Abatement and degradation pathways of toluene in indoor air
by positive corona discharge
J.Van Durme a ,J.Dewulf
a,*
,W.Sysmans a ,C.Leys b ,H.Van Langenhove
a
a
Rearch Group EnVOC,Faculty of Bioscience Engineering,Ghent University,Coupure Links 653,B-9000Ghent,Belgium
b
Department of Applied Physics,Faculty of Engineering,Ghent University,Rozier 44,B-9000Ghent,Belgium
啊组词四声
数据库结构Received 9November 2006;received in revid form 13March 2007;accepted 20March 2007
Available online 8May 2007
Abstract
Indoor air concentrations of volatile organic compounds often exceed outdoor levels by a factor of 5.There is much interest in devel-oping 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 10l min À1containing 0.50±0.02ppm toluene was treated by the plasma reactor in atmospheric conditions.Tol-uene removal proved to be achievable with a characteristic energy density e 0of 50J l À1.Removal efficiencies were higher for 26%relative humidity (e 0=35J l À1),compared with tho at incread humidities (50%relative humidity,e 0=49J l À1).
Reaction products such as formic acid,benzaldehyde,benzyl alcohol,3-methyl-4-nitrophenol,4-methyl-2-nitrophenol,4-methyl-2-propyl furan,5-methyl-2-nitrophenol,4-nitrophenol,2-methyl-4,6-dinitrophenol are identified by means of mass spectrometry.Bad on the by-products a toluene degradation mechanism is propod.Ó2007Elvier Ltd.All rights rerved.
Keywords:Indoor air quality;Corona discharge;Degradation products;Toluene oxidation;Ozone
1.Introduction
Over the last 25years,health complaints related to indoor climate have incread (Isbell et al.,2005).Poor indoor air quality can be attributed to physical (humidity),chemical (organic and inorganic),physical–chemical (par-ticulate matter)and biological (molds)agents.Buildings are being aled more tightly to reduce thermal energy loss.Concentrations of indoor pollutants can build up becau there is a low turnover rate of indoor air.Jones (1998)proved that the level of indoor pollutants is 2–10times higher than outdoor.In general volatile organic com-pounds (VOCs)can be given offby office products,insulat-ing materials,synthetic furniture,cleaning and maintenance products,presd may originate from tobacco
smoke (Aguado et al.,2004).Much effort has been devoted to characterize the levels of indoor air pollutants.Concen-trations are known to be random variables becau of their dependence on veral sources and the fluctuations of emis-sion variables (Park and Ikeda,2004).
Associations to adver health effects such as allergic reactions,headache,eye,no and throat irritation,dry cough,dizziness and naua,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).The symptoms affect human health verely and lead to economic loss.Fanger (2000)estimated that the ‘sick building syndrome’results in a productivity decrea of average 6.5%in offices.
Mechanical or electronical filters can effectively trap par-ticulate contaminants and remove them from the circulating air.Ionic air purifiers emit ions enhancing the agglomera-tion of smaller particles into larger ones,which then gravi-tationally ttle.Ionization may also cau attraction
0045-6535/$-e front matter Ó2007Elvier Ltd.All rights rerved.doi:10.1016/j.chemosphere.2007.03.053
*
Corresponding author.Tel.:+3292645949;fax:+3292646243.E-mail address:jo.dewulf@ugent.be (J.Dewulf).
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Chemosphere 68(2007)
1821–1829
between particles and grounded surfaces resulting in elec-trostatic precipitation(Grinshpun et al.,2005).It has been proven that accumulation of pollutants,may rve as nutri-ent source for moulds and bacteria.The organisms are able to produce VOCs such as aldehydes,ketones,alcohols, furans,esters and acids(Jelen et al.,1995;Schleibinger and Ruben,1999).Sorption ,activated carbon)only transfer VOCs to another pha rather than eliminating them(Zhao and Yang,2003).The u of adsorbents implies the change,disposal or regeneration of the adsorbents (Pichat et al.,2000).Ozone generators are sold as air clean-ers,however the levels of ozone produced by ozone genera-tors can be harmful:concentrations range up to500ppbv (Hubbard et al.,2005).Scientific rearch has shown that the levels of ozone are too low to remove indoor VOCs (Boeniger,1995).
In conclusion,available techniques prove to be not effi-cient in treating gasflows 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 t of technologies that offers opportu-nities for indoor air cleaning are advanced oxidation pro-cess(AOP).In the process highly reactive,oxidizing species are produced such as ozone,atomic oxygen or hydroxyl radicals.The active species subquently initiate the VOC de
gradation.Examples include ultraviolet,pho-tolysis,direct ozonation,high-energy irradiation and ultra-sonification(He et al.,2005)and photocatalytic oxidation (PCO)(Zhao and Yang,2003).Unlike earlier discusd control methods,PCO actually oxidizes pollutants to CO2and H2O.However,Kim and Hong(2002)showed that the rate of PCO decread with decreasing pollutant concentration.In addition,at high humidity levels,water vapor competed with TiO2for adsorption sites which fur-ther decread the rate of PCO(Ao et al.,2003,2004).
冲浪的原理
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 the advantages compared to other purification technologies is its non-lec-tivity.Plasma designs for removing VOCs include electron beam,surface discharge,dielectric barrier discharge,ferro-electric packed-bed,puld corona,DC discharge and microwave discharge process(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 andfluorinated hydro-carbons as well as inorganic pollutants such as SO2,H2S and NO x(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 stresd 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 con-ductor 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 electricfield is high enough(>3·106V/m)to produce electrons and ions(Gasparik et al.,2000;Chen and Davidson,2002). When the number of electrons in an ionized avalanche increas up to an amount where their spatialfield is able to shield the electricfield,streamers will be formed.A strea-mer-like discharge is produced by the ionized waves of spa-tial charge,so it takes place in the space between electrodes (Sigmond,1994).Streamer discharges have a higher effi-ciency 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 v
isual aspect of the discharge is that of a luminous plasma column completelyfilling the inter-electrode space.
The objective is to investigate the ability of this technol-ogy 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 between5and50ppbv (Tilborghs et al.,2005),with maximum concentrations up to9ppm(AERIAS,2006).To our knowledge,little is known about chemical reactions of toluene in a corona dis-charge or in non-thermal plasma discharges in general. Reactions are numerous due to the prence of veral 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 tup
The plasma reactor developed consists of a cylindrical tube with an inner diameter of40mm(Fig.1).T
he cathode is an inox mesh,the anode consists of four crenellated inox pins.The pin tips are constructed in such a way that the dis-charge connects to typically eight anode spots on each pin. Each pin is ballasted with a1.5M X resistance.The fraction of the total electrical power that is dissipated in the resis-tors amounts to10%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 dis-tance of20mm was applied.Gas can freelyflow along the
1822J.Van Durme et al./Chemosphere68(2007)1821–1829
pins.The gas flow pattern and velocities in both plasma units are comparable.The discharge is powered by a 40kV/5mA DC high voltage supply,in dry air applied voltages exceeding the inception voltage of 14kV were nec-essary to initiate the corona discharge.The plasma reactor is created in a modular way.Several plasma source units can be placed in ries along the gas flow.In this work two 4-pins-to-mesh modules were placed in ries 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,<3ppm H 2O)is humidified by a temperature controlled water bubble column.A cond
stream is loaded with tol-uene after passing through a toluene rervoir.For removal experiments a capillary diffusion system was ud (Schoene and Steinhans,1989).When concentrations >1ppm were necessary (e.g.,to detect degradation products),a bubble column was ud.Toluene 99.5+%was delivered from Aldrich (Steinheim,Germany).Ozone was measured by an ozone analyzer (Anros)equipped with Picolog Data Acquisition software.Temperature and humidity monitor-ing was conducted with a TESTO 110device.For current and voltage measurements two multimeters (Velleman DVM 92)were ud.
巴黎公社的意义
For the experimental determination of Kova
苹果手机照片导入电脑´ts indices 3-methyl-4-nitrophenol,4-methyl-2-nitrophenol,5-methyl-2-nitrophenol were obtained from Aldrich (Steinheim,Germany),n-alkanes were delivered by Polyscience Corp (Niles,US).泊船瓜洲的诗意
2.2.Chemical analysis
Inlet and outlet air samples were taken by solid-pha 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 asmbly.Xiong et al.(2004)did not detect ozone when a 100l m polydimethylsiloxane (PDMS)fiber was expod to high concentrations of ozone (>100ppm),indicatin
g that the coating is not efficient in extracting ozone.They also proved that the absorption capacity of the PDMS 100l m SPME coating was not affected by repeated expo-sure to ozone and oxygen.
Chemical analys were carried out with an Agilent 6890Series GC,equipped with a flame ionization detector (FID)and a HPCORE integration system.The FID detector (250°C)was fed by 400ml min À1air and 40ml min À1hydrogen.The carrier gas was helium with a flow rate of 3l min À1.A SPME inlet liner was installed and placed at a temperature of 200°C.Separation was done on a 30m ·0.53mm cross linked methylsiloxane capillary col-umn with a film thickness of 5.0l m (HP-1)(HP,USA).Analys were carried out isothermally (140°C).Sampling with a 100l m PDMS SPME fiber (Supelco)combined with GC-FID,resulted in a limit of detection of 67ppbv.
Analysis of degradation products was done in two ways:First,for the identification of degradation products,a Trace DSQ GC/MS was ud (Thermo Finnigan).The temperature program started at 35°C and incread to 40°C with a rate of 1.0°C min À1.Next,the temperature was incread to 60°C at 2°C min À1,followed by an increa to 160°C at 8°C min À1.Finally,the oven temper-ature reached 220°C with a temperature increa rate of 12°C min À1.This final oven temperature was maintained during 10min.A SPME liner was installed at the inlet.The injector had a constant temperature of 200°C and was ud in a splitless mode.The injector was connected to a capillary col
umn VARIAN VF-1ms (FactorFour
TM
Fig.1.Experimental tup with detail of developed DC corona reactors.
J.Van Durme et al./Chemosphere 68(2007)1821–18291823
GC columns),30m length,diameter0.25mm and coating thickness of1l m.Ionization was achieved by electron impact(70eV),detection of fragments by an electron mul-tiplier(Trace DSQ Thermo Finnigan).For the identifica-tion of the unknown compounds obtained mass spectra the NIST v.2.0.databank(NIST/EPA/NIH Mass Spectral Library)was ud.Secondly,confirmation of identified compounds was done by determination of Kova´ts indices on a GC-FID(4890)equipped with a capillary HP-5col-umn(HP,USA),the indices were determined isother-mally at35,90or170°C.The Kova´ts retention index (KI)has received wide acceptance and is defined as Girard (1996):
KI¼100zþ100
ln t RðiÞÀln t RðzÞln t Rðzþ1ÞÀln t RðzÞ
where t R(z)<t R(i)<t R(z+1),t R(i)is the retention time of a component I,and t R(z)and t R(z+1)are the retention times of n-alkanes with z and z+1carbon atoms.
NO x concentrations were measured by a chemilumines-cence NO–NO2–NO x Analyzer(Model42C,Environmen-tal Instruments,Inc.,Franklin).
3.Results and discussion
3.1.Plasma characterization:current–voltage characteristics and ozone production
Fouad and Elhazek(1995)reported that the corona inception voltage increas as the relative humidity(RH) increas up to a certain limit when this inception voltage decreas for higher RH.Indoor environments have vari-able humidity levels:Maki and Aoki(2006)for example, measured humidities ranging from38%to78%in concrete building rooms.In this work the effect of water molecules on the discharge characteristics was examined.Fig.2a illus-trates the influence of humidity on the plasma characteris-tics.With increasing humidity,lower currents are measured for a given voltage.For an applied voltage of18.4kV,a current of0.6mA is measured in dry air,while this is only 0.3mA for60%RH.Fouad and Elhazek(1995)concluded that for corona discharges,the attachment coefficient,g h,is function of both thefield strength and water vapor content of the gas mixture.For higher RH,an incread attach-ment coefficient results in a shift of the ionization equilibrium.
One of the most important by-products in a non-ther-mal plasma discharge is ozone.Oxygen radicals are gener-ated 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 process with excited molec-ular states of nitrogen(N2(A3R)and N2(B3P)(Morent and Leys,2005).Atomic oxygen is a strong oxidizer,but its sta-bility is very limited.Due to fast
recombination process, the lifetime is only a few microconds at atmospheric pres-sure(Oda et al.,2005).Ozone is mainly formed by a three-body collision by the following reactions:
eÀþO2!eÀþOÅþOÅ
OÅþO2þM!O3þM
In air,M can be either molecular oxygen or molecular nitrogen(Magureanu et al.,2005).Humidity affects the ozone production,our results show a decrea of ozone production with increasing humidity(Fig.2b).
如何找回删除的文件The ozone outlet concentration for an energy density of 40J lÀ1is130ppm for dry air,while this decread to 90ppm at20%RH and75ppm at60%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 initialfield strength,the mobility of charge carriers and the plasma composition explaining the reduced ozone out-let concentrations.
3.2.Toluene oxidation experiments
霍桑探案集
A gas stream containing0.5ppm toluene was treated by the non-thermal plasma reactor.Fig.3shows the removal
1824J.Van Durme et al./Chemosphere68(2007)1821–1829
efficiency(%)of toluene for a constantflow rate of 10l minÀ1.Experiments were done at room temperature, atmospheric pressure and for different humidities(dry air, 26%and50%RH).The detection limit of the100l m PDMS-SPME combined with GC-FID measurements was67ppbv.Ozone exposure did not result in visual dete-rioration 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,O2or H2O are more probable, resulting in active species such as atomic oxygen or hydro-xyl radicals.The most powerful toluene oxidizing species are in fact the hydroxyl radicals(k=5.7·10À12mole-cule cmÀ3sÀ1)followed by atomic oxygen(k=7.6·10À14molecule cmÀ3sÀ1)(NIST,2006).The reaction rate is smaller for ozone(k=3.9·10À22molecule cmÀ3sÀ1), resulting in a minor role in the oxidation kinetics of toluene (NIST,2006).Next,excited nitrogen los 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 air46%toluene is oxidized at28.8J lÀ1,while this is57%when the gas has a RH of26%.Water molecules dissociate to form HÅand OHÅby collision with electrons or reaction with OÅ, O(1D).From the reaction kinetics of toluene with available oxidizing species,it can be concluded that OH induced de-gradation 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 concen
tration strongly influences the hydroxyl radical con-centrations.Hydroxyl radical production saturates or diminishes as humidity increas.In the prence of higher water concentrations,incread plasma attachment pro-cess result in a reduced hydroxyl radical production.It can be concluded that two opposite phenomena are en:water partially dissociates in the plasma,producing reac-tive species,but humidity also negatively influences the plasma characteristics.
The concentration of all the reactive species(OHÅ,OÅ, O3,charged species)can be linked with the energy density of the plasma.The relationship between energy density and removal efficiency can be expresd as Vertriest et al. (2003)
N out
N in
¼eÀ
e
e0
with N in(N out)the density of toluene molecules in inlet (outlet)gas stream(molecule cmÀ3),the energy density e (J lÀ1)and the characteristic energy e0(J lÀ1).This para-meter e0is often ud to express the energy efficiency of the ud discharge reactor and is defined as the energy nec-essary to reduce the concentration of pollutants by a factor e.The characteristic energy for the oxidation of0.5ppm toluene in dry air is49.5J lÀ1.For26%RH,the character-istic energy e0is35J lÀ1,while it was49J lÀ1for60%RH.
Mok et al.(2002)found that alkenes and substituted alkenes have higher decomposition rates than aromatics and substituted alkanes in puld corona and dielectric bar-rier discharges.Vertriest et al.(2003)tested veral VOCs, they proved that toluene was the most difficult to remove with a negative DC glow ,e0,toluene= 160J lÀ1>e0,octane=125J lÀ1>e0,1-octene=30J lÀ1).Since in this work toluene was efficiently removed,it may be expected that the positive streamer discharge reactor is able to oxidize veral indoor VOCs in a rather efficient way.
3.3.Intermediates:toluene degradation products
In order to obtain measurable concentrations of degra-dation products,higher inlet concentrations of150ppm were ud.The plasma reactor effluent of the oxidized tol-uene-air gas stream is analyzed
in order to identify degra-dation products.It is known that non-thermal plasma has high potential in air cleaning technology,but a disad-vantage could be that in some cas 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.4shows that conversion of toluene into CO2is not complete at the inlet concentrations and partially oxidized intermediates can be formed.Analysis of the degradation products was done with SPME sam-pling 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.Calcu-lated and measured KI values of products identified by mass spectrometry are given in Table1.For3-methyl-4-nitrophenol,4-methyl-2-nitrophenol and5-methyl-2-nitro-phenol,KI were not found in literature;they were determined experimentally.
J.Van Durme et al./Chemosphere68(2007)1821–18291825
