Removal of elemental mercury by clays impregnated with KI and KBr

Chemical Engineering Journal 241 (2014) 19–27

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Removal of elemental mercury by clays impregnated with KI and KBr Ji Cai, Boxiong Shen ⇑, Zhuo Li, Jianhong Chen, Chuan He College of Environmental Science & Engineering, Nankai University, Tianjin 300071, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Clay impregnated with halogen (Br/I)

enhanced the capacity of Hg0 removal.  KI-clay performed better than KBr-clay in Hg0 removal under the same conditions.  The reaction of KI and O2 to form I2 was an key step in the chemisorption of Hg0.  O2 and SO2 showed a promotion whereas H2O showed an inhibition on Hg0 removal.

a r t i c l e

i n f o

Article history: Received 20 September 2013 Received in revised form 23 November 2013 Accepted 26 November 2013 Available online 4 December 2013 Keywords: Clays Potassium iodine Potassium bromide Elemental mercury removal

Hg0 O2, SO2, H2O

KBr-clay

KBr-clay

Hg0 KI-clay KI-clay O2, SO2, H2O

a b s t r a c t The removal of elemental mercury (Hg0) by KI and KBr modified clays (KI-clays and KBr-clays) was studied under simulated flue gas conditions. The physicochemical properties of these catalysts were investigated by a variety of characterization methods. The effects of KI and KBr loading, adsorption temperatures and the flue gas components (such as O2, SO2 and H2O (g)) on Hg0 removal efficiency were investigated. A pseudo-second-order model simulation was also introduced to reveal the mechanisms of Hg0 removal. The results showed that the Hg0 removal efficiency for the clays was significantly enhanced by KI or KBr modification, and the KI-clays performed much better than the KBr-clays in terms of Hg0 removal under the same conditions. An increase in KI or KBr loading and adsorption temperatures improved the Hg0 removal efficiency, which indicated that chemisorption occurred. The presence of O2 and SO2 promoted Hg0 removal, whereas the presence of H2O inhibited Hg0 removal by these modified clays. The formation of I2 from the reaction (2KI + 1/2O2 M I2 + K2O) was considered to be an important step in the chemisorption of Hg0 on the surfaces of the KI-clays. The lower extent of Hg0 removal exhibited by the KBr-clays than by the KI-clays was due to the difficulty of Br2 formation on the surfaces. The pseudo-second-order model was demonstrated to simulate with the removal of Hg0 by KI-clays and KBrclays well. The KI-clays displayed much a higher of the initial adsorption rate (H) and Hg0 removal capacity (qe) than the KBr-clays under the same conditions, which demonstrated that KI-clays are more active than KBr-clays with respect to Hg0 removal. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction The toxicity of mercury in the environment and its effects on human health have garnered world-wide attention due to mercury’s propensity for bio-accumulation in the aquatic food chain [1]. The total amount of mercury discharged to the atmosphere by human activities is approximately 1000–6000 tons ⇑ Corresponding author. Tel.: +86 022 23508319; fax: +86 022 23508807. E-mail address: [email protected] (B. Shen). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.11.072

each year [2]. The emission of mercury from coal-fired power plant accounts for approximately one-third of the total atmospheric mercury [3]. As one of the main sources of mercury pollution, coal-fired power plants should be constrained by more rigorous discharge criteria of mercury emission. The Environmental Protection Agency (EPA) in the USA announced the Clean Air Mercury Rule (CAMR) in March 2005 emphasizing mercury emission control for coal-fired power plants [4]. The mercury species in the coal combustion flue gas always contains elemental mercury (Hg0), oxidized mercury (Hg2+) and particle-bound mercury (Hgp) [5].

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Oxidized mercury is soluble in water; thus, it can be removed easily by the system of a wet flue gas desulfurization (WFGD) system. Particle-bound mercury enriched on the surfaces of fly ash can be captured mostly by electrostatic precipitators or baghouses. However, Hg0 is considered to be difficult to be captured with the typical air pollution control devices (APCDs) because Hg0 is highly volatile and insoluble. It has been reported that Hg0 can be oxidized by selective catalytic reduction (SCR) catalysts and then be captured by the following APCDs [6–9], but the oxidation effect is weak when there is no halogen present in the flue gas [10,11]. Direct injection of sorbents upstream of a particulate control system is a potential technology for capturing mercury from utility flue gases. Various sorbents have been reported for the removal of mercury from flue gas, such as activated carbons [12,13], petroleum coke [14], coal tar, fly ash [15], zeolites [16], and calcium sorbents [17], with activated carbons demonstrating the best performance among these adsorbents. Activated carbon powder has been commercially used in the flue gas for the adsorption of mercury in the United States in recent years. Due to the low concentration of Hg in flue gas, a large carbon/Hg ratio is required to achieve a high removal efficiency for mercury and the cost for activated carbon injection is great. This high cost of activated carbon powder will restrict the development of the technology in china. Modified activated carbons have been studied to improve the activity of mercury capture in recent years. S, I2, and Cl2 impregnated activated carbons have been reported to improve the adsorption of Hg0 [18–20]. Sun et al. [19] observed that brominated activated carbon was effective in removing mercury, with the adsorption capacity increasing 80-fold at a bromination loading of 0.33%. De et al. [20] studied the mercury adsorption by KI impregnated activated carbons, observed that Hg–Br and Hg–I compound were formed on the modified activated carbons by complex chemisorption. Eswaran and Stenger [21] studied the effects of halogen acids such as HCl, HBr and HI on Hg0 conversion in the flue gas, and it suggested that HCl was the least effective oxidant, whereas HBr and HI demonstrated better abilities to oxidize Hg0. The presence of halogens on the surfaces of sorbents or in the fuel gas may benefit to oxidation of Hg0, but the impregnation of halogens on the surfaces of the sorbents may be more feasible for the application. Clay, a type of natural Si–Al-based material, is highly abundant in China. The price of clay is approximately 1/20 of that of activated carbon. Due to their layered structure, high surface acidity, good thermal stability and abundance of mesopores, clays and the modified clays have been used as adsorbents and catalyst supports for treating waste gas [22–24]. Until now, however, there have been almost no papers reported on the use of clays/modified clays to capture Hg in flue gas. Therefore, it is interesting to investigate the characteristics of Hg removal using clays and modified clays. In this study, the halide salts of KBr and KI were used to modify clays because KBr and KI are more stable than Br2 and I2. KCl was not considered as a modifier during the experiment due to its imperceptible effect on mercury removal, according to previous studies on KCl-modified activated carbon [12,26]. Therefore, the characteristics and mechanisms of the Hg0 removal by KBrclays and KI-clays in flue gas will be studied comparatively taking account of the kinetic simulation in this paper.

tonite in our study is as follows: SiO2 64.07%, Al2O3 25.37%, CaO 2.63%, MgO 2.71%, Fe2O3 2.90%, and K2O 2.31%, which was analyzed by Energy Dispersive Spectrometer (Bruker AXS Microanalysis GmbH Berlin, Germany). Halide-impregnated clays were prepared by an impregnation method using a mixed solution of KI (or KBr) and bentonite under vigorous stirring at room temperature for 3 h. The weight loading of KI or KBr was varied over the range of 0.5–3 wt.% to the clays. The final sorbents were dried at 100 °C for 12 h before being crushed and sieved to form particles with a mesh size of 40–60 mesh. The sorbents are denoted as KX(z)-clays, where z represents the weight ratio (%) of KX (X = I and Br) to clays. 2.2. Characterization of sorbents The specific surface area, pore volume and pore size distribution of the samples were determined by N2 adsorption at 196 °C on a NOVA 2000 automated gas sorption system. The total surface area and pore volume were evaluated by the Brunauer–Emmett–Teller (BET) method. The pore size distributions were calculated by the Barrett–Joiner–Halenda (BJH) method using the adsorption data of the isotherm. SEM was performed using an S-3500N scanning electron microanalyzer with an accelerating voltage of 15 kV. Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a Nicolet Magna-560 infrared spectrometer (US Bio-rad company) over the range of 4000–400 cm1. 2.3. Sorbents activity test The experiments were carried out in a down-flow fixed bed reactor. The schematic diagram of the system is shown in Fig. 1. The system was composed of the following main parts: simulated flue gas generator, vapor generator, Hg0 generator, gas pre-heater, fixed bed reactor, online elemental mercury analyzer, exhaust gas purification devices and system of temperature control devices. The simulated flue gas generator featured various gases, such as N2, O2 and SO2 (2, 3 and 4 in Fig. 1, respectively). The vapor generator (5 in Fig. 1) was a system that injected water by a micropump to the gas pre-heater (7 in Fig. 1). The Hg0 generator included a part charged with evaporating Hg0 from the mercury permeation tube (11 in Fig. 1), which was placed in a U-type tube (10 in Fig. 1). The U-type tube was immersed in a water bath (6 in Fig. 1). The concentration of mercury in the simulated gas was controlled by varying the temperature in the water bath and the flow rate of N2 (1 in Fig. 1) through the U-type tube. All of the simulated gases, including Hg0 gas and water vapor, were led to the adsorbed reactor (8 in Fig. 1), and the exhaust gas from the reactor was then introduced into the carbon trap and expelled into the atmosphere. The reactor was a quartz glass reactor with an inner diameter of

2. Experimental 2.1. Preparation of sorbents As a native clay in China [25], bentonite was selected as the starting material in this study. The chemical composition of ben-

Fig. 1. The schematic diagram of the experimental system. (1) N2, (2) N2, (3) O2, (4) SO2, (5) vapor generator, (6) water batch, (7) gas pre-heater, (8) reactor, (9) mercury analyzer, (10) U-type pipe and (11) mercury permeation tube.

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10 mm and a length of 400 mm. A screen was fitted 100 mm from the bottom of the tube to support the sorbent materials. The reactor was fitted into an aluminum block with a 50 mm diameter, outside which the insulator was wrapped and the system was externally heated by an electric stove wire placed in the grooves on the outside of the system to provide a uniform temperature throughout the reactor. The system was then wrapped with a layer of insulation to prevent heat loss. The temperature of the reactor was controlled by a temperature controller with K type thermocouple arranged inside the reactor. The range of mercury concentrations in actual flue gas from coal combustion is 3–30 lg/Nm3, falling mostly within the range 5–10 lg/Nm3 [27]. The mercury concentration in the simulated flue gas was controlled to be approximately 26 ± 2 lg/Nm3 during the experiments. The simulated flue gas mainly composed of N2 (99.99%) and O2 (99.99%). The N2 flow gas flowed through two branches. One branch (600 mL/min, 2 in Fig. 1) was mixed with O2 (3 in Fig. 1) to form the main gas flow, and the other branch (200 mL/min, 1 in Fig. 1) passed through the mercury permeation tube in the U-type tube to introduce Hg0 vapor into the system. SO2 and H2O were added to the gas when used. In all of the runs, a certain 20 mg sample was placed in the fixed-bed reactor (8 in Fig. 1) at a certain temperature. The inlet and outlet Hg0 concentrations were measured using an online mercury analyzer (9 in Fig. 1, H11-QM201H, Suzhou Qinan Co. LTD.). The duration for the adsorption was designed to be 3 h in our experiments. The total amount of removed Hg0 per gram of sorbent Q (lg/g) was calculated by Eq. (1) to evaluate the Hg0 adsorption capacity. The activity of the sorbent was calculated in terms of the Hg0 removal efficiency (g) by Eq. (2).

Rt

gds

Q¼

F  C in  W

g¼

C in  C out  100% C in

0

ð1Þ

ð2Þ

where F is the gas flow rate (Nm3/min), W is the mass of sorbent (g),

g is the Hg0 removal efficiency, Cin and Cout are the Hg0 concentrations at the inlet and outlet of the fixed-bed reactor, respectively, and t was the adsorption time. 3. Results and discussion

mesopores, capillary condensation will occur during adsorption and is preceded by a metastable fluid state (‘‘cylindrical meniscus’’), whereas capillary vaporization during desorption occurs via a hemispherical meniscus, separating the vapor and the capillary condensed phase. This process results in hysteresis, because pores of a specific size are filled at higher pressures and emptied at lower pressures. In Fig. 2, it shows that the desorption branch was different from the adsorption branch and displayed an obvious inflection ‘‘knee’’ at a P/P0 of approximately 0.40–0.50. The type IV adsorption/desorption isotherms displayed marked H3 hysteresis loops for all of the sorbents according to the IUPAC classification. This phenomenon has been observed for many different types of layered materials when N2 is used as the adsorbent. In Fig. 3, the pore size distributions of the sorbents derived from the adsorption branch of the isotherm (BJH) are displayed. The pore size distributions (dV/dW) of the materials derived from the adsorption branch indicated a broader distribution of pore diameters (2–100 nm). Both the unmodified clay and the modified clays were observed to contain a significant number of mesopores (2–20 nm) but with almost no micropore (<2 nm) distribution in these materials. As shown in Fig. 3, the impregnation of KI and KBr decreased the mesopore content (especially for pores <10 nm) distribution and the incremental pore volumes, which was due to the partially blockage of mesopores by KI or KBr clusters in the materials. The KI-clays exhibited more significant blockage of mesopores than the KBrclays. Ghorishi et al. [28] assessed the effect of pore size distribution on Hg0 removal for Ca-based sorbents. Their results revealed that Hg0 capture required a greater number of small pores; however, Zhao et al. [29] indicated that pore size distribution is not a key factor affecting the efficiency of Hg0 removal. The BET specific surface areas, pore volumes, and average pore size of the unmodified clay and the modified clays are shown in Table 1. The BET specific surface area of the unmodified clay was 77.37 m2/g. After the impregnation with KI and KBr, the BET specific surface areas decreased to 76.70 m2/g and 76.82 m2/g for the KI(1)-clay and the KBr(1)-clay, respectively. The same modification change trends for total pore volumes and average pore sizes were observed for the KI(1)-clay and the KBr(1)-clay. The impregnation resulted in the decrease in the BET specific surface areas and pore volumes, which is considered to be unfavorable for the physisorption of mercury. The SEM of the materials shown in Fig. 4 suggested that the particle morphologies of the modified and unmodified materials were slightly different. Among these samples, the profile for the

100 75 50 25 0 100 75 50 25 0 100 75 50 25 0 0.0

1

2

3

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

P/P0 Fig. 2. Adsorption/desorption isotherms. (1) Clay, (2) KBr(1)-clay and (3) KI(1)-clay.

BJH adsorption dV/dw pore volume (cc/g-nm) ×10

Adsorption/desorption/cc/g

The N2 adsorption/desorption isotherms and the pore size distributions of the modified clays are shown in Figs. 2 and 3. For

-4

3.1. Characterization of sorbents 10

1

9

2

8

3

7 6 5 4 3 2 1 0 -1

1

10

100

pore diameter /nm Fig. 3. Pore distribution of the samples. (1) Clay, (2) KBr(1)-clay and (3) KI(1)-clay.

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Table 1 Pore structure analysis of different adsorbent materials. Samples

BET surface area (m2/g)

Clay KBr(1)-clay KI(1)-clay

77.37 76.82 76.70

Pore volume (cm3/g)

Average pore diameter (nm)

Micropre

Mesopore

Macropore

Total pore

0.00148 0.00341 0.00149

0.11599 0.10944 0.11193

0.03957 0.03690 0.02859

0.15704 0.14976 0.14201

KI(1)-clay seemed to be the tightest and the profile for the unmodified clay seemed to be the loosest. The surface functional groups on the unmodified clay and modified clays were compared (shown in Fig. 5). The KI(1)-clay and the KBr(1)-clay exhibited two more distinct peaks at approximately 3622 and 3404 cm1 in the -OH stretching region than those observed for the unmodified clay [30]. These two peaks were due to the Al–O–H stretching vibration of the structural hydroxyl groups on the surfaces and the water molecules presented in the interlayer. Moreover, at 470–520 cm1 wavenumber, the peak became sharper for the modified clays, which was caused by the bending vibration of Si–O–Si and stretching vibration of Al–O. All of these differences may be caused by the addition of I and Br into the bentonite. Moreover, the changes of the functional groups may affect the capacity of mercury removal capacity of the modified sorbents.

10.15 9.23 8.67

3

Transmittance/%

2517

1875

1797

2 3622

3409

1629

1451

2517

717 920 793 1020

1796 1875

3623

1

3401

920 1874 1630

3404

3500

920

3000

2500

2000

793

1040

1451

1021

4000

453

719

2517

3622

511

1630 1449

1500

793

1000

516

470

515 453

500

Wavenumber /cm-1 Fig. 5. FTIR analysis. (1) Clay, (2) KBr(1)-clay and (3) KI(1)-clay.

3.2. Sorbents activity test 3.2.1. The effects of halide loading The effects of KI and KBr loading on Hg0 accumulated adsorption capacity and removal efficiency are shown in Figs. 6 and 7. All of the experiments were carried out at 80 °C over an adsorption time at 3 h. It can be observed that the unmodified clay could hardly capture Hg0; however, a clear improvement in Hg0 capture was observed for the KI(1)-clay, with the accumulative adsorption amount reaching 82.82 lg/g at 3 h, but only 1.59 lg/g for the unmodified clay. The capability of the KBr-clay for Hg0 capture was promoted but not as highly as that of the KI-clay. The results also suggested that Hg0 removal efficiency (g) improved remarkably with the increase in KI loading. For the KI(1)-clay, the removal efficiency was maintained at 38–62% during 3 h, whereas for the KI(3)-clay the removal efficiency was maintained at 78–98%. For the KBr(1)-clay, g was maintained at 5–42%, and the variation in KBr loading did not affect g greatly. The unmodified clay with a large BET specific surface area showed very low Hg0 capture when compared to that exhibited by the impregnated clays, which indicated that the physisorption of Hg0 on the surfaces of the KI-clays or the KBr-clays was not so great. Research by Lee et al. also indicates that Hg0 is removed by halide-impregnated activated carbon when gas-phase mercury reacts with the surface halide to form mercury iodides [31]. It has been shown that an increase in KI or KBr loading improves the capability of clays for Hg0 removal. This result is similar to that reported by De et al. regarding Hg0 removal

1

160 1 3 5 7

Hg0 adsorption capacity/µg/g

140 120

2 4 6

100 80 60 40 20 0

0

20

40

60

80

100

120

140

160

180

200

Time/min Fig. 6. Effects of KI/KBr loading on Hg0 adsorption capacity. (1) KI(0.5)-clay, (2) KI(1)-clay, (3) KI(3)-clay, (4) KBr(0.5)-clay, (5) KBr(1)-clay, (6) KBr(3)-clay and (7) clay.

with bio-char based modified activated carbons [20]. Therefore, it is expected that a higher loading leads to better contact with the vapor-phase mercury and thus a higher Hg0 removal efficiency.

2

3

Fig. 4. The SEM of the samples. (1) Clay, (2) KBr(1)-clay and (3) KI(1)-clay.

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J. Cai et al. / Chemical Engineering Journal 241 (2014) 19–27

100

80 1 3 5 7

60

Hg0 removal efficiency/%

Hg0 removal efficiency/%

100

2 4 6

40

20

0

0

20

40

60

80

100

120

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200

80

2 4 6

60

40

20

0

Time/min

1 3 5

0

20

40

60

80

100

120

140

160

180

200

Time/min Fig. 7. Effects of KI/KBr loading on Hg0 removal efficiency. (1) KI(0.5)-clay, (2) KI(1)-clay, (3) KI(3)-clay, (4) KBr(0.5)-clay. (5) KBr(1)-clay, (6) KBr(3)-clay and (7) clay.

3.2.2. The effect of adsorption temperature The effects of different adsorption temperatures (80 °C, 120 °C and 180 °C) on the Hg0 adsorption capacity and Hg0 removal efficiency for the KI-clays and KBr-clays are shown in Figs. 8 and 9. Figs. 8 and 9 show that the Hg0 removal efficiency of both KI-clays and KBr-clays increased with the rise of adsorption temperatures. The Hg0 removal efficiency for the KI(1)-clay decreased from 89% to 57% in 3 h at 180 °C, from 81% to 49% at 120 °C, and from 62% to 39% at 80 °C, respectively. The Hg0 removal efficiency of the KBr(1)-clay decreased from 62% to 31% in 3 h at 180 °C, from 46% to 10% at 120 °C, and from 38% to 10% at 80 °C, respectively. The enhancement in Hg0 removal efficiency from 120 °C to 180 °C was much greater than that from 80 °C to 120 °C for the KBr-clays. However, this effect was not observed for the KI-clays. It is supposed that the reaction between KI (on the surfaces of the KI-clays) and Hg0 proceeds more easily than that between KBr (on the surfaces of the KBr-clays) and Hg0. When the reaction temperatures increased from 80 °C to 120 °C, the reaction was still chemically controlled for the KBr-clays but when the temperatures increased to 180 °C, the extent of the reaction increased greatly. It is known that the adsorption of Hg0 on the surfaces of the materials can be typically classified into two types of processes: physisorption and chemisorption. The adsorption of Hg0 through physisorption is considered to be reversible and favorable at low temperature, and the BET specific surface area is an important factor that deter-

Fig. 9. Effects of temperature on Hg0 removal efficiency. (1) KI-clay(80 °C), (2) KIclay(120 °C), (3) KI-clay(180 °C). (4) KBr-clay(80 °C), (5) KBr-clay(120 °C) and (6) KBr-clay(180 °C).

mines the extent of physisorption. On the other hand, chemisorption is associated with a certain activation energy and proceeds at a limited rate which increases with rise in temperature [32]. The clear enhancement in Hg0 removal with temperature suggests that the removal of Hg0 on these modified clays occurred mainly by chemisorption. 3.2.3. The effect of O2 on Hg0 removal O2 is another important component in flue gas. Thus, considering the effect of O2 concentration on Hg0 adsorption was necessary. Experiments concerning the effect of O2 on Hg0 adsorption were carried out with the KI(1)-clay and the KBr(1)-clay at 80 °C using different O2 concentrations. Figs. 10 and 11 show the effect of O2 concentration on the Hg0 adsorption capacity and Hg0 removal efficiency. The KI(1)-clay exhibited a low efficiency of Hg0 removal in the absence of O2. The Hg0 removal efficiency for the KI(1)-clay decreased rapidly from the initial 47% to 26% in the absence of O2 over 3 h. The Hg0 removal efficiency increased as the O2 concentration increased from 3% to 10%. It was observed that the increase in the O2 concentration from 0% to 3% greatly enhanced the Hg0 removal efficiency, whereas the enhancement was less when the O2 concentration increased from 3% to 10%. With the introduction of O2 into the flue gas, the improvement was improved. It is concluded that the presence of O2 favors the chemisorption reaction 120

1 3 5

120 100

Hg0 adsorption capacity/µg/g

Hg0 adsorption capacity/µg/g

140 2 4 6

80 60 40 20 0

0

20

40

60

80

100

120

140

160

180

200

Time/min Fig. 8. Effects of temperature on Hg0 adsorption capacity. (1) KI-clay(80 °C), (2) KIclay(120 °C), (3) KI-clay(180 °C). (4) KBr-clay(80 °C), (5) KBr-clay(120 °C) and (6) KBr-clay(180 °C).

100

1 3

2 4

5

6

80 60 40 20 0

0

20

40

60

80

100

120

140

160

180

200

Time/min Fig. 10. Effects of O2 on Hg0 adsorption capacity. (1) KI(1)-clay 0% O2, (2) KI(1)-clay 3% O2, (3) KI(1)-clay 10% O2, (4) KBr(1)-clay 0% O2, (5) KBr(1)-clay 3% O2, (6) KBr(1)clay 10% O2.

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100

70

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3

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Hg0 removal efficiency/%

Hg0 removal efficiency/%

80

50 40 30 20 10 0

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Time/min

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80

between mercury and halogen-modified clays. However, the degree of this improvement for the KI(1)-clay and KBr(1)-clay was different. The effects of O2 on mercury capture for the KI(1)-clay were more distinct than those observed for the KBr(1)-clay. 3.2.4. The effect of SO2 and H2O SO2 and H2O always existed in the real flue gas. Thus, effects of SO2 and H2O on the Hg0 removal were investigated in our study. The effects of SO2 and H2O on the Hg0 adsorption capacity are shown in Fig. 12, and the effects of SO2 and H2O on the Hg0 removal efficiency are shown in Fig. 13. It was observed that the introduction of SO2 enhanced the Hg0 removal efficiency of the sorbents at 80 °C. For the KI(1)-clay, the mercury accumulated adsorption capacity increased from 82.82 lg/g to 90.18 lg/g after 3 h when 0.01% SO2 was added, and the Hg0 removal efficiency increased from 38% to 42% accordingly. For the KBr(1)-clay, the Hg0 accumulated adsorption capacity increased from 6.51 lg/g to 15.45 lg/g after 3 h, and the Hg0 removal efficiency increased from 11% to 35%. It is clear that the Hg0 capture enhancement by the KBr(1)clay with the addition of SO2 was greater than that observed for the KI(1)-clay. This effect may be due to the decrease in the chemical control of the reaction between the KBr(1)-clay and Hg0 induced by SO2. It has been previously reported that SO2 inhibits

Hg0 accumulated adsorption capacity/µg/g

100 1 3 5

80

2 4 6

100

120

140

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180

200

Time/min

0

Fig. 11. Effects of O2 on Hg removal efficiency. (1) KI(1)-clay 0% O2, (2) KI(1)-clay 3% O2, (3) KI(1)-clay 10% O2, (4) KBr(1)-clay 0% O2, (5) KBr(1)-clay 3% O2, (6) KBr(1)-clay 10% O2.

2 4 6

Fig. 13. Effects of SO2 and H2O on Hg0 removal efficiency. (1) KI(1)-clay, without SO2 and H2O, (2) KI(1)-clay,with 0.01% SO2, (3) KI(1)-clay, with 0.01%SO2 and 3%H2O, (4) KBr(1)-clay, without SO2 and H2O, (5) KBr(1)-clay,with 0.01% SO2, (6) KBr(1)-clay,with 0.01% SO2 and 3% H2O.

the carbon impregnation with I2 for Hg0 removal [33]. The competitive adsorption of SO2 that can chemically bond with Hg0 is considered to be a main factor in reducing Hg0 removal efficiency. However, the enhancement in mercury removal induced by SO2 is supported in most of the literatures [34,35]. It is supposed that SO2 interacts with activated carbon via physisorption or chemisorption to form sulfur groups on the surface of activated carbon. These sulfur groups constituted chemisorption sites for the adsorption of Hg0, because sulfur atoms can accept the two electrons present on Hg0, oxidizing Hg0 into Hg2+ [36]. Additionally, the probability that Hg0 and sulfur dioxide compete the surface of activated is minimized, because SO2 is typically adsorbed on activated carbons with a large amount of basic groups. More specifically, the presence of pyronic or pyronic-like structures greatly enhances the adsorption of SO2 on activated carbon, whereas Hg0 prefers lactones and carbonyls [37,38]. It is supposed that the presence of 2 SO2 may introduce anions such as SO2 on the surfaces of 3 =SO4 the modified clays, favoring the chemisorption of Hg0, which requires further study in the future. The inhibitory effect on Hg0 removal was extremely significant when 0.01% SO2 and 3% H2O coexisted at 80 °C. With the addition of 0.01% SO2 and 3% H2O, the mercury accumulated adsorption capacity of the KI(1)-clay decreased to 19.04 lg/g after 3 h and the Hg0 removal efficiency decreased to 6%. However, with the addition of 0.01% SO2 and 3% H2O, the mercury accumulated adsorption capacity of the KBr(1)clay decreased to 2.49 lg/g after 3 h and the Hg0 removal efficiency decreased to 4%. The competitive adsorption of H2O on the active sites is considered to be a main factor in reducing Hg0 removal efficiency by Li et al. [40].

60

3.3. Mechanism discussion and kinetic model 40

20

0

0

20

40

60

80

100

120

140

160

180

200

Time/min Fig. 12. Effects of SO2 and H2O on Hg0 adsorption capacity. (1) KI(1)-clay,without SO2 and H2O, (2) KI(1)-clay, with 0.01% SO2, (3) KI(1)-clay,with 0.01%SO2 and 3%H2O, (4) KBr(1)-clay, without SO2 and H2O, (5) KBr(1)-clay,with 0.01% SO2, (6) KBr(1)-clay, with 0.01% SO2 and 3% H2O.

In the capture process, Hg0 was adsorbed on the surfaces of the sorbents first and then reacted with the halogens (or halogen ions, expressed as X) that impregnated on the surfaces of the sorbents, a process referred to as chemisorption. The reactions of Hg0 and the halogens (or halogen ions) can be described by following chemical Eqs. (3–8) [39]:

Hg þ X2 þ 2KX $ K2 HgX4

ð3Þ

Hg þ X2 þ 2KX $ KHgX3

ð4Þ

2KX þ HgX þ 1=2X2 $ K 2 HgX4

ð5Þ

25

J. Cai et al. / Chemical Engineering Journal 241 (2014) 19–27 Table 2 Gibbs free energy (DGf) of formation of mercuric halides from elemental mercury and molecular halogen at different temperatures. Gibbs free energy (DGf)

25 °C (KJ/mol)

127 °C (KJ/mol)

227 °C (KJ/mol)

HgBr2 HgI2

162 100

140 101

126 92

KX þ HgX þ 1=2X 2 $ KHg X 3

ð6Þ

Hg0 þ 1=2X2 $ HgX

ð7Þ

Hg0 þ X2 ! HgX2

ð8Þ

Hutson et al. [41] discussed the mechanism of mercury removal by brominated activated carbon. The authors showed that Br2 reacts with Hg0 in a manner similar to that in which Hg0 reacts with I2. It was reported that mercury bromine compounds such as HgBr or HgBr2 forms on the surface of sorbents. The same mechanism of the Hg removal was supposed for the KI-clays and KBr-clays with Hg. The formation of the mercury halides was thermodynamically favorable from 80 °C to 180 °C as indicated by the negative values for the Gibbs free energy (4Gf) of formation shown in Table 2, with the favorability of formation ranking: HgBr2 > HgI2. However, the reverse order observed in Hg0 removal efficiency for HgBr2 and HgI2 may be explained based on the exposure of the sorbent surfaces. At the same molar loading, the iodide ions were expected to exhibit higher surface exposure [20]. Therefore, it was expected that iodide ions would have better contact with the vapor phase mercury and thereby result in higher Hg removal efficiency. Chemical Eq. (3)–(8) indicate that the presence of halogen molecules of I2 and Br2 is favorable for the reactions. To provide enough I2 to react with Hg on the surfaces of the KI-clays, the dynamic equilibrium between KI and I2 will be established automatically. The reaction was supposed to occur as indicated by the following equation:

2KI þ 1=2O2 $ I2 þ K2 O

ð9Þ

It is known that KI can be oxidized by O2 to form I2 much more easily than in the case in which KBr is oxidized. The oxidation of KBr by O2 to form Br2 is believed to be slow such that there may be a deficiency of Br2 on the surfaces of KBr-clays, which will inhibit the reaction between the KBr-clays and Hg. This assumption is supported by the fact that the increases in KBr loading and the O2 concentrations did not greatly enhance the capture of Hg0, which may be the main reason why the KBr-clays displayed a lower Hg0 capture ability than that of the KI-clays. To determine the reaction rate for the KI-clays and KBr-clays with Hg0, kinetic models were established to calculate the reaction rates of the KI-clays and KBr-clays with Hg0. Based on the experi-

mental data, kinetic studies were performed to investigate the mechanism of adsorption and the rate controlling steps. The kinetic models include the Fick’s intraparticle diffusion equation, the pseudo-first order model, the pseudo-second order model and Elovich kinetic equation, as proposed by Skodras et al. to simulate Hg0 adsorption by activated carbon or modified activated carbon [42]. The most accurate prediction of the experimental data was achieved by the pseudo-second order model, suggesting that the chemisorption rate is the controlling step in the reaction. From the above discussed experimental results, it is concluded that the KI-clays and KBr-clays chemisorbed for Hg0; therefore, the pseudo-second order model was used to simulate the adsorption of mercury by the KI-clays and KBr-clays. The pseudo-second order kinetic model constitutes a mass action rate model in which the surface diffusivity is inversely proportional to the square of the concentration of vacant sites on the sorbent surface. The rate of the adsorption is described by the following equation [43,44]:

dq ¼ k1 ðqe  qÞ2 dt

ð10Þ

According to the boundary conditions t = 0 to t = t and q = 0 to q = qt, the integrated form of the above equation can be expressed as follows:

t 1 1 ¼ þ t qt k1 q2e qe qt ¼

ð11Þ

t ð1=ðk1 q2e ÞÞ þ ðt=qe Þ

ð12Þ

where qe and qt are the amounts of Hg0 absorbed at equilibrium and at time t (lg g1), respectively. k1 is the rate constant of pseudo-second-order sorption (g lg1 min1). k1 q2e can be regarded as the initial adsorption rate H [45]. Based on the analysis of t/qt linearly fitting with t, the pseudo-second-order kinetic parameters of qe, k1 and H can be obtained. The advantage of this model is that the amount of equilibrium adsorption does need to be known but can be calculated directly. The kinetic parameters and the correlation coefficient (R2) for the KI-clays and KBr-clays at different loading and adsorption temperatures are shown in Table 3. The results suggested that the correlation coefficient (R2) for the unmodified clay was only 0.74 (not shown in Table 3), whereas the correlation coefficient (R2) for the KI-clays and KBr-clays were greater than 0.93. The results demonstrate that pseudo-second-order model can be well used to describe the Hg0 adsorption on the KI(1)-clay and KBr(1)-clay as shown in Fig. 14. As shown in Table 3, the initial adsorption rate H for the KBr-clays increased with KBr loading, but the Hg0 removal capacity qe did not improve greatly. With the increase in the reaction temperature, the initial adsorption rate H for the KBr-clays remained at nearly the same level, but the Hg0 removal capacity qe improved greatly. It was proven from the above assumption that the formation of Br2 from KBr and O2 was the chemically controlling step of

Table 3 The kinetic parameters of the pseudo-second-order model by KBr-clays and KI-clays. Halide loading

1

1

Temperature

(0.5)

(1)

(3)

80 °C

120 °C

180 °C

KBr-clays

H(g lg min ) qe (lg g1) k1  104(g lg1 min1) R2

0.117 8.88 14.9 0.99

0.138 12.46 8.92 0.98

0.176 11.44 13.5 0.99

0.138 12.46 8.92 0.98

0.119 15.75 4.80 0.99

0.127 52.96 0.456 0.99

KI-clays

H(g lg1 min1) qe (lg g1) k1  106(g lg1 min1) R2

0.589 78.25 96.3 0.99

0.648 263.16 9.36 0.95

0.971 698.62 0.38 0.97

0.648 263.15 9.66 0.95

0.906 307.69 9.58 0.95

0.944 487.80 3.97 0.93

26

J. Cai et al. / Chemical Engineering Journal 241 (2014) 19–27

References

90 1

Hg0 adsorption capacity/µg/g

80

2

70

3 4

60 50 40 30 20 10 0 0

20

40

60

80

100

120

140

160

180

200

Time/min Fig. 14. Simulation of mercury removal by pseudo-second-order model. (1) clay: experimental data, (2) KBr(1)-clay: experimental data. (3) KI(1)-clay: experimental data and (4) calculation by the model.

Hg0 adsorption. The increase in KBr loading and the O2 concentration does not change the chemically controlling step for KBr and O2, whereas the increase in the adsorption temperature may accelerate the formation of Br2 to enhance Hg0 chemisorption. However, both the initial adsorption rate H and the Hg0 removal capacity qe of the KI-clays are enhanced significantly with the increase in of KI loading, O2 concentration and adsorption temperature. The formation of I2 from KI and O2 is considered to be an important step for Hg0 adsorption by KI-clays. Compared to those of the KBr-clays, the initial adsorption rate H and the Hg0 removal capacity qe for the KIclays were much greater under the same conditions. Thus, the KIclays demonstrated much greater Hg0 capture than KBr-clays.

4. Conclusions In order to evaluate the possibility of the low-cost material (clay) using as a sorbent to remove elemental mercury (Hg0) in flue gas, the removal of Hg0 by KI and KBr modified clays (KI-clays and KBr-clays) under simulated flue gas conditions was investigated. The results show that the Hg0 removal efficiencies of the clays were significantly enhanced by halogen modification, and the KIclays exhibited much better Hg0 removal performance than the KBr-clays under the same conditions. The Hg0 removal efficiency increased with KI or KBr loading and adsorption temperature, indicating that Hg0 removal by these modified clays occurred mainly by chemisorption. O2 and SO2 promoted Hg0 removal, whereas H2O inhibited it. The reaction 2KI + 1/2O2 M I2 + K2O is considered to be an important step in the chemisorption of Hg0 on the surfaces of KI-clays. The KBr-clays exhibiting a lower extent of Hg0 removal than the KI-clays may be due to the difficulty of Br2 formation by KBr with O2 on the surface of the KBr-clays. The pseudo-second-order model was demonstrated to simulate Hg0 removal by KI-clays and KBr-clays well. The KI-clays showed a much higher initial adsorption rate H and Hg0 removal capacity qe than the KBr-clays under the same conditions using the kinetic model. The KI and KBr modification clays are demonstrated to be effective sorbents for Hg0 removal in flue gas. Acknowledgements This project was financially supported by the National Natural Science Foundation of China (No. 51176077, 50976050), and the Research Fund for International Young Scientists (No. 51350110229).

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