competitive adsorption for chemical active sites between SO2 and Hg0 (Cai et al., 2014). Moreover, the adsorbed SO2 also can react with the surface oxygen on the adsorbents, thus suppressing the reaction between Hg0 and surface oxygen (Xie et al., 2015). 3.5.4. Effects of H2O on Hg0 removal Water vapor (H2O) is one of the main components in flue gas (Li et al., 2012). To study the effect of H2O on Hg0 removal, four different H2O concentrations (0%, 1.5%, 5%, 8%) were prepared. Fig. 6(g) and (h) show the Hg0 removal efficiency of E8C5 and E8B5, respectively. For E8C5, when H2O concentration increased from 0% to 1.5%, the average Hg0 removal efficiency slightly increased from 84.13% to 84.51%. However, when H2O concentration further increased to 8%, the average Hg0 removal efficiency decreased to 61.25%. Similarly, the average Hg0 removal efficiency of E8B5 increased from 75.33% to 92.15% when H2O concentration increased from 0% to 1.5%, and then decreased to 72.26% when H2O concentration further increased to 8%. The results indicate that H2O promotes Hg0 removal at lower H2O concentrations, but inhibits Hg0 removal when H2O concentrations are higher. Li et al. (2002) indicated that the decomposition of water molecules on the

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100

(a)

Hg0 removal efficiency (%)

90 80

0 5 10 15

E8C5 70

O2 concentrations (%)

60 50 40

0

10

20

30

40

50

60

70

80

Hg0 removal efficiency (%)

100

(b) 90

O2 concentrations (%)

70 60 50

90 100

0

10

20

30

Time (min)

0 400 600 800

E8C5 NO concentrations (ppm)

70 60

0

10

20

30

40

50

60

70

80

Hg0 removal efficiency (%)

Hg0 removal efficiency (%)

(c)

80

80

90 100

80

0 400 600 800

E8B5

70

NO concentrations (ppm)

0

10

20

30

40

50

60

70

80

90 100

Time (min) 100

(e)

90 80 70

0 600 900 1200

E8C5 SO2 concentrations (ppm)

60 50 0

10

20

30

40

50

60

70

80

Hg0 removal efficiency (%)

Hg0 removal efficiency (%)

70

90

60

90 100

100

80 70

0 600 900 1200

E8B5 SO2 concentrations (ppm)

60 50

90 100

(f)

90

0

10

20

30

40

Time (min)

50

60

70

80

90 100

Time (min) 100

90

(g)

80

Hg0 removal efficiency (%)

Hg0 removal efficiency (%)

60

(d)

Time (min)

70 60 50

0 1.5 5 8

E8C5 H2O concentrations (vol.%)

40 30

50

100

90

40

40

Time (min)

100

50

0 5 10 15

E8B5

80

0

10

20

30

40

50

60

Time (min)

70

80

90 100

90

(h)

80 70 60

H2O concentrations (vol.%)

50 40

0 1.5 5 8

E8B5

0

10

20

30

40

50

60

70

80

90 100

Time (min)

Fig. 6. The effects of (a,b) O2, (c,d) NO, (e,f) SO2 and (g,h) H2O on Hg0 removal efficiency. Conditions: O2 concentration, 5%; NO concentration, 400 ppm; SO2 concentration, 600 ppm; H2O concentration, 1.5 vol%; Hg0 concentration, 50 mg/m3; Adsorbents: E8C5 or E8B5; Reaction temperature, 130
C.

W. Xu et al. / Journal of Cleaner Production 216 (2019) 277e287

FTIR analysis is a powerful technique to characterize organic functional groups. Fig. 7 shows the FTIR spectra of the original chars (E8), the modified chars (E8C5, E8B5) and the used chars (E8C5u, E8B5u). The Hg0 adsorption experiment conditions for E8C5 and E8B5 are as follows: reaction temperature of 130
C, 5% O2, 400 ppm NO, 600 ppm SO2, 1.5 vol% H2O, 50 mg/m3 Hg0 and balanced N2. From Fig. 7, it can be seen that all the samples show a distribution of functional groups, including aliphatic and aromatic CeH vibrational groups at 468 and 604 cm1, the stretching vibration of SieOeSi at 785 cm1, CeO stretching vibrations for unsaturated alcohols at 1009 cm1, the phenol, or the CeO, C]O bending vibrations at 1090 cm1, the aromatic skeletal stretching band at 1597 cm1 and stretching vibrations of hydroxyl (eOH) and amine (NeH) groups at 3440 cm1 (Li et al., 2014; Li and Wu, 2006; Johari et al., 2016). After modification by NH4Cl/NH4Br, a variety of peaks of E8C5/ E8B5 are added and some peak intensity has been enhanced. The peak at 872 cm1 represents CeH bending/wag, while the peak at 1404 cm1 is due to the asymmetric variable angle vibration of NHþ 4 (Li et al., 2014; Wu et al., 2015). The peak intensity of E8C5 and E8B5 at 785 cm1 and 1090 cm1 has been enhanced after modification. Peak at 3450 cm1, corresponding to OeH stretching vibration, is present in the modified chars (Li et al., 2017a,b). The results indicate that the reaction between NH4Cl/NH4Br and chars occurs during the adsorbent preparation, and new functional groups which promote Hg0 removal are introduced to E8 chars by NH4Cl/NH4Br modification. Some functional groups such as CeO and C]O increase which may also be beneficial for Hg0 removal. For E8C5, the peak at 1630 cm1 which represents CeCl bond is also added, showing that the Cl had reacted with the C atoms on the surface of

3.7. Discussion of Hg0 removal mechanism To analyze the element valence state on the surface of adsorbents and explore the mercury removal mechanism, fresh and used E8C5 and E8B5 were investigated by XPS technique. The corresponding Cl 2p, Br 3d and Hg 4f XPS spectra are displayed in Fig. 8. The XPS spectra of Hg 4f for used E8C5 and E8B5 are shown in Fig. 8(a) and (b). The strong peak at 102.7 ± 0.1 eV represents the Si 2p electron, while the peak at 101.8 ± 0.2 eV for Hg 4f7/2 and the peak at 104.0 ± 0.1 eV for Hg 4f5/2 are attributed to the oxidized mercury (Hg2þ) (Xu et al., 2013; He et al., 2011). The presence of oxidized mercury indicates that the adsorbed Hg0 was oxidized by the active sites on the surface of samples. As shown in Fig. 8(c) and (d), for fresh and used E8C5, three obvious peaks of Cl 2p are showed at 197.5 ± 0.1 eV, 198.7 ± 0.1 eV and 200.1 ± 0.1 eV. The peak at 197.5 ± 0.1 eV is attributed to Cl 2p 3/2 of Cl, the peak at 198.7 ± 0.1 eV represents the overlap between Cl 2p 1/2 of Cl and Cl 2p 3/2 of CeCl, and the peak at 200.1 ± 0.1 eV belongs to Cl 2p 1/2

(a) used E8C5

(b) used E8B5 Hg 4f

Hg 4f Intensity (a.u.)

3.6. FTIR analysis

the chars (Shen et al., 2015). At the same time, the peak at 1450 cm1 corresponding to aliphatic CeH bending vibration and peak at 1525 cm1 related to the C]C stretching are observed on the surface of E8B5 chars (Xu et al., 2017; Sikkanthar et al., 2015). It can also be found that the peaks of E8B5 show higher intensities than the peaks of E8C5, indicating that compared with Cl, Br is easier to produce covalent halide groups that can oxidize Hg0 to mercury halide, and thus leading to the Hg0 removal efficiency of E8B5 higher than that of E8C5. For E8C5u/E8B5u, the intensities of peaks are weaker and some peaks disappeared when the adsorbents are used. The results suggest that the functional groups react with Hg0 to form HgO or mercury halides on the surface of adsorbents.

Intensity (a.u.)

surface of adsorbents could produce OH radicals. The produced OH radicals could promote Hg0 removal by oxidizing Hg0 to form HgO at lower H2O concentrations. Nevertheless, it was also reported that excessive water vapor would compete with Hg0 for the available active sites on the adsorbents (Xie et al., 2015; Li and Wu, 2006). Besides, when the H2O concentrations are higher, the adsorbed water vapor might form water film on the surface of the adsorbents, which will block the contact of Hg0 and active sites on the surface of the adsorbents, and thereby inhibits the Hg0 removal.

283

98

100

102

104

98

106

E8B5

1525 1450

785 604468

872

Cl 2p

Cl 2p

1090

E8C5u

195

3450

198

201

204

195

198

201

Binding energy (eV)

Binding energy (eV)

E8C5

204

1630 (f) used E8B5

(e) fresh E8B5 Br 3d

E8

Br 3d Intensity (a.u.)

Intensity (a.u.)

Transmittance (a.u.)

1009

106

Intensity (a.u.)

15971404

104

(d) used E8C5

Intensity (a.u.)

E8B5u

102

Binding Energy (eV)

(c) fresh E8C5

3440

100

Binding Energy (eV)

1630

1525

4000 3600 3200 2800 2400 2000 1600 1200 -1

Wave number (cm )

Fig. 7. FTIR spectra of adsorbents.

800

400

66

68

70

Binding Energy (eV)

72

66

68

70

72

Binding Energy (eV)

Fig. 8. XPS spectra of fresh and used E8C5 and E8B5 for (a, b) Hg 4f, (c, d) Cl 2p and (e, f) Br 3d.

284

W. Xu et al. / Journal of Cleaner Production 216 (2019) 277e287

of CeCl (Tan et al., 2015). In Fig. 8(e) and (f), there are also three peaks appearing at the 67.94 eV, 68.6 eV and 69.3 ± 0.1 eV on the surface of fresh and used E8B5. The peak at 67.94 eV belongs to Br 3d 5/2 of Br, the peak at 68.6 eV refers to the overlap between Br 3d 3/2 of Br and Br 3d 5/2 of CeBr, and the peak at 69.3 ± 0.1 eV represents Br 3d 3/2 of CeBr (Cai et al., 2014; Xu et al., 2013). From Fig. 8(c)e(f), it can be seen that the peak area ratio of both ionic and covalent groups changed before and after adsorption experiments. Table 4 shows the peak area ratio of different halides valence states. As shown in Table 4, the ionic (Cl 2p 3/2 of Cl or Br 3d 5/2 of  Br ) peak ratios increase, whereas the covalent halide group (Cl 2p 1/2 of CeCl or Br 3d 3/2 of CeBr) peak ratios decrease after Hg0 removal reactions. The peak ratios of the overlap between Cl 2p 1/2 of Cl and Cl 2p 3/2 of CeCl or the overlap between Br 3d 3/2 of Br and Br 3d 5/2 of CeBr only increase slightly. The result indicates that during the Hg0 adsorption process, some covalent halides groups are translated into ionic groups. Based on this result and the existence form of oxidation state mercury (in Fig. 8(a) and (b)), it can be inferred that the covalent halide groups may oxidize Hg0 to mercuric halides, such as HgCl2 and HgBr2. Hence, the covalent halide groups are considered to be the major chemical active sites that oxidize Hg0 to mercury halide during the Hg0 removal process. The similar conclusions can also be found in other literature (Li et al., 2017a; Cai et al., 2014; Xu et al., 2013). According to above characterization analysis and experimental results, the Hg0 removal mechanism of halide modified Enteromorpha chars can be described as follows: (a) The gas-phase Hg0 (Hg0(g)) was adsorbed on the surface of adsorbents to form the adsorbed mercury (Hg0(ad)). (b) The halide ions reacted with carbon atom to form the halide covalent bonds. (c) Hg0 can be oxidized by the chemisorbed oxygen (O(ad)) and oxygen functional groups to form HgO. And the consumed chemisorbed oxygen and oxygen functional groups can be regenerated by gas-phase O2. (d) The halide covalent groups oxidized Hg0(ad) into mercury halides (Fang et al., 2010). (e) According to the FTIR analysis results, there are many carbonyl groups (C]O) which can be transformed into CeO groups on the surface of adsorbents. These function groups can be further transformed into CeH bond and CeC bond during the Hg0 removal process (Shen et al., 2015). (f) According to the XRF and XRD analysis of Enteromorpha ash, the chars contains some metal oxides that can be used as catalysts to adsorb Hg0, such as CaO, MgO, Al2O3, Fe2O3 and P2O5. However, due to the little contact area and low activity between Hg0 and metal oxides, the catalysis/ adsorption is very weak (the low Hg0 capture efficiency in Fig. 5(a) proves this speculation). The adsorption process can be explained by Mars-Maessen reaction mechanism (Fang et al., 2010; Granite et al., 2000). (g) When NO, SO2 and H2O were present in the flue gas, some side reactions and the other competitive reactions for active sites between Hg0(ad) and NO/SO2/H2O would also take place in the Hg0 removal process. Furthermore, the actual coal-fired flue gas contains extremely complex compositions or ingredients, such

Table 4 Peak area ratio of different halides valence states. Groups

Position (eV)

E8C5

E8B5

Fresh

Used

Fresh

Used

Cl 2p 3/2 of Cl Cl 2p 1/2 of Cl and Cl 2p 3/2 of CeCl Cl 2p 1/2 of CeCl Br 3d 5/2 of Br Br 3d 3/2 of Br and Br 3d 5/2 of CeBr Br 3d 3/2 of CeBr

197.5 ± 0.1 198.7 ± 0.1

31.78% 35.1%

34.26% 36.17%

e e

e e

200.1 ± 0.1 67.94 68.6

33.12% e e

29.57% e e

e 30.19% 34.78%

e 35.37% 35.03%

69.3 ± 0.1

e

e

35.03%

29.6%

as heavy metals, halides, hydrogen sulfide, alkaline substances, volatile organic compounds, etc (Wang et al., 2019; Liu et al., 2010, 2016, 2017, 2018a,b; Wang and Xu, 2019; Liu and Wang, 2016). Therefore, the more research works under the actual coal-fired flue gas atmosphere should be implemented in the future works. 3.8. Adsorption kinetic simulation Adsorption kinetic studies are important for revealing adsorption mechanism and determining kinetic parameters. The pseudofirst-order model, based on mass balance, can be applied to describe the external mass transfer process (Cai et al., 2014; Hsi et al., 2011; Skodras et al., 2008). The pseudo-second-order model, based on the Langmuir adsorption isotherm equation, can be used to express the chemisorption process (Cai et al., 2014; Hsi et al., 2011; Skodras et al., 2008). In this study, the pseudo-firstorder kinetic model and pseudo-second-order kinetic model are used to study the adsorption kinetic process of Hg0 over E8C5 and E8B5, and determine the adsorption mechanism and kinetic parameters. The pseudo-first-order equation can be expressed as follows:

dqt ¼ k1 ðqe  qt Þ dt

(2)

where qt and qe represent the amount of adsorbate (mg/g) at time t and at the equilibrium time, respectively, and k1 represents rate constant of pseudo-first order equation (min1). The integration of Eq. (2) with the initial condition, qt ¼ 0 at t ¼ 0 leads to:

lgðqe  qt Þ ¼ lgqe 

k1 t 2:303

(3)

The pseudo-second-order can be described by Eq. (4):

dqt ¼ k2 ðqe  qt Þ2 dt

(4)

where qt and qe represent the amount of adsorbate (mg/g) at time t and at the equilibrium time, respectively, and k2 represents the adsorption reaction rate constant (g/mg∙min). According to the boundary conditions t ¼ 0 to t ¼ t and q ¼ 0 to q ¼ qt, the integrated form of Eq. (4) can be expressed as follows:

t 1 1 ¼ þ t qt k2 qe 2 qe

(5)

The k2 and qe values were calculated from the intercept and slope of the linear plot t/qt against t, respectively, and h ¼ k2$q2e was considered as the initial adsorption rate as qt/t/0. The experimental data for E8C5 and E8B5 in different temperatures were fitted with the pseudo-first-order and pseudo-secondorder models, and the results are shown in Fig. 9(a)e(d). From Fig. 9(a)e(d), it can be seen that the pseudo-second-order model can fit better with the experimental data than the pseudo-firstorder model at four temperatures. The calculated parameters of the pseudo-first-order model and the pseudo-second- order model are listed in Table 5. As shown in Table 5, for the pseudo-first-order model, the correlation coefficient (R2) values for E8C5 are only 0.67e0.93 at four temperatures, while those for E8B5 only reach 0.75e0.89. For the pseudo-second-order model, all the R2 values are >0.96 at four temperatures for E8C5 and E8B5. The results suggest that both of the adsorption processes of Hg0 over E8C5 and E8B5 can be well described by the pseudo-second-order kinetic model. Thus, the adsorption processes of Hg0 over E8C5 and E8B5 are controlled by the chemisorption process.

W. Xu et al. / Journal of Cleaner Production 216 (2019) 277e287 2.5

2.5

(b)

(a)

2.0

2.0

lg(qe-qt)

lg(qe-qt)

1.5 1.0 0.5 0.0 -0.5

70 90 110 130

E8C5

0

20

40

60

80

1.5 1.0

70 90 110 130

E8B5 0.5

100

120

140

0.0

160

0

20

40

t (min)

60

80

100

120

140

(d)

(c)

3.0

70 90 110 130

1.4

70 90 110 130

E8C5

of 5 to 40 kJ/mol represent physisorption while that in the scope of 40 to 800 kJ/mol mean chemisorption (Liu et al., 2018c). The Ea calculated by Eq. (6) are 62.61 kJ/mol for E8C5 and -108.57 kJ/ mol for E8B5, respectively, which are both in the range of 40 to 800 kJ/mol. The results indicate that Hg0 removal on the surface of E8C5 and E8B5 are chemisorption processes, which are agreed with the results of “Adsorption kinetic simulation” in Section 3.8. The thermodynamic parameters of enthalpy (DH), entropy (DS) and the Gibbs free energy (DG) can be obtained by Eqs. (7) and (8) (Shen et al., 2015).

E8B5

DG ¼ RT ln KD

t/qt

t/qt

2.4

160

t (min)

1.2

1.8

ln KD ¼

1.2 0

20

40

60

80

100

120

140

t (min)

160

1.0

0

20

40

285

60

80

t (min)

100

120

140

160

Fig. 9. Kinetic analysis by (a,b) pseudo-first-order model and (c,d) pseudo-secondorder model.

Besides, for the pseudo-second-order model, the adsorption equilibrium (qe) has a negative correlation with the reaction rate constant (k2). Hsi and Chen (2012) reported that most adsorbents with larger qe presented the poorer k2. In this study, for E8C5 and E8B5, the qe values under four temperatures followed the order: 70
C < 90
C < 110
C < 130
C, while the k2 followed the order: 70
C > 90
C > 110
C > 130
C inversely. The analysis results fits well with the results in Section 3.4. In addition, the qe values for E8B5 are higher than those for E8C5 under the same temperatures and tested conditions, showing that the adsorption capacity of NH4Br modified chars is stronger than that of NH4Cl modified chars. The results are also consistent with those in Fig. 5(b) and the XPS analysis in Section 3.7.

DS R



(7)

DH

(8)

RT

where KD is the distribution coefficient of the adsorbent, and is equal to qe/Ce; qe denotes the Hg0 accumulative adsorption amount of adsorbent at equilibrium time (mg/g); Ce represents the Hg0 equilibrium concentration at the adsorption reactor outlet (mg/m3); DH, DS and DG represent enthalpy (kJ/mol), entropy (J/mol$K) and Gibbs free energy (kJ/mol), respectively. The calculation results of DH, DS and DG are listed in Table 6. From Table 6, it can be seen that the values of DG are negative, indicating that the Hg0 adsorption processes on E8C5 and E8B5 are spontaneous and thermodynamically achievable. Besides, DG values decrease with the increase of reaction temperatures, showing that higher adsorption can be obtained at higher temperatures. Moreover, both enthalpy (DH) and entropy (DS) values are positive, demonstrating that the adsorption processes of Hg0 removals on E8C5 and E8B5 are endothermic. It further suggests that the increase of reaction temperature is advantage for adsorption reaction of Hg0 (Wang and Li, 2005), which is consistent with the results of “Effects of reaction temperature on Hg0 removal” in Section 3.4.

3.9. Thermodynamic analysis 3.10. The comparison between E8C5/E8B5 and other adsorbents Based on the reaction rate constant k2 in the pseudo-secondorder model, the activation energy of Hg0 removal for E8C5 and E8B5 can be obtained by Eq. (6) (Liu et al., 2018c).

ln k2 ¼ ln A 

Ea RT

(6)

where k2 represents the reaction rate constant of pseudo-secondorder kinetic model; A represents the Arrhenius equation factor; R represents the molar gas constant, 8.314 J/mol$K; T represents the reaction temperature, K; Ea represents the activation energy of adsorption, kJ/mol. The research shows that the activation energies in the range

The comparison of Hg0 removal between E8C5/E8B5 and other adsorbents are carried out and summarized in Table 7. From Table 7,

Table 6 Thermodynamic parameters for Hg0 adsorption on E8C5 and E8B5 at different temperatures. Samples

E8C5 E8B5

DH (kJ/mol)

67.98 31.81

DS (J/mol$K)

212.55 118.95

DG (kJ/mol) 70
C

90
C

110
C

130
C

4.26 8.89

10.11 11.77

13.14 13.27

17.29 16.32

Table 5 Kinetic parameters of pseudo-first-order and pseudo-second-order models. Temperature (
C)

Kinetic models

E8C5

Pseudo-first order model

Pseudo-second order model

qe (mg/g) k1 (/min) R2 qe (mg/g) k2 (g/mg$min) h R2

E8B5

70

90

110

130

70

90

110

130

20.58 0.0134 0.93 11.14 5.21E-3 0.6471 0.97

23.56 0.0523 0.68 71.28 1.28E-4 0.6483 0.99

66.53 0.0679 0.71 155.02 3.03E-5 0.7301 0.98

191.66 0.0702 0.67 235.35 1.54E-5 0.8524 0.99

74.20 0.0138 0.81 56.49 2.73E-4 0.8700 0.98

134.04 0.1561 0.81 123.46 5.68E-5 0.8658 0.98

138.92 0.1064 0.89 161.29 3.45E-5 0.8987 0.99

304.16 0.0511 0.75 326.46 8.54E-6 0.9098 0.99

286

W. Xu et al. / Journal of Cleaner Production 216 (2019) 277e287

Table 7 The comparison of Hg0 removal between E8C5/E8B5 and other adsorbents. Adsorbents

Temperature (
C)

Simulated flue gas

Maximum Hg0 removal efficiency (%)

References

(1)KBr-clay Fly ash 2% HBr-fly ash Ben/Na 30% Br-Ben/Na T6N5 E8 E8C5 E8B5

80 150

N2þO2þSO2þH2O þ Hg0 N2þHg0

Cai et al., (2014) Zhang et al., (2015)

140

N2þHg0

42 11 44 22 97.7 83.1 34.2 88.7 94.6

120 130

0

N2þO2þHg N2þO2þSO2þH2O þ Hg0

it can be seen that same with other raw adsorbents, the raw chars (E8) also shows low Hg0 removal efficiency. However, E8 exhibits a slight higher Hg0 removal capacity (34.2%) than fly ash (11%) and Ben/Na (22%). Hg0 removal capacity of adsorbents commonly increases after modification, and E8C5/E8B5 also shows higher Hg0 removal efficiency. Although the adsorbent of 30% Br-Ben/Na achieves higher Hg0 removal efficiency of 97.7%, the NH4Br reagent dosage up to 30%. E8B5 also shows high Hg0 removal efficiency of 94.6% and only uses 5 wt.% NH4Br reagent. Therefore, E8C5/E8B5 could be a promising Hg0 adsorbent in industrial utility considering the high Hg0 removal capacity and low cost. 4. Conclusions In order to evaluate the feasibility of resource utilization of Enteromorpha, NH4Cl and NH4Br modified Enteromorpha chars (E8C5 and E8B5) were developed and applied in this study to remove Hg0 from flue gas. The results showed that Hg0 removal efficiency of Enteromorpha chars was significantly enhanced by halides modification, and NH4Br modified chars exhibited better Hg0 removal performance than NH4Cl modified chars. Optimal pyrolysis temperature and reaction temperature were 800
C and 130
C, respectively. 5 wt.% was considered as the optimal loading value for both NH4Cl and NH4Br. Both O2 and NO obviously strengthened Hg0 removal, but SO2 inhibited Hg0 removal. Low concentrations of H2O promoted Hg0 removal, while high concentrations of H2O inhibited Hg0 removal. The pseudo-second-order model can simulate the Hg0 removal processes over E8C5 and E8B5 well. Hg0 removal mechanism, adsorption kinetic models and thermodynamic analysis show that Hg0 adsorption processes over E8C5 and E8B5 are endothermic, and chemisorption is the dominant process. The Enteromorpha chars modified by NH4Cl and NH4Br are proved to be economically effective adsorbents for Hg0 removal from flue gas. Acknowledgements This study was supported by National Natural Science Foundation of China (Nos. U1710108; 51576094), Jiangsu ‘‘Six Personnel Peak” Talent-Funded Projects (GDZB-014). References Cai, J., Shen, B.X., Li, Z., Chen, J.H., He, C., 2014. Removal of elemental mercury by clays impregnated with KI and KBr. Chem. Eng. J. 241, 19e27. De, M., Azargohar, R., Dalai, A.K., Shewchuk, S.R., 2013. Mercury removal by bio-char based modified activated carbons. Fuel 103, 570e578. Dranga, B.A., Lazar, L., Koeser, H., 2012. Oxidation catalysts for elemental mercury in flue gases-A review. Catalysts 2, 139e170. Du, W., Yin, L.B., Zhuo, Y.Q., Xu, Q.S., Zhang, L., Chen, C.H., 2015. Performance of CuOx-neutral Al2O3 sorbents on mercury removal from simulated coal combustion flue gas. Fuel Process. Technol. 131, 403e408. Fang, P., Cen, C.P., Chen, D.S., Tang, Z.X., 2010. Carbonaceous adsorbents prepared from sewage sludge and its application for Hg0 adsorption in simulated flue gas.

Li et al., (2014) Li et al., 2017a,b Present study

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