Abatement and degradation pathways of toluene in indoor air by positive corona discharge

Chemosphere 68 (2007) 1821–1829 www.elsevier.com/locate/chemosphere

Abatement and degradation pathways of toluene in indoor air by positive corona discharge J. Van Durme a, J. Dewulf a

a,*

, W. Sysmans a, C. Leys b, H. Van Langenhove

a

Research Group EnVOC, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, B-9000 Ghent, Belgium b Department of Applied Physics, Faculty of Engineering, Ghent University, Rozier 44, B-9000 Ghent, Belgium Received 9 November 2006; received in revised form 13 March 2007; accepted 20 March 2007 Available online 8 May 2007

Abstract Indoor air concentrations of volatile organic compounds often exceed outdoor levels by a factor of 5. There is much interest in developing new technologies in order to improve indoor air quality. In this work non-thermal plasma (DC positive corona discharge) is explored as an innovative technology for indoor air purification. An inlet gas stream of 10 l min1 containing 0.50 ± 0.02 ppm toluene was treated by the plasma reactor in atmospheric conditions. Toluene removal proved to be achievable with a characteristic energy density e0 of 50 J l1. Removal efficiencies were higher for 26% relative humidity (e0 = 35 J l1), compared with those at increased humidities (50% relative humidity, e0 = 49 J l1). Reaction products such as formic acid, benzaldehyde, benzyl alcohol, 3-methyl-4-nitrophenol, 4-methyl-2-nitrophenol, 4-methyl-2propyl furan, 5-methyl-2-nitrophenol, 4-nitrophenol, 2-methyl-4,6-dinitrophenol are identified by means of mass spectrometry. Based on these by-products a toluene degradation mechanism is proposed.  2007 Elsevier Ltd. All rights reserved. Keywords: Indoor air quality; Corona discharge; Degradation products; Toluene oxidation; Ozone

1. Introduction Over the last 25 years, health complaints related to indoor climate have increased (Isbell et al., 2005). Poor indoor air quality can be attributed to physical (humidity), chemical (organic and inorganic), physical–chemical (particulate matter) and biological (molds) agents. Buildings are being sealed more tightly to reduce thermal energy losses. Concentrations of indoor pollutants can build up because there is a low turnover rate of indoor air. Jones (1998) proved that the level of indoor pollutants is 2–10 times higher than outdoor. In general volatile organic compounds (VOCs) can be given off by office products, insulating materials, synthetic furniture, cleaning and maintenance products, pressed wood, etc. or may originate from tobacco

*

Corresponding author. Tel.: +32 9 264 59 49; fax: +32 9 264 62 43. E-mail address: [email protected] (J. Dewulf).

0045-6535/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2007.03.053

smoke (Aguado et al., 2004). Much effort has been devoted to characterize the levels of indoor air pollutants. Concentrations are known to be random variables because of their dependence on several sources and the fluctuations of emission variables (Park and Ikeda, 2004). Associations to adverse health effects such as allergic reactions, headache, eye, nose and throat irritation, dry cough, dizziness and nausea, concentration problems, tiredness (Mo et al., 2005) and even cancer (Luo et al., 1996) have been made to a poor indoor air quality (Jones, 1999). These symptoms affect human health severely and lead to economic losses. Fanger (2000) estimated that the ‘sick building syndrome’ results in a productivity decrease of average 6.5% in offices. Mechanical or electronical filters can effectively trap particulate contaminants and remove them from the circulating air. Ionic air purifiers emit ions enhancing the agglomeration of smaller particles into larger ones, which then gravitationally settle. Ionization may also cause attraction

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between particles and grounded surfaces resulting in electrostatic precipitation (Grinshpun et al., 2005). It has been proven that accumulation of pollutants, may serve as nutrient source for moulds and bacteria. These organisms are able to produce VOCs such as aldehydes, ketones, alcohols, furans, esters and acids (Jelen et al., 1995; Schleibinger and Ruben, 1999). Sorption media (e.g., activated carbon) only transfer VOCs to another phase rather than eliminating them (Zhao and Yang, 2003). The use of adsorbents implies the change, disposal or regeneration of the adsorbents (Pichat et al., 2000). Ozone generators are sold as air cleaners, however the levels of ozone produced by ozone generators can be harmful: concentrations range up to 500 ppbv (Hubbard et al., 2005). Scientific research has shown that these levels of ozone are too low to remove indoor VOCs (Boeniger, 1995). In conclusion, available techniques prove to be not efficient in treating gas flows containing low concentrations of VOCs. In recent years, great attention has been given to the development of new indoor air treatment techniques. A more appropriate set of technologies that offers opportunities for indoor air cleaning are advanced oxidation processes (AOP). In these processes highly reactive, oxidizing species are produced such as ozone, atomic oxygen or hydroxyl radicals. These active species subsequently initiate the VOC degradation. Examples include ultraviolet, photolysis, direct ozonation, high-energy irradiation and ultrasonification (He et al., 2005) and photocatalytic oxidation (PCO) (Zhao and Yang, 2003). Unlike earlier discussed control methods, PCO actually oxidizes pollutants to CO2 and H2O. However, Kim and Hong (2002) showed that the rate of PCO decreased with decreasing pollutant concentration. In addition, at high humidity levels, water vapor competed with TiO2 for adsorption sites which further decreased the rate of PCO (Ao et al., 2003, 2004). Non-thermal plasma processing is an AOP that has been considered as an effective and energy-saving method to remove VOCs due to its unique properties (Futamura et al., 2002; Morent et al., 2006). One of these advantages compared to other purification technologies is its non-selectivity. Plasma designs for removing VOCs include electron beam, surface discharge, dielectric barrier discharge, ferroelectric packed-bed, pulsed corona, DC discharge and microwave discharge processes (Li et al., 2002). The more or less effective removal of a wide variety of contaminants has already been described in literature: aliphatic, aromatic (Rudolph et al., 2002), chlorinated and fluorinated hydrocarbons as well as inorganic pollutants such as SO2, H2S and NOx (Ao et al., 2004). Despite the importance of indoor air quality, only a few studies (Zhu et al., 2005) have reported on the feasibility of applying non-thermal plasma technology for the removal of indoor air pollutants. In this work, a DC positive corona reactor is developed and characterized. This type of plasma, sometimes referred as a unipolar discharge, occurs in a region of high electric field near electrically stressed sharp points, edges, or wires (Sigmond, 1978; Chen and Davidson, 2002). It is generally

known that DC corona appears in two modes: glow and streamer. Glow corona is an electrical discharge near a conductor with a small curvature radius. The region of corona discharge can be divided into two parts: ionized and drift. In the ionized region, which appears in the vicinity of small curvature radius conductors, the intensity of electric field is high enough (>3 · 106 V/m) to produce electrons and ions (Gasparik et al., 2000; Chen and Davidson, 2002). When the number of electrons in an ionized avalanche increases up to an amount where their spatial field is able to shield the electric field, streamers will be formed. A streamer-like discharge is produced by the ionized waves of spatial charge, so it takes place in the space between electrodes (Sigmond, 1994). Streamer discharges have a higher efficiency in the generation of chemical active species. In this work, the discharge was operated in the positive streamer regime. Discharge current waveforms recorded with an oscilloscope feature repetitive current spikes which are indicative of the positive streamer mode. It was verified that the ozone levels measured in this regime are higher than when the same discharge module was operated in the negative (glow) mode (Ma and Qiu, 2003). The visual aspect of the discharge is that of a luminous plasma column completely filling the inter-electrode space. The objective is to investigate the ability of this technology for the abatement of low levels of toluene from indoor air. Toluene, together with benzene, ethylbenzene and o-xylene are grouped as BTEX compounds. BTEX is the major group found in indoor environments in different countries (Van Winkle and Scheff, 2001). Average indoor air concentrations of toluene vary between 5 and 50 ppbv (Tilborghs et al., 2005), with maximum concentrations up to 9 ppm (AERIAS, 2006). To our knowledge, little is known about chemical reactions of toluene in a corona discharge or in non-thermal plasma discharges in general. Reactions are numerous due to the presence of several types of reactive species in the plasma zone. In this work the plasma source will be characterized and the effect of humidity will be examined. Further, the focus is to identify reaction products, allowing a better understanding of the degradation pathway of toluene in a non-thermal plasma. 2. Experimental 2.1. Experimental setup The plasma reactor developed consists of a cylindrical tube with an inner diameter of 40 mm (Fig. 1). The cathode is an inox mesh, the anode consists of four crenellated inox pins. The pin tips are constructed in such a way that the discharge connects to typically eight anode spots on each pin. Each pin is ballasted with a 1.5 MX resistance. The fraction of the total electrical power that is dissipated in these resistors amounts to 10% at most. The pins and mesh are mounted in the tube in such a way that the inter-electron gap can be varied, for this manuscript an inter-electron distance of 20 mm was applied. Gas can freely flow along the

J. Van Durme et al. / Chemosphere 68 (2007) 1821–1829

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Fig. 1. Experimental setup with detail of developed DC corona reactors.

pins. The gas flow pattern and velocities in both plasma units are comparable. The discharge is powered by a 40 kV/5 mA DC high voltage supply, in dry air applied voltages exceeding the inception voltage of 14 kV were necessary to initiate the corona discharge. The plasma reactor is created in a modular way. Several plasma source units can be placed in series along the gas flow. In this work two 4pins-to-mesh modules were placed in series along the gas flow. The units were electrically connected in parallel. Two gas streams are controlled by mass flow controllers (Brooks Instrument BV). Dry air (Alphagas 1, Air Liquide, <3 ppm H2O) is humidified by a temperature controlled water bubble column. A second stream is loaded with toluene after passing through a toluene reservoir. For removal experiments a capillary diffusion system was used (Schoene and Steinhanses, 1989). When concentrations >1 ppm were necessary (e.g., to detect degradation products), a bubble column was used. Toluene 99.5+% was delivered from Aldrich (Steinheim, Germany). Ozone was measured by an ozone analyzer (Anseros) equipped with Picolog Data Acquisition software. Temperature and humidity monitoring was conducted with a TESTO 110 device. For current and voltage measurements two multimeters (Velleman DVM 92) were used. For the experimental determination of Kova´ts indices 3-methyl-4-nitrophenol, 4-methyl-2-nitrophenol, 5-methyl2-nitrophenol were obtained from Aldrich (Steinheim, Germany), n-alkanes were delivered by Polyscience Corp (Niles, US). 2.2. Chemical analysis Inlet and outlet air samples were taken by solid-phase microextraction (SPME) (Martos and Pawliszyn, 1999).

The SPME device was supplied by Supelco equipped with a 57330-U SPME holder containing a plain hub fiber assembly. Xiong et al. (2004) did not detect ozone when a 100 lm polydimethylsiloxane (PDMS) fiber was exposed to high concentrations of ozone (>100 ppm), indicating that the coating is not efficient in extracting ozone. They also proved that the absorption capacity of the PDMS 100 lm SPME coating was not affected by repeated exposure to ozone and oxygen. Chemical analyses were carried out with an Agilent 6890 Series GC, equipped with a flame ionization detector (FID) and a HPCORE integration system. The FID detector (250 C) was fed by 400 ml min1 air and 40 ml min1 hydrogen. The carrier gas was helium with a flow rate of 3 l min1. A SPME inlet liner was installed and placed at a temperature of 200 C. Separation was done on a 30 m · 0.53 mm cross linked methylsiloxane capillary column with a film thickness of 5.0 lm (HP-1) (HP, USA). Analyses were carried out isothermally (140 C). Sampling with a 100 lm PDMS SPME fiber (Supelco) combined with GC-FID, resulted in a limit of detection of 67 ppbv. Analysis of degradation products was done in two ways: First, for the identification of degradation products, a Trace DSQ GC/MS was used (Thermo Finnigan). The temperature program started at 35 C and increased to 40 C with a rate of 1.0 C min1. Next, the temperature was increased to 60 C at 2 C min1, followed by an increase to 160 C at 8 C min1. Finally, the oven temperature reached 220 C with a temperature increase rate of 12 C min1. This final oven temperature was maintained during 10 min. A SPME liner was installed at the inlet. The injector had a constant temperature of 200 C and was used in a splitless mode. The injector was connected to a capillary column VARIAN VF-1ms (FactorFourTM

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0.3 mA for 60% RH. Fouad and Elhazek (1995) concluded that for corona discharges, the attachment coefficient, gh, is function of both the field strength and water vapor content of the gas mixture. For higher RH, an increased attachment coefficient results in a shift of the ionization equilibrium. One of the most important by-products in a non-thermal plasma discharge is ozone. Oxygen radicals are generated by dissociation of molecular oxygen after impact with accelerated electrons in the corona discharge. It is also known that a substantial part of the atomic oxygen formed in air discharges results from processes with excited molecular states of nitrogen (N2(A3R) and N2(B3P) (Morent and Leys, 2005). Atomic oxygen is a strong oxidizer, but its stability is very limited. Due to fast recombination processes, the lifetime is only a few microseconds at atmospheric pressure (Oda et al., 2005). Ozone is mainly formed by a threebody collision by the following reactions:

GC columns), 30 m length, diameter 0.25 mm and coating thickness of 1 lm. Ionization was achieved by electron impact (70 eV), detection of fragments by an electron multiplier (Trace DSQ Thermo Finnigan). For the identification of the unknown compounds obtained mass spectra the NIST v.2.0. databank (NIST/EPA/NIH Mass Spectral Library) was used. Secondly, confirmation of identified compounds was done by determination of Kova´ts indices on a GC-FID (4890) equipped with a capillary HP-5 column (HP, USA), these indices were determined isothermally at 35, 90 or 170 C. The Kova´ts retention index (KI) has received wide acceptance and is defined as Girard (1996): KI ¼ 100z þ 100

ln tRðiÞ  ln tRðzÞ ln tRðzþ1Þ  ln tRðzÞ

where tR(z) < tR(i) < tR(z+1), tR(i) is the retention time of a component I, and tR(z) and tR(z+1) are the retention times of n-alkanes with z and z + 1 carbon atoms. NOx concentrations were measured by a chemiluminescence NO–NO2–NOx Analyzer (Model 42C, Environmental Instruments, Inc., Franklin).

e þ O2 ! e þ O þ O O þ O2 þ M ! O3 þ M In air, M can be either molecular oxygen or molecular nitrogen (Magureanu et al., 2005). Humidity affects the ozone production, our results show a decrease of ozone production with increasing humidity (Fig. 2b). The ozone outlet concentration for an energy density of 40 J l1 is 130 ppm for dry air, while this decreased to 90 ppm at 20% RH and 75 ppm at 60% RH. Chen and Wang (2005) modified existing ozone prediction models taking into account the water effect. Produced hydroxyl radicals and water molecules react with oxygen radicals and ozone. Next, as already mentioned earlier water affects the initial field strength, the mobility of charge carriers and the plasma composition explaining the reduced ozone outlet concentrations.

3. Results and discussion 3.1. Plasma characterization: current–voltage characteristics and ozone production Fouad and Elhazek (1995) reported that the corona inception voltage increases as the relative humidity (RH) increases up to a certain limit when this inception voltage decreases for higher RH. Indoor environments have variable humidity levels: Maki and Aoki (2006) for example, measured humidities ranging from 38% to 78% in concrete building rooms. In this work the effect of water molecules on the discharge characteristics was examined. Fig. 2a illustrates the influence of humidity on the plasma characteristics. With increasing humidity, lower currents are measured for a given voltage. For an applied voltage of 18.4 kV, a current of 0.6 mA is measured in dry air, while this is only

U (kV)

19.50 18.50 17.50

dry air RH=20%

16.50

RH=40% 15.50 14.50 0.00

RH=60%

0.50

I (mA)

1.00

A gas stream containing 0.5 ppm toluene was treated by the non-thermal plasma reactor. Fig. 3 shows the removal

Ozone concentration (ppm)

20.50

3.2. Toluene oxidation experiments

350.00 300.00 250.00 200.00 150.00 100.00 dry air RH=40%

50.00 0.00 0.00

20.00

40.00

60.00

RH=20% RH=60%

80.00

100.00

Energy density (J/l)

Fig. 2. Effect of indoor air humidity on plasma characteristics: (a) voltage–current curves and (b) ozone outlet concentration (ppm) as function of energy density (J l1) (Qair = 10 l min1, P = 101.3 kPa, T = 298 K).

J. Van Durme et al. / Chemosphere 68 (2007) 1821–1829

Removal efficiency (%)

100 90 80 70 60 50 40 30 50% RH 26% RH dry air

20 10 0 0

20

40

60

80

Energy density (J/l)

Fig. 3. Removal efficiency (%) of 0.5 ppm toluene by DC positive corona discharge and the effect of humidity (0%, 26% and 50% RH) (Qair = 10 l min1, P = P = 101.3 kPa, T = 298 K) (n = 3).

efficiency (%) of toluene for a constant flow rate of 10 l min1. Experiments were done at room temperature, atmospheric pressure and for different humidities (dry air, 26% and 50% RH). The detection limit of the 100 lm PDMS-SPME combined with GC-FID measurements was 67 ppbv. Ozone exposure did not result in visual deterioration and change of sorption capability of the PDMS coating during our experiments. Toluene degradation can be the result of direct electron impact dissociation. Since toluene concentrations are rather low, the probability for direct ionization, however, is very small. Collisions of accelerated electrons with other gas molecules such as N2, O2 or H2O are more probable, resulting in active species such as atomic oxygen or hydroxyl radicals. The most powerful toluene oxidizing species are in fact the hydroxyl radicals (k = 5.7 · 1012 molecule cm3 s1) followed by atomic oxygen (k = 7.6 · 1014 molecule cm3 s1) (NIST, 2006). The reaction rate is smaller for ozone (k = 3.9 · 1022 molecule cm3 s1), resulting in a minor role in the oxidation kinetics of toluene (NIST, 2006). Next, excited nitrogen loses its excess of energy by emitting UV light which could also result in photolysis (Duten et al., 2002). Humidity clearly has a significant effect on removal efficiency of toluene by plasma. In dry air 46% toluene is oxidized at 28.8 J l1, while this is 57% when the gas has a RH of 26%. Water molecules dissociate to form H and OH by collision with electrons or reaction with O, O(1D). From the reaction kinetics of toluene with available oxidizing species, it can be concluded that OH induced degradation is important (NIST, 2006). Ono and Oda (2002) explain that for low water concentrations (<1% RH) the OH density is proportional to water concentration. This positive effect, however, is counteracted for higher humidities. For a constant applied voltage, the water concentration strongly influences the hydroxyl radical concentrations. Hydroxyl radical production saturates or diminishes as humidity increases. In the presence of higher water concentrations, increased plasma attachment processes result in a reduced hydroxyl radical production. It can be concluded that two opposite phenomena are seen:

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water partially dissociates in the plasma, producing reactive species, but humidity also negatively influences the plasma characteristics. The concentration of all these reactive species (OH, O, O3, charged species) can be linked with the energy density of the plasma. The relationship between energy density and removal efficiency can be expressed as Vertriest et al. (2003)   ee N out ¼e 0 N in with Nin (Nout) the density of toluene molecules in inlet (outlet) gas stream (molecule cm3), the energy density e (J l1) and the characteristic energy e0 (J l1). This parameter e0 is often used to express the energy efficiency of the used discharge reactor and is defined as the energy necessary to reduce the concentration of pollutants by a factor e. The characteristic energy for the oxidation of 0.5 ppm toluene in dry air is 49.5 J l1. For 26% RH, the characteristic energy e0 is 35 J l1, while it was 49 J l1 for 60% RH. Mok et al. (2002) found that alkenes and substituted alkenes have higher decomposition rates than aromatics and substituted alkanes in pulsed corona and dielectric barrier discharges. Vertriest et al. (2003) tested several VOCs, they proved that toluene was the most difficult to remove with a negative DC glow discharge (e.g., e0,toluene = 160 J l1 > e0,octane = 125 J l1 > e0,1-octene = 30 J l1). Since in this work toluene was efficiently removed, it may be expected that the positive streamer discharge reactor is able to oxidize several indoor VOCs in a rather efficient way. 3.3. Intermediates: toluene degradation products In order to obtain measurable concentrations of degradation products, higher inlet concentrations of 150 ppm were used. The plasma reactor effluent of the oxidized toluene-air gas stream is analyzed in order to identify degradation products. It is known that non-thermal plasma has high potential in air cleaning technology, but a disadvantage could be that in some cases unwanted degradation products are formed which could be more harmful than the original VOC (Magureanu et al., 2005). Identifying degradation products is done in order to understand reaction mechanism in a DC positive corona discharge. Fig. 4 shows that conversion of toluene into CO2 is not complete at these inlet concentrations and partially oxidized intermediates can be formed. Analysis of the degradation products was done with SPME sampling combined with GC/MS analysis. Confirmation of identified products with a relative abundance of more then 10% was done on GC-FID by KI determination. Calculated and measured KI values of products identified by mass spectrometry are given in Table 1. For 3-methyl-4nitrophenol, 4-methyl-2-nitrophenol and 5-methyl-2-nitrophenol, KI were not found in literature; they were determined experimentally.

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Fig. 4. Chromatogram of GC/MS analysis for the identification of toluene degradation products (dry air (<1% RH), Q = 10.8 l min1, Cin = 150 ± 1 ppm, e = 54 J l1, P = 101.3 kPa).

Table 1 Identified and confirmed most abundant oxidation products by mass spectrometry and Kova´ts index determination (n = 3) RT (min)

Compound

KI (–)exp

KI (–)lit

Reference

3.82 24.00 26.21 28.37 28.95 31.05 31.45 35.58 38.20

Formic acid Benzaldehyde Benzyl alcohol 3-Methyl-4-nitrophenol 4-Methyl-2-propyl furan 4-Methyl-2-nitrophenol 5-Methyl-2-nitrophenol 4-Nitrophenol 2-Methyl-4,6-dinitrophenol

582.0 ± 11.6 960.8 ± 19.2 1023.6 ± 20.5 1222.3 ± 24.4 1236.5 ± 24.7 1236.4 ± 24.7 1247.0 ± 24.9 1544.8 ± 30.9 1634.6 ± 32.7

580.0 960.0 1039.0 1225.1 1241.0 1241.3 1247.4 1539.0 1624.0

NIST (2006) Flavornet (2006) Flavornet (2006) This work NIST (2006) This work This work NIST (2006) NIST (2006)

Atkinson et al. (2000) showed that aromatic compounds react with OH radicals by two pathways: hydrogen atom abstraction and OH addition to the aromatic ring. This results in a complex oxidation mechanism of toluene via several pathways, producing either ring-retaining or ringopening products (Fig. 5). (Pathways I–XII are illustrated in Fig. 5.) 3.3.1. Ring-retaining products after H atom abstraction Benzaldehyde, benzoic acid and benzyl alcohol were identified in the outlet gas stream. An important pathway in toluene (I) oxidation, partially resulting in these degradation products, is a hydrogen abstraction from the methyl group leading to a benzyl radical (II) (D’hennezel et al., 1998). In the non-thermal plasma, this H-abstraction could be the result of (a) direct electron impact, or (b) abstraction by hydroxyl radicals. The benzyl radical reacts with oxygen to form a benzyl peroxy radical (III). The benzyl peroxy radicals can couple to form a tetraoxide (IV). This intermediate can decompose according the Russell-mechanism to yield non-radical products: alcohols, ketones and singlet oxygen. The benzyl peroxy radical can also react with a hydroperoxy radical to form a monoalkyltetroxide (V) that decomposes into benzaldehyde, molecular oxygen, and water. In the gas phase, Seuwen and Warneck (1996) pro-

posed that the benzyl peroxy radicals undergo self-reaction to form either benzaldehyde via a benzoxy radical or benzaldehyde and benzyl alcohol. Benzaldehyde oxidizes easily into benzoic acid. Benzene-1-methyl-3-(phenylmethyl) (retention time (RT) = 36.49 min) was tentatively identified (not confirmed by KI) in the outlet gas stream (VI). The formation of this compound can be explained by benzyl radicals who can also react with an incoming toluene molecule, initiating a polymerization reaction. D’hennezel et al. (1998) proved that during toluene oxidation, a methyl group attack with subsequent coupling could result in the production of oand p-methyl-diphenyl-methane isomers and bibenzyl (VI). 3.3.2. Ring-opening products During our measurements 4-methyl-propyl furan and formic acid were identified. These compounds indicate that the aromatic ring has been opened. The ring-opening pathways of the toluene oxidation may proceed by a sequential addition of OH and O2, resulting in hydroxycyclohexadienyl type peroxy radical (VII). Bartolotti and Edney (1995) wrote that in the atmosphere 85% of the time OH radical reactions with toluene lead to this intermediate. This unstable, reactive compound can form a peroxide bridged radical which is the precursor for both the carbonyl and the

J. Van Durme et al. / Chemosphere 68 (2007) 1821–1829

H3C

H3C

( VIII )

CH3

*

CH3 ( VII )

O2

1827 H

CH2

O O

(I)

OH

OH

OH

O2

O

( II )

HO

( III ) O2

HO

O

O

HO 2

Polymerisation products ( VI )

( IX )

•

CH3

OH

− HO 2

CH3 O2

H2O

•

O2

CH3

•

+ HO 2

OH

O4H

OH O

H

O4

H

O O

(V)

•

O2

HO 2

CH 3

cresol

O

( IV )

•

O2

HO 2

OH

First generation products

O2 O

H3C

O

O

2(5H)-furanone-5-methyl

H

O2

OH H

H2O

Tetraoxide compound

CH3

( XII )

O

H

OH CH3 H H

O N

H O

O

O

O

( XI )

+

O

+

N

-

O

-

2-methyl-4, 6-dinitrophenol OH

Benzaldehyde H3C

O

CH3

O•

Benzyl alcohol HO

4-methyl-2-propyl furan

O

+

N O

O

-

4-nitrophenol

O CH3

O

O

Benzoic acid

O

O HCOOH

(X)

CH2O

formic acid

O

O CO

CO2 H2O

Fig. 5. Degradation pathway of toluene in non-thermal plasma discharge (Qair = 10 l min1, P = 101.3 kPa, T = 298 K, 0% humidity). Compounds enclosed by full frame are identified and confirmed; compounds in dotted frame are tentatively identified.

epoxide route. The dicarbonyl reaction route describes a ring opening via a series of peroxide–bicyclic intermediates (VIII) eventually forming a-dicarbonyls and conjugated cdicarbonyls. Furanones (4-methyl-2-propyl furan) are proposed products of glyoxal and methyl glyoxal formed as primary products of the decomposition of the peroxidebicyclic radical (Wagner et al., 2003). Bartolotti and Edney (1995) described the epoxide route in which epoxides are the dominant reaction products during OH initiated oxidation of toluene. A relatively stable epoxide (IX) (Fig. 5) can be formed by addition of O2 to the ring or by multistep processes. This structure is unstable and undergoes further reactions. Based on analogies with less complex alkoxy radicals, the epoxide radical (IX) could undergo carbon–carbon scission and reaction

with O2. A first reaction pathway could be the scission of the 2,3-carbon bond in the presence of O2, finally resulting in CHOCHO (X) and a reactive unsaturated epoxide dicarbonyl compound (XI). A second reaction pathway is the abstraction of H by O2 producing an epoxide carbonyl ring compound (XII) which itself is subject to further oxidation reaction. 3.3.3. Ring retaining products after OH-addition Another possible chemical pathway of toluene oxidation can be via an isomerization and subsequent release of HO2 to form cresol. This cresol route can explain partial conversion into nitro-aromatics by reaction with NO2 and NO3 (Hamilton et al., 2005). Reaction products of NOx and cresol were identified in the outlet gas stream;

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methyl-nitrophenols (3-methyl-4-nitrophenol, 4-methyl-2nitrophenol, 5-methyl-2-nitrophenol), methyl-dinitrophenols (2-methyl-4,6-dinitrophenol) and others (4-nitrophenol) (Table 1). Because of the presence of these nitro compounds as degradation products, NO and NO2 levels were measured in the outlet stream of the corona discharge. Concentrations of nitric oxide in the outlet gas stream were smaller then 10 ppbv, these molecules are efficient oxidized into NO2 when an excess of O3, O2 and O is present. This trend is consistent with other experimental studies on coronastreamer like discharges (Cooray and Rahman, 2005). Experiments showed that NO2 production increases linearly with applied energy density. This relationship is confirmed by Cooray and Rahman (2005). NOx outlet concentrations for an energy density of 15 J l1 were 2.5 ppm, while this was 5 ppm for 30 J l1. 4. Conclusions Non-thermal plasma technology is proposed as a useful technology for indoor air purification. A DC positive corona is used to oxidize low toluene levels in gas streams. Humidity strongly influences the plasma characteristics, lower electron densities and ozone levels are measured when humidity increases. The removal of toluene is achievable with a characteristic energy density of 50 J l1 (Ctoluene = 0.5 ppm, Q = 10 l min1). Degradation products are benzaldehyde, formic acid, nitrophenols, furans, and others. Hydroxyl radicals play a major role in the oxidation kinetics due to initiation by H-abstraction or OH-addition. Generally, it can be concluded that chemical pathways are similar to those in atmospheric chemistry, however, strongly accelerated. Acknowledgements The author acknowledges financial support by the Institute for the Promotion of Innovation through Science and Technology in Flanders (IWT-Vlaanderen). The author also thanks the Flemish Environment Agency (VMM – Vlaamse Milieumaatschappij) for the use of the NOx-analyzer. References Aguado, S., Polo, A.C., Bernal, M.P., Coronas, J., Santamaria, J., 2004. Removal of pollutants from indoor air using zeolite membranes. J. Membr. Sci. 240, 159–166. Air Quality Sciences (AERIAS), 2006. . Ao, C.H., Lee, S.C., Mak, C.L., Chan, L.Y., 2003. Photodegradation of volatile organic compounds (VOCs) and NO for indoor air purification using TiO2: promotion versus inhibition effect of NO. Appl. Catal. B: Environ. 42, 119–129. Ao, C.H., Lee, S.C., Yu, J.Z., Xu, J.H., 2004. Photodegradation of formaldehyde by photocatalyst TiO2: effects on the presences of NO, SO2 and VOCs. Appl. Catal. B: Environ. 54, 41–50. Atkinson, R., Baulch, D.L., Cox, R.A., Hampson, R.F., Kerr, J.A., Rossi, M.J., Troe, J., 2000. Evaluated kinetic and photochemical data for atmospheric chemistry: supplement VIII, halogen species-IUPAC

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