Photocatalytic Process of Simultaneous Desulfurization and

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Photocatalytic Process of Simultaneous Desulfurization and Denitrification of Flue Gas by TiO2−Polyacrylonitrile Nanofibers Chunyan Su,*,† Xu Ran,† Jianglei Hu,† and Changlu Shao*,‡ †

School of Chemical Engineering, Changchun University of Technology, 2055 Yanan Street, Changchun 130012, People’s Republic of China ‡ Center for Advanced Optoelectronic Functional Materials Research, and Key Laboratory for UV-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, Changchun 130024, People’s Republic of China S Supporting Information *

ABSTRACT: TiO2 nanoparticles were successfully fabricated on electrospun polyacrylonitrile (PAN) nanofibers via the coupling of electrospinning and hydrothermal pathway. A straightforward photocatalysis oxidation process has been developed for simultaneous desulfurization and denitrification of flue gas using the TiO2−PAN photocatalyst. Also, the influences of some important operating parameters, such as titanium loading content of catalyst, flue gas humidity, flue gas flow, and inlet flue gas temperature on removal efficiencies of SO2 and NO were investigated. The results demonstrated that removal efficiencies of 99.3% for SO2 and 71.2% for NO were attained under the following optimal experiment conditions: titanium loading content, 6.78 At %; gas flow rate, 200 mL/ min; flue gas humidity, 5%; inlet flue gas temperature, 40 °C. Furthermore, the presumed reaction mechanism of SO2 and NO removal using TiO2−PAN photocatalyst under UV light was proposed. and hydroperoxyl (HO2·) radicals to remove pollutants by an oxidation reaction.7,8 Among available technologies, such interesting advanced oxidation processes as plasma oxidation,9,10 sonochemical oxidation,11,12 and Fenton oxidation13 have been developed to achieve removal of multiple gaseous pollutants from flue gas. However, so far, most of them cannot yet favorably substitute for the combination removal technology due to the current difficulties in economic or technical implementation. The photocatalytic oxidation process, with unique advantages of low energy consumption and high efficiency, as well as simple and pollution-free operation, has proved to be a practical advanced oxidization technique for disposal of the pollutants.14,15 Also, it is more attractive over other systems in that it can efficiently eliminate many environmental contaminants under mild conditions. Therefore, many researchers have gotten involved with trying to find more effective photocatalytic oxidation technologies for air cleanup and water purification.16−18 For a few years, the TiO2 photocatalytic oxidation process has been well underway to oxidize and remove various pollutants in the aquatic environment.19,20 As such, it is also worth considering to investigate simultaneous removal of SO2

1. INTRODUCTION Sulfur dioxide (SO2) and nitrogen oxides (NOx), as noxious gases, are commonly derived from anthropogenic sources such as power plants, incinerators, and boilers during coal combustion. Once released into the atmospheric environment from these pollution sources, they can cause acid rain, photochemical smog, stratospheric ozone destruction, and fine particulates,1,2 resulting in serious hazardous effects on ecosystems and human health. Therefore, much effort has been focused on developing eligible technologies for the elimination of these air pollutants from the coal-fired flue gas. Typically, wet limestone−gypsum flue gas technology is adopted to remove SO2, after which denitrification equipment for selective catalytic reduction removal of NOx is installed, namely, a combination removal technology.3,4 However, there exist some shortcomings in this combined removal process, which include large and complex systems, expensive investment, and running costs.5,6 Evidently, this stage-treatment approach of combined removal of SO2 and NOx is difficult to commercialize any more. To this end, exploring more novel and effective technologies and equipment of simultaneous desufurization and denitrification in one process has become one of the most attractive topics in the field of coal-fired flue gas purification. Currently, there has been much attention to the use of advanced oxidation technologies to prevent and remediate environmental contamination, mainly because they can produce highly reactive species including hydroxyl (·OH), oxygen(·O), © 2013 American Chemical Society

Received: Revised: Accepted: Published: 11562

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and NOx from flue gas based on the TiO2 photocatalytic technique. The potentiality on simultaneous removal of SO2 and NOx from flue gas by utilizing the TiO2 photocatalytic system has recently been addressed by two research groups.21,22 Nonetheless, the study on the application of the TiO2 photocatalysis oxidation process for simultaneous desulfurization and denitrification is still not sufficient so far. For example, immobilization on TiO2 photocatalyst under simulated practical conditions, lifespan and regeneration of photocatalyst, as well as rational design of equipment, are all associated with the industrial application and realization of the TiO2 photocatalytic technique. Therefore, there is a need for much deeper exploration of the use of the TiO2 photocatalytic process for removing SO2 and NOx from flue gas simultaneously, not merely for the theoretical significance but also for the industrial practicality. In this study, we reported a successful immobilization of nanostructured TiO2 on the support of electrospun polyacrylonitrile (PAN) nanofibers based on easy electrospinning and hydrothermal methods. Further, this kind of TiO2-based catalyst was used for the simultaneous desulfurization and denitrification of simulated flue gas by a self-designed photocatalytic apparatus. Herein, the electrospun PAN is explored as a carrier for the TiO2 catalyst owing to its advantages of ready accessibility and fine stability as well as large surface areas.23 In addition, because it is flexible in structure, the TiO2−PAN nanofiber catalyst can be placed on the eligible position of the reactor so as to make maximum use of the UV light source. Furthermore, the novel photocatalysis system of this study produced very large gas−solid interfacial areas, and therefore, high removal efficiencies were observed using a very low content of photocatalyst under mild conditions. In short, this system can surmount some drawbacks of conventional gaseous pollutants removal systems. Significantly, the experimental investigation also evaluated explicitly the factors influencing the removal efficiencies of SO2 and NO, such as titanium loading content of catalyst, flue gas humidity, flue gas flow, and inlet flue gas temperature, etc. Meanwhile, the life evaluation and regeneration for the catalyst were examined. In the end, the mechanism of SO2 and NO removal by the TiO2 photocatalyst under UV light was also proposed. It is hoped that the conclusions deduced from this study could provide some academic guidance for the development and application of this simultaneous desulfurization and denitrification technology in the deep-purification of polluted air.

Herein, the three TiO2−PAN nanofibers mats resulting from 0.3, 0.5, and 0.8 mL of Ti(OBu)4 contained in the electrospinning solution were correspondingly denoted as S1, S2, and S3. 2.2. Experimental Procedures and Apparatus. The selfdesigned experimental apparatus is showed in Figure 1. The

Figure 1. Schematic diagram of the experimental setup for simultaneous desulfurization and denitrification by photocatalysis oxidation processing: (1) SO2 gas cylinder; (2) NO gas cylinder; (3) air compressor; (4−6) rotameters; (7) gas-mixing chamber; (8) compressor; (9) evaporator; (10) outer layer of photoreactor; (11) photoreactor inner; (12) steel mesh wrapped with photocatalyst; (13) quartz tube of UV lamp; (14) mercury thermometer; (15) electrical resistance heater; (16) temperature controller and thermocouple; (17) UV lamp; (18) gas dryer; (19) tail gas absorber; (20) flue gas analyzer.

apparatus comprises a simulated flue gas system, a stationary bed photocatalysis reactor, and an analytical system. The photocatalysis reactor, as the body apparatus, is constructed of a bubble column reactor covered with stainless steel. A heating tape covered the outside of the reactor in order to control the temperature of inlet flue gas. A UV lamp (PL-L36W; main wavelength, 254 nm) covered by a quartz tube was vertically placed at the center of the reactor. A stainless steel column mesh is located in the gap from the inner wall of the reactor to the outer wall of the quartz tube, which is wrapped with two pieces of TiO2−PAN photocatalyst. SO2, NO, and air were used to make the simulated flue gas. The compositions and flow rates of the simulated gas were regulated by the rotameters. The flue gas humidity was controlled by modulation of the water vapor coming from an evaporator. The inlet and outlet concentrations of SO2 and NO were measured using a flue gas analyzer through the gas bypass. 2.3. Characterization. The morphologies of the asfabricated samples were observed by scanning electron microscope (SEM; XL-30 ESEM FEG, Micro FEI Philips) and transmission electron microscopy (TEM; Hitachi 600). The crystal structure of the samples was determined by X-ray diffraction (XRD) pattern recorded on a Siemens D5005 diffractometer using Cu Kα radiation at a scan rate of 2° min−1 in the range of 20−80°. The inlet and outlet concentrations of SO2 and NO were measured using a flue gas analyzer (M-9000, China). The tail gas absorption solution was analyzed using ion chromatography (IC) on ICS-90 ion chromatography system.

2. EXPERIMENTAL SECTION 2.1. Fabrication of Photocatalyst. In a typical procedure, the electrospinning was first conducted in an air atmosphere. A 1.30 g sample of polyacrylonitrile (PAN, Mw ca. 60000) was dissolved in 10 mL of N,N-dimethyl formamide (DMF), then 0.3, 0.5, and 0.8 mL of tetrabutyl titanate [Ti(OBu)4] and a certain amount of acetic acid were slowly dropped into the above solution. The final homogeneous solution was subjected to electrospinning, thereby aquiring the Ti(OBu)4−PAN nanofibers mats. Second, in a hydrothermal process, a certain amount of phosphoric acid and ethanol were added into three beakers containing 80 mL of deionized water under stirring. A piece of Ti(OBu)4−PAN fibers mats from mats with different Ti(OBu)4 feeding capacity was respectively added. Subsequently, the resulting mixtures were transferred into 100 mL Teflon-lined stainless steel autoclaves and kept at 180 °C for 10 h. Ultimately, TiO2−PAN nanofibers mats were attained. 11563

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3.2. TEM of the Fabricated Sample. The more detailed microstructure characteristics of the TiO2−PAN nanofibers (S2) were further examined by TEM. The TEM image shown in Figure 3A indicates that the entire surface of the PAN

3. RESULTS AND DISCUSSION 3.1. XRD Patterns and SEM of the Fabricated Samples. The crystalline phase of the as-fabricated samples were analyzed by X-ray power diffraction (XRD) (see Figure S1, Supporting Information). It can be seen that the diffraction peaks centered at 2θ = 25.1°, 37.6°, 47.9°, 53.7°, 54.5°, and 62.7° appear in all samples, which agree well with the crystal planes of anatase TiO2 (JCPDS file No. 21-1272). The morphologies of the asfabricated TiO2−PAN nanofibers were investigated by scanning electron microscopy (SEM). It can be clearly observed from high-magnification SEM micrographs shown as Figure 2A−C

Figure 3. TEM images of TiO2−PAN nanofibers for S2. (A) SAED image of TiO2 nanoparticles shown inset part; (B) the recorded pink circle and blue arrows point to TiO2 nanoparticles.

nanofibers is occupied uniformly by the TiO2 nanoparticles, the light areas represent PAN nanofibers and the dark areas represent TiO2 nanoparticles. From the image of inset part in Figure 3A, it can be seen that the corresponding ringlike selected-area electron diffraction (SAED) pattern is manifested from the polycrystalline structure of the TiO2 nanoparticles, and the diffraction rings from inside to outside could be relevant to the (101), (004), (200), (102) and (211) planes of anatase TiO2, respectively. As shown in Figure 3B, it is easy to observe that the sizes of the TiO2 nanoparticles are about 10− 40 nm, which is in good agreement with that from the SEM observations above. 3.3. Simultaneous Desulfurization and Denitrification by Photocatalytic Processing. 3.3.1. Comparison of Different Catalysts on Removal Efficiencies of SO2 and NO. To search out the optimum catalyst for simultaneous desulfurization and denitrification, comparative experiments were performed under identical experimental conditions. Figure 4 shows the removal efficiencies of SO2 and NO as a function of the irradiation time for the different catalysts as well as the adsorption curves of SO2 and NO in the dark. Obviously, both the SO2 and NO removal efficiencies from adsorption are significantly lower than their photocatalytic efficiencies, demonstrating that the photocatalytic oxidation is the main reaction in the whole desulfurization and denitrification process. In addition, S2 exhibits better catalytic performance than the other photocatalysts for simultaneous desulfurization and denitrification, and the removal efficiencies of SO2 and NO can reach 98.6% and 69.7%, respectively, with ultraviolet irradiation of 15 min. These statements imply that photocatalytic activity is governed by the TiO2 loading amount. Within the low range TiO2 amount, TiO2 particles loaded on nanofibers can act as the separation centers of electrons and holes and the photocatalytic efficiency of TiO2 is improved with increasing TiO2 amount. Within the high range TiO2 amount, however, more TiO2 amount could favor the recombination of the electrons and holes so that the photocatalytic efficiency of the catalyst is decreased to some extent. Therefore, the optimal TiO2 loading amount for S2 is used as the photocatalyst for the subsequent experiments of SO2 and NO removal. 3.3.2. Effect of UV Irradiation on SO2 Removal Efficiency. To verify the necessity of UV irradiation for the occurrence of photocatalytic reaction, the continuous tests of alternating UV irradiation (turning on UV lamp) and dark (turning off UV

Figure 2. Composition and morphology of the as-fabricated samples. FESEM images: (A) S1, (B) S2, and (C) S3. (D) Photograph of the electrospun nonwoven mats of S1. EDX spectra: (E) S1, (F) S2, and (G) S3.

that the diameters of the nonwoven nanofibers are in the range of 100−250 nm, and the lengths could run up to several micrometers. Additionally, it also reveals that numerous TiO2 nanoparticles are highly dispersed on the PAN nanofibers, and the diameters of the TiO2 nanoparticles range from 10 to 40 nm. Figure 2D shows a photograph of a piece of nanofibers mat catalyst with ca. 23 cm in edge length. It is ca. 0.40 g in weight, 70−80 μm in thickness, and 10.8 m2 g−1 in surface area of the TiO2−PAN photocatalyst nanofibers mats. Moreover, the surface chemical composition of the as-fabricated samples were further determined by energy dispersive X-ray (EDX) measurement as described in Figure 2E−G. The EDX spectra demonstrate that the TiO2−PAN nanofibers are mainly composed of titanium (Ti), oxygen (O), carbon (C), and nitrogen (N) elements; the Au element originates in the conductive coating while operating the SEM. As expected, the atomic ratios of O to Ti are close to 2:1, confirming the existence of TiO2. And, the loading amount of TiO2 dispersed into the nanofibers (from S1 to S2 and S3) are increased with the increase of Ti(OBu)4 concentration contained in the electrospinning solution. The sizes of the TiO2 nanoparticles are also increased correspondingly. 11564

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Figure 4. Comparison of different catalysts on simultaneous desulfurization (A) and denitrification (B). Conditions: catalysts, ca. 0.80 g; SO2, 1700 mg/m3; NO, 650 mg/m3; gas flow rate, 200 mL/min; ambient temperature; flue gas humidity, 3%.

lamp) for desulfurization were carried out, as exhibited in Figure 5. The results show that, with the turning on (full line)

Figure 6. Effect of photocatalyst on SO2 removal efficiency.

singlet 1SO2 and three line 3SO2. Among them, the 3SO2 plays a crucial role and can be directly oxidated into SO3:

Figure 5. Effect of UV irradiation on SO2 removal efficiency.

3

SO2 + O2 → SO3 + O

and turning off (dotted line) of the UV lamp, the SO2 removal efficiency changes greatly. Specifically, the desulfurization efficiency under the dark condition is only up to 18.8%, which mainly results from the physical adsorption for SO2 over the photocatalyst. Then, after turning on the UV lamp for 10 min after the adsorption equilibrium, the SO2 removal efficiency shows a rapid upward trend and runs up to 95.6%. It may be explained by the fact that when UV is introduced, some hydroxyl free radicals (·OH) and other active oxygen species are produced by TiO2 photocatalysis. Finally, when the UV lamp is turned off, the removal efficiency of SO2 decreases rapidly to the initial adsorption equilibrium level in dark. On the basis of this description, it can be concluded that there is a significant effect of UV irradiation on the SO2 removal, which means that UV light is an essential factor in the photocatalytic reaction of flue gas removal. 3.3.3. Effect of Catalyst on SO2 Removal Efficiency. To verify that the catalyst was another necessary factor for the occurrence of the photocatalytic reaction, the SO2 removal efficiencies with and without catalyst were investigated under UV irradiation. As shown in Figure 6, the SO2 removal efficiency in the presence of photocatalyst is increased with UV irradiation time being prolonged. The degradation efficiency tends to be stable at 10 min, and the maximum removal efficiency runs up to 96.2%. The SO2 removal efficiency without catalyst is also positively correlated with the irradiation time, and the maximum removal efficiency achieved is 59.3% at 10 min. Obviously, the presence of catalyst is beneficial for flue gas decontamination. As a relevant study suggested,24 the socalled homogeneous photocatalytic reaction for SO2 can occur without catalyst under UV irradiation, which can generate

(1)

Regarding the presence of a catalyst under UV irradiation, both homogeneous and heterogeneous photocatalytic reactions have occurred. In short, ·OH and ·O2− free radicals can be produced in addition to 3SO2, resulting in the acceleration of the photocatalytic oxidation reaction for the SO2 removal. The results show that there is a significant cooperative effect between UV and the TiO2−PAN catalyst. 3.3.4. Effect of Flue Gas Flow on SO2 and NO Removal Efficiencies. The effect of the gas flow rate on the SO2 and NO removal efficiencies is shown in Figure 7. With an increase of the flue gas flow rate from 200 to 1000 mL/min, the removal efficiency of SO2 rapidly decreases from 98.6 to 62.5%, and that of NO also decreases markedly from 69.7 to 40.2%. These results show that the SO2 and NO removal efficiencies decrease

Figure 7. Effect of flue gas flow on SO2 and NO removal efficiencies. Conditions: catalysts, ca. 0.80 g; SO2, 1700 mg/m3; NO, 650 mg/m3; ambient temperature; flue gas humidity, 3%. 11565

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with the increase of the flue gas flow. This point of view is mainly based on the following meditations. The increase in the flue gas flow means the residence time (reaction time) reduction of SO2 and NO in the reactor; that is, the amount of SO2 and NO through the reactor per unit time greatly increases,13,25 thereby being able to decrease the relative molar ratios of ·OH to SO2 or NO. Therefore, the oxidation of SO2 to SO3 and NO to NO2 are restrained, leading to a reduction of SO2 and NO removal efficiencies. Herein, a gas flow rate of 200 mL/min is optimal for the completion of simultaneous removal of SO2 and NO. 3.3.5. Effect of Inlet Flue Gas Temperature on SO2 and NO Removal Efficiencies. The relationship between the SO2 and NO removal efficiencies and the inlet gas temperature is depicted in Figure 8. As seen, by increasing the temperature

from 41.8 to 71.2%. This occurs mainly because the water molecules provide hydroxyl for trapping photogenerated holes, thereby generating hydroxyl radical (·OH) with very strong oxidability, which could accelerate the oxidation rates of SO2 and NO. When the humidity is further increased from 5% to 9%, the NO removal efficiency decreases by 17.5%, indicating that photocatalysis activity is lowered by degrees. The reason might be that water molecules would occupy more active cites of the photocatalyst, because of the increase of humidity, and compete with NO, thus affecting the removal efficiency of NO. Moreover, it is shown that the desulfurization efficiency is almost not affected with increasing vaper content when the humidity is greater than 5%. This phenomenon can occur because SO2 is dissolved in water easily. So, when the flue gas humidity is less than 5% an increase of flue gas humidity is beneficial for the removal of SO2 and NO in the photocatalysis process. The maximum NO removal efficiency is obtained with a humidity of 5%, the photocatayst activity for denitrification might be inhibited if the flue gas humidity exceeds 5%. Hence, the optimal flue gas humidity is at 5%. 3.3.7. Life Evaluation and Regeneration on Catalyst. Under the optimum reaction conditions, the experiments for life evaluation of TiO2−PAN nanofibers catalyst were carried out by continuous irradiation of UV light. The removal efficiencies of SO2 and NO as a function of the irradiation time are shown in Figure 10A. It can be found that the ability of the photocatalyst for desulfurization and denitrification remains high at all times in the course of continuous UV irradiation of 7 h. At 10 h the efficiencies of desulfurization and denitrification decrease to 53.1, and 37.8%, accordingly, which could mean that the catalyst has been poisoned so that it is inactive. The catalyst was regenerated for long-term use by being leached with deionized water, then exposed to infrared lamp irradiation for 2 h. Then, the activity of the catalyst after regeneration, under continued UV irradiation of 7 h, at the identical conditions as the first for desulfurization and denitrification, was examined. This is denoted as the second test. As shown in Figure 10B, the desulfurization efficiencies of 95.1, and 91.8% at 7 h correspond to first and second test, respectively. And, the denitrification efficienciesis of 62.6%, and 59.2% at 7 h correspond to first and second test. In short, the experiments of catalyst lifespan and regeneration for desulfurization and denitrification indicate that the catalyst is stable and its active recovery is very excellent. 3.4. Reaction Pathways of SO2 and NO Removal by Photocatalytic Process. To further understand the reaction mechanism of desulfurization and denitrification by the TiO2− PAN photocatalyst, the tail gas absorption solution under the optimum experiment conditions were analyzed using ion chromatography (IC). Clearly, SO42− and NO3− are the major anion products in solution, and the noxious byproducts NO2− and SO32− are not measured (see Figure S2, Supporting Information). Combined our investigation with other previous reports,27−30 the reaction pathways of SO2 and NO removal from flue gas by the TiO2−PAN photocatalysis could be inferred as follows: (I) At first, the SO2 and NO is adsorbed on the surface of the photocatalyst. (II) Photocatalytic oxidation process:

Figure 8. Effect of inlet flue gas temperature on SO2 and NO removal efficiencies.

from 40 to 120 °C, the removal efficiency of SO2 decreases greatly from 98.2 to 56.1%, and that of NO decreases markedly from 69.3 to 33.2%. The effect of inlet gas temperature on SO2 and NO removal could be explained by the quenching of reactive oxygen species with temperature rise. As reported, many reactive oxygen species are lost in the reactor wall when the temperature increases due to the increase of kinetic energies of the gas molecules, which leads to greater thermal motions.26 The present study indicates that the removal efficiencies of NO and SO2 are higher at lower flue gas temperatures. 3.3.6. Effect of Flue Gas Humidity on SO2 and NO Removal Efficiencies. The effect of flue gas humidity on the SO2 and NO removal efficiencies was investigated in detail. The results are shown Figure 9. When the humidity of flue gas increases from 0 to 5%, the removal efficiency for SO2 increases from 80.2 to 99.3%, and that for NO is also enhanced rapidly

Figure 9. Effect of flue gas humidity on SO2 and NO removal efficiencies. Conditions: catalysts, ca. 0.80 g; SO2, 1700 mg/m3; NO, 650 mg/m3; gas flow rate, 200 mL/min; inlet flue gas temperature, 40 °C. 11566

TiO2 + hv → TiO2 *(h+ + e−)

(2)

h+ + e−(recombined) → heat

(3)

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Figure 10. (A) Relation between catalytic efficiency and irradiation time. (B) SO2 and NO removal efficiencies at 7 h with respect to first test (from panel A) and that of second test (after catalyst regeneration).

H 2O + h+ → ·OH + H+

(4)

OH− + h+ → ·OH

(5)

O2 + e− → ·O2− + H 2O → ·OOH + OH−

(6)

2·OOH → O2 + H 2O2

(7)

−

·OOH + H 2O + e → H 2O2 + OH

−

H 2O2 + e− → ·OH + OH−

calcium based organic compounds. Chem. Eng. J. 2011, 168 (1), 255− 261. (2) Man, C. K.; Gibbins, J. R.; Witkamp, J. G.; Zhang, J. Coal characterization for NOx prediction in air−staged combustion of pulverised coals. Fuel 2005, 84 (17), 2190−2195. (3) Hutson, N. D.; Krzyzynska, R.; Srivastava, R. K. Simultaneous removal of SO2, NOx, and Hg from coal flue gas using a NaClO2enhanced wet scrubber. Ind. Eng. Chem. Res. 2008, 47 (16), 5825− 5831. (4) Liu, Y. X.; Zhang, J.; Wang, Z. L.; Du, M. Simultaneous removal of NO and SO2 from flue gas by UV/H2O2/CaO. Chem. Eng. Technol. 2012, 35 (10), 1879−1884. (5) Wu, Z. B.; Wang, H. Q.; Liu, Y.; Jiang, B. Q.; Sheng, Z. Y. Study of a photocatalytic oxidation and wet absorption combined process for removal of nitrogen oxides. Chem. Eng. J. 2008, 144 (2), 221−226. (6) Liu, Y. X.; Zhang, J.; Sheng, C. D.; Zhang, Y. C.; Zhao, L. Simultaneous removal of NO and SO2 from coal-fired flue gas by UV/ H2O2 advanced oxidation process. Chem. Eng. J. 2010, 162 (3), 1006− 1011. (7) Liu, Y. X.; Zhang, J.; Sheng, C. D.; Zhang, Y. C.; Zhao, L. Wet removal of sulfur dioxide and nitric oxide from simulated coal-fired flue gas by UV/H2O2 advanced oxidation process. Energy Fuels 2010, 24 (9), 4931−4936. (8) Wang, Z. H.; Zhou, J. H.; Zhu, Y. Q. Simultaneous removal of NOx/SO2 and Hg in nitrogen flow in a narrow reactor by ozone injection: Experimental results. Fuel Process. Technol. 2007, 88 (8), 817−823. (9) Xu, F.; Luo, Z. Y.; Cao, W.; Wang, P. Simultaneous oxidation of NO, SO2, and Hg° from flue gas by pulsed corona discharge. Fuel Process. Technol. 2008, 89 (5), 540−548. (10) Nasonova, A.; Pham, H. C.; Kim, D. J.; Kim, K. S. NO and SO2 removal in non-thermal plasma reactor packed with glass beads-TiO2 thin film coated by PCVD process. Chem. Eng. J. 2010, 156 (3), 557− 561. (11) Owusu, S. O.; Adewuyi, Y. G. Sonochemical removal of nitric oxide from flue gases. Ind. Eng. Chem. Res. 2006, 45 (13), 4475−4485. (12) Adewuyi, Y. G.; Owusu, S. O. Ultrasound-induced aqueous removal of nitric oxide from flue gases: Effects of sulfur dioxide, chloride, and chemical oxidant. J. Phys. Chem. A 2006, 110 (38), 11098−11107. (13) Liu, Y. X.; Zhang, J.; Pan, J. F.; Tang, A. K. Investigation on the removal of NO from SO2-containing simulated flue gas by an ultraviolet/Fenton-like reaction. Energy Fuels 2012, 26 (9), 5430− 5436. (14) Xamena, F. X. L.; Calza, P.; Lamberti, C.; Prestipino, C.; Damin, A.; Bordiga, S.; Pelizzetti, E.; Zecchina, A. Enhancement of the ETS-10 titanosilicate activity in the shape-selective photocatalytic degradation of large aromatic molecules by controlled defect production. J. Am. Chem. Soc. 2003, 125 (8), 2264−2271. (15) Alibegic, D.; Tsuneda, S.; Hirata, A. Kinetics of tetrachloroethylene (PCE) gas degradation and byproducts formation during UV/H2O2 treatment in UV-bubble column reactor. Chem. Eng. Sci. 2001, 56 (21−22), 6195−6203.

(8) (9)

NO + ·OOH → NO2 + · OH

(10)

NO + 2·OH → NO2 + H 2O

(11)

NO2 + ·OH → HNO3

(12)

SO2 + 2·OH → SO3 + H 2O

(13)

SO2 + ·O2− + H 2O → SO3 + 2OH−

(14)

(III) Solution process:

■

SO3 + H 2O → H 2SO4

(15)

NO2 + H 2O → HNO3

(16)

ASSOCIATED CONTENT

S Supporting Information *

Two additional figures as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +86 431 85716328. *E-mail: [email protected]. Tel.: +86 431 85098803. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work is supported financially by the National Natural Science Foundation of China (No. 51272041, and 51201020), the Natural Science Foundation of Jilin Province of China (No. 201215125), and the Science and Technology Program Project of Changchun City of China (No. 2011190, and 2011297).

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REFERENCES

(1) Niu, S. L.; Han, K. H.; Lu, C. M. Release of sulfur dioxide and nitric oxide and characteristic of coal combustion under the effect of 11567

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dx.doi.org/10.1021/es4025595 | Environ. Sci. Technol. 2013, 47, 11562−11568