AgBr-Ag2CO3 hybrids with enhanced photocatalytic removal of elemental mercury driven by visible light

Journal of Hazardous Materials 314 (2016) 78–87

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Facile synthesis of ternary Ag/AgBr-Ag2 CO3 hybrids with enhanced photocatalytic removal of elemental mercury driven by visible light Anchao Zhang a,∗ , Lixiang Zhang a , Hao Lu a , Guoyan Chen a , Zhichao Liu a , Jun Xiang b , Lushi Sun b a b

School of Mechanical and Power Engineering, Henan Polytechnic University, Jiaozuo, 454000, China State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, 430074, China

h i g h l i g h t s

g r a p h i c a l

• A novel technique on Hg0 removal

Schematic illustration for the charge transfer in the Ag/AgBr(0.7)-Ag2 CO3 system.

a b s t r a c t

using visible-light-driven Ag/AgBrAg2 CO3 hybrids was proposed. • Ag/AgBr-Ag2 CO3 hybrids were synthesized by a simple modified coprecipitation method. • Hg0 was mainly removed by the photogenerated holes (h+ ). • The possible reaction mechanism for superior Hg0 removal was proposed.

a r t i c l e

i n f o

Article history: Received 28 January 2016 Received in revised form 28 March 2016 Accepted 13 April 2016 Available online 14 April 2016 Keywords: Photocatalytic removal Ternary Ag/AgBr-Ag2 CO3 hybrids Elemental mercury Visible light

http://dx.doi.org/10.1016/j.jhazmat.2016.04.032 0304-3894/© 2016 Elsevier B.V. All rights reserved.

a b s t r a c t A novel technique for photocatalytic removal of elemental mercury (Hg0 ) using visible-light-driven Ag/AgBr-Ag2 CO3 hybrids was proposed. The ternary Ag/AgBr-Ag2 CO3 hybrids were synthesized by a simple modified co-precipitation method and characterized by N2 adsorption-desorption, scanning electron microscope (SEM), X-ray diffraction (XRD), UV–vis diffused reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) techniques. The effects of AgBr content, fluorescent lamp (FSL) irradiation, solution temperature, SO2 and NO on Hg0 removal were investigated in detail. Furthermore, a possible reaction mechanism for higher Hg0 removal was proposed, and the simultaneous removal of Hg0 , SO2 and NO was studied. The results showed that a high efficiency of Hg0 removal was obtained by using Ag/AgBr-Ag2 CO3 hybrids under fluorescent lamp irradiation. The AgBr content, FSL irradiation, solution temperature, and SO2 all exhibited significant effects on Hg0 removal, while NO had slight effect on Hg0 removal. The addition of Ca(OH)2 demonstrated a little impact on Hg0 removal and could significantly improve the SO2 -resistance performance of Ag/AgBr(0.7)-Ag2 CO3 hybrid. The characterization results exhibited that hydroxyl radical (• OH), superoxide radical (• O2 − ), hole (h+ ), and Br0 , were reactive species responsible for removing Hg0 , and the h+ played a key role in Hg0 removal. © 2016 Elsevier B.V. All rights reserved.

A. Zhang et al. / Journal of Hazardous Materials 314 (2016) 78–87

1. Introduction Mercury has attracted wide attention due to its high toxicity to human and other organisms [1]. Coal combustion has been considered as one of the largest anthropogenic mercury emission sources in the world [2]. In flue gases, mercury usually contains three forms: elemental mercury (Hg0 ), oxidized mercury (Hg2+ ) and particlebound mercury (Hgp ) [3]. Hg2+ and Hgp can be easily removed by wet flue gas desulfurization (WFGD) system and dust removing devices, respectively [4,5]. In comparison with Hg2+ and Hgp , it is hard to remove Hg0 from flue gas because of its volatility and indissolubility [6]. Thus, the removal of mercury will be enhanced when Hg0 is converted to its oxidized form [7]. Many technologies about Hg0 removal via catalytic oxidation of Hg0 into Hg2+ have been extensively developed recently [8]. On the one hand, heterogeneous catalytic oxidation of Hg0 using modified carbon-based catalysts, selective catalytic reduction (SCR) catalysts and metals or metal oxides had been largely reported in the literatures [8–12]. On the other hand, lots of oxidizing techniques such as photochemical oxidation [13,14], plasma oxidation [15,16], ozonation [17,18], sodium chlorite [19,20] and K2 S2 O8 /Ag+ /Cu2+ or H2 O2 /K2 S2 O8 solution [21,22] were researched in laboratory to control Hg0 emission. The development of high efficiency, low pollution and economic technology for Hg0 removal is still necessary and urgent [23]. To date, the advanced oxidation processes (AOPs), which can produce strong oxidizing hydroxyl radical (• OH), have been widely studied, and researchers have focused on this novel method to remove Hg0 [7,24–27]. The reaction mechanism of Hg0 removal was proposed as follows: hv •

H2 O2 →2 OH catalyst•

H2 O2 →

(1)

OH + OH−

(2)

Hg + • OH → HgOH

(3)

HgOH + • OH → Hg(OH)2

(4)

0

HgOH + • OH → Hg(OH)2

(4) +

HgOH + O2 + H2 O → Hg(OH)2 + H + O2

−

(5)

The above method is attractive since it can be performed even at room temperature. However it should be noted that the approach requires plenty of UV light irradiation and some amount of H2 O2 , and the cost of providing continuous UV light in power plant remains a challenging issue. Therefore, it is appropriate for us to synthesize a photocatalyst that can produce free radicals with strong oxidation under visible light or free sunlight irradiation. Ag2 CO3 with a band gap of 2.30 eV has been recognized as one of the most promising visible-light-driven photocatalyst to photodegrade organic dye for its efficient photocatalytic oxidation performance [28,29]. However, Ag2 CO3 material was frequently modified by halogenide to improve its unstable property [28–31]. Compared with pure Ag2 CO3 and silver halide, Mehraj et al. [32] and Xu et al. [33] observed the enhanced photocatalytic activities of AgX/Ag2 CO3 (X = Cl, Br, I) due to the efficient separation of electron–hole pairs. Yao et al. [31] confirmed that holes (h+ ) and superoxide ions (• O2 − ) were the two main active species in the photocatalytic process of methylene blue (MB) by Ag2 CO3 /Ag/AgCl hybrid. Cao et al. [34] observed that reactive • OH and h+ played the major role for the methyl orange (MO) degradation over AgBr/Ag3 PO4 hybrids. Moreover, Hu et al. [35] verified that due

∗ Corresponding author. E-mail address: [email protected] (A. Zhang).

79

to the surface plasmon resonance (SPR) effect of metallic silver nanoparticles (Ag0 NPs), Ag0 NPs can act as a sinker for photoinduced charge carriers, promoting charge separation to enhance the photocatalytic efficiency. Until now, although wet photocatalytic technology under visible light had been systematically studied in the field of waste water treatment [28–35], the mechanism of Hg0 removal by visible-light-responsive silver-based photocatalyst was still unclear. To the best of our knowledge, no research in the literatures has yet paid attention to the synthesis of Ag2 CO3 with AgBr and Ag0 NPs material to remove Hg0 under visible light. Since solar radiation contains more visible light (∼43%), the appropriate use of this fraction through the utilization of efficient visible-light photocatalysts to remove Hg0 is promising. Thus, in this study, the Hg0 removal activity over Ag/AgBr(X)-Ag2 CO3 (X = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1) hybrid photocatalysts were studied in detail under different experimental conditions. The properties of as-synthesized hybrids were characterized by N2 adsorptiondesorption, scanning electron microscope (SEM), X-ray diffraction (XRD), UV–vis diffused reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) techniques. A possible mechanism was proposed to explain the superior activity of Hg0 removal. 2. Experimental 2.1. Synthesis of Ag/AgBr(X)-Ag2 CO3 hybrids The Ag/AgBr(X)-Ag2 CO3 hybrid photocatalysts were synthesized by a simple modified co-precipitation method and the molar ratio X of AgBr/Ag2 CO3 was controlled by changing the content of additive KBr and Na2 CO3 . In a typical procedure, firstly, 9.86 g of AgNO3 was dissolved in 300 mL of distilled water to obtain 0.2 M AgNO3 solution. 0.7 g of KBr and 2.77 g of Na2 CO3 were dissolved in 150 mL distilled water. The obtained KBr/Na2 CO3 solution was slowly added to the above AgNO3 solution under vigorous magnetic stirring for 20 min until a faint yellow AgBr-Ag2 CO3 hybrid suspension was achieved. Secondly, the suspension was cultivated at 60 ◦ C with continuous stirring for 5 min, afterwards 5 mL of ethylene glycol (EG) was dropwise added and kept at 60 ◦ C for another 25 min. Finally, after placed overnight, the brownish black material was filtered and washed by distilled water for several times, and dried at 60 ◦ C for 24 h. The obtained material was denoted as Ag/AgBr(0.1)-Ag2 CO3 hybrid photocatalyst. Similarly, Ag2 CO3 , Ag/Ag2 CO3 , Ag/AgBr(0.3)-Ag2 CO3 , Ag/AgBr(0.5)-Ag2 CO3 , Ag/AgBr(0.7)-Ag2 CO3 , Ag/AgBr(0.9)- Ag2 CO3 , and Ag/AgBr photocatalysts were synthesized in the same manner. 2.2. Photocatalytic activity test The experimental setup consisted of a simulated flue gas system, a photochemical system, and an analytical system. As shown in Fig. 1, the baseline (BL) flue gas, which only contained 6% O2 , 12% CO2 , and balance N2 , 600 ppm of SO2 (when employed), and 300 ppm of NO (when employed) were considered as carrier gas. Hg0 vapor was produced from a mercury permeating tube (VICI Metronics Inc., USA) that was heated in a constant temperature water-bath. The photochemical system was composed of a cylindrical glass reactor, a magnetic stirring water-bath heater equipped with a constant-temperature function, a circular-type gas distributor, a quartz tube, a circulating cooling water device, and a fluorescent lamp. The volume of glass reactor (i.d. 100 mm) was 2 L and the power of fluorescent lamp (FSL) employed was 11 W (YDN11-␲.RR, Foshan Electrical and Lighting Co., Ltd., China). The circulating water device was employed to take away the heat produced by FSL. The analytical system, including a VM-3000

80

A. Zhang et al. / Journal of Hazardous Materials 314 (2016) 78–87

Fig. 1. Schematic diagram of the experimental setup.

 = (1−C out /C in ) × 100

(6)

100

Hg0 removal efficiency (%)

online measuring mercury analyzer (Mercury Instruments GmbH, Germany) and a KM9106 flue gas analyzer (Kane International Limited, U.K.), was used to monitor the Hg0 vapor concentration and the concentrations of SO2 and NO. Moreover, to protect the detection cell of the mercury analyzer, one bottle of NaOH solution (20 wt%) and a low-temperature (about −10 ◦ C) tank were used to absorb acid gases and to condensate water vapor carried by mixed gas from the bubbling reactor, respectively. The total gas flow of simulated flue gas was approximately 1.50 L/min. The inlet Hg0 concentration (Cin ) was about 55 ␮g/m3 . 0.80 g of photocatalyst and 1 L of deionized water were employed as reaction solution. During each test, the Hg0 gas first bypassed the bubbling reactor and then was introduced into the photocatalytic system until a desired Hg0 concentration had been established for about 20 min. To ensure the accuracy, the mercury analyzer was calibrated before each test. The Hg0 (or SO2 /NO) removal efficiency () was calculated by the following equation:

90 80 Ag2CO3 Ag/Ag2CO3

70

Ag/AgBr(0.1)-Ag2CO3 Ag/AgBr(0.3)-Ag2CO3

60

Ag/AgBr(0.5)-Ag2CO3 Ag/AgBr(0.7)-Ag2CO3 Ag/AgBr(0.9)-Ag2CO3

50

Ag/AgBr

0

10

20

30

40

50

60

Reaction time (min) Fig. 2. Effect of AgBr content on Hg0 removal efficiency under BL condition.

3. Results and discussion 3.1. Photocatalytic activity

where  corresponds to the Hg0 (or SO2 /NO) removal efficiency (%), and where Cin and Cout represent the Hg0 (or SO2 /NO) concentrations corresponding to the influent and effluent (␮g/m3 ), respectively.

2.3. Photocatalyst characterization N2 adsorption-desorption, scanning electron microscope (SEM), X-ray diffraction (XRD), UV–vis diffused reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) techniques were used to characterize the photocatalysts. Detailed information is given in the Supplementary material.

3.1.1. Effect of AgBr content and stability The photocatalytic activities of Ag/AgBr-Ag2 CO3 hybrids are shown in Fig. 2. It was clear that the ternary Ag/AgBr(X)-Ag2 CO3 and binary Ag/Ag2 CO3 and Ag/AgBr hybrids all displayed much better photocatalytic performances than that of Ag2 CO3 . This was attributed to the SPR effect of Ag0 NPs under visible light [36,37]. Moreover, the Hg0 removal efficiency over ternary Ag/AgBr(X)Ag2 CO3 (X = 0.1, 0.3, 0.5, 0.7) hybrids increased with the increasing of AgBr content, and then decreased when the AgBr content rose to 0.9. Thus the optimal molar ratio X of AgBr/Ag2 CO3 was 0.7, and the highest photocatalytic activity was about 93%. Two reasons can be responsible for the promotional activity. One was due to the formation of heterojunction between AgBr and Ag2 CO3 , which could effectively decrease the recombination rate of electron–hole

A. Zhang et al. / Journal of Hazardous Materials 314 (2016) 78–87

81

Hg0 removal efficiency (%)

Ag2CO3 Ag/AgBr(0.7)-Ag2CO3

100 80 60 40 20 0

3 rd

2 nd

1 st

Fig. 3. Hg0 removal efficiency of consecutive experiments by Ag2 CO3 and Ag/AgBr(0.7)-Ag2 CO3 under BL condition. Error bars represent standard deviation of means, n = 3.

FSL+Ag/AgBr(0.7)-Ag2CO3 only with Ag/AgBr(0.7)-Ag2CO3

80 Hg0 removal efficiency (%)

Hg0 removal efficiency (%)

100

60 40 20 0

100 90

10

20

Light off

80 Light on

70

Light on

60 50 0

FSL

0

Light off

30

20

40 60 80 100 Reaction time (min)

40

50

60

Reaction time (min)

Fig. 5. Effect of solution temperature on Hg0 removal efficiency under BL condition.

nism might be taken place even in the absence of FSL irradiation as follows: 4AgBr + O2 → 2Ag2 O + 2Br2

(7)

Hg0 + Br2 → HgBr2

(8)

Besides, the switch light experiment was also carefully performed. As shown from the inset of Fig. 4, when the FSL was turned off, the Hg0 removal efficiency over Ag/AgBr(0.7)-Ag2 CO3 slowly dropped from 92% to 70% in 25 min, but once the FSL was turned on, the Hg0 removal efficiency quickly increased to 90% within 10 min. The Hg0 removal efficiency had no obvious decrease after another repeated test, indicating that the ternary Ag/AgBr(0.7)Ag2 CO3 hybrid was stable under visible light and the presence of visible light was very necessary for higher Hg0 removal.

(e− –h+ ) pairs [32,34]. The other one might be that when the amount of AgBr increased to 0.7, much more Ag0 NPs were formed on the surface of photocatalyst, which would trap the excited electrons and hence facilitate the separation of photoinduced electrons and holes [36,38]. A similar phenomenon was also found during the photocatalytic decomposition of Rhodamine B under visible light over AgBr-BiOBr [39]. The stability of a novel photocatalyst is crucial for its possible commercial application. Thus the stability tests of Ag2 CO3 and optimal Ag/AgBr(0.7)-Ag2 CO3 on photocatalytic oxidation of Hg0 were also investigated. From Fig. 3, it was clear that after three consecutive cycles, the Hg0 removal efficiency over Ag2 CO3 decreased from 80% to 73%, while the Hg0 removal efficiency over Ag/AgBr(0.7)-Ag2 CO3 hybrid showed little changes. The excellent and stable photocatalytic activity of Ag/AgBr(0.7)-Ag2 CO3 hybrid on Hg0 removal could be owed to the formation of heterojunction between AgBr and Ag2 CO3 [32,34].

3.1.3. Effect of solution temperature It has been known that the solution temperature has significant influences on gas solubility, mass rate and chemical reaction rate. Although increasing the temperature of reaction solution would enhance the chemical reaction rate by lowering the activation energy [26] according the famous Arrhenius equation, sometimes higher solution temperature would show a negative effect. Fig. 5 shows the effect of reacting solution temperature on Hg0 removal efficiency over Ag/AgBr(0.7)-Ag2 CO3 under FSL irradiation. When the reacting solution temperature increased from 25 ◦ C to 65 ◦ C, the Hg0 removal efficiency decreased from 97% to 78%, suggesting that increasing the solution temperature was not favor to remove Hg0 . Similar results were reported by Zhan et al. [7] and Zhou et al. [25]. The reason might be that with the increase of solution temperature, the solubility of gas-phase O2 would decrease in solution, which led to lower content of reactive species (• O2 − ) produced. Furthermore, the energy barrier of a reaction required to overcome was relatively low [25], a further increase of reaction temperature would slightly result in less photoinduced electrons and holes produced under visible light, thereby reducing the Hg0 removal efficiency.

3.1.2. Effect of FSL irradiation The effect of FSL irradiation on Hg0 removal efficiency was studied by optimal Ag/AgBr(0.7)-Ag2 CO3 . As shown in Fig. 4, only 12% of Hg0 removal efficiency appeared under FSL irradiation. However, when the Ag/AgBr(0.7)-Ag2 CO3 was employed in combination with a FSL, much higher Hg0 removal efficiency (93%) was obtained. It should be noted that due to a lower BET surface area of Ag/AgBr(0.7)-Ag2 CO3 (vide infra), the superior performance would not be physical adsorption but chemical reaction. Zhuang et al. [40] indicated that iron bromide (FeBr2 ) can react with O2 to form ferric oxide and free bromine (Br2 ), therefore, a similar reaction mecha-

3.1.4. Effect of SO2 without and with Ca(OH)2 addition Coal-fired flue gas often inevitably contains a small amount of SO2 and NO though the existing air pollution control technologies, such as wet flue gas desulfurization (WFGD) and selective catalytic reduction (SCR) de-NOx , are employed. Thus, it is important to investigate the effect of SO2 and NO on Hg0 removal over the novel photocatalyst. As demonstrated in Fig. 6, when 600 ppm of SO2 was introduced into the reaction solution, Hg0 removal efficiency of Ag/AgBr(0.7)-Ag2 CO3 slowly decreased from 92% to 85% in 15 min due to the slow decrease of reactive species, then sharply dropped to 20% in 5 min because of the exhaustion of reactive species, and

Fig. 4. Effect of FSL irradiation on Hg0 removal efficiency under BL condition.

82

A. Zhang et al. / Journal of Hazardous Materials 314 (2016) 78–87

100

100

Hg0 removal efficiency (%)

Hg0 removal efficiency (%)

NO on

80 SO2 on SO2 on

60 40

SO2 off

SO2 off

20 0

Ag/AgBr(0.7)-Ag2CO3 Ag/AgBr Ag/AgBr(0.7)-Ag2CO3 + Ca(OH)2

0

20

40

60

80

100

120

140

NO on

80 NO off

NO off

60 40 20

Ag/AgBr(0.7)-Ag2CO3

160

Ag/AgBr(0.7)-Ag2CO3 + Ca(OH)2

Reaction time (min)

0

Fig. 6. Effect of SO2 on Hg0 removal efficiency over Ag/AgBr(0.7)-Ag2 CO3 and Ag/AgBr in the absence and presence of Ca(OH)2 .

0

20

40

60

80

100

120

140

160

Reaction time (min) Fig. 7. Effect of NO on Hg0 removal efficiency in the absence and presence of Ca(OH)2 over Ag/AgBr(0.7)-Ag2 CO3 .

afterwards tardily rose to 50% in 10 min owing to the recovery of some reactive species. Once the SO2 gas was turned off, the Hg0 removal efficiency quickly returned to its original level in 10 min, suggesting a poor nature of the Ag/AgBr(0.7)-Ag2 CO3 photocatalyst on SO2 resistance. Another similar manner was performed in the 100th minute to guarantee the repeatability of the data. This might be due to the reaction between Ag2 CO3 and H2 SO4 . Moreover, to validate this speculation, a similar test was carried out by using Ag/AgBr as photocatalyst. As displayed in Fig. 6, compared with Ag/AgBr(0.7)-Ag2 CO3 , the effect of SO2 over Ag/AgBr exhibited little effect on Hg0 removal, which successfully confirmed the above ratiocination. In coal-fired power plant, SO2 is widely removed by wet flue gas desulfurization (WFGD) using Ca(OH)2 as adsorbent, therefore it is appropriate to eliminate the effect of SO2 by adding a small amount of Ca(OH)2 powder into the reaction solution, which had been adopted in the experiments by other researchers [26,41]. The effect of SO2 on Hg0 removal was performed in the presence of 1 g Ca(OH)2 and the corresponding result is also given in Fig. 6. It can be clearly seen that although SO2 gas was continuously introduced into the bubbling reactor from 40 to 160 min, almost no decrease in Hg0 removal efficiency occurred, implying that the addition of Ca(OH)2 exhibited very little impact on Hg0 removal efficiency. We knew that SO2 can be absorbed by Ca(OH)2 to produce CaSO3 and further be oxidized to CaSO4 precipitation by O2 , thus its inhibitive effect on Hg0 removal was restrained.

3.1.5. Effect of NO without and with Ca(OH)2 addition The effect of NO on Hg0 removal efficiency in the absence and presence of Ca(OH)2 is shown in Fig. 7. Obviously, when 300 ppm of NO was introduced in the ranges of 40–70 min and 100–130 min, the Hg0 removal efficiency over Ag/AgBr(0.7)-Ag2 CO3 slowly decreased. Once NO gas was turned off, the Hg0 removal efficiency slowly rose to the initial values. A similar result also occurred when 1 g of Ca(OH)2 was added into the reaction solution. The slightly inhibition can be explained by the following reaction equations [42]: NO + 3• OH → NO3 − + H2 O + H+

(9)

NO + • O2 − → 2NO3 −

(10)

2NO + 2 h+ + O2 + 2H2 O → 2NO3 − + 4H+

(11)

It was clear that some reactive species (• OH, • O2 − and h+ ) was consumed by NO to generate stable nitrate.

Table 1 Physical properties of the samples. Sample

BET surface area (m2 /g)

Total pore volume (cm3 /g)

Average pore diameter (nm)

Ag/Ag2 CO3 Ag/AgBr(0.7)/Ag2 CO3 Ag/AgBr

2.27 1.09 0.79

9.64 × 10−3 2.96 × 10−3 2.13 × 10−3

16.81 9.80 9.16

3.2. Characterization of photocatalysts 3.2.1. N2 adsorption-desorption and SEM The physical properties of three selective samples are listed in Table 1. The BET surface area, total pore volume, and average pore diameter of Ag/Ag2 CO3 hybrid were 2.27 m2 /g, 9.64 × 10−3 cm3 /g, and 16.81 nm, respectively. With the AgBr content increasing, the three parameters all decreased, indicating the agglomeration and microspore block of Ag2 CO3 . The typical SEM images of as-prepared samples are presented in Fig. 8. It can be observed that the polyhedral structure particle was Ag2 CO3 material (see blue square marks, Fig. 8a). On the smooth surface of Ag2 CO3 particle, some granules were detected (see red annular marks), corresponding to Ag0 NPs reduced by EG agent. As shown in Fig. 8b, the as-prepared Ag/AgBr(0.7)-Ag2 CO3 composites were irregular and agglomerated. The formed AgBr can be evidently observed on the surface of Ag2 CO3 particle. Moreover, significant agglomerations appeared on Ag/AgBr material, which was in accordance with the result of BET analysis. Also, it should be pointed out that Ag0 NPs reduced by EG reagent in Ag/AgBr(0.7)-Ag2 CO3 was highly dispersed and bigger than that in Ag/Ag2 CO3 and Ag/AgBr, which may be significant for its higher activity of Hg0 removal. 3.2.2. XRD and DRS Fig. 9 presents the typical XRD pattern of three hybrids. In Fig. 9a, almost all the diffraction peaks can be assigned to monoclinic phase Ag2 CO3 (JCPDS 26-0339) [36]. A weak diffraction peak at 38.1◦ was attributed to the (111) facet of the face-centered cubic phase Ag (JCPDS 65-2871) [43] due to the addition of reducing agent. Seven major diffraction peaks coinciding with the standard face-centered cubic AgBr phase (JCPDS 06-0438) [44] were clearly detected in Ag/AgBr (Fig. 9c). While the characteristic diffraction peak of Ag0 was not observed in Ag/AgBr due to the high dispersion or low content of Ag0 NPs [45,46]. The XRD pattern of Ag/AgBr(0.7)-Ag2 CO3 photocatalyst exhibited a coexistence pattern of both Ag/Ag2 CO3 and Ag/AgBr materials (Fig. 9b), which indicated that Ag/AgBr(0.7)Ag2 CO3 photocatalyst was successfully synthesized by the simple modified co-precipitation method.

A. Zhang et al. / Journal of Hazardous Materials 314 (2016) 78–87

83

Fig. 8. SEM images of (a) Ag/Ag2 CO3 , (b) Ag/AgBr(0.7)-Ag2 CO3 , and (c) Ag/AgBr.

0.7 Ag/AgBr(0.7)-Ag2CO3

Absorption (a.u.)

0.6 Ag plasmon absorption

0.5

Ag/Ag2CO3

Ag/AgBr

0.4

0.3 200

300

400

500

600

700

800

Wavelength (nm) Fig. 9. XRD pattern of the photocatalysts.

Fig. 10. UV–vis DRS spectra of the samples.

the amount of Ag0 NPs formed on Ag/AgBr(0.7)-Ag2 CO3 was much higher than that in other two hybrids. The UV–vis diffuse reflectance spectra (DRS) taken from three hybrids are presented in Fig. 10. The three photocatalysts all showed absorption bands in the visible light region ( ≥ 420 nm). Ag/AgBr sample displayed the strongest absorption in the ultraviolet light region ( < 420 nm), while its absorption intensity was the lowest in the visible light region. Ag/AgBr(0.7)-Ag2 CO3 hybrid exhibited a much more intense absorption in the visible light region. Moreover, in comparison with Ag/Ag2 CO3 and Ag/AgBr, Ag/AgBr(0.7)-Ag2 CO3 showed a strong absorption response from 450 to 600 nm (see the shadow region), which can be attributed to the localized SPR effect of Ag0 NPs [36,37]. This also suggested that

3.2.3. XPS and ESR Fig. 11 presents the XPS spectra of three photocatalysts. The samples were evidently composed of Ag, Br, C, and O elements (Fig. 11a). The C 1s spectra of Ag/Ag2 CO3 displayed two obvious peaks at 284.7 and 288.7 eV, while Ag/AgBr(0.7)-Ag2 CO3 sample showed a distinct peak at 284.7 eV and a smaller peak at 288.7 eV (Fig. 11b). For Ag/AgBr, only an apparent peak at 284.7 eV appeared. The peak at 284.7 eV can be ascribed to adventitious contaminants from the environment [47], while the peak at 288.7 eV was consistent with the characteristic peak of carbon from CO3 2−

A. Zhang et al. / Journal of Hazardous Materials 314 (2016) 78–87

(b) C 1s

Intensity (a.u.)

O KLL

284.7 eV

Ag/Ag2CO3

Ag 3p

Br 3s

Br 3p

Ag 3s

O 1s Ag 3p

(a) survey spectra

C 1s

Ag 4d Br 3d

Intensity (a.u.)

Ag 3d

84

Ag/AgBr(0.7)-Ag2CO3

288.7 eV Ag/AgBr

Ag/AgBr(0.7)-Ag2CO3 Ag/Ag2CO3

Ag/AgBr

0

200

400

600

800

1000

282

285

Binding energy (eV)

288

291

294

Binding energy(eV)

530.8 eV

(c) O 1s

(d) Br 3d Br 3d 5/2

532.8 eV

Intensity (a.u.)

Intensity (a.u.)

Br 3d3/2

Ag/Ag2CO3

Ag/AgBr

Ag/AgBr(0.7)-Ag2CO 3

Ag/AgBr(0.7)-Ag2CO3

Ag/AgBr

528

530

532

534

536

538

66

68

Binding energy (eV)

70

72

74

Binding energy (eV)

(e) Ag 3d

Ag 3d5/2 Ag 3d3/2 0

Ag

Intensity (a.u.)

Ag/Ag2CO3 +

Ag

Ag/AgBr(0.7)-Ag2CO3

Ag/AgBr

366

368

370

372

374

376

378

Binding energy (eV) Fig. 11. XPS spectra of as-synthesized Ag/Ag2 CO3 , Ag/AgBr(0.7)-Ag2 CO3 and Ag/AgBr: (a) the survey spectra, (b) C 1s, (c) O 1s, (d) Br 3d, and (e) Ag 3d.

[28,32,48]. From Fig. 11c, it can be observed that the XPS spectra of O 1s for Ag/Ag2 CO3 and Ag/AgBr(0.7)-Ag2 CO3 presented an intense peak and a small peak at 530.8 eV, respectively, which corresponded to the oxygen from CO3 2− . Whereas Ag/AgBr(0.7)-Ag2 CO3 and Ag/AgBr materials all exhibited another peak at 532.8 eV corresponding to adsorbed hydroxyl groups (or water) from the environment [47,49]. The binding energies at 68.7 eV and 69.7 eV belonged to Br 3d5/2 and Br 3d3/2 (Fig. 11d), respectively [50]. In Fig. 11e, the Ag 3d5/2 and Ag 3d3/2 peaks can be further deconvoluted into four peaks at about 367.6, 368.4, 373.6, and 374.4 eV, respectively. According to the literatures [31,51], the peaks at 367.6 and 373.6 eV were attributed to Ag+ , whereas the peaks at 368.4 and 374.4 eV were assigned to Ag0 . The XPS analysis of the Ag 3d spectra confirmed the existence of Ag0 , which was in agreement with the results of SEM and DRS. It was accepted that the dyes and organic pollutants can be successfully degraded via photocatalytic oxidation (PCO) process driven by visible light [30–37]. The photoinduced e− –h+ pairs are separated and then migrate to the surface of photocatalyst, which

will be trapped by the absorbed O2 and H2 O to form reactive species, such as • O2 − and • OH. To prove the existence of the reactive radicals, the ESR technique was employed to detect • O2 − and • OH radicals. As shown in Fig. 12, there was no ESR signals in the dark, but a gradually increasing evolution of ESR signals of DMPO• O − and DMPO-• OH adducts was detected with the time of visible 2 light irradiation increasing, which was consistent with the former studies for • O2 − and • OH [52,53]. The ESR results confirmed that visible light illumination was an essential factor for the presence of • O − and • OH. 2 3.3. Possible photocatalytic mechanism To further ascertain the main reactive species in Hg0 removal over Ag/AgBr(0.7)-Ag2 CO3 hybrid, several scavengers were employed to quench the relevant reactive species during the photocatalytic process. Benzoquinone (BQ) [54,55] and isopropyl alcohol (IPA) [52] were added as • O2 − and • OH scavengers, respectively, whereas the ethylenediamine teraacetic acid disodium salt

A. Zhang et al. / Journal of Hazardous Materials 314 (2016) 78–87

(a)

(b)

Irradiated 6 min

Irradiated 6 min

Irradiated 4 min

Irradiated 4 min

Irradiated 2 min

Irradiated 2 min

In dark

318.3

318.4

85

In dark

318.5

318.6

318.7

318.8

318.9 318.4

318.5

Magnetic (mT)

318.6

318.7

318.8

Magnetic (mT)

Fig. 12. ESR spectra for (a) • O2 − and (b) • OH radicals of Ag/AgBr(0.7)-Ag2 CO3 hybrid.

80

60

40

0

Hg removal efficiency (%)

100

20

0

Without

BQ

IPA

EDTA-2Na

Scavenger Fig. 13. Effect of scavengers on Hg0 removal over Ag/AgBr(0.7)-Ag2 CO3 .

(EDTA-2Na) [33] was employed to scavenge holes (h+ ). The dosages of BQ, IPA, and EDTA-2Na were 1.0 g, 10 mL, and 2 g, respectively. Fig. 13 displays that the additions of BQ and IPA had much little effects on Hg0 removal, while when EDTA-2Na was added, a more predominant inhibition appeared. It can be concluded that compared with • O2 − and • OH, the holes (h+ ) should be the main reactive species of Ag/AgBr(0.7)-Ag2 CO3 photocatalyst for higher Hg0 removal. Based on the above experimental results and characterization, the proposed reaction mechanism in Ag/AgBr(0.7)-Ag2 CO3 photocatalytic system for Hg0 removal is schematically illustrated in Fig. 14, and the possible reactions during photocatalytic process are proposed by Eqs. (12)–(27). According to the previous report [31,32], the conduction band (CB) and valence band (VB) edges of AgBr (2.55 eV and 0.07 eV) are lower than that of Ag2 CO3 (2.67 eV and 0.37 eV), respectively. Under visible light, Ag2 CO3 , AgBr and Ag0 NPs can be simultaneously excited to generate photoinduced electron–hole (e− –h+ ) pairs (Eqs. (12)−(14)). Due to the suitable CB and VB energy levels of Ag2 CO3 and AgBr, the photogenerated electrons transfer to Ag2 CO3 while holes transfer to AgBr (Eqs. (15) and (16)), thus, the electrons and holes can be effectively separated. Additionally, due to the excellent conductivity of Ag0 NPs, partially photogenerated electrons can be transferred from the CB bottom of AgBr to Ag0 NPs (Eq. (17)) so as to enhance the interfacial charge transfer and inhibit the recombination of e− –h+ pairs efficiently [56]. Moreover, because the SPR effect of Ag0 NPs induced local electromagnetic field, the separation efficiency of photogenerated charge carriers on Ag0 NPs can be enhanced [37,57,58].

The photogenerated electrons in the CB bottom of Ag2 CO3 cannot be trapped by adsorbed O2 to generate • O2 − reactive species because the CB edge potential of Ag2 CO3 (0.37 eV vs SHE) is more positive than the single electron reduction potential of oxygen (E0 (O2 /• O2 − ) = −0.046 eV vs SHE) [31]. Whereas plenty of electrons gathered on the surface of Ag0 NPs can be captured by absorbed O2 to form • O2 − (Eq. (18)). The reactive holes generated at the VB of AgBr and on Ag0 NPs surface can combine with H2 O, OH− , and Br− to generate • OH and Br0 atoms (Eqs. (19)−(22)), respectively [57]. In addition, other active radicals (• HO2 ) also appeared and participated in the chemical reaction (Eqs. (23) and (24)). From the photoelectronchemistry point of view, all of • OH, • O2 − , h+ and Br0 , were reactive species responsible for the superior photocatalytic activity of Hg0 removal (Eqs. (25) and (26)). Among these reactive species, the h+ could play a key role in Hg0 removal. In addition, as Br0 atoms were powerful radical species, they could oxidize Hg0 and hence were reduced to Br− ions again (Eq. (27)) [57]. Ag2 CO3 + h → Ag2 CO3 (e− + h+ ) −

+

(12)

AgBr + h → AgBr(e + h )

(13)

Ag0 NPs + h → AgNPs∗ → Ag(e− + h+ )

(14)

AgBr(e− ) + Ag2 CO3 → Ag2 CO3 (e− ) + AgBr

(15)

Ag2 CO3 (h+ ) + AgBr → AgBr(h+ ) + Ag2 CO3

(16)

AgBr(e− ) + Ag0 NPs → AgBr + Ag0 NPs(e− )

(17)

Ag0 NPs(e− ) + O2 → • O2 −

(18)

AgBr(h+ ) + H2 O → H+ + • OH

(19)

Ag0 NPs(h+ ) + H2 O → H+ + • OH

(20)

AgBr(h+ ) + Br− → Br0

(21)

Ag0 NPs(h+ ) + Br− → Br0

(22)

•O − 2

(23)

+ H+ → • HO2

2e− + • HO2 + H+ → • OH + OH−

(24)

• OH, • O − , h+ 2

(25)

+ Hg0 → Hg2+

2Br0 + Hg0 → Hg2+ + 2Br− 0

−

Br + OH →

• OH

+ Br

−

(26) (27)

3.4. Simultaneous removal of Hg0 /SO2 /NO The above experiments had been carried out to investigate the influences of SO2 and NO on Hg0 removal in the absence and presence of Ca(OH)2 . But the performance of Hg0 removal with

86

A. Zhang et al. / Journal of Hazardous Materials 314 (2016) 78–87

Fig. 14. Schematic illustration for the charge transfer in the Ag/AgBr(0.7)-Ag2 CO3 system.

4. Conclusions

Removal efficiency (%)

100 80

60 40 0

Hg SO2

20

0

NO

0

30

60

90

120

150

180

Reaction time (min) Fig. 15. Simultaneous removal of Hg0 /SO2 /NO with Ag/AgBr(0.7)-Ag2 CO3 and Ca(OH)2 .

both NO and SO2 presented in simulated flue gas was not studied. More importantly, due to the addition of Ca(OH)2 and presence of large amounts of active radicals, further investigation about simultaneous removal of Hg0 , SO2 and NO was much meaningful for the commercial application of the novel method. As exhibited in Fig. 15, SO2 removal efficiency was nearly 98%, and Hg0 removal efficiency was about 90%. However, NO removal efficiency was only 40%, far less than that of SO2 and Hg0 because of its inert nature and insolubility in water. It was evident that Ag/AgBr(0.7)Ag2 CO3 photocatalyst accompanied with Ca(OH)2 showed much excellent performance on simultaneous removal of SO2 and Hg0 . In view of the fact that the current mature SCR technology was effective to remove NOx and was widely adopted by coal-fired power plants, therefore, the integration of desulfurization and Hg0 removal driven by visible light or even sunlight could be very promising in the future.

The ternary Ag/AgBr-Ag2 CO3 hybrids were successfully synthesized by a simple modified co-precipitation method. The Hg0 removal efficiency over the ternary Ag/AgBr(X)-Ag2 CO3 (X = 0.1, 0.3, 0.5, 0.7, 0.9) hybrids increased with the increasing of AgBr content, whereas the photocatalytic activity decreased when the AgBr content rose to 0.9. The optical molar ratio X of AgBr/Ag2 CO3 was 0.7. The Hg0 removal efficiency over Ag/AgBr(0.7)-Ag2 CO3 hybrid can reach as high as 93%. The enhanced activity for Hg0 removal can be well explained by the synergistic effect of Ag/AgBr plasmonic photocatalysis and Ag2 CO3 semiconductor photocatalysis. An excellent performance of simultaneous removal of SO2 and Hg0 over Ag/AgBr(0.7)-Ag2 CO3 and Ca(OH)2 under FSL irradiation provides a new insight into multi-pollutants simultaneous control from coal-fired flue gas. Acknowledgments This study was financially supported by the National Natural Science Foundation of China (No. 51306046, 51376073, and 21176098), and the Fundamental Research Funds for the Universities of Henan Province (No.NSFRF140204) and the Doctoral Fund of Henan Polytechnic University (No.B2011-089). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2016.04. 032. References [1] A.P. Dastoor, Y. Larocque, Global circulation of atmospheric mercury: a modelling study, Atmos. Environ. 38 (2004) 147–161. [2] E.G. Pacyna, J.M. Pacyna, K. Sundseth, J. Munthe, K. Kindbom, S. Wilson, F. Steenhuisen, P. Maxson, Global emission of mercury to the atmosphere from anthropogenic sources in 2005 and projections to 2020, Atmos. Environ. 44 (2010) 2487–2499.

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