Application of Dielectric Barrier Discharge Reactor Immersed in Wastewater to the Oxidative Degradation of Organic Contaminant

Dielectric barrier discharge (DBD) is an effective method available for the production of ozone and ultraviolet light. The wastewater treatment system of this study was designed to utilize both ozone and ultraviolet light produced in the DBD reactor
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  Abstract  Dielectric barrier discharge (DBD) is an effective method available forthe production of ozone and ultraviolet light. The wastewater treatment system of this study was designed to utilize both ozone and ultraviolet light produced in theDBD reactor for the degradation of organic contaminant. The DBD reactor con-sisted of a quartz cylinder and a coaxial ceramic tube inside of which a steel rod wasplaced. The DBD reactor was immersed in the wastewater that was grounded. In thiscase, the wastewater acted not only as an electrode but also as the cooling mediumfor the DBD reactor. An azo dye, Acid Red 27, was used as the organic contaminant.In this system, the organic contaminant was degraded by two oxidation pathwaysinduced by ozone and ultraviolet light. The concentration of ozone, the ultravioletradiation intensity and the degradation efficiency of the organic contaminant weremeasured by varying the discharge. The results showed that the present system wasvery effective for the degradation of the organic contaminant. The energy require-ment for the degradation was found to be 0.654 kJ/mg, which is much smaller valuethan those obtained with an ultraviolet/photocatalytic process. Keywords  Dielectric barrier discharge  Æ  Ozone  Æ  Ultraviolet light  Æ Organic contaminant Introduction The increases in the demand for clear water and ever-strengthening environmentalregulations have caused interests in economical post-treatment processes capable of  Y. S. Mok ( & )  Æ  J.-O. JoDepartment of Chemical Engineering, Cheju National University, Jeju 690-756, South Koreae-mail: LeeDepartment of Energy Engineering, Cheju National University, Jeju 690-756, South KoreaH. T. Ahn  Æ  J. T. KimDepartment of Architectural Engineering, Kyung Hee University, Yongin-si 446-701, Korea  1 3 Plasma Chem Plasma Process (2007) 27:51–64DOI 10.1007/s11090-006-9043-1ORIGINAL PAPER Application of Dielectric Barrier Discharge ReactorImmersed in Wastewater to the Oxidative Degradationof Organic Contaminant Young Sun Mok   Jin-Oh Jo   Heon-Ju Lee   Hyun Tae Ahn   Jeong Tai Kim Received: 10 October 2006/Published online: 13 January 2007   Springer Science+Business Media, LLC 2007  effectively dealing with wastewater. The use of various advanced oxidation processes(AOPs) including supercritical water oxidation [1], electrochemical method [2], direct ozonation [3], electron-beam irradiation [4], ultrasonification [5], photocata- lytic oxidation [6] and pulsed electrical discharge [7, 8] to remove organic contam- inants from wastewater has been an active field of research and development. Inaddition, there have been many studies on the combination of an AOP process withanother to improve the removal of the organic contaminants [7, 9–11]. For instance, the combination of ultraviolet light with a photocatalyst, the combination of pulsedelectrical discharge with ozonation, or the combination of ultrasonification withozonation can be an effective method to remove organic contaminants fromwastewater.In the various AOPs mentioned above, most of the attention has been focused onhydroxyl radical  ð OH  Þ  and ozone (O 3 ) because they are very powerful oxidants todegrade organic contaminants in wastewater via oxidation pathways. The hydroxylradical can be produced in water by several technologies including electron beamirradiation, ultraviolet light and photocatalyst [10–13]. Many applications of these technologies to wastewater treatment have been reported so far [9–15]. According to the literature [16–18], electrical discharge such as pulsed corona discharge and dielectric barrier discharge (DBD) can be an available means to produce ultravioletlight of various wavelengths in the range of 300–400 nm. As well known, the DBD isan effective method to generate ozone [19, 20]. Thus, if the DBD system is properly designed in order to take advantage of both ozone and ultraviolet light for thedegradation of organic contaminants, it results in the equivalent of the combinationof the ozonation and ultraviolet photolysis processes.In this study, we made an attempt to produce oxidative species such as ozone andhydroxyl radical by using a cylindrical DBD reactor, which consisted of a quartzcylinder and a coaxial ceramic tube inside of which a steel rod was placed. The DBDreactor was immersed in the wastewater that was grounded, in order to irradiate thewastewater using the ultraviolet light produced. The ozone-containing gas generatedin the DBD reactor was sparged throughout the wastewater. In the present system,either ozone or ultraviolet light can contribute to the oxidation of the organiccontaminant. The performance of the present system was evaluated with a simulatedwastewater formed with distilled water and an azo dye Acid Red 27. Experimental Experimental ApparatusFigure 1 shows the schematic diagram of the experimental apparatus for thedecomposition of organic contaminant. The apparatus includes a DBD reactor toproduce ozone and ultraviolet light, AC high voltage power supply, and a reactorvessel containing the wastewater. The experiments were performed in a batch mode.The reactor vessel with 55 mm inner diameter and 200 mm length was made of glass,which was water-jacketed to maintain the wastewater at a constant temperature of 20   C. A cylindrical porous diffuser was placed at the bottom of the reactor vessel totransfer ozone-containing gas from the DBD reactor into the wastewater.The DBD reactor was made with a quartz cylinder (inner diameter: 14 mm; outerdiameter: 18 mm) and a coaxial ceramic tube (outer diameter: 6 mm) inside of which  1 3 52 Plasma Chem Plasma Process (2007) 27:51–64  a 2.3 mm thick steel rod was placed. The AC high voltage was applied to the steelrod while the wastewater was grounded. The volume of the wastewater contained inthe reactor vessel was 200 mL. The DBD reactor was immersed in the wastewater,as in Fig. 1. The flow rate of dry air fed to the DBD reactor was 8 L/min. The depthof the wastewater was 105 mm, but it increased to about 120 mm due to the gasbubbles when the ozone-containing gas was sparged in the wastewater. Since thewastewater itself functioned as the ground electrode, the depth of the wastewaterdefined the discharge region, which was about 120 mm. A 1.0  l F-capacitor con-nected to the DBD reactor in series was for measuring the discharge power. The AChigh voltage applied to the DBD reactor was varied from 17 to 29 kV (peak value)to change the discharge power, eventually to change the concentration of ozonegenerated and the intensity of ultraviolet light. So as to utilize the ultravioletemission from the DBD reactor for the degradation of the organic contaminant,aluminum wire meshes (120 mm  ·  120 mm) coated with titanium dioxide photo-catalyst (Tioz, Co., Korea) were rolled and put in the wastewater. The thickness of the aluminum wire was 0.6 mm, and there were 36 meshes per 100 mm. For acomparison, powdery titanium oxide (anatase, Degussa P-25) was also used in anexperiment. To exclude any possible interference due to the indoor electrical light,the reactor system was shielded with a black opaque material.Experimental MethodsThe simulated wastewater was prepared by dissolving a given amount of an azo dyeAcid Red 27 (C 20 H 11 N 2 Na 3 O 10 S 3 , molecular weight: 604.48, Sigma-Aldrich Corp.,USA) in 200 mL distilled water. The concentration of the organic contaminant was50 mg/L. The pH of the wastewater was adjusted to 3.0 by sulfuric acid. The con-centration of hydrogen peroxide added to the wastewater was 0.98 g/L. Hydrogen AC high voltagepower supplyAirAirWastewaterTiO 2 -coatedaluminum meshesOzoneDiffuserMFCQuartz tubeReactorvessel1 µ F Fig. 1  Schematic diagram of the experimental apparatus  1 3 Plasma Chem Plasma Process (2007) 27:51–64 53  peroxide produces hydroxyl radical in the presence of photocatalyst and ultravioletlight. All experiments were performed at room temperature (20   C). The electricalconductivity of the wastewater was measured to be about 1,800  l S/cm by a con-ductivity meter (DIST-3, Hanna Instruments, USA). The ozone-containing gas fromthe DBD reactor was sparged throughout the reactor vessel using the porous diffuser(distributor).Samples were taken from the wastewater at regular time intervals for analyses.The quantification of the organic contaminant was done using an ultraviolet/visiblespectrophotometer (Model UV-2500, Labomed, Inc., USA) at a wavelength of 521 nm. The chemical oxygen demand (COD) was measured by acidified potassiumpermanganate method using a COD meter (Model COD-60A, TKK-TOA Corp.,Japan). The concentration of ozone in the gas phase was analyzed by a portable gasanalyzer (Porta Sens II, Analytical Technology, Inc., USA). For the measurement of the voltage applied to the DBD reactor, a 1000:1 high voltage probe (PVM-4, NorthStar Research, Corp., USA) and a digital oscilloscope (TDS 3032, Tektronix) wereused. The power dissipated in the DBD reactor (discharge power) was estimated bythe so-called Lissajous figure (charge-voltage plot), which is described in detail in theliterature [19, 21]. The ultraviolet radiation intensity was measured with a radiom- eter (VLX-365, Vilber Lourmat, France), which covers the ultraviolet intensity inthe UVA band. Results and Discussion Characteristics Of The Dielectric Barrier Discharge ReactorThe power dissipated in the DBD reactor was determined by measuring the voltagesacross the electrodes of the DBD reactor and across the 1  l F capacitor. The voltageacross the capacitor multiplied by its capacitance (1  l F) corresponds to the charge,which is, in principle, equal to the charge deposited on the electrodes of the DBDreactor. Note that the 1  l F capacitor and the DBD reactor is a series circuit. Figure 2ashows example waveforms of the voltage applied to the DBD reactor and chargedeposited on the electrodes of the DBD reactor. The energy per cycle delivered to theDBD reactor was obtained using the so-called Lissajous figure that plots the chargeversus the voltage, as shown in Fig. 2b. The area of the parallelogram is equal to theenergy dissipated in the DBD reactor per cycle, and the discharge power can becalculated by multiplying the energy per cycle by the operating frequency (60 Hz). Inthis case, the power dissipated in the DBD reactor was found to be 3.4 W.Figure 3 presents the effect of the discharge power on the generation of ozone inthe DBD reactor. The energetic electrons (e) produced during the electricaldischarge dissociate oxygen molecules by collisions, and the oxygen atoms formedreact with oxygen molecules to produce ozone (O 3 ) as follows:O 2  þ  e  !  O  þ  O  þ  e  ð 1 Þ O  þ  O 2  !  O 3  ð 2 Þ Ozone is a very strong oxidant capable of oxidizing organic contaminants. As can beseen, the increase in the discharge power increased the generation of ozone.  1 3 54 Plasma Chem Plasma Process (2007) 27:51–64  In Fig. 3, the energy yield for the generation of ozone is also presented. The averagevalue of the energy yield was about 6.4  ·  10 –3 g/kJ. Note that the present DBDreactor was not optimized for the generation of ozone. If the parameters of the DBDreactor were optimized and oxygen was used instead of dry air, the energy yield forthe generation of ozone would increase.Figure 4 presents the ultraviolet radiation intensity measured by varying thedischarge power. As expected, the radiation intensity rapidly increased with thedischarge power. The ultraviolet radiation intensity is a function of electric field andthe number of energetic electrons, and thus it is deeply related to the power dissi-pated in the DBD reactor. Nitrogen molecules in the dry air fed to the DBD reactorare excited from N 2  X 1 P þ  g    (ground state) to N 2  C 3 P u    by the impact of elec-trons with energies higher than 11 eV. Radiative transition of N 2  C 3 P u  !  B 3 P g   emits photons with 337.1 and 357.7 nm wavelengths [17]:N 2 ð C 3 P u Þ v ¼ 0  !  N 2 ð B 3 P g Þ v ¼ 0  þ  light 337 : 1 nm ð Þ ð 3 Þ Fig. 2  Waveforms of voltageand charge ( a ), and charge-voltage plot (Lissajousfigure) ( b )  1 3 Plasma Chem Plasma Process (2007) 27:51–64 55
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