Destruction of Toluene by the Combination of High Frequency Discharge Electrodeless Lamp and Manganese Oxide-Impregnated Granular Activated Carbon Catalyst

Xu, Jianhui; Li, Chaolin; Zhang, Qian; He, Di; Liu, Peng; Ren, Yong

doi:https://doi.org/10.1155/2014/365862

International Journal of Photoenergy

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Abstract Introduction Materials and Methods Results Discussion Conclusion Acknowledgments Supplementary Materials References Copyright Related Articles

Research Article | Open Access

Volume 2014 | Article ID 365862 | https://doi.org/10.1155/2014/365862

Destruction of Toluene by the Combination of High Frequency Discharge Electrodeless Lamp and Manganese Oxide-Impregnated Granular Activated Carbon Catalyst

Jianhui Xu,¹Chaolin Li,¹Qian Zhang,^1,2Di He,³Peng Liu,¹and Yong Ren¹

Academic Editor: Hongtao Yu

Received28 Jun 2014

Revised21 Aug 2014

Accepted08 Sept 2014

Published13 Oct 2014

Abstract

The destruction of low concentration of toluene (0–30 ppm) has been studied under the UV/photogenerated O₃/MnO₂-impregnated granular activated carbon (MnO₂-impregnated GAC) process by the combination of self-made high frequency discharge electrodeless lamp (HFDEL) with MnO₂-impregnated GAC catalyst. Experimental results showed that the initial toluene concentration can strongly affect the concentration of photogenerated O₃ from HFDEL and the efficiency and mass rate of destruction of toluene via HFDEL/MnO₂-impregnated GAC system. Active oxygen and hydroxyl radicals generated from HFDEL/MnO₂-impregnated GAC system played a key role in the decomposition of toluene process and the intermediates formed by photolysis are more prone to be mineralized by the subsequent MnO₂-impregnated GAC catalyst compared to the original toluene, resulting in synergistic mineralization of toluene by HFDEL/MnO₂-impregnated GAC system. The role of MnO₂-impregnated GAC catalyst is not only to eliminate the residual O₃ completely but also to enhance the decomposition and mineralization of toluene.

1. Introduction

Volatile organic compounds (VOCs) as widespread air pollutants can be found in both outdoor and indoor environments. The majority of VOCs originate from the exhausts of motor vehicle and solvent utilization and VOCs from the former case can react with to form tropospheric O₃ which results in smog in urban air [1]. Exposure to VOCs might cause toxic effects on central nervous system and internal organs, and the related symptoms, such as headache, respiratory tract irritation, dizziness, and nausea, are known as the sick building syndrome (SBS) [2]. For high concentration (several hundreds of ppm) of VOCs emission sources, catalytic incineration and combustion (200–900°C) have been well developed and successfully operated but are not cost-effective for low concentration of VOCs [3]. Among those available potential air-cleaning technologies for contamination of lower concentration of VOCs, photocatalytic oxidation (PCO) has caused extensive concern recently, but the problems of the poisoning or deactivation of photocatalyst caused by accumulation of organic products on the surface of catalyst have not been well solved [4, 5], which may render PCO as a technology for controlling contaminations of low concentration of VOCs inefficiently and uneconomically.

High frequency (HF) discharge electrodeless lamp (HFDEL), a typical UV light source, has been invented over 100 years and started to apply into the photolysis of organic compounds in solution and the irradiation of gases since 1970s [6]. Due to the leakage of electromagnetic radiation and ozone, however, the application of HFDEL became restricted afterwards. Unlike the conventional lamps energized by the electric field between the electrodes, the principle of HFDELs is that the gas or materials in the lamp are excited by HF electromagnetic field to form stable UV-emitting discharge plasma. Such unique discharge pattern endows the HFDELs with lots of advantages compared to the conventional UV lamps (i.e., low pressure mercury lamps), including long lifetime of UV output, high UV/vacuum UV (VUV) radiant power, VUV-mediated generation of O₃ in conjunction with UV to create hydroxyl radicals (HO^•), and adaptable lamp shapes. Compared to microwave discharge electrodeless lamps (MDELs), HFDELs possess higher energy conversion efficiency, have no need for resonant cavity, and overcome the short lifetime of magnetron equipped on MDELs. Employing mercury MDELs to water sterilization [7, 8] and photodissociation of organic pollutants in aqueous solution [9–12] have been studied extensively in the past few years, as mercury is the most readily to be excited, and even a domestic microwave oven may act as the microwave power supply reactor [13]. However, little has been done on the photolysis of VOCs using MDELs as the UV light source probably due to practical limitations in reactor design and operation for the treatment of gas pollutants [14]. Recently, we have investigated the photolysis of H₂S using HFDEL showing much higher removal efficiency compared to MDELs, gained some insights into the possible mechanisms for photolysis, and confirmed the feasibility of application of HFDEL into the decomposition of air pollutants [15]. To further extend the application of HFDEL, the photolysis of VOCs by HFDEL has been investigated in this study. To cope with electromagnetic radiation generated from HFDEL, stainless steel reactors have been applied in this study to minimize the negative effects of electromagnetic wave on human bodies. Meanwhile, the stainless steel reactor is also beneficial in the reflection of UV light and resistant to corrosion of corrosive gas.

Although photogenerated O₃ by HFDEL can induce advanced oxidation processes (AOPs) such as UV/O₃ to produce HO^• for effectively decomposing VOCs, excess O₃, as an air pollutant, in the effluent gas stream should be reduced to a safe level. Therefore, a reactor containing an O₃-decomposition catalyst (ODC) needs to be set up following the photoreactor to treat excess O₃ in the effluent gas stream. The O₃/granular activated carbon (GAC) method is relatively common in which the GAC performs dual roles: adsorption of residual O₃ and VOCs (their organic products) and decomposition of O₃ over the surface to yield HO^•, which in turn are able to quickly mineralize VOCs and their organic products adsorbed on the surface of GAC and/or in gas phase [16]. To further enhance the O₃ decomposition and generation of HO^• for mineralization of VOCs and their organic products, the impregnation of MnO₂ on GAC has been applied in this study as MnO₂ shows an excellent simultaneous elimination of VOCs and O₃ [17]. In addition to the elimination of excess O₃ and enhanced removal of VOCs, the combination of HFDEL and ODC may also have the following advantages: (i) the photolysis of VOCs results in the partial mineralization of VOCs, which may reduce the accumulation of organic intermediates on the surface of ODC and extend the lifetime of ODC, (ii) photogenerated O₃ may have a positive effect on the regeneration of ODC considering that O₃ photogenerated from HFEDL can regenerate the photocatalyst [18], and (iii) the organic intermediates of photolysis may be more subject to be mineralized by MnO₂-impregnated/O₃ compared to the original VOCs.

In this study, a preliminary study on the removal of VOCs with the combination of HFDEL and MnO₂-impregnated GAC was investigated. Filled with binary mixtures of Hg-Ar, HFDEL was found to emit intense atomic lines of mercury in both UV and VUV region (mainly atomic Hg emission lines at 185 nm (6s6p(¹P₁)–6s6p(¹S₀)) and 253.7 nm (6²P₁–6²S₀) [15] and MnO₂-impregnated GAC was confirmed to decompose O₃ and induce the formation of O^• and HO^• following the exposure to O₃. A relatively low concentration level of toluene as the target VOC, in the range of 0–30 ppm, was selected for this work, in the consideration of ubiquitousness of indoor and outdoor environments. The performances of removal and mineralization of toluene were examined under different conditions with photolysis by HFDEL, MnO₂-impregnated GAC-mediated catalyzed ozonation, and the combination of HFDEL and MnO₂-impregnated GAC. The analysis of intermediates and possible mechanisms for photolysis and catalyzed ozonation were also evaluated in this study.

2. Materials and Methods

2.1. HFDEL

HFDEL consisted of an HF power supply and electrodeless lamps. The HF power supply was operated to produce a current with a fixed frequency of 2.45 MHz, which was transmitted to a coupling fixture to generate an HF electromagnetic field. The mercury atoms were excited by the HF electromagnetic field followed by returning to ground state to emit UV light [13, 19]. The working power for HFDEL was 80 W which was sufficient to ignite the quartz lamp. The electrodeless lamps were made of quartz bulb with the height of 15 cm (the volume is ~550 mL).

2.2. Characterization of HFDEL

The emission spectrum of UV radiation emitted by HFDEL was detected by an Acton VM-505 VUV monochromator. The stability of light intensity of HFDEL at 253.7 nm was monitored at the outflow of the photolysis reactor by an irradiatometer (TN-2254, Taiwan Taina Instrument) while the distribution of light intensity at 253.7 nm as a function of the distance to the bottom center of the lamp was measured by the same irradiatometer. More details have been shown in Supplementary Material available online at http://dx.doi.org/10.1155/2014/365862 (SM) (Figures SM1–3).

2.3. MnO₂-Impregnated GAC

Wood-based GAC (cylinder shape with a diameter of 4 cm) was selected as supporting material in this study (Calgon Carbon Corporation in Tianjin, China). Firstly, GAC was acid-treated in a 5% solution of hydrochloric acid for 6 h to reduce ash content, washed with Milli-Q water (MQ) repeatedly to reach neutrality, and dried at 105°C for 24 h. Then 500 g GAC and certain amounts of Mn(NO₃)₂ were mixed with 2 L MQ and constantly shaken for 24 hours at 25°C for the preparation of MnO₂-impregnated GAC with different mass loading of MnO₂ from 0 to 10%. The concentration of Mn²⁺ in the supernatant was characterized by inductively coupled plasma-optical emission spectrometry (ICP-OES) (Varian AX, Varian) confirming ~100% Mn²⁺ has been adsorbed on GAC. Afterwards, the solids were filtrated and dried overnight at 105°C for 12 h in an air oven and calcinated at 450°C in muffle furnace for 6 h. Mineralogy of the MnO₂ was characterized by X-ray diffraction (Rigaku D/MAX 2500). The surface morphology of MnO₂-impregnated GAC was obtained by scanning electron microscope (Hitachi S-4700).

2.4. Experimental Setup

The experimental setup designed for the evaluation of toluene removal efficiency by HFDEL/MnO₂-impregnated GAC is shown in Figure 1. The system consisted of an experimental parameter control system, a continuous flow gas generation system, HFDEL stainless steel reactor, and MnO₂-impregnated GAC system. The toluene gas was pumped via the bubbling of air into the toluene solution (1) and the humidity was adjusted to 74.1% through the experimental parameter control system (2). The mixture of toluene was transferred through a mixing tube (3) before introducing into the photolysis area (4), which is followed by MnO₂-impregnated GAC system (5). The ozone generator (6) was only applied to investigate the effect of O₃/MnO₂-impregnated GAC on the removal of toluene. The gas stream passed through the reactor for 15 min to allow the system to reach the steady state. Power was then applied to inspire the lamp for another 10 min to make sure the steady state of light intensity had been achieved before the measurement of concentration of toluene in the gas stream. The initial toluene concentration ranged from 0 to 30 ppm. All the experiments were carried out at fixed gas flow rate of 4 m³ h⁻¹ at room temperature (25 ± 2°C).

2.5. Chemical Analysis

The concentration of toluene and its final product CO₂ in the air stream was analyzed by a gas chromatograph (GC, Thermo Finnigan) equipped with flame ionization detector (FID), respectively. The concentration of O₃ was monitored spectrophotometrically at 254 nm where O₃ possesses a molar absorptivity of 3292 ± 70 M⁻¹ cm⁻¹ [20]. The gaseous intermediates in the outlet gas were collected by an absorption bottle filled with HPLC grade methanol (Dima Technology, USA) for 1 h after the reaction reached equilibrium. The solution was analyzed by GC (Agilent 6890A)-MSD (Agilent 5975C with Triple-Axis Detector). The carrier gas was ultrahigh purity helium at a constant flow rate of 1 mL min⁻¹. The injector and detector temperatures were set at 230 and 270°C, respectively. The GC column was DB-5 (30 m × 0.25 mm × 0.25 mm, Agilent technology). The temperature of the GC oven was initially set at 80°C for 1 min and then raised at 5°C min⁻¹ to 250°C for 3 min with a subsequent increase to a final 300°C at a rate of 10°C min⁻¹ for 5 min. 1.0 μL of sample was injected in the splitless mode.

3. Results

3.1. Generation of O₃

As photogenerated O₃ can induce AOPs including UV/O₃ and MnO₂-impregnated GAC/O₃ to create HO^• for effective decomposition of toluene, the concentration of generated O₃ as a function of inlet toluene concentration was investigated. Figure 2 shows that the concentration of photogenerated O₃ decreased from 130 to 41 ppm with an increase in the inlet toluene concentration from 0 to 30 ppm. The possible pathway of O₃ formation during the UV photolysis process is as follows (see (1)–(3)): O^• is commonly regarded as the main oxidant in the catalytic ozonation [21]. The primary pathway of toluene oxidation by O^• is the abstraction of hydrogen atoms from the methyl group, directly resulting in the production of benzyl alcohol or/and benzaldehyde, which were further attacked by an O^• leading to benzoic acid or the direct opening of the aromatic ring followed by mineralization of intermediates by O^• [22]. With an increase in the inlet concentration of toluene, therefore, the consumption of O^• by toluene increases, which may result in a decrease in the generation of O₃ during photolysis process (see (3)).

3.2. Optimization of MnO₂-Impregnated GAC

In addition to the elimination of excess O₃, MnO₂-impregnated GAC was also applied to enhance the removal of toluene through the catalyzed-ozonation process. The XRD pattern of MnO₂-impregnated GAC is shown in Figure 3. Besides two basic diffraction peaks (2θ = 24° and 43°), there is a sharp and intense peak at about 2θ = 26°, which is identified with the typical spectrum of β-MnO₂ phase [23], the most stable structure in a variety of Mn (IV) oxides structural forms at low temperatures [23]. In addition, significant instrumental noises indicate that MnO₂ impregnated on the surface is amorphous in structure. Figure 4(a) shows the removal efficiency of toluene as a function of the loading of MnO₂ impregnated on GAC (0–10%), where the inlet concentrations of toluene and O₃ are 9 and 40 ppm, respectively, and the depth of GAC is fixed at 1.6 cm. To evaluate the “real” decomposition of toluene by MnO₂-impregnated GAC/O₃, all adsorption sites of MnO₂-impregnated GAC were saturated with toluene followed by the introduction of O₃ (the breakthrough curve for the adsorption of toluene by MnO₂-impregnated GAC has been shown as in Figure SM-4). Since it has been confirmed previously that molecular O₃ reacts very slowly with toluene (1.4 M⁻¹ s⁻¹) [24], the observed reduction of toluene by MnO₂-impregnated GAC/O₃ in Figure 4 could be largely the result of a partial formation of HO^• and O^• (the rate constants for the reaction of toluene with HO^• and O^• are 3.0 × 10⁹ [25] and 2.1 × 10⁹ M⁻¹ s⁻¹ [26], resp.), which is consistent with other reports that the decomposition of toluene and its intermediates mainly result from the presence of HO^• and O^• produced during O₃ decomposition over MnO₂ layer [17, 27, 28]. With an increase in the loading of MnO₂ from 0 to 5%, the impregnation of MnO₂ on GAC indeed enhanced the decomposition of toluene from 49 to 76% while the degradation of toluene remained unchanged (76%) with a further increase in the loading of MnO₂ from 5 to 10%, which could be attributed to (i) the complete consumption of O₃ which halts the initiation of HO^• and O^• formation and/or (ii) overloading of MnO₂ which could block the access of O₃ to surface sites within the pores of GAC, as shown in Figure 5. Therefore, the optimal loading of MnO₂ is selected as 5% when the inlet concentration of O₃ is 40 ppm.

(a)

(b)

(c)

(d)

Similarly, the effect of depth of GAC layer on the decomposition of toluene was also investigated. The MnO₂- (5%) impregnated GACs with different layer depths were saturated with toluene followed by the introduction of 40 ppm O₃ and 20 ppm toluene, respectively. Figure 4(b) demonstrates that, with an increase in the depth of GAC layer from 1.6 to 3.6 cm, the decomposition of toluene increased from 47 to 58% while a further increase in the depth of GAC layer from 3.6 to 5.6 cm did not result in a further removal of toluene, which could be attributed to the complete elimination of O₃ by MnO₂-impregnated GACs. As the photogenerated O₃ increased from 41 to 130 ppm with a decrease in the inlet toluene concentration from 30 to 0 ppm, the ODC system filled with MnO₂-impregnated GACs must be capable of removing 130 ppm O₃ completely. No detection of O₃ in the effluent gas stream has been confirmed when the loading of MnO₂ is 5% and the depth of GAC layer is 3.6 cm. As the inlet O₃ concentration varies with the initial concentration of toluene, it might be unrealistic to optimize the loading of MnO₂ and depth of GAC for each concentration of toluene. For consistency, therefore, the loading of MnO₂ and the depth of GAC layer were selected as 5% and 4.0 cm in the following experiments, respectively.

3.3. Decomposition and Mineralization of Toluene

Figures 6(a) and 7(a) show the removal and mineralization efficiency of toluene as a function of inlet toluene concentration from 5 to 30 ppm, respectively. The efficiencies of toluene decomposition by HFDEL, MnO₂-impregnated GAC/O₃, and HFDEL/MnO₂-impregnated GAC were observed (Figure 6(a)) to decrease from 90 to 46%, 94 to 30%, and 100 to 55%, respectively, while the efficiencies of toluene mineralization by HFDEL, MnO₂-impregnated GAC/O₃, and HFDEL/MnO₂-impregnated GAC (Figure 7(a)) decreased from 74 to 12%, 86 to 7%, and 100 to 23%, respectively, as the inlet toluene concentration increases from 5 to 30 ppm. It is noted that the inlet O₃ concentration in the case of MnO₂-impregnated GAC/O₃ is consistent with the concentration of photogenerated O₃ from HFDEL as a function of inlet toluene concentration. Since the number and energy of photons and active radicals in the reaction area did not change at a fixed input HF power and a constant dosage of catalyst, with constant gas flow rate, unit toluene obtains less energy as inlet toluene concentration increases, which results in relatively lower removal and mineralization efficiencies of higher concentration of inlet toluene. The HFDEL/MnO₂-impregnated GAC possesses higher removal efficiency compared to HFDEL only, which implies the combination of HFDEL with MnO₂-impregnated GAC indeed enhances the removal of toluene. More importantly, the synergistic effects of the mineralization of toluene by HFDEL/MnO₂-impregnated GAC were found in Figure 7(a) in which the mineralization efficiency of toluene by HFDEL/MnO₂-impregnated GAC is higher than the summation of the efficiency by HFDEL only and MnO₂-impregnated GAC/O₃ under the circumstances of the inlet toluene concentration between 20 and 30 ppm, suggesting the photolysis of toluene by HFDEL enhances the subsequent mineralization of toluene by MnO₂-impregnated GAC/O₃. These synergistic effects could be attributed to the formation of intermediates during the photolysis process which are subjected to be mineralized by HO^• and O^• formed during O₃ decomposition over MnO₂-impregnated GAC compared to the original toluene [22].

(a)

(b)

(a)

(b)

Figures 6(b) and 7(b) demonstrate the removal and mineralization mass rate of toluene as a function of inlet toluene concentration from 5 to 30 ppm, respectively. The mass rates of toluene decomposition and mineralization by HFDEL, MnO₂-impregnated GAC/O₃, and HFDEL/MnO₂-impregnated GAC followed a similar profile which increased initially followed by a decrease with an increase in the inlet toluene (Figures 5(b) and 6(b)). During the photolysis process, O^• and HO^• formed via the absorbance of ( nm) by O₂ and H₂O, respectively (see (1) and (2)), are sufficient for low concentration of inlet toluene and thus the removal and mineralization mass rate increases with the inlet toluene concentration. With the inlet toluene concentration approaching to a certain concentration, however, toluene starts to compete with O₂ and H₂O to consume UV light at 185 nm, resulting in a lower production rate of O^• and HO^•. Although the direct photolysis of toluene by UV light at 185 nm can occur, the conversion rate of toluene by direct photolysis is much lower than that by O^• and HO^•, respectively [17], thus resulting in a decrease in the removal and mineralization mass rate with a further increase in the inlet toluene concentration. For the catalyzed ozonation process (MnO₂-impregnated GAC/O₃), a decrease in the removal and mineralization mass rate of toluene with a further increase in the inlet toluene concentration was due to the decrease in the inlet O₃ concentration with increasing in the inlet toluene concentration as the inlet O₃ concentration in this study needs to be consistent with the photogenerated O₃ concentration (Figure 2). For the low concentration of inlet toluene, an increase in removal and mineralization mass rate with the inlet toluene concentration can be observed since the high concentration of inlet O₃ combined with MnO₂-impregnated GAC results in the sufficient amount of O^• and HO^• for decomposition of toluene.

4. Discussion

In this study, we have shown that the removal efficiency of toluene using HFDEL depends on the inlet toluene concentration and the combination of MnO₂-impregnated GAC with HFDEL can not only eliminate the residual O₃ but also enhance the removal of toluene. More importantly, the synergistic effects of HFDEL/MnO₂-impregnated GAC on the mineralization of toluene have also been confirmed, showing the intermediates formed during the photolysis process are prone to be mineralized by the following MnO₂-impregnated GAC catalyzed ozonation process. Possible mechanisms for the removal of toluene by UV/O₃/MnO₂-impregnated GAC will be discussed in this section.

Under oxygen environment, the photo energy of VUV light at 185 nm produced by HFDEL is capable of destroying the bond of O=O (491 kJ mol⁻¹), resulting in formation of O^• (see (1)) with a subsequent generation of HO^• and O₃ (see (2) and (3)) [29, 30]. Photogenerated O₃ can efficiently be decomposed into O^• by UV irradiation (see (4)) and O^• can further react with H₂O and O₃ to generate HO^•, respectively (see (2) and (5)) [31, 32]: For O₃ decomposition over the layer of MnO₂-impregnated GAC, O₃ can be also decomposed to form O^• on the active sites of MnO₂-impregnated GAC surface (see (6)) and O^• can further react with H₂O and O₃ to generate HO^•, respectively (see (2) and (5)) [17]: where represents active sites on MnO₂-impregnated GAC surface. Due to the limited direct photolysis of toluene by VUV at 185 nm, the primary pathway of toluene oxidation was the H-abstraction from the methyl group by HO^• and O^•, resulting in two pathways of toluene destruction in the UV/O₃/MnO₂-impregnated GAC process.

The primary pathway of toluene oxidation by HO^•/O^• was the H-abstraction from the methyl group, resulting in the production of a benzyl radical and then the formation of benzyl alcohol and/or benzaldehyde [22], which were further attacked by HO^•/O^• leading to benzoic acid followed by the opening of the aromatic ring [33–35]. The compounds generated after the ring opening were substances with low molecular mass, such as formic acid, acetic acid, and CO, with a subsequent formation of harmless CO₂ and H₂O by the attack of HO^•/O^•. In our study, benzyl alcohol and benzaldehyde dimethyl acetal (BDA) as intermediates were detected by GC-MS analysis (Figure SM5). The presence of BDA, the product of aldol condensation of benzaldehyde and methanol under acidic conditions, suggests that benzaldehyde is formed during the photolysis of toluene. Due to the lack of detection of benzoic acid, the acidic environment for the formation of BDA could be attributed to the formation of low molecular-weight acid (e.g., formic acid and acetic acid). The low boiling point of these small organic acids leads to no direct evidence to confirm their presence by GC-MS. In summary, it can be proposed that the intermediates including benzyl alcohol and benzaldehyde are produced from the HFDEL system followed by the direct opening of the aromatic ring without formation of benzoic acid by the attack of HO^•/O^•. Compared to the original toluene compounds, these intermediates are more subjected to be decomposed to small molecules followed by the formation of CO₂ and H₂O via the subsequent O₃/MnO₂-impregnated GAC process, resulting in the synergistic mineralization of toluene using the HFDEL/MnO₂-impregnated GAC process.

5. Conclusion

The destruction of low concentration of toluene (0–30 ppm) has been studied under the advanced photooxidation processes by the combination of self-made HFDEL with MnO₂-impregnated GAC catalyst. The conclusions are as follow:(1)The concentration of photogenerated O₃ from HFDEL decreased from 130 to 41 ppm with an increase in the inlet toluene concentration from 0 to 30 ppm.(2)The efficiency of decomposition of toluene by HFDEL decreased from 90 to 46% as the inlet toluene concentration increases from 5 to 30 ppm. The introduction of MnO₂-impregnated GAC catalyst is not only to eliminate the residual O₃ (41–130 ppm) completely but also to enhance the decomposition of toluene by ~10%. (The mass loading of MnO₂ and the depth of GAC layer were 5% and 4.0 cm, resp.)(3)Active oxygen and hydroxyl radicals generated from HFDEL/MnO₂-impregnated GAC system played a key role in the decomposition of toluene process. The intermediates formed by photolysis are more prone to be mineralized by the subsequent MnO₂-impregnated GAC catalyst compared to the original toluene, resulting in synergistic mineralization of toluene by HFDEL/MnO₂-impregnated GAC system.

In summary, the combination of HFDEL and MnO₂-impregnated GAC efficiently enhances the toluene destruction process, eliminates the residual O₃, and, more importantly, fulfills the synergistic mineralization of toluene, demonstrating that HFDEL/MnO₂-impregnated GAC system will be a promising air-cleaning technology for contamination of lower concentration of VOCs.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant no. 51178135) and China Postdoctoral Science Foundation (Grant no. 2014M560267).

Supplementary Materials

More details on emission spectrum of HFDEL and stability and distribution of light intensity of HFDEL have been shown in Supplementary Material available.

Supplementary Material

References

L. Zou, Y. Luo, M. Hooper, and E. Hu, “Removal of VOCs by photocatalysis process using adsorption enhanced TiO₂-SiO₂ catalyst,” Chemical Engineering and Processing: Process Intensification, vol. 45, no. 11, pp. 959–964, 2006.
View at: Publisher Site | Google Scholar
Y. M. Kim, S. Harrad, and R. M. Harrison, “Concentrations and sources of VOCs in urban domestic and public microenvironments,” Environmental Science and Technology, vol. 35, no. 6, pp. 997–1004, 2001.
View at: Publisher Site | Google Scholar
J. Peral, X. Domenech, and D. F. Ollis, “Heterogeneous photocatalysis for purification, decontamination and deodorization of air,” Journal of Chemical Technology and Biotechnology, vol. 70, pp. 117–140, 1997.
View at: Google Scholar
T. X. Liu, X. Z. Li, and F. B. Li, “AgNO₃-induced photocatalytic degradation of odorous methyl mercaptan in gaseous phase: Mechanism of chemisorption and photocatalytic reaction,” Environmental Science and Technology, vol. 42, no. 12, pp. 4540–4545, 2008.
View at: Publisher Site | Google Scholar
T.-X. Liu, F.-B. Li, and X.-Z. Li, “Effects of peptizing conditions on nanometer properties and photocatalytic activity of TiO₂ hydrosols prepared by H₂TiO₃,” Journal of Hazardous Materials, vol. 155, no. 1-2, pp. 90–99, 2008.
View at: Publisher Site | Google Scholar
I. E. Den Besten and J. W. Tracy, “Electrodelessly discharged photochemical lamps,” Journal of Chemical Education, vol. 50, no. 4, p. 303, 1973.
View at: Publisher Site | Google Scholar
S. Iwaguch, K. Matsumura, Y. Tokuoka, S. Wakui, and N. Kawashima, “Sterilization system using microwave and UV light,” Colloids and Surfaces B: Biointerfaces, vol. 25, no. 4, pp. 299–304, 2002.
View at: Publisher Site | Google Scholar
I. Pandithas, K. Brown, A. I. Al-Shamma'a, J. Lucas, and J. J. Lowke, “Biological applications of a low pressure microwave plasma UV lamp,” in Proceedings of the 14th IEEE International Pulsed Power Conference (Ppc '03), vol. 1-2, pp. 1112–1115, 2003.
View at: Google Scholar
S. Horikoshi, H. Hidaka, and N. Serpone, “Environmental remediation by an integrated microwave/UV-illumination method, 1. Microwave-assisted degradation of rhodamine-B dye in aqueous TiO₂ dispersions,” Environmental Science and Technology, vol. 36, no. 6, pp. 1357–1366, 2002.
View at: Publisher Site | Google Scholar
S. Horikoshi, H. Hidaka, and N. Serpone, “Environmental remediation by an integrated microwave/UV illumination technique. 3. A microwave-powered plasma light source and photoreactor to degrade pollutants in aqueous dispersions of TiO₂ illuminated by the emitted UV/visible radiation,” Environmental Science and Technology, vol. 36, no. 23, pp. 5229–5237, 2002.
View at: Publisher Site | Google Scholar
S. Horikoshi, H. Hidaka, and N. Serpone, “Environmental remediation by an integrated microwave/UV illumination technique VI. A simple modified domestic microwave oven integrating an electrodeless UV-Vis lamp to photodegrade environmental pollutants in aqueous media,” Journal of Photochemistry and Photobiology A, vol. 161, no. 2-3, pp. 221–225, 2004.
View at: Publisher Site | Google Scholar
S. Horikoshi, F. Hojo, H. Hidaka, and N. Serpone, “Environmental remediation by an integrated microwave/UV illumination technique. 8. Fate of carboxylic acids, aldehydes, alkoxycarbonyl and phenolic substrates in a microwave radiation field in the presence of TiO₂ particles under UV irradiation,” Environmental Science and Technology, vol. 38, no. 7, pp. 2198–2208, 2004.
View at: Publisher Site | Google Scholar
P. Klan, J. Literak, and M. Hajek, “The electrodeless discharge lamp: a prospective tool for photochemistry,” Journal of Photochemistry and Photobiology A, vol. 128, pp. 145–149, 1999.
View at: Google Scholar
L.-Y. Xia, D.-H. Gu, J. Tan, W.-B. Dong, and H.-Q. Hou, “Photolysis of low concentration H₂S under UV/VUV irradiation emitted from microwave discharge electrodeless lamps,” Chemosphere, vol. 71, no. 9, pp. 1774–1780, 2008.
View at: Publisher Site | Google Scholar
J. Xu, C. Li, P. Liu, D. He, J. Wang, and Q. Zhang, “Photolysis of low concentration H₂S under UV/VUV irradiation emitted from high frequency discharge electrodeless lamps,” Chemosphere, vol. 109, pp. 202–207, 2014.
View at: Publisher Site | Google Scholar
P. M. Álvarez, J. P. Pocostales, and F. J. Beltrán, “Granular activated carbon promoted ozonation of a food-processing secondary effluent,” Journal of Hazardous Materials, vol. 185, no. 2-3, pp. 776–783, 2011.
View at: Publisher Site | Google Scholar
J. Jeong, K. Sekiguchi, and K. Sakamoto, “Photochemical and photocatalytic degradation of gaseous toluene using short-wavelength UV irradiation with TiO₂ catalyst: comparison of three UV sources,” Chemosphere, vol. 57, no. 7, pp. 663–671, 2004.
View at: Publisher Site | Google Scholar
K.-P. Yu and G. W. M. Lee, “Decomposition of gas-phase toluene by the combination of ozone and photocatalytic oxidation process (TiO₂/UV, TiO₂/UV/O₃, and UV/O₃),” Applied Catalysis B: Environmental, vol. 75, no. 1-2, pp. 29–38, 2007.
View at: Publisher Site | Google Scholar
P. Müller, P. Klán, and V. Ćrkva, “The electrodeless discharge lamp: A prospective tool for photochemistry Part 4: temperature- and envelope material-dependent emission characteristics,” Journal of Photochemistry and Photobiology A: Chemistry, vol. 158, no. 1, pp. 1–5, 2003.
View at: Publisher Site | Google Scholar
E. J. Hart, K. Sehested, and J. Holcman, “Molar absorptivities of ultraviolet and visible bands of ozone in aqueous solutions,” Analytical Chemistry, vol. 55, no. 1, pp. 46–49, 1983.
View at: Publisher Site | Google Scholar
H. Einaga and S. Futamura, “Catalytic oxidation of benzene with ozone over alumina-supported manganese oxides,” Journal of Catalysis, vol. 227, no. 2, pp. 304–312, 2004.
View at: Publisher Site | Google Scholar
H. Huang and W. Li, “Destruction of toluene by ozone-enhanced photocatalysis: performance and mechanism,” Applied Catalysis B: Environmental, vol. 102, no. 3-4, pp. 449–453, 2011.
View at: Publisher Site | Google Scholar
B. Liu, A. Li, M. Xia, and Z. Zhu, “Preparation of manganese oxide supported on activated carbon and its application in catalytic ozonation of 4-chlorophenol,” Advanced Materials Research, vol. 538–541, pp. 2285–2288, 2012.
View at: Publisher Site | Google Scholar
J. Hoigne and H. Bader, “Rate constants of reactions of ozone with organic and inorganic compounds in water, 1. Non-dissociating organic compounds,” Water Research, vol. 17, no. 2, pp. 173–183, 1983.
View at: Publisher Site | Google Scholar
L. M. Dorfman, I. A. Taub, and D. A. Harter, “Rate constants for the reaction of the hydroxyl radical with aromatic molecules,” The Journal of Chemical Physics, vol. 41, no. 9, pp. 2954–2955, 1964.
View at: Publisher Site | Google Scholar
H. C. Christensen, K. Sehested, and E. J. Hart, “Formation of benzyl radicals by pulse radiolysis of toluene in aqueous solutions,” The Journal of Physical Chemistry, vol. 77, no. 8, pp. 983–987, 1973.
View at: Publisher Site | Google Scholar
J. Ma and N. J. D. Graham, “Degradation of atrazine by manganese-catalysed ozonation: influence of humic substances,” Water Research, vol. 33, no. 3, pp. 785–793, 1999.
View at: Publisher Site | Google Scholar
S. Futamura, A. Zhang, H. Einaga, and H. Kabashima, “Involvement of catalyst materials in nonthermal plasma chemical processing of hazardous air pollutants,” Catalysis Today, vol. 72, no. 3-4, pp. 259–265, 2002.
View at: Publisher Site | Google Scholar
J. H. Seinfeld and I. Pandis, Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, John Wiley & Sons, 2nd edition, 2006.
D. He, A. M. Jones, S. Garg, A. N. Pham, and T. D. Waite, “Silver nanoparticle-reactive oxygen species interactions: application of a charging-discharging model,” Journal of Physical Chemistry C, vol. 115, no. 13, pp. 5461–5468, 2011.
View at: Publisher Site | Google Scholar
X. Huang, J. Yuan, J. Shi, and W. Shangguan, “Ozone-assisted photocatalytic oxidation of gaseous acetaldehyde on TiO₂/H-ZSM-5 catalysts,” Journal of Hazardous Materials, vol. 171, no. 1–3, pp. 827–832, 2009.
View at: Publisher Site | Google Scholar
M. Kogoma, Y. Miki, K. Tanaka, and K. Takahashi, “Highly efficient VOC decomposition using a complex system (OH radical, ozone-UV, and TiO₂),” Plasma Processes and Polymers, vol. 3, no. 9, pp. 727–733, 2006.
View at: Publisher Site | Google Scholar
M. Sleiman, P. Conchon, C. Ferronato, and J. M. Chovelon, “Photocatalytic oxidation of toluene at indoor air levels (ppbv): towards a better assessment of conversion, reaction intermediates and mineralization,” Applied Catalysis B: Environmental, vol. 86, no. 3-4, pp. 159–165, 2009.
View at: Publisher Site | Google Scholar
T. J. Frankcombe and S. C. Smith, “OH-initiated oxidation of toluene. 1. Quantum chemistry investigation of the reaction path,” Journal of Physical Chemistry A, vol. 111, no. 19, pp. 3686–3690, 2007.
View at: Publisher Site | Google Scholar
T. J. Frankcombe and S. C. Smith, “OH-initiated oxidation of toluene. 2. Master equation simulation of toluene oxide isomerization,” The Journal of Physical Chemistry A, vol. 111, no. 19, pp. 3691–3696, 2007.
View at: Publisher Site | Google Scholar

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Copyright © 2014 Jianhui Xu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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International Journal of Photoenergy

Destruction of Toluene by the Combination of High Frequency Discharge Electrodeless Lamp and Manganese Oxide-Impregnated Granular Activated Carbon Catalyst

Abstract

1. Introduction

2. Materials and Methods

2.1. HFDEL

2.2. Characterization of HFDEL

2.3. MnO2-Impregnated GAC

2.4. Experimental Setup

2.5. Chemical Analysis

3. Results

3.1. Generation of O3

3.2. Optimization of MnO2-Impregnated GAC

3.3. Decomposition and Mineralization of Toluene

4. Discussion

5. Conclusion

Conflict of Interests

Acknowledgments

Supplementary Materials

References

Copyright

2.3. MnO₂-Impregnated GAC

3.1. Generation of O₃

3.2. Optimization of MnO₂-Impregnated GAC