Characteristics of gas-liquid pulsed discharge plasma reactor and dye decoloration efficiency

Availableonlineatwww.sciencedirectcomScienceDirectENTALournal of Environmental Sciences 24(2012)840-845Characteristics of gas-liquid pulsed discharge plasma reactor and dyedecoloration eficiencyBing Sun, Nyein Nyein Aye, Zhiying Gao, Dan Lv, Xiaomei Zhu, Masayuki SatoEnvironmentalScienceandEngineeringCollege,DalianMaritimeUniversityDalian116026,ChinaE-mail:sunb88@dlmu.edu.cnReceived 30 June 2011; revised 25 October 2011; accepted 02 November 2011AbstractThe pulsed high-voltage discharge is a new advanced oxidation technology for water treatment. Methyl Orange ( MO)dye wastewaterwas chosen as the target object. Some investigations were conducted on mo decoloration including the discharge characteristics of themulti-needle reactor, parameter optimization, and the degradation mechanism. The following results were obtained. The color groupof the azo dye MO was effectively decomposed by water surface plasma. The decoloration rate was promoted with the increase oftreatment time, peak voltage, and pulse frequency. When the initial conductivity was 1700 u S/cm, the decoloration rate was the highest.The optimum distance between the needle electrodes and the water surface was 1 mm, the distance between the grounding electrodeand the water surface was 28 mm, and the number of needle electrodes and spacing between needles were 24 and 7.5 mm, respectivelyThe decoloration rate of MO was affected by the gas in the reactor and varied in the order oxygen >air argon >nitrogen, and theenergy yield obtained in this investigation was 0.45 g/kwh.Key words: multi-needle plasma reactor; water surface plasma; decoloration efficiency; Methyl Orange decolorationDOI:10.106S10010742(1)60837-1Introductiontemperature and pressure, and low power consumption,thus is a promising technology for wastewater treatmentWith the rapid development of the production of textile Decoloration of dye solutions or decomposition of organicprinting and dyeing, petrochemical, fine chemical, pharma- compounds using plasma on a water surface has been re-ceutical and food industry, etc, large quantities of harmful ported (Sato et aL., 2008: Lukes and Locke, 2005; Hoebensubstances were released into the environment. Conse- et al., 1999; Hayashi et aL, 2000). Corona discharge plasmaquently, the development of efficient and environmentally on the water surface by the application of dc, ac, or pulsesfriendly elimination methods for refractory organic pol- produces active species which diffuse in the gas phase andlutants is a matter of great concem. Advanced oxidation dissolve in the water through the surface layer to reacttechnology effectively promotes the degradation or the with the organic materials in water. It takes some time formineralization of organic pollutants in water, which has the active species to diffuse from the plasma region to thereceived extensive attention. Recently, some advanced water surface. In that time period, short-lived active speciesoxidation technologies, such as Fenton reaction(Fenton, such as radicals would disappear before reaching the water1894), ozonation(Lei and Wang, 2001), UV/O3 oxida- surface. Therefore, filamentary streamer discharge thattion(Olson and Barbier, 1994), H2O/O3(Zhong et al., produces plasma near the water surface was tried to de-1998), UV/H2O2/O3(Kurbus, 2003), wet oxidation(Fu color the dye in water( Sato et al., 2008; Lukes and Locke,et al., 2007), electrochemical oxidation Johnson, 2000), 2005). Sato et al. (2008)reported that the water surfacesupercritical oxidation(Ding et al, 1995), photochemical plasma(wSP)with bright filamentary streamer dischargeoxidation( Nogueira and Guimaraes, 2000; Zhang et al., mode had a higher decoloration rate and a shorter treatment2006), E-beam irradiation( Cooper et al., 2002)and high- time than that with the corona mode Lukes and Lockevoltage pulse discharge plasma have been considered to (2005)investigated the degradation of phenol in a hybridbe promising alternatives for water treatment (Sun et gas-liquid electrical discharge reactor under different gasal., 1999, 2000). High-voltage pulsed discharge plasma condit中国煤化工 oosphere) and liqfor the degradation of organic pollutants in water has uidcharacteristics such as a non-selective high degradation phenolCNMHG in oxygen than inrate for organic pollutants, no secondary pollution, nornargon atmosphere in alkaline solution.In this study, a multi-needle plasma reactor was pro-sCorrespondingauthor.E-mail:sunb88@dlmu.edu.cnposed for decolorizing organic dye solution using a pulsedNo 5Characteristics of gas-liquid pulsed discharge plasma reactor and dye decoloration efficiencyhigh voltage discharge directly on the water surface with solution was adjusted from 500 to 3000 uS/cm by additiona filamentary streamer discharge mode. Some factors of potassium chloride. A UV-Visible Spectrophotometerinfluencing the dye elimination were studied, including (JASCO V-550, Japan) was used to check the decolorationthe pulse parameters, such as peak voltage and pulse of Mo before and after the treatment processing. Thefrequency, conductivity of the solution, spacing among decoloration rate(R, %)of Mo was calculated by theneedles, and the electrode distance between the needle tip following Eq. (1 ):and the water surfaceR=(4100%1 Materials and methodswhere, Ao is the initial absorbance before reaction; A is theA multi-needle to water surface discharge plasma reactor absorbance after reactionwas used as previously reported( Sun et al., 2007). Theschematic diagram of the experimental apparatus is shown 2 Results and discussionin Fig. 1. The reactor is made of aPlexiglas cylindercontaining a needle to water surface geometry electrode 21 Plasma formation on water surfacesystem. The inner diameter and the height of the dischargereactor are 90 and 150 mm, respectively. The needle The water surface plasma that was generated between theelectrodes for producing plasma are composed of 2 to needle tips and water surface was seen as a bright streamer24 stainless steel syringe needles attached to a 50 mm discharge, as shown in Fig. 2. It was a uniform coronadiameter stainless steel disc. The ground plate electrode discharge from each needle tip. Between each needle tipis a stainless steel disc of 50 mm diameter which is and the water surface, an umbrella discharge zone can besubmerged in the water phase. A pulsed power supply formed. Thus a reasonable needle-to-needle distancewith a rotating spark-gap switch was used to generate high form a uniform discharge zone on the water surfacevoltage pulses. The pulse voltage, the trequency, and the 2.2 Effects of pulse peak voltage and liquid conductivitycapacity of the storage capacitor were 0-60 kV, 0-300 Hzand 3 nF, respectivelThe effect of pulse peak voltage on Mo decolorationDuring the experiment, the pulse peak voltage was set at efficiency is shown in Fig. 3a. During the experiment, the22, 26, and 30 kV; the pulse frequencies was varied from pulse peak voltage was varied at levels of 22, 26, and 3020 to 100 Hz: the treatment time was varied from 3 to 15 kV. The pulse frequency was set at 50 Hz, the conductivitysurface(ds )was varied from I to 10 mm and the distance time was 3, 6,9, 12 and 15 min. As shown in Fig 3a, thebetween grounding electrode and water surface(d,)was effect of pulse peak voltage on the decoloration of MO isvaried from 13 to 28 mm. The number of needle electrodes evident, that is, the decoloration rate of Mo was increasedwas varied from 2 to 24. The needle electrode spacing(dn) with increasing pulse peak voltage. The reason is thatwas varied from 7.5 to 30 mmwhen the pulse peak voltage increased, the average electricIn each reaction, 450 mL of Methyl Orange dye(Mo) field strength and the number of plasma channels betweensolution with a initial concentration of 10 mg/L(pH= the electrodes also increased, which led to generating a6.6)was circulated by a peristaltic pump at a flow rate stronger discharge, more active species (i.e. .OH, ozone,of 130 mL/min, The electrical conductivity of the Mo and hydrogen peroxide)and stronger ultraviolet light.In particular, the ozone concentration in the gas phaseincreased when the pulse peak voltage increased. Theozone in the gas phase transferred into the water, and partPulse power supply of the dissolved ozone in the liquid phase formed hydrogenperoxide(203 H2O -OH.+ O2+ HO2), therefore,oltage probPeristalticelectrodesDiCurrent probe中国煤化工CNMHGFig. 1 Schematic diagram of experimental apparatus.Fig. 2 Reactor photo when plasma was present (22 kV, 50 Hz, ds 1 mm,Journal of Environmental Sciences 2012, 24(5)840-845/ Bing Sun et alVoL 2490-o--22KV26kVer30kv20一3min6min+9min一12min-O15min10003000Conductivity (uS/cm)Fig 3 Effect of peak voltage(a)and liquid conductivity (b)on decoloration rate of MO 50 Hz, 1700 uS/cmthe concentrations of ozone and hydrogen peroxidesurface at the liquid conductivity of 1700 Hs/cm. As thethe liquid phase increased with increasing pulse peak liquid conductivity increased, the discharge spots gradual-voltage. Figure 3a also shows that the treatment time has ly decreased. In the case of high conductivity such as 3000in important effect on the Mo decoloration rate. The uS/cm, the discharge spots moved among the needles ducMo decoloration rate increased with increasing treatment to the fluctuation of the water surface, which resulted intime, especially in the initial 3 min, when the decoloration a lower decoloration rate. The maximum decoloration raterate increased rapidly, then increased slowly after 3was obtained when the initial conductivity of the MO dyeActually, the residence time of the solution in the reactor solution was 1700 uS/cm, which could be due to achievingincreases with increasing treatment time, and the treatment an optimum matching condition in the discharge circuit.effect changes with treatment time with an exponential Experimentally, it was found that the energy injected intodependence, approximately( Sun et al., 2000)the reactor was different under the conditions of differenThe effect of initial liquid conductivity on MO decol- conductivities. The highest energy was injected into theoration rate is shown in Fig. 3b. The liquid conductivity reactor at 1700 uS/cm, and the energy injected into thewas varied from 500 to 3000 uS/cm. The highest decol- reactor decreased for higher conductivity due to oscillationoration rate of MO was obtained at the initial conductivity of the circuit currentof 1700 uS/ cmThe gas-liquid hybrid discharge system, which consists 2.3 Optimization of electrode configurationof the plasma channel and bulk solution, is considered tosurface plasmabe a series connection of resistors(including a small ca- 2.3.1 Distance between needle electrodes and waterpacitance effect). The increase of the solution conductivityurface(ds)decreases the impedance of the circuit, leading to a larger As shown in Fig. 4a, the ds clearly influenced the decol-current flow in the circuit. Stronger plasma can formed due oration rate of the Mo solution. As d. varied from 1 to 10to the larger current, but the total electric energy is the mm. the decoloration rate of the Mo solution decreasedsame because of the use of a capacitor discharge type of from 84.3% to 70.3%. The amount of the active speciespulse generator. As shown in Fig. 2, uniform filamentary generated in the plasma channels including-OH,H,odischarges from every needle tip were formed on the water HO, and O, was greatly influenced by the electric field in-80中国煤化工CNMHGFig. 4 Effect of distance between needle-electrodes and water surface (d, )(a) and ground electrode and water surface(do)(b)on decoloration rate ofMO (22 kV, 50 Hz, 1700 uS/cm, and treatment time 15 min)Characteristics of gas-liquid pulsed discharge plasma reactor and dye decoloration efficiencytensity between the needle electrodes and the solution. The 2. 4 Introducing four kinds of gasesincrease of the electric field resulted in the enhancement ofthe discharge on the water surface and a large amount ofWhen different gases were introduced into the reactoractive species being injected into the liquid. Therefore, the system, different decoloration processes were observed.Asdecoloration rate was increased by the decrease of d, in the shown in Fig. 6, the order of the decoloration efficiencyoxidation of the Mo solutionwas: oxygen >air>argon>nitrogen. The decolorationrate of the Mo dye solution was highest when oxygen2.3.2 Distance between ground electrode and water gas was introduced The gas phase discharge generated neonly .OH but also other active species, such as ozone(O3),The effect of dg on the decoloration rate of MO at 22kIroxide(O2-)and singlet oxygen(O)50 Hz, conductivity of MO solution 1700 uS/cm and d, ofThe ionization energy of argon was lower than that1 mm is shown in Fig 4b. The decoloration rate of Mo of nitrogen, so it was more easily ionized. High energybecame higher when d increased. This was probably due electrons could be generated by the gas phase pulsedto the fact that the effective volume in the electric fieldharge when argon gas was used. In addition, argon isincreased with the increase of d which thus increased the a noble gas so it has a lower ability to capture electronsoverall removal rate. When dg increased, more discharge compared to nitrogen. Therefore, the high energy electronscurrent flow passed through the MO solution in the electric generated in the plasma can collide with water moleculesfield,which led to an increase of the reaction probability to produce more hydroxyl radicals and hydrogen atomsof the MO solution acted on by the electric field. The Under this condition, the decoloration rate was higher withdecoloration rate of Mo was almost proportional to the argon than with nitrogen. The Mo decoloration rate wasIcrease of the distance dgthe lowest in nitrogen as there are small amounts of strongchemically active substances, such as ozone. When oxyge2.3.3 Number of needle electrodesing between gas or air was used, a high concentration of ozone wasneedlesgenerated and dissolved into the water to produce moreFigure 5 shows the effect of the number of needle elec- bydroxyl radicals and additional active substances(Al-trodes on the decoloration rate of the MO solution. When Qaradawi and Salman, 2002). Therefore, the decolorationthe number of needle electrodes increased, the decol- rate of MO was the highest when oxygen was used, andoration rate of the Mo solution was effectively increased. the value of the reaction rate constant k(1. 38x10-Imin-)with the increase of the number of needle electrodes, was higher when using oxygen than with other gases(airthe spacing between needles decreased. When the number 1.14x10-Imin-, argon=3.09x10-2 minand nitrogenof needle electrodes was 24 and the spacing was 7.5 mm, =2.51x10-2 min-)the decoloration rate was the highest. When the number of The active species generated by discharge in the gasneedle electrodes was reduced or the spacing increased, the phase include ozone(O3), OH radical, peroxide(02-1decoloration rate declinedand atomic oxygen(O). Some of them, such as O andUnder the same conditions of electrode distance(ds+ OH radical, can dissolve into the water solution and formdg)and applied voltage, the growth in needle electrode hydrogen peroxide in the solution. It was found thatnumber increased the plasma channels between the needle the concentration of hydrogen peroxide in the solutiontips and the water surface, which resulted in the generation increased when the O3 concentration in the gas phaseof more active species and a higher Mo decoloration increased. Consequently, these active species generated atrate. It was shown that the reactor system had a suitable the gas-liquid interface helped the rapid decoloration of theconfiguration when the number of needle electrodes was organic dye.24 and the spacing was 7.5 mm.2.5 UV-Visible spectrumUv-Visible absorption spectra for the MO solution beforeand after the treatment are shown in Fig 7e中国煤化工Number of needle electrodeCNMHFlg. 5 Effect of the number of needIe-electrodes on decoloration of Mo Fig. 6 Effect of different source gases on MO decoloration(22 kV, 50(22 kV, 50 Hz, 1700 w/cm, treatment time 15 min, ds 1 mm, dg 28 mm)Hz, 1700 uS/cm, treatment time 15 min, ds l mm, dg 28 mm)Joumal of Environmental Sciences 2012, 24(5)840-845/Bing Sun et alelection of discharge type must be considered first; thenthe space homogenization of electric discharge for a multi-be considered. The umbrella shapeH, 02+ dischargecorona streamer discharge for each needle electrode isbetter than a column shaped spark dischargeOnly discharge3 ConclusionsIn this study, water surface discharge plasma was pro-for the discoloration of MO dythe followconclusions were reached. (1)When the pulse peak volt--0-220 270 320 370 420 470 520 570 age, frequency and the treatment time increased, theFig. 7 UV- Vis spectra of Mo before and after pulsed discharge(22 KV, conductivity of the mo dye solution was 1700 us/cm,50 Hz, 1700 uS/cm, and treatment time 15 min)the highest energy was injected into the reactor and themaximum decoloration rate was obtained. (3)The highestThe characteristic MO absorption peak for the n-I* decoloration rate was obtained when the distance betweentransition is at 464 nm. This absorption peak arises from needle electrodes and the water surface(d,)was 1 mm, thethe strong chromophore azo double bond (-N=N-)which distance between the ground electrode and the water sur-is present in the entire group of dye molecules. The face(dg)was 28 mm, and the number of needle electrodesabsorption peak at 280 nm occurs in a number of aromatic was 24(spacing between needles was 7.5 mm).(4)Theor polycyclic aromatic hydrocarbons and the chromophore decoloration rate of MO was changed when introducinggroup interaction peaks around 220 nm can be attributed different gases(oxygen, air, argon and nitrogen) into theto the absorption of aromatic or polycyclic aromatic hy- reactor system, and the decoloration rate varied in the orderdrocarbons in the intrinsic absorption(Kulikovsky, 1997). oxygen air >argon nitrogen. (5)The energy yieldAfter pulse discharge treatment, the visible region of the obtained in this investigation was 0.45 g/kWh, which wasabsorption peaks was weakened. This is because the azo higher than in other reports.group(-N=N-)in the MO molecule which was linked withn amino benzene ring was an active site and susceptible toAcknowledgementsattack. During discharge, a large number of strong oxidiz- This work was supported by the National Natural Scienceing species, especially -OH, was produced and attacked the Foundation of China (No, 10875019), and the Funda-dye molecules. The OH radical decomposed the-N=N- mental Research Funds for the Central Universities. Thebond conjugated with the benzene ring resulting in the authors express their appreciation to Professor B.R.bleaching of the solution. The absorption of the methyl Locke, Florida State University, for his valuable sugges-orange solution at 300 nm was also reduced, indicating tionthat the aromatic ring structure was damaged with thedestruction of double bonds, producing small organic acid References2.6 Energy yield for decolorationAl-Qaradawi S, Salman S R, 2002. Photocatalytic degrada-on of methyl orange as a model compound. Journal ofThe energy yield is an important parameter for appraisingPhotochemistry and Photobiology A: Chemistry, 148(1-3)the treatment method. The energy yield of the water16l-168surface pulsed discharge reactor was calculated by Eq. 2): Cooper W J, Nickelsen M G, Green RV, Mezyk s P, 2002. Themoval of naphthalene from aqueous solutions using high-I CoVoG50-2 Ptsmergy electron beam irradiation. Radiation Physics andChemistry,65(4-5):57-577Ding Z Y, Aki SNVK, Abraham M A, 1995. Catalytic stwhere, Gso(g/kWh)is the energy yield, Co(molL)is thepercritical water oxidation: phenol conversion and productmolar concentration of the pollutant at t=0, Vo (L) isselectivity. Emvironmental Science &r Technology, 29(11)volume of treated solution in liters, P(W)is power of the27482753.reactor,and tso is the time in seconds required for 50% Fenton HJH, 1894. 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