872 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 37, NO. 6, JUNE 2009

An Application of AC Glow Discharge Stabilized byFast Air Flow for Water Treatment

Anton Yurievich Nikiforov

Abstract—A scheme for a plasma-solution reactor on the basisof a glow discharge stabilized by a fast air flow is suggested as anovel water treatment technique. The discharge ignition voltageis about 2.7 kV, and the input power can be as low as 3 W. It isfound that plasma is generated in pulsed mode where the durationof each individual pulse is 15–20 μs. A maximal destruction rate ofmethylene blue dye of 3.27 × 10−9 mol/l · s has been received inan acidic medium at a discharge power level of 6 W. A comparisonof the efficiency of plasma interaction in different solutions hasbeen carried out. The chemical effectiveness of the plasma treat-ment decreases with an increase of solution pH. It is shown thatthe developed glow discharge two phase reactor can be effectivelyused for destruction of water pollutants.

Index Terms—Atmospheric plasma, gas flow, glow discharge,water treatment.

I. INTRODUCTION

IN THE LAST decades, the quantity of works devoted tounderwater electric discharges (UWED) has considerably

increased. These systems attract significant attention of manyresearch groups as new effective technology for solution pu-rification and material modification. All types of underwaterdischarges can be divided into two large groups on the basisof the plasma generation method. These are pulse discharges(corona, underwater streamer discharge, etc.) formed by shortpulses of a high voltage with duration from nanosecond tomicrosecond and currents up to kiloampere and dc or ac dis-charges, generated in gas-to-steam bubbles [1]. All underwaterdischarges are effective sources of radicals and active particlessuch as OH∗, O∗

2, H∗, O∗, HO2, hydrogen peroxide, ozone,UV radiation, and also shock waves in some cases [2], [3]. Anessential advantage of UWED is the possibility of the combi-nation of plasma action in objects under treatment with highselectivity of conventional chemical processes in solutions.The available experimental data devoted to the applicationof UWED reveals a demand of such systems in technologyof water purification and sterilization. As shown in [4] and[5], most of reactions for organic compound destruction areinitiated by the interaction of pollutants with the secondary

Manuscript received November 24, 2008; revised January 19, 2009 andMarch 4, 2009. First published April 24, 2009; current version publishedJune 10, 2009. This paper was presented in part of “Plasma Generation”(WPKI) in the framework of the Interuniversity attraction poles program,Belgian state—Belgian Science Policy.

The author is with the Laboratory of Nonlinear Plasma Processes andTechnologies, Institute of Solution Chemistry, Russian Academy of Science,153045 Ivanovo, Russia, and also with the Department of Applied Physics,Ghent University, 9000 Gent, Belgium (e-mail: anton-nik@yandex.ru).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPS.2009.2017747

active particles, generated under discharge action. Chemicaleffects of UWED in solution treatment processes were shownby a number of authors [6]–[8] to be directly connected withthe efficiency of the formation of hydrogen peroxide and OH∗

radicals under plasma action. In most of the published works,this effect of plasma action is estimated by the destruction rateof a number of model compounds. First of all, these are dyes(methylene blue, orange, etc.) and phenol, which have beenused for a long time by many group of researches as modelaromatic compounds for investigation of water purificationprocesses under plasma action. Unfortunately, a wide applica-tion of plasma-chemical methods for purification of aqueousmediums in real technologies becomes complicated due to anumber of technical difficulties. Pulse discharges character-ized by high effectiveness required in most cases complicatedequipment for plasma generation. A further disadvantage ofpulse discharge reactors is the destruction of electrode materialsand reactor parts. Another group of underwater discharges arethose that use direct and alternating current (diaphragm and anumber of its versions, capillary discharge, contact glow, andmicrodischarges [1], [9], [10]). They do not demand the difficultequipment for initiation, and discharge maintenance can beeasily scaled up to commercial level by the increase in numberof working sections and increasing discharge current. On theother hand, dc and ac underwater discharges can be limited tothe use for the treatment of solutions with small conductivity.Moreover, a significant part of input power applied to electrodesis spent in Joule heating of the electrolyte, generation of a gas-to-steam bubble in which discharge is generated and a numberof other undesirable processes.

This paper presented here is devoted to the investigation ofthe process of methylene blue destruction in electrolyte solutionwith an initial pH in the range of 2 to 11 by means of newtype of atmospheric pressure glow discharge with an alternatingcurrent stabilized by a fast gas flow. The system is characterizedby the absence of Joule heating of solution and requires lowinput power for plasma generation.

II. EXPERIMENTAL TECHNIQUE

The principal scheme of the setup for the generation of adischarge stabilized by a fast gas flow is shown in Fig. 1.

The electrode system consists of two stainless steel diskswith a thickness of 1 and 0.2 mm separated by a 1-mm ceramicdisk. In the center of the disks, there is a pinhole 0.8 mm indiameter. Gas flow in the system is supplied by compressor.Flow rates from 0.1 to 50 l · min−1 are controlled by a needlevalve. A floating-type rotameter is used for measurement of

0093-3813/$25.00 © 2009 IEEE

NIKIFOROV: APPLICATION OF AC GLOW DISCHARGE STABILIZED BY FAST AIR FLOW FOR WATER TREATMENT 873

Fig. 1. Principal scheme of the setup for plasma treatment of solutions.

Fig. 2. Visual appearance of the discharge (a view from above).

gas flow velocity. Gas passes in the volume of the electrolytesolution filling the upper beaker through the pinhole in theelectrode system.

The power supply for the discharge comes from the sec-ondary winding of the high-voltage transformer (transfer ratio70), through a ballast resistor 30 kΩ. The primary input lineis fed by a 50-Hz 220-V ac-regulated transformer, and theelectrodes are connected to the secondary output of a neontransformer. The applied voltage is varied from 2 to 15 kVrms.The electrode placed in contact with the liquid is groundedthrough a shunting resistance of 100 Ω. The discharge currentand discharge voltage are registered by means of a digital oscil-loscope GDS806S with a bandpass of 60 MHz. The appearanceof the discharge (photo from above) at an applied voltage of2.7 kV is shown in Fig. 2.

For visual improvement, the system is presented withoutelectrolyte solution. Diameter of the plasma zone generated inthe reactor was estimated to be about 2 mm. Plasma jet length(from the edge of the top electrode) is in the size of 3–4 mm,and plasma volume is 5 · 10−9 m3.

It is well known [11], [12] that the action of atmosphericpressure discharges on electrolyte results in significant changeof pH and electric conductivities of solutions. The effects

of the discharge action at solution properties for three typesof electrolytes were under study: Na2SO4 with initial pH =6.03 and χ = 1.77 mS/cm; H2SO4 with pH = 2.095 and χ =2.77 mS/cm; NaOH with pH = 11.47 and χ = 1.343 mS/cm.Measurements of pH and electric conductivities were carriedout on an ionomer I-160MI and a conductometer inoLabCond L1. Solution volumes in all experimental series were80 ml. The investigation of decomposition efficiencies of or-ganic compounds by high-voltage glow discharge stabilized bya gas flow was carried out on an example of a model organiccompound—methylene blue dye. The UV-Vis (200–900 nm)spectrum of the solution was scanned by a Bruker OpticsUV-VIS 2000 spectrometer. The dye concentration was mea-sured at its maximum absorbance (663 nm). The molecularabsorption coefficient of methylene blue is 4.89 · 105, and theinitial dye concentration was 0.3 · 10−5 mol · l−1. Typically,4-ml samples were taken from the reactor vessel at 2.5, 5, 10,15, 20, 30, and 40 min during each run. Posteffect of plasmatreatment was measured after the exposition of three days.

III. RESULTS AND DISCUSSION

The discharge generated between two metal electrodes is astrongly nonstationary glow discharge at atmospheric pressure.The use of a ballast resistance in series with the interelectrodegap and the use of a fast air flow prevents the glow dischargefrom transforming to spark or arc [13], [25]. It allows sustainingthe discharge at low input power. The latter is spent basicallyon the formation of active particles in contrast to spark or arcdischarges where a significant part of energy dissipates in gasheating. The discharge between electrodes spaced at 1 mm(Fig. 1) starts at the applied voltage 2.7–3 kV. This valuecorresponds to the voltage necessary for air breakdown−30 kV/cm determined in [14]. A typical voltage waveformof the discharge is shown in Fig. 3.

As the resistance in the plasma channel is considerably lessthan in the gas, the voltage across the discharge gap dropssharply after ignition of the discharge in the pinhole, up to

874 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 37, NO. 6, JUNE 2009

Fig. 3. Voltage waveform across discharge gap (time resolution is 10 ms).

Fig. 4. Current and voltage waveform across discharge gap (time resolutionis 50 μs).

500–600 V. At the moment prior to the ignition of the dischargeshort peaks of the discharge voltage are registered (Figs. 3and 4). Similar voltage peaks have been observed in plasma-solution ac discharges by others researchers [15]. To explainthis effect, additional theoretical and experimental investiga-tions are required.

The existence of the discharge is observed until the appliedvoltage becomes too small to sustain plasma, then extinctionof the discharge is observed. The described process of break-down and plasma formation is repeated fixed number of timesduring every half-cycle of the applied voltage. The averagedischarge lifetime is about 6–8 ms in the voltage range from 2.7to 15 kV. The energy, dissipated during one plasma pulse, isabout 0.03–0.05 J, and the maximum current density throughthe pinhole is 60 A/cm2. The discharge current and appliedvoltage waveforms with a time resolution of 50 μs are shownin Fig. 4. One can see that after breakdown and generation ofplasma, the discharge exists in the form of separate short im-pulses with duration of 15–20 μs. These impulses are repeatedthrough 50–250 μs. The described regime of the discharge canbe explained by the influence of the fast air flow on the plasmain the interelectrode gap, and by the ablation of discharge-generated particles by the air stream.

TABLE IINFLUENCE OF DISCHARGE TREATMENT ON CHEMICAL

PROPERTIES OF SOLUTIONS

The application of HV to the electrode system immersed in agas gives rise to high-energy electrons which, in turn, produceions and free radicals. Depending on the gas mixture compo-sition the following reactions have been suggested widely tooccurring. In these reactions, N∗, O∗, OH∗ radicals are producedin the presence of water vapor [16], [17]

e + N2 → e + N + N (1)

e + N2 → 2e + N+ + N (2)

e + O2 → e + O(3P) + O(3P) (3)

e + O2 → 2e + O + O+ (4)

e + O2 → e + O(3P) + O(1D) (5)

e + H2O → e + H + OH (6)

e + H2O → 2e + OH + H+. (7)

OH∗, N∗, O∗(3P) [18] radicals generated under plasma ac-tion in the discharge zone (reactions 1–7) are quickly carriedaway by the gas flow and take part in secondary chemical reac-tions resulting in the formation of NO∗, O3, and H2O2. Radicalsand ozone are strong oxidizers and can interact directly withorganic compounds, destroying their structure. The primaryactive particle, hydroxyl radical, takes part in dimerizationreactions and interacts with water molecules forming hydrogenperoxide which is also a strong oxidizer. In that way, thedischarge has a chemical effect at the liquid phase that resultsin the change of the solution chemical composition and in theelectrochemical properties. In Table I, data on the effect ofexposition of pure solutions (without dye) to plasma action withvarious initial pH values and conductivities were presented.

The discharge effect at all types of solutions results in afast pH decrease, this can be explained by accumulation ofH2O2 and HNO3 in a liquid phase [24]. Conductivity of neutralsolutions under plasma action increases for the same reasons:accumulation of products of secondary reactions taking placein the solution under the discharge action. In case of NaOHand H2SO4 solutions, the electric conductivity decreases duringplasma exposition. The reason for this is the following: HNO3

interacts with the alkali forming NaNO3 whose conductivitydiffers from the conductivity of the initial compound. In thecase of H2SO4 treatment, a destruction of sulfuric acid underdischarge action can results in a decrease of χ.

From a technological point of view, these results are ofsignificant interest due to the effect of discharge influence on

NIKIFOROV: APPLICATION OF AC GLOW DISCHARGE STABILIZED BY FAST AIR FLOW FOR WATER TREATMENT 875

Fig. 5. UV-Vis spectra of methylene blue solution at different plasma exposi-tion times.

organic water pollutions. Investigation of discharge action ondye solution (methylene blue) was carried out by spectropho-tometry techniques. In the range of UV-Vis light, methyleneblue solution has four absorption peaks at wavelengths of 244,291, 609, and 663 nm, respectively. The strong absorptionbands of the H-chromophore at 609 and 663 nm are responsiblefor the blue color of the solution. The temporal decrease ofthe spectral features during the degradation of methylene bluesolutions is observed at 291, 609, and 663 nm under dischargeplasma influence (see Fig. 5).

Any reaction that leads to the destruction of the chromophorein the methylene blue molecule results in decoloration of thetreated solution. High-energy electrons and active species suchas O3, OH∗, H2O2, etc., selectively react with chemical bondsin dye molecules to form aldehydes, ketones, phenolic residues,and other low molecular mass organic species [19]–[21]. Thus,the absorption bands at 609 and 663 nm decrease rapidlyand, finally, disappear after 40-min treatment time in an acidicmedium. Fifty percent of dye is found to be destroyed after8-min plasma treatment, the energy consumption of the systembeing 3.5 · 10−8 g/W · s. The occurrence of the peak in therange of 245 nm at short treatment times is connected withthe formation of H2O2 in the solution. Hydrogen peroxideis well known to have an absorption peak at 220–250 nm[22], [23]. Observable fast growth of absorption in the UVregion 200–230 nm is explained, first of all, by fast accumu-lation of HNO3 (absorption peak of 200–220 nm [23]) andalso by the formation of dye destruction products absorbingin the same UV region. The analysis of intermediate productsof dye destruction with the development of a kinetic model ofchemical processes taking place in the fluid phase is supposedto be carried out hereafter.

In solutions with initial pH > 6, the same regularities ofpollution destruction are observed as well as in the acid solu-tion. Data on destruction efficiencies with different exposuretime are presented in Table II. One can see that in the caseof Na2SO4 and NaOH, the destruction efficiency of methyleneblue molecules is lower. Such an effect is connected with higherchemical stability of these kinds of dyes in solutions withpH > 6, and the oxidative ability of plasmas is stronger inacidic conditions (e.g., the oxidative potential of the hydroxyl

TABLE IIEFFECTIVENESS OF PLASMA TREATMENT OF METHYLENE BLUE

SOLUTION IN ACID, NEUTRAL, AND ALKALINE SOLUTIONS

radical is 2.70 V at pH 3.0 and 2.34 V at pH 9.0). Results on theposteffect of the discharge treatment are presented in Table IIas well. In general, posteffect process is well known for mostplasma-solution systems [1].

Apparently, it can be explained by a slow oxidizing action ofthe hydrogen peroxide formed under the action of the dischargeon organic molecules.

IV. CONCLUSION

A new type of discharge stabilized by a fast gas flow isdeveloped. The scheme of discharge allows applications foreffective purification of organic impurities in water solutions inthe range of any solution conductivities. Input power requiredfor plasma is 3–6 W. Chemical efficiency of the dischargeaction with a modeling compound—the methylene blue dye isestimated. Time of dye total destruction (solution pH 2.09) isabout 40 min, destruction rate constant is 3.27 · 10−9 mol/l · sand energy consumption is 3.5 · 10−8 g/W · s. Fifty-percentdestruction of the compound under study was found to bereached after only 8 min of plasma exposition.

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Anton Yurievich Nikiforov was born in Perm,Russia, on March 27, 1978. He received the B.E.and M.E. degrees from Ivanovo State University ofChemistry and Technology, Ivanovo, Russia, in 1999and 2001, respectively, and the D.E. degree from theInstitute of Solution Chemistry, Russian Academy ofScience, Ivanovo, in 2004.

During period of 2005–2006, he investigatedplasma-solution systems within the bounds of post-doc project with the Department of Applied Physics,Ghent University, Gent, Belgium. His research in-

terests include atmospheric pressure discharges, underwater plasma for waterpurification and modification of polymer materials. He is currently with theDepartment of Applied Plasma Physics, Ghent University within the boundsof IAP project in the field of the plasma–liquid interface investigations. Since2004, he has also been with the Institute of Solution Chemistry, RussianAcademy of Science, where he is currently a Senior Researcher.