Journal of Hazardous Materials 289 (2015) 235–243
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Enhanced photocatalytic oxidation of gaseous elemental mercury by TiO2 in a high temperature environment Huazhen Shen a , Iau-Ren Ie a , Chung-Shin Yuan a,∗ , Chung-Hsuang Hung b , Wei-Hsiang Chen a , Jinjing Luo c , Yi-Hsiu Jen a a
Institute of Environmental Engineering, National Sun Yat-Sen University No. 70, Lian-Hai Road, Kaohsiung 804, Taiwan Department of Safety, Health and Environmental Engineering, National Kaohsiung First University of Science and Technology No. 2, Juoyue Road, Nantz District, Kaohsiung 811, Taiwan c College of the Environmental & Ecology, Xiamen University, Xiamen, Fujian, PR China b
h i g h l i g h t s • • • •
The photo-oxidation efficiency of Hg0 by TiO2 at high temperatures was investigated. The irradiation of 254 nm promoted the photo-oxidation efficiency of TiO2 at high temperatures. The best calcination of TiO2 for photo-oxidation of Hg0 was 400 ◦ C. Increasing irradiation strength enhanced the photo-oxidation of Hg0 .
a r t i c l e
i n f o
Article history: Received 2 October 2014 Received in revised form 21 December 2014 Accepted 11 February 2015 Available online 12 February 2015 Keywords: Photo-oxidation of Hg0 Titanium dioxide Calcination temperature Irradiation strength Oxygen content
a b s t r a c t The photo-oxidation of Hg0 in a lab-scale reactor by titanium dioxide (TiO2 ) coated on the surface of glass beads was investigated at high temperatures. TiO2 was calcinated at four different temperatures of 300 ◦ C, 400 ◦ C, 500 ◦ C and 600 ◦ C (noted as Ti300, Ti400, Ti500 and Ti600) and characterized for its physicochemical properties. The calcinated TiO2 coating on the glass beads was then tested to compare the photo-oxidation efficiencies of Hg0 with an incident light of 365 nm. The results showed that the oxidation efficiencies of Hg0 for Ti400 and Ti500 were higher than those of Ti300 and Ti600. To enhance the photo-oxidation efficiency of Hg0 , Ti400 was selected to examine the wave lengths () of 254 nm, 365 nm and visible light with various influent Hg0 concentrations. The effects of irradiation strength and the presence of oxygen on the photo-oxidation efficiency of Hg0 were further investigated, respectively. This study revealed that the wave length () of 254 nm could promote the photo-oxidation efficiency of Hg0 at 140 and 160 ◦ C, while increasing the influent Hg0 concentration and could enhance the photooxidation rate of Hg0 . However, the influence of 5% O2 present in the flue gas for the enhancement of Hg0 oxidation was limited. Moreover, the intensity of the incident wave length of 365 nm and visible light were demonstrated to boost the photo-oxidation efficiency of Hg0 effectively. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Due to the toxicity effect on ecological safety and human health, mercury and its derivatives have attracted more attention and international co-operation from governmental and Non-Governmental Organizations (NGO) all over the world. The International Minamata Convention already signed an agreement on strictly limiting mercury emission on October 9, 2013 [1].
∗ Corresponding author. Tel.: +886 7 5252000x4409; fax: +886 7 52524409. E-mail address: [email protected] (C.-S. Yuan). http://dx.doi.org/10.1016/j.jhazmat.2015.02.033 0304-3894/© 2015 Elsevier B.V. All rights reserved.
According to the convention, the new treaty will control mercury emissions from various large industrial facilities, such as coal-fired power plants and industrial boilers to certain kinds of smelter handling, for example zinc and gold processing. The gaseous elemental mercury (Hg0 ) emission from coal-fired power plants in particular will be restricted. To date, most coal-fired power plants have not installed independent mercury removal device(s), however, the existing particle or acid gas removal devices are found to eliminate the elemental mercury (Hg0 ) or mercury containing compounds (Hg2+ and Hg+ ) [2–5]. The removal efficiency of these mercury containing chemicals vary greatly by air pollution control devices, such as selective catalytic reduction (SCR), electrostatic
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precipitator (ESP) and wet flue gas desulfurization (WFGD) [6]. It is more efficient to capture Hg2+ than Hg0 in flue gas with solubility of 20 g/L and 60 g/L, respectively [7–9]. The conventional methods to remove Hg0 include injecting activated carbon, zeolite, fly ash, metal oxide and their corresponding modified form. TiO2 -based photocatalysis has been investigated since Fujishima and Honda’s first report in the early 1970s on UVinduced redox chemistry on TiO2 . The researches on TiO2 application have flourished in green chemistry for a long time, including photocatalytic water splitting, hydrogen production, dye sensitization, solar energy conversion and photochemical air and water treatments. The effectiveness and mechanism on the reduction of Hg0 emission by TiO2 -based technology have caused great concern due to the wide application of commercial TiO2 . The photo-oxidation of mercury by TiO2 has been demonstrated to be an excellent method [10], due to its high removal efficiency, lower cost than activated carbons injection, which could be great potential to exploit in the coal-fired power plants. However, there remain difficulties of applying the photo-oxidation by TiO2 in fire power plant to be solved by the environmental experts [10], among which the high temperature of the ESP ranging from 100 to 180 ◦ C [5] that deteriorated the photo oxidation efficiency seriously and yet received less attention. An abundance of research has focused on the photo-oxidation of Hg0 at temperatures lower than 100 ◦ C during which the photooxidation efficiency and mechanism of Hg0 by TiO2 have been comprehensively investigated. The gas–solid photo-oxidation reaction process includes inter-media adsorption and photo-oxidation reaction. Adamson pointed that [11], the adsorption rate constant of kad is inversely proportional to the square root of temperature (T) and the desorption constant of kde increase exponentially as the temperature increase. Therefore, the adsorption equilibrium of K decrease as the temperature increase, which is not favor of the reaction between the gas–solid reactions. However, very few studies have paid attention to the performance of TiO2 at temperatures between 100 and 200 ◦ C systematically upon the photo-oxidation of Hg0 at ESP, which is the only irradiation source in the coalfired power plants. At room temperature, it becomes more effective on the photo-oxidation efficiency of Hg0 under UV or visible irradiation [12–18]. It has been observed that the photo-oxidation efficiency of Hg0 by TiO2 under fluorescent light reaches up to 90% in a lab-scale control system and could be maintained at 90% when conducted in a pilot-scale control system with a flow rate of 30 m3 /h [19,20]. The rate constant (k) of a photo-oxidation reaction by TiO2 was 7.4 ± 2.5 × 1014 molecules/(min cm2 ) at 24 ± 2 ◦ C in a batch reactor [12]. However, Lee et al. [15] observed that, at temperatures below 80 ◦ C, the overall rate of the initial mercury (Hg0 ) uptake increases as the temperature increases, while at 110 and 130 ◦ C, the increase in temperature results in a decrease in the overall reaction rate, indicating a relatively low oxidation efficiency of Hg0 at high temperatures. Thus, the investigation on the enhancement of Hg0 photo-oxidation at high temperatures is crucial. The photochemical mechanism shows that hydroxyl radicals (OH• ) are very reactive and originate from the dissociation of H2 O oxidation by the photo-generated holes, which could oxidize Hg0 to HgO on the surface of photo-irradiated TiO2 [12,13]. Since O2 and H2 O are trapped by photo-generated electrons and holes to form oxidants O2 − and OH• , the addition of O2 and H2 O seems to be vital for the photo-oxidation of Hg0 [21,22]. However, Tsai et al. found that the enhancement is limited to the addition of 6% O2 in a photocatalytic reaction system, which suggests that O2 − is unimportant for the oxidation of Hg0 on the surface of TiO2 under UV irradiation [23]. The effect of O2 seems insignificant on the efficient improvement for photo-oxidation of Hg0 . In this study, the photo-oxidation efficiency of Hg0 by TiO2 calcinated at four different temperatures was compared to select
Fig. 1. Schematic diagram of the experimental setup.
the most effective photocatalysts. Moreover, in order to investigate the effect of reaction temperatures higher than 100 ◦ C on photo-oxidation, five different influent Hg0 concentrations (20–100 g/m3 ) and incident lights with a wavelength () of 254 nm, 365 nm and 370–720 nm (visible light) were selected for the Hg0 oxidation experiments. The reaction rate and photooxidation mechanism dependent on reaction temperature were further discussed to ascertain the photo-oxidation efficiency of Hg0 at high temperatures of 100–120 ◦ C. Finally, the effects of O2 and irradiation light strengthened on the photo-oxidation efficiency of Hg0 were investigated, respectively. 2. Experimental methods 2.1. Preparation and characterization of photocatalysts The photocatalysts used for this particular study were prepared by coating the calcinated TiO2 onto the surface of 2.6 mm glass beads based on our previous description [24]. The glass beads were rinsed in 0.1 M sodium hydroxide solution and further neutralized in sulfuric acid. The glass beads were cleaned to remove residual chemicals in DI water twice and then dried. Prior to coating, the glass beads were heated at 150 ◦ C for 30 min on standby. TiO2 was dissolved at 1 g of Degusa-P25 to 10 mL DI water and stirred for 30 min at 60–70 ◦ C. The TiO2 solution was then sprayed onto the heated glass beads. After being dried at 100 ◦ C for 30 min, the TiO2 coated glass beads were further heated to 300 ◦ C, 400 ◦ C, 500 ◦ C and 600 ◦ C for 4 h and denoted as Ti300, Ti400, Ti500 and Ti600, respectively. The characterization of TiO2 was performed by the following methods. The BET specific surface area of TiO2 powder was carried out by a nitrogen adsorption apparatus at 77 K (Micromeritics, ASAP 2020). Each sample was outgassed at 105 ◦ C prior to BET measurement. The X-ray diffraction (XRD) pattern was recorded on an X-ray Diffractometer (Siemens D5000) using CuK␣ radiation as the X-ray source. A scanning angle range was initiated from 20◦ to 80◦ with the step time of 0.5◦ /s. The accelerating voltage and the applied current were 40 KV and 40 mA, respectively. The UV–visible adsorption spectra, ranging from 300 to 800 nm, were further determined with a spectrophotometer (PerkinElmer Lambda 35, UV-35). 2.2. Photocatalytic reaction measurement The schematic of the photo-catalytic oxidation experimental system is illustrated in Fig. 1. A dynacalibrator (VICI Metronics Model 450) with a permeation tube of Hg0 was used to generate Hg0 with nitrogen as the carrier gas. The overall influent flow was set as
H. Shen et al. / Journal of Hazardous Materials 289 (2015) 235–243
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Table 1 Operating parameters for photo-oxidation experimental tests. Tests
Calcinated TiO2
UV lamps nm/W
Reaction temperature (◦ C)
Influent Hg0 conc. (g/m3 )
Carrier gas
1
Ti300 Ti400 Ti500 Ti600 Ti400 Ti400 Ti400 Ti400 Ti400 Ti400 Ti400 Ti400
365/15 365/15 365/15 365/15 365/15 254/15 Visible/15 365/15 254/15 Visible/15 365/20 Visible/20
100,120,140,160 100,120,140,160 100,120,140,160 100,120,140,160 100,120,140,160 100,120,140,160 100,120,140,160 140,160 140,160 140,160 140,160 140,160
20–100 20–100 20–100 20–100 20–100 20–100 20–100 20 20 20 20 20
N2 N2 N2 N2 N2 N2 N2 N2 + 5%O2 , N2 N2 + 5%O2 , N2 N2 + 5%O2 , N2 N2 N2
2
3
4
Table 2 The surface characteristics of Degussa P-25 calcinated at various temperatures. Calcination temp. (◦ C)
BET surface area (m2 /g)
Mass fraction of anatase % 1 f= I
BJH adsorption cumulative pores volume (cm3 /g)
Crystallite diametere (nm)
1+1.26 R
Band gap (ev)
IA
Anatase 300 400 500 600
46.7 43.9 43.0 35.5
0.36 0.36 0.343 0.28
76 80 75 70
850 mL/min, with a retention time of 0.69 s, and the flow ratios of Hg0 laden nitrogen and dilution nitrogen were controlled by mass flow controllers to obtain Hg0 concentrations of 0–100 g/m3 . The continuous cylindrical reactor was placed vertically with a length of 400 mm, a diameter of 25 mm, a central bump of length and diameter of 100 mm and 35 mm created by filling with photocatalyts. A lamp with a specific wave number was placed in the middle of the photocatalytic reactor, leaving a slit to hold the TiO2 coating glass beads between the glass’s inner wall and the lamp. The irradiation wave number of the light source was determined by making use of lamps with different wave numbers. The lamps used in this study included a 254 nm UV light, a 365 nm black UV light and a visible fluorescent light with powers of 15 W and 20 W, respectively. At the end of the photocatalytic reactor, the light was jacketed with rubber O-rings and plastic plugs to avoid leakage from the end. The outer wall of the photocatalytic reactor was wrapped with heating tape to control elevated temperatures if a K-type thermocouple touched the TiO2 . The tube connected between the mix chamber and reactor was also preheated with heating tape to avoid the condensation of water vapor. In order to investigate the influence of temperature on the photo-oxidation reaction, the mass of TiO2 coating on the surface of glass beads was set at 20 g with a total surface area of 0.00218 m2 . Since the total mass of TiO2 coated was 0.15 g with a coverage ratio of 0.0075 g TiO2 per gram of glass beads, the average thickness (d) of TiO2 coated on the surface of glass beads was determined as 1.72 m. TiO2 was made to exert the photo-oxidation reaction upon the irradiation, while the removal process was ineffective for all without irradiation at all experimental temperatures. An elemental mercury analyzer (NIC, EMP-2,) was used to detect Hg0 downstream on line. The decrease of Hg0 concentration through the photocatalysts was mainly due to the photo-oxidation of Hg0 , and the photo-oxidation efficiency (po ) is defined as Eq. (1):
po (%) =
Hg0in
=
Hg0in − Hg0out Hg0in
× 100%
21.1 21.5 21.5 21.1
29.9 28.5 28.9 29.4
3.01 3.03 3.03 3.10
where Hg0in and Hg0out represent the concentration of Hg0 at the inlet and outlet of the photocatalytic reactor, respectively. It is noted that the fate of Hg0 cannot be distinguished by this measurement system since Hg0 could be adsorbed onto the surface of photocatalysts or converted to gaseous oxidized mercury. Therefore, the photooxidation efficiency was determined by the difference in influent and effluent Hg0 concentrations. The blank experiments were initially carried out to validate the mass balance of the experimental setup to exclude errors resulting from leakage and adsorption of glass beads. The operating parameters for a photo-oxidation reaction (Tests 1–4) are summarized in Table 1. First, Test 1 investigated the effects of calcination temperature on the photo-oxidation efficiency of Hg0 . The photo-oxidation efficiency of Hg0 for different TiO2 calcinated at 300 ◦ C, 400 ◦ C, 500 ◦ C and 600 ◦ C were compared under the irradiation of 365 nm to select the best photocatalysts. Second, the effects of the reaction temperature and illumination wave length
A(101)
R(110)
A(004) R(101)
A(200)
A(105) A(211)
o
A(204)
A(116) 600 A(220) A(215)
C
500oC
400oC
300oC 20
Hg0
Rutile
24
28
32
36
40
44
48
52
56
60
64
68
72
2 Theta angle (1) Fig. 2. The XRD pattern of TiO2 on the surface of glass beads.
76
80
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3. Results and discussion
16 14
3.1. Surface characteristics of TiO2
Ti300 Ti400
12
(Ahv)2 (ev2)
Ti500 Ti600
10 8 6 4 2 0 1.5
2.0
2.5
3.0
3.5
4.0
hv(ev) Fig. 3. Ultraviolet–visible absorption of TiO2 coated on the surface of glass beads.
on the photo-oxidation efficiency were investigated in Test 2. The photo-oxidation efficiency of Hg0 was compared at the reaction temperatures of 100 ◦ C, 120 ◦ C, 140 ◦ C and 160 ◦ C for the wavelengths of irradiation ranging from 254 nm to visible light. Finally, to enhance the photo-oxidation efficiency at higher reaction temperatures, the influence of oxygen in the flue gas (Test 3) and the increase of irradiation strength (Test 4) on the photo-oxidation reaction were further examined, respectively.
a
The BET specific surface area and the pore volume of TiO2 are summarized in Table 2. The increasing calcination temperature reduced both the BET surface area and the pore volume of TiO2 . As calcination temperatures increased from 300 ◦ C to 600 ◦ C, the BET specific surface area decreased from 46.7 m2 /g to 35.5 m2 /g and the pore volume also declined from 0.36 cm3 /g to 0.28 cm3 /g. This was possibly due to fast intra-agglomerate densification or intercrystallite sintering within the agglomerates [25]. Compared to the activated carbons with huge specific surface areas, such a low BET surface area for TiO2 indicated that the overall oxidation efficiency was dependent upon the photo-oxidation of Hg0 since the physical adsorption of Hg0 was limited. The XRD patterns of TiO2 calcinated at different temperatures are illustrated in Fig. 2. The results indicate that the main peaks were distributed at 2 identical positions for all the photocatalysts, while the peaks’ intensity ratios were slightly different between 2 = 25.36◦ anatase (1 0 1) and 2 = 27.46◦ rutile (1 1 0) due to different calcination temperatures. According to Eq. (2) [26], IR represents the intensity of rutile at 2 = 27.46◦ and IA represents the intensity of anatase at 2 = 25.36◦ . The mass fraction of anatase in the Degussa P-25 can be determined by the intensity ratio between 25.36◦ and 27.46◦ (IR /IA ):
fA =
c
1 1 + 1.26(IR /IA )
(2)
100
100
80
60
40
100oC 120oC 140oC 160oC
20
0 20
40
60
80
100
120
Photo-oxidation Efficiency %
Photo-oxidation Efficiency %
90 80 70 60 50 40 30
100oC 120oC 140oC 160oC
20 10 0 20
140
40
60
100
120
140
d
b 100
100
90
90
80 70 60 50 40 30 20 10 0 0
20
40
60
80 0
3
Influent Conc.of Hg (µg/m )
100
100oC 120oC 140oC 160oC
Photo-oxidation Efficiency %
Photo-oxidation Efficiency %
80
Influent Conc. of Hg0 (µg/m3)
Influent Conc. of Hg0 (µg/m3)
80 70 60 50 40 30
100oC 120oC 140oC 160oC
20 10 0 20
40
60
80
100
120
Influent Conc. of Hg0 (µg/m3)
Fig. 4. (a) The effect of reaction temperature on the photo-oxidation efficiency of Hg0 by Ti300. (b) The effect of reaction temperature on the photo-oxidation efficiency of Hg0 by Ti400. (c) The effect of reaction temperature on the photo-oxidation efficiency of Hg0 by Ti500. (d) The effect of reaction temperature on the photo-oxidation efficiency of Hg0 by Ti600.
H. Shen et al. / Journal of Hazardous Materials 289 (2015) 235–243
0.89 B cosϑ
(3)
where is the wavelength of X-ray, is the diffraction angle and B is the corrected half width of the crystal. The results show that calcination led to a small discrepancy among the crystal diameters. The diameters of anatase in Ti400 and Ti500 were 21.5 nm and 21.5 nm, respectively, which were larger than those of 20.1 nm and 20.1 nm in Ti300 and Ti600. On the contrary, the diameters of rutile in Ti400 and Ti500 were 28.5 nm and 28.9 nm, which were smaller than those of 29.9 nm and 29.4 nm in Ti300 and Ti600 (see Table 2). Fig. 3 shows the absorption of UV–visible light by different calcinated TiO2 coated on the surface of glass beads. The optical band gap can be calculated by the following formula equation: n
˛hv ∝ (E − Eg )
(4)
where Eg is the band gap energy, E is the photon energy (E = hv), n = 2 for the direct transition band gap and ˛ (absorption coefficient) is proportional to A (absorbance). The band gap of the TiO2 can be calculated by extrapolating the linear portion of (˛hv)2 versus hv plot to ˛ = 0. The binding energies were 3.01, 3.03, 3.03 and 3.10 ev for Ti300, Ti400, Ti500 and Ti600, respectively, ranging between 3.2 eV for pure anatase and 3.00 eV for pure rutile [27]. The slight difference in binding energy indicated that the influence of the calcination temperature on the crystal transformation was insignificant for Degussa P-25 calcinated at 300–600 ◦ C because a large proportion of rutile crystals were not generated in the TiO2 until the calcination temperature was elevated above 700 ◦ C [25]. 3.2. The effect of TiO2 calcination temperature on the photo-oxidation of Hg0 The photo-oxidation efficiencies of Hg0 by calcinated TiO2 at 300 ◦ C, 400 ◦ C, 500 ◦ C and 600 ◦ C varied with various influent Hg0 concentrations under the irradiation of 365 nm and are illustrated in Fig. 4, respectively. Each curve stands for the removal efficiency of Hg0 at reaction temperatures from 100 ◦ C to 160 ◦ C with the influent Hg0 concentration ranging from 20 to 140 g/m3 . The results show that the photo-oxidation efficiency of Hg0 for various photocatalysts was significantly influenced by the reaction temperature. At 100 ◦ C and 120 ◦ C, the photo-oxidation of Hg0 by calcinated TiO2 was effective allowing for a high removal efficiency of Hg0 that could be sustained for longer than 72 h. This concurred in studies reported in previous literature [20,28]. However, as the temperature was set at 140 ◦ C and 160 ◦ C, the photo-oxidation efficiency of Hg0 dropped dramatically compared to those at 100 ◦ C and 120 ◦ C. The experimental results showed that the photo-oxidation efficiency of Hg0 for Ti400 was the highest compared with Ti300, Ti500 and Ti600 at all the reaction temperatures. When the reaction temperature was fixed at 100 ◦ C, the photo-oxidation efficiencies of Hg0 for Ti400 and Ti500 could reach as high as 100%, while those for Ti300 and Ti600 were relatively lower and yet kept more than 90% of its photo-oxidation efficiency. When the reaction temperatures increased from 100 ◦ C to 120 ◦ C, the photo-oxidation efficiencies of Hg0 for Ti400 and Ti500 maintained a higher than 90% photooxidation efficiency, while those for Ti300 and Ti600 decreased to 80%. It was noted that at 140 ◦ C and 160 ◦ C, the photo-oxidation efficiencies of Hg0 for all the calcinated photocatalysts decreased
100 90
Photo-oxidation Efficiency %
D=
a
80 70 60 50 40 30
100oC 120oC 140oC 160oC
20 10 0 0
10
20
30
40
50
60
70
80
90
Influent Conc. of Hg0(µg/m3)
b 100
Photo-oxidation Efficiency %
The mass fraction of the anatase and rutile crystallite in TiO2 is summarized in Table 2. The largest content of anatase (1 0 1) up to 80% was observed in Ti400 amongst all the calcinated TiO2 , while the mass fractions of anatase were 76%, 75% and 70% for Ti300, Ti500 and Ti600, respectively. The diameters of anatase (2 = 25.36◦ ) and rutile (2 = 27.46◦ ) in the TiO2 crystallite can be estimated by the Scherrer Equation 25 (Eq. (3)) and are summarized in Table 2:
239
90 80 70 60 50 40 30
100 120 140 160
20 10 0 20
40
60
80
100
Influent Conc. of Hg0 (µg/m3) Fig. 5. (a) The photo-oxidation efficiency of Hg0 with various reaction temperatures under the irradiation of visible light by Ti400. (b) The photo-oxidation efficiency of Hg0 with various reaction temperatures under the irradiation of 254 nm by Ti400.
sharply, and the descending Hg0 photo-oxidation efficiencies were ordered as Ti400 > Ti500 > Ti600 > Ti300. Factors attributing to high photo-oxidation efficiency included high mass ratio of anatase to rutile, high surface area, high pore volume, high adsorption affinity and low recombination rates, all of which could be influenced by the TiO2 calcination temperature [25,29]. In this study, although the calcination temperature resulted in a slight difference for the physical and chemical characteristics of TiO2 , the photo-oxidation efficiencies were different to some extent. This was probably due to a relatively higher mass fraction of anatase and higher specific surface area of Ti400 among the calcinated photocatalysts. Yet it is still unknown why Ti300 had the lowest photo-oxidation efficiency but had the largest specific surface area and a relatively higher mass fraction of anatase. Consequently, Ti400 was chosen as the sole photocatalyst for conducting the following experimental tests. 3.3. The photo-oxidation of Hg0 at 365 nm irradiation Fig. 4(b) summarizes the photo-oxidation efficiencies of Hg0 by photocatalyst Ti400 at the irradiation of 365 nm for reaction temperatures from 100 ◦ C to 160 ◦ C. The results indicate that a reaction temperature was essential for the Hg0 photo-oxidation process. The photo-oxidation efficiencies decreased from 98% to 30% as the reaction temperatures increased from 100 ◦ C to 160 ◦ C. At 100 ◦ C, the photo-oxidation efficiency of Hg0 was above 98%. When the reaction temperature increased to 120 ◦ C, the photooxidation efficiency of Hg0 was preserved at 89–95%. However, the photo-oxidation efficiency of Hg0 was sharply reduced to about
240
a
H. Shen et al. / Journal of Hazardous Materials 289 (2015) 235–243
a
1.0
1.0
Normalized Hg0 Concentration
Normalized Hg0 Concentration
0.9 0.8
0.6
0.4
0.2
o
140 C 160oC
0.0 0
100
200
300
400
500
0.8 0.7 0.6 0.5 0.4 0.3 0.2
without O2
0.1
with 5% O2
0.0
600
0
100
200
Time (min)
b
300
400
500
600
Time (min)
b
1.0
1.1
0.8
0.6
0.4
140oC 160oC
0.2
0
100
200
300
400
500
600
0.8 0.7 0.6 0.5 0.4 0.3 0.2
without O2
0.1
with 5% O2 0
50
100
150
200
250
300
350
Time (min)
Time (min) 1.2
Fig. 7. (a) The influence of 5% O2 on the photo-oxidation of Hg0 at 140 ◦ C with influent conc. of 20 g/m3 under the irradiation of 365 nm by Ti400. (b) The influence of 5% O2 on the photo-oxidation of Hg0 at 160 ◦ C with influent conc. of 20 g/m3 under the irradiation of 365 nm by Ti400.
1.0
Normalized Hg0 Concentration
0.9
0.0
0.0
c
Normalized Hg0 Concentration
Normalized Hg0 Concentration
1.0
0.8
0.6 Time (min)
0.4
140oC
0.2
0.0 0
200
400
600
800
1000
1200
1400
1600
Time (min) Fig. 6. (a) The photo-oxidation of Hg0 with influentconc. of 20 g/m3 at 140 ◦ C and 160 ◦ C under the irradiation of 365 nm by Ti400. (b) The photo-oxidation of Hg0 with influent conc. of 20 g/m3 at 140 ◦ C and 160 ◦ C under the irradiation of 254 nm by Ti400. (c) The photo-oxidation of Hg0 with influent conc. of 17 g/m3 at 140 ◦ C and 160 ◦ C under the irradiation of visible light by Ti400.
50–56% when the reaction temperature rose to 140 ◦ C, and further decreased to approximately 28–41% at 160 ◦ C. Moreover, at 140 ◦ C and 160 ◦ C the maximum photo-oxidation efficiency could be not only maintained as stable but also dropped continuously. As a result, the effluent concentration of Hg0 could finally break through within 10 h. To further understand how the increasing reaction temperature decreased the photo-oxidation efficiency of Hg0 , we investigated the variation of effluent concentrations of Hg0 without irradiation during the photo-oxidation periods. At 140–160 ◦ C, a small amount of the adsorbed mercury was desorbed from the surface of TiO2 when the influent Hg0 was shutdown. Turning off the irradiation light resulted in a rapid increase of effluent concentration of the Hg0 concentration desorbed from the surface of TiO2 at 140 ◦ C and 160 ◦ C. The results imply that the removal efficiency of Hg0 could be obtained by a balance between photo-oxidation and thermal desorption. However, the removal efficiency of Hg0 declined sharply as the temperature increased from 120 ◦ C to 140 ◦ C, at which the thermal desorption rate was enhanced to exceed the photo-oxidation rate in Table 3. For instance, when the influent concentration of Hg0 was fixed at 38 g/m3 , the difference in reaction rate between 100 ◦ C (247 ng Hg0 /m2 s) and 120 ◦ C (234 ng Hg0 /m2 s) was 13 ng Hg0 /m2 s; however, the difference expanded to 104 ng Hg0 /m2 s between 120 ◦ C (234 ng Hg0 /m2 s) and 140 ◦ C (130 ng Hg0 /m2 s).
H. Shen et al. / Journal of Hazardous Materials 289 (2015) 235–243
241
Table 3 Photo-oxidation of Hg0 by Ti400 irradiated by a wavelength of 254 nm, 365 nm, visible light. = 365 nm
= 254 nm
Visible light
Temp. (◦ C)
Influent Hg0 (g/m3 )
Photo-oxidation rate (ng Hg0 /m2 s)
Influent Hg0 (g/m3 )
Photo-oxidation rate (ng Hg0 /m2 s)
Influent Hg0 (g/m3 )
Photo-oxidation rate (ng Hg0 /m2 s)
100 100 100 100 100 120 120 120 120 120 140 140 140 140 140 160 160 160 160 160
18 38 60 78 102 18 38 60 78 102 16 38 60 78 102 17 38 60 80 102
117 247 383 500 650 104 234 364 474 611 52 130 201 260 370 45 91 117 143 188
16 33 49 67 82 16 33 49 67 82 16 33 49 67 82 16 33 49 67 82
104 195 286 383 455 84 175 234 312 351 71 143 188 240 247 52 91 117 156 149
19 38 57 78 95 19 40 60 80 100 20 40 60 80 100 20 40 80 100 –
123 227 338 455 539 104 221 305 396 468 97 188 260 325 364 78 136 240 266 –
Table 4 The photo-oxidation capacity of Hg0 (g/g TiO2 ) compared with the effect of 5% O2 and the irradiation strength of 15 W and 20 W at 140 ◦ C and 160 ◦ C. Lamps ()
140 15 W 5%O2 + N2
UV (254 nm) Near-UV (365 nm) Visible (370–720 nm)
– 20.1 –
N2
160 15 W 5%O2 + N2
20.5 17.2 16.4
– 4.7 1.5
N2
140 20 W 5%O2 + N2
160 20 W N2
4.1 2.4 1.1
– 44.6 28.7
– 11.3 –
Unit: g Hg0 /g TiO2 .
The increase in influent concentration of Hg0 can raise the photo-oxidation rate at all tested reaction temperatures. For instance, at 100 ◦ C the photo-oxidation rate of 117 ng Hg0 /m2 s increased to 650 ng Hg0 /m2 s as the influent concentration of Hg0 rose from 18 g/m3 to 102 g/m3 . The results indicate that the calcinated TiO2 could enhance the photo-oxidation rate as the influent concentration of Hg0 increased, which fit the L–H model [15]. 3.4. Variation of photo-oxidation efficiency of Hg0 with wave number of irradiation The effects of visible light on the photo-oxidation efficiency of Hg0 by TiO2 are shown in Fig. 5(a). Although the wave number of visible light was not as concentrated at 365 nm as the black UV light, the irradiation of visible light on TiO2 can enhance the photo-oxidation efficiency of Hg0 , which concurred with previous researches [19,20,23,30,31]. Similar to the effect of near UV light ( = 365 nm), the photo-oxidation efficiency gradually declined when the reaction temperature elevated, and the photo-oxidation rate went up as the influent Hg0 concentration rose. Although both the photo-oxidation efficiency and rate were relatively lower compared to near UV light irradiation, the fluorescent light could also play a critical role in facilitating the photo-oxidation of Hg0 on the surface of TiO2 above 100 ◦ C. Furthermore, the ultraviolet light ( = 254 nm) irradiation was applied to the photo-oxidation of Hg0 , as shown in Fig. 5(b). At 100 and 120 ◦ C, the photo-oxidation efficiency of Hg0 over TiO2 irradiated with UV light was lower than that with the near UV light. However, at 140 ◦ C and 160 ◦ C, the photo-oxidation efficiency under 254 nm irradiation was the highest among various light sources. Actually, the effects of UV light irradiation on the
photo-oxidation of Hg0 over TiO2 included two mechanisms, one was the heterogeneous oxidation of Hg0 to Hg2+ by TiO2 coated on the surface of glass beads, contributing to 90% of the overall photo-oxidation efficiency, and the other was the homogeneous photo-induced oxidation reaction of Hg0 to form Hg+ or Hg2+ which was then adsorbed onto the surface of TiO2 or the inner wall of the photocatalytic reactor, contributing to 10% of the overall photooxidation efficiency [32]. The effects of various wavelength irradiations on the photooxidation of Hg0 at 140 ◦ C and 160 ◦ C with the influent Hg0 concentration of 16–20 g/m3 are also illustrated in Fig. 6. The amount of Hg0 oxidized by TiO2 was determined by integrating the difference between the photo-oxidation curve and influent Hg0 concentration curve versus time. The results show that the amount of Hg0 was 20.5 g Hg0 /g TiO2 at 140 ◦ C and 4.1 g Hg0 /g TiO2 at 160 ◦ C under the irradiation of UV light ( = 254 nm), which surpassed the corresponding 17.2 and 2.4 g Hg0 /g TiO2 under the irradiation of the near UV light ( = 365 nm) and 16.4 g Hg0 /g TiO2 and 1.1 g Hg0 /g TiO2 under the irradiation of visible light, respectively (see Table 4). 3.5. The enhancement of Hg0 photo-oxidation by oxygen and irradiation strength The role of O2 was essential to the photo-oxidation of Hg0 by TiO2 as the separator of the photo-generated electrons and holes to boost the production of the OH radical [31–35]. Therefore, the influence of 5% O2 on the photo-oxidation of Hg0 in the simulated gas at 140 ◦ C and 160 ◦ C was investigated. The effect of O2 on the photo-oxidation efficiency is shown in Fig. 7. The addition of O2 could neither prolonged the breakthrough time of TiO2 nor improve
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Table 4 show the influences of increasing irradiation strength from 15 to 20 W on the photo-oxidation of Hg0 . The photo-oxidized capacity of Hg0 could be significantly improved. At 140 ◦ C, the photo-oxidation capacity of Hg0 was 44.5 g Hg0 /g TiO2 under the irradiation of 20 W, which was 2.5 times higher than 17.2 g Hg0 /g TiO2 under the irradiation of 15 W. At 160 ◦ C and under the irradiation of 20 W, the Hg0 photo-oxidation reaction lasted for 300 min without breaking through. The amount of Hg0 oxidized rose to 8.5 g Hg0 /g TiO2 , which was 3.5 times higher than 2.4 g Hg0 /g under the irradiation of 15 W. The photo-oxidation of Hg0 was greatly improved by increasing the strength of irradiation, suggesting that enhancement of incident fluorescent energy could be beneficial in increasing the photo-reactivity of TiO2 .
a
Normalized Hg0 Concentration
1.0
0.8
0.6
0.4
0.2
20 W 15 W
4. Conclusions
0.0 0
100
200
300
400
Time (min)
Normalized Hg0 Concentration
b
This study investigated the photo-oxidation efficiency of Hg0 at high temperatures of 100–160 ◦ C by calcinated TiO2 under different irradiations of wave number (254 nm, 365 nm and visible light) and various influent concentrations of Hg0 (20–100 g/m3 ). The calcination temperature of 400 ◦ C and 500 ◦ C brought about higher photo-oxidation efficiency than that at 300 ◦ C and 600 ◦ C. The increasing reaction temperature, ranging from 120 ◦ C to 140 ◦ C, decreased the photo-oxidation efficiency significantly, in which the thermal desorption rate was enhanced to exceed the photooxidation rate. The approach to enhancing the photo-oxidation efficiency at high temperatures showed that the increase of influent Hg0 concentration was not likely to raise the photo-oxidation efficiency of Hg0 , but was beneficial in improving its photooxidation rate. Results obtained from the influence of incident lights with different wave lengths on the photo-oxidation efficiency indicated that irradiation of 254 nm could effectively raise the photo-oxidation efficiency of Hg0 at 140 ◦ C and 160 ◦ C. The strength of irradiation from 15 W to 20 W was verified to significantly enhance the photo-oxidation efficiency of Hg0 at 140 ◦ C and 160 ◦ C. However, the effect of 5% O2 present in the flue gas on the photo-oxidation capacity of Hg0 at 140 ◦ C and 160 ◦ C was limited.
1.0
0.8
0.6
0.4
0.2
15W 20W
0.0 0
100
200
300
400
500
600
Time (min)
c
1.0
Normalized Hg0 Concentration
0.9 0.8
Acknowledgements
0.7
This study was performed under the auspices of the Ministry of Science and Technology, Republic of China, under contract number 101-2221-E-110-058-MY3. The authors are grateful to the Ministry of Science and Technology of Taiwan for its financial support.
0.6 0.5 0.4 0.3
References
15 W 20 W
0.2 0.1 0.0 0
50
100
150
200
250
300
350
400
Time (min) Fig. 8. (a) The influence of irradiation strength of 365 nm on the photo-oxidation of Hg0 with influent conc. of 20 g/m3 at 140 ◦ C by Ti400. (b) The influence of irradiation strength of 365 nm on the photo-oxidation of Hg0 with influent conc. of 20 g/m3 at 140 ◦ C by Ti400. (c) The influence of irradiation strength of 365 nm on the photo-oxidation of Hg0 with influent conc. of 20 g/m3 at 160 ◦ C by Ti400.
the efficiency during the whole photo-oxidation process. However, the addition of O2 could improve the photo-oxidation capacity of Hg0 from 17.2 g Hg0 /g TiO2 to 20.1 g Hg0 /g TiO2 at 140 ◦ C and from 2.4 g Hg0 /gTiO2 to 4.7 g Hg0 /gTiO2 and 160 ◦ C, respectively, (see Table 4 and Fig. 7). Previous literature reported that increasing the strength of visible light can enhance the photo-oxidation of Hg0 [20]. Fig. 8 and
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Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Enhanced photocatalytic oxidation of gaseous elemental mercury by TiO2 in a high temperature environment Huazhen Shen a , Iau-Ren Ie a , Chung-Shin Yuan a,∗ , Chung-Hsuang Hung b , Wei-Hsiang Chen a , Jinjing Luo c , Yi-Hsiu Jen a a
Institute of Environmental Engineering, National Sun Yat-Sen University No. 70, Lian-Hai Road, Kaohsiung 804, Taiwan Department of Safety, Health and Environmental Engineering, National Kaohsiung First University of Science and Technology No. 2, Juoyue Road, Nantz District, Kaohsiung 811, Taiwan c College of the Environmental & Ecology, Xiamen University, Xiamen, Fujian, PR China b
h i g h l i g h t s • • • •
The photo-oxidation efficiency of Hg0 by TiO2 at high temperatures was investigated. The irradiation of 254 nm promoted the photo-oxidation efficiency of TiO2 at high temperatures. The best calcination of TiO2 for photo-oxidation of Hg0 was 400 ◦ C. Increasing irradiation strength enhanced the photo-oxidation of Hg0 .
a r t i c l e
i n f o
Article history: Received 2 October 2014 Received in revised form 21 December 2014 Accepted 11 February 2015 Available online 12 February 2015 Keywords: Photo-oxidation of Hg0 Titanium dioxide Calcination temperature Irradiation strength Oxygen content
a b s t r a c t The photo-oxidation of Hg0 in a lab-scale reactor by titanium dioxide (TiO2 ) coated on the surface of glass beads was investigated at high temperatures. TiO2 was calcinated at four different temperatures of 300 ◦ C, 400 ◦ C, 500 ◦ C and 600 ◦ C (noted as Ti300, Ti400, Ti500 and Ti600) and characterized for its physicochemical properties. The calcinated TiO2 coating on the glass beads was then tested to compare the photo-oxidation efficiencies of Hg0 with an incident light of 365 nm. The results showed that the oxidation efficiencies of Hg0 for Ti400 and Ti500 were higher than those of Ti300 and Ti600. To enhance the photo-oxidation efficiency of Hg0 , Ti400 was selected to examine the wave lengths () of 254 nm, 365 nm and visible light with various influent Hg0 concentrations. The effects of irradiation strength and the presence of oxygen on the photo-oxidation efficiency of Hg0 were further investigated, respectively. This study revealed that the wave length () of 254 nm could promote the photo-oxidation efficiency of Hg0 at 140 and 160 ◦ C, while increasing the influent Hg0 concentration and could enhance the photooxidation rate of Hg0 . However, the influence of 5% O2 present in the flue gas for the enhancement of Hg0 oxidation was limited. Moreover, the intensity of the incident wave length of 365 nm and visible light were demonstrated to boost the photo-oxidation efficiency of Hg0 effectively. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Due to the toxicity effect on ecological safety and human health, mercury and its derivatives have attracted more attention and international co-operation from governmental and Non-Governmental Organizations (NGO) all over the world. The International Minamata Convention already signed an agreement on strictly limiting mercury emission on October 9, 2013 [1].
∗ Corresponding author. Tel.: +886 7 5252000x4409; fax: +886 7 52524409. E-mail address: [email protected] (C.-S. Yuan). http://dx.doi.org/10.1016/j.jhazmat.2015.02.033 0304-3894/© 2015 Elsevier B.V. All rights reserved.
According to the convention, the new treaty will control mercury emissions from various large industrial facilities, such as coal-fired power plants and industrial boilers to certain kinds of smelter handling, for example zinc and gold processing. The gaseous elemental mercury (Hg0 ) emission from coal-fired power plants in particular will be restricted. To date, most coal-fired power plants have not installed independent mercury removal device(s), however, the existing particle or acid gas removal devices are found to eliminate the elemental mercury (Hg0 ) or mercury containing compounds (Hg2+ and Hg+ ) [2–5]. The removal efficiency of these mercury containing chemicals vary greatly by air pollution control devices, such as selective catalytic reduction (SCR), electrostatic
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precipitator (ESP) and wet flue gas desulfurization (WFGD) [6]. It is more efficient to capture Hg2+ than Hg0 in flue gas with solubility of 20 g/L and 60 g/L, respectively [7–9]. The conventional methods to remove Hg0 include injecting activated carbon, zeolite, fly ash, metal oxide and their corresponding modified form. TiO2 -based photocatalysis has been investigated since Fujishima and Honda’s first report in the early 1970s on UVinduced redox chemistry on TiO2 . The researches on TiO2 application have flourished in green chemistry for a long time, including photocatalytic water splitting, hydrogen production, dye sensitization, solar energy conversion and photochemical air and water treatments. The effectiveness and mechanism on the reduction of Hg0 emission by TiO2 -based technology have caused great concern due to the wide application of commercial TiO2 . The photo-oxidation of mercury by TiO2 has been demonstrated to be an excellent method [10], due to its high removal efficiency, lower cost than activated carbons injection, which could be great potential to exploit in the coal-fired power plants. However, there remain difficulties of applying the photo-oxidation by TiO2 in fire power plant to be solved by the environmental experts [10], among which the high temperature of the ESP ranging from 100 to 180 ◦ C [5] that deteriorated the photo oxidation efficiency seriously and yet received less attention. An abundance of research has focused on the photo-oxidation of Hg0 at temperatures lower than 100 ◦ C during which the photooxidation efficiency and mechanism of Hg0 by TiO2 have been comprehensively investigated. The gas–solid photo-oxidation reaction process includes inter-media adsorption and photo-oxidation reaction. Adamson pointed that [11], the adsorption rate constant of kad is inversely proportional to the square root of temperature (T) and the desorption constant of kde increase exponentially as the temperature increase. Therefore, the adsorption equilibrium of K decrease as the temperature increase, which is not favor of the reaction between the gas–solid reactions. However, very few studies have paid attention to the performance of TiO2 at temperatures between 100 and 200 ◦ C systematically upon the photo-oxidation of Hg0 at ESP, which is the only irradiation source in the coalfired power plants. At room temperature, it becomes more effective on the photo-oxidation efficiency of Hg0 under UV or visible irradiation [12–18]. It has been observed that the photo-oxidation efficiency of Hg0 by TiO2 under fluorescent light reaches up to 90% in a lab-scale control system and could be maintained at 90% when conducted in a pilot-scale control system with a flow rate of 30 m3 /h [19,20]. The rate constant (k) of a photo-oxidation reaction by TiO2 was 7.4 ± 2.5 × 1014 molecules/(min cm2 ) at 24 ± 2 ◦ C in a batch reactor [12]. However, Lee et al. [15] observed that, at temperatures below 80 ◦ C, the overall rate of the initial mercury (Hg0 ) uptake increases as the temperature increases, while at 110 and 130 ◦ C, the increase in temperature results in a decrease in the overall reaction rate, indicating a relatively low oxidation efficiency of Hg0 at high temperatures. Thus, the investigation on the enhancement of Hg0 photo-oxidation at high temperatures is crucial. The photochemical mechanism shows that hydroxyl radicals (OH• ) are very reactive and originate from the dissociation of H2 O oxidation by the photo-generated holes, which could oxidize Hg0 to HgO on the surface of photo-irradiated TiO2 [12,13]. Since O2 and H2 O are trapped by photo-generated electrons and holes to form oxidants O2 − and OH• , the addition of O2 and H2 O seems to be vital for the photo-oxidation of Hg0 [21,22]. However, Tsai et al. found that the enhancement is limited to the addition of 6% O2 in a photocatalytic reaction system, which suggests that O2 − is unimportant for the oxidation of Hg0 on the surface of TiO2 under UV irradiation [23]. The effect of O2 seems insignificant on the efficient improvement for photo-oxidation of Hg0 . In this study, the photo-oxidation efficiency of Hg0 by TiO2 calcinated at four different temperatures was compared to select
Fig. 1. Schematic diagram of the experimental setup.
the most effective photocatalysts. Moreover, in order to investigate the effect of reaction temperatures higher than 100 ◦ C on photo-oxidation, five different influent Hg0 concentrations (20–100 g/m3 ) and incident lights with a wavelength () of 254 nm, 365 nm and 370–720 nm (visible light) were selected for the Hg0 oxidation experiments. The reaction rate and photooxidation mechanism dependent on reaction temperature were further discussed to ascertain the photo-oxidation efficiency of Hg0 at high temperatures of 100–120 ◦ C. Finally, the effects of O2 and irradiation light strengthened on the photo-oxidation efficiency of Hg0 were investigated, respectively. 2. Experimental methods 2.1. Preparation and characterization of photocatalysts The photocatalysts used for this particular study were prepared by coating the calcinated TiO2 onto the surface of 2.6 mm glass beads based on our previous description [24]. The glass beads were rinsed in 0.1 M sodium hydroxide solution and further neutralized in sulfuric acid. The glass beads were cleaned to remove residual chemicals in DI water twice and then dried. Prior to coating, the glass beads were heated at 150 ◦ C for 30 min on standby. TiO2 was dissolved at 1 g of Degusa-P25 to 10 mL DI water and stirred for 30 min at 60–70 ◦ C. The TiO2 solution was then sprayed onto the heated glass beads. After being dried at 100 ◦ C for 30 min, the TiO2 coated glass beads were further heated to 300 ◦ C, 400 ◦ C, 500 ◦ C and 600 ◦ C for 4 h and denoted as Ti300, Ti400, Ti500 and Ti600, respectively. The characterization of TiO2 was performed by the following methods. The BET specific surface area of TiO2 powder was carried out by a nitrogen adsorption apparatus at 77 K (Micromeritics, ASAP 2020). Each sample was outgassed at 105 ◦ C prior to BET measurement. The X-ray diffraction (XRD) pattern was recorded on an X-ray Diffractometer (Siemens D5000) using CuK␣ radiation as the X-ray source. A scanning angle range was initiated from 20◦ to 80◦ with the step time of 0.5◦ /s. The accelerating voltage and the applied current were 40 KV and 40 mA, respectively. The UV–visible adsorption spectra, ranging from 300 to 800 nm, were further determined with a spectrophotometer (PerkinElmer Lambda 35, UV-35). 2.2. Photocatalytic reaction measurement The schematic of the photo-catalytic oxidation experimental system is illustrated in Fig. 1. A dynacalibrator (VICI Metronics Model 450) with a permeation tube of Hg0 was used to generate Hg0 with nitrogen as the carrier gas. The overall influent flow was set as
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Table 1 Operating parameters for photo-oxidation experimental tests. Tests
Calcinated TiO2
UV lamps nm/W
Reaction temperature (◦ C)
Influent Hg0 conc. (g/m3 )
Carrier gas
1
Ti300 Ti400 Ti500 Ti600 Ti400 Ti400 Ti400 Ti400 Ti400 Ti400 Ti400 Ti400
365/15 365/15 365/15 365/15 365/15 254/15 Visible/15 365/15 254/15 Visible/15 365/20 Visible/20
100,120,140,160 100,120,140,160 100,120,140,160 100,120,140,160 100,120,140,160 100,120,140,160 100,120,140,160 140,160 140,160 140,160 140,160 140,160
20–100 20–100 20–100 20–100 20–100 20–100 20–100 20 20 20 20 20
N2 N2 N2 N2 N2 N2 N2 N2 + 5%O2 , N2 N2 + 5%O2 , N2 N2 + 5%O2 , N2 N2 N2
2
3
4
Table 2 The surface characteristics of Degussa P-25 calcinated at various temperatures. Calcination temp. (◦ C)
BET surface area (m2 /g)
Mass fraction of anatase % 1 f= I
BJH adsorption cumulative pores volume (cm3 /g)
Crystallite diametere (nm)
1+1.26 R
Band gap (ev)
IA
Anatase 300 400 500 600
46.7 43.9 43.0 35.5
0.36 0.36 0.343 0.28
76 80 75 70
850 mL/min, with a retention time of 0.69 s, and the flow ratios of Hg0 laden nitrogen and dilution nitrogen were controlled by mass flow controllers to obtain Hg0 concentrations of 0–100 g/m3 . The continuous cylindrical reactor was placed vertically with a length of 400 mm, a diameter of 25 mm, a central bump of length and diameter of 100 mm and 35 mm created by filling with photocatalyts. A lamp with a specific wave number was placed in the middle of the photocatalytic reactor, leaving a slit to hold the TiO2 coating glass beads between the glass’s inner wall and the lamp. The irradiation wave number of the light source was determined by making use of lamps with different wave numbers. The lamps used in this study included a 254 nm UV light, a 365 nm black UV light and a visible fluorescent light with powers of 15 W and 20 W, respectively. At the end of the photocatalytic reactor, the light was jacketed with rubber O-rings and plastic plugs to avoid leakage from the end. The outer wall of the photocatalytic reactor was wrapped with heating tape to control elevated temperatures if a K-type thermocouple touched the TiO2 . The tube connected between the mix chamber and reactor was also preheated with heating tape to avoid the condensation of water vapor. In order to investigate the influence of temperature on the photo-oxidation reaction, the mass of TiO2 coating on the surface of glass beads was set at 20 g with a total surface area of 0.00218 m2 . Since the total mass of TiO2 coated was 0.15 g with a coverage ratio of 0.0075 g TiO2 per gram of glass beads, the average thickness (d) of TiO2 coated on the surface of glass beads was determined as 1.72 m. TiO2 was made to exert the photo-oxidation reaction upon the irradiation, while the removal process was ineffective for all without irradiation at all experimental temperatures. An elemental mercury analyzer (NIC, EMP-2,) was used to detect Hg0 downstream on line. The decrease of Hg0 concentration through the photocatalysts was mainly due to the photo-oxidation of Hg0 , and the photo-oxidation efficiency (po ) is defined as Eq. (1):
po (%) =
Hg0in
=
Hg0in − Hg0out Hg0in
× 100%
21.1 21.5 21.5 21.1
29.9 28.5 28.9 29.4
3.01 3.03 3.03 3.10
where Hg0in and Hg0out represent the concentration of Hg0 at the inlet and outlet of the photocatalytic reactor, respectively. It is noted that the fate of Hg0 cannot be distinguished by this measurement system since Hg0 could be adsorbed onto the surface of photocatalysts or converted to gaseous oxidized mercury. Therefore, the photooxidation efficiency was determined by the difference in influent and effluent Hg0 concentrations. The blank experiments were initially carried out to validate the mass balance of the experimental setup to exclude errors resulting from leakage and adsorption of glass beads. The operating parameters for a photo-oxidation reaction (Tests 1–4) are summarized in Table 1. First, Test 1 investigated the effects of calcination temperature on the photo-oxidation efficiency of Hg0 . The photo-oxidation efficiency of Hg0 for different TiO2 calcinated at 300 ◦ C, 400 ◦ C, 500 ◦ C and 600 ◦ C were compared under the irradiation of 365 nm to select the best photocatalysts. Second, the effects of the reaction temperature and illumination wave length
A(101)
R(110)
A(004) R(101)
A(200)
A(105) A(211)
o
A(204)
A(116) 600 A(220) A(215)
C
500oC
400oC
300oC 20
Hg0
Rutile
24
28
32
36
40
44
48
52
56
60
64
68
72
2 Theta angle (1) Fig. 2. The XRD pattern of TiO2 on the surface of glass beads.
76
80
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3. Results and discussion
16 14
3.1. Surface characteristics of TiO2
Ti300 Ti400
12
(Ahv)2 (ev2)
Ti500 Ti600
10 8 6 4 2 0 1.5
2.0
2.5
3.0
3.5
4.0
hv(ev) Fig. 3. Ultraviolet–visible absorption of TiO2 coated on the surface of glass beads.
on the photo-oxidation efficiency were investigated in Test 2. The photo-oxidation efficiency of Hg0 was compared at the reaction temperatures of 100 ◦ C, 120 ◦ C, 140 ◦ C and 160 ◦ C for the wavelengths of irradiation ranging from 254 nm to visible light. Finally, to enhance the photo-oxidation efficiency at higher reaction temperatures, the influence of oxygen in the flue gas (Test 3) and the increase of irradiation strength (Test 4) on the photo-oxidation reaction were further examined, respectively.
a
The BET specific surface area and the pore volume of TiO2 are summarized in Table 2. The increasing calcination temperature reduced both the BET surface area and the pore volume of TiO2 . As calcination temperatures increased from 300 ◦ C to 600 ◦ C, the BET specific surface area decreased from 46.7 m2 /g to 35.5 m2 /g and the pore volume also declined from 0.36 cm3 /g to 0.28 cm3 /g. This was possibly due to fast intra-agglomerate densification or intercrystallite sintering within the agglomerates [25]. Compared to the activated carbons with huge specific surface areas, such a low BET surface area for TiO2 indicated that the overall oxidation efficiency was dependent upon the photo-oxidation of Hg0 since the physical adsorption of Hg0 was limited. The XRD patterns of TiO2 calcinated at different temperatures are illustrated in Fig. 2. The results indicate that the main peaks were distributed at 2 identical positions for all the photocatalysts, while the peaks’ intensity ratios were slightly different between 2 = 25.36◦ anatase (1 0 1) and 2 = 27.46◦ rutile (1 1 0) due to different calcination temperatures. According to Eq. (2) [26], IR represents the intensity of rutile at 2 = 27.46◦ and IA represents the intensity of anatase at 2 = 25.36◦ . The mass fraction of anatase in the Degussa P-25 can be determined by the intensity ratio between 25.36◦ and 27.46◦ (IR /IA ):
fA =
c
1 1 + 1.26(IR /IA )
(2)
100
100
80
60
40
100oC 120oC 140oC 160oC
20
0 20
40
60
80
100
120
Photo-oxidation Efficiency %
Photo-oxidation Efficiency %
90 80 70 60 50 40 30
100oC 120oC 140oC 160oC
20 10 0 20
140
40
60
100
120
140
d
b 100
100
90
90
80 70 60 50 40 30 20 10 0 0
20
40
60
80 0
3
Influent Conc.of Hg (µg/m )
100
100oC 120oC 140oC 160oC
Photo-oxidation Efficiency %
Photo-oxidation Efficiency %
80
Influent Conc. of Hg0 (µg/m3)
Influent Conc. of Hg0 (µg/m3)
80 70 60 50 40 30
100oC 120oC 140oC 160oC
20 10 0 20
40
60
80
100
120
Influent Conc. of Hg0 (µg/m3)
Fig. 4. (a) The effect of reaction temperature on the photo-oxidation efficiency of Hg0 by Ti300. (b) The effect of reaction temperature on the photo-oxidation efficiency of Hg0 by Ti400. (c) The effect of reaction temperature on the photo-oxidation efficiency of Hg0 by Ti500. (d) The effect of reaction temperature on the photo-oxidation efficiency of Hg0 by Ti600.
H. Shen et al. / Journal of Hazardous Materials 289 (2015) 235–243
0.89 B cosϑ
(3)
where is the wavelength of X-ray, is the diffraction angle and B is the corrected half width of the crystal. The results show that calcination led to a small discrepancy among the crystal diameters. The diameters of anatase in Ti400 and Ti500 were 21.5 nm and 21.5 nm, respectively, which were larger than those of 20.1 nm and 20.1 nm in Ti300 and Ti600. On the contrary, the diameters of rutile in Ti400 and Ti500 were 28.5 nm and 28.9 nm, which were smaller than those of 29.9 nm and 29.4 nm in Ti300 and Ti600 (see Table 2). Fig. 3 shows the absorption of UV–visible light by different calcinated TiO2 coated on the surface of glass beads. The optical band gap can be calculated by the following formula equation: n
˛hv ∝ (E − Eg )
(4)
where Eg is the band gap energy, E is the photon energy (E = hv), n = 2 for the direct transition band gap and ˛ (absorption coefficient) is proportional to A (absorbance). The band gap of the TiO2 can be calculated by extrapolating the linear portion of (˛hv)2 versus hv plot to ˛ = 0. The binding energies were 3.01, 3.03, 3.03 and 3.10 ev for Ti300, Ti400, Ti500 and Ti600, respectively, ranging between 3.2 eV for pure anatase and 3.00 eV for pure rutile [27]. The slight difference in binding energy indicated that the influence of the calcination temperature on the crystal transformation was insignificant for Degussa P-25 calcinated at 300–600 ◦ C because a large proportion of rutile crystals were not generated in the TiO2 until the calcination temperature was elevated above 700 ◦ C [25]. 3.2. The effect of TiO2 calcination temperature on the photo-oxidation of Hg0 The photo-oxidation efficiencies of Hg0 by calcinated TiO2 at 300 ◦ C, 400 ◦ C, 500 ◦ C and 600 ◦ C varied with various influent Hg0 concentrations under the irradiation of 365 nm and are illustrated in Fig. 4, respectively. Each curve stands for the removal efficiency of Hg0 at reaction temperatures from 100 ◦ C to 160 ◦ C with the influent Hg0 concentration ranging from 20 to 140 g/m3 . The results show that the photo-oxidation efficiency of Hg0 for various photocatalysts was significantly influenced by the reaction temperature. At 100 ◦ C and 120 ◦ C, the photo-oxidation of Hg0 by calcinated TiO2 was effective allowing for a high removal efficiency of Hg0 that could be sustained for longer than 72 h. This concurred in studies reported in previous literature [20,28]. However, as the temperature was set at 140 ◦ C and 160 ◦ C, the photo-oxidation efficiency of Hg0 dropped dramatically compared to those at 100 ◦ C and 120 ◦ C. The experimental results showed that the photo-oxidation efficiency of Hg0 for Ti400 was the highest compared with Ti300, Ti500 and Ti600 at all the reaction temperatures. When the reaction temperature was fixed at 100 ◦ C, the photo-oxidation efficiencies of Hg0 for Ti400 and Ti500 could reach as high as 100%, while those for Ti300 and Ti600 were relatively lower and yet kept more than 90% of its photo-oxidation efficiency. When the reaction temperatures increased from 100 ◦ C to 120 ◦ C, the photo-oxidation efficiencies of Hg0 for Ti400 and Ti500 maintained a higher than 90% photooxidation efficiency, while those for Ti300 and Ti600 decreased to 80%. It was noted that at 140 ◦ C and 160 ◦ C, the photo-oxidation efficiencies of Hg0 for all the calcinated photocatalysts decreased
100 90
Photo-oxidation Efficiency %
D=
a
80 70 60 50 40 30
100oC 120oC 140oC 160oC
20 10 0 0
10
20
30
40
50
60
70
80
90
Influent Conc. of Hg0(µg/m3)
b 100
Photo-oxidation Efficiency %
The mass fraction of the anatase and rutile crystallite in TiO2 is summarized in Table 2. The largest content of anatase (1 0 1) up to 80% was observed in Ti400 amongst all the calcinated TiO2 , while the mass fractions of anatase were 76%, 75% and 70% for Ti300, Ti500 and Ti600, respectively. The diameters of anatase (2 = 25.36◦ ) and rutile (2 = 27.46◦ ) in the TiO2 crystallite can be estimated by the Scherrer Equation 25 (Eq. (3)) and are summarized in Table 2:
239
90 80 70 60 50 40 30
100 120 140 160
20 10 0 20
40
60
80
100
Influent Conc. of Hg0 (µg/m3) Fig. 5. (a) The photo-oxidation efficiency of Hg0 with various reaction temperatures under the irradiation of visible light by Ti400. (b) The photo-oxidation efficiency of Hg0 with various reaction temperatures under the irradiation of 254 nm by Ti400.
sharply, and the descending Hg0 photo-oxidation efficiencies were ordered as Ti400 > Ti500 > Ti600 > Ti300. Factors attributing to high photo-oxidation efficiency included high mass ratio of anatase to rutile, high surface area, high pore volume, high adsorption affinity and low recombination rates, all of which could be influenced by the TiO2 calcination temperature [25,29]. In this study, although the calcination temperature resulted in a slight difference for the physical and chemical characteristics of TiO2 , the photo-oxidation efficiencies were different to some extent. This was probably due to a relatively higher mass fraction of anatase and higher specific surface area of Ti400 among the calcinated photocatalysts. Yet it is still unknown why Ti300 had the lowest photo-oxidation efficiency but had the largest specific surface area and a relatively higher mass fraction of anatase. Consequently, Ti400 was chosen as the sole photocatalyst for conducting the following experimental tests. 3.3. The photo-oxidation of Hg0 at 365 nm irradiation Fig. 4(b) summarizes the photo-oxidation efficiencies of Hg0 by photocatalyst Ti400 at the irradiation of 365 nm for reaction temperatures from 100 ◦ C to 160 ◦ C. The results indicate that a reaction temperature was essential for the Hg0 photo-oxidation process. The photo-oxidation efficiencies decreased from 98% to 30% as the reaction temperatures increased from 100 ◦ C to 160 ◦ C. At 100 ◦ C, the photo-oxidation efficiency of Hg0 was above 98%. When the reaction temperature increased to 120 ◦ C, the photooxidation efficiency of Hg0 was preserved at 89–95%. However, the photo-oxidation efficiency of Hg0 was sharply reduced to about
240
a
H. Shen et al. / Journal of Hazardous Materials 289 (2015) 235–243
a
1.0
1.0
Normalized Hg0 Concentration
Normalized Hg0 Concentration
0.9 0.8
0.6
0.4
0.2
o
140 C 160oC
0.0 0
100
200
300
400
500
0.8 0.7 0.6 0.5 0.4 0.3 0.2
without O2
0.1
with 5% O2
0.0
600
0
100
200
Time (min)
b
300
400
500
600
Time (min)
b
1.0
1.1
0.8
0.6
0.4
140oC 160oC
0.2
0
100
200
300
400
500
600
0.8 0.7 0.6 0.5 0.4 0.3 0.2
without O2
0.1
with 5% O2 0
50
100
150
200
250
300
350
Time (min)
Time (min) 1.2
Fig. 7. (a) The influence of 5% O2 on the photo-oxidation of Hg0 at 140 ◦ C with influent conc. of 20 g/m3 under the irradiation of 365 nm by Ti400. (b) The influence of 5% O2 on the photo-oxidation of Hg0 at 160 ◦ C with influent conc. of 20 g/m3 under the irradiation of 365 nm by Ti400.
1.0
Normalized Hg0 Concentration
0.9
0.0
0.0
c
Normalized Hg0 Concentration
Normalized Hg0 Concentration
1.0
0.8
0.6 Time (min)
0.4
140oC
0.2
0.0 0
200
400
600
800
1000
1200
1400
1600
Time (min) Fig. 6. (a) The photo-oxidation of Hg0 with influentconc. of 20 g/m3 at 140 ◦ C and 160 ◦ C under the irradiation of 365 nm by Ti400. (b) The photo-oxidation of Hg0 with influent conc. of 20 g/m3 at 140 ◦ C and 160 ◦ C under the irradiation of 254 nm by Ti400. (c) The photo-oxidation of Hg0 with influent conc. of 17 g/m3 at 140 ◦ C and 160 ◦ C under the irradiation of visible light by Ti400.
50–56% when the reaction temperature rose to 140 ◦ C, and further decreased to approximately 28–41% at 160 ◦ C. Moreover, at 140 ◦ C and 160 ◦ C the maximum photo-oxidation efficiency could be not only maintained as stable but also dropped continuously. As a result, the effluent concentration of Hg0 could finally break through within 10 h. To further understand how the increasing reaction temperature decreased the photo-oxidation efficiency of Hg0 , we investigated the variation of effluent concentrations of Hg0 without irradiation during the photo-oxidation periods. At 140–160 ◦ C, a small amount of the adsorbed mercury was desorbed from the surface of TiO2 when the influent Hg0 was shutdown. Turning off the irradiation light resulted in a rapid increase of effluent concentration of the Hg0 concentration desorbed from the surface of TiO2 at 140 ◦ C and 160 ◦ C. The results imply that the removal efficiency of Hg0 could be obtained by a balance between photo-oxidation and thermal desorption. However, the removal efficiency of Hg0 declined sharply as the temperature increased from 120 ◦ C to 140 ◦ C, at which the thermal desorption rate was enhanced to exceed the photo-oxidation rate in Table 3. For instance, when the influent concentration of Hg0 was fixed at 38 g/m3 , the difference in reaction rate between 100 ◦ C (247 ng Hg0 /m2 s) and 120 ◦ C (234 ng Hg0 /m2 s) was 13 ng Hg0 /m2 s; however, the difference expanded to 104 ng Hg0 /m2 s between 120 ◦ C (234 ng Hg0 /m2 s) and 140 ◦ C (130 ng Hg0 /m2 s).
H. Shen et al. / Journal of Hazardous Materials 289 (2015) 235–243
241
Table 3 Photo-oxidation of Hg0 by Ti400 irradiated by a wavelength of 254 nm, 365 nm, visible light. = 365 nm
= 254 nm
Visible light
Temp. (◦ C)
Influent Hg0 (g/m3 )
Photo-oxidation rate (ng Hg0 /m2 s)
Influent Hg0 (g/m3 )
Photo-oxidation rate (ng Hg0 /m2 s)
Influent Hg0 (g/m3 )
Photo-oxidation rate (ng Hg0 /m2 s)
100 100 100 100 100 120 120 120 120 120 140 140 140 140 140 160 160 160 160 160
18 38 60 78 102 18 38 60 78 102 16 38 60 78 102 17 38 60 80 102
117 247 383 500 650 104 234 364 474 611 52 130 201 260 370 45 91 117 143 188
16 33 49 67 82 16 33 49 67 82 16 33 49 67 82 16 33 49 67 82
104 195 286 383 455 84 175 234 312 351 71 143 188 240 247 52 91 117 156 149
19 38 57 78 95 19 40 60 80 100 20 40 60 80 100 20 40 80 100 –
123 227 338 455 539 104 221 305 396 468 97 188 260 325 364 78 136 240 266 –
Table 4 The photo-oxidation capacity of Hg0 (g/g TiO2 ) compared with the effect of 5% O2 and the irradiation strength of 15 W and 20 W at 140 ◦ C and 160 ◦ C. Lamps ()
140 15 W 5%O2 + N2
UV (254 nm) Near-UV (365 nm) Visible (370–720 nm)
– 20.1 –
N2
160 15 W 5%O2 + N2
20.5 17.2 16.4
– 4.7 1.5
N2
140 20 W 5%O2 + N2
160 20 W N2
4.1 2.4 1.1
– 44.6 28.7
– 11.3 –
Unit: g Hg0 /g TiO2 .
The increase in influent concentration of Hg0 can raise the photo-oxidation rate at all tested reaction temperatures. For instance, at 100 ◦ C the photo-oxidation rate of 117 ng Hg0 /m2 s increased to 650 ng Hg0 /m2 s as the influent concentration of Hg0 rose from 18 g/m3 to 102 g/m3 . The results indicate that the calcinated TiO2 could enhance the photo-oxidation rate as the influent concentration of Hg0 increased, which fit the L–H model [15]. 3.4. Variation of photo-oxidation efficiency of Hg0 with wave number of irradiation The effects of visible light on the photo-oxidation efficiency of Hg0 by TiO2 are shown in Fig. 5(a). Although the wave number of visible light was not as concentrated at 365 nm as the black UV light, the irradiation of visible light on TiO2 can enhance the photo-oxidation efficiency of Hg0 , which concurred with previous researches [19,20,23,30,31]. Similar to the effect of near UV light ( = 365 nm), the photo-oxidation efficiency gradually declined when the reaction temperature elevated, and the photo-oxidation rate went up as the influent Hg0 concentration rose. Although both the photo-oxidation efficiency and rate were relatively lower compared to near UV light irradiation, the fluorescent light could also play a critical role in facilitating the photo-oxidation of Hg0 on the surface of TiO2 above 100 ◦ C. Furthermore, the ultraviolet light ( = 254 nm) irradiation was applied to the photo-oxidation of Hg0 , as shown in Fig. 5(b). At 100 and 120 ◦ C, the photo-oxidation efficiency of Hg0 over TiO2 irradiated with UV light was lower than that with the near UV light. However, at 140 ◦ C and 160 ◦ C, the photo-oxidation efficiency under 254 nm irradiation was the highest among various light sources. Actually, the effects of UV light irradiation on the
photo-oxidation of Hg0 over TiO2 included two mechanisms, one was the heterogeneous oxidation of Hg0 to Hg2+ by TiO2 coated on the surface of glass beads, contributing to 90% of the overall photo-oxidation efficiency, and the other was the homogeneous photo-induced oxidation reaction of Hg0 to form Hg+ or Hg2+ which was then adsorbed onto the surface of TiO2 or the inner wall of the photocatalytic reactor, contributing to 10% of the overall photooxidation efficiency [32]. The effects of various wavelength irradiations on the photooxidation of Hg0 at 140 ◦ C and 160 ◦ C with the influent Hg0 concentration of 16–20 g/m3 are also illustrated in Fig. 6. The amount of Hg0 oxidized by TiO2 was determined by integrating the difference between the photo-oxidation curve and influent Hg0 concentration curve versus time. The results show that the amount of Hg0 was 20.5 g Hg0 /g TiO2 at 140 ◦ C and 4.1 g Hg0 /g TiO2 at 160 ◦ C under the irradiation of UV light ( = 254 nm), which surpassed the corresponding 17.2 and 2.4 g Hg0 /g TiO2 under the irradiation of the near UV light ( = 365 nm) and 16.4 g Hg0 /g TiO2 and 1.1 g Hg0 /g TiO2 under the irradiation of visible light, respectively (see Table 4). 3.5. The enhancement of Hg0 photo-oxidation by oxygen and irradiation strength The role of O2 was essential to the photo-oxidation of Hg0 by TiO2 as the separator of the photo-generated electrons and holes to boost the production of the OH radical [31–35]. Therefore, the influence of 5% O2 on the photo-oxidation of Hg0 in the simulated gas at 140 ◦ C and 160 ◦ C was investigated. The effect of O2 on the photo-oxidation efficiency is shown in Fig. 7. The addition of O2 could neither prolonged the breakthrough time of TiO2 nor improve
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H. Shen et al. / Journal of Hazardous Materials 289 (2015) 235–243
Table 4 show the influences of increasing irradiation strength from 15 to 20 W on the photo-oxidation of Hg0 . The photo-oxidized capacity of Hg0 could be significantly improved. At 140 ◦ C, the photo-oxidation capacity of Hg0 was 44.5 g Hg0 /g TiO2 under the irradiation of 20 W, which was 2.5 times higher than 17.2 g Hg0 /g TiO2 under the irradiation of 15 W. At 160 ◦ C and under the irradiation of 20 W, the Hg0 photo-oxidation reaction lasted for 300 min without breaking through. The amount of Hg0 oxidized rose to 8.5 g Hg0 /g TiO2 , which was 3.5 times higher than 2.4 g Hg0 /g under the irradiation of 15 W. The photo-oxidation of Hg0 was greatly improved by increasing the strength of irradiation, suggesting that enhancement of incident fluorescent energy could be beneficial in increasing the photo-reactivity of TiO2 .
a
Normalized Hg0 Concentration
1.0
0.8
0.6
0.4
0.2
20 W 15 W
4. Conclusions
0.0 0
100
200
300
400
Time (min)
Normalized Hg0 Concentration
b
This study investigated the photo-oxidation efficiency of Hg0 at high temperatures of 100–160 ◦ C by calcinated TiO2 under different irradiations of wave number (254 nm, 365 nm and visible light) and various influent concentrations of Hg0 (20–100 g/m3 ). The calcination temperature of 400 ◦ C and 500 ◦ C brought about higher photo-oxidation efficiency than that at 300 ◦ C and 600 ◦ C. The increasing reaction temperature, ranging from 120 ◦ C to 140 ◦ C, decreased the photo-oxidation efficiency significantly, in which the thermal desorption rate was enhanced to exceed the photooxidation rate. The approach to enhancing the photo-oxidation efficiency at high temperatures showed that the increase of influent Hg0 concentration was not likely to raise the photo-oxidation efficiency of Hg0 , but was beneficial in improving its photooxidation rate. Results obtained from the influence of incident lights with different wave lengths on the photo-oxidation efficiency indicated that irradiation of 254 nm could effectively raise the photo-oxidation efficiency of Hg0 at 140 ◦ C and 160 ◦ C. The strength of irradiation from 15 W to 20 W was verified to significantly enhance the photo-oxidation efficiency of Hg0 at 140 ◦ C and 160 ◦ C. However, the effect of 5% O2 present in the flue gas on the photo-oxidation capacity of Hg0 at 140 ◦ C and 160 ◦ C was limited.
1.0
0.8
0.6
0.4
0.2
15W 20W
0.0 0
100
200
300
400
500
600
Time (min)
c
1.0
Normalized Hg0 Concentration
0.9 0.8
Acknowledgements
0.7
This study was performed under the auspices of the Ministry of Science and Technology, Republic of China, under contract number 101-2221-E-110-058-MY3. The authors are grateful to the Ministry of Science and Technology of Taiwan for its financial support.
0.6 0.5 0.4 0.3
References
15 W 20 W
0.2 0.1 0.0 0
50
100
150
200
250
300
350
400
Time (min) Fig. 8. (a) The influence of irradiation strength of 365 nm on the photo-oxidation of Hg0 with influent conc. of 20 g/m3 at 140 ◦ C by Ti400. (b) The influence of irradiation strength of 365 nm on the photo-oxidation of Hg0 with influent conc. of 20 g/m3 at 140 ◦ C by Ti400. (c) The influence of irradiation strength of 365 nm on the photo-oxidation of Hg0 with influent conc. of 20 g/m3 at 160 ◦ C by Ti400.
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