Decolorization of Aqueous Acid__Red B Solution during the Cathode Process in Abnormal Glow Discharge

Decolorization of Aqueous Acid Red BSolution During the Cathode Processin Abnormal Glow DischargeDilai Lu,Jierong Chen,Aihua Gao,Georges Zissis,Senior Member,IEEE,Shangbin Hu,and Zhiguo LuAbstract—This paper investigated some chemical and physical factors which probably affect the decolorization of aqueous acid red B solution by atmospheric abnormal glow discharge.When the aqueous solution served as the cathode,effective decolorization occurred in the inert gas discharges as well as in the active gas discharges.The hydroxyl radicals were mainly responsible for the decolorization in the solution phase.The gas ionization potential may influence the decolorization effect.Index Terms—Abnormal glow discharge,cathode,decoloriza-tion,ionization potential.I.I NTRODUCTIONT HE DISCHARGES and chemical reactions in plasma wa-ter treatment are fairly complicated.When plasma comes in contact with water,various physical and chemical processes are initiated[1],[2];as a result,positive ions,electrons,and UV radiations as well as active species,such as active molecules (nitrogen oxides,ozone,and hydrogen peroxide)and radicals (hydrogen atom,hydroxyl,etc.),may be created[3].In the past decade,attempts have been made to understand the mechanism of the discharge processes occurring above the surface of water [4].However,for a thorough understanding of the degradation mechanism,it is necessary to investigate the properties of the discharge gases used for the treatment.Both chemical and physical factors can affect the chemical oxidation reactions occurring during the discharge process. Aside from the conductivity effects,pH,input power,and characterization of the water-phase products[5],the charge particles in the discharge phase may influence the degradation of the organic pollutant as well.In this paper,some chemical active products(hydrogen peroxide,radicals,etc.)were created in the solution phase,and their effect on decolorization hasManuscript received January14,2010;revised June24,2010,May15,2010, and March25,2010;accepted July15,2010.Date of publication August19, 2010;date of current version October8,2010.This work was supported in part by the National Science Foundation of China under Grant20877062and in part by the Scientific Technique Innovation Engineering of Shaanxi Province through the Specialized Project of“13115”under Grant2008ZDKG-78.D.L.Lu and J.R.Chen are with the Department of Environmen-tal Science and Engineering,School of Energy and Power Engineering, Xi’an Jiaotong University,Xi’an710049,China(e-mail:ludilai531@; Jrchen@).A.H.Gao,S.B.Hu,and Z.G.Lu are with the Department of Physics, Northwest University,Xi’an710069,China(e-mail:gaoaihua@; hushangbin110@;luzg@).G.Zissis is with the Laboratoire Plasma et Conversion d’Energie (LAPLACE),Universite Paul Sabatier-Toulouse III,31062Toulouse Cedex9, France(e-mail:georges.zissis@laplace.univ-tlse.fr).Digital Object Identifier10.1109/TPS.2010.2061871Fig.1.Schematic diagram of our experimental setup.been discussed.The authors also focus on some physical factorsin various discharge gases.The abnormal glow discharge wasemployed for our studies because the mechanism has been wellestablished in some literatures[6].In this paper,we examined the decolorization of acid red B(CAS Number:3567-69-9)as a model pollutant in wastewater(see Fig.1).In our experiment,the current intensity was variedin the range of27–28mA,and the discharge pressure wascontrolled at1atm or so(750–760torr).The acid red B aqueoussolution was used as the cathode during the discharge process,the so-called liquid cathode process,which was conducted toinvestigate the interrelationship between the decolorization ef-fect and the physical and chemical factors during the dischargeprocess.II.E XPERIMENTALThe concentration and volume of the aqueous solution ofacid red B were100mg/L and50ml,respectively.The pH andconductivity of the initial solution were6–7and approximately0.12mS/cm,respectively.The electrical discharges in the puregases(N2,O2,Ar,and Ne with gas purity≥99.995%)wereadopted in our experiment.The experimental reactor is shown in Fig.1.The reactorwas hermetically isolated to ambient air,and both gas-phaseand solution-phase gaseous impurities were removed from thereactor with a vacuum pump.A high-voltage(HV)dc powersupply which can provide a discharge voltage of0–13kV wasemployed to generate a discharge in the reactor.A HV wasapplied between the aqueous solution(cathode)and a pointedtungsten electrode(anode;diameter:1.5mm).The HV tungstenanode was placed above the solution surface,and the distancebetween the HV tip of the anode and the cathode was varied 0093-3813/$26.00©2010IEEEFig.2.Decolorization rate in N 2,O 2,Ar,and Ne discharges (discharge gap:3mm;discharge pressure:1atm or so).from 1to 5mm.This distance could be adjusted by changing the solution volume.The negative electrode of the dc power supply was connected to a platinum (Pt)electrode immersed in the solution.A milliammeter and an electrostatic voltmeter were used to measure the discharge current and the potential fall,respectively.As per the method reported in [7],we perform electric probe measurements to determine the potential fall and the spatial potential distribution in the air discharge channel.Since the electric probe measurement device is exposed to ambient air,it is used only for the air discharge experiment.A tungsten filament (diameter:≤0.1mm)covered by a ceramic sheath is used as the diagnostic probe;the probe position can be precisely controlled by a screw micrometer.In fact,the potential fall (U t )measured in this diagnostic device is the sum of the potential fall at a certain point (U p )in the discharge channel and the voltage drop (U s )in the solution.Hence,(U p )can be obtained by subtracting (U s )from (U t ).(U s )can be obtained when the diagnostic probe is made to touch the solution surface.In the initial stage of the discharge (solution conductivity:∼0.12mS/cm),(U s )is approximately 150V .A compact dual-channel monochromator (AH430)was em-ployed to test the decolorization effect.The absorbance of the acid redB solution at 507nm was measured.The decolorization rate of the solution,which is denoted as η,can be calculated as follows:η=A 0−A tA 0×100%(1)A 0absorbance of the solution before treatment;A t absorbance of the solution after treatment.Optical emission spectroscopy measurements were performed by putting the tungsten electrode to the window of a mono-chrometer at a distance of 9–10cm.The optical emission from the discharge region was detected by a GWD-3monochrometer (the wavelength resolution is 0.1nm at a 0.2-mm slit width).III.R ESULTS AND D ISCUSSIONA.Experimental ResultsAs Fig.2presents,after an 8-min discharge,the decoloriza-tion rate ηreaches over 97%in the N 2discharge.The decol-orization rate was much higher when using N 2as thedischargeFig.3.Emission spectrum of (a)N 2,(b)O 2,(c)Ar,and (d)Ne discharges (discharge gap:3mm;760torr).gas than using the other gases.It was more interesting that the value of ηreached over 40%in the Ne and Ar discharges;how-ever,after a 35-min discharge,the value of ηeven could reach over 97%in both the discharges.The results suggested that there was an evident decolorization effect no matter whether the discharge gases were activated (N 2)or inert (Ar and Ne).After a discharge time of10min,ηexceeds60%in the Ar discharge but approaches70%in the Ne discharge(see Fig.3). After16min of discharge,ηcould reach85%in the Ar dis-charge and90%in the Ne discharge.Thus,the decolorization rate in the Ne discharge was better than that in the Ar discharge. O2is one kind of strong oxidizer;a lot of strong oxygen-based oxidizers(ozone ions,ozone,atom oxygen,hydroxyl radicals,etc.)can be produced in an electrical discharge at an oxygen–water interface[8].Therefore,it should be expected that the decolorization effect would be much better when using O2as the discharge gas than the other gas discharges.However, as Fig.2shows,the decolorization rate in the pure O2discharge is unexpectedly lower than that in the other gases:after10min of discharge,ηcould reach50%in the O2discharge but exceed 60%in the Ar discharge,and after16min of discharge,ηcan approach70%in the O2discharge but reach over80%in the Ar discharge.Consequently,an overall evaluation of the decolorization rate in the four gas discharges adopted in our experiments could be obtained,and theηvalues in the different gas dis-charges can be listed in the following order:η(N2)>η(Ne)>η(Ar)>η(O2).Fig.3(a)–(d)shows the emission spectra recorded at the gas–solution interface in the N2,O2,Ar,and Ne discharges. From the emission spectra,we can observe a number of emis-sion lines of positive ions,atoms,and some emission bands of molecules or molecule ions.Moreover,we canfind the emission spectra intensity of positive ions often stronger than the other species.It seems to suggest that the production rate of the positive ions was much higher than that of the other active particles in the discharge phase.B.Effects of Chemical FactorsThe decolorization of dye is a complex chemical process [1]–[5];many chemical products forming in aqueous solution, such as hydrogen atoms(H.),hydrogen peroxides(H2O2), ozone(O3),nitrogen oxides,and hydroxyl radicals(OH),could affect the decolorization.Nitrogen oxides can be generated in the air discharges and affect the decolorization rate.Due to the absence of oxygen, the production rate of nitrogen oxides in the pure nitrogen discharge must be less than that in the air discharge.It was confirmed by the emission spectra measurements:The emission bands of the nitrogen oxides in the nitrogen discharge were hardly detected.However,the decolorization rate in the nitro-gen discharge is higher than that in the air discharge(see Fig.4). Moreover,the evident decolorization also occurred in some electrical discharges without nitrogen(in the Ne,Ar,and O2 discharges).These results demonstrate that the nitrogen oxides are not a necessary requirement to the decolorization of dye. H2O2which is formed by the recombination of hydroxyl radicals in the water phase can be generated in the four gas discharges considered in this paper;it also could influence the decolorization of dye owing to its strong oxidizing ability. The authors have tested the effect of H2O2(concentration is about30%)by mixing an initial acid red B solution with H2O2 (1:1,v/v).However,the apparent decolorization did notoccur Fig.4.Decolorization rates in the nitrogen and airdischarges.Fig.5.Decolorization rate in the Ar discharge(after30-min discharge)versus concentration of Na2CO3(mol/L).after2h of stirring.It shows that H2O2can hardly influence the decolorization in our experiments.Both H and OH can be formed at the gas–water interface when the four gases adopted in this paper were used as the discharge gases.The formation of OH radicals is desired to the decolorization of dye since they are one of the strongest oxidizers[8],and H.can affect the decolorization also.Sodium carbonate(Na2CO3)is well known as a radical scavenger[9]; OH can be scavenged by the carbonate anions easily,whereasH.reacts with this radical scavenger in a lower reaction rate[10].To avoid disturbing the nitrides and ozone,the discharge in the pure Ar was employed for testing the effect of the radicals.As Fig.5presents,the decolorization rate decreases significantly with an increasing concentration of sodium car-bonate.If H.is a main factor to the decolorization reactions, the apparent decolorization rate(may be lower than that at a low concentration of Na2CO3)would have also taken place at a higher concentration of Na2CO3;however,it was not the case in our experiments.As mentioned earlier,a large number of OH radicals can be scavenged at a high concentration of Na2CO3,and the lower decolorization rate occurred at the higher concentration of Na2CO3.Therefore,it was concluded that it was the OH radicals that are mainly responsible for the decolorization of dye in the aqueous solution.Fig.6.Fitting curve for spatial potential distribution in air discharge channel (760torr).C.Effects of Physical FactorsSome physical factors in the discharge phase,such as the potential fall of the discharge channel(U),discharge power,and charge particle characteristics,can contribute to the decoloriza-tion in our experiments.To determine the spatial potential distribution in the air dis-charge channel,the gap between the electrodes isfixed at5mm, and the spatial potential fall at different space positions is measured by using an electric probe.Then,the spatial potential distribution in the air discharge channel is obtained by curve fitting(see Fig.6).The electricfield at the solution surface is very strong when the aqueous solution served as the cathode,as is evident from the large slope of the voltage drop(see Fig.6).As a result,the positive ions are pulled by the electric force and swarm to the cathode.At the same time,most of the electrons and negative ions are repulsed by the electricfield and hardly reach the cathode.Hence,only the positive ions and neutral particles can reach the cathode surface.The neutral particles can also obtain energy from the electricfield because of the charge exchange effect.When the gas pressure isfixed,the kinetic energy of the gas ions should be influenced by the electricfield[11].Denaro and Hough[12]proposed that the positive ion(⊕)collide with water molecule(H2O+)and ionize H2O into H2O+with its kinetic energy and that H2O+decompose into H+and OH [see(2)].According to their view,in some degree,the decol-orization may be determined by the potential fall U since it is known that the potential fall affects the electricfield⊕+(n+1)H2O→(n+1)H2O++ne−aq→nH+(n+1)OH+H+.(2) As Fig.7presents,the potential fall U in the N2discharge is much higher than that in the Ne,Ar,and O2discharges as well as theηvalues in the N2discharge are remarkably higher than that in the other gas discharges.It also suggests that the potential fall U may affect the decolorization.The input discharge power can also influenceη.In our experiment,the input discharge power for N2(P N≈84W) was considerably higher than that for Ne(P Ne≈16W)and Fig.7.Potential fall U of O2,Ar,Ne,and N2(discharge gap:3mm;760torr; measured by electrostatic voltmeter).Ar(P Ar≈16W).Hence,the decolorization rate in the N2 discharge was higher than that in the Ar and Ne discharges.As we know,the input discharge power depends on the discharge current and the potential fall U.In this experiment,the input power depends on the potential fall U solely because the dis-charge current is maintained at27–28mA.Therefore,the effect of the input power was led by the potential fall U substantially. However,the potential fall U in the O2discharge is evidently higher than that in the inert gas discharges,whereas theηvalues in the O2discharge is lower than that in the inert gas discharges. Thus,aside from the potential fall U and the input power,there should be other factors that can affect the decolorization of dye. The gas discharge channel of the stable abnormal glow discharge can be simply classified as the anode zone,the plasma region,the negative-glow region,and the cathode fall region. The mechanism of the glow discharge has been well studied [11],and it has been reported that the secondary electron ejection at the cathode surface is mainly caused by the effect of the positive ion[11].When the positive ions come to approach the cathode surface,because of the potential ejection from the positive ion[13],some electrons in the aqueous solution can gain sufficient energy to overcome the electron work function of aqueous solution eϕ(6.1eV)as a result,and two electrons may be ejected:One neutralizes the positive ion,and the other desorbs from the cathode surface,forming the secondary electron.At the cathode surface,the currentflow in the gas phase is mainly carried by the positive gas ions;however,a lot of the positive gas ions are deionized at the cathode surface [11].As a result,aqueous electrons(e−aq)[14]and some cations (H2O+,H3O+,etc.)are formed in the solution phase.These water-phase charge particles act as the current carriers in the aqueous solution.In fact,we can predict the decolorization results(η(N2)>η(Ne)>η(Ar)>η(O2))even before carrying out the experi-ments;this was because of the potential ejection of the positive ions that were mentioned earlier.During this process,a positive ion with a higher ionization potential can transfer more ioniza-tion energy to the cathode surface.Thus,it is possible that the positive ion with the higher ionization potential can produce more OH radicals.As Fig.3shows,in optical emission spectra experiments,evidently,the emission spectra which belong to the positiveions were markedly stronger than that belonging to the otherspecies.The most prominent emission spectra lines or bandswere observed as follows:N2+(336.1nm,312.5nm,etc.)in theN2discharge[see Fig.3(a)],O+2(A2Πu−X2Πg system[15], 321.1nm,323.1nm,etc.)in the O2discharge[see Fig.3(b)],Ar+(321.3nm,329.3nm,etc.)in the Ar discharge[seeFig.3(c)],and Ne+(337.1nm,357.7nm,etc.)in the Nedischarge[see Fig.3(d)].These results demonstrate that themajority excitation particles were positive ions and the mainlypositive ions were N2+,O+2,Ar+,and Ne+in the N2,Ne,O2,Ar,and Ne discharges,respectively.According to theliterature[16],the ionization potential values of the mainlyposition ions in the four different gas discharges consideredin this paper decrease in the following order:N2+(29.5eV)>Ne+(21.5eV)>Ar+(15.8eV)>O+2(12.2eV).Interestingly,the sequence is in accordance with the sequence of the decol-orization rate:η(N2)>η(Ne)>η(Ar)>η(O2).Therefore,the decolorization rate could be thought to depend on theionization potential in some degree.Additionally,it is possible that the metastable potential ofthe gases affect the decolorization of dye.For example,themetastable potential of Ne(16.53eV)is higher than that ofAr(11.49eV)as well as the decolorization rate in the Nedischarge is higher than that in the Ar discharge.Owing tothe long lifetime of metastable states(10−3s),the metastableatoms have enough time to go through the cathode fall regionand transfer their energy to the solvent molecules.The excitation potential of Ne(16.58eV)is higher thanthat of Ar(11.56eV),and this might affect the decolorizationalso.However,many excitation atoms unlikely transfer theirenergy to the solvent molecules due to the shorter lifetime ofthe excitation states(10−8∼10−10s).Hence,not all of the excitation atoms are involved in the decolorization effect.IV.C ONCLUSIONIn this paper,some chemical and physical factors have been investigated during the liquid cathode process of the decol-orization of acid red B in aqueous solution.It was found that the decolorization effect has been obtained no matter whether the discharge gas is active or inert and that the decoloriza-tion efficiency is lower in the pure O2discharge than that in the other gas discharges.The experimental results have also demonstrated that the hydroxyl radicals are mainly responsible for the decolorization in the solution phase and that the ioniza-tion potential,the metastable potential of the discharge gases, probably affects the decolorization also.The results of our study suggest that the positive ion can affect the decolorization or degradation of the organic dyes in aqueous solution and that taking advantage of the deionization of the positive ions can be useful to improve the decolorization/degradation efficiency.R EFERENCES[1]J.-Z.Gao,X.Wang,Z.Hu,H.Deng,J.Hou,X.Lu,and J.Kang,“Plasmadegradation of dyes in water with contact glow discharge electrolysis,”Water Res.,vol.37,no.2,pp.267–272,Jan.2003.[2]L.Wang,“Aqueous organic dye discoloration induced by contact glowdischarge electrolysis,”J.Hazard.Mater.,vol.171,no.1–3,pp.577–581, Nov.2009.[3]P.Lukes and B.R.Locke,“Plasmachemical oxidation processes in hy-brid reactor,”J.Phys.D.Appl.Phys.,vol.38,no.22,pp.4074–4081, Nov.2005.[4]P.Lukes,M.Clupek,V.Babicky,and P.Sunka,“UV radiation from thepulsed corona discharge in water,”Plasma Sources Sci.Technol.,vol.17, no.2,pp.24012-1–24012-11,May2008.[5]A.Hickling,“Electrochemical 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ed.Oxford,U.K.:Clarendon,1965,pp.222–233.[12]A.R.Denaro and K.O.Hough,“Glow-discharge electrolysis of sulphuricacid solutions,”Electrochim.Acta,vol.17,no.3,pp.549–559,Mar.1972.[13]H.D.Hagstrum,“The development of ion-neutralization spectroscopy,”J.Vac.Sci.Technol.,vol.12,no.1,pp.7–16,Jan.1975.[14]S.M.Thagard,K.Takashima,and A.Mizuno,“Chemistry of the posi-tive and negative electrical discharges formed in liquid water and abovea gas–liquid surface,”Plasma Chem.Plasma Process.,vol.29,no.6,pp.455–473,Dec.2009.[15]R.W.B.Pearse and A.G.Gaydon,The Identification of MolecularSpectra,4th ed.London,U.K.:Chapman&Hall,1976,pp.260–263.[16]G.Herzberg,Atomic Spectra and Atomic Structure.New York:Dover,1944.Dilai Lu was born in Shaanxi,China.He receivedthe B.S.and M.S.degrees in electric circuit andsystem from Northwest University,Xi’an,China.Heis currently working toward to the Ph.D.degree inthe Department of Environmental Science and Engi-neering,School of Energy and Power Engineering,Xi’an Jiaotong University,Xi’an.His current research interests include plasmachemistry and plasma wastewatertreatment.Jierong Chen was born in Shanghai,China,onFebruary18,1949.She received the Ph.D.degreefrom Kyoto Institute of Technology,Kyoto,Japan,in1991.She is currently with the Xi’an Jiaotong Univer-sity,Xi’an,China,where she is a Professor of plasmachemistry and the Head of the Department of Envi-ronmental Science and Engineering and where shehas been doing research in low-temperature plasmachemistry and plasma application in the medicalfieldsince1999,and,for the pastfive years,has been studying the main chemical features of the complex gas media excited by a different kind of gas discharge.Dr.Chen is a syndic of the polymer and a member of the American Association for the Advancement of Science.Aihua Gao was born in Shaanxi,China.She re-ceived the B.S.and M.S.degrees in physics from Northwest University,Xi’an,China,in1987and 1996,respectively.She is currently an Associate Professor with the Department of Physics,Northwest University.Her research interests include gas laser characterizationand gas discharge plasmaapplications.Georges Zissis(M’82–SM’06)was born in Athens,Greece,in1964.He received the degree in generalphysics from the Department of Physics,Universityof Crete,Crete,Greece,in1986,and the M.Sc.andPh.D.degrees in plasma science from Toulouse3University,Toulouse,France,in1987and1990,respectively.He is currently a Full Professor with the Depart-ment of Electrical Engineering,Toulouse3Univer-sity.He is also the Deputy Director of the Laboratoryof Plasma and Conversion of Energy,a joint labora-tory between the Toulouse3University,the National Polytechnic Institute of Toulouse,Toulouse,and the French National Council of Research.He acted as the Chairman of the European Union COST-529“efficient lighting for the21st century”network.He is working in the domain of light sources science and technology.Prof.Zissis was a Chairman of the Industrial Lighting and Displays Tech-nical Committee of the IEEE Industry Applications Society,and a National Secretary and a President of the Regional Branch of the French Illuminating Society(AFE).Shangbin Hu was born in1984.He is currently working toward the degree at Northwest University, Xi’an,China.His research interests include plasmaapplications. Zhiguo Lu was born in Henan,China.He received the B.S.degree from the Department of Physics, Northwest University,Xi’an,China.He is currently a Professor with Northwest Uni-versity.His research interests include plasma appli-cation,gas laser,and lighting technology.Prof.Lu is the Chairman of the Laser and Infrared Society in Xi’an and the Vice-Chairman of the Illu-minating Engineering Society in Shaanxi,China.。